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SiO2 interface-amorphous silicon revisited
Applications of Surface Science 22/23 (1985) 879~890 North-Holland, Amsterdam
879
NATIVE DEFECTS AT THE Si/SiO 2 I N T E R F A C E - AMORPHOUS SILICON REVISITED D.K. B I E G E L S E N , N.M. J O H N S O N and M. S T U T Z M A N N Xerox Palo Alto Research Center, 3333 Coyote Hill Road, Palo Alto, California 94304, USA
and
E.H. P O I N D E X T E R and P.J. C A P L A N US Army Electronics Technology and Devices Laboratory, Fort Monmouth, New Jersey 07703, USA
Received 27 August 1984; accepted for publication 31 October 1984
We review here work which demonstrates that silicon dangling bonds, Si3, are the predominant, electrically active deep states associated with the clean crystalline silicon/amorphous SiO2 interface. Si3 exists in three charge states in the silicon band gap with E+,0- Ev + 0.3 eV and E0. = Ev+ 0.9eV, where E represents a demarcation level between charge states. We discuss the structural and electronic characteristics of Si3 at the Si/SiO2 interface, including degree of charge localization, effective correlation energies, role of hydrogen passivation, etc. We argue, from the strongly analogous behavior in amorphous silicon, that the electronic density of states in the gap is dominated by the characteristic effects of disorder in covalently bonded semiconductors. The states consist of two topologically distinct entities: distorted, fully-bonded network configurations giving rise to shallow silicon band tails, and three-fold coordinated, amphoteric silicon defects.
1. Introduction T h e p i ct u re e m e r g i n g f r o m most m e a s u r e m e n t s of the i n t er f ace b e t w e e n crystalline silicon and glassy silicon d i o x i d e s e e m s to be o n e of nearly ideal b o n d i n g . T h a t is, the silicon crystal t e r m i n a t e s a b r u p t l y [1] with only a l i m i t ed region of m o d e r a t e strain [2]. T h e silica n e t w o r k is almost p er f ect l y m e r g e d with the crystal and m i s m a t c h of the a v e r a g e silicon-silicon spacing is a c c o m m o d a t e d in the flexible S iO 2 n e t w o r k . This is of c o u r s e technologically f o r t u i t o u s b e c a u s e the i n t e r f a c e naturally tends to be stable and electrically inactive. A perfectly b o n d e d , u n s t r a i n e d S i / S i O 2 i n t e r f a c e netw o r k w o u l d h a v e no states in the silicon b a n d gap. A s we will try to d e m o n s t r a t e below, t h e e l e c t r o n i c p r o p e r t i e s of the i n t e r f a c e are d o m i n a t e d by the residual d i s o r d e r and manifest p r o p e r t i e s c h a r a c t e r i s t i c of d i s o r d e r e d 0378-5963/85/$03.30 O E l s e v i e r S c i e n c e Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing D iv is i o n )
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D.K. Biegelsen et al. / Native dejects at Si/NiO, inter/fiwe
covalently-bonded semiconductors. The states introduced into the silicon gap arise from strained fully-coordinated bonding configurations and isolated, topologically distinct three-bonded silicon atoms, Si:~.
2. Experiment We review here recent experimental work concerning the electrical and structural properties of native defects of the pure, as-grown Si/SiO 2 interface. We will focus on the (111) surface of silicon because the defect density is highest and the defects have only one spatial orientation. Although conceptually simpler for the (11 I) interface, the results are nonetheless true for interface states on the (100) and (110) surfaces. 2. 1. t ~ S R c h a r a c t e r i z a t i o n
Caplan et al. [3] showed from electron spin resonance (ESR) studies that the prominent paramagnetic center, Pb [4]. occurs at the interface, is cylindrically symmetric and aligned along [111] directions. For (111) silicon these defects are directed only perpendicular to the interface, i.e. pointing into the oxide. These defects are neutral dangling bonds, Si~, where i) denotes the charge state. The dangling bonds are readily passivated if atomic hydrogen is present [5] so care must be exercised in sample preparation to avoid hydrogenation. The shifts in the principal gyromagnetic (g-tensor) c o m p o n e n t s from the free electron value of g,. 2.0023 are to first order a g - ~ - g,. -
a~ ) 2 ( L ~ . ~ / / ( E ~
E~),
(1)
where the summation is over all band states, b, asj is the atomic spin-orbit coupling factor, '(Ld,b) iS the angular m o m e n t u m matrix element for transitions between the defect level at energy E'd and band states at energy F~,. By symmetry, ag, i~ 0 when the applied magnetic field H , is parallel to the defect cylinder axis. :lg, is expected to be positive for an sp -~ hybrid stale in silicon. This follows from the fact that the top of the valence band is strongly p-like and the bottom of the conduction band s-like [6]. s-like states have zero angular m o m e n t u m ((Ld.b)--0). E d /'7_~, is negative for coupling Io valence band states, so ,Ag,± is positive. Brower [7] has measured the hyperfine interaction between the Si'i electron and 2~Si nuclear spins. With a simple molecular orbital model of Sii~ he found that the paramagnetic electron wave function dJ(r) is - 8 0 % localized on the Si atom and the non-bonding hybrid is ~--12"/,, s-like and 88% p-like. The hyperfinc split lines arc broadened from the unshiftcd linewidth. This is most likely due to variations in the electron wavefunction arising from a distribution of local environmental parameters. The unshifted
D.K. Biegelsen et al. / Native defects at Si/Si02 interface
