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A study of the interface states in MIS-structures with thin SiO2 and SiOxNy layers using deep level transient spectroscopy
PERGAMON
Microelectronics Reliability 39 (1999) 297±302
A study of the interface states in MIS-structures with thin SiO2 and SiOxNy layers using deep level transient spectroscopy R. Beyer a, *, H. Burghardt a, E. Thomas a, R. Reich a, D.R.T. Zahn b, T. Geûner a a
Institut fuÈr Halbleiter-und Mikrosystemtechnik, Mikrotechnologie, Technische UniversitaÈt Chemnitz, D-09107 Chemnitz, Germany b Institut fuÈr Physik, Halbleiterphysik, Technische UniversitaÈt Chemnitz, D-09107 Chemnitz, Germany Received 20 April 1998; received in revised form 17 August 1998
Abstract Deep level transient spectroscopy (DLTS) and quasistatic CV measurements were used for the study of the interface states of thin SiO2 and SiOxNy layers of 6±9 nm thickness, grown by rapid thermal processing in O2 or N2O ambient. DLTS was applied either in the saturating pulse or in the small pulse (energy resolved) mode. From an Arrhenius evaluation of the peaks obtained by temperature-scan measurements with small pulse excitation we derived the distribution of the capture cross section sn in the upper half of the gap, which exhibits a drastic decrease towards the conduction band edge. # 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction In recent years considerable work has been done on thin siliconoxynitride layers grown on silicon in N2O atmosphere [1±7]. Compared with SiO2, a reduced charge trapping, a lower interface state generation during charge injection, and higher Qbd and Ebd values are referred to. It is well accepted, that the substitution of weak Si±H or Si±OH bonds by more stable Si±N bonds at the semiconductor±insulator interface contributes to the improved electrical behaviour. Furthermore, the incorporation of nitrogen may reduce the interfacial strain. Consequently, the knowledge of the electronic structure of the interface and the near interfacial region is a key to the understanding of the electrical reliability and the degradation due to high ®eld stress. Unfortunately the role of nitrogen for the formation or removing of interface states is still not clear in detail. In order to reveal these mechan-
* Corresponding author. Tel.: +49-371-531-4807; fax: 49371-531-3131; e-mail: [email protected].
isms, further studies of the electronic states at the interface are required. This is particularly emphasized by the fact that the interface, i.e. the transition region between the semiconductor substrate and the stochiometric bulk insulator, becomes more and more a dominant part of the whole MIS structure when the insulator thickness decreases below 10 nm. Surprisingly, there are only a few papers known using deep level transient spectroscopy (DLTS) to examine nitrous oxide grown SiOxNy. While Belkough and coworkers [8] applied the conventional DLTS using a saturating pulse excitation, Thurzo et al. [9] performed energy resolved charge transient DLTS measurements using the selective small pulse excitation. However, both groups assumed a constant capture cross section of the interface states in order to determine the interface state density Dit. In this study we apply DLTS as well as CV measurements in order to determine the interface state density distribution Dit versus Ec ÿ Et of MIS structures with thin SiO2 and SiOxNy dielectric grown by rapid thermal processing (RTP) in O2 and N2O. From small pulse DLTS measurements we obtained the
0026-2714/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 6 - 2 7 1 4 ( 9 8 ) 0 0 2 3 5 - 2
298
R. Beyer et al. / Microelectronics Reliability 39 (1999) 297±302
distribution of the capture cross section sn(E) of the interface states for both oxides and oxynitride layers. 2. Experimental Thin oxynitride layers were grown on (100) oriented n-type silicon in N2O atmosphere at high temperatures (11008 and 11508C) using rapid thermal processing. For comparison, SiO2 layers were grown at 11008C in O2. MIS-capacitors with 7.510 ÿ 4, 2.510 ÿ 3 and 510 ÿ 3cm2 area were formed by the deposition of a gate stack of 80 nm polycrystalline silicon and 400 nm aluminium on top of the insulator. The insulator thickness was in the range of 6±9 nm for all samples studied. We intentionally performed no anneal in the hydrogen-rich atmosphere after the metallization. Thus, the electrical active states due to growth-related defects at the semiconductor±insulator interface are not removed by hydrogen saturation. CV measurements of the MIS-structures were carried out using a HP 4275A multi-frequency LCR meter and a HP 4140B picoammeter. The distribution of the interface states Dit versus Ec ÿEt was derived from the dierence between the measured quasistatic CV characteristic and a calculated theoretical curve. DLTS measurements were performed in the temperature range between 77 and 400 K. Our DLTS setup is based on a fast capacitance meter Boonton 72B. The capacitance transients due to the emission of trapped carriers were digitally stored and could be treated with dierent weighting functions according to the model for the time-dependent process assumed. A Fourier transform procedure corresponding to the application of sinusoidal weighting functions was mainly used to form the DLTS signal [10, 11]. We applied both large pulse DLTS, where the ®lling pulse saturates all interface states with carriers as well as small pulse (SP)-DLTS, where the surface potential is kept constant (in depletion), superimposed by a small ®lling pulse DVp, which can selectively populate a small fraction of states within a section DE near the Fermi level.
