【International Papers】p-Type β-Ga₂O₃ Homoepitaxial Films with Superior Electrical Transport Properties

      Researchers from the Université Paris-Saclay have published a dissertation titled " p-Type β-Ga₂O₃ Homoepitaxial Films with Superior Electrical Transport Properties" in Advanced Photonics Research.

Background

      To realize the full functionality of any emerging electronic technology based on UWBG semiconductors, both n- and p-type conductivity (i.e., bipolarity) should be attained. Till recently, a drawback with bipolar Ga₂O₃ technology was related to the absence of robust homoepitaxial p-type layers possessing high conductivity at room temperature. This obstacle has led to heterostructures mainly using polycrystalline p-NiO as p-type oxides. It is worth emphasizing that significant advancements have been made in the growth of high quality n-type homoepitaxial β-Ga₂O₃ thin-films on substrates with (–201), (100), (010), and (001) crystallographic orientations using MOCVD, MOVPE, LPCVD, HVPE, MBE. However, demonstrations of Ga₂O₃ homo p-n junctions are still handful. To efficiently address the urgent demand for p-n homojunction realization, the first necessary step is to grow high-quality p-type β-Ga₂O₃ layers with hole conductivity measurable at or near room temperature.

      This work demonstrates high structural quality undoped p-type β-Ga₂O₃ epitaxial layers with distinct electrical transport properties grown on (–201)- and (010)-oriented gallium oxide substrates. Remarkable high hole concentration and mobility from 370 K for (010) β-Ga₂O₃ films opens a way for homojunction fabrication, simultaneously revealing the nature of electrical anisotropy related to crystallographic orientations. These findings are crucial for the realization of p-n homojunctions – essential building blocks for a wide range of power electronic and opto-electronic devices.

Abstract

      This work reports high structural quality and exceptional electrical transport properties of homoepitaxial β-Ga₂O₃ thin films grown by Metal–Organic Chemical Vapor Deposition (MOCVD) on (010)- and (–201)-oriented substrates. (010) β-Ga₂O₃ samples exhibit mobility of up to 69.4 cm² (V·s)⁻¹ and stable hole concentrations ≈2.4 × 10¹⁷ cm⁻³ from 370 to 700 K. Structural and morphological studies, including XRD, AFM, and STEM, confirm high epitaxial quality, absence of extended defects and minimal strain. (–201) β-Ga₂O₃ layer, which is simultaneously grown, exhibits typical p-Ga₂O₃ behavior with observed deep level defects. The hole mobility ranging from 26 to 36 cm² V⁻¹·s⁻¹ is measured between 420 and 700 K. Comparison of (010) and (–201) orientations reveals distinct anisotropic electrical properties. The findings emphasize the free motion of holes in β-Ga₂O₃ and the critical role of crystallographic orientation.

Conclusion

      In this work, we demonstrated that β-Ga₂O₃ (010) and (–201) layers simultaneously grown by MOCVD exhibit p-type conductivity with pronounced anisotropic electrical transport properties, strongly dependent on the substrate's crystallographic orientation. Our measurements revealed that the (–201) layer has typical p-Ga₂O₃ behavior with deep level defects. At the same time, the hole mobility for this layer ranges from 26 to 36 cm² V⁻¹·s⁻¹ for the temperatures between 420 and 700 K. The (010) layer, on the other hand, exhibited high free hole concentrations (2.4 × 10¹⁷ cm⁻³) at 370 K with high mobility of 69.4 cm² V⁻¹·s⁻¹. High crystalline quality and the absence of extrinsic impurities and extended defects for the layers, grown on the substrates of both orientations, were confirmed by structural and compositional studies.