GaAs interface

Nuclear Instruments and Methods North-Holland, Amsterdam

in Physics

Research

B13 (1986)

HEAVY ION RUTHERFORD BACKSCATTERING THE ALLOYED METAL/GaAs INTERFACE A. CHEVARIER,

N. CHEVARIER

and

207-212

207

ANALYSIS

M. STERN

Institut de Physique Nu&aire (and IN2P3), 69622 Villeurbanne Cedex, France

Universitt! Claude Bernard Lyon-l,

D. LAMOUCHE,

J.R.

Laboratoire

P. CLECHET,

Physico-Chimique

des Interfaces,

USED TO STUDY

MARTIN

and

43, Bd du 11 Novembre

1981,

P. PERSON

Ecole Centrale de Lyon, 69130 Ecully Cedex, France

Heavy ion backscattering analysis of the near surface Pd/GaAs and Au: IniPdiGaAs chemically prepared systems have been investigated as a function of annealing temperature. We performed RBS analysis using a 7 MeV incident energy nitrogen beam connected with time-of-flight spectrometer detection. Using RBS random spectra, such a method can resolve the composition of the alloyed deposited films (less 5 nm) and can profile As and Ga over 40 nm. The two main results obtained from such analysis are: first, chemical palladium deposition is connected with a lack of arsenic which is preferentially exchanged to gallium atoms. Second, palladium diffusion is enhanced when using Au: IniPd alloyed

contacts.

1. Introduction The possibility to obtain low resistance ohmic contacts on n-GaAs using AulPd and Au:IniPd electroless deposits has recently been demonstrated [l]. Such results are superior to those obtained with evaporated contacts and suggest important differences in interfacial structures. We performed RBS analysis in order to obtain interfacial characterisation. The aim of the present study is twofold: (i) as Pd is deposited here by a chemical displacement reaction, it seemed to be important to check whether the corresponding PdiGaAs interface is similar to the one obtained with evaporated Pd deposits, (ii) to learn about the annealing behavior of the so obtained PdlGaAs and Au:IniPdlGaAs alloyed heterostructure. Heavy ion backscattering analysis using a 7 MeV nitrogen incident beam and a time-of-flight system was performed. Such methods can resolve the composition of the alloyed deposited film and identity the arsenic and gallium contribution. The performance and the restriction of the method will be presented before considering the analysis.

2. Experimental

2.1. Sample preparation GaAs

samples

* RTC: Radiotechnique

obtained

from

Compelec,

RTC*

were

92156 Suresne,

n-type

2.2.

Heavy-ion

backscattering

The gold, palladium

France.

0168-583X/86/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)

Si-doped crystals in the 10” cm-j range. As the chemical deposition reaction was very sensitive to the surface cleanliness, great care was taken in the preparation of the wafers. The samples were first vigorously degreased with hot organic solvents using Soxhlet cycles (successively with trichloroethylene, acetone, isopropanol, methanol). After cleaning, the damaged surface layer was removed by a slight etch in a H,PO,/H,O,IH,O (I/I:25 v/v) mixture at 15°C for 1 min. The remaining native oxide layer was dissolved by dipping into a HCI/CH,OH(I:I) solution. A thin film of palladium was then deposited on GaAs using a chemical displacement reaction between the substrate and Pd*+ ions [2]: the sample was mounted onto a Teflon tip with negative photoresist (Kodak KTFR) and dipped into a stirred aqueous acidic solution containing PdCli- . We used a deposition time of 30-40 min, leading to a uniform Pd coverage [4]. For some samples, a Au: In film was deposited afterwards using an electroless Au : In plating solution that we have developed [l]. In order to better resolve the As and Ga edges on the RBS spectra, we used very thin Au: In films whose thickness might consequently be slightly inhomogeneous; the Au : In plating time was fixed to 10 min, corresponding to an average film thickness of about 1Onm in our experimental conditions. The annealing was performed in an open-tube furnace under N, atmosphere for a fixed time of 2 min.

