Nikolai Faleev

Impact of stress relaxation in GaAsSb cladding layers on quantum dot creation in InAs/GaAsSb structures grown on GaAs (001)

We describe InAs quantum dot creation in InAs/GaAsSb barrier structures grown on GaAs (001) wafers by molecular beam epitaxy. The structures consist of 20-nm-thick GaAsSb barrier layers with Sb content of 8%, 13%, 15%, 16%, and 37% enclosing 2 monolayers of self-assembled InAs quantum dots. Transmission electron microscopy and X-ray diffraction results indicate the onset of relaxation of the GaAsSb layers at around 15% Sb content with intersected 60° dislocation semi-loops, and edge segments created within the volume of the epitaxial structures. 38% relaxation of initial elastic stress is seen for 37% Sb content, accompanied by the creation of a dense net of dislocations. The degradation of In surface migration by these dislocation trenches is so severe that quantum dot formation is completely suppressed. The results highlight the importance of understanding defect formation during stress relaxation for quantum dot structures particularly those with larger numbers of InAs quantum-dot layers, such as those proposed for realizing an intermediate band material.

Strain and crystal defects in thin AlN/GaN structures on (0001) SiC

High-resolution x-ray diffraction was used to compare strain relaxation and defect populations in thin GaN/AlN heterostructures (total thickness ≈480 nm) grown on (0001) SiC using metalorganic chemical vapor deposition (MOCVD) and hydride vapor epitaxy (HVPE) techniques. The results of high-resolution x-ray diffraction measurements (rocking curves and reciprocal space mapping) were corroborated using transmission electron microscopy. Differently grown films exhibited dissimilar strain relaxation and defect populations that were related to specific growth conditions. In the MOCVD films, grown under lower deposition rates, the elastic strain in the AlN and GaN layers was fully relaxed at the initial stages of the epitaxial growth yielding nearly similar densities of threading dislocation segments (TDS) in layer volumes. Additional, “secondary” elastic stresses in these layers were attributed to the excess of point defects. In the HVPE films, grown under higher (five to ten times) deposition rates, these lay...

Critical thickness investigation of MBE-grown GaInAs/GaAs and GaAsSb/GaAs heterostructures

GaInAs/GaAs and GaAsSb/GaAs heterostructures were grown by molecular beam epitaxy with different In/Sb compositions and thicknesses in order to obtain samples with different amounts of initial strain. High resolution x-ray diffraction was used to extract the alloys composition, specify the presence of dislocations, and determine the extent of relaxation while transmission electron microscopy and x-ray topography were used to observe these dislocations and characterize their type and density. The onset for the formation of misfit dislocations was found to be in agreement with the equilibrium theory. However, the films remained coherently strained for thicknesses far beyond this value. The onset for strain relaxation was found by considering the kinetics of plastic deformation using the approach proposed by Tsao and coworkers [Phys. Rev. Lett. 59, 2455 (1987)]. The mechanism of extended defect creation leading to measurable strain relief is described as a multistage process related with the structural stabi...

Quantitative analysis of the quantum dot superlattice by high-resolution x-ray diffraction

A new high-resolution x-ray diffraction approach for quantitative analysis of superlattice structures (SLs) with self-assembled quantum dots (QDs) was developed. For numerical simulations of the 2D angular distribution of diffracted x-ray radiation, both the coherent and diffuse scattering components have been calculated. Direct comparison of simulated patterns and experimental results revealed good agreement of the calculated intensity distribution with experimental reciprocal space maps for the superlattice GaAs(001)-AlGaAs-{InAs QDs-GaAs}SL with 20 periods of quantum dots. The simulation procedure allows one to obtain data about the shape, average size, elastic strains around the QDs, average density of the QDs, the presence of short- or long-range order in the arrangement of QDs in the semiconducting matrix, the vertical and lateral correlation lengths of the ensemble of quantum dots, and the parameters of the intermediate GaAs and AlGaAs layers.

Growth and characterization of GaAs1−x−ySbxNy/GaAs heterostructures for multijunction solar cell applications

The GaAsSbN dilute-nitride alloy can be grown lattice-matched to GaAs with a bandgap of 1u2009eV, making it an ideal candidate for use in multijunction solar cells. In this work, using molecular beam epitaxy in conjunction with a radio-frequency nitrogen plasma source, the authors focus first on the growth optimization of the GaAsSb and GaAsN alloys in order to calibrate the Sb and N compositions independently of each other. After the optimum growth conditions to maintain two-dimensional growth were identified, the growth of GaAsSbN films was demonstrated. Both a GaAsSb0.076N0.018/GaAs heterostructure (100u2009nm thick) and a GaAsSb0.073N0.015/GaAs quantum well (11u2009nm thick) were grown. X-ray diffraction analysis reveals quite high crystal quality with a small lattice mismatch of 0.13%–0.16%. Secondary ion mass spectrometry profiling revealed that nitrogen was unintentionally incorporated in the GaAs buffer layer during the plasma ignition and stabilization. Nevertheless, a low temperature photoluminescence peak ...