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line is also inhomogeneously broadened. The natural linewidth of the line, l/yT~ for isolated spins, is much less than the observed linewidth. The broadening is also greater than a width corresponding to unresolved hyperfine interactions (Resolved hyperfine structure will be discussed below.) This can be seen empirically from the magnetic field dependence of the linewidth, AH. (The hyperfine broadening is independent of field.) For example, Brower finds, at H 0 - 7100 G, A H is slightly less than twice that of Caplan et al. [3] at - 3 2 0 0 G. Furthermore, the linewidth is approximately proportional to zig. Both of these effects are indicative of a relatively broad distribution of g-values arising from local environmental fluctuations and concomitant modifications of the Si ° wavefunction. Another useful finding that Brower [8] has made is that the dangling bond defects are spatially anticorrelated. Dipolar interactions between nearest D~ centers should be observable for a random distribution out to third nearest neighbor lateral separations. Brower has found that these lines are not present. Thus there are far fewer near pairs than would exist in a random distribution, and certainly no pairing. In an MOS structure the occupation of interface states can be changed by varying the band bending-induced charge accumulation or depletion [9]. Corona charging of an unmetallized, oxidized sample can be used to the same effect [10,11]. In fig. 1 we show results for the change in ESR strength with external bias voltage in a p-type sample [12]. Flat band, determined from low frequency capacitance-voltage ( C - V ) measurements, occurs at -30V. The signal height drops for high values of forward and reverse biasing indicating a paramagnetic to diamagnetic transition at high bias levels. The most straightforward interpretation is that we are observing three charge states of the Si 3 site with Si 3 and Si~ being spin-paired and unoccupied, respectively, and Si ° being paramagnetic. The energy difference between the I
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D.K. Biege&en et al. I Native defects at Si/Si02 interface
transition levels Si~*--~Si!~ and SiI~-,Si~ is the effective correlation energy, Uctr. Uca is the extra energy required to place a second electron in the same Si 3 orbital, and is principally the sum of the Coulombic repulsion term, /_./¢. and any elastic energy reduction due to lattice relaxation, U w The conversion from applied bias to interface potential is derived using the C - V measurements. From the energies at which the ESR drops to half height, we ascertain that Ue~-0.6-+0.1 eV. This result has been verified in gammairradiated material by Lenahan and Dressendorfer [11]. It is interesting to note that at high bias levels majority or minority carriers accumulate at the interface. The spin lattice relaxation time, T r, is reduced by approximately an order of magnitude due to spin interactions with the mobile carriers. However, in the voltage range of the measurements above, T~ is still far too long to produce lifetime broadening (and apparent signal height reduction) of the Si ° line. 2.2. E l e c t r i c a l c h a r a c t e r i z a t i o n
There have been many reports of a hole trap at 0.3-4).4eV above the valence band edge. The integrated density depends strongly on sample processing. Our ESR results, implying an amphoteric defect, compelled us to search for a second transition of the dangling bond in C - V and DLTS measurements. Fig. 2 shows results of current transient spectroscopy on identically processed p- and n-type silicon samples [12]. (All our samples consisted of epitaxial silicon layers on doped s u b s t r a t e s - f o r C - V - o r undoped s u b s t r a t e s - f o r ESR. Thermal oxides were grown in dry 02 at 1000°C ~-""
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constant capture cross sections.
D.K. Biegelsen et al. / Native defects at Si/Si02 interface
883
1.2
MOS C a p a c i t o r 1.0 -- Si:(111). P-Type Si02:194 nm
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Fig. 3. Interface potential determined from low frequency (10Hz) capacitance-voltage measurements indicating the two dangling bond peaks in the same sample.
to thicknesses of - 150 nm.) As will be discussed below, both peaks anneal in a similar fashion. They have approximately the same integrated density which is furthermore (within an absolute experimental uncertainty of - 2 ) the same as the maximum ESR spin density. In fig. 3 we show the interface potential derived from low frequency C - V measurements on a p-type sample [13]. The interface density of states is essentially the derivative of the curve. This can be seen intuitively because the band bending with gate voltage is least in regions of high densities of states. In particular, we see here that both levels can be observed in a single sample.
2.3. Passivation Removal of electronic states from the gap by chemical bonding is quite informative, as well as technologically useful. We have demonstrated, using atomic deuterium diffusions and SIMS analysis [5], that the interface acts as a deuterium sink until the interface states (dangling bonds) are saturated. In fig. 4 we show vacuum annealing behavior of spin density and hole trap peak density (measured by quasistatic C - V ) in identically processed aluminum gate n-type silicon epitaxial MOS samples. (The passivation is hypothesized to involve atomic hydrogen introduced into the oxide from the metal-oxide interface during processing [12].) The common annealing behavior is strong evidence that the dangling bond center is equivalent to the hole trap at E - E V+ 0.3 eV. Annealing in p-type samples produces similar results. The electron trap at E - E 0 - 0 . 2 5 e V also anneals in a similar fashion. However, because the peak is close to the conduction band tail and because the conduction band tail states, unlike the valence band tail states, are reduced far less by hydrogenation than the defect states, the upper peak coexists with a large background. This leads to relatively large uncertainties in the defect peak deconvolution. We also note here that the above
884
D.K. Biegelsen et al. / Native defects at Si/Si02 interjku'e
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Fig. 4. Normalized peak heights for ESR and E, ~ 0.3 cV ( ' - V peak :filer 15 rain anncab,.
c h a r a c t e r i s t i c s s e e m to explain the fact that o t h e r g r o u p s have not o b s e r v e d this b a n d of d e e p levels. A high i n t e r f a c e s t a t e d e n s i t y - n o t a c c i d e n l a l l y o r i n t e n t i o n a l l y p a s s i w ~ t e d - must be a c h i e v e d to r e s o l v e the b r o a d b a n d of t r a p states from the c o n d u c t i o n b a n d tail states.
3.
