Chemical interaction of pulsed laster deposited Co with the MoS2(0001) surface

surface s c i e n c e ELSEVIER

Surface Science 365 (1996)411-421

Chemical interaction of pulsed laser deposited Co with the MoS2(0001) surface J. B u l i c z a, L . M o r a l e s

de la Garza

b. , , S. F u e n t e s

b

a Centro de Investigacidn Cientifica y de Educaci6n Superior de Ensenada, Ensenada, M~xico b Instituto de Fisica, Universidad Nacional Aut6noma de M~xico, Laboratorio de Ensenada, Apdo. Postal 2681, 22800 Ensenada, B.C. M~xico

Received 18 September 1995; accepted for publication 26 March 1996

Abstract The interaction between M o S 2 (0001) and pulsed laser deposited Co was studied with X-ray photoelectron spectroscopy, Auger electron spectroscopy, glancing angle high energy electron diffraction and scanning electron microscopy. 150 shots from a KrF excimer laser of wavelength 248 nm with fluence 10 J c m - 2 aimed at a solid Co target resulted in an energetic source of Co which produced an overlayer of randomly oriented crystallites at the MoS2(0001) surface. The deposition of Co was accompanied by the formation of metallic Mo and a net depletion of S at the substrate surface. Annealing to 770 K induced the formation of a limited quantity of metallic Mo. We interpret the formation of metallic Mo after Co deposition and annealing to 770 K as caused by surface damage suffered by the substrate due to sputtering by the energetic ions and neutrals produced by pulsed laser deposition. Annealing did not drive a reaction between Co and MoS2(0001) to completion indicating that Co is not strongly reactive with MoS2(0001). The relative concentrations of species calculated from XPS and AES intensities after deposition were consistent with S incorporated in the overlayer film and metallic Mo situated at the overlayer/substrate interface. Annealing between 770 and 1070 K caused agglomeration of the Co overlayer, resulting in particles of average size 1000 A.distributed on the basal plane, and a high concentration of Co at the step edges. Keywords: Auger electron spectroscopy; Cobalt; Molybdenum disulphide; Pulsed laser deposition (PLD); Reflection high-energy electron diffraction (RHEED); X-ray photoelectron spectroscopy

1. Introduction T h e m e t a l / M o S 2 ( 0 0 0 1 ) interface has a t t r a c t e d a t t e n t i o n b e c a u s e of the u n u s u a l surface reactivity it exhibits as a c o n s e q u e n c e of the structure of the s u b s t r a t e [1--8]. M o S 2 has a highly a n i s o t r o p i c structure consisting of stacks of layers. W i t h i n each layer is a p l a n e of m o l y b d e n u m a t o m s s a n d w i c h e d by two planes of s u l p h u r atoms. T h e b o n d i n g within the layers is covalent, with all the b o n d s * Corresponding author.

saturated. Between consecutive layers there exists a relatively w e a k van d e r W a a l s a t t r a c t i o n . C l e a v i n g MoS2 between the layers results in a relatively inert (0001), o r basal plane, surface. It is f a v o r a b l e for m e t a l films to g r o w as three d i m e n sional particles on the inert MoS2(0001) surface [ 2 , 3 , 5 , 6 ] . T h e (0001) surface of MoS2 crystals has been used to s t u d y the reactivity of the m e t a l / s u b s t r a t e interface a n d defect sites relating to the a p p l i c a t i o n s of m o l y b d e n u m d i s u l p h i d e for electronics [ 1 , 4 ] , catalysis I-2,3,9,10,12] a n d lubrication [7,8].

