Explosive crystallization (EC) takes place during flash lamp annealing in micrometer-thick amorphous Si (a-Si) films deposited on glass substrates. The EC starts from the edges of the a-Si films due to additional heating from flash lamp light. This is followed by lateral crystallization with a velocity on the order of m/s, leaving behind periodic microstructures in which regions containing several hundreds of nm-ordered grains and regions consisting of only 10-nm-sized fine grains alternatively appear. The formation of the dense grains can be understood as explosive solid-phase nucleation, whereas the several hundreds of nanometer-sized grains, stretched in the lateral direction, are probably formed through explosive liquid-phase epitaxy. This phenomenon will be applied to the high-throughput formation of thick poly-Si films for solar cells.

Explosive crystallization of amorphous silicon films by flash lamp annealing

In this paper we present a review on major advances achieved over the past ten years in the field of fabrication of semiconductor quantum wires (QWRs) using epitaxial growth techniques and investigation of their optical properties. We begin the review with a brief summary on typical epitaxial QWRs developed so far. We next describe the state-of-the-art structural qualities of epitaxial QWRs in terms of (i) size uniformity between wires, (ii) heterointerface uniformity, (iii) crystal purity, and (iv) strength of lateral quantum confinement. Several prominent breakthroughs have been accomplished concerning the improvements of wire qualities, including (i) realization of V-shaped GaAs∕AlGaAs QWRs in the “real one-dimensional” (1D) regime in which exciton states can extend coherently over distances exceeding 1μm, (ii) reduction of residual impurity concentrations in V-shaped GaAs∕AlGaAs QWRs to a level comparable to that in an equivalent quantum well (QWL), which resulted in the semiconductor QWR with room-temperature photoluminescence efficiency exceeding that of a QWL, and (iii) reduction of the multimonolayer (ML) interface fluctuations on the second-grown arm QWL surface, in old-generation T-shaped GaAs∕AlGaAs QWRs, to the single-ML level. The second part of this article is devoted to the discussion of optical properties of epitaxial QWRs, such as exciton dynamics, fine structure of exciton levels, and nonlinear effects, studied by means of high-spatial resolution spectroscopy, i.e., microphotoluminescence experiments. We will concentrate our discussions on V-shaped GaAs∕AlGaAs QWRs and put an emphasis on demonstrating how the interface quality influences wire’s optical properties. The properties of QWRs in the “zero-dimensional quantum box regime” and QWRs in the real 1D regime will be presented in separate sections. We will show that the realization of QWRs in the real 1D regime makes possible the investigation of intrinsic 1D effects by focusing on a single perfect 1D wire region using microscopic techniques. This has led to important results, for instance, (i) the demonstration of the square-root dependence of 1D exciton radiative recombination lifetimes down to a temperature as low as 10K (limited by the experimental setup) and (ii) the clear demonstration of the existence of Mott transition in a 1D exciton system which is a fundamental problem under long debate.

Epitaxial growth and optical properties of semiconductor quantum wires

https://iopscience.iop.org/article/10.1088/0268-1242/31/10/103001/pdf

A review of thermal processing in the subsecond range: semiconductors and beyond

GaAs/AlAs quantum wires (QWRs) were found to be naturally formed by a regularly corrugated AlAs/GaAs interface and a flat GaAs/AlAs interface in an AlAs/GaAs/AlAs quantum well with a well width of 3.3 nm grown on a (775)B GaAs substrate by molecular beam epitaxy. The lateral period and vertical amplitude of the AlAs/GaAs interface corrugation were 12 nm and 1.2 nm, respectively. The QWRs were formed side by side with an extremely high density of 8×105 QWRs/cm. A photoluminescence peak at λ=715 nm from the QWRs with a cross section of about 12×3 nm2 showed a polarization degree of (I∥ - I⊥) / (I∥ + I⊥) = 0.11 and a very small full width at half maximum of 15 meV at 4.2 K.

High-Density GaAs/AlAs Quantum Wires Grown on (775)B-Oriented GaAs Substrates by Molecular Beam Epitaxy

Intense pulsed light processing for photovoltaic manufacturing

We report the formation of GaAs quantum wires using giant step structure formed during molecular beam epitaxial growth of AlGaAs on vicinal (110) GaAs surfaces. Atomic force microscope observation indicates that the steps extend to over several µm and are coherently aligned. The growth of an AlGaAs/GaAs quantum well (QWL) on the giant step structure forms quantum wires (QWRs) along the step edges. Carrier confinement into the QWRs is caused by the increase of well width (well-width modulation) and the decrease of Al composition in the AlGaAs barriers (barrier-compositional modulation), which are confirmed by transmission electron microscope observation. Redshift and strong polarization parallel to the wire direction in the photoluminescence spectra support carrier confinement into the GaAs QWRs.

https://iopscience.iop.org/article/10.1143/JJAP.34.4411/pdf

Formation and Characterization of GaAs Quantum Wires at Giant Step Edges on Vicinal (110) GaAs Surfaces

https://dspace.jaist.ac.jp/dspace/bitstream/10119/8167/1/Thin_Solid_Films_517_3472_2009.pdf

Precursor Cat-CVD a-Si films for the formation of high-quality poly-Si films on glass substrates by flash lamp annealing

Polycrystalline silicon (poly-Si) films 4.5 µm thick, formed on glass substrates by flash lamp annealing (FLA) of precursor amorphous Si (a-Si) films, show remarkably long minority carrier lifetimes of approximately 100 µs after post-furnace annealing under N2 ambient. Even after crystallization by FLA, there remain a large number of H atoms, on the order of 1021/cm3, which probably effectively act to passivate dangling bonds in the poly-Si films. A minority carrier diffusion length of approximately 60 µm, estimated using the lifetime value, indicates high feasibility of realizing thin-film solar cells using this material.

/pdf/drastic-improvement-of-minority-carrier-lifetimes-observed-2t83co9lxi.pdf

Drastic Improvement of Minority Carrier Lifetimes Observed in Hydrogen-Passivated Flash-Lamp-Crystallized Polycrystalline Silicon Films

Flash lamp annealing (FLA), a rapid annealing technique with a millisecond-order duration, can form polycrystalline silicon (poly-Si) films of a few µm thickness on glass substrates by crystallizing precursor amorphous Si (a-Si) films without serious thermal damage to the substrates. We attempt to use several kinds of metal films for adhesion layers inserted between the Si films and the glass substrates to prevent Si film peeling during FLA. One of the requirements for the insertion metals is a melting point (Tmelt) higher than 1414 °C, to which the metal films could be heated during the crystallization induced by FLA. Of the metal films attempted, only Cr films can prevent Si film peeling from soda lime glass substrates, which have a much larger thermal expansion coefficient than quartz, indicating the necessity of sufficient adhesiveness to glass and Si as well as of a high Tmelt. Actual solar cell operation is demonstrated using a flash-lamp-crystallized poly-Si film as an absorber layer and a Cr film as a back electrode.

Kazuhiro Shiba

Papers

Formation and Characterization of GaAs Quantum Wires at Giant Step Edges on Vicinal (110) GaAs Surfaces

Precursor Cat-CVD a-Si films for the formation of high-quality poly-Si films on glass substrates by flash lamp annealing

Drastic Improvement of Minority Carrier Lifetimes Observed in Hydrogen-Passivated Flash-Lamp-Crystallized Polycrystalline Silicon Films

Selection of Material for the Back Electrodes of Thin-Film Solar Cells Using Polycrystalline Silicon Films Formed by Flash Lamp Annealing

Drastic suppression of the optical reflection of flash-lamp-crystallized poly-Si films with spontaneously formed periodic microstructures