- Email: [email protected]
Evaluation of chemical cleaning for Ga1−xAlxAs photocathode by spectral response
Optics Communications 309 (2013) 323–327
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Evaluation of chemical cleaning for Ga1 xAlxAs photocathode by spectral response Xinlong Chen, Benkang Chang n, Jing Zhao, Guanghui Hao, Muchun Jin, Yuan Xu School of Electronic and Optical Engineering, Nanjing University of Science and Technology, Nanjing 210094, China
art ic l e i nf o
a b s t r a c t
Article history: Received 8 May 2013 Received in revised form 15 July 2013 Accepted 4 August 2013 Available online 19 August 2013
The spectral response has been used to evaluate the chemical cleaning for Ga1 xAlxAs photocathode by an on-line spectral response measurement system. The spectral response curves of Ga1 xAlxAs photocathodes treated by different chemical cleaning methods are measured and analyzed in detail. We use the quantum efficiency formulas to fit the experimental curves transforming from the spectral response curves, and obtain the related performance parameters such as the surface electron escape probability, the back-interface recombination velocity, the electron diffusion length, and the thickness of the etching GaAs layer. The results show that the GaAs photocathode cleaned by the HF solution could obtain a good photoemission effect, while the Ga0.37Al0.63As photocathode could be well cleaned by the solution of sulfuric acid and hydrogen peroxide. & 2013 Elsevier B.V. All rights reserved.
Keywords: Spectral response Chemical cleaning Ga1 xAlxAs photocathode Quantum efficiency
1. Introduction Negative electron affinity (NEA) GaAs photocathodes have been widely used in various optoelectronic devices, including night vision detection, electron sources for future light sources and high-energy nuclear physics because of high electron average current and low thermal emittance [1–4]. As a special photocathode, the NEA GaAlAs photocathode is a potential photocathode for the next-generation electron accelerators and the marine detection due to the long lifetime and the controlled threshold wavelength in comparison with the conventional GaAs photocathode [5,6]. The performance of the devices depends significantly on having an atomically clean surface. The preparation process of NEA photocathodes mainly includes three steps: chemical cleaning, annealing, and (Cs, O) activation [7]. The surface of the grown photocathode sample is often contaminated by the grease and oxides, so the chemical cleaning process, which offers an effective and practical method for cleaning the surface of semiconductor, is a critical step in preparing a NEA semiconductor photocathode [8–10]. Most etchants used to prepare GaAs photocathode contain an oxidizing agent such as Br2 and H2O2, and a complexing agent such as NH4OH, H2SO4 and H3PO4. Various surface analytical techniques has been used in the evaluation of chemical cleaning, such as auger electron spectroscopy (AES), x-ray photoelectron spectroscopy (XPS), and low-energy electron spectroscopy (LEED)[11]. However, there are no reports about the
n
Corresponding author. Tel.: +86 02584315177. E-mail address: [email protected] (B. Chang).
0030-4018/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optcom.2013.08.006
evaluation of chemical cleaning for Ga1 xAlxAs photocathodes with different Al composition by spectral response. Spectral response is generally used to study the photoemission characteristics of NEA photocathode [12]. In this work, we use the spectral response to evaluate the chemical cleaning for Ga1 xAlxAs photocathodes. The GaAs photocathode and Ga0.37Al0.63As photocathode samples are activated by cesium and oxygen after chemical cleaning and annealing, and the spectral response curves of the photocathodes are measured and analyzed. The detailed performance parameters of reflection-mode Ga1 xAlxAs photocathodes are obtained by fitting the quantum efficiency formulas. 2. Experiments In order to study the chemical cleaning methods of Ga1 xAlxAs photocathodes, we designed two reflection-mode photocathode samples with different epitaxial layers. Both of the 2-inchdiameter samples were grown on the high quality n-type GaAs (1 0 0) substrate by metal organic chemical vapor deposition (MOCVD), the epitaxial layers of the samples were all the p-type zinc (Zn)-doped. The epitaxial layers of one sample consisted of a Ga0.37Al0.63As buffer layer and a GaAs emission layer, as shown in Fig. 1(a). The epitaxial layers of another sample consisted of a Ga0.21Al0.79As buffer layer, a Ga0.37Al0.63As emission layer, and a GaAs protection layer, as shown in Fig. 1(b). The GaAs emission layer of the sample in Fig. 1(a) and the Ga0.37Al0.63As emission layer of the sample in Fig. 1(b) are both exponential doping ranging from 1019 cm 3 to 1018 cm 3, which could increase the photoemission capacity of the Ga1 xAlxAs photocathodes [13]. In an exponential
324
X. Chen et al. / Optics Communications 309 (2013) 323–327
Fig. 1. Structure diagram of two Ga1 xAlxAs photocathode samples.
doping Ga1 xAlxAs photocathode, a downward band bending region from the Ga1 xAlxAs bulk to the surface is formed by the Fermi-level leveling effect. The electric field in the band bending region could improve photoelectron movement toward the surface, and therefore the photoelectrons reaching the surface would be increased. The 0.1 μm-thick GaAs protection layer of the sample in Fig. 1(b) could prevent Ga0.37Al0.63As emission layer from oxidizing in the air. The as-grown samples were cut into some 10 10 mm ones. One photocathode sample as Fig. 1(a) and six photocathode samples as Fig. 1(b) were studied in our experiments, which were named from Sample 1 to Sample 7 in sequence. First, they were all cleaned by carbon tetrachloride, acetone, and absolute ethyl alcohol orderly. Then, Sample 1 and Sample 2 were cleaned by a HF solution for 10 min. The Sample 3, Sample 4, Sample 5 and Sample 6 were respectively cleaned by a H2SO4(98%): H2O2(30%): H2O solution with the ratio of 4:1:100 for 60 s, 90 s, 120 s, 150 s. The Sample 7 was cleaned by two steps including a H2SO4(98%): H2O2(30%): H2O solution with the ratio of 4:1:100 for 150 s and a HF solution for 10 min. After chemical cleaning, the samples were transferred into an ultra-high vacuum (UHV) chamber with a base pressure less than 10 7 Pa and heated for removing the surface contamination such as carbon and oxides. The heat cleaning was performed on the backside of samples by the thermal irradiation from a halogen tungsten filament lamp, and the temperature was measured by a thermocouple. The samples were heated at the temperature about 650 1C for 20 min. When the samples cooled down to room temperature, the same (Cs, O) activation for the samples were performed in the UHV chamber, in which the cesium source was kept continuous and the oxygen source was introduced periodically [14,15]. During the activation process, the photocurrent was monitored when the photocathode was illuminated by a halogen tungsten lamp. The Cs source used in the activation is in the form of a mixture of cesium chromate and zirconium–aluminum alloy powder in a thin-walled nickel tube and the O source is in the form of barium peroxide in a nickel tube. The Cs or O is released into the activation chamber by heating the nickel tube, and the Cs or O flux is accurately controlled by regulating the heating current. The photocurrent of the photocathode were controlled and monitored by a self-developed multiinformation measurement system [16,17]. After the activation, the spectral response curves of the photocathodes were measured in situ by the on-line spectral response
Fig. 2. Schematic of the on-line spectral response measurement system.
