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Electro- and photo-luminescence spectra from a-Si:H and a-SiGe p–i–n solar cells
Journal of Non-Crystalline Solids 266±269 (2000) 1119±1123
www.elsevier.com/locate/jnoncrysol
Electro- and photo-luminescence spectra from a-Si:H and a-SiGe p±i±n solar cells Guozhen Yue a, Xunming Deng b, G. Ganguly c, Daxing Han a,* a
Department of Physics and Astronomy, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599-3255, USA b Department of Physics and Astronomy, University of Toledo, Toledo, OH 43606, USA c BP Solarex, 3601, LaGrange Parkway, Toano, VA 23168, USA
Abstract Electroluminescence (EL) and photoluminescence (PL) spectroscopies were used to study the eects of hydrogen dilution on a-Si:H and of varying Ge content on a-SiGe p±i±n cells. We found that (a) in the case of hydrogen dilution, the EL peak energy (ELpeak ) increased with H-dilution that correlated with an increase of open circuit voltage (Voc ), but the corresponding PL peak energy (PLpeak ) did not change; and (b) dierent from observations that ELpeak < PLpeak , the ELpeak can be the same or larger than the PLpeak . The results were explained by the model of dispersive-transportcontrolled recombination for EL. We suggest that the Voc is not only related to the optical gap and the tail states but also to the transport processes. Ó 2000 Elsevier Science B.V. All rights reserved.
1. Introduction Photoluminescence (PL) and electroluminescence (EL) spectroscopies have been used as a tool to study the density of localized states in amorphous silicon [1]. Dierences in the features of the EL and PL were observed [1,2]. Generally, at low temperature (80 K), the EL peak energy, ELpeak , is 0.2 eV less than that of the PLpeak , and the EL eciency is two or three orders of magnitude less than the PL if the forward current density is used as the generation rate. Spear et al. [1] argued that the EL peak energy was modulated by interference fringes, and the low EL eciency is due to the EL generation only near the p contact. Recently, the
* Corresponding author. Tel.: +1-919 962 5002; fax: +1-919 962 0480. E-mail address: [email protected] (D. Han).
above unique EL features have been explained by a new model of dispersive-transport-controlled recombination by Han et al. [2]. In the new model, the ELpeak is determined by the carrier-transport level rather than the demarcation level determined by carrier thermalization; and the EL generation rate must be the recombination current rather than the forward current, in which a gain factor of 100 to 1000 must be taken into account. The previous studies [2] suggested that the EL spectroscopy gives important information about the localized states in the intrinsic layer of p±i±n cells. In this paper, we studied a series of hydrogen diluted a-Si:H cells and a-SiGe p±i±n cells with a varied Ge content. Hydrogen dilution of reaction gas during ®lm growth has been used to improve solar cell performance [3,4]; and the SiGe alloy has been used as the i-layer in the narrow-gap cells. To understand the device/material physics, we need to know the density of localized states in the intrinsic layer. Therefore, a comparison study of EL and
0022-3093/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 9 9 ) 0 0 9 1 4 - X
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G. Yue et al. / Journal of Non-Crystalline Solids 266±269 (2000) 1119±1123
Table 1 Deposition conditions and cell performances for the H-diluted cellsa Jsc 0:25 (mA/cm2 )
FF 0:5 (%)
0.931 0.926 0.95 0.948
11.2 10.3 9.7 7.4
71.1 66.2 71.6 66.3
186 189 195 153
0.936 0.944 0.951 0.971
9.1 8.9 9.1 8.1
72.1 73.0 71.8 70.3
33 50 100
201 156 160
0.948 0.985 0.966
8.9 9.7 6.8
73.3 71.1 71.3
100
159
0.972
6.4
69.6
Sample
Ts (°C)
R
d 5 (nm)
GD288 GD290 GD299 GD300
250 250 250 250
20 25 50 100
207 215 172 170
GD289 GD291 GD292 GD298
200 200 200 200
25 33 50 100
GD293 GD304 GD294
175 175 175
GD295
150
Voc 0:01 (V)
a
Ts , R, d, Voc , Jsc , and FF are the substrate temperature, H-dilution ratio H2 :Si2 H6 , i-layer thickness, open circuit voltage, short circuit current and ®ll factor, respectively.
