Carrier-Density-Dependent Lattice Stability in InSb

Abstract

The ultrafast decay of the x-ray diffraction intensity following laser excitation of an InSb crystal has been utilized to observe carrier dependent changes in the potential energy surface. For the first time, an abrupt carrier dependent onset for potential energy surface softening and the appearance of accelerated atomic disordering for a very high average carrier density have been observed. Inertial dynamics dominate the early stages of crystal disordering for a wide range of carrier densities between the onset of crystal softening and the appearance of accelerated atomic disordering. DOI: 10.1103/PhysRevLett.98.125501 PACS numbers: 63.20.Kr, 61.10.ÿi, 64.70.Dv, 78.47.+p First-order phase transitions and chemical reactions require crossing a transition state on the potential energy surface (PES). Characterizing the topography of the energy landscape in the vicinity of the transition state represents the key step to understanding the pathway followed during a chemical reaction or first-order phase transition. The experimental and theoretical characterization of these far from equilibrium regions of the PES has proven to be very difficult because of the vanishingly short time spent near the transition state and the multitude of degrees of freedom that influence chemical and physical transformations in the condensed phase. Time-resolved x-ray scattering experiments provide a window for observing the structural dynamics that occur during certain physical transformations. This is achieved by using femtosecond (fs) x-ray pulses to monitor laser initiated dynamics, selectively track the time-dependent evolution of nonequilibrium atomic structures, and extract the shape of a photoinduced PES [1][2][3][4].This approach has proven crucial to investigating the influence of carrier excitation on the stability of tetrahedrally bonded semiconductors. Theoretical, experimental, and simulation studies of these systems indicate that extreme carrier densities destabilize the crystal structure and lead to nonthermal melting [3,[5][6][7][8][9][10][11][12][13][14][15][16][17]. Theoretical studies predict a rapid reduction in the shear restoring force when the excited carrier density exceeds a few percent of the valence band electron density [5][6][7]. A further doubling of the carrier density eliminates the shear restoring force, transforms the room temperature potential energy minimum into a saddle point, and leads to accelerated atomic disordering.The initial ultrafast x-ray diffraction studies of laserexcited InSb at the Sub-Picosecond Pulse Source (SPPS) determined that inertial atomic displacements on a lasersoftened potential energy surface dominate the response to intense optical excitation during the first 500 fs for a range of laser fluences [3]. This predominance of inertial dynamics in a wide fluence range had not been predicted by either theory or simulation. While the studies of Lindenberg et al. [3] and Gaffney et al. [15] covered the mean carrier density range over which theory predicted crystal stability to r...

Detector response to high repetition rate ultra-short laser pulses. I

“…Direct observations of lattice changes following femto-second optical excitations were reported with X-ray diffraction measurements [4] and, those results shows that lattice changes start in ~ 1 pico-second after pumping laser pulse -see Figure 2, b. minuoiszaimm Bleaching time or transient absorption time measurement has been done by direct measurement of changes in semiconductor optical characteristics. Time-resolved semiconductor transmission measurement provides information excited electrons dynamics after the excitation by ultra-short pulse.…”

“…Free carriers generated in such a way posses an excess of energy determined by the difference between energies of continuum of photon excited energies and very specific band gap energy -(hν−Εg) -see Figure 2, a. Carrier-carrier interaction leads to equalization of free carriers energies inside the both bands: free electrons in the conduction band and holes (vacancies) in the valence band. Equalization is quite rapid process and it lasts up to tens of femto-seconds [4]. However, energy-equalized continuum of free carriers still has effective temperature Te significantly higher than lattice temperature TL.…”

“…That process calls thermalization and the timing of thermalization is presented in Figure 2,a. Thermalization or energy-redistribution that takes place in the conduction band may last much longer than excitation time -measured in Ge and InSb that energy redistribution time is in few picoseconds [4,6]. Pump-probe technique has been used to investigate processes in solids and to detect changes in the lattice due to energy transfer via either X-ray diffraction on lattice [4].…”

“…Timing of energy transfer (a) and thermalization experiment[4] with X-ray diffraction following light-induced transient absorption (b) in optically excited bulk semiconductor -Ge.…”

Non-Thermal Collapse of the Silicon Lattice Observed with Femtosecond Electron Diffraction

“…It has been proposed that above a certain excitation threshold disorder occurs non-thermally due to weakening of the covalent bonding [2] before the electronic energy dissipates thermally into the lattice through electron-phonon coupling. Recently, non-thermal collapse of the InSb lattice has been revealed by femtosecond X-ray diffraction [3].The silicon membranes were excited at an absorbed fluence of 70 mJ/cm 2 corresponding to a carrier density of 2.8×10 22 cm -3 , which is equivalent to 14% of the valence electrons. Diffraction images were taken before, during, and after excitation at a series of time delays.…”