Electron paramagnetic resonance study of defects in oxidized and nitrided porous Si and Si1−xGex

COLLOIDS AND - ' ~ ~

ELSEVIER

Colloids and Surfaces A: Physicochemical and Engineering Aspects 11511996) 277 289

A

SURFACES

Electron paramagnetic resonance study of defects in oxidized and nitrided porous Si and Si _,:Ge H.J. yon Bardeleben

*, M . S c h o i s s w o h l ,

J.L. Cantin

Groupe de Physique des Solides, Universitbs Paris 6&7, U RA au C N RS I ~, 2, place dussieu. 75251 Paris Cedex 05, France

Received 6 October 1995: accepted 30 January 1996

Abslraet

Point defects in bulk Si have been studied for over thirty years and a very detailed understanding of their microscopic structure and electronic properties has been achieved. The recent development of porous Si has allowed a further advancement in Si related defect studies, which will be reviewed here. Two aspects of porous Si have turned out to be of particular importance for defect studies by the magnetic resonance techniques: the larger number of nanocrystals contained in a cubic millimeter sized sample and the high internal surface area with values up to 1000 m ecm ~. l h e first allows classical EPR studies of defects in nanocrystals and the second the observation of interracial defects under highly improved conditions. We present recent results on donor defects in nanocrystalline Si, detailed EPR and ENDOR measurements of the Pb center in Si/SiO 2 prepared by native, thermal and anodic oxidation, the study of the t:O superhyperfine interaction of the Pt, center in isotopically enriched samples, the study of the Pbl center, the dominant interface defect at (100) Si/SiO 2 interfaces, as well as of the native defect structure in very thin I v 20 ei) thermal oxides and nitrides. First results on interfacial defects in oxidized SiGe epitaxia[ layers will also be reported.

Keywords: Defects: Electron paramagnetic resonance 1. Introduction The recent discovery [ 1] of r o o m temperature visible photoluminescence and electroluminescence in p o r o u s silicon (PSi (Fig. I) has initiated a wide ranging research activity on the correlation between the structural transformation of the monocrystaIline Si induced by the pore formation process and the resulting modified optical properties [ 2 ] . It is generally assumed that the basic mechanism for the energy shift in the optical absorption and emission are q u a n t u m confinement related effects, which become of importance for

* Corresponding author. 0927-7757/96/$15.00
nanocrystals with sizes below ~ 30 A [ 3 ] (Fig. 2). It has further been shown that, in addition to the size of the nanocrystals, their geometrical form as well as their surface passivation are also of importance for the carrier recombination processes. Quantitative information about the surface oftenrations, their distributions, their geometrical form and the relationship with the pore structure are however difficult to obtain. The q u a n t u m confinement effect is also expected to modify the electronic structure of point defects in the nanocrystals (Fig. 3/ as c o m p a r e d to those known from bulk crystal studies [4,5]. As a macroscopic porous layer can actually be considered as a large ensemble of oriented and connected nanocrystals, defect studies with magnetic

278

H.J. yon Bardeleben et al./'Colloids Su~ faces A." Physicochem. Eng. Aspects 115 (1996) 277-289

Photon energy (eV) 2.0 t . 8

I.G

11.

1.2

1.0

I I

I

I

I

I 4.2K

PSI ~

lOWcm-2

514.5nm °~ e-

PS2

BETo

/

l.

77°/o

PS1

PS2 0.5

1,5

e-

2.5 Cristallite

e"

~

Fig. 2. Optical band gap as a function of the diameter of crystallites ( + ) and for wires with different crystallographic orientations, x,[ I00]; o,[ 111 ]; *,[ 1103; after Ref. [3 ].

64°/o 30°/o

3.5 (nm)

Diameter

x 4 0 ~

BETo 3.0

1

i

I

I

1

1

I

1

O.G

0.7

0.8

0.9

1.0

1.1

1.2

1.3

Wavelength

1.0

1.L

(nm)

Fig. 1. Low temperature photoluminescence spectra of porous Si as a function of porosity; for comparison bulk Si (0% porosity) is shown below, after Ref. [44]. r e s o n a n c e spectroscopy, which require high defect n u m b e r s - - in excess of 10 ~° spins in cubic millimeter sized samples - - s h o u l d be feasible. Such studies have n o t yet been p e r f o r m e d in n o n - p o r o u s semic o n d u c t o r microstructures. A n o t h e r distinct p r o p e r t y of p o r o u s semicond u c t o r s is their high internal surface a r e a [ 6 ] . It is k n o w n from gas a d s o r p t i o n m e a s u r e m e n t s that this surface a r e a can take values of up to 1 0 0 0 m 2 c m -3. C o n s i d e r i n g a cubic s a m p l e of 1 cm 3, this c o r r e s p o n d s to an increase of the surface from 6 to 107 cm2; such n u m b e r s show i m m e d i a t e l y the p o t e n t i a l of p o r o u s s e m i c o n d u c t o r s for surface related defect studies with techniques such as electron p a r a m a g n e t i c r e s o n a n c e s p e c t r o s c o p y ( E P R ) a n d electron nuclear d o u b l e r e s o n a n c e

