Recombination centers identification in very thin silicon epitaxiallayers via lifetime measurements

A new test structure for the recombination lifetime profile measurement has been designed and applied, for the first time, to characterize very thin (4 μm) silicon epitaxial layers. The results of our analysis have shown how the lifetime behavior, at room temperature, is clearly position dependent its value being influenced by different recombination centers. Moreover, two distinct recombination centers have been identified, the first one related to the dopant (arsenic in our case) and the second one induced by the process steps required to realize the test structure itself     

Recombination lifetime in oxygen-precipitated silicon

The recombination lifetime degrades when interstitial oxygen precipitates in Czochralski-grown silicon. We have observed a more severe degradation in p-type than in n-type material. Based on recombination lifetime, deep-level transient spectroscopy, Fourier-transform infra-red transmission, and transmission electron microscopy measurements, we attribute the degradation mainly to interface states at the precipitate-silicon interface acting as recombination centers. Positive charge in the oxygen precipitates (OP's), causing an electron-attractive space-charge region (scr) around the precipitate in p-Si and a hole-repulsive accumulation layer in n-Si, is proposed to explain the lifetime differences between n-type and p-type silicon.     

Innovative localized lifetime control in high-speed IGBTs

An innovative method to control carrier lifetime locally and efficiently in Insulated Gate Bipolar Transistors (IGBTs) is presented. It is based on the formation of void layers by low-energy and high-dose He implants and annealing. Voids introduce two well-defined midgap trap levels in silicon. HFIELDS simulations demonstrate the increase of surface hole concentration when a well localized recombination region is introduced in the buffer layer. High-speed IGBTs were fabricated both with voids in the buffer layer or with unlocalized recombination centres. Devices with localized bandgap centres show a lower on-resistance with a fast turn-off behavior     

Charge-carrier lifetime in Hg1−x CdxTe (x=0.22) structures grown by molecular-beam epitaxy

Measurements of the charge carrier lifetime in epitaxial structures based on narrow-gap Hg_1−x Cd_xTe (x=0.22), grown by molecular-beam epitaxy with pulsed excitation using radiation at different wavelengths, are reported. It is shown that in p-type epitaxial films the lifetime is determined by the Auger recombination mechanism at temperatures corresponding to the impurity conductivity, and for n-type epitaxial films recombination via local centers is characteristic. Fiz. Tekh. Poluprovodn. 31, 774–776 (July 1997)     

Characteristics of TIL GTO thyristors using no lifetime killers

The investigation reported in this work was focused on the main characteristics of the recently developed two interdigitation level (TIL) gate turn-off (GTO) thyristors, which use neither lifetime killers nor anode shorts. The advantages of these TIL GTO's with low on-state losses are outlined in comparison with their identical, yet gold-doped, counterparts. It is shown that, except for the turn-off time, the main electrical characteristics of TIL GTO's using no induced recombination centers are superior to those of similar gold-doped devices. The current-handling capability of the former under tough electrothermal ratings is also better than that of gold-doped devices up to a commutation frequency of 5 kHz. The results of this work demonstrated that sought-for benefits could be obtained in TIL GTO's which use neither induced recombination centers nor anode shorts.     

Determination of the minority carrier lifetime in crystalline silicon thin‐film material

The effective minority carrier lifetimes on epitaxial silicon thin‐film material have been measured successfully using two independent microwave‐detected photoconductivity decay setups. Both measurement setups are found to be equally suited to determine the minority carrier lifetime of crystalline silicon thin‐film (cSiTF) material. The different measurement conditions to which the sample under investigation is exposed are critically analyzed by both simulations and measurements on a large number of lifetime samples. No systematic deviation between the lifetime results from different measurement setups could be observed, underlining the accuracy of the determined lifetime value. Subsequently, a method to separate the epitaxial bulk lifetime and the total recombination velocity, consisting of front surface and interface recombination between the epitaxial layer and the substrate, is presented. The method, based on different thicknesses of the epitaxial layer, is applied to all batches of this investigation. Each batch consists of samples with the same material quality but different epitaxial layer thicknesses whereas different batches differ in their material quality. In addition, the same method is also successfully applied on individual cSiTF samples. From the results, it can be concluded that the limiting factor of the effective minority carrier lifetime for the investigated solar‐grade cSiTF material is the elevated recombination velocity at the interface between epitaxial layer and the substrate compared with microelectronic‐grade material. In contrast, the samples cannot be classified into different material qualities by their epitaxial bulk lifetimes. Even on multicrystalline substrate, solar‐grade material can exhibit high epitaxial bulk lifetimes comparable to microelectronic‐grade material. Copyright © 2012 John Wiley & Sons, Ltd.     

