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Capture cross-section of hole traps in reoxidized nitrided oxide measured by irradiation
Solid-Qare Ekctronics Vol. 38. No. IO, pp. 1851-1853, 1995 Copyright 0 1995Elsevier Science Ltd Printed in Great Britain. All rights reserved 0038-I lOI/ $9.50+ 0.00
Pergamon
NOTE CAPTURE CROSS-SECTION OF HOLE TRAPS IN REOXIDIZED NITRIDED OXIDE MEASURED BY IRRADIATION (Received
17 October
1994; in revised form 20 February 1995)
1. INTRODUCIION
centroid, peak density and standard deviation as variable parameters, is assumed. Our technique assumes that the hole traps generated by radiation are negligible, so that irradiation has the effect only of filling up the existing traps, whose u, is being measured.
In the search for an alternative insulator to silicon dioxide for radiation-hard applications, Reoxidized Nitrided Oxide (RNO) has shown great promise[l-6]. Experiments by Dunn[5] indicate that the hole traps in RN0 are located near the gat+SiO, interface in RNO. We have shown earlier[‘l] that, in addition, their energy distribution in RN0 is different from that in conventional oxides (dry pure SiO,). These results indicate a different origin of the hole traps in RN0 as compared to conventional oxides. If this is indeed true, then the value of the capture cross-section of these traps in RN0 is also likely to be different. However, no reports of the measurement of capture cross-section of hole traps in RN0 are available in the literature. The capture cross-sections of oxide traps are normally measured by the avalanche injection technique[l]. However, this technique can only be applied to hole traps with difficulty as discussed in Section 2. In this paper, we suggest a simple radiation-based technique for estimating the capture cross-section of hole traps. The capture cross-section of the traps in RN0 as well as in conventional dry oxides are measured by this technique, and found to be quite different.
3. EXPERIMENTAL
2. A RADIATION-BASED TECHNIQUE FOR ESTIMATION OF CAPTURE CROSSSECTION OF HOLE TRAPS
Avalanche injection has been traditionally used to calculate capture cross-section c,, of hole traps in MOS insulators[9]. This method assumes that the current which flows in the MOS structure is due only to holes injected from the substrate. This may often not be true. The smaller barrier for electrons at the gate-oxide interface can cause substantial electron injection from the gate electrode into the oxide along with hole injection from the substrate, resulting in a large electron current. This results in a value of u,, which may be quite incorrect. We suggest a new technique to calculate the capture cross-section of the hole traps. In this technique, we measure the midgap voltage shift At’,,,* of a MOS capacitor or transistor as a function of radiation dose. The rate of build-up of AV,,,g gives an idea of u,,. To get a good quantitative value of tr , we fit the data with the results of a simulator which pre d!acts AV,,,* as a function of radiation dose. The simulator solves the one-dimensional continuity and trap rate equations for holes as well as for electrons plus Poisson’s equation, in both the oxide and semiconductor. The details of the simulator can be found elsewhere[9]. Processes like geminate recombination and hole detrapping are taken into account in this simulator. The values of electron and hole mobility are taken as 20 and 10e5 cm*/V.s[lO] respectively. A constant trap distribution is taken for electron traps. The values of trap density and capture cross-section of the electron traps are taken as 2 x 10” and 4 x 10-i* cm2 respectively, which are reasonable[6,10]. For the hole traps, a Gaussian distribution with
The n-channel MOS transistors with W = 149 pm and J&= 5.4pm were fabricated on 4-6 G-cm (100) boron doped p-type wafers. The fabrication details are described elsewhere[rl]. Gate oxides 40 nm thick were made using both conventional dry oxide and RNO. The threshold voltage (V,) was measured from the I,-VGs plot for a low value of V,, (40 mV). A straight line through the point of maximum slope of the I,-VGs plot was drawn and its intercept with the V,, axis gives V,. The interface-state density (Ni,) was measured by using a standard two-level charge pumping (CP) technique. A 100 kHz 6 V peak-to-peak square wave was applied to the gate and the substrate current (1,) was measured by a Keithley 619 electrometer. The devices were irradiated floating using a @‘Cogammaray source with a dose rate of 360 krad(Si)/h. The midgap voltage shift (AV,) for each radiation dose was derived from the corresponding AV, and AN,, as follows. It is widely accepted that the interface states are acceptor-type above midgap and donor-type below midgap. Assuming a uniform distribution, we get:
where C,,, is the gate capacitance per unit area. Note that we have assumed there is no transformation between trapped holes and interface states. 4. RESULTS AND DISCUSSIONS
It is well-known that the hole traps in conventional oxides are located close to the Si-SiO, interface. These traps are popularly known as the E’ centers. In our simulation, therefore, we assume a Gaussian distribution of the hole traps with the centroid 50 8, away from the Si-SiO, interface and with a standard deviation of 5019 in the case of conventional dry oxide. On the other hand, the dominant hole traps for RN0 are found to be located near the gate-SiO, interface[5,7]. Therefore, for RN0 devices, we assume a Gaussian distribution of these traps with the centroid 5OA from the gat&iO, interface and with the same standard deviation as stated earlier. The peak value and a,, are found so as to obtain best fits of the simulator output to the experimental data, as described in more detail below. In Fig. 1, AV,, is plotted against radiation dose. The open and filled circles represent the experimental values of AV,,,, respectively for conventional dry oxide and RN0 devices.
