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Photoemission from advanced heterostructured AlxGa1-xAs/GaAs photocathodes under multilevel built-in electric field Cheng Feng,1 Yijun Zhang,1,* Yunsheng Qian,1 Benkang Chang,1 Feng Shi,2 Gangcheng Jiao,2 and Jijun Zou3 Institute of Electronic Engineering and Optoelectronic Technology, Nanjing University of Science and Technology, Nanjing 210094, China 2 Science and Technology on Low-Light-Level Night Vision Laboratory, Xi'an 710065, China 3 Engineering Research Center of Nuclear Technology Application (East China Institute of Technology), Ministry of Education, Nanchang 330013, China * [email protected]

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Abstract: A heterostructured AlxGa1-xAs/GaAs photocathode consisting of a composition-graded buffer layer and an exponential-doped emission layer is developed to improve the photoemission performance over the wavelength region of interest. The theoretical quantum efficiency models for reflection-mode and transmission-mode AlxGa1-xAs/GaAs photocathodes are deduced based on one-dimensional continuity equations, respectively. By comparison of simulated results with conventional quantum efficiency models, it is found that the multilevel built-in electric field can effectively improve the quantum efficiency, which is related to the buffer layer parameters and cathode thicknesses. This special graded bandgap structure arising from the compositional grade in the buffer layer and doping grade in the emission layer would bring about the reduction of back interface recombination losses and the efficient collection of photons generating photoelectrons. Moreover, a best fit of the experimental quantum efficiency data can be achieved with the aid of the deduced models, which would provide an effective approach to evaluate internal parameters for the special graded bandgap photoemitters. ©2015 Optical Society of America OCIS codes: (160.2100) Electro-optical materials; (040.5160) Photodetectors; (250.0250) Optoelectronics; (300.6470) Spectroscopy, semiconductors.

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Received 3 Jun 2015; revised 15 Jul 2015; accepted 15 Jul 2015; published 17 Jul 2015 27 Jul 2015 | Vol. 23, No. 15 | DOI:10.1364/OE.23.019478 | OPTICS EXPRESS 19478

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1. Introduction From the standpoint of their high quantum efficiency, low dark current, low energy spread for emitted electrons, good long-wavelength response and high spin polarization, III-V group

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GaAs-based heterojunction photocathodes are nearly optimum candidates in the fabrication of various optoelectronic devices such as image intensifier tube, polarized electron source, and solar cell [1–5]. With the development of material growth techniques and the goal of improving the device performance, extensive efforts based on bandgap engineering have been made to realize the expected spectral characteristics, and different models and structures have been proposed in recent years [6–12]. Among them, applying a built-in electric field in the photocathode interior was proposed to increase collection of generated minority carriers, which was accomplished by varying the energy band gap [5,6,8,11]. As for various photocathodes in practice, the quantum efficiency is distinctly a very important parameter utilized to evaluate the performance. The graded band gap in AlxGa1-xAs material and AlxGa1-xAs/GaAs material has been considered previously especially in solar cells [13–16]. By varying the Al proportion in the AlxGa1-xAs layer, the bandgap gradation is achieved, and the built-in electric field is introduced to encourage the transport of excited photoelectrons to the interface. Moreover, the contact interface recombination action between AlxGa1-xAs buffer layer and GaAs emission layer can be reduced and lead to the improved quantum efficiency. On the other hand, introducing a structure of high-to-low varying doping concentration from bulk to surface in GaAs emission layer is a novel concept to achieve a higher photoemission capability [15,17,18]. To sum up, the improvement in the cathode performance can be attributed to the built-in electric field, which effectively increases the electron transport efficiency and escape probability. In view of the function of built-in electric field for photoemitters, the complex AlxGa1xAs/GaAs photocathodes consisting of the composition-graded AlxGa1-xAs layer and dopinggraded GaAs layer have been grown and activated to negative electron affinity (NEA) state with the goal of maximizing quantum efficiency [19]. However, it still lacks of proper quantum efficiency theoretical models for evaluating these complex photocathodes. Accordingly, how the induced multilevel built-in electric field acts on quantum efficiency is not clear, and the photoemission mechanism is still not well understood. In this paper, the theoretical quantum efficiency models for reflection-mode (r-mode) and transmission-mode (t-mode) AlxGa1-xAs/GaAs heterostructured photocathodes are deduced by solving from the one dimensional continuity equations combined with the three-step model [20]. According to the proposed quantum efficiency models, we explain the superiority in quantum efficiency over the conventional exponential-doped photocathode, and analyze some related cathode performance parameters acting on the quantum efficiency characteristics, which would provide theoretical guidance for the optimum design of graded bandgap photocathodes. Moreover, in order to verify the theoretical practicality, the r-mode and t-mode graded bandgap AlxGa1-xAs/GaAs photocathodes are prepared respectively, and the experimental quantum efficiency curves are well fitted with the theoretical models. 2. Structure and model To prevent deterioration of cathode performance arising from lattice mismatch at the interface between buffer layer and emission layer, and combine the advantages of both compositiongraded and exponential-doped photocathodes, a special structural design regarding the heterostructured AlxGa1-xAs/GaAs photocathode is proposed, and the multilayer energy band structure is shown in Fig. 1. The main difference between r-mode and t-mode photocathodes is the direction of incident light.

