Phonons In Germanium Research Articles

Междолинные процессы релаксации состояний мелких доноров в германии

The role of the intervalley processes of electron-phonon interaction in the relaxation of excited shallow arsenic donors in germanium is analyzed. The rates of intracenter inter-valley transitions with emission of TA phonons in ger-manium are calculated in dependence on the uniaxial compression stress along {111} crystallographic direction. It is shown that inter-valley transitions to the ground state of the donor with emission of phonons can play a significant role in the relaxation of excited impurities only upon uniaxial stress of the crystal, since at zero stress, there are no exact resonances between impurity transitions and intervalley phonons. There are also transitions from highly excited states lying in a narrow band of energies (~ 0.5 meV) under very bottom of the conduction band to the first excited state 1s(3)(Г5) (in stressed germanium crystal to 1s(3)(Г3) state). The average rate of these transitions is estimated at 0.3×109 s-1.

Laser-excited acoustical phonons probed by ultrashort pulses from a laser-driven x-ray diode

We demonstrate that an ultrashort-pulse laser-driven x-ray diode can be used for time-resolved experiments on a picosecond timescale. Hence, acoustical phonons in germanium are observed after ultrashort laser-excitation and the results are compared with calculations according to a microphysical model. We also show the advantages of this kind of picosecond x-ray source compared to other sources on the basis of its properties.

Self-energy of zone-boundary phonons in germanium: Ab initiocalculations versus neutron spin-echo measurements

We present ab initio calculations and spin-echo measurements of the anharmonic self-energy of phonons at the X point of the Brillouin zone for germanium. The mechanisms that contribute to these self-energies are different from those usually displayed by phonons at the center of the Brillouin zone. Each mode shows trends representative of the variety of anharmonic mechanisms that are responsible for the scattering and the decay of phonons. The transverse acoustic phonons at the X point are very long lived at low temperatures; i.e., their probability of decay approaches zero, as a consequence of an unusual decay mechanism allowed by energy conservation. We have identified the microscopic processes that determine these self-energies. The calculated self-energies and their temperature dependence agree well with experiment.

Photoluminescence of GeO2 films containing germanium nanocrystals

Germanium nanocrystals were formed in a GeO2 film during the process of germanium monoxide gas-phase deposition onto a sapphire substrate and studied by photoluminescence (PL) and Raman scattering spectroscopy. A PL peak in this heterosystem was observed in the visible region at room temperature. The sizes of Ge nanocrystals were estimated from the position of a Raman peak corresponding to scattering by localized optical phonons in germanium. The PL peak position calculated with allowance for the electron and hole size quantization in Ge nanocrystals coincides well with the experimentally observed position of this peak.

Anharmonic Lattice Dynamics in Germanium Measured with Ultrafast X-Ray Diffraction

Damping of impulsively generated coherent acoustic oscillations in a femtosecond laser-heated thin germanium film is measured as a function of fluence by means of ultrafast x-ray diffraction. By simultaneously measuring picosecond strain dynamics in the film and in the unexcited silicon substrate, we separate anharmonic damping from acoustic transmission through the buried interface. The measured damping rate and its dependence on the calculated temperature of the thermal bath is consistent with estimated four-body, elastic dephasing times (T2) for 7-GHz longitudinal acoustic phonons in germanium.

Effects of isotope disorder on energies and lifetimes of phonons in germanium

The effects of isotope disorder on phonon energies and lifetimes have been measured by means of high-resolution inelastic neutron scattering on a nearly maximally disordered ${}^{70/76}\mathrm{Ge}$ single crystal (mass variance $g=1.52\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$). Two theoretical approaches have been used to understand the observed effects: calculations based on the coherent potential approximation and on large supercells, respectively. For the mass variance of our sample, the two methods lead to rather similar results that agree with the experimental data. This agreement is marginally better for the supercell calculations. Calculations performed for larger mass variances $g$ revealed, however, that the two approaches yield significantly different results when $g$ is larger than $3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}.$

Pressure Induced Frequency Shifts of Transverse Acoustic Phonons in Germanium to 9.7 GPa

A complete set of transverse acoustic phonon frequencies of Ge has been measured at pressures up to 9.7 GPa by inelastic neutron scattering and studied by ab initio calculations using density functional perturbation theory. At the X and L points, the pressure dependence of the phonon frequency \ensuremath{\omega} in the cubic structure is shown to be that of a classical soft mode with ${\mathrm{\ensuremath{\omega}}}^{2}$ strictly linear in pressure P. This finding is in contrast with previous interpretations that associated the pressure dependence of the zone boundary modes with the occurrence of the first order transition at $\ensuremath{\sim}10$ GPa and represented the pressure dependence of its frequencies by a polynomial in P.

Effects of isotope disorder on phonons in germanium determined from bound exciton luminescence

The effects of isotope disorder on the energies of the transverse acoustic (TAL), longitudinal acoustic (LAL) and transverse optical (TOL) phonons at the L point have been investigated using the luminescence of excitons bound to copper acceptors. In a sample with approximately equal 70Ge and 78Ge the TOL phonon is 0.14+or-0.03 meV higher in energy than expected for a crystal of the same mean atomic mass, while the LAL phonon is 0.07+or-0.03 meV lower in energy; the TOL phonon is also broadened by 0.4 meV. In this disordered sample the Cu no-phonon line is 0.05+or-0.02 meV higher than expected.

