- Email: [email protected]
Comparative research on reflection-mode GaAs photocathode with graded AlxGa1−xAs buffer layer
Optics Communications 355 (2015) 186–190
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Comparative research on reflection-mode GaAs photocathode with graded AlxGa1 xAs buffer layer Liang Chen a,b,n, Yang Shen a, Shuqin Zhang a, Yunsheng Qian b, Sunan Xu a 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
art ic l e i nf o
a b s t r a c t
Article history: Received 18 April 2015 Received in revised form 16 June 2015 Accepted 21 June 2015 Available online 24 June 2015
The graded Al compositional AlxGa1 xAs buffer layer can not only form continuous internal electric field from buffer layer to active layer but also optimize the interface properties by decreasing the misfit dislocations and stacking faults arising from lattice mismatch. By measuring the spectral response current (SRC) for two reflection-mode (r-mode) designed samples of graded and stationary Al compositional structure, we can find the special phenomenon that the graded structure had quite influence at the middle wavelength band from 550 nm to 850 nm, but not the short wavelength band from 400 nm to 550 nm, though the buffer layer can only absorb photon energy at the short wavelength band. Through the comparative research for designed samples through SPV before Cs–O activation and SRC after Cs–O activation, the graded structure can well optimize the key parameters such as LD, Ln, Sv and P. For the photon absorption lengths are relative little at the short wavelength band and relative long at the middle wavelength band, so the optimizations of key parameters have little influence on photo-excited electrons at the short wavelength band which are mainly excited from the region in active layer near surface barriers. The optimizations of key parameter, mainly the back interface recombination velocity (Sv), can have quite impact on photo-excited electrons at the middle short wavelength band which are mainly excited from the internal active layer near the back interface. This comparative research can help to well study the photo-emission theory and structure design on graded Al compositional design for r-mode GaAs photocathodes in the future research. & 2015 Published by Elsevier B.V.
OCIS codes: 160.2100 250.0250 Keywords: GaAs photocathode Surface photovoltage Spectral response current Surface escape probability AlxGa1 xAs buffer layer
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 low thermal emittance. The quantum efficiency of GaAs photocathodes mainly depends on the key parameters such as: the electron diffusion length (LD), the back interface recombination velocity (Sv) and the surface escape probability (P). In early research, the reflection-mode structure was epitaxial grown of GaAs substrate, AlGaAs buffer layer and GaAs active layer by metal-organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE) [1–4]. The stationary Al compositional and uniform doping AlGaAs buffer layer can form strong electric field at the AlGaAs/GaAs interface to reflect the photo-excited electrons arriving at the back interface in active n Corresponding author at: Institute of Optoelectronics Technology, China Jiliang University, 310018 Hangzhou, China. E-mail address: [email protected] (L. Chen).
http://dx.doi.org/10.1016/j.optcom.2015.06.053 0030-4018/& 2015 Published by Elsevier B.V.
layer and finally enhance the emitting quantum efficiency, so the AlGaAs buffer layer can finally well increase the key parameters for GaAs active layer including LD, Sv and P. In recent research, the graded Al compositional structure design was carried out in AlxGa1 xAs buffer layer, which can well increase the quantum efficiency and interface properties. But the influence of AlxGa1 xAs buffer layer has not been well studied. In this paper, in order to deeply study the impact of AlxGa1 xAs buffer layer for reflectionmode GaAs photocathodes, we carried out the comparative research between the surface photovoltage (SPV) before Cs–O activation and spectral response current (SRC) after Cs–O activation. The fitting equations and comparative research results can help to optimize the structure design and study the influence of graded Al compositional AlxGa1 xAs buffer layer in later research [2–6].
