PHONONS OF THE METAL/AMORPHOUS SILICON INTERFACE STUDIED BY INTERFERENCE ENHANCED RAMAN SCATTERING

(1)

HAL Id: jpa-00221328

https://hal.archives-ouvertes.fr/jpa-00221328

Submitted on 1 Jan 1981

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

PHONONS OF THE METAL/AMORPHOUS SILICON INTERFACE STUDIED BY INTERFERENCE

ENHANCED RAMAN SCATTERING

R. Nemanich, C. Tsai

To cite this version:

R. Nemanich, C. Tsai. PHONONS OF THE METAL/AMORPHOUS SILICON INTERFACE STUD-

IED BY INTERFERENCE ENHANCED RAMAN SCATTERING. Journal de Physique Colloques,

1981, 42 (C6), pp.C6-822-C6-824. �10.1051/jphyscol:19816242�. �jpa-00221328�

(2)

JOURNAL DE

PHYSIQUE

Colloque C6, supple'ment au n o

1 2 ,

Tome

42,

ddcembre 1981 page

C6-822

PHONONS OF THE METAL/AMORPHOUS s I LI CON INTERFACE STUDIED BY INTERFERENCE ENHANCED RAMAN SCATTERING

R . J . Nemanich and C . C . Tsai

Xerox Pa20 A l t o Research Center, 3333 Coyote H i 2 2 Road, Pa20 Alto, C a l i f o r n i a 94304, U.S.A.

Abstract. - The interference enhanced Raman scattering (IERS) configuration is used to study the initial interfacial interactions of thin films of Pd or Pt on hydrogenated amorphous silicon (a-Si:H). Sharp spectral features are observed in the Raman spectrum of as-deposited Pd on a-Si:H which are attributed to crystalline P3,Si. In contrast, for as-deposited Pt on a-Si:H, broad spectral features are observed which are attributed to an intermixed Pt-Si phase.

Silicide formation at metal-semiconductor interfaces is an important aspect of future technologies and relates to the nature of Schottky barrier formation. Most work has been concerned with interactions at metal-crystalline silicon interfaces.L2 However, since hydrogenstsd amorphous silicon (a-Si:H) has exhibited true semiconductor properties, interactions at the metal/a-Si:H interface will also be important and could be quite different from interactions on crystalline Si. In this s:udy Raman scattering is used to probe the initial interactions at the interface of Pd and Pt on a-Si:H.

Although light scattering has proved a very useful probe of lattice vibrations of insulators, semiconductors and even some mstals, this probe has only recently achieved any success in studying the physical interactions at interfaces and surfaces. In particular, the metal-semiconductor interface has provsn to be a particularly difficult configuration, but the interactions at a metal-semiconductor interface are both physically varied and technologically important. There are several major experimental problems with light scattering in a standard backscattering configuration. Firstly, the interface region is a small portion of the sample, and excitations of the region are often "masked" by those of the semiconductor. In addition, because of the high reflectivity and absorption of visible light from metals (or semiconductors), it is difficult to illuminate the "buried" interface.

Furthermore, the scattering that does occur is strongly absorbed. While there has been recent success using standard Raman backscattering techniques to explore the metal- semiconductor interface, a new technique called interference enhanced Raman scattering (IERS)3 holds the possibility of routine application to many problems in this area.

The IERS configuration utilizes a multilayer sample configuration which enhances the signal by optical interference properties. For scattering from a metal/a-Si:H interface, a four-layer structure is used and a schematic of this structure is shown in Fig. la. This

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19816242

(3)

DESTRUCTIVE

SiO,

DISTANCE ALONG Z

Fig. 1 : (a) A schematic of the four-layer sample configuration used to obtain interference enhanced Raman scattering (IERS); (b) the electric field intensity due to 514.5 nm light impinging on the multilayer sample shown in (a) at normal incidence. The dashed lined represents the light intensity if no sample were present.

