- Email: [email protected]
Growth and optical characteristics of Mg-doped GaAs epitaxial layers by molecular beam epitaxy
Microelectronic Engineering 89 (2012) 6–9
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Growth and optical characteristics of Mg-doped GaAs epitaxial layers by molecular beam epitaxy Hyun Young Choi a, Min Young Cho a, Kwang Gug Yim a, Min Su Kim a, Dong-Yul Lee b, Jin Soo Kim c, Jong Su Kim d, Jae-Young Leem a,⇑ a
Department of Nano Systems Engineering, Center for Nano Manufacturing, Inje University, Gimhae 621-749, Republic of Korea Epi-Manufacturing Technology, Samsung LED Co., Ltd., Suwon 443-373, Republic of Korea Division of Advanced Materials Engineering, Chonbuk National University, Jeonju 561-756, Republic of Korea d Department of Physics, Yeungnam University, Gyeongsan 712-749, Republic of Korea b c
a r t i c l e
i n f o
Article history: Available online 29 April 2011 Keywords: Gallium arsenide Molecular beam epitaxy Photoluminescences Negative thermal quenching
a b s t r a c t Photoluminescence (PL) measurement is used to study the optical properties of Mg-doped GaAs epitaxial layers grown by molecular beam epitaxy (MBE) with various growth temperatures. Four dominant PL peaks are observed, which may be associated with free-to-bound (e–A), exciton-bound to neutral acceptor (AoX), and two kinds of acceptor associated (g, g–g) transitions. The g and g–g peaks are especially prominent in a sample with a carrier concentration of 2.3 1017 cm3. To investigate the behavior of each peak as a function of temperature, PL measurements were carried out over a temperature range of 18–152 K. The AoX peak position follows the Varshni model for GaAs with increasing temperature. For the g and AoX peaks, we observe an increase in PL intensity with increasing temperature from 18 to 28 K. This phenomenon is known as ‘‘negative thermal quenching (NTQ)’’, and it is observed in the g peak for the first time in this paper. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Over the past few decades, considerable effort has been devoted to the research of p-type dopants in III–V compound semiconductors. Group II and IV elements such as magnesium (Mg) [1,2], beryllium (Be) [3], carbon (C) [4], and silicon (Si) [5] are known as p-type dopants in III–V compound semiconductor. In particular, the doping behavior of these materials in epitaxial layers is one of the most important areas of research in the entire field of III–V compound semiconductors [6]. Be has been widely used as the most convenient and controllable acceptor species for GaAs. Although Mg is less toxic and less carcinogenic, it has not been a useful dopant because the incorporation coefficient of Mg varies considerably with substrate temperature. The sticking coefficient of Mg in GaAs is constant at substrate temperatures below 500 °C, but, decreases exponentially above 500 °C. Therefore, Mg has not been a useful p-type dopant in compound semiconductors grown at high temperatures [7]. Currently, GaAs layers grown by molecular beam epitaxy (MBE) at low temperatures (LT, from 200–250 °C) are attracting considerable interest for high-speed device applications. This technique improves several common device problems such as backgating and sidegating [8]. Low-dimensional systems have also been widely ⇑ Corresponding author. Tel.: +82 55 320 3716; fax: +82 55 320 3631. E-mail address: [email protected] (J.-Y. Leem). 0167-9317/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2011.04.011
studied, including quantum dots (QDs) grown at low temperature. Mg is also commonly used as a p-type dopant for GaN and related materials, such as InGaN or AlGaN, even at high growth temperatures [9–11]. Recently, Leem et al. [12] reported growth of high quality Mg-doped GaAs epitaxial layers on GaAs substrates by MBE, even at low growth temperature. The carrier concentration for the GaAs epitaxial layer grown at a substrate temperature of 460 °C was 1.2 1020 cm3 [1]. Based on these examples, Mg may be considered as the most acceptable p-type dopant for III– V compound semiconductors. The optical properties of Mg-doped GaAs as realized by ion implantation [13], liquid phase-epitaxy (LPE) [14], and organometallic vapor phase epitaxy (OM-VPE) [15] have been reported, and there have also been some reports on the optical properties of Mgdoped GaAs grown by MBE [1,12]. To the best of our knowledge, no experimental results on temperature dependent photoluminescence (PL) have yet been presented for Mg-doped GaAs grown by MBE. In this paper, Mg-doped GaAs epitaxial layers were grown by MBE at various growth temperatures. The optical properties of the Mg-doped GaAs epitaxial layers were probed by investigating the temperature dependence of PL. 2. Experimental procedure The samples studied in this work were grown by MBE on semiinsulating GaAs (1 0 0) substrates. The MBE growth system was
H.Y. Choi et al. / Microelectronic Engineering 89 (2012) 6–9
also equipped with a 12 keV reflection high energy electron diffraction (RHEED) system. After desorption of resident oxide on GaAs (1 0 0), 3000 Å undoped GaAs buffer layers were grown on all samples at a substrate temperature of 580 °C in As atmosphere. 5000 Å Mg-doped GaAs epitaxial layers were then grown at 500, 520, and 540 °C, while As and Ga beam equivalent pressures (BEP) were kept at 8.6 106 and 4.8 108 Torr, respectively. The Mg cell temperature was kept at 300 °C. At the end of the growth process, RHEED analysis on all samples clearly showed a streaky (2 4) pattern. To determine the respective carrier concentrations, Hall measurements were carried out on the Mg-doped GaAs samples using the Van der Pauw method at room temperature. In order to investigate the optical properties of the Mg-doped GaAs, PL measurements were carried out in a closed cycle He cryostat with a focused He–Ne gas laser beam (k = 632.8 nm) with a maximum excitation power of 30 mW. The luminescence signal was dispersed by a 0.75 m monochromator, and subsequently detected by a water-cooled photomultiplier tube using lock-in techniques. Temperature dependent PL measurements were carried out for temperatures ranging from 18 to 153 K.
