Low Thermal Budget Heteroepitaxial Gallium Oxide Thin Films Enabled by Atomic Layer Deposition

Abstract

This work explores the applicability of atomic layer deposition (ALD) in producing highly oriented crystalline gallium oxide films on foreign substrates at low thermal budgets. The effects of substrate, deposition temperature, and annealing process on formation of crystalline gallium oxide are discussed. The Ga 2 O 3 films exhibited a strong preferred orientation on the c-plane sapphire substrate. The onset of formation of crystalline gallium oxide is determined, at which only two sets of planes, i.e., α-Ga 2 O 3 (006) and β-Ga 2 O 3 (4̅ 02), are present parallel to the surface. More specifically, this work reports, for the first time, that epitaxial gallium oxide films on sapphire start to form at deposition temperatures ≥ 190 °C by using an optimized plasma-enhanced ALD process such that α-Ga 2 O 3 (006)∥α-Al 2 O 3 (006) and β-Ga 2 O 3 (2̅ 01)∥α-Al 2 O 3 (006). Both α-Ga 2 O 3 (006) and β-Ga 2 O 3 (2̅ 01) planes are polar planes (i.e., consisting of only one type of atom, either Ga or O) and, therefore, favorable to form by ALD at such low deposition temperatures. Ellipsometry and van der Pauw measurements confirmed that the crystalline films have optical and electrical properties close to bulk gallium oxide. The film grown at 277 °C was determined to have superior properties among as-deposited films. Using TEM to locate α-Ga 2 O 3 and β-Ga 2 O 3 domains in the as-deposited crystalline films, we proposed a short annealing scheme to limit the development of α-Ga 2 O 3 domains in the film and produce pure β-Ga 2 O 3 films via an energy-efficient process. A pure β-Ga 2 O 3 phase on sapphire with β-Ga 2 O 3 (2̅ 01)∥α-Al 2 O 3 (006) was successfully achieved by using the proposed process at the low annealing temperature of 550 °C preceded by the low deposition temperature of 190 °C. The results of this work enable epitaxial growth of gallium oxide thin films, with superior material properties offered by ALD, not only with potential applications as a high-performance material in reducing global energy consumption but also with an energyefficient fabrication process.

Atomic‐Layer‐Ti‐Doped Ga2O3 Thin Films with Tunable Optical Properties and Wide Ultraviolet Optoelectronic Responses

“…In contrast to sputtering, [ 22 ] pulsed laser deposition (PLD), [ 23 ] electron beam evaporation (EBE), [ 24 ] and molecular beam epitaxy (MBE), [ 25 ] ALD is a self‐saturating and self‐limiting method and used for achieving conformal and uniform coverage, precisely controllable thickness, easy and accurate incorporation, large‐scale fabrication, and a low growth temperature. [ 17–19,26 ] In our case, the average growth rate was ≈0.46 Å/cycle for the amorphous Ga 2 O 3 thin films deposited during PE‐ALD processes. Subsequently, crystalline β ‐Ga 2 O 3 thin films were prepared by annealing the as‐deposited amorphous Ga 2 O 3 films at 900 °C in air.…”

“…The optical bandgap and dielectric properties are vital to effectively design and fabricate optical and electronic devices. Typically, spectroscopic ellipsometry (SE) [ 15–19 ] and absorption spectroscopy [ 20 ] have been successfully applied to evaluate and analyze the optical constants of Ga 2 O 3 bulk materials and thin films with various phases. More efforts in examining the optical constants of amorphous and nanocrystalline Ga 2 O 3 thin films are expected because these constants are significant in practical applications.…”

High Performance β‐Ga2O3 Schottky Barrier Transistors with Large Work Function TMD Gate of NbS2 and TaS2

“…Although β-Ga 2 O 3 is prepared by heating nitrate, acetate, oxalate or other organic derivatives above 1000 °C, epitaxial thin films of β-Ga 2 O 3 can be deposited on sapphire substrate at temperatures between 190 and 550 °C. [12] Based on aforementioned properties, high temperature and high display almost the same low leakage performance, maintaining the OLED brightness over 58% of initial state for 100 s after onstate switching. It means that our MESFET devices as one of highly efficient energy-saving devices can be applied in a variety of circuits requiring low power consumption.…”

“…Although β‐Ga 2 O 3 is prepared by heating nitrate, acetate, oxalate or other organic derivatives above 1000 °C, epitaxial thin films of β‐Ga 2 O 3 can be deposited on sapphire substrate at temperatures between 190 and 550 °C. [ 12 ] Based on aforementioned properties, high temperature and high power transistors have been reported using β‐Ga 2 O 3 active channel. [ 13–17 ] Most of the reported β‐Ga 2 O 3 FETs are metal oxide semiconductor field effect transistors (MOSFETs) [ 13–21 ] and show very negative threshold voltages ( V th ) while those of metal semiconductor field effect transistor (MESFETs) [ 1,22–26 ] seem less negative in general.…”

Low-Temperature As-Grown Crystalline β-Ga2O3 Films via Plasma-Enhanced Atomic Layer Deposition

“…To date, Ga 2 O 3 films have been grown by using deposition techniques such as sputtering, pulsed laser deposition (PLD), the sol–gel method, molecular beam epitaxy (MBE), , metal–organic chemical vapor deposition (MOCVD), and atomic layer deposition (ALD). ,,− Although Ga 2 O 3 layers can be deposited at relatively low substrate temperatures via sputtering and PLD, these techniques significantly lack from high crystal quality, large-area uniformity, and 3D conformality on high-aspect-ratio device structures . On the other hand, device quality epitaxial Ga 2 O 3 thin films are mainly produced on non-Si substrates by using MBE and MOCVD, which require substantially higher substrate temperatures (700–1000 °C). ,− However, such high-temperature harsh processing environments limit the application space of high-quality Ga 2 O 3 layers, particularly its direct monolithic integration on temperature-sensitive CMOS and flexible platforms (Si and amorphous glass/polymeric substrates), which necessitate low-temperature, conformal, and precisely controlled film growth with high crystallinity …”