Magneto-optical determination of the electron-solid phase-boundary

We have obtained a two-dimensional electron-solid phase diagram in the extreme magnetic quantum limit by studying the temperature dependence of the radiative recombination of electrons in a GaAs/AlxGa1-xAs heterojunction with holes bound to a delta-layer, 250 A away in the GaAs, of Be acceptors. The low-energy shoulder to the luminescence line, indicating the presence of the electron solid, is seen to disappear at a filling-factor-dependent critical temperature. We observe no shoulder above a filling factor of 0.25, and the critical temperature falls to below 0.4 K at filling factors 1/5 and 1/7

Photoelectrolysis Using Type-II Semiconductor Heterojunctions

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

Simulations of ultra-low power non-volatile cells for random access memory

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

Magneto-optical study of electron occupation and hole wave functions in stacked self-assembled InP quantum dots

We have studied the magnetophotoluminescence of doubly stacked layers of self-assembled InP quantum dots in a GaInP matrix. 4.0±0.1 monolayers of InP were deposited in the lower layer of each sample, whereas in the upper layer 3.9, 3.4, and 3.0 monolayers were used. Low-temperature photoluminescence measurements in zero magnetic field are used to show that, in each case, only one layer of dots is occupied by an electron, and imply that when the amount of InP in both layers is the same, the dots in the upper layer are larger. High-field photoluminescence data reveal that the position and extent of the hole wave function are strongly dependent on the amount of InP in the stack. ©2001 American Institute of Physics

Phonon bottleneck in GaAs/AlxGa1-xAs quantum dots

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

A detailed comparison of measured and simulated optical properties of a short-period GaAs/AlxGa1-xAs distributed Bragg reflector

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

Hydrogenation of GaSb/GaAs quantum rings

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

Electron localization by self-assembled GaSb/GaAs quantum dots.

We have studied the photoluminescence from type-II GaSb/GaAs self-assembled quantum dots in magnetic fields up to 50 T. Our results show that at low laser power, electrons are more weakly bound to the dots than to the wetting layer, but that at high laser power, the situation is reversed. We attribute this effect to an enhanced Coulomb interaction between a single electron and dots that are multiply charged with holes

Demonstration of a Fast, Low-Voltage, III-V Semiconductor, Non-Volatile Memory

ULTRARAM™ is a III-V semiconductor memory technology which exploits resonant tunneling to allow ultra-low-energy memory logic switching (per unit area), whilst retaining non-volatility. Single-cell memories developed on GaAs substrates with a revised design and atomic-layer-deposition Al 2 O 3 gate dielectric demonstrate significant improvements compared to prior prototypes. Floating-gate (FG) memories with 20-μm gate length show 0/1 state contrast from 2.5-V program-read-erase-read (P/E) cycles with 500-μs pulse duration, which would scale to sub-ns switching speed at 20-nm node. Nonvolatility is confirmed by memory retention tests of 4×10 3 s with both 0 and 1 states completely invariant. Single cells demonstrate promising endurance results, undergoing 10 4 cycles without degradation. P/E cycling and disturbance tests are performed using half-voltages (±1.25 V), validating the high-density random access memory (RAM) architecture proposed previously. Finally, memory logic is retained after an equivalent of >10 5 P/E disturbances

Nanometre scale 3D nanomechanical imaging of semiconductor structures from few nm to sub-micrometre depths

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