PL spectra in a temperature range 80±200 K was carried out. 2. Experimental Device quality a-Si:H and a-SiGe p±i±n solar cells were prepared by glow-discharge chemical vapor deposition (GD-CVD) on stainless steel (ss) substrates 1 and on glass substrates. 2 Disilane (Si2 H6 ) was used for deposition on ss substrates. The i-layer thickness was 0.2 lm for a-Si:H solar cells and 0.5 lm for a-SiGe solar cells. The top contact area was 0.2 cm2 . The luminescence signal was dispersed by a grating monochromator (the spectral resolution was 0.3 nm at 546.074 nm) and then collected by a liquid-nitrogen-cooled Ge detector. The lock-in technique was used with a light chopper at a frequency of 17 Hz. All spectra were calibrated for the system response. A microrefrigerator operating between 80±340 K was used to mount the sample. Between consecutive measurements, the sample was kept at the same position and the experimental error was <10 meV in the experimental range of photon energy. For EL, a voltage of 5 V was supplied by a pulse generator. The forward bias current density was in the range 1 2
The cells were made at University of Toledo. The cells were made at Solarex.
of 50±500 mA/cm2 in the temperature range 80± 200 K. We used a 632.8 nm laser excitation for PL. The penetration depth of the 632.8 nm light in aSi:H was 1 lm which is greater than the sample thickness. A bandpass ®lter at 632.8 nm was used to avoid the IR component of the He±Ne laser. The incident laser power was 2 mW. For a-SiGe cells, 514.5 nm Ar laser excitation was used, the incident laser power was 6 mW.
3. Results The preparation conditions and the cell performances for H-diluted cells are listed in Table 1. Four group of cells deposited at 150°C, 175°C, 200°C, and 250°C were studied. We will pay special attention to the H-dilution eect on the open circuit voltage, Voc . We note that Voc is greater than 0.9 V in all the cells and it increases with increasing H-dilution. For the same H-dilution, on the other hand, Voc increases with decreasing deposition temperature, Ts . The luminescence spectrum obtained at 80 K originates from tail-to-tail transitions [5], hence it gives information about the optical gap and the energy distribution of the tail states. We expect the same spectral lineshape for the EL and PL from the same sample. In Fig. 1, typical EL and PL spectra at 80 K are shown for a group of cells
G. Yue et al. / Journal of Non-Crystalline Solids 266±269 (2000) 1119±1123
1121
294 5 to 206 5 meV as shown in Fig. 1(a) whereas, the PL spectral lineshapes do not change much as shown in Fig. 1(b). More interestingly, at the H-dilution of R 100 the EL peak energy (1:43 0:05 eV) is 20 meV greater than PLs (1:41 0:05 eV). To our knowledge, this phenomenon of ELpeak > PLpeak has not been observed in a-Si based diodes [1,2]. We observed the same tendency in the three groups of cells listed in Table 1, so we assume that the results cannot be due to experimental error. We further studied the EL and PL spectra of aSiGe cells with a dierent Ge content (the ¯ow ratio of GeH4 :Si2 H6 45:55 and 50:50). The typical EL and PL spectra from a pair of a-SiGe cells are shown in Fig. 2 (a) and (b), respectively. The result shows that (a) the EL peak energy, ELpeak , is the same as the PLpeak within the experimental error of 10 meV, and (b) both the ELpeak and PLpeak decrease from 1.14 to 1.04 eV with increasing Ge content due to the reduction of the optical gap [6]. Finally, we summarize all the data of ELpeak and PLpeak at 80 K as a function of Voc for all the a-Si:H and a-SiGe cells studied. In Fig. 3(a), we note that higher the luminescence peak energy, the larger the Voc obtained. However, there are two interesting points: First, the ELpeak is NOT always less than the PLpeak for the same sample. It can be the same or even larger as shown on the right side in Fig. 3(b) (the largest Voc ). Second, the ELpeak shifts 50 meV from 1:28 0:05 to 1:43 0:05 eV in the same direction as Voc does; whereas, the PLpeak at 1.4 eV is independent of Voc . 4. Discussion Fig. 1. Typical luminescence spectra for a group of cells deposited at Ts 200°C with dierent H-dilution ratio from R 25 to 100, (a) EL, and (b) PL, the inset is EL and PL from the sample of R 100.
