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Correlation of electronic, leed, and auger diagnostics on ZnO surfaces
SURFACE
SCIENCE 29 (1972) 144164 0 North-Holland
CORRELATION AND AUGER
Publishing Co.
OF ELECTRONIC,
DIAGNOSTICS
LEED,
ON ZnO SURFACES
JULES D. LEVINE and A. WILLIS David
Samoff'Research Center, RCA Laboratories,
Princeton,
New Jersey 08540, U.S.A.
W. R. BOTTOMS and PETER MARK* Princeton
University,
Princeton,
New Jersey 08540,
U.S.A.
Received 1 July 1971; revised manuscript received 6 October 1971 Surface electrical properties of ZnO single crystals were measured and were correlated with complementary LEED and Auger measurements. For the crystal faces (OOOl),(OOOi), (1 lzO), and (10iO) that were prepared by ion bombardment and anneal in ultrahigh vacuum, the above surfaces were found to be: (a) atomically regular as judged by bright LEED spots, (b) free of chemisorbates as judged by Auger spectroscopy, and (c) electrically inert, as judged by the inability of the surface to become charged when immersed in a flux of thermalized negative ions. Similar observations were made on the (0001) face, prepared only by chemical polishing in HCI. All the above phenomena persisted for remarkably extended periods without reprocessing, even after exposure to room air for 3 months. On the other hand, the (OOOi)(1120), and (lOi0) surfaces that were prepared only by chemical polishing were found to be: (1) atomically irregular as judged by LEED, (2) nominally flat as judged by SEM, (3) contaminated with strongly bound chlorine as judged by Auger, and (4) electrically active in the following sense; when negative thermalized ions were deposited from a corona source in air, the ZnO surfaces easily became charged and a giant band bending of N 70 V was sustained in the dark. On these chargeable surfaces, photodischarge spectroscopy was carried out with less-than-bandgap-light, and discrete energy levels within the ZnO bandgap of the extrinsic surface states were inferred. Depending on crystal face, the surface state levels below the conduction band were from 1.1 to 1.7 eV. On the basis of these and other observations, it was possible to deduce various bulk and surface discharge mechanisms and to correlate the charging ability with the presence of strongly bound chlorine.
1. Introduction Single crystals of ZnO offer unique possibilities for surface studies. We have found that for properly prepared ZnO surfaces, the electrical properties, low-energy-electron-diffraction (LEED) patterns, and Auger-electron-spectroscopy (AES) patterns persist, even after exposure to room air for periods of days,
and in some
cases,
months.
This persistence
indicates
a remarkable
* Supported by Office of Naval Research and National Science Foundation. 144
ELECTRONIC, LEED
AND AUGERDL~GNO~TICS~N
ZnO
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surface atomic stability for those crystal faces studied: COOOl),(OOOi), (1 lZO), and (IOTO). Energy levels can sometimes be inferred from negative charges that are gently deposited on the surface by thermalized ions. This is possible even in the presence of air. In this respect, ZnO differs markedly from most metals, semiconductors, and insulators, where easily interpretable surface studies must normally be carried out in ultra-high vacuuml-3). When negatively charged adsorbates are introduced on ZnO surfaces in the dark, it is knowna-6) that the surface voltage greatly increases with respect to the bulk, so that the band bending is -50 V. This relatively high voltage is easily measured with a Kelvin probe and it readily decreases to zero in the presence of light of moderate intensity. By contrast, most semiconductors have much smaller band bendingsl-3) ranging from millivolts to tenths of volts; and these band bendings can be reduced by the photovoltaic effect to near-zero only with difficulty and with very intense lightl32). It is known that most insulators can sustain large band-bendings; but these cannot be decreased upon exposure to light. The bonding of ZnO is partly ionic, and simple electrostatic stability arguments7) have been used to theoretically predict surface structures responsible for the extra reconstruction spots observed on the LEED pattern of the polar faces. ZnO is distinguished among other n-type semiconductors in having no appreciable intrinsic surface states 8,s) ; this feature makes ZnO a preferred choice for studying extrinsic states introduced by adding the charged absorbates. Also, ZnO seems to have very few of the bulk traps normally expected for a large bandgap semiconductor, which may be the reason why ZnO in powder form is so useful for certain electrophotographic applicationslo). Many other interesting aspects of ZnO surfaces are not covered in this paper, but are described by Heiland et al.ll), and by Morrisont2). In fact, ZnO is sometimes considered to be a semiconductor in a class by itself, and is not even treated in Aven and Prener’s otherwise comprehensive book, “Physics and Chemistry of II-VI Compounds”rs). To study the physical, chemical, and electrical properties of this unusual ZnO surface, we have used three different ZnO surface preparations, four different crystal faces, and we have performed a photodischarge spectroscopic analysis using less-than-bandgap light, as a function of the number of tests (runs) made. Initially, the results of the analysis were sensitive to crystal face and surface
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For those surface preparations which allowed the ZnO to become charged, an experimental diagnostic technique called photodischarge spectroscopy (PDS) was carried out to determine the surface energy level. The PDS technique was first introduced by Williams and Willis4). They used the (0001) and (OOOi) ZnO faces prepared by polishing and etching, in H,PO,. They charged these in a corona, and found a complicated (at least bimodal) energy distribution of presumably extrinsic surface states across the forbidden gap of ZnO. The present paper is arranged as follows. Crystal preparation is discussed in section 2. LEED and AES studies carried out by the Princeton University group are reported in section 3. Photodischarge spectroscopy carried out by the RCA group is described in section 4. Finally a general discussion is presented in section 5, where a correlation is made between the various experimental methods used in this study. It should be noted that a nearly parallel sequence of measurements were carried out for CdS (OOOI), (OOOi), (1120) and (IOiO). Some comparisons of CdS with ZnO will be described in section 5.
2. Crystal The various
preparation
techniques used to process ZnO surfaces will be A, and C. are summarized in the consequences of are also tabulated. the Companyl4) in thin plates, -4 x 4 x 1 mm. In A, the ZnO crystal is etched 85% H,PO, to 30 material the surface. be called for ZnO single crystal 4). Under the optical the nominal plane is not flat, but the surface consists of numerous mountains and valleys. Clearly, there are many patches of different crystallographic planes. In preparation B, the ZnO surface is chemicaZlypolishedl5), using a rotating wheel and a slowly dripping aqueous solution of fresh HCl. Each ZnO face requires a specific etch concentration and accumulated time of etching. Previous to the chemical polishing, mechanical polishing was necessary. The materials used were 0.3 urn Linde A powder on (0001) (1 120) and (IOiO). The resulting surface is found to be nominally flat. There are no mountains and valleys, as judged both by optical microscopy 1000 x and by scanning electron microscopy (SEM) 10000 x magnification. However, this surface could have physical microscopic defects (such as steps) and/or chemical defects (such as adsorbed chlorine from the HCI etchant). In preparation C, the surfaces were cleaned in ultrahigh vacuum, using argon bombardment and annealing, until in situ LEED observations showed a
Auger electron spectroscopy
LEED observations without additional processing
Surface state energy levels
Surface texture by optical microscopy and SEM Charges in a negative corona
Faces studied with corona
Description
Surface preparations
TABLE
1
~
A
_
(ioio)
Strongly bound chlorine persisting to 1200°C
No spots
Diffuse sixfold spots, no reconstruction
Cannot be detected
No
Flat
(0001)
polished and chemically polished
B
1.1 * 0.2 eV for (oooi) 1.3 ho.2 eV for (1120) 1.7 * 0.2 eV for (ioio)
Yes
Yes
1.0 * 0.2 eV, 2.0 * 0.2 eV, and 2.5 f 0.2 eV for (OOOi). 1.O i 0.2 eV and 2.5 -i 0.2 eV for (0001).
Flat
coooi),(iiZo),
Mechanically
Rough
(OOOI),(oooi)
Mechanically polished and chemically etched
-___
Physisorbed chlorine pumped off at 25°C
Sharp spots and reconstruction spots for (OOOI),(OOOi),and (1120
Cannot be detected
No
Flat
Preparation B plus ion bombarded and annealed to optimize for brightness of LEED spots (ooo~), moi), (1120), (1010)
Summary of physical, chemical, and electrical diagnostics of certain ZnO surfaces and preparations
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maximum in pattern intensity. This will be called “LEED processing” and a detailed table of the processing schedule is given in section 3. Preparation B yielded LEED patterns, without bombardment or anneal, on& for the (0001) face. In preparation C, the ZnO sample is removed from the LEED instrument, and tested (usually within one day) for its electrical and optoelectronic properties. These surfaces are highly ordered on an atomic scale. Preparations A, B, C give distinctly different electrical, LEED, and AES properties, as judged from the studies described below, which are summarized in table 1.
