Effect of nitridation on crystallinity of GaN grown on GaAs by MBE
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Materials Chemistry and Physics 100 (2006) 457–459
![Materials Chemistry and Physics 100 (2006) 457–459
Effect of nitridation on crystallinity of GaN grown on GaAs by MBE
O. Maksimova,∗, P. Fisherb, M. Skowronskib, V.D. Heydemanna
a Electro-Optics Center, Pennsylvania State University, 559A Freeport Road, Freeport, PA 16229, United States
b Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213, United States
Received 6 October 2005; received in revised form 29 December 2005; accepted 23 January 2006
Abstract
GaN films are grown on [0 0 1] GaAs substrates by plasma-assisted molecular beam epitaxy using a three-step process that consists of a substrate
nitridation, deposition of a low-temperature buffer layer, and a high-temperature overgrowth. Films are evaluated by X-ray diffraction and the
dependence of crystalline quality on the nitridation temperature is studied. It is demonstrated that nitridation has to be performed at low-temperature
to achieve c-oriented ␣-GaN. Higher nitridation temperature promotes formation of mis-oriented domains and -GaN inclusions
© 2006 Elsevier B.V. All rights reserved.
Keywords: Molecular beam epitaxy; GaN; GaAs
GaN materials are technologically important for a variety of
device application [1,2]. They are ideal candidates for fabri-
cation of high power microwave devices, high frequency field
effect transistors, high electron mobility transistors, light emit-
ters and detectors operating in the visible to UV spectral range.
High quality hexagonal ␣-GaN films and heterostructures are
usually grown either by metal organic chemical vapor deposition
(MOCVD) or by molecular beam epitaxy (MBE) on sapphire
and 6H-SiC substrates [3,4]. Growth on [0 0 1] GaAs is much
less studied, although these substrates provide several advan-
tages, such as, low cost, easy cleavage along [0 1 1] direction,
closer thermal expansion coefficient matching, and possibility
to stabilize cubic -GaN.
We have reported that direct deposition on a thermally des-
orbed GaAs results in the growth of a polycrystalline poorly ori-
ented ␣-GaN containing mis-oriented domains and large cubic
inclusions. However, a significant improvement of the crys-
tallinity is achieved by adopting the growth procedure that con-
sists of a substrate nitridation, deposition of a low-temperature
buffer layer, and epitaxial overgrowth at elevated temperature
[5]. The nitridation conditions are extremely critical for this pro-
cess and have to be carefully controlled to achieve high-quality
film.
Here we investigate the influence of the substrate tempera-
ture during nitridation on the structural properties of GaN film.
∗ Corresponding author. Tel.: +1 724 295 6624; fax: +1 724 295 6617.
E-mail address: Maksimov@netzero.net (O. Maksimov).
We observe that low-temperature (400 ◦C) nitridation promotes
growth of c-oriented ␣-GaN. When nitridation is performed
at higher temperature, crystalline quality degrades and film
becomes polycrystalline.
The samples are fabricated in a custom-built MBE system
equipped with a Ga effusion cell, a radio frequency (RF) excited
plasma source (SVT Associates, Inc.), a retractable ion gauge for
flux calibration, and a reflection high-energy electron diffraction
(RHEED) system. GaN is grown on semi-insulating epi-ready
[0 0 1] GaAs substrates indium-mounted to molybdenum hold-
ers. The substrate temperature is measured by a thermocouple
in contact with the backside of the mounting block. To prevent
As incorporation in the GaN alloy, the oxide layer is desorbed at
500 ◦C in the absence of As flux. The GaAs wafer is exposed to
sub-monolayer Ga pulses to facilitate oxide desorption through
the conversion of Ga2O3 to a more volatile Ga2O [6]. This pro-
cess results in a slightly distorted GaAs surface. Kikuchi lines
are clearly visible in a RHEED pattern, indicating that GaAs
surface is free of oxide layer, Fig. 1A.
After oxide desorption wafer temperature is adjusted to the
desired setting and nitridation is performed by exposing sub-
strate to nitrogen plasma. The nitridation rate is controlled with
a mass flow controller through which a high purity (6N) N2
gas (gas flow is ∼2.5 sccm) is introduced into the RF-plasma
source (input power is ∼400 W). The nitridation is performed
for 15 min with the substrate temperature kept constant.
