Abnormal energy dependence of photoluminescence decay time in InGaN epilayer

We employ photoluminescence (PL) and time-resolved PL to study exciton localization effect in InGaN epilayers. By measuring the exciton decay time as a, function of the monitored emission energy at different temperatures, we have found unusual behaviour of the energy dependence in the PL decay process. At low temperature, the measured PL decay time increases with the emission energy. It decreases with the emission energy at 200K, and remains nearly constant at the intermediate temperature of 120K. We have studied the dot size effect on the radiative recombination time by calculating the temperature dependence of the exciton recombination lifetime in quantum dots, and have found that the observed behaviour can be well correlated to the exciton localization in quantum dots. This suggestion is further supported by steady state PL results.

Alloy compositional fluctuation in InAlGaN epitaxial films

The quaternary InAlGaN films were grown by metal-organic vapor phase epitaxy (MOVPE) at various temperatures and the optical and structural properties of the quaternary films were investigated by temperature-dependent photoluminescence (PL), high-resolution X-ray diffraction (HRXRD) and high-resolution electron microscopy (HREM). The results show that the temperature-dependent PL intensity of the InAlGaN film is similar to that of the disordered alloys, which is thought to be due to local alloy compositional fluctuations (ACF) in the epilayer. HRXRD measurement reveals there are In-rich and In-poor phases in the film and HREM observation, on the other hand, demonstrates that nanoclusters formed in the epilayer. Therefore the experimental results support the existence of ACF in the epilayers.

Strain-induced in-plane optical anisotropy in (001) GaAs/AlGaAs superlattice studied by reflectance difference spectroscopy

In-plane optical anisotropy (IPOA) in (001) GaAs/AlGaAs superlattice induced by uniaxial strain has been investigated by reflectance difference spectroscopy (RDS). Uniaxial strain on the order of 10(-4) was introduced by bending a strip sample with a stress apparatus. The IPOA of all interband transitions shows a linear dependence on strain. The birefringence and dichroism spectra induced by strain are obtained by RDS on the basis of a three-phase model, which is in good agreement with the reported results. (c) 2006 American Institute of Physics.

Structural characterization of AlGaN/GaN superlattices by x-ray diffraction and Rutherford backscattering

We report on structural characterization of AlGaN/GaN superlattices grown on sapphire. The superlattice formation is evidenced by high-resolution x-ray diffraction and transmission electron microscopy. The high resolution x-ray diffraction spectra exhibit a pattern of satellite peaks. The in-plane lattice constants of the superlattices indicate the coherent growth of the AlGaN layer onto GaN. The average At composition in the superlattices is determined to be 0.08 by Rutherford backscattering spectroscopy. The average parallel and perpendicular elastic strains for the SLs are determined to be (e(parallel to)) = +0.25% and (e(perpendicular to)) = -0.17%. (c) 2006 Elsevier Ltd. All rights reserved.

Narrow stripe selective growth of oxide-free InGaAlAs/InGaAlAs MQWs by ultra-low pressure MOVPE

Narrow stripe selective growth of oxide-free InGaAlAs/InGaAlAs multiple quantum wells (MQWs) has been successfully performed on patterned InP substrates by ultra-low pressure MOVPE. Flat and clear interfaces were obtained for the narrow stripe selectively grown MQWs under optimized growth conditions. These selectively grown MQWs were covered by specific InP layers, which can keep the MQWs from being oxidized during the fabrication of the devices. The characteristics of selectively grown MQWs were strongly dependent on the mask stripe width. In particular, a PL peak wavelength shift of 73 nm, a PL intensity of more than 57% and a PL FWHM of less than 102 meV were observed simultaneously with a small mask stripe width varying from 0 to 40 mu m. The results were explained by considering the migration effect from the masked region (MMR) and the lateral vapour diffusion effect (LVD).

