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GaAs multiple quantum well
Materials Letters 57 (2003) 2932 – 2935 www.elsevier.com/locate/matlet
Influence of annealing condition on photoluminescence characteristics of AlGaAs/GaAs multiple quantum well Minju Ying a,*, Yueyuan Xia b, Pijun Liu c, Xiangdong Liu b, Mingwen Zhao b, Zhuo Wang d a
Department of Optoelectronics, Shandong University, Jinan, Shandong 250100, China b Department of Physics, Shandong University, Jinan, Shandong 250100, China c Research Institute for Micro/Nanometer Science and Technology, Shanghai Jiao Tong University, Shanghai 200030, China d Institute of Crystal Materials, Shandong University, Jinan, Shandong 250100, China Received 16 September 2002; accepted 19 November 2002
Abstract The energy levels in AlGaAs/GaAs multiple quantum well (MQW) were calculated with transfer matrix method (TMM) and compared with experimental results from photoluminescence (PL) measurements. The comparison shows good agreement. A thin layer of sputtered SiO2 film was deposited on AlGaAs/GaAs MQW samples and a subsequent rapid thermal annealing (RTA) was carried out. Photoluminescence (PL) measurements showed that the PL characteristics of the wells at different depths beneath the surface of the AlGaAs/GaAs MQW behave differently with the change of RTA condition. The PL intensities of the wells close to the surface and the bottom of the AlGaAs/GaAs MQW are sensitive to the RTA condition, while those of the wells in the middle of the MQW show little variations with the change of RTA condition. D 2002 Elsevier Science B.V. All rights reserved. PACS: 78.55.Cr; 66.30.Jt; 81.15.Cd Keywords: AlGaAs/GaAs MQW; Rapid thermal annealing; Photoluminescence
Heteroepitaxy of III –V semiconductors has a wide range of applications such as in high-efficiency tandem solar cells and optoelectronic integrated circuits (OEIC) [1]. The arsenide-based material system, AlGaAs alloy material, now becomes an attractive alternative to the phosphide-based material system [2]. Investigation of this kind of heteroepitaxial layers is a field of still growing physical and technological
* Corresponding author. Tel.: +86-531-8364655; fax: +86-5318565167. E-mail address: [email protected] (M. Ying).
interest [3]. In this paper, we calculated the locations of the energy levels in AlGaAs/GaAs multiple quantum well (MQW) with transfer matrix method (TMM) and compared the calculated results with the experimental values from photoluminescence (PL) measurement. The influence of the RTA condition on PL characteristics of the wells at different depths beneath the surface of the AlGaAs/GaAs MQW was also studied after annealing the samples at 650– 780 jC for 20 to 80 s. The AlGaAs/GaAs MQW samples used here are grown on (100) oriented semi-insulating GaAs substrates by metallo-organic chemical vapor deposition
0167-577X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0167-577X(02)01399-X
M. Ying et al. / Materials Letters 57 (2003) 2932–2935
(MOCVD). The structure of the samples was shown in Fig. 1. All the quantum wells in the MQW have the same barrier width, 30 nm, while the widths of the wells are 1.4 nm (#4), 2.8 nm (#3), 5.6 nm (#2), and 11.2 nm (#1), respectively. The optical confining layer and the barrier material are all AlxGa1 xAs with x values shown in Fig. 1. A 200-nm sputtered SiO2 film was deposited upon the samples with a JG-PF3B high-frequency sputter at an rf power of 240 W and a dc bias of 1.4 kV. The Ar pressure was 1 10 2 Torr with the background pressure of 9 10 6 Torr in the chamber. The samples were then annealed in a rapid thermal annealing tube at 650 –780 jC for 20 to 80 s. A subsequent PL measurement was carried out using an Ar+ laser (514.5 nm). The photoluminescence of the samples was dispersed by a SPEX monochromator and detected by two CCD detectors with the wavelength range between 600 – 900 and 600– 1100 nm, respectively. Theoretical calculations of the energy level structures or energy bands for III – V heteroepitaxial layers are very important for better understanding of the physical properties of optoelectronic devices based on quantum well structures and can provide a guideline for design of quantum well device with expected performance. Several methods have been developed to obtain the energy level structures of quantum wells, including WKB approximation [4], the variational calculations [5], Monte Carlo method [6], the finite element method [7], and transfer matrix method (TMM) [8]. The advantage of TMM is that it can give fast and accurate results and is easy to take the variation of the effective mass into account. Furthermore, it can be used to calculate energy levels of quantum well structures with an applied static electric field. In the present work, we use TMM to calculate
