Photoconductive decay

NHE OCP ONO OPS PCD PDS PL PLE PMMA PP PP PS PSG PSL PTFE PVC PVDF normal hydrogen electrode (= SHE) open circuit potential oxide-nitride-oxide dielectric oxidized porous silicon photoconductive decay photothermal displacement spectroscopy photoluminescence photoluminescence excitation spectroscopy polymethyl methacrylate passivation potential polypropylene porous silicon phosphosilicate glass porous silicon layer polytetrafluoroethylene polyvinyl chloride polyvinylidene fluoride... 

Fig. 7. Photoconductivity-decay lifetime, as a function of depth removed by etching, after fine polish. Note change in scale at 5p.

There are numerous techniques to measure the recombination lifetime. Some of the better known are photoconductive decay (13). diode reverse recovery (14). diode open circuit voltage decay (15). surface photovoltage (JL ) and forward-biased pn junction I-V characteristic (17. I will describe one particular photoconductive decay method, because it is a relatively new, non-contact method that requires no junctions. This makes it very suitable for a large number of measurements as for a process sequence characterization tool. 

What is impUed by unequal generation and recombination lifetimes The lifetime measurement techniques and the resulting lifetimes measured with them must be clearly understood to avoid confusion. For example, the recombination lifetime is measured by such methods as photoconductive decay, open-circuit voltage decay, diode reverse-recovery, surface photo voltage, electron-beam induced current and others. 

The photoconductive decay (PCD) lifetime measuring technique was one of the first lifetime characterization methods to be used. [70] As the name implies it uses optical excitation of e-h pairs. The carrier decay is monitored as a function of time following the termination of the optical pulse. Traditionally the sample is provided with contacts and the current is measured as a function of time. More recently, non-contacting techniques have been developed that make the method attractive because it is fast and non-destructive. 

A comparison of lifetimes measured with the conventional photoconductive decay and the microwave PCD methods has shown the conventional PCD derived lifetimes to be always higher then those obtained from microwave PCD. [75] This discrepancy was found to increase for longer lifetimes. 

The question whether amorphous semiconductors fulfil the condition for relaxation-case semiconductors was raised (Fagen (1972)) because the photoconductive decay time is found to be much longer than the lifetime assumed by van Roosbroeck and coworkers (about 10 sec). However, Ryvkin (1964) and also van Roosbroeck pointed out that in the relaxation regime the photoconductive decay is not governed by the lifetime but by the much longer relaxation time. 

The abihty to accept and hold the electrostatic charge in the darkness. The photoconductive layer should support a surface charge density of approximately 0.5-2 x 10 C/cm. The charge also has to be uniformly distributed along the surface, otherwise nonuniformities can print out as spot defects. The appHed surface potential should be retained on the photoreceptor until the time when the latent electrostatic image is developed and transferred to paper or, if needed, to an intermediate belt or dmm. In other words, the "dark decay" or conductivity in the dark must be very low. The photoconductor materials must be insulators in the dark. 

Fig. 4. Energy below the conduction band of levels reported in the literature for GaP. States are arranged from top to bottom chronologically, then by author. At the left is an indication of the method of sample growth or preparation liquid phase epitaxy (LPE), liquid encapsulated Czochralski (LEC), irradiated with 1-MeV electrons (1-MeV e), and vapor phase epitaxy (VPE). Next to this the experimental method is listed photoluminescence (PL), photoluminescence decay time (PLD), junction photocurrent (PCUR), photocapacitance (PCAP), transient capacitance (TCAP), thermally stimulated current (TSC), transient junction dark current (TC), deep level transient spectroscopy (DLTS), photoconductivity (PC), and optical absorption (OA).

When a semiconductor is illuminated, electrons may be excited into the conduction band and/or holes into the valence band, producing photoconductivity. This excited condition is not generally permanent, and when the illumination ceases, the excess current carriers will decay, or recombine. The average time which a photoelectron remains in the conduction band is termed the lifetime. As the lifetime increases, the photocurrent, for a given intensity of illumination, increases. 