measurement system, as shown in Fig. 2. The light source generated by a DC power supply of 12 V and a 100 W halogen tungsten lamp is focused and modulated into a monochromatic light by the grating monochromator. Then the monochromatic light is transmitted by an optical fiber to illuminate the flange window of the UHV activation chamber, and irradiate on the photocathode surface. The generated weak photocurrent signal is transferred to the signal process module by a lead-out line. The weak photocurrent signal is amplified and converted into a digital signal by the analog-to-digital converter. Finally, the digital signal is fed back to the computer, which is in charge of processing the spectral response data.
3. Results and analysis After the (Cs, O) activation, the spectral response curves of all photocathodes are measured. The spectral response curves of Sample 1 and Sample 2 are shown in Fig. 3. As we can see, the spectral responses of the two photocathodes at the shortwavelength region are similar, but the spectral responses show an obvious difference at the middle-wavelength and longwavelength regions. The cut-off wavelength of Sample 2 is close to that of Sample 1, which shows that the GaAs protection layer of Sample 2 has not been removed by the HF solution. We can also find that the photocathode has a large spectral response at the short-wavelength region though the GaAs protection layer is thin enough. The shorter cut-off wavelength of Sample 2 shows that the photoemission induced by the long-wavelength light is
X. Chen et al. / Optics Communications 309 (2013) 323–327
325
Fig. 5. Spectral response curves of Sample 5, Sample 6, and Sample 7. Fig. 3. Spectral response curves of Sample 1 and Sample 2.
Fig. 4. Spectral response curves of Sample 2, Sample 3, and Sample 4.
unconspicuous because of low absorption in the GaAs layer. The results also prove that a thicker emission layer could adequately absorb the visible light for GaAs photocathode, especially the longwavelength light, which could enhance the photoemission capacity of GaAs photocathode with the large enough electron diffusion length [18]. The above experimental results show that the HF solution could not etch the GaAs layer. In order to remove the GaAs layer, a H2SO4(98%), H2O2(30%), and H2O solution was used. The spectral response curves of Sample 3 and Sample 4 are measured after activation, as shown in Fig. 4. As can be clearly seen, the spectral response curves of Sample 3 and Sample 4 are lower than that of Sample 2, which shows that the solution of sulfuric acid and hydrogen peroxide could etch the GaAs protection layer, and the reaction equation is given by [19] GaAs+12H2O2+3H+ ¼Ga2(HAsO4)3↓+12H2O+Ga3+
(1)
Ga2(HAsO4)3+6H+-2Ga3++3H3AsO4
(2)
However, the cut-off wavelengths of Sample 3 and Sample 4 are close to that of Sample 2, which indicates that the GaAs protection layers of the two samples are not removed fully. In Fig. 4, it could be also found that the three photocathodes all have a larger spectral response at the short-wavelength region, where the three spectral response curves have a little difference because the shortwavelength light has a small absorption length compared with the
long-wavelength light. The thin GaAs layer cannot absorb the long-wavelength light adequately. The spectral response curves of Sample 5, Sample 6, and Sample 7 are shown in Fig. 5. The cut-off wavelengths of the three spectral response curves all appear at about 570 nm which corresponds to the direct bandgap of Ga0.37Al0.63As [20]. The experimental results show that the 0.1 μm-thick GaAs protection layer could be etched fully in the H2SO4(98%): H2O2(30%): H2O solution with the ratio of 4:1:100 for 120 s. The spectral response curve of Sample 6 etched for 150 s is similar to that of Sample 5, which shows that a slightly longer time scarcely influences on the Ga0.37Al0.63As surface. The spectral response curve of Sample 7 is obviously lower than that of Sample 5 and Sample 6, which demonstrates that the Ga0.37Al0.63As could be etched by HF solution. The actual reaction equation of Ga1 xAlxAs etched by the HF solution is still unknown, but some studies about this has been reported, the Ga1 xAlxAs with a higher Al composition may be etched faster [21]. From the above, the GaAs photocathode cleaned by the HF solution could obtain a good photoemission effect, while the HF solution etches the Ga0.37Al0.63As. The solution of sulfuric acid and hydrogen peroxide could etch the GaAs protection layer, but has less effect on the photoemission capacity of Ga0.37Al0.63As photocathode.
4. Discussion The band structures of the exponential-doping reflection-mode Ga1 xAlxAs photocathode and the uniform-doping reflectionmode GaAs photocathode with thin GaAs emission layer are shown in Fig. 6. Fig. 4 indeed shows the spectral response curves of the GaAs photocathodes with thin GaAs emission layer, the GaAs protection layer in Fig. 1(b) acts a role of emission layer in these photocathodes. The photoemission process for the normal reflection-mode Ga1 xAlxAs photocathode follows the three-step photoemission model [22]. But for the reflection-mode GaAs photocathode with thin GaAs emission layer, the GaAlAs layer could act the role of emission layer and join the photoemission [23]. However, the emission layers of photocathodes absorb the photons and generate the photoelectrons which diffuse to the surface and emit into vacuum after tunneling the surface barrier (Cs–O layer). Quantum efficiency is the most important characteristic parameter to evaluate the performance of photocathodes. The quantum efficiency curves can be obtained by transforming the spectral
326
X. Chen et al. / Optics Communications 309 (2013) 323–327
Fig. 7. Experimental (solid lines) and fitting (dashed lines) quantum efficiency curves of reflection-mode Ga1 xAlxAs photocathodes. Curves 1 7 are corresponding to Samples 1 7, respectively.
