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Effect of Be codoping on the photoluminescence spectra of GaMnAs
Current Applied Physics 11 (2011) 735e739
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Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Effect of Be codoping on the photoluminescence spectra of GaMnAs Fucheng Yu a, c, P.B. Parchinskiy a, b, Dojin Kim a, *, Hyojin Kim a, Young Eon Ihm a, Duck-Kyun Choi c a
School of Nano Science and Technology, Chungnam National University, 220 Gung-dong, Yuseong-gu, Daejeon 305-764, Republic of Korea National University of Uzbekistan, Tashkent, 700174, Uzbekistan c Information Display Research Institute, Hanyang University, Seoul 133-791, Republic of Korea b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 July 2009 Received in revised form 16 November 2010 Accepted 16 November 2010 Available online 25 November 2010
The measurements of the photoluminescence (PL) spectra have been performed on GaAs, GaMnAs, GaAs:Be, and GaMnAs:Be samples to study the effect of Be codoping on the PL spectra of GaMnAs layers grown via low temperature molecular beam epitaxy. Based on the temperature dependence of the exciton-related transitions energy, it was shown that doping GaAs with Mn and Be leads to modification of the temperature dependence of the band gap. It was shown that although Be itself weakly affected the PL spectra of GaAs, codoping with Be significantly modified the PL spectra of GaMnAs. Ó 2010 Elsevier B.V. All rights reserved.
Keywords: Photoluminescence spectra GaMnAs Be codoping MBE Ferromagnetic semiconductor
1. Introduction Since the discovery by the Ohno Group of ferromagnetism in InMnAs and GaMnAs grown via low temperature molecular beam epitaxy (LT MBE), IIIeV semiconductors doped with magnetic ion impurities known as diluted magnetic semiconductor (DMS) have been considered basic materials for the development of spintronics [1,2]. From the conventional models, ferromagnetism in DMS occurs as a result of interaction between localized spins of magnetic ions and delocalized or slightly localized holes [3]. Mn ions in GaMnAs occupy the sites of Ga atoms and act as both originators of spins and acceptors. However, the low solubility of Mn in GaAs [1] and the reduction of hole concentration owing to the existence of Mn interstitial defects and As anti-site defects [4e7] limit the temperature of paramagneticeferromagnetic transition in GaMnAs. Nowadays Be, as additional acceptor in GaMnAs, is widely used to enhance the hole concentration at the valence band in semiconductors [8e10]. On the other hand, the IIIeV compounds grown by LT MBE are very sensitive to the growth condition and alloy composition. Even a small change in impurities concentration could significantly modify the defect structures of the semiconducting matrix and its physical properties.
* Corresponding author. E-mail address: [email protected] (D. Kim). 1567-1739/$ e see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2010.11.049
It is well known that low temperature photoluminescence (PL) is one of the most promising techniques to obtain information about electronic properties and defect structures of semiconductors [11,12]. In this context, we have studied the effects of codoping of Be on the PL spectra in GaMnAs. 2. Experiment Samples used in the present study were grown on semi-insulating GaAs (001) substrate. The growth chamber was equipped with a reflection high-energy electron diffraction (RHEED) gun for in-situ monitoring of the surface reconstruction during the growth. The background pressure with flowing liquid nitrogen is in the low 1010 Torr range. Prior to main layers deposition a ∼250 nm-thick buffer GaAs layer was grown at Ts ¼ 580 C at a growth rate of 0.5 mm/h with an As4 pressure of ∼2.5 106 Torr, where the highest specular beam RHEED intensity was observed during growth for the smoothest growth front. The main layers of GaMnAs, GaMnAs codoped with Be, and GaAs doped with Be were grown at the substrate temperature of 300 C under an As4 pressure of 1.4 106 Torr. The thickness of the main layers were ∼250 nm with the growth rate of 0.25 mm/h. Be flux was set with TBe ¼ 1175 C, at which temperature, metallic Be-doped GaAs could be realized. The Mn cell temperature was fixed at TMn ¼ 910 C. GaAs layer grown under the same LT MBE conditions but without any impurities was used as a reference sample. The
F. Yu et al. / Current Applied Physics 11 (2011) 735e739
