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Angle-dependent XPS study of the mechanisms of “high–low temperature” activation of GaAs photocathode
Applied Surface Science 251 (2005) 267–272 www.elsevier.com/locate/apsusc
Angle-dependent XPS study of the mechanisms of ‘‘high–low temperature’’ activation of GaAs photocathode Xiaoqing Du *, Benkang Chang Lab 442, Institute of Electron Engineering and Photoelectric Technology, Nanjing University of Science and Technology, Nanjing 210094, PR China Available online 5 July 2005
Abstract The surface chemical compositions, atomic concentration percentage and layer thickness after ‘‘high-temperature’’ singlestep activation and ‘‘high–low temperature’’ two-step activation were obtained using quantitative analysis of angle-dependent X-ray photoelectron spectroscopy (XPS). It was found that compared to single-step activation, the thickness of GaAs–O interface barrier had a remarkable decrease, the degree of As–O bond became much smaller and the Ga–O bond became dominating, and at the same time the thickness of (Cs, O) layer also had a deduction while the ratio of Cs to O had no change after two-step activation. The measured spectral response curves showed that a increase of 29% of sensitivity had been obtained after two-step activation. To explore the inherent mechanisms of influences of the evolution of GaAs(Cs, O) surface layers on photoemission, surface electric barrier models based on the experimental results were built. By calculation of electron escape probability it was found that the decrease of thickness of GaAs–O interface barrier and (Cs, O) layer is the main reasons, which explained why higher sensitivity is achieved after two-step activation than single-step activation. # 2005 Elsevier B.V. All rights reserved. Keywords: Angle-dependent X-ray photoelectron spectroscopy (XPS); GaAs; Photocathode; Activation; Escape probability
1. Introduction The state of negative electron affinity (NEA) can be formed by co-adsorption of Cs and O on heavily p-type doped GaAs surface, and the method of ‘‘high– * Corresponding author. Tel.: +86 25 84315870; fax: +86 25 84315177. E-mail address: [email protected] (X. Du).
low temperature’’ two-step activation [1], has been commonly applied for the preparation of NEA. Whilst GaAs NEA photocathodes with high sensitivity have been obtained by this activation technique, there is not an agreed mechanism to fully explain why the sensitivity of photocathodes generated using the ‘‘high–low temperature’’ two-step activation is usually 30–40% higher than those generated by only ‘‘high-temperature’’ single-step activation. Much work has been done
0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.03.220
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X. Du, B. Chang / Applied Surface Science 251 (2005) 267–272
to research on this subject [1–6]. At present more surface information during NEA formation has been obtained by advanced surface chemical analysis technology and the dependences of sensitivity on some certain surface electronic parameters have been on researched [4–6]. Nonetheless, the relationship between chemical information and electronic parameters of surface layer during a two-step activation process has not been studied in detail with a view to elucidating the mechanism. In this paper, the surface chemical compositions, atomic concentration percentage and layer thickness after ‘‘high-temperature’’ single-step activation and ‘‘high–low temperature’’ two-step activation were determined using quantitative analysis of angledependent X-ray photoelectron spectroscopy (XPS), and surface electric barrier models based on the experimental results were built. By calculation of electron escape probability the inherent mechanisms of influences of the evolution of GaAs(Cs, O) surface layers on surface photoelectrons properties, and there on photoemission were explored.
2. Experiment The experimental sample is MOCVD-grown GaAs(1 0 0), with Zn p-type doping, at a concentration is 1019 cm 3. Before activation GaAs was sequentially degreased in carbon tetrachloride,
acetone, ethanol solutions and distilled water for respective 5-min ultrasonic cleaning. Then it was etched in a mixture of H2SO4, H2O2 and distilled water with 3:1:1 (v/v) for 15 s to remove the surface oxides. After drying with nitrogen, it was transferred into an ultra-high vacuum (UHV) system. By XPS analysis of GaAs surface after chemical cleaning little C and O was observed. The heating cleaning was carried in base pressure of 10 8 Pa. The degree of hightemperature cleaning is 600 8C for 10 min and the second step low temperature cleaning is 530 8C, for 10 min. The single-step and two-step activation were carried by ‘‘yo–yo’’ activation means in room temperature. The Ga2p3/2, As2p3/2 spectra of GaAs photocathode surface after single-step and two-step activation under eight take-off angles were measured by XPS to calculate the chemical compositions on surface.
