Infrared photodetectors have wide applications in both military and civilian areas. The current infrared detectors can be mainly divided into three categories by their sensing wavelengths: short-wave infrared (SWIR, 1.5-3 µm), mid-wave infrared (MWIR, 3-5 µm) and long-wave infrared (LWIR, 8-12 µm). As promising alternatives to bulk materials, low-dimensional nanomaterials have been explored successfully as tunable infrared-sensitive materials. This thesis aims to develop low-dimensional nanomaterial-based multispectral photodetectors, and to understand and control the photocarrier generation, transport and collection mechanism. Mercury chalcogenide colloidal quantum dots (CQDs), graphene and CQDs-graphene nanocomposites have been investigated in this dissertation. The main contributions of this thesis are summarized as follows:
First, graphene/silicon-based infrared photodetectors have been developed. Since the interband transitions in graphene were governed by Pauli blocking, the spectral absorbance of graphene could be tuned by controlling the Fermi level shifts. Graphene samples with different spectral absorbance were assembled with electrodes by poly(methyl methacrylate)(PMMA)-assisted transfer process. The photodetectors consisted of three pixels with cut-off wavelength at 4.75 µm, 6.4 µm and 9 µm. The interfacial photocarrier transport mechanism was experimentally studied and the device structure was optimized to enhance the carrier collection efficiency. The responsivity of the fabricated graphene/silicon junction based photodetectors was in the ranges of 0.2-1 mA/W.
Second, the performance of graphene/silicon-based photodetectors could be enhanced by the addition of mercury telluride (HgTe) CQDs. Similar to graphene, the optical absorption of HgTe CQDs was dominated by interband transitions. In this case, any photon with energy higher than or equal to the interband gap energy of HgTe CQDs can be absorbed, leading to broadband absorption. The spectral absorption and photoresponse of HgTe CQDs were characterized and demonstrated tunable cut-off wavelength of 4.8 µm, 6.0 µm and 9.5 µm. Furthermore, by transferring the HgTe CQDs films onto graphene/silicon junction, HgTe CQDs film/graphene/silicon-based photodetectors were fabricated. The results showed ~300% enhancement ratio of responsivity. The enhanced responsivity was attributed to the increased number of photocarriers transferred from HgTe CQDs.
Third, mercury selenium (HgSe) CQDs with narrowband absorption was investigated to provide better selectivity in different wavelengths. The narrowband absorption of HgSe CQDs originated from intraband transitions from 1Se to 1Pe. In this case, only photons with energy matching the intraband gap could be absorbed. Four narrowband photodetectors based on HgSe CQDs film with tunable peak wavelengths of 4.2 µm, 6.4 µm, 7.2 µm and 9.0 µm were fabricated and demonstrated peak responsivity in the ranges of 10-30 mA/W. The peak responsivity was further enhanced by the integration with the plasmonic nanodisk array, reaching up to 145 mA/W@ 4.2 μm, 92.3 mA/W@ 6.4 μm, 88.6 mA/W@ 7.2 μm, and 86 mA/W@ 9.0 μm, respectively. Moreover, we developed a method to synthesize HgSe CQDs-twisted graphene nanocomposites. In the nanocomposites, HgSe CQDs were tightly attached to the surface of graphene and the photocarriers excited in HgSe CQDs could directly transfer into graphene without hopping process, leading to higher collection efficiency. HgSe CQDs-twisted graphene nanocomposites/silicon junction based photodetectors were fabricated by drop-casting method. Compared with HgSe CQDs/graphene/silicon junction, over 2700% enhancement ratio of spectral responsivity was achieved, reaching up to 31.5 mA/W@ 9µm. The interfacial energy band diagram was deduced for a better understanding of the photocarrier transfer process.
Fourth, eight-pixel multispectral infrared photodetector was fabricated by using HgTe CQDs and each pixel could response to different bands of infrared. Based on the wavelength multiplexing principle, a mathematical reconstruction algorithm was developed to reconstruct the spectrum of the incident infrared. The fabricated multispectral infrared photodetector was able to reconstruct the spectrum in the range from 2 μm to 10 µm with resolution of 1 µm.
In summary, we have systematically investigated the fundamental properties, fabrication methods and potential applications of low-dimensional nanomaterials for the detection of SWIR, MWIR and LWIR. The fabricated multispectral photodetectors demonstrated tunable sensing wavelengths. We believe that this study could shed light on the development of high performance multispectral infrared photodetectors for potential application in multicolor focal plane camera.