B.V.

analysis

and arsenic

V. HIGH ENERGY

distributions

SURFACE

in the

INTERACTIONS

208

A. Chevarier et at. i

The alloyed metaiiCaAs

Pd/GaAs and Au: In/Pd/GaAs systems were investigated as functions of the annealing temperature. For this purpose we performed RBS analysis of these samples using a nitrogen beam at 7 MeV incident energy which corresponds to the maximum stopping power values. In order to optimise detection resolution we used a time-of-flight spectrometer [3]. The experiments were performed at the 4MV Van de Graaff facility of the Institut de Physique Nucleaire de Lyon. Time-of-flight measurements were performed at 150 laboratory angle using a 1.06 m flight path. The nitrogen beam impinges with normal incidence on the target. We did not take advantage of channeling effects. Targets were turned around the beam axis in order to obtain random spectra. Start and stop detectors made from thin carbon foil were combined with channel plate electron multipliers. The detection efficiency measured as a ratio of the counting rate observed in the surface barrier detector and the time-offlight spectrometer was 93% for 3-6MeV backscattered nitrogen ions. The relationship between time-offlight and energy is: t = 2kl (MlE3)“2 where k is a dimensional constant, 1 the flight path length, M the detection particle mass and E, its scattered energy. Because the mass of backscattered particles is well known, the energy determination can be deduced from time-of-flight measurements. In order to obtain energy to a spectra a transformation corresponding 2 kevichannel energy scale was performed. Energy calibration was obtained from 7MeV nitrogen backscattering measurements on gold, indium, palladium and GaAs bulk samples, The interesting features of such methods are the good depth and mass resolutions. As shown in fig. 1 the arsenic profile is probed over the first 40 nm of a GaAs virgin sample. Using the GaAs height HGaAs as a reference, the measurements of the step height H,, of arsenic allow the ratios between As and Ga to be calculated. The shaded parts in fig. 1 represent the measured areas of As and GaAs corresponding to the same energy interval: H,,=H

-H Al,1 G&.x\

where Z is the atomic number, S is the backscattered stopping cross section parameter, n the number of atoms per gram. An example of (the time) spectra observed for an Au: IniPdiGaAs sample is given in fig. 2, where the dependence of the time (and correspondent energy) are given. The cut-off observed at the low energy part of the GaAs distribution is due to the limited analysis range of the time amplifier converter. A good separation of the In and Pd distribution is obtained. It allows

t’

146

interface

144

142

1

4

140

138

L

I

TOF(ns)

3.0

3.1

3.3

3.2

E(M&)

Fig. 1. RBS spectrum from a 7 MeV N” incident particle onto a GaAs amorphous sample. The shaded parts of the spectrum show where the HAS and H,%,, areas were measured.

160

189

128

tins1 3

6

4

E(M&

Fig. 2. RBS time spectrum from the 7MeV N” particle

onto

corresponding

a Au:In/Pd/GaAs

unannealed

incident The

sample.

energy is given.

the concentration of palladium to be determined from a comparison of the Pd peak to GaAs random spectrum plateau height at a corresponding depth in the sample. Anyhow such analysis is restrictive concerning the metal overlayer thickness which can be deposited onto GaAs for two reasons. First, in order to correctly profile the arsenic the depth resolution must not be washed out by the straggling effect. Second, the mass differences between the elements (Au, In, Pd, GaAs) limit the gold and palladium distribution depths which can be examined. These depths are shown to be 450 nm for the Au distribution and 220 for the Pd one.

A. Chevarier et al. I

The alloyed metallGaAs

NwIN,,a,s (N = number of atoms g-‘) have been calculated. The results are summarized in table 1. For the virgin GaAs monocrystal we obviously find case we find NA\INCi& = 0.5 but in the PdlGaAs = 0.33 and N,+,IN,,,,, = 0.15 which leads N,,IN,,,\ one to believe that the deposition of palladium occurs by the removal of arsenic atoms preferentially to the gallium ones. Fig. 4 displays the comparison between the experimental and calculated spectra. The calculation is performed assuming a 30 pglcm’ overlayer thickness and a weighted ratio of 25% Pd and 75% GaAs. Such a ratio is deduced from the comparison of the counting rate between the RBS palladium peak in the PdiGaAs spectrum and the plateau height of bulk palladium. The quite good fit of the palladium peak confirms that the smoothness of the experimental palladium edge is mainly due to the contribution of numerous palladium isotopes. The experimental amount of As in the 40 nm depth range profile, which roughly corresponds to 30 pg cm-‘, is shown to be much lower than the calculated one. This result confirms that the deposition of palladium occurs by the removal of the arsenic atoms preferentially to the gallium ones. Such a preferential displacement has already been observed on an AuiCdTe contact [7] prepared by a similar method. Let us now discuss the case of the 360” annealed PdiGaAs sample. After annealing, only a slight Pd in-diffusion is observed. The whole palladium peak is shifted to a lower energy which points to the fact that some out-diffusion of Ga or As has taken place. That feature is underlined on fig. 5 on which superposed

The results cannot be obtained by 2 MeV alpha RBS analysis. In order to improve the mass resolution, the alpha-energy has to be increased up to 7 MeV which then worsened the depth resolution [4]. Promising results have been obtained using 30 MeV oxygen ion backscattering. In this work [5] a clear separation of Ga to As front edges is observed over 1 pm near surface depth. The only problem is that such a method requires the use of a tandem facility. For high annealing temperatures an out-diffusion of gallium takes place. The gallium is quickly oxidized as soon as the samples are put in air. This oxide layer thickness was measured using first the N2+ RBS spectra (the As front edge being shifted to lower energy), second the oxygen resonance IhO(~,a’) as is described in ref. [6].

