AN ATOM-PROBE STUDY OF SEMICONDUCTOR-METAL INTERFACES

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AN ATOM-PROBE STUDY OF

SEMICONDUCTOR-METAL INTERFACES

A. Jimbo, T. Hashizume, T. Sakata, T. Sakurai

To cite this version:

A. Jimbo, T. Hashizume, T. Sakata, T. Sakurai. AN ATOM-PROBE STUDY OF

SEMICONDUCTOR-METAL INTERFACES. Journal de Physique Colloques, 1986, 47 (C2), pp.C2-

321-C2-327. �10.1051/jphyscol:1986249�. �jpa-00225683�

JOURNAL DE PHYSIQUE

Colloque C 2 , supplement au n03, Tome 47, mars 1986 page c2-321

W ATOM-PROBE STUDY OF SEMICONDUCTOR-METAL INTERFACES

A. JIMBO, T. HASHIZUME, T. SAKATA' and T. SAKURAI

The Institute for Solid State Physics, The University of Tokyo, Minato-ku, Tokyo, Japan

'College of A r t and Science, Osaka Prefecture University, Osaka, Japan

Abstract - A ToF atom-probe study of GaAs-metal (Ti, Pd, Ni etc.) interfaces were carried out using both high-voltage and laser pulses.

I - INTRODUCTION

The movement of semiconductor and metal atoms across a

metal-semiconductor interface has been investigated intensively/l/.

The interdiffusion is of particular importance for compound

semiconductors such as GaAs since it is directly connected with the elecronic properties of the interface, namely the Schottky barrier formation/2/. It has been said that significant interdiffusion takes place at the interfaces exhibiting no strong chemical reaction/3/.

There have been, however, few truly microscopic investigations on the interfaces. A high-performance atom-probe should be an ideal

technique to study such a system on a truly atomic scale, since it enables us to view the surface atomic structure and to determine the chemical composition without any assumption at a11/4/. Laser-pulsed evaporation, instead of high-voltage pulse trigger, may add an

additional flexibilty and power to the atom-probe study of semiconductors since high resistivity of the semiconductors now causes no problem/5/.

I1 - EXPERIMENTAL

GaAs tips with various orientations were prepared as follows:

At first the rod of 1 X lmm was cut from a GaAs wafer commercially available (from Sumitomo Electric Industries., Itami, Japan). Those GaAs wafers are Si doped and have the resisitivity of 10-3 -cm.

The rod specimen was chemically etched into a sharp needle-shape tip by H2SO4 + H202 + H ~ O / ~ / .

A clean tip surface was obtained by field evaporation in hydrogen gas at 20 K and several kinds of metals were deposited on it for the interface study. The metals studied are Pd, Ti, and Ni and the deposition was performed by evaporation of the metal from a W filament. After deposition of the metal(s), the tip was heated to achieve an equilibriated interface by joule heating of the 140 loop in-situ of the main FIM chamber. In the cases of Ni and Pd, we have

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986249

JOURNAL DE PHYSIQUE

found that islands were formed by heat treatments, therefore the analysis by an atom-probe were made after observing F1 image and repositioning the probe-hole onto the interface. In other case, the metals were found to be deposited uniformly on the surface. Thus no adjustment of the probe-hole was necessary once the probing area was selected during the F1 observation.

The focusing-type ToF atom-probe used in this experiment has the unique capability of the 1 0 0 % detectability and is highly suitable for the analysis of the compositions of surfaces and interfaces, where the number of atoms of interest is limited and small in some cases/7, 8/ (see Ref. 8 for the details of the discussion on the 1 0 0 % detection efficiency).

I11 - RESULT I: METAL/G~AS

(A)GaAs-Pd system: The most reactive metal among those metals studied (Pd, Al., Ni and Ti) is Pd. We have found that Pd begins to react with GaAs even at 50K. Fig. 1 shows the depth profile of as-deposited ~ d / G a ~ s surface. The cumulative number of detected ions is plotted against the number of applied trigger pulses. The

probe-hole was set at the (111) plane and the layer-by-layer field

Depth p r o f i l e of Ga/As/Pd

n Pd a s d e p o s i t e d on G a A s ( l l l )

W I--

0

Probed a t 4 5 ~ i n 4 x l 0 - ' ~ o r r H, Fig. 1. Depth profile

W

k-

of the interface of

W

a ^Pd/GaAs without any

6-2 heat treatment.

