X-ray diffraction analysis and scanning micro-Raman spectroscopy of structural irregularities and strains deep inside the multilayered InGaN/GaN heterostructure

High-resolution X-ray diffraction analysis and scanning confocal Raman spectroscopy are used to study the spatial distribution of strains in the In _x Ga_{1 − x} N/GaN layers and structural quality of these layers in a multilayered light-emitting diode structure produced by metal-organic chemical vapor deposition onto (0001)-oriented sapphire substrates. It is shown that elastic strains almost completely relax at the heterointerface between the thick GaN buffer layer and In _x Ga_{1 − x} N/GaN buffer superlattice. It is established that the GaN layers in the superlattice are in a stretched state, whereas the alloy layers are in a compressed state. In magnitude, the stretching strains in the GaN layers are lower than the compressive strains in the InGaN layers. It is shown that, as compared to the buffer layers, the layers of the superlattice contain a smaller number of dislocations and the distribution of dislocations is more randomly disordered. In micro-Raman studies on scanning through the thickness of the multilayered structure, direct evidence is obtained for the asymmetric gradient distributions of strains and crystal imperfections of the epitaxial nitride layers along the direction of growth. It is shown that the emission intensity of the In _x Ga_{1 − x} N quantum well is considerably (more than 30 times) higher than the emission intensity of the GaN barrier layers, suggesting the high efficiency of trapping of charge carriers by the quantum well.     

Radiation effects and interphase interactions in ohmic and barrier contacts to indium phosphide as induced by rapid thermal annealing and irradiation with γ-ray 60Co photons

The radiation resistance of Au-Pd-Ti-Pd-n ⁺⁺-InP ohmic contacts and Au-TiB _x -n-n ⁺-n ⁺⁺-InP barrier contacts—both initial and subjected to a rapid thermal annealing and irradiated with ⁶⁰Co γ-ray photons with doses as high as 10⁹ R—has been studied. Before and after external effects, the electrical characteristics of the barrier and ohmic contacts, distribution profiles for components, and phase composition in the metallization layers have been measured. In ohmic Pd-Ti-Pd-Au contacts subjected to rapid thermal annealing and irradiation, a significant distortion of the layered structure of metallization occurs; this distortion is caused by the thermal and irradiation-stimulated transport of Pd over the grain boundaries in polycrystalline Ti and Au films. However, the specific contact resistance ρ _c does not change appreciably, which is related to a comparatively unvaried composition of the contact-forming layer at the Pd-n ⁺-InP interface. In the initial sample and the sample subjected to the rapid thermal annealing at T = 400°C with the Au-TiB _x -n-n ⁺-n ⁺⁺-InP barrier contacts and irradiated with the dose as high as 2 × 10⁸ R, a layered structure of metallization is retained. After irradiation with the dose as high as 10⁹ R, in the samples subjected to a rapid thermal annealing at T = 400°C, the layered structure of metallization becomes completely distorted; however, this structure is retained in the initial sample. The electrical properties of the contact structure appreciably degrade only after irradiation of the sample preliminarily subjected to a rapid thermal annealing at T = 400°C.     

A new type of structural defects in CdZnSe/ZnSe heterostructures

The changes of structural and photoluminescence (PL) characteristics of MBE-grown CdZnSe/ZnSe single quantum well (QW) structures caused by Cd/Zn interdiffusion were studied by high-resolution X-ray diffraction (HRXRD) and low-temperature PL methods. The PL investigations showed that the increase of Cd content in the QW resulted in the increase of the depth of potential fluctuations in the QW as well as in the decrease of ZnSe cap layer band gap (up to several meV). The HRXRD scans as well as reciprocal space maps measured for symmetrical and asymmetrical diffractions revealed the formation of CdZnSe layers near the QW/ZnSe interface with Cd content of order of several percents. It is found that in some samples, the Cd profile in these layers is rather smooth while in others it is quite abrupt. In the latter case, the partial strain relaxation can occur in these layers.     

Effect of p-n junction overheating on degradation of silicon high-power pulsed IMPATT diodes

The thermal limits of the two-drift impact avalanche and transit-time (IMPATT) diode operating in the pulsed mode in the 8-mm wavelength region with a microwave power as high as 30–35 W have been estimated. It is shown that p-n junction overheat at an operating pulse length of 300 ns and a supply current amplitude of 11.3–15 A amounts to 270–430°C relative to an ambient medium. The temperature limit of junction overheating, above which IMPATT diodes rapidly degrade, was determined as 350°C. The presented results of X-ray phase analysis and depth profiles of Au-Pt-Ti-Pd-Si ohmic contact components confirm thermal limits of the IMPATT diode operating in the pulsed mode.

     

On the nature of doping effect of methane in ZnO thin films deposited by RF-magnetron sputtering

Journal of Materials Science: Materials in Electronics - A comparative study of the effects of methane and hydrogen as reactive agents on the structural, optical, and electrical properties of ZnO...     

