- Email: [email protected]
Isolated hydrogen center in wide gap semiconductors studied by μSR
ARTICLE IN PRESS
Physica B 376–377 (2006) 444–446 www.elsevier.com/locate/physb
Isolated hydrogen center in wide gap semiconductors studied by mSR K. Shimomuraa,, R. Kadonoa, K. Nishiyamaa, I. Watanabeb, T. Suzukib, F. Prattc, K. Ohishid, M. Mizutae, M. Saitof, K.H. Chowg, B. Hittih, R.L. Lichtii, H. Miyaderaj, Y. Ikedoa, H. Tanakaj, K. Nagaminea,j a
Muon Science Laboratory, Institute of Material Structure Science, KEK, Tsukuba, Ibaragi 305-0801, Japan b Advanced Meson Science Laboratory, RIKEN, Saitama 351-0198, Japan c Rutherford Appleton Laboratory, Didcot, Oxfordshire OX11 0QX, UK d Advanced Science Research Center, JAERI, Ibaragi 319-1195, Japan e Nippon EMC LTD., Tokyo 206-0001, Japan f Graduate School of Natural Science and Technology, Kanazawa Univ., Ishikawa 920-1192, Japan g Department of Physics, University of Alberta, Edmonton, Alt., Canada T6G 2J1 h TRIUMF, Vancouver, Canada V6T 2A3 i Department of Physics, Texas Tech University, Lubbock, TX 79409-1051, USA j Department of Physics, University of California, Riverside, CA 92521, USA
Abstract Recent progresses of the studies of isolated hydrogen center in ZnO and GaN by muon spin rotation methods are reported. Preliminary experimental results for shallow muonium hunting as an acceptor in InSb and GaSb are also discussed. r 2006 Elsevier B.V. All rights reserved. PACS: 61.82.Fk; 76.75.+i Keywords: ZnO; GaN; InSb; GaSb; Hydrogen; Muonium
1. Introduction Hydrogen is a ubiquitous impurity in most semiconductors, including elemental (e.g., Si), compound (e.g., GaAs). In these systems, hydrogen is known to be amphoteric, forming an acceptor level in n-type and a donor level in ptype materials. In contrast, hydrogen can lead to electron conduction in some wide gap semiconductors such as ZnO [1,2]. These observations raise the question of what is the basic systematic at work here: if H can be incorporated into some materials, which one will be doped by H (i.e., become conductive) and which will not? Currently considerable theoretical studies were performed in this subject [3–5]. Muonium Spin Rotation Methods is now considered to be one of the most powerful tools to experimentally investigate these theoretical predictions. The muonium center Corresponding author. Tel.: +81 29 864 5624; fax: +81 29 864 5623.
E-mail address: [email protected] (K. Shimomura). 0921-4526/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2005.12.114
(Mu; an analog of isolated hydrogen whose proton is substituted by a positive muon) is readily observed in a wide variety of semiconductors after positive muon implantation, and has been serving as a unique source of information on the electronic structure of isolated hydrogen centers. While the dynamical aspect (e.g., diffusion property) may be considerably different between Mu and H due to the light mass of Mu (1/9), the local electronic structure of Mu is virtually equivalent to that of H after a small correction due to the difference in the reduced mass (0.4%). In this contribution, our recent results of isolated hydrogen center in several semiconductors by muon spin rotation methods are reported. 2. Radio frequncy lSR in ZnO In ZnO, we have reported in the previous paper [6], two species of Mu centers with extremely small hyperfine (HF) parameters have been observed below 40 K. Both Mu
ARTICLE IN PRESS K. Shimomura et al. / Physica B 376–377 (2006) 444–446
centers have an axial-symmetric HF structure along with the c-axis, indicating that they are located at the antibonding and bond-center sites. It is inferred from their small ionization energy (6 and 50 meV) and HF parameters (10 4 times the vacuum value) that these centers behave as shallow donors, strongly suggesting that hydrogen is one of the primary origins of n-type conductivity in as-grown ZnO, which is predicted by the theoretical studies. However, the other group reported only a single species of muonium existence in ZnO [7,8]. For the further studies of the existence of the second species, we preformed muon spin resonance experiments, which is similar to the normal NMR methods. Because the weak radio frequency (RF) field (1 G) measurements might achieve the better frequency resolution to distinguish the signals from these muoniums. Fig. 1 shows our preliminary results of RF measurements in ZnO, where, 50,000 MHz RF and longitudinal magnetic field were applied perpendicular and parallel to [11–20] axis, respectively. Center peak is corresponding to the diamagnetic muon resonance. One satellite, which is symmetrically located around the center peak, is corresponding to the shallow muonium resonance. Because 370 mT already corresponds to the high-field or Paschen–Back limit in which the muon–electron hyperfine interaction is effectively decoupled. The deduced hyperfine parameter at this orientation is determined to 360 kHz. This value is fairly consistent with the parameter of the muonium which ionization energy is 6 mV in the previous report [6]. Unfortunately, the preliminary analysis cannot distinguish two species of muonium, might be due to the
Fig. 1. Diamagnetic muon and muonium resonance sprectra in ZnO at 2.0 K with 50 MHz of radio-frequency field and about 0.37 T of static field parallel to [11–20] axis.
