Experimental corrections for the hot hole distribution function in germanium in crossed electric and magnetic fields

A procedure for finding corrections for the hot hole distribution functions obtained from absorption measurements on intersubband transitions of hot heavy and light holes in germanium in crossed electric and magnetic fields is proposed. This procedure is based on the multiple-valued dependence of the absorption on the photon energies of transitions from light holes to a subband split off as a result of the spin-orbit interaction. Taking these corrections into account improves the agreement between the gain for direct optical transitions between the light and heavy hole subbands calculated from measurements of the absorption in the near-infrared and direct measurements in the far-infrared. Fiz. Tekh. Poluprovodn. 32, 1197–1199 (October 1998)     

Resonance donor states in quantum wells

The energies and wave functions of the resonance states of shallow donors in quantum wells (QWs) are calculated. The calculations are performed in a model of an isolated impurity center using the example of a GaAs/AlGaAs heterostructure. A formula for the probability of a spontaneous emission of polar optical (LO) phonons is derived. It is shown that, in the vicinity of the resonance-state energies, polar electron-phonon interaction is modified. This modification is produced by a hybridization of confinement subbands. Generally, due to hybridization, an electron interacts with phonons simultaneously in two channels (subbands). The absorption cross section for infrared radiation is calculated, taking into account both homogeneous (in the mid-infrared range) and inhomogeneous broadening (in the far-infrared range). The absorption of radiation whose electric field is normal to the heterointerfaces is related to optical transitions to the states near the resonances. Homogeneous broadening of the absorption lines, as well as the LO-phonon scattering rate, depends on the width of the resonance states (the degree of subband hybridization).     

Terahertz emission and photoconductivity in n-type GaAs/AlGaAs quantum wells: the role of resonant impurity states

Terahertz emission spectra in a longitudinal electric field and lateral photoconductivity spectra under terahertz illumination have been studied in structures with GaAs/AlGaAs quantum wells (QWs). It is shown that the spectra contain features associated with electron transitions involving resonant impurity states related to the second quantum-well subband. Calculations of the energy spectrum of impurity states and matrix elements of optical transitions made by taking into account various positions of the impurity relative to the QW center confirm the assumptions made.     

Impact ionization and light emission in InAlAs/InGaAsheterostructure field-effect transistors

We present measurements on impact ionization effects, real space transfer of holes and electrons, and light emission occurring in n-channel InAlAs/InGaGs heterostructure Field-Effect Transistors based on InP operated at high electric fields and at different temperatures. The channel electrons heated by the lateral electric field give rise to impact ionization and light emission. By comparing the electrical characteristics and the integrated light intensity in different energy ranges and at different temperatures, we were able to identify two main different light emission mechanisms: conduction to conduction-band transitions for low energy photons and conduction to valence-band transitions for high energy photons. The correlation between the gate current and the light intensity allowed us to separately evaluate the electron and hole components of the gate current     

Donor impurity-related optical absorption coefficients and refractive index changes in a rectangular GaAs quantum dot in the presence of electric field

Within the quasi-one-dimensional effective potential model and effective mass approximation, we obtain the wavefunctions and energy eigenvalues of the ground (j=1) and first 2 excited states (j=2 and 3) of a donor impurity in a rectangular GaAs quantum dot in the presence of electric field. The donor impurity-related linear and nonlinear optical absorption as well as refractive index changes for the transitions j=1-2 and j=2-3 are investigated. The results show that the impurity position, incident optical intensity and electric field play important roles in the optical absorption coefficients and refractive index changes. We find that the impurity effect induces the blueshift for j=1-2 and redshift for j=3-2 in the absence of the electric field, but it leads to redshift for j=1-2 and blueshift for j=3-2 in the existence of the field. Also, the optical coefficient for the higher energy transitions j=2-3 is insensitive to variation of impurity positions, while that for the low energy transition j=1-2 depends significantly on the positions of impurity. In addition, the saturation and splitting phenomenon of the optical absorption are observed as the incident optical intensity increases.     

Optical and electrical effects of charges on defects in non-metals

Electrically charged defects, which are unavoidable in solids, create very strong electric fields that affect many properties of the host. In an insulator with ∿ 10 ppm charged impurities, for example, the average field strength is F ∿ 50 kV/cm. The polar lattice vibrations, which are defects of a different sort, create fields in ionic solids that are typically lO⁶V/cm at room temperature. Perhaps the most fundamental effect of these charges is the alteration of the density of electronic states. Near energy band edges this produces “band tails” that have important consequences in most electron transport properties quite apart from ordinary impurity scattering. Furthermore, the matrix elements for optical transitions among states near band edges are also affected with several significant consequences in the optical properties. Aside from these fundamental physical consequences of charged defects, a number of important practical effects occur, including the domination of semiconductor laser properties, limitation of light transmission in quartz fibers and behavior of amorphous semi-conductors.     

