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Comparative research on activation technique for GaAs photocathodes
Optics Communications 285 (2012) 1264–1268
Contents lists available at SciVerse ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Comparative research on activation technique for GaAs photocathodes Liang Chen a, b, Yunsheng Qian b,⁎, Benkang Chang b, Xinlong Chen b, Rui Yang b a b
Institute of Optoelectronics Technology, China Jiliang University, 310018, Hangzhou, China Institute of Electronic Engineering & Optoelectronics Technology, Nanjing University of Science and Technology, 210094, Nanjing, China
a r t i c l e
i n f o
Article history: Received 4 July 2011 Received in revised form 18 November 2011 Accepted 28 November 2011 Available online 13 December 2011 OCIS codes: 160.2100 250.0250 Keywords: GaAs photocathode Surface photovoltage Surface escape probability Activation technique
a b s t r a c t The properties of GaAs photocathodes mainly depend on the material design and activation technique. In early researches, high-low temperature two-step activation has been proved to get more quantum efficiency than high-temperature single-step activation. But the variations of surface barriers for two activation techniques have not been well studied, thus the best activation temperature, best Cs–O ratio and best activation time for two-step activation technique have not been well found. Because the surface photovoltage spectroscopy (SPS) before activation is only in connection with the body parameters for GaAs photocathode such as electron diffusion length and the spectral response current (SRC) after activation is in connection with not only body parameters but also surface barriers, thus the surface escape probability (SEP) can be well fitted through the comparative research between SPS before activation and SEP after activation. Through deduction for the tunneling process of surface barriers by Schrödinger equation, the width and height for surface barrier I and II can be well fitted through the curves of SEP. The fitting results were well proved and analyzed by quantitative analysis of angle-dependent X-ray photoelectron spectroscopy (ADXPS) which can also study the surface chemical compositions, atomic concentration percentage and layer thickness for GaAs photocathodes. This comparative research method for fitting parameters of surface barriers through SPS before activation and SRC after activation shows a better real-time in system method for the researches of activation techniques. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Negative-electron-affinity (NEA) GaAs photocathodes activated by cesium (Cs) and oxygen (O) have already been found widespread applications in night vision image intensifiers, photomultiplier tubes and polarized electron sources because of their high quantum efficiency and good long-wavelength response. The quantum efficiency of NEA GaAs photocathodes mainly depends on the body performance of the cathode material and the Cs–O activation technique of preparation [1,2]. For material design, varied doping structure design has been a research hotspot because of driving more photoelectrons to surface barriers not only through diffusion but also through electric field drift caused by the Fermi-level effect between different active layers of varied doping concentration. For activation technique, the two-step activation technique has been proved to get more quantum efficiency than the single-step activation technique. The two-step activation technique means that the materials are cleaned by high temperature of about 600 °C and low temperature of about 400 °C, and then the materials are further activated by “yo-yo” processes after returning to normal temperature at about 60 °C behind each cleaning
⁎ Corresponding author. E-mail address: [email protected] (Y. Qian). 0030-4018/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2011.11.113
process. Many researches have been carried out on the two-step activation technique by means of SRC and ADXPS after Cs–O activation. Because SRC is influenced by not only the activation techniques and structures of surface barriers but also the body properties of cathode materials, and ADXPS is not an in-system measuring system for activation process, thus many parameters of GaAs photocathode such as electron diffusion length, surface escape probability and so on cannot be exactly measured only by SRC after activation, furthermore many key parameters for activation such as best activation time, best Cs–O ratio and so on can also not be well studied. For SPS is only in connection with the body properties of cathode materials, thus the body parameters of cathode materials can be well studied by SPS before activation avoiding the influence of Cs–O activation and surface barriers. Furthermore, the curve of SEP can be also well fitted through the comparative research between SPS before activation and SRC after activation. Through the calculation for photo-excited electrons to tunnel through surface barriers by Schrödinger equation, the calculation equation between SEP and the width and height of surface barriers can be well deduced. Thus the parameters of surface barriers can be well fitted through this comparative research method. For the surface chemical compositions, atomic concentration percentage and layer thickness for surface barriers can also be well fitted by ADXPS, the fitting results by SEP can be well proved and analyzed by ADXPS. In this paper, through experiments and theoretical fitting calculations, the variations of surface barriers through two-step activation and single-step activation were well discussed by the comparative research method and ADXPS [3–5].