Fig. 6. Band structures of the (a) exponential-doping reflection-mode Ga1 xAlxAs photocathode and (b) uniform-doping reflection-mode GaAs photocathode with thin GaAs emission layer. Ec is the conduction band minimum, Ev is the valence band maximum, EF is the Fermi level, Evac is the vacuum level, and Te is the thickness of emission layer.
response curves according to the following equation [18] YðhυÞ 1:24Sλ =λ
ð3Þ
where Y(hv) is the quantum efficiency, 1.24 is an empirical value, and Sλ is the spectral response value of the corresponding wavelength λ. The quantum efficiency curves of the reflection-mode Ga1 xAlxAs photocathodes are shown in Fig. 7, which are corresponding to the spectral response curves. The quantum efficiency formula of the exponential-doping reflection-mode Ga1 xAlxAs photocathodes could be obtained by solving one-dimensional continuity equations with the given boundary conditions, which is given by [6] h i 2 αhv LD Q ð4Þ Y RE ¼ Pð1RÞ NðSαhv Dn ÞexpðLME T e =2LD αhv T e ÞM þ αhv LD 2 2 αhv LD αhv LE 1
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi N ¼ L2E þ 4L2D S ¼ μjEj þ Sv LE ¼ μjEjτ ¼
qjEj 2 LD k0 T
M ¼ ðNDn =LD Þ cos hðNT e =2LD 2 Þ þ ð2SLD Dn LE =LD Þ sin hðNT e =2LD 2 Þ Q ¼ SN cos hðNT e =2LD 2 Þ þ ðSLE þ 2Dn Þ sin hðNT e =2LD 2 Þ where P is the surface electron escape probability, R is the reflectivity on the surface of photocathode, αhv is the absorption coefficient of Ga1 xAlxAs emission layer, LD is the electron
diffusion length in the Ga1 xAlxAs emission layer, Sv is the backinterface recombination velocity, Te is the thickness of the Ga1 xAlxAs emission layer, Dn is the diffusion coefficient of electron, τ is the minority carrier lifetime, and LE is the electron drift length under the built-in electric field E. The average distance of photoelectrons moving toward the surface in the exponentialdoping Ga1 xAlxAs layer is defined the electron diffusion and drift length LDE qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 LDE ¼ ð LE 2 þ 4LD 2 þ LE Þ ð5Þ 2 kB T lnðN 0 =Ns Þ E¼ q Te
ð6Þ
where kB is the Boltzmann constant, T is the absolute temperature, q is the electron charge, N0 is the doping concentration at the back-interface of the Ga1 xAlxAs emission layer, and Ns is the doping concentration at the surface. The quantum efficiency formula for the reflection-mode GaAs photocathode with thin GaAs emission layer could be solved by considering the photoemission of GaAlAs emission layer, which is given by [23] Pð1RÞα1 LD ðSv α1 Dn Þexpðα1 TÞ YR ¼ ðDn =LD Þ cos hðT e =LD Þ þ Sv sin hðT e =LD Þ α1 2 LD 2 1 Sv cos hðT e =LD Þ þ ðDn =LD Þ sin hðT e =LD Þ þ α 1 LD ðDn =LD Þ cos hðT e =LD Þ þ Sv sin hðT e =LD Þ Pð1RÞSv Ln α2 expðα1 T e Þ 1 þ U ðDn =LD Þ cos hðT e =LD Þ þ Sv sin hðT e =LD Þ ð1 þ α2 Ln Þð1 þ Sv Ln =Dn Þ ð7Þ where Ln is the electron diffusion length in the GaAlAs emission layer, and LD is that in the GaAs emission layer, α1 is the absorption coefficient of GaAs, α2 is the absorption coefficient of GaAlAs, and some other symbols are the same as that in Eq. (4). Though the GaAlAs emission layer in the derivation process of Eq. (7) is uniform doping, but we think Eq. (7) is also suitable for the quantum efficiency in this study. The quantum efficiency curves of Sample 1, Sample 5, and Sample 6 in Fig. 7 could be respectively fitted by Eq. (4), while the quantum efficiency curves of Sample 2, Sample 3, and Sample 4 could be respectively fitted by Eq. (7). The fitting quantum efficiency curves are shown in Fig. 7, and the fitted performance parameters for the photocathode samples are listed in Table 1. The Ga0.37Al0.63As surface could be damaged by the HF solution, which
X. Chen et al. / Optics Communications 309 (2013) 323–327
Table 1 Fitted performance parameters for photocathode samples. Sample
Sv/(cm/s)
P
LD/μm
LDE/μm
Te/μm
1 2 3 4 5 6
2 106 1 106 1 106 1 106 1 106 1 106
0.59 0.60 0.60 0.60 0.40 0.39
– 2.0 2.0 2.0 – –
3.3 – – – 0.8 0.78
2.0 0.1 0.054 0.025 1.2 1.2
directly affects the surface electron escape probability of the photocathode. Besides, the thickness of Ga0.37Al0.63As layer etched by the HF solution is unknown. So, the performance parameters of Sample 7 are hard to be fitted and not given here. As we can see from Table 1, the back-interface recombination velocity Sv of these photocathodes are similar, which mainly depends on the growth quality of the interface. The electron diffusion and drift length LDE of Ga0.37Al0.63As photocathode is obviously smaller than that of GaAs photocathode, which may be caused by Al composition. According to the quantum efficiency curves in Fig. 7, we can conclude that the prepared GaAs photocathodes may have similar NEA surfaces because of the approximate quantum efficiency at the high-energy region. In Table 1, it could be found that the surface electron escape probability P of Sample 3 and Sample 4 are equal to that of Sample 2 though the chemical cleaning methods are different, which shows