Discussion
T h e e l e c t r o n i c states in the silicon b a n d g a p a s s o c i a l e d with the p u r e t h e r m a l l y o x i d i z e d silicon i n t e r f a c e are thus easily c h a r a c t e r i z e d . A n a r r o w d i s t r i b u t i o n of states tail into the g a p from each b a n d edge. A single e l e c t r i c a l l y active defect, the silicon d a n g l i n g b o n d , d o m i n a t e s the d e e p g a p levels, it has its two e n e r g y levels in the gap a p p r o x i m a t e l y s y m m e t r i c a l l y d i s p o s e d r e l a t i v e to m i d g a p . T h e effective c o r r e l a t i o n e n e r g y is positive and e q u a l to ~ E J 2 . A t o m i c h y d r o g e n r e a d i l y b o n d s to the d a n g l i n g o r b i t a l s and is also i n s e r t e d into the m o r e n u m e r o u s b a n d tail states. This shifts the states o u t of the g a p t r a n s f e r r i n g the b o n d i n g and a n t i - b o n d i n g - l i k e states d e e p into the b a n d s . D u r i n g the last ten y e a r s an e n o r m o u s a m o u n t has b c c n l e a r n e d a b o u t a n o t h e r d i s o r d e r e d silicon s y s t e m - a m o r p h o u s h y d r o g e n a t e d silicon, aSi : H. W e will discuss s o m e of the results here b e c a u s e we b c l i e v c that t h c r c are c o m m o n physical m e c h a n i s m s u n d e r l y i n g the c h a r a c t e r i s t i c a t t r i b u t c s of the two systems. W e think each a r e a can benefit from the c o m p a r i s o n . A s e x a m p l e s , the Si 3 c h a r a c t e r i z a t i o n can be best o b t a i n e d in the S i / S i O 2 system w h e r e the site a n i s o t r o p y is not spatially a v c r a g e d as in the a m o r p h o u s
D.K. Biegelsen et al. / Native defects at Si/Si02 interface
885
network. Conversely, information about the effects of hydrogen bonding on tail states, for example, is more readily obtained in bulk disordered materials. The electronic states in the gap of amorphous silicon, as for the Si/SiO 2 interface, are surprisingly easy to describe [13]. We show a simplified model in fig. 5. Tails of localized states falling in density approximately exponentially extend into the gap between the mobility edges. Again, dangling bonds are the predominant electrically active defect. Si3, with a positive correlation energy of 0.3 + 0.1 eV, has its two energy levels approximately symmetrically disposed about midgap. The upper Hubbard band has been characterized most definitively. It has a width - 50% larger than the corresponding states at the Si/SiO: interface. In both material systems in the absence of hydrogen the dangling bonds are isolated with similar average separations ( - 1 0 nm). The density of tail states and dangling bonds depends strongly on hydrogen concentration. Interestingly in passivating the dangling bonds, approximately one hundred times more hydrogen is bonded into the silicon network [15]. The excess hydrogen is thought to be bonded preferentially at the most strongly strained bonds. This is clearly shown in the Si/SiO 2, interface system [5] where again the amount of hydrogen bonded at the interface is - 1 0 0 times the dangling bond concentration, yet almost no hydrogen is inserted into the strongly bonded crystal silicon matrix of the substrate. It has also been shown in a-Si : H that hydrogenation removes the valence band tail states to a greater degree than the conduction band tail states [16]. The similarities of these effects in a - S i : H and Si/SiO 2 we think follow from the bond energetics of covalent, tetrahedrally coordinated semiconductors. The potential increases strongly with bond length variations so that strain energy is minimized by defect (dangling bond) creation, as opposed to strain distribution among many bonds [17]. Furthermore, dangling bonds
\ Dangling Bonds
20 Ue~ ~ 0.4eV
Ev
/
/
Ec
Fig. 5. Schematic density of states diagram for hydrogenated amorphous silicon. Ec and E~ denote the conduction and valence mobility edges respectively. The arrows denote electron spins and indicate spin-pairing in the doubly-occupied Si3 state.
D.K. Biegelsen et al. / Native defects at Si/Si02 interface
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should be separated (anticorrelated) because they form only where network strain exceeds a critical level. This simple c h e m i c a l - t o p o l o g i c a l picture readily explains the o v e r w h e l m i n g t e n d e n c y in these disordered systems to have a single kind of electronic d e f e c t - S i 3. All existing structural defects d e c o m p o s e into m o r e or less weakly b o n d e d (tail) states and dangling bonds. This separation is meaningful if, and only if, the electronic wavefunction on Si~ is strongly localized. W e note that point defects in crystalline silicon generally c o n f o r m to this description. J a h n - T e l l e r distortions lead to reconstruction and weak b o n d i n g and isolated, localized dangling b o n d s [6]. In the next paragraph we c o m p a r e the extent of Si ° localization in the two systems. In fig. 6 we show the E S R data from [7] for H 0 parallel (O = 0 °) through p e r p e n d i c u l a r (0 = 90 °) to the dangling b o n d axis. T h e two satellite lines arise from interactions with the 29Si nuclear spins in the m~ = -+ J2 states. The natural a b u n d a n c e of 29Si is 4.7% and the two hyperfine lines t o g e t h e r ( 1 1 1 ) Si/SiO 2 interface Pb spectrum
v =19.939 GHz (a) ~ = 0o
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(c)
]/
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//
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Fig. 6. Dangling bond ESR data of Brower [7] showing the unshifted ~Si central line and satellite peaks corresponding to neutral dangling bonds o n 29Si atoms with nuclear spins in M] - + ~state.
D.K. Biegelsen et al. / Native defects at SilSiO~ interface
887
contribute > 3% experimentally. From the width of the hyperfine peaks we can infer that there is a distribution in the degree of localization and/or the variation of hydbridization values of - 1 0 % . This is consistent with the notion of a strained silicon interracial region. In fig. 7a we show a calculated X-band powder pattern spectrum based on Brower's data. The powder pattern is an orientational average of the anisotropic lines which one might expect for a powdered crystal. We have used a 1 0 G environmental broadening for the 290.o 3t 3 hyperfine tensor, a 20 G smaller splitting (due to a slightly weaker localization or a more p-like character in a - S i : H ) and a Ag-dependent broadening for the 28$13 .0 central line, as observed in the Si/SiO 2 data. In fig. 7b we show the ESR spectrum for a-Si : H. The features
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MAGNETIC FIELD Fig. 7. (a) Powder pattern ESR signal calculated from the data of Brower [7]. The hyperfine shifts of [7] have been arbitrarily reduced by 20 G to obtain a better likeness to the a-Si : H data in (b). (b) ESR spectrum of undoped a-Si : H, Note that the asymmetry of the central line (arising from the Ag-rclated line broadening) and the hyperflne widths are simialr to those in (a).
