Carrier relaxation dynamics in defect states of epitaxial GaN/AlN/Si using ultrafast transient absorption spectroscopy

The relaxation dynamics of the carriers through the defect levels in an epitaxial GaN film grown with an AlN buffer layer on Si has been performed on the femto-picosecond timescale, using ultrafast transient absorption spectroscopy (UFTS). The sample was pumped above and below the band gap and probed with a white light continuum (480-800 nm). A combination of bi and triple exponential decay functions at different probe wavelengths were used to fit the kinetic profile of the carriers in the defect continuum. Based on the UFTS measurements, a model is proposed which explains the dynamics in the shallow traps and deep level defects. Furthermore, to determine the role of the lattice in the relaxation dynamics, the experiment was conducted at a low lattice temperature of 4.2 K. The relaxation constants from the UFTS measurements confirm not only the presence of shallow and deep level defects but also the involvement of phonons in one of the relaxation processes

Facile synthesis and photoluminescence spectroscopy of 3D-triangular GaN nano prism islands

We report a strategy for fabrication of 3D triangular GaN nano prism islands (TGNPI) grown on Ga/Si(553) substrate at tow temperature by N-2(+) ions implantation using a sputtering gun technique. The annealing of Ga/Si(553) (600 degrees C) followed by nitridation (2 key) shows the formation of high quality GaN TGNPI cross-section. TGNPI morphology has been confirmed by atomic force microscopy. Furthermore, these nano prism islands exhibit prominent ultra-violet luminescence peaking at 366 nm upon 325 nm excitation wavelength along with a low intensity yellow luminescence broad peak at 545 nm which characterizes low defects density TGNPI. Furthermore, the time-resolved spectroscopy of luminescent TGNPI in nanoseconds holds promise for its futuristic application in next generation UV-based sensors as well as many portable optoelectronic devices

Correlation of current-voltage-temperature analysis with deep level defects in epitaxial GaN films

The effect of temperature on the nature of metal-semiconductor system in a Au contact deposited on c-plane and a-plane GaN film was investigated by current-voltage (I-V) measurements. The I-V measurements have been obtained systematically at different temperatures ranging from room temperature (300 K) to low temperature (78 K). Photoluminescence measurements were obtained to investigate correlation between the growth conditions, the substrate used for the growth of GaN film, and the presence of deep level defects therein by equating with the yellow band luminescence. The resistance-voltage-temperature analysis indicates that a gradual shift of the nature of contact towards Schottky behavior takes place while moving from room temperature to low temperature. Additionally, memory effect like aberration is present at low temperature, which can be attributed to the presence of deep-level defects and carrier recombination therein

Band alignment and Schottky behaviour of InN/GaN heterostructure grown by low-temperature low-energy nitrogen ion bombardment

InN/GaN heterostructure based Schottky diodes are fabricated by reactive Low Energy Nitrogen Ion (LENI) bombardment at low substrate temperature (300 degrees C). The valence band offset (VBO) of the nitrogen ion induced In-polar InN/GaN hetero-interface has been analyzed by X-ray photoelectron spectroscopy and it is determined to be 0.72 +/- 0.28 eV, a type-I straddled band alignment is formed at the InN/GaN interface. Fermi level pinning is observed to be 1.3 +/- 0.1 eV above the conduction band minimum resulting in a strong downward band bending. Valence band maxima of InN/GaN show that the surface electron accumulation occurs due to the presence of In adlayer on the film. Atomic force microscopy analysis divulged the formation of a step like InN structure on the GaN surface. I-V characteristic showed that the junction between InN and GaN exhibits a Schottky type behaviour. The room temperature barrier height and the ideality factor of the InN/GaN Schottky diodes are calculated by using the thermionic emission (TE) model and found to be 0.72 eV and 20.8 respectivel

Influence of metallic surface states on electron affinity of epitaxial AlN films

The present article investigates surface metallic states induced alteration in the electron affinity of epitaxial AlN films. AlN films grown by plasma-assisted molecular beam epitaxy system with (30% and 16%) and without metallic aluminium on the surface were probed via photoemission spectroscopic measurements. An in-depth analysis exploring the influence of metallic aluminium and native oxide on the electronic structure of the films is performed. It was observed that the metallic states pinned the Fermi Level (FL) near valence band edge and lead to the reduction of electron affinity (EA). These metallic states initiated charge transfer and induced changes in surface and interface dipoles strength. Therefore, the EA of the films varied between 0.6-1.0 eV due to the variation in contribution of metallic states and native oxide. However, the surface barrier height (SBH) increased (4.2-3.5 eV) adversely due to the availability of donor-like surface states in metallic aluminium rich films

