Growth and fabrication of AlGaN/GaN HEMT ... - Semantic Scholar
ARTICLE IN PRESS
Microelectronics Journal 39 (2008) 1108–1111 www.elsevier.com/locate/mejo
Growth and fabrication of AlGaN/GaN HEMT based on Si(1 1 1) substrates by MOCVD Weijun Luo, Xiaoliang Wang, Hongling Xiao, Cuimei Wang, Junxue Ran, Lunchun Guo, Jianping Li, Hongxin Liu, Yanling Chen, Fuhua Yang, Jinmin Li Key Laboratory of Semiconductor Materials Science, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China Received 27 November 2007; accepted 25 January 2008 Available online 18 April 2008
Abstract AlGaN/GaN high electron mobility transistor (HEMT) hetero-structures were grown on the 2-in Si (1 1 1) substrate using metalorganic chemical vapor deposition (MOCVD). Low-temperature (LT) AlN layers were inserted to relieve the tension stress during the growth of GaN epilayers. The grown AlGaN/GaN HEMT samples exhibited a maximum crack-free area of 8 mm 5 mm, XRD GaN (0 0 0 2) full-width at half-maximum (FWHM) of 661 arcsec and surface roughness of 0.377 nm. The device with a gate length of 1.4 mm and a gate width of 60 mm demonstrated maximum drain current density of 304 mA/mm, transconductance of 124 mS/mm and reverse gate leakage current of 0.76 mA/mm at the gate voltage of 10 V. r 2008 Published by Elsevier Ltd. Keywords: AlGaN/GaN; High electron mobility transistor (HEMT); Si (1 1 1)
1. Introduction The AlGaN/GaN compound semiconductor material system has been extensively of interest for its large band gap (GaN 3.4 eV, AlN 6.2 eV), high breakdown electric strength (1–3 1010 V/cm), high electron saturated drift velocity (2.2 1010 cm/s) and good thermal stability. At the same time, AlGaN/GaN hetero-junction has large conduction band discontinuity and strong spontaneous and piezoelectric, polarization effect of which results in high concentration of 2-dimensional electron gas (2DEG) near the interface. Consequently, with the excellent epitaxy material and improved process techniques, the power density demonstrated by GaN-based devices is 5–10 times larger than that of GaAs-based devices. So, AlGaN/GaN high electron mobility transistor (HEMT) has a larger potential application in the high-frequency, high-temperature and large-power areas [1–3]. However, sapphire substrate has the bottleneck of bad thermal conduction, Corresponding author. Tel.: +86 10 82305342; fax: +86 10 82304232.
E-mail address: [email protected] (W. Luo). 0026-2692/$ - see front matter r 2008 Published by Elsevier Ltd. doi:10.1016/j.mejo.2008.01.083
which limits the improvement of the power density of the device, and SiC substrate is so expensive that it hinders the application of GaN material. Therefore, Si is the best alternative substrate for its low cost, good thermal conductivity and integrating with the mature Si-based processing techniques [4,5]. Because of the large differences of lattice mismatch and thermal expansion coefficients between GaN epilayer and Si substrate, the cracking and high defect density problems are serious when the GaN is grown on Si [6–9]. It was reported [10–12] that LT AlN interlayer could be used to relieve the tension stress in the GaN films grown on the Si substrate and then reduce the cracks in the grown GaN epilayer. Besides, it could also stop the threading dislocation propagating from the substrate to the GaN epilayer. This article reports the growth and fabrication of the AlGaN/GaN HEMT based on 2-in Si (1 1 1) substrate through inserting the LT AlN layers. With the gate length of 1.4 mm and gate width of 60 mm, the device exhibits the maximum drain current density of 304 mA/mm, transconductance of 124 mS/mm and reverse gate leakage current of 0.76 mA/mm at the gate voltage of 10 V.
ARTICLE IN PRESS W. Luo et al. / Microelectronics Journal 39 (2008) 1108–1111
1109
2. Experiments
3. Results and discussion Optical microscopy, atomic force microscopy (AFM) and double crystal X-ray diffraction (DCXRD) were performed to characterize the surface morphology and
Fig. 1. The layer structure of the Si (1 1 1)-based AlGaN/GaN HEMT.
Fig. 2. AFM image of the surface of the Si (1 1 1)-based AlGaN/GaN HEMT.
