Structural and optical properties of Ga2O3 films on sapphire substrates by pulsed laser deposition
Introduction
Ga2O3 is a material that has a high dielectric constant, good thermal stability, but most obviously, it shows superior performance as a wide-bandgap semiconductor material [1]. Thus Ga2O3 can be used as green light-emitting diode [2], ultraviolet photodetector [3], [4], deep-ultraviolet transparent electrode [5], [6], metal oxide semiconductor field-effect transistors [7], and high dielectric oxide [8] or active material for FET device [9]. Ga2O3 films have been prepared by various methods such as sputtering [10], [11], chemical vapor deposition [1], [12], spray pyrolysis [13], sol–gel method [3], molecular beam epitaxy (MBE) [4], [14], and pulsed laser deposition (PLD) [15], [16]. Among them, PLD has many advantages due to completely compositional consistency between a target and a deposited film, and is especially suitable for low temperature growth of thin films for the relative high kinetic energies that the ablated species have [17]. Moreover, PLD has another advantage for growing oxide films such as Ga2O3 because of the carriers generated by oxygen vacancies can be controlled by oxygen pressure in PLD [5].
It is known that the structure and properties of Ga2O3 films depend highly on growth conditions. For example, for the phase of Ga2O3, five polymorphs: α, β, γ, δ and ε phases, are known so far [18]. Among them, the β phase with a monoclinic structure is commonly observed in Ga2O3 after annealing at elevated temperatures. However, a recent first-principles study on the energetics of the Ga2O3 polymorphs suggested that the differences in free energy between α, β and ε phases are small and, therefore, the polymorph of Ga2O3 is sensitively selected depending upon the preparation conditions [15]. Thus systematically investigation of growth factors such as oxygen pressure and substrate temperature influence on the structure and properties of Ga2O3 films is highly required. However, most of the reported Ga2O3 films grown by PLD contained impurities such as Sn and Mn, and appeared in different phases as have been reported [6]. To the best of our knowledge, no temperature effect on the pure Ga2O3 films grown by PLD has been reported up to now, although Lee et al. have reported the influence of oxygen pressure [8]. In this paper, we have grown high oriented β-Ga2O3 films on (0001) sapphire substrates by PLD. The influences of substrate temperature on crystal quality, surface morphology and transmittance were systematically investigated.
Section snippets
Experiment
Ga2O3 films were prepared by PLD using a KrF excimer laser source (λ=248 nm) on (0001) sapphire substrates. Prior to deposition, the sapphire substrates were cleaned ultrasonically in organic solvents, chemically etched in a hot H3PO4:H2SO4 (1:3) solution, rinsed in deionized water, and then blown dry with nitrogen gas before they were introduced into the growth chamber. Facing the substrate, Ga2O3 (99.99%) was set as the target. The pulsed laser with a frequency of 1 Hz was irradiated with a
Results and discussion
Fig. 1 shows the XRD patterns of Ga2O3 films deposited on (0001) sapphire substrates with different temperature. When the substrate temperature is lower than 400 °C, there is no peak except the (0006) reflection from sapphire substrate. However, when the substrate temperature is increased to 500 °C, three different diffraction peaks located at 2θ value of 18.89°, 38.05° and 58.93° appear. The peaks match well with those of the PDF card (No.: 43-1012), and can be assigned as (−201), (−402) and
Conclusions
In conclusion, we have systematically investigated the substrate temperature influence on the structure and optical properties of Ga2O3 films on (0001) sapphire substrates grown by PLD. XRD measurements revealed that (−201) oriented β-Ga2O3 can be obtained at substrate temperatures of 500 °C. All the films have high transmittance and smooth surface, indicating PLD is a promising growth technology for growing high quality β-Ga2O3 films at low substrate temperature.
Acknowledgments
This work was partially supported by the Partnership Project for Fundamental Technology Researches of Ministry of Education, Culture, Sports, Science and Technology, Japan.
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