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15 Paramagnetic Defects in the Surface Region of Processed

Magnetic Resonance in Colloid and Interface Science Downloaded from pubs.acs.org by UNIV OF MICHIGAN ANN ARBOR on 11/20/18. For personal use only.

Silicon PHILIP J. CAPLAN US Army Electronics Technology & Devices Laboratory (ECOM), Fort Monmouth, N.J. 07703

One of the most significant trends in the evolution of electronics has been the development of large-scale integrated circuits based on the metal-oxide-semiconductor (MOS) structure. During the growth of this technology many physical and chemical processes have been perfected which make it possible to fabricate and evaluate satisfactory devices. As a result much has been learned about the nature of the silicon or oxidized silicon surface (1,2). Sensitive electrical measurements such as capacitance-voltage measurements reveal the presence of very small concentrations of impurities at the Si-SiO2 interface. Yet much of the information is qualitative and more specific spectroscopic techniques such as magnetic resonance have been explored to throw additional light on this important area. As long ago as 1960 (3), it was proposed that electron spin resonance (ESR), nuclear magnetic resonance (NMR), and even dynamic polarization experiments might add significantly to the knowledge of the silicon surface. Process Steps Generating ESR Signals Let us consider some basic preliminary steps in the processing of a MOS device. A slice of monocrystalline silicon with a given crystallographic orientation is cut off from a boule and given a high polish. It is etched with a mixture of HF, HNO3, and acetic acid, which removes the remaining damage region of the surface. It is known that very shortly after the sample is etched and exposed to air, an oxide layer forms of the order of 20 Åin thickness. Further processing requires, however, a thicker oxide layer of perhaps 2000 Å, which is grown by heating in oxygen in a furnace at 1000° to 1200°C. We will not go further with the device fabrication, but will now show that the aforementioned processes display associated ESR signals, as shown in Table I. Before etching, the damaged surface region produces an ESR signal with g = 2.0055, and about 5 gauss wide, which is readily observable even at room temperature. After etching, the original damage signal disappears, but a photoinduced ESR signal having 173

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Table I Processes and ESR S i g n a t u r e s Process

g-value

Other c h a r a c t e r i s t i c s

Surface Damage

2.0055

Damage r e g i o n i s 1 micron deep

Standard S i E t c h

2.007-2.008 2.003-2.004

O p t i c a l l y stimulated

Thermal Oxide

P 2.000 P 2.002-2.010 PC 2.06-2.07

In Si02 or S i near s u r f a c e In Si02 l a y e r near i n t e r f a c e Extends many microns i n t o silicon

A

B

two components appears a t an o b s e r v a t i o n temperature of 77°. A f t e r the o x i d a t i o n procedure as many as three new resonances may appear. They were denoted by N i s h i , who f i r s t described them, as A> B and Pç. They a l s o were observed a t 77° and were shown t o depend on the d e t a i l s of the thermal o x i d a t i o n . P

