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ULTRARAMTM is a novel floating-gate nonvolatile memory in which the oxide barrier of flash is replaced by a triple-barrier resonant tunneling structure comprising of multiple InAs/AlSb heterojunctions. The quality of the triple barrier resonant tunneling heterostructure of an ULTRARAMTM device in terms of interface sharpness and the presence of defects was analyzed by cross-sectional scanning tunneling microscopy. We observed two different types of defects: stacking faults originating in the layers below the triple barrier resonant tunneling structure and AlSb accumulations at the interface between the lower AlSb layer of the triple barrier resonant tunneling structure and the InGaAs channel. The InGaAs surface of a second sample was measured by atomic force microscopy in order to investigate whether its unevenness is caused by deposition of the AlSb layer or it is already present before the AlSb deposition process.

ULTRARAM is a nonvolatile memory with the potential to achieve fast, ultralow-energy electron storage in a floating gate accessed through a triple-barrier resonant tunneling heterostructure. Here its implementation is reported on a Si substrate; a vital step toward cost-effective mass production. Sample growth using molecular beam epitaxy commences with deposition of an AlSb nucleation layer to seed the growth of a GaSb buffer layer, followed by the III–V memory epilayers. Fabricated single-cell memories show clear 0/1 logic-state contrast after ≤10 ms duration program/erase pulses of ≈2.5 V, a remarkably fast switching speed for 10 and 20 µm devices. Furthermore, the combination of low voltage and small device capacitance per unit area results in a switching energy that is orders of magnitude lower than dynamic random access memory and flash, for a given cell size. Extended testing of devices reveals retention in excess of 1000 years and degradation-free endurance of over 107 program/erase cycles, surpassing very recent results for similar devices on GaAs substrates.

The aim of this work is to establish the feasibility of designing and producing telecoms wavelength vertical-cavity structures using GaSb quantum rings (QRs) embedded in GaAs - AlxGa1−xAs resonators. Additionally, various methods of characterising the optical and material properties of the produced samples are assessed, to determine the most suitable techniques in terms of accuracy, ease of use, cost, and measurement time. Extensive optical modelling is carried out to design and optimise vertical-cavity surface-emitting lasers (VCSELs), and single-photon light-emitting diodes (SPLEDs). Optical models are used to check sample growth quality, and provide sample selection criteria when used in conjunction with measured material parameters and optical transmission spectra. Various measurement techniques are compared for the quantification of AlxGa1−xAs in many-layered structures, including secondary-ion mass spectroscopy (SIMS), X-ray photoelectron spectroscopy (XPS), and electron energy-loss spectroscopy (EELS). SIMS proves to be effective for thick samples, particularly in terms of measurement speed and layer clarity, with XPS and EELS more suited for thinner samples, or where high resolution over a small area is required. Distributed Bragg reflectors (DBRs) grown at IQE using metalorganic vapour-phase epitaxy (MOVPE) are successfully combined with the QR active region and upper DBR overgrowth using molecular beam epitaxy (MBE) at Lancaster University. The graded IQE DBRs are shown to be very consistent their optical properties, and reduce the electrical resistance and growth time for the remainder of the optical device. SPLED devices are shown to emit light, operating around 1310nm and 1550 nm. At this time no single-photon emission was observed. VCSEL devices are shown to operate at around 1270 nm, demonstrating the telecoms-wavelength capabilities of the system, while device performance is limited by high resistance and other potential factors. Further improvements

ULTRARAM is a nonvolatile memory with the potential to achieve fast, ultralow-energy electron storage in a floating gate accessed through a triple-barrier resonant tunneling heterostructure. Here its implementation is reported on a Si substrate; a vital step toward cost-effective mass production. Sample growth using molecular beam epitaxy commences with deposition of an AlSb nucleation layer to seed the growth of a GaSb buffer layer, followed by the III–V memory epilayers. Fabricated single-cell memories show clear 0/1 logic-state contrast after ≤10 ms duration program/erase pulses of ≈2.5 V, a remarkably fast switching speed for 10 and 20 µm devices. Furthermore, the combination of low voltage and small device capacitance per unit area results in a switching energy that is orders of magnitude lower than dynamic random access memory and flash, for a given cell size. Extended testing of devices reveals retention in excess of 1000 years and degradation-free endurance of over 107 program/erase cycles, surpassing very recent results for similar devices on GaAs substrates.

