Positron beam profiling study of the metal-GaAs interface

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Applied Surface Science 116 (1997) 256-262

Positron beam profiling study of the metal-GaAs interface C.C. Ling a,*, H.M, Weng b, Y.F. Hu a, C.D. Beling a, S. Fung a a Department of Physics, The University ofHong Kong, Pokfulam Road, Hong Kong b Department of Modern Physics, The University of Science and Technology of China, Hefei, PR China

Received 2 June 1996; accepted 15 July 1996

Abstract

Properties of metal contacts on III-V semiconductors are dependent on the electronegativity of the metal. Generally metals have been classified into three categories according to their behaviour upon heat treatment. Au, Ni and W are the three metals belonging to different categories with the descending order of electronegativity. In this study a low energy positron beam has been used to investigate as-grown Au/GaAs (SI), Ni/GaAs (SI) and W/GaAs (SI) samples with different thicknesses of metal overlayers. The method that has been employed is that of monitoring the Doppler broadening of annihilation radiation through S parameter measurements as a function of the beam energy. The S - E data were analysed by the program VEPFIT with different models and different implantation profiles. It is found that the interfacial information able to be extracted from the fitting is limited by present uncertainties in the implantation profile.

1. Introduction

It is found that metals making contact with GaAs have different reactions according to their electronegativity [1]. For metals with highest electronegativity (e.g. Au, Ag and Cu), the annealing causes Ga outdiffusion into the metal. In the case of metals with moderate electronegativity (e.g. Ni, Pd and Pt), the metal forms stable compounds with both Ga and As upon annealing. For metals with small electronegativity (e.g. W, AI and Cr), the interface is fairly stable upon annealing. Conventional techniques, like Auger electron spectroscopy (AES), have been employed to study the atomic interdiffusion at

* Corresponding author. Tel.: +852-2859-2358; fax: +8522559-9152; e-mail: [email protected].

the interface by obtaining the chemical profile of the system [2]. Such methods usually involve physical damage to the sample because they rely on Ar ion sputtering to successively remove material from the exposed surface. Moreover the sputtering process can also disrupt the atomic distribution [3]. Positron annihilation spectroscopy (PAS), however, because of its sensitivity towards open volume defects, has recently been employed as a non-destructive probe to investigate the metal-semiconductor interface [4-6]. Ling et al. [7,8] have recently investigated the A1/GaAs and A u / G a A s systems with different thicknesses of metal films and under different annealing conditions by measuring the S parameter as a function of the positron implantation energy. The measured data were found to be well fitted by the positron absorbing extended interface (PAEI) model consisting of three layers, namely the metal over-

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C.C. Ling et al. /

Applied SurfaceScience 116 (1997)256-262

layer, an extended interfacial region (which is perfectly absorbing to positrons) and the GaAs bulk region. Thus, while elemental specificity was not available from such studies some information on the physical structure of the interface was extractable. The present study is a continuation of this previous work, in which S - E measurements have been performed on the A u / G a A s , N i / G a A s and W / G a A s interfaces. Its motivation behind lies in the fact that, since these systems belong to three different categories of electronegativity on the classification scheme of Sinha and Poate [1], a depth profiling PAS study might independently reveal the different degrees of atomic intermixing.

2. Experimental The GaAs (100) wafers used in this study were CZ grown single crystals obtained from Atraomet (USA). The material was semi-insulating with a resistivity larger than 2 X 107 ~-~ cm. The wafer was cut into substrates having dimensions of 1 cm X 1 cm. These were then degreased with acetone and ethanol, and chemically etched by NH4OH:H202:H20 (1 : 1:10) and H2SOa:H202:H20 (10:1:1) solutions. Metal films with different thicknesses were then deposited by thermal evaporation for Au and by electron beam evaporation for Ni and W. The pressure during the evaporation process was kept at 10 5 mbar. The film thickness was monitored by a quartz crystal thickness monitor. The depth profiling Doppler broadening measurements on the metal-GaAs interfaces were performed with the slow positron beam facility at the University of Hong Kong [9]. The shape of the 511 keV annihilation line was measured by a gamma ray spectroscopy system comprising of an 30% ORTEC HPGe detector, an ORTEC 572 spectroscopy amplifier and an ORTEC 918 multichannel buffer (MCB) interfaced to an 486 PC. The resolution of the system was 1.25 keV (FWHM). The lineshape was monitored in the usual way through the S parameter, defined in the normal way as the ratio of the area of the central region of the 511 keV peak to the total area. Each spectrum was accumulated with 106 counts. The S - E data were then analyzed by the program VEPFIT [10] assuming a positron implanta-

tion

profile

of

257 the

form

p(x)

=

-(d/dx){exp[-(z(x)/zo)m]} with z ( x ) = f~ P( ~ ) / Po d~, Zo being defined by z0 = zi/ F( 1 + l / m ) , F ( y ) the gamma function, zi = AE"/pi being the mean positron implantation depth and Pi the position dependent density of the ith layer [1 l]. The positron diffusion length in the GaAs bulk was measured previously on a GaAs sample with no metal coverage and was found to be 227 nm.

