Realization of p-type Ultra-Wide Bandgap Ga2O3 via Band Structure Engineering and Doping for High Power and Optoelectronic Applications

Abstract

Ultra-wide band gap (UWBG) semiconductors have shown tremendous potential for applications in high-power and optoelectronic devices. Their bandgap values of >4 eV leads to an increased breakdown electric field and short wavelength emission, with direct consequences for deep UV photodetection and solid-state lightings, including laser diodes (LDs) and light-emitting diodes (LEDs) in the deep UV regions. Beta phase Ga₂O₃ (β-Ga₂O₃) with a band gap value of ~4.5-5 eV has recently gained much interest as a potential UWBG material for high-power electronics and UV/deep-UV optoelectronic devices. In addition to its UWBG, relatively inexpensive bulk β-Ga₂O₃ are available, which enables the homo-epitaxial growth of high-quality β-Ga₂O₃ thin films. The monoclinic β-Ga₂O₃ has high thermal stability and can be doped n-type over a wide conductivity range. N-type β-Ga₂O₃ with high respective mobility of 194 cm²/Vs and ~9500 cm²/Vs at room temperature and 45K, and exceptionally high electron concentration up to 10²¹ cm^-3 has been achieved with Si, Sn, and Ge dopants grown by Molecular beam epitaxy and Metalorganic vapor phase epitaxy. Despite numerous attempts, p-type doping in Ga₂O₃ has proven extremely challenging. This unipolar nature is commonly observed in many wide-gap oxides (e.g., ZnO, CdO, In₂O₃), which severely limits the functionalities of these materials.

The difficulty in achieving p-type Ga₂O₃, and also for many wide gap transition metal oxides, can be attributed to the very low position of its valence band maximum (VBM) (~9 eV below the vacuum level E_vac), and the more localized nature of its valence band (VB) states due to its major contribution from the O 2p orbital. Hence, most native acceptors (Ga vacancies and O interstitials) have relatively high formation and ionization energies. Consequently, most extrinsic acceptors also have high ionization energy and will be readily compensated by native donors. The VB structure and position must be modified to overcome this challenge. Specifically, an effective way to achieve p-type conductivity in most wide-gap semiconductors is to uplift their VB. Noting that alloying is an established method of modifying the electronic structures of semiconductors, we expect that p-type doping of Ga₂O₃ can become feasible by raising its VBM through alloying with another oxide with a high VB. Motivated by the unique properties of Ga₂O₃ and its diverse potential applications, this work aims to achieve p-type Ga₂O₃ by modifying its electronic band structure via alloying. This work demonstrates that when alloyed with NiO, a p-type oxide with high VB at ~4.8 eV below Evac, Ga₂O₃ can exhibit p-type conductivity due to an uplift of its VB. It further shows that the p-type performance of the Ga₂O₃-NiO alloys can be improved with Li and Zn doping.

This dissertation begins with an introduction to the background of oxide semiconductors and the material properties and device applications ofGa₂O₃. We first investigate how the band structures of Ga₂O₃ can be modified by alloying with NiO. This is achieved by sputter depositing a series of Ga₂O₃-NiO alloys over the entire composition range using both pure Ar (stoichiometric) and (Ar+O₂) (O-rich) sputtering gases. The systematic study compares the structural, electrical, and electronic properties of stoichiometric (Ni_xGa_1-xO) and O-rich Ga₂O₃-NiO alloys (Ni_xGa_1-xO_1+δ). We find that room temperature sputtered Ga-rich Ni_xGa_1-xO alloys (x<0.2) are amorphous, and a phase transition to polycrystalline rocksalt (RS) structure occurs at a Ni composition x ≳0.2 for both the Ni_xGa_1-xO and Ni_xGa_1-xO_1+δ alloys. While the stoichiometric alloys and all amorphous alloys with x<0.2 are highly resistive (ρ >10⁵ Ω-cm), p-type conductivity is observed in the O-rich RS crystalline alloys with ρ decreasing from ~3000 to ~1.3 Ω-cm as x increases from 0.2 to 1. Moreover, as the alloy transforms from the amorphous to the RS crystalline phase at x~0.2, its VBM exhibits a stepwise upshift of ~1.8 eV (from ~8 to 6.2 eV below E_vac). This rise in the VBM closer to the Fermi level stabilization energy E_FS or the charge neutrality level (CNL) located at ~4.9 eV below E_vac suggests that the formation of native acceptors becomes energetically more favorable in RS-Ga₂O₃-NiO alloys, which is responsible for their p-type conductivity.

