- Email: [email protected]
Characterization of porous silicon by NRA, RBS and channeling
622
Nuclear Instruments and Methods in Physics Research B45 (1990) 622-626 North-Holland
C~~~~~ON C. ORTEGA,
OF POROUS J. SIEJKA
SILICON BY NRA, RBS AND CONING
and G. VIZKELETHY
*
**
Groupe de Physique des Solides, lJniversit4 Paris VII, Tour 23, 2 Place Jussieu, 7.5221 Paris Cedex OS, France
Porous silicon could be an interesting material for microelectronic applications if the quality of porous films is significantly improved. This material has the ability to be easily oxidized at relatively low temperatures ( 4 850 o C). Moreover, it preserves the single-crystal characteri$ics of the substrate, allowing epitaxial growth of Si and GaAs at moderate temperat~~. The aim of this work was to determine the impurity content of porous films and to test their crystalline quality, as a function of preparation conditions, using ion beam analysis: NRA, ERDA, RBS and channeling. Relatively good channeling was observed in some porous materials ( xmi,, = 15%) in spite of the adsorbed impurities and the disorder due to the enormous surface area of the pores: 400 times higher than the geometrical surface area for a 2 urn thick porous layer. The surface density of the impurities is relatively low and can be decreased significantly by adequate surface preparation. Typical results are: 0.6~10’~ 0-at./cm2, 3~10” C-at./cm’, 1Or5 H-at./cm’ and 0.1 X lOI F-at/cm’.
1. In~uetion In the continous research for new materials for microelectronic applications, porous Si occupies a particular situation due to two interesting properties: the crystalline orientation of the substrate is preserved and, because of its high porosity (lo-70%), it is very rapidly oxidized at relatively low temperatures (7” < 850 o C) to form a thick oxide layer (2-10 pm). An excellent-q~lity Si epilayer has recently been grown by chemical vapour deposition (ND) at a low temperature (T < 83O’C) [l]. For these reasons it can be very attractive for silicon insulator technology (SOI). Porous Si can also be used as substrate for hetero-epitaxial growth. It is believed [2] that porous Si can accommodate, more easily than compact Si, the epitaxial growth of materials having different lattice parameters (GaAs, Ge, . - ’ ) and also different lateral expansion coefficients (oxide, fluoride, . ..)_ Recently, good-quality GaAs epilayers have been obtained on porous Si by molecular beam epitaxy @IRE) 13941. Even when there has been extensive work over the past several years in the characterization of the poroussilicon microstructure [5,6], only a few studies have been made to determine the atomic composition of the porous layers 171.The purpose of our work is twofold: (i) to determine the amount of impurities adsorbed at the pore surface and test the crystalline quality of the layer, and (ii) to find experimental conditions leading to the formation of layers as little contaminated as possible, while still preserving a good crystal quality. * Work supported by GDR 86 of CNRS. **
Present address: Central Research Institute Budapest, P.O. Box 49, H-1525, Hungary.
0168-583X/90/$03.50 (North-Holland)
for Physics,
0 Elsevier Science Publishers B.V.
In this paper we present a ch~acte~~~on of porous silicon layers formed in currently used conditions, using nuclear reaction analysis (NRA), proton recoil analysis (ERDA), Rutherford backscattering spectrometry (RIB) in combination with computer simulation, and channeling.
2. section
of porous Si layers
Ail wafers were p+-type single-crystalline silicon, boron-doped, with 2 in. diameter, low2 !J cm resistivity, (lOO)-oriented. Porous Si was prepared by anodic dissolution in a HF electrolyte (1 : 1 : 2 HF : II,0 : C,H,OH) under a current of 50 mA/cm’ (porosity 45%) or 20 mA/cm2 (porosity 40%). The anodic reaction time was varied from 1 to 17 min to obtain a porous layer of about 2, 10 or 30 pm thickness. According to ref. [S] the mean value of the radius of the pores increases (25-45 A) with the formation current density (lo-240 mA/cm’) but the surface of the pores, measured by gas adsorption, remains practically constant at 200 m2/cm3. This corresponds to a pore surface area 200 times higher than the geometrical surface area, for a 1 pm thick porous layer.
