Patterned GaAs Substrates Research Articles

Extending the propagation length of graphene plasmons via nonlinear frequency conversion

Graphene plasmons (GPs) are broadband and electrically tunable mid-infrared (MIR)/terahertz (THz) excitations, exhibiting high confinement factors exceeding two orders of magnitude. Such highly confined modes are extremely attractive for nonlinear frequency conversion owing to the large inherent field enhancement. However, this high confinement is also accompanied by losses, and together with the centrosymmetric nature of graphene practical usage of its properties in second-order nonlinear processes remains hindered. In this paper, we introduce an approach for realizing quasi-phase-matching (QPM) of propagating GPs, by placing the graphene on an orientationally patterned GaAs substrate—a transparent material in the MIR/THz range with a large second-order nonlinear coefficient. We analyze the complete frequency/Fermi-level space for QPMed second-harmonic generation of GPs in the MIR and THz and demonstrate GP amplification and loss compensation. We find that our approach provides extended GP propagation lengths that are more than twice larger than the state-of-the-art cryogenic temperature propagation lengths. The approach is general to all second-order nonlinear processes, such as sum and difference frequency generation, thus opening a path for efficient and electrically tunable QPM nonlinear processes at the atomic scale.

Kinetic calculation analysis of Ga deposition on the morphology evolution of GaAs quantum ring

GaAs Quantum Ring (QR) was gained on GaAs (001) by Droplet Epitaxy (DE), and the microscopic morphology of the GaAs samples was observed by Scanning Tunnel Microscope (STM). The kinetic model of Local Droplet Etching (LDE) was mainly used to study the influence of Ga deposition on the morphology evolution from Ga droplets to GaAs QR. Comparing experimental data with the theoretical value, it can be seen that the increase of Ga deposition will cause the reduction of surface density of Ga droplets, but the volume, height and diameter of Ga droplets will increase. Geometric dimension of GaAs QR increased also with the increase of Ga deposition. In addition, it found that the rate at which the substrate was etched was affected by Ga deposition and As pressure from experiment. The more Ga deposition, the deeper GaAs nano hole. However, GaAs nano hole became shallower under high As pressure. These results are consistent with theoretical calculation analysis. Under high substrate temperature, GaAs double rings finally evolved into a single ring. Above all results have certain guiding significance for the preparation of patterned GaAs substrates and the controlled growth of GaAs QR morphology.

Transferrable dielectric DBR membranes for versatile GaN-based polariton and VCSEL technology

We present an innovative process to realize planar GaN microcavities, based on transferrable high-quality oxide-based Distributed-Bragg-Reflector (t-DBR) membranes. Unlike the widely used direct DBR deposition, which is not always feasible, the concept of t-DBR membranes allows their versatile use as top mirrors in all kinds of vertical-cavity surface emitting geometry devices, including light emitting diodes and lasers. The process illustrated here starts by evaporating first a 4-pair SiO2/Ta2O5 DBR centered around 400 nm on a lithographically patterned GaAs substrate. The GaAs substrate is subsequently removed by wet-etching, and the released DBR membranes are transferred onto other templates for their use as top mirrors in microcavities. By transferring a 4-pair top DBR on an 8-pair bottom SiO2/Ta2O5 DBR, a distinct cavity mode appears due to the formed λ/2 oxide cavity with a Q-factor of 110. If the 4-pair top DBR is transferred on 200 nm-thick GaN/AlGaN quantum well-containing membranes sitting on 10-pair bottom DBRs, complete all-dielectric nitride polariton structures are fabricated with pronounced strong-coupling characteristics.

