/ Takashi Iizuka / Professor
/ Victor I. Ryzhii / Professor
/ Irina I. Khmyrova / Assistant Professor
/ 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
A novel infrared photodetector utilizing intersubband electron transitions and plasma waves excitation in a quantum well (QW) - the QW intersubband hot-electron phototransistor (IHEPT) - is proposed and evaluated. It is shown that the excitation of standing plasma waves in the QW--base by incident modulated infrared radiation can result in resonant response of IHEPT's. The plasma resonance peaks of the linear responsivity of IHEPT's with base contacts spacing about 1 micrometer correspond to the terahertz range of modulation (signal) frequencies. The resonant peaks of the resposivity can be tuned by biasing voltage. The peak value of the IHEPT resonant responsivity can be of the order of the steady-state responsivity of standard QW intersubband infrared photodetectors, significantly exceeding the high-frequency performance of the latters. Relatively large values of the IHEPT resonant responsivity are associated with strong injected current stimulated by the plasma waves in the QW-base. Finally, it is shown that nonlinear dependence of injected current on the potential of the QW-base results in the variation of the dc current with changing signal amplitude. The nonlinear response current as a function of the modulation frequency also exhibits resonant behavior with the peaks at the plasma resonance frequencies and can be used for the detection of the signal component envelope.
We present the results of a new self-consistent analytical model for the calculation of the dark current and the electric-field and space-charge distributions in quantum well infrared photodetectors (QWIPs). This model takes into account thermionic emission from the QWs, tunneling injection of electrons from the emitter contact. transport of the excited and injected electrons across an active region of a QWIP and their capture into the QWs. It is shown that the electric-field and space-charge distributions in the QW structure are nonuniform in general. The character of their nonuniformity is determined by the relationships between the structural parameters, parameters of elementary processes, and bias voltage.
An analytical model of integrated quantum well infrared photodetector (QWIP) and light-emitting diode (LED) operating as a pixelless image up-converter is developed and used to estimate the device performance. It is shown that the reabsorption of the near-infrared photons trapped in the LED due to total internal reflection and their reemission can significantly influence the quality.
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.
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 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 review the occurence of periodic or near periodic electric-field domains stimulated by bound-to-continuum transitions under influence of infrared radiation in multiple uncoupled quntum wells. The conditions of formation of such domains and their stability are studied theoretically using the proposed model.
An analytical device model for hot-electron transistors (HETs) is proposed and used to study plasma effects in a two-dimensional electron gas in their base.
The performance of pixelless imaging device based on an integrated quantum-well photodetector and light-emitting diode is studied theoretically. It is shown that the photon reabsorption and reemission processes substantially influence the electron spatial distribution and limit the quality of output image.
The comments deal with the discussion of a recent paper by Ershov, where he reported analytical calculations of the response of single quantum well photodetectors (QWIPs) with tunnel injection of electrons to transient spatially nonuniform photon flux. It is shown that the paper commented contains incorrect results.
We study the response of the electron system in quantum well infrared photodetectors under illumination by infrared radiation to small electric-field and charge fluctuations using the proposed analytical model. It is shown that uniform electric-field distributions corresponding to quasi-neutral quantum wells can be unstable with regard to the excitation of the recharging waves in a wide range of infrared radiation intensities.
The results of an ensemble Monte Carlo particle modeling of vertical electron transport and capture processes in AlGaAs/GaAs multiple quantum well infrared photodetectors (QWIPs) are presented. It is shown that the heating of free electrons and their redistribution over the conduction band valleys under the influence of electric field play an essential role in both the transport processes and the capture into bound states in the QWs. The electron drift velocity and macroscopic capture parameter are calculated as functions of the structural parameters and applied electric field. The capture parameter exhibits a pronounced drop with increasing electric field due to a significant decrease of the fraction of electrons with energies less than the optical phonon energy.
The effect of donor space charge on electron capture processes in AlGaAs/GaAs multiple quantum well infrared photodetectors (QWIPs) is studied using an ensemble Monte Carlo particle modeling. It is shown that the corrugation of the conduction band edge due to donor charges in the inter-QW barriers strongly influences the electron distribution over energies and the electric-field dependence of the electron capture rate. Vertical nonuniformity of the donor distributions in QWs results in an asymmetry in the electric-field dependence of the capture parameter. This effect can essentially contribute to the asymmetry in the QWIP current-voltage characteristics in the dark condition and under illumination.
