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Fluoride layers and superlattices grown by MBE on Si(111): dynamic RHEED and Sm2+ photoluminescence studies
Applied Surface Science fin/hi (1992)421 425 Norlh*llolland
aphid surface science
Fluoride layers and superlattices grown by MBE on Si( 111): dynamic R H E E D and Sm 2+ photoluminescence studies N.S. Sokelov, LC.
Alvarez
t and
N . L ¥akovlev
A.E lath" Phy~h'tJ.7~'chnical hJ.sttlt~re. ~t .P,,ter~hw;~ IC~4021, R.x~ht
Received 20 No~ember 19gl: accepted for puhiicatkm 311Jantulr~ lets2
Tile oscillatiuns of ruflectil~n high-energy, cleclron ditfractitln inlen,;ityduring CaF2 and SrF. epilaxial growth on the 1111) suUuce of silicoll antl the fluorides II~w¢been investigated. Fronl these slndies the critical thickries::of the fluoride pseudomorphie grt~wth mode ~as measured fiw different gr~lwfllcondititms. The~e data alhlwcd us It~ grtlw sta}~l/~l~'strained ~herent fluoride layers ~nd superlulfices. Sludies of Sm-~' ion photolumincscence in Ihes¢ fluoride structulcs at liquid-helium Icmperature have been shown to be useful flJr characterizationof Ihe cl3'~tallinequalityof the films and ftlFthe nleaSuremcnl tit ehlMicstrain. I ligh quantum yic(d of the luminescence enabled us to explore doped layers as thin as several nltmnlayers.
1. lntrnd~tctinn Epitaxial CaF~. and SrFe layers on semiconductors attract the attention of researchels [1,2] owing to various potential applications of these hcterostructurcs in opto- and microelectronics including creation of 3D integral circuits, semiconductor-on-insulator structures and MISFETs on GaAs. There exist unique opportunities for fundamental research of the interface between two crystals with different type~ of bonding: the ionic bonding in fluorides and the covalent one in semiconductors. The successful application of molecular beam epitaxy ( M B E ) for the fluoride layer growth revealed interesting possibilities for research in the growth processes and creation of new structures for basic research in the low-dimension physics. It is well known that reflection high-energy electron diffraction ( R H E E D ) intensity oscillation measurements during M B E growth are informative lc,r ,studiEs of growth processes. ]t was demonstrated that this powerful technique could also be applied to the investigation of fluoride growth [3]. i perman~nt addles~: hlslautc tff Materialsanti Reagentsfilr Electronics. Physics Faculty. ]~lvanll Universil%Cuha.
As the OaAs lattice constant lies between those of CaF., and SrF z, the (Ca,Sr)F 2 solid solutions were used [4] to grow lattice-matched GaAs-fluoride heterostruetures. It would be attractive in some cases to use strained CaF2-SrF 2 superlattiees (SLs) for the same purpose. However because of the large lattice mismatch in this fluoride couple (h.2%) until recently it has been ullelear whether it is possible to grow a coherent fluoride SL. Taking into account the technological difficulties of the preparation of the clean G a A s ( l l l ) B surface [5], it seems reasonable to study the growth and properties of such SLs on a more technologically convenient Si(! l l) surface. Optical properties of rare-earth ions in bulk fluoride single crystals were the subject of the thorough fundamcutal research [6]. Low-temperature photolumineseence of Eu 2+ and Sm z+ ions in epitaxial CaF~ and SrF 2 layers was recently used to characterize the crystalline quality and to measure the strain in these films [7,8]. In the next part of the paper we present the results of dynamic R H E E D studies during M B E growth of CaF~, SrF 2 strained layers and fluoride SLs on Si(ll 1). The third part of the paper deals with Sm ~* photoluminesccnee studies of these structures.
nlhg-4332/q"/$1)5.0a ( 1992- Elsevier Science Puhlisher~ B.V. All rights reserved
422
N.S, Svkolor el aL / Fluoride lay('~ and sltperlaences grown or1 Sifl l l )
2. MBE g r e ' ~ h and dynamic F.~EED studies ~:pitaxial C a F 2 and SrFg layers on Si(t I [ ) were grown in an M B E system for research with a 20 L deposition chamber [9]. T h e system was equipped by 5 molecular cells with pure or Sin-doped (0.10.3 tool%) C a F : and S r F 2 crystals. T h e rate of fluoride deposition was calibrated by measuring the period of R H E E D intensity oscillations [3] and typically was 1-5 n m / m i n . The electron energy in the R H E E D gun was 15 keV, the incidence angle of the electron beam on the layer surface being about 40'. This angle corresponds to the so-called off-Bragg condition, which gives the maximum sensitivity of lira spot (specular beam) intensity to the densily of munolayer steps [10]. T h e intensity of the spot in the pattern was measured using a photomultiplier and a D C amplifier. Jn the majority of the experiments described in this paper the R H E E D patterns were streaked with bright specular spots. This indicated the flatness of the fluoride layer surface on the atomic scale. Fig. 1 shows 00-spot inte,lsity oscination~ during CaF 2 growth on Si(l I 1) at different substrate temperatures. O n both curves one can see an oscillating periodic component with a period •~vhleh e o r r c s p o n ' l s to the growth of one triple F - C a - F layer 0,315 nm thick. Such l ~ ( t ) dependence convincingly proves the two-dimensional layer-by-layer fluorite growth mode on Si(111). The complicated shape of R H E E D intensity os-
,,~ro i
•
....... -c °~, °
"
1
i
~.sec
Fig. I. Depel~dencesot spt;cularbeam illlellsit~,'in the RHEED pattern versu,~ ~he time of CaF2 deposition on Si(lll) at 600°C (a) ~nd 65ff'C (b). Hereafter BD is the breakdown of pseutlume,phic growth mode.