250000
GaN (0002)
200000 Intensity
The epilayers were prepared by using vertical metalorganic chemical vapor deposition (MOCVD) on (1 1 1) oriented 2-in Si substrate with the resistivity of 2000 O cm. The growth pressures were 100 Torr for GaN layers and 50 Torr for LT AlN interlayers. Trimethylgallium (TMGa), trimethylaluminum (TMAl) and anmonia (NH3) were used as Ga, Al and N precursors, respectively. N2 and H2 were used as carrier gases. In order to obtain an oxide-free surface, the Si (1 1 1) substrate was cleaned by the HF solution before being loaded into the reactor. The un-doped AlGaN/GaN HEMT epitaxy structure is depicted in Fig. 1. The layer structure contains 100 nm high temperature AlN nucleation layer, a total thickness of 1.2 mm GaN layers, 20 nm AlGaN barrier layer with Al content of 20%, 3 nm GaN cap layer and three LT AlN interlayers. The thicknesses of the three LT AlN interlayers at different growth temperatures are 20 nm (600 1C), 15 nm (700 1C) and 10 nm (800 1C), respectively. The three LT AlN interlayers were designed to alleviate the tension stress in the GaN epilayers as well as prevent the threading dislocations propagating from the substrate to the upper layers. Detailed discussions will be reported later. The device fabrication followed the standard optical lithography process. The device isolation was realized by implanted He ions with high energy. The ohm-contact metals were Ti/Al/Ti/Au layers and exhibited good surface morphology with specific ohm-contact resistance of 10 5 O cm2 level after being annealed in the 830 1C N2 atmosphere for 35 s, and the source–drain distance is 5 mm. The gate metals are Ni/Au with gate length and width of 1.4 and 60 mm, respectively. Finally, the device was passivated with the Si3N4 film by using the plasmaenhanced chemical vapor deposition (PECVD).
150000 FWHM = 661 arcsec 100000 50000 0 -6000
-4000
-2000
0 arcsec
2000
4000
6000
Fig. 3. GaN (0 0 0 2) XRD spectrum of the Si (1 1 1)-based AlGaN/GaN HEMT structure.
structure properties of the Si (1 1 1)-based AlGaN/GaN HEMT structure. The crack-free area of the surface of the 2-in Si (1 1 1)-based sample is 8 mm 5 mm, characterized by the optical microscopy. The surface morphology of the sample measured by AFM is depicted in Fig. 2. The sample exhibited a smooth surface with clear atomic steps and the root-mean-square (RMS) roughness was 0.377 nm for a scan area of 5 mm 5 mm. The crystal quality of the sample was characterized by DCXRD described in Fig. 3. The full-width at half-maximum (FWHM) of GaN (0 0 0 2) peak is 661 arcsec from the rocking curve measurement. These results show that the grown Si (1 1 1)-based AlGaN/GaN HEMT structure has good crystal quality and surface morphology, which demonstrates the advantage of designed layer structures. The DC characteristics of the device were measured by the HP4142 semiconductor parameter analyzer instrument and the results are shown in Figs. 4–6. As shown in Fig. 4, the conditions of the I–V characteristic measurement are 2 VoVgso+1.5 V and 0 VoVdso10 V. The maximum saturated current density of the device is 304 mA/mm at the gate voltage of 1.5 V. The knee voltage is about 3 V, which shows the excellent Ohmic contact characteristics of the device. Besides, it can be seen that the drain current of the Si (1 1 1)-based AlGaN/GaN HEMT
ARTICLE IN PRESS W. Luo et al. / Microelectronics Journal 39 (2008) 1108–1111
1110
350
Idsmax = 304mA/mm
Vgs = 1.5V 0.04
300 Vgs = 1.0V Igs mA/mm
Ids mA/mm
250 200 150
0.02
Igs = 0.76 uA/mm 100 0.00 50 0
-12 0
2
4
6 Vds V
8
Gmmax = 124ms/mm
Idsmax = 338mA/mm
140 120
Ids (mA/mm)
80 60 150 40 100
Gm (ms/mm)
100 200
20 50 Lg = 1.4um Wg = 60um
0
0 -20
-2
0 Vg (V)
-6
-4
-2
0
2
Fig. 6. The Schottcky characteristics of Si (1 1 1) based on the AlGaN/ GaN HEMT.
4. Conclusions
300 250
-8
Vgs V
Fig. 4. The I–V characteristics of the Si (1 1 1)-based AlGaN/GaN HEMT.
