Continuous-wave InGaN MQW Laser Diodes On Copper And Diamond Substrates (Forum Nitride Lasers)
An effective means for improving blue laser performance can be achieved by thin-film lift-off and transfer of pre-fabricated, fully functional devices from sapphire onto another host substrate. Although high-performance CW (In,Ga,Al)N laser diodes possessing lifetimes greater than 10,000 hours have been realized on sapphire substrates , a major impediment to the development of III-nitride lasers 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 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 either unfeasible or result in poor-quality thin films. Thin-film laser lift-off (LLO) techniques have recently been established as an effective tool for the integration of GaN thin films with a variety of dissimilar substrates . The LLO process has successfully been used to fabricate vertical-structure InGaN-based LEDs on Si while showing no degradation to the device performance after the LLO and transfer process [3,4]. In addition, the integration of InGaN-based blue lasers with copper substrates have been demonstrated for lasers under pulsed operation . Here, we describe the transfer of pre-fabricated CW InGaN MQW ridge-waveguide lasers from sapphire onto Cu and diamond substrates in order to enhance device performance through materials integration. Device Fabrication and Transfer The laser structures were grown on (0001) c-plane sapphire substrates by MOCVD. A 2 m-thick GaN film was deposited, followed by a 100 nm thick SiO2 layer, which was subsequently patterned. The patterning resulted in an 8 m-wide stripe with a period of 11 m parallel to the GaN direction. MOCVD growth was then resumed with a 15 m-thick, Si-doped GaN layer, followed by a standard laser heterostructure . The device consisted of a 100 nm-thick Si-doped InGaN defect reducing layer; a 1 m-thick Si-doped AlGaN/GaN strained-layer-superlattice cladding layer; an active region comprised of three 3.5-nm-thick InGaN QWs sandwiched between 100 nm-thick GaN waveguiding layers; a 20 nm Mg-doped AlGaN tunnel barrier layer; a 500 nm-thick Mg-doped AlGaN cladding layer; and finally, a 50 nm-thick Mg-doped GaN contact layer. After MOCVD growth, 2 m ridge-waveguide structures were formed by etching into the AlGaN cladding layer using CAIBE, which was also used to fabricate mirrors. Metal contacts were then deposited on the top p-type GaN layer through openings in a silicon oxynitride overlayer. Finally, a highly reflective dielectric coating (R=90%) was deposited on the backside facet in order to minimize the mirror loss. The InGaN MQW laser structures were then bonded onto the surface of a B-doped, p-type Si(100) wafer using an ethyl cyanoacrylate (C6H7NO2)-based adhesive [see ]. Lift-off and transfer of the laser structures from sapphire onto the receptor Si substrate were accomplished by using a single 20 ns, 500 mJ/cm2 pulse from a XeCl pulsed-excimer laser (308 nm) directed through the transparent sapphire substrate. After the laser irradiation, a low-temperature (40C) anneal completed the separation process by melting the Ga-rich interface. In order to improve the structural rigidity and minimize the mechanical failure of the laser membranes and the facet mirror coatings, a secondary supporting layer consisting of a 5 m-thick indium film was deposited onto the exposed GaN interface before removal of the Si support substrate. Immersion of the In/laser/adhesive/Si structure in acetone, to dissolve the adhesive bond, then completed the fabrication of 1 cm2 free-standing membranes. The membranes were then transferred and bonded at 200C onto a Cu or diamond substrate by using the indium layer first as a structural support and subsequently as a bond interface. An advantage to removing the sapphire growth substrate is the ability to easily cleave mirror facets on the lasers before transfer . shows a cross- sectional scanning electron micros-copy view of a cleaved mirror facet for a typical ridge waveguide laser. The micrograph shows a very smooth topography along the surface of the facet with a surface roughness less than 0.5 nm measured by AFM over a 11 m2 area. The surface roughness was an order of magnitude smoother than typical CAIBE-etched facets. Laser Performance is a plot of the light output versus current and voltage (L-I-V) characteristics for a typical 2800 m2 ridge waveguide device under room-temperature CW operation after transfer from sapphire onto diamond. The threshold current for a transferred device on diamond was 87 mA at 5.2 V, with a 0.5 W/A differential slope efficiency. These devices showed no measurable degradation after the laser processing, which suggests that the optical properties of the transferred lasers were unaffected, and damage to the waveguide due to micro-cracking or deterioration of the mirror facet coatings had not occurred. The effectiveness of using a thermally and electrically conductive Cu substrate to improve InGaN-based laser performance is demonstrated in the laser output-power performance. shows the L-I curve for a typical 3500 m2 laser on sapphire compared to a laser on Cu with a backside n-contact. By using the Cu substrate as an efficient heat sink, it was possible to reduce the thermal resistance of the laser diode and raise the CW light output to more than 100 mW . The enhanced device performance further establishes the use of LLO as a tool for integrating GaN-based optoelectronics with virtually any substrate material, allowing one to independently optimize the growth conditions and diode performance that can later be combined with other platforms. References  S. Nakamura et al., Jpn. J. Appl. Phys., Part 2 37, L627 (1998).  W. Wong et al., J. Electron. Mater. 28,1409 (1999).  W. Wong et al., Appl. Phys. Lett. 77, 2822 (2000).  W. Wong et al., Appl. Phys. Lett. 75, 1360 (1999).  W. Wong et al., Jpn. J. Appl. Phys. 39, L1203 (2000).  M. Kneissl et al., Appl. Phys. Lett. 77, 1931 (2000).  M. Kneissl et al., submitted to IEEE J. Sel. Top. Quantum Eletron. (2001).