High reverse breakdown voltage Schottky rectifiers without edge termination on Ga₂O₃

There is strong interest in developing wide bandgap semiconductor rectifiers as an advance on Si rectifiers and switches in power electronic applications operating at high temperatures or voltages and currents beyond the capabilities of Si.^1–5 Alternatives to date have included GaN (3.4 eV), diamond (5.5 eV), and the different polytypes of SiC (4H-SiC 3.3 eV and 6H-SiC 3.0 eV), and impressive performance has been reported for power rectifiers and transistors with low on-resistance (R_on), high breakdown voltage (V_BR), and fast switching times.^1–5 These materials have high critical electrical fields than Si, as well as reasonable-to-excellent thermal conductivities and low on-state resistances. Advances in use of diamond as a heat sink on GaN and SiC have further pushed the performance limits.^6–9 

There is also interest in developing materials with wider bandgaps than GaN or SiC for extreme environment applications. While diamond has a high Baliga figure of merit (BFOM) for power electronics and excellent thermal conductivity,^2,7 n-type doping remains difficult. Monoclinic β-phase Ga₂O₃ has outstanding potential for power electronics, with a large direct bandgap of ∼4.6 eV and the commercial availability of high quality, large diameter bulk crystals and epitaxial layers with a range of controllable n-type doping levels.^2,10–25 It has a high theoretical breakdown electric field (∼8 MV/cm), leading to a Baliga figure-of-merit almost four times higher than that for GaN.^2,11,15 Experimentally obtained breakdown field values up to 3.8 MV/cm in Sn-doped Ga₂O₃ metal-oxide-semiconductor field-effect transistors (MOSFETs) grown by Metal Organic Chemical Vapor Deposition (MOCVD) on (100) semi-insulating substrates are already higher than the bulk critical field strengths of both GaN and SiC.¹⁶ Power Ga₂O₃ Schottky diode rectifiers, metal-semiconductor field-effect transistors (MESFETs), and metal-oxide-semiconductor field-effect transistors (MOSFETs) fabricated on either bulk or thin film β-Ga₂O₃ have been reported.^11–28 The MOSFETs exhibited breakdown voltages >750 V with field-plate edge termination.¹⁹ 

Schottky rectifiers are ideal devices for establishing material quality and have fast switching speed and low on-state losses, and as unipolar devices, they are good candidates for high power and high frequency applications because they do not suffer from minority-carrier storage effects that limit the switching speed.^1,4,29,30 Compared with lateral diodes grown on insulating substrates, vertical Schottky diodes on conducting substrates can deliver higher power with full back side Ohmic electrodes. Sasaki et al.²⁴ reported rectifiers with a reverse breakdown voltage (V_BR) of ∼150 V on n-type homoepitaxial β-Ga₂O₃ and on single-crystal substrates, while Oh et al. reported the temperature-dependent performance of 210 V Ni/β-Ga₂O₃ vertical Schottky diodes up to 225 °C.²⁵ Konishi et al.³¹ reported 1 kV (actually 1076 V) vertical field-plated Schottky diodes with an excellent specific on-resistance of 5.1 mΩ cm² for anode diameters of 200- or 400 μm. The breakdown voltage (V_BR) is a crucial parameter for power electronic applications. The V_BR depends on the doping concentration, doping gradient, junction depth, device design, and the dielectric constant, bandgap, and impact ionization coefficients.³⁰ The V_BR is proportional to the (bandgap)^3/2 × (doping concentration)^−3/4.³⁰ 

In this letter, we show that Schottky rectifiers without edge termination on epitaxial layers of β-Ga₂O₃ on bulk conducting substrates can achieve V_BR values in the range of 920–1016 V for 105 μm diameter diodes and that the reverse currents are dependent on the diode diameter.

The starting samples were bulk β-phase Ga₂O₃ single crystal wafers (∼650 μm thick) with the (001) surface orientation (Tamura Corporation, Japan) grown by the edge-defined film-fed growth method. Hall effect measurements showed that the Sn-doped samples had a carrier concentration of 3.6 × 10¹⁸ cm⁻³.³² Epitaxial layers (initially ∼20 μm thick) of lightly Si-doped n-type Ga₂O₃ (∼2 × 10¹⁶ cm⁻³) were grown on these substrates by Hydride Vapor Phase Epitaxy (HVPE) at Novel Crystal Technology. After growth, the epi surface is subjected to Chemical Mechanical Polishing (CMP) to remove pits. The final epi layer thickness was ∼10 μm. Figure 1 shows Nomarski images before and after the CMP process. The X-ray diffraction full width at half maximum of the (402) peak was ∼10 arc sec, and the dislocation density from etch pit observation was of the order of 10³ cm⁻². This is the same basic process as in the Konishi samples,³¹ but the epi was carried out in different labs, an important distinction since it shows that the basic HVPE/CMP process is transferrable and robust.

