the role of impurities in the stability of zno surfaces

Surface Science 48 (1975) 486-496 © North-Holland Publishing Co.

THE ROLE OF IMPURITIES IN THE STABILITY OF ZnO SURFACES

B.J. HOPKINS, R. LEYSEN* and P.A. TAYLOR Surface Physics, University of Southampton, Southampton S09 5NH, England

Received 20 August 1974; revised manuscript received 7 October 1974

Both "as-grown" and "real" etched prism and (0003) oxygen surfaces have been studied by LEED and Auger electron spectroscopy. Heat treatment up to 800 K was sufficient to remove impurities other than calcium on all surfaces and potassium on the polar "real" surface. These could only be removed by ion bombardment. The Ca was associated with a (3 X 1) superstructure on the prism surface and a (x/r3 × x/3) on the polar surface. On the "as-grown" polar surface it was also possible to see (3 × 3) structure associated with re- duced amounts of Ca. The especially strong binding of the electropositive elements on the negative oxygen polar surface is due to charge transfer, i.e. impurity stabilisation, this in turn can lead to chemical shifts in some of the Zn Auger transitions and to changes in the oxygen peak shape.

1. Introduction

The most comprehensive investigation of the role of impurities on ZnO single crys-

tal surfaces was made by Fiermans et al. [1 ] using the techniques of LEED, Auger spectroscopy and X-ray photoelectron spectroscopy. Attent ion was restricted to chemically etched polar surfaces and some attempts made to relate a range of surface structures to the surface impurities detected. Of these, some were clearly associated with the etch, but the presence of calcium was not and the possibility that this was diffusing from the bulk was suggested. There was an indication that a stable (x/3 x x/3) superstructure, also recorded by Levine et al. [2], was associated with this calcium. An ion-bombardment~anneal treatment ultimately produced a clean (1 x 1) surface, although Levine et al. [2] obtained only a (x/~ xx/'3) by similar treatment. Both Levine et al. and Chung and Farsworth [3] formed a (1 x 1) structure by ion bombardment and annealing on the prism surfaces. Such (1 x 1) polar surfaces have also been seen on UHV cleaved crystals by Mtiller [4] and by Van Hove and Leysen [5]. In the earlier Auger study of Levine et al. [2] carbon was reported to be the only significant surface impurity.

The presence of impurities on ZnO surfaces is of particular interest for the polar

* Gillette International Fellow, present address: Fysico-Chemisch Laboratorium, Celestynenlaan 200 G, 3030 Heverlee, Belgium.

B.J. Hopkins et al./Impurities and stability o f ZnO surfaces 487

surfaces. Unlike the prism surface, these consist of sheets of ions of only one sign which are therefore unbalanced. The electrostatic stability of such surfaces can be appreciably enhanced [6] by the formation of a superstructure of appropriate electropositive or electronegative adatoms. Levine et al. [2] used an argument based on Zn or O vacancies to explain the extreme stability (up to 1300 K) of the (x/~ xx/r~) structure observed on both of their polar surfaces. The presence of foreign atoms or a deficiency of the parent atoms are not the only ways the polar surfaces can be sta- bilized. Intrinsic surface states achieve this function, as in the (1 x 1) cleaved polar surface of Van Hove and Leysen [5] and of Glachant et al. [7] for the clean polar surface of GaP. In the case of the polar Zn surface, Leysen et al. [8] have shown that of the possibilities, the most favoured energetically, when suitable alien atoms are available, is the impurity stabilisation.

The object of the present work is therefore to establish a link between the various superstructures, their stability on the surfaces and their chemical nature. Although the polar surfaces are likely to be of most interest, we have also studied the prism surfaces. Since the polishing and etching process can introduce impurities [ 1 ] the studies were made on both "as-grown" crystals, which had only been lightly cleaned, and on "real" surfaces that had been both polished and etched. In the final stages ion-bombardment/anneal cleaned surfaces were investigated.

