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GaMnAs with high Mn composition grown in Stranski–Krastanov mode
Materials Letters 63 (2009) 337–339
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
GaMnAs with high Mn composition grown in Stranski–Krastanov mode F. Xu a,b, P.W. Huang b, J.H. Huang b,⁎, W.N. Lee b, T.S. Chin b, H.C. Ku c a b c
Department of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing 210094, China Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan Department of Physics, National Tsing Hua University, Hsinchu 30013, Taiwan
a r t i c l e
i n f o
Article history: Received 29 September 2008 Accepted 23 October 2008 Available online 30 October 2008 Keywords: Epitaxial growth Magnetic materials GaMnAs Diluted magnetic semiconductor (DMS)
a b s t r a c t GaMnAs diluted magnetic semiconductor (DMS) thin films with high Mn compositions were grown on (001) GaAs substrate by low-temperature molecular beam epitaxy (LT-MBE) after inserting an InAs wetting layer onto the GaAs buffer. The growth follows the Stranski–Krastanov mode, which brings about special magnetic characteristics of Ga0.88Mn0.12As. The insulating-DMS-like M(T) relation of the as-grown sample indicates the existence of a high proportion of interstitial Mn atoms. The magnetization is greatly improved after annealing, and the low Curie temperature is suggested to be due to the surface hole-localization effect. The investigation on the magnetic anisotropy shows an almost isotropic characteristic of the magnetic properties, which is ascribed to the S–K growth mode. © 2008 Elsevier B.V. All rights reserved.
1. Introduction As a famous diluted magnetic semiconductor (DMS) material, GaMnAs has attracted lots of research interests [1]. Most of these films are epitaxially grown on (001) GaAs substrate by low-temperature molecular beam epitaxy (LT-MBE). The Mn composition is limited (usually less than 8%) because of the lattice mismatch between the buffer and GaMnAs layer, and the compressive strain in GaMnAs layer. In order to increase the Mn incorporation, researchers have attempted to grow GaMnAs films on the modulated buffer layers with a larger lattice constant. However, only ultrathin films (~5 nm) have been successfully obtained in GaMnAs with high Mn compositions (N10%) [2]. Therefore, it is necessary to make further studies to obtain thicker GaMnAs films with high Mn compositions. In this work, we report our investigation on the growth of GaMnAs with high Mn compositions on (001) GaAs substrate by LT-MBE and its magnetic properties. The Mn composition is increased because the compressive strain is relaxed with an InAs wetting layer, and the growth of the samples is found following the Stranski–Krastanov mode [3]. The magnetic properties of Ga0.88Mn0.12As before and after LT-annealing are investigated, showing special characteristics corresponding to the growth mode. 2. Experimental The samples were grown on (001) semi-insulating, epiready GaAs substrates by a Varian Modular GEN-II MBE system. Following the native oxide desorption, a 250 nm GaAs buffer layer was grown at 580 °C to smoothen the surface. In order to relax the compressive strain
⁎ Corresponding author. Tel.: +886 3 5162228; fax: +886 3 5722366. E-mail address: [email protected] (J.H. Huang). 0167-577X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2008.10.040
in GaMnAs layer, and to incorporate more Mn atoms, an InAs wetting layer of about 1–2 monolayers was subsequently grown at 500 °C, in imitation of the growth processes of InMnAs quantum dots [4]. Then, Ga1 − xMnxAs (0.09 ≤ x ≤ 0.14) was successfully grown at 250 °C in a discontinuous way with 2 min interval per 10-second growth. The estimated thickness of Ga0.88Mn0.12As layer after 80 s growth is about 12 nm. The growth was in-situ monitored with reflection high energy electron diffraction (RHEED). During the growth of GaMnAs layer, a 1 × 1 streaky pattern was initially identified, instead of the typical 1 × 2 streaky pattern in the conventional LT-GaMnAs growth [5]. With the growth, the streaky pattern gradually changed into a 1 × 1 spotty pattern, indicating a Stranski–Krastanov growth mode [3,6]. After growth, the surface morphology was characterized by a Digital Instruments Nanoscope 3100 atomic force microscopy (AFM). And the magnetic properties were measured with a commercial superconducting quantum interference device magnetometer (SQUID). 