3. Results 3.1. Au / GaAs (SI) S parameter profiling studies were performed on A u / G a A s (SI) samples with metal coverages of 50, 100, 150 and 200 nm. The S-E data for samples with different film thicknesses are shown in Fig. 1 from which it is observed that the data are similar to those previously obtained [7]. The S parameter initially decreases with increasing positron incident energy as more positrons annihilate in the Au overlayer and then increases as more positrons are implanted into the GaAs bulk.

0.540 0.535 t 0.530 0.525 I i 0.520 -t u~ 0.515 0.510

l

0.505 0.500 I

V

0.495

zuu am

,

0.490

i

l

I 5

I 10 15 E (keV)

I 20

I -25

I

-~ /

I 0

30

Fig. 1. S parameter versus positron implantation energy for Au/GaAs (SI) samples with different Au coverages. The solid lines representthe PAEI model curves taken with the implantation profile for Au suggestedby Baker et al. [13].

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C.C. Ling et al. /Applied Surface Science 116 (1997) 256-262

In our previous study [7] we performed similar S parameter depth profiling studies on identical Au/GaAs systems and observed that good fits could not be obtained with the assumption of a two-layer model, but a three layer PAEI (perfectly absorbing extended interface) model was able to give a good fit to the data. This PAEI model assumes that the sample system consists of three layers, namely the metal overlayer, the positron absorbing interfacial region and the GaAs bulk. The positron absorbing interfacial region (produced by taking a positron diffusion length in the region close to zero) was attributed to the atomic intermixing and the AuGa alloy formation occurring at the interface. In fitting the present data we first take the conventional values 2, 1.6 and 400 ,~ g cm -3 keV-" of the positron implantation profile parameters m, n and A respectively [11]. The fitted S parameters of the Au overlayer and the interfacial region normalised to the GaAs bulk are found to be 0.9412 (62) and 0.9780 (87), respectively. These values compare well with the values 0.9405 and 0.9822, respectively obtained in the previous study [7]. The fitted results of the boundary positions T~ and T2 for different samples are shown by empty circles and solid circles, respectively, in Fig. 2a and the straight line represents the ideal boundary position T. In good agreement with our previous observations [7], it is found that, (i) the majority of the interfacial width is on the Au side of the ideal boundary position and (ii) the interfacial width T2 T~ increases with increasing metal coverage and spans from about 14 to 60 nm. Recent experimental studies and Monte Carlo simulations of the positron implantation profile have revealed that the conventional density scaling factor cannot be used to give good description on the material dependence of the profile [12-15]. AsokaKumar and Lynn [12] proposed an energy dependent equation for the A and a value for the parameter n = 1.64. On the other hand Baker et al. [13] and Coleman et al. [15] have performed experiments and Monte Carlo simulations on Au, which indicate that values of A = 831 ,~ g cm -3 keV-" and n = 1.42 are more appropriate for this metal. Although all these studies were performed on a homogeneous Au system and thus are not strictly applicable to the present Au/GaAs data we re-fitted our data taking

250

(a)

200

!1

Au/GaAs (St) A=400Agcm%eV -~ n=1.6

./

150

!

g

T

1oo

±

T

LZ 50

i

F

i

i

50

100

150

200

250

Metal Coverage (nm)

2~o (b) 200 i

Au/GaAs (St) A=831Agcm3keV -n n=1.42

_O j

~ 150-

g

@

o_

©

~ 100

u~ 5(1

50

100

150

200

250

Metal Coverage (nm)

Fig. 2. (a) Fitted upper boundary position (solid circles) and the lower boundary position (empty circles) of the interfacial region versus metal coverage for the A u / G a A s samples. The widely used implantation profile parameters (i.e. A = 4 0 0 ,~ g cm --x keV-" and n = 1.6) and the PAEI model are assumed. The solid line shows the ideal boundary (the film thickness). It is observed that the interfacial widths increase with increasing metal coverages and the interfacial region occurs mainly on the Au side. (b) Fitted boundary positions as a function of the metal coverage taking the Baker et al. implantation profile (i.e. A = 831 ,~ g cm -3 keV-" and n = 1.42) [13]. It is noted that the interfacial width is independent of the metal coverage and that the interface is centered at the ideal boundary position.