2. Experiments and results In order to carry out the comparative research, two reflectionmode GaAs photocathodes were grown on the high-quality n-type GaAs (100)-oriented substrate by MOCVD. The epitaxial layers
L. Chen et al. / Optics Communications 355 (2015) 186–190
both consist of a GaAs substrate, an AlGaAs buffer layer and a GaAs active layer. Both of the AlGaAs layer and the GaAs layer are p-type zinc (Zn)-doped concentration. The GaAs active layers of two samples have the same uniform doping concentration of 1 1018 cm 3 and the same thickness of 0.2 μm. The AlxGa1 xAs buffer layers of two samples both have the same doping concentration of 1 1018 cm 3 and the main differences for buffer layers are that the graded compositional sample has the graded Al composition from 0.6 to 0, while the standard sample has the stationary Al composition of 0.63. The structure diagrams for two designed samples are shown as Fig. 1 [4–8]. The absorption coefficient of AlxGa1 xAs buffer layer is shown in Fig. 2, and the surface photo voltage (SPV) is mainly depended on the electron–hole separation while absorbing photon energy. Thus in order to shield off the influence of AlxGa1 xAs buffer layer at the short wavelength band from 400 nm to 550 nm, the SPV curve was only measured form 550 nm to 950 nm, where the buffer layer can be seemed as fully window layer with no photon absorption [2–6]. The designed samples were firstly ultrasonically washed in acetone, hydrofluoric acid and absolute ethyl alcohol, in sequence, in order to wipe off the surface oxidation layer and impurities for measuring the SPV curves. The normalized SPV curves were measured by the measurement system designed by our own, and the SPV curves for active layers while shielding off the influences of the buffer layers and substrates are shown in Fig. 3 [2,3,8–10]. After measuring SPV curves, the designed samples were again through chemically etched in the hydrofluoric acid solution of 40% for 10 min and being rinsed by de-ionized water for drying. Finally the samples were transferred into the ultrahigh vacuum chamber (base pressure r1 10 9 Pa) for heating cleaning of 20 min at the temperature of 650 °C. After the chemical and heating cleaning, the samples can have the atomically clean crystal surface for Cs–O activation. The Cs–O activation was carried out according to the principle that the Cs source is continuous and the O source is discontinuous. After the high temperature Cs–O activation, the samples were again heated at the low temperature of 600 °C for same 20 min and carried out activation in the same way. During the activation process, the photocurrent was monitored under the illumination of the halogen tungsten lamp. After the Cs–O activation, the SRC curves for two samples were measured in situ by the on-line spectral response measurement system from 400 nm to 950 nm. The normalized measuring SRC curves for two designed samples are shown in Fig. 4 [11–13]. From Fig. 4, we can find the special phenomenon that the graded structure has quite influence at the middle wavelength band from 550 nm to 850 nm but not the short wavelength band from 400 nm to 550 nm, though the AlxGa1 xAs buffer layer can
GaAs active layer 0.2µ m, 1.0×1018cm-3
GaAs active layer 0.2µ m, 1.0×1018cm-3
AlxGa1-xAs buffer layer x=0.6~0 0.5µ m, 1.0×1018cm-3
AlxGa1-xAs window layer x=0.63 0.5µ m, 1.0×1018cm-3
n-GaAs (100) substrate
n-GaAs (100) substrate
Fig. 1. Structures for two designed reflection-mode GaAs photocathode module samples: (a) the graded compositional sample; (b) the standard sample.
187
Fig. 2. The absorption coefficient curve for AlxGa1 xAs buffer layer.
Fig. 3. The experimental SPV curves for two designed samples.
only absorb the photon energy at the short wavelength band. So in order to well study the influence of graded structure for r-mode photocathode, we carried out the comparative research by the SPV curve before activation and the SRC curve after activation.
3. Analyses and discussions For the graded Al compositional design in buffer layer can form internal electric field and decrease the band slope near the interface, thus the energy band structures for two designs can be shown in Fig. 5. When the GaAs active layer is relatively thin, the photon energy can not be sufficiently absorbed in active layer and subsequently absorbed in active layer, the photo-excited electrons in buffer layer with high doping concentration can diffuse into active layer and
Fig. 4. Experimental SRC curves for designed samples after activation.
188
L. Chen et al. / Optics Communications 355 (2015) 186–190
finally arrive at the surface barriers for emitting. Thus, the quantum efficiency for graded Al composition or stationary Al composition should both take the photo-excited electrons in buffer layer in consideration, while the active layer is relatively thin [2– 4,12–14]. According to diode theory, the connections for SPV and SRC with n(x) are shown as:
ΔV =
⎛ jw ⎞ KT dn (x) dn (x) | x = Te , J = P . Dn | x = Te ln ⎜1 + ⎟, Jw = Dn q c⎠ dx dx ⎝
(1)
where ΔV is surface photovoltage for GaAs photocathode, Jw is the photo-electron current flowing to surface barriers from the internal body, J is the spectral response current emitting into vacuum by NEA after Cs–O activation, and n(x) is the density of electron minority carrier [12–15]. The one-dimensional continuity equation for n(x) is shown as:
Dn
d2n (x) n (x) − + g (x) = 0 τ dx2
(2)
where τ is the lifetime of minority carrier, Dn is the diffusion coefficient of electron, and g(x) is the generation rate of photoexcited electrons. The generation rate of photo-excited electrons in GaAs active layer and AlxGa1 xAs buffer layer can be given by:
gGaAs (x) = (1 − R) I0 α1 exp ( − α1x); g Al x Ga1 − x As (x)
x ∈ [0, Te ] x ∈ ⎡⎣Te, Te + d⎤⎦
= (1 − R) I0 exp ( − α1Te ) α2 exp [−α2 (x − Te ) ]; where I0 is the incident light intensity,
(3)
α1 and α2 are the