that the cleaning effects of the H2SO4(98%): H2O2(30%): H2O solution with the ratio of 4:1:100 and the HF solution on the surface of GaAs photocathodes are almost the same. Besides, the thickness of GaAs layer for Sample 3 and Sample 4 could be fitted by the quantum efficiency formula, respectively. In Fig. 7, the quantum efficiency curves of GaAs photocathodes are obviously higher than that of Ga0.37Al0.63As photocathodes, and the fitted surface electron escape probability of GaAs photocathodes are larger than that of Ga0.37Al0.63As photocathodes. The lower surface electron escape probability of Ga0.37Al0.63As photocathode indicates that the formative NEA surface is worse than that of GaAs photocathode, this phenomenon is corresponding to the study of Martinelli and Ettenberg, [24] which could be due to the difficult operation of removing the surface oxides from the Ga1 xAlxAs with high Al composition. 5. Conclusion In this paper, the spectral response is used to evaluate chemical cleaning for the Ga1 xAlxAs photocathodes. Two kinds of the reflection-mode Ga1 xAlxAs photocathode samples are cleaned by the H2SO4(98%): H2O2(30%): H2O solution with the ratio of 4:1:100 and the HF solution, respectively. The spectral response curves of different photocathodes are measured after the Cs/O activation by the on-line spectral response measurement system, and detailed analysis is performed on the spectral response curves. The quantum efficiency formulas are used to fit the quantum
327
efficiency curves which are transformed from the spectral response curves, and some related performance parameters including the thickness of the etching GaAs layer for the Ga1 xAlxAs photocathodes are given. The results show that the GaAs photocathode cleaned by the HF solution could obtain a good photoemission effect, while the HF solution is not suitable for the Ga0.37Al0.63As photocathode. The solution of sulfuric acid and hydrogen peroxide could etch the GaAs layer, but has less effect on the Ga0.37Al0.63As layer. The evaluation method of spectral response could be applied to the preparation of Ga1 xAlxAs photocathodes, even extend to other photocathodes.
Acknowledgements This work is supported by the National Natural Science Foundation of China (grants 61171042 and 61301023), the Research and Innovation Plan for Graduate Students of Jiangsu Higher Education Institutions, China (grant CXZZ12_0193). References [1] S. Karkare, D. Dimitrov, W. Schaff, L. Cultrera, A. Bartnik, X.H. Liu, E. Sawyer, T. Esposito, I. Bazarov, Journal of Applied Physics 113 (2013) 104904. [2] L. Chen, Y.S. Qian, B.K. Chang, X.L. Chen, R. Yang, Optics Communications 285 (2012) 1264. [3] T. Rao, A. Burrill, X.Y. Chang, J. Smedley, T. Nishitani, C.H. Garcia, M. Poelker, E. Seddon, F.E. Hannon, C.K. Sinclair, J. Lewellen, D. Feldmang, Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 557 (2006) 124. [4] I.V. Bazarov, D.G. Ouzounov, B.M. Dunham, S.A. Belomestnykh, Y.L. Li, X.H. Liu, R.E. Meller, J. Sikora, C.K. Sinclair, Physical Review Special Topics—Accelerators and Beams 11 (2008) 040702. [5] T. Nishitani, M. Tabuchi, Y. Takeda, Y. Suzuki, K. Motoki, T. Meguro, Japanese Journal of Applied Physics 48 (2009) 06FF02. [6] X.L. Chen, J. Zhao, B.K. Chang, M.C. Jin, G.H. Hao, Y. Xu, Proceedings of SPIE, 8555, 85550 R. [7] J.J. Zou, B.K. Chang, H. Wang, Z. Yang, P. Gao, Proceedings of SPIE, 6352, 635239. [8] D. Kikuchi, Y. Matsui, S. Adachi, Journal of the Electrochemical Society. 147 (2000) 1973. [9] D. Kikuchi, S. Adachi, Materials Science and Engineering: B 76 (2000) 133. [10] A. Guivarch, H. Lharidon, G. Pelous, G. Hollinger, P. Pertosa, Journal of Applied Physics 55 (1984) 1139. [11] Z. Liu, Y. Sun, F. Machuca, P. Pianetta, W.E. Spicer, R.F.W. Pease, Journal of Vacuum Science & Technology A 21 (2003) 212. [12] Y.J. Zhang, J. Niu, J.J. Zou, B.K. Chang, Y.J. Xiong, Applied Optics 49 (2010) 3935. [13] Y.J. Zhang, J. Niu, J. Zhao, J.J. Zou, B.K. Chang, F. Shi, H.C. Cheng, Journal of Applied Physics 108 (2010) 093108. [14] X.Q. Du, B.K. Chang, Applied Surface Science 251 (2005) 267. [15] B.J. Stocker, Surface Science 47 (1975) 501. [16] B.K. Chang, X.Q. Du, L. Liu, Z.Y. Zong, R.G. Fu, Y.S. Qian, Proceedings of SPIE, 5209, 209. [17] Y.S. Qian, Z.Y. Zong, B.K. Chang, Proceedings of SPIE, 4580, 486. [18] J.J. Zou, B.K. Chang, Optical Engineering 45 (2006) 054001. [19] Z. Liu, Y. Sun, F. Machuca, P. Pianetta, W.E. Spicer, R.F.W. Pease, Journal of Vacuum Science and Technology B 21 (2003) 1953. [20] S. Adachi, Journal of Applied Physics 58 (1985) R1. [21] M.M.A.J. Voncken, J.J. Schermer, G. Maduro, G.J. Bauhuis, P. Mulder, P.K. Larsen, Materials Science and Engineering B 95 (2002) 242. [22] W.E. Spicer, A. Herrera-Gómez, Proceedings of SPIE, 2022, 18. [23] X.L. Chen, Y.J. Zhang, B.K. Chang, J. Zhao, M.C. Jin, G.H. Hao, Y. Xu, Optics Communication 287 (2013) 35. [24] R.U. Martinelli, M. Ettenberg, Journal of Applied Physics 45 (1974) 3896.
