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Effects of BIAS on radiation induced defects in mos oxides: An ESR study
383
Nuclear Instruments and Methods in Physics Research Bl (1984) 383-386 North-Holland, Amsterdam
EFFEmS W.E.
OF BIAS ON RADIATION
INDUCED
DEFECTS
IN MOS OXIDES: AN ESR STUDY
CARLOS
Naval Research Laboratory,
Washington, DC 20375, USA
Electron spin resonance is used to study oxide damage centers generated by ionizing radiation in MOS capacitors as a function of bias and temperature during irradiation. Two centers are observed, an E’ and an oxygen hole center. The concentration of E’ centers is about three times as high for sampJes irradiated under positive gate bias as for those irradiated under negative bias. The concentration of E’ centers is independent of sample temperature during irradiation indicating that some charge carriers created by irradiation are still mobile at low temperatures. The oxygen hole center density is not a function of bias indicating that they are relatively uniformly distributed throughout the bulk of oxide.
1. Introduction It is well established that ionizing radiation generates a variety of damage centers in SiO, [1,2]. In MOS devices this is seen as a build-up of positive oxide charge and can be observed as a shift in the flat band voltage on a C-F/ curve [3]. This flat band shift is strongly dependent on the bias applied to the device during irradiation. A much larger shift is seen when the metal gate is positive than if the gate is negative or shorted to the substrate. This bias dependence is explained [4] by assuming a concentration of hole traps within the oxide near the Si-SiOa interface and relatively few electron traps. When the gate bias is positive, the holes move toward the interface and are trapped, while under negative bias they are pulled away from Si-SiO, interface toward the metal-oxide interface where there are fewer traps. A number of inferences about the nature of these traps have been drawn. SIMS measurements have shown an accumulation of impurities such as hydrogen and nitrogen [5] near the interface leading to the conclusion that at least some of these traps are impurity related. From chemical arguments one might expect oxygen deficient defects such as Si-Si bonds to play a role. In this study electron spin resonance (ESR) is employed to study the nature of these defects. ESR is particularly well suited for this problem since it gives information about structure of the traps and is sensitive to only the total number of traps and not their location within the oxide. ESR has proved to be a powerful tool in understanding the nature of irradiation induced defects in bulk SiO, [1,2]. A wide range of paramagnetic centers, relating to stoichiometric imperfections as well as to impurities, has been observed and characterized. In this work I will draw on this “catalogue” of known defects to help 0168-583X/84/$03.00 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
identify the nature of the defects in the MOS devices. After pioneering work by Nishi [6], ESR has become an increasingly powerful probe of the Si-SiO, interface. Poindexter [7] and coworkers have done extensive studies of the “Pa” center (a trivalent silicon atom at the interface). By studying the effects of optical excitation, doping and applied surface potential they have established the correspondence between the charge states of this center and trapping levels at the interface. Marquardt and Sigel studied the effect of y-irradiation on E’ centers (a trivalent Si bonded to three oxygen atoms) in oxides grown on silicon films on sapphire [8]. They found that these centers were concentrated near the Si-SiO, interface and that in one sample irradiated under bias the population increased significantly. Lenahan and coworkers have recently looked at defects induced by y-irradiation at both liquid helium temperature [9] and room temperature [lo]. For low temperature irradiation they observed interstitial hydrogen and an oxygen hole center. Both of these centers annealed out by - 150 K. They also observed a decrease in the Pa signal which inversely correlated with the interstitial hydrogen. In later experiments on samples prepared differently, they observed an increase in the Pn signal and a weak but stable oxygen center as well as a weak E’ signal after a room temperature irradiation [lo]. In this paper we begin a systematic study of effects of bias and temperature during irradiation on the ESR signal induced in metal-oxide-semiconductor structures.