to 5 mV/s in order to meet the quasi-equilibrium condition, the measured current can be as low as in the sub-pA range, even in accumulation or inversion. Since the measured current Im contains a capacitive component Ic and an ohmic leakage current part Il the in¯uence of the latter one has to be taken into account. From a separate IV measurement Il was determined and subtracted from Im. Assuming that the exponential tails towards the band edges are of dierent origin from the states in the vicinity of midgap, the exponential ®t of the tails is subtracted from the total distribution of the interface states. The remaining part of the distribution stretches from approximately 0.25 to 0.95 eV above the valence band edge. For the silicon± silicon dioxide interface a Gaussian ®t with two peaks describes the shape of the distribution fairly well, with maxima positions at Ev + 0.32 eV and Ev + 0.76 eV. These features probably originate from Pb centres since those were often found to exhibit peaks centred at about 0.3 eV above the valence band edge and 0.3 eV below the conduction band edge. For the N2Ogrown oxynitride layers on silicon a third peak had to be added for a satisfactory ®t; thus yielding maxima positions of 0.33, 0.63 and 0.83 eV above the valence band edge, respectively. It is noticable that the midgap values of Dit of all samples grown in N2O at 11008 and 11508C are in the range between 2.51011 and 6.51011 cm ÿ 2eV ÿ 1, which is signi®cantly lower than the value of the thermally grown SiO2. The lower overall interface state density of the oxynitride involves a diminished concentration of Pb centres. The interfacial nitrogen concentration, obtained by auger electron spectroscopy, is about 4 at% and we have good reasons to suppose that only a fraction of Pb defects is healed according to strain reduction and Si±N bond formation, whereas the other part is still present in the sample. The additional level, deduced from the ®tting procedure, might be an indication of a defect related
3. Results and discussion A comparison of the interface state densities Dit versus the energy position within the gap of the silicon for SiO2 and SiOxNy is displayed in Fig. 1. The calculation of Dit from the CV curve for the whole gap, including the region near the band edges requires a careful determination of the oxide capacitance Cox. ÿ1 Using a plot of Cm versus Vgÿ 1 we obtained Cox from the intercept with the 1/C axis, which corresponds to the value for Vg 41. However, since the sweep rate dU/dt in the quasistatic CV measurement was reduced
Fig. 1. Interface state density Dit versus E for Si±SiO2 and Si± SiOxNy samples. For the Si±SiO2 interface the evaluation procedure is demonstrated.
R. Beyer et al. / Microelectronics Reliability 39 (1999) 297±302
to the interfacial nitrogen, as pointed out previously [8, 9]. Unfortunately, a microscopic description of such a kind of defect has been hitherto lacking. The distribution of Dit was alternatively determined from saturating pulse DLTS experiments. Therefore, the interface was ®rst driven into accumulation during the ®lling pulse (+2 V, 10 ÿ 1 s) in order to populate all interface traps with carriers captured from the conduction band. Subsequently, the reverse bias was applied, driving the MIS structures into depletion or weak inversion and the emission of the previously trapped carriers was observed by monitoring the transient behaviour of the capacitance C(t). A typical DLTS spectrum for the Si-SiOxNy interface is displayed in Fig. 2. As can be seen, the DLTS signal S(T) has a quite featureless and smooth shape, which is due to the contribution of the continuum of states at the interface. Thus, a peak maximum shift owing to a variation of the emission rate window is not detectable and the classical Arrhenius evaluation [12] is not applicable. The increase of S(T) at 300 K corresponds to the build-up of the inversion layer, i.e. both the emission of trapped carriers from interface states as well as the build-up of the inversion layer contribute to the capacitance transient and consequently to the DLTS signal, too. While the ®rst mechanism is governed by a multiexponential or logarithmic time law, the latter one is known to have linear dependence on time. With the Fourier±DLTS system used, a change of the dominant time law can be recognized by monitoring the ratio between the discrete Fourier coecients an, bn (n = 1,2,3 . . . ) corresponding to the DLTS signal S(T) for dierent rate windows. A detailed description of this procedure is given by Weiss [13]. As a consequence, already the slight increase of S(T) above 275 K in Fig. 2 requires a critical evaluation, when connecting the interface state density Dit with the DLTS signal. However, the weak point of the evalu-
Fig. 2. DLTS signal S(T) of an oxynitride layer on silicon, measured with a rate window en = 23.15 s ÿ 1. The reverse bias was ÿ2 V, the ®lling pulse voltage +2 V.
