MOCVD Growth of AlN/GaN DBR Structure Under Various Ambient Conditions
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MOCVD Growth of AlN/GaN DBR Structure under Various Ambient Conditions H. H. Yao,C. F. Lin, and S. C. Wang, Institute of Electro-optical Engineering, National Chiao Tung University, Hsinchu 300, Taiwan, R. O. C.
Abstract High reflectivity AlN/GaN DBR structures were grown by MOCVD under three ambient gas conditions during the AlN layer growth. Highest peak reflectivity of about 94.5% with a stopband width of 18 nm at a center wavelength of 442nm was obtained under pure N2 gas ambient growth condition. The center wavelength of the DBR structures blue-shifted to 418 nm and 371 nm and the peak reflectivity also showed a reduction to 92 % and 79 % under the ambient gas conditions of a N2/H2 mixture and pure H2 respectively. Surface roughness showed slight increase from 8nm to 12nm, and the groove depth shows an increase from 25nm to 60nm with increasing the H2 ambient gas ratio. TEM results showed that AlN grown rate was reduction with the increasing H2 content in the ambient gas. The simulation result indicated that optical loss of AlN layers was increased from 450 cm-1 to 1850 cm-1 with the increasing H2 content in the ambient gas.
INTRODUCTION Gallium nitride is a direct wide bandgap semiconductor, which has attracted great attentions for application to light sources of short wavelength. In particular, GaN-based semiconductor laser diodes (LDs) and light emitting diodes (LEDs) have applications in displays, traffic signals, and high density DVD. Although there are still many issues on blue edge emitting LDs and LEDs, the research interest has gradually shifted to the development and demonstration of GaN-based vertical cavity surface emitting lasers (VCSELs)
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and Resonant-Cavity Light-Emitting Diodes
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(RC-LEDs) . An important requirement for the operation of such devices is the use of high reflectance mirrors, usually in the form of distributed Bragg reflectors (DBRs). The DBR structures are particular important for GaN VCSELs in two aspects. First, according to Honda and co-workers,5 the threshold current density of a GaN VCSEL can be reduced by an order of magnitude with an increase of the DBR peak reflectance from 90% to 99%. The second aspect is that the DBRs should have large stopband width. This is important because the active region of the GaN based VCSEL is typical made of InGaN multiple quantum wells (MQWs), and the emission peak of the InGaN MQWs tends to fluctuate with small variations in either the growth conditions or the process parameters.6 The DBRs with wide stopband can provide sufficient coverage of such spectral variation in emission wavelength.
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Several GaN/AlGaN based DBR structures have been reported either grown by molecular beam epitaxy (MBE)7 or by metal organic chemical vapor deposition (MOCVD)8. The main difficulty in fabrication GaN-ba
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