Calculation on hot-electron noise in semiconductors

Under hot-electron conditions, noise arises both from fluctuations in the carrier velocity and the carrier collision time. The magnitudes of these two contributions are calculated by the Monte-Carlo method for InSb at 77°K.     

Theory of hot-electron quantum diffusion in semiconductors

In the linear response regime close to equilibrium, the fluctuation-dissipation theorem relates linear transport coefficients via the well-known Green–Kubo or Einstein relation. The latter embodies a deep connection between fluctuations causing diffusion and dissipation, which are responsible for a finite mobility. Far from equilibrium, however, the Einstein relation is no longer valid so that both the mobility and diffusivity gain their own physical integrity. Consequently, beyond a linear response, both quantities have to be described by different approaches. Unfortunately, there is a strong imbalance of research activities devoted to the study of both transport mechanisms in semiconductors. On one hand, the rich physics of high-field quantum drift in semiconducting structures has a long history and has reached a high level of sophistication. On the other hand, there are only comparatively few and unsystematic studies that cover quantum diffusion of carriers under high-field conditions. This review aims at reducing this gap by presenting a unified approach to quantum drift and quantum diffusion. Starting from a semi-phenomenological basis, a quantum theory of transport coefficients is developed for one- as well as multi-band models. Physical implications are illustrated by selected applications whereby the quantum character of the approach is emphasized. Furthermore, the basic unified treatment of transport coefficients is extended by accounting for the two-time dependence of one-particle correlation functions in quantum statistics. As an application, a phononless transport mechanism is identified, which solely originates from the double-time nature of the evolution. Finally, additional examples are presented that illustrate the important role played by quantum diffusion in semiconductor physics.     

Measurement of the hot-electron conductivity in semiconductors using ultrafast electric pulses

Semiconductor response to ultrafast electric pulses was investigated both theoretically and experimentally. The possibilities for hot-electron drift velocity estimation from a pulsed electric conductivity measurement were analysed. An optoelectronic arrangement with time resolution of 20 ps was used to perform such measurements on then-InSb andn-InAs single crystals. Negative differential mobility (n.d.m.) was observed in both semiconductors at high electric fields.     

Bulk modulus and microhardness of tetrahedral semiconductors

Using the plasma oscillations theory of solids, simple relations have been proposed for the calculation of bulk modulus (B) and microhardness (H) of group IV, II-VI, III-V, I-III-VI₂ and II-IV-V₂ semiconductors with tetrahedral structure. We find that B=K₁ (?ω_p)^2.3333 and H=K₂ (?ω_p)^2.3333−K₃, where K₁, K₂ and K₃ are the constants. The numerical values of K₁, K₂ and K₃ are respectively, 0.141, 0.036 and 12.895 for group IV, 0.109, 0.0037 and 0.782 for II-VI, 0.125, 0.0202 and 5.743 for III-V, 0.109, 0.0065 and 1.160 for I-III-VI₂, and 0.125, 0.0359 and 15.310 for II-IV-V₂ semiconductors. The calculated values of B and H are compared with the experimental values and the values reported by different workers. Reasonably good agreement has been obtained between them.     

Free carrier Voigt magneto-absorption in cubic semiconductors

A compensation bridge for measuring Voigt magneto-absorption in semiconductors has been setup for X-band. Room temperature measurements for phase and amplitude, as a function of magnetic field up to 9.8 kOe, have been carried on n-type single crystals of germanium, at 9.02 GHz. Theoretical calculations have been done for the amplitude and phase measurements as a function of magnetic field taking into account lattice scattering. There is a good agreement between the experimental and the theoretical results.     

Excitons in direct band gap cubic semiconductors

Perturbation is applied to study of the Wannier-Mott excitons in direct band gap cubic semiconductors with a fourfold degenerate highest valence band. The fine structure of exciton energy levels is investigated. General formulae are derived for the matrix elements of the perturbation. From these expressions it is straightforward to obtain values of the fine structure splittings of the energy levels and the wave functions of corresponding staes in any order of perturbation theory. A comparison with results of previous works is made.     

Higher excited states of acceptors in cubic semiconductors

For the first time, higher excited states of shallow acceptors up to the 3s and 4s states are calculated based on the Balderschi and Lipari theory including the cubic correction. The eigenvalues and eigenvectors of the effective mass Hamiltonian for shallow acceptor states were obtained by the finite element method. The resultant sparse matrix is diagonalized by a newly developed Saad's method based on Arnoldi's algorithm. Comparison with experimental spectra on ZnTe:Li and ZnTe:P gives best valence band parameters for ZnTe; μ = 0.60 and δ = 0.12.     

Bulk and surface ordering in cubic crystals

The order-disorder transition of face and body centred cubic crystals is studied by a Monte Carlo simulation of a two-component lattice gas with nearest neighbour interactions, and of different compositions. It is shown that the degree of surface order may both be higher and lower than that of the bulk. General guide lines are drawn from the results to predict such behaviour in experimental situations.     

Magnetic moment of an acceptor center in cubic semiconductors

The magnetic field-induced splitting of the ground-state spin levels of an acceptor center, described by superposition of the Coulomb and the central-cell potentials, has been calculated for diamond-type. An analytic expression for the g factor obtained in the zero-radius potential approximation depends only on the light to heavy hole mass ratio. It is shown that the differences between the values of the g factor for the limiting cases of purely Coulomb and zero-radius potentials do not exceed 5%, thus permitting one to use this analytic expression for estimates. Fiz. Tverd. Tela (St. Petersburg) 39, 58–60 (January 1997)     

Intrinsically minimal thermal conductivity in cubic I-V-VI2 semiconductors

We report measurements of the thermal conductivity of high-quality crystals of the cubic I-V-VI2 semiconductors AgSbTe2 and AgBiSe2. The thermal conductivity is temperature independent from 80 to 300 K at a value of approximately 0.70 W/mK. Heat conduction is dominated by the lattice term, which we show is limited by umklapp and normal phonon-phonon scattering processes to a value that corresponds to the minimum possible, where the phonon mean free path equals the interatomic distance. Minimum thermal conductivity in cubic I-V-VI2 semiconductors is due to an extreme anharmonicity of the lattice vibrational spectrum that gives rise to a high Grüneisen parameter and strong phonon-phonon interactions. Members of this family of compounds are therefore most promising for thermoelectric applications, particularly as p-type materials.     

Acceptor states in cubic semiconductors having a large hole-mass ratio

In cubic semiconductors the hole-mass ratio is small, which makes it possible to use the zero light-hole-mass limit. It was found that variational methods in this popular limit are not necessary to solve the Luttinger equation and not only the entire energy spectrum for the bound states of an acceptor and the eigenfunctions, including in momentum space, but also the behavior of the eigenfunctions at large radii can be determined to a high degree of accuracy. This approach made it possible to suggest comparatively simple relations for the lowest states of each series having different angular momenta, covering the entire range of possible mass ratios for semiconductors. Fiz. Tverd. Tela (St. Petersburg) 41, 1556–1563 (September 1999)