- Email: [email protected]
Electron spectroscopy at Diamond (100)2 × 1:H with a scanning tunnelling microscope
Vacuum/volume
46humbers
8-IO/pages
Pergamon
1097 to 1100/1995 Elsevier Science Ltd Printed in Great Britain
0042-207x/95
$9.50+.00
0042-207)<(95)00114-X
Electron spectroscopy at Diamond scanning tunnelling microscope Hans-Gerd Busmann, Bremen, Germany
Fraunhofer-Institute
for Applied Materials
(100)2 x 1: H with a
Research, Lesumer HeerstraQe 36, D-2471 7
and Wolfgang Zimmermann-Edling and Riidiger Brenn, Freiburg Materials 3la, D-79104 Freiburg LBrsg., Germany
Research Centre, Stefan-Meier-StraQe
Topographic scanning tunnelling microscopy on an undoped 2 x l-reconstructed diamond surface of a polycrystalline film was possible in air, but not in ultra-high vacuum. Elastic electron tunnelling spectroscopy with the scanning tunnelling microscope was when applied to study electronic states at the surface. A surface bandgap was not found for the surface in air, whereas a gap approximately 3.5 eV wide was found in ultrahigh vacuum. This gives strong evidence for the monohydrogenated form of the 2 x I reconstruction, /100)2x 1: H. The Fermi level appeared within the gap about 0.5 eV below the upper band edge which thus explained the impossibility of topographic scanning tunnelling microscopy in ultra-high vacuum. Comparison with data for ultraviolet photoelectron spectroscopy taken from the literature indicates that the surface states form one single, approximately2-eV-wide band to filled states located just above the valence band edge. The Fermi level is accordingly located approximately 0.5 eV below the conduction band edge and not pinned by surface states.
Introduction The study of electronic structures at surfaces of chemical-vapourdeposited (CVD) diamond !ilms is important for basic surface science and the development of diamond thin-film applications. Photoelectron spectra have been reported for natural diamond Ill-surfaces by Pate’ and Kubiak et al’, and for lOO-faces by Hamza et aZ3. All show the presence of electronic surface states within the bulk bandgap of diamond. Whereas photoelectron spectroscopy and related methods give global spatial information, scanning tunnelling microscopy (STM) can principally be used to obtain atom-resolved surface state spectra as has been demonstrated for a semiconductor by Salvan et al4 and reviewed for example, by Hamers’. After the practicability of STM on CVD-diamond surfaces was demonstrated about three years ago6, 2 x l-reconstructed IOO-surfaces were the first examples of atom-resolved STM-topographs of diamond surfaces7.8. These were followed by reports on (1 1 1)-faces with evidence for 1 x l- and 2 x l-structures’ and novel observation of a (,/3 x 43) R30” reconstruction for a diamond (111) surface lo. All these STM measurements were taken in air. However, the nature of the electronic states that in one respect dominate the tunnelling current in air and on the other hand reflect the periodic atomic structure of the diamond surface is still unknown. First relations between specific surface topographies and associated electronic states were recently analysed by Fraunheim et al”, who drew comparisons between molecular
dynamic annealing studies based on quantum mechanically derived interatomic forces and the above-mentioned STMresults. In particular, different STM-topographs from lOO-surfaces grown under different conditions’* were associated with the atomic structures of the hydrogen-free and monohydrogenated 2 x l-reconstructions (100)2 x 1 and (100)2 x 1 : H, respectively. This paper reports on the first experimental access to electronic states of a 2 x l-reconstructed lOO-surface by elastic electron tunnelling spectroscopy (EETS) performed in air in ultra-high vacuum (UHV) on the same film and with the same apparatus.
