Ville Polojärvi

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

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.

Effects of (NH4)2S and NH4OH surface treatments prior to SiO2 capping and thermal annealing on 1.3 μm GaInAsN/GaAs quantum well structures

We report the influence of (NH4)2S and NH4OH surface treatments prior to SiO2 capping and subsequent rapid thermal annealing, on optical properties of GaInAsN/GaAs quantum-well (QW) structures. We observed an increase in QW photoluminescence (PL) emission for the (NH4)2S treated samples as compared to the untreated sample. After annealing, also the NH4OH treated sample showed significant improvement in PL. The treatments were also found to decrease the In out-diffusion and reduce the blueshift upon annealing. The PL results are discussed with x-ray diffraction and x-ray photoemission data from SiO2/GaAs, in particular, with changes found in Ga 3d spectra.

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.

Stacked GaAs quantum dots fabricated by refilling of self-organized nanoholes: optical properties and post-growth annealing

We study the photoluminescence and impact of post-growth annealing of stacked, strain-free GaAs quantum dots fabricated by refilling of self-organized nanoholes using molecular beam epitaxy. Temperature- and power-dependent photoluminescence studies reveal an excellent optical quality of the quantum-dot stack. After high-temperature post-growth annealing only slight blueshifts and an increase in full width at half-maximum of the photoluminescence peak are observed, indicating very high-temperature stability and crystalline quality of the stacked GaAs quantum-dot structure.

Properties of the SiO2- and SiNx-capped GaAs(100) surfaces of GaInAsN/GaAs quantum-well heterostructures studied by photoelectron spectroscopy and photoluminescence

SiO2 and SiNx layers are routinely deposited onto III-V(100) surfaces at different device processing steps. We elucidate these insulator-interface properties with photoemission and photoluminescence (PL) of SiO2- and SiNx-capped GaAs(100) surfaces of GaInAsN/GaAs quantum wells (QWs). Post-growth annealing led to an increase of the QW-PL intensity, of which origin can be clearly linked to the SiO2 and SiNx interfaces. Concomitantly, Ga2O–related photoemission increased, indicating useful formation of Ga2O at both insulator interfaces. Furthermore, higher Ga-oxidation-state emission, identified with Ga diffused into SiO2 and SiNx, correlates with the blue-shift of the QW-PL wavelength. Also, interfacial As-As related photoemission was identified.

Suppression of annealing-induced In diffusion in Be-doped GaInAsN/GaAs quantum well

The authors report on an interesting observation regarding thermal annealing of a beryllium-doped Ga0.65In0.35As0.99N0.01/GaAs quantum well (QW) grown by molecular beam epitaxy. A QW doped at 6×1019 cm−3 exhibited superior thermal properties and about six times larger photoluminescence than an undoped QW of the same structure. X-ray diffraction and secondary ion mass spectrometry provided evidence that beryllium suppressed indium diffusion and stabilized (metastable) dilute nitride heterostructure upon annealing.