Surface structure of MgO(001): a medium energy ion scattering study

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Surface

Science 302 (1994) 350-362

Surface structure of MgO( 001) : a medium energy ion scattering study J.B. Zhou a,1, H.C. Lu ‘, T. Gustafsson

a,*, P. Hiiberle

b

pDepartment of Physics and Astronomy and Laboratory for Surface Modification, Rutgers, P.O. Box 849, The State University of New Jersey, Piscataway, NJ 08855-0849, USA ’ Department0 de Fisica, Universidad Tknica Federico Santa Maria, Casilla 110-K Valparaiso, Chile (Received

7 July 1993; accepted

for publication

15 September

1993)

Abstract We have used medium-energy ion-scattering with channeling and blocking to study the surface structure of MgO(001). The ion scattering data show that UHV-cleaved crystals result in well ordered surfaces, while sputtered/ annealed samples show structural disorder. Surface blocking dips in both the 0 and Mg spectra are found to occur at angles very close to the corresponding bulk blocking directions, indicating very small surface relaxation and rumpling. A detailed R-factor analysis comparing the experimental spectra with results of Monte Carlo simulations for different structures gives a surface relaxation of - 1.0% + 1.0% and a rumpling of 0.5% + 1.0%. These results are in good agreement with preliminary results from all-electron total-energy calculations and imply that the surface 02- ions have small polarizations.

1. Introduction The bulk crystal structure of MgO is the rocksalt structure with each anion (cation) surrounded by six cations (anions) (Fig. 1). The (001) surface consists of equal number of cations and anions. Interest in the MgO(001) surface structure originated in the prediction by Madelung [ll and the subsequent calculation by Verwey [2] who concluded that the vertical displacements of the anion and cation atoms on the (001) surfaces of alkali halides are different. Their argument was

* Corresponding author. * Present address: Department of Chemical Engineering, University of California, Berkeley, CA 94720-9989, USA. 0039-6028/94/$07.00 0 1994 Elsevier SSDI 0039-6028(93)E0561-8

Science

that different polarizabilities of the cations and anions should lead to differential relaxations of the two species, a quantity often called “surface rumpling” (Fig. 1). Therefore, the structure of the MgO(001) surface has been extensively studied during the past two decades [3-291. Past studies have shown that the (001) surface of the crystalline MgO is bulk terminated. Results on the surface relaxation and rumpling, however, often differ. Shell model calculations [7,8,19,20], total-energy calculations based on Hartree-Fock [21] and tight-binding [29] approximations, dynamic LEED studies [4,11,14,28] and RHEED analyses [18] indicated a relaxation of no more than *3% with reference to the bulk nearestneighbor spacing. Despite this large number of reports consistently showing a small relaxation,

B.V. All rights reserved

J.B. Zhou et al. /Surface M~(OOl) 000)

ciio)

Rumpling

Side View (010) plane

side view in the (Ol!) plane of MgO(001). The bulk inter-layer spacing d (2.105 A), the first

Fig. 1. (a) Top view, and (b)

and second layer spacing d,,, and surface rumpling are indicated in (b).

an impact collision ion scattering spectroscopy (ICISS) study [23] and a surface extended energy loss fine structure (SEELFS) [24] study reported - 15% and - 17% inward relaxation, respectively. There has also been considerable debate over the magnitude of the rumpling. RHEED studies of the UHV-cleaved surfaces by Murata and coworkers [6,9,10] reported an irreversible transfo~ation of the surface structure upon heating to 300°C which resulted in a 6% rumpling in the first layer. Dynamic LEED studies of aircleaved/vacuum-annealed surfaces by WeltonCook and Berndt [ll] reported only a 2% rumpling. Careful LEED studies of air-cleaved/ vacuum-annealed, UHV-cleaved, and UHVcleaved/annealed surfaces by Urano et al. [141 reported 0% ~mpling independent of surface preparation. Yet more recently, Blanchard et al. [28] reported a 5% k 2.5% rumpling. Theoretical calculations based on shell model approximations [7,8,19,20], Hartree-Fock 1211 and tight-binding [29] methods reported even more diverse values

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of surface rumpling, ranging from 0% [8] to 11% [20]. Most shell model calculations have used formalisms ‘developed in calculations of bulk properties, which often yields accurate descriptions there. In an attempt to clarify the Mg0(001) surface structure, both experimental and theoretical investigations using medium-energy ion-scattering (MEIS) and total-energy calculations were initiated in our laboratory. This paper will describe the results from MEIS studies of the surface structure. Our results will then be compared to those obtained with various other techniques. In particular, our results on the surface relaxation and rumpling are in excellent agreement with the prelimina~ results from the total-energy calculation based on local density approximations (LDA) [30]. These calculations show that the polarizations of the surface 02- ion are significantly less than those obtained by fitting the shell model parameters to bulk phonon dispersion data. These smaller polarizations are responsible for the observed small rumpling. We believe that this is the first structural study of an oxide surface using medium-energy ionscattering. MEIS has earlier proven to be a powerful tool for surface structural analyses of metal and semi~nductor surfaces. The present experiment is therefore also a test for the application of MEIS to the study of oxide surfaces.

