The Multi-spectrum Vision Technique Studies On Heterojunction GaN Of Thin Film Materials

Nowadays,direct bandgap ?-? semiconductor materials have significant advantages in optoelectronic applications compared to traditional semiconductors such as Si and Ge.Since the successful development of single crystal thin film GaN by Maruska et al.,it has become the most important optoelectronic material in ?-? semiconductors.GaN combines with In or Al,it could cover a near-ultraviolet,visible and near-infrared spectrum(0.7-6.2 eV,).The efficiency is the most attractive topic in the research of GaN-based LED.The main reason of the low internal quantum efficiency of GaN-based LEDs is the lattice mismatch between the GaN epitaxial layer/substrate and the InGaN well/GaN barrier.Therefore,based on the microcavity model of the heteroepitaxial refractive index sudden step,the interface of the heteroepitaxial GaN thin film material is studied by novel fitting function of the PL spectrum with the Fabry-Perot oscillation.The oscillation coefficient is related to the interface,which is extracted from the fitting function.The oscillation coefficient is a function of the interface reflectivity,which is verified by spectroscopic ellipsometry.The oscillation spectra were separated from PL spectra and the oscillation peaks and periods were analyzed.In the last,Raman and time-resolved spectroscopy was studied.The main research content of the paper is the following aspects,such as:(1)Heteroepitaxial materials are conventional LED sample(S1)grown on c-plane sapphire substrate and p-GaN thin film structure sample(S2).Both are based on a step of refractive index at the interfaces of air/GaN and GaN/sapphire.The sandwich structure was deemed to the microcavity.F-P oscillation is due to the interference of light field E11 and El.A new method was proposed to studying the quality of heteroepitaxial interface.A function of with oscillation phenomenon is used to match the PL curve.The oscillation coefficient was used to describe the oscillation strength.The oscillation coefficient is a key parameter of interface for qualitative analysis of interface quality.(2)The method was successfully verified by the measurement and analysis of spectroscopic ellipsometry.Then,the relationship between the oscillation coefficient and the interface reflectivity was obtained,which was applied to quantitatively analyze the dielectric properties of interface with defects and deformations.The oscillation coefficient was extracted from the fitting function,not only reveals the optical properties of the interface,but also very sensitive to interface quality,the oscillation coefficient varies 20 times,reflectivity changes 400 times.Finally,different luminous thickness is simulated.(3)The variation of the oscillation coefficients at different PL wavelengths was obtained by fitting the samples S1 and S2 of GaN heterostructure.No-oscillation in temperature dependence PL spectra is attributed to the sudden grounding of the reflectivity at the interface,and no-oscillation at the band gap is attributed to absorption.The oscillation coefficient decreases from 0.13 to 0.05.The oscillation peak number,oscillation peak spacing and film thickness were also analyzed.(4)The spectroscopic ellipsometry and Raman of sample S2 were measured for the characterization of optical properties,growth quality and interfacial stress.Time-resolved spectroscopy measurements were performed on sample S1,and information of the carrier dynamics process was obtained by the PL decaytime.The photoluminescence lifetimes are 8 ns(fast)and 60 ns(slow).