GaAs Spacer Research Articles

GaAs/AlGaAs resonant tunneling diodes with a GaInNAs absorption layer for telecommunication light sensing

Al0.6Ga0.4As/GaAs/Al0.6Ga0.4As double-barrier resonant-tunneling diodes (RTD) were grown by molecular beam epitaxy with a nearby, lattice-matched Ga0.89In0.11N0.04As0.96 absorption layer. RTD mesas with ring contacts and an aperture for optical excitation of charge carriers were fabricated on the epitaxial layers. Electrical and optical properties of the RTDs were investigated for different thicknesses of a thin GaAs spacer layer incorporated between the AlGaAs tunnel barrier adjacent to the GaInNAs absorption layer. Illumination of the RTDs with laser light of 1.3 μm wavelength leads to a pronounced photo-effect with a sensitivities of around 103 A/W.

Investigation of superlattices based on ferromagnetic semiconductor GaMnAs by planar Hall effect

Two ferromagnetic semiconductor GaMnAs-based superlattices (SLs) were investigated by measuring the planar Hall effect (PHE) with the external magnetic field applied in the plane of the sample. The two GaMnAs/GaAs SLs differed only by the Be doping of the nonmagnetic GaAs spacer layers. Both SLs showed a typical two-step transition behavior in PHE field scans at 4.0 K, essentially the same as that normally observed on single GaMnAs ferromagnetic layers with two in-plane magnetic easy axes. As the temperature increased to 30 K, the behaviors of the PHE changed differently in the two SL samples. The PHE in the undoped SL can be described simply by the temperature dependence of the magnetic anisotropy within the film plane of a GaMnAs film. However, the Be-doped SL revealed a completely different behavior, showing a transition of magnetization with a negative coercive field. The observation of this feature in a ferromagnetic multilayer indicates the presence of spontaneous anti-parallel interlayer exchange coupling between the GaMnAs magnetic layers.

Improved performance of multilayer InAs/GaAs quantum-dot solar cells using a high-growth-temperature GaAs spacer layer

The use of high-growth-temperature GaAs spacer layers is demonstrated to significantly enhance the performance of multilayer InAs/GaAs quantum-dot solar cells. Threading dislocations are observed for a 30-layer quantum-dot structure with GaAs spacer layers grown at low temperature (510 °C). The formation of threading dislocations is suppressed by growing the GaAs spacer layer at high temperature (580 °C), leading to enhanced quantum-dot optical and structural characteristics. Incorporation of the high-growth-temperature GaAs spacer layers into a 30-layer InAs/GaAs quantum-dot solar cell results in a dramatic increase in the short-circuit current compared to the one without the high-growth-temperature spacer layers and an increase in the short-circuit current compared to the reference GaAs solar cell.

Magnetotransport properties of ferromagnetic semiconductor GaMnAs-based superlattices

Two series of GaMnAs/GaAs superlattices (SSs) comprised of ferromagnetic semiconductor GaMnAs layers and non-magnetic GaAs spacers were investigated by the electronic transport measurements with an external magnetic field applied in the plane of the sample. The two SL series consisted of specimens that were structurally the same, but the GaAs spacers layers of one series were doped by Be, while in the second series the spacers were undoped. Although in field scans taken at 4 K all SLs showed a typical anisotropic magnetoresistance (MR) behavior dominated by magnetic anisotropy, similar to that normally observed in single GaMnAs ferromagnetic layers grown on GaAs (001) substrates, some of the SLs showed signatures of interaction between the GaMnAs layers in the form of broadening of the MR hysteresis. The effect of inter-layer exchange coupling (IEC) between the GaMnAs magnetic layers became clear in the MR data taken at 30 K, where the strength of the magnetic anisotropy was reduced to the level of the inter-layer interaction. Specifically, MR measurements on two of the Be-doped SLs (BD2 and BD3) exhibited conspicuously large values of resistance at zero field, along with transitions of magnetization with negative coercive fields. The observation of these features in GaMnAs/GaAs SLs indicates the presence of spontaneous antiferromagnetic (AFM) inter-layer exchange coupling (IEC) between the GaMnAs magnetic layers. The study further revealed that the IEC in the GaMnAs multilayers strongly depended on the properties of the non-magnetic GaAs spacers, such as their thickness and the density of carriers in the layers. Importantly, these IEC effects occurred on a longer range than that expected from current theoretical studies.