3. Ion beam analysis 3.1. oxygen measurement The oxygen content was measured using the i6G(d, p1)r70 nuclear reaction at a deuteron energy of 850 keV and a detection angle 8 = 150 O. At these
623
C. Ortega et al. / Characterization of porous silicon
200 _
**** 2 jhrn porous ++ + + + 30 pm porous
Si Si
l
. * * *** ***
16~(d,p)170
E&=850
l
tl
keV
$150:
u
** 1
*
‘i
1 1 I l
: ,+++ ’ t +++++* l+ +% :“+ *+* + ++ Z + i *g4+
1001
50:
100
Channels Fig. 1. Proton spectra from i6Q(d, p1)i70 of a 2 pm and a 30 Frn thick porous Si layer formed at 20 mA/cm2: 8 = 150 O; absorber: 12 pm Mylar.
conditions the cross section varies slowly until 780 keV. The energy loss of 850 keV deuterons in a 2 pm thick porous Si layer of 40% porisity is about 80 keV. For oxygen reference, we used anodie tantalum oxides very well calibrated by coulometry (3%). Fig. 1 shows the proton spectra respectively ~~es~ond~g to 2 and 30
+ ** ** 2 j.hm porous + + + + + 30 pm porous
Ed = 850 keV,
pm thick layers formed at 20 mA/cm2. The 160 content in the 2 pm thick layer is equal to 230 X 1015 cm-*. The oxygen content in the first 2 pm of the 30 ym thick layer is here lower than that of the 2 pm thick layer. Similar values were found in a layer grown at 50 mA/cm~.
Si Si
12C(d,p)t3C Ed=910 keV
* *
f
l
L
l
*
4 * ,*+++++++ j * + t + I
2000
4
+++ l
i+
+ t +
1
Fig. 2. Proton spectra from “C(d, p)13C of a 2 pm and a 30 pm thick porous Si layer formed at 20 mA,/cm2: Ed = 970 keV, B = 150 “; absorber: 12 pm Mylar. X. COMPLEMENTARY
TECHNIQUES
624
C. Ortega et al. / Characterization of porous silicon
3.2. Carbon measurement
3.3. Fluorine measurement
For carbon measurement we used the 12C(d, p)r3C nuclear reaction at a deuteron energy of 970 keV (150 o detection angle). The cross section does not vary appreciably in the range 970-900 keV, which allowed us to analyze the 2 urn thick porous layers. This reaction has been calibrated relative to the r60(d, p1)170 reaction [S]: at 970 keV the ratio of the cross sections, 12C(d, p)13C/160(d, p1)170, is equal to 1.91 + 0.02. This allows one to measure the 12C content using oxygen references, which are more easily obtained than a carbon reference. The energy of 970 keV corresponds to a strong resonance peak of the nuclear reaction 160(d, p1)170. Consequently the oxygen reference used in such measurements has to be thin enough. Fig. 2 shows the proton spectra corresponding to the samples described above. The 2 urn thick layer contains 1100 X 10" carbon atoms/cm*. By comparing the counts from the two layers we estimated that the carbon contamination is about two times lower for the 30 urn thick layer. A slightly higher carbon contamination was found in porous layers formed at 50 mA/cm* [(1200-1400) x 1015 at./cm* for a 2 pm thick layer], but the same phenomenon was observed: thicker films are less contaminated by carbon.
The fluorine content has been determined using the 19F(p, o,,)160 nuclear reaction at a proton energy of 1260 keV (detection angle 150 O, Mylar absorber - 30 urn). The stationary part of the cross section extends down to 1100 keV [9]. The depth corresponding to the quasi-plateau is typically 4 pm in silicon (8 urn in porous Si of - 50% porosity). The reference used was a CaF, layer on Si, the thickness of which was determined by RBS. The 19F content in a 2 urn thick layer is about 40 X 1015 cm-*, and 140 x 1015 cm-* for a 10 urn thick layer. 3.4. Hydrogen measurement To estimate the hydrogen content we used proton recoil spectra analysis. The samples were tilted from a 4He beam of 2 MeV energy with an angle of 75O and the protons were detected at 30 O. The reference used for absolute determination was 1017 H/cm* implanted in silicon. Fig. 3 shows proton recoil spectra corresponding to a 2 urn and a 10 urn thick porous Si layer formed at 50 mA/cm*. For the 2 urn thick layer the hydrogen content was estimated to be equal to 10% of
Channel Fig. 3. Proton recoil spectra: incident particles: 4He; EdHe= 2 MeV; tilt angle: 75 o ; 0 = 30 O; 3.2 keV/channel; absorber: 10 pm Mylar. + : 10” H-at./cm’ implanted in Si; 0: 2 pm thick porous Si layer formed at 50 mA/cm2; W: 10 pm thick porous Si layer formed at 50 mA/cm2.
