Many-valley Semiconductors Research Articles

A Valleytronic Diamond Transistor: Electrostatic Control of Valley Currents and Charge-State Manipulation of NV Centers.

The valley degree of freedom in many-valley semiconductors provides a new paradigm for storing and processing information in valleytronic and quantum-computing applications. Achieving practical devices requires all-electric control of long-lived valley-polarized states, without the use of strong external magnetic fields. Because of the extreme strength of the carbon–carbon bond, diamond possesses exceptionally stable valley states that provide a useful platform for valleytronic devices. Using ultrapure single-crystalline diamond, we demonstrate electrostatic control of valley currents in a dual-gate field-effect transistor, where the electrons are generated with a short ultraviolet pulse. The charge current and the valley current measured at the receiving electrodes are controlled separately by varying the gate voltages. We propose a model to interpret experimental data, based on drift-diffusion equations coupled through rate terms, with the rates computed by microscopic Monte Carlo simulations. As an application, we demonstrate valley-current charge-state modulation of nitrogen-vacancy centers.

Spin pump induced inverse spin Hall effect observed in Bi-doped n -type Si

An inverse spin Hall effect (ISHE) in n-type silicon was observed experimentally when conduction electrons were scattered on the spin-orbit potential of bismuth. The spin current in the silicon layer was generated by excitation of the magnetization precession during ferromagnetic resonance in a thin permalloy (Py) layer deposited on a Si layer doped by phosphor and bismuth. From the angular dependences of the dc voltage for different Py/n-Si:Bi structures aligned along the [011] or [100] crystal axes, we were able to distinguish the planar Hall effect (PHE) and ISHE contributions. The ISHE dc voltage signal was proportional to sin{\theta}*sin2{\theta} product for the structure aligned to the [011] crystal axis and to sin{\theta}*cos2{\theta} for the [100] direction. In addition, the PHE dc voltage was observed for the angles corresponded to the sin2{\theta} dependence. It means that for silicon as a many-valley semiconductor, the scattering of spins due to the spin-orbit potential induced by shallow donor in n-type material is dependent on the orientation of the valley axes relative to the direction of the magnetic field.

Determination of concentrations of donors and acceptors in many-valley semiconductors

A possibility of separate determination of concentrations of donors and acceptors in manyvalley semiconductors on the basis of temperature dependence of total concentration of charge carriers is shown. As distinct from a single-valley semiconductor, it is necessary to take into account that effective mass of density of states and effective ionization energy are not constant, but depend on temperature.

Possibility of experimental determination of band structure parameters in many-valley semiconductors

We show possibility in principle to determine some parameters (density-of-states effective mass of different valleys, energy distances between valleys, energies of ionization of impurity levels split from different valleys) of the band structure in many-valley semiconductors, based on the experimental curve of temperature dependence of overall concentration of charge carriers.

Band mass anisotropy and the intrinsic metric of fractional quantum Hall systems

It was recently pointed out that topological liquid phases arising in the fractional quantum Hall effect (FQHE) are not required to be rotationally invariant, as most variational wave functions proposed to date have been. Instead, they possess a geometric degree of freedom corresponding to a shear deformation that acts like an intrinsic metric. We apply this idea to a system with an anisotropic band mass, as is intrinsically the case in many-valley semiconductors such as AlAs and Si or in isotropic systems like GaAs in the presence of a tilted magnetic field, which breaks the rotational invariance. We perform exact diagonalization calculations with periodic boundary conditions (torus geometry) for various filling fractions in the lowest, first, and second Landau levels. In the lowest Landau level, we demonstrate that FQHE states generally survive the breakdown of rotational invariance by moderate values of the band mass anisotropy. At 1/3 filling, we generate a variational family of Laughlin wave functions parametrized by the metric degree of freedom. We show that the intrinsic metric of the Laughlin state adjusts as the band mass anisotropy or the dielectric tensor is varied, while the phase remains robust. In the $n=1$ Landau level, mass anisotropy drives transitions between incompressible liquids and compressible states with charge density wave ordering. In $n\ensuremath{\ge}2$ Landau levels, mass anisotropy selects and enhances stripe ordering with compatible wave vectors at partial 1/3 and 1/2 fillings.

Voltage sensitivity of a point-contact GaAs/AlGaAs heterojunction microwave detector

The detection properties of a microwave detector containing a point-contact GaAs/AlGaAs heterojunction were investigated in the X to Ka frequency range. The detector had a small-area GaAs/AlGaAs heterojunction connected in series with small- and large-area n–n+ homojunctions. Both theoretically and experimentally, the frequency dependence of the voltage sensitivity of the microwave detector was established, which we relate with the intervalley electromotive force arising in Al0.3Ga0.7As. The intervalley electromotive force is expressed using phenomenological electron transport theory in a many-valley semiconductor, and an intervalley relaxation time of 20 ps is estimated using the frequency dependence of the detector voltage sensitivity.

