Influence of swift heavy ion irradiation on electrical characteristics of β-Ga₂O₃ Schottky barrier diode

Ga₂O₃ crystals exhibit polymorphism with five confirmed polytypes (α, β, γ, δ, ). β-Ga₂O₃ is believed to be the most stable and, hence, the most extensively used phase [1, 2]. β-Gallium oxide has emerged as a promising material for power electronics, owing to superior material properties such as an ultrawide bandgap of 4.9 eV and a high breakdown electric field of up to 8 MV cm⁻¹ [3], which enable β-Ga₂O₃ based devices to operate at high temperature and high voltage bias [4]. Another important advantage of β-Ga₂O₃ is that large, high-quality bulk single crystals can be grown by melt growth methods [5–8]. Furthermore, various types of devices, including Schottky barrier diodes (SBDs), metal-oxide-semiconductor field effect transistors and solar-blind photodetectors have been fabricated and the device characteristics carefully investigated. Since the expected application of β-Ga₂O₃ devices in artificial satellites, launch vehicles, and spacecraft, there are much interests in their ability to withstand high radiation fluences consisting of high-energy protons, electrons, neutrons, gamma rays, and heavy ions (HIs) [9, 10].

These particles produce various effects on semiconductor devices, including the accumulation of ionizing dose deposition over a long period, known as the total ionizing dose effect and the accumulation of non-ionizing dose deposition due to lattice defects generated by protons or high energy electrons, leading to displacement damage (DD) effects [11, 12]. Although some effects of proton, electron, gamma ray and neutron irradiation on β-Ga₂O₃ material and device properties have been reported recently, there is few investigations of the influence of HIs irradiation on β-Ga₂O₃ SBDs [13]. In 2020, Chen et al reported for the first time the influence of x-ray irradiation under bias voltage on the field effect transistors prepared by mechanically stripped β-Ga₂O₃ nano strips [14]. In 2022, N. Manikanthababu et al reported the situ electrical characteristics and defect dynamics induced by swift HI irradiation of 120 MeV Au⁹⁺ in a fluence range of 1 × 10¹⁰–2 × 10¹² ions cm⁻² in Pt/PtO_x /β-Ga₂O₃ vertical SBDs [15].

In this paper, we reported the effects of swift HIs (16 MeV ¹⁸¹Ta) radiation on vertical β-Ga₂O₃ Schottky diodes under the fluence of 1 × 10⁸, 3 × 10⁸ and 3 × 10⁹ cm⁻². The trap densities introduced at the Au/Ni/β-Ga₂O₃ interface were extracted to be 2.89 × 10¹⁵–2.49 × 10¹⁶ cm⁻² eV⁻¹and 2.18 × 10¹⁵–4.98 × 10¹⁶ cm⁻² eV⁻¹ for 1 × 10⁸ and 3 × 10⁸ cm⁻², respectively, with the trap energy of 0.85–0.87 eV below the conduction band edge.

Figure 1 presents the schematic cross section of β-Ga₂O₃ SBD. The β-Ga₂O₃ drift layer with a n⁻ doping concentration of 1 × 10¹⁷ cm⁻³ and thickness of 8 μm was grown on the β-Ga₂O₃ substrate with n⁺ doping concentration of 1 × 10¹⁸ cm⁻³ by hydride vapor phase epitaxy, which was purchased from Novel Crystal Technology, Inc. After ultrasonic cleaning in acetone, ethanol and deionized water for 10 min, respectively, the metal stack of Ti (60 nm)/Au (120 nm) was deposited on the backside of β-Ga₂O₃ drift layer by e-beam deposition, then thermal annealed at 500 °C for 60 s to reduce ohmic contact resistance. Schottky Ni (50 nm)/Au (80 nm) circular contacts with the diameter of 100 μm were deposited using e-beam evaporation.

