Access the full text.
Sign up today, get DeepDyve free for 14 days.
Loading next page...
/lp/hindawi-publishing-corporation/pecvd-silicon-nitride-passivation-on-boron-emitter-the-analysis-of-6dC5keJ48s
References
References for this paper are not available at this time. We will be adding them shortly, thank you for your patience.
- Publisher
- Hindawi Publishing Corporation
- Copyright
- Copyright © 2010 Natalita M. Nursam et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
- ISSN
- 1687-563X
- eISSN
- 1687-5648
- DOI
- 10.1155/2010/487406
- Publisher site
- See Article on Publisher Site
Abstract
Hindawi Publishing Corporation Advances in OptoElectronics Volume 2010, Article ID 487406, 8 pages doi:10.1155/2010/487406 Research Article PECVD Silicon Nitride Passivation on Boron Emitter: The Analysis of Electrostatic Charge on the Interface Properties Natalita M. Nursam, Yongling Ren, and Klaus J. Weber Centre for Sustainable Energy System, School of Engineering, The Australian National University, Building 32 North Road, Acton, Canberra 0200, Australia Correspondence should be addressed to Natalita M. Nursam, natalita.nursam@anu.edu.au Received 9 March 2010; Accepted 10 June 2010 Academic Editor: Chang Sun Copyright © 2010 Natalita M. Nursam et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The dependence of surface recombination of boron diffused and undiffused silicon surfaces passivated with a-SiN :H on the net charge density is investigated in detail. The films are deposited by plasma-enhanced chemical vapour deposition using a 2.45 GHz microwave remote plasma system. The surface charge density on the samples is varied by depositing charge using a corona discharge chamber. Excess carrier lifetime, capacitance-voltage, and Kelvin probe measurements are combined to determine the surface recombination velocity and emitter saturation current density as a function of net charge density. Our results show that the application of negative charge causes a substantial reduction in the surface recombination of samples with boron diffused 19 −3 emitters, even for high boron surface concentrations of 5× 10 cm . The significant difference observed in surface recombination between boron diffused and undiffused sample under accumulation implies that the presence of boron diffusion has results in some degradation of the Si-SiN interface. Further, (111) oriented surfaces appear more sensitive to the boron surface concentration than (100) oriented surfaces. 1. Introduction generally undesirable on heavily doped p type (p )surfaces, such as boron diffused emitters, as the positive charge results The incorporation of silicon nitride (SiN ) films into x in a reduction in hole concentration at the Si surface and semiconductor devices, and in particularly solar cells, has consequently an increase in surface recombination velocity. become an increasingly important topic. This is partly due While recent studies have demonstrated that PECVD nitride to their favorable characteristics, which allow them to act could actually provide excellent passivation of B diffused as antireflectioncoatingsaswellaspassivation layers.For emitter structures when an appropriate postannealing pro- the case of PECVD deposited nitride, the film properties cess was carried out [2], the paper did not provide details of can be varied over a broad range, and the films contain a the charge density in the SiN film, or the B surface concen- high concentration of hydrogen atoms that can be used-to- tration. Hence, the reason for the excellent reported surface passivate bulk defects. Given the growing interest in solar passivation has not been clearly established. Moreover, the cells based on n-type silicon substrates [1], the passivation of exact dependence of the surface recombination rate on the p-type emitters has become more important and, therefore, combination of charge density in the PECVD nitride film the application of PECVD SiN films on this type of emitter x and the p diffused emitter profile has not been explored has emerged as a topic of interest as well. in detail. Neither does much information exist on whether PECVD SiN generally contains a moderate to high the presence of a B diffusion results in significant changes density of positive fixed charge located at or near the Si- to the Si-SiN interface region (such as the creation of SiN interface. This positive charge is beneficial for the more interface defects). This paper aims to investigate these passivation of lowly doped or n doped surfaces but is issues. 