3. Leed and AES studies As indicated by table 1, crystal surfaces treated according to preparation A were not examined in the LEED apparatus. These surfaces were highly irregular under optical microscopy and were very likely strongly contaminated by etch residues since more carefully processed surfaces were also so contaminated. Preparation B produced the following results according to LEED and AES, which are summarized in table 1. A crystal processed by preparation B with the (0001) Zn face exposed was inserted into the LEED apparatus and the diffraction pattern reproduced in fig. 1 was obtained without any vacuum processing other than room temperature pump-down. The system was pumped overnight without bake-out to a pressure of 2 x 10-l’ torr. However, subsequent work indicated that 10m6 torr is sufficient to observe the same LEED pattern with this surface. Although atomic structure is evident, featuring hexagonal spots with the threefold symmetry of the ideal (0001) structure, the poor quality of the diffraction pattern and the high background intensity indicates that the surface is not atomically clean. This was subsequently substantiated with AES which indicated significant amounts of chlorine, sulfur and carbon in the spectrum. The chlorine is presumably a residue from the etchant. Similar experiments performed with the other surfaces [(lIZO), (lOi0) and (OOOi)] treated according to preparation B revealed no LEED patterns after the same pumpdown procedure. Strong Kikuchi diffraction patterns were observed on all these crystals faces for incident energies below 900 V indicating that the crystals are well ordered after preparation B except for a surface layer a few atomic layers thick. AES measurements with these surfaces also revealed strong chlorine, carbon and sulfur signals two of which (chlorine and carbon) persisted after heat treatment in vucuo to 1200°C. The surfaces subjected to preparation B prior to insertion into the vacuum system were then subjected to argon ion bombardment and subsequent annealing in the LEED chamber. This sequence constitutes preparation C. The (0001) and (OOOi) surfaces were subjected to the schedule listed in Ap-
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149
pendix
A. No appreciable change in the pattern symmetry was evident until the 1140°C anneal, although the sharpness of the diffraction spots improved qualitatively with each treatment step. Beyond this anneal, additional diffraction spots of index + x + were observed. This is similar to the reconstruction observed by Chung and Farnsworthls) where they reported fairly intense
Fig. 1.
LEED pattern of virgin (0001) surface after preparation incident electrons.
B: 195 V
(3, 0) and (+, 3) spots for the clean (0001) ZnO surface. In no case, however, were we able to observe the (+, 0) spots. Our pattern, reproduced in fig. 2, is consistent with the superlattice of fig. 3, which was suggested by electrostatic stability arguments. To be specific, in this surface superlattice, the zinc ions from a basic hexagon with 4 zinc ions on a side. This is the repeat cell and contains 27 zinc ions. To attain electrostatic stability’), one needs, on the average, 4 x 27=6$ zinc vacancies or 6$ excess oxygen ions per basic cell. This can be accommodated by having a cluster of either 6 or 7 zinc vacancies as shown in fig. 3. One out of 4 such cells has 6 vacancies, but the cells are
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arranged in a random manner. This superlattice is presented only as an illustration of the compatibility of the criteria of the electrostatic stability of the observed LEED pattern. It is also possible to satisfy the electronic stability criteria by adopting fractional charges for surface atoms. We have no experimental evidence that either method is physically correct. In addition
Fig. 2.
Reconstructed
LEED pattern of (0001) surface after argon ion bombardment and lZOO*C anneal: Jo0 V incident electrons.
to the consistency with the observed diffraction pattern, the proposed superlattice predicts the existence of & order spots which were not observed. A similar lack of predicted reflections in LEED has been observed with other materials, and this absence of particular spots may be due to very Iow intensities resulting from dynamical diffraction effects. High temperatures were necessary to establish the reconstruction pattern of fig. 2. However, no LEED evidence of faceting on a macroscopic scale was detected. This phase
ELECTRONIC,
was never observed
LEED
without
AND AUGER
DIAGNOSTICS
high vacuum
ON ZIlO
annealing
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151
above 600°C and iden-
tical results were obtained for crystals mounted in crystal holders made of MO or Ta. The possibility that this reconstruction phase might be due to the migration of impurities from the bulk or from the sample holder cannot be entirely ruled out. However, no evidence of impurities other than carbon was observed with AES on the reconstructed surface. The agreement of this 0 0
0
0 0
0
0 0
0 @ 0
0 0
0
0 0 0
0
0
Fig. 3. Superlattice in real space consistent with LEED pattern of fig. 2. Only zinc ions are shown in this unit hexagonal cell. Electrostatic stability arguments suggest that 6 central zinc ions are missing, to form a vacancy cluster. In a random manner, three out of every four of such cells has an additional vacancy shown as the double circle.
structure with stability arguments based on partial ionic bonding and its independence of both the geometry and composition of the sample holder lend support to the speculation that we are observing an intrinsic reconstruction phase. The (0001) crystals were mounted to allow access to the reverse side, and the (OOOi) oxygen surface was also examined in situ. The diffraction pattern observed after the final treatment step is reproduced in fig. 4 and shows the same reconstruction pattern as the (0001) surface. One might expect homopolar bonding contributionsl7) to differ between these two surfaces, but the ionic bonding contributions must, by classical electrostatic symmetry, be the same. The identical LEED pattern observed from these two surfaces therefore supports the use of an ionic bonding model to account for the reconstruction of the polar (0001) and (OOOi) surfaces. Both the (0001) and (OOOi) LEED structures depicted in figs. 2 and 4, respectively, were in no measurable way affected by up to IO5 torr set exposure to dry oxygen at 500 torr, neither were they irreversably altered by extended exposure, up to 3 months to room ambient conditions. Photo-
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discharge spectroscopy measurements (section 4) indicated that neither reconstructed surface could sustain a charge. The LEED pattern could be destroyed by a chemical etch and the ability to hold a corona-induced charge was restored. A similar argon ion bombardment and high vacuum annealing procedure, that is, preparation C, was applied to the chemical-polished (IOiO) and (1120)
Fig. 4.
Reconstructed
LEED pattern of (OOOi) surface after argon ion bombardment and 1200°C anneal: 88 V incident electrons.
surfaces, according to the treatment schedule of Appendixes B and C, respectively. No LEED patterns were evident until steps 4 and 7 for the (lOi0) and (1120) surfaces, respectively. After the final step, the (IOiO) surface diffraction pattern was consistent with an unreconstructed surface while the (1120) surface diffraction pattern displayed integral order spots in register with the bulk and additional + order spots in the (0001) azimuth. The same
ELECTRONIC,LEED
reconstruction
AND AUGERDIAGNOSTICS
phase has been reported
0~ ZnO SURFACES
by Chung and Farnsworthls)
153
for the
(1120) CdS surface. Our surfaces were also stable against exposure to room ambient conditions for as long as three months (that is, after such exposure, the original diffraction patterns were observed after reinsertion into the LEED chamber and a room temperature pump-down), and they were not able to sustain a corona charge. AES analysis revealed chlorine peaks right after pump-down which disappeared below the noise level after 24 hr pumping at 25°C. Here too, the LEED patterns could be destroyed and the charge holding properties restored by mild chemical etching and no evidence of macrofaceting was detected. The ideally terminated (IOiO) surface is electrostatically neutral and, using ionic argumentsr*), it is theoretically expected to be unreconstructed, as observed.
4. Photodischarge
spectroscopy
The experimental apparatus used for determining the electrical properties of ZnO surfaces is identical to that described in some detail by Williams and Willisa). Ohmic contacts4) are made to the backs of the ZnO crystals. The crystals are not intentionally doped. They are semiconducting with a donor density which is typically4) ND= 1.2 x 1017 cmp3. The crystals are mounted on a pedestal which is free to be moved inside a light-tight box from a negative corona-charging station (- 5000 V) to a measuring station, where an oscillating Kelvin probe (1 Hz) is located. Dessicants are placed in the bottom of the box. Otherwise, the crystals are normally exposed to room air. Because of the very short mean free path of ions in one atmosphere of air, most ions generated at the corona center (a fine wire brush) are essentially thermalizedlg) after traversing the 4 cm path to the crystal. Some ions may have somewhat higher energies than thermal, and they may, under certain conditions, introduce cumulative changes in the surface and in the observed PDS. The latter has been observed for certain samples with preparation B. Less-thanbandgap-light enters the box through a shutter. The variation of ZnO surface voltage with photon energy, intensity and time-exposure is then determined. A typical surface voltage is 50 V. Light sources included: (a) an argon laser giving either the 4880 8, line or the 5140 A line, (b) a Bausch and Lomb Monochromator #33-86-02 with slits adjusted to give a light output of width 20 nm (-0.1 eV) at half maximum in the visible optical range; and (c) a tungsten microscope light with a series of high energy cutoff filters roughly equally spaced in filter energy E, in the range 0.7 eV< E,t3.4eV. These filters were Ge, Si, and GaAs wafers, and Corning glass filters #7-69, 2-64, 2-62,2-73,3-67,3-70,3-72, and 3-75. Sufficient light intensity permits votlage decay in the light to dominate over voltage decay in the dark. No attempt has
1.54
J.D.
LEVINE
ET AL.
been made to unravel the many contributions responsible for the dark decay. The study of the decay of the giant surface voltage with less-than-bandgaplight is called photodis~harge spectroscopy (PDS). It is explainable with the use of two previously proposed mechanisms. In both mechanisms, the negative surface charge of whatever energy distribution within the bandgap is assumed to be exactly balanced by space charge of positive sign in the adjacent Schottky depletion layer of the ZnO n-type semiconductor. This gives rise to the band bending diagrams shown in figs. 5a and 5b.
Fig. 5. Schematic band bending diagrams of ZnO single crystals that were charged in a corona, and some possible discharge mechanisms with less-than-bandgap-light. (a) Surface mechanism. (b) Bulk mechanism. Here EF is the ZnO Fermi level, and the bandgap is -3.2 eV.
