A cathodoluminescence study on the diffusion length in AlGaInP/InGaP/AlInP solar cell heterostructures

Producción CientíficaThe diffusion length of minority carriers in a p-doped InGaP layer is derived from the cathodoluminescence (CL) intensity profiles. Two procedures are used. First, the CL profile is recorded along a line crossing the intersection between a thin metallic mask and the semiconductor; a second approach consists of the measurement of the intensity profile around an intentional scratch on the surface of the sample. A longer diffusion length is measured when using the metallic mask as compared with the scratch. We discuss the role of non-radiative recombination centers in the reduction of the diffusion length around the scratch. The temperature dependence of the diffusion length is also measured, and the length is found to decrease with temperature.Junta de Castilla y León (project VA283P18)Ministerio de Economía, Industria y Competitividad (project ENE2017- 89561-C4-3-R

Electrical activity of crystal defects in multicrystalline Si

Producción CientíficaUpgraded metallurgical-grade silicon solar cells with different ranges of efficiencies have been characterized by light-beam-induced current (LBIC) measurements. The interaction between grain boundaries and metallic impurities is studied for cells fabricated on wafers from different solidification heights of the ingot. A tight relation is observed between the electrical activity of the grain boundaries and the position of the wafer in the ingot, which is related to the impurity contamination. The presence of a large amount of metallic impurities enhances the electrical activity of the grain boundaries. The main features of the LBIC images are discussed in relation to the presence of metallic impurities.Junta de Castilla y León (project VA283P18)Ministerio de Economía, Industria y Competitividad (project ENE2017-89561-C4-3-R

GaAs/GaP quantum dots: Ensemble of direct and indirect heterostructures with room temperature optical emission

Producción CientíficaWe describe the optical emission and the carrier dynamics of an ensemble of self-assembled GaAs quantum dots embedded in GaP(001). The QD formation is driven by the 3.6% lattice mismatch between GaAs and GaP in the Stranski-Krastanow mode after deposition of more than 1.2 monolayers of GaAs. The quantum dots have an areal density between 6 and 7.6 × 1010 per cm−2 and multimodal size distribution. The luminescence spectra show two peaks in the range of 1.7 and 2.1 eV. The samples with larger quantum dots have red emission and show less thermal quenching compared with the samples with smaller QDs. The large QDs luminescence up to room temperature. We attribute the high energy emission to indirect carrier recombination in the thin quantum wells or small strained quantum dots, whereas the low energy red emission is due to the direct electron-hole recombination in the relaxed quantum dots.Comisión Europea (project FP7-ICT-2013-613024-GRASP

