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Total yield energy dependence of low-energy electrons from surfaces of ZnO crystals
Surface .Science 162 (1985) 209-216 North-Holland, Amsterdam
209
T O T A L YIELD ENERGY D E P E N D E N C E OF LOW-ENERGY E L E C T R O N S F R O M S U R F A C E S OF ZnO C R Y S T A L S Preben J. MOLLER and Jian-Wei HE Department of Physical Chemistry, H.C. Orsted Institute, University of Copenhagen. 5 Unioersitetsparken, DK- 2100 Copenhagen, Denmark Received 1 April 1985; accepted for publication 6 May 1985
The energy dependence of the reflection of low-energy incident electrons (of the range 0-18 eV above the vacuum level) from the polar zinc (0001) face, the non-polar (10i0) and the (1120) faces of ZnO crystals is explained in terms of the onset of inelastic electron-electron scattering accompanied by interband transitions. Based on the spectra a revision of the band structure diagram for energies near the band gap is proposed.
I. Introduction ZnO is a well known semiconductor and a widely used catalyst. Electron energy loss spectroscopy (ELS) experiments have given information on its electronic structure through energy loss features which have been interpreted as interband transitions [1-3]. Froitzheim and lbach [1] furthermore observed a c-axis orientation dependence for the (10]0) surface. In ultraviolet photoemission spectroscopy (UPS) experiments [4-5] and X-ray photoelectron spectroscopy (XPS) experiments [6] the density-of-states structure of the valence band both for the O 2p and the Zn 3d bands have been determined. Also, theoretical calculations [7] have proposed a band structure of the O 2p valence band and of the conduction band. Recently [8] photoconductivity measurements have suggested additional low density surface states within the forbidden band very near the conduction band edge. To further investigate the question of crystal surface anisotropy and to investigate the electronic structure for the low energies in the region near the band gap it would be useful to apply the Total Current Spectroscopy (TCS) method, which may give experimental information both for the conduction band and for the valence band of ZnO. The iterative calculation is facilitated in this case through the use of the available experimental UPS and XPS data for the valence band. TCS is a recently developed technique [9-12] in which an electron beam of low energy is directed onto a target surface. The total (or target) current J in the target circuit is measured with respect to the incident primary energy E 1, 0039-6028/85/$03.30 ~;' Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
210
P.J. Moiler, Jtan-Wet He / Surface.g of ZnO crv~'tal~"
the latter typically being 0 15 eV above the vacuum level. The energy structure of the total yield resulting from the impingement of electrons onto surfaces can be related both to interband transitions and to collective oscillations. Both elastically and inelastically reflected electrons contribute to the total current. It has been indicated theoretically [10] and experimentally, e.g. refs. [11-131, that at very low incident energy (less than 20 eV) the fine structure in the TCS spectra, d J ( E i ) / d E I versus E l, is dominated by inelastic electron-electron scattering processes, resulting in interband transitions which can be analyzed through a convolution over density of states in the approximation of constant transition matrix element, energy conservation and wave vector non-conservation in the scattering process. Since a quite high density of unoccupied states below the vacuum level has been suggested for ZnO [2,8] then a sharp change in the efficiency of inelastic scattering, which causes fluctuations in the intensity of the elastically reflected current, and hence in that of the yield, would be expected, and the obtained fine structure TCS spectrum may then be described by the above name model. In this paper we present experimental TCS results for three Z n O surfaces: the polar zinc (0001) surface and the non-polar (1010) and (1120) Z n O surfaces, and we conclude that the main structures in the TCS spectra are not angular dependent for primary energies below 18 eV, and that the structures can be interpreted as due to electron and band transitions. Further we propose a revised band structure diagram for ZnO for the energies near the band gap, as determined from TCS experiments.
