Electronic Structure Of GaAs Research Articles

Band folding, strain, confinement, and surface relaxation effects on the electronic structure of GaAs and GaP: from bulk to nanowires

It is common to find materials that show strikingly different properties between its bulk and nanometric forms. In this paper we show how to link the electronic structures of two III-V systems, one a direct gap material, GaAs, and the other an indirect gap material, GaP, from their bulks right down to the shape of thin nanowires. The understanding of how these changes occur represents a scientific and technological challenge and is relevant for the design and prediction of novel nanostructured materials. GaAs and GaP bulk and nanowire systems are studied in the zinc-blende and wurtzite structures both free of strain and subjected to biaxial strains perpendicular to the [111]/[0001] direction (the kind of strain that the materials are subjected to when grown one on top of the other). We provide an interpretation of the band structure of nanowires, grown along the [111] (zinc-blende structure) and the [0001] (wurtzite structure) directions, in terms of the bulk band structures of the corresponding binary compounds. The procedure reveals the origin of the valence and conduction valleys relevant to determine the nature (direct or indirect) of the band gaps and the kind (direct and pseudodirect) of the valence to conduction transitions. Thus, by calculating only the bulk bands it is possible to describe the behavior of the nanowire bands even for very thin nanowires. The effects on the band structures due to biaxial strain are analogously analyzed, providing for bulk GaP the first results in literature. The role of confinement, and surface relaxation, in determining the nanowire electronic structure of thin nanowires are analyzed separately revealing that the change in the nature of the band gap is due mainly to surface relaxation effects, not confinement. We show that the change of the gap (indirect/direct) from the bulk to the 1D systems is mainly due to the competition between the energies of bulk conduction valleys which are differently influenced by confinement and strain. This effect is shown also by other low dimensional materials like the 2D materials extending only few atomic layers in one dimension. The competing valleys are already present in the bulk band structure. While the main effect of confinement is to open all gaps, it is not necessarily the main cause of the direct/indirect change in the nature of the electronic gap as instead is usually claimed in the literature. Our study can be used to understand and engineer the structure of many nanostructures systems by just better analysing the behavior of the bulk bands.

Effect of N interstitial complexes on the electronic properties of GaAs1−xNx alloys from first principles

Several approaches have been used to investigate the impact of Nitrogen (N) on the electronic structure of GaAs$_{1-x}$N$_{x}$ alloys, however, there is no agreement between theory and experiments about the importance of the different N interstitial defects in these alloys, and their nature is still unknown. Here we analyze the impact of five different N defects on the electronic structure of GaAs$_{1-x}$N$_{x}$ alloys, using density-functional methods: we calculate electronic states, formation energies and charge transition levels. The studied defects include N$_{As}$, As$_{Ga}$, As$_{Ga}$-N$_{As}$ substitutional defects, and (N-N)$_{As}$, (N-As)$_{As}$ split-interstitial complex defects. Our calculated defect formation energies agree with those reported by S.B. Zhang et al. [Phys. Rev. Lett. 86, 1789 (2001)], who predicted these defects. Among the interstitial defects, we found that (N-As)$_{As}$ emerges as the lowest energy configuration in comparison with (N-N)$_{As}$, agreeing with recent experiments [T. Jen, et al., Appl. Phys. Lett. 107, 221904 (2015)]. We also calculated the levels induced in the electronic structure due to each of these defects: defect states may occur as deep levels in the gap, shallow levels close to the band edges, and as levels between bulk states. We find that the largest changes in the band structure are produced by an isolated N atom in GaAs, which is resonant with the conduction band, exhibiting a strong hybridization between N and GaAs states. Deeper levels in the bandgap are obtained with (N-N)$_{As}$ split-interstitial defects. Our results confirm the formation of highly localized states around the N sites, which is convenient for photovoltaics and photoluminescence applications.