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.
Using ensemble Monte Carlo particle modeling we demonstrate that monotonic electric-field distributions in multiple QWIPs can be unstable. The instability can lead to oscillatory electric-field and charge domains with a period equal to twice the QW structure period. This effect is due to heating of the electron gas in continuum states resulting in a nonlocal dependenceof the capture rate on the electric field.
We demonstrate using an ensemble Monte Carlo particle modeling that periodic electric-field domains can arise in optically excited multiple quantum well structures under applied voltage. In particular, the formation of electric-field distributions with the period equal to twice the structure period is possible.
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. We show also that monotonic electric-field distributions can be unstable with formation of quasi-periodic domain structures.
We describe the modeling of quantum-well infrared photodetectors (QWIP's) using an ensemble Monte Carlo (MC) particle method and present the results of the study of transient recharging effects in QWIP's 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 QWIP's under illumination.
The current-voltage characteristics of multiple quantum well (QW) structures are calculated using the proposed analytical model. It is shown that the electron transport mechanisms in the QW structures under consideration can be different in three lovtage ranges.
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 this work we show that the main mechanism of the image smearing limiting the QWIP-LED pixelless imager performance can be associated with the lateral transport of the emitted near-infrared (NIR) photons, their reabsorption in the LED active region with the generation of the electro-hole pairs, and reemission from this region caused by the recombination of such pairs. Due to low efficiency of the photon escape from the LED active region the reabsorption and reemission of NIR photons can take place far enough from the site where the injected electrons recombine. The developed model yields the contrast transfer characteristic of the imager as a function of the scale of nonuniformity of the incident image as well as electrical and optical parameters of the device structure.
We propose and evaluate a novel detector and harmonic multiplier-barrier-injection heterostructure transistor (BIHT)- - with the structure similar to a heterostructure field-effect transistor. The device utilizes thermionic injection of electrons to the BIHT channel in which the standing plasma waves are excited by incoming signal. The combination of strong nonlinearity of the dependence of the injected current on the channel potential with high amplitude of the oscillation of the latter at resonant frequencies results in a significant enhancement of the responsivity and harmonic multiplication efficiency. The BIHT detectors and multipliers can be effective in the terahertz range of frequencies surpassing the performance of Schottky-diode based devices.
An analysis of quantum well microwave detectors (QWMDs) utilizing the effect of electron heating by absorbed radiation is carried out using an analytical device model. The dark current and the responsivity of a QWMD are calculated as functions of the structural parameters, temperature and bias voltage. It is shown that QWMDs can effectively operate as detectors of electromagnetic radiation in the gigahertz and terahertz ranges.
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 300GHz. 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.
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 charge domains.
The concept of an integrated quantum well infrared photodetector (QWIP) and light-emitting diode (LED) pixelless imaging device was proposed, discussed, and fabricated recently. This work deals with the theory of the electron spreading effects in the LED part of the QWIP-LED pixelless imager. The smearing of the output near-infrared (NIR) image is determined by the lateral diffusion of the photoelectons in the LED active region and the processes of the NIR photons reabsorption and their reemission. It is shown that the photon reabsorption and reemission processes substantially affect the electron spreading and markedly deteriorate the QWIP-LED pixelless imager characteristics.
This paper deals with the study of the electron spreading effects in an integrated quantum well infrared photodetector (QWIP) and light-emitting diode (LED) pixelless imaging device. The QWIP-LED performance is limited by the electron injection from the QWIP emitter and lateral spreading of the photoelectrons in both the QWIP and LED parts. The developed analytical QWIP-LED device model takes into account both the photoelectrons recombination and their generation by the reabsorbed radiation, and the propagation of near-infrared radiation along the LED active region.
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 present an ensemble Monte Carlo particle modeling of transverse transport of electrons and their capture in AlGaAs/GaAs multiple quantum well (QW) structures under influence of electric field. The transverse electron drift velocity and the macroscopic capture parameter (determining the electron capture rate) are calculated as functions of the QW structure parameters (thicknesses of the QWs and barriers) and applied electric field.
We report ensemble Monte Carlo particle modeling of 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 occur in QWIPs. The spatio-temporal variations of the electric-field distributions are accompanied with pronounced pulsing of the photocurrent.
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.