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TIME RESOLVED RAMAN SPECTROSCOPY OF
OPTICALLY GENERATED PHONONS IN III-V
SEMICONDUCTORS
J. Tsang, J. Kash, J. Hvam
To cite this version:
JOURNAL DE PHYSIQUE
Colloque C7, supplément au n°10, Tome 46, octobre 1985 page C7-235
TIME RESOLVED RAMAN SPECTROSCOPY OF OPTICALLY GENERATED PHONONS IN I I I - V SEMICONDUCTORS
J . C . Tsang, J . A . Kash and J.M. Hvam
IBM T.J. Watson Research Center, P.O. Box 818, Yorktown Heights, 10598 New York, U.S.A.
Résumé - Grâce à un laser sub-picoseconde et à un système de diffusion Raman muni d'1 détecteur multicanaux ultra-sensible, nous avons obtenu des spectres de diffusion Raman résolus en temps à l'échelle de la sub-picoseconde et mesure ainsi la dynamique des phonons LO hors d'équilibre dans GaAs et InAs. La dépendance en temps de la popula-tion de phonons en excès les variapopula-tions du spectre avec le temps et le niveau d'excitapopula-tion ont été étudiés.
Abstract - A sub-picosecond laser and a highly sensitive multichannel detector Raman scattering system have been combined to perform sub-picosecond, time resolved Raman scattering experiments on the dynamics of non-equilibrium LO phonons in GaAs and InAs. The time dependence of the excess phonon population and the changes in the spectral content with both time and excitation level have been studied.
I - INTRODUCTION
The relaxation of energetic carriers in polar semiconductors such as GaAs and InAs is dominated by the emission of longitudinal optical phonons when the hot carriers are more than a few LO phonons above the band edges./1/ Because of the long range dipole fields associated with these vibrational modes, the generation process strongly emphasizes the emission of phonons with small wavevectors. These non-equilibrium phonons can be generated by carriers excited by a visible laser and also directly observed by Raman spectroscopy using the same laser./2/ We have combined a laser system emitting 0.5 picosecond optical pulses with a highly sensitive, multichannel optical detector Raman scattering system to directly study the non-equilibrium LO phonons generated by hot carriers in polar semiconductors./3/ We have resolved the time dependence of the non-equilibrium LO phonon scattering following the creation of the "hot" carriers. We directly measure the changes in the non-equilibrium vibrational spectrum as a function of time and excitation intensity. This allows us to characterize both the generation process for hot phonons in III-V semiconductors and the changes introduced into the vibrational spectrum by the presence of optically injected carriers. Our results show the power of Raman spectroscopy as a tool for the study of non-equilibrium phonons in these materials.
II - EXPERIMENTAL METHODS
In Fig. 1, we schematically describe the experimental system used to obtain Raman scattering from GaAs and InAs with a temporal resolution of better than 1 picosecond. We use the pump/probe technique, taking advantage of its superior time resolution and the well defined polarization selection rules for LO phonon scattering from the (100) surfaces of GaAs and InAs to measure the time dependent changes introduced into the vibrational spectra of these materials by optically excited carriers.
We derive both the pump and probe beams from a mode locked Ar ion pumped dye laser. The 5 picosecond, 76 MHz dye laser pulses are focussed into a1 single mode, polarization preserving, optical fiber which frequency broadens and chirps the transform limited dye laser pulses./4,5/ The
Permanent address : Fysisk I n s t i t u t , Odense University, DK-5230 Odense M, Denmark
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fiber grating I I 76 MHzFig. 1. A schematic representation of a sub-picosecond time resolved pump probe Raman scattering system.
linear chirp is matched to the linear dispersion of a diffraction grating used in a multiple pass geometry to produce a temporal compression of the chirped pulse because of the difference in beam path length for different frequency components of the pulse. This produces a 76
MHz
train of 0.5 ps wide pulses. The spectral width of these pulses is about 3 0 cm-', somewhat greater than the transform limited minimum of 15 cm-I expected for a pulse of this duration. The compressed pulses are accompanied by a weak continuum background due to co-propagating luminescence in the optical fiber. The intensity of this luminescence background is several orders of magnitude weaker than the peak intensity of the compressed pulses. However, it is strong enough to make it impossi- ble to observe the Raman scattering from GaAs and InAs because of the small values of the Raman scattering cross sections in these opaque semiconductors. We remove the continuum background by taking advantage of the dispersion of the grating compressor. A slit assembly was placed in one arm of the compressor and adjusted only to pass the chirped pulses and to cut off the continuum background. This reduced the continuum sufficiently to allow for measurements to be made on samples with smooth optical surfaces. The results presented in this paper were obtained for a pump and probe wavelength of 588 nm.The 0.5 picosecpnd pulses were divided by a beam splitter into a pump and a probe beam. The pump beam passed through a variable optical delay line so the two beams could interact with the sample at different times. Typical pump power levels were about 10 mW while the probe power levels were usually below 2 mW. For most of the experiments described here, the two beams were focused on the sample surface with a single lens. The pump and probe spot sizes on the sample varied between 100 and 200 microns in diameter. For our powers, such spot sizes produced optically injected carrier densities in GaAs of 1016 to 1 0 ' ~ / c m ~ . The optically injected electrons i n GaAs have about 0.6 eV of excess energy per carrier. Therefore these hot carriers can emit about 15 LO phonons before reaching the band gap. In InAs, the carrier densities are 3x higher because of the higher absorption coefficient. The excess energy per carrier is almost 1.7 eV so about 60 LO phonons are emitted before the bandgap is reached. At 588 nm, the probe beam working in the backscattering geometry can couple to phonons with wavevectors centered about 8x10' cm-' in both materials.
