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The variation of spectral response of transmission-type GaAs photocathode in the seal process
Applied Surface Science 251 (2005) 273–277 www.elsevier.com/locate/apsusc
The variation of spectral response of transmission-type GaAs photocathode in the seal process Liu Lei *, Chang BenKang, Du YuJie, Qian YunSheng, Gao Pin School of Electronic Engineering and Optoelectronic Technology, Nanjing University of Science and Technology, Nanjing, Jiangsu 210094 China Available online 27 July 2005
Abstract In this paper, firstly the spectral response of transmission-type GaAs photocathode is measured online by the spectral response-testing instrument. Then the cathode is sealed in the third generation intensifier and put into the instrument again to get another spectral response curve. The variation of spectral response curves was compared. The results show that through the seal process, the spectral response in the long wavelength decrease. Based on these curves, the spectral matching factors of GaAs photocathode for green vegetation and rough concrete are calculated. The calculated performance parameters show that the variation of the spectral response in the seal process is an important influence factor on the performance of the intensifier in the use of night vision. # 2005 Elsevier B.V. All rights reserved. Keywords: GaAs photocathode; Spectral response; Spectral matching factor; Visual range
1. Introduction Transmission-type GaAs photocathodes are emerging from the laboratory as practical photosensing devices with vastly improved sensitivity and spectral range compared with other types of photocathode. They are widely used in many fields such as semiconductor devices, optical radiation measurement, camera devices and low-light-level night vision. Measurement of parameters of photocathodes and especially their * Corresponding author. Tel.: +86 25 4315437; fax: +86 25 4315177. E-mail address: [email protected] (L. Lei).
spectral response, is of great importance in understanding the photocathode’s performance. During fabrication of these photocathodes, the technique of online spectral response measurement enables us to determine spectral responses quickly and accurately. Through analyzing and comparing those measured response curves, a great deal of information about photocathodes can be derived which is useful in both the research and the fabrication of photocathodes, for example, the sensitivity, the photoelectron surface escape probability, the thickness of the active layer, the diffusion length of electrons and the back interface recombination velocity of electrons can all be derived from the response curves [1,2].
0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.03.219
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L. Lei et al. / Applied Surface Science 251 (2005) 273–277
Fig. 1. The block diagram of spectral response measurement.
The apparatus used for the spectral response measurements has already been described in a previous publication [3]. As shown in Fig. 1, the system consists of light source, power source, detecting circuits, data acquisition module, control module and the software, etc. The halogen tungsten lamp whose power supply is dc supply of 12 V/120 W is used as the light source. The light from the source is focused and modulated. Through the grating monochromator controlled by computer, the white light is changed into monochromatic light that is transmitted to irradiate the photocathode surface through an optical fiber and quartz crystal. The wavelength of monochromatic light is within range 400–1800 nm. The photocathode emission induced by the monochromatic light is amplified and converted into digital
signal by the A/D converter. The computer acquires and processes the signal and the spectral response is recorded. The scanner is also used to drive the grating monochromator. From the spectral response, the integral sensitivity can be calculated. In this paper, we have studied a transmittedtype GaAs photocathode with a thickness of 1.5 mm and diameter of 18 mm. The sample was sealed in the third generation intensifier and put into the spectral response-testing instrument to obtain the response curve. In order to guarantee the precision, the experiment was repeated and the two curves are illustrated in Fig. 2. We can see that the two curves are coincident. Data derived from the spectral response property parameters and sensitivity of curves are summarised in Table 1. By these spectral response curves, quantum yield curves for the GaAs photocathode are shown in Figs. 3 and 4. Through a combination of calculation and
Fig. 2. The measured spectral response curves.
Fig. 3. Simulation results of curve 1.
2. Measurements on the photocathode sealed in a GenIII intensifier
L. Lei et al. / Applied Surface Science 251 (2005) 273–277
275
Table 1 Spectral response performance of GenIII imaging intensifier Type
Curve
Start wavelength (mm)
Cut-off wavelength (mm)
Peak response (mA/W)
Peak wavelength (mm)
Integrated sensitivity (mA/lm)
GenIII Intensifier
1 2
0.405 0.400
0.955 0.955
97.8 99.0
0.770 0.770
800 809
3. Online measurements
Fig. 4. Simulation results of curve 2.
simulation on these quantum yield curves, several performance parameters can be derived, such as surface escape probability, electron diffusion length and interface recombination velocity, which are useful in both the research and the fabrication of GaAs photocathodes. In Figs. 3 and 4, the curve marked with 1 is the experiment one, the other is the theoretical one.