2933
the energy levels of AlGaAs/GaAs MQW. In the calculation with TMM, it is of great importance to use appropriate parameters for the effective mass, the band gap Eg and band splitting offset Q. However, the Q values reported yet are quite controversial. Dingle [1] first gave the Q value of 0.85 F 0.03 based on photon absorption measurement, while Miller suggested that the Q value should be 0.57 F 0.03. In this work, both the two values of Q were used in our calculation in order to estimate to what degree this parameter can affect the final result. The PL spectrum of a quantum well usually originated from the transition between energy level E1 of the ground state electron to that of heavy vacancy Eh1. Thus the peak energy of the PL spectrum is given by: Epk ¼ Eg þ E1 þ Eh1 Eex ;
ð1Þ
where Eg is the band gap of GaAs and Eex is the binding energy of the exciton with the value usually being 4 – 10 meV [9]. Eex = 6 meV is used in this work. Calculations of the locations of the energy levels E1 and Eh1 in the AlGaAs/GaAs MQW with different Q values were carried out. Epk were then obtained from Eq. (1). The experimentally measured PL spectrum of the AlGaAs/GaAs MQW is shown in Fig. 2, from which Epk can be also determined. The calculated Epk value and that of the PL measurement were shown in Table 1. It is clear from Table 1 that the calculated results agree quite well with the experimental ones. The Q values have some influence on the calculated results. However, the error resulted from different Q values is within 1%, and it decreases with the increase of the
Fig. 1. The structure of AlGaAs/GaAs multiple quantum well.
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M. Ying et al. / Materials Letters 57 (2003) 2932–2935
layer of sputtered SiO2 film deposited on the samples to prevent possible desorption of the group V elements during the annealing process. Fig. 3(a) shows the PL emission intensities of the four wells as a function of annealing time at 700 jC. It is clear that the intensities of the wells close to the surface (#1) and bottom (#4) of the MQW fluctuate with annealing time, while those of the wells in the middle of the MQW (#2, #3) show little variations. The PL characteristics of the wells close to the surface and the bottom of the MQW are much more sensitive to the RTA condition than those of the wells in the middle of the MQW. PL spectra of the AlGaAs/GaAs samples after RTA at different temperature for 60 s also confirm this conclusion. The result is shown in Fig. 3(b). It is clear that the Fig. 2. PL spectrum of AlGaAs/GaAs MQW at 70 K.
well width Lz. The discrepancy between the experimental results and the calculation ones using TMM also decreases with the increase of Lz. For #1 (Lz = 11.2 nm), the experiment result and the calculated value agrees very well, with an error of about 1%. It is easy to understand this phenomenon since the nonparabolic effect decreases with the increase of the well width, and the effective mass approximation is getting better with the increase of the well width, and thus the TMM is more accurate when the well width is increased. The four wells were treated as independent of each other in the TMM calculation. This is true only when the barrier layer is thick enough. The definite thickness of the barrier width may cause additional error in the TMM calculation. To see the influence of RTA condition on PL characteristics of the wells at different depth beneath the surface of the sample, we annealed the AlGaAs/ GaAs MQW at 650– 780 jC for 20 to 80 s with a thin Table 1 The calculated peak locations of the AlGaAs/GaAs MQW and the experimental ones from PL measurement Well
#4 #3 #2 #1
Well ˚) width (A
EPK (eV) Q = 0.6
Q = 0.85
Experimental results
14 28 56 112
1.774 1.674 1.588 1.538
1.794 1.689 1.594 1.540
1.854 1.725 1.618 1.522
Fig. 3. (a) PL intensities of the four wells in AlGaAs/GaAs MQW as a function of annealing time at the temperature of 700 jC. (b) PL intensities of the four wells in AlGaAs/GaAs MQW as a function of annealing temperature for a given annealing time of 60 s.
