Polar Optical Phonon Emission Research Articles

Electron-phonon interaction and velocity oscillations in quantum wire structures

A 1D Monte Carlo simulation of a multisubband quantum wire indicates the presence of spatial velocity oscillations dependent on the strength of a longitudinally applied electric field. This effect results from the quasi-coherent relaxation of the electron ensemble upon reaching the threshold for polar optical phonon emission. Although these oscillations have been predicted for bulk semiconductors, the peaked scattering rates and lack of angular randomization existing in quasi-1D systems make these structures ideally suited for detecting this phenomenon. Both the field and temperature dependence are explored.

Real space transfer rates for polar optical phonon scattering from asymmetric quantum wells

We calculate the rate at which electrons bound in asymmetric semiconductor quantum wells are scattered out of the well by absorption or emission of polar optical phonons. The 2-D to 3-D nature of the scattering is included. The final states after scattering are states which carry current either to the right or left. We find that rates for scattering out of the well can be significantly smaller than bulk scattering rates. We also show that asymmetries in the well shape result in a directional dependence for the final state current. That is, electrons scatter out preferentially to the left or to the right depending on the details of the well potential.

UHF generation in sub-micron layers of GaAs

Mathematical modelling of n+-n-n+ GaAs structures using the macroparticle method and the Monte Carlo procedure for simulating scattering events indicates that at a lattice temperature of 4.2 K ballistic and quasiballistic electron transport can lead to electron plasma instabilities. If the main scattering mechanism is the emission of polar optical phonons, plasma instability leads to UHF current oscillations at frequencies near 460 GHz.

Real-space transfer from quantum wells by polar optical phonon scattering

We calculate the rate at which electrons bound in a semiconductor quantum well are scattered out of the well by absorption or emission of polar optical phonons. This escape rate is important in real-space transfer devices and as a loss mechanism in high electron mobility transistors. Both quantum confinement effects and the two-dimensional (2D) to three-dimensional (3D) nature of the scattering are included. For all but very shallow wells, the real-space transfer rate is much smaller than either the bulk (3D) scattering rate or the confined (2D) scattering rate. Quantum resonances in the final state result in oscillations in the scattering rate as a function of electron energy.

2D hot electron transport in a modulation-doped GaAs/AlGaAs interface

Direct evidence of polar optical phonon emission and intersubband transfer of 2 D electrons has been observed in GaAs/Al xGa 1−xAs ( x = 0.3) heterostructures. Magnetophonon resonance as well as SdH effect were observed simultaneously in magneto-resistance data at 4.2 K. These electron scatterings are responsible for the cooling process of high mobility 2D electrons heated by applied electric fields. The electron temperature and velocity were also measured to reveal hot electron effects of the 2 DEG.

The electron-phonon interaction in quasi-two-dimensional semiconductor quantum-well structures

Approximate analytic expressions are obtained for the scattering rates and momentum-relaxation rates of an electron in a quasi-two-dimensional quantum well interacting with acoustic, optical and intervalley modes via the deformation potential and with longitudinal optical modes via the polar interaction. These analytic expressions are obtained using a momentum-conservation approximation. The threshold for optical phonon emission, unlike the case in the bulk, is abrupt. All scattering rates are energy-independent and are inversely proportional to L, the thickness of the well. The momentum-relaxation rate associated with the absorption of polar optical phonons, on the other hand, proves to be proportional to L. These properties are shown to lead to a negative differential resistance for pure polar mode scattering, and to the existence of a runaway field for deformation-potential scattering. The self-energy associated with the emission of polar optical phonons at absolute zero is shown to be divergent unless the polar interaction is screened, and some consequences of this for laser and other optical processes are pointed out. The description of scattering by perturbation theory breaks down in very narrow wells.