3. Results and discussion Fig. 1 shows the PL spectra of GMn, GB, and GMB samples and reference GaAs layer measured at 4 K. For all samples, three major PL peaks, labeled as “FX”, “CX”, and “AX” for transition energy 1.51 eV, 1.492 eV, and 1.462 eV, respectively, have been observed. Peak FX is interpreted as exciton-related transition [13e15] and more detailed examination showed that this peak consists of at least two closed lines at energy 1.514 and 1.510 eV, which were attributed to free excitons and donor-related exciton transitions [16]. Peaks CX and AX are associated with recombination via carbon acceptor level (carbon is well known as the most important unintentional impurities in the MBE growth process) and Ga anti-site transition, respectively [12e15]. In the GMn sample, we have also observed a wide peak with maximum at approximately 1.425 eV, which could be attributed to recombination via Mn acceptor level [12,14,16]. The anomalously large width of this peak, in our opinion, indicates that Mn-related levels form an impurities sub-band in the band gap of GaMnAs. The absence of Mn-related transition in the PL spectra of GMB sample e in which Mn concentration, according to the EPMA data, is the same as in the GMn sample e could reflect the fact that codoping with Be, on one side, did not change the total Mn concentration. On the other side, however, increased concentration of Mn interstitials (MnI) and, possibly, Mn-based precipitate, decreased Mn acceptor (MnGa) levels in the GaAs matrix [8]. The conclusion that MnIeMnGa ratio in GMB sample is lower than in GMn sample was also supported by the data of magnetotransport measurements. Fig. 2 shows the temperature dependence of resistivity for GMn and GMB samples, measured at zero magnetic field. It is clear that for both samples local maxima of the resistivity exist. The position of the maximum in GaMnAs and other III-Mn-V ferromagnetic alloys is associated with onset of the magnetic ordering in the semiconducting matrix [17e19], and it allows us to estimate a paramagneticeferromagnetic transition temperature (Tc) as Tc z 55 K for GMn sample and Tc z 45 K for GMB sample. The Hall effect measurements showed that in GMn samples the contribution of the anomalous Hall effect (AHE) was still predominant at 50 K (Fig. 3a), whereas in GMB sample the temperature, above which the contribution of AHE disappeared, is 45 K (Fig. 3b). Thus, both of the resistivity and Hall effect measurements demonstrate that the temperature of paramagneticeferromagnetic transition in Be codoped samples is lower than in undoped GaMnAs layer. As mentioned earlier Be is an effective acceptor and codoping element and Be should increase free hole concentration in GaMnAs. Indeed, the hole concentration in GMn and GMB samples, estimated from Hall effect measurements at room temperature, was 0.8 1020 cm3 and 1.8 1020 cm3, respectively. We were not able Table 1 The summary of growth conditions of each sample. Sample ID
Mn flux (oC)
GB GMn GMB
910 910
Be flux (oC)
Description
1175
GaAs:Be GaMnAs GaMnAs:Be
1175
1 - GMB
PL intensity (arc.units)
details of growth conditions for each sample are listed in Table 1. The atomic concentration of Mn in GM and GMB samples, measured by electron probe X-ray microanalysis (EPMA), was about 3 atomic percent. PL properties were characterized by the Raman/PL Measurement System with an argon laser operating at the wavelength of 514 nm and excitation power density of 50e100 V/cm2.
CX
2 - GB 3 - GaAs
FX
4 - GMn
1
AX
2
MX
3 4
1.40
1.44
1.48
1.52
Energy (eV) Fig. 1. PL spectra of the investigated samples, measured at the temperature of 4 K.
to estimate the hole concentration in the immediate vicinity of Tc from Hall effect data because of the large contribution of AHE. However, with the assumption that the same mechanisms are responsible for temperature dependence of hole mobility in GaMnAs and Be codoped GaMnAs, temperature dependences of resistivity allows us to conclude that hole concentration in GMB samples is 2e5 times higher than in GMn sample in all the investigated temperature range. Thus, in spite of the higher free hole concentration, the Tc value in GMB sample is lower than in GMn sample in accordance with [3,8], which could be a result of the decreasing MnGa concentration in Be codoped GaMnAs epilayer. Returning to the data of the PL measurements, note that we have not detected any peaks, aside from the peaks observed in the GaAs layer in the GB samples. It indicates that the intensity of radiative recombination through Be levels in GaAs, grown by LT MBE, is much less than the intensity of exciton-related transition and recombination through defect levels generated in the epitaxy process. Note also that peak AX in the PL spectra of GMB sample is masked by a broad low energy shoulder of the CX peak. A comparison of the PL spectra, measured for different samples, allows us to suggest that the broad shoulder of CX peak is a result of the overlapping of AX and CX peaks with an additional new peak located between the AX and CX peaks. The clear evidence for the existence of this additional peak in the GMB sample was obtained from measurements of the temperature dependence of the PL spectra. The PL spectra of GMB, GB, and GM samples, measured in the temperature range of 4e100 K, are shown in Fig. 4. It is clearly
0.01
GMB GMn
Resistivity ( cm)
736
0
50
100 150 200 250 300 T (K)
Fig. 2. Temperature dependence of the resistivity for GaMnAs and Be codoped GaMnAs epitaxial layers.
F. Yu et al. / Current Applied Physics 11 (2011) 735e739
0
20K 30K 40K 50K 60K
-10
-8000
Hall Resistance
20
b
-4000
0 H (Oe)
4000
10 20K 30K 40K 45K 50K
0 -10
-4000
0
4000
AX
4K 50K 80K 100K
1.45
b
1.50
1.55
PL intensity (arc.units)
FX
4K 50K 80K 100K
H (Oe) Fig. 3. Hall resistance for (a) GaMnAs and (b) Be codoped GaMnAs epitaxial layers.