3. Results and discussions Some RHEED patterns were recorded after growth. Fig. 1 selectively presents the RHEED patterns of Ga0.88Mn0.12As at 250 °C along both [110] and [11̄0] azimuths of the substrate, showing a 1 × 1 spotty pattern and indicating a Stranski–Krastanov growth mode. The S–K growth mode is unusual in the growth of GaMnAs, and it should be ascribed to the InAs wetting layer, which partially relaxes the compressive strain in Ga0.88Mn0.12As layer from the GaAs buffer layer and consequently avoids the formation of polycrystalline structure. The careful control of InAs wetting layer is necessary, because if the InAs nanodots are formed, they will bring about more complex and inhomogeneous strains inside the GaMnAs layer [7]. Compared to the growth of InAs on (001) GaAs [6,7], the transition of the RHEED pattern from the streaky to the spotty pattern is much slower. The slow transition indicates a small surface roughness of the sample. This indication and the S–K growth mode are further supported by the surface morphology, investigated with AFM. As shown in Fig. 2, the monodisperse nanoislands with a high density of about 3 × 1011 cm− 2 were formed on the surface of Ga0.88Mn0.12As sample. The roughness is much smaller than the average thickness, and it is ascribed to the effectively-relaxed compressive strain in GaMnAs layer.
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F. Xu et al. / Materials Letters 63 (2009) 337–339
Fig. 1. RHEED patterns of Ga0.88Mn0.12As sample.
The S–K growth mode is supposed to bring about special magnetic properties. The temperature dependences of the magnetization [M(T)] were measured during warming after zero-field-cooling, with a 100 Oe measurement field applied along [110], [100] and [11̄0] axes, respectively. Fig. 3 presents these M(T) relations along [110] axes of the sample before and after LT-annealing as a comparison. For the as-grown sample, the M(T) relation is of a concave shape. The concave shape of M(T) relations is not common in metallic GaMnAs systems, but often observed in the insulating DMS materials [8]. And it indicates that the hole-density in the DMS material is much lower than the active Mn spin density [8]. The LT-annealing is performed isothermally at 250 °C for 30 min in air. After annealing, the magnetization was greatly enhanced at low temperatures. And the M(T) curves show a convex shape, with a Curie temperature (TC) at about 30–40 K (where the tail of the curve at T N TC reflects the influence of the measurement field). One possible reason for the low TC even after annealing is suggested to be due to the surface hole-localization effect, which is obvious in very thin epilayers [9], and can be even enhanced in the case of S–K growth. The large difference between the two M(T) curves before and after annealing indicates the existence of a large amount of interstitial Mn atoms (MnI) inside the asgrown sample [10,11]. And the proportion of the MnI should be much higher than that of the common GaMnAs thin films, since so many holes are compensated by the electrons from these double donors (MnI) that the material turns to be an insulating DMS material. The high proportion of MnI is not only due to the high Mn composition, but also mainly due to the S–K growth mode at low growth temperature, which usually brings about more crystalline defects [3]. The M(H) loops of the samples are presented as the inset of Fig. 3. The coercivity of Ga0.88Mn0.12As decreases after LT-annealing. Different from the common GaMnAs thin films, however, for the samples before and after annealing, the saturation fields are much higher, and the shapes of the loops are far from rectangular. For the as-grown sample, the M(H) curve shows a ferromagnetic characteristic with a coercivity of about
Fig. 2. AFM image of Ga0.88Mn0.12As sample.
Fig. 3. M(T) relations of Ga0.88Mn0.12As before (open circle) and after (solid circle) annealing, measured along [110] axes. The inset shows the hysteresis loops of the asgrown and annealed samples, and the loop of as-grown sample measured at 125 K is enlarged as the inner inset.
