The most common triple-junction solar cell design which has been commercially available to date utilizes a germanium
bottom cell with an (In)GaAs and InGaP middle and top cell respectively. This type of device has a well-known
efficiency limitation somewhere around 40% at 500 suns. Higher efficiencies can be obtained by changing the effective
bandgaps of the three junctions, but the choice of materials and approaches to do so is very limited. We at Solar Junction
have adopted the dilute nitride material system to obtain these new bandgaps, and break through the 40% efficiency
barrier. The unique advantage of the dilute nitrides is that the bandgap and lattice constant can be tuned independently,
allowing bulk material lattice matched to Germanium or GaAs over a wide range of bandgaps. The dilute nitride
technology in our first commercial product has enabled us to maximize the efficiency of a triple junction solar cell by
using the optimal set of bandgaps (including one around 1eV). Commercial Solar Junction concentrator cells with
efficiencies of 43.5% have been independently verified by NREL and Fraunhofer. These higher efficiencies are
generally the result of higher output voltage, not higher current, which keeps system-level resistive wiring losses in
The dilute-nitride GaInNAs shows great promise in becoming the next choice for long-wavelength (0.9 to 1.6 μm) photodetector applications due to the ability for it to be grown lattice-matched on GaAs substrates. GaAs-based devices have several advantages over InP-based devices, such as substrate cost, convenience of processing, and optoelectronic band parameters. This paper will present results from the first high-quality thick GaInNAs films grown by solid state molecular beam epitaxy with a nitrogen plasma source and the first high efficiency photodetectors which have been fabricated from those materials. GaInNAs films up to 2 microns thick have been grown coherently on GaAs substrates. These films exhibit reasonable photoluminescence intensities at peak wavelengths of 1.22 to 1.13 μm before and after a rapid thermal anneal at a series of temperatures. PIN photodiodes with these thick GaInNAs films in the intrinsic regions show responsivity (better than 0.5 A/W at 1.064 μm), dark current (200 nA at -2 V), and signal-to-noise ratio (greater than 105) approaching those of commercially available InGaAs/InP devices. Furthermore, it will be shown that these devices show significantly lower dark current and higher signal-to-noise ratio than similar metamorphic InGaAs/GaAs structures.
We achieved 1.5-um CW SQW GaInNAsSb lasers with GaNAs barriers grown by MBE on GaAs substrates with typical room temperature threshold densities below 600A/cm2, external quantum efficiencies above 50%, and output powers exceeding 200mW from both facets for 20x1222um devices tested epitaxial-side up. In pulsed mode, 450A/cm2, 50%, and 1100mW were realized. Longer devices yielded over 425mW of total CW power and thresholds below 450A/cm2. These results are comparable to high quality GaInNAs/GaAs lasers at 1.3um. Z-parameter measurements revealed that these improvements in the performance metrics of approximately 40-60% over previous results are primarily due to reduced monomolecular recombination. The large differential gain of GaInNAsSb/GaNAs/GaAs lasers at 1.5um of approximately 1.2x10-15cm2 was mostly squandered in previous devices due to large quantities of monomolecular recombination. The characteristic temperatures for threshold current, T0, and for efficiency, T1, were 66K and 132K, respectively. These reduced values, compared to prior measurements of 106K and 208K, respectively, indicate carrier leakage. Since monomolecular recombination is temperature insensitive, the temperature stability of device operation was adversely affected.