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To exploit the promising properties of semiconductor nanowires and ensure the uniformity required to achieve device integration, their position on the growth substrate must be controlled. This work demonstrates the direct patterning of a SiO2/Si substrate using focused ion beam (FIB) patterning to control self-catalyzed GaAsSb nanowire growth in molecular beam epitaxy (MBE). Besides position control, FIB patterning parameters influence nanowire yield, composition and structure. Total ion dose per hole is found to be the most important parameter. Yield of single nanowires ranges from ≈ 34% to ≈ 83%, with larger holes dominated by multiple nanowires per hole. Areas exposed to low ion beam doses are selectively etched by routine pre-MBE HF cleaning, enabling patterning and nanowire nucleation with minimal damage to the Si substrate. The optical and electronic properties of nanowires are found to depend on the ion dose used during patterning, indicating the potential for FIB patterning to tune nanowire properties. These findings demonstrate the possibility for a FIB lithography protocol which could provide a rapid and direct patterning process for flexible controlled nanowire growth.

We developed a new technique to fabricate single nanowire devices with reliable graphene/nanowire contacts using a position-controlled microtransfer and an embedded nanowire structure in a planar junction configuration. A thorough study of electrical properties and fabrication challenges of single p-GaAs nanowire/graphene devices was carried out in two different device configurations: (1) a graphene bottom-contact device where the nanowire-graphene contact junction is formed by transferring a nanowire on top of graphene and (2) a graphene top-contact device where the nanowire-graphene contact junction is formed by transferring graphene on top of an embedded nanowire. For the graphene top-contact devices, graphene-nanowire-metal devices, where graphene is used as one electrode and metal is the other electrode to a nanowire, and graphene-nanowire-graphene devices, where both electrodes to a nanowire are graphene, were investigated and compared with conventional metal/p-GaAs nanowire devices. Conventional metal/p-GaAs nanowire contact devices were further investigated in embedded and nonembedded nanowire device configurations. A significantly improved current in the embedded device configuration is explained with a "parallel resistors model" where the high-resistance parts with the metal-semiconductor Schottky contact and the low-resistance parts with noncontacted facets of the hexagonal nanowires are taken into consideration. Consistently, the nonembedded nanowire structure is found to be depleted much easier than the embedded nanowires from which an estimation for a fully depleted condition has also been established.

The many outstanding properties of graphene have impressed and intrigued scientists for the last few decades. Its transparency to light of all wavelengths combined with a low sheet resistance makes it a promising electrode material for novel optoelectronics. So far, no one has utilized graphene as both the substrate and transparent electrode of a functional optoelectronic device. Here, we demonstrate the use of double-layer graphene as a growth substrate and transparent conductive electrode for an ultraviolet light-emitting diode in a flip-chip configuration, where GaN/AlGaN nanocolumns are grown as the light-emitting structure using plasma-assisted molecular beam epitaxy. Although the sheet resistance is increased after nanocolumn growth compared with pristine double-layer graphene, our experiments show that the double-layer graphene functions adequately as an electrode. The GaN/AlGaN nanocolumns are found to exhibit a high crystal quality with no observable defects or stacking faults. Room-temperature electroluminescence measurements show a GaN related near bandgap emission peak at 365 nm and no defect-related yellow emission.

Here we demonstrate a more effective use of III–V photoconversion material to achieve an ultrahigh power-per-weight ratio from a solar cell utilizing an axial p-i-n junction GaAs/AlGaAs nanowire (NW) array grown by molecular beam epitaxy on a Si substrate. By analyzing single NW multicontact devices, we first show that an n-GaAs shell is self-formed radially outside the axial p- and i-core of the GaAs NW during n-core growth, which significantly deteriorates the rectification property of the NWs in the axial direction. When employing a selective-area ex situ etching process for the n-GaAs shell, a clear rectification of the axial NW p-i-n junction with a high on/off ratio was revealed. Such a controlled etching process of the self-formed n-GaAs shell was further introduced to fabricate axial p-i-n junction GaAs NW array solar cells. Employing this method, a GaAs NW array solar cell with only ∼1.3% areal coverage of the NWs shows a photoconversion efficiency of ∼7.7% under 1 Sun intensity (AM 1.5G), which is the highest achieved efficiency from any single junction GaAs NW solar cell grown on a Si substrate so far. This corresponds to a power-per-weight ratio of the active III–V photoconversion material as high as 560 W/g, showing great promise for high-efficiency and low-cost III–V NW solar cells and III–V NW/Si tandem solar cells.

GaN nanocolumns were synthesized on single-layer graphene via radio-frequency plasma-assisted molecular beam epitaxy, using a thin migration-enhanced epitaxy (MEE) AlN buffer layer as nucleation sites. Due to the weak nucleation on graphene, instead of an AlN thin-film we observe two distinguished AlN formations which affect the subsequent GaN nanocolumn growth: (i) AlN islands and (ii) AlN nanostructures grown along line defects (grain boundaries or wrinkles) of graphene. Structure (i) leads to the formation of vertical GaN nanocolumns regardless of the number of AlN MEE cycles, whereas (ii) can result in random orientation of the nanocolumns depending on the AlN morphology. Additionally, there is a limited amount of direct GaN nucleation on graphene, which induces non-vertical GaN nanocolumn growth. The GaN nanocolumn samples were characterized by means of scanning electron microscopy, transmission electron microscopy, high-resolution X-ray diffraction, room temperature micro-photoluminescence, and micro-Raman measurements. Surprisingly, the graphene with AlN buffer layer formed using less MEE cycles, thus resulting in lower AlN coverage, has a lower level of nitrogen plasma damage. The AlN buffer layer with lowest AlN coverage also provides the best result with respect to high-quality and vertically-aligned GaN nanocolumns. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.