(Al,Ga)(As,P) structures in the GaP matrix

GaP ist ein Halbleiter mit einer großen Bandlückenenergie und infolgedessen transparent im größten Teil der sichtbaren Wellenlängen. GaP hat außerdem die kleinste Gitterfehlanpasung zu Si (weniger als 0.4%). Das macht GaP ein interessantes Material für monolithische Integration zu III–V Lichtsender auf Si. Diese Arbeit ist eine Untersuchung über die strukturellen und optische Eigenschaften von (Al, Ga) (As, P) Heterostrukturen auf GaP (001) -Substrat aufgewachsen. Die Einflüsse des PH3 Fluss und Wachstumstemperatur untersucht auf dem Kristallqualität und Oberflächenqualität von AlGaP/GaP Heterostructure. Experimentelle Ergebnisse deuten darauf hin, dass eine Wachstumstemperatur von 490 oC und ein geknackter (engl. cracked) PH3 Fluss von 2.7 sccm zur besten AlGaP Qualität und gleichzeitig zur guten GaP Qualität führen. Um die ineffiziente Lichtemission von GaP zu überwinden wurde GaAs in der GaP-Matrix gewachsen. Die Entstehung der Quantenpunkte wurde durch die 3.7% Gitterfehlanpassung zwischen GaAs und GaP für GaAs Nenndicke über 1,2 ML. Die optischen Messungen zeigen zwei Peaks im Bereich von 1,7 bis 2,1 eV und die Lumineszenz auf Raumtemperatur für 2,7 und 3,6 ML-Proben. Die hohe Energieemission wird der indirekten Rekombination in den dünnen Quantentröge oder kleine gespannte Quantenpunkte zurückzuführen, Während die niedrige Energie Emission ist aufgrund der direkten Elektron-Loch- Rekombination in der entspannten Quantenpunkte. Die Wirkung von Al wird untersucht auf die energetische Bandausrichtung und auf die elektronische Struktur der (Al,Ga)As Quantenstrukturen. Die optische Spektren zeigten einen blaue Verschiebung (engl. blue shift) mit wachsendem Al-Inhalt und die höchste missionsenergie für die (Al,Ga)As/GaP- Heterostruktur war 2.17 eV die zum indirekten Typ-II-Rekombination zusammenhängt.Transparency of GaP due to the large indirect bandgap energy and its small lattice mismatch with Si make GaP an interesting candidate for optoelectronic devices in visible wavelength. This thesis is an investigation on the structural and optical characteristics of (Al,Ga)(As,P) heterostructures grown on GaP (001) substrates. The influences of the PH3 flux and growth temperature are studied on the crystal and surface quality of AlGaP/GaP heterostructure. The results indicate the narrow growth window of PH3 = 2.7 sccm and growth temperature = 490oC as the optimized conditions. To overcome the inefficient light emission of indirect GaP, direct bandgap GaAs was grown as the quantum structures in the GaP matrix. The QD formation is driven by the 3.7% lattice mismatch between GaAs and GaP for GaAs nominal thickness above 1.2 ML. The optical measurements show two peaks in the range of 1.7 to 2.1 eV and the luminescence up to room temperature for 2.7 and 3.6 ML samples. The high energy emission is attributed to indirect carrier recombination in the thin quantum wells or small strained quantum dots, whereas the low energy red emission is due to the direct electron-hole recombination in the relaxed quantum dots. The influence of the Al content on the band alignment and electronic structure of (Al,Ga)As quantum structures is studied. The optical spectra illustrate the blueshift of the radiative emission with increasing the Al content and the highest emission energy of 2.17 eV is observed for the (Al,Ga)As/GaP system that is related to the indirect type-II radiative recombination

CL as a tool for device characterisation: the case of laser diode degradation

Producción CientíficaCathodoluminescence is a powerful technique for the characterization of semiconductors. Due to its high spatial resolution, it is emerging as a suitable method for the study of semiconductor devices. The reduced dimension of the devices and the multilayer structure of their active parts demand experimental means with high lateral resolution and probe depth tunability for characterising the different layers forming the device structure. Degradation is a crucial technological issue for high power devices. In particular, the failures of laser diodes are due to the formation of defects during the laser operation. Those defects can be imaged by cathodoluminescence; furthermore, its spectroscopic capabilities permit to go beyond the mere observation of the non-luminescent area morphology, allowing a better understanding of the physical mechanisms of degradation. We present herein an overview of the cathodoluminescence analysis of catastrophically degraded high power laser diodes, both single mode and multimode broad emitter lasers. The study of the defects responsible of the degradation is a step forward to establish models of degradation, necessary to improve the laser power and durability.Junta de Castilla y León - Fondo Europeo de Desarrollo Regional (project VA283P18)Ministerio de Economía, Industria y Competitividad (project ENE-2017-89561-C4-3-R

Point-Defects Assisted Zn-Diffusion in AlGaInP/GaInP Systems During the MOVPE Growth of Inverted Multijunction Solar Cells

