J. Fraser

Ultraviolet photoenhanced wet etching of GaN in K2S2O8 solution

The mechanism of the UV photoenhanced wet etching of GaN is determined. The UV photoenhanced wet etching does not require an electrical contact to be made to the sample, and nitrides deposited on insulating substrates (such as sapphire) can be etched, unlike photoelectrochemical (PEC) wet etching. The present technique relies on adding an appropriate oxidizing agent, in this case, peroxydisulfate (S2O82−), to KOH solutions. In a similar mechanism to PEC wet etching, the regions of low defect density are preferentially etched, leaving regions of high electron recombination such as threading dislocations relatively intact. The threading dislocations may be physically broken off, either by stirring or by a postetch sonication of the sample in KOH solution. Smoothly etched surfaces can be obtained under the proper conditions. A noble metal mask acts in a catalytic manner, yielding etch rates approximately one order of magnitude greater than those observed using inert masks. The essential role of the free radi...

InAs self-assembled quantum-dot lasers grown on (100) InP

Five stacked layers of InAs quantum dots (QDs) embedded in quaternary InGaAsP are grown on (100) InP substrate to form a laser diode. The QD ensemble has a density of 1.5×1010 cm−2 and emits light at ∼1.6 μm at 77 K. Lasing wavelength and threshold current density can be shifted by changing the cavity length of the laser diode and the latter reaches a value as low as 49 A/cm2 at 77 K for a gate size of 2000 μm×150 μm. Temperature dependence of the threshold current is observed implying the presence of thermionic emission increasing with temperature.

Formation of visible light emitting porous GaAs micropatterns

Pore growth on n-type GaAs (100) can be initiated in 1 M HCl solution by electrochemical polarization of the material anodic to a critical potential value—the pore formation potential (PFP). At surface defects, however, the PFP is significantly lower (shifted cathodically). Focused ion beam, implantation of Si++ was used to create defined patterns in the substrate. At these implant sites, the growth of porous GaAs was selectively achieved by polarization below the overall PFP. From the porous GaAs patterns visible photoluminescence at green-yellow wavelengths can be observed. This technique, thus, allows the production of light emitting porous GaAs micropatterns of arbitrary shape by a direct writing process.

Predefined initiation of porous GaAs using focused ion beam surface sensitization

Pore formation on n-type GaAs(100) under anodic polarization in I M HCI has been studied. Focused ion beam implantation of Si 2+ into GaAs was used to write defined surface damage/implant patterns into the substrate. These implant sites represent initiation sites for pore growth and show pore formation at potentials significantly cathodic to the intact surface. Hence, pore growth within the patterns can be achieved selectively if anodic polarization is kept below the pore formation potential of the unimplanted surface. The results show that both the polarization potential and the implantation dose strongly influence the morphology and the photoluminescence behavior of the porous structures. Monte Carlo simulations of the implantation process revealed that the morphology of the etch process and its kinetics are both strongly connected to the implant/damage profile. Furthermore, it is demonstrated that green-light-emitting porous GaAs lift-off layers can be produced.

Formation and properties of porous GaAs

Porous structures on n-type GaAs (100) can be grown electrochemically in chloride-containing solutions. Crystallographic etching of the sample is a precursor stage of the attack. Polarization curves reveal the existanece of a critical onset potential for por formation (PFP). PFP is strongly dependent on the doping level of the sample and presence of surface defects. Good agreement between PFP and breakdown voltage of the space charge layer is found. Surface analysis by EDX, AES, and XPS show that the porous structure consists mainly of GaAs and that anion uptake in the structure can only observed after attackhas been initiated. Photoluminescence measurements reveal (under certain conditions) visible light emission from the porous structure.

Surface topology of GaAs(100) after focused ion beam implantation of Si

GaAs(100) was implanted with Si++ doses ranging from 3×1013 to 3×1016 cm−2 using a focused ion beam. The surface topology and roughness of implanted lines and squares was studied by atomic force microscopy. Above a threshold dose, protrusions of the ion beam treated areas in the range of 1–15 nm in heights and an increase in surface roughness were found. The height of the protrusions and surface roughness increase with increasing implantation dose up to a saturation level. Both the onset of substrate bulging and saturation of the effect are both dependent on the linewidth of the implant. Different causes for the protrusions are discussed. From Monte Carlo simulations, it is deduced that the volume expansion is most likely due to the creation of vacancies during implantation.

Influence of the Anodic Oxide Film on Pitting of Iron

The fundamental processes leading to the initiation of pitting on pure Fe in acetate and borate solutions containing halide ions are discussed. The results of potentiostatic step experiments show that pit initiation is associated with a critical stage in the development of the passive oxide film, likely corresponding to a certain critical oxide thickness. The critical oxide thickness depends on the identity (chloride or bromide) and the concentration of the halide ion in solution, and on the supporting electrolyte, but not on the anodic potential. Acetate is shown to inhibit pitting

Selective growth of GaN on a SiC substrate patterned with an AlN seed layer by ammonia molecular-beam epitaxy

Highly selective growth of GaN on 4H–SiC using the SiC substrate as a pseudomask has been demonstrated using the ammonia molecular-beam-epitaxy technique. A total lack of nucleation on the bare SiC surface was observed under typical GaN growth conditions. The nucleation of the GaN layer occurred preferentially from a patterned thin (300 A) AlN seed layer, which had been predeposited on the SiC surface using the magnetron-sputter-epitaxy technique and patterned into parallel stripes by photolithography and chemically assisted ion-beam etching. Evidence of lateral overgrowth was observed by scanning electron microscopy and x-ray diffraction studies. The GaN stripes grown show extremely smooth side facets due to the lateral growth.

Selective area growth of GaN on SiC substrate by ammonia-source MBE

The feasibility of selective area growth of GaN by ammonia-MBE has been demonstrated on SiC substrates. Under typical growth conditions for ammonia-MBE, GaN was unable to nucleate on the bare SiC surface. However, GaN nucleation could occur instantly if the SiC surface was first seeded with a thin (300 A) AlN layer prepared by magnetron sputter epitaxy. Thus, GaN growth could occur selectively from a patterned, pre-deposited thin AlN seed layer, and effectively utilizing the exposed SiC surface as a pseudo mask. Evidence of lateral overgrowth was observed by scanning electron microscopy and X-ray diffraction studies. The selectively grown GaN patterns exhibited a strong tendency to form {1011} or {1012} type of facets with excellent smoothness.

Multimodal plasmonic nanosensor for the detection of pathogenic bacteria

Multi-modal sensing scheme significantly improves the detection accuracy but can also introduce extra complexity in the overall design of the sensor. We overcome this difficulty by utilizing the plasmonic properties of metallic nanoparticles. In this study, we will present a simple dual optical sensing mechanism which harvests signals of the resonantly excited metallic nanostructure in the form of surface enhanced Raman scattering (SERS) and resonant Rayleigh scattering. Silver and gold nanoparticles labeled with appropriate antibodies act as signal transduction units and upon exposure to the targeted pathogen render the targeted species optically active. We demonstrate that detection of a single pathogen cell is easily attainable with the dual detection scheme. Furthermore, we explore the markedly different SERS intensity observed from the use of two very different antibody recognition units during the pathogen labeling process.