P-type β-gallium oxide: A new perspective for power and optoelectronic devices

Highlights

• Evidence of p-type conduction in the undoped wide band gap β-Ga₂O₃.

• The ionization energy of the acceptor level was measured to be 1.1eV above the valence band edge.

• p and n type Ga₂O₃, opens perspective for power and deep UV-optoelectronic devices.

Abstract

Wide-bandgap semiconductors (WBG) are expected to be applied to solid-state lighting and power devices, supporting a future energy-saving society. Here we present evidence of p-type conduction in the undoped WBG β-Ga₂O₃. Hole conduction, established by Hall and Seebeck measurements, is consistent with findings from photoemission and cathodoluminescence spectroscopies. The ionization energy of the acceptor level was measured to be 1.1eV above the valence band edge. The gallium vacancy was identified as a possible acceptor candidate based on thermodynamic equilibrium Ga₂O₃ (crystal) – O₂ (gas) system calculations (Kroger theory) which revealed a window without oxygen vacancy compensation. The possibility of fabricating large diameter wafers of β-Ga₂O₃ of p and n type nature, provides new avenues for high power and deep UV-optoelectronic devices.

Introduction

Ga₂O₃ has recently attracted considerable interest for its unique combination of material properties [1], [2] and relevance to many present and future application fields: extreme (also referred to as so-called “ultra”) wide bandgap semiconductors (≥4.8 eV) for deep-ultraviolet optoelectronics, very large breakdown electrical field (E_br = 8 × 10⁶ V cm⁻¹) for high voltage and power electronics. Indeed recent breakthroughs in material quality have led to a “rediscovery” of β-Ga₂O₃ as an ultra-wide bandgap transparent conductor [3]. Demonstrators include transparent field-effect transistors [4], photodetectors [5], [6], use as a material for microwave and optical maser [7], as well as a material for electroluminescent devices [8] and chemical sensing [9]. Moreover, its very wide bandgap and large disruptive critical electrical field has allowed Ga₂O₃ to emerge as the fourth generation material platform for power electronics [10], [11], (after silicon, silicon carbide and gallium nitride) [12], [13].

Nevertheless, all the Ga₂O₃ devices demonstrated thus far are unipolar (only n-type) [2], [14], [15], [16], [17], [18]. In order to realize the full potential for WBG (opto)electronics β-Ga₂O₃, will need bipolar junction based devices, for which p-type doping will be required. The bipolar junction would be engineered by combining p-type transparent TCOs with either, n-type Ga₂O₃ or the common n-type ones (namely In₂O₃, SnO₂ and ZnO and its alloys) into transparent p–n heterojunctions in a range of thin-film transistor applications. P-type requirement is also important in the power electronics context, where a high current carrying capability is desirable when considering applications such as grid-level converters. P-type Ga₂O₃ would allow the definition of Ga₂O₃ p-n junction building blocks and therefore any traditional Silicon-type devices would be engineered; including metal-oxide-semiconductor transistor field-effect devices (MOSFETs), (complementary) CMOS logic or bipolar devices such as pin diodes or insulator gate bipolar transistors (IGBT).

The current approach to achieve hole conductivity in Ga₂O₃ devices is by the definition of heterostructures of known p-type semiconductors such as p-type oxides (Ir₂O₃ [19], NiO_x [20]) or acceptor doped semiconductors (Si [21], SiC [22], [23] or graphene [5]), with known disadvantages of crystalline and electronic band structure miss-matches. P doping in Ga₂O₃ is hugely challenging as an oxide (oxide usually has tendency of formation donor type oxygen vacancies, causing n-type conduction) and as an ultra-wide bandgap material, intrinsic conductivity is rare and even doping (“p” and “n”) is normally not symmetrical. This lack of hole conductivity is probably the main limitation of emerging gallium oxide technology.

Currently, p-type wide bandgap oxides are in the form of binary copper oxides [24]. Cu-based delafosites [25], tin monoxide [26], nickel oxide [27] or layered oxide-chalcogenides [28]. Each of these have a valence band made of eep localized oxygen 2p orbitals, which are responsible for poor hole transport in these materials [29]. Owing to its extreme wide bandgap, doping p-type β-Ga₂O₃ has been considered practically challenging-if not impossible. Usually nominal undoped as grown β-Ga₂O₃ single crystals and thin films are generally n-type, because of the existence of unintentional impurities [30], [31]. P-type conductivity was theoretically predicted in gallium oxide by doping group I and II metals from the Mendeleyev table [32], [33].

However, there is no experimental demonstration of this in the literature up to date. Some of the challenges to be overcome in order to realize hole conductivity are [34]:/i/very wide bandgap,/ii/high formation energy of point defects that are hole producers, e.g. native acceptors such as cation vacancies,/iii/small ionization energy for these defects so as to readily release holes, i.e., a shallow acceptor level with respect to the host valence band; and most crucially/iv/low energy of hole killer native defect donors such as cation interstitials and anion vacancies. In particular oxygen vacancies act as compensating donors and both as grown β-Ga₂O₃ films and bulk crystals are invariably found to be n-type in the literature [30], [31].

This work reveals (via a range of characterization techniques) that intrinsic majority hole conduction can exist and emerge in nominally undoped β-Ga₂O₃, when compensation by background native donors is reduced.