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The HIs irradiation was performed using the Heavy Ion Research Facility in Lanzhou at the Institute of Modern Physics, Chinese Academy of Sciences. Samples were irradiated by 16 MeV u^{−1 181}Ta ions beam with fluences of 1 × 10⁸ cm⁻², 3 × 10⁸ cm⁻² and 3 × 10⁹ cm⁻² (labeled as sample A, sample B and sample C) at room temperature, and the beam spot size was 4 cm × 4 cm.

The forward current density–voltage (J_F –V), reverse current density–voltage (J_R –V), and high-frequency capacitance–voltage (C–V) characteristics were measured by Keithley 4200A-SCS and Agilent B1500A semiconductor parameter analyzers. The frequency-dependent conductance measurement (G–ω) was performed to evaluate the interface states density introduced by irradiation. In addition, the Ta ions transportation process was simulated by the stopping and range of ions in matter (SRIM) software and the device characteristics were also simulated by Sentaurus with the average vacancies density calculated from SRIM and the trap level extracted by G–ω used in the model.

Figure 2(a) shows the SRIM simulation result of 16 MeV Ta ions irradiation in β-Ga₂O₃ SBD. Ta ions transferred energy to Ga atoms and O atoms in β-Ga₂O₃ along their incident path, and deposited their own energy in the target material with the slowdown process after ions incidence. Generally, there are two energy loss mechanisms before the incident ions' energy are completely lost. One is named electron energy loss (S_e), which means that high-energy incident ions transfer energy to electrons to excite or ionize electrons. The other is named nuclear energy loss (S_n), which means the elastic collision of ions with atoms during ions incidence, resulting in vacancies, interstitial atoms and Frenkel defects, etc. In this simulation, the depth of 16 MeV Ta ions in the device was about 2.51 µm, and thus, ions would not stop at the interface between the metal stack (Au/Ni) and the semiconductor (β-Ga₂O₃) but reach the drift layer instead. As the incident ions entered the drift layer, S_e decreased while S_n increased with the increase of the incident depth and S_e (7.08 KeV nm⁻¹) was 10 times that of S_n (0.67 KeV nm⁻¹). The dominant energy loss mechanism was S_e in the whole range of radiation damage, which excited or ionized electrons and induced deep defect levels. These defects served as the trap centers, deteriorating the electrical performance of the β-Ga₂O₃ SBD,

$$\begin{align}{C_{{\text{va}}}}\left( {{\text{c}}{{\text{m}}^{ - 3}}} \right) &= {\text{fluence}}\left( {{\text{c}}{{\text{m}}^{ - 2}}} \right) \nonumber \\ &\quad \times {\text{Vacancies}}\left( {{\text{vacancies}}/{{{\unicode{x00C5}} - \textrm{ion}}}} \right) \times {10^8}.\end{align} \tag{ 1 }$$

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The vacancy densities produced in β-Ga₂O₃ SBD irradiated by a single Ta ion of 16 MeV is shown in figure 2(b). The average value is 1.57/Å-ion. According to equation (1), the average vacancy densities, determined by SRIM simulation were 1.57 × 10¹⁶, 4.71 × 10¹⁶ and 4.71 × 10¹⁷ cm⁻³ for three samples after radiation, which were included in the Sentaurus model to analyze the influence on the electrical characteristics of SBD with the trap energy of ∼0.85 eV below the conduction band edge (described in the following).

Figure 3 shows the measurement and simulation results of forward J–V curves for three samples before and after 1 × 10⁸ cm⁻², 3 × 10⁸ cm⁻² and 3 × 10⁹ cm⁻² 16 MeV u^{−1 181}Ta irradiation. The simulation results were in good agreement with the experiment. With the increase of the irradiation dose, the current density decreased monotonously as the bias larger than turn-on voltage of 0.52 V (defined as the voltage @J= 10⁻³ A cm⁻²). At the forward bias of 5 V, the current density remains almost constant for sample A and decreased from 1216.3 A cm⁻² to 1027.1 A cm⁻² for sample B. However, for sample C, the forward current density was seriously decreased to 0.19 A cm⁻² and the device performance was deteriorated significantly.