Application of the equivalent circuit model for semiconductors to the study of Au-doped p-n junctions under forward bias

The Green-Shewchun method of solution is applied to the transmission line circuit model of Sah to obtain the forward current-, capacitance-, and conductance-voltage characteristics of semiconductor p-n junctions. Numerical solutions are obtained for diffused dopant impurity profiles and several position dependent concentration profiles of gold recombination centers to illustrate the variation of the reciprocal slope parameter m in the dc current, exp (qV/mkT). A new behavior of m = 2 is observed for many decades of current in the low-level range when the recombination centers are concentrated at the edge of the space-charge layer as expected from ion implantation. The theoretical calculations are compared with experimental forward current-, conductance-, and capacitance-voltage data from 10 to 10⁶Hz and 77 to 300 K. Excellent agreements are obtained without adjustable parameters for boron and gold diffused p⁺-n silicon diodes from low to high injection levels. A twenty-five fold increase of the steady-state hole lifetime from low to high injection level is both observed and predicted. Agreements are also obtained for phosphorus- and gold-diffused n⁺-p silicon diodes from low to intermediate injection levels where the steady-state electron lifetime is nearly constant and controlled by electron capture into the positively charged gold donor centers.     

Interface-recombination-controlled minority-carrier lifetime in n-type gap

The effects of interface recombination on measured minority-carrier lifetime in n-type liquid-phase epitaxial GaP layers have been studied quantitatively. Results are presented for a number of different interface conditions, and analysis shows that the interface recombination velocity is high in all cases. This results in a unique dependence of interface-recombination-controlled lifetime ?s, on layer thickness t for all values of t > ~0.5 ?m. In most layers, the value of ?s is low enough to influence the measured lifetime. A value for the minority hole diffusion coefficient, Dh = 4.0±0.3 cm2 s?1, is also deduced from the ?s data.     

Measurement of recombination lifetime profiles in epilayers using a conductivity modulation technique

A new experimental techique is described to accurately measure the recombination lifetime profile in lightly doped epitaxial layers of thickness less than the minority-carrier diffusion length. This technique requires the use of a particular "test" structure composed of a lateral p⁺-n-n⁺diode on the surface of the epilayer and a control electrode on the substrate layer. Using a conductivity modulation technique, the proposed measurement method is independent of recombination effects in the highly doped regions needed for every test structure. Moreover, the evaluation of lifetime profiles along the epilayer is made possible by varying the width of the conductivity modulated region through the control electrode bias. The measurement theory is developed for the high-injection regime, usually applicable to the lightly doped layers of bipolar power devices.     

Silicon epitaxial layer recombination and generation lifetime characterization

We have made recombination and generation lifetime measurements on silicon p-epitaxial layers on p/sup +/ and on p-substrates. The recombination lifetimes are dominated by surface/interface recombination for layers only a few microns thick. By coupling measurements of p/p with those of p/p/sup +/ samples, it is possible to extract the epi-layer lifetime. For p/p/sup +/ samples, recombination lifetimes are poorly suited to characterize epi-layers. Generation lifetime measurements are eminently suitable for epi-layer characterization, since carrier generation occurs in the space-charge region confined to the epitaxial layer, and when coupled with corona charge/Kelvin probe, allow contact-less measurements.     

Mapping charge carrier density in organic thin-film transistors by time-resolved photoluminescence lifetime studies

The device performance of organic transistors is strongly influenced by the charge carrier distribution. A range of factors effect this distribution, including injection barriers at the metal-semiconductor interface, the morphology of the organic film, and charge traps at the dielectric/organic interface or at grain boundaries. In our comprehensive experimental and analytical work we demonstrate a method to characterize the charge carrier density in organic thin-film transistors using time-resolved photoluminescence spectroscopy. We developed a numerical model that describes the electrical and optical responses consistently. We determined the densities of free and trapped holes at the interface between the organic layer and the SiO₂ gate dielectric by comparison to electrical measurements. Furthermore by applying fluorescence lifetime imaging microscopy we determine the local charge carrier distribution between source and drain electrodes of the transistor for different biasing conditions. We observe the expected hole density gradient from source to drain electrode.     

Electron irradiation induced recombination centers in silicon-minority carrier lifetime control

Recombination centers introduced in silicon p⁺-n-n⁺structures by irradiation with 2-MeV electrons are studied by measuring minority carrier lifetime and annealing kinetics. The approximate location of these recombination centers in the forbidden gap and their densities are obtained by the thermally stimulated current method. The results identify one defect as a divacancy with an energy level of Ev+ 0.26 eV. Possible identities of other deep levels are discussed. The technique of minority carrier lifetime control by electron irradiation has been developed into a reliable manufacturing process for power devices.     