Phonon Band Filling and Photon Emission by Phonons

Phonon generation by electrons is supplied in n-type Si crystals in electric fields E≤100 kV/cm at the lattice temperature of 80 K employing the ensemble Monte Carlo technique. Electron transfer between equivalent energy valleys is accounted for the g-type- and f-type phonon absorption and emission. Acoustic phonons are accounted for the quasi-elastic scattering of electrons within the energy valleys. Excess phonon number is determined using numerical data on phonon generation rate and experimental values of phonon lifetimes. The feasibility of stimulated emission of infrared-range photons due to direct optical transitions between the phonon bands is discussed.     

Low density photoexcitation phenomena in semiconductors: Aspects of theory and experiment

The basic mechanisms related to the photoexcitation of electron-hole pairs in semiconductors under conditions of low excitation density, low temperature and high crystal purity are reviewed. The use of high-resolution emission spectroscopy of band-to-impurity optical transitions in GaAs to measure the energy distribution functions $\text{?(E)}$ of electrons and holes in optically excited carrier plasmas of well defined densities (10¹⁰ cm^?3≤n≤ 10¹³ cm^?3) is described. With this experimental method (i) the energy relaxation of initially hot carrier distributions after pulsed photoexcitation ($\text{h}\text{?}\text{ω ? E}{}_{\mathrm{g}}$), (ii) stationary non-equilibrium distributions of electrons in the conduction band under cw photoexcitation ($\text{h}\text{?}\text{ω ? E}{}_{\mathrm{g}}$) and (iii) the transport properties of resonantly excited carrier plasmas in low electric fields ($\text{0≤|}\text{F}\text{|≤20}\text{V/cm}$) are investigated. The observed distribution functions are compared with theoretical results on the basis of the known band structure data of GaAs, taking into account polar optic and acoustic phonon scattering, the interaction among the carriers, ionized impurity scattering, and using approximate solutions of the appropriate transport equation.     

Modulation of intersubband absorption in tunnel-coupled quantum wells in electric fields

Modulation of absorption of middle-infrared radiation in double tunneling-coupled quantum wells in longitudinal electric fields is studied. A specific feature of the quantum wells is the small separation in energy between the two lower levels. As a consequence, the levels may exhibit “anticrossing” even in low transverse electric fields. An interpretation of the change in intersubband absorption is suggested. The interpretation is based on the assumption that a transverse electric-field component may appear in the structure. The change in the absorption coefficient is calculated taking into account the redistribution of electrons between size-quantization subbands and the changes in the temperature of electrons in the subbands in the longitudinal electric field, as well as the changes in the optical matrix elements, the energies of transitions, and the concentrations of electrons in the subbands in the transverse electric field. The possibility of using the structure for the efficient modulation of middle-infrared light with the photon energy 136 meV is shown.     

Emission and capture of electrons in multiquantum-well structures

The mechanisms of unipolar emission and capture of electrons are studied in multiquantum-well structures in relation with the quantum-well infrared photoconductors (QWIP's). We clarify the roles played by the physical parameters which appear in the different QWIP photoresponse models, i.e., the photoconductive and the photoemissive ones. We then describe the experimental procedures which allow us to independently determine these different parameters: deep level optical spectroscopy for the electron emission probability, impedance spectroscopy for the quantum-well capture velocity and thermal emission time, as well as the infrared photoconductive gain for the unipolar electron capture time. The measured dependence of these parameters on photon energy and electric field sheds light on the microscopic physical phenomena which are involved in quantum-well infrared photodetection. Theoretical results on optical phonon mediated transitions in an applied electric field from barrier to well states show good agreement with experiment at low fields but less dependence on the field. Finally, this theoretical approach allows us to connect the different parameters and solve the apparent discrepancy between the QWIP photoresponse models     

Magneto-absorption of donor impurities in quantum-well wires

We show calculations for far-infrared absorption of shallow-donor impurities in quantum well wires in the presence of an external magnetic field. The wave functions and the eigenvalues are obtained in the effective-mass approximation by using an extended variational approach in which the ground and excited magneto-impurity states are simultaneously obtained. We investigate the allowed intra-donor transitions for radiation circularly polarized in the plane perpendicular to the magnetic field. We present results for the absorption coefficient as a function of the photon energy for several field strengths and arbitrary impurity positions.     

Transport of hot carriers in semiconductor quantized inversion layers

The transport of warm and hot carriers in quantized inversion layers has recently become of considerable interest, due in part to the quasi-two-dimensional nature of the carrier system and to the multitude of subbands present. Generally, the number of carriers in the inversion layer is sufficiently large that carrier-carrier scattering maintains a quasi-Maxwellian for the isotropic part of the distribution function, but the inter-subband interactions are sufficiently weak that each subband possesses a separate electron temperature. The treatment of carrier transport can be naturally separated into two regimes. In the first, the carriers are hot. In this regime, the transport can be found from energy and momentum balance equations and the transport differs little from a classical three-dimensional model, except in the field region in which inter-subband transfer of carriers is important. In this field range, subtle changes in the velocity-field curve are observed and significant effects are found in the microwave conductivity at frequencies on the order of the inter-subband repopulation rate. In the warm electron regime, however, for low and moderate electric fields, the degenerate nature of the carrier distribution function must be considered. Although the electron temperature concept remains valid in this regime, the agreement between theory and experiment is not good and the lack of this agreement makes it difficult to assess the physical processes occurring. The situation is complicated at low temperatures where many of the scattering mechanisms are not fully understood and the carrier densities and transport can show activation behavior. This lack of understanding is especially true in warm carrier magneto-transport. For this reason, care must be exercised in evaluating the role played by the electric field. In this paper, these various regimes are discussed and compared to the available experimental data.     

