Differences In Crystal Quality Research Articles

Local augmentation of phonon transport at GaInN/GaN heterointerface by introducing a graded variation of InN mole fraction

The pump and probe technique in Raman spectroscopy of the E2 (high) mode is exploited to uncover the enhancing factor of the phonon transport across Ga1−xInxN/GaN interfaces. Two samples are investigated: one with a uniform x of 0.09 and another one with a graded variation in x from 0.17 to 0 along the depth direction. Lateral phonon transport is obtained by scanning the 532-nm probing laser from the irradiation position of the 325-nm heating laser. No difference in the lateral diffusion length is observed between the two samples, while the transport probability across the interface is higher for the sample with the graded variation in x than the sample with the uniform x of 0.09. The microscopic images of the decrease in the mode energy or the increase in temperature of the GaN layer reveal that the local phonon transport across the heterointerface is enhanced in regions with low differences in the phonon mode energy between the GaN and GaInN rather than the difference in crystal quality.

Effect of Defects on Strain Relaxation in InGaN/AlGaN Multiple‐Quantum‐Well Near‐Ultraviolet Light‐Emitting Diodes

Three similar‐structure InGaN/AlGaN multiple‐quantum‐well near‐ultraviolet (NUV) light‐emitting diodes (LEDs) are utilized to investigate the microscopic effect of defects on strain relaxation. Consistent correlations among the crystal quality, the piezoelectric field (FPZ), the internal quantum efficiency (IQE), and the bandgap shrinkage of NUV LEDs are obtained by investigating the macroscopic characterizations. The difference in crystal quality (or the defect density) of NUV LEDs is found by the ideality factor, the emission microscope image, the Shockley–Read–Hall coefficient, and the IQE. Electroreflectance spectra are used to calculate FPZ of NUV LEDs. FPZ, the IQE, and the peak‐wavelength shift at driving currents are increased with the samples’ crystal quality compared to the reference sample. Also, FPZ, the IQE, and the peak‐wavelength shift are decreased with the increase in samples’ defect densities. A similar result is found for the bandgap shrinkage. This effect significantly indicates that the strain relaxation is induced by defects. Herein, a model that systematically explains the observed changes in macroscopic properties of NUV LEDs is proposed.

Spectroscopic characterization and photocatalytic activity of vanadosilicate AM-6 towards the degradation of 2,5-Dichlorophenol

Vanadosilicate AM-6 crystals were synthesized using two methods – a structure-directing agent (SDA) method (i.e., non-porous), and a non-SDA method (i.e., porous). As a result, two types of AM-6 crystal products were obtained as evidenced by XRD, SEM, BET surface area measurements, and Raman and UV–vis spectroscopic analyses. These porous and non-porous AM-6 crystal products were used to determine the rate of photodegradation of a selected organic compound, 2,5 dichlorophenol (2,5-DCP). Both crystal products showed enhanced photocatalytic activity as compared to photolysis. Furthermore, the rate of degradation was found to be higher for the porous AM-6 products as compared to the non-porous crystals under UV light irradiation. This variance was attributed to differences in crystal quality. Raman and diffuse reflectance UV–vis spectroscopies showed more disorder along the ⋯V–O–V⋯ chains for porous AM-6 products. Thus, the higher photocatalytic degradation rate of 2,5-DCP is likely due to the presence of more accessible VOH sites.

Lattice Parameters a-, c-, Strain-Stress Analysis and Thermal Expansion Coefficient of InGaN/GaN Solar Cell Structures Grown by MOCVD

Structural properties of InGaN/GaN solar cells (SCs) grown by metal organic chemical vapor deposition (MOCVD) technique are investigated by high resolution X-ray diffraction (HR-XRD) method. It is noticed that a- and c- lattice parameters of the structures showed small differences according to examined (hkl) planes. Fault percentage of the a- and c- lattice parameters are also calculated. It is seen that fault percentage is smaller than %2 for all samples. Investigations have been made for three different samples. Differences in crystal quality caused by growth conditions are seen in all three samples. At the same time, properties such as crystal size, strain and stress are determined. During determination of stress, two different methods including elastic constants, Young module and Poisson ratio are used. Results gained from these two methods are compared with each other. Thermal expansion coefficients of InGaN are calculated for (002), (004), (006) and (121) planes for 100 oC temperature difference (300-400 oC). It is seen that peak positions gained from HR-XRD are nearly the same with the ones in database. All the results obtained from calculations are given in tables in the following sections of this article. It can be seen that all these results are in accordance with previous works done by different authors and with the real values.