ULTRARAM™ is a III–V semiconductor memory technology which allows non-volatile logic switching at ultra-low energy (per unit area). This is achieved by exploiting triple-barrier resonant tunnelling (TBRT) through a series of InAs/AlSb heterojunctions specifically engineered for this purpose. Electrons tunnelling through the barriers at low bias are trapped in a floating gate, in which the presence or absence of charge defines the memory logic. Here, we report detailed non-equilibrium Green's functions simulations of the InAs/AlSb TBRT heterostructure, which is the principal source of ULTRARAM™'s extraordinary performance benefits. The effects of variations to the heterostructure layer thickness are investigated for performance optimization, and for assessing growth and process tolerances for commercial implementation on 12'' Si wafers. Trade-offs between power, speed, logic disturbance and data retention time are identified. Importantly, most one monolayer alterations to the tunnelling region show the required characteristics for ULTRARAM™ memory operation, thus some tolerance in any future commercial fabrication process is identified.

ULTRARAM™ is a III-V compound semiconductor memory concept which exploits quantum resonant tunneling to achieve non-volatility at extremely low switching energy per unit area. Prototype devices are fabricated in a 2x2 memory array formation on GaAs substrates. The devices show 0/1 state contrast from program/erase (P/E) cycles with 2.5 V pulses of 500-µs duration, a remarkable switching speed for a 20 µm gate length. Memory retention is tested for 8x104 s, whereby the 0/1 states show adequate contrast throughout, whilst performing 8x104 readout operations. Further reliability is demonstrated via program-read-erase-read endurance cycling for 106 cycles with 0/1 contrast. A half-voltage array architecture proposed in our previous work is experimentally realized, with an outstandingly small disturb rate over 105 half-voltage cycles.

In this thesis, a novel memory based on III-V compound semiconductors is studied, both theoretically and experimentally, with the aim of developing a technology with superior performance capabilities to established and emerging rival memories. This technology is known as ULTRARAM™. The memory concept is based on quantum resonant tunnelling through InAs/AlSb heterostructures, which are engineered to only allow electron tunnelling at precise energy alignment(s) when a bias is applied. The memory device features a floating gate (FG) as the storage medium, where electrons that tunnel through the InAs/AlSb heterostructure are confined in the FG to define the memory logic (0 or 1). The large conduction band offset of the InAs/AlSb heterojunction (2.1 eV) keeps electrons in the FG indefinitely, constituting a non-volatile logic state. Electrons can be removed from the FG via a similar resonant tunnelling process by reversing the voltage polarity. This concept shares similarities with flash memory, however the resonant tunnelling mechanism provides ultra-low-power, low-voltage, high-endurance and high-speed switching capability. The quantum tunnelling junction is studied in detail using the non-equilibrium Green’s function (NEGF) method. Then, Poisson-Schrödinger simulations are used to design a high-contrast readout procedure for the memory using the unusual type-III band-offset of the InAs/GaSb heterojunction. With the theoretical groundwork for the technology laid out, the memory performance is modelled and a high-density ULTRARAM™ memory architecture is proposed for random-access memory applications. Later, NEGF calculations are used for a detailed study of the process tolerances in the tunnelling region required for ULTRARAM™ large-scale wafer manufacture. Using interfacial misfit array growth techniques, III-V layers (InAs, AlSb and GaSb) for ULTRARAM™ were successfully implemented on both GaAs and Si substrates. Single devices and 2×2 arrays were then fabricated usin

ULTRARAM™ is a III-V compound semiconductor memory concept which exploits quantum resonant tunneling to achieve non-volatility at extremely low switching energy per unit area. Prototype devices are fabricated in a 2x2 memory array formation on GaAs substrates. The devices show 0/1 state contrast from program/erase (P/E) cycles with 2.5 V pulses of 500-µs duration, a remarkable switching speed for a 20 µm gate length. Memory retention is tested for 8x104 s, whereby the 0/1 states show adequate contrast throughout, whilst performing 8x104 readout operations. Further reliability is demonstrated via program-read-erase-read endurance cycling for 106 cycles with 0/1 contrast. A half-voltage array architecture proposed in our previous work is experimentally realized, with an outstandingly small disturb rate over 105 half-voltage cycles.

