Continuous-wave InGaN laser diodes on copper and diamond substrates
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InGaN-based optoelectronics were integrated with dissimilar substrate materials using a novel thin-film laser lift-off (LLO) process. The LLO process was employed to integrate InGaN-based laser diodes (LDs) with Cu and diamond substrates. Separation of InGaN-based thin-film devices from their typical sapphire growth substrates was accomplished using a pulsed excimer laser in the ultraviolet regime incident through the transparent substrate. Characterization of the LDs before and after the sapphire substrate removal revealed no measurable degradation in device performance. Reduced threshold currents and increased differential quantum efficiences were measured for LDs on Cu due to a 50% reduction of the thermal impedence. Light output for LDs on Cu was two times greater than comparable LDs on sapphire with a maximum output of 100 mW. Increased light output for LDs on diamond was also measured with a maximum output of 80 mW.
I. INTRODUCTION
An effective means for improving blue laser performance can be achieved by thin-film lift-off and transfer of prefabricated, fully functional devices from sapphire onto another host substrate. Although high-performance continuous-wave (In,Ga,Al)N laser diodes possessing lifetimes greater than 10,000 h have been realized on sapphire substrates,1 a major impediment to the development of III-nitride laser diodes (LDs) still remains: the efficient dissipation of heat generated from the active area of the device. The high thermal resistance of the sapphire substrate and the relatively high current densities combine to degrade the device performance and reduce the lifetimes due, in part, to excessive heating during operation. Substrates such as copper or diamond would be more ideal in terms of thermal and electrical conductivity, although direct deposition and fabrication of III-nitride-based laser devices on these materials are unfeasible or result in poor-quality thin films. A less-direct integration method, but a more flexible process, is thin-film lift-off and transfer to join dissimilar materials. A lift-off and transfer process allows integration of the highest quality materials available, prefabricated in separate process steps, to create one wholly optimized system. Removing the transferred film from its original growth substrate completes the final product, created by bonding and lift-off onto the desired platform. This integration method has been demonstrated by several methods. 890
http://journals.cambridge.org
J. Mater. Res., Vol. 17, No. 4, Apr 2002 Downloaded: 18 Mar 2015
One such technique for thin-film lift-off is the use of a thin sacrificial layer between the growth substrate and the desired thin film. The method has been described and demonstrated for the AlxGa1−xAs system using an AlAs release layer grown on a GaAs substrate.2 – 4 The epitaxial lift-off (ELO) process takes advantage of the extreme etch selectivity between AlxGa1−xAs and AlAs in a hydrofluoric (HF) acid.5 In this materials system, the selectivity for etching has been measured to be >107 between th
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