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

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±5μm for one set and 25±5μm for the second. The carrier concentrations were ∼8×10 ¹⁶cm ^-3 and ∼1.5×10 ¹⁷cm ^-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 (V _dc) being the only variable processing parameter (125-500 V). The thermal conductivity near the surface, κ, was found to exhibit a linear decrease with V _dc 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 Cl ₂ and Ar ⁺ gas flow and V _dc ^′ of 50 V. For the samples with V _dc in the range 125V<V _dc≤250V, 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 250V<V _dc<500V, κ showed a tendency to recover somewhat.