Growth and characterization of wide bandgap semiconductor oxide thin films
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Wide bandgap semiconductors are receiving extensive attention due to their exceptional physical and chemical properties making them useful for high efficiency and high power electronic devices. Comparing other conventional wide bandgap materials, monoclinic β-Ga2O3 also represents an outstanding semiconductor oxide for next generation of UV optoelectronics and high temperature sensors due to its wide band gap (~4.9eV). This new semiconductor material has higher breakdown voltage (8MV/cm) and n-type conductivity which make it more suitable for potential application as high power electronics. The properties and potential applications of these wide bandgap materials have not yet fully explored. In this study, the growth and characterization of single crystal β-Ga2O3 thin films grown on c-plane sapphire (Al2O3) substrate using two different techniques; molecular beam epitaxy (MBE) and pulsed laser deposition (PLD) techniques has been investigated. The influence of the growth parameters of MBE and PLD on crystalline quality and surface has been explored. Two methods have been used to grow Ga2O3 using MBE; one method is to use elemental Ga and the second is the use of a polycrystalline Ga2O3 compound source with and without an oxygen source. Using the elemental Ga source, growth rate of β-Ga2O3 thin films was limited due to the formation and desorption of Ga2O molecules. In order to mitigate this problem, a compound Ga2O3 source has been introduced and used for the growth of crystalline β-Ga2O3 thin films without the need for additional oxygen since this source produces Ga-O molecules and additional oxygen. Two different alloys (InGa)2O3 and (AlGa)2O3 has been grown on c-plane sapphire substrate by pulsed laser deposition technique to tune the bandgap of the oxide thin films from 3.5-8.6 eV suitable for applications such as wavelength-tunable optical devices, solid-state lighting and high electron mobility transistors (HEMTs).
The crystallinity, chemical bonding, surface morphology and optical properties have been systematically evaluated by a number of in-situ and ex-situ techniques. The crystalline Ga2O3 films showed pure phase of (2 ̅01) plane orientation and in-plane XRD phi-scan exhibited the six-fold rotational symmetry for β-Ga2O3 when grown on sapphire substrate. The alloys exhibit different phases has been stabilized depending on the compositions. Finally, a metal-semiconductor-metal (MSM) structure deep-ultraviolet (DUV) photodetector has been fabricated on β-Ga2O3 film grown with an optimized growth condition has been demonstrated. This photodetector exhibited high resistance as well as small dark current with expected photoresponse for 254 nm UV light irradiation suggesting β-Ga2O3 thin films as a potential candidate for deep-UV photodetectors. While the grown Ga2O3 shows high resistivity, the electrical properties of (In0.6Ga0.4)2O3 and (In0.8Ga0.2)2O3 alloys show low resistivity with a high carrier concentration and increasing mobility with In content.