0039-6028/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved PII S0039-6028 (96) 00723-6

412

J. Bulicz et al./Surfaee Science 365 (1996) 411-421

Bulk thermodynamics predicts that a particular metal-MoS2 system should be reactive at room temperature for negative or small positive (<0.5 eV) heats of reaction, •HR, resulting in the formation of metallic molybdenum and a metalsulphide [,1,2]. It has been found, however, that this condition is not sufficient in practice to predict the reactivity of particular transition metals with MoS2(0001) at room temperature. The study of particular metal/MoS2(0001) systems has identified three types of chemical behavior. Certain metals deposited on MoS2(0001) react strongly with the substrate forming bulk sulphides and metallic molybdenum, in agreement with bulk thermodynamics. This type of reaction is easily identifiable using X-ray photoelectron spectroscopy (XPS) by an extra peak which appears in the Mo 3d core level spectrum after metal deposition. This peak is shifted about 1.2 eV to lower binding energy with respect to the substrate Mo peak [ 1,5,8]. Identification of the sulphides formed in this type of reaction is more challenging, as the shifts in binding energy of the products are relatively small compared with the width of the S 2p core level peaks observed by XPS. High resolution photoelectron spectroscopy (or soft X-ray photoelectron spectroscopy, SXPS) has been effective in identifying the sulphur species formed in this type of reaction [5,8]. Annealing these systems drives the reaction to completion. This is detected in a broadening in the width of the peaks in the deposited metal XPS core level spectrum due to the formation of a sulphide with the deposited metal. The metals which have shown strong reactivity with MoS2 (0001) at room temperature are Mg [-1], Yi [,1], Cr [-8] and Mn [-5], Cr and Mn reacting to completion with annealing. Other metals show a minimal chemical reactivity with the substrate. The most well studied example has been Fe [,2,6], in which a Fe-S surface phase and sulphur vacancy formation is observed after deposition. The identification of this type of reactivity has been most successful using SXPS in order to measure the small shifts in binding energy of the S 2p core levels of the S species formed in this reaction. Annealing does not drive the reaction further. Reports of the reactivity of Ni place it either in the minimally reactive group [, 1] or the

non-reactive group [-3] of metals o n M o S 2 (0001). There are many metals (A1 [,1], V [,5], Co [,5], Cu [,1], Rh [,5], Pd [5], Ag E5], In [1] and Au [5]) which, when deposited on MoS2 (0001) at room temperature, have been reported not to react with the substrate notwithstanding a negative or small positive value of the heat of reaction. The effect of annealing this last set of metals o n M o S 2 (0001) has not been studied. The reactivity of MoS2(0001) after bombardment of the surface by noble gas ions has also been studied [,9-13]. It is well known that defect sites in the basal plane are active in catalysis, for example, of hydrodesulphurization processes ([9,12] and references therein). A detailed study of Ne + bombardment of MoS2(0001) by Lince et al. [13] using SXPS reported that an amorphous surface layer is produced following high fluence bombardment showing S depletion compared with p r e - b o m b a r d e d M o S 2. In addition a reduced Mo species was identified by a shift of 0.6 eV to lower binding energy as compared to the substrate Mo 3d core level peak. This Mo species corresponds to Mo atoms implanted into the lattice by the ion-surface impacts. Annealing the ion-bombarded substrate to 750 K induced the formation of metallic molybdenum, detected by the presence of a Mo peak shifted 1.1 eV to lower binding energy with respect to the substrate Mo peak. Deposition by evaporation has been the technique used by others to form metallic overlayers on MoS2(0001). This technique produces a low energy source of material unlikely to damage the MoS2(0001 ) surface during deposition. On the other hand, pulsed laser deposition (PLD) can produce a deposition source of energetic ions and neutrals highly directed perpendicular to the laser ablated surface, capable of sputtering effects [ 14]. In this investigation we deposited Co onto MoS z (0001) by PLD to determine the effect of this deposition technique on surface reactivity and subsequent behavior on annealing the system. The heat of reaction for the formation of CoS is + 0.31 eV, satisfying the condition stated above for the formation of bulk sulphides [1,5]. Co deposited on MoS2(0001) by evaporation has been reported to be non-reactive, while Co on sputtered MoS2(0001) causes a shift in binding energy of the

J. Bulicz et al./Surface Science 365 (1996) 411 421

S 2p core level attributed to a possible chemical reaction [5]. In this study XPS was used as the primary measurement tool to identify chemical species. The combination of XPS and Auger electron spectroscopy (AES) was used to estimate the distribution of species in the overlayer and at the interface formed with the substrate. The growth mode of the overlayer and its behavior at high annealing temperatures was observed by grazing angle high energy electron diffraction ( R H E E D geometry). Scanning electron microscopy (SEM) showed the size and distribution of metal particles at the surface after annealing the overlayer/ substrate system to 1270 K.

