Carrier Transit Time Research Articles

Effect of carrier transition mechanisms and gain compression on relative intensity noise of 1.55 µm QD lasers

The impacts of gain compression and direct carrier transition on relative intensity noise (RIN) characteristics of 1.55µm QD lasers have been investigated theoretically. The auto and cross correlation coefficients are calculated through small signal linearization of rate equations for carriers and photon numbers in presence of Langevin noise sources. Calculations reveal that gain nonlinearity reduces RIN level of QD lasers while increases damping factor. These results show good agreement with experimental measurements of RIN spectrum in 1.55µm QD lasers which is flat up to 10GHz. Evaluations demonstrate that among wetting layer, Quantum and Photon shot noises, the second and third one have essential role in overall value of RIN in both linear and nonlinear QD lasers. Moreover, calculations indicate that by increasing the direct carrier transition time, RIN level of auto and cross correlations resulted from wetting layer and ground state carriers increases. Finally, it is shown that the RIN level declines in nonlinear QD lasers by working at higher injection currents which is in confirmation with recent experimental reports. Though, it is demonstrated that considering nonlinear gain and direct carrier transition play essential role on correct modeling of RIN characteristics in 1.55µm QD lasers.

Characterization of Interfaces Between Contacts and Active Layer in Organic Photovoltaics Using Impedance Spectroscopy and Equivalent Circuit Model

This study investigates the interface between the active layer and contacts in organic photovoltaics (OPVs) since the contact materials strongly affect the energy barrier at the interfaces. The interfacial characteristics are simply defined as a resistance–capacitance ( R–C ) shunt pair and extracted by fitting the impedance spectra to the equivalent circuit model. A change in the energy barrier is found to affect the values of R and C at the interface and the carrier transition time. In addition, the effect of electron buffer layer (TiO2) thickness on the interfacial characteristics is analyzed using an impedance spectroscopy. The interfacial area between the hole buffer layer (MoO3), and the active layer affects the values of R and C at the interface.

Gain characteristics and femto-second optical pulse response of 1550 nm-band multi-stacked QD-SOA grown on InP(311)B substrate

In this paper, we demonstrated 155nm-band multi-stacked QD-SOA grown by the strain-compensation technique on an InP(311)B substrate, and evaluated the fundamental gain characteristics and the femto-second optical pulse response, for the application to ultra-fast all-optical logic gate devices. The device length was 1650μm, and a maximum gain of 35dB was obtained under an injection current of 500mA. We also input two serial femto-second duplicated pulses into the QD-SOA by changing the duration and observed the output auto-correlation waveforms. As a result, an effective carrier transition time was estimated to be about 1ps.

Imaging ultrafast carrier transport in nanoscale field-effect transistors.

In the present study, we visualize ultrafast carrier dynamics in one-dimensional nanoscale devices, such as Si nanowire and carbon nanotube transistors using femtosecond photocurrent microscopy. We investigate transit times of ultrashort carriers that are generated near one metallic electrode and subsequently transported toward the opposite electrode based on drift and diffusion motions. Conversely, pure diffusion motion is observed when the pump pulse is located in the middle of the nanowires. Carrier dynamics have been addressed for various working conditions, in which we found that the carrier velocity and pulse width can be manipulated by the external electrodes. In particular, the carrier velocities extracted from transit times increase for a larger negative gate bias because of the increased field strength at the Schottky barrier.

SiGe HBTs in 90nm BiCMOS Technology Demonstrating fT/fMAX 285GHz/475GHz through Simultaneous Reduction of Base Resistance and Extrinsic Collector Capacitance

Development of SiGe HBTs in BiCMOS technology with both high f T and f MAX faces significant challenges. To increase f T, thinning the base and collector thickness is generally the first step to reduce the carrier transit times, but this increases the base resistance and the collector-base capacitance, which impacts f MAX negatively. Increasing collector doping is also often employed to increase f T, but this increases collector-base capacitance, which drives f MAX down. To overcome these limits, millisecond anneal techniques, low temperature silicide and low temperature contact processes are employed to reduce the base resistance. Concurrently a novel approach to reduce the extrinsic collector-base capacitance is developed, without affecting the manufacturability and integration with CMOS. The simultaneous reduction of both base resistance and collector capacitance enables high performance SiGe HBT devices in 90nm BiCMOS Technology with operating frequencies of 285/475GHz f T/f MAX.