2. Experimental methods The samples were made from wafers of (111) boron doped ( - 30 s2. cm) silicon wafers which had been polished on both sides to double the surface area (0.7 V. DEVICE APPLICATIONS
384
W.E. Carh / Effects sf bi0s.m dsfects
cm’). Approximately 1000 A of dry oxide was grown on both sides and then annealed in N, at 1200°C for 20 mm. Aluminum gates, about 500 A thick were evaporated on each side and wires were bonded to the gate and substrate for applying bias. The samples were X-irradiated (Cu target, 100 kV) at either 77 K or 300 K with an applied gate bias of -9 V to +9 V. ESR measurements were made using a standard Varian xband spectrometer. During a measurement the sample was held at - 7 K. Care was taken to be sure that those which had been irradiated at 77 K were not allowed to warm up in being transferred to the spectrometer for measurement and to be sure that all samples were positioned the same in the cavity. The metal gates and the silicon caused &nificant changes in the resonant frequency of the cavity so that any error in positioning a sample was immediately apparent. At such low temperatures the normal absorption mode signal was in all cases severely saturated and it was necessary to take measurements in the dispersion mode under rapid adiabatic passage conditions [ll]. Under these conditions the dispersion signal is proportional to the undifferentiated absorption signal, x”. The relative signal intensities can be measured reasonably accurately for these conditions; however, the absolute number of spins cannot be as easily determined. For this work a sample of high quality quartz was irradiated and used as a standard for estimating the total number of spins. In addition to the ESR measurements high frequency capacitance measurements were done on samples irradiated under the same conditions to determine the flat band voltage shifts induced by the build-up of charge in the oxide.
H(kG)
1
Fig. 1. The ESR spectrum after approximately 10 Mrad of irradiation for samples biased at + 9 V and - 9 V. The two traces have been offset for display purposes.
could be deconvolved into its two components, as shown in fig. 2, and each analyzed separately. Fig. 3 shows the increase in the E’ with increasing radiation dose. The number of such centers is about a factor of three higher for those irradiated under positive bias than for those irradiated under negative bias over the range of total dosage shown. Flat band voltage shifts were also measured for capacitors made on the same wafers. Unfortunately the shifts induced at the higher doses were much too large to be measured without breaking down the oxides. However, at low dosage the difference between positive and negative bias is about a factor of 7 and the density of traps is of the same order of magnitude indicating that the E’ centers are related to a significant fraction of the hole traps in the oxide. This result supports the basic model [4] of the bias
3. Experimental results and discussion Fig. 1 shows a comparison of the ESR signal for irradiation under f 9 V bias on the metal gate. The line is composed of at least two components. The narrow line centered at g E 2.001 (H = 3.28 kG) is due to E’ centers, while the broad line centered around g = 2.0076 (H = 3.27 kG) is due to oxygen hole centers. In order to analyze these lines separately, a sample of bulk spectrosil quartz was irradiated to produce a large number of E’ centers. Initially, the ESR spectrum of this standard sample was taken with a MOS sample also in the cavity to maintain nearly the same cavity loading and resonant frequency as with the MOS capacitor. Later experiments showed that the precaution was unnecessary as the sensitivity of the cavity was not significantly altered by the MOS capacitor. The ESR signal for this sample was then used to fit the E’ signal for the thin films and to provide a standard for counting the number of E centers in the MOS capacitor. Using this the total signal
3.25 H (kG) Fig. 2. The FSR signal and its two components, and the oxygen hole center (OHC).
the E’ center
W. E. Carh / Effects of bias on defects
.L-----l
0’
1
0
t
I
4 6 DOSE (Mrod)
6
I
2 TOTAL
Fig. 3. The build-up of the E’ center as a function of radiation dose for positive and negative bias.
