Nanomechanical characterizations of InGaN thin films
Introduction
The III-nitride wide-band-gap semiconductors are the most promising materials for constructing optoelectronic and electronic devices [1], [2]. Among the materials, ternary InGaN alloys which cover the direct-band-gap energy ranging from 1.9 eV (InN) to 3.4 eV (GaN) are crucial and functional materials for GaN-based blue/UV light emitters/lasers and high-power electronic devices [3], [4]. Although many works have been focused on the optoelectronic characteristics of InGaN alloys, however, their mechanical properties are almost ignored. InGaN alloys have been used in the form of thin films in optoelectronic and electronic devices. Mechanical properties of materials are size-dependent. Thin films may have different mechanical properties from their bulk materials. A precise measurement of the mechanical properties of InGaN thin films is required to use them as structural/functional elements in devices. Nanoindentation has been widely used for charactering the mechanical properties of solid surfaces and thin films [5], [6]. Among the mechanical properties of interest, hardness, Young's modulus, elastic/plastic deformation behavior and creep resistance can be obtained from nanoindentation testing.
Recently, the indentation-induced plastic behaviors and mechanisms of III–V semiconductors have been discussed [7] in more detail by means of atomic force microscopy (AFM), plane-view and cross-sectional transmission electron microscopy (XTEM); in particular, the effects of alloy strengthening of InxGa1−xAs/GaAs were also investigated. To our best knowledge, however, there are no reports on the mechanical properties of InGaN thin films have been made until now.
In this study, InGaN thin films with different indium concentrations were deposited by low-pressure metal-organic chemical vapor deposition (MOCVD). We report, for the first time to our knowledge, the hardness, Young's modulus and creep resistance of InGaN thin films. Nanoindentation deformation behavior of the deposited films was studied by analyzing the nanoindentation load–displacement curves and post-indentation imaging of the indentation impressions. Changes in mechanical properties for the deposited films are discussed in conjunction with deposition temperature, crystalline structure, grain size and surface morphology.
Section snippets
Experimental details
The InxGa1−xN films were grown on sapphire substrate by horizontal low-pressure MOCVD. Prior to material growth, the sapphire substrate was annealed/cleaned to remove surface residual stress and impurities in H2 gas at 1120 °C for 10 min. Trimethylgallium (TMGa), trimethylindium (TMIn) and ammonia (NH3) were used as source precursors for gallium, indium and nitrogen, respectively. A 25 nm thick GaN nucleation layer was first deposited at 520 °C for 4 min on the sapphire substrate. The substrate
Structural and surface characterization
Fig. 1 shows the XRD spectra and AFM images of the InGaN films deposited at 790, 760 and 730 °C, respectively. The InGaN and GaN peaks are observed in the XRD spectra and these two peaks do not shift while the In concentration is increased. The In concentrations in the InGaN films deposited at 790, 760 and 730 °C are 25, 30 and 34%, respectively. The concentrations were obtained by separating the GaN and InGaN peaks in XRD spectra using Vegard's law [8]. Broadness of the InGaN peak is attributed
Conclusions
The structural features and nanomechanical properties of the InGaN thin films with different In-content produced by MOCVD were investigated using the XRD, AFM and nanoindentation techniques. The following results were obtained:
- (1)
XRD analysis and AFM micrographs show no evidence of phase separation for InGaN thin films; also with a relatively smooth surface.
- (2)
Nanoindentation results indicate that Young's modulus has no varied significantly and, at lower indentation depths, a slightly increasing
Acknowledgement
Authors would like to thank Prof. X. Li, Department of Mechanical Engineering, University of South Carolina, USA, for his helpful discussions, and this work was partially supported by the National Science Council of Taiwan, under Grant Nos. NSC93-2218-E218-005 and NSC94-M2120-009-002.
References (32)
Int. J. Eng. Sci.
(1965)- et al.
J. Mech. Phys. Solids
(1998) - et al.
Acta Mater.
(2002) - et al.
J. Cryst. Growth
(2000) - et al.
Appl. Phys. Lett.
(1993) - et al.
Appl. Phys. Lett.
(1996) - et al.
The Blue Laser Diode
(1997) - et al.
Int. Mater. Rev.
(2003) - et al.
Mater. Charact.
(2002)