Jiancheng Yang

A review of Ga2O3 materials, processing, and devices

Gallium oxide (Ga2O3) is emerging as a viable candidate for certain classes of power electronics, solar blind UV photodetectors, solar cells, and sensors with capabilities beyond existing technologies due to its large bandgap. It is usually reported that there are five different polymorphs of Ga2O3, namely, the monoclinic (β-Ga2O3), rhombohedral (α), defective spinel (γ), cubic (δ), or orthorhombic (e) structures. Of these, the β-polymorph is the stable form under normal conditions and has been the most widely studied and utilized. Since melt growth techniques can be used to grow bulk crystals of β-GaO3, the cost of producing larger area, uniform substrates is potentially lower compared to the vapor growth techniques used to manufacture bulk crystals of GaN and SiC. The performance of technologically important high voltage rectifiers and enhancement-mode Metal-Oxide Field Effect Transistors benefit from the larger critical electric field of β-Ga2O3 relative to either SiC or GaN. However, the absence of clear demonstrations of p-type doping in Ga2O3, which may be a fundamental issue resulting from the band structure, makes it very difficult to simultaneously achieve low turn-on voltages and ultra-high breakdown. The purpose of this review is to summarize recent advances in the growth, processing, and device performance of the most widely studied polymorph, β-Ga2O3. The role of defects and impurities on the transport and optical properties of bulk, epitaxial, and nanostructures material, the difficulty in p-type doping, and the development of processing techniques like etching, contact formation, dielectrics for gate formation, and passivation are discussed. Areas where continued development is needed to fully exploit the properties of Ga2O3 are identified.

High reverse breakdown voltage Schottky rectifiers without edge termination on Ga2O3

Vertical geometry Ni/Au-β-Ga2O3 Schottky rectifiers were fabricated on Hydride Vapor Phase Epitaxy layers on conducting bulk substrates, and the rectifying forward and reverse current-voltage characteristics were measured at temperatures in the range of 25–100 °C. The reverse breakdown voltage (VBR) of these β-Ga2O3 rectifiers without edge termination was a function of the diode diameter, being in the range of 920–1016 V (average value from 25 diodes was 975 ± 40 V, with 10 of the diodes over 1 kV) for diameters of 105 μm and consistently 810 V (810 ± 3 V for 22 diodes) for a diameter of 210 μm. The Schottky barrier height decreased from 1.1 at 25 °C to 0.94 at 100 °C, while the ideality factor increased from 1.08 to 1.28 over the same range. The figure-of-merit (VBR2/Ron), where Ron is the on-state resistance (∼6.7 mΩ cm2), was approximately 154.07 MW·cm−2 for the 105 μm diameter diodes. The reverse recovery time was 26 ns for switching from +5 V to −5 V. These results represent another impressive advanc...

1.5 MeV electron irradiation damage in β-Ga2O3 vertical rectifiers

Vertical rectifiers fabricated on epi Ga2O3 on bulk β-Ga2O3 were subject to 1.5 MeV electron irradiation at fluences from 1.79 × 1015 to 1.43 × 1016 cm−2 at a fixed beam current of 10−3 A. The electron irradiation caused a reduction in carrier concentration in the epi Ga2O3, with a carrier removal rate of 4.9 cm−1. The 2 kT region of the forward current–voltage characteristics increased due to electron-induced damage, with an increase in diode ideality factor of ∼8% at the highest fluence and a more than 2 order of magnitude increase in on-state resistance. There was a significant reduction in reverse bias current, which scaled with electron fluence. The on/off ratio at −10 V reverse bias voltage was severely degraded by electron irradiation, decreasing from ∼107 in the reference diodes to ∼2 × 104 for the 1.43 × 1016 cm−2 fluence. The reverse recovery characteristics showed little change even at the highest fluence, with values in the range of 21–25 ns for all rectifiers.

Point defect induced degradation of electrical properties of Ga2O3 by 10 MeV proton damage

Deep electron and hole traps in 10 MeV proton irradiated high-quality β-Ga2O3 films grown by Hydride Vapor Phase Epitaxy (HVPE) on bulk β-Ga2O3 substrates were measured by deep level transient spectroscopy with electrical and optical injection, capacitance-voltage profiling in the dark and under monochromatic irradiation, and also electron beam induced current. Proton irradiation caused the diffusion length of charge carriers to decrease from 350–380 μm in unirradiated samples to 190 μm for a fluence of 1014 cm−2, and this was correlated with an increase in density of hole traps with optical ionization threshold energy near 2.3 eV. These defects most likely determine the recombination lifetime in HVPE β-Ga2O3 epilayers. Electron traps at Ec-0.75 eV and Ec-1.2 eV present in as-grown samples increase in the concentration after irradiation and suggest that these centers involve native point defects.

Annealing of dry etch damage in metallized and bare (-201) Ga2O3

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 A 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 b...

Inductively coupled plasma etch damage in (-201) Ga2O3 Schottky diodes

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 A 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 ...

Ohmic contacts on n-type β-Ga2O3 using AZO/Ti/Au

AZO interlayers between n-Ga2O3 and Ti/Au metallization significantly enhance Ohmic contact formation after annealing at ≥ 300°C. Without the presence of the AZO, similar anneals produce only rectifying current-voltage characteristics. Transmission Line Measurements of the Au/Ti/AZO/Ga2O3 stacks showed the specific contact resistance and transfer resistance decreased sharply from as-deposited values with annealing. The minimum contact resistance and specific contact resistance of 0.42 Ω-mm and 2.82 × 10-5 Ω-cm2 were achieved after a relatively low temperature 400°C annealing. The conduction band offset between AZO and Ga2O3 is 0.79 eV and provides a favorable pathway for improved electron transport across this interface.

Improvement of Ohmic contacts on Ga2O3 through use of ITO-interlayers

The use of ITO interlayers between Ga2O3 and Ti/Au metallization is shown to produce Ohmic contacts after annealing in the range of 500–600 °C. Without the ITO, similar anneals do not lead to linear current–voltage characteristics. Transmission line measurements were used to extract the specific contact resistance of the Au/Ti/ITO/Ga2O3 stacks as a function of annealing temperature. Sheet, specific contact, and transfer resistances all decreased sharply from as-deposited values with annealing. The minimum transfer resistance and specific contact resistance of 0.60 Ω mm and 6.3 × 10−5 Ω cm2 were achieved after 600 °C annealing, respectively. The conduction band offset between ITO and Ga2O3 is 0.32 eV and is consistent with the improved electron transport across the heterointerface.

Inductively coupled plasma etching of bulk, single-crystal Ga2O3

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 A 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...

10 MeV proton damage in β-Ga2O3 Schottky rectifiers

The electrical performance of vertical geometry Ga2O3 rectifiers was measured before and after 10 MeV proton irradiation at a fixed fluence of 1014 cm−2, as well as subsequent annealing up to 450 °C. Point defects introduced by the proton damage create trap states that reduce the carrier concentration in the Ga2O3, with a carrier removal rate of 235.7 cm−1 for protons of this energy. The carrier removal rates under these conditions are comparable to GaN-based films and heterostructures. Even annealing at 300 °C produces a recovery of approximately half of the carriers in the Ga2O3, while annealing at 450 °C almost restores the reverse breakdown voltage. The on/off ratio of the rectifiers was severely degraded by proton damage and this was only partially recovered by 450 °C annealing. The minority carrier diffusion length decreased from ∼340 nm in the starting material to ∼315 nm after the proton irradiation. The reverse recovery characteristics showed little change with values in the range 20–30 ns before...