Eighteen mega-electron-volt alpha-particle damage in homoepitaxial β-Ga₂O₃ Schottky rectifiers

There is currently significant interest in the use of β-Ga₂O₃ for ultrahigh power electronics and solar blind photodetectors.^1–17 The β-polymorph can be readily grown in the form of high-crystalline-quality bulk crystals and thick epitaxial films.^1–5 This polymorph has a bandgap of ∼4.85 eV, with a breakdown field of 8 MV/cm, and electron saturation velocity of 2 × 10⁷ cm/s. For device applications, controllable n-type doping can be achieved with Sn or Si donors. It is not yet clear if extrinsic p-type doping can be achieved due to the formation of self-trapped holes. Large area bulk substrates are commercially available, and various types of devices, including Schottky diode power rectifiers, field effect transistors with Schottky or insulated gates, and high-sensitivity solar-blind photodetectors have been demonstrated. Since many of the expected applications of Ga₂O₃ involve space-borne deployment, there is interest in its ability to withstand high radiation fluences of the type encountered in satellite or space applications and how it compares with materials like GaN.^18–26 These wide bandgap semiconductors have high bond strengths and hence vacancy formation energies. This means that they will have fewer atomic displacements per incoming nonionizing radiation particle than lower gap semiconductors.^27–34 The radiation hardness also depends on the type of defects created and their electrical activity.¹⁹ For example, in n-type layers, the creation of compensating acceptors like Ga vacancies has a strong influence on the remaining net carrier concentration.¹⁹ There are already significant concentrations (>10¹⁸ cm⁻³) of ionized Ga vacancies in currently available Ga₂O₃, as determined by positron annihilation spectroscopy and electron paramagnetic resonance.^35,36

There have been some recent reports on the effect of proton, electron, gamma ray, and neutron irradiation on devices and material properties of n-type β-Ga₂O₃ under conditions relevant to space exposure conditions.^{22–27,37–39} In general, the carrier concentration decreases due to trap formation as a result of radiation damage, the electron mobility degrades and this leads to changes in the performance of the rectifiers and UV photodetectors tested. The few reports of carrier removal rates in Ga₂O₃ as a result of radiation exposure show them to be roughly comparable to those in GaN of similar doping levels for the same types of fluences.^22–27 

In this paper, we report the effect of 18 MeV alpha particle irradiation on vertical geometry β-Ga₂O₃ Schottky rectifiers. The carrier removal rate is found to be ∼406–728 cm⁻¹ for this energy, the highest reported for any of the radiation types examined to date.

The starting samples were bulk β-phase Ga₂O₃ single crystal wafers (∼650 μm thick) with (001) surface orientation grown by the edge-defined film-fed growth method.¹ Hall measurements showed these Sn-doped wafers had carrier concentration of 2.2 × 10¹⁸ cm⁻³. Epitaxial layers (initially ∼20 μm thick) of lightly Si-doped n-type Ga₂O₃ (∼8.3 × 10¹⁵ cm⁻³) were grown on these substrates by hydride vapor phase epitaxy. After growth, the episurface was subjected to chemical mechanical polishing to planarize the surface, with a final epitaxial layer thickness of ∼7 μm.

Vertical geometry, homoepitaxial diodes were fabricated by depositing a full area back Ohmic contacts of Ti/Au (20 nm/80 nm) by e-beam evaporation. The front sides were patterned by lift-off of electron-beam deposited circular Schottky contacts Ni/Au (20 nm/80 nm) with the diameter of 210 μm. Figure 1 shows a schematic of the rectifier layer structure. Current–voltage (I-V) characteristics were recorded at 25 °C on an Agilent 4145B parameter analyzer. The 18-MeV proton beam was generated using a MC-50 Cyclotron at the Korea Atomic Energy Research Institute. The alpha particle beam was injected into a low-vacuum chamber, where the β-Ga₂O₃-based devices were loaded, facing the beam. The average beam-current, measured by Faraday-cup, was 100 nA during the proton irradiation process. Fluences were fixed at 10¹² and 10¹³ cm⁻². The projected range of the 18-MeV alpha particle beam was calculated using the stopping and range of ions in matter (SRIM) program and is 80 μm, as shown in Fig. 2. This means that the alpha particles completely traverse the drift region of the rectifiers and come to rest in the substrate. In other words, the damage is mainly beyond the drift region.

