Materials Science in Semiconductor Processing 121 (2021) 105430 Available online 12 September 2020 1369-8001/© 2020 Published by Elsevier Ltd. Full length article Electrical and photocurrent properties of a polycrystalline Sn-doped β-Ga 2 O 3 thin film Youngbin Yoon a , Sunjae Kim a , In Gyu Lee a , Byung Jin Cho b , Wan Sik Hwang a, * a Department of Materials Engineering, Korea Aerospace University, Goyang, 10540, Republic of Korea b School of Electrical Engineering, KAIST, Daejeon, 34141, Republic of Korea A R T I C L E INFO Keywords: Ga 2 O 3 Thin film Spin-on-glass Metal-oxide semiconductor field-effect transis- tors Solar-blind photodetectors Sputtering ABSTRACT Polycrystalline n-type β-Ga 2 O 3 thin films with a thickness of 100 nm are demonstrated via the sputtering process followed by the spin-on-glass Sn-doping technique. The thin films are used as an active layer for power elec- tronics and ultraviolet optoelectronics. In the present study, they are implemented in back-gated metal-oxide- semiconductor field-effect transistors with a 300-nm thick SiO 2 gate dielectric. The fabricated device shows typical power electronic properties with high breakdown voltages (as high as 216 V). The device also shows a clear photoresponse to the 254-nm light illuminations, indicating that the polycrystalline β-Ga 2 O 3 thin film is also suitable for solar-blind photodetectors. 1. Introduction Ga 2 O 3 semiconductors with an ultra-wide bandgap (UWB) in the range of 4.6–4.9 eV have been studied for more than a half-century [1], but they have recently gained significant attention due to the huge de- mand for the efficient conversion of electrical energy [2,3]. Though Ga 2 O 3 has been observed to exist in five polymorphs (α-, β-, γ-, δ-, and ε-phases) [1], device applications have primarily employed β-Ga 2 O 3 because it is the most thermodynamically stable phase [4]. The energy band structure of β-Ga 2 O 3 reveals that it exhibits UWB characteristics as well as almost direct bandgap behavior, both of which are beneficial to a wide range of applications from switching high-power conversion to converting photons to electrons and vice-versa. However, intrinsic β-Ga 2 O 3 exhibits poor conductivity and high contact resistance due to an ultra-wide bandgap of 4.9 eV, making it impractical for device appli- cations at room temperature. To realize Ga 2 O 3 -based power electronics and optoelectronics [5–10], the conductivity and contact resistance of the Ga 2 O 3 semiconductor can be improved by incorporating dopants. The depletion layer width between the metal-semiconductor interface of power electronics and ultraviolet optoelectronics becomes smaller as the doping concentration increases, which can enhance device performance. In addition, the trapping of free carriers by the dopant-induced defect states can reduce the dark current and enhance the sensitivity of the optoelectronics [11]. Although p-type doping in Ga 2 O 3 has proven to be a significant challenge [12], n-type doping has been actively studied [13–15] using group IV elements such as Si, Ge and Sn. Among the Si, Ge, and Sn dopants, Sn is promising because the radii of Sn 4+ ions (55 ~ 81 p.m.) are comparable to those of Ga 3+ (62 p.m.) [16,17]. Accord- ingly, Sn dopants can serve as a substitute for Ga atoms in an octahedral site without noticeable deformation [18] and form a shallow donor level [19–21]. Conventionally, the doping process is conducted using an ion im- plantation system in Si-based semiconductors because it allows precise control of the dopant profile. However, it is difficult to dope a very shallow region in a thin film. In this work, polycrystalline Sn-doped Ga 2 O 3 thin films with a thickness of 100 nm were demonstrated. To form the n-type Ga 2 O 3 , the Sn was diffused in the Ga 2 O 3 via the spin-on- glass (SOG) technique. Unlike a single-crystal Ga 2 O 3 film, poly- crystalline Ga 2 O 3 could currently be prepared with a high throughput at a low-cost wafer-scaled size. In addition, the disorder at the grain boundaries (GB) in polycrystalline Ga 2 O 3 thin films provides high- diffusivity paths along with dopants [22], which can lower the dopant diffusion temperature compared to that of single-crystal Ga 2 O 3 [15]. The formed polycrystalline n-type Ga 2 O 3 thin film was then used as an active layer for power electronics and ultraviolet (UV) optoelectronics. 2. Experimental details First, amorphous Ga 2 O 3 thin films with a thickness of 100 nm were deposited on a SiO 2 (300 nm)/highly doped p-type Si substrate using a * Corresponding author. E-mail address: whwang@kau.ac.kr (W.S. Hwang). Contents lists available at ScienceDirect Materials Science in Semiconductor Processing journal homepage: http://www.elsevier.com/locate/mssp https://doi.org/10.1016/j.mssp.2020.105430 Received 15 May 2020; Received in revised form 31 July 2020; Accepted 27 August 2020