625
C. Ortegaet al. / Characterizationofporous silicon
Energy _r 0.2
0.4
0.6
(MeV) 0.8
1.0
1.2
1.4
20 -0 -r; .+15 'U E D El0 6
Z
5
0 200
Channel Fig. 4. 4He RBS:
2.2 MeV, 8 = 165 ‘, 3 keV/channel; solid line = RUMP simulation; l : compact Si, aligned; 0: compact Si, random; 0: 2 pm thick porous Si layer, aligned; + : 2 pm thick porous Si layer, random.
E4,, =
the silicon atoms of the porous layer. For the thicker film the hydrogen content is about 20% lower. During beam irradiation the hydrogen content decreases. For example, it decreases by 25% for a dose of 1.2 mCb/cm’. By changing the impact point of the beam one can find the initial value. 3.5. RBS and channeling experiments Fig. 4 shows the spectra of 4He backscattered from a 2 pm thick porous layer formed at 50 mA/cm’ and from compact Si. The channeling conditions were the same for the two samples. For the porous Si, a minimum yield (Xti) calculated from the ratio of the yield aligned through the (100) axis to random yield is equal to 20%. Better values were found in some samples (15%). These relatively good values were measured near the periphery of the wafer where the porous layer is not formed. From preliminary results it seems that the Xti -value is higher in the central region of the wafer. This effect could be due to the strains produced in the substrate by the porous layer [6]. Similar X&-values were found for a 10 or 30 pm thick porous layer. The X&-value decreases slightly with the energy of the beam. This seems to indicate that the relatively high value of Xmin is mainly due to lattice plane m&orientations in the porous layer. More detailed studies will be carried out in the near future.
The yield of the aligned spectrum increases faster with depth in the case of porous Si than in the case of compact Si. This is caused by dechanneling in the pores and scattering by the surface of the pores and impurities. In fig. 4 the solid line represents a computer simulation (RUMP) corresponding to the following composition: 4.5 X 1Or8 Si-at/cm’, 1.26 X lOI C-at./cm*, 0.2 X 1Or8 0-at/cm’, 0.45 X 1Ol8 H-at./cm* and 0.04 X 10” F-at./cm’. The above values correspond to a porous layer of 0.241 mg/cm2 weight. This is in good agreement with the value we found by weighing: 0.245 mg/cm2. The fact that the random yield corresponding to the porous Si is lower than that corresponding to compact Si is hence due to the presence of impurities on the pore walls.
4. Discussion The above results are summarized in table 1. In this table we have represented typical values of the total impurity contents in the porous layer, deduced from IBA. Assuming that these impurities are uniformly distributed on the surface of the pores, we have also represented their surface density, taking into consideration that for a 2 l_~rnthick porous film the pore surface area is equal to about 400 times the geometrical surface. X. COMPLEMENTARY
TECHNIQUES
C. Ortega et al. / Characterization of porous silicon
626
Table 1 Typical values of impurity contents in porous silicon. The impurity contents reported here were measured by IBA and are relative to the first 2 pm for 160, “C and ‘H. For fluorine, 140X lOI represents the impurity content in the first 8 pm. For a 2 km thick layer the real surface is 400 times the geometrical surface. Formation conditions 2 30 2 10
pm, pm, pm, pm,
20 mA/cm2 20 mA/cm2 50 mA/cm2 50 mA/cm2
I60 f1015cm-‘]
“C f1015cmm2]
‘H [lOi cm-‘]
19F [lOI cmp2]
Geometrical surface
Real surface
Geometrical surface
Real surface
Geometrical surface
Real surface
200 180 350 500
0.5 0.45 0.875 1.25
1000 500 1300 650
2.5 1.25 3.25 1.6
450 360
_ 1.1 0.9
This gives a better idea of the intonation. The porous-Si contamination is compared to that of compact Si. Some treatments were carried out to decrease the impurity content. For instance, porous Si layers were annealed in a dry 0, atmosphere during 15 min: the carbon content is divided by 3 while the oxygen surface density is increased f:om 0.5 to 3 x 1015 cm-‘. This corresponds to a 6.6 A thick SiO, layer covering the pore walls. This layer may hinder further adsorption. It is surprising that the impurity contamination is much lower for porous Si than for compact Si. From preliminary results (to be published) it seems that the impurity content decreases with the radius of the pores, while the surface area of the pores remains practically constant (51. The size of the pores seems to have influence on the impurity adsorption. Further studies will be carried out to find conditions leading to layers as little contaminated as possible. We thank G. Bomchil, R. Herino and A. Halimaoui for their important help in starting these studies.
