Phototransport under the presence of a small steady-state photocarrier grating

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Abstract
By taking into account space-charge effects, general expressions for carrier distributions and photocurrent are derived for the case of a small steady-state photocarrier grating (SSPG) in a finite electric field. Conditions are specified under which the continuity equations can be linearized and solved analytically. It is shown that the approach of spatially averaging local resistivity in the previous treatments of SSPG transport can be flawed. It is argued that the drift motion of electrons and holes is predominantly bipolar rather than ambipolar as asserted earlier. The prerequisites for determining the ambipolar diffusion length L by the SSPG technique are identified and compared to those obtained before. It is demonstrated that under the weak-field condition, L can be evaluated without the lifetime-regime or ambipolarity restriction, particularly when the recombination lifetime τ is known. When the dielectric relaxation time is far shorter than τ, the transport equation is reduced to the familiar formula derived by assuming space-charge neutrality. Also illustrated is the possibility of deducing the ratio of electron-to-hole drift mobility through the electric field dependence of photocurrent under the strong-field condition.