Recombination-generation lifetime

Before discussing DLTS, it is appropriate to talk about some device parameters that are affected by these impurities. The main parameters are the recombination and generation lifetimes because they affect junction leakage current, device switching speed, light emitting diode efficiency and a number of other device performance indicators. 

Deep levels influence a variety of device parameters. For example, in minority carrier devices they influence the recombination and generation lifetimes. The lifetime in turn controls junction currents and refresh times in dynamic random access memories. For this reason we discuss lifetimes. Defects... 

We will classify the carrier lifetimes into two broad categories recombination lifetimes and generation l etimes [60]. The recombination lifetime, applies when there are excess carriers in the semiconductor and recombination dominates. The generation lifetime, x, obtains when thermal generation dominates. Occasionally the lifetimes are further subdivided. For example, it is usefiil to distinguish between low-level and high-level injection lifetimes. But such finer subdivision is not done here. For a more detailed treatment of lifetime concepts and their measurements, the reader is referred to ref.61. 

Multiphonon generation is the inverse of multiphonon recombination. Multiphonon generation is a thermally activated process. Electrons are diermally excited from G-R centers into the conduction band and holes are similarly thermally excited from G-R centers into the valence band. The key feature of SRH generation is thermal excitation. A thermally activated process is very dependent on the temperature and on the activation energy. We define a generation lifetime as [60]... 

AVhat is Tg Just as Xj. represents the average time for ehp to recombine, so X represents the average time for ehp to be generated. It is intuitively obvious mat the mechanisms responsible for recombination and generation are quite different. We should, therefore, expect the recombination and generation lifetimes to be quite different as well. If in fact they are different what do they mean, how are they measured and how do they apply to device operation One must be very careful in the interpretation of lifetime measurements. 

The generation lifetime is measured with the pulsed MOS capacitor or the gate-controlled diode technique. It is important to understand that the lifetime measured by this technique can, and generally does, give very different values from the recombination lifetime measured by one of the techniques indicated above. 

Especially with LTG GaAs, materials became available that were nearly ideal for time-resolved THz spectroscopy. Due to the low growth temperature and the slight As excess incorporated, clusters are fonned which act as recombination sites for the excited carriers, leading to lifetimes of <250 fs [45], With such recombination lifetunes, THz radiators such as dipole anteimae or log-periodic spirals placed onto optoelectronic substrates and pumped with ultrafast lasers can be used to generate sub-picosecond pulses with optical bandwidths of 2-4 THz. Moreover, coherent sub-picosecond detection is possible, which enables both... 

The equihbtium lever relation, np = 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 energy levels in the energy gap. The recombination rate in extrinsic semiconductors is limited by the lifetime of minority carriers which, according to the equihbtium lever relation, have much lower concentrations than majority carriers. Thus, for a -type semiconductor where electrons are the minority carrier, the recombination rate is /S n/z. An = n — is the increase of the electron concentration over its value in thermal equihbtium, and... 

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. 

The mobile charge carrier species may either recombine or reach the semiconductor surface, where they can be trapped by the surface adsorbates or other sites. The lifetime of electron-hole (e /h+) pairs that are generated is important in determining the reaction yield. The holes are mainly trapped by water molecules or hydroxyl ions, giving rise to very reactive hydroxyl radicals ... 

Here (dPk/dt)9 is a rate of generation of defects, and 77 is lifetime of a defect with respect to recombination. It was assumed in [109] as an approximation that 77 is determined by the concentration of defects solely within... 

In contrast, one need not consult an Aldrich catalog to see if r-butyl radical is commercially available. Nor can one generate a significant quantity of r-butyl radical in solution prior to the addition of a reagent with which it is destined to react. This is because r-butyl radical, like nearly every other radical that is employed in synthesis, is transient. The lifetime of a transient radical rarely exceeds 1 ps and is limited by the rate of self reaction. This self reaction, which can be either a disproportionation or a recombination (Scheme 2), typically occurs at the diffusion-controlled limit the enthalpy of activation of most radical-radical reactions = 0 kcal mol-1. 

---