Hole subbands in freestanding nanowires: six-band versus eight-band k⋅p modelling

The electronic structure of GaAs, InAs and InSb nanowires is studied using the six-band and the eight-band k⋅p models. The effect of the different Luttinger-like parameters (in the eight-band model) on the hole band structure is investigated. Although GaAs nanostructures are often treated within a six-band model because of the large bandgap, it is shown that an eight-band model is necessary for a correct description of its hole spectrum. The camel-back structure usually found in the six-band model is not always present in the eight-band model. This camel-back structure depends on the interaction between light and heavy holes, especially the ones with opposite spin. The latter effect is less pronounced in an eight-band model, but could be very sensitive to the Kane inter-band energy (EP) value.

Electronic structure of Mn-doped III-V semiconductor quantum dots

The electronic structure of GaAs and InAs quantum dots (QDs) containing a single substitutional Mn impurity is investigated in the envelope-function formalism. The Mn impurity in these compounds is known to be a shallow acceptor in the configuration ${d}^{5}+h$ and characterized by a strong antiferromagnetic $sp\text{\ensuremath{-}}d$ exchange interaction between the hole $(h)$ and the Mn ion. Our model for the hole states is based on the Luttinger Hamiltonian and the Coulomb potential with a central-cell correction that accounts for the observed binding energy and the effective $g$ factor in the bulk. The binding energy as well as the exchange contribution is found to increase with decreasing QD size. However, in contrast with the case of spherical nanocrystals (NCs), the binding energy in lens-shaped self-assembled QDs in the low-confinement limit is lower than that in the bulk because of their highly anisotropic shape. With an on-center impurity, NCs retain the bulk ${T}_{d}$ symmetry and the ground state is a $j=3/2$-like ${\ensuremath{\Gamma}}_{8}({T}_{d})$ level. In self-assembled QDs it splits into two doublets: ${\ensuremath{\Gamma}}_{6}$ $(|{j}_{z}|=1/2)$ and ${\ensuremath{\Gamma}}_{7}$ $(|{j}_{z}|=3/2)$ of ${D}_{2d}$, which mix in the presence of in-plane asymmetry, both belonging to ${\ensuremath{\Gamma}}_{5}$ of the reduced symmetry ${C}_{2v}$. The order and the splitting between the doublets depend on the degree of confinement and the strain-induced separation between the light- and heavy-hole valence bands. In lattice-matched GaAs/(Ga,Al)As QDs the ground-state doublet is $|{j}_{z}|=3/2$-like in the low-confinement limit. As the lateral size decreases there is a rapid crossover to a $|{j}_{z}|=1/2$-like ground state in QDs of typical sizes. On the other hand, in strained InAs/GaAs QDs the ground state is always $|{j}_{z}|=3/2$-like and the splitting relatively large. The $sp\text{\ensuremath{-}}d$ coupling with the Mn spin $S=5/2$ finally leads to a splitting of the ground-state doublet into six doubly-degenerate levels. The components are close to one another as the effective exchange parameters are an order of magnitude smaller than in the bulk. Our results thoroughly contradict the previously adopted picture based on treating the confinement potential as a small perturbation to the bulk impurity levels. We also consider the lowest two-hole states: the ground state in InAs/GaAs QDs is a singlet almost uncoupled to the Mn spin. We deduce the zero-field fine structure of the excitonic transitions and compare the results with the recently reported photoluminescence spectra.

Pressure-induced phase transitions and electronic structure of GaAs

The structural and electronic properties of GaAs under high pressure have beeninvestigated using an ab initio pseudo-potential approach within the framework of densityfunctional theory. We use the local density approximation based on exchange–correlationenergy optimization for calculating the total energy. The phase transition in bulk GaAs is investigated. Results of the first-principles calculation of thestructural phase transition and the electronic properties of GaAs in the three differentcrystallographic structures are reported. GaAs is found to undergo structural transitionfrom B3 to B1 phase, demonstrating a reasonably good agreement with experimentaldata. The equation of state for these transformations also shows good agreementwith experimental results. The calculated value of volume collapse is close to theobserved data. The energy of the B2 phase is found to be slightly higher thanthat of the B1 phase. A possible mechanism for the transition characterized by the space group is discussed. The calculated values show that there is a relatively small value of enthalpyfor the transition as compared to the transition, which provides the transition behaviour of GaAs from the kineticviewpoint. The electronic structures have also been computed at different volumes.

