Quantum interference effect and electric field domain formation in quantum well infrared photodetectors

Abstract

An observation of quantum interference effect in photocurrent spectra of a weakly coupled bound-to-continuum multiple quantum well photodetector is reported. This effect persists even at high biases where the Kronig-Penney minibands of periodic superlattice potential in the continuum are destroyed. Our results show that electrons remain coherent over a distance of 40-50 nm. The observation was used to investigate electric field domain formation induced by sequential resonant tunneling in the superlattice. A large off-resonant energy level alignment between two neighboring wells in the high field domain was observed. © 1995 American Institute of Physics.Recently, there has been great interest in studying optical and transport properties of multiple quantum well ͑MQW͒ structures. In these ''artificial molecules,'' energy quantization and wave nature of carriers have been used to design new devices and to demonstrate some basic laws of quantum mechanics, e.g., to observe minibands in the continuum of periodic potential superlattice, 1 to observe suppression of optical absorption in coupled potential wells, 2 and to make quantum well infrared photodetectors ͑QWIPs͒ by using minibands in the continuum.3 In this letter, we report on a new observation of a quantum interference effect in the photocurrent spectra in bound-to-continuum QWIPs. Using this effect, we analyze the electric field domain formation in the superlattice.In a weakly coupled MQW structure with two bound states in each well ͑i.e., a bound-to-bound QWIP͒, 3 the absorption spectrum is a Lorentzian shape peak corresponding to a transition between the ground state and the first excited state. The contribution of other states in the continuum above the barrier is negligible because of the oscillator strength sum rule ͑one-to-two transition, both states being localized in the well, has a much more significant transition dipole matrix element͒.When the quantum well parameters allow only one state in the well ͑i.e., a bound-to-continuum QWIP͒, the absorption spectrum is not Lorentzian any more, the states above the barriers have a strong contribution to the absorption. Because these continuum states are extended over the barriers and several neighboring wells ͑depending on the coherence length of electrons͒, electron interference effects can be observed in the absorption spectrum. At zero bias, due to the potential translation symmetry there are well-known minibands in the continuum states of the superlattice 1 which can be calculated using, for example, the Kronig-Penney model. The miniband energy gaps, depending on the overlap of the states between neighboring wells, can be designed large enough to be observable in the absorption spectrum. However, under an applied bias, such that the voltage drop per period is bigger than these energy gaps, the miniband structure is destroyed. We will show that in a QWIP even at large biases, one can still see some features in the photocurrent spectrum which are due to electron interference effects over one or two periods ...

The effect of temperature on the resonant tunneling and electric field domain formation in multiple quantum well superlattices

“…1͑b͒. 4 Assuming electron coherence length of a few periods of the superlattice and an electric field strength of ϳ31 kV/cm in the HFD, one can deduce the positions of the peaks in good agreement with the experimental photocurrent spectrum.The current-voltage characteristics are shown in Fig. 2.…”

“…3, is more evidence for the existence of EFDs in the superlattice. 4 Figure 3 shows also the sweep-up and the sweep-down measurements of the I -V characteristics, which supports the multistability observation of Ref. 5, and is an indication of the complexity of the growth of electric field domains in the device.…”

“…7 The self-consistent simulation gives an electric field strength of ϳ26 kV/cm in the HFD. One also notices the presence of side peak at ϳ182 meV in the photocurrent spectrum at Tϭ50 K. This peak, being an indication of electron coherence over the QW structure, 4 is, however, less pro- nounced than the 10 K one. This suggests an increased contribution of incoherent processes to the photocurrent spectrum at higher temperatures.…”

“…As discussed in Ref. 4, the sample was designed to have one bound state in the well, and a first resonant state highly in the continuum, about 24 meV above the barrier. Devices with 200 m diameter were fabricated by standard photolithography and wet chemical etching techniques.…”

“…Recently, the sensitivity of the photocurrent spectra in single-bound state MQW structures to well spacings and to the electric field was used to investigate electron coherence effects and the electric field distribution in the device. 4 In this letter, we report on the effect of the temperature on current transport in MQW devices, showing how the transport changes from being dominated by sequential resonant tunneling at low temperatures to domination by nonresonant processes at higher temperatures. Evidences for these nonresonant transport mechanisms which lead to strong changes in the electric field distribution in the device are also presented.…”