CX
AX
1.40
8000
shown in Fig. 4 that the intensity of the CX peak, which dominates in the PL spectra at 4 K for all samples, decays rapidly with an increase in temperature, and exciton-related transition becomes dominant in the PL spectra at temperature above 50 K. Such rapid degeneration of carbon-related transition may be caused by a decrease in carriers capture coefficient of the carbon-related recombination centers with the increase in temperature. As shown in Fig. 4a at the temperature 50 K, when the intensity of the CX peak decreased significantly, a new broad peak, labeled as BMX with maximum intensity at 1.487 eV, was observed in the PL spectra of the GMB sample. With further increase in temperature, carbonrelated transition disappeared completely and only the BMX peak was observed in the PL spectra. Although the intensity of the BMX peak also decays, this peak is relatively stable and has been traced in the PL spectra of the GMB sample at temperature as high as 150 K with the transition energy of about 1.44 eV. Now let us identify the origin of the BMX peak. Since this peak is observed in the samples codoped with Mn and Be, we have to consider the possibility of its origin from impurities-related transition. If we suggest that the temperature dependence energy of the BMX peak is close to the temperature dependence energy of the other peak, we can estimate that at 4 K, the energy position of the BMX peak must be at approximately 1.482e1.484 eV. From our knowledge, we have not received any information that Be-related transition energy in GaAs-based semiconductor materials is located at such energies in the temperature range of 4e50 K. In the study of Karasyuk [20], Be-related transition energy in Be-doped GaAs is from 1.511 eV to 1.512 eV. Pavesi and Guzzi [11] reported energy of 1.4915 eV for conduction bandeneutral Be acceptor transition and 1.488 for donor eBe acceptor transition at 4 K, that is, very close to analogical carbon-related transition (1.493 eV and 1.489 eV for
FX
Energy (eV)
-20
-8000
CX BMX
1.40
8000
(GMB)
a
PL intensity (arc.units)
Hall Resistance
(GMn)
PL intensity (arc.units)
a 10
737
1.45 1.50 Energy (eV)
c
CX
1.55
FX
AX 4K 50K
BMX
80K 100K
1.40
1.45
1.50
1.55
Energy (eV) Fig. 4. The temperature dependent PL spectra for sample (a) GMB, (b) GB, and (c) GMn.
carbon conduction band and carbonedonor transition respectively). Moreover, although the CX peaks’ intensity also rapidly decays in the GB samples, in this sample no trace of BMX peak has been detected (Fig. 4b). These facts allow us to rule out recombination through the impurities level of Be as a possible reason for the appearance of the BMX peak in the GMB sample. The wide shape of the BMX peak could indicate that it originates from defects or impurities-related sub-band. Also, it should be noticed that an additional peak, similar with the BMX peak, has been observed on the low energy shoulder of the CX peak in the PL spectra of the GMn sample with increasing temperature (Fig. 4c). Hence, we have detected the BMX peak in both Mn-doped GaAs epilayers. Although the occurrence of this peak cannot be attributed to the recombination through MnGa acceptors level (as it was mentioned above, the energy for MnGa-related transition is 1.41e1.42 eV), it might be possible that the BMX peak related to MnI
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F. Yu et al. / Current Applied Physics 11 (2011) 735e739
defects and increasing MnI concentration in GaMnAs, codoped with Be, makes it visible in the PL spectra of the GMB sample even at 4 K. However, further studies are required for definitive identification of the BMX peak’s nature. Complementary to decreasing intensity of the PL peaks, for all samples, shift of the peaks’ positions towards the lower energy (red shift) with an increase in temperature has been observed. It is well known that the semiconductors temperature behavior of the PL peaks, especially for the exciton-related transitions, in general, is determined by temperature dependence of the band gap (Eg) [11,12]. Hence, the temperature dependence of the PL spectra allows us to study the effect of the dopants on the temperature dependence of Eg in GaAs. The red shifts in the peak positions in the PL spectra for all samples are summarized in Table 2. Here we determined red shift as DE ¼ E(4K)E(T), where DE is the shifted energy, E(4K) is the transition energy at temperature 4 K, and E(T) is the transition energy at temperature T. To determine the effect of dopants, we compared DEx (the red shift determined for exciton-related transition in GMn, GB, and GMB samples) with the red shift of the FX line in the control GaAs layer, and with DEg value for pure crystalline GaAs as determined by Varshini’s semi-empirical relation [11,21].
EgðTÞ ¼ 1:519 5:405$104 $T2 =ðT þ 204Þ
(1)
First, note that the calculated DEg value is different from the DEx value determined in the GaAs control sample that, in our opinion, reflects the effect of the defects generated during the LT MBE process, on the temperature dependence of the Eg value in GaAs. Besides, it is clearly seen that the DEx values for the GMn and GB samples are significantly different. This result indicates that the temperature dependence of the band gap in GaAs grown by LT MBE is modified by the introduction of impurities. In the Be-doped GaAs samples, the temperature dependence of DEx is very close to the same value observed in the GaAs control sample and, first of all, we had to answer the following question: if the Be concentration in the semiconducting matrix is enough to modify the temperature dependence of the band gap on GaAs. In the measurements of the Hall Effect for the GB sample, a hole concentration about 2.1 1020/ cc at 300 K was determined. As far as the GaAs grown by LT-MBE demonstrates n-type conductivity, we can attribute the whole hole concentration to the Be acceptor, which occupies the Ga sites and estimates a lower limit of Be in the GaAs matrix. (In fact, the hole concentration is lower than the Be acceptor concentration due to the compensation by donor-like defects, the concentration of which in GaAs-based materials grown by LT MBE is about 1020/cc Table 2 Temperature dependence of the red shift of the PL spectra peaks of each sample. T (K)
