Semiconductor Lifetime Research Articles

Activationless charge transport across 4.5 to 22 nm in molecular electronic junctions

In this work, we bridge the gap between short-range tunneling in molecular junctions and activated hopping in bulk organic films, and greatly extend the distance range of charge transport in molecular electronic devices. Three distinct transport mechanisms were observed for 4.5-22-nm-thick oligo(thiophene) layers between carbon contacts, with tunneling operative when d < 8 nm, activated hopping when d > 16 nm for high temperatures and low bias, and a third mechanism consistent with field-induced ionization of highest occupied molecular orbitals or interface states to generate charge carriers when d = 8-22 nm. Transport in the 8-22-nm range is weakly temperature dependent, with a field-dependent activation barrier that becomes negligible at moderate bias. We thus report here a unique, activationless transport mechanism, operative over 8-22-nm distances without involving hopping, which severely limits carrier mobility and device lifetime in organic semiconductors. Charge transport in molecular electronic junctions can thus be effective for transport distances significantly greater than the 1-5 nm associated with quantum-mechanical tunneling.

Optical and Other Measurement Techniques of Carrier Lifetime in Semiconductors

In this paper, various methods for characterization of semiconductor charge carrier lifetime are reviewed and an optical technique is described in detail. This technique is contactless, all-optical and based upon measurements of free carrier absorption transients by an infrared probe beam following electron-hole pair excitation by a pulsed laser beam. Main features are a direct probing of the excess carrier density coupled with a homogeneous carrier distribution within the sample, enabling precision studies of different recombination mechanisms. The method is capable of measuring the lifetime over a broad range of injections (10 13 -10 18 cm -3 ) probing the minority carrier lifetime, the high injection lifetime and Auger recombination, as well as the transition between these ranges. Performance and limitations of the technique, such as lateral resolution, are addressed while application of the technique for lifetime mapping and effects of surface recombination are also outlined. Results from detailed studies of the injection dependence yield good agreement with the Shockley-Read-Hall theory, whereas the coefficient for Auger recombination shows an apparent shift to a higher value, with respect to the traditionally accepted value, at carrier densities below 2×10 17 cm -3 . Data also indicate an increased value of the coefficient for bimolecular recombination from the generally accepted value. Measurement on an electron irradiated wafer and wafers of exceptionally high carrier lifetimes are also discussed within the framework of different recombination mechanisms.

Determination of free polaron lifetime in organic bulk heterojunction solar cells by transient time analysis

A transient response technique that is widely used to measure the minority carrier lifetime in inorganic semiconductors is proposed to measure the lifetime of free polarons in a polymer:fullerene bulk heterojunction (BHJ) solar cell. A numerical model that can be used to describe the transient behavior of BHJ devices has been developed. Using the proposed method, the lifetime of free polarons in poly (3-hexylthiophene) and [6, 6]-phenyl C61-butyric acid methyl ester blend film is estimated to be in the range of 300–400 ns.

A new lifetime diagnostic system for photovoltaic materials

We have developed a new minority-carrier lifetime measurement apparatus for measuring the recombination lifetime in semiconductors. We describe the technique as transmission modulated photoconductive decay (TMPCD). This is a contactless, non-invasive technique that produces transient photoconductive lifetime data. The measurement procedure is very sensitive to small signals and has a superior time response for measurement of short carrier lifetimes. This technology has several advantages over resonant coupled photoconductive decay (RCPCD) and transient microwave reflection photoconductive decay (μPCD). The response time advantage provides a capability to measure very short lifetimes in thin film materials, such as nanocrystalline silicon films and nanowire composites. This is accomplished while maintaining a sensitivity that is at least comparable to RCPCD. The new technique has been successfully applied to silicon wafers, compound semiconductor thin films, nano-crystalline silicon films, and II–VI nanowires.

Calculation of positron lifetime of compound semiconductors

On the basis of of local density approximation (LDA) and general gradient approximation (GGA), positron annihilation information has been calculated for five types of compound semiconductors, which are ZnO, GaN, GaAs, SiC and InP. The calculated information includes distribution of positron density, distribution of positron annihilation rate density, positron bulk lifetime, positron monovacancy lifetime and positron divacancy lifetime.

Pulsed photo-conductivity and carrier recombination lifetime spectroscopy of metal doped germanium

A spectroscopic technique is presented, based on pulsed photo-conductivity and carrier lifetime measurements. The technique is implemented by using excitation light pulses with variable wavelength and duration, to generate excess carriers and to study deep levels as well as their impact on carrier lifetime in semiconductors. The carrier lifetime is measured simultaneously with the photo-ionization response as a function of the excitation photon energy. The technique is based on the analysis of amplitude and decay rate characteristics as a function of the wavelength of the light used for excitation. The photoconductivity transients are probed by microwaves. A calibration technique is discussed to relate the photo-conductivity response to the photo-ionization cross-section and the activation energy of deep levels present in the material. As the technique can be applied at room temperature, measurement conditions can be chosen which are relevant for device operation. The possibilities are illustrated for metal implanted and annealed Ge wafers. The obtained results are compared with those of capacitance-deep level transient spectroscopy analyses on similar samples, and with published results.

