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Physica B 170 (1991) 507-512 North-Holland

Reversible H 2 passivation of * S i - Si 3 interface defects in (1 1 1)Si/SiO 2 A. S t e s m a n s ~ a n d G . Van Gorp Department Natuurkunde, Katholieke Universiteit Leuven, 3001 Leuven, Belgium

K-band electron spin resonance (ESR) at 4.3 K has revealed the dipole-dipole (DD) interaction effects between [1 1 liP b centers (*Si = Si~ defects with unpaired sp ~ hybrid ]] [1 1 1]) at the 2 dimensional (1 1 1)Si/SiO= interface. This has been enabled by the perfectly reversible H= passivation of Pb, which affects the defect's spin state. Sequential hydrogenation at 253-353°C and degassing treatments in high vacuum at 743-835°C allowed to vary the Pt~ density in the range 5 × 1()~'< [Pb] ~< (1.14 -+ 0.06) x 10 ~ cm -'. With increasing [Ph] fine structure doublets are clearly resolved. It is found that (1 1 1)Si/SiO z interfaces, dry thermally grown at ~920°C, naturally comprise a *Si Si, defect density passivated or not - of 1.14 x 1()~ cm -~.

1. Introduction At the Si/SiO, interface defects are built in naturally to relieve strain. Electrically, these defects lead to fast interface states (density D~t ) behaving as trapping and recombination centers, thus degrading device performance. By the electron spin resonance (ESR) technique the dominant defect - denoted as Pb and PNI when observed at the ( l l l ) S i / S i O 2 and ( 1 0 0 ) S i / S i O 2 interface, r e s p e c t i v e l y - h a s structurally been identified as an unpaired sp 3 orbital ("dangling b o n d " ) on a trivalently-bonded interfacial Si atom (schematically denoted as *Si---Si3) (see, e.g., ref. [1]). This center, of C3,, point symmetry, is illustrated in fig. 1. Only the neutral, paramagnetic state is ESR active. Typically, on as-oxidized (1 1 1)Si/SiO 2, a fraction f - - = [ P b ] / N~, ~ 0.5% (where N~, = 7.830 x 10 ~4 cm-2, the density of lattice sites in a (1 1 1) plane) of the Si surface atoms are the sites of Pb defects [1-3]. It has long been recognized from electrical observations that H 2 plays a prominent role in the thermochemical properties of interface centers [4, 5]. The beneficial effect of H~ on defect ~ Supported by the National Fund for Scientific Research, Belgium.

inactivation is well known as, e.g., exemplified by the generally-applied post oxidation heat treatments in forming gas (70% N2; 30% H~). In particular, it is known [6] that the Pb defect is readily passivated by bonding to H resulting in a diamagnetic entity symbolized as HP b. Interestingly, HP b centers dissociate in vacuum at a temperature T ~ 550°C leading to the reappearance of paramagnetic P~s. Provided that subsequent thermal treatments are carried out in situ, the total defect density [P~] =-[Pb] + [HPb] (comprising both ESR-active and passivated defects) appears stable up to 850°C. Also. from a detailed ESR study [6], evidence has been provided that the passivation occurs through direct reaction of Pb with the H~ m o l e c u l e - n o t with atomic H. This H-related kinetics has recently gained renewed interest in an endeavour to understand the fundamental chemistry of the Si/ S i O 2 interface growth and trap formation. It is believed that in the rush for "Du-free'" d e v i c e s - i n t e n t i o n a l l y or n o t - H , has been the ubiquitous passivating agent. This is evidenced by the ease of passivation. But as it is almost equally easy to dissociate a HP h complex one can hardly be enthusiastic about such passivation: it provides little protection against ambient radia-

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SAMPLE S~HAPE Fig. 1. Ball-and-stick model of the (I 11)Si/SiO, interface showing the basic structure of the [111]P, defect (cntil} A) and the applied sample geometr?~. tion (radiation hardness), electron injection ticross the interface, etc., suggesting to search for other passivating means. Hence the importance to understand passivation on atomic scale which is tied to the question whether a radiation-hard Si device can ever be obtained if sticking to chemically pure Si/SiO~ transitions? The present work reports on a systematic analysis of the passivation/dissociation behaviour of Ph/HPb defects resulting from interaction with H , using ESR as a diagnostic probe. This is then used as a tool to disclose the d i p o l e - d i p o l e ( D D ) interactions within the 2-dimensional (2D) Pb spin system, which is of fundamental importance.

