Papers by Keyword: GaN

Abstract: This paper presents a study of the vertical current transport in a graphene (Gr) heterostructure with Al_xGa_1-xN/GaN, which represent the main building block of a novel high frequency device, the hot electron transistor (HET) with Gr base. The morphological and electrical properties of this heterostructures have been investigated at nanoscale by atomic force microscopy (AFM) and conductive atomic force microscopy (CAFM). In particular, local current-voltage measurements by the CAFM probe revealed the formation of a Schottky contact with low barrier height (∼0.41 eV) and excellent lateral uniformity between Gr and AlGaN. Basing on the electrical parameters extracted from this characterization, the theoretical performances of a HET formed by a metal/Al₂O₃/Gr/AlGaN/GaN stack have been evaluated.

Abstract: This paper fabricates rare earth(RE) La, Ce and Pr doped GaN film by sol-gel method and uses X-ray diffraction analysis (XRD), scanning electron microscopy (SEM) and photoluminescence spectroscopy (PL) to characterize and analyze their dimensions, morphology and optical properties. The results show that doped GaN film is hexagonal wurtzite structure and has good crystallinity. With the increasing of the Ce doping amounts, the particle size of GaN increases gradually. While augmenting the doping amounts of La or Ce can cause particle size to be smaller. Several RE doped GaN have inhibitory effects on yellow luminescence and generate red emission peaks. The GaN:La don’t appear photoluminescence peak except for the characteristic peak of GaN. Ce doped GaN appear emission peaks with narrow FWHM between 500~600nm. 5% Pr doping enhances the intrinsic excitation and blue band edge luminescence of GaN. Through the analysis, it can be found that La, Ce and Pr doping influence the grain growth and photoluminescence spectrum of GaN film effectively.

Abstract: GaN epitaxial layers were successfully grown by hydride vapour phase epitaxy (HVPE) on β-Ga₂O₃ substrates produced by cleaving. The initial stages of GaN epitaxial growth on β-Ga₂O₃ were studied by scanning electron microscopy (SEM) and x-ray diffraction analysis (XRD). The nucleation and the transition from the nucleation layer to a continuous GaN film were studied. It was found that the growth starts with formation of small crystallites on the substrate surface. As the growth continues, crystallites transform into pyramidal islands which increase in size and merge together. It was found that the structural quality of the GaN layers rapidly improves with increasing thickness. The full width at half maximum of x-ray ω rocking curves for (0002) peak decreased from 1370 to 540 arcsec as the deposition time was increased from 30 to 120 sec. This corresponds to the variation of the nominal layer thickness from 250 nm to 1000 nm.

Abstract: In this study, we investigated the possibility of smoothing a GaN substrate utilizing ultraviolet (UV) assisted polishing method in potassium hydroxide (KOH) solution. In this polishing method, GaN substrate was excited by an UV radiation, and then an oxide layer on the GaN substrate was formed by photochemical reaction. Simultaneously, generated oxide layer was removed by synthetic quartz tool and chemically etched by KOH solution. Finally, smoothed GaN surface could be realized. The surface quality and removal depth were measured and evaluated using a scanning white light interferometer and Normalski type differential interference microscopy. Obtained results show that the surface morphology and the removal rate are strongly dependent on the existence of the UV irradiation. Moreover, the processed surface has revealed that many scratches on the preprocessed GaN surface could be completely removed. The microroughness of the processed GaN surface profile was improved to be 0.18 nm (Rms), 1.06 nm (Rz).

Abstract: We previously reported a unipolar mode p⁺-polycrystalline silicon (poly-Si)/4H-SiC heterojunction diode (SiC-HJD) [1-3]. In this work, we demonstrate a poly-Si/GaN vertical unipolar heterojunction diode (GaN-HJD) based on numerical simulation and experimental results. The GaN-HJD is expected to control the electrical characteristics of both Schottky action with a p-type poly-Si and ohmic action with an n-type poly-Si. We investigated the detailed physics of the GaN-HJD between p⁺ Si and n⁺ Si by numerical simulation. The GaN-HJD was also fabricated with p⁺-type polycrystalline silicon on an n^--type epitaxial layer on bulk GaN substrates. The measured barrier height of the GaN-HJD was 0.79 eV and the ideality factor was 1.10.

