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Structure characterization of AlN buffer layers grown on (0 0 0 1) sapphire by magnetron sputter epitaxy
Journal of Crystal Growth 244 (2002) 1–5
Structure characterization of AlN buffer layers grown on (0 0 0 1) sapphire by magnetron sputter epitaxy H. Tang*, J.B. Webb, S. Moisa, J.A. Bardwell, S. Rolfe Institute for Microstructural Sciences, National Research Council, M-50, Montreal Road, Ottawa, Ont., Canada K1A 0R6 Received 23 March 2002; accepted 3 June 2002 Communicated by C.R. Abernathy
Abstract AlN layers grown by magnetron sputter epitaxy (MSE) are effective buffer layers for the growth of high quality GaN materials and devices. The structural and morphological properties of AlN layers with two different thicknesses (20 and 200 nm) grown by this technique were analyzed by atomic force microscopy and X-ray diffraction measurements. The root-mean-square surface roughness of the MSE AlN layers was 0.625 nm for the 20 nm thick AlN layer, and 0.249 nm for the 200 nm thick one. In the latter, however, scattered hexagonal plateaus (up to 200 nm in diameter and 10 nm in height) were found embedded in the otherwise atomically smooth surface. The MSE AlN layers were found to be single crystalline epilayers in c-axis orientation, with crystalline quality better or comparable to AlN layers grown by ammonia-molecular-beam epitaxy (MBE) with similar thicknesses. Compared with the MBE counterparts, the MSE AlN layers showed smaller compressive strain at the 20 nm thickness, and crossed into tensile strain at the 200 nm thickness. r 2002 Elsevier Science B.V. All rights reserved. PACS: 68.55.a; 68.55.Jk; 81.15.z; 81.15.Cd Keywords: A1. Atomic force microscopy; A1. Crystal structure; A3. Magnetron sputter epitaxy; B1. Nitrides
1. Introduction The need of an appropriate buffer layer in order to grow good quality GaN epilayers on commonly used non-GaN substrates has been well documented. For GaN growth by metal-organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE), the buffer layer (either a thin GaN *Corresponding author: Tel.: +1-613-998-7636; fax: +1613-990-0202. E-mail address: [email protected] (H. Tang).
or AlN layer) is usually deposited by the same technique at low temperatures prior to the growth of the GaN layer [1–5]. The low temperature buffer layer is normally annealed in situ to restore the crystalline order. It is also practical to deposit the AlN buffer layers at high temperatures, thus relieving the annealing step during the growth [6–8]. Various studies have revealed that the structural properties of this buffer layer has a very marked effect on the crystal quality, morphology and electronic properties of the subsequently grown GaN layers [1–8].
0022-0248/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 2 ) 0 1 6 0 2 - 0
2
H. Tang et al. / Journal of Crystal Growth 244 (2002) 1–5
We have been using magnetron sputter epitaxy (MSE) to deposit AlN buffer layers for the subsequent growth of GaN by ammonia-MBE for a number of years. The magnetron sputter source, containing an Al target, was mounted on the same MBE chamber, thus allowing in situ deposition of the buffer layer. With this combination of an MSE-grown AlN buffer layer and the ammonia-MBE, high mobility values of 560 cm2/ V s (at room temperature and n ¼ 1:2 1017 cm3) for a bulk GaN layer, and 11,000 cm2/V s (at 77 K and ns ¼ 6 1012 cm2) for a AlGaN/GaN heterostructure have been obtained [9,10]. MSE grown AlN buffer layers are also found to be effective for both sapphire and SiC substrates [10,11]. The good stability of the magnetron sputter source also contributes to a remarkable reproducibility of the high mobility AlGaN/GaN HFET structures grown [12,13]. However, there has been no data available on the structural properties of the AlN buffer layer itself. In this letter we report on a comparative study of the structural characteristics of the AlN buffer layers grown by MSE versus those grown by the conventional MBE method.
