InAs Conduction Band Research Articles

Fermi-Level Pinning in ErAs Nanoparticles Embedded in III-V Semiconductors.

Embedding rare-earth monopnictide nanoparticles into III-V semiconductors enables unique optical, electrical, and thermal properties for THz photoconductive switches, tunnel junctions, and thermoelectric devices. Despite the high structural quality and control over growth, particle size (<3 nm), and density, the underlying electronic structure of these nanocomposite materials has only been hypothesized. Structural and electronic properties of ErAs nanoparticles with different shapes and sizes (cubic to spherical, 1.14, 1.71, and 2.28 nm) in AlAs, GaAs, InAs, and their alloys are investigated using first-principles calculations, revealing that spherical nanoparticles have lower formation energies. For the lowest-energy nanoparticles, the Fermi level is pinned near midgap in GaAs and AlAs but resonant in the conduction band in InAs. The Fermi level is shifted down as the particle size increases and is pinned on an absolute energy scale considering the band alignment at AlAs/GaAs/InAs interfaces, offering insights into the rational design of these nanomaterials.

Spin-Resolved Magneto-Tunneling and Giant Anisotropic g-Factor in Broken Gap InAs-GaSb Core-Shell Nanowires.

We experimentally and computationally investigate the magneto-conductance across the radial heterojunction of InAs-GaSb core-shell nanowires under a magnetic field, B, up to 30 T and at temperatures in the range 4.2-200 K. The observed double-peak negative differential conductance markedly blue-shifts with increasing B. The doublet accounts for spin-polarized currents through the Zeeman split channels of the InAs (GaSb) conduction (valence) band and exhibits strong anisotropy with respect to B orientation and marked temperature dependence. Envelope function approximation and a semiclassical (WKB) approach allow to compute the magnetic quantum states of InAs and GaSb sections of the nanowire and to estimate the B-dependent tunneling current across the broken-gap interface. Disentangling different magneto-transport channels and a thermally activated valence-to-valence band transport current, we extract the g-factor from the spin-up and spin-down dI/dV branch dispersion, revealing a giant, strongly anisotropic g-factor in excess of 60 (100) for the radial (tilted) field configurations.

Evaluation of interface traps inside the conduction band of InAs-on-insulator nMOSFET by self-consistent Hall-QSCV method

Interface trap density (Dit) inside the conduction band of (111)-oriented InAs-on-insulator (InAs-OI) n-channel metal-oxide-semiconductor field-effect-transistor (nMOSFET) was experimentally evaluated by developing a method through a combination of a Hall measurement and quasi-static split C–V (Hall-QSCV). The surface potential and Dit of the InAs-OI nMOSFET were self-consistently calculated by numerically solving the Schrödinger–Poisson equation. The energy distributions of Dit were found to be almost independent of the ultra-thin-body channel thickness and the quantization energy, indicating the validity of the proposed Hall-QSCV evaluation. The energy position of the Dit minimum is in good agreement with the theoretically predicted position of the charge neutrality level, which locates deeply inside the conduction band of InAs. The experimental maximum surface electron density Nsmax at the InAs MOS interface, limited by Fermi level pinning, is 1.2 × 1013 cm−2, which is 2–3 times higher than Nsmax at the In0.53Ga0.47As MOS interfaces, owing to the lower Dit inside the InAs conduction band.

Coherent Epitaxial Semiconductor-Ferromagnetic Insulator InAs/EuS Interfaces: Band Alignment and Magnetic Structure.

Hybrid semiconductor-ferromagnetic insulator heterostructures are interesting due to their tunable electronic transport, self-sustained stray field, and local proximitized magnetic exchange. In this work, we present lattice-matched hybrid epitaxy of semiconductor-ferromagnetic insulator InAs/EuS heterostructures and analyze the atomic-scale structure and their electronic and magnetic characteristics. The Fermi level at the InAs/EuS interface is found to be close to the InAs conduction band and in the band gap of EuS, thus preserving the semiconducting properties. Both neutron and X-ray reflectivity measurements show that the overall ferromagnetic component is mainly localized in the EuS thin film with a suppression of the Eu moment in the EuS layer nearest the InAs and magnetic moments outside the detection limits on the pure InAs side. This work presents a step toward realizing defect-free semiconductor-ferromagnetic insulator epitaxial hybrids for spin-lifted quantum and spintronic applications without external magnetic fields.