445
inhomogenity of the applied static field. Further improvements are required. 3. Transverse field lSR in GaN More interestingly, a shallow Mu center has been discovered in GaN, while no such state has been predicted in the latest theoretical studies. Above 22.5 K, only a single (diamagnetic) precession signal is observed at the muon Larmor frequency The muon spin rotation signal changes drastically below 22.5 K, which strongly suggests the shallow muonium is formed. Fig. 2 shows the angular and temperature dependences of the frequency spectra obtained by Fourier transform, in which one pair of satellite lines is clearly seen with their positions situated symmetrically around the central line, which corresponds to the precession of diamagnetic muons. The splitting of these satellites remained unchanged when the applied field was increased from 1.5 up to 5 T (see Figs. 2(a) and (c)), a result that is important in identifying the spectra as due to the hyperfine interaction of a Mu0 center. The splitting decreases when the [0 0 0 1] axis is tilted with respect to the external magnetic field as in Fig. 2(d). Moreover, an equivalent frequency spectrum was observed when the [11–20] axis was rotated by 901 around [0 0 0 1], which was oriented at 351 to the applied magnetic field. These observations demonstrate the presence of a paramagnetic muonium state in GaN. The resulting hyperfine interaction is extremely small, about 10 4 times the vacuum value for a Mu atom, and is axially symmetric with respect to [0 0 0 1]. Due to the muonium electron polarization, the satellite peak has an asymmetric feature, which is shown in Figs. 2(a) and (c). These results inform us the hyperfine constant is the positive for the displayed orientations. For the detailed analysis, the parallel and perpendicular component of the hyperfine tensor is deduced to be +337(10) and 243(30) kHz, respectively and their ionization energy is less than 14 meV [9].
Fig. 2. Frequency spectra obtained for GaN at (a) 2.5 K and (b)22.5 K with 3.0 T parallel to [0 0 0 1] axis, and with 1.5 T at 2.5 K, where [0 0 0 1] is parallel to B (c) or tiltrd by 351 from B (d).
ARTICLE IN PRESS K. Shimomura et al. / Physica B 376–377 (2006) 444–446
446
GaSb diamagnetic fraction 0.2
0.15
0.1
0.05
0
0
50
100
200 150 Temperature (K)
250
300
GaSb LF decoupling 0.2
0.15
pointed out that hydrogen (H) might behave as a shallow acceptor in InSb and GaSb [2]. To examine these predictions, Transverse and Longitudinal Field mSR measurements were performed at KEK–MSL. At first, low transverse field (8 mT) measurements were performed at 5 K with enough high statistics (100 M), however, no significant satellite peaks, which are situated symmetrically around the diamagnetic component, were obtained in fast Fourier transformation (FFT) spectrum in both cases. On the other hand, clear decrease of diamagnetic fraction were observed below about 150 K, which is much higher than the typical temperature of the other shallow muonium observed wide-gap semiconductors like ZnO or GaN. Also, longitudinal field (LF) measurements show the typical decoupling pattern by the existence of muonium, which hyperfine constant is a few tens % of the vacuum. These observations suggest only deep muonium exists at these conditions. Temperature dependence of diamagnetic muon fraction and longitudinal decoupling pattern at 5 K in GaSb are shown in Fig. 3. Detail will be reported elsewhere.