2. Experimental This experiment was performed in a LDM-32 laser ablation system custom built by RIBER S.A., consisting of three interconnected U H V chambers with transfer mechanisms for transport of samples in situ. Introduction and cleaning of samples was carried out in the preparation chamber which reached a base pressure of 1 x 10 -8 Torr. Film growth and monitoring with R H E E D were carried out in the deposition chamber at a base pressure of l x l 0 - 1 ° T o r r . XPS and AES data were obtained at a base pressure of 2 x 10 -1° Torr in the analysis chamber. Natural molybdenite samples were cleaved in air with a razor blade and mounted in the introduction chamber on a sample holder capable of heating the sample up to 1270 K. MoS2 is thermally stable up to 1300 K in vacuum [6]. The sample was heated in UHV to 920 K for 15 rain to clean the surface. This cleaning procedure resulted in AES and XPS spectra showing no contamination. Cobalt was deposited onto the MoS2(0001) surface by laser ablation of a presputtered 99.99% pure metallic Co target using a LEX 200 excimer laser (KrF) by Lambda Physik (wavelength 248nm) under the following conditions: 400 mJ energy, 10 J cm 2 fluence, 20 ns pulse duration and a 1 Hz repetition rate. XPS spectra were obtained using the CAMECA CX 700 A1 X-ray source and CAMECA MAC 2 electron energy analyzer with 0.8 eV resolution.

413

The samples were positioned such that the angle between the surface normal and analyzer was 20 °. Core levels were approximately deconvoluted with Gaussian functions. XPS spectra of the Mo 3d, S 2p and Co 2p core levels were obtained before and after Co deposition by 150 laser pulses. An estimate of film thickness was made from the changes in relative peak intensities of the core levels of Mo and S after deposition, using the formula I = Ioexp ( - t/)~cos~9),

( 1)

where t is the thickness of the film, )~ is the photoelectron escape depth for the element, O is the angle between the detector and the surface normal and I o is the peak intensity before evaporation [5]. Electrons from the Mo 3d core level have a kinetic energy of ~ 1300 eV while those from the S 2p level have an energy of ~ 1250 eV from which an inelastic mean free path in Co of about 2 nm was estimated [ 19]. The formula we used is strictly valid for overlayers of uniform thickness while metal films on inert surfaces such as MoS2(0001) tend to grow as three dimensional particles leading, in this case, to an estimate of the minimum coverage of the substrate equal to 40 ~,. Particular attention was paid to the Mo core level spectra, as the appearance of peaks due to various chemical species could be easily identified. The production of S species results in core level peaks shifted by small amounts of energy with respect to each other and which are unresolvable with the present spectrometer. As will be discussed in the following section the binding energies of the Co core levels did not change with annealing. Thus we present a series of Mo core level spectra which clearly show changes in surface chemistry under various conditions. Atomic concentrations were calculated from the peak areas using sensitivity factors. AES spectra were obtained, using a RIBER CER101 electron gun with a primary electron energy of 3000 eV and the above mentioned electron energy analyzer, after each of the annealing cycles. AES peak intensities were also used to calculate atomic concentrations at the surface. 10 keV electrons incident at glancing angle to the sample ( R H E E D geometry) were used to moni-

414

J. Bulicz et al./Surface Science365 (1996) 411-421

tor the structural changes at the surface during deposition and annealing. The diffraction patterns obtained were due to transmission through steps and particles at the surface, rather than reflection from a smooth surface. With this in mind we refer to the resultant patterns as being due to grazing incidence electron diffraction rather than reflection high energy electron diffraction. The size and distribution of Co particles at the surface was studied ex situ with a JEOL JSM-5300 scanning electron microscope.

