High Density Electron-hole Plasma Research Articles

Luminescence of High Density Electron-Hole Plasma in GaAs

Spontaneous luminescence and optical gain of high density electron-hole phasma (EHP) in GaAs are observed at 4.2 K under nanosecond pulse excitation. Time-resolved luminescence spectra are observed under picosecond pulse excitation. The chemical potential and reduced band-gap energy of the EHP are obtained from the spectral shape analysis. These values and also their time changes are explained qualitatively by the calculation of the EHP ground-state energy. After picosecond pulse excitation, hot carriers cool down rapidly to 50 K within the first 150 ps, but the thermal equilibrium with the lattice is not attained up to 400 ps. The observed gaseous EHP is considered to exist only in a transient state and the electron-hole liquid state is not realized.

Time-resolved spectra of spontaneous luminescence from high density electron-hole plasma in CdS

Pico-second time-resolved spectra of the spontaneous luminescence from high density electron-hole plasma in CdS are measured at 4.2 K suppressing the stimulation effect. It is found that after the pulse excitation hot carriers are cooled rapidly for the first 100 psec, and that therafter up to the 400 psec delay time the shape of spectra hardly changes but the spectral width depends on excitation density. Although it seems as if some kind of state like electron-hole liquid is formed, one cannot easily regard that state as the condensed electron-hole drop state.

Spontaneous luminescence due to high density electron-hole plasma in GaAs under nano- and pico-second pulse excitation

Spontaneous luminescence due to high density electron-hole plasma in GaAs is observed at 4.2 K under nano- and pico-second pulse excitation. From the pico-second time-resolved spectra, it is found that the hot carriers are cooled down rapidly within 150 psec, and the changes of spectra are not appreciable in the later stage. One may consider, together with results of the spectral shape analysis, that the electron-hole liquid formation is improbable at least within the time range observed.

Efficient generation of an electron-hole plasma in electron-beam-excited GaAs

An investigation was made of the laser energy and spectral characteristics of specially pure epitaxial GaAs films, having a carrier density of 7×1013–3×1015 cm–3, excited by a pulsed (τ≈100 nsec, ƒ=50Hz) 50 keV electron beam at temperatures 92–300°K. It was found that coherent radiation is generated as a result of the recombination of carriers which form a high-density electron-hole plasma. At 92°K, the lasing threshold was fairly low, ~0.4 A/cm2, but the differential quantum efficiency was high, ~20–25%. As the temperature increased (up to 300°K), the efficiency decreased negligibly, by a factor of 1.5–2, and jth varied as ~T3/2 . Waveguide modes, in which the radiation was completely polarized, were observed in the emission spectra of these lasers. The reasons for the formation of a waveguide on excitation of undoped GaAs are discussed.

Luminescence of high density electron-hole plasma in CdSe at elevated temperature

Luminescence of high density electron-hole plasma in CdSe is observed in the 77–300 K temperature range by picosecond pulse excitation. With increasing temperature from 77 K the stimulated emission band is replaced by the spontaneous emission band. Temperature changes of spectral features of these two bands and also their time dependence after pulse excitation are consistent with the theoretical consideration.

Picosecond spectroscopy of highly excited direct-gap semiconductors

Recent results of studies by the author's group on picosecond time-resolved spectroscopy of highly excited direct-gap semiconductors are reviewed. Studies on excitonic molecules in CuCl, especially on the molecules resonantly excited, and those on high density electron-hole plasma produced under intense picosecond pulse excitation in CdSe, CdS and GaAs are described.

Optical gains associated with exciton collision processes and high density electron-hole plasma in CdSe

Optical gain spectra for CdSe are measured at 4.2 K changing excitation density. The gain in the 683–689 nm region is concluded to be due to exciton collision processes, in disagreement with the assignment to electron-hole liquid by other investigators. The gain due to high density electron-hole plasma, which has been found recently by the authors to be generated under very high excitation density, is observed in the 688–691 nm region.