50 Oe even at 125 K. And the ferromagnetism is suggested to be mainly from the contribution of the active Mn spins, instead of the carrier-mediated magnetization [8]. The magnetic anisotropy is an interesting physical issue in the conventional GaMnAs films, and it is dependent on both hole concentration and temperature. In the present case, in order to investigate the magnetic anisotropy, we measured both the M(T) curves and the M(H) loops along the [110], [100] and [11̄0] axes of the substrate. And they are presented in Fig. 4 and the insets. Different from most of the conventional GaMnAs thin films, where the transition of anisotropy from a biaxial anisotropy along [100] axes at the low temperature range to a uniaxial anisotropy along [110] axis at the high temperature range is observed at the as-grown state [5,12,13], no obvious transition of the anisotropies with increasing temperature can be observed in Fig. 4 (a), and even no distinct anisotropy of Ga0.88Mn0.12As can be identified, neither before nor after annealing. Such a special isotropic characteristic of magnetic properties should be ascribed to the 1 × 1 growth of Ga0.88Mn0.12As nanoislands, while the uniaxial anisotropy along [110] in most of the common GaMnAs material is suggested to be caused by preferential Mn incorporation arising due to the 1 × 2 reconstructed GaMnAs surface [2].
Fig. 4. M(T) curves of Ga0.88Mn0.12As before (a) and after (b) annealing. (Inset) Parts of M(H) loops measured at 10 K.
F. Xu et al. / Materials Letters 63 (2009) 337–339 A similar change from a uniaxial magnetic anisotropy to an isotropic magnetization was ever reported in MnAs/ErAs/GaAs heterostructure, where the ErAs template layer was controlled within few monolayers [14]. In our Ga0.88Mn0.12As material, only a very slight uniaxial anisotropy contribution along the [110] direction seems to exist, especially in the as-grown sample, reflecting the influence of (001) GaAs buffers [5].
References [1] [2] [3] [4]
4. Conclusion [5]
By inserting an InAs wetting layer onto (001) GaAs buffer, we successfully grow GaMnAs with high Mn compositions. The growth follows Stranski–Krastanov mode, which is confirmed by RHEED pattern and the surface morphology. The insulating-DMS-like M(T) relation of the as-grown Ga0.88Mn0.12As indicates a high proportion of MnI, which is mainly ascribed to S–K growth. The magnetization is greatly improved by LT-annealing, and the low TC is suggested to be from the surface hole-localization effect. The investigation on the magnetic anisotropy of both the as-grown and annealed Ga0.88Mn0.12As, indicates an isotropic characteristic of the magnetic properties, as a result of the S–K growth mode. Acknowledgment This work was supported by the National Science Council, Taiwan, under Contract No. NSC 94-2120-M-007-012.
339
[6] [7] [8] [9] [10] [11] [12] [13] [14]
Ohno H. Science 1998;281:951–6. Chiba D, Nishitani Y, Matsukura F, Ohno H. Appl Phys Lett 2007;90:122503. Ovid'ko IA, Sheinerman AG. Adv Phys 2006;55:627–89. Chen YF, Huang JH, Lee WN, Chin TS, Huang RT, Chen FR, et al. Appl Phys Lett 2007;90:022505. Welp U, Vlasko-Vlasov VK, Menzel A, You HD, Liu X, Furdyna JK, et al. Appl Phys Lett 2004;85:260–2. Kita T, Yamashita K, Tango H, Nishino T. Physica E 2000;7:891–5. Honma T, Tsukamoto S, Arakawa Y. Jpn J Appl Phys 2006;45:L777–9. Sarma SD, Hwang EH, Kaminski A. Phys Rev B 2003;67:155201. Hamaya K, Kitamoto Y, Yamazaki Y, Taniyama T, Moriya R, Munekata H. J Appl Phys 2005;97:10D301. Yu KM, Walukiewicz W, Wojtowicz T, Kuryliszyn I, Liu X, Sasaki Y, et al. Phys Rev B 2002;65:201303. Bouzerar G, Ziman T, Kudrnovský J. Phys Rev B 2005;72:125207. Hamaya K, Taniyama T, Kitamoto Y, Fuji T, Yamazaki Y. Phys Rev Lett 2005;94: 147203. Wang KY, Sawicki M, Edmonds KW, Campion RP, Maat S, Foxon CT, et al. Phys Rev Lett 2005;95:217204. Tanaka M, Harbison JP, Rothberg GM. J Cryst Growth 1995;150:1132–8.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
GaMnAs with high Mn composition grown in Stranski–Krastanov mode F. Xu a,b, P.W. Huang b, J.H. Huang b,⁎, W.N. Lee b, T.S. Chin b, H.C. Ku c a b c
