Investigating the effect of O2 plasma treatment on the operational characteristics of Schottky-gate AlGaN/GaN HEMT

This study investigates the effect of O2 plasma treatment on the physical and electrical properties of the surface region in Schottky-gate AlGaN/GaN high electron mobility transistor (HEMT). We demonstrate that O2 plasma treatment significantly reduces the gate leakage current and enhances the on/off ratio by three orders of magnitude compared to devices without treatment. The O2 plasma treatment removes organic chemical residue and forms Ga–O bonds on the AlGaN surface beneath the gate metal. X-ray photoelectron spectroscopy results indicate that the treatment effectively forms a Ga–O compound oxide layer, which provides surface passivation. Furthermore, atomic force microscope analysis reveals a 50% reduction in surface roughness after the O2 plasma treatment. Using O2 plasma oxidation treatment caused a shift in the threshold voltage (VTH ) of Schottky-gate AlGaN/GaN HEMT. Initially measured at −5.26 V, the VTH value shifted to +0.5 V. Furthermore, we also employ TCAD simulation to assist in the process developed during the manufacturing process. It is worth noting that the drain current decreases as the Ga–O compound oxide layer increases. This is due to effectively depleted the polarization charges at the AlGaN/GaN interfaces during E-mode operation when reducing the thickness of the AlGaN layer beneath the gate metal. Our results demonstrate the importance of O2 plasma surface treatment in achieving optimal device performance. This study systematically discusses the effect of O2 plasma on AlGaN/GaN surface properties and the formation of Ga–O bonding. It offers insights into developing high-performance Schottky-gate AlGaN/GaN HEMT.


Introduction
As a semiconductor material with a wide band gap, gallium nitride (GaN) holds a promising future in the realm of power semiconductor devices.It boasts several advantages over silicon material, including the wide band gap, high breakdown electric field and superior electron mobility.The strong polarization effect in III-V compound semiconductors enables the formation of a two-dimensional electron gas with a high concentration and high mobility at the AlGaN/GaN interface.GaN-based high electron mobility transistors (HEMTs) offer distinct benefits, such as low on-resistance, rapid switching speed and a formidable power handling capability, particularly advantageous in power-switching applications and emerging wireless base station technologies [1][2][3].In practical scenarios, ensuring the attainment of the low gate leakage current characteristic during each fabrication process is crucial for integrating AlGaN/GaN HEMTs into low-loss power control circuits or high-efficiency microwave integrated circuits.To reduce gate leakage current, several methods have been proposed, including thermal oxidation, electrochemical oxidation (using a mixture of propylene glycol and tartaric acid) and oxygen plasma oxidation in an oxygen-rich environment.However, the oxidation of the AlGaN surface beneath the gate electrode results in the formation of a thin composite insulator consisting of Al 2 O 3 and Ga 2 O 3 [4,5].The presence of plasma-induced interfacial defects and fixed charges can detrimentally impact the performance of AlGaN/GaN HEMTs, particularly under conditions of increasing currents and temperatures.Numerous gate dielectrics, such as Al 2 O 3 [6], SiO 2 [7], SiN X [8], HfO 2 [9], La 2 O 3 [10], MgCaO [11] and LaHfO X [12] have been explored to reduce gate leakage current, but the interface qualities of the dielectric and the barrier continue to be a challenge [13,14].
This study investigates the influence of the surface quality of AlGaN/GaN on the operational characteristics of O 2 plasma-treated AlGaN/GaN HEMT.Our findings from atomic force microscope (AFM) and X-ray photoelectron spectroscopy (XPS) elemental bonding studies demonstrate that O 2 plasma treatment effectively removes organics from the AlGaN/GaN surface and produces slightly Ga-O bonds with gallium, resulting in a flatter AlGaN surface.

Sample structure and experiment
In the experiment, O 2 plasma treatment was utilized to remove organic compounds and slightly surface passivate the AlGaN/GaN surface before depositing the Schottky metal, as shown in figure 1(a).After the crucial O 2 plasma treatment process, the AlGaN/GaN HEMT was fabricated by a standard Schottky-gate HEMT process flow that included gate metal deposition, SiNx passivation, the opening of contact holes and metal pad formation, as shown in figure 1(b).The wafer consisted of a 1000 µm Si substrate, a 4 µm buffer layer, 300 nm u-GaN, 1 nm AlN, 25 µm AlGaN and 1 nm GaN cap.The mesa was then isolated using BCl 3 plasma.Work-In-Process HEMT with Schottky-gate designs was created by initially depositing a Ti/Al/Ni/Au metal stack for ohmic contact, followed by annealing at 825 • C for 30 s in an N 2 environment to generate source and drain ohmic connections.The backend process involved depositing a Ni/Au metal stack as a Schottky-gate, followed by SiNx passivation.Contact openings were made to serve as pad electrodes for the Ni/Au metal stack.The specific characteristics of the Schottky-gate AlGaN/GaN HEMT that were fabricated included the gate (L g ), gate-source (L gs ), gate-drain (L gd ) lengths and width gate (W g ) of 2, 5, 10 and 100 µm.

