β-Ga₂O₃ has attracted great attention in power electronics because of its ultra-wide bandgap (E_g) of 4.9 eV and high critical electric field of 8 MV/cm. Due to the excellent material properties, the Baliga’s figure of merit (BFOM) is 3444x Si, which is much higher than other wide bandgap materials like SiC and GaN^{[1, 2]}. In recent years, β-Ga₂O₃ power field-effect transistors (FETs) have achieved tremendous progress in high BFOM^{[3, 4]}. However, the absence of p-type β-Ga₂O₃ makes the breakdown voltage (BV) and the BFOM far lower than its theoretical limit. To overcome this challenge and obtain the advantages of p-n junction devices, various p-type oxides have been hetero-integrated with β-Ga₂O₃^{[5, 6]}, among which NiO with E_g of 3.8-4 eV and controllable doping concentration is proved to be greatly suitable for β-Ga₂O₃ power devices^{[7, 8]}. Moreover, great advances have been achieved in BFOM of NiO/β-Ga₂O₃ power heterojunction FETs^{[9, 10]}. However, the abrupt NiO/β-Ga₂O₃ heterojunction may produce severe trap effects. These effects include degradation of the threshold voltage (V_TH) and current under the negative bias stress (NBS)^{[11, 12]}. Few studies have focused on the critical issue of gate instability in NiO/β-Ga₂O₃ heterojunction FETs. Therefore, to promote β-Ga₂O₃ high-power applications in the future, it is important to further our investigation on the degradation mechanism of NiO/β-Ga₂O₃ heterojunction FETs.

In this work, a NiO/β-Ga₂O₃ heterojunction-gate FET (HJ-FET) was fabricated and its instability was analyzed at different gate stress voltage (V_G,s) and stress times (t_s) under negative bias stress (NBS). Two different degradation mechanisms were found: the first is a recoverable degradation owing to NiO bulk traps on the condition of a low V_G,s and short t_s; and the second is a permanent degradation caused by the ionized interface dipoles in the NiO/β-Ga₂O₃ heterojunction with large V_G,s or t_s values.

Fig. 1(a) shows the cross-sectional view and process flow of the fabricated NiO/β-Ga₂O₃ HJ-FET. A 200 nm thick Si-doped β-Ga₂O₃ epitaxial layer with a concentration of 1 × 10¹⁸ cm⁻³ was grown on the Fe-doped semi-insulating (010) β-Ga₂O₃ substrate with Agilis R&D MOCVD system. After that, the mesa isolation was realized by an inductively coupled plasma (ICP) process. Using SiO₂ as a hard mask, the source and drain N⁺ regions were selectively regrown by MOCVD with a heavy doping concentration of 1 × 10¹⁹ cm⁻³. Then, to form source/drain ohmic contacts, Ti/Al/Ni/Au (20/160/40/80 nm) were evaporated and followed by rapid thermal annealing (RTA) at 470 °C for 60 s in nitrogen. The p-type NiO was sputtered at room temperature and followed by a lift-off process. The NiO concentration and thickness are 1 × 10¹⁹ cm⁻³ and 50 nm, respectively. Finally, the gate metal Ni/Au (20/80 nm) was fabricated using an e-beam evaporation and lift-off process. The fabricated NiO/β-Ga₂O₃ HJ-FETs have a gate length (L_G) of 2 μm, gate-source separation (L_GS) of 2 μm, and gate-drain separation (L_GD) of 3 μm.

The NBS-induced instability of NiO/β-Ga₂O₃ HJ-FET was experimentally investigated using the Agilent 4155B. The NBS with a constant stress time t_s of 10 s experiments were first performed with V_G,s = −5 V to −20 V and V_DS = 0 V. After the stress was removed, the device enters the recovery process immediately with the gate/source/drain grounded, and the total recovery time (t_r) is 1000 s, as shown in Fig. 1(b). Fast measurements are performed during interrupting the recovery processes at pre-set time nodes (from 1 s to 1000 s) to record the I-V characteristics. The NBS with prolonged t_s experiments, are characterized by the stress V_G,s = −10 V and V_DS = 0 V, with the t_s from 1 µs to the total t_s of 1000 s. After the stress was removed, the device repeats the recovery experiment described above with a total t_r of 2000 s, as shown in Fig. 1(c).