L. Chen et al. / Optics Communications 285 (2012) 1264–1268
Thus the equations for SPS and SRC can be well deduced from Eqs. (1) to (3) as:
2. Principles Photoemission from GaAs NEA photocathodes is described as a three-step process of optical absorption, electron transport to the surface barriers and escaping across the surface barriers into vacuum. The band structure of reflection-mode cathode material of uniform doping concentration is shown as Fig. 1, where EC is the bottom energy level of conduction band, EV is the top energy level of valence band, EF is Fermi energy level, δs and d are the height and width of surface band bending region, b and c are the width of surface barriers I and II and V2 and V3 are the bottom energy level of two surface barriers [6,7]. When the cathode materials are irradiated by high light intensities, the dynamic filling or depleting of surface bending region (SBR) will lead to the sublinear increase of photoemission current. This phenomenon has been found and discussed by B.W. Boreham and other researchers [8]. Because the GaAs photocathodes are firmly being used in low-light level night vision and the luminous flux of activating light is at about 100 lx level, thus the photo-excited electrons will not cause the dynamic filling or depleting for SBR. The main effect of SBR is to cause the variation of energy distribution for emitting electrons, which will finally cause the variation of SEP. Thus the effect of SBR is firmly being embedded in the variation of SEP. Thus the one-dimensional diffusion equation of no equilibrium minority carrier for reflection-mode uniform doping GaAs photocathode can be given as [9]: d2 nðxÞ nðxÞ þ αI0 ð1−RÞ exp½ð−α ðT e −xÞ ¼ 0 − τ dx2
1265
KTI0 ð1−RÞα hv LD qc α 2hv L2D −1
ΔV ¼
J¼
f
−
SV coshðT e =LD Þ þ ðDn =LD Þ sinhðT e =LD Þ þ α hv LD ðDn =LD Þ coshðT e =LD Þ þ SV sinhðT e =LD Þ
ðSV −α hv Dn Þ expð−α hv T e Þ ðDn =LD Þ coshðT e =LD Þ þ SV sinhðT e =LD Þ
ð4Þ
g
PI0 ð1−RÞα hv LD α 2hv L2D −1 ðSV −α hv Dn Þ expð−α hv T e Þ ðDn =LD Þ coshðT e =LD Þ þ SV sinhðT e =LD Þ
f
−
SV coshðT e =LD Þ þ ðDn =LD Þ sinhðT e =LD Þ þ α hv LD ðDn =LD Þ coshðT e =LD Þ þ SV sinhðT e =LD Þ
ð5Þ
g
Thus from Eqs. (4) to (5), we can find that the curve of SEP for GaAs photocathode can be well fitted through the comparative research between SPS before activation and SRC after activation. 3. Experiments and results
Where ΔV is surface photovoltage for GaAs photocathode, Jw is the photoelectron current flow to surface barriers from photocathode body, P is the value of SEP, and J is the value of SRC after activation for photoelectrons tunneling through surface barriers into vacuum.
For experiments, the reflection-mode GaAs photocathode material was grown on a high-quality GaAs wafer (100) by molecular beam epitaxy (MBE) with p-type beryllium doping, the doping concentration was 1 × 10 19 cm − 3 and the active layer thickness was 1.6 μm. During growth procedure, the substrate was put into an MBE growth chamber and heated to a temperature of 620 °C to remove the surface oxide coating before growth which was monitored with an in situ Auger electron spectrometer. After cleaning, the substrate temperature was held at 580 °C under rich arsenic circumstance with the growth rate of about 1.0 μm/h. The crystalline quality of the epitaxial film was also monitored during the growth procedure with in situ reflection high-energy electron diffraction [11]. Before measuring the surface photovoltage curve, the cathode material was passed through acetone, hydrofluoric acid, absolute ethyl alcohol in turn for ultrasonic washing at about 5 min separately in order to wipe off the surface oxide coating and impurities. Then the material was sandwiched between two indium–tin oxide (ITO) glasses and measured by the SPS measuring system designed by our own. In order to avoid saturation, the light power illuminating on the cathode material was kept to about 200 nW and the light frequency was set to 30 Hz. The experimental curve of SPS is shown as Fig. 2 and the values of SPS were normalized [12,13].
Fig. 1. Band structure and surface potential barriers of GaAs:Cs–O photocathodes for reflection-mode GaAs photocathodes.
Fig. 2. Experimental curve of SPS for GaAs photocathode.
Dn
ð1Þ
For reflection-mode GaAs photocathodes, the boundary conditions are shown as: Dn
dnðxÞ ¼ SV nðxÞjx¼0 ; nðT e Þ ¼ 0 dx
ð2Þ
Through the calculations for the above equations, we can well get the values of photoelectrons arriving at surface barriers. According to the diode theory, the relationships between SPS before activation and SRC after activation for photoelectrons at surface barriers are shown as [10]: ΔV ¼
KT j dnðxÞ dnðxÞ ln 1 þ w ; J w ¼ Dn j ; J ¼ PDn j q dx x¼0 dx x¼0 c
ð3Þ
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L. Chen et al. / Optics Communications 285 (2012) 1264–1268
For Cs–O activation, the cathode material was again chemically cleaned in order to remove the carbon traces from the surface. Then the material was flushed by distilled water and quickly put into the vacuum chamber. When the vacuum pressure was up to 1 × 10− 7 Pa, the material was heated at the high temperature of 600 °C for cleaning to about 50 min. After the temperature of material being naturally cooled, the material was activated through a “yo-yo” process which kept the Cs source to be continuous and controlled the O source to be active or inactive through monitoring the photocurrent of cathode material irradiated by the halogen tungsten lamp of 12 V/50 W. As the activation process ended, the material was again heated at a low temperature of 410 °C for cleaning to about 40 min. When the temperature was naturally cooled, the material was again activated through a new “yo-yo” process [14,15]. The two activation processes after high and low temperature cleaning are shown as Fig. 3. From Fig. 3, we can find that the main differences of high and low temperature activation processes are that the first rising time of low temperature was shorter than that of high temperature and the bounding scope on photocurrent of low temperature was higher than that of high temperature. The differences on photocurrent of two activation processes show that the two-step activation process can help to form better NEA surface Cs-O surface barriers which help more photoelectrons to tunnel into vacuum. After two activation processes, the curves of SRC were also measured in situ by the spectral response measurement system, which are shown in Fig. 4 and the values of SRC were also normalized. From Fig. 4, we can find that the values of SRC were increased in the whole wavelength band for two-step temperature activation than only high-temperature single-step activation which means that the value of SEP can be well increased by the two-step activation [16]. Through the fitting calculation between SPS curve before activation and SRC curve after activation as shown in Eqs. (4)—(5), the fitting curves of SEP for two activation processes were well gotten as shown in Fig. 5 and the values of SEP were also normalized. Because the variations of SEP are mainly determined by the energy of photoelectrons which is in connection with the properties of body material and the width and height of surface barriers which are in connection with activation techniques, thus the variations on SEP of two activation processes for the same cathode material must lie on the differences of surface barriers caused by two temperature activation processes. For the relationships between SEP and surface barriers can be well deduced through one-dimensional stationary Schrödinger equation, thus the parameters of surface barriers can be well fitted by the curve of SEP. As is shown in Fig. 1, the surface potential energy
Fig. 4. Experimental curves of SPS for GaAs photocathode.