3. Experimental results The angle-dependent XPS spectra of GaAs photocathode after single-step and two-step activation are shown in Figs. 1 and 2, respectively. It is shown in Figs. 1 and 2 that there exists Ga and As oxides on both GaAs surfaces after single-step and two-step activation, and the oxidation of As is more obvious than the oxidation of Ga. After two-step
Fig. 1. Angle-dependent XPS spectral after ‘‘high-temperature’’ single-step activation.
X. Du, B. Chang / Applied Surface Science 251 (2005) 267–272
269
Fig. 2. Angle-dependent XPS spectral after ‘‘high–low temperature’’ two-step activation.
activation the Ga oxide has a little decrease, while the full wave at half maximum (FWHM) of peak As2p3/2 has an obvious decrease compared to single-step. By use of angle-dependent XPS quantitative analysis procedure [7], the surface layer structure of GaAs photocathode was calculated from the XPS spectra, and is given in Table 1. In calculation, the surface was layered into (Cs, O) activation layer, GaAs–O layer and GaAs relaxation layer. From Table 1, it can be seen that after two-step activation the thickness of (Cs, O) layer becomes smaller, while the ratio of Cs to O do not change, the ratio is near 1.78. For the GaAs–O layer, the thickness becomes much smaller, As–O bonds decrease greatly and Ga–O bonds dominate compared to single-step. The changes of surface layer structure of GaAs photocathode have direct effects on photoemission. By use of self-developing spectral response measureTable 1 Calculation results of surface layers structure of GaAs photocathode ˚) Activation phase Layer Thickness (A
ment instrument [8], the spectral response curves of GaAs photocathode after single-step and two-step activation were obtained, and are shown in Fig. 3. A comparison of spectral response performance parameters is given in Table 2. From Fig. 3 and Table 2 the sensitivity of GaAs photocathode after two-step activation has increased 29% compared to the single-step, and both the cut-off wavelength and peak wavelength have a small move towards long-wave. The variation of simulated electron escape probability from experimental curves corresponds with the variation of sensitivity.
4. Discussions From the above XPS analysis results, it is found that after the two-step activation both the thickness of
Atomic Concentration percentage (%) O
Cs
O–Ga
O–As
Single-step activation
(Cs, O) GaAs–O
8.0 3.5
35.3 60
64.7 –
– 17.1
– 22.9
Two-step activation
(Cs, O) GaAs–O
7.0 2.0
36.1 41.2
63.9 –
– 40.6
– 18.2
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X. Du, B. Chang / Applied Surface Science 251 (2005) 267–272
Fig. 4. Surface electronic barrier of GaAs(Cs, O) photocathode.
Fig. 3. On-line spectral response measured curves. Curve 1: after ‘‘high-temperature’’ single-step activation, curve 2: after ‘‘high–low temperature’’ two-step activation.