2. Experimental procedure The MBE/MSE dual-mode growth system with ammonia as the nitrogen source has been described previously [9]. The AlN buffer layers studied in this work were all grown on sapphire substrates. The substrates were degreased in boiling chloroform for 10 min, dipped in 10% HF for 1 min, thoroughly rinsed and blown dry with nitrogen. AlN layers of two different thicknesses (20 and 200 nm) were grown using either the MSE or the MBE technique. For the MSE growth, the sapphire substrate was first held at 10001C and 100 sccm NH3 for 10 min for nitridation. Following nitridation, the AlN layer was grown at 8601C, 15 sccm NH3, and 60 sccm Ar. The Al magnetron sputter source was operated with 50 W DC power, ( For which yielded an AlN growth rate of 0.5 As. the MBE growth, following the same nitridation procedure as above, the AlN layers were grown at 8601C and 15 sccm NH3. The Al effusion cell
temperature was set to 10701C, yielding an AlN ( growth rate of about 0.5 A/s. Atomic force microscopy (AFM) and X-ray diffraction analysis were used to characterize the morphological and structural properties. The surface morphology of the investigated samples was imaged with an atomic force microscope operating in a tapping mode. The X-ray diffraction measurements have been performed using a monochromatized Cu-Ka source and a receiving slit width of 0.45 mm on both the 20 and 200 nm thick AlN layers. The spectra of the thinner films show more noise due to the smaller signal counts. The y angle values were calibrated using the (0 0 0 6) reflection of the sapphire substrate as the reference angle (y ¼ 20:84231).
3. Results and discussion The surface morphologies of the two 20 nm thick AlN layers grown by MSE and MBE are shown by AFM images in Fig. 1(a) and (b), respectively. The two samples exhibit similar coalesced grains forming relatively smooth surfaces. The grain sizes are similar in both samples, with an average diameter of about 20 nm. Interestingly, the average grain size is comparable to the layer thickness. The root-mean-square (RMS) roughness is evaluated to be 0.625 nm in the case of the MSE grown sample, and 0.779 nm for the MBE grown sample. The surface morphologies of the two 200 nm AlN layers grown by MSE and MBE are shown by AFM images in Fig. 2(a) and (b), respectively. At this thickness, the AlN layers grown by both techniques smoothened considerably compared with the 20 nm thick layers. The surfaces no longer show any granular structures, but a very flat surface front with scattered small pits. Embedded on such a flat surface, however, scattered, protruding hexagonal plateaus with diameters up to 200 nm are observed on both samples. The hexagonal plateaus show very flat tops, and protrude above the flat surface by as much as 10 nm, as revealed by section analysis of the AFM data. The density of such hexagonal protrusions is found to be smaller on the MSE grown sample
H. Tang et al. / Journal of Crystal Growth 244 (2002) 1–5
Fig. 1. AFM images of 20 nm thick AlN layers grown by (a) MSE and (b) MBE.
than on the MBE grown sample. The clear hexagonal shape and oriented nature of the protrusions allude to the single crystalline quality of the AlN films. Non-hexagonal protrusions are also observed especially on the MBE grown AlN layer, indicating possible formation of disoriented or cubic phases. Excluding the large protrusions, the RMS roughness of the flat area of the surface is
3
Fig. 2. AFM images of 200 nm thick AlN layers grown by (a) MSE and (b) MBE.
