Alloy Potential Fluctuations Research Articles

Influence of electron irradiation and rapid thermal annealing on photoluminescence from GaAsNBi alloys

We have examined the influence of electron irradiation and rapid thermal annealing on photoluminescence emission from GaAsNBi alloys. Electron irradiation of a 1-eV compressively strained GaNAsBi-on-GaAs epilayer, grown by molecular beam epitaxy and subsequently rapidly thermally annealed, is found to induce much stronger photoluminescence than what is observed for an identical as-grown sample upon annealing. At the same time, annealing of the irradiated sample caused a negligible spectral blueshift and reduced alloy potential energy fluctuations. These irradiation-related phenomena occurred without a change in the alloy macroscopic composition as revealed by x-ray diffraction and are mainly related to the nitrogen incorporated into non-substitutional sites in the quaternary alloy.

Single-junction solar cells based on p-i-n GaAsSbN heterostructures grown by liquid phase epitaxy

In this paper, we present single heterojunction p-i-n GaAsSbN/GaAs solar cells grown by low-temperature liquid-phase epitaxy (LPE) – this is of interest as a component of multi-junction solar cell devices. The quaternary absorber layer was characterized by low excitation power photoluminescence to give the temperature dependence of the bandgap. This conformed to the Varshni function at low temperatures to within 10 meV, indicating relatively small alloy potential fluctuations. The absorption properties and the transport of the photogenerated carriers in the heterostructures were investigated using surface photovoltage method. A power conversion efficiency of 4.15% (AM1.5, 1000 W·m−2) was measured for p-i-n GaAsSbN/GaAs solar cells, which is comparable to the efficiency of MOCVD grown devices of this type. This is promising for the first report of LPE grown GaAsSbN/GaAs solar cells since the current record efficiency for the cells based on these compounds grown by MBE stands just at 6%. The long-wavelength photosensitivity of the cells determined from external quantum efficiency and surface photovoltage measurements was shown to be extended to 1040 nm.

Three dimensional simulation on the transport and quantum efficiency of UVC-LEDs with random alloy fluctuations

The active regions of ultraviolet light emitting diodes (UVLEDs) for UVB and ultra-violet band C wavelengths are composed of AlGaN alloy quantum barriers (QBs) and quantum wells (QWs). The use of alloy QBs and QWs facilitates the formation of percolative paths for carrier injection but also decreases carrier confinement within the QWs. We applied the recently developed Localization Landscape (LL) theory for a full 3D simulation of the LEDs. LL theory describes the effective quantum potential of the quantum states for electrons and holes in a random disordered system with a high computational speed. The results show that the potential fluctuations in the n-AlGaN buffer layer, QWs, and QBs provide percolative paths for carrier injection into the top (p-side) QW. Several properties due to compositional disorder are observed: (1) The peak internal quantum efficiency is larger when disorder is present, due to carrier localization, than for a simulation without fluctuations. (2) The droop is larger mainly due to poor hole injection and weaker blocking ability of the electron blocking layer caused by the fluctuating potentials. (3) Carriers are less confined in the QW and extend into the QBs due to the alloy potential fluctuations. The wave function extension into the QBs enhances TM emission as shown from a k·p simulation of wave-functions admixture, which should then lead to poor light extraction.

Exciton localization and structural disorder of GaAs1−xBix/GaAs quantum wells grown by molecular beam epitaxy on (311)B GaAs substrates

In this work, we have investigated the structural and optical properties of GaAs(1−x)Bix/GaAs single quantum wells (QWs) grown by molecular beam epitaxy on GaAs (311)B substrates using x-ray diffraction, atomic force microscopy, Fourier-transform Raman (FT-Raman) and photoluminescence spectroscopy techniques. The FT-Raman results revealed a decrease of the relative intensity ratio of transverse and longitudinal optical modes with the increase of Bi concentration, which indicates a reduction of the structural disorder with increasing Bi incorporation. In addition, the PL results show an enhancement of the optical efficiency of the structures as the Bi concentration is increased due to important effects of exciton localization related to Bi defects, nonradiative centers and alloy disorder. These results provide evidence that Bi is incorporated effectively into the QW region. Finally, the temperature dependence of the PL spectra has evidenced two distinct types of defects related to the Bi incorporation, namely Bi clusters and pairs, and alloy disorder and potential fluctuation.