3. Results and discussion The analysis of the spectra is done in three steps: _ comparison between RBS spectra of the unannealed samples and RBS spectra of GaAs, Pd, Au bulk samples used as references, - fit of the RBS spectra of the unannealed samples with the calculated ones, - comparison between experimental distributions for annealed and unannealed samples. PdlGaAs

3.1.

The random energy spectrum of backscattered N” ions on PdlGaAs unannealed samples is shown in fig. 3. The overlap of Ga and As energy position edges between RBS spectra on Pd/GaAs and virgin GaAs suggests there is no well-defined PdlGaAs layered structure but more likely an indetermined PdiGaAs overlayer. The arsenic profile from the Pd/GaAs spectrum provides evidence for a lack of arsenic connected to palladium deposition. The ratios N,,IN,;,,, and I -__-

(a.u.

Table 1 As/GaAs

and Pd/GaAs

atomic ratio in the surface laver

Atomic ratio

Monocrystal virgin sample

PdiGaAs unannealed

PdiGaAs 360°C annealed

NJN,;,,> N,.,lN,,....

0.50 f 0.01

0.33 + 0.02 0.15 t- 0.03

0.22 +-0.02 0.23 + 0.03

I

I

1

N

209

interface

Bulk GaAs

Pd

x0.25

virgin

Pd/ GaAs

monocryst

unannealed

5

E(MeV)

Fig. 3. Comparison between RBS spectra from a 7 MeV N ” incident particle onto a PdiGaAs unannealed sample and onto Pd and GaAs bulk samples. V. HIGH ENERGY SURFACE

INTERACTIONS

210

A. Chevarier

et al.

I

The alloyed

I

I

I

3

4

5

Fig. 4. Comparison between experimental

from virgin Pd/GaAs and 360” PdiGaAs anneafed samples are represented on a logarithmic scale. The energy position of the gallium front edge argues out-diffusion for gallium. As the gallium is known to quickly oxidize we confirm the gallium out-diffusion by oxygen profiling of the gallium oxide 181. To summarize, the RBS anaiysis gives evidence that the chemical deposition of Pd is connected with a lack of arsenic which is preferentially exchanged and that an out-diffusion of gallium afterwards readily oxidized, takes place after 360°C annealing.

3.2. Au : 1niPdJGaAs The random energy spectra of backscattered N2’ ions from Au:In/Pd/GaAs unannealed samples are shown in fig. 6. The gold, Pd, In, As, Ga position edges suggest one more time that we are dealing with an intermixed overlayer. The amount of deposited indium is small and the proximity of the Pd peak does not allow information to be obtained from RBS analysis of the indium behavior. The experimental spectrum is compared to the calculated one assuming a 10 @g/cm’

.. .. . .. . .._.

4

interface

w E (Met!)

and calcuiated spectra concerning a PdiGaAs

spectra

3

~et~~iGaAs

unannealed sample.

overlayer on bulk GaAs (fig. 7). The respective weighted ratios are 1% In, 25% Au, 4% Pd, 70% GaAs. The calculated arsenic profile is then divided in two steps: the first one corresponds to the 10 @g/cm’ overlayer, the second one to the bulk GaAs. This feature explains the smoothed front edge of the experimental GaAs spectrum and prevents us from obtaining more information on the Ga to As ratio. Concerning the gold peak a small low energy tail confirms that a slight gold diffusSian occurs even at room temperature [9]. Information on gold and palladium annealing behavior is obtained from comparison of the RBS spectra on unannealed and as-deposited Au : In/Pd/GaAs samples (fig. 8). The nitrogen RBS spectra on Au: IniPdiGaAs after 360°C annealing (fig. 8a) shows that gallium does not appear at the sample surface as we noticed on the PdiGaAs structure annealed under the same conditions. Such an observation is confirmed by the low surface oxygen content which is less than 1016 atoms/cm* and by the fact that less than 1.7 nm Ga,O, is evident. Moreover, a large diffusion of palladium is observed which did not take place in the Pd/GaAs case.

