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Oxide-layer formation on the ZnSe (110) surface studied by LEED intensity measurements
Surface Science 105 (1981) 475-488 North-Ho~and ~b~sh~g Company
OXIDE-LAYER FORMATION ON THE ZnSe (110) SURFACE STUDIED BY LEED INTENSITY MEASUREMENTS * T. TAKAHASHI, H. TAKIGUCHI and A. EBINA Research Institute of Electrical Communication,
Tohoku University, Sendai 980. Japan
Received 2 September 1980; accepted for pubIication 1 November 1980
Oxidation of the vacuum cleaved ZnSe (110) surface by electron-beam irradiation during the course of LEED measurements leads to a formation of buiklike ZnO layers. The LEED pattern and the I-V profile from the oxide overlayer confirm the presence of the ZnO (OOOi) layer __which is inclined at an angle of 30’ from the (110) substrate and has a near orientation of the (11 I) plane of the substrate crystal.
1. Introduction
We have been investigating oxidation properties of the II-VI compounds of ZnSe, ZnTe, CdSe, and CdTe with the method of electron spectroscopy. In our early work eIectron~ner~-loss spectroscopy (ELS) together with Auger-electron spectroscopy (AES) were used [l-4], and recently photoemission spectroscopy was included [5,6]. The studies showed that the oxygen uptake on the ordered surface is very slow when the surface is exposed to molecular oxygen; the initial sticking coefficient is of the order of 10~“‘-10~14, and the oxygen uptake tends to saturate at a coverage less than or equal to about one monolayer. We also found that electron-beam irradiation during the course of ELS or AES measurements enhanced the oxygen uptake greatly and true buiklike oxides were prepared by the beam-irradiation technique. The formation of oxide overlayers on semiconductor surfaces is of technical interest in device applications. In general, disordered (or amorphous) oxide layers are formed, being judged from disappearances of the LEED pattern after exposing the surface at high doses. A LEED intensity analysis for a cleaved GaAs (110) surface reported by Kahn et al. shows the presence of a disordered oxide overlayer on the ordered substrate [7]. Our previous studies with ELS and AES revealed the formation of a zinc oxide polar layer probably, (OOOi), on the oxidation of ZnSe and ZnTe surfaces by the electron-beam-irradiation tech-
* Work supported by Grants in Aid for Scientific Research from the Ministry of Education, Science and Culture of Japan, and by the Toray Science Foundation of Japan.
0039-6028/8 l/0000-0000/$02.50
0 Noah-HoIland Publishing Company
476
T. Ta~a~as~i et al. j Oxide-layer formation on .&Se [I 101
nique. In this work, a LEED technique was employed in order to monitor changes in surface structure with oxygen uptakes and to identify the formation of oxide overlayers. We will report on measurements of the LEED pattern and the diffraction intensity as a function of primary electron energies (the 1-V profile) from the cleaved ZnSe (110) substrate. During the course of the LEED’ measurements, electronbeam irradiation caused oxidation of the ZnSe surface and bulklike oxide overlayers were prepared. When the diffraction spots from the (110) substrate disappeared, broad spots appeared newly and they grew in intensity with further beam irradiation, indicating a formation of somewhat ordered oxide layers. To the author’s knowledge, the observation of the ordered oxide has not been reported on the II-VI compound surfaces as well as III-V compound surfaces.
2. Experimental The LEED experiments were performed on the cleaved ZnSe (110) surface with a stainless steel chamber equipped with a 4-grid LEED optics and a crystal cleaving mechanism. The LEED optics was also used as a monitor of Auger spectrum. The base pressure was 2 X 10 -lo Torr and the working p ressure was 6 X lo-” Ton. The diffraction intensity of a spot was recorded by a TV camera display~g the diffraction pattern from the LEED fluorescent screen in combination with electric circuits consisting of a beam-spot selecting circuit, a signal averaging circuit, and a background-intensity subtracting circuit. By adding a spot tracing circuit which traces the moving spot with changes in primary electron energy, E,, and a constantprimary-electron-beam-current circuit, we were able to record an 1-V profile in the range of E, from 30 to 200 eV within 2 min. The LEED measurement was performed at the primary electron beam current, 1n of 0.4-1.0 PA. The 1-V profile was mostly taken at the possible low I, of 0.4 PA, which corresponds to the current density of 0.5 pA/cm’, in order to minimize electron-beam-irradiation effects such as damages and conta~ations with oxygen. In the present work, we address ourselves to the measurement of the I-V profrie rather than to the observation of the LEED pattern. Three pairs of Helmholtz coils were set mutually pe~endicular to let the diffraction space free of magnetic field. Some details in experimental techniques are described in separate papers [ 81. Single crystals of ZnSe grown by a vapor-phase method were employed. The crystal was treated in molten Zn to make it conductive (-10 a cm). The (110) surface was prepared by cleaving the single crystal free of twin boundaries and in situ the LEED measurement was performed on the several cleaved surfaces at primaryelectron-beam-normal and off-normal incidence on the sample surface. In this work, we focus our attention on the normal-incidence I- V profile taken continuously on the cleaved surface as a function of beam irradiation times during the course of the LEED mea~rement. The procedure of the measurement is as folIows. After
T. Takahashi et al. / Oxide-layer formation on ZnSe (I 10)
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cleaving the crystal, the sample surface is set in correct geometry so that the normalincidence LEED pattern having a correct symmetry can be found. Once this setting is achieved, then the I-V profile is continuously recorded, apart from a time required to take photographs of the LEED pattern, for more than about 20 h after cleaving till the bulklike oxide overlayer was formed. Besides the vapor-phase grown crystals, melt grown ones were examined also. They tend to crystallize in twinned structure although sometimes twin free single-crystal brocks were obtained. In the case of the twinned crystal, a LEED pattern consists of two sets of 1 X 1 beams from the twinned domains. However, no difference in Z-G’ profile was detected between the twinned and twin-free melt grown crystals. Also it should be noted that the I-V profde from the melt grown crystal agrees with that from the vaporphase crystal, indicating that there is no difference in surface structure between the crystals grown by two different methods.
