- Email: [email protected]
Study of low-frequency excess noise in GaN materials
Optical Materials 23 (2003) 203–206 www.elsevier.com/locate/optmat
Study of low-frequency excess noise in GaN materials B.H. Leung, W.K. Fong, C. Surya
*
Department of Electronic and Information Engineering and Photonics Research Centre, The Hong Kong Polytechnic University, Yuk Choi Road, Hung Hom, Kowloon, Hong Kong, China
Abstract We report detailed investigations of low-frequency excess noise in GaN-based metal–semiconductor–metal devices fabricated on GaN thin films deposited by RF-plasma assisted molecular beam epitaxy on different types of buffer structures. Our experimental data indicate two orders of magnitude reduction in flicker noise for samples grown on double buffer layers that consist of a GaN intermediate temperature buffer layer on top of a thin AlN high temperature buffer layer. Experimental results on the temperature dependencies of the current noise power spectra stipulate that the noise arises from thermally activated trapping and detrapping of carriers. Based on the thermal activation model for 1=f noise, we computed the energy distribution of the traps responsible for the observed flicker noise. We also performed systematic studies on the hot-electron degradation of the devices through the application of a large voltage bias. The data demonstrate substantial improvement in the hot-electron hardness for devices fabricated on the double buffer layer structures. Ó 2003 Elsevier Science B.V. All rights reserved.
1. Introduction Gallium nitride and its alloys are the materials of choice for many optoelectronic and electronic applications. While a number of high-quality blue– green light emitters, UV detectors as well as microwave transistors have been commercialized, there is still much room for material improvement in order to realize the full potential of GaN-based devices. It is well documented that the lack of native substrate is the cause for the highly defective GaN materials [1,2]. An important breakthrough in the GaN technology is the development of AlN or GaN low-temperature buffer layer
*
Corresponding author. Tel.: +852-2766-6220; fax: +8522362-8439. E-mail address: [email protected] (C. Surya).
(LTBL) grown at 500 °C of thickness about 20 nm. This has led to substantial improvements in the film quality for metal organic chemical vapor deposition-grown films. Extensive investigations on the role of the LTBL have shown that the technique enhances two-dimensional growth and the density of nucleation for the epitaxial films. This is attributed to the reduction in the interfacial energy for the AlN or GaN/buffer system compared to the GaN/sapphire system. The beneficial effects of LTBL is less pronounced in molecular beam epitaxy (MBE)-grown materials. Recent work by the authors have shown that the use of an intermediate temperature buffer layer (ITBL) resulted in substantial improvements in both the optical and electronic properties GaN epitaxial thin films [3,4]. It is important to gain further insights on the effects of the ITBL on the noise properties of GaN-based optoelectronic devices.
0925-3467/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0925-3467(03)00084-3
B.H. Leung et al. / Optical Materials 23 (2003) 203–206
This is because low-frequency noise is an important figure-of-merit for optoelectronic devices such as sensors and detectors, since the noise most crucial factor that determines the minimum detectable signal of the receiver. Furthermore, lowfrequency noise characterization has shown to be a powerful technique for the study of defect properties in semiconductor devices [5]. In the following sections we will present detailed experimental investigations of low-frequency excess noise in GaN-based metal-semiconductor-metal (MSM) UV detectors. The GaN thin films were deposited by RF-plasma assisted MBE. Two different types of buffer layers were used in our studies. Type I buffer layer consists of a 20 nm-thick AlN high temperature buffer layer (HTBL). Type II buffer layer consists of an additional 800 nm-thick GaN ITBL grown on top of a 20 nm-thick AlN HTBL. We will also report studies on the degradation of the flicker noise properties due to electrical stress. This is an important study since the devices typically operate under high voltage biases, it is important to examine how the different buffer layer structures may affect the degradation process.