Growth of high quality ZnO thin films with a homonucleation on sapphire

ZnO thin films were epitaxially grown on c-plane sapphire substrates by plasma-assisted molecular beam epitaxy. A low temperature homonucleation ZnO layer was found crucial at the interfacial region to absorb the defects formed by the lattice mismatch between the sapphire and ZnO, resulting in a smooth surface that enables smooth 2D epitaxial growth. High quality ZnO films were achieved after careful optimization of critical growth conditions: the sequence of Zn and O source shutters, growth temperature for both the ZnO nucleation and growth layer, and Zn/O ratio. Oxygen plasma pretreatment was not applied prior to the growth, thus shortening the growth time and reducing oxidation of the metallic sources. Resultant epitaxial ZnO films on sapphire demonstrated a root-mean-square surface roughness of 0.373u2009nm for 1u2009μmu2009×u20091u2009μm atomic force microscope images with clear hexagonal structure and terrace steps. The x-ray diffraction full width at half maximum (FWHM) for ω and ω-2θ ZnO (0002) triple-crystal rocking...

Carrier localization effects in GaAs1−xSbx/GaAs heterostructures

We investigated the structural and optical properties of GaAs1−xSbx/GaAs heterostructures grown by molecular beam epitaxy on GaAs (001) substrates for Sb concentration up to 12% by means of high-resolution X-ray diffraction and photoluminescence. The correlation between our structural and optical analysis revealed that compositional fluctuations induced localized states which trap carriers at low temperature. Under low excitation power, the photoluminescence (PL) spectra are composed of two competing peaks in the temperature range of 30–80u2009K. The lower energy peak is associated with transitions from localized states in the band-tail of the density of states while the higher energy peak corresponds to transitions from free carriers. A model based on a redistribution process of localized excitons was used to reproduce the S-shape behavior of the temperature dependent PL. Reducing the growth temperature from 500u2009°C to 420u2009°C suppressed the S-shape behavior of the PL indicating a reduction in compositional va...

1-eV GaNAsSb for multijunction solar cells

We report on the growth of 1-eV GaNAsSb lattice- matched to GaAs as an alternative material to the most commonly used GaInNAs(Sb). GaNAsSb layers were grown by plasma assisted molecular beam epitaxy with different N and Sb compositions. The electronic and optical properties of the layers were probed using photoluminescence and photoreflectance spectroscopy and compared to the band anticrossing model. The incorporation mechanism of the group-V elements were investigated using secondary ion mass spectrometry. It was found that Sb does not affect the N incorporation. Moreover increasing the N flux increased the N composition at the expense of the Sb composition. Post-growth annealing was investigated and found to greatly improve the photoluminescence intensity.

MBE growth of Sb-based nBn photodetectors on large diameter GaAs substrates

The GaSb-based family of materials and heterostructures provides rich bandgap engineering possibilities for a variety of infrared (IR) applications. Mid-wave and long-wave IR photodetectors are progressing toward commercial manufacturing applications, but to succeed they must move from research laboratory settings to general semiconductor production and they require larger diameter substrates than the current standard 2-inch and 3-inch GaSb. Substrate vendors are beginning production of 4-inch GaSb, but another alternative is growth on 6-inch GaAs substrates with appropriate metamorphic buffer layers. We have grown generic MWIR nBn photodetectors on large diameter, 6-inch GaAs substrates by molecular beam epitaxy. Multiple metamorphic buffer architectures, including bulk GaSb nucleation, AlAsSb superlattices, and graded GaAsSb and InAlSb ternary alloys, were employed to bridge the 7.8% mismatch gap from the GaAs substrates to the GaSb-based epilayers at 6.1 Å lattice-constant and beyond. Reaching ~6.2 Å extends the nBn cutoff wavelength from 4.2 to <5 µm, thus broadening the application space. The metamorphic nBn epiwafers demonstrated unique surface morphologies and crystal properties, as revealed by AFM, high-resolution XRD, and cross-section TEM. GaSb nucleation resulted in island-like surface morphology while graded ternary buffers resulted in cross-hatched surface morphology, with low root-mean-square roughness values of ~10 Å obtained. XRD determined dislocation densities as low as 2 × 107 cm-2. Device mesas were fabricated and dark currents of 1 × 10-6 A/cm2 at 150K were measured. This work demonstrates a promising path to satisfy the increasing demand for even larger area focal plane array detectors in a commercial production environment.

Hetero-emitter GaP/Si solar cells with high Si bulk lifetime

III-V/silicon solar cells which have an active silicon bottom solar cell are promising for multi-junction solar cell applications. In such solar cell structures, a high minority carrier lifetime in the bulk silicon substrate is necessary. Annealing silicon wafers at high temperature (> 500oC) in the molecular beam epitaxy (MBE) high-vacuum chamber revealed significant lifetime degradation. In this work, we developed a practical method to maintain high Si bulk lifetime. SiNx layer deposited on Si back side helps maintain millisecond level minority carrier lifetime. By this procedure high minority carrier lifetime in the Si substrate is preserved while high quality thin GaP layer is achieved. We demonstrate GaP as a hetero-emitter layer with high Si bulk lifetime in GaP/Si structure solar cell with 524mV open circuit voltage.