GaAs
GB
GMn
GMB
CalculatedDE
30 50 100 30 50 100 30 50 100 30 50 100
DE ¼ E (4K)E (T), (eV) FX
CX
AX
0 0.0022 0.0148 0.0002 0.0020 0.0146 0.0006 0.002 0.0135 0.0005 0.002 0.0145
0.0005 0.0022 e 0.0011 0.0025 e 0 0.0017 0.0102 0.0006 0.002 e
0.0008 e e 0.0006 e e 0.0005 0.0026 e e e e
T (K)
DEg, (eV)
30 50 100
0.0020 0.0052 0.0177
[22]). As shown in our previous study, the doping GaAs with similar concentration of Mn notably modified the temperature dependence of DEx [13,16]. Hence, we can make a conclusion that doping GaAs with Be almost does not affect the temperature dependence of the band gap. Temperature dependent behaviors of the Ex value in GMn and GMB samples are significantly different in spite of similar total Mn concentration. For GMn sample the temperature dependence of Ex is weaker than in the control GaAs. As was demonstrated in our previous study [16], the difference becomes more significant with the increase in Mn concentration for homogeneous GaMnAs. The dependence of DEx on MnGa concentration is also supported by the fact that precipitation of Mn acceptors into MnAs clusters, caused by high temperature annealing, reduces the difference between the temperature dependences of Ex values in GaMnAs and GaAs [13]. On the contrary, temperature dependence of the red shift in GMB sample is very close to the temperature dependence of DEx observed in the GB sample and control GaAs epilayer. This result indicates that codoping with Be can effectively compensate for the effects of Mn doping on the temperature dependence of the band gap energy in the GaMnAs grown via LT MBE. Taking into account the correlations between DEx and MnGa we can suggest that the compensation effect as well as the disappearing of the Mn-related transition in the PL spectra of the GMB sample, might be related to the decrease in the concentration of a substitutional Mn acceptor due to the introduction of an additional Be acceptor into the GaAs matrix. Thus, the results of our study show that although doping with Be almost did not affect the PL spectra of GaAs grown by LT MBE, codoping with Be significantly modified the PL spectra of GaMnAs. Such modification, in our opinion, could be related with the influence of Be on the MnIeMnGa ratio into semiconducting GaAs matrix. 4. Conclusions The PL spectra of the GaAs, GaAs:Be, GaMnAs, and GaMnAs:Be, grown by LT MBE have been studied. A new peak in the PL spectra of the GaMnAs codoping with the Be sample has been observed. At 4 K, this peak is manifested as a low energy shoulder of carbonrelated transition. However, the relatively weak temperature dependence intensity of this transition makes the peak visible at temperature higher than 50 K. It was shown that introduction of the additional Be impurity in GaMnAs could compensate the effect of Mn doping on the temperature behavior of the band gap of GaAs grown by LT MBE. This compensation effect might be a result of decreasing MnIeMnGa ratio in Be codoped GaMnAs epitaxial layers. Acknowledgments This work was done with the support by the National Research Laboratory program, Korea. References [1] H. Ohno, F. Matsukura, T. Penny, S. Von Molnar, L.L. Chang, Phys. Rev. Lett. 68 (1992) 2664. [2] H. Ohno, A. Shen, F. Matsukura, A. Oiwa, A. Endo, S. Katsumoto, Y. Iye, Appl. Phys. Lett. 69 (1996) 363. [3] T. Dietl, H. Ohno, F. Matsukura, J. Cibert, D. Ferrand, Science 287 (2000) 1019. [4] N. Samarth, S.H. Chun, K.C. Ku, S.J. Potashnik, P. Schiffer, Solid State Commun. 127 (2003) 173. [5] K.M. Yu, W. Walukiewicz, Phys. Rev. B. 65 (2002) 201303. [6] K.M. Yu, W. Walukiewicz, L.Y. Chan, R. Leon, E.E. Haller, J.M. Jaklevic, C.M. Hanson, J. Appl. Phys. 74 (1993) 86. [7] K.W. Edmonds, P. Bogus1awski, K.Y. Wang, R.P. Campion, S.N. Novikov, N.R.S. Farley, B.L. Gallagher, C.T. Foxon, M. Sawicki, T. Dietl, M. Buongiorno Nardelli, J. Bernholc, Phys. Rev. Lett. 92 (2004) 037201. [8] S. Lee, S.J. Chung, I.S. Choi, Sh.U. Yuldashev, H. Im, T.W. Kang, W.-L. Lim, Y. Sasaki, X. Liu, T. Wojtowicz, J.K. Furdyna, J. Appl. Phys. 93 (2003) 8307.
F. Yu et al. / Current Applied Physics 11 (2011) 735e739 [9] T. Wojtowicz, W.L. Lim, X. Liu, M. Dobrowolska, J.K. Furdyna, K.M. Yu, W. Walukiewicz, I. Vurgaftman, J.R. Meyer, Appl. Phys. Lett. 83 (2003) 4220. [10] Koji Onomitsu, Hideo Fukui, Takashi Maed, Yoshiro Hirayama, Yoshiji Horikoshi, J. Crys. Growth 278 (2005) 699. [11] Lorenzo Pavesi, Mario Guzzi, J. Appl. Phys. 75 (1994) 4779. [12] K.S. Zhuravlev, T.S. Shamirzaev, N.A. Yakusheva, Semiconductors 32 (1998) 43. [13] P.B. Parchinskiy, Fu Cheng Yu, Se Young Jeong, Cunxu Gao, Dojin Kim, Hyojin Kim, Young Eon Ihm, Appl. Surf. Sci. 253 (2006) 515. [14] Th. Hartmann, M. Lampalerzer, W. Stolz, K. Megges, J. Lorberth, P.J. Klar, W. Yeimbrodt, Thin Solid Films 364 (2000) 209. [15] K. Wagenhuber, H.-P. Traintz, M. Reiwald, W. Wegscheder, Appl. Phys. Lett. 85 (2004) 1190.