A

e-

-1.0

-3.0

1

[

I

1.0 2.0 Cristallite

I

I

3.0 Diameter

I

I

4.0 (nm)

Fig, 3. Variation of the band gap (solid line) and the energy levels of hydrogen impurities as a function of the crystallite diameter. For comparison the gap of bulk Si is shown (horizontal lines); after Ref. [45]. s p e c t r o s c o p y ( E N D O R ) . In the case of bulk crystals such studies were rendered difficult ( E P R ) o r i m p o s s i b l e ( E N D O R ) due to the low available spin numbers. Just as in the case of bulk Si samples, the surface p a s s i v a t i o n of the a s - p r e p a r e d p o r o u s samples can be m o d i f i e d by different o x i d a t i o n processes in

H.J. yon Bardeleben et al./Colloids" Sur/~tces A." Physicochem. Eng. 4specls 115 (1996) 277 2,W

order to transform the large internal surface into a Si/SiO2 interface. The results obtained on such samples have shown [ % 10] that oxidized porous Si is a material of choice for the experimental study of interlace defects by magnetic resonance techniques. Due to the limit imposed by the pore dimension some 10 to 100A (Fig. 4 ) the SiO2 layers formed will be necessarily thin with typical thicknesses of 5-40 ~,; this fact should be kept in mind, when the results obtained in PS are compared with the previous ones obtained in bulk samples, where generally oxides of much higher thickness ( ~ 1000 ~,) were used. The interface defects in the bulk Si/SiO2 system have been studied before by magnetic resonance techniques; for a review of the first results see Refs. [11 14]. Many detailed results concerning the nature of the defects, their concentrations, their localization in the interface plane, their electrical activity and their hydrogen passivation properties have been obtained by the groups of Poindexter [11], Brower [12,13] and Stesmans [14]. More

279

recently further studies of the dependence of tile Pb center parameters on the oxide type, interface strain and defect concentration have been reported [15,16]. The most detailed information was obtained for the ( l l l ) S i / S i 0 2 interface: for the technologically more important ( 100)Si/SiO2 interface, however, the results are much less complete. The high specific surface area of porous semiconductors also opens the way for the study 0t" defects in ultra thin dielectric SiO2 or Si3N a layers using PS as a substrate material [17,18]. The defect properties and electrical properties of such layers are currently of importance for the realization of microelectronic devices employing dielectric layers with thicknesses of less than 100 A. As compared to "'conventional" magnetic resonance studies on bulk substrates the use of porous layers can improve the sensitivity limitations bv ~ 103, setting the minimal defect concentrations at ~ 1014 cm 3 The formation of porous semiconductors is not restricted to the case of Si. Indeed the elaboration of porous SiGe and SiC has been reported recently.

~;

b Fig. 4. TEM cross-sectionand top view of a porous Si laver Ip , ( 1001substratt,'), from Rcf. 146 I.

280

H.J. von Bardeleben et al./Colloids SurJdees A: Physicochem. Eng. Aspects 115 (1996) 277 289

We will present here recent results on oxidized porous SiGe where Ge Pb centers were observed [19].

2. Porous Si: formation and characterization The starting materials for the preparation of porous Si are conventional Si substrates of p, p+, n or n + conductivity. Porous layers can be prepared by different dissolution processes such as electrochemical [20] or purely chemical (stain etching) dissolution [21]. In the first case the substrate is placed in an electrochemical cell with suitable electrolyte such as hydrofluoric acid/alcohol mixtures with the substrate serving as the anode. The porosities and pore structures obtained depend on the electrolyte composition and the current densities. In the generally used configuration, where a sealed metal contact is made to the backside of the wafer, a porous layer with typical thicknesses between 1 and 100 gm and porosities between 50 and 80% will be formed on one side of the substrate only. The porosity is defined as the ratio of the pore volume to the total volume; this means in a 80% porosity layer that only 20% of the original silicon content is left. The detailed dissolution mechanism is still under debate; Lehman and G6sele [22] for example have proposed a successive fluoridation and hydrogen passivation process. Most of our magnetic resonance studies have been performed on layers prepared from (100) oriented p + substrates; in this case, a vertical pore system following the [ 100] crystallographic directions with smaller lateral deviations is formed (Fig. 4). The pore sizes and the size of the remaining Si structure depend on the porosity; typical values are 100 A and 20-50 A respectively. Porous samples with or without quantum confinement effects can be produced, a porosity of ~ 70% being the limit for the two cases. The interface porous layer/substrate is flat and parallel to the external surface. An interesting point is that anodic dissolution allows the elaboration of homogeneous porous layers over thicknesses of several hundreds of microns, which makes it possible to prepare free standing porous layers.