2. Experimental

The ZnO crystals were undoped, uniformly grown, twin-free samples having a re- sistivity of about 5 -10 ohm cm. Pencil-like in shape they were a few cm in length and a few mm across the hexagonal section. Crystals could be found with a well de- veloped "point" at one end and good fiat surface at the other. The application of standard etching techniques [9] to a suitable crystal showed that, as expected, the point was a Zn or (0001) surface so that the flat at the opposite end corresponds to a naturally grown oxygen or (000~[) surface. The crystal was mounted in tungsten clips with Pt foil insert to ensure good ohmic contacts, such that by rotation of the crystal manipulator, either the end (000I) O polar surface, or the side (10]0) prism surface could be examined. The crystal could be heated resistively and a P t -P t 10% Rh thermocouple pressed against the side permitted the temperature to be measured.

An ion-pumped, stainless steel vacuum chamber, fitted with a sublimation pump and quadrupole mass analyser, was used throughout the work and was capable, after the usual bake-out, of ultimate pressures better than 2 x 10 -10 Torr. The system also incorporated a 4-grid LEED optics, a grazing incidence Auger electron gun and an ion gun. A low noise Auger electronic detection system [10] allowed the use of relatively low Auger beam currents (< 6 #A) while maintaining a reasonably fast scan rate.

The only surface preparation given to the "as-grown" crystals was a brief cleaning in H3PO 4 followed by extensive washing in distilled water. The "real" crystal surfaces, on the other hand, were given a full treatment of mechanical polishing with diamond

488 B.J. Hopkins et aL/Impurities and stability o f ZnO surfaces

paste followed by chemical etching. The polar surface was first etched with HC1 and H3PO 4 and then protected while the prism surface was further etched with HC1 [9]. The whole crystal was finally rinsed in distilled water.

After mounting the crystal into the vacuum chamber and pumping into the 10 -9 Tort range without bake-out, a brief Auger/LEED examination was made. In all cases a very large carbon peak dominated the Auger and there were no LEED diffraction features other than a very high background. An interesting observation at this stage was that exposure to the electron beam caused a progressive fall in the carbon Auger signal [11]. A significant reduction could be obtained after several hundred microamp minutes of exposure. All the results in the next section were made after bake-out and the achievement of an ultra-high vacuum.

3. Results

3.1. "As-grown "' surfaces

Following the bake-out there was no detectable change in the state of the surfaces until the crystal had been heated to 750 K for about 1 hr.

Fig. 1 shows the (3 x 1) superstructure formed by this treatment on the prism surface together with the Auger spectrum showing as well as Zn, O and C, there are peaks due to S, C1 and Ca. Further heating to 800 K caused the removal of all the impurities with the exception of the calcium while the (3 × 1) structure remained. Raising the temperature to 1000 K did not change the LEED pattern but resulted in a slight increase in the Ca Auger peak height, suggesting the possibility of out-diffusion from the crystal bulk to the surface (fig. 2).

The Auger spectrum from the polar (0001) surface after heating to 750 K was very similar to that of fig. 1 for the prism surface except that the 107,926,991 and 1014 eV Zn peaks now appeared displaced in energy by some +4 eV. The LEED pattern (fig. 3) corresponding to this surface was now indicative of a (X/~ x x/~) superstructure. As on the prism surface, at 800 K all impurities other than Ca were removed, while, the LEED pattern was changed to a (3 x 3) structure (fig. 4).

The only difference between the Auger spectra corresponding with the LEED pat- terns of figs. 3 and 4 is the peak height ratio of the Ca/O. Further heating up to 1100 K did not cause any further change in either Auger or the LEED.

No attempts were made to use temperatures higher than 1100 K to remove the two terminal superstructures as there was appreciable gas evolution from the crystal even at 1000 K. Since much of this gas was oxygen we suspected that it originated from the decomposition of the crystal. Further crystal cleaning was therefore by argon ion bombardment.

B.J. Hopkins et al./Impurities and stability of ZnO surfaces 489

IOO1 2~o 3~o 4~o 5ool ENERGY EV

(a)

(b)

Fig. 1 (a) Auger spectrum of the (1010) prism ZnO surface after heating to 750 K for 1 hr. Con- ditions: beam current = 4 #A, primary energy = 2.5 keV, modulation 10 V ptp, time constant = 0.3 s. (b) Corresponding (3 X 1) LEED pattern 74 eV.

