ATOM PROBE MICROANALYSIS OF THE COLD-GALLIUM ARSENlDE INTERFACE

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ATOM PROBE MICROANALYSIS OF THE COLD-GALLIUM ARSENlDE INTERFACE

H.-O. Andrén

To cite this version:

H.-O. Andrén. ATOM PROBE MICROANALYSIS OF THE COLD-GALLIUM ARSENlDE INTER- FACE. Journal de Physique Colloques, 1987, 48 (C6), pp.C6-463-C6-468. �10.1051/jphyscol:1987676�.

�jpa-00226884�

ATOM PROBE MICROANALYSIS OF THE GOLD-GALLIUM ARSENlDE INTERFACE

Department of Physics, Chalmers University of Technology, S-412 96 Goteborg, Sweden

Resume - Des m6thodes pour analyser de l'arseniure de gdium en utilisant la sonde atomique B d6ficits en 6nergie compens6s sont dtbites. On a trouv6 que la couche d'oxyde qui se fonne dans l'air sec consiste en gallium et en oxyg2ne. L'or d6pod sur la couche d'oxyde forme des ilots d'or pur. Le d6pot d'or sur de l'ars6niure de gallium pur conduit B la formation de couches d'un m6lange d'or de gallium et d'arsenic, d'une 6paisseur comprise entre une fraction de monocouche et plusieurs couches atorniques.

Abstract

-

Methods for analysis of gallium arsenide using an energy compensated atom probe are described. The oxide layer formed in dry air on gallium arsenide was found to consist of gallium and oxygen, and gold deposited onto the oxide formed small islands of pure gold.

Deposition of gold onto clean gallium arsenide resulted in the formation of intermixed gold- gallium-arsenic layers, between a fraction of a rnonolayer and several monolayers in thickness.

Gallium arsenide is used for high speed and opto-electronic devices because of a high charge carrier mo- bility and a suitable direct band gap (1.4 eV). One problem with these devices is that good metallic con- tacts are more difficult to make onto GaAs than onto silicon. Conducting (ohmic) contacts are often made by depositing eutectic gold-germanium alloy on the GaAs device [I]. However, these contacts are non-uniform both laterally and in depth, m d this gives problems when the lateral and active layer di- mensions of GaAs devices are reduced. In addition, stability and reliability problems occur [2].

As a first step in an attempt to better understand contact formation, atom probe analyses of pure gold- GaAs contacts are at present being made. These contacts are of the Schottky type and during the last ten years their microstructure have attracted considerable interest. Investigations have been made with dif- ferent surface sensitive spectroscopies (e.g. 13,4]),and several models explaining their electrical behav- iour have been suggested, such as the unified defect model [5], the metal induced gap states model [6], and disordered induced gap state model [7].

I1 - EXPERIMENTAL

Specimens of n-type GaAs (doped with 3.9~1018 lcm3 Si; room temperature resistivity 0.7 mRcm) were prepared by cleaving wafers into square cross section rods and chemically etching the rods in 50%

sulphuric acid in hydrogen peroxide at 85 OC (Figure 1). The energy compensated atom probe instru- ment used for this investigation has been described before [8,9]. Imaging was performed at 90 K using neon as imaging gas, and analysis was made by electric pulsing at temperatures between 45 and 90 K.

Gold was deposited onto the specimen tips by thermal evaporation in ultra high vacuum. The evapor- ation source, a droplet of gold on a molybdenum filament, was mounted inside a separate field ion microscope. The gold evaporation was controlled using a 10 MHz quartz crystal film thickness monitor with a resolution better than 0.1

A.

The procedure used for the gold deposition was as follows: F i t the specimen was field evaporated in neon at 80 K. The nwn gas was then pumped out, but the specimen

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987676

C6-464 JOURNAL DE PHYSIQUE

voltage was not reduced to zero until the pressure was lower than 3.10-'Pa. The specimen was then immediately moved to the deposition position, and the desired thickness of gold was deposited at a tem- perature of or slightly above 80 K. The time between voltage removal and completion of the gold de- position was less than three minutes. Finally the specimen was allowed to warm up to room tempera- ture, exposed to air, transferred to the atom probe instrument and analysed, usually by pulsing from zero voltage without previous field ion imaging.

Specimens could be imaged in neon at a temperature of 80 K, although the image intensity was rather low. From measurements of field evaporation radii (Figure 1) and from comparisons with the ionisation field of neon (35 Vfnm), the evaporation field was estimated to about 30 Vlnm. However, the apparent evaporation field calculated from the tip radius before gold deposition was considerably higher, indi- cating some voltage losses in the uncoated specimens.

Atom probe spectra of pure GaAs obtained under different conditions are shown in Figure 2. Clearly, the large energy deficits (long tails on the mass peaks towards the high mass end) present during elec- trical pulsing make the use of an energy compensator mandatory. With the energy compensator the stoi- chiomemcally correct analysis was obtained, but only when ^using a low temperature (45 K) and a low pulse amplitude (15% of the standing voltage). An interesting observation was that under these con- ditions a large proportion of the arsenic atoms appeared as molecular ions (21% as A$+, 25% as As2+

and 12% as As3+). Consequently the mean charge of arsenic (but not gallium) decreased at low temp- eratures. Another observation was that many gallium atoms appeared as molecular GaH+ ions, and some probably also as Ga-H20+ ions, in spite of the fact that the partial pressure of both Hz and H20 was less than 108 Pa.