Abstract: AlGaN/GaN heterostructures are important materials for the fabrication of high power and high frequency devices. However, the mechanisms of Ohmic contacts formation on these systems are continuously under scientific debate. In this paper, a structural and electrical investigation of Ti/Al/Ni/Au Ohmic contacts to AlGaN/GaN heterostructures is reported. In particular, the behavior of Ti/Al/Ni/Au multilayers was monitored at different annealing temperatures. The contacts became Ohmic after annealing at 750°C and showed a decreasing temperature behavior of the specific contact resistance R_C, described by a thermionic field emission mechanism. On the other hand, annealing at 850°C led to a further reduction of R_C , with a slightly increasing dependence of R_C on the measurement temperature (here regarded as a “metal-like” behavior). The microstructural analysis of the interfacial region allowed to explain the results with the formation of metallic intrusions contacting directly the two dimensional electron gas.

Abstract: GaN High Electron Mobility Transistors (HEMTs) on SiC have gained remarkable attention as these devices are revolutionizing the power and radio frequency (RF) electronics markets. Although significant advances have been made in transistor technology innovations, innovations pertaining to the backside process technology have been noticeably few. This paper will address and focus on innovations in the backside processing of GaN on SiC devices. A series of innovations in backside processing enable through SiC and GaN via etch rates to exceed 1.5 micron/minute. Wafer dicing process innovations afforded a >4x improvement in sawing throughput and a >6x improvement in blade lifetime through the novel addition of ultrasonic power to a conventional sawing tool.

Abstract: In this paper, we propose a physics-based analytical model of novel InAlN/GaN High Electron Mobility Transistor (HEMT) by considering the quasi-triangular quantum well with minimal empirical parameters. The derived model is compared for different short and long gate length devices. The results are calibrated and verified with experimental data over a full range for gate and drain applied voltages. Significant improvement in n_s, drain Current, and transconductance are observed for InAlN HEMT making it suitable for nanoscale and microwave analysis in circuit design. Therefore, the proposed model can deal directly with device/physical parameters, and it can be expressed by a very small number of model parameters.

Abstract: A method to grow gallium nitride (GaN) films directly on the graphene layers by hydride vapor phase epitaxy (HVPE) method is reported in this work. We used a chemical vapor deposition (CVD) method to grow graphene on a copper foil, and the test results showed the presence of monolayer graphene at most regions. GaN films were grown on the graphene/MO-GaN substrate (GaN which was grown by metal organic vapor phase deposition) and MO-GaN template by HVPE method. Raman Spectroscopy, Scanning Electron Microscopy (SEM), and X-Ray Diffraction (XRD) were adopted for characterization. By comparing with the MO-GaN substrate, the crystalline quality of the GaN films were both increased. However, the crystalline quality of the HVPE-GaN with graphene interlayer is slightly lower than that directly grown on MO-GaN.

Abstract: Time-resolved photoluminescence spectra of vertical cavity surface emitting laser (VCSEL) structures under different excitation intensity are investigated. The effect of the distributed Bragg reflectors (DBR) on the laser emission and the mechanism of multi-longitudinal-mode are analysed. A broad peak around 2.80eV is observed from the structure without DBR cavity when pumped under low excitation intensity. At higher excitation density up to 21.4kW/cm², a lasing peak appears at 2.86eV, and exhibits a rapid growth and red shift with the increase of the excitation density. The decay time of the peak is about 66ps. When the spectrum is measured after the deposition of DBR on the top side of the grown nitride structure, the number of peaks increases to 7, and the free spectral range 0.21×10¹⁴ Hz is close to the value that calculated by the Fabry-Perot cavity length. After depositing the second DBR, multi PL peaks around 2.87eV are observed. The relationship of laser emission intensity and angle of the polarizer shows nearly a cosine square variation. However, the polarization characteristics of the four main peaks are different, which indicates that these peaks occurr by the oscillation of different optical cavities.