Producción CientíficaWe investigate the dynamics of Zn diffusion in MOVPE-grown AlGaInP/GaInP systems by the comparison of different structures that emulate the back-surface field (BSF) and base layers of a GaInP subcell integrated into an inverted multijunction solar cell structure. Through the analysis of secondary ion mass spectroscopy, electrochemical capacitance-voltage and spectrally resolved cathodoluminescence measurements, we provide experimental evidence that 1) the Zn diffusion is enhanced by point defects injected during the growth of the tunnel junction cathode layer; 2) the intensity of the process is determined by the cathode doping level and it happens for different cathode materials; 3) the mobile Zn is positively charged; and 4) the diffusion mechanism reduces the CuPt ordering in GaInP. We demonstrate that using barrier layers the diffusion of point defects can be mitigated, so that they do not reach Zn-doped layers, preventing its diffusion. Finally, the impact of Zn diffusion on solar cells with different Zn-profiles is evaluated by comparing the electrical I-V curves at different concentrations. The results rule out the introduction of internal barriers in the BSF but illustrate how Zn diffusion under typical growth condition can reach the emitter and dramatically affect the series resistance, among other effects.Ministerio de Economía, Industria y Competitividad (project TEC2017-83447-P)Comunidad de Madrid (project Y2018/EMT-4892)Ministerio de Educación, Cultura y Deporte (grant FPU15/03436)Programa Estatal de Promoción del Talento y su Empleabilidad through a Ramon y Cajal grant (grant RYC-2014-15621)Junta de Castilla y León - Fondo Europeo de Desarrollo Regional (project VA283P18

Defects-Recognition, Imaging and Physics in Semiconductors (DRIP) XVIII

- Carrier recombination is critical for the operation of many devices, e.g. solar cells, light emitting diodes, and laser diodes. - The diffusion length depends on the band structure of the material, the crystal quality, alloy scattering, doping, and the presence of defectsProyecto de Investigación ENE2017-89561-C4-3-R (MCIN)Proyecto de Investigación VA283P18 (Junta de Castilla y León

Thinned GaInP/GaInAs/Ge solar cells grown with reduced cracking on Ge|Si virtual substrates

Producción CientíficaReducing the formation of cracks during growth of GaInP/GaInAs/Ge 3-junction solar cells on Ge|Si virtual substrates has been attempted by thinning the structure, namely the Ge bottom cell and the GaInAs middle cell. The theoretical analysis performed using realistic device parameters indicates that the GaInAs middle cell can be drastically thinned to 1000 nm while increasing its In content to 8% with an efficiency loss in the 3-junction cell below 3%. The experimental results show that the formation of macroscopic cracks is prevented in thinned GaInAs/Ge 2-junction and GaInP/GaInAs/Ge 3-junction cells. These prototype crack-free multijunction cells demonstrate the concept and were used to rule out any possible component integration issue. The performance metrics are limited by the high threading dislocation density over 2·107cm−2 in the virtual substrates used, but an almost current matched, crack-free, thinned 3-junction solar cell is demonstrated, and the pathway towards solar cells with higher voltages identified.Agencia Estatal de Investigación (project RTI2018-094291-B-I00)Ministerio de Educación, Cultura y Deporte (project FPU-15/03436)Programa Estatal de Promoción del Talento y su Empleabilidad (grant RYC-2014- 15621)Junta de Castilla y León - Fondo Europeo de Desarrollo Regional (project VA283P18

Luminescence in undoped and Nb-doped SrTiO3 crystals: Bulk and surface emission

Producción CientíficaThe luminescence of SrTiO3 depends on the sample type, either doped or stoichiometric, as grown or treated, the excitation conditions, and temperature. The origin of the luminescence emissions, blue, green, and infrared, remains controversial. In particular, the role played by defects, mainly oxygen vacancies, impurities, and self-trapped holes and electrons on the different emissions are far to be elucidated. We present a cathodoluminescence (CL) and photoluminescence (PL) study of undoped and Nb-doped samples. The different excitation conditions of CL and PL permit to distinguish the luminescence emission from the bulk (CL) and from a surface skin region (PL). Significant differences between both techniques are seen for the undoped sample, while the Nb-doped sample presents less differences, highlighting the role played by the surface defects and the doped electrons. The study is complemented by the temperature dependence of the luminescence spectra and the emission due to defects generated by plastic deformation.Junta de Castilla y León (project VA283P18)Ministerio de Economía, Industria y Competitividad - Fondo Europeo de Desarrollo Regional (project ENE2017-89561-C4-3-R