GaAs–O layer and Cs–O layer had a remarkable decrease compared to the surface after the single-step activation, and the degree of As–O bond become much smaller while the Ga–O bond become dominating. The obvious variation of the ratio of Cs to O during the single-step activation and the two-step activation has not been observed. The variations of surface layer structure of GaAs photocathode have direct effects on the surface electrons escape probability and then on the sensitivity, and the behaviors were observed in our experiment results, which has been given in Fig. 3. The theoretical relation between surface layer structure and photoemission ability can be explored by surface electronic potential barrier, and the different influences of the different parameters of surface layer structure on electrons escape probability can be further compared. According to previous XPS analysis results, the surface electronic barrier is made of GaAs–O–Cs layer
and Cs–O layer, which is the surface model of double dipole layers [9]. The surface electronic barrier of GaAs(Cs, O) photocathode is shown in Fig. 4. The GaAs–O–Cs layer corresponds to the I-barrier and the Cs–O layer corresponds to the II-barrier in Fig. 4. Ibarrier is thin and its start height V2 is much higher than the conductance band minimum (CBM) of bulk GaAs. It reduces the vacuum level to bulk conduction band minimum (CBM) by Cs–Ga dipoles or Cs–As dipoles and therefore its final height V1 is equal to the CBM. It is semi-transparent and photoelectrons traverse through by tunneling. II-barrier further reduces the vacuum level to arrive at the negative electron affinity state by formation of Cs–O dipoles and the value of its final height V0 is equal to the final surface escape work function. The II-barrier is thicker and part of the escaping photoelectrons would be scattered by Cs–O layer. In fact the thickness of I-barrier x1 is the sum of the thickness of GaAs–O layer and the radius of Cs ion, and the thickness of II-barrier x2 x1 is deduced from the thickness of Cs–O layer and the radius of Cs ion. The barrier height V0 and V2 can be simulated from electron escape probability in Table 2 [10]. So, the shapes of surface potential barrier after single-step activation and two-step activation can be built from
Table 2 Performance parameters of spectral response curves Curves
Start wavelength (mm)
Cut-off wavelength (mm)
Peak response (mA/W)
Peak position (mm)
Integral sensitivity (mA/lm)
Electrons escape probability (simulation)
1 2
0.500 0.500
0.920 0.925
93.9 122.7
0.580 0.585
588 828
0.3 0.42
X. Du, B. Chang / Applied Surface Science 251 (2005) 267–272
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Table 3 Shapes of surface potential barriers after single-step activation and two-step activation Activation step
Single-step Two-step
I-barrier
II-barrier
˚) Thickness x1 (A
Height V2 (eV)
Thickness x1
5.1 3.6
4.0 4.55
6.4 5.4
the previous XPS analysis results, which is shown in Table 3. From Table 3 we find that the XPS analysis results in our experiment imply that the GaAs–O layer and Cs–O layer after ‘‘high–low temperature’’ two-step activation became thinner than the one after ‘‘high-temperature’’ single-step activation, which lead to the decrease of thickness of I-barrier and II-barrier. It is also noted that both the height of I-barrier and II-barrier increased after two-step activation, which indicates that the escape work function has not been optimized. For a certain surface barrier model the electron escape probability P can be calculated [10] and the P is the function of these surface parameters (ds, d, V0, V1, V2, x1, x2) in Fig. 4, where ds is the amount of band
˚) x2 (A
Height V0 (eV) 0.9 0.96
bending region (BBR), d is the width of BBR, V0, V1, V2 have been explained, x1 is the thickness of I-barrier, x2 is the thickness of the two barriers, and x2 x1 is the thickness of II-barrier. In fact the variations of parameters of ds and d during activation process are much smaller and can be ignored, and therefore the variations of parameters of V0, V2, x1 and x2 during activation process are the main influencing factors on photoemission. By calculation of electron escape probability, the relationships between these parameters and electron escape probability are given in Fig. 5. From Fig. 5 it is found that the electron escape probability is more sensitive to thickness than to height of the surface potential barriers, and the
Fig. 5. Relationships between the parameters of surface electron barriers and electron surface escape probability. (a) The height of I-barrier; (b) the height of II-barrier; (c) the thickness of I-barrier; (d) the thickness of II-barrier.
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X. Du, B. Chang / Applied Surface Science 251 (2005) 267–272
thickness of I-barrier has the greatest influence on electron escape probability. Considering the decrease of thickness of I-barrier and II-barrier in Table 3, it can be indicated that the decrease of thickness of surface layer is the main reasons to explain why greater sensitivity can be obtained after two-step activation. It can be thought that the decrease of thickness of surface layers also decreases the electrons scattering and therefore increases the transmission probability of tunneling electrons. The decrease of thickness of surface layers after two-step activation may be achieved by the more regulation of Cs–O dipoles distribution on surface and the more sufficient of the first Cs overlayer, because the GaAs surface after low temperature heat cleaning behaves better surface clean and smooth than the one after high-temperature heat cleaning, and therefore can attract more amount of Cs to adsorption on the surface during the first Cs exposure process.
