GaN Homoepitaxy for Device Applications

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Cite this article as: MRS Internet J. Nitride Semicond. Res. 4S1, G10.2(1999) Abstract Epitaxial growth on GaN single bulk crystals sets new standards in GaN material quality. The outstanding properties provide new insights into fundamental material parameters (e.g. lattice constants, exciton binding energies, etc.) being not accessible by heteroepitaxial growth on sapphire or SiC. With MOVPE and MBE we realized unstrained GaN layers with dislocation densities about six orders of magnitude lower than in heteroepitaxy. Those layers revealed an exceptional optical quality as determined by a reduction of the photoluminescence linewidth from 5 to 0.1 meV and a reduced XRD rocking curve width from 400 to 20 arcsec. Only recently, progress in surface preparation allowed morphologies of the layers suitable for device applications. We report on InGaN/GaN MQW structures as well as the first GaN pnand InGaN/GaN double heterostructure LEDs on GaN single bulk crystals. Those LEDs are twice as bright as their counterparts grown on sapphire. In addition they reveal an improved high power characteristics, which is attributed to an enhanced crystal quality and an increased p-doping. Time resolved electroluminescence measurements proof that band/band recombination is the dominant emission mechanism for the InGaN/GaN LEDs.

Introduction Due to its excellent optical and electrical properties, GaN attracts worldwide attention for devices and fundamental research. The wide direct bandgap, the high luminescence efficiency and the thermal, mechanical, and chemical robustness make group IlI-nitride semiconductors the superior material system for optoelectronic devices in the UV to visible range. Despite exceptional progress, group III-nitrides technology still suffers from mismatched heteroepitaxial growth. Mismatch in lattice constants and thermal expansion coefficients between substrate (mostly sapphire or SiC) and epitaxial layer inhibit perfect crystal formation, resulting in 109 to 1010 threading dislocations per cm2 . Homoepitaxial growth of GaN has proven its tremendous potential to achieve superior material quality resulting in extremely narrow photoluminescence (PL) linewidths [1],[21 and a reduction of the dislocation densities by six orders of magnitude. These material qualities can only be attained using a substrate which is identical in crystal structure, lattice parameter and thermal expansion coefficient. Under those conditions, two-dimensional layer-by-layer growth can be obtained and the generation of dislocations can be inhibited. Additional process steps such as nitridation and nucleation layers, mandatory in heteroepitaxy of GaN, are no longer

required, thus significantly simplifying the growth process. Besides the fundamental advantages of homoepitaxy, GaN substrates have a high thermal conductivity facilitating high power applications. Since they are electrically conductive, too, they provide additional freedom for the device design (e.g. vertical current transport) and simplify the device processing. Beside above G

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