An investigation of the role of defect levels on the radiation response of synthetic diamond crystals when used as sensors for the detection of mammography X-rays

This study evaluates the role of defects on the performances of synthetic diamond sensors on exposure to mammography X-rays. Through systematic investigations, the main cause of instability of response of examined sensors necessitating pre-irradiation was isolated and ascribed to the presence of ambient light which has the effect of emptying shallow trapping levels. The changes in response between measurements in light and dark conditions varied from 2.8 ± 1.2% to 63.0 ± 0.3%. Sensitivities between 0.4 and 6.7nCGy−1mm−3 determined for the sensors varied with defect levels. The study indicates that differences in crystal quality due to the presence and influence of defects would cause a discrepancy in the dosimetric performances of various diamond detectors. Once a sensor plate is selected (based on the influence of defect levels) and coupled to the probe housing with the response of the diamond sensor stabilised and appropriately shielded from ambient light, daily priming is not needed.

The influence of defect levels on the dose rate dependence of synthetic diamond detectors of various types on exposures to high-energy radiotherapy beams

Abstract The linear response of a radiation dosimeter with absorbed dose rate is a principal requirement in radiotherapy. Fowler's model for electrical conductivity, σ of a solid-state detector and absorbed dose rate, Dr is of the form σ ∝ D r Δ where Δ is the linearity index that can take on a range of values around unity. Utilising synthetic diamond detectors of various types as sensors, this study investigates the influence of defect levels on the Δ values of the sensors and the dependence of Δ on bias voltage, beam energy and type in the dosimetry of high-energy photon and electron therapy beams. One main objective of the study was to establish whether for a given diamond detector, Δ could be determined only once for any given beam energy and then used for other beam energies of clinical interest. In order to attain the ICRU overall ±5% uncertainty of absorbed dose delivery in radiotherapy, ±2% accuracy was considered. The study was conducted on one HPHT and eight CVD synthesised diamonds of optical grade (OG) and detector grade (DG) qualities using 6 and 15 MV photon, and 7 and 12 MeV electron energies. Values of Δ ranging from 0.79–1.03 to 0.85–0.96 were obtained for the electron and photon beams, respectively for all the diamond sensors at 1 kV/cm. The Δ values were found to change with various defect levels present within the crystals as characterised by Raman spectroscopy, ESR, FTIR spectroscopy and TL emission, and it was observed that the Δ values of crystals with high defect levels varied strongly with bias voltage. Whereas the Δ values of the HPHT diamond were found not vary with the electron and photon energies, only those of three CVD samples of a given class showed a variation within 2% between the two energies of each beam type. However, for all the crystals tested Δ showed a maximum variation of 3.4% between the photon energies unlike the electron energies where a very strong variation (>5%) was observed for three OG CVD crystals. The results of this study have suggested that differences in crystal quality due to the presence of defects could cause Δ to vary with bias voltage, beam energy and type.

The electrical and structural properties of n-type InAs nanowires grown frommetal–organic precursors

The electrical and structural properties of⟨111⟩B-oriented InAs nanowires grown using metal–organic precursors have been studied.On the basis of electrical measurements it was found that the trends in carbonincorporation are similar to those observed in the layer growth, where an increasedAs/In precursor ratio and growth temperature result in a decrease in carbon-related impurities.Our results also show that the effect of non-intentional carbon doping is weakerin InAs nanowires compared to bulk, which may be explained by lower carbonincorporation in the nanowire core. We determine that differences in crystal quality, herequantified as the stacking fault density, are not the primary cause for variations inresistivity of the material studied. The effects of some n-dopant precursors (S, Se, Si,Sn) on InAs nanowire morphology, crystal structure and resistivity were alsoinvestigated. All precursors result in n-doped nanowires, but high precursor flowsof Si and Sn also lead to enhanced radial overgrowth. Use of the Se precursorincreases the stacking fault density in wurtzite nanowires, ultimately at high flowsleading to a zinc blende crystal structure with strong overgrowth and very lowresistivity.

Nanosecond laser induced damage in RbTiOPO_4: The missing influence of crystal quality

Nanosecond laser induced damage in RbTiOPO(4) (RTP) an isomorphic material to the more widely known KTiOPO(4) (KTP) is studied in crystals with varying properties. The ionic conductivity along the z-axes of the tested crystals ranged from 1.5 10(-9) S/cm to 1.1 10(-12) S/cm. Further, different growth sectors with different absorption in the range of hundreds of ppm/cm and differing zones in inhomogeneous crystals have been investigated. Despite these important differences in crystal quality, no significant difference could be observed in the laser damage resistance at 1064 nm. Thus growth induced defects only play a minor role in nanosecond laser induced damage in RTP. Transient, laser induced defects are discussed in analogy with KTP as possible laser damage precursors.

Thermal Stability of MOCVD and HVPE GaN Layers in H2, HCl, NH3 and N2

This work represents a complete study of GaN annealed in H 2 , HCl, NH 3 and N 2 . The GaN thermal behavior was evaluated by comparison of MOCVD and HVPE samples. The MOCVD films were found to obey a dissociative sublimation mechanism with only gaseous species forming, while the HVPE films reacted with ambient gases to form condensed Ga in addition to the gaseous species. Differences in crystal quality for MOCVD and HVPE samples resulting from the different growth mechanisms account for the observed difference. This analysis of the thermal stability of GaN is extended to include how polarity affects the thermal behavior of GaN during growth and annealing of MOCVD and HVPE films.