function by piecewise linearization for surface barriers I and II can be shown as [17]: V ðxÞ ¼
V iþ1 −V i a þ Vi x− i ; i ¼ 1; 2; ai bxbaiþ1 aiþ1 −ai Fi
Thus the one-dimensional stationary Schrödinger equation for V (x) can be shown as: d2 ψðxÞ 2m − 2 ½V ðxÞ−EψðxÞ ¼ 0 ℏ d2 x
ð7Þ
So the solution for the Schrödinger equation between the section (ai, ai + 1) can be expressed by the combination of Airy function as the following equation: þ
−
ψi ðxÞ ¼ C i Aiðzi Þ þ C i Biðzi Þ
ð8Þ
2 1=3 ; ci ¼ ai þ ðV i −EÞ=F i zi ¼ r i ðx−ci Þ; r i ¼ − 2mF i =ℏ Where Ai and Bi are the Airy functions, Ci+ and Ci− are the coefficients to be determined, m is the electron mass and E is the incident electron energy. As the photoelectron wave function is continuous, the boundary conditions at each interface of surface barriers can be shown as: ψi ðxÞjx¼aiþ1 ¼ ψiþ1 ðxÞjx¼ai þ1 ;
Fig. 3. Activation processes after high and low temperature cleaning for GaAs photocathode.
ð6Þ
dψiþ1 ðxÞ dψi ðxÞ x¼aiþ1 x¼aiþ1 ¼ dx dx
ð9Þ
Fig. 5. Fitting curves of SEP after two activation processes for GaAs photocathode.
L. Chen et al. / Optics Communications 285 (2012) 1264–1268 Table 1 Fitting parameters of surface barriers after two activation processes for GaAs photocathode. Activation
V1 (eV)
V2 (eV)
V3 (eV)
b1 (Å)
b2 (Å)
Single step Two step
4.9 5.1
1.42 1.42
0.9 0.9
3.4 2.1
8.1 6.9
Thus the final iterative matrix for photoelectrons tunneling through two barriers can be deduced through Eqs. (7)–(9) as the following equation [18,19]:
þ
C0 − C0
2
3 i
1 − 16 Aiðr 1 ða1 −c1 ÞÞ Biðr 1 ða1 −c1 ÞÞ k0 7 7 ¼ 6 0 0 4 5 i r 1 Ai ðr 1 ða1 −c1 ÞÞ r 1 Bi ðr 1 ða1 −c1 ÞÞ 2 1 k0
Biðr 1 ða2 −c1 ÞÞ −1 Aiðr 1 ða2 −c1 ÞÞ 0 0 r 1 Ai ðr 1 ða2 −c1 ÞÞ r 1 Bi ðr 1 ða2 −c1 ÞÞ
Aiðr 2 ða2 −c2 ÞÞ r 2 Ai0 ðr 2 ða2 −c2 ÞÞ
Biðr2 ða2 −c2 ÞÞ r2 Bi0 ðr2 ða2 −c2 ÞÞ
ð10Þ
Biðr 2 ða3 −c2 ÞÞ −1 1 Aiðr 2 ða3 −c2 ÞÞ 0 0 ik3 r 2 Ai ðr 2 ða3 −c2 ÞÞ r 2 Bi ðr 2 ða3 −c2 ÞÞ
1 −ik3
þ
C3 C3 −
So the final equation of SEP can be well deduced as:
P¼
pffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi k3 1 2 ; k ¼ 2mE=ℏ; k3 ¼ 2mðE−V 3 Þ=ℏ k0 M11 0
1267
4. ADXPS analyses In order to well study the variations of surface barriers after two activation processes, we used the ADXPS to measure the quantity of Ga and As oxides on surface barriers after high-temperature activation and low temperature activation, which are shown in Fig. 6 [20]. Thus we can find that there are all oxides on Ga and As after two activation processes, but in contrast to high-temperature activation, the oxides on Ga were being reduced slightly and the full width at half maximum of As2p3/2 was also being evidently reduced. For the structures of surface barriers can be separated into three layers from surface to body as Cs–O activation layer, GaAs–O layer and GaAs relaxation layer, so through quantitative analysis on the results of ASXPS, we well fitted the parameters of each layer, which are shown in Table 2. From Table 2, we can find that in contrast to single-step activation, the twostep activation can superiorly reduce the combinations of As–O and turn to lie on the combinations of Ga–O. Though the proportion of Cs/O was kept at the same level of 1.77, the depth of Cs–O layer and GaAs–O layer can be evidently reduced from 8.2 Å and 3.6 Å to 7.2 Å and 1.9 Å [21]. These parameters for surface barriers are almost same with the fitting values by the comparative research between SPS before activation and SRC after activation. The main reason for the variations of surface barriers is the variations of oxides after two temperature cleaning process. When the cathode material is being cleaned at 600 °C, GaAs combinations will be separated following the equation as below: 4GaAs→4Ga þ 2As2 ðorAs4 Þ↑; Ga2 O3 þ 4Ga→3Ga2 O↑
ð11Þ
For Eq. (11), M11 is the value of the row 1/column 1 element in the transfer matrix. Thus the parameters for surface barriers by Fig. 5 and Eq. (11) were well fitted, which are shown in Table 1.