3. Results The 1-V profiles were measured at normal incidence for the (Ol), (Oi), (10) = (TO), (11) = (II), and (li) = (ii) beams as a function of beam-irradiation times after the start of the LEED measurement. The intensity of the (hk) and (hk) beams
INCIDENT BEAM
0 iii)
0
0
(101
(ll!
Fig. 1. Schematic of the arrangements of the surface Zn and Se atoms and the LEED normal --incidence spot pattern for the (110) surface of ZnSe, and the orientation of the (110) and (111) planes. The orientation of the incident electron beam is defined by the angles of 0 and q5.
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T. ~~kffh~sh~ et al. / Oxide-layer formation on .&Se (i IQ)
v
I
I
I
/
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---I
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&Se
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IliI-BEAM
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I
i I
1
I,
200
ENERGY ieVi Fig. 2. I-V profiles of (a) the (01) beam, fb) the (IO) beam, and (c) the (li) ZnSe (110) surface as a function of electron-beam-irradiation times.
beam from the
T. Takahashiet al. / Oxide-layerformation on ZnSe (110)
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A ,O
iii)
c
Ax
B -
100
I
200 TIME
I
300 (MN)
Fig. 3. Changes in diffraction intensity of representative peaks appearing in the I-V profiles of the (Ol), (lo), and (17) beams from the ZnSe (110) surface as a function of electron-beamirradiation times.
agreed within 5%. The arrangements of the diffraction spots and the Zn and Se atoms on the (110) surface are shown in fig. 1. The orientation of the incident beam defined by the angle 8 and $I and that of the (110) and (iii) planes are included also in the figure. Fig. 2 represents the Z-V profiles for the (01) (lo), and (li) beams as a function of beam-irradiation times. The changes in intensity are summarized in fig. 3 for representative peaks .appearing in the Z-V profile. The diffraction spots reduced in magnitude with beam-irradiation times accompanied by increases in background intensities, but no new peaks appeared in the I- V profile. It is noticed that in spite of the decrease in diffraction intensity the diffraction spots are still very sharp. After beam irradiation for about 500 min, some diffraction spots appearing at Ep = 100 ev lost their intensity significantly, and at this stages of the oxidation very broad spots started to appear. The intensity of these new spots increased gradually with subsequent beam irradiation upto 1.5 h and the spots from the ZnSe substrate diminished totally. After that, the diffraction pattern remained unchanged for further beam irradiation. We recognized six sets of facet beams. The most intense one came from the facet inclined at 30°C with respect to the [liO] crystal axis on the (110) surface; the normal incidence of the primary
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T. Taka~ash~ et al. / ~~i~e-laye~ for~a~orl
on .&Se (I i0)
Fig. 4. Normal-incidence LEED. patterns (a-e) from the Z&e (1 IO) substrates and (0 from the oxide facet inclined at 30’ with respect to the substrate, after the start of the LEED measurement of (a) 410, (b) 570, (c) 730, (d) 742, and (e-f) 1960 min;E, = 62 eV for a-d, 61 eV for e, 67 eV for f.
T. Takahashiet al. / Oxide-layerformation on Z&e fll@
L-+-----1-----‘-~ 100
481
150
ENERGY (eV)
ib) &Se (110) HEAVILY OXIDIZED1 A 6=27" i-90" I
ENERGY (eV1 Fig, 5. I-V prodes for (a) the (10) beam and (b) the (00) beam from the oxide facet formed on the ZnSe (110) substrate. For comparison, the I- V profdes from the ZnO {OOOl} surfaces (from ref. [ 10 J) are included also.
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T. Takahashi et al. / Oxide-layer formation on ZnSe (1 IOj
electron beam to the facet is produced at 0 = 30” and ct,= 90” (see fig. 1). We could not identify the orientation of the other facets because of their weak diffraction intensities. Fig. 4 shows the normal-incidence LEED pattenrs from the ZnSe (110) surface (photographs a-e) and that from the facet inclined at @= 90” (photograph f). The LEED pattern shows that the 1 X 1 structure remains unchanged but the back~ound intensity increases with increases in beam-irradiation times and that the diffraction pattern from the facet taken at normal incidence is not of three-fold rotational symmetry but of six-fold symmetry (see photograph f), indicating a presence of steps. The size of the diffraction spot from the facet is about five times as large as that from the ZnSe substrate. The Auger spectrum from the oxide layers exhibited the Zn and 0 signals only. The nearest neighbor distance deduced from the diffraction pattern was about 2.03 A, which should be compared with a value of 1.973 A of ZnO [9]. The I- T/profile for the (10) beam taken at the normal incidence and the (00) beam taken at the polar angle 0’ = 7” are shown in fig. 5. For comparison, the I- I’ profrles from the ZnO (000 1) surfaces reported by Change and Mark [lo] are also included. The dotted line in the I- I’ profile for the (IO) beam is due to the presence of the other facets. The smaller magnitude of the peak at about 30 eV in the 1-I’ profile for the (00) beam is caused by the broadened spot size; the spot size is over the size of the window measuring the spot intensity. The intensity from the area over the window size causes the intensity appearing in the background-intensity curve. Thus, fig. 5 indicates the formation of the ZnO (OOOi)layer on the oxidation of the ZnSe (110) surface by electron-beam irradiation.
4. Discussion The 1-I profife from the surface free of the oxygen atoms agrees fairly well with a result reported by Duke et al. on an Ar’-bombarded-annealed surface [ 1I]. However, there are some discrepancies between the two sets of the spectra, as we can see in fig. 6. At present, it seems to be difficult to mention whether these discrepancies should be associated with a difference in surface structure or not. It is noticed that the energy-loss spectrum from the cleaved surface shows some differences in structure from that of an Ar’-bombarded-annealed surface. An Augerelectron-spectro~opy (AES) study reveals that although the beg-to-peg-heist ratio of the Zn signal at 55 eV to the Se signal at 42 eV is the same for the cleaved and Ar+-bombarded-annealed surfaces, the peak height itself of the Ar’-bombarded-annealed surface is about 75% of that of the cleaved surface, suggesting that the bombarded-annealed surface may be stoichiometric in concentration of Zn and Se when the cleaved surface is stoichiometric [12]. In the case of ZnTe, there is no essential difference in loss spectrum between the cleaved and bombarded-annealed surfaces [6]. The 1-V profile of the cleaved surface of ZnTe agrees fairly well with that reported by Duke et al. on a bombarded-annealed surface [ 131, as presented in fig. 7, where the 1-V profile from a cleaved ZnTe (110)
T. Takahashiet al. / Oxide-layerformation on ZnSe [I IO)
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ZnSe(ll0) NORMAL
INCIDENCE
-
VACUUM CLEAVED
---
P.MARK
et al.