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Fig. 1. Energy band structure diagram of the graded bandgap AlxGa1-xAs/GaAs heterojunction photocathode. Ec is the conduction-band minimum, Ev is the valence band maximum, Eg is the bandgap, EF is the Fermi level, E0 is the vacuum level.

The composition-graded AlxGa1-xAs layer can serve as a passivation layer that reduces defects at the back interface, and the larger bandgap of the AlxGa1-xAs layer presents a potential barrier to reflect the electrons away from the back interface [16]. Moreover, the AlxGa1-xAs layer is of a graded Al composition varying with thickness. In this case, it is clear that some basic parameters of electrical properties will also change as a function of thickness. To simplify the calculation, the AlxGa1-xAs layer can be divided into a series of parts with different Al composition, and each of them has uniform parameters of electrical properties. As shown in Fig. 1, there is a band-bending region at each interface forming by the Fermilevel leveling effect between two different Al composition sections. As proved in previous research [21,22], a buffer layer is usually not only used to match with the lattice of emission layer, but also participate in photoemission of the heterostructured photocathodes and enhance the photoemission performance. Consequently, there is need to figure out the number of photoelectrons nbuf in buffer layer that can contribute to the emission layer. For the graded bandgap sections of AlxGa1-xAs layer for r-mode and t-mode heterostructured photocathodes, the build-in electric field is induced by varying bandgap energy. Accordingly, based on the three-step model of photoemission [20], the transport of photoexcited electrons, including diffusion and drift, follows the one dimensional continuity equation, which is given by Dni

d 2 ni ( x) dn ( x) ni ( x) + μi E i − + g i ( x) = 0, i = 1, 2,3, ,n 2 τi dx dx

(1)

where gi(x) is the generation rate of photoelectrons in AlxGa1-xAs layer. For r-mode AlxGa1photocathode,

xAs/GaAs

gi ( x) = (1 − Rhv ) I 0α hvi exp(−α hv 0Te ) exp[−α hvi ( x − Te )], i = 1

(2)

 i −1  gi ( x) = (1 − Rhv ) I 0α hvi exp(−α hv 0Te ) ∏ exp(−α hvm Twm )  exp[−α hvi ( x − Tdi−1 )],  m =1  i = 2,3, , n (3) While for t-mode AlxGa1-xAs/GaAs photocathode irradiated by incident light in the opposite direction,

 n  gi ( x) = (1 − Rhv ) I 0α hvi  ∏ exp(−α hvm Twm )  exp[−α hvi (Tdi − x)], i = 1, 2, , n − 1 (4)  m = i +1  gi ( x) = (1 − Rhv ) I 0α hvi exp[−α hvi (Tdi − x)], i = n

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(5)