An electrochemical method based on OCP fluctuations for anti-corrosion alloy composition optimization

Purpose An electrochemical method based on the open circuit potential (OCP) fluctuations was put forward. It can be used to optimize the alloy compositions for improving the corrosion resistance of rust layer. Design/methodology/approach The potential trends and potential fluctuations of carbon steels in seawater were separated by Hodrick–Prescott filter. The Spearman correlation coefficient and max information coefficient were used to explore the correlation of alloy compositions and potential fluctuations. Findings After long-term immersion, potential fluctuation resistance (PFR) can be used to characterize the corrosion resistance of metals and its rust layers. In the 1,500 to 2,500 h exposure period, Fe, C and S compositions have strong negative correlations, whereas PFR and P composition have weak negative correlations. Mn, Cu and Ti alloy compositions help the rust layer of carbon steels have higher PFRs. These elements that exhibit higher PFRs in this period have been confirmed to have the effect on improving the corrosion resistance of rust layer. Originality/value A new computing method for alloy composition optimization of carbon steels based on the OCP fluctuations was put forward. This method combines electrochemical monitoring with the long-term actual seawater environmental tests of various carbon steels.

Recombination mechanisms in heteroepitaxial non-polar InGaN/GaN quantum wells

We report on the low temperature photoluminescence and photoluminescence excitation spectroscopy of non-polar InGaN/GaN multiple quantum well (QW) structures, grown on r-plane sapphire, containing average indium concentrations of 3%, 4.5%, and 6.5%. In the sample with the lowest indium content, two distinct emission processes are identified which are attributed to recombination from planar areas of the QWs and regions of the QWs intersected by basal-plane stacking faults (BSFs). The marked reduction in the strength of the QW exciton observed in the excitation spectra when detecting BSF-related emission is attributed to the electronic disorder in the QW due to alloy potential fluctuations which inhibits exciton transfer from the planar regions of the QW. The magnitude of this disorder increases rapidly with increasing indium fraction as is evidenced by the progressive broadening of the emission and absorption features.

Valence band tail states in disordered superlattices embedded in wide parabolic AlGaAs well

Optical properties of intentionally disordered multiple quantum well (QW) system embedded in a wide AlGaAs parabolic well were investigated by photoluminescence (PL) measurements as functions of the laser excitation power and the temperature. The characterization of the carriers localized in the individual wells was allowed due to the artificial disorder that caused spectral separation of the photoluminescence lines emitted by different wells. We observed that the photoluminescence peak intensity from each quantum well shifted to high energy as the excitation power was increased. This blue-shift is associated with the filling of localized states in the valence band tail. We also found that the dependence of the peak intensity on the temperature is very sensitive to the excitation power. The temperature dependence of the photoluminescence peak energy from each QW was well fitted using a model that takes into account the thermal redistribution of the localized carriers. Our results demonstrate that the band tails in the studied structures are caused by alloy potential fluctuations and the band tail states dominate the emission from the peripheral wells.

Raman scattering of InxAl1‐xN alloys with 0.2 < x < 0.9

AbstractInxAl1‐xN (0001) wurtzite layers with 0.2 < x < 0.9 that are grown by molecular beam epitaxy on AlN/Al2O3 templates are studied using Raman spectroscopy. In the high‐frequency part of the spectra (450‐850 cm‐1), the two EH2 modes (one AlN‐ and one InN‐like) and one A1(LO) are observed in the backscattering geometry. The three peaks shift towards higher frequencies as the Al concentration increases. In the low‐ frequency part, three peaks are also observed. The InN‐like EL2 mode is present in the whole composition range while the AlN‐like EL2 mode appears for x > 0.45. In the case of the Al rich samples, an additional broad feature appears. The Raman spectra are fitted using the spatial correlation model providing the extent of the phonon confinement imposed by the alloy potential fluctuations is estimated. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Exciton localization effect in Mn-implanted GaN by photoluminescence measurements