2. Experiment Gallium nitride thin films were deposited on sapphire (0 0 0 1) substrates. The substrate was first cleaned using a standard cleaning procedure. It was then outgassed at 850 °C. Nitridation of the sapphire wafer was then carried out inside the growth chamber at 500 °C. The GaN epitaxial layers, of thickness 1.8 lm, were grown on different buffer layers at 740 °C. For sample A, the epitaxial layer was grown on top of a type I buffer layer. For sample B, a type II buffer layer was used. Interdigitated structures of dimension 2 60 lm2 were fabricated by e-beam deposition of Ni on GaN. Prior to the deposition of the metallic fingers, a thin layer of SiOx passivation layer was deposited on the surface of the sample. The devices were placed inside a continuous-flow cryostat and the device temperature was controlled by a Lakeshore 91C temperature controller with a silicon
diode sensor. The devices were voltage biased using a passive voltage source. The fluctuating current was converted to a voltage signal using an Analog Device AD549 current-to-voltage converter and was subsequently amplified using a PAR 113 low-noise preamplifier. The amplified noise was then coupled to an HP3561A dynamic signal analyzer for the measurement of voltage noise power spectra [6,7].
3. Experimental results and discussion The experimental current noise power spectra, SI ðf Þ, for devices A and B are shown in Fig. 1. The data indicate close to two orders of magnitude reduction in the low-frequency excess noise for devices fabricated on type II buffer layers. It is noteworthy that these are highly repeatable results. Our data show that the current noise power spectra measured from five different devices deviate by less than 3 dB from each other. To investigate hot-electron degradation phenomenon of the devices. The devices were subjected to electrical stress through the application of a 7 V constant stressing voltage at room temperature for a period up to 45 min. Subsequent to the electrical stressing process the low-frequency excess noise from the devices were measured again
Log10 SI ( f ) (A2/Hz)
204
-15.5 -16.0 -16.5 -17.0 -17.5 -18.0 -18.5 -19.0 -19.5 -20.0 -20.5 -21.0 -21.5 1.5
2.0
2.5
3.0
3.5
Log10 f (Hz) Fig. 1. Room temperature current noise power spectra, SI ðf Þ, for device A before stressing ðjÞ, device B before stressing ðNÞ, device A after stressing ðdÞ, and device B after stressing ð.Þ.
B.H. Leung et al. / Optical Materials 23 (2003) 203–206
and the results are shown in Fig. 1. Comparing the data before and after the electrical stressing process we observe an increase in the current noise power spectra for both devices. For device A, there is a roughly 30-fold increase in SI ðf Þ, whereas a 3-fold increase is observed for device B. The experimental results clearly demonstrate significant improvement in the hot-electron hardness for devices fabricated with the use of ITBLs. To determine the physical mechanism underlying the fluctuation process, we have conducted detailed characterization of the low-frequency noise properties over a wide range of temperatures. Our experimental data demonstrate that the current noise power spectra are proportional to 1=f c where 0:8 6 c 6 1:3. Detailed analyses of the low-frequency noise for both devices A and B clearly demonstrate systematic variations of c as a function of the device temperature as shown in Fig. 2(a) and (b) respectively. The error involved in the measurement of c is about 0.02. This indicates that the observed low-frequency noise originates from the thermally activated trapping and detrapping process. Based on this model the noise power spectral density of the trapping and detrapping process is given by [8,9] Z Z Z Z Sðf Þ ¼ NT ðEÞ s dx dy dz dE; 1 þ 4p2 f 2 s2
ð1Þ
205
in which s is the thermally activated fluctuation time constant. From Eq. (1) above one observes that the Lorentzian is a sharply peaked function of energy at Ep ¼ kT lnð2pf s0 Þ [10], where s0 is usually taken as the inverse phonon frequency. This stipulates that the observed low-frequency noise is dominated by the capture and emission activities by the traps located at energy Ep . The frequency exponent, c, of the noise power spectral density is dependent on the energy distribution of the localized states, NT ðEÞ [11–13]. From Fig. 3 we observe significant changes in the values of c as a function of temperature for both devices A and B after subjecting to the electrical stressing experiment. This indicates that hot-electron stressing results in changes in the distribution of the trap density responsible for the noise. According to the thermal activation model it has been shown that the trap density can be expressed in terms of current noise power spectral density as given below [14]: NT ðEp Þ /
f SI ðf Þ : kTI 2
ð2Þ
It has been suggested that flicker noise in Schottky barriers arises from the modulation of the tunneling barriers due to the carrier capture and emission by the localize states at the depletion region. However, based on our experimental results alone it is not sufficient to confirm this. Nevertheless, 1.4
1.3
γ
γ
1.3 1.2
1.2
100
γ
300
1.1 (a) 100
300
1.3 1.2 1.1 1.0 0.9 (b) 0.8 100
(a)
1.2 1.1 1.0 0.9 (b) 0.8 100
150
200 250 Temperature (K)
150 200 250 Temperature (K)
300
150
300
γ
1.1
150
200 250 Temperature (K)
Fig. 2. Variations of g as a function of the device temperature for device A ðjÞ and device B ðNÞ before stressing.