Department of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing 210094, China Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan Department of Physics, National Tsing Hua University, Hsinchu 30013, Taiwan
a r t i c l e
i n f o
Article history: Received 29 September 2008 Accepted 23 October 2008 Available online 30 October 2008 Keywords: Epitaxial growth Magnetic materials GaMnAs Diluted magnetic semiconductor (DMS)
a b s t r a c t GaMnAs diluted magnetic semiconductor (DMS) thin films with high Mn compositions were grown on (001) GaAs substrate by low-temperature molecular beam epitaxy (LT-MBE) after inserting an InAs wetting layer onto the GaAs buffer. The growth follows the Stranski–Krastanov mode, which brings about special magnetic characteristics of Ga0.88Mn0.12As. The insulating-DMS-like M(T) relation of the as-grown sample indicates the existence of a high proportion of interstitial Mn atoms. The magnetization is greatly improved after annealing, and the low Curie temperature is suggested to be due to the surface hole-localization effect. The investigation on the magnetic anisotropy shows an almost isotropic characteristic of the magnetic properties, which is ascribed to the S–K growth mode. © 2008 Elsevier B.V. All rights reserved.
1. Introduction As a famous diluted magnetic semiconductor (DMS) material, GaMnAs has attracted lots of research interests [1]. Most of these films are epitaxially grown on (001) GaAs substrate by low-temperature molecular beam epitaxy (LT-MBE). The Mn composition is limited (usually less than 8%) because of the lattice mismatch between the buffer and GaMnAs layer, and the compressive strain in GaMnAs layer. In order to increase the Mn incorporation, researchers have attempted to grow GaMnAs films on the modulated buffer layers with a larger lattice constant. However, only ultrathin films (~5 nm) have been successfully obtained in GaMnAs with high Mn compositions (N10%) [2]. Therefore, it is necessary to make further studies to obtain thicker GaMnAs films with high Mn compositions. In this work, we report our investigation on the growth of GaMnAs with high Mn compositions on (001) GaAs substrate by LT-MBE and its magnetic properties. The Mn composition is increased because the compressive strain is relaxed with an InAs wetting layer, and the growth of the samples is found following the Stranski–Krastanov mode [3]. The magnetic properties of Ga0.88Mn0.12As before and after LT-annealing are investigated, showing special characteristics corresponding to the growth mode. 2. Experimental The samples were grown on (001) semi-insulating, epiready GaAs substrates by a Varian Modular GEN-II MBE system. Following the native oxide desorption, a 250 nm GaAs buffer layer was grown at 580 °C to smoothen the surface. In order to relax the compressive strain
⁎ Corresponding author. Tel.: +886 3 5162228; fax: +886 3 5722366. E-mail address: [email protected] (J.H. Huang). 0167-577X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2008.10.040
in GaMnAs layer, and to incorporate more Mn atoms, an InAs wetting layer of about 1–2 monolayers was subsequently grown at 500 °C, in imitation of the growth processes of InMnAs quantum dots [4]. Then, Ga1 − xMnxAs (0.09 ≤ x ≤ 0.14) was successfully grown at 250 °C in a discontinuous way with 2 min interval per 10-second growth. The estimated thickness of Ga0.88Mn0.12As layer after 80 s growth is about 12 nm. The growth was in-situ monitored with reflection high energy electron diffraction (RHEED). During the growth of GaMnAs layer, a 1 × 1 streaky pattern was initially identified, instead of the typical 1 × 2 streaky pattern in the conventional LT-GaMnAs growth [5]. With the growth, the streaky pattern gradually changed into a 1 × 1 spotty pattern, indicating a Stranski–Krastanov growth mode [3,6]. After growth, the surface morphology was characterized by a Digital Instruments Nanoscope 3100 atomic force microscopy (AFM). And the magnetic properties were measured with a commercial superconducting quantum interference device magnetometer (SQUID). 