Impurities In GaAs Research Articles

Si‐Doping of Low‐Temperature‐Grown GaAs Heterostructures on (100) and (111)A GaAs Substrates

Utilizing the amphoteric property of silicon impurity in GaAs (111)A for acceptor doping of low‐temperature‐grown (LTG‐) GaAs films and LTG‐GaAs‐based heterostructures for THz photoconductive applications is proposed and realized. The electronic properties of superlattice structures {LTG‐GaAs/GaAs:Si} grown by molecular beam epitaxy on (100) and (111)A GaAs substrates are experimentally investigated, where GaAs:Si are silicon‐doped GaAs layers grown at standard conditions. The use of GaAs substrates with surface orientation (111)A results in spatially indirect p‐type doping of LTG‐GaAs layers. In addition, the charge redistribution at the LTG‐GaAs/GaAs:Si interfaces, due to the presence of silicon atoms in GaAs:Si layers and antisite point defects in adjacent LTG‐GaAs layers, is also analyzed by means of band structure modeling.

Scattering mechanisms in state-of-the-art GaAs/AlGaAs quantum wells

Motivated by recent breakthrough in molecular beam epitaxy of GaAs/AlGaAs quantum wells [Y. J. Chung \textit{et al.}, Nature Materials \textbf{20}, 632 (2021)], we examine contributions to mobility and quantum mobility from various scattering mechanisms and their dependencies on the electron density. We find that at lower electron densities, $n_e \lesssim 1 \times 10^{11}$ cm$^{-2}$, both transport and quantum mobility are limited by unintentional background impurities and follow a power law dependence, $\propto n_e^{\alpha}$, with $\alpha \approx 0.85$. Our predictions for quantum mobility are in reasonable agreement with an estimate obtained from the resistivity at filling factor $\nu= 1/2$ in a sample of Y. J. Chung \textit{et al.} with $n_e = 1 \times 10^{11}$ cm$^{-2}$. Consideration of other scattering mechanisms indicates that interface roughness (remote donors) is a likely limiting factor of transport (quantum) mobility at higher electron densities. Future measurements of quantum mobility should yield information on the distribution of background impurities in GaAs and AlGaAs.

Assessing site selectivity of Si-Ge in GaAs by isotopic dependent vibrational modes

In doubly doped GaAs with silicon (Si) and germanium (Ge) defects, the observed localized vibrational modes (LVMs) by high resolution infrared absorption spectroscopy are carefully assessed in terms of a comprehensive Green’s function (GF) theory. Accurate values of phonons generated by a realistic lattice-dynamical model are meticulously integrated to simulate the GFs and perturbation matrix elements for attaining the site selectivity of Si and Ge impurities in GaAs. By considering two nearest-neighbor (NN) donor–acceptor prototypes: (A)SiGa+-GeAs- and (B) GeGa+-SiAs- of C3v-symmetry with strict sets of perturbation parameters, we have cautiously appraised the two models by calculating their impurity vibrational modes. Careful analysis of the experimental vibrational traits has convincingly established the observed split modes at 403 and 373 cm−1 as radial (a1+) and transverse (e+) modes, respectively of a NN SiGa+-GeAs-pair where Ge acts as an acceptor. For this pair structure, the high frequency broad band at 403 cm−1 with several phonon features is justified by relating it to the five naturally occurring Ge isotopes. Although, the sharp line at 373 cm−1 overlaps with the 30SiGa isotopic LVM we, assigned it, however, to the transverse vibration of the pair-defect. In the two NN Ge-Si defect structures, the accuracy of calculated impurity modes is checked by using a simple model which correctly predicted isotopic shifts of the triply degenerate (F2) phonons of isolated SiGa+ and SiAs- impurities involved in the (SiGa+-GeAs- and GeGa+-SiAs-) donor–acceptor pairs.____________________________________________________________________________

Atomic-scale characterization of highly doped Si impurities in GaAs using scanning tunneling microscopy

Characterizing the atomic arrangement and distribution of dopant atoms is important for the fundamental understanding of the effects of doping on the electronic properties of semiconductors. Here, we used scanning tunneling microscopy (STM) to image the dopant species in highly Si-doped GaAs. Based on the distinct features in the STM images, we identified the most common Si species, which enabled the accurate evaluation of the local donor and acceptor concentrations and their in-plane variation across different regions. Systematic analysis of the variation of these properties with the doping concentration and the changes after the post-annealing process allowed atomic-scale information about the detailed behaviors of the Si species to be directly extracted. Our findings provided strong insights into the possible roles of each Si species in modifying the macroscopic electronic properties of GaAs.

Design and Characterization of a Sharp GaAs/Zn(Mn)Se Heterovalent Interface: A Sub-Nanometer Scale View.