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Evaluation of chemical cleaning for Ga1 xAlxAs photocathode by spectral response Xinlong Chen, Benkang Chang n, Jing Zhao, Guanghui Hao, Muchun Jin, Yuan Xu School of Electronic and Optical Engineering, Nanjing University of Science and Technology, Nanjing 210094, China
art ic l e i nf o
a b s t r a c t
Article history: Received 8 May 2013 Received in revised form 15 July 2013 Accepted 4 August 2013 Available online 19 August 2013
The spectral response has been used to evaluate the chemical cleaning for Ga1 xAlxAs photocathode by an on-line spectral response measurement system. The spectral response curves of Ga1 xAlxAs photocathodes treated by different chemical cleaning methods are measured and analyzed in detail. We use the quantum efficiency formulas to fit the experimental curves transforming from the spectral response curves, and obtain the related performance parameters such as the surface electron escape probability, the back-interface recombination velocity, the electron diffusion length, and the thickness of the etching GaAs layer. The results show that the GaAs photocathode cleaned by the HF solution could obtain a good photoemission effect, while the Ga0.37Al0.63As photocathode could be well cleaned by the solution of sulfuric acid and hydrogen peroxide. & 2013 Elsevier B.V. All rights reserved.
Keywords: Spectral response Chemical cleaning Ga1 xAlxAs photocathode Quantum efficiency
1. Introduction Negative electron affinity (NEA) GaAs photocathodes have been widely used in various optoelectronic devices, including night vision detection, electron sources for future light sources and high-energy nuclear physics because of high electron average current and low thermal emittance [1–4]. As a special photocathode, the NEA GaAlAs photocathode is a potential photocathode for the next-generation electron accelerators and the marine detection due to the long lifetime and the controlled threshold wavelength in comparison with the conventional GaAs photocathode [5,6]. The performance of the devices depends significantly on having an atomically clean surface. The preparation process of NEA photocathodes mainly includes three steps: chemical cleaning, annealing, and (Cs, O) activation [7]. The surface of the grown photocathode sample is often contaminated by the grease and oxides, so the chemical cleaning process, which offers an effective and practical method for cleaning the surface of semiconductor, is a critical step in preparing a NEA semiconductor photocathode [8–10]. Most etchants used to prepare GaAs photocathode contain an oxidizing agent such as Br2 and H2O2, and a complexing agent such as NH4OH, H2SO4 and H3PO4. Various surface analytical techniques has been used in the evaluation of chemical cleaning, such as auger electron spectroscopy (AES), x-ray photoelectron spectroscopy (XPS), and low-energy electron spectroscopy (LEED)[11]. However, there are no reports about the
n
Corresponding author. Tel.: +86 02584315177. E-mail address: [email protected] (B. Chang).
0030-4018/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optcom.2013.08.006
evaluation of chemical cleaning for Ga1 xAlxAs photocathodes with different Al composition by spectral response. Spectral response is generally used to study the photoemission characteristics of NEA photocathode [12]. In this work, we use the spectral response to evaluate the chemical cleaning for Ga1 xAlxAs photocathodes. The GaAs photocathode and Ga0.37Al0.63As photocathode samples are activated by cesium and oxygen after chemical cleaning and annealing, and the spectral response curves of the photocathodes are measured and analyzed. The detailed performance parameters of reflection-mode Ga1 xAlxAs photocathodes are obtained by fitting the quantum efficiency formulas. 2. Experiments In order to study the chemical cleaning methods of Ga1 xAlxAs photocathodes, we designed two reflection-mode photocathode samples with different epitaxial layers. Both of the 2-inchdiameter samples were grown on the high quality n-type GaAs (1 0 0) substrate by metal organic chemical vapor deposition (MOCVD), the epitaxial layers of the samples were all the p-type zinc (Zn)-doped. The epitaxial layers of one sample consisted of a Ga0.37Al0.63As buffer layer and a GaAs emission layer, as shown in Fig. 1(a). The epitaxial layers of another sample consisted of a Ga0.21Al0.79As buffer layer, a Ga0.37Al0.63As emission layer, and a GaAs protection layer, as shown in Fig. 1(b). The GaAs emission layer of the sample in Fig. 1(a) and the Ga0.37Al0.63As emission layer of the sample in Fig. 1(b) are both exponential doping ranging from 1019 cm 3 to 1018 cm 3, which could increase the photoemission capacity of the Ga1 xAlxAs photocathodes [13]. In an exponential
324
X. Chen et al. / Optics Communications 309 (2013) 323–327
Fig. 1. Structure diagram of two Ga1 xAlxAs photocathode samples.
doping Ga1 xAlxAs photocathode, a downward band bending region from the Ga1 xAlxAs bulk to the surface is formed by the Fermi-level leveling effect. The electric field in the band bending region could improve photoelectron movement toward the surface, and therefore the photoelectrons reaching the surface would be increased. The 0.1 μm-thick GaAs protection layer of the sample in Fig. 1(b) could prevent Ga0.37Al0.63As emission layer from oxidizing in the air. The as-grown samples were cut into some 10 10 mm ones. One photocathode sample as Fig. 1(a) and six photocathode samples as Fig. 1(b) were studied in our experiments, which were named from Sample 1 to Sample 7 in sequence. First, they were all cleaned by carbon tetrachloride, acetone, and absolute ethyl alcohol orderly. Then, Sample 1 and Sample 2 were cleaned by a HF solution for 10 min. The Sample 3, Sample 4, Sample 5 and Sample 6 were respectively cleaned by a H2SO4(98%): H2O2(30%): H2O solution with the ratio of 4:1:100 for 60 s, 90 s, 120 s, 150 s. The Sample 7 was cleaned by two steps including a H2SO4(98%): H2O2(30%): H2O solution with the ratio of 4:1:100 for 150 s and a HF solution for 10 min. After chemical cleaning, the samples were transferred into an ultra-high vacuum (UHV) chamber with a base pressure less than 10 7 Pa and heated for removing the surface contamination such as carbon and oxides. The heat cleaning was performed on the backside of samples by the thermal irradiation from a halogen tungsten filament lamp, and the temperature was measured by a thermocouple. The samples were heated at the temperature about 650 1C for 20 min. When the samples cooled down to room temperature, the same (Cs, O) activation for the samples were performed in the UHV chamber, in which the cesium source was kept continuous and the oxygen source was introduced periodically [14,15]. During the activation process, the photocurrent was monitored when the photocathode was illuminated by a halogen tungsten lamp. The Cs source used in the activation is in the form of a mixture of cesium chromate and zirconium–aluminum alloy powder in a thin-walled nickel tube and the O source is in the form of barium peroxide in a nickel tube. The Cs or O is released into the activation chamber by heating the nickel tube, and the Cs or O flux is accurately controlled by regulating the heating current. The photocurrent of the photocathode were controlled and monitored by a self-developed multiinformation measurement system [16,17]. After the activation, the spectral response curves of the photocathodes were measured in situ by the on-line spectral response
Fig. 2. Schematic of the on-line spectral response measurement system.