200 250 Temperature (K)
Fig. 3. Variations of g as a function of the device temperature for device A ðdÞ and device B ð.Þ after stressing.
206
B.H. Leung et al. / Optical Materials 23 (2003) 203–206
frequency noise properties over a wide range of temperatures. Our experimental data show that the noise arises from the thermal activated capture and emission of carriers by localized states. Detailed analyses of the data yielded the relative distribution of the trap densities for the devices. The experimental results show that devices fabricated on type I buffer layers demonstrate substantially larger increase in the trap density due to electrical stressing.
Acknowledgements Fig. 4. Relative trap density distribution for devices A ðdÞ and B ðjÞ before stressing (a) and after stressing (b).
using Eq. (2) one can determine the relative distributions of the localized states responsible for the observed low-frequency noise. Fig. 4(a) and (b) show the trap density distribution, for the devices before and after the electrical stress respectively. The results clearly demonstrate the significant changes in the trap distribution due to the electrical stressing experiment [15]. The data clearly demonstrates improved hot-electron hardness for devices fabricated utilizing ITBLs.
4. Conclusion We have investigated low-frequency noise properties for GaN-based MSM devices fabricated on two different buffer layers. Type I buffer layer consists of a single 20 nm-thick AlN HTBL, whereas type II buffer layer consists of a 20 nmthick AlN HTBL and an 800 nm-thick GaN ITBL. Our experimental results indicate that devices fabricated on type II buffer layers exhibit over two order of magnitude reduction in the flicker noise level. Furthermore, when subjecting the devices to electrical stress, it is found that devices fabricated on type II buffer layers demonstrate much improved hot-electron hardness. We have conducted systematic investigation of the low-
The work described in this paper was partially supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project no. CRC4/98). Additional support was provided by a University Research Grant from The Hong Kong Polytechnic University.
References [1] J.C. Carrano, T. Li, P.A. Grudowski, C.J. Eiting, R.D. Dupuis, J.C. Campbell, J. Appl. Phys. 83 (11) (2000) 6148. [2] S.C. Jain, M. Willander, J. Narayan, R. Van Overstraeten, J. Appl. Phys. 87 (3) (2000) 965. [3] W.K. Fong, C.F. Zhu, B.H. Leung, C. Surya, MRS Int. J. Nitride Semicond. Res. 5, article 12, 2000. [4] C.F. Zhu, W.K. Fong, B.H. Leung, C. Surya, Appl. Phys. A 72 (4) (2001) 495. [5] B.H. Leung, W.K. Fong, C.F. Zhu, C. Surya, IEEE Trans. Electron Dev. 48 (2001) 2400. [6] L.K.J. Vandamme, IEEE Trans. Electron Dev. 41 (11) (1994) 2176. [7] C. Claeys, E. Simoen, J. Electrochem. Soc. 145 (1998) 2058. [8] F.N. Hooge, T.G.M. Kleinpenning, L.K.J. Vandamme, Rep. Prog. Phys. 44 (1981) 479. [9] C.M. Van Vliet, IEEE Trans. Electron Dev. 41 (11) (1994) 1902. [10] A.L. McWhorter, in: R.H. Kinston (Ed.), Semiconductor Surface Physics, University of Pennsylvania Press, Philadelphia, 1956, p. 207. [11] S.H. Ng, C. Surya, E.R. Brown, P.A. Maki, Appl. Phys. Lett. 62 (18) (1993) 2262. [12] P. Dutta, P.M. Horn, Rev. Mod. Phys. 53 (1981) 497. [13] J.W. Eberhard, P.M. Horn, Phys. Rev. B 18 (1978) 6681. [14] P. Dutta, P. Dimon, P.M. Horn, Phys. Rev. Lett. 43 (1979) 646. [15] C. Surya, T.Y. Hsiang, Solid-State Electron. 31 (5) (1988) 959.
