Joel Salmi

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.

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.

Dilute nitride and GaAs n-i-p-i solar cells

We demonstrate for the first time the operation of GaInNAs and GaAs n-i-p-i doping solar cells with ion-implanted selective contacts. Multiple layers of alternate doping are grown by molecular beam epitaxy to form the n-i-p-i structure. After growth, vertical selective contacts are fabricated by Mg and Si ion implantation, followed by rapid thermal annealing treatment and fabrication into circular mesa cells. As means of characterisation, spectral response and illuminated current–voltage (I-V) were measured on the samples. The spectral response suggests that all horizontal layers are able to contribute to the photocurrent. Performance of the devices is discussed with interest in the n-i-p-i structure as a possible design for the GaInP/GaAs/GaInNAs tandem solar cell.

High-power 1550 nm tapered DBR lasers fabricated using soft UV-nano-imprint lithography

Paper reports the DBR-RWG surface grating design, the fabrication process, and the output characteristics of tapered DBR laser diodes for the applications, like for example LIDAR and range finding, that require eye-safe high-power single-mode coherent light sources. The fabricated regrowth-free DBR AlGaInAs/InP lasers exhibited a CW output power as high as 560 mW in single-mode operation at room temperature. At maximum output power the SMSR was 38 dB, proving the excellent behavior of the surface gratings. The tapered section enabled scaling the maximum CW power at room temperature from 125 mW to 560 mW, by increasing its length from 0.5 mm to 4.0 mm. The paper discusses the limitations and performance variation associated to the power scaling by using the tapered section length as a scaling parameter.

Difference frequency modulation of multi-section dual-mode lasers with nanoscale surface gratings

Dual-mode multi-section quantum-well distributed feedback lasers with surface gratings have been fabricated, without regrowth, at 1310 and 1550 nm using UV nano-imprint lithography. Several laser and grating sections have been employed to control and stabilize the dual-mode emission and to reduce mode competition. Frequency differences between 15 GHz and 1 THz were achieved for different longitudinal structures. Frequency difference variations of several GHz have been measured under bias modulation with rates up to a few GHz. Higher frequency difference modulation rates are expected from improved measurement setups and from employing quantum dot active regions for further reduction of mode competition.

Difference Frequency Modulation in Dual-Mode Multi-Section DFB Lasers

Dual-longitudinal-mode multi-section DFB lasers with surface gratings were fabricated at 1310 and 1550 nm. Frequency differences between 15 and 1000 GHz were achieved and were modulated by several GHz with rates up to 10 GHz.

Dual-mode multi-section lasers with nanoscale surface gratings

Dual-mode multi-section distributed-feedback lasers with surface gratings have been fabricated using UV nanoimprint lithography. Frequency differences from 14GHz to 1.3THz for different longitudinal structures and frequency difference modulation speed up to 500MHz have been measured.