- Email: [email protected]
p-Si heterojunction and solar cells
Solid-State Electronics 44 (2000) 1471±1475
Recombination center in C60/p-Si heterojunction and solar cells Aurangzeb Khan *, Masafumi Yamaguchi, N. Kojima Semiconductor Laboratory, Toyota Technological Institute, 2-12-1 Hisakata, Tempaku, Nagoya 468-8511, Japan Received 9 November 1999; received in revised form 22 February 2000
Abstract Deep level transient spectroscopy (DLTS) has been used to detect the contribution of interface-related states, responsible for minority carrier lifetime reduction in C60 /p-Si solar cells. The very low eciency of C60 based solar cells is found to be due to the presence of one hole level EV 0:70 eV at the interface of C60 /p-Si heterojunction. The hole capture cross-section and the variation with temperature have been determined, to account for minority carrier lifetime of the material. A large barrier for hole capture, implying a large electron±phonon interaction, characterizes the observed hole level. A pronounced reduction in DLTS peak of the dominant level (0:70 eV) due to injection of electrons provides evidence of electron±hole recombination. A detailed characterization indicates that a dominant deep level, with comparable electron and hole emission rates, acts as a recombination center, which governs the minority-carrier lifetime Ó 2000 Elsevier Science Ltd. All rights reserved. Keywords: Interface states; C60 based solar cells; Deep levels; Minority carrier diusion length
1. Introduction C60 thin ®lms have the possibility of photovoltaic device application because of its band gap energy being in the range 1.6±1.7 eV. Recently, the photovoltaic and photoconductive properties of C60 /p-Si heterojunction solar cells have been demonstrated by the authors [1] and other researchers [2,3]. It has been found that C60 interacts strongly with Si substrates and shows strong rectifying behavior. However, eciency of C60 based solar cells (C60 /p-Si) was very low. In order to achieve high-eciency devices, a detailed understanding of different aspects regarding electronic and structural properties of these materials is necessary. In particular, characterizing the deep level states/interface states of the C60 /p-Si heterojunction is an important step in re®ning our understanding of the role of deep states and improving the photovoltaic parameters of the cell.
*
Corresponding author. Tel.: +81-52-809-1877; fax: +81-52809-1879. E-mail address: [email protected] (A. Khan).
In this study, we have evaluated interface-related states of C60 /p-Si heterojunction solar cells, by using deep level transient spectroscopy (DLTS), that clarify the role of the deep states to the minority-carrier recombination (minority-carrier lifetime) that has a direct in¯uence on conversion eciency of the solar cells. 2. Experimental C60 /p-Si heterojunction cells were fabricated by the molecular beam epitaxy (MBE) method. A schematic cross-section of the cell used in this study is shown in Fig. 1. Undoped C60 acts as an n-type semiconductor. Therefore, 380 lm thick p-type Si (1 0 0) wafers (2±3 X cm) were used as a substrates, so that n±p heterojunction was formed with ideality factor, n 1:45. A large value of n (1.45) indicates the presence of an interfacial layer. Further details of the samples, together with detailed analysis of the photovoltaic properties of the devices, are included in a previous publication [1]. The capacitance±voltage (C±V) analysis of the cell shows that plotting 1/C2 versus reverse voltage (VR ) plot gives a straight line, as shown in Fig. 2, indicating
0038-1101/00/$ - see front matter Ó 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 3 8 - 1 1 0 1 ( 0 0 ) 0 0 0 6 2 - 9
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A. Khan et al. / Solid-State Electronics 44 (2000) 1471±1475
Fig. 1. A schematic diagram of the C60 /p-Si heterojunction cell used in this study.