3. Results and discussion 3.1. Post annealed MoS2(O001 ) surface

The AES spectrum obtained for the postannealed substrate showed that the surface was free of contamination. Grazing angle electron diffraction yielded the pattern shown in Fig. 1 due to transmission of the electron beam through steps at the substrate surface. The XPS spectrum of the Mo 3d core level is shown in Fig. 2a. Two peaks appear, corresponding to the Mo 3d spin-orbit doublet for MoS2. This spectrum was fit approximately by a sum of two Gaussian functions. The constraint used for subsequent fitting was to keep the widths and relative heights of the peaks in each Mo 3d doublet constant, based on the values obtained for the post-annealed surface. 3.2. Co deposited on MOS2(0001) 3.2.1. Chemical reactivity

Two core level doublets of approximately equal intensity were observed in the Mo 3d spectrum following deposition of Co as shown in Fig. 2b. The Mo substrate doublet was shifted 0.2 eV to lower binding energy and a second doublet at lower binding energy was observed. The energy separation between the two doublets was 1.2 eV, consistent with the presence of metallic, bulk Mo, the formation of which has been associated with strong reactivity at the metal/MoS2(0001) interface [ 5,8]. The S 2p doublet showed a slight broadening and a shift of 0.6 eV to lower binding energy following Co deposition. Equal shifts in binding

energy of the S and Mo core levels could be explained by band bending, while the unequal shifts in binding energy of the S and Mo core levels suggests that the sulphur detected was in a chemical state different from that of the substrate as will be discussed below I-5,]. The Co core level spectrum showed two very well defined peaks of a single chemical state, metallic Co. Annealing at 370, 470, 570 and 670 K resulted in minimal variation in the relative intensities and binding energy positions of all species in the XPS spectra, as shown in Fig. 3. Annealing at 770 K yielded an increase in the relative intensity of the metallic Mo core level shown in Fig. 2c. Annealing at 870, 970 and 1070 K resulted in the relative core level intensities of the substrate Mo and S increasing, Co decreasing and metallic Mo remaining constant. The relative intensities of the core levels of the various species approached a plateau as the sample was annealed at 1170 and 1270 K. No shifts in binding energy were observed for the Co core level, which maintained the sharp peak shape observed after deposition at room temperature. The limited reactivity of the Co/MoS2(0001) interface with annealing is not consistent with the behavior of strongly reacting metal/MoS2(0001) interfaces, such as Mn 1-5] and Cr [8-]. On the other hand, minimally reactive metals, such as Fe, do not show any evidence of reactivity as the metal/substrate system is annealed. However, the reaction products resulting from Fe deposition on MoS2(0001) at room temperature are a surface Fe-S phase and sulphur vacancy defects while metallic Mo is not produced I-6]. That the reaction between Co and MoS2(0001) was not driven to completion by annealing is evidence that bulk sulphide formation is not favorable for this combination of materials, placing Co either with Fe as minimally reactive or with the large group of transition metals which show no reactivity with this substrate. This agrees with the work of Lince et al. I-5] in which Co deposited by evaporation onto MoS2(0001) showed no evidence of reactivity with the substrate. We attribute the presence of a metallic Mo doublet in the core level spectrum after Co deposition to be a consequence of the deposition technique. As has been mentioned, laser

,L Bulicz et al./Surface Science 365 (1996) 411-421

415

(a)

(b) Fig. 1. Transmission diffraction patterns of glancing angle 10 keV energy electrons (RHEED geometry) penetrating through features at the MoSz(0001) surface. (a) After cleaving the sample and annealing at 920 K for 15 min to clean the surface, transmission of electrons through steps at the surface gives rise to the observed diffraction pattern. (b) After depositing cobalt using 50 pulses from a KrF laser to ablate material from a high purity Co target, a diffraction pattern due to transmission of electrons through randomly oriented Co crystallites is observed.