~.
°'l
to. /a0
o
eo
12o
~BO ~.~ec
Fig, 2. RHEED intensity oscillations during SrF~ growth, (a) im St(I I I I lit (}lgl°c,(b) on CItF2/~;i( I 11) structure ut 100°t".
cillations at early stages of C a F 2 growth is probably connected with the influence of the S K i l l ) (7 × 7) sunerstrueture and bonding at the interface i l l ] on the interference of the electrons, scattered by the fluorite layer and the silicon substrate. Slow change of the average intensity at the later stages of the fluoride growth could originate in the variations of the step density on the fluorite layer surface at different growth modes. T h e decrease of the average specular beam intensity after the growth of 4 - 5 monolayers (fig. lb) is likely to be due to the breakdown of the pseudomorphic growth mode and the roughening of the surface. The increase of the average intensity after ~ l 0 monolayer deposition could be caused by the subsequent flattening of the growing surface. It is worthwhile to rtote that th~ lattice mismatch for CaFz-Si couple is 1.7% at 600°C. In fig. 2a the l~m(t) dependence during S r F z growth on S i ( l l l ) at 600°C is presented. One can see that the shape of this dependence is quite similar to the l ~ ( t ) dependence shown in fig. lb. The main difference is that in the SrF2 case the breakdown of the pseudomorphic growth occurs after ~ 3 monolayers. Taking into account the large value of lattice mismatch ( ~ 7%) in the S r F z / S i heteroeouple, the small critical thickness of the SrF 2 coherent growth on Si(II1) is not unexpected. T h e observed 2D growth mode at the large mismatch can be explained by considerably less fluoride ( I l l ) surface free enerlff compared with this value 0¢ S i ( ! l l ) surfa¢,: [1!.
423
N,S Sokolor et aL / Fhmridt hlyers and supedattice.v grolwl on Si(I I I) s ~ ~*
"Z
levels
in
,'
CaF 2
b i
708.5
~oo
5oe
ooo
Specular beam intensity behavior in the R H E E D pattern during S r F , / C a F 2 / S i ( I 1 l) heterostructure growth is shown in fig. 2b. One can see that the first two SrF2 monolayers on the pseudomorphie C a F 2 layer on S i ( l l l ) also grow coherently, the breakdown of the pseudom0rphie growth mode occurring during the grt,wth of the third S r F 2 monolayer. Taking into consideration the above results we have grown strained fluoride Sl.,s on Sit111). Fig. 3 shows the ltd,(t) dependences during [SrF2(2 M L ) / C a F 2:Sm(5 ML)] 4 (a) and [SrF2(I M L ) / CaF2:Sm(10 ML)] w (b) fluoride SLs growth. The damping of the oscillations in fig. 3a is probably due to the coarsening of the surface because of the low growth temperature of the SL. O n e can see that ltm(t) behavior during SL growth at 500°C is periodic (fig. 3b), which evidences for reproducibility of the growth conditions in each period of the structure.
fluoride layers and
of Sm 2+ ions SLs
98
in S r F z
V I I
rim
i
5 nm
---L_
Fig. 4. Schematic diagram *If excited Sm"~" ion levcl~ in CaF, pad SrF2, The wavelengUlof radiation is shown at the arrows indicating electron transitions.
sint;le crystals the lowest energy level important for optical transitions is the 5D 0 state, belonging to th~ same 4f" shell as the ground 7F 1 state. Deca,' time of the luminescence which is duo to these forbidden intraeonflguration transitions is usually about 10 2 s in bulk crystals at liquidhelium temperature. Piezo.-.peetroseopie studies [12] showed that the energy of 4f t' shell levels depends but little up strain in the fluoride crystals. Meanwhile, for the levels of 4¢55d mixed electronic configuration a well pronounced linear dependence of their energy on uniaxial stress along the [111] direction has been o~served. T h e relation between the wavelength shift AA of the zero phonon emission line in the strained fluorite layer (3.) from that in unstressed bull,: crystal (3.n) on the strain in the plane of the interface E was found to he AA,m = 3. - 3-o ~ - 6 × 10zcll 8 . This relation was used to put the strain scale in figs. 5 and 6.
