Quantum states trap types

The last class of defects considered here are volume defects. These are due to precipitates and domains of materials different from the matrix in which they lie. There is little new to add concerning these as the majority of the problems associated with them are due to interface states at the boundary between one material and another. An interface between different materials, even if perfect structurally, will generally have a contact potential that will produce an electric field and trap one type of carrier. If it has a lower energy gap it may trap both types of carriers. Such a second-phase region is used to advantage in a laser diode, in which the active quantum well traps both types of carriers (see Chapter 3). 

The occurrence and deactivation of excited states of the first type are schematically shown in Fig. 35. Let the minority carriers (holes) be injected into the semiconductor in the course of an electrode reaction (reduction of substance A). The holes recombine with the majority carriers (electrons). The energy, which is released in the direct band-to-band recombination, is equal to the energy gap, so that we have the relation ha> = Eg for the emitted light quantum (case I). More probable, however, is recombination through surface or bulk levels, lying in the forbidden band, which successively trap the electrons and holes. In this case the excess energy of recombined carriers is released in smaller amounts, so that hco < Eg (case II in Fig. 35). Both these types of recombination are revealed in luminescence spectra recorded with n-type semiconductor electrodes under electrochemical generation of holes (Fig. 

As the name suggests, shape-type resonances result from the shape of the potential at hand. But, what attributes must a potential have in order to trap the particle for a finite time and thus form a metastable state The wave nature of particles in quantum mechanics provides two typical ways for a... 

The net photocurrent and the quantum yield are a function of a number of competing processes " as shown in Fig. 1.22. For an n-type semiconductor, the externally measurable current i is the difference between the photocurrent and the forward current of electrons. The electron current is decreased to zero under certain anodic bias. While the flux of holes to the surface is exclusively controlled by the solid-state properties, all the other reaction steps depend on the surface properties of the semiconductor. The holes arriving at the surface can either (i) transfer to an electron donor in the solution, (ii) be trapped at the surface states, or (iii) recombine with electrons in the conduction band in the depletion region or at the surface. Process (iii) does not generate current in the external circuit, whereas process (ii) produces only transient current charging up the surface states. Only process (i) produces steady photocurrent. The measured photocurrent /ph can therefore be different from the flux of holes to the surface due to these processes. 

Another area of interest in quantum interference effects, which has been studied extensively, is the response of a V-type three-level atom to a coherent laser field directly coupled to the decaying transitions. This was studied by Cardimona et al. [36], who found that the system can be driven into a trapping state in which quantum interference prevents any fluorescence from the excited levels, regardless of the intensity of the driving laser. Similar predictions have been reported by Zhou and Swain [5], who have shown that ultrasharp spectral lines can be predicted in the fluorescence spectrum when the dipole moments of the atomic transitions are nearly parallel and the fluorescence can be completely quenched when the dipole moments are exactly parallel. 

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