dependence of radiation induced charge in the oxide. That is that the X-rays produce electron-hole pairs and under positive bias the holes are pulled toward the Si-SiO, interface and trapped in that region, while the electrons are swept out of the oxide and into the metal since there are few electron gaps in the oxide. Under negative bias the holes are swept out of the oxide and into the metal. There is considerable evidence that the hole traps are concentrated near the interface as this would require. Marquardt and Sigel’s [8] etch back experiments also indicate a build-up of E’ centers near the Si-SiO, interface. The ESR signal simply measures the total number of centers in the oxide, while the voltage shift is sensitive to both the total charge and the position of the centroid of the charge distribution in the oxide. That is Av,aa
Q,,/d,
385
capture of a hole by a strained bond. The direct process would result in a uniform distribution of such centers in the oxide, while the indirect process would result in a distribution skewed in the direction of hole current. One might also expect to find more strained bonds near the Si-SiO, interface. The distribution of E’ centers in these oxides is probably due to a combination of these two processes. The samples irradiated at 77 K showed the same response as those irradiated at room temperature. This might indicate that holes created at 77 K are still mobile enough to move toward silicon interface and be trapped there. Although care was taken to transfer the sample from the liquid nitrogen to the liquid helium as quickly as possible, it is possible that the sample could have warmed up to - 100 K resulting in some annealing. On the other hand, electrical measurements [12] indicate that the holes are quite immobile at temperatures below - 100 K. It is generally agreed that electrons in SiO, are still quite mobile at low temperatures. This suggests a second explanation for the bias dependence of the E concentration. Neutral paramagnetic E’ centers could be created directly by X-irradiation. These centers could then act as electron traps and after trapping an electron would be diamagnetic and not detected by ESR. If we again assume that the E’ centers are concentrated near the Si-SiO, interface, more electrons will be captured if the metal is negative than if it is positive, thereby reducing the ESR signal. Clearly, more work needs to be done to determine the role of centers such as E’ in trapping oxide charge. The oxygen hole center seen in these experiments is shown in fig. 4. The line is centered at g 5 2.0076. There may also be some structure around g = 2.001; however, this is masked by the E’ resonance. This structure corresponds quite well to the peroxy radical (Si-O-O) seen in bulk SiO, [13] with g, = 2.0014, g, = 2.0074, g,
(1)
where AF,, is the flat band voltage shift, Q,, is the oxide charge and d is the distance from the semiconductor to the center of the oxide charge. If we assume that all of the oxide charge is trapped at E’ centers then the center of this charge is approximately twice as close to the semiconductor for samples irradiated under positive bias as for those irradiated under negative bias. This argument, of course rests on the assumption that the paramagnetic E’ centers are involved in the trapping of all the positive charge. However, it is entirely possible that other diamagnetic sites also play a significant role and must be considered in any quantitative comparison of results. The E’ centers could originate directly from the breaking of a bond by irradiation or indirectly from the
H (kG1
Fig. 4. The oxygen hole centers for positive and negative bias. The traces are offset for display purposes. The solid arrow indicates g = 2.0074 and the dashed arrow, g = 2.0095. V. DEVICE APPLICATIONS
386
W.E. Carlos / Effects of bias on defects
= 2.067. Since the structure is broad and the signal to noise is poor, it is not possible to rule out the “wet” oxygen hole center at g = 2.0095 which is generally attributed to an impurity site. The concentration of these centers saturates for very low exposures. Furthermore, their concentration is not dependent on bias during irradiation indicating that these centers are not concentrated near the interface as the E’ centers are. The concentration of these centers is difficult to determine in the dispersion mode. While the integrated area under this resonance is - 20-508 of that of the E signal, the signal amplitude is inversely proportional to T,, the spin lattice relaxation time. At room temperature, T1 for the oxygen hole center is about three orders of magnitude smaller than I’, for the E’ center. If this were also true at low temperatures, the number of oxygen hole centers would be about three orders of magnitude lower than the concentration of E’ centers. In summary, two radiation induced ESR sites have been observed in MOS capacitors. Of these only the E’ center is seen to depend on bias during irradiation and a positive correlation between the number of E’ centers and the flat-band voltage shift is observed, indicating that this center is involved in the trapping of positive oxide change. The oxygen hole center is lower in concentration than the E’ center and distributed more uniformly throughout the oxide since its amplitude does not depend on applied bias. It is quite reasonable to expect that this center which is oxgen rich would be more uniformly distributed than the Si rich E’ center.
D.L. Griscom and E.J. Friebele are thanked for the use of their spectrometer and for helpful discussions on
defects in bulk SiO,. H.L. Hughes is thanked for discussions on defects as seen by other techniques.