0.
(oi) 0,
0,
0,
0.
I *
0
I.1
I
50
*
1
II
1
’
(
“1
ENERGY
* 150
100
1
*
”
* 200
’
*
1
0
(eV)
Fig. 6. Comparison in I-V profile from the vacuum cleaved (110) surface and the Ar+-bombarded-annealed (110) surface (from ref. [ 111) of ZnSe.
surface, which was taken by the same setup used in this work, is compared with the I- Y profile from a polished-bombarded-annealed ZnTe (110) surface reported in ref. 1131, for the (01, (Oil, (ll), (li), (O?), and (12) beams. Each profde in the corresponding set has been normalized at its m~um intensity. The fairly good
484
T. T~kaha~i et al. / Oxide-layer for~a~~o~ on ZnSe (110) I
I
I
I
-
VACUUM
----
POLISHED,
I
CLEAVED FROM REF. 1:
Fig. 7. Comparison in I- Y profiie from the vacuum cleaved (110) surface and the Ar+-bombarded-annealed (110) surface (from ref. [ 131) of ZnTe.
agreement in feature appearing in I-V profile from the two different research groups should be emphasized. All major peaks are observed in both profiles and they coincide in position within 5 eV. The agreement between the two respective profiles is, in generally speaking, more good for ZnTe than for ZnSe, as a comparison of fig. 7 with fig. 6 indicates. These results lead us to conclude that in ZnTe there seems to be no essential difference in surface structure between the two surfaces prepared by the two different methods. No significant difference in I- ‘v pro-
T. Takahashiet al. / Oxide-layerformation
On .k%
(110)
485
file between
the cleaved and Ar’-bombarded-annealed surfaces has been reported by Mark et al. [ 141 and by Kahn et al. [7] on one of typical III-V compounds of GaAs. From their LEED 1-V analyses, they have concluded that there in no essential difference in surface structure of the ordered regions of the two surfaces prepared by the two different methods. A similar conclusion has been very recently drawn by Tsang et al. [1.5] for another III-V compound of InP. In addition to LEED I-lr profiles, they have examined Auger spectrum with cleaved and bombarded-annealed InP (110) surfaces. The LEED and AES data reveal that while Ar’-bombarded-annealed surfaces do not quite reach stoichiometry, the LEED data from the cleaved and In-rich surfaces are essentially similar, providing that the surface (reconstructed) structure of the ordered portions of the two surfaces are identical [ 151. All the LEED studies described above for ZnTe, GaAs, and InP show that the ordered surface prepared by the conveniently used technique consisting of Ar’ bombardment and annealing exhibits the same surface (reconstructed) structure of the surface prepared by cleaving. Thus, also in the case of ZnSe it is very likely that there is no significant difference in surface structure between the cleaved and bombarded-annealed surfaces. Nevertheless, it is hoped to perform the LEED measurement on the two surfaces prepared by the two different methods by using the same setup before the conclusion of the surface structure will be drawn. We point out that the loss spectrum may be more sensitive to the presence of surface defects and that a misalignment in orientation of the sample surface in the experiments for ZnSe, if it would occur, may also cause some changes in intensity of peaks in the I- I’ profile between the two sets of the LEED data. In the visual observation, it became to be rather difficult to recognize the diffraction spots, appearing in the region E, larger than about 40 eV, from the ZnSe (110) substrate due to increases in intensity of the background when the
Fig. 8. Normal-incidence LEED patterns from the ZnSe (110) surface after the start of the LEED measurement of (a) 733 and (b) 7.50 min, at Ep = 102 eV.
beam irradiation time reached to about 500 min. The spots, however, were still clearly seen in photographic observation at this time of beam irradiation, as photographs a and b in fig. 4 show. A dramatic increase in background intensity accompanied by the disappearance of the diffraction spots from the substrate occurred when the time of beam irradiation increased from -720 to 750 mm. We show in fig. 8 the normal-incidence LEED pattern taken at the times of 735 and 750 min at E, = 102 eV. The sharp diffraction spots appearing at 735 min are totally diminished at 750 min. A comparison of photograph c at 713 min with photograph d at 742 mm in fig. 4 also shows the rather dramatic changes in intensity on the diffraction spots and the background. Thus, the oxygen uptake seems to be very fast at this stage of oxidation, probably, at an oxygen coverage of about one monolayer. A dramatic increase in oxygen adsorption and a disappearance of the LEED pattern have been observed for a ZnTe (110) cleaved surface [6]. The diffraction spot is still sharp even when it loses intensity with the absorption of oxygen. The same behavior was observed for the cleaved ZnTe (110) surface [6]. This situation is similar to the case of the GaAs (1 IO) surface, which is interpreted by a model of a thin layer,highly disordered by the oxidation, on the ordered GaAs substrate [7]. On the basis of this model and together with the result from ELS [S], we will propose an oxidation model for the ZnSe (110) surface. At the initial stages of the oxygen absorption (the oxygen coverage less than about 0.5 of one monolayer), the oxygen atoms bind with the surface Se atoms randomly without breaking the back bonds, leading to the decrease in diffraction intensity. When the oxygen absorption proceeds further, the back bonds are broken and the oxide of SeOz may be formed, which sublimes away from the surface because of its high vapor pressure. Thus, the oxygen, in turn, may occupy the vacant Se sits, and the bulklike ZnO layer is formed. The decrease in diffraction intensity with beamirradiation times is the greatest for the peak at E, = 100 eV. It