- Email: [email protected]
Gold–hydrogen complexes in silicon
Materials Science and Engineering B58 (1999) 126 – 129
Gold–hydrogen complexes in silicon L. Rubaldo a, P. Deixler b, I.D. Hawkins b, J. Terry b, D.K. Maude a, J.-C. Portal a, J.H. Evans-Freeman a, L. Dobaczewski c, A.R. Peaker a,b,* b
a Laboratoire des Champs Magne´tiques Intenses MPI-CNRS, 25, a6e des Martyrs, Grenoble 38042 France Centre for Electronic Materials, Uni6ersity of Manchester Institute of Science and Technology, PO Box 88, Manchester, M60 1QD, UK c Institute of Physics, Aleja Lotnikow 32 /46, 02 -668 Warsaw, Poland
Abstract Electron emission from gold and gold–hydrogen complexes in n-type silicon have been studied using high resolution (Laplace) DLTS. This technique permits a clear separation of defects which have very similar carrier emission characteristics. At low hydrogen concentrations our results confirm those inferred previously from conventional DLTS. However by using ‘Laplace’ DLTS it has been possible to study the gold acceptor and G4 defect independently. G4 has an activation energy of 542 98 meV. By directly measuring the electron capture cross-section of G4 we conclude that it is acceptor like. At high hydrogen concentrations additional complexes are formed, notably a defect with emission characteristics similar to G4 (referred to as G4%) with an activation energy of 578 910 meV, and a state with an activation energy for electron emission of 276 5 meV. We put forward the hypothesis that these may be charge states of AuH2 or AuH3 complexes. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Transition metal impurities; Hydrogen; Silicon; Complexes; DLTS
1. Introduction It is well established that atomic hydrogen introduced by plasma discharge [1] or wet etching [2] forms complexes with gold in silicon. These complexes have different electronic properties to the constituent species. This can result in the creation of new energy levels in the gap or in passivation if the energy level of the state is moved out of the gap. In the last few years the effect of hydrogenation of gold in silicon resulting from silicon cleaning processes and etching in CP4 (HNO3:HF:CH3COOH in the ratio 5:3:3) has been studied in some detail [3 – 5]. It has been proposed that four electrically active deep levels exist (referred to as G1, G2, G3, G4) resulting from the formation of Au-H complexes. It is believed that G1, G2 and G4 are different charge states of the Au-H pair [3,4] although the detailed structure of the complex has not yet been resolved. * Corresponding author. Tel.: +33-476887489; 476855610; e-mail: [email protected].
fax
+33-
Ab initio calculations of the properties of gold hydrogen complexes support the key aspects of this interpretation [6]. The G4 level appears to be very close in energy to the gold acceptor and seems to have almost identical electron emission characteristics [3,4]. Consequently it is very difficult to characterise the G4 level, primarily because of the limitations of resolution of conventional deep level transient spectroscopy (DLTS). We have applied a new, high resolution DLTS technique called Laplace DLTS (LDLTS) [7,8] to the problem, and separated the gold acceptor and gold– hydrogen (G4) thermal emission signatures. This has enabled us to determine the activation energy and capture cross section of G4 [9]. In the work described in the present paper we have extended this previous study to silicon containing considerably higher hydrogen concentrations than are normally used in an attempt to increase the concentration of multi-hydrogen complexes (AuH2, AuH3 and AuH4) which have been predicted to exist [6] and to study their properties using ‘Laplace’ DLTS.
