H. B. Yuen

43.5% Efficient Lattice Matched Solar Cells

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 check.

High-efficiency multijunction solar cells employing dilute nitrides

Solar Junction has developed a set of dilute nitride compound semiconductors with antimony that offer tunable absorption between the GaAs and Ge bandedges, while retaining lattice matching to GaAs or Ge substrates. By replacing the Ge junction in a conventional triple junction solar cell with a GaInNAsSb junction, world record cell efficiencies of 43.5% have been achieved, and CPV module efficiencies (DC) exceeding 35% may now be possible in the near future.

Long-wavelength GaInNAs(Sb) lasers on GaAs

The boom in fiber-optic communications has caused a high demand for GaAs-based lasers in the 1.3-1.6-/spl mu/m range. This has led to the introduction of small amounts of nitrogen into InGaAs to reduce the bandgap sufficiently, resulting in a new material that is lattice matched to GaAs. More recently, the addition of Sb has allowed further reduction of the bandgap, leading to the first demonstration of 1.5-/spl mu/m GaAs-based lasers by the authors. Additional work has focused on the use of GaAs, GaNAs, and now GaNAsSb barriers as cladding for GaInNAsSb quantum wells. We present the results of photoluminescence, as well as in-plane lasers studies, made with these combinations of materials. With GaNAs or GaNAsSb barriers, the blue shift due to post-growth annealing is suppressed, and longer wavelength laser emission is achieved. Long wavelength luminescence out to 1.6 /spl mu/m from GaInNAsSb quantum wells, with GaNAsSb barriers, was observed. In-plane lasers from these samples yielded lasers operating out to 1.49 /spl mu/m, a minimum threshold current density of 500 A/cm/sup 2/ per quantum well, a maximum differential quantum efficiency of 75%, and pulsed power up to 350 mW at room temperature.

GaInNAsSb for 1.3-1.6-/spl mu/m-long wavelength lasers grown by molecular beam epitaxy

High-efficiency optical emission past 1.3 /spl mu/m of GaInNAs on GaAs, with an ultimate goal of a high-power 1.55-/spl mu/m vertical-cavity surface-emitting laser (VCSEL), has proven to be elusive. While GaInNAs could theoretically be grown lattice-matched to GaAs with a very small bandgap, wavelengths are actually limited by the N solubility limit and the high In strain limit. By adding Sb to the GaInNAs quaternary, we have observed a remarkable shift toward longer luminescent wavelengths while maintaining high intensity. The increase in strain of these new alloys necessitates the use of tensile strain compensating GaNAs barriers around quantum-well (QW) structures. With the incorporation of Sb and using In concentrations as high as 40%, high-intensity photoluminescence (PL) was observed as long as 1.6 /spl mu/m. PL at 1.5 /spl mu/m was measured with peak intensity over 50% of the best 1.3 /spl mu/m GaInNAs samples grown. Three QW GaIn-NAsSb in-plane lasers were fabricated with room-temperature pulsed operation out to 1.49 /spl mu/m.

Interference effects in electromodulation spectroscopy applied to GaAs-based structures: A comparison of photoreflectance and contactless electroreflectance

In this letter, we show that the oscillation features (OFs) usually observed in photoreflectance (PR) spectra of GaAs-based structures grown on the n-type GaAs substrate below the GaAs fundamental gap could be eliminated completely by applying the contactless electroreflectance (CER) instead of PR. This finding confirms that the origin of OFs is the modulation of the refractive index in the sample due to the generation of additional carriers by the modulated pump beam. In the case of CER spectroscopy, any additional carriers are not generated during the modulation hence CER spectra are free of OFs. This advantage of CER spectroscopy is very important in investigations of all structures for which OFs are present in PR spectra. In order to illustrate this advantage of CER spectroscopy we show PR and CER spectra measured first for the GaAs epilayer and next for more complicated steplike GaInNAsSb∕GaNAs∕GaAs quantum well structures.