SiGe HBTs in 90nm BiCMOS Technology Demonstrating fT/fMAX 285GHz/475GHz through Simultaneous Reduction of Base Resistance and Extrinsic Collector Capacitance

Development of SiGe HBTs in BiCMOS technology with both high fT and fMAX faces significant challenges. To increase fT, thinning the base and collector thickness is generally the first step to reduce the carrier transit times, but increases the base resistance and the collector-base capacitance, which impacts fMAX negatively. Increasing collector doping is also often employed to increase fT, but again increases collector-base capacitance, which drives fMAX down. To overcome these limits, a series of process and device structure innovations have been made in recent years. Millisecond anneal techniques, low temperature silicide and low temperature contact processes are shown to be effective in reducing the base resistance. A novel approach to reduce the extrinsic collector-base capacitance is developed, concurrent to the reduction of base resistance, without affecting the manufacturability and integration with CMOS. The simultaneous reduction of both base resistance and collector capacitance enables high performance SiGe HBT devices in 90nm BiCMOS Technology with operating frequencies of 285/475GHz fT/fMAX.

Electronic Pulse Shape Formation in Transient Charge and Transient Current Detection Approach in (CdZn)Te Detectors

The transient-charge technique (TChT) and transient-current technique (TCT) belong to basic methods routinely used for the determination of transport properties of semiconductor radiation detectors. In spite of apparently definite correspondence between the charge and current transients via simple differentiation or integration in time, the passing of the signal through real electronic circuits interferes with the evaluation of the transients and hampers the direct relation between TChT and TCT transients. In this paper we use in parallel both TChT and TCT for the characterization of (CdZn)Te detector and compare their output. We show how real TChT and TCT signals deviate from their ideal shape and complicate the evaluation of electron mobility from detected pulses. We conclude that whilst TCT may conveniently be used for the analysis of current waveforms and determination of carrier transit time and mobility in rather highly biased detectors, TChT is mainly useful for determining the total charge collection both at high and low biasing. We also suggest the way to improve the transit time readout in case of TChT method.

A high-speed avalanche photodiode

High-speed avalanche photodiodes are widely used in optical communication systems. Nowadays, separate absorption charge and multiplication structure is widely adopted. In this article, a structure with higher speed than separate absorption charge and multiplication structure is reported. Besides the traditional absorption layer, charge layer and multiplication layer, this structure introduces an additional charge layer and transit layer and thus can be referred to as separate absorption, charge, multiplication, charge and transit structure. The introduction of the new charge layer and transit layer brings additional freedom in device structure design. The benefit of this structure is that the carrier transit time and device capacitance can be reduced independently, thus the 3 dB bandwidth could be improved by more than 50% in contrast to the separate absorption charge and multiplication structure with the same size.

Low-temperature processed ZnO and CdS photodetectors deposited by pulsed laser deposition

UV-VIS photodetectors using an interdigital configuration, with zinc oxide (ZnO) and cadmium sulfide (CdS) semiconductors deposited by pulsed laser deposition, were fabricated with a maximum processing temperature of 100 °C. Without any further post-growth annealing, the photodetectors are compatible with flexible and transparent substrates. Aluminum (Al) and indium tin oxide (ITO) were investigated as contacts. Focusing on underwater communications, the impact of metal contact (ITO versus Al) was investigated to determine the maximum responsivity using a laser with a 405 nm wavelength. As expected, the responsivity increases for reduced metal finger separation. This is a consequence of reduced carrier transit time for shorter finger separation. For ITO, the highest responsivities for both films (ZnO and CdS) were ∼3 A W−1 at 5 V. On the other hand, for Al contacts, the maximum responsivities at 5 V were ∼0.1 A W−1 and 0.7 A W−1 for CdS and ZnO, respectively.

Enhanced photocurrent and dynamic response in vertically aligned In₂S₃/Ag core/shell nanorod array photoconductive devices.

Enhanced photocurrent values were achieved through a semiconductor-core/metal-shell nanorod array photoconductive device geometry. Vertically aligned indium sulfide (In2S3) nanorods were formed as the core by using glancing angle deposition technique (GLAD). A thin silver (Ag) layer is conformally coated around nanorods as the metallic shell through a high pressure sputter deposition method. This was followed by capping the nanorods with a metallic blanket layer of Ag film by utilizing a new small angle deposition technique combined with GLAD. Radial interface that was formed by the core/shell geometry provided an efficient charge carrier collection by shortening carrier transit times, which led to a superior photocurrent and gain. Thin metal shells around nanorods acted as a passivation layer to decrease surface states that cause prolonged carrier lifetimes and slow recovery of the photocurrent in nanorods. A combination of efficient carrier collection with surface passivation resulted in enhanced photocurrent and dynamic response at the same time in one device structure. In2S3 nanorod devices without the metal shell and with relatively thicker metal shell were also fabricated and characterized for comparison. In2S3 nanorods with thin metal shell showed the highest photosensitivity (photocurrent/dark current) response compared to two other designs. Microstructural, morphological, and electronic properties of the core/shell nanorods were used to explain the results observed.

