Growth Of Nonpolar Gan-based Material By MOCVD And Research Of MSM UV Photodetector

As direct and wide bandgap, GaN-based material is one of the most promisingmaterial. It's good electrieal, optical characteristics and excellent mechnicalproperties make it one of the most ideal choices for short wave photo electrondevices,such as ultraviolet photodetectors (UV-PDs), blue/green LED, solar cell,laser diodes, and High Electron Mobility Transistor (HEMTs). AlGaN has emergedas the most ideal material for application in UV-PDs because of its direct band gapwhich can be tuned from solar-blind to visible blind by varying Al content.Especially, the solar-blind photodetector has attracted widely attention in recentyears. Nowadays, most nitride-based devices are grown on (0001)-oriented (c-plane)sapphire substrates. The nonpolar GaN-based material will eliminate the largeinternal electric fields caused by polarization-effect at the heterointerfaces.Nevertheless, the nonpolar GaN-based material epilayers are characterized by highdefect density which will affect the following epilayers quality and the surfacemorphology. Moreover, unlike c-plane GaN on c-plane sapphire, the in-planethermal expansion and lattice mismatches are anisotropic lead to the strain in thea-plane nitride heterostructure is anisotropically biaxial in-plane and complex, whichsignificantly affect optical and electrical properties of the epitaxial films. To date,various types of AlGaN-based photodetectors with different structures have beenreported. Among them, the MSM photodetector is a good candidate for this application. Based on these high Al content AlGaN epilayers, metal-semiconductor-metal (MSM) solar-blind UV photodetectors were fabricated. The results in ourwork are listed as follows:1)The effects of TMAl preflow on the properties of AlN films grown on (0001)sapphire substrates via metalorganic chemical vapor deposition (MOCVD) usinghigh-temperature treatments. Short TMAl preflow treatments had little effect onthe surface morphology of the AlN films, but hexagonal islands appeared on thesurface when the TMAl preflow time was increased. As the preflow timeincreased, the crystalline quality decreased and the stress state of the AlN filmsalso changed.2)The effect of using nitridation or the low temperature (LT) AlN buffer growthmethod on the properties and strain of a-plane GaN epilayers grown on r-planesapphire by low pressure metalorganic chemical vapor deposition (LP-MOCVD)has been investigated by high resolution X-ray diffraction (HRXRD) andpolarized Raman scattering spectra in backscattering configurations. Comparedwith the sample with nitridation, the LT-AlN buffer is beneficial to improve thequality, reduce stress and benefit the reduction of the structural and stressanisotropic distribution of the a-plane GaN. Different stress in the two in-planedirections is attributed to the elastic and thermal anisotropy of the GaN and thesapphire.3) The stress and its effect on optical properties of a-AlGaN epilayers grown withAlN interlayer by using metal organic chemical vapor deposition (MOCVD)method are investigated. The stress is characterized by the frequency shift basedon Raman spectroscopy measurement. It shows that residual stress in a-AlGaNepilayer grown on the AlN interlayer is relaxed. Accordingly, the near band edgeemission (NBE) peak shows red shift in room temperature photoluminescence (PL)measurement. We assume that the AlN interlayer modulates and reduces themismatch between a-plane AlGaN and sapphire is the main reason to reduce the residual stress and influence corresponding optical properties.4)The a-plane AlGaN metal-semiconductor-metal (MSM)ultraviolet photodetectors(UV-PDs) were investigated by depositing SiO2nanoparticles (SNPs) on thea-plane AlGaN. It was shown that the dark current of the detectors with SNPswas more than one order of magnitude lower than that without SNPs and thepeak responsivity was enhanced more than two orders of magnitude afterdeposition of the SNPs.