350
-10
10
2
The AlGaN/GaN HEMT device was designed and fabricated on the 2-in Si (1 1 1) substrate. With the insertion of LT AlN layers, the grown sample exhibited good surface morphology with a crack-free area of 8 mm 5 mm and an RMS of 0.377 nm. The FWHM of the GaN (0 0 0 2) rocking curve is 661 arcsec. The fabricated device, with gate length of 1.4 mm and gate width of 60 mm, demonstrated maximum source–drain current density of 304 mA/mm, transconductance of 124 mS/mm, and reverse bias leakage current of 0.76 mA/mm at the gate voltage of 10 V. These results show that a high-performance Si (1 1 1)-based AlGaN/GaN HEMT device can be obtained by optimizing the inserted LT AlN layers.
Fig. 5. The transfer and transconductance characteristics of the Si (1 1 1)based AlGaN/GaN HEMT.
Acknowledgments device does not reduce with the increase of the source– drain voltage, while this phenomenon is clear in the sapphire-based devices. The reason is that the Si substrate has better thermal conduction characteristic than that of the sapphire substrate, which results in small thermal resistance. Thus, then the performance of the device can be improved. The transconductance characteristic of the device is shown in Fig. 5. The source–drain voltage is 7 V and the gate voltage changes from 2 to 2 V. As can be seen in Fig. 5, the passivated device exhibits maximum transconductance of 124 mS/mm at the gate voltage of 0 V nearby. From the transfer curve of the device, we can see that the pinched-off voltage is about 2 V. As described in Fig. 6, the Si (1 1 1)-based AlGaN/GaN HEMT device exhibits good Schottcky characteristics. The reverse bias leakage current is 0.76 mA/mm at the gate voltage of 10 V, which has potential in high-voltage application.
This work has been supported by the Key Innovation Program of the Chinese Academy of Sciences (no. KGCX2-SW-107-1), National Natural Science Foundation of China (no. 60606002) and Special Funds for Major State Basic Research Projects (nos. 2002CB311903, 2006CB604905 and 513270605). References [1] Y.-f . Wu, D. Kapolnek, J.P. Ibbetson, P. Parikh, U.K. Mishra, Veryhigh power density AlGaN/GaN HEMT, IEEE Trans. Electron Devices 48 (3) (2001) 586–590. [2] S.T. Sheppard, K. Doverspike, W.L. Pribble, S.T. Allen, J.W. Palmour, L.T. Kehias, T.J. Jenkins, High-power microwave GaN/ AlGaN HEMT’s on silicon carbide, IEEE Electron Device Lett. 20 (4) (1999) 161–163. [3] X.L. Wang, T.S. Cheng, Z.Y. Ma, G.X. Hu, H.L. Xiao, J.X. Ran, C.M. Wang, W.J. Luo, 1-mm gate periphery AlGaN/AlN/GaN HEMTs on SiC with output power of 9.39 W at 8 GHz, Solid-State Electron. 51 (3) (2007) 428–432.
ARTICLE IN PRESS W. Luo et al. / Microelectronics Journal 39 (2008) 1108–1111 [4] Masayoshi Umeno, Takashi Egawa, Hiroyasu Ishikawa, GaN-based optoelectronic devices on sapphire and Si substrates, Mater. Sci. Semicond. Process. 4 (6) (2001) 459–466. [5] Kun-Ta Wu, P.H. Chang, S.T. Lien, N.C. Chen, et al., Growth and characterization of GaN/AlGaN high-electron mobility transistors grown on p-type Si substrates, Physica E: Low-dimensional Syst. Nanostruct. 32 (1–2) (2006) 566–568. [6] X. Weng, S. Raghavan, J.D. Acord, et al., J. Cryst. Growth 300 (2007) 822–825. [7] Doek kyu Kim, Choon Bae Park, J. Korean Phys. Soc. 49 (4) (2006) 1497–1500.
1111
[8] A. Dadgar, P. Veit, F. Schulze, J. Bla¨sing, et al., Thin Solid Films 515 (2007) 4356–4361. [9] Zhe Liu Liu, Xiaoliang Wang–, Junxi Wang, et al., J. Cryst. Growth 298 (2007) 281–283. [10] Srinivasan. Raghavan, Xiaojun. Weng, Elizabeth. Dickey, Joan. M. Redwing, Appl. Phys. Lett. 87 (142101) (2005). [11] A. Reiher, J. Blasing, A. Dadgar, A. Diez, A. Krost, J. Cryst. Growth 248 (2003) 563–567. [12] A. Dadgar, J. Bla¨sing, A. Diez, et al., Jpn. J. Appl. Phys. 39 (2000) L1183–L1185.