Strain dependence of electron-phonon energy loss rate in many-valley semiconductors

We demonstrate significant modification of the electron-phonon energy loss rate in a many-valley semiconductor system due to lattice mismatch induced strain. We show that the thermal conductance from the electron system to the phonon bath in strained n+Si, at phonon temperatures between 200 and 480 mK, is more than an order of magnitude lower than that for a similar unstrained sample.

Electron–acoustic-phonon energy-loss rate in multicomponent electron systems with symmetric and asymmetric coupling constants

We consider electron-phonon (\textit{e-ph}) energy loss rate in 3D and 2D multi-component electron systems in semiconductors. We allow general asymmetry in the \textit{e-ph} coupling constants (matrix elements), i.e., we allow that the coupling depends on the electron sub-system index. We derive a multi-component \textit{e-ph}power loss formula, which takes into account the asymmetric coupling and links the total \textit{e-ph} energy loss rate to the density response matrix of the total electron system. We write the density response matrix within mean field approximation, which leads to coexistence of\ symmetric energy loss rate $F_{S}(T)$ and asymmetric energy loss rate $F_{A}(T)$ with total energy loss rate $ F(T)=F_{S}(T)+F_{A}(T)$ at temperature $T$. The symmetric component F_{S}(T) $ is equivalent to the conventional single-sub-system energy loss rate in the literature, and in the Bloch-Gr\"{u}neisen limit we reproduce a set of well-known power laws $F_{S}(T)\propto T^{n_{S}}$, where the prefactor and power $n_{S}$ depend on electron system dimensionality and electron mean free path. For $F_{A}(T)$ we produce a new set of power laws F_{A}(T)\propto T^{n_{A}}$. Screening strongly reduces the symmetric coupling, but the asymmetric coupling is unscreened, provided that the inter-sub-system Coulomb interactions are strong. The lack of screening enhances $F_{A}(T)$ and the total energy loss rate $F(T)$. Especially, in the strong screening limit we find $F_{A}(T)\gg F_{S}(T)$. A canonical example of strongly asymmetric \textit{e-ph} matrix elements is the deformation potential coupling in many-valley semiconductors.

Intervalley-Scattering-Induced Electron-Phonon Energy Relaxation in Many-Valley Semiconductors at Low Temperatures

We report on the effect of elastic intervalley scattering on the energy transport between electrons and phonons in many-valley semiconductors. We derive a general expression for the electron-phonon energy flow rate at the limit where elastic intervalley scattering dominates over diffusion. Electron heating experiments on doped n-type Si samples with electron concentrations (3.5-16.0) x 10(25) m(-3) are performed at sub-Kelvin temperatures. We find a good agreement between the theory and the experiment.

Piezoresistance consideration on n-type 6H SiC for MEMS-based piezoresistance sensors

Piezoresistance in n-type 6H SiC was analyzed on the basis of electron transfer and mobility shift mechanisms for hexagonal many-valley semiconductors. Three important gauge factors or coefficients for piezoresistive sensor application, i.e., the longitudinal, transverse and shear, were calculated by using band parameters. The calculation was compared with experimental results taken from the literature. It was shown that incorporation of the electron transfer and the mobility shift mechanisms gives reasonable interpretation for the piezoresistance in n-type 6H SiC within the temperature range from 300 K to 773 K, and impurity concentration n = 2 × 1019 to 3.3 × 1019 cm−3. These conditions correspond to typical operation ranges of state-of-the-art MEMS-based piezoresistive sensors.

Analysis of piezoresistance in n-type β-SiC for high-temperature mechanical sensors

Piezoresistance in n-type β-SiC was analyzed on the basis of electron transfer and mobility shift mechanisms for cubic many-valley semiconductors. Gauge factors were calculated by using shear deformation potential constant Ⅺu. The calculation was compared with experimental results taken from the literature. It was shown that incorporation of the electron transfer and the mobility shift mechanisms gives reasonable interpretation for piezoresistance in n-type β-SiC within the temperature range from 300 to 673 K, and impurity concentration range from 1018 to 1020 cm−3. These conditions correspond to typical operation ranges of high-temperature piezoresistive sensors. The effect of the intervalley scattering on piezoresistance can be neglected from the evidence that gauge factor is inversely proportional to temperature within the abovementioned conditions.

Special features of electron drift in submicrometer GaAs structures

A quasi-hydrodynamic model of a submicrometer field-effect transistor was used to study the effect of the drift-velocity overshoot on the characteristics of drain-current saturation. It is shown that, in submicrometer transistor structures based on many-valley semiconductors, the current saturation is controlled by levelling off of the electron-drift velocity in the channel, with this levelling off being caused by the effective intervalley scattering. It is also shown that the highest electron-drift velocity in the channel is inversely proportional to the transistor gate length.