2. Experimental MEIS [31] is a quantitative surface structural and compositional probe that utilizes ion beams in the energy range of 50-300 keV. For all the data shown below, we have used 97 keV H+as a probing ion beam. From the kinematics of atomic scattering, Hf ions scattered off Mg atoms lose less energy than those scattered off 0 atoms resulting in two distinctive surface peaks in an energy spectrum. This effect allows for the separation of scattering signals from different elements in the surface (mass specificity). For an incident ion beam aligned with a low-index cryst~lographic direction, all atoms in the crystal lie in rows parallel to the ion beam. Scattering by the

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first atom in each of these rows results in a shadow cone that reduces the probability of atoms further along the row being hit by the ion beam. The vast majority of the incident ions will penetrate deep into the crystal. Only a few monolayers of the surface atoms will dominate the ion scattering process. The ion scattering yield from the bulk is greatly reduced due to this “channeling” effect [31-331. Thermal vibrations, surface reconstructions or surface and bulk defects make shadowing less effective. Those ions that scatter off subsurface atoms will be strongly blocked along crystallographic directions - resulting in distinctive blocking dips in the angular spectra of the backscattered ion yield. Surface reconstruction or relaxation will either induce an overall change in the spectra (extra or missing blocking dips) or shifts in the blocking dip positions. By measuring the angular distribution of the backscattered ion yield, one can therefore extract information on surface structure. While in many circumstances one can get qualitative structural information by mere inspection of the angular spectra, quantitative information of the surface structure relies on more detailed analyses of experimental spectra by means of Monte Carlo simulations. In order to perform computer simulations, one needs to either know or assume a particular model for: (i> the scattering potential, (ii> the surface as well as bulk structure of the target, and (iii) the vibration amplitudes of all atoms in the target. In the MEIS energy regime GO-300 keV), the scattering potential is well known and can be viewed as a screened Coulomb potential [31]. We have used the Moliere potential [34] in our simulations. This is the most commonly used form in RBS analyses. An important strength of MEIS is that both the simulations and the experiments can be performed in absolute units. This means that effects like sample misalignment and surface disorder can be detected as they give rise to increases in backscattered yields. While the overall angular structure of the backscattered yield is mainly determined by the bulk and surface structure of the crystal, the total yield depends sensitively on the atomic vibrations. Larger vibrations will lead to more deep

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layer atoms becoming visible to the ion beam. The vibration amplitudes of the bulk Mg and 0 atoms we use in our simulations are taken from an earlier theoretical calculation [35] to be 0.062 and 0.065 A, respectively (isotropic). Vibration amplitudes of the surface atoms are usually found to be larger than those of the bulk atoms due to the reduced coordination numbers. We characterize the surface vibrations with two parameters, no(= UG;/rr,“) and qMvrp(=U&./U&), defined as the ratio of the vibration amplitudes of the surface atoms (Us> with that of the bulk atoms (Ub) for 0 and Mg, respectively, both assumed to be isotropic for simplicity. These quantities are known as the surface vibrational enhancement of the 0 and Mg atoms. The surface vibration amplitudes are allowed to decay exponentially to the bulk value with a decay constant of 1.9 and 4.9 A for 0 and Mg, respectively, chosen to give the best matches of the blocking dip shapes prior to the detailed R-factor analysis. The total number of parameters we have allowed for our model of the surface is four: Ad,, (relaxation), rumpling, 770 and TMg. In a search of a surface structure that best describes the experimental spectra, one usually tries several different surface structures and optimizes the surface structural parameters in each case. The structure that gives simulations closest to the experiment is usually deemed the result of the experiment. An objective assessment of the goodness-of-fit between the experimental and simulated yields is realized with an R-factor (x2) calculation:

GIoo/i,

(1)

where N is the number of data points, and YeXP is the experimental (simulated) yield [36]. Due to the extremely insulating nature of the MgO crystal, an electron flood gun had to be used to keep the sample at a charge-neutral state during the experiments. This prevented the measurement of the ion current directly at the sample. A new ion current monitor was therefore conceived and constructed. The monitor consists