P

Surface-Damage S i g n a l A f t e r t h i s b r i e f survey, we s h a l l now consider the i n d i v i dual resonances i n d e t a i l . The surface-damage s i g n a l was the f i r s t to be discovered (4,5). I t occurred when a s i l i c o n s u r f a c e was s a n d b l a s t e d , p o l i s h e d , or crushed. By c r u s h i n g i n t o f i n e r powder an i n c r e a s i n g l y l a r g e s i g n a l i s obtained. The s i g n a l was independent of dopants i n the s i l i c o n , o c c u r r i n g e q u a l l y w e l l i n n- or p-type. I t disappeared, however, when about 1 micron of the s u r f a c e was removed by a chemical e t c h , thus i n d i c a t i n g i t s s u r f a c e nature. A s e r i e s of c a r e f u l l y designed experiments were c a r r i e d out t o i n v e s t i g a t e the atomic nature of these ESR c e n t e r s , p a r t i c u l a r l y by Haneman (6,J7,8., 9) . I n order to s t a r t out w i t h a c h e m i c a l l y simple system, h i g h - p u r i t y s i n g l e - c r y s t a l specimens of s i l i c o n were sealed o f f together w i t h a g l a s s s l u g i n a h i g h vacuum system w i t h pressures i n the range 10~® - 10"^ t o r r , and they were crushed i n vacuum by shaking the g l a s s s l u g w i t h i n the enclosure. The r e s u l t i n g powder, about 5 microns i n s i z e , d i s played the t y p i c a l strong surface-damage resonance. Subsequently, v a r i o u s gases were introduced i n t o the vacuum chamber, and s i g n i f i c a n t i n c r e a s e s i n s i g n a l amplitude were noted f o r H£ adsorbed (^50%) and f o r adsorbed O2 (^100%), whereas water vapor caused a decrease of ^20%. From t h i s s e n s i t i v i t y to the i n t r o d u c t i o n of gases, Haneman proposed that most of the s p i n s c o n t r i b u t i n g t o the surface-damage s i g n a l were l o c a l i z e d a t the s u r f a c e , and that they were i n f a c t the d a n g l i n g bonds to be expected a t the s u r f a c e of the s i l i c o n c r y s t a l l i t e s .

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How then can one explain the often observed fact that in order to remove the surface-damage signal you have to etch a distance of 1 micron into the s i l i c o n surface? This i s due to the fact that within this damaged region there i s a vast labyrinth of cracks and fissures a l l contributing to a much greater effective surface area. Thus etching this away reduces the signal to a negligible level. Pursuing this hypothesis further, Haneman constructed an apparatus for cleaving s i l i c o n crystals i n high vacuum and studying the weak ESR signal from the bare s i l i c o n surfaces thus generated. From this work he estimated that about 1 spin per 10 surface atoms was paramagnetic. Other workers have, however, not been convinced that the s i g nal observed i n crushed samples i s explained simply by a large effective surface, but attribute the ESR signal to defect structures. A recent interesting contribution (10) to this controvers i a l question was made when evidence was found that the ESR signals obtained by Haneman from cleaved samples were spurious. It was shown that i n fact cleaving created a fine dust at the cleavage surface and when this was wiped off the signal became considerably weaker. The ESR signal was thus attributed to amorphous regions in severely damaged powders rather than dangling surface bonds. An interesting by-product of the ultrahigh vacuum experiments on crushed s i l i c o n has l i t t l e to do with s i l i c o n i t s e l f , but i s worth mentioning i n the general context of this conference. Heat treatments i n vacuum of the s i l i c o n powder produced a sharp resonance at g = 2.0028, and the properties of this signal as a function of ambient and temperature were studied (11,12). Hypotheses in terms of the structure of the s i l i c o n surface were proposed to account for this signal, but i t eventually turned out that the identical signal was seen when other materials aside from S i were studied. The cause was traced to leakage of small amounts of carbon compounds from the o i l i n the vacuum systems which adsorbed on the s i l i c o n surface and at elevated temperatures formed radical compounds. Indeed the properties of this signal were found to be similar to those of the signal from heated carbonaceous materials (13). Effects of Thermal Oxidation We now take up the important area of thermal oxidation of the s i l i c o n surface. There are several types of defects that have been identified as influencing the behavior of MOS devices (I). First there are positively charged defects permanently located i n the oxide near the oxide-silicon interface. These are thought to be associated with the nonstoichiometry at the interface region. A second type of defect also consists of positive charges i n the oxide. However, these are due to mobile a l k a l i metal ions, and their distribution i n the oxide can be altered by the application of electric f i e l d s . Great care i s taken to eliminate sodium