Infrared absorption in a high density electron-hole plasma in germanium at low temperatures

Measurements of the absorption in the temperature range 1.8–9 K have been made for wavelengths between 3.1 and 4.2μm, in germanium highly excited by a 1.06μm laser. The shape of the absorption curve is noted to be similar to that observed by Pokrovskii and Svistunova at lower excitation levels, and somewhat broader than that calculated by Smith, Chen and McGill for exciton absorption. A cross section of 4.5 (±1.1) × 10 -17 cm 2 at 3.39μm is deduced from normalization with the theoretical curves of Smith et al.

Electrical conductivity of germanium as a function of optically injected carrier density and temperature

The effects of carrier-carrier scattering on the transport properties of high-density electron-hole plasmas in germanium were studied both experimentally and theoretically. Experimentally, the dc electrical conductivities of nearly uniform optically injected bipolar plasmas in high-purity single crystals of germanium were measured for a broad range of carrier densities (\ensuremath{\sim} ${10}^{14}$ - \ensuremath{\sim} 5 \ifmmode\times\else\texttimes\fi{} ${10}^{17}$ ${\mathrm{cm}}^{\ensuremath{-}3}$) and temperatures (21-298 K). Concurrent measurements of the absorption of 3.39-\ensuremath{\mu}m radiation due to inter-hole-band transitions aided the calibration of the carrier densities at some temperatures. The previous theory of Appel, which used Kohler's variational principle to solve the Boltzmann equation, was generalized with respect to the inclusion of interband hole scattering, electron mass anisotropy, degeneracy, and use of the RPA $\stackrel{\ensuremath{\rightarrow}}{\mathrm{q}}$-dependent screening approximation (although not with respect to all at once). The most crucial generalization was the accounting for the presence of nonconducting excitons in the plasma. The scattering of free carriers by excitons was also considered, using a neutral-impurity scattering theory similar to that of Erginsoy. Incorporation of all the above considerations led to excellent agreement between experiment and theory at densities below the Mott criterion for transition to the exciton-free phase. The transition itself appeared to be gradual, however, and behavior of the conductivity in the high-density region was consistent with the recent assertion by Thomas and Rice that excitons are present at densities significantly above the Mott criterion.

Pico-second spectroscopy of highly excited CdSe and CdS—II high density electron-hole plasma

A new luminescence line characterized by a broad spectral width appears under intense pico-second pulse excitation at 4.2 K in both CdSe and CdS. With increasing excitation density the peak of the line shifts to lower energies and the spectral width becomes broader. Just opposite changes are observed with the lapse of time after excitation. It is concluded that the line is due to high density electron-hole plasma.

Luminescence Due to High-Density Electron-Hole Plasma in GaAs

Spontaneous and stimulated emissions due to high-density metallic plasma state of electrons and holes have been observed in GaAs under intense optical excitation. The experiment has been performed on high-purity epitaxial layers in the 4.2–17 K temperature range using a pulsed nitrogen laser and a dye laser as exciting sources, Luminescence band shape fits well with that calculated by taking account of the band-gap reduction and energy broadening. Dependence of the emission spectra on excitation intensity, temperature, and applied electric and magnetic fields gives support to the above interpretation. The energetics and thermodynamical properties of the high-density plasma state are discussed.

Parametric Coupling in an Optically Excited Plasma in Ge

Recent measurements of apparent absorption saturation and ultrafast relaxation in Ge have been reinterpreted as a parametric coupling between optical beams due to an index grating produced by a high-density electron-hole plasma. A sharp spike in probe-beam transmission of width 2 psec observed near zero time delay between a strong pumping and a weak probing beam has been shown to be the autocorrelation of the optical electric fields. In addition, evidence of a true saturation effect with a much longer recovery time is observed.

Gigahertz-rate 100-V pulse generator

A high-density electron-hole plasma generated within a high-efficiency avalanche diode induces a sharp, high-voltage pulse on a transmission line. A novel gigahertz-rate pulse generator has been constructed utilizing the high- efficiency microwave avalanche diode. An impedance discontinuity within the circuit gives rise to a sharp pulse with undistorted waveform, which keeps retriggering the diode for a self-excited pulse generation. The peak- to-peak amplitude of the pulse is 125 V into a 50-/spl Omega/ load. The pulsewidth is 400 ps, the rise time 100 ps, and the fall time 200 ps.