2.0
I ~L, 3. Phntoluminescence
,"
[4t e] r~"
'F,
Fig, 3. Behaviour ot the RHEED specular beam intensils during the growth of SrF,/CaF z superlattices: (a) [S~Fz(2 ML)/CaF2:Sm(5 MLI]~/CaF~(2 ML)/Si(III). and Ib) [SrF:(I ML)/CuF2:Sm{10 MLJ]In/SrF_~llaML)/Si(I It), The brackets correspond to die intervalswhen a source is open.
m
/'
1.5
c
1.0
Q.S
b
0.0
d
-0.5 C,
o
in thin
The structure of Sm 2+ ion energy levels in Ca and Sr fluorides is schematically shown in fig. 4. The most intense photoluminescence (PL) of these ions in bulk CaF 2 crystals originates from the allowed optical transitions 4fsSd ~ 4f ¢'. The decay time of the luminescence is ~ 10 -~' s at liquid-helium temperature. In the S r F 2 : S m 2+
700
704
n
Fig. S. Photolorninescence spectra (at 1.7 K ) of Sm2--doped CaF~ layers on Si(I I I) grown at different ~onditions: (a) 6 n m
at 560°C, (b) 12 nm at 7U~J'C,(c) 20 nm at 770°C with rapid annealing a! 950~C ~hr 30 s, (d) spectrum of the CaF,:Sm single crystal.
N.X Sokokn' q'r aL / Huoruh, layers and superlauk'e~ grown vn Sill I l J 2.0
pt,
69g
1.5
1.0
b
c
698
70
05
0.0
d
-O.g gm,~
a
Fig, 6. PL spectra of ISrF:( I ML)/CaF:.Sm{It) Ml,~h, supeJi~nlices g¢own al 5011°C. (at on C a F : / S i ( l l l k (bt on SrF:/Sill lit lille dashed cuwc is the luminc~:nce delayed o~er I ms alter tile ued of escitalkm), (¢) CaF,:Sm 3() nm layer on SrF~(la ML)/Si( I I 1). td ) speclrum nf Ihe CaF: : Sm single cr~stal.
T o excite the PL of Sm -'~ ions in fluoride structures a H e - N e laser (A = 632.8 nm) was used. The samples were put in a p u m p e d ( F - 1.7 K) liquid-helium glass cryostat. The PL spectra were measured by a double-grating monochromator, a cooled photomultiplier and a photon counting system. T h e afterglow spectra were obtained with a mechanical chopper at a delay over 1 ms after the end of every excitation impulse. Curves Ca), (b) and (e) in fig. 5 show Sin: + PL spectra in thin epitaxia[ CaF: layers grown on SiC111 ) at diffcrent conditions. Curve (d) presents PL spectrum of the bulk CaF, :Sin -'÷ crystal in the region of zero phonon llne of the 4fS5d ~ 4f* transition. T h e fluorite and silicon lattice mismatch at liquid-helium temperature is about 0.3%. thus in coherent heterostructures the fluorite layer should have a compressive strain of - 0 . 3 % . Such s~:rain as well as a relatively narrow emission line presented by curve (at shows that the layer grown at 560':C is coherent with the ~ilicon substrate [8]. The leusilc strain and the broad emission line (curve b) in the layer grown at 700°C are due 1o the breakdown of pseudomorphic growth mode, accommodation of mismatch stress and a larger value of the CaF~ thermal expansion coefficient compared with Si [7]. Curve (e) in this figure shows that in the 20 nm fluorite layer the average value ef tensile strain can reach 1.7% after rapid thermal annealing •~950°C. 30 s) in situ. It considerably exceeds
the largest strain values which can be obtained in bulk crystals in uniaxial stress experiments [12]. PL spectra of [SrF.,(I M L ) / C a F . , : S m ( I t l ML)]j. superlattiees grown at T,=5fl0°C on CaF2(2 M L ) / S i ( I I I ) and SrF2(II) M L ) / S i ( I I 1 ) are shown by curves Ca) and (b) in fig. 6, respectively. The Sin" ~ line position evidences the coherence of the former SL with sdicon substrate. However, the PL spectrum of the latter SL, grown on fhc inlt, rmcdiate SrF: layer, has an unusual shape with two peaks (curve b). The afterglow spectrum presented in the figure hy the dashed curve allows one to separate the component with long decay time. This component is obviously due to the forbidden 5D. ~ 7F I transition, The spectrum with a short wave peak, having a short decay time, can be naturnlly associated with the 4fsgd 4f* transition in CaF: . One can find by its energy position that the tensile strain in CaF 2 layers in the SL exceeds 2%. The spectrum shape observed in fig. 611 =ould be explained by the inverse order of 4f55d and 5Dl~(4f*) levels in these strongly strained fluorite layers. Because of its lower energy position the forbidden transition from SDD level could become observable. The intensity of this new line also may be influenced by the vicinity of C a F , / S r F 2 interfaces and dislocations which should be present in the SLs grown on SrF2/Si(III}. Curve (c) in fig. 6 shows the PL spectrum of a C a F z : S m layer grown on SrF2(10 M L ) / S i ( l l l ) . The latter has 30 am thickness as the total thickness of SL and with PL spectrum is presented by curve (bk Comparing these two spectra one can come to the conclusion that the presence of S r F 2 monoIayers in this SL results in a strain increase in C a F 2 layers by ~ 0.8%, d o s e to the expected value in coherent SL with such a ratio (10 : I) of CaF, and SrF, thicknesses. In fig. 7 PL spectra of Sm ~÷ ions in the zerophonon region of the ~D 0 ~ 7F I transition in epitaxial S r F 2 layers, 120 and 6 nm thick - grown at 701~°C, and of bulk crystal are shown by curves (at, (b) and (c), respectively. Taking into consideration that a shift of the emission line on strain in SrF, is about 25 times smaller than that in CaF~ crystal [12] one can deduce that the films have a tensile strain of about 1%. This value can be caused by the residual thermal stress induced by
N.S. Sokolor e! aL / Fluodde luyvr~ a~ld ~uperlattices grown oJi Si( l 111
la
o
Pl ¢
. . . . . . .
