Short Notes K137

phys. stat. sol. (b) fg, K137 (1975) Subject classification: 17.22.4.3

Department of Physics, University of Aston in Birmingham

Field Emission from Lead Telluride

BY D.E. SYKES and E. BRAUN

Introduction The theory of field emission from semiconductors has recently

been improved (1) and is now in good agreement with experimental results on the

energy distribution of electrons field emitted from germanium (2). On the other

hand, some theoretical difficulties remain in explaining the behaviour of wide band-

gap materials. In order to clarify this area further, an experimental investigation

into the field emission characteristics of lead telluride, a material with a very

small band-gap ( r~ 0.3eV) was carried out. A preliminary study of field emission

from gallium phosphide, a wide band-gap material, i s also reported.

Experimental The lead telluride emitters were prepared from 1x1~15 mm

(111) oriented single crystal slices supplied by the Royal Radar Establishment,

Malvern. The square sections were formed into cylinders 200 to 300 microns in

diameter by rotating the crystals in a mixture of 2Ogms of KOH dissolved in 45 cm 3 3 of water, 20 cm of glycerol and 20 cm of methanol, whilst passing a current of

density 0. 2A/cm2, after the method of Norr (3). The final etching of the tip was

performed in a drop of the same electrolyte suspended from a loop of gold wire

which acted as the cathode. A single gallium phosphide emitter was prepared from

a 1x1~15 mm blank by polishing in a mixture of 2 parts H 0 : 2 parts HC1 : 1 part

HN03 : 1 part H SO

about 100ym and then allowed to form a neck and left until the end dropped

Off.

3

2 The crystal was rotated in the etch until its diameter reached

2 4 '

The emitters were mounted on a loop of tungsten wire with a drop of colloidal

silver, and attached to a specimen manipulator. The horizontal specimen could be

translated in the vertical plane and rotated round a vertical axis. The emitters

were positioned, by use of the manipulator, on the axis of a retarding potential

analyser similar to that of van Oostrom (4), built from drawings kindly supplied

by Mr. B.Singh of Salford University. The analyser was mounted within a stainless

Kl38 physica status solidi (b) 69

retarding potential U, ( V ) retarding potential Uc (Vl-

Fig. 1 Fig. 2

Fig. 1. Electron energy distribution for tungsten

Fig. 2. Electron energy distribution for lead telluride

steel vacuum chamber pumped by a trapped oil diffusion pump, typical operating

pressures were around 2xlO-l' T o r r after an overnight bake at 250 C. 0

The tips were cleaned by field desorption until a stable symmetrical emission

pattern was produced. This usually consisted of three stable spots at the points

of an equilateral triangle. The spots correspond to emission from (100) planes.

Results Before investigating the field emission characteristics of the semi-

conductor materials, the performance of the analyser was tested with a tungsten

emitter. A typical tungsten distribution from the (111) plane is shown in Fig. 1.

This distribution corresponds exactly to that obtained by Young and Muller (5) at

room temperature. This work with the tungsten specimen not only demonstrated

that the analyser was working well but also enabled the work function of the collec-

tor to be measured and hence the position of the Fermi level, within the emitter,

fixed on the retarding potential axis. The Fermi level ,remained constant in repeat-

ed experiments. A typical energy distribution from a field desorbed lead telluride

emitter i s shown in Fig. 2, halfwidths of distributions measured from several spec-

imens varied between 0.20 and 0.26 eV. A preliminary result for the energy dis-

tribution of electrons emitted from gallium phosphide is shown in Fig. 3. Consi-

derable difficulty was experienced in making this measurement as, although the

Short Notes I

Kl39

Fig. 3. Electron energy distribution for gallium phosphide

t 8oI

surface had been cleaned by field desorption, it contaminated rapidly causing the

total emitted current to fluctuate and hence the distribution to shift along the re-

tarding potential axis due to the high resistance ( * 10 SZ) of the specimen. This

i s always a problem when working with high resistivity materials. The high resis-

tance of the specimen also makes it difficult to position the distribution with re-

spect to the level.

8

Discussion The emission characteristics observed for lead telluride must be

interpreted a s emission from surface states within the forbidden gap. Recently it

has been shown, experimentally by Shepherd and Peria (2) and theoretically by

Modinos (1), that surface state emission plays an important role in the emission

process from germanium. Our results for lead telluride resemble the prediction

by Modinos (for germanium) for the case when the surface state emission dominates

the energy distribution. Empirically, by comparison of parameters, it would seem

reasonable that this should be the case.

It i s our contention that surface state emission makes a significant contribution

to the distribution of electrons field emitted from semiconductors generally. This

is borne out by our preliminary measurement of the energy distribution for gallium

phosphide, which indicates that the width of the energy distribution i s related to the

band gap. Other workers (Hughes and White (6), Salmon and Braun (7), have not

discussed surface state emission in detail, tending to associate it with contamina-

tion effects. Their results, however, a re in keeping with our hypothesis that the

width of the distribution is a function of the width of the forbidden gap.

References

(1) A. MODINOS, Surface Sci. 42, 205 (1974). (2) W.B. SHEPHERD and W.T. PERU, Surface Sci. 38, 461 (1973).

11 physica (b)

K140

(3) M.K. NORR, J. Electrochem. SOC. 109, 433 (1962).

(4) A.G.J . VAN OOSTROM, Phillips R e s . Rep. Supplement No. 1 (1966).

(5) R.D. YOUNG and E.W. MULLER, Phys. Rev. 113, 119 (1959) .

(6) O.H. HUGHES and P . M . WHITE, phys. stat. sol. (b) 33, 309 (1969).

(7) L .T.J . SALMON and E . BRAUN, phys. stat. sol. (a) Is, 527 (1973).

physica status solidi (b) 69

(Received April 22 , 1975)