1851
Note Table 1. Results of the simulation for dry oxide with different centroid and standard deviation
0.6 .
5:
-
E” > a
Standard deviation
Centroid (A) from gate
0.4
350 350 370
RN0 (Expt.) Oxide(Simulated) RNO(Simulated)
,
1
Dose
1200
900
600
300
(krad
(Si))
Fig. 1. Midgap voltage shift as a function of radiation dose for conventional dry oxide and RN0 devices. The continuous and dotted lines represent respectively the simulated results for dry oxide and RNO. The hole trap distributions are shown in Fig. 2. The peak value of hole density determines the final (saturation) value of A Vme,and the up affects how quickly (as a function of dose) tt gets there. The values of the capture cross-section of the hole traps obtained for best fit are 1.1 x lo-” and 7.9 x 10-t* cm* respectively in conventional dry oxide and in RNO. There is a large difference in the value of capture cross-section of the hole traps in conventional oxide and in RNO. This is a reflection of the experimental data of Fig. 1 where we see that AV, saturates much faster in RN0 devices as compared to conventional oxide devices. The value of u,, obtained for hole traps in oxide is quite comparable to values quoted in the literature[lO,ll]. This confirms the validity of our new technique. The value of cy of hole traps in RN0 is obtained here for the first ttme. It has been shown earlier that the location as well as the energy distribution of the hole traps in RN0 are different to these in conventional oxides[7]. The electron spin resonance study by Chaiyasena et a[.[121 also indicates that the dominant hole traps in RN0 are not the well-known E’ centers. The difference in the value of capture cross-section is yet one more piece of evidence that the origin of the hole traps in RN0 is different from that in conventional oxides.
50 80 50
3.75 x IO” 2.96 x IO” 3.67 x IO”
Oxide RN0
Capture cross-section (cm? I.1 x lo-” I.1 x lo-” 1.3 x lo-”
Since the hole traps are located near the gate_SiO, interface, one may suspect the existence of a very high density of hole traps in RNO. Dunn et a1.[1,5], in fact, suggested the presence of a large number of hole traps near the gate-SiO, interface. However, we see from Fig. 2 that the hole trap density is less in RN0 devices. The values of the peak hole trap density obtained for the best fit are 3.75 x lOi*and 9.2 x lO”cm- respectively in dry oxide and RNO. We conclude that not only are the hole traps in RN0 located away from the Si-SiO, interface and close to the gate.-SiO, interface, but also their density is less as compared to conventional dry oxides. There is a degree of arbitrariness in the exact values of centroid and standard deviation assumed in our simulation. To verify the effect of variation in these parameters, we carried out various simulations by assuming different values of the centroid and the standard deviation. The results for oxide are shown in Table 1. We see from this table that the value of capture cross-section is relatively insensitive to variation in these parameters, as expected. Similar results are obtained for RN0 also. 5.