Modulation of intersubband light absorption and interband photoluminescence in double GaAs/AlGaAs quantum wells under strong lateral electric fields

The effect of a lateral electric field on the mid-infrared absorption and interband photoluminescence spectra in double tunnel-coupled GaAs/AlGaAs quantum wells is studied. The results obtained are explained by the redistribution of hot electrons between quantum wells and changes in the space charge in the structure. The hot carrier temperature is determined by analyzing the intersubband light absorption and interband photoluminescence modulation spectra under strong lateral electric fields.     

Charge accumulation effects in quantum-well structures

The effect of accumulation of charges produced owing to the absorption of light in quantum-well structures under longitudinal electric fields is considered. It is shown that, in such two-dimensional structures, the charge accumulation process differs substantially from that in three-dimensional systems. The charge distributions in the structures are calculated for various electric fields and total numbers of accumulated charges. It is also shown that, at considerable amount of accumulated charges, the Fermi-type degeneracy begins to play an important part in the distributions of charges and potentials. Such accumulation of charges can result in nontrivial luminescence in which the processes of charge carrier generation and subsequent recombination can be essentially separated in time from each other. In addition, this luminescence is not masked by emission on the transitions involving the tails of the density of states in the band gap and by emission from the substrate.     

Different mechanisms affecting the inversion layer transient response

The transient response of the MOS capacitance after the application of a large depleting voltage can be caused by three different mechanisms depending on the distribution of the electric field between the silicon and the oxide. The three different cases were distinguished by measurements of the temperature dependence and the voltage dependence of the relaxation behavior. Relaxation due to thermal generation of carriers prevails at low electric fields, relaxation via oxide states occurs at medium fields, and relaxation by avalanche effects predominates at high fields. From the case where thermal relaxation is predominant the effective lifetime of minority carriers in the depletion region and the surface recombination velocity could be determined. At low temperatures (&sim100°K) where most frequently no relaxation occurs in the dark, light-induced relaxation phenomena were studied. Only light with energies larger than the Si band gap causes relaxation.     

The distribution function of hot charge carriers under conditions of resonance scattering

A simple analytical method for solving the kinetic equation for charge carriers in the presence of resonance states in the streaming mode is suggested. The Breit-Wigner isotropic model for resonance scattering was used in an analysis of both the anisotropic energy distribution of charge carriers, which arises under the effect of an external electric field in a two-and three-dimensional gas of charge carriers, and the occupancy of the resonance state. The conditions for the origination of the intracenter population inversion are considered.     

A numerical estimate of transport properties in degenerate silicon p-n junctions

Magnitudes of the transport terms describing hole and electron motion in extrinsic degenerate silicon with large impurity gradients are calculated using Fermi-Dirac statistics. The absolute error between these transport terms using Boltzmann statistics and Fermi-Dirac statistics is tabulated for the reduced Fermi energy level from - 15 kT units to + 15 kT units. An accurate equilibrium state is assumed, and the effects of impurity scattering and electric field on the mobility are examined. The necessity for including impurity scattering in steady and transient states has been demonstrated. It was found necessary to maintain better than a 1:10⁶accuracy in the Fermi-Dirac integrals in order to obtain a highly accurate equilibrium state.     

Features of impurity-photoconductivity relaxation in boron-doped silicon

A series of studies of the impurity-photoconductivity relaxation in Si:B is carried out under pulse optical excitation by a narrow-band tunable radiation source in low and ??heating?? (10?C500 V/cm) electric fields. It is shown that the dependence of the carrier-capture time in a band on the applied electric field is nonmonotonic and, in high fields (>75 V/cm), the capture time decreases with increasing field intensity, which is related to initiating the relaxation processes with optical-phonon emission within the band. The dependence of the relaxation rate for the carriers on the excitation-radiation wavelength is investigated, and a decrease in the carrier-capture time in the band is revealed in the vicinity of the Breit-Wigner-Fano resonances caused by direct capture at an impurity with optical-phonon emission.     

Inelastic light scattering from electronic excitations in quantum dots

A review is given of progress in our understanding of the electronic excitations in semiconductor quantum dots as studied by inelastic light scattering spectroscopy. Such experiments have revealed the characteristics of single particle, charge density, and spin density excitations of many-electron dots in zero and applied magnetic fields. Theoretical calculations, which reveal an electronic shell structure, are in general accord with the experimental results. Although the behaviour in a magnetic field is extremely complex, it is now feasible to study collective excitations from a range of strongly correlated ground states using this technique.