0.249 nm for the MSE grown layer, and 0.220 nm for the MBE grown layer. Such small roughness values form a huge contrast with the roughness of GaN layers grown by ammonia-MBE, which is typically more than 5 nm in RMS values. Reflection high-energy electron diffraction (RHEED) studies of the AlN layers prepared by both techniques invariably showed streaky patterns at both 20 and 200 nm thickness. These results
H. Tang et al. / Journal of Crystal Growth 244 (2002) 1–5
10 MSE 8 MBE 6 4 2 0 -4000
(a)
0 4000 ∆ω (arcsec)
X-ray intensity (counts/s)
X-ray intensity (counts/s)
4
12 10
MSE
8
MBE
6 4 2 0 17.8
(b)
18.0 18.2 θ (degree)
(a)
400 350 300 250 200 150 100 50 0
MSE
MBE
400 350 300 250 200 150 100 50 0
X-ray intensity (counts/s)
X-ray intensity (counts/s)
Fig. 3. X-ray diffraction (a) o scan and (b) y=2y scan around the (0 0 0 2) reflection of the 20 nm thick AlN layers grown by MSE (thicker curves) and by MBE (thinner curves).
-6000 -3000 0 3000 6000 ∆ω (arcsec) (b)
17.8
MBE
MSE
18.0 θ (degree)
18.2
Fig. 4. X-ray diffraction (a) o scan and (b) y=2y scan around the (0 0 0 2) reflection of the 200 nm thick AlN layers grown by MSE (thicker curves) and by MBE (thinner curves).
indicate that the mode of growth for AlN by both MSE and MBE techniques tends to be twodimensional. Fig. 3 shows the o-scan and y=2y scan curves of the (0 0 0 2) reflection for the two 20 nm thick samples grown by MSE and MBE. The o-scan full-width at half-maximum (FWHM) values are found to be 3200 arcsec for the MSE grown layer compared to 4500 arcsec for the MBE grown layer. This indicates that at this thickness, the AlN grown by MSE has a better crystallinity than that grown by MBE. From the y=2y scan spectra, the (0 0 0 2) peaks of both samples (y ¼ 17:980 for the MBE layer and y ¼ 18:006 for the MSE layer) are shifted to lower angles from that of totally relaxed AlN which is at y ¼ 18:023 as calculated from the ( This translates lattice constant of c ¼ 4:9792 A.
into a larger in-plane compressive strain (Dc=c ¼ 0:231%) in the MBE AlN layer than in the MSE AlN layer (Dc=c ¼ 0:0911%). Fig. 4 shows the o-scan and y=2y scan curves of the (0 0 0 2) reflection for the two 200 nm thick samples grown by MSE and MBE. Both thicker films show reduced o-scan FWHM values, i.e. 2088 arcsec for the MSE grown layer, and 1980 arcsec for the MBE grown layer. Therefore, at this thickness, both the MSE and MBE grown AlN layers are of similar crystalline quality with the c-axis oriented parallel to the surface normal. In the y=2y scans, the (0 0 0 2) peak of the AlN layer grown by MSE at y ¼ 18:040 is shifted to higher angle that of unstrained AlN, indicating that the film is now under a tensile strain of Dc=c ¼ 0:0911%: In contrast, the (0 0 0 2) peak of
H. Tang et al. / Journal of Crystal Growth 244 (2002) 1–5
the MBE AlN layer at y ¼ 18:018 is still lower than the unstrained value, indicating of a residual compressive strain of Dc=c ¼ 0:0268%: Used as a buffer for GaN growth on non-GaN substrates, the role of the AlN layer is to provide better nucleation sites with reduced lattice mismatch to GaN. Experiments have proved that MSE grown AlN layers at a range of thicknesses, including those used in this study, are effective buffer layers that have resulted in high mobility GaN as well as GaN/AlGaN heterostructures [9–13].
4. Conclusions The present study concludes that the AlN layers grown by MSE on (0 0 0 1) sapphire are single crystalline films with c-axis orientation, high smoothness, and comparable or better crystalline quality than the MBE grown AlN layers. The ease of operation and high reproducibility of a magnetron sputter source makes the MSE technique an advantageous alternative to MBE for the key process of buffer layer deposition.