J.B. Zhou et al. /Surface Science 302 (1994) 350-362

of two coaxial cylinders with small openings in both ends, allowing the ions to pass through. Inside the inner cylinder is a fine conductive mesh with N 50% transmission. The relative current was measured from the mesh. The inner cylinder is kept at u 300 V with a battery to suppress secondary electron emissions from the mesh, and the outer cylinder is kept at ground. The details of the monitor are described in Ref. [37]. The accuracy of the current measurement was tested for ion beams under different focusing conditions. An insulating material may be subject to a very high rate of radiation damage. This is mainly due to electronic excitation induced bond breaking, as well as the lack of self-annealing in compound materials. Since the electronic contribution to the stopping power is typically lo2 greater than the nuclear contribution in the MEIS energy regime, a “worst case” estimate of the electronic excitation induced damage (EEID) rate can be lo2 higher than that induced by nuclear recoil, assuming ail the dissipated energy is converted to atomic displacements. However, in the case of MgO, the predominant EEID mechanism for ionic materials - the Knotek-Feibelman [38] mechanism - is known to be very inefficient due to structural effects [39]. It is generally difficult to predict the magnitude of EEID due to the many possible relaxation channels for an electronically excited species. On the other hand, damage induced by nuclear recoil can be well predicted by the Kinchin-Pease theory [31,40]. According to this theory, the displacement cross section for a primary recoil atom, a,, and the total number of displaced atoms in the cascade per primary knock-on, Nd, are given by: Ud =

4M IZ2Z2E2 1 2 R M2

Ed

&

(2)

and

where Et”‘= is the maximum recoil energy (at 180” scattering), Ed the displacement energy (20 eV>, E, the Rydberg energy (13.6 eV), and a0

353

the Bohr radius. The total number of displaced atoms per incident ion is: N, = Na,N,,

(4)

where N is the area1 density of the target. For 97 keV protons incident on MgO, N = 2.3 X 1015 N -4 3 and N atoms/ cm2, a, = 1x10-‘9cm2 yd = 1 X 10d3. A damage of 1% of a monolaye: (2.3 x lOi atomsfcm2) corresponds to a beam dose of w 2 X 1016 ions/cm2. We estimate the damage situation by observing the change in the integrated Mg yield in a channeling experiment in a 20” range of scattering angles, and in an energy range which contains the dispersing Mg surface peaks. This provides an efficient method to examine the damage in the near surface region. The yield was found to be nearly constant up to a dose of 2 x 10” ions/ cm2, followed by an abrupt increase corresponding to the start of si~ificant damage. For a dose of 2 x 1017 ions/cm2, the damage is N 8% of a monolayer according to the Kinchin-Pease calculation. It is not unreasonable to expect a damage cascade to start in this situation. On the other hand, this tells us that EEID must be insignificant in MgO. The experiments described below were performed using a beam dose less than 4 x 10’” ion/cm2, with which a damage of less than 2% of a monolayer can be assured according to the above calculations and the experimental observations. The UHV ion-scattering chamber is equipped with a high-resolution toroidal electrostatic ion energy analyzer, LEED, double pass CMA for Auger analysis and a mass spectrometer. The ion analyzer has recently been upgraded to a two-dimensional detector which simultaneously measures a 20” angular range and a N 2 keV energy range for 100 keV ions. The relative energy resolution of the detector (AE/E) is N 1.5 X 10e3. Descriptions of a similar detector can be found in Ref. [41]. Since the electrostatic analyzer will filter out those backscattered ions that have been neutraiized upon leaving the target, we must measure the ratio of the charged particles to the sum of charged and neutral particles in the backscattered yield, and correct the measured yield ac-

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cordingly. This is done by using a solid-state detector which is preceded by a pair of parallel electrostatic deflection plates. With a proper voltage on the plates turned on and off, the neutralized and the total backscattered yield can be measured. The fraction of the charged particles is found to be 0.76 and 0.81 at 75 and 100 keV, respectively. Two differently prepared ~g~OO1) surfaces were studied. The first was a 10 x 10 x 1 mm3 wafer cleaved in air (Atomergic Chemetals, Inc., 99.99%). The sample was repeatedly sputtered and annealed in UHV until a clean surface was obtained as monitored with both Auger electron spectroscopy and MEIS. Several different wafers were studied with this preparation; all exhibited sharp LEED patterns. The most common contaminants found on these surfaces were C and Ca, which were successfully removed by the cleaning procedure. The second type of surface studied was prepared by cleaving in UHV. Bulk MgO crystals were obtained from Metron, Inc. (99.99%). The UHV-cleaved surfaces showed very sharp LEED spots, with much less diffuse background than the sputtered/ annealed surfaces.