2 Advances in OptoElectronics 2. Impact of Charge on Surface Recombination where U is the recombination rate at the surface and Δn is s s the excess surface carrier concentration. Using the Shockley- The surface recombination velocity (S) is defined by the Read-Hall (SRH) equation, S can be expressed as [3] following expression: U ≡ S(Δn ) · Δn,(1) s s s D (E ) n + p + Δn it t 0 0 s S(Δn ) = v dE,(2) S th t v 1/ σ (E ) n + n exp((E − E )/kT ) +1/(σ (E )) p + n exp−((E − E )/kT ) p t S i t i n t S i t i where υ is the thermal velocity, n and p are the electron results has been obtained. Few publications report on similar th 0 0 and hole concentration under equilibrium, n is the intrinsic studies on the Si-PECVD SiN system, but the results are i x carrier concentration, n and p are the electron and hole s s similar to those obtained on Si-SiO [11]. An important concentrations at the surface, E is the interface defects and consistently observed feature in all the experimental energy level, D (E ) is the energy-dependent interface defects it t results is the saturation (rather than a continued reduction, density, and σ (E )and σ (E ) are the energy-dependent n t p t as predicted by models) of surface recombination for for electrons and holes, respectively. Examination of (2) sufficiently large densities of positive and negative charge. shows that, while S is in general a function of Δn , S can For Si-SiO , saturation occurs at a net charge density of less 12 −2 be considered to be independent of Δn under low level than 5 × 10 cm , or a majority carrier concentration of injection conditions at the surface. Furthermore, for high 19 −3 5× 10 cm [4, 10, 12]. The majority carrier concentration 18 −3 surface majority carrier concentrations (>10 cm ), S is here means the concentration of the majority carrier taking expected to be only weakly dependent on p if the interface into account the effect of the application of surface charge. properties do not change with p . For example, for a B diffused surface with a very high positive Surface recombination can be minimized by reducing the charge density, the majority carriers would be electrons. concentration of one type of carrier at the surface, either electrons or holes. This can be achieved by creating either accumulation or strong inversion conditions at the surface. 3. Experimental Further examination of the SRH equation shows that, for Samples used for lifetime measurements were Float Zoned defects with energy levels that are not close to the band edges (FZ), n-type, (100), 40–60 ohm.cm, ∼500 um thick c-Si and with a capture cross section ratio R = σ /σ , the peak n p in recombination will occur when the carrier concentration wafers. These samples were etched (removing at least 15 μm per side) in HNO and HF solution to remove any saw at the surface is p /n = R,where p and n are the S S S S 3 surface hole and electron concentrations, respectively [4]. damage. After receiving a standard RCA clean, selected For approximately equal capture cross sections, the peak will samples were subjected to a B diffusion using a liquid BBr source in two different groups to form a symmetrical occur when p ≈ n , that is, when the surface is in depletion. S S 3 + + Surface recombination as a function of charge density can structure of p /n/p for lifetime measurement. The emitter profiles were determined using spreading resistance and are be modelled using the extended SRH formalism developed shown in Figure 2. The surface concentrations obtained after by Girisch et al. [5]and Aberle et al.[6]. An example 19 −3 of this model is given in Figure 1, showing the effect of deglaze for groups A and B are approximately 5 × 10 cm 18 −3 and 6 × 10 cm with corresponding sheet resistances R surface charge upon surface recombination rate for samples sh with different surface doping concentrations (N ). This of 200 Ω/ and 150 Ω/, respectively. All samples received model predicts that, under strong inversion or accumulation, a clean HF dip prior to nitride depositions. surface recombination will become independent of the All samples received subsequent silicon nitride deposi- surface doping, and relatively weakly dependent on charge tions on both sides, one side at a time. The film deposition density. This suggests that a comparison of samples under was performed in Roth & Rau high-frequency microwave deep inversion or accumulation conditions can be used to remote PECVD systems. For most samples, the deposition directly compare the interface quality (in terms of interface process was carried out at a temperature of 450 C, a defect density distribution and capture cross sections) of processing pressure of 0.2 mbar, a microwave frequency of different samples. 