absorption coefficient of GaAs active layer and AlxGa1 xAs buffer layer respectively, R is the surface reflectivity, Te is the thickness of the GaAs active layer, and d is the thickness of the AlxGa1 xAs buffer layer [12–15]. For solving Eqs. (1)–(3), the boundary conditions for GaAs active layer are simplified as:
n (x)| x = 0 =0, − Dn
dn (x) dx
x = Te−
= Sv ⎡⎣n (Te−) − n (Te+) ⎤⎦
(4)
And the boundary conditions for AlxGa1 xAs buffer layer are simplified as:
Dn
dn (x) dx
x = Te+
= Sv n (x) x = T + , n (x) x =∞ = 0 e
(5)
where n(Te ) represents the photo-excited electrons arriving at the back interface from active layer, n(Te þ ) represents the photoexcited electrons arriving at the front interface from the AlxGa1 xAs buffer layer [15–18]. Thus through deductions, the SRC equation for two designed structures taking consideration of the GaAs active layer and the AlxGa1 xAs buffer layer is given by [12–18]:
J=
P (1 − R) α1L D α12 L D2 − 1 ⎤ ⎡ (Sv − α1Dn ) exp ( − α1Te ) ⎥ ⎢ D L T L S T L / cosh / sinh / ( ) ( ) + ( ) e D v e D ⎥ ⎢ n D ⎢ S cosh (T /L ) + (D /L ) sinh (T /L ) ⎥ v e D n D e D ⎥ ⎢− ⎢ (Dn /L D ) cosh (Te/L D ) + Sv sinh (Te/L D ) ⎥ ⎥ ⎢+α L ⎦ ⎣ 1 D
(6)
The SPV equation for active layer taking no account of the AlxGa1 xAs buffer layer is given by:
⎤ ⎡ (Sv − α1Dn ) exp ( − α1Te ) ⎥ ⎢ D L T L S T L / cosh / sinh / ( ) ( ) + ( ) e D v e D ⎥ ⎢ n D KT (1 − R) α1L D ⎢ ΔV = Sv cosh (Te/L D ) + (Dn /L D ) sinh (Te/L D ) ⎥ 2 2 ⎥ qc (α1 L D − 1) ⎢− ⎢ (Dn /L D ) cosh (Te/L D ) + Sv sinh (Te/L D ) ⎥ ⎥ ⎢+ α L ⎦ ⎣ 1 D
Fig. 5. Energy band structures for (a) the standard structure design and (b) the graded compositional structure design. Ec is the conduction band minimum, Ev is the valence band maximum, Eg is the band gap, EF is the Fermi level, Evac is the vacuum level, and δs and ds are the height and width of the surface band-bending region (BBR).
(7)
where P is the surface escape probability for photo-excited electrons tunneling through surface barriers into vacuum, Ln is the electron diffusion length of AlxGa1 xAs buffer layer, and LD is the electron diffusion length of GaAs active layer. For r-mode GaAs photocathode, the reflection coefficient curve of cathode material was firstly measured before fitting calculations of SPV and SRC curves. So through the fitting calculations, the normalized experimental and fitting SPV curves for two designed samples are shown in Fig. 6. The key fitting parameters for the active layers of two designed cathode samples before Cs–O activation were well fitted by Eq. (7). Furthermore, we can also find that using the fitting parameters by SPV for active layers, the values of P and Ln will be easily fitted. The final fitting parameters for GaAs active layer and AlxGa1 xAs buffer layer are shown in Table 1. As we know, the misfit dislocations and stacking faults arising from lattice mismatch at the back interface will primarily affect the interface properties, such as Sv and finally affect the electron diffusion length. So from Table 1, we can find that the graded Al compositional structure can well increase the values of LD and Ln, especially well decrease the value of Sv from 1.1 109 μm/s to 1.2 1010 μm/s. Thus the grade structure can not only form internal electric field to increase electron diffusion length, but also optimize the interface properties to further
L. Chen et al. / Optics Communications 355 (2015) 186–190
Fig. 6. Experimental and fitting SPV curves for designed samples. Table 1 Key fitting parameters for two designed samples. Sample
Dn (μm2/s)
Sv (μm/s)
LD (μm)
Ln (μm)
Graded Standard
1.2 1010 1.2 1010
1.1 109 1.2 1010
1.80 1.65
0.85 0.70
decrease the interface recombination velocity [12–16]. The normalized experimental and fitting SRC curves for two designed samples are shown in Fig. 7. Through the comparative fitting calculation between SPV and SRC curves, the normalized values of P along wavelength can be well fitted, which are shown in Fig. 8. From Fig. 8, we can find that the curves of P have little differences only at middle and long wavelength band. The reason is that for thin active layer, the stationary and graded Al compositional structure can also form strong electric field at the back interface and reflect photo-excited electrons to surface barriers for emitting [14–18]. From Fig. 7, we can find that though the buffer layer can only absorb photon energy at short wavelength band, the graded Al compositional structure have more influence on the middle wave band from 550 nm to 850 nm, while minor influence on the short wave band. The reason is that for short wavelength band, the absorption length of high-energy photons is shorter and the first absorption layer is active layer, so the thermalized electrons excited by short-wave photons are mainly generated from the region in active layer near surface and can easily tunnel through surface barriers with high energy after transferring short distance. Though the photo-excited electrons of AlxGa1 xAs buffer layer can have higher diffusion length Ln and shorter interface recombination velocity Sv for graded sample than standard sample, the increasing
189
Fig. 8. Surface escape probability curves for two designed samples.
electrons from buffer layer occupy very little part on the total short wavelength electrons from buffer layer and active layer. Thus the SRC values have little differences at short wave band for two samples for that the promotions of Ln, Sv have little influence on photo-excited electrons at short wave band. While for middle wavelength band, the absorption length of middle energy photons is relative long, so the thermalized electrons are mainly from the internal active layer. Thus more photo-excited electrons will arrive at the back interface, the variation of Sv has deep influence on the middle wavelength band. And the main improvement of graded structure for r-mode GaAs photocathode with thin active layer is the major optimization of Sv, which will further increase the final quantum efficiency at middle wavelength band [15–20].