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Assuming standard thermionic emission (TE) theory, the forward current density can be expressed in the following form [16]

$$\begin{equation}J = {A^ * }{T^2}\exp \left( { - \frac{{q\phi }}{{{k_0}T}}} \right)\exp \left( { - \frac{{qV}}{{n{k_0}T}}} \right) = {J_{\text{s}}}\exp \left( { - \frac{{qV}}{{n{k_0}T}}} \right)\end{equation} \tag{ 2 }$$

$$\begin{equation}{J_{\text{s}}} = {A^ * }{T^2}\exp \left( { - \frac{{q\phi }}{{{k_0}T}}} \right)\end{equation} \tag{ 3 }$$

$$\begin{equation}{A^ * } = \frac{{4\pi qm_n^ * k_0^2}}{{{h^3}}}\end{equation} \tag{ 4 }$$

where J is the current density, T the absolute temperature, q = 1.6 × 10⁻¹⁹ C is electron charge, k₀ the Boltzmann constant, and n the ideality factor representing the difference between the experimental Schottky diode and the ideal Schottky diode, J_s the reverse saturation current density, φ the Schottky barrier height, A* is the Richardson constant of 41 A cm⁻² K⁻² [17].

Figure 4(a) presents the lnJ–V curves before and after radiation for three samples, respectively. The linear part of the lnJ–V curves was taken for fitting to extract n and φ, as shown in figures 4(d) and (c). For the radiation doses of 1 × 10⁸ cm⁻² and 3 × 10⁸ cm⁻², the n and φ remain almost constant, about 1.01–1.03 and 1.08–1.11 eV. When the radiation dose increasing to 3 × 10⁹ cm⁻², φ decreased from 1.11 eV to 0.94 eV and n increased from 1.01 to 1.29, illustrating that the TE was the dominant transport mechanism but the trap-assisted tunneling and the field emission were also involved, which may be related to the introduction of trap centers at (Au/Ni)/β-Ga₂O₃ interface and the defects formation in the epilayer during Ta ions radiation [18]. When a large number of Ta ions reached the (Au/Ni)/β-Ga₂O₃ interface, much more atoms would be ionized and act as trap centers to increase the electric field at the interface, leading to the decrease of Schottky barrier height [18]. In addition, the defects also could be formed along the incident path in the β-Ga₂O₃ epilayer and introduce energy levels in the bandgap, enhancing the electron tunneling and the higher n. On the other hand, after radiation, the turn-on voltage (V_on) increased from 0.52 V to 0.54 V, 0.56 V and 0.80 V for three samples, respectively.

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Figure 5 shows the variation of the differential resistance with the forward bias for three samples before and after radiation. For sample A and B, the on-resistance (R_on) were 3.14 Ω cm² and 3.51 Ω cm² after radiation, which only has slight increment compared to the value of 2.73 Ω cm² before radiation. With the radiation dose of 3 × 10⁹ cm⁻², R_on significantly increased from 2.73 Ω cm² to 5.0 × 10⁴ Ω cm², which may be attributed to the DDs generated in β-Ga₂O₃ film after irradiation and the substantial decrease in carrier concentration and mobility [19].