In Eqs. (1)-(5), Dni is the diffusion coefficient of electrons, ni(x) is the concentration of minority carrier (i.e., electrons) in AlxGa1-xAs buffer layer, μi is electron mobility, E is the built-in electric field, τi is the lifetime of electrons in buffer layer, I0 is the intensity of incident light, αhvi is the absorption coefficient of the material in AlxGa1-xAs layer, Rhv is the reflectivity on the surface of photocathode, Te and Twi are the thickness of GaAs emission layer and each part of AlxGa1-xAs buffer layer, respectively, Tdi = Te +  i =1 Twi , and i values n

from 1 to n which corresponds to each divided part with different Al composition. The electron diffusion coefficient as well as the electron mobility of AlxGa1-xAs indeed depend on the Al proportion x, which are respectively given by [23] 200 − 550 x + 250 x 2 (cm 2 /s), 0 < x < 0.45 Dni =  2 2 −6.4 + 29 x − 18 x (cm /s), 0.45 < x < 1

(6)

8000 − 22000 x + 10000 x 2 (cm 2 V −1s −1 ), 0 < x < 0.45

μi = 

(7) 2 2 −1 −1 −255 + 1160 x − 720 x (cm V s ), 0.45 < x < 1 The built-in electric field E induced by the compositional grade in the AlxGa1-xAs buffer layer can be treated as the following uniform distribution [14]: E=

Egn − Eg1

(8) Tdn − Te In addition, the energy band gap of AlxGa1-xAs also varies with Al proportion x, and the relation is given by [24]

(9) Eg ( x) = 1.424 + 1.594 x + x(1 − x)(0.127 − 1.310 x) In order to obtain nbuf, for both r-mode and t-mode AlxGa1-xAs/GaAs photocathodes, the boundary conditions suitable for each divided sublayer considering the electron contribution from the former sublayer in AlxGa1-xAs buffer layer are given as follows: When i = 1, 2, , n − 1 , dni ( x)    Dni dx + μi E ni ( x)   

x =Tdi

= − Svi+1 ni ( x)

dni ( x)    Dni dx + μi E ni ( x)   

x =Tdi−1

x =Tdi

+ Svi+1 ni +1 ( x)

= Svi ni ( x)

x =Tdi −1

x =Tdi

(10)

(11)

When i = n , ni ( x)

x =Te +Tw

dni ( x)    Dni dx + μi E ni ( x)   

=0

x =Tdi−1

(12) = Svi ni ( x)

x =Tdi −1

(13)

where Svi denotes the interface recombination velocity. By recursive calculation to solve the series of aboved equations, the electron concentration at the back interface of the buffer layer nbuf = n1(Te) can finally be figured out. In GaAs emission layer, an internal built-in electric field arising from the nonuniform doping structure has been introduced to increase emission performance of the photocathodes, in which the doping concentration from the GaAs bulk to the surface is exponentially distributed from high to low [25]. Similar to the electrons transport mechanism in AlxGa1-xAs layer, the photoelectrons in GaAs layer are promoted towards surface by way of both diffusion and directional drift, and the one dimensional continuity equations used for the

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emission layer of r-mode and t-mode exponential-doped photocathodes are respectively given by Dn 0

Dn 0

d 2 n0 ( x) dn ( x) n0 ( x) + μ0 E0 0 − + (1 − Rhv ) I 0α hv 0 exp(−α hv 0 x) = 0, x ∈ [0, Te ] (14) 2 dx dx τ0

d 2 n0 ( x) dn ( x) n0 ( x)  n  + μ0 E0 0 − + (1 − Rhv ) I 0α hv 0 ∏ exp(−α hvmTwm )  2 dx τ0 dx  m =1  × exp[−α hv 0 (Te − x)] = 0, x ∈ [0, Te ]

(15) where Dn0 is the diffusion coefficient of electrons in emission layer, n0(x) is the concentration of minority carrier (i.e., electrons) in emission layer, μ0 is the electron mobility of GaAs, E0 is the built-in electric field formed in emission layer, τ0 is the lifetime of electrons in emission layer, and αhv0 is the absorption coefficient of GaAs material. When defining the boundary conditions as the solution to Eqs. (14) and (15), the electron contributions from the AlxGa1-xAs buffer layer should be taken into account. Accordingly, the boundary conditions suitable for the GaAs emission layer are given by n0 ( x)

x=0

=0

(16)

dn0 ( x)   (17)  Dn 0 dx + μ0 E0 n0 ( x)  x =Te = − Sv1n0 ( x) x =Te + Sv1n1 ( x) x =Te   Upon applying the boundary conditions to (14) and (15), we can obtain the concentration of electrons n0(x) in the emission layer. After n0(x) is substituted into dn ( x) Y (hv) = PDn 0 0 x = 0 / I 0 , where P is the surface electron escape probability, the quantum dx efficiency distributions with photon wavelength are finally deduced for both r-mode and tmode heterostructured AlxGa1-xAs/GaAs photocathodes assisted by multilevel electric field.