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contamination from the p r o c e s s i n g , s i n c e such a v a r i a b l e charge system causes the parameters of the f i n a l device t o be u n s t a b l e , A t h i r d category i s the s o - c a l l e d f a s t s u r f a c e s t a t e s which e x i s t at the i n t e r f a c e . These a r e t r a p s which can r a p i d l y be charged or discharged by e l e c t r o n s from the s i l i c o n . These are assumed to be due e i t h e r t o s t r u c t u r a l d e f e c t s a t the i n t e r f a c e , i m p u r i t y atoms i n t h i s r e g i o n , or both. F i n a l l y , there i s the s p e c i a l case of p o s i t i v e l y charged d e f e c t s i n the oxide caused by i o n i z i n g radiation. I t i s thus important t o study the ESR s i g n a l s (see F i g u r e 1) that a r e generated by thermal o x i d a t i o n (14,15), to i n t e r p r e t them w i t h respect to the s t r u c t u r e of the o x i d i z e d s i l i c o n s u r f a c e and, i f p o s s i b l e t o r e l a t e them t o the e l e c t r i c a l l y observed d e f e c t s . N i s h i (15), having observed three d i s t i n c t resonances, wanted to determine t h e i r l o c a t i o n w i t h i n the s u r f a c e . He made s u c c e s s i v e slow etches i n t o the oxide l a y e r and then i n t o the s i l i c o n , and i n between etches recorded the ESR s i g n a l s t r e n g t h s . I n t h i s way he determined that the P and Pg s i g n a l s were i n the oxide. He succeeded i n p r o f i l i n g the stronger Pg s i g n a l , and determined t h a t the peak of the Pg d i s t r i b u t i o n i s near the o x i d e - s i l i c o n i n t e r f a c e . The Pç s i g n a l was a l s o p r o f i l e d , and i t extended about 10 microns i n t o the s i l i c o n bulk. I n our r e p e t i t i o n s of t h i s work (16), we found VQ a c t u a l l y extending much f u r t h e r i n t o the bulk. Various a u x i l i a r y experiments l e d us to conclude that Ρς was ac­ t u a l l y due t o i r o n i m p u r i t i e s (Fe°) that were d i s t r i b u t e d through the s i l i c o n . The heat treatment e i t h e r converted p r e v i o u s l y pres­ ent i r o n ions to the Fe^ s t a t e , or caused i r o n atoms from the o u t s i d e t o d i f f u s e i n t o the m a t e r i a l . The Pg s i g n a l appeared t o be the most s i g n i f i c a n t w i t h regard to i t s relevance t o the i n t e r f a c e s t r u c t u r e . I t was noted (15) that the s p a t i a l d i s t r i b u t i o n of the Pg center i s the same as that measured f o r the f i x e d s u r f a c e change, i . e . concentrated near the o x i d e - s i l i c o n i n t e r f a c e . J u s t as these e l e c t r i c a l l y charged species were a t t r i b u t e d to d e f e c t s i n the s t o i c h i o m e t r y near the i n t e r f a c e , so i t seemed l i k e l y that Pg r e s u l t e d from a defect i n the Si02 s t r u c t u r e . N i s h i concluded that i t was due to a d e f e c t where a s i l i c o n atom has only three e l e c t r o n s bonded and one hanging l o o s e . This c o n c l u s i o n was r e i n f o r c e d by comparison w i t h the resonance e x h i b i t e d by s i l i c o n monoxide (17) . This amorphous m a t e r i a l has many d e f e c t s i n i t s b u l k s t r u c t u r e , and i t s strong resonance w i t h g « 2.0055 has been a s c r i b e d to t r i v a l e n t s i l i c o n w i t h one unpaired e l e c t r o n . Now the Pg s i g n a l i n the o x i d e - s i l i ­ con t r a n s i t i o n r e g i o n i s a n i s o t r o p i c w i t h respect to the angle be­ tween the magnetic f i e l d and the s i l i c o n s u r f a c e , ranging from about g » 2.002 t o g - 2.010, the average value being ^2.005. Thus the resonance a t the o x i d i z e d s i l i c o n s u r f a c e may be the same as t h a t i n bulk s i l i c o n monoxide. In our l a b o r a t o r y we were concerned w i t h one f l a w i n t h i s i d e n t i f i c a t i o n . The s i l i c o n monoxide resonance i s r e a d i l y observ­ able a t room temperature, although of course a few times weaker A

15.

