Self-Organized Superlattices in GaInAsSb Grown on Vicinal Substrates

Self-organized superlattices are observed in GaInAsSb epilayers grown lattice matched to vicinal GaSb substrates. The natural superlattice (NSL) is oriented at a slight angle of about 4{sup o} with respect to the vicinal (001) GaSb substrate. This vertical composition modulation is detected at the onset of growth. Layers in the NSL are continuous over the lateral extent of the substrate. Furthermore, the NSL persists throughout several microns of deposition. The NSLs have a period ranging from 10 to 30 nm, which is dependent on deposition temperature and GaInAsSb alloy composition. While the principle driving force for this type of phase separation is chemical, the mechanism for the self-organized microstructure is related to local strains associated with surface undulations. By using a substrate with surface undulations, the tilted NSL can be induced in layers with alloy compositions that normally do not exhibit this self-organized microstructure under typical growth conditions. These results underscore the complex interactions between compositional and morphological perturbations

Effect of Substrate Orientation on Phase Separation in Epitaxial GaInAsSb

The effect of substrate misorientation on phase separation in Ga{sub 1-x}In{sub x}As{sub y}Sb{sub 1-y} nominally lattice-matched to GaSb is reported. The layers were grown at 575 C by organometallic vapor phase epitaxy on vicinal (001) GaSb substrates, miscut 2{sup o} {yields} (-111)A, (1-11)B, or (101). Ga{sub 1-x}In{sub x}As{sub y}Sb{sub 1-y} (x {approx} 0.1, y {approx} 0.09) layers, which have 300-K photoluminescence (PL) peak emission at {approx}2.1 {micro}m, grow step-bunched and exhibit minimal phase separation. The full width at half maximum of 4-K PL spectra is slightly smaller at 7 meV for layers grown on substrates miscut toward (1-11)B compared to 9 meV for layers grown on substrates miscut toward (-1-11)A and (101). Ga{sub 1-x}In{sub x}As{sub y}Sb{sub 1-y} layers with higher alloy composition (0.16 {le} x {le} 0.19, 0.14 {ge} y {le} 0.17), which have 300-K PL peak emission at {approx}2.4 {micro}m, have significant phase separation. These layers are characterized by increased lattice constant variations and epitaxial tilt, broad PL spectra with significant band tailing, and strong contrast modulation in transmission electron microscopy. The degree of decomposition depends on substrate miscut direction: Ga{sub 1-x}In{sub x}As{sub y}Sb{sub 1-y} layers grown on (001) 2{sup o} {yields} (1-11)B substrates are more homogeneous than those grown on (001) 2{sup o} {yields} (-1-11)A and (001) 2{sup o} {yields} (101) substrates. The results are attributed to the smaller adatom diffusion length on substrates miscut toward (1-11)B

Subpicosecond carrier lifetime in GaAs grown by molecular beam epitaxy at low temperatures

Epitaxial GaAs grown by molecular beam epitaxy (MBE) at low substrate temperatures is observed to have a significantly shorter carrier lifetime than GaAs grown at normal substrate temperatures. Using femtosecond time‐resolved‐reflectance techniques, a sub‐picosecond (<0.4 ps) carrier lifetime has been measured for GaAs grown by MBE at ∼200°C and annealed at 600 °C. With the same material as a photoconductive switch we have measured electrical pulses with a full‐width at half‐maximum of 0.6 ps using the technique of electro‐optic sampling. Good responsivity for a photoconductive switch is observed, corresponding to a mobility of the photoexcited carriers of ∼120–150 cm2/V s. GaAs grown by MBE at 200 °C and annealed at 600 °C is also semi‐insulating, which results in a low dark current in the switch application. The combination of fast recombination lifetime, high carrier mobility, and high resistivity makes this material ideal for a number of subpicosecond photoconductive applications.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/71318/2/APPLAB-59-25-3276-1.pd

Phase-controlled, heterodyne laser-induced transient grating measurements of thermal transport properties in opaque material

