CW InGaN Multiple-Quantum-Well Laser Diodes on Copper Substrates

Abstract

Continuous-wave (cw) InGaN multiple-quantum-well laser diodes grown on sapphire substrates by metalorganic chemical vapor deposition (MOCVD) were successfully transferred onto copper using an excimer laser lift-off technique. For the laser diodes on copper substrates improved device performance was observed with room-temperature cw threshold currents as low as 68 mA and threshold voltages of 5.9 V. Differential quantum efficiencies of 0.7 W/A were obtained with a laser emission wavelength near 400 nm. GaN-based laser structures transferred onto copper substrates show a significantly reduced thermal resistance resulting in a more than twofold increase in cw output power to more than 100 W. High quality cleaved facets have been obtained for freestanding GaN laser membranes after sapphire substrate removal.Introduction Since the first demonstration of a pulsed InGaN multiple-quantum-well (MQW) laser diode grown on sapphire substrate in 1995 [1], GaN-based laser diodes have reached extraordinary levels of performance with the currently most advanced devices operating under cw conditions and lifetimes greater than 15000 h [2-4]. Less than two years ago the first commercialization of a violet laser diodes on sapphire substrate has been announced [5] aiming towards a number of applications, particularly optical storage (e.g. DVD-RW or DVR) and laser printing. Although the current levels of performance may already be sufficient for some of these applications, a number of important issues still remain to be solved to enable a wider range of applications.One of the critical issues for GaN-based laser devices on sapphire is efficient heat dissipation. The relatively high operating currents and voltages of InGaN MQW laser diodes together with the poor thermal conductivity of sapphire are causing the device to heat during operation and as a consequence the device performance degrades. This is particularly problematic for applications, which require a high output power or applications in which little or no fluctuations of the output power can be tolerated. For example, transient heating remains a very critical issue for laser diodes in printing applications. When a laser diode is forward biased at a constant current above threshold, laser operation is obtained very quickly (typically $ns), while the device temperature continues to increase until it reaches equilibrium (typically several hundred ms). Because of this transient heating effect the output power of the laser diode drops as the threshold current of the device increases. For red and IR laser diodes in printing applications the allowed drop in the transient laser light output power (also known as "droop") is typically in the order of <5% [6, 7] and those requirements would be simi-