Geometrical surface _ 40 140
Real surface _ 0.1 0.09
References [l] C. Oules, A. Halimaoui, J.L. Regohni, R. Herino, A. Perio, D. Bensahel and G. Bomchil, Extended Abstract B-XI.5 of the E-MRS Conf., May 30, 1989, Strasbourg, France. PI S. Luryi and E. Suhir, Appl. Phys. Lett. 48 (1986) 140. c31 T.L. Lin, L. Sadwick, K.L. Wang, Y.C. Kao, P. Hull, C.W. Nieh, D.N. Jamieson and J.K. Lin, Appl. Phys. Lett. 51 (1987) 814. E41 S. Hasegawa, K. Maehashi and H. Nakasima, Proc. 5th Molecular Beam Epitaxy Conf., Sapporo, Japan, 1988 (Tokyo Inst. of Technology, 1988) p, 153. 151 R. Herino, G. Born&l, K. Barla, C. Bertrand and J.L. Ginoux, J. Electrochem. Sot. 134 (1987) 1994. I61 K. Barla, G. Bomchil, R. Herino, J.C. Pfister and J. Baruchel, J. Cryst. Growth 68 (1984) 721. 171 L.C. Eat-waker, J.P.G. Farr, P.E. Grzeszczyk, I. Sturland and J.M. Keen, Nucl. Instr. and Meth. B9 (1985) 317. I81 J.A. Davies and P.R. Norton, Nucl. Instr. and Meth. 168 (1980) 611. [91 D. Dieumegard, B. Maurel and G. Amsel, Nucl. Instr. and Meth. 168 (1980) 93.
Nuclear Instruments and Methods in Physics Research B45 (1990) 622-626 North-Holland
C~~~~~ON C. ORTEGA,
OF POROUS J. SIEJKA
SILICON BY NRA, RBS AND CONING
and G. VIZKELETHY
*
**
Groupe de Physique des Solides, lJniversit4 Paris VII, Tour 23, 2 Place Jussieu, 7.5221 Paris Cedex OS, France
Porous silicon could be an interesting material for microelectronic applications if the quality of porous films is significantly improved. This material has the ability to be easily oxidized at relatively low temperatures ( 4 850 o C). Moreover, it preserves the single-crystal characteri$ics of the substrate, allowing epitaxial growth of Si and GaAs at moderate temperat~~. The aim of this work was to determine the impurity content of porous films and to test their crystalline quality, as a function of preparation conditions, using ion beam analysis: NRA, ERDA, RBS and channeling. Relatively good channeling was observed in some porous materials ( xmi,, = 15%) in spite of the adsorbed impurities and the disorder due to the enormous surface area of the pores: 400 times higher than the geometrical surface area for a 2 urn thick porous layer. The surface density of the impurities is relatively low and can be decreased significantly by adequate surface preparation. Typical results are: 0.6~10’~ 0-at./cm2, 3~10” C-at./cm’, 1Or5 H-at./cm’ and 0.1 X lOI F-at/cm’.