299
ation of saturating pulse DLTS data is the relation between the T-axis of the DLTS spectrum and the energy axis in the distribution Dit(E), since it requires an assumption about a constant capture cross section sn. We calculated Dit using the relation Dit ÿ
E Er A Ns Cox bn C3R k T b~n
1
where E is the vacuum permittivity, Er the dielectric constant, A the area of the MOS capacitor, Ns the doping concentration in the semiconductor, Cox the oxide capacitance, CR the reverse bias capacitance in depletion, k is the Boltzmann constant and T the absolute temperature, respectively. The quantity bn is the discrete Fourier coecient, which corresponds to the DLTS signal S(T), whereas bÄn are continuous coecients, for details see ref. [13]. Fig. 3 shows the distribution of Dit versus E in the upper half of the bandgap obtained from both CV and DLTS measurements for thin oxide and oxynitride layers. The values determined by DLTS (assuming sn = 10 ÿ 17cm2) are up to 12 order of magnitude below those obtained from quasistatic CV measurements. Autran et al. [14] reported on a similar or even larger disagreement between the two methods, whereas Bauza and coworkers [15] found a good coincidence of Dit near midgap for thin oxides. It is noted that the signi®cant peaks 0.36 eV below the conduction band edge for the SiO2±Si interface and 0.29 eV below CB for the SiOxNy±Si interface found by CV are not detectable by DLTS, which can be explained by the better sensitivity of the quasistatic CV measurement to a fast response of the interface states according to the inherent current integration of the method. On the other hand,
Fig. 3. Comparison of the distribution of Dit in the upper half of the gap for SiO2 (11008C, O2, 20 s, ti = 8.7 nm) and SiOxNy (11508C, N2O, 90 s, ti = 7.7 nm) obtained by CV and DLTS: SiO2, CV (1), SiOxNy, CV (2); SiO2, DLTS (3); SiOxNy, DLTS (4). The curves (5) belong to the ®t procedure explained in the text.
300
R. Beyer et al. / Microelectronics Reliability 39 (1999) 297±302
the sensitivity of the DLTS is roughly restricted to emission processes with a time constant te which depends on the rate window en adjusted during the observation of the transient behaviour. Since we applied rate windows between 1 and 200 s ÿ 1, only processes with a time constant between 10 ÿ 2 and 1 s were caught remarkably well. Considering a sample to sample comparison of the interface state density distribution as well as of the the midgap levels of Dit, the results obtained by saturating pulse DLTS and CV measurements generally agree. In order to overcome the lack of the unknown capture cross section, additional SP-DLTS experiments were performed. The method, often designated as energy-resolved (ER) DLTS was introduced by Johnson [16] and Yamasaki et al. [17]. It allows a proof of the interface states either by temperature-scan under constant voltage conditions or by isothermal bias-scan, providing information about the triples (Dit, Et, sn) of interface states via Arrhenius evaluation of the SP-DLTS peaks. Despite of the advantages of the SP-DLTS method, it is only rarely used, obviously due to the time-consuming measurement and evaluation procedure, which requires, for instance, supplementary measurements of the temperature-dependent CV characteristic in order to determine the band bending at the maximum temperature of the SP-DLTS peaks. Moreover, surface potential ¯uctuations [18], the charge feedback eect [19] and the complicated kinetics of the electron capture process in the depletion mode have to be kept in mind when interpreting the data. Fig. 4 shows a series of small pulse DLTS peaks of a MIS structure with a SiO2 dielectric of 8.7 nm thickness. The quiescent voltage Vq was changed from ÿ0.90 V to ÿ0.50 V in steps of 50 mV for the dierent temperature scans, whereas the amplitude of the ®lling
Fig. 4. A series of small pulse DLTS signals related to the Si± SiO2 interface. The parameter is the quiescent voltage Vq, changed from ÿ0.5 to ÿ0.9 V in steps of 50 mV. The ®lling pulse amplitude was 100 mV and the rate window en = 23.1 s ÿ 1.
pulse voltage DVp was 100 mV for all cycles. The curve with Vq =ÿ 0.50 V corresponds to a smaller band bending at the semiconductor surface. Consequently, the Fermi level is closer to the conduction band edge, thus giving rise to a peak maximum at lower temperatures. Even in the case of small pulse DLTS the emission signal comes from a continuous distribution of states located within a small section DE where the traps are charged and discharged by applying and removing the ®lling pulse voltage DVp. Therefore, an eective trap energy Et,e exists, related to the eective emission time constant te,e for emptying the traps within DE. The energy Et,e, which describes the thermal activation energy of carrier transition from the traps in the vicinity of the Fermi level to the conduction band can be obtained alternatively by an Arrhenius evaluation of the sharp SP-DLTS peaks or by the simple relation [22] Ec ÿ Et;eff Cs Ec ÿ EF ÿ 0:5 DE
2
where Cs is the surface band bending and DE is the energy section near the Fermi level contributing to the capture and emission process. Thus, the peak at 98 K in Fig. 4 can be attributed to states located 0.09 eV below the conduction band edge, whereas the peak at approximately 240 K originates from interface states 0.51 eV below Ec. It is evident that the peak shape becomes broader when probing interface states close to midgap. Considering the band bending as a function of the gate voltage, as can be obtained from CV measurements and a solution of the Berglund integral, we can estimate the energetic resolution of the SPDLTS experiment. Obviously, the section DE for a given small pulse amplitude DVp is smaller for gate voltages close to the ¯atband condition and broader for gate voltages close to Vmidgap. Since a broad peak shape may give rise to an incorrect determination of Et,e and the capture cross section sn from an Arrhenius evaluation of the peak maxima, the enhancement of the resolution is a decisive task. Therefore, the amplitude for the small pulse excitation DVp was reduced from 100 mV down to 10 mV. As can be seen in Fig. 5, even for DVp = 10 mV an evaluable DLTS signal is obtained. This holds good for both the shallow states as well as for the deep states at the interface. However, the high sensitivity is dedicated to the rather high concentration of interface traps in our samples. If the interface state density is in the lower 1010 cm ÿ 2 eV ÿ 1 range, small ®lling pulses as low as 10 mV are not applicable. It is noticable that the DLTS peaks are not much in¯uenced by surface potential ¯uctuations, caused by the HF voltage, which had in our case an amplitude of 100 mV. Only the peak positions change slightly according to the
R. Beyer et al. / Microelectronics Reliability 39 (1999) 297±302
301
above midgap [23]. The dispersion of sn is actually closely related to the method and the model used for the measurement and the evaluation. The data from SP-DLTS peaks obtained by a 100 mV pulse excitation are con®rmed by measurements with DVp = 50 mV.