Microelectronics Journal 39 (2008) 1108–1111 www.elsevier.com/locate/mejo
Growth and fabrication of AlGaN/GaN HEMT based on Si(1 1 1) substrates by MOCVD Weijun Luo, Xiaoliang Wang, Hongling Xiao, Cuimei Wang, Junxue Ran, Lunchun Guo, Jianping Li, Hongxin Liu, Yanling Chen, Fuhua Yang, Jinmin Li Key Laboratory of Semiconductor Materials Science, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China Received 27 November 2007; accepted 25 January 2008 Available online 18 April 2008
Abstract AlGaN/GaN high electron mobility transistor (HEMT) hetero-structures were grown on the 2-in Si (1 1 1) substrate using metalorganic chemical vapor deposition (MOCVD). Low-temperature (LT) AlN layers were inserted to relieve the tension stress during the growth of GaN epilayers. The grown AlGaN/GaN HEMT samples exhibited a maximum crack-free area of 8 mm 5 mm, XRD GaN (0 0 0 2) full-width at half-maximum (FWHM) of 661 arcsec and surface roughness of 0.377 nm. The device with a gate length of 1.4 mm and a gate width of 60 mm demonstrated maximum drain current density of 304 mA/mm, transconductance of 124 mS/mm and reverse gate leakage current of 0.76 mA/mm at the gate voltage of 10 V. r 2008 Published by Elsevier Ltd. Keywords: AlGaN/GaN; High electron mobility transistor (HEMT); Si (1 1 1)
1. Introduction The AlGaN/GaN compound semiconductor material system has been extensively of interest for its large band gap (GaN 3.4 eV, AlN 6.2 eV), high breakdown electric strength (1–3 1010 V/cm), high electron saturated drift velocity (2.2 1010 cm/s) and good thermal stability. At the same time, AlGaN/GaN hetero-junction has large conduction band discontinuity and strong spontaneous and piezoelectric, polarization effect of which results in high concentration of 2-dimensional electron gas (2DEG) near the interface. Consequently, with the excellent epitaxy material and improved process techniques, the power density demonstrated by GaN-based devices is 5–10 times larger than that of GaAs-based devices. So, AlGaN/GaN high electron mobility transistor (HEMT) has a larger potential application in the high-frequency, high-temperature and large-power areas [1–3]. However, sapphire substrate has the bottleneck of bad thermal conduction, Corresponding author. Tel.: +86 10 82305342; fax: +86 10 82304232.
E-mail address: [email protected] (W. Luo). 0026-2692/$ - see front matter r 2008 Published by Elsevier Ltd. doi:10.1016/j.mejo.2008.01.083
which limits the improvement of the power density of the device, and SiC substrate is so expensive that it hinders the application of GaN material. Therefore, Si is the best alternative substrate for its low cost, good thermal conductivity and integrating with the mature Si-based processing techniques [4,5]. Because of the large differences of lattice mismatch and thermal expansion coefficients between GaN epilayer and Si substrate, the cracking and high defect density problems are serious when the GaN is grown on Si [6–9]. It was reported [10–12] that LT AlN interlayer could be used to relieve the tension stress in the GaN films grown on the Si substrate and then reduce the cracks in the grown GaN epilayer. Besides, it could also stop the threading dislocation propagating from the substrate to the GaN epilayer. This article reports the growth and fabrication of the AlGaN/GaN HEMT based on 2-in Si (1 1 1) substrate through inserting the LT AlN layers. With the gate length of 1.4 mm and gate width of 60 mm, the device exhibits the maximum drain current density of 304 mA/mm, transconductance of 124 mS/mm and reverse gate leakage current of 0.76 mA/mm at the gate voltage of 10 V.
ARTICLE IN PRESS W. Luo et al. / Microelectronics Journal 39 (2008) 1108–1111
1109
2. Experiments
3. Results and discussion Optical microscopy, atomic force microscopy (AFM) and double crystal X-ray diffraction (DCXRD) were performed to characterize the surface morphology and
Fig. 1. The layer structure of the Si (1 1 1)-based AlGaN/GaN HEMT.
Fig. 2. AFM image of the surface of the Si (1 1 1)-based AlGaN/GaN HEMT.
