J Mater Sci: Mater Electron (2006) 17: 321–324

DOI 10.1007/s10854-006-6951-z

Fractal character of in situ heat treated metal-compoundsemiconductor contactsL. David · L. Dobos · B. Kovacs · I. Mojzes · B. Pecz

Received: 26 August 2005 / Accepted: 30 September 2005C© Springer Science + Business Media, LLC 2006

Abstract The heat treatment of metallized (Au) compound

semiconductors (InP) was studied by in situ scanning electron

microscopy combined with mass spectrometry. Correlation

was found between the change in the surface morphology and

the volatile component loss caused by the material interac-

tions taking place during the heat treatment. Our experiments

proved that the surface morphology can be characterized by

its fractal dimension at the maximum value of the volatile

component loss. In this paper the dependence of the frac-

tal dimension of the surface pattern on the heat treatment

temperature (in a given temperature range), on the volatile

component loss and on metal thickness is described. Changes

of the surface morphology on the analyzed samples begin at

different temperatures. The evaluated patterns were created

describing contour lines of the metal islands on the surface.

This island formation known as balling-up phenomenon is

due to the heating up of the samples. In the case of the 10 and

30 nm Au/InP(100) samples the fractal behavior appeared

nearly at the same temperature (470◦C ÷ 490◦C). The ex-

amined thick Au(85 nm)/InP(100) contact showed a fractal

character at a lower (385◦C) temperature. Although fractal

dimension values could be obtained in a rather wide tempera-

ture range the surface had a real fractal character at only those

temperatures where volatile component loss took place.

L. David (�)Institute of Microelectronics and Technology, Kando KalmanPolytechnic, P.O.B. 112 Budapest H-1431, Hungary

L. Dobos . B. PeczResearch Institute for Technical Physics and Materials Science,Hungarian Academy of Sciences, P.O.B. 49 Budapest H-1525,Hungary

B. Kovacs . I. MojzesDept. of Electronics Technology Budapest University ofTechnology and Economics, Budapest H-1521, Hungary

1. Introduction

Heat treatment of metal–compound semiconductor struc-

tures is an important step of the compound semiconductor

device technology [1]. The relationship between the surface

morphology and the volatile component loss during the heat

treatment of the Au/InP contacts was analyzed using in situ

scanning electron microscope equipped with a quadrupole

mass spectrometer [2].

There is an interaction between the components of the

compound semiconductor (InP) and the thin metallic film

(Au). Table 1 shows the phases identified by other authors

at a given temperature range. During annealing the con-

tact layer of the sample is enriched with the AIII com-

ponent (In) while the BV component (P) evaporates. This

volatile component loss is very high at certain characteristic

temperatures.

The surface morphology of the samples undergoes a dras-

tic transformation reaching a characteristic temperature cor-

responding to the intensity peaks of volatile component loss

vs. temperature curve [2]. The temperature and the inten-

sity of the peak of volatile component loss are function of

the thickness of the evaporated metal layer [3]. The surface

changes at characteristic temperatures can be described by

fractal mathematics [4, 5]. In our laboratory a method was de-

veloped and used characterizing the surface with the contour

lines of metal drops [6].

In this paper the relationship between the fractal di-

mension of surface morphology and the volatile compo-

nent loss during heat treatment of Au/InP contacts was an-

alyzed. The investigated samples were heat treated in the

vacuum chamber of a SEM equipped with a mass spec-

trometer measuring the volatile component loss during the

heat treatment while the surface was fixed at characteristic

temperatures. Moreover, the effect of the metal layer

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322 J Mater Sci: Mater Electron (2006) 17: 321–324

Table 1 Summary of the phase analysis for gold contacts on InP

Metallization and thickness (nm) Semiconductor Technology Annealing Identified phase References

Au(45nm) InP(100) TEM, SEM, RBS, 320◦C, 30 min α-Au(InP) Piotrowska

(1981)

