SiH 4 exposure of GaN surfaces: A useful tool for highlighting dislocations
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SiH4 exposure of GaN surfaces: A useful tool for highlighting dislocations Rachel A. Oliver, Menno J. Kappers, Joy Sumner, Ranjan Datta, Colin J. Humphreys Department of Materials Science and Metallurgy, Pembroke Street, Cambridge CB2 3QZ, UK ABSTRACT Fast-turnaround, accurate methods for the assessment of threading dislocation densities in GaN are an essential research tool. Here, we present an in-situ surface treatment for use in MOVPE (metal-organic vapour phase epitaxy) growth, in which GaN is exposed to a SiH4 flux at 860 °C in the presence of NH3. Subsequent characterisation by atomic force microscopy shows that the treatment is effective in increasing edge and mixed/screw dislocation pit sizes on both n- and p-type material, and on partially coalesced GaN layers. INTRODUCTION GaN-based light-emitting diodes (LEDs) and laser diodes (LDs) grown on unlike substrates (e.g. sapphire or silicon carbide) contain high densities of threading dislocations (TDs) which may reduce their output and lifetime. Research aiming to reduce the density of these defects is hampered by a lack of fast-turnaround, accurate techniques to determine the TD density and the proportions of edge, screw and mixed TDs in a sample. Transmission electron microscopy (TEM) provides accurate data, providing the sample is orientated appropriately [1], but requires painstaking sample preparation, expensive equipment not widely available in an industrial context, and significant skill. Additionally, only very limited areas may be imaged. Atomic force microscopy (AFM) is a more convenient technique, but tends to underestimate the TD density, since, whilst pits associated with screw- or mixed-type TDs are quite easily imaged, the smaller pits associated with edge-type TDs are difficult to resolve with standard AFM probes. To aid dislocation imaging by AFM or scanning electron microscopy (SEM), a variety of etching methods have been developed, with limited success. Common techniques include etching in a hot mixture of phosphoric and sulphuric acid [2], etching in molten bases (such as KOH) [2] and photoelectrochemical (PEC) etching in aqueous KOH [3]. Of these, only PEC etching has been consistently reported to reveal all types of dislocations, and unfortunately this technique is only applicable to conductive materials, and does not provide data on the proportion of edge, mixed and screw TDs in the sample [2]. Here, we report a treatment employed in situ in our metal-organic vapour phase epitaxy (MOVPE) system, that highlights all types of TDs and allows discrimination between screw/mixed and edge-type TDs using AFM. The method is effective on doped and undoped material, and on samples in which a range of facets are present. EXPERIMENTAL DETAILS GaN pseudo-substrates consisting of ca. 4.5 µm of GaN on c-plane (0001) sapphire were grown by MOVPE in a 6 × 2″ Thomas Swan close-coupled showerhead reactor. The precursors trimethylgallium (TMG), silane (SiH4) at 50 ppm in H2, and ammonia (NH3) were employed, with H2 as a carrier gas. Growth wa
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