Guangyi Xi

Enhancement of electron-longitudinal optical phonon coupling in highly strained InGaN/GaN quantum well structures

An anomalously strong phonon replica is observed in the photoluminescence (PL) spectra of In0.23Ga0.77N/GaN multiple quantum well (MQW) samples at 10 K. The Huang-Rhys factor for the two-well sample reaches 0.95, in contrast to the value of 0.45 for the five-well sample obtained with the same growth conditions. Narrow linewidth and clearly resolved emission peak splitting are also exhibited in the two-well structure. The strong enhancement of electron-longitudinal optical (LO) phonon coupling is attributed to the interface features and the high residual strain in the quantum well structure. A model is proposed to account for the experimental results that takes the interface optical phonon mode as well as the impact of the residual strain into consideration.

Study on Injection Efficiency in InGaN/GaN Multiple Quantum Wells Blue Light Emitting Diodes

The dependence of the electrical efficiency on the injection current was studied in detail in InGaN/GaN multiple quantum wells (MQWs) blue light emitting diodes. When the InGaN quantum well thickness increased from 2 to 3.5 nm, or the Al component in p-AlGaN changed from 0.1 to 0.2, it was found that the electrical efficiency decreased dramatically, while a thin Mg-doped GaN layer inserted between p-AlGaN and MQWs with optimized Mg concentration can enhance the electrical efficiency effectively. Analysis shows that the injection efficiency was dramatically affected by the interface states due to the strong stress at the interface between p-AlGaN blocking layer and MQWs active region and the capability of electrons arriving at the interface.

Determination of the Twist Angle of GaN Film by High Resolution X-ray Diffraction

As an azimuthal scan method, Φ-scans preformed by X-ray diffraction are usually used to determine the twist angle in GaN films. However, both twist and tilt contribute to full width at half maximum (FWHM) of Φ-scan curves. So far no model has been proposed to distinguish the contributions of tilt and twist to FWHM of these curves. A geometrical model is presented to distinguish their contributions, which is very important to precisely determine the dislocation densities in GaN. Based on this model, FWHM of Φ-scans can be expressed as a simple explicit function of the original twist angle and an additional twist angle induced by tilt. These twist angles are then simply and effectively determined for various GaN films according to this simple function, and are in good agreement with those deduced from ω-scan data fitting.

Splitting of valance subbands in the wurtzite c-plane InGaN/GaN quantum well structure

Peak splitting in the low temperature photoluminescence (PL) spectra of c-plane InGaN/GaN single quantum well samples was observed. For the k∥c configuration, the splitting peaks show a variation in relative intensity as the excitation power is tuned. For the k⊥c configuration, a strong polarization dependence of the luminescence distribution and intensity was spotted. The PL spectra was analyzed with a calculation model based on the k⋅p effective mass theory, and the splitting peaks were identified as free-exciton transitions between the conduction subband C1 and two groups of valence subbands, the {HH1,LH1} and the {HH2,LH2,CH1}, respectively.

Determination of the mosaic structure of GaN films by high resolution-ray diffraction

Using Pseudo-Voigt functions instead of Gauss or Lorentz functions to fit HR-XRD curves, the widely used Williamson-Hall plot was modified. Then the mosaic structure parameters of various GaN-films were precisely determined by this method.

Improvement of thick InGaN template by thin InGaN buffer grown at low temperature

Thick InGaN templates with an without InGaN buffer were grown using MOCVD method. It is demonstrated that the crystal quality of the InGaN template can be efficiently improved by the InGaN buffer layer.

Studies on Uniformity of MOVPE-Grown AlGaN/GaN HEMT Structures by Spectroscopic Ellipsometry

The uniformities of AIGaN/GaN HEMT structures grown by MOVPE have been studied by using spectroscopic ellipsometry. The results show that the total flow rate is the most critical growth parameter for the uniformity.

Effect of Mg-doping adjacent to active region on the efficiency of InGaN blue MQW LED

The effect of Mg-doping adjacent to active region on the efficiency of InGaN blue MQW LED was investigated. The tunneling recombination and the Mg diffusion models were used to explain the optimized Mg concentration.

Microstructure Evolution in Metal-organic Vapor Phase Epitaxy-Grown GaN with Different Low-Temperature Buffer Layer Annealing Time

Microstructure evolution in MOVPE-grown GaN with different low-temperature-buffer annealing time was investigated. Low-temperature-buffer annealing process was found to effectively modulate the in-plane and out-of-plane misorientation of mosaic blocks, and simultaneously diminish the in-plane crystallite size.

Influence of low-V/III intermediate layer on GaN grown on patterned sapphire substrate by MOVPE

By inserting a low-V/III intermediate layer between bulk GaN and initial growth layer, the crystal quality of GaN grown on patterned sapphire substrate (PSS) by MOVPE was improved, which can be explained by dislocation bending mechanism.