d
'
"
'
"
'7 " 6'
",rim
Fig. 7. PL spectra (if fluoride/Si(I I I ) heterostruetures includills SrFz:Sm: ' : (al 121)nrn SrF 2, (b) fl nfa SrF:, (ct SrF~ st(isle crystal, (d) CaF2tlU MLt/SrFzlI0 ML)/CaF2tlO ML). (c) CaFz(10 MLI/SrF2(3 MLI/CaF_,( Ill MLL Larger fluoride t h e r m a l expansiovt c o m p a r e d with that of silicon. T h e decay time of the luminescence in the spectra shown by curves (at, (b) and (c) was 9, 3 and 13 ms, respectively. A w e a k d e p e n d e n c e of the emission line position on strain in SrF 2 a p p e a r e d to be favorable for detection o f Sin -'÷ ions in t h e films, as thin as several monolayers. P L spectra o f two CaF2(1O M L ) / S r F 2 : S m / C a F z ( I0 ML)/Si(,I 1 I) structures grown at 100°C a r e presented in fig. 7, with SrF, :Sin layer thicknesses of I0 (curve d) and 3 M L (curve e). T h e short wave line in the latter curve could be associated with Sm z+ ions located n e a r C a F J S r F 2 interfaces.
4.
Conclusion
From the d a t a presented above it is clear that the breakdown of pseudomorphie growth mode can he d e t e c t e d in most cases by R H E E D intensity oscillation studies during CaF2 and SrF 2 growth. It allows us to find technological conditions of the fluoride pseudomorphic growth mode on S t ( l i l t as well as one fluoride on t h e other. Based on these d a t a the first strained cohercnt C a F 2 / S r F 2 superlattices have been grown on SKill). It has also been d e m o n s t r a t e d that Sm 2+ ions could be used as sensitive photolumincsccnt probes, which can be applied to evaluate crystalline quality and to m e a s u r e elastic strain in very thin CAP', layers. Usually it is very difficult
425
or even impossible to obtain such d a t a by the conventional m e a s u r e m e n t s with X-ray diffractometry or Rutherford b a e k s c a t t e r i n g / g h a n n e l ins o f H e + ions. The analys~s of Sm-'+-ion P L spectra in [ S r F 2 / C a F z : Sm] superlattices, carried o u t in this paper, showed that highly strained and coherent layers and superlattices could be grown by m e a n s of molecular beam epitaxy. Insignificant influence of strain on the Sm z* emission wa,~elength in SrF 2 la~ers wa~ iavorable to d e t e c t the ions in the fluoride layecs as thin as three ntonolaygrs.
Acknowledgements The authors are indebted to E.L. Ivchcnko, A.A. Kapl,Janskii and S.V. N o v i k ~ for a useful discussion.
References [1] LJ. Schov.raller and R.W. Fathauer. J. Vac. Sci. Techno]. A 4 { 1986) IG2h.
[2] H. Ishiwara. "['. Asano. K. Tstttsui. It.C. Lee and S. Furukawa. Mater. Res. Suc. Prec. In2 (1988) 343. [3] S.V. Novikov. N.S. Sokohlv and N.L Yakovlev, 5or. Tezh. Phys. Lea. 13 (I987)fi03: S.V. Novikuv. N.S. Sokllllw arid N i . Yakov[ev. Ext, Ahstr. 6th Eur, Conf. Molecular Beam Epita).2~and Related Growth Methods, Tamper¢, 1991. p. Fp$. [4] T. Waho and H. SaekL Jpn. J. Appl. Phys. 30 ( 1~)91) 221. [5l tL lmamoto. F. Sate. K. tmanaka and M. Shimura, Appl. Phys. Lea. 5511989) 115. [Sl C.II. Atldei'~n. in; C~stals with 1he Ftuodte Structure. Ed W. Hayes (Clarendon. Oxford. 1974) eh. 5. p. 281. [7] N.S. Sokolov. E. ViGil, S.V. G~tstev. S.V. Novikov and N.L. Yakovlev, Soy, Phys. Solid State 31 11989) 216. [8] N.S. Soknklv, N.L. Yakovlcv and J. Aline(dr, Solid State Comnmn. 76 11991})883. [9] S.V. Gastev. S.V. Nc*vikov, M.S. Sokolov and N.L Yakcwlev. S~'. Tech. Phys. LeU. I3 11987) 4/11. [ln] P. Chen, J.Y. Kim, A. Madhuhar and N.M. Clio. J. Vac. Sci. Technol. B 4 (1986) 891). [11] M.A. Olmstead, R.I.G. UhrherG. R.D. Briagans and R.Z. gachrach. Phys. Rev. B 35 (1987) 7526. [12] #.A. Kaplyanskii and A.K. Przevuskii, Opt. Spektmsk. 2a 11966) I1145: I,V. Ignat'ev arid V,V, Ovsyankln, Opt, Spektrosk, 49 ( 198111538.
aphid surface science
Fluoride layers and superlattices grown by MBE on Si( 111): dynamic R H E E D and Sm 2+ photoluminescence studies N.S. Sokelov, LC.