A study of the growth and optical properties of AlInGaN alloys

We have studied the growth and optical properties of AlInGaN alloys in this article. By the measurement of three samples, we found that the incorporation of In decreases with the increase of temperature, while there is nearly no change for the incorporation of Al. The sample grown at the lowest temperature had the best material and optical properties, which owes to the high In component, because the In component can reduce defects and improve the material quality. We also used the time-resolved photoluminescence(PL) to study the mechanism of recombination of carriers, and found that the time dependence of PL intensity was not in exponential decay, but in stretched-exponential decay. Through the study of the character of this decay, we come to the conclusion that the emission comes from the recombination of localized excitons. Once more, this localization exhibites the character of quantum dots, and the stretched, exponential decay results from the hopping of carriers between different localized states. In addition, we have used the relation of emission energy dependence of carrier's lifetime and the character of radiative recombination and non-radiative combination to confirm our conclusion.

Void formation and failure in InGaN/AlGaN double heterostructures

Condensed clusters of point defects within an InGaN/AlGaN double heterostructure grown by metal-organic vapor phase epitaxy on sapphire substrate have been observed using transmission electron microscopy. The existence of voids results in failure of the heterostructure in electroluminescence. The voids are 50-100 nm in diameter and are distributed inhomogeneously within In0.25Ga0.75N/AlGaN active layers. The density of the voids was measured as 10(15) cm(-3), which corresponds to a density of dangling bonds of 10(20) cm(-3). These dangling bonds may fully deplete free carriers in this double heterostructure and result in the heterostructure having high resistivity as confirmed by electrical measurement. (C) 2003 Elsevier Science B.V. All rights reserved.

Localized exciton dynamics in AlInGaN alloy

Carrier recombination dynamics in AlInGaN alloy has been studied by photoluminescence (PL) and time-resolved PL (TRPL) at various temperatures. The fast red-shift of PL peak energy is observed and well fitted by a physical model considering the thermal activation and transfer processes. This result provides evidence for the exciton localization in the quantum dot (QD)-like potentials in our AlInGaN alloy. The TRPL signals are found to be described by a stretched exponential function of exp[(-t/,tau)13], indicating the presence of a significant disorder in the material. The disorder is attributed to a randomly distributed QDs or clusters caused by indium fluctuations. By studying the dependence of the dispersive exponent beta on temperature and emission energy, we suggest that the exciton hopping dominate the diffusion of carriers localized in the disordered QDs. Furthermore, the localized states are found to have 0D density of states up to 250 K, since the radiative lifetime remains almost unchanged with increasing temperature. (C) 2003 Elsevier Science Ltd. All rights reserved.

Structural and optical properties of InAlGaN films grown directly on low-temperature buffer layer with (0001)sapphire substrate

Quaternary InAlGaN film has been grown directly on top of low-temperature-deposited GaN buffer layer by low-pressure metalorganic vapor phase epitaxy. High-resolution X-ray diffraction and photoluminescence (PL) results show that the film has good crystal quality and optical property. Temperature-dependent PL and time-resolved PL (TRPL) have been employed to study the carriers recombination dynamics in the film. The TRPL signals can be well fitted as a stretched exponential function exp[-(t/tau)(beta)] from 14 to 250 K, indicating that the emission is attributed to the radiative recombination of excitons localized in disorder quantum nanostructures such as quantum disks originating from indium (In) clusters or In composition fluctuation. The cross-sectional high-resolution electron microscopy measurement further proves that there exist the disorder quantum nanostructures in the quaternary. By investigating the dependence of the exponential parameter beta on the temperature, it is shown that the multiple trapping-detrapping mechanism dominates the diffusion among the localized states. The localized states are considered to have two-dimensional density of states (DOS) at 250 K, since radiative recombination lifetime tau(r) increases linearly with increasing temperature. (C) 2002 Elsevier Science B.V. All rights reserved.

Annealing-induced evolution of defects in low-temperature-grown GaAs-related materials

