Recently, β-Ga₂O₃ has been extensively studied for power, optical, and RF electronics due to its large bandgap of 4.9 eV and high breakdown field of 8 MV/cm. However, most of the demonstrated devices are unipolar due to the lack of p-type Ga₂O₃, such as FETs and SBDs. This is primarily attributed to the absence of shallow acceptors in Ga₂O₃. As a solution, other p-type materials, such as NiO_x, have been utilized to produce β-Ga₂O₃ based p-n heterojunctions. Several NiO_x/β-Ga₂O₃ devices, such as p-n diodes and junction barrier Schottky diodes, have been demonstrated with excellent electrical properties. Moreover, due to its highly asymmetric monoclinic crystal structure, β-Ga₂O₃ exhibits anisotropic properties along different crystal orientations. However, the impacts of different β-Ga₂O₃ crystal orientations on NiO_x/β-Ga₂O₃p-n heterojunction are still unclear. In this work, we perform a systematic study of NiO_x/β-Ga₂O₃p-n heterojunctions on (-201), (001), and (010) crystal orientations. The EFG-grown β-Ga₂O₃ substrates were acquired with a similar n-type doping concentration and thickness. First, the substrates were cleaned using acetone, IPA, and DI water. The Ti/Au (20/130 nm) back contacts were deposited by E-beam evaporation followed by rapid thermal annealing at 500 °C in N₂. Then, standard photolithography was performed to define patterns for deposition of NiO_x and the anode. 200 nm NiO_x and the anode Ni/Ti/Au (20/15/100 nm) were deposited using E-beam evaporation, followed by a liftoff process to isolate devices. I-V and C-V measurements were performed using a 4200 SCS semiconductor parameter analyzer. All devices show an excellent rectification with on/off ratio >10⁹. I-V measurements indicate a turn-on voltage of 2.09, 2.22, and 2.50 V, an ideality factor of 1.95, 2.03, and 2.13, and an on-resistance of 2.92, 1.55, and 6.50 mΩ.cm² for (-201), (001), and (010) devices, respectively. C-f measurements indicated an interface state density of 4.3 × 10¹⁰, 7.4 × 10¹⁰, and 1.6 × 10¹¹ eV^–1cm^–2 for (-201), (001), and (010) plane devices, respectively. Furthermore, the reverse recovery of the diodes shows a slight difference between (-201) [or (001)] and (010) devices due to the anisotropic nature of β-Ga₂O₃. These differences in the electrical properties are attributed to the different atomic configurations, the density of dangling bonds, and conductivity-modulated hole injection into β-Ga₂O₃. Further investigation through temperature-dependent measurements will reveal more information about the anisotropic nature of NiO_x/β-Ga₂O₃p-n heterojunctions.

In addition to stable and meta-stable crystalline phases of Ga₂O₃, its amorphous phase is also being explored for various applications owing to its technological advantage like room temperature and large area growth for next generation solar-blind photodetectors .[1] But being often sub-stoichiometric and replete with oxygen vacancies, performance of amorphous thin film PDs is far below their crystalline counterparts. Hence, novel methods and techniques need to be adopted to improve performance of such devices.[2] Herein, we report one such method where ultra-thin films of amorphous Ga₂O₃ are deposited on nanopatterned SiO₂ coated Si substrate. Controlled nanopatterns or ripple formation is carried out by irradiating SiO₂/Si substrate with 500eV Ar⁺ ions for variable times. Morphology of ripples formed on substrate is studied using AFM images to calculate the characteristic wavelength and Power Spectral Density. Amorphous Ga₂O₃ (~5nm) is then sputtered onto these rippled substrates using RF Magnetron Sputtering at room temperature. Uniformity of Ga₂O₃ is confirmed by EDX elemental mapping studies. Reflectance measurements were carried out using UV-Vis spectroscopy and results showed lower reflectance as compared to non-rippled because of the enhanced absorption due to multiple scattering by the substrate and enhanced surface area. FDTD was used to simulate reflectance measurements which are well in agreement with experimental results.

The performance of subsequently fabricated photodetectors showed that conformally coated devices had an enhanced performance as compared to non-rippled “bare” device. The solar-blind PDs show an increase in the responsivity from ~3 mA W^-1to 433 mA W^-1 – an increment of more than 140 times at +5V. For the response times, bare device shows slow rise/fall time of 0.37s /0.39s whereas the conformal devices showed an ultrafast rise time of 896µs and fall time of 710µs, respectively, to 254 nm incident light. The detailed analysis showed that the device performance can be attributed to the incorporation of elemental Si from the substrate below, the presence of which is confirmed by XPS. This study shows how amorphous Ga₂O₃ films can be used to fabricate ultra-fast devices, especially for next-generation solar-blind PD applications.

References:

1. Kaur, D. and M. Kumar, A Strategic Review on Gallium Oxide Based Deep-Ultraviolet Photodetectors: Recent Progress and Future Prospects. Advanced Optical Materials, 2021. 9(9): p. 2002160.
2. Kaur, D., et al., Surface nanopatterning of amorphous gallium oxide thin film for enhanced solar-blind photodetection. Nanotechnology, 2022. 33(37): p. 375302.