A 6-period GaAs/Al$_{0.9}$Ga$_{0.1}$As distributed Bragg reflector (DBR) has been grown and its optical properties have been both measured and simulated. Incremental improvements were made to the simulation, allowing it to account for internal consistency error, incorrect layer thicknesses, and absorption due to substrate doping to improve simulation accuracy. A compositional depth profile using secondary-ion mass spectrometry (SIMS) has been taken and shows that the Al fraction averages 88.0$\pm0.3\%$. It is found that the amplitude of the transmission is significantly affected by absorption in the n-doped GaAs substrate, even though the energy of the transmitted light is well below the GaAs band gap. The wavelength of the features on the transmission spectrum are mostly affected by DBR layer thicknesses. On the other hand, the transmission spectrum is found to be relatively tolerant to changes to Al fraction.

Dynamic random-access memory (DRAM), which represents 99% of random access memory (RAM), is fast and has excellent endurance, but suffers from disadvantages such as short data retention time (volatility) and loss of data during readout (destructive read). As a consequence, it requires persistent data refreshing, increasing energy consumption, degrading performance and limiting scaling capacity. It is therefore desirable that the next generation of RAM will be non-volatile (NVRAM), low power, high endurance, fast and non-destructively read. Here, we report on a new form of NVRAM: a compound-semiconductor charge-storage memory that exploits quantum phenomena for its operational advantages. Simulations show that the device is extremely low power, with 100 times lower switching energy per unit area than DRAM, but with similar operating speeds. Non-volatility is achieved due to the extraordinary band offsets of InAs and AlSb, providing a large energy barrier (2.1 eV) which prevents the escape of electrons. Based on the simulation results, an NVRAM architecture is proposed for which extremely low disturb-rates are predicted as a result of the quantum-mechanical resonant-tunnelling mechanism used to write and erase.

The work in this thesis reports on the design, growth, fabrication, measurement and analysis of n-GaSb/Al0.2Ga0.8Sb heterostructures. Initially a growth study was completed in order to minimise the native n-type GaSb defects (usually of p ~ × 1017cm−3 ) and to calibrate the n-type doping (achieved using Te). This growth study then informed simulations of the band structure and the transport properties via Schrödinger-Poisson and transport lifetime modelling, allowing an investigation for the design of a high mobility n-GaSb/Al0.2Ga0.8Sb structure, to ensure correct confinement. The optimum designs from the simulation study were then grown and fabricated into 8-pad geometry non-gated Hall bars. The metallisation recipe researched further as initial recipes were deemed unsuitable. The resulting devices were then measured as a function of magnetic field and temperature to obtain the transport properties (mobility and carrier density).It has been shown that an optimum set of growth conditions (Tg= 475 °C, V/III=1.3) reduces the p-type native defects resulting in a hole concentration of 5 × 1016 cm−3 at room temperature. Simulations of AlGaSb/GaSb heterostructures were completed, investigating the conduction band and the effect of the p-type defects. It was found that doping below the well is necessary for confinement in these structures. Both square and triangular quantum wells were grown on semi-insulating GaAs substrates with an interfacial misfit array to prevent strain. These samples were fabricated where the metallisation recipe Pd/In/Pd/Au was used, allowing cold temperature measurements. Measurements of the transport properties across a temperature range of (3-300) K at lowfield (< 3 T) resulted in a peak mobility of 9030 cm2 /Vs, where background impurity scattering was shown to be the limiting scattering rate. High field measurements of a doped heterojunction resulted in Shubnikov-de Haas oscillations visible up to 100 K. As the first transport measurements of

Excitonic complexes in type-II quantum-ring heterostructures may be considered as artificial atoms due to the confinement of only one charge-carrier type in an artificial nucleus. Binding energies of excitons, trions, and biexcitons in these nanostructures are then effectively ionization energies of these artificial atoms. The binding energies reported here are calculated within the effective-mass approximation using the diffusion quantum Monte Carlo method and realistic geometries for gallium antimonide rings in gallium arsenide. The electrons form a halo outside the ring, with very little charge density inside the central cavity of the ring. The de-excitonization and binding energies of the complexes are relatively independent of the precise shape of the ring.

Mobilities and carrier densities of modulation doped Al0.2Ga0.8Sb/GaSb heterostructures are presented for the first time. The structures studied were grown by molecular beam epitaxy and consisted of a single heterojunction with Te compensation doping to reduce the intrinsic p-type background. Hall measurements were performed from 30–300 K, giving p-type mobilities peaking at 3240 cm2/Vs, a considerable improvement over previous reported bulk mobilities for samples with compensation doping. Growth trials on bulk material have also been carried out to investigate the optimum growth conditions for future structures, with the aim of minimising the occurrence of natural growth defects in GaSb, which act as acceptors. Together these measurements lay the ground work for (magneto)transport studies of two-dimensional charge-carriers in AlxGa1-xSb/GaSb heterostructures, which has not been previously reported.