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[16] P.B. Parchinskiy, A.Yu. Bobylev, S.I. Vlasov, Fucheng Yu, Dojin Kim, Semiconductors 41 (2007) 1145. [17] G. Borghs, J. DeBoeck, Mat. Sci. Eng. B. 84 (2001) 75. [18] Y. Iye, A. Oiwa, A. Endo, S. Katsumoto, F. Matsukura, A. Shen, H. Ohno, H. Munekata, Mat. Sci. Eng. B. 63 (1999) 88. [19] Sh.U. Yuldashev, H. Im, V.Sh. Yalishev, C.S. Park, T.W. Kang, S. Lee, Y. Sasaki, X. Liu, J.K. Furdina, Appl. Phys. Lett. 82 (2003) 1206. [20] V.A. Karasyuk, M.L.W. Thewalt, A.J. SpringThorpe, Phys. Stat. Sol. (b) 210 (1998) 353. [21] S.M. Sze, Physics of Semiconductor Devises. John Wiley & Sons, New York, 1981. [22] A.G. de Oliviera, G.M. Ribeiro, D.A.V. Soares, J.A. . Corea, F.M.I.N. da Silva, H. Chacham, J. Appl. Phys. 78 (1996) 2659.
Contents lists available at ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Effect of Be codoping on the photoluminescence spectra of GaMnAs Fucheng Yu a, c, P.B. Parchinskiy a, b, Dojin Kim a, *, Hyojin Kim a, Young Eon Ihm a, Duck-Kyun Choi c a
School of Nano Science and Technology, Chungnam National University, 220 Gung-dong, Yuseong-gu, Daejeon 305-764, Republic of Korea National University of Uzbekistan, Tashkent, 700174, Uzbekistan c Information Display Research Institute, Hanyang University, Seoul 133-791, Republic of Korea b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 July 2009 Received in revised form 16 November 2010 Accepted 16 November 2010 Available online 25 November 2010
The measurements of the photoluminescence (PL) spectra have been performed on GaAs, GaMnAs, GaAs:Be, and GaMnAs:Be samples to study the effect of Be codoping on the PL spectra of GaMnAs layers grown via low temperature molecular beam epitaxy. Based on the temperature dependence of the exciton-related transitions energy, it was shown that doping GaAs with Mn and Be leads to modification of the temperature dependence of the band gap. It was shown that although Be itself weakly affected the PL spectra of GaAs, codoping with Be significantly modified the PL spectra of GaMnAs. Ó 2010 Elsevier B.V. All rights reserved.
Keywords: Photoluminescence spectra GaMnAs Be codoping MBE Ferromagnetic semiconductor
1. Introduction Since the discovery by the Ohno Group of ferromagnetism in InMnAs and GaMnAs grown via low temperature molecular beam epitaxy (LT MBE), IIIeV semiconductors doped with magnetic ion impurities known as diluted magnetic semiconductor (DMS) have been considered basic materials for the development of spintronics [1,2]. From the conventional models, ferromagnetism in DMS occurs as a result of interaction between localized spins of magnetic ions and delocalized or slightly localized holes [3]. Mn ions in GaMnAs occupy the sites of Ga atoms and act as both originators of spins and acceptors. However, the low solubility of Mn in GaAs [1] and the reduction of hole concentration owing to the existence of Mn interstitial defects and As anti-site defects [4e7] limit the temperature of paramagneticeferromagnetic transition in GaMnAs. Nowadays Be, as additional acceptor in GaMnAs, is widely used to enhance the hole concentration at the valence band in semiconductors [8e10]. On the other hand, the IIIeV compounds grown by LT MBE are very sensitive to the growth condition and alloy composition. Even a small change in impurities concentration could significantly modify the defect structures of the semiconducting matrix and its physical properties.