5. Conclusions GaAs(Cs, O) surface layer structures after ‘‘hightemperature’’ single-step activation and ‘‘high–low temperature’’ two-step activation were obtained using quantitative analysis of angle-dependent XPS, and the surface potential barrier was built on basis the XPS analysis results. By calculation of electron escape probability it is found that the effects of thickness of surface barrier on escape probability are more obvious than the effects of height of surface barrier, and the
decrease of thickness of GaAs–O interface barrier and (Cs, O) layer are the main reasons which explained why higher sensitivity can be achieved after ‘‘high– low’’ two-step activation.
Acknowledgement The author would like to thank the Prof. Wang providing the experimental results at the surface analysis laboratory.
Reference [1] B.J. Stocker, Surf. Sci. 47 (1975) 501. [2] C.C. Phillips, A.E. Hughes, W. Sibbett, J. Phys. D: Appl. Phys. 17 (1984) 1713. [3] D.C. Rodway, M.B. Allenson, J. Phys. D: Appl. Phys. 19 (1986) 1353. [4] V.L. Alperovich, A.G. Paulish, H.E. Scheibler, A.S. Terekhov, Appl. Phys. Lett. 66 (1995) 2122. [5] K. Yamada, J. Asanari, M. Naitoh, S. Nishigaki, Surf. Sci. 402– 404 (1998) 683. [6] S. More´ , S. Tanaka, S. Tanaka, Y. Fujii, M. Kamada, Surf. Sci. 454–456 (2000) 161. [7] G.H. Wang, W.Y. Yang, B.K. Chang, Vac. Cryog. (China) 5 (1999) 242. [8] B.K. Chang, H.B. Fang, Y.Z. Liu, Vac. Sci. Tech. (China) 16 (1996) 364. [9] C.Y. Su, W.E. Spicer, I. Lindau, J. Appl. Phys. 54 (1983) 1413. [10] Z.Y. Zong, R.G. Fu, Y.S. Qian, B.K. Chang, Infra. Tech. (China) 24 (2002) 27.
Angle-dependent XPS study of the mechanisms of ‘‘high–low temperature’’ activation of GaAs photocathode Xiaoqing Du *, Benkang Chang Lab 442, Institute of Electron Engineering and Photoelectric Technology, Nanjing University of Science and Technology, Nanjing 210094, PR China Available online 5 July 2005
Abstract The surface chemical compositions, atomic concentration percentage and layer thickness after ‘‘high-temperature’’ singlestep activation and ‘‘high–low temperature’’ two-step activation were obtained using quantitative analysis of angle-dependent X-ray photoelectron spectroscopy (XPS). It was found that compared to single-step activation, the thickness of GaAs–O interface barrier had a remarkable decrease, the degree of As–O bond became much smaller and the Ga–O bond became dominating, and at the same time the thickness of (Cs, O) layer also had a deduction while the ratio of Cs to O had no change after two-step activation. The measured spectral response curves showed that a increase of 29% of sensitivity had been obtained after two-step activation. To explore the inherent mechanisms of influences of the evolution of GaAs(Cs, O) surface layers on photoemission, surface electric barrier models based on the experimental results were built. By calculation of electron escape probability it was found that the decrease of thickness of GaAs–O interface barrier and (Cs, O) layer is the main reasons, which explained why higher sensitivity is achieved after two-step activation than single-step activation. # 2005 Elsevier B.V. All rights reserved. Keywords: Angle-dependent X-ray photoelectron spectroscopy (XPS); GaAs; Photocathode; Activation; Escape probability
1. Introduction The state of negative electron affinity (NEA) can be formed by co-adsorption of Cs and O on heavily p-type doped GaAs surface, and the method of ‘‘high– * Corresponding author. Tel.: +86 25 84315870; fax: +86 25 84315177. E-mail address: [email protected] (X. Du).