4. Conclusions Through the comparative research between SPV before Cs–O activation and SRC after Cs–O activation for the AlxGa1 xAs buffer layer with graded or stationary Al compositional structure, the graded structure can well increase the quantum efficiency for the middle wavelength band from 550 nm to 850 nm than the short wavelength band from 400 nm to 550 nm, though the buffer layer can only absorb the photon energy at the short wavelength band. The graded structure can not only form continuous internal electric field from buffer layer to active layer, but also optimize the interface properties by decreasing the misfit dislocations and stacking faults arising from lattice mismatch. So the graded structure can well optimize the key parameters such as LD, Ln, Sv and P. For the absorption length of high-energy photon energy at the short wavelength band is relative little, so the graded structure has less influence on the photo-excited electrons at the short wavelength band which are mainly excited at the region near surface. But the graded structure can well increase the quantum efficiency at the middle wavelength band by the high optimization of Sv for that the major photo-excited electrons are excited near the back interface by relative long absorption length. This comparative research can help to well study the photo-emission theory and structure design for graded Al compositional design in the later research.
Acknowledgments
Fig. 7. Experimental and fitting SRC curves for two designed samples.
This work was supported by the National Natural Science Foundation of China under Grant (Nos. 61308089 and 61440065), Public Technology Applied Research Project of Zhejiang Province (No. 2013C31068), Science and Technology on Night Vision
190
L. Chen et al. / Optics Communications 355 (2015) 186–190
Laboratory Foundation (No. J20110105). Applied Research Project of Zhejiang Provincial Education Department (Nos. Y201432598 and 201328587) and China Postdoctoral Science Foundation Funded Project (No. 2014M551596).
References [1] Z. Liu, Y. Sun, S. Peterson, P. Pianetta, Appl. Phys. Lett. 92 (2008) 241107. [2] L. Chen, Y.S. Qian, B.K. Chang, X.L. Chen, R. Yang, Opt. Commun. 285 (2012) 1264. [3] Y.J. Zhang, B.K. Chang, J. Niu, J. Zhao, J.J. Zou, F. Shi, Appl. Phys. Lett. 99 (2011) 101104. [4] W. Enloe, R. Scheldon, L. Reed, A. Amith, Proc. SPIE 1655 (1992) 41. [5] Y.Z. Liu, J.L. Moll, W.E. Spicer, Appl. Phys. Lett. 17 (1970) 60. [6] Y.Z. Liu, C.D. Hollish, W.W. Stein, J. Appl. Phys. 44 (1973) 56192. [7] D. Durek, F. Frommberger, T. Reichelt, M. Westermann, Appl. Surf. Sci. 143 (1999) 319. [8] Y.T. Cheng, Y.S. Huang, D.Y. Lin, F.H. Pollak, K.R. Evans, Physics E, 14, (2002) 2.
[9] A. Kenta, S. Takushi, Kakiuchi, Hiroaki, Morita, Mizuho, Physica B 376 (2006) 893. [10] H. Liu, L. Zhang, Q. Zeng, H. Ren, K. Guan, Q. Liu, L. Cheng, Solid State Commun. 151 (2011) 61. [11] V.L. Alperovich, A.G. Paulish, H.E. Scheibler, A.S. Terekhov, Appl. Phys. Lett. 66 (1995) 2122. [12] J.J. Zou, B.K. Chang, H.L. Chen, L. Liu, J. Appl. Phys. 101 (2007) 033121. [13] C.H. Chan, J.D. Wu, Y.S. Huang, H.P. Hsu, K.K. Tiong, Y.K. Su, Mater. Chem. Phys. 124 (2010) 2. [14] D.G. Fisher, IEEE Trans. Electron Devices ED-21 (1974) 541. [15] X.L. Chen, Y.J. Zhang, B.K. Chang, J. Zhao, M. Jin, G.H. Hao, Y. Xu, Opt. Commun. 287 (2013) 35. [16] D.E. Aspnes, S.M. Kelso, R.A. Logan, R. Bhat, J. Appl. Phys. 60 (1986) 754. [17] Y. Sun, R.E. Kirby, T. Maruyama, G.A. Mulhollan, J.C. Bierman, P. Pianetta, Appl. Phys. Lett. 95 (2009) 174109. [18] J. Niu, Y.J. Zhang, B.K. Chang, Y.J. Xiong, Chin. Phys. B 20 (2011) 044209. [19] L.W. James, G.A. Antypas, J. Edgecumbe, R.L. Moon, R.L. Bell, J. Appl. Phys. 42 (1971) 4976. [20] Y.J. Zhang, J. Niu, J.J. Zou, B.K. Chang, Y.J. Xiong, Appl. Opt. 49 (2010) 3935.
