p-GaN Selective Passivation via H Ion Implantation to Obtain a p-GaN Gate Normally off AlGaN/GaN HEMT

Abstract

A dependable and robust technique for nanomachining is ion implantation. In this work, hydrogen (H) ion implantation was used, for the first time, to passivate p-GaN, except for the gate area, in order to create a normally off p-GaN/AlGaN/GaN high-electron-mobility transistor (HEMT). Ion implantation passivation reduces H ion diffusion in p-GaN, allowing it to withstand temperatures above 350 °C. Through experiments and analyses, the H ion implantation energy and dosage required to passivate p-GaN, by generating Mg-H neutral complexes, were determined to be 20 keV and 1.5 × 10¹³ cm⁻², respectively. After conducting annealing procedures at various temperatures, we discovered that 400 °C was the ideal temperature to effectively obtain a normally off p-GaN HEMT. A threshold voltage of 0.8 V was achievable. The p-GaN HEMT also had a breakdown voltage of 642 V at a gate voltage of 0 V, maximum transconductance of 57.7 mS/mm, an on/off current ratio of 10⁸, an on-resistance of 8.4 mm, and a maximum drain current of 240.0 mA/mm at a gate voltage of 6 V after being annealed at 400 °C.

1. Introduction

Due to its wide bandgap and high electron mobility, AlGaN/GaN high-electron-mobility transistors (HEMTs) are advantageous for power electronics [1,2,3,4]. The conventional HEMT is a normally on device [3,4,5,6]. However, a normally off device is more secure [7,8,9,10]. The p-GaN gate [11,12,13] is one of the most common methods of preparing a normally off HEMT. In some studies, etching p-GaN [14,15] and passivating p-GaN using hydrogen plasma [16,17] are effective techniques to realize the p-GaN gate via the selective shedding of p-GaN. Nevertheless, etching can easily cause interface damage. Additionally, hydrogen will diffuse at high temperatures after hydrogen plasma treatment. There are still some difficulties associated with the stability and controllability of using these techniques in practice [18].

In this work, to obtain a p-GaN gate, H ion implantation, which is a reliable and stable method, was conducted to selectively shed the p-GaN layer. The Mg acceptors in p-GaN can form Mg-H neutral complexes [16,17] via H ion implantation. Then, the p-GaN becomes a high-resistance GaN. H ion implantation also can be applied to introduce spin carriers or p-GaN-based spintronics [19,20,21]. The H concentration is accumulated on the surface after H plasma treatment. H is easily diffused into the p-GaN gate. When exposed to high temperatures, the device will fail [16]. Due to the modicum of H ion on the surface due to ion implantation passivation, p-GaN can withstand temperatures above 350 °C. After conducting our experiments and analyses, the factors of H ion implantation containing the implantation energy, ion dose, and annealing conditions to passivate p-GaN were ascertained. The p-GaN HEMT with H ion implantation successfully achieved normally off operation. A maximum drain current of 240 mA/mm was found to be larger than that of p-GaN gate HEMTs using selective plasma etching and metallization technologies [12].

2. Materials and Methods

The p-GaN/AlGaN/GaN heterojunction was grown via metal-organic chemical vapor deposition on a 6-inch Si substrate. As shown in Figure 1a, the epitaxy material was composed of a GaN buffer layer, a GaN channel layer, an AlN space layer, an Al_0.2Ga_0.8N barrier layer, and a p-GaN cap layer with 3 × 10¹⁹ cm⁻³ Mg doping. The device was isolated first. Before source (S) and drain (D) deposition, the p-GaN was graphically etched using inductively coupled plasma. Then, Ti/Al/Ni/Au metals were deposited on the source and drain regions, and ohmic contacts were formed after annealing at 875 ℃ for 30 s. Additionally, Ni/Au metals were evaporated at the gate electrode (G) [16,17]. The devices were coated with silicon nitride (SiN_x) as the sacrificial layer. After that, the p-GaN cap layer was deactivated via H ion implantation, except for the gate regions, as shown in Figure 1a. Finally, the SiN_x was removed via reactive ion etching, and the p-GaN HEMT shown in Figure 1b was prepared. Figure 1c displays an optical image of the p-GaN HEMT.

