Comparative Study of GaN Based Light Emitting Devices Grown on Sapphire and GaN Substrates
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Comparative Study of GaN Based Light Emitting Devices Grown on Sapphire and GaN Substrates Stephan Figge, Jens Dennemarck, Gabriela Alexe, Detlef Hommel University of Bremen, Institute of Solid State Physics, Semiconductor Epitaxy, 28359 Bremen, Germany ABSTRACT The homoeptaxial growth of GaN based devices has advantages against the heteroepitaxial realization on substrates such as sapphire or SiC, since heteroepitaxy implies a lot of problems like lattice mismatch, different thermal expansion coefficients, and needs an extensive optimization of the growth at the heterointerface. In this paper we will discuss GaN based light emitting devices grown by homoepitaxy in comparison to devices grown on sapphire. We will show the differences in device performance, device processing and the influence of the thermal resistivity on the devices. INTRODUCTION The interest in GaN based devices raised in the beginning of the 90ths with the development of the 3-step nucleation scheme by Akasaki et al. [1], which made it for the first time possible to grow GaN with defect densities below 1010 1/cm2 . The achieved layer quality has been sufficient for the development of optoelectronic devices. But still the threshold current density of these devices was in the range of 10 kA/cm2 [2] such that the life time of the devices was to short to fabricate commercially usable lasers [3]. Further improvements in defect density reduction with techniques like laterally enhanced overgrowth (LEO/ELOG) [4, 5] were needed to gain sufficient material quality with local defect densities below 107 1/cm2 for the production of laser structures with an output power up to 100 mW and a life time beyond 10000 hours [6, 7]. Although these growth techniques are satisfactory for low power devices there is still a need for superior substrates enabling high power devices needed for lightning or laser projection. By an optimization of the GaN nucleation one can reduce the threading dislocation density of GaN layer on sapphire substrates down to 8×107 cm−2 , but this is opposed by high layer thicknesses, which are needed for coalescence of such layers [8]. The best choice for high power devices is therefore the use of GaN substrates using the advantages of homoepitaxy. However, there is still no growth method available to grow bulk GaN crystals in adequate size for wafer production. Instead GaN wafers are fabricated from thick GaN layers grown by hydride vapor phase epitaxy (HVPE) on sapphire substrates. In this paper the growth and processing of GaN based light emitting devices on sapphire and GaN substrates will be discussed and the advantages of the lower thermal conductivity of GaN substrates in comparison to sapphire will be analyzed. EXPERIMENTAL Laser structures were grown on GaN as well as sapphire substrates by metal organic
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vapor phase epitaxy (MOVPE) in close coupled shower-head reactor. Both samples were grown simultaneously in one growth run on a commercial available GaN substrate supplied by ATMI (now Cree) and a template of GaN g
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