3. Results and discussion Fig. 1 shows the PL spectra measured at 20 K for Mg-doped GaAs epilayers with different carrier concentrations and it is on a logarithmic scale to magnify the weak emissions. The carrier concentrations of Mg-doped GaAs grown at substrate temperatures of 500, 520, and 540 °C are 1 1018 cm3, 2 1017 cm3, and 6 1016 cm3, respectively. The sample with a carrier concentration of 6 1016 cm3 exhibits three emission peaks at 1.4926, 1.5098, and 1.5134 eV. As the carrier concentration increases from 6 1016 to 2 1017 cm3, one additional peak appears at 1.5034 eV. However, with further increase of carrier concentration to 1 1018 cm3, two peaks totally vanish due to peak broadening. As carrier concentration increases, each peak shifts slightly to lower energy. This may be explained by the shrinkage of the band gap with increasing carrier concentration due to the many body effect [12,16]. The broadening of the full width at half maximum (FWHM) with increasing carrier concentration is due to the evolu-
Fig. 1. PL emission spectra on a logarithmic scale obtained from Mg-doped GaAs as grown by MBE, with carrier concentrations of 6.6 1016 cm3, 2.3 1017 cm3, and 1.5 1018 cm3.
7
tion of impurity levels into an impurity band [17]. Four emission peaks shown in the sample with a carrier concentration of 2 1017 cm3 are located at 1.4925, 1.5034, 1.5084, and 1.5136 eV. The lowest (1.4925 eV) and highest (1.5136 eV) energy emission may be respectively attributed to recombination in a free-to-bound transition (e–A) and exciton-bound to neutral acceptor (AoX) [1]. Between these two peaks, g and g–g peaks are observed, which have been reported by Leem et al. [12] and Takeuchi et al. [18]. These two peaks are produced by ion implantation of well established acceptor impurities and dependent on carrier concentration. Thus it can consider the g and g–g peaks are strongly related to acceptor atoms in GaAs. To investigate the behavior of the four peaks with temperature, the temperature dependent PL measurement at the sample with a carrier concentration of 2 1017 cm3 was carried out, and it is shown in Fig. 2. The PL intensity of the e–A peak is higher than that of the AoX peak in low temperature region, and as increasing temperature, the AoX peak intensity become higher than the e–A peak intensity. Generally, PL intensity was quenched as increasing temperature due to some reasons, such as increasing of the non-radiative recombination rate [19]. The intensity quenching rate of the e– A emission is higher than that of the g and AoX emission, leading to the fact that the intensity of e–A emission is lower than that of the AoX emission above 40 K. Feng et al. reported similar phenomenon in Be doped GaAs grown by metal-organic chemical vapor deposition (MOCVD) and this phenomenon was occurred due to the low doping density [20]. In low temperature region, the intensity of e– A peak was higher than that of the band-to-band transition (B–B). However, the intensity of e–A peak became lower than that of the B–B peak. Because the all acceptors can be ionized in low doping density as increasing temperature, the number of unionized acceptor hole was decreased which can be recombined with electrons. So the recombination of electrons through the available acceptor holes can be negligible at high temperature and this means that the intensity of e–A peak relative to the AoX peak was decreased
Fig. 2. The temperature dependent PL spectrum of Mg-doped GaAs with a carrier concentration of about 2.3 1017 cm3, for temperatures in the range from 18 to 152 K.
8
H.Y. Choi et al. / Microelectronic Engineering 89 (2012) 6–9
with increasing temperature. In our experiments, the carrier concentration was 2 1017 cm3, which was similar value with the Be doping concentration reported by the Feng et al. So it can consider that this behavior may be caused by the ionization of acceptor, and it leads that the intensity of e–A emission became lower than that of the AoX emission in high temperature region. Fig. 3 shows the PL peak positions as a function of temperature. It can be seen that four peaks corresponding to AoX, g, g–g, and e–A monotonically red shifted as increasing temperature with different rates. The traditional method of describing the temperature dependence of a band to band transition has been using the Varshni equation given by equation [21]:
Eg ðTÞ ¼ Eg ð0Þ aT 2 =ðb þ TÞ
Fig. 3. Shifts in the PL peak positions as a function of temperature for Mg-doped GaAs with a carrier concentration of about 2.3 1017 cm3. The dotted lines represent the Varshni curve for GaAs.
