Properties Of β-Ga2O3 Thin Films Research Articles

Deep Ultraviolet Optical Anisotropy of β-Gallium Oxide Thin Films.

β-Crystalline phase gallium oxide (β-Ga2O3) is an ultrawide bandgap material with prospective applications in electronics and deep ultraviolet (DUV) optoelectronics and optics. The monoclinic crystal structure of β-Ga2O3 results in optical anisotropy to incident light with different polarization states. This attribute can lead to different optical applications in the DUV. In this article, we investigated the optical properties of β-Ga2O3 thin films grown by pulsed laser deposition technique on sapphire substrates with different crystallographic orientations. Marked in-plane polarization anisotropy, determined by reflectance and Raman spectroscopy, was observed for β-Ga2O3 films deposited on an r-cut sapphire substrate. In contrast, isotropic optical properties were observed in β-Ga2O3 films deposited on a c-cut sapphire substrate.

Microstructure and optical properties of β-Ga2O3 thin films fabricated by pulsed laser deposition

β-Ga2O3 has attracted extensive attention in the fields of optoelectronics and high-power electric devices due to its excellent electrical properties and thermal stability. Growth of epitaxial β-Ga2O3 films with good crystallinity is challenging since it is difficult to effectively control the impurities in the films. Here, β-Ga2O3 thin films were epitaxially grown on MgO (100) substrates via pulsed laser deposition system. The microstructure and optical properties of the β-Ga2O3 thin films were systematically characterized by combining high-resolution X-ray diffraction, X-ray photoelectron spectroscopy, ultraviolet–visible spectrophotometer and advanced transmission electron microscopy. Effects of the growth temperature and O2 partial pressure on the crystallinity, valance states of Ga ions and band gap of the β-Ga2O3 thin films were investigated. The light absorption wavelength of the β-Ga2O3 thin films ranges from 220 nm to 260 nm, which is close to the theoretical value. The epitaxial orientation between β-Ga2O3 film and MgO substrate is β-Ga2O3 (100) [02¯1] // MgO (100) [010]. A thin layer of γ-Ga2O3 exists in the interfacial region.

Structural Properties of β-Ga2O3 Thin Films Obtained on Different Substrates by Sol-Gel Method

β-Ga2O3 thin films were obtained by the sol-gel method on sapphire and quartz substrates, as well as on Cu-O buffer layers. It was shown that the sol-gel method allowed to obtain β-Ga2O3 thin films with good optical and structural properties by using X-ray diffraction, scanning electron microscopy and optical spectroscopy. The energy of the optical band gap of Ga2O3 films calculated by the Tauc plot varied from 4.39 to 4.59 eV.

Solar-blind photodetectors fabricated on β-Ga2O3 films deposited on 6° off-angled sapphire substrates

In this paper, the β-Ga2O3 films were deposited on 6° off-angled (towards <11–20> direction) (0001) sapphire substrates by MOCVD, and the MSM structure solar-blind photodetectors were fabricated. The effects of growth temperature on the surface morphology, elemental composition, crystalline quality and optical properties of β-Ga2O3 thin films were investigated. As the substrate temperature increased from 650 to 850 °C, the surface RMS roughness of the film gradually increased, and the bandgap was 4.71, 4.73 and 4.76 eV, respectively. In addition, the photodetector fabricated on β-Ga2O3 film at 850 °C exhibited a large photocurrent (Iphoto) of 0.59 mA, a high responsivity (R) of 1.26 A/W, a high photo-to-dark current ratio (PDCR) of 9.3 × 104, a detectivity (D*) of 2.5 × 1012 Jones, and a fast response recovery time of 0.38 s.

