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Register a new userCapacitance-Voltage (C-V) Characterization of Graphene-Silicon Heterojunction Photodiodes
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Heterostructures of two-dimensional (2D) and three-dimensional (3D) materials form efficient devices for utilizing the properties of both classes of materials. Graphene/silicon (G/Si) Schottky diodes have been studied extensively with respect to their optoelectronic properties. Here, we introduce a method to analyze measured capacitance-voltage data of G/Si Schottky diodes connected in parallel with G/silicon dioxide/Si (GIS) capacitors. We also demonstrate the accurate extraction of the built-in potential ($Phi$$_{bi}$) and the Schottky barrier height from the measurement data independent of the Richardson constant.
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Unparalleled strength, chemical stability, ultimate surface-to-volume ratio and excellent electronic properties of graphene make it an ideal candidate as a material for membranes in micro- and nanoelectromechanical systems (MEMS and NEMS). However, the integration of graphene into MEMS or NEMS devices and suspended structures such as proof masses on graphene membranes raises several technological challenges, including collapse and rupture of the graphene. We have developed a robust route for realizing membranes made of double-layer CVD graphene and suspending large silicon proof masses on membranes with high yields. We have demonstrated the manufacture of square graphene membranes with side lengths from 7 micro meter to 110 micro meter and suspended proof masses consisting of solid silicon cubes that are from 5 micro meter multiply 5 micro meter multiply 16.4 micro meter to 100 micro meter multiply 100 micro meter multiply 16.4 micro meter in size. Our approach is compatible with wafer-scale MEMS and semiconductor manufacturing technologies, and the manufacturing yields of the graphene membranes with suspended proof masses were greater than 90%, with more than 70% of the graphene membranes having more than 90% graphene area without visible defects. The graphene membranes with suspended proof masses were extremely robust and were able to withstand indentation forces from an atomic force microscope (AFM) tip of up to ~7000 nN. The measured resonance frequencies of the realized structures ranged from tens to hundreds of kHz, with quality factors ranging from 63 to 148. The proposed approach for the reliable and large-scale manufacture of graphene membranes with suspended proof masses will enable the development and study of innovative NEMS devices with new functionalities and improved performances.
We propose a new triple-junction solar cell structure composed of a III-V heterojunction bipolar transistor solar cell (HBTSC) stacked on top of, and series-connected to, a Si solar cell (III-V-HBTSC-on-Si). The HBTSC is a novel three-terminal device, whose viability has been recently experimentally demonstrated. It has the theoretical efficiency limit of an independently-connected double-junction solar cell. Here, we perform detailed balance efficiency limit calculations under one-sun illumination that show that the absolute efficiency limit of a III-V-HBTSC-on-Si device is the same as for the conventional current-matched III-V-on-Si triple-junction (47% assuming black-body spectrum, 49% with AM1.5G). However, the range of band-gap energies for which the efficiency limit is above 40% is much wider in the III-V-HBTSC-on-Si stack case. From a technological point of view, the lattice-matched GaInP/GaAs combination is particularly interesting, which has an AM1.5G efficiency limit of 47% with the HBTSC-on-Si structure and 39% if the current-matched III-V-on-Si triple junction is considered. Moreover, we show that interconnecting the terminals of the HBTSC to achieve a two-terminal GaInP/GaAs-HBTSC-on-Si device only reduces the efficiency limit by three points, to 43%. As a result, the GaInP/GaAs-HBTSC-on-Si solar cell becomes a promising device for two-terminal, high-efficiency one-sun operation. For it to also be cost-effective, low-cost technologies must be applied to the III-V material growth, such as high-throughput epitaxy or sequential growth.
Graphene / silicon (G/Si) heterostructures have been studied extensively in the past years for applications such as photodiodes, photodetectors and solar cells, with a growing focus on efficiency and performance. Here, a specific contact pattern scheme with interdigitated Schottky and graphene/insulator/silicon (GIS) structures is explored to experimentally demonstrate highly sensitive G/Si photodiodes. With the proposed design, an external quantum efficiency (EQE) of > 80 % is achieved for wavelengths ranging from 380 to 930 nm. A maximum EQE of 98% is observed at 850 nm, where the responsivity peaks to 635 mA/W, surpassing conventional Si p-n photodiodes. This efficiency is attributed to the highly effective collection of charge carriers photogenerated in Si under the GIS parts of the diodes. The experimental data is supported by numerical simulations of the diodes. Based on these results, a definition for the true active area in G/Si photodiodes is proposed, which may serve towards standardization of G/Si based optoelectronic devices.
Silicon heterojunction (SHJ) solar cells represent a promising technological approach towards higher photovoltaics efficiencies and lower fabrication cost. While the device physics of SHJ solar cells have been studied extensively in the past, the ways in which nanoscopic electronic processes such as charge-carrier generation, recombination, trapping, and percolation affect SHJ device properties macroscopically have yet to be fully understood. We report the study of atomic scale current percolation at state-of-the-art a-Si:H/c-Si heterojunction solar cells under ambient operating conditions, revealing the profound complexity of electronic SHJ interface processes. Using conduction atomic force microscopy (cAFM), it is shown that the macroscopic current-voltage characteristics of SHJ solar cells is governed by the average of local nanometer-sized percolation pathways associated with bandtail states of the doped a-Si:H selective contact leading to above bandgap open circuit voltages ($V_{mbox{OC}}$) as high as 1.2 V ($V_{mbox{OC}}>e E_{mbox{gap}}^{mbox{Si}}$). This is not in violation of photovoltaic device physics but a consequence of the nature of nanometer-scale charge percolation pathways which originate from trap-assisted tunneling causing dark leakage current. We show that the broad distribution of local photovoltage is a direct consequence of randomly trapped charges at a-Si:H dangling bond defects which lead to strong local potential fluctuations and induce random telegraph noise of the dark current.
We report vertically-illuminated, resonant cavity enhanced, graphene-Si Schottky photodetectors (PDs) operating at 1550nm. These exploit internal photoemission at the graphene-Si interface. To obtain spectral selectivity and enhance responsivity, the PDs are integrated with an optical cavity, resulting in multiple reflections at resonance, and enhanced absorption in graphene. Our devices have wavelength-dependent photoresponse with external (internal) responsivity~20mA/W (0.25A/W). The spectral-selectivity may be further tuned by varying the cavity resonant wavelength. Our devices pave the way for developing high responsivity hybrid graphene-Si free-space illuminated PDs for free-space optical communications, coherence optical tomography and light-radars
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