Shihyun Ahn

Effect of front and back gates on β-Ga2O3 nano-belt field-effect transistors

Field effect transistors (FETs) using SiO2 and Al2O3 as the gate oxides for the back and front sides, respectively, were fabricated on exfoliated two-dimensional (2D) β-Ga2O3 nano-belts transferred to a SiO2/Si substrate. The mechanical exfoliation and transfer process produced nano-belts with smooth surface morphologies and a uniform low defect density interface with the SiO2/Si substrate. The depletion mode nanobelt transistors exhibited better channel modulation with both front and back gates operational compared to either front or back-gating alone. The maximum transconductance was ∼4.4 mS mm−1 with front and back-gating and ∼3.7 mS mm−1 with front-gating only and a maximum drain source current density of 60 mA mm−1 was achieved at a drain-source voltage of 10 V. The FETs had on/off ratios of ∼105 at 25 °C with gate-source current densities of ∼2 × 10−3 mA mm−1 at a gate voltage of −30 V. The device characteristics were stable over more than a month for storage in air ambient and the results show the ...

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...

Effect of 5 MeV proton irradiation damage on performance of β-Ga2O3 photodetectors

Planar thin film β-Ga2O3 photodetectors were irradiated with 5 MeV protons at doses from 1013 to 1015 cm−2, and the resulting effects on photocurrent, responsivity, quantum efficiency, and photo-to-dark current ratio at 254 nm wavelength were measured at both 25 and 150 °C. The photocurrent increased with dose due to the introduction of damage from nonionizing energy loss by the protons. The total calculated vacancy concentration increased from 5 × 1015 to 5 × 1017 cm−3 over the dose range investigated. The dark current increased in proportion with the implant dose, leading to a decrease in the ratio of photocurrent to dark current. The photocurrent induced by 254 nm illumination increased with dose, from ∼0.3 × 10−7 A at 25 °C for a dose of 1013 cm−2 to ∼10−6 A at a dose of 1015 cm−2 at a fixed light intensity of 760 μW/cm2. The photo-to-dark current ratio decreased from ∼60 in the control samples to ∼9 after proton doses of 1015 cm−2, with corresponding external quantum efficiencies of ∼103% in control ...

Characteristics of gate leakage current and breakdown voltage of AlGaN/GaN high electron mobility transistors after postprocess annealing

The effects of postprocess annealing on the gate leakage current and breakdown voltage characteristics of AlGaN/GaN high electron mobility transistors (HEMTs) was investigated. The fabricated AlGaN/GaN HEMTs were postannealed at 250, 300, 350, 400, or 450 °C under a nitrogen (N2) atmosphere by using rapid thermal annealing, and both direct current (dc) and pulsed measurements were performed to characterize the changes in device performance. The reverse gate leakage current (IG) at VG = −10 V was reduced by one order of magnitude and the off-state drain breakdown voltage (Voff) increased by over three-fold after postprocess annealing at 450 °C. The reverse gate leakage current was found to be independent of gate-to-drain potential after annealing. The gate pulse measurements revealed the activation of deep traps during the postannealing at elevated temperatures.

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 ...

Effect of proton irradiation energy on AlGaN/GaN metal-oxide semiconductor high electron mobility transistors

The effects of proton irradiation energy on dc characteristics of AlGaN/GaN metal-oxide semiconductor high electron mobility transistors (MOSHEMTs) using Al2O3 as the gate dielectric were studied. Al2O3/AlGaN/GaN MOSHEMTs were irradiated with a fixed proton dose of 5 × 1015 cm−2 at different energies of 5, 10, or 15 MeV. More degradation of the device dc characteristics was observed for lower irradiation energy due to the larger amount of nonionizing energy loss in the active region of the MOSHEMTs under these conditions. The reductions in saturation current were 95.3%, 68.3%, and 59.8% and reductions in maximum transconductance were 88%, 54.4%, and 40.7% after 5, 10, and 15 MeV proton irradiation, respectively. Both forward and reverse gate leakage current were reduced more than one order of magnitude after irradiation. The carrier removal rates for the irradiation energies employed in this study were in the range of 127–289 cm−1. These are similar to the values reported for conventional metal-gate high-...

High breakdown voltage in AlN/GaN metal–insulator–semiconductor high-electron-mobility transistors

The breakdown characteristics of AlGaN/GaN based metal–insulator–semiconductor high-electron-mobility transistors (MISHEMTs) using a 10 nm thick AlN gate insulator and passivation layer deposited plasma enhanced atomic layer deposition. The AlN was effective in significantly reducing gate leakage current relative to Schottky gate devices and showed only small decreases in drain current during gate lag measurements. The devices exhibited a strong dependence of gate breakdown voltage on source–drain distance, reaching a value of 2000 V for a source–drain distance of 40 μm limited by the measurement instrument. The specific on-state resistance was 1.3 and 10.9 mΩ cm2 for the devices with the gate–drain distance of 7.5 and 37.5 μm, respectively. The saturation drain current was inversely dependent on source–drain distance and the on–off ratios were in excess of 108 due to the low gate leakage current in the MISHEMTs.

Low dose 60Co gamma-irradiation effects on electronic carrier transport and DC characteristics of AlGaN/GaN high-electron-mobility transistors

ABSTRACT The impact of internal irradiation with secondary Compton electrons, generated by gamma-photons, on the characteristics of III-N/GaN-based devices was explored. N-channel AlGaN/GaN high-electron-mobility transistors (HEMTs) were exposed to gamma-radiation from a 60Co source for doses up to 600 Gy. Temperature-dependent electron beam-induced current (EBIC) was employed to measure minority carrier transport properties. For low doses below ∼250 Gy, the minority carrier diffusion length in AlGaN/GaN HEMTs is shown to increase by about 40%. This increase is likely due to longer minority carrier lifetime induced by internal Compton electron irradiation. An associated decrease in activation energy, extracted from temperature-dependent EBIC, was also found. The obtained increase in transconductance and decrease in gate leakage current indicate an improvement in performance of the devices after low doses of irradiation. For high doses of gamma-irradiation, above ∼300 Gy, the performance of HEMTs showed a deterioration. The deterioration results from the onset of increased carrier scattering due to additional radiation-induced defects, as is translated in a decrease of minority carrier diffusion length.

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...

Elevated temperature performance of Si-implanted solar-blind β-Ga2O3 photodetectors

The temperature dependent photoresponse of planar photodetectors fabricated on β-Ga2O3 films grown on Al2O3 by metalorganic chemical vapor deposition to 254 nm wavelength, and blindness to 365 nm light, are reported over the range of 25–350 °C. Ohmic contacts were formed by Si-implantation and annealing at 900 °C, followed by deposition of Ti/Au metallization. The photocurrent induced by 254 nm illumination increased monotonically with temperature, from ∼2.5 × 10−7 A at 25 °C to ∼2.2 × 10−6 A at 350 °C at a fixed 254 nm light intensity of 760 μW/cm2. The photosensitivity decreases at high temperatures in many photoconductors (thermal quenching), in sharp contrast to the photosensitivity increase with high temperatures in this study. This is ascribed to the presence of states in the gap of Ga2O3, whose presence was proven by exposure to below band-gap energy. In this case, the current still increased due to the presence of defect levels in the band gap and the generation of photocurrent is due to a transit...