Tunneling spectroscopy of an indium adatom precisely manipulated on the cross-sectional surface of InAs/GaSb quantum structures

Indium atom manipulation was achieved on the (110) cross-sectional surface of a quantum structure consisting of InAs/GaSb multilayer using a scanning tunneling microscope (STM) at low temperature. Positioning, removing, and lateral displacement by means of tip-induced hopping was performed on both InAs and GaSb layers. Electron tunneling via the electron orbital of the manipulated atom (adatom) was observed not only on the InAs layer but also on the GaSb layer, even when the energy of the orbital was in the GaSb band gap. The tunneling process from the STM tip to the InAs conduction band via the adatom and the GaSb band gap is explained well by the single electron tunneling model through the double-barrier tunneling junction. In the case of the adatom being very close to the InAs/GaSb heterointerface, interference with the electron tunneling to the InAs conduction electronic states through the GaSb barrier with/without passing the adatom was observed. The peak energy in the tunneling spectrum corresponding to the orbital of the adatom depends on the adatom position on the GaSb, demonstrating the fundamental interactions between the surface atomic-scale structures and semiconductor heterostructures.

Extracting band structure characteristics of GaSb/InAs core-shell nanowires from thermoelectric properties

Nanowires with a GaSb core and an InAs shell (and the inverted structure) are interesting for studies of electron-hole hybridization and interaction effects due to the bulk broken band-gap alignment at the material interface. We have used eight-band $\mathbf{k\cdot p}$ theory together with the envelope function approximation to calculate the band structure of such nanowires. For a fixed core radius, as a function of shell thickness the band structure changes from metallic (for a thick shell) to semiconducting (for a thin shell) with a gap induced by quantum confinement. For intermediate shell thickness, a different gapped band structure can appear, where the gap is induced by hybridization between the valence band in GaSb and the conduction band in InAs. To establish a relationship between the nanowire band structures and signatures in thermoelectrical measurements, we use the calculated energy dispersions as input to the Boltzmann equation and to ballistic transport equations to study the diffusive limit and the ballistic limit, respectively. Our theoretical results provide a guide for experiments, showing how thermoelectric measurements in a gated setup can be used to distinguish between different types of band gaps, or tune the system into a regime with few electrons and few holes, which can be of interest for studies of exciton physics.

Enhancement of giant magnetoresistance and oscillation by wave-vector filtering in Fe/Ag/Fe/InAs/Ag

The performance of a giant magnetoresistance (GMR) heterostructure Fe/Ag/Fe/InAs/Ag(100) in the current-perpendicular-to-plane geometry is presented. Calculations are based on a realistic tight-banding model with full $spd$ bands and the recursive Green's function algorithm. Results show that the system's GMR can reach values above 1000%. This GMR enhancement mainly is a result of the wave-vector filtering effect imposed by the InAs layer, restricting conductance within a small region around the $\mathrm{\ensuremath{\Gamma}}$ point in the 2D Brillouin zone. Calculations also reveal that when the Fermi level sits in the InAs band gap, MR gradually saturates as a function of InAs thickness with a smooth plateau; whereas when the Fermi level is in the InAs conduction band and close to the band bottom, GMR exhibits an oscillatory behavior with a large period. GMR oscillations are also observed with respect to Ag thickness, with oscillation amplitude determined by the Fermi level position relative to the InAs conduction band edge. The oscillation periods in both cases can be well explained by the concept of quantum-well states, and are determined by the spanning vector of the Fermi surface belly of the material whose thickness is varied. The observed GMR oscillations are due to the quantum interference of conduction electrons near the $\mathrm{\ensuremath{\Gamma}}$ point. The GMR and area-resistance (RA) product profiles at a wide range of Fermi energy positions relative to InAs bands are also compared. Near the GMR peak (with GMR above 1000%) in the conduction band, RA product can be as low as 8.$8\mathrm{\ensuremath{\Omega}}\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}{\text{m}}^{2}$. This feature of large GMR but small RA product results from the wave-vector filtering effect of doped InAs, and it makes the structure under study distinct from conventional GMR systems (small GMR, small RA) or magnetic tunnel junctions (large GMR, large RA).

Ballistic modeling of InAs nanowire transistors

In this work, the intrinsic performance of InAs nanowire transistors is evaluated in the ballistic limit. A self-consistent Schrödinger–Poisson solver is utilized in the cylindrical geometry, while accounting for conduction band non-parabolicity. The transistor characteristics are derived from simulations of ballistic transport within the nanowire. Using this approach, the performance is calculated for a continuous range of nanowire diameters and the transport properties are mapped. A transconductance exceeding 4S/mm is predicted at a gate overdrive of 0.5V and it is shown that the performance is improved with scaling.Furthermore, the influence from including self-consistency and non-parabolicity in the band structure simulations is quantified. It is demonstrated that the effective mass approximation underestimates the transistor performance due to the highly non-parabolic conduction band in InAs. Neglecting self-consistency severely overestimates the device performance, especially for thick nanowires. The error introduced by both of these approximations gets increasingly worse under high bias conditions.

