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LEED study of the (0001) face of YMnO3 in order to epitaxy CdSe on this surface
SURFACE
SCIENCE
LEED
14 (1969) 121-140 o North-Holland
STUDY
IN ORDER
OF THE
TO
(0001) FACE
EPITAXY
D. ABERDAM,
Publishing Co., Amsterdam
OF YMnO,
CdSe ON THIS SURFACE
G. BOUCHET,
P. DUCROS
Laboratoire de Spectrome’trie Physique, Faculte’ des Sciences de Grenoble, Domaine Universitaire, 38 - Saint-Martin d’H&es, France
and J. DAVAL, Laboratoire d’Electronique Centre d’Etudes Nuclkaires
G. GRUNBERG et Technologie de i’lnformatique, de Grenoble 38 - Grenoble, France
Received 9 August 1968 Using the technique of low-energy electron diffraction the structure of the (0001) face of yttrium manganite has been studied with a view to subsequent epitaxying of the semiconducting material cadmium selenide upon this ferroelectric crystal. The heat treatments necessary for obtaining an ordered surface of YMnOs have been determined, and a possible explanation for the appearance of a 2 x 2 surface structure is given. As a result of this study the epitaxy of CdSe on YMnOs has been achieved in the best possible conditions. 1. Introduction
The recent development
of certain electronic
devices has brought
about the
need for combining a semi-conductor and a ferroelectric in a single electronic element l-5). The series of experiments reported here was undertaken in order to study, purely from a crystallographic point of view, the epitaxying of a deposit of cadmium selenide (a semiconductor) on the surface of a monocrystal of yttrium manganite, YMnO, (a ferroelectric). A thorough knowledge of the surface conditions of the YMnO, crystal chosen as the substrate was therefore desirable. The first part of this report is concerned with the low-energy electron diffraction (LEED) observations which were used to determine the various treatments needed to obtain an ordered surface structure and also the nature of the structures so obtained. The second part is concerned with theepitaxy experiments of cadmium selenide on the surface prepared using the information from the LEED experiments. Yttrium manganite crystallises in the hexagonal system, its space group being P6, cm (ref. 6). Its bulk structure (fig. 1) is characterised by the pres121
D.ABERDAM
122
ET AL.
ence of polyhedra with fivefold coordination around the manganese atoms, and sevenfold around the atoms of yttrium. The bulk lattice unit cell contains six units of the chemical formula, and its parameters are: a =6.125A
c=
11.41 A.
An important aspect of this substance is its ferroelectric nature. The polarisation appears in this structure parallel to the c-axis, which thus allows
l/2
lfi
0
Fig.
I.
Bulk
0
structure of YMn03.
for two possible orientations of the polarisation vector (domains at 180”). The crystals used for these studies were in the form of thin platelets or wafers grown by a flux process using two different procedures in which the flux agents were respectively : - an excess of manganese oxide (Mn,0,)7) - a bismuth oxide (Bi,O,)*) The surface studies were performed on crystals prepared by both processes.
LEED STUDY 0F THE(OOI)FACE
0F
YMn03
123
2. Experimental procedure The experiments were carried out using a modified Varian LEED apparatus in which the lowest attainable pressure is of the order of 2 x lOWi Torr. The specimen assembly was mounted on a Varian specimen holder which provides the basic movements necessary for low-energy electron diffraction studies. The crystal specimen was supported on a tantalum foil which had been moulded to the shape of the crystal. Electrical resistance heating of the foil enabled heat treatment of the specimen (fig. 2). This form for the crystal support was chosen because it allowed the crystal to be heated as homogeneously as possible, a condition which is particularly difficult to achieve in the case of an insulator. The homogeneity of the specimen temperature is important for two reasons : - firstly, it provides a well-defined surface temperature - secondly, it minimises internal strains in the crystal. A Pt-Pt/Rh thermocouple spot-welded to the back face of the foil provided an approximate measurement of the temperature, the measured temperature
Connectins collar.
Fig. 2.
Crystal holder and heater.
124
D.ABERDAM
being greater two tantalum
ET AL.
than the specim.en temperature. The foil was supported rods which were also used as the electrical connections
by for
heating the foil. The above system was able to provide a maximum specimen temperature of about 1400°C. In addition, the vacuum chamber pressure was monitored by an ionisation gauge, and a quadrupole mass spectrometer mounted in direct view of the specimen was used to analyse the residual gas content. 3. Experimental results 3.1.
SPECIMEN
PREPARED
FROM THE MANGANESE
OXIDE FLUX
A summary of the results obtained after different heat treatments is given in table I. The type of diffraction pattern obtained and the residual gas analysis is given for each treatment undergone by the crystal. The table indicates that two principal stages in the decontamination process of the crystal may be defined. The first stage, corresponding to heating to 910°C for about 20 min, gives rise to the appearance of the specularly reflected beam (fig. 3) and a structured background scattering at high incident electron energy. After this initial stage the crystal surface is still not completely clean, there being a physi- or chemisorbed layer whose thickness scarcely exceeds one or two monolayers. The second stage is reached by heating to 1200°C for 5 min. In this
Fig. 3.
Electron beam voltage: 590 V.
20 min
30 min
1 min 1 min 5 min
1 min
5 min
910°C
l@OO”C
1060°C 1100°C 1100°C
1200°C
1200°C
when cold high contrast with 2
area observed x
2 superstructure
(fig. 4)
- Pattern obtained at lower temperature than before - Voltage for the appearance of space charge depends on the
very slight improvement
space charge when crystal hot little improvement in diffraction pattern
* Appearance of (00) spot at fairly high temperature * Appearance of a pattern when the crystal is cold (fig. 3) - Structured background at 965 V
Space charge - no pattern
Diffraction pattern
Peaks are arranged in order of increasing intensity.