The solar-powered production of hydrogen for use as a renewable fuel is highly desirable for the world’s future energy infrastructure. However, difficulties in achieving reasonable efficiencies, and thus cost-effectiveness, have hampered significant research progress. Here we propose the use of semiconductor nanostructures to create a type-II heterojunction at the semiconductor–water interface in a photoelectrochemical cell (PEC) and theoretically investigate it as a method of increasing the maximum photovoltage such a cell can generate under illumination, with the aim of increasing the overall cell efficiency. A model for the semiconductor electrode in a PEC is created, which solves the Schrödinger, Poisson and drift–diffusion equations self-consistently. From this, it is determined that ZnO quantum dots on bulk n-InGaN with low In content is the most desirable system, having electron-accepting and -donating states straddling the oxygen- and hydrogen-production potentials for , though large variance in literature values for certain material parameters means large uncertainties in the model output. Accordingly, results presented here should form the basis for further experimental work, which will in turn provide input to refine and develop the model.

We report a comprehensive study of exciton confinement in self-assembled InAs quantum dots (QDs) in strain-engineered metamorphic InxGa1-xAs confining layers on GaAs using low temperature magneto-photoluminescence. As the lattice mismatch (strain) between QDs and confining layers (CLs) increases from 4.8% to 5.7% the reduced mass of the exciton increases, but saturates at higher mismatches. At low QD-CL mismatch there is a clear evidence of spillover of the exciton wave-function due to small localisation energies. This is suppressed as the In content, x, in the CLs decreases (mismatch and localisation energy increasing). The combined effects of low effective mass and wave-function spillover at high x result in a diamagnetic shift coefficient that is an order of magnitude larger than for samples where In content in the barrier is low (mismatch is high and localisation energy is large). Finally, an anomalously small measured Bohr radius in samples with the highest x is attributed to a combination of thermalisation due to low localisation energy, and its enhancement with magnetic field, a mechanism which results in small dots in the ensemble dominating the measured Bohr radius.

In this work, the effect of the application of a thermal annealing on the structural properties of GaSb/GaAs quantum dots (QDs)1 is analyzed by aberration corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM)2 and electron energy loss spectroscopy (EELS)3. Our results show that the GaSb/GaAs QDs are more elongated after the annealing, and that the interfaces are less abrupt due to the Sb diffusion. We have also found a strong reduction in the misfit dislocation density with the annealing. The analysis by EELS of a threading dislocation has shown that the dislocation core is rich in Sb. In addition, the region of the GaAs substrate delimited by the threading dislocation is shown to be Sb-rich as well. An enhanced diffusion of Sb due to a mechanism assisted by the dislocation movement is discussed.

In this work, the effect of the application of a thermal annealing on the structural properties of GaSb/GaAs quantum dots (QDs)1 is analyzed by aberration corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM)2 and electron energy loss spectroscopy (EELS)3 Our results show that the GaSb/GaAs QDs are more elongated after the annealing, and that the interfaces are less abrupt due to the Sb diffusion. We have also found a strong reduction in the misfit dislocation density with the annealing. The analysis by EELS of a threading dislocation has shown that the dislocation core is rich in Sb. In addition, the region of the GaAs substrate delimited by the threading dislocation is shown to be Sb-rich as well. An enhanced diffusion of Sb due to a mechanism assisted by the dislocation movement is discussed.

We report the investigation of exciton dynamics in type-II self-assembled GaSb/GaAs quantum dots. The GaSb/GaAs quantum dots (QDs) were grown using a modified liquid phase epitaxy technique. Statistical size distributions of the uncapped QDs were investigated experimentally by field-emission scanning electron microscopy (SEM) and atomic force microscopy (AFM), and theoretically by an eight-band k . p calculation, which demonstrated a dissolution effect. Furthermore, the low-temperature luminescence spectra of type-II GaSb/GaAs QDs with a thick capping layer exhibit well-resolved emission bands and LO-phonon-assisted transitions in the GaSb wetting layer. However, the luminescence lines quench at temperatures above 250 K, which is attributed to the weak quantum confinement of electrons participating in indirect exciton recombination. It was demonstrated that the room temperature stability of the excitons in type-II GaSb/GaAs QDs could be achieved by growing thin a capping layer, which provides strong quantum confinement in the conduction band and enhances the electron-hole Coulomb interaction, stabilizing the excitons.

