The UV photodissociations of dimethyltelluride (CH3)2Te and diethyltelluride (C2Hs)2Te have been investigated by state-selective detection. Nascent populations of Te(5p Pjw) atoms produced by the photodissociation process were monitored using laser-induced fluorescence following excimer laser (248 nm and 193 nm) photolysis. Experimental results for both molecules indicate that the photolysis at 248 nm produces Te atoms via a single photon process and that the liberated Te atoms populate only the 5p3P2 state. The photolysis of diethyltelluride at 193 nm has also been found to be a single photon process producing Te atoms distributed over the3P2,3P1, and Pø states in a ratio of 13:2:1, respectively.

Excimer laser-assisted MOVPE has been used to deposit thin films of CdTe (111) on GaAs (100) substrates. Auger spectroscopy indicates that the films are stoichiometric and that carbon and oxygen contamination are below the levels of detectability. Good crystalline quality of the layers is confirmed by x-ray diffractometry. Growth rates up to 2 /μm/hr have been recorded in real time using time-resolved reflectivity. The CdTe growth rate is found to have a linear dependence on reactant partial pressure and laser average power, but was independent of substrate temperature in the range of 150° C to 300* C. Laser-induced fluorescence spectroscopy has been used to determine that photolysis of diethyltellurium at 248 nm is a one photon process which produces ground state Te atoms. The flux of reactive species to the substrate surface has been probed by mass spectrometry. Evidence for the generation of monoethyltelluride, either by recombination or direct photolysis, is presented.

A compact, differential, hydrogen-specific sensor based on the lattice expansion of LaAl0.3Ni4.7 metal hydride thin films has been fabricated and characterized. Characterization of LaAl0.3Ni4.7 films performed using a capacitance dilatometer revealed that the lattice expansion was proportional to the partial pressure of H2 over the range 0.01–1.3atm used in this experiment. The films were mechanically robust to cycling between vacuum and partial pressures of H2 up to 1.3atm and were not poisoned by exposure to atmosphere's containing up to 24% carbon monoxide. A wafer level process has been established for the fabrication of the differential hydrogen sensor which includes both an active LaAl0.3Ni4.7 sensing capacitor and an inert Au reference capacitor. A minimum sensitivity of 400ppm hydrogen is calculated for the differential device.

Manipulation of electron spin is a critical component of many proposed semiconductor spintronic devices. One promising approach utilizes the Rashba effect by which an applied electric field can be used to reduce the spin lifetime or rotate spin orientation through spin–orbit interaction. The large spin–orbit interaction needed for this technique to be effective typically leads to fast spin relaxation through precessional decay, which may severely limit device architectures and functionalities. An exception arises in [110]-oriented heterostructures where the crystal magnetic field associated with bulk inversion asymmetry lies along the growth direction and in which case spins oriented along the growth direction do not precess. These considerations have led to a recent proposal of a spin-FET that incorporates a [110]-oriented, gate-controlled InAs quantum well channel. We report measurements of the electron spin lifetime as a function of applied electric field in a [110]-InAs 2DES. Measurements made using an ultrafast, mid-IR pump-probe technique indicate that the spin lifetime can be reduced from its maximum to minimum value over a range of less than 0.2V per quantum well at room temperature.

We report the demonstration of room temperature gate control over the electron spin dynamics using the Rashba effect in a (110) InAs∕AlSb two-dimensional electron gas. Our calculations predict that the strong spin–orbit interaction in this system produces pseudomagnetic fields exceeding 1 T when only 140 mV is applied across a single quantum well. Using this large pseudomagnetic field, we demonstrate low-power spin manipulation on a picosecond time scale. Our findings are promising for the prospect of nonmagnetic low-power, high-speed spintronics.

Growth of high-quality Sb-based resonant tunneling diodes in the (110) orientation is demonstrated. The room-temperature current–voltage characteristics of the diodes are studied as a function of GaSb well width. Electronic band structure calculations including spin support the conclusion that the position of the GaSb light hole band with respect to the InAs conduction band is responsible for the strength of the negative differential resistance observed. The spin splitting of the heavy hole band is calculated to be larger than the light hole band suggesting that the observation of negative differential resistance may not be necessary or desirable for spin transport in these structures.

We report a prototype side-gated asymmetric resonant interband tunneling device (RITD) fabricated with an AlSb∕InAs∕GaSb∕AlSb heterostructure for Rashba spin filter applications. This device features independent control gates along the sides of an RITD mesa structure that can be used to provide an electric field orthogonal to the resonant tunneling current. The distribution of lateral wave vectors for electrons approaching the RITD active layers obtains a nonzero average through the application of the orthogonal electric field, a requirement for Rashba spin filter operation. Measured current–voltage curves show a modulation of the tunneling current with a side gate bias, demonstrating lateral electric-field projection into the resonant tunneling mesa structure. The prototype device was fabricated to submicron dimensions utilizing a conformal and scalable processing scheme.

W-band direct detection circuits have been designed and fabricated for use in a passive millimeter wave camera. The circuits are based on the recently developed Sb-heterostructure diode. We measure record voltage responsivities in test circuits, up to 8,000 mV/mW from 75 to 93 GHz, with input power from -50 to -30 dBm. Performance was similar in an actual camera frequency processor board with 128 tuned channels. 72% of detectors showed responsivity at or above 6,000 mV/mW and 3% of channels were above 10,000 mV/mW. Since tens of thousands of Sb-heterostructure diodes can be reproducibly and inexpensively fabricated, this demonstrates for the first time the feasibility of large-scale detector arrays utilizing zero bias direct detection circuitry.

