Impact ionization induced negative far-infrared photoconductivity inn-GaAs

Abstract

Far-infrared photoconductivity of n-GaAs epitaxial layers showing impact ionization breakdown has been investigated by molecular lasers at photon energies below the 1 s -2p shallow donor transition energy. Negative photoconductivity was observed if a magnetic field was applied to the crystals and if impact ionization of donors by the electric bias field was the dominant electron excitation mechanism. The experimental results are qualitatively explained on the basis of the generation-recombination kinetics of electrons bound to donors. Negative photoconductivity is attributed to optically induced free to bound transitions of electrons from the N=0 Landau band to donor levels shifted by the magnetic field above the low energy edge of the conduction band.

Far-infrared radiation controlled chaos in n-GaAs

“…The observed phenomena might be caused by the same microscopic mechanisms which lead to negative fax-infrared photoconductivity in n-GaAs subjected to a magnetic field [14]. This effect has been explained by the assumption that the free electron concentration is depleted by optically induced'free-to-bound transitions from the highly populated N = 0 Landau level into donor bound states shifted into the conduction band by the magnetic field.…”

Nonlinear fir magneto-photoconductivity on quasi-bound Coulomb states of light holes in high purity p-Ge

“…Thus photoconductivity measurements, which require a bias voltage across the sample, do not necessarily reproduce the intrinsic line shape of optical impurity transitions. Furthermore we point out that with the parameter s = 1 no reasonable fit of the data could be obtained; s = 1 has been frequently used to model the impact ionization probability [4,7,8,17]. From the kinetic coefficients we obtain the excited state thermal ionization probability i* = X~(T2 s + X~) -1 ..~ 6.4.10-3, thus in the limit of vanishing impac~ ionization at zero or very low bias voltages not more than about 0.6pc of the population of the metastable state is thermally transfered to the valence band.…”

“…Assuming that the hole mobility is not much changed, it follows that the voltage change is proportional to the optically generated free carrier concentration Ap. The bias voltage was kept well below the impact ionization threshold of shallow acceptors to avoid impact ionization nonlinearities [8].…”

Zeeman splitting of excited boron states in p-Ge

“…The photothermal conductivity spectrum of the sample was measured in Faraday configuration with the magnetic field normal to the [111] faces of the crystal, using a commercial far-infrared Fourier-spectrometer in the frequency range between 45 and 200 cm -1. A standard load resistor circuit was used and the bias voltage was kept well below the critical voltage of impurity breakdown to avoid impact ionisation instabilities in the photoconductive response [7].…”