Bulk, single-crystal Ga2O3 was etched in BCl3/Ar inductively coupled plasmas as a function of ion impact energy. For pure Ar, the etch rate (R) was found to increase with ion energy (E) as predicted from a model of ion enhanced sputtering by a collision-cascade process, R ∝(E0.5 – ETH0.5), where the threshold energy for Ga2O3, ETH, was experimentally determined to be ∼75 eV. When BCl3 was added, the complexity of the ion energy distribution precluded, obtaining an equivalent threshold. Electrically active damage introduced during etching was quantified using Schottky barrier height and diode ideality factor measurements obtained by evaporating Ni/Au rectifying contacts through stencil masks onto the etched surfaces. For low etch rate conditions (∼120 Å min−1) at low powers (150 W of the 2 MHz ICP source power and 15 W rf of 13.56 MHz chuck power), there was only a small decrease in reverse breakdown voltage (∼6%), while the barrier height decreased from 1.2 eV to 1.01 eV and the ideality factor increased from 1.00 to 1.06. Under higher etch rate (∼700 Å min−1) and power (400 W ICP and 200 W rf) conditions, the damage was more significant, with the reverse breakdown voltage decreasing by ∼35%, the barrier height was reduced to 0.86 eV, and the ideality factor increased to 1.2. This shows that there is a trade-off between the etch rate and near-surface damage.
The surface of single-crystal (-201) oriented β-Ga2O3 was etched in BCl3/Ar inductively coupled plasmas under conditions (an excitation frequency of 13.56 MHz, a source power of 400 W, and a dc self-bias of −450 V) that produce removal rates of ∼700 Å min−1. Annealing at 400 and 450 °C was carried out after etching on Ni/Au Schottky diodes formed on the surface either before or after the annealing step. Current–voltage (I–V) measurements were used to extract the Schottky barrier height (Φ), diode ideality factor (n), and reverse breakdown voltage (VRB) for plasma damaged diodes after annealing. Annealing at 450 °C was found to essentially restore the values of Φ, n, and VRB to their reference (unetched) values on samples metallized after etching and annealing. Thermal annealing at either temperature of metallized diodes degraded their reverse breakdown voltage, showing that Ni/Au is not stable on β-Ga2O3 at these temperatures. Photoluminescence revealed a decrease in total emission intensity in the near band-edge region after the introduction of etch damage. Electron beam-induced current measurements showed a decrease in the minority carrier diffusion length from 350 μm in the control sample to 311 μm in the etched sample.
High ion density dry etching of bulk single-crystal β-Ga2O3 was carried out as a function of source power (100–800 W), chuck power (15–400 W), and frequency (13.56 or 40 MHz) in inductively coupled plasma (ICP) systems using Cl2/Ar or BCl3/Ar discharges. The highest etch rate achieved was ∼1300 Å min−1 using 800 W ICP source power and 200 W chuck power (13.56 MHz) with either Cl2/Ar or BCl3/Ar. This is still a comfortably practical set of conditions, where resist reticulation does not occur because of the effective He backside cooling of the sample in the tool and the avoidance of overly high powers in systems capable of 2000 W of source power. The etching is ion-assisted and produces anisotropic pattern transfer. The etched surface may become oxygen-deficient under strong ion-bombardment conditions. Schottky diodes fabricated on these surfaces show increased ideality factors (increasing from 1.00 to 1.29 for high power conditions) and reduced barrier heights (1.1 on reference diodes to 0.86 eV for etched surfaces). This electrically active damage is dependent on ion energy and flux during the etching. An obvious strategy is to reduce plasma powers toward the end of an etch sequence to reduce the disruption to the Ga2O3 surface.
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