Polarized photoreflectance and photoluminescence spectroscopy of InGaAs/GaAs quantum rods grown with As2 and As4 sources

We report photoreflectance (PR) and photoluminescence (PL) investigations of the electronic and polarization properties of different aspect ratio (height/diameter) InGaAs quantum rods (QRs) embedded in InGaAs quantum wells (QWs). These nanostructures were grown by molecular beam epitaxy using As2 or As4 sources. The impact of the As source on the spectral and polarization features of the QR- and QW-related interband transitions was investigated and explained in terms of the carrier confinement effects caused by variation of composition contrast between the QR material and the surrounding well. Polarized PR and PL measurements reveal that the polarization has a preferential direction along the [ 110] crystal axis with a large optical anisotropy of about 60% in the (001) plane for high aspect ratio (4.1:1) InGaAs QRs. As a result, in PL spectra, the transverse magnetic mode dominated (110)-cleaved surfaces (TM[001] > TE[110]), whereas the transverse electric mode prevailed for (110)-cleaved surfaces (TM[001] < TE[110] ¯ ). This strong optical anisotropy in the (001) plane is interpreted in terms of the hole wavefunction orientation along the [ 110] direction for high aspect ratio QRs

Temperature-dependent modulated reflectance of InAs/InGaAs/GaAs quantum dots-in-a-well infrared photodetectors

We present a photoreflectance (PR) study of multi-layer InAs quantum dot (QD) photodetector structures, incorporating InGaAs overgrown layers and positioned asymmetrically within GaAs/AlAs quantum wells (QWs). The influence of the back-surface reflections on the QD PR spectra is explained and a temperature-dependent photomodulation mechanism is discussed. The optical interband transitions originating from the QD/QW ground- and excited-states are revealed and their temperature behaviour in the range of 3–300 K is established. In particular, we estimated the activation energy (∼320 meV) of exciton thermal escape from QD to QW bound-states at high temperatures. Furthermore, from the obtained Varshni parameters, a strain-driven partial decomposition of the InGaAs cap layer is determined

Low processing temperatures explored in Sb2S3 solar cells by close-spaced sublimation and analysis of bulk and interface related defects

This study was funded by the Estonian Research Council project PRG627 “Antimony chalcogenide thin films for next-generation semi-transparent solar cells applicable in electricity producing windows”, the Estonian Research Council project PSG689 “Bismuth Chalcogenide Thin-Film Disruptive Green Solar Technology for Next Generation Photovoltaics”, the Estonian Centre of Excellence project TK141 (TAR16016EK) “Advanced materials and high-technology devices for energy recuperation systems”, and the European Union's Horizon 2020 ERA Chair project 5GSOLAR (grant agreement No. 952509). The article is based upon work from COST Action Research and International Networking project "Emerging Inorganic Chalcogenides for Photovoltaics (RENEW-PV)," CA21148, supported by COST (European Cooperation in Science and Technology); Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD 01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART2.Antimony trisulfide (Sb2S3) is a promising photovoltaic absorber, which has so far been fabricated mainly by chemical deposition methods. Despite its aptness for congruent sublimation, less research efforts have been made on low-temperature Sb2S3 processing by physical methods. In this regard, recent studies show large variation in the processing temperature of Sb2S3 films, which overall brings into question the need for higher substrate temperatures (>350 °C). Furthermore, in-depth analysis of defect structure of Sb2S3 employing temperature-dependent admittance spectroscopy (TAS) and photoluminescence (PL) remains largely unexplored. In this work, we systematically study the effect of close-spaced sublimation (CSS) substrate temperature on Sb2S3 absorber growth, employing a wide temperature range of 240–400 °C. Temperatures above 320 °C caused cracking phenomena in the Sb2S3 absorber film, proving the unviability of higher processing temperatures. CSS processing temperature of 300 °C was found optimal, producing crack-free Sb2S3 films with increased presence of (hk1) planes, and achieving the best CdS/Sb2S3 device with photoconversion efficiency of 3.8%. TAS study revealed two deep defects with activation energies of 0.32 eV and 0.37 eV. Low-temperature PL measurement revealed a band-to-band emission at 1.72 eV and a broad band peaked at 1.40 eV, which was assigned to a donor-acceptor pair recombination. Temperature-dependent I-V analysis showed that recombination at CdS–Sb2S3 interface remains a large limitation for the device efficiency. --//-- R. Krautmann, N. Spalatu, R. Josepson, R. Nedzinskas, R. Kondrotas, R. Gržibovskis, A. Vembris, M. Krunks, I. Oja Acik, Low processing temperatures explored in Sb2S3 solar cells by close-spaced sublimation and analysis of bulk and interface related defects, Solar Energy Materials and Solar Cells, Volume 251, 2023, 112139, ISSN 0927-0248, https://doi.org/10.1016/j.solmat.2022.112139. (https://www.sciencedirect.com/science/article/pii/S0927024822005566) Published under the CC BY licence.Estonian Research Council project PRG627; Estonian Research Council project PSG689; Estonian Centre of Excellence project TK141 (TAR16016EK); European Union's Horizon 2020 ERA Chair project 5GSOLAR (grant agreement No. 952509; Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD 01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART2

