Aymeric Maros

III-Nitride Double-Heterojunction Solar Cells With High In-Content InGaN Absorbing Layers: Comparison of Large-Area and Small-Area Devices

This paper investigates the molecular beam epitaxy (MBE) growth, material characterization, and performance testing of indium gallium nitride (InGaN)/GaN double-heterojunction solar cells. Structures with varying thicknesses and compositions of the InGaN absorbing layer are studied. The N-rich MBE growth at low temperatures enables the growth of thick 10% and 20% InGaN films with minimal strain relaxation and defect generation. The characteristics of both large- and small-area devices are compared. While leakage current and high ideality factors associated with the double-heterojunction structure remain issues as detected by I-V and concentration effect measurements, the double-heterojunction cell with a record-high In content of 22% shows a promising photovoltaic response.

High temperature characterization of GaAs single junction solar cells

We report temperature-dependent characterization of the electrical and optical properties of GaAs single junction solar cells up to 450 °C. An external quantum efficiency (EQE) of 75% was maintained at temperatures up to 300 °C, with a corresponding increase in the absorption edge as a function of wavelength due to the decrease in band gap with temperature, in agreement with theory. Above 300 °C, the EQE decreased strongly as the temperature was further increased. This drop in EQE resulted in a corresponding decrease in short-circuit current, also observed in the I-V characteristics as a function of temperature. When cooled back to room temperature the short-circuit current nearly fully recovers whereas the open-circuit voltage is found to be irreversibly degraded. The origin of the degradation is discussed. Modeling is used to support the experimental results.

Critical thickness investigation of MBE-grown GaInAs/GaAs and GaAsSb/GaAs heterostructures

GaInAs/GaAs and GaAsSb/GaAs heterostructures were grown by molecular beam epitaxy with different In/Sb compositions and thicknesses in order to obtain samples with different amounts of initial strain. High resolution x-ray diffraction was used to extract the alloys composition, specify the presence of dislocations, and determine the extent of relaxation while transmission electron microscopy and x-ray topography were used to observe these dislocations and characterize their type and density. The onset for the formation of misfit dislocations was found to be in agreement with the equilibrium theory. However, the films remained coherently strained for thicknesses far beyond this value. The onset for strain relaxation was found by considering the kinetics of plastic deformation using the approach proposed by Tsao and coworkers [Phys. Rev. Lett. 59, 2455 (1987)]. The mechanism of extended defect creation leading to measurable strain relief is described as a multistage process related with the structural stabi...

Growth and characterization of GaAs1−x−ySbxNy/GaAs heterostructures for multijunction solar cell applications

The GaAsSbN dilute-nitride alloy can be grown lattice-matched to GaAs with a bandgap of 1 eV, making it an ideal candidate for use in multijunction solar cells. In this work, using molecular beam epitaxy in conjunction with a radio-frequency nitrogen plasma source, the authors focus first on the growth optimization of the GaAsSb and GaAsN alloys in order to calibrate the Sb and N compositions independently of each other. After the optimum growth conditions to maintain two-dimensional growth were identified, the growth of GaAsSbN films was demonstrated. Both a GaAsSb0.076N0.018/GaAs heterostructure (100 nm thick) and a GaAsSb0.073N0.015/GaAs quantum well (11 nm thick) were grown. X-ray diffraction analysis reveals quite high crystal quality with a small lattice mismatch of 0.13%–0.16%. Secondary ion mass spectrometry profiling revealed that nitrogen was unintentionally incorporated in the GaAs buffer layer during the plasma ignition and stabilization. Nevertheless, a low temperature photoluminescence peak ...

Carrier localization effects in GaAs1−xSbx/GaAs heterostructures

We investigated the structural and optical properties of GaAs1−xSbx/GaAs heterostructures grown by molecular beam epitaxy on GaAs (001) substrates for Sb concentration up to 12% by means of high-resolution X-ray diffraction and photoluminescence. The correlation between our structural and optical analysis revealed that compositional fluctuations induced localized states which trap carriers at low temperature. Under low excitation power, the photoluminescence (PL) spectra are composed of two competing peaks in the temperature range of 30–80 K. The lower energy peak is associated with transitions from localized states in the band-tail of the density of states while the higher energy peak corresponds to transitions from free carriers. A model based on a redistribution process of localized excitons was used to reproduce the S-shape behavior of the temperature dependent PL. Reducing the growth temperature from 500 °C to 420 °C suppressed the S-shape behavior of the PL indicating a reduction in compositional va...

