Minority Carrier Trapping Research Articles

Analysis of generation and annihilation of deep level defects in a silicon-irradiated bipolar junction transistor

A commercial bipolar junction transistor (2 N 2219 A, npn), irradiated with 120 MeV Si9+ ions with a fluence of the order of 1012 ions cm−2, is studied for radiation-induced gain degradation and deep level defects. I–V measurements are made to study the gain degradation as a function of ion fluence. Properties such as activation energy, trap concentration and capture cross section of deep levels are studied by deep level transient spectroscopy (DLTS). Minority carrier trap energy levels with energies ranging from EC − 0.160 eV to EC − 0.581 eV are observed in the base-collector junction of the transistor. Majority carrier trap levels are also observed with energies ranging from EV + 0.182 eV to EV + 0.401 eV. The identification of the defect type is made on the basis of its finger prints such as activation energy, annealing temperature and capture cross section by comparing with those reported in the literature. New energy levels for the defects A-center, di-vacancy and Si-interstitial are also observed. The irradiated transistor is subjected to isothermal and isochronal annealing. The defects are seen to anneal above 250 °C. The defects generated in the base region of the transistor by displacement damage appear to be responsible for transistor gain degradation.

Defect imaging in multicrystalline silicon using a lock-in infrared camera technique

We image the lifetime distribution of multicrystalline silicon wafers by means of calibrated measurements of the free-carrier emission using an infrared camera. The spatially resolved lifetime measurements are performed as a function of the light-generated excess carrier density, showing a pronounced increase in lifetime with decreasing injection density at very low injection levels. Two theoretical models are applied to describe the abnormal lifetime increase: (i) minority-carrier trapping and (ii) depletion region modulation around charged bulk defects. The trapping model is found to give better agreement with the experimental data. By fitting the trapping model to each point of the lifetime image recorded at different injection levels, we generate a trap density mapping. On multicrystalline silicon wafers we find a clear correlation between trap and dislocation density mappings.

DLTS study of deep level defects in Li-ion irradiated bipolar junction transistor

Commercial npn transistor (2N 2219A) irradiated with 50MeV Li3+-ions with fluences ranging from 3.1×1013ionscm−2 to 12.5×1013ionscm−2, is studied for radiation induced gain degradation and minority carrier trap levels or recombination centers. The properties such as activation energy, trap concentration and capture cross section of induced deep levels are studied by deep level transient spectroscopy (DLTS) technique. Minority carrier trap levels with energies ranging from 0.237eV to 0.591eV were observed in the base–collector junction of the transistor. In situ I–V measurements were made to study the gain degradation as a function of ion fluence. Ion induced energy levels result in increase in the base current through Shockley Read Hall (SRH) or multi-phonon recombination and subsequent transistor gain degradation.

Photogenerated minority carrier trapping and inversion layer formation in polymer field-effect structures

We describe the results of a photocapacitance study of Metal-Insulator-Semiconductor (MIS) capacitors formed from semiconducting polymers. In this technique, capacitance-voltage (C-V) plots of the devices are obtained prior, during and after illumination with light of energy greater than the optical band gap of the semiconductor. When the capacitors are biased into depletion and simultaneously exposed to light, optically-generated minority electrons drift to and become trapped in states at the semiconductor-insulator interface, resulting in a significant shift of the C-V plot to more positive voltages. For devices employing a polysilsesquioxane insulator clear and unambiguous evidence is obtained for the formation of an inversion layer at the interface. Upon terminating the illumination, the devices relax back to the initially-obtained dark C-V plots as the trapped electrons are thermally excited from their trapping states. By tracking the voltage required to maintain a constant capacitance the dynamics of charge detrapping can be followed leading to an estimate for the energy of the interface state

Photogenerated minority carrier trapping and inversion layer formation in polymer field-effect structures

Photogenerated minority carrier trapping and inversion layer formation in polymer field-effect structures

Minority-carrier trapping in Ga-doped multicrystalline Si wafers

Abstract B- and Ga-doped multicrystalline-silicon (mc-Si) wafers with different resistivity and different positions of grown ingots have been used to evaluate the stability, quality improvement and thermal annealing effects on carrier lifetimes. The effective carrier lifetimes are improved and high lifetimes as high as 200 μs are realized after P-diffusion and SiN x coating. Ga-doped wafers show a certain stability after thermal annealing up to 250 °C which insures the possibilities of eliminating the light-induced degradation effects generated in p-type mc-Si wafers.

Photoluminescence imaging of silicon wafers

Photoluminescence imaging is demonstrated to be an extremely fast spatially resolved characterization technique for large silicon wafers. The spatial variation of the effective minority carrier lifetime is measured without being affected by minority carrier trapping or by excess carriers in space charge regions, effects that lead to experimental artifacts in other techniques. Photoluminescence imaging is contactless and can therefore be used for process monitoring before and after individual processing stages, for example, in photovoltaics research. Photoluminescence imaging is also demonstrated to be fast enough to be used as an in-line tool for spatially resolved characterization in an industrial environment.

