Pamela Jurczak

Influence of Droplet Size on the Growth of Self-Catalyzed Ternary GaAsP Nanowires

The influences of droplet size on the growth of self-catalyzed ternary nanowires (NWs) were studied using GaAsP NWs. The size-induced Gibbs-Thomson (GT) effect makes the smaller catalytic droplets have lower effective supersaturations and hence slower nucleation rates than the larger ones. Large variation in droplet size thus led to the growth of NWs with low uniformity, while a good size uniformity of droplets resulted in the production of highly uniform NWs. Moreover, thinner NWs were observed to be richer in P, indicating that P is more resistant to the GT effect than As because of a higher chemical potential inside Ga droplets. These results provide useful information for understanding the mechanisms of self-catalyzed III-V NW nucleation and growth with the important ternary III-V material systems.

Solid solution strengthening in GaSb/GaAs: A mode to reduce the TD density through Be-doping

The need for a low bandgap semiconductor on a GaAs substrate for thermophotovoltaic applications has motivated research on GaSb alloys, in particular, the control of plastic relaxation of its active layer. Although interfacial misfit arrays offer a possibility of growing strain-free GaSb-based devices on GaAs substrates, a high density of threading dislocations is normally observed. Here, we present the effects of the combined influence of Be dopants and low growth temperature on the threading dislocation density observed by Transmission Electron Microscopy. The Be-related hardening mechanism, occurring at island coalescence, is shown to prevent dislocations to glide and hence reduce the threading dislocation density in these structures. The threading density in the doped GaSb layers reaches the values of seven times less than those observed in undoped samples, which confirms the proposed Be-related hardening mechanism.

Doping of Self-Catalyzed Nanowires under the Influence of Droplets

Controlled and reproducible doping is essential for nanowires (NWs) to realize their functions. However, for the widely used self-catalyzed vapor-liquid-solid (VLS) growth mode, the doping mechanism is far from clear, as the participation of the nanoscale liquid phase makes the doping environment highly complex and significantly different from that of the thin film growth. Here, the doping mechanism of self-catalyzed NWs and the influence of self-catalytic droplets on the doping process are systematically studied using beryllium (Be) doped GaAs NWs. Be atoms are found for the first time to be incorporated into NWs predominantly through the Ga droplet that is observed to be beneficial for setting up thermodynamic equilibrium at the growth front. Be dopants are thus substitutional on Ga sites and redundant Be atoms are accumulated inside the Ga droplets when NWs are saturated, leading to the change of the Ga droplet properties and causing the growth of phase-pure zincblende NWs. This study is an essential step toward the design and fabrication of nanowire devices.

Efficiency of GaInAs thermophotovoltaic cells: the effects of incident radiation, light trapping and recombinations

The radiative limit model, based on the black body theory extended to semiconductors and the flow equilibrium in the cell, has been adapted for Ga(x)In(1-x)As thermophotovoltaic devices. The impact of the thermal emitter temperature and the incident power density on the performance of cells for different Ga/In ratios has been investigated. The effects of the thickness of the cell and of light trapping have been investigated as well. A theoretical maximum efficiency of 24.2% has been calculated for a dislocation-free 5-μm-thick cell with a 0.43 eV bandgap illuminated by a source at 1800 K. The model also takes into account Auger recombinations and threading dislocations-related Shockley-Read-Hall recombinations.

Ten-Fold Enhancement of InAs Nanowire Photoluminescence Emission with an InP Passivation Layer

In this Letter, we demonstrate that a significant improvement of optical performance of InAs nanowires can be achieved by capping the core InAs nanowires with a thin InP shell, which successfully passivates the surface states reducing the rate of nonradiative recombination. The improvements have been confirmed by detailed photoluminescence measurements, which showed up to a 10-fold increase in the intensity of room-temperature photoluminescence from the capped InAs/InP nanowires compared to the sample with core-only InAs nanowires. Moreover, the nanowires exhibit a high stability of total photoluminescence emission strength across temperature range from 10 to 300 K as a result of strong quantum confinement. These findings could be the key to successful implementation of InAs nanowires into optoelectronic devices.

Growth of Pure Zinc-Blende GaAs(P) Core–Shell Nanowires with Highly Regular Morphology

The growth of self-catalyzed core-shell nanowires (NWs) is investigated systematically using GaAs(P) NWs. The defects in the core NW are found to be detrimental for the shell growth. These defects are effectively eliminated by introducing beryllium (Be) doping during the NW core growth and hence forming Be-Ga alloy droplets that can effectively suppress the WZ nucleation and facilitate the droplet consumption. Shells with pure zinc-blende crystal quality and highly regular morphology are successfully grown on the defect-free NW cores and demonstrated an enhancement of one order of magnitude for room-temperature emission compared to that of the defective shells. These results provide useful information on guiding the growth of high-quality shell, which can greatly enhance the NW device performance.

GaSb and GaSb/AlSb Superlattice Buffer Layers for High-Quality Photodiodes Grown on Commercial GaAs and Si Substrates

The objective of this work is the integration of InGaAs/GaSb/GaAs heterostructures, with high indium content, on GaAs and Si commercial wafers. The design of an interfacial misfit dislocation array, either on GaAs or Si substrates, allowed growth of strain-free devices. The growth of purposely designed superlattices with their active region free of extended defects on both GaAs and Si substrates is demonstrated. Transmission electron microscopy technique is used for the structural characterization and plastic relaxation study. In the first case, on GaAs substrates, the presence of dopants was demonstrated to reduce several times the threading dislocation density through a strain-hardening mechanism avoiding dislocation interactions, while in the second case, on Si substrates, similar reduction of dislocation interactions is obtained using an AlSb/GaSb superlattice. The latter is shown to redistribute spatially the interfacial misfit dislocation array to reduce dislocation interactions.

Influence of droplet size on the growth of high-quality self-catalyzed GaAsP nanowires

Nanowires (NWs) have better functionality and superior performance as compared with the traditional thin film counterparts. However, NW growth is highly complicated and the growth mechanism is far from clear, especially when it is grown by vapor-liquid-solid mode. In this work, the influences of droplet size on the growth of self-catalyzed ternary NWs were studied using GaAsP NWs. The size-induced Gibbs−Thomson (GT) effect is observed for the first time in the self-catalyzed growth mode, which can make the smaller catalytic droplets have lower effective supersaturations. Thus, the droplet size can significantly influence the uniformity and composition of NWs. By carefully control the droplet size, the growth of highly uniform NW arrays are demonstrated. These results provide useful information for understanding the mechanisms of self-catalyzed III−V NW nucleation and growth with the important ternary III−V material systems.

Growth of high-quality self-catalyzed core-shell GaAsP nanowires on Si substrates

Self-catalyzed GaAsP nanowires (NWs) have a band gap that is capable of covering the working wavelengths from green to infrared. However, the difficulties in controlling P and the complexities of the growth of ternary NWs make it challenging to fabricate them. In this work, self-catalyzed GaAsP NWs were successfully grown on Si substrates by solid-source molecular beam epitaxy and demonstrated almost stacking fault free zinc blend crystal structure, Growth of high-quality shell has been realized on the core NWs. In the shell, a quasi-3-fold composition symmetry has been observed for the first time. Moreover, these growth techniques have been successfully applied for growth on patterned Si substrates after some creative modifications such as high-temperature substrate cleaning and Ga pre-deposition. These results open up new perspectives for integrating III−V nanowire photovoltaics and visible light emitters on the silicon platform using self-catalyzed GaAsP core−shell nanowires.