low temperature’’ two-step activation [1], has been commonly applied for the preparation of NEA. Whilst GaAs NEA photocathodes with high sensitivity have been obtained by this activation technique, there is not an agreed mechanism to fully explain why the sensitivity of photocathodes generated using the ‘‘high–low temperature’’ two-step activation is usually 30–40% higher than those generated by only ‘‘high-temperature’’ single-step activation. Much work has been done
0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.03.220
268
X. Du, B. Chang / Applied Surface Science 251 (2005) 267–272
to research on this subject [1–6]. At present more surface information during NEA formation has been obtained by advanced surface chemical analysis technology and the dependences of sensitivity on some certain surface electronic parameters have been on researched [4–6]. Nonetheless, the relationship between chemical information and electronic parameters of surface layer during a two-step activation process has not been studied in detail with a view to elucidating the mechanism. In this paper, the surface chemical compositions, atomic concentration percentage and layer thickness after ‘‘high-temperature’’ single-step activation and ‘‘high–low temperature’’ two-step activation were determined using quantitative analysis of angledependent X-ray photoelectron spectroscopy (XPS), and surface electric barrier models based on the experimental results were built. By calculation of electron escape probability the inherent mechanisms of influences of the evolution of GaAs(Cs, O) surface layers on surface photoelectrons properties, and there on photoemission were explored.
2. Experiment The experimental sample is MOCVD-grown GaAs(1 0 0), with Zn p-type doping, at a concentration is 1019 cm 3. Before activation GaAs was sequentially degreased in carbon tetrachloride,
acetone, ethanol solutions and distilled water for respective 5-min ultrasonic cleaning. Then it was etched in a mixture of H2SO4, H2O2 and distilled water with 3:1:1 (v/v) for 15 s to remove the surface oxides. After drying with nitrogen, it was transferred into an ultra-high vacuum (UHV) system. By XPS analysis of GaAs surface after chemical cleaning little C and O was observed. The heating cleaning was carried in base pressure of 10 8 Pa. The degree of hightemperature cleaning is 600 8C for 10 min and the second step low temperature cleaning is 530 8C, for 10 min. The single-step and two-step activation were carried by ‘‘yo–yo’’ activation means in room temperature. The Ga2p3/2, As2p3/2 spectra of GaAs photocathode surface after single-step and two-step activation under eight take-off angles were measured by XPS to calculate the chemical compositions on surface.
3. Experimental results The angle-dependent XPS spectra of GaAs photocathode after single-step and two-step activation are shown in Figs. 1 and 2, respectively. It is shown in Figs. 1 and 2 that there exists Ga and As oxides on both GaAs surfaces after single-step and two-step activation, and the oxidation of As is more obvious than the oxidation of Ga. After two-step
Fig. 1. Angle-dependent XPS spectral after ‘‘high-temperature’’ single-step activation.
X. Du, B. Chang / Applied Surface Science 251 (2005) 267–272
269
Fig. 2. Angle-dependent XPS spectral after ‘‘high–low temperature’’ two-step activation.
activation the Ga oxide has a little decrease, while the full wave at half maximum (FWHM) of peak As2p3/2 has an obvious decrease compared to single-step. By use of angle-dependent XPS quantitative analysis procedure [7], the surface layer structure of GaAs photocathode was calculated from the XPS spectra, and is given in Table 1. In calculation, the surface was layered into (Cs, O) activation layer, GaAs–O layer and GaAs relaxation layer. From Table 1, it can be seen that after two-step activation the thickness of (Cs, O) layer becomes smaller, while the ratio of Cs to O do not change, the ratio is near 1.78. For the GaAs–O layer, the thickness becomes much smaller, As–O bonds decrease greatly and Ga–O bonds dominate compared to single-step. The changes of surface layer structure of GaAs photocathode have direct effects on photoemission. By use of self-developing spectral response measureTable 1 Calculation results of surface layers structure of GaAs photocathode ˚) Activation phase Layer Thickness (A
ment instrument [8], the spectral response curves of GaAs photocathode after single-step and two-step activation were obtained, and are shown in Fig. 3. A comparison of spectral response performance parameters is given in Table 2. From Fig. 3 and Table 2 the sensitivity of GaAs photocathode after two-step activation has increased 29% compared to the single-step, and both the cut-off wavelength and peak wavelength have a small move towards long-wave. The variation of simulated electron escape probability from experimental curves corresponds with the variation of sensitivity.