Alvarez
t and
N . L ¥akovlev
A.E lath" Phy~h'tJ.7~'chnical hJ.sttlt~re. ~t .P,,ter~hw;~ IC~4021, R.x~ht
Received 20 No~ember 19gl: accepted for puhiicatkm 311Jantulr~ lets2
Tile oscillatiuns of ruflectil~n high-energy, cleclron ditfractitln inlen,;ityduring CaF2 and SrF. epilaxial growth on the 1111) suUuce of silicoll antl the fluorides II~w¢been investigated. Fronl these slndies the critical thickries::of the fluoride pseudomorphie grt~wth mode ~as measured fiw different gr~lwfllcondititms. The~e data alhlwcd us It~ grtlw sta}~l/~l~'strained ~herent fluoride layers ~nd superlulfices. Sludies of Sm-~' ion photolumincscence in Ihes¢ fluoride structulcs at liquid-helium Icmperature have been shown to be useful flJr characterizationof Ihe cl3'~tallinequalityof the films and ftlFthe nleaSuremcnl tit ehlMicstrain. I ligh quantum yic(d of the luminescence enabled us to explore doped layers as thin as several nltmnlayers.
1. lntrnd~tctinn Epitaxial CaF~. and SrFe layers on semiconductors attract the attention of researchels [1,2] owing to various potential applications of these hcterostructurcs in opto- and microelectronics including creation of 3D integral circuits, semiconductor-on-insulator structures and MISFETs on GaAs. There exist unique opportunities for fundamental research of the interface between two crystals with different type~ of bonding: the ionic bonding in fluorides and the covalent one in semiconductors. The successful application of molecular beam epitaxy ( M B E ) for the fluoride layer growth revealed interesting possibilities for research in the growth processes and creation of new structures for basic research in the low-dimension physics. It is well known that reflection high-energy electron diffraction ( R H E E D ) intensity oscillation measurements during M B E growth are informative lc,r ,studiEs of growth processes. ]t was demonstrated that this powerful technique could also be applied to the investigation of fluoride growth [3]. i perman~nt addles~: hlslautc tff Materialsanti Reagentsfilr Electronics. Physics Faculty. ]~lvanll Universil%Cuha.
As the OaAs lattice constant lies between those of CaF., and SrF z, the (Ca,Sr)F 2 solid solutions were used [4] to grow lattice-matched GaAs-fluoride heterostruetures. It would be attractive in some cases to use strained CaF2-SrF 2 superlattiees (SLs) for the same purpose. However because of the large lattice mismatch in this fluoride couple (h.2%) until recently it has been ullelear whether it is possible to grow a coherent fluoride SL. Taking into account the technological difficulties of the preparation of the clean G a A s ( l l l ) B surface [5], it seems reasonable to study the growth and properties of such SLs on a more technologically convenient Si(! l l) surface. Optical properties of rare-earth ions in bulk fluoride single crystals were the subject of the thorough fundamcutal research [6]. Low-temperature photolumineseence of Eu 2+ and Sm z+ ions in epitaxial CaF~ and SrF 2 layers was recently used to characterize the crystalline quality and to measure the strain in these films [7,8]. In the next part of the paper we present the results of dynamic R H E E D studies during M B E growth of CaF~, SrF 2 strained layers and fluoride SLs on Si(ll 1). The third part of the paper deals with Sm ~* photoluminesccnee studies of these structures.
nlhg-4332/q"/$1)5.0a ( 1992- Elsevier Science Puhlisher~ B.V. All rights reserved
422
N.S, Svkolor el aL / Fluoride lay('~ and sltperlaences grown or1 Sifl l l )
2. MBE g r e ' ~ h and dynamic F.~EED studies ~:pitaxial C a F 2 and SrFg layers on Si(t I [ ) were grown in an M B E system for research with a 20 L deposition chamber [9]. T h e system was equipped by 5 molecular cells with pure or Sin-doped (0.10.3 tool%) C a F : and S r F 2 crystals. T h e rate of fluoride deposition was calibrated by measuring the period of R H E E D intensity oscillations [3] and typically was 1-5 n m / m i n . The electron energy in the R H E E D gun was 15 keV, the incidence angle of the electron beam on the layer surface being about 40'. This angle corresponds to the so-called off-Bragg condition, which gives the maximum sensitivity of lira spot (specular beam) intensity to the densily of munolayer steps [10]. T h e intensity of the spot in the pattern was measured using a photomultiplier and a D C amplifier. Jn the majority of the experiments described in this paper the R H E E D patterns were streaked with bright specular spots. This indicated the flatness of the fluoride layer surface on the atomic scale. Fig. 1 shows 00-spot inte,lsity oscination~ during CaF 2 growth on Si(l I 1) at different substrate temperatures. O n both curves one can see an oscillating periodic component with a period •~vhleh e o r r c s p o n ' l s to the growth of one triple F - C a - F layer 0,315 nm thick. Such l ~ ( t ) dependence convincingly proves the two-dimensional layer-by-layer fluorite growth mode on Si(111). The complicated shape of R H E E D intensity os-
,,~ro i
•
....... -c °~, °
"
1
i
~.sec
Fig. I. Depel~dencesot spt;cularbeam illlellsit~,'in the RHEED pattern versu,~ ~he time of CaF2 deposition on Si(lll) at 600°C (a) ~nd 65ff'C (b). Hereafter BD is the breakdown of pseutlume,phic growth mode.