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Figure 6(a) depicts the reverse J–V curves of β-Ga₂O₃ SBD before and after 1 × 10⁸ cm⁻², 3 × 10⁸ cm⁻² and 3 × 10⁹ cm⁻² 16 MeV u^{−1 181}Ta irradiation. The reverse current (I_R) of β-Ga₂O₃ SBD increased with the irradiation dose until the device broke down. The breakdown voltage was decreased from −405 V to −375 V, −350 V and −255 V for three samples, respectively. The simulation results were also presented in figure 6(b), In addition to the Shockley–Read–Hall, Auger recombination and field-dependent mobility model [20], the tunnel mode was also included in the simulation process with the deep acceptor energy level of E_C-0.85 eV (extracted by G–ω curves in the following) and the concentration of 1.57 × 10¹⁶, 4.71 × 10¹⁶ and 4.71 × 10¹⁷ cm⁻³ (the average vacancy densities determined by SRIM simulation in the above). The simulation results were in coincidence with the experiment data. As shown in figure 6(b), at −250 V, I_R increased to 5.8 × 10⁻² A cm⁻² for sample C after radiation with a dose of 3 × 10⁹ cm⁻², about three orders of magnitude higher than that before irradiation. This phenomena can be explained as the following, with φ decreasing from 1.11 eV to 0.94 eV for sample C, the reverse saturation current density is 5.8 × 10⁻² A cm⁻², much higher than 3.0 × 10⁻⁵ A cm⁻² before radiation and resulting in the reverse leakage current increase. In addition, much electrons tunneling from anode to cathode through the interface states at (Au/Ni)/β-Ga₂O₃ interface leading to the increase of reverse leakage current.

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Figure 7 presents the high frequency (1 MHz) C–V curves and 1/C² –V curves at the AC frequency of 1 MHz of three samples before and after Ta ion radiation. The C–V relationship for a Schottky barrier is [21]

$$\begin{equation}\frac{{{A^2}}}{{{C^2}}} = \frac{2}{{q{\varepsilon _0}{\varepsilon _{\text{r}}}{N_{\text{D}}}}}\left( {{V_{{\text{bi}}}} - \frac{{kT}}{q} - V} \right)\end{equation} \tag{ 5 }$$

$$\begin{equation}{N_{\text{D}}} = \frac{2}{{q{\varepsilon _0}{\varepsilon _{\text{r}}}{A^2}\frac{{{\text{d}}\left( {1/{C^2}} \right)}}{{{\text{d}}V}}}}\end{equation} \tag{ 6 }$$

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where, ₀ = 8.85 × 10⁻¹² F m⁻¹ is the vacuum dielectric constant, _r = 10 is the relative permittivity of β-Ga₂O₃, N_D is the electron concentration of the drift layer, V_bi is the built-in potential, and A (πr², r= 50 μm) is the area of SBD. The electron concentration N_d of β-Ga₂O₃ drift layer could be determined by the slope of the 1/C² –V curve, 4.92 × 10¹⁷ cm⁻³ before radiation and 3.41 × 10¹⁷ cm⁻³ for sample A, 2.91 × 0¹⁷ cm⁻³ for sample B, respectively. The electron concentration decreased by 30% (sample A) and 40% (sample B) after radiation. Moreover, we have obtained a doping profile (concentration versus depth) as shown in figure 7(c). Thus, we assumed that during Ta ion irradiation, the defects introduced in the β-Ga₂O₃ drift layer could act as deep acceptor traps to capture electrons and the carrier concentration was decreased, space charge region widened and the capacitance was also reduced. However, as the radiation dose further increasing to 3 × 10⁹ cm⁻², a large number of defects were introduced that the electrical conductivity of the β-Ga₂O₃ epitaxial layer dramatically fell down. The variation of capacitance with bias could not be obtained, as shown in figure 7(b). Furthermore, the carrier removal rate (R_C) was calculated to be 1.5 × 10⁹ cm⁻¹ and 6.7 × 10⁸ cm⁻¹ for sample A and sample B, respectively. R_C increased rapidly after low-dose irradiation (sample A), when the irradiation dose was further increased (sample B), R_C decreased for the most of the carriers in the drift layer had been removed by low-dose irradiation and the number of carriers that could be removed was limited and R_C no longer increased.