888
I).K. Hiege&en et al. / Native defects at Si/Si02 interface
in the wings of the central line are remarkably close to the Si/SiO 2 values. (Studies on isotopically-enriched a-29Si : H are in progress.) From various observations in a - S i : H it can be inferred that the Si!~ electron is strongly localized on the atom. The hyperfine shift is proportional to the magnitude of the wavefunction at the nuclear site. For the dangling bond electron interaction with the parent nuclear spin, I~(())12 and thus the hyperfine shift depends strongly on the extent of localization and the degree of hybridization. The observation of a resolvable Fermi contact hyperfine pair in an amorphous material is therefore remarkable. It implies that the Si~ sites are relaxed so that a relatively small distribution in 4if0) exists. The similarity of the hyperfine linewidth in the two material systems is further indication of the characteristic, relaxed nature of the Sis defect in the disordered silicon network. The smaller splitting in a-Si: H implies either less admixture in the sp hybridization or slightly greater delocalization, or both. The final analogous feature that we discuss here is the effective correlation energy. In a-Si : H ( U - 0.3 eV) the dangling bond must point into a silicon network void. At the (1 1 1) Si/SiO 2 interface, the dangling bond must extend into a network void in the oxide. The relatively lower dielectric screening of the oxide leads to a stronger Coulomb repulsion (larger U(,). Furthermore, the higher average coordination of the crystalline network back bonded to the Si°3 allows less charge-induced relaxation of the site (larger UR). It is thus to be expected that U~.~(Si/SiO2) should be greater than U ~ ( a - S i : H ) . The relative size of U ~ is consistent with the reduced hyperfine shift in a-Si: H as discussed above. The one theoretical estimate [18] that we know of finds U~(Si/SiO2) ~ 0.8 ± 0.2 eV. Before concluding we feel compelled to c o m m e n t on a recent paper disagreeing with our findings for U(Si/Si02). Chen and Lang [19] have found in samples of lower dangling bond density: (1) no observable peak near the conduction band tail; and (2) a 10 6 spin-dependent modulation of the D L T S emission spectrum. They invoke a model of close pairs of neutral dangling bonds with a positive U - 0.1 eV. First, as discussed above, resoNability of the electron trap level from the band tail is greatly reduced at reduced Si~ densities. We believe the peak was simply missed in ref. [19] where samples of considerably lower interface state density were used. Second, if the E v + 0.3 eV band consists of the overlap of the Si~ and Si~ peaks, then the integral of the charge density would be much greater than the peak spin density. In fact, within the experimental uncertainties, the two quantities arc equal. Finally, as discussed in the E S R section above, the majority of •Si 3'~ a r e anticorrelated spatially. Thus, if the 10 ~ modulation effect of [19] is not artifactual~ then the spatially paired, low U states observed do not represent the majority of centers.
D.K. Biegelsen et al. / Native defects at Si/Si02 interface
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4. Conclusion In summary, we have described the gap states associated with the Si/SiO 2 interface which are intrinsic to the pure thermal oxidation process. By analogy with a-Si : H we find that the electrical properties of the interface are dominated by the effects of disorder in a tetrahedrally coordinated, covalently-bonded network. The states in the gap divide into band tailsarising from strained, but fully-coordinated atomic configurations-and three-coordinated silicon defects. The interface dangling bond has its two levels in the gap with an effective correlation energy of - 0 . 6 eV, slightly larger than that in a-Si: H. The degree of charge localization is high and apparently higher than in a - S i : H . The effects of hydrogenation are also similar in the two systems. We infer that these characteristics are, in fact, applicable to other disordered silicon systems, e.g. grain boundaries [20].
Acknowledgements We wish to thank M.D. Moyer for state of the art sample preparation. We also thank K.L. Brower for sharing with us his results (ref. [8]) before publication. Work reported here was supported in part by the US Army ERADCOM.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
F.A. Ponce and T. Yamashita, Appl. Phys. Letters 43 (1983) 1051. R. Haight and L.C. Feldman, J. Appl. Phys. 53 (1982) 4884. P.J. Caplan, E.H. Poindexter, B.E. Deal and R.R. Razouk. J. Appl. Phys. 50 (1979) 5847. Y. Nishi, Japan. J. Appl. Phys. 10 (1971) 52. N.M. Johnson, D.K. Biegelsen, M.D. Moyer. V.R. Deline and C.A. Evans, Jr., Appl. Phys. Letters 38 (1981)995. G.D. Watkins and J.W. Corbett, Phys. Rev. 134 (1964) A1359. K.L. Brower, Appl. Phys. Letters 43 (1983) 111. K.L. Brower, J. Electron. Mater., in press. E.H. Nicollian and J.R. Brews, MOS Physics and Technology (Wiley, New York, 1982). C. Brunstrom and C. Svensson, Solid State Commun. 37 (1981) 399. P.M. Lenahan and PN. Dressendorfer, Appl. Phys. Letters 41 (1982) 542. N.M. Johnson, D.K. Biegelsen, M.D. Moyer, S.T. Chang, E.H. Poindexter and P.J. Caplan, Appl. Phys. Letters 43 (1983) 563. E.H. Poindexter, G.J. Gerardi, M.-E. Reuckel, P.J. Caplan, N.M. Johnson, D.K. Biegelsen and M.D. Moyer, J. Appl. Phys., in press. R.A. Street and D.K. Biegelsen, Spectroscopy of localized states, in: Topics in Applied Physics, Vol. 56, Eds. J.D. Joannopoulos and G. Lucovsky (Springer, Berlin, 1984). D.K. Biegelsen, R.A. Street, C.C. Tsai and J.C. Knights, Phys. Rev. B20 (1979) 4839.
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[16] [17] [18] [19] [20]
1,. Ley, Photoemission and optic~d properties, in ref. [14]. J. Robertson, Phys, (-!hem. Glasses 23 (1982) 1. A.H. Edwards, J. Electron. Mater., in press. M.C. Chen and D.V. Lang, Phys. Rev. Letters 51 (1'-t83) 427. W.B. Jackson, N.M. Johnson and D.K. Bictaelsen, Appl. Phys. Letters 43 (1983) IriS.