Characterization of High Mobility InAs/InGaAs/InAlAs Composite Channels by THz Magneto-Photoresponse Spectroscopy

Inserted narrow InAs quantum wells in InAs/InGaAs/InAlAs heterostructures have been used to achieve higher mobility for high-electron-mobility transistors (HEMTs) with ultra-low-power and low-noise amplification characteristics and for spin-based devices. Due to the large nonparabolicity of the conduction band of InAs and the penetration of the confined electronic envelope function into the adjacent layer(s), accurate calculations of effective mass and g-factor of charge carriers can be problematic. Methods of making precise determinations of the mass and other electronic parameters are thus of interest. We have applied magneto-photoresponse and -transmissions measurements at several THz laser frequencies in concert with dc magnetotransport measurements at low temperature (T = 1.6 K) to determine various electronic parameters (effective mass, carrier density, g-factor, mobility and the quantum scattering time) of the 2DEG in an InAs/In0.75Ga0.25As/In0.75Al0.25As inserted channel structure. This characterization method can also be used to probe the effect of strain, Rashba field, etc on the properties of charge carriers in such structures.

Emission of photoexcited charge carriers from InAs/GaAs quantum dots grown by gas-phase epitaxy

A model describing the emission of photoexcited electrons and holes from an array of InAs quantum dots into the GaAs matrix is suggested. The analytical expression obtained for the emission efficiency takes into account the thermal emission of charge carriers into the GaAs matrix and two-dimensional states of the InAs wetting layer, tunneling and thermally activated tunneling escape, and electron transitions between the quantum-confinement levels in the conduction band of InAs. The temperature dependences of the photosensitivity in the regions of the ground-state and first excited-state optical transitions in InAs/GaAs quantum dots grown by gas-phase epitaxy are investigated experimentally. A number of quantum dot parameters are determined by fitting the results of a theoretical calculation to the experimental data. Good agreement between the theoretical and experimental results is obtained in this way.

Electrical design of InAs-Sb/GaSb superlattices for optical detectors using full bandstructure sp3s* tight-binding method and Bloch boundary conditions

InAs-Sb/GaSb type-II strain compensated superlattices (SLS) are currently being used in mid-wave and long-wave infrared photodetectors. The electronic bandstructure of InSb and GaSb shows very strong anisotropy and non-parabolicity close to the Γ-point for the conduction band (CB) minimum and the valence band (VB) maximum. Particularly around the energy range of 45–80 meV from band-edge we observe strong non-parabolicity in the CB and light hole VB. The band-edge dispersion determines the electrical properties of a material. When the bulk materials are combined to form a superlattice we need a model of bandstructure which takes into account the full bandstructure details of the constituents and also the strong interaction between the conduction band of InAs and valence bands of GaSb. There can also be contact potentials near the interface between two dissimilar superlattices which will not be captured unless a full bandstructure calculation is done. In this study, we have done a calculation using second nearest neighbor tight binding model in order to accurately reproduce the effective masses. The calculation of mini-band structure is done by finding the wavefunctions within one SL period subject to Bloch boundary conditions ψ(L)=ψ(0)eikL. We demonstrate in this paper how a calculation of carrier concentration as a function of the position of the Fermi level (EF) within bandgap(Eg) should be done in order to take into account the full bandstructure of broken-bandgap material systems. This calculation is key for determining electron transport particularly when we have an interface between two dissimilar superlattices.

Electrical properties and band diagram of InSb-InAs nanowire type-III heterojunctions

The electrical properties of nanowire-based n-InSb-n-InAs heterojunctions were investigated theoretically and experimentally. Analysis of the current-voltage characteristics showed that the current through the heterojunction is caused mostly by generation-recombination processes in the InSb and at the heterointerface. Due to the partially overlapping valence band of InSb and the conduction band of InAs, the second process is fast and activationless. Theoretical analysis showed that, depending on the heterojunction parameters, the flux of non-equilibrium minority carriers may have a different direction, explaining the experimentally observed non-monotonic coordinate dependence of the electron beam induced current.

The cyclotron resonance in Heterostructures with the InSb/AlInSb quantum wells

The absorption of two-dimensional electrons in InSb-based quantum wells in the quantized magnetic fields in the terahertz spectral region are studied. A p-Ge-based cyclotron laser was used as the radiation source. The effective mass of carriers at the Fermi level equal to 0.0219m0 (m0 is the mass of a free electron) is determined from the cyclotron resonance spectra. It is shown that the electron spectrum is described by the Kane model in a wide range of magnetic fields. An anomalously pronounced splitting of the cyclotron resonance line not associated with the nonparabolicity of the conduction band of InAs is observed in low magnetic fields, which can be attributed to the effect of the spin–orbit interaction.