The forward and reverse bias I-V characteristics shown in Fig. 3 demonstrate that the alpha irradiation-induced damage is already measurable for the lowest dose. We can summarize the effects on the I-V characteristics as follows:

1. The barrier height and ideality factors showed little change at either dose, but the on-state resistance, R_ON, increased from 4 to 62 mΩ cm⁻² at the higher dose. The reverse breakdown voltage increased from 60 V in the reference to 80 and 110 V, respectively, in the 10¹² and 10¹³ cm⁻² dose samples.

2. The carrier concentration in the drift region decreased as a result of alpha particle damage and led to an increase in reverse breakdown voltage. The decrease in net electron density in the epilayer results from the deep trap formation by nonionizing energy loss that compensates the initial donor doping. Previous experiments on proton irradiated β-Ga₂O₃ nanobelts have shown that the electron mobility also decreases as a result of radiation damage.²⁶ 

Quantification of the carrier loss can be obtained from 1/C²-V plots for the rectifiers after alpha particle irradiation shown in Fig. 4. The calculated carrier removal rate was 406 cm⁻¹ for the 10¹² cm⁻² dose and 728 cm⁻¹ for the 10¹³ cm⁻² dose. The initial carrier density of 8.3 × 10¹⁵ cm⁻³ was reduced to 7.9 × 10¹⁵ cm⁻³ after a dose of 10¹² cm⁻² and to 1.03 × 10¹⁵ cm⁻³ after the higher dose of 10¹³ cm⁻².

Figure 5 shows the rectifier on/off ratio when switching from +1 V forward bias to the reverse voltages shown on the x-axis. The unirradiated rectifiers showed on/off ratios of >10⁶ across the entire voltage range investigated. These values were degraded by alpha particle irradiation, as summarized in Table I. This is due to the reduction of forward current as the carrier density is reduced by the alpha particle damage-induced trap introduction. These results show that some parameters of the rectifiers are more degraded by exposure to high energy alpha particle fluences than others, due to the specific loss of carriers and electron mobility.

Figure 6 shows a compilation of reported carrier removal rates for Ga₂O₃. The carrier removal rates for alpha particle irradiation of 406–728 cm⁻¹ are the much higher than for protons, neutrons, or electrons, reported previously. For example, carrier removal rates of ∼4.9 cm⁻¹ for 1.5 MeV electron irradiation and ∼300 cm⁻¹ for 10 MeV protons were reported for the same type of rectifiers.²⁵ Note that alpha particles exhibit the highest carrier removal rates of the four types of radiation represented. The results for Ga₂O₃ are also generally comparable to those for GaN (Refs. 20 and 21) and indicate that the former is a good candidate for space-borne applications.

Finally, we also measured the reverse recovery characteristics when switching from +1 V to a range of reverse biases and found recovery times of order 20 ns for both control and alpha particle irradiated rectifiers, as shown in Fig. 7. The oscillations are due to the decay of stored charge upon switching bias polarities. This is consistent with both electron and proton irradiated rectifiers in which the reverse recovery showed little change with radiation dose,²⁵ since the minority carrier lifetime (which controls the carrier storage time in the intrinsic layer) is already small in Ga₂O₃.

Table I and the data shown in this paper emphasize that the diode parameters most affected by alpha particle irradiation are the on-state resistance, reverse breakdown voltage, and on-off ratio, while the diode ideality factor, barrier height, and reverse recovery do not show significant changes for the doses we investigated. Rectifiers are a convenient platform for investigating radiation effects since they have a simple structure but yield a large number of measurable device parameters.

Ga₂O₃ rectifiers were irradiated with 18 MeV alpha particles at fluences of 10¹²–10¹³ cm⁻². The carrier removal rate in the drift region of the rectifiers was 406–728 cm⁻¹ under these conditions. The reverse breakdown voltage increases in response to a reduction in channel carrier density, and the on/off ratio is also degraded. The carrier removal rates in Ga₂O₃ are comparable to those in GaN under similar conditions.

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 Korea University was supported by the New and Renewable Energy Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant from the Ministry of Trade, Industry and Energy, Republic of Korea (20173010012970 and 20172010104830). Part of the 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. The authors also thank Kohei Sasaki from Tamura Corporation for fruitful discussions.