1. In~uetion In the continous research for new materials for microelectronic applications, porous Si occupies a particular situation due to two interesting properties: the crystalline orientation of the substrate is preserved and, because of its high porosity (lo-70%), it is very rapidly oxidized at relatively low temperatures (7” < 850 o C) to form a thick oxide layer (2-10 pm). An excellent-q~lity Si epilayer has recently been grown by chemical vapour deposition (ND) at a low temperature (T < 83O’C) [l]. For these reasons it can be very attractive for silicon insulator technology (SOI). Porous Si can also be used as substrate for hetero-epitaxial growth. It is believed [2] that porous Si can accommodate, more easily than compact Si, the epitaxial growth of materials having different lattice parameters (GaAs, Ge, . - ’ ) and also different lateral expansion coefficients (oxide, fluoride, . ..)_ Recently, good-quality GaAs epilayers have been obtained on porous Si by molecular beam epitaxy @IRE) 13941. Even when there has been extensive work over the past several years in the characterization of the poroussilicon microstructure [5,6], only a few studies have been made to determine the atomic composition of the porous layers 171.The purpose of our work is twofold: (i) to determine the amount of impurities adsorbed at the pore surface and test the crystalline quality of the layer, and (ii) to find experimental conditions leading to the formation of layers as little contaminated as possible, while still preserving a good crystal quality. * Work supported by GDR 86 of CNRS. **
Present address: Central Research Institute Budapest, P.O. Box 49, H-1525, Hungary.
0168-583X/90/$03.50 (North-Holland)
for Physics,
0 Elsevier Science Publishers B.V.
In this paper we present a ch~acte~~~on of porous silicon layers formed in currently used conditions, using nuclear reaction analysis (NRA), proton recoil analysis (ERDA), Rutherford backscattering spectrometry (RIB) in combination with computer simulation, and channeling.
2. section
of porous Si layers
Ail wafers were p+-type single-crystalline silicon, boron-doped, with 2 in. diameter, low2 !J cm resistivity, (lOO)-oriented. Porous Si was prepared by anodic dissolution in a HF electrolyte (1 : 1 : 2 HF : II,0 : C,H,OH) under a current of 50 mA/cm’ (porosity 45%) or 20 mA/cm2 (porosity 40%). The anodic reaction time was varied from 1 to 17 min to obtain a porous layer of about 2, 10 or 30 pm thickness. According to ref. [S] the mean value of the radius of the pores increases (25-45 A) with the formation current density (lo-240 mA/cm’) but the surface of the pores, measured by gas adsorption, remains practically constant at 200 m2/cm3. This corresponds to a pore surface area 200 times higher than the geometrical surface area, for a 1 pm thick porous layer.
3. Ion beam analysis 3.1. oxygen measurement The oxygen content was measured using the i6G(d, p1)r70 nuclear reaction at a deuteron energy of 850 keV and a detection angle 8 = 150 O. At these
623
C. Ortega et al. / Characterization of porous silicon
200 _
**** 2 jhrn porous ++ + + + 30 pm porous
Si Si
l
. * * *** ***
16~(d,p)170
E&=850
l
tl
keV
$150:
u
** 1
*
‘i
1 1 I l
: ,+++ ’ t +++++* l+ +% :“+ *+* + ++ Z + i *g4+
1001
50:
100
Channels Fig. 1. Proton spectra from i6Q(d, p1)i70 of a 2 pm and a 30 Frn thick porous Si layer formed at 20 mA/cm2: 8 = 150 O; absorber: 12 pm Mylar.
conditions the cross section varies slowly until 780 keV. The energy loss of 850 keV deuterons in a 2 pm thick porous Si layer of 40% porisity is about 80 keV. For oxygen reference, we used anodie tantalum oxides very well calibrated by coulometry (3%). Fig. 1 shows the proton spectra respectively ~~es~ond~g to 2 and 30
+ ** ** 2 j.hm porous + + + + + 30 pm porous
Ed = 850 keV,
pm thick layers formed at 20 mA/cm2. The 160 content in the 2 pm thick layer is equal to 230 X 1015 cm-*. The oxygen content in the first 2 pm of the 30 ym thick layer is here lower than that of the 2 pm thick layer. Similar values were found in a layer grown at 50 mA/cm~.