2h lh 1h 20 min
500°C 600°C 680°C 830°C
Heat treatment
TABLE 1
Specimen prepared from the manganese oxide flux
increase of peaks 44, 32 at start of heating increase of peak 28 at the end of heating
increase of peaks 28, 16, 20, 32, 44
peak 36 disappears
increase of peaks 16, 30, 32
increase of peaks 16, 44, 28, 18, 32, 36
increase of peaks 28, 44, 18 increase of peaks 16, 18, 20, 28, 44 increase of peak 18 no change of peaks 16, 18, 20
Residual gas analysis
126
D.ABERDAM
ET AL.
case an ordered surface structure is found, since a diffraction pattern with high contrast is observed (fig. 4) indicating a 2 x 2 superstructure. A possible explanation of this structure is given below. Two important conclusions may be drawn from the manner in which the heat treatments were carried out in order to decontaminate the surface.
Fig.
4.
Electron
beam
voltage:
153 V.
- The heat treatment giving rise to a clean surface seems to be only a function of the temperature rather than the product temperature-time, which shows that it is necessary to overcome a certain activation energy in order to obtain a clean surface. - The temperature required for decontamination is higher than the Curie temperature for the ferroelectric, and no evident relation seems to exist between these two temperatures. 3.2.
SPEClMEN
PREPARED
FROM
THE BISMUTH
OXIDE
FLUX
For this specimen no particularly well-defined treatment was found which produced the expected clean surface conditions. After heating the crystal to 700°C for 2 hours the diffraction pattern was found to contain the Kikuchi patterns above about 1500 V, but only an intense background was observed at Iower voltages. it should be remarked that the diffraction spots were only visibleathighenergiesandthat nothreshold value was found below which the crystal surface becomes negatively charged.
LEED STUDY OF THE(OOI)FACE
OF
YMn03
127
Heating successively to 1000°C and 1100°C produced a slightly improved surface. Even so, the diffraction patterns observed (fig. 5) indicated that there were still one or more impurity layers on the surface. It is possible that these impurity layers were conducting, giving rise to a surface conductivity which would explain the absence of space charge effects.
Fig. 5.
Electron beam voltage: 284 V.
This surface layer must, however, be quite stable since the same diffraction pattern was obtained after opening the apparatus to “reagent pure” nitrogen gas, once the system had been pumped-out and rebaked. A second series of treatments was tried in which the crystal was heated successively to 1200 “C, 125O”C, 13OO”C,and 1350 “C. After heating to 1200°C for 5 min a pattern was obtained which contained a large number of diffraction spots (fig. 6). Heating to the three higher temperatures did not bring about any important changes in the pattern, the only significant change being a resolution of the streaks in the form of a star which was observed about the main diffraction spots (fig. 7). It may be seen from table 2, that it is difficult to propose a well-defined treatment which will decontaminate the crystal surface. 3.3
DIFFRACTED INTENSITIES, KIKUCHI
BANDS
The following results were obtained using the specimen prepared from bismuth oxide.
128
D.ABERDAM
ET AL.
Fig. 6.
Electron beam voltage: 108 V.
Fig. 7.
Electron beam voltage:
I16 V.
The only observed spots are those due to epitaxied layer (fig. 5) High background No evident space charge
5 min
1100°C
Slight sharpening (00) (fig. 7)
3 min
1300°C
1350°C
Peaks are arranged in order of increasing intensity.
The streaks forming the star around the (00) spot are replaced by spots in hexagonal arrangement
2 min
1250°C
of spots forming hexagon around
Space charge * No change in the pattern is observed
5 min
1200°C
Pattern with numerous diffraction spots (fig. 6) Space charge
5 min
No change in the pattern is observed
No evident space charge Little improvement in diffraction pattern
6 min
1oOO”c
Chamber filled with nitrogen. Subsequent pump-down followed by bake-out 12 h at 250°C
High background Low contrast No evident space charge
2h
Diffraction pattern
700°C
Heat treatment
TABLE2 Specimen prepared from the bismuth oxide flux
T= 1180°C Peaks 44,28,2,18,16,30,17,40
T= 700°C Peaks 2,28,44,20,16,18 T= 950°C Peaks 2,44,28,18,16,20
T=250"C Peaks 44,28,20,16,40,18 T= 450°C Peaks 44,18,28,20,17, 16
Residual gas analysis
130
D.ABERDAM
3.3.1.
Curves qfI,,fV/
E‘I‘ AL.
and Z,,(T)
The variation of the intensity of the specularly reflected beam as a function of the accelerating voltage (fig. 8) shows a series of maxima for which the observed periodicity is twice that expected for the (00) reflection. This result can be explained; in effect, examination of the crystal structure shows that a trans-
I,
0
la7 Fig. 8.
(00)
200
300
Loo
5al
700
600
beam intensity versus electron beam voltage; incidence:
v
0 - 2.75”.
lation of -.$Cbrings into coincidence two planes with the same diffusion power. The agreement between the observed and calculated voltage values for these maxima seems to be dependent on the state of the crystal surface: however, the possibility of a defined inner potential was eliminated. For the maxima of the (00) spot at 104 V and 210 V the variation of the intensity lo0 was studied as a function of the temperature. The plot of log I versus the temperature was found to be linear, consistent with the expected exponential variation of the intensity with temperature. It was thus possible to define a Debye temperature (On) in terms of the usual expression 9) for the Debve-Wailer factor W I = I, exp(For the maximum
2W);
W = +sin’O* ikTz
at 210 V it was found
[*(O/T)
the 0,=756”K.
+
q(OjT)].
LEED
STUDY
OF
THE
(001)
FACE
OF
YMn03
131
3.3.2. Kikuchi lines and bands In the diffraction pattern shown in fig. 9 it was found that the bands with the greatest contrast were due to the (OliO) type planes, the diffraction cones generating these bands were therefore third-order. It is known that the bands corresponding to first-order cones should be observed preferentially to any higher order. However, on examining the crystal structure it was remarked that only very slight displacements involving principally the Yttrium atoms prevent the lattice from having a centrosymmetric unit cell, smaller than the real unit cell (fig. 10). In indexing the above Kikuchi bands on the basis of
Fig. 9.
Kikuchi bands. Electron beam voltage: 1700 V.
b
Fig. 10.
Relative orientation
and size of both meshes.