ð12Þ
Because Ga2O can be easily removed from the surface by thermal desorption, so the composition of GaAs–O layer is mainly on As–O oxides. But when the cathode material is being cleaned at the temperature of 400 °C, the following combination will occur: As2 O3 þ 2GaAs→Ga2 O3 þ 2As2 ðorAs4 Þ↑
ð13Þ
1)
2)
3)
4)
Fig. 6. Experimental curves by ADXPS for GaAs photocathode after activation. Curves for single-step activation are shown in parts (1) and (2); curves for two-step activation are shown in parts (3) and (4).
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L. Chen et al. / Optics Communications 285 (2012) 1264–1268
Table 2 Fitting values of each layer surface barriers after two activation processes. Activation
Single step Two step
Layer
Cs–O GaAs–O Cs–O GaAs–O
Depth (Å)
Percent of atom (%) O
Cs
8.2 3.6 7.2 1.9
35.2 59.8 36.2 41.2
64.8
Ox–Ga
Ox–As
17.2
23.0
40.6
18.2
research method of fitting surface barriers by SPS before activation and SRC after activation can form an in-system measuring method to carry out researches on the Cs–O source ratio, activation time, vacuum degree, best cleaning temperature and so on for activation techniques of GaAs photocathodes in the future. Acknowledgement
63.8
The Ga2O3 combinations can remain at the cathode surface stably, so the main composition of GaAs–O layer can be converted to Ga–O oxides after high-low temperature two-step activation as is shown in Table 2. So the two-step activation process can further optimize the surface barriers by the reason that Ga–O combination in contrast to As–O combination can achieve better interface characteristics and thinner interface barriers and furthermore the two-step activation can achieve higher cleanness on atom level and more ordering surface structure, thus Cs can be absorbed on the surface more uniformly and sufficiently in order to decrease the final depth of Cs–O activation layer. The variations of surface barriers by two cleaning temperatures directly affect the surface escape probability for photoelectrons to tunnel into vacuum which finally influences the photocurrents in two “yo-yo” processes are shown in Fig. 3 and the spectral response currents after activations are shown in Fig. 4. From the above analyses, the comparative research between SPS before activation and SRC after activation can well fit the variations of surface barriers and the results were well proved by ADXPS measurements. Because the ADXPS measurement cannot real-time monitor the variations of surface barriers during the activation process, the comparative research method can form a real-time measuring system for activation process and will help us to further carry out researches on activation techniques [22,23]. 5. Conclusions Through the comparative research between SPS before activation and SRC after activation, the SEP curves for high-temperature single-step activation and high-low temperature two-step activation were well fitted. Through deduction for the tunneling process of surface barriers by Schrödinger equation, the width and height for surface barriers I and II were well fitted through the curves of SEP. The results by comparative research were well proved and discussed by the measuring of ASXPS for two activation processes. This comparative
This work was supported by the National Natural Science Foundation of China under Grant Nos. 60678043 and 60801036, General Administration for Quality Supervision of China under Grant No.2008QK328 and Zhejiang Provincial Natural Science Foundation of China under Grant No.Y5090150. References [1] H.K. Pollehn, Advances in Electronics and Electron Physics 64A (1995) 63. [2] W. Enloe, R. Scheldon, L. Reed, A. Amith, Proceedings of SPIE 1655 (1992) 41. [3] P. Tomkiewicz, S. Arabasz, B. Adamowicz, M. Miczek, J. Mizsei, D.R.T. Zahn, H. Hasegawa, J. Szuber, Surface Science 603 (2009) 3. [4] C.H. Chan, J.D. Wu, Y.S. Huang, H.P. Hsu, K.K. Tiong, Y.K. Su, Materials Chemistry and Physics 124 (2010) 2. [5] Y.T. Cheng, Y.S. Huang, D.Y. Lin, F.H. Pollak, K.R. Evans, Physics E 14 (2002) 2. [6] Y.Z. Liu, J.L. Moll, W.E. Spicer, Applied Physics Letters 17 (1970) 60. [7] Y.Z. Liu, C.D. Hollish, W.W. Stein, Journal of Applied Physics 44 (1973) 56192. [8] B.W. Boreham, D.S. Newman, Journal of Applied Physics 78 (1995) 5848. [9] Z. Yang, B.K. Chang, J.J. Zou, J.L. Qiao, P. Gao, Y.P. Zeng, Applied Optics 46 (2007) 7035. [10] A. Kenta, S. Takushi, Hiroaki Kakiuchi, Mizuho Morita, Physica B: Condensed Matter 376 (2006) 893. [11] Y.P. Zeng, X. Cao, L.J. Cui, M.Y. Kong, L. Pan, B.Q. Wang, Journal of Crystal Growth 210 (2001) 227. [12] V. Donchev, K. Kirilov, Ts Ivanov, K. Germanova, Materials Science and Engineering B 129 (2006) 186. [13] D. Durek, F. Fromm Berger, T. Reichelt, M. Westermann, Applied Surface Science 143 (1999) 319. [14] J.J. Zou, B.K. Chang, X.Q. Du, Journal of Vacuum Science and Technology 25 (2005) 401. [15] J.J. Zou, Z. Yang, J.L. Qiao, P. Gao, B.K. Chang, Proceedings of SPIE 6782 (2007). [16] Y.J. Zhang, B.K. Chang, Z. Yang, J. Niu, Applied Optics 48 (2009) 1715. [17] J. Niu, Y.J. Zhang, B.K. Chang, Y.J. Xiong, Chinese Physics B 20 (2011) 044209. [18] J. Niu, Y.J. Zhang, B.K. Chang, Y.J. Xiong, Acta Physics Sinica 60 (2011) 044210. [19] Y.J. Zhang, J. Niu, J. Zhao, J.J. Zou, B.K. Chang, F. Shi, H.C. Chen, Journal of Applied Physics 108 (2010) 093108. [20] D.G. Fisher, IEEE Transactions on Electron Devices ED-21 (1974) 541. [21] X.Q. Du, B.K. Chang, Applied Surface Science 251 (2005) 267. [22] J.J. Zou, B.K. Chang, H.L. Chen, L. Liu, Journal of Applied Physics 101 (2007) 033121. [23] V.L. Alperovich, A.G. Paulish, H.E. Scheibler, A.S. Terekhov, Applied Physics Letters 66 (1995) 2122.