Z

,--,

L

W

m X 3 Z

NUMBER OF PULSES ⁹⁶⁰⁰⁰

evaporation is evident in this figure. The (111) surface of GaAs is constructed by double-layer of pure Ga and pure As atoms. Ga and As atoms are indeed detected in this sequence in the case of the

atom-probe analysis of a clean GaAs surface. However upon the deposition of Pd, the outermost surface layer always consisted of As instead of Ga. Pd was not found at the topmost layers, but beneath this As layer, suggesting that Pd atoms begin to diffuse into the bulk, substitutionally with As atoms. This interdiffusion became more evident when the system is heated. After annealing this surface at approximately 470K

2 0 0 ° C ) for 1 0 min, Ga and Pd were detected in 1:l ratio without any trace of As atoms at the surface

(Fig. 2). The F1 images of this surface suggest that GaPd layer formed after annealing has a sturucture similar to that of GaAs.

GaPd compound layers also evaporate by double-layer as is seen in Fig. 2. The interface between the newly formed GaPd phase and GaAs substrate is completely abrupt without any mixing of As and Pd.

Annealing at a higher temperature, such as 680K ( % 4 1 O o C ) , or for a longer period, has resulted in a slightly different depth profile

(Fig. 3). The topmost region consists of mainly Ga and a small

F i g . 2. Depth p r o f i l e of t h e i n t e r f a c e of Pd d e p o s i t e d on GaAs upon h e a t i n g a t 200 C f o r 1 0 min. The f o r m a t i o n of GaPd p h a s e i s e v i d e n t .

4

a U

\

m -X

1 1

O NUMBER OF Ga8As8Pd ATOMS 200 Pd/GaAs

Annealed a t 200'C f o r 10 m i t i

amount of Pd and As. The next region beneath Ga layer is GaPd phase and almost no As is seen, which interfaces with the original GaAs.

At much higher annealing temperatures, such as 900K (

630"~), The topmost layer becomes GaPd phase, resulting in G a ~ d / mixed-region/

GaAs .

\ ^<f'.

~d

PdGa--GaAs

,-..=*

c l

L L i.~-'

0 ^'

cn X

0 .

Depth p r o f ~ l e o f Ga. As K Pd m

4

Annealing et 6BOK(410'C) F i g . 3 . Depth p r o f i l e

' .

m f o r 10 mir. i n vac. o f t h e i n t e r f a c e of Pd

a on GaAs upon h e a t i n g

' .

U a t 410-C f o r 10 min.

a

Pd d i f f u s e s i n t o t h e

!.L

0 '

bulk.

0 NUMBER OF TOTAL ATOMS 660

These observations strongly suggest the substitutional

interdiffusion of Pd with As (Fig. 4). The behavior of the mixing of Pd deposited on GaAs surface is schematically des,cribed as a function of annealing temperature, (To < T1 < T2J. Annealing time, t, is also plotted on the same axis. At the lower temperature, To, Pd diffuses gradually into GaAs substrate and substitutionlly replaces As. At a slightly higher temperature, Tl, this reaction takes place more rigorously and GaPd compound is formed. And As atoms, which were substituted by Pd, duffuse to the surface. For a longer annealing period at the same annealing

temperature, interdiffusion of Pd proceeds into the bulk leaving even

~ 2 - 3 2 4 JOURNAL DE PHYSIQUE

Ga behind. As atoms which diffused the surface readily evaporated because of its higher vopor pressure. At a

higher annealing temperature, D e p o s i t i o n o f Pd on GaAs T2 (T2 _>

~ 1 ) , ^{Ga atoms,} Temp. 0

left behind bv pd _{[ .} ^I

(CIGaAs-Ti system: A

surprising observation is that interdif fusion, also thermally

evaporate at the surface. To This mechanism is interestingly

similar to the behavior of A1 on the surface of GaAs, where A1 diffuses into the bulk

substitutionally not with As T~

but with ~a/9/.