2.45 GHz, a total microwave power of 1000 W, and SiH to Investigations of surface recombination as a function of NH gas mixture ratio of 1 : 1. This process gave ∼70 nm surface charge density can be performed by applying a gate thick films with a refractive index of nearly 2. For some voltage on a suitable metal-insulator-semiconductor (MIS) samples, a slightly different set of deposition parameters was structure, or by depositing corona charges on the surfaces used, with a deposition temperature of 400 C,amicrowave of samples. Several such investigations have been carried power of 750 W and a slightly lower SiH to NH ratio, 4 3 out for the Si-SiO interface [7–10], where generally good resulting in ∼90 nm thick films with an almost identical qualitative agreement between experimental and modelled refractive index. Advances in OptoElectronics 3 measured surface potential change, using appropriate values for the dielectric constants ε determined separately from SiN capacitance-voltage measurements. Czochralski (Cz), n-type, (100), 2–9 ohm·cm, ∼500 um thick c-Si wafers were used for Capacitance-Voltage (C- V) measurements. Theses samples received a standard saw damage etch, RCA clean, and clean HF dip prior to PECVD nitride deposition. Nitride films with thickness around 100 nm were subsequently deposited on one polished side of the wafer, followed by Al metal dots (with diameter ∼ 5 700 um) evaporated on top of the surface to form a Metal- Insulator-Semiconductor structure. Gallium-indium paste was applied on the rear side to allow electrical contact to −2 −1 012 Si bulk. Quasistatic (QS) and high-frequency (HF) C-V −2 ×10 Surface charge density (cm ) measurements were carried out. 14 −3 N = 10 cm 19 −3 N = 10 cm 4. Results and Discussion Figure 1: Calculated dependence of surface recombination rate 4.1. Charge Density. Figure 3 shows typical quasi-static and as a function of applied surface charge upon p-type surface with high frequency C-V curves of an as-deposited nitride film. different surface doping. This calculation was performed using the The curves shown were measured with two different sweep 12 −3 extended Girisch model [5, 6] with a bulk injection level 10 cm directions. In addition to some hysteresis which is commonly and assuming a single level defect at midgap with equal capture observed, the shape of the curves shown is significantly cross sections. different for the two sweep directions. A possible explanation for this unusual behavior could be the presence of interface defects with relatively small capture cross sections, and 20 additionally with σ σ . As the Fermi level is swept from p n near the valence band to the conduction band edge, filling of the traps with electrons is sufficiently rapid to result in high QS capacitances and a stretch-out of the HF curve. However, as the Fermi level is swept in the other direction, emission from the traps is much slower, resulting in the modified shape of the curve. These traps are mostly removed following an anneal in N at 400 C, resulting in the curves shown in 17 Figure 4. The values of the accumulation capacitance (equal to the insulator capacitance C ) and the flat band voltage V allow i fb determination of the charge density Q in the as-deposited 00.10.20.3 and annealed films, using: Depth (microns) ⎡ ⎤ E − E c f ⎣ ⎦ Δϕ = ϕ − χ + , Sample A ms m Si Sample B (3) Figure 2: Boron emitter profiles of the samples used in this paper i Q = Δϕ − V , f ms fb as determined by spreading resistance analysis. Aq where A is the capacitor area (0.0039 cm ), ϕ is the Al- ms siliconworkfunctiondifference, ϕ is the work function of The samples were characterized by effective lifetime Al (4.1 eV), χ is the electron affinity of Si (4.05 eV), q is the Si measurements determined from photoconductance mea- electronic charge, and E –E is the difference between the c f surements in high level injection. The emitter saturation conduction band edge and fermi level in the material used current density was extracted from the slope of the Auger (0.24 eV). Given the flat band voltage (V ) obtained from corrected inverse lifetime as a function of injection level fb C-V data, the fixed charge density within the nitride films [13]. This was followed by surface charge deposition using used in this paper was calculated using (3). The results are a corona discharge chamber. The variation in the density of summarized in Table 1. As can be seen, both the as-deposited the deposited charge was ±20%. Kelvin probe measurements and annealed films display a relatively low-charge density. were performed immediately following charge deposition to monitor the surface potential change and thus allow calculation of deposited charge density at the surface. The 4.2. The Effect