Semiconductor-Metal Transition in Many-Valley Semiconductors

The common conclusion that the Thomas-Fermi approximation for screening leading to a poor estimate of the critical donor concentration for a metal-insulator transition in many-valley semiconductors is critically examined. The many-body effects are introduced in a simple way, partly in the kinetic energy term as a mass renormalization in the spirit of the Fermi liquid theory and partly in the potential energy term when random distribution of impurities leading to Anderson localization is considered. In the absence of localization we obtain a*N c 1/3 = 0.36v -1/3 for the value of Mott constant, where v refers to the number of equivalent conduction band minima and a * is the orbit size of the donor electron in the semiconductor. When an impurity distribution is included we lose a simple expression as given above; instead we get an expression for the energy containing two parameters, the mass in the kinetic energy and the distribution parameter a in the potential energy. The mass is fixed choosing the ionization energy of an isocoric impurity in the low doping regime, and the value of a is fixed demanding the vanishing of the donor binding energy. Thus we are able to account for the variation of the binding energy with the donor concentration for several donors in Si and Ge. Our results are compared with the existing data, and the agreement is good.

Cascade capture of electrons by dislocations in many-valley semiconductors

The process of electron capture in many-valley semiconductors in a quantizing magnetic field is investigated theoretically. An analytical expression is obtained for the cross section of electron capture by dislocations. It is found that the electron capture cross section depends substantially on the orientation of the valley relative to the magnetic field.

Excitonic molecules trapped by quantum dots and isoelectronic impurities in many-valley semiconductors

Excitonic molecules with two to six excitons trapped by quantum dots or isoelectronic impurity centers are studied theoretically and experimentally for the first time. Such excitonic molecules can exist only in many-valley semiconductors. A model of an excitonic molecule, valid for a large number of valleys in the conduction and valence bands, is examined. The formation kinetics of excitonic molecules is discussed. A material feature of excitonic molecules bound on quantum dots is the existence of tunneling transitions of excitons between quantum dots.

Size effects in fluctuation spectra of many-valley semiconductors

We present the results of theoretical investigation of inhomogeneous fluctuations in submicrometer active layers of many-valley semiconductors with equivalent valleys (Ge, Si type), where the layer dimension, $2d$, is comparable to or less than the intervalley diffusion relaxation length, ${L}_{\mathrm{iv}}.$ The study is based on the Boltzmann-Langevin kinetic equation. Boundary conditions for the fluctuations on the layer surfaces are derived. It is shown that for arbitrary orientations of the valley axes (crystal axes) with respect to the surfaces, the fluctuation spectra depend on the applied small electric field. Some physical phenomena are reported: unlike bulk samples, intravalley fluctuation processes cause the intervalley fluctuations in thin layers; the spectra of fluctuations depend on the layer thickness; with $2d\ensuremath{\lesssim}{L}_{\mathrm{iv}},$ a considerable suppression of the fluctuations arises for the fluctuation frequency $\ensuremath{\omega}\ensuremath{\ll}{\ensuremath{\tau}}_{\mathrm{iv}}^{\ensuremath{-}1}$, where ${\ensuremath{\tau}}_{\mathrm{iv}}$ is the characteristic intervalley relaxation time.

Exciton molecules and exciton liquids in semiconductors

Excitons in many-valley semiconductors form molecules consisting of four and more excitons. The degeneracy factor g of the conduction band in germanium is 8, and in silicon g=12. As in acceptors, the hole ground state in excitons is fourfold degenerate. The same is valid for exciton molecules, because they are quantum objects with spherical symmetry. The exciton binding energy in molecules is close to that in exciton-liquid droplets. Experimental evidence is considered for the existence, besides biexcitons, of stable exciton molecules consisting of three and four, and, possibly, 11 and 12 excitons. Molecules containing from five to ten excitons are apparently unstable.

Phonon growth in a many valley model semiconductor at low lattice temperatures

It is shown how the traditional method of neglecting the energy of acoustic phonons and approximating their distribution by the equipartition law leads to significant errors in the phonon growth rate in a many-valley model semiconductor when the lattice temperature is low.

Electron correlation effects in screened hydrogenic impurity states in many-valley semiconductors.

The critical concentration of donor hydrogenic impurities in a screened Coulomb potential is calculated, considering the effects of exchange, correlations, and mass anisotropy of the bound electron. We used the random-phase approximation and a self-consistent scheme proposed by Singwi, Tosi, Land, and Sj\"olander with variational solutions in the effective mass equation. Our results are compared with other theoretical calculations and with experimental measurements of the metal-semiconductor transition in n-type impurities of silicon and germanium. The agreement with the experimental results let us conclude that in many semiconductor systems the short-range correlations need to be taken into account in a more effective way.

Binding energy of D− ions in Si and Ge

A formulation is presented to find the electronic states of a D− centre in a many-valley semiconductor. Parameterizing the intervalley scattering terms and using a model potential in the central cell region the D− binding energies in Si and Ge doped with group V elements are calculated variationally, including electron correlation. The binding energy obtained for Si : P and Ge : Sb are in good agreement with experimental results. D− formed by As donors appears to require a more detailed consideration for the central cell potential.