Improvement in Uniformity of Type-II p-GaAsSb/n-InAs Nanowires on GaAs Substrates by the Insertion of GaAs Segments in Position-controlled VLS Method

AbstractTo improve the uniformity of type-II GaAsSb/InAs nanowires (NWs) grown on GaAs substrate using the position-controlled vapor-liquid-solid (VLS) method, the insertion of GaAs NW segments was investigated. In conventional InAs NWs grown directly on patterned GaAs substrate at 430°C, anomalous growth occurred in some pattern holes, and the proportion of holes having anomalous growth was in particular pronounced when a large Au catalyst diameter (80 nm) was employed, which indicates that the strain coming from the lattice-mismatched system affected the NW growth. With the insertion of GaAs segments, the yield of vertical NWs drastically improved for all pattern conditions. The uniformity of NWs was maintained after the growth of p-GaAsSb segments, and well-organized, two-dimensional arrays of type-II GaAsSb/InAs diode NWs were obtained. Moreover, the formation of p-n junctions at the GaAsSb/InAs interface was clearly observed using scanning capacitive microscopy. These results show that the investigated growth strategy is promising for the development of high-performance NW tunnel diodes.

Invited: Approaches to the Formation and Integration of Large Lattice Mismatched Materials: Metamorphic and Non-Conventional 'buffer' Layers

The growth of large lattice mismatched materials has been studied for many years in order to understand mismatch-derived defects, their mode of introduction, and their mitigation. Traditional approaches focused on the formation of ‘buffer’ layers. ‘Buffer’ layers are intermediate layers within a growing structure which allow a chemical or structural transition between two epitaxial materials. The choice and design of these layers depend critically on the nature of the material transition. Controllably varying the composition across the buffer layer has been the approach most often used in this context. As the composition is graded from the substrate or a lattice matched compound to the substrate to a material which has the desired lattice parameter, the structurally required mismatch dislocations are introduced with a minimum of residual threading dislocations. The approach of transitional or graded layers has undergone continual development with new effects being realized which can reduce the buffer layer thickness and achieve the dislocation density desired for growth of device layers. Composition grading in compound semiconductors can be accomplished by the change in the cation composition, as in the grading of InxGa1-xAs to transition from GaAs to InAs, or on the anion sublattice using InAsyP1-y to transition from InP to InAs. Often these graded layers are too rough to use in device applications due to the presence of a surface cross-hatch derived from the introduction of mismatch dislocations. Chemical-mechanical polishing has offered a means to recover the required planar surface. There are alternative approaches to graded buffer layers which are beginning to be studied and utilized. The intent of all these approaches is to generate the required misfit dislocations, have them reside at an appropriate interface and, finally, leave no threading dislocation line segments residing within the device layer. A self-assembled block co-polymer approach to nanoscale patterning, which offers rapid and cost-effective full wafer patterning at the 20-nm length scale, was used to achieve improvements in heteroepitaxial growth of large lattice mismatched materials. The x-ray spectra taken from a 600-nm or less thickness GaSb film grown on a patterned GaAs substrate show a sharp reduction in the full width at half maximum (FWHM). The FWHM of the GaSb peak in this initial study was reduced by at least a factor of two as compared to the film grown to the same thickness on a non-patterned wafer. A reduction of this magnitude is extremely significant for films of this thickness. This improvement in materials properties through nano-patterned growth should be applicable to a wide range of materials and lattice mismatch situations. Views on the strengths, challenges, similarities and applicability of these various approaches will be presented.

In-assisted deoxidation of GaAs substrates for the growth of single InAs/GaAs quantum dot emitters

We report a systematic study of the In-assisted deoxidation (IAD) of epitaxial GaAs(100) substrates. Optimized IAD conditions resulting in a pit-free and smooth GaAs surface are found. Photoluminescence lines from single quantum dots (QDs) with linewidths in the range of 250–400 μeV are observed from low-density InAs QDs grown at a distance of 10 nm from a GaAs surface deoxidized under an optimized condition. Our study shows that IAD is very promising for application in the growth of nanostructures on patterned GaAs substrates.

