NUSOD Blog

Connecting Theory and Practice in Optoelectronics

Category Archives: material properties

NUSOD 2017 Preview: Strain tuning of absorption spectrum in a topological insulator

absorbanceTopological insulators have attracted a huge amount of attention in the field of condensed matter physics. This new state of matter is characterized by a bulk band gap and conducting surface states. The surface states have linear dispersion resembling relativistic Dirac fermions, in analogy with graphene. In contrast to graphene the 2D fermions on the surface are non-degenerate, whereas electrons in graphene are spin and valley degenerate. The linear dispersion and 2D nature of the electrons in graphene leads to the universal optical absorbance απ≈2.3% given by the fine structure constant , independent on the material parameters and the photon energy. Due to this relatively large absorption for a single atomic layer, graphene is a promising material for optoelectronic applications e.g. photodetectors, which has been demonstrated [1]. Similarly a topological insulator has an absorbance of απ/2≈1.1% for photon energies below the bulk band gap, due to the surface states at the top and bottom surface. It has been shown that the signal-to-noise ratio of photodetector based on a thin slab of the topological insulator Bi2Secan be significantly larger than for a graphene based device[2]. For a slab thickness below 6 nm the surface states on opposing sides interact leading to a band gap, which can be tuned by varying the thickness. However, this way of manipulating the optical properties is not very flexible since Bi2Se3 has a layered structure with five atomic layers strongly bound in a quintuple layer, limiting the possible thicknesses to only integer numbers of quintuple layers (QL). If instead strain is used to tune the optical properties, this can be done continuously and dynamically.

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How to get your simulation paper accepted

Looking back at 2016, I just realized that my yearly load of peer reviews has increased to almost 80 journal papers, mainly in the field of optoelectronic device simulation. The rising number of such paper submissions to top journals is certainly good news, but the paper quality is often insufficient. Unfortunately, I have to propose rejection of most papers after a detailed assessment of essential mistakes. A fundamental mistake in my view is the unproven assumption that simulations represent the real world. Authors often don’t seem to understand that computer simulations lead us into a virtual reality in which many unreal effects can happen – depending on their choice of mathematical models and  material parameters.

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Call for Papers: Special Journal Issue on Multiscale Modeling of Solar Cells

In the last two decades, there has been an increasing interest in multiscale modeling applied to electronic devices. Several factors are driving this trend. On the one hand, device dimensions of “classical” devices like MOSFETs have continuously been scaled down in order to increase device performance. On the other hand, specific properties of quantum structures are systematically utilized in modern devices. The embedding of the active device region in its environment including access regions and contacts, and the mutual interaction between different aspects like optics, thermal heating, strain and carrier transport requires an involved multiscale/multiphysics simulation approach which can handle different physical models and different length or time scales.

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NUSOD 2016 Preview: Modeling of light soaking effect in CdTe Solar Cells

light soakNearly all photovoltaic technologies exhibit changes in device performance under extended illumination, or “light soaking”. Experiments on both commercial modules and research cells based on CdTe technology have shown improvement of cell performance under light soaking conditions for up to 20 hours. Many accredited such phenomena to the passivation of traps and migration of Cu ions. In this work, we employed a self-consistent one-dimensional (1D) diffusion-reaction simulator to study the migration and passivation of Cu related dopants in CdTe solar cell as a function of soaking conditions. Read more of this post

NUSOD 2016 Preview: GaAs-based dilute bismide semiconductor lasers – theory vs. experiment

GaBiAs_band_gapsTwo long sought-after goals for the semiconductor community have been (i) to develop long-wavelength semiconductor lasers on GaAs substrates, to enable exploitation of vertical-cavity architectures as well as monolithic integration with GaAs-based high-speed microelectronics, and (ii) to realise uncooled operation of semiconductor lasers, whereby the external cooling equipment typically required to maintain operational stability in long-wavelength devices can be removed in order to significantly reduce energy consumption without degrading the device performance.

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