carrier injection pulses. The minority carrierÕs capture properties of the observed deep level were studied using extended capacitance transient spectroscopy (double carrier pulse DLTS). The activation energy of the observed levels were obtained from the slopes of plot of ln e=T 2 vs 1=T , where e is the thermal emission rate of electron or hole and T is the temperature. The measurements of capture cross-section were performed by using the standard procedure [4], i.e. variable width single pulse method. The reduction of the DLTS peak height S was monitored with the decrease in the excitation pulse width tp . The data were plotted as log 1 ÿ S=S0 vs tp at a ®x emission rate window, S0 being the saturated DLTS peak height. The semilog plot was usually found to be signi®cantly deviated from the theoretically expected exponential behavior, due to the presence of additional deep levels. In the capture cross-section measurements, undertaken here, the DLTS peak reduction with tp was found to ®t a straight line on the log 1 ÿ S=S0 vs tp plots. The slope of the straight lines obtained from the semilog plot was used to calculate capture rates at the temperature of the measurements. To investigate the temperature dependence of the capture cross-section, these experiments were repeated for dierent DLTS emission rates, each window corresponding to a DLTS peak at dierent temperature.
3. Results and discussion
Fig. 2. Capacitance±voltage characteristics on a C60 /p-Si heterojunction solar cell. The line inside the ®gure is a least square ®t of the data which show abrupt heterojunction.
abruptness of the C60 /Si heterojunction. The curve in the inset shows a linear relationship over the measured voltage range, also indicating uniformity of a shallow doping concentration. C±V measurements at room temperature and at 77 K were taken in order to monitor the variation, if any, of the shallow level concentration with temperature due to possible freeze-out eects. The shallow level concentration determined from C±V analysis at peak temperature is 4 ´ 1014 cmÿ3 . The depletion width, at typical reverse bias (VR ÿ6 V), was 1.4 lm. DLTS has been employed to investigate the defect characteristics using a deep level spectrometer Model MI416B (Sanwa) and bio Rad DL8000. Excitation of the deep levels was performed using the usual reduce reverse biased majority carrier pulses and by minority
The DLTS spectrum of C60 /p-Si sample clearly reveals the presence of one dominant hole level with the activation energy of 0.70 eV as shown in Figs. 3 and 4. The trap density NT and capture cross-section r (at 300 K) measured are 4:53 1013 cmÿ3 and 3:52 10ÿ14 cm2 , respectively. The DLTS spectrum shows a very interesting feature. It can be seen that the DLTS peak heights increase with temperature. Note that the peak heights are not constant with ®xed (t1=t2) ratio (Fig. 3). The variation of the peak height indicates a level in the lower half of the band with non-negligible electron emission rates (provided that this assumption is only possible when deep level has large minority carrier emission rates). This anomalous behavior displayed by our sample can be explained by assuming temperature dependent capture cross-section with comparable electron and hole emission rates. The knowledge of capture cross-section of deep levels for both electrons and holes is necessary to establish whether a deep level is acting as a recombination, generation or trapping center. In addition, such knowledge leads to a deeper insight into the electronic structure of the deep level centers and the defect±lattice interaction.
A. Khan et al. / Solid-State Electronics 44 (2000) 1471±1475
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In order to clarify the role of the observed deep states in reduction of minority-carrier lifetime, the detailed capture cross-sections were measured over the temperature range 300±360 K and were found to be strongly temperature dependent according to the relation rp r1 exp ÿEb =KT ;
1
where Eb is the activation energy (thermal energy barrier) for non-radiative multiphonon capture. The multiphonon capture process involves the movement of an electron above the conduction band curves crossover point, surmounting an energy barrier Eb . Thermal emission is the reverse of this, with dierent barrier Ea . The large capture barrier (0.30 eV) observed for this level suggests a strong lattice relaxation within the multiphonon emission model of capture (this mechanism is favored whenever there is a large change in the equilibrium lattice coordinates as well as in the energy of the defect state during capture or ejection of an electron or hole). In case of multiphonon capture, the activation energy Eb for carrier capture must be subtracted from the empirical activation energy Ea , for thermal emission to obtained the true deep level depth ET , using the detailed balance formula en;p rn;p V NC;V exp ÿET =KT ; where ET Ea ÿ Eb : Fig. 3. DLTS spectra of C60 /Si junction, with (t1, t2) set as (1 ms, 2 ms), (0.5 ms, 1 ms), (0.3 ms, 0.6 ms), (0.2 ms, 0.4 ms), and (0.1 ms, 0.2 ms), respectively.