The structural quality of GaSb/GaAs quantum dots (QDs) has been analyzed at atomic scale by aberration-corrected high-angle annular dark-field scanning transmission electron microscopy. In particular, we have studied the misfit dislocations that appear because of the high lattice mismatch in the heterostructure. Our results have shown the formation of Lomer dislocations at the interface between the GaSb QDs and the GaAs substrate, but also at the interface with the GaAs capping layer, which is not a frequent observation. The analysis of these dislocations point to the existence of chains of dislocation loops around the QDs. The dislocation core of the observed defects has been characterized, showing that they are reconstructed Lomer dislocations, which have less distortion at the dislocation core in comparison to unreconstructed ones. Strain measurements using geometric phase analysis (GPA) show that these dislocations may not fully relax the strain due to the lattice mismatch in the GaSb QDs.

Charge-carrier confinement in nanoscale In-rich agglomerations within a lateral InGaAs quantum well (QW) formed from stacked submonolayers (SMLs) of InAs in GaAs is studied. Low-temperature photoluminescence (PL) and magneto-PL clearly demonstrate strong vertical and weak lateral confinement, yielding two-dimensional (2D) excitons. In contrast, high-temperature (400 K) magneto-PL reveals excited states that fit a Fock-Darwin spectrum, characteristic of a zero-dimensional (0D) system in a magnetic field. This paradox is resolved by concluding that the system is heterodimensional: the light electrons extend over several In-rich agglomerations and see only the lateral InGaAs QW, i.e., are 2D, while the heavier holes are confined within the In-rich agglomerations, i.e., are 0D. This description is supported by single-particle effective-mass and eight-band k⋅p calculations. We suggest that the heterodimensional nature of nanoscale SML inclusions is fundamental to the ability of respective optoelectronic devices to operate efficiently and at high speed.

In this thesis, semiconductor nanostructures are studied, both experimentally and theoretically, to help aid the development of two diverse and important technologies. Firstly, charge-carrier confinement in stacked sub-monolayer (SML) InAs in GaAs: SML deposition results in the formation of In-rich agglomerations within a lateral InGaAs quantum well (QW) with lower In content. Low-temperature photoluminescence (PL) and magneto-PL reveals strong vertical and weak lateral confinement, indicative of a two-dimensional (2D) excitons. Paradoxically, high-temperature magneto-PL allows excited-state peaks to become resolved, which can be fitted by a Fock-Darwin spectrum, characteristic of a zero-dimensional (0D) system. To solve this contradiction, we postulate that stacked SML InAs in GaAs forms a heterodimensional system, in which electrons are 2D, and see only the lateral InGaAs QW, whilst the heavier holes are 0D, and are confined in the In-rich agglomerations. This description is fully supported by single-particle effective-mass and eight-band k · p calculations, which show heterodimensional confinement is probable for a large variation in In content. SML vertical-cavity surface-emitting lasers (VCSELs) — which prove to be one of the most promising candidates for datacoms applications — have been demonstrated at >20 Gb/s, and we postulate that heterodimensionality is fundamental to this high-speed operation. Efficient carrier injection is achieved by the lack of a wetting layer, along with the 2D electrons coupling to several In-rich agglomerations, making them quickly available to states that are lasing. Furthermore, the shallow confining potential of the In-rich agglomerations means that excess holes cannot build up in states that aren't lasing. Secondly, semiconductor photoelectrolysis for the solar-powered generation of renewable hydrogen by water splitting is researched. The novel use of nanostructures at the semiconductor-electrolyte interface (SEI) in a photoele

Electrodispensing is a technique that can be used to precisely fabricate tunable microspheroids, of various shapes and sizes. Recent investigations have demonstrated their use as solid immersion lenses (SIL’s), with large numerical apertures, and short focal lengths; this makes them perfect for coupling light into and out of semiconductor devices. Furthermore fabricating SIL’s from UV cure epoxy has significant advantages over conventional glass formed SIL’s, allowing their size, location and shape to be perfectly tuned to the individual requirements of the system, greatly increasing the extraction efficiency and resolution, a boon to many optoelectronic applications. GaSb Quantum rings (QR) provide strong zero-dimensional confinement, which may make them suitable as quantum light sources at telecoms wavelengths. However, when these nanostructures are embedded in a semiconductor structure most of the light emitted by them is lost to total internal reflection. Here we show that electrodispensed UV-cured SIL’s can be deterministically coupled to quantum rings to massively increase the output coupling efficiency of these structures. This promises a simple and cheap solution to the development of useful quantum light sources.