Online experiments to measure the spectral response during the activation of the GaAs photocathode have also been undertaken. The technique of online spectral response measurement has been studied by a number of workers. The conventional procedure for determining spectral response is a pointby-point measurement. This method is both troublesome and time-consuming. With respect to the conventional method, automatic recording of the spectral response is not only convenient; its main advantage is in reducing the measurment time. This is important, because when a photocathode is operating, the time available for measurement is limited, especially in the case of online measurement. In addition, the resolution of response curves recorded using an automated system is greatly improved. By reason of these advantages, from the 1960s to the 1990s, researchers in America, the Former Soviet Union, Switzerland, India and China each developed automated spectral response recording systems. Now our laboratory has also developed an automated spectral response recording system as shown in Fig. 5. The system is different from the works of Sydney et al. When the online experiment is performed, the cathode
Fig. 5. Sketch of interface of on-line spectral response measurement.
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L. Lei et al. / Applied Surface Science 251 (2005) 273–277
Fig. 6. The spectral response and quantum field curves of transmission-type NEA photocathode.
is put in the activation chamber for processing. Firstly, the radiation light is turned into monochromatic light by the modulator and the grating monochromator. Then it is transmitted and attenuated by the optical fibre so as to incident on the flange windows of the activation chamber. The light is then reflected by the copper reflector and irradiates on the surface of the photocathode. The output signal is detected by the microsignal detection apparatus and after convertion
by analog-to-digital converter, is fed into the computer, which completes the data processing. Figs. 6 and 7 illustrates the online spectral response curves obtained from the experiment. The performance parameters obtained by simulation are summarised in Table 2. Table 2 Spectral response performance of GaAs photocathode in online measurement Spectral response performance Start wavelength (nm) Cut-off wavelength (nm) Peak response (mA/W) Peak wavelength (nm)
530 950 108.8 840
Sensitivity (mA/lm)
717.9
Evaluation of parameters Diffusion length (mm) Interface recombine (cm/s) Escape probability (%)
2.1 1 107 0.55
Table 3 Spectral matching factors of NEA photocathodes for scenes Object
Fig. 7. The comparison of two relative spectral response curves.
Night-sky radiation
Spectral matching factors a Sealed in the tube
Online
Green vegetation
Full moon Clear star
0.562905 0.584189711
0.672091 0.589365179
Rough concrete
Full moon Clear star
0.381982 0.49723
0.559963 0.589356
L. Lei et al. / Applied Surface Science 251 (2005) 273–277 Table 4 The detecting range of the LLL night vision goggle for drive P(l)
Clear star Transparenta Man
RL online (m) RL sealed in the tube (m) a b
78 78
b
Grassa b
Car
Manb
Carb
202 199
63 60
168 163
Background. Object.
4. Conclusions Comparison of online measurement curves with the spectral response curve of a GaAs photocathode (sealed in the third generation imaging intensifier) indicates that the process of sealing of the imaging intensifier has led to a decrease in the long wavelength response of the photocathode. Based on these curves, the spectral matching factors of GaAs photocathode for green vegetation and rough concrete have been calculated [4,5] as shown in Tables 3 and 4. The calculated performance parameters show that the variation of the spectral response in the sealing process
277
has influenced the spectral matching factor of the photocathode object combination and detecting distance [6] of the imaging intensifier and has resulted in the practical use of low-light-level night vision instruments.
References [1] Z. Zhiyuan, C. Benkang, A study on computer simulation of S25 series photocathodes spectral response, J. Nanjing Univ. Sci. Technol. 22 (3) (1998) 228–231. [2] J.P. Andre´ , P. Giittard, J. Hallais, C. Piaget, GaAs photocathodes for low light level imaging, J. Cryst. Growth 55 (1981) 235–245. [3] C. Benkang, F. Hongbing, H. Yuanzhen, Investigation application of the automatic survey instrument of dynamic spectral response of the photoelectric material, Vac. Sci. Technol. 16 (5) (1996) 364–366. [4] L. Wei, C. Benkang, Spectral matching factors between GaAs and multialkali photocathodes and reflective radiation of objects, Opt. Eng. 40 (5) (2001) 674–678. [5] L. Lei, C. Benkang, Spectral response characterization of super S25 and new S25 photocathodes, Appl. Opt. 43 (3) (2004) 616– 619. [6] L. Lei, C. Benkang, L. Wei, Visual range of low-light-level night vision goggles for driving, Opt. Eng. 42 (7) (2003) 2056–2061.