3. Results

Science 302 (1994) 3X-362

Angle

90

91

YS

90

85

92

0%)

93

Energy (kcV) Fig. 2. (a) Bulk bIocking spectra (in arbitrary units) taken in the (010) scattering plane with a--97 keV H+ ion beam. The primary beam is incident along [loll and simultaneously detected in a range of + lo” near (iOl]. The energy of the detected ions is - 65 keV (scattered from the bulk). (bI MEIS energy spectra (in arbitrary units) of Cu taken from a 10 ML (average thickness) Cu thin film deposited on Mg~~l). Spectra are shown with (solid lines) and without (dots) a neutralizing electron beam on the surface.

3.1. Charge-induced features on oxide surfaces

Pure MgO is an extremely good insulator which has a 7.8 eV energy gap in the bulk [42]. In applying any spectroscopic technique based on charged particles to insulator surfaces, one has to take special precautions of the possibility of charge build-up on the surface. We found that the MgO surface can be highly charged by the incident positive ions, which in turn distorts ion trajectories near the surface. An electron flood gun was therefore used to neutralize the surface. The charge-induced effects in the MEIS spectra can be seen in Fig. 2. In Fig. 2a, two bulk blocking spectra are shown. These spectra are taken in the (010) scattering plane, with the ion beam incident along the [iOil direction and detected in a k 10” range around the [iOll direction. The detector pass energy is set at 65 keV, correspond-

ing to detection of ions that are scattered from the bulk. The solid curve shows the spectra with the electron flood gun turned on, and the dotted curve with the gun off. In the solid curve, a blocking dip at 90” is observed, as expected. When the electron gun is turned off, a 2.5” shift for the blocking dip is observed. The total yield also increases by more than an order of magnitude. Both the blocking dip shift and the increase in yield can be explained by charge-induced trajectory distortion. The MgO surface is highly charged by the ion beam which creates an electrostatic field mainly along the surface normal. An exiting ion is pushed along this direction on its way out to the vacuum, which results in an shift towards a larger scattering angle. The electric field affects the incident ion in the same way and greatly

J.B. Thou et al. /Surface

reduces the channeling effect, increasing the yield from bulk scattering. Fig. 2b shows two Cu spectra taken from a MgO substrate with 10 ML (average thickness) of Cu deposited. A shift in the Cu peak energy (N 500 eV), and an increase in the peak area are observed when the electron gun is turned off. Both of these features are consistent with the observed trajectory distortion. When the electron gun is turned off, the detected ions are scattered at an angle smaller than the apparent (detected) angle. The kinematic energy loss is smalier, and the scattering cross section is larger at this smaller angle. The energy shift is also partly due to the lowering of the ion kinetic energy at collision since the MgO surface is positively charged. The kinematic energy loss is proportional to the ion kinetic energy, and therefore the energy loss is also reduced.

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355

n

I

(a)

II 82

.

I

I

I

84

86

88

IQMWJ 90 92

Energy (keV) (b)

3.2. Comparison between the two surfaces In Fig. 3a, we show the 0 and Mg MEIS spectra taken from both the sputtered/annealed and UHV-cleaved MgO(001) surfaces in the (010) scattering plane. The dotted curve is taken from a wafer after Ne+ion sputtering and annealing to 1050°C for 15 min; the solid curve is taken from a UHV-cleaved sample. In the spectrum from the cleaved surface, the three isotopes, 24Mg, “Mg and 26Mg are resolved, something that could onIy be observed after the detector was upgraded. A clear difference in the peak shape between the two surfaces can be seen. The UHV-cleaved surface results in sharp and well defined Mg and 0 surface peaks, and the sputtered/annealed surface gives broad, diffuse peaks with increased areas. Broadened MEIS peaks and increased peak areas indicate that more ions are scattered from deep layer atoms. These data indicate a higher surface structural disorder for the sputtered/ annealed surface. We attribute this to surface roughness on the sputtered/ annealed surface. As MgO has a melting temperature of 28OO”C,it is likely that the atoms are not mobile at 1050°C and large-scale etch-pits created during sputtering are not annealed out. Ions scattered from the bottom of these etch-pits will have to travel a