Modeling of Multiple-Quantum-Well p-i-n Photodiodes

A model for multiple-quantum well (MQW) p-i-n photodiode (PD) is presented. The model accounts for the responsivity spectrum and the polarization-dependent loss. The important physical effects for modeling MQW PDs are the carrier transit time through the MQW layers and the free carrier density in the quantum wells. Furthermore, both effects influence the internal quantum efficiency and saturation of the optical absorption. The model is verified by comparing with the measurement results of a waveguide integrated MQW p-i-n PD. It is suitable for optimizing the design and describing the properties of MQW p-i-n PDs.

Physics of fully depleted CCDs

In this work we present simple, physics-based modelsfor two effects that have been noted in the fully depleted CCDsthat are presently used in the Dark Energy Survey Camera. Thefirst effect is the observation that the point-spread functionincreases slightly with the signal level. This is explainedby considering the effect on charge-carrier diffusion due to the reduction in the magnitude of the channel potential as collected signal charge acts to partially neutralize the fixed charge in the depleted channel. The resulting reduced voltage drop across thecarrier drift region decreases the vertical electric field andincreases the carrier transit time. The second effect is the observation of low-level, concentric ring patterns seen in uniformly illuminated images. This effect is shown to be most likely due to lateral deflection of charge during the transit of the photo-generated carriers to the potential wells as a result of lateral electric fields. The lateral fields are a result of space charge in the fully depleted substrates arising from resistivity variations inherent to the growth of the high-resistivity silicon used to fabricate the CCDs.

Simulation of the Metal-Semiconductor-Metal photodetector based on InGaAs for the photodetection at the wavelength 1.55 μm

In this paper we present the simulation of Metal-Semiconductor-Metal photodetector (MSM-PD) of interdigitated planar structure based on InAlAs/InGaAs adapted for photodetection at the wavelength 1.55μm. We use the theoretical models to plot the variations of the dark current, the photocurrent, the capacity, and the cut-off frequency of the photodetector as a function of bias voltage and the interelectrode distance. The obtained results show a very low dark current, mainly due to the introduction of a thin layer to increase the Schottky barrier based on In0.52Al0.48As in the epitaxial structure of component. The obtained photocurrent and cut-off frequencies are very appreciable, these latter are mainly limited by the transit time of the photo-generated carriers given the low component capacity obtained by simulation.

Extracting the charge carrier mobility from the nanosecond photocurrent response of organic solar cells and photodiodes

We present two simple methods to estimate the effective mobility of the faster charge carrier species from the transient nanosecond photoresponse of an organic solar cell or photodiode. In combination with detailed numerical drift-diffusion simulations in the framework of the multiple-trapping model, we identify the energetic relaxation of the charge carriers and hence a decrease of the effective charge carrier mobility while drifting towards the electrodes. From the characteristic shape of the transient current density, the temperature as well as the nonlinear voltage dependence of the charge carrier transit time, we can quantify an exponential trap distribution. In addition, the nonlinearity of the transit time, as also known from comparable time-of-flight measurements, can be explained by charge carrier relaxation processes in the presence of trap states. The effective charge carrier mobility is shown to be field independent but highly temperature dependent.

Time‐resolved electroabsorption measurement of carrier velocity in inverted polarity In1‐xGax N/GaN heterostructures due to internal electric fields

AbstractCarrier transport measurements were made in a c ‐plane inverted‐polarity n ‐GaN/i ‐In1–xGax N/p ‐GaN heterostructure with a 200 nm thick In1–xGax N layer using ultrafast spectroscopy techniques. Time‐domain THz measurements indicate that the direction of electron transport is dominated by drift towards the n ‐GaN layer. Time‐resolved electroabsorption measurements were used to determine carrier velocities by monitoring the change in transmission of a probe beam tuned to the In1‐xGax N absorption edge due to the transport of photogenerated carriers under the built‐in internal electric field. Carrier transit times associated with screening of the electroabsorption are resolved at the lowest injection level. The signal rise time shows two distinct time scales, which correspond to an electron velocity of 3.3 ± 0.5 x 107 cm/s and a hole velocity of 6.7 ± 0.3 x 106 cm/s in an internal electric field of ∼150 kV/cm. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Application of capacitance spectrum and the imaginary part of impedance spectrum to study carrier dynamics of N,N′-diphenyl-N,N′bis(1,1′-biphenyl)-4,4′-diamine