C.C. Ling et al. /Applied Surface Science 116 (1997) 256-262

the values of A and n obtained in Refs. [13,15]. As the fitted parameters relating to the Au overlayer and the interfacial region are mainly influenced by the implantation profile in the Au rather than in the GaAs side this was thought to be a reasonable procedure. The fitted curves are shown in Fig. 1 by the solid lines and are found to give good representation of the S - E data. The normalised S parameters of the Au overlayer and the interface are found to be 0.928 (13) and 0.974 (12), respectively. The fitted results of the boundaries positions are shown in Fig. 2b. Two observations may be made. Firstly, with the new parameters, contrary to the findings of the earlier analysis which placed the interfacial region on the Au side, the interfacial region now spans in both directions from the ideal boundary position. Secondly, the interfacial width is found to be less dependent on metal coverage increasing only from 16.5 to 33.5 nm over the range studied (compared to a previous increase from 14 to 60 nm). These new values are closer to the values obtained in the depth profiling AES studies of Chye et al. [16] and Hiraki et al. [17], in which interfacial widths of 200 and 170 ,~, respectively, were obtained. These results show that the choice of implantation profile can give significant effects on the analysis and interpretation of the buried metal-semiconductor interface as studied using the depth profiling PAS technique.

259

0.54 7 ij

i

Ni/GaAs (Sl)

0.53

i o.52 ~

0.51 ~

'

i

~

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:

%

:

,

0.49 - -

I

~

i

]

0

5

10

15

~

100 nm

~

200 nm

20

25

30

E (keV)

Fig. 3. S parameter versus positron implantation energy for N i / G a A s (SI) samples with 100, 150 and 200 nm Ni coverages. The solid lines are the fitted curves with the PAE1 model taking the conventional implantation profile parameters.

average width of the N i / G a A s interface is found to be 17.5 nm. Other than this three layer PAEI model, the assumption of a two layer model having a positron absorbing interface (i.e. the PAl model) was also

260

3.2. Ni / GaAs (SI) 240

Depth profiling S parameter measurements have been carried out on N i / G a A s (SI) samples with Ni film thickness of 100, 150 and 200 nm. The S - E data are plotted in Fig. 3 and were first analyzed assuming the PAEI model and the widely/ used implantation profile parameters A = 400 A g cm -3 k e V - " and n = 1.6. The fitted curves are shown by the solid lines. The bulk normalised S parameters of the Ni overlayer and the interface are found to be 0.9490 (53) and 0.956 (54), respectively. The fitted positions of the boundaries as a function of the metal coverage are plotted in Fig. 4, In sharp contrast to the large coverage dependent interfacial width found for the A u / G a A s system using the conventional implantation parameters, it is noted that the width of the interfacial region is found to be fairly small and constant for all of the three samples studied. The

-

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/ //

200

i//

,

Ijy

T

i

/

/

-

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120 100

80

÷,

/

f

-

i I

80

100

120

140

160

180

200

220

240

26O

Metal Coverage (nm)

Fig. 4. Fitted positions of boundaries as a function of the Ni coverage for the N i / G a A s samples. The upper and lower boundaries are represented by the empty squares and the empty circles, respectively. The solid line shows the ideal boundary position.

C.C. Ling et al. /Applied Surface Science 116 (1997) 256-262

260

tested. The interfacial width was taken as 0.1 nm as a zero width layer is not allowed in the VEPFIT program. The boundary position was fixed at the mid position of the interfacial region obtained from the PAEI (i.e. at (T 2 + Tl)/2). It was found that the PAl and PAEI fits were equally as good, showing no significant change in the chi-square value. Since it is not possible to distinguish between the models, the implication is that either the interfacial width is small, or the S parameter of the interface is close to that of the Ni overlayer.

260

I(a)

240

W/GaAs (Sl) A==4.0OAgcm-3kev"

220 200 180 160 140 120 0.