416

J. Bulicz et aL/Surface Science 365 (1996) 411-421

•

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BINDING ENERGY (eV) (b) Fig. 2. X-ray photoelectron spectra of the M • 3d doublet of the MoS2(0001) surface, before and after Co deposition and annealing. The circles show the raw data. The S 2s core level is also shown. (a) The clean surface (b) after a 4 nm Co film was deposited and after annealing the 40 ~. Co film to (c) 770 K (d) 1070 K. The peaks are fit approximately using Gaussian functions. Following cobalt deposition the doublet appearing at lower binding energy corresponds to metallic molybdenum.

J. Buliez et al./Surface Science 365 (1996) 411-421

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Fig. 2. (Continued). ablation of a target produces a highly directional source of energetic neutrals and ions, which are capable of implanting into or sputtering the substrate surface. Sputtering alone produces M o atoms

and small clusters implanted into the substrate lattice and a generally disordered surface. The appearance of a metallic Mo core level peak after P L D might be due to a combination of sputtering

418

J. Bulicz et aL/Surface Science 365 (1996) 411-421 90

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Fig. 3. The composition of the surface derived from XPS as a function of temperature as the 40 A Co film on MoS2(0001) was annealed. Sensitivity factors were used to determine the relative concentration of chemical species from the relative peak areas of the core levels. a n d the presence of energetic C o a t o m s b o n d i n g to the M o atoms. I o n b o m b a r d e d MoS2(0001) is k n o w n to form metallic M o when a n n e a l e d at 750 K [-13], which is consistent with the increase in the metallic M o core level intensity we observe after a n n e a l i n g to 770 K. D e p l e t i o n of S at the surface is characteristic of ion b o m b a r d e d MoS2(0001) [ 13 ]. After C o d e p o s ition a n d for annealings b e l o w 770 K, the concent r a t i o n of S relative to the t o t a l S a n d M o signal i.e., the sum of the relative c o n c e n t r a t i o n s of S, s u b s t r a t e M o a n d metallic M o , was 5 to 11% lower c o m p a r e d with the clean s u b s t r a t e as d e t e r m i n e d from X P S core level intensities. O n the o t h e r hand, the c o n c e n t r a t i o n of S relative to the sum of S a n d s u b s t r a t e M o s h o w e d an excess of S c o m p a r e d with clean MoS2(0001 ) as seen in Fig, 4. These two results indicate t h a t while P L D did induce a net d e p l e t i o n of S n e a r the surface of the substrate, there was m o r e S t h a n c o u l d be a c c o u n t e d for b y the presence of o n l y MoS2 a n d metallic M o . This "excess" S resulted in the shift to lower b i n d i n g energy of the S 2p core level after C o deposition. W h i l e we c o u l d n o t identify the chemical state of the "excess" S we refer the r e a d e r to o t h e r studies which have r e p o r t e d t h a t several species of S occur at the MoS2(0001) surface after ion b o m b a r d m e n t [ 1 3 ] . T h e a m o u n t of "excess" S p e a k e d after