Nitrogen (N) is a common element added to GaAs for band gap engineering and strain compensation. However, detection of small amounts of N is difficult for electron microscopy as well as for other chemical analysis techniques. In this work, N in GaAs is examined by using different transmission electron microscopy (TEM) techniques. While both dark-field TEM imaging using the composition sensitive (002) reflections and selected area diffraction reveal a significant difference between the doped thin-film and the GaAs substrate, spectroscopy techniques such as electron energy loss and energy dispersive X-ray spectroscopy are not able to detect N. To quantify the N content, quantitative convergent beam electron diffraction (QCBED) is used, which gives a direct evidence of N substitution and As vacancies. The measurements are enabled by the electron energy-filtered scanning CBED technique. These results demonstrate a sensitive method for composition analysis based on quantitative electron diffraction.

We demonstrate GaN nanocolumn growth on fused silica glass by plasma-assisted molecular beam epitaxy. The effect of the substrate temperature, Ga flux and N2 flow rate on the structural and optical properties are studied. At optimum growth conditions, GaN nanocolumns are vertically aligned and well separated with an average diameter, height and density of 72 nm, 1.2 μm and 1.6 × 109 cm−2, respectively. The nanocolumns exhibit wurtzite crystal structure with no threading dislocations, stacking faults or twinning and grow in the [0 0 0 1] direction. At the interface adjacent to the glass, there is a few atom layers thick intermediate phase with ABC stacking order (zinc blende). Photoluminescence measurements evidence intense and narrow excitonic emissions, along with the absence of any defect-related zinc blende and yellow luminescence emission.

Incorporation of beryllium (Be) and tellurium (Te) dopants in epitaxially grown Al0.9Ga0.1As0.06Sb0.94 layers was investigated. Carrier concentrations and mobilities of the doped layers were obtained from room temperature Hall effect measurements, and dopant densities from secondary ion mass spectrometry depth profiling. An undoped Al0.3Ga0.7As cap layer and side wall passivation were used to reduce oxidation and improve accuracy in Hall effect measurements. The measurements on Be-doped samples revealed high doping efficiency and the carrier concentration varied linearly with dopant density up to the highest Be dopant density of 2.9 × 1019 cm−3, whereas for Te doped samples the doping efficiency was in general low and the carrier concentration saturated for Te-dopant densities above 8.0 × 1018 cm−3. The low doping efficiency in Te-doped Al0.9Ga0.1As0.06Sb0.94 layer was studied by deep-level transient spectroscopy, revealing existence of deep trap levels and related DX-centers which explains the low doping efficiency. This is a submitted manuscript of an article published by Elsevier Ltd in Journal of Crystal Growth, 10 February 2017.

Epitaxially grown ternary III-arsenide-antimonide (III-As-Sb) nanowires (NWs) are increasingly attracting attention due to their feasibility as a platform for the integration of largely lattice-mismatched antimonide-based heterostructures while preserving the high crystal quality. This and the inherent bandgap tuning flexibility of III-As-Sb in the near- and mid-infrared wavelength regions are important and auspicious premises for a variety of optoelectronic applications. In this review, we summarize the current understanding of the nucleation, morphology-change and crystal phase evolution of GaAsSb and InAsSb NWs and their characterization, especially in relation to Sb incorporation during growth. By linking these findings to the optical properties in such ternary NWs and their heterostructures, a brief account of the ongoing development of III-As-Sb NW-based photodetectors and light emitters is also given.

Ternary semiconductor nanowire arrays enable scalable fabrication of nano-optoelectronic devices with tunable bandgap. However, the lack of insight into the effects of the incorporation of Vy element results in lack of control on the growth of ternary III-V(1-y)Vy nanowires and hinders the development of high-performance nanowire devices based on such ternaries. Here, we report on the origins of Sb-induced effects affecting the morphology and crystal structure of self-catalyzed GaAsSb nanowire arrays. The nanowire growth by molecular beam epitaxy is changed both kinetically and thermodynamically by the introduction of Sb. An anomalous decrease of the axial growth rate with increased Sb2 flux is found to be due to both the indirect kinetic influence via the Ga adatom diffusion induced catalyst geometry evolution and the direct composition modulation. From the fundamental growth analyses and the crystal phase evolution mechanism proposed in this Letter, the phase transition/stability in catalyst-assisted ternary III-V-V nanowire growth can be well explained. Wavelength tunability with good homogeneity of the optical emission from the self-catalyzed GaAsSb nanowire arrays with high crystal phase purity is demonstrated by only adjusting the Sb2 flux.

