Deqiu Zhou

Effect of AlN/GaN superlattice buffer on the strain state in GaN-on-Si(111) system

The effect of AlN/GaN superlattice (SL) buffer on the strain state in a GaN-on-Si(111) system was studied in detail by room-temperature micro-Raman scattering measurement. An abnormal satellite peak attached to a GaN E2 peak was observed, which was verified to stem from the compressively strained GaN in SLs. The results indicate that the strain-sensitive GaN E2 (high) peak in the GaN-on-Si system with AlN/GaN SLs splits into two peaks because the GaN stress state in the top GaN layer is different from that in SLs. The compressive stress in the GaN layer in SLs was introduced by the AlN layer in each SL period because of the lattice mismatch between GaN and AlN, which ultimately counterbalanced the tensile stress in the top GaN during cooling. Such a counterbalance interaction is strongly dependent on the stiffness coefficient of AlN/GaN SLs, which is proportional to the number of periods of SLs and the relative thickness of AlN in SLs. Such two E2 peaks from GaN enable us to monitor the strain state in the GaN-on-Si system with AlN/GaN SLs quantitatively.

Current transport mechanism of AlGaN/GaN Schottky barrier diode with fully recessed Schottky anode

Both the forward and reverse-bias current transport mechanisms of an AlGaN/GaN Schottky barrier diode with a fully recessed Schottky anode (recessed-SBD) are investigated for the first time. A two-dimensional (2D) device simulation gives insight into the electronic transport. The difference between the forward and reverse conduction paths enables the reduction in Von without sacrificing the low reverse leakage current properties. The results of temperature-dependent current–voltage (T–I–V) measurements show that thermionic field emission (TFE) is the dominant current transport mechanism for forward bias. In the reverse-bias region above the pinch-off voltage, two mechanisms codetermine leakage currents, which contain Frenkel–Poole emission from the overlapped planar contact and tunneling from the recessed sidewall contact. Below the pinch-off voltage, the leakage currents are observed to have exponential temperature dependence, which may be consistent with trap-assisted tunneling (TAT).

The suppression of background doping in selective area growth technique for high performance normally-off AlGaN/GaN MOSFET

In this paper, the selective area growth (SAG) technique is used to regrow thin AlGaN/GaN heterostructure on access region for realizing trench gate normally-off AlGaN/GaN MOSFET. Heavy background doping is found in SAG AlGaN/GaN heterostructure, which is not expected for its degradation on device performance. The background doping originates from SiO2 residuals at SAG interface. Through reducing the deposition temperature of SiO2 mask, background doping can be efficiently suppressed. As a result, the 2 dimensional electron gas transport property of SAG AlGaN/GaN heterostructure improves greatly, which is as good as the as-grown AlGaN/GaN heterostructure. Moreover, the performance of normally-off SAG AlGaN/GaN MOSFET improves greatly by suppression the Si residual on GaN template.

Selective Area Growth: A Promising Way for Recessed Gate GaN MOSFET With High Quality MOS Interface

Based on the selective area growth (SAG) technique, an enhancement mode GaN recessed gate MOSFET was fabricated successfully with negligible gate trapping effect, presenting an extremely small threshold voltage (Vth) hysteresis of 50 mV at a gate bias swing up to 10 V. Compared with the larger Vth hysteresis of a recessed gate GaN MOSFET fabricated by dry-etching, the correlation between the Vth hysteresis and the lattice damage related traps caused by plasma dry etching process has been confirmed. Furthermore, the SAG recessed MOSFET shows a lower turn-ON resistance due to the higher MOS channel mobility. We believe that all of these superior performance of SAG MOSFET are attributed to the damage free high quality GaN surface at the Al2O3/GaN MOS interface, which indicates that the SAG is a promising alternative technique toward stable GaN MOSFET for the power switching applications.

Influence of the carbon-doping location on the material and electrical properties of a AlGaN/GaN heterostructure on Si substrate

The influence of different C-doping locations in a GaN/Si structure with a GaN/AlN superlattice (SL) buffer on the material and electrical properties of GaN/Si was studied. The introduction of C doping can remarkably degrade the crystal quality of the buffer. C-doping of a top GaN buffer can introduce compressive stress into the top GaN due to the size effect, while C-doping in a SL buffer can impair the compressive stress provided from the SL buffer to the top GaN. It is found that introducing high-density carbon into the whole buffer can result in a more strain-balanced GaN/Si system with small deterioration of the 2DEG channel. Furthermore, the whole buffer C-doping method is an effective and easy way to achieve a thin buffer with low leakage current and high breakdown voltage (266 V@1 nA mm−1; 698 V@10 μA mm−1; 912 V@1 mA mm−1). By using the whole-buffer C-doping method, a 2.5 μm-thick AlGaN/GaN HFET with a breakdown voltage higher than 900 V was achieved, and the breakdown voltage per unit buffer thickness can reach 181 V μm−1.

Influence of the Aln/Gan superlattices buffer thickness on the electrical properties of Algan/Gan HFET on Si substrate

The influence of the total thickness (periods of AlN/GaN SLs) of AlN/GaN superlattices (SLs) buffer on the static electrical properties of AlGaN/GaN HFETs is study for the purpose of improving devices breakdown voltage (BV) and reducing its on-resistance (R<inf>ON</inf>). It is found that for top-GaN layer with a constant thickness of lμm, a proper thickness of AlN/GaN SLs buffer (such as 2.5 μm with 100 period AlN/GaN SLs) could obtain better crystal quality of top-GaN layer as well as more robust devices with more superior performance in all aspects. The most robust HFETs in this work have achieved a specific on-resistance (R<inf>ON, SP</inf>) of 0.68 mΩ-cm² (@ L<inf>GD</inf>=4μm), a maximum on-state drain current (I<inf>D, max</inf>) of 430 mA/mm (@ L<inf>GD</inf>=4μm), an On/Off ratio of 2×10⁸, a BV of 552V at a drain leakage current of lμA/mm (@ L<inf>GD</inf>=15μm), and a figure-of-merit (FOM=BV²/R<inf>ON, SP</inf>) of 168 MW/cm² (@ L<inf>GD</inf>=8μm).

The breakdown behavior of GaN epitaxial material on silicon

In this paper, the leakage path and breakdown behavior of GaN on silicon substrate were studied systematically. Three terminal breakdown voltage characteristics of the samples with various ohmic contacts spacing were evaluated. With increasing the spacing between the contacts, the breakdown voltage increased linearly first and then saturated. In order to clarify the breakdown behavior, leakage path after breakdown test was further analyzed and the breakdown behaviors were identified. Furthermore, the burnt buffer layer was observed by FIB SEM after device breakdown intuitively. It manifested that the spacing dependent breakdown characteristics of the epitaxial layer was ascribe to that different leakage paths dominated the breakdown at different spacing.

Low-leakage current and high-breakdown voltage GaN-on-Si (111) System with an AlGaN impurity blocking layer

The influence of different AlGaN Impurity Blocking Layer (IBL) thickness in the GaN/Si (111) structure with GaN/AlN SLs buffer on the material and electrical properties of GaN/Si (111) system was studied in detail. It is found that the insertion of AlGaN IBL can increase the (102) FWHM and decrease the (002) FWHM. Meanwhile, AlGaN IBL with an optimized thickness can further improve the surface roughness and strain-state of GaN-Si (111) system. By using Secondary Ion Mass Spectroscopy, it is found that AlGaN IBL have a strong effect in blocking the Si donor impurities originating from the Si substrate during the high temperature growth, which can decrease the leakage current while the breakdown voltage can be dramatically increased.