Article pubs.acs.org/JPCC
Structure, Morphology, and Optical Properties of Amorphous and Nanocrystalline Gallium Oxide Thin Films S. Sampath Kumar,†,‡ E.J. Rubio,† M. Noor-A-Alam,† G. Martinez,† S. Manandhar,§ V. Shutthanandan,§ S. Thevuthasan,§ and C.V. Ramana*,† †
Department of Mechanical Engineering, University of Texas at El Paso, El Paso, Texas 79968, United States Department of Electrical and Computer Engineering, University of Texas at El Paso, El Paso, Texas 79968, United States § Environmental Molecular Sciences Laboratory (EMSL), Pacific Northwest National Laboratory (PNNL), Richland, Washington 99352, United States ‡
ABSTRACT: Gallium oxide (Ga2O3) thin films were produced by sputter deposition by varying the substrate temperature (Ts) in a wide range (Ts = 25−800 °C). The structural characteristics and optical properties of Ga2O3 films were evaluated using Xray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectrometry (EDS), Rutherford backscattering spectrometry (RBS), and spectrophotometric measurements. The effect of growth temperature is significant on the chemistry, crystal structure, and morphology of Ga2O3 films. XRD and SEM analyses indicate that the Ga2O3 films grown at lower temperatures were amorphous, while those grown at Ts ≥ 500 °C were nanocrystalline. RBS measurements indicate the well-maintained stoichiometry of Ga2O3 films at Ts = 300−800 °C. The spectral transmission of the films increased with increasing temperature. The band gap of the films varied from 4.96 to 5.17 eV for a variation in Ts in the range 25−800 °C. A relationship between microstructure and optical property is discussed.
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INTRODUCTION There has been a great deal of interest in the wide band gap oxide materials for electronics, electro-optics, opto-electronics, and magneto-electronics. These materials find a wide range of applications in photoresistors, photodiodes, luminescent phosphors, electronic switches, and gas sensors. Gallium oxide (Ga2O3), which is a stable oxide of Ga, is a wide band gap material. 1−6 Ga2O3 finds attractive applications in luminescent phosphors,7 high-temperature sensors,8−11 antireflection coatings, and solar cells.10 Ga2O3 has been recognized as a deep ultraviolet transparent conducting oxide (UV− TCO),1−5,13 which makes the material a potential candidate for transparent electrode applications in UV optoelectronics.6,14−16 While conventional transparent oxides such as In2O3, SnO2, ZnO, and ITO are opaque in the UV region due to a small band gap (∼3 eV), Ga2O3 exhibits a wide band gap (∼5 eV) and deep transparency to the UV region.4,6,12−16 Gallium oxide can crystallize in five different crystal structures: α, β, γ, δ, and ε phases.17,18 Among these phases, β-Ga2O3 is the most stable form with thermal and chemical stability. The melting point of β-Ga2O3 is 1780 °C10,17 which is useful to readily integrate the material in high-temperature applications. The crystal structure of β-Ga2O3 is base-centered monoclinic (space group C2/m)17,18 where the oxygen ions are in a distorted cubic packing arrangement and the gallium ions are in distorted tetrahedral and octahedral sites.18 The lattice parameters of β-Ga2O3 are a = 12.2140(3) Å, b = 3.0371(9) Å, c = 5.7981(9) Å, and β = 103.83°.19−23 Despite great promise, studies on β-Ga2O3 are rather scarce compared to other well© 2013 American Chemical Society
explored wide band gap oxides. While there are a number of reasons, the most important and challenging ones are: (a) difficulties in preparing single crystals, thin layers and/or their surfaces with sufficient quality, and (b) complications due to a large band gap, which is a major hindrance for the powerful electron-based spectroscopic methods.2 Additionally, for thin films grown using either physical or chemical vapor deposition methods, properties and phenomena of β-Ga2O3 films depend on the processing conditions such as base pressure,9 growth temperature,24,25 reactive pressure (if any), deposition rate, and annealing conditions. Therefore, the controlled growth and manipulation of specific crystal structures of β-Ga2O3 at the nanoscale dimensions has important technological implications. The fundamental, microscopic characteristics such as the optical properties and electrical conductivity mechanism of βGa2O3 films are not well understood, specifically at the reduced dimensions, compared to the best studied wide band gap transparent conducting oxides. Sin-Liang Ou et al.25 have fabricated gallium oxide thin films by pulsed laser deposition (PLD) under variable substrate temperature. The structural, optical, and etching properties of the films were investigated. The phase transition of the films was observed; crystallinity of gallium oxide was enhanced; and the etching rate decreased with increasing temperature. Baban et al.26 have reported the oxygen sensing mechanism of gallium oxide thin films prepared Received: November 15, 2012 Revised: December 29, 2012 Published: February 15, 2013 4194
