Overcoming the Fermi-Level Pinning Effect in the Nanoscale Metal and Silicon Interface

Silicon-based photodetectors are attractive as low-cost and environmentally friendly optical sensors. Also, their compatibility with complementary metal-oxide-semiconductor (CMOS) technology is advantageous for the development of silicon photonics systems. However, extending optical responsivity of silicon-based photodetectors to the mid-infrared (mid-IR) wavelength range remains challenging. In developing mid-IR infrared Schottky detectors, nanoscale metals are critical. Nonetheless, one key factor is the Fermi-level pinning effect at the metal/silicon interface and the presence of metal-induced gap states (MIGS). Here, we demonstrate the utilization of the passivated surface layer on semiconductor materials as an insulating material in metal-insulator-semiconductor (MIS) contacts to mitigate the Fermi-level pinning effect. The removal of Fermi-level pinning effectively reduces the Schottky barrier height by 12.5% to 16%. The demonstrated devices exhibit a high responsivity of up to 234 μA/W at a wavelength of 2 μm, 48.2 μA/W at 3 μm, and 1.75 μA/W at 6 μm. The corresponding detectivities at 2 and 3 μm are 1.17 × 108 cm Hz1/2 W-1 and 2.41 × 107 cm Hz1/2 W-1, respectively. The expanded sensing wavelength range contributes to the application development of future silicon photonics integration platforms.

Ultra-thin Ag/Si heterojunction hot-carrier photovoltaic conversion Schottky devices for harvesting solar energy at wavelength above 1.1 µm

Traditional silicon solar cells can only absorb the solar spectrum at wavelengths below 1.1 μm. Here we proposed a breakthrough in harvesting solar energy below Si bandgap through conversion of hot carriers generated in the metal into a current using an energy barrier at the metal-semiconductor junction. Under appropriate conditions, the photo-excited hot carriers can quickly pass through the energy barrier and lead to photocurrent, maximizing the use of excitation energy and reducing waste heat consumption. Compared with conventional silicon solar cells, hot-carrier photovoltaic conversion Schottky device has better absorption and conversion efficiency for an infrared regime above 1.1 μm, expands the absorption wavelength range of silicon-based solar cells, makes more effective use of the entire solar spectrum, and further improves the photovoltaic performance of metal-silicon interface components by controlling the evaporation rate, deposition thickness, and annealing temperature of the metal layer. Finally, the conversion efficiency 3.316% is achieved under the infrared regime with a wavelength of more than 1100 nm and an irradiance of 13.85 mW/cm².

Mid-Infrared Response from Cr/n-Si Schottky Junction with an Ultra-Thin Cr Metal

Infrared detection technology has been widely applied in many areas. Unlike internal photoemission and the photoelectric mechanism, which are limited by the interface barrier height and material bandgap, the research of the hot carrier effect from nanometer thickness of metal could surpass the capability of silicon-based Schottky devices to detect mid-infrared and even far-infrared. In this work, we investigate the effects of physical characteristics of Cr nanometal surfaces and metal/silicon interfaces on hot carrier optical detection. Based on the results of scanning electron microscopy, atomic force microscopy, and X-ray diffraction analysis, the hot carrier effect and the variation of optical response intensity are found to depend highly on the physical properties of metal surfaces, such as surface coverage, metal thickness, and internal stress. Since the contact layer formed by Cr and Si is the main role of infrared light detection in the experiment, the higher the metal coverage, the higher the optical response. Additionally, a thicker metal surface makes the hot carriers take a longer time to convert into current signals after generation, leading to signal degradation due to the short lifetime of the hot carriers. Furthermore, the film with the best hot carrier effect induced in the Cr/Si structure is able to detect an infrared signal up to 4.2 μm. Additionally, it has a 229 times improvement in the signal-to-noise ratio (SNR) for a single band compared with ones with less favorable conditions.

Role of free electrons in phosphorescence in n-type wide bandgap semiconductors

Long persistent phosphorescence is generally known as a phenomenon involving carrier traps induced by defects or impurities in crystals. In this paper, phosphorescence sustained for tens of minutes was found in intentionally undoped ZnO and it was proposed to be a universal phenomenon in wide bandgap semiconductors upon satisfying several conditions. A new model was built to understand this attractive phenomenon within the framework of the traditional trapping-detrapping model but it was modified by considering the free electrons in the conduction band as a significant contributor to the long persistent phosphorescence besides the electrons trapped by shallow donors. This model, explicitly expressed as I(t) ∝ [1 + M(1 - Fe^-γt)^-2]e^-γt, is not only capable of giving a quantitative description of the non-exponential decay of phosphorescence in a wide temperature range but also enables one to determine the depth of shallow donors in semiconductors. The participation of free electrons in phosphorescence was further confirmed by another carefully designed experiment. Thus, this study may represent significant progress in understanding phosphorescence.

Transition of radiative recombination channels from delocalized states to localized states in a GaInP alloy with partial atomic ordering: a direct optical signature of Mott transition?