Study on Preparing β-Ga2O3 Films with Temperature-Controlled Buffer Layer by RF Magnetron Sputtering

β-Ga2O3 thin films were prepared on (0006) sapphire substrates by RF magnetron sputtering. Under the conditions of sputtering power of 80 W, time of 10 min, and total flow rate of 40 sccm in oxygen and argon atmosphere (2.5 % oxygen ratio). Different preparation temperatures were used to conduct layering by temperature modulation. A homogenous β-Ga2O3 buffer layer was grown first, and then the second β-Ga2O3 film was grown on top of it. When the stratified sputtering of different temperature combinations was completed, high-temperature thermal annealing with the same parameters was performed. The effects on the structure, surface morphology, and optical properties of β-Ga2O3 thin films were compared and analyzed when using the preparation sequence of the homogenous buffer layer and the top layer at different temperatures after annealing. Finally, based on the stratified preparation temperature parameters, the optimal stratified temperature parameters were summarized.

Effects of Drying Temperature and Molar Concentration on Structural, Optical, and Electrical Properties of β-Ga2O3 Thin Films Fabricated by Sol–Gel Method

In this study, β-Ga2O3 films were fabricated on a quartz substrate by the sol–gel method using different drying temperatures and solutions of different molar concentrations, and their structural, optical, and electrical properties were evaluated. The as-fabricated films exhibited a monoclinic β-Ga2O3 crystal structure, whose crystallinity and crystallite size increased with increasing molar concentration of the solutions used and increasing drying temperature. Scanning electron microscopy of the as-prepared samples revealed dense surface morphologies and that the thickness of the films also depended on the deposition conditions. The average transmittance of all the samples was above 8% in visible light, and the calculated optical bandgap energy was 4.9 eV. The resistivity measured using a 4-point probe system was 3.7 × 103 Ω cm.

The effect of annealing temperature on Ga2O3 film properties

Using Radio Frequency Magnetron Sputtering (RFMS) equipment, gallium oxide (Ga2O3) films were deposited on sapphire substrates. Then the samples were annealed at 700 °C, 900 °C, and 1100 °C for 120 min in the air atmosphere to convert them into β-Ga2O3 films with different crystalline quality. The effects of annealing temperature on the properties of β-Ga2O3 thin films were investigated. The crystal structure and surface morphology were tested by X-ray Diffraction (XRD), absorption spectroscopy, and Atomic Force Microscopy (AFM). The results obtained show that, within a certain range, as the annealing temperature increases, the intensity of the XRD peaks increases, the Full Width at Half Maximum (FWHW) – decreases. The crystal grains “engorge” as the temperature rises, and the degree of crystallinity of the samples becomes better.

Impact of Cr2O3 additives on the gas-sensitive properties of β-Ga2O3 thin films to oxygen, hydrogen, carbon monoxide, and toluene vapors

High-temperature β-Ga2O3:Cr2O3-based sensors sensitive to oxygen- and hydrogen-containing gases have been developed and studied. Magnetron cosputtering is the method of choice for the thin film synthesis as an industry-compatible technique. The composition-structure-properties relationship has been revealed. An introduction of 0.04–0.14 wt. % Cr leads to a significant increase in the response of the O2 sensors over the temperature range 250–400 °C. The highest response in the above-mentioned temperature range has been achieved for a Cr addition of 0.14 wt. %. An increase in the Cr content from 0.04 to 0.22 wt. % leads to a decrease in the β-Ga2O3-based sensors’ response time, especially for low O2 concentrations (≤10 vol. %). Reliable control of the β-Ga2O3:Cr2O3-based sensors’ selectivity to industry-relevant reducing gases—hydrogen, carbon monoxide, and toluene—is demonstrated. β-Ga2O3 films with a Cr incorporation content of 0.04 and 0.06 wt. % have a high response to toluene at operating temperatures 300–500 °C, while the films with 0.14 and 0.22 wt. % Cr have a high response to H2 in the range 400–500 °C. Regardless of the Cr content in β-Ga2O3 thin films, all sensors considered demonstrate a weak response to CO within the operating temperature range 250–500 °C. The results attained are of certain technological importance, i.e., in terms of the development of cost-effective methods for the synthesis of materials and systems for monitoring and control of industry-relevant gases for an environmentally friendly and sustainable growth.