(B)GaAs-Ni system: In the case of Ni deposition on GaAs,

annealing above 470K resulted ^T~

in the formation of many islands/lO/. The thickness of the island is large, amounting

to as much as -30-40 times the l., deposition. The analysis by

the atom-probe showed that the islands are Ni2(Ga, As).

annealing up to approximately

1130K caused no mixing at all ^t To<T, <T2 when Ti was deposited. The

surface layer consisted only of

Ti atoms. Mixing of Ti with _{Fig. 4.} Schematic of the mixing GaAs was noticed only after behaviour of Pd deposited on GaAs annealing at 1130K. Under this as a function of heating temperature condition, the surface layer and heating duration. Two underlying consisted of a mixture of Ti, mechanisms are (1) Pd substitutes As Ga and As, roughly 1: 1: 1, and (2) As desorbs from the surface.

suggesting that interdiffusion of Ti begins only at this temperature11 l 1.

(D)Co-deposition of two metal elements on GaAs: When two species of metals are deposited together, the behavior of mixing is not

straightforward and fairly complicated.

In the case of Ti and Pd, Ti always remained on the top of the surface regardless of the order of the deposition of two elements.

Ti was observed right at the surface, upon annealing at 670K ( * 400 C) for 5 min., even Pd was deposited on the top of Ti (Fig. 5). There were only few Pd atoms in this region and almost all Pd were

accumulated in the region beneath. It is interesting to note that in this second region the number of Pd atoms is roughly equal to the difference between Ga atoms and As, suggesting the formation of Ga(Pd,, Asl-,). Since there were roughly the equal amount of Ga and As atoms in the surface layer, some As must have evaporated from the surface at 400°C in the presence of Pd and Ti.

In the case of Ni and Pd, Pd is not able to diffuse through Ni sublayers. There appear three regions distinctly different in composition when Ni and Pd are deposited in this order and annealed at 620 K ( % 3 5 0 " ~ ) for 30 min.: from the surface to the bulk, Pd rich region, Ni rich region and GaAs substrate (Fig. 6). In the first region, Pd and Ga were found in a ratio of 1 : 2 and small number of As and Pd only toward the interface. In the second region, where no

( Pd L a A s

-:,c,;

. , . .. L P d i G a A s As

, ,

GaPd GaAs

$:..L I

A'[ Ga

: ~ a GaAs ~ d

;,?:-+- ,

! I ^!

of the diffusivity of the metal must also be considered.

The melting temperature of a compound is a measure of the stability and strength of the chemical bond of the compound.

Table I lists some of the compounds of our interest and Pd atoms were

detected, Ni signal 400.

their melting temperature.

is approximately equal to the sum of

L 0 NUMBER OF TOTAL ATOMS

Deposit i o n o f T L Pd

I I

Fig. 6. CO-deposition of Pd and Ni on GaAs.

Ga and As Annealing a t 400'C _!or 5 min i

signals. As soon as Ni started to m

deplete, Ga X

appeared and became even stronger than

As signal. This

depth profile may o suggest that Ni

reacts with As

_m first and diffuses 5

into the bulk, Z

leaving Ga atoms at the surface and

those Ga atoms left

.

alone may react 0 NUMBER OF TOTAL ATOMS 1000 with Pd atoms.

IV - DISCUSSION Fig. 5. CO-deposition of Pd and Ti on the GaAs surface.

The reactivity of a metal at the GaAs surface can be 1000

measured by the Deposit i o n of N ^L 8 Pd

interface. The rate t a

The temperatures at which reaction begins to take place at the interface are 50K, 620K and 1130K for Pd, Ni and Ti, respectively, suggesting that Pd

reacts most strongly. Our data also suggest that Ga tends to form a compound with Ga while A1 forms with As. Ti and Ni appear to form compounds with both Ga and As. These results should be combined with those on Schottky barrier formation in order to advance our knowledge of the metal-semiconductor interfaces.

V - Result 11: LASER-PULSED DATA

~t is difficult, as we had reported at last International

FESymposium, Paris, France, to obtain a stoicheometrically correct

composition using a conventional high voltage pulse mode, although it

can be done by introducing a small amount of hydrogen gas in the

system during atom-probe analysis/9/. We also reported that

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deposition of thin layers of a metal

(Ti, Au etc.) on GaAs( I I I )

the semiconductor

emitter promotes W VE =9.0-9.8KV

ordinary field

eva~oration from Q PULSE RATIO=O. 08

kink sites, suggesting that high resistivity of the semiconductor emitter may prevent a proper

transmission of high-voltage nanosecond pulses.

