Ion-beam processing effects on the thermal conductivity of n-GaN/sapphire (0001)

Abstract

We have measured high spatial/depth resolution (2–3 μm) thermal conductivity (κ) at 300 K before and after plasma-induced effects on two series of n-GaN sapphire (0001) samples fabricated by hydride vapor phase epitaxy using scanning thermal microscopy. The sample thicknesses were 50±5 μm for one set and 25±5 μm for the second. The carrier concentrations were ∼8×1016 cm−3 and ∼1.5×1017 cm−3, respectively, as determined by Hall effect measurements. The thermal conductivity before treatment was similar to that previously reported for hydride vapor phase epitaxy material with comparable carrier concentration and thickness [D. I. Florescu et al., J. Appl. Phys. 88, 3295 (2000)]. Damage was induced by ion-beam processing the samples under constant Ar+ gas flow and pressure for a fixed period of time (5 min), with the dc bias voltage (Vdc) being the only variable processing parameter (125–500 V). The thermal conductivity near the surface, κ, was found to exhibit a linear decrease with Vdc in the investigated range after this procedure. A second process was then applied in order to remove some damage. In this case the samples were processed under a constant mixture of Cl2 and Ar+ gas flow and Vdc′ of 50 V. For the samples with Vdc in the range 125 V<Vdc⩽250 V, κ was found to be actually lower after the damage removal process. The minimum κ was found at 250 V. This is probably due to Ar+ beam channeling [O. Breitschadel et al., Appl. Phys. Lett. 76, 1899 (2000)], which has been reported on similar structures at this voltage. When the initial processing voltage was 250 V<Vdc<500 V, κ showed a tendency to recover somewhat.