We investigated the temperature-dependent and excitation power-dependent photoluminescence spectra of Mn-implanted (Ga,Mn)N samples with five Mn-implantation doses. The near-band-energy emission was observed and was attributed to the Mn-related exciton transition, which exhibits localized exciton behaviour resulting from alloy potential fluctuations and demonstrates a special temperature-dependence characteristic of alloy disorder. In terms of the integrated photoluminescence intensity as a function of temperature, the activation energy of the localized exciton was obtained. All these results show strong dependence on the Mn concentration of (Ga,Mn)N epilayers.

The correlation effect of carrierinduced magnetic solitons in diluted magnetic semiconductors

AbstractWe have discussed the localization mechanism in the transport property in the random potentials due to the alloy potential fluctuations in diluted magnetic semiconductors, taking into account the long‐range interactions among magnetic solitons, by using the effective Lagrangian of diffusion modes. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Raman study of alloy potential fluctuations inMgxZn1−xO nanopowders

The blueshift of near-band-edge emission and excitonic absorption indicate thatZn2+ ions are successfullysubstituted by Mg2+ ions in MgxZn1−xO nanopowders for 0≤x≤0.14. The changes in Raman spectral linewidth and the asymmetry of theE2 (high) mode for various Mg contents can be well described by a modified spatial correlationmodel that considers the grain size distribution. With increasing Mg concentration, the alloypotential fluctuations lead to a decrease in the grain size, which is induced by the surplusMg2+ that could form MgO clusters surrounding the crystalline MgZnO.

Kinetics of persistent photoconductivity in InGaN epitaxial films grown by MOCVD

In this study, the spectral and temperature dependence of persistent photoconductivity (PPC) in InGaN, which was grown by metal-organic chemical vapor deposition (MOCVD), were discussed. The related optical and electrical properties of the InGaN epilayers have been investigated by PPC and photoluminescence (PL). The metastability was observed in InGaN epilayers with indium contents of 0.12 and 0.2, and show that the PPC effect arises from alloy potential fluctuations in InGaN layer. PPC decay behavior can be well described by a stretched-exponential function and the relaxation time constant τ and decay exponent β both increase with increasing In content. Moreover, the carrier capture energy barrier for free carrier capture by defects Δ E was found to increase with the increase of indium concentration as well. All these observations show clearly a relation between localized exciton in InGaN-based optoelectronic devices, and the deep levels significantly influence the carrier lifetime of minority carrier devices.

Plateau-to-plateau transitions in random-alloy scattering dominated 2DEGs of different densities

We report a study of the integer quantum Hall plateau-to-plateau transitions in the two-dimensional electron system in Al x Ga 1 - x As – Al 0.32 Ga 0.68 As heterostructures. For two-dimensional electron systems with disorder dominated by short-ranged alloy potential fluctuations, a power-law temperature scaling is established with the critical exponent obtained to be κ = 0.42 ± 0.01 , disregarding the density of the samples.

The effect of exciton localization on the optical and electrical properties of undoped and Si-doped AlxGa1−xN

The properties of undoped and Si-doped AlxGa1−xN layers grown by metalorganic vapour-phase epitaxy have been investigated by photoluminescence and Hall effect measurements. The variable temperature photoluminescence properties of the layers were typical of AlxGa1−xN, with the temperature dependence of the peak energy showing the often-observed S-shape. The low and variable temperature photoluminescence properties of undoped material could be well described using alloy potential fluctuation theory. However, this was not the case for moderately Si-doped material, for which much larger S-shape behaviour, and thus larger exciton localization, was observed for samples with x > 0.3. The temperature dependence of the PL peak intensity of the Si-doped AlxGa1−xN layers, as well as their Hall activation energies, were then studied in order to obtain possible origins for the large increase in the exciton localization energy. It is tentatively proposed that the increase in the exciton localization could be related to the Γ9 → Γ7 valence band crossover, which occurs at roughly the same composition (0.2 < x < 0.3).