SUMMARY
AND CONCLUSIONS
In this paper we have suggested a simple and robust radiation-based technique to estimate capture cross-section of hole traps in MOS insulators. The capture cross-section is measured by comparing data of AV,,,, vs dose with results of numerical simulation. The capture cross-sections of the traps in RN0 as well as in conventional dry oxide were measured by this technique. We found a large difference in the values of op in RN0 and in conventional oxides which corroborates the suggestion that the origin of the hole traps in RN0 is quite different from that in conventional oxides. Acknowledgements-This work was supported by the Department of Electronics, Government of India. The authors would like to thank Dr S. J. Patrikar for help with the simulator and Professor R. La1 for stimulating discussions. Department of Electrical Engineering Indian Institute of Technology Powai, Bombay -400 076 India
4x 10’6 ----
(A)
Peak hole density (cm-‘)
A. Mallik A. N. Chandorkar J. Vasi
3x 10’0
REFERENCES
2x1018
1 XlO’6
, 0
‘.
100 Distance
I
I
200 from
300 gate
400 C&
Fig. 2. Hole trap distribution assumed in conventional dry oxide and RN0 for the simulated results of Fig. 1.
1. G. J. Dunn and P. W. Wyatt, IEEE Trans. Nucl. Sci. NS-36, 2161 (1989). 2. N. Bhat and J. Vasi. IEEE Trans. Nucl. Sci. NS-39.2230 (1992). 3. G. J. Dunn, R. Jayaraman, W. Yang and C. G. Sodini, ADDI. Phvs. L&t. 52, 1713 (1988). 4. A.-Malhk, J. Vasi and A.‘N. Chandorkar, Solid-St. Electron. 36, 1359 (1993). 5. G. J. Dunn, J. uppl. Phys. 66, 4879 (1989). 6. A. Mallik, J. Vasi and A. N. Chandorkar, IEEE Trans. Nucl. Sci. NS-40, 1380 (1993). 7. A. Mallik, J. Vasi and A. N. Chandorkar, J. appl. Phys. 74, 2665 (1993). 8. T. H. Ning, J. uppl. Phys. 47, 3203 (1976). 9. V. Vasudevan and J. Vasi, J. appl. Phys. 70,449O (1991).
Note 10. F. B. McLean, H. E. Boesch Jr and T. R. Oldham, in Ionising Radiation Effects in MOS Devices and Circuits
(Edited by T. P. Ma and P. V. Dressendorfer), p. 87. Wiley, New York (1989).
18.53 11. J. M. Aitken and D. R. Young, IEEE Trans. Nucl. Sci. NS-24, 2128 (1977).
12. I. A. Chaiyasena, P. M. Lenahan and G. J. Dunn, J. appl. Phys. 72, 820 (1992).
Pergamon
NOTE CAPTURE CROSS-SECTION OF HOLE TRAPS IN REOXIDIZED NITRIDED OXIDE MEASURED BY IRRADIATION (Received
17 October
1994; in revised form 20 February 1995)
1. INTRODUCIION
centroid, peak density and standard deviation as variable parameters, is assumed. Our technique assumes that the hole traps generated by radiation are negligible, so that irradiation has the effect only of filling up the existing traps, whose u, is being measured.
In the search for an alternative insulator to silicon dioxide for radiation-hard applications, Reoxidized Nitrided Oxide (RNO) has shown great promise[l-6]. Experiments by Dunn[5] indicate that the hole traps in RN0 are located near the gat+SiO, interface in RNO. We have shown earlier[‘l] that, in addition, their energy distribution in RN0 is different from that in conventional oxides (dry pure SiO,). These results indicate a different origin of the hole traps in RN0 as compared to conventional oxides. If this is indeed true, then the value of the capture cross-section of these traps in RN0 is also likely to be different. However, no reports of the measurement of capture cross-section of hole traps in RN0 are available in the literature. The capture cross-sections of oxide traps are normally measured by the avalanche injection technique[l]. However, this technique can only be applied to hole traps with difficulty as discussed in Section 2. In this paper, we suggest a simple radiation-based technique for estimating the capture cross-section of hole traps. The capture cross-section of the traps in RN0 as well as in conventional dry oxides are measured by this technique, and found to be quite different.