Effects of growth pressure on the characteristics of the β-Ga2O3 thin films deposited on (0001) sapphire substrates

β-Ga2O3 films were grown on (0001) sapphire substrates by low-pressure MOCVD. The effects of growth pressure on the growth rate, surface morphology and optical properties of β-Ga2O3 thin films were investigated. XRD showed that the β-Ga2O3 films grown preferentially along the (-201) crystal plane family were obtained, and the increase in growth pressure led to a decrease in growth rate, consistent with the SEM results. AFM measurements indicated that the increase in growth pressure reduced the grain size and RMS roughness. Optical transmission spectroscopy demonstrated that the average transmittance of all films in the visible range exceeded 75%, and the optical bandgap at 20, 30 and 40 Torr were 4.95, 4.93 and 4.91 eV, respectively. XPS revealed that the Ga 2p peak and O 1s peak blue shift as the growth pressure. Through RT-Raman spectroscopy, the low-frequency and high-frequency Raman vibration modes of β-Ga2O3 were detected.

Improved structural and optical properties of β-Ga2O3 films by face-to-face annealing

Face-to-face annealing (FTFA) technology has been used to investigate the structural and optical properties of β-Ga2O3 thin films. The scanning electron microscope (SEM) results show that the top β-Ga2O3 film by using FTFA technology has a flatter surface than that of the conventional annealed film; on the contrary, the bottom film appears noticeable cracks. X-ray diffraction (XRD) and Raman scattering spectrum (Raman) studies further indicate that the top β-Ga2O3 film has a higher crystal quality, while the preferred orientation of the bottom one has changed significantly. Compared with the conventional annealed β-Ga2O3 film, the average transmittance in the near-ultraviolet to visible region of the top β-Ga2O3 film increases up to 96.4%, whereas the bottom one dropped sharply to 86.6%. It is demonstrated that the FTFA technology could effectively improve the structural and optical properties of β-Ga2O3 films, which shows a great potential for fabrication of high-performance β-Ga2O3 films optoelectronic devices.

Effects of Energetic Ion Irradiation on β-Ga2O3 Thin Films

In the present work, effect of swift heavy ion (SHI) irradiation on structural and optical properties of β-Ga2O3 thin films was investigated. Different ion fluences (ϕ) of 120 MeV Ag9+ ions ranging from 1 × 1011 ions-cm−2 to 5 × 1012 ions-cm−2 were employed. The films were grown at room temperature (RT) using electron beam evaporation method and post-deposition annealing was done at 900 °C in oxygen atmosphere. X-ray diffraction (XRD) and UV–visible (UV-Vis) spectroscopy data confirmed the formation of polycrystalline β-Ga2O3 phase having a bandgap of ∼5.14 eV. An increase in the structural disorder, and decrease in the average crystallites size of β-Ga2O3 with increasing ϕ was also revealed by XRD. Ga2O3 thin films showed high transparency in the UV (upto 280 nm) and visible range with average transmittance of ∼80%. Rutherford backscattering spectrometry (RBS) revealed that the thin films were slightly O deficient. A low frequency vibration mode at 170 cm−1 arising from liberation and translation of tetrahedra-octahedra chains in β-Ga2O3 was observed through Raman spectroscopy. Scanning electron micrograph (SEM) images suggested that the films were fairly smooth.

The further investigation of N-doped β-Ga2O3 thin films with native defects for Schottky-barrier diode

The structural, electronic and optical properties of β-Ga2O3 thin films with different N ion concentrations are investigated. With N concentrations increasing, the crystallization degrades and even polycrystalline appears. After Ar annealing treatment, the crystallization of N-doped β-Ga2O3 thin films improved owing to recovery of the implantation-damaged crystals. The N-doped Ga2O3 thin films were used to construct vertical Schottky diodes comprising Ga2O3:N/n-Ga2O3. The net carrier concentrations of these devices degrade one-two orders due to the compensation effect of more N implantation, which suggests that hole could be mobile although N is a deep acceptor dopant. The phenomenon could be explained by the decrease of the ionization energy of N dopant due to the complex defects NGa2O3VO (N-doped β-Ga2O3 with O vacancy) and NGa2O3VGa (N-doped β-Ga2O3 with Ga vacancy). Thus, the development of N ion implantation for β-Ga2O3 has its potential opportunity for β-Ga2O3 power devices.