Shubnikov‐de Haas Effect and Energy Relaxation of Hot Carriers in p‐Type Tellurium at 4.2 K

AbstractThe influence of electric fields on the amplitude of the Shubnikov‐de Haas (SdH) oscillations of p‐type tellurium has been studied for the configuration j ⊥ B and the trigonal c‐axis oriented perpendicular to the magnetic field B and to the current direction. From a comparison of the temperature dependence of the amplitudes of the ohmic SdH data and the electric‐field dependence of the amplitudes at 4.2 K the relation between applied electric field and resulting hole temperature is deduced. The measured energy loss rate of the holes to the lattice is compared with calculated values assuming scattering by acoustic and polar optical phonons. Piezoelectric interaction is responsible for the energy transfer below 6 K. An estimation of the contribution of deformation‐potential scattering allows to derive an approximate value for the valence‐band deformation potential constant EV ≈︁ 1.2 eV. Above 6 K emission of polar optical phonons becomes the dominant energy dissipation mechanism.

Hot‐Electron Magnetic Susceptibility of Degenerate Semiconductors at Low Temperatures: Effect of Non‐Parabolicity

AbstractThe dependence of hot‐electron magnetic susceptibility (χ) in a non‐parabolic conduction band on external dc electric field (E) has been investigated theoretically. The non‐parabolicity is accounted for by Kane's model for the conduction band and the “effective‐electron‐temperature model” has been used to describe the electron distribution function. Ionized impurity scattering has been assumed to be the dominant momentum relaxation process while energy loss was assumed to be accounted for by scattering due to acoustic phonons via the screened deformation potential, the screened piezoelectric interaction and scattering by polar optical phonons. Individual energy loss rates have been calculated using the corresponding analytical expressions which take account of the non‐parabolic nature of the energy bands both in the density of states and electron wave functions. Numerical results are presented for the typical case of an n‐InSb sample for fields up to ≈ 2 V/cm. The plot of susceptibility (χ) as a function of the dc electric field (E) leads to a kink at a field strength of ≈ 370 mV/cm corresponding to an electron temperature ≈︁ 18.5 °K. This is attributed to the onset of the spontaneous emission of polar optical phonons. Results are compared with those obtained on assuming a parabolic model for the energy bands. Non‐parabolicity, in general, leads to a lower value of the hot‐carrier magnetic susceptibility (absolute magnitude) as compared to its value in the parabolic band model for the same value of the electric field.

Hot-electron magnetic susceptibility of degenerate semiconductors at low temperatures

An analytical investigation of the effect of a dc electric field on the magnetic susceptibility of conduction electrons in a degenerate semiconductor at low temperatures has been made. From energy balance considerations, the electron temperature (Te) as a function of the strength of the dc electric field has been determined. Ionized impurity scattering has been assumed to be the dominant momentum relaxation process; the energy loss has been attributed mainly to the scattering by acoustic phonons (via both screened deformation and screened piezoelectric potentials), and by polar optical phonons. Numerical results have been presented for a typical case of an n-type InAs sample for fields up to 10 V/cm. It is found that susceptibility decreases with increasing field strengths. The plot of susceptibility (χ) as a function of dc electric field (E) leads to a kink at a field strength of about 450 mV/cm. This is explained on the basis of the spontaneous emission of polar optical phonons. It is further suggested that the measurement of susceptibility may be usefully employed as a tool for investigation of hot-electron phenomena.

Energy-Diffusion Equation in Hot-Electron Problems: Disturbance of the Polar-Optical-Phonon Distribution

In polar semiconductors, the emission of polar optical phonons by electrons with $\mathcal{E}>\ensuremath{\hbar}{\ensuremath{\omega}}_{0}$ is a major energy-relaxation process. At low temperatures, the large number of emitted phonons disturbs the polar mode distribution. The energy-diffusion equation for carrier flow in energy space can be used to solve the energy distribution function of the carriers. This function is then used to solve for the disturbed phonon distribution. A phonon quasitemperature is introduced to aid in the calculation of the disturbed phonon density and the hot-electron energy distribution function. At high values of the electric field, tailing of the energy distribution function occurs primarily due to the phonon disturbance. The theories are applied to $n$-type InSb. All optical-phonon-emission and -absorption processes are considered, even at low temperatures since the absorption processes are important for determining a balance in the distribution function. Scattering by acoustic deformation potential, piezoelectric, and ionized impurities are also considered.