Semiconductor Lifetime Research Articles

Determination of free carrier recombination lifetime in amorphous semiconductors: application to the study of iodine doping effect in arsenic triselenide

A method for measuring the monomolecular recombination lifetime in amorphous semiconductors is described. The present method is based on the measurement of localized-state distributions from transient photoconductivity data and on the fitting of computer-generated photocurrent transients using the measured localized-state distributions to experimental data, by which the monomolecular recombination lifetime is determined. The present method is applied to the study of the iodine doping effects in amorphous arsenic triselenide, and it is found that the iodine doping does not change the localized-state distribution or monomolecular recombination lifetime.

Contactless measurements of the photocarrier lifetime in amorphous silicon by a heterodyne experiment

We describe a heterodyne detection scheme for an optical measurement of the photocarrier lifetime in semiconductors. The setup utilizes the Doppler shift of a laser beam that is diffracted from a periodic modulation of the optical constants of a sample, when it is illuminated with a moving interference grating. The advantage of heterodyne detection is its high sensitivity and the extraction of phase information of the diffracted beam. We present measurements on amorphous hydrogenated silicon.

New life in detecting defects

Carrier in semiconductors have been recently rediscovered by the silicon IC community. This is an opportune time to discuss this topic since lifetime is emerging as an important parameter for describing material, process, and equipment cleanliness after being ignored for many years. This article tries to shed some light on lifetimes in semiconductors, which is a topic that has caused much confusion in the past. Various recombination mechanisms are discussed and the concept of recombination and generation lifetime is presented. We will show that surface recombination/generation plays an important role in today's high-purity Si and will become yet more important as bulk impurity densities in Si are reduced further.

Determination of two-photon-generated free-carrier lifetime in semiconductors by a single-beam Z-scan technique

Determination of two-photon-generated free-carrier lifetime in semiconductors by a single-beam Z-scan technique

Capillary zone electrophoretic determination of ionic impurities in silicone products used for electronic applications

Ionic contamination has a very negative impact on the lifetime of semiconductors and electronic devices. The ionic purity of the polymeric materials that remain in contact with active chip surfaces is regarded as a key factor to long term reliability. Currently, ion chromatography (IC) is utilised to measure and routinely control cationic and anionic contaminants (in the μg/l range) of silicone coatings and encapsulants used for electronic applications. However, this technique is expensive, time consuming and requires intensive maintenance. Capillary zone electrophoresis (CZE) has been identified as an excellent alternative to IC for the determination of extractable ionic impurities in silicone products. Main advantages of CZE for routine industrial operations are short analysis time, high sensitivity, simpler operation than IC and reduced maintenance cost.

Carrier lifetimes in silicon

Carrier lifetimes in semiconductors are being rediscovered by the Si IC community, because the lifetime is a very effective parameter to characterize the purity of a material or device. It has become a process and equipment characterization parameter. The various recombination mechanisms are discussed and the concept of recombination and generation lifetime is presented. We show that surface recombination/generation plays an important role in today's high purity Si and will become yet more important as bulk impurity densities in Si are reduced further. Furthermore, the dependence of lifetime on impurity energy level and minority carrier injection level is discussed. Concepts are stressed in the paper, with the necessary equations to clarify these concepts. Wherever possible, the concepts are augmented with experimental data, with particular emphasis on the case of iron in silicon, because Fe is one of the most important impurities in Si today. We have used Si in the examples because lifetime measurements are most commonly made in Si.

Contactless determination of current–voltage characteristics and minority-carrier lifetimes in semiconductors from quasi-steady-state photoconductance data

A simple method for implementing the steady-state photoconductance technique for determining the minority-carrier lifetime of semiconductor materials is presented. Using a contactless instrument, the photoconductance is measured in a quasi-steady-state mode during a long, slow varying light pulse. This permits the use of simple electronics and light sources. Despite its simplicity, the technique is capable of determining very low minority carrier lifetimes and is applicable to a wide range of semiconductor materials. In addition, by analyzing this quasi-steady-state photoconductance as a function of incident light intensity, implicit current–voltage characteristic curves can be obtained for noncontacted silicon wafers and solar cell precursors in an expedient manner.