Elastic electron tunnelling spectroscopy The determination of electronic state densities from currentvoltage (1-L’) characteristics of tunnelling junctions dates back to the work of I Giaver13 and was newly motivated by the introduction of the STM by Binning and Rohrer14. Figure 1 shows a graphic representation of a simplified electron energy band diagram (a) and a schematic of the density of states at the diamond surface (b) to illustrate the following short synopsis of EETS. More detailed information can be found in the review given by Hamers4. For the band diagram, it is assumed that the wave functions of the metal tip and semiconductor surface partially overlap in such a way that electrons at the interfaces have a relevant probability to tunnel through the barrier of height @. The density of states at the semiconductor surface, p,(E), is 1097
Hans-Gerd Busmann et al: Electron spectroscopy at diamond (100)2 x 1 : H b)
However, because of the large variation of V, this approach may be less appropriate for wide-bandgap semiconductors than for, example supraconducting devices with a much lower bandgap. Thus, if one uses the logarithmic derivative to deduce the density of states of diamond, one has particularly to consider its possible modification by T(E, V) and p,(E). Beside calculations, the density of states obtained from the logarithmic derivative can be examined by for example, UPS-measurements. Experimental and results
0 metal
WP
diamond
2E14
denstity
4E14
of states
314 [ l/cm2
I
Figure 1. (a) Graphic description of a simplified energy band diagram for a metal-diamond tunnelling junction : ET_’ and I$ are the Fermi levels at the metal and diamond surfaces, respectively ; CD is the height and 3 the width of the tunnelling barrier ; CBE and VBE are the lower conduction and upper valence band edge, respectively ; Vis the voltage applied from outside to the junction (bias). (b) Simplified and schematic representation of the electromc density of states at a diamond surface. The density of states of the valence and conduction band were obtained by projection of corresponding bulk states. An integrated density of one state per (lOO)surface atom is assumed for the supposed surface states within the bulk bandgap.
specified by intrinsic bulk states of the semiconductor valence and conduction bands and by surface states due to reconstruction, impurities, adsorbates, etc. The non-intrinsic states are normally unknown and are strongly dependent on surface preparation. In the scheme, a band of surface states above the valence band edge has been drawn for demonstration purposes, referring back to results of ultraviolet photoelectron spectroscopy (UPS) on a natural diamond lOO-surface3. Possible band bending is neglected. The parameter V is the voltage applied to the electrodes of the junction, whereby the present negative V-value corresponds to the tunnelling of electrons from the semiconductor into the tip (negative bias). The transmission probability T for electrons of energy E in the semiconductor can be written within the WKBapproximation as
T(E,V) = exp(aJ@-E-1
Az=0_8nm
eV/2 I),
where a is a constant and proportional to the width of the tunnelling junction z. From this equation, it can clearly be seen that the tunnelling probability decreases with decreasing E and is thus highest for electrons at the Fermi level Er. However, T depends also on V, which makes the determination of p,(E) from tunnelling measurements difficult. Since eV has to be varied over an energy range than the width of the bandgap, a major influence of T(E, v) on the tunnelling current is expected, particularly for wide-bandgap semiconductors such as diamond. The elastic tunnelling current from the semiconductor into the tip for a particular value of E can be written as dZ(V) - p,(E)p,(E+eV)T(E,V)dE, where p,(E) is the density of states in the semiconductor and pr (E+ e V) that of the opposing isoenergetic tip-states. The total tunnelling current is obtained by an integration over all currents dZ(F) within the energy range [O,ev] in the semiconductor, whereby, p,(E) is commonly approximated by its value at Ey, I(V) - p,(E~)~ov p,(E) T(E, V)dE. Because dZ/dV and Z/V should exhibit similar T(E, V)-dependence, the profile of the logarithmic derivation d ln(Z( V)/d In (V) (=(dZ/d V) (Z/V)-‘) should be proportional to p,(E), smoothly modulated by the influence of T(E,V)“. 1098
