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Terahertz Emission Spectroscopy and Microscopy on Ultrawide Bandgap Semiconductor β-Ga2O3

Terahertz Emission Spectroscopy and Microscopy on Ultrawide Bandgap Semiconductor β-Ga2O3 hv photonics Article Terahertz Emission Spectroscopy and Microscopy on Ultrawide Bandgap Semiconductor -Ga O 2 3 1 1 2 1 1 , 3 Hao Jiang , Chen Gong , Tatsuhiko Nishimura , Hironaru Murakami , Iwao Kawayama , 2 1 , Hidetoshi Nakanishi and Masayoshi Tonouchi * Institute of Laser Engineering, Osaka University, Osaka 565-0871, Japan; jiang-h@ile.osaka-u.ac.jp (H.J.); gong-c@ile.osaka-u.ac.jp (C.G.); hiro@ile.osaka-u.ac.jp (H.M.); kawayama.iwao.3a@kyoto-u.ac.jp (I.K.) SCREEN Holdings CO., Ltd., Kyoto 612-8486, Japan; tat.nishimura@screen.co.jp (T.N.); nakanisi@screen.co.jp (H.N.) Graduate School of Energy Science, Kyoto University, Kyoto 606-8501, Japan * Correspondence: tonouchi@ile.osaka-u.ac.jp; Tel.: +81-6-6879-7981 Received: 18 August 2020; Accepted: 11 September 2020; Published: 14 September 2020 Abstract: Although gallium oxide Ga O is attracting much attention as a next-generation ultrawide 2 3 bandgap semiconductor for various applications, it needs further optical characterization to support its use in higher-performance devices. In the present study, terahertz (THz) emission spectroscopy (TES) and laser THz emission microscopy (LTEM) are applied to Sn-doped, unintentionally doped, and Fe-doped -Ga O wafers. Femtosecond (fs) laser illumination generated THz waves based on 2 3 the time derivative of the photocurrent. TES probes the motion of ultrafast photocarriers that are excited into a conduction band, and LTEM visualizes their local spatiotemporal movement at a spatial and temporal resolution of laser beam diameter and a few hundred fs. In contrast, one observes neither photoluminescence nor distinguishable optical absorption for a band-to-band transition for Ga O . 2 3 TES/LTEM thus provides complementary information on, for example, the local mobility, surface potential, defects, band bending, and anisotropic photo-response in a noncontact, nondestructive manner. The results indicated that the band bends downward at the surface of an Fe-doped wafer, unlike with an n-type wafer, and the THz emission intensity is qualitatively proportional to the product of local electron mobility and di usion potential, and is inversely proportional to penetration depth, all of which have a strong correlation with the quality of the materials and defects/impurities in them. Keywords: terahertz emission spectroscopy; laser terahertz emission microscopy; ultrawide bandgap semiconductor; -Ga O 2 3 1. Introduction Gallium oxide (Ga O ) is an attractive ultrawide-bandgap semiconductor for the use in a variety 2 3 of applications such as gas sensors, high-power electronics, and deep-ultraviolet (UV) photo-detectors [1–3]. In recent years, tremendous efforts have been made to improve material quality and device performance. However, there remain many unsolved problems in the areas of defect reduction, passivation improvement, impurity-doping, device processes, carrier dynamics, etc. Contributions to resolve these issues require a new type of characterization tool, particularly one that operates in a noncontact, nondestructive manner. Terahertz (THz) emission spectroscopy (TES) and laser THz emission microscopy (LTEM) are emerging technologies that can detect ultrafast photocarrier dynamics and responses in materials and devices [4,5] that are a ected by electron mobility, surface potential, defects, and band bending. Photonics 2020, 7, 73; doi:10.3390/photonics7030073 www.mdpi.com/journal/photonics Photonics 2020, 7, 73 2 of 11 Femtosecond (fs) laser illumination to materials generates THz waves due to the charge displacement that occurs when the photons create free carriers, or when the materials have nonlinear optical coecients. In the former case, excitation via acceleration due to the built-in field inside the material or carrier di usion due to a concentration gradient causes excited electrons to translate rapidly and generate transient photocurrents, which are the source of THz radiation. The THz waveforms monitored in the time domain include early-stage carrier dynamics information on a time scale of less than a picosecond. The dynamics-related information provided is di erent from that provided by typical photoluminescence, electroluminescence, and laser-induced photocurrent characterization methods [6]. We have reported several examples in which defect analysis [7], noncontact surface potential estimation [8,9], and nondestructive evaluation of solar cells [10] are demonstrated and shown that TES and LTEM are practical tools for semiconductor research and development. In the present study, we employ TES and LTEM to study local ultrafast photocarrier dynamics in -Ga O , 2 3 which has an ultrawide bandgap of approximately 4.7–4.9 eV [3,11–13]. As discussed later, the wide bandgap semiconductors have narrow depletion layers with large surface potentials, which mean that the THz emission strongly depends on the surface conditions and the dynamic behavior of the excited carriers. Such information is essential to develop high-performance devices. 2. Terahertz Emission from Semiconductor Surfaces Photons with energies larger than the bandgap create electron–hole pairs within the optical penetration depth. The primary mechanism of THz wave generation in standard and widegap semiconductors is attributed to the drift current generated by carrier acceleration from the built-in field, whereas wave generation in narrow bandgap semiconductors is explained by ballistic hot carrier di usion [14,15]. The drift current is defined primarily by the built-in field, carrier mobility, and the number of photocarriers. Here, one can assume that the only electrons contribute to generate the THz waves, and the built-in field is approximated by the field at the surface E . The THz amplitude E Max THz is then written as [16] @J DnDv E / / /  E I (1) THz e Max P @t Dt where J is photocurrent, Dn is the number of the excited carrier, Dv is the carrier velocity after the transient time Dt, which is typically less than 500 fs,  is the electron mobility and I is the number of e P photons injected. When the optical penetration depth  is close to or longer than the depletion layer thickness w, this can be simplified as [16] E /  I , (2) THz e P where  is the di usion potential. The sign changes with the sign of the potential. This formula indicates that TES includes information on the photocarrier mobility and traveling direction, defects close to the surface, and impurities at a resolution similar to that of the laser spot size. Thus, one can interrogate such physics by combining TES/LTEM results with those from other characterization tools such as the Kelvin force microscope (KFM), photoluminescence (PL), Raman spectroscopy, and optical absorption. 3. Samples and Experiments The five types of -Ga O crystals examined here are Sn-doped (010), Sn-doped (201), 2 3 unintentionally doped (UID) (010), UID (201), and Fe-doped (010), which were grown by Tamura Corporation using edge-defined film-fed growth, and are labeled as #SnD-1, #SnD-2, #UID-1, #UID-2, and #FeD, respectively. The UV-vis absorption study and terahertz time domain spectroscopy have been reported in [17,18]. The sample specifications are listed in Table 1, together with other information such as electron mobility, surface potential  , and dielectric constants from the literature [19–23] and S Photonics 2020, 7, x; doi: FOR PEER REVIEW 3 of 11 Figure 1 shows the schematics of typical n-type and semi-insulating Ga2O3 energy band structures. Ga2O3 has various types of defects, such as oxygen vacancies, Ga vacancies, and their complexes. There exist many levels, such as those from self-trapped holes, self-trapped excitons, and shallow donors Sn and Si in n-type samples. These have activation energies from a few meV to several tens of meV [22,24,25]. Although the information on Fe-doped Ga2O3 remains limited, the Fermi energy is pinned at Ec-0.85 eV due to its deep acceptors [26]. However, no clear picture of the energy Photonics 2020, 7, 73 3 of 11 band structure near the surface of Fe-doped β-Ga2O3 crystals has been provided thus far. Therefore, there are two possibilities as depicted in Figure 1b. One is similar to that of an n-type semiconductor, while the other is similar to that of Fe-doped InP. These two possibilities would cause positive and our estimates of depletion layer thickness w, E , and  . It is worth noting that the thickness of Max e negative diffusion potentials, respectively. We examine this question for the present case. The optical the depletion layers is in the order of 100 nm or shorter. penetration depth at a photon energy of 4.8 eV is estimated to be approximately 125 nm based on the Figure 1 shows the schematics of typical n-type and semi-insulating Ga O energy band structures. 2 3 absorption measurement in [27]. Thus, Equation (1) is always valid. Photoluminescence spectra for Ga O has various types of defects, such as oxygen vacancies, Ga vacancies, and their complexes. 