132
D.ABERDAM
ET AL.
the centro-symmetric cell it was found that they were due to first order reflections from planes of the type (2110). The other rectilinear bands are third-order reflections from (I OTO) type planes. Nevertheless, consideration of the structure leads to the conclusion that these bands are more intense. In fact the order of the (IOiO) planes is as shown in fig. Il. Using the kinematic approximation one may write: N
F=
C {FA[l +exp(4~i~d,~.cos8)] n=O
+ FB exp
871i ~IOTO .- mP.cOse a
3
47rni ~~ -*d,o~o~cose A
.
_!!d! a+ 0
a
-i
krlEI ;---I---
1 dloio
a__&--Fig. Il.
Arrangement
of atomic planes parallel to (lOi0).
It may thus be expected that the maxima of order 3 will be more intense. The indexing of the circles is much less certain. These are possibly due to (lOi2) planes (using centro-symmetric cell) the reflections being first-order. 4. Discussion Two possible explanations for the origin of the observed 2 x 2 structure may be proposed. The first is based on the ferroelectric character and the second on the chemical character of the specimen. 4.1. FERROELECTRIC ORIGIN It is known that the normal way a ferroelectric function is to take on a domain structure.
crystal minirnises
its Gibbs
LEED
STUDY
OF THE
(001)
FACE
OF
YMn03
133
Nevertheless, it may be asked if, close to the surface, the polarisation in a given domain would relax about an average value Pro). This would be in order to minimise the surface energy 1’). The amplitude of this relaxation should however decrease with increasing depth away from the surface, and finally become zero lz), whereas at the same time P would be varying, finally to take on the usual bulk value. As a first approach to this problem, we may consider a simplified model consisting of a linear chain of dipoles, along which the polarisation may be written : P, = P + 6P cos (Z~~/~),
(0
M being the relaxation period. The energy in each system (relaxed and nonrelaxed) is the sum of two terms, elastic and electrostatic. Electrostatic term : E,=
1
n,m
E:,,,,i--*
FlPf?l Ir,, J
(2)
’
Elastic term : Eq = &,
EP,” + PP,” + QP,”+ y (P” - P,+ $
;
(3)
with P, = P for the non-relaxed system, and P, given by eq. (1) for the relaxed system. The energy difference d U between the two systems may be written in the form: d U = (E, + E,) relaxed - (E, + E,) non-relaxed. (4) It is therefore necessary to find the values of M which will minimise this expression, The calculation shows that only three values of M make AU negative, the most favourable case being M=2. It may be concluded that if the elastic term is sufficiently small a relaxation may appear with M=2. In the opposite situation the system will keep a uniform polarisation. 4.2. CHEMICAL ORIGIN The possibility of a surface layer having a chemical composition different from the bulk of the crystal cannot be ruled out. First of all, one may consider the presence of impurities which, after diffusing from the bulk, form an ordered chemisorbed layer. Secondly, it is possible that close to the surface substitutions of various atoms take place during crystal formation. It seems that the most probable substitutions would be those involving the atoms of the rare earth element (yttrium) being substituted by atoms of manganese.
134
D.ABERDAM
5. Application:
ET AL.
epitaxy of cadmium selenide on YMnO,
5.1. INTRODUCTION As has been previously stated, it was considered of interest to take maximum advantage of the insulating, monocrystalline, and above all, ferroelectric, properties of yttrium manganite. The main objective is to combine this ferroelectric with a semi-conductor to produce electronic devices such as: A field-effect
transistor on a ferroelectric
13) (fig. 12)
In this device the different stable states of the polarisation of the Y MnO, crystal are able to modify the conduction paths in the semiconductor between the emitter and base, thus allowing the fabrication of an analog memory unit with a non-destructive read-out. Device of the type “Ferrotron”
(fig. 13)
In this device a semiconductor which is made locally conducting by a light beam produces a localised polarisation of the ferroelectric under an applied field, thus producing a genuine memory store. In the two above-mentioned devices, one has made use of the same phenomenon of conduction in a semiconductor. To achieve a high conductivity the carrier mobility must be high, consequently it is necessary to produce a semiconductor which is as perfect as possible; that is, monocrystalline and epitaxially grown on the ferroelectric. 5.2.
PREPARATION OF THE YMnO,
CRYSTAL
Before depositing the semiconductor, the surface of the YMnO, crystal must be made sufficiently “clean” to allow epitaxying to take place.
b
A-
polarization
P
pulse
ferroelectric
+
I Fig.
semiconductor
12.
Field-effect
J transistor
on a ferroelectric.
LEEDSTUDYOFTHE
(001)
FACEOF
YMn03
135
The YMnO, monocrystals were obtained in the form of thin wafers. This being the form in which they crystallised from the bismuth oxide bath, the best epitaxy results were obtained with crystals that were subsequently mechanically polished. The surface damage thus produced was removed by a chemical treatment (the only way possible with YMnO,), using warm orthophosphoric acid. It should be noted that this procedure could also be used to reduce the thickness of the wafers sufficiently to allow the crystals to be examined by transmission electron microscopy. The crystals were then cleaned in a solution of warm sulpho-chromic acid, followed by a warm bath of methyl alcohol. These treatments, however, were not sufficient to transparent
electrode
semiconductor
b
polarization Duke rroelectric
Fig. 13. “Ferrotron” type device. remove physi- or chemisorbed layers on the YMnO, surface, which is therefore in an unfavourable state for epitaxying. It was also considered that the presence of an electric polarisation perpendicular to the surface of the crystal could consequently produce a compensating charge on the crystal surface. It was supposed that by heating the crystal under vacuum to its transition temperature (Curie temperature: 660*C)r*), the ferroelectric polarisation would disappear, and the com~nsa~ng surface charges would be eliminated, thus leaving a clean surface. These considerations suggested the process of subjecting the YMnO, crystals to a heat treatment in ultra-high vacuum and observing the corresponding changes in the LEED diffraction pattern originating from the (0001) face of these crystals; this would provide a continuous monitoring of the effect of the heat treatment on the surface cleanliness throughout the treatment. The resul+s of the LEED study are described in the first part of the
136
D.ABERDAM
ET AL.
present report. They have provided an indication of the treatment needed to obtain the appropriate surface conditions; that is, heating to 1200°C for about 5 min under a residual pressure which is lower than lo-” Torr, these being the conditions necessary for obtaining the pattern shown in fig. 6. 5.3. CHOICE OF THE SEMI-CONDUCTOR Cadmium selenide was selected as the semi-conductor in view of {a) its crystallographic properties (symmetry elements in common with YMnO, in the g-hexagonal phase), (b) its electronic properties (it has a considerable photoconductivity), (c) its particularly convenient chemical properties (readily obtainable in stoichiometric thin films, easily deposited, and stable chemically, thermally and mechanically). It should be noted that for the presumed epitaxying, with the (0001) planes in the CdSe layer parallel to the substrate surface, the misfit is large (17%) if one considers the sub-lattice in the (0001) YMnO, planes formed by the atoms of the same type, manganese, yttrium or oxygen. (The same misfit is obtained if one considers the parameter a= 5.15 8, of the surface monolayer found to be present on the crystals prepared from bismuth oxide.)