3. EXPERIMENTAL
2. A RADIATION-BASED TECHNIQUE FOR ESTIMATION OF CAPTURE CROSSSECTION OF HOLE TRAPS
Avalanche injection has been traditionally used to calculate capture cross-section c,, of hole traps in MOS insulators[9]. This method assumes that the current which flows in the MOS structure is due only to holes injected from the substrate. This may often not be true. The smaller barrier for electrons at the gate-oxide interface can cause substantial electron injection from the gate electrode into the oxide along with hole injection from the substrate, resulting in a large electron current. This results in a value of u,, which may be quite incorrect. We suggest a new technique to calculate the capture cross-section of the hole traps. In this technique, we measure the midgap voltage shift At’,,,* of a MOS capacitor or transistor as a function of radiation dose. The rate of build-up of AV,,,g gives an idea of u,,. To get a good quantitative value of tr , we fit the data with the results of a simulator which pre d!acts AV,,,* as a function of radiation dose. The simulator solves the one-dimensional continuity and trap rate equations for holes as well as for electrons plus Poisson’s equation, in both the oxide and semiconductor. The details of the simulator can be found elsewhere[9]. Processes like geminate recombination and hole detrapping are taken into account in this simulator. The values of electron and hole mobility are taken as 20 and 10e5 cm*/V.s[lO] respectively. A constant trap distribution is taken for electron traps. The values of trap density and capture cross-section of the electron traps are taken as 2 x 10” and 4 x 10-i* cm2 respectively, which are reasonable[6,10]. For the hole traps, a Gaussian distribution with
The n-channel MOS transistors with W = 149 pm and J&= 5.4pm were fabricated on 4-6 G-cm (100) boron doped p-type wafers. The fabrication details are described elsewhere[rl]. Gate oxides 40 nm thick were made using both conventional dry oxide and RNO. The threshold voltage (V,) was measured from the I,-VGs plot for a low value of V,, (40 mV). A straight line through the point of maximum slope of the I,-VGs plot was drawn and its intercept with the V,, axis gives V,. The interface-state density (Ni,) was measured by using a standard two-level charge pumping (CP) technique. A 100 kHz 6 V peak-to-peak square wave was applied to the gate and the substrate current (1,) was measured by a Keithley 619 electrometer. The devices were irradiated floating using a @‘Cogammaray source with a dose rate of 360 krad(Si)/h. The midgap voltage shift (AV,) for each radiation dose was derived from the corresponding AV, and AN,, as follows. It is widely accepted that the interface states are acceptor-type above midgap and donor-type below midgap. Assuming a uniform distribution, we get:
where C,,, is the gate capacitance per unit area. Note that we have assumed there is no transformation between trapped holes and interface states. 4. RESULTS AND DISCUSSIONS
It is well-known that the hole traps in conventional oxides are located close to the Si-SiO, interface. These traps are popularly known as the E’ centers. In our simulation, therefore, we assume a Gaussian distribution of the hole traps with the centroid 50 8, away from the Si-SiO, interface and with a standard deviation of 5019 in the case of conventional dry oxide. On the other hand, the dominant hole traps for RN0 are found to be located near the gate-SiO, interface[5,7]. Therefore, for RN0 devices, we assume a Gaussian distribution of these traps with the centroid 5OA from the gat&iO, interface and with the same standard deviation as stated earlier. The peak value and a,, are found so as to obtain best fits of the simulator output to the experimental data, as described in more detail below. In Fig. 1, AV,, is plotted against radiation dose. The open and filled circles represent the experimental values of AV,,,, respectively for conventional dry oxide and RN0 devices.
1851
Note Table 1. Results of the simulation for dry oxide with different centroid and standard deviation
0.6 .
5:
-
E” > a
Standard deviation
Centroid (A) from gate
0.4
350 350 370
RN0 (Expt.) Oxide(Simulated) RNO(Simulated)
,
1
Dose
1200
900
600
300
(krad
(Si))
Fig. 1. Midgap voltage shift as a function of radiation dose for conventional dry oxide and RN0 devices. The continuous and dotted lines represent respectively the simulated results for dry oxide and RNO. The hole trap distributions are shown in Fig. 2. The peak value of hole density determines the final (saturation) value of A Vme,and the up affects how quickly (as a function of dose) tt gets there. The values of the capture cross-section of the hole traps obtained for best fit are 1.1 x lo-” and 7.9 x 10-t* cm* respectively in conventional dry oxide and in RNO. There is a large difference in the value of capture cross-section of the hole traps in conventional oxide and in RNO. This is a reflection of the experimental data of Fig. 1 where we see that AV, saturates much faster in RN0 devices as compared to conventional oxide devices. The value of u,, obtained for hole traps in oxide is quite comparable to values quoted in the literature[lO,ll]. This confirms the validity of our new technique. The value of cy of hole traps in RN0 is obtained here for the first ttme. It has been shown earlier that the location as well as the energy distribution of the hole traps in RN0 are different to these in conventional oxides[7]. The electron spin resonance study by Chaiyasena et a[.[121 also indicates that the dominant hole traps in RN0 are not the well-known E’ centers. The difference in the value of capture cross-section is yet one more piece of evidence that the origin of the hole traps in RN0 is different from that in conventional oxides.
