Interface structure between Bi and CdTe in molecular beam epitaxially Grown Bi/CdTe and Bi/Bi1−xSbx superlattices

We report cross-sectional high resolution transmission electron microscopy studies of the interface structure in Bi/CdTe and Bi/Bi_1−x Sb_x superlattice films deposited on (111) CdTe by molecular beam epitaxy. Results show that the interface between Bi and CdTe is coherent. Two kinds of interfaces were observed. One is the so-called twin- or allo-interface, where the stacking sequence of Bi is different from or does not follow continuously that of CdTe. The lattice orientation across this kind of interface is similar to a (111) rotation twin as is commonly present in cubic semiconductors. The other is the so-called isointerface, where the stacking sequence of Bi follows the CdTe without any change. However, the presence of one kind of Bi twin boundary with translational characteristics indicates that it fits a model of random nucleation and two-dimensional growth.     

The role of lattice mismatch in the deposition of CdTe thin films

Cadmium telluride (CdTe) is the most well-established II–VI compound largely due to its use as a photonic material. Existing applications, as well as those under consideration, are demanding increasingly stringent control of the material properties. The deposition of high-quality thin films is of utmost importance to such applications. In this regard, we present a report detailing the role of lattice mismatch in determining the film quality. Thin films were deposited on a wide variety of substrate materials using the pulsed laser deposition (PLD) technique. Common to all substrates was the strong tendency toward the preferential alignment of CdTe’s (111) planes parallel to the substrate’s surface. X-ray diffraction analysis, however, revealed that the crystalline quality varied dramatically depending upon the substrate used with the best results yielding a single-crystal film. This tendency also manifested itself in the surface morphology with higher structural perfection yielding smoother surfaces. The film quality showed a strong correlation with lattice mismatch. Texture analysis using the [111] pole figure confirmed that improvements in the lattice mismatch led to a higher degree of in-plane alignment of the (111) grains.     

Gunn effect in CdTe

We have been examining the response of n-type CdTe to pulsed electric fields. In our best units, prepared to have a uniform cross section, the current remains ohmic, to good approximation, almost to threshold. Above the threshold field of (13±2) kV/cm well-defined Gunn oscillations are observed with a spike amplitude 35-50 percent of the total current. We estimate a domain drift velocityv_{dd}=7times10^{6}cm/s, a field outside the domainE_{1} = 7kV/cm, and a domain fieldE_{2}gsim37kV/cm. After a few nanoseconds of operation, however, current runaway occurs in units showing the spiking mode of oscillation, presumably because of carrier ionization induced by the moving high-field domains.     

CdTe和CdTe/CdS量子点的量子尺寸效应

利用水热法合成了三种不同尺寸的单核CdTe量子点和核壳CdTe/CdS量子点。应用Top-hat Z-scan技术在纳秒、皮秒、飞秒激光脉冲作用下研究了三种不同尺寸单核CdTe量子点的非线性吸收特性。实验结果表明:在不同激光脉冲作用下三种不同尺寸的CdTe量子点的非线性吸收特性均表现为饱和吸收,并且均呈现出随着量子点尺寸的减小,其非线性吸收特性增大的趋势。为了进一步研究量子点尺寸的变化对非线性吸收特性的影响,又在飞秒激光脉冲作用下研究了核壳CdTe/CdS量子点的非线性吸收特性;随着包壳时间的增加,壳层厚度增加,量子点尺寸增加,其非线性吸收特性呈减小趋势,并且核壳CdTe/CdS量子点的非线性吸收特性明显优于单核CdTe量子点;分析讨论了单核CdTe量子点与核壳CdTe/CdS量子点的非线性吸收特性和量子尺寸效应机制,实验结果表明合成的量子点样品均具有良好的量子尺寸效应。     

On the Role of Boron in CdTe and CdZnTe Crystals

It is well known that group III elements act as donors if they play a substitutional role at the metallic site in II-tellurides; nevertheless, several studies report both on the creation of complexes with vacancies, named A-centers, and on the involvement in self-compensation mechanisms, especially for indium. The boron concentration in II-tellurides is negligible, and its contribution to transport mechanisms has not been studied yet. For the last few years the authors have been developing a new technique to grow CdZnTe by the vertical Bridgman technique, taking advantage of encapsulation by means of boron oxide. In this way, crystals characterized by large single grains, low etch pit density, and high resistivity have been obtained. Recently, x-ray detectors with state-of-the-art performance have been produced from such crystals. Boron contamination, as a consequence of this growth method, is quite low but at least one order of magnitude above values obtained with other growth techniques. Besides being a low-cost technique which is also suitable for large-scale mass production, the encapsulated vertical Bridgman technique is quite useful to prevent dislocations, grain boundaries, and stacking faults; for these reasons, careful characterization was performed to understand the effect of boron both on the electrical properties and on the spectroscopic performance of the final crystals. Our characterization is mainly based on low-temperature photoluminescence in addition to electrical current–voltage measurements, photostimulated current, and x-ray spectroscopy. The results indicate that boron behaves like other group III elements; in fact, boron forms a complex that does not affect the good performance of our x-ray detectors, even if it shows some properties which are typical of A-centers.     

