Photoconductivity persistent

A3.5 Time-resolved photoluminescence studies of GaN A3.6 Persistent photoconductivity in GaN A3.7 Electrical transport in wurtzite and zincblende GaN A3.8 Characterisation of III-V nitrides by capacitance transient spectroscopy... 

FIGURE 1 (a) A typical behaviour of persistent photoconductivity (PPC) in Mg-doped p-GaN grown by reactive MBE. (b) The dark conductivity as a function of temperature. The bottom curve (solid circles) represents data taken with the sample cooling down in the dark, while the top curve (open triangles) is for data taken with the sample illuminated at 10 K for about ten minutes and then warming up in the dark. After [5],... 

P PAE PD PDS PEC PL PLE PMBE PPC PPPW PR PV PWP PWPP pi-MODFET precipitate power added efficiency photodetector photothermal deflection spectroscopy photoelectrochemical photoluminescence photoluminescence excitation spectroscopy plasma-assisted molecular beam epitaxy persistent photoconductivity pseudo-potential plane-wave photoreflectance photovoltage plane-wave pseudo-potential plane-wave pseudo-potential piezoelectric modulation doped field effect transistor... 

The photoconductivity response of a-Si H nipi structures has an extremely long recombination lifetime. A brief exposure to illumination causes an increase in the conductivity which persists almost indefinitely at room temperature (Kakalios and Fritzsche 1984). An example of this persistent photoconductivity is shown in Fig. 9.32. The decay time exceeds 10 s at room temperature and decreases as the temperature is raised, with an activation energy of about 0.5 eV. 

E>efect equilibration provides a plausible explanation of the persistent photoconductivity (Kakalios and Street 1987). According to this model, defect generation is enhanced by the long lifetime of holes in the p-type region of the nipi structure. Holes in the p-type layers... 

Fig. 9.32. (a) Time dependence of the conductivity of a nipi multilayer after a brief exposure to light, showing persistent photoconductivity (i) temperature dependence of the relaxation time, x, of the persistent photoconductivity (Kakalios and Fritzsche 1984). 

The origin of these effects is not at all clear. The bias effects and the room temperature persistent photoconductivity have similar annealing properties. There is also an obvious similarity between the annealing curves and those for the frozen-in excess conductivity of bulk doped a-Si H (e.g. Fig. 6.3). It is therefore probable that carrier-induced defect creation is the origin of the changes in conductivity and that annealing to the equilibration temperature restores the initial state. However there is as yet no complete explanation for the non-ohmic behavior and why it depends on the applied bias. 

In this section, we focus on the uses of metaphor in a contemporary research lab. Heather Graves spent 7 months studying the rhetoric of inquiry in a solid state physics lab with a seasoned researcher and his graduate students. She focused much of her time on research about amorphous semiconductors, specifically, persistent photoconductivity. What Graves learned about the function of metaphor in this context can be readily transferred to our study of chemistry. 

Metastable states, which are found in the conduction band tail region of the irradiated samples, are created by irradiation. Street and Winer [172] gave one mechanism for the persistent photoconductivity in a Si and a Si-H. Similarly, for carbon there is a chemical equilibrium between weak C—C or C=C bonds and neutral dangling bonds (C—) in the amorphous network. After irradiation a new equilibrium is reached, which can be described by a reduction of energy to form a single defect [183,184]. The formation of dangling bonds takes place in the following way ... 

Many semiconductors exhibit below 80 K a metastable increase in dark conductivity caused by brief exposure to light [139,140]. The increase can be a small fraction of the pre-illumination conductivity value or many orders of magnitude larger, and the relaxation time can vary between several minutes and many weeks. The excess conductivity, however, can be quickly eliminated in most cases by increasing the sample temperature above about 100 K. This photo-induced excess conductivity, commonly referred to as persistent photoconductivity (PPC), is observed primarily in compound semiconductors [139] and layered structures [141,142], although it has also been observed at low temperatures in doped Se and specially prepared Ge and Si samples [139]. 

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