In this report, the hole-only device was fabricated in order to study the carrier mobility of N,N′-diphenyl-N,N′bis(1,1′-biphenyl)-4,4′-diamine through the space charge limited current model. Then the capacitance spectrum and the corresponding imaginary part of impedance spectrum were measured. When the frequency-dependent mathematical physics equations were directly used to fit the capacitance spectrum, it was found that there was obviously error in the curve between the range of ω~τt−1. Considering the distribution of the carrier transit time, two models, the exponential distribution and the Gaussian distribution, were added to fit the experimental curve. Through such distribution functions, the curve could be more satisfactorily to be fitted. And from the logic thinking, we think the Gaussian distribution is more reasonable and more close to the real transit time distribution. Further, we used the imaginary part of the impedance spectra for research and also achieved satisfactory results. Finally, the four kinds of carrier mobility tested by the above four research methods were all found to be in accordance with the Poole–Frenkel model, with only small difference in the values.

Vertical organic spin valves in perpendicular magnetic fields

We report the results of magnetoresistance measurements in vertical organic spin valves with the magnetic field oriented perpendicular to the layer stack. The magnetoresistance measurements were performed after carefully preparing either parallel or antiparallel in-plane magnetization states of the magnetic electrodes in order to observe traces of Hanle precession. Due to the low mobility in organic semiconductors, the transit time of spin-polarized carriers should allow for precession of the spins in perpendicular fields which in statistical average would quench the magnetoresistance. However, in none of the experiments do we observe any change in resistance while sweeping the perpendicular field, up to the point where the electrode's magnetization starts to reorient. This absence of Hanle-type effects indicates that the magnetoresistance is not based on the injection of spin-polarized electrons into the organic semiconductor but rather on tunneling through pinholes superimposed with tunneling anisotropic magnetoresistance. These measurements suggest that even for clear spin-valve signatures, control experiments should be established which can confirm or disprove the electrical detection of injected spin-polarized carriers, similar to the Hanle precession in inorganic spin valves.

Influence of Carrier Lifetime, Transit Time, and Operation Voltages on the Photoresponse of Visible-Blind AlGaN Metal–Semiconductor–Metal Photodetectors

We investigated the influence of lifetime and transit time of photogenerated carriers on the performance of visible-blind Al0.25Ga0.75N metal–semiconductor–metal photodetectors by a combination of experimental studies and numerical simulations. Good agreement between simulated and measured current–voltage (I–V) characteristics was achieved for several geometries of the interdigitated contact structure. Simulations of the external quantum efficiency (EQE) at low bias voltages showed that a long hole lifetime in the AlGaN absorption layer significantly influences the EQE due to the slow carrier transit in weak electric fields. At 1 V the EQE can be enhanced by a factor of 3 by increasing the hole lifetime from 10 ps to 1 ns. Reducing the electrode spacing from 10 to 1 µm as well as operating the device at higher voltages additionally increases the ratio between carrier lifetime and transit time, resulting in an enhancement of the EQE at a fixed carrier lifetime by one order of magnitude.

CVD diamond as a position sensitive detector using charge carrier transition time

A concept of position sensitive single crystal chemical vapour deposition (scCVD) diamond detector is proposed and tested using the lateral time resolved ion beam induced current (TRIBIC) using 4.5MeV proton microbeam. Simulation of charge transients confirmed that it is possible to correlate the length of the signal pulse to the position of the proton impact. The simulation also shows that the position sensitivity of such device is improved when the mobility and the effective lifetime values of the electrons and the holes are significantly different. A proof of concept was confirmed using lateral TRIBIC performed on a 1×1×0.5mm scCVD diamond detector.

Semiconductor detectors and focal plane arrays for far-infrared imaging

AbstractThe detection of far-infrared (far-IR) and sub-mm-wave radiation is resistant to the commonly employed techniques in the neighbouring microwave and IR frequency bands. In this wavelength detection range the use of solid state detectors has been hampered for the reasons of transit time of charge carriers being larger than the time of one oscillation period of radiation. Also the energy of radiation quanta is substantially smaller than the thermal energy at room temperature and even liquid nitrogen temperature. The realization of terahertz (THz) emitters and receivers is a challenge because the frequencies are too high for conventional electronics and the photon energies are too small for classical optics.Development of semiconductor focal plane arrays started in seventies last century and has revolutionized imaging systems in the next decades. This paper presents progress in far-IR and sub-mm-wave semiconductor detector technology of focal plane arrays during the past twenty years. Special attention is given on recent progress in the detector technologies for real-time uncooled THz focal plane arrays such as Schottky barrier arrays, field-effect transistor detectors, and microbolometers. Also cryogenically cooled silicon and germanium extrinsic photoconductor arrays, and semiconductor bolometer arrays are considered.