硅异质结光电探测器的光谱灵敏度

Abstract—We have studied the optical properties of two-dimensional (2D) Schottky photodiode  heterojunctions made of chemical vapor deposited (CVD) graphene on n- and p-type Silicon (Si)  substrates. Much better rectification behavior is observed from the diodes fabricated on n- Si  substrates in comparison with the devices on p-Si substrates in dark condition. Also, graphene – n-Si  photodiodes show a considerable responsivity of 270 mAW-1 within the silicon spectral range in DC  reverse bias condition. The present results are furthermore compared with that of a molybdenum  disulfide (MoS2) – p-type silicon photodiodes.

Molybdenum-disulfide (MoS2), a layered transition metal dichalcogenide (TMD), is an important member in the family of two-dimensional (2D) materials. In contrast to graphene, MoS2 is a semiconductor and its  bandgap varies depending on the number of the layers. Therefore, MoS2 has the possibility to detect light at  different wavelengths. Monolayer MoS2 has a direct bandgap of ~1.8 eV, while bulk MoS2 has an indirect  bandgap of ~1.3 eV. In addition, MoS2 has high transparency and mechanical flexibility and it is easy to  process . All of these exceptional properties make MoS2 another promising material for electronic and  optoelectronic applications.

In this work, we report on CVD graphene-based Schottky barrier photodiodes of simple architecture  ,which are also scalable, reproducible and of low-cost. To fabricate the diodes, we have adopted a similar  process as in, where large area CVD-grown graphene films were transferred onto pre-patterned nand p-type Si-substrates. Afterwards, we performed electrical and optical characterizations on the  photodiodes. The optical data obtained from the measurements are further compared to that of MoS2 – p-type  silicon photodiodes, which have been presented in detail in our previous work.

Electrical measurements on the diodes were performed on a Karl Süss probe station connected to a  Keithley semiconductor analyzer (SCS4200) under ambient conditions. The voltage for all devices was swept  from 0 to +2 V for forward and from 0 to -2 V for reverse biasing. A white light source (50 W halogen lamp)  with a dimmer to control the light intensity was used to measure the photoconductivity of the diodes. The  variable incident light intensity has been modulated between 0 and 3 mWcm-2 . A CA 2 laboratory thermopile  was used to measure the intensity of the light source.  

Fig1

Henceforth, further measurements in this work were performed on the graphene – n-Si diodes. The  forward J-V characteristic of the diode can be described by the single-exponential Shockley equation [30] allowing the extraction of the ideality factor and the Schottky barrier height (SBH). The ideality factor, the  Schottky barrier height (SBH) and the series resistance of 1.52 ± 0.1, 0.65 ± 0.08 eV and 7.5 ± 0.7 kΩ,  respectively, were extracted for the diodes in Fig. 3a. The extraction method is described in detail in [31].  The corresponding energy band diagrams for graphene – n-Si and p-Si diodes at zero bias voltage in the dark  is shown in Fig. 3c and 3d, respectively.

Graphene based Schottky barrier photodiodes were fabricated using a simple, scalable and reproducible  technology. We observed that diodes fabricated on an n-Si substrate exhibit a rectifying behavior, whereas  the devices fabricated on a p-type Si substrate act as photoresistors. This behavior can be attributed to p-type  graphene-doping due to its exposure to the ambient atmosphere leading to a higher Schottky barrier to the ntype silicon. Graphene – n-Si diodes exhibit a broad SR with responsivity up to 270 mAW-1 . Nevertheless,  the main photoexcitation takes place in the n-type Si substrate due to low optical absorption by single layer  graphene. Unlike in graphene-based diodes, the SR is limited to the band structure of MoS2 in the MoS2based photodiodes. This study promotes SR measurements as an excellent tool to probe the electronic  properties of novel 2D-materials.