Effects of confinements on morphology of InxGa1−xAs thin film grown on sub-micron patterned GaAs substrate: Elastoplastic phase field model

An elastoplastic phase field model is developed to investigate the role of lateral confinement on morphology of thin films grown heteroepitaxially on patterned substrates. Parameters of the model are chosen to represent InxGa1−xAs thin films growing on GaAs patterned with SiO2. We determined the effect of misfit strain on morphology of thin films grown in 0.5 μm patterns with non-uniform deposition flux. Growth of islands inside patterns can be controlled by non-uniformity of deposition flux, misfit strain between film and the substrate, and also strain energy relaxation due to plastic deformation. Our results show that the evolution of island morphology depends non-monotonically on indium content and associated misfit strain due to coupling between the plastic relaxation and the confinements effects. Low indium concentration (0%–40%) causes formation of instabilities with relatively long wavelengths across the width of the pattern. Low surface diffusion (due to low indium concentration) and fewer islands across the pattern (due to small misfit strain) lead to formation and growth of islands near the walls driven by overflow flux. Further increase in indium concentration (40%–75%) increases the lattice mismatch and surface diffusivity of the film, and also activates plastic deformation mechanism, which leads to coalescence of islands usually away from the edges. By further increasing the indium concentration (up to 100%), plastic deformation relaxes most of the strain energy density of the film, which prevents formation of instabilities in the film. Hence, in this case, islands are only formed near the walls.

Study of site controlled quantum dot formation on focused ion beam patterned GaAs substrate

In this project, we combine Ga+ focused ion beam (FIB) patterning and self-assembly of InGaAs quantum dots in order to produce regular quantum dot (QD) arrays. The GaAs substrate is patterned before transporting into molecular beam epitaxy growth chamber to do anneal and overgrowth. The thickness of deposited InAs is precisely controlled. Quantum dots are expected to nucleate at specific locations, where the ion beam has previously implanted gallium atoms. The properties of the quantum dots formed are related to the FIB beam parameters, which include accelerating voltage, probe current, dwell time and pitch. We studied and statistically analyzed the relationship, focusing on their diameter, height and distribution. Scanning electronic microscope (SEM) and atomic force microscope (AFM) images have been used to analyze the patterns and QDs before and after overgrowth. Micro-PL study was performed to test the QD opto-electronic property. Scanning transmission electron microscope (STEM) cross sectional analysis and X-ray mapping is performed.

Site-controlled Ag nanocrystals grown by molecular beam epitaxy—Towards plasmonic integration technology

We demonstrate site-controlled growth of epitaxial Ag nanocrystals on patterned GaAs substrates by molecular beam epitaxy with high degree of long-range uniformity. The alignment is based on lithographically defined holes in which position controlled InAs quantum dots are grown. The Ag nanocrystals self-align preferentially on top of the InAs quantum dots. No such ordering is observed in the absence of InAs quantum dots, proving that the ordering is strain-driven. The presented technique facilitates the placement of active plasmonic nanostructures at arbitrarily defined positions enabling their integration into complex devices and plasmonic circuits.

Fabrication of ridge‐shaped AlGaInP/GaInP quantum well structure for observation of evanescent wave coupling effect

AbstractA ridge‐shaped AlGaInP/GaInP quantum well (QW) structure with a ridge top width in the sub‐wavelength region (500–600 nm) is successfully fabricated by selective area metal organic vapor phase epitaxial growth on patterned GaAs substrates, as the first step towards the realization of high‐efficiency light‐emitting diodes (LEDs) utilizing the evanescent wave coupling effect. It is confirmed by an X‐ray diffraction study that AlGaInP layers grown on all the different facets of the ridge structure satisfy the lattice matching requirements for high‐efficiency LED applications. A considerable enhancement of the light‐extraction efficiency in the fabricated ridge‐shaped QW structure is demonstrated by means of a photoluminescence analysis. The realization of the evanescent wave coupling effect is suggested by theoretical simulation. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Carrier tunneling effects on the temperature dependent photoluminescence of InAs/GaAs quantum dot: Simulation and experiment