2. Experimental details These experiments were performed using the experimental procedures and UHV apparatus previously described [11 ]. A fixed accelerating voltage (of 176 V) was applied such that the essential requirement of energy-independent intensity of a well-collimated beam was achieved. The beam was oriented to perpendicular incidence onto the target. The differentiation of the target current J with respect to incident energy was obtained through a modulating voltage with a peak-to-peak amplitude of 0.15 V and a frequency of 430 Hz. Pressures were always less than 1 × 10 - 9 Torr. The ZnO crystals were cut to thin 0.6 mm thick disks oriented along the (0001), (1010) and (119-0) planes respectively to within 0.5 ° as determined by Laue back reflection. The size of the crystals were approximately 3.5 mm × 3.5 mm, 5 mm × 5mm and 2.5 mm × 3 mm in area, respectively. The samples were then diamond-paste polished to 1 ~m followed by chemical etching in a 5 wt% diluted H N O 3 solution for the (1010) surface and in an 8 wt% diluted HCI solution for both the (0001) and the (119-0) surfaces. The (1010) surfaces were annealed at 600°C for 15 min and then flashed for
P.J. Moiler, Jian- Wei tie / Surfaces of ZnO crystals
211
1 min at approximately 860°C while the (0001) and the (1120) surfaces were annealed at 500°C for 15 min and flashed at 600 and 540°C for 30 s, respectively. When the sample had reached room temperature TCS spectra were recorded. The target heater was placed inside the target holder manifold, at a position such that no contamination of the sample was possible. The temperature was measured by a chromel-alumel thermocouple spot-welded onto the target holder. When recording high temperature TCS spectra the target heater was switched off to avoid any influence from the heater current. The temperatures were determined through blank experiment calibration curves. The TCS spectra were reproducible. 3. Results and discussion The experimental TCS spectra, d J ( E 1 ) / d E t, in the 0-.18 eV incident primary energy range (E~ is measured with respect to the vacuum level) from the three surfaces (0001), (1010) and (1120) respectively, all measured at room temperature, are shown in fig. 1. Table 1 compares the values of the positions of the peaks and throughs of the TCS spectra with those known from ELS experiments. It is seen that the spectra of the three different surfaces are rather identical with regard to peak positions and, especially at the lower energy range, also to intensities. Fig. 2a shows a series of TCS spectra presented as a function of temperature in the range 150 to 350°C for the (0001) and the (1120) surfaces. We observe that these show no detectable Debye-Wailer temperature dependence either in the energy range under consideration. Fig. 2b shows a series of TCS spectra for the (1010) surface presented as a function of angular incidence of the primary beam. Apart from a slight variation in intensity of the lowest energies no angular dependence is detected. The good agreement of the low energy TCS spectra for the three different surfaces, fig. 1, also agrees with the observation of no angular dependence for the investigated low energies of the incident primary beam. We have further calculated a theoretical TCS spectrum by applying the model of Komolov [10], which was expanded [12] to an iterative process from which it is possible to deduce a density-of-states diagram. Starting the iterative calculation from the proposed band structure of Dorn et al. [2] we propose to add two peaks in the density-of-states diagram at 3.6 and 5.8 eV (assigned a and b respectively in fig. 3) above the top of the valence band. Our calculated TCS spectrum corresponding to this diagram is inserted in fig. 1 (curve D). It is seen that the spectral features for all the different crystal faces are occurring at the same individual primary electron energies. Such agreement between theory and experiment suggests that the TCS characters for Z n O at these low incident electron energies can be described as resonances of one-electron excitation events.
212
P.J. Moiler. Jian-Wei He / Surfaces of ZnO crv~'tal~ 13
//i .m
2
i.,.7
D 1
0
l
I
1
I
I
1
I
I
10 Incident energy E(eVI
Fig. 1. TCS spectrum: S(E~) = d J( E 1)~dE l versus incident energy E 1 from ZnO crystal surfaces at rc<3m temperature. Curves are for: (A) (0001) surface; (B) (1070) surface; ((7') (1 l P-0) surface: (I)) theoretical calculated TCS spectrum. The electron beam is incident perpendicular to the target surface.