Effects of the lateral ordering of self-assembled SiGe nanoislands grown on strained Si<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Ge<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> buffer layers

Atomic-force microscopy, micro-Raman spectroscopy, and high resolution X-ray diffraction are applied to study the spatial ordering in single layers of SiGe nanoislands grown on a strained Si_{1 − x} Ge _x buffer sublayer. It is shown that, apart from stimulating the spatial ordering of nanoislands, the introduction of a Si_{1 − x} Ge _x sublayer leads to an enhanced role for interdiffusion processes. An unusually high increase in the volume of nanoislands in the process of the epitaxy is related to the anomalously strong diffusion from the buffer sublayer into the islands that is induced by nonuniform fields of elastic strains. The anisotropy of the islands shape and spatial ordering is discussed in terms of the anisotropy of the diffusion processes in spatially nonuniform fields of elastic strains.     

Si-rich Al2O3 films grown by RF magnetron sputtering: structural and photoluminescence properties versus annealing treatment

Silicon-rich Al₂O₃ films (Si_x(Al₂O₃)_1−x) were co-sputtered from two separate silicon and alumina targets onto a long silicon oxide substrate. The effects of different annealing treatments on the structure and light emission of the films versus x were investigated by means of spectroscopic ellipsometry, X-ray diffraction, micro-Raman scattering, and micro-photoluminescence (PL) methods. The formation of amorphous Si clusters upon the deposition process was found for the films with x ≥ 0.38. The annealing treatment of the films at 1,050°C to 1,150°C results in formation of Si nanocrystallites (Si-ncs). It was observed that their size depends on the type of this treatment. The conventional annealing at 1,150°C for 30 min of the samples with x = 0.5 to 0.68 leads to the formation of Si-ncs with the mean size of about 14 nm, whereas rapid thermal annealing of similar samples at 1,050°C for 1 min showed the presence of Si-ncs with sizes of about 5 nm. Two main broad PL bands were observed in the 500- to 900-nm spectral range with peak positions at 575 to 600 nm and 700 to 750 nm accompanied by near-infrared tail. The low-temperature measurement revealed that the intensity of the main PL band did not change with cooling contrary to the behavior expected for quantum confined Si-ncs. Based on the analysis of PL spectrum, it is supposed that the near-infrared PL component originates from the exciton recombination in the Si-ncs. However, the most intense emission in the visible spectral range is due to either defects in matrix or electron states at the Si-nc/matrix interface.     

Structural transformations in ZnS:Cu in the course of thermal annealing

The influence of annealing at 800°C on the photoluminescence, electron spin resonance, and X-ray diffraction spectra of powder-like ZnS:Cu, obtained by the self-propagating high-temperature synthesis of a charge, consisting of Zn, S, and CuCl, is studied. It is shown that variation in the material’s heating rate up to the annealing temperature leads to a nonmonotonic variation in the spectral location and full-width at half-maximum of the photoluminescence band in the blue-green spectral region, as well as in the Mn²⁺ paramagnetic center concentration. It is established that the cubic and hexagonal ZnS phases, as well as the ZnO and CuZn phases, are present in the powder after synthesis. It is shown that annealing of the obtained powder at 800°C leads to three processes: the transformation of the hexagonal ZnS phase into the cubic phase, the oxidation of ZnS and CuZn, and the diffusion of Cu into the bulk of the ZnS microcrystals from the CuZn phase. A model attributing the observed variations in luminescence and electron spin resonance spectra to the diffusion of Cu and Mn impurities into the microcrystal bulk, particularly from the CuZn phase, and to their accumulation at extended defects is suggested.     

Evolution of the deformation state and composition as a result of changes in the number of quantum wells in multilayered InGaN/GaN structures

The methods of high-resolution X-ray diffraction have been used to study the multilayered structures in an In _x Ga_{1 − x} N/GaN system grown by the method of metal-organic chemical-vapor deposition. A correlation between the strain state (relaxation) of the system, the indium content within quantum wells, the ratio of the barrier/well thicknesses, and the number of quantum wells in the active superlattice is established. It is shown that partial relaxation is observed even in a structure with one quantum well. The results we obtained indicate that the relaxation processes are bound to appreciably affect the optical characteristics of devices.     

Interphase interactions and the mechanism of current flow in Au-TiB x -AuGe-n-GaP ohmic contacts

Au-TiB _x -AuGe-n-GaP ohmic contacts have been investigated before and after rapid thermal annealing at T = 723, 773, and 873 K for 60 s in a hydrogen atmosphere. It is shown that the contact resistivity decreases with an increase in temperature in the range 77–232 K due to the thermionic nature of current flow in inhomogeneous ohmic contacts, while in the range 232–386 K the contact resistivity increases, which can be related to the conduction through metal shunts.