/ Takashi Iizuka / Professor
/ Victor I. Ryzhii / Professor
/ Irina I. Khmyrova / Assistant Professor
/ Wen Xu / Visiting Researcher
/ Maxim V. Ryzhii / Research Associate
The research activity of the Computer Solid State Physics Laboratory is aimed at investigation of semiconductor quantum functional electronic and optoelectronic devices as a base for prospective computer hardware, intercomputer links and future communication systems. It is expected that by the end of the decade about 20 percent of the components in high performance computer systems will utilize quantum electron and photonic phenomena and this could progress further.
The efforts of the members of the laboratory are focused on
Refereed Journal Papers
We describe the modeling of quantum-well infrared photodetectors (QWIPs) using en ensemble Monte Carlo particle method and present the results of the study of transient recharging effects in QWIPs obtained by the developed technique. It is shown that the excitation of recharging waves can strongly affect the transient photocurrent and can result in the formation of stable or pulsating electric-field and charge domain structures in QWIPs under illumination.
We study steady-state and transient electron transport and capture effects in n-AlGaAs/GaAs multiple quantum well infrared photodetectors utilizing bound-to-continuum transitions using an ensemble Monte Carlo particle modeling. It is shown that nonequilibrium distributions of electrons over the conduction band valleys essentially determine the characteristics of these devices. The macroscopic capture parameter is calculated as a function of the electric field. We show also that monotonic electric-field distributions can be unstable with the excitation of the wave of quantum well recharging. As a result of the instability, quasi-periodic domain structures in quantum well infrared photodetectors can be formed.
In recent paper, Ershov considered an artificial situation of photoexcitation of only one quantum well in a multiple quantum well infrared photodetector using the drift-diffusion model of electron transport. We show that the results obtained in this work can be interpreted as an artifact.
We review the occurrence of periodic or near periodic electric-field domains stimulated by bound-to-continuum transitions under the influence of infrared radiation in multiple uncoupled quantum wells. The conditions of formation of such domains and their stability are studied theoretically using the proposed phenomenological model.
We study the spatiotemporal variations of electric-field distributions in multiple quantum well structures excited by infrared radiation due to bound-to-continuum electron transitions using ensemble Monte Carlo particle simulations. The occurence of self-sustained pulsing and the formation of spatially periodic electric-field domains with weakly oscillating amplitudes demonstrated in the simulations are associated with the excitation of the recharging waves. It is shown that the dynamics of electric-field distributions and photocurrent can be different in the structures with even and odd numbers of quantum wells.
We investigate the spatio-temporal electric-field distributions in multiple quantum well infrared photodetectors (QWIPs) excited by infrared radiation using ensemble Monte Carlo particle modeling. It is shown that self-organized stable periodic electric-field structures can occur in QWIPs excited by infrared radiation. These electric-field distributions have a period equal to twice the period of the QWIP structure. The periodic distributions are superimposed by relatively weak spatio-temporal oscillations. The transition between two stable periodic structures in response to step-like variation of the applied voltage is demonstrated. The periodicity of the electric field structures is conserved after the cessation of illumination for a long time. This time is determined by relatively slow thermoexcitation processes. The occurrence of the electric-field distributions can pronouncedly affect the QWIP steady-state and noise characteristics.
We review the occurrence of periodic electric-field and charge domains in multiple quantum well infrared photodetectors (QWIPs) under relatively strong illumination. The analysis of the formation of periodic domains and their features is based on both ensemble Monte Carlo particle simulations and original analytical models.
The response of multiple quantum well (QW) infrared photodetectors to the photoexcitation of one QW selected from many identical QWs was recently modeled by M.Ershov. We point out here that the presented analysis based on the use of drift-diffusion model for a system with a few electrons is incorrect.
We present the results of a new analytical model for the analysis of the dark current in realistic quantim dot infrared photodetectors (QDIPs). This model includes the effect of the space charge formed by electrons captured in QDs and donors, the self-consistent electric potential in the QDIP active region, the activation character of the electron capture and its limitation by the Pauli principle, the thermionic electron emission from QDs and thermionic injection of electrons from the emitter contact into the QDIP active region, and the existence of the punctures between QDs. The developed model yields the dark current as a function of the QDIP structural parameters, applied voltage, and temperature. It explains some features of the dark current characteristics observed experimentally.
