【International Papers】Electrostatic effects of self-trapped holes in β-Ga2O3 devices

      Researchers from the Ohio State University have published a dissertation titled " Electrostatic effects of self-trapped holes in β-Ga₂O₃ devices" in Applied Physics Letters.

Background

      β-Ga₂O₃ is a promising new semiconductor material. It has several attractive attributes-a Baliga Figure of Merit that is higher than that of its competitors SiC and GaN, several shallow n-type dopants with usable solubilities, an ultra-wide bandgap of 4.5-4.8 eV, and a critical breakdown field of 6-8 MV/cm. Despite its many strengths, β-Ga₂O₃ is not without limitations. Two principal challenges hinder its development: intrinsically low thermal conductivity and lack of p-type material. The lack of shallow acceptor dopants precludes the formation of conventional p-n junctions, thereby requiring device designers to use unique architectures to perform those functions.

Abstract

      β-Ga₂O₃ is an ultra-wide bandgap semiconductor with exceptional properties for power electronics and UV-C optoelectronics, but its behavior under illumination remains poorly understood. In this work, we investigate how optically generated self-trapped holes influence electrostatics and current conduction in β-Ga₂O₃ devices. Using a vertical Schottky photodiode with a semi-transparent Ni anode, we performed capacitance–voltage (C–V), current–voltage (I–V), and temperature-dependent I–V measurements under dark and above-bandgap illumination. Analysis of photocurrent gain reveals that conventional image-force barrier-lowering models require unrealistically high interfacial electric fields, suggesting the presence of an alternative mechanism. By applying the Fowler–Nordheim tunneling theory, we reconcile measured photocurrents and photo-capacitance results with physically plausible fields and quantify the two-dimensional concentration of self-trapped holes. Our findings demonstrate that illumination-induced charge significantly alters device electrostatics. Understanding this tunneling-based photocurrent gain mechanism is critical for designing β-Ga₂O₃ devices for UV-C detectors and power electronics.

Highlights

      The traditional image-force barrier-lowering model is overturned, and the photoconductive gain of β-Ga₂O₃ Schottky detectors is confirmed to originate from Fowler-Nordheim tunneling.

      The areal density and centroid position of illumination-induced self-trapped holes are quantitatively extracted, revealing their mechanism of reshaping internal electric field and energy band.

      Temperature-dependent electrical measurements exclude contributions from interface states and thermionic emission, providing direct experimental evidence for electrostatic modulation by self-trapped holes.

Conclusion

      In summary, we have shown how charge, field, and energy band profiles of β-Ga₂O₃ photodetectors change upon illumination with above-bandgap light because of excess charges introduced by optically generated self-trapped holes based on the evidence provided by I–V, C–V, and I–V–T measurements in dark and illuminated conditions. β-Ga₂O₃ provides a rather unique case of a semiconductor with excellent transport properties which also has self-trapped holes. While the presence of self-trapped holes in illuminated β-Ga₂O₃ devices is well established, this work shows the impact of polarons on device electrostatics and current characteristics. Our results show that the photoconductive gain in β-Ga₂O₃ Schottky photodetectors originates from electron tunneling from the metal into the semiconductor, rather than from thermionic emission enhanced by image-force barrier lowering. We show that hole-induced tunneling uniquely explains the current–voltage behavior typically reported in Ga₂O₃ optoelectronic devices, and we provide a quantitative, experimentally validated, physics-based rationale for this mechanism. By adopting a model that includes tunneling of electrons from the metal, we reconcile measured photocurrents with physically plausible electric fields, extract a two-dimensional concentration of self-trapped holes which reshape the internal field profile, and estimate the energy band diagram at a range of voltages. Understanding this tunneling-based gain mechanism and, more fundamentally, the behaviors and impact of self-trapped holes on the electrostatic conditions of β-Ga₂O₃ devices is essential for understanding the characteristics of electronic and photonic β-Ga₂O₃ devices. This work is relevant for understanding the behavior of Ga₂O₃ devices in the presence of holes, whether they are generated by impact ionization or optical generation. Furthermore, the ability to introduce a two-dimensional charge concentration as large as 2 ×10¹³ cm^-2 by illuminating the material with above-bandgap light could enable novel device structures. With β-Ga₂O₃’s other notable advantages as a material for UV-C optoelectronics and power electronics, this phenomenon could be exploited for several new and exciting device applications.