2 3 all samples are given in the Supplemental Material. There exist many levels, such as those from self-trapped holes, self-trapped excitons, and shallow donors Sn and Si in n-type samples. These have activation energies from a few meV to several tens Table 1. Sample specifications. of meV [22,24,25]. Although the information on Fe-doped Ga O remains limited, the Fermi energy 2 3 is pinned at Ec-0.85 eV due to its deep acceptors [26]. However, no clear picture of the energy band Thickness n μe w EMax S str am uctur ples e O near rienthe tatiosurface n Dop of an Fe-doped t -Ga O crystals has been provided thus far. Therefore, there 2 3 -3 2 (mm) (cm ) (cm /Vs) (eV) (nm) (MV/cm) (a.u) are two possibilities as depicted in Figure 1b. One is similar to that of an n-type semiconductor, 18 a b #SnD-1 (010) Sn 0.65 6.8 × 10 45 1.63 16 1.9 13.66 while the other is similar to that of Fe-doped InP. These two possibilities would cause positive and 18 a b #SnD-2 (201) Sn 0.65 1.8 × 10 50 1.14 26 0.85 10.61 negative di usion potentials, respectively. We examine this question for the present case. The optical 17 a b #UID-1 (010) UID 0.5 1.4 × 10 80 1.63 113 0.28 24.38 penetration depth at a photon energy of 4.8 eV is estimated to be approximately 125 nm based on the 17 a b #UID-2 (201) UID 0.5 3.8 × 10 60 1.14 57 0.39 12.74 absorption measurement in [27]. Thus, Equation (1) is always valid. Photoluminescence spectra for all #FeD (010) Fe 0.5 - - - - - - samples are given in the Supplemental Material. a b [28], [29]. Figure 1. Typical energy band structures for (a) an n-type and (b) an Fe-doped sample. Figure 1. Typical energy band structures for (a) an n-type and (b) an Fe-doped sample. Table 1. Sample specifications. Figure 2a shows an experimental setup for the THz emission measurements. A Ti:sapphire femtosecond laser (pulse width: 100 fs, center wavelength: 800 nm, repetition rate: 80 MHz  ) is used Thickness n   w E e S Max | | Samples Orientation Dopant L (mm) (cm ) (cm /Vs) (eV) (nm) (MV/cm) as a laser source. The femtosecond laser pulses are divided into pump pulses and trigge (a.u) r pulses by 18 a b #SnD-1 (010) Sn 0.65 6.8 10 45 1.63 16 1.9 13.66 using a beam splitter. To generate THz pulse, the surfaces of the samples are irradiated with the 18 a b #SnD-2 (201) Sn 0.65 1.8 10 50 1.14 26 0.85 10.61 pump pulses at an incidence angle of 45 degrees through an optical lens, and the radiated terahertz 17 a b #UID-1 (010) UID 0.5 1.4 10 80 1.63 113 0.28 24.38 waves are detected at an opposing 45 degrees angle of emission. The samples are mounted on 17 a b #UID-2 UID 0.5 60 57 0.39 12.74 (201) 3.8 10 1.14 computer-controlled x-y stage. The PL detector is placed at 90 degrees of prospect angle to the #FeD (010) Fe 0.5 - - - - - - surface. The schematic drawing of those geometries is given in Figure 2b. The emitted THz waves are a b [28], [29]. collimated and focused onto a detector by a pair of parabolic mirrors. We use a spiral type photoconductive antenna (PCA) fabricated on the low-temperature-grown (LTG) GaAs substrate Figure 2a shows an experimental setup for the THz emission measurements. A Ti:sapphire (Hamamatsu Photonics) to detect the THz waves. THz signals in time-domain are acquired by femtosecond laser (pulse width: 100 fs, center wavelength: 800 nm, repetition rate: 80 MHz) is used as a varying the delay between the pump and trigger pulses. The amplitude of the THz wave emission is laser source. The femtosecond laser pulses are divided into pump pulses and trigger pulses by using a monitored using a lock-in amplifier. By fixing the time delay at the maximum amplitude of the THz beam splitter. To generate THz pulse, the surfaces of the samples are irradiated with the pump pulses at emission and scanning the sample by the pump beam, the THz emission image can be obtained. The an incidence angle of 45 degrees through an optical lens, and the radiated terahertz waves are detected at an opposing 45 degrees angle of emission. The samples are mounted on computer-controlled x-y stage. The PL detector is placed at 90 degrees of prospect angle to the surface. The schematic drawing of those geometries is given in Figure 2b. The emitted THz waves are collimated and focused onto a detector by a pair of parabolic mirrors. We use a spiral type photoconductive antenna (PCA) fabricated on the low-temperature-grown (LTG) GaAs substrate (Hamamatsu Photonics) to detect the THz waves. Photonics 2020, 7, 73 4 of 11 THz signals in time-domain are acquired by varying the delay between the pump and trigger pulses. Photonics 2020, 7, x; doi: FOR PEER REVIEW 4 of 11 The amplitude of the THz wave emission is monitored using a lock-in amplifier. By fixing the time delay at the maximum amplitude of the THz emission and scanning the sample by the pump beam, laser beam diameters for TES and LTEM/PL are 500 mm and 50 mm, respectively. The details are the THz emission image can be obtained. The laser beam diameters for TES and LTEM/PL are 500 mm reported elsewhere [20,30–32]. and 50 mm, respectively. The details are reported elsewhere [20,30–32]. (a) (b) Figure 2. Schematic drawing of (a) the whole experimental setup and (b) the geometrical relationship Figure 2. Schematic drawing of (a) the whole experimental setup and (b) the geometrical relationship among the incident laser beam, photoluminescence (PL) and terahertz (THz) detector positions. among the incident laser beam, photoluminescence (PL) and terahertz (THz) detector positions. 4. Results and Discussion 4. Results and Discussion Figure 3 shows the THz emission properties of -Ga O . We observe THz emissions from all of the Figure 3 shows the THz emission properties of β-Ga2O3. We observe THz emissions from all of 2 3 samples that we examined, although their amplitudes di er substantially. The sign of the amplitude of the samples that we examined, although their amplitudes differ substantially. The sign of the #FeD is opposite those of the doped samples. This indicates that the excited electrons in the doped amplitude of #FeD is opposite those of the doped samples. This indicates that the excited electrons in samples travel inward to the substrates, whereas those in #FeD travel to the surface. This answers the doped samples travel inward to the substrates, whereas those in #FeD travel to the surface. This the first question for the surface di usion potential of the Fe-doped -Ga O by clarifying that the answers the first question for the surface diffusion potential of the Fe-doped β-Ga2O3 by clarifying 2 3 that the conduction band near the surface of #FeD bends downward. After the main pulses, there are certain differences, which is attributed to the charge oscillation near the surfaces related to the capacitance of the depletion layers. In the Fourier spectra, the lower frequency components are enhanced because we used the spiral type PCAs. Since the electromagnetic waves propagate along Photonics 2020, 7, 73 5 of 11 conduction band near the surface of #FeD bends downward. After the main pulses, there are certain di erences, which is attributed to the charge oscillation near the surfaces related to the capacitance of the depletion layers. In the Fourier spectra, the lower frequency components are enhanced because we Photonics 2020, 7, x; doi: FOR PEER REVIEW 5 of 11 used the spiral type PCAs. Since the electromagnetic waves propagate along the edge of the antenna, resulting in the integration of the waveform in time domain [33,34], we cannot discuss the intrinsic the edge of the antenna, resulting in the integration of the waveform in time domain [33,34], we carrier dynamics in the -Ga O with the frequency spectra. 2 3 cannot discuss the intrinsic carrier dynamics in the β-Ga2O3 with the frequency spectra. (a) (b) (c) Figure 3. (a) Time-domain waveforms of THz amplitude (ETHz) excited at 245 nm and corresponding Figure 3. (a) Time-domain waveforms of THz amplitude (E ) excited at 245 nm and corresponding THz Fourier spectra for #UID-1 and #FE at a laser power of 30 mW. (c) The laser power dependence of the Fourier spectra for #UID-1 and #FE at a laser power of 30 mW. (c) The laser power dependence of the intensity intensit of y oEf ETHwith z with fits fitto s to the thdata. e dataThe . The intensity intensity is idefined s defined at athe t thmaximum e maximum point poin of t o the f thwaveforms e waveforms THz at a ar t ound aroun5 dpsec. 