0.1
Acknowledgments 0.05
0
0
500
1000
1500 2000 LF (G)
2500
3000
3500
Fig. 3. Temperature dependence of diamagnetic muon fraction in GaSb (Upper). Longitudinal decoupling pattern in GaSb at 5 K (Lower).
We would like to thank the staff of KEK–MSL, TRIUMF and RIKEN–RAL Muon Facility for their technical support. Partially, this work is supported by COE budget of Ministry of Education, Culture, Sports, Science and Technology and by Grant-in-Aid for Scientific Research of Japan Society for the Promotion of Science (JSPS) and JSPS Core to core program. References
The observed Mu center has an extremely small HF parameter with an axially symmetric HF structure, suggesting that it is located either near a nitrogenantibonding or a bond-centered site oriented parallel to the c-axis. Its small ionization energy (o14 meV) and small HF parameter (10 4 times the vacuum value) indicate again that a shallow electronic state is associated with the Mu center, suggesting the possibility that the analogous hydrogen center could be a source of n-type conductivity in as-grown GaN. 4. Shallow muonium hunting in InSb and GaSb Inspired by the progress of shallow hydrogen impurity studies in semiconductors, it was recently theoretically
[1] [2] [3] [4] [5] [6] [7] [8] [9]
C.G. Van de Walle, Phys. Rev. Lett. 85 (2000) 1012. C.G. Van de Walle, et al., Nature 423 (2003) 626. C. Etin Kılıc, et al., Appl. Phys. Lett. 81 (2002) 73. P.W. Peacock, et al., Appl. Phys. Lett. 83 (2003) 2025. S.F.J. Cox, J. Phys.: Condens. Matter 15 (2003) R1727. K. Shimomura, et al., Phys Rev. Lett. 89 (2002) 255505. S.F.J. Cox, et al., Phys Rev. Lett. 86 (2001) 2601. J.M. Gil, et al., Phys Rev. B 64 (2001) 075205. K. Shimomura, et al., Phys. Rev. Lett. 92 (2004) 135505.
Physica B 376–377 (2006) 444–446 www.elsevier.com/locate/physb
Isolated hydrogen center in wide gap semiconductors studied by mSR K. Shimomuraa,, R. Kadonoa, K. Nishiyamaa, I. Watanabeb, T. Suzukib, F. Prattc, K. Ohishid, M. Mizutae, M. Saitof, K.H. Chowg, B. Hittih, R.L. Lichtii, H. Miyaderaj, Y. Ikedoa, H. Tanakaj, K. Nagaminea,j a
Muon Science Laboratory, Institute of Material Structure Science, KEK, Tsukuba, Ibaragi 305-0801, Japan b Advanced Meson Science Laboratory, RIKEN, Saitama 351-0198, Japan c Rutherford Appleton Laboratory, Didcot, Oxfordshire OX11 0QX, UK d Advanced Science Research Center, JAERI, Ibaragi 319-1195, Japan e Nippon EMC LTD., Tokyo 206-0001, Japan f Graduate School of Natural Science and Technology, Kanazawa Univ., Ishikawa 920-1192, Japan g Department of Physics, University of Alberta, Edmonton, Alt., Canada T6G 2J1 h TRIUMF, Vancouver, Canada V6T 2A3 i Department of Physics, Texas Tech University, Lubbock, TX 79409-1051, USA j Department of Physics, University of California, Riverside, CA 92521, USA
Abstract Recent progresses of the studies of isolated hydrogen center in ZnO and GaN by muon spin rotation methods are reported. Preliminary experimental results for shallow muonium hunting as an acceptor in InSb and GaSb are also discussed. r 2006 Elsevier B.V. All rights reserved. PACS: 61.82.Fk; 76.75.+i Keywords: ZnO; GaN; InSb; GaSb; Hydrogen; Muonium
1. Introduction Hydrogen is a ubiquitous impurity in most semiconductors, including elemental (e.g., Si), compound (e.g., GaAs). In these systems, hydrogen is known to be amphoteric, forming an acceptor level in n-type and a donor level in ptype materials. In contrast, hydrogen can lead to electron conduction in some wide gap semiconductors such as ZnO [1,2]. These observations raise the question of what is the basic systematic at work here: if H can be incorporated into some materials, which one will be doped by H (i.e., become conductive) and which will not? Currently considerable theoretical studies were performed in this subject [3–5]. Muonium Spin Rotation Methods is now considered to be one of the most powerful tools to experimentally investigate these theoretical predictions. The muonium center Corresponding author. Tel.: +81 29 864 5624; fax: +81 29 864 5623.