ð1Þ
where a is a constant, b is approximately the Debye temperature, and Eg(0) is the energy band gap at 0 K. The temperature dependence of AoX peak energy was higher than GaAs band gap energy calculated by Varshni equation in low temperature region due to the small binding energy. However AoX peak position followed the Varshni curve in high temperature region. This behavior is due to the thermal dissociation of AoX, so the origin of this peak changed from AoX to band to band transition as increasing temperature. The e–A peak position also red shifted from 1.4925 to 1.4763 eV as increasing temperature from 18 to 77 K. Above 77 K, it was not observed. The e–A peak position was changed linearly compared with the AoX emission, and it was lower than the Varshni curve for GaAs due to the ionization energy. The reasons for this linear behavior are not clear at this moment, but may be due to different acceptor related level. The g peak position red shifted and the g– g peak position was almost remained at 1.503 eV with increasing temperature. As the temperature increases from 18 to 52 K, the g
Fig. 4. Plots of PL intensity on a logarithmic scale versus 1000/T from the four PL emission. The dotted lines represent fits to an Arrhenius equation and the indicated numbers are the Ea.
H.Y. Choi et al. / Microelectronic Engineering 89 (2012) 6–9
and g–g peaks became weaker and disappeared due to the broadening of the e–A and AoX emissions. Also, disappearance of the g and g–g peaks can be caused by the thermal release of holes to other states. So it was needed to investigate the activation energy (Ea) of each emission in Fig. 4. Fig. 4 shows the temperature dependence of the PL intensity of the e–A, g–g, g, and AoX peaks to investigate the mechanism governing the quenching behavior. Variations in the PL intensity can be explained in terms of competition between radiative and nonradiative recombination. The thermal Ea for each peak is obtained by using an Arrhenius equation [22]:
IðTÞ ¼
I0 1 þ C expðEa =kB TÞ
ð2Þ
where C is a constant, kB is the Botzmann constant, and I(T) and I0 are the intensities at temperature T and 0 K, respectively. The PL intensity of the e–A emission decreases rapidly, and that of the g– g emission decreases with increasing temperature and vanishes above 28 K. Based on a fit to the experimental data as indicated by the dotted line, the Ea of the e–A and g–g peaks are estimated to be 8.612 and 0.375 meV, respectively. In general, the PL intensity in semiconductors usually shows thermal quenching, i.e., a reduction in the PL intensity is observed with increasing temperature. This phenomenon is ascribed to a temperature-induced increase in the non-radiative recombination probability of electrons and holes with the increasing temperature [23,24]. On the other hand, the g and AoX peaks, an increase in the PL intensity with increasing temperature was observed in the range of temperatures between 18 and 28 K. This is unusual phenomenon and it is known as ‘‘negative thermal quenching’’ (NTQ) [25,26]. Shibata proposed that the principal mechanism of the NTQ is the thermal excitation of electrons to the initial (conduction) states of the PL transition from intermediated states [26]. With increasing temperature, carriers are thermally excited from the intermediated states to the conduction states, so the radiative recombination is enhanced. It was observed in some semiconducting materials such as ZnS [25], GaAs [27], and ZnO [28]. Bebb et al. observed the NTQ in n-type GaAs and they explained that this phenomenon for the e–A emission was due to the thermal dissociation of the exciton-neutral donor (DoX) emission, which resulted in the ejection of a free electron into the conduction band [27]. In this study, the NTQ for the g and AoX are observed with increasing temperature up to 28 K, while the g-g emission disappeared above 28 K, so it assumes that the NTQ is caused by the thermal dissociation of the g–g emission. Above 28 K, the g and AoX peaks show usual thermal quenching as increasing temperature. The g emission is disappeared at 57 K, and the slope of the line for the AoX emission is changed at 57 K. Some carriers trapped in the g band are thermally excited to the AoX band. When the number of thermally liberated carriers from the AoX is larger than that of the excited carriers from the g to AoX band, the intensity of AoX emission decreased with increasing temperature. Thus, we assume that the slope of the line for the AoX emission was affected by the thermal dissociation of the g emission. Although the equation (2) has been generally used for obtaining the Ea, the Ea needs to be reconsidered when the NTQ is observed in low temperature region. Through this work, we firstly observed the NTQ at the acceptor associated emissions. Although further studies are needed to demonstrate the cause of the NTQ for the g and AoX emission accurately, it is evident that the Ea of the acceptor related peak is affected by the other peaks’ dissociation.