Undoped fihns were grown in a microwave plasma on a silicon substrate at a temperature of 1000°C with a gas mixture of 1% CH4 and 99% H, and then directly handed over to a UHV-STM station (Omicron) via a UHV-transfer system. We were not able to perform stable STM and to obtain reproducible topographies from films as they were deposited and transferred. However, when the films were exposed to air, reproducible topographies appeared. Figure 2 shows a topographic STM-image obtained from a loo-facet at air of a freshly prepared polycrystalline diamond film. It shows a commonly observed dimerised 2 x l-reconstructiorrd9 of an atom-flat 30 nm x 30 nm large area with rather asymmetric islands of dimer-rows. After this measurement, UHV was re-established, and the sample was heated for several minutes up to 200°C. Topographic STM was again impossible after this, although the sample exhibited a somewhat enhanced tunnelling current and the 2 x l-reconstruction was sometimes recognisable on small areas. Several sequences of air- and UHV-measurements were performed, and these always showed the alternation between stable and rather unstable resolving power. For STM, good electrical conductivity is needed along the entire path of the current from the tip through the probe to the counter electrode of the microscope. Thus electron tunnelling into or from the facets of the polycrystalline films investigated alone is not sufficient for STM. The kind of conductivity behaviour observed can be understood if we assume that the first
nm Figure 2. Topographic
image of a 2 x l-reconstructed diamond 100 -face taker I with a scanning tunnelling microscope. The tunnelling currenl . was 4nA and the bras voltage 1 V.
Hans-Gerd
Busmann
et al: Electron spectroscopy
at diamond
(100)2x
exposure of air to the surface leads to molecular adsorption at all regions of the polycrystalline film inclusive of, but not confined to, grain boundaries. This adsorption enhances the conductivity of the polycrystalline film. When UHV is now re-established, desorption of at least part of the adsorbates makes topographic UHV-STM impossible. About 20 locations statistically distributed over an area of approximately 6 mn by 6 nm, Z-V curves were measured at constant horizontal and vertical positions of the tip and finally averaged. Figures 3(a) and 3(b) show curves for measurements in air and in UHV, respectively, and Figures 3(c) and 3(d) the corresponding logarithmic derivatives. The I-Vcharacteristic for air slowly increases from about - 10 eV to about 0 eV, whereupon it rapidly increases in the range from approximately 0 eV to 2 eV. The corresponding logarithmic derivation does not indicate a surface bandgap. It is reasonable to assume, but not certain, that the two maxima in the profile around approximately - 5 eV and 1 eV correspond to the valence and conduction band edge, respectively. The Z--V characteristic for the measurements in UHV-Figure 3(c)+xhibits remarkable differences compared with those in air. The tunnelling current is significantly lower over the bias range from approximately - 10 eV to approximately + 3 eV. For V > 3 eV, a strong increase is observed. However, the current scale in Figure 3(c) is compressed by a factor of ten as compared with Figure 3(a), and fine structures below 3 eV are certainly not visible. The logarithmic derivative exhibits a pronounced 3.5-eVwide bandgap, which is approximately 2 eV smaller than that of the bulk approximately (5.5 eV). This reduction of the bandgap can be either due to filled states above the valence band edge and/or due to empty states at the lower edge of the conduction band. Fraunheim et al” investigated the monohydrogenated (100)2 x 1 diamond surface, C-(100)2 x 1 : H and demonstrated its stability in the presence of atomic hydrogen. They also calculated the density of states locally at various surface atoms and did not find any indication of states within the bulk bandgap due to the reconstruction. In a similar way, they investigated the hydrogen-free 2 x l-surface and found states distributed over the
.d
4
1: H
entire bandgap due to the n-like bonding structure of the dimers. The lower energy of the monohydrogenated as compared with the hydrogen-free 2 x 1 structure in the presence of hydrogen” and the appearance of the 3.5-eV-wide surface bandgap provide strong evidence to assign our investigated surface to the monohydrogenated form, (100)2 x 1: H. The parameter EF corresponds to zero bias, so it is located within the bandgap approximately 0.5 eV below the upper edge. Hamza et a? investigated (100)2 x l-surfaces of natural diamonds with regard to the hydrogen coverage and electronic structure with the natural bandgap. A probe heated up to approximately 1000°C and thus comparable with our film exhibited filled states in a approximately 1.5-eV-wide band above the valence band edge as monitored by UPS. Together with their investigations with respect to hydrogen coverage, they assigned their investigated surface also to the monohydrogenated 2x 1 structure, (100)2 x 1 : H.
