Xu Shengrui

Fabrication and Characteristics of AlInN/AlN/GaN MOS-HEMTs with Ultra-Thin Atomic Layer Deposited Al2O3 Gate Dielectric

Al 0.85In0.15N/AlN/GaNmetal-oxide-semiconductor high electron mobility transistors (MOS-HEMTs) employing a 3-nm ultra-thin atomic-layer deposited (ALD) Al2O3 gate dielectric layer are reported. Devices with 0.6μm gate lengths exhibit an improved maximum drain current density of 1227mA/mm at a gate bias of 3 V, a peak transconductance of 328mS/mm, a cutoff frequency fT of 16 GHz, a maximum frequency of oscillation fmax of 45 GHz, as well as significant gate leakage suppression in both reverse and forward directions, compared with the conventional Al0.85In 0.15N/AlN/GaN HEMT. Negligible C — V hysteresis, together with a smaller pinch-off voltage shift, is observed, demonstrating few bulk traps in the dielectric and high quality of the Al2O3/AlInN interface. It is most notable that not only the transconductance profile of the MOS-HEMT is almost the same as that of the conventional HEMT with a negative shift, but also the peak transconductance of the MOS-HEMT is increased slightly. It is an exciting improvement in the transconductance performance.

Comparative Study of the Characteristics of the Basal Plane Stacking Faults of Nonpolar a −Plane and Semipolar (11<span style=

Nonpolar (112?0) and semipolar (112?2) GaN are grown on r-plane and m-plane sapphire by MOCVD to investigate the characteristics of basal plane stacking faults (BSFs). Transmission electron microscopy reveals that the density of BSFs for the semipolar (112?2) and nonpolar a-plane GaN template is 3?105 cm?1 and 8?105 cm?1, respectively. The semipolar (112?2) GaN shows an arrowhead-like structure, and the nonpolar a-plane GaN has a much smoother morphology with a streak along the c-axis. Both nonpolar (112?0) and semipolar (112?2) GaN have very strong BSF luminescence due to the optically active character of the BSFs.

Characterization of GaN grown on 4H-SiC and sapphire by Raman spectroscopy and high resolution XRD ∗

The crystal quality, stress and strain of GaN grown on 4H-SiC and sapphire are characterized by high resolution X-ray diffraction (HRXRD) and Raman spectroscopy. The large stress in GaN leads to the generation of a large number of dislocations. The Raman stress is determined by the results of HRXRD. The position and line shape of the A1 longitudinal optical (LO) phonon mode is used to determine the free carrier concentration and electron mobility in GaN. The differences between free carrier concentration and electron mobility in GaN grown on sapphire and 4H-SiC are analyzed.

Influence of compressive strain on the incorporation of indium in InGaN and InAlN ternary alloys

In order to investigate the influence of compressive strain on indium incorporation in InAlN and InGaN ternary nitrides, InAlN/GaN heterostructures and InGaN films were grown by metal?organic chemical vapor deposition. For the heterostructures, different compressive strains are produced by GaN buffer layers grown on unpatterned and patterned sapphire substrates thanks to the distinct growth mode; while for the InGaN films, compressive strains are changed by employing AlGaN templates with different aluminum compositions. By various characterization methods, we find that the compressive strain will hamper the indium incorporation in both InAlN and InGaN. Furthermore, compressive strain is conducive to suppress the non-uniform distribution of indium in InGaN ternary alloys.

Stress and morphology of a nonpolar a -plane GaN layer on r -plane sapphire substrate

The anisotropic strain of a nonpolar (110) a-plane GaN epilayer on an r-plane (102) sapphire substrate, grown by low-pressure metal-organic vapour deposition is investigated by Raman spectroscopy. The room-temperature Raman scattering spectra of nonpolar a-plane GaN are measured in surface and edge backscattering geometries. The lattice is contracted in both the c- and the m-axis directions, and the stress in the m-axis direction is larger than that in the c-axis direction. On the surface of this sample, a number of cracks appear only along the m-axis, which is confirmed by the scanning electron micrograph. Atomic force microscopy images reveal a significant decrease in the root-mean-square roughness and the density of submicron pits after the stress relief.