Preliminary study for the effects of temperatures on optoelectrical properties of β-Ga2O3 thin films

The thermal issue concerning β-Ga2O3 based device performances is one key index because β-Ga2O3 has a low thermal conductivity. The accumulated heat may degenerate the devices if there is no effective heat dissipation mechanism. In this paper, we focus on the effects of high temperatures (298 K < T < 623 K) on the optoelectrical properties of β-Ga2O3 thin films. Which is valuable for the potential applications of β-Ga2O3 especially in places where operating temperatures are substantially higher than room temperature. We have performed dark current (Idark) and light current (Ilight) measurements on these devices in a temperature range from 298 K to 623 K. With increasing temperature, Idark exhibited a monotonic increasing trend. However, the light current (Ilight) first increases and then decrease with the increase of temperature. This supralinear photoconductivity behaviors are related to self-trapped holes. Photocurrent (Ilight-Idark) decrease then increase and decrease again with the increasing temperatures. The photosensitivity (Ilight/Idark) is decreasing as the temperature goes up mainly due to the higher dark conductance. The results obtained in this work may accelerate the β-Ga2O3 devices development related to temperatures and deepen the understanding of the thermal affected characteristics of β-Ga2O3.

The influence of sputtering power on the structural, morphological and optical properties of β-Ga2O3 thin films

In this work, Ga2O3 thin films were grown on Al2O3 (0001) substrate by radio frequency magnetron sputtering. The influence of sputtering power on crystalline structure, morphology, transmittance, band gap, and photoluminescence were investigated in detail. X-ray diffraction results showed that the β-Ga2O3 films were oriented along (2¯01) plane, and the crystalline quality improved with increasing sputtering power. Scanning electron microscope images revealed that the proportion of large-size grain increased with increasing sputtering power. The β-Ga2O3 films displayed very high transmittance close to 100% in visible region. Photoluminescence spectra showed that all the films exhibited intense blue and green emission centered at approximately 430 nm and 550 nm originated from the donor-acceptor pair recombination.

Photoelectric properties of β-Ga2O3 thin films annealed at different conditions

In this work, metal–semiconductor–metal solar-blind ultraviolet photoconductors were fabricated based on the β-Ga2O3 thin films which were grown on the c-plane sapphire substrates by molecular beam epitaxy. Then, the effects of β-Ga2O3 annealing on both its material characteristics and the device photoconductivity were studied. The β-Ga2O3 thin films were annealed at 800, 900, 1000, and 1100 °C, respectively. Moreover, the annealing time was fixed at 2 h, and the annealing ambients were oxygen, nitrogen, and vacuum (4.9 × 10−4 Pa), respectively. The crystalline quality and texture of the β-Ga2O3 thin films before and after annealing were investigated by X-ray diffraction (XRD), showing that higher annealing temperature can result in a weaker intensity of \(\left( {\bar{4}02} \right)\) diffraction peak and a lower device photoresponsivity. Furthermore, the vacuum-annealed sample exhibits the highest photoresponsivity compared with the oxygen- and nitrogen-annealed samples at the same annealing temperature. In addition, the persistent photoconductivity effect is effectively restrained in the oxygen-annealed sample even with the lowest photoresponsivity.

Growth and structural properties of β-Ga₂O₃ thin films on GaN substrates by an oxygen plasma treatment

Growth and structural properties of β-Ga₂O₃ thin films on GaN substrates by an oxygen plasma treatment