The Raman scattering due to the pump beam is always much stronger than the scattering due to me probe. By adjusting the polarization of the pump and probe beams to be orthogonal, the polariza- tions of the pump and probe scattered light from the LO phonons in GaAs will also be orthogonal for light polarized along either the 100 and 110 axes. By restricting the light incident on the monochromator to only one polarization, we can discriminate against the scattered light arising from the pump, substantially improving our statistics for the detection of the pump induced changes in the probe scattering.
A triple monochromator was used to both filter the elastically scattered light and analyze the inelastically scattered light from the probe. A cooled microchannel plate photomultiplier with a position sensitive resistive anode is used to detect the dispersed scattered light./6/ With a dark count per channel of about 0.01 counts per second, a spatial resolution of about 60 microns over a diameter of 1 inch and a photocathode quantum efficiency in the visible of almost 0.1, this detector is at least two orders of magnitude more sensitive than a conventional photomultiplier. Signals were detectable at count rates below 0.1 count per second, making the use of probe beams with a strength of less than 2 mW quite practical in the Raman studies of GaAs and InAs. Our spectral resolution at 0.5 picoseconds was determined by the spectral width of our laser pulses.
Most of our results were obtained at room temperature with the sample either in air or a flowing He ambient. The GaAs samples were intrinsic MBE or MOCVD grown oriented (100). The surfaces were used as grown although they were washed in organic solvents to remove any possible organic contaminants arising from the laboratory environment. The InAs samples studied were polished (100) wafers, n type with 3 x 1 0 ' ~ carriers/cm3 at 300 K.
With the sensitivity and stability of our experimental system, we were able to study pump induced changes in the phonon scattering intensity that were an order of magnitude smaller than the normal equilibrium room temperature Raman scattering. Within this experimental regime, the analysis of the data is considerably simplified since substantial changes in the dynamics are not expected unless the non-equilibrium phonon population is significant compared to the equilibrium population./7,8/ The time resolved Raman spectra were obtained in a difference mode. The Raman spectrum was measured for a particular delay time between the pump and the probe. A second spectrum was obtained in a configuration where the probe preceded the pump by several picoseconds. Since we are interested in only the pump induced effects on the probe Raman scattering, subtraction of the two spectra allowed us to discriminate against the normal, thermal, pump and probe driven Raman spectra. In addition, this procedure allowed us to discriminate against pump induced luminescence from the samples since this emission is also independent of the delay time between the pump and the probe.
I11 - RESULTS
In Fig. 2, we show time resolved anti-Stokes Raman spectra obtained from InAs at 300 K. The anti-Stokes scattering is preferred for studying pump induced changes in the phonon population since its intensity is simply proportional to the occupation number of the Raman active modes. We show the results obtained for -1, 0 and 7 picosecond delays of the probe with respect to the pump and compare all three with the Raman scattering obtained when the pump does not perturb the sample before the probe interrogates it (dotted line spectra). The peculiar shape and width of the
LO phonons shown in Figure 2 arise from the spectral shape of of our 0.5 picosecond pulses. The single strong line at 240 cm-', in Fig. 2, is the small wavevector LO phonon of InAs. For the scattering geometry used here, only LO phonon scattering from the probe is symmetry allowed. In Fig. 2a, the solid and dotted curves overlap strongly, showing that when the probe preceeds the pump on the sample by about 1 psec., there is very little if any, extra, pump induced scattering. Within 1 picosecond however, the LO phonon scattering becomes almost twice as strong as the scattering in the absence of the pump perturbation. For 7 ps. delay, Fig. 2c shows that the extra
LO phonon scattering is less than a third of its value in Fig. 2b. In all three spectra, the only observable change due to the pump is the increase in the LO phonon scattering intensity. We have also measured the Stokes scattering as a function of delay time to confirm that the increase in LO
phonon scattering is due to a pump induced increase in the LO phonon population.