In the Williams and Willis model of surface discharge, shown in fig. 5a, an incident photon excites an electron from the surface energy level distribution (shown shaded) to the conduction band, whereby it is immediately swept away by the high electric field (- IO6 V/cm) of the Schottky barrier. If a discrete energy level is present at energy Ei below the conduction band, it can be excited by light whenever hv 2 E,. The Williams and Willis surface-discharge model can be extended, if desired, by including the absorption cross section cri(hv) which is dependent on hv, and which has a threshold at hv=&. Theoretical estimates of cam are available2*,ar); in these models, ~i(~v)passes through a maximum. If desired, corrections can also be
ELECTRONIC,
made for the Jahn-Teller
LEED
AND AUGER
DIAGNOSTICS
ON ZllO
effect, and for tunnel-assisted
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155
optical transitions,
In the Kiess6) model of bulk discharge, shown in fig. 5b, an incident photon hv, first excites an electron from the valence band into a bulk trap, thereby releasing a mobile hole which neutralizes a negatively charged surface state. A subsequent photon of similar energy hv, may re-excite the trapped electron into the conduction band where it is swept away by the electric field at the Schottky barrier. It can be shown that the models corresponding to both figs. 5a and 5b give nearly linear plots of log surface voltage V versus light exposure time 1. To explain most of our data, the models in figs. Sa and 5b seem to be adequate. Naturally, PDS cannot be carried out for those surfaces prepared in such a way that they do not charge at all. These surfaces include all faces of preparation C and the (0001) face of preparation B. PDS data taken with preparations C, B, and A will now be described, in that order. Chronologically, A was studied first and C last. The surface with preparation C is the most carefully defined on the atomic scale, however, and that of preparation A is the least defined. 4.1. PREPARATION C - LEED-PROCESSED ZnO crystals with faces (OOOI), (OOOi), (1120) and (lOi0) were first given preparation B, and then were placed in a LEED system. Argon ion bombardment and temperature anneals to N 1200°C were carried out so as to give very bright spots. In addition to the expected spots from the ideally terminated lattice, extra “reconstruction” spots were observed on the zinc face (OOOI), the oxygen face (OOOi) and one prism face (1120). These spots are described in section 3. Repeated attempts to charge any of these highly regular and persistent surfaces in the corona were unsuccessful up to the highest corona voltage tried: 10000 V. No surface voltage was detected at the Kelvin probe in the time (-3 set) needed to move the crystal from the corona station to the Kelvin probe station aa). One interpretation of this firm negative result is that the surface might be too regular. It might lack physical and/or chemical defects which might bind surface charge with energies deep enough to hold a surface charge for > 3 set against thermionic or field emission to the conduction band. In this interpretation, either there are no surface acceptor levels on these faces, or their energies Eiare shallower than - 0.8eV. Another possible interpretation of the same data is that the ion-bombardment-and-anneal procedure would introduce an excess of surface donors. This would produce a strongly conducting skin on the crystal. Arguments against the latter interpretation are given in section 5.
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4.2. PREPARATION B - CHEMKAL POLISHED The zinc face, (0001), prepared with preparation B behaved similar to the LEED-processed surfaces described above. It would not hold a charge, even after repeated attempts to charge it in the corona; and it yielded a LEED spot pattern (thought not as sharp as LEED-processed (0001)) upon the first pumpdown in ultrahigh vacuum, without heating. This behavior was repeatable with 4 separate chips of (0001) with the same preparation B. The other faces (OOOT), (1120) and (IOiO) prepared with preparation B charged easily in the corona. Typically, the surface voltage was -50 V negative with respect to the grounded ohmic contact at the back of the crystal. Typical PDS plots for the first two runs on each face are shown in fig. 6. Here log voltage V is plotted versus filter energy E,; each filter ideally
Ef (eV1
Fig. 6. Photodischarge spectroscopy of various faces of ZnO which had received preparation B. First and second runs are indicated by the dark and light symbols, respectively. From the knees of the curves, one can determine their surface state levels, according to the surface discharge model.
ELECTRONIC,LEED
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157
absorbs all light with hv 2 Ef. Points are taken at one-minute intervals. In the first half minute, the tungsten microscope light is on with one filter in place; in the second half minute, the light is off and V is determined by the Kelvin probe. The filter is then changed to the next higher Ef. The process is repeated at succeeding higher Ef. The first two points from the left in fig. 6, however, are taken in the dark, in order to show the extent of dark decay. For each crystal face, the curves through the dark and light symbols indicate the first and second runs on these samples, respectively. Each run constitutes a complete corona-charging-process and a complete, systematic discharge with light. Typically, V slowly decreases (due to dark decay) up to a certain filter energy Ef = El (the knee of the curve), after which V begins to rapidly decrease with succeeding Ef. Characteristically, the second run is slightly shifted to the right of the first run in fig. 6. This shift is seen best in fig. 7, where PDS data for 21 consecutive
l(j
1
I I
I
I 2
I
Ef (ev) Fig. 7. Photodischarge spectroscopy of ZnO (1010) as a function of the number of successive runs. After N 100 runs, the crystal becomes less sensitive to light and ‘stabilizes’.
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runs on ZnO (lOi0) is shown. Note that with successive runs, structure appears in the curves, possibly associated with microplanes formed from the charging process. Also, the sensitivity of the crystal to less-th~~-bandgaplight decreases. After a great many runs (> 100) further shift ceases and a “stabilization” is established as shown by the dashed curve in the figure. Strongly bound chlorine was detected on these surfaces, as judged from Auger spectroscopy. The large chlorine signal persisted even after annealing the crystal to 1200°C. The shift to the right, and the appearance of structure can be considerably speeded up, or simulated, by chemical etching in HCI. Even the (0001) face with preparation B can be made to charge after 5 set of chemical etching in SN HCl. Before “stabilization” the dependence of V on E, is sensitive to crystal face and number of runs; this may indicate a surface discharge mechanism. In that case, the knee of the curves in fig. 7 corresponds to the threshold energy levels E, for each face. These levels, measured below the conduction band edge, are given in table 1 for (0004) (1120), and (10iO). After “stabilization”, the monochromator described in Section II was used, and the voltage decay with exposure time in light (stray UV filtered out) followed the laws V = V, exp (- t/t), z -‘=oJ
7
(1) (2)
over one order in magnitude in V. Here I’,, is the voltage at t=O (typically 70 V), r -r is the rate constant, G is the cross section at energy hv, and J is the photon flux at the same hv. A plot of G versus hv is shown in fig. 8, for the (OOOi)and (1120) faces. Here g is given on a relative scale because of the change in optical alignment between the tests on the different faces. Note that the plot follows the relation ci = A exp (Bhv),
(3)
where, for both faces, B=3.L 40.2 eV_‘. For the monochromator with hv= 2.5 eV, z = 113 sec. For the more intense laser light of 2.54 eV, r = 2.7 sec. An estimate of the absolute value of LFwas made using this laser light and the stabilized (OOOi)face. The result was G= 3 _t 1 x IO-‘* cm2/photon, after the stray UV light from the laser was removed. The optical trans~ssion of a bulk ZnO crystal was carried out on another ZnO crystal. A plot of relative optical absorption AA (after a uniform background of absorption independent of hv was subtracted) versus hv gives the dashed line in fig. 8. It is seen that all lines in the figure are sufficientIy parallel to infer that one mechanism may be operable in all cases. This mechanism could be the bulk discharge mechanism, since it is insensitive to surface crystal face.
.J.D.LEVINE
Fig. 8.
ET AL.
Relative cross sections 1 for the photodischarge process after ‘stabilization’, a function of hv. The relative optical absorption BA is also shown.
159
as
4.3 PREPARATION A - CHEMICAL ETCHED The first PDS studies were made with the simplest preparation, preparation A. The PDS analysis for (0001) and (OOOi) assuming the surface discharge mechanismd) indicated two energy levels for each, as given in table 1. We have repeated the experiment and have also found two energy levels at about the same energies. These two energy levels are measured with respect to the conduction band edge of ZnO, whose bandgap is 3.2 eV. The existence of these two levels (or bands of relatively narrow width), and the visual observation of surface macroplanes suggested to us at an early stage that a “patch” effect might be present. That is, each microplane patch might give rise to a distinct energy level. For this reason little attention was subsequently paid to samples with preparation A. (It seemed unlikely that the internal structure of the PO: ion could generate the two levels, because red light discharged
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only the upper or “red” levels, while blue light discharged both the “red” and “blue” levels.) From the electrical measurements (see e.g. fig. 6) and the optical and scanning-electron microscope observations, it seems that the “patches” produced in preparation A were removed in preparation B.