Due to large scattering cross-sections, multiple scattering effects might occur at these low energies and hence lead to low energy reflections at energies below integral Bragg reflections. However, it appears that the rather simple model used here quite well describes the spectra solely in terms of i n t e r b a n d transitions at these energies. The influence u p o n the m a g n i t u d e of the intensities of the low energy specular beam reflection thus seems to be insignificant in c o m p a r i s o n with the said effect of the i n t e r b a n d transitions. Now let us discuss the obtained TCS spectra in relation to the proposed
P.J. Moiler, Jian-Wei He / Surfaces of ZnO cr)'stal.s
213
Table 1 Comparison of the energies, E (in eV). required for transitions in Z n O shown in fig. 3 with values obse~'ed by electron energy-loss spectroscopy (ELS) Structure No+
TCS (this study) (0001)
Pl
(10]0)
0.5
P7 Ps t~
(1120)
0.6 1.4
0.6
2.3 4.2 6.2
4.2 6.4
4.1 6.4
8.4
8.4
8.6
11.2 13.2 15.3 9.8
11.6 13.2 15.6 9.3
11.4 13.2 15.2 9.6
ELS [1]
ELS [21
(1]00)
(0001)
3.9 5.5 7.8
4.3
ELS [3] (1i00)
4.3
7.6
7.8
9.1 11.8
9.1 12.6
15.3
15.8
Ep = 60eV
Ep = 1 0 0 e V
(0001)
(0001)
(1]00)
4.2 6.1
4.7 6.2 7.9
4.2 6.2
9.1 10.5 13.0 15.8
(1]00)
6.3 8.0
9.1 11.9 13.8
13.6 15.8
15.3
+m +~_ m v
l.u
(A)593 K
=..,
I
0
I
i
I
I
I
10
I
I
I
l
l
l
0
Incident energy El
L
l
l
l
t
10
(eV)
Fig. 2. TCS spectrum: S ( E I) = d J ( E 1)/d E t versus incident energy Et from Z n O crystals. (a) (0001) (A) and (1120) (C) surfaces as function of target temperature. (b) (10]0) surface as function of angles of incidence (perpendicular incidence = 0°).
P.J. Moiler. Jtan- Wei He / Surfaces of ZnO crv.~'taL~
214
P~
P:
I? ,,~ .'7
~ Zn-dangling bond surface states
i L . . . . .
i//
20
p~ P~ 15
P';
~-=.
P' i --'
~ b
i
:,
10
,
.i,,y, ~ ,
,
I J :
.t a
O-dangling bond surface states
Evac
5 . _ - - i Ec O:E v -5
Zn- 3d
~------~ fO t.~
-15
0-2s
~ ~ ~"
-20 25
t
Bulk density- of-states
( arbitrary units )
Fig. 3. Density-of-states diagram for ZnO: ( ) our proposed diagram: ( . ) Dorn et al.'s (1977) theoretically proposed diagram (ref. [2], with permission). E~ valence band edge; E~: conduction band edge; El:: Fermi energy; E,a,:: v a c u u m level. Our proposed interband transition Pl -P~ and t 1, also shown in fig. 1, have been marked. The lengths of the arrows correspond to the energy losses. e l e c t r o n i c b a n d structure. O u r p r o p o s e d i n t e r b a n d t r a n s i t i o n s c o r r e s p o n d i n g to the T C S spectra are m a r k e d in the d e n s i t y - o f - s t a t e s d i a g r a m of fig. 3 w h i c h also i n c l u d e s the literature d a t a of D o r n et al. [2]. T h e c a l c u l a t e d states m a y i n c l u d e a c o n t r i b u t i o n from surface states which is u n f o r t u n a t e l y p r e s e n t l y n o t well d i s t i n g u i s h e d by the c a l c u l a t i o n process. T h e peak of 1.4 eV of the (1010) curve m a y be a s s i g n e d to a t r a n s i t i o n from a n o x y g e n d a n g l i n g b o n d to the peak m a r k e d a in the b o t t o m of the c o n d u c t i o n b a n d . Since it has b e e n p r o p o s e d [2,8] that the o x y g e n d a n g l i n g
P.J. Moiler. Jtan-Wet He / Surfaces of ZnO crystals
215
bond is located approximately 2 eV below the top of the valence band we propose that the energy difference required for the transition from the oxygen dangling bond to the peak near the bottom of the conduction band is 3.6 eV + 2 eV = 5.6 eV. This difference is just equal to the energy loss of primary electrons of energy 1.4 eV above the vacuum level during a transition to the peak marked a. The peak at 2.3 eV in the (0001) curve may correspond to a transition from the first peak in the valence band to the zinc dangling bond. This seems reasonable since the state density of the zinc dangling bond ot~ the (0001) surface should be quite large. The trough at 9.3 eV may be explained as the threshold emission [14] of electrons from the O 2p band. It is of interest to note that the calculated TCS spectrum indicates a peak at 8.4 eV which does not appear very clearly in experimental TCS spectrum of the (1070) surface, but it does so on that of the (0001) and (less clearly) the (1120) surfaces, and in the above referred ELS experiments this peak was seen also and thus demonstrates the anisotropic effects in ELS. The density of the Zn 3d band might be several times larger [2,8] than that proposed on the basis of the TCS calculation, since when the energy of incident electrons is larger than 9.3 eV the Zn 3d band begins to contribute heavily to the target current, but at the same time the threshold emission from the O 2p band will diminish the target current, i.e. the 13.2 eV peak contains both a contribution of increasing intensity of the Zn 3d band and of decreasing intensity of the O 2p band.