In this paper a model for hot-electron transport and capture phenomena in semiconductor heterostructures with multiple uncoupled quantum wells (QW) excited by infrared radiation is presented. The model takes into account the nonlocal character of the electron transport and capture of mobile electrons propagating over the barriers. The model is used for the calculation of steady-state spatial distributions of the electric field and the average electron energy in multiple QW structures. The conditions of the formation of periodic electric-field domains revealed recently in ensemble Monte Carlo particle simulations are found. The developed analytical model highlights the origin of periodic electric-field domains in multiple QW structures and explains some features of such domain behavior.
We demonstrate that the self-excitation of plasma oscillations in a resonant-tunneling transistor structure results in the generation of transient current at frequencies in the terahertz range. The amplitudes and the frequencies of different modes of the generated oscillations can be tuned by the voltage.
We calculate the photocurrent in realistic quantum dot infrared photodetectors (QDIPs) using a developed device model. This model takes into account the space charge and the self-consistent electric potential in the QDIP active region, the activation character of the electron capture and its limitation by the Pauli principle, the thermionic electron emission from QDs and thermionic injection of electrons from the emitter contact into the QDIP active region, and the existence of the punctures between QDs. The developed model yields the photocurrent in a QDIP as a function of its structural parameters, applied voltage, and intensity. The calculated dependences are in agreement with available experimental data. The obtained results point the way for the optimization of QDIPs in order to realize their potential advantages.
We present an analytical model for the photon recycling effect associated with the reabsorption of generated photons in a light-emitting diode (LED) driven by nonuniform current injected from a quantum well infrared photodetector (QWIP). The model is applied to the evaluation of the modulation transfer function and the external quantum efficiency in pixelless imaging devices based on the integration of a QWIP with a LED. The modulation transfer function and the signal external quantum efficiency of a QWIP-LED pixelless imager are calculated as functions of the structural physical and geometrical parameters. It is shown that photon recycling is a significant factor that limits the spatial resolution of images and increases the external quantum efficiency.
We study the effect of photon recycling in double heterostructure light emitting diodes (LEDs) with relatively small area contact providing nonuniform injection of electrons. A simple phenomenological model of the electron and photon transport in the LED is used to calculate the spatial distributions of electrons and output radiation as well as the external quantum efficiency as functions of device parameters. It is shown that photon recycling is the governing factor of the operation of LEDs with nonuniform injection.
We propose the use of a resonant-tunneling transistor with high mobility of electrons in its quantum-well channel as a detector and a frequency multiplier of terahertz radiation and analyze its characteristics. The combination of strong nonlinearity of the resonant-tunneling current, negative differential conductivity, and excitation of plasma waves in the channel can provide higher performance of detectors and frequency multipliers based on the resonant-tunneling transistor compared to those considered previously.
We review the occurrence of periodic or near periodic electric-field domains stimulated by bound-to-continuum transitions under the influence of infrared radiation in multiple uncoupled quantum wells. The conditions of formation of such domains and their stability are studied theoretically using the proposed phenomenological model.
This paper presents the recent developments of device models for quantum dot infrared photodetectors (QDIPs) and for imagers based on the integration of these photodetectors with light emitting diodes (LEDs). We derive analytical formulas for the dark current and the responsivity in QDIPs based on different QD structures and the QDIP-LED contrast transfer characteristic as functions of the structural parameters and the bias voltage. It is shown that the characteristics of QDIPs are strongly affected by the effect of electron accumulation in QDs close to the emitter contact. The main effect limiting QDIP-LED imager resolution is associated with the processes of photon reabsorption and reemission in the device LED part.
We developed an analytical model for quantum well infrared photodetectors (QWIPs) with a superlattice serving as the absorption region and a blocking barrier. The explicit analytical expression for the frequency-dependent responsivity is derived as a function of the QWIP structural parameters. Using this expression we calculate the 3dB cut-off frequency and bandwidth-response efficiency. The bandwidth of QWIPs with a moderate number of QWs can exceed 100~GHz exhibiting fairly large responsivity. We show that a fast tunneling filling of the absorption region by the electrons injected from the emitter contact results in a significant modification of the high-frequency properties of the QWIP under consideration compared to the standard QWIPs, in particular, in the elimination of the low-frequency peak of the responsivity.