Interaction of microwaves with photoelectrons in semiconductors

Microwave photoconductive decay is widely used to measure the recombination lifetime in semiconductors and is generally considered to be a low-injection technique. Here, the authors examine the technique in detail from both a theoretical and experimental perspective. When the skin or penetration depth is much less than the sample thickness, the reflectance-conductivity relationship is nonlinear when the injected carrier density exceeds about 5% of background doping. However, when the penetration depth is much larger than the sample thickness, the transmitted signal is linear over several orders of magnitude of sample conductivity. In the transmission mode of operation, high-injection lifetime measurements are accurate.

Microwave probed photoconductivity spectroscopy of deep levels in Ni doped Ge

A transient technique to simultaneously measure carrier lifetime and deep levels in semiconductors is proposed based on microwave probed photoconductivity spectroscopy in the wavelength range between 0.5 and 16μm. The application of this noncontacting technique is illustrated by a study of carrier lifetime and deep levels in Ni implanted and annealed Ge wafers. The activation energies of the deep levels are determined at room temperature and compared with those extracted by means of capacitance deep level transient spectroscopy.

The investigation of a novel approach in the simulation of the trap levels effect on the conduction in a GaAs P +νN + diode

A numerical simulation using C ++ language, allowing the store of big size information (presented as ( X × Y × Z) matrix) is realized, and a theoretical modelling of the charges transport in the semiconductor devices made in materials presenting a significant concentration of impurities whose energy levels are deep is implemented. The results are validated on a semi-insulating gallium arsenide (SI-GaAs) PIN structure. The numerical simulation is made by the resolution of the phenomenological transport's equations by adopting a new approach permitting to study the conduction phenomena through all the structure. From the 3D modelling, the variation of the potential profile, the free carriers and the space charge distributions, and the electrical field inside the structure are simulated. The effects of the traps are taken into account through the mechanisms of recombination and storage of a significant space charge, where the influence of the presence of these levels on the above factors is seen. Through this work a physical model intended for the study of the influence of the trap levels on the potential and the free carrier density distributions for weak lifetime semiconductors, more particularly on semi-insulating GaAs is introduced. Taking into account the trap centres is essential for a good comprehension and explanation of the transport phenomena. The present study consists to do a simulation and a modelisation of structures based on materials presenting a deep and a trap centres. A trapping model based on a combined numerical method Gummel and Newton is adopted to reduce both the run time and to reach convergence in the process when working not with one junction but with two.

Precession trajectories of the hole spin in zinc-blende semiconductors

Precession of the hole spin in zinc-blende-type semiconductors in the absence of magnetic field has been investigated. In a noncentrosymmetric lattice the precession is caused by spin-splitting of the valence band. The hole spin quantization axis is shown to depend on the wave vector magnitude and direction, and the spin precession trajectories that run around the quantization axis are found to be ellipses. Analytical solutions are given for high symmetry directions of the wave vector. For a general case, the numerical algorithm is presented. The algorithm takes into account the warping and nonparabolicity of the bands, and may be used to simulate time-dependent precession of the hole spin in semiconductor devices by the Monte Carlo method, for example in the analysis of spin transport in spin-FETs or spin lifetime in semiconductors with complex band structure.

Theoretical Study of Spin Lifetimes in [110] Strained GaAs

We present first-principles studies of the zero field spin splitting of conduction band in [110] strained GaAs that determine spin lifetimes in semiconductors. Our calculations reveal strong anisotropy of the linear-k spin splitting in the (110) plane of the Brillouin zone and very minor in the (001) plane. This provides a qualitative understanding of the difference in the spin lifetimes in the GaAs/AlAs heterostructures grown along [100] and [110] crystallographic directions.

Publisher’s Note: “General theory of carrier lifetime in semiconductors with multiple localized states” [J. Appl. Phys. 96, 6454 (2004)

First Page

General theory of carrier lifetime in semiconductors with multiple localized states

The Shockley-Read-Hall rate equations determine the average carrier transitions via a single-level defect in the band gap of a nondegenerate semiconductor. In the present work the differential rate equations for multiple levels, or localized states systems, are derived from first principles. These multiple level systems comprise the multiple discrete defects system and the coupled or excited states system. The solution for the single-level rate equations, developed recently for transient decay, is represented by an infinite series of monoexponential terms, the frequencies or inverse time constants of which are a linear combination of the fundamental frequencies ω=1∕τ. For the multiple localized state solution expressions for the fundamental time constants τ1+k are derived for m states with k=1,2,…,m without an approximation at a given temperature for an excess carrier concentration below nondegenerate doping, arbitrary uniform doping concentration NA,D, defect level concentration Nk, cross section σnk,pk, and energy level Ek. Verification of the set of rate equations for each system is performed by comparing the analysis of the numerical solution for component time constants with the prediction of the theory. The variation of the fundamental time constant τ1 with excess carrier concentration indicates the behavior of minority carrier trapping.

Accurate first-principles detailed-balance determination of auger recombination and impact ionization rates in semiconductors.