Polarization dependences of electroluminescence and absorption of vertically correlated InAs/GaAs QDs

The results of experimental studies concerning the optical polarization anisotropy of electroluminescence and absorption spectra of systems with a varied number of tunnel-coupled vertically correlated In(Ga)As/GaAs quantum dots (QDs), built into a double-section laser with equal-length sections, are presented. One such system is a QD superlattice exhibiting the Wannier-Stark effect. The involvement of heavyhole ground states in optical transitions for light polarized both in the plane perpendicular to the growth axis (X-Y) and along the growth direction Z of the structure was observed. The degree of polarization anisotropy depends on the height of vertically correlated QDs and the QD superlattice: the total thickness of all In(Ga)As QD layers and GaAs spacers between the QDs, which is related to the Z component of the wave function of heavy-hole ground states for vertically correlated QDs and for the QD superlattice.

Single-particle and collective excitations in quantum wires made up of vertically stacked quantum dots: Zero magnetic field

We report on the theoretical investigation of the elementary electronic excitations in a quantum wire made up of vertically stacked self-assembled InAs/GaAs quantum dots. The length scales (of a few nanometers) involved in the experimental setups prompt us to consider an infinitely periodic system of two-dimensionally confined (InAs) quantum dot layers separated by GaAs spacers. The resultant quantum wire is characterized by a two-dimensional harmonic confining potential in the x-y plane and a periodic (Kronig-Penney) potential along the z (or the growth) direction within the tight-binding approximation. Since the wells and barriers are formed from two different materials, we employ the Bastard's boundary conditions in order to determine the eigenfunctions along the z direction. These wave functions are then used to generate the Wannier functions, which, in turn, constitute the legitimate Bloch functions that govern the electron dynamics along the direction of periodicity. Thus, the Bloch functions and the Hermite functions together characterize the whole system. We then make use of the Bohm-Pines' (full) random-phase approximation in order to derive a general nonlocal, dynamic dielectric function. Thus, developed theoretical framework is then specified to work within a (lowest miniband and) two-subband model that enables us to scrutinize the single-particle as well as collective responses of the system. We compute and discuss the behavior of the eigenfunctions, band-widths, density of states, Fermi energy, single-particle and collective excitations, and finally size up the importance of studying the inverse dielectric function in relation with the quantum transport phenomena. It is remarkable to notice how the variation in the barrier- and well-widths can allow us to tailor the excitation spectrum in the desired energy range. Given the advantage of the vertically stacked quantum dots over the planar ones and the foreseen applications in the single-electron devices and in the quantum computation, it is quite interesting and important to explore the electronic, optical, and transport phenomena in such systems.

Optical properties of multi-layer type II InP/GaAs quantum dots studied by surface photovoltage spectroscopy

We present a low-temperature (73 K) study of the optical properties of multi-layer type II InP/GaAs self-assembled quantum dots by means of surface photovoltage (SPV) spectroscopy, taking advantage of its high sensitivity and contactless nature. The samples contain 10 periods of InP quantum dot planes separated by 5 nm GaAs spacers. The SPV amplitude spectra reveal two major broad peaks, situated at low and high energies, respectively. These features are analyzed taking into account the type II character of the structure, the quantum coupling effects, the spectral behavior of the SPV phase, and the photoluminescence spectra. As a result they have been attributed to optical transitions in the quantum dots and the wetting layers, respectively. The main mechanism for carrier separation in the SPV generation process is clarified via the analysis of the SPV phase spectra. The influence of the substrate absorption on the SPV spectra is discussed in details.