The capture barrier of Eb 0:30 eV, along with the hole emission activation energy Ea 0:70 eV, means that the position of the level ET 0:40 eV is above the valence band edge. This means that the dominant deep level observed might be acting as recombination center, which governs the minority carrier lifetime in the cell and hence low conversion eciency of C60 /Si heterojunction solar cells. Using the measured NT and rp values, minoritycarrier lifetime sp in C60 was estimated by Eq. (3) 1=sp rp vNT ;
Fig. 4. Emission rate signatures for the dominant deep level observed in the C60 /Si heterojunction cell.
2
3
where v is the thermal velocity of the carrier. The estimated average sp is 65 ns. The minority-carrier diusion length Lp , estimated by Lp (Dsp )1=2 , is 0.018 lm. We were able to discount the possibility of the observed hole trap (0.7 eV) located in the solid C60 or p-Si. We have recorded a spectrum using a large reverse bias and only a small decrease in bias to ®ll the traps, so that the occupancy of the interface state is not perturbed signi®cantly, and transient is due to the state in the bulk of the semiconductor. Fig. 5(a) corresponds to the DLTS scan excluding junction region of the depletion layer, and Fig. 5(b) includes the junction region of the depletion layer. The most interesting feature of this measurement is the marked decrease in concentration of the dominant hole level EV 0:70 eV, where the occupancy of the interface state is not perturbed Fig. 5(a),
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A. Khan et al. / Solid-State Electronics 44 (2000) 1471±1475
Fig. 5. Comparison of DLTS signals under two dierent carrier pulse conditions: (a) DLTS scan excluding junction region of the depletion layer. The reverse bias and ®lling pulse were 6 and 5 V, respectively and only thin region of the depletion layer was sampled. (b) 0 V single pulse to ®ll the traps by hole including junction region from the surface was sampled.
and only a thin region of the depletion layer was sampled. The reverse bias and ®lling pulse were 6 and 5 V, respectively. This leads to the fact that the observed hole trap is just located at the C60 /Si heterojunction interface. Furthermore, if hole trap 0.7 eV is located in solid C60 , then this level must not be observed due to reduce reverse bias (without injection pulse), because C60 is a weak n-type semiconductor. In addition, to discount the possibility of observed deep level in p-Si, DLTS measurements have also been done on Schottky diode (Ti/pSi) made on the same substrate as the C60 /p-Si, which shows complete absence of hole trap 0.7 eV. Direct evidence was obtained to demonstrate the role of the 0.70 eV level as a recombination center using double pulse DLTS measurements. We have carried out a systematic study of the variation of the majority carrier emission spectra following injection with the strength of minority-carrier (electrons) injection pulses. The traps are ®lled with majority carriers with shortcircuit pulse (or 0 V pulse) of 1 ms duration, so that all the traps are occupied by holes, then a forward bias minority carrier injection pulse of duration 50 ls is applied to induce the capture of minority carriers. Interestingly, a pronounced decrease in the DLTS spectra have been observed following the amplitude of minoritycarrier injection pulse. This indicates that the deep level
state (EV 0:70 eV) is also much more ecient in capturing electron. This means that rn P rp , leading to acts as recombination center. It should be noted that under forward bias minority carrier injection pulses, the depletion layer is almost collapsed and if the minority carrier capture cross-section of the interface state is high enough then there is a probability of capturing the minority carriers (electrons) at the interface state even at a high electric ®eld region. A typical DLTS spectra for such evidence are shown in Fig. 6. Fig. 7 shows the concentration of the deep level (0.70 eV) as a function of the respective injection pulse voltage. These changes continue with increase of injection up to about 4 V, beyond which saturation is observed. The pronounced reduction of the DLTS peak, when electrons are present, can be explained by electron±hole recombination. If the capture rates of the electrons by the defect studied is in the same order or higher, then the capture rates of the hole during the injection pulse, a reduction of DLTS peak associated with hole traps will occur, since a defect will only be partly ®lled with holes after the ®lling pulse. The pronounced reduction in DLTS peak corresponds
Fig. 6. DLTS spectra showing the reduction in the peak height of 0.70 eV deep level on the injection pulse voltage: (a) 1 ms single carrier pulse (hole) with a reverse bias of 2 V and pulse height of 0 V to ®ll the trap by hole and (b) 50 ls minority carrier injection pulse (ÿ2 V double carrier pulse) with a reverse bias of 2 V and a current density of 400 mA/cm2 .