Study of low-frequency excess noise in GaN materials B.H. Leung, W.K. Fong, C. Surya
*
Department of Electronic and Information Engineering and Photonics Research Centre, The Hong Kong Polytechnic University, Yuk Choi Road, Hung Hom, Kowloon, Hong Kong, China
Abstract We report detailed investigations of low-frequency excess noise in GaN-based metal–semiconductor–metal devices fabricated on GaN thin films deposited by RF-plasma assisted molecular beam epitaxy on different types of buffer structures. Our experimental data indicate two orders of magnitude reduction in flicker noise for samples grown on double buffer layers that consist of a GaN intermediate temperature buffer layer on top of a thin AlN high temperature buffer layer. Experimental results on the temperature dependencies of the current noise power spectra stipulate that the noise arises from thermally activated trapping and detrapping of carriers. Based on the thermal activation model for 1=f noise, we computed the energy distribution of the traps responsible for the observed flicker noise. We also performed systematic studies on the hot-electron degradation of the devices through the application of a large voltage bias. The data demonstrate substantial improvement in the hot-electron hardness for devices fabricated on the double buffer layer structures. Ó 2003 Elsevier Science B.V. All rights reserved.
1. Introduction Gallium nitride and its alloys are the materials of choice for many optoelectronic and electronic applications. While a number of high-quality blue– green light emitters, UV detectors as well as microwave transistors have been commercialized, there is still much room for material improvement in order to realize the full potential of GaN-based devices. It is well documented that the lack of native substrate is the cause for the highly defective GaN materials [1,2]. An important breakthrough in the GaN technology is the development of AlN or GaN low-temperature buffer layer
*
Corresponding author. Tel.: +852-2766-6220; fax: +8522362-8439. E-mail address: [email protected] (C. Surya).
(LTBL) grown at 500 °C of thickness about 20 nm. This has led to substantial improvements in the film quality for metal organic chemical vapor deposition-grown films. Extensive investigations on the role of the LTBL have shown that the technique enhances two-dimensional growth and the density of nucleation for the epitaxial films. This is attributed to the reduction in the interfacial energy for the AlN or GaN/buffer system compared to the GaN/sapphire system. The beneficial effects of LTBL is less pronounced in molecular beam epitaxy (MBE)-grown materials. Recent work by the authors have shown that the use of an intermediate temperature buffer layer (ITBL) resulted in substantial improvements in both the optical and electronic properties GaN epitaxial thin films [3,4]. It is important to gain further insights on the effects of the ITBL on the noise properties of GaN-based optoelectronic devices.
0925-3467/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0925-3467(03)00084-3
B.H. Leung et al. / Optical Materials 23 (2003) 203–206
This is because low-frequency noise is an important figure-of-merit for optoelectronic devices such as sensors and detectors, since the noise most crucial factor that determines the minimum detectable signal of the receiver. Furthermore, lowfrequency noise characterization has shown to be a powerful technique for the study of defect properties in semiconductor devices [5]. In the following sections we will present detailed experimental investigations of low-frequency excess noise in GaN-based metal-semiconductor-metal (MSM) UV detectors. The GaN thin films were deposited by RF-plasma assisted MBE. Two different types of buffer layers were used in our studies. Type I buffer layer consists of a 20 nm-thick AlN high temperature buffer layer (HTBL). Type II buffer layer consists of an additional 800 nm-thick GaN ITBL grown on top of a 20 nm-thick AlN HTBL. We will also report studies on the degradation of the flicker noise properties due to electrical stress. This is an important study since the devices typically operate under high voltage biases, it is important to examine how the different buffer layer structures may affect the degradation process.