0921-5107/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 9 8 ) 0 0 2 7 8 - 5
L. Rubaldo et al. / Materials Science and Engineering B58 (1999) 126–129
2. Experimental issues Compared to conventional DLTS, LDLTS sacrifices sensitivity for resolution and so can only be used with transients displaying high signal to noise ratios. For the samples used in this study the deep state concentration is between 0.3 and 20% of the shallow dopant concentration. In LDLTS computation based on regularisation techniques produces a plot of the spectral density as a function of emission rate rather than time filtered capacitance change vs temperature as in conventional DLTS. The result is delta-like peak(s) in the spectra for mono- and multi-exponential transients and broadened peaks for non-exponential transients. The area under each peak (integrated signal intensity) is proportional to the charge released from the defect (not the height). This is a very important issue in Laplace measurements as the width of the peak varies because of physical effects in the defect environment. In conventional DLTS the peak width is usually defined by the correlator function and is so wide that broadening due to defect parameters is rarely observed for point defects. Consequently it has become common practice to calculate the concentration of a defect from the DLTS peak height. In Laplace the area under the peak MUST be used. We undertake the measurement isothermally and in this paper present our data on a log scale of emission rate. This must be taken into account when visualising the concentration. The starting materials used in this work were singlecrystal phosphorus doped Czochralski silicon with electron concentrations of 6.4× 1015cm − 3 and 4.5× 1014 cm − 3. A thin gold film was evaporated on the unpolished side of the wafer and diffused in at temperatures of either 875, 900 or 1000°C for 2 h under a dry nitrogen atmosphere This resulted in electrically active gold concentrations of 3× 1013, 5× 1013 or 1015cm − 3 respectively. The concentration is almost uniform throughout the wafer apart from the near surface regions. These regions of high gold doping (: 100 mm) were etched away at room temperature using a CP4 etch, (HNO3:HF:CH3COOH in the ratio 5:3:3). This procedure releases atomic hydrogen which diffuses into the surface region of the silicon. One millimeter diameter gold Schottky diodes were evaporated on the polished side of the wafer. A large area aluminum contact, which acted as an Ohmic connection, was applied to the rear face without heating the sample. The amount of hydrogen present in the silicon depends on the CP4 etch times but as this removes additional material we have produced samples with different hydrogen concentrations by soaking in HF after the CP4 etch. An estimate of the relative concentrations of hydrogen present was made by determining the carrier loss using conventional CV profile measurements shortly after sample preparation. This provides a
127
measure of the hydrogen passivation of the phosphorous donors plus any deep acceptor states created by the hydrogenation. This will underestimate the total hydrogen concentration in the silicon as it will not detect deep donors or neutral hydrogen complexes. In this paper we report work on two groups of samples which we refer to as low hydrogen concentration (loH) and high hydrogen concentration (hiH). The loH samples were prepared with a 30 s CP4 etch and no HF soak while the hiH samples had the same CP4 etch but followed by a 30 min soak in HF. The carrier loss in the hiH samples was about 4× 1015 atoms cm − 3 and in the loH samples around 1015 atoms cm − 3 (measured in material with n= 6.4× 1015 cm − 3).
3. The gold acceptor and G4 Fig. 1 shows a comparison of the LDLTS spectra obtained from two Si:Au,H samples the spectrum on the left is from the loH group and on the right from the hiH group. Both spectra were taken with 5 V reverse bias and a 1 ms filling pulse of 0 V. The conventional DLTS spectrum of both samples show almost identical broad featureless peaks in the region of 260 K as reported previously [3,4]. The LDLTS spectrum of the loH sample shows that there are two separate and quite distinct bound to free electron emission rates. This is consistent with the previous interpretation of de-convoluted DLTS spectra. We have shown by annealing and profiling experiments that the higher emission rate peak is from the gold acceptor and the lower one from the gold–hydrogen complex G4 [9].
Fig. 1. ‘Laplace’ DLTS spectra of hydrogenated silicon containing gold. The spectrum on the left shows the emission rates of the gold acceptor and G4 to be distinctly different. The spectrum on the right shows data taken from a sample containing higher gold and higher hydrogen concentrations. An addition defect, G4%, can be seen.
128
L. Rubaldo et al. / Materials Science and Engineering B58 (1999) 126–129
Recent ab-initio calculations [10] have placed the (-/0) level of AuH2 close in energy to G4 and the gold acceptor. This combined with the association with high hydrogen concentrations and the shifts in concentration between G4 and G4% on annealing lead us to make a tentative assignment of the acceptor state of AuH2 to G4%. Work is now in progress to try to ascertain if this is correct.
4. G1 and states around 280meV
Fig. 2. Arrhenius plots, obtained from Laplace DLTS measurements, of the thermal emission rates of electrons from the gold-acceptor level and gold – hydrogen levels G4 and G4%. Activation energies derived from the slope of the least mean squares fit are 542 meV (G4), 558 meV (gold-acceptor) and 578 meV (G4%).