X-ray diffraction 320◦C, 90 min α-Au(InP),

Au3In, Au2P3

320◦C, Au3In, Au2P3

120 min

360◦C, Au3In, Au2P3

15–120 min

Au(45nm) InP(100) TEM, SEM, RBS, ≥ 400◦C Au2P3, AuaInb Piotrowska

(1982)

X-ray diffraction

Au(50nm) InP(100) SIMS, SEM, RBS 320◦C, Au2P3+Au3In Piotrowska

120 min (1985)

420◦C,

3 min Au2P3+Au9In4

Au(100nm) X-ray diffraction 320–340◦C α-Au(In) Vandenberg

(1983)

335–420◦C Au4In

390–460◦C Au9In4

450◦C AuIn2

320–400◦C, Au3In, Au2P3 Auvray (1985)

120 min

Au(1500A–2.2μm) InP(001) SIMS, SEM, AES 450◦C, Au9In4, Au2P3

15–120 min

> 450◦C Au3In, Au2P3 Wada (1985)

X-ray diffraction < 450◦C AuIn2

Au(55nm) InP(100) SEM, AES, QMS 350–500◦C Au2P3 Veresegyhazy

(1990)

450–510◦C, AuIn2, Au7In3,

85 min Au9In4, γ -AuIn

Au(2000A) InP(100) SEM, XPS 470◦C Au2P3 →P loss Fatemi (1989)

InP(100) XPS 340-450◦C, Au3In, Au9In4 Weizer (1990)

10s (Au2P3 no

found)

Au(100nm) InP(111) TEM 375-475oC Au9In4, Au2P3 Pecz (1991,

1992a)

Au(100nm) InP(111) TEM, X-ray diffr. 500◦C AuIn2, Au9In4, Pecz (1992b)

thickness on the fractal dimension values obtained was also

described.

2. Experimental details

Single crystalline (100) InP wafers doped by Sn with the

carrier concentration of 1×1024 m−3 were used as substrates.

After mechanical and chemical polishing to a mirror-like

surface, the wafers were chemically cleaned and etched [2].

10 nm, 30 nm, 85 nm thick gold metallization were deposited

in a vacuum evaporator at a pressure lower than 10−4 Pa.

The investigated samples were annealed up to 630◦C

in a JEOL JSM-T20 type SEM (Scanning Electron Mi-

croscopy) applying a home-made heater developed by the

authors [2]. The heating rate was about 30◦C/min. The

volatile component loss during the heat treatment was mon-

itored by a quadrupole mass spectrometer and SEM-BEI

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J Mater Sci: Mater Electron (2006) 17: 321–324 323

(Backscattered Electron Image) pictures of the surface mor-

phology were taken at every 10 centigrades from 350◦C to

630◦C.

The contour-line of the surface patterns were obtained by

digitizing and thresholding the SEM-BEI pictures [6]. The

fractal dimension of the pattern was obtained applying the

box counting method. In this case, the slope of the ln-ln plot

of average pixel number (M) of the pattern versus linear box

size (L) gives the fractal dimension value [4]. The sizes of

the investigated surface regions were 229 μm × 178 μm,

900 × 700 pixels. The magnification was 500, the resolution

of the digitizer was 78.74 pixel/cm (200 pixel/inch).

3. Results

In this work Au(10 nm)/InP(100), Au(30 nm)/InP(100),

Au(85 nm)/InP(100) samples were investigated. Change in

the surface morphology in the analyzed samples began at

different temperatures.

During the in situ heat treatment of the Au(10

nm)/InP(100) samples up to 350◦C the surface was intact,

no special morphology change was observed. AuaInb drops

[7–11] covered the surface (Fig. 1(a)) approximately with the

same sizes in the temperature range of 350◦C ÷ 460◦C. In

this temperature range there was no phosphorus loss (Fig. 2).

The absence of the phosphorus loss showed that the formed

Au2P3 crystalline phase [10 ,12] did not dissolve yet.

