Charles W. Seabury

Growth and doping of GaAsSb via metalorganic chemical vapor deposition for InP heterojunction bipolar transistors

GaAsSb is a low band gap, lattice matched to InP, alternative to GaInAs. Growth and doping using diethyltellurium and carbon tetrachloride were investigated. Hole concentrations up to 1.3×1020 cm−3 have been achieved in as‐grown carbon‐doped GaAsSb [i.e., no postgrowth annealing was necessary for dopant activation, a key requirement for n‐p‐n heterojunction bipolar transistor (HBT) structures]. This is a sevenfold improvement over the best carbon‐doped InGaAs reported by metalorganic chemical vapor deposition. Hall measurements indicate that GaAsSb’s hole mobility is 55%–60% of GaInAs’s, for a given carrier concentration. InP HBTs with carbon‐doped GaAsSb base are demonstrated.

Mechanical modeling of fluid-driven polymer lenses

A finite-element model (FEM) is employed to study the pressure response of deformable elastic membranes used as tunable optical elements. The model is capable of determining in situ both the modulus and the prestrain from a measurement of peak deflection versus pressure. Given accurate values for modulus and prestrain, it is shown that the two parameters of a standard optical shape function (radius of curvature and conic constant) can be accurately predicted. The effects of prestrain in polydimethylsiloxane (PDMS) membranes are investigated in detail. It was found that prestrain reduces the sensitivity of the membrane shape to the details of the edge clamping. It also reduces the variation of the conic constant with changes in curvature. Thus the ability to control the prestrain as well as thickness and modulus is important to developing robust optical designs based on fluid-driven polymer lenses.

Thin film resonators as mass transducers for explosives detection

Sub-miniature thin film resonators (TFR) operating near 2 GHz are being developed as mass transducers for high- sensitivity vapor detection. The TFR sensor are coated with species selective vapor absorbing polymers to develop pattern response to the target species for detection and identification. Eight member arrays of TFR sensor have been fabricated and tested for the detection of characteristics explosive vapors including trinitrotoluene, dinitrotoluene, and dinitrobenzene. The TFR sensor use aluminum nitride as the active piezoelectric element and have ben fabricated with resonator quality factors greater than 200. Response patterns and sensitivity measurements are being made using pure vapors, water solutions of the target species, and solid contaminated with the target vapor species.

Thin film resonators for TNT vapor detection

Progress on the development of a miniature mass transducer for low level vapor detection is described. Thin-film bulk acoustic wave resonators (TFRs) are fabricated with AlN as the piezoelectric layer onto an acoustically reflecting solid substrate. Qs of 200 at 2 GHz have been measured. Comparison of the sensitivity of these 2 GHz TFRs with 250 MHz surface acoustic wave resonators is made. A set of TFRs with species selective coatings has the potential of detecting TNT vapor at the sub PPT level. Detection of 5 ppb TNT with a signal to noise of 200:1 is demonstrated.

Carbon doped InP/GaAsSb HBTs via MOCVD

Heterojunction bipolar transistors (HBTs) need heavily doped p-type regions. However, at high concentrations (>10/sup 19/ cm/sup -3/), most p-type dopants diffuse into other regions of the device, ruining performance and preventing stable, reliable operation. The discovery that carbon as a p-type dopant does not significantly diffuse has lead to reliable operation of GaAs-based HBTs. For the first time, carbon-doped double heterojunction InP/GaAsSb/InP HBTs have been fabricated (emitter area of approximately 70/spl times/70 /spl mu/m/sup 2/). The base doping was P=4/spl times/10/sup 19/ cm/sup -3/. Base thickness was varied from 350 /spl Aring/ to 1500 /spl Aring/, giving sheet resistances of 850 to 200 /spl Omega//sq. The devices had DC current gains ranging from 5 to 45 that scaled sheet resistance. While these gains are low, they are comparable to the best InP/GaInAs HBTs fabricated, where the gain is limited due to Auger recombination in highly doped bases. BV/sub ceo/ is on the order of 6 volts. The typical turn-on voltage for both emitter-base and base-collector junctions is approximately 0.4 V, even with the emitter-base junction grown nominally abrupt: i.e., no undoped setbacks or intentionally graded layers were used for these structures. The authors report on the growth, fabrication, and properties of the MOCVD-grown carbon-doped InP/GaAsSb HBTs.