Valley current characterization of high current density resonant tunnelling diodes for terahertz-wave applications

We report valley current characterisation of high current density InGaAs/AlAs/InP resonant tunnelling diodes (RTDs) grown by metal-organic vapour phase epitaxy (MOVPE) for THz emission, with a view to investigate the origin of the valley current and optimize device performance. By applying a dual-pass fabrication technique, we are able to measure the RTD I-V characteristic for different perimeter/area ratios, which uniquely allows us to investigate the contribution of leakage current to the valley current and its effect on the PVCR from a single device. Temperature dependent (20 – 300 K) characteristics for a device are critically analysed and the effect of temperature on the maximum extractable power (PMAX) and the negative differential conductance (NDC) of the device is investigated. By performing theoretical modelling, we are able to explore the effect of typical variations in structural composition during the growth process on the tunnelling properties of the device, and hence the device performance

Study of electro-absorption effects in 1300nm In(Ga)As/GaAs quantum dot materials

No abstract available

A GaAs-based self-aligned stripe distributed feedback laser

We demonstrate operation of a GaAs-based self-aligned stripe (SAS) distributed feedback (DFB) laser. In this structure, a first order GaInP/GaAs index-coupled DFB grating is built within the p-doped AlGaAs layer between the active region and the n-doped GaInP opto-electronic confinement layer of a SAS laser structure. In this process no Al-containing layers are exposed to atmosphere prior to overgrowth. The use of AlGaAs cladding affords the luxury of full flexibility in upper cladding design, which proved necessary due to limitations imposed by the grating infill and overgrowth with the GaInP current block layer. Resultant devices exhibit single-mode lasing with high side-mode-suppression of >40 dB over the temperature range 20 °C–70 °C. The experimentally determined optical profile and grating confinement correlate well with those simulated using Fimmwave

Quantum well and dot self-aligned stripe lasers utilizing an InGaP optoelectronic confinement layer

We demonstrate and study a novel process for fabrication of GaAs-based self-aligned lasers based upon a single over-growth. A lattice-matched n-doped InGaP layer is utilized for both electrical and optical confinements. Single-lateral-mode emission is demonstrated initially from an In0.17Ga0.83 As double quantum well laser emitting similar to 980 nm. We then apply the fabrication technique to a quantum dot laser emitting similar to 1300 nm. Furthermore, we analyze the breakdown mechanism in our devices and discuss the limitations of index guiding in our structures

Simulation of Broad Spectral Bandwidth Emitters at 1060 nm for Optical Coherence Tomography

The simulation of broad spectral bandwidth light sources (semiconductor optical amplifiers (SOA) and superluminescent diodes (SLD)) for application in ophthalmic optical coherence tomography is reported. The device requirements and origin of key device parameters are outlined, and a range of single and double InGaAs/GaAs quantum well (QW) active elements are simulated with a view to application in different OCT embodiments. We confirm that utilising higher order optical transitions is beneficial for single QW SOAs, but may introduce deleterious spectral modulation in SLDs. We show how an addition QW may be introduced to eliminate this spectral modulation, but that this results in a reduction of the gain spectrum width. We go on to explore double QW structures where the roles of the two QWs are reversed, with the narrow QW providing long wavelength emission and gain. We show how this modification in the density of states results in a significant increase in gain-spectrum width for a given current. © (2016) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only

Optimization of High Current Density Resonant Tunneling Diodes for Terahertz Emitters

We discuss the numerical simulation of high current density InGaAs/AlAs/InP resonant tunneling diodes with a view to their optimization for application as THz emitters. We introduce a figure of merit based upon the ratio of maximum extractable THz power and the electrical power developed in the chip. The aim being to develop high efficiency emitters as output power is presently limited by catastrophic failure. A description of the interplay of key parameters follows. We propose an optimized structure utilizing thin barriers paired with a comparatively wide quantum well

Book Reviews: MARRINER-TOMEY, A. (1989). Nursing Theorists and Their Work, 2nd ed. St. Louis: Mosby

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/68645/2/10.1177_089431849000300211.pd

Direct seeding of chenopod shrubs for saltland and rangeland environments

There are currently two ways of establishing chenopod shrubs: sowing from seed using a niche seeder, or planting nursery-raised seedlings with a tree planter. Planting seedlings is the more reliable method, but is relatively expensive (in excess of $450 per hectare). On the other hand, direct seeding using the specialised “niche seeder” is much less expensive ($100-150 per hectare), but is also less reliable. This project aimed to investigate alternative methods of direct seeding chenopod shrubs for saltland and rangeland areas by developing a greater understanding of their seed biology and agronomic requirements. Our aspiration was that shrubs should be established using more conventional farm machinery. This bulletin reports on a combination of seed biology and agronomic research to develop reliable, low-cost direct seeding options for chenopod shrubs. Experiments into the impact of changing environmental conditions on seeds were studied in the laboratory, and field experiments were conducted to test the applicability of these insights in the field using conventional modified farm seeding machinery. As a result of this work, a successful direct seeding package using farm seeding equipment (modified for wide row spacings and depth control) was developed for Atriplex nummularia (old man saltbush), the most widely planted saltbush species across southern Australia. The nine key elements of the package are: 1. Select suitable paddocks for introduction of new shrubs 2. Prepare a weed-free seedbed using two knockdown herbicide applications (4-6 weeks and 1-2 weeks before seeding) and commence control of rabbits and kangaroos 3. Sow the best seed, by ensuring: a. Large fruits, with a high proportion of viable seeds, have been selected b. Seed is of subspecies nummularia (not subsp. spathulata) c. Fruits have been harvested within the previous six months and stored in a cool, dry environment d. Bracts are retained around the seeds 4. Sow into moisture in late winter - early spring (depending on district) a. If the area to be sown is waterlogged, defer sowing until later in spring b. If insufficient soil moisture, defer sowing until the following year 5. Use a sowing rate of ~10 fruits/m (if germination rate is 15%) to provide at least one plant for every 2 m of row; use higher rates for seed of lower germination 6. Set the seeder up to sow into furrows with trailing press wheels 7. Sow to a depth of 5-10 mm (very critical) 8. Control weeds and pests (insects, mites, kangaroos and rabbits) 9. Defer grazing until seedlings are well established This establishment method has also been shown to work for Rhagodia preissii (mallee saltbush). This project was not able to develop reliable direct seeding packages for other Atriplex species, including A. amnicola and A. undulata. Further work is needed to understand the triggers for their germination, before these species can be direct-seeded with conventional machinery. Direct sowing of M. brevifolia and M. pyramidata appears to be problematic in much of southern Australia, due to their requirement for temperatures >30°C for germination, which do not occur within the normal winter growing season. An exception to this would be areas with more reliable summer rainfall, such as northern New South Wales, where sowing could be deferred until late spring-early summer. An alternative strategy for establishing M. brevifolia, is to encourage natural recruitment of seedlings from seed produced on surrounding bushes (if it is already present in the area), or to transplant a low density of nursery-raised seedlings, which could then act as a seed source for natural recruitment (if it is not already present)

Strain balancing of MOVPE InAs/GaAs quantum dots using GaAs0.8P0.2

MOVPE growth of stacked InAs/ GaAs QDs with and without GaAs 0.8 P 0.2 strain balancing layers has been studied. The GaAsP layers reduce the accumulated strain whilst maintaining the electrical characteristics. This should enable closer stacking of QD layers leading to higher gain and improved laser performance