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Silicon clusters as photoactive traps in buried oxide layers of SIMOX structures
Microelectronic Elsevier
Silicon oxide
Engineering
22 (1993) 367-370 367
clusters layers of
as photoactive traps SIMOX structures
S.I. Fedoseenko, V.K. Adamchuk, Institute of Physics, St.-Petersburg
V.V. Afanas’ev University, St.-Petersburg
in
buried
198904 Russia
Charge trapping in buried oxide layers (BOX) of SIMOX structures was studied using electron photoinjection in UV and VUV spectral ranges. In addition to highly efficient trapping of injected electrons the direct photo-excitation of defects to the positive charged state and photodepopulation of trapped electrons were observed. Silicon clusters in BOX are proposed as photoactive traps with large cross section. 1. INTRODUCTION Despite the importance of charge trapping in buried oxide (BOX) layers of SIMOX structures, there are very few data on the trap parameters. We have studied the traps in BOX layers by electron photoinjection and found a neutral trap with anomalously large cross section. Contrary to thermally grown SiOa films, which do not contain photoactive traps, these traps in BOX may be photo-ionized. They are very likely related to Si clusters as the position of their energy levels relative to the conduction band of the oxide is the same as silicon band edges at Si/Si02 interface. 2. EXPERIMENTAL MOS capacitors for photoinjection experiments were fabricated from single (SI) and triple implanted (TI) SIMOX structures that are similar to those used previously for studying the interaction between deuterium and BOX [ 11. The top Si layer was removed by wet or dry etch and semi-transparent Al or Au electrodes were deposited on the 400 nm thick BOX and the control thermal oxide of the same thickness. The Al covered capacitors received the standard PMA in forming gas, while the Au metallized samples remain unannealed. In contrast to the thermal oxide layers of the same thickness, in the BOX layers several processes contribute to carrier injection and trapping/detrapping under optical excitation. The long-term transients ( 102s) of electron photoinjection current from silicon were observed that reveal the presence of shallow electron traps in BOX near the silicon surface. These traps seem to govern the conductivity of the insulator. The present report is focussed on deep traps responsible for fixed charge generation. The charge trapping experiments were performed in several spectral ranges. This provides the possibility to make distinction between various mechanisms of carrier excitation in the Si/BOX system and to reveal their contributions to the charge accumulation in the oxide. In UV range (hv c6eV) the oxide layer is transparent and both photoinjection from the electrodes of the MOS capacitor and optical excitation of defects in the oxide are possible. The contribution of photoinjection may be easily revealed using the sensitivity of the process to the direction of electric field in the oxide and by replacing the metal electrode material, because the barrier heights for electron injection are different for Al and Au by 1 eV [2]. In the case of WV illumination (hv =lOeV) the photons are absorbed within the thin (lo-20 nm) oxide layer near the metal electrode and electrons or holes may be extracted into oxide bulk under negative or positive metal bias, respectively. In this case no optical excitation of the oxide defects is expected and the trapping may be studied in pure form.
0167-9317/93/$06.00
0 1993 - Elsevier Science Publishers B.V. All rights reserved.
368
S.I. Fedoseenko et al. I Silicon clusters as photoactive traps
b Thermal -1 0
I 200
100 12 o(lnj)ie, 10
0
10 12
-2 cm
O(lnJ)/e, 10
20 -2 cm
Fig. 1.a - Trapped positive charge versus injected electrons for UV illumination (hv>4.3eV) of MOS capacitors with various oxides under 100 V positive bias on Au electrode (2.5 MV/cm oxide field). b - Positive charge generation kinetics in MOS capacitor with SI BOX layer under UV illumination (hv >4.3eV) at various negative biases on Au field electrode. 