Anderson localization is a predominant phenomenon in condensed matter and materials physics. In fact, localized and delocalized states often co-exist in one material. They are separated by a boundary called the mobility edge. Mott transition may take place between these two regimes. However, it is widely recognized that an apparent demonstration of Anderson localization or Mott transition is a challenging task. In this article, we present a direct optical observation of a transition of radiative recombination dominant channels from delocalized (i.e., local extended) states to Anderson localized states in the GaInP base layer of a GaInP/GaAs single junction solar cell by the means of the variable-temperature electroluminescence (EL) technique. It is found that by increasing temperature, we can boost a remarkable transition of radiative recombination dominant channels from the delocalized states to the localized states. The delocalized states are induced by the local atomic ordering domains (InP/GaP monolayer superlattices) while the localized states are caused by random distribution of indium (gallium) content. The efficient transfer and thermal redistribution of carriers between the two kinds of electronic states was revealed to result in both a distinct EL mechanism transition and an electrical resistance evolution with temperature. Our study gives rise to a self-consistent precise picture for carrier localization and transfer in a GaInP alloy, which is an extremely technologically important energy material for fabricating high-efficiency photovoltaic devices.

First Report of a Leaf Spot on Goldthread (Coptis chinensis) Caused by Phoma aquilegiicola in China

Goldthread (Coptis chinensis) is an important herbaceous plant in traditional Chinese medicine (3). Annual production of goldthread root is ~3,000 tons (dry weight) in China. The plant is cultivated extensively in Shizhu Co., Chongqing (29.98°E, 108.13°N), where goldthread yields account for more than 60% of total world production. A foliar disease was first observed on goldthread plants in 2008 in Shizhu County (5). In 2011 and 2012, about 10 ha of goldthread fields in different townships of Shizhu Co. were surveyed. The results demonstrated that the disease present in the fields was widespread at incidences of 30 to 100%, with yield losses of 15 to 75%. Typical symptoms included irregular, purple brown lesions on leaves, beginning usually at the leaf margin and extending to the central leaf blade. The lesions coalesced and turned deep purple. Black pycnidia were visible on the lesions, and severely diseased plants were usually wholly blighted. To identify the pathogen, infected leaves were collected from goldthread fields in different townships of Shizhu Co. and small pieces of symptomatic tissue were cut from each leaf. The leaf pieces were surface-disinfected for 1 min in 1.5% sodium hypochlorite, rinsed in sterilized water, air-dried, and transferred onto potato dextrose agar (PDA) plates with 0.5 g/liter of streptomycin sulfate. Thirty-three fungal isolates with similar colony morphology were obtained. On oatmeal agar plates, each colony was circular with a smooth edge, initially cream, and then pale-brown. Pycnidia were dark brown, spherical, with or without papillae, and 100 to 112 × 189 to 222 μm. Conidia were produced on short, straight, and aseptate conidiophores in the pycnidia; they were monocellular, hyaline, ellipsoidal or clavate, and 2.01 to 2.50 × 4.20 to 5.55 μm. Three isolates (SZ-9, SZ-10, and SZ-13) were selected randomly from all 33 isolates, and genomic DNA of each isolate was extracted following the CTAB method (4). The rDNA ITS region of each isolate was amplified with V9G/ITS4 primers and sequenced (1). The ITS sequences of the three isolates (GenBank Accession Nos. KF692355.2 [SZ-9], KF985236.1 [SZ-10], and KF985237.1 [SZ-13]) were identical, and BLAST revealed 100% identity with the ITS sequence of an isolate of Phoma aquilegiicola (CBS 107.96, GU237735.1). Based on the morphological characteristics and ITS sequences, all three isolates were identified as P. aquilegiicola. Pathogenicity test of 10 isolates was conducted by placing a 5-mm-diameter mycelial agar plug (from the margin of a 5-day-old PDA culture) on each of 10 fully-expanded leaves of healthy goldthread plants/isolate. Ten leaves were treated similarly with sterilized PDA plugs as a control. Inoculated and control plants were incubated in the dark for 24 h at 25 ± 2°C and >90% RH, and then maintained in a growth chamber at 25 ± 2°C, 3,100 lux, and >90% RH. The pathogenicity test was carried out three times. Symptoms developed on all inoculated leaves for all 10 isolates, but not on the control plants. Lesions were first visible 48 h after inoculation, and typical irregular lesions similar to those observed on field plants were seen after 6 days. The same pathogenic fungus was re-isolated from the infected leaves but not from the non-inoculated leaves. A disease caused by P. aquilegiicola was first reported on Aquilegia flabellata plants of the cv. Fan Columbine in a perennial garden in Italy (2). This is the first report of leaf spot on goldthread caused by P. aquilegiicola in China. Studies on the epidemiology and control of the disease are necessary owing to the economic significance of the host and destructiveness of the disease. References: (1) M. M. Aveskamp et al. Mycologia 101:363, 2009. (2) A. Garibaldi et al. Plant Dis. 95:880, 2011. (3) B. Liu et al. J. Pharmaceut. Biomed. 41:1056, 2006. (4) M. A. Saghai-Maaroof et al. Proc. Natl. Acad. Sci. USA. 81:8014, 1984. (5) X. R. Zhou et al. J. Shizhen Medicine Res. 23:471, 2012.