Ab initiostudy of atomic structure and Schottky barrier height at theGaAs/Ni0.5Pt0.5Geinterface

We report a study of the atomic and electronic structures of GaAs(001)/NiPtGe(001) interfaces. By using density functional theory, we study the dependence of the Schottky barrier on the interface stoichiometry. The calculated $p$-type Schottky barrier heights vary by as much as 0.18 eV around the average value of 0.5 eV, which corresponds to a strongly pinned interface. We relate the As-Ge bonds at the interface with a strong Fermi level pinning.

Optical fine structure of II‐VI and III‐V nanocrystals: the role of d orbitals in a tight‐binding approach

AbstractThe electronic structure of GaAs and CdSe nanocrystals is studied by using a nearest neighbor tight‐binding model extended to include d orbitals. Our results show that the inclusion of d orbitals in a minimal basis set is necessary for a proper description of the lowest electron states, especially in the strong confinement regime. We have checked that simpler sp 3s * models yield an incorrect description of electron states made from bulk band‐states away from the Brillouin zone center. For GaAs nanocrystals with sizes below 2.5 nm, the d model predicts that the ground electron state is primarily built of L‐point band‐states, giving the nanocrystal an indirect‐gap character. The sp 3d 5 model also explains successfully the evolution of the absorption peaks in CdSe nanocrystals with confinement. We provide an interpretation of the fine band‐edge structure based on an orbital‐forbidden dark exciton mechanism, which is in agreement, for the first time, with both the resonant and non‐resonant experimental Stokes shifts. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Atomic and electronic structures of GaAs (001) surface

The atomic and electronic structures of α, β, β2, and ζ reconstructions for the Ga-terminated GaAs (001)-(4×2) surface are investigated in the framework of the pseudopotential approach. Total, surface, and local densities of electron states, electron-energy spectra, and relative surface energies of the structures under consideration are calculated.

Electronic structure of GaAs 1− xN x under pressure

The electronic band structures of GaAs 1− x N x for x=0.009, 0.016, 0.031 and 0.062 are calculated ab initio using a supercell approach in connection with the full-potential linear muffin–tin orbital method. Corrections for the ‘LDA gap errors’ are made by adding external potentials which are adjusted to yield correct gaps in pure GaAs. Even small amounts of nitrogen modify significantly the conduction bands, which become strongly non-parabolic. The effective mass in the lowest conduction band thus exhibits strong k-vector dependence. Calculated variations of gaps and effective masses with x and externally applied pressure are presented and compared to a variety of experimental data. There are significant error bars on our results due to the use of the supercell approach. These are estimated by examining the effects of varying the geometrical arrangement of the N-atoms substituting As. However, the calculations show that the electron mass for x>0.009 is much larger than that of pure GaAs, and that it decreases with x.