For the development of electronic nanoscale structures, feedback on its electronic properties is crucial, but challenging. Here, we present a comparison of various in situ methods for electronically probing single, p-doped GaAs nanowires inside a scanning electron microscope. The methods used include (i) directly probing individual as-grown nanowires with a sharp nano-manipulator, (ii) contacting dispersed nanowires with two metal contacts and (iii) contacting dispersed nanowires with four metal contacts. For the last two cases, we compare the results obtained using conventional ex situ litho-graphy contacting techniques and by in situ, direct-write electron beam induced deposition of a metal (Pt). The comparison shows that 2-probe measurements gives consistent results also with contacts made by electron beam induced deposition, but that for 4-probe, stray deposition can be a problem for shorter nanowires. This comparative study demonstrates that the preferred in situ method depends on the required throughput and reliability.

© 2018. This is the authors’ accepted and refereed manuscript to the article. Locked until 1.2.2019 due to copyright restrictions.

Device configurations that enable a unidirectional propagation of carriers in a semiconductor are fundamental components for electronic and optoelectronic applications. To realize such devices, however, it is generally required to have complex processes to make p-n or Schottky junctions. Here we report on a unidirectional propagation effect due to a self-induced compositional variation in GaAsSb nanowires (NWs). The individual GaAsSb NWs exhibit a highly reproducible rectifying behavior, where the rectifying direction is determined by the NW growth direction. Combining the results from confocal micro-Raman spectroscopy, electron microscopy, and electrical measurements, the origin of the rectifying behavior is found to be associated with a self-induced variation of the Sb and the carrier concentrations in the NW. To demonstrate the usefulness of these GaAsSb NWs for device applications, NW-based photodetectors and logic circuits have been made.

Geometric phase analysis has been applied to high resolution aberration corrected (scanning) transmission electron microscopy images of InAs/GaAs quantum dot (QD) materials. We show quantitatively how the lattice mismatch induced strain varies on the atomic scale and tetragonally distorts the lattice in a wide region that extends several nanometers into the GaAs spacer layer below and above the QDs. Finally, we show how V-shaped dislocations originating at the QD/GaAs interface efficiently remove most of the lattice mismatch induced tetragonal distortions in and around the QD., Comment: 11 pages, 3 figures

By applying a correlated micro-photoluminescence spectroscopy and transmission electron microscopy (TEM) approach, we have utilized molecular beam epitaxy grown self-catalysed GaAs nanowires (NWs) with an axial GaAsSb insert to determine the band offsets at the crystal phase heterojunction between zinc blende (ZB) and wurtzite (WZ) GaAs. Two distinct PL emission bands originating from the ZB GaAsSb insert were identified. The lower energy PL emission allowed an independent verification of the maximum Sb molar fraction to be ∼30%, in agreement with quantitative high-angle annular dark field scanning TEM performed on the same single NW. The higher energy PL emission revealed a low temperature ZB/WZ band offset of 120 meV at the interface between the two GaAs crystal phases occurring at the upper boundary of the insert. Separate conduction and valence band offsets develop at a higher temperature due to the different temperature dependence of the ZB and WZ GaAs band gaps, but both offset values show a relatively little variation in the range of 10–150 K. Published by AIP Publishing. Locked until 28.12.2018 due to copyright restrictions. This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. The following article appeared in Applied Physics Letters and may be found at http://aip.scitation.org/doi/10.1063/1.4960193http://aip.scitation.org/doi/10.1063/1.4991884

Semiconductor nanowire lasers can produce guided coherent light emission with miniaturized geometry, bringing about new possibilities for a variety of applications including nanophotonic circuits, optical sensing, and on-chip and chip-to-chip optical communications. Here, we report on the realization of single-mode and room-temperature lasing from 890 nm to 990 nm utilizing a novel design of single nanowires with GaAsSb-based multiple axial superlattices as gain medium under optical pumping. The control of lasing wavelength via compositional tuning with excellent room-temperature lasing performance is shown to result from the unique nanowire structure with efficient gain material, which delivers a low lasing threshold of ~ 6 kW/cm2 (75 μJ/cm2 per pulse), a lasing quality factor as high as 1250 and a high characteristic temperature of ~ 129 K. These results present a major advancement for the design and synthesis of nanowire laser structures, which can pave the way towards future nanoscale integrated optoelectronic systems with superior performance. © American Chemical Society 2018. This is the authors accepted and refereed manuscript to the article. Locked until 4.3.2019 due to copyright restrictions.

Atomic structure models of semiconductor surfaces consider usually ideal models based on density functional theory calculations. In reality, however, semiconductor surfaces exhibit a variety of defects which deviate from this ideal surface structure. Depending on their specific nature and amount, these defects can contribute significantly to the total energy of the surface. Furthermore, the electronic properties and consequently their specific reactivity towards adsorption processes can be modified significantly due to the existence of surface defects. Here, we present an analysis of different kinds of defects at the GaAs(001)- c (4 × 4) surface reconstruction. The surfaces were prepared by thermal decapping of GaAs(001) epilayers grown by molecular beam epitaxy and capped by an amorphous As cap. High resolution measurements with scanning tunneling microscopy were performed at room temperature and allowed the identification and atomic analysis of several different kinds of surface defects. Apart from other defects we found indications that approximately 3% of the surface dimers are incomplete, consisting only of one As atom instead of two.

