Strategies towards realization of high performance solar-blind photodetectors based on gallium oxide thin films

Abstract

The UV-C (< 280 nm of wavelength) part of the solar spectrum is highly dangerous to living organisms, with exposure often causing changes in their genetic make-up. Thankfully, the ozone layer in the atmosphere absorbs almost all of this radiation, thereby providing us with natural protection. The complete absorption leads to a very low background signal of UV-C on earth. This unique characteristic can be exploited in the fabrication of highly sensitive photodetectors which are responsive to UV-C only from other sources on earth, leading to the development of the so-called solar-blind photodetectors (SBPD). Therefore, SBPDs have applications ranging from civil to medical uses which includes ozone-hole monitoring, flame detection, space exploration, UV sterilization systems, non-line-of-sight communications and hydrogen flame detection. Gallium oxide is a new, ultra-wide Band Gap (UWBG) material with properties that are promising for its use as a functional material in the fabrication of SBPDs. It has an apt band gap of 4.6-5.3 eV (depending on the polymorph) making it intrinsically solar-blind. Moreover, it is highly chemically and thermally stable (m.p. ~ 1730°C), has a high breakdown voltage and is extremely radiation hardened and hence it can be employed for use in extreme environment applications (high temperature and high radiation conditions). For photodetection, however, the trade-off between two key parameters viz. responsivity and response time has become one of the main bottlenecks in achieving high performance. Herein, we aim to develop high performance gallium oxide based SBPDs and study its use in extreme environment applications. The thesis begins by optimizing the active layer of gallium oxide. First, polycrystalline thin films are optimized on different orientations of Si substrate and the strain induced differences in photodetector performance were studied. Polycrystalline films were also optimized on the sapphire c-plane sapphire substrate which has a lower lattice mismatch (~6.6%) with Ga2O3 by depositing varying thicknesses via RF magnetron and it was found that thicker films have a lower number of oxygen vacancies and a better stoichiometric ratio and hence can be utilized for fabricating better and faster optoelectronic devices. We have also found room temperature crystallinity in Ga2O3 by using a facile fabrication with conventional sputtering technique. The subsequently grown photodetectors show an ultra-high photo-to-dark current ratio of over 105 at a moderate bias of 10 V bias under 254 nm illumination. In the second part of the thesis, the speed of the amorphous Ga2O3 photodetectors were enhanced by using surface modification. First, we have modified the surface of a 300 nm amorphous Ga2O3 film by nanopatterning it using 500 eV Ar+ ion beam sputtering. The defects introduced in the system act as recombination centers for the charge carriers bringing about a reduction in the decay time of the devices, even at zero-bias. Second, instead of modifying the active layer, we have modified the surface of the substrate used underneath and conformally coated an ultra-thin amorphous Ga2O3 on top. Ion-beam sputtering (500eV Ar+) is utilized to nanopattern SiO2 coated Si substrate leaving the topmost part rich in elemental Si, which enhances the carrier conduction by increasing n-type doping of the subsequently coated Ga2O3 films. The metal-semiconductor-metal (MSM) photoconductor devices fabricated on doped, rippled films show superior properties with responsivity increasing from 6 mAW-1 to 433 mAW-1 while having fast detection speeds of 861 µs/710 µs (rise/fall time). The third part of the present thesis focusses on utilizing heterostructures for improving the performance. The amalgamation of two different materials is highly dependent on the interface between the two materials. The end result of how two different materials will integrate is highly dependent on their interface chemistry. This is shown by interfacing amorphous Ga2O3 with two different sulfide materials – CdS and PtS. For the CdS- Ga2O3 heterostructure, the resultant devices remain solar-blind and outperform the singular bare photodetectors. For the PtS- Ga2O3 heterostructure, even though the band alignment still remains type I, the resultant devices show a broadband photoresponse instead of the solar-blind nature. Thus, this shows that the interfacing of two materials for enhanced photodetection requires a thorough and complete understanding of the interfacial dynamics and charge transfer from one material to another and therefore, warrants careful consideration of optimization parameters before implementation. In the last part of the thesis, we try to look at the impact of extreme environments such as high temperatures and swift heavy ion radiation on the fabricated photodetectors, since the active layer itself is extremely resilient to extreme temperatures and high radiation doses. In the first part, we study the origin of the near-failure of conventional Au contacts to β-Ga2O3 at high temperatures using interfacial studies. Next, an unconventional transparent conducting oxide contact of indium zinc oxide (IZO) to β-Ga2O3 is studied under high temperatures. The devices show a unique conversion from Schottky to Ohmic by annealing at an optimized temperature of 650°C, while changing back to Schottky at higher temperatures. Finally, the radiation hardness of gallium oxide is checked against swift heavy ions Ag7+ of 100 MeV and the device performance judged under control and irradiated conditions.