Atomic and electronic structures of N interstitials in GaAs

We have carried out electronic structure calculations on N interstitials in GaAs alloys using the first-principles plane-wave pseudopotential (PWPP) and projector augmented-wave (PAW) electronic structure methods in the framework of the density functional theory (DFT). Both the ultrasoft pseudopotential (USPP) method in connection with the generalized gradient approximation (GGA) and the PAW method in connection with the local density approximation (LDA) have been employed. Effects of the single nitrogen atom and nitrogen dimer related interstitial defects on the atomic and electronic structures of GaAs have been studied. Total energies, electronic band structures, and local densities of states have been evaluated. In general, energies of the defects with the NN dimer at the center of the Ga or As tetrahedron are more than 2 eV lower per nitrogen atom than those with a single N impurity at the same sites. We have also found that there are metastable defect candidates with a single N impurity in the middle of a particular edge of the Ga or As tetrahedra. Considering the modifications of the atomic structure of GaAs, our calculations show that the relaxations of the nearest neighbor atoms around the NN dimer at the center of the Ga or As tetrahedron, are essentially smaller than those around the single N impurity at the center of an edge of the Ga tetrahedron. Finally, these defect states induce drastic modifications into the electronic structure of GaAs. Interestingly, the NN dimer related defects cause noticeable changes only to the conduction bands near the conduction band edge, while the single N impurity related defects mainly modify the valence band edge and also induce localized and delocalized states into the band gap. Notably, the NN dimer and N impurity related defects lead to redshift and blueshift behavior of the band gap, respectively. The blueshift behavior is tentatively supported by the recent photoluminescence experiments.

Interpretation of orientational contrast in STM images of GaAs(1 1 0) cleavage surface

The electronic structure of GaAs(1 1 0) surface is analyzed using Density Functional Theory (DFT-GGA) in atomic orbital basis (LCAO). The surface orbitals and the corresponding local density of electronic states (LDOS) are calculated for purposes of interpreting STM images. We show how local atomic orbitals of surface atoms are related to tunneling channels for electrons in STM imaging. A destructive interference between orbitals of two neighbouring atoms increases the contrast between the two atoms, and this is reflected in directionality of STM patterns of GaAs(1 1 0) surfaces. We also discuss how the basic formalism of Tersoff–Hamann approach to STM simulation can be reformulated to reveal the role of phase difference between tunneling channels.

Photoemission study of LT-GaAs

The electronic structure of GaAs (100) grown by low-temperature molecular beam epitaxy was investigated by means of photoemission spectroscopy. Slight differences are found in the valence band spectra of GaAs layers grown at different As2/Ga flux ratios. Analysis of As 3d core level spectra does not indicate qualitative differences in respect to high temperature grown GaAs (100) layers. The Ga 3d core level spectra include a new component in comparison to the spectra of high temperature grown GaAs. The origin of this component is attributed to high density of As antisites in low-temperature grown GaAs.

Effects of hydrogen on the electronic properties of dilute GaAsN alloys.

Nitrogen has profound effects on the electronic structure of GaAs, as only a few percent of N can drastically lower the band gap. It is, however, not recognized that the same amount of N can also qualitatively alter the electronic behavior of hydrogen: First-principles calculations reveal that, in GaAsN, a H atom bonds to N and can act as a donor in its own right, whereas in GaAs and GaN, H is amphoteric, causing passivation instead. At high Fermi energy and H concentration, a N complex with two H was found to have lower energy than the single-H configuration. By removing the effect of N, this electrically inactive complex restores the gap of GaAs.

GaAs:N vs GaAs:B alloys: Symmetry-induced effects

Electronic structure of GaAs with a 3% addition of B and N is studied by ab initio molecular dynamics. We find that the influence of B on the band gap and on its pressure dependence is weaker than that of N. Hydrostatic pressure strongly reduces the oscillator strength of the fundamental transition in GaAs:B, but not in GaAs:N. Group-theoretical analysis shows that the differences are symmetry induced, and stem from the different substitution sites of B (cation sublattice) and N (anion sublattice). For both alloys we find an additional optical transition at about 0.5 eV above the band gap involving a state derived from ${L}_{1c};$ its features agree with these of the ${E}_{+}$ transition in GaAs:N.

Geometry and electronic structure of GaAs(001)(2 x 4) reconstructions.

Geometry and electronic structure of GaAs(001)(2 x 4) reconstructions.

Single-mode quantum wires.