The data treatment influence on the spectra decomposition in positron lifetime spectroscopy Part 2: The effect of source corrections

We investigate the reliability and self-consistency of source corrections in positron lifetime spectroscopy using a 22Na e +-emitter covered by thin metal foils. Furthermore, we study the influence of different source corrections on the decomposability of positron lifetime spectra. In positron lifetime spectroscopy, examining metals and semiconductors, we find that it is possible to obtain consistent source corrections for a broad variety of sample materials. The experimental results were verified by Monte-Carlo simulations of corresponding spectra. In the case that only one instead of two identical samples is available, e.g. in non-destructive testing geometry, it is important which material is used to cover the backside of the source. We find differences in the stability of the decomposition of lifetime spectra if one uses reference materials with longer or shorter bulk lifetime as cover.

The quasineutrality condition in amorphous semiconductors: Reformulation of the ‘lifetime/relaxation’ criterion

The concepts of lifetime and relaxation semiconductors introduced by van Roosbroeck and Casey are reconsidered for amorphous semiconductors and the effect of localized states on the lifetime/relaxation criterion specified: The quantity to be considered is τ ρ/ T d, where T d is (as before) the dielectric relaxation time, but τ ρ is now the lifetime of the total charge including charge stored in localized states. τ ρ is equal to the free carrier lifetime times a correction factor that is much larger than unity for amorphous semiconductors. Bandtail states and dangling bonds act differently on the criterion. The correction factor is frequency-dependent and decreases at higher frequencies. Two illustrative experimental examples (low substrate temperature a-Si:H, a-SiC:H alloys) are given, showing borderline cases.

Determining carrier lifetime using frequency dependence in contactless photoelectromagnetic investigations of semiconductors

A new contactless method of determining the carrier lifetime in semiconductors is presented. It uses the dependence of the magnetic field evoked by photoelectromagnetic current in an illuminated semiconductor placed in an external magnetic field on the frequency of chopping the illumination intensity. This method was used to determine the electron lifetime in p-type crystalline silicon. It is suitable for rapid and simple inspection of a semiconductor wafer for laboratory and industrial purposes. The using of this method in investigations of superlattices and multi-quantum wells is proposed.

Platinum as Recombination-Generation Centers in Silicon

The general equations of steady-state lifetime in semiconductors with multiple deep impurity levels are derived based on the recombination theory. From the obtained equations the six capture cross sections of three Pt-induced levels in silicon are experimentally determined. The behaviors of minority carrier lifetime and leakage current in Pt-diffused devices are also discussed. It is found that the minority carrier lifetime is influenced by the three Pt-related levels and that the major contribution to the leakage current arises from the level of E c-0.52 eV.

Anharmonic Phonon Lifetimes in Semiconductors from Density-Functional Perturbation Theory.

The anharmonic lifetimes of zone-center optical phonons in C, Si, and Ge are calculated along with their temperature and pressure dependences, using third-order density-functional perturbation theory. Our basic ingredients are by-products of a standard linear-response calculation of phonon dispersions in the harmonic approximation, resulting in a similarly good agreement with experiments. The microscopic mechanisms responsible for the decay are revealed and shown to be different for different materials and to depend sensitively on the applied pressure.

Sensitivity and transient response of microwave reflection measurements

Microwave reflection measurements are widely used for the characterization of minority-carrier lifetimes in semiconductors. A theoretical description of the technique is presented. The approach is based on a dielectric multilayer model that accounts for experimental parameters such as microwave frequency, sample thickness and doping, and the distance to an optional reflector behind the sample. With a new definition of the sensitivity in transient microwave reflection measurements, the most sensitive configuration is investigated for a given semiconductor thickness and conductivity. Good agreement between the theoretical simulation and measurements is demonstrated. The model is also used for calculating microwave reflection transients from the excess carrier decay after pulsed laser excitation. It is found that the reflected microwave power mirrors the carrier decay if three criteria are fulfilled: The carrier generation must be homogeneous; low-injection conditions are required; and the reflector must be positioned appropriately for linear dependence of the microwave reflection on the carrier density.