Infrared absorption spectroscopy, optical transient current spectroscopy (OTCS), and photoluminescence (PL) spectroscopy are used to investigate the annealing induced evolution of defects in low-temperature (LT)-grown GaAs-related materials. Two LT samples of bulk GaAs (sample A) and GaAs/AlxGa1-xAs multiple-quantum-well. (MQW) structure (sample B) were grown at 220 and 320 degreesC on (001) GaAs substrates, respectively. A strong defect-related absorption band has been observed in both as-grown samples A and B. It becomes weaker in samples annealed at temperatures above 600 degreesC. In sample A, annealed in the range of 600-800 degreesC, a large negative decay signal of the optical transient current (OTC) is observed in a certain range of temperature, which distorts deep-level spectra measured by OTCS, making it difficult to identify any deep levels. At annealing temperatures of 600 and 700 degreesC, both As-Ga antisite and small As cluster-related deep levels are identified in sample B. It is found that compared to the As cluster, the As-Ga antisite has a larger activation energy and carrier capture rate. At an annealing temperature of 800 degreesC, the large negative decay signal of the OTC is also observed in sample B. It is argued that this negative decay signal of the OTC is related to large arsenic clusters. For sample B, transient PL spectra have also been measured to study the influence of the, defect evolution on optical properties of LT GaAs/AlxGa1-xAs MQW structures. Our results clearly identify a defect evolution from AS(Ga) antisites to arsenic clusters after annealing.

Anomaly of the current self-oscillation frequency in the sequential tunneling of a doped GaAs/AlAs superlattice

An anomalous behavior of the current self-oscillation frequency is observed in the dynamic de voltage bands, emerging from each sawtoothlike branch of the current-voltage characteristic of a doped GaAs/A1As superlattice in the transition process from static to dynamic electric field domain formations. Varying the applied de voltage at a fixed temperature, we find that the frequency increases while the averaged current decreases. Inside each voltage band, the frequency has a strong voltage dependence in the temperature range where the averaged current changes with the applied de voltage. This dependence can be understood in terms of motion of the system along a limit cycle.

Observation of the third subband population in modulation-doped InGaAs/InAlAs heterostructure

The population of the third (n = 3) two-dimensional electron subband of InGaAs/InAlAs modulation-doped structures has been observed by means of Fourier transform photoluminescence (PL). Three well resolved PL peaks centred at 0.737, 0.908, and 0.980eV are observed, which are attributed to the transitions from the lowest three electron subbands to the n = 1 heavy-hole subband. The subband separations clearly exhibiting the features of the stepped quantum well with triangle and square potentials are consistent with numerical calculation. Thanks to the presence of Fermi cutoff, the population ratio of these three subbands can be estimated. Temperature- and excitation-dependent luminescences are also analyzed.

The third subband population in modulation-doped InGaAs/InAlAs heterostructures

We have observed the population of the third (n=3) two-dimensional electron subband of InGaAs/ InAlAs modulation-doped structures with very dense sheet carrier density by means of Fourier transform photoluminescence (PL). Three well-resolved PL peaks centered at 0.737, 0.908, and 0.980 eV are observed, which are attributed to the recombination transitions from the lowest three electron subbands to the n=1 heavy-hole subband. The subband separations clearly exhibit the features of the stepped quantum well with triangle and square potential, consistent with numerical calculation. Thanks to the presence of the Fermi cutoff, the population ratio of these three subbands can be estimated. Temperature and excitation intensity dependence of the quantum well luminescence intensity is also analyzed. (C) 1997 American Institute of Physics.

Tandem electroabsorption modulators integrated with DFB laser by ultra-low-pressure selective-area-growth MOCVD for 10 GHz optical short pulse generation - art. no. 60200O

A novel device of tandem multiple quantum wells (MQWs) electroabsorption modulators (EAMs) monolithically integrated with DFB laser is fabricated by ultra-low-pressure (22 mbar) selective area guowth (SAG) MOCVD technique. Experimental results exhibit superior device characteristics with low threshold of 19 mX output light power of 4.5 mW and over 20 dB extinction ratio when coupled into a single mode Fiber. Moreover, over 10 GHz modulation bandwidth is developed with a driving voltage of 2 V. Using I this sinusoidal voltage driven integrated device, 10GHz repetition rate pulse with a width of 13.7 ps without any compression elements is obtained.

PHOTOELECTROCHEMICAL BEHAVIOR OF THE GAAS/ALXGA1-XAS SUPERLATTICE ELECTRODE/ELECTROLYTE INTERFACE

Single and multiple quantum wells of lattice-matched superlattices material GaAs/AlxGa1-xAs have been studied as photoelectrodes in photoelectrochemical cells containing nonaqueous electrolyte. Structural photocurrent spectra in the potential range of -1.8 to 1.0 V (vs standard calomel electrode) were obtained. The quantum yields for both superlattice electrodes were estimated and compared.