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Comparative research on reflection-mode GaAs photocathode with graded AlxGa1 xAs buffer layer Liang Chen a,b,n, Yang Shen a, Shuqin Zhang a, Yunsheng Qian b, Sunan Xu a 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
art ic l e i nf o
a b s t r a c t
Article history: Received 18 April 2015 Received in revised form 16 June 2015 Accepted 21 June 2015 Available online 24 June 2015
The graded Al compositional AlxGa1 xAs buffer layer can not only form continuous internal electric field from buffer layer to active layer but also optimize the interface properties by decreasing the misfit dislocations and stacking faults arising from lattice mismatch. By measuring the spectral response current (SRC) for two reflection-mode (r-mode) designed samples of graded and stationary Al compositional structure, we can find the special phenomenon that the graded structure had quite influence at the middle wavelength band from 550 nm to 850 nm, but not the short wavelength band from 400 nm to 550 nm, though the buffer layer can only absorb photon energy at the short wavelength band. Through the comparative research for designed samples through SPV before Cs–O activation and SRC after Cs–O activation, the graded structure can well optimize the key parameters such as LD, Ln, Sv and P. For the photon absorption lengths are relative little at the short wavelength band and relative long at the middle wavelength band, so the optimizations of key parameters have little influence on photo-excited electrons at the short wavelength band which are mainly excited from the region in active layer near surface barriers. The optimizations of key parameter, mainly the back interface recombination velocity (Sv), can have quite impact on photo-excited electrons at the middle short wavelength band which are mainly excited from the internal active layer near the back interface. This comparative research can help to well study the photo-emission theory and structure design on graded Al compositional design for r-mode GaAs photocathodes in the future research. & 2015 Published by Elsevier B.V.
OCIS codes: 160.2100 250.0250 Keywords: GaAs photocathode Surface photovoltage Spectral response current Surface escape probability AlxGa1 xAs buffer layer
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 low thermal emittance. The quantum efficiency of GaAs photocathodes mainly depends on the key parameters such as: the electron diffusion length (LD), the back interface recombination velocity (Sv) and the surface escape probability (P). In early research, the reflection-mode structure was epitaxial grown of GaAs substrate, AlGaAs buffer layer and GaAs active layer by metal-organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE) [1–4]. The stationary Al compositional and uniform doping AlGaAs buffer layer can form strong electric field at the AlGaAs/GaAs interface to reflect the photo-excited electrons arriving at the back interface in active n Corresponding author at: Institute of Optoelectronics Technology, China Jiliang University, 310018 Hangzhou, China. E-mail address: [email protected] (L. Chen).
http://dx.doi.org/10.1016/j.optcom.2015.06.053 0030-4018/& 2015 Published by Elsevier B.V.
layer and finally enhance the emitting quantum efficiency, so the AlGaAs buffer layer can finally well increase the key parameters for GaAs active layer including LD, Sv and P. In recent research, the graded Al compositional structure design was carried out in AlxGa1 xAs buffer layer, which can well increase the quantum efficiency and interface properties. But the influence of AlxGa1 xAs buffer layer has not been well studied. In this paper, in order to deeply study the impact of AlxGa1 xAs buffer layer for reflectionmode GaAs photocathodes, we carried out the comparative research between the surface photovoltage (SPV) before Cs–O activation and spectral response current (SRC) after Cs–O activation. The fitting equations and comparative research results can help to optimize the structure design and study the influence of graded Al compositional AlxGa1 xAs buffer layer in later research [2–6].