5 psec The . Thlines e line in s i(n b ) (b ar ) e ar eye e ey guides. e guides. Sample #FeD is found to emit the strongest THz radiation at high influences. At low fluences, Sample #FeD is found to emit the strongest THz radiation at high influences. At low fluences, its its emission is similar to that of #UID-1. The amplitudes of samples other than #FeD increase with emission is similar to that of #UID-1. The amplitudes of samples other than #FeD increase with saturation. This is often explained by the screening e ect [35]. Since the electrons and holes move in saturation. This is often explained by the screening effect [35]. Since the electrons and holes move in opposite directions in the depletion layer, the built-in field is screened. The fits are obtained using the opposite directions in the depletion layer, the built-in field is screened. The fits are obtained using screening e ect formula, E = (E F)/(F + F ), where E is the amplitude of THz radiation, E is the screening effect formul THaz, 0 = (E0F)/(sat F + Fsat), wheT rH e z is the amplitude of THz radiatio 0n, E0 the THz amplitude at the high fluence limit, F is the optical fluence, and F is the saturation fluence. is the THz amplitude at the high fluence limit, F is the optical fluence, and sat Fsat is the saturation fluence. The estimated values for E0 and Fsat are listed in the Table 2. However, those parameters include no important physical meaning because the emission mechanism is complicated in the case of the semiconductor surface. For instance, #FeD exhibits complex dynamics. On the one hand, the depletion layer of #FeD is thicker than those of the doped samples. This produces a weaker . On Max the other hand, is expected to be high because of the lower free electron scattering rate. Furthermore, the Fe acceptors capture electrons from point defect donors, which may be excited. The Photonics 2020, 7, 73 6 of 11 The estimated values for E and F are listed in the Table 2. However, those parameters include 0 sat no important physical meaning because the emission mechanism is complicated in the case of the semiconductor surface. For instance, #FeD exhibits complex dynamics. On the one hand, the depletion layer of #FeD is thicker than those of the doped samples. This produces a weaker E . On the other Max hand,  is expected to be high because of the lower free electron scattering rate. Furthermore, the Fe Photonics 2020, 7, x; doi: FOR PEER REVIEW 6 of 11 acceptors capture electrons from point defect donors, which may be excited. The excited electrons are blocked at the surface, while those in the doped materials can travel inward. All of these issues excited electrons are blocked at the surface, while those in the doped materials can travel inward. All a ect the dynamic screening e ect and should be examined via more precise models and time-domain of these issues affect the dynamic screening effect and should be examined via more precise models simulations with a Monte Carlo simulation considering real parameters. and time-domain simulations with a Monte Carlo simulation considering real parameters. Table 2. Estimated parameters at an excitation wavelength of 245 nm. Table 2. Estimated parameters at an excitation wavelength of 245 nm. Samples #SnD-1 #SnD-2 #UID-1 #UID-2 #FeD Samples #SnD-1 #SnD-2 #UID-1 #UID-2 #FeD E 0.69 0.18 1.60 0.16 36.0 E0 0.69 0.18 1.60 0.16 36.0 F 0.28 0.07 0.12 0.06 3.5 sat Fsat 0.28 0.07 0.12 0.06 3.5 The wavelength dependences of the waveforms are depicted in Figure 4. As the photon energy The wavelength dependences of the waveforms are depicted in Figure 4. As the photon energy crossing the bandgap increases, one can clearly see THz emission enhancement. TES thus discloses crossing the bandgap increases, one can clearly see THz emission enhancement. TES thus discloses the the ultrafast nature of the photocarriers excited from the valence band to the conduction band. Note ultrafast nature of the photocarriers excited from the valence band to the conduction band. Note that that the PL and optical absorption measurements merely show the direct band-to-band transitions. the PL and optical absorption measurements merely show the direct band-to-band transitions. The PL The PL results are given in Figure S1 in the Supplementary Material. Details of these dynamics are results are given in Figure S1 in the Supplementary Material. Details of these dynamics are discussed in discussed in later sections. Although weak emission is observed below the bandgap, this is explained later sections. Although weak emission is observed below the bandgap, this is explained by wavelength by wavelength broadening due to the short pulse width of the fs laser. broadening due to the short pulse width of the fs laser. Figure 4. THz emission waveforms of (a) #UID-1 and (b) #FeD at various wavelengths. Figure 4. THz emission waveforms of (a) #UID-1 and (b) #FeD at various wavelengths. The maximum intensities of E for #UID-1 and #FeD are plotted at the measured three The maximum intensities of THz for #UID-1 and #FeD are plotted at the measured three THz wavelengths in Figure 5. Here to avoid the strong screening e ect, the amplitudes measured at a laser wavelengths in Figure 5. Here to avoid the strong screening effect, the amplitudes measured at a laser fluence of 0.04 mJ/cm are used. The intensity increases rapidly with decreasing wavelength. The fluence of 0.04 mJ/cm are used. The intensity increases rapidly with decreasing wavelength. The transition is explained by the broadening of the fs optical pulses. The laser pulse, in general, is regarded transition is explained by the broadening of the fs optical pulses. The laser pulse, in general, is to have a shape of hyperbolic secant function, and wavelength ()-dependent laser intensity I () is regarded to have a shape of hyperbolic secant function, and wavelength ( )-dependent laser inP tensity expressed by ( ) is expressed by I () / sech (A(  )), (3) P P ( ) ( ) ∝ sech − , (3) P P where  is the center wavelength of the laser, and A is a fitting parameter corresponding to the pulse where is the center wavelength of the laser, and is a fitting parameter corresponding to the width. Thus, the intensity of E is expressed by THz pulse width. Thus, the intensity of is expressed by THz | | ∝ sech 2 ( − ) , (4) THz P P jE j / I sech (A(  ))d, (4) THz P P where is the bandgap wavelength of the laser. The broken line in Figure 4 is the fit to the | | experimental data of with of 0.25 assuming the wavelength width of 7 nm at a half maximum THz for of 257 nm, which corresponds to an energy of 4.82 eV. The value agrees with the β-Ga2O3 bandgap energy. The fit quantitatively explains that the THz emission amplitudes are defined by the number of the photocarriers excited into the conduction bands, i.e., THz waves are emitted by the photocarrier excitation. In other words, for the present case, the THz emission spectroscopy is a direct local measure of the Ga2O3 bandgap for the surface layer within a thickness of about 100 nm. Photonics 2020, 7, 73 7 of 11 where  is the bandgap wavelength of the laser. The broken line in Figure 4 is the fit to the experimental data of jE j with A of 0.25 assuming the wavelength width of 7 nm at a half maximum for  of THz 257 nm, which corresponds to an energy of 4.82 eV. The value agrees with the -Ga O bandgap 2 3 energy. The fit quantitatively explains that the THz emission amplitudes are defined by the number of the photocarriers excited into the conduction bands, i.e., THz waves are emitted by the photocarrier excitation. In other words, for the present case, the THz emission spectroscopy is a direct local measure Photonics 2020, 7, x; doi: FOR PEER REVIEW 7 of 11 of the Ga O bandgap for the surface layer within a thickness of about 100 nm. 2 3 0.6 0.5 0.4 0.3 0.2 0.1 230 240 250 260 270 280 Wavelength (nm) Figure 5. THz emission intensities at di erent wavelengths. Closed circles and triangles are for #UID-1 Figure 5. THz emission intensities at different wavelengths. Closed circles and triangles are for #UID- and #FeD, respectively. The dashed line is the calculated fit by Equation (3). 1 and #FeD, respectively. The dashed line is the calculated fit by Equation (3). The anisotropic bandgap of (010) -Ga O has been reported by observing the anisotropic optical 2 3 The anisotropic bandgap of (010) β-Ga2O3 has been reported by observing the anisotropic optical absorption with the polarization of the laser [17]. Thus it is expected that the THz emission amplitude absorption with the polarization of the laser [17]. Thus it is expected that the THz emission amplitude depends on the polarization of the fs laser. Figure 6 gives the optical polarization angle dependence of depends on the polarization of the fs laser. Figure 6 gives the optical polarization angle dependence the emission amplitude. We observed the sinusoidal modulation of the THz emission amplitudes. Here, of the emission amplitude. We observed the sinusoidal modulation of the THz emission amplitudes. we rotate the pump polarization angle from 0 to 360 (0 and 90 correspond to p- and s-polarization, Here, we rotate the pump polarization angle α from 0° to 360° (0° and 90° correspond to p- and s- respectively) using a half-wave plate instead of rotating the sample to keep the THz E field always polarization, respectively) using a half-wave plate instead of rotating the sample to keep the THz E aligned in the same direction to the detector. Note that the spiral PCA has a strong anisotropic response field always aligned in the same direction to the detector. Note that the spiral PCA has a strong function to the THz E-field. Typically, this modulation is explained by nonlinear THz generation anisotropic response function to the THz E-field. Typically, this modulation is explained by nonlinear through optical rectification (OR) based on the second-order nonlinear optical susceptibility, which is THz generation through optical rectification (OR) based on the second-order nonlinear optical observed in many semiconductors [36]. However, (010) -Ga O has the centrosymmetric plain [3], and 2 3 susceptibility, which is observed in many semiconductors [36]. However, (010) β-Ga2O3 has the no strong THz generation due to the second order nonlinear susceptibility is expected [37]. In addition. centrosymmetric plain [3], and no strong THz generation due to the second order nonlinear A small wavelength change on the order of a few nm strongly modulates the THz amplitude. Thus, susceptibility is expected [37]. In addition. A small wavelength change on the order of a few nm one can conclude that TES can measure the anisotropic bandgap of -Ga O directly. 2 3 strongly modulates the THz amplitude. Thus, one can conclude that TES can measure the anisotropic The built-in surface field of doped Ga O is much larger than those of conventional semiconductors. 