Jay S. Brown

Electron cyclotron resonance etching characteristics of GaN in SiCl4/Ar

Electron cyclotron resonance (ECR) plasma etching characteristics of gallium nitride (GaN) are investigated using low pressure (4–10 mTorr) SiCl4/Ar ECR discharges. The purpose of this effort is to examine the dry etching processes of GaN that do not require hydrogen, which is known to cause carrier compensation in GaN. A maximum etching rate of 960 A/min and good surface morphologies are obtained. The etch rate is found to increase near‐linearly with increasing dc bias, and a minimum dc bias of 100 V is required to initiate etching. Enhanced etching rates are obtained as the fraction of active chemical etchant species (SiCl4) in the discharge is increased. We have also found that the material quality significantly affects the etch rate. The latter decreases with x‐ray rocking curve half‐width and increases with defect density.

Single bump, two-color quantum dot camera

The authors report a two-color, colocated quantum dot based imaging system used to take multicolor images using a single focal plane array (FPA). The dots-in-a-well (DWELL) detectors consist of an active region composed of InAs quantum dots embedded in In.15Ga.85As quantum wells. DWELL samples were grown using molecular beam epitaxy and fabricated into 320×256 focal plane arrays with indium bumps. The FPA was then hybridized to an Indigo ISC9705 readout circuit and tested. Calibrated blackbody measurements at a device temperature of 77K yield midwave infrared and long wave infrared noise equivalent difference in temperature of ∼55 and 70mK.

Comparison of Quantum Dots-in-a-Double-Well and Quantum Dots-in-a-Well Focal Plane Arrays in the Long-Wave Infrared

Our previous research has reported on the development of the first generation of quantum dots-in-a-well (DWELL) focal plane arrays (FPAs), which are based on InAs quantum dots (QDs) embedded in an InGaAs well having GaAs barriers, which have demonstrated spectral tunability via an externally applied bias voltage. More recently, technologies in DWELL devices have been further advanced by embedding InAs QDs in InGaAs and GaAs double wells with AlGaAs barriers, leading to a less strained InAs/InGaAs/GaAs/AlGaAs heterostructure. These lower strain quantum dots-in-a-double-well devices exhibit lower dark current than the previous generation DWELL devices while still demonstrating spectral tunability. This paper compares two different configurations of double DWELL (DDWELL) FPAs to a previous generation DWELL detector and to a commercially available quantum well infrared photodetector (QWIP). All four devices are 320 × 256 pixel FPAs that have been fabricated and hybridized with an Indigo 9705 read-out integrated circuit. Radiometric characterization, average array responsivity, array uniformity and measured noise equivalent temperature difference for all four devices is computed and compared at 60 K. Overall, the DDWELL devices had lower noise equivalent temperature difference and higher uniformity than the first-generation DWELL devices, although the commercially available QWIP has demonstrated the best performance.

Comparison of Long-Wave Infrared Quantum-Dots-in-a-Well and Quantum-Well Focal Plane Arrays

This paper reports on a comparison between a commercially available quantum-well infrared focal plane array (FPA) and a custom quantum-dot (QD)-in-a-well (DWELL) infrared FPA in the long-wave infrared (LWIR). The DWELL detectors consist of an active region composed of InAs QDs embedded in In0.15Ga0.85As quantum wells. DWELL samples were grown using molecular beam epitaxy and fabricated into 320 times 256 pixels FPA with a flip-chip indium bump technique. Both the DWELL and QmagiQ commercial quantum-well detector were hybridized to an Indigo ISC9705 readout circuit and tested in the same camera system. Calibrated blackbody measurements at a device temperature of 60 K with LWIR optics yield a noise equivalent change in temperature of 17 mK and 91 mK for quantum-well and DWELL FPAs operating at 0.95- and 0.58-V biases, respectively. The comparison of the DWELL and quantum-well FPA when imaging a 35degC black body showed that the DWELL had a signal-to-noise ratio of 124 while the quantum-well FPA showed 1961. As well, the quantum-well FPA showed a higher collection efficiency of 1.3 compared to the DWELL.