2. Experiment Gallium nitride thin films were deposited on sapphire (0 0 0 1) substrates. The substrate was first cleaned using a standard cleaning procedure. It was then outgassed at 850 °C. Nitridation of the sapphire wafer was then carried out inside the growth chamber at 500 °C. The GaN epitaxial layers, of thickness 1.8 lm, were grown on different buffer layers at 740 °C. For sample A, the epitaxial layer was grown on top of a type I buffer layer. For sample B, a type II buffer layer was used. Interdigitated structures of dimension 2 60 lm2 were fabricated by e-beam deposition of Ni on GaN. Prior to the deposition of the metallic fingers, a thin layer of SiOx passivation layer was deposited on the surface of the sample. The devices were placed inside a continuous-flow cryostat and the device temperature was controlled by a Lakeshore 91C temperature controller with a silicon
diode sensor. The devices were voltage biased using a passive voltage source. The fluctuating current was converted to a voltage signal using an Analog Device AD549 current-to-voltage converter and was subsequently amplified using a PAR 113 low-noise preamplifier. The amplified noise was then coupled to an HP3561A dynamic signal analyzer for the measurement of voltage noise power spectra [6,7].
3. Experimental results and discussion The experimental current noise power spectra, SI ðf Þ, for devices A and B are shown in Fig. 1. The data indicate close to two orders of magnitude reduction in the low-frequency excess noise for devices fabricated on type II buffer layers. It is noteworthy that these are highly repeatable results. Our data show that the current noise power spectra measured from five different devices deviate by less than 3 dB from each other. To investigate hot-electron degradation phenomenon of the devices. The devices were subjected to electrical stress through the application of a 7 V constant stressing voltage at room temperature for a period up to 45 min. Subsequent to the electrical stressing process the low-frequency excess noise from the devices were measured again
Log10 SI ( f ) (A2/Hz)
204
-15.5 -16.0 -16.5 -17.0 -17.5 -18.0 -18.5 -19.0 -19.5 -20.0 -20.5 -21.0 -21.5 1.5
2.0
2.5
3.0
3.5
Log10 f (Hz) Fig. 1. Room temperature current noise power spectra, SI ðf Þ, for device A before stressing ðjÞ, device B before stressing ðNÞ, device A after stressing ðdÞ, and device B after stressing ð.Þ.
B.H. Leung et al. / Optical Materials 23 (2003) 203–206
and the results are shown in Fig. 1. Comparing the data before and after the electrical stressing process we observe an increase in the current noise power spectra for both devices. For device A, there is a roughly 30-fold increase in SI ðf Þ, whereas a 3-fold increase is observed for device B. The experimental results clearly demonstrate significant improvement in the hot-electron hardness for devices fabricated with the use of ITBLs. To determine the physical mechanism underlying the fluctuation process, we have conducted detailed characterization of the low-frequency noise properties over a wide range of temperatures. Our experimental data demonstrate that the current noise power spectra are proportional to 1=f c where 0:8 6 c 6 1:3. Detailed analyses of the low-frequency noise for both devices A and B clearly demonstrate systematic variations of c as a function of the device temperature as shown in Fig. 2(a) and (b) respectively. The error involved in the measurement of c is about 0.02. This indicates that the observed low-frequency noise originates from the thermally activated trapping and detrapping process. Based on this model the noise power spectral density of the trapping and detrapping process is given by [8,9] Z Z Z Z Sðf Þ ¼ NT ðEÞ s dx dy dz dE; 1 þ 4p2 f 2 s2
ð1Þ
205
in which s is the thermally activated fluctuation time constant. From Eq. (1) above one observes that the Lorentzian is a sharply peaked function of energy at Ep ¼ kT lnð2pf s0 Þ [10], where s0 is usually taken as the inverse phonon frequency. This stipulates that the observed low-frequency noise is dominated by the capture and emission activities by the traps located at energy Ep . The frequency exponent, c, of the noise power spectral density is dependent on the energy distribution of the localized states, NT ðEÞ [11–13]. From Fig. 3 we observe significant changes in the values of c as a function of temperature for both devices A and B after subjecting to the electrical stressing experiment. This indicates that hot-electron stressing results in changes in the distribution of the trap density responsible for the noise. According to the thermal activation model it has been shown that the trap density can be expressed in terms of current noise power spectral density as given below [14]: NT ðEp Þ /
f SI ðf Þ : kTI 2
ð2Þ
It has been suggested that flicker noise in Schottky barriers arises from the modulation of the tunneling barriers due to the carrier capture and emission by the localize states at the depletion region. However, based on our experimental results alone it is not sufficient to confirm this. Nevertheless, 1.4
1.3
γ
γ
1.3 1.2
1.2
100
γ
300
1.1 (a) 100
300
1.3 1.2 1.1 1.0 0.9 (b) 0.8 100
(a)
1.2 1.1 1.0 0.9 (b) 0.8 100
150
200 250 Temperature (K)
150 200 250 Temperature (K)
300
150
300
γ
1.1
150
200 250 Temperature (K)
Fig. 2. Variations of g as a function of the device temperature for device A ðjÞ and device B ðNÞ before stressing.