A. Khan et al. / Solid-State Electronics 44 (2000) 1471±1475
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sion activation energy of 0.70 eV, using the capture barrier height 0.30 eV, the true energy position of the level EV 0:70 eV was found to be 0.40 eV. It has been observed that the amplitude of the DLTS peak was signi®cantly reduced when both holes and electrons were injected into the space charge region. This reduction of the peak is only possible if the observed center act as the ecient hole±electron recombination center. The temperature dependence of capture cross-section variation of peak height with electron injection, and, the variation of peak height with a ®xed t1=t2 ratio showing comparable electron and hole emission rates of the deep level (0.70 eV) suggest that this center acts as a recombination center which may governed the minority-carrier lifetime.
Acknowledgements Fig. 7. Dependence of the concentration of the deep level shown in Fig. 6 as a function of the minority-carrier injection pulse voltage.
to the observed level, when electrons are present suggests that the observed interface state acts as a recombination center. The origin of observed recombination center is not clear. 4. Conclusion DLTS was used to clarify the correlation between the interface states and the minority carrier recombination. It has been found that the dominant hole level, with activation energy 0.70 eV, has a strongly temperature dependent capture cross-section and the data ®t the theory of multiphonon emission. Correcting the emis-
This work was partially supported by the Japan Society for the promotion of Science (Research for the Future Program: JSPS-RFTF97P00902), and by the Ministry of Education as a Private University HighTechnology Research Center Program.
References [1] Kojima N, Ishikawa N, Yang MJ, Yamaguchi M. In: Proceedings of the Second World Conference on Photovoltaic Energy Conversion. Wien, Austria, July 6±10, 1998. p. 246. [2] Kita K, Wen C, Ihara M, Yamada K. J Appl Phys 1996; 79:2798. [3] Chen KM, Jia YQ, Jin SX, Wu K, Zhao WB, Li CY, Gu ZN, Zhou XH. J Phys Conds Matter 1995;7:L201. [4] Henry CH, Lang DV. Phys Rev B 1977;15:989.
Recombination center in C60/p-Si heterojunction and solar cells Aurangzeb Khan *, Masafumi Yamaguchi, N. Kojima Semiconductor Laboratory, Toyota Technological Institute, 2-12-1 Hisakata, Tempaku, Nagoya 468-8511, Japan Received 9 November 1999; received in revised form 22 February 2000
Abstract Deep level transient spectroscopy (DLTS) has been used to detect the contribution of interface-related states, responsible for minority carrier lifetime reduction in C60 /p-Si solar cells. The very low eciency of C60 based solar cells is found to be due to the presence of one hole level EV 0:70 eV at the interface of C60 /p-Si heterojunction. The hole capture cross-section and the variation with temperature have been determined, to account for minority carrier lifetime of the material. A large barrier for hole capture, implying a large electron±phonon interaction, characterizes the observed hole level. A pronounced reduction in DLTS peak of the dominant level (0:70 eV) due to injection of electrons provides evidence of electron±hole recombination. A detailed characterization indicates that a dominant deep level, with comparable electron and hole emission rates, acts as a recombination center, which governs the minority-carrier lifetime Ó 2000 Elsevier Science Ltd. All rights reserved. Keywords: Interface states; C60 based solar cells; Deep levels; Minority carrier diusion length
1. Introduction C60 thin ®lms have the possibility of photovoltaic device application because of its band gap energy being in the range 1.6±1.7 eV. Recently, the photovoltaic and photoconductive properties of C60 /p-Si heterojunction solar cells have been demonstrated by the authors [1] and other researchers [2,3]. It has been found that C60 interacts strongly with Si substrates and shows strong rectifying behavior. However, eciency of C60 based solar cells (C60 /p-Si) was very low. In order to achieve high-eciency devices, a detailed understanding of different aspects regarding electronic and structural properties of these materials is necessary. In particular, characterizing the deep level states/interface states of the C60 /p-Si heterojunction is an important step in re®ning our understanding of the role of deep states and improving the photovoltaic parameters of the cell.