GaInNAsSb Solar Cells Grown by Molecular Beam Epitaxy

The first GaInNAsSb solar cells are reported. The dilute nitride antimonide material, grown by molecular beam epitaxy, has a bandgap of 0.92 eV and maintains excellent carrier collection efficiency. Internal quantum efficiency of nearly 80% at maximum is obtained in the narrow bandgap GaInNAsSb cells. The short-circuit current density produced by the GaInNAsSb cells underneath a GaAs sub-cell in a multijunction stack, determined from the overlap of the quantum efficiency and the low-AOD spectrum, is 14.8 mA/cm2. This is sufficient to current match the GaInNAsSb sub-cell to the other sub-cells in a GaInP/GaAs/GaInNAsSb solar cell. However, the open-circuit voltage and fill factor of the antimonide devices, 0.28 V and 0.61, are somewhat reduced when compared to GaInNAs devices with 1.03 eV bandgaps. The GaInNAsSb devices had wider depletion regions, which improves the collection efficiency but adversely affects the fill-factor and dark current by increasing depletion region recombination

Temperature dependence of diffusion length, lifetime and minority electron mobility in GaInP

The mobility of electrons in double heterostructures of p-type Ga0.50In0.50P has been determined by measuring minority carrier diffusion length and lifetime. The minority electron mobility increases monotonically from 300 K to 5 K, limited primarily by optical phonon and alloy scattering. Comparison to majority electron mobility over the same temperature range in comparably doped samples shows a significant reduction in ionized impurity scattering at lower temperatures, due to differences in interaction of repulsive versus attractive carriers with ionized dopant sites. These results should be useful in modeling and optimization for multi-junction solar cells and other optoelectronic devices.

Contactless electroreflectance of GaInNAsSb/GaAs single quantum wells with indium content of 8%-32%

Interband transitions in GaInNAsSb∕GaAs single quantum wells (SQWs) with nominally identical nitrogen and antimony concentrations (2.5% N and 7% Sb) and varying indium concentrations (from 8% to 32%) have been investigated by contactless electroreflectance (CER). CER features related to optical transitions between the ground and excited states have been clearly observed. Energies of the QW transitions extracted from CER measurements have been matched with those obtained from theoretical calculations performed within the effective mass approximation for various conduction-band offsets (QC) and various electron effective masses. It has been found that the QC increases from 40% to 80% with the rise of the indium content from 8% to 32% and the electron effective mass is close to 0.09m0. The results show that the band gap discontinuity in GaInNAsSb∕GaAs SQWs can be broadly tuned with a change in the indium concentration.

Band gap discontinuity in Ga0.9In0.1N0.027As0.973−xSbx∕GaAs single quantum wells with 0⩽x<0.06 studied by contactless electroreflectance spectroscopy

Contactless electroreflectance (CER) spectroscopy has been applied to study optical transitions in Ga0.9In0.1N0.027As0.973−xSbx∕GaAs single quantum well (QW) with antimony content varying from 0% to 5.4%. CER features related to optical transitions between the ground and excited states have been clearly observed. Energies of the QW transitions have been matched with those obtained from theoretical calculations. It has been determined that the conduction band offset decreases from ∼55% to ∼45% with the increase in Sb content from 0% to 5.4%. This result demonstrates that the band gap discontinuity for Ga0.9In0.1N0.027As0.973−xSbx∕GaAs system can be simply tuned by a change in antimony content.

GaInNAs(Sb) vertical-cavity surface-emitting lasers at 1.460 μm

We demonstrate a top emitting, electrically pumped, GaInNAsSb vertical-cavity surface-emitting laser (VCSEL) grown monolithically on GaAs, lasing pulsed at a wavelength of 1.460 μm, at a chuck temperature of −10 °C, with a threshold current of 550 mA (16 kA/cm2) and a duty cycle of 0.1% for large mesas. Dilute nitrides, such as GaInNAs, have proven effective for lasers operating at 1.31 μm, but reaching longer wavelengths has proven difficult due to defects from low-temperature growth, surface roughening, and nitrogen-related defects. Reduction of oxygen contamination and careful attention to plasma conditions allow a similar extension to laser wavelength, by minimizing crystal defects introduced during growth. This is the first VCSEL on GaAs beyond 1.31 μm to date.