3. Results and discussions Some RHEED patterns were recorded after growth. Fig. 1 selectively presents the RHEED patterns of Ga0.88Mn0.12As at 250 °C along both [110] and [11̄0] azimuths of the substrate, showing a 1 × 1 spotty pattern and indicating a Stranski–Krastanov growth mode. The S–K growth mode is unusual in the growth of GaMnAs, and it should be ascribed to the InAs wetting layer, which partially relaxes the compressive strain in Ga0.88Mn0.12As layer from the GaAs buffer layer and consequently avoids the formation of polycrystalline structure. The careful control of InAs wetting layer is necessary, because if the InAs nanodots are formed, they will bring about more complex and inhomogeneous strains inside the GaMnAs layer [7]. Compared to the growth of InAs on (001) GaAs [6,7], the transition of the RHEED pattern from the streaky to the spotty pattern is much slower. The slow transition indicates a small surface roughness of the sample. This indication and the S–K growth mode are further supported by the surface morphology, investigated with AFM. As shown in Fig. 2, the monodisperse nanoislands with a high density of about 3 × 1011 cm− 2 were formed on the surface of Ga0.88Mn0.12As sample. The roughness is much smaller than the average thickness, and it is ascribed to the effectively-relaxed compressive strain in GaMnAs layer.
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F. Xu et al. / Materials Letters 63 (2009) 337–339
Fig. 1. RHEED patterns of Ga0.88Mn0.12As sample.
The S–K growth mode is supposed to bring about special magnetic properties. The temperature dependences of the magnetization [M(T)] were measured during warming after zero-field-cooling, with a 100 Oe measurement field applied along [110], [100] and [11̄0] axes, respectively. Fig. 3 presents these M(T) relations along [110] axes of the sample before and after LT-annealing as a comparison. For the as-grown sample, the M(T) relation is of a concave shape. The concave shape of M(T) relations is not common in metallic GaMnAs systems, but often observed in the insulating DMS materials [8]. And it indicates that the hole-density in the DMS material is much lower than the active Mn spin density [8]. The LT-annealing is performed isothermally at 250 °C for 30 min in air. After annealing, the magnetization was greatly enhanced at low temperatures. And the M(T) curves show a convex shape, with a Curie temperature (TC) at about 30–40 K (where the tail of the curve at T N TC reflects the influence of the measurement field). One possible reason for the low TC even after annealing is suggested to be due to the surface hole-localization effect, which is obvious in very thin epilayers [9], and can be even enhanced in the case of S–K growth. The large difference between the two M(T) curves before and after annealing indicates the existence of a large amount of interstitial Mn atoms (MnI) inside the asgrown sample [10,11]. And the proportion of the MnI should be much higher than that of the common GaMnAs thin films, since so many holes are compensated by the electrons from these double donors (MnI) that the material turns to be an insulating DMS material. The high proportion of MnI is not only due to the high Mn composition, but also mainly due to the S–K growth mode at low growth temperature, which usually brings about more crystalline defects [3]. The M(H) loops of the samples are presented as the inset of Fig. 3. The coercivity of Ga0.88Mn0.12As decreases after LT-annealing. Different from the common GaMnAs thin films, however, for the samples before and after annealing, the saturation fields are much higher, and the shapes of the loops are far from rectangular. For the as-grown sample, the M(H) curve shows a ferromagnetic characteristic with a coercivity of about
Fig. 2. AFM image of Ga0.88Mn0.12As sample.
Fig. 3. M(T) relations of Ga0.88Mn0.12As before (open circle) and after (solid circle) annealing, measured along [110] axes. The inset shows the hysteresis loops of the asgrown and annealed samples, and the loop of as-grown sample measured at 125 K is enlarged as the inner inset.