5
References [1] S. Nakamura, T. Mukai, M. Senoh, Appl. Phys. Lett. 64 (1994) 1686. [2] H. Amano, M. Kito, K. Hiramatsu, I. Akasaki, Jpn. J. Appl. Phys. Part 2 28 (1989) L2112. [3] V. Bousquet, J. Heffernan, J. Barnes, S. Hooper, Appl. Phys. Lett. 78 (2001) 754. [4] S.D. Hersee, J. Ramer, K. Zheng, C. Kranenberg, K. Malloy, M. Banas, M. Goorsky, J. Electron. Mater. 24 (1995) 1519. [5] K. Doverspike, L.B. Rowland, D.K. Gaskill, J.A. Freitas Jr., J. Electron. Mater. 24 (1995) 269. [6] M.J. Murphy, K. Chu, H. Wu, W. Yeo, W.J. Schaff, O. Ambacher, J. Smart, T.J. Eustis, J.R. Shealy, L.F. Eastman, J. Vac. Sci. Technol. B 17 (1999) 1252. [7] T.W. Weeks Jr., M.D. Bremser, K.S. Ailey, E. Carlson, W.G. Perry, R.F. Davis, Appl. Phys. Lett. 67 (1995) 401. [8] W. Kim, A. Botchkarev, A. Salvador, G. Popovici, H. Tang, H. Morkoc, J. Appl. Phys. 82 (1997) 219. [9] H. Tang, J.B. Webb, Appl. Phys. Lett. 74 (1999) 2373. [10] J.B. Webb, H. Tang, J.A. Bardwell, P. Coleridge, Appl. Phys. Lett. 78 (2001) 3845. [11] J.B. Webb, H. Tang, J.A. Bardwell, S. Rolfe, Ying Liu, J. Lapointe, P. Marshall, T.W. MacElwee, Phys. Stat. Sol. A 188 (2001) 271. [12] H. Tang, J.B. Webb, J.A. Bardwell, S. Rolfe, T. MacElwee, Solid State Electron. 44 (2000) 2177. [13] J.B. Webb, H. Tang, S. Rolfe, J. Bardwell, Appl. Phys. Lett. 75 (1999) 953.
Structure characterization of AlN buffer layers grown on (0 0 0 1) sapphire by magnetron sputter epitaxy H. Tang*, J.B. Webb, S. Moisa, J.A. Bardwell, S. Rolfe Institute for Microstructural Sciences, National Research Council, M-50, Montreal Road, Ottawa, Ont., Canada K1A 0R6 Received 23 March 2002; accepted 3 June 2002 Communicated by C.R. Abernathy
Abstract AlN layers grown by magnetron sputter epitaxy (MSE) are effective buffer layers for the growth of high quality GaN materials and devices. The structural and morphological properties of AlN layers with two different thicknesses (20 and 200 nm) grown by this technique were analyzed by atomic force microscopy and X-ray diffraction measurements. The root-mean-square surface roughness of the MSE AlN layers was 0.625 nm for the 20 nm thick AlN layer, and 0.249 nm for the 200 nm thick one. In the latter, however, scattered hexagonal plateaus (up to 200 nm in diameter and 10 nm in height) were found embedded in the otherwise atomically smooth surface. The MSE AlN layers were found to be single crystalline epilayers in c-axis orientation, with crystalline quality better or comparable to AlN layers grown by ammonia-molecular-beam epitaxy (MBE) with similar thicknesses. Compared with the MBE counterparts, the MSE AlN layers showed smaller compressive strain at the 20 nm thickness, and crossed into tensile strain at the 200 nm thickness. r 2002 Elsevier Science B.V. All rights reserved. PACS: 68.55.a; 68.55.Jk; 81.15.z; 81.15.Cd Keywords: A1. Atomic force microscopy; A1. Crystal structure; A3. Magnetron sputter epitaxy; B1. Nitrides
1. Introduction The need of an appropriate buffer layer in order to grow good quality GaN epilayers on commonly used non-GaN substrates has been well documented. For GaN growth by metal-organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE), the buffer layer (either a thin GaN *Corresponding author: Tel.: +1-613-998-7636; fax: +1613-990-0202. E-mail address: [email protected] (H. Tang).