3. RESULTS Photoinjection of electrons from Al (hv ~3.5 eV) results in negative charge buildup with trapping efficiencies ranging from 0.02 for TI to 0.15 for SI samples. This situation corresponds to the case of monopolar electron injection into oxide, but the efficiency of electron trapping is much higher, than that for thermal oxide (10-4). However, if the photon energy exceeds 4.3 eV the positive charge accumulation is observed in BOX layers independently of bias polarity (Fig. 1) rather than the expected trapping of electrons. This behaviour corresponds to direct optical excitation of defects in BOX because only in this case the barrier for excited carrier is independent on the field direction. The photoionization efficiency in SI samples is higher than in TI due to larger cross section (in order of 10-18cm2) and higher density of centres, but the main features of the process are the same. The generated positively charged centres may be neutralized by electrons injected into oxide from Al at photon energies below 3.5 eV or by non-penetrating VUV photons. The neutralization is characterized by the same field dependent cross section as for holes trapped in Si02 revealing the creation of Coulomb attractive traps. The neutralization process takes place simultaneously with the photoionization and controls the field dependent positive charge accumulation kinetics, shown in Fig.lb. Continuation of electron injection results in filling neutral electron traps with 2x10-l7 cm2 cross section and leads to compensation of positive charge by the negative (the “turn-around” of the charge accumulation kinetics). Electron injection into BOX under VUV illumination leads to electron trapping in the oxide bulk and near the Si/SiO, interface with the same efficiency as in the case of Al photoinjection. It is difficult to fit with single cross section the kinetics shown in Fig.2a and the data may indicate continuous cross section spectrum. The maximal section of the traps is about lo-13 cm2 for SI and lo-14 cm* for TI layers. These sections weakly depend on electric field in the range of 0.2-2.5 MV/cm (Fig.2b) indicating sharp edges of the trap potential well. Its size may be determined in this case from the cross section value and it is about several nanometers for SI and one nanometer for TI BOX. These traps were found to be photoactive as well: the trapped electrons may be easily removed from them by optical excitation in UV range. For this reason the trapping of negative charge during electron photoinjection from Si (hv >4.3 eV) does not contribute to the charge accumulation kinetics. Moreover, the trapped electrons are eliminated by negative bias stress (2-2.5 MV/cm), but they are stable against the positive bias stress. This result may indicate the close location of electron traps to silicon substrate surface and the possibility of charge exchange between traps and semiconductor.
S.I. Fedoseenko et al. I Silicon clusters as photoactive traps
VUV,
0
100
hv=
10 eV,
200
300 12
Q~lnive.10
-40
369
V
400
500
0
1
-2
-I 3
2
Field,
MVlcm
cm
Fig. 2. a- Electron trapping kinetics in SI and TI BOX layers under VUV illumination with negative bias on metal electrode. b- The electron capture cross sections in BOX layers for various oxide fields. Circular symbols correspond to Au electrodes (no anneal), triangles to Al ones after PMA in forming gas at 4OOOC. In order to determine the energy position of the traps levels relative the oxide band edges experiments with spectral resolution were performed. The influence of optical interference was reduced by oxide thinning in buffered HF down to 200 nm. The thinning did not reduce the density of photoactive traps and the centres responsible for generation of positive charge; accordingly, the defects are not located in the bulk of oxide. The relative quantum yields of the charge generation (elimination) processes were determined from the shift of C-V curve after 30 s illumination of the sample by monochromatic light of a known intensity. Figure 3 shows the spectral characteristics of the yield of positive charge generation (curve 1) and the yield of removal of trapped electrons (curve 2) from SI oxide. The sign of charge variation directly indicates that these transitions correspond to the excitation of electrons from the defect level to the oxide conduction band. The energy positions of the levels are shown in the insert. Their comparison with the energy band diagram of Si/SiO:! interface [2] indicates remarkably close energy positions of the trap levels and silicon band edges relative to the oxide conduction band. It is easily seen, that the threshold of positive charging corresponds to the top of Si valence states (4.3 eV), while the electron detrapping threshold is close to the barrier height for electrons in Si conduction band (3.2 eV). 20 SI,
Fig. 3. Spectral dependences of relative quantum yield of positive generation (1) and charge photodepopulation of electron traps (2) in SI BOX layer. The insert shows the positions of trap levels relative to SiO2 conduction band (energies are given in eV).
F=
lMV/cm
1
% $ ,. z *
4 Photon
Energy,
5 eV
S.I. Fedoseenko
370
4.