Suppression of planar defects in the molecular beam epitaxy of GaAs/ErAs/GaAs heterostructures

We present a growth method that overcomes the mismatch in rotational symmetry of ErAs and conventional III-V semiconductors, allowing for epitaxially integrated semimetal/semiconductor heterostructures. Transmission electron microscopy and reflection high-energy electron diffraction reveal defect-free overgrowth of ErAs layers, consisting of >2× the total amount of ErAs that can be embedded with conventional layer-by-layer growth methods. We utilize epitaxial ErAs nanoparticles, overgrown with GaAs, as a seed to grow full films of ErAs. Growth proceeds by diffusion of erbium atoms through the GaAs spacer, which remains registered to the underlying substrate, preventing planar defect formation during subsequent GaAs growth. This growth method is promising for metal/semiconductor heterostructures that serve as embedded Ohmic contacts to epitaxial layers and epitaxially integrated active plasmonic devices.

Room-temperature optical absorption in the InAs/GaAs quantum-dot superlattice under an electric field

Electroluminescence and absorption spectra of a ten-layer InAs/GaAs quantum dot (QD) superlattice built in a two-section laser with sections of equal length is experimentally studied at room temperature. The thickness of the GaAs spacer layer between InAs QD layers, determined by transmission electron microscopy, is ∼6 nm. In contrast to tunnel-coupled QDs, QD superlattices amplify the optical polarization intensity and waveguide absorption of the TM mode in comparison with the TE mode. It is found that variations in the multimodal periodic spectrum of differential absorption of the QD superlattice structure are strongly linearly dependent on the applied electric field. Differential absorption spectra exhibit the Wannier-Stark effect in the InAs/GaAs QD superlattice, in which, in the presence of an external electric field, coupling of wave functions of miniband electron states is suppressed and a series of discrete levels called the Wannier-Stark ladder states are formed.

Investigation of weak interlayer exchange coupling in GaMnAs/GaAs superlattices with insulating nonmagnetic spacers

A robust long-range antiferromagnetic coupling between ferromagnetic Ga0.97Mn0.03As layers has previously been realized via insertion of nonmagnetic Be-doped GaAs spacers between the magnetic layers. In this paper we report the observation of weak antiferromagnetic coupling between Ga0.97Mn0.03As layers through undoped GaAs spacers with thicknesses as large as 25 monolayers. The field and the temperature dependences of the sample magnetization suggest that the interlayer coupling in these systems substantially deviates from typical ferromagnetic behavior. Polarized neutron reflectivity measurements reveal antiferromagnetic alignment between Ga0.97Mn0.03As layers when a weak field is applied perpendicular to the magnetic easy axis during cooling below TC. The strength of the observed coupling between the magnetic layers is estimated to be weaker than 0.05 mT.

Atomic structure and optical properties of InAs submonolayer depositions in GaAs

Using cross-sectional scanning tunneling microscopy and photoluminescence spectroscopy, the atomic structure and optical properties of submonolayer depositions of InAs in GaAs are studied. The submonolayer depositions are formed by a cycled deposition of 0.5 monolayers InAs with GaAs spacer layers of different thicknesses between 1.5 and 32 monolayers. The microscopy images exhibit InAs-rich agglomerations with widths around 5 nm and heights of up to 8 monolayers. A lateral agglomeration density in the 1012 cm−2 range is found. During the capping of the InAs depositions a vertical segregation occurs, for which a segregation coefficient of ∼0.73 was determined. In the case of thin GaAs spacer layers, the observed segregation forms vertically connected agglomerations. The photoluminescence spectra exhibit peaks with linewidths below 10 meV and show a considerable dependence of the peak energy on the spacer thickness, even up to 32 monolayers GaAs, indicating a long range electronic coupling.