200 250 Temperature (K)
Fig. 3. Variations of g as a function of the device temperature for device A ðdÞ and device B ð.Þ after stressing.
206
B.H. Leung et al. / Optical Materials 23 (2003) 203–206
frequency noise properties over a wide range of temperatures. Our experimental data show that the noise arises from the thermal activated capture and emission of carriers by localized states. Detailed analyses of the data yielded the relative distribution of the trap densities for the devices. The experimental results show that devices fabricated on type I buffer layers demonstrate substantially larger increase in the trap density due to electrical stressing.
Acknowledgements Fig. 4. Relative trap density distribution for devices A ðdÞ and B ðjÞ before stressing (a) and after stressing (b).
using Eq. (2) one can determine the relative distributions of the localized states responsible for the observed low-frequency noise. Fig. 4(a) and (b) show the trap density distribution, for the devices before and after the electrical stress respectively. The results clearly demonstrate the significant changes in the trap distribution due to the electrical stressing experiment [15]. The data clearly demonstrates improved hot-electron hardness for devices fabricated utilizing ITBLs.
4. Conclusion We have investigated low-frequency noise properties for GaN-based MSM devices fabricated on two different buffer layers. Type I buffer layer consists of a single 20 nm-thick AlN HTBL, whereas type II buffer layer consists of a 20 nmthick AlN HTBL and an 800 nm-thick GaN ITBL. Our experimental results indicate that devices fabricated on type II buffer layers exhibit over two order of magnitude reduction in the flicker noise level. Furthermore, when subjecting the devices to electrical stress, it is found that devices fabricated on type II buffer layers demonstrate much improved hot-electron hardness. We have conducted systematic investigation of the low-
The work described in this paper was partially supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project no. CRC4/98). Additional support was provided by a University Research Grant from The Hong Kong Polytechnic University.
References [1] J.C. Carrano, T. Li, P.A. Grudowski, C.J. Eiting, R.D. Dupuis, J.C. Campbell, J. Appl. Phys. 83 (11) (2000) 6148. [2] S.C. Jain, M. Willander, J. Narayan, R. Van Overstraeten, J. Appl. Phys. 87 (3) (2000) 965. [3] W.K. Fong, C.F. Zhu, B.H. Leung, C. Surya, MRS Int. J. Nitride Semicond. Res. 5, article 12, 2000. [4] C.F. Zhu, W.K. Fong, B.H. Leung, C. Surya, Appl. Phys. A 72 (4) (2001) 495. [5] B.H. Leung, W.K. Fong, C.F. Zhu, C. Surya, IEEE Trans. Electron Dev. 48 (2001) 2400. [6] L.K.J. Vandamme, IEEE Trans. Electron Dev. 41 (11) (1994) 2176. [7] C. Claeys, E. Simoen, J. Electrochem. Soc. 145 (1998) 2058. [8] F.N. Hooge, T.G.M. Kleinpenning, L.K.J. Vandamme, Rep. Prog. Phys. 44 (1981) 479. [9] C.M. Van Vliet, IEEE Trans. Electron Dev. 41 (11) (1994) 1902. [10] A.L. McWhorter, in: R.H. Kinston (Ed.), Semiconductor Surface Physics, University of Pennsylvania Press, Philadelphia, 1956, p. 207. [11] S.H. Ng, C. Surya, E.R. Brown, P.A. Maki, Appl. Phys. Lett. 62 (18) (1993) 2262. [12] P. Dutta, P.M. Horn, Rev. Mod. Phys. 53 (1981) 497. [13] J.W. Eberhard, P.M. Horn, Phys. Rev. B 18 (1978) 6681. [14] P. Dutta, P. Dimon, P.M. Horn, Phys. Rev. Lett. 43 (1979) 646. [15] C. Surya, T.Y. Hsiang, Solid-State Electron. 31 (5) (1988) 959.