G.L. Olson

Kinetics of laser‐induced solid phase epitaxy in amorphous silicon films

The kinetics of laser and furnace‐induced solid phase epitaxial crystallization of As‐implanted amorphous layers (∼0.16 μm) on Si (100) substrates was studied using a time‐resolved optical reflectivity technique. Epitaxial rates from 10−10 to 0.2 cm/sec were measured over a temperature range from 750 to 1550 °K at two different As concentrations, ∼2×1019 cm−3 and ∼4×1019 cm−3. Temperatures achieved during cw‐laser heating were calculated using a three‐dimensional steady‐state thermal analysis. The crystal growth rates are accurately described by the Arrhenius equation v = v0e−Ea/kT , with Ea dependent on the dopant concentration; Ea = 2.62 and 2.52 eV for the lower and the higher concentrations, respectively. Additionally, the rate measurements at high temperatures imply that the melt temperature of amorphous Si is greater than 1550 °K.

Zone refining and enhancement of solid phase epitaxial growth rates in Au‐implanted amorphous Si

Epitaxial crystallization of Au‐implanted amorphous Si layers has been studied over the temperature range 515–735 °C. During crystallization, Au is zone refined into the remaining amorphous layer, resulting in an Au concentration that increases as the layer becomes thinner. The rate of solid phase epitaxy increases rapidly with Au concentration over the range from 0.15 to approximately 0.50 at. %. At higher concentrations the rate enhancement diminishes and above 0.70 at. % severe retardation of epitaxy is observed to occur.

Laser-Induced Solid Phase Crystallization in Amorphous Silicon Films

We review recent work on the kinetics of laser-induced solid phase epitaxial crystallization of silicon as determined from time-resolved reflectivity measurements. Specific topics which are addressed include: the intrinsic kinetics of solid phase epitaxy (SPE) in ion-implanted and UHV-deposited films; SPE rate enhancement by implanted dopant atoms and the effects of electrical compensation on the SPE rate; and the temperature dependence of SPE and competing processes in samples containing impurity atoms at concentrations exceeding the solid solubility limit. The high temperature kinetics results are compared with predictions from transition state theory and are discussed with respect to a proposed depression in the amorphous Si melting temperature.

Kinetics and Mechanisms of Solid Phase Epitaxy and Competitive Processes in Silicon

Recent progress in studies of temperature dependent kinetic competition during solid phase crystallization of silicon is reviewed. Specific areas which are emphasized include: the enhancement of solid phase epitaxial growth rates by impurity-induced changes in electronic properties at the crystal/amorphous interface, the influence of impurity diffusion and precipitation in amorphous silicon on the kinetics of epitaxial growth, the effects of impurities on the kinetic competition between solid phase epitaxy and random crystallization, and the kinetics of solid phase crystallization at very high temperatures in silicon.

Kinetic limitations of the Mg2Si system for reversible hydrogen storage

Despite the promising thermodynamics and storage capacities of many destabilized metal hydride hydrogen storage material systems, they are often kinetically limited from achieving practical and reversible behavior. Such is the case with the Mg2Si system. We investigated the kinetic mechanisms responsible for limiting the reversibility of the MgH2+Si system using thin films as a controlled research platform. We observed that the reaction MgH2 + 1/2Mg2Si + H2 is limited by the mass transport of Mg and Si into separate phases. Hydrogen readily diffuses through the Mg2Si material and nucleating MgH2 phase growth does not result in reaction completion. By depositing and characterizing multilayer films of Mg2Si and Mg with varying Mg2Si layer thicknesses, we conclude that the hydrogenation reaction consumes no more than 1 nm of Mg2Si, making this system impractical for reversible hydrogen storage.

CW Laser Annealing Of Ion-Implanted Silicon

Experimental measurements and theoretical calculations of recrystallization induced by single laser scans over amorphous silicon are reported and quantitatively compared. The details of laser-induced crystal regrowth of high dose ion implanted silicon studied over a wide range of experimental parameters are accurately predicted from three dimensional thermal transport considerations and solid phase epitaxial crystal growth. Best agreement between theory and experiment is obtained with the value of 2.24 eV for the activation energy of recrystallization.

ANNEALING OF IMPLANTED LAYERS IN COMPOUND SEMICONDUCTORS BY LOCALIZED BEAM HEATING TECHNIQUES

A comparative study of the transient annealing of Si- and Se-implanted GaAs has been performed using pulsed and cw lasers and pulsed electron beams. Both low (10 13 cm –2 ) and high (5×10 14 cm –2 ) implant fluences were studied. Activation of low fluence implants was achieved only in electron beam annealed samples co-implanted with Ga and As. Pulsed laser annealing yielded layers with low electron mobilities ( 2 V –1 s –1 ) and apparent dopant activation of 4 to 7%. Much higher mobilities (˜2000 cm 2 V –1 s –1 ) and lower activation were observed in cw laser annealed samples. Pulsed electron beam annealing produced the highest apparent dopant activation (20–35%) and intermediate mobilities (900–1000 cm 2 V –1 s –1 ). Some possible explanations for the differences observed between the various types of annealing are discussed.

CW LASER ANNEALING STUDIES OF DEPOSITED FILMS

CW laser annealing of thin (0.05, 0.1, 0.2, 0.5 μ m) silicon films deposited in UHV on silicon produces high quality epitaxial layers when the annealing is conducted in ultrahigh vacuum (UHV). However, incomplete crystallization occurs when cw laser annealing of these films is carried out in air, and a layer of polycrystalline silicon is formed at the surface. The depth of this disordered region increases monotonically with the thickness of the original deposited layer. Auger analysis of deposited films after air exposure shows the presence of significant amounts of oxygen. We postulate that the rate of solid phase epitaxy (SPE) is reduced sufficiently by oxygen to allow spontaneous nucleation of polycrystallites to compete with SPE and eventually totally inhibit planar epitaxial crystallization of the deposited films.

ELECTRICAL CHARACTERISTICS OF LASER-ANNEALED POLYSILICON RESISTORS FOR DEVICE APPLICATIONS

Laser annealing of polysilicon was studied and used to improve the control and reproducibility of resistors fabricated from ion-implanted polysilicon films (0.5 μ m). We found that, with particular laser annealing conditions, a ±10% variation in the doping concentration results in only a factor of two change in the resistivity, which is at least a factor of five improvement over thermally annealed polysilicon resistors. This improvement is attributed not only to increased polysilicon grain sizes but also to a significant reduction in the grain boundary trap density. Analysis of resistivity data in terms of the carrier trapping model shows that the grain boundary trap density is reduced from 8.5 × 10 12 cm –2 for thermally annealed resistors to 1.1 × 10 12 cm –2 for laser-annealed resistors.

Solid-phase-epitaxial growth of ion implanted silicon using CW laser and electron beam annealing

The nature of residual damage in As/sup +/, Sb/sup +/, and In/sup +/ implanted silicon after CW laser and e/sup -/ beam annealing has been studied using plan-view and cross-section electron microscopy. Lattice location of implanted atoms and their concentrations were determined by Rutherford backscattering and channeling techniques. Maximum substitutional concentrations achieved by furnace annealing in a temperature range of 500-600/sup 0/C have been previously reported and greatly exceeded the retrograde solubility limits for all dopants studied. Higher temperatures and SPE growth rates characteristic of electron or CW laser annealing did not lead to greater incorporation of dopant within the lattice and often resulted in dopant precipitation. Dopant segregation at the surface was sometimes observed at higher temperatures.