Atomic Structure of Grain Boundaries and Interfaces in III-Nitrides Epitaxial Systems
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bulk and homoepitaxial
crystals14 -16 were shown to contain defects 17 including stacking faults
(SFs), dislocations and grain boundaries. Defects effect electronic properties18 ,19 and, hence, device performance. 5 Therefore reduction of their density is one of the problems in GaN technology. In order to improve the quality of GaN crystals, a better understanding of defect formation and their atomic structure in this material is highly desirable. In the present paper, defects formed in GaN epitaxial layers grown by MOVPE on (0001) sapphire are reviewed in comparison with those in MBE grown materials. High resolution electron microscopy was used to evaluate atomic structure of the GaN-based heterostructures in both cross-section and plan view projections. EXPERIMENTAL GaN layers were grown by MOVPE on (0001) A120 3 with an AIN buffer layer as described elsewhere. 20 The AIN buffer layers were grown at 400 °C on sapphire. GaN layers were either undoped or doped by Si (up to 4x1018 cm- 3 ) and Mg. Mg-doped samples were annealed at 800 0 C for 30 min. TEM studies were carried out on Topcon 002B and JEOL 200CX microscopes operated at 200 kV. Both plan view and cross-sectional specimens were prepared for TEM study by dimpling followed by ion milling.
387 Mat. Res. Soc. Symp. Proc. Vol. 482 01998 Materials Research Society
RESULTS AND DISCUSSION Epitaxial GaN layers were shown to contain a wide variety of crystalline defects (Table 1) which are also typical in other III-V materials. However some defects like nanotubes 21-23 are specific for wurtzite III-nitrides. The major defects in epitaxially grown GaN layers are dislocations, grain boundaries (GBs), stacking faults (SFs), nanopipes and pinholes. Table 1.Crystalline defects and interfaces in GaN
Extrinsic
Intrinsic 0D
point defects, small voids and clusters
impurities, complexes and clusters of foreign atoms
ID
dislocations, nanotubes
ordered arrays of foreign atoms or clusters
2D
SFs, IDB, GB
interfaces with substrate, QWs
3D
pinholes, politype inclusions
alloy decomposition in QWs, QDs
Fig. I (a) shows a cross-sectional TEM image of an epitaxial GaN layer grown on sapphire with an AIN buffer layer. The dislocation density is high at the interface and then drastically decreases over 0.2 gim toward the GaN top surface ranging typically from 109 to 1011 cm -2 near the surface. Threading dislocations are mostly generated at the interface with AIN probably during coalescence of three dimensional (3D) islands formed at initial stages of GaN growth, but also propagate from the AIN buffer layer.
Fig. 1, a-c. Cross-sectional (a,b) and plan view (c) TEM images of epitaxial GaN layer grown on sapphire using AIN buffer layer. The AIN buffer layer typically has a "mosaic" structure24 ,25 with a high density of defects lb). It contains crystalline grains slightly misoriented around the c-axis, but perfectly (Fig. oriented in the c-plane. 3D growth is typical for both AIN/sapphire and GaN/AIN systems due to 2). Due high mismatch in crystalline lattices which ar
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