2. Experiments and results In order to carry out the comparative research, two reflectionmode GaAs photocathodes were grown on the high-quality n-type GaAs (100)-oriented substrate by MOCVD. The epitaxial layers
L. Chen et al. / Optics Communications 355 (2015) 186–190
both consist of a GaAs substrate, an AlGaAs buffer layer and a GaAs active layer. Both of the AlGaAs layer and the GaAs layer are p-type zinc (Zn)-doped concentration. The GaAs active layers of two samples have the same uniform doping concentration of 1 1018 cm 3 and the same thickness of 0.2 μm. The AlxGa1 xAs buffer layers of two samples both have the same doping concentration of 1 1018 cm 3 and the main differences for buffer layers are that the graded compositional sample has the graded Al composition from 0.6 to 0, while the standard sample has the stationary Al composition of 0.63. The structure diagrams for two designed samples are shown as Fig. 1 [4–8]. The absorption coefficient of AlxGa1 xAs buffer layer is shown in Fig. 2, and the surface photo voltage (SPV) is mainly depended on the electron–hole separation while absorbing photon energy. Thus in order to shield off the influence of AlxGa1 xAs buffer layer at the short wavelength band from 400 nm to 550 nm, the SPV curve was only measured form 550 nm to 950 nm, where the buffer layer can be seemed as fully window layer with no photon absorption [2–6]. The designed samples were firstly ultrasonically washed in acetone, hydrofluoric acid and absolute ethyl alcohol, in sequence, in order to wipe off the surface oxidation layer and impurities for measuring the SPV curves. The normalized SPV curves were measured by the measurement system designed by our own, and the SPV curves for active layers while shielding off the influences of the buffer layers and substrates are shown in Fig. 3 [2,3,8–10]. After measuring SPV curves, the designed samples were again through chemically etched in the hydrofluoric acid solution of 40% for 10 min and being rinsed by de-ionized water for drying. Finally the samples were transferred into the ultrahigh vacuum chamber (base pressure r1 10 9 Pa) for heating cleaning of 20 min at the temperature of 650 °C. After the chemical and heating cleaning, the samples can have the atomically clean crystal surface for Cs–O activation. The Cs–O activation was carried out according to the principle that the Cs source is continuous and the O source is discontinuous. After the high temperature Cs–O activation, the samples were again heated at the low temperature of 600 °C for same 20 min and carried out activation in the same way. During the activation process, the photocurrent was monitored under the illumination of the halogen tungsten lamp. After the Cs–O activation, the SRC curves for two samples were measured in situ by the on-line spectral response measurement system from 400 nm to 950 nm. The normalized measuring SRC curves for two designed samples are shown in Fig. 4 [11–13]. From Fig. 4, we can find the special phenomenon that the graded structure has quite influence at the middle wavelength band from 550 nm to 850 nm but not the short wavelength band from 400 nm to 550 nm, though the AlxGa1 xAs buffer layer can
GaAs active layer 0.2µ m, 1.0×1018cm-3
GaAs active layer 0.2µ m, 1.0×1018cm-3
AlxGa1-xAs buffer layer x=0.6~0 0.5µ m, 1.0×1018cm-3
AlxGa1-xAs window layer x=0.63 0.5µ m, 1.0×1018cm-3
n-GaAs (100) substrate
n-GaAs (100) substrate
Fig. 1. Structures for two designed reflection-mode GaAs photocathode module samples: (a) the graded compositional sample; (b) the standard sample.
187
Fig. 2. The absorption coefficient curve for AlxGa1 xAs buffer layer.
Fig. 3. The experimental SPV curves for two designed samples.
only absorb the photon energy at the short wavelength band. So in order to well study the influence of graded structure for r-mode photocathode, we carried out the comparative research by the SPV curve before activation and the SRC curve after activation.
3. Analyses and discussions For the graded Al compositional design in buffer layer can form internal electric field and decrease the band slope near the interface, thus the energy band structures for two designs can be shown in Fig. 5. When the GaAs active layer is relatively thin, the photon energy can not be sufficiently absorbed in active layer and subsequently absorbed in active layer, the photo-excited electrons in buffer layer with high doping concentration can diffuse into active layer and
Fig. 4. Experimental SRC curves for designed samples after activation.
188
L. Chen et al. / Optics Communications 355 (2015) 186–190
finally arrive at the surface barriers for emitting. Thus, the quantum efficiency for graded Al composition or stationary Al composition should both take the photo-excited electrons in buffer layer in consideration, while the active layer is relatively thin [2– 4,12–14]. According to diode theory, the connections for SPV and SRC with n(x) are shown as:
ΔV =
⎛ jw ⎞ KT dn (x) dn (x) | x = Te , J = P . Dn | x = Te ln ⎜1 + ⎟, Jw = Dn q c⎠ dx dx ⎝
(1)
where ΔV is surface photovoltage for GaAs photocathode, Jw is the photo-electron current flowing to surface barriers from the internal body, J is the spectral response current emitting into vacuum by NEA after Cs–O activation, and n(x) is the density of electron minority carrier [12–15]. The one-dimensional continuity equation for n(x) is shown as:
Dn
d2n (x) n (x) − + g (x) = 0 τ dx2
(2)
where τ is the lifetime of minority carrier, Dn is the diffusion coefficient of electron, and g(x) is the generation rate of photoexcited electrons. The generation rate of photo-excited electrons in GaAs active layer and AlxGa1 xAs buffer layer can be given by:
gGaAs (x) = (1 − R) I0 α1 exp ( − α1x); g Al x Ga1 − x As (x)
x ∈ [0, Te ] x ∈ ⎡⎣Te, Te + d⎤⎦
= (1 − R) I0 exp ( − α1Te ) α2 exp [−α2 (x − Te ) ]; where I0 is the incident light intensity,
(3)
α1 and α2 are the