is not so significant for the low-energy peak (&, < 50 eV) at the early stages of beam irradiation (see fig. 3). This behavior is the reflection of the electron-mean-free path; in ZnSe the minimum in mean-free path seems to be at the kinetic energy of about 100 eV. It occurs at -60 eV for the GaAs (110) surface [ 17f. The formation of the ZnO (OOOi) Layer is consistent with the result deduced from ELS. The loss spectrum from the heavily oxidized ZnSe surfaces is similar in shape to that from the ZnO (OOOi) surface [l ,S]. However, the identification of the (OOOi) layer was still open, because of a somewhat similar shape in loss spectrum between the (OOOi) and (0001) surfaces. In this work, we have been able to confirm the presence of the (OOOi) layer and, in addition, to identify its orientation. Considering that the (iii) plane of the fee structure corresponds to the (OOOi) plane of the hcp structure, we expect that the formation of the (OOOi) plane inclined 6’= 35” with respect to the substrate (see fig. l), when the lattice distance would be the same for ZnSe and ZnO and the ZnO layer is formed by substitution of the 0 atoms for the Se atoms. The angle of 30” detected in this work may be related with the smaller lattice distance in ZnO than in ZnSe. At first sight
T. Takahashi et al. / Oxide-layer formation on ZnSe (110)
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the lattice mismatch as large as 20% between ZnSe and ZnO would not lead to a well ordered facet. We did confnm the presence of the facet with somewhat ordered area, although the spots from the facet were broadened. The broadened spot may be due to a small size of the facet as compared with the effective coherence diameter ]17]. To estimate the length of the ordered area, we consider a surface net of dimensions Nrar XNzaz, where aI and a2 are the unit vectors for the net. From an elementary consideration of the interference function of X-rays [ 181, we can say that for the (hk) beam the diffraction intensity I(& r)) is to be appreciable when e and n lie within the limits h f l/N, and k + l/N;, respectively. The size of the (10) beam provides l/N, * l/iv, = 0.19; the length of the ordered area is about 17 A. The 3% dilation in surface net deduced from the LEED pattern is within the experimental accuracy, but the lattice distance might be elongated in the facet. The dilation is favorable for the formation of the ordered facet. As we can see from fig. 5, the Z-V profile of the most well developed facet is in good agreement with the curve from the ZnO (OOOi) surface, when the energy scale of our data is shifted by about 6 eV to the lower energies. There seems to be no systematic shift in energy scale in our experimental system, as we can see from fig. 6, where I-V profiles from the two different experimental systems are presented. The shift in energy scale might be associated with a dilation in layer spacing of the (OOOi) layer on the ZnSe substrate. A change in inner potently V, from the ZnO bulk crystal affects on the energy scale. However, this effect seems to be very small, since the V, value of 10 eV for ZnO [9,17] is close to that of 8 eV for ZnSe
[111. Oxidation of the cleaved ZnTe surface by the similar technique used in this work provided no new diffraction spots. The ELS results [2,5,6] indicate the oxide overlayer consisting of the ZnO (OOOi)layers for the heavily oxidized surface. However, in this case, during the processes of oxidation, we detected a formation of Te02 in addition to ZnO [2,6]. Thus, subl~ation of Te02 and the lattice mismatch as large as 25% between ZnTe (ae = 6.089 .&) and ZnO prevent a formation of the ordered or well oriented ZnO (OOOi)layer on the ZnTe substrate.
References [I] [2] [3] [4] [S] [6]
A. Ebina and T. Takahashi, Phys: Rev. B16 (1977) 2676; Surface Sci. 74 (1978) 667. A. Ebina, K, Asauo and T. Takahasbi, Phys. Rev. 318 (1978) 4332,434l. A. Ebina, K. Asano and T. Takahashi, Surface Sci. 86 (1979) 803. A. Ebina, K. Asano and T. Takahashi, Phys. Rev. B22 (1980) 1980. A. Ebina, K. Asano, Y. Suda and T. Takahashi, J. Vacuum Sci. Technol. 17 (1980) 1074. K. Asano, Y. Suda, M. Komatsu, A. Ebiua and T. Tak~a~i, J. Phys. Sot. Japan 49, Suppl. A (1980) 1105. [7] A. Kahan, D. Kanani, P. Mark, P.W. Chye, C.Y. Su, 1. Lindau and W.E. Spicer, Surface Sci. 87 (1979) 325; A. Kahn, D. Kanani and P. Mark, Surface Sci. 94 (1980) 547.
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[ 81 H. Takiguchi, K. Uchibori, A. Ebina and T. Takahashi, Record of Electrical and Communication Engineering Conversation, Tohoku University 47 (1978) 111 (in Japanese); H. Takiguchi, Ph.D. Thesis, Tohoku University (1979) (in Japanese). [9] C.B. Duke, R.J. Meyer, A. Paton and P. Mark, Phys. Rev, B18 (1978) 4225. [lo] S.C. Chang and P. Mark, Surface Sci. 46 (1974) 293. [ 1 l] C.B. Duke, A.R. Lubinsky, M. Bonn, G. Cisneros and P. Mark, J. Vacuum Sci. Technol. 14 (1977) 294; P. Mark, G. Cisneros, M. Bonn, A. Kahn, C.B. Duke, A Paton and A.R. Lubinsky, J. Vacuum Sci. Technol. 14 (1977) 910. [ 121 A. Ebina, J. Surface Sci. Sot. Japan 1 (1980) 19 (in Japanese). [ 131 C.B. Duke, R.J. Meyer, A. Taton, P. Mark, E. So and J.-L. Yeh, J. Vacuum Sci. Technol. 16 (1979) 647. R.J. Meyer, C.B. Duke, A. Paton, E. So, J.-L. Yeh, A. Kahn and P. Mark, Phys. Rev. B, to be published. [ 141 P. Mark, P. Pianetta, I. Lindau and W.E. Spicer, Surface Sci. 69 (1977) 735. [ 1.51J.C. Tsang, A. Kahn and P. Mark, Surface Sci. 97 (1980) 119. f 161 P. Pianetta, I. Lindau,C.M. Garner and W.E. Spicer, Phys. Rev. B18 (1978) 2792. f 171 P.L. Estrup and E.G. McRae, Surface Sci. 25 (1971) 1. (181 R.W. James, The Optical Principles of the Diffraction of X-Rays (Cornell Univ. Press, New York, 1965) p. 8. [19] C.B. Duke and A.R. Lubinsky, Surface Sci. 50 (1975) 605.