As a direct consequence of the uplift of the VBM, many originally deep acceptors in Ga₂O₃ may become shallow in RS-Ga₂O₃-NiO alloys so that their p-type doping may be feasible. Hence, we further explore p-type doping of Ni_xGa_1-xO alloys with possible acceptors, including Li and Zn. Li doping was achieved by co-sputtering a Ga₂O₃ and a Li-doped NiO target. Similar to undoped alloys, Li-doped Ga₂O₃-NiO alloys also show a transition from amorphous to polycrystalline phase with a Ni content x ≳0.2. In contrast to undoped alloys in which p-type conductivity arises from native acceptors in O-rich growth, Li-doped stoichiometric alloys (Ni_xGa_1-xO:Li) with x>0.4 also exhibit p-type conductivity, and the ρ decreases with increasing x from 3.7x10³ (x~0.4) to 24 Ω-cm (x=1). This suggests that Li is an effective acceptor in Ni_xGa_1-xO alloys. However, when grown in an O-rich environment (Ni_xGa_1-xO_1+δ:Li), the ρ of Li-doped alloys is further reduced due to the enhanced substitution of Li in the cation sublattice (Li_cat). Compared to the O-rich undoped alloys, the p-type resistivity is lower by about an order of magnitude with Li doping for alloys with similar composition. The lowering of the ρ is even more dramatic as x increases, e.g., for Ni-rich alloys with x~0.8, ρ for the O-rich undoped and Li-doped alloys are ~11 and ~0.8 Ω-cm, respectively. This improvement in the p-type conductivity can be attributed to a lowering in the ionization energy of Li_cat and the increase in Li doping concentration with increasing x.

Finally, Zn is also explored as a possible acceptor in the Ga₂O₃-NiO alloys. Zn doping was achieved by co-sputteringGa₂O₃, NiO and a ZnO target using Ar+O₂ sputtering gas for a range of Zn dopant concentration from 0 to 10%. Similar to the undoped and Li-doped alloys, Zn-doped alloys also show the phase transition from amorphous to crystalline at Ni content x ≳0.2. All amorphous Zn-doped alloys are also highly resistive but become p-type conducting in the RS phase with the lowest ρ occurring at a Zn concentration of ~9.2%. Similar to Li, Zn is also found to be an effective acceptor in RS-Ni_xGa_1-xO alloys. For instance, for the alloy with x~0.6, ρ of the undoped film is ~22 Ω-cm, whereas that of the Zn-doped film is ~5 Ω-cm. It is interesting to note that Zn-doped alloys show an even more drastic reduction in ρ for Ga-rich alloys. For example, for Ga-rich alloys with x~0.37, ρ for the undoped and Zn doped Ni_xGa_1-xO_1+δ are ~2200 and ~150 Ω-cm, respectively. This may suggest that the substitution of Zn for Ga (Zn_Ga) is more energetically favorable than for Ni sites (Zn_Ni). In general, we find that the doping efficiency of Li and Zn is similar in RS-Ni_xGa_1-xO1+δ with mid to high Ni content.

In addition to the p-type conductivity, a comprehensive study on the optical properties and electronic band structures of undoped and doped RS-Ni_xGa_1-xO alloys were also carried out and the results were correlated with their electrical behaviors. Overall, this study demonstrates that by modifying the VB of Ga₂O₃ via alloying with NiO, p-type conductivity can be achieved. Furthermore, Li and Zn are shown to be effective acceptor dopants in Ga₂O₃-NiO alloys, which can further improve their p-type performance. We believe that this work is not only significant for enhancing the functionality of Ga₂O₃, but this band structure modification approach can also be used to overcome the p-type challenge in other wide gap oxides.

Date of Award	26 Jun 2023
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Kin Man YU (Supervisor)