Contents lists available at SciVerse ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Comparative research on activation technique for GaAs photocathodes Liang Chen a, b, Yunsheng Qian b,⁎, Benkang Chang b, Xinlong Chen b, Rui Yang b a b
Institute of Optoelectronics Technology, China Jiliang University, 310018, Hangzhou, China Institute of Electronic Engineering & Optoelectronics Technology, Nanjing University of Science and Technology, 210094, Nanjing, China
a r t i c l e
i n f o
Article history: Received 4 July 2011 Received in revised form 18 November 2011 Accepted 28 November 2011 Available online 13 December 2011 OCIS codes: 160.2100 250.0250 Keywords: GaAs photocathode Surface photovoltage Surface escape probability Activation technique
a b s t r a c t The properties of GaAs photocathodes mainly depend on the material design and activation technique. In early researches, high-low temperature two-step activation has been proved to get more quantum efficiency than high-temperature single-step activation. But the variations of surface barriers for two activation techniques have not been well studied, thus the best activation temperature, best Cs–O ratio and best activation time for two-step activation technique have not been well found. Because the surface photovoltage spectroscopy (SPS) before activation is only in connection with the body parameters for GaAs photocathode such as electron diffusion length and the spectral response current (SRC) after activation is in connection with not only body parameters but also surface barriers, thus the surface escape probability (SEP) can be well fitted through the comparative research between SPS before activation and SEP after activation. Through deduction for the tunneling process of surface barriers by Schrödinger equation, the width and height for surface barrier I and II can be well fitted through the curves of SEP. The fitting results were well proved and analyzed by quantitative analysis of angle-dependent X-ray photoelectron spectroscopy (ADXPS) which can also study the surface chemical compositions, atomic concentration percentage and layer thickness for GaAs photocathodes. This comparative research method for fitting parameters of surface barriers through SPS before activation and SRC after activation shows a better real-time in system method for the researches of activation techniques. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Negative-electron-affinity (NEA) GaAs photocathodes activated by cesium (Cs) and oxygen (O) have already been found widespread applications in night vision image intensifiers, photomultiplier tubes and polarized electron sources because of their high quantum efficiency and good long-wavelength response. The quantum efficiency of NEA GaAs photocathodes mainly depends on the body performance of the cathode material and the Cs–O activation technique of preparation [1,2]. For material design, varied doping structure design has been a research hotspot because of driving more photoelectrons to surface barriers not only through diffusion but also through electric field drift caused by the Fermi-level effect between different active layers of varied doping concentration. For activation technique, the two-step activation technique has been proved to get more quantum efficiency than the single-step activation technique. The two-step activation technique means that the materials are cleaned by high temperature of about 600 °C and low temperature of about 400 °C, and then the materials are further activated by “yo-yo” processes after returning to normal temperature at about 60 °C behind each cleaning
⁎ Corresponding author. E-mail address: [email protected] (Y. Qian). 0030-4018/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2011.11.113
process. Many researches have been carried out on the two-step activation technique by means of SRC and ADXPS after Cs–O activation. Because SRC is influenced by not only the activation techniques and structures of surface barriers but also the body properties of cathode materials, and ADXPS is not an in-system measuring system for activation process, thus many parameters of GaAs photocathode such as electron diffusion length, surface escape probability and so on cannot be exactly measured only by SRC after activation, furthermore many key parameters for activation such as best activation time, best Cs–O ratio and so on can also not be well studied. For SPS is only in connection with the body properties of cathode materials, thus the body parameters of cathode materials can be well studied by SPS before activation avoiding the influence of Cs–O activation and surface barriers. Furthermore, the curve of SEP can be also well fitted through the comparative research between SPS before activation and SRC after activation. Through the calculation for photo-excited electrons to tunnel through surface barriers by Schrödinger equation, the calculation equation between SEP and the width and height of surface barriers can be well deduced. Thus the parameters of surface barriers can be well fitted through this comparative research method. For the surface chemical compositions, atomic concentration percentage and layer thickness for surface barriers can also be well fitted by ADXPS, the fitting results by SEP can be well proved and analyzed by ADXPS. In this paper, through experiments and theoretical fitting calculations, the variations of surface barriers through two-step activation and single-step activation were well discussed by the comparative research method and ADXPS [3–5].