OXIDE-LAYER FORMATION ON THE ZnSe (110) SURFACE STUDIED BY LEED INTENSITY MEASUREMENTS * T. TAKAHASHI, H. TAKIGUCHI and A. EBINA Research Institute of Electrical Communication,
Tohoku University, Sendai 980. Japan
Received 2 September 1980; accepted for pubIication 1 November 1980
Oxidation of the vacuum cleaved ZnSe (110) surface by electron-beam irradiation during the course of LEED measurements leads to a formation of buiklike ZnO layers. The LEED pattern and the I-V profile from the oxide overlayer confirm the presence of the ZnO (OOOi) layer __which is inclined at an angle of 30’ from the (110) substrate and has a near orientation of the (11 I) plane of the substrate crystal.
1. Introduction
We have been investigating oxidation properties of the II-VI compounds of ZnSe, ZnTe, CdSe, and CdTe with the method of electron spectroscopy. In our early work eIectron~ner~-loss spectroscopy (ELS) together with Auger-electron spectroscopy (AES) were used [l-4], and recently photoemission spectroscopy was included [5,6]. The studies showed that the oxygen uptake on the ordered surface is very slow when the surface is exposed to molecular oxygen; the initial sticking coefficient is of the order of 10~“‘-10~14, and the oxygen uptake tends to saturate at a coverage less than or equal to about one monolayer. We also found that electron-beam irradiation during the course of ELS or AES measurements enhanced the oxygen uptake greatly and true buiklike oxides were prepared by the beam-irradiation technique. The formation of oxide overlayers on semiconductor surfaces is of technical interest in device applications. In general, disordered (or amorphous) oxide layers are formed, being judged from disappearances of the LEED pattern after exposing the surface at high doses. A LEED intensity analysis for a cleaved GaAs (110) surface reported by Kahn et al. shows the presence of a disordered oxide overlayer on the ordered substrate [7]. Our previous studies with ELS and AES revealed the formation of a zinc oxide polar layer probably, (OOOi), on the oxidation of ZnSe and ZnTe surfaces by the electron-beam-irradiation tech-
* Work supported by Grants in Aid for Scientific Research from the Ministry of Education, Science and Culture of Japan, and by the Toray Science Foundation of Japan.
0039-6028/8 l/0000-0000/$02.50
0 Noah-HoIland Publishing Company
476
T. Ta~a~as~i et al. j Oxide-layer formation on .&Se [I 101
nique. In this work, a LEED technique was employed in order to monitor changes in surface structure with oxygen uptakes and to identify the formation of oxide overlayers. We will report on measurements of the LEED pattern and the diffraction intensity as a function of primary electron energies (the 1-V profile) from the cleaved ZnSe (110) substrate. During the course of the LEED’ measurements, electronbeam irradiation caused oxidation of the ZnSe surface and bulklike oxide overlayers were prepared. When the diffraction spots from the (110) substrate disappeared, broad spots appeared newly and they grew in intensity with further beam irradiation, indicating a formation of somewhat ordered oxide layers. To the author’s knowledge, the observation of the ordered oxide has not been reported on the II-VI compound surfaces as well as III-V compound surfaces.
2. Experimental The LEED experiments were performed on the cleaved ZnSe (110) surface with a stainless steel chamber equipped with a 4-grid LEED optics and a crystal cleaving mechanism. The LEED optics was also used as a monitor of Auger spectrum. The base pressure was 2 X 10 -lo Torr and the working p ressure was 6 X lo-” Ton. The diffraction intensity of a spot was recorded by a TV camera display~g the diffraction pattern from the LEED fluorescent screen in combination with electric circuits consisting of a beam-spot selecting circuit, a signal averaging circuit, and a background-intensity subtracting circuit. By adding a spot tracing circuit which traces the moving spot with changes in primary electron energy, E,, and a constantprimary-electron-beam-current circuit, we were able to record an 1-V profile in the range of E, from 30 to 200 eV within 2 min. The LEED measurement was performed at the primary electron beam current, 1n of 0.4-1.0 PA. The 1-V profile was mostly taken at the possible low I, of 0.4 PA, which corresponds to the current density of 0.5 pA/cm’, in order to minimize electron-beam-irradiation effects such as damages and conta~ations with oxygen. In the present work, we address ourselves to the measurement of the I-V profrie rather than to the observation of the LEED pattern. Three pairs of Helmholtz coils were set mutually pe~endicular to let the diffraction space free of magnetic field. Some details in experimental techniques are described in separate papers [ 81. Single crystals of ZnSe grown by a vapor-phase method were employed. The crystal was treated in molten Zn to make it conductive (-10 a cm). The (110) surface was prepared by cleaving the single crystal free of twin boundaries and in situ the LEED measurement was performed on the several cleaved surfaces at primaryelectron-beam-normal and off-normal incidence on the sample surface. In this work, we focus our attention on the normal-incidence I- V profile taken continuously on the cleaved surface as a function of beam irradiation times during the course of the LEED mea~rement. The procedure of the measurement is as folIows. After
T. Takahashi et al. / Oxide-layer formation on ZnSe (I 10)
477
cleaving the crystal, the sample surface is set in correct geometry so that the normalincidence LEED pattern having a correct symmetry can be found. Once this setting is achieved, then the I-V profile is continuously recorded, apart from a time required to take photographs of the LEED pattern, for more than about 20 h after cleaving till the bulklike oxide overlayer was formed. Besides the vapor-phase grown crystals, melt grown ones were examined also. They tend to crystallize in twinned structure although sometimes twin free single-crystal brocks were obtained. In the case of the twinned crystal, a LEED pattern consists of two sets of 1 X 1 beams from the twinned domains. However, no difference in Z-G’ profile was detected between the twinned and twin-free melt grown crystals. Also it should be noted that the I-V profde from the melt grown crystal agrees with that from the vaporphase crystal, indicating that there is no difference in surface structure between the crystals grown by two different methods.
3. Results The 1-V profiles were measured at normal incidence for the (Ol), (Oi), (10) = (TO), (11) = (II), and (li) = (ii) beams as a function of beam-irradiation times after the start of the LEED measurement. The intensity of the (hk) and (hk) beams
INCIDENT BEAM
0 iii)
0
0
(101
(ll!