We report on the development of a compensated, integrated, differential capacitive hydrogen sensor based on the lattice expansion of LaNi 5-x M x thin films.Fabricated devices demonstrate a stable response to hydrogen as measured over 0.1 to 1 atm.We have measured the intrinsic response time of LaNi 4.4 Al 0.6 films as τ 90 =10 s.We have determined a Sieverts type dependence of the LaNi 4.4 Al 0.6 film response to hydrogen as a function of partial pressure.Additionally, we have shown the integrated sensor sensitivity to be better than 0.01 atm with the ability to detect hydrogen at pressures > 2 atm.

Nearly lattice-matched InAs/AlSb/GaSb-based heterostructure backward diodes for zero-bias millimeter wave detection were fabricated and measured. A record-high curvature, gamma = 39.1 V-1, at zero bias was measured. On-wafer sensitivity measurements from 1 to 110 GHz gave a record-high average sensitivity of 3687 V/W for zero-bias operation. Further enhancement of detector sensitivity was observed with applied dc bias, with a sensitivity of 7996 V/W obtained for a 0.9 muA bias. Extrapolating the conjugately-matched measured sensitivity suggests that 1000 V/W should be achievable at a record-high 541 GHz. The temperature dependence of detector sensitivity was evaluated from measured dc current-voltage characteristics and gave expected sensitivities ranging from 3910 V/W at 293 K to 7740 V/W at 4.2 K.

A 25 times enhancement of the electron spin lifetime is observed in a [110] InAs/GaSb superlattice relative to the corresponding [001] superlattice, an effect that is primarily attributed to suppression of native interface asymmetry.

A method for measuring the V/III ratio for epitaxial growth using threshold photoemission is demonstrated. The variation in photoyield observed during reconstruction changes of InAs (001), attributed to the variation of As dimer concentration on the surface, can be used to determine the As incorporation rate, and hence the V/III ratio in situ, in a manner analogous to As-limited reflection high energy electron diffraction oscillation measurements. Although As-limited photoyield oscillations are observed and reported here, the photoyield method for determining the V/III ratio is not dependent on the measurement of oscillations. The photoyield measurement approach is still applicable at high temperatures where step flow growth competes with island nucleation and oscillations are difficult to resolve.

Summary form only given. Molecular beam epitaxial growth on III-V substrates has been optimized for [100] oriented growth. However, spintronic devices may benefit from growth of quantum structures on [110] oriented substrates because the D'yakanov-Perel spin relaxation mechanism is suppressed or absent in this orientation for 2-D systems. Consistent with this hypothesis, we have recently measured a significant enhancement of the spin relaxation time in a [110]-oriented, short period, InAs/GaSb superlattice relative to a similar [001]-oriented structure. The spin dynamics were measured in the two superlattices using 100 fs mid-infrared pulses from a synchronously pumped PPLN optical parametric oscillator and polarization-sensitive, differential transmission. Results at /spl sim/120 K, indicate that the spin relaxation time in the [110] superlattice is twenty times longer than that measured for the [001] structure. We are currently investigating the growth and optimization of 6.1 /spl Aring/ heterostructures on InAs [110] and GaSb [110] oriented substrates. We have noted interesting differences in the growth behavior between the [100] and [110] orientations with respect to group V dependence and antimonide on arsenide terminated interfaces. In addition, the bandgap of corresponding InAs/GaSb superlattices are consistently observed at much lower energies on [110] versus [100] orientations. The origin of these differences will be discussed with reference to band structure calculations.

Combining theoretical and experimental methods, we investigate the influence of group V fluxes of As2 and Sb2 on the InAs(001) surface. We find that equilibrated surfaces under As2 flux change their surface stoichiometry continuously for InAs in the technologically relevant α2(2×4)-β2(2×4) reconstruction region and that the As-dimer density increases with increasing As2 flux. The change of the surface morphology under Sb2 exposure is also studied and discussed specifically in the context of interface formation. The existence of a common α2(2×4) reconstruction allows for the possibility of keeping the In sublattice unchanged when switching from As2 to Sb2 flux.

The structure of InAs(001)-(2×4) surfaces equilibrated under typical MBE conditions is studied by scanning tunneling microscopy (STM). Depending on the magnitude of the As flux, typical surfaces are found to contain a mixture of α2(2×4) and β2(2×4) reconstructions. The relative populations of the α2 and β2 reconstructions are found to depend on substrate temperature and the magnitude of the As flux. The atomic-scale details of the reconstructed units on these mixed-phase surfaces are definitively determined by comparing atomic-resolution dual-bias STM images to first-principles calculations. The imaging mechanism for revealing atomic-scale details, particularly the trench dimer, is found to be qualitatively similar to that for GaAs, although the effect is less pronounced. Additionally, a significant population of ad-atom related structures are observed on quenched surfaces, apparently unrelated to any equilibrium ad-atom population.

Microscopic mechanisms of the (2x4) to (4x2) surface phase transition on InAs(001) are identified using a combination of theoretical and experimental methods. Two distinct transition stages are found, both rate limited by As-2 desorption. The unusually high prefactors observed experimentally are traced back to the microscopic As-2 desorption processes. Calculated interactions between As dimers explain the observed disorder of the mixed alpha2(2x4) and beta2(2x4) reconstructions and are not responsible for observed first order behavior of the transition.