The distribution of magnetic impurities (Mn) across a GaAs/Zn(Mn)Se heterovalent interface is investigated combining three experimental techniques: Cross-Section Scanning Tunnel Microscopy (X-STM), Atom Probe Tomography (APT), and Secondary Ions Mass Spectroscopy (SIMS). This unique combination allowed us to probe the Mn distribution with excellent sensitivity and sub-nanometer resolution. Our results show that the diffusion of Mn impurities in GaAs is strongly suppressed; conversely, Mn atoms are subject to a substantial redistribution in the ZnSe layer, which is affected by the growth conditions and the presence of an annealing step. These results show that it is possible to fabricate a sharp interface between a magnetic semiconductor (Zn(Mn)Se) and high quality GaAs, with low dopant concentration and good optical properties.

Studying the Effect of Electrically Active Impurities Coming from Trimethylgallium Synthesized by Different Means on the Electrophysical Characteristics of Gallium Arsenide Epitaxial Layers

Chemical deposition from vapors of metalorganic compounds and volatile hydrides (metalorganic vapor phase epitaxy) is the main way of growing devices’ heteroepitaxial structures based on gallium arsenide (GaAs) and solid solutions of it (AlxGa1 – xAs and InyGa1 – yAs) on an industrial scale. Electrically active impurities deposited uncontrollably on epitaxial layers during growth worsen the electrophysical parameters of devices’ structures. Sources of deposited background impurities include the initial material. In this work, ways of synthesizing trimethylgallium (TMG) are compared from the viewpoint of impurities, especially electrically active GaAs, being incorporated into the final product. Ways of preparing TMG that include the exchange reaction between gallium trichloride and trimethylaluminum (TMA) and magnesium–organic syntheses in which metallic magnesium is used as the initial reagent are selected as the objects of study. The behavior of electrically active impurities during the purification of TMG via rectification is studied. The investigations are performed by means of spectral analysis and functional control over the growth of electrophysical parameters of GaAs epitaxial layers from TMG and arsine. It is established that the qualitative and quantitative composition of impurities in TMG depends on their content in the initial reagents. TMG prepared using the exchange reaction between gallium trichloride and TMA is the source of n-type impurities in GaAs. These are mainly Group IV elements (silicon, tin, and lead), with silicon impurities predominating. TMG fabricated using metallic magnesium is a source of both p- (preferentially zinc) and n-type (preferentially silicon) impurities. Irrespective of the quality of the initial raw TMG, the use of reduced-pressure rectification allows the fabrication of TMG with low impurity contents. Epitaxial GaAs layers grown using purified TMG (TMG rectificate) have n-type conduction with a low level of background doping (0.7–4) × 1014 cm–3 and high carrier mobilities of 7300–8500 cm2/(V s) at 300 K and 90 000–156 000 cm2/(V s) at 77 K. These values correspond to purest samples made with TMG and arsine using MOC-hydride epitaxy. Based on the data on functional control, the content of impurities that display electrical activity in GaAs is on the level of 10−7–10−6 at % in fabricated TMG-rectificate samples.

Energy reversal of light- and heavy-hole excitons bound to isoelectronic centers

Isoelectronic centers in semiconductor hosts offer convenient optical access to both heavy- and light-hole states, thus providing powerful resources for the ultrafast control of electron or hole spins. From a detailed analysis of the fine structure of excitons bound to isoelectronic defects of ${C}_{s}$ symmetry formed by two nitrogen impurities in GaAs, we demonstrate that the lowest energy states, normally exhibiting a strongly dominant heavy-hole character, can be reversed to a dominant light-hole character by the strain found in the vicinity of the emitter. This reversal suggests that strain could be used to engineer the orbital composition and ordering of excitonic states, allowing enhanced optical control over spin states.

Observation of the symmetry of core states of a single Fe impurity in GaAs

We report the direct observation of two mid-gap core d-states of differing symmetry for a single Fe atom embedded in GaAs. These states are distinguished by the strength of their hybridization with the surrounding host electronic structure. The mid-gap state of Fe that does not hybridize via sigma-bonding is strongly localized to the Fe atom, whereas the other, which does, is extended and comparable in size to other acceptor states. Tight-binding calculations of these mid-gap states agree with the spatial structure of the measured wave functions, and illustrate that such measurements can determine the degree of hybridization via pi-bonding of impurity d-states. These single-dopant mid-gap states with strong d-character, which are intrinsically spin-orbit-entangled, provide an opportunity for probing and manipulating local magnetism and may be of use for high-speed electrical control of single spins.

Binding energy of the donor impurities in GaAs–Ga1−xAlxAs quantum well wires with Morse potential in the presence of electric and magnetic fields**Project supported by the Turkish Science Research Council (TÜBİTAK) and the Financial Supports from Akdeniz and Nigde Universities.