3. Theoretical simulation

By combination of the two different approaches to produce multilevel built-in electric fields, the graded bandgap structure would be conducive to drive the excited electrons in the buffer layer towards the heterostructured interface, thus enhancing the photoemission. We believe that the enhanced spectral response mainly induced by the contribution of photoelectrons in the composition-graded AlxGa1-xAs buffer layer. To confirm the conjecture and reveal the effect of photoelectrons generated in AlxGa1-xAs buffer layer on spectral characteristics of heterostructured AlxGa1-xAs/GaAs photocathodes, we analyze the critical factors including the thicknesses of GaAs emission layer and AlxGa1-xAs buffer layer, in combination with the conventional models of exponential-doped photocathodes considering the photoelectrons generated in the buffer layer with fixed Al proportion [18]. Moreover, the effect of different thickness distribution with graded Al proportion in AlxGa1-xAs layer on the quantum efficiency is analyzed by utilizing the deduced models. When simulating the quantum efficiency performance of the cathode, the compositiongraded AlxGa1-xAs layer with the Al proportion range of 0.9-0 is simplified into five parts (i.e. n = 5), and each of them is of a uniform bandgap with Al proportion of 0.9, 0.675, 0.45, 0.225, and 0, respectively, from the bulk to the back interface. In order to hightlight the differences of the graded bandgap quantum efficiency models, we choose the typical Al0.59Ga0.41As/GaAs photocathode as the comparison object. In the two structures, the GaAs emission layer is an exponential-doped structure with the doping concentration of 1 × 1019 to

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1 × 1018 cm−3. Besides, some parameters are also fixed, wherein P = 0.5, Rhv = 0.31 [26], and the absorption coefficients of GaAs and AlxGa1-xAs are referred to [27] and [24]. We first investigate the effect of GaAs emission layer with various thicknesses on quantum efficiency, and the changes of theoretical quantum efficiency curves are shown in Fig. 2. The solid lines and dashed lines correspond to the simulated quantum efficiency curves using the deduced models with multilevel built-in electric field and the conventional exponential-doped models, respectively. As shown in Fig. 2(a) assuming Svi≤103 cm/s (i = 1, …, 5) and Tw = 0.5 μm (Twi = 0.1 μm, i = 1, 2, …, 5), the quantum efficiency of r-mode AlxGa1-xAs/GaAs photocathode is improved by the increased Te, especially in the longwave response region. For r-mode AlxGa1-xAs/GaAs photocathode, when Te is thin enough, most photons can pass through the GaAs emission layer, and only a small part of longwave light is absorbed by GaAs to excite photoelectrons contributing to the final quantum efficiency. If Te increases, it will provide more ample space to absorb photons and generate more electrons, until Te is thick enough that all incident photons are completely absorbed. In addition, it is noted that the quantum efficiency values denoted by solid lines are always higher than those denoted by dashed lines, especially in the longwave region until Te is more than 1 μm. It is because that, the longwave photons are mainly absorbed in the bulk for r-mode photocathode, the AlxGa1-xAs sublayer of low Al proportion can abort the longwave photons on one hand, and the built-in electric field induced by the compositional grade can accelerate the transport of photoelectrons excited by longwave light on the other hand. When Te is more than 1 μm, most photons are absorbed in GaAs emission layer, and the solid line will coincide with the dashed line. For t-mode AlxGa1-xAs/GaAs photocathode as shown in Fig. 2(b) assuming Svi≤103 cm/s (i = 1, …, 5) and Tw = 0.5 μm (Twi = 0.1μm, i = 1, 2, …, 5). In practical applications, the thickness of GaAs emission layer is usually no more than 2 μm [28]. While the thickness of emission layer increases within the range of 2 μm, the longwave response induced in GaAs emission layer is improved evidently because of the more available space to absorb longwave photons. When Te is thin, the longwave response of the solid line is higher than that of the dashed line due to the lower Al composition near the GaAs back interface. If Te continues to increase, for t-mode AlxGa1-xAs/GaAs photocathode with backside illumination, the quantum efficiency in the shortwave region will decrease slightly, because the excited photoelectrons generated in AlxGa1-xAs layer cannot completely pass through GaAs emission layer and escape to vacuum. It demonstrates that for t-mode photocathode, the additional built-in electric field caused by the composition-graded structure can distinctly improve the quantum efficiency in the shortwave response region, which is consistent with our original intention of this graded bandgap model.