Summary
and conclusions
Topographic STM on 2 x l-reconstructed lOO-faces of undoped polycrystalline films grown at 1000°C was possible in air, but not in UHV. The Z-V characteristics for the surface in air do not exhibit any evidence for a surface bandgap. However, a 3.5-eVwide gap appeared for the surface in UHV. Because of this bandgap, the surface investigated can be assigned to the monohydrogenated form (100)2x 1: H. The parameter Ep is found within the gap approximately 0.5 eV below its upper edge. With reference to UPS data from literature, the surface states reducing the bulk bandgap to the observed surface bandgap are filled and located just above the valence band edge, as is schematically indicated in Figure 3 by the dotted line. The approximately 3.5-eV-wide surface bandgap lies above these filled states, i.e. empty surface states between the upper edge of the filled states and the conduction band edge do not exist. The parameter EF is approximately 0.5 eV below the conduction band edge and thus not pinned by surface states. This situation is schematically shown in Figure 1 (b). It explains
surface bandgap
b)
............................. .......*......
Voltage [V]
Voltage [VJ
Figure 3. (a) and (b) : Z-V characteristic of a tunnelling junction (metal tip 1diamond (100)2 x 1) as obtained with a scanning tunnelling microscope. The curves are averaged I-V curves obtained from about 20 locations statistically distributed over an area of 6 nm by 6 nm. (a) Measurement in air (b) Measurement in ultra-high vacuum. (c) and (d) Logarithmic derivatives d ln(Z)/d ln( v) of th curves shown in (a) and (b), respectively. 1099
Hans-Gerd
Bosmann et al: Electron spectroscopy
at diamond
(100)2 x 1 : H
why topographic STM on this surface in UHV was impossible. To obtain a tunnelling current that is large enough for topographic STM, either the tip would have to be positioned close to the surface or bias voltages would have to be applied that are too high for our STM system. Thus the lack of surface states in approximately 3.5eV-wide band below the conduction band edge and the location of EF within this band made topographic STM impossible. The reasons for the appearance of reproducible topographies in the case of the air measurements are less clear, but they are possibly related to physisorbed adsorbates that modify the band structure and charge distribution at the diamond surface. However, much more information is necessary to come to a detailed understanding of the reported tunnelling behaviour. Acknowledgements The authors wish to thank H J Giintherodt and I V Hertel for encouragement and support. They also gratefully acknowledge financial support of the present work by the Deutsche Forschungsgemeinschaft (DFG, Project No. Bu 781/1-l) carried out under the auspices of the trinational D-A-CH co-operation of Germany, Austria, and Switzerland on the Synthesis of Superhard Materials, and by the DFG-project No. Br 761/5-2
1100
References ‘B B Pate, Surf&i 165,83 (1986). *G D Kubiakand K W Kolasinski, Phys Rev, B39, 1381 (1989). 3A V Hamza. G D Kubiak and R H Stulen. Surf Scz. 237, 3 5 ( 1990). “F Salvan, H’Fuchs, A Baratoff and G Binnig, hrf Sci, li2,634 (1685). ‘R J Hamers, Annu, Rev Phys Chem, 40,531 (1989). 6 H-G Busmann, H Sprang, I V Hertel, W Zimmermann-Edling and H-J Gtintherodt, Appl Phys Lett, 59,295 (1991). ‘T Tsuno, T Imai, Y Nishibayashi, K Hamada and N Fujimori, Jpn J Appl Phvs 30, 1063 (1991). ‘W Zimmermann-Edling, H-G Busmann, H Sprang and I V Hertel, Ultramicroscopy, 42-44 1366 (1992). 9H-G Busmann, W Zimmermann-Edling, H Sprang, H-J Gtintherodt and I V Hertel, Diamond Related Mater, 01, 979 (1992). “H-G Busmann, S Lauer, I V Hertel, W Zimmermann-Edling, H-J Gtintherodt, Th Frauenheim, P Blaudeck and D Porezack, Surf Sci, 295, 340 (1993). I’ Th Frauenheim, U Stephan, B Blaudeck, D Porezag, H-G Busmann, W Zimmermann-Edling and S Lauer, Phys Rev, B48, 18, 189 (1993) “H-G Busmann, W Zimmermann-Edling and S Lauer, In Procedures oj 3rd IUMRS International Conference, Tokyo, (Edited by M Wakatsuki). Elsevier (1993). I31 Giaver, Phys Rev Lett, 5, 147 (1960). 14G Binning, C F Quate and C Gerber, Phys Rev Lett, 56,930 (1986). I5 R M Feenstra, J A Stroscio and A P Fein, Surf Scz, 181,295 (1987).