4. Discussions From the above XPS analysis results, it is found that after the two-step activation both the thickness of
Atomic Concentration percentage (%) O
Cs
O–Ga
O–As
Single-step activation
(Cs, O) GaAs–O
8.0 3.5
35.3 60
64.7 –
– 17.1
– 22.9
Two-step activation
(Cs, O) GaAs–O
7.0 2.0
36.1 41.2
63.9 –
– 40.6
– 18.2
270
X. Du, B. Chang / Applied Surface Science 251 (2005) 267–272
Fig. 4. Surface electronic barrier of GaAs(Cs, O) photocathode.
Fig. 3. On-line spectral response measured curves. Curve 1: after ‘‘high-temperature’’ single-step activation, curve 2: after ‘‘high–low temperature’’ two-step activation.
GaAs–O layer and Cs–O layer had a remarkable decrease compared to the surface after the single-step activation, and the degree of As–O bond become much smaller while the Ga–O bond become dominating. The obvious variation of the ratio of Cs to O during the single-step activation and the two-step activation has not been observed. The variations of surface layer structure of GaAs photocathode have direct effects on the surface electrons escape probability and then on the sensitivity, and the behaviors were observed in our experiment results, which has been given in Fig. 3. The theoretical relation between surface layer structure and photoemission ability can be explored by surface electronic potential barrier, and the different influences of the different parameters of surface layer structure on electrons escape probability can be further compared. According to previous XPS analysis results, the surface electronic barrier is made of GaAs–O–Cs layer
and Cs–O layer, which is the surface model of double dipole layers [9]. The surface electronic barrier of GaAs(Cs, O) photocathode is shown in Fig. 4. The GaAs–O–Cs layer corresponds to the I-barrier and the Cs–O layer corresponds to the II-barrier in Fig. 4. Ibarrier is thin and its start height V2 is much higher than the conductance band minimum (CBM) of bulk GaAs. It reduces the vacuum level to bulk conduction band minimum (CBM) by Cs–Ga dipoles or Cs–As dipoles and therefore its final height V1 is equal to the CBM. It is semi-transparent and photoelectrons traverse through by tunneling. II-barrier further reduces the vacuum level to arrive at the negative electron affinity state by formation of Cs–O dipoles and the value of its final height V0 is equal to the final surface escape work function. The II-barrier is thicker and part of the escaping photoelectrons would be scattered by Cs–O layer. In fact the thickness of I-barrier x1 is the sum of the thickness of GaAs–O layer and the radius of Cs ion, and the thickness of II-barrier x2 x1 is deduced from the thickness of Cs–O layer and the radius of Cs ion. The barrier height V0 and V2 can be simulated from electron escape probability in Table 2 [10]. So, the shapes of surface potential barrier after single-step activation and two-step activation can be built from
Table 2 Performance parameters of spectral response curves Curves
Start wavelength (mm)
Cut-off wavelength (mm)
Peak response (mA/W)
Peak position (mm)
Integral sensitivity (mA/lm)
Electrons escape probability (simulation)
1 2
0.500 0.500
0.920 0.925
93.9 122.7
0.580 0.585
588 828
0.3 0.42
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271
Table 3 Shapes of surface potential barriers after single-step activation and two-step activation Activation step
Single-step Two-step
I-barrier
II-barrier
˚) Thickness x1 (A
Height V2 (eV)
Thickness x1
5.1 3.6
4.0 4.55
6.4 5.4
the previous XPS analysis results, which is shown in Table 3. From Table 3 we find that the XPS analysis results in our experiment imply that the GaAs–O layer and Cs–O layer after ‘‘high–low temperature’’ two-step activation became thinner than the one after ‘‘high-temperature’’ single-step activation, which lead to the decrease of thickness of I-barrier and II-barrier. It is also noted that both the height of I-barrier and II-barrier increased after two-step activation, which indicates that the escape work function has not been optimized. For a certain surface barrier model the electron escape probability P can be calculated [10] and the P is the function of these surface parameters (ds, d, V0, V1, V2, x1, x2) in Fig. 4, where ds is the amount of band
˚) x2 (A
Height V0 (eV) 0.9 0.96
bending region (BBR), d is the width of BBR, V0, V1, V2 have been explained, x1 is the thickness of I-barrier, x2 is the thickness of the two barriers, and x2 x1 is the thickness of II-barrier. In fact the variations of parameters of ds and d during activation process are much smaller and can be ignored, and therefore the variations of parameters of V0, V2, x1 and x2 during activation process are the main influencing factors on photoemission. By calculation of electron escape probability, the relationships between these parameters and electron escape probability are given in Fig. 5. From Fig. 5 it is found that the electron escape probability is more sensitive to thickness than to height of the surface potential barriers, and the
Fig. 5. Relationships between the parameters of surface electron barriers and electron surface escape probability. (a) The height of I-barrier; (b) the height of II-barrier; (c) the thickness of I-barrier; (d) the thickness of II-barrier.