Temperature-dependent modulated reflectance and photoluminescence of InAs-GaAs and InAs-InGaAs-GaAs quantum dot heterostructures

Optical transitions and electronic properties of epitaxial InAs quantum dots (QDs) grown with and without InGaAs strain-relieving capping layer within GaAs/AlAs quantum well (QW) are investigated. Modulated reflectance and photoluminescence spectroscopy is used to probe the QD- and QW-related interband optical transitions over the temperature range of 3–300 K. The observed spectral features in QDs are identified using numerical calculations in a framework of 8-band k⋅p k⋅p method. It is found that covering the dots by a 5 nm-thick InGaAs layer yields the energy red-shift of ground-state transition by ∼150 meV ∼150 meV . Moreover, the analysis of interband transition energy dependence on temperature using Varshni expression shows that material composition of InAs QDs significantly changes due to Ga/In interdiffusion. A comparison of emission- and absorption-type spectroscopy applied for InAs–GaAs QDs indicates a Stokes shift of ∼0.02 meV ∼0.02 meV above 150 K temperature

Optical features of InAs quantum dots-in-a-well structures

Electronic energy structure and features of optical interband transitions of InAs quantum dots-in-a-well structures are studied via photoreflectance (PR) and temperature-dependent photoluminescence (PL) spectroscopy. InAs dots were grown with and without InGaAs capping layer and embedded in GaAs/AlAs quantum wells. Experimental results revealed that the 5 nm thick InGaAs capping layer significantly improves PR and PL signal intensity. Moreover, a shift of the quantum dot ground-state optical transition to lower energy by about 120 meV was observed. The red-shift of the ground-state transition is associated mainly with an increase of dot size and decrease of strain within quantum dots. Furthermore, the origin of PL intensity quenching with temperature is discussed in terms of electronic energy structure revealed from PR spectra and calculations performed within effective mass approximation

Photoreflectance of Epitaxial InGaAs Quantum Rods

Photoreflectance spectroscopy and photoluminescence have been used to study the optical properties and electronic structure of InGaAs quantum rods grown by molecular beam epitaxy. Spectral features associated with interband optical transitions localized in the quantum rod and the surrounding quantum well regions are examined. Experimental results are compared with calculations performed within the envelope function approximation. A red shift of the quantum rod- and a blue shift of the quantum well-related optical transitions, along with a significant increase in PL intensity have been observed if an $As_4$ source is used instead of an $As_2$ source during the molecular beam epitaxial growth

The response rate of room temperature terahertz InGaAs-based bow-tie detector with broken symmetry

A bow-tie InGaAs with broken symmetry has been designed for terahertz detection at room temperature. An active part of the detector consists of a two-dimensional electron gas which is heated non-uniformly with incident radiation. Main detector performances are operation in a passive scheme, flat frequency response up to 1 THz, the voltage sensitivity of about 5 V/W, the noise equivalent power of roughly 10 nW/Hz(exp 1/2), and the response time less then 7 ns

Energy Spectrum of InAs Quantum Dots in GaAs/AlAs Superlattices

Photo- and contactless electroreflectance spectroscopies were applied to study optical properties and electronic structure of GaAs/AlAs superlattice systems with embedded InAs quantum dots. The observed interband transitions related to the quantum dot ground and excited states, as well as optical transitions in the combined system formed by the InAs wetting layer and GaAs/AlAs superlattice are discussed