Zinc-blende GaAs Research Articles

Controlled wurtzite inclusions in self-catalyzed zinc blende III–V semiconductor nanowires

The control of the Ga droplet during the MBE growth and its impact on the crystal structure of self-catalyzed GaAs nanowires (NWs) were investigated. The consumption of the droplet proceeds in two steps. First, the contact angle decreases to 90° keeping the NW diameter constant. The crystal structure changes from zinc blende (ZB) to wurtzite (WZ). Then, the contact angle keeps constant while the top radius of the NW decreases and the NW grows again in ZB configuration. During the last step, {110}, {211}B and {100} facets develop at the top. Calculations show that the Ga desorption from the droplet has to be taken into account during its consumption. With this information, several WZ segments of different lengths were placed into ZB GaAs NWs via partial droplet consumption. For this purpose, we supplied As and Ga separately, in order to partially consume and refill the Ga droplet. The same mechanism was applied to self-catalyzed InAs NWs resulting in short WZ segments inserted in a ZB twinning superlattice.

The half-metallicity of zinc-blende CaC/GaAs(001) heterojunction: A density functional theory study

First principles calculations, based on spin density functional theory (SDFT) as implemented in the PWscf code of the QUANTUM-ESPRESSO package, are performed to study electronic and magnetic properties of CaC/GaAs(001) heterojunction. We simulate the interface of CaC in Zinc-Blende (ZB) structure and GaAs semiconductor to see whether this electrode could be a good choice for spin current injection or not. For this purpose, we calculate the spin-polarized density of states for ZB–CaC, in-plane strained ZB–CaC (tetragonal phase) and also for ZB–CaC/GaAs(001) heterojunction. It is shown that ZB–CaC, is a half-metal ferromagnet with large half-metallic gap. The half-metallicity is found to be robust with respect to the lattice deformation to the tetragonal phase and is maintained in the range of 0.59< c/a <1.22. Calculations with supercell scheme for simulation ZB-CaC/GaAs(001) heterojunction show that the spin polarization at the Fermi energy reduced at the CaC/GaAs(001) heterojunction. But due to large difference of density of states of spin channels around the Fermi energy, the spin polarization is large yet. So the CaC/GaAs(001) heterojunction can be used in spintronics applications as a spin current injector but it is not 100% spin-polarized.

A detailed characterization of the transient electron transport within zinc oxide, gallium nitride, and gallium arsenide

A three-valley Monte Carlo simulation approach is used in order to probe the transient electron transport that occurs within bulk wurtzite zinc oxide, bulk wurtzite gallium nitride, and bulk zinc-blende gallium arsenide. For the purposes of this analysis, we follow the approach of O'Leary et al. [O'Leary et al., Solid State Commun. 150, 2182 (2010)], and study how electrons, initially in thermal equilibrium, respond to the sudden application of a constant applied electric field. Through a determination of the dependence of the transient electron drift velocity on both the time elapsed since the onset of the applied electric field and the applied electric field strength, a complete characterization of the transient electron transport response of these materials is obtained. We then apply these results in order to estimate how the optimal cut-off frequency and the corresponding operating device voltage vary with the device length. We find that while the cut-off frequency found for the case of zinc-blende gallium arsenide, 637 GHz for a device length of 100 nm, is marginally less than that found for the cases of wurtzite zinc oxide and wurtzite gallium nitride, 1.05 and 1.32 THz, respectively, the corresponding operating voltage found for the case of zinc-blende GaAs, 0.08 V, precludes the use of this material for the operation of devices in the terahertz frequency range if higher powers are required; the corresponding operating voltages for the cases of wurtzite ZnO and wurtzite GaN are found to be 8 and 4 V, respectively. These results clearly demonstrate the compelling advantage offered by wurtzite zinc oxide and wurtzite gallium nitride, as opposed to zinc-blende gallium arsenide, for electron devices operating in the terahertz frequency range if higher powers are required.