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X. Du, B. Chang / Applied Surface Science 251 (2005) 267–272
thickness of I-barrier has the greatest influence on electron escape probability. Considering the decrease of thickness of I-barrier and II-barrier in Table 3, it can be indicated that the decrease of thickness of surface layer is the main reasons to explain why greater sensitivity can be obtained after two-step activation. It can be thought that the decrease of thickness of surface layers also decreases the electrons scattering and therefore increases the transmission probability of tunneling electrons. The decrease of thickness of surface layers after two-step activation may be achieved by the more regulation of Cs–O dipoles distribution on surface and the more sufficient of the first Cs overlayer, because the GaAs surface after low temperature heat cleaning behaves better surface clean and smooth than the one after high-temperature heat cleaning, and therefore can attract more amount of Cs to adsorption on the surface during the first Cs exposure process.
5. Conclusions GaAs(Cs, O) surface layer structures after ‘‘hightemperature’’ single-step activation and ‘‘high–low temperature’’ two-step activation were obtained using quantitative analysis of angle-dependent XPS, and the surface potential barrier was built on basis the XPS analysis results. By calculation of electron escape probability it is found that the effects of thickness of surface barrier on escape probability are more obvious than the effects of height of surface barrier, and the
decrease of thickness of GaAs–O interface barrier and (Cs, O) layer are the main reasons which explained why higher sensitivity can be achieved after ‘‘high– low’’ two-step activation.
Acknowledgement The author would like to thank the Prof. Wang providing the experimental results at the surface analysis laboratory.
Reference [1] B.J. Stocker, Surf. Sci. 47 (1975) 501. [2] C.C. Phillips, A.E. Hughes, W. Sibbett, J. Phys. D: Appl. Phys. 17 (1984) 1713. [3] D.C. Rodway, M.B. Allenson, J. Phys. D: Appl. Phys. 19 (1986) 1353. [4] V.L. Alperovich, A.G. Paulish, H.E. Scheibler, A.S. Terekhov, Appl. Phys. Lett. 66 (1995) 2122. [5] K. Yamada, J. Asanari, M. Naitoh, S. Nishigaki, Surf. Sci. 402– 404 (1998) 683. [6] S. More´ , S. Tanaka, S. Tanaka, Y. Fujii, M. Kamada, Surf. Sci. 454–456 (2000) 161. [7] G.H. Wang, W.Y. Yang, B.K. Chang, Vac. Cryog. (China) 5 (1999) 242. [8] B.K. Chang, H.B. Fang, Y.Z. Liu, Vac. Sci. Tech. (China) 16 (1996) 364. [9] C.Y. Su, W.E. Spicer, I. Lindau, J. Appl. Phys. 54 (1983) 1413. [10] Z.Y. Zong, R.G. Fu, Y.S. Qian, B.K. Chang, Infra. Tech. (China) 24 (2002) 27.