E-mail address: [email protected] (K. Shimomura). 0921-4526/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2005.12.114
(Mu; an analog of isolated hydrogen whose proton is substituted by a positive muon) is readily observed in a wide variety of semiconductors after positive muon implantation, and has been serving as a unique source of information on the electronic structure of isolated hydrogen centers. While the dynamical aspect (e.g., diffusion property) may be considerably different between Mu and H due to the light mass of Mu (1/9), the local electronic structure of Mu is virtually equivalent to that of H after a small correction due to the difference in the reduced mass (0.4%). In this contribution, our recent results of isolated hydrogen center in several semiconductors by muon spin rotation methods are reported. 2. Radio frequncy lSR in ZnO In ZnO, we have reported in the previous paper [6], two species of Mu centers with extremely small hyperfine (HF) parameters have been observed below 40 K. Both Mu
ARTICLE IN PRESS K. Shimomura et al. / Physica B 376–377 (2006) 444–446
centers have an axial-symmetric HF structure along with the c-axis, indicating that they are located at the antibonding and bond-center sites. It is inferred from their small ionization energy (6 and 50 meV) and HF parameters (10 4 times the vacuum value) that these centers behave as shallow donors, strongly suggesting that hydrogen is one of the primary origins of n-type conductivity in as-grown ZnO, which is predicted by the theoretical studies. However, the other group reported only a single species of muonium existence in ZnO [7,8]. For the further studies of the existence of the second species, we preformed muon spin resonance experiments, which is similar to the normal NMR methods. Because the weak radio frequency (RF) field (1 G) measurements might achieve the better frequency resolution to distinguish the signals from these muoniums. Fig. 1 shows our preliminary results of RF measurements in ZnO, where, 50,000 MHz RF and longitudinal magnetic field were applied perpendicular and parallel to [11–20] axis, respectively. Center peak is corresponding to the diamagnetic muon resonance. One satellite, which is symmetrically located around the center peak, is corresponding to the shallow muonium resonance. Because 370 mT already corresponds to the high-field or Paschen–Back limit in which the muon–electron hyperfine interaction is effectively decoupled. The deduced hyperfine parameter at this orientation is determined to 360 kHz. This value is fairly consistent with the parameter of the muonium which ionization energy is 6 mV in the previous report [6]. Unfortunately, the preliminary analysis cannot distinguish two species of muonium, might be due to the
Fig. 1. Diamagnetic muon and muonium resonance sprectra in ZnO at 2.0 K with 50 MHz of radio-frequency field and about 0.37 T of static field parallel to [11–20] axis.
445
inhomogenity of the applied static field. Further improvements are required. 3. Transverse field lSR in GaN More interestingly, a shallow Mu center has been discovered in GaN, while no such state has been predicted in the latest theoretical studies. Above 22.5 K, only a single (diamagnetic) precession signal is observed at the muon Larmor frequency The muon spin rotation signal changes drastically below 22.5 K, which strongly suggests the shallow muonium is formed. Fig. 2 shows the angular and temperature dependences of the frequency spectra obtained by Fourier transform, in which one pair of satellite lines is clearly seen with their positions situated symmetrically around the central line, which corresponds to the precession of diamagnetic muons. The splitting of these satellites remained unchanged when the applied field was increased from 1.5 up to 5 T (see Figs. 2(a) and (c)), a result that is important in identifying the spectra as due to the hyperfine interaction of a Mu0 center. The splitting decreases when the [0 0 0 1] axis is tilted with respect to the external magnetic field as in Fig. 2(d). Moreover, an equivalent frequency spectrum was observed when the [11–20] axis was rotated by 901 around [0 0 0 1], which was oriented at 351 to the applied magnetic field. These observations demonstrate the presence of a paramagnetic muonium state in GaN. The resulting hyperfine interaction is extremely small, about 10 4 times the vacuum value for a Mu atom, and is axially symmetric with respect to [0 0 0 1]. Due to the muonium electron polarization, the satellite peak has an asymmetric feature, which is shown in Figs. 2(a) and (c). These results inform us the hyperfine constant is the positive for the displayed orientations. For the detailed analysis, the parallel and perpendicular component of the hyperfine tensor is deduced to be +337(10) and 243(30) kHz, respectively and their ionization energy is less than 14 meV [9].
