The Use of AIN Interlayers to Improve GaN Growth on A-Plane Sapphire
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The Use of AlN Interlayers to Improve GaN Growth on A-Plane Sapphire D.D. KOLESKE, M.E. TWIGG, A.E. WICKENDEN, R.L. HENRY, R.J. GORMAN, J.A. FREITAS, JR., AND M. FATEMI Code 6800, Electronics Science and Technology Division, Naval Research Laboratory, Washington, D.C. 20375 ABSTRACT The lack of a suitable, lattice matched substrate for the growth of the group III nitrides typically restricts GaN film growth to substrates such as sapphire or SiC, despite the large lattice and thermal mismatch. With the use of AlN or GaN nucleation layers (NL), GaN films of sufficient quality have been produced for blue LEDs. However, for laser and large-area microwave device applications, the large number of dislocations (> 108 cm-2) limit device performance, and techniques are desired to reduce dislocation density during the growth process. Here, we demonstrate how low temperature AlN interlayers (IL) sandwiched between high temperature (HT) GaN layers can be used to improve the electrical, optical, and structural properties of Si doped GaN films. A nearly two-fold increase in mobility is observed in Si doped GaN grown using 5 AlN IL compared to GaN grown on a single AlN NL. For GaN films grown on multiple AlN IL, cross-sectional transmission electron microscopy images reveal a significant reduction in the screw dislocation density and photoluminescence spectra reveal a reduction in yellow band intensity. An analysis of the electrical data based on a single donor/single acceptor model suggests that the improved electron mobility is the result of a reduced acceptor concentration in the top GaN film. The reduction in the calculated acceptor concentration may be associated with the reduction of the screw dislocation density. INTRODUCTION Many approaches are currently being pursued to reduce defects in GaN epitaxial films. These approaches include the development of bulk substrates for homoepitaxy [1], the growth of thick “bulk-like” heteroepitaxial GaN layers for homoepitaxy [2], the lateral growth of GaN over Si oxide or nitride masked regions [3], and the growth low temperature interlayers (IL) on top of GaN heteroepitaxial layers [4]. The recent success of lasers fabricated on laterally overgrown GaN [5], suggests that the high dislocation density in GaN does still limit the fabrication and commercialization of some types of devices. For instance, in the fabrication of both lasers [5] and p-n diodes [6] on dislocation free GaN grown, both the threshold current density and leakage current are significantly reduced. Diodes grown on bulk GaN crystals using molecular beam epitaxy (MBE) are twice as bright and have higher p-type doping compared to their counterparts grown on sapphire [7]. Recent theoretical models suggest that dislocations play a role in limiting electron transport in GaN films, since the dislocations may contain trapping-type defects [8,9] or compensating acceptors [10]. Low dislocation densities have been obtained in bulk GaN grown in high pressure, thick GaN films grown by hydride vapor phase epitaxy, and in laterall
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