The variation of spectral response of transmission-type GaAs photocathode in the seal process Liu Lei *, Chang BenKang, Du YuJie, Qian YunSheng, Gao Pin School of Electronic Engineering and Optoelectronic Technology, Nanjing University of Science and Technology, Nanjing, Jiangsu 210094 China Available online 27 July 2005
Abstract In this paper, firstly the spectral response of transmission-type GaAs photocathode is measured online by the spectral response-testing instrument. Then the cathode is sealed in the third generation intensifier and put into the instrument again to get another spectral response curve. The variation of spectral response curves was compared. The results show that through the seal process, the spectral response in the long wavelength decrease. Based on these curves, the spectral matching factors of GaAs photocathode for green vegetation and rough concrete are calculated. The calculated performance parameters show that the variation of the spectral response in the seal process is an important influence factor on the performance of the intensifier in the use of night vision. # 2005 Elsevier B.V. All rights reserved. Keywords: GaAs photocathode; Spectral response; Spectral matching factor; Visual range
1. Introduction Transmission-type GaAs photocathodes are emerging from the laboratory as practical photosensing devices with vastly improved sensitivity and spectral range compared with other types of photocathode. They are widely used in many fields such as semiconductor devices, optical radiation measurement, camera devices and low-light-level night vision. Measurement of parameters of photocathodes and especially their * Corresponding author. Tel.: +86 25 4315437; fax: +86 25 4315177. E-mail address: [email protected] (L. Lei).
spectral response, is of great importance in understanding the photocathode’s performance. During fabrication of these photocathodes, the technique of online spectral response measurement enables us to determine spectral responses quickly and accurately. Through analyzing and comparing those measured response curves, a great deal of information about photocathodes can be derived which is useful in both the research and the fabrication of photocathodes, for example, the sensitivity, the photoelectron surface escape probability, the thickness of the active layer, the diffusion length of electrons and the back interface recombination velocity of electrons can all be derived from the response curves [1,2].
0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.03.219
274
L. Lei et al. / Applied Surface Science 251 (2005) 273–277
Fig. 1. The block diagram of spectral response measurement.
The apparatus used for the spectral response measurements has already been described in a previous publication [3]. As shown in Fig. 1, the system consists of light source, power source, detecting circuits, data acquisition module, control module and the software, etc. The halogen tungsten lamp whose power supply is dc supply of 12 V/120 W is used as the light source. The light from the source is focused and modulated. Through the grating monochromator controlled by computer, the white light is changed into monochromatic light that is transmitted to irradiate the photocathode surface through an optical fiber and quartz crystal. The wavelength of monochromatic light is within range 400–1800 nm. The photocathode emission induced by the monochromatic light is amplified and converted into digital
signal by the A/D converter. The computer acquires and processes the signal and the spectral response is recorded. The scanner is also used to drive the grating monochromator. From the spectral response, the integral sensitivity can be calculated. In this paper, we have studied a transmittedtype GaAs photocathode with a thickness of 1.5 mm and diameter of 18 mm. The sample was sealed in the third generation intensifier and put into the spectral response-testing instrument to obtain the response curve. In order to guarantee the precision, the experiment was repeated and the two curves are illustrated in Fig. 2. We can see that the two curves are coincident. Data derived from the spectral response property parameters and sensitivity of curves are summarised in Table 1. By these spectral response curves, quantum yield curves for the GaAs photocathode are shown in Figs. 3 and 4. Through a combination of calculation and
Fig. 2. The measured spectral response curves.
Fig. 3. Simulation results of curve 1.
2. Measurements on the photocathode sealed in a GenIII intensifier
L. Lei et al. / Applied Surface Science 251 (2005) 273–277
275
Table 1 Spectral response performance of GenIII imaging intensifier Type
Curve
Start wavelength (mm)
Cut-off wavelength (mm)
Peak response (mA/W)
Peak wavelength (mm)
Integrated sensitivity (mA/lm)
GenIII Intensifier
1 2
0.405 0.400
0.955 0.955
97.8 99.0
0.770 0.770
800 809
3. Online measurements
Fig. 4. Simulation results of curve 2.
simulation on these quantum yield curves, several performance parameters can be derived, such as surface escape probability, electron diffusion length and interface recombination velocity, which are useful in both the research and the fabrication of GaAs photocathodes. In Figs. 3 and 4, the curve marked with 1 is the experiment one, the other is the theoretical one.