The wafers of yttrium manganite were placed in an oven capable of reaching the required temperature of 12OO”C, this oven being mounted inside an ultra-high vacuum chamber (limiting pressure 2 x 10m9 Torr). At 1200°C the chamber pressure was never greater than 5 x 10m7 Torr. The cadmium selenide was evaporated from a quartz crucible supported in a tantalum foil basket which served as the heater. The source-substrate distance was 20 cm. The substrate temperature and the deposition rate were measured using, respectively a thermocouple placed close to the YMnO, substrate and the frequency of a piezoelectric quartz oscillator. The following sequence of operations was performed: {a) chamber pumped out to 2 x 10m9 Torr. (b) heat treatment of the YMnO, substrate: 1200°C for 30 min, and degassing of the source. (c) substrate cooled and its temperature stabilised at that required for deposition (about 200°C). (d) deposition of the CdSe.
Analysis of the deposits The orientation with respect to the substrate of deposits with thicknesses up to 4~ was found using high-energy reflection electron diffraction (80 keV electrons).
LED
STUDY oFT~(OOI)FACE
0F
YMn03
137
The principal results are as follows: - The CdSe always cristallised in the hexagonal phase (wurtzite structure: a=4.30 A, c=7.01 A). - The composition was stoichiometric; no excess of either component was ever found. - Whenthedepositwasordered, the[OOOl] direction wasalwaysperpendicular
Fig. 14a. Reflection high energy electron diffraction pattern (80 keV) of a CdSe deposit on YMnOa. Incidence along [lOTO] direction. Substrate temperature: 210°C. Deposition rate: 3 Ajsec.
I
&- incident ClOOl
Fig. 14b.
Geometrical
arrangement
leading to fig. 14a pattern.
138
D.ABERDAM
ET AL.
to the substrate surface, that is parallel to the [OOOl] direction in YMnO,. - The degree of orientation was greatest for the lowest deposition rates, the deposition temperatures all being in the range 185 “C to 250°C. Fig. 14a shows diffraction pattern from a well-epitaxied deposit that had been obtained using a substrate temperature of 210°C and a deposition rate of 3 Wlsec. The diffraction arrangement is shown in fig. 14b. The pattern shown in fig. 15 indicates that the deposit is strongly disoriented, with, however, a tendency to epitaxy. The experimental conditions were : substrate temperature = 25O”C, deposition rate = 25 &sec.
Fig. 15. RHEED pattern of a CdSe deposit on YMn03. This deposit is partly monocrystalline: incidence along [IOTO]direction. Substrate temperature: 250°C. Depositerate: 25 @sec.
Fig. 16 shows the diffraction pattern obtained from the YMnO, surface after removing the last-mentioned CdSe deposit from the surface, the orientation of the specimen with respect to the incident electron beam being unchanged. It was thus determined that the CdSe epitaxies on the (0001) surface of Y MnO, with the [ IOiO] direction in the CdSe parallel to the [ 1I?01 . . . du-ectron m YMnO,. 6. Conclusion The LEED study of the (0001) face of YMnO, essential points to be clarified.
has allowed
the following
LEED STUDY OF THE(~~)FACEOF
YMnOs
139
The importance of the influence of the procedure used to fabricate the specimen on the definition of the surface state of the crystal. The observed 2 x 2 structure may be due to the ferroelectric nature of the crystal, depending on the importance of the elastic terms in the expression for the energy of the system.
Fig. 16.
RHEED
pattern of the (~I) face of YMn03 after removing deposit; incidence along [1 IZO] direction.
the CdSe
The observed Kikuchi phenomena show that the atomic planes seem to be equivalent from the point of view of LEED even if they are of different composition. This study has allowed the experimental conditions necessary to obtain a clean surface to be accurately determined. If full account is taken of these conditions, as well as the parameters for the deposition of CdSe, this latter material may be perfectly epitaxied upon YMnO,.
-
Acknowledgements
The authors are grateful to Dr. E. F. Bertaut, Directeur de Recherches and Dr. G. Buisson, for the preparation of the YMnO, crystals used in these experiments.
140
D.ABERDAM
ET AL.
References 1) J. L. Moll, Stanford Electronics Laboratory, Report no. 4, II, 1, (1963). 2) J. L. Moll and Y. Tarvi, Solid State Device Rescarels Conference, University of Michigan (1963). 3) P. M. Heyman and G. H. Heilmeier, Proc. IEEE 54 (1966) 842. 4) R. Zuleeg and H. H. Wieder, Solid State Electronics 9 (1966) 657. 5) A. M. Renard and C. W. Hastings, Honeywell’s System and Research Center, Private communication. 6) H. L. Yakel, W. C. Koehler, E. F. Bertaut and F. Forrat, Acta Cryst. 16 (1963) 957. 7) E. F. Bertaut and G. Buisson, Brevet C.E.A. no. PV 962 681. 8) E. F. Bertaut and F. Forrat, Brevet C.E.A. 9) A. Guinier, Thkorie et Technique de la Radiocristallographie (Dunod, 1956). 10) P. Ducros, Surface Sci. 10 (1968) 118. 11) G. C. Benson, P. I. Freeman and E. Dempsey, Advan. Chem. 33 (1961) 26. 12) T. E. Feuchtwang, Phys. Rev. 155 (1967) 715. 13) Ph. Coeure, Note technique no. 364, CEA-CENG. 14) I. G. Ismailzade and S. A. Kizhaev, Sov. Phys. Solid State 7 (1965) 236.