Table 1 Impurity analysis of a PS 80% sample; values are given in atomic fractions Sample

H/Si

O/Si

F/Si

C/Si

p + 80%

0.66

0.02

0.02

~<0.1

In the second approach, stain etching, porous layers are formed without the necessity of electrical contacts. This second approach, which is more adapted for technological applications, produces samples of maximum thicknesses of ~<1 gm only, due to a continuous dissolution of the top porous layer. In the as-prepared state the surface of the porous layers is hydrogen passivated with some minor contamination by fluorine. When the sample is exposed to ambient conditions oxygen and carbon contamination can readily occur. Table 1 shows a typical chemical analysis by ion beam analysis of a 10pro thick, aged p+-type PS layer of 80% porosity [23]. The H content of > 50% reflects the high fraction of Si surface atoms in nanocrystals but it includes equally possible contamination by hydrocarbon molecule adsorption. Most of the C contamination is attributed to the adsorption of atmospheric contaminants. X-ray diffraction measurements have shown that the PS samples are monocrystalline with a porosity-dependent shift of the lattice constant in the range Aa/a~ 10 -4 to 10 -3. The origin of the lattice expansion has been attributed to the Si-H bond interaction at the surface of the nanocrystals.

3. EPR Studies of P donors in nanocrystalline porous silicon Among the possible defects in Si nanocrystals to be studied by magnetic resonance techniques the P donor seemed to be an ideal candidate. This defect, which is a shallow effective mass donor in bulk Si, is expected to become a deep donor in nanocrystals due to the quantum confinement effects [5] (Fig. 3). As a consequence, the central hyperfine interaction with the 100% spin l/2 P

281

H.J. yon Bardeleben et aL / Colloids SmJaces A : Physicochem. Eng. Aspects 115 ( 1996i 277 2,~9

nucleus is expected to vary with the size of the nanocrystals. This would give direct access to the size distribution in a macroscopic PS layer. We have studied n + doped porous Si layers at 300 K and in the 20 K temperature range. Contrary to the case of bulk n + samples free standing n + doped porous layers no longer show a free carrier resonance signal even at room temperature. The absence of free carriers is attributed to a highly increased ionization energy of the P donor and the compensating action of unpassivated Pb centers at the crystallite surfaces. Low temperature photoexcitation leads to the creation of free carriers as detected by the associated free electron resonance signal with lifetimes in excess of seconds; but no EPR signals associated with the P donor - - displaying I = 1/2 hyperfine interaction - - could be detected. Additional studies on improved passivated layers are under way in order to limit carrier recombination via surface/interface states.

4. EPR Studies in oxidized porous silicon

Most of our EPR measurements have been performed with an X-band spectrometer at room temperature or in the 4-20 K range, on single samples of a few square millimeters in area; typical sample characteristics are p+ doped (100) substrates, a porosity of 66% and thicknesses of 10 pan. Such samples have a specific surface of 300 cm 2, which multiplies their external surface by ~ 1000. Microwave powers of ~0.5 mW can be applied at room temperature without saturation effects. The spectra are generally taken in the absorption mode under slow passage conditions, which allow quantitative concentration measurements.

Si/SiQ interfaces: the results obtained show that the g values and linewidths are not unique for these defects but depend in addition on the conditions under which the oxide is formed. However no correlation between the EPR parameters and specific interface configurations could be made. In spite of numerous complementary studies, the exact microscopic structure of the Si/SiO2 interface is still a matter of debate [24]. Additional experimental results, which might give more insight into the microscopic structure of the interface and the adjacent Si and SiO2 regions are clearly required. Magnetic resonance techniques applied to oxidized porous Si can be expected to be particularly useful for such studies as the high signal t o noise ratios obtainable should allow the determination of the ligand hyperfine interactions. Figure 5 shows typical EPR spectra obtained for an oxidized sample prepared from a {100) oriented substrate. For a rotation of the magnetic field in the (110) plane we observe a three line spectrum, which simplifies for BII [ 110] and BI! [ 111 ] into a two line spectrum and for B I [001] into a single line spectrum. The orientation and principal values of the g-tensor gli ~tL~1=2.0020+0.0002 and g± lt~1=2.0087 +0.0002 are identical to those previously reported for the Pb defect at the (lll)Si/native SiO, interface [25] !Fig. 6/ The trigonal Pu center is also known to exist at (100)Si/high temperature SiO e interfaces, where it