Using temperature dependent X-ray diffraction on two InSb single crystalline substrates, the bulk lattice constant of InSb was determined between 32 and 325 °C. A polynomial function was fitted to the data: a ( T ) = 6.4791 + 3.28 × 10 − 5 × T + 1.02 × 10 − 8 × T 2 A ( T in °C), which gives slightly higher values than previously published (which go up to 62 °C). From the fit, the thermal expansion of InSb was calculated to be α ( T ) = 5.062 × 10 − 6 + 3.15 × 10 − 9 × T K − 1 ( T in °C). We found that the thermal expansion coefficient is higher than previously published values above 100 °C (more than 10% higher at 325 °C).

We report on the growth, structural and electrical characterizations of Be-doped GaAs nanowires (NWs) grown by the Au- and Ga-assisted vapour–liquid–solid techniques using molecular beam epitaxy. The growth rate of Be-doped GaAs NWs grown by the Au-assisted technique is observed to be lower as compared to the growth rate of undoped GaAs NWs grown under identical conditions. However, no effect on either the growth rate or the morphology of NWs has been observed for Be-doped GaAs NWs grown by the Ga-assisted technique with the same Be flux as used for the Au-assisted technique. Electrical characterization reveals that the Ga-assisted grown NWs show more consistent, symmetric current–voltage (I–V) characteristics with higher electrical current than the Au-assisted grown NWs. Finally, we show that ohmic contacts to Be-doped Ga-assisted NWs can be achieved either by post-annealing the metal-contacted NW or increasing the doping concentration during the NW growth.

Accomplishing control of the crystal phases in III–V semiconductor nanowires (NWs) is important for applications in future advanced nano-devices. In this work, we report on the growth of both zinc blende (ZB) and wurtzite (WZ) GaAs in self-catalyzed GaAs and GaAs/GaAsSb axial heterostructured NWs on Si(111) substrates by the vapor–liquid–solid technique using molecular beam epitaxy. The self-catalyzed GaAs NWs usually adopt the ZB phase. However, by growing GaAs NWs with short GaAsSb axial inserts, the crystal phase of GaAs can be changed from ZB below the GaAsSb insert, to WZ above. This crystal phase change in GaAs can be explained in terms of a change in the contact angle of the Ga droplet which changes the fraction of the triple-phase-line in contact with the edge of the NW top facet and therefore affects the probability of nucleation for ZB and WZ phases. In addition, by growing ZB GaAs after the insert using a flux interruption, we demonstrate the growth of all combinations of crystal phases of the GaAs segments on both sides of the GaAsSb insert. This understanding has also enabled us to achieve the growth of WZ phase in GaAs NWs as well as a ZB–WZ–ZB GaAs NW heterostructure, by changing the Ga droplet contact angle, without growing a GaAsSb insert. The contact angle was controlled by introducing different flux interruptions and tuning the V/III flux ratio.

Here we report on the heat-induced solid-state replacement of GaAs by Au in nanowires. Such replacement of semiconductor nanowires by metals is envisioned as a method to achieve well-defined junctions within nanowires. To better understand the mechanisms and dynamics that govern the replacement reaction, we performed in situ heating studies using high-resolution scanning transmission electron microscopy. The dynamic evolution of the phase boundary was investigated, as well as the crystal structure and orientation of the different phases at reaction temperatures. In general, the replacement proceeds one GaAs(111) bilayer at a time, and no fixed epitaxial relation could be found between the two phases. The relative orientation of the phases affects the replacement dynamics and can induce growth twins in the Au nanowire phase. In the case of a limited Au supply, the metal phase can also become liquid.

Aluminum-based contacts could be a good alternative to conventional gold-based contacts for a number of GaSb-based devices. In this study, the use of some Al-based contacts in GaSb-based diode lasers was investigated via the measurement of specific contact resistivity and laser output characteristics. The Al-based contacts to p-type GaSb(001) exhibited lower specific contact resistivities than the conventional Au-based contacts, whereas the opposite was the case for contacts to n-type GaSb(001). The good performance of GaSb-based laser diodes using Al-based contacts shows the applicability of this type of contact in GaSb-based devices. The contact between Al only and p-type GaSb(001), however, could suffer from a reliability problem when used in diode lasers, due to interdiffusion, in which case a diffusion barrier should be included. This is the authors' accepted and refereed manuscript to the article. This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. The following article appeared in Journal of Vacuum Science and Technology B and may be found at http://avs.scitation.org/doi/10.1116/1.4967300

This paper examines and models the effect of temperature on the mode-locking capability of monolithic two-section InAs/GaAs quantum dot passively mode-locked lasers. A set of equations based on an analytic net-gain modulation phasor approach is used to model the observed mode-locking stability of these devices over temperature. The equations used rely solely on static material parameters, measured on the actual device itself, namely, the modal gain and loss characteristics, and govern the limit describing the onset of mode-locking. Employment of the measured gain and loss characteristics of the gain material over temperature, wavelength and current injection in the model provides a physical insight as to why the mode-locking shuts down at elevated temperatures. Moreover, the model enables a temperature-dependent prediction of the range of cavity geometries (absorber to gain length ratios) where mode-locking can be maintained. Excellent agreement between the measured and the modeled mode-locking stability over a wide temperature range is achieved for an 8-stack InAs/GaAs quantum dot mode-locked laser. This is an attractive tool to guide the design of monolithic passively mode-locked lasers for applications requiring broad temperature operation.

