Abstract

Double barrier tunneling heterostructures are part of an emerging class of devices that rely on quantum-mechanical effects for their operation. A deeper understanding of these devices is possible when diagnostic methods that augment electrical measurements are employed. In our project, we have used differential absorption spectroscopy (DAS) to investigate the operation of the double barrier diode (DBD). This technique, whose experimental realization is detailed in Fig. 1, relies on measurements of normalized changes in optical transmission, ΔT/T = [T(V) - T(V₀)]/T(V₀) where T(x) is the transmission at voltage x, arising from electrical modulation imposed upon the device.[l,2] For small signals ΔT/T = lΔα, where Δα = α(V₀)-α(V) is the change in absorption and I is the length over which the change occurs. Lock-in signal extraction allows the observation of ΔT/T signals as small as ~ 10⁻⁴, permitting observation of events on length scales ~ l0Å with low probe intensities (we use an incandescent lamp). DBD depletion and accumulation regions can be seen, as can signals originating in the QW. This signal can be calibrated to obtain the QW stored charge Q. Other workers have reported measurements of Q using photoluminescence,[3] excitation spectroscopy in conjunction with photoluminescence,[4] as well as methods involving magnetic fields.[5] Since we can use low light intensities and rely on optical absorption-a technique allowing absolute calibration-we believe DAS to be both a significantly more accurate[3] and a much less invasive method of determining Q. In addition, we can determine the energy distribution of the electrons stored inside the QW, without the confusion caused by creating large numbers of highly excited electron-hole pairs in the measurement process. All samples were grown by MBE on conducting InP substates and show profound negative differential resistance (NDR). Geometries are similar to that illustrated in Fig. 2, with variable barrier thicknesses.[1] The barrier material is In_0.52Al_0.48As, with QW and electrodes being In_0.53Ga_0.47As. Measurements were performed on 150 μm² mesas at 10K.

© 1991 Optical Society of America