Relationship between trapping density and open circuit voltage in multicrystalline silicon solar cells

Minority carrier trapping frequently exists in solar grade multicrystalline silicon. At low illumination levels, the effect of trapping centers on open circuit voltage of multicrystalline silicon solar cells is dependent on the trap density and illumination level. In this paper, the relation between trapping density and open circuit voltage of multicrystalline silicon solar cells at different illumination levels is studied by a series of experiments. The experimental evidence suggests that the effect of trapping on open circuit voltage of multicrystalline silicon solar cells is obvious at carrier injection levels equal to and below the trap density, the trapping effect of multicrystalline silicon can be reflected by measuring open circuit voltage at low illumination levels, instead of complicated lifetime measurements, and some multicrystalline silicon solar cells with higher trap densities have higher open-circuit voltages at weak illumination levels. The measurement and analysis of the trapping effect is a relative tool to diagnose the quality of multicrystalline silicon, so a new method is presented to analyze relative quality of multicrystalline silicon by measuring open circuit voltage at weak illumination levels.

Trapping artifacts in quasi-steady-state photoluminescence and photoconductance lifetime measurements on silicon wafers

Photoluminescence and photoconductance lifetime measurements on multicrystalline silicon wafers are presented. It is demonstrated experimentally that the large overestimation of the lifetime at low carrier concentrations due to trapping that is observed in photoconductance measurements is not found in photoluminescence data. This is explained theoretically by the dependence of photoluminescence and photoconductance on the product and the sum, respectively, of the minority and majority carrier densities. Based on this analysis, it is shown that photoluminescence lifetime measurements are not significantly affected by minority carrier trapping in most practical cases while implied current-voltage curves obtained from photoluminescence are completely unaffected.

Mapping of trap densities and energy levels in semiconductors using a lock-in infrared camera technique

We examine nonrecombination active minority-carrier trapping centers in crystalline silicon using a lock-in infrared camera technique. Application of a simple trapping model to the injection-dependent lifetime data obtained from the infrared emission signal results in high-resolution mappings (spatial resolution=170μm) of the trap density and the energy level. Measurements on Czochralski-grown silicon wafers show striation-related inhomogeneities of the trap density and a very homogeneous distribution of energy levels.

Observed trapping of minority-carrier electrons in p-type GaAsN during deep-level transient spectroscopy measurement

Deep-level transient spectroscopy measurements on a reverse-biased p-type GaAsN Schottky diode grown by metalorganic chemical vapor deposition show a minority-carrier trap signal corresponding to an electron trap having an activation energy of about 0.2eV. The proportion of trapped electrons agrees with that of modeled defect states near the Schottky-barrier metal interface whose occupation is affected by changing reverse-bias conditions. Estimates of thermionic emission provide adequate filling of the traps with electrons from the metal. The inclusion of a GaAs layer between the metal and GaAsN layer reduces the electron-trapping signal.

Characterization of a Dominant Electron Trap in GaNAs Using Deep-Level Transient Spectroscopy

ABSTRACTDilute-nitrogen GaNAs epitaxial layers grown by metal-organic chemical vapor deposition were characterized by deep-level transient spectroscopy (DLTS). For all samples, the dominant DLTS signal corresponds to an electron trap having an activation energy of about 0.25 to 0.35 eV. The minority-carrier trap density in the p-type material is quantified based on computer simulation of the devices. The simulations show that only about 2% of the traps in the depleted layer are filled during the transient. The fraction of the traps that are filled depends strongly on the depth of the trap, but only weakly on the doping of the layers and on the conduction-band offset. The simulations provide a pathway to obtain semi-quantitative data for analysis of minority-carrier traps by DLTS.

Laplace-DLTS analysis of the minority carrier traps in the Cu(In,Ga)Se 2-based solar cells

In this paper, we summarize our investigations of the minority carrier traps in Cu(In,Ga)Se 2 devices by use of the Laplace-Deep Level Transient Spectroscopy (Laplace-DLTS) method. We explain the nonlinearities of the Arrhenius plots in terms of the thermally assisted tunneling (TAT) effect and indicate that the standard analysis by use of admittance spectroscopy (AS) may lead to significant errors in trap parameters. We show that the position of the peak in the DLTS and admittance spectra depends on activation energy as well as on electric field. The values of the activation energies after correction for the electric field agree very well with the transition energies predicted by theoretical calculations for In Cu ++, In Cu +, (In Cu+V Cu) and V Se ++ defects. Thus, we conclude that the minority carrier signal is not only due to a continuous distribution of traps at the CdS/Cu(In,Ga)Se 2 interface but also to the close-to-interface bulk donor states. Basing on these findings and using the concept of the defect layer, we propose a model which explains the metastabilities in the minority carrier trap spectra induced by illumination and reverse bias.