2 3 bandgap of β-Ga2O3 directly. The field E is roughly estimated by dividing  by the depletion layer thickness w, which is Max D defined by, 2" " r 0 D w = (5) eN where  of a doped semiconductor can be approximated by the surface potential measured values via KFM. The values of w are typically about 100 nm or less, which results in built-in fields that are much stronger than those of the conventional semiconductors [38]. As given in Equation (2), the emission amplitude is proportional to  . All these parameters are intricately related to each other with the surface states and impurities near the surfaces. The parameters discussed above are listed in Table 1. |E | (a. u.) THz Photonics 2020, 7, 73 8 of 11 They suggest that #UID-1 has the strongest emission of the doped Ga O samples. The values agree 2 3 quantitatively with the intensities observed in Figure 2. The results indicate that TES is a useful tool for nondestructive, noncontact analysis of the local electron mobility, surface potential, defects, etc. Photonics 2020, 7, x; doi: FOR PEER REVIEW 8 of 11 Figure 6. Pump polarization angle dependence of the THz radiation amplitude for Fe-doped (010) at Figure 6. Pump polarization angle dependence of the THz radiation amplitude for Fe-doped (010) at various pump powers and wavelengths. (a) illustrates the schematic configuration of the various pump powers and wavelengths. (a) illustrates the schematic configuration of the measurements. measurements. The angle starts from the parallel to the [102] direction. (b) and (c) are measured at a The angle starts from the parallel to the [102] direction. (b) and (c) are measured at a wavelength of wavelength of 245 nm and 260 nm, respectively. 245 nm and 260 nm, respectively. The built-in surface field of doped Ga2O3 is much larger than those of conventional Finally, we evaluate the #UID-1 wafer by LTEM. With a beam diameter of 50 m at an fs laser semiconductors. The field is roughly estimated by dividing by the depletion layer power of 50 mW, we scanned the laser beam on #UID-1 for a 5 mm 5 mm area. The LTEM and PL Max thickness , which is defined by, images are given in Figure 7a and 7b, respectively. The PL image was obtained at an fs laser power of 20 mW at a wavelength of 365 nm, which corresponds to the UV luminescence as described in Figure 1a [39]. Both images show a similar intensity distribution. The distribution is possibly attributed = (5) to the sample holder tilt. The LTEM image of the normalized intensity divided by the PL intensity is shown in Figure 7c. This suggests that the LTEM image corresponds to almost the self-trapped hole where of a doped semiconductor can be approximated by the surface potential measured values distribution. A faint feature is also seen in the LTEM image, which might be caused by the surface via KFM. The values of are typically about 100 nm or less, which results in built-in fields that are scratch marks. much stronger than those of the conventional semiconductors [38]. As given in Equation (2), the emission amplitude is proportional to . All these parameters are intricately related to each other with the surface states and impurities near the surfaces. The parameters discussed above are listed in Table 1. They suggest that #UID-1 has the strongest emission of the doped Ga2O3 samples. The values agree quantitatively with the intensities observed in Figure 2. The results indicate that TES is a useful tool for nondestructive, noncontact analysis of the local electron mobility, surface potential, defects, etc. Finally, we evaluate the #UID-1 wafer by LTEM. With a beam diameter of 50 µm at an fs laser power of 50 mW, we scanned the laser beam on #UID-1 for a 5 mm × 5 mm area. The LTEM and PL images are given in Figure 7a and 7b, respectively. The PL image was obtained at an fs laser power of 20 mW at a wavelength of 365 nm, which corresponds to the UV luminescence as described in Figure 1a [39]. Both images show a similar intensity distribution. The distribution is possibly attributed to the sample holder tilt. The LTEM image of the normalized intensity divided by the PL intensity is shown in Figure 7c. This suggests that the LTEM image corresponds to almost the self- trapped hole distribution. A faint feature is also seen in the LTEM image, which might be caused by the surface scratch marks. Photonics 2020, 7, 73 9 of 11 Photonics 2020, 7, x; doi: FOR PEER REVIEW 9 of 11 Figure 7. (a) Laser THz emission microscopy (LTEM) image and (b) PL image. (c) is the distribution Figure 7. (a) Laser THz emission microscopy (LTEM) image and (b) PL image. (c) is the distribution of of the LTEM amplitude divided by the PL intensity. the LTEM amplitude divided by the PL intensity. 5. Conclusions 5. Conclusions In summary, TES and LTEM have been applied to -Ga O . The polarity change in the THz In summary, TES and LTEM have been applied to β-Ga2 2O3 3. The polarity change in the THz amplitude revealed that the band near the surface of Fe-doped -Ga O bends downward, unlike amplitude revealed that the band near the surface of Fe-doped β-Ga2 2O33 bends downward, unlike similar bands in n-type materials. It was also found that the emission intensity from the n-type -Ga O similar bands in n-type materials. It was also found that the emission intensity from the n-type2 β- 3 samples was proportional to  . The wavelength and polarization dependences of the THz emission Ga2O3 samples was proporti oe nal to . The wavelength and polarization dependences of the also confirmed that anisotropic optical excitation from the valence to the conduction band played an THz emission also confirmed that anisotropic optical excitation from the valence to the conduction essential role in THz emission. It is also shown that the LTEM visualizes the self-trapped hole and a faint band played an essential role in THz emission. It is also shown that the LTEM visualizes the self- surface potential distribution. These results have proven that TES and LTEM provide local information trapped hole and a faint surface potential distribution. These results have proven that TES and LTEM on the mobility, surface potential, defects, and anisotropic photoresponse within the diameter of the fs provide local information on the mobility, surface potential, defects, and anisotropic photoresponse laser beam in a nondestructive, noncontact manner. PL and UV-vis sometimes provide less information within the diameter of the fs laser beam in a nondestructive, noncontact manner. PL and UV-vis on band-to-band transitions [40,41]. Thus, one can probe local ultrafast photocarrier dynamics by sometimes provide less information on band-to-band transitions [40,41]. Thus, one can probe local combining TES/LTEM with other characterization techniques such as KFM, nano-Raman spectroscopy, ultrafast photocarrier dynamics by combining TES/LTEM with other characterization techniques nano-PL, and pump-and-probe measurements [42]. This is essential to semiconductor research and such as KFM, nano-Raman spectroscopy, nano-PL, and pump-and-probe measurements [42]. This is development, especially with regard to wide bandgap semiconductors [43]. essential to semiconductor research and development, especially with regard to wide bandgap semiconductors [43]. Supplementary Materials: The following are available online at http://www.mdpi.com/2304-6732/7/3/73/s1, Figure S1: Photoluminescence of each sample. Supplementary Materials: The following are available online at www.mdpi.com/xxx/s1, Figure S1: Author Contributions: I.K. and M.T. conceptualized the work; H.J., C.G., and T.N. carried out the terahertz Photoluminescence of each sample. measurements; all authors (H.J., C.G., T.N., H.M., I.K., H.N., M.T.) discussed the results; H.J. and M.T. analyzed A data utho and r Cwr ontr ote ibthe utio paper; ns: I.KI.K. . anand d MH.N. .T. cocommented nceptualized on th the e w manuscript. ork; H.J., C.All G., authors and T.N have . carr riead ed o and ut tagr he eed terah to er the tz published version of the manuscript. measurements; all authors (H.J., C.G., T.N., H.M., I.K., H.N., M.T.) discussed the results; H.J. and M.T. analyzed data and wrote the paper; I.K. and H.N. commented on the manuscript. All authors have read and agreed to the Funding: This work was partially supported by JSPS KAKENHI, Grant Numbers JP18KK0140, and JP18K18861. published version of the manuscript. Conflicts of Interest: The authors declare no conflict of interest. Funding: This work was partially supported by JSPS KAKENHI, Grant Numbers JP18KK0140, and JP18K18861. Conflicts of Interest: The authors declare no conflict of interest. Photonics 2020, 7, 73 10 of 11 References 1. Ahmadi, S.E.; Oshima, Y. Materials issues and devices of -and -Ga O . J. Appl. Phys. 2019, 126, 160901. 2 3 [CrossRef] 2. Pearton, S.J.; Ren, F.; Tadjer, M.; Kim, J. Perspective: Ga O for ultra-high power rectifiers and MOSFETS. 2 3 J. Appl. Phys. 2018, 124, 220901. [CrossRef] 3. Higashiwaki, M.; Sasaki, K.; Murakami, H.; Kumagai, Y.; Koukitu, A.; Kuramata, A.; Masui, T.; Yamakoshi, S. Recent progress in Ga O power devices. Semicond. Sci. Technol. 2016, 31, 034001. [CrossRef] 2 3 4. Rana, D.S.; Tonouchi, M. 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Ma, N.; Tanen, N.; Verma, A.; Guo, Z.; Luo, T.; Xing, H.; Jena, D. Intrinsic electron mobility limits in -Ga O . 2 3 Appl. Phys. Lett. 2016, 109, 212101. [CrossRef] © 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Photonics Multidisciplinary Digital Publishing Institute