3. 2. "Real" surfaces

The main difference between the "real" surfaces and those "as-grown" was the presence of extra peaks in the Auger spectrum. After the heating to 750 K, peaks due to S, P, Ca and C1 were observed as for the "as-grown" crystal together with new peaks which could be identified as K. The new peaks showed up best after heating to 800 K

490 B.J. Hopkins et al./Impurities and stability o f ZnO surfaces

I I I I I

: o

I

_ _ ~ ~ O

d ~ " s , 2 ~

'~° ~t ° ' t . . . . . . . , , ENERGY EV

Fig. 2. Auger spectrum of the (1010) prism ZnO surface after heating to 1000 K showing Ca impurity and high energy spectrum of the main Zn peaks. Conditions: beam current = 6 ~A, primary energy = 2.5 keV, modulation = 10 V ptp, time constant = 0.3 s.

when all the impurities other than Ca were removed, fig. 5. An unusual effect on the potassium contaminated surfaces was noticed for the polar surface only. Whereas on the prism surface the oxygen triplet was normal in appearance, on the potassium contaminated polar surface the oxygen shape had become similar to that commonly observed [ 11 ] for chemisorbed oxygen.

4

Fig. 3. (x/~ × x/~) structure on the (000~) O surface; 70 eV.

B.J. Hopkins et aL /lmpurities and stability of ZnO surfaces 491

Fig. 4. (3 × 3) structure on the same (0001) O surface as in fig. 3 after fur ther heating; 70 eV.

z"

I 300 I

ENERGY EV

4OO 5OO ! I

Fig. 5. Auger spect rum of the' (0007) O polar "real" surface~after heating to 800 K, showing K- impurities. Condit ions: beam current = 6/aA, primary energy = 2.5 keV, modula t ion = 10 V ptp, t ime cons tant = 1 s.

492 B.J. Hopkins et al./hnpurities and stability of ZnO surfaces

I ! I I I

800 900 I000

200 30O 400 500 L L I !

ENERGY EV

F xl

x4

I

(a)

(b)

Fig. 6. (a) Auger spectrum of the "clean" (0001) O polar surface after argon bombardment and annealing. Auger conditions are the same as in fig. 5. (b) Corresponding (1 X 1) structure; 81 eV.

B.J. Hopkins et aL/lmpurities and stability of ZnO surfaces 493

Removal of the potassium could be achieved on the prism surface by simply heating to about 900 K, the only remaining alien element was then calcium - as for the "as-grown" crystal. On the polar surface the potassium was much more tenaciously bound and one hour at 1000 K was ineffective; complete cleaning again required ion bombardment. On the prism surface the final surface again gave rise to a (3 x l) superstructure, while on the K/Ca contaminated final polar surface a (x/~ x x/~) structure was observed as in fig. 3.

3.3. Argon ion bombarded/annealed surfaces

The Auger spectrum was monitored after each cycle of ion bombardment/anneal which comprised: 1 hr in 2 x 10 -5 Torr Ar with 2 to 3/aA cm -2 beam at 400 V followed by annealing at 900 K for 15 min. Three such cycles were necessary to produce the clean polar surface as shown in fig. 6. The prism surface could be similarly cleaned to give a (1 x 1) surface.

On both surfaces heating to more than about 100 ° above the annealing temperature caused the re-appearance of calcium on the cleaned surface together with streaking in the LEED. With increasing time at an elevated temperature the streaks became resolved into the (3 x 1) pattern on the prism surface, and the (3 x 3) on the polar surface. An example of the streaking is shown in fig. 7 for the prism surface formed after heating to 1000 K for about 15 min.

3.4. Auger peak changes

Mention has already been made of changes in both peak shape and peak position

Fig. 7. LEED pattern of the (1010) prism surface after excessive heating till about 1000 K for 15 min showing streaking in the third order direction [perpendicular to the (0001) azimuth] ; 70 eV.