Fig. 1. Schematic of the arrangements of the surface Zn and Se atoms and the LEED normal --incidence spot pattern for the (110) surface of ZnSe, and the orientation of the (110) and (111) planes. The orientation of the incident electron beam is defined by the angles of 0 and q5.
478
T. ~~kffh~sh~ et al. / Oxide-layer formation on .&Se (i IQ)
v
I
I
I
/
-i---r-
---I
T--
znse (11Oi (lObBEAM
ENERGY ieV1
&Se
!
1 IO!
IliI-BEAM
~+-i’--_.L.-ii-i-
I
i I
1
I,
200
ENERGY ieVi Fig. 2. I-V profiles of (a) the (01) beam, fb) the (IO) beam, and (c) the (li) ZnSe (110) surface as a function of electron-beam-irradiation times.
beam from the
T. Takahashiet al. / Oxide-layerformation on ZnSe (110)
479
A ,O
iii)
c
Ax
B -
100
I
200 TIME
I
300 (MN)
Fig. 3. Changes in diffraction intensity of representative peaks appearing in the I-V profiles of the (Ol), (lo), and (17) beams from the ZnSe (110) surface as a function of electron-beamirradiation times.
agreed within 5%. The arrangements of the diffraction spots and the Zn and Se atoms on the (110) surface are shown in fig. 1. The orientation of the incident beam defined by the angle 8 and $I and that of the (110) and (iii) planes are included also in the figure. Fig. 2 represents the Z-V profiles for the (01) (lo), and (li) beams as a function of beam-irradiation times. The changes in intensity are summarized in fig. 3 for representative peaks .appearing in the Z-V profile. The diffraction spots reduced in magnitude with beam-irradiation times accompanied by increases in background intensities, but no new peaks appeared in the I- V profile. It is noticed that in spite of the decrease in diffraction intensity the diffraction spots are still very sharp. After beam irradiation for about 500 min, some diffraction spots appearing at Ep = 100 ev lost their intensity significantly, and at this stages of the oxidation very broad spots started to appear. The intensity of these new spots increased gradually with subsequent beam irradiation upto 1.5 h and the spots from the ZnSe substrate diminished totally. After that, the diffraction pattern remained unchanged for further beam irradiation. We recognized six sets of facet beams. The most intense one came from the facet inclined at 30°C with respect to the [liO] crystal axis on the (110) surface; the normal incidence of the primary
480
T. Taka~ash~ et al. / ~~i~e-laye~ for~a~orl
on .&Se (I i0)
Fig. 4. Normal-incidence LEED. patterns (a-e) from the Z&e (1 IO) substrates and (0 from the oxide facet inclined at 30’ with respect to the substrate, after the start of the LEED measurement of (a) 410, (b) 570, (c) 730, (d) 742, and (e-f) 1960 min;E, = 62 eV for a-d, 61 eV for e, 67 eV for f.
T. Takahashiet al. / Oxide-layerformation on Z&e fll@
L-+-----1-----‘-~ 100
481
150
ENERGY (eV)
ib) &Se (110) HEAVILY OXIDIZED1 A 6=27" i-90" I
ENERGY (eV1 Fig, 5. I-V prodes for (a) the (10) beam and (b) the (00) beam from the oxide facet formed on the ZnSe (110) substrate. For comparison, the I- V profdes from the ZnO {OOOl} surfaces (from ref. [ 10 J) are included also.
482
T. Takahashi et al. / Oxide-layer formation on ZnSe (1 IOj
electron beam to the facet is produced at 0 = 30” and ct,= 90” (see fig. 1). We could not identify the orientation of the other facets because of their weak diffraction intensities. Fig. 4 shows the normal-incidence LEED pattenrs from the ZnSe (110) surface (photographs a-e) and that from the facet inclined at @= 90” (photograph f). The LEED pattern shows that the 1 X 1 structure remains unchanged but the back~ound intensity increases with increases in beam-irradiation times and that the diffraction pattern from the facet taken at normal incidence is not of three-fold rotational symmetry but of six-fold symmetry (see photograph f), indicating a presence of steps. The size of the diffraction spot from the facet is about five times as large as that from the ZnSe substrate. The Auger spectrum from the oxide layers exhibited the Zn and 0 signals only. The nearest neighbor distance deduced from the diffraction pattern was about 2.03 A, which should be compared with a value of 1.973 A of ZnO [9]. The I- T/profile for the (10) beam taken at the normal incidence and the (00) beam taken at the polar angle 0’ = 7” are shown in fig. 5. For comparison, the I- I’ profrles from the ZnO (000 1) surfaces reported by Change and Mark [lo] are also included. The dotted line in the I- I’ profile for the (IO) beam is due to the presence of the other facets. The smaller magnitude of the peak at about 30 eV in the 1-I’ profile for the (00) beam is caused by the broadened spot size; the spot size is over the size of the window measuring the spot intensity. The intensity from the area over the window size causes the intensity appearing in the background-intensity curve. Thus, fig. 5 indicates the formation of the ZnO (OOOi)layer on the oxidation of the ZnSe (110) surface by electron-beam irradiation.
4. Discussion The 1-I profife from the surface free of the oxygen atoms agrees fairly well with a result reported by Duke et al. on an Ar’-bombarded-annealed surface [ 1I]. However, there are some discrepancies between the two sets of the spectra, as we can see in fig. 6. At present, it seems to be difficult to mention whether these discrepancies should be associated with a difference in surface structure or not. It is noticed that the energy-loss spectrum from the cleaved surface shows some differences in structure from that of an Ar’-bombarded-annealed surface. An Augerelectron-spectro~opy (AES) study reveals that although the beg-to-peg-heist ratio of the Zn signal at 55 eV to the Se signal at 42 eV is the same for the cleaved and Ar+-bombarded-annealed surfaces, the peak height itself of the Ar’-bombarded-annealed surface is about 75% of that of the cleaved surface, suggesting that the bombarded-annealed surface may be stoichiometric in concentration of Zn and Se when the cleaved surface is stoichiometric [12]. In the case of ZnTe, there is no essential difference in loss spectrum between the cleaved and bombarded-annealed surfaces [6]. The 1-V profile of the cleaved surface of ZnTe agrees fairly well with that reported by Duke et al. on a bombarded-annealed surface [ 131, as presented in fig. 7, where the 1-V profile from a cleaved ZnTe (110)
T. Takahashiet al. / Oxide-layerformation on ZnSe [I IO)
483
ZnSe(ll0) NORMAL
INCIDENCE
-
VACUUM CLEAVED
---
P.MARK
et al.
