F. MEYER
2922
AND
J. nl.
MORABITO
Adsorption of Organic Gases on Clean Germanium Surfaces
by F. Meyer and J. M. Morabito Philips Research Laboratories, N . V . Philips' Gloeilampenfabrieken? Eindhoven, The Netherlands
(Received December 89,29'70)
Publication costs assisted by Philips Research Laboratories
The chemical adsorption of a number of organic compounds (CHsOH, CHaSH, CHaC1, CH3Br, and CH30CH3) on clean germanium surfaces has been investigated by means of gas volumetric measurements on powder samples and ellipsometric measurements on single-crystal faces. The reactions appear to be dissociative and plane-specific, compensating the so-called dangling bonds of the germanium surface atoms, Structures for the adsorption complexes have been proposed and evidence is presented for shifts in the positions of the surface atoms due to the bonding in the adsorption complex.
I. Introduction The clean surfaces of germanium and silicon appear to be highly reactive to many gases, which can be ascribed to the presence of uncompensated (dangling) bonds at the surface atoms. This was demonstrated for a number of inorganic hydrides1 (HCI, HzS,NHa,etc.) and for some organic molecules (methanol, ethanol, and others).2 These adsorption reactions have been interpreted as being dissociative, compensating the dangling bonds of the surface atoms.* It is the purpose of this paper to obtain information on the structure of adsorption complexes on germanium, since this is an essential step in the understanding of the chemical behavior of these clean surfaces and their catalytic properties. We have combined several techniques for surface study in this investigation. Gas volumetric adsorption measurements on powders with a large surface area (-1 m2)yield the amount adsorbed and subsequent desorption experiments as a function of temperature give decomposition products, which can be analyzed mass spectrometrically. The inherent difficulty of the powder measurements is the presence of more than one crystallographic orientation. The cleavage plane (111) will be the main but not the only constituent of the powder surface. Ellipsometry ties in very well with the gas volumetric measurements. Ellipsometry is an accurate optical method, which measures, in principle, surface coverages on single-crystal surfaces with an accuracy of approximately 0.05 of a monolayer. A previously reported study, in which ellipsometric and gas volumetric data have been ~ o m p a r e dled , ~ to the conclusion that in the special case of silicon and germanium ellipsometry also gives information about the number of dangling bonds compensated by the adsorbed molecules. This gives direct information on the structure of the adsorption complexes. Auger electron spectroscopy (aes) has been used to check surface cleanliness and to give further support to the ellipsometric measurements since both methods The Journal of Physical Chemistry, Vol. '76,No. 19, 1971
have approximately the same sensitivity and can be used to study single-crystal faces. Low-energy electron diffraction has been tried, but no ordered structures for the organic adsorption complexes have been observed. The only structure which has been found, was a Ge(ll1) 2 X 1 after hydrogen sulfide adsorption at 250".6
11. Experimental Section The adsorption and desorption experiments on powders were performed in an all-glass apparatus. A low pressure of lo-' Torr was obtained with a mercury diffusion pump provided with a liquid nitrogen cold trap, Pressure readings of the organic gases were taken with a R4cLeod manometer. The germanium powder, obtained by crushing a highohmic single crystal in air, was cleaned by heating to 650" at Torr for 15 hr.' After cooling to room temperature, the gas was admitted and the pressure decrease was recorded. The excess gas was pumped off at room temperature. Then the temperature was raised by using an electric furnace around the reaction tube. Temperature readings were taken with a Pt-Pt10% Rd thermocouple imbedded in the powder. The decomposition products were analyzed with an Atlas R486 mass spectrometer connected to the reaction tube. The sensitivity of the mass spectrometer has been calibrated separately for the different gases in the appropriate pressure range. The total pressure calculated from the mass spectra agreed within 3% with (1) A . H. Roonstra and J. van Ruler, Surface Sci., 4 , 141 (1966); A . H. Boonstra, Philips Res. Rep., Suppl., No. 3 (1968). (2) F. Meyer, J . Phya. Chem., 73, 3844 (1969). (3) We will use the term "dangling" bond throughout this paper, without having any special electronic configuration in mind. Prior to adsorption the electrons in the nonbonding orbitals will probably form some type of bond a t the surface, which may induce the shifts in atomic positions associated with the surface structures observed by LEED. I n the adsorption reactions these bonds will dissociate again and react as uncompensated (radical-like) bonds with the adsorbate. (4) G. A. Bootsma and F. Meyer, Surface Sci., 14, 52 (1969). (5) A. J. van Bommel and F. Meyer, ibid., 6, 391 (1967).
ADSORPTIONOF ORGANIC GASESON CLEANGERMANIUM SURFACES the total pressure in the desorption system as measured with the RlcLeod manometer. The total surface area of the germanium powder was obtained by the BET methodj6utilizing krypton as the adsorbate at liquid nitrogen temperature. For the tross-sectional area of one krypton atom a value of 19.5 A2 was taken.' The total clean surface can be determined by the chemical adsorption of oxygen, which adsorbs at room temperature to an extent of one 0 atom per surface atom both on the (111) and (100) faces at an exposure of approximately 1 Torr-min. l , * The further reaction, Le., the adsorption on a covered surface, is much slower. The surface areas from BET calculations and from oxygen adsorption measurements agreed well for freshly prepared powders after the heat treatment at 650". If the powder had been often exposed to carboncontaining gases and subsequently heated under vacuum, the clean surface area decreased, probably due to carbon contamination, which could not be removed by heating under vacuum nor by heating in oxygen. The determination of the clean surface area by oxygen adsorption can also be used if the surface contains some preadsorbed organic gas. This has been used in cases where no complete adsorption layers of the organic compound could be obtained. The specific surface area of the powder was 0.1 m2/g and total surface areas of -1 m2have been used. The ellipsometric measurements mere carried out on polished single-crystalline germanium samples which had been oriented within 0.5' of the desired crystallographic orientation. The crystals were clamped in tantalum strips and cleaned by resistive heating to 800" for several hours in vacuo (10-9 Torr). The surface cleanliness was checked by measurements of the ellipsometric effects upon chemical adsorption of a test gas (HCl), of which the effects can be predicted within certain assumptions (compare ref 3 and section IIIA of this paper). These measurements suggested that the samples could be cleaned to at least SO-SO%. Aes measurements indicated that some carbon persists on the surface, which cannot be removed by heating under high vacuum or in oxygen. LEED measurements gave the diffraction patterns Ge (111)-8 and Ge (100)-2, which have been ascribed to the clean surfaces.@ The reaction chamber for the ellipsometric memurements is connected to a 50-l./sec lVIullard Vac Ion pump, and the base pressure as measured by the pump current or by an ionization manometer directly connected to the reaction chamber is - 1 0 - 9 T ~ ~ . The Vac Ion pump was turned off prior to the adsorption measurements. No adsorption on the sample from the residual gas could be detected ellipsometrically (