References [l] D.L. Griscom, J. Non-Cryst. Sol. 40 (1980) 211. (21 D.L. Griscom, M. Stapelbroek and E.J. Friebele, J. Chem. Phys. 78 (1983) 1638. (31 For a review see J.N. Churchill, F.E. Homstrom and T.W. Collins, Adv. Electron. Electron Phys. 58 (1981) 1. [4] E.H. Snow, A.S. Grove and D.J. Fitzgerald, Proc. IEEE 55 (1967) 1168. [5] H.L. Hughes, private communication. [6] Y. Nishi, Jap. J. of Appl. Phys. 5 (1966) 333 and 10 (1971) 52. [7] P.J. Caplan and E.H. Poindexter, J. Appl. Phys. 52 (1981) 522; E.H. Poindexter and P.J. Caplan, J. Appl. Phys. 52 (1981) 879; P.J. Caplan, E.H. Poindexter and S.R. Morrison, J. Appl. Phys. 53 (1982) 541. [S] CL. Marquardt and G.H. Sigel, Jr., IEEE Trans. Nucl. Sci. NS-22 (1975) 2234. [9] K.L. Brower, P.M. Lenehan and P.V. Dressendorfer, Appl. Phys. Lett. 41 (1982) 251. [lo] P.M. Lenahan and P.V. Dressendorfer, IEEE Trans. Nucl. Sci. NS-29 (1982) 1459. [ll] For a detailed review of passage effects in ESR, see M. Weger, Bell Sys. Tech. J. 39 (1960) 1013. The conditions used in this work satisfy Weger’s case 7. [12] P.S. Winokur and H.E. Boesch, IEEE Trans. Nucl. Sci. NS-28 (1981) 4088. [13] M. Stapelbroek, D.L. Griscom, E.J. Friebele and G.H. Sigel, Jr., J. Non-Cryst. Sol. 32 (1979) 313; E.J. Friebele, D.L. Griscom and M. Stapelbroek, Phys. Rev. Lett. 42 (1979) 1346.
Nuclear Instruments and Methods in Physics Research Bl (1984) 383-386 North-Holland, Amsterdam
EFFEmS W.E.
OF BIAS ON RADIATION
INDUCED
DEFECTS
IN MOS OXIDES: AN ESR STUDY
CARLOS
Naval Research Laboratory,
Washington, DC 20375, USA
Electron spin resonance is used to study oxide damage centers generated by ionizing radiation in MOS capacitors as a function of bias and temperature during irradiation. Two centers are observed, an E’ and an oxygen hole center. The concentration of E’ centers is about three times as high for sampJes irradiated under positive gate bias as for those irradiated under negative bias. The concentration of E’ centers is independent of sample temperature during irradiation indicating that some charge carriers created by irradiation are still mobile at low temperatures. The oxygen hole center density is not a function of bias indicating that they are relatively uniformly distributed throughout the bulk of oxide.
