Bidirectional photocurrent of holes in layers of Ge/Si quantum dots

Abstract

Intensive studies of the fundamental processes of light absorption and the appearance of a photocurrent in low dimensional systems are stimulated by the necessity of the implementation of effective pho totransducers of the mid and far infrared (IR) radia tion ranges. Recently, particular interest was noted in photon detectors containing layers of quantum dots as an active element. The complete removal of the for biddance of optical transitions polarized in the growth plane of structures with quantum dots provided the possibility of the absorption of normally incident elec tromagnetic radiation by holes and electrons in con trast to the structures with quantum wells [1]. Arrays of quantum dots are characterized by the lifetime of the nonequilibrium charge carriers higher than that of the higher dimensional systems owing to the suppression of scattering on phonons. The thermal generation rate of charge carriers decreases owing to the discreteness of the energy spectrum in them, and the oscillator strength for the intraband and exciton transitions increases as a result of the localization of the wave function in all three directions. Silicon photodetectors with Ge quantum dots are of particular interest [2, 3]. The main advantage of such structures is that they can be manufactured on Si substrates; i.e., there is the pos sibility of manufacturing very large size matrices as a result of the monolith integration with silicon field transistors and high speed signal processing electron ics. Ge/Si quantum dots refer to heterostructures of the second kind. Here, the breakups of the valence band and the conduction band at the heterobound aries have the same sign. As a result, only holes are localized in the Ge regions, and electrons are in the states of the continuous spectrum of the Si conduction band. The binding energy of holes in Ge nanoclusters depends weakly on the conditions of formation of the Ge/Si structures and is 200-400 meV, providing a photoresponse in the middle transparency window of the Earth's atmosphere of 3-5 μm [2, 4-6].The possibility for the detector to function in the photovoltaic mode without an external voltage source is the most advantageous when producing matrices of the focal plane with the limiting low energy consump tion and power dissipation. At zero or low bias, the detector noise is minimal and the detecting capability of the photoreceptor devices is limited only by the thermal fluctuations. Under such conditions, the presence of the electric fields built in near quantum dots, which are caused by the asymmetry of the shape of quantum dots, the inhomogeneous distribution of elastic deformations in the medium, and the space charge of the ionized impurity atoms [2,4,7,8], is a significant physical factor. In this respect, fundamen tal problems arise concerning the establishment of the mechanisms and characteristics of formation of the photocurrent at low applied bias voltages. In this work, we measured the spectra of the photoconductivity in boron δ doped silicon structures with Ge quantum do...

Cotunneling and polaronic effect in granular systems

“…The onset of Arrhenius-like behaviour of conductivity at low temperatures in granular materials is reported far less often. For example, in experiments [21] the Arrhenius behaviour was observed below ∼ 5K in twodimensional arrays of semiconducting Ge/Si quantum dots. Above that temperature the conductivity followed Efros-Shklovskii law.…”