BII [111

4.1. The Pb center at (111) Si/Si02 interlaces and the Pho centre at (i O 0 ) S i / S i O 2 interlaces The Pb centers are intrinsic defects of the Si/SiO 2 interface, which reflect the structural misfit between the monocrystalline Si and the SiO2 surface layer. They are specific for the interface orientation. The Pb defects, which are paramagnetic in the neutral charge state, have been studied previously by magnetic resonance techniques for (111) and (100) bulk

3425

3430

3435

3440

3445

3450

3455

Magnetic Field B (G)

Fig. 5. EPR spectra of a typical porous p ' type layer after 4 0 0 ' C vacuum annealing (dotted linel and its decomposition in the central and 29Si SHF lines ot' the f'b center.

282

H.J. von Bardeleben et al./Colloids Surfaces A: Physicochem. Eng. Aspects 115 (1996) 277-289

o

E O &

50 30

'~

100

~ . . . . .

2001

/

/

i ......

/ I

2003

,

/ ,

2005

,

;

....

2007

2.009

g value

Fig. 6. Angular dependence of the g factor of the Pb center for a rotation of the magnetic field in the (110) plane.

is called the Pbo center. As PB and Pbo centers have the same symmetry, C3v, and similar g-tensor values it is not possible to distinguish them in the absence of additional information. Initial suggestions for a lower symmetry of the Pbo center were not confirmed by later results 1-14]. If(111) facetting of the (100) interface is at the origin of the presence of Pbo centers the distinction between Pb and Pbo might actually only be a semantic one. Another way to describe this situation is to say that the observation of Pb a n d / o r Pbo centers is proof of the existence of local or extended (i 11) interfaces. Nevertheless, the non-observation of Pbt centers, which are specific fingerprints for (100) interfaces, is in favor of the assignment of the interfaces observed to ( 11 I) interfaces. Whereas in EPR studies on flat bulk (111) interfaces only one dominant defect orientation, the one oriented normal to the macroscopic interface plane, is observed, in porous Si prepared from

p+ (100) substrates all four possible defect orientations are observed with the same probability due to the simultaneous presence o f ( l l l ) , (111), (1]1), (111) interfaces. At X-band frequencies the lineshape of the individual lines is Lorentzian; their orientation dependent linewidths are equally sample dependent and vary between z~Bpp-1.2 to 1.7G (B jJ [111]) and ABpp = 3.2G (BII [110]). Such values are comparable to those reported for Pb centers in high quality thermal oxides. The absence of any line broadening demonstrates that the distortion of the interface planes in PS relative to the substrate must be small. An estimation of the spin concentration from the total spin number and the internal surface area, shows that only a small fraction (< 1%) of the total surface is oxidized in these as-prepared samples. This is not surprising, given that the "normal" H passivation of the surfaces should lead to the absence of any paramagnetic defect. Whereas an analysis of as-prepared layers is of interest for the structural characterization of porous Si, the full potential of this material for interface defect studies requires controlled oxidation procedures leading to a complete oxidation of the surface. We have explored three different oxidation processes, which are (i) vacuum annealing in the 300-650°C range followed by exposure to oxygen after cooling the sample back to room temperature, (ii) anodic oxidation at room temperature and (iii) high temperature oxidation at 1050°C. After such treatments the Pb a n d / o r Pbo center concentration is increased by two orders of magnitude allowing a detailed study of the hyperfine interactions in non-isotopically enriched samples. In addition, some of the oxidation

Table 2 Principal values of the g-tensor and peak to peak linewidths AB (X-band) for Brl[ 111] and B± [ 111] for the Pb center Material

g,,

g±

ABpl(G)

ABj_ (G)

Ref.

Pb-native oxide Pb-HT Pbo-HT PS-native oxide (66% porosity) PS-native oxide (75% porosity) PS-HT (1000°C)

2.00186 2.0013-2.0016 2.0015 2.0019 2.0020 2.0022

2.0087 2.0087-2.0090 2.0087 2.0089 2.0084 2.0087

2.3 1.3

3.1

Stesmans [25] Brower [26] Poindexter [27]

1.2-1.7 1.7-1.9 1.7 1.9

3.2 3.2-3.4 3.2-3.4

H.J. yon Bardeleben et al./'Colloids Sulfates A: Physicochem. Eng. Aspects 115 (1996) 277 2<~?