0.
(oi) 0,
0,
0,
0.
I *
0
I.1
I
50
*
1
II
1
’
(
“1
ENERGY
* 150
100
1
*
”
* 200
’
*
1
0
(eV)
Fig. 6. Comparison in I-V profile from the vacuum cleaved (110) surface and the Ar+-bombarded-annealed (110) surface (from ref. [ 111) of ZnSe.
surface, which was taken by the same setup used in this work, is compared with the I- Y profile from a polished-bombarded-annealed ZnTe (110) surface reported in ref. 1131, for the (01, (Oil, (ll), (li), (O?), and (12) beams. Each profde in the corresponding set has been normalized at its m~um intensity. The fairly good
484
T. T~kaha~i et al. / Oxide-layer for~a~~o~ on ZnSe (110) I
I
I
I
-
VACUUM
----
POLISHED,
I
CLEAVED FROM REF. 1:
Fig. 7. Comparison in I- Y profiie from the vacuum cleaved (110) surface and the Ar+-bombarded-annealed (110) surface (from ref. [ 131) of ZnTe.
agreement in feature appearing in I-V profile from the two different research groups should be emphasized. All major peaks are observed in both profiles and they coincide in position within 5 eV. The agreement between the two respective profiles is, in generally speaking, more good for ZnTe than for ZnSe, as a comparison of fig. 7 with fig. 6 indicates. These results lead us to conclude that in ZnTe there seems to be no essential difference in surface structure between the two surfaces prepared by the two different methods. No significant difference in I- ‘v pro-
T. Takahashiet al. / Oxide-layerformation
On .k%
(110)
485
file between
the cleaved and Ar’-bombarded-annealed surfaces has been reported by Mark et al. [ 141 and by Kahn et al. [7] on one of typical III-V compounds of GaAs. From their LEED 1-V analyses, they have concluded that there in no essential difference in surface structure of the ordered regions of the two surfaces prepared by the two different methods. A similar conclusion has been very recently drawn by Tsang et al. [1.5] for another III-V compound of InP. In addition to LEED I-lr profiles, they have examined Auger spectrum with cleaved and bombarded-annealed InP (110) surfaces. The LEED and AES data reveal that while Ar’-bombarded-annealed surfaces do not quite reach stoichiometry, the LEED data from the cleaved and In-rich surfaces are essentially similar, providing that the surface (reconstructed) structure of the ordered portions of the two surfaces are identical [ 151. All the LEED studies described above for ZnTe, GaAs, and InP show that the ordered surface prepared by the conveniently used technique consisting of Ar’ bombardment and annealing exhibits the same surface (reconstructed) structure of the surface prepared by cleaving. Thus, also in the case of ZnSe it is very likely that there is no significant difference in surface structure between the cleaved and bombarded-annealed surfaces. Nevertheless, it is hoped to perform the LEED measurement on the two surfaces prepared by the two different methods by using the same setup before the conclusion of the surface structure will be drawn. We point out that the loss spectrum may be more sensitive to the presence of surface defects and that a misalignment in orientation of the sample surface in the experiments for ZnSe, if it would occur, may also cause some changes in intensity of peaks in the I- I’ profile between the two sets of the LEED data. In the visual observation, it became to be rather difficult to recognize the diffraction spots, appearing in the region E, larger than about 40 eV, from the ZnSe (110) substrate due to increases in intensity of the background when the
Fig. 8. Normal-incidence LEED patterns from the ZnSe (110) surface after the start of the LEED measurement of (a) 733 and (b) 7.50 min, at Ep = 102 eV.
beam irradiation time reached to about 500 min. The spots, however, were still clearly seen in photographic observation at this time of beam irradiation, as photographs a and b in fig. 4 show. A dramatic increase in background intensity accompanied by the disappearance of the diffraction spots from the substrate occurred when the time of beam irradiation increased from -720 to 750 mm. We show in fig. 8 the normal-incidence LEED pattern taken at the times of 735 and 750 min at E, = 102 eV. The sharp diffraction spots appearing at 735 min are totally diminished at 750 min. A comparison of photograph c at 713 min with photograph d at 742 mm in fig. 4 also shows the rather dramatic changes in intensity on the diffraction spots and the background. Thus, the oxygen uptake seems to be very fast at this stage of oxidation, probably, at an oxygen coverage of about one monolayer. A dramatic increase in oxygen adsorption and a disappearance of the LEED pattern have been observed for a ZnTe (110) cleaved surface [6]. The diffraction spot is still sharp even when it loses intensity with the absorption of oxygen. The same behavior was observed for the cleaved ZnTe (110) surface [6]. This situation is similar to the case of the GaAs (1 IO) surface, which is interpreted by a model of a thin layer,highly disordered by the oxidation, on the ordered GaAs substrate [7]. On the basis of this model and together with the result from ELS [S], we will propose an oxidation model for the ZnSe (110) surface. At the initial stages of the oxygen absorption (the oxygen coverage less than about 0.5 of one monolayer), the oxygen atoms bind with the surface Se atoms randomly without breaking the back bonds, leading to the decrease in diffraction intensity. When the oxygen absorption proceeds further, the back bonds are broken and the oxide of SeOz may be formed, which sublimes away from the surface because of its high vapor pressure. Thus, the oxygen, in turn, may occupy the vacant Se sits, and the bulklike ZnO layer is formed. The decrease in diffraction intensity with beamirradiation times is the greatest for the peak at E, = 100 eV. It