SCIENCE
LEED
14 (1969) 121-140 o North-Holland
STUDY
IN ORDER
OF THE
TO
(0001) FACE
EPITAXY
D. ABERDAM,
Publishing Co., Amsterdam
OF YMnO,
CdSe ON THIS SURFACE
G. BOUCHET,
P. DUCROS
Laboratoire de Spectrome’trie Physique, Faculte’ des Sciences de Grenoble, Domaine Universitaire, 38 - Saint-Martin d’H&es, France
and J. DAVAL, Laboratoire d’Electronique Centre d’Etudes Nuclkaires
G. GRUNBERG et Technologie de i’lnformatique, de Grenoble 38 - Grenoble, France
Received 9 August 1968 Using the technique of low-energy electron diffraction the structure of the (0001) face of yttrium manganite has been studied with a view to subsequent epitaxying of the semiconducting material cadmium selenide upon this ferroelectric crystal. The heat treatments necessary for obtaining an ordered surface of YMnOs have been determined, and a possible explanation for the appearance of a 2 x 2 surface structure is given. As a result of this study the epitaxy of CdSe on YMnOs has been achieved in the best possible conditions. 1. Introduction
The recent development
of certain electronic
devices has brought
about the
need for combining a semi-conductor and a ferroelectric in a single electronic element l-5). The series of experiments reported here was undertaken in order to study, purely from a crystallographic point of view, the epitaxying of a deposit of cadmium selenide (a semiconductor) on the surface of a monocrystal of yttrium manganite, YMnO, (a ferroelectric). A thorough knowledge of the surface conditions of the YMnO, crystal chosen as the substrate was therefore desirable. The first part of this report is concerned with the low-energy electron diffraction (LEED) observations which were used to determine the various treatments needed to obtain an ordered surface structure and also the nature of the structures so obtained. The second part is concerned with theepitaxy experiments of cadmium selenide on the surface prepared using the information from the LEED experiments. Yttrium manganite crystallises in the hexagonal system, its space group being P6, cm (ref. 6). Its bulk structure (fig. 1) is characterised by the pres121
D.ABERDAM
122
ET AL.
ence of polyhedra with fivefold coordination around the manganese atoms, and sevenfold around the atoms of yttrium. The bulk lattice unit cell contains six units of the chemical formula, and its parameters are: a =6.125A
c=
11.41 A.
An important aspect of this substance is its ferroelectric nature. The polarisation appears in this structure parallel to the c-axis, which thus allows
l/2
lfi
0
Fig.
I.
Bulk
0
structure of YMn03.
for two possible orientations of the polarisation vector (domains at 180”). The crystals used for these studies were in the form of thin platelets or wafers grown by a flux process using two different procedures in which the flux agents were respectively : - an excess of manganese oxide (Mn,0,)7) - a bismuth oxide (Bi,O,)*) The surface studies were performed on crystals prepared by both processes.
LEED STUDY 0F THE(OOI)FACE
0F
YMn03
123
2. Experimental procedure The experiments were carried out using a modified Varian LEED apparatus in which the lowest attainable pressure is of the order of 2 x lOWi Torr. The specimen assembly was mounted on a Varian specimen holder which provides the basic movements necessary for low-energy electron diffraction studies. The crystal specimen was supported on a tantalum foil which had been moulded to the shape of the crystal. Electrical resistance heating of the foil enabled heat treatment of the specimen (fig. 2). This form for the crystal support was chosen because it allowed the crystal to be heated as homogeneously as possible, a condition which is particularly difficult to achieve in the case of an insulator. The homogeneity of the specimen temperature is important for two reasons : - firstly, it provides a well-defined surface temperature - secondly, it minimises internal strains in the crystal. A Pt-Pt/Rh thermocouple spot-welded to the back face of the foil provided an approximate measurement of the temperature, the measured temperature
Connectins collar.
Fig. 2.
Crystal holder and heater.
124
D.ABERDAM
being greater two tantalum
ET AL.
than the specim.en temperature. The foil was supported rods which were also used as the electrical connections
by for
heating the foil. The above system was able to provide a maximum specimen temperature of about 1400°C. In addition, the vacuum chamber pressure was monitored by an ionisation gauge, and a quadrupole mass spectrometer mounted in direct view of the specimen was used to analyse the residual gas content. 3. Experimental results 3.1.
SPECIMEN
PREPARED
FROM THE MANGANESE
OXIDE FLUX
A summary of the results obtained after different heat treatments is given in table I. The type of diffraction pattern obtained and the residual gas analysis is given for each treatment undergone by the crystal. The table indicates that two principal stages in the decontamination process of the crystal may be defined. The first stage, corresponding to heating to 910°C for about 20 min, gives rise to the appearance of the specularly reflected beam (fig. 3) and a structured background scattering at high incident electron energy. After this initial stage the crystal surface is still not completely clean, there being a physi- or chemisorbed layer whose thickness scarcely exceeds one or two monolayers. The second stage is reached by heating to 1200°C for 5 min. In this
Fig. 3.
Electron beam voltage: 590 V.
20 min
30 min
1 min 1 min 5 min
1 min
5 min
910°C
l@OO”C
1060°C 1100°C 1100°C
1200°C
1200°C
when cold high contrast with 2
area observed x
2 superstructure
(fig. 4)
- Pattern obtained at lower temperature than before - Voltage for the appearance of space charge depends on the
very slight improvement
space charge when crystal hot little improvement in diffraction pattern
* Appearance of (00) spot at fairly high temperature * Appearance of a pattern when the crystal is cold (fig. 3) - Structured background at 965 V
Space charge - no pattern
Diffraction pattern
Peaks are arranged in order of increasing intensity.
2h lh 1h 20 min
500°C 600°C 680°C 830°C
Heat treatment
TABLE 1
Specimen prepared from the manganese oxide flux
increase of peaks 44, 32 at start of heating increase of peak 28 at the end of heating
increase of peaks 28, 16, 20, 32, 44
peak 36 disappears
increase of peaks 16, 30, 32
increase of peaks 16, 44, 28, 18, 32, 36
increase of peaks 28, 44, 18 increase of peaks 16, 18, 20, 28, 44 increase of peak 18 no change of peaks 16, 18, 20
Residual gas analysis
126
D.ABERDAM
ET AL.
case an ordered surface structure is found, since a diffraction pattern with high contrast is observed (fig. 4) indicating a 2 x 2 superstructure. A possible explanation of this structure is given below. Two important conclusions may be drawn from the manner in which the heat treatments were carried out in order to decontaminate the surface.