M. Ying et al. / Materials Letters 57 (2003) 2932–2935
PL intensities of wells #1 and #4, especially #1, vary greatly with the annealing temperature, while those of wells #2 and #3 remain almost constant. As well #1 is close to the surface of the AlGaAs/GaAs MQW sample, it may endure greater stress caused by the lattice mismatch between the AlGaAs/GaAs MQW and the sputtered SiO2 film. This is also true for well #4, which suffers greater stress from the lattice mismatch between the AlGaAs/GaAs MQW and the GaAs substrate. As it is still not clear how the stress influence the PL characteristics of the wells, further investigations is necessary and is going on now in our group. We calculated the locations of the energy levels in AlGaAs/GaAs MQW with TMM and compared the calculated results with the experimental values from PL measurement. The comparison shows little discrepancy. A thin layer of sputtered SiO2 film was deposited on AlGaAs/GaAs MQW samples with a JG-PF3B high-frequency sputter. After annealing the samples at different condition, PL measurement was carried out. It is found that the emission intensities of the wells close to the surface and the bottom of the MQW are sensitive to the RTA condition, while those of the wells in the middle of the MQW show little variations with the change of RTA condition.
2935
Acknowledgements This work is supported by the National Natural Science Foundation of China under Grant No. 19975030 and Natural Science Foundation of Shandong Province under Grant No. Y2001A02.
References [1] R. Dingle, Semiconductors and Semimetals, vol. 24, Academic Press, New York, 1987. [2] A. Ramam, S.J. Chua, J. Cryst. Growth 175 (1997) 1294. [3] K. Jeganathan, V. Ramakrishnan, J. Kumar, Cryst. Res. Technol. 34 (1999) 1293. [4] E. Merzbacher, Quantum Mechanics, 2nd ed., Wiley, New York, 1970, ch. 7. [5] D. Ahn, S.L. Chuang, Appl. Phys. Lett. 49 (1986) 1450. [6] J. Singh, Appl. Phys. Lett. 48 (1986) 434. [7] K. Hayata, M. Koshiba, K. Nakamra, A. Shimizu, Electron. Lett. 24 (1988) 614. [8] B. Jonsson, S.T. Eng, IEEE J. Quantum Electron. 26 (1990) 2025. [9] G. Bastard, E.E. Mendez, et al., Phys. Rev., B 26 (1982) 1974.
Influence of annealing condition on photoluminescence characteristics of AlGaAs/GaAs multiple quantum well Minju Ying a,*, Yueyuan Xia b, Pijun Liu c, Xiangdong Liu b, Mingwen Zhao b, Zhuo Wang d a
Department of Optoelectronics, Shandong University, Jinan, Shandong 250100, China b Department of Physics, Shandong University, Jinan, Shandong 250100, China c Research Institute for Micro/Nanometer Science and Technology, Shanghai Jiao Tong University, Shanghai 200030, China d Institute of Crystal Materials, Shandong University, Jinan, Shandong 250100, China Received 16 September 2002; accepted 19 November 2002
Abstract The energy levels in AlGaAs/GaAs multiple quantum well (MQW) were calculated with transfer matrix method (TMM) and compared with experimental results from photoluminescence (PL) measurements. The comparison shows good agreement. A thin layer of sputtered SiO2 film was deposited on AlGaAs/GaAs MQW samples and a subsequent rapid thermal annealing (RTA) was carried out. Photoluminescence (PL) measurements showed that the PL characteristics of the wells at different depths beneath the surface of the AlGaAs/GaAs MQW behave differently with the change of RTA condition. The PL intensities of the wells close to the surface and the bottom of the AlGaAs/GaAs MQW are sensitive to the RTA condition, while those of the wells in the middle of the MQW show little variations with the change of RTA condition. D 2002 Elsevier Science B.V. All rights reserved. PACS: 78.55.Cr; 66.30.Jt; 81.15.Cd Keywords: AlGaAs/GaAs MQW; Rapid thermal annealing; Photoluminescence
Heteroepitaxy of III –V semiconductors has a wide range of applications such as in high-efficiency tandem solar cells and optoelectronic integrated circuits (OEIC) [1]. The arsenide-based material system, AlGaAs alloy material, now becomes an attractive alternative to the phosphide-based material system [2]. Investigation of this kind of heteroepitaxial layers is a field of still growing physical and technological
* Corresponding author. Tel.: +86-531-8364655; fax: +86-5318565167. E-mail address: [email protected] (M. Ying).
