【Member News】Zhang Hongliang from Xiamen University and Qi Hongji team from Shanghai Institute of Optics Fine Mechanics have made progress in the study of deep ultraviolet transparent conductive Si doped Gallium Oxide heteroepitaxial films

      In recent years, Gallium Oxide (Ga₂O₃) semiconductor has been widely concerned by scientific research and industry all over the world. Gallium Oxide has an ultra-wide band gap of 4.9 eV, which is higher than the 3.2 eV of Silicon Carbide (SiC) and 3.39 eV of Gallium Nitride (GaN) of third-generation semiconductors. The wider band gap means that electrons need more energy to transition from valence band to conduction band, so Gallium Oxide has the characteristics of high withstand voltage (extremely strong critical field strength), high efficiency (lower on-resistance), high power, radiation resistance and so on. Power electronic devices based on Gallium Oxide have potential applications in new energy vehicles, rail transit and other fields, and photodetectors based on Gallium Oxide show important potential in missile early warning, corona detection of high-voltage power grids and other fields, as shown in Figure 1 below.

Figure 1. Applications of ultra-wide band gap Gallium Oxide semiconductors in power electronics, solar blind ultraviolet photodetection and deep ultraviolet transparent electrodes.

      On the other hand, the absorption limit corresponding to the bandgap width of Gallium Oxide is in the sun-blind deep ultraviolet region (253 nm), and it is transparent in the visible to deep ultraviolet spectral range, and Gallium Oxide can obtain high conductivity by doping Si, Sn and other elements. Therefore, Gallium Oxide is a promising candidate material for deep ultraviolet transparent conductive electrodes. Light sources in the deep ultraviolet wavelength range (such as deep ultraviolet LED, deep ultraviolet solid-state lasers, etc.) are widely used in sterilization, water purification and biomedical fields. However, for the device application of deep ultraviolet emission sources, traditional commercial transparent electrode materials such as indium tin oxide (ITO), fluorine doped tin oxide (FTO) and other band gap is small, it is difficult to meet the needs of deep ultraviolet light transmittance. Therefore, Gallium Oxide films have inherent material advantages in this respect.

      At present, most AlGaN deep ultraviolet photoappliances are developed based on sapphire (Al₂O₃) substrate, which is mainly due to the mature production technology, good stability, low price and excellent deep ultraviolet transparency of sapphire substrate. However, due to the large lattice mismatch between Ga₂O₃₃ and Al₂O₃, the conductivity of GaO films grown on sapphire substrate heteroepitaxy is not ideal, usually less than 1 S∙cm. ^-1Recently, Hongliang Zhang of Xiamen University and Qi Hongji team of Shanghai Institute of Optics Fine Mechanics have significantly improved the conductivity of Ga₂O₃₃ films by optimizing the chamfer angle of Si doped GaO and sapphire substrate, up to 37 S∙cm. ^-1The research team used pulsed laser deposition (PLD) technology to epitaxial grow Si-doped Ga₂O₃ films on chamfer sapphire substrate. It was found that appropriately increasing the chamfer angle of the substrate can provide higher step current density, accelerate the nucleation layer growth during epitaxial process, effectively inhibit the in-plane domain structure, and reduce the in-plane rotation symmetry of the film (FIG. 2). Si doped Ga₂O₃ films grown on a 6° chamfer substrate showed better growth orientation and step flow growth mode, and significantly increased their mobility and conductivity while maintaining excellent deep ultraviolet phototransmittance, which is conducive to their application in deep ultraviolet transparent electrodes (FIG. 3).

FIG. 2. (a) Growth morphology of Si doped Ga₂O₃ film on a 0° and 6° chamfer Al₂O₃(0001) substrate; (b) XRD Phi scans of Si doped Ga₂O₃ films show that the chamfer substrate significantly reduces the in-plane rotation and domain density of Gallium Oxide films. (c) Schematic growth patterns of Ga₂O₃ films on a non-chamfer angle substrate (left: island growth) and a 6° chamfer substrate (right: step flow growth).

      In addition, the research team also studied the surface electronic properties of Si doped GaO₃ films based on X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS), and found that the β-Ga₂O₃ surface has an upward band bending of more than 0.35 eV, which prevents the formation of ohmic contact. On the other hand, the low work function of about 3.3 eV on the surface of the Ga₂O₃ film makes it promising as an efficient electron injection material in deep ultraviolet LED.

Figure 3. (a) Si: UV transmittance of Ga₂O₃ electrode as a function of wavelength; (b) Si grown on a 0° and 6° chamfer Al₂O₃(0001) substrate: Hall mobility of Ga₂O₃ films with Si doping concentrations.

      The results of this study provide important experimental reference for the heteroepitaxy growth of Ga₂O₃ films and the development of deep ultraviolet transparent conductive materials. The related work was published in the well-known Applied Physics journal Applied Physics Letters under the title "Deep UV transparent conductive Si-doped Ga₂O₃ thin films grown on Al₂O₃ substrates". The first author of this paper is Jenny Yang, PhD, co-trained by Xiamen University and Hangzhou Institute of Optics and Machinery. This work was funded and supported by the National Natural Science Foundation of China and the Natural Science Foundation of Shenzhen.

Paper Link：https://pubs.aip.org/aip/apl/article/122/17/172102/2886721

About Zhang Hongliang's team at Xiamen University:

      Zhang Hongliang Team of Xiamen University (Home: https://khlzhang.xmu.edu.cn/) long-term commitment to the oxide semiconductor film epitaxial growth and photoelectric device, In combination with advanced methods such as synchrotron radiation photoelectron spectroscopy, absorption spectroscopy and theoretical calculation, the electronic structure, doping and defect mechanism, surface interface structure and other microscopic mechanisms of semiconductor materials and devices are deeply studied. So far in Phys. Rev. Lett., Nat. Commun., Adv. Mater., J. Am. Chem. Soc., Phys. Rev. B. He has published more than 150 papers and applied for 15 patents. He has presided over 10 projects of national key research and development plan, National Natural Science Foundation of China, and enterprise cooperative research and development. He has won the National high-level "Young" Talent, Herchel Smith Research Fellowship of Cambridge University, and the Best International postgraduate Research Award of Taiwan Integrated Circuit Manufacturing Company (TSMC).