5. Discussion The surface electrical properties of single crystal ZnO, together with SEM, LEED and Auger diagnostics, have been carried out for the crystal faces (OOOl), (OOOi), (1120) and (lOiO), for three different surface preparations, A, B, C, and as a function of the number of runs made on each. For those crystais which did charge in the presence of the corona, see table 1, the following atomistic model is offered. We use the fact that these crystal surfaces are known to have chemisorbed (or otherwise strongly bound) chlorine, as judged by Auger spectroscopy. Suppose that the chlorine (or chlorine complex} acts as the surface acceptor center at energy level Ei below the conduction band. Then a negatively charged ion from the corona, such as CO; l9), can drift to the surface of the crysta1, and deposit its charge into the supposedly-more-electronegative surface-chlorine-complex. Subsequently, CO; becomes a neutral molecule and it can be readily desorbed according to the reaction: 2C03 (s)-+2CO, (g)+O, (g). In this manner, the electron transfer reaction continues and the surface voltage increases in the Schottky barrier as the square of the charge deposited. The terminal charge and voltage is reached, most probably, when the internal breakdown field, - 2 x 10’ V/cm, of the Schottky barrier in ZnO is attained. Thereafter, the charge from the corona ion is deposited directly in the ZnO conduction band and is collected at the ohmic contact at the back of the crystal. Four corollaries
of the above atomistic
model will now be described,
and
some evidences presented for their validity. (1) If chemisorbed chlorine (or same similar adsorbate) were absent, the crystal would not charge. This corollary is substantiated by LEED and Auger spectroscopy data taken on crystals with preparation C. The surface was atomically regular, chemisorbed chlorine was not present, and the surfaces did not charge in the corona. (2) Physical defects, in the absense of chemical acceptorssuch as chemisorbed chlorine, are not suficient for holding a charge. TO introduce such physical defects on the surface in the absence of chlorine, we have used ion bombardment in the LEED system, but with no anneal. There is a poor LEED pattern and the surface is thought to be irregular on an atomic scale. An ohmic
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contact was made to the previously diffused indium contact by applying a drop of gallium, at room temperature. It was found that this surface with certain physical defects did not charge. Subsequently, the surface could be made to charge by mild etching in HCl. (3) Etch solutions might be found where the etch anion is much less electronegative than chlorine; surfaces etched this way might not charge in a corona. Such an etch was found. It is organic in nature and its properties will be described in a future publication. (4) If the chlorine surface acceptors were locatedatpreferential atomic sites, their p-like wave function orientations might be detected using PDS with polarized light, normally incident on the crystal. This was found to be the case on (1120) using preparation B with < 100 runs. The rate constant r- 1 of PDS was a maximum for an angle 0 = 60’ f 10” of the photon E-vector with respect to the c-axis. Also 7-l was a minimum for 8 = 30”, with an appreciable ratio of 3 between the maximum and minimum of 2- ‘. This can be explained using the concepts of surface atomic isomers which are identical in the unit cell of the ideally terminated crystal, but which are differentiated in a reconstructed crystal surface. It is known from LEED that ZnO (1120) has such a reconstruction, and with only one isomer active, the predicted angle would be half the tetrahedral angle or 54.7”. This angle lies within the error limits of the angle observed. The atomistic model and the four corollaries described above suggest that a ZnO surface free of chemical defects (such as chemisorbed chlorine) is electrically inert with respect to the charging action of a corona. This observation is consistent with the so-called “active site” theory of catalysis, wherein only a few isolated chemical and/or physical defects on the surface are presumed to be responsible for the catalytic process. In our case, it is not known if the electrically-active chlorine resides at a physical defect or not. From the polarization studies in (4) above, there is a tentative indication that the electrically-active chlorine has a specific orientation with respect to the c-axis of the wurtzite crystal. Consider an alternate explanation for the observed lack of charging ability in some of the crystals. Suppose that the processing of certain ZnO surfaces is such as to form nearly degenerate surface layers. Then this makes it difficult to attain large voltages with the corona. As a numerical example, suppose that the Schottky barrier law applies, and the breakdown field is independent of doping ND (cme3) or initial voltage V,. Then the product V,N, is a constant. Typically Tr,=70 V, Nn=2 x 1O1’ cmd3, and our resolution is
162
donors
J.D.LEVINE
ET AL.
given by 23) ND = 1.1 x 10” exp (-0.52/kT).
(4)
This formula applies over the temperature range 800 “C < T-c1350 “C. Using the highest temperature of our processing, T- 1200°C for the (OOOl), (OOOi), and (lOi0) faces as used in preparation C, one gets from (4) that No = 2 x IO'8 cmF3, about an order of magnitude too small to account for a resolution of < 1 V. Furthermore, one might expect this value of 2 x 101* to be an upper limit, as our crystals with preparation C are heated in ultrahigh vacuum, not in saturated zinc vapor. It is also significant that the maximum processing temperature is only 400 “C in the case of ZnO (1120) and yet this face exhibits the same lack of charging ability. Also the (0001) crystals that were chemically polished, but not heated at all, refused to charge. The probability thus seems remote that surface layers of ND> 1019 cme3 exist for each of these greatly different temperature conditions and processes. A nearly parallel study was made of the PDS observations on CdS 24). The faces tested were (OOOl), (OOOi), and (1120) prepared by chemical etching, similar to preparation B. Insulating and semiconducting crystals of CdS were charged to-2000 V and - 100 V, respectively, in a negative corona of 5000 V. They all discharged rapidly with less-than-bandgap light. A variation in crystal face and even a variation in ambients did not produce any noticeable variation in the PDS. This pronounced insensitivity of PDS to the surface region is consistent with an assumed bulk discharge mechanism similar to that shown in fig. 5b. The CdS crystals were estimated to be lo-100 times more sensitive than the ZnO crystals to less-than-bandgap light. This difference between CdS and ZnO can be explained if one presumes a relative abundance and scarcity, respectively, of bulk traps which are responsible for the bulk discharge mechanism. Acknowledgements The authors are indebted to A. J. Tocci for chemically polishing our ZnO crystals, and to R. Williams, A. M. Goodman, T. Freund, H. Kiess, D. Redfield, F. H. Nicoll, and D. Meyerhofer, for offering many helpful suggestions. Note added in proof Baidyaroy, Bottoms and Mark 24) have carried out a complementary study of the chemisorption of O2 by CdS. They found that highly regular surfaces of CdS did not adsorb significant amounts of 02, as judged from conductivity measurements. This observation supports the finding in this paper that active sites are necessary to make certain semiconductors electronically active.
ZnO
ELECTRONIC,LEEDANDAUGERDIAGN~~TICS~N
163
SURFACES
Appendix A VACUUM SURFACE PREPARATION
PROCEDURES FOR THE
(0001) AND (0001) ZnO
SURFACES
(1) (2) (3) (4) (5) (6) (7) (8) (9) *(lo) (11) ** (12) (13) (14)
System evacuated to 1 x 10d9 torr without bakeout Overnight bakeout at 25O”C, pressure 2 x lo-” torr after cooling 300°C anneal for 10 min 400°C anneal for 10 min 500°C anneal for 10 min 600°C anneal for 10 min 700°C anneal for 10 min 800°C anneal for 10 min 900°C anneal for 10 min Argon ion bombardment 5 PA; 225 V for 30 min 1140°C anneal for 10 min Argon ion bombardment 5.6 pA ; 225 V for 30 min 1140°C anneal for 10 min 1300 “C anneal for 5 min Appendix B
VACUUM
(1) (2) (3) (4) (5) (6) (7)
SURFACE PREPARATION
PROCEDURES FOR THE (10x0)
ZtlO
SURFACE
System evacuated to 1 x 10e9 torr without bakeout Overnight bakeout at 250°C pressure 2 x IO-” torr after cooling Argon ion bombardment 5 PA; 225 V for 45 min 300°C anneal for 2+ hr 650°C anneal for 35 min 800°C anneal for 35 min 1000°C anneal for 10 min Appendix C
VACUUM
SURFACE PREPARATION
(1)
System
(2)
2OO“C anneal
for 2 hr
(3)
35O’C
anneal
for
1 hr
(4)
400°C
anneal
for
1
(5)
evacuated
PROCEDURES FOR THE (1 120)
to 2 x 10m9 without
hr ; maximum
by indium
contacts
on this sample
Overnight
bakeout
at 25O”C,
* Only for the (0001) surface. ** Only for the (OOOT)surface.
bakeout
annealing
pressure
ZnO SURFACE
temperatures
2 x 10-l’
torr after
were cooling
limited
164
(6) (7) (8) (9)
J. D. LEVINE
Argon 400°C Argon 400°C
ET AL.
ion bombardment 7 pA; 225 V for 30 min anneal for 10 min ion bombardment 5 pA; 225 V for 3 hr anneal for 10 min References
1) S. G. Davison and J. D. Levine, Surface States, in: Solid State Physics, Vol. 25, Eds. F. Seitz and D. Turnbull (Academic Press, New York, 1970) pp. 1-149. 2) P. Mark, Methods of Determining Surface State Energies, in: Modern Methods of Surface Analysis, Eds. P. Mark and J. D. Levine (North-Holland, Amsterdam, 1971) pp. 192-223. Published in Surface Sci. 25 (1971). 3) M. Henzler, Surface Sci. 25 (1971) 650. 4) R. Williams and A. Willis, J. Appl. Phys. 39 (1968) 3731. 5) H. Kiess, Applied Optics, Suppl. 3 (Electrophotography) (1969) 100. 6) H. Kiess, J. Appl. Phys. 40 (1969) 4054. 7) R. Nosker, P. Mark and J. D. Levine, Surface Sci. 19 (1970) 291. 8) R. K. Swank, Phys. Rev. 153 (1967) 844. 9) J. D. Levine, J. Vacuum Sci. Technol. 6 (1969) 549. 10) R. M. Schaffert, EIectrophotogruphy (The Focal Press, New York, 1965). 11) G. Heiland and P. Kunstmann, Surface Sci. 13 (1969) 72; G. Heiland, J. Phys. Chem. Solids 22 (1961) 227. 12) S. R. Morrison, Surface Sci. 13 (1969) 85. 13) M. Aven and J. S. Prener, Eds., Physics and Chemistry of II-VI Compounds (NorthHolland, Amsterdam, 1964). 14) We have been formally advised that the 3M Company has since discontinued their line of ZnO single crystals. 15) See e.g. W. H. Strehlow, J. Appl. Phys. 40 (1969) 2928. 16) M. F. Chung and H. E. Farnsworth, Surface Sci. 22 (1970) 93. 17) H. C. Gatos, J. Appl. Phys. 32 (1961) 1232: R. Seiwatz, Surface Sci. 2 (1964) 473; S. G. Davison and J. D. Levine, Ref. 1, p. 81 ff. 18) J. D. Levine and P. Mark, Phys. Rev. 144 (1966) 751. 19) M. M. Shahin, Applied Optics Suppl. 3 (Electrophotography) (1969) 106. 20) D. M. Eagles, J. Phys. Chem. Solids 16 (1960) 76. 21) H. B. Bebb, Phys. Rev. 185 (1969) 1116. 22) Actually there was a small residual voltage of -2 V of the opposite sign. This did not vary with light. Most probably it is caused either by a difference in work functions or a small amount of negative charge fixed in the wax used for mounting the crystal to the pedestal. 23) F. A. Kriiger, The Chemistry of Imperfect Crystals (North-Holland, Amsterdam, 1964) p. 697. 24) S. Baidyaroy, W. R. Bottoms and P. Mark, Surface Sci. 29 (1972) 165.
SCIENCE 29 (1972) 144164 0 North-Holland
CORRELATION AND AUGER
Publishing Co.
