Nonpolar a-plane GaN epilayers are grown on several r-plane sapphire substrates by metal organic chemical vapour deposition using different nucleation layers:(A) a GaN nucleation layer deposited at low temperature(LT);(B) an AlN nucleation layer deposited at high temperature;or(C) an LT thin AlN nucleation layer with an AlN layer and an AlN/AlGaN superlattice both subsequently deposited at high temperature.The samples have been characterized by Xray diffraction(XRD),atomic force microscopy and photoluminescence.The GaN layers grown using nucleation layers B and C show narrower XRD rocking curves than that using nucleation layer A,indicating a reduction in crystal defect density.Furthermore,the GaN layer grown using nucleation layer C exhibits a surface morphology with triangular defect pits eliminated completely.The improved optical property,corresponding to the enhanced crystal quality,is also confirmed by temperature-dependent and excitation power-dependent photoluminescence measurements.
This paper calculates the wavelengths of the interband transitions as a function of the Al mole fraction of Al x Ga 1 x N bulk material.It is finds that when the Al mole fraction is between 0.456 and 0.639,the wavelengths correspond to the solar-blind (250 nm to 280 nm).The influence of the structure parameters of Al y Ga 1 y N/GaN quantum wells on the wavelength and absorption coefficient of intersubband transitions has been investigated by solving the Schro¨dinger and Poisson equations self-consistently.The Al mole fraction of the Al y Ga 1 y N barrier changes from 0.30 to 0.46,meanwhile the width of the well changes from 2.9 nm to 2.2 nm,for maximal intersubband absorption in the window of the air (3 μm < λ < 5 μm).The absorption coefficient of the intersubband transition between the ground state and the first excited state decreases with the increase of the wavelength.The results are finally used to discuss the prospects of GaN-based bulk material and quantum wells for a solar-blind and middle infrared two-colour photodetector.
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.
This paper finds that the two-dimensional electron gas density in high Al-content AlGaN/GaN heterostructures exhibits an obvious time-dependent degradation after the epitaxial growth.The degradation mechanism was investigated in depth using Hall effect measurements,high resolution x-ray diffraction,scanning electron microscopy,x-ray photoelectron spectroscopy and energy dispersive x-ray spectroscopy.The results reveal that the formation of surface oxide is the main reason for the degradation,and the surface oxidation always occurs within the surface hexagonal defects for high Al-content AlGaN/GaN heterostructures.
Electromagnetic field distribution in the vertical metal organic chemical vapour deposition (MOCVD) reactor is simulated by using the finite element method (FEM).The effects of alternating current frequency,intensity,coil turn number and the distance between the coil turns on the distribution of the Joule heat are analysed separately,and their relations to the value of Joule heat are also investigated.The temperature distribution on the susceptor is also obtained.It is observed that the results of the simulation are in good agreement with previous measurements.