Fig.
4.
Electron
beam
voltage:
153 V.
- The heat treatment giving rise to a clean surface seems to be only a function of the temperature rather than the product temperature-time, which shows that it is necessary to overcome a certain activation energy in order to obtain a clean surface. - The temperature required for decontamination is higher than the Curie temperature for the ferroelectric, and no evident relation seems to exist between these two temperatures. 3.2.
SPEClMEN
PREPARED
FROM
THE BISMUTH
OXIDE
FLUX
For this specimen no particularly well-defined treatment was found which produced the expected clean surface conditions. After heating the crystal to 700°C for 2 hours the diffraction pattern was found to contain the Kikuchi patterns above about 1500 V, but only an intense background was observed at Iower voltages. it should be remarked that the diffraction spots were only visibleathighenergiesandthat nothreshold value was found below which the crystal surface becomes negatively charged.
LEED STUDY OF THE(OOI)FACE
OF
YMn03
127
Heating successively to 1000°C and 1100°C produced a slightly improved surface. Even so, the diffraction patterns observed (fig. 5) indicated that there were still one or more impurity layers on the surface. It is possible that these impurity layers were conducting, giving rise to a surface conductivity which would explain the absence of space charge effects.
Fig. 5.
Electron beam voltage: 284 V.
This surface layer must, however, be quite stable since the same diffraction pattern was obtained after opening the apparatus to “reagent pure” nitrogen gas, once the system had been pumped-out and rebaked. A second series of treatments was tried in which the crystal was heated successively to 1200 “C, 125O”C, 13OO”C,and 1350 “C. After heating to 1200°C for 5 min a pattern was obtained which contained a large number of diffraction spots (fig. 6). Heating to the three higher temperatures did not bring about any important changes in the pattern, the only significant change being a resolution of the streaks in the form of a star which was observed about the main diffraction spots (fig. 7). It may be seen from table 2, that it is difficult to propose a well-defined treatment which will decontaminate the crystal surface. 3.3
DIFFRACTED INTENSITIES, KIKUCHI
BANDS
The following results were obtained using the specimen prepared from bismuth oxide.
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D.ABERDAM
ET AL.
Fig. 6.
Electron beam voltage: 108 V.
Fig. 7.
Electron beam voltage:
I16 V.
The only observed spots are those due to epitaxied layer (fig. 5) High background No evident space charge
5 min
1100°C
Slight sharpening (00) (fig. 7)
3 min
1300°C
1350°C
Peaks are arranged in order of increasing intensity.
The streaks forming the star around the (00) spot are replaced by spots in hexagonal arrangement
2 min
1250°C
of spots forming hexagon around
Space charge * No change in the pattern is observed
5 min
1200°C
Pattern with numerous diffraction spots (fig. 6) Space charge
5 min
No change in the pattern is observed
No evident space charge Little improvement in diffraction pattern
6 min
1oOO”c
Chamber filled with nitrogen. Subsequent pump-down followed by bake-out 12 h at 250°C
High background Low contrast No evident space charge
2h
Diffraction pattern
700°C
Heat treatment
TABLE2 Specimen prepared from the bismuth oxide flux
T= 1180°C Peaks 44,28,2,18,16,30,17,40
T= 700°C Peaks 2,28,44,20,16,18 T= 950°C Peaks 2,44,28,18,16,20
T=250"C Peaks 44,28,20,16,40,18 T= 450°C Peaks 44,18,28,20,17, 16
Residual gas analysis
130
D.ABERDAM
3.3.1.
Curves qfI,,fV/
E‘I‘ AL.
and Z,,(T)
The variation of the intensity of the specularly reflected beam as a function of the accelerating voltage (fig. 8) shows a series of maxima for which the observed periodicity is twice that expected for the (00) reflection. This result can be explained; in effect, examination of the crystal structure shows that a trans-
I,
0
la7 Fig. 8.
(00)
200
300
Loo
5al
700
600
beam intensity versus electron beam voltage; incidence:
v
0 - 2.75”.
lation of -.$Cbrings into coincidence two planes with the same diffusion power. The agreement between the observed and calculated voltage values for these maxima seems to be dependent on the state of the crystal surface: however, the possibility of a defined inner potential was eliminated. For the maxima of the (00) spot at 104 V and 210 V the variation of the intensity lo0 was studied as a function of the temperature. The plot of log I versus the temperature was found to be linear, consistent with the expected exponential variation of the intensity with temperature. It was thus possible to define a Debye temperature (On) in terms of the usual expression 9) for the Debve-Wailer factor W I = I, exp(For the maximum
2W);
W = +sin’O* ikTz
at 210 V it was found
[*(O/T)
the 0,=756”K.
+
q(OjT)].
LEED
STUDY
OF
THE
(001)
FACE
OF
YMn03
131
3.3.2. Kikuchi lines and bands In the diffraction pattern shown in fig. 9 it was found that the bands with the greatest contrast were due to the (OliO) type planes, the diffraction cones generating these bands were therefore third-order. It is known that the bands corresponding to first-order cones should be observed preferentially to any higher order. However, on examining the crystal structure it was remarked that only very slight displacements involving principally the Yttrium atoms prevent the lattice from having a centrosymmetric unit cell, smaller than the real unit cell (fig. 10). In indexing the above Kikuchi bands on the basis of
Fig. 9.
Kikuchi bands. Electron beam voltage: 1700 V.
b
Fig. 10.
Relative orientation
and size of both meshes.
132
D.ABERDAM
ET AL.
the centro-symmetric cell it was found that they were due to first order reflections from planes of the type (2110). The other rectilinear bands are third-order reflections from (I OTO) type planes. Nevertheless, consideration of the structure leads to the conclusion that these bands are more intense. In fact the order of the (IOiO) planes is as shown in fig. Il. Using the kinematic approximation one may write: N
F=
C {FA[l +exp(4~i~d,~.cos8)] n=O
+ FB exp
871i ~IOTO .- mP.cOse a
3
47rni ~~ -*d,o~o~cose A
.