deposited at Ts 200°C with increasing H-dilution from R 25 to 100. Interestingly, we ®nd that the EL peak energy shifts from 1:28 0:05 to 1:43 0:05 eV and the band width decreases from
Both the luminescence peak energy and the Voc are relevant to the optical gap of the i-layer material [6], so it is reasonable to observe a correlation between the luminescence peak energy and Voc . The PL feature has been explained by the thermalization model [7,8], i.e., the PLpeak depends on the position of the demarcation level, ED . The carrier transport is not involved in PL processes. As long as the optical gap and the band tails have
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G. Yue et al. / Journal of Non-Crystalline Solids 266±269 (2000) 1119±1123
Fig. 3. Correlation between Voc and the ELpeak (d) and PLpeak (s) at 80 K: (a) for a-Si:H and a-SiGe cells, and (b) for aSi:H cells with dierent H-dilution. The lines are drawn as guides for the eye. Fig. 2. Typical luminescence spectra for a group of a-SiGe cells with dierent Ge ¯ow ratio from GeH4 :Si2 H6 45:55 to 50:50, (a) EL, and (b) PL.
not changed, the PL lineshape would not change. We explained the dierences between the ELpeak and PLpeak by using the dispersive-transport-controlled recombination model [2]. At low tempera-
ture (80 K), hopping transport in the exponential band tail must be considered [9]. When the carrier lifetime is longer than its transit time, the carriers would hop down to the transport level, Et , then recombine. In the case of smaller H-diluted a-Si:H cells, the carrier's recombination lifetime in the ilayer is much longer than the transit time. In other
G. Yue et al. / Journal of Non-Crystalline Solids 266±269 (2000) 1119±1123
words, Et is deeper than ED , hence PLpeak > ELpeak was observed. However, for a-SiGe cells, the mobility lifetime product (ls) is 2 or 3 orders of magnitude less than that for a-Si:H [6], so the carriers would recombine right after injecting into the i-layer. When Et is equal to ED , PLpeak ELpeak was observed in a-SiGe cells. The carrier lifetime could be further shortened due to the increase of defect density by microcrystallinity in the high Hdiluted cells [10]. Therefore, the energy of Et could be higher than ED , and then ELpeak > PLpeak was obtained in highly H-diluted cells.
1123
(c) We observed ELpeak > PLpeak in highly Hdiluted a-Si:H p±i±n cells. Acknowledgements This work is supported by NREL sub-subcontract XAK-8-17619-11 and the thin ®lm PV partnership. Han is partially supported by CGP Fund, NSF-Int-9802430. Yue is partially supported by NSF-Int-960495. Deng is supported by NREL subcontract ZAF-8-17619-14 and work at BP Solarex is supported by NREL under subcontract No. ZAK-8-17619-02.
5. Conclusion EL and PL spectroscopies were used to study the eects of H-dilution in a-Si:H as well as the eects of varying Ge content in a-SiGe solar cells. Statistically, higher the luminescence peak energies, ELpeak and PLpeak , the larger the Voc . However, the Voc is not only relevant to the optical gap but also to the transport processes. This relevancy was demonstrated in the EL spectra. (a) The Voc increased with increasing H-dilution. When R increased from 20 to 100, the ELpeak shifted from 1.28 to 1.43 eV whereas, PLpeak at 1.40 eV did not change. (b) For a-SiGe p±i±n cells GeH4 :Si2 H6 45:55 and 50:50), the peak energy positions of the EL and PL are the same. Both the ELpeak and PLpeak decreased from 1.14 to 1.04 eV with increasing Ge content due to the reduction of the optical gap.
References [1] T.S. Nashashibi, I.G. Austin, T.M. Searle, R.A. Gibson, W.E. Spear, P.G. Le Comber, Philos. Mag. B 45 (1982) 553. [2] D. Han, K. Wang, B. Von Roedern, Phys. Rev. Lett. 77 (1996) 4410. [3] R. Arya, A. Catalano, M. Bennett, J. Newton, B. Fieselman, L. Yang, Y.M. Li, R. D'Aiello, in: Proceedings of the 11th EC PVSEC, 1992, p. 199. [4] D.V. Tsu, B.S. Chao, S.R. Ovshinsky, S. Guha, J. Yang, Appl. Phys. Lett. 71 (1997) 1317. [5] F. Boulitrop, D.J. Dunstan, Phys. Rev. B 28 (1983) 5923. [6] M. Stutzmann, R.A. Street, C.C. Tsai, J.B. Boyce, S.E. Ready, J. Appl. Phys. 66 (1989) 569. [7] R.W. Collins, M.A. Paesler, W. Paul, Solid State Commun. 34 (1980) 833. [8] R.W. Collins, W. Paul, Phys. Rev. B 25 (1982) 5257. [9] D. Monroe, Phys. Rev. Lett. 54 (1985) 146. [10] D. Han, H. Habuchi, T. Hori, A. Nishibe, T. Namioka, Jing Lin, Guozhen Yue, these Proceedings, p. 274.
