/ Takashi Iizuka / Professor
/ Victor I. Ryzhii / Professor
/ Irina I. Khmyrova / Assistant
Professor
/ Leonid E. Fedichkin /
Visiting Researcher
/ Maxim V. Ryzhii / Research Associate
The research activity of the Computer Solid State Physics Laboratory is aimed atinvestigation of semiconductor quantum functional electronic and optoelectronic devices as a base for prospective communication and imaging 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
Theory and computer modeling of quantum-well, -wire and -dot structures for quantum functional electron devices. Theory and computer modeling of photonic functional devices, particularly, photodetectors for mid- and far-infrared radiation as well as for terahertz radiation. Development of novel mathematical models for simulation of quantum functional electronic and optoelectronic devices.
The results of the research have been included in 11 refereed articles published/accepted in the following journals:
Refereed Journal Papers
The harnessing of induced base hot electron transistors (IBHETs) with high mobility of electrons in the base as detectors and mixers o f electromagnetic radiation in the range of terahertz frequencies is proposed. The theory of physical effects associated with the operation of detectors and mixers based on IBHETs is developed. It is shown that the resonant excitation of the standing plasma waves in the IBHET base is possible under the influence of high-frequency electromagnetic signals. At resonant frequencies the amplitude of the base potential oscillation significantly exceeds the amplitude of incoming signals provided the electron mobility is high enough. This results in the IBHET responsivity at resonant frequencies much higher than that of Schottky diodes. For IBHETs with micrometer or near submicrometer lateral sizes the resonant frequencies fall into the terahertz range. The resonant frequencies are tuned by biasing voltages.
The high--frequency properties of quantum well infrared photodetectors (QWIPs) based on a double-barrier single QW structure are studied theoretically. An analytical model of the QWIP is developed. The model takes into account the main processes responsible for the QWIP operation, namely: the electron tunneling from the emitter, capture of the electrons into the QW, their photoexcitation from the QW and electron drift or ballistic transport across the QWIP structure. Analytical expressions for the QWIP responsivity as functions of the modulation frequency of infrared radiation, its power density and the QWIP structural parameters are obtained from the rigorous self--consistent small signal analysis. It is shown that there are two distinct ranges where the frequency dispersion of the responsivity is strong. At low frequences the responsivity dispersion is associated with the inertia of the process of recharging of the QW while at very high frequences the dispersion is due to the electron transit--time effect. The influence of the electron transit--time effect on the QWIP admittance is also evaluated. The derivation of the QWIP high--frequency performance and, in particular, the estimates of 3dB bandwidth show that the QWIPs have a great potential for devices utilizing both infrared radiation and millimeter or submillimeter wavelength microwave signals.
A self-consistent ensemble Monte Carlo simulation is used to study ultrafast electron transport in quantum well infrared photodetectors (QWIPs). It is shown that transient photocurrent triggered by a short infrared radiation pulse reveals a sharp peak followed by a relatively slow decay. The photocurrent peak is associated with the electron velocity overshoot effect while the slow component of the photocurrent is determined by the electron transit time and capture effects. The velocity overshoot effect results in the existence of a plateau in the QWIP frequency-dependent responsivity in the range of terahertz frequencies.
An analytical model of multiple quantum well photodetectors (QWIPs) is used to calculate their frequency--dependent responsivity and bandwidth. The device model takes into account all electron processes important for the QWIP high--frequency operation. It is shown that the roll--off of the frequency--dependent responsivity and, consequently, the bandwidth are determined by the electron transit time across the inter--QW barrier, the capture probability of electrons in the QWs and the number of the latters. The bandwidth can be expanded by using the QW structures specially designed to have large electron capture probability.
Calculations by ensemble Monte Carlo particle method on ultra--high--frequency response of metal--semiconductor--metal (MSM) photodetectors are reported. It is shown that the MSM photodetectors with relatively large contact spacing (0.2 - 1.0$\mu$m) with lateral light input into a thin absorbing layer reveal rather high frequency response in terahertz range of signals due to the velocity overshoot effect manifested by the photoexcited electrons. The response--signal frequency product exhibits a pronounced maximum at terahertz frequencies being significantly higher than that evaluated without the inclusion of the velocity overshoot effect. The obtained numerical results are interpreted invoking the developed analytical device model.
In this paper a two-dimensional ensemble Monte Carlo particle method is used to simulate the metal-semiconductor-metal (MSM) photodetector response in the terahertz range of signal frequencies. We consider planar MSM photodetectors consisting of a GaAs absorbing layer with a system of Schottky contacts made ``back-to-back'' on the above layer. The model takes into account the features of the carrier energy spectra, mechanisms of their scattering and a self-consistent electric field. The intrinsic transient response triggered by an ultra-short light pulse is calculated. The MSM frequency response is calculated using the Fourier transform of the obtained temporal dependences. It is shown that due to velocity overshoot effect exhibited by the photoelectrons, the MSM photodetector reveals rather high response to terahertz signals even if the contact spacing is relatively large. The frequency response of the MSM photodetectors utilizing the photoelectron velocity overshoot effect is compared with that of the MSM photodetectors with ultra-short carrier lifetime.