IR spectroscopy of lattice vibrations and comparative analysis of the ZnTe/CdTe quantum-dot superlattices on the GaAs substrate and with the ZnTe and CdTe buffer layers

A comparative analysis of multiperiod ZnTe/CdTe superlattices with the CdTe quantum dots grown by molecular beam epitaxy on the GaAs substrate with the ZnTe and CdTe buffer layers is carried out. The elastic-stress-induced shifts of eigenfrequencies of the modes of the CdTe- and ZnTe-like vibrations of materials forming similar superlattices but grown on different buffer ZnTe and CdTe layers are compared. The conditions of formation of quantum dots in the ZnTe/CdTe superlattices on the ZnTe and CdTe buffer layers differ radically.     

Temperature behavior and compensation of light-emitting diode

This letter offers a fresh insight into the behavior of light-emitting diode (LED) over temperature. Theoretically and by measurement, it has been shown that equi-intensity curves in the diodes current-voltage plane are nearly a straight line over a very wide range of temperatures. Based on this property, bias voltage and resistance value of a bias circuit have been realized and practical measurement shows the peak-to-peak light intensity variation decreases from 99 % (in case of fixed current bias) to 6 % over the temperature range of -20 C to +80 C for the LED IN6092. This circuit uses no separate temperature sensor or compensating mechanism, but responds directly to the junction temperature of the diodes. This prevents any error caused by temperature gradient, or by self-heating due to power dissipation in the diode.     

Phenomenon of the inverse adsorption piezoelectric effect in CdTe and CdHgTe semiconductors

The structure and chemical composition, and also the adsorption, electrical, and optical properties of the surface of Cd _x Hg_{1 − x} Te system solid solutions have been studied; the existence of the previously unknown phenomenon of the inverse adsorption piezoelectric effect has been established. Using the semiconductor compounds CdTe and Cd_0.2Hg_0.2Te as an example, it is shown that adsorption-induced charging of the surface causes the relaxation of charges localized at slow surface states which manifests itself in the form of voltage pulses and governs structural reconstructions and surface relaxation.     

Electrooptic effect in CdTe at 23.35 and 27.95 microns

The unlamped electrooptic coefficient r41of high-purity CdTe has been measured at 23.35 and 27.95 micros in the far infrared. The values obtained for n0³r41are 9.4 × 10^-11m/V at 23.35 µ and 8.1 × 10^-11m/V at 27.95 µ. Using previously reported values for n0, the electrooptic coefficients are found to be 5.5 × 10^-12m/V at 23.35 µ and 5.0 × 10^-12m/V at 27.95 µ. These measurements extend the region of observed electrooptic effect from 16 µ, previously obtained using GaAs, to 28 µ using CdTe.     

CdTe as a passivating layer in CdTe/HgCdTe heterostructures

CdTe/Hg_{1 ? x} Cd _x Te heterostructures are studied. In the structures, CdTe is used as a passivating layer deposited as a polycrystal or single crystal on a single-crystal Hg_{1 ? x} Cd _x Te film. The film and a passivating layer were obtained in a single technological process of molecular beam epitaxy. The structure of passivating layers was studied by reflection high-energy electron diffraction, and the effect of the structure of the passivating layer on the properties of the active layer was studied by X-ray diffractometry. Mechanical properties of heterostructures were studied by the microhardness method. Electrical and photoelectrical parameters of the Hg_{1 ? x} Cd _x Te films are reported.     

Calibration and compensation of cogging effect in a permanent magnet linear motor

The cogging force significantly deteriorates positioning accuracy and tracking accuracy in the direct-drive system with a permanent magnet linear motor (PMLM) that has widely applied in electromechanical manufacturing industries. At present, the optimization design of motor construction and adequate control strategies are main measures to overcome the cogging force of PMLM. This paper proposes an experimental method with iterative process to calibrate the cogging compensation force that is used to counteract the cogging force effect. A compensation approach using linear interpolation is presented to make a real time feedforward compensation for the cogging force. The calibration of the cogging compensation force and the compensation of the cogging force are alternately executed. The experiments obtain gradually accurate cogging compensation force. The tracking errors of the motion system are compared with and without cogging force compensation. The results show that the positioning accuracy and tracking accuracy are improved greatly by applying the cogging compensation with the calibrated cogging compensation force. This demonstrates the effectiveness and feasibility of the proposed methods.     

