High-stability 4H-SiC avalanche photodiodes for UV detection at high temperatures

Xingye Zhou; Yuanjie Lü; Hongyu Guo; Xubo Song; Yuangang Wang; Shixiong Liang; Aimin Bu; Zhihong Feng

doi:10.3788/COL202321.032502

Abstract

In this work, high-stability 4H-SiC avalanche photodiodes (APDs) for ultraviolet (UV) detection at high temperatures are fabricated and investigated. With the temperature increasing from room temperature to 150°C, a very small temperature coefficient of 7.4 mV/°C is achieved for the avalanche breakdown voltage of devices. For the first time, the stability of 4H-SiC APDs is verified based on an accelerated aging test with harsh stress conditions. Three different stress conditions are selected with the temperatures and reverse currents of 175°C/100 µA, 200°C/100 µA, and 200°C/500 µA, respectively. The results show that our 4H-SiC APD exhibits robust high-temperature performance and can even endure more than 120 hours at the harsh aging condition of 200°C/500 µA, which indicates that 4H-SiC APDs are very stable and reliable for applications at high temperatures.

Keywords

avalanche Geiger mode high temperature photodiode silicon carbide UV detector

1. Introduction

Due to the high-quality and chemically stable material, wide bandgap semiconductor avalanche photodiodes (APDs) based on silicon carbide (SiC) have advantages of being photosensitive, visible-blind, reliable, small, and easily integrated, which makes them powerful competitors for ultraviolet (UV) detectors in plenty of applications, such as corona detection, astronomical research, UV communication, and missile plume detection^[1–5]. In the past few years, with the improvement of crystal material growth and device fabrication technology, 4H-SiC APDs have achieved great advancements with high gain, high quantum efficiency (QE), low dark current, and even excellent Geiger-mode single-photon-detection performance^[6–13]. Related arrays based on 4H-SiC APDs have also been demonstrated for UV imaging in the future^[14–18]. However, most of the reported research work has been focused on 4H-SiC APDs at low temperatures. For such critical applications as flame detetion in gas turbines and UV detection for space exploration, high-stability UV detectors at high temperatures are in great demand. The temperature-dependent performance of 4H-SiC APD-based UV detectors has been analyzed by few research groups in the world^[19–21]. The stability and reliability of 4H-SiC APDs with harsh stress conditions have not yet been reported.

In this Letter, high-stability 4H-SiC APDs at high temperatures are fabricated and investigated for UV detections. Based on the variable-temperature $I - V$ test, junction temperature test, and accelerated aging test, the stability of 4H-SiC APDs is extensively analyzed and verified. With the temperature increasing from room temperature to 150°C, a stable avalanche breakdown voltage is obtained with a very small temperature coefficient of 7.4 mV/°C for 4H-SiC APD devices. In addition, the high-temperature stability of our 4H-SiC APDs was verified with a high junction temperature of $\sim 270 ° C$ by using an infrared microscope. Finally, for the first time (to the best of our knowledge), the stability of 4H-SiC APDs is further proved based on an accelerated aging test under harsh stress conditions. Three different stress conditions are selected with the temperatures and reverse currents of 175°C/100 µA, 200°C/100 µA, and 200°C/500 µA, respectively. Our 4H-SiC APDs can endure more than 120 h, even at the aging condition of 200°C/500 µA. The results in this work indicate that 4H-SiC APDs are very stable and reliable in harsh environments at high temperatures.

2. Device Design and Fabrication

First, the device structure of 4H-SiC APDs is carefully designed, which determines the performance under dark current, avalanche breakdown voltage, QE, etc. As shown in Fig. 1(a), a separate absorption charge multiplication (SACM) epilayer structure is used for our 4H-SiC APDs in this work. From bottom to top, the wafer is composed of a $p^{+}$ layer with a thickness of 3 µm and a doping concentration of $N_{a} = 1 \times 10^{19} {cm}^{- 3}$ , a 0.5 µm-thick $n^{-}$ multiplication layer with a doping concentration of $N_{d} = 1 \times 10^{15} {cm}^{- 3}$ , an $n$ charge layer with a thickness of 0.2 µm and a doping concentration of $N_{d} = 1 \times 10^{18} {cm}^{- 3}$ , a 0.5 µm-thick $n^{-}$ absorption layer with a doping concentration of $N_{d} = 1 \times 10^{15} {cm}^{- 3}$ , and a 0.3 µm-thick $n^{+}$ contact layer with a doping concentration of $N_{d} = 1 \times 10^{19} {cm}^{- 3}$ . By chemical vapor deposition (CVD), the epimaterial was grown on an $n^{+}$ 4H-SiC substrate according to the optimized epistructure. During the growth process of the 4H-SiC epilayers, nitrogen donors and aluminum acceptors were used for n-type and p-type in situ doping, respectively. The doping concentration profile of epilayers is obtained by secondary ion mass spectroscopy (SIMS), as illustrated in Fig. 1(b). It should be noted that the low doping concentration is limited to $1 \times 10^{16} {cm}^{- 3}$ by the measurement system of SIMS. It can be seen that the doping concentration profile of the grown crystal material matches that of the designed structure.

