Strain-stress study of AlxGa1-xN/AlN heterostructures on c-plane sapphire and related optical properties

This work presents a systematic study of stress and strain of AlxGa1-xN/AlN with composition ranging from GaN to AlN, grown on a c-plane sapphire by metal-organic chemical vapor deposition, using synchrotron radiation high-resolution X-ray diffraction and reciprocal space mapping. The c-plane of the AlxGa1-xN epitaxial layers exhibits compressive strain, while the a-plane exhibits tensile strain. The biaxial stress and strain are found to increase with increasing Al composition, although the lattice mismatch between the AlxGa1-xN and the buffer layer AlN gets smaller. A reduction in the lateral coherence lengths and an increase in the edge and screw dislocations are seen as the AlxGa1-xN composition is varied from GaN to AlN, exhibiting a clear dependence of the crystal properties of AlxGa1-xN on the Al content. The bandgap of the epitaxial layers is slightly lower than predicted value due to a larger tensile strain effect on the a-axis compared to the compressive strain on the c-axis. Raman characteristics of the AlxGa1-xN samples exhibit a shift in the phonon peaks with the Al composition. The effect of strain is also discussed on the optical phonon energies of the epitaxial layers. The techniques discussed here can be used to study other similar materials.Comment: 14 pages, 5 figures, 2 table

Characterization of low conductivity wide band gap semiconductors

This thesis covers research on low electric conductivity wide band gap semiconductors of the group-III nitride material system. The work presented focussed on using multi-mode scanning electron microscope (SEM) techniques to investigate the luminescence properties and their correlation with surface effects, doping concentration and structure of semiconductor structures.The measurement techniques combined cathodoluminescence (CL) for the characterization of luminescence properties, secondary electron (SE) imaging for imaging of the morphology and wavelength dispersive X-ray (WDX) spectroscopy for compositional analysis. The high spatial resolution of CL and SE-imaging allowed for the investigation of nanometer sized features, whilst environmental SEM allowed the characterisation of low conductivity samples.The investigated AlₓGa₁₋ₓN samples showed a strong dependence on the miscut of the substrate, which was proven to influence the surface morphology and the compositional homogeneity. Studying the influence of the AlₓGa₁₋ₓN sample thickness displayed a reduced strain in the samples with increasing thickness as well as an increasing crystalline quality. The analysis of AlₓGa₁₋ₓN:Si samples showed the incorporation properties of Si in AlₓGa₁₋ₓN, the correlation between defect luminescence, Si concentration and resistivity as well as the influence of threading dislocations on the luminescence properties and incorporation of point defects.The characterization of UV-LED structures demonstrated that a change in the band structure is one of the main reasons for a decreasing output power in AlₓGa₁₋ₓN based UV-LEDs. In addition the dependence of the luminescence properties and crystalline quality of InₓAl₁₋ₓN based UV-LEDs on various growth parameters (e.g. growth temperature, quantum well thickness) was investigated.The study of nanorods revealed the influence of the template on the compositional homogeneity and luminescence of InₓAl₁₋ₓN nanorod LEDs. Furthermore,the influence of optical modes in these structures was studied and found to provide an additional engineering parameter for the design of nanorod LEDs.This thesis covers research on low electric conductivity wide band gap semiconductors of the group-III nitride material system. The work presented focussed on using multi-mode scanning electron microscope (SEM) techniques to investigate the luminescence properties and their correlation with surface effects, doping concentration and structure of semiconductor structures.The measurement techniques combined cathodoluminescence (CL) for the characterization of luminescence properties, secondary electron (SE) imaging for imaging of the morphology and wavelength dispersive X-ray (WDX) spectroscopy for compositional analysis. The high spatial resolution of CL and SE-imaging allowed for the investigation of nanometer sized features, whilst environmental SEM allowed the characterisation of low conductivity samples.The investigated AlₓGa₁₋ₓN samples showed a strong dependence on the miscut of the substrate, which was proven to influence the surface morphology and the compositional homogeneity. Studying the influence of the AlₓGa₁₋ₓN sample thickness displayed a reduced strain in the samples with increasing thickness as well as an increasing crystalline quality. The analysis of AlₓGa₁₋ₓN:Si samples showed the incorporation properties of Si in AlₓGa₁₋ₓN, the correlation between defect luminescence, Si concentration and resistivity as well as the influence of threading dislocations on the luminescence properties and incorporation of point defects.The characterization of UV-LED structures demonstrated that a change in the band structure is one of the main reasons for a decreasing output power in AlₓGa₁₋ₓN based UV-LEDs. In addition the dependence of the luminescence properties and crystalline quality of InₓAl₁₋ₓN based UV-LEDs on various growth parameters (e.g. growth temperature, quantum well thickness) was investigated.The study of nanorods revealed the influence of the template on the compositional homogeneity and luminescence of InₓAl₁₋ₓN nanorod LEDs. Furthermore,the influence of optical modes in these structures was studied and found to provide an additional engineering parameter for the design of nanorod LEDs

Infrared photocurrent spectroscopy of deep levels in magnesium-doped gallium nitride and aluminum gallium nitride