et al. I
Silicon clusters as photoactive traps
DISCUSSION
There are several indications of the common nature of the centres responsible for positive charge photogeneration and the photoactive electron traps with large cross section. The first one is the same range of densities of these defects. It is clearly seen from Figs 1 and 2 for SI sample. For the TI sample the apparent density of positive charge is lower, but the saturation level of the kinetics in Fig. 1 is controlled by neutralization of the positive charges by electrons simultaneously injected from silicon rather than by the ionization of all the defects. Rough estimate of neutralization probability, using the neutralization cross section for a given field and the density of electron injection current, indicates that the density of photoionized defects is several times higher than the measured charge density. Another feature is the correlated reduction of photoionization cross section and electron trapping cross section in TI samples compared with the SI ones. It is well understandable from the viewpoint of cluster size reduction in TI samples. The centres of positive charge generation and neutral electron traps are located near the Si/BOX interface. Both of them may be directly photoexcited; this feature is unique for the intrinsic defects in SiOZ. These correlations indicate that a single amphoteric defect is responsible for the observed effects. We think that they are due to Si clusters in BOX because the energy positions of the defect levels agree very well with those known for the Si/Si02 interface. This proposition agrees with the observation of E’-centres (unpaired electron on Si dangling bond in SiOZ) generation in BOX by light with h>200 nm [3]. Moreover, the elaborate studies of E’ centres in BOX indicate the additional component of the ESR signal corresponding to the electron delocalized among several Si atoms. These defects have never been observed in thermal SiOZ or in reoxidized BOX and supposed to be related to Si clusters [4]. This case corresponds to the excitation of electron from valence states of the cluster or trapping of hole during irradiation.The trapping of electron by neutral cluster results in filling the conduction state; the energy necessary to remove an electron corresponds to the position of silicon conduction band edge relative that of the oxide. From this viewpoint the “red” shift of photoionization threshold in TI samples (90 % of trapped electrons may be removed by illumination by photons hv<3 eV) becomes understandable as well. It is due to quantum confinement of electron wave in cluster of about 1 nm size for TI, and several nm for SI oxides, as was discussed above. Consequently, in TI samples the electron levels in the clusters are shifted upwards. 5.
CONCLUSIONS
The results of present study indicate the presence in BOX layers of neutral electron traps which have never been observed before in thermal oxide layers. These traps demonstrate high trapping efficiency and ability to direct photo-excitation. Silicon clusters created near silicon/BOX interface during SIMOX structure fabrication seem to be the origin of the traps. ACKNOWLEDGEMENTS The authors would like to thank Dr. A.G.Revesz for numerous stimulating discussions and consulting. Valuable discussions with Dr. A.Stesmans on ESR studies as well as providing of unpublished data are gratefully appreciated. REFERENCES 1. A.G.Revesz, S.Myers, G.Brown, and H.Hughes, Proc. IEEE SO1 Conference, Vedra,FL,( 1992)40. 2. V.K.Adamchuk and V.V.Afanas’ev, Progr.Surf.Sci. 41( 1992) 111. 3. R.A.B.Devine, J-L.Leray, and J.Margail, Appl.Phys.Lett.,59( 1991)2275. 4. A.Stesmans, private communication.
Ponte
Silicon oxide
Engineering
22 (1993) 367-370 367
clusters layers of
as photoactive traps SIMOX structures
S.I. Fedoseenko, V.K. Adamchuk, Institute of Physics, St.-Petersburg
V.V. Afanas’ev University, St.-Petersburg
in
buried
198904 Russia
Charge trapping in buried oxide layers (BOX) of SIMOX structures was studied using electron photoinjection in UV and VUV spectral ranges. In addition to highly efficient trapping of injected electrons the direct photo-excitation of defects to the positive charged state and photodepopulation of trapped electrons were observed. Silicon clusters in BOX are proposed as photoactive traps with large cross section. 1. INTRODUCTION Despite the importance of charge trapping in buried oxide (BOX) layers of SIMOX structures, there are very few data on the trap parameters. We have studied the traps in BOX layers by electron photoinjection and found a neutral trap with anomalously large cross section. Contrary to thermally grown SiOa films, which do not contain photoactive traps, these traps in BOX may be photo-ionized. They are very likely related to Si clusters as the position of their energy levels relative to the conduction band of the oxide is the same as silicon band edges at Si/Si02 interface. 2. EXPERIMENTAL MOS capacitors for photoinjection experiments were fabricated from single (SI) and triple implanted (TI) SIMOX structures that are similar to those used previously for studying the interaction between deuterium and BOX [ 11. The top Si layer was removed by wet or dry etch and semi-transparent Al or Au electrodes were deposited on the 400 nm thick BOX and the control thermal oxide of the same thickness. The Al covered capacitors received the standard PMA in forming gas, while the Au metallized samples remain unannealed. In contrast to the thermal oxide layers of the same thickness, in the BOX layers several processes contribute to carrier injection and trapping/detrapping under optical excitation. The long-term transients ( 102s) of electron photoinjection current from silicon were observed that reveal the presence of shallow electron traps in BOX near the silicon surface. These traps seem to govern the conductivity of the insulator. The present report is focussed on deep traps responsible for fixed charge generation. The charge trapping experiments were performed in several spectral ranges. This provides the possibility to make distinction between various mechanisms of carrier excitation in the Si/BOX system and to reveal their contributions to the charge accumulation in the oxide. In UV range (hv c6eV) the oxide layer is transparent and both photoinjection from the electrodes of the MOS capacitor and optical excitation of defects in the oxide are possible. The contribution of photoinjection may be easily revealed using the sensitivity of the process to the direction of electric field in the oxide and by replacing the metal electrode material, because the barrier heights for electron injection are different for Al and Au by 1 eV [2]. In the case of WV illumination (hv =lOeV) the photons are absorbed within the thin (lo-20 nm) oxide layer near the metal electrode and electrons or holes may be extracted into oxide bulk under negative or positive metal bias, respectively. In this case no optical excitation of the oxide defects is expected and the trapping may be studied in pure form.