4. Conclusions
Fig. 5. Isothermal bias-scan with small pulse excitation of a Si±SiO2 interface at 90 K and 250 K. The ®lling pulse amplitude DVp is varied from 100 to 10 mV.
shift of the eective emission time constant within the small section DE (DVp). Using the well known detailed balance equation EC ÿ Et en sn vth NC exp ÿ 3 kT with en being the emission rate, vth the thermal velocity of the carriers, Nc the eective density of states in the conduction band and Ec ÿ Et the thermal activation energy we calculated the electron capture cross section sn. The energetic distribution of sn is plotted in Fig. 6. A signi®cant decrease towards the band edge is evident for the SiO2 as well as for the SiOxNy interface. Within the error bars both curves coincide. The general shape of sn versus E for the semiconductor±insulator interface is in good agreement with a variety of other papers [19±21]. Some authors, however, reported on much higher values of the electron capture cross section with a maximum at approximately 0.2 eV
We used DLTS with saturating pulse excitation, small pulse DLTS and CV measurements in order to characterize the electrically active states at the semiconductor±insulator interface of MIS structures with thin oxide and oxynitride layers. The interface state density Dit provided by DLTS is lower compared with the data extracted from CV measurements. The analysis of the distribution of Dit obtained by CV reveals two peaks for the SiO2 and three peaks for the SiOxNy interface, DLTS provides a smooth continuous distribution within the upper half of the gap. Implementating the SP-DLTS, the dispersion of the capture cross section s(E) was determined, which exhibits a strong decrease towards the conduction band edge. However, in order to avoid an incorrect interpretation of the data obtained from small pulse DLTS experiments, further studies are required with an appropriate consideration of the real kinetics of the capture and emission of carriers at interface states, including the interaction with insulator traps. Moreover, a quantitative description of the generation process leading to the build-up of the inversion layer and consequently a distinction between trap emission and generation seems to be promising for the extension of the energetic range covered by the SP-DLTS method.
References
Fig. 6. Distribution of the capture cross section sn versus E of the Si±SiO2 and Si±SiOxNy interface states, obtained from small pulse DLTS measurements.
[1] Fukuda H, Endoh T, Nomura S. In: The Physics and Chemistry of SiO2 and the Si±SiO2 InterfaceÐ3. Pennington, NJ: The Electrochemical Society. 1996. p. 15±27. [2] Hegde RI, Maiti B, Tobin PJ. J Electrochem Society 1997;144:1081±6. [3] Fleetwood DM, Saks NS. J Appl Phys 1996;79:1583±94. [4] Fukuda H, Yasuda M, Iwabuchi T, Ohno S. IEEE Electron Device Lett 1991;12:587±9. [5] Hwang H, Ting W, Maiti B, Kwong DL, Lee J. Appl Phys Lett 1990;57:1010±1. [6] Joshi AB, Yoon G, Kim J, Lo GQ, Kwong DL. IEEE Trans Electron Devices 1993;40:1437±43. [7] Beyer R, Burghardt H, Reich R, Thomas E, Geûner T, Zahn DRT. Microelectron Reliab 1998;38:243±7. [8] Belkouch S, Jean C, Aktik C, Ameziane EL. Appl Phys Lett 1995;67:530±2.
302
R. Beyer et al. / Microelectronics Reliability 39 (1999) 297±302
[9] Thurzo I, Gmucova K, Csabay O, Harmatha L, Burghardt H, Beyer R. Phys Stat Solid (a) 1995;151:345± 53. [10] Ikeda K, Takaoka H. Jap J Appl Phys 1982;21:462±6. [11] Weiss S, Kassing R. Solid State Electron 1988;31:1733±9. [12] Lang DV. J Appl Phys 1974;45:3014±3022;3023±3032. [13] Weiss S. Ph.D. Thesis. University of Kassel, 1991. [14] Autran JL, Seigneur F, Plossu C, Balland B. J. Appl. Phys. 1993;74:3932±5. [15] Bauza D, Morfouli P, Pananakakis G. Solid State Electron 1991;34:933±6. [16] Johnson NM. Appl Phys Lett 1979;34:802±4.
[17] Yamasaki K, Yoshida M, Sugano T. Jap J Appl Phys 1979;18:113±22. [18] OÈzder S, Atilgan I, Katircioglu B. Solid State Electron 1996;39:243±249. [19] Goguenheim D, Vuillaume D, Vincent G, Johnson NM. J Appl Phys 1990;68:1104±13. [20] Aberle AG, Glunz S, Warta W. J Appl Phys 1992;71:4422±31. [21] Tredwell TJ, Viswanathan CR. Solid State Electron 1980;23:1171±8. [22] Katsube T, Kakimoto K, Ikoma T. J Appl Phys 1981;52:3504±8. [23] Eades WD, Swanson RM. J Appl Phys 1984;56:1744±51.