The spectrum on the right of Fig. 1 is taken from a high hydrogen sample, this sample contained 20 times as much gold as the low hydrogen sample as well as much more hydrogen. The spectral density function scale is ten times more sensitive for the spectrum on the left. It can be seen that an additional peak is evident indicative of the presence of a third defect which we have labeled G4%. From the area under the peak the concentration of G4% is about the same as G4. If the high hydrogen samples are stored for 6 months at room temperature the concentration of G4 is observed to reduce to about half of the original value. However the concentration of G4% increases by a factor of three and the gold acceptor concentration also increases slightly. The peak associated with G4% is broadened after this room temperature anneal possibly indicating some variation in local structure. Fig. 2 shows Arrhenius plots, derived from the Laplace spectra, of G4, G4% and the gold acceptor, over temperatures in the range 245 to 300 K. The activation energies derived from these plots are 5589 8 meV (gold acceptor), 5429 8 meV (G4) and 578 9 10 meV (G4%). We have made direct measurements of the electron capture cross section of the gold acceptor and G4 and find the values to be similar. Our measurements show that sn(G4) = 0.6sn(gold acceptor) so taking a typical value for sn(gold acceptor) we expect that sn(G4) will be around 6× 10 − 17 cm2. This result is significant in the sense that this value is typical of capture into a neutral centre. Consequently we can say with some certainty that we are measuring the (-/0) charge transitions and so G4 is an acceptor. We have not yet been able to measure the capture cross section of G4% because in all our hiH samples the trap to carrier ratio is too high for reliable measurements.
There is strong evidence from previous work [3,4] that the DLTS peak at around 120 K (referred to as G1) is the double acceptor charge state (--/-) of the same AuH complex as G4. All our measurements on low hydrogen material (concentration, annealing and cross section measurements) are consistent with this assignment. In hiH material we see a large increase in the apparent G1 concentration compared to loH material but the thermal emission characteristics remain unchanged. We observe two additional defect signatures in hiH layers which are quite undetectable in loH material. On annealing at room temperature for six months G1 reduces in magnitude very considerably, one of the new defects disappears but the other increases substantially. This is shown in Fig. 3. The Arrhenius plots of the electron emission from these defects are shown in Fig. 4. The letters on the lines cross reference with Fig. 3.The Arrhenius plots of G1 and (a) are derived from Laplace data, (b) and (c) are from conventional DLTS measurements. The activation energy of G1 is 1939 3 meV, in close agreement with previously publish values [3,4]. The new defect after anneal has an activation energy of 27695 meV. We associate the new defects with hydrogen but have
Fig. 3. Conventional DLTS measurement of the (--/-) state of the gold hydrogen pair (G1) and unknown defects detected only when high hydrogen concentrations are present.
L. Rubaldo et al. / Materials Science and Engineering B58 (1999) 126–129
129
has an activation energy for electron emission of 542 meV. The electron capture cross section of G4 is measured to be 0.6 of that of the gold acceptor. This confirms its assignment as the (-/0) transition of the gold hydrogen pair. In material with high hydrogen content additional states are observed, notably a state very similar to G4 and the gold acceptor which we refer to as G4% this has an activation energy for electron emission of 5789 10 meV. It is postulated that this is the (-/0) charge state of AuH2.
Acknowledgements
Fig. 4. Arrhenius plots of the thermal emission rates of electrons from the gold – hydrogen level G1 and from unknown hydrogen related defects (a), (b) and (c) cross referenced with Fig. 3. Data from reference [11] is shown for comparison.
not yet measured control samples with high hydrogen but without gold so we cannot say if the defect state is gold related. However previous work on ion implanted gold [11] but with no intentional hydrogen reported a state with almost identical emission characteristics (shown in Fig. 4 as a dotted line). A previous publication on silicon doped with gold during float zone growth and subsequently ‘saturated’ with hydrogen also reported DLTS peaks in this region of the spectrum but did not quantify the result [12]. Consequently there is some slight evidence for considering that this state may be gold–hydrogen related. If this is the case its energy fits very well with the ab-initio calculation of the level of the (-/0) state of AuH3 [10].
5. Conclusions In conclusion, we have applied the high resolution technique of Laplace DLTS to silicon diffused with gold and wet-etched to introduce hydrogen. In material with low hydrogen content our results confirm previously published work but we have been able to separate G4 from the gold acceptor and report that the G4 state
.