~.
°'l
to. /a0
o
eo
12o
~BO ~.~ec
Fig, 2. RHEED intensity oscillations during SrF~ growth, (a) im St(I I I I lit (}lgl°c,(b) on CItF2/~;i( I 11) structure ut 100°t".
cillations at early stages of C a F 2 growth is probably connected with the influence of the S K i l l ) (7 × 7) sunerstrueture and bonding at the interface i l l ] on the interference of the electrons, scattered by the fluorite layer and the silicon substrate. Slow change of the average intensity at the later stages of the fluoride growth could originate in the variations of the step density on the fluorite layer surface at different growth modes. T h e decrease of the average specular beam intensity after the growth of 4 - 5 monolayers (fig. lb) is likely to be due to the breakdown of the pseudomorphic growth mode and the roughening of the surface. The increase of the average intensity after ~ l 0 monolayer deposition could be caused by the subsequent flattening of the growing surface. It is worthwhile to rtote that th~ lattice mismatch for CaFz-Si couple is 1.7% at 600°C. In fig. 2a the l~m(t) dependence during S r F z growth on S i ( l l l ) at 600°C is presented. One can see that the shape of this dependence is quite similar to the l ~ ( t ) dependence shown in fig. lb. The main difference is that in the SrF2 case the breakdown of the pseudomorphic growth occurs after ~ 3 monolayers. Taking into account the large value of lattice mismatch ( ~ 7%) in the S r F z / S i heteroeouple, the small critical thickness of the SrF 2 coherent growth on Si(II1) is not unexpected. T h e observed 2D growth mode at the large mismatch can be explained by considerably less fluoride ( I l l ) surface free enerlff compared with this value 0¢ S i ( ! l l ) surfa¢,: [1!.
423
N,S Sokolor et aL / Fhmridt hlyers and supedattice.v grolwl on Si(I I I) s ~ ~*
"Z
levels
in
,'
CaF 2
b i
708.5
~oo
5oe
ooo
Specular beam intensity behavior in the R H E E D pattern during S r F , / C a F 2 / S i ( I 1 l) heterostructure growth is shown in fig. 2b. One can see that the first two SrF2 monolayers on the pseudomorphie C a F 2 layer on S i ( l l l ) also grow coherently, the breakdown of the pseudom0rphie growth mode occurring during the grt,wth of the third S r F 2 monolayer. Taking into consideration the above results we have grown strained fluoride Sl.,s on Sit111). Fig. 3 shows the ltd,(t) dependences during [SrF2(2 M L ) / C a F 2:Sm(5 ML)] 4 (a) and [SrF2(I M L ) / CaF2:Sm(10 ML)] w (b) fluoride SLs growth. The damping of the oscillations in fig. 3a is probably due to the coarsening of the surface because of the low growth temperature of the SL. O n e can see that ltm(t) behavior during SL growth at 500°C is periodic (fig. 3b), which evidences for reproducibility of the growth conditions in each period of the structure.
fluoride layers and
of Sm 2+ ions SLs
98
in S r F z
V I I
rim
i
5 nm
---L_
Fig. 4. Schematic diagram *If excited Sm"~" ion levcl~ in CaF, pad SrF2, The wavelengUlof radiation is shown at the arrows indicating electron transitions.
sint;le crystals the lowest energy level important for optical transitions is the 5D 0 state, belonging to th~ same 4f" shell as the ground 7F 1 state. Deca,' time of the luminescence which is duo to these forbidden intraeonflguration transitions is usually about 10 2 s in bulk crystals at liquidhelium temperature. Piezo.-.peetroseopie studies [12] showed that the energy of 4f t' shell levels depends but little up strain in the fluoride crystals. Meanwhile, for the levels of 4¢55d mixed electronic configuration a well pronounced linear dependence of their energy on uniaxial stress along the [111] direction has been o~served. T h e relation between the wavelength shift AA of the zero phonon emission line in the strained fluorite layer (3.) from that in unstressed bull,: crystal (3.n) on the strain in the plane of the interface E was found to he AA,m = 3. - 3-o ~ - 6 × 10zcll 8 . This relation was used to put the strain scale in figs. 5 and 6.