Diodes were fabricated by depositing full area back Ohmic contacts of Ti/Au (20 nm/80 nm) by E-beam evaporation. Ohmic behavior was achieved without the need for dry etching. The front sides were patterned by lift-off of E-beam deposited Schottky contacts Ni/Au (20 nm/80 nm) on the epitaxial layers.³¹ The diameter of these contacts was 105 or 210 μm. Figure 2 shows schematics (top) and optical images of the completed diodes (bottom). Current-voltage (I-V) characteristics were recorded in air from 25 to 100 °C on an Agilent 4145B parameter analyzer or a Tektronix 370 A curve tracer using a heated probe station. For these moderately doped layers, the basic current transport processes in Schottky contacts will be thermionic emission that generally dominates for moderately doped semiconductors. The I-V is then given by³⁰ 

where I_S is the saturation current given by

and A is the contact area, A* is the effective Richardson constant (33.7 A cm⁻²K⁻²),³⁰ Φ_B is the effective barrier height, n is the ideality factor, e is the electronic charge, k is Boltzmann's constant, and T is the absolute temperature. If the current flow is dominated by thermionic emission, then the ideality factor n should be close to unity, with a small increase from unity due to the image force effect.^28,30

Figure 3 shows the forward current density (J-V) characteristics as a function of measurement temperature. We see higher turn-on current, which is explained by the lowering of the barrier height. For extraction of the barrier height from these characteristics, we fitted the linear portions that obeyed the ideal thermionic-emission behavior.^32,33 Figure 4 shows that the barrier height decreased from 1.1 at 25 °C to 0.94 at 100 °C, while the ideality factor increased from 1.08 to 1.28 over the same range. This is expected since pure thermionic emission would lead to a reduced barrier at elevated temperatures.^25,28,30 Higher barrier heights can be achieved with a Pt metal.^31,33 As the operating temperature increases, the turn-on voltage continually decreased, consistent with previous reports.^25,33

Figure 5 shows some reverse I-V characteristics for two different diameter diodes. The V_BR at room temperature was in the range of 920–1016 V for the smaller diode (we show the extreme values here for a total of 25 diodes measured over the wafer area, with the average value from 25 diodes being 975 ± 40 V, with 10 of the diodes over 1 kV) for diameters of 105 μm and consistently 810 V (810 ± 3 V for 22 diodes for a diameter of 210 μm.). This trend is typical of newer material technologies which are still being optimized in terms of defect density.³⁴ The on-resistance (R_on) values obtained from the I–V curves was approximately 6 mΩ·cm². The figure-of-merit (V_BR²/R_on) was ∼154.3 W·cm⁻² at room temperature. Note that in this simple rectifier design, there will be electrical field crowding at the contact edges, which is where breakdown is likely occurring.³¹ The use of field-plate or guard-ring structures as edge termination would increase the V_BR of the β-Ga₂O₃ rectifiers.^18,25,31 Konishi et al.³¹ noted that their 1 kV field-plated diodes in a slightly thinner drift region (7 μm versus 10 μm used here, with similar doping) showed breakdown voltages a factor of 2 larger than non-field-plated devices. The fact that our diodes achieved high breakdown without field plating is indicative of good material quality.

Figure 6 (top) shows the reverse current density as a function of temperature for two different reverse biases. The magnitude of the reverse leakage current determines the reverse power dissipation. For thermionic emission dominated leakage current, there is an increase in reverse leakage current of around one order of magnitude for every 25 °C increase in the diode junction temperature. In our diodes, the reverse leakage current only increased around 2–4 times. Thus, the reverse leakage current was also affected by the tunneling mechanism, which might be due to ionization of different trap states in the epilayer, as found in deep level transient spectroscopy measurements of bulk Ga₂O_3.³⁵ We also measured the reverse recovery characteristics when switching from +5 V to −5 V and found recovery times of order 26 ns (Fig. 6, bottom). These results show the potential of β-Ga₂O₃ as a promising material for high-performance power devices under elevated temperature conditions.

In summary, vertical β-Ga₂O₃ Schottky rectifiers without field plates fabricated on high quality epilayers on bulk substrates show V_BR values in the range of 920–1016 V at room temperature for 105 μm diameters and lower values (810 V) for larger diameter rectifiers. The Schottky barrier height decreased with temperature and was consistent with one dominant conduction mode. The present results show that β-Ga₂O₃ Schottky rectifiers are promising candidates for high power devices.

This project was sponsored by the Department of the Defense, Defense Threat Reduction Agency, HDTRA1–17-1–011, monitored by Jacob Calkins. The content of the information does not necessarily reflect the position or the policy of the federal government, and no official endorsement should be inferred. The research at Dankook was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2015R1D1A1A01058663) and Nano Material Technology Development Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT, and Future Planning (2015M3A7B7045185). This work at Korea University was supported by a Korea University grant, the LG Innotek-Korea University Nano-Photonics Program, the Korea Institute of Energy Technology Evaluation and Planning (KETEP), and the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea (No. 20163010012140). Part of this work at Tamura was supported by “The research and development project for innovation technique of energy conservation” of the New Energy and Industrial Technology Development Organization (NEDO), Japan. We also thank Dr. Kohei Sasaki from Tamura Corporation for fruitful discussions.