Quantum Wells Grown Research Articles

Thermal orientation of electron spins

It is shown that the spin orientation of free electrons occurs in low-symmetry semiconductor structures if the electron gas is driven out of thermal equilibrium with the crystal lattice. The proposed mechanism of such a thermal orientation of electron spins is based on spin dependence of the electron-phonon interaction, which tends to restore equilibrium. The microscopic theory of the effect is developed here for asymmetric (110)-grown quantum wells where the electron gas heating leads to the spin orientation along the [\( 1\bar 10 \)] axis in the quantum well plane.

Optical Properties of Dilute Nitride InN(As)Sb Quantum Wells and Quantum Dots Grown by Molecular Beam Epitaxy

We report the growth and characterization of a new dilute nitride infrared material: InN(As)Sb. InNAsSb single quantum wells (SQWs) and InNSb self-assembled quantum dots (QDs) were grown on both InAs and GaAs substrates by solid-source molecular beam epitaxy. High-quality InNAsSb epilayers were realized by optimizing the nitrogen incorporation growth conditions. Both secondary-ion mass spectroscopy and x-ray diffraction measurements confirmed a nitrogen incorporation of 1%. Temperature- and power-dependent photoluminescence measurements were conducted and revealed a luminescence emission at 4.03 μm from localized states and ∼4.3 μm from the ground-state transition in InNAsSb SQWs. InNSb QDs exhibited a 10 K photoluminescence peak at 3.6 μm.

Symmetry and Spin Dephasing in (110)-Grown Quantum Wells

Symmetry and spin dephasing in (110)-grown GaAs quantum wells (QWs) are investigated applying magnetic field induced photogalvanic effect and time-resolved Kerr rotation. We show that magnetic field induced photogalvanic effect provides a tool to probe the symmetry of (110)-grown quantum wells. The photocurrent is only observed for asymmetric structures but vanishes for symmetric QWs. Applying Kerr rotation we prove that in the latter case the spin relaxation time is maximal; therefore, these structures set the upper limit of spin dephasing in GaAs QWs. We also demonstrate that structure inversion asymmetry can be controllably tuned to zero by variation of delta-doping layer positions.

Characteristics of Strained GaAsSb(N)/InP Quantum Wells Grown by Metalorganic Chemical Vapor Deposition on InP Substrates

AbstractGaAsSb (N) superattices (SLs) grown on InP substrates using metalorganic vapor phase epitaxy are investigated by high resolution X-ray diffraction (XRD), low temperature photoluminescence (PL), and high resolution transmission electron microscopy (TEM). XRD shows very sharp satellite peaks and pendellosung fringes, which indicates excellent crystalline quality and abrupt interfaces in the GaAsSb (N)/InP SL, with Sb varies with 0.2 to 0.7. Low temperature PL shows clearly different features between the 25% Sb and 44% Sb samples. A band alignment difference is proposed to explain these behaviors. Experimental data establishes that the transition from a type-I to a type-II heterostructure occurs for a Sb-content of approximately 40%, which agrees well with the prediction by Model Solid Theory. While N incorporation degrades the PL intensity, it also provides the greater electron confinement needed to achieve mid-IR emission from GaAsSbN/GaAsSb type-II QWs

Classical memory effects on spin dynamics in two-dimensional systems

We discuss classical dynamics of electron spin in two-dimensional semiconductors with a spin-split spectrum. We focus on a special case, when spin-orbit induced random magnetic field is directed along a fixed axis. This case is realized in III-V-based quantum wells grown in [110] direction and also in [100]-grown quantum wells with equal strength of Dresselhaus and Bychkov-Rashba spin-orbit couplings. We show that in such wells the long-time spin dynamics is determined by non-Markovian memory effects. Due to these effects the non-exponential tail $1/t^2$ appears in the spin polarization.