absorption coefficient of GaAs active layer and AlxGa1 xAs buffer layer respectively, R is the surface reflectivity, Te is the thickness of the GaAs active layer, and d is the thickness of the AlxGa1 xAs buffer layer [12–15]. For solving Eqs. (1)–(3), the boundary conditions for GaAs active layer are simplified as:
n (x)| x = 0 =0, − Dn
dn (x) dx
x = Te−
= Sv ⎡⎣n (Te−) − n (Te+) ⎤⎦
(4)
And the boundary conditions for AlxGa1 xAs buffer layer are simplified as:
Dn
dn (x) dx
x = Te+
= Sv n (x) x = T + , n (x) x =∞ = 0 e
(5)
where n(Te ) represents the photo-excited electrons arriving at the back interface from active layer, n(Te þ ) represents the photoexcited electrons arriving at the front interface from the AlxGa1 xAs buffer layer [15–18]. Thus through deductions, the SRC equation for two designed structures taking consideration of the GaAs active layer and the AlxGa1 xAs buffer layer is given by [12–18]:
J=
P (1 − R) α1L D α12 L D2 − 1 ⎤ ⎡ (Sv − α1Dn ) exp ( − α1Te ) ⎥ ⎢ D L T L S T L / cosh / sinh / ( ) ( ) + ( ) e D v e D ⎥ ⎢ n D ⎢ S cosh (T /L ) + (D /L ) sinh (T /L ) ⎥ v e D n D e D ⎥ ⎢− ⎢ (Dn /L D ) cosh (Te/L D ) + Sv sinh (Te/L D ) ⎥ ⎥ ⎢+α L ⎦ ⎣ 1 D
(6)
The SPV equation for active layer taking no account of the AlxGa1 xAs buffer layer is given by:
⎤ ⎡ (Sv − α1Dn ) exp ( − α1Te ) ⎥ ⎢ D L T L S T L / cosh / sinh / ( ) ( ) + ( ) e D v e D ⎥ ⎢ n D KT (1 − R) α1L D ⎢ ΔV = Sv cosh (Te/L D ) + (Dn /L D ) sinh (Te/L D ) ⎥ 2 2 ⎥ qc (α1 L D − 1) ⎢− ⎢ (Dn /L D ) cosh (Te/L D ) + Sv sinh (Te/L D ) ⎥ ⎥ ⎢+ α L ⎦ ⎣ 1 D
Fig. 5. Energy band structures for (a) the standard structure design and (b) the graded compositional structure design. Ec is the conduction band minimum, Ev is the valence band maximum, Eg is the band gap, EF is the Fermi level, Evac is the vacuum level, and δs and ds are the height and width of the surface band-bending region (BBR).
(7)
where P is the surface escape probability for photo-excited electrons tunneling through surface barriers into vacuum, Ln is the electron diffusion length of AlxGa1 xAs buffer layer, and LD is the electron diffusion length of GaAs active layer. For r-mode GaAs photocathode, the reflection coefficient curve of cathode material was firstly measured before fitting calculations of SPV and SRC curves. So through the fitting calculations, the normalized experimental and fitting SPV curves for two designed samples are shown in Fig. 6. The key fitting parameters for the active layers of two designed cathode samples before Cs–O activation were well fitted by Eq. (7). Furthermore, we can also find that using the fitting parameters by SPV for active layers, the values of P and Ln will be easily fitted. The final fitting parameters for GaAs active layer and AlxGa1 xAs buffer layer are shown in Table 1. As we know, the misfit dislocations and stacking faults arising from lattice mismatch at the back interface will primarily affect the interface properties, such as Sv and finally affect the electron diffusion length. So from Table 1, we can find that the graded Al compositional structure can well increase the values of LD and Ln, especially well decrease the value of Sv from 1.1 109 μm/s to 1.2 1010 μm/s. Thus the grade structure can not only form internal electric field to increase electron diffusion length, but also optimize the interface properties to further
L. Chen et al. / Optics Communications 355 (2015) 186–190
Fig. 6. Experimental and fitting SPV curves for designed samples. Table 1 Key fitting parameters for two designed samples. Sample
Dn (μm2/s)
Sv (μm/s)
LD (μm)
Ln (μm)
Graded Standard
1.2 1010 1.2 1010
1.1 109 1.2 1010
1.80 1.65
0.85 0.70
decrease the interface recombination velocity [12–16]. The normalized experimental and fitting SRC curves for two designed samples are shown in Fig. 7. Through the comparative fitting calculation between SPV and SRC curves, the normalized values of P along wavelength can be well fitted, which are shown in Fig. 8. From Fig. 8, we can find that the curves of P have little differences only at middle and long wavelength band. The reason is that for thin active layer, the stationary and graded Al compositional structure can also form strong electric field at the back interface and reflect photo-excited electrons to surface barriers for emitting [14–18]. From Fig. 7, we can find that though the buffer layer can only absorb photon energy at short wavelength band, the graded Al compositional structure have more influence on the middle wave band from 550 nm to 850 nm, while minor influence on the short wave band. The reason is that for short wavelength band, the absorption length of high-energy photons is shorter and the first absorption layer is active layer, so the thermalized electrons excited by short-wave photons are mainly generated from the region in active layer near surface and can easily tunnel through surface barriers with high energy after transferring short distance. Though the photo-excited electrons of AlxGa1 xAs buffer layer can have higher diffusion length Ln and shorter interface recombination velocity Sv for graded sample than standard sample, the increasing
189
Fig. 8. Surface escape probability curves for two designed samples.
electrons from buffer layer occupy very little part on the total short wavelength electrons from buffer layer and active layer. Thus the SRC values have little differences at short wave band for two samples for that the promotions of Ln, Sv have little influence on photo-excited electrons at short wave band. While for middle wavelength band, the absorption length of middle energy photons is relative long, so the thermalized electrons are mainly from the internal active layer. Thus more photo-excited electrons will arrive at the back interface, the variation of Sv has deep influence on the middle wavelength band. And the main improvement of graded structure for r-mode GaAs photocathode with thin active layer is the major optimization of Sv, which will further increase the final quantum efficiency at middle wavelength band [15–20].