We calculate the self-consistent electronic structure of GaAs-${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As split-gate quantum wires taking into account the ordering in the donor layer which occurs when the ionization fraction F is less than unity. Ordered ion distributions are generated via a Monte Carlo simulation and show glassy behavior as well as formation of a Wigner-like crystal without long-range order for sufficiently small F. We study the structure of the wire modes and show that ordering substantially reduces the standard deviation of mode energies, making single-mode wires possible. \textcopyright{} 1996 The American Physical Society.

Electronic structure of GaAs with an InAs (001) monolayer.

The effect on the electronic structure of an InAs monomolecular plane inserted in bulk GaAs is investigated theoretically. The (InAs${)}_{1}$(GaAs${)}_{\mathit{n}}$ (001) strained superlattice is studied via ab initio self-consistent pseudopotential calculations. Both electrons and holes are localized nearby the inserted InAs monolayer, which therefore acts as a quantum well for all the charge carriers. The small thickness of the inserted InAs slab is responsible for high confinement energies of the charge carriers, and therefore the interband electron--heavy-hole transition energy is close to the energy gap of the bulk GaAs, in agreement with recent experimental data.

Determination of the valence-band offset of GaAs-(Ga,In)P quantum wells by photoreflectance spectroscopy

This work reports on growth, characterization, and calculation of the electronic structure of GaAs-(Ga,In)P quantum wells, where the two semiconductors share neither a common anion nor a common cation. Metal organic molecular-beam epitaxy was the growth method that we used. The composition of the disordered alloy was close to 50 at. % indium. We have determined the valence-band offset in such structures. We have found ΔE c /ΔE υ =0.4, and have calculated the influence of the spin-orbit split-off states on the light-hole confinement. We found that these deep valence states significantly reduce the light-hole confinement

Surface atomic structure and bonding of GaAs(110)-p(1×1)–Bi (1 ML)

The surface atomic and electronic structure of GaAs(110)‐p(1×1)–Bi (1 ML) has been examined using a tight‐binding total‐energy (TBTE) model. Two structural candidates were considered; the epitaxial continued‐layer structure (ECLS) and the epitaxial on‐top structure (EOTS). The TBTE computations indicate that the ECLS is more stable than the EOTS by ∼0.06 eV/surface atom/unit cell in agreement with a recent low‐energy electron diffraction (LEED) intensity analysis which selected the ECLS as the best‐fit structure. The predicted TBTE structural parameters for the ECLS are in good agreement with the best‐fit LEED results. The energy difference between the ECLS and EOTS is much smaller for the Bi/GaAs interface than for the Sb/GaAs interface (∼0.11 eV/surface atom/unit cell) in agreement with the proposition that the relative stability of the ECLS and EOTS is a function of the overlayer‐substrate mismatch; with the EOTS being favored for larger overlayer‐substrate mismatch. The primary difference between the ...

Temperature-dependent chemical and electronic structure of reconstructed GaAs (100) surfaces

Low-energy electron diffraction, soft x-ray photoemission, cathodoluminescence (CL), and Auger electron spectroscopies have been performed to investigate the geometric, chemical, and electronic properties of GaAs (100) surfaces as a function of annealing temperature and surface reconstruction. These measurements indicate gradual changes in surface geometry, composition, deep level CL features, and Fermi-level (EF) position with increasing temperature of surface preparation. In contrast, it was observed that pronounced changes in the surface ionization potential and work function between different surface reconstructions. For most of the desorption temperatures and surface reconstructions, the secondary electron emission exhibits characteristic double onsets, possibly due to the existence of differently reconstructed patches on the surface. The implications of these variations in the surface chemical and electronic structure of GaAs (100) surfaces on their metal contact properties. It was concluded that (a) unique characterization of these surfaces requires measurements of geometric ordering, chemical composition and bonding, and deep level emission in the band gap, and (b) the correlation of the surface geometry with chemical and electronic surface and interface structure points to the central role of surface preparation in achieving controlled Schottky barrier behavior.