Thermal Stability of MOCVD and HVPE GaN Layers in H2, HCl, NH3 and N2

This work represents a complete study of GaN annealed in H2, HCl, NH3 and N2. The GaN thermal behavior was evaluated by comparison of MOCVD and HVPE samples. The MOCVD films were found to obey a dissociative sublimation mechanism with only gaseous species forming, while the HVPE films reacted with ambient gases to form condensed Ga in addition to the gaseous species. Differences in crystal quality for MOCVD and HVPE samples resulting from the different growth mechanisms account for the observed difference. This analysis of the thermal stability of GaN is extended to include how polarity affects the thermal behavior of GaN during growth and annealing of MOCVD and HVPE films.

Variation in the lattice parameter and crystal quality of commercially available Si-doped GaAs substrates

Abstract Variations in the lattice parameter and crystal quality of commercially available GaAs (n-type, Si: 1−7 × 1018 cm−3 substrates have been studied by high-resolution X-ray diffraction (HRXRD) and asymmetric crystal topography (ACT). Crystals grown by vertical Bridgman (VB) and horizontal Bridgman (HB) typically have a lattice parameter that is either the same as or slightly larger ( ∼ 0.0001 A ) than semi-insulating GaAs(SI) while crystals grown by vertical gradient freeze (VGF) had a lattice parameter that could be as much as ∼ 0.0002 A smaller than GaAs(SI). ACT topography confirmed also fundamental differences in crystal quality. While VB and HB crystals showed a faint cellular dislocation network related to threading dislocations, VGF samples showed almost flawless crystal quality with only a faint swirl pattern commonly associated with small amounts of segregation at the growth front. Topographs of liquid-encapsulated Czochralski (LEC) samples showed extensive cross-hatching probably caused by the presence of precipitates. The observed lattice parameter differences can be explained by the presence of boron impurities and not by other defects as previously postulated. SIMS measurements found that VGF GaAs(Si, n-type) contained boron in the 3−4 × 1018 cm−3 concentration range and VB crystals contained boron only in the low 1016 cm−3 range while HB crystals contained no detectable boron. In VGF crystals the boron concentration is so high (and the boron atom so small) that the lattice contraction can be entirely attributed to boron size effects. On the other hand, the larger lattice constant of HB crystals compared to GaAs(SI) crystals, is consistent with the dilation of the lattice driven by the free electron density which outweighs the contraction of the lattice by the Si-dopant size effect.

Comparison of Relaxation Process of Compressive and Tensile Strains in InGaAs Lattice-Mismatched Layers on InP Substrates

This paper investigates the difference in crystal quality between strained-layer multiple quantum wells with compressive (+0.5%) and tensile strains (-0.5 %). For the compressive strain, the photoluminescence intensity decreased and the length of fringe bending increased from 250 Å to 500 Å when the number of periods increased from 5 to 15. The amount of fringe bending increased when the InP thickness decreased, especially when the strain was compressive. We also investigated the relaxation process in an InGaAs layer as a function of the layer thickness (from 25 nm to 2 µ m). For a compressive strain (+1.1%), misfit dislocations were observed near the interface between InGaAs and InP substrate. On the other hand, for a tensile strain (-1.1 %), we observed cracks instead of misfit dislocations. Moreover, the cracks were considered to increase the X-ray full width at half maximum of both the InGaAs lattice-mismatched layer and the InP substrate.

Photocurrent kinetics of group III metal–(phthalocyaninato)–halogen complexes in vacuum and oxidising gas atmospheres

The photoconductivity and photocurrent kinetics of the metal–(phthalocyaninato)chloro complexes, M(pc)Cl (M = Al, Ga or In), are reported in high vacuum (10–6 Torr), oxygen and nitrogen dioxide. In vacuum it is shown that the photocurrent rise and decay kinetics of Al(pc)Cl are slow compared to those of the Ga(pc)Cl due to the disorder of the Al(pc)Cl molecule on its lattice site. The electrical properties of the M(pc)Cl complexes are compared to those of the homopolymeric [M(pc)F]n(M = Al or Ga) complexes. The photocurrent rise times of [Ga(pc)F]n in vacuum are longer than the corresponding rise times of Ga(pc)Cl, due to a difference in crystal quality. Nitrogen dioxide is shown to increase the conductivity and photoconductivity of the M(pc)X complexes. Oxygen has a similar, but smaller, effect on the M(pc)Cl (M = Al or In) and [M(pc)F]n(M = Al or Ga) complexes. However, oxygen was found to decrease the conductivity of the Ga(pc)Cl complex. The photocurrent rise times of the complexes were found to be relatively unaffected by the presence of adsorbed species. The photocurrent decay times were, however, significantly increased by the presence of the gases; this is due to an increase in the number and depth of surface charge-carrier trapping sites.