9
4. Conclusion Mg-doped GaAs epitaxial layers were grown by MBE at different growth temperatures. Carrier concentrations in these materials varied from 6 1016 to 1 1018 cm3 with decreasing growth temperature. Four PL peaks are observed in the sample with a carrier concentration of 2 1017 cm3, especially, and these peaks are attributed to emission from a free-to-bound transition (e–A), two acceptor related emission (g, g–g), and the transition of a exciton bound to neutral acceptor (AoX). The temperature dependent behavior of the PL intensities for the four peaks is investigated, and particularly the intensities of the g and AoX peaks show interesting variations, named as NTQ. This phenomenon was occurred by the other peaks’ dissociation. So the Ea of the peaks can be affected by the other emissions’ dissociation, especially the g and AoX peaks which are acceptor associated emissions. Acknowledgement This work was supported by the 2010 Inje University research grant. References [1] J.S. Kim, D.Y. Lee, I.H. Bae, J.I. Lee, S.K. Noh, J.S. Kim, G.H. Kim, S.G. Ban, S.K. Kang, S.M. Kim, J.Y. Leem, M.H. Jeon, J.S. Son, J. Korean Phys. Soc. 39 (2001) S518. [2] M.S. Kim, D.Y. Kim, T.H. Kim, G.S. Kim, H.Y. Choi, M.Y. Cho, S.M. Jeon, H.H. Ryu, W.W. Park, J.Y. Leem, J.S. Kim, J.S. Kim, D.Y. Lee, J.S. Son, J. Korean Phys. Soc. 54 (2009) 673. [3] P. Krispin, M. Asghar, H. Kostial, R. Hey, Physica B 273–274 (1999) 693. [4] M.S. Liang, T.J. Bullough, T.B. Joyce, Solid State Electron. 52 (2008) 1256. [5] K. Shinohara, T. Motokawa, K. Kasahara, S. Shimomura, N. Sano, A. Adachi, S. Hiyamizu, Semicond. Sci. Technol. 11 (1996) L125. [6] J.Y. Leem, C.R. Lee, S.K. Noh, J.S. Son, J. Crystal Growth 197 (1999) 84. [7] C.E.C. Wood, D. Desimone, K. Singer, G.W. Wicks, J. Appl. Phys. 53 (1982) 4230. [8] K. Ma, R. Urata, D.A.B. Miller, J.S. Harris, IEEE J. Quantum Electron. 40 (2004) 800. [9] D. Xu, H. Yang, D.G. Zhao, S.F. Li, R.H. Wu, J. Appl. Phys. 87 (2000) 2064. [10] S. Yamasaki, S. Asami, N. Shibata, M. Koike, K. Manabe, T. Tanaka, H. Amano, I. Akasaki, Appl. Phys. Lett. 66 (1995) 1112. [11] P. Kozodoy, Y.P. Smorchkova, M. Hansen, H. Xing, S.P. Denbaars, U.K. Mishra, A.W. Saxler, R. Perrin, W.C. Mitchel, Appl. Phys. Lett. 75 (1999) 2444. [12] J.S. Kim, I.H. Bae, J.Y. Leem, S.K. Noh, J.I. Lee, J.S. Kim, S.M. Kim, J.S. Son, M.H. Jeon, J. Crystal Growth 226 (2001) 52. [13] J. Simonson, S.B. Qadri, M.V. Rao, R. fischer, J. Grun, M.C. Ridgway, Appl. Phys. A 81 (2005) 601. [14] M. Milanova, V. Khvostikov, J. Cryst. Growth 219 (2000) 193. [15] C.R. Lewis, W.T. Dietze, M.J. Ludowise, J. Electron. Mater. 12 (1983) 507. [16] M.S. Kim, D.Y. Kim, H.J. Park, J.S. Kim, J.S. Kim, D.Y. Lee, J.S. Son, J.Y. Leem, Jpn. J. Appl. Phys. 48 (2009) 041103. [17] J.I. Pankove, Optical Processes in Semiconductors, 10, Dover Publications Inc., 1971. [18] Y. Takeuchi, Y. Makita, K. Kudo, T. Nomura, Appl. Phys. Lett. 48 (1986) 59. [19] M. Leroux, N. Grandjean, B. Beaumont, G. Nataf, F. Semond, J. Massies, P. Gibart, Phys. stat. sol. (b) 216 (1999) 605. [20] M.S. Feng, C.S.A. Fang, H.D. Chen, Mater. Chem. Phys. 42 (1995) 143. [21] Y.P. Varshni, Physica 34 (1967) 149. [22] C.H. Zang, D.X. Zhao, Y. Tang, Z. Guo, J.Y. Zhang, D.Z. Shen, Y.C. Liu, Chem. Phys. Lett. 452 (2008) 148. [23] A.St. Amour, J.C. Sturn, Y. Lacroix, M.L.W. Thewalt, Appl. Phys. Lett. 65 (1994) 3344. [24] S. Fukatsu, Y. Shiraki, J. Cryst. Growth 150 (1995) 1025. [25] T. Yokogawa, T. Taguchi, S. Fujita, M. Satoh, IEEE Trans. Electron Devices 30 (1983) 271. [26] H. Shibata, Jpn. J. Appl. Phys. 37 (1998) 550. [27] E.W. Williams, H.B. Bebb, in: R.K. Willardson, C. Beer (Eds.), Semiconductors and Semimetals, Academic, New York, 1972, p. 321. [28] Y.H. Tong, Y.C. Liu, L. Dong, L.X. Lu, D.X. Zhao, J.Y. Zhang, Y.M. Lu, D.Z. Shen, X.W. Fan, Mater. Chem. Phys. 103 (2007) 190.