Considering the direct quantum tunneling of carrier, we propose a new carrier rate equation model to simulate the temperature dependent photoluminescence (TDPL) of InAs/GaAs quantum dots (QDs). The TDPL showed abnormal variations: the peak rapid redshift, linewidth shrinkage, and thermal activation energy all decreased with increasing tunneling strength. A criterion, which could be used to evaluate the tunneling strength, has been developed. That is, smaller tunneling strength coefficient α indicates higher carrier tunneling strength. Meanwhile, the criterion is also demonstrated via comparative experimental results of InAs QDs grown on different patterned GaAs substrates. It is found that, to some extent, the tunneling strength would be enhanced by decreasing the dot-dot distance for closely arranged QDs ensembles.

Ordered InAs nanodots formed on the patterned GaAs substrate by molecular beam epitaxy

Ordered indium arsenide (InAs) nanodots are formed by molecular beam epitaxy (MBE) on patterned gallium arsenide (GaAs) substrates, which are prepared by implanting manganese (Mn) ions through anodic aluminum oxide (AAO) membranes into the GaAs wafers. Morphology and structure of the patterned GaAs substrate is determined both by the oxygen desorption and the Mn ion diffusion. Suitable patterned GaAs substrates with the same dosage of Mn ions for the following epitaxy can be obtained by controlling the deoxidization As 4 pressures during the oxygen desorption. Images of samples with different Mn ion implantation dosages and different molecular beam epitaxial conditions for the following deposition of InAs nanodots on the patterned GaAs substrates are characterized by atomic force microscopy (AFM). The order of the InAs nanodots is determined both by the AAO membrane and dosage of Mn ions. The density of InAs nanodots has great relation to the pore density of the AAO.

Hybrid conjugated polymer solar cells using patterned GaAs nanopillars

In this work, we study hybrid solar cells based on poly(3-hexylthiophene)-coated GaAs nanopillars grown on a patterned GaAs substrate using selective-area metal organic chemical vapor deposition. The hybrid solar cells show extremely low leakage currents (I≅45 nA @−1V) under dark conditions and an open circuit voltage, short circuit current density, and fill factor of 0.2 V, 8.7 mA/cm2, and 32%, respectively, giving a power conversion efficiency of η=0.6% under AM 1.5 G illumination. Surface passivation of the GaAs results in further improvement, yielding η=1.44% under AM 1.5 G illumination. External quantum efficiency measurements of these polymer/inorganic solar cells are also presented.

High optical quality InAs site-controlled quantum dots grown on soft photocurable nanoimprint lithography patterned GaAs substrates

InAs site-controlled quantum dots (SCQDs) have been demonstrated on GaAs substrates patterned by soft photocurable nanoimprint lithography. The substrates are patterned with a dense array of 100 nm nanopores and then InAs is deposited by molecular beam epitaxy to form uniform single QD in each patterned nanopore. Strong room temperature photoluminescence (PL) obtained from uncapped sample with a single SCQD layer reveals a defect-free interface and high quality growth of QDs. A narrower PL linewidth indicates more uniformly distributed SCQDs on the patterned substrate than self-assembled QDs on an unpatterned substrate.

Effect of an applied electric current in epitaxial growth of GaAs layer on patterned GaAs substrate

Selective epitaxial growth of a GaAs layer on SiNx masked Si-doped semi-insulating (100) GaAs substrate was performed by current-controlled liquid-phase epitaxy (CCLPE) in the conventional liquid-phase epitaxy. Experiments were carried out with and without the application of electric current. Surface morphology of (100) facet of the grown layer and the vertical and lateral growth rates were significantly improved under applied electric current. A thick layer of about 330μm was achieved at relatively low growth time of 6h with a current density of 20Acm−2. The epitaxial growth is realized by both electromigration of the solute and supercooling under a constant rate of furnace cooling. The dislocation density of the grown layer was significantly reduced, compared with that of the substrate (4×104cm2).

