Adjacent GaAs Research Articles

Anomalous spectral dependence of optical polarization and its impact on spin detection in InGaAs/GaAs quantum dots

We show that circularly polarized emission light from InGaAs/GaAs quantum dot (QD) ensembles under optical spin injection from an adjacent GaAs layer can switch its helicity depending on emission wavelengths and optical excitation density. We attribute this anomalous behavior to simultaneous contributions from both positive and negative trions and a lower number of photo-excited holes than electrons being injected into the QDs due to trapping of holes at ionized acceptors and a lower hole mobility. Our results call for caution in reading out electron spin polarization by optical polarization of the QD ensembles and also provide a guideline in improving efficiency of spin light emitting devices that utilize QDs.

Step-terraced morphology of GaAs(001) substrates prepared at quasiequilibrium conditions

“Step and terrace” surface morphology with regular arrays of atomically flat terraces separated by monatomic steps was obtained by annealing GaAs(001) substrates in the conditions close to equilibrium. These conditions were realized under molecular hydrogen ambient in a narrow slit (“capillary”) between two adjacent GaAs substrates or in a graphite cassette containing saturated Ga–GaAs melt. The evolution of the step and terrace morphology was characterized by the length of monatomic steps, which was proved to be a more adequate characteristic of flattening process as compared to the root mean square roughness.

MBE growth of self-assembled InGaAs quantum dots aligned along quasi-periodic multi-atomic steps on a vicinal (1 1 1)B GaAs surface

Growth mechanisms of InGaAs three-dimensional (3D) islands formed on vicinal (1 1 1)B GaAs surfaces with multi-atomic steps are studied by examining surface morphologies of In x Ga 1− x As layers having various In contents x and/or nominal thickness d. When an InGaAs layer with x = 0.29 and d = 3 nm was deposited by molecular beam epitaxy (MBE), 3D islands with lateral sizes of about 40 nm, almost twice as much as the average period (∼20 nm) of underlying GaAs steps, were formed and densely aligned along the steps. A nearly flat surface was formed, when an InGaAs layer with x = 0.10 and d = 3 nm was deposited. In case of d = 2 nm and x = 0.30 , smaller 3D islands were densely formed and their lateral size perpendicular to the steps was typically about 20 nm. Such 3D islands were hardly found when d = 2 nm and x = 0.55 . These results support our hypothetical model that the 3D island formation is induced by the abrupt increase in strain energy resulting from the coherent connection of a pair of InGaAs wire-like structures on a pair of adjacent GaAs multi-atomic steps.

Generic degradation mechanism for 980 nm InxGa1−xAs/GaAs strained quantum-well lasers

We have observed In out diffusion from strained InxGa1−xAs quantum wells into the adjacent GaAs barriers in degraded 980-nm-wavelength strained quantum-well lasers. A previous calculation on misfit stress-induced compositional instability indicates that this material system is stable with respect to misfit strain. Therefore, the out diffusion of In from an InxGa1−xAs quantum well is mainly driven by the compositional discontinuity across the well/barrier heterointerfaces, and is believed to be activated by the nonradiative recombination of injected carriers.

Carrier transport and photoluminescence affected by Γ–X resonance in GaAs–AlAs type-I superlattices

We observed two peaks (fast and slow components) in time-resolved photocurrents for type-I GaAs/AlAs superlattices (SLs). The calculations of the envelop function for the SLs showed that the X 1 state in the AlAs barriers resonates with the Γ 2 state in the adjacent GaAs wells at the bias voltage where the slow components become larger. The photoluminescence measurements revealed that some electrons transfer from the Γ 1 to X 1 states and flow through the X 1– Γ 2–X 1– Γ 2 channel at the near X 1– Γ 2 resonance voltage. The slow component in the transient photocurrents originates from the X 1– Γ 2–X 1– Γ 2 electron transport.