Si Si
12C(d,p)t3C Ed=910 keV
* *
f
l
L
l
*
4 * ,*+++++++ j * + t + I
2000
4
+++ l
i+
+ t +
1
Fig. 2. Proton spectra from “C(d, p)13C of a 2 pm and a 30 pm thick porous Si layer formed at 20 mA,/cm2: Ed = 970 keV, B = 150 “; absorber: 12 pm Mylar. X. COMPLEMENTARY
TECHNIQUES
624
C. Ortega et al. / Characterization of porous silicon
3.2. Carbon measurement
3.3. Fluorine measurement
For carbon measurement we used the 12C(d, p)r3C nuclear reaction at a deuteron energy of 970 keV (150 o detection angle). The cross section does not vary appreciably in the range 970-900 keV, which allowed us to analyze the 2 urn thick porous layers. This reaction has been calibrated relative to the r60(d, p1)170 reaction [S]: at 970 keV the ratio of the cross sections, 12C(d, p)13C/160(d, p1)170, is equal to 1.91 + 0.02. This allows one to measure the 12C content using oxygen references, which are more easily obtained than a carbon reference. The energy of 970 keV corresponds to a strong resonance peak of the nuclear reaction 160(d, p1)170. Consequently the oxygen reference used in such measurements has to be thin enough. Fig. 2 shows the proton spectra corresponding to the samples described above. The 2 urn thick layer contains 1100 X 10" carbon atoms/cm*. By comparing the counts from the two layers we estimated that the carbon contamination is about two times lower for the 30 urn thick layer. A slightly higher carbon contamination was found in porous layers formed at 50 mA/cm* [(1200-1400) x 1015 at./cm* for a 2 pm thick layer], but the same phenomenon was observed: thicker films are less contaminated by carbon.
The fluorine content has been determined using the 19F(p, o,,)160 nuclear reaction at a proton energy of 1260 keV (detection angle 150 O, Mylar absorber - 30 urn). The stationary part of the cross section extends down to 1100 keV [9]. The depth corresponding to the quasi-plateau is typically 4 pm in silicon (8 urn in porous Si of - 50% porosity). The reference used was a CaF, layer on Si, the thickness of which was determined by RBS. The 19F content in a 2 urn thick layer is about 40 X 1015 cm-*, and 140 x 1015 cm-* for a 10 urn thick layer. 3.4. Hydrogen measurement To estimate the hydrogen content we used proton recoil spectra analysis. The samples were tilted from a 4He beam of 2 MeV energy with an angle of 75O and the protons were detected at 30 O. The reference used for absolute determination was 1017 H/cm* implanted in silicon. Fig. 3 shows proton recoil spectra corresponding to a 2 urn and a 10 urn thick porous Si layer formed at 50 mA/cm*. For the 2 urn thick layer the hydrogen content was estimated to be equal to 10% of
Channel Fig. 3. Proton recoil spectra: incident particles: 4He; EdHe= 2 MeV; tilt angle: 75 o ; 0 = 30 O; 3.2 keV/channel; absorber: 10 pm Mylar. + : 10” H-at./cm’ implanted in Si; 0: 2 pm thick porous Si layer formed at 50 mA/cm2; W: 10 pm thick porous Si layer formed at 50 mA/cm2.
625
C. Ortegaet al. / Characterizationofporous silicon
Energy _r 0.2
0.4
0.6
(MeV) 0.8
1.0
1.2
1.4
20 -0 -r; .+15 'U E D El0 6
Z
5
0 200
Channel Fig. 4. 4He RBS:
2.2 MeV, 8 = 165 ‘, 3 keV/channel; solid line = RUMP simulation; l : compact Si, aligned; 0: compact Si, random; 0: 2 pm thick porous Si layer, aligned; + : 2 pm thick porous Si layer, random.
E4,, =
the silicon atoms of the porous layer. For the thicker film the hydrogen content is about 20% lower. During beam irradiation the hydrogen content decreases. For example, it decreases by 25% for a dose of 1.2 mCb/cm’. By changing the impact point of the beam one can find the initial value. 3.5. RBS and channeling experiments Fig. 4 shows the spectra of 4He backscattered from a 2 pm thick porous layer formed at 50 mA/cm’ and from compact Si. The channeling conditions were the same for the two samples. For the porous Si, a minimum yield (Xti) calculated from the ratio of the yield aligned through the (100) axis to random yield is equal to 20%. Better values were found in some samples (15%). These relatively good values were measured near the periphery of the wafer where the porous layer is not formed. From preliminary results it seems that the Xti -value is higher in the central region of the wafer. This effect could be due to the strains produced in the substrate by the porous layer [6]. Similar X&-values were found for a 10 or 30 pm thick porous layer. The X&-value decreases slightly with the energy of the beam. This seems to indicate that the relatively high value of Xmin is mainly due to lattice plane m&orientations in the porous layer. More detailed studies will be carried out in the near future.