494 B.J. Hopkins et aL/Impurities and stability of ZnO surfaces

associated with the polar surface. The "normal" oxygen Auger peak shape is shown in fig. 6 and the positions in energy of Zn peaks are also characteristic of the clean surface. The inclusion in the surface of large amounts of electropositive elements, such as Ca and K, fig. 5, caused first an alteration in the oxygen transitions and second changes in position of the Zn peaks. The higher Zn transitions at 926,991 and 1014 eV (fig. 6), were displaced some 4 eV towards higher energy on the contaminated surfaces. At the low energy end, the single 107 eV peak of the clean surface gives place to two peaks, at 95 and 112 eV (fig. 5). The surface structure in this state is (x/3 x x/~). The 112 eV peak would appear to be the 107 eV peak displaced by the same amount as the high energy peaks, while the 95 eV peak is probably an entirely new feature of the electropositive contaminated surface. Reduction of the surface Ca or K restores the oxygen peak shape, reduces the 95 eV transition and removes the 4 eV Zn peak displacement.

4. Discussion

Regardless of the pre-vacuum treatment, both the prism and the polar oxygen surfaces are grossly carbon contaminated when first examined by Auger. The carbon is easily removed by heating or by the action of the electron beam. Other impurities are S, C1 and Ca on the "as-grown" crystals and S, C1, Ca and K on the "real" surfaces. Ca and K are tenaciously held on the surfaces and are responsible for (3 x 1), (x/~ x x/~) superstructures, the other elements a~-e easily removed by heating. Ion bombardment is necessary to remove the Ca and K to give the clean (1 × 1) surface except for the "real" prism surface where heating alone will remove K.

The stability of the (x/3 x x/~) structure on the (000]) surface was first reported by Levine et al. [2] for crystal cleaning temperatures as high as 1600 K. Fiermans et al. [ 1 ] found "some indication of possible correlation" of the structure with Ca which they suggested could be out-diffusing from the crystal bulk. There is some descrepancy concerning the role of carbon, Fiermans et al. are in agreement with our observations, carbon is easily removed by heating to above 800 K. On the other hand, Levine et al. find that carbon was the only impurity remaining on the surface after ion bombardment and annealing to as high as 1600 K. No Auger traces were repro- duced by Levine et al. and it might be possible that the carbon transition was confused with that due to calcium. Further, at pressures in the low tens we do not find re-ad- sorption of carbon from the residual to be a problem over periods of many hours, clearly the sticking probability of CO/CO 2 on either clean or contaminated ZnO surfaces must be very low.

The link between the Ca on the surface as revealed by the Auger and the (3 × 1), (vr3 x x/~) and (3 x 3) superstructures is fairly conclusive although clearly this could only be followed in the present experiments during the removal of the Ca. An attrac- tive confirmation would be to add the electropositive metal to the clean (1 x 1) ZnO surface. Heating the clean surface can produce the same effect by out-diffusion of the

B.Z Hopkins et aL/Impurities and stability o f ZnO surfaces 495

Ca, unfortunately by the time the clean surface has been obtained there is insufficient Ca to produce well defined surface structures and only streaking in the direction of the ~r order features is observed.

A difference on the polar surfaces between the "as-grown" and the "real" crystals, other than in their impurity content, was in the number of surface structures. On the "real" surface the (x/~ x x/~) superstructure was immediately formed and was perfect- ly stable throughout the temperature treatment. The "as-grown" surface on the other hand also showed a (3 x 3) structure as higher heating temperatures removed some calcium. These structures would correspond to respectively ] , and ~ of a monolayer of Ca. This would suggest that the "as-grown" surface is better able to stabilise by compensation from surface states than the "real" surface which requires impurity stabilisation. The effect might be a consequence of differences in the microscopic surface structure, roughness, defects concentration etc. The electrostatic theory [6] for impurity stabilisation of the polar surfaces indicates that ¼ monolayer of unit positive electrostatic charges is necessary on the oxygen surface. Since the stable (x/r3 x X/~) surface is due to ~ monolayer of calcium, each Ca atom is contributing less than unit positive charge to stabilise the surface. Clearly there ought to be condiserable changes in the charge distribution on the surface depending on the nature, size and electropositivity of the impurity atom stabilising the surface. These might, for example, show up in work function changes, but they certainly show up in the Auger oxygen line shape fig. 5, when K is the impurity. Although on the prism surface K caused no change in the oxygen triplet, on the polar surface this large electropositive element caused a change giving rise to the shape common on studies of adsorbed oxygen on active metals [12].