1. Introduction It is well established that ionizing radiation generates a variety of damage centers in SiO, [1,2]. In MOS devices this is seen as a build-up of positive oxide charge and can be observed as a shift in the flat band voltage on a C-F/ curve [3]. This flat band shift is strongly dependent on the bias applied to the device during irradiation. A much larger shift is seen when the metal gate is positive than if the gate is negative or shorted to the substrate. This bias dependence is explained [4] by assuming a concentration of hole traps within the oxide near the Si-SiOa interface and relatively few electron traps. When the gate bias is positive, the holes move toward the interface and are trapped, while under negative bias they are pulled away from Si-SiO, interface toward the metal-oxide interface where there are fewer traps. A number of inferences about the nature of these traps have been drawn. SIMS measurements have shown an accumulation of impurities such as hydrogen and nitrogen [5] near the interface leading to the conclusion that at least some of these traps are impurity related. From chemical arguments one might expect oxygen deficient defects such as Si-Si bonds to play a role. In this study electron spin resonance (ESR) is employed to study the nature of these defects. ESR is particularly well suited for this problem since it gives information about structure of the traps and is sensitive to only the total number of traps and not their location within the oxide. ESR has proved to be a powerful tool in understanding the nature of irradiation induced defects in bulk SiO, [1,2]. A wide range of paramagnetic centers, relating to stoichiometric imperfections as well as to impurities, has been observed and characterized. In this work I will draw on this “catalogue” of known defects to help 0168-583X/84/$03.00 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
identify the nature of the defects in the MOS devices. After pioneering work by Nishi [6], ESR has become an increasingly powerful probe of the Si-SiO, interface. Poindexter [7] and coworkers have done extensive studies of the “Pa” center (a trivalent silicon atom at the interface). By studying the effects of optical excitation, doping and applied surface potential they have established the correspondence between the charge states of this center and trapping levels at the interface. Marquardt and Sigel studied the effect of y-irradiation on E’ centers (a trivalent Si bonded to three oxygen atoms) in oxides grown on silicon films on sapphire [8]. They found that these centers were concentrated near the Si-SiO, interface and that in one sample irradiated under bias the population increased significantly. Lenahan and coworkers have recently looked at defects induced by y-irradiation at both liquid helium temperature [9] and room temperature [lo]. For low temperature irradiation they observed interstitial hydrogen and an oxygen hole center. Both of these centers annealed out by - 150 K. They also observed a decrease in the Pa signal which inversely correlated with the interstitial hydrogen. In later experiments on samples prepared differently, they observed an increase in the Pn signal and a weak but stable oxygen center as well as a weak E’ signal after a room temperature irradiation [lo]. In this paper we begin a systematic study of effects of bias and temperature during irradiation on the ESR signal induced in metal-oxide-semiconductor structures.
2. Experimental methods The samples were made from wafers of (111) boron doped ( - 30 s2. cm) silicon wafers which had been polished on both sides to double the surface area (0.7 V. DEVICE APPLICATIONS
384
W.E. Carh / Effects sf bi0s.m dsfects
cm’). Approximately 1000 A of dry oxide was grown on both sides and then annealed in N, at 1200°C for 20 mm. Aluminum gates, about 500 A thick were evaporated on each side and wires were bonded to the gate and substrate for applying bias. The samples were X-irradiated (Cu target, 100 kV) at either 77 K or 300 K with an applied gate bias of -9 V to +9 V. ESR measurements were made using a standard Varian xband spectrometer. During a measurement the sample was held at - 7 K. Care was taken to be sure that those which had been irradiated at 77 K were not allowed to warm up in being transferred to the spectrometer for measurement and to be sure that all samples were positioned the same in the cavity. The metal gates and the silicon caused &nificant changes in the resonant frequency of the cavity so that any error in positioning a sample was immediately apparent. At such low temperatures the normal absorption mode signal was in all cases severely saturated and it was necessary to take measurements in the dispersion mode under rapid adiabatic passage conditions [ll]. Under these conditions the dispersion signal is proportional to the undifferentiated absorption signal, x”. The relative signal intensities can be measured reasonably accurately for these conditions; however, the absolute number of spins cannot be as easily determined. For this work a sample of high quality quartz was irradiated and used as a standard for estimating the total number of spins. In addition to the ESR measurements high frequency capacitance measurements were done on samples irradiated under the same conditions to determine the flat band voltage shifts induced by the build-up of charge in the oxide.
H(kG)
1
Fig. 1. The ESR spectrum after approximately 10 Mrad of irradiation for samples biased at + 9 V and - 9 V. The two traces have been offset for display purposes.