360°C

5

annealed

E (WV)

Fig. 5. RRS spectra from a 7 MeV N 2c incident particle onto virgin PdlGaAs and 360°C PdlGaAs annealed samples (logarithmic scale).

A. Chevarier et al. I

211

The alloyed meta1lGaA.y interface

N-

(a.u.)

I

I

1

3

4

5

Fig. 6. Comparison between In, Pd, GaAs bulk samples.

I

N

RBS spectra

I

I

from a 7 MeV N”

incident

particle

b

E( MeV)

onto Au: IniPdiGaAs

I

I

unannealed

samples

and Au,

I

.

.

. .

. . . . .

.

. : .

b

. . .

3 Fig. 7. Comparison

5

4 between

experimental

and calculated

spectra

Analysis of RBS spectra on the 550°C annealed Au:InlPdiGaAs sample shows that a large Ga outdiffusion occurs: the large shift of the RBS energy gold peak is connected with the observation of an extra amount of gallium near the sample surface (dashed part of the spectra) (fig. 8c) and also with a large oxygen contamination corresponding to 6 x lO’(’ atoms/cm’ (i.e. 10nm Ga,O,). The 450°C annealed sample corresponds to an intermediate case (fig. 8b) where we already notice a slight shift on the gold front edge. In summary the presence of an Au: In overlayer enhances palladium diffusion and prevents surface oxidation up to 360°C annealing. The gold and pal-

concerning

*

E(MeV)

a Au :IniPd/GaAs

unannealed

sample.

ladium diffusion depth scales are given in table 2. They are determined by assuming that a small amount of metal diffuses in GaAs and therefore the stopping power of GaAs can be used. The increase of palladium diffusion in the Au : InlPdlGaAs case can be explained by the enhancement of the following reaction rate: Pd + 2( GaAs) + (PdAs,) initiated

+ 2 Ga

by the well known

Au + GaAsT Complementary which allows

(AuGa)

process: + (OAs)

information from the ESCA technique the study of the chemical compound

V. HIGH

ENERGY

SURFACE

INTERACTIONS

A. Chevarier et al. I

212

The alloyed meraliGaAs structure, this

interface

are

reaction

planned.

They

are

necessary

to support

scheme.

4. Conclusion

PdiGaAs and Au: IniPdlGaAs chemically prepared systems have been analysed as functions of annealing temperature with heavy ion backscattering measurements. The results can be summarized as follows: the chemical deposition of palladium is connected with arsenic deficiency. The annealing of PdiGaAs samples gives a slight palladium in-diffusion and a gallium out-diffusion which is connected with oxidation. In the Au : InlPdiGaAs case, the presence of gold does help palladium in-diffusion and prevents gallium out-diffusion. This feature may probably explain why the Au: IniPdiGaAs structure leads to a lower resistive contact than the PdiGaAs one.

References 3

4

5

E (MeV)

Fig. 8. Comparison between RBS spectra from a 7 MeV N*’ incident particle onto unannealed and annealed Au :Ini Pd/GaAs samples. A logarithmic scale is chosen to emphasize the diffusion process. Table 2 Gold and palladium C” of *lo% Sample Annealing temperature Palladium diffusion depth (nm) Gold diffusion deoth (nm)

diffusion

depths

estimated

PdlGaAs

Au:In/PdlGaAs

360°C

360°C

62

with an accura-

450°C

550°C

96

110

220

170

225

450

111 D. Lamouche,

These Doct.-Ing., Lyon (1984). L.A. D’Asaro, S. Nakahara and Y. Okinaka, .I. Electrothem. Sot. 127 (1980) 1935. Nucl. Instr. and Meth. [31 A. Chevarier and N. Chevarier, 218 (1983) 1. [41 J.P. Thomas, M. Fallavier, H. Carchano and L. Alimoussa, Nucl. Instr. and Meth. 218 (1983) 579. [51 M. Gstling and C.S. Peterson, Nucl. Instr. and Meth. B4 (1984) 88. VI S. Peterson, H. Norde, G. Possnert and B. Orre, Nucl. Instr. and Meth. 149 (1978) 285. [71 A.M. Mancini, A. Quirini, L. Vasanelli, E. Perillo, E. Rosato, G. Spadaccini and E. Barbarino, J. Appl. Phys. 53 (1982) 5785. A. Chevarier, N. Chevarier, P. Clechet, 181 D. Lamouche, J.R. Martin, P. Person and M. Stern, submitted to Thin Solid Films. [9] J. Gyulai, J.W. Mayer, V Rodriguez, A.Y.C. Hu and H.J. Gopen, J. Appl. Phys. 42 (1971) 3578.

PI