www.elsevier.com/locate/jnoncrysol
Electro- and photo-luminescence spectra from a-Si:H and a-SiGe p±i±n solar cells Guozhen Yue a, Xunming Deng b, G. Ganguly c, Daxing Han a,* a
Department of Physics and Astronomy, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599-3255, USA b Department of Physics and Astronomy, University of Toledo, Toledo, OH 43606, USA c BP Solarex, 3601, LaGrange Parkway, Toano, VA 23168, USA
Abstract Electroluminescence (EL) and photoluminescence (PL) spectroscopies were used to study the eects of hydrogen dilution on a-Si:H and of varying Ge content on a-SiGe p±i±n cells. We found that (a) in the case of hydrogen dilution, the EL peak energy (ELpeak ) increased with H-dilution that correlated with an increase of open circuit voltage (Voc ), but the corresponding PL peak energy (PLpeak ) did not change; and (b) dierent from observations that ELpeak < PLpeak , the ELpeak can be the same or larger than the PLpeak . The results were explained by the model of dispersive-transportcontrolled recombination for EL. We suggest that the Voc is not only related to the optical gap and the tail states but also to the transport processes. Ó 2000 Elsevier Science B.V. All rights reserved.
1. Introduction Photoluminescence (PL) and electroluminescence (EL) spectroscopies have been used as a tool to study the density of localized states in amorphous silicon [1]. Dierences in the features of the EL and PL were observed [1,2]. Generally, at low temperature (80 K), the EL peak energy, ELpeak , is 0.2 eV less than that of the PLpeak , and the EL eciency is two or three orders of magnitude less than the PL if the forward current density is used as the generation rate. Spear et al. [1] argued that the EL peak energy was modulated by interference fringes, and the low EL eciency is due to the EL generation only near the p contact. Recently, the
* Corresponding author. Tel.: +1-919 962 5002; fax: +1-919 962 0480. E-mail address: [email protected] (D. Han).
above unique EL features have been explained by a new model of dispersive-transport-controlled recombination by Han et al. [2]. In the new model, the ELpeak is determined by the carrier-transport level rather than the demarcation level determined by carrier thermalization; and the EL generation rate must be the recombination current rather than the forward current, in which a gain factor of 100 to 1000 must be taken into account. The previous studies [2] suggested that the EL spectroscopy gives important information about the localized states in the intrinsic layer of p±i±n cells. In this paper, we studied a series of hydrogen diluted a-Si:H cells and a-SiGe p±i±n cells with a varied Ge content. Hydrogen dilution of reaction gas during ®lm growth has been used to improve solar cell performance [3,4]; and the SiGe alloy has been used as the i-layer in the narrow-gap cells. To understand the device/material physics, we need to know the density of localized states in the intrinsic layer. Therefore, a comparison study of EL and
0022-3093/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 9 9 ) 0 0 9 1 4 - X
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G. Yue et al. / Journal of Non-Crystalline Solids 266±269 (2000) 1119±1123
Table 1 Deposition conditions and cell performances for the H-diluted cellsa Jsc 0:25 (mA/cm2 )
FF 0:5 (%)
0.931 0.926 0.95 0.948
11.2 10.3 9.7 7.4
71.1 66.2 71.6 66.3
186 189 195 153
0.936 0.944 0.951 0.971
9.1 8.9 9.1 8.1
72.1 73.0 71.8 70.3
33 50 100
201 156 160
0.948 0.985 0.966
8.9 9.7 6.8
73.3 71.1 71.3
100
159
0.972
6.4
69.6
Sample
Ts (°C)
R
d 5 (nm)
GD288 GD290 GD299 GD300
250 250 250 250
20 25 50 100
207 215 172 170
GD289 GD291 GD292 GD298
200 200 200 200
25 33 50 100
GD293 GD304 GD294
175 175 175
GD295
150
Voc 0:01 (V)
a
Ts , R, d, Voc , Jsc , and FF are the substrate temperature, H-dilution ratio H2 :Si2 H6 , i-layer thickness, open circuit voltage, short circuit current and ®ll factor, respectively.
