Optical properties of GaInNAs/GaAs quantum well structures

The radiative recombination in GaInNAs/GaAs quantum well structures was investigated by low temperature optical spectroscopy. In the temperature region, below 100 K, we found that the observed transition energies strongly depend on the excitation intensity and the temperature, which is indicative of carrier localization. The degree of carrier localization depends on the In-concentration but is not significantly influenced by the N-concentration when the N-concentration exceeds 1.6%. Photoluminescence studies indicate that the degree of the carrier localization decreases with increasing In-concentration at a constant N-concentration. In addition, the experimental results show that carrier localization is strongly correlated to deep level emission. Through post-growth thermal treatment at 650 °C both carrier localization and deep level emission can be eliminated.     

Electrooptical properties of GaNAs/GaAs multiple quantum well structures

The electrooptical properties of the GaNAs/GaAs multiple quantum well structures have been studied using the photoreflectance spectroscopy from 20 K to room temperature. Above the band gap energy of GaAs, Franz–Keldysh oscillations were observed. The period of the Franz–Keldysh oscillations decreased slightly with decreasing temperature, and indicated that the corresponding space charge distribution varied slowly with temperature. The modulated quantum well transition features were observed below the band gap energy of GaAs. A matrix transfer algorithm was used to calculate the quantum well subband energies numerically. The band gap energy and the electron effective mass of the GaNAs/GaAs system were adjusted to obtain the subband energies to best fit the observed quantum well transition energies.     

Piezoelectric and deformation potential effects of strain-dependent luminescence in semiconductor quantum well structures

The mechanism of strain-dependent luminescence is important for the rational design of pressure-sensing devices. The interband momentum-matrix element is the key quantity for understanding luminescent phenomena. We analytically solved an infinite quantum well (IQW) model with strain, in the framework of the 6 × 6 k·p Hamiltonian for the valence states, to directly assess the interplay between the spin-orbit coupling and the strain-induced deformation potential for the interband momentum-matrix element. We numerically addressed problems of both the infinite and IQWs with piezoelectric fields to elucidate the effects of the piezoelectric potential and the deformation potential on the strain-dependent luminescence. The experimentally measured photoluminescence variation as a function of pressure can be qualitatively explained by the theoretical results.

Open image in new window

     

Potential barrier study at the InGaAs/InAs quantum dot interface in asymmetric multi quantum well structures

The photoluminescence and its temperature dependences have been investigated for the InAs quantum dots embedded in the asymmetric GaAs/In_xGa_1−xAs/In_0.15Ga_0.85As/GaAs quantum wells (dot-in-a-well, DWELL structures) as a function of the In content x (x = 0.10-0.25) in the capping In_xGa_1−xAs layer. The study of PL temperature dependences in the range of 80-120 K has revealed the potential barrier for electrons at the capping In_xGa_1−xAs/InAs QD interface. The value of mentioned barrier has varied in QD structures with the different In_xGa_1−xAs capping layer compositions and it was estimated in the studied asymmetric GaAs/In_xGa_1−xAs/In_0.15Ga_0.85As/GaAs DWELL structures. It is shown that the barrier for electrons equal to 48 and 24 meV appears in the InGaAs conduction band at the formation of InGaAs QW/InAs QD interface for the In compositions of x = 0.10 and 0.15, respectively. This barrier has not been detected in DWELLs with the In compositions x = 0.20 and 0.25. The energy gap offsets at the InAs/In_xGa_1−xAs interface in studied structures has been estimated and discussed as well.     

Modeling of In segregation, stress and strain in InGaAs/GaAs(100) quantum well heterostructures

An advanced model for simulation of In segregation phenomena, stress and strain distribution during metal-organic chemical vapor deposition of InGaAs/GaAs(100) quantum well (QW) heterostructures based on representation of boundary gas layer as “quasi-liquid” has been suggested. Elastic energy was taken into account by considering epitaxy as a sequence of growth acts each resulted in the formation of ultrathin imaginary layers. The assumption that elastic influence is not distributed throughout the whole thickness of the substrate but affects only its near-surface layer has been postulated. Results of calculations of In profiles and stress distribution for heterostructures with single and multiple QWs for varied epitaxy conditions are provided. Various options of exploring the developed model for other materials and the limitations of applicability are discussed.     