Terahertz Emission Spectroscopy and Microscopy on Ultrawide Bandgap Semiconductor β-Ga2O3

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Abstract

hv photonics Article Terahertz Emission Spectroscopy and Microscopy on Ultrawide Bandgap Semiconductor -Ga O 2 3 1 1 2 1 1 , 3 Hao Jiang , Chen Gong , Tatsuhiko Nishimura , Hironaru Murakami , Iwao Kawayama , 2 1 , Hidetoshi Nakanishi and Masayoshi Tonouchi * Institute of Laser Engineering, Osaka University, Osaka 565-0871, Japan; jiang-h@ile.osaka-u.ac.jp (H.J.); gong-c@ile.osaka-u.ac.jp (C.G.); hiro@ile.osaka-u.ac.jp (H.M.); kawayama.iwao.3a@kyoto-u.ac.jp (I.K.) SCREEN Holdings CO., Ltd., Kyoto 612-8486, Japan; tat.nishimura@screen.co.jp (T.N.); nakanisi@screen.co.jp (H.N.) Graduate School of Energy Science, Kyoto University, Kyoto 606-8501, Japan * Correspondence: tonouchi@ile.osaka-u.ac.jp; Tel.: +81-6-6879-7981 Received: 18 August 2020; Accepted: 11 September 2020; Published: 14 September 2020 Abstract: Although gallium oxide Ga O is attracting much attention as a next-generation ultrawide 2 3 bandgap semiconductor for various applications, it needs further optical characterization to support its use in higher-performance devices. In the present study, terahertz (THz) emission spectroscopy (TES) and laser THz emission microscopy (LTEM) are applied to Sn-doped, unintentionally doped, and Fe-doped -Ga O wafers. Femtosecond (fs) laser illumination generated THz waves based on 2 3 the time derivative of the photocurrent. TES probes the motion of ultrafast photocarriers that are excited into a conduction band, and LTEM visualizes their local spatiotemporal movement at a spatial and temporal resolution of laser beam diameter and a few hundred fs. In contrast, one observes neither photoluminescence nor distinguishable optical absorption for a band-to-band transition for Ga O . 2 3 TES/LTEM thus provides complementary information on, for example, the local mobility, surface potential, defects, band bending, and anisotropic photo-response in a noncontact, nondestructive manner. The results indicated that the band bends downward at the surface of an Fe-doped wafer, unlike with an n-type wafer, and the THz emission intensity is qualitatively proportional to the product of local electron mobility and di usion potential, and is inversely proportional to penetration depth, all of which have a strong correlation with the quality of the materials and defects/impurities in them. Keywords: terahertz emission spectroscopy; laser terahertz emission microscopy; ultrawide bandgap semiconductor; -Ga O 2 3 1. Introduction Gallium oxide (Ga O ) is an attractive ultrawide-bandgap semiconductor for the use in a variety 2 3 of applications such as gas sensors, high-power electronics, and deep-ultraviolet (UV) photo-detectors [1–3]. In recent years, tremendous efforts have been made to improve material quality and device performance. However, there remain many unsolved problems in the areas of defect reduction, passivation improvement, impurity-doping, device processes, carrier dynamics, etc. Contributions to resolve these issues require a new type of characterization tool, particularly one that operates in a noncontact, nondestructive manner. Terahertz (THz) emission spectroscopy (TES) and laser THz emission microscopy (LTEM) are emerging technologies that can detect ultrafast photocarrier dynamics and responses in materials and devices [4,5] that are a ected by electron mobility, surface potential, defects, and band bending. Photonics 2020, 7, 73; doi:10.3390/photonics7030073 www.mdpi.com/journal/photonics Photonics 2020, 7, 73 2 of 11 Femtosecond (fs) laser illumination to materials generates THz waves due to the charge displacement that occurs when the photons create free carriers, or when the materials have nonlinear optical coecients. In the former case, excitation via acceleration due to the built-in field inside the material or carrier di usion due to a concentration gradient causes excited electrons to translate rapidly and generate transient photocurrents, which are the source of THz radiation. The THz waveforms monitored in the time domain include early-stage carrier dynamics information on a time scale of less than a picosecond. The dynamics-related information provided is di erent from that provided by typical photoluminescence, electroluminescence, and laser-induced photocurrent characterization methods [6]. We have reported several examples in which defect analysis [7], noncontact surface potential estimation [8,9], and nondestructive evaluation of solar cells [10] are demonstrated and shown that TES and LTEM are practical tools for semiconductor research and development. In the present study, we employ TES and LTEM to study local ultrafast photocarrier dynamics in -Ga O , 2 3 which has an ultrawide bandgap of approximately 4.7–4.9 eV [3,11–13]. As discussed later, the wide bandgap semiconductors have narrow depletion layers with large surface potentials, which mean that the THz emission strongly depends on the surface conditions and the dynamic behavior of the excited carriers. Such information is essential to develop high-performance devices. 2. Terahertz Emission from Semiconductor Surfaces Photons with energies larger than the bandgap create electron–hole pairs within the optical penetration depth. The primary mechanism of THz wave generation in standard and widegap semiconductors is attributed to the drift current generated by carrier acceleration from the built-in field, whereas wave generation in narrow bandgap semiconductors is explained by ballistic hot carrier di usion [14,15]. The drift current is defined primarily by the built-in field, carrier mobility, and the number of photocarriers. Here, one can assume that the only electrons contribute to generate the THz waves, and the built-in field is approximated by the field at the surface E . The THz amplitude E Max THz is then written as [16] @J DnDv E / / /  E I (1) THz e Max P @t Dt where J is photocurrent, Dn is the number of the excited carrier, Dv is the carrier velocity after the transient time Dt, which is typically less than 500 fs,  is the electron mobility and I is the number of e P photons injected. When the optical penetration depth  is close to or longer than the depletion layer thickness w, this can be simplified as [16] E /  I , (2) THz e P where  is the di usion potential. The sign changes with the sign of the potential. This formula indicates that TES includes information on the photocarrier mobility and traveling direction, defects close to the surface, and impurities at a resolution similar to that of the laser spot size. Thus, one can interrogate such physics by combining TES/LTEM results with those from other characterization tools such as the Kelvin force microscope (KFM), photoluminescence (PL), Raman spectroscopy, and optical absorption. 3. Samples and Experiments The five types of -Ga O crystals examined here are Sn-doped (010), Sn-doped (201), 2 3 unintentionally doped (UID) (010), UID (201), and Fe-doped (010), which were grown by Tamura Corporation using edge-defined film-fed growth, and are labeled as #SnD-1, #SnD-2, #UID-1, #UID-2, and #FeD, respectively. The UV-vis absorption study and terahertz time domain spectroscopy have been reported in [17,18]. The sample specifications are listed in Table 1, together with other information such as electron mobility, surface potential  , and dielectric constants from the literature [19–23] and S Photonics 2020, 7, x; doi: FOR PEER REVIEW 3 of 11 Figure 1 shows the schematics of typical n-type and semi-insulating Ga2O3 energy band structures. Ga2O3 has various types of defects, such as oxygen vacancies, Ga vacancies, and their complexes. There exist many levels, such as those from self-trapped holes, self-trapped excitons, and shallow donors Sn and Si in n-type samples. These have activation energies from a few meV to several tens of meV [22,24,25]. Although the information on Fe-doped Ga2O3 remains limited, the Fermi energy is pinned at Ec-0.85 eV due to its deep acceptors [26]. However, no clear picture of the energy Photonics 2020, 7, 73 3 of 11 band structure near the surface of Fe-doped β-Ga2O3 crystals has been provided thus far. Therefore, there are two possibilities as depicted in Figure 1b. One is similar to that of an n-type semiconductor, while the other is similar to that of Fe-doped InP. These two possibilities would cause positive and our estimates of depletion layer thickness w, E , and  . It is worth noting that the thickness of Max e negative diffusion potentials, respectively. We examine this question for the present case. The optical the depletion layers is in the order of 100 nm or shorter. penetration depth at a photon energy of 4.8 eV is estimated to be approximately 125 nm based on the Figure 1 shows the schematics of typical n-type and semi-insulating Ga O energy band structures. 2 3 absorption measurement in [27]. Thus, Equation (1) is always valid. Photoluminescence spectra for Ga O has various types of defects, such as oxygen vacancies, Ga vacancies, and their complexes. 