(Invited) Exploration of Process Techniques for Ga2O3 Based Electronics
Authors: Ren Fan , Pearton Stephen J. , Yang Jiancheng , Carey Patrick , Ahn Shihyun , Khanna Rohit , Bevlin Kristen , Geerpuram Dwarakanath , Kuramata Akito 
Published: February 2020

Monoclinic β-phase Ga2O3 has outstanding potential for power electronics, and high quality, large diameter bulk crystals and epitaxial layers of Ga2O3 are already available with a range of controllable n-type doping levels by edge-defined film-fed (EFG) growth using iridium crucibles, by Czochralski or by float zone. The direct energy bandgap of Ga2O3, ∼4.9 eV, yields a very high theoretical breakdown electric field (∼8 MV/cm). For power electronics, the Baliga figure-of-merit proportional carrier mobility, critical electric field and breakdown voltage, is almost four times higher for Ga2O3 than for GaN. Currently, the major limitation for Ga2O3 based device fabrication is the lack of low resistance Ohmic contacts, low damage dry etching process, and thermally stable Schottky contacts. In this work, we report a technique by employing Aluminum Zinc Oxide (AZO) to improve Ohmic contacts on Ga2O3, studies of etch rates and etching induced damages with Cl2/Ar and BCl3/Ar based discharge as well as Ni/Au and Pt/Au Schottky contacts.

High quality Ohmic contacts are a prerequisite for any device to deliver low contact resistance at moderate anneal temperatures. Additional contact resistance leads to slower device switching speeds as well as reliability issues due to local contact heating from high contact resistance during device operation. AZO thin films have been widely studied for transparent and flexible device applications such as liquid crystal displays, plasma display panels, electronic paper displays, organic light emitting diode, solar cells, touch panels, gas sensors and other optoelectronics devices. To be used as a contact layer on Ga2O3, a band alignment to ensure a less barrier to electron transport from the contact metal layer via AZO into the Ga2O3.is a prerequisite. The AZO/Ga2O3 heterojunction was realized with a nested gap alignment of

band offsets with a valence band offset of 0.61eV ± 0.06eV and a conduction band offset of 0.79 eV ± 0.80 eV determined from x-RayPhotoelectron Spectroscopy (XPS) measurements. This is useful information for Ohmic contact schemes on n-type Ga2O3 devices. A minimum transfer resistance and specific contact resistance of 0.42 Ω-mm and 2.82 ´ 10-5 Ω-cm2 were achieved after a relative low annealing temperature of 400°C. It is essential to generate high resolution pattern transfer processes on semiconductor with wet chemical or dry etching. While numerous wet etchants been reported, these etching processes involve strong acids and elevated temperatures, and the standard positive photoresists will not be able to use as the etching mask. For dry etching, although, the etching rates are generally low, and the resultant oxygen deficient surfaces, standard device patterning and process are compatible with Ga2O3 based device fabrication. Both Cl2/Ar and BCl3/Ar discharges were used to etch high quality bulk Ga2O3 with a Plasma-Therm Inductively Coupled Plasma (ICP) system in this work. The etching is ion-enhanced and leads to a reduction of Schottky barrier height and diode reverse breakdown voltage in diodes fabricated on the etched surfaces depending on the rf and IPC powers. For low rf and ICP power conditions, the reverse breakdown voltage decreased around 6%, while barrier height reduced from 1.2 to 1.01 eV. Under higher power conditions, the reverse breakdown voltage decreased around 35%, and the barrier height was reduced to 0.86 eV. For Schottky diodes, Ni/Au or Pt/Au were used to form Schottky contact on bulk or epitaxial β- Ga2O3. The barrier heights were 1.07 and 1.04 eV for Ni/Au and Pt/Au, respectively, at 25°C. The diode on-state resistances (Ron) decreased with increasing temperature. The temperature coefficient of reverse breakdown voltage (VB), β, was −4 mV/K for Ni/Au and −0.1 mV/K for Pt/Au. The figure-of-merit (VB
2/RON) was above 3 MW/cm2 at 25°C for Ni/Au diodes and was still ∼1 MW/cm2 at 200°C. The reverse recovery times were also measured as a function of temperature and were of the order of 21–28 ns over the range 25–150°C.