Search Results (4 total)

We present a comprehensive report of pump-probe reflectivity and transmission measurements on highly oriented pyrolytic graphite with 50 fs time resolution. The experiments trace the generation, relaxation, and recombination of nonequilibrium carriers in a quasi-two-dimensional semimetallic solid over a wide range of experimental parameters. The fluence of excitation at hν=2.0 eV was varied between 10^-6 and 10^-2 J/cm², below the threshold for optical damage, while probe pulses in the photon energy range 1.5<hν<4.0 eV were used. On a subpicosecond time scale we observe a strong, initial, broadband absorption saturation caused by state filling by a hot, dense π-band electron population, which recovers with a fluence- and probe-wavelength-dependent time constant as the carriers cool and recombine in less than 1 ps. Later dynamics reflect the generation and diffusion of heat in the lattice, and are consistent with previous picosecond reflectivity measurements.

We have studied the time-resolved reflectivity and transmission changes induced by femtosecond laser pulses in hydrogenated and nonhydrogenated amorphous silicon thin films, a-Si:H and a-Si, respectively. By varying the pump power, and hence the photoexcited free-carrier densities, by several orders of magnitude, a quadratic, nonradiative recombination process has been identified that controls the density of free carriers on a picosecond time scale for excitation levels above 5×10¹⁸ cm^-3 in a-Si:H and above 5×10¹⁹ cm^-3 in a-Si. At lower free-carrier densities, the reflectivity transients display the dynamics expected from a trapping mechanism. We suggest that the process that dominates for the higher free-carrier densities may result from Auger recombination but with a dependence on the carrier density that is different from that which has been observed in crystalline semiconductors where k selection prevails.

The effect of indium incorporation on the concentration of deep traps in a series of GaAs epitaxial layers has been investigated by performing quantitative photoluminescence (PL) and capacitance [deep-level transient spectroscopy (DLTS)] spectroscopic studies. All samples were epitaxial layers of n-type GaAs:In, grown by organometallic vapor-phase epitaxy (OMVPE) on liquid-encapsulated Czochralski (LEC) -grown GaAs:Cr substrates. The calibrated indium concentration ranged between 0 and 6.5×10¹⁹ atoms cm^-3, which is about 0.3% in alloy composition. We have investigated (i) the bands associated with chromium in both the epitaxial layers and the original substrates; (ii) a large recombination band, associated with an unidentified (D-V_Ga) complex, at about 1.2 eV; and (iii) the DLTS signal associated with the well-known deep trap EL2. We find the following. First, there is a one-to-one correspondence between the PL intensity associated with Cr²⁺, at 0.84 eV, and the D-V_Ga signal at 1.2 eV. This is true for both the epitaxial layers and the original substrates and suggests identification of the unknown donor participating in the D-V_Ga complex as Cr⁴⁺.

Second, we find all PL intensities to decrease with increasing indium concentration, while the concentration and depth profile of EL2 are not affected. In contrast to the near-band-edge PL intensity, which increased with increasing indium content, there is a drop by about 1 order of magnitude for all chromium-related features when going from indium-free to about 0.3% indium-rich sample. Moreover, there is a one-to-one correspondence between the increase in the near-band-edge PL intensity and the decrease in the chromium-related signals. This establishes, on a fully experimental basis, the relative roles played by indium and chromium in our epitaxial samples: both compete to incorporate on gallium sites in the strain field of neighboring vacancies but, because of a higher incorporation rate, increasing the indium concentration in the gas phase, one lowers the amount of residual chromium present at those specific sites. This results in closing low-energy recombination paths and increases the near-band-edge PL efficiency.

The influence of indium incorporation in GaAs organometallic–vapor-phase-epitaxy (OMVPE) layers has been investigated in great detail. The results obtained concern the change in band-gap energy, the concentration of residual impurities, and the low-temperature (2-K) photoluminescence (PL) efficiency. For In concentrations ranging between 0 and 6.5×10¹⁹ cm^-3, both A⁰X and D⁰X bound-exciton lines could be resolved. Together with the near-band-gap transitions involving shallow impurities (DA- and eA-related recombination lines), they shift toward lower energies versus indium content. This indicates the formation of a ternary compound Ga_1-xIn_xAs, even at these extremely dilute indium concentrations. After a quantitative calibration of the indium content, linear relations have been found which connect the PL emission line energies and the indium concentration. They make low-temperature PL measurements the most quantitative, and nondestructive, tool for precise composition studies. In this case, care should be taken that the slope parameters are line dependent. For instance, we find a slight, but finite, discrepancy between the slope parameters corresponding to substitutional acceptors on Ga and As sites, respectively. This is discussed in terms of the two different sublattices by using a simple cluster model of 17 atoms. Lastly, we find the absolute PL intensities to increase versus indium concentration: This indicates an improvement in the optical quality of our samples. Since, on a relative scale, the PL signals involving Zn_Ga and/or Mg_Ga residual acceptors are not significantly affected by the amount of indium incorporated, but depend mainly on the growth sequence, we feel that indium in GaAs acts primarily by closing nonradiative-recombination paths which are not necessarily associated with gallium vacancies.