could be deconvolved into its two components, as shown in fig. 2, and each analyzed separately. Fig. 3 shows the increase in the E’ with increasing radiation dose. The number of such centers is about a factor of three higher for those irradiated under positive bias than for those irradiated under negative bias over the range of total dosage shown. Flat band voltage shifts were also measured for capacitors made on the same wafers. Unfortunately the shifts induced at the higher doses were much too large to be measured without breaking down the oxides. However, at low dosage the difference between positive and negative bias is about a factor of 7 and the density of traps is of the same order of magnitude indicating that the E’ centers are related to a significant fraction of the hole traps in the oxide. This result supports the basic model [4] of the bias
3. Experimental results and discussion Fig. 1 shows a comparison of the ESR signal for irradiation under f 9 V bias on the metal gate. The line is composed of at least two components. The narrow line centered at g E 2.001 (H = 3.28 kG) is due to E’ centers, while the broad line centered around g = 2.0076 (H = 3.27 kG) is due to oxygen hole centers. In order to analyze these lines separately, a sample of bulk spectrosil quartz was irradiated to produce a large number of E’ centers. Initially, the ESR spectrum of this standard sample was taken with a MOS sample also in the cavity to maintain nearly the same cavity loading and resonant frequency as with the MOS capacitor. Later experiments showed that the precaution was unnecessary as the sensitivity of the cavity was not significantly altered by the MOS capacitor. The ESR signal for this sample was then used to fit the E’ signal for the thin films and to provide a standard for counting the number of E centers in the MOS capacitor. Using this the total signal
3.25 H (kG) Fig. 2. The FSR signal and its two components, and the oxygen hole center (OHC).
the E’ center
W. E. Carh / Effects of bias on defects
.L-----l
0’
1
0
t
I
4 6 DOSE (Mrod)
6
I
2 TOTAL
Fig. 3. The build-up of the E’ center as a function of radiation dose for positive and negative bias.
dependence of radiation induced charge in the oxide. That is that the X-rays produce electron-hole pairs and under positive bias the holes are pulled toward the Si-SiO, interface and trapped in that region, while the electrons are swept out of the oxide and into the metal since there are few electron gaps in the oxide. Under negative bias the holes are swept out of the oxide and into the metal. There is considerable evidence that the hole traps are concentrated near the interface as this would require. Marquardt and Sigel’s [8] etch back experiments also indicate a build-up of E’ centers near the Si-SiO, interface. The ESR signal simply measures the total number of centers in the oxide, while the voltage shift is sensitive to both the total charge and the position of the centroid of the charge distribution in the oxide. That is Av,aa
Q,,/d,
385
capture of a hole by a strained bond. The direct process would result in a uniform distribution of such centers in the oxide, while the indirect process would result in a distribution skewed in the direction of hole current. One might also expect to find more strained bonds near the Si-SiO, interface. The distribution of E’ centers in these oxides is probably due to a combination of these two processes. The samples irradiated at 77 K showed the same response as those irradiated at room temperature. This might indicate that holes created at 77 K are still mobile enough to move toward silicon interface and be trapped there. Although care was taken to transfer the sample from the liquid nitrogen to the liquid helium as quickly as possible, it is possible that the sample could have warmed up to - 100 K resulting in some annealing. On the other hand, electrical measurements [12] indicate that the holes are quite immobile at temperatures below - 100 K. It is generally agreed that electrons in SiO, are still quite mobile at low temperatures. This suggests a second explanation for the bias dependence of the E concentration. Neutral paramagnetic E’ centers could be created directly by X-irradiation. These centers could then act as electron traps and after trapping an electron would be diamagnetic and not detected by ESR. If we again assume that the E’ centers are concentrated near the Si-SiO, interface, more electrons will be captured if the metal is negative than if it is positive, thereby reducing the ESR signal. Clearly, more work needs to be done to determine the role of centers such as E’ in trapping oxide charge. The oxygen hole center seen in these experiments is shown in fig. 4. The line is centered at g 5 2.0076. There may also be some structure around g = 2.001; however, this is masked by the E’ resonance. This structure corresponds quite well to the peroxy radical (Si-O-O) seen in bulk SiO, [13] with g, = 2.0014, g, = 2.0074, g,