4. Conclusions Experimental TCS spectra of the (0001), (1010) and (1120) surfaces of Z n O crystals have been obtained. The rather close similarity of the spectra form the different surfaces, at different temperatures and at different angles of incidence suggests that the TCS structural features to a quite large extent for low incident energies (0-18 eV) are due to inelastic electron-electron scattering processes resulting from interband transitions. The main features of the TCS spectrum for Z n O have been explained, and we have proposed a revised band structure diagram which may be explained by interband transitions corresponding to bulk and surfaces states.
Acknowledgements We are grateful to the Danish Natural Science Research Council and to Physics Laboratory II of this Institute for their continued support. One of us
216
P.J. Moiler, Jian-Wet tie / Surface,~ of ZnO ~rvstal~
(P.J.M.) sity for J u l i e v. Letters,
e x p r e s s e s his g r a t i t u d e to P r o f e s s o r E.1. S o l o m o n at S t a n f o r d U n i v e r f r u i t f u l d i s c u s s i o n s a n d for t h e gift o f Z n O s i n g l e c r y s t a l s , a n d to t h e Miallen F o u n d a t i o n , T h e R o y a l D a n i s h A c a d e m y o f S c i e n c e s a n d for t h e i r s u p p o r t .
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
H. Froitzheim and H. Ibach, Z. Physik 269 (1974) 17. R. Dorn, H. Li~th and M. Bi~chel, Phys. Rev. B16 (19771 4675. J. Onsgaard. S.M. Barlow and T.E. Gallon, 3. Phys. ('12 (1979) 925. W. Ranke, Solid State Commun. 19 (1976} 685. R.R. Gay, M.H. Nodine, V.E. Henrich, H.J. Zeiger and E.I. Solomon, J. Am. Chem. Soc. 102 (1980} 6753. L. Ley, R.A. Pollak, FR. McFeely, S.P. Kowalczyk and D.A. Shirley, Phys. Rev. B9 (1974) 600. S. Bloom and I. Ortenburger, Phys. Status Solidi (b) 58 (1973) 561. G. Heiland and H. L~th, in: The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis, Vol. 3B, Eds. I).A. King and D.P. Woodruff. (Elsevier, Amsterdam, 1984) p. 156. S.A. Komolov and L.T. Chadderton, ~'~lid State Commun. 20 (1976) 765. S.A. Komolov, Soviet Phys.-Tech. Phys. 49 (1979) 1318. M.H. Mohamed and P.J. Moiler, Radiation Effects 55 (1981) 39. P.J. M~ller and M.H. Mohamed. Vacuum 35 (1985) 29. P.J. Moiler and M.H. Mohamed. J. Phys. C15 (1982) 6457. M.II. Mohamed and P.J. M~ller, Phys. Scripta 25 (1982) 765.
209
T O T A L YIELD ENERGY D E P E N D E N C E OF LOW-ENERGY E L E C T R O N S F R O M S U R F A C E S OF ZnO C R Y S T A L S Preben J. MOLLER and Jian-Wei HE Department of Physical Chemistry, H.C. Orsted Institute, University of Copenhagen. 5 Unioersitetsparken, DK- 2100 Copenhagen, Denmark Received 1 April 1985; accepted for publication 6 May 1985
The energy dependence of the reflection of low-energy incident electrons (of the range 0-18 eV above the vacuum level) from the polar zinc (0001) face, the non-polar (10i0) and the (1120) faces of ZnO crystals is explained in terms of the onset of inelastic electron-electron scattering accompanied by interband transitions. Based on the spectra a revision of the band structure diagram for energies near the band gap is proposed.
