In recent years, there have been many studies on the preparation of high-quality hexagonal boron nitride (hBN) materials and the application of hBN ultraviolet (UV) photodetectors. Cubic boron nitride (cBN) has a higher band gap compared with hBN [cBN: (6.4±0.5) eV, hBN: (5.9±1.0) eV], and a higher hardness and melting point, which makes cBN-based UV photodetectors more advantageous. However, on one hand, due to a large number of spontaneous defects inside cBN and the non-uniform process, which result in poor doping efficiency of the prepared devices; on the other hand, different doped impurities exhibit different optical and electrical properties, both making the poor performance of the detectors. Additionally, different photodetector structures such as pin, APD, and heterostructure can also bring about performance differences. Silvaco TCAD software is based on a series of physical models and physical equations that rely on well-established solid-state and semiconductor physics theories or on some empirical formulas to accurately predict the electrical, thermal, and optical results of semiconductor devices. Meanwhile, mesa pin photodetectors feature low dark current and high internal quantum efficiency. Therefore, a numerical model of cBN-based mesa structured pin photodetector is built by Silvaco TCAD software, and the effects of different doping concentrations and thicknesses of the cBN layer on photocurrent, dark current, and internal quantum efficiency of this model are calculated.

The numerical calculation model of cBN-based mesa-structured pin is built by Silvaco TCAD software (Fig. 2). As the intrinsic layer is n-type by default in the undoped case, it is replaced by a n-type cBN with a doping concentration of 1×10¹⁵ cm^-3 and a thickness of 0.6 μm, and p-type and n-type background carrier concentrations are set as 1×10¹⁴ cm^-3 and 1×10¹⁹ cm^-3 with thicknesses of 0.1 μm and 2.0 μm respectively. Based on the constant low-field mobility model (conmob), parallel electric field-dependent mobility model (fldmob), Auger recombination, Shockley-Reed-Hall (SRH) recombination, and basic semiconductor equations of Poisson's equation, carrier transport equations, and carrier continuity equations, the effects of doping concentrations of each layer and thicknesses of each layer on the photocurrent, dark current and internal quantum efficiency are simulated and calculated by the "control variate" method. Firstly, the spectral response of the initial structure is obtained in the deep UV band (Fig. 3), which indicates that the device has a strong response to deep ultraviolet. Secondly, on this basis, the doping concentrations of p-type, i-type, and n-type layers are varied to analyze the changes in performance parameters and select the better doping concentration values with sound device performance. Finally, the thicknesses of p-type, n-type, and intrinsic layers are changed to analyze the performance and select better values.

The doping concentration of p-type rises, and the photocurrent, dark current, and internal quantum efficiency firstly increase and then decrease (Figs. 4-6). This is because the concentration of holes in the p-type region is higher and the probability of recombination increases, resulting in fewer electron-hole pairs generated by photoexcitation. The photocurrent and dark current decrease with the increasing doping concentration of the i-type layer, while internal quantum efficiency is hardly affected (Figs. 7-9). The possible reason is that the intrinsic layer is replaced by a n-type layer and the rising electron concentration increases the recombination probability, leading to the decreased photocurrent. The dark current decreases with the rising doping concentration which enhances the built-in electric field. The dark current increases with the increasing doping concentration of the n-type layer (Fig. 11), but the photocurrent and internal quantum efficiency decrease with it (Figs. 10 and 12). The possible reason is that the heavy doping concentration of the n-type layer increases the diffusion current inside the region, which causes decreased photocurrent and internal quantum efficiency and increased dark current. The photocurrent and internal quantum efficiency decrease while the dark current increases with the rising thickness of the p-type layer (Figs. 13-15). Many photogenerated carriers will be absorbed by the p-type layer, which is too thick to allow carriers to diffuse into the electric field region and form photocurrent. The dark current increases with the thickness of the intrinsic layer while the internal quantum efficiency decreases with it (Figs. 17 and 18). Differently, the thicker intrinsic layer thickness leads to a smaller photocurrent at low bias,but the photocurrent is positively correlated with thickness at high bias(Fig. 16). Thus, the effect of bias voltage on photocurrent should be considered. An increase in the n-type layer thickness increases the photocurrent but causes a little decrease in the internal quantum efficiency, without clear regularity of thickness and dark current (Figs. 19-21). The increasing thickness of the n-type layer means rising light absorption area, and the rising volume/area ratio decreases the recombination. Those could be the possible factors for the photocurrent increase, which causes the current crowding phenomenon, then local heat concentration, and higher thermal energy obtained by electrons. Finally, Auger recombination is enhanced to reduce the internal quantum efficiency.

The increase in the doping concentration of the p-type layer makes all the parameters increase first and then decrease, but the overall change has little effect. The increasing doping concentration of the i-type layer decreases the photocurrent and dark current and has little effect on the internal quantum efficiency. The rising doping concentration of the n-type layer makes the photocurrent and internal quantum efficiency increase, and the dark current greatly decreases to around 10^-20 A. Increasing the thickness of the p-type layer exerts almost no effect on the dark current, but decreases the photocurrent and internal quantum efficiency. The rise in intrinsic layer thickness will increase the dark current and decrease the internal quantum efficiency. The photocurrent change with the thickness of the intrinsic layer may also be controlled by the bias voltage. The larger thickness of the n-type layer leads to larger photocurrent, but it has little effect on the dark current and internal quantum efficiency. Since there are no defects in the material during the simulation and the impurities are uniformly distributed, the calculation results are ideal. However, the actual experimental preparation of the device is influenced by the process factors, and the various defects introduced in the material and the non-uniform distribution of impurities make the actual value worse than the simulated calculation value. Finally, the doping concentrations of p-type, i-type, and n-type layers are set as 1×10¹⁷ cm^-3, 1×10¹⁵ cm^-3, and 1×10¹⁵ cm^-3, and the thicknesses of p-type layer, i-type layer, and n-type layer are 0.1 μm, 0.8 μm, and 2.2 μm respectively, the performance obtains the photocurrent is 3. 910×10^-8 A, with a maximum dark current of 8.177×10^-20 A and internal quantum efficiency of 98.565%.