measurement system, as shown in Fig. 2. The light source generated by a DC power supply of 12 V and a 100 W halogen tungsten lamp is focused and modulated into a monochromatic light by the grating monochromator. Then the monochromatic light is transmitted by an optical fiber to illuminate the flange window of the UHV activation chamber, and irradiate on the photocathode surface. The generated weak photocurrent signal is transferred to the signal process module by a lead-out line. The weak photocurrent signal is amplified and converted into a digital signal by the analog-to-digital converter. Finally, the digital signal is fed back to the computer, which is in charge of processing the spectral response data.
3. Results and analysis After the (Cs, O) activation, the spectral response curves of all photocathodes are measured. The spectral response curves of Sample 1 and Sample 2 are shown in Fig. 3. As we can see, the spectral responses of the two photocathodes at the shortwavelength region are similar, but the spectral responses show an obvious difference at the middle-wavelength and longwavelength regions. The cut-off wavelength of Sample 2 is close to that of Sample 1, which shows that the GaAs protection layer of Sample 2 has not been removed by the HF solution. We can also find that the photocathode has a large spectral response at the short-wavelength region though the GaAs protection layer is thin enough. The shorter cut-off wavelength of Sample 2 shows that the photoemission induced by the long-wavelength light is
X. Chen et al. / Optics Communications 309 (2013) 323–327
325
Fig. 5. Spectral response curves of Sample 5, Sample 6, and Sample 7. Fig. 3. Spectral response curves of Sample 1 and Sample 2.
Fig. 4. Spectral response curves of Sample 2, Sample 3, and Sample 4.
unconspicuous because of low absorption in the GaAs layer. The results also prove that a thicker emission layer could adequately absorb the visible light for GaAs photocathode, especially the longwavelength light, which could enhance the photoemission capacity of GaAs photocathode with the large enough electron diffusion length [18]. The above experimental results show that the HF solution could not etch the GaAs layer. In order to remove the GaAs layer, a H2SO4(98%), H2O2(30%), and H2O solution was used. The spectral response curves of Sample 3 and Sample 4 are measured after activation, as shown in Fig. 4. As can be clearly seen, the spectral response curves of Sample 3 and Sample 4 are lower than that of Sample 2, which shows that the solution of sulfuric acid and hydrogen peroxide could etch the GaAs protection layer, and the reaction equation is given by [19] GaAs+12H2O2+3H+ ¼Ga2(HAsO4)3↓+12H2O+Ga3+
(1)
Ga2(HAsO4)3+6H+-2Ga3++3H3AsO4
(2)
However, the cut-off wavelengths of Sample 3 and Sample 4 are close to that of Sample 2, which indicates that the GaAs protection layers of the two samples are not removed fully. In Fig. 4, it could be also found that the three photocathodes all have a larger spectral response at the short-wavelength region, where the three spectral response curves have a little difference because the shortwavelength light has a small absorption length compared with the
long-wavelength light. The thin GaAs layer cannot absorb the long-wavelength light adequately. The spectral response curves of Sample 5, Sample 6, and Sample 7 are shown in Fig. 5. The cut-off wavelengths of the three spectral response curves all appear at about 570 nm which corresponds to the direct bandgap of Ga0.37Al0.63As [20]. The experimental results show that the 0.1 μm-thick GaAs protection layer could be etched fully in the H2SO4(98%): H2O2(30%): H2O solution with the ratio of 4:1:100 for 120 s. The spectral response curve of Sample 6 etched for 150 s is similar to that of Sample 5, which shows that a slightly longer time scarcely influences on the Ga0.37Al0.63As surface. The spectral response curve of Sample 7 is obviously lower than that of Sample 5 and Sample 6, which demonstrates that the Ga0.37Al0.63As could be etched by HF solution. The actual reaction equation of Ga1 xAlxAs etched by the HF solution is still unknown, but some studies about this has been reported, the Ga1 xAlxAs with a higher Al composition may be etched faster [21]. From the above, the GaAs photocathode cleaned by the HF solution could obtain a good photoemission effect, while the HF solution etches the Ga0.37Al0.63As. The solution of sulfuric acid and hydrogen peroxide could etch the GaAs protection layer, but has less effect on the photoemission capacity of Ga0.37Al0.63As photocathode.
4. Discussion The band structures of the exponential-doping reflection-mode Ga1 xAlxAs photocathode and the uniform-doping reflectionmode GaAs photocathode with thin GaAs emission layer are shown in Fig. 6. Fig. 4 indeed shows the spectral response curves of the GaAs photocathodes with thin GaAs emission layer, the GaAs protection layer in Fig. 1(b) acts a role of emission layer in these photocathodes. The photoemission process for the normal reflection-mode Ga1 xAlxAs photocathode follows the three-step photoemission model [22]. But for the reflection-mode GaAs photocathode with thin GaAs emission layer, the GaAlAs layer could act the role of emission layer and join the photoemission [23]. However, the emission layers of photocathodes absorb the photons and generate the photoelectrons which diffuse to the surface and emit into vacuum after tunneling the surface barrier (Cs–O layer). Quantum efficiency is the most important characteristic parameter to evaluate the performance of photocathodes. The quantum efficiency curves can be obtained by transforming the spectral
326
X. Chen et al. / Optics Communications 309 (2013) 323–327
Fig. 7. Experimental (solid lines) and fitting (dashed lines) quantum efficiency curves of reflection-mode Ga1 xAlxAs photocathodes. Curves 1 7 are corresponding to Samples 1 7, respectively.