The behavior of a donor in the GaAs–Ga1−xAlxAs quantum well wire represented by the Morse potential is examined within the framework of the effective-mass approximation. The donor binding energies are numerically calculated for with and without the electric and magnetic fields in order to show their influence on the binding energies. Moreover, how the donor binding energies change for the constant potential parameters (De, re, and a) as well as with the different values of the electric and magnetic field strengths is determined. It is found that the donor binding energy is highly dependent on the external electric and magnetic fields as well as parameters of the Morse potential.

Spin dynamics of Mn impurities and their bound acceptors in GaAs

We present results of tight-binding spin-dynamics simulations of individual and pairs of substitutional Mn impurities in GaAs. Our approach is based on the mixed quantum-classical scheme for spin dynamics, with coupled equations of motions for the quantum subsystem, representing the host, and the localized spins of magnetic dopants, which are treated classically. In the case of a single Mn impurity, we calculate explicitly the time evolution of the Mn spin and the spins of nearest-neighbors As atoms, where the acceptor (hole) state introduced by the Mn dopant resides. We relate the characteristic frequencies in the dynamical spectra to the two dominant energy scales of the system, namely the spin-orbit interaction strength and the value of the p-d exchange coupling between the impurity spin and the host carriers. For a pair of Mn impurities, we find signatures of the indirect (carrier-mediated) exchange interaction in the time evolution of the impurity spins. Finally, we examine temporal correlations between the two Mn spins and their dependence on the exchange coupling and spin-orbit interaction strength, as well as on the initial spin-configuration and separation between the impurities. Our results provide insight into the dynamic interaction between localized magnetic impurities in a nano-scaled magnetic-semiconductor sample, in the extremely dilute (solotronics) regime.

Investigation of Mn incorporation in fifteen-period InGaAs/GaAs quantum well system

A ferromagnetic ordering with a Curie temperature of 50 K of fifteen layer of InGaMnAs/GaAs multi quantum wells (MQWs) structure grown on high resistivity (100) p-type GaAs substrates by molecular beam epitaxy (MBE) was found. It is likely that the ferromagnetic exchange coupling of sample with Curie temperature of 50 K is hole-mediated resulting in Mn substituting In or Ga sites. Temperature and excitation power dependent PL emission spectra of InGaMnAs MQWs sample grown at temperature of 170 °C show that an activation energy of Mn ion on the first quantum confinement level in InGaAs quantum well is 36 meV and impurity Mn is partly ionized. It is found that the activation energy of 36 meV of Mn ion in the QW is lower than the activation energy of 110 meV for a substitutional Mn impurity in GaAs. These measurements provide strong evidence that an impurity band existing in the bandgap due to substitutional Mn ions and it is the location of the Fermi level within the impurity band that determines Curie temperature.

Segregation of Impurities in GaAs and InAs Nanowires

Using ab initio methods based on the density functional theory, we investigate the segregation and formation energies for various dopants (Si, Be, Zn, Sn), commonly used to obtain p- or ntype conductivity in GaAs and InAs nanowires. The distribution of Au and O atoms, which may be unintentionally incorporated during the wire growth, is also studied. The calculations performed for nanowires of zinc blende and wurtzite structure show that the distribution of most of the impurities depends on the crystal structure of the wires. For example, it is shown that the same growth conditions can lead to lower energy for Si substituting Ga (donor) in the wire of wurtzite structure and substituting As (acceptor) in the wire of zinc blende structure. In contrast, we obtain that gold and oxygen atoms always tend to stay at the lateral surfaces of GaAs and InAs nanowires, in agreement with experimental findings, while for beryllium the lowest energies are found when the impurities are located in sites in the center of the wurtzite wire or along the [1; 0; 1] axis from surface to the center of the zinc blende wire, what can explain the recently observed diffusion of this impurity into the volume of GaAs wires.

Optical and Magnetic Properties of Ten-Period InGaMnAs/GaAs Quantum Wells

Ten layers of InGaMnAs/GaAs multiquantum wells (MQWs) structure were grown on high resistivity (100) p-type GaAs substrates by molecular beam epitaxy (MBE). A presence of the ferromagnetic structure was confirmed in the InGaMnAs/GaAs MQWs structure, and have ferromagnetic ordering with a Curie temperature, T C=50 K. It is likely that the ferromagnetic exchange coupling of the sample with T C=50 K is hole-mediated resulting in Mn substituting In or Ga sites. PL emission spectra of the InGaMnAs MQWs sample grown at a temperature of 170 °C show that an activation energy of the Mn ion on the first quantum confinement level in InGaAs QW is 32 meV and impurity Mn is partly ionized. The fact that the activation energy of 32 meV of Mn ion in the QW is lower than an activation energy of 110 meV for a substitutional Mn impurity in GaAs, indicating an impurity band existing in the bandgap due to substitutional Mn ions.