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than a t 7 7 ° . Therefore, i t was p u z z l i n g t h a t N i s h i reported t h a t none of h i s ESR s i g n a l s were v i s i b l e a t room temperature. We t h e r e f o r e r a n some of our samples c o n t a i n i n g P R a t room tempera­ t u r e , and indeed found that the P R s i g n a l s were v i s i b l e and of the expected magnitude ( 1 1 ) . Thus t h i s o b j e c t i o n a g a i n s t the i d e n t i ­ f i c a t i o n of the P R centers and SiO centers i s removed. I t i s remarkable that although the thermal oxide r e g i o n i s c h a r a c t e r i z e d as n o n c r y s t a l l i n e , the P R s i g n a l i s a n i s o t r o p i c w i t h respect t o the s u r f a c e d i r e c t i o n . This i n d i c a t e s that a t l e a s t the p o r t i o n of the oxide near t h e i n t e r f a c e r e g i o n has a macroscopic symmetry determined by the s i l i c o n s u b s t r a t e . Photoinduced ESR Due t o E t c h i n g About three years ago a new and unusual k i n d o f s i l i c o n s u r ­ face resonance was announced by a group a t Tohoku U n i v e r s i t y ( 1 8 ) . I t had been assumed t h a t e t c h i n g s i l i c o n merely serves t o remove the damage r e g i o n and i t s attendant ESR s i g n a l . However, i t was found that a f t e r e t c h i n g , a new resonance appeared i f the sample was simultaneously exposed t o l i g h t (see F i g u r e 2 ) w i t h energy g r e a t e r than the s i l i c o n band gap. The resonance appeared t o have two components and d i s p l a y e d some a n i s o t r o p y w i t h respect t o the angle between the s u r f a c e and the e x t e r n a l magnetic f i e l d . I t was a l s o noted that the p h o t o s i g n a l response depended on the con­ d u c t i v i t y type, η or p, and the e f f e c t was not observed i n samples w i t h too h i g h a c o n d u c t i v i t y . I t was suggested t h a t these resonances were due t o water being i n c o r p o r a t e d during the e t c h i n g and r i n s i n g process. I n order t o t e s t the h y p o t h e s i s , we decided i n our l a b o r a t o r y t o per­ form v a r i o u s chemical treatments on the s u r f a c e ( 1 6 ) . F i r s t we v e r i f i e d that treatment w i t h HF alone d i d not produce the photos i g n a l . This was t r u e whether the HF was used on a f r e s h s i l i c o n s u r f a c e o r a t h e r m a l l y o x i d i z e d s u r f a c e . Of course HF does not etch i n t o the s i l i c o n i t s e l f . Therefore, we t r i e d other types o f chemical etch that do a t t a c k s i l i c o n . These i n c l u d e d hot potas­ sium hydroxide, an etch c o n s i s t i n g of HF mixed w i t h Η2Ο2» and a procedure of t r e a t i n g the sample s u c c e s s i v e l y i n HF and hydrazine. Although these etches encompass a v a r i e t y of chemical types, none of them were e f f e c t i v e i n producing the p h o t o s i g n a l . Since i n a l l of them water was i n contact w i t h the s u r f a c e , the s u p p o s i t i o n that water was r e s p o n s i b l e f o r the p h o t o s i g n a l seemed d o u b t f u l . We a d d i t i o n a l l y checked t h a t a c e t i c a c i d was not necessary t o pro­ duce the p h o t o s i g n a l , so we suggested t h a t the presence o f HNO3 was the c r u c i a l f a c t o r , and that some k i n d of nitrogenous r a d i c a l was r e s p o n s i b l e f o r the resonance. Another q u e s t i o n we addressed was the l o c a t i o n of the ESR centers w i t h respect t o the s u r f a c e . One would guess t h a t they are s u p e r f i c i a l l y adsorbed atoms. One would l i k e t o s t r i p away the outer atomic l a y e r s g r a d u a l l y and check the ESR every time. C e r t a i n l y one cannot use f o r t h i s purpose a weak v e r s i o n of the