2.0
I ~L, 3. Phntoluminescence
,"
[4t e] r~"
'F,
Fig, 3. Behaviour ot the RHEED specular beam intensils during the growth of SrF,/CaF z superlattices: (a) [S~Fz(2 ML)/CaF2:Sm(5 MLI]~/CaF~(2 ML)/Si(III). and Ib) [SrF:(I ML)/CuF2:Sm{10 MLJ]In/SrF_~llaML)/Si(I It), The brackets correspond to die intervalswhen a source is open.
m
/'
1.5
c
1.0
Q.S
b
0.0
d
-0.5 C,
o
in thin
The structure of Sm 2+ ion energy levels in Ca and Sr fluorides is schematically shown in fig. 4. The most intense photoluminescence (PL) of these ions in bulk CaF 2 crystals originates from the allowed optical transitions 4fsSd ~ 4f ¢'. The decay time of the luminescence is ~ 10 -~' s at liquid-helium temperature. In the S r F 2 : S m 2+
700
704
n
Fig. S. Photolorninescence spectra (at 1.7 K ) of Sm2--doped CaF~ layers on Si(I I I) grown at different ~onditions: (a) 6 n m
at 560°C, (b) 12 nm at 7U~J'C,(c) 20 nm at 770°C with rapid annealing a! 950~C ~hr 30 s, (d) spectrum of the CaF,:Sm single crystal.
N.X Sokokn' q'r aL / Huoruh, layers and superlauk'e~ grown vn Sill I l J 2.0
pt,
69g
1.5
1.0
b
c
698
70
05
0.0
d
-O.g gm,~
a
Fig, 6. PL spectra of ISrF:( I ML)/CaF:.Sm{It) Ml,~h, supeJi~nlices g¢own al 5011°C. (at on C a F : / S i ( l l l k (bt on SrF:/Sill lit lille dashed cuwc is the luminc~:nce delayed o~er I ms alter tile ued of escitalkm), (¢) CaF,:Sm 3() nm layer on SrF~(la ML)/Si( I I 1). td ) speclrum nf Ihe CaF: : Sm single cr~stal.
T o excite the PL of Sm -'~ ions in fluoride structures a H e - N e laser (A = 632.8 nm) was used. The samples were put in a p u m p e d ( F - 1.7 K) liquid-helium glass cryostat. The PL spectra were measured by a double-grating monochromator, a cooled photomultiplier and a photon counting system. T h e afterglow spectra were obtained with a mechanical chopper at a delay over 1 ms after the end of every excitation impulse. Curves Ca), (b) and (e) in fig. 5 show Sin: + PL spectra in thin epitaxia[ CaF: layers grown on SiC111 ) at diffcrent conditions. Curve (d) presents PL spectrum of the bulk CaF, :Sin -'÷ crystal in the region of zero phonon llne of the 4fS5d ~ 4f* transition. T h e fluorite and silicon lattice mismatch at liquid-helium temperature is about 0.3%. thus in coherent heterostructures the fluorite layer should have a compressive strain of - 0 . 3 % . Such s~:rain as well as a relatively narrow emission line presented by curve (at shows that the layer grown at 560':C is coherent with the ~ilicon substrate [8]. The leusilc strain and the broad emission line (curve b) in the layer grown at 700°C are due 1o the breakdown of pseudomorphic growth mode, accommodation of mismatch stress and a larger value of the CaF~ thermal expansion coefficient compared with Si [7]. Curve (e) in this figure shows that in the 20 nm fluorite layer the average value ef tensile strain can reach 1.7% after rapid thermal annealing •~950°C. 30 s) in situ. It considerably exceeds
the largest strain values which can be obtained in bulk crystals in uniaxial stress experiments [12]. PL spectra of [SrF.,(I M L ) / C a F . , : S m ( I t l ML)]j. superlattiees grown at T,=5fl0°C on CaF2(2 M L ) / S i ( I I I ) and SrF2(II) M L ) / S i ( I I 1 ) are shown by curves Ca) and (b) in fig. 6, respectively. The Sin" ~ line position evidences the coherence of the former SL with sdicon substrate. However, the PL spectrum of the latter SL, grown on fhc inlt, rmcdiate SrF: layer, has an unusual shape with two peaks (curve b). The afterglow spectrum presented in the figure hy the dashed curve allows one to separate the component with long decay time. This component is obviously due to the forbidden 5D. ~ 7F I transition, The spectrum with a short wave peak, having a short decay time, can be naturnlly associated with the 4fsgd 4f* transition in CaF: . One can find by its energy position that the tensile strain in CaF 2 layers in the SL exceeds 2%. The spectrum shape observed in fig. 611 =ould be explained by the inverse order of 4f55d and 5Dl~(4f*) levels in these strongly strained fluorite layers. Because of its lower energy position the forbidden transition from SDD level could become observable. The intensity of this new line also may be influenced by the vicinity of C a F , / S r F 2 interfaces and dislocations which should be present in the SLs grown on SrF2/Si(III}. Curve (c) in fig. 6 shows the PL spectrum of a C a F z : S m layer grown on SrF2(10 M L ) / S i ( l l l ) . The latter has 30 am thickness as the total thickness of SL and with PL spectrum is presented by curve (bk Comparing these two spectra one can come to the conclusion that the presence of S r F 2 monoIayers in this SL results in a strain increase in C a F 2 layers by ~ 0.8%, d o s e to the expected value in coherent SL with such a ratio (10 : I) of CaF, and SrF, thicknesses. In fig. 7 PL spectra of Sm ~÷ ions in the zerophonon region of the ~D 0 ~ 7F I transition in epitaxial S r F 2 layers, 120 and 6 nm thick - grown at 701~°C, and of bulk crystal are shown by curves (at, (b) and (c), respectively. Taking into consideration that a shift of the emission line on strain in SrF, is about 25 times smaller than that in CaF~ crystal [12] one can deduce that the films have a tensile strain of about 1%. This value can be caused by the residual thermal stress induced by
N.S. Sokolor e! aL / Fluodde luyvr~ a~ld ~uperlattices grown oJi Si( l 111
la
o
Pl ¢
. . . . . . .
