Microscopic insights into metal diffusion and ohmic contact formation in delta-doped GaAs/(Al,Ga)As core/shell nanowires.

Semiconductor nanowires (NWs) are promising candidates for use in electronic and optoelectronic applications, offering numerous advantages over their thin film counterparts. Their performance relies heavily on the quality of the contacts to the NW, which should exhibit ohmic behavior with low resistance and should be formed in a reproducible manner. In the case of heterostructure NWs for high-mobility applications that host a two-dimensional electron gas, ohmic contacts are particularly challenging to implement since the NW core constituting the conduction channel is away from the NW surface. We investigated contact formation to modulation-doped GaAs/(Al,Ga)As core/shell NWs using scanning transmission electron microscopy, energy dispersive x-ray spectroscopy and electron tomography to correlate microstructure, diffusion profile and chemical composition of the NW contact region with the current-voltage (I-V) characteristics of the contacted NWs. Our results illustrate how diffusion, alloying and phase formation processes essential to the effective formation of ohmic contacts are more intricate than in planar layers, leading to reproducibility challenges even when the processing conditions are the same. We demonstrate that the NW geometry plays a crucial role in the creation of good contacts. Both ohmic and rectifying contacts were obtained under nominally identical processing conditions. Furthermore, the presence of Ge in the NW core, in the absence of Au and Ni, was found as the key factor leading to ohmic contacts. The analysis contributes to the current understanding of ohmic contact formation to heterostructure core/shell NWs offering pathways to enhance the reproducibility and further optimization of such NW contacts.

Tailoring of AlAs/InAs/GaAs QDs Nanostructures via Capping Growth Rate.

The use of thin AlA capping layers (CLs) on InAs quantum dots (QDs) has recently received considerable attention due to improved photovoltaic performance in QD solar cells. However, there is little data on the structural changes that occur during capping and their relation to different growth conditions. In this work, we studied the effect of AlA capping growth rate (CGR) on the structural features of InAs QDs in terms of shape, size, density, and average content. As will be shown, there are notable differences in the characteristics of the QDs upon changing CGR. The Al distribution analysis in the CL around the QDs was revealed to be the key. On the one hand, for the lowest CGR, Al has a homogeneous distribution over the entire surface, but there is a large thickening of the CL on the sides of the QD. As a result, the QDs are lower, lenticular in shape, but richer in In. On the other hand, for the higher CGRs, Al accumulates preferentially around the QD but with a more uniform thickness, resulting in taller QDs, which progressively adopt a truncated pyramidal shape. Surprisingly, intermediate CGRs do not improve either of these behaviors, resulting in less enriched QDs.

Suppressing the Effect of the Wetting Layer through AlAs Capping in InAs/GaAs QD Structures for Solar Cells Applications.

Recently, thin AlAs capping layers (CLs) on InAs quantum dot solar cells (QDSCs) have been shown to yield better photovoltaic efficiency compared to traditional QDSCs. Although it has been proposed that this improvement is due to the suppression of the capture of photogenerated carriers through the wetting layer (WL) states by a de-wetting process, the mechanisms that operate during this process are not clear. In this work, a structural analysis of the WL characteristics in the AlAs/InAs QD system with different CL-thickness has been made by scanning transmission electron microscopy techniques. First, an exponential decline of the amount of InAs in the WL with the CL thickness increase has been found, far from a complete elimination of the WL. Instead, this reduction is linked to a higher shield effect against QD decomposition. Second, there is no compositional separation between the WL and CL, but rather single layer with a variable content of InAlGaAs. Both effects, the high intermixing and WL reduction cause a drastic change in electronic levels, with the CL making up of 1-2 monolayers being the most effective configuration to reduce the radiative-recombination and minimize the potential barriers for carrier transport.

Axial GaAs/Ga(As, Bi) nanowire heterostructures.