C7-238 J O U R N A L D E PHYSIQUE
Fig. 3. The Stokes and anti-Stokes scatter- Fig. 2. Anti-Stokes Raman spectrum of ing from GaAs under 5 ps laser excitation. InAs for three different delay times of the a)Low power densities. b) Optically inject- probe with respect to the pump beam. a) ed carrier densities in excess of 10''/crn3. -1 ps. b) 0 ps. c) 7 ps. The dotted curve Each pair of Stokes and anti-Stokes spec- is the Raman scattering excited by the tra are correctly scaled but a) and b) have probe when it precedes the pump by 10 not been corrected for relative differences psec. in integration time and pump power
with a lifetime of about 3-4 picoseconds at room temperature and 6-7 picoseconds at 77 K or lower. von der Linde et a1./2/ demonstrated that this decay is due to the finite lifetime of the LO phonons. Collins and Yu/9/ studied the dependence on photon energy above the band gap of the Raman scattering in GaAs and verified the applicability of the simple cascade model for the relaxation of photoexcited carriers with excess energies of less than 0.5 eV. We have shown that the 2-3 picoseconds required in our GaAs experiments for the non-equilibrium LO phonon population to reach its maximum value reflects the period of time during which the optically injected electrons emit LO phonons as they relax to the bottom of the conduction band. The longer relaxation times measured by Shank et a1./10/ and others reflect the time required for the relaxed carriers to come into thermal equilibrium with the lattice. Since the optically excited electrons emit about 12 Raman active LO phonons before relaxing to the band edges and this takes between 2 and 3 picoseconds, we obtain an estimate of the scattering time for the generation of LO phonons by hot carriers in GaAs of about 160 fsec, in good agreement with theoretical estimates./l/
velocities reported for 111-V semiconductors raise the possibility that a substantial fraction of the optically injected carriers can move into the bulk of the sample over 1 or 2 picoseconds, reducing the rate of increase of the phonon population at later times. Substantial band structure effects may also be present due to the proximity of the E , gap.
In Fig. 3, we show the Stokes and anti-Stokes spectra of GaAs at 300 K. We use 5 picosecond laser pulses to excite these spectra. A single beam is used so the same pulse injects hot carriers and detects the LO phonons. For the excitation power levels and spot sizes in this experiment, the injected carrier densities are in excess of 1 0 ' ~ / c m ~ . A pinhole was used to restrict the scattered light that was detected to that emitted from the center of the focus on the sample in order to minimize the effects of lateral inhomogeneities in the optically induced carrier densities. The low power scattering shown in Fig. 3a is consistent with cw results. The high power scattering is characterized by an order of magnitude decrease in the scattering efficiency from the LO phonon and the growth of a new mode near 268 cm-'. This behavior is due to the coupling of the LO phonons to the optically induced carriers./9/ It points out that the increase in the LO phonon population due to hot carrier relaxation occurs only at relatively low carrier concentrations ( < 1 0 ' ~ / c m ~ ) and that at higher concentrations, the normal modes of the system can be significantly perturbed by hot carriers. The scattering near 268 cm-' arises from the w- mode which is strongly phonon like at these concentrations. The observation of LO phonon scattering is due to the time required to build up the carrier density during the duration of the pulse and the variation in the carrier density with distance into the sample. The w + is not observed due probably to heavy damping of the plasmon./ 1 1
/
IV - CONCLUSION
We have demonstrated the ability of Raman scattering to study the dynamics of phonons excited by hot carriers in 111-V semiconductors. Both the decay time of the excess LO phonon population which is a measure of the LO phonon lifetime and the rise time of the population which is a measure of the relaxation time of the optically injected carriers can now be resolved on the sub-picosecond time scale. The experimental results can be directly interpreted when the carrier concentrations are small enough not to perturb the LO phonons and the optical penetration length large enough that carrier diffusion into the bulk is not significant on this time scale.
ACKNOWLEDGEMENTS We thank W. Wang and T.Kuech for GaAs samples, D. Pettit for InAs samples and Professor S. S. Jha for many helpful discussions.
REFERENCES
/1/ E. M. Conwell and M. 0. Vassel, IEEE Tran. Electron Devices m 3 , 22 (1966) /2/ D. von der Linde, J. Kuhl and H. Klingenburg, Phys. Rev. Lett.
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1505 (1980) /3/ J. A. Kash, J. C. Tsang and J. M. Hvam, Phys. Rev. Lett. 2151 (1985). /4/ H. Nakatsuka, D. Grischkowsky and A. C. Balant, Phys. Rev. Lett.a,
910, (1981) /5/ B. Nikolaus and D. Grischkowsky, Appl. Phys. Lett. 43, 228 (1983)./6/ J. C. Tsang in Dynamics on Surfaces ed. by B. Pullman, J. Jortner, A. Nitzan and B. Gerber, (D. Reidel, Dordrecht, 1984) p. 329.
/7/ J. Shah, R. C. C. Leite, and J. F. Scott, Solid State Commun.
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1089 (1970). /8/ P. J. Price, Superlattice ~icrostructuresl, 225 (1985)/9/ C. Collins and P. Y. Yu, Phys. Rev.