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Growth and optical characteristics of Mg-doped GaAs epitaxial layers by molecular beam epitaxy Hyun Young Choi a, Min Young Cho a, Kwang Gug Yim a, Min Su Kim a, Dong-Yul Lee b, Jin Soo Kim c, Jong Su Kim d, Jae-Young Leem a,⇑ a
Department of Nano Systems Engineering, Center for Nano Manufacturing, Inje University, Gimhae 621-749, Republic of Korea Epi-Manufacturing Technology, Samsung LED Co., Ltd., Suwon 443-373, Republic of Korea Division of Advanced Materials Engineering, Chonbuk National University, Jeonju 561-756, Republic of Korea d Department of Physics, Yeungnam University, Gyeongsan 712-749, Republic of Korea b c
a r t i c l e
i n f o
Article history: Available online 29 April 2011 Keywords: Gallium arsenide Molecular beam epitaxy Photoluminescences Negative thermal quenching
a b s t r a c t Photoluminescence (PL) measurement is used to study the optical properties of Mg-doped GaAs epitaxial layers grown by molecular beam epitaxy (MBE) with various growth temperatures. Four dominant PL peaks are observed, which may be associated with free-to-bound (e–A), exciton-bound to neutral acceptor (AoX), and two kinds of acceptor associated (g, g–g) transitions. The g and g–g peaks are especially prominent in a sample with a carrier concentration of 2.3 1017 cm3. To investigate the behavior of each peak as a function of temperature, PL measurements were carried out over a temperature range of 18–152 K. The AoX peak position follows the Varshni model for GaAs with increasing temperature. For the g and AoX peaks, we observe an increase in PL intensity with increasing temperature from 18 to 28 K. This phenomenon is known as ‘‘negative thermal quenching (NTQ)’’, and it is observed in the g peak for the first time in this paper. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Over the past few decades, considerable effort has been devoted to the research of p-type dopants in III–V compound semiconductors. Group II and IV elements such as magnesium (Mg) [1,2], beryllium (Be) [3], carbon (C) [4], and silicon (Si) [5] are known as p-type dopants in III–V compound semiconductor. In particular, the doping behavior of these materials in epitaxial layers is one of the most important areas of research in the entire field of III–V compound semiconductors [6]. Be has been widely used as the most convenient and controllable acceptor species for GaAs. Although Mg is less toxic and less carcinogenic, it has not been a useful dopant because the incorporation coefficient of Mg varies considerably with substrate temperature. The sticking coefficient of Mg in GaAs is constant at substrate temperatures below 500 °C, but, decreases exponentially above 500 °C. Therefore, Mg has not been a useful p-type dopant in compound semiconductors grown at high temperatures [7]. Currently, GaAs layers grown by molecular beam epitaxy (MBE) at low temperatures (LT, from 200–250 °C) are attracting considerable interest for high-speed device applications. This technique improves several common device problems such as backgating and sidegating [8]. Low-dimensional systems have also been widely ⇑ Corresponding author. Tel.: +82 55 320 3716; fax: +82 55 320 3631. E-mail address: [email protected] (J.-Y. Leem). 0167-9317/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2011.04.011
studied, including quantum dots (QDs) grown at low temperature. Mg is also commonly used as a p-type dopant for GaN and related materials, such as InGaN or AlGaN, even at high growth temperatures [9–11]. Recently, Leem et al. [12] reported growth of high quality Mg-doped GaAs epitaxial layers on GaAs substrates by MBE, even at low growth temperature. The carrier concentration for the GaAs epitaxial layer grown at a substrate temperature of 460 °C was 1.2 1020 cm3 [1]. Based on these examples, Mg may be considered as the most acceptable p-type dopant for III– V compound semiconductors. The optical properties of Mg-doped GaAs as realized by ion implantation [13], liquid phase-epitaxy (LPE) [14], and organometallic vapor phase epitaxy (OM-VPE) [15] have been reported, and there have also been some reports on the optical properties of Mgdoped GaAs grown by MBE [1,12]. To the best of our knowledge, no experimental results on temperature dependent photoluminescence (PL) have yet been presented for Mg-doped GaAs grown by MBE. In this paper, Mg-doped GaAs epitaxial layers were grown by MBE at various growth temperatures. The optical properties of the Mg-doped GaAs epitaxial layers were probed by investigating the temperature dependence of PL. 2. Experimental procedure The samples studied in this work were grown by MBE on semiinsulating GaAs (1 0 0) substrates. The MBE growth system was
H.Y. Choi et al. / Microelectronic Engineering 89 (2012) 6–9
also equipped with a 12 keV reflection high energy electron diffraction (RHEED) system. After desorption of resident oxide on GaAs (1 0 0), 3000 Å undoped GaAs buffer layers were grown on all samples at a substrate temperature of 580 °C in As atmosphere. 5000 Å Mg-doped GaAs epitaxial layers were then grown at 500, 520, and 540 °C, while As and Ga beam equivalent pressures (BEP) were kept at 8.6 106 and 4.8 108 Torr, respectively. The Mg cell temperature was kept at 300 °C. At the end of the growth process, RHEED analysis on all samples clearly showed a streaky (2 4) pattern. To determine the respective carrier concentrations, Hall measurements were carried out on the Mg-doped GaAs samples using the Van der Pauw method at room temperature. In order to investigate the optical properties of the Mg-doped GaAs, PL measurements were carried out in a closed cycle He cryostat with a focused He–Ne gas laser beam (k = 632.8 nm) with a maximum excitation power of 30 mW. The luminescence signal was dispersed by a 0.75 m monochromator, and subsequently detected by a water-cooled photomultiplier tube using lock-in techniques. Temperature dependent PL measurements were carried out for temperatures ranging from 18 to 153 K.