Microelectronics Reliability 39 (1999) 297±302
A study of the interface states in MIS-structures with thin SiO2 and SiOxNy layers using deep level transient spectroscopy R. Beyer a, *, H. Burghardt a, E. Thomas a, R. Reich a, D.R.T. Zahn b, T. Geûner a a
Institut fuÈr Halbleiter-und Mikrosystemtechnik, Mikrotechnologie, Technische UniversitaÈt Chemnitz, D-09107 Chemnitz, Germany b Institut fuÈr Physik, Halbleiterphysik, Technische UniversitaÈt Chemnitz, D-09107 Chemnitz, Germany Received 20 April 1998; received in revised form 17 August 1998
Abstract Deep level transient spectroscopy (DLTS) and quasistatic CV measurements were used for the study of the interface states of thin SiO2 and SiOxNy layers of 6±9 nm thickness, grown by rapid thermal processing in O2 or N2O ambient. DLTS was applied either in the saturating pulse or in the small pulse (energy resolved) mode. From an Arrhenius evaluation of the peaks obtained by temperature-scan measurements with small pulse excitation we derived the distribution of the capture cross section sn in the upper half of the gap, which exhibits a drastic decrease towards the conduction band edge. # 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction In recent years considerable work has been done on thin siliconoxynitride layers grown on silicon in N2O atmosphere [1±7]. Compared with SiO2, a reduced charge trapping, a lower interface state generation during charge injection, and higher Qbd and Ebd values are referred to. It is well accepted, that the substitution of weak Si±H or Si±OH bonds by more stable Si±N bonds at the semiconductor±insulator interface contributes to the improved electrical behaviour. Furthermore, the incorporation of nitrogen may reduce the interfacial strain. Consequently, the knowledge of the electronic structure of the interface and the near interfacial region is a key to the understanding of the electrical reliability and the degradation due to high ®eld stress. Unfortunately the role of nitrogen for the formation or removing of interface states is still not clear in detail. In order to reveal these mechan-
* Corresponding author. Tel.: +49-371-531-4807; fax: 49371-531-3131; e-mail: [email protected].
isms, further studies of the electronic states at the interface are required. This is particularly emphasized by the fact that the interface, i.e. the transition region between the semiconductor substrate and the stochiometric bulk insulator, becomes more and more a dominant part of the whole MIS structure when the insulator thickness decreases below 10 nm. Surprisingly, there are only a few papers known using deep level transient spectroscopy (DLTS) to examine nitrous oxide grown SiOxNy. While Belkough and coworkers [8] applied the conventional DLTS using a saturating pulse excitation, Thurzo et al. [9] performed energy resolved charge transient DLTS measurements using the selective small pulse excitation. However, both groups assumed a constant capture cross section of the interface states in order to determine the interface state density Dit. In this study we apply DLTS as well as CV measurements in order to determine the interface state density distribution Dit versus Ec ÿ Et of MIS structures with thin SiO2 and SiOxNy dielectric grown by rapid thermal processing (RTP) in O2 and N2O. From small pulse DLTS measurements we obtained the
0026-2714/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 6 - 2 7 1 4 ( 9 8 ) 0 0 2 3 5 - 2
298
R. Beyer et al. / Microelectronics Reliability 39 (1999) 297±302
distribution of the capture cross section sn(E) of the interface states for both oxides and oxynitride layers. 2. Experimental Thin oxynitride layers were grown on (100) oriented n-type silicon in N2O atmosphere at high temperatures (11008 and 11508C) using rapid thermal processing. For comparison, SiO2 layers were grown at 11008C in O2. MIS-capacitors with 7.510 ÿ 4, 2.510 ÿ 3 and 510 ÿ 3cm2 area were formed by the deposition of a gate stack of 80 nm polycrystalline silicon and 400 nm aluminium on top of the insulator. The insulator thickness was in the range of 6±9 nm for all samples studied. We intentionally performed no anneal in the hydrogen-rich atmosphere after the metallization. Thus, the electrical active states due to growth-related defects at the semiconductor±insulator interface are not removed by hydrogen saturation. CV measurements of the MIS-structures were carried out using a HP 4275A multi-frequency LCR meter and a HP 4140B picoammeter. The distribution of the interface states Dit versus Ec ÿEt was derived from the dierence between the measured quasistatic CV characteristic and a calculated theoretical curve. DLTS measurements were performed in the temperature range between 77 and 400 K. Our DLTS setup is based on a fast capacitance meter Boonton 72B. The capacitance transients due to the emission of trapped carriers were digitally stored and could be treated with dierent weighting functions according to the model for the time-dependent process assumed. A Fourier transform procedure corresponding to the application of sinusoidal weighting functions was mainly used to form the DLTS signal [10, 11]. We applied both large pulse DLTS, where the ®lling pulse saturates all interface states with carriers as well as small pulse (SP)-DLTS, where the surface potential is kept constant (in depletion), superimposed by a small ®lling pulse DVp, which can selectively populate a small fraction of states within a section DE near the Fermi level.
