Investigation of Processing and Annealing Conditions of Ohmic and Schottky Contacts on β-Ga2O3 with Enhanced Electrical Characteristics

The prospects for growth of gallium oxide make it a promising wide band gap semiconductor. During the last 10 years, β-Ga₂O₃ has developed at a rapid rate, and a variety of new data has been produced on metal/ Ga₂O₃ interactions. When designing high-power electronic devices, it is crucial to understand the electrical properties of Schottky and ohmic contacts on β-Ga₂O₃. With an emphasis on improved thermal stability and electrical qualities, Schottky and ohmic contacts are studied. 

We investigate the electrical properties of Co/Au, Ni/Au and a Ni-based alloy as Schottky contacts on single-crystal n-type β-Ga₂O₃ substrates as a function of extended annealing time at 300 °C and 500 °C in air. Co and Ni were chosen because of their high work functions (5 eV and 5.25 eV, respectively) and previous work by our group that showed near-ideal Schottky diode characteristics with an ideality factor (n) of ~1.05 for these metals on (100) 𝛽-Ga₂O₃¹. In addition, positive free energies of reaction to form metal-oxides (NiO, Co₃O₄, and CoO) from Ga₂O₃ were calculated at 25–500 °C using the Factsage Database.² This analysis indicates that Ni or Co should not reduce Ga₂O₃ to form Ni-oxide or Co-oxide at the interface, although it does not preclude the possibility of other reactions and reaction products. An Inconel 600 alloy that is based on Ni was also investigated because of its high-temperature corrosion resistance. Electrical properties of the contacts were determined from current-voltage (I-V) and capacitance-voltage (C-V) measurements conducted after sequential annealing treatments. Selected samples were characterized using high-resolution transmission electron microscopy (TEM), energy dispersive x-ray (EDX) spectroscopy, and scanning electron microscopy (SEM) to investigate changes in the morphology, microstructure, and chemical composition of the contacts after annealing.

Thin (40–150 nm), highly-doped n+ (10¹⁹ – 10²⁰ cm^-3) Ga₂O₃ layers deposited using pulsed laser deposition (PLD) were incorporated into Ti/Au ohmic contacts on (001) and (010) β-Ga₂O₃ substrates with carrier concentrations between 2.5–5.1 × 10¹⁸ cm^-3. Specific contact resistivity values were calculated for contact structures both without and with a PLD layer having different thicknesses up to 150 nm. With the exception of a 40 nm PLD layer on the (001) substrate, the specific contact resistivity values decreased with increasing PLD layer thickness: up to 8x on (001) Ga₂O₃ and up to 17x on (010) Ga₂O₃ compared to the samples without a PLD layer. The lowest average specific contact resistivities were achieved with 150 nm PLD layers: 3.48 × 10^-5 Ω cm² on (001) Ga₂O₃ and 4.79 × 10^-5 Ω cm2 on (010) Ga₂O₃. Cross-section TEM images revealed differences in the microstructure and morphology of the PLD layers on the different substrate orientations. This study describes a low-temperature process that could be used to reduce the contact resistance in Ga₂O₃ devices. 

Lastly, the electrical properties of Ti/Au ohmic contacts on (001) and (010) single-crystal n-type Sn-doped β-Ga₂O₃ substrates with and without a 150 nm highly-doped n+ (10¹⁹ – 10²⁰ cm³) Ga₂O₃ layers deposited using PLD as a function of extended annealing time at 150 °C and 300 °C in air was investigated. Electrical properties of the contacts were determined from currentvoltage (I-V) and specific contact resistivity values were calculated for contact structures both without and with a PLD layer. The results indicated that the use of a 150 nm PLD layer could improve the electrical properties of Sn-doped β- Ga₂O₃ substrates, resulting in lower specific contact resistivity. However, the PLD layer ceased to enhance the device's electrical characteristics after 150 hours of annealing at 300°C. The specific contact resistivity remained stable for the (001) Sn-doped substrate and for the (010) Sn-doped substrate with a 150 nm of n+ PLD layer after 500 and 400 hrs. of annealing at 150°C. Results suggest that the use of a PLD layer can improve the electrical properties of Sn-doped β-Ga₂O₃ substrates, leading to a lower specific contact resistivity.