Photocurrent Analysis Procedure

The data required from photocurrent analysis experiments was the differential photocurrent response of a sample to a given wavelength, as discussed in §5. Thus for every wavelength (grating position) set on the sine drive o f the monochromator two current readings were obtained: one in darkness and one under the selected illumination, so that the difference between these two values could be plotted as the response of the device to that excitation. The attraction and validity of reasserting the dark current at each wavelength in this way lies in the practical necessity to define what is meant by a working dark current. In §5.3 it is shown that the post-illumination conductivity decay of a photoconductor takes the form o f an inverse exponential, and §3.2.2 demonstrated that the duration of this decay can be strongly influenced by the perfection and purity of the sample under test, such that the full decay time o f the conductivity in a poor quality sample can be in the order of days [4.20]. To allow such a device to reset fully after each exposure would entail waiting perhaps a week between recording the response to each wavelength, meaning that a full responsivity spectrum could take months to obtain. Even if collecting such a data set were to be econom ically and logistically viable, its validity would be undermined over the extended time period by drift in the output o f the light source and possible charging effects in the device itself.

A practical solution is to specify a fixed time interval between current readings, and to define the working dark current as being whatever the current has decayed to within this

Chapter 4: Experimental Methods

allowed period. This is acceptable because the exponential decay format means that the initial period o f turn-on or turn-off accounts for the most rapid part o f the response characteristic: the part o f the cycle which is ignored is the part during which least happens. Any systematic errors introduced in this way w ill tend to understate the response o f a slow sample to the next exposure wavelength, so the risk o f exaggerating a device's response due to residual conductivity from a previous exposure is avoided. A sampling interval o f 1 minute per wavelength setting allows for 20s dark exposure, 20s light exposure and 20s reset time. This results in an overall experiment time o f 38 minutes to obtain a complete spectral response curve, which is long enough to gain a reliable insight into the behaviour o f the sample and short enough to be executed accurately and comfortably by a manual operator. When such an experiment is accompanied by dark current measurements and a speed o f response test, as described elsewhere in this chapter, a complete picture o f the sample's photoconductive properties will be obtained.

An additional complication in the process o f obtaining photoresponse data over a wide range o f wavelengths arises due to the nature o f the grating optics which lie at the heart o f the monochromator. When the grating angle is set to allow a pass band centred on a given wavelength Ap, the 2"^, 4^^ „th orders o f the harmonics (Ap/2, Ap/3, Ap/4 Xpin) o f that wavelength will also be present. Thus, if the passband is centred upon, for example, SOOnm (to which intrinsic diamond has negligible sensitivity) the monochromator output w ill also contain diminished intensities o f 400nm , 266nm and 200nm light, the last o f which corresponds closely to bandgap illumination and so excites a significant increase in the conductivity o f the sample. To avoid the erroneous conclusion that the sample is sensitive to SOOnm excitation it is necessary to employ low (energy) pass filters to remove the unwanted harmonics.

The system constructed used a pair of filters with cut-offs o f 400nm and 630nm along with an opaque shutter (equivalent to o°nm cut-off) and the absence o f any filter or shutter (equivalent to a Onm cut-off). The procedure by which these filters were used is best described by reference to figure (4.8) in which the broadband light source is represented at the base o f the diagram. Taking a vertical direction o f photon flux, the polychromatic beam passes through one o f the four available filters and is then incident upon the diffraction grating o f the monochromator. The passband, as determined by the angle of incidence via the sine drive mechanism, is then projected through the exit slit o f the monochromator and onto the target.

Chapter 4: Experimental Methods Light Output TARGET 2 0 0 n m 2 6 6 n m 400nm 4^' o rd e r S'" o rd e r 2"" o rd er SOOnm 1" o rd er G rating (at SOOnm) / Onm S (no filter) 400 n m « I 630 n m oonm (o p aq u e) Light S o u rc e ^ 200

t l ü ü t î î î t î î î î î î î î

Figure 4.8: Schematic diagram outlining the use and function of filtering to remove unwanted harmonics from the light transmitted by the monochromator grating, see text for full description.

To obtain a full spectral responsivity curve the monochromator grating would be first set for a passband centred on SOOnm. The two required current readings would be taken using the filter regime indicated by the heavy dotted box at the bottom right of the figure, ie: the dark current would be measured with the shutter closed (oonm filter), this filter would then be replaced by the 630nm filter and twenty seconds later the photocurrent would be recorded. Following the measurement the shutter is returned and the device is allowed to reset for twenty seconds whilst the grating is rotated to select a passband centred on the next required wavelength, 780nm in this case. It can be seen from the diagram that under these conditions, the higher order passbands of the monochromator grating are excluded from both the dark and light exposure so that a true response current is obtained. For wavelengths in the range 400nm<Ap<630nm the filter regime is that indicated by the central dotted box on the diagram, and for Ap<400nm the left hand boxed combination is used.

Chapter 4: Experimental Methods