Elsevier

Superlattices and Microstructures

Volume 36, Issues 4–6, October–December 2004, Pages 509-515
Superlattices and Microstructures

Microstructural assessment of InN-on-GaN films grown by plasma-assisted MBE

https://doi.org/10.1016/j.spmi.2004.09.011Get rights and content

Abstract

The structural properties of InN thin films, grown by rf plasma-assisted molecular beam epitaxy on Ga-face GaN/Al2O3(0001) substrates, were investigated by means of conventional and high resolution electron microscopy. Our observations showed that a uniform InN film of total thickness up to 1 μm could be readily grown on GaN without any indication of columnar growth. A clear epitaxial orientation relationship of [0001]GaN[0001]InN, (101̄0)GaN(101̄0)InN was determined. The quality of the InN film was rather good, having threading dislocations as the dominant structural defect with a density in the range of 109–1010 cm−2. The crystal lattice parameters of wurtzite InN were estimated by electron diffraction analysis to be a=0.354 nm and c=0.569 nm, using Al2O3 as the reference crystal. Heteroepitaxial growth of InN on GaN was accomplished by the introduction of a network of three regularly spaced misfit dislocation arrays at the atomically flat interface plane. The experimentally measured distance of misfit dislocations was 2.72 nm. This is in good agreement with the theoretical value derived from the in-plane lattice mismatch of InN and GaN, which indicated that nearly full relaxation of the interfacial strain between the two crystal lattices was achieved.

Introduction

In recent years, III-nitride compound semiconductors such as AlN, GaN, InN and their ternary and quaternary alloys have gained significant interest due to their potential use in high-power, high-frequency electronics and ultraviolet (UV) to infrared (IR) optoelectronic devices. In particular, growth and optimization of InN-based thin films and devices is of crucial importance, since InN was found to have the smallest effective mass and the highest electron drift velocity [1]. In addition, its use along with GaN and AlN makes it possible to extend the light emission of nitride-based LEDs from the ultraviolet to the near-infrared region [2]. However, fabrication and a clear understanding of the physical properties of InN thin films were hindered due to the low dissociation temperature of InN and the high equilibrium vapor pressure of nitrogen [3].

During the previous decade, growth of InN thin films has been remarkably improved, by Metalorganic Vapor Phase Epitaxy (MOVPE) and Molecular Beam Epitaxy (MBE) growth techniques. Si, Al2O3, GaAs and GaN have been utilized as substrates in InN film fabrication. However, the large lattice mismatch of InN with sapphire and the large difference in the thermal expansion coefficient values between sapphire and InN leads to the development of a significant number of structural defects. Surface pre-treatment of the substrate and the introduction of a buffer layer such as AlN, InN or GaN prior to the InN film growth have been employed to overcome this obstacle [4]. Furthermore, the value of the lattice parameters of InN is a matter of ongoing controversy, since no exact values have been defined yet [4], [5].

In the present study, a structural assessment of InN films grown by MBE on GaN/Al2O3 templates is presented, employing conventional and high-resolution transmission electron microscopy (TEM-HRTEM). We have investigated the growth mode of InN on GaN as well as the crystallization of the InN film. Electron diffraction analysis was used for the determination of the orientation relationship between the two crystal lattices and the lattice parameters of InN. The density of all types of threading dislocations and the interfacial structure were anticipated directly from the micrographs. Interfacial defects such as misfit dislocations arising from the mismatch of the two lattices were also investigated. Moreover, the degree of relaxation introduced by the network of misfit dislocations was estimated, in terms of the difference between the experimentally measured and the calculated value of the equidistance of misfit dislocations that corresponds to a useful approximation of the relaxed configuration.

Section snippets

Experimental details

The InN thin films were grown by rf plasma-assisted MBE on Ga-face 2.5 μm thick GaN/Al2O3(0001) templates. Details for the RFMBE system have been given elsewhere [6]. Prior to InN deposition, a 0.1 μm thick GaN layer was typically grown under Ga-rich conditions on the GaN template with the aim of improving surface purity and smoothness, and favoring the step-flow growth mode. At the end, the GaN surface was exposed for sufficient time to the nitrogen beam to consume any accumulated Ga atoms. A

Results and discussion

A XTEM micrograph of the InN film on top of the GaN template is shown in Fig. 1a. The observations readily confirmed that the InN film has a total thickness of around 0.9 μm. The images revealed that InN is uniformly grown on top of GaN, showing no indication of columnar growth, which is a common feature during the deposition of InN thin films on various substrates [3], [4]. Fig. 1b is the corresponding diffraction pattern, revealing the epitaxial orientation relationship between InN and GaN.

Conclusions

The structural properties of InN thin films grown by rf plasma-assisted MBE on GaN/Al2O3 templates have been investigated by means of TEM and HRTEM. InN was found to be solely crystallized in the hexagonal wurtzite structure, having lattice parameters a=0.354 nm and c=0.569 nm and with a clearly determined epitaxial relationship of [0001]GaN[0001]InN and (101̄0)GaN(101̄0)InN. The crystal quality of the film was good, having threading dislocations as the dominant structural defect with

Acknowledgements

The authors acknowledge the General Secretariat of Research and Technology (GSRT) for financial support through the PENED 01ED481 contract.

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