is not so significant for the low-energy peak (&, < 50 eV) at the early stages of beam irradiation (see fig. 3). This behavior is the reflection of the electron-mean-free path; in ZnSe the minimum in mean-free path seems to be at the kinetic energy of about 100 eV. It occurs at -60 eV for the GaAs (110) surface [ 17f. The formation of the ZnO (OOOi) Layer is consistent with the result deduced from ELS. The loss spectrum from the heavily oxidized ZnSe surfaces is similar in shape to that from the ZnO (OOOi) surface [l ,S]. However, the identification of the (OOOi) layer was still open, because of a somewhat similar shape in loss spectrum between the (OOOi) and (0001) surfaces. In this work, we have been able to confirm the presence of the (OOOi) layer and, in addition, to identify its orientation. Considering that the (iii) plane of the fee structure corresponds to the (OOOi) plane of the hcp structure, we expect that the formation of the (OOOi) plane inclined 6’= 35” with respect to the substrate (see fig. l), when the lattice distance would be the same for ZnSe and ZnO and the ZnO layer is formed by substitution of the 0 atoms for the Se atoms. The angle of 30” detected in this work may be related with the smaller lattice distance in ZnO than in ZnSe. At first sight
T. Takahashi et al. / Oxide-layer formation on ZnSe (110)
487
the lattice mismatch as large as 20% between ZnSe and ZnO would not lead to a well ordered facet. We did confnm the presence of the facet with somewhat ordered area, although the spots from the facet were broadened. The broadened spot may be due to a small size of the facet as compared with the effective coherence diameter ]17]. To estimate the length of the ordered area, we consider a surface net of dimensions Nrar XNzaz, where aI and a2 are the unit vectors for the net. From an elementary consideration of the interference function of X-rays [ 181, we can say that for the (hk) beam the diffraction intensity I(& r)) is to be appreciable when e and n lie within the limits h f l/N, and k + l/N;, respectively. The size of the (10) beam provides l/N, * l/iv, = 0.19; the length of the ordered area is about 17 A. The 3% dilation in surface net deduced from the LEED pattern is within the experimental accuracy, but the lattice distance might be elongated in the facet. The dilation is favorable for the formation of the ordered facet. As we can see from fig. 5, the Z-V profile of the most well developed facet is in good agreement with the curve from the ZnO (OOOi) surface, when the energy scale of our data is shifted by about 6 eV to the lower energies. There seems to be no systematic shift in energy scale in our experimental system, as we can see from fig. 6, where I-V profiles from the two different experimental systems are presented. The shift in energy scale might be associated with a dilation in layer spacing of the (OOOi) layer on the ZnSe substrate. A change in inner potently V, from the ZnO bulk crystal affects on the energy scale. However, this effect seems to be very small, since the V, value of 10 eV for ZnO [9,17] is close to that of 8 eV for ZnSe
[111. Oxidation of the cleaved ZnTe surface by the similar technique used in this work provided no new diffraction spots. The ELS results [2,5,6] indicate the oxide overlayer consisting of the ZnO (OOOi)layers for the heavily oxidized surface. However, in this case, during the processes of oxidation, we detected a formation of Te02 in addition to ZnO [2,6]. Thus, subl~ation of Te02 and the lattice mismatch as large as 25% between ZnTe (ae = 6.089 .&) and ZnO prevent a formation of the ordered or well oriented ZnO (OOOi)layer on the ZnTe substrate.
References [I] [2] [3] [4] [S] [6]
A. Ebina and T. Takahashi, Phys: Rev. B16 (1977) 2676; Surface Sci. 74 (1978) 667. A. Ebina, K, Asauo and T. Takahasbi, Phys. Rev. 318 (1978) 4332,434l. A. Ebina, K. Asano and T. Takahashi, Surface Sci. 86 (1979) 803. A. Ebina, K. Asano and T. Takahashi, Phys. Rev. B22 (1980) 1980. A. Ebina, K. Asano, Y. Suda and T. Takahashi, J. Vacuum Sci. Technol. 17 (1980) 1074. K. Asano, Y. Suda, M. Komatsu, A. Ebiua and T. Tak~a~i, J. Phys. Sot. Japan 49, Suppl. A (1980) 1105. [7] A. Kahan, D. Kanani, P. Mark, P.W. Chye, C.Y. Su, 1. Lindau and W.E. Spicer, Surface Sci. 87 (1979) 325; A. Kahn, D. Kanani and P. Mark, Surface Sci. 94 (1980) 547.
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T. Takuhash~ et al. / Oxide-layer formation on ZnSe (110)
[ 81 H. Takiguchi, K. Uchibori, A. Ebina and T. Takahashi, Record of Electrical and Communication Engineering Conversation, Tohoku University 47 (1978) 111 (in Japanese); H. Takiguchi, Ph.D. Thesis, Tohoku University (1979) (in Japanese). [9] C.B. Duke, R.J. Meyer, A. Paton and P. Mark, Phys. Rev, B18 (1978) 4225. [lo] S.C. Chang and P. Mark, Surface Sci. 46 (1974) 293. [ 1 l] C.B. Duke, A.R. Lubinsky, M. Bonn, G. Cisneros and P. Mark, J. Vacuum Sci. Technol. 14 (1977) 294; P. Mark, G. Cisneros, M. Bonn, A. Kahn, C.B. Duke, A Paton and A.R. Lubinsky, J. Vacuum Sci. Technol. 14 (1977) 910. [ 121 A. Ebina, J. Surface Sci. Sot. Japan 1 (1980) 19 (in Japanese). [ 131 C.B. Duke, R.J. Meyer, A. Taton, P. Mark, E. So and J.-L. Yeh, J. Vacuum Sci. Technol. 16 (1979) 647. R.J. Meyer, C.B. Duke, A. Paton, E. So, J.-L. Yeh, A. Kahn and P. Mark, Phys. Rev. B, to be published. [ 141 P. Mark, P. Pianetta, I. Lindau and W.E. Spicer, Surface Sci. 69 (1977) 735. [ 1.51J.C. Tsang, A. Kahn and P. Mark, Surface Sci. 97 (1980) 119. f 161 P. Pianetta, I. Lindau,C.M. Garner and W.E. Spicer, Phys. Rev. B18 (1978) 2792. f 171 P.L. Estrup and E.G. McRae, Surface Sci. 25 (1971) 1. (181 R.W. James, The Optical Principles of the Diffraction of X-Rays (Cornell Univ. Press, New York, 1965) p. 8. [19] C.B. Duke and A.R. Lubinsky, Surface Sci. 50 (1975) 605.