Alexander James Robson

Low Leakage-Current InAsSb Nanowire Photodetectors on Silicon.

Axially doped p-i-n InAs0.93Sb0.07 nanowire arrays have been grown on Si substrates and fabricated into photodetectors for shortwave infrared detection. The devices exhibit a leakage current density around 2 mA/cm(2) and a 20% cutoff of 2.3 μm at 300 K. This record low leakage current density for InAsSb based devices demonstrates the suitability of nanowires for the integration of III-V semiconductors with silicon technology.

High-accuracy analysis of nanoscale semiconductor layers using beam-exit ar-ion polishing and scanning probe microscopy.

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.

GaSb quantum rings in GaAs/AlxGa1−xAs quantum wells

We report the results of continuous and time-resolved photoluminescence measurements on type-II GaSb quantum rings embedded within GaAs/AlxGa1−xAs quantum wells. A range of samples were grown with different well widths, compensation-doping concentrations within the wells, and number of quantum-ring layers. We find that each of these variants have no discernible effect on the radiative recombination, except for the very narrowest (5 nm) quantum well. In contrast, single-particle numerical simulations of the sample predict changes in photoluminescence energy of up to 200 meV. This remarkable difference is explained by the strong Coulomb binding of electrons to rings that are multiply charged with holes. The resilience of the emission to compensation doping indicates that multiple hole occupancy of the quantum rings is required for efficient carrier recombination, regardless of whether these holes come from doping or excitation.

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.

Optical identification using imperfections in 2D materials

The ability to uniquely identify an object or device is important for authentication. Imperfections, locked into structures during fabrication, can be used to provide a fingerprint that is challenging to reproduce. In this paper, we propose a simple optical technique to read unique information from nanometer-scale defects in 2D materials. Flaws created during crystal growth or fabrication lead to spatial variations in the bandgap of 2D materials that can be characterized through photoluminescence measurements. We show a simple setup involving an angle-adjustable transmission filter, simple optics and a CCD camera can capture spatially-dependent photoluminescence to produce complex maps of unique information from 2D monolayers. Atomic force microscopy is used to verify the origin of the optical signature measured, demonstrating that it results from nanometer-scale imperfections. This solution to optical identification with 2D materials could be employed as a robust security measure to prevent counterfeiting.

Improving accuracy of nanothermal measurements via spatially distributed scanning thermal microscope probes

Advances in material design and device miniaturization lead to physical properties that may significantly differ from the bulk ones. In particular, thermal transport is strongly affected when the device dimensions approach the mean free path of heat carriers. Scanning Thermal Microscopy (SThM) is arguably the best approach for probing nanoscale thermal properties with few tens of nm lateral resolution. Typical SThM probes based on microfabricated Pd resistive probes (PdRP) using a spatially distributed heater and a nanoscale tip in contact with the sample provide high sensitivity and operation in ambient, vacuum, and liquid environments. Although some aspects of the response of this sensor have been studied, both for static and dynamic measurements, here we build an analytical model of the PdRP sensor taking into account finite dimensions of the heater that improves the precision and stability of the quantitative measurements. In particular, we analyse the probe response for heat flowing through a tip to the sample and due to probe selfheating and theoretically and experimentally demonstrate that they can differ by more than 50%, hence introducing significant correction in the SThM measurements. Furthermore, we analyzed the effect of environmental parameters such as sample and microscope stage temperatures and laser illumination, which allowed reducing the experimental scatter by a factor of 10. Finally, varying these parameters, we measured absolute values of heat resistances and compared these to the model for both ambient and vacuum SThM operations, providing a comprehensive pathway improving the precision of the nanothermal measurements in SThM.

Probing thermal transport and layering in disk media using scanning thermal microscopy

With the advent of heat-assisted magnetic recording (HAMR) [1] the thermal transport properties of magnetic recording media have become a key performance characteristic.

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.