/ Takashi Iizuka / Professor
/ Victor I. Ryzhii / Professor
/ Irina I. Khmyrova / Assistant Professor
/ Maxim Yu. Ershov / Research Associate
/ 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
The results of the research were published in 16 refereed articles in the following journals: Semiconductor Science and Technology - 1, Journal of Applied Physics - 2, Applied Physics Letters - 5, IEEE Journal of Quantum Electronics - 1, Physica B - 1, Japanese Journal of Applied Physics - 5, Lithuanian Journal of Physics - 1.
The results also have been presented at the following conferences: 9th International Conference on Superlattices, Microstructures and Microdevices (Belgium) - 3, 23rd International Symposium on Compound Semiconductors (Russia) - 1, 26th European Solid State Device Research Conference (Italy) - 1, European Quantum Electronics Conference (Germany) - 2, International Conference on Mid-infrared Optoelectronics. Materials and Devices (England) - 3, 7th China-Japan Symposium on Science and Technology of Crystal Growth and Materials (China) - 1, International Conference on Semiconductor Electronics ( Malasia) - 1, Conference on Optoelectronic and Microelectronic Materials and Devices'96 (Australia) - 2, Fourth International Workshop on Femtosecond Technology (Japan) - 1, SPIE's International Conference Photodetectors: Materials and Devices - Photonics West'97 (USA) - 1, 2nd International Conference on Low Dimensional Structures and Devices (Portugal) - 1, Pacific Rim Conferense on Lasers and Electro-Optics (Japan) - 1, 10th International Conference on Superlattices, Microstructures and Microdevices - ICSMM-10 (USA) - 2, 8th Annual Conference on Modulated Semiconductor Structures (MSS'8) (USA) - 1, 1997 Joint International Meeting (France) - 3, International Topical Meeting on Microwave Photonics - MWP'97 (Germany) - 1, Second International Symposium on Silicon Heterostructures: from Physics to Devices (Italy) - 1, 27th European Solid State Device Research Conference - ESSDERC'97 (Germany) - 1, International Workshop on Nano-Physics and Electronics (Japan) - 1.
Refereed Journal Papers
A novel unipolar infrared photodetector -- the Darlington infrared phototransistor (DIPT) -- is proposed and evaluated. The DIPT is the integration of a quantum--well infrared photodetector, utilizing the electron intersubband transitions, and a hot-electron transistor. The operation principle of the DIPT is considered. Its performance is estimated using an analytical model. It is shown that the DIPT can exhibit very large responsivity. DIPTs may be useful for new detectors of infrared radiation of wavelength longer than 2 um.
A model is presented for the performance of quantum well infrared photodetectors (QWIPs) utilizing intersubband electron transitions and tunneling injection electrons. The dark current and the responsivity are derived as functions of the QWIP parameters, including the number of the QWs, in an analytical form. Nonlinear effects in the QWIP operation at high infrared power are considered and the threshold value of power density is estimated.
The excitation of the standing plasma waves in induced-base hot-electron transistors (IBHETs) and its impact on the IBHET high--frequency operation are considered for the first time. It is shown that the plasma waves result in resonant behaviour of the IBHET performance at high--frequencies. The frequency dependent small-signal transconductance of the IBHET is calculated using an analytical model. The resonant frequencies and the sharpness of the resonant peaks are found as functions of structural parameters and bias voltage. The resonant frequencies can correspond to the terahertz range. The resonant transconductance can significantly exceed the steady--state transconductances of the IBHET.
A novel infrared photodetector - the quantum-dot infrared phototransistor - based on a multiple quantum dot structure is considered theoretically. An analytical model of the device is developed to evaluate its performance. The dark current and sensitivity are calculated. It is shown that the quantum-dot infrared phototransistor performance can surpass that of the quantum-well photodetectors.
Novel three-terminal devices - laser-transistors with a quantum-well active region and a resonant-tunneling collector - are considered theoretically. The optical gain of the active region of the LTs is controlled by the collector voltage due to the electron extraction via the resonant-tunneling structure. The current-voltage and light-voltage characteristics are calculated. Both of them reflect possible bistability in the collector current and output optical power in lasing mode in a certain range of the collector voltage. The electron heating due to the electron injection and extraction is vital for the voltage control of the output optical power.
Nonlinear photoconductivity effects at high excitation power in quantum well infrared photodetectors (QWIPs) are studied both experimentally and theoretically. The photoconductivity nonlinearity is mainly caused by a redistribution of the electric potential at high power, which leads to a decrease of electric field in the bulk of the QWIP. As a result of the decreased field, the photoexcited electron escape probability and drift velocity decrease result in a decrease of responsivity.