*
Corresponding author. Tel.: +81-52-809-1877; fax: +81-52809-1879. E-mail address: [email protected] (A. Khan).
In this study, we have evaluated interface-related states of C60 /p-Si heterojunction solar cells, by using deep level transient spectroscopy (DLTS), that clarify the role of the deep states to the minority-carrier recombination (minority-carrier lifetime) that has a direct in¯uence on conversion eciency of the solar cells. 2. Experimental C60 /p-Si heterojunction cells were fabricated by the molecular beam epitaxy (MBE) method. A schematic cross-section of the cell used in this study is shown in Fig. 1. Undoped C60 acts as an n-type semiconductor. Therefore, 380 lm thick p-type Si (1 0 0) wafers (2±3 X cm) were used as a substrates, so that n±p heterojunction was formed with ideality factor, n 1:45. A large value of n (1.45) indicates the presence of an interfacial layer. Further details of the samples, together with detailed analysis of the photovoltaic properties of the devices, are included in a previous publication [1]. The capacitance±voltage (C±V) analysis of the cell shows that plotting 1/C2 versus reverse voltage (VR ) plot gives a straight line, as shown in Fig. 2, indicating
0038-1101/00/$ - see front matter Ó 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 3 8 - 1 1 0 1 ( 0 0 ) 0 0 0 6 2 - 9
1472
A. Khan et al. / Solid-State Electronics 44 (2000) 1471±1475
Fig. 1. A schematic diagram of the C60 /p-Si heterojunction cell used in this study.
carrier injection pulses. The minority carrierÕs capture properties of the observed deep level were studied using extended capacitance transient spectroscopy (double carrier pulse DLTS). The activation energy of the observed levels were obtained from the slopes of plot of ln e=T 2 vs 1=T , where e is the thermal emission rate of electron or hole and T is the temperature. The measurements of capture cross-section were performed by using the standard procedure [4], i.e. variable width single pulse method. The reduction of the DLTS peak height S was monitored with the decrease in the excitation pulse width tp . The data were plotted as log 1 ÿ S=S0 vs tp at a ®x emission rate window, S0 being the saturated DLTS peak height. The semilog plot was usually found to be signi®cantly deviated from the theoretically expected exponential behavior, due to the presence of additional deep levels. In the capture cross-section measurements, undertaken here, the DLTS peak reduction with tp was found to ®t a straight line on the log 1 ÿ S=S0 vs tp plots. The slope of the straight lines obtained from the semilog plot was used to calculate capture rates at the temperature of the measurements. To investigate the temperature dependence of the capture cross-section, these experiments were repeated for dierent DLTS emission rates, each window corresponding to a DLTS peak at dierent temperature.
3. Results and discussion
Fig. 2. Capacitance±voltage characteristics on a C60 /p-Si heterojunction solar cell. The line inside the ®gure is a least square ®t of the data which show abrupt heterojunction.
abruptness of the C60 /Si heterojunction. The curve in the inset shows a linear relationship over the measured voltage range, also indicating uniformity of a shallow doping concentration. C±V measurements at room temperature and at 77 K were taken in order to monitor the variation, if any, of the shallow level concentration with temperature due to possible freeze-out eects. The shallow level concentration determined from C±V analysis at peak temperature is 4 ´ 1014 cmÿ3 . The depletion width, at typical reverse bias (VR ÿ6 V), was 1.4 lm. DLTS has been employed to investigate the defect characteristics using a deep level spectrometer Model MI416B (Sanwa) and bio Rad DL8000. Excitation of the deep levels was performed using the usual reduce reverse biased majority carrier pulses and by minority
The DLTS spectrum of C60 /p-Si sample clearly reveals the presence of one dominant hole level with the activation energy of 0.70 eV as shown in Figs. 3 and 4. The trap density NT and capture cross-section r (at 300 K) measured are 4:53 1013 cmÿ3 and 3:52 10ÿ14 cm2 , respectively. The DLTS spectrum shows a very interesting feature. It can be seen that the DLTS peak heights increase with temperature. Note that the peak heights are not constant with ®xed (t1=t2) ratio (Fig. 3). The variation of the peak height indicates a level in the lower half of the band with non-negligible electron emission rates (provided that this assumption is only possible when deep level has large minority carrier emission rates). This anomalous behavior displayed by our sample can be explained by assuming temperature dependent capture cross-section with comparable electron and hole emission rates. The knowledge of capture cross-section of deep levels for both electrons and holes is necessary to establish whether a deep level is acting as a recombination, generation or trapping center. In addition, such knowledge leads to a deeper insight into the electronic structure of the deep level centers and the defect±lattice interaction.