or AlN layer) is usually deposited by the same technique at low temperatures prior to the growth of the GaN layer [1–5]. The low temperature buffer layer is normally annealed in situ to restore the crystalline order. It is also practical to deposit the AlN buffer layers at high temperatures, thus relieving the annealing step during the growth [6–8]. Various studies have revealed that the structural properties of this buffer layer has a very marked effect on the crystal quality, morphology and electronic properties of the subsequently grown GaN layers [1–8].
0022-0248/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 2 ) 0 1 6 0 2 - 0
2
H. Tang et al. / Journal of Crystal Growth 244 (2002) 1–5
We have been using magnetron sputter epitaxy (MSE) to deposit AlN buffer layers for the subsequent growth of GaN by ammonia-MBE for a number of years. The magnetron sputter source, containing an Al target, was mounted on the same MBE chamber, thus allowing in situ deposition of the buffer layer. With this combination of an MSE-grown AlN buffer layer and the ammonia-MBE, high mobility values of 560 cm2/ V s (at room temperature and n ¼ 1:2 1017 cm3) for a bulk GaN layer, and 11,000 cm2/V s (at 77 K and ns ¼ 6 1012 cm2) for a AlGaN/GaN heterostructure have been obtained [9,10]. MSE grown AlN buffer layers are also found to be effective for both sapphire and SiC substrates [10,11]. The good stability of the magnetron sputter source also contributes to a remarkable reproducibility of the high mobility AlGaN/GaN HFET structures grown [12,13]. However, there has been no data available on the structural properties of the AlN buffer layer itself. In this letter we report on a comparative study of the structural characteristics of the AlN buffer layers grown by MSE versus those grown by the conventional MBE method.
2. Experimental procedure The MBE/MSE dual-mode growth system with ammonia as the nitrogen source has been described previously [9]. The AlN buffer layers studied in this work were all grown on sapphire substrates. The substrates were degreased in boiling chloroform for 10 min, dipped in 10% HF for 1 min, thoroughly rinsed and blown dry with nitrogen. AlN layers of two different thicknesses (20 and 200 nm) were grown using either the MSE or the MBE technique. For the MSE growth, the sapphire substrate was first held at 10001C and 100 sccm NH3 for 10 min for nitridation. Following nitridation, the AlN layer was grown at 8601C, 15 sccm NH3, and 60 sccm Ar. The Al magnetron sputter source was operated with 50 W DC power, ( For which yielded an AlN growth rate of 0.5 As. the MBE growth, following the same nitridation procedure as above, the AlN layers were grown at 8601C and 15 sccm NH3. The Al effusion cell
temperature was set to 10701C, yielding an AlN ( growth rate of about 0.5 A/s. Atomic force microscopy (AFM) and X-ray diffraction analysis were used to characterize the morphological and structural properties. The surface morphology of the investigated samples was imaged with an atomic force microscope operating in a tapping mode. The X-ray diffraction measurements have been performed using a monochromatized Cu-Ka source and a receiving slit width of 0.45 mm on both the 20 and 200 nm thick AlN layers. The spectra of the thinner films show more noise due to the smaller signal counts. The y angle values were calibrated using the (0 0 0 6) reflection of the sapphire substrate as the reference angle (y ¼ 20:84231).