* Corresponding author. E-mail address: [email protected] (D. Kim). 1567-1739/$ e see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2010.11.049
It is well known that low temperature photoluminescence (PL) is one of the most promising techniques to obtain information about electronic properties and defect structures of semiconductors [11,12]. In this context, we have studied the effects of codoping of Be on the PL spectra in GaMnAs. 2. Experiment Samples used in the present study were grown on semi-insulating GaAs (001) substrate. The growth chamber was equipped with a reflection high-energy electron diffraction (RHEED) gun for in-situ monitoring of the surface reconstruction during the growth. The background pressure with flowing liquid nitrogen is in the low 1010 Torr range. Prior to main layers deposition a ∼250 nm-thick buffer GaAs layer was grown at Ts ¼ 580 C at a growth rate of 0.5 mm/h with an As4 pressure of ∼2.5 106 Torr, where the highest specular beam RHEED intensity was observed during growth for the smoothest growth front. The main layers of GaMnAs, GaMnAs codoped with Be, and GaAs doped with Be were grown at the substrate temperature of 300 C under an As4 pressure of 1.4 106 Torr. The thickness of the main layers were ∼250 nm with the growth rate of 0.25 mm/h. Be flux was set with TBe ¼ 1175 C, at which temperature, metallic Be-doped GaAs could be realized. The Mn cell temperature was fixed at TMn ¼ 910 C. GaAs layer grown under the same LT MBE conditions but without any impurities was used as a reference sample. The
F. Yu et al. / Current Applied Physics 11 (2011) 735e739
3. Results and discussion Fig. 1 shows the PL spectra of GMn, GB, and GMB samples and reference GaAs layer measured at 4 K. For all samples, three major PL peaks, labeled as “FX”, “CX”, and “AX” for transition energy 1.51 eV, 1.492 eV, and 1.462 eV, respectively, have been observed. Peak FX is interpreted as exciton-related transition [13e15] and more detailed examination showed that this peak consists of at least two closed lines at energy 1.514 and 1.510 eV, which were attributed to free excitons and donor-related exciton transitions [16]. Peaks CX and AX are associated with recombination via carbon acceptor level (carbon is well known as the most important unintentional impurities in the MBE growth process) and Ga anti-site transition, respectively [12e15]. In the GMn sample, we have also observed a wide peak with maximum at approximately 1.425 eV, which could be attributed to recombination via Mn acceptor level [12,14,16]. The anomalously large width of this peak, in our opinion, indicates that Mn-related levels form an impurities sub-band in the band gap of GaMnAs. The absence of Mn-related transition in the PL spectra of GMB sample e in which Mn concentration, according to the EPMA data, is the same as in the GMn sample e could reflect the fact that codoping with Be, on one side, did not change the total Mn concentration. On the other side, however, increased concentration of Mn interstitials (MnI) and, possibly, Mn-based precipitate, decreased Mn acceptor (MnGa) levels in the GaAs matrix [8]. The conclusion that MnIeMnGa ratio in GMB sample is lower than in GMn sample was also supported by the data of magnetotransport measurements. Fig. 2 shows the temperature dependence of resistivity for GMn and GMB samples, measured at zero magnetic field. It is clear that for both samples local maxima of the resistivity exist. The position of the maximum in GaMnAs and other III-Mn-V ferromagnetic alloys is associated with onset of the magnetic ordering in the semiconducting matrix [17e19], and it allows us to estimate a paramagneticeferromagnetic transition temperature (Tc) as Tc z 55 K for GMn sample and Tc z 45 K for GMB sample. The Hall effect measurements showed that in GMn samples the contribution of the anomalous Hall effect (AHE) was still predominant at 50 K (Fig. 3a), whereas in GMB sample the temperature, above which the contribution of AHE disappeared, is 45 K (Fig. 3b). Thus, both of the resistivity and Hall effect measurements demonstrate that the temperature of paramagneticeferromagnetic transition in Be codoped samples is lower than in undoped GaMnAs layer. As mentioned earlier Be is an effective acceptor and codoping element and Be should increase free hole concentration in GaMnAs. Indeed, the hole concentration in GMn and GMB samples, estimated from Hall effect measurements at room temperature, was 0.8 1020 cm3 and 1.8 1020 cm3, respectively. We were not able Table 1 The summary of growth conditions of each sample. Sample ID
Mn flux (oC)
GB GMn GMB
910 910
Be flux (oC)
Description
1175
GaAs:Be GaMnAs GaMnAs:Be
1175
1 - GMB
PL intensity (arc.units)
details of growth conditions for each sample are listed in Table 1. The atomic concentration of Mn in GM and GMB samples, measured by electron probe X-ray microanalysis (EPMA), was about 3 atomic percent. PL properties were characterized by the Raman/PL Measurement System with an argon laser operating at the wavelength of 514 nm and excitation power density of 50e100 V/cm2.
CX
2 - GB 3 - GaAs
FX
4 - GMn
1
AX
2
MX
3 4
1.40
1.44
1.48
1.52
Energy (eV) Fig. 1. PL spectra of the investigated samples, measured at the temperature of 4 K.
to estimate the hole concentration in the immediate vicinity of Tc from Hall effect data because of the large contribution of AHE. However, with the assumption that the same mechanisms are responsible for temperature dependence of hole mobility in GaMnAs and Be codoped GaMnAs, temperature dependences of resistivity allows us to conclude that hole concentration in GMB samples is 2e5 times higher than in GMn sample in all the investigated temperature range. Thus, in spite of the higher free hole concentration, the Tc value in GMB sample is lower than in GMn sample in accordance with [3,8], which could be a result of the decreasing MnGa concentration in Be codoped GaMnAs epilayer. Returning to the data of the PL measurements, note that we have not detected any peaks, aside from the peaks observed in the GaAs layer in the GB samples. It indicates that the intensity of radiative recombination through Be levels in GaAs, grown by LT MBE, is much less than the intensity of exciton-related transition and recombination through defect levels generated in the epitaxy process. Note also that peak AX in the PL spectra of GMB sample is masked by a broad low energy shoulder of the CX peak. A comparison of the PL spectra, measured for different samples, allows us to suggest that the broad shoulder of CX peak is a result of the overlapping of AX and CX peaks with an additional new peak located between the AX and CX peaks. The clear evidence for the existence of this additional peak in the GMB sample was obtained from measurements of the temperature dependence of the PL spectra. The PL spectra of GMB, GB, and GM samples, measured in the temperature range of 4e100 K, are shown in Fig. 4. It is clearly
0.01
GMB GMn
Resistivity ( cm)
736
0
50
100 150 200 250 300 T (K)
Fig. 2. Temperature dependence of the resistivity for GaMnAs and Be codoped GaMnAs epitaxial layers.