Result and discussion
Prior to depositing the Ni/Au Schottky-gate contacts, O 2 plasma treatment was applied to the AlGaN/GaN gate region.The surface roughness of the gate region was analyzed using AFM and the results are shown in figure 2. To confirm the effectiveness of the O 2 plasma treatment in reducing roughness, the surface roughness of the AlGaN surface was evaluated.The root mean square (RMS) surface roughness reduced after the O 2 plasma treatment, with RMS values of 0.27 and 0.17-0.18nm observed without and with O 2 plasma, respectively, indicating a 50% improvement in surface roughness reduction.
Figures 3(a) and (b) and table 1 show the spectra of Ga-Ga, Al-Ga-N,and Ga-O obtained from the AlGaN surface after plasma oxidation of the AlGaN surface with O 2 plasma by XPS analysis.During all O 2 plasma oxidations, the Ga oxide peaks increased, while the Ga-Ga and Al-Ga-N peaks decreased.This indicates that O 2 elements can react with Ga to generate Ga-O bonds on the AlGaN surface after performing the surface treatment.As a result, the AlGaN/GaN surface was modified by O 2 plasma, producing a flat and uniform surface due to the organics removed and simultaneously, the Ga-O bond formed to surface passivation function [15][16][17].
The L g , L gs , L gd and W g of the Schottky-gate AlGaN/GaN HEMT feature were 2, 5, 10 and 100 µm respectively.Figure 4 and table 2 show the I G and I D -V G characteristics of Schottkygate AlGaN/GaN HEMT without and with O 2 plasma surface treatment.The gate leakage current (I G ) in the O 2 plasma treatment device was 5.34 × 10 −4 mA mm −1 , lower than the untreated device (I G = 2.66 × 10 −1 mA mm −1 ) at the V G of 10 V measured condition.An on/off ratio of 1.26 × 10 6 was achieved in the O 2 plasma device, as shown in table 2, which was three orders of magnitude higher than without the O 2 plasma device.V TH observed after O 2 plasma surface treatment.Significantly, this study provides valuable process information for the semiconductor industry by delivering the trade-off between the slight reduction in I D current and the improvement in gate leakage achieved through variations in O 2 plasma duration.

Conclusion
This study used GaN-on-Si wafers to build high-performance Schottky-gate AlGaN/GaN HEMT.Before depositing Ni/Au Schottky-gate contacts, O 2 plasma treatment was applied to the AlGaN/GaN gate region.Our findings demonstrate that this surface treatment improved the gate leakage current and achieved an on/off ratio of 1.26 × 10 6 , three orders of magnitude higher than without O 2 plasma treatment.Additionally, the breakdown voltage measurement indicated a significant improvement in the device treated with O 2 plasma, with nearly double the value compared to the untreated device.
The observed improvement can be attributed to the effective removal of organic chemical residue and the formation of Ga-O bonds on the AlGaN surface beneath the gate metal through O 2 plasma treatment.Additionally, applying O 2 plasma oxidation treatment resulted in a significant shift in the V TH of Schottky-gate AlGaN/GaN HEMT.Initially measured at −5.26 V, the V TH value shifted to +0.5 V.The TCAD simulation results of I D -V G strongly agreed with our study, indicating a rightward (positive) shift in V TH for devices treated with O 2 plasma.The reduced thickness of the AlGaN layer beneath the gate metal effectively depleted the polarization charges at the AlGaN/GaN interfaces during E-mode operation.Overall, our study makes a valuable contribution to advancing Schottkygate AlGaN/GaN HEMT technology by explicitly investigating the effect of O 2 plasma treatment duration on AlGaN surfaces.Our analysis, which weighs a slight reduction in I D current against an improvement in gate leakage, yields unique insights that can enhance existing knowledge and impact semiconductor device manufacturing, establishing its importance in the field.

Figure 1 .
Figure 1.(a) Two functional surface treatments by O 2 plasma on AlGaN/GaN surfaces and (b) Schottky-gate AlGaN/GaN HEMT process flow.

Figure 2 .
Figure 2. AFM analysis to compare the surface roughness of AlGaN without 0s, with 100s and with 180s O 2 plasma treatment.

Table 1 .
XPS analysis results without 0s, with 100s and with 180s O 2 plasma treatment.

Figure 4 . 2 .
Figure 4.I D , I G -V G of Schottky-gate AlGaN/GaN HEMT without 0s and with 100s O 2 plasma treatment.

Figure 5 .
Figure 5.I D -V G TCAD simulation of Schottky-gate AlGaN/GaN HEMT without and with O 2 plasma treatment.