Ralph L. Chapman

Efficient Si solar cells by laser photochemical doping

An ArF excimer laser has been used to form p‐n junctions in Si. The laser produces dopant molecules by gas‐phase photolysis of an organometallic molecule and simultaneously heats the substrate to allow incorporation of the dopant. Solar cells having conversion efficiencies of 9.6% at AM1 without the use of antireflection coatings have been fabricated from these junctions.

Solid‐phase growth of large aligned grains during scanned laser crystallization of amorphous Ge films on fused silica

Well‐aligned grains of dimensions up to 2–3×100 μm have been obtained by laser crystallization of amorphous Ge films on fused‐silica substrates. A theoretical model has been developed for the dynamics of the crystallization process, a solid‐phase transformation that is greatly accelerated by liberation of the latent heat of transformation, resulting in either periodic or runaway motion of the crystallization front. We believe that scanning of amorphous films with an energy beam of large aspect ratio may be developed into an effective process for preparing large‐grained or even single‐crystal sheets of Ge and other semiconductors, including Si and GaAs.

Simplified fabrication of GaAs homojunction solar cells with increased conversion efficiencies

Conversion efficiencies as high as 20% of AM1 have been obtained for single‐crystal GaAs shallow‐homojunction solar cells without Ga1−xAlxAs layers. These cells, which are fabricated by a simplified technique that does not require any vacuum processing steps, utilize an n+/p/p+ structure with an antireflection coating prepared by anodic oxidation of the n+ layer.

Transient annealing of selenium‐implanted gallium arsenide using a graphite strip heater

A graphite strip heater has been used for transient annealing, at temperatures of 900–1140 °C, of GaAs wafers implanted at 300 °C with a 1×1015 cm−2 dose of 400‐keV Se+ ions. The electrical activation of the implant produced by annealing at 1140 °C for 10 s yields a sheet resistivity of 25 Ω/⧠ and sheet carrier concentration of 1.8×1014 cm−2, compared with values of 35 Ω/⧠ and 1.1×1014 cm−2 obtained by conventional furnace annealing at 950 °C for 30 min.

Crystallization‐front velocity during scanned laser crystallization of amorphous Ge films

An optical transmission technique has been used to measure the propagation velocity of the crystallization front during scanned laser crystallization of amorphous Ge films on fused‐silica substrates. The measurements confirm our theoretical model for the solid‐phase amorphous‐to‐crystalline transformation. According to this model, the periodic structural features of the crystallized films are formed because the crystallization front moves in a series of periodic jumps between rest positions, with the velocity of the front during each jump much higher than the laser scanning velocity. The measured values of the crystallization‐front velocity range from 140 to 260 cm/sec, compared with the laser scanning velocity of only 0.5 cm/sec.

Zone‐melting recrystallization of 3‐in.‐diam Si films on SiO2‐coated Si substrates

We have constructed a scaled‐up graphite strip‐heater system that permits routine zone‐melting recrystallization of 3‐in.‐diam Si films on SiO2‐coated Si wafers. The recrystallized films are similar in crystal quality to those obtained previously in a smaller system, except that they contain higher densities of small protrusions along the subgrain boundaries. Seeded recrystallization has been accomplished by scribing a stripe opening, that extends through the Si and SiO2 films to the Si wafer.

Efficient GaAs solar cells formed by UV laser chemical doping

ArF and XeF excimer laser radiation has been used to form p‐n junctions in GaAs. The laser releases S atoms by the dissociation of H2S gas and simultaneously heats the substrate to allow incorporation of the S dopant. Solar cells having AM1 efficiencies of 10.8% have been fabricated from these junctions. The process can also produce doped GaAs layers with sheet resistances as low as 30 Ω/⧠.

Ion-implanted laser-annealed GaAs solar cells

Conversion efficiencies up to 12% at AM1 have been obtained for ion‐implanted laser‐annealed (IILA) GaAs solar cells utilizing a shallow‐homojunction n+/p/p+ structure without a GaAlAs window. The n+ layer was formed by Se+‐ion implantation into the p layer, which was grown epitaxially by chemical vapor deposition on a single‐crystal p+ substrate. The implanted layer was annealed, without encapsulation, by scanning with a cw Nd : YAG laser. Cell metallization was performed by electroplating, and an antireflection coating was formed by anodic oxidation of the n+ layer.

Topographic Imperfections in Zone Melting Recrystallized Si Films on SiO2

Examen des principales imperfections topographiques et discussion de leur variation avec les profils thermiques presents durant le processus de recristallisation. Observation de vides formes dans les films cristallises par contamination lors de la preparation de la pastille

Efficient shallow‐homojunction GaAs solar cells by molecular beam epitaxy

Conversion efficiencies up to 16% at AM1 have been obtained for molecular beam epitaxy (MBE) GaAs solar cells utilizing a shallow‐homojunction n+/p/p+ structure without a GaAlAs window. The n+, p, and p+ GaAs layers were all grown by MBE on single‐crystal p+ GaAs substrates. Cell metallization was performed by electroplating, and an antireflection coating was formed by anodic oxidation of the n+ layer. These cells are the first efficient MBE solar cells of any type to be reported.