3. Results and discussion The surface morphologies of the two 20 nm thick AlN layers grown by MSE and MBE are shown by AFM images in Fig. 1(a) and (b), respectively. The two samples exhibit similar coalesced grains forming relatively smooth surfaces. The grain sizes are similar in both samples, with an average diameter of about 20 nm. Interestingly, the average grain size is comparable to the layer thickness. The root-mean-square (RMS) roughness is evaluated to be 0.625 nm in the case of the MSE grown sample, and 0.779 nm for the MBE grown sample. The surface morphologies of the two 200 nm AlN layers grown by MSE and MBE are shown by AFM images in Fig. 2(a) and (b), respectively. At this thickness, the AlN layers grown by both techniques smoothened considerably compared with the 20 nm thick layers. The surfaces no longer show any granular structures, but a very flat surface front with scattered small pits. Embedded on such a flat surface, however, scattered, protruding hexagonal plateaus with diameters up to 200 nm are observed on both samples. The hexagonal plateaus show very flat tops, and protrude above the flat surface by as much as 10 nm, as revealed by section analysis of the AFM data. The density of such hexagonal protrusions is found to be smaller on the MSE grown sample
H. Tang et al. / Journal of Crystal Growth 244 (2002) 1–5
Fig. 1. AFM images of 20 nm thick AlN layers grown by (a) MSE and (b) MBE.
than on the MBE grown sample. The clear hexagonal shape and oriented nature of the protrusions allude to the single crystalline quality of the AlN films. Non-hexagonal protrusions are also observed especially on the MBE grown AlN layer, indicating possible formation of disoriented or cubic phases. Excluding the large protrusions, the RMS roughness of the flat area of the surface is
3
Fig. 2. AFM images of 200 nm thick AlN layers grown by (a) MSE and (b) MBE.
0.249 nm for the MSE grown layer, and 0.220 nm for the MBE grown layer. Such small roughness values form a huge contrast with the roughness of GaN layers grown by ammonia-MBE, which is typically more than 5 nm in RMS values. Reflection high-energy electron diffraction (RHEED) studies of the AlN layers prepared by both techniques invariably showed streaky patterns at both 20 and 200 nm thickness. These results
H. Tang et al. / Journal of Crystal Growth 244 (2002) 1–5
10 MSE 8 MBE 6 4 2 0 -4000
(a)
0 4000 ∆ω (arcsec)
X-ray intensity (counts/s)
X-ray intensity (counts/s)
4
12 10
MSE
8
MBE
6 4 2 0 17.8
(b)
18.0 18.2 θ (degree)
(a)
400 350 300 250 200 150 100 50 0
MSE
MBE
400 350 300 250 200 150 100 50 0
X-ray intensity (counts/s)
X-ray intensity (counts/s)
Fig. 3. X-ray diffraction (a) o scan and (b) y=2y scan around the (0 0 0 2) reflection of the 20 nm thick AlN layers grown by MSE (thicker curves) and by MBE (thinner curves).
-6000 -3000 0 3000 6000 ∆ω (arcsec) (b)
17.8
MBE
MSE
18.0 θ (degree)
18.2
Fig. 4. X-ray diffraction (a) o scan and (b) y=2y scan around the (0 0 0 2) reflection of the 200 nm thick AlN layers grown by MSE (thicker curves) and by MBE (thinner curves).
indicate that the mode of growth for AlN by both MSE and MBE techniques tends to be twodimensional. Fig. 3 shows the o-scan and y=2y scan curves of the (0 0 0 2) reflection for the two 20 nm thick samples grown by MSE and MBE. The o-scan full-width at half-maximum (FWHM) values are found to be 3200 arcsec for the MSE grown layer compared to 4500 arcsec for the MBE grown layer. This indicates that at this thickness, the AlN grown by MSE has a better crystallinity than that grown by MBE. From the y=2y scan spectra, the (0 0 0 2) peaks of both samples (y ¼ 17:980 for the MBE layer and y ¼ 18:006 for the MSE layer) are shifted to lower angles from that of totally relaxed AlN which is at y ¼ 18:023 as calculated from the ( This translates lattice constant of c ¼ 4:9792 A.