50 80 50
3.75 x IO” 2.96 x IO” 3.67 x IO”
Oxide RN0
Capture cross-section (cm? I.1 x lo-” I.1 x lo-” 1.3 x lo-”
Since the hole traps are located near the gate_SiO, interface, one may suspect the existence of a very high density of hole traps in RNO. Dunn et a1.[1,5], in fact, suggested the presence of a large number of hole traps near the gate-SiO, interface. However, we see from Fig. 2 that the hole trap density is less in RN0 devices. The values of the peak hole trap density obtained for the best fit are 3.75 x lOi*and 9.2 x lO”cm- respectively in dry oxide and RNO. We conclude that not only are the hole traps in RN0 located away from the Si-SiO, interface and close to the gate.-SiO, interface, but also their density is less as compared to conventional dry oxides. There is a degree of arbitrariness in the exact values of centroid and standard deviation assumed in our simulation. To verify the effect of variation in these parameters, we carried out various simulations by assuming different values of the centroid and the standard deviation. The results for oxide are shown in Table 1. We see from this table that the value of capture cross-section is relatively insensitive to variation in these parameters, as expected. Similar results are obtained for RN0 also. 5.
SUMMARY
AND CONCLUSIONS
In this paper we have suggested a simple and robust radiation-based technique to estimate capture cross-section of hole traps in MOS insulators. The capture cross-section is measured by comparing data of AV,,,, vs dose with results of numerical simulation. The capture cross-sections of the traps in RN0 as well as in conventional dry oxide were measured by this technique. We found a large difference in the values of op in RN0 and in conventional oxides which corroborates the suggestion that the origin of the hole traps in RN0 is quite different from that in conventional oxides. Acknowledgements-This work was supported by the Department of Electronics, Government of India. The authors would like to thank Dr S. J. Patrikar for help with the simulator and Professor R. La1 for stimulating discussions. Department of Electrical Engineering Indian Institute of Technology Powai, Bombay -400 076 India
4x 10’6 ----
(A)
Peak hole density (cm-‘)
A. Mallik A. N. Chandorkar J. Vasi
3x 10’0
REFERENCES
2x1018
1 XlO’6
, 0
‘.
100 Distance
I
I
200 from
300 gate
400 C&
Fig. 2. Hole trap distribution assumed in conventional dry oxide and RN0 for the simulated results of Fig. 1.
1. G. J. Dunn and P. W. Wyatt, IEEE Trans. Nucl. Sci. NS-36, 2161 (1989). 2. N. Bhat and J. Vasi. IEEE Trans. Nucl. Sci. NS-39.2230 (1992). 3. G. J. Dunn, R. Jayaraman, W. Yang and C. G. Sodini, ADDI. Phvs. L&t. 52, 1713 (1988). 4. A.-Malhk, J. Vasi and A.‘N. Chandorkar, Solid-St. Electron. 36, 1359 (1993). 5. G. J. Dunn, J. uppl. Phys. 66, 4879 (1989). 6. A. Mallik, J. Vasi and A. N. Chandorkar, IEEE Trans. Nucl. Sci. NS-40, 1380 (1993). 7. A. Mallik, J. Vasi and A. N. Chandorkar, J. appl. Phys. 74, 2665 (1993). 8. T. H. Ning, J. uppl. Phys. 47, 3203 (1976). 9. V. Vasudevan and J. Vasi, J. appl. Phys. 70,449O (1991).
Note 10. F. B. McLean, H. E. Boesch Jr and T. R. Oldham, in Ionising Radiation Effects in MOS Devices and Circuits
(Edited by T. P. Ma and P. V. Dressendorfer), p. 87. Wiley, New York (1989).
18.53 11. J. M. Aitken and D. R. Young, IEEE Trans. Nucl. Sci. NS-24, 2128 (1977).
12. I. A. Chaiyasena, P. M. Lenahan and G. J. Dunn, J. appl. Phys. 72, 820 (1992).