4. Conclusions Through the comparative research between SPV before Cs–O activation and SRC after Cs–O activation for the AlxGa1 xAs buffer layer with graded or stationary Al compositional structure, the graded structure can well increase the quantum efficiency for the middle wavelength band from 550 nm to 850 nm than the short wavelength band from 400 nm to 550 nm, though the buffer layer can only absorb the photon energy at the short wavelength band. The graded structure can not only form continuous internal electric field from buffer layer to active layer, but also optimize the interface properties by decreasing the misfit dislocations and stacking faults arising from lattice mismatch. So the graded structure can well optimize the key parameters such as LD, Ln, Sv and P. For the absorption length of high-energy photon energy at the short wavelength band is relative little, so the graded structure has less influence on the photo-excited electrons at the short wavelength band which are mainly excited at the region near surface. But the graded structure can well increase the quantum efficiency at the middle wavelength band by the high optimization of Sv for that the major photo-excited electrons are excited near the back interface by relative long absorption length. This comparative research can help to well study the photo-emission theory and structure design for graded Al compositional design in the later research.
Acknowledgments
Fig. 7. Experimental and fitting SRC curves for two designed samples.
This work was supported by the National Natural Science Foundation of China under Grant (Nos. 61308089 and 61440065), Public Technology Applied Research Project of Zhejiang Province (No. 2013C31068), Science and Technology on Night Vision
190
L. Chen et al. / Optics Communications 355 (2015) 186–190
Laboratory Foundation (No. J20110105). Applied Research Project of Zhejiang Provincial Education Department (Nos. Y201432598 and 201328587) and China Postdoctoral Science Foundation Funded Project (No. 2014M551596).
References [1] Z. Liu, Y. Sun, S. Peterson, P. Pianetta, Appl. Phys. Lett. 92 (2008) 241107. [2] L. Chen, Y.S. Qian, B.K. Chang, X.L. Chen, R. Yang, Opt. Commun. 285 (2012) 1264. [3] Y.J. Zhang, B.K. Chang, J. Niu, J. Zhao, J.J. Zou, F. Shi, Appl. Phys. Lett. 99 (2011) 101104. [4] W. Enloe, R. Scheldon, L. Reed, A. Amith, Proc. SPIE 1655 (1992) 41. [5] Y.Z. Liu, J.L. Moll, W.E. Spicer, Appl. Phys. Lett. 17 (1970) 60. [6] Y.Z. Liu, C.D. Hollish, W.W. Stein, J. Appl. Phys. 44 (1973) 56192. [7] D. Durek, F. Frommberger, T. Reichelt, M. Westermann, Appl. Surf. Sci. 143 (1999) 319. [8] Y.T. Cheng, Y.S. Huang, D.Y. Lin, F.H. Pollak, K.R. Evans, Physics E, 14, (2002) 2.
[9] A. Kenta, S. Takushi, Kakiuchi, Hiroaki, Morita, Mizuho, Physica B 376 (2006) 893. [10] H. Liu, L. Zhang, Q. Zeng, H. Ren, K. Guan, Q. Liu, L. Cheng, Solid State Commun. 151 (2011) 61. [11] V.L. Alperovich, A.G. Paulish, H.E. Scheibler, A.S. Terekhov, Appl. Phys. Lett. 66 (1995) 2122. [12] J.J. Zou, B.K. Chang, H.L. Chen, L. Liu, J. Appl. Phys. 101 (2007) 033121. [13] C.H. Chan, J.D. Wu, Y.S. Huang, H.P. Hsu, K.K. Tiong, Y.K. Su, Mater. Chem. Phys. 124 (2010) 2. [14] D.G. Fisher, IEEE Trans. Electron Devices ED-21 (1974) 541. [15] X.L. Chen, Y.J. Zhang, B.K. Chang, J. Zhao, M. Jin, G.H. Hao, Y. Xu, Opt. Commun. 287 (2013) 35. [16] D.E. Aspnes, S.M. Kelso, R.A. Logan, R. Bhat, J. Appl. Phys. 60 (1986) 754. [17] Y. Sun, R.E. Kirby, T. Maruyama, G.A. Mulhollan, J.C. Bierman, P. Pianetta, Appl. Phys. Lett. 95 (2009) 174109. [18] J. Niu, Y.J. Zhang, B.K. Chang, Y.J. Xiong, Chin. Phys. B 20 (2011) 044209. [19] L.W. James, G.A. Antypas, J. Edgecumbe, R.L. Moon, R.L. Bell, J. Appl. Phys. 42 (1971) 4976. [20] Y.J. Zhang, J. Niu, J.J. Zou, B.K. Chang, Y.J. Xiong, Appl. Opt. 49 (2010) 3935.