Observation of Γ-X resonances in type-I GaAs/AlAs semiconductor superlattices: Anomaly in photoluminescence

We report the direct observation of \ensuremath{\Gamma}-X resonances in type-I GaAs/AlAs semiconductor superlattices under an electric field. Photoluminescence (PL) associated with the first ${\mathrm{X}}_{\mathrm{z}}$ state in the AlAs and the first \ensuremath{\Gamma} heavy-hole state in the GaAs was found in type-I superlattices. The PL strongly increased its intensity due to \ensuremath{\Gamma}-X mixing, by tuning the resonance between the ${\mathrm{X}}_{\mathrm{z}}$ state and the second \ensuremath{\Gamma}-electron state in the adjacent GaAs layer. This observation gives clear evidence that \ensuremath{\Gamma}-X scattering and mixing often occur even in type-I superlattices under an electric field.

Ion damage buildup and amorphization processes in GaAs–AlxGa1−xAs multilayers

The nature of ion damage buildup and amorphization in GaAs–AlxGa1−xAs multilayers at liquid-nitrogen temperature is investigated for a variety of compositions and structures using Rutherford backscattering-channeling and cross-sectional transmission electron microscopy techniques. In this multilayer system, damage accumulates preferentially in the GaAs layers; however, the presence of AlGaAs enhances the dynamic annealing process in adjacent GaAs regions and thus amorphization is retarded close to the GaAs–AlGaAs interfaces even when such regions suffer maximum collisional displacements. This dynamic annealing in AlGaAs and at GaAs–AlGaAs interfaces is more efficient with increasing Al content; however, the dynamic annealing process is not perfect and an amorphous phase may be formed at the interface above a critical defect level or ion dose. Once an amorphous phase is nucleated, amorphization proceeds rapidly into the adjacent AlGaAs. This is explained in terms of the interplay between defect migration and defect trapping at an amorphous–crystalline or GaAs–AlGaAs interface. In addition, enhanced recrystallization of the amorphous GaAs at the interface may occur during heating if an amorphous phase is not formed in the adjacent AlGaAs layer. This is most likely the result of mobile defects injected from the AlGaAs layer during heating.

The effect of excess gallium vacancies in low-temperature GaAs/AlAs/GaAs:Si heterostructures

This article shows that the presence of low-temperature-grown GaAs (LT-GaAs) in LT-GaAs/AlAs/GaAs:Si heterostructures increases the Al/Ga interdiffusion at the heterostructure interfaces. The interdiffusion enhancement is attributed to the presence of Ga vacancies (VGa) in the As-rich LT-GaAs, which diffuses from a supersaturation of VGa frozen-in during sample growth. Chemical mapping, which distinguishes between the AlAs and GaAs lattices at an atomic scale, is used to measure the Al concentration gradient in adjacent GaAs:Si layers. A correlation is observed between the Al/Ga interdiffusion and the gate breakdown voltage in metal-insulator field-effect transistor structures containing LT-GaAs.

Delayed photocurrent affected by Γ-X resonance in GaAs/AlAs type-I short-period superlattices

Delayed photocurrents were observed in GaAs/AlAs type-I short-period superlattices by measuring time-resolved photoresponses under ultrashort optical pulse excitation. According to the envelope function calculations, the X1 state in AlAs barriers resonates with the Γ2 state in the adjacent GaAs wells at a bias voltage where the delayed photocurrents were conspicuous. These results strongly suggest that the dynamic carrier transport process is significantly influenced by X1-Γ2 resonance effects in the superlattices.

Gamma -X intervalley tunnelling in a GaAs/AlAs resonant tunnelling diode under uniaxial stress

Longitudinal uniaxial stress (O<S<36 kbar) at 300 K and 77 K was used to experimentally examine intervalley tunnelling processes in a GaAs/AlAs resonant tunnelling diode (RTD) by lowering quasibound states confined in AlAs X-point quantum wells with respect to the Gamma -point conduction band profile. Intervalley resonances were observed that arose from tunnelling through: (i) large effective mass longitudinal X-states, (ii) a small-mass transverse X-state, and (iii) longitudinal X-states in AlAs aligned with the Gamma -point ground state confined in the adjacent GaAs quantum well, resulting in a resonance with a bias voltage position that increased (decreased) with increasing stress for X-states in the emitter (collector) AlAs layer. At 77 K, intervalley tunnelling can provide resonances of magnitude comparable with the intravalley Gamma resonance. Intervalley resonances remained observable in the differential conductance at 300 K. Self-consistent calculations of device characteristics agreed with the data and established voltage-stress domains for all possible intervalley resonances in the RTD.