50 Oe even at 125 K. And the ferromagnetism is suggested to be mainly from the contribution of the active Mn spins, instead of the carrier-mediated magnetization [8]. The magnetic anisotropy is an interesting physical issue in the conventional GaMnAs films, and it is dependent on both hole concentration and temperature. In the present case, in order to investigate the magnetic anisotropy, we measured both the M(T) curves and the M(H) loops along the [110], [100] and [11̄0] axes of the substrate. And they are presented in Fig. 4 and the insets. Different from most of the conventional GaMnAs thin films, where the transition of anisotropy from a biaxial anisotropy along [100] axes at the low temperature range to a uniaxial anisotropy along [110] axis at the high temperature range is observed at the as-grown state [5,12,13], no obvious transition of the anisotropies with increasing temperature can be observed in Fig. 4 (a), and even no distinct anisotropy of Ga0.88Mn0.12As can be identified, neither before nor after annealing. Such a special isotropic characteristic of magnetic properties should be ascribed to the 1 × 1 growth of Ga0.88Mn0.12As nanoislands, while the uniaxial anisotropy along [110] in most of the common GaMnAs material is suggested to be caused by preferential Mn incorporation arising due to the 1 × 2 reconstructed GaMnAs surface [2].
Fig. 4. M(T) curves of Ga0.88Mn0.12As before (a) and after (b) annealing. (Inset) Parts of M(H) loops measured at 10 K.
F. Xu et al. / Materials Letters 63 (2009) 337–339 A similar change from a uniaxial magnetic anisotropy to an isotropic magnetization was ever reported in MnAs/ErAs/GaAs heterostructure, where the ErAs template layer was controlled within few monolayers [14]. In our Ga0.88Mn0.12As material, only a very slight uniaxial anisotropy contribution along the [110] direction seems to exist, especially in the as-grown sample, reflecting the influence of (001) GaAs buffers [5].
References [1] [2] [3] [4]
4. Conclusion [5]
By inserting an InAs wetting layer onto (001) GaAs buffer, we successfully grow GaMnAs with high Mn compositions. The growth follows Stranski–Krastanov mode, which is confirmed by RHEED pattern and the surface morphology. The insulating-DMS-like M(T) relation of the as-grown Ga0.88Mn0.12As indicates a high proportion of MnI, which is mainly ascribed to S–K growth. The magnetization is greatly improved by LT-annealing, and the low TC is suggested to be from the surface hole-localization effect. The investigation on the magnetic anisotropy of both the as-grown and annealed Ga0.88Mn0.12As, indicates an isotropic characteristic of the magnetic properties, as a result of the S–K growth mode. Acknowledgment This work was supported by the National Science Council, Taiwan, under Contract No. NSC 94-2120-M-007-012.
339
[6] [7] [8] [9] [10] [11] [12] [13] [14]
Ohno H. Science 1998;281:951–6. Chiba D, Nishitani Y, Matsukura F, Ohno H. Appl Phys Lett 2007;90:122503. Ovid'ko IA, Sheinerman AG. Adv Phys 2006;55:627–89. Chen YF, Huang JH, Lee WN, Chin TS, Huang RT, Chen FR, et al. Appl Phys Lett 2007;90:022505. Welp U, Vlasko-Vlasov VK, Menzel A, You HD, Liu X, Furdyna JK, et al. Appl Phys Lett 2004;85:260–2. Kita T, Yamashita K, Tango H, Nishino T. Physica E 2000;7:891–5. Honma T, Tsukamoto S, Arakawa Y. Jpn J Appl Phys 2006;45:L777–9. Sarma SD, Hwang EH, Kaminski A. Phys Rev B 2003;67:155201. Hamaya K, Kitamoto Y, Yamazaki Y, Taniyama T, Moriya R, Munekata H. J Appl Phys 2005;97:10D301. Yu KM, Walukiewicz W, Wojtowicz T, Kuryliszyn I, Liu X, Sasaki Y, et al. Phys Rev B 2002;65:201303. Bouzerar G, Ziman T, Kudrnovský J. Phys Rev B 2005;72:125207. Hamaya K, Taniyama T, Kitamoto Y, Fuji T, Yamazaki Y. Phys Rev Lett 2005;94: 147203. Wang KY, Sawicki M, Edmonds KW, Campion RP, Maat S, Foxon CT, et al. Phys Rev Lett 2005;95:217204. Tanaka M, Harbison JP, Rothberg GM. J Cryst Growth 1995;150:1132–8.