Stretched exponential profiles of photoluminescence decays related to localized states in InGaAsN/GaAs single-quantum wells

We have investigated photoluminescence (PL) dynamics related to localized states in In x Ga 1− x As 1− y N y /GaAs single-quantum wells (SQWs) with the constant In content of x = 0.32 and various N contents of y = 0 , 0.004 , and 0.008 . In order to determine the intrinsic band-edge energy, we used photoreflectance (PR) spectroscopy that is sensitive to the optical transitions at critical points. From systematic measurements of the PL and PR spectra, it is demonstrated that the slight incorporation of nitrogen considerably disorders the band-edge states of the InGaAsN SQWs, resulting from formation of localized states, so-called band-tail states. We find that the PL-decay profile related to the localized states generally exhibits a stretched exponential behavior peculiar to a disordered system at low temperatures, which means that randomness of alloy potential fluctuations including nitrogen dominates the PL dynamics.

Metal–insulator transition and magnetic solitons in diluted magnetic semiconductors

The carrier-induced magnetic solitons in diluted magnetic semiconductors have been introduced by using the gauge-invariant effective Lagrangian. We have discussed the localization mechanism in the transport property by the random potentials due to the alloy potential fluctuations and long-range interactions.

Photo-induced magnetic-solitons and spin dynamics in diluted magnetic semiconductor quantum wells

The localization mechanism of transport property in the randomly distributed system of the hole-induced magnetic solitons with the alloy potential fluctuations in diluted magnetic semiconductors has been proposed, by using the effective Lagrangian of diffusion modes. The mechanism of the long relaxation of the spin dynamics below Curie temperature in diluted magnetic semiconductor wells and the bulk system has been discussed.

Scaling and Universality of Integer Quantum Hall Plateau-to-Plateau Transitions

We have investigated the integer quantum Hall plateau-to-plateau transition in two-dimensional electrons confined to AlxGa(1-x)As-Al0.33Ga0.67As heterostructures over a broad range of Al concentration x. For x between 0.65% and 1.6%, where the dominant contribution to disorder is from the short-range alloy potential fluctuations, we observe a perfect power-law scaling in the temperature range from 30 mK to 1 K with a critical exponent kappa = 0.42 +/- 0.01.

Absence of local-potential fluctuation effects in Si-doped In xGa 1−xN epilayers studied by optical characterizations

We have studied the characteristics of In x Ga 1− x N epilayers grown on sapphire substrates by using photoluminescence (PL), optical absorption (OA), photocurrent (PC) and persistent photoconductivity (PPC) measurements. For the undoped In x Ga 1− x N epilayers containing high In composition, we observed the Stokes shift, S-shaped temperature-dependent PL emission, and PPC effect. These results show a strong dependence on In contents in the properties of In x Ga 1− x N epilayers. The decay kinetics of the PPC effect has been investigated, from that, the depth of the localization caused by alloy potential fluctuations (APFs) in In x Ga 1− x N epilayers is determined. However, for Si-doped In x Ga 1− x N epilayers, the Stokes shift, S-shaped temperature-dependent PL peak shift and PPC effect have not been observed.

Electron-irradiation enhanced photoluminescence from GaInNAs∕GaAs quantum wells subject to thermal annealing

Electron irradiation of a 1.3-μm-GaInNAs∕GaAs multi-quantum-well heterostructure, grown by molecular beam epitaxy and subsequently rapid-thermal annealed, is found to induce much stronger photoluminescence than what is observed for an identical as-grown sample upon annealing. Annealing of the irradiated sample also causes a small additional spectral blueshift and reduces alloy potential energy fluctuations at the conduction band minimum. These irradiation-related phenomena are accompanied by small but discernable changes in x-ray diffraction features upon annealing, which indicate compositional and∕or structural changes in the quantum wells.