of a Boron Diffused Surface. Figure 5 shows surface charge density was subsequently calculated from the curves of Auger corrected inverse lifetime as a function of −2 −1 −3 U (cm s ) Carrier concentration (cm ) S 4 Advances in OptoElectronics −4 −20 2 −6 −4 −20 2 Voltage (V) Voltage (V) Quasi-static Quasi-static High-frequency High-frequency (a) (b) Figure 3: Quasi-static and high-frequency C-V curves of a PECVD nitride film deposited at 450 C. The voltage during the measurements was swept: (a) from inversion to accumulation, and (b) from accumulation to inversion. Table 1: Flat band voltage and fixed charge density of the PECVD silicon nitride films used in this study as determined from C-V measurements. As Deposited Annealed Deposition Temperature −2 −2 V (V) Q (cm ) V (V) Q (cm ) fb f fb f ◦ 11 11 450 C −1.92 +6.5× 10 −1.54 +5.8× 10 ◦ 11 11 400 C −1.22 +2 × 10 −1.9 +4.3× 10 obtained from the sum of Q (from C-V measurements) and the applied surface charge density Q , obtained from the change in surface potential following corona charging as measured using a Kelvin probe. However, the actual value −6 −4 −20 2 4 of net charge density may be overestimated by this method, Voltage (V) particularly for large Q . This is because the application As deposited of large charge densities on the surface of the nitride can Annealed induce the injection of the opposite type of carrier into the nitride film. As a result of such injection, it is observed Figure 4: High-frequency C-V curves measured for different sweep in Figure 5 that the removal of negative surface charge by directions on as-deposited (open symbols) and annealed (filled rinsing in isopropyl alcohol (IPA) results in a lifetime curve symbols) nitride films deposited at 450 C. which approaches that following positive corona charging (due to the injection and storage of positive charge in the excess carrier density for a 40–60 Ω·cm, planar, n-type undif- nitride) while the removal of positive surface charge results fused (100) sample following PECVD nitride deposition, in a lifetime curve which approaches that following negative as well as after subsequent corona charging. Following the corona charging (due to the injection and storage of negative deposition of high negative or positive charge densities, the charge in the nitride). surface is expected to be in low level injection. The net charge The emitter saturation current density (J )can be 0e density under these conditions is estimated to be −4.7 × determined from the slope of the Auger-corrected inverse 12 12 −2 10 and +5.9 × 10 cm . These net charge densities were lifetime curves. As shown in Figure 5,bothafter negative Capacitance (pF) Capacitance (pF) Capacitance (pF) Advances in OptoElectronics 5 ×10 achieved for low-net charge densities, (less than about ±1 × 60 12 −2 10 cm ). However, for large densities of either negative or positive charge, the presence of an accumulation or inversion layer should result in a meaningful extraction of J . 1 0e The second method is to use the relationship between bulk lifetime, effective (measured) lifetime, and surface recombination [14]which is givenby: Before charging 1 1 = + β D,(5) amb τ τ 3 eff b where β is expressed by βW S eff tan = . (6) 2 βD amb 24 6 8 10 ×10 Minority carrier density (cm ) By combining (5)and (6), S can be calculated by eff 1. negatively charged 3. positively charged 1 1 W 1 1 1 S = D − tan − . 2. IPA rinse post 4. IPA rinse post eff amb τ τ 2 D τ τ eff b amb eff b negative charge positive charge (7) Figure 5: Plots of Auger corrected-inverse lifetime as a function of injection level measured on a PECVD SiN passivated n-type, (100) Here, D is the ambipolar diffusion constant (cm /s), W x amb oriented Si sample. The films were deposited at 400 C. The inserted is wafer thickness, and τ is the bulk lifetime. This equation numbers represent the sequence of the charging steps. Black- assumed that all recombination can be attributed to either straight line represents the initial measurement which is followed the bulk or the surface region, and that no other recom- by consecutive negative charge deposition (red-circle), rinse in bination source (e.g., depletion recombination) is present. IPA (black-circles), positive charge deposition (blue-squares), and Equation (7) requires determination of τ . We estimate another rinse in IPA (black squares). τ for each sample using (8)inwhich S is calculated b eff with (4) using the highest τ which was obtained under eff accumulation conditions, with positive applied charge. and positive charging, the Auger corrected inverse lifetime 1 1 2S plots yield reasonably straight lines at high injection levels, eff = + . (8) as would be expected for a sample in which the surface τ τ W eff b is in low-level injection while the bulk is in high-level Equation (7) is used to calculate S as a function of τ , eff eff injection. Nevertheless, following negative corona charging, rather than simpler and more commonly used expression in an unusual feature was observed. While J extracted from 0e (8). This is done since, for several of the samples analysed, the slope of inverse lifetime is very low, (in fact, J following 0e the relatively high-surface recombination velocities lead to a negative corona charge application is significantly lower than significant (>10%) underestimation of S if (8)isused. eff following positive charge application) the effective lifetime is As shown in Figure 6, it is clear that the application of significantly lower as well. This unusual behaviour cannot be negative charge leads to significant discrepancy between the explained by recombination in the depletion region created two calculations due to the opposing trends of decreasing J 0e by the surface inversion, since, under conditions of high level and decreasing τ , which cannot be reconciled with a simple eff injection, no depletion region exists in the samples. model of a sample with a uniform bulk and surfaces in low In order to analyse the results from minority carrier level injection. Similar results have been observed in a range lifetime measurements, two methods were used to extract of PECVD nitride passivated, undiffused samples deposited S over the range of applied charge densities of the same eff using different PECVD systems and on different substrates, sample as in Figure 5. The first is to determine S from the eff including both lightly and moderately doped, p type and n measurement of J : 0e type substrates. It is confined to undiffused substrates with J (N + Δn) a PECVD nitride layer, following the deposition of negative 0e A S = ,(4) eff charge. The reason for this phenomenon is currently not qn well understood and will require further investigation. In this where Δn is the excess carrier density in the sample bulk paper, we will not concern ourselves further with measure- and N is the background doping concentration. S was ments on undiffused substrates following the application of A eff 15 −3 determined at an injection level 4 × 10 cm and thus, N negative charge. In the subsequent discussions, (6)willbe is small compared to Δn. The determination of S from J used for S extraction, since this method does not require eff 0e eff is not expected to be accurate over the entire range of applied the surface to be in low level injection. charge densities. The condition for valid J measurements— The variation of S with net charge density for a 0e eff that the surface must be in low level injection—will not be Bdiffused sample with a high surface B concentration −1 Auger-corrected 1/τ (s ) eff 6 Advances in OptoElectronics −2 −10 1 2 −4 −20 2 4 −2 ×10 Charge density (cm ) −2 ×10 Charge density (cm ) Negative charge Determined from (4) Positive charge Determined from (7) Figure 7: S as a function of applied charge density for a B eff Figure 6: S as a function of applied charge density calculated from eff 19 −3 diffused annealed sample with N ≈ 5 × 10 cm . PECVD SiN s x J (4)and τ (7) with respect to applied surface charge. PECVD 0e eff ◦ was deposited at 450 C and the sample was subsequently annealed SiN was deposited at 400 C and the sample was subsequently x ◦ (30 mins 400 CinN ). Negative and positive charge application annealed (30 mins 400 CinN ). Negative and positive charge is denoted by filled and empty symbols, respectively. The black application is denoted by filled and empty symbols, respectively. All solid line is a fit to the data, while the red-dashed line shows the S values were extracted from data measured at an injection level eff modelling results obtained from the Girisch model [5, 6], assuming 15 −3 4 × 10 cm . a continuum of defect levels with midgap ratio σ /σ ≈10 and D n p itd 10 −2 −1 = 2 × 10 cm eV . This density and capture cross sections of the defects were adjusted to fit the experimental data. is depicted in Figure 7. Negative charges were initially deposited, followed by charge removal in IPA solution and deposition of positive charge (in according with the sequence numbers given on the curve). The starting point (point 1), 11 −2 n prior to any charging, is given by Q (+5.8× 10 cm ). The −14 results of Figure 6 show a reduction and eventual saturation of S as the surface is pushed into accumulation. This trend eff is consistent with other reports in the literature [8–12]. −16 We applied the extended Girisch model [5, 