Variations in quantum dot density across 16 different 2 in quarter wafers have been studied. The InAs/GaAs quantum dots (QDs) were grown on n-type GaAs(0 0 1) substrates by molecular beam epitaxy and studied by scanning electron microscopy (SEM). The SEM study reveals large variations in QD density and size homogeneity across the 16 quarter wafers. A representative sample was studied in great detail. For this sample the QD density is lower in the middle of the 1/4-wafer than along the rims, and also considerably higher in one of the corners compared to the other two corners. The QD diameters and size homogeneity are reduced in areas with high QD density compared to areas with low QD density. The above variations are believed to be mainly due to temperature variations across the 1/4-wafer during growth of the QDs.

We have investigated the modification of the surface electric field (SEF) of the GaAs (001)-c(4 × 4) surface during the adsorption of cyclopentene and 1,4-cyclohexadiene molecules by reflectance anisotropy spectroscopy (RAS) in the spectral range from 1.5 to 5.0 eV. At around 3eV the RAS line shape originates from the so-called linear electro-optic effect (LEO). The amplitude of the LEO oscillation scales linearly with the SEF within the RAS penetration depth and is thus a measure for it. For the separation of the LEO from the RAS spectra several different methods are described in the literature. Here, we present a modified method which allows the LEO separation in particular for interfaces between GaAs and organic molecules. The results obtained this way show a significant increase of the LEO effect upon molecule adsorption indicating a higher SEF. The observed changes of the surface electronic properties are probably related to a modification of surface strain upon molecule adsorption.

Synchrotron surface X-ray diffraction has been used to investigate in situ the morphology and epitaxy of monolayer amounts of copper electrodeposited from aqueous electrolyte onto ultra-high vacuum prepared, smooth, Ga- or As-terminated GaAs(0 0 1) surfaces. The fcc lattice of the epitaxial Cu islands is rotated by ∼5° and tilted by about ∼9° with respect to the GaAs substrate lattice, leading to eight symmetry equivalent domains of Cu islands terminated by {1 1 1} facets.

The understanding of self–assembly and bonding mechanisms of organic molecules on semiconductor surfaces represents a central research aspect in the investigation of novel organic/inorganic interfaces and their technological applicability. Here, we investigated the adsorption and bond formation of cyclopentene and 1,4-cyclohexadiene on a GaAs(001)- c(4 × 4) surface in order to clarify the influence of the number of intra-molecular C=C double bonds on the respective adsorption sites. For a determination of the adsorption configuration, the interfaces were characterized electronically and optically by synchrotron based X-ray photoelectron spectroscopy (SXPS), low energy electron diffraction (LEED) and reflectance anisotropy spectroscopy (RAS). The results reveal significantly different adsorption configurations for the two molecules. While cyclopentene bonds with a single covalent bond to the surface, 1,4-cyclohexadiene adsorbs onto the surface by the formation of multiple covalent bonds, e.g. bridge bonds. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

GaAs nanowire (NW) heterostructures with four GaAsSb inserts were grown on GaAs(1 1 1)B substrates by Au-assisted molecular beam epitaxy (MBE). Structural characterization of the NWs showed that the transition between the GaAs and GaAsSb heterojunction is atomically abrupt and that the GaAs crystallographic phase is mainly wurtzite with some stacking faults, whereas the GaAsSb phase is zinc blende and free from twinning defects. Interestingly, the growth rates of the GaAs segments and GaAsSb inserts were observed to vary with growth time. We attribute this behavior to the effect of the inclined molecular beams with respect to the substrate surface in the MBE growth chamber. Further, the mole fraction of Sb in the GaAsSb insert was determined by energy-dispersive X-ray spectrometry and was found to increase with the inserts position along the NW.

The adsorption of copper deposited from an effusion cell in ultra high vacuum and electrodeposited from aqueous solution on As-terminated GaAs(001) is investigated by scanning tunneling microscopy, X-ray diffraction, as well as X-ray standing waves in combination with X-ray fluorescence spectroscopy. The analysis of the adsorbate is performed in situ with the samples either in ultra high vacuum or in aqueous electrolyte under potential control. Deposited as Cu ions from sulphuric acid solution, submonolayer coverages of Cu diffuse into the GaAs interface region assumimg predominantly Ga substitutional positions. Different positions with lower symmetry are occupied by Cu if submonolayer are deposited at room temperature in ultra high vacuum. At higher coverage, epitaxial Cu clusters grow by electrodeposition on the As-terminated GaAs(001), but with the Cu lattice of the islands rotated and inclined with respect to the GaAs substrate lattice, leading in total to eight equivalent domains. At comparable coverage, no epitaxial growth was observed for Cu deposited in ultra high vacuum.