46humbers
8-IO/pages
Pergamon
1097 to 1100/1995 Elsevier Science Ltd Printed in Great Britain
0042-207x/95
$9.50+.00
0042-207)<(95)00114-X
Electron spectroscopy at Diamond scanning tunnelling microscope Hans-Gerd Busmann, Bremen, Germany
Fraunhofer-Institute
for Applied Materials
(100)2 x 1: H with a
Research, Lesumer HeerstraQe 36, D-2471 7
and Wolfgang Zimmermann-Edling and Riidiger Brenn, Freiburg Materials 3la, D-79104 Freiburg LBrsg., Germany
Research Centre, Stefan-Meier-StraQe
Topographic scanning tunnelling microscopy on an undoped 2 x l-reconstructed diamond surface of a polycrystalline film was possible in air, but not in ultra-high vacuum. Elastic electron tunnelling spectroscopy with the scanning tunnelling microscope was when applied to study electronic states at the surface. A surface bandgap was not found for the surface in air, whereas a gap approximately 3.5 eV wide was found in ultrahigh vacuum. This gives strong evidence for the monohydrogenated form of the 2 x I reconstruction, /100)2x 1: H. The Fermi level appeared within the gap about 0.5 eV below the upper band edge which thus explained the impossibility of topographic scanning tunnelling microscopy in ultra-high vacuum. Comparison with data for ultraviolet photoelectron spectroscopy taken from the literature indicates that the surface states form one single, approximately2-eV-wide band to filled states located just above the valence band edge. The Fermi level is accordingly located approximately 0.5 eV below the conduction band edge and not pinned by surface states.
Introduction The study of electronic structures at surfaces of chemical-vapourdeposited (CVD) diamond !ilms is important for basic surface science and the development of diamond thin-film applications. Photoelectron spectra have been reported for natural diamond Ill-surfaces by Pate’ and Kubiak et al’, and for lOO-faces by Hamza et aZ3. All show the presence of electronic surface states within the bulk bandgap of diamond. Whereas photoelectron spectroscopy and related methods give global spatial information, scanning tunnelling microscopy (STM) can principally be used to obtain atom-resolved surface state spectra as has been demonstrated for a semiconductor by Salvan et al4 and reviewed for example, by Hamers’. After the practicability of STM on CVD-diamond surfaces was demonstrated about three years ago6, 2 x l-reconstructed IOO-surfaces were the first examples of atom-resolved STM-topographs of diamond surfaces7.8. These were followed by reports on (1 1 1)-faces with evidence for 1 x l- and 2 x l-structures’ and novel observation of a (,/3 x 43) R30” reconstruction for a diamond (111) surface lo. All these STM measurements were taken in air. However, the nature of the electronic states that in one respect dominate the tunnelling current in air and on the other hand reflect the periodic atomic structure of the diamond surface is still unknown. First relations between specific surface topographies and associated electronic states were recently analysed by Fraunheim et al”, who drew comparisons between molecular
dynamic annealing studies based on quantum mechanically derived interatomic forces and the above-mentioned STMresults. In particular, different STM-topographs from lOO-surfaces grown under different conditions’* were associated with the atomic structures of the hydrogen-free and monohydrogenated 2 x l-reconstructions (100)2 x 1 and (100)2 x 1 : H, respectively. This paper reports on the first experimental access to electronic states of a 2 x l-reconstructed lOO-surface by elastic electron tunnelling spectroscopy (EETS) performed in air in ultra-high vacuum (UHV) on the same film and with the same apparatus.
