Photovoltaic (PV) devices have been extensively studied for applications in solar cells. However, the grid-parity of solar powers is not yet achieved due to the expensive cost of electricity compared to the cost with fossil fuels [1]. Designs of highly efficient light absorption layers with earth-abundant materials, is thus critical as they can reduce the cost of PV cells by increasing the power-per-unit-area of solar panels. In consequence, optical properties of various materials and structures have been the central topic for research in nanoelectronics. III-V semiconductors have been known as excellent materials for light absorption layers. In particular, InAs/GaAs quantum dots (QDs) have been great interest to researchers as they exhibit high efficiency in light absorption, and have tunable optical gaps [2]. But the expensive material cost becomes the limiting factor for the grid-parity. Earth-abundant materials such as Iron disulphide and Tin monosulphide have been studied due to their appropriate optical gaps [3][4], but the strong surface-recombination limits the device performance.
The combination of scanning tunneling microscope lithography and molecular beam epitaxy enabled a precise control of dopant placements in silicon (Si), a representative earth-abundant material. Various phosphorus (P) monolayer devices embedded in Si (Si:P) thus have been realized with the atomically precise incorporation of dopants, including planar electrodes, ultra-thin interconnects and donor-based QDs for advanced logic applications [5-7]. In this work, we expand the application scope of Si:P devices to optoelectronics with a focus on Si:P QDs. Electronic structures are calculated with the sp3d5s* tight-binding model that has shown its validity in describing Si:P devices [5-7]. We report promising properties of Si:P QDs as light absorption layers, particularly against self-assembled InAs/GaAs QDs: (I) The optical gap of single-dopant QDs is good for absorbing the natural light at ~1100um wavelengths, and is widely tunable by changing the number of P atoms in the dopant cluster. (II) The inter-band optical transition rate is much more insensitive to directions of polarization, which represents the strong potential of Si:P QDs in efficient absorption of the natural light injected from arbitrary directions. Establishing a theoretical framework of optical properties of Si:P QDs that have been rarely explored for realistically sized devices with a full atomistic model, this work presents a detailed analysis useful for potential designs of light absorption layers with Si:P QDs.
References
[1] M. Bazillan et al., Renewable Energy 53, 329 (2013)
[2] D. Forbes et al., Proceedings of SPIE (2014)
[3] P. Lazic et al., Journal of Physics: Condensed Matters 25, 465801 (2013)
[4] P. Sinsermsuksakul et al., Applied Physics Letters 102, 053901 (2013)
[5] H. Ryu et al., Nanoscale 8, 053901 (2013)
[6] H. Ryu et al., Small 11, 374 (2014)
[7] H. Ryu et al., Nano Letters 15, 450 (2015)