Controlled synthesis of AlN/GaN multiple quantum well nanowire structures and their optical properties

We report the controlled synthesis of AlN/GaN multi-quantum well (MQW) radial nanowire heterostructures by metal-organic chemical vapor deposition. The structure consists of a single-crystal GaN nanowire core and an epitaxially grown (AlN/GaN)(m) (m = 3, 13) MQW shell. Optical excitation of individual MQW nanowires yielded strong, blue-shifted photoluminescence in the range 340-360 nm, with respect to the GaN near band-edge emission at 368.8 nm. Cathodoluminescence analysis on the cross-sectional MQW nanowire samples showed that the blue-shifted ultraviolet luminescence originated from the GaN quantum wells, while the defect-associated yellow luminescence was emitted from the GaN core. Computational simulation provided a quantitative analysis of the mini-band energies in the AlN/GaN superlattices and suggested the observed blue-shifted emission corresponds to the interband transitions between the second subbands of GaN, as a result of quantum confinement and strain effect in these AlN/GaN MQW nanowire structures.     

Selective modification of the band gaps of GaInNas/GaAs structures by quantum well intermixing techniques

We report the unambiguous demonstration of controlled quantum well intermixing (QWI) in the technologically important GaInNAs/GaAs 1.3 μm material system. QWI is a key technique to selectively modify the band gap of quantum wells, which has found broad application in semiconductor lasers and photonic integrated circuits (PICs). Extending such technology to GaInNAs/GaAs structures is highly desirable due to the technologically advantageous properties of this material system. Here, we investigate well-characterized GaInNAs quantum well material which has been annealed “to saturation” before QWI processing to allow unambiguous interpretation of results. After RTA at 700 °C for ∼180 s, controlled shifts in band-gap at room temperature of up to 200 nm have been observed in sputtered SiO₂-capped samples, whilst uncapped and PECVD SiO₂-capped samples demonstrated negligible shift. This selective modification of the band gap has been confirmed by detailed photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopy. Analysis of composition profile by SIMS revealed that the QWI is due to the interdiffusion of In–Ga between the quantum wells and the barriers enhanced by the point defects generated during the sputtering process. Investigation of a series of samples of differing N concentrations will be presented, which provides extra information about the intrinsic properties of GaInNAs.     

Near-room-temperature mid-infrared quantum well photodetector

We demonstrate InGaAs mid-infrared quantum well infrared photodetectors (MIR PV-QWIPs) that enable cost-effective mature GaAs-based detection and imaging technologies, with exceptional material uniformity, reproducibility, and yield, over a large area, with high spectral selectivity, innate polarization sensitivity, radiation hardness, high detectivity, and high speed operation at TEC temperatures without bias.     

Multiple quantum well AlGaAs nanowires

This letter reports on the growth, structure, and luminescent properties of individual multiple quantum well (MQW) AlGaAs nanowires (NWs). The composition modulations (MQWs) are obtained by alternating the elemental flux of Al and Ga during the molecular beam epitaxy growth of the AlGaAs wire on GaAs (111)B substrates. Transmission electron microscopy and energy dispersive X-ray spectroscopy performed on individual NWs are consistent with a configuration composed of conical segments stacked along the NW axis. Microphotoluminescence measurements and confocal microscopy showed enhanced light emission from the MQW NWs as compared to nonsegmented NWs due to carrier confinement and sidewall passivation.     

Auger recombination rate in quantum well lasers

Auger recombination is the dominant non-radiative process in InGaAsP quantum well lasers and is responsible for the poor temperature dependence of the threshold current density. In all recent calculations of the Auger rate the electron-electron interaction potential is taken to be either of the bulk form or an approximate form derived from it. In the present work, the rate is calculated by taking an appropriate potential valid for quasi two-dimensional electrons and the expected changes are pointed out. The calculated Auger life-time is in agreement with the values reported in the literature.     

Photoluminescence of Al0.4Ga0.6As/GaAs quantum well structures prepared by molecular beam epitaxy

The effect of growth conditions, in particular the substrate temperature, on the quality of Al_0.4Ga_0.6As/GaAs quantum well heterostructures grown by molecular beam epitaxy was investigated. Characterization by low temperature photoluminescence indicates that the quality of the Al_xGa_{1 − x}As confining layer is optimized at 720 °C; above 720 °C it becomes very difficult to maintain sufficient arsenic coverage on the surface because of rapid arsenic desorption. The quality of the quantum wells is dominated by the interfacial properties which are dependent on the quality of the Al_xGa_{1 − x}As and to some extent on the quality of the GaAs. Our experiments show that a substrate temperature of 720 °C also optimizes the quantum wells. The photoluminescence spectra of the Al_0.4Ga_0.6As layers show excitonic processes with a width of 7 meV.     

