Photo-excited semiconductor superlattices as constrained excitable media: Motion of dipole domains and current self-oscillations

A model for charge transport in undoped, photo-excited semiconductor superlattices, which includes the dependence of the electron-hole recombination on the electric field and on the photo-excitation intensity through the field-dependent recombination coefficient, is proposed and analyzed. Under dc voltage bias and high photo-excitation intensities, there appear self-sustained oscillations of the current due to a repeated homogeneous nucleation of a number of charge dipole waves inside the superlattice. In contrast to the case of a constant recombination coefficient, nucleated dipole waves can split for a field-dependent recombination coefficient in two oppositely moving dipoles. The key for understanding these unusual properties is that these superlattices have a unique static electric-field domain. At the same time, their dynamical behavior is akin to the one of an extended excitable system: an appropriate finite disturbance of the unique stable fixed point may cause a large excursion in phase space before returning to the stable state and trigger pulses and wave trains. The voltage bias constraint causes new waves to be nucleated when old ones reach the contact.Comment: 19 pages, 8 figures, to appear in Phys. Rev.

Picosecond optical time-of-flight studies of carrier transport in α-Si:H/α-SiNx:H multilayers

Journal ArticleWe report time-of-flight experiments in the time range from 0.2 psec to 1.8 nsec in 0-Si:H-a-SiNx:H multilayer structures using a purely optical technique. The transport mechanism of photoexcited carriers is shown to be dispersive and its characteristic parameters are determined in the temperature range 70-300 K

Dynamics of Electric Field Domains and Oscillations of the Photocurrent in a Simple Superlattice Model

A discrete model is introduced to account for the time-periodic oscillations of the photocurrent in a superlattice observed by Kwok et al, in an undoped 40 period AlAs/GaAs superlattice. Basic ingredients are an effective negative differential resistance due to the sequential resonant tunneling of the photoexcited carriers through the potential barriers, and a rate equation for the holes that incorporates photogeneration and recombination. The photoexciting laser acts as a damping factor ending the oscillations when its power is large enough. The model explains: (i) the known oscillatory static I-V characteristic curve through the formation of a domain wall connecting high and low electric field domains, and (ii) the photocurrent and photoluminescence time-dependent oscillations after the domain wall is formed. In our model, they arise from the combined motion of the wall and the shift of the values of the electric field at the domains. Up to a certain value of the photoexcitation, the non-uniform field profile with two domains turns out to be metastable: after the photocurrent oscillations have ceased, the field profile slowly relaxes toward the uniform stationary solution (which is reached on a much longer time scale). Multiple stability of stationary states and hysteresis are also found. An interpretation of the oscillations in the photoluminescence spectrum is also given.Comment: 34 pages, REVTeX 3.0, 10 figures upon request, MA/UC3M/07/9

Interchain photogeneration of charged solitons in trans-(CH)x

Journal ArticleIn a recent Letter1 Rothberg, Jedju, Etemad, and Baker (RJEB) used a novel photoinduced-absorption (PA) technique to study charged-soliton (S ?) dynamics in trans-(CH)X with picosecond resolution by measuring transient PA at 0.45 eV. Both intrachain and interchain photogeneration of S ? are reported. The former process occurs instantaneously, while the latter process is delayed by about 30 ps and was interpreted1 as neutral soliton (S?) to S ? conversion. In this Comment we would like to experimentally support and complement RJEB's measurements and to further discuss the interchain photogeneration process

Picosecond dynamics of photoexcitations in amorphous multilayer structures

Journal ArticleWe report on measurements of ultrafast relaxation processes in transmission and reflection in amorphous multilayer structures consisting of a-Si:H, a-SiNx:H, a-SiOx:H, and a-Ge:H. The decays recorded in transmission in the a-Si:H/a-SiNx :H and a-Si:H/a-SiOx :H multilayers depend strongly on the silicon sublayer thickness and are interpreted in terms of carrier transport to and trapping at interfacial defects. In the a-Si:H/a-Ge:H multilayers we observe oscillations in reflectivity due to standing acoustic waves with a frequency that depends on the repeat distance of the multilayer