879
NATIVE DEFECTS AT THE Si/SiO 2 I N T E R F A C E - AMORPHOUS SILICON REVISITED D.K. B I E G E L S E N , N.M. J O H N S O N and M. S T U T Z M A N N Xerox Palo Alto Research Center, 3333 Coyote Hill Road, Palo Alto, California 94304, USA
and
E.H. P O I N D E X T E R and P.J. C A P L A N US Army Electronics Technology and Devices Laboratory, Fort Monmouth, New Jersey 07703, USA
Received 27 August 1984; accepted for publication 31 October 1984
We review here work which demonstrates that silicon dangling bonds, Si3, are the predominant, electrically active deep states associated with the clean crystalline silicon/amorphous SiO2 interface. Si3 exists in three charge states in the silicon band gap with E+,0- Ev + 0.3 eV and E0. = Ev+ 0.9eV, where E represents a demarcation level between charge states. We discuss the structural and electronic characteristics of Si3 at the Si/SiO2 interface, including degree of charge localization, effective correlation energies, role of hydrogen passivation, etc. We argue, from the strongly analogous behavior in amorphous silicon, that the electronic density of states in the gap is dominated by the characteristic effects of disorder in covalently bonded semiconductors. The states consist of two topologically distinct entities: distorted, fully-bonded network configurations giving rise to shallow silicon band tails, and three-fold coordinated, amphoteric silicon defects.
1. Introduction T h e p i ct u re e m e r g i n g f r o m most m e a s u r e m e n t s of the i n t er f ace b e t w e e n crystalline silicon and glassy silicon d i o x i d e s e e m s to be o n e of nearly ideal b o n d i n g . T h a t is, the silicon crystal t e r m i n a t e s a b r u p t l y [1] with only a l i m i t ed region of m o d e r a t e strain [2]. T h e silica n e t w o r k is almost p er f ect l y m e r g e d with the crystal and m i s m a t c h of the a v e r a g e silicon-silicon spacing is a c c o m m o d a t e d in the flexible S iO 2 n e t w o r k . This is of c o u r s e technologically f o r t u i t o u s b e c a u s e the i n t e r f a c e naturally tends to be stable and electrically inactive. A perfectly b o n d e d , u n s t r a i n e d S i / S i O 2 i n t e r f a c e netw o r k w o u l d h a v e no states in the silicon b a n d gap. A s we will try to d e m o n s t r a t e below, t h e e l e c t r o n i c p r o p e r t i e s of the i n t e r f a c e are d o m i n a t e d by the residual d i s o r d e r and manifest p r o p e r t i e s c h a r a c t e r i s t i c of d i s o r d e r e d 0378-5963/85/$03.30 O E l s e v i e r S c i e n c e Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing D iv is i o n )
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D.K. Biegelsen et al. / Native dejects at Si/NiO, inter/fiwe
covalently-bonded semiconductors. The states introduced into the silicon gap arise from strained fully-coordinated bonding configurations and isolated, topologically distinct three-bonded silicon atoms, Si:~.
2. Experiment We review here recent experimental work concerning the electrical and structural properties of native defects of the pure, as-grown Si/SiO 2 interface. We will focus on the (111) surface of silicon because the defect density is highest and the defects have only one spatial orientation. Although conceptually simpler for the (11 I) interface, the results are nonetheless true for interface states on the (100) and (110) surfaces. 2. 1. t ~ S R c h a r a c t e r i z a t i o n
Caplan et al. [3] showed from electron spin resonance (ESR) studies that the prominent paramagnetic center, Pb [4]. occurs at the interface, is cylindrically symmetric and aligned along [111] directions. For (111) silicon these defects are directed only perpendicular to the interface, i.e. pointing into the oxide. These defects are neutral dangling bonds, Si~, where i) denotes the charge state. The dangling bonds are readily passivated if atomic hydrogen is present [5] so care must be exercised in sample preparation to avoid hydrogenation. The shifts in the principal gyromagnetic (g-tensor) c o m p o n e n t s from the free electron value of g,. 2.0023 are to first order a g - ~ - g,. -
a~ ) 2 ( L ~ . ~ / / ( E ~
E~),
(1)
where the summation is over all band states, b, asj is the atomic spin-orbit coupling factor, '(Ld,b) iS the angular m o m e n t u m matrix element for transitions between the defect level at energy E'd and band states at energy F~,. By symmetry, ag, i~ 0 when the applied magnetic field H , is parallel to the defect cylinder axis. :lg, is expected to be positive for an sp -~ hybrid stale in silicon. This follows from the fact that the top of the valence band is strongly p-like and the bottom of the conduction band s-like [6]. s-like states have zero angular m o m e n t u m ((Ld.b)--0). E d /'7_~, is negative for coupling Io valence band states, so ,Ag,± is positive. Brower [7] has measured the hyperfine interaction between the Si'i electron and 2~Si nuclear spins. With a simple molecular orbital model of Sii~ he found that the paramagnetic electron wave function dJ(r) is - 8 0 % localized on the Si atom and the non-bonding hybrid is ~--12"/,, s-like and 88% p-like. The hyperfinc split lines arc broadened from the unshiftcd linewidth. This is most likely due to variations in the electron wavefunction arising from a distribution of local environmental parameters. The unshifted
D.K. Biegelsen et al. / Native defects at Si/Si02 interface