3. Results and discussion Fig. 1 shows the PL spectra measured at 20 K for Mg-doped GaAs epilayers with different carrier concentrations and it is on a logarithmic scale to magnify the weak emissions. The carrier concentrations of Mg-doped GaAs grown at substrate temperatures of 500, 520, and 540 °C are 1 1018 cm3, 2 1017 cm3, and 6 1016 cm3, respectively. The sample with a carrier concentration of 6 1016 cm3 exhibits three emission peaks at 1.4926, 1.5098, and 1.5134 eV. As the carrier concentration increases from 6 1016 to 2 1017 cm3, one additional peak appears at 1.5034 eV. However, with further increase of carrier concentration to 1 1018 cm3, two peaks totally vanish due to peak broadening. As carrier concentration increases, each peak shifts slightly to lower energy. This may be explained by the shrinkage of the band gap with increasing carrier concentration due to the many body effect [12,16]. The broadening of the full width at half maximum (FWHM) with increasing carrier concentration is due to the evolu-
Fig. 1. PL emission spectra on a logarithmic scale obtained from Mg-doped GaAs as grown by MBE, with carrier concentrations of 6.6 1016 cm3, 2.3 1017 cm3, and 1.5 1018 cm3.
7
tion of impurity levels into an impurity band [17]. Four emission peaks shown in the sample with a carrier concentration of 2 1017 cm3 are located at 1.4925, 1.5034, 1.5084, and 1.5136 eV. The lowest (1.4925 eV) and highest (1.5136 eV) energy emission may be respectively attributed to recombination in a free-to-bound transition (e–A) and exciton-bound to neutral acceptor (AoX) [1]. Between these two peaks, g and g–g peaks are observed, which have been reported by Leem et al. [12] and Takeuchi et al. [18]. These two peaks are produced by ion implantation of well established acceptor impurities and dependent on carrier concentration. Thus it can consider the g and g–g peaks are strongly related to acceptor atoms in GaAs. To investigate the behavior of the four peaks with temperature, the temperature dependent PL measurement at the sample with a carrier concentration of 2 1017 cm3 was carried out, and it is shown in Fig. 2. The PL intensity of the e–A peak is higher than that of the AoX peak in low temperature region, and as increasing temperature, the AoX peak intensity become higher than the e–A peak intensity. Generally, PL intensity was quenched as increasing temperature due to some reasons, such as increasing of the non-radiative recombination rate [19]. The intensity quenching rate of the e– A emission is higher than that of the g and AoX emission, leading to the fact that the intensity of e–A emission is lower than that of the AoX emission above 40 K. Feng et al. reported similar phenomenon in Be doped GaAs grown by metal-organic chemical vapor deposition (MOCVD) and this phenomenon was occurred due to the low doping density [20]. In low temperature region, the intensity of e– A peak was higher than that of the band-to-band transition (B–B). However, the intensity of e–A peak became lower than that of the B–B peak. Because the all acceptors can be ionized in low doping density as increasing temperature, the number of unionized acceptor hole was decreased which can be recombined with electrons. So the recombination of electrons through the available acceptor holes can be negligible at high temperature and this means that the intensity of e–A peak relative to the AoX peak was decreased
Fig. 2. The temperature dependent PL spectrum of Mg-doped GaAs with a carrier concentration of about 2.3 1017 cm3, for temperatures in the range from 18 to 152 K.
8
H.Y. Choi et al. / Microelectronic Engineering 89 (2012) 6–9
with increasing temperature. In our experiments, the carrier concentration was 2 1017 cm3, which was similar value with the Be doping concentration reported by the Feng et al. So it can consider that this behavior may be caused by the ionization of acceptor, and it leads that the intensity of e–A emission became lower than that of the AoX emission in high temperature region. Fig. 3 shows the PL peak positions as a function of temperature. It can be seen that four peaks corresponding to AoX, g, g–g, and e–A monotonically red shifted as increasing temperature with different rates. The traditional method of describing the temperature dependence of a band to band transition has been using the Varshni equation given by equation [21]:
Eg ðTÞ ¼ Eg ð0Þ aT 2 =ðb þ TÞ
Fig. 3. Shifts in the PL peak positions as a function of temperature for Mg-doped GaAs with a carrier concentration of about 2.3 1017 cm3. The dotted lines represent the Varshni curve for GaAs.