(1)
where AF,, is the flat band voltage shift, Q,, is the oxide charge and d is the distance from the semiconductor to the center of the oxide charge. If we assume that all of the oxide charge is trapped at E’ centers then the center of this charge is approximately twice as close to the semiconductor for samples irradiated under positive bias as for those irradiated under negative bias. This argument, of course rests on the assumption that the paramagnetic E’ centers are involved in the trapping of all the positive charge. However, it is entirely possible that other diamagnetic sites also play a significant role and must be considered in any quantitative comparison of results. The E’ centers could originate directly from the breaking of a bond by irradiation or indirectly from the
H (kG1
Fig. 4. The oxygen hole centers for positive and negative bias. The traces are offset for display purposes. The solid arrow indicates g = 2.0074 and the dashed arrow, g = 2.0095. V. DEVICE APPLICATIONS
386
W.E. Carlos / Effects of bias on defects
= 2.067. Since the structure is broad and the signal to noise is poor, it is not possible to rule out the “wet” oxygen hole center at g = 2.0095 which is generally attributed to an impurity site. The concentration of these centers saturates for very low exposures. Furthermore, their concentration is not dependent on bias during irradiation indicating that these centers are not concentrated near the interface as the E’ centers are. The concentration of these centers is difficult to determine in the dispersion mode. While the integrated area under this resonance is - 20-508 of that of the E signal, the signal amplitude is inversely proportional to T,, the spin lattice relaxation time. At room temperature, T1 for the oxygen hole center is about three orders of magnitude smaller than I’, for the E’ center. If this were also true at low temperatures, the number of oxygen hole centers would be about three orders of magnitude lower than the concentration of E’ centers. In summary, two radiation induced ESR sites have been observed in MOS capacitors. Of these only the E’ center is seen to depend on bias during irradiation and a positive correlation between the number of E’ centers and the flat-band voltage shift is observed, indicating that this center is involved in the trapping of positive oxide change. The oxygen hole center is lower in concentration than the E’ center and distributed more uniformly throughout the oxide since its amplitude does not depend on applied bias. It is quite reasonable to expect that this center which is oxgen rich would be more uniformly distributed than the Si rich E’ center.
D.L. Griscom and E.J. Friebele are thanked for the use of their spectrometer and for helpful discussions on
defects in bulk SiO,. H.L. Hughes is thanked for discussions on defects as seen by other techniques.
References [l] D.L. Griscom, J. Non-Cryst. Sol. 40 (1980) 211. (21 D.L. Griscom, M. Stapelbroek and E.J. Friebele, J. Chem. Phys. 78 (1983) 1638. (31 For a review see J.N. Churchill, F.E. Homstrom and T.W. Collins, Adv. Electron. Electron Phys. 58 (1981) 1. [4] E.H. Snow, A.S. Grove and D.J. Fitzgerald, Proc. IEEE 55 (1967) 1168. [5] H.L. Hughes, private communication. [6] Y. Nishi, Jap. J. of Appl. Phys. 5 (1966) 333 and 10 (1971) 52. [7] P.J. Caplan and E.H. Poindexter, J. Appl. Phys. 52 (1981) 522; E.H. Poindexter and P.J. Caplan, J. Appl. Phys. 52 (1981) 879; P.J. Caplan, E.H. Poindexter and S.R. Morrison, J. Appl. Phys. 53 (1982) 541. [S] CL. Marquardt and G.H. Sigel, Jr., IEEE Trans. Nucl. Sci. NS-22 (1975) 2234. [9] K.L. Brower, P.M. Lenehan and P.V. Dressendorfer, Appl. Phys. Lett. 41 (1982) 251. [lo] P.M. Lenahan and P.V. Dressendorfer, IEEE Trans. Nucl. Sci. NS-29 (1982) 1459. [ll] For a detailed review of passage effects in ESR, see M. Weger, Bell Sys. Tech. J. 39 (1960) 1013. The conditions used in this work satisfy Weger’s case 7. [12] P.S. Winokur and H.E. Boesch, IEEE Trans. Nucl. Sci. NS-28 (1981) 4088. [13] M. Stapelbroek, D.L. Griscom, E.J. Friebele and G.H. Sigel, Jr., J. Non-Cryst. Sol. 32 (1979) 313; E.J. Friebele, D.L. Griscom and M. Stapelbroek, Phys. Rev. Lett. 42 (1979) 1346.