Optoelectronic properties of GaN epilayers in the region of yellow luminescence (original) (raw)
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Physical Review B, 2005
Heteroepitaxial GaN films in general have very high concentration o f defects and at the same time show broad photoluminescence (PL) peaks at energies below the band-gap energy (3.4 eV), i.e., the so-called yellow (YL) and blue luminescence (BL) centered at 2.2 and 2.9 eV, respectively. The origin o f these PL peaks is commonly attributed to defects. The present paper studies the relationship o f the yellow and blue luminescence with the defect structure by a combination o f photoetching and photoluminescence. Nominally undoped (n-type) GaN layers were grown by metal organic chemical vapor deposition (MOCVD). By photoetching the perfect material between defects is removed and defect-related nanocolumns are formed. PL measurements o f samples etched to various degrees allow the identification o f the different PL features. A fully etched sample shows no PL signature related to any o f the band edge features and BL is quenched completely, while the yellow lum inescence is attenuated only to the degree to w hich the volume amount o f GaN has been removed. Such behavior suggests that defects causing YL are not related to dislocations but are rather homogenously distributed throughout the layer in the form o f point-like defects. The decrease o f the BL intensity at 4.4 K with excitation dose (bleaching) and the simultaneous increase in YL intensity for as-grown samples and the lack o f such effects for etched samples imply a direct connection between the defect centers responsible for YL and BL.
Manifestation of edge dislocations in photoluminescence of GaN
Physica B-condensed Matter, 2005
A GaN layer was grown by molecular beam epitaxy on a freestanding GaN template prepared by hydride vaporphase epitaxy. Two characteristic areas have been found in the overgrown layer: a region nearly free from dislocations and a region with the density of the edge dislocations of $5 Â 10 9 cm À2 , as determined by transmission electron microscopy. Low-temperature photoluminescence spectrum from the former contained only well-known exciton lines, whereas the spectrum of the defective area contained additional lines at 3.21 and 3.35 eV. These lines are attributed to unidentified point defects trapped by the edge threading dislocations. r
GaN epitaxial layers on inhomogeneous buffer layer: electrical and optical properties
The European Physical Journal Applied Physics, 2004
This paper reports on first results obtained on GaN epitaxial layer grown on a single wafer with a lateral variation in defect density. The chemically homogeneous GaN epitaxial layer was deposited by metalorganic chemical vapour deposition onto a specially prepared buffer layer. A chemical gradient in the nitrogen contents of the precursor flux was induced during the growth of the GaN buffer layer. This condition leads a corresponding gradient of the dislocation density within the epilayer. The electrical and optical properties of the GaN epilayer have been analysed by means of electron beam induced current, photoluminescence and photocurrent techniques. All our measurements reveal lateral gradients in the epilayer properties as concern (i) the density of recombining centres in the GaN film and (ii) their recombination activity, both radiative and nonradiative. This paper shows that a proper combination of beam based techniques can contribute to the detailed analysis of the well know yellow luminescence band which in the GaN epilayer here investigated consist of four well distinct peaks.
Influence of dislocation density on photoluminescence intensity of GaN
Journal of Crystal Growth, 2005
The influence of dislocation density on photoluminescence intensity is investigated experimentally and compared to a model. GaN samples were grown by molecular beam epitaxy and metal-organic chemical vapour deposition. Different growth parameters and thicknesses of the layers resulted in different dislocation densities. The threading dislocation density, measured by atomic force microscopy, scanning electron microscopy and X-ray diffraction, covered a range from 5 Â 10 8 to 3 Â 10 10 cm À2. Carrier concentration was measured by capacitance-voltage-, and Hall effect measurements and photoluminescence at 2 K was recorded. A model which accounts for the photoluminescence intensity as a function of dislocation density and carrier concentration in GaN is developed. The model shows good agreement with experimental results for typical GaN dislocation densities, 5 Â 10 8-1 Â 10 10 cm À2 , and carrier concentrations 4 Â 10 16-1 Â 10 18 cm À3 .
Physica Status Solidi (a), 1999
The spatial distribution of the long-wavelength luminescence in thick InezGaesAs/GaAs multiple quantum wells (MQWs) has been investigated using cathodoluminescence (CL) imaging and spectroscopy. The CL spectra show defect-induced broad bands between 1000 5 J.5 1600 nm. These bands exhibit spatial variations which correlate with the dark line defects (DLDs) observed in the A=950 nm exciton luminescence imaging. Transmission electron microscopy showed that [l IO]-oriented misfit dislocations occur primarily at the substrate-to-MQW and GaAs capping layer-to-MQW interfaces. The large spatial variation of the luminescence intensities indicates that the DLDs observed in CL images are caused by the presence of nonradiative recombination centers occurring in the MQW region located between the interface misfit dislocations. This study provides new information describing the origin and nature of DLDs and differs from previous models which have regarded the electronic nature of dislocation cores as the primary mechanism for inducing DLD radiative contrast in luminescence imaging of strained InGaAs/GaAs.
2013
III-nitride materials are very promising for high speed electronics/optical applications but still suffer in performance due to problems during high quality epitaxial growth, evolution of dislocation and defects, less understanding of fundamental physics of materials/processing of devices etc. This thesis mainly focus on GaN based heterostructures to understand the metal-semiconductor interface properties, 2DE(H)G influence on electrical and optical properties, and deep level states in GaN and InAlN, InGaN materials. The detailed electrical characterizations have been employed on Schottky diodes at GaN and InAl(Ga)N/GaN heterostructures in order to understand the metal-semiconductor interface related properties in these materials. I have observed the occurrence of Schottky barrier inhomogenity, role of dislocations in terms of leakage and creating electrically active defect states within energy gap of materials. Deep level transient spectroscopy method is employed on GaN, InAlN and ...
Inclination of screw dislocations on the performance of homoepitaxial GaN based UV photodetectors
Materials Science and Engineering: B, 2021
We analyze the dislocation-assisted charge carrier transport and effective photocurrent generation in the photodetection devices. Specifically, the impact of screw dislocations on the performance of GaN based metal-semiconductor-metal ultraviolet photodetector is investigated. The experiments reveal that reducing screw dislocations had a strong impact on dark current (~3x decrement) of the devices as well as on the photogenerated current (~20x enhancement) upon illumination (ultraviolet, 325 nm). The responsivity of the photodetection device has been increased from 85.05 mA/W to 130.17 mA/W with decreasing dislocation density. Because, higher dislocations created leakage paths that develop trap states which enhances the possibility of recombination process of photo-generated electron-hole pairs leading to lower charge collection. Further, the external quantum efficiency increases from 32.51% to 49.76% by reducing dislocation density. The work in this study proposes that reduction of defects/dislocations will be an effective approach to enrich the III-nitride semiconductor system for advancement in optoelectronic devices.