Generation and detection of coherent optical phonons in germanium.

Coherent optical phonons are generated impulsively in Ge by irradiation with femtosecond laser pulses. They are detected via reflectivity modulations in the time domain in a pump-probe configuration. The deformation potential of the optical phonon is responsible both for the generation and for the detection process. The excitation of the coherent optical phonons is explained by anisotropic weakening of bonding valence orbitals.

Isotopic disorder-effects on the phonons in germanium

High-resolution (±0.02 cm −1) Raman and infrared transmission (ir) measurements were used to study the effect of isotopic disorder on the vibrational properties of Ge crystals. Calculations using the coherent potential and the self-consistent Born approximations were performed and compared with the experiment. The linewidth of the Raman modes is predominantly determined by the anharmonic decay of the optical phonons into lower-energy phonons, with a characteristic decay time of 6.1(±0.1) ps in natural Ge. The phonon energies of the Raman active phonon and the transverse optical ir modes at the Brillouin zone boundary ( L,K,W,X-critical point) are shifted by the isotopic disorder in the material. The frequency shifts and the line broadenings are in good agreement with the theoretical calculations.

Picosecond Raman scattering from non-equilibrium LO and TO phonons in germanium

The temporal evolution of non-equilibrium LO and TO phonon populations in photo-excited Ge was monitored at 77K and 300K using a picosecond Raman scattering technique. In addition to demonstrating that significant non-equilibrium populations are produced in non-polar semiconductors, the results also indicate that carrier-TO phonon interactions are of similar strength to carrier-LO phonon interactions. The phonon lifetimes and non-equilibrium populations are discussed and compared with data obtained by others in GaAs.

Transverse acoustic phonons in germanium near the melting point

The transverse acoustic phonon dispersion in germanium has been studied along [111] and [100] up to 935°C, only 2 degrees below the melting point T m . No evidence of softening is found, neither near the Brillouin Zone boundary, nor uniformly throughout the BZ. The phonon energies decrease slowly and linearly with increasing T from 100 K to T m . This suggests that the free electron density at T m is less than 10 21 cm −3 . The phonon linewidths are very narrow up to T m : ∼0.035±0.015 THz, corresponding to lifetimes of the order of 10 ps, although large densities of phonons and free carriers are present in the crystal near the melting point Etude experimentale de la dispersion de ces phonons, dans les directions [111] et [100], jusqu'a 935°C (deux degres au-dessous du point de fusion T m ). Absence d'amollissement de facon uniforme dans la zone de Brillouin ou pres du bord de zone. Observation d'une decroissance, lente et lineaire, des energies de phonons quand la temperature augmente de 100 K jusqu'a T m . Interpretation par une densite d'electrons libres inferieure a 10 21 cm −3 . Raies de phonons tres etroites jusqu'a T m (durees de vie ∼10 ps) bien que les densites d'electrons libres et de phonons soient elevees pres de T m

Anisotropic scattering of holes by acoustic and optical phonons in germanium

The anisotropic scattering of holes by acoustic and optical phonons in germanium is investigated with allowance for the complex structure of the energy spectrum and the specifics of the scattering mechanisms. The temperature dependence of the hole mobility is analyzed in the temperature interval 10°K ⩽ T ⩽ 300°K. The results are in good agreement with the experimental data.

Directional dependence of boundary-scattering mean free path of phonons in germanium and silicon

Directional dependence of boundary-scattering mean free path of phonons in germanium and silicon

Electron-phonon interaction in germanium

Abstract The interaction of electrons with short wavelength phonons in germanium is calculated from an empirical pseudopotential. A result for the longitudinal acoustic phonon is compared with the observed indirect optical transition probability.

Amplification of Microwave Phonons in Germanium

Amplification of Microwave Phonons in Germanium

Amplification of Microwave Phonons in Germanium

Amplification of Microwave Phonons in Germanium

Propagation of microwave phonons in germanium

The effects of various kinds of interactions on the propagation of 9 GC microwave phonons in germanium have been studied experimentally. We will discuss phonon interactions 1) with conduction electrons in degenerate n - Ge, which produces attenuation of waves of specific polarization in particular directions of propagation, 2) with thermal phonons, which gives temperaturedependent ultrasonic attenuation, 3) with microwave phonons, which gives nonlinear effects.

Propagation of Microwave Phonons in Germanium

Propagation of Microwave Phonons in Germanium

The lattice dynamics of ionic and covalent crystal

First-order perturbation theory is used to derive the equations governing the normal inodes of vibrations of a crystal in which the atoms are polarizable. The method is based on the harmonic, electrostatic and adiabatic approximations, it is thus assumed that all phonon frequencies are much lower than the frequencies of electron transitions; the results therefore do not apply to metals. In the dipolar approximation the equations derived are found to be identical with those which have been obtained for the `shell model ’ of a crystal, and which are known to give results in rather good agreement with the measured dispersion relations for phonons in germanium and sodium iodide. The method is extended to include the interaction of quadrupole and higher moments of the electron distribution of anatom or ion.