Fig. 2. Theoretical quantum efficiency curves with the change of Te for AlxGa1-xAs/GaAs photocathodes operating in the (a) r-mode and (b) t-mode, respectively.

Figure 3 shows the effect of AlxGa1-xAs buffer layer with various thicknesses Tw (Twi = Tw/5 μm, i = 1, 2, …, 5) on the final quantum efficiency. The solid lines and dashed lines correspond to the deduced models with multilevel built-in electric field and the conventional

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exponential-doped models, respectively. In Fig. 3(a) assuming Svi≤103 cm/s (i = 1, …, 5) and Te = 0.1 μm, the dashed lines are coincident, because the longwave photons passing through the GaAs emission layer cannot be absorbed by the Al0.59Ga0.41As buffer layer due to its larger band gap threshold. Different from the conventional exponential-doped model, the longwave response corresponding to the solid line is always higher, which is attributed to the role of the graded bandgap in AlxGa1-xAs buffer layer. When the GaAs emission layer is thin enough, the photons cannot be absorbed adequately, so parts of the longwave photons are absorbed by the AlxGa1-xAs buffer layer. As Tw increases, there is more sufficient space to generate photoelectrons. On the other hand, the built-in electric field will decrease while Tw is increasing, which is unfavorable for the transport of photoelectrons in the AlxGa1-xAs buffer layer . The two factors are gradually balanced when Tw is more than 1 μm. As a result of the large absorption coefficient of shortwave light in AlxGa1-xAs materials, the thickness of buffer layer is usually no more than 1 μm in practical applications for t-mode photocathodes [27]. As shown in Fig. 3(b) assuming Svi≤103 cm/s (i = 1, …, 5) and Te = 1 μm, The quantum efficiency is improved in the shortwave region as Tw decreases because of the short-wave constraint factor in the two models [29]. When Tw is thin, only a small part of shortwave photons are absorbed by the buffer layer, instead, a great majority of photons penetrating through the buffer layer are absorbed by the GaAs emission layer. Because of the superior photoemission efficiency of GaAs in contrast to that of AlxGa1-xAs, the GaAs emission layer has more dominant effect on the shortwave response. However, it is noted from Fig. 3(b) that the quantum efficiency denoted by solid lines are always higher than those denoted by dashed lines, especially in the shortwave region. As Tw decreases, the improvement of shortwave response becomes more evident. It is because that, for t-mode photocathode, the shortwave photons with short absorption length are preferentially absorbed by the buffer layer, and the enhanced built-in electric field induced by the compositional grade can facilitate these shortwave photoelectrons towards surface.

Fig. 3. Theoretical quantum efficiency curves with the change of Tw for AlxGa1-xAs/GaAs photocathodes operating in the (a) r-mode and (b) t-mode, respectively.

