Arto Aho

Performance assessment of multijunction solar cells incorporating GaInNAsSb

We have measured the characteristics of molecular beam epitaxy grown GaInNAsSb solar cells with different bandgaps using AM1.5G real sun illumination. Based on the solar cell diode characteristics and known parameters for state-of-the-art GaInP/GaAs and GaInP/GaAs/Ge cells, we have calculated the realistic potential efficiency increase for GaInP/GaAs/GaInNAsSb and GaInP/GaAs/GaInNAsSb/Ge multijunction solar cells for different current matching conditions. The analyses reveal that realistic GaInNAsSb solar cell parameters, render possible an extraction efficiency of over 36% at 1-sun AM1.5D illumination.PACS88.40.hj; 88.40.jm; 88.40.jp; 81.15.Hi

Study of nitrogen incorporation into GaInNAs: The role of growth temperature in molecular beam epitaxy

GaInNAs has an important impact on developing GaAs-based optoelectronics and multijunction solar cells, but the complex nature of the nitrogen incorporation into GaInAs is still not fully understood. By combining x-ray diffraction, photoluminescence, reflection high-energy electron diffraction, and photoelectron spectroscopy measurements, we show that nitrogen incorporation is enhanced with increasing growth temperature in the range of 300–450 °C. We study the growth front and show that the surface reconstruction is (1 × 3) regardless of growth temperature in this range. The enhanced nitrogen incorporation can be modeled as a thermally activated process with activation energy of about 0.1 eV.

Dynamics of time-resolved photoluminescence in GaInNAs and GaNAsSb solar cells

We report a time-resolved photoluminescence study for GaInNAs and GaNAsSb p-i-n bulk solar cells grown on GaAs(100). In particular, we studied the extent to which the carrier lifetime decreases with the increase of N content. Rapid thermal annealing proved to significantly increase the decay times by a factor of 10 to 12 times, for both GaInNAs and GaNAsSb heterostructures, while for the 1-eV bandgap GaNAsSb structure, grown at the same growth conditions as the GaInNAs, the photoluminescence decay time remained slightly below 100 ps after annealing; the approximately 1.15-eV GaInNAs p-i-n solar cell exhibited a lifetime as long as 900 ps.PACS78.47.D; 78.55.Cr; 88.40.hj

High current generation in dilute nitride solar cells grown by molecular beam epitaxy

We review our recent work concerning the development of dilute nitride solar cells by molecular beam epitaxy. This epitaxial technology enables a high level of control of the growth conditions and alleviates known issues related to epitaxy of dilute nitrides ultimately enabling to achieve high quality materials suitable for solar cell developments. In particular, we focus on discussing the mechanisms linking the epitaxial and annealing conditions to the operation of dilute nitride solar cells. We also report operation of a single junction dilute nitride solar cell with a short circuit current density as high as ~39 mA/cm2 under 1 sun illumination.

Comparison of GaInNAs and GaInNAsSb solar cells grown by plasma-assisted molecular beam epitaxy

We compare dilute nitride GaInNAs and GaInNAsSb solar cells grown by molecular beam epitaxy. Single junction p-i-n diode solar cells were fabricated to test the dilute nitride and antimonide material fabrication process. Triple-junction solar cells were fabricated to test the behavior of single GaInNAs(Sb) junctions in multi-junction configuration. When nitrogen was added to the growth of GaInNAs, good crystal quality was maintained up to 4% of nitrogen at the used growth conditions. Short circuit current densities of the devices could be increased by adding Sb to the growth but at the same time the open circuit voltages decreased due to bandgap shrinkage induced by Sb. In multijunction configuration, the samples with Sb showed inferior properties to ones without it. Lower currents and voltages of Sb-containing cells may be linked to segregation of Sb and transfer to the upper junctions.

Annealing of self-assembled InAs/GaAs quantum dots: A stabilizing effect of beryllium doping

We investigated the effects of postgrowth thermal annealing on optical properties of beryllium-doped InAs quantum dot (QD) heterostructures grown by molecular beam epitaxy. Thermal annealing induced a blueshift of up to 200 meV in light emission from an undoped sample, while a sample having GaAs layer heavily doped with beryllium on top of the QD region exhibited a much smaller blueshift. This phenomenon is interpreted as due to suppression of annealing-induced In/Ga interdiffusion.

Removal of strain relaxation induced defects by flushing of InAs quantum dots

We report the effect of indium flushing on the electrical and optical properties of strain-relaxed InAs quantum dots (QDs) embedded in GaAs Schottky diodes. The InAs QDs were intentionally grown beyond the critical thickness to induce plastically relaxed QDs. The samples were fabricated by molecular beam epitaxy on GaAs(1 0 0) substrates using continuous and cycled InAs deposition. Deep level transient spectroscopy (DLTS) experiments show broad dislocation-induced defects in non-flushed samples. We show by DLTS that indium flushing after QD deposition decreases remarkably the amount of electron traps in the QD layer and suppresses the defect formation into GaAs capping layer. Using capacitance–voltage measurements we observed that the flushed samples exhibit a recovery of electron confinement in the QD states. Furthermore, we used photoluminescence experiments to analyse the effects of direct versus cycled InAs deposition and indium flushing steps.

Temperature coefficients for GaInP/GaAs/GaInNAsSb solar cells

We report the temperature coefficients for MBE-grown GaInP/GaAs/GaInNAsSb multijunction solar cells and the corresponding single junction sub-cells. Temperature-dependent current-voltage measurements were carried out using a solar simulator equipped with a 1000 W Xenon lamp and a three-band AM1.5D simulator. The triple-junction cell exhibited an efficiency of 31% at AM1.5G illumination and an efficiency of 37–39% at 70x real sun concentration. The external quantum efficiency was also measured at different temperatures. The temperature coefficients up to 80°C, for the open circuit voltage, the short circuit current density, and the conversion efficiency were determined to be −7.5 mV/°C, 0.040 mA/cm2/°C, and −0.09%/°C, respectively.

Comparative study of defect levels in GaInNAs, GaNAsSb, and GaInNAsSb for high-efficiency solar cells

Background doping and defect levels in GaInNAs, GaNAsSb, and GaInNAsSb solar cells with 1 eV band-gap are reported. Localized point defect induced traps were observed showing broadest defect distribution in GaInNAsSb. Incorporation of Sb reduced the unintentional p-type background doping by an order of magnitude, but increased the capture cross sections of deep levels by three orders of magnitude. The thermal activation energy of the dominating hole trap was increased from 350 meV for GaInNAs to 560 meV for GaNAsSb. Annealing of GaNAsSb solar cells improved the open circuit voltage from 280 mV to 415 mV, owing to the reduction in trap density.

Incorporation model of N into GaInNAs alloys grown by radio-frequency plasma-assisted molecular beam epitaxy

We present a Maxwell-Boltzmann electron energy distribution based model for the incorporation rate of nitrogen into GaInNAs grown by molecular beam epitaxy (MBE) using a radio frequency plasma source. Nitrogen concentration is predicted as a function of radio-frequency system primary resistance, N flow, and RF power, and group III growth rate. The semi-empirical model is shown to be repeatable with a maximum error of 6%. The model was validated for two different MBE systems by growing GaInNAs on GaAs(100) with variable nitrogen composition of 0%–6%.