In epitaxial layers with a large area to thickness ratio, surface and interface space charge regions behave as transverse resistance-capacitance systems modulating the effective volume that takes part in the electrical conductivity. In this work it is shown that in photoconductivity spectroscopy experiments these surface and interface photovoltaic effects can become dominant, therefore giving information about the sample surface and interface band-bending. In photoconductivity spectroscopy, procedures to identify the signatures of such band bending are described. The present technique to assess surface bending is first applied to gated and ungated GaAs samples, to validate the present model. N-type AlGaAs and undoped InGaAs strained buffer layers, with various degrees of strain and surface roughness, are also characterized by this technique.
Room-temperature time-resolved photocurrent measurements have been performed on novel InGaAdGaAs multiple-quantum-wet1 (MQW) P-CN diodes on (1 1 l ) B GaAs substrates, having an average electric field of opposite sign to the built-in field. Direct evidence for dipole formation, displacement photocurrents and out-of-well screening by photocarriers is found. In these piezoelectric structures the sign of the internal field can be controlled by an external modulating voltage and the optically pumped charges stored at the extremes of the diode MQW active region can be exiracted by appropriate puised biasing. Tie generated dipoie moment and its time evolution have been determined by an optical pump and electrical probe technique the spatial separation of electrons and holes accounts for the observation of slow recombination processes. The piezoelectric constant is evaluated for In,Ga,-,As (x = 17% In) from the zero-average-field condition.
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