Time and frequency resolved photoconductivity studies on amorphous semiconductors

  • Dennis Webb

    Student thesis: Doctoral Thesis


    Time and frequency domain photoconductivity experiments and analysis assuming the multiple trapping transport model are used in a complementary fashion to investigate the density of states (DOS) and associated trapping and recombination parameters of amorphous semiconductors. The solution of the linearised multiple trapping rate equations is straightforward in the frequency domain, and it is simple to invert the modulated photoconductivity (MPC) response to obtain an accurate DOS. It is proposed that a novel, wide energy range, accurate DOS probe is realisable by applying the Frequency domain inversion to the Fourier transform of transient photoconductivity (TPC) data. A numerical Fourier integral procedure appropriate to the nature of TPC data sets is described and tested.

    Application of the Fourier transform method to TPC measurements on undoped a-Si:H produces a calculated DOS distribution covering half the mobility gap. The DOS consists of an 0.03 eV exponential band-tail, and a bump of 4.4x1016cm-3 states at 0.65eV below Ec, attributed to dangling bonds. The method detects the bump despite there being no related emission feature in the TPC decay. The accuracy of the calculated DOS is demonstrated to depend on the size of the missing contribution to the Fourier integral from the signal at times shorter than the experimentally observed minimum.
    An undershoot or sign reversal of TPC measured with optical bias on undoped a-Si:H for steady state generation rate Gss ≥1018cm-3s-1 is observed at a time tus which varies inversely and sublinearly with Gss. From the linearised frequency domain analysis such an undershoot is expected for bimolecular recombination and recombination coefficient CR>Cn, the band-tail trapping coefficient. The analysis predicts a related sign reversal in the imaginary part of the Fourier transform spectrum at a frequency ω∝=CR, which is also observed. Values for CR mostly in the range 1-3x10-8cm3s-1 are calculated.
    Undoped a-Si:H MPC spectra exhibit a phase peak at a frequency ωpeak which varies superlinearly with Gss. The phase peak accompanies a transition in the MPC response from DOS dependent to DOS independent behaviour for ω<ωpeak, in which régime the calculated DOS varies as ω0.43. For a DOS consisting of a single trap species it is expected that the transition frequency be around lOOx smaller than observed and the calculated DOS be proportional to co at low frequency. Assigning a trapping coefficient of 10-100Cn to defect states accounts for the transition frequency but not the variation of the calculated DOS for ω<ωpeak. The calculated DOS for ω<ωpeak  matches the DOS calculated from TPC data.
    Steady state photoconductivity measurements on a-As2Se3 are interpreted to indicate a set of defects sited at 0.66eV above Ev. TPC measurements exhibit no related emission feature. The DOS calculated from the Fourier transform method is a featureless exponential but is unreliable because of the limited measurement bandwidth. The magnitude of the TPC is too small for the DOS to consist solely of an exponential tail extending to the band edge. The DOS calculated from MPC measurements is also a featureless exponential but it is suggested that the MPC response is in the DOS independent régime.
    Date of AwardMay 1995
    Original languageEnglish

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