250000
GaN (0002)
200000 Intensity
The epilayers were prepared by using vertical metalorganic chemical vapor deposition (MOCVD) on (1 1 1) oriented 2-in Si substrate with the resistivity of 2000 O cm. The growth pressures were 100 Torr for GaN layers and 50 Torr for LT AlN interlayers. Trimethylgallium (TMGa), trimethylaluminum (TMAl) and anmonia (NH3) were used as Ga, Al and N precursors, respectively. N2 and H2 were used as carrier gases. In order to obtain an oxide-free surface, the Si (1 1 1) substrate was cleaned by the HF solution before being loaded into the reactor. The un-doped AlGaN/GaN HEMT epitaxy structure is depicted in Fig. 1. The layer structure contains 100 nm high temperature AlN nucleation layer, a total thickness of 1.2 mm GaN layers, 20 nm AlGaN barrier layer with Al content of 20%, 3 nm GaN cap layer and three LT AlN interlayers. The thicknesses of the three LT AlN interlayers at different growth temperatures are 20 nm (600 1C), 15 nm (700 1C) and 10 nm (800 1C), respectively. The three LT AlN interlayers were designed to alleviate the tension stress in the GaN epilayers as well as prevent the threading dislocations propagating from the substrate to the upper layers. Detailed discussions will be reported later. The device fabrication followed the standard optical lithography process. The device isolation was realized by implanted He ions with high energy. The ohm-contact metals were Ti/Al/Ti/Au layers and exhibited good surface morphology with specific ohm-contact resistance of 10 5 O cm2 level after being annealed in the 830 1C N2 atmosphere for 35 s, and the source–drain distance is 5 mm. The gate metals are Ni/Au with gate length and width of 1.4 and 60 mm, respectively. Finally, the device was passivated with the Si3N4 film by using the plasmaenhanced chemical vapor deposition (PECVD).
150000 FWHM = 661 arcsec 100000 50000 0 -6000
-4000
-2000
0 arcsec
2000
4000
6000
Fig. 3. GaN (0 0 0 2) XRD spectrum of the Si (1 1 1)-based AlGaN/GaN HEMT structure.
structure properties of the Si (1 1 1)-based AlGaN/GaN HEMT structure. The crack-free area of the surface of the 2-in Si (1 1 1)-based sample is 8 mm 5 mm, characterized by the optical microscopy. The surface morphology of the sample measured by AFM is depicted in Fig. 2. The sample exhibited a smooth surface with clear atomic steps and the root-mean-square (RMS) roughness was 0.377 nm for a scan area of 5 mm 5 mm. The crystal quality of the sample was characterized by DCXRD described in Fig. 3. The full-width at half-maximum (FWHM) of GaN (0 0 0 2) peak is 661 arcsec from the rocking curve measurement. These results show that the grown Si (1 1 1)-based AlGaN/GaN HEMT structure has good crystal quality and surface morphology, which demonstrates the advantage of designed layer structures. The DC characteristics of the device were measured by the HP4142 semiconductor parameter analyzer instrument and the results are shown in Figs. 4–6. As shown in Fig. 4, the conditions of the I–V characteristic measurement are 2 VoVgso+1.5 V and 0 VoVdso10 V. The maximum saturated current density of the device is 304 mA/mm at the gate voltage of 1.5 V. The knee voltage is about 3 V, which shows the excellent Ohmic contact characteristics of the device. Besides, it can be seen that the drain current of the Si (1 1 1)-based AlGaN/GaN HEMT
ARTICLE IN PRESS W. Luo et al. / Microelectronics Journal 39 (2008) 1108–1111
1110
350
Idsmax = 304mA/mm
Vgs = 1.5V 0.04
300 Vgs = 1.0V Igs mA/mm
Ids mA/mm
250 200 150
0.02
Igs = 0.76 uA/mm 100 0.00 50 0
-12 0
2
4
6 Vds V
8
Gmmax = 124ms/mm
Idsmax = 338mA/mm
140 120
Ids (mA/mm)
80 60 150 40 100
Gm (ms/mm)
100 200
20 50 Lg = 1.4um Wg = 60um
0
0 -20
-2
0 Vg (V)
-6
-4
-2
0
2
Fig. 6. The Schottcky characteristics of Si (1 1 1) based on the AlGaN/ GaN HEMT.
4. Conclusions
300 250
-8
Vgs V
Fig. 4. The I–V characteristics of the Si (1 1 1)-based AlGaN/GaN HEMT.
