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Register a new userIssues related to the usage of nitrogen as carrier gas for the MOVPE growth of GaSb/InAs heterostructures on InAs pseudosubstrates
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Added by
Emanuele Pelucchi Dr
Publication date
2016
fields
Physics
and research's language is
English
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GaSb/InAs/GaSb layer stacks have been grown on InAs metamorphic substrates (pseudosubstrates) by MOVPE, using nitrogen as major carrier gas. We demonstrate that GaSb growth by nitrogen MOVPE on InAs metamorphic substrates (and on InAs wafers) is possible in a very narrow range of growth parameters. As demonstrators, GaSb/InAs/GaSb structures were grown for electron mobility tests, obtaining (unintentional) 2D electron gas densities in the order of 9/5x10e12 cm(e-2) and mobilities up to 1.2/1.8x10e4 cm(e2)/Vs at room and liquid nitrogen temperature respectively. We show that it is beneficial to have some hydrogen in the carrier gas mixture for GaSb growth to achieve good crystal quality, morphology and electrical properties. Furthermore, an unexpected and previously unreported decomposition process of GaSb is observed at relatively low growth temperatures under the supply of the precursors for InAs epitaxial overgrowth. This nevertheless gets suppressed at even lower growth temperatures.
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We present a detailed x-ray diffraction study of the strain in InAs/GaSb superlattices grown by molecular beam epitaxy. The superlattices were grown with either InSb or GaAs interfaces. We show that the superlattice morphology, either planar or nanostructured, is dependent on the chemical bonds at the heterointerfaces. In both cases, the misfit strain has been determined for the superlattice layers and the interfaces. We also determined how the magnitude and sign of this strain is crucial in governing the morphology of the superlattice. Our analysis suggests that the growth of self-assembled nanostructures may be extended to many systems generally thought to have too small a lattice mismatch.
The cross-plane thermal conductivity of a type II InAs/GaSb superlattice (T2SL) is measured from 13 K to 300 K using the 3{omega} method. Thermal conductivity is reduced by up to 2 orders of magnitude relative to the GaSb bulk substrate. The low thermal conductivity of around 1-8 W/mcdotK may serve as an advantage for thermoelectric applications at low temperatures, while presenting a challenge for T2SL quantum cascade lasers and high power light emitting diodes. We introduce a power-law approximation to model non-linearities in the thermal conductivity, resulting in increased or decreased peak temperature for negative or positive exponents, respectively.
We study quantum point contacts in two-dimensional topological insulators by means of quantum transport simulations for InAs/GaSb heterostructures and HgTe/(Hg,Cd)Te quantum wells. In InAs/GaSb, the density of edge states shows an oscillatory decay as a function of the distance to the edge. This is in contrast to the behavior of the edge states in HgTe quantum wells, which decay into the bulk in a simple exponential manner. The difference between the two materials is brought about by spatial separation of electrons and holes in InAs/GaSb, which affects the magnitudes of the parameters describing the particle-hole asymmetry and the strength of intersubband coupling within the Bernevig-Hughes-Zhang model. We show that the character of the wave function decay impacts directly the dependence of the point contact conductance on the constriction width and the Fermi energy, which can be verified experimentally and serve to determine accurately the values of relevant parameters. In the case of InAs/GaSb heterostructures the conductance magnitude oscillates as a function of the constriction width following the oscillations of the edge state penetration, whereas in HgTe/(Hg,Cd)Te quantum wells a single switching from transmitting to reflecting contact is predicted.
Self-assisted growth of InAs nanowires on graphene by molecular beam epitaxy is reported. Nanowires with diameter of ~50 nm and aspect ratio of up to 100 were achieved. The morphological and structural properties of the nanowires were carefully studied by changing the substrate from bilayer graphene through buffer layer to quasi-free-standing monolayer graphene. The positional relation of the InAs NWs with the graphene substrate was determined. A 30{deg} orientation configuration of some of the InAs NWs is shown to be related to the surface corrugation of the graphene substrate. InAs NW-based devices for transport measurements were fabricated, and the conductance measurements showed a semi-ballistic behavior. In Josephson junction measurements in the non-linear regime, Multiple Andreev Reflections were observed, and an inelastic scattering length of about 900 nm was derived.
We report an enhancement of the electron spin relaxation time (T1) in a (110) InAs/GaSb superlattice by more than an order of magnitude (25 times) relative to the corresponding (001) structure. The spin dynamics were measured using polarization sensitive pump probe techniques and a mid-infrared, subpicosecond PPLN OPO. Longer T1 times in (110) superlattices are attributed to the suppression of the native interface asymmetry and bulk inversion asymmetry contributions to the precessional Dyakonov Perel spin relaxation process. Calculations using a nonperturbative 14-band nanostructure model give good agreement with experiment and indicate that possible structural inversion asymmetry contributions to T1 associated with compositional mixing at the superlattice interfaces may limit the observed spin lifetime in (110) superlattices. Our findings have implications for potential spintronics applications using InAs/GaSb heterostructures.
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