PL spectra in a temperature range 80±200 K was carried out. 2. Experimental Device quality a-Si:H and a-SiGe p±i±n solar cells were prepared by glow-discharge chemical vapor deposition (GD-CVD) on stainless steel (ss) substrates 1 and on glass substrates. 2 Disilane (Si2 H6 ) was used for deposition on ss substrates. The i-layer thickness was 0.2 lm for a-Si:H solar cells and 0.5 lm for a-SiGe solar cells. The top contact area was 0.2 cm2 . The luminescence signal was dispersed by a grating monochromator (the spectral resolution was 0.3 nm at 546.074 nm) and then collected by a liquid-nitrogen-cooled Ge detector. The lock-in technique was used with a light chopper at a frequency of 17 Hz. All spectra were calibrated for the system response. A microrefrigerator operating between 80±340 K was used to mount the sample. Between consecutive measurements, the sample was kept at the same position and the experimental error was <10 meV in the experimental range of photon energy. For EL, a voltage of 5 V was supplied by a pulse generator. The forward bias current density was in the range 1 2
The cells were made at University of Toledo. The cells were made at Solarex.
of 50±500 mA/cm2 in the temperature range 80± 200 K. We used a 632.8 nm laser excitation for PL. The penetration depth of the 632.8 nm light in aSi:H was 1 lm which is greater than the sample thickness. A bandpass ®lter at 632.8 nm was used to avoid the IR component of the He±Ne laser. The incident laser power was 2 mW. For a-SiGe cells, 514.5 nm Ar laser excitation was used, the incident laser power was 6 mW.
3. Results The preparation conditions and the cell performances for H-diluted cells are listed in Table 1. Four group of cells deposited at 150°C, 175°C, 200°C, and 250°C were studied. We will pay special attention to the H-dilution eect on the open circuit voltage, Voc . We note that Voc is greater than 0.9 V in all the cells and it increases with increasing H-dilution. For the same H-dilution, on the other hand, Voc increases with decreasing deposition temperature, Ts . The luminescence spectrum obtained at 80 K originates from tail-to-tail transitions [5], hence it gives information about the optical gap and the energy distribution of the tail states. We expect the same spectral lineshape for the EL and PL from the same sample. In Fig. 1, typical EL and PL spectra at 80 K are shown for a group of cells
G. Yue et al. / Journal of Non-Crystalline Solids 266±269 (2000) 1119±1123
1121
294 5 to 206 5 meV as shown in Fig. 1(a) whereas, the PL spectral lineshapes do not change much as shown in Fig. 1(b). More interestingly, at the H-dilution of R 100 the EL peak energy (1:43 0:05 eV) is 20 meV greater than PLs (1:41 0:05 eV). To our knowledge, this phenomenon of ELpeak > PLpeak has not been observed in a-Si based diodes [1,2]. We observed the same tendency in the three groups of cells listed in Table 1, so we assume that the results cannot be due to experimental error. We further studied the EL and PL spectra of aSiGe cells with a dierent Ge content (the ¯ow ratio of GeH4 :Si2 H6 45:55 and 50:50). The typical EL and PL spectra from a pair of a-SiGe cells are shown in Fig. 2 (a) and (b), respectively. The result shows that (a) the EL peak energy, ELpeak , is the same as the PLpeak within the experimental error of 10 meV, and (b) both the ELpeak and PLpeak decrease from 1.14 to 1.04 eV with increasing Ge content due to the reduction of the optical gap [6]. Finally, we summarize all the data of ELpeak and PLpeak at 80 K as a function of Voc for all the a-Si:H and a-SiGe cells studied. In Fig. 3(a), we note that higher the luminescence peak energy, the larger the Voc obtained. However, there are two interesting points: First, the ELpeak is NOT always less than the PLpeak for the same sample. It can be the same or even larger as shown on the right side in Fig. 3(b) (the largest Voc ). Second, the ELpeak shifts 50 meV from 1:28 0:05 to 1:43 0:05 eV in the same direction as Voc does; whereas, the PLpeak at 1.4 eV is independent of Voc . 4. Discussion Fig. 1. Typical luminescence spectra for a group of cells deposited at Ts 200°C with dierent H-dilution ratio from R 25 to 100, (a) EL, and (b) PL, the inset is EL and PL from the sample of R 100.