This work was funded under a European Commission COPERNICUS project and a Foresight Award from the Royal Academy of Engineering. We would like to thank Dr J.A. Davidson (UMIST) for his advice and comments.
References [1] S.J. Pearton, A.J. Tavendale, Phys Rev B 26 (1982) 7105. [2] E.O8 . Sveinjbo¨rnsson, O. Engstro¨m, Appl. Phys. Lett. 61 (1992) 2323. [3] E.O8 . Sveinjbo¨rnsson, O. Engstro¨m, Phys. Rev. B 52 (1995) 4884. [4] J.A. Davidson, J.H. Evans, Semicond. Sci. Technol. 11 (1996) 1704. [5] A. Mesli, E. Courcelle, T. Zundel, P. Siffert, Phys. Rev. B 36 (1987) 8049. [6] A. Resende, J. Goss, P.R. Briddon, S. O8 berg, R. Jones, in: Gordon Davies, Maria Helena Nazare´ (Eds.), Defects in Semiconductors ICDS-19, Materials Science Forum 258–263 (1997) 295. [7] L. Dobaczewski, P. Kaczor, I.D. Hawkins, A.R. Peaker, J. Appl. Phys. 76 (1994) 194. [8] Detailed information is available on the World Wide Web http://www.mcc.ac.uk/cem/laplace/ [9] P. Deixler, J. Terry, I.D. Hawkins, J.H. Evans-Freeman, A.R. Peaker, L. Rubaldo, D.K. Maude, J.-C. Portal, L. Dobaczewski, K. Bonde Nielsen, A. Nylandsted Larsen, A. Mesli, Appl. Phys. Lett. 73 (1998) 3126. [10] R. Jones, A. Resende, S O8 berg, P.R. Briddon, Mater. Sci. Eng. B this issue. [11] K.L. Wang, A.O. Evwaraye, J. Appl. Phys. 47 (1976) 4574. [12] A.L. Parakhonskiıˆ, O.V. Feklisova, S.S. Karelin, N.A. Yarykin, Semiconductors 30 (1996) 362.
Gold–hydrogen complexes in silicon L. Rubaldo a, P. Deixler b, I.D. Hawkins b, J. Terry b, D.K. Maude a, J.-C. Portal a, J.H. Evans-Freeman a, L. Dobaczewski c, A.R. Peaker a,b,* b
a Laboratoire des Champs Magne´tiques Intenses MPI-CNRS, 25, a6e des Martyrs, Grenoble 38042 France Centre for Electronic Materials, Uni6ersity of Manchester Institute of Science and Technology, PO Box 88, Manchester, M60 1QD, UK c Institute of Physics, Aleja Lotnikow 32 /46, 02 -668 Warsaw, Poland
Abstract Electron emission from gold and gold–hydrogen complexes in n-type silicon have been studied using high resolution (Laplace) DLTS. This technique permits a clear separation of defects which have very similar carrier emission characteristics. At low hydrogen concentrations our results confirm those inferred previously from conventional DLTS. However by using ‘Laplace’ DLTS it has been possible to study the gold acceptor and G4 defect independently. G4 has an activation energy of 542 98 meV. By directly measuring the electron capture cross-section of G4 we conclude that it is acceptor like. At high hydrogen concentrations additional complexes are formed, notably a defect with emission characteristics similar to G4 (referred to as G4%) with an activation energy of 578 910 meV, and a state with an activation energy for electron emission of 276 5 meV. We put forward the hypothesis that these may be charge states of AuH2 or AuH3 complexes. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Transition metal impurities; Hydrogen; Silicon; Complexes; DLTS
1. Introduction It is well established that atomic hydrogen introduced by plasma discharge [1] or wet etching [2] forms complexes with gold in silicon. These complexes have different electronic properties to the constituent species. This can result in the creation of new energy levels in the gap or in passivation if the energy level of the state is moved out of the gap. In the last few years the effect of hydrogenation of gold in silicon resulting from silicon cleaning processes and etching in CP4 (HNO3:HF:CH3COOH in the ratio 5:3:3) has been studied in some detail [3 – 5]. It has been proposed that four electrically active deep levels exist (referred to as G1, G2, G3, G4) resulting from the formation of Au-H complexes. It is believed that G1, G2 and G4 are different charge states of the Au-H pair [3,4] although the detailed structure of the complex has not yet been resolved. * Corresponding author. Tel.: +33-476887489; 476855610; e-mail: [email protected].