We report how the presence of electrophilic surface sites influences the adsorption mechanism of pyrrole on GaAs(001) surfaces. For this purpose, we have investigated the adsorption behavior of pyrrole on different GaAs(001) reconstructions with different stoichiometries and thus different surface chemistries. The interfaces were characterized by x-ray photoelectron spectroscopy, scanning tunneling microscopy, and by reflectance anisotropy spectroscopy in a spectral range between 1.5 and 5 eV. On the As-rich c(4 × 4) reconstruction that exhibits only nucleophilic surface sites, pyrrole was found to physisorb on the surface without any significant modification of the structural and electronic properties of the surface. On the Ga-rich GaAs(001)-(4 × 2)/(6 × 6) reconstructions which exhibit nucleophilic as well as electrophilic surface sites, pyrrole was found to form stable covalent bonds mainly to the electrophilic (charge deficient) Ga atoms of the surface. These results clearly demonstrate that the existence of electrophilic surface sites is a crucial precondition for the chemisorption of pyrrole on GaAs(001) surfaces.

We have previously shown that rapid thermal annealing (RTA) of Te-doped GaSb can increase the n-type carrier concentration (Bugge et al 2004 13th Int. Conf. on Mol. Beam Epitaxy (Edinburgh)). In order to see whether this process can be used on a complete laser structure, we have studied the effect of RTA on the active layers and cladding layers of a GaSb-based laser, as well as on the defect structure in the material. Multiple-quantum-well (MQW) structures of InGaAsSb/AlGaAsSb and InGaAsSb/AlInGaAsSb were grown by molecular beam epitaxy (MBE) on GaSb substrates. The MQW structures were examined with photoluminescence (PL) and x-ray diffraction prior to and after RTA at 625°C and at 650°C for 10 s to look for differences in the (QW) structure. Highly Te-doped layers of GaSb grown by MBE were examined with PL to yield some information on the change in defect structure during RTA. The results show that atomic interdiffusion across the QW heterointerfaces plays a major role during RTA and that it can reduce interface strain for InGaAsSb/AlInGaAsSb MQWs. The diffusion length of group-III atoms was found to be about 2 nm after annealing at 650°C for 10 s.

Molecular beam epitaxy has been used to grow GaInAsSb/AlGaAsSb multiple-quantum-well (MQW) structures. Growth has been interrupted at the interfaces between the wells and the barriers. During the growth interruptions, the interfaces have been exposed to Sbx(x=1, 2) and As2 fluxes. The structures have been studied using high-resolution x-ray diffraction (HRXRD) and photoluminescence (PL). The As content in the interface layers has been found to have a large impact on the HRXRD curves. The As content in the interface layers has been determined by simulation of HRXRD rocking curves. We also show how highly strained interfaces cause more satellite peaks to appear in HRXRD rocking curves. PL spectra show that interrupting growth at the interfaces between wells and barriers and exposing the interfaces to an Sb soak result in flatter interfaces.

The surface or bulk origin of the optical anisotropies detected by reflectance anisotropy spectroscopy (RAS) at GaAs(001)(2 x 4) surfaces has been extensively investigated in the last years and a quite general agreement has been reached that the dominating character would be bulk-like. Nevertheless, a very recent paper [F. Arciprete et al., Phys. Rev. B 69, 081308(R) (2004)] has again issued the presence of surface states contributions in optical anisotropies, revealing a structure at 2.5 eV due to surface states, in addition to the well known features around 2.9 eV and 4.5 eV related to the bulk critical points E 1 and E' 0 . We have carried out a new experiment to prove this conclusion by following the changes in the optical anisotropy of a GaAs(001)(2 x 4) surface in the range 2.0-5.0 eV induced by Ag/Sb-codeposition. The interface Ag/GaAs(001) is known to be not reactive. Due to its surfactant effect, codeposition of Sb leads to a nearly epitaxial growth of the Ag overlayer. We show that at the early stages of deposition (nominally at 0.25 monolayer) an evident modification of the RAS spectrum is detected at 2.5 eV, well below the photon energy (2.9 eV) where bulk-like anisotropies appear. We relate this modification to the disappearance of surface states characteristic of the (2 x 4) reconstruction, in excellent agreement with previous conclusions and experiments.

Electrochemical deposition of Cu and Co in monolayer amounts on hydrogen terminated Si(1 1 1) was studied ex situ and in situ by X-ray techniques. The X-ray beam was found to have a strong effect on the deposit causing desorption under the beam. Cu deposition on GaAs(0 0 1) from UHV is compared with electrodeposited Cu on the same surface, elucidating similarities and differences of electrochemical and UHV deposition. Roughening due to corrosion of Cu3Au(1 1 1) is observed by crystal truncation scattering. The observed behaviour of passivation of this surface is explained by the formation of Au clusters, increasingly covering the surface at higher oxidation potential.

To efficiently evaluate the novel approach of focused ion beam (FIB) direct patterning of substrates for nanowire growth, a reference matrix of hole arrays has been used to study the effect of ion fluence and hole diameter on nanowire growth. Self-catalyzed GaAsSb nanowires were grown using molecular beam epitaxy and studied by scanning electron microscopy (SEM). To ensure an objective analysis, SEM images were analyzed with computer vision to automatically identify nanowires and characterize each array. It is shown that FIB milling parameters can be used to control the nanowire growth. Lower ion fluence and smaller diameter holes result in a higher yield (up to 83%) of single vertical nanowires, while higher fluence and hole diameter exhibit a regime of multiple nanowires. The catalyst size distribution and placement uniformity of vertical nanowires is best for low-value parameter combinations, indicating how to improve the FIB parameters for positioned-controlled nanowire growth.