When the wafer is exposed to nitrogen plasma, surface
reconstruction disappears during the first few minutes sug-
gesting formation of an amorphous GaAsN layer. We observe
0254-0584/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.matchemphys.2006.01.024](https://image.slidesharecdn.com/mcp100457-130516104531-phpapp01/85/Effect-of-nitridation-on-crystallinity-of-GaN-grown-on-GaAs-by-MBE-1-320.jpg)
![458 O. Maksimov et al. / Materials Chemistry and Physics 100 (2006) 457–459
Fig. 1. RHEED patterns for: (A) GaAs substrate after oxide desorption at 500 ◦C, (B) GaAs substrate after 5 min of nitridation at 400 ◦C, (C) GaN buffer layer
deposited at 600 ◦C, (D) GaN film grown at 750 ◦C.
an arc pattern after approximately 5 min indicating develop-
ment of a preferred out-of-plane orientation in a disordered
layer, Fig. 1B. Since it does not change when the wafer is
rotated around the surface normal, layer is not oriented in-
plane. Spot-like features with hexagonal symmetry develop
after, approximately, 10 min. This reconstruction does not sig-
nificantly change when the wafer is exposed to nitrogen plasma
for a longer period of time. Therefore, we limit nitridation
to 15 min.
In the next step we close nitrogen plasma source shutter
and increase wafer temperature to 600 ◦C. Annealing, ∼30 min,
sharpens diffraction spots demonstrating recrystallization of ␣-
GaN phase, Fig. 1C. The diffraction spots are broad signifying
that very defective GaN layer forms at the beginning. How-
ever, they become significantly sharper and elongated during
the growth of a relatively thin (50-nm) buffer layer, indicating
that GaN buffer has a better crystalline quality and a smoother
surface.
Finally, wafer temperature is raised to 750 ◦C for GaN
growth. A slightly diffused (1 × 1) reconstruction is observed
during the film growth, Fig. 1D.
Crystalline quality of the GaN films is studied by X-ray
diffraction (XRD). All the films are deposited in one growth
run under identical conditions and differ only in the nitridation
temperature (A 400 ◦C, B 500 ◦C, C 550 ◦C, D 600 ◦C). A XRD
θ–2θ scan demonstrates that low temperature nitridation pro-
motes growth of c-oriented ␣-GaN, Fig. 2A. Mis-oriented grains
( 1 0 1 1 , 1 0 1 2 , 1 1 2 0 , 1 0 1 3 ) and cubic -GaN inclu-
sions ( 0 0 2 ) develop when nitridation is performed at 500 ◦C,
Fig. 2B. The intensity of 0 0 0 2 diffraction decreases while
other peaks become more pronounced with the further increase
of nitridation temperature indicating degradation of crystalline
quality of the film, Fig. 2C and D. This trend is, most prob-
ably, due to the surface etching that is activated by substrate
temperature during nitridation [7]. It results in a rough defec-
tive epilayer/substrate interface and can promote polycrystalline
growth.
In conclusion, we demonstrate that crystalline quality of GaN
films grown on [0 0 1] GaAs substrates is extremely sensitive to
nitridation conditions. Nitridation has to be performed at low-
temperature (400 ◦C) to achieve c-oriented ␣-GaN. Higher sub-
strate temperature promotes formation of mis-oriented domains
and -GaN inclusions.
Fig. 2. XRD θ–2θ scans of ∼2 m thick GaN films grown on a GaAs substrate.