The methodology for a heterodyned laser-induced transient thermal grating technique for non-contact, non-destructive measurements of thermal transport in opaque material is presented. Phase-controlled heterodyne detection allows us to isolate pure phase or amplitude transient grating signal contributions by varying the relative phase between reference and probe beams. The phase grating signal includes components associated with both transient reflectivity and surface displacement whereas the amplitude grating contribution is governed by transient reflectivity alone. By analyzing the latter with the two-dimensional thermal diffusion model, we extract the in-plane thermal diffusivity of the sample. Measurements on a 5 {\mu}m thick single crystal PbTe film yielded excellent agreement with the model over a range of grating periods from 1.6 to 2.8 {\mu}m. The measured thermal diffusivity of 1.3 \times 10-6 m2/s was found to be slightly lower than the bulk value.Comment: 19 pages, 6 figure

Evolution of Surface Structure and Phase Separation in GaInAsSb

Atomic force microscopy was used to study changes in the surface step structure of GaInAsSb layers with varying degrees of phase separation. The layers were grown by organometallic vapor phase epitaxy on (001) GaSb substrates with 2{sup o} miscut angles toward (-1-11)A, (1-11)B, and (101). Alloy decomposition was observed by contrast modulations in plan-view transmission electron microscopy, and broadening in x-ray diffraction and photoluminescence peaks. GaInAsSb layers with a minimal degree of phase separation exhibit a step-bunched step structure. A gradual degradation in the periodicity of the step structure is observed as the alloy decomposes into GaAs- and InSb-rich regions. The surface eventually develops trenches to accommodate the local strain associated with composition variations, which are on the order of a few percent. The surface composition is affected by substrate miscut angle, and although phase separation cannot be eliminated, its extent can be reduced by growing on substrates miscut toward (1-11)B

AlGaAsSb/GaSb Distributed Bragg Reflectors Grown by Organometallic Vapor Phase Epitaxy

The first AlGaAsSb/GaSb quarter-wave distributed Bragg reflectors grown by metallic vapor phase epitaxy are reported. The peak reflectance is 96% for a 10-period structure

TUNABLE LASER MEASUREMENTS OF WATER VAPOR TRANSITIONS IN THE VICINITY OF 5 μ\mum

This work was sponsored by the Department of the Air Force. $^{1}$ F. A. Blum, K.W. Nill, P.L. Kelley, A.R. Calawa and T.C. Harman, Science 177, 694 (1972). $^{2}$ W. S. Benedict and R.F. Calfee, Line Parameters for the 1.9 and 6.3 Micron Water Bands (Government Printing Office, Washington, D.C., 1967).Author Institution: Lincoln Laboratory, Massachusetts Institute of TechnologySelected line parameters of a number of infrared water vapor transitions in the 6.3 $\mu$m $\nu_{2}$ band have been measured using tunable semiconductor lasers with resolution better than $10^{-5}$$cm^{-1}$. The results further confirm earlier tunable laser work $^{1}$ which showed that high J rotational lines were substantially narrower than previously $estimated.^{2}$ The rotational quantum number J^{\prime\prime} of the lines observed range from 5 to 15. For all lines with $J^{\prime\prime} \geq 11$, the experimental widths have been found to be narrower than previous calculations, with the discrepancy becoming greater as J increases. For the measured lines where $J^{\prime\prime} \leq 8$, the experimental results still show some deviation from the calculated values. In addition, accurate values for pressure shifts, self-broadening, nitrogen and oxygen broadening coefficients are given for the first time. An experimental line shape study was also made on a number of lines and the shape was found to be Lorentzian over a range of up to three linewidths. A number of relative positions were also measured using a frequency scale generated by tuning the laser frequency through the transmission peaks of a Fabry-Perot interferometer. The data give further experimental checks on theoretical models for atmospheric transmission under a wide variety of atmospheric conditions

375‐GHz‐bandwidth photoconductive detector

We report the development of a new, integrable photoconductive detector, based on low‐temperature‐grown GaAs, that has a response time of 1.2 ps and a 3‐dB bandwidth of 375 GHz. The responsivity is 0.1 A/W. Signal amplitudes up to 6 V can be produced with virtually no degradation in response time.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/71199/2/APPLAB-59-16-1984-1.pd