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line is also inhomogeneously broadened. The natural linewidth of the line, l/yT~ for isolated spins, is much less than the observed linewidth. The broadening is also greater than a width corresponding to unresolved hyperfine interactions (Resolved hyperfine structure will be discussed below.) This can be seen empirically from the magnetic field dependence of the linewidth, AH. (The hyperfine broadening is independent of field.) For example, Brower finds, at H 0 - 7100 G, A H is slightly less than twice that of Caplan et al. [3] at - 3 2 0 0 G. Furthermore, the linewidth is approximately proportional to zig. Both of these effects are indicative of a relatively broad distribution of g-values arising from local environmental fluctuations and concomitant modifications of the Si ° wavefunction. Another useful finding that Brower [8] has made is that the dangling bond defects are spatially anticorrelated. Dipolar interactions between nearest D~ centers should be observable for a random distribution out to third nearest neighbor lateral separations. Brower has found that these lines are not present. Thus there are far fewer near pairs than would exist in a random distribution, and certainly no pairing. In an MOS structure the occupation of interface states can be changed by varying the band bending-induced charge accumulation or depletion [9]. Corona charging of an unmetallized, oxidized sample can be used to the same effect [10,11]. In fig. 1 we show results for the change in ESR strength with external bias voltage in a p-type sample [12]. Flat band, determined from low frequency capacitance-voltage ( C - V ) measurements, occurs at -30V. The signal height drops for high values of forward and reverse biasing indicating a paramagnetic to diamagnetic transition at high bias levels. The most straightforward interpretation is that we are observing three charge states of the Si 3 site with Si 3 and Si~ being spin-paired and unoccupied, respectively, and Si ° being paramagnetic. The energy difference between the I
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D.K. Biege&en et al. I Native defects at Si/Si02 interface
transition levels Si~*--~Si!~ and SiI~-,Si~ is the effective correlation energy, Uctr. Uca is the extra energy required to place a second electron in the same Si 3 orbital, and is principally the sum of the Coulombic repulsion term, /_./¢. and any elastic energy reduction due to lattice relaxation, U w The conversion from applied bias to interface potential is derived using the C - V measurements. From the energies at which the ESR drops to half height, we ascertain that Ue~-0.6-+0.1 eV. This result has been verified in gammairradiated material by Lenahan and Dressendorfer [11]. It is interesting to note that at high bias levels majority or minority carriers accumulate at the interface. The spin lattice relaxation time, T r, is reduced by approximately an order of magnitude due to spin interactions with the mobile carriers. However, in the voltage range of the measurements above, T~ is still far too long to produce lifetime broadening (and apparent signal height reduction) of the Si ° line. 2.2. E l e c t r i c a l c h a r a c t e r i z a t i o n
There have been many reports of a hole trap at 0.3-4).4eV above the valence band edge. The integrated density depends strongly on sample processing. Our ESR results, implying an amphoteric defect, compelled us to search for a second transition of the dangling bond in C - V and DLTS measurements. Fig. 2 shows results of current transient spectroscopy on identically processed p- and n-type silicon samples [12]. (All our samples consisted of epitaxial silicon layers on doped s u b s t r a t e s - f o r C - V - o r undoped s u b s t r a t e s - f o r ESR. Thermal oxides were grown in dry 02 at 1000°C ~-""
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Energy (eV) Fig. 2. Density of gap states derived from current-mode D L T S measurements, assuming
constant capture cross sections.
D.K. Biegelsen et al. / Native defects at Si/Si02 interface
883
1.2
MOS C a p a c i t o r 1.0 -- Si:(111). P-Type Si02:194 nm
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j
/
-
Low-Frequency C-V I
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Fig. 3. Interface potential determined from low frequency (10Hz) capacitance-voltage measurements indicating the two dangling bond peaks in the same sample.
to thicknesses of - 150 nm.) As will be discussed below, both peaks anneal in a similar fashion. They have approximately the same integrated density which is furthermore (within an absolute experimental uncertainty of - 2 ) the same as the maximum ESR spin density. In fig. 3 we show the interface potential derived from low frequency C - V measurements on a p-type sample [13]. The interface density of states is essentially the derivative of the curve. This can be seen intuitively because the band bending with gate voltage is least in regions of high densities of states. In particular, we see here that both levels can be observed in a single sample.
2.3. Passivation Removal of electronic states from the gap by chemical bonding is quite informative, as well as technologically useful. We have demonstrated, using atomic deuterium diffusions and SIMS analysis [5], that the interface acts as a deuterium sink until the interface states (dangling bonds) are saturated. In fig. 4 we show vacuum annealing behavior of spin density and hole trap peak density (measured by quasistatic C - V ) in identically processed aluminum gate n-type silicon epitaxial MOS samples. (The passivation is hypothesized to involve atomic hydrogen introduced into the oxide from the metal-oxide interface during processing [12].) The common annealing behavior is strong evidence that the dangling bond center is equivalent to the hole trap at E - E V+ 0.3 eV. Annealing in p-type samples produces similar results. The electron trap at E - E 0 - 0 . 2 5 e V also anneals in a similar fashion. However, because the peak is close to the conduction band tail and because the conduction band tail states, unlike the valence band tail states, are reduced far less by hydrogenation than the defect states, the upper peak coexists with a large background. This leads to relatively large uncertainties in the defect peak deconvolution. We also note here that the above
884
D.K. Biegelsen et al. / Native defects at Si/Si02 interjku'e
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1.0
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o
~
o Characteristic Peak Density
0.8-I I Pb Spin Density d 0.6
0.4 O Z
0.2 \ 100
200 300 Anneal Temperature (°C)
400
Fig. 4. Normalized peak heights for ESR and E, ~ 0.3 cV ( ' - V peak :filer 15 rain anncab,.
c h a r a c t e r i s t i c s s e e m to explain the fact that o t h e r g r o u p s have not o b s e r v e d this b a n d of d e e p levels. A high i n t e r f a c e s t a t e d e n s i t y - n o t a c c i d e n l a l l y o r i n t e n t i o n a l l y p a s s i w ~ t e d - must be a c h i e v e d to r e s o l v e the b r o a d b a n d of t r a p states from the c o n d u c t i o n b a n d tail states.
3.