On the injection level dependence of the minority carrier lifetime in defected silicon substrates

In the literature several reports have indicated a strong increase of the minority carrier lifetime with injection level in defected single crystal and semi-crystalline silicon. These types of potentially low-cost materials are being developed for the photovoltaic industry. The lifetime increase, mainly observed at low injection levels, has usually been explained by the saturation of active traps with increasing injection level. However, a detailed experimental analysis of the recombination losses in materials showing this behavior, points to a strong local variation in the trap density, presumably associated with crystal microdefects. We demonstrate in this paper that the non-linearity of the recombination current at low illuminations observed in defected materials, can be explained by considering the recombination at grain boundaries and microdefects. The conditions are detailed under which the lifetime increases with injection level.     

Quadratic recombination in silicon and its influence on the bulk lifetime

The coefficient of nonradiative excitonic recombination by the Auger mechanism involving deep-level centers in n-Si was determined by comparing the theoretical dependence of the effective bulk lifetime on the doping level with the experimental dependence. It is shown that this mechanism controls the bulk lifetime in silicon at doping levels on the order of or above 10¹⁶ cm^?3. This mechanism is more pronounced at shorter bulk lifetimes τ _v0 and low doping levels. The dependences of the bipolar diffusion length in n-Si on the doping level (using the parameter τ _v0) were calculated.     

Recombination level selection criteria for lifetime reduction in integrated circuits

In the past, lifetime control in integrated circuits has been done on an empirical basis. This paper introduces selection criteria for recombination centers which are to be used for reducing minority carrier lifetime in integrated circuits. It is shown that the recombination level should have a large lifetime ratio (τ_SC/τ_LL) in order to obtain minority carrier lifetime reduction with minimal increase in the leakage current, and should possess large capture cross section values in order to minimize compensation effects. Using these criteria, preferred locations for the recombination center have been defined for both p and n type silicon, and the trade-off between reduction of lifetime and increase in leakage current has been shown to degrade with increase in resistivity and ambient temperature. These criteria have also allowed a quantitative comparison between various lifetime control techniques for the first time, and platinum doping has been identified as the most favorable lifetime control process at the present time.     

A measurement technique to obtain the recombination lifetime profile in epi layers at any injection level

The measurement technique proposed in [1] to obtain a high injection recombination lifetime profile is extended to any injection level in order to obtain both the majority- and minority-carrier lifetimes as well as its profile along the epi layer. The technique is simulated by numerically solving the relevant equations and the extraction of a varying lifetime profile is demonstrated both at low- and high-injection levels. Some experimental results on n-n⁺epi layers of different doping and thicknesses will be reported to demonstrate the possibilities of this measurement technique, and the extraction of both minority- and majority-carrier lifetime is obtained from the measurements of lifetime as a function of the injection level.     

Collector recombination lifetime from the quasi-saturation analysisof high-voltage bipolar transistors

A novel method of measuring the collector recombination lifetime, which is independent of emitter effects, is presented by extending the quasi-saturation analysis of high-voltage bipolar transistors to the high-current-density regime. The technique is supported by theory, and experimental results are presented on transistors fabricated with different emitter properties. This is a nondestructive method and gives the lifetime values at the current densities normally encountered when the transistor is in actual operation. The values for the collector recombination lifetime obtained by the present method are independent of the properties of the emitter region     

Possibilities and limits of axial lifetime control by radiation induced centers in fast recovery diodes

Device simulation, based on an extended recombination model, is used as a design tool for lifetime-controlled power diodes with different lifetime profiles. Homogenous and local recombination center profiles are considered. The sensitivity of important device properties, such as the trade-off between stationary and dynamical characteristics, to the recombination center peak position is investigated. The occurrence of dynamic impatt oscillations is analyzed.     

Determination of recombination lifetime in MOS structures by a sine voltage-sweep technique

A sine-voltage technique for measurements of recombination lifetime in metal oxide semiconductor (MOS) structures is proposed. When a fast sine-voltage sweep ramp is applied to the gate of an MOS capacitor a non-equilibrium depletion layer is formed and electron–hole generation starts in the space–charge–region and in the bulk. If the measurements are performed at elevated temperature so that quasi-neutral region generation rather than space charge region generation dominates, then the diffusion length, consequently the recombination lifetime, can be determined.     

Determination of minority-carrier lifetime and surface recombination velocity with high spacial resolution

Quantitative analysis of the electron beam induced current in conjunction with high-resolution scanning makes it possible to evaluate the minority-carrier lifetime three dimensionally in the bulk and the surface recombination velocity two dimensionally, with a high spacial resolution. The analysis is based on the concept of the effective excitation strength of the carriers which takes into consideration all possible recombination sources. Two-dimensional mapping of the surface recombination velocity of phosphorus-diffused silicon diodes is presented as well as a three-dimensional mapping of the changes in the minority-carrier lifetime in ion-implanted silicon.