to 5 mV/s in order to meet the quasi-equilibrium condition, the measured current can be as low as in the sub-pA range, even in accumulation or inversion. Since the measured current Im contains a capacitive component Ic and an ohmic leakage current part Il the in¯uence of the latter one has to be taken into account. From a separate IV measurement Il was determined and subtracted from Im. Assuming that the exponential tails towards the band edges are of dierent origin from the states in the vicinity of midgap, the exponential ®t of the tails is subtracted from the total distribution of the interface states. The remaining part of the distribution stretches from approximately 0.25 to 0.95 eV above the valence band edge. For the silicon± silicon dioxide interface a Gaussian ®t with two peaks describes the shape of the distribution fairly well, with maxima positions at Ev + 0.32 eV and Ev + 0.76 eV. These features probably originate from Pb centres since those were often found to exhibit peaks centred at about 0.3 eV above the valence band edge and 0.3 eV below the conduction band edge. For the N2Ogrown oxynitride layers on silicon a third peak had to be added for a satisfactory ®t; thus yielding maxima positions of 0.33, 0.63 and 0.83 eV above the valence band edge, respectively. It is noticable that the midgap values of Dit of all samples grown in N2O at 11008 and 11508C are in the range between 2.51011 and 6.51011 cm ÿ 2eV ÿ 1, which is signi®cantly lower than the value of the thermally grown SiO2. The lower overall interface state density of the oxynitride involves a diminished concentration of Pb centres. The interfacial nitrogen concentration, obtained by auger electron spectroscopy, is about 4 at% and we have good reasons to suppose that only a fraction of Pb defects is healed according to strain reduction and Si±N bond formation, whereas the other part is still present in the sample. The additional level, deduced from the ®tting procedure, might be an indication of a defect related
3. Results and discussion A comparison of the interface state densities Dit versus the energy position within the gap of the silicon for SiO2 and SiOxNy is displayed in Fig. 1. The calculation of Dit from the CV curve for the whole gap, including the region near the band edges requires a careful determination of the oxide capacitance Cox. ÿ1 Using a plot of Cm versus Vgÿ 1 we obtained Cox from the intercept with the 1/C axis, which corresponds to the value for Vg 41. However, since the sweep rate dU/dt in the quasistatic CV measurement was reduced
Fig. 1. Interface state density Dit versus E for Si±SiO2 and Si± SiOxNy samples. For the Si±SiO2 interface the evaluation procedure is demonstrated.
R. Beyer et al. / Microelectronics Reliability 39 (1999) 297±302
to the interfacial nitrogen, as pointed out previously [8, 9]. Unfortunately, a microscopic description of such a kind of defect has been hitherto lacking. The distribution of Dit was alternatively determined from saturating pulse DLTS experiments. Therefore, the interface was ®rst driven into accumulation during the ®lling pulse (+2 V, 10 ÿ 1 s) in order to populate all interface traps with carriers captured from the conduction band. Subsequently, the reverse bias was applied, driving the MIS structures into depletion or weak inversion and the emission of the previously trapped carriers was observed by monitoring the transient behaviour of the capacitance C(t). A typical DLTS spectrum for the Si-SiOxNy interface is displayed in Fig. 2. As can be seen, the DLTS signal S(T) has a quite featureless and smooth shape, which is due to the contribution of the continuum of states at the interface. Thus, a peak maximum shift owing to a variation of the emission rate window is not detectable and the classical Arrhenius evaluation [12] is not applicable. The increase of S(T) at 300 K corresponds to the build-up of the inversion layer, i.e. both the emission of trapped carriers from interface states as well as the build-up of the inversion layer contribute to the capacitance transient and consequently to the DLTS signal, too. While the ®rst mechanism is governed by a multiexponential or logarithmic time law, the latter one is known to have linear dependence on time. With the Fourier±DLTS system used, a change of the dominant time law can be recognized by monitoring the ratio between the discrete Fourier coecients an, bn (n = 1,2,3 . . . ) corresponding to the DLTS signal S(T) for dierent rate windows. A detailed description of this procedure is given by Weiss [13]. As a consequence, already the slight increase of S(T) above 275 K in Fig. 2 requires a critical evaluation, when connecting the interface state density Dit with the DLTS signal. However, the weak point of the evalu-
Fig. 2. DLTS signal S(T) of an oxynitride layer on silicon, measured with a rate window en = 23.15 s ÿ 1. The reverse bias was ÿ2 V, the ®lling pulse voltage +2 V.