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standard s i l i c o n e t c h , s i n c e i t i s t h i s etch that generates the p h o t o s i g n a l . However, one can s a f e l y t r y HF alone, s i n c e we know that by i t s e l f i t does not generate the s i g n a l . I t was found that e t c h i n g i n concentrated HF does not v i s i b l y reduce the photos i g n a l , even though i t presumably d i s s o l v e s the outer 20-angstrom l a y e r o f oxide. I t was found, however, that repeated c y c l e s of HF treatment and a i r exposure, i . e . , o x i d a t i o n and d i s s o l u t i o n of s u c c e s s i v e 20-angstrom l a y e r s , do d i m i n i s h the p h o t o s i g n a l r e markably. Thjs i n d i c a t e s that the p h o t o s i g n a l center may extend to about 100 A i n t o the s i l i c o n . In a recent p u b l i c a t i o n (19) the Tohoku U n i v e r s i t y group have explained t h i s photoinduced s i g n a l as being q u i t e d i f f e r e n t from the normal ESR phenomenon. To understand the mechanism l e t us r e f e r t o a previous study by Lepine (20). He etched a s i l i c o n wafer, i n s e r t e d i t i n t o an ESR c a v i t y , and simultaneously i r r a d i ated i t w i t h l i g h t and w i t h a microwave f i e l d . As the magnetic f i e l d was v a r i e d he monitored the change i n r e s i s t i v i t y o f the sample (a few p a r t s i n 10^) as the resonant f i e l d was t r a v e r s e d , and a resonance i n the r e s i s t i v i t y curve was traced out. H i s e x p l a n a t i o n was that the o p t i c a l r a d i a t i o n , by generating e l e c t r o n - h o l e p a i r s , c o n t r i b u t e d t o the c o n d u c t i v i t y o f the sample. The recombination r a t e o f the e l e c t r o n s and h o l e s , however, i s i n f l u e n c e d by recombination centers a t the s u r f a c e o f the s i l i c o n . These centers are a l s o paramagnetic, and the recombination r a t e depends on the r e l a t i v e s p i n o r i e n t a t i o n between the recombination centers and conduction e l e c t r o n s . Thus when these surface d e f e c t s are paramagnetically s a t u r a t e d , the c o n c e n t r a t i o n o f e l e c t r o n s and holes i s a l t e r e d and a change i n the r e s i s t i v i t y i s observed. Now we r e t u r n t o our photoinduced s i g n a l observed by ESR.

Figure 2. Optically-generated ESR signal in single-crystal silicon etched in HNO HF-HAc with reformed native oxide: (a) light off, (b) light on s