2 3 all samples are given in the Supplemental Material. There exist many levels, such as those from self-trapped holes, self-trapped excitons, and shallow donors Sn and Si in n-type samples. These have activation energies from a few meV to several tens Table 1. Sample specifications. of meV [22,24,25]. Although the information on Fe-doped Ga O remains limited, the Fermi energy 2 3 is pinned at Ec-0.85 eV due to its deep acceptors [26]. However, no clear picture of the energy band Thickness n μe w EMax S str am uctur ples e O near rienthe tatiosurface n Dop of an Fe-doped t -Ga O crystals has been provided thus far. Therefore, there 2 3 -3 2 (mm) (cm ) (cm /Vs) (eV) (nm) (MV/cm) (a.u) are two possibilities as depicted in Figure 1b. One is similar to that of an n-type semiconductor, 18 a b #SnD-1 (010) Sn 0.65 6.8 × 10 45 1.63 16 1.9 13.66 while the other is similar to that of Fe-doped InP. These two possibilities would cause positive and 18 a b #SnD-2 (201) Sn 0.65 1.8 × 10 50 1.14 26 0.85 10.61 negative di usion potentials, respectively. We examine this question for the present case. The optical 17 a b #UID-1 (010) UID 0.5 1.4 × 10 80 1.63 113 0.28 24.38 penetration depth at a photon energy of 4.8 eV is estimated to be approximately 125 nm based on the 17 a b #UID-2 (201) UID 0.5 3.8 × 10 60 1.14 57 0.39 12.74 absorption measurement in [27]. Thus, Equation (1) is always valid. Photoluminescence spectra for all #FeD (010) Fe 0.5 - - - - - - samples are given in the Supplemental Material. a b [28], [29]. Figure 1. Typical energy band structures for (a) an n-type and (b) an Fe-doped sample. Figure 1. Typical energy band structures for (a) an n-type and (b) an Fe-doped sample. Table 1. Sample specifications. Figure 2a shows an experimental setup for the THz emission measurements. A Ti:sapphire femtosecond laser (pulse width: 100 fs, center wavelength: 800 nm, repetition rate: 80 MHz  ) is used Thickness n   w E e S Max | | Samples Orientation Dopant L (mm) (cm ) (cm /Vs) (eV) (nm) (MV/cm) as a laser source. The femtosecond laser pulses are divided into pump pulses and trigge (a.u) r pulses by 18 a b #SnD-1 (010) Sn 0.65 6.8 10 45 1.63 16 1.9 13.66 using a beam splitter. To generate THz pulse, the surfaces of the samples are irradiated with the 18 a b #SnD-2 (201) Sn 0.65 1.8 10 50 1.14 26 0.85 10.61 pump pulses at an incidence angle of 45 degrees through an optical lens, and the radiated terahertz 17 a b #UID-1 (010) UID 0.5 1.4 10 80 1.63 113 0.28 24.38 waves are detected at an opposing 45 degrees angle of emission. The samples are mounted on 17 a b #UID-2 UID 0.5 60 57 0.39 12.74 (201) 3.8 10 1.14 computer-controlled x-y stage. The PL detector is placed at 90 degrees of prospect angle to the #FeD (010) Fe 0.5 - - - - - - surface. The schematic drawing of those geometries is given in Figure 2b. The emitted THz waves are a b [28], [29]. collimated and focused onto a detector by a pair of parabolic mirrors. We use a spiral type photoconductive antenna (PCA) fabricated on the low-temperature-grown (LTG) GaAs substrate Figure 2a shows an experimental setup for the THz emission measurements. A Ti:sapphire (Hamamatsu Photonics) to detect the THz waves. THz signals in time-domain are acquired by femtosecond laser (pulse width: 100 fs, center wavelength: 800 nm, repetition rate: 80 MHz) is used as a varying the delay between the pump and trigger pulses. The amplitude of the THz wave emission is laser source. The femtosecond laser pulses are divided into pump pulses and trigger pulses by using a monitored using a lock-in amplifier. By fixing the time delay at the maximum amplitude of the THz beam splitter. To generate THz pulse, the surfaces of the samples are irradiated with the pump pulses at emission and scanning the sample by the pump beam, the THz emission image can be obtained. The an incidence angle of 45 degrees through an optical lens, and the radiated terahertz waves are detected at an opposing 45 degrees angle of emission. The samples are mounted on computer-controlled x-y stage. The PL detector is placed at 90 degrees of prospect angle to the surface. The schematic drawing of those geometries is given in Figure 2b. The emitted THz waves are collimated and focused onto a detector by a pair of parabolic mirrors. We use a spiral type photoconductive antenna (PCA) fabricated on the low-temperature-grown (LTG) GaAs substrate (Hamamatsu Photonics) to detect the THz waves. Photonics 2020, 7, 73 4 of 11 THz signals in time-domain are acquired by varying the delay between the pump and trigger pulses. Photonics 2020, 7, x; doi: FOR PEER REVIEW 4 of 11 The amplitude of the THz wave emission is monitored using a lock-in amplifier. By fixing the time delay at the maximum amplitude of the THz emission and scanning the sample by the pump beam, laser beam diameters for TES and LTEM/PL are 500 mm and 50 mm, respectively. The details are the THz emission image can be obtained. The laser beam diameters for TES and LTEM/PL are 500 mm reported elsewhere [20,30–32]. and 50 mm, respectively. The details are reported elsewhere [20,30–32]. (a) (b) Figure 2. Schematic drawing of (a) the whole experimental setup and (b) the geometrical relationship Figure 2. Schematic drawing of (a) the whole experimental setup and (b) the geometrical relationship among the incident laser beam, photoluminescence (PL) and terahertz (THz) detector positions. among the incident laser beam, photoluminescence (PL) and terahertz (THz) detector positions. 4. Results and Discussion 4. Results and Discussion Figure 3 shows the THz emission properties of -Ga O . We observe THz emissions from all of the Figure 3 shows the THz emission properties of β-Ga2O3. We observe THz emissions from all of 2 3 samples that we examined, although their amplitudes di er substantially. The sign of the amplitude of the samples that we examined, although their amplitudes differ substantially. The sign of the #FeD is opposite those of the doped samples. This indicates that the excited electrons in the doped amplitude of #FeD is opposite those of the doped samples. This indicates that the excited electrons in samples travel inward to the substrates, whereas those in #FeD travel to the surface. This answers the doped samples travel inward to the substrates, whereas those in #FeD travel to the surface. This the first question for the surface di usion potential of the Fe-doped -Ga O by clarifying that the answers the first question for the surface diffusion potential of the Fe-doped β-Ga2O3 by clarifying 2 3 that the conduction band near the surface of #FeD bends downward. After the main pulses, there are certain differences, which is attributed to the charge oscillation near the surfaces related to the capacitance of the depletion layers. In the Fourier spectra, the lower frequency components are enhanced because we used the spiral type PCAs. Since the electromagnetic waves propagate along Photonics 2020, 7, 73 5 of 11 conduction band near the surface of #FeD bends downward. After the main pulses, there are certain di erences, which is attributed to the charge oscillation near the surfaces related to the capacitance of the depletion layers. In the Fourier spectra, the lower frequency components are enhanced because we Photonics 2020, 7, x; doi: FOR PEER REVIEW 5 of 11 used the spiral type PCAs. Since the electromagnetic waves propagate along the edge of the antenna, resulting in the integration of the waveform in time domain [33,34], we cannot discuss the intrinsic the edge of the antenna, resulting in the integration of the waveform in time domain [33,34], we carrier dynamics in the -Ga O with the frequency spectra. 2 3 cannot discuss the intrinsic carrier dynamics in the β-Ga2O3 with the frequency spectra. (a) (b) (c) Figure 3. (a) Time-domain waveforms of THz amplitude (ETHz) excited at 245 nm and corresponding Figure 3. (a) Time-domain waveforms of THz amplitude (E ) excited at 245 nm and corresponding THz Fourier spectra for #UID-1 and #FE at a laser power of 30 mW. (c) The laser power dependence of the Fourier spectra for #UID-1 and #FE at a laser power of 30 mW. (c) The laser power dependence of the intensity intensit of y oEf ETHwith z with fits fitto s to the thdata. e dataThe . The intensity intensity is idefined s defined at athe t thmaximum e maximum point poin of t o the f thwaveforms e waveforms THz at a ar t ound aroun5 dpsec. 