conditions lead to the observation of a second EPR spectrum, which we have attributed to the Pbl defect [28], the defect characteristic of the (100)Si/SiO2 interface. Fig. 5 shows a typical EPR spectrum of a 4 0 0 ' C vacuum annealed oxidized sample. The excellent signal to noise ratio allows a quantitative decomposition of the spectrum revealing the 29Si superhyperfine (SHF) interaction. The g-values are not modified as compared to the as-prepared samples. The 2')Si SHF probes the microscopic structure of the Si lattice near the Pb defect. Previously in bulk samples, it had not been possible to study the SHF splitting for sensitivity reasons. Only one spectrum for the particular orientation Btl [111] has been published [31 ]; it had led to an estimation of a splitting of 15 G and 18G respectively in agreement with later calculations. More recently the angular variation of the 298i SHF had been measured by Carlos et al. [29] in SIMOX material, where the Si/SiO2 interface is thought to be formed by SiO 2 precipitates in the bulk Si matrix. In Table 3, we compare the SHF tensors published for porous Si with the values measured in bulk silicon and theoretical values calculated by Edwards for the trigonal Si dangling bond defect at the (111) interface. The agreement between experimental results and theory is fair and can be taken as further proof of the Pb center model. The central hyperfine interaction (HE) with the 4.7% abundant >St nucleus can equally be detected

Table 3 >St SHF interaction tensor l~r the Pb center at bulk (lll)Si/SiO2 interfaces, in SIMOX material and at porous Si/SiO2 interfaces Parameter

>St SHF interaction tensor Theory

Bulk silicon

Porous silicon

[31]

[8]

[29]

in porous Si (Figs. 7, 8). After an increase of the gain by ~ 5 0 we observe the hyperfine doublets associated with each of the central EPR lines. The first obserwttion of the 29Si HE interaction in bulk Si was reported by Brower [12] for the Pb center at ( 111 ) interfaces. For (100) interfaces the angular variation of the hyperfine interaction of the Pbo center has not been reported: only the value for Bll [001 ] has been published. As the measurements on the bulk samples were performed at low temperature under fast passage conditions quantitative intensity measurements were difficult: the use of porous Si overcomes this difficulty and the intensity ratio IHv/Icentra 1 c a n be determined quantitatively [32]. The intensity ratio measured confirms the assignment of the Pb and/or Ph,, defect to a Si dangling bond defect, a model recently questioned by Stallinga et al. [33]. An analysis of the isotropic s-part of the HF tensor and the anisotropic p-part shows that the differences in the values relate to differences in the localization of the electron wave function of :v 10% at the central nucleus. Whereas the H F interaction constant is expected to be more sensitive to the local microscopic structure of the interface around the Pb center than the g-tensor, the peak to peak width of the HF lines of 8 G in porous St, is lower than that observed in high temperature bulk oxidized samples, indicating a comparable or smaller distribution of local strain. In particular, we never observed different sets of hyperfine lines simultaneously in one samples, as one might have expected

B//{O0li

. . . .

This paper

..........

B//[llO]

7 ; [ l l 1] (G) Tj J i l l ] (G) Isill~/lcentra

I

27 21

Thermal oxide 15, 18

SIMOX 16 11.6 0.12-0.14

Native oxide 15.9 15.8 12.0 10.8 0.14 0.12

-

_

...........

+ ~

.-

. . . . . . . . . . .

.

.

.

.

A ,

Comment:

..r

7-

B//[III]

[30]

283

.

.....

.

..

~ , ,

.

A

H [vlT~'i i ill C

I, "

3340

c~,

\l~t"llCt

c. h t ' l , !

]?

~540

:,.~at t,

Fig. 7. EPR spectra of the Ph center displaying the ~"+Sicentral hyperfine interaction: note the change in gain for ElF and central lines

284

H.J. yon Bardeleben et al./Colloids Surfaces A." Physicochem. Eng. Aspects 115 (1996) 277 289 90 80 I_d k~J 6 0

~50 ~--~ 4 0 CI2 I--Z 3 0 ELI

c~20 o

10 i

i

.009

i

i

.O11

.013

i

.015

HYPERFINE CONSTRNT

.012

(10 5 cm -I )

Fig. 8. Angular variation - - (110) plane - - of the 298i central hyperfine interaction constant (0 ~ corresponds to BII [ 110].