_!!d! a+ 0
a
-i
krlEI ;---I---
1 dloio
a__&--Fig. Il.
Arrangement
of atomic planes parallel to (lOi0).
It may thus be expected that the maxima of order 3 will be more intense. The indexing of the circles is much less certain. These are possibly due to (lOi2) planes (using centro-symmetric cell) the reflections being first-order. 4. Discussion Two possible explanations for the origin of the observed 2 x 2 structure may be proposed. The first is based on the ferroelectric character and the second on the chemical character of the specimen. 4.1. FERROELECTRIC ORIGIN It is known that the normal way a ferroelectric function is to take on a domain structure.
crystal minirnises
its Gibbs
LEED
STUDY
OF THE
(001)
FACE
OF
YMn03
133
Nevertheless, it may be asked if, close to the surface, the polarisation in a given domain would relax about an average value Pro). This would be in order to minimise the surface energy 1’). The amplitude of this relaxation should however decrease with increasing depth away from the surface, and finally become zero lz), whereas at the same time P would be varying, finally to take on the usual bulk value. As a first approach to this problem, we may consider a simplified model consisting of a linear chain of dipoles, along which the polarisation may be written : P, = P + 6P cos (Z~~/~),
(0
M being the relaxation period. The energy in each system (relaxed and nonrelaxed) is the sum of two terms, elastic and electrostatic. Electrostatic term : E,=
1
n,m
E:,,,,i--*
FlPf?l Ir,, J
(2)
’
Elastic term : Eq = &,
EP,” + PP,” + QP,”+ y (P” - P,+ $
;
(3)
with P, = P for the non-relaxed system, and P, given by eq. (1) for the relaxed system. The energy difference d U between the two systems may be written in the form: d U = (E, + E,) relaxed - (E, + E,) non-relaxed. (4) It is therefore necessary to find the values of M which will minimise this expression, The calculation shows that only three values of M make AU negative, the most favourable case being M=2. It may be concluded that if the elastic term is sufficiently small a relaxation may appear with M=2. In the opposite situation the system will keep a uniform polarisation. 4.2. CHEMICAL ORIGIN The possibility of a surface layer having a chemical composition different from the bulk of the crystal cannot be ruled out. First of all, one may consider the presence of impurities which, after diffusing from the bulk, form an ordered chemisorbed layer. Secondly, it is possible that close to the surface substitutions of various atoms take place during crystal formation. It seems that the most probable substitutions would be those involving the atoms of the rare earth element (yttrium) being substituted by atoms of manganese.
134
D.ABERDAM
5. Application:
ET AL.
epitaxy of cadmium selenide on YMnO,
5.1. INTRODUCTION As has been previously stated, it was considered of interest to take maximum advantage of the insulating, monocrystalline, and above all, ferroelectric, properties of yttrium manganite. The main objective is to combine this ferroelectric with a semi-conductor to produce electronic devices such as: A field-effect
transistor on a ferroelectric
13) (fig. 12)
In this device the different stable states of the polarisation of the Y MnO, crystal are able to modify the conduction paths in the semiconductor between the emitter and base, thus allowing the fabrication of an analog memory unit with a non-destructive read-out. Device of the type “Ferrotron”
(fig. 13)
In this device a semiconductor which is made locally conducting by a light beam produces a localised polarisation of the ferroelectric under an applied field, thus producing a genuine memory store. In the two above-mentioned devices, one has made use of the same phenomenon of conduction in a semiconductor. To achieve a high conductivity the carrier mobility must be high, consequently it is necessary to produce a semiconductor which is as perfect as possible; that is, monocrystalline and epitaxially grown on the ferroelectric. 5.2.
PREPARATION OF THE YMnO,
CRYSTAL
Before depositing the semiconductor, the surface of the YMnO, crystal must be made sufficiently “clean” to allow epitaxying to take place.
b
A-
polarization
P
pulse
ferroelectric
+
I Fig.
semiconductor
12.
Field-effect
J transistor
on a ferroelectric.
LEEDSTUDYOFTHE
(001)
FACEOF
YMn03
135
The YMnO, monocrystals were obtained in the form of thin wafers. This being the form in which they crystallised from the bismuth oxide bath, the best epitaxy results were obtained with crystals that were subsequently mechanically polished. The surface damage thus produced was removed by a chemical treatment (the only way possible with YMnO,), using warm orthophosphoric acid. It should be noted that this procedure could also be used to reduce the thickness of the wafers sufficiently to allow the crystals to be examined by transmission electron microscopy. The crystals were then cleaned in a solution of warm sulpho-chromic acid, followed by a warm bath of methyl alcohol. These treatments, however, were not sufficient to transparent
electrode
semiconductor
b
polarization Duke rroelectric
Fig. 13. “Ferrotron” type device. remove physi- or chemisorbed layers on the YMnO, surface, which is therefore in an unfavourable state for epitaxying. It was also considered that the presence of an electric polarisation perpendicular to the surface of the crystal could consequently produce a compensating charge on the crystal surface. It was supposed that by heating the crystal under vacuum to its transition temperature (Curie temperature: 660*C)r*), the ferroelectric polarisation would disappear, and the com~nsa~ng surface charges would be eliminated, thus leaving a clean surface. These considerations suggested the process of subjecting the YMnO, crystals to a heat treatment in ultra-high vacuum and observing the corresponding changes in the LEED diffraction pattern originating from the (0001) face of these crystals; this would provide a continuous monitoring of the effect of the heat treatment on the surface cleanliness throughout the treatment. The resul+s of the LEED study are described in the first part of the
136
D.ABERDAM
ET AL.
present report. They have provided an indication of the treatment needed to obtain the appropriate surface conditions; that is, heating to 1200°C for about 5 min under a residual pressure which is lower than lo-” Torr, these being the conditions necessary for obtaining the pattern shown in fig. 6. 5.3. CHOICE OF THE SEMI-CONDUCTOR Cadmium selenide was selected as the semi-conductor in view of {a) its crystallographic properties (symmetry elements in common with YMnO, in the g-hexagonal phase), (b) its electronic properties (it has a considerable photoconductivity), (c) its particularly convenient chemical properties (readily obtainable in stoichiometric thin films, easily deposited, and stable chemically, thermally and mechanically). It should be noted that for the presumed epitaxying, with the (0001) planes in the CdSe layer parallel to the substrate surface, the misfit is large (17%) if one considers the sub-lattice in the (0001) YMnO, planes formed by the atoms of the same type, manganese, yttrium or oxygen. (The same misfit is obtained if one considers the parameter a= 5.15 8, of the surface monolayer found to be present on the crystals prepared from bismuth oxide.)