Discussion
T h e e l e c t r o n i c states in the silicon b a n d g a p a s s o c i a l e d with the p u r e t h e r m a l l y o x i d i z e d silicon i n t e r f a c e are thus easily c h a r a c t e r i z e d . A n a r r o w d i s t r i b u t i o n of states tail into the g a p from each b a n d edge. A single e l e c t r i c a l l y active defect, the silicon d a n g l i n g b o n d , d o m i n a t e s the d e e p g a p levels, it has its two e n e r g y levels in the gap a p p r o x i m a t e l y s y m m e t r i c a l l y d i s p o s e d r e l a t i v e to m i d g a p . T h e effective c o r r e l a t i o n e n e r g y is positive and e q u a l to ~ E J 2 . A t o m i c h y d r o g e n r e a d i l y b o n d s to the d a n g l i n g o r b i t a l s and is also i n s e r t e d into the m o r e n u m e r o u s b a n d tail states. This shifts the states o u t of the g a p t r a n s f e r r i n g the b o n d i n g and a n t i - b o n d i n g - l i k e states d e e p into the b a n d s . D u r i n g the last ten y e a r s an e n o r m o u s a m o u n t has b c c n l e a r n e d a b o u t a n o t h e r d i s o r d e r e d silicon s y s t e m - a m o r p h o u s h y d r o g e n a t e d silicon, aSi : H. W e will discuss s o m e of the results here b e c a u s e we b c l i e v c that t h c r c are c o m m o n physical m e c h a n i s m s u n d e r l y i n g the c h a r a c t e r i s t i c a t t r i b u t c s of the two systems. W e think each a r e a can benefit from the c o m p a r i s o n . A s e x a m p l e s , the Si 3 c h a r a c t e r i z a t i o n can be best o b t a i n e d in the S i / S i O 2 system w h e r e the site a n i s o t r o p y is not spatially a v c r a g e d as in the a m o r p h o u s
D.K. Biegelsen et al. / Native defects at Si/Si02 interface
885
network. Conversely, information about the effects of hydrogen bonding on tail states, for example, is more readily obtained in bulk disordered materials. The electronic states in the gap of amorphous silicon, as for the Si/SiO 2 interface, are surprisingly easy to describe [13]. We show a simplified model in fig. 5. Tails of localized states falling in density approximately exponentially extend into the gap between the mobility edges. Again, dangling bonds are the predominant electrically active defect. Si3, with a positive correlation energy of 0.3 + 0.1 eV, has its two energy levels approximately symmetrically disposed about midgap. The upper Hubbard band has been characterized most definitively. It has a width - 50% larger than the corresponding states at the Si/SiO: interface. In both material systems in the absence of hydrogen the dangling bonds are isolated with similar average separations ( - 1 0 nm). The density of tail states and dangling bonds depends strongly on hydrogen concentration. Interestingly in passivating the dangling bonds, approximately one hundred times more hydrogen is bonded into the silicon network [15]. The excess hydrogen is thought to be bonded preferentially at the most strongly strained bonds. This is clearly shown in the Si/SiO 2, interface system [5] where again the amount of hydrogen bonded at the interface is - 1 0 0 times the dangling bond concentration, yet almost no hydrogen is inserted into the strongly bonded crystal silicon matrix of the substrate. It has also been shown in a-Si : H that hydrogenation removes the valence band tail states to a greater degree than the conduction band tail states [16]. The similarities of these effects in a - S i : H and Si/SiO 2 we think follow from the bond energetics of covalent, tetrahedrally coordinated semiconductors. The potential increases strongly with bond length variations so that strain energy is minimized by defect (dangling bond) creation, as opposed to strain distribution among many bonds [17]. Furthermore, dangling bonds
\ Dangling Bonds
20 Ue~ ~ 0.4eV
Ev
/
/
Ec
Fig. 5. Schematic density of states diagram for hydrogenated amorphous silicon. Ec and E~ denote the conduction and valence mobility edges respectively. The arrows denote electron spins and indicate spin-pairing in the doubly-occupied Si3 state.
D.K. Biegelsen et al. / Native defects at Si/Si02 interface
886
should be separated (anticorrelated) because they form only where network strain exceeds a critical level. This simple c h e m i c a l - t o p o l o g i c a l picture readily explains the o v e r w h e l m i n g t e n d e n c y in these disordered systems to have a single kind of electronic d e f e c t - S i 3. All existing structural defects d e c o m p o s e into m o r e or less weakly b o n d e d (tail) states and dangling bonds. This separation is meaningful if, and only if, the electronic wavefunction on Si~ is strongly localized. W e note that point defects in crystalline silicon generally c o n f o r m to this description. J a h n - T e l l e r distortions lead to reconstruction and weak b o n d i n g and isolated, localized dangling b o n d s [6]. In the next paragraph we c o m p a r e the extent of Si ° localization in the two systems. In fig. 6 we show the E S R data from [7] for H 0 parallel (O = 0 °) through p e r p e n d i c u l a r (0 = 90 °) to the dangling b o n d axis. T h e two satellite lines arise from interactions with the 29Si nuclear spins in the m~ = -+ J2 states. The natural a b u n d a n c e of 29Si is 4.7% and the two hyperfine lines t o g e t h e r ( 1 1 1 ) Si/SiO 2 interface Pb spectrum
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(c)
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Fig. 6. Dangling bond ESR data of Brower [7] showing the unshifted ~Si central line and satellite peaks corresponding to neutral dangling bonds o n 29Si atoms with nuclear spins in M] - + ~state.
D.K. Biegelsen et al. / Native defects at SilSiO~ interface
887
contribute > 3% experimentally. From the width of the hyperfine peaks we can infer that there is a distribution in the degree of localization and/or the variation of hydbridization values of - 1 0 % . This is consistent with the notion of a strained silicon interracial region. In fig. 7a we show a calculated X-band powder pattern spectrum based on Brower's data. The powder pattern is an orientational average of the anisotropic lines which one might expect for a powdered crystal. We have used a 1 0 G environmental broadening for the 290.o 3t 3 hyperfine tensor, a 20 G smaller splitting (due to a slightly weaker localization or a more p-like character in a - S i : H ) and a Ag-dependent broadening for the 28$13 .0 central line, as observed in the Si/SiO 2 data. In fig. 7b we show the ESR spectrum for a-Si : H. The features
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MAGNETIC FIELD Fig. 7. (a) Powder pattern ESR signal calculated from the data of Brower [7]. The hyperfine shifts of [7] have been arbitrarily reduced by 20 G to obtain a better likeness to the a-Si : H data in (b). (b) ESR spectrum of undoped a-Si : H, Note that the asymmetry of the central line (arising from the Ag-rclated line broadening) and the hyperflne widths are simialr to those in (a).