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Figure 1.Device structure of 4H-SiC APDs. (a) Schematic cross-sectional structure; (b) doping concentration profile of epilayers obtained by SIMS.

Device fabrication was based on an optimized manufacturing process, and the detailed fabrication process can be referred to in Ref. [22]. For isolation and passivation, a ${SiN}_{x}$ layer with a thickness of 500 nm was deposited by dual-frequency plasma-enhanced chemical vapor deposition (PECVD). The ${SiN}_{x}$ in the active area of 4H-SiC APD was removed to open an optical window. The active area diameter of the fabricated 4H-SiC APDs is 150 µm. Finally, the chip was attached into a TO46 package for the accelerated aging test.

3. Results and Discussion

The room-temperature performance of 4H-SiC APDs was first investigated. As shown in Fig. 2(a), the dark current and photocurrent at $λ = 280 nm$ as well as the multiplication gain $M$ of 4H-SiC APDs were characterized as a function of the reverse voltage. The photo image of the fabricated 4H-SiC APD is shown in the inset of Fig. 2(a). It is shown that the dark current maintains at a very low level of about 10 pA until the avalanche breakdown happens near 163 V. With the definition of a unity gain at the reverse bias voltage of $V_{r} = 10 V$ , a high avalanche multiplication gain of $M > 10^{6}$ is calculated for our 4H-SiC APDs. The spectral responsivity of 4H-SiC APDs at the unity gain was measured based on a monochromator, and a silicon UV photodetector was used to calibrate the power of a xenon lamp light source. As shown in Fig. 2(b), a peak responsivity of 0.124 A/W at the wavelength of 285 nm is obtained, corresponding to the maximum external QE of 54%.

Figure 2.Characteristics of 4H-SiC APDs at room temperature. (a) I-V and I-M curves; (b) spectral responsivity. Here, the photo image of the fabricated 4H-SiC APD is shown in the inset of Fig. 2(a).

In order to investigate the high-temperature device performances, the $I - V$ characteristics of our 4H-SiC APDs were measured, with the temperatures ranging from room temperature to 150°C. The dark currents as a function of reverse voltage near the breakdown voltage ( $V_{b}$ ) are provided in Fig. 3(a), and it is shown that the breakdown voltage shifts positively when the temperature increases. The result suggests that hard avalanche breakdown happens in our devices, which is expected for high-gain APDs. In order to evaluate the $V_{b}$ shift more clearly, the dependence of $V_{b}$ shift on the temperature is illustrated in Fig. 3(b), and here $V_{b}$ is defined as the reverse voltage at which a multiplication gain is 1000. It can be seen clearly from Fig. 3(b) that the avalanche breakdown voltage shifts positively and linearly with the temperature increase. This is because the impact ionization coefficient decreases at high temperatures^[22], and a higher reverse voltage is desired to create a stronger electric field for the avalanche effect. For practical usage, a small temperature coefficient is expected for the avalanche breakdown voltage of APDs. With the optimized epitaxial structure, our 4H-SiC APDs exhibit a stable avalanche breakdown voltage with a small temperature coefficient of $C_{T} = 7.4 mV / K$ . As is known, the dark current, breakdown voltage, gain, and temperature coefficient of $V_{b}$ are determined by the epitaxial structure, which has to be carefully designed for a trade-off between the device performance parameters. The comparison results of different APDs are shown in Table 1, and it can be seen that a very small temperature coefficient of $V_{b}$ is achieved for our 4H-SiC APD devices. Meanwhile, a small breakdown voltage, low dark current, high gain, and high QE are also obtained. The excellent device performance demonstrates that our 4H-SiC APDs are very stable at high temperatures due to the high-quality epitaxial material, optimized device structure, and fabrication process.