We propose a device model for quantum dot infrared photodetectors (QDIPs) with relatively large lateral spacing between QDs as occurs in QDIPs fabricated and experimentally investigated recently. The developed model accounts for the self-consistent potential distribution and features of the electron capture and transport in realistic QDIDs in dark conditions. The model is used for the calculation of the dark-current as a function of the structural parameters, applied voltage, and temperature. It explains a rather sharp increase in the dark current with increasing applied voltage and its strong sensitivity to the density of QDs and the doping level of the active region. The calculated dependences are in good agreement with available experimental data. The obtained characteristics of QDIPs are compared to those of QWIPs with similar parameters.
A device model for quantum dot infrared photodetectors (QDIPs) with the blocking layer (BL) between quantum dots is presented. Explicit analytical formulas for the dark current and the responsivity in the QDIP of this type are obtained as functions of device parameters, including the doping level of the QDIP active region, and the applied bias voltage. It is shown that there are three voltage ranges in which the dark current and the responsivity of QDI Psreveal different behavior. The obtained results qualitatively explainsome features of the QDIPs with BL observed experimentally. The characteristics of the QDIPs under consideration are compared with those of other detectors.
We developed an analytical device model for quantum well lateral p-n~junction photodiodes (LJPD's). The model takes into account the features of the carrier transport in LJPD's and their geometry, which ensure short transit times and a low capacitance. This model is used for the calculation of the LJPD characteristics as functions of the signal frequency, bias voltage, and structural parameters and for the estimation of the LJPD ultimate performance.
We present a simple model for quantum-dot infrared photodetectors (QDIPs) describing nontrivial (decreasing) dependences of the photocurrent on the QD density and the applied voltage. It is shown that recent experiments demonstrating a negative differential photoconductivity in QDIPs can be interpreted in terms of this model. The effects under consideration can be attributed to the repulsive potential of charged quantum dots and heating of mobile electrons influencing the rate of the electron capture. Qualitative distinctions between the QDIP photocurrent-voltage and dark current-voltage characteristics are explained as well.
We report on the analysis of thermally-limited operation of quantum-dot infrared photodetectors (QDIPs). A device model is developed and used to calculate the QDIP detectivity as a function of the structural parameters, temperature, and applied voltage, as well as to determine the conditions for the detectivity maximum. The QDIP detectivity is compared with that of quantum-well infrared photodetectors (QWIPs). This work clarifies why the existing QDIPs are still inferior to QWIPs and shows that a significant improvement in the QDIP performance can be accomplished by the utilization of dense QD arrays with small QDs.
We present a theoretical study on transport and optical properties of a semiconductor-based electron gas subjected simultaneously to quantizing magnetic fields and intense terahertz (THz) laser fields in Voigt geometry. It is found that the presence of the THz radiation can result in an enhanced magneto-phonon resonance effect and a resonant-absorption peak can be observed at about f ~ 1 THz foe GaAs in high magnetic fields. the results are pertinent to experiments where THz free-electron lasers are employed as intense radiation sources.
We report ensemble Monte Carlo particle modeling on nonequilibrium self-organization phenomena in QWIPs associated with the excitation of QW recharging waves. It is shown that stable periodic or near periodic electric-field domains and prolonged chaotic spatio-temporal pulsations of the electric field can occure in QWIPs. The spatio-temporal variations of the electric-field distributions are accompanied with pronounced pulsing of the photocurrent.
We discuss different aspects of a new effect in multiple quantum well infrared photodetectors (QWIPs) - the formation of periodic and near periodic electric-field and chargs domains.
We investigate transient processes in optically excited quantum well infrared photodetectors (QWIPs) using the ensemble Monte Carlo (MC) particle simulations. We demonstrate different modes of the periodic-electric-field domains formation, irregular (chaotic) oscillations of such domains, and the transformation of the domains by varying bias voltage.
We report ensemble Monte Carlo (MC) particle modeling of nonequilibrium phenomena in multiple quantum well (MQW) structures associated with their excitation by infrared radiation. The response of MQW structures to step-like pulses of high power infrared radiation at constant bias voltage and to step-like pulses of applied voltage at constant intensity of radiation is studied.