Although the obtained values of the fractal dimension

were slightly smaller than 2 (Fig. 2) the second condition,

the existence of the self-similarity, was not fulfilled since

the examined surface was almost totally covered with uni-

form small AuaInb islands (Table 1). It means that the fractal

character is absent.

Phosphorus evaporation from the sample was detected

(Fig. 2) and the Au2P3 compound formed [13] above 460◦C,

which means the InP–metal interface decomposes. Differ-

ent sizes of AuaInb (Table 1) drops formed on the surface

(Fig. 1(b)), they were self-similar, and the obtained value of

the fractal dimension decreased to 1.6 (Fig. 2). In summary,

the appearance of the real fractal pattern coincided with the

beginning of the volatile component loss.

During the heat treatment of the Au(30 nm)/InP(100)

samples the phenomenon was very similar. In the lowest tem-

perature range (25◦C ÷ 380◦C) the continuous metal layer

does not change. Between 380◦C ÷ 480◦C approximately

equal-sized AuaInb drops covered the surface (Fig. 1(c)). In

this sample the original metal layer was thicker (30 nm),

the drops were somewhat larger and the average distance

between the closest points of their respective circumference

was also larger than in the case of 10 nm Au. Volatile com-

ponent loss did not take place in this temperature range (Fig.

2). The self-similarity condition was not fulfilled, in this way

Fig. 1 SEM-BEI photos of the in situ heat treated Au/InP samples.Magnification 500, angle 0◦. (a) (10 nm)Au/InP(100) surface at 450◦C;(b) (10 nm)Au/InP(100) surface at 500◦C; (c) (30 nm)Au/InP(100)surface at 450◦C; (d) (30 nm)Au/InP(100) surface at 500◦C; (e) (85nm)Au/InP(100) surface at 450◦C; (f) (85 nm)Au/InP(100) surface at500◦C

the obtained fractal dimension values were nearly equal to 2,

which does not indicate real fractals (Fig. 2).

The AuaInb drops become larger above 490◦C (Fig. 1(d))

due to condensation, consequently their density decreased

while the fractal dimension decreased towards 1.6, as in the

case of 10 nm Au layer. Significant P evaporation was ob-

served in this temperature range. The real fractal behavior of

the Au(30 nm)/InP(100) sample appeared at the beginning

of the phosphorus loss (Fig. 2).

During the in situ heat treatment of the Au(85

nm)/InP(100) sample the volatile component loss began at

a temperature below 350◦C. The volatile component loss in-

tensity had two peaks, at 385◦C and about 470◦C (Fig. 2). At

these characteristic temperatures the lowest fractal values of

the examined sample were obtained (Fig. 2).

In the investigated temperature range the surface pattern

was self-similar as well.

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324 J Mater Sci: Mater Electron (2006) 17: 321–324

Fig. 2 Volatile component loss and fractal dimension versus heat treat-ment temperature of the investigated samples

In spite of the processes experienced in the case of 10

nm and 30 nm Au, investigating the 85 nm Au metallization

the real fractal dimension value were increasing in the whole

studied temperature range, especially above the temperature

of the second evaporation peak (Figs. 1(e) and 1(f)).

Comparing the fractal behavior of 10 nm, 30 nm, 85 nm

Au/InP structures (Fig. 2) the fractal dimension values in-

creased with the increase of the Au layer thickness above

490◦C.

4. Conclusions

By heating up the metal/compound semiconductor structures

the surface morphology transformation has two distinct steps.

At lower temperature the metal are forming drops whose sizes

are controlled by the surface and capillary tension values.

Increasing the temperature, volatile component loss is taking

place and the metal islands show a typical fractal pattern.

These patterns are the results of the interaction between the

metallization and the semiconductor compound taking place

during the heat treatment.

Acknowledgments This work was supported by the Hungarian Na-tional Scientific Foundation (OTKA) through Grant T37509.

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