in the case of the simultaneous presence of P b and Pbo centers. In samples submitted to different oxidation conditions the 29Si H F splittings are found to vary between 107 and 115 G for Bit [001]. These values include those reported by Brower for the Pb0 and P b centers respectively. Our results show however, that the hyperfine interaction constant is related to the oxidation conditions; it can not be taken as a fingerprint for a specific interface orientation. The high signal to noise ratio observed in the oxidized PS samples suggested that E N D O R measurements might be successfully performed in order to obtain a more detailed image of the immediate surroundings of the P b defect. Whereas in most models isolated P b defects in a cage structure at the interface are assumed, the experimental verification for such a model is still lacking and one cannot exclude a priori the existence of a more extended defect structure around the P b defects. E N D O R measurements can be expected to provide such information. Two E N D O R studies on as-prepared aged and vacuum annealed PS have been reported. In a first study [32], only two structureless E N D O R lines at the Larmor frequencies of 1H and 19F nuclei were observed. From their linewidths the minimum distances of the H and F nuclei from the Pb center have been esti-

mated to be ~ 13 A. This distance corresponds to the average separation of Pb centers of concentration ~1013cm -2. In a second refined study [-34], an additional angular dependent hyperfine interaction with three 1H shells was resolved (Fig. 9). The results have been successfully modeled by interactions with 1H nuclei situated at the second, third and fourth nearest neighbor sites in the (111) interface plane. This model implies

900 OO9

/f°~6

~'-'" °'~" -.

1

700 600 "~

500

.....

4OO :300 200 1OO O

30

60

90

'4) ( d e g ) Fig. 9. Experimental values and simulation of the hydrogen ligand hyperfine constants Aefr for three different neighbor shells (second, third and fourth) around the Pb defect [34].

285

H.J. yon Bardeleben et al./'Colloids Surlaces A: Physicochem. Eng. A.~'pects 115 (1996J 2 77 2,~'9

directly an extended defect structure for at least a fraction of the Pb centers. 4.2. The

/

,j,"\

_

P

lines

Phi center at ( lO0)Si/Si02 intetfaces

The ( 100)Si/SiO 2 interfaces have been shown to contain two different paramagnetic defects which have been labeled Pb0 and Pb~ respectively [27,35]. Whereas the Pbo center has been attributed to the ( l l l l oriented Si dangling bond defect, the Pb] center could not be modeled due to the lack of sufficient experimental information [36]. These difficulties were related to the low defect concentrations encountered at the (100) interfaces, the lower point symmetry of this defect and the simultaneous presence of Pbt and Pbo defects at comparable concentrations. We have recently shown [28], that high temperature (650~'C} vacuum annealing of PS can lead to the dominant formation of Pb~ defects with typical Pb~/Pbo concentration ratios of up to 4. Correlated hydrogen effusion and infrared measurements have shown this annealing process to be associated with the desorption of hydrogen from Sill surface atoms. From XPS measurements performed on the EPR samples it was deduced that the native oxide film had a thickness of only 5 A. The interfacial Si suboxides ( 1 + ,2 + ,3), the weights of which represent ~, 36% of the total Si 2p spectral density, have been observed with conventional XPS. The intensity ratio obtained, Si * :Si 2+ :Si 3" = 37:37:26, is typical for smooth ( 100t interfaces [37]. Fig. 10 shows a typical EPR spectrum obtained on such a sample. Whereas before the vacuum annealing the

N

~ / f ' Pt~o

ua

ines

Parameter

i

I(b)

34"15

3425

3435 3445 3455 Magnetic Field B (G)

spectrum was determined by the P b / P b o spectrum, increasing annealing temperatures lead to the formation of an additional second EPR spectrum ( Pbl }, which at the final stage of 650 C has become dominant. A thorough analysis of the angular dependence of the total EPR spectrum allowed us to determine the point symmetry, C~ monoclinic [, the orientation and principal values of its g-tensor. The results are shown in Table 5 and compared to the two previous results reported for bulk samples. The angular variation of the EPR spectra for the Pbo and Pbl centers superposed in Fig. 11 underlines the difficult in separating the two multiplet spectra in particular at K-band frequen-

41[lll]llO 4cm tl .t~[111] (10 4cm ~) A~[ 10o] (G)

146 85 116

Pb

Pb Pbo

141 7s

139

105

107

/Ill. /central Comment

SIMOX

[11]

Ill)Si,Si() 2 interfaces, at bulk

Porous silicon Pb0

[11]

3465

Fig. 10, EPR spectrum (B I [1 l i t ) of a 650 (" annealed stainetched PS layer and: (a) its decomposition into three Pb and two Puo lines: (b) the difference between the cxpermaen~.al and simulated spec! ra,

Bulk silicon Pb

"~ ~' ,

i

Table 4 Principal values of the central 29Si hyperfine interaction tensor A for the Pb center at bulk t 1001Si'SiO 2 interfaces, in SIMOX material and at porous Si,'SiO2 interfaces

Ref.