350
-10
10
2
The AlGaN/GaN HEMT device was designed and fabricated on the 2-in Si (1 1 1) substrate. With the insertion of LT AlN layers, the grown sample exhibited good surface morphology with a crack-free area of 8 mm 5 mm and an RMS of 0.377 nm. The FWHM of the GaN (0 0 0 2) rocking curve is 661 arcsec. The fabricated device, with gate length of 1.4 mm and gate width of 60 mm, demonstrated maximum source–drain current density of 304 mA/mm, transconductance of 124 mS/mm, and reverse bias leakage current of 0.76 mA/mm at the gate voltage of 10 V. These results show that a high-performance Si (1 1 1)-based AlGaN/GaN HEMT device can be obtained by optimizing the inserted LT AlN layers.
Fig. 5. The transfer and transconductance characteristics of the Si (1 1 1)based AlGaN/GaN HEMT.
Acknowledgments device does not reduce with the increase of the source– drain voltage, while this phenomenon is clear in the sapphire-based devices. The reason is that the Si substrate has better thermal conduction characteristic than that of the sapphire substrate, which results in small thermal resistance. Thus, then the performance of the device can be improved. The transconductance characteristic of the device is shown in Fig. 5. The source–drain voltage is 7 V and the gate voltage changes from 2 to 2 V. As can be seen in Fig. 5, the passivated device exhibits maximum transconductance of 124 mS/mm at the gate voltage of 0 V nearby. From the transfer curve of the device, we can see that the pinched-off voltage is about 2 V. As described in Fig. 6, the Si (1 1 1)-based AlGaN/GaN HEMT device exhibits good Schottcky characteristics. The reverse bias leakage current is 0.76 mA/mm at the gate voltage of 10 V, which has potential in high-voltage application.
This work has been supported by the Key Innovation Program of the Chinese Academy of Sciences (no. KGCX2-SW-107-1), National Natural Science Foundation of China (no. 60606002) and Special Funds for Major State Basic Research Projects (nos. 2002CB311903, 2006CB604905 and 513270605). References [1] Y.-f . Wu, D. Kapolnek, J.P. Ibbetson, P. Parikh, U.K. Mishra, Veryhigh power density AlGaN/GaN HEMT, IEEE Trans. Electron Devices 48 (3) (2001) 586–590. [2] S.T. Sheppard, K. Doverspike, W.L. Pribble, S.T. Allen, J.W. Palmour, L.T. Kehias, T.J. Jenkins, High-power microwave GaN/ AlGaN HEMT’s on silicon carbide, IEEE Electron Device Lett. 20 (4) (1999) 161–163. [3] X.L. Wang, T.S. Cheng, Z.Y. Ma, G.X. Hu, H.L. Xiao, J.X. Ran, C.M. Wang, W.J. Luo, 1-mm gate periphery AlGaN/AlN/GaN HEMTs on SiC with output power of 9.39 W at 8 GHz, Solid-State Electron. 51 (3) (2007) 428–432.
ARTICLE IN PRESS W. Luo et al. / Microelectronics Journal 39 (2008) 1108–1111 [4] Masayoshi Umeno, Takashi Egawa, Hiroyasu Ishikawa, GaN-based optoelectronic devices on sapphire and Si substrates, Mater. Sci. Semicond. Process. 4 (6) (2001) 459–466. [5] Kun-Ta Wu, P.H. Chang, S.T. Lien, N.C. Chen, et al., Growth and characterization of GaN/AlGaN high-electron mobility transistors grown on p-type Si substrates, Physica E: Low-dimensional Syst. Nanostruct. 32 (1–2) (2006) 566–568. [6] X. Weng, S. Raghavan, J.D. Acord, et al., J. Cryst. Growth 300 (2007) 822–825. [7] Doek kyu Kim, Choon Bae Park, J. Korean Phys. Soc. 49 (4) (2006) 1497–1500.
1111
[8] A. Dadgar, P. Veit, F. Schulze, J. Bla¨sing, et al., Thin Solid Films 515 (2007) 4356–4361. [9] Zhe Liu Liu, Xiaoliang Wang–, Junxi Wang, et al., J. Cryst. Growth 298 (2007) 281–283. [10] Srinivasan. Raghavan, Xiaojun. Weng, Elizabeth. Dickey, Joan. M. Redwing, Appl. Phys. Lett. 87 (142101) (2005). [11] A. Reiher, J. Blasing, A. Dadgar, A. Diez, A. Krost, J. Cryst. Growth 248 (2003) 563–567. [12] A. Dadgar, J. Bla¨sing, A. Diez, et al., Jpn. J. Appl. Phys. 39 (2000) L1183–L1185.