I. Introduction ZnO is a well known semiconductor and a widely used catalyst. Electron energy loss spectroscopy (ELS) experiments have given information on its electronic structure through energy loss features which have been interpreted as interband transitions [1-3]. Froitzheim and lbach [1] furthermore observed a c-axis orientation dependence for the (10]0) surface. In ultraviolet photoemission spectroscopy (UPS) experiments [4-5] and X-ray photoelectron spectroscopy (XPS) experiments [6] the density-of-states structure of the valence band both for the O 2p and the Zn 3d bands have been determined. Also, theoretical calculations [7] have proposed a band structure of the O 2p valence band and of the conduction band. Recently [8] photoconductivity measurements have suggested additional low density surface states within the forbidden band very near the conduction band edge. To further investigate the question of crystal surface anisotropy and to investigate the electronic structure for the low energies in the region near the band gap it would be useful to apply the Total Current Spectroscopy (TCS) method, which may give experimental information both for the conduction band and for the valence band of ZnO. The iterative calculation is facilitated in this case through the use of the available experimental UPS and XPS data for the valence band. TCS is a recently developed technique [9-12] in which an electron beam of low energy is directed onto a target surface. The total (or target) current J in the target circuit is measured with respect to the incident primary energy E 1, 0039-6028/85/$03.30 ~;' Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
210
P.J. Moiler, Jtan-Wet He / Surface.g of ZnO crv~'tal~"
the latter typically being 0 15 eV above the vacuum level. The energy structure of the total yield resulting from the impingement of electrons onto surfaces can be related both to interband transitions and to collective oscillations. Both elastically and inelastically reflected electrons contribute to the total current. It has been indicated theoretically [10] and experimentally, e.g. refs. [11-131, that at very low incident energy (less than 20 eV) the fine structure in the TCS spectra, d J ( E i ) / d E I versus E l, is dominated by inelastic electron-electron scattering processes, resulting in interband transitions which can be analyzed through a convolution over density of states in the approximation of constant transition matrix element, energy conservation and wave vector non-conservation in the scattering process. Since a quite high density of unoccupied states below the vacuum level has been suggested for ZnO [2,8] then a sharp change in the efficiency of inelastic scattering, which causes fluctuations in the intensity of the elastically reflected current, and hence in that of the yield, would be expected, and the obtained fine structure TCS spectrum may then be described by the above name model. In this paper we present experimental TCS results for three Z n O surfaces: the polar zinc (0001) surface and the non-polar (1010) and (1120) Z n O surfaces, and we conclude that the main structures in the TCS spectra are not angular dependent for primary energies below 18 eV, and that the structures can be interpreted as due to electron and band transitions. Further we propose a revised band structure diagram for ZnO for the energies near the band gap, as determined from TCS experiments.
2. Experimental details These experiments were performed using the experimental procedures and UHV apparatus previously described [11 ]. A fixed accelerating voltage (of 176 V) was applied such that the essential requirement of energy-independent intensity of a well-collimated beam was achieved. The beam was oriented to perpendicular incidence onto the target. The differentiation of the target current J with respect to incident energy was obtained through a modulating voltage with a peak-to-peak amplitude of 0.15 V and a frequency of 430 Hz. Pressures were always less than 1 × 10 - 9 Torr. The ZnO crystals were cut to thin 0.6 mm thick disks oriented along the (0001), (1010) and (119-0) planes respectively to within 0.5 ° as determined by Laue back reflection. The size of the crystals were approximately 3.5 mm × 3.5 mm, 5 mm × 5mm and 2.5 mm × 3 mm in area, respectively. The samples were then diamond-paste polished to 1 ~m followed by chemical etching in a 5 wt% diluted H N O 3 solution for the (1010) surface and in an 8 wt% diluted HCI solution for both the (0001) and the (119-0) surfaces. The (1010) surfaces were annealed at 600°C for 15 min and then flashed for
P.J. Moiler, Jian- Wei tie / Surfaces of ZnO crystals
211