L

[29]

0.048 PS native oxide

[-]

14t~ 7S 114 PS native oxide This ~vork

286

H.J. yon Bardeleben et aL/Colloids Surfaces A." Physicochem. Eng. Aspects 115 (1996) 277 289

Table 5 Principal values, principal axes of the g-tensor of the Pb~ center in PS and bulk (100)Si/SiO; interfaces; effective g-values for Bll[001 ] and Bll[ll0 ] Component

Orientation

Porous silicon Bulk silicon This work ReC [35] Re[ [27] Ag = ± 0.0003

gx g3

[071] [211] [111]

2.0058 2.0029 2.0069

2.0058 2.0020 2.0084

2.0052 2.0012 2.0081

gtj[001]

[001]

2.0041

2.0035

gll[ll0]

[110]

2.0041 2.0058 2.0055 2.0058 2.0064 2.0037

2.0063 2.0058

2.0058 2.0052

gz

I

o

80

8

70 60

E

50 4O

1

]

I

1

I

l

1

'

/ /

\~ \ \

3O

Fig. 12. Microscopic structure of the strained bond (SB1) defect as proposed by Edwards [36]. Full and empty circles represent Si and oxygen atoms respectively. The defect atom is larger and colored grey.

least a factor two smaller than that of the Pb and Pbo defects, implying a delocalization of its electron wave function over more than one Si nucleus. This issue is of importance as the hyperfine splitting measured by Brower was one of the key arguments for eliminating defect models proposed by Edwards. The properties of the Pbt defect observed in PS are compatible with the dangling bond defect on a strained bond Si dimer (SBI) previously proposed by Edwards [36] (Fig. 12). Future studies are desirable to evaluate whether the Pb~ center parameters do depend equally on the oxidation conditions. 4.3. Pb centers at porous SiGe/SiGe02 interfaces

f

2.001

[

2.003

]

,

F

,

I

2.005

,

f

2.007

I

2.009

geif v a l u e

Fig. 1l. Angular dependence of the g-factors of the Pbo (dashed lines) and Pbl (solid lines) centers for the rotation of the magnetic field in the (110) plane.

cies, where the Pbo signal linewidth becomes comparable to that of Pbl. Unlike the Pb and Pb0 defects the linewidth of 4.5 G of the Pb~ center is isotropic and frequency independent, indicating an atomic configuration not influenced by stress distributions. In spite of a very favorable Pbl/Pbo ratio we were unable to observe the 29Si central hyperfine splitting for the Pb~ center reported by Brower [11]. Our results seem to show that the central 29Si hyperfine interaction of the Pb~ defect is at

Porous SiGe layers have recently been shown to be an alternative porous semiconductor material for room temperature visible photoluminescence with lifetimes in the nanosecond range [19,38]. The samples were prepared from epitaxial SiGe layers of ~< 1 pm thickness deposited on a Si substrate. Ge alloy compositions of 5 and 20% have been studied. The interface defects of SiGe/SiO2 bulk samples have never been studied before by magnetic resonance techniques, apart from the case of SIMOX samples [39]. Whereas the as-prepared porous SiGe samples showed only a low intensity Si Pb center EPR spectrum, after low temperature thermal oxidation samples with a Ge composition x = 0.2 present in addition a spin S = 1/2 multiplet spectrum with trigonal point symmetry and principal g-values gij=2.0008 and g_c=2.0210 (Fig. 13). This spectrum has been attributed to the Ge Pb

287

H.J. yon Bardeleben et al. ,,'Colhfi&" Su(face.s A." Physicochem. Eng. As'peers 115 (1996) 2 77 2,~'9

9 0 i~ _

' ' ~

~-,

, ,~7-

....

,--, 80~

i

\\

B, [ooi]

'-" 70 60 I .o. S0 30

i

~, 2oL < loL

tl_

0i . 2

g11=2.0008

g/=2.0210~ 2.005

2.01

geff

2.015

2.02

2.025

value

Fig. 13. Angular variation of the g-factor of the GePb ccnler observed in oxidized porous Si( sGe02. center at the SiGe/SiO2 interlace [19]. The gvalues vary slightly with the oxidation conditions (native/thermal oxidel.

