Impact ionization induced negative far-infrared photoconductivity inn-GaAs

Far-infrared photoconductivity ofn-GaAs epitaxial layers showing impact ionization breakdown has been investigated by molecular lasers at photon energies below the 1s-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 theN=0 Landau band to donor levels shifted by the magnetic field above the low energy edge of the conduction band

High power splitting of the cyclotron resonance induced photoconductivity in n-GaAs

In summary our results show that photoconductivity and optical absorption under cyclotron resonance conditions in high purity n-GaAs are more complex than it has been assumed previously. As cyclotron resonance can only be measured with non zero free electron concentration, and because the 2p – shallow donor level remains below the N=0 Landau level for all magnetic field strengths, a certain population of 2p – states must be present and thus the interference between both absorption processes is unavoidable as long as 2p –rarr2p + electric dipole transitions are activated by ionized impuritics. This interference gets most drastically manifest at high intensities causing an apparent splitting of the photoconductivity line. Even at lower intensities however when the dip in the photoconductivity line is not observable, all optical characteristics previously attributed to cyclotron resonance are affected by the shallow donor absorption. The energy separationE 2p+–E 2p– of shallow donors in a magnetic field is exactly equal to hstrokw CR only for isolated impurities in the effective mass approximation. The same electric stray field of ionized impurities which cause the activation of 2p –rarr2p + absorption may shift the donor energy levels due to stark effect. For magnetic field strengthsBE 2 +–E 1p – from the cyclotron resonance quantum energy was observed [15] amounting up to 8 pc. In this case we expect that the peak positions of photoconductivity and absorption do not spectrally coincide and do not occur at the resonance magnetic field strength of 0rarr1 Landau level transitions. Thus effective masses determined by standard cyclotron resonance methods at low magnetic fields may be incorrect by a few percent