Controlling crystal phases in GaAs nanowires grown by Au-assisted molecular beam epitaxy

Control of the crystal phases of GaAs nanowires (NWs) is essential to eliminate the formation of stacking faults which deteriorate the optical and electronic properties of the NWs. In addition, the ability to control the crystal phase of NWs provides an opportunity to engineer the band gap without changing the crystal material. We show that the crystal phase of GaAs NWs grown on GaAs(111)B substrates by molecular beam epitaxy using the Au-assisted vapor–liquid–solid growth mechanism can be tuned between wurtzite (WZ) and zinc blende (ZB) by changing the V/III flux ratio. As an example we demonstrate the realization of WZ GaAs NWs with a ZB GaAs insert that has been grown without changing the substrate temperature.

A Story Told by a Single Nanowire: Optical Properties of Wurtzite GaAs

The optical properties of the wurtzite (WZ) GaAs crystal phase found in nanowires (NWs) are a highly controversial topic. Here, we study high-quality pure WZ GaAs/AlGaAs core-shell NWs grown by Au-assisted molecular beam epitaxy (MBE) with microphotoluminescence spectroscopy (μ-PL) and (scanning) transmission electron microscopy on the very same single wire. We determine the room temperature (294 K) WZ GaAs bandgap to be 1.444 eV, which is ∼20 meV larger than in zinc blende (ZB) GaAs, and show that the free exciton emission at 15 K is at 1.516 eV. On the basis of time- and temperature-resolved μ-PL results, we propose a Γ(8) conduction band symmetry in WZ GaAs. We suggest a method for quantifying the optical quality of NWs, taking into consideration the difference between the room and low temperature integrated PL intensity, and demonstrate that Au-assisted GaAs/AlGaAs core-shell NWs can have high PL brightness up to room temperature.

Spin-splitting calculation for zincblende semiconductors using an atomic bond-orbital model

We develop a 16-band atomic bond-orbital model (16ABOM) to compute the spin splitting induced by bulk inversion asymmetry in zincblende materials. This model is derived from the linear combination of atomic-orbital (LCAO) scheme such that the characteristics of the real atomic orbitals can be preserved to calculate the spin splitting. The Hamiltonian of 16ABOM is based on a similarity transformation performed on the nearest-neighbor LCAO Hamiltonian with a second-order Taylor expansion over at the Γ point. The spin-splitting energies in bulk zincblende semiconductors, GaAs and InSb, are calculated, and the results agree with the LCAO and first-principles calculations. However, we find that the spin–orbit coupling between bonding and antibonding p-like states, evaluated by the 16ABOM, dominates the spin splitting of the lowest conduction bands in the zincblende materials.

Optimized effective potential using the Hylleraas variational method

In electronic structure calculations the optimized effective potential (OEP) is a method that treats exchange interactions exactly using a local potential within density-functional theory. In the exchange-only case, this is commonly called exact exchange. We present a method using density-functional perturbation theory combined with the Hylleraas variational method for finding the OEP by direct minimization, which avoids any sum over unoccupied states. The method has been implemented within the plane-wave, pseudopotential formalism. Band structures for zinc-blende semiconductors Si, Ge, C, GaAs, CdTe, and ZnSe; wurtzite semiconductors InN, GaN, and ZnO; and the rocksalt insulators CaO and NaCl have been calculated using the OEP and compared to calculations using the local-density approximation (LDA), a selection of generalized gradient approximations (GGAs) and Hartree-Fock (HF) functionals. The band gaps found with the OEP improve on the calculated results for the LDA, GGAs, or HF, with calculated values of 1.16 eV for Si, 3.32 eV for GaN, and 3.48 eV for ZnO. The OEP energies of semicore $d$ states are also greatly improved compared to LDA.

Bandgap and band discontinuity in wurtzite/zincblende GaAs homomaterial heterostructure

A wurtzite GaAs epilayer grown on a zincblende GaAs substrate by metalorganic chemical vapor deposition is studied by surface photovoltage spectroscopy. The wurtzite structure of the epilayer is disclosed by scanning electron microscope images of surface pits, where the pits are seen to change their structure from a rectangular into a hexagonal shape. The wurtzite phase is also revealed in x-ray diffraction showing a 〈0002〉 diffraction alongside the main (200) diffraction, suggesting a “c” lattice constant of 0.668 nm. A comparison of room temperature surface photovoltage spectra taken from the epilayer sample and from an epilayer-etched substrate suggests a type II heterostructure with valence band difference of about 15 meV and bandgap difference of about 70 meV between the zincblende and the wurtzite GaAs polytypes.