1-eV GaNAsSb for multijunction solar cells

We report on the growth of 1-eV GaNAsSb lattice- matched to GaAs as an alternative material to the most commonly used GaInNAs(Sb). GaNAsSb layers were grown by plasma assisted molecular beam epitaxy with different N and Sb compositions. The electronic and optical properties of the layers were probed using photoluminescence and photoreflectance spectroscopy and compared to the band anticrossing model. The incorporation mechanism of the group-V elements were investigated using secondary ion mass spectrometry. It was found that Sb does not affect the N incorporation. Moreover increasing the N flux increased the N composition at the expense of the Sb composition. Post-growth annealing was investigated and found to greatly improve the photoluminescence intensity.

Defect Evolution in GaAs-based Low-mismatch Heterostructures

Epitaxial growth of thin films of ternary semiconductors such as In1-xGaxAs on GaAs substrates enables band-gap engineering which is very useful for many device applications such as intermediate-band solar cells, lasers, light-emitting diodes, etc. Due to the difference in lattice parameters between film and substrate, the film/substrate interface is subjected to bi-axial misfit strain leading to the introduction of misfit dislocations which are liable to act as non-radiative recombination centers, that are highly deleterious for device performance. Characterization of interfacial defects in these heterostructures is critical for developing a fundamental understanding of strain relaxation processes and eventually finding a path towards minimizing defect density by optimizing growth conditions, film thickness, etc. In this study, transmission electron microscopy (TEM) was used to investigate defect evolution in molecular beam epitaxy-grown low-mismatch (misfit strain ~ 0.6%) GaAs0.92Sb0.08/GaAs (001) and In0.08Ga0.92As/GaAs(001) heterostructures with film thicknesses in the range of 50 to 4000 nm. All samples had a GaAs capping layer of 50-nm thickness. Plan-view and cross-sectional TEM samples were prepared by polishing, dimpling and liquid-nitrogen argon-ion milling. Philips-FEI CM-200 FEG and JEOL ARM-200F microscopes operated at 200 keV were used for bright-field TEM and aberrationcorrected scanning transmission electron microscopy (STEM) imaging, respectively.

Thickness-Dependent Defect Evolution in GaAs0.92Sb0.08/GaAs Heterostructures

GaAs1-xSbx/GaAs heterostructures are promising candidates for intermediate-band solar cells (IBSCs). Ternary compounds such as GaAs1-xSbx facilitate band-gap tuning by introducing an intermediate band between valence and conduction bands. The efficiency of IBSCs can ideally be as high as 63.7% owing to additional photon absorptions related to electron transitions between valence band to intermediate band and intermediate band to conduction band [1]. A major obstacle to achieving high efficiency is the presence of crystallographic defects which act as non-radiative recombination centers. Understanding defect evolution during crystal growth and subsequently minimizing defects by adjusting growth conditions should improve device performance. Defect-free pseudomorphic growth of mismatched materials enabled by molecular beam epitaxy (MBE) is possible up to some critical thickness as the lattice mismatched epilayer is elastically strained to match the substrate lattice. Beyond the critical thickness the accumulated elastic strain is partially relieved by introduction of misfit dislocations. The nature, distribution and density of heteroepitaxial defects are the result of complex inter-related factors such as growth temperature, lattice mismatch, quality of substrate and thickness of epilayer [2]. This study describes the characterization of defect evolution as a function of epilayer thickness in GaAs1xSbx/GaAs heterostructures grown by MBE using GaAs (001) substrates. The Sb content of the epilayers was measured to be ~8% using high-resolution x-ray diffraction. Cross-sectional TEM samples were prepared using conventional polishing, dimpling and argon ion milling. The milling was carried out at 2.7 keV with samples held at liquid nitrogen temperature in order to minimize any milling damage. A Philips-FEI CM-200 microscope operated at 200 keV was used for imaging. Results are reported for GaAs0.92Sb0.08/GaAs heterostructures with thicknesses in the range of 50 to 4000 nm.

Defect creation in low lattice-mismatched epitaxial structures

The formation of crystalline defects is studied as a function of the epitaxial layer thickness in InGaAs and GaAsSb material systems grown by molecular beam epitaxy on (001) GaAs wafers. The Sb and In composition is roughly 8% in both sets of samples while the nominal thicknesses are respectively 50, 125, 250nm and 500nm for the InGaAs structures and 100, 250 and 500nm for the GaAsSb structures. High-resolution x-ray diffraction results show that similar partial relaxation is obtained in both systems for nearly the same thickness. Consistent structural transformation of point defects into dislocation loops related to the thickness of ternary layers is revealed. This resulted in a partial relaxation of 42 and 46% in the 250 nm thick GaAsSb and InGaAs layers respectively due to a density of secondary 60° dislocation loops of ~ 1 × 109 cm-2. The relaxation increased to 64% in the 500nm thick InGaAs and to 68% for the 500nm thick GaAsSb films even though the density of 60° dislocation loops in the volume was reduced due to intersections of these dislocation loops. Explanation of revealed structural features is suggested.