FIELD EMISSION MICROSCOPY OF GALLIUM ARSENIDE

HAL Id: jpa-00225639

https://hal.archives-ouvertes.fr/jpa-00225639

Submitted on 1 Jan 1986

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

FIELD EMISSION MICROSCOPY OF GALLIUM ARSENIDE

C. Patel

To cite this version:

C. Patel. FIELD EMISSION MICROSCOPY OF GALLIUM ARSENIDE. Journal de Physique Col-

loques, 1986, 47 (C2), pp.C2-53-C2-58. �10.1051/jphyscol:1986208�. �jpa-00225639�

FIELD EMISSION MICROSCOPY OF GALLIUM ARSENIDE

C. PATEL

Louisiana State University, Department of Mechanical Engineering, Division of Material Science, Baton Rouge, Louisiana 70803, U.S.A.

ABSTRACT:

Field Emission technique has been applied to study the behaviour of thin overlayers of gold on GaAs. Using Fowler- Nordheim plots, change in the work function

is examined for temperatures, T=77K and T=300K.

changes slightly for low doses of gold and significantly for larger ones

=4.3 -

3.7 eV]. Desorption of gold is also examined and

results indicate two different adsorbed states in Au-overlayers formed at room temperature. Finally, a brief description of sample preparation is also included.

INTRODUCTION:

In the fabrication of semiconductor devices, the role played by GaAs is becoming more and more important. The first step in its device technology is the preparation of perfect crystals and the last steps are the formation of an electrode and the surface of passivation. Therefore the study of thin film growth is very important technologically. The study of metal adsorption is especially necessary for the understanding of metal-semiconductor interfaces.

The Field Emission Microscope has been extensively used in the study of metal and semiconductor surfaces. The process of Field Emission is, itself, of great interest and a considerable amount of both theoretical and experimental work has been carried out in this field. The Field Emission Microscope also yields useful information of a more practical nature, such as the nature of bulk and surface impurity, diffusion, chemisorption and surface potential barriers.

It is essential that the surface to be studied can be prepared in the form of a high curvature tip, with a radius of 1kT5 cm and can be cleaned sufficiently well for a symmetrical reproducible pattern to be observed.

In recent years, considerable attention has been given to the 111-V compound semiconductors and a number have been studied by means of the FEM ( ~ n S b [ l l , InAs[Z], GaAs[3,4,5,6])

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986208

JOURNAL

DE PHYSIQUE

THEORY:

In experimental measurement, the emission current I (in amperes) is measured as a function of the electrostatic potential V (in volts) between the screen and the tip.

The current I is related to the current density by the relation

where

and z(y) and t(y) are functions of the variable

-4 F lh

y

3.79~10 ----

9 which have been tabulated.

The applied voltage V is related to the electric field F, by the relation

F

,qv ...

where A is the emitting area in cm2 and B a geometrical factor in cm-1 depending on the geometry of the electron emitter and the other electrodes. Combining equations l,

we get

The curve is produced by plotting log (1/v2

v& l@'/" and is called the Fowler-Nordheim plot. The slope at any point is given by

m

-2.97~10'

B

where

The function

(y) has a value close to unity, hence the value of

m is also constant and the F-N plot approaches a straight line.

steel isolating valve. The vacuum was attained upto 2x18'' Torr by baking at 220 OC overnight (12-16 hrs).

The specimens of GaAs were in the form of whiskers (fig 1) which were grown by the vapour phase reaction between gallium suboxide and Arsenic in a close tube system

Whiskers were grown b y o t h e Vapour-Liquid-Solid (VLS) mechanism

in the range of 950-1038 C on the silica plate covered by vapour deposited gold.

A GaAs whisker was adhered by silver paste to a loop of 0.125 mm diameter tungsten wire which was spot-welded to 1 mm nickel rods.

The sharp GaAs field emission tipsP were prepared by dipping the specimens for 2-3 minutes in freshly prepared hot solution of H

S

:H?

:H

3 : : l The prepared tips were then rinsed2 in deionlzed water and methanol taking care not to wet the GaAs-loop- silver paste junction. After inspection in an optical microscope at approximately X400, the loop with GaAs tip was mounted in the microscope.

FIGURE 2: Clean FEM Micrograph of GaAs surface; a) field desorbed at

b) stereographic projection of (a).

CLEANING OF GALLIUM ARSENIDE TIPS:

This at first appeared to be a little difficult, purely from the fact that most of the 111-V compound, such as GaAs, crystallizes in the zinc blend structure, which is related to the diamond structure;

the only difference between the two structures is that, in the diamond structure, all lattice sites are occupied by the atoms of the same kind, whereas in the zinc blend structure, the lattice sites are alternately occupied by A and B atoms, 191. As a result, the bonds are not completely covalent but have partially ionic character and the planes of which, the sum of Miller indices h+k+l is odd, are polarized. For example, for the type I plane, the first layer is occupied by only type 1 atom (group 111 element) and the second layer is occupied by only type

atom (group V element).

Once this is realized, application of the field desorption technique

to clean the GaAs tip proved quite successful and the resulting

images are depicted in fig 2.