interest [3]. In this paper, we calculated the locations of the energy levels in AlGaAs/GaAs multiple quantum well (MQW) with transfer matrix method (TMM) and compared the calculated results with the experimental values from photoluminescence (PL) measurement. The influence of the RTA condition on PL characteristics of the wells at different depths beneath the surface of the AlGaAs/GaAs MQW was also studied after annealing the samples at 650– 780 jC for 20 to 80 s. The AlGaAs/GaAs MQW samples used here are grown on (100) oriented semi-insulating GaAs substrates by metallo-organic chemical vapor deposition
0167-577X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0167-577X(02)01399-X
M. Ying et al. / Materials Letters 57 (2003) 2932–2935
(MOCVD). The structure of the samples was shown in Fig. 1. All the quantum wells in the MQW have the same barrier width, 30 nm, while the widths of the wells are 1.4 nm (#4), 2.8 nm (#3), 5.6 nm (#2), and 11.2 nm (#1), respectively. The optical confining layer and the barrier material are all AlxGa1 xAs with x values shown in Fig. 1. A 200-nm sputtered SiO2 film was deposited upon the samples with a JG-PF3B high-frequency sputter at an rf power of 240 W and a dc bias of 1.4 kV. The Ar pressure was 1 10 2 Torr with the background pressure of 9 10 6 Torr in the chamber. The samples were then annealed in a rapid thermal annealing tube at 650 –780 jC for 20 to 80 s. A subsequent PL measurement was carried out using an Ar+ laser (514.5 nm). The photoluminescence of the samples was dispersed by a SPEX monochromator and detected by two CCD detectors with the wavelength range between 600 – 900 and 600– 1100 nm, respectively. Theoretical calculations of the energy level structures or energy bands for III – V heteroepitaxial layers are very important for better understanding of the physical properties of optoelectronic devices based on quantum well structures and can provide a guideline for design of quantum well device with expected performance. Several methods have been developed to obtain the energy level structures of quantum wells, including WKB approximation [4], the variational calculations [5], Monte Carlo method [6], the finite element method [7], and transfer matrix method (TMM) [8]. The advantage of TMM is that it can give fast and accurate results and is easy to take the variation of the effective mass into account. Furthermore, it can be used to calculate energy levels of quantum well structures with an applied static electric field. In the present work, we use TMM to calculate
2933
the energy levels of AlGaAs/GaAs MQW. In the calculation with TMM, it is of great importance to use appropriate parameters for the effective mass, the band gap Eg and band splitting offset Q. However, the Q values reported yet are quite controversial. Dingle [1] first gave the Q value of 0.85 F 0.03 based on photon absorption measurement, while Miller suggested that the Q value should be 0.57 F 0.03. In this work, both the two values of Q were used in our calculation in order to estimate to what degree this parameter can affect the final result. The PL spectrum of a quantum well usually originated from the transition between energy level E1 of the ground state electron to that of heavy vacancy Eh1. Thus the peak energy of the PL spectrum is given by: Epk ¼ Eg þ E1 þ Eh1 Eex ;
ð1Þ
where Eg is the band gap of GaAs and Eex is the binding energy of the exciton with the value usually being 4 – 10 meV [9]. Eex = 6 meV is used in this work. Calculations of the locations of the energy levels E1 and Eh1 in the AlGaAs/GaAs MQW with different Q values were carried out. Epk were then obtained from Eq. (1). The experimentally measured PL spectrum of the AlGaAs/GaAs MQW is shown in Fig. 2, from which Epk can be also determined. The calculated Epk value and that of the PL measurement were shown in Table 1. It is clear from Table 1 that the calculated results agree quite well with the experimental ones. The Q values have some influence on the calculated results. However, the error resulted from different Q values is within 1%, and it decreases with the increase of the
Fig. 1. The structure of AlGaAs/GaAs multiple quantum well.