We study theoretically the influence of the optical generation of electrons and holes in the base of an induced--base hot--electron transistor (IBHET) on its high--frequency operation. High electron mobility is assumed in the IBHET base, so that the standing plasma waves in the base can substantially affect the induced--base IBHET performance. It is shown that the photogeneration leads to marked transformation of the resonant peaks in the IBHET frequency--dependent transconductance. This is due to the dependence of the resonant frequencies and the electron scattering time on the concentration of the photogenerated electrons and holes. The latter results in the shift of the resonant peaks to higher frequencies and their broadening. This effect can be used for an effective optical control of IBHETs operation in the terahertz range of signal frequencies.
The conversion of images into nonuniform distribution of the output current density in a multiple quantum well structure for infrared pixelless imaging system is considered using an analytical device model. The nonuniform current, which reproduces the incident image, is converted back to a light emitting diode emission image by the device considered here. The developed model takes into account transport processes responsible for the device operation. An explicit expression for the contrast transfer characteristic is derived as a function of the number of quantum wells and the electron capture parameter. It is shown that the quality of the up-converted images (contrast and resolution) is improved with increasing number of quantum wells. The pixelless imaging devices under consideration can effectively convert long-wavelength infrared images into short-wavelength infrared or visible images with contrast transfer ratio close to unity.
Heterostructure bipolar transistors with a quantum-well base incorporated a resonant-tunneling structure in the collector are proposed and studied theoretically. Static characteristics of the transistors are evaluated using the proposed analytical model. It is shown that the current-voltage characteristics and the collector current versus base current dependences can be ambiguous corresponding to bistability operation. The bistability effect is associated with the mobile electron space charge in the collector region. Direct on-off switching by the collector voltage or base current pulses is discussed.
Transient photoconductivity in quantum well infrared photodetectors (QWIPs) is studied using numerical modeling. Transient photocurrent in QWIPs illuminated by steplikeinfrared radiation consists of fast and slow components.
We report experimental and simulation results of capacitance of quantum well infrared photodetectors (QWIPs). We found that the QWIP capaitance displays unusual behavior as a function of voltage and frequency, deviating far from the constant geometric capacitance value. Negative capacitance arises when the transient current in response to a voltage step is nonmonotonic with time. Simulation shows that this effect is due to nonequilibrium transient electron injection from the emitter resulting from the properties of the injection barrier and inertia of the QW recharging processes.
We present an analysis of physical effects responsible for infrared image transformation in integrated Quantum Well Infrared Photodetector - Light Emitting Diode (QWIP-LED). For a large-area device, the spatial smearing of the transformed image is determined by the lateral photocurrent spreading in the QWIP and the lateral diffusion of carriers injected into the LED. For devices with low QWIP photocurrent gain, the spatial resolution is limited by the carrier diffusion lengths in the QWIP and in the LED active region, which are much shorter than radiation wavelength, and hence, the transformed image is practically undistorted.
The spectral density of current fluctuations in single quantum well infrared photodetectors is calculated using Langevin approach. The noise gain and the photocurrent gain are expressed in terms of basic transport parameters. Fluctuations of the incident photon flux are taken into account.
Three-terminal laser-transistors with a quantum well active region and a resonant-tunneling collector are proposed and considered. It is shown that the laser-transistor is controlled by the collector voltage due to the electron extraction via the resonant-tunneling structure. The current-voltage and light-voltage characteristics are calculated. Both of them reflect effective voltage control and possible bistable behaviour of the laser-transistor in a certain range of the collector voltage. The electron heating due to the electron injection and extraction can significantly affect the laser-transistor controllability and bistability.
We have theoretically studied the response of quantum well intersubband photodetectors to high-frequency modulated infrared radiation. The small-signal responsivity dependent on the modulation frequency of infrared radiation and device parameters has been derived using the proposed analytical model. It has been shown that with increasing modulation frequency the roll-off of the small-signal responsivity is associated with the electron transit and capture effects which limit the device bandwidth.
Transient behavior of bipolar transistors with a quantum-well base and a resonant-tunneling structure in the collector is studied using the ensemble Monte Carlo particle method. The switch on/off processes as a response to trigger pulses of the collector voltage and the base current are simulated. The amplitudes and duration of the pulses providing reliable switching are obtained.
Indentation- and scratch-induced mechanical damages on silicon (111) wafers are investigated by using optical microscopy and x-ray diffraction topography. Fractures,residual strain fields and anneal-induced dislocations around the damages represent anisotropies. These anisotropies are correlated each other and discussed in terms of inclined cleavage planes.
Electron extraction and injection effects in three-terminal semiconductor lasers can be used not only for the control of the concentration of the electron gas in the active region but also for its rapid heating. As the optical gain strongly depends on the electron temperature this mechanism results in a high-speed control of laser generation. The three-terminal laser is considered theoretically. It is shown that the electron extraction via the resonant-tunneling structure (RTS) can result in an effective and rapid modulation of the laser radiation. The bistability effect in the lasers under consideration connected with the electron charge accumulation behind the RTS is also investigated.