Substrate nitridation is performed at (A) 400 ◦C, (B) 500 ◦C, (C) 550 ◦C, (D)
600 ◦C.](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
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Effect of nitridation on crystallinity of GaN grown on GaAs by MBE
- 1. Materials Chemistry and Physics 100 (2006) 457–459 Effect of nitridation on crystallinity of GaN grown on GaAs by MBE O. Maksimova,∗, P. Fisherb, M. Skowronskib, V.D. Heydemanna a Electro-Optics Center, Pennsylvania State University, 559A Freeport Road, Freeport, PA 16229, United States b Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213, United States Received 6 October 2005; received in revised form 29 December 2005; accepted 23 January 2006 Abstract GaN films are grown on [0 0 1] GaAs substrates by plasma-assisted molecular beam epitaxy using a three-step process that consists of a substrate nitridation, deposition of a low-temperature buffer layer, and a high-temperature overgrowth. Films are evaluated by X-ray diffraction and the dependence of crystalline quality on the nitridation temperature is studied. It is demonstrated that nitridation has to be performed at low-temperature to achieve c-oriented ␣-GaN. Higher nitridation temperature promotes formation of mis-oriented domains and -GaN inclusions © 2006 Elsevier B.V. All rights reserved. Keywords: Molecular beam epitaxy; GaN; GaAs GaN materials are technologically important for a variety of device application [1,2]. They are ideal candidates for fabri- cation of high power microwave devices, high frequency field effect transistors, high electron mobility transistors, light emit- ters and detectors operating in the visible to UV spectral range. High quality hexagonal ␣-GaN films and heterostructures are usually grown either by metal organic chemical vapor deposition (MOCVD) or by molecular beam epitaxy (MBE) on sapphire and 6H-SiC substrates [3,4]. Growth on [0 0 1] GaAs is much less studied, although these substrates provide several advan- tages, such as, low cost, easy cleavage along [0 1 1] direction, closer thermal expansion coefficient matching, and possibility to stabilize cubic -GaN. We have reported that direct deposition on a thermally des- orbed GaAs results in the growth of a polycrystalline poorly ori- ented ␣-GaN containing mis-oriented domains and large cubic inclusions. However, a significant improvement of the crys- tallinity is achieved by adopting the growth procedure that con- sists of a substrate nitridation, deposition of a low-temperature buffer layer, and epitaxial overgrowth at elevated temperature [5]. The nitridation conditions are extremely critical for this pro- cess and have to be carefully controlled to achieve high-quality film. Here we investigate the influence of the substrate tempera- ture during nitridation on the structural properties of GaN film. ∗ Corresponding author. Tel.: +1 724 295 6624; fax: +1 724 295 6617. E-mail address: Maksimov@netzero.net (O. Maksimov). We observe that low-temperature (400 ◦C) nitridation promotes growth of c-oriented ␣-GaN. When nitridation is performed at higher temperature, crystalline quality degrades and film becomes polycrystalline. The samples are fabricated in a custom-built MBE system equipped with a Ga effusion cell, a radio frequency (RF) excited plasma source (SVT Associates, Inc.), a retractable ion gauge for flux calibration, and a reflection high-energy electron diffraction (RHEED) system. GaN is grown on semi-insulating epi-ready [0 0 1] GaAs substrates indium-mounted to molybdenum hold- ers. The substrate temperature is measured by a thermocouple in contact with the backside of the mounting block. To prevent As incorporation in the GaN alloy, the oxide layer is desorbed at 500 ◦C in the absence of As flux. The GaAs wafer is exposed to sub-monolayer Ga pulses to facilitate oxide desorption through the conversion of Ga2O3 to a more volatile Ga2O [6]. This pro- cess results in a slightly distorted GaAs surface. Kikuchi lines are clearly visible in a RHEED pattern, indicating that GaAs surface is free of oxide layer, Fig. 1A. After oxide desorption wafer temperature is adjusted to the desired setting and nitridation is performed by exposing sub- strate to nitrogen plasma. The nitridation rate is controlled with a mass flow controller through which a high purity (6N) N2 gas (gas flow is ∼2.5 sccm) is introduced into the RF-plasma source (input power is ∼400 W). The nitridation is performed for 15 min with the substrate temperature kept constant. When the wafer is exposed to nitrogen plasma, surface reconstruction disappears during the first few minutes sug- gesting formation of an amorphous GaAsN layer. We observe 0254-0584/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2006.01.024