dx.doi.org/10.1021/jp311300e | J. Phys. Chem. C 2013, 117, 4194−4200
The Journal of Physical Chemistry C
Article
by RF magnetron sputtering. Hyoun Woo Kim et al.24 have deposited gallium oxide on silicon by metal organic chemical vapor deposition (MOCVD) by reacting trimethylgallium with oxygen. The investigations on the growth and structure of the films showed that the films were amorphous with roughness increasing as a function of temperature. Guzman-Navarro et al.27 have discussed the cathodoluminescence study of gallium oxide films grown by thermal evaporation of GaN. Condensation and subsequent oxidation of metallic Ga is suggested as the growth mechanism of gallium oxide nanowires. Thermal annealing of oxygen vacancies is proposed as the responsible mechanism for the observed behavior. Orita et al.1 have fabricated the gallium oxide thin films on silica glass substrates by the PLD method and have demonstrated high conductivity and high transparency by reducing the oxygen pressure in the chamber and increasing the substrate temperature. Mohamed et al.2 have fabricated gallium oxide by the Czochralski method and have shown that the (100) cleavage plane gives a surface of high perfection and the degree of correspondence of the electronic band structure between hybrid density functional theory (DFT) and angle-resolved photoelectron spectroscopy (ARPED) measurements for valence bandwidth, band mass, and individual dispersions is good. Rebien et al.15 have prepared gallium oxide films on GaAs by electron-beam evaporation of gallium oxide pellets and also with RF magnetron sputter deposition. The linear optical functions of the thin film in the ultraviolet−visible−near-infrared (UV−vis−NIR) spectral range were in good agreement. However, microscopic characteristics such as the optical properties and electrical conductivity mechanism of β-Ga2O3 films are still under debate, which calls for further investigations specifically at the reduced dimensions. The present work was, therefore, performed to determine the effect of growth temperature in a wide range on the structural and optical characteristics of Ga2O3 films made by sputter deposition. The results obtained are presented and discussed to derive a structure−property relation.
tometer. All the measurements were made ex situ as a function of growth temperature. XRD patterns were recorded using Cu Kα radiation (λ = 1.54056 Å) at room temperature. Surface imaging analysis was performed using a high-performance and ultra-high-resolution scanning electron microscope (Hitachi S4800). Secondary electron imaging was performed on Ga2O3 films grown on Si wafers using carbon paste at the ends to avoid charging problems. Ion beam analysis of the Ga2O3−Si(100) samples was performed to understand the chemical composition and elemental depth distribution. Rutherford backscattering spectrometry (RBS) experiments were carried out in the accelerator facility at the Environmental Molecular Sciences Laboratory (EMSL) within the Pacific Northwest National Laboratory (PNNL). The RBS experiments were performed at the National Electrostatic Corporation (NEC) RC43 end station. An incident ion probe containing 2 MeV He+ ions with a 7° angle of incidence measured from the sample normal was used. The backscattered ions were detected using a silicon barrier detector at a scattering angle of 150°. Composition profiles were determined by comparing SIMNRA computer simulations of the spectra with the experimental data.29 The detailed procedure on using this simulation to obtain the stoichiometry and atomic concentration in the films has been outlined elsewhere.30,31 The optical properties of Ga2O3 films were evaluated using spectrophotometry measurements employing a Cary 5000 UV−vis−NIR double-beam spectrophotometer. Films grown on optical grade quartz were employed for optical property measurements. The quartz substrates employed extend the transparency range down to ∼190 nm and determined the absorption edge extending into the ultraviolet (UV) region, which is more than sufficient to determine the band gap shift in deficient or stoichiometric or metal incorporated Ga2O3 films. The measurements were made using the film grown on quartz under a sampling beam while keeping the bare quartz substrate under the reference beam.
