TUNGSTEN METALLIZATION FOR STABLE AND SELF-ALIGNED INP-BASED LASER DEVICES

Tungsten thin films on n-InP layers have been investigated for potential use as a refractory ohmic contact for self-aligned In-based etched mesa buried heterostructure laser devices. The W films were rf sputter deposited onto InP substrates, S doped in the range of 1×1018- 1×1019 cm-3. The deposition parameters were optimized to produce films with the lowest possible induced stress, minimum argon content, and best morphology for as-deposited wafers and after undergoing reactive-ion etching and high-temperature thermal cycles (700 °C), which are required for the self-aligned technology. These parameters were obtained for films that were rf sputter deposited at a discharge power of 240 W and under argon pressure of about 10 mTorr. A thermal expansion coefficient of 5.84×10-6 C-1 and a biaxial elastic modulus of 0.97×1012 Pa were measured for the films. The electrical behavior of the W films sputtered onto n-InP was studied by means of I-V and contact resistance measurements, which revealed a linear ohmic contact as-deposited, while sputtered onto n-InP substrates, S doped to the level of 5×1018 cm-3 or higher. Rapid thermal processing at elevated temperatures improved the ohmic contact quality and decreased the specific contact resistance values to a minimum of 3.5×10-6 Ω cm2 as a result of heating the W/InP (S doped 1×1019 cm-3) at 600 °C.