Wavefunction engineering of layered wurtzite semiconductors grown along arbitrary crystallographic directions

The electronic band structure of wurtzite semiconductor heterostructures is investigated theoretically using the envelope function k ⋅ P formalism. We use a Lagrangian formulation for the valence bands so that the order of the derivatives appearing in the multiband description is explicitly specified when Schrödinger’s equations for the envelope functions are generated through the application of the principle of least action. The issues of derivative operator ordering and boundary conditions at material interfaces are examined in detail. The theoretical results are presented for arbitrary growth directions and the spin–orbit interaction is taken into account. This is of interest, for example, in treating A -plane wurtzite heterostructures such as GaN/AlGaN quantum wells grown on R -plane sapphire. Strain effects are included using a general rotation method that diagonalizes strain and is appropriate for pseudomorphic wurtzite structures in any allowed orientation. It is shown that inversion asymmetry in the valence band leads to shifts in the band-edge energies in the [ 11 2 ¯ 0 ]-grown quantum wells. The finite element method is used with the Lagrangian for the composite layered semiconductor structure in order to obtain the energy eigenvalues for multi-quantum well systems. Calculations for quantum wells and superlattices are presented.

Strain-induced enhancement of spin relaxation times in [110] and [111] grown quantum wells

We derive the strain-dependent Hamiltonian of the conduction band for semiconductor quantum wells grown along [110] and [111] directions. We propose and demonstrate theoretically that by properly controlling the strain of $\mathrm{In}\mathrm{Ga}\mathrm{As}∕\mathrm{In}\mathrm{P}$ quantum wells, the spin relaxation from the Dyakonov-Perel mechanism can be suppressed in [110] and [111] quantum wells. The spin relaxation time can be enhanced by more than one order of magnitude at an optimal strain for a given well width.

Photocurrent and Photoluminescence Investigations of GaInNAs and GaInNAs(Sb) Quantum Wells Grown by Molecular Beam Epitaxy

We have investigated photocurrent (PC) and photoluminescence (PL) in sequentially grown GaInNAs/GaAs and GaInNAs(Sb)/GaAsSbN quantum wells. Photocurrent transitions are analyzed by theoretical calculations using envelope function formalism taking into account the strain effect and the strong coupling between nitrogen localized state and the GaInAs band gap. The results are consistent with a type I band alignment and a conduction band offset ratio of about 80 %. Additionally, our results suggest an increase of the electron effective mass by as much as 0.035 m0 resulting from the flattening of the conduction band under nitrogen effect. The temperature evolution of the PL peak energy and the integrated PL intensity of GaInNAsSb QW show evidence of strong localization of carriers. Both, the high delocalization temperature, in the 230 K range and the strong shift between the PC and PL spectra of GaInNAsSb QW, indicate the presence of deeper localized states as compared to that in the GaInNAs QW.

Growth of high-quality ZnMgO epilayers and ZnO∕ZnMgO quantum well structures by radical-source molecular-beam epitaxy on sapphire

We report on a specific growth procedure combining low-temperature growth of ZnMgO and postgrowth annealing at intermediate temperatures. Despite the large lattice misfit induced by the sapphire substrate, layer-by-layer growth is accomplished up to the phase-separation limit found at a c-lattice constant of 0.5136nm and Mg mole fraction of 0.40. The procedure allows us to grow quantum wells with atomically smooth interfaces in a wide range of structural designs exhibiting prominent emission features up to room temperature.

Optical Properties of Highly Strained GaInAs/GaAs QuantumWells Grown by Sb Assistance

Optical properties of highly strained GaInAs/GaAs quantum wells (QWs)grown by molecular beam epitaxy with Sb assistance areinvestigated. The samples grown by Sb incorporation and Sbpre-deposition methods display high room-temperature photoluminescence(PL) intensity at extended long wavelength. This result is explained bythe surfactant effects of Sb during the growth of GaInAs/GaAs QWsystems. An abnormal S-shaped temperature dependence of the PL peakposition is found in the In0.42Ga0.58As/GaAs triple QWssample grown with Sb pre-deposition. By investigating the transmissionelectron microscope images and time-resolved PL spectra, it isfound that the S-shaped temperature dependence of the PL peak positionoriginates from the exciton localization effect brought by the Sb-richclusters on the QW interface.