L. Chen et al. / Optics Communications 285 (2012) 1264–1268
Thus the equations for SPS and SRC can be well deduced from Eqs. (1) to (3) as:
2. Principles Photoemission from GaAs NEA photocathodes is described as a three-step process of optical absorption, electron transport to the surface barriers and escaping across the surface barriers into vacuum. The band structure of reflection-mode cathode material of uniform doping concentration is shown as Fig. 1, where EC is the bottom energy level of conduction band, EV is the top energy level of valence band, EF is Fermi energy level, δs and d are the height and width of surface band bending region, b and c are the width of surface barriers I and II and V2 and V3 are the bottom energy level of two surface barriers [6,7]. When the cathode materials are irradiated by high light intensities, the dynamic filling or depleting of surface bending region (SBR) will lead to the sublinear increase of photoemission current. This phenomenon has been found and discussed by B.W. Boreham and other researchers [8]. Because the GaAs photocathodes are firmly being used in low-light level night vision and the luminous flux of activating light is at about 100 lx level, thus the photo-excited electrons will not cause the dynamic filling or depleting for SBR. The main effect of SBR is to cause the variation of energy distribution for emitting electrons, which will finally cause the variation of SEP. Thus the effect of SBR is firmly being embedded in the variation of SEP. Thus the one-dimensional diffusion equation of no equilibrium minority carrier for reflection-mode uniform doping GaAs photocathode can be given as [9]: d2 nðxÞ nðxÞ þ αI0 ð1−RÞ exp½ð−α ðT e −xÞ ¼ 0 − τ dx2
1265
KTI0 ð1−RÞα hv LD qc α 2hv L2D −1
ΔV ¼
J¼
f
−
SV coshðT e =LD Þ þ ðDn =LD Þ sinhðT e =LD Þ þ α hv LD ðDn =LD Þ coshðT e =LD Þ þ SV sinhðT e =LD Þ
ðSV −α hv Dn Þ expð−α hv T e Þ ðDn =LD Þ coshðT e =LD Þ þ SV sinhðT e =LD Þ
ð4Þ
g
PI0 ð1−RÞα hv LD α 2hv L2D −1 ðSV −α hv Dn Þ expð−α hv T e Þ ðDn =LD Þ coshðT e =LD Þ þ SV sinhðT e =LD Þ
f
−
SV coshðT e =LD Þ þ ðDn =LD Þ sinhðT e =LD Þ þ α hv LD ðDn =LD Þ coshðT e =LD Þ þ SV sinhðT e =LD Þ
ð5Þ
g
Thus from Eqs. (4) to (5), we can find that the curve of SEP for GaAs photocathode can be well fitted through the comparative research between SPS before activation and SRC after activation. 3. Experiments and results
Where ΔV is surface photovoltage for GaAs photocathode, Jw is the photoelectron current flow to surface barriers from photocathode body, P is the value of SEP, and J is the value of SRC after activation for photoelectrons tunneling through surface barriers into vacuum.
For experiments, the reflection-mode GaAs photocathode material was grown on a high-quality GaAs wafer (100) by molecular beam epitaxy (MBE) with p-type beryllium doping, the doping concentration was 1 × 10 19 cm − 3 and the active layer thickness was 1.6 μm. During growth procedure, the substrate was put into an MBE growth chamber and heated to a temperature of 620 °C to remove the surface oxide coating before growth which was monitored with an in situ Auger electron spectrometer. After cleaning, the substrate temperature was held at 580 °C under rich arsenic circumstance with the growth rate of about 1.0 μm/h. The crystalline quality of the epitaxial film was also monitored during the growth procedure with in situ reflection high-energy electron diffraction [11]. Before measuring the surface photovoltage curve, the cathode material was passed through acetone, hydrofluoric acid, absolute ethyl alcohol in turn for ultrasonic washing at about 5 min separately in order to wipe off the surface oxide coating and impurities. Then the material was sandwiched between two indium–tin oxide (ITO) glasses and measured by the SPS measuring system designed by our own. In order to avoid saturation, the light power illuminating on the cathode material was kept to about 200 nW and the light frequency was set to 30 Hz. The experimental curve of SPS is shown as Fig. 2 and the values of SPS were normalized [12,13].
Fig. 1. Band structure and surface potential barriers of GaAs:Cs–O photocathodes for reflection-mode GaAs photocathodes.
Fig. 2. Experimental curve of SPS for GaAs photocathode.