100 80

3.3. W / GaAs (SI)

60 40

For the case of W / G a A s (SI) system, samples with metal coverages of 105, 167 and 230 nm have been prepared. The S - E data for these samples are plotted in Fig. 5. It is found that reasonable fit could not be obtained by taking a two-layer model. With the three-layer PAEI model, the S - E data were first fitted by taking the conventional A and n implantation parameters. The fitted curves, shown by the solid lines in Fig. 5, indicate that the PAEI model represents the S - E data well. The S parameters of the W overlayer and the interracial region are found to be 0.9364 and 0.9697, respectively. The fitted

F

60

80

I

I

I

~-

I

I

I

I

100 120 140 160 180 200 220 240 260 Metal Coverage (nm)

260 (b)w/GaAs (Sl)3

240 220 E ._~ 200 v

"~ 180a3

"6 160 140

8 -o 120 u_ 100

0.540 ,0

0.535

W/GaAs (SI)

60

0.530

I

80

I

I

I

I

r

I

J

I

100 120 140 160 180 200 220 240 260

Metal Coverage (nm)

0.525

Fig. 6. (a) Fitted boundary positions as a function of the W coverage for the W / G a A s samples assuming the PAEI model and the conventional implantation profile. Similar to A u / G a A s , the interfacial widths are found to increase with increasing metal coverage and the interfacial region mainly occurs on the metal side of the ideal interface position. (b) Fitted boundary positions versus W coverage for the W / G a A s samples using the PAEI model and the implantation profile of Baker et al. [13]. The interracial region is found to be evenly distributed about the ideal boundary and the interracial width is independent of metal coverage.

0.520 0.515 0.510 0.505 ~ I E " ' 7 ''',v

0.500

A

167 nm 105 nm

0.495 0.490

t

o0 I

t

I

1

I

r

t

0

5

10

15

20

25

30

E (keV)

Fig. 5. S parameter versus positron implantation energy for W / G a A s (SI) samples with different W thicknesses. The solid lines are the fitted curves for the data with the PAEI model taking the conventional positron implantation profile.

boundary positions Tt and T2 are shown in Fig. 6a. In a similar manner to the Au/GaAs results analysed assuming the conventional implantation parameters,

C. C. Ling et al. / Applied Surface Science 116 (1997) 256-262

the interfacial widths are found to increase from 47 to 87 nm as the W film thickness increases from 105 to 230 nm. Also in similarity to the A u / G a A s results the interfacial region is again found to occur mainly on the metal side of the ideal boundary location. Since the new A and n values are not available for W in the literature, the A and n values obtained for Au (i.e. A = 831 ,~ g cm 3 k e V - " and n = 1.42) were used for fitting because the density and Z of tungsten are close to those of gold. With these new implantation parameters, the S - E data were found to be well fitted with the PAEI model, values of the boundaries of the interfacial region being shown in Fig. 6b. The interfacial widths of 40, 60 and 57.5 nm were found for the 105, 167 and 230 nm coverage samples, respectively. As with the A u / G a A s samples the reduced dependence of interracial width on metal coverage is noted. (Indeed within the statistical error the interracial width is constant.) Moreover, the interfacial region is found to be centered closer to the ideal boundary position. The S parameters for the W overlayer and the interfacial region are found to be 0.9357 and 0.9646, respectively.

4. Discussion and conclusion For the W / G a A s and A u / G a A s systems, the interfacial region was found to occur mostly on the metal side of the expected position of the interface. Moreover the interfacial width was found to increase with increasing metal coverage if the conventional positron implantation parameters (i.e. A = 400 .~ g cm -3 keV -n and n = 1.6) were employed. When the new implantation parameters by Baker et al. [13] and Coleman et al. [ 15] were introduced (i.e. A = 831 ,~ g cm -3 k e V - " and n = 1.42), the interfacial region was found to center around the ideal boundary position and the interfacial width was found to be nearly constant for samples with all thicknesses. Thus one conclusion is that the choice of implantation profile strongly influences the fitted results and the information involving the interfacial region. The importance of obtaining and working with the correct positron implantation profile in the study of buried interfacial layers is thus emphasised by the present study. Since the new implantation profile by

