The Time Response of a Uniformly Doped Transmission-Mode NEA AlGaN Photocathode Applied to a Solar-Blind Ultraviolet Detecting System

Abstract

Due to the excellent quantum conversion and spectral response characteristics of the AlGaN photocathode, it has become the most promising III-V group semiconductor photocathode in solar-blind signal photoconversion devices in the ultraviolet band. Herein, the influence factors of the time-resolved characteristics of the AlGaN photocathode are researched by solving the photoelectron continuity equation and photoelectron flow density equation, such as the AlN/AlGaN interface recombination rate, AlGaN electron diffusion coefficient, and AlGaN activation layer thickness. The results show that the response time of the AlGaN photocathode decreases gradually with the increase in AlGaN photoelectron diffusion coefficient and AlN/AlGaN interface recombination rate, but the response time of the AlGaN photocathode gradually becomes saturated with the further increase in AlN/AlGaN interface recombination rate. When the thickness of the AlGaN photocathode is reduced from 250 nm to 50 nm, the response time of the AlGaN photocathode decreases from 63.28 ps to 9.91 ps, and the response time of AlGaN photocathode greatly improves. This study provides theoretical guidance for the development of a fast response UV detector.

1. Introduction

As a photoelectric detection and countermeasure system, an airborne missile warning system can realize rapid identification of an incoming target by using a high-performance photoelectric detector, which can improve the survivability of combat aircraft in low-altitude penetration and ground attack. Among the numerous airborne missile warning systems, the solar-blind ultraviolet missile warning system uses the strong ultraviolet radiation contained in the jet flame of the engine when the missile is launched or flown to detect and determine the direction and flight time of the incoming missile. Owing to its characteristics of small size, low false alarm rate, no refrigeration, and strong environmental adaptability, it has become one of the most widely used airborne missile warning systems in the world. The core photodetector of the sun-blind UV missile alarm system mostly uses the solar-blind UV image intensifier [1,2], and the photocathodes for solar-blind UV signal detection are mainly Cs₂Te photocathodes and AlGaN photocathodes. Compared with Cs₂Te photocathodes, AlGaN photocathodes have the advantages of high quantum efficiency, good solar-blind characteristics, and adjustable response spectrum design [3,4,5] and are considered to be the most promising III–V group semiconductor photocathodes for realizing solar-blind UV signal photoelectric conversion [6,7,8,9,10,11].

High quantum efficiency and high time resolution are the prerequisites for the application of the AlGaN photocathode in the field of solar-blind UV missile warning systems [12]. At present, researchers have focused on the theoretical research and experimental verification of the quantum efficiency model of the AlGaN photocathode, and the results show that improving photoelectron transport characteristics is an effective method to improve the quantum efficiency of the AlGaN photocathode. The parameters that affect photoelectron transport characteristics mainly include the electron diffusion coefficient, the rear interface recombination rate, and the photoemission layer thickness [13,14,15,16], but the mechanism of the above parameters on the response time of the AlGaN photocathode is still unclear.

In this study, the response time of a uniformly doped AlGaN photocathode is calculated theoretically based on the photoelectron diffusion model, and the effects of electron diffusion coefficient, rear interface recombination rate, and photoemission layer thickness on the response time of a uniformly doped AlGaN photocathode are analyzed, which provides a theoretical basis for further research on improving the response time of the AlGaN photocathode.

2. Physical Model of AlGaN Photocathode Response Time Calculation

According to the “three-step model” of photoemission proposed by Spicer, the response time of a transmission AlGaN photocathode refers to the response time of the photocathode from illumination to electron emission, which is determined by the time of photoelectron diffusion movement. When the transmission AlGaN photocathode is uniformly doped, there is no built-in electric field introduced by band bending in the AlGaN photoemission layer. Therefore, the diffusion mode of the time electrons is concentration gradient diffusion, as shown in Figure 1.

The photoemission layer of AlGaN is a p-type semiconductor, and the photoelectrons excited to the conduction band are non-equilibrium carriers. The excited photoelectrons that transition to the bottom of the conduction band follow the carrier transport equation. The distribution of photoelectrons in the photocathode through diffusion and migration is a function of time. By solving the continuity equation of photoelectrons in the photoemission layer of AlGaN (Formula (1)) and a series of boundary conditions (Formulas (2)–(4)) by matrix difference, the numerical solution curve of the outgoing photoelectron flux density equation of the AlGaN photocathode (Formula (5)) can be obtained. The half-peak width of the curve is the response time of the transmitted uniformly doped AlGaN photocathode.

$${\displaystyle \frac{\partial \Delta \mathrm{n}}{\partial \tau }}={\mathrm{D}}_n{\displaystyle \frac{{\partial }^2\Delta n}{\partial x^2}}-{\displaystyle \frac{\Delta n}{{\tau }_n}}$$

(1)

$${\left[D_n{\displaystyle \frac{\partial \Delta n}{\partial x}}-S_v\Delta n\right]|}_{x=0}=0$$

(2)

$$\Delta n(d,t)=0$$

(3)

$$\Delta n(x,0)=F\cdot e^{-\alpha x}$$

(4)

$${j(t)=-D_n\cdot {\displaystyle \frac{\partial \Delta n(x,t)}{\partial x}}|}_{x=d}$$

(5)

where △n is the distribution of photoelectrons; t is time; D_n is the diffusion coefficient of photoelectrons in the AlGaN photoemission layer; x is the distance between the photoelectron generation position and the AlGaN surface; τ_n is the lifetime of photoelectrons in the AlGaN photoemission layer; S_v is the recombination rate of the AlN/AlGaN interface; d is the thickness of the AlGaN photoemission layer; and F is the value related to the AlGaN absorption coefficient α:

$$F=\eta (1-R)\cdot {\displaystyle \frac{N}{A}}\cdot \alpha \cdot \Delta t$$

(6)

where R is the reflectance of the light surface of the AlGaN photocathode in the incident light band; η is the rate of photoelectron production; N is the number of incident photons; A is the photocathode area; △t is the pulse width of the light source; and α is the absorption coefficient of AlGaN in the incident light band.

