Dr. Shabbir A. Bashar's Ph.D. Thesis

"Study of Indium Tin Oxide (ITO) for Novel Optoelectronic Devices"
Ph.D. thesis by Shabbir A Bashar

This section describes the experimental set-up behind some of the regular optical measurements carried out in the course of this study. These include the optical transmittance measurement of the ITO films and assessment of the absolute responsivities of the optoelectronic devices which were subsequently fabricated.

4.5.1 ITO Film Characterisation

The transmittance of the ITO films were measured using a double beam Perkin Elmer spectrophotometer (Model 330). In this system, the source beam from a filament lamp is first monochromated and then split into two separate coherent beams; a clear glass microscope cover slide is placed in the path of the first beam while an identical cover slide coated with the ITO film to be characterised is placed in the path of the second beam. These monochromatic beams are then converged and converted to a measurable electrical signal. This process is repeated by sweeping the wavelength of the beam over the entire range of the characterisation: 200 - 2000nm in this study. The measured transmittance is then recorded on a X-Y chart recorder or converted to a digital form using a suitable A/D converter.

The benefit of having a double beam spectrophotometer means that there is no need for two separate measurements (one for the uncoated and one for the ITO coated sample) but also the actual transmittance of the ITO films is obtained directly as a result leaving aside any common background artifacts such as interference due to the presence of water vapour or indeed characteristics of the microscope cover slides themselves.

4.5.2 Device Quantum Efficiency and Absolute Responsivity

The quantum efficiency, h, of an optoelectronic device is defined as the fraction of electrons collected over the number of incident photons which are absorbed [61]; this can be expressed as:

h = (rq/rp) x 100%

(eqn. 4.29)

where,
rq = rate of electron collection
rp = rate of photon arrival
As discussed in later chapters, h varies with the wavelength, l, of the photons and is therefore quoted for a specified l.

The absolute responsivity, R, of the device at a given wavelength, is defined as the ratio of the output (photo generated) electrical current, IPh, over the corresponding input optical power, Po:

R = IPh/Po

(eqn. 4.30)

Since the energy of a photon is given by the product of its associated frequency, n, and the Planck constant, h, the rate of photon arrival can be expressed as a function of the corresponding optical power:

rp = Po/hn

(eqn. 4.31)

Substituting (eqn. 4.31) into (eqn. 4.29) yields:

rq = hPo/hn

(eqn. 4.32)

and,

IPh = qhPo/hn

(eqn. 4.33)

where
q = the electronic charge

R, from (eqn. 4.30) can now be expressed as:

R = qh/hn

(eqn. 4.34)

Since, the speed of light, c = nl, the responsivity can also be expressed as:

R = qhl/hc

(eqn. 4.35)

In order to do absolute responsivity measurements, devices are packaged in T05 transistor headers. These are then plugged on a jig, as shown in Figure 4.10, designed to isolate mechanical vibrations (originating from measuring equipment) disturbing the precision optical alignment of the device under test. The jig can be rigidly mounted on a triple axis micro-manipulator situated on an optical bench.

Figure 4.10: Schematic diagram of the jig used for optical characterisation of devices mounted on T05 headers

A schematic block diagram and a photograph of the actual measurement set-up are shown in Figure 4.11 and Figure 4.12 respectively. The responsivity measurement set-up consists of a solid state Sharp LT022MS0 AlGaAs laser with an emission wavelength of 780 nm and a maximum output power of 5mW. The laser is packaged into a collimator and controlled by a dedicated laser driver circuit (available from Hero Electronics, UK). The emergent beam is then launched onto the surface of the detector using a convergence lens with beam diameter of less than 10 mm. Individual XYZ micro-manipulators, mounted on an optical bench, are used for positioning the laser, the lens and the detector to allow maximum flexibility in obtaining an optimum responsivity.

Figure 4.11: Schematic block diagram showing the essential set-up for optical characterisation of a test photodetector

Figure 4.12: Photograph of optical bench being used for measuring the optical output characteristics of a photo detector under test

In case of the responsivity measurements at 1310 nm, the Sharp laser was replaced by a Laser Diode Inc.(of Calif. USA) LD 6171 (BN 17444). This laser is packaged on a chip with an on- board fiber interface. Thus a monomode fiber designed to carry light at this wavelength was pig-tailed to the laser. A fiber with LD interface (model no. LOF121A 000411 - available from E-Tek Dynamics Inc., Calif. USA) and a fitted lens was used to focus the beam on the detector. The lens in this case has a nominal focusing capability of 2 mm diameter output beam.

© 1998: Shabbir A. Bashar (in accordance with paragraph 8.2d, University of London Regulations for the Degrees of M.Phil. and Ph.D., October 1997). The Copyright of this thesis rests with the author, and no quotation from it or information derived from it may be published without the prior written consent of the author.

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