1 min at approximately 860°C while the (0001) and the (1120) surfaces were annealed at 500°C for 15 min and flashed at 600 and 540°C for 30 s, respectively. When the sample had reached room temperature TCS spectra were recorded. The target heater was placed inside the target holder manifold, at a position such that no contamination of the sample was possible. The temperature was measured by a chromel-alumel thermocouple spot-welded onto the target holder. When recording high temperature TCS spectra the target heater was switched off to avoid any influence from the heater current. The temperatures were determined through blank experiment calibration curves. The TCS spectra were reproducible. 3. Results and discussion The experimental TCS spectra, d J ( E 1 ) / d E t, in the 0-.18 eV incident primary energy range (E~ is measured with respect to the vacuum level) from the three surfaces (0001), (1010) and (1120) respectively, all measured at room temperature, are shown in fig. 1. Table 1 compares the values of the positions of the peaks and throughs of the TCS spectra with those known from ELS experiments. It is seen that the spectra of the three different surfaces are rather identical with regard to peak positions and, especially at the lower energy range, also to intensities. Fig. 2a shows a series of TCS spectra presented as a function of temperature in the range 150 to 350°C for the (0001) and the (1120) surfaces. We observe that these show no detectable Debye-Wailer temperature dependence either in the energy range under consideration. Fig. 2b shows a series of TCS spectra for the (1010) surface presented as a function of angular incidence of the primary beam. Apart from a slight variation in intensity of the lowest energies no angular dependence is detected. The good agreement of the low energy TCS spectra for the three different surfaces, fig. 1, also agrees with the observation of no angular dependence for the investigated low energies of the incident primary beam. We have further calculated a theoretical TCS spectrum by applying the model of Komolov [10], which was expanded [12] to an iterative process from which it is possible to deduce a density-of-states diagram. Starting the iterative calculation from the proposed band structure of Dorn et al. [2] we propose to add two peaks in the density-of-states diagram at 3.6 and 5.8 eV (assigned a and b respectively in fig. 3) above the top of the valence band. Our calculated TCS spectrum corresponding to this diagram is inserted in fig. 1 (curve D). It is seen that the spectral features for all the different crystal faces are occurring at the same individual primary electron energies. Such agreement between theory and experiment suggests that the TCS characters for Z n O at these low incident electron energies can be described as resonances of one-electron excitation events.
212
P.J. Moiler. Jian-Wei He / Surfaces of ZnO crv~'tal~ 13
//i .m
2
i.,.7
D 1
0
l
I
1
I
I
1
I
I
10 Incident energy E(eVI
Fig. 1. TCS spectrum: S(E~) = d J( E 1)~dE l versus incident energy E 1 from ZnO crystal surfaces at rc<3m temperature. Curves are for: (A) (0001) surface; (B) (1070) surface; ((7') (1 l P-0) surface: (I)) theoretical calculated TCS spectrum. The electron beam is incident perpendicular to the target surface.
Due to large scattering cross-sections, multiple scattering effects might occur at these low energies and hence lead to low energy reflections at energies below integral Bragg reflections. However, it appears that the rather simple model used here quite well describes the spectra solely in terms of i n t e r b a n d transitions at these energies. The influence u p o n the m a g n i t u d e of the intensities of the low energy specular beam reflection thus seems to be insignificant in c o m p a r i s o n with the said effect of the i n t e r b a n d transitions. Now let us discuss the obtained TCS spectra in relation to the proposed
P.J. Moiler, Jian-Wei He / Surfaces of ZnO cr)'stal.s
213
Table 1 Comparison of the energies, E (in eV). required for transitions in Z n O shown in fig. 3 with values obse~'ed by electron energy-loss spectroscopy (ELS) Structure No+
TCS (this study) (0001)
Pl
(10]0)
0.5
P7 Ps t~
(1120)
0.6 1.4
0.6
2.3 4.2 6.2
4.2 6.4
4.1 6.4
8.4
8.4
8.6
11.2 13.2 15.3 9.8
11.6 13.2 15.6 9.3
11.4 13.2 15.2 9.6
ELS [1]
ELS [21
(1]00)
(0001)
3.9 5.5 7.8
4.3
ELS [3] (1i00)
4.3
7.6
7.8
9.1 11.8
9.1 12.6
15.3
15.8
Ep = 60eV
Ep = 1 0 0 e V
(0001)
(0001)
(1]00)
4.2 6.1
4.7 6.2 7.9
4.2 6.2
9.1 10.5 13.0 15.8
(1]00)
6.3 8.0
9.1 11.9 13.8
13.6 15.8
15.3
+m +~_ m v
l.u
(A)593 K
=..,
I
0
I
i
I
I
I
10
I
I
I
l
l
l
0
Incident energy El
L
l
l
l
t
10
(eV)
Fig. 2. TCS spectrum: S ( E I) = d J ( E 1)/d E t versus incident energy Et from Z n O crystals. (a) (0001) (A) and (1120) (C) surfaces as function of target temperature. (b) (10]0) surface as function of angles of incidence (perpendicular incidence = 0°).
P.J. Moiler. Jtan- Wei He / Surfaces of ZnO crv.~'taL~
214
P~
P:
I? ,,~ .'7
~ Zn-dangling bond surface states
i L . . . . .
i//
20
p~ P~ 15
P';
~-=.