Photoconductivity in zinc oxide, on the other hand, appears to be influenced by the surface through a different effect. Absorption of light effectively excites the electrons trapped in surface levels into the conduction band. This chapter will be primarily devoted to a consideration of this concept, proposed by Melnick (11), that photoconducting electrons are produced through the ionization of surface levels, specifically the adsorbed oxygen levels on zinc oxide. The decay of the photoconductivity. 

Photoconductive response (the rate of creation, or the rise, of the photocurrent, and the rate of decay of the photocurrent) appears to be divided into fast and slow responses. The fast responses, with time constants for rise and decay of the order of a second or less, have been adequately interpreted by Mollwo, et al. (53-55), Weiss (56), and Heiland (47,57) as bulk processes. These authors have concluded that the fast response processes are associated with the double ionization of interstitial zinc, and have proposed that the photon excites electrons from the valence band, and that the hole immediately recombines with the electron from an interstitial Zn+, producing double-ionized zinc ions. 

A few of his experimental results will be briefly described. A typical photoconductive rise at room temperature was found to reach half of the maximum conductivity change in a time of the order of an hour after the light was switched on. A similar period was required for half the decay to occur after the light was switched off. The rates of the rise and decay decreased with increasing time, the conductance approaching equilibrium. ... 

It was observed that the increase of photoconductivity under illumination was retarded by high oxygen pressure that is, the photoconductivity, at a given time and intensity of illumination, has a lower value if the oxygen pressure is high. On the other hand, decay of the induced conductivity was hastened by increased oxygen pressure. Nitrogen, water vapor, or carbon dioxide had no effect. 

Studying the effect of temperature changes, Melnick observed that the photoconductivity could be frozen in. For example, he found, while studying the decay of the photoconductivity at room temperature, that if the temperature was suddenly lowered to 130°K, the slow decay ceased. The photoconductivity was frozen in. If, at some later time, the temperature was raised again to room temperature, the decay recom-... 

The frozen in photoconductivity, as was concluded by Melnick, will arise from effects of the surface barrier layer or, of course, would arise similarly from any other rate-limiting process in the adsorption of oxygen. For our model in this discussion we shall use electron transfer over the surface barrier as the rate-limiting reaction. In this case, the rate at which adsorption occurs is proportional to exp ( —Ei/kT), where E2 is the barrier height. Thus if we measure the decay in photoconductivity (the chemisorption of oxygen) at room temperature, and then suddenly quench the sample to 130°K, it is obvious that the rate of decay in photoconductivity will decrease considerably. The change in the rate will be dependent on Ei and the temperature to which the sample is quenched. 

From an analysis of the rate of decay of the photoconductance as a function of temperature, Melnick concluded that the barrier height was 0.43 e.v., as compared to Morrison s (31) estimate on a different sample of about 0.5 e.v., the latter being based on measurements of the reversible portion of the time-dependent conductivity. 

In this expression, q is the surface concentration of the adsorbed gas, ci and C2 are constants, and t is the time. By relating the conductance to the amount of adsorption, he derived an expression for the decay of the photoconductance due to adsorption... 

Melnick has suggested that even the fast photoconductive rise and decay is a surface phenomenon, involving the same reactions and model as the slow responses. Again he shows agreement between the theoretical predictions and the experimental results. 

The impurity generation route is shown in Fig. 5 to the right of the line. The interaction of the relaxed states formed from m ami unexcited impurity t results in exiplex t formation. This exiplex can decay thermally or form coupled ion-radical pairs t, which may dissociate in the electric field. For explanation of the absorption and photoconductivity spectrum correlation it is necessary to assume a very high concentration of exiplex sites. 

Transient Photoconductivity. A solution of neutral molecules in a polar solvent shows only ohmic conductivity, but if ions are formed by the action of the photolytic flash these charge carriers generate an additional current which is proportional to the ion concentration. The observation of such transient photocurrents is the most direct experimental evidence for the formation of free, solvated ions in electron transfer reactions. The quantum yield of ion formation can be obtained through proper calibration procedures and the kinetics of ion recombination can be determined. Figure 7.37 gives an example of such transient photocurrent rise and decay. 

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