OF ELECTRONIC,
DIAGNOSTICS
LEED,
ON ZnO SURFACES
JULES D. LEVINE and A. WILLIS David
Samoff'Research Center, RCA Laboratories,
Princeton,
New Jersey 08540, U.S.A.
W. R. BOTTOMS and PETER MARK* Princeton
University,
Princeton,
New Jersey 08540,
U.S.A.
Received 1 July 1971; revised manuscript received 6 October 1971 Surface electrical properties of ZnO single crystals were measured and were correlated with complementary LEED and Auger measurements. For the crystal faces (OOOl),(OOOi), (1 lzO), and (10iO) that were prepared by ion bombardment and anneal in ultrahigh vacuum, the above surfaces were found to be: (a) atomically regular as judged by bright LEED spots, (b) free of chemisorbates as judged by Auger spectroscopy, and (c) electrically inert, as judged by the inability of the surface to become charged when immersed in a flux of thermalized negative ions. Similar observations were made on the (0001) face, prepared only by chemical polishing in HCI. All the above phenomena persisted for remarkably extended periods without reprocessing, even after exposure to room air for 3 months. On the other hand, the (OOOi)(1120), and (lOi0) surfaces that were prepared only by chemical polishing were found to be: (1) atomically irregular as judged by LEED, (2) nominally flat as judged by SEM, (3) contaminated with strongly bound chlorine as judged by Auger, and (4) electrically active in the following sense; when negative thermalized ions were deposited from a corona source in air, the ZnO surfaces easily became charged and a giant band bending of N 70 V was sustained in the dark. On these chargeable surfaces, photodischarge spectroscopy was carried out with less-than-bandgap-light, and discrete energy levels within the ZnO bandgap of the extrinsic surface states were inferred. Depending on crystal face, the surface state levels below the conduction band were from 1.1 to 1.7 eV. On the basis of these and other observations, it was possible to deduce various bulk and surface discharge mechanisms and to correlate the charging ability with the presence of strongly bound chlorine.
1. Introduction Single crystals of ZnO offer unique possibilities for surface studies. We have found that for properly prepared ZnO surfaces, the electrical properties, low-energy-electron-diffraction (LEED) patterns, and Auger-electron-spectroscopy (AES) patterns persist, even after exposure to room air for periods of days,
and in some
cases,
months.
This persistence
indicates
a remarkable
* Supported by Office of Naval Research and National Science Foundation. 144
ELECTRONIC, LEED
AND AUGERDL~GNO~TICS~N
ZnO
SURFACES
145
surface atomic stability for those crystal faces studied: COOOl),(OOOi), (1 lZO), and (IOTO). Energy levels can sometimes be inferred from negative charges that are gently deposited on the surface by thermalized ions. This is possible even in the presence of air. In this respect, ZnO differs markedly from most metals, semiconductors, and insulators, where easily interpretable surface studies must normally be carried out in ultra-high vacuuml-3). When negatively charged adsorbates are introduced on ZnO surfaces in the dark, it is knowna-6) that the surface voltage greatly increases with respect to the bulk, so that the band bending is -50 V. This relatively high voltage is easily measured with a Kelvin probe and it readily decreases to zero in the presence of light of moderate intensity. By contrast, most semiconductors have much smaller band bendingsl-3) ranging from millivolts to tenths of volts; and these band bendings can be reduced by the photovoltaic effect to near-zero only with difficulty and with very intense lightl32). It is known that most insulators can sustain large band-bendings; but these cannot be decreased upon exposure to light. The bonding of ZnO is partly ionic, and simple electrostatic stability arguments7) have been used to theoretically predict surface structures responsible for the extra reconstruction spots observed on the LEED pattern of the polar faces. ZnO is distinguished among other n-type semiconductors in having no appreciable intrinsic surface states 8,s) ; this feature makes ZnO a preferred choice for studying extrinsic states introduced by adding the charged absorbates. Also, ZnO seems to have very few of the bulk traps normally expected for a large bandgap semiconductor, which may be the reason why ZnO in powder form is so useful for certain electrophotographic applicationslo). Many other interesting aspects of ZnO surfaces are not covered in this paper, but are described by Heiland et al.ll), and by Morrisont2). In fact, ZnO is sometimes considered to be a semiconductor in a class by itself, and is not even treated in Aven and Prener’s otherwise comprehensive book, “Physics and Chemistry of II-VI Compounds”rs). To study the physical, chemical, and electrical properties of this unusual ZnO surface, we have used three different ZnO surface preparations, four different crystal faces, and we have performed a photodischarge spectroscopic analysis using less-than-bandgap light, as a function of the number of tests (runs) made. Initially, the results of the analysis were sensitive to crystal face and surface
146
J.D.LEVINE
ET AL.
For those surface preparations which allowed the ZnO to become charged, an experimental diagnostic technique called photodischarge spectroscopy (PDS) was carried out to determine the surface energy level. The PDS technique was first introduced by Williams and Willis4). They used the (0001) and (OOOi) ZnO faces prepared by polishing and etching, in H,PO,. They charged these in a corona, and found a complicated (at least bimodal) energy distribution of presumably extrinsic surface states across the forbidden gap of ZnO. The present paper is arranged as follows. Crystal preparation is discussed in section 2. LEED and AES studies carried out by the Princeton University group are reported in section 3. Photodischarge spectroscopy carried out by the RCA group is described in section 4. Finally a general discussion is presented in section 5, where a correlation is made between the various experimental methods used in this study. It should be noted that a nearly parallel sequence of measurements were carried out for CdS (OOOI), (OOOi), (1120) and (IOiO). Some comparisons of CdS with ZnO will be described in section 5.
2. Crystal The various
preparation
techniques used to process ZnO surfaces will be A, and C. are summarized in the consequences of are also tabulated. the Companyl4) in thin plates, -4 x 4 x 1 mm. In A, the ZnO crystal is etched 85% H,PO, to 30 material the surface. be called for ZnO single crystal 4). Under the optical the nominal plane is not flat, but the surface consists of numerous mountains and valleys. Clearly, there are many patches of different crystallographic planes. In preparation B, the ZnO surface is chemicaZlypolishedl5), using a rotating wheel and a slowly dripping aqueous solution of fresh HCl. Each ZnO face requires a specific etch concentration and accumulated time of etching. Previous to the chemical polishing, mechanical polishing was necessary. The materials used were 0.3 urn Linde A powder on (0001) (1 120) and (IOiO). The resulting surface is found to be nominally flat. There are no mountains and valleys, as judged both by optical microscopy 1000 x and by scanning electron microscopy (SEM) 10000 x magnification. However, this surface could have physical microscopic defects (such as steps) and/or chemical defects (such as adsorbed chlorine from the HCI etchant). In preparation C, the surfaces were cleaned in ultrahigh vacuum, using argon bombardment and annealing, until in situ LEED observations showed a
Auger electron spectroscopy
LEED observations without additional processing
Surface state energy levels
Surface texture by optical microscopy and SEM Charges in a negative corona
Faces studied with corona
Description
Surface preparations
TABLE
1
~
A
_
(ioio)
Strongly bound chlorine persisting to 1200°C
No spots
Diffuse sixfold spots, no reconstruction
Cannot be detected
No
Flat
(0001)
polished and chemically polished
B
1.1 * 0.2 eV for (oooi) 1.3 ho.2 eV for (1120) 1.7 * 0.2 eV for (ioio)
Yes
Yes
1.0 * 0.2 eV, 2.0 * 0.2 eV, and 2.5 f 0.2 eV for (OOOi). 1.O i 0.2 eV and 2.5 -i 0.2 eV for (0001).
Flat
coooi),(iiZo),
Mechanically
Rough
(OOOI),(oooi)
Mechanically polished and chemically etched
-___
Physisorbed chlorine pumped off at 25°C
Sharp spots and reconstruction spots for (OOOI),(OOOi),and (1120
Cannot be detected
No
Flat
Preparation B plus ion bombarded and annealed to optimize for brightness of LEED spots (ooo~), moi), (1120), (1010)
Summary of physical, chemical, and electrical diagnostics of certain ZnO surfaces and preparations
148
J.D.
LEVINE
ET AL.
maximum in pattern intensity. This will be called “LEED processing” and a detailed table of the processing schedule is given in section 3. Preparation B yielded LEED patterns, without bombardment or anneal, on& for the (0001) face. In preparation C, the ZnO sample is removed from the LEED instrument, and tested (usually within one day) for its electrical and optoelectronic properties. These surfaces are highly ordered on an atomic scale. Preparations A, B, C give distinctly different electrical, LEED, and AES properties, as judged from the studies described below, which are summarized in table 1.
3. Leed and AES studies As indicated by table 1, crystal surfaces treated according to preparation A were not examined in the LEED apparatus. These surfaces were highly irregular under optical microscopy and were very likely strongly contaminated by etch residues since more carefully processed surfaces were also so contaminated. Preparation B produced the following results according to LEED and AES, which are summarized in table 1. A crystal processed by preparation B with the (0001) Zn face exposed was inserted into the LEED apparatus and the diffraction pattern reproduced in fig. 1 was obtained without any vacuum processing other than room temperature pump-down. The system was pumped overnight without bake-out to a pressure of 2 x 10-l’ torr. However, subsequent work indicated that 10m6 torr is sufficient to observe the same LEED pattern with this surface. Although atomic structure is evident, featuring hexagonal spots with the threefold symmetry of the ideal (0001) structure, the poor quality of the diffraction pattern and the high background intensity indicates that the surface is not atomically clean. This was subsequently substantiated with AES which indicated significant amounts of chlorine, sulfur and carbon in the spectrum. The chlorine is presumably a residue from the etchant. Similar experiments performed with the other surfaces [(lIZO), (lOi0) and (OOOi)] treated according to preparation B revealed no LEED patterns after the same pumpdown procedure. Strong Kikuchi diffraction patterns were observed on all these crystals faces for incident energies below 900 V indicating that the crystals are well ordered after preparation B except for a surface layer a few atomic layers thick. AES measurements with these surfaces also revealed strong chlorine, carbon and sulfur signals two of which (chlorine and carbon) persisted after heat treatment in vucuo to 1200°C. The surfaces subjected to preparation B prior to insertion into the vacuum system were then subjected to argon ion bombardment and subsequent annealing in the LEED chamber. This sequence constitutes preparation C. The (0001) and (OOOi) surfaces were subjected to the schedule listed in Ap-
ELECTRONIC, LEESAND AUGER01.40~0sTICs0~ ZnO SURFACES
149
pendix
A. No appreciable change in the pattern symmetry was evident until the 1140°C anneal, although the sharpness of the diffraction spots improved qualitatively with each treatment step. Beyond this anneal, additional diffraction spots of index + x + were observed. This is similar to the reconstruction observed by Chung and Farnsworthls) where they reported fairly intense
Fig. 1.