Parameters	Ref. [19]	Ref. [20]	This Work
V_b (V)	634	186	163
I_dark $@ 95 %$ V_b ( $A / {μm}^{2}$ )	$5 \times 10^{- 14}$	$1.7 \times 10^{- 17}$	$1.1 \times 10^{- 15}$
Gain	2500	$> 10^{6}$	$> 10^{6}$
Maximum QE (%)	45%@290 nm	53%@290 nm	54%@285 nm
C_T of V_b (mV/°C)	110	14	7.4

Table 1. Comparison of Different 4H-SiC APDs

View all Tables

Figure 3.Performance of 4H-SiC APDs at different temperatures. (a) Dark currents near the breakdown voltage; (b) breakdown voltage shift as a function of temperature.

In addition, the high-temperature stability of our 4H-SiC APDs was verified with a high junction temperature, as shown in Fig. 4. The two-dimensional mapping of junction temperature was obtained for our fabricated 4H-SiC APDs based on an infrared microscope, during which the device was stressed with large power consumption and high temperatures. Figures 4(a) and 4(b) provide the junction temperature mapping of 4H-SiC APD with different maximum junction temperature of 145°C and 270°C, respectively. It should be noted that the distribution of junction temperature is nonuniform for 4H-SiC APDs, which can be explained by the asymmetric carrier lateral drift in the off-orientated 4H-SiC^[7,22–25]. The results demonstrate that the fabricated 4H-SiC APD can bear a junction temperature of $\sim 270 ° C$ [see Fig. 4(b)], proving the stability of our devices at high temperatures.

Figure 4.Two-dimensional mapping of the junction temperature for 4H-SiC APDs with different maximum junction temperatures. (a) 145°C; (b) 270°C.

Finally, for the first time, an accelerated aging test with harsh stress conditions was performed to further confirm the stability of 4H-SiC APDs in this work. Three different stress conditions are selected with the temperatures and reverse currents of 175°C/100 µA, 200°C/100 µA, and 200°C/500 µA, respectively. The stress condition for 4H-SiC APDs in this work is much harsher than that of the aging test for InGaAs APDs in Ref. [26]. Device failure is defined as a state in which the dark current at a bias of 0.5 V over the avalanche breakdown voltage increases more than 0.01 µA compared with the initial value. A pre-aging test of 24 h at the stress condition of 175°C/100 µA was done to check the bonding of the device sample, and the first 24 h of pre-aging did not result in changes in the dark-current characteristics.

After the pre-aging, a long-time aging test was conducted. The dark-current variation of 4H-SiC APD as a function of accelerated aging time with different stress conditions is shown in Fig. 5. The dark current was measured at room temperature with a reverse bias voltage, where a gain of $M = 1000$ was obtained. It demonstrates that there is almost no degradation in the dark current of 4H-SiC APD after 120 h of aging at the condition of 175°C/100 µA and 200°C/100 µA. With the aim to accelerate the degradation of the device sample, a harsher stress condition with a reverse bias current of 500 µA at the temperature of 200°C was chosen. The fabricated 4H-SiC APD can endure more than 120 h, even at the aging condition of 200°C/500 µA. After 129 h of aging under the harsh condition of 200°C/500 µA, the device sample failed, with hard breakdown. In practical usage, 4H-SiC APDs usually operate with a low-level dark current of a few pico- or nanoamperes. Therefore, our 4H-SiC APDs can actually endure many more hours at 200°C. To our knowledge, this is the first report that verifies the stability of 4H-SiC APDs based on an accelerated aging test. The reliability and lifetime of 4H-SiC APDs will be further investigated based on more device samples in our ongoing work in the future.

Figure 5.Dark current of 4H-SiC APD as a function of aging time with different stress conditions. (a) 175°C/100 µA; (b) 200°C/100 µA; (c) 200°C/500 µA.

4. Conclusion

In conclusion, high-stability 4H-SiC APDs are fabricated and investigated for UV detection at high temperatures. The high-temperature stability of our 4H-SiC APDs was verified based on the variable temperature $I - V$ test, a high junction temperature, and the accelerated aging test under harsh stress conditions. A stable avalanche breakdown voltage with a very small temperature coefficient of 7.4 mV/°C is achieved from room temperature to 150°C. The fabricated 4H-SiC APDs can bear a high junction temperature of $\sim 270 ° C$ and can endure more than 120 h at the aging condition of 200°C/500 µA. The results in this work indicate that 4H-SiC APDs have the capability to operate in harsh environment at high temperatures.

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