Core-state manipulation of single Fe impurities in GaAs with a scanning tunneling microscope

We demonstrate that a scanning tunneling microscope tip can be used to manipulate the tightly bound core ($d$-electron) state of single Fe ions embedded in GaAs. Increasing tip-sample voltage removes one $d$ electron from the core of a single Fe, changing the dopant from the (Fe${}^{2+}$)${}^{\ensuremath{-}}$ ionized acceptor state to the (Fe${}^{3+}$)${}^{0}$ isoelectronic state, which alters the spin moment and dramatically modifies the measured local electronic contrast in topographic maps of the surface. Evidence of internal transitions among the $d$ states of the Fe core is also seen in topographic maps where dark anisotropic features emerge from the interference between two paths: the direct tip-sample tunneling and tunneling which excites a $d$-state core exciton of the Fe dopant.

Binding energy of a donor impurity in GaAs [formula omitted] systems under electric and magnetic fields, and hydrostatic pressure

The problem of the impurity binding energy in GaAs n-type delta-doped systems is studied taking the field effect transistor as an archetypical structure. The theoretical investigation considers the effects of externally applied electric and magnetic fields as well as of hydrostatic pressure. The description of the one-dimensional potential profile is made including Hartree and exchange effects via a Thomas–Fermi-based local density approximation. The allowed energy levels are calculated within the effective mass and envelope function approximations by means of an expansion over an orthogonal set of infinite well eigenfunctions. The results for the impurity binding energy are presented also for different configurations of the impurity position within the system.

Resonance Coulomb scattering by shallow donor impurities in GaAs and InP

We report a theoretical study on characteristics of the Coulomb scattering of conduction electrons by donor centers in GaAs and InP, taking into account the influence of resonance states. The resonance states occur through the polar interaction of electrons with the longitudinal optical phonons. The obtained spectra of the total and transport Coulomb scattering cross-sections exhibit asymmetric resonance peculiarities in the vicinity of the resonance energy. It is shown that these peculiarities are no small effect: near the resonance energy the cross-sections may largely differ from the corresponding values for a regular Rutherford scattering.

Intense THz laser effects on off-axis donor impurities in GaAs–AlGaAs coaxial quantum well wires

The binding energy of an off-axis shallow-donor impurity in a GaAs coaxial cylindrical quantum well-wire under the action of an intense, high-frequency laser field is calculated using a variational procedure within the effective-mass approximation. We take into account the laser dressing effects on both the impurity Coulomb potential and the confinement potential. Numerical calculations of the ground state subband energy levels based on a finite element method are performed for different barrier thicknesses and laser field parameters. Our model indicates a possible tuning of the well potential shape along the laser polarization direction. Strong distortions of the electron probability density under intense laser field conditions are also predicted. The study proves that the presence of the laser field partially breaks down the degeneracy of the states for donors symmetrically positioned within the structure. The results obtained show that the impurity energy levels in coaxial quantum wires can be significantly modified and controlled by intense laser fields.

Electric field perturbation due to impurities in GaAs through single electron transistor

Abstract The present work shows the presence of inevitable impurities in the semi-insulating GaAs domains when one is developing a single electron transistor (SET) and alters the quantization mechanism of single electron tunneling through the island. It is also indicated that these impurities decrease the amount of energy required to change the number of electrons on the island, which leads to a drastic reduction of SET quality. A theoretical model has been presented for elucidating the I–V characteristics of GaAs nano-crystals. It is found that this proposed model fits well the experimental data.

Point defects in covalent semiconductors: A molecular cluster model

A multiple-scattering-Xα cluster model of tetrahedrally coordinated covalent semiconductors is discussed. In order to simulate the rest of the crystal at the cluster boundaries, all electrons filling dangling bonds are eliminated from the self-consistent calculations and transferred to a Watson sphere. This manner of simulating the “saturating effects” of the rest of the crystal is more practical than the usual procedure of terminating the atoms at the surface of the cluster with hydrogen atoms. The model is applied to perform the first self-consistent calculations of the electronic structure of vacancies and substitutional impurities in GaAs. The efficiency and accuracy of the model is demonstrated by calculating the well-studied states of an ideal vacancy in Si. The obtained results are in good agreement with other self-consistent calculations.

Photoionization cross-section and binding energy of donor impurities in GaAs, Ge and Si quantum wires with infinite barriers

Within the effective-mass approximation, we have investigated the binding energies of donor impurities as a function of the wire dimensions and the photoionization cross-section for a hydrogenic donor impurity placed on the center of the quantum well-wire as a function of the normalized photon energy in the GaAs, Ge and Si quantum wires with infinite barriers. The calculations are performed by the variational method based on a two-parametric trial wave function. The results show that the impurity binding energy and the photoionization cross-section depend strongly on both wire dimensions and material parameters.