As shown in figure 8, the frequency dependent C–V curves were obtained for sample A and sample B after radiation. The capacitance changed slowly at low voltage, then increased rapidly to reach the peak capacitance at bias voltage of 0.7 V. During radiation, the high energy ions broke the atomic bonds at the metal–semiconductor interface easily and a large number of interface states were generated, capturing and releasing carriers with the bias and additional interface state capacitance was introduced [22]. At low frequencies, most of the interface states could follow the AC voltage and a high capacitance was achieved. As the frequency further increasing, the interface states could not follow the change of the AC voltage, and the capacitance gradually decreased.

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Moreover, the frequency dependent conductance measurements were employed to characterize the interface state at the metal-semiconductor contact interface, as shown in figure 9. If we assumed that the energy of the trap was continuously distributed in the whole β-Ga₂O₃ forbidden band, the equivalent parallel interface trap conductance G/ω and the trap energy below the conduction band E_T could be expressed as [23]

$$\begin{equation}\frac{G}{\omega } = \frac{{q{D_{{\text{it}}}}}}{{2\omega {\tau _{\text{T}}}}}\ln \left[ {1 + {{\left( {\omega {\tau _{\text{T}}}} \right)}^2}} \right]\end{equation} \tag{ 7 }$$

$$\begin{equation}{E_{\text{T}}} = kT\ln \left( {{\sigma _{\text{T}}}{N_{\text{C}}}{\nu _{\text{t}}}{\tau _{\text{T}}}} \right)\end{equation} \tag{ 8 }$$

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where D_it, τ_T and ω are the trap density, time constant and the radio frequency. σ_T = 1 × 10⁻¹⁵ cm² is the trapping cross section of the trap state [24], N_C = 5.2 × 10¹⁸ cm⁻³ is the density of states in the conduction band [25], ν_t = (3 kT m^{−1 *})^1/2 is the average thermal velocity of carriers, m^* = 0.28m₀ is the effective mass of electrons [26]. According to equations (7) and (8), D_it, τ_T and E_T were determined for sample A and B after radiation. The interface trap density was about 2.89 × 10¹⁵–2.49 × 10¹⁶ cm⁻² eV⁻¹ and 2.18 × 10¹⁵–4.98 × 10¹⁶ cm⁻² eV⁻¹ with the energy level of 0.85–0.87 eV below the conduction band edge for sample A and B, respectively.

In this work, Au/Ni/β-Ga₂O₃ vertical SBDs were fabricated and irradiated with swift HIs (16 MeV ¹⁸¹Ta) at the ion fluence of 1 × 10⁸, 3 × 10⁸ and 3 × 10⁹ ions cm⁻². With the radiation dose of 3 × 10⁹ cm⁻², φ decreased from 1.11 eV to 1.05 eV and n increased from 1 to 1.35. The breakdown voltage was decreased from −405 V to −375 V, −350 V and −255 V, respectively. The C–V measurement results indicated that irradiation introduced acceptor defects at interface and in the drift layer, trapping carriers and reducing the carrier concentration of the epitaxial layer. The trap density at the Ni/β-Ga₂O₃ interface were extracted to be 2.89 × 10¹⁵–2.49 × 10¹⁶ cm⁻² eV⁻¹ and 2.18 × 10¹⁵–4.98 × 10¹⁶ cm⁻² eV⁻¹ with the energy level of 0.85–0.87 eV below the conduction band edge for sample A and B, respectively.

Our results indicate that Au/Ni/β-Ga2O3 vertical SBDs has an intrinsically low susceptibility to radiation-induced material degradation. However, it will still have a negative effect on the characteristics of the Schottky junction and, hence, the performance and reliability of β-Ga₂O₃-based high power rectifiers, metal-semiconductor field effect transistor (MESFETs) and high electron mobility transistor (HFETs) operating in high-energy radiation environments.

This work was supported by the National Natural Science Foundation of China (NSFC) under Grant Nos. U21A20503, 61974112 and 61974115.

The data that support the findings of this study are available from the corresponding author upon reasonable request.

The data that support the findings of this study are available upon reasonable request from the authors.