d
'
"
'
"
'7 " 6'
",rim
Fig. 7. PL spectra (if fluoride/Si(I I I ) heterostruetures includills SrFz:Sm: ' : (al 121)nrn SrF 2, (b) fl nfa SrF:, (ct SrF~ st(isle crystal, (d) CaF2tlU MLt/SrFzlI0 ML)/CaF2tlO ML). (c) CaFz(10 MLI/SrF2(3 MLI/CaF_,( Ill MLL Larger fluoride t h e r m a l expansiovt c o m p a r e d with that of silicon. T h e decay time of the luminescence in the spectra shown by curves (at, (b) and (c) was 9, 3 and 13 ms, respectively. A w e a k d e p e n d e n c e of the emission line position on strain in SrF 2 a p p e a r e d to be favorable for detection o f Sin -'÷ ions in t h e films, as thin as several monolayers. P L spectra o f two CaF2(1O M L ) / S r F 2 : S m / C a F z ( I0 ML)/Si(,I 1 I) structures grown at 100°C a r e presented in fig. 7, with SrF, :Sin layer thicknesses of I0 (curve d) and 3 M L (curve e). T h e short wave line in the latter curve could be associated with Sm z+ ions located n e a r C a F J S r F 2 interfaces.
4.
Conclusion
From the d a t a presented above it is clear that the breakdown of pseudomorphie growth mode can he d e t e c t e d in most cases by R H E E D intensity oscillation studies during CaF2 and SrF 2 growth. It allows us to find technological conditions of the fluoride pseudomorphic growth mode on S t ( l i l t as well as one fluoride on t h e other. Based on these d a t a the first strained cohercnt C a F 2 / S r F 2 superlattices have been grown on SKill). It has also been d e m o n s t r a t e d that Sm 2+ ions could be used as sensitive photolumincsccnt probes, which can be applied to evaluate crystalline quality and to m e a s u r e elastic strain in very thin CAP', layers. Usually it is very difficult
425
or even impossible to obtain such d a t a by the conventional m e a s u r e m e n t s with X-ray diffractometry or Rutherford b a e k s c a t t e r i n g / g h a n n e l ins o f H e + ions. The analys~s of Sm-'+-ion P L spectra in [ S r F 2 / C a F z : Sm] superlattices, carried o u t in this paper, showed that highly strained and coherent layers and superlattices could be grown by m e a n s of molecular beam epitaxy. Insignificant influence of strain on the Sm z* emission wa,~elength in SrF 2 la~ers wa~ iavorable to d e t e c t the ions in the fluoride layecs as thin as three ntonolaygrs.
Acknowledgements The authors are indebted to E.L. Ivchcnko, A.A. Kapl,Janskii and S.V. N o v i k ~ for a useful discussion.
References [1] LJ. Schov.raller and R.W. Fathauer. J. Vac. Sci. Techno]. A 4 { 1986) IG2h.
[2] H. Ishiwara. "['. Asano. K. Tstttsui. It.C. Lee and S. Furukawa. Mater. Res. Suc. Prec. In2 (1988) 343. [3] S.V. Novikov. N.S. Sokohlv and N.L Yakovlev, 5or. Tezh. Phys. Lea. 13 (I987)fi03: S.V. Novikuv. N.S. Sokllllw arid N i . Yakov[ev. Ext, Ahstr. 6th Eur, Conf. Molecular Beam Epita).2~and Related Growth Methods, Tamper¢, 1991. p. Fp$. [4] T. Waho and H. SaekL Jpn. J. Appl. Phys. 30 ( 1~)91) 221. [5l tL lmamoto. F. Sate. K. tmanaka and M. Shimura, Appl. Phys. Lea. 5511989) 115. [Sl C.II. Atldei'~n. in; C~stals with 1he Ftuodte Structure. Ed W. Hayes (Clarendon. Oxford. 1974) eh. 5. p. 281. [7] N.S. Sokolov. E. ViGil, S.V. G~tstev. S.V. Novikov and N.L. Yakovlev, Soy, Phys. Solid State 31 11989) 216. [8] N.S. Soknklv, N.L. Yakovlcv and J. Aline(dr, Solid State Comnmn. 76 11991})883. [9] S.V. Gastev. S.V. Nc*vikov, M.S. Sokolov and N.L Yakcwlev. S~'. Tech. Phys. LeU. I3 11987) 4/11. [ln] P. Chen, J.Y. Kim, A. Madhuhar and N.M. Clio. J. Vac. Sci. Technol. B 4 (1986) 891). [11] M.A. Olmstead, R.I.G. UhrherG. R.D. Briagans and R.Z. gachrach. Phys. Rev. B 35 (1987) 7526. [12] #.A. Kaplyanskii and A.K. Przevuskii, Opt. Spektmsk. 2a 11966) I1145: I,V. Ignat'ev arid V,V, Ovsyankln, Opt, Spektrosk, 49 ( 198111538.