Fig. 3. (a) MEIS energy spectra of 0 and Mg taken in the (010) scattering plane for both the UHV-cleaved (solid line) and the sputtered/annealed (dots) Mg~~l) surfaces. The ion beam (97 keV Hf ) is incident along [TOi] and detected along a random exit direction (84” scattering angle). The 24Mg, 25Mg and 26Mg isotopes are resolved. The small peak between the 0 and Mg peaks is due to the adsorption of small amounts of F from the residual gas (this sample had been kept in the UHV chamber for 10 h). (b) A roughened-surface model for the sputtered/annealed samples that accounts for the broadened peaks in (a).

long distance before they exit from the target. Fig. 3b presents such a roughened-surface model which accounts for the increased peak widths and peak areas. Ions scattered from a nucleus located below the surface experience an additional energy loss due to electronic excitations. This energy loss is proportional to t, the total distance traveled inside the scattering material, and thus proportional to the scattering depth. The FWHM increase from UHV-cleaved to sputtered/ annealed surface is w 500 eV. A calculation of energy losses based on this model gives a,n estimated rms surface roughness of - 15-20 A. Nak~atsu et al. [43] observed that when MgO is heated to 1380-141O”C, the surface is almost

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Science 302 (1994) 3X-362

free of defects. We were able to support their conclusion based on our MEIS energy spectra for samples annealed at different temperatures. We observed at higher annealing temperatures that both the 0 and Mg peaks became narrower. However, at temperatures above 125o”C, difficulties arose due to impurity segregation (mostly Ca and Sr), and sampie fracture. Therefore, our analyses on the surface structure are all performed on the data taken from UHV-cleaved samples. 3.3. Surface structure determination To determine the surface structure, we have taken MEIS spectra in both the (li0) and (010) plane (cf. Fig. 1). In the (110) plane, there are two inequivalent planes containing alternating rows of 0 and Mg atoms (Fig. 4a). One plane is terminated with a row of 0 atoms and the other with a row --- of Mg atoms. The ion beam is incident along [Ill] and detected near [al]. The second layer 0 (Mg) atom is blocked by the first layer Mg (01 atom. The bulk blocking dip along the [ii11 direction occurs at a scattering angle of 70.53”. Since the scattering probabili~ quickly decays to zero after a few layers, the blocking dip position is mainly determined by the relative position of the first and second layer atoms. An inward (outward) first layer relaxation (assuming no rumpling) of the surface would shift both blocking dips below (above) 70.53”. On the other hand, a rumpling (assuming no relaxation) of the surface would shift the 0 (Mg) blocking dip below (above) 70.53”. The experimental spectra and simulations in the (1iO) plane are shown in Figs. 4b and 4c. Both the 0 and Mg blocking dips are very close to 70.53” (the bulk blocking direction), indicating that both the relaxation and rumpling are small. A slight shift towards lower angles in both the 0 and Mg blocking dips can be seen, with the 0 blocking dip shifted slightly more than the Mg dip. A very small degree of inward relaxation and rumpling therefore exists. Simulated results for our best model (see below) are shown. For comparison, simulated spectra with a -5% (inward) relaxation and a 0% rumpling are also shown.

60 1.61-1

65 I Scattering

/ 70 75 Angle (Deg)

1 so

Fig. 4. (a) Schematic picture of the (10) scattering plane which consists of two inequivalent planes, terminated with a row of 0 (right) and Mg (left) atoms, respectively. (b), (c) The angular distribution of the normalized backscattered flux from 0 (circles, (b)) and Mg (squares, (c)I. The spectra were taken with 97 keV incident protons. The solid lines are simulations of our best structure; the dashed lines are simulations using Ad,, /d,, = - 5% and rumpling 0%.

The simulation results in 0 and Mg blocking dips both, with a large shift towards a lower angle, which does not agree with the experimental spectra. By changing the value in rumpling (still using 5% relaxation), either the 0 or Mg spectrum can be made to fit the experiment better, but only at the expense of the other blocking dip being further shifted away from the experimental position. Therefore, the spectra in this scattering plane clearly indicate that the surface relaxation must be smaller than 5%. In principle, since there are four parameters for our surface model, the angular spectra in the (1iO) plane alone are sufficient. The overall yields for 0 and Mg (Figs. 4b and 4~) impose two constraints on the vibration amplitudes. The two blocking dips in the 0 and Mg spectra impose another two constraints on the relaxation and

J.3. Zhou et al. /Surface

[ioli

(a)

rioi]

0 Tl 2.2 2.0 1.8 1.6 2.d== 2.4 2.2 2.0 1.8 1.6 80

85

90

95

100

Scattering Angle (De@ Fig. 5. (a) Schematic picture of the (010) scattering plane. (b), (c) The normalized angular distribution of the backscattered flux from 0 (circles, (b)) and Mg (squares, Cc)).The solid lines are simulations of our best structure; the dashed lines are simulations using Ad,, /d,, = - 1% and rumpling 6%.