Dn
ð1Þ
For reflection-mode GaAs photocathodes, the boundary conditions are shown as: Dn
dnðxÞ ¼ SV nðxÞjx¼0 ; nðT e Þ ¼ 0 dx
ð2Þ
Through the calculations for the above equations, we can well get the values of photoelectrons arriving at surface barriers. According to the diode theory, the relationships between SPS before activation and SRC after activation for photoelectrons at surface barriers are shown as [10]: ΔV ¼
KT j dnðxÞ dnðxÞ ln 1 þ w ; J w ¼ Dn j ; J ¼ PDn j q dx x¼0 dx x¼0 c
ð3Þ
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For Cs–O activation, the cathode material was again chemically cleaned in order to remove the carbon traces from the surface. Then the material was flushed by distilled water and quickly put into the vacuum chamber. When the vacuum pressure was up to 1 × 10− 7 Pa, the material was heated at the high temperature of 600 °C for cleaning to about 50 min. After the temperature of material being naturally cooled, the material was activated through a “yo-yo” process which kept the Cs source to be continuous and controlled the O source to be active or inactive through monitoring the photocurrent of cathode material irradiated by the halogen tungsten lamp of 12 V/50 W. As the activation process ended, the material was again heated at a low temperature of 410 °C for cleaning to about 40 min. When the temperature was naturally cooled, the material was again activated through a new “yo-yo” process [14,15]. The two activation processes after high and low temperature cleaning are shown as Fig. 3. From Fig. 3, we can find that the main differences of high and low temperature activation processes are that the first rising time of low temperature was shorter than that of high temperature and the bounding scope on photocurrent of low temperature was higher than that of high temperature. The differences on photocurrent of two activation processes show that the two-step activation process can help to form better NEA surface Cs-O surface barriers which help more photoelectrons to tunnel into vacuum. After two activation processes, the curves of SRC were also measured in situ by the spectral response measurement system, which are shown in Fig. 4 and the values of SRC were also normalized. From Fig. 4, we can find that the values of SRC were increased in the whole wavelength band for two-step temperature activation than only high-temperature single-step activation which means that the value of SEP can be well increased by the two-step activation [16]. Through the fitting calculation between SPS curve before activation and SRC curve after activation as shown in Eqs. (4)—(5), the fitting curves of SEP for two activation processes were well gotten as shown in Fig. 5 and the values of SEP were also normalized. Because the variations of SEP are mainly determined by the energy of photoelectrons which is in connection with the properties of body material and the width and height of surface barriers which are in connection with activation techniques, thus the variations on SEP of two activation processes for the same cathode material must lie on the differences of surface barriers caused by two temperature activation processes. For the relationships between SEP and surface barriers can be well deduced through one-dimensional stationary Schrödinger equation, thus the parameters of surface barriers can be well fitted by the curve of SEP. As is shown in Fig. 1, the surface potential energy
Fig. 4. Experimental curves of SPS for GaAs photocathode.
function by piecewise linearization for surface barriers I and II can be shown as [17]: V ðxÞ ¼
V iþ1 −V i a þ Vi x− i ; i ¼ 1; 2; ai bxbaiþ1 aiþ1 −ai Fi
Thus the one-dimensional stationary Schrödinger equation for V (x) can be shown as: d2 ψðxÞ 2m − 2 ½V ðxÞ−EψðxÞ ¼ 0 ℏ d2 x
ð7Þ
So the solution for the Schrödinger equation between the section (ai, ai + 1) can be expressed by the combination of Airy function as the following equation: þ
−
ψi ðxÞ ¼ C i Aiðzi Þ þ C i Biðzi Þ
ð8Þ
2 1=3 ; ci ¼ ai þ ðV i −EÞ=F i zi ¼ r i ðx−ci Þ; r i ¼ − 2mF i =ℏ Where Ai and Bi are the Airy functions, Ci+ and Ci− are the coefficients to be determined, m is the electron mass and E is the incident electron energy. As the photoelectron wave function is continuous, the boundary conditions at each interface of surface barriers can be shown as: ψi ðxÞjx¼aiþ1 ¼ ψiþ1 ðxÞjx¼ai þ1 ;
Fig. 3. Activation processes after high and low temperature cleaning for GaAs photocathode.
ð6Þ
dψiþ1 ðxÞ dψi ðxÞ x¼aiþ1 x¼aiþ1 ¼ dx dx
ð9Þ
Fig. 5. Fitting curves of SEP after two activation processes for GaAs photocathode.