Bi-containing III-V semiconductors constitute an exciting class of metastable compounds with wide-ranging potential optoelectronic and electronic applications. However, the growth of III-V-Bi alloys requires group-III-rich growth conditions, which pose severe challenges for planar growth. In this work, we exploit the naturally-Ga-rich environment present inside the metallic droplet of a self-catalyzed GaAs nanowire (NW) to synthesize metastable GaAs/GaAs1-x Bi x axial NW heterostructures with high Bi contents. The axial GaAs1-x Bi x segments are realized with molecular beam epitaxy by first enriching only the vapor-liquid-solid (VLS) Ga droplets with Bi, followed by exposing the resulting Ga-Bi droplets to As2 at temperatures ranging from 270 °C to 380 °C to precipitate GaAs1-x Bi x only under the NW droplets. Microstructural and elemental characterization reveals the presence of single crystal zincblende GaAs1-x Bi x axial NW segments with Bi contents up to (10 ± 2)%. This work illustrates how the unique local growth environment present during the VLS NW growth can be exploited to synthesize heterostructures with metastable compounds.

Efficient methodology to correlate structural with optical properties of GaAs nanowires based on scanning electron microscopy.

Twin boundaries and boundaries between zincblende (ZB) and wurtzite (WZ) segments of GaAs-related nanowires (NWs) form intrinsic heterointerfaces with essential consequences for the application of such nanomaterials in optoelectronic devices. We show that for GaAs and GaAs/(Al, Ga)As core/shell NWs, crystal twinning along the NW axis can be imaged with a spatial resolution of 10 nm using secondary electrons in a scanning electron microscope (SEM). Changes of the crystal structure from the ZB to the WZ phase have been investigated by electron backscatter diffraction. In addition to these methods, we employ spectrally and spatially resolved cathodoluminescence measurements in the same SEM to study the correlation between the structural and optical properties in single NWs. Two GaAs/AlAs/GaAs core/shell/shell NWs differing significantly in the crystal structure along their axis have been investigated combining these three techniques in order to demonstrate the strength of the employed methodology. Our experiments show that based on commonly available SEM methods, an overview of the structural properties along an entire NW and their impact on the spectral and spatial luminescence distribution can be efficiently obtained providing a quick feedback for the optimization of growth conditions.

GaAsBi/GaAs multi-quantum well LED grown by molecular beam epitaxy using a two-substrate-temperature technique.

We report a GaAs0.96Bi0.04/GaAs multiple quantum well (MQW) light emitting diode (LED) grown by molecular beam epitaxy using a two-substrate-temperature (TST) technique. In particular, the QWs and the barriers in the intrinsic region were grown at the different temperatures of [Formula: see text] = 350 °C and [Formula: see text] respectively. Investigations of the microstructure using transmission electron microscopy (TEM) reveal homogeneous MQWs free of extended defects. Furthermore, the local determination of the Bi distribution profile across the MQWs region using TEM techniques confirm the uniform Bi distribution, while revealing a slightly chemically graded GaAs-on-GaAsBi interface due to Bi surface segregation. Despite this small broadening, we found that Bi segregation is significantly reduced (up to 18% reduction) compared to previous reports on Bi segregation in GaAsBi/GaAs MQWs. Hence, the TST procedure proves as a very efficient method to reduce Bi segregation and thus increase the quality of the layers and interfaces. These improvements positively reflect in the optical properties. Room temperature photoluminescence and electroluminescence (EL) at 1.23 µm emission wavelength are successfully demonstrated using TST MQWs containing less Bi content than in previous reports. Finally, LED fabricated using the present TST technique show current-voltage (I-V) curves with a forward voltage of 3.3 V at an injection current of 130 mA under 1.0 kA cm-2 current excitation. These results not only demonstrate that TST technique provides optical device quality GaAsBi/GaAs MQWs but highlight the relevance of TST-based growth techniques on the fabrication of future heterostructure devices based on dilute bismides.

Observation of Dielectrically Confined Excitons in Ultrathin GaN Nanowires up to Room Temperature.

The realization of semiconductor structures with stable excitons at room temperature is crucial for the development of excitonics and polaritonics. Quantum confinement has commonly been employed for enhancing excitonic effects in semiconductor heterostructures. Dielectric confinement, which gives rises to much stronger enhancement, has proven to be more difficult to achieve because of the rapid nonradiative surface/interface recombination in hybrid dielectric-semiconductor structures. Here, we demonstrate intense excitonic emission from bare GaN nanowires with diameters down to 6 nm. The large dielectric mismatch between the nanowires and vacuum greatly enhances the Coulomb interaction, with the thinnest nanowires showing the strongest dielectric confinement and the highest radiative efficiency at room temperature. In situ monitoring of the fabrication of these structures allows one to accurately control the degree of dielectric enhancement. These ultrathin nanowires may constitute the basis for the fabrication of advanced low-dimensional structures with an unprecedented degree of confinement.

