Study On Interfacial Engineering For Ⅲ-Nitride MIS-HEMTs

GaN-based metal-insulator-semiconductor high-electron-mobility transistors (MIS-HEMTs) have attracted increasing attention for microwave/mm-wave power amplifiers and high-voltage switch applications, due to the suppressed leakage current and enlarged gate swing compared with the traditional Schottky-gate HEMTs. However, the large amount of interface charges between insulator and barrier layers were the critical issues to device performance, which caused serious reliability problems such as Vth instability. In this dissertation, GaN-based MIS-HEMTs with improved interface properties were fabricated by using AlN gate dielectric instead of oxide gate dielectrics, and then the interface quality was characterized using equivalent circuit and analytic models.Firstly, the high-quality AlN thin films were grown at low temperature by using plasma-enhanced ALD technique in this dissertation. The deposition recipe was optimized with TMA and NH3 as the precursors, and self-limiting growth behavior was observed at growth temperature range from 100℃ to 300℃. The growth rate per cycle for PEALD-grown AlN was 0.081nm/cycle, indicating the atomic level control of thin film deposition. At the optimized growth temperature of 300℃, AlN thin films exhibited well optical properties and surface morphology, with band gap of 5.8eV and RMS value smaller than 0.5nm. Chemical composition analysis indicated 13% of oxygen containment in AlN thin films and Al/N atomic ratio of 1.6:1. In bulk film, C impurity was effectively removed during atomic layer deposition. The structural and electrical properties of AlN thin films were improved with rapid thermal annealing, and the optimized annealing temperature was 450℃.Secondly, this dissertation proposed low-damage interface pre-treatment process and novel surface passivation techniques for GaN-based MIS-HEMTs. A significant decrease in sheet resistance and degeneration of Ohmic contact resistance for AlGaN/GaN structures was observed after soaking in HF solution, so soaking in KOH solution, which had little effects on the sheet and contact resistance, was adopted as the wet cleaning process in this work. Hall measurement indicated that, alkali treatment, plasma treatment in N2 ambition, and plasma treatment in O2/N2 ambition resulted in an increase in 2DHG density by 10% and a decrease in carrier mobility by 10-20%. While during surface plasma treatment in NH3/N2 ambition, the hydrogen groups resulted in a decrease in surface donors and 2DliG density by 10%, with an increase in mobility by 10%. Based on the surface treatment results, the in situ pre-treatment process for GaN-based MIS-HEMTs was studied, and NH3/N2 plasma pre-treatment was the optimized process to improve interface and 2DEG transport properties. After surface passivation with 10nm AIN, Al2O3, or SiN, the sheet resistance of AlGaN/GaN heterostructure was reduced by 10%, with an increase in carrier mobility and great change in 2DEG density. The additional stresses applied to AlGaN/GaN by surface pasisvation layer was measured with raman spectrum, indicating negligible influence on 2DEG properties. The -OH group in ALD-grown Al2O3 led to an increase in On surface donors and 2DEG density, while there existed NH3 during the reaction for PEALD-grown AIN and PECVD-grown SiN, causing a dramatic decrease in 2DEG density by 20-50% and an increase in carrier mobility by two times. Pulsed I-V measurement for HEMTs with PEALD-grown AIN passivation layer exhibited light current slump of 6%, much less than that of 26% for devices with PECVD-grown SiN passivation layer.Thirdly, high-performance AlGaN/GaN MIS-HEMTs were fabricated by using PEALD-grown AIN gate dielectris and the optimized interface pre-treatment process. MOS-HEMTs with 20nm ALD-grown Al2O3 gate insulator shew large negative Vth shift of 5.2V, compared with that of Schottky-gate HEMTs, while the Vth shift of MIS-HEMTs with PEALD-grown AIN gate insulator was only 0.8V, indicating that the interface charges between insulator and nitride semiconductor was reduced by several times. The C-VVth hysteresis of MIS-gate heterostructures was decreased from 0.6V to 50mV, and interface trap states estimated from multi-frequency C-V analysis was reduced from 4.61×1012cm-2 to 2.78x1012cm-2 by using AIN gate dielectric. The AIN gate dielectric effectively improved the interface and 2DEG transport properties, and resulted in an increase in peak transconductance from 203mS/mm to 289mS/mm, which was even larger than 270mS/mm for Schottky-gate HEMTs. Improved high-frequency properties for insulator-gate HEMTs were achieved by using AIN gate insulator. MIS-HEMTs with WG=100μm exhibited increased fr/fmax of 13.4GHz/16.1GHz, compared with that of 10.8GHz/11.6GHz for MOS-HEMTs. The stability factor of GaN-based HEMTs was also decreased by using AIN gate insulator. Then, high-performance GaN-based MIS-HEMTs with 5nm-Al2O3/lnm-AlN gate stack and recess-gate were fabricated on SiC substrate. Gate-recessed Al2O3/AlN/AlGaN/GaN MIS-HEMTs exhibited large saturated output current of 1.24 A/mm, high peak transconductance of 413mS/mm, high ON/OFF-state current ratio of 10-10, and breakdown voltage larger than 150V. Gate-recessed MIS-HEMTs with 0.5μm gate length exhibited high fr/fmax of 24GHz/102GHz. With biased at AB-class operation voltage,>7W/mm CW output power and>40% PAE were obtained at 5 GHz.Fourthly, the interface states and fixed charges in GaN-based MIS-HEMTs (or MOS-HEMTs) were characterized quantitatively in this dissertation. The equivalent circuit model with interface states taken into consideration for GaN-based insulator-gate heterostructures were formed, and then interface states between insulator and nitride semiconductor were mapped with conductance method. The derived trap level between PEALD-grown AIN insulator and nitride semiconductor was 0.45-0.67eV, with trap density of 0.97-2.2×1013cm-2eV-1, while ALD-grown Al2O3 led to deeper interface states at 0.52-0.72eV below conduction band, and the trap density increased from 1.6×1013cm-2eV-1 to 9.0×1013cm-2eV-1 with traps moving to deeper levels. In comparison with the traditional multi-frequency C-V measurement, conductance method was much more precise to analyze interface states. The interface fixed charges were then figured out by using the analytical model of flat-band voltage for Schottky-and insulator-gate structures, which was formed based on the schematic draw of energy band and distribution of interface charges. The interface charges between Al2O3 and barrier layer were estimated to be as much as 8.98 × 1012cm-2, causing a negative voltage shift of 3.78V, while that for MIS-gate structures was-1.18x1012cm-2, indicating that there existed few interface fixed charges, and ultra-deep level interface states dominated the positive voltage shift of 0.32V.