Recently, there is an increasing demand for III-nitride semiconductor based optoelectronic devices, and especially for ultraviolet (UV) and Deep UV sources and sensors; however, the development and the performance of such devices is fundamentally limited by low carrier concentration, especially in p-type GaN and its alloys with aluminum, p-type AlxGa1-xN. Common acceptors such as magnesium (Mg), which function adequately in GaN, are often too deep in AlGaN alloys to allow significant acceptor ionization at room temperature. Various strategies such as short period superlattices are often incorporated into device architectures in order to enhance carrier levels in p-AlGaN. In this work, IR photocurrent spectroscopy in Mg doped GaN, and Mg-doped AlxGa1-xN (0.15 < x < 0.52) was conducted by means of a YAG-pumped OPO/OPA system tunable from 250 meV to 1.75 eV with the goal of observing and identifying energy levels associated with acceptor atoms of Mg in GaN and AlxGa1-xN. Infrared photocurrent spectra are presented from a variety of GaN and AlxGa1-xN test structures. Non-zero background response is associated with shallow extrinsic impurities and/or a continuum of shallow levels, photocurrent response peaks observed and associated with deep level donors. No evidence of acceptor ionization associated with Mg in magnesium-doped GaN and AlxGa1-xN is observed. A number of deep levels are observed in photocurrent spectra, including several 400 meV in GaN and low aluminum alloys, and one around 800 meV in higher aluminum alloys. Thermal analysis of Mg:GaN photocurrent data is consistent with the deep levels being electron donors. Finally, the effects of IR radiation on the UV optical output of forward biased commercial 365nm UV LEDs is investigated and reported

Study of simulations of double graded InGaN solar cell structures

The performances of various configurations of InGaN solar cells are compared using nextnano software. Here we compare a flat base graded wall GaN/InGaN structure, with an InxGa1-xN well with sharp GaN contact layers, and an InxGa1-xN structure with InxGa1-xN contact layers, i.e. a homojunction. The doping in the graded structures are the result of polarization doping at each edge (10 nm from each side) due to the graded structure, while the well structures are intentionally doped at each edge (10 nm from each side) equal to the doping concentration in the graded structure. The solar cells are characterized by their open-circuit voltage, V_oc, short circuit current, I_sc, solar efficiency, and energy band diagram. The results indicate that an increase in I_sc and efficiency results from increasing both the fixed and the maximum indium compositions, while the V_oc decreases. The maximum efficiency is obtained for the InGaN well with 60% In.Comment: 14 pages, 12 figure

Strain-stress study of AlxGa1−xN/AlN heterostructures on c-plane sapphire and related optical properties

This work presents a systematic study of stress and strain of AlxGa1−xN/AlN with composition ranging from GaN to AlN, grown on a c-plane sapphire by metal-organic chemical vapor deposition, using synchrotron radiation high-resolution X-ray diffraction and reciprocal space mapping. The c-plane of the AlxGa1−xN epitaxial layers exhibits compressive strain, while the a-plane exhibits tensile strain. The biaxial stress and strain are found to increase with increasing Al composition, although the lattice mismatch between the AlxGa1−xN and the buffer layer AlN gets smaller. A reduction in the lateral coherence lengths and an increase in the edge and screw dislocations are seen as the AlxGa1−xN composition is varied from GaN to AlN, exhibiting a clear dependence of the crystal properties of AlxGa1−xN on the Al content. The bandgap of the epitaxial layers is slightly lower than predicted value due to a larger tensile strain effect on the a-axis compared to the compressive strain on the c-axis. Raman characteristics of the AlxGa1−xN samples exhibit a shift in the phonon peaks with the Al composition. The effect of strain on the optical phonon energies of the epitaxial layers is also discussed

Luminescence properties of MOCVD grown Al0.2Ga0.8N layers implanted with Tb

AlxGa1-xN (x = 0.20) layers grown on (0001) sapphire substrates by metal organic chemical vapour deposition were implanted with terbium (Tb) ions at 150 keV with a fluence of 7 × 1014 Tb cm−2 at different temperatures. After thermal annealing, all layers evidenced the Tb-related 5D4-7FJ intra-4f8 transitions, demonstrating an enhancement of their intensity with increasing implantation temperature. A detailed spectroscopic analysis of the optical properties of these layers was conducted using luminescence techniques. An atypical behaviour for the relative intensity of both the broad visible band and the intraionic lines was found as a function of temperature and its origin is discussed based on potential fluctuation phenomena and energy transfer processes. The 5D4-7FJ intra-4f8 transitions exhibit thermal population with increasing temperature between ∼100 K and ∼200–230 K, with a subsequent decrease up to RT due to further competitive non-radiative recombination paths. The values calculated for the population energies of each sample are in good agreement with the ones obtained for the activation energies of the de-excitation of the yellow broad band also present in the spectra, suggesting a correlation between the host defect de-excitation processes and the population of the ion emitting levels.publishe

In GaN Double Heterostructure (DH) Laser Diode Performance And Optimization.

The laser performances of the blue DH InGaN laser diode (LD) structures have been numerically investigated by using ISE TCAD software