LEED pattern of virgin (0001) surface after preparation incident electrons.
B: 195 V
(3, 0) and (+, 3) spots for the clean (0001) ZnO surface. In no case, however, were we able to observe the (+, 0) spots. Our pattern, reproduced in fig. 2, is consistent with the superlattice of fig. 3, which was suggested by electrostatic stability arguments. To be specific, in this surface superlattice, the zinc ions from a basic hexagon with 4 zinc ions on a side. This is the repeat cell and contains 27 zinc ions. To attain electrostatic stability’), one needs, on the average, 4 x 27=6$ zinc vacancies or 6$ excess oxygen ions per basic cell. This can be accommodated by having a cluster of either 6 or 7 zinc vacancies as shown in fig. 3. One out of 4 such cells has 6 vacancies, but the cells are
150
J.D.
LEVINE
ET AL,
arranged in a random manner. This superlattice is presented only as an illustration of the compatibility of the criteria of the electrostatic stability of the observed LEED pattern. It is also possible to satisfy the electronic stability criteria by adopting fractional charges for surface atoms. We have no experimental evidence that either method is physically correct. In addition
Fig. 2.
Reconstructed
LEED pattern of (0001) surface after argon ion bombardment and lZOO*C anneal: Jo0 V incident electrons.
to the consistency with the observed diffraction pattern, the proposed superlattice predicts the existence of & order spots which were not observed. A similar lack of predicted reflections in LEED has been observed with other materials, and this absence of particular spots may be due to very Iow intensities resulting from dynamical diffraction effects. High temperatures were necessary to establish the reconstruction pattern of fig. 2. However, no LEED evidence of faceting on a macroscopic scale was detected. This phase
ELECTRONIC,
was never observed
LEED
without
AND AUGER
DIAGNOSTICS
high vacuum
ON ZIlO
annealing
SURFACES
151
above 600°C and iden-
tical results were obtained for crystals mounted in crystal holders made of MO or Ta. The possibility that this reconstruction phase might be due to the migration of impurities from the bulk or from the sample holder cannot be entirely ruled out. However, no evidence of impurities other than carbon was observed with AES on the reconstructed surface. The agreement of this 0 0
0
0 0
0
0 0
0 @ 0
0 0
0
0 0 0
0
0
Fig. 3. Superlattice in real space consistent with LEED pattern of fig. 2. Only zinc ions are shown in this unit hexagonal cell. Electrostatic stability arguments suggest that 6 central zinc ions are missing, to form a vacancy cluster. In a random manner, three out of every four of such cells has an additional vacancy shown as the double circle.
structure with stability arguments based on partial ionic bonding and its independence of both the geometry and composition of the sample holder lend support to the speculation that we are observing an intrinsic reconstruction phase. The (0001) crystals were mounted to allow access to the reverse side, and the (OOOi) oxygen surface was also examined in situ. The diffraction pattern observed after the final treatment step is reproduced in fig. 4 and shows the same reconstruction pattern as the (0001) surface. One might expect homopolar bonding contributionsl7) to differ between these two surfaces, but the ionic bonding contributions must, by classical electrostatic symmetry, be the same. The identical LEED pattern observed from these two surfaces therefore supports the use of an ionic bonding model to account for the reconstruction of the polar (0001) and (OOOi) surfaces. Both the (0001) and (OOOi) LEED structures depicted in figs. 2 and 4, respectively, were in no measurable way affected by up to IO5 torr set exposure to dry oxygen at 500 torr, neither were they irreversably altered by extended exposure, up to 3 months to room ambient conditions. Photo-
152
J.D.LEVINE
ET AL.
discharge spectroscopy measurements (section 4) indicated that neither reconstructed surface could sustain a charge. The LEED pattern could be destroyed by a chemical etch and the ability to hold a corona-induced charge was restored. A similar argon ion bombardment and high vacuum annealing procedure, that is, preparation C, was applied to the chemical-polished (IOiO) and (1120)
Fig. 4.
Reconstructed
LEED pattern of (OOOi) surface after argon ion bombardment and 1200°C anneal: 88 V incident electrons.
surfaces, according to the treatment schedule of Appendixes B and C, respectively. No LEED patterns were evident until steps 4 and 7 for the (lOi0) and (1120) surfaces, respectively. After the final step, the (IOiO) surface diffraction pattern was consistent with an unreconstructed surface while the (1120) surface diffraction pattern displayed integral order spots in register with the bulk and additional + order spots in the (0001) azimuth. The same
ELECTRONIC,LEED
reconstruction
AND AUGERDIAGNOSTICS
phase has been reported
0~ ZnO SURFACES
by Chung and Farnsworthls)
153
for the
(1120) CdS surface. Our surfaces were also stable against exposure to room ambient conditions for as long as three months (that is, after such exposure, the original diffraction patterns were observed after reinsertion into the LEED chamber and a room temperature pump-down), and they were not able to sustain a corona charge. AES analysis revealed chlorine peaks right after pump-down which disappeared below the noise level after 24 hr pumping at 25°C. Here too, the LEED patterns could be destroyed and the charge holding properties restored by mild chemical etching and no evidence of macrofaceting was detected. The ideally terminated (IOiO) surface is electrostatically neutral and, using ionic argumentsr*), it is theoretically expected to be unreconstructed, as observed.
4. Photodischarge
spectroscopy
The experimental apparatus used for determining the electrical properties of ZnO surfaces is identical to that described in some detail by Williams and Willisa). Ohmic contacts4) are made to the backs of the ZnO crystals. The crystals are not intentionally doped. They are semiconducting with a donor density which is typically4) ND= 1.2 x 1017 cmp3. The crystals are mounted on a pedestal which is free to be moved inside a light-tight box from a negative corona-charging station (- 5000 V) to a measuring station, where an oscillating Kelvin probe (1 Hz) is located. Dessicants are placed in the bottom of the box. Otherwise, the crystals are normally exposed to room air. Because of the very short mean free path of ions in one atmosphere of air, most ions generated at the corona center (a fine wire brush) are essentially thermalizedlg) after traversing the 4 cm path to the crystal. Some ions may have somewhat higher energies than thermal, and they may, under certain conditions, introduce cumulative changes in the surface and in the observed PDS. The latter has been observed for certain samples with preparation B. Less-thanbandgap-light enters the box through a shutter. The variation of ZnO surface voltage with photon energy, intensity and time-exposure is then determined. A typical surface voltage is 50 V. Light sources included: (a) an argon laser giving either the 4880 8, line or the 5140 A line, (b) a Bausch and Lomb Monochromator #33-86-02 with slits adjusted to give a light output of width 20 nm (-0.1 eV) at half maximum in the visible optical range; and (c) a tungsten microscope light with a series of high energy cutoff filters roughly equally spaced in filter energy E, in the range 0.7 eV< E,t3.4eV. These filters were Ge, Si, and GaAs wafers, and Corning glass filters #7-69, 2-64, 2-62,2-73,3-67,3-70,3-72, and 3-75. Sufficient light intensity permits votlage decay in the light to dominate over voltage decay in the dark. No attempt has
1.54
J.D.
LEVINE
ET AL.
been made to unravel the many contributions responsible for the dark decay. The study of the decay of the giant surface voltage with less-than-bandgaplight is called photodis~harge spectroscopy (PDS). It is explainable with the use of two previously proposed mechanisms. In both mechanisms, the negative surface charge of whatever energy distribution within the bandgap is assumed to be exactly balanced by space charge of positive sign in the adjacent Schottky depletion layer of the ZnO n-type semiconductor. This gives rise to the band bending diagrams shown in figs. 5a and 5b.
Fig. 5. Schematic band bending diagrams of ZnO single crystals that were charged in a corona, and some possible discharge mechanisms with less-than-bandgap-light. (a) Surface mechanism. (b) Bulk mechanism. Here EF is the ZnO Fermi level, and the bandgap is -3.2 eV.