The modulation of laser radiation in a three-terminal laser incorporated a resonant-tunneling controlling structure is studied using an analytical model. It is shown that the modulation method under consideration can be very efficient and fast, in particular, due to the electron heating effects and high sensitivity of the resonant-tunneling current to the controlling voltage.
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.
High-speed performance of quantum well infrared photodetectors (QWIPs) are studied using numerical modeling. QWIP's response consists of two transients; a fast transient is due to combination of carrier transit and capture effects, and a slow transient is is governed by QWs recharging processes.
We consider theoretically the three-terminal lasers with a resonant-tunneling structure (RTS). Electron extraction and injection can provide a fast heating of the electron gas in the active region of such lasers. The developed analytical model includes the equations for the electron concentration, their effective temperature and the equation for photon density of a lasing mode. Using this model the frequency dependence of modulation efficiency is calculated. It is shown that the electron extraction via the RTS can result in an effective and rapid modulation of the laser radiation.
High-frequency characteristics of a quantum well (QW) diode with thermionic electron transport are reported in this paper. The frequency-dependent admittance of the QW diode is obtained in an analytical using the rigorous self-consistent small-signal analysis. The frequency dependencies of the QW diode capacitance and conductance are governed by a characteristic time of the recharging of the QW, which depends strongly on temperature and device structural parameters. Experimental data on conductance and capacitance of the QW diode as functions of temperature and frequency can be used to extract the parameters of the QW, such as QW recombination velocity and ionization energy.
We study the screening of external potential in uniform quantum wires. Self-consistent field approximation is used to find the expression for the response of electrons in quantum channels to time-dependent longitudinal electric field. The asymptotic screening of Coulomb potential in one-dimensional electron gas is investigated.
We report on theoretical and experimental investigations of the frequency-dependent admittance of quantum well infrared photodetectors (QWIPs). The effect of negative capacitance has been predicted theoretically and confirmed experimentally. Capacitance measurements under different applied voltages and frequencies have been performed for a series of QWIPs with different number of QWs. Frequency and voltage dependencies of the measured capacitance show a good agreement with the results of computer simulation.
The conversion of infrared images in multiple quantum well infrared photodetector (QWIP) structure into nonuniform distribution of the current driving a light--emitting diode (LED) is considered using an analytical model. The developed model of pixelless QWIP--LED imagers takes into account transport processes determining the device operation. The contrast transfer characteristic is derived as a function of the number of quantum wells and the electron capture parameter. It is shown that the quality of the up--converted images is improved with increasing number of quantum wells. The pixelless imaging devices under consideration can effectively convert long--wavelength infrared images into short--wavelength infrared or visible images.
The paper deals with a theoretical study of physical phenomena limiting the high-frequency performance of QWIPs. The proposed analytical device model accounts the electron photoexcitation from QWs and capture processes, tunneling of the electrons through the emitter barrier and their drift over the barriers. The QWIP responsivity is derived in explicit analytical form as a function of the modulation frequency and structure parameters.
The paper deals with a theoretical study of the high-frequency resonant effects in induced-base hot-elecron transistors (IBHETs) associated with the excitation of standing plasma waves with a linear dispersion law. Under AC voltage applied between the emitter and base contacts with a frequency close to one of resonant frequencies the plasma waves in the base channel can lead to relatively large periodic oscillations of the barrier controlling the electron injection from the IBHET emitter to collector. As a result, the IBHET transconductance at the resonant frequencies can significantly exceed its value in low frequency limit. It is shown that the transconductance reveals gigantic resonances in the terahertz range. Due to dependence of the resonant frequencies on the biasing voltages the peak positions can be voltage tunable.
This paper is concerned with an in-depth study of the high-frequency performance of quantum well infrared photodetectors (QWIPs) with different number of quantum wells (QWs) using the developed original analytical self-consistent model. We are able to derive for the first time the expression for the small--signal frequency dependent responsivity of the QWIPs in an explicit analytical form as functions of device structural parameters including the number of the QWs, intensity of the steady-state component of the incident infrared radiation, capture parameter, and electron transit time.
Ultrahigh--speed effects in quantum--well infrared photodetectors utilizing intersubband electron transitions are studied theoretically. It is shown that their ultrahigh--speed operation is limited by the electron transit--time and capture effects.
Lateral base resistance in bipolar transistors is one of the factors substantially limiting their potentials as ultrafast devices. The implementation of the base channel with carriers (electrons or holes) induced by a dopant layer located at some distance from the channel promotes lowering of the lateral resistance of the latter. This is due to the enhancement of the electron or hole mobility associated with the exclusion of their scattering on impurities. In this paper we show that high mobility of the carriers in induced--base heterostructure bipolar transistors (IBHBTs) can account for resonant effects caused by the excitation of standing plasma waves in the base channel.