6]tosimulate the behaviour. Figure 7 shows that a reasonably good fit to the data can be obtained when a suitable set of modelling parameters is chosen. The distribution of electrons and −18 holes capture cross sections and the density of dopant- itd like defects over the bandgap energy that were used in this model are given in Figure 8.However,itisnot possibleto −20 10 10 make definitive statements concerning the variation of defect 00.20.40.60.81 density or capture cross sections from the available data. As E − E (eV) T v mentioned before, the Girisch model predicts a continuing Figure 8: Modelled energy dependence of the capture cross sections decrease in S as the negative charge density is increased, eff of electrons (σ ) and holes (σ ) and dopant-like defect density (D ) while in practice, saturation of S is observed. n p itd eff that were used to fit the experimental data as given in Figure 7. It is interesting to note from the experimental results shown in Figure 7 that, even for a very heavily doped surface, the application of charge can still result in a significant 19 −3 further reduction in surface recombination. Previous studies 5 × 10 cm . The main conclusion that can be drawn from [10, 12] on undiffused surfaces have found that surface Figure 7 is that even heavily doped surfaces are sensitive to recombination tends to saturate for applied surface charge the presence of charge. 12 −2 densities of 5× 10 cm or less, with corresponding surface The best J of the B diffused sample shown in Figure 7 0e 19 −3 majority carrier concentrations of 5 × 10 cm or less. following negative charge application is approximately 2 2 For the above sample, S is observed to saturate for charge 41 fA/cm . This can be compared to a J of ∼6fA/cm for an eff 0e 12 2 densities of ∼5 × 10 cm , but the corresponding surface identically prepared, undiffused sample under accumulation. majority carrier concentrations are significantly higher than The difference in J values is partly due to the contribution 0e S (cm/s) eff 2 S (cm/s) eff Capture cross section (cm ) −2 −1 D (cm eV ) it Advances in OptoElectronics 7 −2 −1.5 −1 −0.50 −1.5 −1 −0.50 −2 ×10 13 Charge density (cm ) −2 ×10 Charge density (cm ) <100> low SBC <111> low SBC As deposited <100> high SBC <111> high SBC Annealed in N Annealed in FGA Figure 10: J as a function of charge density of (100) and (111) B 0e diffused planar samples with different surface boron concentration (SBC). “Low” and “high” SBC refers to N = 6 × 10 and 5 × Figure 9: S as a function of charge density for a B diffused sample eff 19 −3 18 −3 ◦ 10 cm on (100) samples, respectively. All samples were annealed with N ≈ 6× 10 cm and PECVD SiN was deposited at 450 C. s x 15 −3 in N prior to charge deposition. Measurements were taken at an The measurements were taken at injection level 4 × 10 cm in as- 15 −3 injection level 4 × 10 cm . deposited condition and after annealing in different ambient. The lines are fit to the data. to J from the emitter region so this factor should be orientations as a function of net charge density is plotted in 0e accounted. Results on oxide passivated B diffused emitters Figure 10. These samples received anneals in N at 400 C. with similar sheet resistances have shown that this kind of At low charge density where Q = 0, J for both (100) and s 0e emitter contributes less than 6 fA/cm [15]. The remainder (111) samples is dependent on surface doping concentration, of the difference shown in this experiment (∼29 fA/cm )is with a lower B surface concentration resulting in a higher likely to indicate that the B diffused interface is not as good as J due to a lower hole concentration (and hence a higher 0e that of undiffused (i.e., has a higher interface defect density, minority carrier electron concentration), which results in or defects with greater capture cross sections), as has also higher recombination. For the case of (100), the J values for 0e been observed for samples passivated with a thermal oxide a high applied density of negative charge are almost identical; [16]. suggesting that the difference in B surface concentration Figure 9 shows the S extractions for a B diffused sample has not significantly affected the interface defect properties, eff with nitride film deposited at 450 C before and after anneals such as their density, distribution, or capture cross sections. at a temperature of 400 C for 30 minutes. These anneals were Given that the presence of a B diffusion clearly degrades the performed in N . This was followed by an annealing in FGA interface, it may be that the extent of this degradation is on the same sample. Annealing in N wasabletoimprove more closely related