Due to the various optical properties as Al composition changes, the different thickness distribution of the divided sublayer of AlxGa1-xAs layer can influence the quantum efficiency as well, as shown in Fig. 4. For the r-mode photocathode, as shown in Fig. 4(a) assuming Svi≤103 cm/s (i = 1, …, 5) and Te = 0.1 μm, when Twi with low Al composition is thicker than that with high Al composition, the quantum efficiency of r-mode AlxGa1-xAs/GaAs photocathode is enhanced in longwave region. According to [27], the AlxGa1-xAs sublayer with high Al composition absorbs less longwave photons. When the AlxGa1-xAs sublayer with high Al composition part becomes relatively thinner, more longwave photons are absorbed by AlxGa1-xAs, which can generate more photoelectrons. Likewise, as is shown in Fig. 4(b) assuming Svi≤103 cm/s (i = 1, …, 5) and Te = 1 μm, when Twi with high Al composition is thinner than that with low Al composition, the quantum efficiency is increased in the range of 400-500 nm for t-mode AlxGa1-xAs/GaAs photocathode. In this case, more shortwave photons

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can be absorbed by the AlxGa1-xAs sublayer with low Al composition. Because of the larger diffusion length inversely proportional to the Al component, the photoelectrons generated in the AlxGa1-xAs sublayer with low Al composition has more contributions to the shortwave response. On the other hand, in the range of 500-720 nm, the quantum efficiency is enhanced while Twi with high Al composition is relatively thicker. After passing through the thicker AlxGa1-xAs sublayer with high Al composition, the longwave photons can be absorbed nearby the back interface of GaAs layer, which is convenient for these photoelectrons to escape from bulk to surface and would lead to more contributions of longwave response. In a word, the effect of sublayer thickness distribution with compositional grade in AlxGa1-xAs layer on quantum efficiency should be distinguished between r-mode and t-mode photocathodes.

Fig. 4. Theoretical quantum efficiency curves with different Al proportion distribution for AlxGa1-xAs/GaAs photocathodes operating in the (a) r-mode and (b) t-mode, respectively.

4. Experiment and analysis

In order to verify the quantum efficiency models for r-mode and t-mode graded bandgap AlxGa1-xAs/GaAs photocathode with multilevel built-in electronic field, two types of heterostructured AlxGa1-xAs/GaAs photocathodes samples operating in r-mode and t-mode were prepared, respectively. The two heterostructured AlxGa1-xAs/GaAs epilayers were grown on the high quality Si-doped GaAs (100)-oriented substrates with a low dislocation density by the metal-organic chemical vapor deposition (MOCVD) technique, and the doping concentration of the substrate is (1-5) × 1018 cm−3. The p-type AlxGa1-xAs buffer layer doped to 1 × 1019 cm−3 is of a graded Al composition varying from pure GaAs to Al0.6Ga0.3As for rmode photocathode and to Al0.9Ga0.1As for t-mode photocathode. The thickness of the buffer layer for r-mode and t-mode samples is 1.0 μm and 0.5 μm, respectively. The GaAs emission layer of 0.2 μm and 1.0 μm in total thickness corresponding to r-mode and t-mode photocathodes is quasi-exponentially doped with a graded p-type zinc doping concentration from 1 × 1019 cm−3 to 1 × 1018 cm−3 with the limitation of epitaxial grown technique. While growing the multi-epitaxial layers, the metalorganic compounds trimethylgallium and trimethylalumium are utilized to be the group III sources, the AsH3 is used as the group V source and diethylzinc is used as the dopant source. Additionally, the high-purity H2 is used as a carrier gas. During the growth procedure, the V/III ratio was kept at 10–15, and the growth temperature was 680 °C for GaAs and 710 °C for AlxGa1-xAs. According to the proposed process of fabricating t-mode GaAs photocathodes by Antypas et al [30], the t-mode AlxGa1-xAs/GaAs sample was made into the cathode modules with a glass/Si3N4/AlxGa1-xAs/GaAs structure after thermal bonding and selective etching, which is more complex than the process of r-mode sample. Prior to activation, both the r-mode and tmode samples underwent a two-step surface preparation including a wet chemical cleaning process and a heat treatment process in vacuum to eliminate the surface contamination, especially oxides and carbon. Following is the NEA activation for the cathode module performing in an ultrahigh vacuum chamber with base pressure better than 10−8 Pa by using

