Recombination carrier lifetime

Radiative recombination of minority carriers is tlie most likely process in direct gap semiconductors. Since tlie carriers at tlie CB minimum and tlie VB maximum have tlie same momentum, very fast recombination can occur. The radiative recombination lifetimes in direct semiconductors are tlius very short, of tlie order of tlie ns. The presence of deep-level defects opens up a non-radiative recombination patli and furtlier shortens tlie carrier lifetime. 

The situation is very different in indirect gap materials where phonons must be involved to conserve momentum. Radiative recombination is inefficient, resulting in long lifetimes. The minority carrier lifetimes in Si reach many ms, again in tire absence of defects. It should be noted tliat long minority carrier lifetimes imply long diffusion lengtlis. Minority carrier lifetime can be used as a convenient quality benchmark of a semiconductor. 

If tlie level(s) associated witli tlie defect are deep, tliey become electron-hole recombination centres. The result is a (sometimes dramatic) reduction in carrier lifetimes. Such an effect is often associated witli tlie presence of transition metal impurities or certain extended defects in tlie material. For example, substitutional Au is used to make fast switches in Si. Many point defects have deep levels in tlie gap, such as vacancies or transition metals. In addition, complexes, precipitates and extended defects are often associated witli recombination centres. The presence of grain boundaries, dislocation tangles and metallic precipitates in poly-Si photovoltaic devices are major factors which reduce tlieir efficiency. 

The variations in D and D and the much larger value for In show the limitations of a simple hydrogen atom model. Other elements, particularly transition metals, tend to introduce several deep levels in the energy gap. For example, gold introduces a donor level 0.54 eV below D and an acceptor level 0.35 eV above D in siHcon. Because such impurities are effective aids to the recombination of electrons and holes, they limit carrier lifetime. 

Ion implantation generates many dangling bonds that form centers for nonradiative recombination. These centers decrease the carrier lifetime and compete effectively with radiative transitions. However, after hydrogenation, since hydrogen ties dangling bonds, the luminescence process becomes more efficient. Furthermore, since the 1.0-eV emission is obtained even before hydrogen is introduced, the new radiative center may be formed due to residual hydrogen in the c-Si that combines with the implantation-induced defects. 

Gold has been used for many years as a minority carrier lifeline controller in Si. As such, it is introduced in a controlled manner, usually by diffusion into transistor structures to decrease the carrier lifetime in the base region in order to increase the switching speed (Ravi, 1981). Conversely, the uncontrolled presence of Au is clearly deleterious to the performance of devices, both because of the increased recombination within the structure and the increase of pipe defects, which can cause shorting of the device. These pipe defects consist of clusters of metallic impurities at dislocations bounding epitaxial stacking faults. 

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. 

The equilibrium lever relation, np= n , can be regarded from a chemical kinetics perspective as the result of a balance between the generation and recombination of electrons and holes (21). In extrinsic semiconductors recombination is assisted by chemical defects, such as transition metals, which introduce new eneigy levels in the energy gap. The recombination rate in extrinsic semiconductors is limited by the lifetime of minority carriers which, according to the equilibrium lever relation, have much lower concentrations than majority carriers. Thus, for a >-type semiconductor where electrons are the minority carrier, the recombination rate is A /t. An = n — n0 is the increase of the electron concentration over its value in thermal equilibrium, nQy and Xn is the minority carrier lifetime. This assumes low level injection where An is much smaller than pQy the equilibrium majority carrier concentration. 

Hamilton et al. (1979) have shown that a level at 0.75 eV above the valence band (M1 in Figs. 5 and 7) is the dominant recombination center in epitaxial layers and controls the minority-carrier lifetime in -type GaP. Another state at v + 0.92 eV has been shown to be caused by the persistent presence of Ni in vapor phase epitaxial (VPE) GaP (Dean et al, 1977). 

Chapter 1 focuses on the characteristics of deep states in wide band-gap III-V compound semiconductors, particularly the recombination properties which control minority-carrier lifetime and luminescence efficiency. These properties are significant for many optoelectronic devices, including lasers, LEDs, and solar cells. While this review emphasizes areas of extensive recent development, it also provides references to previous comprehensive reviews. The compilation of levels reported in GaAs and GaP since 1974 is an important contribution, as is the discussion of the methods used to characterize these levels. 

The short carrier lifetimes together with time, field, intensity and temperature dependences of the photocurrent indicate an intrinsic generation mechanism with the carrier yield limited by geminate recombination. 

Moreover, we shall keep in mind that the incorporation of dopants creates additional states for electrons and holes within the energy gap (often called impurity states). As a consequence, it unavoidably allows transitions to and from these states. This introduces additional recombination processes, which reduce the minority carrier lifetimes compared with lifetimes in the undoped material. 

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... 

Illumination creates excess electrons and holes which populate the extended and localized states at the band edges and give rise to photoconductivity. The ability to sustain a large excess mobile carrier concentration is crucial for efficient solar cells and light sensors and depends on the carriers having a long recombination lifetime. The carrier lifetime is a sensitive function of the density and distribution of localized gap states, so that the study of recombination in a-Si H gives much information about the nature of the gap states as well as about the recombination mechanisms. 

At temperatures above 100 K, the defect recombination mechanism changes gradually from tunneling to direct capture of a mobile electron or hole at a defect. The capture rate defines the capture cross-section such that the free carrier lifetime is given by... 

The carrier lifetime is longer in the nipi structures than for bulk a-Si H, but the excess carrier lifetimes decrease below a few minutes when the temperature is raised above 50 K. It is concluded that the tunneling recombination mechanism is present at low temperatures, but is obscured by the defect creation mechanism at elevated temperatures. 

It is known that clean dislocations without decoration reveal almost no recombination activity [55], but increasing decoration with impurities leads to recombination centres deep in the band gap, which significantly reduce carrier lifetime [56]. It can, therefore, be concluded that one of the most detrimental defects in EFG and SR apart from recombination active large angle grain boundaries are decorated dislocations. 

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). 

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