deposited at Ts 200°C with increasing H-dilution from R 25 to 100. Interestingly, we ®nd that the EL peak energy shifts from 1:28 0:05 to 1:43 0:05 eV and the band width decreases from
Both the luminescence peak energy and the Voc are relevant to the optical gap of the i-layer material [6], so it is reasonable to observe a correlation between the luminescence peak energy and Voc . The PL feature has been explained by the thermalization model [7,8], i.e., the PLpeak depends on the position of the demarcation level, ED . The carrier transport is not involved in PL processes. As long as the optical gap and the band tails have
1122
G. Yue et al. / Journal of Non-Crystalline Solids 266±269 (2000) 1119±1123
Fig. 3. Correlation between Voc and the ELpeak (d) and PLpeak (s) at 80 K: (a) for a-Si:H and a-SiGe cells, and (b) for aSi:H cells with dierent H-dilution. The lines are drawn as guides for the eye. Fig. 2. Typical luminescence spectra for a group of a-SiGe cells with dierent Ge ¯ow ratio from GeH4 :Si2 H6 45:55 to 50:50, (a) EL, and (b) PL.
not changed, the PL lineshape would not change. We explained the dierences between the ELpeak and PLpeak by using the dispersive-transport-controlled recombination model [2]. At low tempera-
ture (80 K), hopping transport in the exponential band tail must be considered [9]. When the carrier lifetime is longer than its transit time, the carriers would hop down to the transport level, Et , then recombine. In the case of smaller H-diluted a-Si:H cells, the carrier's recombination lifetime in the ilayer is much longer than the transit time. In other
G. Yue et al. / Journal of Non-Crystalline Solids 266±269 (2000) 1119±1123
words, Et is deeper than ED , hence PLpeak > ELpeak was observed. However, for a-SiGe cells, the mobility lifetime product (ls) is 2 or 3 orders of magnitude less than that for a-Si:H [6], so the carriers would recombine right after injecting into the i-layer. When Et is equal to ED , PLpeak ELpeak was observed in a-SiGe cells. The carrier lifetime could be further shortened due to the increase of defect density by microcrystallinity in the high Hdiluted cells [10]. Therefore, the energy of Et could be higher than ED , and then ELpeak > PLpeak was obtained in highly H-diluted cells.
1123
(c) We observed ELpeak > PLpeak in highly Hdiluted a-Si:H p±i±n cells. Acknowledgements This work is supported by NREL sub-subcontract XAK-8-17619-11 and the thin ®lm PV partnership. Han is partially supported by CGP Fund, NSF-Int-9802430. Yue is partially supported by NSF-Int-960495. Deng is supported by NREL subcontract ZAF-8-17619-14 and work at BP Solarex is supported by NREL under subcontract No. ZAK-8-17619-02.
5. Conclusion EL and PL spectroscopies were used to study the eects of H-dilution in a-Si:H as well as the eects of varying Ge content in a-SiGe solar cells. Statistically, higher the luminescence peak energies, ELpeak and PLpeak , the larger the Voc . However, the Voc is not only relevant to the optical gap but also to the transport processes. This relevancy was demonstrated in the EL spectra. (a) The Voc increased with increasing H-dilution. When R increased from 20 to 100, the ELpeak shifted from 1.28 to 1.43 eV whereas, PLpeak at 1.40 eV did not change. (b) For a-SiGe p±i±n cells GeH4 :Si2 H6 45:55 and 50:50), the peak energy positions of the EL and PL are the same. Both the ELpeak and PLpeak decreased from 1.14 to 1.04 eV with increasing Ge content due to the reduction of the optical gap.
References [1] T.S. Nashashibi, I.G. Austin, T.M. Searle, R.A. Gibson, W.E. Spear, P.G. Le Comber, Philos. Mag. B 45 (1982) 553. [2] D. Han, K. Wang, B. Von Roedern, Phys. Rev. Lett. 77 (1996) 4410. [3] R. Arya, A. Catalano, M. Bennett, J. Newton, B. Fieselman, L. Yang, Y.M. Li, R. D'Aiello, in: Proceedings of the 11th EC PVSEC, 1992, p. 199. [4] D.V. Tsu, B.S. Chao, S.R. Ovshinsky, S. Guha, J. Yang, Appl. Phys. Lett. 71 (1997) 1317. [5] F. Boulitrop, D.J. Dunstan, Phys. Rev. B 28 (1983) 5923. [6] M. Stutzmann, R.A. Street, C.C. Tsai, J.B. Boyce, S.E. Ready, J. Appl. Phys. 66 (1989) 569. [7] R.W. Collins, M.A. Paesler, W. Paul, Solid State Commun. 34 (1980) 833. [8] R.W. Collins, W. Paul, Phys. Rev. B 25 (1982) 5257. [9] D. Monroe, Phys. Rev. Lett. 54 (1985) 146. [10] D. Han, H. Habuchi, T. Hori, A. Nishibe, T. Namioka, Jing Lin, Guozhen Yue, these Proceedings, p. 274.