In order to achieve high sensitivity and fast response of the AlGaN photocathode in the 275 nm band, the Al group of the AlGaN photoemission layer was set to 0.3, and the calculated incident light wavelength was 275 nm. Further considering the structural design of the P-type heavily doped AlGaN photoemission layer and the technical difficulty of AlN/AlGaN epitaxial growth, the typical value of the photoelectron diffusion coefficient of the AlGaN photoemission layer is set as D_n = 60 cm²/s, the AlN/AlGaN interface recombination rate is S_v = 10⁵ cm/s, and the thickness of the AlGaN optical emission layer is d = 150 nm [17,18,19]. One of the parameters was calculated and analyzed, and the other two parameters remained unchanged.

3. Results and Analysis

3.1. Effect of Photoelectron Diffusion Coefficient on the Response Time of the AlGaN Photoemission Layer

When the thickness of the AlGaN photoemission layer is 150 nm and the interface recombination rate after AlN/AlGaN is $S_v$ = 10⁵ cm/s, the relationship between the outflow photoelectron flux density and response time of the AlGaN photocathode and the photoelectron diffusion coefficient of the AlGaN photoemission layer can be obtained through calculation, and the obtained results are shown in Figure 2. It can be seen from Figure 2a that with the increase in the photoelectron diffusion coefficient of the AlGaN photoemission layer, the time required for the AlGaN photocathode to reach the peak photoelectron flux density gradually decreases. This is because the larger the photoelectron diffusion coefficient is, the faster the photoelectrons in the AlGaN photoemission layer move under the action of the concentration gradient [20]; thus, the same diffusion thickness is conducive to shortening the response time of the AlGaN photocathode. As can be seen from Figure 2b, when the photoelectron diffusion coefficient of the AlGaN photoemission layer gradually increases, the response time of the AlGaN photocathode decreases continuously. When the photoelectron migration rate reaches 80 cm²/s, the response time of the AlGaN photocathode reaches 18.87 ps. This may be related to the gradual saturation of the photoelectron migration rate. Relation between peak response time and response time half-peak width and electron diffusion coefficient

3.2. Effect of Different Photoelectron Rear Interface Recombination Rates on the Response Time of the AlGaN Photoemission Layer

When the thickness of the AlGaN photoemission layer is 150 nm and the photoelectron diffusion coefficient D_n of the AlGaN photoemission layer is 60 cm²/s, the relationship between the outgoing photoelectron flux density and response time of the AlGaN photocathode and the recombination rate of the AlN/AlGaN rear interface can be obtained through calculation, and the obtained results are shown in Figure 3. From Figure 3a, one can observe that with the increase in the interface recombination rate after AlN/AlGaN, the time required for the AlGaN photocathode to reach the peak photoelectron flux density also gradually decreases. This is because the higher the recombination rate of AlN/AlGaN after the interface, the higher the concentration of electron–hole pair recombination centers at the AlN/AlGaN interface, and a large number of photoelectrons will be recombined by the recombination center, thus generating a local potential conducive to photoelectron diffusion at the rear interface and accelerating the diffusion and migration movement of photoelectrons [21]. As demonstrated in Figure 3b, when the interface recombination rate after AlN/AlGaN reaches 106 cm/s, it can be calculated by solving Formulas (1)–(6) that the response time of the AlGaN photocathode reaches 16.88 ps, and the subsequent response time will not decrease significantly with the further increase in the interface recombination rate after AlN/AlGaN. Therefore, considering both quantum efficiency and response time, the interface recombination rate after AlN/AlGaN should not be greater than 10⁶ cm/s.

3.3. Effect of AlGaN Photoemission Layer Thickness on Response Time

When the photoelectron diffusion coefficient D_n of the AlGaN photoemission layer is 60 cm²/s and the interface recombination rate S_v after AlN/AlGaN is 10⁵ cm/s, the relationship between the outgoing photoelectron flux density and response time of the AlGaN photocathode and the thickness of the AlGaN photoemission layer can be obtained through calculation, and the obtained results are shown in Figure 4. It can be seen from Figure 4a that with the increase in AlGaN photoemission layer thickness, the time required for the AlGaN photocathode to reach the peak photoelectron flux density increases substantially. This confirms that the greater the thickness of the AlGaN photoemission layer, the greater the diffusion and migration distance of photoelectrons [22,23], and the longer the diffusion time of photoelectrons from the AlN/AlGaN back interface to the AlGaN/vacuum interface. As shown in Figure 4b, when the thickness of the AlGaN photoemission layer is reduced from 250 nm to 50 nm, the response time of the AlGaN photocathode decreases from 63.28 ps to 9.91 ps, and the response time of the AlGaN photocathode is greatly improved [24].

4. Conclusions

In conclusion, the influence of the electron diffusion coefficient, the recombination rate at the back interface, and the thickness of the optical emission layer on the outgoing photoelectron flux density and response time of the AlGaN photocathode were analyzed using a photoelectron diffusion model. The results show that the increase in the electron diffusion coefficient and rear interface recombination rate is beneficial to reducing the response time of the AlGaN photocathode, and the thickness of the photoemission layer has the greatest effect on the response time of the AlGaN photocathode, with the lowest effect of 9.91 ps, which can meet the needs of solar-blind UV missile alarm systems. The research results in this paper will provide necessary theoretical guidance for the design and preparation of high-performance, fast-response AlGaN photocathodes in the future.