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the improved co-deposition technique, in which cesium was supplied continuously and oxygen was introduced periodically [17] [31]. After the usual “high-low temperature” twostep activation [32], the spectral response curve of the AlxGa1-xAs/GaAs r-mode photocathode was measured in situ by an on-line quantum efficiency measurement system, while the tmode AlxGa1-xAs/GaAs photocathode sealed in the image tube was measured after transferred into ambient air [17]. The experimental quantum efficiency curves for r-mode and t-mode AlxGa1-xAs/GaAs samples are presented in Figs. 5 and 6, respectively, which are denoted by the solid line. It is apparent that the quantum efficieny curve exhibits an arc shape rather than a linear shape in the longwave region for r-mode sample and in the shortwave region for t-mode sample, respectively, which could be ascribed to the sufficient photon absorption by the compositiongraded buffer layer. As we know, the actual Al component distribution in the compositiongraded buffer layer is hard to be measured after growth, thereby, the deduced quantum efficiency models for AlxGa1-xAs/GaAs photocathodes with multilevel built-in electric field may provide an approach to obtain cathode parameters by curve fitting.

Fig. 5. Experimental quantum efficiency curve of the r-mode AlxGa1-xAs/GaAs sample and the fitted curve via the deduced model.

Fig. 6. Experimental quantum efficiency curve of the t-mode AlxGa1-xAs/GaAs sample and the fitted curve via the deduced model.

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As for both r-mode and t-mode samples, five Al component values within 0.6 and 0.9 are evenly chosen respectively, wherein the optical properties is exactly referred to [27]. For photocathodes operating whether in the r-mode or in the t-mode, it is apparent that the theoretical quantum efficiency curves can match well with the experimental curves. The fitted values of the photocathode performance parameters through the derived quantum efficiency models are listed in Table 1. It is seen from Table 1 that the thickness of each sublayer in rmode AlxGa1-xAs layer is distributed equally, while the sublayer of t-mode sample is thick in the middle, and thin on both sides. In addition, it is apparent that the interface recombination velocity between GaAs emission layer and AlxGa1-xAs buffer layer is greatly reduced, while that usually exceeds 105 cm/s [14,33]. It is known that the interface recombination velocity is affected by cathode material quality itself, like impurity, carrier concentration and misfit dislocation at AlxGa1−xAs/GaAs interface. The design of graded Al component in the buffer layer would smooth the steepness arising from the lattice mismatch at the interface, which consequently diminishes the interface recombination velocity. As mentioned above, with the aid of the deduced models, a best fit of the experimental quantum efficiency data can be achieved, and would provide us useful internal parameters, which are difficult to be measured directly. Table 1. Fitted performance parameters of the quantum efficiency curves Sample type

r-mode

t-mode

Al component of each AlxGa1-xAs sublayer 0.590 0.450 0.315 0.198 0 0.900 0.675 0.450 0.225 0

Thickness of each AlxGa1-xAs sublayer (μm) 0.19 0.22 0.21 0.18 0.20 0.05 0.15 0.13 0.08 0.09

Interface recombination velocity (cm/s)

Surface escape probability

1000

0.54

1000

0.54

5. Conclusion

In summary, we have deduced the theoretical quantum efficiency models of the graded bandgap heterostructured photocathode with a composition-graded buffer layer and an exponential-doped emission layer, based on one dimensional continuity equations. Through theoretical simulation using the deduced models, it is found that the graded bandgap structure is beneficial to the increase of quantum efficiency over the wavelength region of interest for respective r-mode and t-mode photocathodes, and the improvement of quantum efficiency is related to the buffer layer parameters and cathode thicknesses. Besides, the experimental quantum efficiency data for r-mode and t-mode AlxGa1-xAs/GaAs photocathodes are well fitted, which demonstrates that the multilevel built-in electric field induced by the bandgap gradation reduces the interface recombination and enhances the photoelectron collection. The models would help to well understand the photoemission mechanism for various heterostructured photocathodes with the composition-graded structure and provide guidance for better design of superior photocathodes for a particular wavelength range. Acknowledgments

This work is supported by the National Natural Science Foundation of China (Grant Nos. 61301023, 91433108 and 61261009), Specialized Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20133219120008), the Research and Innovation Plan for Graduate Students of Jiangsu Higher Education Institutions (Grant KYLX15_0372) and Science and Technology on Low-Light-Level Night Vision Laboratory Foundation (Grant No. BJ2014001). #242134 © 2015 OSA