Epitaxial growth of 20 nm InAs and GaAs quantum dots on GaAs through block copolymer templated SiO2 masks

We report on selective area growth of InAs and GaAs quantum dots (QDs) on GaAs through ∼20 nm SiO2 windows prepared by block copolymer lithography. We discuss the mechanisms of growth through these masks, highlighting the variation of the resulting morphology (dot size, spacing, uniformity, and areal density) as a function of growth parameters. We have obtained highly uniform arrays of InAs and GaAs QDs with mean diameters and areal densities of 20.6 nm and 1×1011 cm−2, respectively. We have also investigated the optical characteristics of these QDs as a function of temperature and drawn correlations between the optical response and their crystalline quality.

Ohmic contacts to Si-implanted and un-implanted n-type GaN

We report on ohmic contacts to Si-implanted and un-implanted n-type GaN on sapphire. A ring shaped contact design avoids the need to isolate the contact structures by additional implantation or etching. Metal layers of Al and Ti/Al were investigated. On un-implanted GaN, post metalization annealing was performed in an RTA for 30 seconds in N{sub 2} at 700, 800, and 900 C. A minimum specific contact resistance (r{sub c}) of 1.4{times}10{sup -5} {Omega}{minus}cm{sup 2} was measured for Ti/Al at an annealing temperature of 800 C. Although these values are reasonably low, variations of 95% in specific contact resistance were measured within a 500 {mu}m distance on the wafer. These results are most likely caused by the presence of compensating hydrogen. Specific contact resistance variation was reduced from 95 to 10% by annealing at 900 C prior to metalization. On Si-implanted GaN, un-annealed ohmic contacts were formed with Ti/Al metalization. The implant activation anneal of 1120 C generates nitrogen vacancies that leave the surface heavily n-type, which makes un-annealed ohmic contacts with low contact resistivity possible.

Electron cyclotron resonance etching characteristics of GaN in plasmas with and without hydrogen

Electron cyclotron resonance (ECR) plasma etching characteristics of gallium nitride (GaN) are investigated using low pressure (4--10 mTorr) SiCl{sub 4}/Ar and Cl{sub 2}/H{sub 2}/Ar ECR discharges. The purpose of this effort is to develop a dry etching process for making laser mirrors on GaN and to examine dry etching processes of GaN that do not require hydrogen, which is known to cause carrier compensation in GaN. The etch rate is found to increase near-linearly with increasing DC bias, and a minimum DC bias of 100V is required to initiate etching in SiCl{sub 4}/Ar. The authors have also found that the material quality significantly affects the etch rate. The latter decreases with x-ray rocking curve half-width and increases with defect density. A reasonable etch rate of 660 {angstrom}/min and good surface morphologies obtained in SiCl{sub 4}/Ar ECR etching make this process suitable for gate recess of an FET. An etch rate of 5,270 {angstrom}/min has been achieved in Cl{sub 2}/H{sub 2}/Ar plasmas. This is the highest reported etch rate of GaN so far. The smooth and vertical etch sidewalls (etch to mask selectivity of 16 is obtained) make this process promising for dry-etched laser mirrors on GaN.

The Establishment Of Markets For Owner-Occupation Within Public Sector Communities

Urban regeneration strategies in the UK have placed considerable emphasis on the development of homeownership and particularly low-cost homeownership. The paper assesses the long-term viability of local homeownership initiatives in public sector communities by reference to case studies in four UK cities. In particular, the research chronicles the housing market experience of localized initiatives and assesses whether in the long term a sustainable market is achieved. The research finds similar market characteristics for both public sector community and inner city homeownership initiatives. Internal demand and externalities create fragile markets. Initial purchasers of these houses acquired in general a poor long-term investment relative to opportunities elsewhere in the housing market. The research suggests that private housing developments within public sector communities are at least as successful as inner city estates for sale. However, while resale markets have developed in most of these estates prices are not high enough to establish sustainable markets.

Demonstration of a two-color 320 x 256 quantum dots-in-a-well focal plane array

We report the first successful demonstration of a two color, co-located infrared focal plane array based on novel InAs/InGaAs quantum dots-in-a-well photodetectors. Two distinct responses (¿1~4.5um and ¿2~8.5um) were observed under 300K f2 irradiance.

Demonstration of a two color 320 × 256 Quantum Dots-in-a well focal plane array

We report the first successful demonstration of a two color, co-located infrared focal plane array based on novel InAs/InGaAs quantum dots-in-a-well photodetectors. Two distinct responses (lambda iota ~ 4.5 mum and lambda 2 ~8.5 mum) were observed under 300 Kf2 irradiance.