Hanle Effect Driven by Weak Localization

The influence of weak localization on the Hanle effect in a two-dimensional system with a spin-split spectrum is considered. We show that weak localization drastically changes the dependence of a stationary spin polarization S on an external magnetic field B. In particular, the nonanalytic dependence of S on B is predicted for III-V-based quantum wells grown in the [110] direction and for the [100]-grown quantum wells having equal strengths of Dresselhaus and Bychkov-Rashba spin-orbit coupling. It is shown that in a weakly localized regime the components of S are discontinuous at B = 0. At low B, the magnetic field-induced rotation of the stationary polarization is determined by quantum interference effects. This implies that the Hanle effect in such systems is totally driven by weak localization.

Studies of Ultrathin InGaAs Quantum Wells Grown at Different Temperatures by Magneto-Photoluminescence

Here, the ultrathin InGaAs/GaAs single-quantum-well (QW) structures grown at different temperatures are studied using magneto-photoluminescence in magnetic fields of up to 15 T both parallel (B|| ) and perpendicular (B⊥) to the growth axis. The dimensionalities and the effective diameters are deduced from the low-field diamagnetic shift. The dimensionalities are approximately the same, 0.3, and the effective diameters obtained in B|| are almost the same for the QWs grown at different temperatures. The effective diameter measured in B⊥ is larger than that in B|| for the QW grown at high temperature while it becomes smaller than that in B|| for the QW grown at the lowest temperature due to the exciton localization. The anomalous diamagnetic shift shows a cusp at approximately 4 T in B⊥, and the high-field diamagnetic shift can be well-described as B4/3 dependence based on the effective width well model.

Low Threshold Current Density 1.5 μm Strained-MQW Laser by n-Type Modulation-Doping

1.5µm n-type modulation-doping InGaAsP/InGaAsP strained multiple quantum wells grown by low pressure metalorganic chemistry vapor decomposition technology is reported for the first time in the world. N-type modulation-doped lasers exhibit much lower threshold current densities than conventional lasers with undoped barrier layers. The lowest threshold current density we obtained was 1052.5 A/cm2 for 1000 µm long lasers with seven quantum wells. The estimated threshold current density for an infinite cavity length was 94.72A/cm2/well, reduced by 23.3% compared with undoped barrier lasers. The n-type modulation doping effects on the lasing characteristics in 1.5µm devices have been demonstrated.

The Mode Competition, Instability, and Second Harmonic Generation in Dual-Frequency InGaAs∕GaAs∕InGaP Lasers

InGaAs/GaAs/InGaP-based heterolasers with asymmetrically grown quantum wells of two types are developed. For the first time, dual-wavelength operation and second-harmonic generation are realized in these lasers over a wide range of injection currents: from 0.2 A in CW mode up to 10 A in injection with 200-ns-long pulses. Previously unknown special features of such a generation are experimentally revealed and interpreted in terms of the competition and coexistence of various short-and long-wavelength modes, including the “whispering-gallery” modes.

Effect of Micro-twins on InSb Quantum Wells Grown on GaAs (001) Substrates

Extended abstract of a paper presented at Microscopy and Microanalysis 2004 in Savannah, Georgia, USA, August 1–5, 2004.

Luminescence in Highly Excited InGaN/GaN Multiple Quantum Wells Grown on GaN and Sapphire Substrates

N/GaN multiple quantum wells grown by MOCVD oversapphire and bulk GaN substrates. High excitation conditions enabled usto achieve a screening of the built-in fleld by free carriers. This allowed forthe evaluation of the in°uence of band potential °uctuations due to thevariation in In-content on e–ciency of spontaneous and stimulated emission.InGaN/GaN multiple quantum wells grown on bulk GaN substrate exhibit asigniflcantly lower stimulated emission threshold and thus enhanced lateralemission. Transient and dynamic properties of luminescence indicate asigniflcant reduction in compositional disorder in homoepitaxially grownstructures.PACS numbers: 78.45.+h, 78.47.+p, 78.67.De

Improvement in Photoluminescence Efficiency of GaInNAs/GaAs Quantum Wells Grown by Metalorganic Chemical Vapor Deposition for Low-Threshold 1.3 µm Range Lasers