5 psec The . Thlines e line in s i(n b ) (b ar ) e ar eye e ey guides. e guides. Sample #FeD is found to emit the strongest THz radiation at high influences. At low fluences, Sample #FeD is found to emit the strongest THz radiation at high influences. At low fluences, its its emission is similar to that of #UID-1. The amplitudes of samples other than #FeD increase with emission is similar to that of #UID-1. The amplitudes of samples other than #FeD increase with saturation. This is often explained by the screening e ect [35]. Since the electrons and holes move in saturation. This is often explained by the screening effect [35]. Since the electrons and holes move in opposite directions in the depletion layer, the built-in field is screened. The fits are obtained using the opposite directions in the depletion layer, the built-in field is screened. The fits are obtained using screening e ect formula, E = (E F)/(F + F ), where E is the amplitude of THz radiation, E is the screening effect formul THaz, 0 = (E0F)/(sat F + Fsat), wheT rH e z is the amplitude of THz radiatio 0n, E0 the THz amplitude at the high fluence limit, F is the optical fluence, and F is the saturation fluence. is the THz amplitude at the high fluence limit, F is the optical fluence, and sat Fsat is the saturation fluence. The estimated values for E0 and Fsat are listed in the Table 2. However, those parameters include no important physical meaning because the emission mechanism is complicated in the case of the semiconductor surface. For instance, #FeD exhibits complex dynamics. On the one hand, the depletion layer of #FeD is thicker than those of the doped samples. This produces a weaker . On Max the other hand, is expected to be high because of the lower free electron scattering rate. Furthermore, the Fe acceptors capture electrons from point defect donors, which may be excited. The Photonics 2020, 7, 73 6 of 11 The estimated values for E and F are listed in the Table 2. However, those parameters include 0 sat no important physical meaning because the emission mechanism is complicated in the case of the semiconductor surface. For instance, #FeD exhibits complex dynamics. On the one hand, the depletion layer of #FeD is thicker than those of the doped samples. This produces a weaker E . On the other Max hand,  is expected to be high because of the lower free electron scattering rate. Furthermore, the Fe Photonics 2020, 7, x; doi: FOR PEER REVIEW 6 of 11 acceptors capture electrons from point defect donors, which may be excited. The excited electrons are blocked at the surface, while those in the doped materials can travel inward. All of these issues excited electrons are blocked at the surface, while those in the doped materials can travel inward. All a ect the dynamic screening e ect and should be examined via more precise models and time-domain of these issues affect the dynamic screening effect and should be examined via more precise models simulations with a Monte Carlo simulation considering real parameters. and time-domain simulations with a Monte Carlo simulation considering real parameters. Table 2. Estimated parameters at an excitation wavelength of 245 nm. Table 2. Estimated parameters at an excitation wavelength of 245 nm. Samples #SnD-1 #SnD-2 #UID-1 #UID-2 #FeD Samples #SnD-1 #SnD-2 #UID-1 #UID-2 #FeD E 0.69 0.18 1.60 0.16 36.0 E0 0.69 0.18 1.60 0.16 36.0 F 0.28 0.07 0.12 0.06 3.5 sat Fsat 0.28 0.07 0.12 0.06 3.5 The wavelength dependences of the waveforms are depicted in Figure 4. As the photon energy The wavelength dependences of the waveforms are depicted in Figure 4. As the photon energy crossing the bandgap increases, one can clearly see THz emission enhancement. TES thus discloses crossing the bandgap increases, one can clearly see THz emission enhancement. TES thus discloses the the ultrafast nature of the photocarriers excited from the valence band to the conduction band. Note ultrafast nature of the photocarriers excited from the valence band to the conduction band. Note that that the PL and optical absorption measurements merely show the direct band-to-band transitions. the PL and optical absorption measurements merely show the direct band-to-band transitions. The PL The PL results are given in Figure S1 in the Supplementary Material. Details of these dynamics are results are given in Figure S1 in the Supplementary Material. Details of these dynamics are discussed in discussed in later sections. Although weak emission is observed below the bandgap, this is explained later sections. Although weak emission is observed below the bandgap, this is explained by wavelength by wavelength broadening due to the short pulse width of the fs laser. broadening due to the short pulse width of the fs laser. Figure 4. THz emission waveforms of (a) #UID-1 and (b) #FeD at various wavelengths. Figure 4. THz emission waveforms of (a) #UID-1 and (b) #FeD at various wavelengths. The maximum intensities of E for #UID-1 and #FeD are plotted at the measured three The maximum intensities of THz for #UID-1 and #FeD are plotted at the measured three THz wavelengths in Figure 5. Here to avoid the strong screening e ect, the amplitudes measured at a laser wavelengths in Figure 5. Here to avoid the strong screening effect, the amplitudes measured at a laser fluence of 0.04 mJ/cm are used. The intensity increases rapidly with decreasing wavelength. The fluence of 0.04 mJ/cm are used. The intensity increases rapidly with decreasing wavelength. The transition is explained by the broadening of the fs optical pulses. The laser pulse, in general, is regarded transition is explained by the broadening of the fs optical pulses. The laser pulse, in general, is to have a shape of hyperbolic secant function, and wavelength ()-dependent laser intensity I () is regarded to have a shape of hyperbolic secant function, and wavelength ( )-dependent laser inP tensity expressed by ( ) is expressed by I () / sech (A(  )), (3) P P ( ) ( ) ∝ sech − , (3) P P where  is the center wavelength of the laser, and A is a fitting parameter corresponding to the pulse where is the center wavelength of the laser, and is a fitting parameter corresponding to the width. Thus, the intensity of E is expressed by THz pulse width. Thus, the intensity of is expressed by THz | | ∝ sech 2 ( − ) , (4) THz P P jE j / I sech (A(  ))d, (4) THz P P where is the bandgap wavelength of the laser. The broken line in Figure 4 is the fit to the | | experimental data of with of 0.25 assuming the wavelength width of 7 nm at a half maximum THz for of 257 nm, which corresponds to an energy of 4.82 eV. The value agrees with the β-Ga2O3 bandgap energy. The fit quantitatively explains that the THz emission amplitudes are defined by the number of the photocarriers excited into the conduction bands, i.e., THz waves are emitted by the photocarrier excitation. In other words, for the present case, the THz emission spectroscopy is a direct local measure of the Ga2O3 bandgap for the surface layer within a thickness of about 100 nm. Photonics 2020, 7, 73 7 of 11 where  is the bandgap wavelength of the laser. The broken line in Figure 4 is the fit to the experimental data of jE j with A of 0.25 assuming the wavelength width of 7 nm at a half maximum for  of THz 257 nm, which corresponds to an energy of 4.82 eV. The value agrees with the -Ga O bandgap 2 3 energy. The fit quantitatively explains that the THz emission amplitudes are defined by the number of the photocarriers excited into the conduction bands, i.e., THz waves are emitted by the photocarrier excitation. In other words, for the present case, the THz emission spectroscopy is a direct local measure Photonics 2020, 7, x; doi: FOR PEER REVIEW 7 of 11 of the Ga O bandgap for the surface layer within a thickness of about 100 nm. 2 3 0.6 0.5 0.4 0.3 0.2 0.1 230 240 250 260 270 280 Wavelength (nm) Figure 5. THz emission intensities at di erent wavelengths. Closed circles and triangles are for #UID-1 Figure 5. THz emission intensities at different wavelengths. Closed circles and triangles are for #UID- and #FeD, respectively. The dashed line is the calculated fit by Equation (3). 1 and #FeD, respectively. The dashed line is the calculated fit by Equation (3). The anisotropic bandgap of (010) -Ga O has been reported by observing the anisotropic optical 2 3 The anisotropic bandgap of (010) β-Ga2O3 has been reported by observing the anisotropic optical absorption with the polarization of the laser [17]. Thus it is expected that the THz emission amplitude absorption with the polarization of the laser [17]. Thus it is expected that the THz emission amplitude depends on the polarization of the fs laser. Figure 6 gives the optical polarization angle dependence of depends on the polarization of the fs laser. Figure 6 gives the optical polarization angle dependence the emission amplitude. We observed the sinusoidal modulation of the THz emission amplitudes. Here, of the emission amplitude. We observed the sinusoidal modulation of the THz emission amplitudes. we rotate the pump polarization angle from 0 to 360 (0 and 90 correspond to p- and s-polarization, Here, we rotate the pump polarization angle α from 0° to 360° (0° and 90° correspond to p- and s- respectively) using a half-wave plate instead of rotating the sample to keep the THz E field always polarization, respectively) using a half-wave plate instead of rotating the sample to keep the THz E aligned in the same direction to the detector. Note that the spiral PCA has a strong anisotropic response field always aligned in the same direction to the detector. Note that the spiral PCA has a strong function to the THz E-field. Typically, this modulation is explained by nonlinear THz generation anisotropic response function to the THz E-field. Typically, this modulation is explained by nonlinear through optical rectification (OR) based on the second-order nonlinear optical susceptibility, which is THz generation through optical rectification (OR) based on the second-order nonlinear optical observed in many semiconductors [36]. However, (010) -Ga O has the centrosymmetric plain [3], and 2 3 susceptibility, which is observed in many semiconductors [36]. However, (010) β-Ga2O3 has the no strong THz generation due to the second order nonlinear susceptibility is expected [37]. In addition. centrosymmetric plain [3], and no strong THz generation due to the second order nonlinear A small wavelength change on the order of a few nm strongly modulates the THz amplitude. Thus, susceptibility is expected [37]. In addition. A small wavelength change on the order of a few nm one can conclude that TES can measure the anisotropic bandgap of -Ga O directly. 2 3 strongly modulates the THz amplitude. Thus, one can conclude that TES can measure the anisotropic The built-in surface field of doped Ga O is much larger than those of conventional semiconductors. 