The noncontact transient grating technique under constant electron-hole pumping to the density 4×1019 cm−3 was used to characterize the ambipolar diffusion coefficient Da in Si:Al, Si:In, and Si:Sb molecular-beam-epitaxy-grown layers. Da was found to be almost constant at a value of ≊8 cm2/s up to an equilibrium carrier density of 2×1019 cm−3 in the layer and was independent of the doping type. At higher doping density, evidence for a sharp increase in Da was observed. For example, Da increased to a value of 20 cm2/s at a doping density of about 1020 cm−3. The Da behavior is in reasonable agreement with results of the high-density ambipolar diffusion theory of Young and van Driel and is incompatible with majority-carriers diffusion coefficients according to the formula Da = 2DnDp/(Dn + Dp). An explanation for this behavior is given.

Subpicosecond timeresolved luminescence spectroscopy of highly excited CuCl Appl. Phys. Lett. 45, 993 (1984); 10.1063/1.95443Temporal measurement of the gain of a CuCl laser

(a), M. PETRAUSKAS (b), V. NETIR~IS (b), G. TAMULAITIS (b), and J. HALFPAP (a) From the measured erasure times of transient gratings created in the volume and on the surface of non-intentionally doped p-type CdTe and Zn,Cdl-,Te (z 5 0.11) single crystals the freecarrier lifetime and the surface recombination velocity a t room temperature are determined, respectively. The erasure time of the surface grating in CdTe i s found to be 130 ps, which is one order smaller than that of the volume grating. A lifetime shortening is observed in mixed crystal. Moreover, using cw-laser excitation the samples are characterized additionally by photoluminescence investigation a t low temperatures to test the quality, to determine the mole fraction, and to identify impurities. The AoX-line observed is interpreted as due t o a single acceptor on Te site.In the mixed crystals at z 2 0.09 the two-mode behaviour of LO-phonons and an u p to now not observed splitting of the AoX-line are found.Aus der Messung der Loschzeiten von im Volumen bzw. auf der Oberflache von undotierten pleitenden CdTe-und Zn,Cdl-,Te (z 5 0,ll)-Einkristallen erzeugten transienten Gittern werden die Lebensdauer der freien Triiger bzw. die Oberflachenrekombinationsgeschwindigkeit bei Raumtemperatur ermittelt. Die Loschzeit des Oberflachengitters in p-CdTe ist um ca. eine GroBenordnung kleiner als die des Volumengitters und betragt 130 ps. I n den Mischkristallen wird eine Verkiirzung der Lebensdauer der freien TrLger heobachtet. Dardberhinaus wird die Photolumineszenz der Proben bei niedrigen Temperaturen und cw-Laser Anregung untersucht, um die Probenqualitiit zu testen, das Mischungsverhaltnis zu bestimmen und Verunreinigungen zu identifizieren. Als Ursache fur die beobachteten AOX-Linien wird ein einfacher Akzeptor auf Te-Platz angenommen. In den Mischkristallen mit r 2 0,09 wird das Zwei-Moden-Verhalten der LO-Phononen und eine bisher nicht bekannte Aufspaltung der AOX-Linie gefunden.

Carrier dynamics in strained Si1−xGex layers and Si/Si1−xGex superlattices, grown by molecular beam epitaxy with different Ge concentrations, were investigated by a transient grating method. The ambipolar diffusion coefficient Da of carrier transport parallel to the layer plane was determined at high-density carrier excitation. An increase to Da values by a factor of up to 1.5 was observed for the strained alloy films compared to the value for moderately doped molecular beam epitaxy silicon layers. This is the first experimental evidence for enhanced performance of parallel carrier transport in Si1−xGex alloy layers.

The spectral and temporal behavior of absorption bleaching and of self-diffraction in Zn$!d,-,Te polycrystalline films with a nominal composition x=0.87 have been investigated. It has been found that the optical transmission increases 15 times at a wavelength of 575 nm for a laser light intensity of 0.2 GW/cm*. In a pump and test experiment, the buildup and the relaxation times T, of the bleaching were measured. Their values, between 10 and 40 and 20 and 45 ps, respectively, depend on the wavelengths of the pump and the test beams. The saturation intensity 1, was also determined (between 1.5 and 6 GW/cm2). This absorption saturation effect can be related to the high density of crystallite grain surface states as well as to other imperfections in the polycrystalline films studied. The high density of these states can explain the large linear absorption and the ultrafast relaxation of the photogenerated carriers. Then, in self-diffraction experiments, the phase relaxation time T2= (lOA5) ps of the localized surface states has been determined. The long duration of T, and the short energy lifetime T1 indicate that elastic scattering processes are highly suppressed in polycrystalline material when compared to monocrystals, while their coherence time is increased. 5729