The displacement of the Zn Auger transitions by some + 4 eV is also associated with the presence of the two electropositive elements, Ca and K in the surface. Charge will be transferred from the Ca or K to the lattice Zn atoms causing a decrease in the binding energies of the remaining electrons and the displacement of the Auger lines to high energy. Comparable, although opposite in size chemical shifts have been observed by Castle et al. [13] on a number of metals and by Haas et al. [12] in the ad- sorption of oxygen onto active metals (Ta, Nb, Mo). There the electronegative surface oxygen results in a displacement towards lower energies.

The appearance of the transition at 95 eV and its increase in amplitude when there are impurities on the surface we tentatively attribute to a loss mechanism [1].

5. Conclusions

Ca is the most persistent of impurities in the ZnO crystals used in the present study and cannot be removed from any surface by heat treatment alone but requires ion bombardment. On real surfaces K plays a similar role to Ca, the two being associated. Like Ca, it is very tightly bound to the polar face but less so on the prism face, from which it can be removed by heating. The Ca is responsible for a series of superstruc-

496 B.J. Hopkins et al./Impurities and stability of ZnO surfaces

tures: on the prism surface a single (3 x 1) structure is observed, on the polar surface a difference between the "real" and "as-grown" crystal surfaces was seen. On the lat- ter, not only is the (x/~ × x/'3) structure of the "real" polar surface observed, but also (3 × 3) between the (x/~ × X/~) and the clean (1 × 1). These structures reflect diminish- ing amounts of Ca on the surface and show the take-over in the role of surface stabili- zation between impurities and intrinsic surface states.

The strong binding of the electropositive elements Ca and K on the polar surface is associated with the important role these elements play in stabilising the out-of- balance charge distribution. The electron contr ibution of an excess of Ca and K to the ZnO gives rise to both chemical shifts in the Zn Auger transitions and also a change in the oxygen peak shape.

Acknowledgements

We would like to thank the Science Research Council for their financial support in this work. One of us, (R.L.) is indebted to the Gillette Company for the provision of an International Fellowship and also to the Royal Society and the Belgian National Foundat ion of Scientific Research (N.F.W.O.) for their support. Finally it is a pleasure to thank Dr. R. Helbig of Erlangen University, Germany for the generous provision of ZnO crystals.

References

[ 1] L. Fiermans, E. Arys, J. Vennik and W. Maenhout-Van der Vorst, Surface Sci. 39 (1973) 357. [2] J.D. Levine, A. Willis, W.R. Bottoms and P. Mark, Surface Sci. 29 (1972) 144. [3] M.F. Chung and H.E. Farnsworth, Surface Sci. 22 (1970) 93. [4] K. Miiller, in: The Structure and Chemistry of Solid Surfaces, Ed. G.A. Somorjai (Wiley,

New York, 1969). [5] H. van Hove and R. Leysen, Phys. Status Solidi (a) 9 (1972) 361. [6] R. Nosker, P. Mark and J.D. Levine, Surface Sci. Surface 19 (1970) 291. [7] A. Galchant, J. Derrien and M. Bienfait, Surface Sci. 40 (1973) 683. [8] R. Leysen, G. Van Orshaegen, H. van Hove and A. Neyens, Phys. Status Solidi (a) 18 (1973)

613. [9] A. Klein, Z. Physik 188 (1965) 352.

[10] R. Nathan and B.J. Hopkins, J. Phys. E 6 (1973) 1040. [11] B. Goldstein, Surface Sci. 39 (1973) 261. [12] T.W. Haas, J.T. Grant and G.J. Dooley, J. Appl. Phys. 43 (1972) 1853. [13] J.E. Castle and D. Epler, Proc. Roy. Soc. (London) A 339 (1974) 49.

the role of impurities in the stability of zno surfaces

Documents