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.
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 of the output image.
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.
A simplified quasi--two--dimensional analytical model of planar interdigitated metal--semiconductor--metal (MSM) photodetectors is presented. The model includes the transit--time and velocity overshoot effects of photogenerated electrons and holes and their recombination. The response is derived as a function of the signal frequency and the device structural parameters. It is shown that MSM photodetectors with relatively large contact spacing can exhibit rather high frequency response in terahertz range due to electron velocity overshoot. The obtained MSM photodetector response is in a good agreement with the results of a Monte Carlo simulation. The response--frequency product reveals a high maximum corresponding to a frequency about few terahertz. The MSM photodetectors exploiting the velocity overshoot effect can exhibit advantages in comparison to the MSM photodetectors with ultra--short lifetime of the photogenerated electrons and holes.
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.
The operation bandwidth of metal-semiconductor-metal (MSM) photodetectors with planar interdigitated Schottky contacts is usually assumed to be limited by the electron and hole transit time effects. This leads to a tendency to reduce the contact spacing to the scale of about few hundredth of micrometer. However, the velocity overshoot effect exhibiting by the photoexcited electrons (holes) can result in marked responsivity of MSM photodetectors with the contact spacing of few tenth of micrometer to optical radiation modulated by terahertz signals. This paper deals with a Monte Carlo particle study of MSM photodetectors operation in terahertz range of signal frequencies. The obtained numerical results are discussed invoking the developed analytical model of the MSM photodetectors.
Interdigitated metal--semiconductor--metal (MSM) photodiodes have received increasing interest in optical fiber communication systems, optical heterodyne conversion and other applications. This paper deals with a two--dimensional ensemble Monte Carlo particle simulation of the MSM photodiode operation in terahertz range of signal frequencies.
Planar metal-semiconductor-metal photodetectors are very attractive for many optoelectronic applications, in particular, for optical fiber communication systems, chip-to-chip connections, a nd optical heterodyne conversion. In this paper a two--dimensional ensemble Monte Carlo particle method is used to simulate the MSM photodetector response. The MSM photodetector frequency response associated with the velocity overshoot effect reveals also the existence of a sharp maximum of the response-frequency produc. The maximum of the product is much larger than the response-bandwidth efficiency of MSM photodetectors if the velocity overshoot effect is supressed.
In this paper a novel quantum well (QW) intersubband device - the QW intersubband hot-electron phototransistor (IHEPT) is proposed and evaluated. The IHEPT has a structure similar to the QWIP structure with a single undoped QW. However, in contrast to normal QWIP the QW is supplied by two side contacts. It is assumed that the mobility of electrons induced in the QW is high, so that the standing plasma waves in a two--dimensional electron gas can be excited by incoming modulated infrared radiation. The IHEPT operation is associated with the injection of hot electrons from the emitter to the collector controlled by the oscillating potential of the QW base. It is shown that if the modulation (signal) frequency is in the vicinity of the plasma resonances the IHEPT responsivity exhibits very high maxima.
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 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.
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.
Metal-semiconductor-metal (MSM) photodiodes with planar structure possess features making them attractive for many optoelectronic applications. Usually it is assumed that the extension of high--frequency performance of such MSM photodiode with interdigitated Schottky contacts to higher frequencies is associated with the diminishing of contact spacing $W$ resulting in shorter carrier transit time or using materials with ultra--short lifetime. In this paper we present the developed analytical model for the MSM photodiodes taking into account dynamic (transit time and velocity overshoot) as well as recombination effects. It is shown that high--frequency response of MSM photodiodes based on relatively perfect materials is limited by the electron transit time and velocity overshoot effects, i.e. by electron dynamic effects.
Unipolar hot--electron transistors (HETs) based on vertical heterostructures are considered as very promissing for ultra high frequency applications. Recently, it has been shown that HETs can exhibit a resonant behaviour under influence of electromagnetic signals in terahertz range of frequencies if the mobility of a two--dimensional electron gas (2DEG) in the HET base is high enough. The resonances are associated with the excitation of the standing plasma waves with a linear dispersion law in the 2DEG. As a result of the plasma wave excitation the electron and displacement current in HETs reach sharp maxima at resonance frequencies. Such HET property can be used in terahertz oscillators, detectors, mixers and other devices. In this paper a quasi-three-dimensional analytical model of HETs with high electron mobility is developed to calculate the resonant frequencies as functions of the device structural parameters and its form.
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.
In this paper, experimental results of n-type QWIP are given which show obvious evidence of intersubband transition under normal incidence condition. Theoretical models are also examined to understand the intrinsic mechanisms of normal incidence absorption.
Heterostructures are the elements of many of the most advanced semiconductor devices currently being developed and fabricated. They are essential parts of modern optoelectronic devices, such as semiconductor lasers, light-emiting diodes, and photodetectors with highest performance. Heterostructures are being employed increasingly in high-speed digital and high-frequency analog devices. The advantages of heterostructures are that they provide effective control over the states and motions of charge carriers in active regions of heterostructure-based devices.