fax
+33-
Ab initio calculations of the properties of gold hydrogen complexes support the key aspects of this interpretation [6]. The G4 level appears to be very close in energy to the gold acceptor and seems to have almost identical electron emission characteristics [3,4]. Consequently it is very difficult to characterise the G4 level, primarily because of the limitations of resolution of conventional deep level transient spectroscopy (DLTS). We have applied a new, high resolution DLTS technique called Laplace DLTS (LDLTS) [7,8] to the problem, and separated the gold acceptor and gold– hydrogen (G4) thermal emission signatures. This has enabled us to determine the activation energy and capture cross section of G4 [9]. In the work described in the present paper we have extended this previous study to silicon containing considerably higher hydrogen concentrations than are normally used in an attempt to increase the concentration of multi-hydrogen complexes (AuH2, AuH3 and AuH4) which have been predicted to exist [6] and to study their properties using ‘Laplace’ DLTS.
0921-5107/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 9 8 ) 0 0 2 7 8 - 5
L. Rubaldo et al. / Materials Science and Engineering B58 (1999) 126–129
2. Experimental issues Compared to conventional DLTS, LDLTS sacrifices sensitivity for resolution and so can only be used with transients displaying high signal to noise ratios. For the samples used in this study the deep state concentration is between 0.3 and 20% of the shallow dopant concentration. In LDLTS computation based on regularisation techniques produces a plot of the spectral density as a function of emission rate rather than time filtered capacitance change vs temperature as in conventional DLTS. The result is delta-like peak(s) in the spectra for mono- and multi-exponential transients and broadened peaks for non-exponential transients. The area under each peak (integrated signal intensity) is proportional to the charge released from the defect (not the height). This is a very important issue in Laplace measurements as the width of the peak varies because of physical effects in the defect environment. In conventional DLTS the peak width is usually defined by the correlator function and is so wide that broadening due to defect parameters is rarely observed for point defects. Consequently it has become common practice to calculate the concentration of a defect from the DLTS peak height. In Laplace the area under the peak MUST be used. We undertake the measurement isothermally and in this paper present our data on a log scale of emission rate. This must be taken into account when visualising the concentration. The starting materials used in this work were singlecrystal phosphorus doped Czochralski silicon with electron concentrations of 6.4× 1015cm − 3 and 4.5× 1014 cm − 3. A thin gold film was evaporated on the unpolished side of the wafer and diffused in at temperatures of either 875, 900 or 1000°C for 2 h under a dry nitrogen atmosphere This resulted in electrically active gold concentrations of 3× 1013, 5× 1013 or 1015cm − 3 respectively. The concentration is almost uniform throughout the wafer apart from the near surface regions. These regions of high gold doping (: 100 mm) were etched away at room temperature using a CP4 etch, (HNO3:HF:CH3COOH in the ratio 5:3:3). This procedure releases atomic hydrogen which diffuses into the surface region of the silicon. One millimeter diameter gold Schottky diodes were evaporated on the polished side of the wafer. A large area aluminum contact, which acted as an Ohmic connection, was applied to the rear face without heating the sample. The amount of hydrogen present in the silicon depends on the CP4 etch times but as this removes additional material we have produced samples with different hydrogen concentrations by soaking in HF after the CP4 etch. An estimate of the relative concentrations of hydrogen present was made by determining the carrier loss using conventional CV profile measurements shortly after sample preparation. This provides a
127
measure of the hydrogen passivation of the phosphorous donors plus any deep acceptor states created by the hydrogenation. This will underestimate the total hydrogen concentration in the silicon as it will not detect deep donors or neutral hydrogen complexes. In this paper we report work on two groups of samples which we refer to as low hydrogen concentration (loH) and high hydrogen concentration (hiH). The loH samples were prepared with a 30 s CP4 etch and no HF soak while the hiH samples had the same CP4 etch but followed by a 30 min soak in HF. The carrier loss in the hiH samples was about 4× 1015 atoms cm − 3 and in the loH samples around 1015 atoms cm − 3 (measured in material with n= 6.4× 1015 cm − 3).