Correlated development of a (2 × 2) reconstruction and a charge accumulation layer on the InAs(111)–Bi surface

We have studied the formation of a Bi-induced (2 × 2) reconstruction on the InAs(111) B surface. In connection to the development of the (2 × 2) reconstruction, a two dimensional charge accumulation layer located at the bottom of the InAs conduction band appears as seen through a photoemission structure at the Fermi level. Not well ordered Bi layers do not induce a charge accumulation. The Bi-induced reconstruction reduces the polarization of the pristine surface and changes the initial charge distribution. InAsBi alloying occurs below the surface where Bi acts as charge donor leading to the charge accumulation layer.

Magneto-photoluminescence in a type-II broken-gap n-GaInAsSb/p-InAs heterojunction

Magneto-photoluminescence in a single type-II broken-gap n-Ga0.94In0.06As0.13Sb0.87/p-InAs heterostructure with a 2D electron channel at the heterointerface containing two occupied electron subbands has been studied in the spectral range 0.3–0.8 eV in high magnetic fields of up to 10 T at low temperatures (T = 7 K). At photon energies in the range 0.5–0.8 eV, bulk photoluminescence from the layer of the n-GaInAsSb alloy was observed. In the low-energy part of the spectrum (0.3–0.45 eV), three narrow emission bands with photon energies hνa = 0.419 eV, hνb = 0.404 eV, and hνc = 0.384 eV and full widths at half-maximum FWHM = 4–7 meV were observed. These bands are due to radiative transitions of 2D electrons localized in the quantum well on the InAs side near the type-II heterointerface. The electron effective mass in the occupied subband E2 was estimated to be m2 = 0.027m0, which is close to the effective mass at the bottom of the InAs conduction band.

Imaging of electron standing waves in InAs/GaSb superlattices by low‐temperature scanning tunneling spectroscopy

The local density of states at cleaved cross-sectional surfaces of InAs/GaSb superlattices were investigated by low-temperature scanning tunneling microscopy and spectroscopy (LT-STM/STS) with atomic resolution. Broken-gap energy band profiles (the overlap of the conduction band of InAs and the valence band of GaSb) were clearly seen in the LT-STS spectra as a function of growth direction. Clear images of electron standing waves confined in the superlattice were also obtained. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Resonant tunneling in (110) oriented interband diodes

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.

PROBING THE LOCAL DENSITY OF STATES OF DILUTE ELECTRON SYSTEMS IN DIFFERENT DIMENSIONS

Scanning tunneling spectroscopy at T = 6 K is used to investigate the local density of states (LDOS) of electron systems belonging to the bulk conduction band of InAs. In particular, the three-dimensional electron system (3DES) of the n-doped material, an adsorbate-induced two-dimensional electron system (2DES) and the tip-induced quantum dot (0DES) are investigated at B = 0 T and B = 6 T. It is found that the 3DES at B = 0 T can be described by Bloch states weakly interacting with the potential disorder provided by ionized dopants. The 2DES at B = 0 T exhibits much stronger LDOS corrugations, stressing the tendency for weak localization in the potential disorder. In a magnetic field, 3DES and 2DES show drift states, which are expected in 2D, but are surprising in 3D, where they point to a new electron phase consisting of droplets of quasi-2D systems. The 0DES at B = 0 T reveals quantized states in accordance with Hartree calculations. At B = 6 T it exhibits Landau states with exchange enhanced spin splitting. These states are used to investigate the influence of potential disorder on the exchange enhancement, which visualizes the nonlocality of the exchange interaction.

Low Density Two-Dimensional Electron Systems Studied by Scanning Tunneling Spectroscopy

A two-dimensional electron system (2DES) belonging to the InAs conduction band has been prepared by depositing tiny amounts of adsorbates on the InAs(110) surface. Photoemission has been used to determine the resulting 2DES subband energies. Since the 2DES is close to the surface, it could be probed by low-temperature scanning tunneling spectroscopy. In zero magnetic field we find strong and rather irregular corrugations of the local density of states (LDOS), which are interpreted as due to the tendency of a 2DES to weakly localize. Applying a magnetic field leads to Landau quantization and to a dramatic change of the LDOS, which is now composed of drift states running along equipotential lines.

Comparing the local density of states of three- and two-dimensional electron systems by low-temperature scanning tunneling spectroscopy

Scanning tunneling spectroscopy performed at T=6 K is used to investigate the local density of states (LDOS) of electron systems in the bulk conduction band of InAs. In particular, the 3DES of the n-doped material and an adsorbate-induced 2DES located at the surface are investigated at B=0 and 6 T . It is found that the 3DES at B=0 T can be described by Bloch states weakly interacting with the potential disorder. The 2DES at B=0 T exhibits much stronger LDOS corrugations revealing the tendency of weak localization. In a magnetic field both systems show drift states, which are expected in 2D, but are surprising in 3D, where they point to a new electron phase consisting of droplets of quasi 2D-systems.