Measurement and simulation of anisotropic magnetoresistance in single GaAs/MnAs core/shell nanowires

We report four probe measurements of the low field magnetoresistance (MR) in single core/shell GaAs/MnAs nanowires (NWs) synthesized by molecular beam epitaxy, demonstrating clear signatures of anisotropic magnetoresistance that track the field-dependent magnetization. A comparison with micromagnetic simulations reveals that the principal characteristics of the magnetoresistance data can be unambiguously attributed to the nanowire segments with a zinc blende GaAs core. The direct correlation between magnetoresistance, magnetization, and crystal structure provides a powerful means of characterizing individual hybrid ferromagnet/semiconductor nanostructures.

Magnetic properties of (Ga,Mn)N ternaries and structural, electronic, and magnetic properties of cation-mixed (Ga,Mn)(As,N) and (In,Mn)(As,N) quaternaries

ABINIT simulation package with built in local density, generalized gradient, and spin local density approximations was used to investigate the structural, electronic, and magnetic properties of cation mixed (Ga,Mn)(As,N) and (In,Mn)(As,N) quaternaries with equal and fixed compositions of Ga, In, and Mn atoms. In particular, total energy minimization approach was used to compute the equilibrium structural parameters of zinc-blende (GaAs, InAs, and MnAs), wurtzite (GaN, InN, and MnN) binary parent compounds, as well as, the corresponding equilibrium parameters of (Ga,Mn)(As,N) and (In,Mn)(As,N) quaternary systems. The band structures of zinc-blende GaAs, InAs, and MnAs binary parent compounds were computed and analyzed. Spin polarized band structures of the cation mixed (Ga,Mn)(As,N) and (In,Mn)(As,N) quaternaries with equal compositions of Ga, In, and Mn cations were computed and analyzed using spin local density approximation based calculations. Moreover, the magnetic properties of (Ga,Mn)(As,N) and (In,Mn)(As,N) quaternaries with equal concentration of Ga, In, and Mn cations were investigated. Our results suggest that the two quaternary systems are nonmagnetic. An interpretation of our results is presented. In addition, the magnetic properties of (Ga,Mn)N nanocrystal ternaries constructed from doping GaN with one or two Mn atoms were investigated using Vienna Ab-initio Simulation Package (VASP) and compared with those of (Ga,Mn)(As,N) quaternaries.

Lattice parameter accommodation between GaAs(111) nanowires and Si(111) substrate after growth via Au-assisted molecular beam epitaxy

Using out-of-plane and in-plane X-ray diffraction techniques, we have investigated the structure at the interface between GaAs nanowires [NWs] grown by Au-assisted molecular beam epitaxy and the underlying Si(111) substrate. Comparing the diffraction pattern measured at samples grown for 5, 60, and 1,800 s, we find a plastic strain release of about 75% close to the NW-to-substrate interface even at the initial state of growth, probably caused by the formation of a dislocation network at the Si-to-GaAs interface. In detail, we deduce that during the initial stage, zinc-blende structure GaAs islands grow with a gradually increasing lattice parameter over a transition region of several 10 nm in the growth direction. In contrast, accommodation of the in-plane lattice parameter takes place within a thickness of about 10 nm. As a consequence, the ratio between out-of-plane and in-plane lattice parameters is smaller than the unity in the initial state of growth. Finally the wurtzite-type NWs grow on top of the islands and are free of strain.

Luminescence of GaAs nanowires consisting of wurtzite and zinc-blende segments

GaAs nanowires (NWs) grown by molecular-beam epitaxy may contain segments of both the zincblende (ZB) and wurtzite (WZ) phases. Depending on the growth conditions, we find that optical emission of such NWs occurs either predominantly above or below the band gap energy of ZB GaAs [E(g,ZB)]. This result is consistent with the assumption that the band gap energy of wurtzite GaAs [E(g,WZ)] is larger than E(g,ZB) and that GaAs NWs with alternating ZB and WZ segments along the wire axis establish a type II band alignment, where electrons captured within the ZB segments recombine with holes of the neighboring WZ segments. Thus, the corresponding transition energy depends on the degree of confinement of the electrons, and transition energies exceeding E(g,ZB) are possible for very thin ZB segments. At low temperatures, the incorporation of carbon acceptors plays a major role in determining the spectral profile as these can effectively bind holes in the ZB segments. From cathodoluminescence measurements of single GaAs NWs performed at room temperature, we deduce a lower bound of 55 meV for the difference E(g,WZ)-E(g,ZB).