0

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Fig. 4. The ratio of S concentration, derived from the area of the S 2p core level peak, to the S + Mo concentration, using the Mo concentration derived from the area of the Mo 3d substrate core level peak. The dashed line shows the ratio for MoS2 after annealing to 920 K to clean the surface. The squares correspond to values from the sample with a 40 .~ Co film, as it was annealed. The excess S concentration for low annealing temperatures was due to sulphur species produced during the reaction between Co and the substrate yielding metallic Mo. Annealing to 770 K shows a peak on the relative concentration of metallic Mo. a n n e a l i n g at 770 K, a l o n g with the f o r m a t i o n of metallic M o suggests the d e c o m p o s i t i o n of a M o - S species, the n a t u r e of which is unclear from this experiment. In Fig. 5 the relative c o n c e n t r a t i o n s as d e t e r m i n e d from A E S p e a k intensities show that after a n n e a l i n g to 870 K a n d higher the a m o u n t of M o signal was in excess of that for clean MoS2(0001) as a result of the d e p l e t i o n of S. S d e p l e t i o n was n o t noticeable in the A u g e r electron spectra until after a n n e a l i n g to 870 K at which p o i n t the b r e a k u p of the o v e r l a y e r film e x p o s e d areas of the s u b s t r a t e surface as discussed in the following section. 3.2.2. Distribution o f species at the surface C o b a l t d e p o s i t e d on MoS2(O001 ) f o r m e d rand o m l y o r d e r e d three d i m e n s i o n a l crystallites o b s e r v e d b y the diffraction of t r a n s m i t t e d electrons incident at grazing angle to the s u b s t r a t e surface, f o r m i n g a p a t t e r n which was i n d e p e n d e n t of azim u t h a l angle (Fig. 1). This is consistent with the

J. Bulicz et aL/Surface Science 365 (1996) 411-421 40 - - - = - - Mo/(Mo+S): C o + MoS 2

............ Mo/(Mo+S): clean MoS 2 30 O I.< n,"

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,

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,

i 800

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A N N E A L I N G T E M P E R A T U R E (K)

Fig. 5. The ratio of Mo concentration, derived from the peak to peak intensity of the Mo M N N Auger transition at 186 eV, to the S + M o concentration, using the S concentration derived from the peak to peak intensity of the L M M Auger transition at 152 eV. The dashed line shows the value for clean MoS2. The circles correspond to values from the sample with a 4 n m Co film, as it was annealed. Below annealing at 670 K no Mo signal was observed. The excess Mo concentration observed for the sample with the Co film above 870 K is due to the contribution of metallic Mo and a net depletion of S at the substrate surface.

Volmer-Weber growth mode of other transition metals on this substrate [2,3,5,6,8]. An overlayer of Cobalt, of minimum thickness 40 ~,, was deposited contributing to 97% of the total concentration of species at the surface as determined by XPS and 92% as determined by AES. AES is the more surface sensitive of the two spectroscopies used, since the inelastic mean free path of the Auger electrons was about 5 ,~, compared to 20 ,~ for the core level electrons used in XPS. Some carbon and oxygen contamination, 5 and 2% respectively, was picked up by AES after Co deposition, while XPS showed none indicating that the contamination was at the surface of the overlayer. The Mo 3d core levels appearing in the XPS spectrum corresponded to 1% relative concentration, while AES showed no Mo peak, consistent with the observation for other metal/MoS2(0001) systems that metallic Mo tends to be localized at the overlayer/substrate interface [5,7,8]. A small sulphur peak was observed in the AES spectrum corresponding to a relative concentration of 1% while XPS showed 2% and an excess of S compared

419

with MoS2. This could be explained by a tendency of S to incorporate into metallic overlayers on MoSz(0001) [6,8]. As mentioned above, the binding energy position of the sulphur peak was shifted to lower binding energy suggesting that it was not due to MoS 2. There were minimal changes in the relative concentrations of species measured by XPS and AES for annealing at 370, 470, 570 and 670 K, indicating a lack of strong reactivity as discussed above, and minimal coalescence of the overlayer. While annealing to 770 K did induce the formation of metallic Mo, the relative intensities of all the species changed slightly. A notable change in relative concentrations began with annealing to 870 K and continued for annealing at 970 and 1070 K. In the range of annealings from 670 to 1070 K the XPS signals from Co dropped from 96 to 29%, S increased from 3 to 49% and the substrate Mo increased from 0.5 to 18%. Relative concentrations calculated from AES showed the same trend, shown in Fig. 6. These changes are consistent with agglomeration of the Co overlayer into larger three dimensional particles accompanied by the uncovering of large areas of the substrate. In addition, there may have been desorption of Co from the surface, as has been reported for Fe [2], Ni [3] and Cs [17], however the measurement of this effect was not part of this experiment. The effect of agglomeration was also observed with glancing angle electron diffraction. After 870 K a sharp transmission diffraction pattern from Co crystallites was observed, whereas after annealing at 1070 K the transmission diffraction pattern from M o S 2 steps reappeared while the observation of the Co crystallite pattern was dependent on sample position and orientation as large particles of Co through which the electron beam could not penetrate predominated at the surface. The change in relative concentration of metallic Mo also increased in this annealing range but not to the extent observed for the other species. Between annealings at 670 and 870 K the relative concentration of metallic Mo increased from 0.5 to 4%. Annealing to 970, 1070 K and higher did not change the relative concentration of metallic Mo by more than 1%. The formation of metallic Mo did not proceed with annealing to higher temperature. This behavior is consistent with a

