SnO/β-Ga2O3 vertical pn heterojunction diodes

Abstract

As a contribution to (transparent) bipolar oxide electronics, vertical pn heterojunction diodes were prepared by plasma-assisted molecular beam epitaxy of unintentionally doped p-type SnO layers with hole concentrations ranging from p=1018 to 1019 cm−3 on unintentionally doped n-type β-Ga2O3(−201) substrates with an electron concentration of n=2.0×1017 cm−3. The SnO layers consist of (001)-oriented grains without in-plane epitaxial relation to the substrate. After subsequent contact processing and mesa-etching (which drastically reduced the reverse current spreading in the SnO layer and associated high leakage), electrical characterization by current–voltage and capacitance–voltage measurement was performed. The results reveal a type-I band alignment and junction transport by thermionic emission in forward bias. A rectification of 2×108 at ±1 V, an ideality factor of 1.16, a differential specific on-resistance of 3.9 m Ω cm2, and a built-in voltage of 0.96 V were determined. The pn-junction isolation prevented parallel conduction in the highly conductive Ga2O3 substrate during van-der-Pauw Hall measurements of the SnO layer on top, highlighting the potential for decoupling the p-type functionality in lateral transport devices from that of the underlying n-type substrate. The measured maximum reverse breakdown voltage of the diodes of 66 V corresponds to a peak breakdown field of 2.2 MV/cm in the Ga2O3-depletion region and suggests the low bandgap of the SnO (≈0.7 eV) not to be the limiting factor for breakdown. Higher breakdown voltages that are required in high-voltage devices could be achieved by reducing the donor concentration in the β-Ga2O3 toward the interface to increase the depletion width, as well as improving the contact geometry to reduce field crowding.

Silane‐Mediated Expansion of Domains in Si‐Doped κ‐Ga2O3 Epitaxy and its Impact on the In‐Plane Electronic Conduction

“…[ 65 ] Ohmic Ti/Au contacts (20 nm/100 nm) with different geometries and spacings had been deposited by electron‐beam evaporation and a lift‐off process on top of the κ‐Ga 2 O 3 /insulating GaN epitaxial layers to allow in‐plane transport measurements through the transfer length TLM method (linear TLM 200 µm wide, gaps: 70, 60, 50, 40, 30, and 20 µm–circular TLM inner contact diameter 80 µm, gap spacing: 5, 10, 15, 25, 50, 75, and 100 µm). A mesa structure with well‐defined and isolated κ‐Ga 2 O 3 squared columns (200 × 200, 150 × 150, 100 × 100, and 70 × 70 µm 2 ) had been obtained via dry etching processing the samples (inductively couple plasma – reactive ion etching ICP‐RIE process details reported in reference, [ 5 ] 1.1 nm s −1 etch rate for κ‐Ga 2 O 3 ) deposited on top of the conducting GaN epitaxial templates. Ohmic Ti/Au contacts had been deposited around the mesa (on the Si:GaN layer) as well as on top of the κ‐Ga 2 O 3 mesas (180 × 180, 130 × 130, 80 × 80, and 55 × 55 µm 2 ) so to allow 2‐point‐probe out‐of‐plane resistance measurements to provide information on the possible structurally‐driven conduction anisotropy (i.e., in‐ versus out‐of‐plane) in the material.…”

“…The monoclinic β-Ga 2 O 3 is the thermodynamically stable crystal structure; for this reason, it has been so far the most investigated one with different proposed device architectures, most of them based on homoepitaxy [1] due to the possibility to synthesize bulk β-Ga 2 O 3 from the melt. [2,4,5] Moreover, the ability for i) tuning its bandgap through Al-and In-alloying, [6] ii) controlling its electrical properties through n-type extrinsic doping (e.g., Si, [7] Sn [8] ), and iii) depositing epitaxial layers with low densities of structural defects and smooth surfaces (because of the presence of perfectly matched substrates, i.e., β-Ga 2 O 3 ), [9][10][11][12] concurrently has allowed the experimental demonstration of the confinement of 2D electron gases (2DEGs) at β-Ga 2 O 3 / Al x Ga 2-x O 3 interfaces. [13] Nonetheless, the low symmetry of the monoclinic cell poses serious obstacles for future device development, i.e., the challenging heteroepitaxial [14,15] as well as homoepitaxial [9,10,16,17] growth of high quality β-Ga 2 O 3 thin films and the intrinsic anisotropy of some of its functional properties (e.g., thermal conductivity).…”

A Comprehensive Study of Defects in Gallium Oxide by Density Functional Theory

“…The Ga2O3 is an UWBG semiconductor with five polymorphs 1 , namely the corundum (α), monoclinic (β), orthorhombic (ε), defective spinel (γ), and cubic (δ) phases having the bandgap of α, β, ε-Ga2O3 exceeding 3.25 eV [8][9][10][11] . These phases of Ga2O3 were grown using a number of different growth methods and have many useful applications 8,[12][13][14][15][16][17][18][19][20] . All phases of Ga2O3 regardless of the growth mechanism have defects that can be studied theoretically and experimentally.…”

Thermodynamic and Kinetic Effects on the Nucleation and Growth of ε/κ- or β-Ga2O3 by Metal–Organic Vapor Phase Epitaxy

“…4 Due to the possibility of growing it from melt and as an epitaxial layer, the monoclinic β-Ga 2 O 3 is by far the most studied and technologically mature polymorph, with many device prototypes already produced. 5,6 Nonetheless, a wide interest is also recently building up on the other polymorphs of this material. 7,8 Apart from a higher symmetry crystal structure with respect to the monoclinic one, which makes them preferable for heteroepitaxial growth and poses fewer problems in film processing and device manufacturing, metastable Ga 2 O 3 polymorphs possess unique physical properties that could be particularly advantageous for device engineering.…”