Direct observation of a scaling effect on effective minority carrier lifetimes

The effective minority carrier lifetime in semiconductors with recombination inhomogeneities depends on the spatial distribution of recombination sites and not only on their absolute number. We use time-resolved microwave reflection measurements to monitor the carrier recombination in silicon wafers with a periodic metallization pattern on one surface. This contact scheme simulates the distribution of sites of enhanced carrier recombination. The experiments reveal a sensitive dependence of the effective minority carrier lifetime on the interdistance and the size—i.e., the scaling—of the metallization pattern at fixed metallization area ratio. This so-called scaling effect occurs whenever the size and the interdistance of recombination sites are comparable to the minority carrier diffusion length. Our experiments are in good agreement with results from a new analytical three-dimensional simulation which is based on the solution of the electronic transport equations in Fourier space. Our model is applicable to the optimization of the backside ohmic contact pattern for high efficiency solar cells.

Theoretical calculation of longitudinal-optical-phonon lifetime in GaAs

The anharmonic decay of longitudinal-optical (LO) phonons in zinc-blende semiconductors has been studied. Based on an approach in which the anharmonic crystal potential is estimated using the theory of elasticity, the lifetime of LO phonons via emission of two acoustic phonons is calculated as a function of lattice temperature and phonon wave vector. Application of this model to bulk GaAs shows an excellent agreement with available experimental data. Since the parameters employed in the model can be obtained experimentally, the approach provides a useful tool to investigate LO-phonon lifetimes in semiconductors.

Excite-probe determination of the intersubband lifetime in wide GaAs/AlGaAs quantum wells using a far-infrared free-electron laser

A direct excite-probe semiconductor lifetime determination in the picosecond regime has been made for the first time in the far infrared. We have used an RF-linac-pumped free-electron laser to determine the relaxation rate associated with intersubband absorption in GaAs/AlGaAs quantum wells having a subband separation smaller than the optical phonon energy. The measurement yields a relaxation lifetime of 40+or-5 ps. This is compared with a variety of other results obtained with less direct techniques.

Carrier lifetime in amorphous semiconductors

In amorphous semiconductors, because of the band-tail and -gap states, the excess carrier lifetime becomes sensitive to many experimental details. An attempt is made to clarify the relationship between carrier lifetime, density of states, and measurement details. The results show that in a steady-state photoconductivity measurement, the loss of carriers by recombination is determined by the density of deep states and the position of the quasi-Fermi level. In a transient measurement, the limited observation time and the contact also play important roles, besides the density of states. The shallow states in the energy band control the drift mobility and thus affect the lifetime indirectly. In an extreme case, the lifetime can become longer as the defect density increases. These points are illustrated with the electron lifetime derived from photoconductivity, time of flight, and delayed-field charge collection for hydrogenated amorphous silicon and silicon-germanium alloys.

Minority carrier accumulation at high-low junctions

The semiconductor transport equations have been solved for lifetime semiconductors under the assumption of quasineutrality which is calculated to be valid up to relatively high current densities. The effective diffusion length of minority carriers is shown to be (strongly) current dependent. Of the four nonequilibrium processes, minority carrier accumulation is treated analytically in greater detail, showing current dependent resistance decreases which are confirmed by both numerical solutions of the transport equations and experiments.

Photothermal rate-window spectrometry for noncontact bulk lifetime measurements in semiconductors

A new noncontact technique for the determination of excess carrier lifetimes in semiconductors is presented. The technique employs a square laser pulse (hν≥Eg) and measures the infrared photothermal radiometric response of the sample. By applying the photothermal rate-window concept, the excess photoexcited carrier bulk lifetime was measured with optimal signal-to-noise (S/N) ratio and simple, unambiguous interpretation from the maximum position of the rate-window signal. The technique has been applied to Au-, Fe-, and Cr-doped Czochralski silicon crystals. The experimental results from boxcar and lock-in rate-window methods were found to agree very well. The results are further mostly in agreement with those from the noncontact laser/microwave detection method.

Photorefractive method of contactless determination of the charge carrier lifetime and diffusion coefficient in semiconductors

A method of contactless characterization of semiconductors is developed which allows the charge carrier lifetime and diffusion coefficient to be determined locally with three-dimensional spatial resolution. The method is based on refraction of the probe beam that intersects the pump beam in the sample volume. In contrast to the conventional mirage-effect technique, the method discussed here uses the deeply penetrating and focused pump beam which allows the surface effect to be substantially weakened while determining the bulk characteristics. The method has been applied to Si wafers, showing a good quantitative agreement with data obtained independently by the conventional photoconductivity decay technique.