into a larger in-plane compressive strain (Dc=c ¼ 0:231%) in the MBE AlN layer than in the MSE AlN layer (Dc=c ¼ 0:0911%). Fig. 4 shows the o-scan and y=2y scan curves of the (0 0 0 2) reflection for the two 200 nm thick samples grown by MSE and MBE. Both thicker films show reduced o-scan FWHM values, i.e. 2088 arcsec for the MSE grown layer, and 1980 arcsec for the MBE grown layer. Therefore, at this thickness, both the MSE and MBE grown AlN layers are of similar crystalline quality with the c-axis oriented parallel to the surface normal. In the y=2y scans, the (0 0 0 2) peak of the AlN layer grown by MSE at y ¼ 18:040 is shifted to higher angle that of unstrained AlN, indicating that the film is now under a tensile strain of Dc=c ¼ 0:0911%: In contrast, the (0 0 0 2) peak of
H. Tang et al. / Journal of Crystal Growth 244 (2002) 1–5
the MBE AlN layer at y ¼ 18:018 is still lower than the unstrained value, indicating of a residual compressive strain of Dc=c ¼ 0:0268%: Used as a buffer for GaN growth on non-GaN substrates, the role of the AlN layer is to provide better nucleation sites with reduced lattice mismatch to GaN. Experiments have proved that MSE grown AlN layers at a range of thicknesses, including those used in this study, are effective buffer layers that have resulted in high mobility GaN as well as GaN/AlGaN heterostructures [9–13].
4. Conclusions The present study concludes that the AlN layers grown by MSE on (0 0 0 1) sapphire are single crystalline films with c-axis orientation, high smoothness, and comparable or better crystalline quality than the MBE grown AlN layers. The ease of operation and high reproducibility of a magnetron sputter source makes the MSE technique an advantageous alternative to MBE for the key process of buffer layer deposition.
5
References [1] S. Nakamura, T. Mukai, M. Senoh, Appl. Phys. Lett. 64 (1994) 1686. [2] H. Amano, M. Kito, K. Hiramatsu, I. Akasaki, Jpn. J. Appl. Phys. Part 2 28 (1989) L2112. [3] V. Bousquet, J. Heffernan, J. Barnes, S. Hooper, Appl. Phys. Lett. 78 (2001) 754. [4] S.D. Hersee, J. Ramer, K. Zheng, C. Kranenberg, K. Malloy, M. Banas, M. Goorsky, J. Electron. Mater. 24 (1995) 1519. [5] K. Doverspike, L.B. Rowland, D.K. Gaskill, J.A. Freitas Jr., J. Electron. Mater. 24 (1995) 269. [6] M.J. Murphy, K. Chu, H. Wu, W. Yeo, W.J. Schaff, O. Ambacher, J. Smart, T.J. Eustis, J.R. Shealy, L.F. Eastman, J. Vac. Sci. Technol. B 17 (1999) 1252. [7] T.W. Weeks Jr., M.D. Bremser, K.S. Ailey, E. Carlson, W.G. Perry, R.F. Davis, Appl. Phys. Lett. 67 (1995) 401. [8] W. Kim, A. Botchkarev, A. Salvador, G. Popovici, H. Tang, H. Morkoc, J. Appl. Phys. 82 (1997) 219. [9] H. Tang, J.B. Webb, Appl. Phys. Lett. 74 (1999) 2373. [10] J.B. Webb, H. Tang, J.A. Bardwell, P. Coleridge, Appl. Phys. Lett. 78 (2001) 3845. [11] J.B. Webb, H. Tang, J.A. Bardwell, S. Rolfe, Ying Liu, J. Lapointe, P. Marshall, T.W. MacElwee, Phys. Stat. Sol. A 188 (2001) 271. [12] H. Tang, J.B. Webb, J.A. Bardwell, S. Rolfe, T. MacElwee, Solid State Electron. 44 (2000) 2177. [13] J.B. Webb, H. Tang, S. Rolfe, J. Bardwell, Appl. Phys. Lett. 75 (1999) 953.