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sure p ~ , . - t t . 2 - 1 . 1 ate for variable times t ~ =: 57-130 min and various oxidation startingup and quenching conditions. (2) H y d r o g c n a l i o n in 99.9999% purc H . ;_it /)ii ~ 1. l a t i n for tilnes

l,,, : 11 43 rain at tenlperatures Tit ' 253-,~.~3(. These treatmcnts wcre quenchcd b\ evacuating the quartz sample tubc and rolling away thc furnacc. Sequential tt 2 annealings arc applied to decrease [Phi in a controlled way stcp by step on onc and the samc samplc. (3) Dcgassin,, (dehydrogenation) at 743-835 + l(l°(" in vacuum ( 17 I(I T o r r } for timcs l~ _. ESR observations were nlade tit 4.3 K in a 20.2 O H z spectromctcr operated in tile absorption mode under adiabatic slow passage. "I'o avoid any saturation distortion at 4.3 K, microwave power levels P incident on tile ~I'E,,~ cylindrical cavity of loaded quality factor Q 13000-16000 as low as --63 dBm (0.5 nW) had to be used. That level m a y wiry somewhat ( +5 dB) as the onset-of-saturation condition may shift slightly owing to changes in transverse relaxation time T, resulting from varying DD interaction strength with varying [Phl. Regarding the optimutn signal-to-noise (S/N) ratio. measuring at 4.3 K emerged as the best choice for the present Si wafer resistivity. Sinusoidal modulation of the externally-applied magnetic induction B at 1211kHz resulted in the detection of absorption-derivative spectra d P , , , ' d B , dt':, representing the microwave power absorbed m the cavity during resonance. Spin densities were measured relative to an A l e O ~ C r 3' susceptibility standard (accuracy + 1 % ) by comparing the respective doublenumerically integrated d P / d B spectra. More details can be found elsewhere [7].

2. Experimental procedures Slices measuring 1 x 9 × I).117 m m 3, which had their 9 m m edge along [1 1 2], were cut from a Cz-grown ( l l l ) S i wafer (p type, 10[~cm) polished to optical quality on both sides. Typically, 10-15 platelets were stacked to enhance the effective interface area in the cavity. Generally, three types of thermal treatments were applied. (1) Dry thermal oxidation at T , - 9 2 0 _ + 15°C in 99.999% pure O, tit a pres-

3. Experimental results and discussion _7.1.

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The impact alternated hydrogenation and degassing steps have on the Ph system has been analysed most reliably by submitting one sample to various thermal steps. The ESR signal for B]I[1 1 11 (B perpendicular to the ( 1 1 1 ) inter-

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passivation and HP b dissociation. These hydrogenation and degassing steps may be arbitrarily alternated; no particular sequence is required. This at once shows that the observed variations in line width do not result from any structural change of the Si/SiO 2 interface induced by the post-oxidation thermal treatments. This would be inconsistent with a reproducible "wandering" over the A B p p v e r s u s [Pb] curve. Instead, the particular A B p p v e r s u s [P~,] variation firstly discloses the DD interaction symptoms among Pb centers. Various conclusions follow. (1) For BIll1 11 ], there appears to exist a u n i v e r s a l ABpp versus [Phi relationship. The line width is only set by the defect concentration, independent of the thermal way followed to reach that [Pt,]. The degree of universality of that curve over different observational frequencies and substrate orientations will depend on the interaction mechanisms determining the residual R natural line w i d t h ABpp, that is, the width devoid of DD broadening. If, for B H [ l l l ], strain broadening of the g factor is considered absent

face) is used as the diagnostic tool, in particular its peak-to-peak w i d t h ABpp. The observed variation o f ABpp with [Pb] is depicted in fig. 2, where the letters labelling the various data points indicate the time sequence of measurement and refer to the last thermal treatment the sample r e c e i v e d - as described in the c a p t i o n - prior to each ESR observation. Point (a) corresponds to the as-oxidized state (920± 15°C; 63min; 0.2 atm O2). Figure 1 exhibits several interesting features. FirsL it is clear that, as expected, H~ treatments indeed do lower [P,] while vacuum annealing at elevated T has the reverse effect. [Pb] may be lowered step by step by varying either the H passivation temperature or time. However, it has been noted that at each T m , [Pb] cannot be decreased below a certain treshold value, even after prolonged hydrogenation. Only by increasing TH, could the treshold value be lowered further. A striking second observation is that, for one sample, one can move up and down t h e ABpp versus f curve very reproducibly by alternate Pb

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Fig. 2. Plot of the peak-to-peak K-band ESR line width (4.3 K; B]I[I I 1]) of the [I I I]P~ defect at the (1 1 l)Si/SiO, interface vs. Pb density. The data were measured sequentially on one sample which received various thermal treatments: a, as oxidized at 920°C in 0.2 atm Oz: b, c, hydrogenated at 253°C for 37 and 43 min, respectively: e, degassed in vacuum ( p < 10 " Torr) for 138 min at 743°C; f - k , m, and n, hydrogenation steps for 11-19 rain at 273 346°C: I, degassed at 835°C for 15(I min. The dashed line is a guide to the eye.