J. Bulicz et al./Surface Science 365 (1996) 411-421

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"

300 400 500 600 700 800 900 1000 1100 1200 ANNEALING TEMPERATURE (K)

Fig. 6. The composition of the surface derived from AES as a function of temperature as the Co overlayer on MoS2(0001) was annealed. Sensitivity factors were used to determine the relative concentrations of the elements from the peak to peak intensities of the differentiated Auger spectra.

limited quantity of metallic Mo localized at the surface of the substrate and not incorporated into the agglomerating overlayer. It has been reported elsewhere, [5,7,8], that metallic Mo produced in the presence of a metal overlayer covering MoS2 (0001) tends to remain at the overlayer/substrate interface. A plateau in the relative concentrations was approached as the sample was annealed to 1170 and 1270 K. Grazing incidence electron diffraction showed a transmission pattern from MoS 2 steps. Ex situ examination of the sample by scanning electron microscopy after the high temperature annealings showed large particles distributed over the surface of average size 1000 A, which explains why grazing angle electron diffraction did not yield a Co crystallite pattern after higher temperature annealing. A higher concentration of Co was observed at the step edges which is a well known behavior of cobalt on molybdenum disulphide 1-17,18]. A SEM image is shown in Fig. 7.

4. Conclusions The primary aim of this study was to investigate the chemical reactivity of pulsed laser deposited Co on MoS2(0001). Evidence is presented that laser ablation of a Co target using a KrF excimer

Fig. 7. SEM shows the distribution of particles at the substrate surface after the sample was removed from vacuum. The average size of particles is 1000A with a distribution of 10 particles/micron.

laser at a fluence of 10 J cm -2 produces a deposition source of sufficient energy to cause surface damage as it interacts with MoS2(0001), producing a limited amount of metallic Mo or a Co-Mo species and a net depletion of S at the substrate surface. Annealing does not drive the production of metallic Mo, except for a limited quantity of which produced after annealing to 770 K, which agrees well with the behavior of sputtered M o S 2 (0001). We conclude that the reaction of Co with MoSz(0001) to form bulk sulphides is not favorable. An overlayer of Co, of minimum thickness 40 ~,, deposited on MoS2(0001) was found to grow as three dimensional particles. Relative concentrations of species at the surface as calculated from XPS and AES intensities are consistent with sulphur incorporated into the overlayer following deposition. Annealing at temperatures between 770 and 1070 K resulted in an agglomeration of the overlayer, uncovering areas of the substrate and a metallic Mo species. After heating to high temperatures Co particles of average size 1000 A were distributed on the basal plane.J15, 16]

Acknowledgements This work was supported by DGAPA-UNAM grant number IN100695. The authors thank Dr.

J. Bulicz et al./Surface Science 365 (1996) 411-421

M. Farias for the very helpful discussions; G. Soto for his technical assistance with the XPS and AES analysis; Dr. J. Siqueiros and J.A. Diaz for assistance with the laser; I. Gradilla for his help in operating the SEM; and G. Vilchis for photographic work. We also thank one of the referees of this article for very helpful comments which led to a clarification of the mechanism involved in producing the chemical species observed.

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