5. EPR Studies of P. centers in 1~O enriched Si/SiOz interfaces The microscopic structure of the SiO 2 layer within the first 10 ,~ from the interface plane is still a matter of debate [24]. Global information about the oxidation states of Si atoms at the interface can be obtained by XPS measurements and have been related to dimer formation and oxygen bridging models. The magnetic resonance techniques can be used to probe the local structure around the Pb defects at this interface via the superhyperfine interaction. In recent calculations different models (Stathis, Stesmans, reactive model) [40] for the {lll)Si/SiO2 interface structure have been considered and the resulting Si and O hyperfine interactions have been calculated. Whereas the Si SH F interaction can be observed without need for isotopic enrichment due to the presence of 4.7% >)Si with spin I = 1,2, the observation of oxygen SHF interaction requires an isotopic enrichment with l ' O (I -- 5/2). The r ' O S H F interaction tensor has not yet been determined. In previous studies on bulk { 11 I)Si/SiO 2 structures the ~vO hyperfine interaction had only been observed for one orientation of the magnetic field, BII [111] [41-43]. The

34215l

13430

3 43 5 ~ 3 4 4 0

3445

34S0~3~45'5

' 13'460

Magnetic Field 8 (G) Fig. 14. Comparison of the Pt~ center EPR spectra obtained in non-enriched atld 51% 1~O enriched oxidized samples. The spectra are normalized to identical amplitudes for a better comparison of the lineshapes. orientation dependent linewidth of the Pb center EPR spectrum, which increases for magnetic field orientations other than [111] has not allowed an analysis of the angular dependence of the l:O S H F structure. The basic question of the anisotropy of this interaction related to a random distribution or ordered distribution of the nearest O neighbor atoms is still open. We have recently started an EPR stud}' on 300 C oxidized porous Si samples, oxidized m 51% 17o enriched dry oxygen. Reference samples taken from the same porous layer were oxidized under identical conditions in non-enriched oxygen. Fig. 14 shows comparative EPR spectra for the three principal orientations of the magnetic field. The effect of the l vO enrichment is clearly observed: the absence of resolved 170 superhyperfine structure does not allow a simple analysis of the spectra. It is however evident that the 170 SHF interaction is weak (hyperfine splitting v l G ) and nearly isotropic. Different decomposition models based on interaction with one or several equivalent/nonequivalent ~vO neighbor atoms in different shells are still being studied. These studies will be completed by lvO E N D O R measurements.

6. EPR studies of defects in thin dielectrics The utilization of thin (< 100 A) dielectric SiOa, Si3N 4 and SiO,N~. layers in modern microelec-

288

H.Z yon Bardeleben et al./Colloids Surfaces A." Physicochem. Eng Aspects 115 (1996) 277-289

tronic devices requires the s t u d y of their intrinsic p o i n t defects such as oxygen v a c a n c y related defects, the so-called E' centers, Si v a c a n c y centers, the EX centers as well as Si a n d N d a n g l i n g b o n d defects. It has been suggested that the c o m p o s i t i o n of such layers is n o t s t o i c h i o m e t r i c within the first few a n g s t r o m s , which s h o u l d lead to high defect c o n c e n t r a t i o n s n e a r the interface. E x p e r i m e n t a l evidence for the presence of such defects has n o t yet been o b t a i n e d by m a g n e t i c r e s o n a n c e techniques due to insufficient defect concentrations. O f p a r t i c u l a r technological interest is also the evolution of these defects u n d e r charge injection or 7 i r r a d i a t i o n k n o w n to induce a d e g r a d a t i o n of the electrical p r o p e r t i e s in these layers. We have r e c e n t l y s h o w n that p o r o u s Si, passiva t e d by thin ( ~ 20 A) SiO2 o r Si3N 4 layers, can be used as a m o d e l s u b s t r a t e m a t e r i a l for such a study. We have detected E'~ a n d Si D B centers ( K centers) as native defects in a n o d i c oxides a n d t h e r m a l nitrides respectively [ 17,18 ]. T h e r m a l high t e m p e r a t u r e oxides d o n o t c o n t a i n native oxygen v a c a n c y defects (E' centers) in the p a r a m a g n e t i c charge state.

7. Conclusion and perspectives The first results o b t a i n e d by m a g n e t i c r e s o n a n c e s p e c t r o s c o p y in p o r o u s Si confirm the high p o t e n tial of p o r o u s s e m i c o n d u c t o r s for the analysis of interfacial defects a n d defects in thin dielectric layers. T h e y are expected to lead to a detailed u n d e r s t a n d i n g of the m i c r o s c o p i c structure of the s e m i c o n d u c t o r / o x i d e interfaces a n d thin dielectric layers. The results o b t a i n e d are of b o t h f u n d a m e n tal a n d t e c h n o l o g i c a l interest given the i m p o r t a n c e of M O S structures in m i c r o e l e c t r o n i c applications. It can be expected that p o r o u s Si a n d SiGe will also be useful reference m a t e r i a l s for the study of d e g r a d a t i o n processes in oxide a n d nitride interface structures.

Acknowledgments We t h a n k A. G r o s m a n a n d V. M o r a z z a n i for the s a m p l e p r e p a r a t i o n s a n d t h e r m a l t r e a t m e n t s a n d F. R o c h e t for the X P S m e a s u r e m e n t s .

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