rumpling. The number of constraints already matches the number of the unknowns in one scattering plane alone. However, it is still advantageous to look at a second scattering geometry. These additional constraints can serve as cross checks for experimental errors. Obviously, if the experiments are performed with absolute accuracy, results from the two scattering planes should yield exactly the same vaIues of the surface structure parameters. If on the other hand, results from the two scattering planes yield structure parameters excluding each other, either the model itself or the experimental accuracy needs to be reexamined. The (010) plane is identical to the (001) plane {Fig. 5a). In this scattering plane, the ion beam is incident along [TOi] and detected near [‘iol]. Ions scattered from the second layer 0 (Mg) atoms are blocked by the first layer 0 (Mg) atoms along the

Science 302 (1994) 350-362

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nOl] direction. Bulk blocking along [iOl] occurs at a 90” scattering angle. An inward first layer relaxation (assuming no rumpling) will result in 0 and Mg blocking dips both shifted below 90”. Rumpling (assuming no relaxation) in the first layer will result in an 0 (Mg) blocking dip shifted above (below) 90”. The experimental and simulated spectra in this scattering plane are shown in Figs, 5b and 5~. Both the 0 and Mg blocking dips are very close to 90”, again indicating relaxation and rumpling to be very small. The simulated spectra using - 1% (inward) relaxation and 6% rumpling are shown for comparison. Simulations using these values result in 0 (Mg) bIocking dips shifted too far above (below) the bulk blocking angle, compared with the experimental spectra. By changing the value of the relaxation, either the 0 or Mg spectrum can be made to fit the experiment better, but only at the expense of further shifting the other blocking dip away from the experimental spectrum. It is obvious from simple geometrical considerations, in a first approximation, that the relaxation determines the average blocking dip positions and the rumpling determines the relative blocking dip positions. Quantitative dete~ination of the relaxation and ~mpIing relies on detailed R-factor analyses. Using the R-factor formula introduced in Section 2 (Eq. Cl)), R-factors for many different sets of structural parameters were evaluated in both scattering planes. In each scattering plane, the results for the 0 and Mg spectra are combined to give one total R-factor. Since there are four independent variables, R-factors are first evaluated for two variables. After a minimum R is found, the remaining two variables are then varied. This procedure is repeated until both sets of variables are optimized consistently. Figs. 6a and 6b show the R-factor plots for the data in Fig. 4, i.e. in the (li0) scattering plane. Minimum values of R are found to exist in the region Ad,, = - 1.5%-O%, vo = 1.15-1.25, rumpling = O%- + 2.0%, TIMg= 1.5-1.6. Figs. 7a and 7b show the R-factor plots for the data in Fig. 5, i.e. in the (010) scattering plane. Minimum values of R are found to exist in the region Ad,, = -2.O%-0.5%, qo = 1.1-1.2, rumpling = -OS%- + 2.0%, 7JMg= 1.45-1.55. The slight dif-

J.B. Zhou et al. /Surface

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ference in the vibrational enhancement values from the two scattering planes are possibly due to different vibrational correlations between neighboring atoms in the two planes. In the present simulation, vibrational correlations are not taken into consideration and therefore, the measured vibration amplitudes refer only to the relative vibration value. The difference may also be due to surface vibrational anisotropies as observed in other systems 1441, since MEIS measures the vibrational amplitudes perpendicular to the ion beam. From the R-factor plots in Figs. 6 and 7, we conclude that the best estimates of the structural parameters and vibrations are Ad,, = - 1.0% f LO%, rumpling 0.5% f LO%, qn = 1.1 and qMg = 1.5 f 0.1. Simulated spectra in

Science 302 (1994) 350-362

O-4.0 -3.0

-2.0 -1.0

0 1.0 Ad,, (%“u)

2.0

3.0

4.0

2.0

3.0

4.0

(b) 2 9

F

Py

--4.0

-3.0

-2.0

-1.0

0

1.0

Rumpling (%)

o -4.0

-3.0

-2.0

-1.0

0 1.0 Ad12 (%I

2.0

3.0

4.0

Fig. 7. R-factor contour plots for the data shown in Fig. 5, as a function of (a) change in the first and second layer spacing, Ad,,, and the oxygen surface vibrational enhancement, no, and (b) surface rumpling and the magnesium surface vibrational enhancement, nMa.

both scattering planes based on these final values are shown in Figs. 4 and 5 (the simulation in Fig. 4 uses qMg = 1.6). The simulated spectra in both scattering planes agree very well with the experimental spectra.