3. Results and Discussion

Firstly, the H ion implantation energy was determined. In this work, 15 keV and 20 keV implantation energies were applied to p-GaN HEMTs. Figure 2a shows the transfer curves of devices with H ion implantation energies of 15 keV and 20 keV, without annealing. The drain current (I_DS) of the device with an implantation energy of 15 keV is 85.5 mA/mm at a gate voltage (V_G) of 6 V, which is 38% more than that with a H ion implantation energy of 20 keV. Additionally, the values of peak transconductance (G_m) are 31.7 mS/mm and 26.7 mS/mm for p-GaN HEMTs with H ion implantation energies of 15 keV and 20 keV, respectively. The average threshold voltages (V_Th) of the p-GaN HEMT, defined as V_G at I_DS = 10 μA/mm, are −0.4 V and −0.2 V after H ion implantation of 15 keV and 20 keV. Additionally, the gate leakage at the negative bias of devices with H ion implantation energies of 15 keV and 20 keV are shown in Figure 2c. The gate leakage current (I_G) of the p-GaN HEMT with a H ion implantation energy of 15 keV is 68% lower than that with a H ion implantation energy of 20 keV at V_G = −8 V. Therefore, 15 keV is more suitable than 20 keV as a H ion implantation energy in the p-GaN HEMT.

Thus, this H ion implantation dose is indispensable for passivating p-GaN completely. During simulation, the H ion dose is about 1.0 × 10¹³ cm⁻² in this work. To further determine the dose in p-GaN HEMTs, H ion doses of 1.0 × 10¹³ cm⁻² and 1.5 × 10¹³ cm⁻² were experimented with. Figure 3a contrasts the transfer characteristics at the log scale of devices without H ion implantation, and with H ion implantation doses of 1.0 × 10¹³ cm⁻² and 1.5 × 10¹³ cm⁻², without annealing. Before H ion implantation, the device is a normally on HEMT. The average value of V_Th is −0.2 V after 1.0 × 10¹³ cm⁻² H ion implantation. This means a H ion implantation dose of 1.0 × 10¹³ cm⁻² cannot passivate p-GaN entirely. Fortunately, as shown in Figure 3b, the average value of V_Th is 0.1 V in the p-GaN HEMT after a H ion implantation dose of 1.5 × 10¹³ cm⁻². This indicates that after a H ion implantation dose of 1.5 × 10¹³ cm⁻², the device is a normally off HEMT. Figure 3c shows the output characteristics at V_G = 6 V of p-GaN HEMTs without H ion implantation, and with H ion implantation doses of 1.0 × 10¹³ cm⁻² and 1.5 × 10¹³ cm⁻². The maximum drain current of the device at V_G = 6 V with a H ion implantation dose of 1.5 × 10¹³ cm⁻² is 53.7 mA/mm, which is less than that with a dose of 1.0 × 10¹³ cm⁻² (81.1 mA/mm) and without H ion implantation (228.9 mA/mm).

To passivate p-GaN completely, sufficient dosage and energy are required. Nevertheless, the lattice damage increases as the energy and dose of ion implantation increase, resulting in an increase in the device’s on-resistance and a decrease in the saturation current. To obtain a high saturation current, lattice damage must be repaired through high-temperature annealing. Three temperatures (350, 375, and 400 °C) were adopted in this work. As shown in Figure 4a, the transfer characteristics of p-GaN HEMTs during H ion implantation, after annealing at three temperatures, can be compared. After annealing at 350, 375, and 400 °C, the average values of the threshold voltages in Figure 4b are 0 V, 0.6 V, and 0.7 V, respectively. In Figure 4c, the gate leakage values of p-GaN HEMTs with H ion implantation at V_G = −8 V, after annealing at 350, 375, and 400 °C, are 1.2 × 10⁻⁴ mA/mm, 6.3 × 10⁻⁶ mA/mm, and 1.3 × 10⁻⁶ mA/mm, respectively. Compared with the three annealing treatments, 400 °C is the optimal temperature.

Finally, the p-GaN HEMT obtained via H ion implantation with energy of 15 keV and a dose of 1.5 × 10¹³ cm⁻², after annealing at 400 °C, was measured. The on/off current ratio is ~10⁶, as shown in Figure 4a. From the transfer curve in Figure 5a, it can be seen that the value of G_m is 57.7 mS/mm. The maximum drain current at V_G = 6 V is 240.0 mA/mm, as shown in Figure 5b. Additionally, the on-resistance (R_ON) is calculated to be 8.4 Ω·mm. As shown in Figure 5c, the breakdown voltage is 642 V at V_G = 0 V.

4. Conclusions

In summary, we investigated the H ion implantation of a p-GaN HEMT to passivate p-GaN, except for the gate region. The H ion implantation energy was 15 keV. Additionally, to passivate p-GaN completely, the H ion implantation dose was 1.5 × 10¹³ cm⁻². Annealing treatment was implemented after H ion implantation to improve the crystal damage caused by implantation. Compared with 350 and 375 °C, 400 °C was an ideal temperature for annealing treatment in this work. After annealing at 400 °C, the normally off device with H ion implantation succeeded, with a threshold voltage of 0.8 V and an on/off current ratio of 10⁸.