Li-Hsin Chien

HOT-ELECTRON TRANSPORT IN QUANTUM-DOT PHOTODETECTORS

Employing Monte-Carlo simulations we investigate effects of an electric field on electron kinetics and transport in quantum-dot structures with potential barriers created around dots via intentional or unintentional doping. Results of our simulations demonstrate that the photoelectron capture is substantially enhanced in strong electric fields and this process has an exponential character. Detailed analysis shows that effects of the electric field on electron capture in the structures with barriers are not sensitive to the redistribution of electrons between valleys and these effects are not related to an increase of drift velocity. Most data find adequate explanation in the model of hot-electron transport in the potential relief of quantum dots. Electron kinetics controllable by potential barriers and an electric field may provide significant improvements in the photoconductive gain, detectivity, and responsivity of photodetectors.

Quantum dot photodetectors based on structures with collective potential barriers

It is known that major restrictions of room-temperature semiconductor photodetectors and some other optoelectronic devices are caused by short photoelectron lifetime, which strongly reduces the photoresponse. Here we report our research on advanced optoelectronic materials, which combine manageable photoelectron lifetime, high mobility, and quantum tuning of localized and conducting states. These structures integrate quantum dot (QD) layers and correlated QD clusters with quantum wells (QWs) and heterointerfaces. The integrated structures provide many possibilities for engineering of electron states as well as specific kinetic and transport properties. Thus, these structures have the strong potential to overcome the limitations of traditional QD and QW structures. The main distinctive characteristic of the QD structures with collective potential barriers is an effective control of photoelectron capture due to separation of highly mobile electrons transferring the photocurrent along heterointerfaces from the localized electron states in the QD blocks (rows, planes, and various clusters). Besides manageable photoelectron kinetics, the advanced QD structures will also provide high coupling to radiation, low generation-recombination noise, and high scalability.

Monte-Carlo modeling of electron kinetics in room temperature quantum-dot photodetectors

Results of our many-particle Monte-Carlo modeling of kinetics and transport of electrons in InAs/GaAs quantum-dot infrared photodetectors are reviewed We studied the dependence of the electron capture time on the electric field at different heights of the potential barriers around the dots The capture time is almost independent on the electric field up to a critical field about 1 kV/cm, and than substantially decreases with the field increase We found that the capture time has exponential dependence on the inverse of the average electron energy, which is in agreement with theory Our results show that controllable kinetics in quantum-dot structures may provide a significant increase in the photoconductive gain, device detectivity, and responsivity.

High performance of IR detectors due to controllable kinetics in quantum-dot structures

To optimize the photodetector based on quantum-dot (QD) structures, we develop and exploit a model of the roomtemperature QD photodetector. Using analytical modeling and Monte-Carlo simulations, we investigate photoelectron kinetics, i.e. capture and transit processes, as functions of selective doping of a QD structure, its geometry, and electric field applied. Results of our simulations demonstrate that the photoelectron capture is substantially enhanced in strong electric fields. Detailed analysis shows that effects of the electric field on electron capture in the structures with barriers are not sensitive to the redistribution of electrons between valleys. Thus, most data find adequate explanation in the model of hot-electron transport in the potential relief of quantum dots. We also show that the photoelectron kinetics is very sensitive to potential barriers of intentionally or unintentionally charged quantum dots. The capture processes can be substantially suppressed by a proper choice of the geometry of a QD structure and modulation doping. The suggested model is in agreement with the available experimental results. Manageable kinetics will allow one to employ QDIP as an adaptive detector with changing parameters.

Photodetectors on structures with vertically correlated dot clusters

Long photocarrier lifetime is a key issue for improving of room-temperature infrared photodetectors. Detectors based on nanostructures with quantum dot clusters have the strong potential to overcome the limitations in quantum well detectors due to various possibilities for engineering of specific kinetic and transport properties. Here we review photocarrier kinetics in traditional QDIPs and present results of our investigations related to the QD structures with vertically correlated dot clusters (VCDC). Modern technologies allow for fabrication of various VCDC with controllable parameters, such as the cluster size, a distance between clusters, dot occupation etc. Modeling of photocarrier kinetics in VCDC structures shows that the photocarrier capture time exponentially increases with increasing of the number of dots in a cluster. It also exponentially increases as the occupation of a dot increases. At the same time, the capture processes are weakly sensitive to geometrical parameters, such as the cluster size and the distance between clusters. Compared with ordinary quantum-dot structures, where the photoelectron lifetime at room temperatures is of the order of 1-10 ps, the VCDC structures allow for increasing the lifetime up to three orders of magnitude. We also study the nonlinear effects of the electric field and optimize operating regimes of photodetectors. Complex investigations of these structures pave the way for optimal design of the room-temperature QDIPs.

Monte-Carlo Modeling of Photoelectron Kinetics in Quantum-Dot Photodetectors

Using Monte-Carlo method, we simulate kinetics and transport of electrons in different types of InAs/GaAs quantum-dot infrared photodetectors. Our simulation program exploits Gamma-L-X model of the conduction band of semiconductor and it includes three major types of electron scattering on: 1) acoustic phonons, 2) polar optical phonons, and 3) intervalley phonons. The results of simulation demonstrate that the combination of local potential barriers around quantum dots and quantum-dot structure with collective barriers can be used to achieve long carrier lifetimes, and therefore high photoconductive gain, responsivity, and detectivity.