Crack Nucleation in AlGaN/GaN Heterostructures
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Crack Nucleation in AlGaN/GaN Heterostructures Peter J. Parbrook, Malcolm A. Whitehead, Richard J. Lynch and Robert T. Murray1 EPSRC National Centre for III-V Technologies, Department of Electronic and Electrical Engineering, University of Sheffield, Mappin Street, Sheffield, S1 3JD, United Kingdom. 1 Materials Science and Engineering, Department of Engineering, University of Liverpool, Liverpool, L69 3GH, United Kingdom. ABSTRACT The tensile strain in AlGaN layers on GaN is well established to lead to cracking if a critical thickness is reached, unless measures such as interlayers are applied to prevent their formation. However in devices, such as HFETs such an approach is impractical. Growth of AlGaN-GaN structures was carried out by MOVPE using a standard two stage process for the growth of the GaN on sapphire. The crack structures were examined by optical and atomic force microscopy. Studies on thin AlGaN layers on GaN close to the crack critical thickness show the stress centres from which the cracks propagate are threading dislocations with cracks often initially forming to link together these stress centres if they are in close proximity. These cracks then extend and "lock" into the generally observed 2110 direction in more highly strained layers. A macroscopically uniform crack array is observed in these thin AlGaN samples. INTRODUCTION The growth of AlGaN is required in a wide variety of III-nitride devices. These include violet laser diodes (LDs), heterojunction field effect transistors (HFETs), ultra-violet emitters, photodetectors and also intersubband devices. However the lattice mismatch of around 3% between AlN and GaN limits the thickness of material that can be grown before relaxation occurs. For AlGaN layers on GaN the observed relaxation is through the formation of a network of cracks [1,2,3]. These cracks, in addition to relieving strain in themselves, can lead to the formation of misfit dislocations in the basal plane [2]. While such crack networks can be avoided through the use of a low temperature interlayer of AlN at the AlGaN-GaN interface for example [4], these strategies are not appropriate in a critical device region (as in an HFET) and can cause an increased threading dislocation density, (which would affect LD performance). Although the above studies have outlined in some detail the propagation and relaxation induced by cracks, none have to our knowledge discussed their nucleation. In this paper evidence is presented that in single layers of AlGaN on GaN a high density of microcracks are nucleated from the dislocations threading up through the GaN epilayer into the AlGaN. EXPERIMENTAL The samples examined in this study were grown by metalorganic vapour phase epitaxy at low pressure in a Thomas-Swan 3x2” close coupled showerhead reactor. They were grown on cplane sapphire substrates, using a standard two-stage growth process with a GaN nucleation layer grown at 525°C followed by the high temperature GaN buffer at 1030°C. Details of the
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nucleation process employed are
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