F. Yu et al. / Current Applied Physics 11 (2011) 735e739
0
20K 30K 40K 50K 60K
-10
-8000
Hall Resistance
20
b
-4000
0 H (Oe)
4000
10 20K 30K 40K 45K 50K
0 -10
-4000
0
4000
AX
4K 50K 80K 100K
1.45
b
1.50
1.55
PL intensity (arc.units)
FX
4K 50K 80K 100K
H (Oe) Fig. 3. Hall resistance for (a) GaMnAs and (b) Be codoped GaMnAs epitaxial layers.
CX
AX
1.40
8000
shown in Fig. 4 that the intensity of the CX peak, which dominates in the PL spectra at 4 K for all samples, decays rapidly with an increase in temperature, and exciton-related transition becomes dominant in the PL spectra at temperature above 50 K. Such rapid degeneration of carbon-related transition may be caused by a decrease in carriers capture coefficient of the carbon-related recombination centers with the increase in temperature. As shown in Fig. 4a at the temperature 50 K, when the intensity of the CX peak decreased significantly, a new broad peak, labeled as BMX with maximum intensity at 1.487 eV, was observed in the PL spectra of the GMB sample. With further increase in temperature, carbonrelated transition disappeared completely and only the BMX peak was observed in the PL spectra. Although the intensity of the BMX peak also decays, this peak is relatively stable and has been traced in the PL spectra of the GMB sample at temperature as high as 150 K with the transition energy of about 1.44 eV. Now let us identify the origin of the BMX peak. Since this peak is observed in the samples codoped with Mn and Be, we have to consider the possibility of its origin from impurities-related transition. If we suggest that the temperature dependence energy of the BMX peak is close to the temperature dependence energy of the other peak, we can estimate that at 4 K, the energy position of the BMX peak must be at approximately 1.482e1.484 eV. From our knowledge, we have not received any information that Be-related transition energy in GaAs-based semiconductor materials is located at such energies in the temperature range of 4e50 K. In the study of Karasyuk [20], Be-related transition energy in Be-doped GaAs is from 1.511 eV to 1.512 eV. Pavesi and Guzzi [11] reported energy of 1.4915 eV for conduction bandeneutral Be acceptor transition and 1.488 for donor eBe acceptor transition at 4 K, that is, very close to analogical carbon-related transition (1.493 eV and 1.489 eV for
FX
Energy (eV)
-20
-8000
CX BMX
1.40
8000
(GMB)
a
PL intensity (arc.units)
Hall Resistance
(GMn)
PL intensity (arc.units)
a 10
737
1.45 1.50 Energy (eV)
c
CX
1.55
FX
AX 4K 50K
BMX
80K 100K
1.40
1.45
1.50
1.55
Energy (eV) Fig. 4. The temperature dependent PL spectra for sample (a) GMB, (b) GB, and (c) GMn.
carbon conduction band and carbonedonor transition respectively). Moreover, although the CX peaks’ intensity also rapidly decays in the GB samples, in this sample no trace of BMX peak has been detected (Fig. 4b). These facts allow us to rule out recombination through the impurities level of Be as a possible reason for the appearance of the BMX peak in the GMB sample. The wide shape of the BMX peak could indicate that it originates from defects or impurities-related sub-band. Also, it should be noticed that an additional peak, similar with the BMX peak, has been observed on the low energy shoulder of the CX peak in the PL spectra of the GMn sample with increasing temperature (Fig. 4c). Hence, we have detected the BMX peak in both Mn-doped GaAs epilayers. Although the occurrence of this peak cannot be attributed to the recombination through MnGa acceptors level (as it was mentioned above, the energy for MnGa-related transition is 1.41e1.42 eV), it might be possible that the BMX peak related to MnI
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F. Yu et al. / Current Applied Physics 11 (2011) 735e739
defects and increasing MnI concentration in GaMnAs, codoped with Be, makes it visible in the PL spectra of the GMB sample even at 4 K. However, further studies are required for definitive identification of the BMX peak’s nature. Complementary to decreasing intensity of the PL peaks, for all samples, shift of the peaks’ positions towards the lower energy (red shift) with an increase in temperature has been observed. It is well known that the semiconductors temperature behavior of the PL peaks, especially for the exciton-related transitions, in general, is determined by temperature dependence of the band gap (Eg) [11,12]. Hence, the temperature dependence of the PL spectra allows us to study the effect of the dopants on the temperature dependence of Eg in GaAs. The red shifts in the peak positions in the PL spectra for all samples are summarized in Table 2. Here we determined red shift as DE ¼ E(4K)E(T), where DE is the shifted energy, E(4K) is the transition energy at temperature 4 K, and E(T) is the transition energy at temperature T. To determine the effect of dopants, we compared DEx (the red shift determined for exciton-related transition in GMn, GB, and GMB samples) with the red shift of the FX line in the control GaAs layer, and with DEg value for pure crystalline GaAs as determined by Varshini’s semi-empirical relation [11,21].