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EXPERIMENTAL SECTION A. Fabrication. Gallium oxide films were deposited onto silicon (Si) wafers and quartz substrates by radio frequency magnetron sputtering. All the substrates were thoroughly cleaned and dried with nitrogen before introducing them into the vacuum chamber, which was initially evacuated to a base pressure of ∼10−6 Torr. Gallium oxide target (Plasmaterials, Inc.) of 2 in. diameter and 99.999% purity was employed for sputtering. The Ga2O3 target was placed on a 2 in. sputter gun, which is placed at a distance of 8 cm from the substrate. A sputtering power of 40 W was initially applied to the target while introducing high-purity argon (Ar) into the chamber to ignite the plasma. Once the plasma was ignited the power was increased to 100 W for reactive deposition. The flow of the Ar was controlled using an MKS mass flow meter. Before each deposition, the Ga2O3 target was presputtered for 10 min with a shutter above the gun closed. The deposition was carried out for 30 min. The thickness of the films was in the range of 40− 33 (±3) nm. The samples were deposited at different temperatures varying in the range of 25−800 °C. Substrate rotation is maintained during the entire deposition time to ensure uniform coverage on the substrate surface. B. Characterization. Ga2O3 films were characterized by performing structural and optical measurements. X-ray diffraction (XRD) measurements on Ga2O3 films grown on Si were performed using a Bruker D8 Advance X-ray diffrac-
RESULTS AND DISCUSSION A. Crystal Structure. X-ray diffraction patterns of Ga2O3 films are shown in Figure 1 as a function of Ts. The XRD curve
Figure 1. XRD patterns of Ga2O3 films. It is evident from the curves that the films grown at RT−400 °C are amorphous, whereas films grown at Ts ≥ 500 °C are nanocrystalline. Ga2O3 films grown at Ts = 800 °C exhibit the presence of an additional small peak, which could be due to the Si−Ga2O3 reaction at the interface. 4195
dx.doi.org/10.1021/jp311300e | J. Phys. Chem. C 2013, 117, 4194−4200
The Journal of Physical Chemistry C
Article
(Figure 1) of Ga2O3 films grown at Ts = RT−400 °C did not show any peaks indicating their characteristic amorphous (aGa2O3) nature. The diffraction peaks began to appear in the XRD pattern when Ts = 500 °C, indicating the film crystallization at this temperature. The appearance of peaks corresponds to the diffraction from β-Ga2O3 as indicated in Figure 1. It is evident (Figure 1) that the intensity of the substrate peak (Si(200)) increases for films grown at Ts = 800 °C, which might be due to interfacial reaction at the substrate− film interface. B. Surface Morphology and Growth Behavior. The scanning electron microscopy (SEM) images of Ga2O3 films are shown in Figure 2. The amorphous nature is clearly evident in
reaction may be occurring at the interface when the temperature is increased to 800 °C. On the basis of SEM and XRD data as a function of Ts, the growth behavior of Ga2O3 films can be conveniently divided into two zones, where the morphology differences are significant. The first category or zone contains the set of Ga2O3 films grown at temperatures