Photomodulated Rayleigh Scattering from Single Semiconductor Nanowires

ABSTRACTWe demonstrate the newly developed technique Photomodulated Rayleigh Scattering spectroscopy in order to probe the electronic band structure of single semiconductor nanowires. We show that both the electronic transition energies and nanowire diameter can be measured simultaneously and with high accuracy in a single non-destructive measurement. We demonstrate our results for zincblende GaAs as well as wurtzite InP nanowires where we probed the band gaps and transition energies at both room and low temperatures. This technique should advance the study of optical properties of single nanowires as well as other types of nanostructures.

Hot phonon effect in Dyakonov-Perel spin relaxation: Ehrenreich’s variational approach

The hot phonon effect in spin relaxation of drifting electrons in the zinc-blende semiconductor GaAs was investigated. Temperature- and doping-density dependent spin relaxations caused by polar optical phonon (POP) scattering were studied. Ehrenreich’s variational approach was used to include the longitudinal POP effect in a model of the scattering process. The electron spin was found to relax with a sub-THz rate and the spin lifetime (τs) was found to decrease with increasing the strength of the drifting field because during transport in an electric field, electrons were accelerated to higher velocities at higher fields. A high-field, however, completely depolarized the electron spin due to an increase of the electron temperature via the longitudinal POP scattering. It was also found that τs increases with increasing, but moderate, n-doping density or decreasing temperature. The results are discussed in detail.

Structural stability of Mn-doped GaInAs and GaInN alloys

Phase stability of pseudo-ternary (Ga,In,Mn)As and (Ga,In,Mn)N alloys with zinc blende (ZB) and rock salt (RS) structures is investigated by means of the first principles full-potential linearized augmented plane-wave method. Calculated results predict that the ordered and disordered states are less favorable with respect to the phase separation. However, the Mn solubility into ZB GaAs increase as the In composition increases. In addition, the Mn-doped GaAs with xMn≲0.44 (and GaN with xMn≲0.63) favors the ZB structure over the RS structure regardless the In incorporation.

Crystal phase and growth orientation dependence of GaAs nanowires on NixGay seeds via vapor-solid-solid mechanism

One of the challenges to utilize high performance III-V compound semiconductor nanowires (NWs) for large-scale technological applications is to control the crystal phase and growth orientation for homogenous nanowire properties. Here, we report the dependence of crystal structure and growth orientation of GaAs NWs on NixGay seeds via vapor-solid-solid mechanism. The crystal structure of catalytic seeds is found to direct the crystal phase of NWs with cubic NiGa seeds yielding zincblende GaAs NWs while hexagonal Ni2Ga3 seeds producing wurtzite GaAs NWs. Furthermore, the seed/nanowire interface plane relationship would dictate the epitaxial growth orientation of NWs, which is independent of the NW diameters and growth conditions. All these suggest the importance of well-controlled phase and orientation of catalysts for the synthesis of homogenous nanowires.

Band structure parameters of wurtzite and zinc-blende GaAs under strain in the GW approximation

Quasiparticle self-consistent GW calculations are used to study the band structure in wurtzite and zinc-blende GaAs. The band-gap change between wurtzite and zinc blende is found to be sensitive to lattice constant and $k$-point convergence of the GW self-energy. Furthermore, the conduction-band minimum can switch between ${\ensuremath{\Gamma}}_{1}$ and ${\ensuremath{\Gamma}}_{3}$ character as a function of strain and the valence-band maximum can cross over from ${\ensuremath{\Gamma}}_{5}$ to ${\ensuremath{\Gamma}}_{1}$ under compressive uniaxial strain. The Kohn-Luttinger and Rashba-Sheka-Pikus effective Hamiltonian band structure parameters of zinc-blende and wurtzite GaAs, respectively, are determined from these first-principles band structure calculations. The uniaxial and homogeneous strain dependence of the band structure are studied and summarized in the appropriate strain deformation potential parameters.