Elastic electron tunnelling spectroscopy The determination of electronic state densities from currentvoltage (1-L’) characteristics of tunnelling junctions dates back to the work of I Giaver13 and was newly motivated by the introduction of the STM by Binning and Rohrer14. Figure 1 shows a graphic representation of a simplified electron energy band diagram (a) and a schematic of the density of states at the diamond surface (b) to illustrate the following short synopsis of EETS. More detailed information can be found in the review given by Hamers4. For the band diagram, it is assumed that the wave functions of the metal tip and semiconductor surface partially overlap in such a way that electrons at the interfaces have a relevant probability to tunnel through the barrier of height @. The density of states at the semiconductor surface, p,(E), is 1097
Hans-Gerd Busmann et al: Electron spectroscopy at diamond (100)2 x 1 : H b)
However, because of the large variation of V, this approach may be less appropriate for wide-bandgap semiconductors than for, example supraconducting devices with a much lower bandgap. Thus, if one uses the logarithmic derivative to deduce the density of states of diamond, one has particularly to consider its possible modification by T(E, V) and p,(E). Beside calculations, the density of states obtained from the logarithmic derivative can be examined by for example, UPS-measurements. Experimental and results
0 metal
WP
diamond
2E14
denstity
4E14
of states
314 [ l/cm2
I
Figure 1. (a) Graphic description of a simplified energy band diagram for a metal-diamond tunnelling junction : ET_’ and I$ are the Fermi levels at the metal and diamond surfaces, respectively ; CD is the height and 3 the width of the tunnelling barrier ; CBE and VBE are the lower conduction and upper valence band edge, respectively ; Vis the voltage applied from outside to the junction (bias). (b) Simplified and schematic representation of the electromc density of states at a diamond surface. The density of states of the valence and conduction band were obtained by projection of corresponding bulk states. An integrated density of one state per (lOO)surface atom is assumed for the supposed surface states within the bulk bandgap.
specified by intrinsic bulk states of the semiconductor valence and conduction bands and by surface states due to reconstruction, impurities, adsorbates, etc. The non-intrinsic states are normally unknown and are strongly dependent on surface preparation. In the scheme, a band of surface states above the valence band edge has been drawn for demonstration purposes, referring back to results of ultraviolet photoelectron spectroscopy (UPS) on a natural diamond lOO-surface3. Possible band bending is neglected. The parameter V is the voltage applied to the electrodes of the junction, whereby the present negative V-value corresponds to the tunnelling of electrons from the semiconductor into the tip (negative bias). The transmission probability T for electrons of energy E in the semiconductor can be written within the WKBapproximation as
T(E,V) = exp(aJ@-E-1
Az=0_8nm
eV/2 I),
where a is a constant and proportional to the width of the tunnelling junction z. From this equation, it can clearly be seen that the tunnelling probability decreases with decreasing E and is thus highest for electrons at the Fermi level Er. However, T depends also on V, which makes the determination of p,(E) from tunnelling measurements difficult. Since eV has to be varied over an energy range than the width of the bandgap, a major influence of T(E, v) on the tunnelling current is expected, particularly for wide-bandgap semiconductors such as diamond. The elastic tunnelling current from the semiconductor into the tip for a particular value of E can be written as dZ(V) - p,(E)p,(E+eV)T(E,V)dE, where p,(E) is the density of states in the semiconductor and pr (E+ e V) that of the opposing isoenergetic tip-states. The total tunnelling current is obtained by an integration over all currents dZ(F) within the energy range [O,ev] in the semiconductor, whereby, p,(E) is commonly approximated by its value at Ey, I(V) - p,(E~)~ov p,(E) T(E, V)dE. Because dZ/dV and Z/V should exhibit similar T(E, V)-dependence, the profile of the logarithmic derivation d ln(Z( V)/d In (V) (=(dZ/d V) (Z/V)-‘) should be proportional to p,(E), smoothly modulated by the influence of T(E,V)“. 1098