This paper deals with a theoretical analysis of the electron effects in pixelless imaging devices limiting their image transformation performance. The obtained results indicate that the quality of the up-converted images is improved with increasing number of the quantum wells having a tendency to saturation. The pixelless imagers based on integrated quantum well infrared photodetectors (QWIPs) and light-emitting diodes (LEDs) can provide the up-conversion of infrared images with contrast transfer ratio close to unity.
Infrared phototransistors utilizing electron photoexcitation from bound states in a two-dimensional quantum dot (QD) array are considered. To evaluate the current-voltage characteristics of the structures in question the developed theoretical model is used. The analytical expression for the current-voltage characteristic is obtained. The effect of the base potential nonuniformity is evaluated. It is shown that the current is distinctly affected by this effect especially at relatively low voltages.
In this paper we evaluate novel infrared phototransistors - the quantum wire infrared phototransistor (QRIP) and the quantum dot infrared phototransistor (QDIP). Their performances are compared with that of the QWIP. The QRIP and QDIP have two features connected with the lower dimensionality of the electrons: sensitivity to normal incident radiation and low dark current. It is shown that the QRIP and QDIP can surpass the QWIP in performance especially at low temperatures.
The effects of QWIP nonlinear photoresponse with infrared power are studied both theoretically using numerical modeling and experimentally. It is shown that responsivity decreases with power for QWIPs with rectangular barriers due to modulation of th electric field in the bulk of QWIP.
High-frequency characteristics of a quantum well (QW) diode with thermionic electron transport are reported in this paper. The frequency-dependent admittance of the QW diode is obtained in an analytical using the rigorous self-consistent small-signal analysis. The frequency dependencies of the QW diode capacitance and conductance are governed by a characteristic time of the recharging of the QW, which depends strongly on temperature and device structural parameters. Experimental data on conductance and capacitance of the QW diode as functions of temperature and frequency can be used to extract the parameters of the QW, such as QW recombination velocity and ionization energy.
In this work we report results of numerical simulation and theory clarifying the physics of ultrafast QWIP operation, and experimental results on heterodyne detection for intermediate frequency up to 82 GHz.
The purpose of this work is theoretical and experimental study of photoresponse of Quantum Well Infrared Photodetectors (QWIPs) at high excitation power. The QWIP photoresponse is shown to be strongly nonlinear in the power range 0.01-100 W/cm2. The responsivity is decreased by more than an order of magnitude for QWIPs with 4 QWs at high voltages. The degradation of responsivity is stronger at low applied voltages. For QWIPs with large number of QWs the responsivity is only slightly dependent on power. The nonlinear behavior of QWIPs is explained using computer simulation based on a self-consistent model. Influence of QWIP structural parameters on nonlinear effects and key factors in designing a QWIP with a suppressed saturation of responsivity are discussed.
This paper presents recent results on numerical simulation of Quantum Well Infrared Photodetectors (QWIPs). Self-consistent simulation of QWIPs highlights the role of the contact and distributed effects in determining both the steady-state and transient or small-signal device characteristics.
We report on theoretical and experimental investigations of the frequency-dependent admittance of Multiple Quantum Well (MQW) semiconductor heterostructures.
The three-terminal lasers controlled by the voltage applied to the resonant-tunneling structure affecting the electron extraction from the active region are considered theoretically. Using the developed analytical model the modulation efficiency is calculated.
We report the results of numerical simulation of the resonant-tunneling bipolar transistors (RTBTs) switching. The RTBT with a quantum well base is considered. The model used for calculation takes into account the resonant-tunneling injection from the base into the collector, self-consistent electric field in the collector depletion layer and non-equilibrium transport of the electrons. It is shown that the RTBT can operate as switching device with delay time about few picoseconds.
Optically controlled hot--electron transistors operating in the terahertz range of signal frequencies are studied theoretically. It is shown that the photogeneration of electrons in the transistor base tunes the plasma resonances strongly affecting the frequency--dependent transconductance.
A novel three-terminal device - laser-transistor - is considered theoretically. It is shown that the heating of the electron gas in the QW significantly affects the LT operation. The delay time of the LT on/off switching by pulses of the collector voltage and the switching energy are also estimated. The LT in question differes from that studied previously by the the incorporation of a QW into the base which makes the bistability effect more pronounced, improving the LT performance.
A quantitative study of the accuracy of carrier mobility values in semiconductors based on Matthiessen's rule is presented, following a systematic comparison with values calculated using the relaxation time approximation. It is found that the resulting error in Matthiessen rule can be as high as 40% in silicon when the scattering rates have different energy dependence and are not predominant.