The yield of the aligned spectrum increases faster with depth in the case of porous Si than in the case of compact Si. This is caused by dechanneling in the pores and scattering by the surface of the pores and impurities. In fig. 4 the solid line represents a computer simulation (RUMP) corresponding to the following composition: 4.5 X 1Or8 Si-at/cm’, 1.26 X lOI C-at./cm*, 0.2 X 1Or8 0-at/cm’, 0.45 X 1Ol8 H-at./cm* and 0.04 X 10” F-at./cm’. The above values correspond to a porous layer of 0.241 mg/cm2 weight. This is in good agreement with the value we found by weighing: 0.245 mg/cm2. The fact that the random yield corresponding to the porous Si is lower than that corresponding to compact Si is hence due to the presence of impurities on the pore walls.
4. Discussion The above results are summarized in table 1. In this table we have represented typical values of the total impurity contents in the porous layer, deduced from IBA. Assuming that these impurities are uniformly distributed on the surface of the pores, we have also represented their surface density, taking into consideration that for a 2 l_~rnthick porous film the pore surface area is equal to about 400 times the geometrical surface. X. COMPLEMENTARY
TECHNIQUES
C. Ortega et al. / Characterization of porous silicon
626
Table 1 Typical values of impurity contents in porous silicon. The impurity contents reported here were measured by IBA and are relative to the first 2 pm for 160, “C and ‘H. For fluorine, 140X lOI represents the impurity content in the first 8 pm. For a 2 km thick layer the real surface is 400 times the geometrical surface. Formation conditions 2 30 2 10
pm, pm, pm, pm,
20 mA/cm2 20 mA/cm2 50 mA/cm2 50 mA/cm2
I60 f1015cm-‘]
“C f1015cmm2]
‘H [lOi cm-‘]
19F [lOI cmp2]
Geometrical surface
Real surface
Geometrical surface
Real surface
Geometrical surface
Real surface
200 180 350 500
0.5 0.45 0.875 1.25
1000 500 1300 650
2.5 1.25 3.25 1.6
450 360
_ 1.1 0.9
This gives a better idea of the intonation. The porous-Si contamination is compared to that of compact Si. Some treatments were carried out to decrease the impurity content. For instance, porous Si layers were annealed in a dry 0, atmosphere during 15 min: the carbon content is divided by 3 while the oxygen surface density is increased f:om 0.5 to 3 x 1015 cm-‘. This corresponds to a 6.6 A thick SiO, layer covering the pore walls. This layer may hinder further adsorption. It is surprising that the impurity contamination is much lower for porous Si than for compact Si. From preliminary results (to be published) it seems that the impurity content decreases with the radius of the pores, while the surface area of the pores remains practically constant (51. The size of the pores seems to have influence on the impurity adsorption. Further studies will be carried out to find conditions leading to layers as little contaminated as possible. We thank G. Bomchil, R. Herino and A. Halimaoui for their important help in starting these studies.
Geometrical surface _ 40 140
Real surface _ 0.1 0.09
References [l] C. Oules, A. Halimaoui, J.L. Regohni, R. Herino, A. Perio, D. Bensahel and G. Bomchil, Extended Abstract B-XI.5 of the E-MRS Conf., May 30, 1989, Strasbourg, France. PI S. Luryi and E. Suhir, Appl. Phys. Lett. 48 (1986) 140. c31 T.L. Lin, L. Sadwick, K.L. Wang, Y.C. Kao, P. Hull, C.W. Nieh, D.N. Jamieson and J.K. Lin, Appl. Phys. Lett. 51 (1987) 814. E41 S. Hasegawa, K. Maehashi and H. Nakasima, Proc. 5th Molecular Beam Epitaxy Conf., Sapporo, Japan, 1988 (Tokyo Inst. of Technology, 1988) p, 153. 151 R. Herino, G. Born&l, K. Barla, C. Bertrand and J.L. Ginoux, J. Electrochem. Sot. 134 (1987) 1994. I61 K. Barla, G. Bomchil, R. Herino, J.C. Pfister and J. Baruchel, J. Cryst. Growth 68 (1984) 721. 171 L.C. Eat-waker, J.P.G. Farr, P.E. Grzeszczyk, I. Sturland and J.M. Keen, Nucl. Instr. and Meth. B9 (1985) 317. I81 J.A. Davies and P.R. Norton, Nucl. Instr. and Meth. 168 (1980) 611. [91 D. Dieumegard, B. Maurel and G. Amsel, Nucl. Instr. and Meth. 168 (1980) 93.