Undoped fihns were grown in a microwave plasma on a silicon substrate at a temperature of 1000°C with a gas mixture of 1% CH4 and 99% H, and then directly handed over to a UHV-STM station (Omicron) via a UHV-transfer system. We were not able to perform stable STM and to obtain reproducible topographies from films as they were deposited and transferred. However, when the films were exposed to air, reproducible topographies appeared. Figure 2 shows a topographic STM-image obtained from a loo-facet at air of a freshly prepared polycrystalline diamond film. It shows a commonly observed dimerised 2 x l-reconstructiorrd9 of an atom-flat 30 nm x 30 nm large area with rather asymmetric islands of dimer-rows. After this measurement, UHV was re-established, and the sample was heated for several minutes up to 200°C. Topographic STM was again impossible after this, although the sample exhibited a somewhat enhanced tunnelling current and the 2 x l-reconstruction was sometimes recognisable on small areas. Several sequences of air- and UHV-measurements were performed, and these always showed the alternation between stable and rather unstable resolving power. For STM, good electrical conductivity is needed along the entire path of the current from the tip through the probe to the counter electrode of the microscope. Thus electron tunnelling into or from the facets of the polycrystalline films investigated alone is not sufficient for STM. The kind of conductivity behaviour observed can be understood if we assume that the first
nm Figure 2. Topographic
image of a 2 x l-reconstructed diamond 100 -face taker I with a scanning tunnelling microscope. The tunnelling currenl . was 4nA and the bras voltage 1 V.
Hans-Gerd
Busmann
et al: Electron spectroscopy
at diamond
(100)2x
exposure of air to the surface leads to molecular adsorption at all regions of the polycrystalline film inclusive of, but not confined to, grain boundaries. This adsorption enhances the conductivity of the polycrystalline film. When UHV is now re-established, desorption of at least part of the adsorbates makes topographic UHV-STM impossible. About 20 locations statistically distributed over an area of approximately 6 mn by 6 nm, Z-V curves were measured at constant horizontal and vertical positions of the tip and finally averaged. Figures 3(a) and 3(b) show curves for measurements in air and in UHV, respectively, and Figures 3(c) and 3(d) the corresponding logarithmic derivatives. The I-Vcharacteristic for air slowly increases from about - 10 eV to about 0 eV, whereupon it rapidly increases in the range from approximately 0 eV to 2 eV. The corresponding logarithmic derivation does not indicate a surface bandgap. It is reasonable to assume, but not certain, that the two maxima in the profile around approximately - 5 eV and 1 eV correspond to the valence and conduction band edge, respectively. The Z--V characteristic for the measurements in UHV-Figure 3(c)+xhibits remarkable differences compared with those in air. The tunnelling current is significantly lower over the bias range from approximately - 10 eV to approximately + 3 eV. For V > 3 eV, a strong increase is observed. However, the current scale in Figure 3(c) is compressed by a factor of ten as compared with Figure 3(a), and fine structures below 3 eV are certainly not visible. The logarithmic derivative exhibits a pronounced 3.5-eVwide bandgap, which is approximately 2 eV smaller than that of the bulk approximately (5.5 eV). This reduction of the bandgap can be either due to filled states above the valence band edge and/or due to empty states at the lower edge of the conduction band. Fraunheim et al” investigated the monohydrogenated (100)2 x 1 diamond surface, C-(100)2 x 1 : H and demonstrated its stability in the presence of atomic hydrogen. They also calculated the density of states locally at various surface atoms and did not find any indication of states within the bulk bandgap due to the reconstruction. In a similar way, they investigated the hydrogen-free 2 x l-surface and found states distributed over the