Determination of composition profile and atomic-scale roughness of GaAs/AlGaAs interfaces by high-resolution transmission electron microscopy

New methods for both qualitative and quantitative analysis of GaAs/AlGaAs interfaces by high-resolution transmission electron microscopy (HREM) at 400 kV in <100> projection are presented. In experimental images of cleaved 90° wedge-shaped specimens recorded under optimum imaging conditions the widths of interfacial transition regions can be easily estimated from the shift of the boundary between the different contrast patterns in the GaAs and AlGaAs layers. With the help of a reliable and efficient image-processing algorithm, composition profiles across individual interfaces can be quantitatively determined from the HREM contrast of the interfaces requiring only the bulk compositions of the adjacent GaAs and AlGaAs layers to be known. Both methods were applied to the investigation of GaAs/AlAs and GaAs/Al 0.6Ga 0.4As heterostructures grown by molecular beam epitaxy (MBE). The obtained results concerning the interface abruptness and roughness were compared to studies by photoluminescence (PL) and electrical transport measurements. Differences in chemical abruptness of normal (AlGaAs on GaAs) and inverted (GaAs on AlGaAs) interfaces were quantitatively determined for conventional cross sections by our algorithm. At the normal GaAs/AlAs interface, a step which is inclined to the <100> direction could be identified by a systematic decrease of the quantitatively determined Al content along the interface. In all heterostructures under investigation, the larger interface diffusiveness (found by HREM) showed to be connected with a larger roughness resulting in variations of local well width (found by PL and transport measurements). Since HREM, PL and transport measurements yield information on different scales, they complement each other favourably.

Model for longitudinal-optical phonons and electron-phonon coupling in GaAs-Ga1-xAlxAs multilayer structures.

A continuum theory is developed to investigate the properties of the long-wavelength longitudinal-optical phonons in GaAs-${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Al}}_{\mathit{x}}$As multilayer structures and the associated electron-phonon interaction. Depending on the layer, the relative ionic displacements are related to GaAs or GaAs-type longitudinal-optical phonons and treated in the framework of the Born-Huang model, generalized to include isotropic dispersion effects in the Brillouin-zone center. For double heterostructures, a finite number of quantized confined modes is found. The interplay between the long-range Coulomb interaction, which couples the vibrations of adjacent GaAs layers, and confinement effects, which prevent the displacements of adjacent GaAs layers to overlap, is elucidated in the case of superlattices. The strength of the electron-phonon coupling in double heterostructures is reduced as compared with the electron--bulk-phonon effective coupling strength for quantum-well widths smaller than 100 \AA{}.

A model for longitudinal optical phonons and electron-phonon coupling in GaAs/GaAlAs multilayer structure

A new model for long-wavelength longitudinal optical phonons in GaAsGaAlAs multi-layer structures is presented. Depending on the layer, the relative ionic displacements are written on the basis of GaAs or GaAs-type longitudinal optical phonons and treated in the framework of the Born-Huang model generalized to include isotropic dispersion effects in the Brillouin zone centre. For double heterostructures, a finite number of quantized confined modes is found. Interplay between the long range Coulomb interaction, which couples the vibrations of adjacent GaAs layers, and confinement effects, which prevent the displacements of adjacent GaAs layers to overlap, is evidenced in the case of superlattices. The strength of the electron-phonon coupling in double heterostructures stays within a factor of 2 of the electron-bulk phonon effective coupling strength for practical values of the parameters.