Laser selective chemical vapor deposition and direct writing of GaAs and its ternary alloys with P have been achieved on GaAs substrates. An Ar+ laser is used to locally heat areas where selective deposition is desired on a substrate which is uniformly biased to a temperature in the range of 25–500 °C. Epitaxial growth was achieved by carefully controlling the deposition parameters to reach growth rates low enough, typically 20 Å/s, for the reaction kinetics of the pyrolitic process to take place. Cross-sectional transmission electron microscopy and photoluminescence results indicate that the quality of the deposited material is comparable to that grown with the conventional metalorganic chemical vapor deposition technique.
The alloy GaInAsN has great potential as a lower-band-gap material lattice matched to GaAs, but there is little understanding of what causes its poor optoelectronic properties and why these improve with annealing. This study provides information about the structural changes that occur when GaInAsN is annealed. The Fourier transform infrared spectra exhibit two primary features: a triplet at ∼470 cm−1 (Ga–N stretch) and two or three bands at ∼3100 cm−1 (N–H stretch). The change in the Ga–N stretch absorption can be explained if the nitrogen environment is converted from NGa4 to NInGa3 after annealing. The N–H stretch is also changed after annealing, implying a second, and unrelated, structural change.
Molecular stream epitaxy allows several molecular beam epitaxy (MBE) concepts to take place in a metalorganic chemical vapor deposition reactor. In this technique, the growth of InGaAs/GaAsP superlattices proceeds by rotating the substrate between two gas streams, one containing trimethylgallium (TMG), triethylindium, and AsH3 and the other containing TMG, PH3, and AsH3. This technique eliminates gas flow transients and provides a method to mechanically shear off the gaseous boundary layer between successive exposures. Ultrathin strained-layer superlattices (SLS’s) with 8-Å-thick films have been obtained. The optical properties of these SLS’s are comparable to those obtained for equivalent superlattices by gas source MBE.
Low cost germanium photodetectors for sensing applications in the 900-1600 nm spectral region have been developed. By varying the Ge substrate resistivity as well as device area, photodetector properties such as reverse leakage current, capacitance, and shunt resistance have been engineered. Low leakage current devices of various sizes up to 1 cm2 have been fabricated and have consistently exhibited exceptionally high shunt resistances and excellent linearity. Over 5000 hours of active stress testing have left the ultra-low leakage currents unchanged. These data were measured in accordance with Telcordia 468-CORE requirements at 85°C, 125°C and 175°C. The results indicate that these mesa photodetectors meet telecommunication industry requirements for reliability. These devices are comparable to commercially available Ge photodetectors, and can be readily substituted for more complex InGaAs photo-detectors in applications such as laser monitor diodes.
GaAs-AlGaAs diode lasers have been fabricated on silicon-on-insulator wafers for the first time. Gain-guided graded-index separate-confinement heterostructure single-quantum-well (GRINSCH-SQW) lasers operated CW at room temperature with threshold current as low as 43 mA, differential quantum efficiency as high as 54%, and output power of more than 60 mW/facet. One device operated CW for 75 min at an output power of 1 mW/facet.< >
Inverted metamorphic (IMM) solar cells based on III–V materials have the potential to achieve solar conversion efficiencies that are significantly higher than today's state of the art solar cells which are based on the 3-junction GaInP/GaInAs/Ge design. The 3J IMM device architecture based on (Al)GaInP/GaInAs/GaInAs, for example, allows for a higher voltage solar cell by replacing the low bandgap Ge (0.67 eV) from the conventional 3J structure with the higher bandgap (∼1 eV) metamorphic GaInAs. The inverted growth simply allows the lattice-matched junctions (i.e., (Al)GaInP/GaInAs) to be grown first on the growth substrate, thereby minimizing or shielding them from the defects that arise from the metamorphic layers. Spectrolab has demonstrated 30.5% AM0 efficiency based on the 3J IMM cell architecture grown on a Ge substrate, with V oc = 2.963V, J sc = 16.9 mA/cm 2 , and FF = 82.5%. In addition, 4J IMM cells have been demonstrated with V oc of 4.072 V and AM0 efficiency approaching 25%. With additional development, demonstrating 33% AM0 efficiency is expected in the near future. However, the IMM devices demand more complex processing requirements than conventional solar cells, and we demonstrate the capability to fabricate large area solar cells from standard Ge solar cell substrates.
We identify a failure mode due to a photoactive back contact for Ge concentrator solar cells. This problem manifests itself as a leveling off and subsequent decrease of open-circuit voltage (V/sub oc/) as the concentration increases above /spl sim/20 suns. Correction of this problem yields a much improved Ge cell for which V/sub oc/ increases in an almost ideal n=1 manner from 0.2 volts at one sun to 0.4 volts at 1400 suns. This cell's fill factor remains at or above its one-sun value up to 500 suns, confirming that this cell is fully suitable for high-concentration use. We show that solving the back-contact problem can significantly improve the high-concentration performance of GaInP/GaAs/Ge three-junction solar cells.
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