MBE growth of one-dimensionally ordered AlInAs quantum dots on a patterned GaAs substrate

The growth of AlxIn1−xAs self-assembled quantum dots (QDs) is studied on a stripe mesa of GaAs by molecular beam epitaxy. The stripe is along the [1 −1 0] direction on a (0 0 1) GaAs substrate and formed with {1 1 1}A and {1 1 4}A facets on the side, and {1 1 5}A facets on the top. The {1 1 5}A facets give a shallow V-groove on the top of the mesa. The dots are grown selectively at the bottom of the V-groove via an AlGaAs interlayer to form one-dimensionally aligned AlInAs QDs. We find that the diameter of an individual dot is elongated along the stripe direction, and the size as well as the density is varied by the sharpness of the V-groove structure. The manner of the QD formation is investigated as a function of the growth temperature and the Al composition x.

Improvement of InAs quantum dots optical properties in close proximity to GaAs(0 0 1) substrate surface

In this work we demonstrate a growth process for obtaining high optical emission efficiency InAs/GaAs(0 0 1) quantum dots (QD) formed at short distance to the interface with the GaAs substrate. In particular, after an initial exposure of the substrate surface to long times of atomic hydrogen flux ( t H up to 45 min) followed by a posterior growth of a GaAs buffer layer by atomic layer molecular beam epitaxy, both steps at low substrate temperature ( T S=450 °C), an enhancement of InAs QD optical emission efficiency is obtained, even at close proximity (3.5 nm) to the substrate interface. This process fulfils the strict requirements in terms of substrate temperature and buffer layer thickness (distance from the QD to the substrate interface) for its possible use as an optimal regrowth protocol on previously patterned GaAs substrates.

Temporal isolation of surface-acoustic-wave-driven luminescence from a lateral p–n junction using pulsed techniques

The authors report surface-acoustic-wave-driven luminescence from a lateral p–n junction formed by molecular-beam epitaxy regrowth of a modulation doped GaAs/AlGaAs quantum well on a patterned GaAs substrate. Pulsed techniques are used to isolate the surface-acoustic-wave-driven emission from any emission due to pick-up of the free-space electromagnetic wave. The luminescence provides a fast probe of the signals arriving at the p–n junction allowing the response of the junction to the surface-acoustic-wave to be studied in the time domain. Oscillations in the surface-acoustic-wave-driven component of the light intensity are resolved at the resonant frequency of the transducer, suggesting that the surface-acoustic-wave is transporting electrons across the junction in packets.

Site control of very low density InAs QDs on patterned GaAs nano-wire surfaces

Self-organized InAs quantum dots (QDs) were grown on patterned GaAs substrates. A 2.5-ML-thick InAs grown on self-organized GaAs nano-wire surfaces showed QD formation with a very low density of 1.5×10 8 cm −2. The self-organized GaAs nano-wires were grown under low V/III ratio, and nano-wires were parallel to the [1 1 0] direction. On the other hand, the 2.5-ML-thick InAs on smooth (0 0 1) GaAs surfaces formed QDs with a density of 4.4×10 9 cm −2. We grew these low density InAs QDs on patterned GaAs, which had small A-planes. The InAs QDs were formed only on the small A-plane, which was positioned at the boundary between the square-patterned and planar areas. Except for the A-planes, no InAs QD formation was observed in a relatively large area. These results indicate that the combination of GaAs nano-wires and pattern substrates with small A-planes can produce InAs QDs at desired areas.

Surface-acoustic-wave-driven luminescence from a lateral p-n junction

The authors report surface-acoustic-wave-driven luminescence from a lateral p-n junction formed by molecular beam epitaxy regrowth of a modulation doped GaAs∕AlGaAs quantum well on a patterned GaAs substrate. Surface-acoustic-wave-driven transport is demonstrated by peaks in the electrical current and light emission from the GaAs quantum well at the resonant frequency of the transducer. This type of junction offers high carrier mobility and scalability. The demonstration of surface-acoustic-wave luminescence is a significant step towards single-photon applications in quantum computation and quantum cryptography.