- 2. 458 O. Maksimov et al. / Materials Chemistry and Physics 100 (2006) 457–459 Fig. 1. RHEED patterns for: (A) GaAs substrate after oxide desorption at 500 ◦C, (B) GaAs substrate after 5 min of nitridation at 400 ◦C, (C) GaN buffer layer deposited at 600 ◦C, (D) GaN film grown at 750 ◦C. an arc pattern after approximately 5 min indicating develop- ment of a preferred out-of-plane orientation in a disordered layer, Fig. 1B. Since it does not change when the wafer is rotated around the surface normal, layer is not oriented in- plane. Spot-like features with hexagonal symmetry develop after, approximately, 10 min. This reconstruction does not sig- nificantly change when the wafer is exposed to nitrogen plasma for a longer period of time. Therefore, we limit nitridation to 15 min. In the next step we close nitrogen plasma source shutter and increase wafer temperature to 600 ◦C. Annealing, ∼30 min, sharpens diffraction spots demonstrating recrystallization of ␣- GaN phase, Fig. 1C. The diffraction spots are broad signifying that very defective GaN layer forms at the beginning. How- ever, they become significantly sharper and elongated during the growth of a relatively thin (50-nm) buffer layer, indicating that GaN buffer has a better crystalline quality and a smoother surface. Finally, wafer temperature is raised to 750 ◦C for GaN growth. A slightly diffused (1 × 1) reconstruction is observed during the film growth, Fig. 1D. Crystalline quality of the GaN films is studied by X-ray diffraction (XRD). All the films are deposited in one growth run under identical conditions and differ only in the nitridation temperature (A 400 ◦C, B 500 ◦C, C 550 ◦C, D 600 ◦C). A XRD θ–2θ scan demonstrates that low temperature nitridation pro- motes growth of c-oriented ␣-GaN, Fig. 2A. Mis-oriented grains ( 1 0 1 1 , 1 0 1 2 , 1 1 2 0 , 1 0 1 3 ) and cubic -GaN inclu- sions ( 0 0 2 ) develop when nitridation is performed at 500 ◦C, Fig. 2B. The intensity of 0 0 0 2 diffraction decreases while other peaks become more pronounced with the further increase of nitridation temperature indicating degradation of crystalline quality of the film, Fig. 2C and D. This trend is, most prob- ably, due to the surface etching that is activated by substrate temperature during nitridation [7]. It results in a rough defec- tive epilayer/substrate interface and can promote polycrystalline growth. In conclusion, we demonstrate that crystalline quality of GaN films grown on [0 0 1] GaAs substrates is extremely sensitive to nitridation conditions. Nitridation has to be performed at low- temperature (400 ◦C) to achieve c-oriented ␣-GaN. Higher sub- strate temperature promotes formation of mis-oriented domains and -GaN inclusions. Fig. 2. XRD θ–2θ scans of ∼2 m thick GaN films grown on a GaAs substrate. Substrate nitridation is performed at (A) 400 ◦C, (B) 500 ◦C, (C) 550 ◦C, (D) 600 ◦C.
- 3. O. Maksimov et al. / Materials Chemistry and Physics 100 (2006) 457–459 459 Acknowledgement This material is based upon work supported by Dr. Colin Wood, ONR under Contract No. N00014-05-1-0238. References [1] S.N. Mohammad, A.A. Salvador, H. Morkoc, Proc. IEEE 83 (1995) 1306. [2] R. Fen, J.C. Zolper, Wide Bandgap Electronic Devices, World Scientific, Singapore, 2003. [3] H.X. Jiang, J.Y. Lin, Opto-Electron. Rev. 10 (2002) 271. [4] O. Brandt, R. Muralihadharan, A. Thamm, P. Waltereit, K.H. Ploog, Appl. Surf. Sci. 175 (2001) 419. [5] O. Maksimov, P. Fisher, H. Du, M. Skowronski, V. D. Heydemann, North American MBE Conference, Santa Barbara, CA, 2005. [6] Z.R. Wasilewski, J.M. Baribeau, M. Beaulieu, X. Wu, G.I. Sproule, J. Vac. Sci. Technol. B 23 (2004) 1534. [7] I. Aksenov, H. Iwai, Y. Nakada, H. Okumura, J. Vac. Sci. Technol. B 17 (1999) 1525.