【Member Papers】Growth of Zn–N Co-Doped Ga₂O₃ Films by a New Scheme with Enhanced Optical Properties

      Researchers from the Xi’an Jiaotong University have published a dissertation titled "Growth of Zn–N Co-Doped Ga₂O₃ Films by a New Scheme with Enhanced Optical Properties" in Nanomaterials.

Project Support

      This research was supported by the National Natural Science Foundation of China (grant numbers 51702253, 51332003, and M0441), the Natural Science Foundation of Shaanxi Province (No. 2022JQ-325) Key Research and Development Program of Shaanxi (2024PT-ZCK-06), and the “111 Project” of China (grant number B14040). Z.-G.Y. acknowledges support from the Natural Science and Engineering Research Council of Canada (NSERC, DG, RGPIN-2013-04416).

Background

      Gallium oxide (Ga₂O₃) has been considered for a wide spectrum of applications in transparent conducting oxides, deep ultraviolet photodetectors, and high-voltage power electronic devices thanks to its fascinating features, such as an ultra-wide bandgap of 4.9 eV a high breakdown voltage (8 MV/cm) and a high Baliga’s figure of merit (BFOM) of around 3444. However, due to the facile presence of oxygen vacancies, Ga₂O₃ generally displays n-type semiconducting properties which limits its practical applications.

      Doping with selective elements is an effective method to control the electronic and optical properties of semiconductors, including Ga₂O₃. In recent years, a series of Ga₂O₃-based materials with excellent properties have been successfully prepared based on this strategy The conductivity of Ga₂O₃ can be adjusted by doping with Sn, Ge, or Si elements and the optical bandgap of Ga₂O₃ semiconductors can be tuned by doping with Zn and Al. Great efforts have been made to improve the results of p-type doping of Ga₂O₃-based materials. Given the similar ionic radii of N³⁻ and O²⁻, nitrogen is considered one of the most promising dopants for regulating the electronic and optical properties of Ga₂O₃-based materials. However, effective high-concentration nitrogen doping is usually difficult to achieve in most studies. Notably, high-quality nitrogen-doped p-type Ga₂O₃ materials were successfully prepared by the thermal oxidation of gallium nitride (GaN) on sapphire substrates at 1000–1100 °C. First-principles calculations showed that tunable optical properties and even p-type conductivity can be achieved through co-doping with Zn and N. However, it is very difficult to achieve co-doping in the experiment. So far, the influence of Zn and N co-doping on the physical properties of Ga₂O₃ films is still unknown experimentally. Therefore, it is rewarding to take on this challenge in this work.

Abstract

      Gallium oxide (Ga₂O₃), as a wide-bandgap semiconductor material, is highly expected to find extensive applications in optoelectronic devices, high-power electronics, gas sensors, etc. However, the photoelectric properties of Ga₂O₃ still need to be improved before its devices become commercially viable. As is well known, doping is an effective method to modulate the various properties of semiconductor materials. In this study, Zn–N co-doped Ga₂O₃ films with various doping concentrations were grown in situ on sapphire substrates by atomic layer deposition (ALD) at 250 °C, followed by post-annealing at 900 °C. The post-annealed undoped Ga₂O₃ film showed a highly preferential orientation, whereas with the increase in Zn doping concentration, the preferential orientation of Ga₂O₃ films was deteriorated, turning it into an amorphous state. The surface roughness of the Ga₂O₃ thin films is largely affected by doping. As a result of post-annealing, the bandgaps of the Ga₂O₃ films can be modulated from 4.69 eV to 5.41 eV by controlling the Zn–N co-doping concentrations. When deposited under optimum conditions, high-quality Zn–N co-doped Ga₂O₃ films showed higher transmittance, a larger bandgap, and fewer defects compared with undoped ones.

Conclusions

      In conclusion, Zn–N co-doped Ga₂O₃ films have been successfully grown on sapphire substrates using ALD followed with a post-annealing process. After being post-annealed at 900 °C, all of the films transform into a highly preferentially oriented β-Ga₂O₃, except for the sample of N-Ga(3)Zn(1), which remains amorphous. The HRTEM results reveal that an atomically sharp interface is formed and partial epitaxial growth takes place at the N-Ga(6)Zn(1) β-Ga₂O₃/α-Al₂O₃ interface. As the Zn doping concentration increases, the surface roughness of the films was reduced from 4.27 nm to 0.154 nm. In the visible wavelength region, all of the films exhibit an ultra-high optical transmittance, and the optical transmittance increases with the Zn doping concentration in the UV wavelength region. The bandgap of the Ga₂O₃ films can be adjusted from 5.07 eV to 5.41 eV by changing the Zn doping concentration. The PL spectra of the films exhibit several emission centers around 327 nm, 445 nm, 473 nm, 511 nm, and 567 nm under the excitation wavelength of 280 nm. A decline in the green luminescence was observed after post-annealing, demonstrating a decrease in oxygen interstitial defects. This work demonstrates that the structural and optical properties of β-Ga₂O₃ film can be well tuned by Zn–N co-doping and post-annealing. These findings facilitate the development of Ga₂O₃-based solar-blind ultraviolet photodetectors and other optoelectronic devices.