Mid-infrared laser diodes have been fabricated and tested, and semiconductor materials related to mid-infrared lasers have been characterized by X-ray diffraction (XRD).The temperature dependent lattice constant of Al0.9 Ga0.1AsySb1−y, GaSb, AlSb and InSb have been examined using XRD measurements. For Al0.9Ga0.1AsySb1−y, GaSb and AlSb, the lattice constants were measured for temperatures up to 546°C, while for InSb it was examined up to 325°C. For AlSb, also the temperature dependent Poisson ratio was determined. It was found that the thermal expansion of Al-containing layers above room temperature was higher than previously reported. An expression for the lattice matching condition for Al0.9Ga0.1AsySb1−y epilayers on GaSb substrates as a function of temperature was presented. For GaSb, it was found that the work of Bublik et al. [1] provided accurate data for the temperature dependent lattice constant, and either our data or Bublik et al. [1]’s data should be used. The measurement technique was validated by measuring the lattice constants of Si and GaAs, where our measured values were found to be in agreement with previously published values. For AlSb it was found that the thermal expansion was larger than previously reported in the literature. For InSb it was found that the lattice constant near room temperature was larger than previously reported, and the thermal expansion above 100°C was larger than previously reported.Laser material was grown using molecular beam epitaxy (MBE). The grown samples were processed into Y-junction laser diodes. The lasers were etched using inductively coupled plasma reactive ion etching (ICP-RIE) and photoresist (PR) ma-N 440 was spun on and baked for use as electrical insulation. The insulation layer was etched using reactive ion etching (RIE) to uncover the top of the etched lasers for contacting. It was found that a O2/CF4 etch gave the best uniformity of the insulation layer. The lasers were contacted and tested.The Y-junction lasers were characterized using power measurements for optical power, multimeters for diode voltage, Fourier transform infrared (FTIR) for spectral measurements, and an infrared camera for near and far field measurements. The measurements suggested that the curved waveguide did not guide the light, most likely due to a low refractive index contrast. This was later supported by scanning electron microscope (SEM) measurements, which showed an etch depth of 1.4 μm, much lower than the etch target of 1.9 μm.The Y-junction waveguides were simulated using the beam propagation method (BPM). Based on 2D BPM simulations, it was found that an effective refractive index contrast of at least 0.03 is required for guiding light in a curved waveguide for our dimensions, and that waveguide roughness due to processing is less important. The simulations support the findings from the laser measurements, and further suggest that a deeper etch is required for functioning Y-junction laser diodes. Suggestions for improvements to the manufacturing mid-infrared laser diodes are presented.

A passively mode-locked laser with a 2.1-GHz fundamental repetition rate is demonstrated using a 20-mm-long monolithic two-section quantum-dot laser. A pulsewidth as short as 15.4 ps was measured using a high-speed sampling scope. The product of the peak power and average power in this laser is treated theoretically and shown experimentally to nearly double by roughly quadrupling the cavity length.

GaSb-based semiconductor diode lasers are promising candidates for light sources working in the mid-infrared wavelength region of 2-5 μm. Using edge emitting lasers with ridge-waveguide structure, light emission with good beam quality can be achieved. Fabrication of the ridge waveguide requires precise etch stop control for optimal laser performance. Simulation results are presented that show the effect of increased confinement in the waveguide when the etch depth is well-defined. In situ reflectance monitoring with a 675 nm-wavelength laser was used to determine the etch stop with high accuracy. Based on the simulations of laser reflectance from a proposed sample, the etching process can be controlled to provide an endpoint depth precision within ± 10 nm.

We report on the epitaxial growth of large-area position-controlled self-catalyzed GaAs nanowires (NWs) directly on Si by molecular beam epitaxy (MBE). Nanohole patterns are defined in a SiO2 mask on 2 in. Si wafers using nanoimprint lithography (NIL) for the growth of positioned GaAs NWs. To optimize the yield of vertical NWs the MBE growth parameter space is tuned, including Ga predeposition time, Ga and As fluxes, growth temperature, and annealing treatment prior to NW growth. In addition, a non-negligible radial growth is observed with increasing growth time and is found to be independent of the As species (i.e., As2 or As4) and the growth temperatures studied. Cross-sectional transmission electron microscopy analysis of the GaAs NW/Si substrate heterointerface reveals an epitaxial growth where NW base fills the oxide hole opening and eventually extends over the oxide mask. These findings have important implications for NW-based device designs with axial and radial p-n junctions. Finally, NIL positioned GaAs/AlGaAs core-shell heterostructured NWs are grown on Si to study the optical properties of the NWs. Room-temperature photoluminescence spectroscopy of ensembles of as-grown core-shell NWs reveals uniform and high optical quality, as required for the subsequent device applications. The combination of NIL and MBE thereby demonstrates the successful heterogeneous integration of highly uniform GaAs NWs on Si, important for fabricating high throughput, large-area position-controlled NW arrays for various optoelectronic device applications.