2934
M. Ying et al. / Materials Letters 57 (2003) 2932–2935
layer of sputtered SiO2 film deposited on the samples to prevent possible desorption of the group V elements during the annealing process. Fig. 3(a) shows the PL emission intensities of the four wells as a function of annealing time at 700 jC. It is clear that the intensities of the wells close to the surface (#1) and bottom (#4) of the MQW fluctuate with annealing time, while those of the wells in the middle of the MQW (#2, #3) show little variations. The PL characteristics of the wells close to the surface and the bottom of the MQW are much more sensitive to the RTA condition than those of the wells in the middle of the MQW. PL spectra of the AlGaAs/GaAs samples after RTA at different temperature for 60 s also confirm this conclusion. The result is shown in Fig. 3(b). It is clear that the Fig. 2. PL spectrum of AlGaAs/GaAs MQW at 70 K.
well width Lz. The discrepancy between the experimental results and the calculation ones using TMM also decreases with the increase of Lz. For #1 (Lz = 11.2 nm), the experiment result and the calculated value agrees very well, with an error of about 1%. It is easy to understand this phenomenon since the nonparabolic effect decreases with the increase of the well width, and the effective mass approximation is getting better with the increase of the well width, and thus the TMM is more accurate when the well width is increased. The four wells were treated as independent of each other in the TMM calculation. This is true only when the barrier layer is thick enough. The definite thickness of the barrier width may cause additional error in the TMM calculation. To see the influence of RTA condition on PL characteristics of the wells at different depth beneath the surface of the sample, we annealed the AlGaAs/ GaAs MQW at 650– 780 jC for 20 to 80 s with a thin Table 1 The calculated peak locations of the AlGaAs/GaAs MQW and the experimental ones from PL measurement Well
#4 #3 #2 #1
Well ˚) width (A
EPK (eV) Q = 0.6
Q = 0.85
Experimental results
14 28 56 112
1.774 1.674 1.588 1.538
1.794 1.689 1.594 1.540
1.854 1.725 1.618 1.522
Fig. 3. (a) PL intensities of the four wells in AlGaAs/GaAs MQW as a function of annealing time at the temperature of 700 jC. (b) PL intensities of the four wells in AlGaAs/GaAs MQW as a function of annealing temperature for a given annealing time of 60 s.
M. Ying et al. / Materials Letters 57 (2003) 2932–2935
PL intensities of wells #1 and #4, especially #1, vary greatly with the annealing temperature, while those of wells #2 and #3 remain almost constant. As well #1 is close to the surface of the AlGaAs/GaAs MQW sample, it may endure greater stress caused by the lattice mismatch between the AlGaAs/GaAs MQW and the sputtered SiO2 film. This is also true for well #4, which suffers greater stress from the lattice mismatch between the AlGaAs/GaAs MQW and the GaAs substrate. As it is still not clear how the stress influence the PL characteristics of the wells, further investigations is necessary and is going on now in our group. We calculated the locations of the energy levels in AlGaAs/GaAs MQW with TMM and compared the calculated results with the experimental values from PL measurement. The comparison shows little discrepancy. A thin layer of sputtered SiO2 film was deposited on AlGaAs/GaAs MQW samples with a JG-PF3B high-frequency sputter. After annealing the samples at different condition, PL measurement was carried out. It is found that the emission intensities of the wells close to the surface and the bottom of the MQW are sensitive to the RTA condition, while those of the wells in the middle of the MQW show little variations with the change of RTA condition.
2935
Acknowledgements This work is supported by the National Natural Science Foundation of China under Grant No. 19975030 and Natural Science Foundation of Shandong Province under Grant No. Y2001A02.
References [1] R. Dingle, Semiconductors and Semimetals, vol. 24, Academic Press, New York, 1987. [2] A. Ramam, S.J. Chua, J. Cryst. Growth 175 (1997) 1294. [3] K. Jeganathan, V. Ramakrishnan, J. Kumar, Cryst. Res. Technol. 34 (1999) 1293. [4] E. Merzbacher, Quantum Mechanics, 2nd ed., Wiley, New York, 1970, ch. 7. [5] D. Ahn, S.L. Chuang, Appl. Phys. Lett. 49 (1986) 1450. [6] J. Singh, Appl. Phys. Lett. 48 (1986) 434. [7] K. Hayata, M. Koshiba, K. Nakamra, A. Shimizu, Electron. Lett. 24 (1988) 614. [8] B. Jonsson, S.T. Eng, IEEE J. Quantum Electron. 26 (1990) 2025. [9] G. Bastard, E.E. Mendez, et al., Phys. Rev., B 26 (1982) 1974.