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Register a new userThermal conductivity for III-V and II-VI semiconductor wurtzite and zinc-blende polytypes: the role of anharmonicity and phase space
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Mart\\'i Raya Moreno
Publication date
2019
fields
Physics
and research's language is
English
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We calculate the lattice thermal conductivity ($kappa$) for cubic (zinc-blende) and hexagonal (wurtzite) phases for 8 semiconductors using $textit{ab initio}$ calculations and solving the Phonon Boltzmann Transport Equation, explaining the different behavior of the ratio $kappa_{rm hex}/kappa_{rm cub}$ between the two phases. We show that this behavior depends on the relative importance of two antagonistic factors: anharmonicity, which we find to be always higher in the cubic phase; and the accessible phase space, which is higher for the less symmetric hexagonal phase. Based on that, we develop a method that predicts the most conducting phase---cubic or hexagonal---where other more heuristic approaches fail. We also present results for nanowires made of the same materials, showing the possibility to tune $kappa_{rm hex}/kappa_{rm cub}$ over a wide range by modifying their diameter, thus making them attractive materials for complex phononic and thermoelectric applications/systems.
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Here, we clarify the central role of the miscut during group III-V/ group IV crystal growth. We show that the miscut first impacts the initial antiphase domain distribution, with two distinct nucleation-driven and terraces-driven regimes. It is then inferred how the antiphase domain distribution mean phase and mean lateral length are affected by the miscut. An experimental confirmation is given through the comparison of antiphase domain distributions in GaP and GaSb/AlSb samples grown on nominal and vicinal Si substrates. The antiphase domain burying step of GaP/Si samples is then observed at the atomic scale by scanning tunneling microscopy. The steps arising from the miscut allow growth rate imbalance between the two phases of the crystal and the growth conditions can deeply modify the imbalance coefficient, as illustrated with GaAs/Si. We finally explain how a monodomain III-V semiconductor configuration can be achieved even on low miscut substrates.
The structural and optical properties of 3 different kinds of GaAs nanowires with 100% zinc-blende structure and with an average of 30% and 70% wurtzite are presented. A variety of shorter and longer segments of zinc-blende or wurtzite crystal phases are observed by transmission electron microscopy in the nanowires. Sharp photoluminescence lines are observed with emission energies tuned from 1.515 eV down to 1.43 eV when the percentage of wurtzite is increased. The downward shift of the emission peaks can be understood by carrier confinement at the interfaces, in quantum wells and in random short period superlattices existent in these nanowires, assuming a staggered band-offset between wurtzite and zinc-blende GaAs. The latter is confirmed also by time resolved measurements. The extremely local nature of these optical transitions is evidenced also by cathodoluminescence measurements. Raman spectroscopy on single wires shows different strain conditions, depending on the wurtzite content which affects also the band alignments. Finally, the occurrence of the two crystallographic phases is discussed in thermodynamic terms.
We report both zinc-blende (ZB) and wurtzite (WZ) crystal phase self-assembled GaAs quantum dots (QDs) embedding in a single GaAs/AlGaAs core-shell nanowires (NWs). Optical transitions and single-photon characteristics of both kinds of QDs have been investigated by measuring photoluminescence (PL) and time-resolved PL spectra upon application of hydrostatic pressure. We find that the ZB QDs are of direct band gap transition with short recombination lifetime (~1 ns) and higher pressure coefficient (75-100 meV/GPa). On the contrary, the WZ QDs undergo a direct-to-pseudodirect bandgap transition as a result of quantum confinement effect, with remarkably longer exciton lifetime (4.5-74.5 ns) and smaller pressure coefficient (28-53 meV/GPa). These fundamentally physical properties are further examined by performing state-of-the-art atomistic pseudopotential calculations.
Polarization dependent Raman scattering experiments realized on single GaAs nanowires with different percentages of zinc-blende and wurtzite structure are presented. The selection rules for the special case of nanowires are found and discussed. In the case of zinc-blende, the transversal optical mode E1(TO) at 267 cm-1 exhibits the highest intensity when the incident and analyzed polarization are parallel to the nanowire axis. This is a consequence of the nanowire geometry and dielectric mismatch with the environment, and in quite good agreement with the Raman selection rules. We also find a consistent splitting of 1 cm-1 of the E1(TO). The transversal optical mode related to the wurtzite structure, E2H, is measured between 254 and 256 cm-1, depending on the wurtzite content. The azymutal dependence of E2H indicates that the mode is excited with the highest efficiency when the incident and analyzed polarization are perpendicular to the nanowire axis, in agreement with the selection rules. The presence of strain between wurtzite and zinc-blende is analyzed by the relative shift of the E1(TO) and E2H modes. Finally, the influence of the surface roughness in the intensity of the longitudinal optical mode on {110} facets is presented.
An ultralow lattice thermal conductivity of 0.14 W$cdot$ m$^{-1} cdot$ K$^{-1}$ along the $vec b$ axis of As$_2$Se$_3$ single crystals was obtained at 300 K by first-principles calculations involving the density functional theory and the resolution of the Boltzmann transport equation. This ultralow lattice thermal conductivity arises from the combination of two mechanisms: 1) a cascade-like fall of the low-lying optical modes, which results in avoided crossings of these with the acoustic modes, low sound velocities and increased scattering rates of the acoustic phonons; and 2) the repulsion between the lone-pair electrons of the As cations and the valence $p$ orbitals of the Se anions, which leads to an increase in the anharmonicity of the bonds. The physical origins of these mechanisms lie on the nature of the chemical bonding in the material and its strong anisotropy. These results, whose validity has been addressed by comparison with SnSe, for which excellent agreement between the theoretical predictions and the experiments is achieved, point out that As$_2$Se$_3$ could exhibit improved thermoelectric properties.
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