Fig. 2(a) represents the transfer characteristic curves of the fabricated NiO/β-Ga₂O₃ HJ-FET, exhibiting a little hysteresis. Because the electrons in the channel are captured by the interface states in the forward sweeping (−10 V to 0 V), while these captured electrons are not released in time in the backward sweeping (0 V to −10 V). The difference in channel electron density during the forward and backward sweeping causes a little hysteresis. The max I_DS ON/OFF ratio of the device is about ~ 10⁹ at V_DS = 8 V. The measured output characteristics with specific on-resistance (R_on,sp) are shown in Fig. 2(b). The instability of NBS experiment with a constant t_s was first investigated at room temperature. Fig. 3(a) shows the recovery shifts of log-scale transfer characteristic curves after NBS with V_G,s = −5 V and t_s= 10 s. As shown in Fig. 3(a), the I_DS decreases after the stress is just withdrawn, and it almost returns to the value before stressed as the t_r increases to 1000 s. The same phenomenon is observed in Fig. 3(b), and the gate current decreases and recovers gradually to its initial after the stress is withdrawn. V_TH is extracted from the V_GS-I_DS curves at I_DS = 1 × 10⁻⁵ A and V_DS = 8 V^[13]. Fig. 3(c) gives the extracted ∆V_TH (= V_TH − V_TH0, V_TH0 is the initial V_TH without gate bias stress) with a slight positive shift, and V_TH almost returns to its initial value after the stress is withdrawn for 1000 s. V_G,s = −10 V leads to a larger V_TH shift than V_G,s = −5 V. The gate off-state current (I_GS,off) is defined as the gate current at V_GS = −10 V from the V_GS-I_GS curves. The I_GS,off degradation ratio is given as (I_GS,off(t) − I_GS,off(0))/I_GS,off(0). Fig. 3(d) shows that I_GS,off decreases by 84% for V_G,s = −5 V and then gradually recovers to its initial value. Also, V_G,s = −10 V causes a larger decrease in I_GS,off of about 87%.

NiO bulk traps could explain the I_GS,off variations. When NBS is applied, electrons injected from the gate are trapped by the acceptor-type bulk traps in NiO, causing the bulk traps to be negatively charged^[14]. After the stress is removed, the negatively charged bulk traps block the gate leakage, resulting in a decrease in I_GS,off. In terms of transfer characteristics, it will lead to a decrease in I_DS and a slight positive shift in V_TH. During the recovery phase, these negatively charged bulk traps gradually release the trapped electrons and return to the initial neutral state^[15].

The mechanism will change for a high V_G,s, as shown in Fig. 4. The NBS experiment in Fig. 4(a) shows an increase in the current of the linear region with the increasing t_r at V_G,s = −20 V, while the current in the off-region is reduced. The current did not recover significantly in the recovery process nevertheless. The extracted ∆V_TH shows that a high V_G,s causes a near-permanent negative shift in V_TH and a higher V_G,s cause a larger shift, as given in Fig. 4(b).

To investigate the instability of NiO/β-Ga₂O₃ HJ-FET under a prolonged t_s, the prolonged t_s NBS experiments were performed on another fresh device with the same structure. Fig. 5(a) shows the shifts of log-scale V_GS-I_DS curves during NBS for different t_s. As t_s increases, the curves steadily shift negatively, and the saturation current significantly increases. The extracted ∆V_TH indicates that the device characteristics are relatively stable at t_s < 20 s, but when t_s ≥ 20 s, V_TH starts to be negatively shifted and eventually decreases by about 2 V, as shown in Fig. 5(b). However, during the recovery process, both the characteristic curves and ∆V_TH are not recovered at t_r = 2000 s, as shown in Figs. 5(a) and 5(b). This non-recoverable degradation is also illustrated by the equivalent heterojunction diode current V_GS-I_GS shown in the inset of Fig. 5(b). The V_GS-I_GS curves shift toward the opposite direction compared to the V_GS-I_DS curves in Fig. 5(a). As the t_s increases, I_GS decreases and does not recover at t_r = 2000 s. The interface recombination current in the NiO/β-Ga₂O₃ diode acts as the dominant current transport mode. The decrease in I_GS is attributed to a decrease in the recombination centers.

Fig. 6 describes the mechanism of the almost permanent degradation under NBS. Due to the energy band discontinuity at the NiO/β-Ga₂O₃ heterojunction interface, the presence of interface dipoles offsets the internal electric field generated by the ionized impurities in the space charge region (SCR)^[16], as shown in Fig. 6(a). These interface dipoles may be considered as charged interface states from two sides of the heterojunction^[17]. The larger V_G,s generates an extremely large electric field at the interface, which causes some interface dipoles ionize into electrons and holes, as given in Fig. 6(b). Under the stress electric field, electrons and holes move towards β-Ga₂O₃ and NiO respectively and recombine with the ionized charges in the SCR, resulting in the thinning of the SCR and the decrease in the on-resistance, as shown in Fig. 6(c). This leads to an increase in current and a negative shift in V_TH. The almost non-recovery of V_TH indicates that this ionization is irreversible. The large potential barrier in the SCR prevents the combined electrons and holes from reorganizing into interface dipoles after the stress is withdrawn. The NiO/β-Ga₂O₃ heterojunction current (I_GS in the inset of Fig. 5(b)) transport is dominated by the interface recombination current^[18], and a reduction in interface dipoles lead to the reduction in the recombination centers, which eventually causes the leakage current in the off-region to decrease. We have also shown that interface dipoles are also ionized for prolonged experiments at relatively small V_G,s.

In summary, the impact of NBS on the electrical characteristics of NiO/β-Ga₂O₃ HJ-FETs has been studied at different stress voltages and stress times. With low V_G,s and t_s values, the NiO bulk traps capture electrons to reduce the gate-injected electrons, resulting in a decrease in leakage current. Fortunately, it will fully recover after a long t_r. With the increasing stress voltage or stress time, the ionization of the interface dipoles expands the current conduction path leading to an increase in the current and a negative shift in V_TH. This is a near-permanent degradation for NiO/β-Ga₂O₃ heterojunctions and should be considered in engineering.