PROBING RESONANT-TUNNELING AND CHARGE ACCUMULATION VIA CAPACITANCE MEASUREMENTS IN [111]-ORIENTED INGAAS GAAS MQW AND SUPERLATTICES/

The features present in the capacitance and conductance-voltage charac teristics of [111]-oriented InGaAs/GaAs MQW and SL, under illumination or dark conditions, are described. The different peaks observed in a p-i-n diode with intrinsically negative average electric field in the MQW region are attributed to sequential tunneling of electrons and hol es between adjacent wells. The capacitance characteristics and their e volution with voltage provide clear evidence for a long-range (out-of- well) screening of the piezoelectric fields by the photogenerated carr iers accumulated at the extremes of the MQW region. This effect is sho wn to be consistent with a self-consistent solution of the Schrodinger and Poisson equations. The voltage corresponding to zero average elec tric field is detected by a feature in the dark device capacitance in a seven period MQW structure and in a 40 period [111] strained superla ttice, for which changes in capacitance and conductance associated wit h miniband formation are shown. Delta n not equal 0 transitions in the quantum wells are observed by photocapacitance spectroscopy even unde r negative average field conditions, where the photocurrent response c annot be detected. X-ray measurements have been used to obtain the per iod thickness and the average In content.