A. Khan et al. / Solid-State Electronics 44 (2000) 1471±1475
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In order to clarify the role of the observed deep states in reduction of minority-carrier lifetime, the detailed capture cross-sections were measured over the temperature range 300±360 K and were found to be strongly temperature dependent according to the relation rp r1 exp ÿEb =KT ;
1
where Eb is the activation energy (thermal energy barrier) for non-radiative multiphonon capture. The multiphonon capture process involves the movement of an electron above the conduction band curves crossover point, surmounting an energy barrier Eb . Thermal emission is the reverse of this, with dierent barrier Ea . The large capture barrier (0.30 eV) observed for this level suggests a strong lattice relaxation within the multiphonon emission model of capture (this mechanism is favored whenever there is a large change in the equilibrium lattice coordinates as well as in the energy of the defect state during capture or ejection of an electron or hole). In case of multiphonon capture, the activation energy Eb for carrier capture must be subtracted from the empirical activation energy Ea , for thermal emission to obtained the true deep level depth ET , using the detailed balance formula en;p rn;p V NC;V exp ÿET =KT ; where ET Ea ÿ Eb : Fig. 3. DLTS spectra of C60 /Si junction, with (t1, t2) set as (1 ms, 2 ms), (0.5 ms, 1 ms), (0.3 ms, 0.6 ms), (0.2 ms, 0.4 ms), and (0.1 ms, 0.2 ms), respectively.
The capture barrier of Eb 0:30 eV, along with the hole emission activation energy Ea 0:70 eV, means that the position of the level ET 0:40 eV is above the valence band edge. This means that the dominant deep level observed might be acting as recombination center, which governs the minority carrier lifetime in the cell and hence low conversion eciency of C60 /Si heterojunction solar cells. Using the measured NT and rp values, minoritycarrier lifetime sp in C60 was estimated by Eq. (3) 1=sp rp vNT ;
Fig. 4. Emission rate signatures for the dominant deep level observed in the C60 /Si heterojunction cell.
2
3
where v is the thermal velocity of the carrier. The estimated average sp is 65 ns. The minority-carrier diusion length Lp , estimated by Lp (Dsp )1=2 , is 0.018 lm. We were able to discount the possibility of the observed hole trap (0.7 eV) located in the solid C60 or p-Si. We have recorded a spectrum using a large reverse bias and only a small decrease in bias to ®ll the traps, so that the occupancy of the interface state is not perturbed signi®cantly, and transient is due to the state in the bulk of the semiconductor. Fig. 5(a) corresponds to the DLTS scan excluding junction region of the depletion layer, and Fig. 5(b) includes the junction region of the depletion layer. The most interesting feature of this measurement is the marked decrease in concentration of the dominant hole level EV 0:70 eV, where the occupancy of the interface state is not perturbed Fig. 5(a),
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A. Khan et al. / Solid-State Electronics 44 (2000) 1471±1475
Fig. 5. Comparison of DLTS signals under two dierent carrier pulse conditions: (a) DLTS scan excluding junction region of the depletion layer. The reverse bias and ®lling pulse were 6 and 5 V, respectively and only thin region of the depletion layer was sampled. (b) 0 V single pulse to ®ll the traps by hole including junction region from the surface was sampled.