3. The gold acceptor and G4 Fig. 1 shows a comparison of the LDLTS spectra obtained from two Si:Au,H samples the spectrum on the left is from the loH group and on the right from the hiH group. Both spectra were taken with 5 V reverse bias and a 1 ms filling pulse of 0 V. The conventional DLTS spectrum of both samples show almost identical broad featureless peaks in the region of 260 K as reported previously [3,4]. The LDLTS spectrum of the loH sample shows that there are two separate and quite distinct bound to free electron emission rates. This is consistent with the previous interpretation of de-convoluted DLTS spectra. We have shown by annealing and profiling experiments that the higher emission rate peak is from the gold acceptor and the lower one from the gold–hydrogen complex G4 [9].
Fig. 1. ‘Laplace’ DLTS spectra of hydrogenated silicon containing gold. The spectrum on the left shows the emission rates of the gold acceptor and G4 to be distinctly different. The spectrum on the right shows data taken from a sample containing higher gold and higher hydrogen concentrations. An addition defect, G4%, can be seen.
128
L. Rubaldo et al. / Materials Science and Engineering B58 (1999) 126–129
Recent ab-initio calculations [10] have placed the (-/0) level of AuH2 close in energy to G4 and the gold acceptor. This combined with the association with high hydrogen concentrations and the shifts in concentration between G4 and G4% on annealing lead us to make a tentative assignment of the acceptor state of AuH2 to G4%. Work is now in progress to try to ascertain if this is correct.
4. G1 and states around 280meV
Fig. 2. Arrhenius plots, obtained from Laplace DLTS measurements, of the thermal emission rates of electrons from the gold-acceptor level and gold – hydrogen levels G4 and G4%. Activation energies derived from the slope of the least mean squares fit are 542 meV (G4), 558 meV (gold-acceptor) and 578 meV (G4%).
The spectrum on the right of Fig. 1 is taken from a high hydrogen sample, this sample contained 20 times as much gold as the low hydrogen sample as well as much more hydrogen. The spectral density function scale is ten times more sensitive for the spectrum on the left. It can be seen that an additional peak is evident indicative of the presence of a third defect which we have labeled G4%. From the area under the peak the concentration of G4% is about the same as G4. If the high hydrogen samples are stored for 6 months at room temperature the concentration of G4 is observed to reduce to about half of the original value. However the concentration of G4% increases by a factor of three and the gold acceptor concentration also increases slightly. The peak associated with G4% is broadened after this room temperature anneal possibly indicating some variation in local structure. Fig. 2 shows Arrhenius plots, derived from the Laplace spectra, of G4, G4% and the gold acceptor, over temperatures in the range 245 to 300 K. The activation energies derived from these plots are 5589 8 meV (gold acceptor), 5429 8 meV (G4) and 578 9 10 meV (G4%). We have made direct measurements of the electron capture cross section of the gold acceptor and G4 and find the values to be similar. Our measurements show that sn(G4) = 0.6sn(gold acceptor) so taking a typical value for sn(gold acceptor) we expect that sn(G4) will be around 6× 10 − 17 cm2. This result is significant in the sense that this value is typical of capture into a neutral centre. Consequently we can say with some certainty that we are measuring the (-/0) charge transitions and so G4 is an acceptor. We have not yet been able to measure the capture cross section of G4% because in all our hiH samples the trap to carrier ratio is too high for reliable measurements.
There is strong evidence from previous work [3,4] that the DLTS peak at around 120 K (referred to as G1) is the double acceptor charge state (--/-) of the same AuH complex as G4. All our measurements on low hydrogen material (concentration, annealing and cross section measurements) are consistent with this assignment. In hiH material we see a large increase in the apparent G1 concentration compared to loH material but the thermal emission characteristics remain unchanged. We observe two additional defect signatures in hiH layers which are quite undetectable in loH material. On annealing at room temperature for six months G1 reduces in magnitude very considerably, one of the new defects disappears but the other increases substantially. This is shown in Fig. 3. The Arrhenius plots of the electron emission from these defects are shown in Fig. 4. The letters on the lines cross reference with Fig. 3.The Arrhenius plots of G1 and (a) are derived from Laplace data, (b) and (c) are from conventional DLTS measurements. The activation energy of G1 is 1939 3 meV, in close agreement with previously publish values [3,4]. The new defect after anneal has an activation energy of 27695 meV. We associate the new defects with hydrogen but have
Fig. 3. Conventional DLTS measurement of the (--/-) state of the gold hydrogen pair (G1) and unknown defects detected only when high hydrogen concentrations are present.