888
I).K. Hiege&en et al. / Native defects at Si/Si02 interface
in the wings of the central line are remarkably close to the Si/SiO 2 values. (Studies on isotopically-enriched a-29Si : H are in progress.) From various observations in a - S i : H it can be inferred that the Si!~ electron is strongly localized on the atom. The hyperfine shift is proportional to the magnitude of the wavefunction at the nuclear site. For the dangling bond electron interaction with the parent nuclear spin, I~(())12 and thus the hyperfine shift depends strongly on the extent of localization and the degree of hybridization. The observation of a resolvable Fermi contact hyperfine pair in an amorphous material is therefore remarkable. It implies that the Si~ sites are relaxed so that a relatively small distribution in 4if0) exists. The similarity of the hyperfine linewidth in the two material systems is further indication of the characteristic, relaxed nature of the Sis defect in the disordered silicon network. The smaller splitting in a-Si: H implies either less admixture in the sp hybridization or slightly greater delocalization, or both. The final analogous feature that we discuss here is the effective correlation energy. In a-Si : H ( U - 0.3 eV) the dangling bond must point into a silicon network void. At the (1 1 1) Si/SiO 2 interface, the dangling bond must extend into a network void in the oxide. The relatively lower dielectric screening of the oxide leads to a stronger Coulomb repulsion (larger U(,). Furthermore, the higher average coordination of the crystalline network back bonded to the Si°3 allows less charge-induced relaxation of the site (larger UR). It is thus to be expected that U~.~(Si/SiO2) should be greater than U ~ ( a - S i : H ) . The relative size of U ~ is consistent with the reduced hyperfine shift in a-Si: H as discussed above. The one theoretical estimate [18] that we know of finds U~(Si/SiO2) ~ 0.8 ± 0.2 eV. Before concluding we feel compelled to c o m m e n t on a recent paper disagreeing with our findings for U(Si/Si02). Chen and Lang [19] have found in samples of lower dangling bond density: (1) no observable peak near the conduction band tail; and (2) a 10 6 spin-dependent modulation of the D L T S emission spectrum. They invoke a model of close pairs of neutral dangling bonds with a positive U - 0.1 eV. First, as discussed above, resoNability of the electron trap level from the band tail is greatly reduced at reduced Si~ densities. We believe the peak was simply missed in ref. [19] where samples of considerably lower interface state density were used. Second, if the E v + 0.3 eV band consists of the overlap of the Si~ and Si~ peaks, then the integral of the charge density would be much greater than the peak spin density. In fact, within the experimental uncertainties, the two quantities arc equal. Finally, as discussed in the E S R section above, the majority of •Si 3'~ a r e anticorrelated spatially. Thus, if the 10 ~ modulation effect of [19] is not artifactual~ then the spatially paired, low U states observed do not represent the majority of centers.
D.K. Biegelsen et al. / Native defects at Si/Si02 interface
889
4. Conclusion In summary, we have described the gap states associated with the Si/SiO 2 interface which are intrinsic to the pure thermal oxidation process. By analogy with a-Si : H we find that the electrical properties of the interface are dominated by the effects of disorder in a tetrahedrally coordinated, covalently-bonded network. The states in the gap divide into band tailsarising from strained, but fully-coordinated atomic configurations-and three-coordinated silicon defects. The interface dangling bond has its two levels in the gap with an effective correlation energy of - 0 . 6 eV, slightly larger than that in a-Si: H. The degree of charge localization is high and apparently higher than in a - S i : H . The effects of hydrogenation are also similar in the two systems. We infer that these characteristics are, in fact, applicable to other disordered silicon systems, e.g. grain boundaries [20].
Acknowledgements We wish to thank M.D. Moyer for state of the art sample preparation. We also thank K.L. Brower for sharing with us his results (ref. [8]) before publication. Work reported here was supported in part by the US Army ERADCOM.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
F.A. Ponce and T. Yamashita, Appl. Phys. Letters 43 (1983) 1051. R. Haight and L.C. Feldman, J. Appl. Phys. 53 (1982) 4884. P.J. Caplan, E.H. Poindexter, B.E. Deal and R.R. Razouk. J. Appl. Phys. 50 (1979) 5847. Y. Nishi, Japan. J. Appl. Phys. 10 (1971) 52. N.M. Johnson, D.K. Biegelsen, M.D. Moyer. V.R. Deline and C.A. Evans, Jr., Appl. Phys. Letters 38 (1981)995. G.D. Watkins and J.W. Corbett, Phys. Rev. 134 (1964) A1359. K.L. Brower, Appl. Phys. Letters 43 (1983) 111. K.L. Brower, J. Electron. Mater., in press. E.H. Nicollian and J.R. Brews, MOS Physics and Technology (Wiley, New York, 1982). C. Brunstrom and C. Svensson, Solid State Commun. 37 (1981) 399. P.M. Lenahan and PN. Dressendorfer, Appl. Phys. Letters 41 (1982) 542. N.M. Johnson, D.K. Biegelsen, M.D. Moyer, S.T. Chang, E.H. Poindexter and P.J. Caplan, Appl. Phys. Letters 43 (1983) 563. E.H. Poindexter, G.J. Gerardi, M.-E. Reuckel, P.J. Caplan, N.M. Johnson, D.K. Biegelsen and M.D. Moyer, J. Appl. Phys., in press. R.A. Street and D.K. Biegelsen, Spectroscopy of localized states, in: Topics in Applied Physics, Vol. 56, Eds. J.D. Joannopoulos and G. Lucovsky (Springer, Berlin, 1984). D.K. Biegelsen, R.A. Street, C.C. Tsai and J.C. Knights, Phys. Rev. B20 (1979) 4839.
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D.K. Biegelsen et al. / Native defects at Si/SiO: interJk~ce
[16] [17] [18] [19] [20]
1,. Ley, Photoemission and optic~d properties, in ref. [14]. J. Robertson, Phys, (-!hem. Glasses 23 (1982) 1. A.H. Edwards, J. Electron. Mater., in press. M.C. Chen and D.V. Lang, Phys. Rev. Letters 51 (1'-t83) 427. W.B. Jackson, N.M. Johnson and D.K. Bictaelsen, Appl. Phys. Letters 43 (1983) IriS.