ð1Þ
where a is a constant, b is approximately the Debye temperature, and Eg(0) is the energy band gap at 0 K. The temperature dependence of AoX peak energy was higher than GaAs band gap energy calculated by Varshni equation in low temperature region due to the small binding energy. However AoX peak position followed the Varshni curve in high temperature region. This behavior is due to the thermal dissociation of AoX, so the origin of this peak changed from AoX to band to band transition as increasing temperature. The e–A peak position also red shifted from 1.4925 to 1.4763 eV as increasing temperature from 18 to 77 K. Above 77 K, it was not observed. The e–A peak position was changed linearly compared with the AoX emission, and it was lower than the Varshni curve for GaAs due to the ionization energy. The reasons for this linear behavior are not clear at this moment, but may be due to different acceptor related level. The g peak position red shifted and the g– g peak position was almost remained at 1.503 eV with increasing temperature. As the temperature increases from 18 to 52 K, the g
Fig. 4. Plots of PL intensity on a logarithmic scale versus 1000/T from the four PL emission. The dotted lines represent fits to an Arrhenius equation and the indicated numbers are the Ea.
H.Y. Choi et al. / Microelectronic Engineering 89 (2012) 6–9
and g–g peaks became weaker and disappeared due to the broadening of the e–A and AoX emissions. Also, disappearance of the g and g–g peaks can be caused by the thermal release of holes to other states. So it was needed to investigate the activation energy (Ea) of each emission in Fig. 4. Fig. 4 shows the temperature dependence of the PL intensity of the e–A, g–g, g, and AoX peaks to investigate the mechanism governing the quenching behavior. Variations in the PL intensity can be explained in terms of competition between radiative and nonradiative recombination. The thermal Ea for each peak is obtained by using an Arrhenius equation [22]:
IðTÞ ¼
I0 1 þ C expðEa =kB TÞ
ð2Þ
where C is a constant, kB is the Botzmann constant, and I(T) and I0 are the intensities at temperature T and 0 K, respectively. The PL intensity of the e–A emission decreases rapidly, and that of the g– g emission decreases with increasing temperature and vanishes above 28 K. Based on a fit to the experimental data as indicated by the dotted line, the Ea of the e–A and g–g peaks are estimated to be 8.612 and 0.375 meV, respectively. In general, the PL intensity in semiconductors usually shows thermal quenching, i.e., a reduction in the PL intensity is observed with increasing temperature. This phenomenon is ascribed to a temperature-induced increase in the non-radiative recombination probability of electrons and holes with the increasing temperature [23,24]. On the other hand, the g and AoX peaks, an increase in the PL intensity with increasing temperature was observed in the range of temperatures between 18 and 28 K. This is unusual phenomenon and it is known as ‘‘negative thermal quenching’’ (NTQ) [25,26]. Shibata proposed that the principal mechanism of the NTQ is the thermal excitation of electrons to the initial (conduction) states of the PL transition from intermediated states [26]. With increasing temperature, carriers are thermally excited from the intermediated states to the conduction states, so the radiative recombination is enhanced. It was observed in some semiconducting materials such as ZnS [25], GaAs [27], and ZnO [28]. Bebb et al. observed the NTQ in n-type GaAs and they explained that this phenomenon for the e–A emission was due to the thermal dissociation of the exciton-neutral donor (DoX) emission, which resulted in the ejection of a free electron into the conduction band [27]. In this study, the NTQ for the g and AoX are observed with increasing temperature up to 28 K, while the g-g emission disappeared above 28 K, so it assumes that the NTQ is caused by the thermal dissociation of the g–g emission. Above 28 K, the g and AoX peaks show usual thermal quenching as increasing temperature. The g emission is disappeared at 57 K, and the slope of the line for the AoX emission is changed at 57 K. Some carriers trapped in the g band are thermally excited to the AoX band. When the number of thermally liberated carriers from the AoX is larger than that of the excited carriers from the g to AoX band, the intensity of AoX emission decreased with increasing temperature. Thus, we assume that the slope of the line for the AoX emission was affected by the thermal dissociation of the g emission. Although the equation (2) has been generally used for obtaining the Ea, the Ea needs to be reconsidered when the NTQ is observed in low temperature region. Through this work, we firstly observed the NTQ at the acceptor associated emissions. Although further studies are needed to demonstrate the cause of the NTQ for the g and AoX emission accurately, it is evident that the Ea of the acceptor related peak is affected by the other peaks’ dissociation.