Flat-Top and Stacking-Fault-Free GaAs-Related Nanopillars Grown on Si Substrates

The VLS (vapor-liquid-solid) method is one of the promising techniques for growing vertical III-V compound semiconductor nanowires on Si for application to optoelectronic circuits. Heterostructures grown in the axial direction by the VLS method and in the radial direction by the general layer-by-layer growth method make it possible to fabricate complicated and functional three-dimensional structures in a bottom-up manner. We can grow some vertical heterostructure nanopillars with flat tops on Si(111) substrates, and we have obtained core-multishell Ga(In)P/GaAs/GaP nanowires with flat tops and their air-gap structures by using selective wet etching. Simulations indicate that a high-<svg style="vertical-align:-1.90608pt;width:12.325px;" id="M1" height="12.925" version="1.1" viewBox="0 0 12.325 12.925" width="12.325" xmlns="http://www.w3.org/2000/svg"> <g transform="matrix(1.25,0,0,-1.25,0,12.925)"> <g transform="translate(72,-61.66)"> <text transform="matrix(1,0,0,-1,-71.95,63.61)"> <tspan style="font-size: 12.50px; " x="0" y="0">𝑄</tspan> </text> </g> </g> </svg> factor of over 2000 can be achieved for this air-gap structure. From the GaAs growth experiments, we found that zincblende GaAs without any stacking faults can be grown after the GaP nanowire growth. Pillars containing a quantum dot and without stacking faults can be grown by using this method. We can also obtain flat-top pillars without removing the Au catalysts when using small Au particles.

Interatomic potentials for the vibrational properties of III-V semiconductor nanostructures

We derive interatomic potentials for zinc blende InAs, InP, GaAs, and GaP semiconductors with possible applications in the realm of nanostructures. The potentials include bond stretching interaction between the nearest and next-nearest neighbors, a three-body term, and a long-range Coulomb interaction. The optimized potential parameters are obtained by (i) fitting to bulk phonon dispersions and elastic properties and (ii) constraining the parameter space to deliver well-behaved potentials for the structural relaxation and vibrational properties of nanostructure clusters. The targets are thereby calculated by density functional theory for clusters of up to 633 atoms. We illustrate the new capability by the calculation Kleinman and Gr\"uneisen parameters and of the vibrational properties of nanostructures with 3 to 5.5 nm diameter.

Effect of biaxial strain on half-metallicity of transition metal alloyed zinc-blende ZnO and GaAs: a first-principles study

The electronic structure, magnetic and half-metallic properties of transitional metal (TM)-alloyed zinc-blende ZnO and GaAs (TM = Cr, Mn, Fe, Co, Ni) thin films with biaxial strains on the (0 0 1) plane are studied by density functional theory and beyond. Here, we focus on two simple layer-by-layer delta doping structures with the TM substituting along the (1 0 0) planes (type-I) and (0 0 1) planes (type-II). We find that the Fe-, Co- and Ni-alloyed GaAs, Mn- and Fe-alloyed ZnO, and Co-alloyed ZnO(II) show antiferromagnetic (AFM) states, while Ni-alloyed ZnO(I) and Cr-alloyed GaAs show ferromagnetic (FM) coupling independent of the biaxial strain within 25% along the (0 0 1) plane. For the systems of Cr-alloyed ZnO, Co-alloyed ZnO(I), Ni-alloyed ZnO(II) and Mn-alloyed GaAs(I, II), the strain from the substrate will induce a phase transition from AFM to FM states. The Co-alloyed ZnO(I), Ni-alloyed ZnO(I, II) and Cr-alloyed GaAs(I, II) systems are demonstrated to be half-metallic from the generalized gradient approximation (GGA) calculations. The Cr-alloyed ZnO and Mn-alloyed GaAs systems also show robust half-metallicity with a large spin-flip gap by a GGA + U description, although their half-metallicity disappears with the standard GGA description.