In the Williams and Willis model of surface discharge, shown in fig. 5a, an incident photon excites an electron from the surface energy level distribution (shown shaded) to the conduction band, whereby it is immediately swept away by the high electric field (- IO6 V/cm) of the Schottky barrier. If a discrete energy level is present at energy Ei below the conduction band, it can be excited by light whenever hv 2 E,. The Williams and Willis surface-discharge model can be extended, if desired, by including the absorption cross section cri(hv) which is dependent on hv, and which has a threshold at hv=&. Theoretical estimates of cam are available2*,ar); in these models, ~i(~v)passes through a maximum. If desired, corrections can also be
ELECTRONIC,
made for the Jahn-Teller
LEED
AND AUGER
DIAGNOSTICS
ON ZllO
effect, and for tunnel-assisted
SURFACES
155
optical transitions,
In the Kiess6) model of bulk discharge, shown in fig. 5b, an incident photon hv, first excites an electron from the valence band into a bulk trap, thereby releasing a mobile hole which neutralizes a negatively charged surface state. A subsequent photon of similar energy hv, may re-excite the trapped electron into the conduction band where it is swept away by the electric field at the Schottky barrier. It can be shown that the models corresponding to both figs. 5a and 5b give nearly linear plots of log surface voltage V versus light exposure time 1. To explain most of our data, the models in figs. Sa and 5b seem to be adequate. Naturally, PDS cannot be carried out for those surfaces prepared in such a way that they do not charge at all. These surfaces include all faces of preparation C and the (0001) face of preparation B. PDS data taken with preparations C, B, and A will now be described, in that order. Chronologically, A was studied first and C last. The surface with preparation C is the most carefully defined on the atomic scale, however, and that of preparation A is the least defined. 4.1. PREPARATION C - LEED-PROCESSED ZnO crystals with faces (OOOI), (OOOi), (1120) and (lOi0) were first given preparation B, and then were placed in a LEED system. Argon ion bombardment and temperature anneals to N 1200°C were carried out so as to give very bright spots. In addition to the expected spots from the ideally terminated lattice, extra “reconstruction” spots were observed on the zinc face (OOOI), the oxygen face (OOOi) and one prism face (1120). These spots are described in section 3. Repeated attempts to charge any of these highly regular and persistent surfaces in the corona were unsuccessful up to the highest corona voltage tried: 10000 V. No surface voltage was detected at the Kelvin probe in the time (-3 set) needed to move the crystal from the corona station to the Kelvin probe station aa). One interpretation of this firm negative result is that the surface might be too regular. It might lack physical and/or chemical defects which might bind surface charge with energies deep enough to hold a surface charge for > 3 set against thermionic or field emission to the conduction band. In this interpretation, either there are no surface acceptor levels on these faces, or their energies Eiare shallower than - 0.8eV. Another possible interpretation of the same data is that the ion-bombardment-and-anneal procedure would introduce an excess of surface donors. This would produce a strongly conducting skin on the crystal. Arguments against the latter interpretation are given in section 5.
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4.2. PREPARATION B - CHEMKAL POLISHED The zinc face, (0001), prepared with preparation B behaved similar to the LEED-processed surfaces described above. It would not hold a charge, even after repeated attempts to charge it in the corona; and it yielded a LEED spot pattern (thought not as sharp as LEED-processed (0001)) upon the first pumpdown in ultrahigh vacuum, without heating. This behavior was repeatable with 4 separate chips of (0001) with the same preparation B. The other faces (OOOT), (1120) and (IOiO) prepared with preparation B charged easily in the corona. Typically, the surface voltage was -50 V negative with respect to the grounded ohmic contact at the back of the crystal. Typical PDS plots for the first two runs on each face are shown in fig. 6. Here log voltage V is plotted versus filter energy E,; each filter ideally
Ef (eV1
Fig. 6. Photodischarge spectroscopy of various faces of ZnO which had received preparation B. First and second runs are indicated by the dark and light symbols, respectively. From the knees of the curves, one can determine their surface state levels, according to the surface discharge model.
ELECTRONIC,LEED
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absorbs all light with hv 2 Ef. Points are taken at one-minute intervals. In the first half minute, the tungsten microscope light is on with one filter in place; in the second half minute, the light is off and V is determined by the Kelvin probe. The filter is then changed to the next higher Ef. The process is repeated at succeeding higher Ef. The first two points from the left in fig. 6, however, are taken in the dark, in order to show the extent of dark decay. For each crystal face, the curves through the dark and light symbols indicate the first and second runs on these samples, respectively. Each run constitutes a complete corona-charging-process and a complete, systematic discharge with light. Typically, V slowly decreases (due to dark decay) up to a certain filter energy Ef = El (the knee of the curve), after which V begins to rapidly decrease with succeeding Ef. Characteristically, the second run is slightly shifted to the right of the first run in fig. 6. This shift is seen best in fig. 7, where PDS data for 21 consecutive
l(j
1
I I
I
I 2
I
Ef (ev) Fig. 7. Photodischarge spectroscopy of ZnO (1010) as a function of the number of successive runs. After N 100 runs, the crystal becomes less sensitive to light and ‘stabilizes’.
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J. D. LEVINE
ET AL.
runs on ZnO (lOi0) is shown. Note that with successive runs, structure appears in the curves, possibly associated with microplanes formed from the charging process. Also, the sensitivity of the crystal to less-th~~-bandgaplight decreases. After a great many runs (> 100) further shift ceases and a “stabilization” is established as shown by the dashed curve in the figure. Strongly bound chlorine was detected on these surfaces, as judged from Auger spectroscopy. The large chlorine signal persisted even after annealing the crystal to 1200°C. The shift to the right, and the appearance of structure can be considerably speeded up, or simulated, by chemical etching in HCI. Even the (0001) face with preparation B can be made to charge after 5 set of chemical etching in SN HCl. Before “stabilization” the dependence of V on E, is sensitive to crystal face and number of runs; this may indicate a surface discharge mechanism. In that case, the knee of the curves in fig. 7 corresponds to the threshold energy levels E, for each face. These levels, measured below the conduction band edge, are given in table 1 for (0004) (1120), and (10iO). After “stabilization”, the monochromator described in Section II was used, and the voltage decay with exposure time in light (stray UV filtered out) followed the laws V = V, exp (- t/t), z -‘=oJ
7
(1) (2)
over one order in magnitude in V. Here I’,, is the voltage at t=O (typically 70 V), r -r is the rate constant, G is the cross section at energy hv, and J is the photon flux at the same hv. A plot of G versus hv is shown in fig. 8, for the (OOOi)and (1120) faces. Here g is given on a relative scale because of the change in optical alignment between the tests on the different faces. Note that the plot follows the relation ci = A exp (Bhv),
(3)
where, for both faces, B=3.L 40.2 eV_‘. For the monochromator with hv= 2.5 eV, z = 113 sec. For the more intense laser light of 2.54 eV, r = 2.7 sec. An estimate of the absolute value of LFwas made using this laser light and the stabilized (OOOi)face. The result was G= 3 _t 1 x IO-‘* cm2/photon, after the stray UV light from the laser was removed. The optical trans~ssion of a bulk ZnO crystal was carried out on another ZnO crystal. A plot of relative optical absorption AA (after a uniform background of absorption independent of hv was subtracted) versus hv gives the dashed line in fig. 8. It is seen that all lines in the figure are sufficientIy parallel to infer that one mechanism may be operable in all cases. This mechanism could be the bulk discharge mechanism, since it is insensitive to surface crystal face.
.J.D.LEVINE
Fig. 8.
ET AL.
Relative cross sections 1 for the photodischarge process after ‘stabilization’, a function of hv. The relative optical absorption BA is also shown.
159
as
4.3 PREPARATION A - CHEMICAL ETCHED The first PDS studies were made with the simplest preparation, preparation A. The PDS analysis for (0001) and (OOOi) assuming the surface discharge mechanismd) indicated two energy levels for each, as given in table 1. We have repeated the experiment and have also found two energy levels at about the same energies. These two energy levels are measured with respect to the conduction band edge of ZnO, whose bandgap is 3.2 eV. The existence of these two levels (or bands of relatively narrow width), and the visual observation of surface macroplanes suggested to us at an early stage that a “patch” effect might be present. That is, each microplane patch might give rise to a distinct energy level. For this reason little attention was subsequently paid to samples with preparation A. (It seemed unlikely that the internal structure of the PO: ion could generate the two levels, because red light discharged
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only the upper or “red” levels, while blue light discharged both the “red” and “blue” levels.) From the electrical measurements (see e.g. fig. 6) and the optical and scanning-electron microscope observations, it seems that the “patches” produced in preparation A were removed in preparation B.
