Ag2S QDs/Si Heterostructure-Based Ultrasensitive SWIR Range Detector

Abstract

In the 20^th century, microelectronics was revolutionized by silicon-its semiconducting properties finally made it possible to reduce the size of electronic components to a few nanometers. The ability to control the semiconducting properties of Si on the nanometer scale promises a breakthrough in the development of Si-based technologies. In this paper, we present the results of our experimental studies of the photovoltaic effect in Ag₂S QD/Si heterostructures in the short-wave infrared range. At room temperature, the Ag₂S/Si heterostructures offer a noise-equivalent power of 1.1 × 10^-10 W/√Hz. The spectral analysis of the photoresponse of the Ag₂S/Si heterostructures has made it possible to identify two main mechanisms behind it: the absorption of IR radiation by defects in the crystalline structure of the Ag₂S QDs or by quantum QD-induced surface states in Si. This study has demonstrated an effective and low-cost way to create a sensitive room temperature SWIR photodetector which would be compatible with the Si complementary metal oxide semiconductor technology.

Keywords: detector; quantum dots; short-wave infrared range; silicon.

Figure 1

Transmission electron microscopy (TEM) images with size histograms of Ag₂S quantum dots (QDs) deposited on an Si substrate (a); X-ray diffraction (XRD) patterns of Ag₂S QDs with different sizes (1–3), curves with dashed numbers (1’–3’) respect to Ag₂S QDs deposed on Si substrate (b).

Figure 2

Optical absorbance spectra of Ag₂S QDs colloidal solutions. The curves have characteristic features at of 1.6 eV, 2.1 eV, and 2.7 eV, for solutions with an Ag₂S QDs average size of 3.2 nm, 2.0 nm, and 1.8 nm, respectively.

Figure 3

Luminescence spectra of Ag₂S QDs colloidal solutions. The maxima of luminescence bands are at 660 nm, 620 nm, and 950 nm for Ag₂S QDs with average size of 1.8 nm, 2.0 nm and 3.2 nm respectively.

Figure 4

(a) An optical image of the inner part of the device; infrared (IR) radiation is focused on the Si surface between the Ti/Au contacts. (b) An image of the device mounted on the Si lens. (c) An IV curve of the Au/Ti/Si/Ti/Au structure; the shape of the IV curve indicates the formation of a spatial charge distribution at the Si/Ti interface.

Figure 5

Spectral response curves of the uncovered Si and Ag₂S/Si heterostructures. The comparison of the curves reflects a sub-band gap IR photon absorption in Si.

Figure 6

Curves of thermoluminescence for Ag₂S QDs colloidal solutions (top curve) and Ag₂S QDs deposed on Si substrate (bottom curve) with average size of 3.2 nm.

Figure 7

The responsivity measured for two groups of devices with QDs of diameters 1.8 and 2 nm at radiation wavelengths of 1.31 and 1.55 μm, which are shown in the upper and lower panels, respectively. The responsivity at λ 1.31 μm for heterostructures with a QD size of 2 nm is noticeably larger than that for heterostructures with a QD size of 1.8 nm. For the same devices, measurements at λ of 1.55 μm do not reveal any dependence on the size of the QD within the experimental error.

Figure 8

Measurements in the case of voltage-biased Ag₂S/Si heterostructures. The IV cure under a positive potential displays a pronounced exponential character. The IV curves of the devices with different QDs were the same and depended on the quality of the Si interface under the Ti/Au contacts and their size. The deviation from a truly exponential shape is caused by the variation of the Schottky contact area, which decreases with the increase of the forward bias voltage. The top insert presents the dependence of the resistance Rxx of Ag₂S/Si heterostructure on the applied voltage. The bottom insert reflects the dependence of the device signal on the applied voltage; the signal increases more than 20-fold in comparison with the signal at zero bias. The maximum sensitivity is achieved in the region with the maximum non-linearity of the IV curve.