Thus we have attempted

laser-pulse

ROOM TEMP.

;Gat

!+ 3 ~ 1 0 - ~ Torr

MASS TO CHARGE RATIO (m/n) evaporation using

GaAs emitters. The Fig. 7 (a). Atom-probe mass histogram of GaAs(ll1)' lasers we used are surface by an ordinary high-voltage pulse mode.

3330 NZ laser (1 mJ Ionsec) and 5030 YAG laser (a few mJ, l Onsec).

Fig. 7 (a) and (b) compare the mass histograms from the

(111) surface obtained by ordinally

high-voltage pulse and laser-pulse modes. Fig. 7(a) is obtained using the following

parameters: pulse ratio = 0.08, tip temperature = R.T., ambient gas = 3 X 1 o - ~ ^{~ o r r} H2.

Fiq. 7(b) is under MASS TO CHARGE RATIO ( m / n ) a similar condition

except that the ^{Fig. 7 ( b ) .} Atom-probe mass histogram o f GaAs(ll1) high-voltage pulse surface by a laser pulse mode. Large As clusters was replaced by are detected abundantly.

laser-pulse. One significant

difference one can immediately notice is that the laser pulse evaporation promotes the evaporation of cluster ions such as As4+. Comparing with the spectra obtained by high-voltage pulse, the spectra in a laser-pulse mode have a slightly larger

distribution, but is still good enough for the presice analysis of semiconductors. Fig. 8 shows the cumulative numbers of Ga and As signals against the cumulative number of trigger pulses for laser pulses. We have demonstrated that the high-voltage trigger mode resulted in the depletion of As signal ( Ga / ~ a + As = 0.6 ) , even though the sample used is identical for both runs, while the laser-pulse mode yielded the stoicheometrically correct Ga

concentration ( close to 50at% ). One can also observe the orderly

layer-by-layer evaporation in the ladder-shape structures. Of

course the outcome depends on the intensity of the laser beam

ase er-pulsed

analysis appeares to be promising to yield a stoicheometrically correct composition even in the case of compound semiconductors.

REFERENCES

/l/ L. J. Brillson, G. M. Margaritondo, and N. G. Stoffel, Phys.

Rev. Lett., 44, 667 (1980).

/2/ J. E. Rowe, G. Margaritondo, and S. B. Christman, Phys. Rev.

m, 2195 (1977).

131 G. Margaritondo, J. E. Rowe, J. E. Rowe and S . B. Christman, Phys. Reb. G , 5396 (1976).

1 4 1 E. W. Mueller, and T. T. Tsong, Progress in Surface Science,

Q, 1 (1973).

151 T. Sakurai, Surf. Sci. 86, 562 (1979).

/6/ T. Sakurai, T. Sakata and A. Jimbo, Japan J. Appl. Phys. 22, L775 (1983).

/7/ T. Sakurai, T. Hashizume and A. Jimbo, J. de Physiq., 45,

C9-343 ( 1 984).

181 T. Sakurai and T. Hashizume, Rev. Sci. Instrum., submitted.

and a paper by T. Sakurai and T. Hashizume, in this Proceedings.

/g/ 0. Nishikawa, 0. Kaneda, M. Shibata, and E. Nomura, Phys.

Rev. Lett. 53, 1252 (1984).

/l01 0. Nishikawa et al. Proceedings of the 30th Intern. Field Emission Symposium, Philadelphia, USA August (1983).

/11/ T. Sakurai, T. Hashizume. A. Jimbo and T. Sakata, J. de Physiq. 45, C9-453 (1984).

relative to the DC holding field. Ga 300

intensity becomes even stronger than that of As if the z laser becomes very 0

intense. Ga signal

becomes less L

intense if the c3

laser power is

reduced and it ^{I Y} W approaches to the m

DC evaporation. E 3 Z V1 - CONCLUSION

We have studied

the compositional 0, ⁷ Jdf---

changes at the

metal-semiconductor 0 NUMBER OF PULSES 180000

interfaces using

the focusing-type ^Fig. 8. Compositional depth profile at GaAs(ll1) using a

ToF atom-probe. laser-pulse mode, yielding roughly 1:l ratio in Ga and As.