Online experiments to measure the spectral response during the activation of the GaAs photocathode have also been undertaken. The technique of online spectral response measurement has been studied by a number of workers. The conventional procedure for determining spectral response is a pointby-point measurement. This method is both troublesome and time-consuming. With respect to the conventional method, automatic recording of the spectral response is not only convenient; its main advantage is in reducing the measurment time. This is important, because when a photocathode is operating, the time available for measurement is limited, especially in the case of online measurement. In addition, the resolution of response curves recorded using an automated system is greatly improved. By reason of these advantages, from the 1960s to the 1990s, researchers in America, the Former Soviet Union, Switzerland, India and China each developed automated spectral response recording systems. Now our laboratory has also developed an automated spectral response recording system as shown in Fig. 5. The system is different from the works of Sydney et al. When the online experiment is performed, the cathode
Fig. 5. Sketch of interface of on-line spectral response measurement.
276
L. Lei et al. / Applied Surface Science 251 (2005) 273–277
Fig. 6. The spectral response and quantum field curves of transmission-type NEA photocathode.
is put in the activation chamber for processing. Firstly, the radiation light is turned into monochromatic light by the modulator and the grating monochromator. Then it is transmitted and attenuated by the optical fibre so as to incident on the flange windows of the activation chamber. The light is then reflected by the copper reflector and irradiates on the surface of the photocathode. The output signal is detected by the microsignal detection apparatus and after convertion
by analog-to-digital converter, is fed into the computer, which completes the data processing. Figs. 6 and 7 illustrates the online spectral response curves obtained from the experiment. The performance parameters obtained by simulation are summarised in Table 2. Table 2 Spectral response performance of GaAs photocathode in online measurement Spectral response performance Start wavelength (nm) Cut-off wavelength (nm) Peak response (mA/W) Peak wavelength (nm)
530 950 108.8 840
Sensitivity (mA/lm)
717.9
Evaluation of parameters Diffusion length (mm) Interface recombine (cm/s) Escape probability (%)
2.1 1 107 0.55
Table 3 Spectral matching factors of NEA photocathodes for scenes Object
Fig. 7. The comparison of two relative spectral response curves.
Night-sky radiation
Spectral matching factors a Sealed in the tube
Online
Green vegetation
Full moon Clear star
0.562905 0.584189711
0.672091 0.589365179
Rough concrete
Full moon Clear star
0.381982 0.49723
0.559963 0.589356
L. Lei et al. / Applied Surface Science 251 (2005) 273–277 Table 4 The detecting range of the LLL night vision goggle for drive P(l)
Clear star Transparenta Man
RL online (m) RL sealed in the tube (m) a b
78 78
b
Grassa b
Car
Manb
Carb
202 199
63 60
168 163
Background. Object.
4. Conclusions Comparison of online measurement curves with the spectral response curve of a GaAs photocathode (sealed in the third generation imaging intensifier) indicates that the process of sealing of the imaging intensifier has led to a decrease in the long wavelength response of the photocathode. Based on these curves, the spectral matching factors of GaAs photocathode for green vegetation and rough concrete have been calculated [4,5] as shown in Tables 3 and 4. The calculated performance parameters show that the variation of the spectral response in the sealing process
277
has influenced the spectral matching factor of the photocathode object combination and detecting distance [6] of the imaging intensifier and has resulted in the practical use of low-light-level night vision instruments.
References [1] Z. Zhiyuan, C. Benkang, A study on computer simulation of S25 series photocathodes spectral response, J. Nanjing Univ. Sci. Technol. 22 (3) (1998) 228–231. [2] J.P. Andre´ , P. Giittard, J. Hallais, C. Piaget, GaAs photocathodes for low light level imaging, J. Cryst. Growth 55 (1981) 235–245. [3] C. Benkang, F. Hongbing, H. Yuanzhen, Investigation application of the automatic survey instrument of dynamic spectral response of the photoelectric material, Vac. Sci. Technol. 16 (5) (1996) 364–366. [4] L. Wei, C. Benkang, Spectral matching factors between GaAs and multialkali photocathodes and reflective radiation of objects, Opt. Eng. 40 (5) (2001) 674–678. [5] L. Lei, C. Benkang, Spectral response characterization of super S25 and new S25 photocathodes, Appl. Opt. 43 (3) (2004) 616– 619. [6] L. Lei, C. Benkang, L. Wei, Visual range of low-light-level night vision goggles for driving, Opt. Eng. 42 (7) (2003) 2056–2061.