Emma J. Renteria

AlGaSb-Based Solar Cells Grown on GaAs: Structural Investigation and Device Performance

GaSb and alloys based on the 6.1 Å family can be grown metamorphically on substrates such as GaAs allowing for the realization of several multijunction solar cell designs. This paper investigates the molecular beam epitaxy growth, crystal quality, and device performance of Al<italic>_x</italic>Ga₁₋<italic>_x</italic>Sb-based single-junction solar cells grown on GaAs substrates. The focus is on the optimization of the growth of Al<italic>_x</italic>Ga₁₋<italic>_x</italic>Sb on GaAs (001) substrates in order to minimize the threading dislocation density resulting from the large lattice mismatch between GaSb and GaAs. Utilizing optimum growth conditions, solar cells with absorbing layers of different Al<italic>_x </italic>Ga₁<italic>_x</italic>Sb compositions are studied and compared to control cells grown on lattice-matched GaSb substrates. GaSb, Al_0.15Ga_0.85Sb, and Al_0.5Ga_0.5Sb solar cells grown on GaAs substrates show open-circuit voltages of 0.16, 0.17, and 0.35 V, respectively. Furthermore, the lattice-mismatched cells demonstrate promising carrier collection with comparable spectral response to lattice-matched control cells grown on GaSb.

Beryllium implant activation and damage recovery study in n-type GaSb

Damage induced by the implantation of beryllium in n-type GaSb and its removal by Rapid Thermal Annealing (RTA) are studied in detail by Atomic Force Microscopy (AFM), Cross Sectional Transmission Electron Microscopy (XTEM) and Energy Dispersive X-ray Spectroscopy (EDS). RTA has been implemented with different times and temperatures in order to optimize ion activation and to avoid Sb outdiffusion during the process. Results indicate a lattice quality that is close to pristine GaSb for samples annealed at 600 °C for 10s using a thick Si3N4 capping layer. Electrical response of the implanted diodes is measured and characterized as function of different annealing conditions.

Pixelated GaSb solar cells on silicon by membrane bonding

We demonstrate thin-film GaSb solar cells which are isolated from a GaSb substrate and transferred to a Si substrate. We epitaxially grow ∼3.3 μm thick GaSb P on N diode structures on a GaSb substrate. Upon patterning in 2D arrays of pixels, the GaSb films are released via epitaxial lift-off and they are transferred to Si substrates. Encapsulation of each pixel preserves the structural integrity of the GaSb film during lift-off. Using this technique, we consistently transfer ∼4 × 4 mm2 array of pixelated GaSb membranes to a Si substrate with a ∼ 80%–100% yield. The area of individual pixels ranges from ∼90 × 90 μm2 to ∼340 × 340 μm2. Further processing to fabricate photovoltaic devices is performed after the transfer. GaSb solar cells with lateral sizes of ∼340 × 340 μm2 under illumination exhibit efficiencies of ∼3%, which compares favorably with extracted values for large-area (i.e., 5 × 5 mm2) homoepitaxial GaSb solar cells on GaSb substrates.We demonstrate thin-film GaSb solar cells which are isolated from a GaSb substrate and transferred to a Si substrate. We epitaxially grow ∼3.3 μm thick GaSb P on N diode structures on a GaSb substrate. Upon patterning in 2D arrays of pixels, the GaSb films are released via epitaxial lift-off and they are transferred to Si substrates. Encapsulation of each pixel preserves the structural integrity of the GaSb film during lift-off. Using this technique, we consistently transfer ∼4 × 4 mm2 array of pixelated GaSb membranes to a Si substrate with a ∼ 80%–100% yield. The area of individual pixels ranges from ∼90 × 90 μm2 to ∼340 × 340 μm2. Further processing to fabricate photovoltaic devices is performed after the transfer. GaSb solar cells with lateral sizes of ∼340 × 340 μm2 under illumination exhibit efficiencies of ∼3%, which compares favorably with extracted values for large-area (i.e., 5 × 5 mm2) homoepitaxial GaSb solar cells on GaSb substrates.

Development of thin film metamorphic GaSb cells by epitaxial lift-off from GaAs substrates

A flexible, lightweight GaSb solar cell has been attempted. The thin-film cell was bonded to a flexible carrier and isolated from the GaAs substrate by using epitaxial lift-off technique. The characteristics of the thin-film metamorphic GaSb cells are compared to regular metamorphic cells and lattice matched GaSb substrate cells. J-V characterization under 1 sun illumination is presented.

Effect of AlSb quantum dots on efficiency of GaAs solar cell(Conference Presentation)

Quantum Dots (QDs) have a broad applications in science and specifically in solar cell. Many research groups show that by adding QDs with lower bandgap respect to host material, the overall absorption of sun spectrum coverage will increase. Here, we propose using QDs with higher band gap respect to host material to improve efficiency of solar cell by improving quantum efficiency. GaAs solar cells have the highest efficiency in single junction solar cells. However, the absorption of GaAs is not good enough in wavelength lower than 550nm. AlSb can absorb shorter wavelength with higher absorption coefficient and also recombination rate should be lower because of higher bandgap of AlSb respect to GaAs. We embed AlSb QDs in GaAs solar cells and results show slight improvement in quantum efficiency and also in overall efficiency. Coverage of AlSb QDs has a direct impact on quality of AlSb QDs and efficiency of cell. In the higher coverage, intermixing between GaAs and AlSb causes to shift bandgap to lower value (having AlGaSb QDs instead of pure AlSb QDs). This intermixing decrease the Voc and overall efficiency of cell. In lower coverage, AlSb can survive from intermixing and overall performance of cell improves. Optimizing growth condition of AlSb QDs is a key point for this work. By using AlSb QDs, we can decrease the thickness of active layer of GaAs solar cells and have a thinner solar cell.

3 µm thick GaSb membrane diodes integrated with CVD diamond heat spreaders for thermally managed TPV cells

We demonstrate the integration of 3μm thick GaSb PN junctions with CVD diamond heat spreaders. The GaSb diodes are grown metamorphically on a GaAs substrate, bonded to CVD diamond by a solder process and isolated from the GaAs substrate by wet etching. Electrical characterization shows good diode behavior with a turn on voltage of 0.5 V and a reverse-bias leakage current of 1.12 mA.

Isolation and characterization of large-area GaSb membranes grown on GaAs substrates

We investigate substrate removal techniques for GaSb epilayers grown on GaAs substrates. The GaSb epilayers are grown metamorphically on the GaAs substrate by inducing large areas of 90° interfacial misfit dislocation arrays at the GaSb/GaAs interface. Three structures have been investigated, each with a different etch stop process such that the GaAs substrate is removed without affecting the GaSb epi-layer. The GaSb membranes upon isolation are characterized for crystal quality using x-ray diffraction and for surface quality using atomic force microscopy.