0167-9317/93/$06.00
0 1993 - Elsevier Science Publishers B.V. All rights reserved.
368
S.I. Fedoseenko et al. I Silicon clusters as photoactive traps
b Thermal -1 0
I 200
100 12 o(lnj)ie, 10
0
10 12
-2 cm
O(lnJ)/e, 10
20 -2 cm
Fig. 1.a - Trapped positive charge versus injected electrons for UV illumination (hv>4.3eV) of MOS capacitors with various oxides under 100 V positive bias on Au electrode (2.5 MV/cm oxide field). b - Positive charge generation kinetics in MOS capacitor with SI BOX layer under UV illumination (hv >4.3eV) at various negative biases on Au field electrode. 3. RESULTS Photoinjection of electrons from Al (hv ~3.5 eV) results in negative charge buildup with trapping efficiencies ranging from 0.02 for TI to 0.15 for SI samples. This situation corresponds to the case of monopolar electron injection into oxide, but the efficiency of electron trapping is much higher, than that for thermal oxide (10-4). However, if the photon energy exceeds 4.3 eV the positive charge accumulation is observed in BOX layers independently of bias polarity (Fig. 1) rather than the expected trapping of electrons. This behaviour corresponds to direct optical excitation of defects in BOX because only in this case the barrier for excited carrier is independent on the field direction. The photoionization efficiency in SI samples is higher than in TI due to larger cross section (in order of 10-18cm2) and higher density of centres, but the main features of the process are the same. The generated positively charged centres may be neutralized by electrons injected into oxide from Al at photon energies below 3.5 eV or by non-penetrating VUV photons. The neutralization is characterized by the same field dependent cross section as for holes trapped in Si02 revealing the creation of Coulomb attractive traps. The neutralization process takes place simultaneously with the photoionization and controls the field dependent positive charge accumulation kinetics, shown in Fig.lb. Continuation of electron injection results in filling neutral electron traps with 2x10-l7 cm2 cross section and leads to compensation of positive charge by the negative (the “turn-around” of the charge accumulation kinetics). Electron injection into BOX under VUV illumination leads to electron trapping in the oxide bulk and near the Si/SiO, interface with the same efficiency as in the case of Al photoinjection. It is difficult to fit with single cross section the kinetics shown in Fig.2a and the data may indicate continuous cross section spectrum. The maximal section of the traps is about lo-13 cm2 for SI and lo-14 cm* for TI layers. These sections weakly depend on electric field in the range of 0.2-2.5 MV/cm (Fig.2b) indicating sharp edges of the trap potential well. Its size may be determined in this case from the cross section value and it is about several nanometers for SI and one nanometer for TI BOX. These traps were found to be photoactive as well: the trapped electrons may be easily removed from them by optical excitation in UV range. For this reason the trapping of negative charge during electron photoinjection from Si (hv >4.3 eV) does not contribute to the charge accumulation kinetics. Moreover, the trapped electrons are eliminated by negative bias stress (2-2.5 MV/cm), but they are stable against the positive bias stress. This result may indicate the close location of electron traps to silicon substrate surface and the possibility of charge exchange between traps and semiconductor.