Fig. 6. Band structures of the (a) exponential-doping reflection-mode Ga1 xAlxAs photocathode and (b) uniform-doping reflection-mode GaAs photocathode with thin GaAs emission layer. Ec is the conduction band minimum, Ev is the valence band maximum, EF is the Fermi level, Evac is the vacuum level, and Te is the thickness of emission layer.
response curves according to the following equation [18] YðhυÞ 1:24Sλ =λ
ð3Þ
where Y(hv) is the quantum efficiency, 1.24 is an empirical value, and Sλ is the spectral response value of the corresponding wavelength λ. The quantum efficiency curves of the reflection-mode Ga1 xAlxAs photocathodes are shown in Fig. 7, which are corresponding to the spectral response curves. The quantum efficiency formula of the exponential-doping reflection-mode Ga1 xAlxAs photocathodes could be obtained by solving one-dimensional continuity equations with the given boundary conditions, which is given by [6] h i 2 αhv LD Q ð4Þ Y RE ¼ Pð1RÞ NðSαhv Dn ÞexpðLME T e =2LD αhv T e ÞM þ αhv LD 2 2 αhv LD αhv LE 1
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi N ¼ L2E þ 4L2D S ¼ μjEj þ Sv LE ¼ μjEjτ ¼
qjEj 2 LD k0 T
M ¼ ðNDn =LD Þ cos hðNT e =2LD 2 Þ þ ð2SLD Dn LE =LD Þ sin hðNT e =2LD 2 Þ Q ¼ SN cos hðNT e =2LD 2 Þ þ ðSLE þ 2Dn Þ sin hðNT e =2LD 2 Þ where P is the surface electron escape probability, R is the reflectivity on the surface of photocathode, αhv is the absorption coefficient of Ga1 xAlxAs emission layer, LD is the electron
diffusion length in the Ga1 xAlxAs emission layer, Sv is the backinterface recombination velocity, Te is the thickness of the Ga1 xAlxAs emission layer, Dn is the diffusion coefficient of electron, τ is the minority carrier lifetime, and LE is the electron drift length under the built-in electric field E. The average distance of photoelectrons moving toward the surface in the exponentialdoping Ga1 xAlxAs layer is defined the electron diffusion and drift length LDE qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 LDE ¼ ð LE 2 þ 4LD 2 þ LE Þ ð5Þ 2 kB T lnðN 0 =Ns Þ E¼ q Te
ð6Þ
where kB is the Boltzmann constant, T is the absolute temperature, q is the electron charge, N0 is the doping concentration at the back-interface of the Ga1 xAlxAs emission layer, and Ns is the doping concentration at the surface. The quantum efficiency formula for the reflection-mode GaAs photocathode with thin GaAs emission layer could be solved by considering the photoemission of GaAlAs emission layer, which is given by [23] Pð1RÞα1 LD ðSv α1 Dn Þexpðα1 TÞ YR ¼ ðDn =LD Þ cos hðT e =LD Þ þ Sv sin hðT e =LD Þ α1 2 LD 2 1 Sv cos hðT e =LD Þ þ ðDn =LD Þ sin hðT e =LD Þ þ α 1 LD ðDn =LD Þ cos hðT e =LD Þ þ Sv sin hðT e =LD Þ Pð1RÞSv Ln α2 expðα1 T e Þ 1 þ U ðDn =LD Þ cos hðT e =LD Þ þ Sv sin hðT e =LD Þ ð1 þ α2 Ln Þð1 þ Sv Ln =Dn Þ ð7Þ where Ln is the electron diffusion length in the GaAlAs emission layer, and LD is that in the GaAs emission layer, α1 is the absorption coefficient of GaAs, α2 is the absorption coefficient of GaAlAs, and some other symbols are the same as that in Eq. (4). Though the GaAlAs emission layer in the derivation process of Eq. (7) is uniform doping, but we think Eq. (7) is also suitable for the quantum efficiency in this study. The quantum efficiency curves of Sample 1, Sample 5, and Sample 6 in Fig. 7 could be respectively fitted by Eq. (4), while the quantum efficiency curves of Sample 2, Sample 3, and Sample 4 could be respectively fitted by Eq. (7). The fitting quantum efficiency curves are shown in Fig. 7, and the fitted performance parameters for the photocathode samples are listed in Table 1. The Ga0.37Al0.63As surface could be damaged by the HF solution, which
X. Chen et al. / Optics Communications 309 (2013) 323–327
Table 1 Fitted performance parameters for photocathode samples. Sample
Sv/(cm/s)
P
LD/μm
LDE/μm
Te/μm
1 2 3 4 5 6
2 106 1 106 1 106 1 106 1 106 1 106
0.59 0.60 0.60 0.60 0.40 0.39
– 2.0 2.0 2.0 – –
3.3 – – – 0.8 0.78
2.0 0.1 0.054 0.025 1.2 1.2
directly affects the surface electron escape probability of the photocathode. Besides, the thickness of Ga0.37Al0.63As layer etched by the HF solution is unknown. So, the performance parameters of Sample 7 are hard to be fitted and not given here. As we can see from Table 1, the back-interface recombination velocity Sv of these photocathodes are similar, which mainly depends on the growth quality of the interface. The electron diffusion and drift length LDE of Ga0.37Al0.63As photocathode is obviously smaller than that of GaAs photocathode, which may be caused by Al composition. According to the quantum efficiency curves in Fig. 7, we can conclude that the prepared GaAs photocathodes may have similar NEA surfaces because of the approximate quantum efficiency at the high-energy region. In Table 1, it could be found that the surface electron escape probability P of Sample 3 and Sample 4 are equal to that of Sample 2 though the chemical cleaning methods are different, which shows