to the total B concentration rather than the interface quality of both diffused and undiffused surfaces, B concentration at the surface. However, further work will be as indicated by lower S regardless of the charge density on required to verify this. eff the surface. This improvement is in good agreement with the For the (111) samples, the dependence of J on surface 0e results shown in Reference [2]. Furthermore, it is also shown concentration under no charge application is much less by the results given in Figure 9 that the introduction of H pronounced. In contrast to the (100) surface, the final molecules did not contribute to the passivation, since there is accumulated J values of (111) samples are found to be 0e no further improvement shown following second annealing dependent on surface concentration, with a higher surface in forming gas. Therefore, it is suggested that the improved concentration resulting in higher saturated J values. This 0e interface quality is more likely due to rearrangements and result suggests that the interface properties of the (111) the removal of some defects at the interface at elevated surface are more susceptible to B surface concentration than temperature, as well as possibly the passivation of interface the (100) surface. For low B surface concentrations, there defects with hydrogen supplied by the SiN film. appears to be no significant difference in the passivation quality of (100) and (111) surfaces, suggesting that, for lowly 4.3. The Effect of Surface Doping and Surface Orientation. and moderately doped surfaces, the PECVD SiN passivation The effect of B surface doping level on different surface is rather independent of surface orientation. S (cm/s) eff J (fA/cm ) 0e 8 Advances in OptoElectronics 5. Conclusion eters using a gate-controlled point-junction diode under illumination,” IEEE Transactions on Electron Devices, vol. 35, In conclusion, we have demonstrated that the application no. 2, pp. 203–222, 1988. of positive surface charge results in a significant increase [6] A. G. Aberle, S. Glunz, and W. Warta, “Impact of illumination level and oxide parameters on Shockley-Read-Hall recombi- in the surface recombination of PECVD SiN passivated B nation at the Si-SiO interface,” Journal of Applied Physics, vol. diffused emitters, even for high B surface concentrations. 2 71, no. 9, pp. 4422–4431, 1992. On the other hand, negative charge causes a substantial [7] D. K. Schroder, “Contactless surface charge semiconductor reduction in surface recombination. Under accumulation, characterization,” Materials Science and Engineering B, vol. 91, the significant difference in J between B diffused and 0e pp. 196–210, 2002. undiffused sample, as much as ∼29 fA/cm , implies that [8] M.Schoefthaler,R.Brendel,G.Langguth, andJ.H.Werner, the B diffusion has resulted in some degradation of the “High-quality surface passivation by corona-charged oxides Si-PECVD SiN interface. The very similar passivation for semiconductor surface characterization,” in Proceedings of quality of (100) samples with different B concentrations the 1st World Conference on Photovoltaic Energy Conversion, at the surface suggests that the degradation is not strongly vol. 2, pp. 1509–1512, Waikoloa, Hawaii, USA, 1994. related to the B surface concentration. However, the interface [9] S. W. Glunz, D. Biro, S. Rein, and W. Warta, “Field-effect properties of (111) surfaces tend to be more susceptible to passivation of the SiO -Si interface,” Journal of Applied Physics, vol. 86, no. 1, pp. 683–691, 1999. the B concentration at the surface rather than (100). [10] W. E. Jellett and K. J. Weber, “Accurate measurement of There are several possible implications for solar cell extremely low surface recombination velocities on charged, design. If PECVD SiN films were to be applied to the pas- oxidized silicon surfaces using a simple metal-oxide- sivation of B diffused emitters, then minimising the positive semiconductor structure,” Applied Physics Letters, vol. 90, no. charge density in the SiN film will be very important in 4, article 042104, 3 pages, 2007. order to obtain good surface passivation, even for a high B [11] S. Dauwe, J. Schmidt, A. Metz, and R. Hezel, “Fixed charge surface concentration. The results suggest that the B surface density in silicon nitride films on crystalline silicon surfaces concentration should be rather high, even though—for some under illumination,” in Proceedings of the 29th IEEE Photo- surfaces at least—a higher B concentration will lead to a voltaic Specialists Conference, pp. 162–165, New Orleans, La, poorer interface. It is likely that textures that avoid the USA, 2002. formation of (111) surfaces will perform better than the [12] K. J. Weber, H. Jin, C. Zhang, N. Nursam, W.E. Jellett, and K. R. McIntosh, “Surface passivation using dielectric films: random pyramid texture. how much charge is enough,” in Proceedings of the 24th European Photovoltaic Solar Energy Conference and Exhibition, Acknowledgments Hamburg, Germany, 2002. [13] D. E. Kane and R. M. Swanson, “Measurement of the The authors wish to thank members of the PV group at the emitter saturation current by a contactless photoconductivity University of New South Wales for carrying out some of the decay method,” in Proceedings of the 18th IEEE Photovoltaic PECVD depositions; Dr. Jason Tan and Mr. Chun Zhang Specialists Conference, pp. 578–583, Las Vegas, Nev, USA. for help and discussion on PECVD nitride depositions; [14] A. W. Stephens,A.G.Aberle, andM.A.Green,“Surface recombination velocity measurements at the silicon-silicon and Mr. Simeon Baker-Finch and Dr. Keith McIntosh for dioxide interface by microwave-detected photoconductance discussions on emitter recombination. Financial support decay,” Journal of Applied Physics, vol. 76, no. 1, pp. 363–370, by the Australian Agency for International Development is gratefully acknowledged. [15] W. E. Jellet, Investigation of recombination at the silicon- silicon dioxide interface, Ph.D. thesis, The Australian National University, Canberra, Australia, 2008. References [16] W. E. Jellett, K. J. Weber, and H. Jin, “Influence of boron [1] J. Schmidt, K. Bothe, R. Bock, C. Schmiga, R. Krain, and diffusion on the Si surface passivation,” in Proceedings of the 22nd European Photovoltaic Solar Energy Conference and R. Brendel, “N-type silicon—the better material choice for industrial high efficiency solar cells,” in Proceedings of the 22nd Exhibition, Milan, Italy, 2007. European Photovoltaic Solar Energy Conference and Exhibition, Milan, Italy, September 2007. [2] F. W. Chen, T.-T. A. Li, and J. E. Cotter, “Passivation of boron emitters on n-type silicon by plasma-enhanced chemical vapor deposited silicon nitride,” Applied Physics Letters, vol. 88, no. 26, Article ID 263514, 2006. [3] W. D. Eades and R. M. Swanson, “Calculation of surface gen- eration and recombination velocities at the Si-SiO interface,” Journal of Applied Physics, vol. 58, no. 11, pp. 4267–4276, 1985. [4] E. Yablonovitch, R. M. Swanson, W. D. Eades, and B. R. Weinberger, “Electron-hole recombination at the Si-SiO interface,” Applied Physics Letters, vol. 48, no. 3, pp. 245–247, [5] R.B.M.Girisch,R.P.Mertens,and R. F. De Keersmaecker, “Determination of Si-SiO interface recombination param- 2 International Journal of Rotating Machinery International Journal of Journal of The Scientific Journal of Distributed Engineering World Journal Sensors Sensor Networks Hindawi Publishing Corporation Hindawi Publishing Corporation Hindawi Publishing Corporation Hindawi Publishing Corporation Hindawi Publishing Corporation http://www.hindawi.com http://www.hindawi.com Volume 2014 http://www.hindawi.com Volume 2014 http://www.hindawi.com Volume 2014 http://www.hindawi.com Volume 2014 Volume 2014 Journal of Control Science and Engineering Advances in Civil Engineering Hindawi Publishing Corporation Hindawi Publishing Corporation http://www.hindawi.com Volume 2014 http://www.hindawi.com Volume 2014 Submit your manuscripts at http://www.hindawi.com Journal of Journal of Electrical and Computer Robotics Engineering Hindawi Publishing Corporation Hindawi Publishing Corporation http://www.hindawi.com Volume 2014 http://www.hindawi.com Volume 2014 VLSI Design Advances in OptoElectronics International Journal of Modelling & Aerospace International Journal of Simulation Navigation and in Engineering Engineering Observation Hindawi Publishing Corporation Hindawi Publishing Corporation Hindawi Publishing Corporation Hindawi Publishing Corporation Volume 2014 http://www.hindawi.com Volume 2014 http://www.hindawi.com Volume 2010 Hindawi Publishing Corporation http://www.hindawi.com Volume 2014 http://www.hindawi.com http://www.hindawi.com Volume 2014 International Journal of Active and Passive International Journal of Antennas and Advances in Chemical Engineering Propagation Electronic Components Shock and Vibration Acoustics and Vibration Hindawi Publishing Corporation Hindawi Publishing Corporation Hindawi Publishing Corporation Hindawi Publishing Corporation Hindawi Publishing Corporation http://www.hindawi.com Volume 2014 http://www.hindawi.com Volume 2014 http://www.hindawi.com Volume 2014 http://www.hindawi.com Volume 2014 http://www.hindawi.com Volume 2014
Journal
Advances in OptoElectronics – Hindawi Publishing Corporation
Published: Jun 29, 2010