The technologically important prediction of Auger recombination lifetimes in semiconductors is addressed by means of a fully first-principles formalism, based on precise energy bands and wave functions provided by the full-potential linearized augmented plane wave code. The minority carrier Auger lifetime is determined by two related approaches: (i) a direct evaluation within Fermi's golden rule, and (ii) an indirect evaluation, based on a detailed balance formulation combining Auger recombination and its inverse process, impact ionization, in a unified framework. Lifetimes determined with the direct and indirect methods show excellent consistency between them (i) for n-doped GaAs and (ii) with measured values for GaAs and InGaAs. This indicates the computational formalism as a new sensitive tool for use in materials performance optimization.

Chemical surface passivation of low resistivity p-type Ge wafers for solar cell applications

The measurement of the bulk minority carrier lifetime of semiconductors requires efficient passivation of the recombination states at the surface. While numerous recipes have been published for Si-surface passivation, no adequate passivation methods are available for Ge. This paper presents a new and straightforward passivation method, based on a solution of iodine in polyvinyl acetate and acetone. The dependence of the carrier lifetime with time after passivation and with Ge resistivity has been investigated. It is found that the lifetime in this low resistivity material is strongly governed by Auger recombination.

Analysis of the moving photocarrier grating technique for semiconductors of high defect density

The moving photocarrier grating (MPG) technique allows us to determine the carrier drift mobilities and the recombination lifetime of semiconductors. This technique utilizes a spatially and temporally modulated light intensity for the generation of photocarriers, which leads to a modulation of the carrier densities. The different mobilities of electrons and holes introduce a phase shift between the charge distributions. The resulting internal electric field produces a short circuit current which depends on the modulation frequency. The theoretical calculation of that short circuit current includes the diffusion and drift currents, and the generation and recombination processes. In the original treatment of the MPG method recombination between electrons and holes was assumed to proceed only through band-to-band transitions. We analyze here the MPG method by explicitly considering recombination through dangling bond states. We simulate short circuit current curves and compare them with measurements. The inclusion of recombination through dangling bonds allows us to explain an apparent contradiction in the evolution of the recombination lifetime as a function of light-induced degradation.

Photoconductive frequency-resolved spectrometer for carrier lifetime determination in semiconductors

A photoconductive frequency-resolved spectrometer (PCFRS) for the determination of carrier lifetime in semiconductors is described. This PCFRS is designed for operation from 1 up to 1000 Hz (lifetime of 0.159 s–0.159 ms). The PCFRS uses an inexpensive superbright light emitting diode, which can be modulated directly, and a system embedded with a 68HC11 microcontroller, which performs all functions of this spectrometer. A sample of the lifetime distribution for a commercial CdS photoresistor is also presented.

Generalized analysis of quasi-steady-state and quasi-transient measurements of carrier lifetimes in semiconductors

Recently, a simple yet powerful carrier lifetime technique for semiconductor wafers has been introduced that is based on the simultaneous measurement of the light-induced photoconductance of the sample and the corresponding light intensity [Appl. Phys. Lett. 69, 2510 (1996)]. In combination with a light pulse from a flash lamp, this method allows the injection level dependent determination of the effective carrier lifetime in the quasi-steady-state mode as well as the quasi-transient mode. For both cases, approximate solutions (those for steady-state and transient conditions) of the underlying semiconductor equations have been used. However, depending on the actual lifetime value and the time dependence of the flash lamp, specific systematic errors in the effective carrier lifetime arise from the involved approximations. In this work, we present a generalized analysis that avoids these approximations and hence substantially extends the applicability of the quasi-steady-state and quasi-transient methods beyond their previous limits.

Enhancement of radiative lifetime in semiconductors using photonic crystals

In this work we present a structure consisting of a semiconductor sample immersed between two one-dimensional photonic crystals, designed with the aim to investigate the possibility to increase radiative lifetime in infrared photodetectors, i.e., to decrease the background radiation limit. Since we used a HgCdTe detector for the detection of CO 2 laser radiation, we formed a defect in the front-side photonic crystal with the transmission peak on 10.6 μm. Our numerical calculations show that the 1-D photonic crystal structures at the front and at the back side of the semiconductor sample significantly increase the radiative lifetime in the photodetector.

Electronic structure of crystals via VLEED

Dispersion relations E(k) of electrons in crystals preserve their meaning in many-electron systems. Interacting electrons become quasiparticles, which apart from energies, are also characterised by lifetimes. As for real systems these characteristics are difficult to evaluate, some experimental evidence is desirable. Attention will be focused on two methods applicable at energies above the vacuum level. Interaction of external beam of slow electrons with crystals in target current spectroscopy (TCS), and in very low energy electron diffraction (VLEED), will be discussed in context with the unoccupied part of the electronic band structure. Knowledge of electron dispersion relations above the vacuum level is important for interpretations of angular resolved photoelectron spectroscopy. Comparison of experimental TCS and VLEED data with theoretical band structure calculations determines positions of energy gaps and more detailed theoretical interpretations of the intensity profiles provides information about electron lifetimes in metals and semiconductors.