PROBLEMA DE INVESTIGACIÓN Enunciado del Problema

Pieces of 0. 5 crystal. Since the crystal faces were plane enough for good spectra to be obtained , the development of polishing apparatus was not attempted.

2. 9 Polarising Microscope Examination of Crystals

ZnO is a uniaxial positive crystal , the C axis being the optic axis. Most plates grown were perpendicular to this axis , so unpolarised light could be used for study. Under the polarising microscope between crossed polaroids crystals were uniformly dark for all stage rotation.

For study in any but the C axis direction, the dendritic growths are useful. These could be found among the many small crystals of a given batch by stage rotation under the microscope. Dendritics gave extinct­ ion at two positions at right angl es. Very few pieces large enough for study were found.

The simpl est method of finding the optic axis direction for these pieces was to study the angl es of intersecting faces. On samples 6.3 and 7 .2 faces were found intersecting at 120°_{, giving an unambiguous axis}

direction.

Checking this by polarisation was difficult as the crystals were too thin for a full off-axis figure to be obtained. When the axis was parallel to the pol ariser

or analyser there was compl ete extinction, but the two positions could not be distinguished. The only test was to use a quartz wedge to compensate for the crystal in the 45o _{positions, and find the position giving the low­}

est order interference colours. When this was obtained the fast axis of the quartz was in the same direction as the slow axis, the optic axis of the ZnO crystal .

THE SPECTROMETERS 3. 1 Facilities Available

Much previous work in the department was done on two Hilger prism spectrographs with photographic detection .

Since no good densitometer was available these were not very convenient for quantitative temperature dependence work, particularly with reasonably wide bands. While this thesis was in progress two further instruments were purchased which as a pair were much more suitable.

The first was a Jarrell-Ash 3. 4 metre grating spectro­ graph using an Ebert mount, with direct-recording output . This gave ample resolution for the sharpest lines in doped zinc oxide, but had low light gathering power . Also, being a single beam instrument, it was inconvenient for study of bands.

A small, low resolution double beam spectrometer also

became available during this project. This was not immediately suitable for single crystal work, but when modified gave a

very convenient means of mapping the main features of the spectra. This is described in the following sections.

3. 2 The Bausch and Lomb Spectronic 505

Figure 3. 1 shows the main features of the double beam instrument. To use the full range of the instrument, an

bandwidth of SA could be used from 2 0 0 0 to 8000 Angstroms . This width corresponds to 1 0 - 16 cm- 1 in the range 5500 -

7000 A , which was used for most spectra. Absorbance could be

read linearly from -0 . 3 to 2 . 0 to within 0 . 0 1 at 0 . 4 . At higher readings accuracy would be much less .

Some features of Figure 3 . 1 will now be explained .

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The chart drum and monochromator are coupled , and can be run at several speeds . When the off-balance signal from the photomultiplier increases as the monochromator sweeps over a line , the brake is actuated to slow the sweep so that the pen can respond in time . Thus in principle fast overall speeds can be used without loss of accuracy .

The beam splitter is a pile of alternately oriented mirrors , sending half the beam down the sample channel and. half down the reference . The occluder is a small mask adjusted between two of these mirrors by a cam on the monochromator , varying the

amount of shadow in the two beams for 100% transmittance adjust­ ment .

Due to the action of the various reflecting surfaces , the beam in the sample chamber is about 80% polarised horizontally .

3 . 3 Use for Solid Samples at Low Temperatures

The beam focus in the sample chamber was about 8 x 1 mm { Figure 3 . 2 ) . Most samples to be studied were smaller than this , resulting in a loss of light . It is desirable if possible to re­ duce the beam size without the need for much loss in the dewar , since if both sample and reference beam are reduced equally ,

less compensation by the balancing system is needed .

With a smaller beam mechanical vibrations are also less troublesome . If a mask passes only part of a highly inhomo­ genous beam , a slight shift in the mask may give a large

variation in the output . If , however , the whole of a smaller

beam passes through the mask with room to spare , the same vibration will have no effect . A small mask and beam are of course more difficult to align initially .

Figure 3 . 3 shows the effect of roughly equal vibrations on the same mask well and badly aligned .

Due to the action of the brake motor , any noise produced in the optical system by vibration slowed down the scan by acting as a continuous out-of-balance signal . This was very inconvenient , since a few spectra for small , heavily doped samples had to be taken by moving the monochromator manually , resulting in a point-by-point spectrum . No signal smoothing was available .

This effect also precluded the use of sample immersion dewars , as bubbles passing through the light beam had the same effect as vibrations .

An attempt to cut down the beam size by reducing the length of the monochromator output slit was unsuccessful . It was found empirically , however , that a smaller , much more homo­ genous and not much weaker beam could be obtained by replacing the mirror condenser system by a lens system . A set of

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finally selected was a 6 diopter spherical convex lens and a 6 d . cylindrical convex lens, axis vertical, the pair being positioned roughly midway between lamp and slit . The resulting beam shape can also be seen in Figure 3 . 2

Since this probably produced large changes in the light distribution inside the monochromator and beam splitter, the

beam occluder was removed . Even without this reason, its

usefulness with masked samples is doubtful.

0

Above 6500 A, second order radiation from the monochromator must be cut out . The filter for doing this is located under

the cover of the instrument, which could not be lifted when the dewar was in place, so a cable for external operation was added . This and the modified condenser housing can be seen in Figure 3 . 4 .

A cover made to surround the dewar when in position was not sufficient to stop stray light from reaching the photomultiplier, and had to be supplemented with several thicknesses of black cloth held down with magnets . To compensate optically for

heavily absorbing samples a holder was made which with external operation could insert a neutral density filter in the reference

beam, (Figure 3 . 5) . This gave readings on the instrument ranges, but as the signals in both channels were at a lower level than normal, accuracy was greatly reduced . Noise was increased, slowing down recording, and stray light reduced the apparent intensity of strong lines . The filters used were the Optics Technology Neutral Density set SA. Normally an absorbance of 1 was used. This did not give excessive noise even with a

cobalt doped dendritic sample studied, a filter absorbance

of 2 had to be used to compensate for the small size and large