Fig. 2. Frequency spectra obtained for GaN at (a) 2.5 K and (b)22.5 K with 3.0 T parallel to [0 0 0 1] axis, and with 1.5 T at 2.5 K, where [0 0 0 1] is parallel to B (c) or tiltrd by 351 from B (d).
ARTICLE IN PRESS K. Shimomura et al. / Physica B 376–377 (2006) 444–446
446
GaSb diamagnetic fraction 0.2
0.15
0.1
0.05
0
0
50
100
200 150 Temperature (K)
250
300
GaSb LF decoupling 0.2
0.15
pointed out that hydrogen (H) might behave as a shallow acceptor in InSb and GaSb [2]. To examine these predictions, Transverse and Longitudinal Field mSR measurements were performed at KEK–MSL. At first, low transverse field (8 mT) measurements were performed at 5 K with enough high statistics (100 M), however, no significant satellite peaks, which are situated symmetrically around the diamagnetic component, were obtained in fast Fourier transformation (FFT) spectrum in both cases. On the other hand, clear decrease of diamagnetic fraction were observed below about 150 K, which is much higher than the typical temperature of the other shallow muonium observed wide-gap semiconductors like ZnO or GaN. Also, longitudinal field (LF) measurements show the typical decoupling pattern by the existence of muonium, which hyperfine constant is a few tens % of the vacuum. These observations suggest only deep muonium exists at these conditions. Temperature dependence of diamagnetic muon fraction and longitudinal decoupling pattern at 5 K in GaSb are shown in Fig. 3. Detail will be reported elsewhere.
0.1
Acknowledgments 0.05
0
0
500
1000
1500 2000 LF (G)
2500
3000
3500
Fig. 3. Temperature dependence of diamagnetic muon fraction in GaSb (Upper). Longitudinal decoupling pattern in GaSb at 5 K (Lower).
We would like to thank the staff of KEK–MSL, TRIUMF and RIKEN–RAL Muon Facility for their technical support. Partially, this work is supported by COE budget of Ministry of Education, Culture, Sports, Science and Technology and by Grant-in-Aid for Scientific Research of Japan Society for the Promotion of Science (JSPS) and JSPS Core to core program. References
The observed Mu center has an extremely small HF parameter with an axially symmetric HF structure, suggesting that it is located either near a nitrogenantibonding or a bond-centered site oriented parallel to the c-axis. Its small ionization energy (o14 meV) and small HF parameter (10 4 times the vacuum value) indicate again that a shallow electronic state is associated with the Mu center, suggesting the possibility that the analogous hydrogen center could be a source of n-type conductivity in as-grown GaN. 4. Shallow muonium hunting in InSb and GaSb Inspired by the progress of shallow hydrogen impurity studies in semiconductors, it was recently theoretically
[1] [2] [3] [4] [5] [6] [7] [8] [9]
C.G. Van de Walle, Phys. Rev. Lett. 85 (2000) 1012. C.G. Van de Walle, et al., Nature 423 (2003) 626. C. Etin Kılıc, et al., Appl. Phys. Lett. 81 (2002) 73. P.W. Peacock, et al., Appl. Phys. Lett. 83 (2003) 2025. S.F.J. Cox, J. Phys.: Condens. Matter 15 (2003) R1727. K. Shimomura, et al., Phys Rev. Lett. 89 (2002) 255505. S.F.J. Cox, et al., Phys Rev. Lett. 86 (2001) 2601. J.M. Gil, et al., Phys Rev. B 64 (2001) 075205. K. Shimomura, et al., Phys. Rev. Lett. 92 (2004) 135505.