261

Baker et al. [13] and Coleman et al. [15] was supported by both experiments and Monte Carlo studies, this implantation profile is expected to be more reliable. Moreover, the interpretation of an interfacial region of fixed width spanning the expected interface position makes more physical sense, thus giving more credibility to the revised A and n values for the heavy metals Au and W. Another observation of the present work is that the interfacial width of A u / G a A s contact is larger than that of N i / G a A s contact as expected from the classification scheme of Sinha and Poate [ 1] in which the former system has the greater atomic interdiffusion. However, it is unexpected to find the interfacial width of W / G a A s interface to be on average larger than that of the N i / G a A s interface. The W contact is expected to be more stable and abrupt than the Ni contact [1]. This may be due to the fact that in the W film evaporation process, the temperature of the GaAs substrate increased significantly by the transfer of heat from the W source to the sample holder through radiation. Such an annealing effect could result in enhancement of atomic interdiffusion across the interface. Further investigations are planned in order to test this hypothesis. Another effect that has not yet been taken into account is the intrinsic band bending that normally occurs at the metal-semiconductor contact. Such induced internal electric fields affect the positron motion and thus the positron effective diffusion length. For the A u / G a A s sample used in the present work, we estimate a band bending of 0.3 eV giving rise to a depletion width of 0.3 /zm and an average electric field of about 2 × l 0 4 V c m - 1 . Preliminary investigations using VEPFIT on such electric field structures shows that the second interfacial boundary moves further from the substrate increasing the interfacial width. Further investigations on this effect are in progress, which require data taken under different contact biases so as to give the true value of the interfacial S parameter, in the presence of strong parameter correlation. In conclusion, the Au/GaAs, N i / G a A s and W / G a A s systems have been studied by measuring the S - E data and fitting the data using the program VEPFIT. It has been found that the fitting parameters and their physical interpretation as regards the buried interface is very dependent on the choice of the

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c.c. Ling et al. / Applied Surface Science 116 (1997) 256-262

i m p l a n t a t i o n profile. It is also f o u n d t h a t the Au/GaAs and W/GaAs s y s t e m s c a n n o t b e repres e n t e d well b y a t w o - l a y e r m o d e l b u t r e q u i r e the t h r e e l a y e r P A E I m o d e l . O n the o t h e r h a n d for the N i / G a A s s y s t e m the i n t e r f a c e a p p e a r s s i g n i f i c a n t l y n a r r o w e r , a n d it is n o t p o s s i b l e to d i s t i n g u i s h bet w e e n a t w o or t h r e e l a y e r m o d e l .

References [1] A.K. Sinha and J.M. Poate, Thin Films-Interdiffusion and Reactions, Eds. J.M. Poate, K.N. Tu and J.W. Mayer (Wiley, New York, 1978) ch. 11. [2] L.J. Brillson, Surf. Sci. Rep. 2 (1982) 123. [3] Z.L. Liau, W.L. Brown, R. Homer and J.M. Poate, Appl. Phys. Lett. 30 (1977) 626. [4] H.M. Weng, X.Z. Guo, R.D. Ran, X.J. Shi, LS. Zhu, T.X. Zhao and X.P. Wang, Nucl. Instr. Meth. A 307 (1991) 577. [5] Y. Tabuki, L. Wei, S. Tanigawa, K. Hinode, N. Kobauashi, T. Onai and N. Onada, Mater. Sci. Forum 105-110 (1992) 1463. [6] (3. Yang, J.H. Kim, S. Yang and A.H. Wesis, Appl. Surf. Sci. 85 (1995) 77.

[7] C.C. Ling, T.C. Lee, S. Fung, C.D. Beling, H. Weng, J. Xu, S. Sun and R. Han, J. Phys.: Condens. Matter 6 (1994) 1133. [8] C.C. Ling, T.C. Lee, S. Fung, C.D. Beling, H. Weng, J. Xu, S. Sun and R. Han, Appl. Surf. Sci. 85 (1995) 305. [9] C.D. Beling, S. Fung, H.M. Weng, C.V. Reddy, S.W. Fan, Y.Y. Shan and C.C. Ling, In: Proc. SLOPOS-5, AIP Proc. 303, Eds. E. Ottewitte and A.H. Wesis (American Institute of Physics. 1994) p. 462. [10] A. van Veen, H. Schut, R.A.J. de Vries, R.A. Hakvoort and M.R. IJpma, Positron Beams for Solids and Surfaces, Eds. P.J. Schultz, G.R. Massoumi and P.J. Simpson (American Institute of Physics, 1990) p. 171. [11] A. Vehanen, K. Saarinen, P. Hautoj~irvi and H. Huomo, Phys. Rev. B 35 (1987) 4606. [12] P. Asoka-Kumar and K.G. Lynn, Appl. Phys. Lett. 57 (1990) 1634. [13] J.A. Baker, N.B. Chiltou, K.O. Jensen, A.B. Walker and P.G. Coleman, Appl. Phys. Lett. 59 (1991) 2962. [14] V.J. Ghosh, Appl. Surf. Sci. 85 (1995) 187. [15] P.G. Coleman, J.A. Baker and N.B. Chilton, J. Phys.: Condens. Matter 5 (1993) 8117. [16] P.W. Chye, I. Lindau, P. Pianetta, C.M. Garner, C.Y. Su and W.E. Spicer, Phys. Rev. B 18 (1978) 5545. [17] A. Hiraki, S. Kim, W. Kammura and M. Iwami, Surf. Sci. 86 (1979) 706.