JOURNAL

DE

RESULTS:

A clean GaAs (111) surface was prepared by field desorption at 8. lkV, (fig

together with its stereographic projection. The clean surfaces of GaAs after field desorption, have a disordered atomic arrangement. At an emission current of more than lg9 amps it was observed that the surface was disturbed.

I - V C h a r a c r e r r s t i c s o f G a A s - A u a t 78K-300K

( a ) o 3 0 0 6 ( b ; 0 7 B K a.

t ^{( C )} A Y - G o A 5 7 8 1

5 d o s e s

dl 1 Q1 a

.I

w a r m t o 3 0 0 X ( = l x r d o s e o n l d l

( 1 ) e 4 doses on

l d )

z ^\

\

G .

S

FIGURE

3:

Fig 3 shows the emission C-V characteristics obtained from the clean surface of GaAs after field desorption and also satisfies a Fowler-Nordheim relation.

GOLD ADSORPTION ON GALIUM ARSENIDE:

We have investigated the effects of Au adsorption on the emission from the GaAs tip at room temperature. The emission C-V characteristics shifted to the lower voltage side with Au adsorption. Their slopes, however, changed complicatedly (fig 3).

Adsorption of Au at room temperature shows a slight decrease in work function , from 4.3 eV (for GaAs clean surface) to about 4.24

(for three doses of Au) , (fig 4). A further increase in the density of Au, results in almost a linear drop in work function upto a point corresponding to about sixteen doses and

eV;

corresponding changes in FEM patterns.

Fig 5 shows desorption of Au from GaAs. It is sought that the Au

atoms deposited at room temperature on GaAs migrate to the center of

the tip surface under the field applied conditions and then field

desorb. Since the positive field strength is highest at the center,

of the tip surface, this implies that the

atoms would have

negative charges. Further study of field desorption of Au from GaAs

shows that there are two different adsorbed states in the Au

overlayers formed at room temperature.

I . . , . . . . . . . . .

1 2 3 4 5 6 7 8 9 1 0 11 12 13 14 15 16 17 1 8 19 M 21 2 2 G O L D DOSES-

FIGURE 4

FIELD DESORPTION O F GOLD FROM GaAs

I

I I

3 0 40 5 . 0 6.0 7 0 8 0 9.0 FIELD DESORPTION VOLTAGE ( k V )

FIGURE 5

CONCLUSION:

Our present exploratory work has given a good indication of the Au- GaAs system in terms of reproducibility of results. Some of the salient features are as mentioned below;

(i) the field emission images of gallium arsenide appears to be unstable for temperatures of T

and

(ii) the

plots for T

and

are linear over the working voltages/current

(iii) work function changes linearly with increasing gold adsorbate density and eventually becomes independent of gold coverage

(iv! the adsorbate covered plots for

and

show some

discontinuity

JOURNAL

DE

i) suggests contaminant activity, but when the sample is cooled from 300K to 77K the activity continues to persist; this can be attributed to the fact that the structure of gallium arsenide having alternating layers (such as Ga and As) both contributing to imaging process.

ii! gives good linearity for both temperatures, although at high emission current, the linearity departs, due to thermal emission and possible phonon-electron contribution [l@].

iii) suggests faster diffusion of gold into GaAs lattice compared to out diffusion of Gallium or ~ r s e n i c , until all sites are occupied when

reaches a value of pure Gallium.

iv) since the outdiffusion effects are plausible, it is highly likely that effects in (iii) are first concluded and then Ga may outdiffuse over the layers as suggested by

value in room temperature

-coverage plot.

We have conducted several extensive experiments to justify these observations [l11 and use of atom probe, probe hole FEM and spectroscopic techniques to further confirm the results.

ACKNOWLEDGEMENT

I (C.P.) wish to thank the United Kingdom Science Engineering Research Council and U.S.Army Research Office (ER0 London) for their financial support: cont. # DAJA 37-81-C00055. Finally, I would like to sincerely thank my esteemed colleague Dr. J.P.Jones for his stimulating discussions and invaluable help.

REFERENCES:

1. W.R.Savage, Solid State Column. 1, 144 (1963)

2. S.G.Troxillo, J.C.Blair, N.G.Einspruch and R.Stratton, J.

Chem. Phys. 44, 1724 (1966)

3. J.R.Arthur, J. Appl. Phys. 37, 3057 (1966)

4. 0.H.Huges and P.M.White, Phys. Stat. Sol. 33, 309 (1969) 5. Y.Ohno, S.Nakamura, T.Kuroda, Sur. Sci. 91, 636-654 (1980)

6.

91, L7 (1980)

7. Gershenzon and R-M-Mikulyak, J. Electrochem Soc. 108, 548 (1961)

8.

R.S.Wagner, W.C.Ellis, K.A.Jackson and S.M.Arnold, J. Appl.

Phys. 35, 2993 (1964)

9. Y.Ohno, S-Nakamura, T.Adachi and T.Kuroda, Surface Science, 69, 521-532 (1977)

10. C-Patel, 'High Field Microscopy Study of Au-GaAs [l111 Interface' submitted to Thin Solid Films

11. C. Pate1 , 'Photo-field Effects on Au-GaAs Interface using

Field Emission Microscopy' submitted to Surface Science