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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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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 °C to 800 °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. [ABSTRACT FROM AUTHOR]

ABSTRACT We review the photoluminescence of semiconductor nanostructures in high magnetic fields, concentrating on the effects of the applied magnetic field on orbital motion (wave function extent), which is probed in experiments on large ensembles. We present an overview of the physics of excitons in high magnetic fields in 3- and 2-D before introducing the zero-dimensional case. We then discuss the physics of quantum-dot excitons in high magnetic fields with particular attention to the approximate analytical models used to interpret experimental results. This is followed by a brief description of a typical high-field magneto-photoluminescence setup. We then present four examples of magneto-photoluminescence experiments on different materials systems chosen from our own research to illustrate how high magnetic fields can be used to reveal new insights into the physics of semiconductor nanostructures. Copyright © 2012 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

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.

A novel method of sample cross sectioning, beam-exit Ar-ion cross-sectional polishing, has been combined with scanning probe microscopy to study thin AlxGa1-xAs/GaAs layers. Additional contrast enhancement via a citric acid/hydrogen peroxide etch allows us to report the observation of layers as thin as 1 nm. Layer thickness measurements agree with transmission electron microscopy (TEM) data to 0.1 ± 0.2 nm, making this a very promising low-cost method for nanoscale analysis of semiconductor heterostructures.

We review the photoluminescence of semiconductor nanostructures in high magnetic fields, concentrating on the effects of the applied magnetic field on orbital motion (wave function extent), which is probed in experiments on large ensembles. We present an overview of the physics of excitons in high magnetic fields in 3- and 2-D before introducing the zero-dimensional case. We then discuss the physics of quantum-dot excitons in high magnetic fields with particular attention to the approximate analytical models used to interpret experimental results. This is followed by a brief description of a typical high-field magneto-photoluminescence setup. We then present four examples of magneto-photoluminescence experiments on different materials systems chosen from our own research to illustrate how high magnetic fields can be used to reveal new insights into the physics of semiconductor nanostructures., (Copyright © 2012 John Wiley & Sons, Ltd.)