High-k praseodymium oxide passivated AlGaN/GaN MOSFETs using P2S5/(NH4)2SX + UV interface treatment

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Abstract

This study elucidates the praseodymium oxide (Pr2O3)-passivated AlGaN/GaN metal–oxide–semiconductor high electron mobility transistors (MOS-HEMTs) with high dielectric constant, and with their AlGaN Schottky layers treated with P2S5/(NH4)2SX + ultraviolet (UV) illumination. An electron-beam evaporated Pr2O3 insulator is used, instead of traditional plasma-assisted chemical vapor deposition (PECVD), to prevent plasma-induced damage on AlGaN. In this work, the HEMTs were pretreated by P2S5/(NH4)2SX solution and UV illumination before the gate insulator (Pr2O3) was deposited. Since stable sulfur that is bound to the Ga species can be efficiently obtained and surface oxygen atoms were reduced by P2S5/(NH4)2SX pretreatment, the lowest leakage current was observed in MOS-HEMT. Additionally, a low flicker noise and a low surface roughness (1.1 nm) were also obtained using this novel process, to demonstrate its ability to reduce the surface states. Low gate leakage current Pr2O3, high-k AlGaN/GaN MOS-HEMTs, under P2S5/(NH4)2SX + UV illumination treatment are suited to low-noise applications because of its electron-beam-evaporated insulator and the new chemical pretreatment.

Introduction

Because of their inherent high breakdown voltage (VBR), high two-dimensional electron gas (2-DEG) concentration, and high saturation velocity [1], [2], AlGaN/GaN high electron mobility transistors (HEMTs) have attracted much interest for high-power and low-noise applications. The major factors that limit the performance and reliability of GaN-based HEMTs at radio frequency (RF) are their high gate leakage current and drain current collapse, which is associated with native oxide-induced surface states [3], [4]. Therefore, AlGaN/GaN metal–oxide–semiconductor HEMTs (MOS-HEMTs) in which SiO2 [5], Si3N4 [6], Ga2O3 [7], Al2O3 [8], and Sc2O3 [9] are used as the gate dielectrics were studied to solve these problems. Related works focus on the formation of a high-k insulator formation, to reduce the Schottky gate leakage current at high input signal swing and to improve channel modulation capacity. However, treatment of the interface between AlGaN and the insulator has not been studied systematically. Pretreatment before the deposition of the passivation layer between the source, drain, and gate terminals dominated the effect of the surface traps, affecting the flicker noise and current collapse problems. For example, (NH4)2SX sulfide treatment is well known to eliminate native Ga2O3 and As2O3 dangling bonds on GaAs- and InP-related semiconductors by forming stable Ga–S and As–S bonds during immersion [10], [11]. In this work, a P2S5/(NH4)2SX + UV treatment was studied to suppress the surface traps. Furthermore, to increase the P2S5/(NH4)2SX treatment efficiency, the treatment was performed in a UV chamber, and both the high-k (ε  10) Pr2O3 gate insulator and the passivation layer were deposited by electron-beam evaporation, effectively preventing the plasma-induced generation of surface states. Comparing the flicker noise performance determined from the pulsed IV of Pr2O3 AlGaN/GaN MOS-HEMT with that of traditional GaN HEMTs indicates that the surface traps can be markedly suppressed by P2S5/(NH4)2SX + UV treatment. The observed lower surface leakage current also improves the DC-RF dispersion of MOS-HEMT. X-ray photoelectron spectroscopy (XPS) measurement and secondary ion mass spectrometry (SIMS) were adopted to study the Ga–S energy bonds and the distributions of depths of oxygen and sulfur atoms following P2S5/(NH4)2SX + UV treatment.

Section snippets

Device structure and fabrication

The AlGaN/GaN HEMT heterostructures in this study were grown by atmospheric pressure metal organic chemical vapor deposition (AP-MOCVD) on 2 in. sapphire wafers. The 4000 nm-thick undoped GaN was grown first to form the buffer and channel layers. Then a 35 nm-thick undoped Al0.25Ga0.75N layer was grown as the Schottky layer. The designed structure had a sheet charge density of 1.65 × 1013 cm−2 and a Hall mobility of 1060 cm2/V s at 300 K. Fig. 1 shows the cross-sections of a Pr2O3/AlGaN/GaN MOS-HEMT

Experimental results and discussions

Table 1 presents the mobility, sheet charge density, and surface roughness of variously treated devices, characterized by Hall measurement at 300 K. The P2S5/(NH4)2SX + UV-treated devices had a sheet charge density of 1.403 × 1013 cm−2 and a Hall mobility of 1150 cm2/V s at 300 K; these values were 1.648 × 1013 cm−2, 1060 cm2/V s and 1.512 × 1013, 1087 cm2/V s for standard treated and (NH4)2SX + UV treated devices, respectively. These results clearly indicate that P2S5/(NH4)2SX + UV treatment improved the channel

Device characteristic comparisons

Fig. 5 plots the gate-to-drain IV curves of standard GaN HEMT, Pr2O3 MOS-HEMTs, and P2S5/(NH4)2SX + UV-treated Pr2O3 MOS-HEMTs. A lowering of the number surface states and leakage current in the sulfide-treated sample, P2S5/(NH4)2SX + UV treated MOS-HEMTs had a higher VON than others. The improved VON of 1.71 V of P2S5/(NH4)2SX + UV treated Pr2O3-gate device significantly reduced the gate leakage current at a high pumped gate voltage, improving the linearity and the reduction of the signal dispersion

Conclusions

In summary, Pr2O3 MOS-HEMTs with low gate leakage current and low flicker noise that underwent P2S5/(NH4)2SX + UV treatment were developed and characterized. An electron-beam-evaporated high-k insulator and a passivation layer were used to prevent formation of surface states by plasma. P2S5/(NH4)2SX + UV treatment represents a simple and efficient means of reducing the number of surface dangling bonds reduction. Based on Hall, XPS, and SIMS measurement results, this P2S5/(NH4)2SX + UV treatment can

Acknowledgements

The authors would like to thank the Nano Device Labs (NDL) for flicker noise and pulse IV measurements. This work is financially supported by the National Science Council (NSC-97-2221-E-182-048-MY3) and Green Technology Research Center of Chang Gung University.

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