P' i --'
~ b
i
:,
10
,
.i,,y, ~ ,
,
I J :
.t a
O-dangling bond surface states
Evac
5 . _ - - i Ec O:E v -5
Zn- 3d
~------~ fO t.~
-15
0-2s
~ ~ ~"
-20 25
t
Bulk density- of-states
( arbitrary units )
Fig. 3. Density-of-states diagram for ZnO: ( ) our proposed diagram: ( . ) Dorn et al.'s (1977) theoretically proposed diagram (ref. [2], with permission). E~ valence band edge; E~: conduction band edge; El:: Fermi energy; E,a,:: v a c u u m level. Our proposed interband transition Pl -P~ and t 1, also shown in fig. 1, have been marked. The lengths of the arrows correspond to the energy losses. e l e c t r o n i c b a n d structure. O u r p r o p o s e d i n t e r b a n d t r a n s i t i o n s c o r r e s p o n d i n g to the T C S spectra are m a r k e d in the d e n s i t y - o f - s t a t e s d i a g r a m of fig. 3 w h i c h also i n c l u d e s the literature d a t a of D o r n et al. [2]. T h e c a l c u l a t e d states m a y i n c l u d e a c o n t r i b u t i o n from surface states which is u n f o r t u n a t e l y p r e s e n t l y n o t well d i s t i n g u i s h e d by the c a l c u l a t i o n process. T h e peak of 1.4 eV of the (1010) curve m a y be a s s i g n e d to a t r a n s i t i o n from a n o x y g e n d a n g l i n g b o n d to the peak m a r k e d a in the b o t t o m of the c o n d u c t i o n b a n d . Since it has b e e n p r o p o s e d [2,8] that the o x y g e n d a n g l i n g
P.J. Moiler. Jtan-Wet He / Surfaces of ZnO crystals
215
bond is located approximately 2 eV below the top of the valence band we propose that the energy difference required for the transition from the oxygen dangling bond to the peak near the bottom of the conduction band is 3.6 eV + 2 eV = 5.6 eV. This difference is just equal to the energy loss of primary electrons of energy 1.4 eV above the vacuum level during a transition to the peak marked a. The peak at 2.3 eV in the (0001) curve may correspond to a transition from the first peak in the valence band to the zinc dangling bond. This seems reasonable since the state density of the zinc dangling bond ot~ the (0001) surface should be quite large. The trough at 9.3 eV may be explained as the threshold emission [14] of electrons from the O 2p band. It is of interest to note that the calculated TCS spectrum indicates a peak at 8.4 eV which does not appear very clearly in experimental TCS spectrum of the (1070) surface, but it does so on that of the (0001) and (less clearly) the (1120) surfaces, and in the above referred ELS experiments this peak was seen also and thus demonstrates the anisotropic effects in ELS. The density of the Zn 3d band might be several times larger [2,8] than that proposed on the basis of the TCS calculation, since when the energy of incident electrons is larger than 9.3 eV the Zn 3d band begins to contribute heavily to the target current, but at the same time the threshold emission from the O 2p band will diminish the target current, i.e. the 13.2 eV peak contains both a contribution of increasing intensity of the Zn 3d band and of decreasing intensity of the O 2p band.
4. Conclusions Experimental TCS spectra of the (0001), (1010) and (1120) surfaces of Z n O crystals have been obtained. The rather close similarity of the spectra form the different surfaces, at different temperatures and at different angles of incidence suggests that the TCS structural features to a quite large extent for low incident energies (0-18 eV) are due to inelastic electron-electron scattering processes resulting from interband transitions. The main features of the TCS spectrum for Z n O have been explained, and we have proposed a revised band structure diagram which may be explained by interband transitions corresponding to bulk and surfaces states.
Acknowledgements We are grateful to the Danish Natural Science Research Council and to Physics Laboratory II of this Institute for their continued support. One of us
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P.J. Moiler, Jian-Wet tie / Surface,~ of ZnO ~rvstal~
(P.J.M.) sity for J u l i e v. Letters,
e x p r e s s e s his g r a t i t u d e to P r o f e s s o r E.1. S o l o m o n at S t a n f o r d U n i v e r f r u i t f u l d i s c u s s i o n s a n d for t h e gift o f Z n O s i n g l e c r y s t a l s , a n d to t h e Miallen F o u n d a t i o n , T h e R o y a l D a n i s h A c a d e m y o f S c i e n c e s a n d for t h e i r s u p p o r t .
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