2.010

2.005

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The proposed theory (19) i s that we are not observing the usual ESR e f f e c t , i . e . a change i n the c a v i t y Q due to paramagnetic l o s s e s , but a change i n the e l e c t r i c l o s s e s due t o the changing sample c o n d u c t i v i t y , which i s i n t u r n caused by s a t u r a t i o n o f the s u r f a c e paramagnetic c e n t e r s . This i n t e r p r e t a t i o n i s supported by c e r t a i n o f our observations (21). F i r s t , we have o f t e n been perplexed by the f a c t that the photoinduced ESR depends on j u s t how the sample i s p o s i t i o n e d i n the dewar v e s s e l . According to the theory that we are a c t u a l l y observing e l e c t r i c l o s s e s i n the c a v i t y t h i s i s reasonable, s i n c e the normal sample p o s i t i o n i s i n a r e g i o n o f maximum magnetic f i e l d and low e l e c t r i c f i e l d , but i f the sample i s d i s p l a c e d i t enters a r e g i o n o f higher e l e c t r i c f i e l d which couples more e f f e c t i v e l y w i t h the changing r e s i s t i v i t y . Another p e r t i n e n t experimental f a c t i s our work w i t h s i l i c o n - o n - s a p p h i r e samples. One of the newer developments i n i n t e g r a t e d current technology i s the f a b r i c a t i o n of c i r c u i t s on very t h i n s i l i c o n s i n g l e - c r y s t a l f i l m s . These f i l m s , which may be a micron t h i c k are deposited on sapphire s u b s t r a t e s . We have obtained samples w i t h a s i l i c o n t h i c k n e s s of about 10 microns, and have etched them and looked f o r the photoinduced s i g n a l , and have not found i t . Now i f we are observing photoinduced resonances d i r e c t l y from the paramagnetic s u r f a c e c e n t e r s , i t should not make any d i f f e r e n c e i f we are u s i n g an unusually t h i n sample. However, i f we are observing an i n d i r e c t e f f e c t o f cond u c t i v i t y which i n v o l v e s the bulk of the m a t e r i a l , i t i s understandable that the t h i n s i l i c o n - o n - s a p p h i r e sample i s i n e f f e c t i v e . NMR R e l a x a t i o n Enhancement by S i Powders F i n a l l y we have explored an i n d i r e c t method o f studying the s u r f a c e o f s i l i c o n by nuclear-magnetic-resonance measurements o f l i q u i d s i n contact w i t h s i l i c o n powders (16). I t i s w e l l known that paramagnetic molecules d i s s o l v e d i n l i q u i d s d r a s t i c a l l y shorten the NMR s p i n - l a t t i c e r e l a x a t i o n of the l i q u i d s . Furthermore, i n such systems one can have the dynamic p o l a r i z a t i o n e f f e c t , where pumping the e l e c t r o n i c paramagnetic l e v e l s produces l a r g e enhancements o f the NMR s i g n a l . I t i s a l s o p o s s i b l e to produce these e f f e c t s by mixing powders of paramagnetic m a t e r i a l s w i t h l i q u i d s that do not d i s s o l v e them, where the e l e c t r o n - n u c l e a r i n t e r a c t i o n only takes p l a c e a t the s u r f a c e o f the powder g r a i n s . For example, granules o f DPPH sieved between 50 and 100 mesh screens, and mixed w i t h water, g i v e a proton dynamic p o l a r i z a t i o n enhancement o f about -20 ( e x t r a p o l a t e d t o i n f i n i t e pumping power) (21). This r e s u l t was obtained w i t h a dynamic nuclear p o l a r i z a t i o n (DNP) apparatus operating a t a low magnetic f i e l d (74 gauss). A corresponding shortening o f T\ a l s o i s observed. We then wanted t o see i f s i m i l a r l y sieved crushed S i powder has enough paramagnetic d e f e c t s a t i t s s u r f a c e t o cause s u r f a c e r e l a x a t i o n and dynamic p o l a r i z a t i o n e f f e c t s . A number of samples were prepared u s i n g powders prepared from two d i f f e r e n t kinds of r a t h e r low p u r i t y s i l i c o n , and these were mixed w i t h v a r i o u s l i q u i d s . A l s o a set o f o x i d i z e d s i l i c o n powders were prepared.

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In a l l cases, the relaxation time became considerably shorter, but there was no evidence of dynamic polarization. The possibil­ i t y was considered that the liquid molecules complexed with the surface so as to form a region of much longer molecular correla­ tion time than in the bulk. However, this hypothesis proved quantitatively unreasonable. The conclusion was thus drawn that paramagnetic defects at the s i l i c o n surface were responsible for the shortening of Τχ, and these defects have a very broad reso­ nance (unlike the g « 2.0055 surface-damage signal) which makes i t unobservable by electron paramagnetic resonance and also pre­ vents the dynamic polarization effect. Further experiments were performed with a single liquid, hexane, in order to study the effects of various processes on the surface relaxation. Three samples of s i l i c o n powder are included in Table II, which gives the change in the hexane Τ χ due to suc­ cessive treatments of these powders. A l l samples were 50-100 mesh size, and were made from Fisher Chemical Co. material. Note that Τ χ of the bulk hexane i s about 9.5 sec. From the results listed i n Table II we concluded that Table II Proton Relaxation Times of Hexane Mixed with Silicon Powders Sample