Detecting lateral composition modulation in dilute Ga(As,Bi) epilayers.

The ability to characterize a structure into the finest details in a quantitative manner is a key issue to understanding and controlling nanoscale phase separation in novel nanomaterials. In this work, we consider the detectability of lateral composition modulation (LCM), a type of nanoscale phase separation in GaAs(1-x)Bix epilayers, by x-ray diffraction (XRD). We show that the satellite peaks due to LCM are hardly detectable in reasonable time with a lab x-ray diffractometer for GaAs(1-x)Bix samples with an average x up to 25% and relative modulation up to 50%. This is in contrast to LCM reported in other III-V combinations, where the intensity of the satellite peak is relatively high and can be easily detected. Our theoretical considerations are complemented experimentally using highly brilliant synchrotron radiation. The results are in good agreement with the predictions. This work provides a guideline for the systematic characterization of LCM in zincblende III-V semiconductor epilayers and points to the critical role of quantitative characterization of nanoscale phase separation.

Investigation of III-V nanowires by plan-view transmission electron microscopy: InN case study.

We discuss observations of InN nanowires (NWs) by plan-view high-resolution transmission electron microscopy (TEM). The main difficulties arise from suitable methods available for plan-view specimen preparation. We explore different approaches and find that the best results are obtained using a refined preparation method based on the conventional procedure for plan-view TEM of thin films, specifically modified for the NW morphology. The fundamental aspects of such a preparation are the initial mechanical stabilization of the NWs and the minimization of the ion-milling process after dimpling the samples until perforation. The combined analysis by plan-view and cross-sectional TEM of the NWs allows determination of the degree of strain relaxation and reveals the formation of an unintentional shell layer (2-3-nm thick) around the InN NWs. The shell layer is composed of bcc In2O3 nanocrystals with a preferred orientation with respect to the wurtzite InN: In2O3 [111] || InN [0001] and In2O3<110>||InN<1120>.

Formation and phase transformation of Bi-containing QD-like clusters in annealed GaAsBi.

We report the formation and phase transformation of Bi-containing clusters in GaAs(1-x)Bi(x) epilayers upon annealing. The GaAs(1-x)Bi(x) layers were grown by molecular beam epitaxy under low (220 °C) and high (315 °C) temperatures and subsequently annealed using different temperatures and annealing times. Bi-containing clusters were identified only in the annealed samples that were grown at low temperature, revealing a relatively homogeneous size distribution. Depending on the annealing temperature and duration, the clusters show different sizes ranging from 5 to 20 nm, as well as different crystallographic phase, being coherently strained zincblende GaAs(1-x)Bi(x) (zb Bi-rich Ga(As, Bi)) clusters or rhombohedral pure Bi (rh-Bi) clusters. We found that: (1) the formation of the zb Bi-rich Ga(As, Bi) clusters is driven by the intrinsic tendency of the alloy to phase separately and is mediated by the native point defects present in the low temperature grown epilayers; (2) the phase transformation from zb Bi-rich Ga(As, Bi) to rh-Bi nucleates in zincblende {111} planes and grows until total consumption of Bi in the GaAs matrix. We propose a model accounting for the formation and phase transformation of Bi-containing clusters in this system. Furthermore, our study reveals the possibility to realize self-organized zb Bi-rich Ga(As, Bi) clusters that can exhibit QD-like features.

Critical role of two-dimensional island-mediated growth on the formation of semiconductor heterointerfaces.

We experimentally demonstrate a sigmoidal variation of the composition profile across semiconductor heterointerfaces. The wide range of material systems (III-arsenides, III-antimonides, III-V quaternary compounds, III-nitrides) exhibiting such a profile suggests a universal behavior. We show that sigmoidal profiles emerge from a simple model of cooperative growth mediated by two-dimensional island formation, wherein cooperative effects are described by a specific functional dependence of the sticking coefficient on the surface coverage. Experimental results confirm that, except in the very early stages, island growth prevails over nucleation as the mechanism governing the interface development and ultimately determines the sigmoidal shape of the chemical profile in these two-dimensional-grown layers. In agreement with our experimental findings, the model also predicts a minimum value of the interfacial width, with the minimum attainable value depending on the chemical identity of the species.