2 3 bandgap of β-Ga2O3 directly. The field E is roughly estimated by dividing  by the depletion layer thickness w, which is Max D defined by, 2" " r 0 D w = (5) eN where  of a doped semiconductor can be approximated by the surface potential measured values via KFM. The values of w are typically about 100 nm or less, which results in built-in fields that are much stronger than those of the conventional semiconductors [38]. As given in Equation (2), the emission amplitude is proportional to  . All these parameters are intricately related to each other with the surface states and impurities near the surfaces. The parameters discussed above are listed in Table 1. |E | (a. u.) THz Photonics 2020, 7, 73 8 of 11 They suggest that #UID-1 has the strongest emission of the doped Ga O samples. The values agree 2 3 quantitatively with the intensities observed in Figure 2. The results indicate that TES is a useful tool for nondestructive, noncontact analysis of the local electron mobility, surface potential, defects, etc. Photonics 2020, 7, x; doi: FOR PEER REVIEW 8 of 11 Figure 6. Pump polarization angle dependence of the THz radiation amplitude for Fe-doped (010) at Figure 6. Pump polarization angle dependence of the THz radiation amplitude for Fe-doped (010) at various pump powers and wavelengths. (a) illustrates the schematic configuration of the various pump powers and wavelengths. (a) illustrates the schematic configuration of the measurements. measurements. The angle starts from the parallel to the [102] direction. (b) and (c) are measured at a The angle starts from the parallel to the [102] direction. (b) and (c) are measured at a wavelength of wavelength of 245 nm and 260 nm, respectively. 245 nm and 260 nm, respectively. The built-in surface field of doped Ga2O3 is much larger than those of conventional Finally, we evaluate the #UID-1 wafer by LTEM. With a beam diameter of 50 m at an fs laser semiconductors. The field is roughly estimated by dividing by the depletion layer power of 50 mW, we scanned the laser beam on #UID-1 for a 5 mm 5 mm area. The LTEM and PL Max thickness , which is defined by, images are given in Figure 7a and 7b, respectively. The PL image was obtained at an fs laser power of 20 mW at a wavelength of 365 nm, which corresponds to the UV luminescence as described in Figure 1a [39]. Both images show a similar intensity distribution. The distribution is possibly attributed = (5) to the sample holder tilt. The LTEM image of the normalized intensity divided by the PL intensity is shown in Figure 7c. This suggests that the LTEM image corresponds to almost the self-trapped hole where of a doped semiconductor can be approximated by the surface potential measured values distribution. A faint feature is also seen in the LTEM image, which might be caused by the surface via KFM. The values of are typically about 100 nm or less, which results in built-in fields that are scratch marks. much stronger than those of the conventional semiconductors [38]. As given in Equation (2), the emission amplitude is proportional to . All these parameters are intricately related to each other with the surface states and impurities near the surfaces. The parameters discussed above are listed in Table 1. They suggest that #UID-1 has the strongest emission of the doped Ga2O3 samples. The values agree quantitatively with the intensities observed in Figure 2. The results indicate that TES is a useful tool for nondestructive, noncontact analysis of the local electron mobility, surface potential, defects, etc. Finally, we evaluate the #UID-1 wafer by LTEM. With a beam diameter of 50 µm at an fs laser power of 50 mW, we scanned the laser beam on #UID-1 for a 5 mm × 5 mm area. The LTEM and PL images are given in Figure 7a and 7b, respectively. The PL image was obtained at an fs laser power of 20 mW at a wavelength of 365 nm, which corresponds to the UV luminescence as described in Figure 1a [39]. Both images show a similar intensity distribution. The distribution is possibly attributed to the sample holder tilt. The LTEM image of the normalized intensity divided by the PL intensity is shown in Figure 7c. This suggests that the LTEM image corresponds to almost the self- trapped hole distribution. A faint feature is also seen in the LTEM image, which might be caused by the surface scratch marks. Photonics 2020, 7, 73 9 of 11 Photonics 2020, 7, x; doi: FOR PEER REVIEW 9 of 11 Figure 7. (a) Laser THz emission microscopy (LTEM) image and (b) PL image. (c) is the distribution Figure 7. (a) Laser THz emission microscopy (LTEM) image and (b) PL image. (c) is the distribution of of the LTEM amplitude divided by the PL intensity. the LTEM amplitude divided by the PL intensity. 5. Conclusions 5. Conclusions In summary, TES and LTEM have been applied to -Ga O . The polarity change in the THz In summary, TES and LTEM have been applied to β-Ga2 2O3 3. The polarity change in the THz amplitude revealed that the band near the surface of Fe-doped -Ga O bends downward, unlike amplitude revealed that the band near the surface of Fe-doped β-Ga2 2O33 bends downward, unlike similar bands in n-type materials. It was also found that the emission intensity from the n-type -Ga O similar bands in n-type materials. It was also found that the emission intensity from the n-type2 β- 3 samples was proportional to  . The wavelength and polarization dependences of the THz emission Ga2O3 samples was proporti oe nal to . The wavelength and polarization dependences of the also confirmed that anisotropic optical excitation from the valence to the conduction band played an THz emission also confirmed that anisotropic optical excitation from the valence to the conduction essential role in THz emission. It is also shown that the LTEM visualizes the self-trapped hole and a faint band played an essential role in THz emission. It is also shown that the LTEM visualizes the self- surface potential distribution. These results have proven that TES and LTEM provide local information trapped hole and a faint surface potential distribution. These results have proven that TES and LTEM on the mobility, surface potential, defects, and anisotropic photoresponse within the diameter of the fs provide local information on the mobility, surface potential, defects, and anisotropic photoresponse laser beam in a nondestructive, noncontact manner. PL and UV-vis sometimes provide less information within the diameter of the fs laser beam in a nondestructive, noncontact manner. PL and UV-vis on band-to-band transitions [40,41]. Thus, one can probe local ultrafast photocarrier dynamics by sometimes provide less information on band-to-band transitions [40,41]. Thus, one can probe local combining TES/LTEM with other characterization techniques such as KFM, nano-Raman spectroscopy, ultrafast photocarrier dynamics by combining TES/LTEM with other characterization techniques nano-PL, and pump-and-probe measurements [42]. This is essential to semiconductor research and such as KFM, nano-Raman spectroscopy, nano-PL, and pump-and-probe measurements [42]. This is development, especially with regard to wide bandgap semiconductors [43]. essential to semiconductor research and development, especially with regard to wide bandgap semiconductors [43]. Supplementary Materials: The following are available online at http://www.mdpi.com/2304-6732/7/3/73/s1, Figure S1: Photoluminescence of each sample. Supplementary Materials: The following are available online at www.mdpi.com/xxx/s1, Figure S1: Author Contributions: I.K. and M.T. conceptualized the work; H.J., C.G., and T.N. carried out the terahertz Photoluminescence of each sample. measurements; all authors (H.J., C.G., T.N., H.M., I.K., H.N., M.T.) discussed the results; H.J. and M.T. analyzed A data utho and r Cwr ontr ote ibthe utio paper; ns: I.KI.K. . anand d MH.N. .T. cocommented nceptualized on th the e w manuscript. ork; H.J., C.All G., authors and T.N have . carr riead ed o and ut tagr he eed terah to er the tz published version of the manuscript. measurements; all authors (H.J., C.G., T.N., H.M., I.K., H.N., M.T.) discussed the results; H.J. and M.T. analyzed data and wrote the paper; I.K. and H.N. commented on the manuscript. All authors have read and agreed to the Funding: This work was partially supported by JSPS KAKENHI, Grant Numbers JP18KK0140, and JP18K18861. published version of the manuscript. Conflicts of Interest: The authors declare no conflict of interest. Funding: This work was partially supported by JSPS KAKENHI, Grant Numbers JP18KK0140, and JP18K18861. Conflicts of Interest: The authors declare no conflict of interest. Photonics 2020, 7, 73 10 of 11 References 1. Ahmadi, S.E.; Oshima, Y. Materials issues and devices of -and -Ga O . J. Appl. Phys. 2019, 126, 160901. 2 3 [CrossRef] 2. Pearton, S.J.; Ren, F.; Tadjer, M.; Kim, J. Perspective: Ga O for ultra-high power rectifiers and MOSFETS. 2 3 J. Appl. Phys. 2018, 124, 220901. [CrossRef] 3. Higashiwaki, M.; Sasaki, K.; Murakami, H.; Kumagai, Y.; Koukitu, A.; Kuramata, A.; Masui, T.; Yamakoshi, S. Recent progress in Ga O power devices. Semicond. Sci. Technol. 2016, 31, 034001. [CrossRef] 2 3 4. Rana, D.S.; Tonouchi, M. 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Photonics – Multidisciplinary Digital Publishing Institute

Published: Sep 14, 2020

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