On the compensation of the nonideal NIC

This letter shows that the criteria for a two-port to be a nonideal NIC in terms of its h parameters should be applied to both orientations of the two-port. Alternatively, a different set of conditions should be used if the two-port is to remain in the same orientation.     

On the tin impurity in the thermoelectric compound ZnSb: Charge-carrier generation and compensation

The technique for measuring the Hall coefficient and electrical conductivity in the thermal cycling mode is used to study the effect of the Sn impurity on the microstructure and properties of pressed ZnSb samples. Tin was introduced as an excess component (0.1 and 0.2 at %) and as a substitutional impurity for Zn and Sb atoms in a concentration of (2–2.5) at % The temperature dependences of the parameters of lightly doped samples are fundamentally like similar curves for ZnSb with 0.1 at % of Cu. The highest Hall concentration, 1.4 × 10¹⁹ cm^–3 at 300 K, is obtained upon the introduction of 0.1 at % of Sn; the dimensionless thermoelectric figure of merit attains its maximum value of 0.85 at 660 K. The experimental data are discussed under the assumption of two doping mechanisms, which are effective in different temperature ranges, with zinc vacancies playing the decisive role of acceptor centers. In two ZnSb samples with SnSb and ZnSn additives, the charge-carrier compensation effect is observed; this effect depends on temperature and markedly changes with doping type. As in p-type A^IV–B^VI materials with a low Sn content, hole compensation can be attributed to atomic recharging Sn²⁺ → Sn⁴⁺. Types of compensating complexes are considered.     

Correlation between the backgating effect of a GaAs MESFET and the compensation mechanism of a semi-insulating substrate

The backgating effect on FET'S fabricated both on a Cr-doped HB and on an undoped LEC substrate is investigated. The deep impurity compensation is found to influence the backgating effect both through the formation of the electric dipole layer at the interface between the n-channel and i-substrate, and through the voltage drop in the semi-insulating substrate.     

Study of defect levels in CdTe using thermoelectric effect spectroscopy

We have studied the defect levels in as grown and post growth processed cadmium telluride (CdTe) using thermoelectric effect spectroscopy (TEES) and thermally stimulated current (TSC) techniques. We have extracted the thermal energy (E_th) and trapping cross section (σ_th) for the defect levels using the initial rise and variable heating rate methods. We have identified 10 different defect levels in the crystals. Thermal ionization energy values obtained experimentally were compared to theoretical values of the transition-energy levels of intrinsic and extrinsic defects and defect complexes in CdTe determined by first-principles band-structure calculations. On the basis of this comparison, we have associated the observed ionization levels with various native defects and impurity complexes.     

Analysis and compensation of rolling shutter effect

Due to the sequential-readout structure of complementary metal-oxide semiconductor image sensor array, each scanline of the acquired image is exposed at a different time, resulting in the so-called electronic rolling shutter that induces geometric image distortion when the object or the video camera moves during image capture. In this paper, we propose an image processing technique using a planar motion model to address the problem. Unlike previous methods that involve complex 3-D feature correspondences, a simple approach to the analysis of inter- and intraframe distortions is presented. The high-resolution velocity estimates used for restoring the image are obtained by global motion estimation, BEzier curve fitting, and local motion estimation without resort to correspondence identification. Experimental results demonstrate the effectiveness of the algorithm.     

Hall effect in CdTe crystals doped with Sn from the vapor phase

The Hall effect in semiinsulating CdTe crystals doped with a Sn impurity from the vapor phase in a closed volume is studied. It is found that the conductivity is due to a donor center with E _t ≈ 0.7 eV and the concentration of electrons and their mobility at 300 K are (4–8) × 10⁶ cm^?3 and 200–300 cm² V^?1 s^?1, respectively.     

On the active compensation of noninverting integrators

A new active compensated balanced time constant (BTC) noninverting integrator is proposed. The compensation is achieved by using a voltage follower in the feedback path, and this result in an extremely high Q-factor, namely Q ≃ |A|³. It is also shown, that the use of a voltage follower with the Deboo noninverting integrator will only double its Q-factor. A novel method is proposed for active compensation of the Deboo integrator, which results in an extremely high Q-factor.     

On the pinched hysteresis behavior in a state-controlled resistor

We show that the pinched hysteresis behavior observed in a voltage-controlled resistor with state-feedback self-control can be attributed to the current flowing in the parasitic capacitance holding the voltage across this resistor. A mathematical model describing this circuit structure is derived and validated numerically. To provide experimental evidence of this proposal, a circuit representing a voltage-controlled resistor is placed in parallel with a capacitor and then the current in this capacitor, which is proportional to the derivative of the applied voltage, is sensed, converted into a voltage and used to control the resistance value. This leads to the appearance of a pinched hysteresis loop as theoretically predicted. This work provides further insight into the origin of this behavior in fabricated solid-state devices that can be shown to follow the proposed circuit structure.