5. Discussion The surface electrical properties of single crystal ZnO, together with SEM, LEED and Auger diagnostics, have been carried out for the crystal faces (OOOl), (OOOi), (1120) and (lOiO), for three different surface preparations, A, B, C, and as a function of the number of runs made on each. For those crystais which did charge in the presence of the corona, see table 1, the following atomistic model is offered. We use the fact that these crystal surfaces are known to have chemisorbed (or otherwise strongly bound) chlorine, as judged by Auger spectroscopy. Suppose that the chlorine (or chlorine complex} acts as the surface acceptor center at energy level Ei below the conduction band. Then a negatively charged ion from the corona, such as CO; l9), can drift to the surface of the crysta1, and deposit its charge into the supposedly-more-electronegative surface-chlorine-complex. Subsequently, CO; becomes a neutral molecule and it can be readily desorbed according to the reaction: 2C03 (s)-+2CO, (g)+O, (g). In this manner, the electron transfer reaction continues and the surface voltage increases in the Schottky barrier as the square of the charge deposited. The terminal charge and voltage is reached, most probably, when the internal breakdown field, - 2 x 10’ V/cm, of the Schottky barrier in ZnO is attained. Thereafter, the charge from the corona ion is deposited directly in the ZnO conduction band and is collected at the ohmic contact at the back of the crystal. Four corollaries
of the above atomistic
model will now be described,
and
some evidences presented for their validity. (1) If chemisorbed chlorine (or same similar adsorbate) were absent, the crystal would not charge. This corollary is substantiated by LEED and Auger spectroscopy data taken on crystals with preparation C. The surface was atomically regular, chemisorbed chlorine was not present, and the surfaces did not charge in the corona. (2) Physical defects, in the absense of chemical acceptorssuch as chemisorbed chlorine, are not suficient for holding a charge. TO introduce such physical defects on the surface in the absence of chlorine, we have used ion bombardment in the LEED system, but with no anneal. There is a poor LEED pattern and the surface is thought to be irregular on an atomic scale. An ohmic
ELECTRONIC,
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161
contact was made to the previously diffused indium contact by applying a drop of gallium, at room temperature. It was found that this surface with certain physical defects did not charge. Subsequently, the surface could be made to charge by mild etching in HCl. (3) Etch solutions might be found where the etch anion is much less electronegative than chlorine; surfaces etched this way might not charge in a corona. Such an etch was found. It is organic in nature and its properties will be described in a future publication. (4) If the chlorine surface acceptors were locatedatpreferential atomic sites, their p-like wave function orientations might be detected using PDS with polarized light, normally incident on the crystal. This was found to be the case on (1120) using preparation B with < 100 runs. The rate constant r- 1 of PDS was a maximum for an angle 0 = 60’ f 10” of the photon E-vector with respect to the c-axis. Also 7-l was a minimum for 8 = 30”, with an appreciable ratio of 3 between the maximum and minimum of 2- ‘. This can be explained using the concepts of surface atomic isomers which are identical in the unit cell of the ideally terminated crystal, but which are differentiated in a reconstructed crystal surface. It is known from LEED that ZnO (1120) has such a reconstruction, and with only one isomer active, the predicted angle would be half the tetrahedral angle or 54.7”. This angle lies within the error limits of the angle observed. The atomistic model and the four corollaries described above suggest that a ZnO surface free of chemical defects (such as chemisorbed chlorine) is electrically inert with respect to the charging action of a corona. This observation is consistent with the so-called “active site” theory of catalysis, wherein only a few isolated chemical and/or physical defects on the surface are presumed to be responsible for the catalytic process. In our case, it is not known if the electrically-active chlorine resides at a physical defect or not. From the polarization studies in (4) above, there is a tentative indication that the electrically-active chlorine has a specific orientation with respect to the c-axis of the wurtzite crystal. Consider an alternate explanation for the observed lack of charging ability in some of the crystals. Suppose that the processing of certain ZnO surfaces is such as to form nearly degenerate surface layers. Then this makes it difficult to attain large voltages with the corona. As a numerical example, suppose that the Schottky barrier law applies, and the breakdown field is independent of doping ND (cme3) or initial voltage V,. Then the product V,N, is a constant. Typically Tr,=70 V, Nn=2 x 1O1’ cmd3, and our resolution is
162
donors
J.D.LEVINE
ET AL.
given by 23) ND = 1.1 x 10” exp (-0.52/kT).
(4)
This formula applies over the temperature range 800 “C < T-c1350 “C. Using the highest temperature of our processing, T- 1200°C for the (OOOl), (OOOi), and (lOi0) faces as used in preparation C, one gets from (4) that No = 2 x IO'8 cmF3, about an order of magnitude too small to account for a resolution of < 1 V. Furthermore, one might expect this value of 2 x 101* to be an upper limit, as our crystals with preparation C are heated in ultrahigh vacuum, not in saturated zinc vapor. It is also significant that the maximum processing temperature is only 400 “C in the case of ZnO (1120) and yet this face exhibits the same lack of charging ability. Also the (0001) crystals that were chemically polished, but not heated at all, refused to charge. The probability thus seems remote that surface layers of ND> 1019 cme3 exist for each of these greatly different temperature conditions and processes. A nearly parallel study was made of the PDS observations on CdS 24). The faces tested were (OOOl), (OOOi), and (1120) prepared by chemical etching, similar to preparation B. Insulating and semiconducting crystals of CdS were charged to-2000 V and - 100 V, respectively, in a negative corona of 5000 V. They all discharged rapidly with less-than-bandgap light. A variation in crystal face and even a variation in ambients did not produce any noticeable variation in the PDS. This pronounced insensitivity of PDS to the surface region is consistent with an assumed bulk discharge mechanism similar to that shown in fig. 5b. The CdS crystals were estimated to be lo-100 times more sensitive than the ZnO crystals to less-than-bandgap light. This difference between CdS and ZnO can be explained if one presumes a relative abundance and scarcity, respectively, of bulk traps which are responsible for the bulk discharge mechanism. Acknowledgements The authors are indebted to A. J. Tocci for chemically polishing our ZnO crystals, and to R. Williams, A. M. Goodman, T. Freund, H. Kiess, D. Redfield, F. H. Nicoll, and D. Meyerhofer, for offering many helpful suggestions. Note added in proof Baidyaroy, Bottoms and Mark 24) have carried out a complementary study of the chemisorption of O2 by CdS. They found that highly regular surfaces of CdS did not adsorb significant amounts of 02, as judged from conductivity measurements. This observation supports the finding in this paper that active sites are necessary to make certain semiconductors electronically active.
ZnO
ELECTRONIC,LEEDANDAUGERDIAGN~~TICS~N
163
SURFACES
Appendix A VACUUM SURFACE PREPARATION
PROCEDURES FOR THE
(0001) AND (0001) ZnO
SURFACES
(1) (2) (3) (4) (5) (6) (7) (8) (9) *(lo) (11) ** (12) (13) (14)
System evacuated to 1 x 10d9 torr without bakeout Overnight bakeout at 25O”C, pressure 2 x lo-” torr after cooling 300°C anneal for 10 min 400°C anneal for 10 min 500°C anneal for 10 min 600°C anneal for 10 min 700°C anneal for 10 min 800°C anneal for 10 min 900°C anneal for 10 min Argon ion bombardment 5 PA; 225 V for 30 min 1140°C anneal for 10 min Argon ion bombardment 5.6 pA ; 225 V for 30 min 1140°C anneal for 10 min 1300 “C anneal for 5 min Appendix B
VACUUM
(1) (2) (3) (4) (5) (6) (7)
SURFACE PREPARATION
PROCEDURES FOR THE (10x0)
ZtlO
SURFACE
System evacuated to 1 x 10e9 torr without bakeout Overnight bakeout at 250°C pressure 2 x IO-” torr after cooling Argon ion bombardment 5 PA; 225 V for 45 min 300°C anneal for 2+ hr 650°C anneal for 35 min 800°C anneal for 35 min 1000°C anneal for 10 min Appendix C
VACUUM
SURFACE PREPARATION
(1)
System
(2)
2OO“C anneal
for 2 hr
(3)
35O’C
anneal
for
1 hr
(4)
400°C
anneal
for
1
(5)
evacuated
PROCEDURES FOR THE (1 120)
to 2 x 10m9 without
hr ; maximum
by indium
contacts
on this sample
Overnight
bakeout
at 25O”C,
* Only for the (0001) surface. ** Only for the (OOOT)surface.
bakeout
annealing
pressure
ZnO SURFACE
temperatures
2 x 10-l’
torr after
were cooling
limited
164
(6) (7) (8) (9)
J. D. LEVINE
Argon 400°C Argon 400°C
ET AL.
ion bombardment 7 pA; 225 V for 30 min anneal for 10 min ion bombardment 5 pA; 225 V for 3 hr anneal for 10 min References
1) S. G. Davison and J. D. Levine, Surface States, in: Solid State Physics, Vol. 25, Eds. F. Seitz and D. Turnbull (Academic Press, New York, 1970) pp. 1-149. 2) P. Mark, Methods of Determining Surface State Energies, in: Modern Methods of Surface Analysis, Eds. P. Mark and J. D. Levine (North-Holland, Amsterdam, 1971) pp. 192-223. Published in Surface Sci. 25 (1971). 3) M. Henzler, Surface Sci. 25 (1971) 650. 4) R. Williams and A. Willis, J. Appl. Phys. 39 (1968) 3731. 5) H. Kiess, Applied Optics, Suppl. 3 (Electrophotography) (1969) 100. 6) H. Kiess, J. Appl. Phys. 40 (1969) 4054. 7) R. Nosker, P. Mark and J. D. Levine, Surface Sci. 19 (1970) 291. 8) R. K. Swank, Phys. Rev. 153 (1967) 844. 9) J. D. Levine, J. Vacuum Sci. Technol. 6 (1969) 549. 10) R. M. Schaffert, EIectrophotogruphy (The Focal Press, New York, 1965). 11) G. Heiland and P. Kunstmann, Surface Sci. 13 (1969) 72; G. Heiland, J. Phys. Chem. Solids 22 (1961) 227. 12) S. R. Morrison, Surface Sci. 13 (1969) 85. 13) M. Aven and J. S. Prener, Eds., Physics and Chemistry of II-VI Compounds (NorthHolland, Amsterdam, 1964). 14) We have been formally advised that the 3M Company has since discontinued their line of ZnO single crystals. 15) See e.g. W. H. Strehlow, J. Appl. Phys. 40 (1969) 2928. 16) M. F. Chung and H. E. Farnsworth, Surface Sci. 22 (1970) 93. 17) H. C. Gatos, J. Appl. Phys. 32 (1961) 1232: R. Seiwatz, Surface Sci. 2 (1964) 473; S. G. Davison and J. D. Levine, Ref. 1, p. 81 ff. 18) J. D. Levine and P. Mark, Phys. Rev. 144 (1966) 751. 19) M. M. Shahin, Applied Optics Suppl. 3 (Electrophotography) (1969) 106. 20) D. M. Eagles, J. Phys. Chem. Solids 16 (1960) 76. 21) H. B. Bebb, Phys. Rev. 185 (1969) 1116. 22) Actually there was a small residual voltage of -2 V of the opposite sign. This did not vary with light. Most probably it is caused either by a difference in work functions or a small amount of negative charge fixed in the wax used for mounting the crystal to the pedestal. 23) F. A. Kriiger, The Chemistry of Imperfect Crystals (North-Holland, Amsterdam, 1964) p. 697. 24) S. Baidyaroy, W. R. Bottoms and P. Mark, Surface Sci. 29 (1972) 165.