.d
4
1: H
entire bandgap due to the n-like bonding structure of the dimers. The lower energy of the monohydrogenated as compared with the hydrogen-free 2 x 1 structure in the presence of hydrogen” and the appearance of the 3.5-eV-wide surface bandgap provide strong evidence to assign our investigated surface to the monohydrogenated form, (100)2 x 1: H. The parameter EF corresponds to zero bias, so it is located within the bandgap approximately 0.5 eV below the upper edge. Hamza et a? investigated (100)2 x l-surfaces of natural diamonds with regard to the hydrogen coverage and electronic structure with the natural bandgap. A probe heated up to approximately 1000°C and thus comparable with our film exhibited filled states in a approximately 1.5-eV-wide band above the valence band edge as monitored by UPS. Together with their investigations with respect to hydrogen coverage, they assigned their investigated surface also to the monohydrogenated 2x 1 structure, (100)2 x 1 : H.
Summary
and conclusions
Topographic STM on 2 x l-reconstructed lOO-faces of undoped polycrystalline films grown at 1000°C was possible in air, but not in UHV. The Z-V characteristics for the surface in air do not exhibit any evidence for a surface bandgap. However, a 3.5-eVwide gap appeared for the surface in UHV. Because of this bandgap, the surface investigated can be assigned to the monohydrogenated form (100)2x 1: H. The parameter Ep is found within the gap approximately 0.5 eV below its upper edge. With reference to UPS data from literature, the surface states reducing the bulk bandgap to the observed surface bandgap are filled and located just above the valence band edge, as is schematically indicated in Figure 3 by the dotted line. The approximately 3.5-eV-wide surface bandgap lies above these filled states, i.e. empty surface states between the upper edge of the filled states and the conduction band edge do not exist. The parameter EF is approximately 0.5 eV below the conduction band edge and thus not pinned by surface states. This situation is schematically shown in Figure 1 (b). It explains
surface bandgap
b)
............................. .......*......
Voltage [V]
Voltage [VJ
Figure 3. (a) and (b) : Z-V characteristic of a tunnelling junction (metal tip 1diamond (100)2 x 1) as obtained with a scanning tunnelling microscope. The curves are averaged I-V curves obtained from about 20 locations statistically distributed over an area of 6 nm by 6 nm. (a) Measurement in air (b) Measurement in ultra-high vacuum. (c) and (d) Logarithmic derivatives d ln(Z)/d ln( v) of th curves shown in (a) and (b), respectively. 1099
Hans-Gerd
Bosmann et al: Electron spectroscopy
at diamond
(100)2 x 1 : H
why topographic STM on this surface in UHV was impossible. To obtain a tunnelling current that is large enough for topographic STM, either the tip would have to be positioned close to the surface or bias voltages would have to be applied that are too high for our STM system. Thus the lack of surface states in approximately 3.5eV-wide band below the conduction band edge and the location of EF within this band made topographic STM impossible. The reasons for the appearance of reproducible topographies in the case of the air measurements are less clear, but they are possibly related to physisorbed adsorbates that modify the band structure and charge distribution at the diamond surface. However, much more information is necessary to come to a detailed understanding of the reported tunnelling behaviour. Acknowledgements The authors wish to thank H J Giintherodt and I V Hertel for encouragement and support. They also gratefully acknowledge financial support of the present work by the Deutsche Forschungsgemeinschaft (DFG, Project No. Bu 781/1-l) carried out under the auspices of the trinational D-A-CH co-operation of Germany, Austria, and Switzerland on the Synthesis of Superhard Materials, and by the DFG-project No. Br 761/5-2
1100
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