The wafers of yttrium manganite were placed in an oven capable of reaching the required temperature of 12OO”C, this oven being mounted inside an ultra-high vacuum chamber (limiting pressure 2 x 10m9 Torr). At 1200°C the chamber pressure was never greater than 5 x 10m7 Torr. The cadmium selenide was evaporated from a quartz crucible supported in a tantalum foil basket which served as the heater. The source-substrate distance was 20 cm. The substrate temperature and the deposition rate were measured using, respectively a thermocouple placed close to the YMnO, substrate and the frequency of a piezoelectric quartz oscillator. The following sequence of operations was performed: {a) chamber pumped out to 2 x 10m9 Torr. (b) heat treatment of the YMnO, substrate: 1200°C for 30 min, and degassing of the source. (c) substrate cooled and its temperature stabilised at that required for deposition (about 200°C). (d) deposition of the CdSe.
Analysis of the deposits The orientation with respect to the substrate of deposits with thicknesses up to 4~ was found using high-energy reflection electron diffraction (80 keV electrons).
LED
STUDY oFT~(OOI)FACE
0F
YMn03
137
The principal results are as follows: - The CdSe always cristallised in the hexagonal phase (wurtzite structure: a=4.30 A, c=7.01 A). - The composition was stoichiometric; no excess of either component was ever found. - Whenthedepositwasordered, the[OOOl] direction wasalwaysperpendicular
Fig. 14a. Reflection high energy electron diffraction pattern (80 keV) of a CdSe deposit on YMnOa. Incidence along [lOTO] direction. Substrate temperature: 210°C. Deposition rate: 3 Ajsec.
I
&- incident ClOOl
Fig. 14b.
Geometrical
arrangement
leading to fig. 14a pattern.
138
D.ABERDAM
ET AL.
to the substrate surface, that is parallel to the [OOOl] direction in YMnO,. - The degree of orientation was greatest for the lowest deposition rates, the deposition temperatures all being in the range 185 “C to 250°C. Fig. 14a shows diffraction pattern from a well-epitaxied deposit that had been obtained using a substrate temperature of 210°C and a deposition rate of 3 Wlsec. The diffraction arrangement is shown in fig. 14b. The pattern shown in fig. 15 indicates that the deposit is strongly disoriented, with, however, a tendency to epitaxy. The experimental conditions were : substrate temperature = 25O”C, deposition rate = 25 &sec.
Fig. 15. RHEED pattern of a CdSe deposit on YMn03. This deposit is partly monocrystalline: incidence along [IOTO]direction. Substrate temperature: 250°C. Depositerate: 25 @sec.
Fig. 16 shows the diffraction pattern obtained from the YMnO, surface after removing the last-mentioned CdSe deposit from the surface, the orientation of the specimen with respect to the incident electron beam being unchanged. It was thus determined that the CdSe epitaxies on the (0001) surface of Y MnO, with the [ IOiO] direction in the CdSe parallel to the [ 1I?01 . . . du-ectron m YMnO,. 6. Conclusion The LEED study of the (0001) face of YMnO, essential points to be clarified.
has allowed
the following
LEED STUDY OF THE(~~)FACEOF
YMnOs
139
The importance of the influence of the procedure used to fabricate the specimen on the definition of the surface state of the crystal. The observed 2 x 2 structure may be due to the ferroelectric nature of the crystal, depending on the importance of the elastic terms in the expression for the energy of the system.
Fig. 16.
RHEED
pattern of the (~I) face of YMn03 after removing deposit; incidence along [1 IZO] direction.
the CdSe
The observed Kikuchi phenomena show that the atomic planes seem to be equivalent from the point of view of LEED even if they are of different composition. This study has allowed the experimental conditions necessary to obtain a clean surface to be accurately determined. If full account is taken of these conditions, as well as the parameters for the deposition of CdSe, this latter material may be perfectly epitaxied upon YMnO,.
-
Acknowledgements
The authors are grateful to Dr. E. F. Bertaut, Directeur de Recherches and Dr. G. Buisson, for the preparation of the YMnO, crystals used in these experiments.
140
D.ABERDAM
ET AL.
References 1) J. L. Moll, Stanford Electronics Laboratory, Report no. 4, II, 1, (1963). 2) J. L. Moll and Y. Tarvi, Solid State Device Rescarels Conference, University of Michigan (1963). 3) P. M. Heyman and G. H. Heilmeier, Proc. IEEE 54 (1966) 842. 4) R. Zuleeg and H. H. Wieder, Solid State Electronics 9 (1966) 657. 5) A. M. Renard and C. W. Hastings, Honeywell’s System and Research Center, Private communication. 6) H. L. Yakel, W. C. Koehler, E. F. Bertaut and F. Forrat, Acta Cryst. 16 (1963) 957. 7) E. F. Bertaut and G. Buisson, Brevet C.E.A. no. PV 962 681. 8) E. F. Bertaut and F. Forrat, Brevet C.E.A. 9) A. Guinier, Thkorie et Technique de la Radiocristallographie (Dunod, 1956). 10) P. Ducros, Surface Sci. 10 (1968) 118. 11) G. C. Benson, P. I. Freeman and E. Dempsey, Advan. Chem. 33 (1961) 26. 12) T. E. Feuchtwang, Phys. Rev. 155 (1967) 715. 13) Ph. Coeure, Note technique no. 364, CEA-CENG. 14) I. G. Ismailzade and S. A. Kizhaev, Sov. Phys. Solid State 7 (1965) 236.