An analytical model for light emitting diodes (LEDs) with nonuniform injection current is developed. The model includes the effect of photon recycling associated with the processes of reabsorption and reemission of photons. The LED external quantum efficiency for different scale of the injected current nonuniformities and different absorption coefficients is calculated as a function of the internal quantum efficiency and the active layer thickness. It is shown that the photon recycling is an important mechanism affecting the performace of LEDs with nonuniform injection.
The photon recycling effect in light emitting diodes with nonuniform injection is studied using the developed phenomenological device model. Analytical formulas for the spatial distribution of output radiation intensity and the external quantum efficiency are derived.
We review the recent studies of physical effects in quantum well and quantum dot infrared photodetectors utilizing intersubband transitions.
We review emerging solid-state technolodies that have promise for generation, detection and modulation of terahertz radiation. Gunn, IMPATT and TUNNETT diodes have achieved operating frequencies close to 300 GHz. Record maximum frequencies of oscillations of transferred-substrate HBTs have reached 800 GHz. Resonant tunneling diodes have also demonstrated operation at 700 GHz or so, albeit at very small power levels. We also discuss new ideas of solid-state devices for application in the terahertz band - from negative effective mass amplifiers and two-stream instability to cyclotron resonance MASERs and plasma wave electronics.
High reliability, low cost and ease of fabrication of light-emitting diodes (LEDs) contributed largely to their key role in the optoelectronic industry. Although being slower than laser diodes, LEDs could be used instead of them in fiber-optic networks at 1-2 Gbit/s. In this paper we study the influence of photon recycling effect on high-frequency performance of double heterostructure LEDs with current confinement aperture using the theoretical model developed previously.
We have proposed the use of a resonant-tunneling transistor as a sourse of terahertz oscillations. Linear and nonlinear stages of the excitation of terahertz oscillations have been studied using the proposed device model.
In this paper the effect of negative differential photoconductivity in monopolar quantum dot (QD) structures is explained invoking the physical model generalized by the inclusion of the heating of mobile (unbound) electrons by electric field. Using the model in question, the photocurrent-voltage characteristics were calculated for QD structures with different parameters, in particular, with different QD densities and QD sizes.
As shown theoretically by one of the authors, infrared photodetectors utilizing the electron (hole) photoexcitation from QD arrays, can exhibit substantial advantages over quantum well infrared photodetectors (QWIPs). However, most of the fabricated and experimentally investigated QD photodetectors (QDIPs) are inferior to QWIPs. Thus, the potential advantages of QDIPs predicted using an idealized model have not been realized yet. In this paper we present a physical model for QD structures similar to those used in realistic QDIPs.
We consider a physical model for monopolar quantum dot (QD) structures. Using this model, we calculate the dark current and photocurrent in QD structures as functions of their structural parameters and the applied voltage. The obtained results clarify some interesting features of in frared photodetectors based on the QD structures observed experimentally, in particular, a rather steep (exponential) rise of the dark current and photocurrent with the applied voltage and the occurrence of the negative differential photoconductivity.
We present an analytical device model for quantum well lateral p-n junction photodiodes (LJPDs) which takes into account the features of the carrier transport in LJPDs and their geometry. These features ensure short transit times and a low capacitance. The developed model is used to calculate the LJPD characteristics as functions of the signal frequency, bias voltage, and structural parameters.
We develop a physical model for quantum dot infrared photodetectors (QDIPs). Using this model, we calculate the dark current and photocurrent in QDIPs as functions of the QDIP structural parameters and the applied voltage. The obtained results clarify some interesting features of the QDIP characteristics observed experimentally, in particular, a rather steep (exponential) rise of the dark current and photocurrent with the applied voltage and the occurrence of the negative differential photoconductivity.
This work represents the development of physical models and computer simulators using the ensemble Monte Carlo technique and their application for the investigation of steady-state and transient processes in heterostructure optoelectronic devices such as quantum-well infrared photodetectors, resonant-tunneling laser-transistors, and metal-semiconductor-metal photodiodes, as well as determination of ultimate performance of the devices. The developed software simulators and obtained results can be useful for the enhancement of the device performance. The predictions made in the work give the possibility of new application of standard devices.