Antiferromagnetic exchange coupling between GaMnAs layers separated by a nonmagnetic GaAs:Be spacer

Interlayer exchange coupling (IEC) between two Ga0.95Mn0.05As layers separated by Be-doped GaAs spacers was investigated using magnetometry and neutron scattering measurements, which indicated the presence of robust antiferromagnetic IEC under certain conditions. We argue that the observed behavior arises from a competition between the IEC field and magnetocrystalline anisotropy fields intrinsic to GaMnAs layers. We estimate the magnitude of the IEC field and show how it decays with increasing temperature.

Photovoltaic response of coupled InGaAs quantum dots

In order to enhance absorption at infrared range for GaAs-based solar cells, multi-stack InGaAs vertical-coupled quantum dots (VCQDs) of 5nm GaAs spacers are grown in the active region. We investigated the photovoltaic response for the solar cells by increasing the layer numbers of VCQDs. The device with nine-layer InGaAs VCQDs shows an enhanced short-circuit current density of 10.5mA/cm2. The value is increased by 42% compared to the GaAs reference device. However, the open-circuit voltage is reduced from 0.88 to 0.54V.

Structural and optical properties of InAs bilayer quantum dots grown at constant growth rate and temperature

Bilayer quantum dots (BQDs) are interesting structures for long wavelength emission due to its ability to tune the areal density and dot size separately. However, wide implementation of BQDs is hindered by the complicated growth procedures, which involve two sets of growth rate and temperature for the respective QD layers. We had recently reported the optical properties of the BQDs grown at constant growth rate and temperature [Appl. Phys. Lett. 95 (2009) 181913]. In this work, we investigate the effects of GaAs spacer thickness and InxGa1−xAs strain-reducing layer on the structural and optical properties of such BQDs.

Observation of antiferromagnetic interlayer exchange coupling in aGa1−xMnxAs/GaAs:Be/Ga1−xMnxAstrilayer structure

Interlayer exchange coupling (IEC) between two ${\text{Ga}}_{0.95}{\text{Mn}}_{0.05}\text{As}$ layers separated by Be-doped GaAs spacers was investigated experimentally using magnetization, magnetotransport and neutron-scattering measurements, which all indicated the presence of robust antiferromagnetic IEC when the GaAs spacer is sufficiently thin. We argue that the observed behavior arises from a competition between the interlayer exchange field and magnetocrystalline anisotropy fields intrinsic to GaMnAs layers.

Increase of spectral width of stacked InAs quantum dots on GaAs by controlling spacer layer thickness

Doubly stacked InAs quantum dots (QDs) /GaAs structures with different thicknesses of a GaAs spacer layer were grown by organometallic vapor phase epitaxy (OMVPE) and optical properties were investigated by photoluminescence (PL) measurements. A PL spectrum showed Gaussian-like shape with a central wavelength of 1144nm and an FWHM of 102nm from doubly stacked InAs QDs with the nominal GaAs spacer layer thickness of 87nm. The central wavelength and the FWHM are almost the same as those from single layer InAs QDs. On the other hand, an asymmetrical PL spectrum was observed from doubly stacked InAs QDs with the nominal GaAs spacer layer thickness of 25nm. The PL spectrum from doubly stacked InAs QDs with the nominal GaAs spacer layer thickness of 55nm showed Gaussian-like shape with the central wavelength of 1276nm and the FWHM of 232nm. This PL spectrum consisted of two emission bands around 1170 and 1300nm with almost the same intensity. These results indicate that the spectral width can be increased by controlling the spacer layer thickness in the stacked InAs QDs/GaAs structure.