Surface chemistry and electronic structure of nonpolar and polar GaN films

Photoemission and microscopic analysis of nonpolar (a-GaN/r-Sapphire) and polar (c-GaN/c-Sapphire) epitaxial gallium nitride (GaN) films grown via RF-Molecular Beam Epitaxy is reported. The effect of polarization on surface properties like surface states, electronic structure, chemical bonding and morphology has been investigated and correlated. It was observed that polarization lead to shifts in core level (CL) as well as valence band (VB) spectra. Angle dependent X-ray Photoelectron Spectroscopic analysis revealed higher surface oxide in polar GaN film compared to nonpolar GaN film. On varying the take off angle (TOA) from 0 degrees to 60 degrees, the Ga-O/Ga-N ratio varied from 0.11-0.23 for nonpolar and 0.17-0.36 for polar GaN film. The nonpolar film exhibited N-face polarity while Ga-face polarity was perceived in polar GaN film due to the inherent polarization effect. Polarization charge compensated surface states were observed on the polar GaN film and resulted in downward band bending. Ultraviolet photoelectron spectroscopic measurements revealed electron affinity and ionization energy of 3.4 +/- 0.1 eV and 6.8 +/- 0.1 eV for nonpolar GaN film and 3.8 +/- 0.1 eV and 7.2 +/- 0.1 eV for polar GaN film respectively. Field Emission Scanning Electron Microscopy measurements divulged smooth morphology with pits on polar GaN film. The nonpolar film on the other hand showed pyramidal structures having facets all over the surface

Electronic structure and chemical state analysis of nanoflowers decorated GaN and AlGaN/GaN heterostructure

The present article reports electronic structure, chemical and defect states analysis of Quasi-continuous GaN film, nanoflowers decorated nanostructured GaN and nanoflowers decorated AlGaN/GaN hetero-structure. The nanostructured GaN and AlGaN surfaces were decorated with nanoflowers having a size variation between 200 and 400 nm. Extensive photoemission analysis was performed to analyse surface chemistry and electronic structure and their correlation with surface morphology. Indication of free electron accumulation was perceived by the observed downwards band bending at the interface of AlGaN/GaN heterostructure. The optical response inveterate defects minimization in nanoflower decorated GaN and AlGaN/GaN heterostructure and the presence of minimum residual stress

Origin of surface electron accumulation and fermi level pinning in low energy ion induced InN/GaN heterostructure

InN/GaN heterostructure was fabricated via reactive low energetic Nitrogen ion (LENI at 300 eV) bombardment at lower substrate temperature (350 degrees C). X-Ray Photoemission spectroscopic (XPS) and Atomic Force Microscopic (AFM) measurements were performed to analyse the electronic structure, surface chemistry, band alignment, and the morphology of the grown heterostructure. XPS analysis revealed the evolution of InN structure with nitridation time, surface electron accumulation, fermi level pinning and the band offset of the grown InN/GaN hetero structure. The valence band and conduction band offsets (VBO & CBO) were calculated to be 0.49 +/- 0.19 eV and 2.21 +/- 0.1 eV and divulged the formation of a type-I heterojunction. A Fermi Level (FL) pinning of 1.5 +/- 0.1 eV above the conduction band minima was perceived and indicated towards strong downward band bending. The analysis of the VB spectra suggested that surface electron accumulation occurred due to the presence of metallic In-adlayer on the surface which resulted in FL pinning and the corresponding downward band bending. Atomic Force Microscopy analysis divulged the formation of smooth surface with granular structure. It was also observed that the growth parameters (e.g. substrate temperature) strongly influence the aforementioned surface and interfacial properties

Electronic structure analysis of GaN films grown on r- and a-plane sapphire

The electronic structure and surface properties of epitaxial GaN films grown on r- and a-plane sapphire substrates were probed via spectroscopic and microscopic measurements. X-ray photoemission spectroscopic (XPS) measurements were performed to analyse the surface chemistry, band bending and valence band hybridization states. It was observed that GaN/a-sapphire display a downward band bending of 0.5 eV and possess higher amount of surface oxide compared to GaN/r-sapphire. The valence band (VB) investigation revealed that the hybridization corresponds to the interactions of Ga 4s and Ga 4p orbitals with N 2p orbital, and result in N2p-Ga4p, N2p-Ga4s*, mixed and N2p-Ga4s states. The energy band structure and electronic properties were measured via ultraviolet photoemission spectroscopic (UPS) experiments. The band structure analysis and electronic properties calculations divulged that the electron affinity and ionization energy of GaN/a-sapphire were 0.3 eV higher than GaN/r-sapphire film. Atomic Force Microscopic (AFM) measurements revealed faceted morphology of GaN/r-sapphire while a smooth pitted surface was observed for GaN/a-sapphire film, which is closely related to surface oxide coverage

Influence of active nitrogen species on surface and optical properties of epitaxial GaN films

Influence of active nitrogen species on surface and optical properties of homoepitaxial GaN films grown on GaN epilayers has been investigated. The epitaxial GaN films were grown at varying plasma powers (350-500 W) under identical growth conditions. High resolution X-Ray diffraction, Field Emission Scanning Electron Microscopy, Atomic Force Microscopy and Photoluminescence measurements were employed to characterize the structural, morphological and optical properties of the grown GaN films. High plasma power (500 W) lead to an increment in active nitrogen radicals and yielded high crystalline quality with reduced dislocations compared to low plasma power (350, 400 W) which divulge the presence of metallic gallium on the surface and low roughness. The valence band maximum position, electron affinity and ionization energy of the films increased with increment in plasma power. PL measurements revealed narrow and intense band to band edge emission with negligible defect related yellow band peak for the sample grown at 500 W. The analysis conveyed that higher amount of active nitrogen species encouraged good optical properties with insignificant defect states which can be employed for the fabrication of high performance optoelectronic & photovoltaic devices