The oxide layer formed in dry air on a clean (field evaporated) specimen was found to consist of about 2

A

of =GaO. When gold was deposited onto an oxidised surface pure gold islands formed, 20-100

k

in diameter (Figure 3).

Gold was also deposited onto specimens which had been cleaned by field evaporation at about 80 K.

After transfer to the atom probe the s p ~ i m e n s were analysed close to their apex (the (1 10) pole) by pulsing from zero voltage. After removal of adsorbed atoms and often some arsenic, gold was detected together with gallium, arsenic and some oxygen (Figure 4). The thickness of the gold containing layer varied from submonolayer (0.2

A)

to monolayer (2

A)

to multilayer (8

A).

In the latter case the gold content of the mixed layer was about 10 at.% and the oxygen content about 3 at.%. After the mixed layers had been field evaporated away, only pure GaAs was recorded.

IV - DISCUSSION

Obviously the electrical conductivity of the gallium arsenide material studied was sufficient also at low temperatures to permit atom probe analysis using electric pulsing. Actually the decrease in charge carrier density at e.g. liquid nitrogen temperature is almost completely compensated by an increase in the carrier mobility. However, the decrease in apparent (DC) evaporation field after gold deposition suggests that some voltage drop occurred in the uncoated specimen.

A rather low temperature (45 K) was required to obtain correct atom probe analysis. This was not unexpected since it is well known that semiconductors of the 111-V type are likely to exhibit preferential field evaporation because of the much lower ionisation potentials of the column 111 atoms [lo]. But also at this low temperature some of the gallium atoms must evaporate during the rise or fall of the pulse, since less than 50% gallium was recorded when a high pulse amplitude (25%) was used. This means that the energy deficits of some of the gallium ions were so large that the energy compensator was unable to transmit them. However, the energy acceptance of the present energy compensator seems to be

The observation that the mean charge of the arsenic ions decreased at low temperature could possibly be explained if it is assumed that at e.g. liquid nitrogen temperature, arsenic is usually evaporated from a gallium depleted surface and thus exposed to a somewhat higher local field. This will increase the pro- bability for post ionisation. By contrast, at lower temperatures most of the gallium atoms are still in their lattice positions when the arsenic atoms field evaporate, and therefore no extra local field enhancement occurs.

It is interesting to compare the present results on the field evaporation of gallium arsenide with those of Cerezo et al. [12]. Using a laser pulsed atom probe they obtained stoichiometrically correct analyses of GaAs at an estimated pulse temperature of 280 K, and they also recorded considerable amounts of molecular arsenic ions (about 65% of the arsenic atoms as compared to 58% in the present study).

However, they did not observe gallium hydride ions or gallium water molecular ions. This may partly be due to the very low residual gas pressure in their laser pulsed atom probe, 3.10-9 Pa, and partly the fact that the laser beam also hits the shank of the specimen and there induce evaporation of adsorbed hydrogen, thus preventing hydrogen diffusion to the specimen tip apex.

In the stoichiometrically correct analysis at 45 K and 15% pulse, when presumably also gallium atoms of large energy deficits were collected, quite a large proportion of the gallium atoms field evaporated as GaH ions and some probably also as Ga-H20. Since the partial pressures of hydrogen and water vapour was very low, this suggests that these gases readily adsorb on Ga atoms in the specimen surface and there further reduce the already low activation energy for field evaporation.

The observation of gold island formation on oxidised specimens, both by electron microscopy (on the shank of the specimens) and by atom probe analysis, is in good agreement with the data of Lii et al.[13].

By photoelectron spectroscopy they found that already a few tenth of a monolayer of oxygen on the gal- lium arsenide surface inhibits any gold-gallium arsenide intermixing and causes an abrupt metal-semi- conductor interface.

All atom probe analyses of gold layers on clean GaAs specimens show that intermixing occurs already at room temperature. Since these analyses were made without imaging, a theoretical possibility exists that pure gold islands were formed on an otherwise clean surface, and that in all analyses both part of an island and part of the surrounding surface were probed. Apart from being highly unlikely, this could not explain the fact that the gallium-arsenic ratio was lower when gold was recorded than when the pure gallium arsenide was analysed.

Consequently, a thin (about one monolayer), intermixed layer probably exists on the surface. Some of the gallium seems to be replaced by gold, but this could well be an effect of a slightly different occurrence of preferential field evaporation in the layer and in pure GaAs, since these analyses were made at a temperature of 90 K. In addition often some little arsenic was detected immediately before the gold containing layer was analysed, which means that some arsenic resides on top of the intermixed layer. The small amount of oxygen found onto and in the intermixed layer was probably introduced during specimen transfer in air from the field ion microscope to the atom probe instrument.