0 I I I I I I

0 200 400 600

FREQUENCY SHIFT (cm-')

Fig. 2: The Raman spectra of (a) a-Si:H, (b) 2 nm Pd on a-Si:H and (c) 5nm Pt on a-Si:H. Polarized spectra are shown in (b) and the symbols refer to the polarization of the incident and scattered light with respect to the scattering plane.

configuration consists of an optically thick aluminum layer, a 40 nm SiO, layer, a 10 nm a-

Si:H layer and a Pd or Pt layer of 2 to 6 nm. Using the bulk optical properties of these films, we can calculate the light intensity inside the sample due to the incident 514.5 nm radiation. The results of this calculation indicated in Fig. ¹ b show that the light intensity is a maximum at the interface and is actually equal to the incident light intensity. This intensity could be 5 to ⁵⁰ times stronger than that at a similar metal interface on a thick semiconductor. In addition, the same interference conditions that cause the optimization of the incident light also causes enhanced normal emission of the Raman scattered light which originates at the interface. This can cause an additional enhancement of up to a factor of 4. Thus, the Raman scattering signal from the interface using IERS may be 200 times stronger than from normal backscatter configuration^.^ The IERS configuration holds the additional benefit of reducing the substrate excitations on two grounds: the semiconductor is very thin and the optical interference is destructive for excitations deeper in the sample.

The a-Si:H used in this study was deposited by plasma decomposition of pure

silane where 1 to 2W of rf power sustained the plasma, and the substrates were held at

the anode at a temperature of 230°C. The resultant material is known to have low defect

densitites and contain -8 at. % H.4 The Pd was thermally evaporated after the a-Si:H

(4)

C6-824

JOURNAL DE P H Y S I Q U E

deposition, and the a-Si:H film suffered an exposure to air for less than 15 min.

The Raman spectrum of a-Si:H in the IERS configuration is shown in Fig. 2a. The broad continuous spectrum obtained from the a-Si:H is essentially identical to that obtained from thick a-Si:H films. And while not shown here, the second order Si network vibrations and Si-H vibrations are also identical to corresponding features from similarly prepared thick a-Si:H films? Thus, the 10 nm a-Si:H film is essentially "bulk-like" in its vibrational excitations.

The deposition of 2 nm of Pd on this film causes dramatic changes in the Raman spectra. Polarized Raman spectra shown in Fig. 2 b display several sharp features. Since Pd has no first order Raman spectrum, these features are attributed to a crystalline Pd- silicide compound which forms at the interface of the Pd and a-Si:H film. By comparison with thick silicide films formed on crystalline Si, these features can be attributed to a form of Pd,Si.6 It should be emphasized that the sharp lines indicate the presence of a crystalline compound. To determine the extent of the initial silicide formation, 3 nm and 6 nm Pd films on a-S~:H were also examined, and it was found that -2 nm of the Pd was initially consumed to form a silicide6

The chemical similarity of Pd and Pt would suggest that a corresponding result might be obtained. As is shown in Fig. 2c, there are no additional sharp spectral features after Pt deposition on a-Si:H. There is, however, a significant broad, low frequency contribution not evident in any of the other spectra. This feature is attributed to an intermixed Pt-silicon phase which lacks long-range order. Annealing of this sample to 200°C causes the appearance of sharp spectral features which are attributed to crystalline compounds Pt,Si and PtSi. It should be noted that annealing both the Pt or Pd/a-Si:H structure leads to improved Schottky barrier pr~perties.~

We are certainly encouraged by the success of the IERS technique. Its major limitation is that thin film structures are required. Thus, it is easier to work with amorphous semiconductors, but with the use of molecular beam epitaxy the scope of the IERS technique may be broadened to examine crystalline interfaces.

References.

1. Tu, K. N. and Mayer, J. W., in Thin Films lnterdiffusion and Reactions, edited by Poate, J. M., Tu, K. N., and Mayer, J. W. (Wiley, New York, 1978), p. 359.