L. Rubaldo et al. / Materials Science and Engineering B58 (1999) 126–129
129
has an activation energy for electron emission of 542 meV. The electron capture cross section of G4 is measured to be 0.6 of that of the gold acceptor. This confirms its assignment as the (-/0) transition of the gold hydrogen pair. In material with high hydrogen content additional states are observed, notably a state very similar to G4 and the gold acceptor which we refer to as G4% this has an activation energy for electron emission of 5789 10 meV. It is postulated that this is the (-/0) charge state of AuH2.
Acknowledgements
Fig. 4. Arrhenius plots of the thermal emission rates of electrons from the gold – hydrogen level G1 and from unknown hydrogen related defects (a), (b) and (c) cross referenced with Fig. 3. Data from reference [11] is shown for comparison.
not yet measured control samples with high hydrogen but without gold so we cannot say if the defect state is gold related. However previous work on ion implanted gold [11] but with no intentional hydrogen reported a state with almost identical emission characteristics (shown in Fig. 4 as a dotted line). A previous publication on silicon doped with gold during float zone growth and subsequently ‘saturated’ with hydrogen also reported DLTS peaks in this region of the spectrum but did not quantify the result [12]. Consequently there is some slight evidence for considering that this state may be gold–hydrogen related. If this is the case its energy fits very well with the ab-initio calculation of the level of the (-/0) state of AuH3 [10].
5. Conclusions In conclusion, we have applied the high resolution technique of Laplace DLTS to silicon diffused with gold and wet-etched to introduce hydrogen. In material with low hydrogen content our results confirm previously published work but we have been able to separate G4 from the gold acceptor and report that the G4 state
.
This work was funded under a European Commission COPERNICUS project and a Foresight Award from the Royal Academy of Engineering. We would like to thank Dr J.A. Davidson (UMIST) for his advice and comments.
References [1] S.J. Pearton, A.J. Tavendale, Phys Rev B 26 (1982) 7105. [2] E.O8 . Sveinjbo¨rnsson, O. Engstro¨m, Appl. Phys. Lett. 61 (1992) 2323. [3] E.O8 . Sveinjbo¨rnsson, O. Engstro¨m, Phys. Rev. B 52 (1995) 4884. [4] J.A. Davidson, J.H. Evans, Semicond. Sci. Technol. 11 (1996) 1704. [5] A. Mesli, E. Courcelle, T. Zundel, P. Siffert, Phys. Rev. B 36 (1987) 8049. [6] A. Resende, J. Goss, P.R. Briddon, S. O8 berg, R. Jones, in: Gordon Davies, Maria Helena Nazare´ (Eds.), Defects in Semiconductors ICDS-19, Materials Science Forum 258–263 (1997) 295. [7] L. Dobaczewski, P. Kaczor, I.D. Hawkins, A.R. Peaker, J. Appl. Phys. 76 (1994) 194. [8] Detailed information is available on the World Wide Web http://www.mcc.ac.uk/cem/laplace/ [9] P. Deixler, J. Terry, I.D. Hawkins, J.H. Evans-Freeman, A.R. Peaker, L. Rubaldo, D.K. Maude, J.-C. Portal, L. Dobaczewski, K. Bonde Nielsen, A. Nylandsted Larsen, A. Mesli, Appl. Phys. Lett. 73 (1998) 3126. [10] R. Jones, A. Resende, S O8 berg, P.R. Briddon, Mater. Sci. Eng. B this issue. [11] K.L. Wang, A.O. Evwaraye, J. Appl. Phys. 47 (1976) 4574. [12] A.L. Parakhonskiıˆ, O.V. Feklisova, S.S. Karelin, N.A. Yarykin, Semiconductors 30 (1996) 362.