Our results are in rather good agreement with the work of Chye et al. [3]. They interpret their spectro- scopic data from the gallium arsenide (1 10) surface in terms of gold-gallium alloyed layer with possibly some elemental arsenic on top of the layer. Keisuke et al. [4] studied the (001) surface spectroscopically and also found a gold-gallium chemisorption reaction at low gold coverage, associated with arsenic re- lease. After about one monolayer of gold had been deposited, fine and probably pure gold islands nucle- ated, which after further gold deposition grew until finally a continuous layer formed at about 50 to 100 monolayers. Clearly further work with the atom probe is required to clarify whether this island form- ation occurs also on field ion specimens, and to study the composition of any such gold islands.

C6-466 JOURNAL DE PHYSIQUE

V - SUMMARY

Stoichiometrically correct analyses of highly doped gallium arsenide were obtained with an electric- ally pulsed energy compensated atom probe, using a specimen temperature of 45 K and a pulse amplitude of 15% of the standing voltage.

The oxide formed in dry air on a clean (field evaporated) gallium arsenide surface was found to con- sist of a 2

A

thick layer of gallium and oxygen.

On an oxidised gallium arsenide specimen, gold deposition resulted in the formation of small (20-100

A)

islands of pure gold.

GoId deposition on a clean (field evaporated) surface resulted in the formation of an intermixed gold- gallium-arsenic layer of thickness between less than one and several monolayers. Some arsenic was often found on top of this layer.

ACKNOWLEDGEMENTS

Financial support was received from the Swedish Natural Science Research Council (NFR) and the National Swedish Board for Technical Development (STU).

REFERENCES

[I] Piotrowska, A., Guivarc'h, A. and Pelous, G., Sol. State Electr. 26 (1983) 179.

[2] Sands, T., J. Metals, October (1986) 31.

[3] Chye, P.W., Lindau, I., Pianetta, P., Garner, C.M., Su, C.Y. and Spicer, W.E., Phys. Rev. B18 (1978) 5545.

[4] Keisuke, L.I.K, Watanabe, N., Narusawa, T. and Nakashima, H., J. Appl. Phys. 58 (1985) 3758.

151 Spicer, W.E., Chye, P.W., Skeath, P.R., Su, C.Y. and Lindau, I., J. Vac. Sci. Technol. 16 (1979) 1422.

[q

Tersoff, J., Phys. Rev. Lett. 52 (1984) 465,

173 Hasegawa, H., in "18th Int. Conf. Phys. Semicond.", ed. Engstriim, O., Stockholm 1986, World Scientific, Singapore (1987) p. 291.

[8] Andrkn, H. 0. and Norden, H., Scand. J. Metall. 8 (1979) 147.

[9] An&&, H. O., J. Physique 47 (1986) 0 - 4 8 3 .

[lo] Ohno, Y., Nakamura, S. and Kuroda, T., Jap. J. Appl. Phys. 17 (1978) 2013.

[ I l l Nishikawa, O., Nomura, E., Kawada, H. and Oida, K., J. de Physique 47 (1986) C2-297.

[12] Cerezo, A., Grovenor, C.R.M. and Smith, G.D.W., J. de Physique 47 (1986) C2-309.

[13] Lii, Z.M., Petro, W.G., Mahowald, P.H., Oshima, M., Lindau, I. and Spicer, W.E., J. Vac. Sci.

Technol. B1 (1983) 598.

Fig. 1 - Gallium arsenide specimen as etched (left) and ajterjield evaporation to 8.9 kV (right). Dark jeld b-am'ssion etectron micrographs.

With compensator 45 K, 15% pulse

50

-

m

z

S

LL 0

3 g

Fig. 2 -Atom probe spectra of gallium arsenide recorded during field evaporation by electric pulsing under different conditions: without energy compensator (above); with compensator at a specimen ternp- erature of 90 K and using 29% pulse amplitude (middle); with compensator at 45 K and 15% pulse (below). Evaluation of the analyses made with the energy compensator gave the following gallium con- centrations: 40.1&1.0% (90 K) and 49.7fl.5% (45 K)

0 . . As2+

With compeqsator 90 K, 29% pulse

As2+

As32+

L..

^As3+

L

_1-

z

100

-

50

-

0 -

GaH+

'

'"""" ^I

Ga-H20+

. .

I , ^{.. .}

^,

.1. ^.

¹

35 70 105 140

t 4 - -

225

JOURNAL DE PHYSIQUE

Fig. 3

-

Gallium arsenide specimen ajter several depositions of gold. Small gold islands have formed on the shank of the specimen, where the oxide had not been removed by freld evaporation. Trans- mission electron micrograph.

0

a , . , -

^-1-,,.

S 750. 1500. 2250.

NUMBER OF IONS

Fig. 4 - Composition profile constructed from an atom probe analysis of a several monolayers thick intermixed layer on gallium- arsenide. About I0 at.% of gold and some oxygen were recorded when analysing the approximately 8

A

thick layer.