4. Discussion -4.0

-3.0

-2.0

-1.0

0

1.0

2.0

3.0

4.0

Rumpling (%f Fig. 6. R-factor contour plots for the data shown in Fig. 4, as a function of (a) change in the first and second layer spacing, Ad,,, and the oxygen surface vibrational enhancement, no, and (h) surface rumpling and the magnesium surface vibrational enhancement, nMs.

Results from the present experiments indicate small values of both relaxation and ~mpling. The small inward relaxation from the present experiment (-- 1.0% + 1.0%) seems to agree with most other experimental and theoretical results. Table 1 lists the results of many experimental and theo-

J.B. Zhou et al. /Surface

retical studies. Considering the uncertainties in data analyses or model approximations in most studies, all studies except the ICISS [23] and SEELFS 1241 have agreed upon such a small relaxation. In the ICISS study, an electron shower from a hot filament was used to neutralize the sample surface. However, a small degree of surface charge must still have existed as evidenced by the < 16 eV shift in the peak energy that was measured even at optimum electron irradiation conditions [231. These surface charges could shift the observed critical angles in the spectra which were used to obtain the surface relaxation (cf. Fig. 2 above). In the surface-extended-ener~-loss-fine-st~cture (SEELFS) study [24], it was not possible to explain the data by a stoichiometric MgO(001) surface. In order to fit to the data, a model which contains extra adsorbed 0 atoms (on top of the

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3.59

Mg atoms) had to be used. Within this model, a - 17% inward relaxation of the first complete layer was inferred. The sample used for this study was cleaved in air and annealed in UHV. Several points with regard to this model are worth pointing out. First, it is clear from a recent report [27] that Mg0(001) surfaces cleaved in air contain structural defects even if they are subsequently annealed in UHV. SEELFS, as a local structural probe, is sensitive to these defects. Also, for both the UHV-cleaved and sputtered/annealed surfaces, the O-adsorbed model does not fit our data. Within this model, the two alternate planes in the (li0) scattering plane (Fig. 4a) would both be terminated with a row of 0 atoms, leading to an increase in the 0 yield by _ 1 atom per unit cell. As a result, the 0 yield would be greater than 2 atoms per unit cell for all scattering angles. This is clearly not the case (Fig. 4b).

Table 1 Results from various experimental and theoretical studies on the MgO@Ol) surface structure; positive (negative) values of the relaxation refer to expansion (contraction) between the first and second layer atoms; positive (negative) values of rumpling refer to outward (inward) displacement of the Ozc ions with respect to the Mg*” ions; both quantities are expressed in percent of the bulk inter-layer spacing (2.105 .Q Technique

Author

LEED

Kinniburgh [4], 1975 Welton-Cook and Berndt [II], 1982 Urano et al. [14], 1983 Blanchard et al. [28], 1991

RHEED

Murata et at. f6], 1977 Maksym [ l&X],1985

Relaxation (Ad&d) - 0.3% f 1.6% -0% 2.5% I%‘oi%

Rumpling -2% - 0% 5% i 2.5% -6%

O%-3%

KISS

Nakamatsu et al. [23], 1988

- -15%

SEELFS

Santoni et al. [24], 1988

Shell model calculations

Welton-Cook and Prutton [7], 1978 Martin and Bliz 181,1979 Lewis and Catlow [ZO],1985 de Wette et al. 1191,1985

Tight-binding calculations

LaFemina and Duke [29], 1991

Hartree-Fock calculations

Causa et al. /21],1986

LDA calculations

Li et al. [30]

-0%

MEIS

Present work

- 1.0% + 1.0%

- 0.3% f 0.9%

- 17% - -0.3% -0.2% -0.7% - -0.6%

5% -0% 11% 2.4%

- 1.5%

2.4%

0%

0.9% -1% 0.5% f 1.0%

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J.B.