We report low-temperature photoluminescence measurements on highly-uniform GaAs/AlxGa1-xAs quantum dots grown by droplet epitaxy. Recombination between confined electrons and holes bound to carbon acceptors in the dots allow us to determine the energies of the confined states in the system, as confirmed by effective mass calculations. The presence of acceptor-bound holes in the quantum dots gives rise to a striking observation of the phonon-bottleneck effect.

Multilayer structures of active semiconductor devices (1), novel memories (2) and semiconductor interconnects are becoming increasingly three-dimensional (3D) with simultaneous decrease of dimensions down to the few nanometres length scale (3). Ability to test and explore these 3D nanostructures with nanoscale resolution is vital for the optimization of their operation and improving manufacturing processes of new semiconductor devices. While electron and scanning probe microscopes (SPMs) can provide necessary lateral resolution, their ability to probe underneath the immediate surface is severely limited. Cross-sectioning of the structures via focused ion beam (FIB) to expose the subsurface areas often introduces multiple artefacts that mask the true features of the hidden structures, negating benefits of such approach. In addition, the few tens of micrometre dimension of FIB cut, make it unusable for the SPM investigation.

True sources of quantum light, where the number of photons in a pulse can be precisely tailored, prove to be a valuable resource in many emerging technologies. These include quantum key distribution, metrology and sub-diffraction limited imaging. Using individual zero-dimensional nanostructures within a structure can allow devices such as quantum light sources, such as single photon emitters1 and high-fidelity entangled photon sources2,3 to be created. The majority of research into nanostructures for quantum light sources has been self-assembled InAs/GaAs quantum dots. This system is, however, limited to low temperature operation. An alternate material system will be presented in this work, looking to overcome this limitation: GaSb/GaAs quantum rings. These heterostructures exhibit a type-II band alignment and have a very large hole confining potential. With its strong hole confinement the rings can emit light at room temperature. Results from micro-photoluminescence measurements on the dot system will be presented alongside scanning probe microscopy.

We present the results of photoluminescence measurements on hydrogenated type-II GaSb/GaAs quantum dot/ring (QD/QR) samples at temperatures ranging from 4.2K to 400 K. Hydrogenation is found to suppress optically induced charge depletion (associated with the presence of carbon acceptors in this system). A redshift of the QD\QR emission energy of a few tens of meV is observed at temperatures 300 K, consistent with a reduction in average occupancy by 1 hole. These effects are accompanied by a reduction in PL intensity post-hydrogenation. We conclude that although hydrogenation may have neutralized the carbon acceptors, multiple hole occupancy of type-II GaSb/GaAs QD/QRs is very likely a precondition for intense emission, which would make extending the wavelength significantly beyond 1300 nm at room temperature difficult.

The effects of rapid thermal annealing on the photoluminescence emission obtained from ten-layer stacks of GaSb/GaAs type-II single monolayer quantum dots and Stranski-Krastanow grown quantum rings have been studied and interpreted. Post-growth rapid thermal annealing was performed with proximity capping at temperatures from 550 degrees C to 800 degrees C, resulting in an increase in photoluminescence emission intensity and a blue shift in peak energy in both types of stacks, together with changes in the activation energy for thermal quenching. This behaviour originates from Sb-As intermixing and changes in morphology of the nanostructures formed using the two different growth mechanisms.

We report the successful molecular-beam epitaxial growth of 10 stacked layers of GaSb/GaAs quantum rings using a new procedure. Exact control of the arsenic flux during capping helps to reduce the strong group-V As-Sb exchange reactions, enabling the rings to be capped at the same growth temperature (480 degrees C) without dissolution. X-ray diffraction and transmission electron microscopy indicate excellent structural quality and uniformity with no threading dislocations. This is due to the reduction in the average strain through the quantum-ring formation. The total ring density in the stacks is 1 x 10(11) cm(-2). An unusually long-wavelength quantum-ring photoluminescence peak of 1.3 mu m is observed at low temperature, which is attributed to a reduction in the quantum-ring charging due to lower unintentional p-doping in the GaAs cap layer. The impact that this effect will have on future device designs in solar cells and lasers is also discussed.