Helping quantum research, Nanjing University achieves the first ultra-high avalanche gain at intrinsic breakdown voltage
Avalanche detectors have very urgent application needs in many fields such as quantum computing, information communication, and astronomical observation. At present, conventional avalanche detectors mainly use non-layer materials, which cannot simultaneously achieve the intrinsic breakdown voltage close to the material band gap and high avalanche gain, and face major challenges such as excessive power consumption and low sensitivity when facing future applications. Exploring and utilizing new materials is considered to be an effective way to solve the above challenges, however, how to design artificial quantum material avalanche detector devices that can meet the above needs is a topic of great interest.
Recently, the team of Shijun Liang and Feng Miao at the School of Physics, Nanjing University, has successfully prepared graphite/indium selenide (graphite/InSe) van der Waals Schottky photodetectors, achieving for the first time experimentally ultra-high avalanche gain at intrinsic breakdown voltage, which provides a feasible technical pathway for the development of new low-energy, highly sensitive weak-light detectors.
Emerging two-dimensional quantum materials show great potential for constructing high-performance photodetectors due to their unique physical properties. In recent years, Prof. Feng Miao's team has focused on the exploration of new physical properties of 2D quantum materials and has made breakthroughs in solid-state quantum simulators, highly robust memristors, ballistic avalanche devices, brain-like vision sensors, etc. by using the unique technology of "Atomic Lego". On this basis, the team has recently constructed graphite/InSe van der Waals Schottky photodetectors by taking advantage of the reduced dimensionality of electro-phonon scattering within the layered InSe material and the strong electric field in the high quality van der Waals Schottky junction region, and achieved an intrinsic breakdown voltage close to the band gap (1.8 V) and ultra-high gain (3*105). This work provides a new technical avenue for developing next-generation high-performance avalanche detectors.
In this work, the collaborative research team constructed van der Waals Schottky avalanche detectors using graphite and InSe van der Waals materials (Figure 1a). The collaborative research team found that when the graphite/InSe Schottky junction was in reverse bias, increasing the bias voltage Vds to 5.5 V was able to produce a current hopping phenomenon of more than five orders of magnitude, which indicated the existence of the avalanche breakdown phenomenon (Figure 1b). Further, the research team used very weak light irradiating the device at a wavelength of 532 nm and a power of 6.9 pW and observed a sharp rise in photocurrent, producing an avalanche gain of up to 3*105. With increasing laser power, the photocurrent response exhibited two distinct characteristics in the two regions where Vds was below and above the breakdown voltage: at Vds below the breakdown voltage, due to the photoconductivity effect, the The photocurrent gradually increases with increasing light intensity at Vds below the breakdown voltage due to the photoconductance effect, while at Vds above the breakdown voltage, the photocurrent in the low power interval hardly changes with increasing light intensity due to the limitation of the series resistance (Figures 1(c) and 1(d)).
Figure 1: (a) Schematic diagram of the graphite/InSe avalanche detector. Inset: optical image of the device. (b) Current-bias (Ids-Vds) characteristic curves at 160 K on a logarithmic scale (black line, dark state; red line, 6.9 pW under 532 nm laser irradiation) and the corresponding avalanche gain curves (blue line), with the corresponding vertical axes marked by arrows. Inset: Ids-Vds characteristics at a linear scale covering the negative bias range (dark state). (c) Ids-Vds characteristic curves measured over the laser power range of 6.9 pW to 69 μW. (d) Photocurrent Iph (red line) in avalanche mode at Vds of 5.5V and photoconductor mode at Vds of 2V (blue line) as a function of laser power.
Fig. 2. (a) Schematic diagram of the reduction of the electric-phonon scattering dimension in van der Waals materials. (b) Schematic diagram of the collisional ionization process involving two types of carriers. (c) Ionization rate and (d) gain as a function of electric field strength and average free range length. The black dashed line indicates the collisional ionization probability p equal to 1, when the gain M diverges. (e) Ionization rate and ionization probability p in two-dimensional material (red solid line) and in three-dimensional material (blue solid line) as a function of electric field strength E. The black dashed line and the blue dashed line are the asymptotic lines of ionization rate in the two-dimensional and three-dimensional materials, respectively. The horizontal black dotted line corresponds to the case of ionization probability p = 1. (f) Gain as a function of electric field strength E in two-dimensional material (red solid line) and three-dimensional material (blue solid line). The vertical gray dashed line corresponds to the case where the gain M is divergent.
The collaborative research team found through theoretical calculations that the excellent performance of the graphite/InSe photodetector stems mainly from the reduction of the electric-phonon scattering dimension due to the super high interlayer potential barrier in the InSe material (Figure 2a). This barrier localizes the carrier transport behavior in a two-dimensional plane, effectively limiting the interlayer scattering process of carriers, resulting in a significant increase in the ionization probability. Unlike the single-carrier ionization process in conventional avalanche materials (Figure 2b), the two-carrier ionization process is characterized by multiple collisional ionization behavior of the carriers only at an intrinsic breakdown voltage close to the band gap, which is fully consistent with the experimentally observed low-voltage high-gain behavior. To further determine the mechanism of the electric-phonon scattering dimension on the collisional ionization probability, the team developed a collisional ionization model for laminar materials and theoretically established the dependence of the collisional ionization probability and gain on the mean free range and electric field, as shown in Figure 2c and Figure 2d. The calculation results show that the collisional ionization probability of two-dimensional laminate materials can reach 100% under the strong electric field (Fig. 2e) and the gain will disperse (Fig. 2f), while the collisional probability and gain of conventional three-dimensional materials will both tend to saturate.
Figure 3: (a) Ids-Vds curves at different temperatures. Inset: three consecutive carrier transport processes (injection, ionization, and collection). (b) Breakdown voltage (Vbd) and gain versus temperature.(c) Ids-Vds characteristic curves and (d) Breakdown voltage (Vbd) and gain versus temperature for InSe/Ti Schottky junction reverse bias.
Further, the collaborative research team achieved high gain at a breakdown voltage close to the intrinsic band gap. As shown in Fig. 3a, the avalanche breakdown voltage decreases continuously as the temperature increases, and at a temperature of 260 K, the avalanche breakdown voltage is 1.8 V, which is consistent with the theoretical predicted value. Meanwhile, the collaborative research team found that the barrier height at the carrier collection end affects the temperature-dependent characteristics of breakdown voltage and gain (Figs. 3b-3d), and pointed out that this characteristic relationship mainly originates from the combined effect of the temperature-dependent collisional ionization process and the thermally assisted carrier collection process.
https://onlinelibrary.wiley.com/doi/10.1002/adma.202206196
