Fullerene Sensitized Silicon for Near‐ to Mid‐Infrared Light Detection

Abstract

Detection of light in the near-to mid-infrared (IR) spectral range is a technology in demand for many applications such as optical data transmission (1.55 mm), contrast enhancement for imaging systems in foggy environments, and quality control. Most of the optoelectronic devices on the market are based on the rather expensive III-V compound technology and, as a result, the monolithic integration into the well-established and cheap silicon-based complementary metal oxide semiconductor (CMOS) production process is still an unachieved goal. Here, we report on a novel light-sensing scheme based on a silicon/fullerene-derivative heterojunction that allows the realization of optoelectronic devices for the detection of near-to mid-IR light, which is fully compatible with CMOS technology. Despite the absence of light absorption by silicon and the fullerene-derivative in the IR. a heterojunction of these materials absorbs and generates a photocurrent (PC) in the near-to mid-IR. In this spectral range it is proposed that the IR PC is caused by an interfacial absorption mechanism.In essence, an inherent disadvantage of silicon for optoelectronic IR applications is its transparency beyond a wavelength of 1.1 mm. To overcome this disadvantage, several technologies such as the heteroepitaxial growth of (polycrystalline) germanium on silicon [1][2][3] or the usage of near-IR (NIR)-photoconductive and soluble nanoparticles have been developed. [4,5] In the latter case, the facile solution processing of a guest material to the silicon-based host is of particular interest. [5] In this work, the soluble C 60 derivative methano-fullerene [6,6] phenyl-C61 butyric acid methyl ester (PCBM) as guest material has been chosen (see inset of Fig. 1, where the chemical structure is depicted). In contrast to pristine C 60 , PCBM exhibits a solubility of up to 5 wt% in common organic solvents due to functionalization of the fullerene cage with a butyric acid methylester sidegroup. [6] For polycrystalline C 60 thin films processed into field effect devices, the electron mobility is of the order of 1 cm 2 Vs À1 [7,8] and for the spin-cast PCBM thin films it is approximately one to two orders lower in magnitude.[9] The microscopic charge-carrier mobility of PCBM is only weakly temperature dependent and a highly conductive state of PCBM under a filamented current density at 15 K has been reported. [10] In this Communication is shown that a p-Si/PCBM heterojunction features a PC for photon energies from $0.55 to 1.1 eV (2.25-1.12 mm). The investigated samples have a layered structure (Fig. 1). On top of a boron-doped p-Si wafer (boron concentration 10 15 -10 16 cm À3 ) the PCBM film is deposited by spin-coating, resulting in a PCBM film thickness of 140 nm. By thermal evaporation of Al the electrical front-and back-contacts to the PCBM thin film and to the p-Si wafer are formed. To ensure an Ohmic contact of the Al to the p-Si wafer, the Al/p-Si contact is alloyed at 580 8C in a nitrogen/hydrogen atmosphere. [11] In Figure 2, the current-density-vo...

Electrical properties of pSi/[6,6] phenyl-C61 butyric acid methyl ester/Al hybrid heterojunctions: Experimental and theoretical evaluation of diode operation

“…The results for the energy level structure across the p-Si/PCBM interface obtained here by electrical methods are in full agreement with the onset energy of the photocurrent as reported in Ref. 2, confirming the previously presented model for photocurrent generation at the interface due to an enhancement of the hole capture rate of the PCBM LUMO level by photon absorption.…”

“…The energy difference between VB of p-Si and LUMO of PCBM is identified as interface barrier height and defines the onset of the spectrally resolved photocurrent reported previously. 2 Based on the detailed understanding of the diode operation principle, further improvements of the device with respect to its application as infrared photodetector compatible with standard Si technology 21 as well as the development of photodetectors based on alternative organic semiconductors becomes possible.…”

“…The device fabrication process was optimized for a PCBM layer thickness of ' 165 nm, since for this thickness the maximum responsivity was observed when the devices were operated as photodetectors in the telecom wavelength region. 2 Figure 1 shows the current density vs voltage (J-V) characteristics of heterostructure diodes measured at 300 K. In this plot, the results obtained from sixteen devices fabricated in five batches are summarized. From each batch, devices with the largest rectification ratio [defined as J(1 V)/J(À1 V)] were chosen in order to minimize the influence of devices degraded by process defects on the data set.…”

“…2 Fullerenes and their derivatives constitute a very special case of organic compounds which can possess semiconducting, metallic 3,4 and even superconducting properties. 5 Due to the large energy gap of fullerenes between the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO), almost no free chargecarriers are available for transport at ambient temperatures.…”

Enhanced near-infrared response of nano- and microstructured silicon/organic hybrid photodetectors

“…Interest in organic/inorganic heterojunction devices began in the 1980s, and since then several models have been developed to understand the electrical characteristics of such junctions. [9][10][11] In 2010, it was reported that a heterojunction of a solution-processed fullerene (C 60 ) derivative and p-Si displayed a photovoltaic effect down to photon energies of around 0.55 eV (2.25 lm), 12 with mediocre NIR responsivity a)…”