Growth Of High Quality GaN Thin Films By MBE On Intermediate-temperature Buffer Layers
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Internet Journal Nitride Semiconductor Research
Growth Of High Quality GaN Thin Films By MBE On Intermediate-temperature Buffer Layers W.K. Fong1, C. F. Zhu1, B. H. Leung1 and Charles Surya 1 1The
Hong Kong Polytechnic University, Department of Electronic and Information Engineering,
(Received Monday, October 16, 2000; accepted Wednesday, December 20, 2000)
We report the growth of high quality GaN epitaxial layers by rf-plasma MBE. The unique feature of our growth process is that the GaN epitaxial layers are grown on top of a double layer that consists of an intermediate-temperature buffer layer (ITBL), which is grown at 690°C and a conventional low-temperature buffer layer deposited at 500°C. It is observed that the electron mobility increases steadily with the thickness of the ITBL, which peaks at 377 cm2V-1s-1 for an ITBL thickness of 800 nm. The PL also demonstrated systematic improvements with the thickness of the ITBL. Our analyses of the mobility and the photoluminescence characteristics demonstrate that the utilization of an ITBL in addition to the conventional low-temperature buffer layer leads to the relaxation of residual strain within the material resulting in improvement in the optoelectronic properties of the films. A maximum electron mobility of 430 cm2V-1s-1 can be obtained using this technique and further optimizing the growth conditions for the low-temperature buffer layer.
1
Introduction
Various groups have attempted different techniques for improving the electrical and optical properties of MBE grown GaN thin films. The nitrogen source is generally provided by an ECR source, an rf-plasma source or by gaseous NH3. It was reported by Tang et al. [1] that electron mobility, up to 560 cm2V-1s-1, can be obtained using a UHV magnetron sputtered AlN buffer layer and a epitaxial GaN layer grown using NH3 as the nitrogen source. High quality GaN was also grown using a GaN template deposited by migration enhanced epitaxy in conjunction with an AlN/GaN superlattice layer. A mobility of 668 cm2V-1s-1 was reported using this growth technique [2]. Recently, Heying et al. [3] reported the highest mobility to date of 1191 cm2V-1s-1 grown by rf-plasma assisted MBE on MOCVD-GaN/ sapphire composite substrates. 2
Experiment And Results
In this paper we report the growth of high quality GaN epilayers by MBE on (0001) oriented sapphire wafers. The activated nitrogen is provided by an EPI UNI-Bulb rf-plasma source. The substrate was first cleaned using a standard cleaning procedure. It was then outgassed at 800°C, followed by a nitridation process within the
growth chamber at 500°C. A conventional low-temperature buffer layer of thickness about 20 nm was grown at 500°C. An intermediate-temperature buffer layer (ITBL) was then grown on top of the conventional buffer layer at 690°C. The thickness of the ITBL was varied up to 1.25 µm. Finally, a 1.8 µm layer of high temperature, slightly n-doped GaN epitaxial layer was grown at 750°C. The surface morphology and optimal III/V ratio were monitored in-situ by the
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