and only a thin region of the depletion layer was sampled. The reverse bias and ®lling pulse were 6 and 5 V, respectively. This leads to the fact that the observed hole trap is just located at the C60 /Si heterojunction interface. Furthermore, if hole trap 0.7 eV is located in solid C60 , then this level must not be observed due to reduce reverse bias (without injection pulse), because C60 is a weak n-type semiconductor. In addition, to discount the possibility of observed deep level in p-Si, DLTS measurements have also been done on Schottky diode (Ti/pSi) made on the same substrate as the C60 /p-Si, which shows complete absence of hole trap 0.7 eV. Direct evidence was obtained to demonstrate the role of the 0.70 eV level as a recombination center using double pulse DLTS measurements. We have carried out a systematic study of the variation of the majority carrier emission spectra following injection with the strength of minority-carrier (electrons) injection pulses. The traps are ®lled with majority carriers with shortcircuit pulse (or 0 V pulse) of 1 ms duration, so that all the traps are occupied by holes, then a forward bias minority carrier injection pulse of duration 50 ls is applied to induce the capture of minority carriers. Interestingly, a pronounced decrease in the DLTS spectra have been observed following the amplitude of minoritycarrier injection pulse. This indicates that the deep level
state (EV 0:70 eV) is also much more ecient in capturing electron. This means that rn P rp , leading to acts as recombination center. It should be noted that under forward bias minority carrier injection pulses, the depletion layer is almost collapsed and if the minority carrier capture cross-section of the interface state is high enough then there is a probability of capturing the minority carriers (electrons) at the interface state even at a high electric ®eld region. A typical DLTS spectra for such evidence are shown in Fig. 6. Fig. 7 shows the concentration of the deep level (0.70 eV) as a function of the respective injection pulse voltage. These changes continue with increase of injection up to about 4 V, beyond which saturation is observed. The pronounced reduction of the DLTS peak, when electrons are present, can be explained by electron±hole recombination. If the capture rates of the electrons by the defect studied is in the same order or higher, then the capture rates of the hole during the injection pulse, a reduction of DLTS peak associated with hole traps will occur, since a defect will only be partly ®lled with holes after the ®lling pulse. The pronounced reduction in DLTS peak corresponds
Fig. 6. DLTS spectra showing the reduction in the peak height of 0.70 eV deep level on the injection pulse voltage: (a) 1 ms single carrier pulse (hole) with a reverse bias of 2 V and pulse height of 0 V to ®ll the trap by hole and (b) 50 ls minority carrier injection pulse (ÿ2 V double carrier pulse) with a reverse bias of 2 V and a current density of 400 mA/cm2 .
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sion activation energy of 0.70 eV, using the capture barrier height 0.30 eV, the true energy position of the level EV 0:70 eV was found to be 0.40 eV. It has been observed that the amplitude of the DLTS peak was signi®cantly reduced when both holes and electrons were injected into the space charge region. This reduction of the peak is only possible if the observed center act as the ecient hole±electron recombination center. The temperature dependence of capture cross-section variation of peak height with electron injection, and, the variation of peak height with a ®xed t1=t2 ratio showing comparable electron and hole emission rates of the deep level (0.70 eV) suggest that this center acts as a recombination center which may governed the minority-carrier lifetime.
Acknowledgements Fig. 7. Dependence of the concentration of the deep level shown in Fig. 6 as a function of the minority-carrier injection pulse voltage.
to the observed level, when electrons are present suggests that the observed interface state acts as a recombination center. The origin of observed recombination center is not clear. 4. Conclusion DLTS was used to clarify the correlation between the interface states and the minority carrier recombination. It has been found that the dominant hole level, with activation energy 0.70 eV, has a strongly temperature dependent capture cross-section and the data ®t the theory of multiphonon emission. Correcting the emis-
This work was partially supported by the Japan Society for the promotion of Science (Research for the Future Program: JSPS-RFTF97P00902), and by the Ministry of Education as a Private University HighTechnology Research Center Program.
References [1] Kojima N, Ishikawa N, Yang MJ, Yamaguchi M. In: Proceedings of the Second World Conference on Photovoltaic Energy Conversion. Wien, Austria, July 6±10, 1998. p. 246. [2] Kita K, Wen C, Ihara M, Yamada K. J Appl Phys 1996; 79:2798. [3] Chen KM, Jia YQ, Jin SX, Wu K, Zhao WB, Li CY, Gu ZN, Zhou XH. J Phys Conds Matter 1995;7:L201. [4] Henry CH, Lang DV. Phys Rev B 1977;15:989.