299
ation of saturating pulse DLTS data is the relation between the T-axis of the DLTS spectrum and the energy axis in the distribution Dit(E), since it requires an assumption about a constant capture cross section sn. We calculated Dit using the relation Dit ÿ
E Er A Ns Cox bn C3R k T b~n
1
where E is the vacuum permittivity, Er the dielectric constant, A the area of the MOS capacitor, Ns the doping concentration in the semiconductor, Cox the oxide capacitance, CR the reverse bias capacitance in depletion, k is the Boltzmann constant and T the absolute temperature, respectively. The quantity bn is the discrete Fourier coecient, which corresponds to the DLTS signal S(T), whereas bÄn are continuous coecients, for details see ref. [13]. Fig. 3 shows the distribution of Dit versus E in the upper half of the bandgap obtained from both CV and DLTS measurements for thin oxide and oxynitride layers. The values determined by DLTS (assuming sn = 10 ÿ 17cm2) are up to 12 order of magnitude below those obtained from quasistatic CV measurements. Autran et al. [14] reported on a similar or even larger disagreement between the two methods, whereas Bauza and coworkers [15] found a good coincidence of Dit near midgap for thin oxides. It is noted that the signi®cant peaks 0.36 eV below the conduction band edge for the SiO2±Si interface and 0.29 eV below CB for the SiOxNy±Si interface found by CV are not detectable by DLTS, which can be explained by the better sensitivity of the quasistatic CV measurement to a fast response of the interface states according to the inherent current integration of the method. On the other hand,
Fig. 3. Comparison of the distribution of Dit in the upper half of the gap for SiO2 (11008C, O2, 20 s, ti = 8.7 nm) and SiOxNy (11508C, N2O, 90 s, ti = 7.7 nm) obtained by CV and DLTS: SiO2, CV (1), SiOxNy, CV (2); SiO2, DLTS (3); SiOxNy, DLTS (4). The curves (5) belong to the ®t procedure explained in the text.
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the sensitivity of the DLTS is roughly restricted to emission processes with a time constant te which depends on the rate window en adjusted during the observation of the transient behaviour. Since we applied rate windows between 1 and 200 s ÿ 1, only processes with a time constant between 10 ÿ 2 and 1 s were caught remarkably well. Considering a sample to sample comparison of the interface state density distribution as well as of the the midgap levels of Dit, the results obtained by saturating pulse DLTS and CV measurements generally agree. In order to overcome the lack of the unknown capture cross section, additional SP-DLTS experiments were performed. The method, often designated as energy-resolved (ER) DLTS was introduced by Johnson [16] and Yamasaki et al. [17]. It allows a proof of the interface states either by temperature-scan under constant voltage conditions or by isothermal bias-scan, providing information about the triples (Dit, Et, sn) of interface states via Arrhenius evaluation of the SP-DLTS peaks. Despite of the advantages of the SP-DLTS method, it is only rarely used, obviously due to the time-consuming measurement and evaluation procedure, which requires, for instance, supplementary measurements of the temperature-dependent CV characteristic in order to determine the band bending at the maximum temperature of the SP-DLTS peaks. Moreover, surface potential ¯uctuations [18], the charge feedback eect [19] and the complicated kinetics of the electron capture process in the depletion mode have to be kept in mind when interpreting the data. Fig. 4 shows a series of small pulse DLTS peaks of a MIS structure with a SiO2 dielectric of 8.7 nm thickness. The quiescent voltage Vq was changed from ÿ0.90 V to ÿ0.50 V in steps of 50 mV for the dierent temperature scans, whereas the amplitude of the ®lling
Fig. 4. A series of small pulse DLTS signals related to the Si± SiO2 interface. The parameter is the quiescent voltage Vq, changed from ÿ0.5 to ÿ0.9 V in steps of 50 mV. The ®lling pulse amplitude was 100 mV and the rate window en = 23.1 s ÿ 1.
pulse voltage DVp was 100 mV for all cycles. The curve with Vq =ÿ 0.50 V corresponds to a smaller band bending at the semiconductor surface. Consequently, the Fermi level is closer to the conduction band edge, thus giving rise to a peak maximum at lower temperatures. Even in the case of small pulse DLTS the emission signal comes from a continuous distribution of states located within a small section DE where the traps are charged and discharged by applying and removing the ®lling pulse voltage DVp. Therefore, an eective trap energy Et,e exists, related to the eective emission time constant te,e for emptying the traps within DE. The energy Et,e, which describes the thermal activation energy of carrier transition from the traps in the vicinity of the Fermi level to the conduction band can be obtained alternatively by an Arrhenius evaluation of the sharp SP-DLTS peaks or by the simple relation [22] Ec ÿ Et;eff Cs Ec ÿ EF ÿ 0:5 DE
2
where Cs is the surface band bending and DE is the energy section near the Fermi level contributing to the capture and emission process. Thus, the peak at 98 K in Fig. 4 can be attributed to states located 0.09 eV below the conduction band edge, whereas the peak at approximately 240 K originates from interface states 0.51 eV below Ec. It is evident that the peak shape becomes broader when probing interface states close to midgap. Considering the band bending as a function of the gate voltage, as can be obtained from CV measurements and a solution of the Berglund integral, we can estimate the energetic resolution of the SPDLTS experiment. Obviously, the section DE for a given small pulse amplitude DVp is smaller for gate voltages close to the ¯atband condition and broader for gate voltages close to Vmidgap. Since a broad peak shape may give rise to an incorrect determination of Et,e and the capture cross section sn from an Arrhenius evaluation of the peak maxima, the enhancement of the resolution is a decisive task. Therefore, the amplitude for the small pulse excitation DVp was reduced from 100 mV down to 10 mV. As can be seen in Fig. 5, even for DVp = 10 mV an evaluable DLTS signal is obtained. This holds good for both the shallow states as well as for the deep states at the interface. However, the high sensitivity is dedicated to the rather high concentration of interface traps in our samples. If the interface state density is in the lower 1010 cm ÿ 2 eV ÿ 1 range, small ®lling pulses as low as 10 mV are not applicable. It is noticable that the DLTS peaks are not much in¯uenced by surface potential ¯uctuations, caused by the HF voltage, which had in our case an amplitude of 100 mV. Only the peak positions change slightly according to the
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above midgap [23]. The dispersion of sn is actually closely related to the method and the model used for the measurement and the evaluation. The data from SP-DLTS peaks obtained by a 100 mV pulse excitation are con®rmed by measurements with DVp = 50 mV.