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.

Defect Evolution in Annealed p-type GaAsN Epilayers Grown by Metalorganic Chemical Vapour Deposition

We have studied the evolution of electrically active defects in rapid thermally annealed p-type GaAsN epitaxial layers using deep level transient spectroscopy (DLTS). A continuous distribution of hole traps and an overlapping minority carrier trap are observed in the layers grown by metalorganic chemical vapour deposition. Rapid thermal annealing (RTA) in the temperature range 600–900°C for 30 s created six hole traps HA1 (EV+0.22 eV), HA2 (EV+0.32 eV), HA3 (EV+0.38 eV), HA4 (EV+0.39 eV), HA5 (EV+0.55 eV), and HA6 (EV+0.78 eV). Most of these defects are stable at 900°C, although their relative concentrations varied over the RTA temperature in this study. We discuss the origin of these hole traps based on previously reported hole traps in the literature. The increase in doping concentration in the annealed samples is also discussed.

Energy level of minority carrier trap centers induced by light illumination in B-doped Cz-Si solar cells

The conversion efficiency of boron (B)-doped Czochralski silicon (Cz-Si) solar cells decreased by light illumination or minority carrier injection. Defects are induced by illumination and they act as trap centers, shorten the minority carrier (electron) lifetime. The energy level of this minority carrier trap center was determined by analyzing the open-circuit voltage ( V OC) changes as a function of substrate temperature. When substrate temperature is low, all electrons which are captured by the trap centers recombine with holes and they do not contribute to the generation of electric power. However, as the substrate temperature is increasing, some of the captured electrons are thermally excited to the conduction band before recombination. Hence, the lifetime of minority carriers are improved and V OC is recovered. Based on this result, the energy level of trap center induced by light illumination is estimated to be 0.26 eV, which corresponds to the boron–oxygen-related defect ( E C-0.26 eV).

Multiple-level defect species evaluation from average carrier decay

An expression for the average decay is determined by solving the the carrier continuity equations, which include terms for multiple defect recombination. This expression is the decay measured by techniques such as the contactless photoconductance decay method, which determines the average or volume integrated decay. Implicit in the above is the requirement for good surface passivation such that only bulk properties are observed. A proposed experimental configuration is given to achieve the intended goal of an assessment of the type of defect in an n-type Czochralski-grown silicon semiconductor with an unusually high relative lifetime. The high lifetime is explained in terms of a ground excited state multiple-level defect system. Also, minority carrier trapping is investigated.

New Junction Capacitance Methods for the Study of Defect Distributions and Carrier Properties in the Copper Indium Diselenide Alloys

AbstractWe have recently been successful utilizing two methods which are new to the study of CIGS thin film samples: drive-level capacitance profiling, and transient photocapacitance spectroscopy. In this paper we review several of the key results that we have obtained by applying these methods to the study of the CIGS alloys over the past 2 years. This has resulted not only in new information concerning the deep defects and their spatial distributions in these materials, but also to more accurate determinations of free carrier densities, and of minority carrier trapping dynamics within the junction region. Light-induced metastable changes in the deep defect properties of these alloys are also documented through the use of these techniques.

Oxygen-related minority-carrier trapping centers in p-type Czochralski silicon

We investigate minority-carrier trapping centers in p-type Czochralski (Cz) silicon by means of the quasi-steady-state photoconductance method. Boron and gallium-doped Cz silicon wafers of varying resistivities and oxygen contamination levels are examined. A clear correlation of the trap density with the interstitial oxygen concentration as well as with the boron and the gallium concentration is detected. The experimental data suggest that oxygen is a direct component of the defect complex responsible for the trapping, while the role of boron and gallium is not fully resolved. Using a simple single-level trapping model, we determine the energy level and the trapping/detrapping time constant ratio of the oxygen-related trapping center.

Weak light effect in multicrystalline silicon solar cells

Minority carrier trapping centers frequently exist in solar grade multicrystalline silicon, such trapping centers cause a drastic increase in photoconductance at carrier injection levels equal to and below the trap density, this phenomenon leads to higher open circuit voltage for multicrystalline silicon solar cells at illumination levels below about 0.2 suns compared to high performance crystalline silicon solar cells. In this paper, the open circuit voltage of multicrystalline silicon solar cells are investigated at low illumination levels, the experiments prove that some multicrystalline silicon solar cells which have higher trap density have higher open circuit voltage at weak illumination levels, and have lower efficiency, so a new method is presented to analyze quality of multicrystalline silicon by measuring open circuit voltage at weak illumination levels in-line, this makes cells manufacturers gain insight into the quality of multicrystalline silicon wafer from different multicrystalline silicon manufacturers easily with the same cell process before screenprinting and firing.