Finite element analysis of the temperature field in a vertical MOCVD reactor by induction heating

The temperature field in the vertical metalorganic chemical vapor deposition (MOCVD) reactor chamber used for the growth of GaN materials is studied using the finite element analysis method (FEM). The effects of the relative position between the coils and the middle section of the susceptor, the radius of the coil, and the height of the susceptor on heating condition are analyzed. All simulation results indicate that the highest heating efficiency can be obtained under the conditions that the coil distributes symmetrically in the middle section of the susceptor and the ratio of the height of the susceptor to that of the coil is three-quarters. Furthermore, the heating efficiency is inversely proportional to the radius of the coil.

Improved crystal quality of GaN film with the in-plane lattice-matched In0.17Al0.83N interlayer grown on sapphire substrate using pulsed metal?organic chemical vapor deposition

We report on an improvement in the crystal quality of GaN film with an In0.17Al0.83N interlayer grown by pulsed metal—organic chemical vapor deposition, which is in-plane lattice-matched to GaN films. The indium composition of about 17% and the reductions of both screw and edge threading dislocations (TDs) in GaN film with the InAlN interlayer are estimated by high resolution X-ray diffraction. Transmission electron microscopy (TEM) measurements are employed to understand the mechanism of reduction in TD density. Raman and photoluminescence measurements indicate that the InAlN interlayer can improve the crystal quality of GaN film, and verify that there is no additional residual stress induced into the GaN film with InAlN interlayer. Atomic force microscopy measurement shows that the InAlN interlayer brings in a smooth surface morphology of GaN film. All the results show that the insertion of the InAlN interlayer is a convenient method to achieve excellent crystal quality in GaN epitaxy.

Influence of double AlN buffer layers on the qualities of GaN films prepared by metal?organic chemical vapour deposition

In this paper we report that the GaN thin film is grown by metal?organic chemical vapour deposition on a sapphire (0001) substrate with double AlN buffer layers. The buffer layer consists of a low-temperature (LT) AlN layer and a high-temperature (HT) AlN layer that are grown at 600 ?C and 1000 ?C, respectively. It is observed that the thickness of the LT-AlN layer drastically influences the quality of GaN thin film, and that the optimized 4.25-min-LT-AlN layer minimizes the dislocation density of GaN thin film. The reason for the improved properties is discussed in this paper.

Temperature dependences of Raman scattering in different types of GaN epilayers

First-order Raman scatterings of hexagonal GaN layers deposited by the hydride vapour phase epitaxy and by metal-organic chemical vapour deposition on SiC and sapphire substrates are studied in a temperature range between 303 K and 503 K. The temperature dependences of two GaN Raman modes (A1 (LO) and E2 (high)) are obtained. We focus our attention on the temperature dependence of E2 (high) mode and find that for different types of GaN epilayers their temperature dependences are somewhat different. We compare their differences and give them an explanation. The simplified formulas we obtained are in good accordance with experiment data. The results can be used to determine the temperature of a GaN sample.

The effect of a HfO2 insulator on the improvement of breakdown voltage in field-plated GaN-based HEMT

A GaN/Al0.3Ga0.7N/AlN/GaN high-electron mobility transistor utilizing a field plate (with a 0.3 μm overhang towards the drain and a 0.2 μm overhang towards the source) over a 165-nm sputtered HfO2 insulator (HfO2-FP-HEMT) is fabricated on a sapphire substrate. Compared with the conventional field-plated HEMT, which has the same geometric structure but uses a 60-nm SiN insulator beneath the field plate (SiN-FP-HEMT), the HfO2-FP-HEMT exhibits a significant improvement of the breakdown voltage (up to 181 V) as well as a record field-plate efficiency (up to 276 V/μm). This is because the HfO2 insulator can further improve the modulation of the field plate on the electric field distribution in the device channel, which is proved by the numerical simulation results. Based on the simulation results, a novel approach named the proportional design is proposed to predict the optimal dielectric thickness beneath the field plate. It can simplify the field-plated HEMT design significantly.