Optical coherence in semiconductor quantum structures

Work in the last year has revealed new coherent behaviours of coupled modes of excitons and light, including the unexpeted influence from disorder. New experiments studying the scattered light from semiconductors demonstrate our incomplete understanding of localisation in quantum wells. Semiconductor coherence now enables directional control of eletron currents.     

Magnetic-field tuning of the alloy-induced disorder in quaternary semimagnetic (Zn, Cd, Mn)Se quantum well structures

The influence of magnetic-field-induced tuning of the disorder on the line width of the lowest excitonic transition in diluted magnetic semiconductor (Zn, Cd, Mn)Se/ZnSe quantum well samples is studied by photoluminescence at 2 K and in magnetic fields up to 7.5 T. It is shown that the dependence of the photoluminescence line width on quantum well thickness can be explained as a sum of contributions of ZnSe barrier, (Zn, Cd, Mn)Se well and the interfaces.     

Recent results on quantum well solar cells

The Quantum Photovoltaic group at Imperial College has pioneered the use of quantum wells in photovoltaic applications, using material supplied by our collaborators in the EPSRC III-V Facility, University of Sheffield, University of Nottingham and the Center for Electronic Materials and Devices. We have shown, in a number of lattice-matched multi-quantum well systems, that quantum wells enhance the output current and efficiency compared to homogeneous cells made from the barrier material and output voltage compared with conventional cells made from the material in the well. This paper discusses recent results in three areas of our work: (1) fundamental studies relevant to the question of whether similar efficiency enhancements will be possible in an ideal cell where radiative recombination dominates, (2) the materials problems we have had to solve in order to enhance the highest efficiency GaAs solar cells with InGaAs wells and (3) the application of quantum well cells in the area of thermophotovoltaics.     

Optically induced multispin entanglement in a semiconductor quantum well

According to quantum mechanics, a many-particle system is allowed to exhibit non-local behaviour, in that measurements performed on one of the particles can affect a second one that is far away. These so-called entangled states are crucial for the implementation of most quantum information protocols and, in particular, gates for quantum computation. Here we use ultrafast optical pulses and coherent techniques to create and control spin-entangled states in an ensemble of non-interacting electrons bound to donors (at least three) and at least two Mn2+ ions in a CdTe quantum well. Our method, relying on the exchange interaction between localized excitons and paramagnetic impurities, can in principle be applied to entangle an arbitrarily large number of spins.     

RF linewidth in passively mode locked quantum well lasers

Investigation of the stability of pulse repetition rate emitted with semiconductor lasers in the regime of passive mode locking has been conducted. It was shown experimentally that the decrease of an overlap integral of the quantum-sized active layer with the waveguide mode and the increase of the time of the carrier capture on the emitting level resulted in narrowing of the radio-frequency linewidth.     

Room temperature ballistic transport in InSb quantum well nanodevices

We report the room temperature observation of significant ballistic electron transport in shallow etched four-terminal mesoscopic devices fabricated on an InSb/AlInSb quantum well (QW) heterostructure with a crucial partitioned growth-buffer scheme. Ballistic electron transport is evidenced by a negative bend resistance signature which is quite clearly observed at 295?K and at current densities in excess of 10(6)?A/cm(2). This demonstrates unequivocally that by using effective growth and processing strategies, room temperature ballistic effects can be exploited in InSb/AlInSb QWs at practical device dimensions.     

Exciton-plasmon interactions in quantum dot-gold nanoparticle structures

We present a self-assembly method to construct CdSe/ZnS quantum dot-gold nanoparticle complexes. This method allows us to form complexes with relatively good control of the composition and structure that can be used for detailed study of the exciton-plasmon interactions. We determine the contribution of the polarization-dependent near-field enhancement, which may enhance the absorption by nearly two orders of magnitude and that of the exciton coupling to plasmon modes, which modifies the exciton decay rate.