L. Chen et al. / Optics Communications 285 (2012) 1264–1268 Table 1 Fitting parameters of surface barriers after two activation processes for GaAs photocathode. Activation
V1 (eV)
V2 (eV)
V3 (eV)
b1 (Å)
b2 (Å)
Single step Two step
4.9 5.1
1.42 1.42
0.9 0.9
3.4 2.1
8.1 6.9
Thus the final iterative matrix for photoelectrons tunneling through two barriers can be deduced through Eqs. (7)–(9) as the following equation [18,19]:
þ
C0 − C0
2
3 i
1 − 16 Aiðr 1 ða1 −c1 ÞÞ Biðr 1 ða1 −c1 ÞÞ k0 7 7 ¼ 6 0 0 4 5 i r 1 Ai ðr 1 ða1 −c1 ÞÞ r 1 Bi ðr 1 ða1 −c1 ÞÞ 2 1 k0
Biðr 1 ða2 −c1 ÞÞ −1 Aiðr 1 ða2 −c1 ÞÞ 0 0 r 1 Ai ðr 1 ða2 −c1 ÞÞ r 1 Bi ðr 1 ða2 −c1 ÞÞ
Aiðr 2 ða2 −c2 ÞÞ r 2 Ai0 ðr 2 ða2 −c2 ÞÞ
Biðr2 ða2 −c2 ÞÞ r2 Bi0 ðr2 ða2 −c2 ÞÞ
ð10Þ
Biðr 2 ða3 −c2 ÞÞ −1 1 Aiðr 2 ða3 −c2 ÞÞ 0 0 ik3 r 2 Ai ðr 2 ða3 −c2 ÞÞ r 2 Bi ðr 2 ða3 −c2 ÞÞ
1 −ik3
þ
C3 C3 −
So the final equation of SEP can be well deduced as:
P¼
pffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi k3 1 2 ; k ¼ 2mE=ℏ; k3 ¼ 2mðE−V 3 Þ=ℏ k0 M11 0
1267
4. ADXPS analyses In order to well study the variations of surface barriers after two activation processes, we used the ADXPS to measure the quantity of Ga and As oxides on surface barriers after high-temperature activation and low temperature activation, which are shown in Fig. 6 [20]. Thus we can find that there are all oxides on Ga and As after two activation processes, but in contrast to high-temperature activation, the oxides on Ga were being reduced slightly and the full width at half maximum of As2p3/2 was also being evidently reduced. For the structures of surface barriers can be separated into three layers from surface to body as Cs–O activation layer, GaAs–O layer and GaAs relaxation layer, so through quantitative analysis on the results of ASXPS, we well fitted the parameters of each layer, which are shown in Table 2. From Table 2, we can find that in contrast to single-step activation, the twostep activation can superiorly reduce the combinations of As–O and turn to lie on the combinations of Ga–O. Though the proportion of Cs/O was kept at the same level of 1.77, the depth of Cs–O layer and GaAs–O layer can be evidently reduced from 8.2 Å and 3.6 Å to 7.2 Å and 1.9 Å [21]. These parameters for surface barriers are almost same with the fitting values by the comparative research between SPS before activation and SRC after activation. The main reason for the variations of surface barriers is the variations of oxides after two temperature cleaning process. When the cathode material is being cleaned at 600 °C, GaAs combinations will be separated following the equation as below: 4GaAs→4Ga þ 2As2 ðorAs4 Þ↑; Ga2 O3 þ 4Ga→3Ga2 O↑
ð11Þ
For Eq. (11), M11 is the value of the row 1/column 1 element in the transfer matrix. Thus the parameters for surface barriers by Fig. 5 and Eq. (11) were well fitted, which are shown in Table 1.
ð12Þ
Because Ga2O can be easily removed from the surface by thermal desorption, so the composition of GaAs–O layer is mainly on As–O oxides. But when the cathode material is being cleaned at the temperature of 400 °C, the following combination will occur: As2 O3 þ 2GaAs→Ga2 O3 þ 2As2 ðorAs4 Þ↑
ð13Þ
1)
2)
3)
4)
Fig. 6. Experimental curves by ADXPS for GaAs photocathode after activation. Curves for single-step activation are shown in parts (1) and (2); curves for two-step activation are shown in parts (3) and (4).
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L. Chen et al. / Optics Communications 285 (2012) 1264–1268
Table 2 Fitting values of each layer surface barriers after two activation processes. Activation
Single step Two step
Layer
Cs–O GaAs–O Cs–O GaAs–O
Depth (Å)
Percent of atom (%) O
Cs
8.2 3.6 7.2 1.9
35.2 59.8 36.2 41.2
64.8
Ox–Ga
Ox–As
17.2
23.0
40.6
18.2
research method of fitting surface barriers by SPS before activation and SRC after activation can form an in-system measuring method to carry out researches on the Cs–O source ratio, activation time, vacuum degree, best cleaning temperature and so on for activation techniques of GaAs photocathodes in the future. Acknowledgement
63.8
The Ga2O3 combinations can remain at the cathode surface stably, so the main composition of GaAs–O layer can be converted to Ga–O oxides after high-low temperature two-step activation as is shown in Table 2. So the two-step activation process can further optimize the surface barriers by the reason that Ga–O combination in contrast to As–O combination can achieve better interface characteristics and thinner interface barriers and furthermore the two-step activation can achieve higher cleanness on atom level and more ordering surface structure, thus Cs can be absorbed on the surface more uniformly and sufficiently in order to decrease the final depth of Cs–O activation layer. The variations of surface barriers by two cleaning temperatures directly affect the surface escape probability for photoelectrons to tunnel into vacuum which finally influences the photocurrents in two “yo-yo” processes are shown in Fig. 3 and the spectral response currents after activations are shown in Fig. 4. From the above analyses, the comparative research between SPS before activation and SRC after activation can well fit the variations of surface barriers and the results were well proved by ADXPS measurements. Because the ADXPS measurement cannot real-time monitor the variations of surface barriers during the activation process, the comparative research method can form a real-time measuring system for activation process and will help us to further carry out researches on activation techniques [22,23]. 5. Conclusions Through the comparative research between SPS before activation and SRC after activation, the SEP curves for high-temperature single-step activation and high-low temperature two-step activation were well fitted. Through deduction for the tunneling process of surface barriers by Schrödinger equation, the width and height for surface barriers I and II were well fitted through the curves of SEP. The results by comparative research were well proved and discussed by the measuring of ASXPS for two activation processes. This comparative
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