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{ 2 J - t h e residual width likely being dominated by unresolved HF splitting - the ~Bpp versus [P,,] curve may be truly universal. This relationship may then be used reversibly, i.e.. accurate measurements of ?~Bpp (at whatever microwave frequency) will unequiw)cally give the P~, concentration, provided, of course, that spectroscopically correct signals are recorded. (2) Post-oxidation thermal cycling (either m vacuum or H~ ambient, at T ~< 835°C) of Si/SiO~ structures thermally grown at T = 920°C has no effect on Ph density, as noticed before [6]. This suggests that [Pt,] is firmly set by the initial oxidation carried out at higher 7". The only effect of the "soft" H passivation and degassing treatments is to switch the defects between the ESRactive paramagnetic Pt, state ( * S i = S i ~ ) and the diamagnetic H S i - Si~ state. (3) In contrast with previous conclusions [6], the various post-oxidation thermal steps need not be sequenced in situ. Each thermal handling was followed by ESR analysis, the sample thus being kept in room temperature ambient for variable times between thermal steps. (4) Degassing for 138-150 min at either 743 or 835°C (cf. (e) and (1) in fig. 2, respectively) results in equal [Phi values. It thus appears that even at 743°C, dehydrogenation is exhausted after ~138 min. (5) After firm degassing (cf. (e) and (1) in fig. 2), [Ph] is much larger than the density [P~,]' observed on the as-oxidized state. This shows that some partial passivation of the Pb system is inherent to the oxidation step: H, is around as impurity in the oxidizing ambient and will further be enhanced by permeation through the walls of the silica tube enclosing the sample. Hence, variations in [P~,]' are to be expected over different oxidation set ups as affirmed by the data in literature [1.3, 8]. (6) Passivation at 353°C for 19 min suffices to lower [Pt,] beyond the detection limit, that is, below 5 × 10 I° cm

3.2. DD interaction among P~,s The effect DD interactions have on the ESR line shape in addition to line broadening is evi-

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t i g . 3. K b a n d I{SR spectra ol lhc Jl I liP,, dclcct (1 I I)Si Si(), i n t c l l a c c ~d~scrxcd al 4 . 3 K with /' d B m and nlll l i II t,,,- xa,,-i,,us p,. densities. Signalr, B aF¢ li[ic s t r u c l u r c d o u b l c l s , while d o u b l e t (" is ascribed s u p e r hypcrfinc stlucturw

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dent from lig. 3, which shows the line shape cvoJution with increasing [Ph]. The signal corrcs p o n d i n g t o f 0.051(~ (cf. (i)illtig. 2 ) , w h i c h i s almost free of DD broadening, nearly represents the residual signal. As f is increased to 1.5d~. the central signal A gradually develops strong "'shoulders- (cf. positions B) and additional doublets (cf. positions C and D) centered around A appear. Doublet C has been interpreted as a >Si superhyperfine (SHF) structure [9]. [lnlikc the SHF signals, the other doublets wiry in intensity relative to the central signal, indicating their DD-interaction origin. This is confirmed by numerical line shape calculations for a 2D dilute spin system, which show that, in contrast with the 3[) case, such "shouldered" spectrum is typi-