A quality improvement of the III–V dilute nitride semiconductor alloy, GaInNAs, grown by metalorganic chemical vapor deposition (MOCVD) on a GaAs substrate is reported for 1.3 µm-wavelength lasers. GaInNAs wafers were grown at various growth temperatures, V/III ratios, and growth rates. The photoluminescence (PL) efficiency of GaInNAs/GaAs quantum wells (QWs) was increased by lowering the growth temperature and increasing the V/III ratio in the growth conditions conventionally used for nitrogen (N)-free GaInAs/GaAs QW growth. These conditions are important for realizing high PL efficiency because they prevent the inhomogeneity of the immiscible alloy of GaInNAs. It was also observed that the optimal window for the growth temperature, V/III ratio, and growth rate for the GaInNAs is narrower than that of N-free GaInAs QWs. After careful optimization of the growth conditions, GaInNAs/GaAs QW lasers with various emission wavelengths were fabricated. Low-threshold current densities of 0.17 kA/cm2/well, 0.18 kA/cm2/well, and 0.44 kA/cm2/well are obtained for emission wavelengths of 1.25 µm, 1.30 µm, and 1.34 µm, respectively. The results obtained for growth conditions and lasing characteristics are useful in further improving 1.3 µm or longer wavelength GaInNAs lasers grown by MOCVD.

Al Contamination in InGaAsN Quantum Wells Grown by Metalorganic Chemical Vapor Deposition and 1.3 µm InGaAsN Vertical Cavity Surface Emitting Lasers

We obtained high-quality InGaAsN quantum wells (QWs) by metalorganic chemical vapor deposition (MOCVD). Our InGaAsN QWs showed low threshold current densities and high-temperature characteristics (550 A/cm2, 150 K at 1275 nm for 3QWs) in broad-area lasers. High output power (2.8 mW at 1280 nm) was also obtained from the room-temperature continuous operation of InGaAsN 3QW vertical cavity surface emitting lasers. We revealed the first solid evidence that unexpected Al incorporation (Al contamination) occurred in InGaAsN QWs continuously grown on GaAs/AlGaAs structures by MOCVD. Our results suggest that Al contamination leads to a rough surface and low photoluminescence intensity of the InGaAsN QWs. By minimizing Al contamination, high-quality InGaAsN QWs were obtained. Al contamination in InGaAsN QWs should be carefully considered in the MOCVD growth of InGaAsN-based devices.

Tuning of the alloy composition of Zn 1− x Cd x Se quantum wells by submonolayer pulsed beam epitaxy (SPBE)

Very often it is desirable to grow quantum wells of ternary alloys such as Zn1−xCdxSe and Zn1−xCdxTe with different composition in the same sample. One way is to stop the growth, change the cell temperatures to modify the cation flux ratio and, after reaching stable conditions, proceed with the sample growth. An alternative is the use of digital methods to produce alloys with an expected average composition. In this work we present a new method, submonolayer pulsed beam epitaxy, that produces homogeneous cationic layers with a given composition that can be varied without stopping the growth and without changing cell temperatures. In this method the sample surface is exposed to Cd-Zn-Se or Cd–Zn–Te cycles with submonolayer cationic coverage. During the exposure of the surface to Zn flux empty cationic sites are filled with Zn and, additionally, due to the stronger Zn–(Se, Te) chemical bond, some Cd atoms are substituted by Zn atoms and evaporate from the surface. The exposure time to Zn plays a main role in the final alloy composition. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Electroluminescence from GalnN Quantum Wells Grown on Non-(0001) Facets of Selectively Grown GaN Stripes

ABSTRACTNon (0001) GalnN QWs have been grown by low pressure MOVPE on side facets of triangular shaped selectively grown GaN stripes. By analysing low temperature photo- and cathodoluminescence and room temperature electroluminescence, we found strong indications, that both, In and Mg are less efficiently incorporated on these side facets compared to the common (0001) plane with even lower efficiency for stripes running along (1–100) compared to (11–20). Nevertheless, we observed strong light emission from these quantum wells, supposed to be at least partly caused by the reduced piezo-electric field.