that the cleaning effects of the H2SO4(98%): H2O2(30%): H2O solution with the ratio of 4:1:100 and the HF solution on the surface of GaAs photocathodes are almost the same. Besides, the thickness of GaAs layer for Sample 3 and Sample 4 could be fitted by the quantum efficiency formula, respectively. In Fig. 7, the quantum efficiency curves of GaAs photocathodes are obviously higher than that of Ga0.37Al0.63As photocathodes, and the fitted surface electron escape probability of GaAs photocathodes are larger than that of Ga0.37Al0.63As photocathodes. The lower surface electron escape probability of Ga0.37Al0.63As photocathode indicates that the formative NEA surface is worse than that of GaAs photocathode, this phenomenon is corresponding to the study of Martinelli and Ettenberg, [24] which could be due to the difficult operation of removing the surface oxides from the Ga1 xAlxAs with high Al composition. 5. Conclusion In this paper, the spectral response is used to evaluate chemical cleaning for the Ga1 xAlxAs photocathodes. Two kinds of the reflection-mode Ga1 xAlxAs photocathode samples are cleaned by the H2SO4(98%): H2O2(30%): H2O solution with the ratio of 4:1:100 and the HF solution, respectively. The spectral response curves of different photocathodes are measured after the Cs/O activation by the on-line spectral response measurement system, and detailed analysis is performed on the spectral response curves. The quantum efficiency formulas are used to fit the quantum
327
efficiency curves which are transformed from the spectral response curves, and some related performance parameters including the thickness of the etching GaAs layer for the Ga1 xAlxAs photocathodes are given. The results show that the GaAs photocathode cleaned by the HF solution could obtain a good photoemission effect, while the HF solution is not suitable for the Ga0.37Al0.63As photocathode. The solution of sulfuric acid and hydrogen peroxide could etch the GaAs layer, but has less effect on the Ga0.37Al0.63As layer. The evaluation method of spectral response could be applied to the preparation of Ga1 xAlxAs photocathodes, even extend to other photocathodes.
Acknowledgements This work is supported by the National Natural Science Foundation of China (grants 61171042 and 61301023), the Research and Innovation Plan for Graduate Students of Jiangsu Higher Education Institutions, China (grant CXZZ12_0193). References [1] S. Karkare, D. Dimitrov, W. Schaff, L. Cultrera, A. Bartnik, X.H. Liu, E. Sawyer, T. Esposito, I. Bazarov, Journal of Applied Physics 113 (2013) 104904. [2] L. Chen, Y.S. Qian, B.K. Chang, X.L. Chen, R. Yang, Optics Communications 285 (2012) 1264. [3] T. Rao, A. Burrill, X.Y. Chang, J. Smedley, T. Nishitani, C.H. Garcia, M. Poelker, E. Seddon, F.E. Hannon, C.K. Sinclair, J. Lewellen, D. Feldmang, Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 557 (2006) 124. [4] I.V. Bazarov, D.G. Ouzounov, B.M. Dunham, S.A. Belomestnykh, Y.L. Li, X.H. Liu, R.E. Meller, J. Sikora, C.K. Sinclair, Physical Review Special Topics—Accelerators and Beams 11 (2008) 040702. [5] T. Nishitani, M. Tabuchi, Y. Takeda, Y. Suzuki, K. Motoki, T. Meguro, Japanese Journal of Applied Physics 48 (2009) 06FF02. [6] X.L. Chen, J. Zhao, B.K. Chang, M.C. Jin, G.H. Hao, Y. Xu, Proceedings of SPIE, 8555, 85550 R. [7] J.J. Zou, B.K. Chang, H. Wang, Z. Yang, P. Gao, Proceedings of SPIE, 6352, 635239. [8] D. Kikuchi, Y. Matsui, S. Adachi, Journal of the Electrochemical Society. 147 (2000) 1973. [9] D. Kikuchi, S. Adachi, Materials Science and Engineering: B 76 (2000) 133. [10] A. Guivarch, H. Lharidon, G. Pelous, G. Hollinger, P. Pertosa, Journal of Applied Physics 55 (1984) 1139. [11] Z. Liu, Y. Sun, F. Machuca, P. Pianetta, W.E. Spicer, R.F.W. Pease, Journal of Vacuum Science & Technology A 21 (2003) 212. [12] Y.J. Zhang, J. Niu, J.J. Zou, B.K. Chang, Y.J. Xiong, Applied Optics 49 (2010) 3935. [13] Y.J. Zhang, J. Niu, J. Zhao, J.J. Zou, B.K. Chang, F. Shi, H.C. Cheng, Journal of Applied Physics 108 (2010) 093108. [14] X.Q. Du, B.K. Chang, Applied Surface Science 251 (2005) 267. [15] B.J. Stocker, Surface Science 47 (1975) 501. [16] B.K. Chang, X.Q. Du, L. Liu, Z.Y. Zong, R.G. Fu, Y.S. Qian, Proceedings of SPIE, 5209, 209. [17] Y.S. Qian, Z.Y. Zong, B.K. Chang, Proceedings of SPIE, 4580, 486. [18] J.J. Zou, B.K. Chang, Optical Engineering 45 (2006) 054001. [19] Z. Liu, Y. Sun, F. Machuca, P. Pianetta, W.E. Spicer, R.F.W. Pease, Journal of Vacuum Science and Technology B 21 (2003) 1953. [20] S. Adachi, Journal of Applied Physics 58 (1985) R1. [21] M.M.A.J. Voncken, J.J. Schermer, G. Maduro, G.J. Bauhuis, P. Mulder, P.K. Larsen, Materials Science and Engineering B 95 (2002) 242. [22] W.E. Spicer, A. Herrera-Gómez, Proceedings of SPIE, 2022, 18. [23] X.L. Chen, Y.J. Zhang, B.K. Chang, J. Zhao, M.C. Jin, G.H. Hao, Y. Xu, Optics Communication 287 (2013) 35. [24] R.U. Martinelli, M. Ettenberg, Journal of Applied Physics 45 (1974) 3896.