Carrier lifetime measurements

G. J. Norga, M. Platero, K. A. Black, A. J. Raddy, J. Michel, and L. C. Kimerling, Detection of metallic contaminants on silicon by surface sensitive minority carrier lifetime measurements, J. Electrochem. Soc. 146, 2602, 1998. 

Under Httle or no illumination,/ must be minimized for optimum performance. The factor B is 1.0 for pure diffusion current and approaches 2.0 as depletion and surface-mode currents become important. Generally, high crystal quality for long minority carrier lifetime and low surface-state density reduce the dark current density which is the sum of the diffusion, depletion, tunneling, and surface currents. The ZM product is typically measured at zero bias and is expressed as RM. The ideal photodiode noise current can be expressed as follows ... 

Contamination of silicon wafers by heavy metals is a major cause of low yields in the manufacture of electronic devices. Concentrations in the order of 1011 cm-3 [Ha2] are sufficient to affect the device performance, because impurity atoms constitute recombination centers for minority carriers and thereby reduce their lifetime [Scl7]. In addition, precipitates caused by contaminants may affect gate oxide quality. Note that a contamination of 1011 cnT3 corresponds to a pinhead of iron (1 mm3) dissolved in a swimming pool of silicon (850 m3). Such minute contamination levels are far below the detection limit of the standard analytical techniques used in chemistry. The best way to detect such traces of contaminants is to measure the induced change in electronic properties itself, such as the oxide defect density or the minority carrier lifetime, respectively diffusion length. 

Parkinson P, Joyce HJ, Gao Q, Tan HH, Zhang X, Zon J, Jagadish C, Heiz LM, Johnston MB (2009) Carrier lifetime and mobility enhancement in nearly defect-free core-shell nanowires measured using time-resolved terahertz spectroscopy. Nano Lett 9 3349... 

Other indirect methods for measuring lifetimes often involve device structures such as p-n junctions. The electron-beam-induced current (EBIC) technique, for example, measures the increase injunction current as an impinging electron beam moves close to the junction, i.e., within a few minority-carrier diffusion lengths. If a diffusion constant can be estimated, say by knowledge of the minority-carrier mobility, then the minority-carrier lifetime can be calculated. However, SI GaAs does not form good junctions, so such methods are really not applicable. 

The rate constants for electron transfer and recombination are readily separated because in the limit (w- 0), equation (8.31) tends to kir/(ktr + krec), and the maximum of the semicircle occurs when ca = 2ir f=kt + krec. In the absence of RC attenuation effects, the high frequency intercept of the IMPS plot (minority carriers. Measurements of gac as a function of potential (band bending) can be used to determine the minority carrier lifetime and absorption coefficient [46]. The main advantage of using the IMPS data rather than dc measurements of... 

Upon exposure to various alkanols and alkyl amines (not exceeding 4 carbons) carrier lifetimes increased up to about 50%, whereas the conductivities increased by 1%. Total conductivity was measured—i.e., bulk plus surface—and, therefore, the magnitude of the change does not reflect the true sensitivity of surface conductivity to the reaction. 

Wagner compared structural and electrical properties of polycrystalline Si layers grown by CVD or LPE (In melt, 947°C, 0.12 pm min-1) with similar grain boundary structures [20]. The measured minority carrier lifetime was always higher and the recombination strength of the defects was smaller in the LPE layers than in the CVD layers. They attributed this to the higher purity of the LPE layer and its lower density of defects (rod-like defects). 

The time-resolved microwave reflectivity (TRMR) techiuque is well established for contactless characterisation of minority-carrier lifetimes in semiconductors. It can be applied to map surface recombination in this case the sample is moved by an X-Y stage to allow spatially resolved measurement of the minority-carrier lifetime. 

Time-resolved photoinduced microwave conductivity measurements can be made as a function of applied potential. It has been shown that the measured minority-carrier lifetime r for moderately fast or slow interfacial charge transfer depends not only on the interfacial rate constant and surface recombination Ukc, but also on the energy band bending (AE) and the Debye length Ld (Tributsch, 1999). 

The actual compensation in a material is more complex than a simple balance between a majority impurity and a minority impurity as the material usually contains a combination of residual impurities, dopant and deep centres, whose concentrations must be estimated to determine the actual degree of compensation in the material. As mentioned before, compensation of the majority impurities by adding opposite type dopant leaves in the material charged ions, which reduce the lifetime of the free carriers. When the lifetime of the carriers in a given pure material is known, a lifetime measurement of an unknown sample of this material can determine the degree of compensation of the sample. 

Optical minority carrier lifetime was also measured in the samples grown at 30 pm h-1. The measurements were performed by F. Yan and W.J. Choyke (University of Pittsburgh). For this, a pulsed NdYAG laser was employed as the excitation source. The values around 1 ps were obtained for both the epilayers on porous and the epilayers on standard surfaces indicating that growing on porous did not change the concentration of nonradiative recombination centers, and the intensity enhancement observed in photoluminescence is due to other reasons. 

What is the advantage of the cross-correlation method in fluorescence lifetime measurements Fluorescence lifetimes are in the order of the ns or / and ps. Thus, high frequencies from the MHz to the GHz are needed to perform fluorescence lifetimes measurements. Still, the accuracy of the measured values is not reached at high frequencies. Therefore, one can translate the high frequency and phase modulation information to lower frequency carrier signal, Aw. In this case, the measured fluorescence lifetimes are highly accurate. 

This characteristic of the material can cause complications in the vicinity of blocking contacts where the conductivity type can be inverted due to band-bending effects. Thus the carrier lifetime is likely to be anomalous near blocking contacts, an effect that is particularly significant in low-quality material and in the samples in which the depletion region is a large fraction of the total thickness. This effect may be responsible for some of the difficulties that have been reported with electron drift mobility measurements on thin (s l- zm) samples (Datta and Silver, 1981). 

The extent to which the carrier mobilities depend on preparation conditions is not well understood. One reason for this is that, unlike optical measurements, for example, the time-of-flight measiu ments can only be made on material for which the carrier lifetimes against deep trapping are sufficiently long for the charges to traverse the sample with a reasonable field, as pointed out earlier. This feature of the time-of-ffight experiment has been exploited by Street (1982) and Steemers et al. (1983), who have used the technique to explore the electronic properties of defects and the influence of preparation conditions on defect density. 

---