4. Conclusions
Fig. 5. Isothermal bias-scan with small pulse excitation of a Si±SiO2 interface at 90 K and 250 K. The ®lling pulse amplitude DVp is varied from 100 to 10 mV.
shift of the eective emission time constant within the small section DE (DVp). Using the well known detailed balance equation EC ÿ Et en sn vth NC exp ÿ 3 kT with en being the emission rate, vth the thermal velocity of the carriers, Nc the eective density of states in the conduction band and Ec ÿ Et the thermal activation energy we calculated the electron capture cross section sn. The energetic distribution of sn is plotted in Fig. 6. A signi®cant decrease towards the band edge is evident for the SiO2 as well as for the SiOxNy interface. Within the error bars both curves coincide. The general shape of sn versus E for the semiconductor±insulator interface is in good agreement with a variety of other papers [19±21]. Some authors, however, reported on much higher values of the electron capture cross section with a maximum at approximately 0.2 eV
We used DLTS with saturating pulse excitation, small pulse DLTS and CV measurements in order to characterize the electrically active states at the semiconductor±insulator interface of MIS structures with thin oxide and oxynitride layers. The interface state density Dit provided by DLTS is lower compared with the data extracted from CV measurements. The analysis of the distribution of Dit obtained by CV reveals two peaks for the SiO2 and three peaks for the SiOxNy interface, DLTS provides a smooth continuous distribution within the upper half of the gap. Implementating the SP-DLTS, the dispersion of the capture cross section s(E) was determined, which exhibits a strong decrease towards the conduction band edge. However, in order to avoid an incorrect interpretation of the data obtained from small pulse DLTS experiments, further studies are required with an appropriate consideration of the real kinetics of the capture and emission of carriers at interface states, including the interaction with insulator traps. Moreover, a quantitative description of the generation process leading to the build-up of the inversion layer and consequently a distinction between trap emission and generation seems to be promising for the extension of the energetic range covered by the SP-DLTS method.
References
Fig. 6. Distribution of the capture cross section sn versus E of the Si±SiO2 and Si±SiOxNy interface states, obtained from small pulse DLTS measurements.
[1] Fukuda H, Endoh T, Nomura S. In: The Physics and Chemistry of SiO2 and the Si±SiO2 InterfaceÐ3. Pennington, NJ: The Electrochemical Society. 1996. p. 15±27. [2] Hegde RI, Maiti B, Tobin PJ. J Electrochem Society 1997;144:1081±6. [3] Fleetwood DM, Saks NS. J Appl Phys 1996;79:1583±94. [4] Fukuda H, Yasuda M, Iwabuchi T, Ohno S. IEEE Electron Device Lett 1991;12:587±9. [5] Hwang H, Ting W, Maiti B, Kwong DL, Lee J. Appl Phys Lett 1990;57:1010±1. [6] Joshi AB, Yoon G, Kim J, Lo GQ, Kwong DL. IEEE Trans Electron Devices 1993;40:1437±43. [7] Beyer R, Burghardt H, Reich R, Thomas E, Geûner T, Zahn DRT. Microelectron Reliab 1998;38:243±7. [8] Belkouch S, Jean C, Aktik C, Ameziane EL. Appl Phys Lett 1995;67:530±2.
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R. Beyer et al. / Microelectronics Reliability 39 (1999) 297±302
[9] Thurzo I, Gmucova K, Csabay O, Harmatha L, Burghardt H, Beyer R. Phys Stat Solid (a) 1995;151:345± 53. [10] Ikeda K, Takaoka H. Jap J Appl Phys 1982;21:462±6. [11] Weiss S, Kassing R. Solid State Electron 1988;31:1733±9. [12] Lang DV. J Appl Phys 1974;45:3014±3022;3023±3032. [13] Weiss S. Ph.D. Thesis. University of Kassel, 1991. [14] Autran JL, Seigneur F, Plossu C, Balland B. J. Appl. Phys. 1993;74:3932±5. [15] Bauza D, Morfouli P, Pananakakis G. Solid State Electron 1991;34:933±6. [16] Johnson NM. Appl Phys Lett 1979;34:802±4.
[17] Yamasaki K, Yoshida M, Sugano T. Jap J Appl Phys 1979;18:113±22. [18] OÈzder S, Atilgan I, Katircioglu B. Solid State Electron 1996;39:243±249. [19] Goguenheim D, Vuillaume D, Vincent G, Johnson NM. J Appl Phys 1990;68:1104±13. [20] Aberle AG, Glunz S, Warta W. J Appl Phys 1992;71:4422±31. [21] Tredwell TJ, Viswanathan CR. Solid State Electron 1980;23:1171±8. [22] Katsube T, Kakimoto K, Ikoma T. J Appl Phys 1981;52:3504±8. [23] Eades WD, Swanson RM. J Appl Phys 1984;56:1744±51.