Peixian Li

Spatial distribution of crystalline quality in N-type GaN grown on patterned sapphire substrate

The epitaxial layers of n-type GaN were grown on both planar and patterned sapphire substrate (PSS) by metal organic chemical vapor deposition. By comparing the epitaxial layers grown on planar substrate, GaN grown on PSS exhibited many improvements both on surface morphology and crystalline quality according to the characterization of atoms force microscopy, and high resolution X-ray diffraction. Spatially resolved micro-Raman scattering results were performed for mapping the spatial variations in crystalline quality of the n-type GaN grown on PSS. According to the variations on the intensity and the full width at half maximum of GaN E2 (high) peaks, the crystalline quality improvement occurred in the lateral growth regions which correspond to center region of the pyramid patterns. We proposed that the bending of dislocations during the lateral growth plays an important role in the spatial variations of GaN crystalline quality. Cross sectional transmission electron microscope and spatial cathodoluminescence mapping results further supported the explanation of the dislocation inhibition during the growth process of GaN grown on PSS.

An InGaN-Based Solar Cell Including Dual InGaN/GaN Multiple Quantum Wells

An InGaN/GaN solar cell including a dual multiple quantum wells (MQWs) structure is investigated. It shows an obvious advantage over the conventional InGaN/GaN cell, which only contains a single MQWs structure. Because the short current density (JSC) increases, the 1 sun power conversion efficiency significantly improves from 0.62% (single MQWs cell) to 1.02% (dual MQWs cell). From the measurement of EQE and PL spectra, the enhancement of effective photoelectric response within the solar spectrum mainly contributes to the high device performance, due to the introduced upper MQWs of higher In content.

Finite element analysis and optimization of temperature field in GaN-MOCVD reactor

The electromagnetic field distribution of the in-home GaN-MOCVD reactor heated by induction was simulated by using finite element method (FEM), and the distributions of the magnetic field and the joule heat in the graphite susceptor were obtained. Then the distribution of joule heat was used as the applied load for the simulation of temperature. Based on heat conduction and heat emission models, the temperature distribution in the reactor and the susceptor were gained. In order to improve the uniformity of the temperature on the top surface of the susceptor, the conventional concentric placement of the susceptor in the reactor was changed into the eccentric placement, which effectively improved the temperature uniformity and increased the heat efficiency.

Improved Semipolar (11

The effect of a self-organized SiNx interlayer on the defect density of (112) semipolar GaN grown on m-plane sapphire is studied by transmission electron microscopy, atomic force microscopy and high resolution x-ray diffraction. The SiNx interlayer reduces the c-type dislocation density from 2.5 × 1010 cm−2 to 5 × 108 cm−2. The SiNx interlayer produces regions that are free from basal plane stacking faults (BSFs) and dislocations. The overall BSF density is reduced from 2.1 × 105 cm−1 to 1.3 × 104 cm−1. The large dislocations and BSF reduction in semipolar (112) GaN with the SiNx interlayer result from two primary mechanisms. The first mechanism is the direct dislocation blocking by the SiNx interlayer, and the second mechanism is associated with the unique structure character of (112) semipolar GaN.

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The epitaxial layers of InGaN/GaN MQWs structure were grown on both planar and vicinal sapphire substrates by metal organic chemical vapor deposition. By comparing the epitaxial layers grown on planar substrate, the sample grown on 4° misoriented from c-plane toward m-plane substrate exhibited many variations both on surface morphology and optical properties according to the scanning electronic microscopy and cathodoluminescence (CL) spectroscopy results. Many huge steps were observed in the misoriented sample and a large amount of V-shape defects located around the boundary of the steps. Atoms force microscopy images show that the steps were inclined and deep grooves were formed at the boundary of the adjacent steps. Phase separation was observed in the CL spectra. CL mapping results also indicated that the deep grooves could effectively influence the localization of Indium atoms and form an In-rich region.

2) GaN Quality Grown on

This is the report on trap states in enhancement-mode AlGaN/GaN/AlGaN double heterostructures high electron mobility transistors by fluorine plasma treatment with different GaN channel layer thicknesses. Compared with the thick GaN channel layer sample, the thin one has smaller 2DEG concentration, lower electron mobility, lower saturation current, and lower peak transconductance, but it has a higher threshold voltage of 1.2 V. Deep level transient spectroscopy measurements are used to obtain the accurate capture cross section of trap states. By frequency dependent capacitance and conductance measurements, the trap state density of (1.98–2.56) × 1012 cm−2 eV−1 is located at ET in a range of (0.37–0.44) eV in the thin sample, while the trap state density of (2.3–2.92) × 1012 cm−2 eV−1 is located at ET in a range of (0.33–0.38) eV in the thick one. It indicates that the trap states in the thin sample are deeper than those in the thick one.

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Temperature dependences of the polarized Raman scattering spectra in the backscattering configuration of the semipolar (11 2¯2) plane GaN thin film are analyzed in the range from 83 K to 563 K. The semipolar GaN film is cut at a tilted angle from polar GaN wafer grown by hydride vapor phase epitaxy. The spectral features of the frequency shift and linewidths of the Raman-active phonon modes Quasi-TO, E1 (TO), E2 (high), and Quasi-LO are prominently revealed, and the temperature coefficients corresponding to the lattice thermal expansion and phonon anharmonic effect, as well as defects and impurities in crystals, are well deduced by the theoretical equations. With the increasing temperature, the Raman scattering peaks would substantially shift to lower frequencies and the linewidths gradually broaden. Our studies will lead to a better understanding of the fundamental physical characteristics of the semipolar (11 2¯2) plane GaN film.

-Plane Sapphire Substrates by Metal Organic Chemical Vapor Deposition Using Self-Organized SiN

The transport mechanisms of the reverse leakage current in the UV light-emitting diodes (380 nm) are investigated by the temperature-dependent current-voltage measurement first. Three possible transport mechanisms, the space-limited-charge conduction, the variable-range hopping and the Poole—Frenkel emission, are proposed to explain the transport process of the reverse leakage current above 295 K, respectively. With the in-depth investigation, the former two transport mechanisms are excluded. It is found that the experimental data agree well with the Poole—Frenkel emission model. Furthermore, the activation energies of the traps that cause the reverse leakage current are extracted, which are 0.05 eV, 0.09 eV, and 0.11 eV, respectively. This indicates that at least three types of trap states are located below the bottom of the conduction band in the depletion region of the UV LEDs.

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A novel and accurate method is proposed to extract the intrinsic elements of the GaN high-electron-mobility transistor (HEMT) switch. The new extraction method is verified by comparing the simulated S-parameters with the measured data over the 5–40 GHz frequency range. The percentage errors Eij within 3.83% show the great agreement between the simulated S-parameters and the measured data.