Engineering topics

Abbas, Ali

Publications and source records attributed to Abbas, Ali.

4 records

DOE OSTI2024

Accommodating a hexagonal ζ- phase Mn 2 N film on a cubic MgO (001) substrate

The thin films of c -plane dominated hexagonal ζ- phase Mn 2 N were successfully grown on a cubic MgO (001) substrate directly using plasma-assisted molecular beam epitaxy. The surface was comprehensively studied through experimental and theoretical approaches. Reflection high energy electron diffraction revealed two pseudo-cubic domains along [100] MgO and [110] MgO , and one hexagonal domain, which is 30° apart from the [100] MgO direction. Scanning tunneling microscopy was used to image and resolve the high-quality surface, revealing a distorted hexagonal surface structure. Furthermore, atomic resolution of the hexagonal domain with a 2 × 2 reconstructed surface is presented. For c-plane surface, theoretical investigations were carried out using first principles studies to determine the surface formation energy for various reconstructed surfaces. The theoretical study shows that the nitrogen terminated 2 × 2 structure with a manganese adatom on the surface as the most stable reconstruction that reproduces the experimental results. A corresponding simulated scanning tunneling microscopy model is also presented, providing strong support for the experimentally observed in-plane lattice structures. Furthermore, the manganese: nitrogen stoichiometry within the bulk and surface is in good agreement with the expected ratio of 2:1.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2024

The effect of remnant CdSe layers on the performance of CdSeTe/CdTe photovoltaic devices

Thin film CdTe-based photovoltaic devices have achieved high efficiency above 22 %. The recent improvement in efficiency is due to Se alloying in the CdTe absorbers to form a CdSeTe/CdTe structure. The subsequent band gap grading increases the short circuit current density. The Se can be introduced by depositing a precursor thin film of either CdSe or a CdSeTe alloy and then diffusing the Se into the CdTe during the high temperature cadmium chloride activation process. Using CdSe is preferred because it is easier to control the Se concentration. However, during fabrication of the CdSeTe/CdTe devices, the CdSe thickness needs to be precisely controlled to prevent the retention of a CdSe remnant layer after the activation treatment. Retention of a remnant CdSe layer causes a dramatic reduction in device efficiency. In this work, we show that the reduction in efficiency is caused by a number of factors. The remnant CdSe layer is n-type which moves the position of the p-n junction. Also, it is widely thought that the CdSe remnants are photo-inactive. In this work, we clarify that the individual CdSe grains are actually highly photo-active. However, the grain sizes in the CdSe remnant and the adjacent CdSeTe layer are very small resulting in a high grain boundary area. Although the grain boundaries are passivated with chlorine, cathodoluminescence imaging and electrical measurements show that this is only partially effective. Also, EQE measurements show that the remnant CdSe causes parasitic absorption. Overall, the remnant CdSe layer causes a reduction in short circuit current density and device efficiency. The thickness of the CdSe precursor layer and the cadmium chloride activation process conditions must be precisely optimised to ensure that all the CdSe is consumed and inter-diffused to form the CdSeTe alloy for highest efficiency devices.

CdSe remnant↗

DOE OSTI2024

Co-deposition of bismuth-nitrogen films on MgO (001) by molecular beam epitaxy

Here, we attempted to grow a thin film of BiN by co-deposition of bismuth and nitrogen on rock-salt structure MgO (001) substrates. Furthermore, we studied the effect of variation of the growth temperature and the nitrogen to bismuth flux ratios on sample growth. For the samples grown and conditions used, we do not find strong evidence for the formation of a bulk Bi-N alloy. Even for very high nitrogen to bismuth flux ratio, we observed only bismuth and no nitrogen using bulk Rutherford back-scattering spectroscopy measurements, and only 1%–2% nitrogen was seen through surface Auger electron spectroscopy measurements. The in-plane lattice measurements show that the resulting Bi (110) samples are strained, which is presumably caused by lattice mismatch between the sample and the substrate when grown without any buffer layer. The use of a high-temperature buffer layer helps to release strain in the sample but only along one axis. Measurements of the atomic layer spacing using x-ray diffraction and also scanning tunneling microscopy confirm the Bi (110) thin film sample structure.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2023

SnO2 Buffer Layers for High Efficiency CdSeTe/CdTe Devices

SnO2 buffer layers of different thickness were deposited onto TEC 15 Fluorine doped tin oxide coated glass substrates using rf magnetron sputtering. The buffer layers were then incorporated into Cu-doped CdSeTe/CdTe devices using a range of CdCl 2 activation treatments and CuCl2 annealing temperatures to determine the effects of buffer layer thickness on device performance. Results show that all devices fabricated with thinner buffer layers resulted in much better J - V characteristics than their thicker counterparts. This was mainly due to a reduced open-circuit voltage (Voc) when using thicker buffer layers. The best device produced a conversion efficiency of 16.59%, fill factor of 71.62%, Jsc of 28.44 mA/cm 2 and Voc of 814.23 mV.

buffer layers↗