We present a systematic study mapping how the As incorporation into GaAs y Sb 1− y and Al x Ga 1− x As y Sb 1− y ( y

We report on the influence of hole pattern pitch lengths in the silicon oxide mask and specific nanowire (NW) locations on the morphology and luminescence properties of self-catalyzed GaAsSb NW arrays grown by molecular beam epitaxy. Due to stronger competition for the limited amount of Ga adatoms, the GaAsSb NWs in the center of arrays with short pitch lengths possess a smaller catalyst droplet contact angle than that of the NWs at the array edge. This smaller contact angle leads to a reduction in the collection of group V flux, bringing about shorter NWs in the center. For pitch lengths beyond the diffusion length of Ga adatoms on the mask, the GaAsSb NWs are taller with larger contact angles than in the case with short pitch lengths. Considering that Sb has a longer diffusion length on the side facets of the NWs than that of As, a reduction/increase of the contact angle will bring about an increase/reduction in the Sb/As ratio of the group-V fluxes collected by the catalyst droplets. By performing micr...

We have studied the adsorption of [( t Bu)GaS] 4 in ultrahigh vacuum on the single-domain Ga-rich GaAs(0 0 1)-(4×2) surface using scanning tunneling microscopy (STM), Auger electron spectroscopy, and low-energy electron diffraction (LEED). This study has focused on deposition at 650 K where adsorption is carbon-free. For submonolayer coverages, STM images show the presence of protrusions on the surface. The height distribution of these protrusions (which is insensitive to bias voltage) is peaked between 0.7 and 1.4 A. Since the expected height of an intact Ga 4 S 4 cubic core of the [( t Bu)GaS] 4 precursor residing on the surface is expected to be between 3 and 4 A, our data suggest that the Ga 4 S 4 cubic core dissociates upon adsorption on the (4×2) surface. Adsorption of a GaS overlayer by exposure of [( t Bu)GaS] 4 at 650 K, results in a reasonably ordered surface exhibiting a (2×1) reconstruction, as judged by LEED and STM. Unlike adsorption on the As-rich GaAs(0 0 1)-(2×4) surface, post-deposition annealing to temperatures in excess of 650 K is not necessary for the fabrication of an ordered GaS overlayer. Possible structural models for the (2×1) surface are discussed along with the practical implications of these results.

Symmetrically modulation doped GaAs/AlxGa1-xAs quantum-well structures, containing a monolayer thickness of single AlAs insertion in the well region, are studied by photoluminescence spectroscopy a ...

We discuss the possibility of using far infrared ellipsometry and synchrotron radiation as a tool in surface and thin-film studies. A portable module that is suitable for measuring the polarization state of reflected radiation between 20 cm −1 and 800 cm −1 is described. A method to deal with the intensity decay after each filling of the storage ring is presented, and polarization and collimation effects in the incoming beam are discussed. The permittivity of GaAs was measured and found to be in excellent agreement with tabulated values. A study was made of vibrational modes in 70 period (GaAs) n /(Al x Ga 1− x As) n superlattice films with n equal to 4, 8, 16, and x =0.3, 1.0. The confined AlAs-like TO phonon modes in the Al 0.3 Ga 0.7 As layers were found to be weakly red-shifted when n increases from 4 to 16. In the (GaAs) n /(AlAs) n films no shifts could be observed.

The influence of antimony as surfactant on the growth mode of indium on GaAs(001) has been investigated for deposition at various temperatures. Clean surfaces of (2 × 4) reconstruction were prepared in UHV by thermal desorption of a protective arsenic cap deposited on top of homoepitaxially grown MBE layers. Before In deposition, 2 ML Sb were pre-deposited on top of the clean GaAs surface of part of the samples. The In growth mode was then monitored by AES and compared with that occurring during In growth on GaAs without an Sb interlayer. After removal of the samples from UHV, SEM investigations show regularly shaped, flat In islands oriented along the 〈110〉 substrate directions. In comparison, In islands grown without an Sb interlayer also show a preferential orientation along the 〈110〉 substrate directions, but differ in shape and size. Low-temperature In deposition (120 K) leads to the formation of a continuous In layer, which tears during warm up to room temperature. An exchange between surfactant material and indium during growth is found for both low- and room-temperature In deposition. X-ray diffraction measurements show a strong influence of the Sb surfactant on the crystallinity of the In film.

The growth of thin indium-layers on the GaAs(100) As-rich (2 × 4) c (2 × 8) surface has been investigated by reflectance anisotropy spectroscopy (RAS), LEED and AES. Clean surfaces of the (2 × 4) c (2 × 8) reconstruction were prepared in UHV by thermal desorption of a protective arsenic layer deposited on homoepitaxially grown MBE layers. Room temperature deposition of indium on the (2 × 4) c (2 × 8) surface and subsequent annealing at 450°C leads to a 90° rotation of symmetry in the LEED pattern at a threshold coverage of 0.5 monolayers, i.e. a change from the (2 × 4) c (2 × 8) to the (4 × 2) c (8 × 2) reconstruction. The RAS spectra show the evolution of a distinct negative feature at 1.8 eV, that shifts to 2.1 eV after annealing, corresponding to optical transitions attributed to In-dimers orientated along the [011]-direction. AES analysis shows a change in growth mode beyond 0.5 ML indicating saturation of all available adsorption sites at this coverage and RAS spectra show a contribution from additional disordered In. The AES spectra display no evidence of a surface exchange reaction between gallium and arsenic atoms, thus indicating a surface termination by In-dimers adsorbed on a layer of As.