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cal for a 2D system o f f = 1%. It is to be ascribed to the less dense statistical density of the surrounding spins in a 2D situation as compared to the 3D one. Doublet D results from the central spin-5th neighbours (6) interaction. From such DD-spectra simulations, much information is expected about the distribution of the Pb defects over the interface plane. This, in turn, could provide the key to understand the physical mechanism of their formation. Gradually lowering of the Pb density discloses a fundamental spectroscopic property of PbS. For f--+0, the natural residual signal is observed, R characterized by ABvr = 1.29 + - 0 . 0 3 G , a width at half height of the absorption spectrum AB R = 1 . 9 8 + 0 . 0 3 G , and line shape factor K = I / R [VD(ABpv) ~] 2.0-+ 0.1 (I and VD represent the intensity and amplitude of the absorption and derivative spectrum, respectively). The AB R value, situated between the values 1.03 and 3.63 of the Gaussian and Lorentzian curves, respectively, would refer to a Voigt-like curve (convolution of Lorentzian and Gaussian broadening). One parameter to test this indication is the line width ratio. For a Voigt curve of K = 2.0_+ 0.1, AB/ABpp = 1.40 _+ 0.02, which is to be compared with the experimental value of 1.53 -+ 0.06. InterL.R preting along a Voigt shape, one finds A Bpp G,R 0.67 G and ABpp : 0.90 G for the peak-to-peak width of the constituent Lorentzian (spin packet) and Gaussian (inhomogeneous) broadening, respectively. This allows an in-depth analysis of the line broadening mechanisms. It suggests that the natural width results from H F broadening, the non-Gaussian shape being the smoothed envelope of the slightly resolved HF structure. Knowing the correct residual shape is crucial in convoluting the calculated histograms to obtain reliable line shape simulations.

3.3. Differently oxidized structures To analyse their influence on the grown-in Pb system, oxidation parameters have been varied over a broad range, i.e. 1.5 x 10 -5~
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bients, that is, either vacuum, 0 2 or N 2 flow (I.1 atm). After degassing at 835°C for ~ 1 5 0 m i n , the maximum Pb density for each oxidation came out amazingly constant, i.e. 8.3 ~< [Pb]m ~< 12.9 × 1012 cm -, the average being given as ([Pb]m) -----(11'4-+0-6) X 10'2 cm 2, which corresponds to f = 1.5%. This strongly suggests that ([Ph]m) = [P~], the total number of structural *Si =- Si 3 defects-passivated or not-built in at the interface. More even, it suggests that ([Pb]m) would emerge as a natural constant, intrinsic to the structural matching of thermal SiO. to (1 1 1)Si.

4. Conclusions

Using ESR as the diagnostic tool, it has been found that the Pb density in (1 1 1)Si/SiO 2 grown at ~-920°C may reversibly be varied over the range 5 x l 0 1 0 < [ p b ] ~ < ( l l . 4 + 0 . 6 ) x l 0 ~ 2 c m 2 by randomly-sequenced, non in situ H 2 and vacuum annealings. This has allowed to uncover the DD interaction among PbS in the ESR spectrum. The natural residual Ph ESR line shape has been separated and fine structure doublets have been clearly resolved. A universal ABpp versus [Pb] relationship exists; ABpp is set by [Pb], independent of thermal history. As sensed by the Pb ESR properties, postoxidation thermal treatments up to 835°C have no influence on the (1 1 1)Si/SiO 2 interface structure. This suggests that the initial oxidation step at 920°C is determinative for the interface configuration and defect density. Comparison of various oxidations at ~920°C under broadly varying circumstances strongly suggest that 11.4 x 10 ~2 cm -2 *Si ~-Si 3 defects (complexed or not) are naturally built in at all clean (1 1 1)Si/ SiO2 interfaces grown at ~920°C. Evidently, that number then would be invaluable to understand the microscopic interface structure. It raises the question whether the Pb d e f e c t s - o c c u p y i n g 1.5% of all interface s i t e s - h a v e to be seen as co-establishing the particular Si/SiO 2 interface structure rather than only being the consequence of strain relief?

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References I1j E.I|. Poindextcr and P.J. ('aplan, Prog. Surf. Sci. 14 (1983) 201. [2] K.L. Brower, Phys. Re','. B 33 (Ig~6) 4471. [31 A. Stesmans, S~_'micond Sci. TechnoI. 4 (19bit,I) 10()(i. [4] E. Kooi, Philips Rcs. Rep. 21 (1966) 477. [5] P. Balk and N. Klein, Thin Solid Films 89 (1982) 329 and references therein.

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[~] K.I.. Browcr, A p p I Phv~,. l.cu. 53 (IgXX) 5(I,";. 171 A. Stcsman~, and ,I Van (}orp. Solid State Comnmn. 74 (19~;l)) I(l()3. [gl K.I,. Browcr and "]..I ttcadlL'\. Ph\s. P,cx. B 34 (19~(~) 3fill). IL)] M. Cook and ('.'I. \Vhitc, Ph~,s. Rex. B ~H {IgOr) 9?~74 A . t t . Edv,,'~LldS, eh\s. Rex. I~ 36 (l~;X7) 963N.