NUSOD Blog

Connecting Theory and Practice in Optoelectronics

Category Archives: material properties

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

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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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NUSOD 2016 Preview: Electronic properties of polytype GaAs superlattices

enBandsGaAs can be considered as the prototype compound semiconductor and is used for a wide range of applications including infrared light emission, acoustic sensing, transistor technology and photovoltaics. Nanowires (NWs) made from GaAs commonly exhibit polytypism, i.e., some segments of the NW crystallize in the zincblende (ZB) and others in the metastable wurtzite (WZ) crystal structure, thus turning the NW into a crystal-phase nanostructure. While the electron states of the ZB phase are principally derived from a single conduction band (CB), two energetically close bands exist in the WZ modification. Read more of this post

How to predict the thermal conductivity of multi-layer structures

DBRWe were puzzled for a long time, as to why our simulated self-heating of vertical-cavity surface-emitting lasers (VCSELs) is always much smaller than the measured temperature rise. After checking all the details of model and measurement, we ended up with the suspicion that the thermal conductivity of the GaAs/AlAs distributed Bragg reflectors (DBRs) is much lower than expected. This was eventually confirmed by direct measurements (see picture). Even 100nm thick layers restrict the phonon mean free path that determines the thermal conductivity of GaAs and AlAs. In other words, the typical averaging of bulk thermal conductivities may lead to a serious underestimation of the thermal resistance of multi-layer structures. In fact, the thermal conductivity of such structures could be strongly anisotropic. Read more of this post

How to calculate the piezoelectric polarization of semi-polar InGaN?

RS_comparison3GaN-based light-emitting devices suffer from strong polarization of the InGaN light-emitting layers when grown along the wurtzite c-axis. That is why researchers worldwide work on growing these devices in different, so-called semi-polar crystal directions that promise less polarization. These promises are typically derived from theoretical polarization models. The most cited model was published by Romanov et al. in 2006 (link). It gives analytical formulas for the piezoelectric polarization as function of the angle between growth direction and c-axis. However, when I tried to use this model, I realized that the given formulas do not produce the results plotted in the paper (see figure). Read more of this post