Growth of GaN from Elemental Gallium and Ammonia via a Modified Sandwich Growth Technique

Abstract

Gallium nitride (GaN) thin films were grown on (0001) sapphire substrates at 1050°C by controlled evaporation of gallium (Ga) metal and reaction with ammonia NH3. The feasibility of the growth process was demonstrated and discussed. One of the biggest challenges of working in the Ga–NH3 system was the instability of molten Ga under NH3 atmosphere at elevated temperatures, especially between 1100–1200°C.

In the first part of the study, transport of Ga species from the source-to-substrate during the GaN growth process and the influence of ammonia—liquid Ga reaction on Ga transport were investigated. Experimental results under different conditions were studied and compared to theoretical predictions to quantify the mechanism of transport in the vapor growth technique. In presence of NH3, Ga transport far exceeded the predicted upper limit for the vapor phase transport. Visual observations confirmed that a significant amount of Ga left the source in a cluster rather than atomic form.

A novel Ga source design was employed in an effort to obtain a stable and high vapor phase transport of Ga species at moderate temperatures. In this design, pure N2 was flowed directly above the molten Ga source. This flow prevented the direct contact and reaction between the molten Ga and NH3 and prevented Ga spattering and GaN crust formation on the source surface. At the same time, it significantly enhanced Ga evaporation rate and enabled control of Ga transport and V/III ratio in the system.

Growth characteristics were described by a mass transport model based on process parameters and experimentally verified. The results showed that the process was mass transport limited and the maximum growth rate was controlled by transport of both Ga and reactive ammonia species to the substrate surface. A growth rate of 1.4 μm/h was obtained at 1050C, 800 Torr, 3 slm of ammonia flow rate, and 1250C Ga source temperature at a 24 mm source-to-substrate distance. It was found that the process required a more effective supply of active NH3 to the substrate in order to increase the crystal quality and growth rate.

The surface morphology of the deposited layers was examined by optical and scanning electron microscopies. XRD analysis was used to determine the crystallinity of deposited films and revealed a full-width at half-maximum (FWHM) of 0.6 deg. for the (0002) GaN peak. EDX analysis was employed for the chemical characterization of the samples and showed that the deposited material contained only Ga an N elements. Room temperature PL spectrum demonstrated the optical quality of the grown samples.