Morphology and Optical Properties of Self-Assembled In 0.5Ga 0.5As Quantum Dots with Different Spacer Layer Thickness

Uncapped double stacked In 0.5Ga 0.5As quantum dots (QDs) with different spacer layer thicknesses were grown using metal-organic chemical vapour deposition (MOCVD). The precursors used for the growth of the GaAs layer and In 0.5Ga 0.5As QDs were trimethylgallium (TMGa), trimethylindium (TMIn), and arsine (AsH 3). The morphology and optical properties of the self-assembled In 0.5Ga 0.5As QDs were investigated and characterized using atomic force microscopy (AFM) and photoluminescence (PL). The AFM images revealed that the sizes of the dots on the topmost were not uniformly distributed. The average size of the dots fluctuated as the GaAs spacer layer thickness increased. A room temperature PL measurement was used to establish the quality and quantity of the stacked QDs. The PL peak position remained at 1148 nm for all samples of QDs; however, the PL intensity increased as spacer layer thickness increased. The structure of the spacer layer in the stacked QD affected the morphology of the topmost surface of the QDs. The PL measurement coherently reflected the AFM characterization, in which the strong PL spectra were caused by the uniformity and high density of the QDs. The surface morphology, structure, and optical properties of the stacked QDs are attributed to seed-layer (first layer) formation of dots and spacer layer structures.

Reducing carrier escape in the InAs/GaAs quantum dot intermediate band solar cell

Intermediate band solar cells (IBSCs) fabricated to date from In(Ga)As/GaAs quantum dot arrays (QD-IBSC) exhibit a quantum efficiency (QE) that extends to below bandgap energies. However, the production of sub-bandgap photocurrent relies often on the thermal and/or tunneling escape of carriers from the QDs, which is incompatible with preservation of the output voltage. In this work, we test the effectiveness of introducing a thick GaAs spacer in addition to an InAlGaAs strain relief layer (SRL) over the QDs to reduce carrier escape. From an analysis of the QE at different temperatures, it is concluded that escape via tunneling can be completely blocked under short-circuit conditions, and that carriers confined in QDs with an InAlGaAs SRL exhibit a thermal escape activation energy over 100 meV larger than in the case of InAs QDs capped only with GaAs.

Investigation of the effect of larger monolayer coverage in the active layer of bilayer InAs/GaAs quantum-dot structure and effects of post-growth annealing

Morphological and optical properties of MBE-grown single and bilayer InAs/GaAs quantum-dot heterostructure with comparatively larger monolayer coverage (∼3.3 ML) are investigated. As compared to the similar single-layer quantum-dot (SQD) structure, the bilayer quantum-dot (BQD) structure showed a more uniform spatial distribution and increased size homogeneity of the dots. It also exhibited longer wavelength photoluminescence (PL) emission at room temperature, with the peak at a wavelength (1.34 μm) in the infrared communication window. The shift seen in the low temperature PL emission peak, even after annealing the BQD samples at temperatures up to 700°C, is negligible. However, the BQD samples showed an activation energy lower than that of the SQD sample, attributed to the In/Ga interdiffusion with the thin (8.5 nm thick) GaAs spacer layer during the growth process. Despite that, the higher thermal stability demonstrated by the BQD structure, along with a lower linewidth, makes the bilayer quantum dots a desirable proposition for efficient lasing devices requiring strict tolerances on operating wavelength.

First-principles studies of interlayer exchange coupling in (Ga, Mn)As-based diluted magnetic semiconductor multilayers

Interlayer exchange coupling (IEC) in a series model diluted magnetic semiconductor multilayers consisting of two magnetic (Ga, Mn)As layers separated by nondoped or Be-doped GaAs nonmagnetic spacers is studied by first-principles calculations. For the GaAs spacers without Be doping, the IEC is always ferromagnetic and is well described by the Ruderman–Kittle–Kasuya–Yoshida interaction based on a two-band model for a gaped system. For the Be-doped GaAs spacers, the IEC is found to be tunable and the antiferromagnetic IEC is achieved via Be-doping in the interfaces between the (Ga, Mn)As layers and the spacers.