EgðTÞ ¼ 1:519 5:405$104 $T2 =ðT þ 204Þ
(1)
First, note that the calculated DEg value is different from the DEx value determined in the GaAs control sample that, in our opinion, reflects the effect of the defects generated during the LT MBE process, on the temperature dependence of the Eg value in GaAs. Besides, it is clearly seen that the DEx values for the GMn and GB samples are significantly different. This result indicates that the temperature dependence of the band gap in GaAs grown by LT MBE is modified by the introduction of impurities. In the Be-doped GaAs samples, the temperature dependence of DEx is very close to the same value observed in the GaAs control sample and, first of all, we had to answer the following question: if the Be concentration in the semiconducting matrix is enough to modify the temperature dependence of the band gap on GaAs. In the measurements of the Hall Effect for the GB sample, a hole concentration about 2.1 1020/ cc at 300 K was determined. As far as the GaAs grown by LT-MBE demonstrates n-type conductivity, we can attribute the whole hole concentration to the Be acceptor, which occupies the Ga sites and estimates a lower limit of Be in the GaAs matrix. (In fact, the hole concentration is lower than the Be acceptor concentration due to the compensation by donor-like defects, the concentration of which in GaAs-based materials grown by LT MBE is about 1020/cc Table 2 Temperature dependence of the red shift of the PL spectra peaks of each sample. T (K)
GaAs
GB
GMn
GMB
CalculatedDE
30 50 100 30 50 100 30 50 100 30 50 100
DE ¼ E (4K)E (T), (eV) FX
CX
AX
0 0.0022 0.0148 0.0002 0.0020 0.0146 0.0006 0.002 0.0135 0.0005 0.002 0.0145
0.0005 0.0022 e 0.0011 0.0025 e 0 0.0017 0.0102 0.0006 0.002 e
0.0008 e e 0.0006 e e 0.0005 0.0026 e e e e
T (K)
DEg, (eV)
30 50 100
0.0020 0.0052 0.0177
[22]). As shown in our previous study, the doping GaAs with similar concentration of Mn notably modified the temperature dependence of DEx [13,16]. Hence, we can make a conclusion that doping GaAs with Be almost does not affect the temperature dependence of the band gap. Temperature dependent behaviors of the Ex value in GMn and GMB samples are significantly different in spite of similar total Mn concentration. For GMn sample the temperature dependence of Ex is weaker than in the control GaAs. As was demonstrated in our previous study [16], the difference becomes more significant with the increase in Mn concentration for homogeneous GaMnAs. The dependence of DEx on MnGa concentration is also supported by the fact that precipitation of Mn acceptors into MnAs clusters, caused by high temperature annealing, reduces the difference between the temperature dependences of Ex values in GaMnAs and GaAs [13]. On the contrary, temperature dependence of the red shift in GMB sample is very close to the temperature dependence of DEx observed in the GB sample and control GaAs epilayer. This result indicates that codoping with Be can effectively compensate for the effects of Mn doping on the temperature dependence of the band gap energy in the GaMnAs grown via LT MBE. Taking into account the correlations between DEx and MnGa we can suggest that the compensation effect as well as the disappearing of the Mn-related transition in the PL spectra of the GMB sample, might be related to the decrease in the concentration of a substitutional Mn acceptor due to the introduction of an additional Be acceptor into the GaAs matrix. Thus, the results of our study show that although doping with Be almost did not affect the PL spectra of GaAs grown by LT MBE, codoping with Be significantly modified the PL spectra of GaMnAs. Such modification, in our opinion, could be related with the influence of Be on the MnIeMnGa ratio into semiconducting GaAs matrix. 4. Conclusions The PL spectra of the GaAs, GaAs:Be, GaMnAs, and GaMnAs:Be, grown by LT MBE have been studied. A new peak in the PL spectra of the GaMnAs codoping with the Be sample has been observed. At 4 K, this peak is manifested as a low energy shoulder of carbonrelated transition. However, the relatively weak temperature dependence intensity of this transition makes the peak visible at temperature higher than 50 K. It was shown that introduction of the additional Be impurity in GaMnAs could compensate the effect of Mn doping on the temperature behavior of the band gap of GaAs grown by LT MBE. This compensation effect might be a result of decreasing MnIeMnGa ratio in Be codoped GaMnAs epitaxial layers. Acknowledgments This work was done with the support by the National Research Laboratory program, Korea. References [1] H. Ohno, F. Matsukura, T. Penny, S. Von Molnar, L.L. Chang, Phys. Rev. Lett. 68 (1992) 2664. [2] H. Ohno, A. Shen, F. Matsukura, A. Oiwa, A. Endo, S. Katsumoto, Y. Iye, Appl. Phys. Lett. 69 (1996) 363. [3] T. Dietl, H. Ohno, F. Matsukura, J. Cibert, D. Ferrand, Science 287 (2000) 1019. [4] N. Samarth, S.H. Chun, K.C. Ku, S.J. Potashnik, P. Schiffer, Solid State Commun. 127 (2003) 173. [5] K.M. Yu, W. Walukiewicz, Phys. Rev. B. 65 (2002) 201303. [6] K.M. Yu, W. Walukiewicz, L.Y. Chan, R. Leon, E.E. Haller, J.M. Jaklevic, C.M. Hanson, J. Appl. Phys. 74 (1993) 86. [7] K.W. Edmonds, P. Bogus1awski, K.Y. Wang, R.P. Campion, S.N. Novikov, N.R.S. Farley, B.L. Gallagher, C.T. Foxon, M. Sawicki, T. Dietl, M. Buongiorno Nardelli, J. Bernholc, Phys. Rev. Lett. 92 (2004) 037201. [8] S. Lee, S.J. Chung, I.S. Choi, Sh.U. Yuldashev, H. Im, T.W. Kang, W.-L. Lim, Y. Sasaki, X. Liu, T. Wojtowicz, J.K. Furdyna, J. Appl. Phys. 93 (2003) 8307.
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