9
4. Conclusion Mg-doped GaAs epitaxial layers were grown by MBE at different growth temperatures. Carrier concentrations in these materials varied from 6 1016 to 1 1018 cm3 with decreasing growth temperature. Four PL peaks are observed in the sample with a carrier concentration of 2 1017 cm3, especially, and these peaks are attributed to emission from a free-to-bound transition (e–A), two acceptor related emission (g, g–g), and the transition of a exciton bound to neutral acceptor (AoX). The temperature dependent behavior of the PL intensities for the four peaks is investigated, and particularly the intensities of the g and AoX peaks show interesting variations, named as NTQ. This phenomenon was occurred by the other peaks’ dissociation. So the Ea of the peaks can be affected by the other emissions’ dissociation, especially the g and AoX peaks which are acceptor associated emissions. Acknowledgement This work was supported by the 2010 Inje University research grant. References [1] J.S. Kim, D.Y. Lee, I.H. Bae, J.I. Lee, S.K. Noh, J.S. Kim, G.H. Kim, S.G. Ban, S.K. Kang, S.M. Kim, J.Y. Leem, M.H. Jeon, J.S. Son, J. Korean Phys. Soc. 39 (2001) S518. [2] M.S. Kim, D.Y. Kim, T.H. Kim, G.S. Kim, H.Y. Choi, M.Y. Cho, S.M. Jeon, H.H. Ryu, W.W. Park, J.Y. Leem, J.S. Kim, J.S. Kim, D.Y. Lee, J.S. Son, J. Korean Phys. Soc. 54 (2009) 673. [3] P. Krispin, M. Asghar, H. Kostial, R. Hey, Physica B 273–274 (1999) 693. [4] M.S. Liang, T.J. Bullough, T.B. Joyce, Solid State Electron. 52 (2008) 1256. [5] K. Shinohara, T. Motokawa, K. Kasahara, S. Shimomura, N. Sano, A. Adachi, S. Hiyamizu, Semicond. Sci. Technol. 11 (1996) L125. [6] J.Y. Leem, C.R. Lee, S.K. Noh, J.S. Son, J. Crystal Growth 197 (1999) 84. [7] C.E.C. Wood, D. Desimone, K. Singer, G.W. Wicks, J. Appl. Phys. 53 (1982) 4230. [8] K. Ma, R. Urata, D.A.B. Miller, J.S. Harris, IEEE J. Quantum Electron. 40 (2004) 800. [9] D. Xu, H. Yang, D.G. Zhao, S.F. Li, R.H. Wu, J. Appl. Phys. 87 (2000) 2064. [10] S. Yamasaki, S. Asami, N. Shibata, M. Koike, K. Manabe, T. Tanaka, H. Amano, I. Akasaki, Appl. Phys. Lett. 66 (1995) 1112. [11] P. Kozodoy, Y.P. Smorchkova, M. Hansen, H. Xing, S.P. Denbaars, U.K. Mishra, A.W. Saxler, R. Perrin, W.C. Mitchel, Appl. Phys. Lett. 75 (1999) 2444. [12] J.S. Kim, I.H. Bae, J.Y. Leem, S.K. Noh, J.I. Lee, J.S. Kim, S.M. Kim, J.S. Son, M.H. Jeon, J. Crystal Growth 226 (2001) 52. [13] J. Simonson, S.B. Qadri, M.V. Rao, R. fischer, J. Grun, M.C. Ridgway, Appl. Phys. A 81 (2005) 601. [14] M. Milanova, V. Khvostikov, J. Cryst. Growth 219 (2000) 193. [15] C.R. Lewis, W.T. Dietze, M.J. Ludowise, J. Electron. Mater. 12 (1983) 507. [16] M.S. Kim, D.Y. Kim, H.J. Park, J.S. Kim, J.S. Kim, D.Y. Lee, J.S. Son, J.Y. Leem, Jpn. J. Appl. Phys. 48 (2009) 041103. [17] J.I. Pankove, Optical Processes in Semiconductors, 10, Dover Publications Inc., 1971. [18] Y. Takeuchi, Y. Makita, K. Kudo, T. Nomura, Appl. Phys. Lett. 48 (1986) 59. [19] M. Leroux, N. Grandjean, B. Beaumont, G. Nataf, F. Semond, J. Massies, P. Gibart, Phys. stat. sol. (b) 216 (1999) 605. [20] M.S. Feng, C.S.A. Fang, H.D. Chen, Mater. Chem. Phys. 42 (1995) 143. [21] Y.P. Varshni, Physica 34 (1967) 149. [22] C.H. Zang, D.X. Zhao, Y. Tang, Z. Guo, J.Y. Zhang, D.Z. Shen, Y.C. Liu, Chem. Phys. Lett. 452 (2008) 148. [23] A.St. Amour, J.C. Sturn, Y. Lacroix, M.L.W. Thewalt, Appl. Phys. Lett. 65 (1994) 3344. [24] S. Fukatsu, Y. Shiraki, J. Cryst. Growth 150 (1995) 1025. [25] T. Yokogawa, T. Taguchi, S. Fujita, M. Satoh, IEEE Trans. Electron Devices 30 (1983) 271. [26] H. Shibata, Jpn. J. Appl. Phys. 37 (1998) 550. [27] E.W. Williams, H.B. Bebb, in: R.K. Willardson, C. Beer (Eds.), Semiconductors and Semimetals, Academic, New York, 1972, p. 321. [28] Y.H. Tong, Y.C. Liu, L. Dong, L.X. Lu, D.X. Zhao, J.Y. Zhang, Y.M. Lu, D.Z. Shen, X.W. Fan, Mater. Chem. Phys. 103 (2007) 190.