No. 1

T| (sec)

freshly crushed treated with HF slow Si etch heated 600°C 1 hr 0 heated 1000°C 2 hr 0 oxidized 1 hr 1050°C etched in HF oxidized 2 hr 1000°C etched i n HF 2

No. 2 No. 3

2

.55 .7 7.5 8.5 10.5 .65 5.2 1.5 5.9

oxidation i n i t s e l f has a minor effect on the s i l i c o n surface re­ laxation. Likewise treatment of freshly crushed s i l i c o n with HF has l i t t l e effect. However, when an appreciable portion of the surface-damage layer i s removed, a large increase in Τ χ occurs. This can be accomplished either by using an etch that attacks s i l i c o n , or by thermal oxidation followed by removal of the oxi­ dized layer with HF. This may mean that the surface paramagnetic centers are associated with damage sites, or that removal of the innumerable irregularities of the damaged surface drastically re­ duces the effective area of the surface and the total number of paramagnetic sites accessible to the molecules of the liquid. In any case, the electron paramagnetic centers must be located at the outer edge of the oxide layer, whether i t i s the native oxide or a thermally grown oxide. Thus, in contrast with the ESR Pg

15.

C A P L A N

Defects in Surface of

Silicon

181

s i g n a l , which i s due to d e f e c t s a t the S i - S i 0 2 i n t e r f a c e , the NMR r e l a x a t i o n measurements r e f l e c t unpaired e l e c t r o n s very c l o s e to the oxide s u r f a c e .

Abstract Several magnetic resonances have been observed in studies of the surface region of processed silicon. A readily detectable resonance occurs when the surface is damaged by abrasion, and it is removable by etching. Treatment with HF-HNO3 etches creates paramagnetic states which become observable upon optical irradiation. Thermal oxidation causes three resonances to appear at different locations with respect to the Si-SiO2 interface. Indirect evidence for a broad ESR resonance center located near the oxide surface has been inferred from NMR relaxation-time measurements of silicon powder mixed with liquids. The relevance of these results for elucidation of the electrical properties of the important Si-SiO2 structure is noted. Literature Cited (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21)

Deal, B.E., J. Electrochem. Soc. (1974) 121 198C. Revesz, A.G. and K.H. Zaininger, RCA Rev. (1968) 29 22. Walters, G.K., J. Phys. Chem. Solids (1960) 14 43. Fletcher, R.C., W.A. Yager, G.L. Pearson, A.N. Holden, W.T. Read, and F.R. Merritt, Phys. Rev. (1954) 94 1392. Feher, G., Phys. Rev. (1959) 114 1219. Chung, M.F. and D. Haneman, J. Appl. Phys. (1966) 37 1879. Chung, M.F., J. Phys. Chem. Solids (1971) 32 475. Haneman, D., Phys. Rev. (1968) 170 705. Lemke, B.P. and D. Haneman, Phys. Rev. Lett. (1975) 35 1379. Kaplan, D., D. Lepine, Y. Petroff and P. Thirry, Phys. Rev. Lett. (1975) 35 1376. Kusumoto, H. and M. Shoji, J. Phys. Soc. Japan (1962) 17 1678. Chan, P. and A. Steinemann, Surface Sci. (1966) 5 267. Miller, D.J. and D. Haneman, Surface Sci. (1970) 19 45. Revesz, A.G. and B. Goldstein, Surface Sci. (1969) 14 361. Nishi, Y., Japan. J. Appl. Phys. (1971) 10 52. Caplan, P.J., J.N. Helbert, B.E. Wagner and E.H. Poindexter, Surface Sci. (1976) 54 33. Mizutani, T., O. Ozawa, T. Wada and T. Arizumi, Japan. J. Appl. Phys. (1970) 9 446. Shiota, I., N. Miyamoto and J. Nishizawa, Surface Sci. (1973) 36 414. Ruzyllo, J., I. Shiota, N. Miyamoto and J. Nishizawa, J. Electrochem. Soc. (1976) 123 26. Lepine, D.J., Phys. Rev. (1972) B6 436. Caplan, P.J., J.N. Helbert and E.H. Poindexter (unpublished).