Zhou

et at. /Surface

The disagreement on rumpling between various experiments seems to be beyond experimental uncertainties. On the one hand, Murata and coworkers [6,9,10] have studied the vacuumcleaved MgO(OO1) surfaces. They observed irreversible changes in both LEED and RHEED Kikuchi patterns when the sample was annealed to 300°C. The authors interpreted these features as a phase transition in the surface layer from a metastable, bulk terminated structure, to a stable and rumpled surface. Their results have stirred much interest. Yet, based on simple thermodynamic considerations, it is not likely that such a phase transition will require a high temperature (3OO’C) since there are no obvious intermediate high-energy-barrier-states to be passed during such a phase transition. LEED Z--I/ studies of Urano et al. [14] show Z-I/ curves that are identical if the surface is air-cleaved/annealed, UHVcleaved or UHV-cleaved/ annealed. We measured MEIS angular spectra of the sputtered/ annealed and UHV-cleaved surfaces that were very similar in shape. The blocking dip positions were close between the two types of surfaces even though the overall yields differed. This indicates that the relaxation and rumpling of the two differently prepared surfaces are quite similar. Different values of the rumpling have been determined in LEED Z-V studies. Welton-Cook and Berndt 1111measured 2%, Urano et al. [14] 0%, and most recently Blanchard et al. [28] 5.0% + 2.5%. It is worth pointing out that the spectra for the (01) and (11) beam taken by Urano et al. [14] and by Blanchard et al. [28] were strikingly similar to each other and yet their results on the rumpling differed significantly. This difference cannot be explained by the fact that only the latter experiment 1281 is R-factor assisted in the data analyses. The difference in the final values on the rumpling could then be due to different approximations in the calculations. Theoretically, a large number of studies of the surface structure have been performed based on shell model appro~mations [7,8,19,20]. Such calculations typically involve a large number of parameters obtained by fitting to existing bulk data. Unfortunately, for many oxides, there is usually insufficient experimental data to determine all

Science 302 (1994)

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the unknown parameters in the shell model potentials. Also, systematic errors are likely to enter in calculations of the surface equilibrium state using the same parameters as in the bulk. Furthermore, it has been suggested that bonding near the MgO surface may not be fully ionic due to the instability of the 02- ions 18,121. It is therefore not surprising that the calculated values of the rumpling are scattered from 0% [8] to 11% [20]. More recent total-energy calculations based on the Hartree-Fock [21] and tight-binding [291 approximations indicated rumplings of 0.9% and 2.4%, respectively, in agreement with our results. The very insulating nature of MgO may add a non-trivial complexity to the inte~retation of various experimental data. In many cases, an electron flood gun or a simple hot fiIament is used to keep the sample in a charge neutral state. Depending on the experimental conditions, a small degree of surface charge may still persist on the surface, which may induce significant changes in various spectroscopic data. In the present experiment, this effect should be far less important due to the use of high energy ions. Finally, preliminary results from a parameterfree, all-electron total-energy LDA calculation 1301 of the MgO(~l) surface are in very good agreement with our results on the relaxation and rumpling. Both quantities have been calculated to be small, indicating a surface close to a bulkterminated structure. For ionic compounds, strong electrostatic field exists in the surfaces. For the Mg~OOl) surface, this field points towards the outside (inside) of the crystal near the 02(Mg”) ions, resulting in attractive forces for both ions. These strong forces tend to polarize the Mg2+ and 02- ions. The outmost shell of the Mg2+ ion (2~‘) is bound tightly to the nucleus and is energetically costly to displace. The outmost shell of the 02- ion (also 2p6) is bound less tightly and is easier to displace. The rumpling of the MgO(0011 surface is therefore mostly due to the polarization of the 02- ions. The recent LDA total-energy calculation I301 reveals that the surface 02- ion polarizations are much smaller than those obtained by fitting shell model parameters to bulk phonon dispersion data.

f.3. Zhou et al. /Surface Science 302 (199413.50-362

5. Conclusions

In summary, we have studied the MgCX0011 surface structure using medium-energy ionscattering with channeling and biocking. Sputtered/ annealed surfaces contain structural disorder as evidenced by increased MEIS peak widths and peak areas even though sharp LEED spots can be obtained. UHV-cleaved surfaces have better structural order. Our results show that surface relaxation and rumpling are - 1.0% * 1.0% and 0.5% f LO%, respectively. The surface is therefore best thought of as having very slight modifications of a bulk terminated structure. We believe that the diverse values on rumpfing from various other experiments are due to either the complexity in data analyses of those experiments or difficulties in working with insulating materials. Our results are in excellent agreement with the recent total-energy calculations based on LDA appro~mations 1301 which reveal that the O*ion polarizations are much smaller than those obtained from shell model cakzulations.

Acknowledgements The authors would like to acknourtedge the support of this work by the National Science Foundation Materials Research Group Program through Grant No. DMR 8907553, NSF Grant No. INT-9121013 and FONDECYT Grant No. 1931126 (Chile). We thank Dr. M. Chester for his help, and Professor D. Langreth and Mr. Y. Li for valuabIe discussions. We aIso thank Dr. H. Nakamatsu for providing us with details of his method of mounting MgO wafers for high-temperature annealing.

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