S.I. Fedoseenko et al. I Silicon clusters as photoactive traps
VUV,
0
100
hv=
10 eV,
200
300 12
Q~lnive.10
-40
369
V
400
500
0
1
-2
-I 3
2
Field,
MVlcm
cm
Fig. 2. a- Electron trapping kinetics in SI and TI BOX layers under VUV illumination with negative bias on metal electrode. b- The electron capture cross sections in BOX layers for various oxide fields. Circular symbols correspond to Au electrodes (no anneal), triangles to Al ones after PMA in forming gas at 4OOOC. In order to determine the energy position of the traps levels relative the oxide band edges experiments with spectral resolution were performed. The influence of optical interference was reduced by oxide thinning in buffered HF down to 200 nm. The thinning did not reduce the density of photoactive traps and the centres responsible for generation of positive charge; accordingly, the defects are not located in the bulk of oxide. The relative quantum yields of the charge generation (elimination) processes were determined from the shift of C-V curve after 30 s illumination of the sample by monochromatic light of a known intensity. Figure 3 shows the spectral characteristics of the yield of positive charge generation (curve 1) and the yield of removal of trapped electrons (curve 2) from SI oxide. The sign of charge variation directly indicates that these transitions correspond to the excitation of electrons from the defect level to the oxide conduction band. The energy positions of the levels are shown in the insert. Their comparison with the energy band diagram of Si/SiO:! interface [2] indicates remarkably close energy positions of the trap levels and silicon band edges relative to the oxide conduction band. It is easily seen, that the threshold of positive charging corresponds to the top of Si valence states (4.3 eV), while the electron detrapping threshold is close to the barrier height for electrons in Si conduction band (3.2 eV). 20 SI,
Fig. 3. Spectral dependences of relative quantum yield of positive generation (1) and charge photodepopulation of electron traps (2) in SI BOX layer. The insert shows the positions of trap levels relative to SiO2 conduction band (energies are given in eV).
F=
lMV/cm
1
% $ ,. z *
4 Photon
Energy,
5 eV
S.I. Fedoseenko
370
4.
et al. I
Silicon clusters as photoactive traps
DISCUSSION
There are several indications of the common nature of the centres responsible for positive charge photogeneration and the photoactive electron traps with large cross section. The first one is the same range of densities of these defects. It is clearly seen from Figs 1 and 2 for SI sample. For the TI sample the apparent density of positive charge is lower, but the saturation level of the kinetics in Fig. 1 is controlled by neutralization of the positive charges by electrons simultaneously injected from silicon rather than by the ionization of all the defects. Rough estimate of neutralization probability, using the neutralization cross section for a given field and the density of electron injection current, indicates that the density of photoionized defects is several times higher than the measured charge density. Another feature is the correlated reduction of photoionization cross section and electron trapping cross section in TI samples compared with the SI ones. It is well understandable from the viewpoint of cluster size reduction in TI samples. The centres of positive charge generation and neutral electron traps are located near the Si/BOX interface. Both of them may be directly photoexcited; this feature is unique for the intrinsic defects in SiOZ. These correlations indicate that a single amphoteric defect is responsible for the observed effects. We think that they are due to Si clusters in BOX because the energy positions of the defect levels agree very well with those known for the Si/Si02 interface. This proposition agrees with the observation of E’-centres (unpaired electron on Si dangling bond in SiOZ) generation in BOX by light with h>200 nm [3]. Moreover, the elaborate studies of E’ centres in BOX indicate the additional component of the ESR signal corresponding to the electron delocalized among several Si atoms. These defects have never been observed in thermal SiOZ or in reoxidized BOX and supposed to be related to Si clusters [4]. This case corresponds to the excitation of electron from valence states of the cluster or trapping of hole during irradiation.The trapping of electron by neutral cluster results in filling the conduction state; the energy necessary to remove an electron corresponds to the position of silicon conduction band edge relative that of the oxide. From this viewpoint the “red” shift of photoionization threshold in TI samples (90 % of trapped electrons may be removed by illumination by photons hv<3 eV) becomes understandable as well. It is due to quantum confinement of electron wave in cluster of about 1 nm size for TI, and several nm for SI oxides, as was discussed above. Consequently, in TI samples the electron levels in the clusters are shifted upwards. 5.
CONCLUSIONS
The results of present study indicate the presence in BOX layers of neutral electron traps which have never been observed before in thermal oxide layers. These traps demonstrate high trapping efficiency and ability to direct photo-excitation. Silicon clusters created near silicon/BOX interface during SIMOX structure fabrication seem to be the origin of the traps. ACKNOWLEDGEMENTS The authors would like to thank Dr. A.G.Revesz for numerous stimulating discussions and consulting. Valuable discussions with Dr. A.Stesmans on ESR studies as well as providing of unpublished data are gratefully appreciated. REFERENCES 1. A.G.Revesz, S.Myers, G.Brown, and H.Hughes, Proc. IEEE SO1 Conference, Vedra,FL,( 1992)40. 2. V.K.Adamchuk and V.V.Afanas’ev, Progr.Surf.Sci. 41( 1992) 111. 3. R.A.B.Devine, J-L.Leray, and J.Margail, Appl.Phys.Lett.,59( 1991)2275. 4. A.Stesmans, private communication.
Ponte