R. Stuck

Silicon solar cells realized by laser induced diffusion of vacuum‐deposited dopants

A technique for solar cell preparation based on vacuum deposition of thin films of dopants on silicon, followed by irradiation with a high‐energy (∼1.5 J/cm2) ruby laser pulse is described. Several dopants like phosphorus, antimony, bismuth, aluminium, gallium, and indium have been investigated. Electrical and Rutherford backscattering measurements indicate that the dopant is dissolved in the silicon and becomes electrically active, as a result of the irradiation. The P‐N junctions which are formed are shallow (depth <4000 A) but heavily doped, since after the laser treatment the solubility of the dopant is generally higher than after a thermal diffusion. Therefore good diode characteristics can be achieved by this simple method, and solar cells up to 14 % efficiency under AM1 (air mass 1) illumination have been realized using dopants like phosphorus, antimony, bismuth, aluminium, or gallium.

Phosphorus diffusion into silicon from a spin‐on source using rapid thermal processing

Diffusion of phosphorus into silicon from a doped spin‐on glass source using rapid thermal processing is described. The structural and electrical characteristics of the resulting shallow junctions including atomic and carrier concentration profiles, sheet resistance, as well as the effects on bulk carrier transport properties were studied and compared to those resulting from the use of conventional furnace heating. The results show that sheet resistance as low as 15 Ω/⧠ and surface carrier concentration higher than 1 × 1020 cm−3 are obtained in the annealed samples. Furthermore, a gettering effect is observed as the minority‐carrier diffusion length measured by the surface photovoltage technique is improved after processing.

Pulsed laser crystallization and doping for the fabrication of high-quality poly-Si TFTs

Abstract We review the various applications of pulsed lasers, working in the nanosecond regime, to prepare high-quality poly-Si TFTs. It is shown that the best device performances (field-effect mobilities in excess of 140 cm2/V·s) are achieved by pulsed excimer laser crystallization of unhydrogenated amorphous Si thin films. In addition, for source and drain formation, we demonstrate that the excimer laser induced diffusion of dopant from a solid source (spin-on phosphorus-doped silicate glass) is very attractive to achieve good electrical properties of the n-channel TFTs.

Formation of thin silicon oxide films by rapid thermal heating

Rapid thermal heating of silicon samples in a dry O2 ambient has been used to form thin SiO2 films. Compared to conventional furnace oxidation, an increased growth rate was observed which is linearly dependent on the square root of time. Activation energies of 1.99 and 2.26 eV for 〈111〉 and 〈100〉 orientation, respectively, have been determined in the range 1000–1200 °C.

Study of trapping in mercuric iodide by thermally stimulated current measurements

Thermally stimulated current measurements have been performed in red mercuric iodide crystals grown by a vapor‐phase technique. They reveal the presence of a hole trap at Ev+0.45 eV with a capture cross section of 10−15 cm2. Two additional centers were found when the crystals were cleaved rather than etched.

Rapid thermal annealing and titanium silicide formation

Titanium silcides have been formed on monocrystalline (111) silicon substrates by rapid thermal annealing (RTA) of Ti layers deposited on Si at 700–800 °C for 1 to 240 s. The phase composition is dependent on the annealing temperature and time: at 700° and 750 °C for short annealing, TiSi and TiSi2 are observed. At 800 °C and by increasing the exposure time at 700 ° and 750 °C, only TiSi2 is detected. The growth of the total silicide thickness is found to be faster for RTA than for conventional furnace annealing and governed by two different mechanisms depending on the phases formed: in the range 700–750 °C, and 750–800 °C, activation-energy values of 2.6 ± 0.2 and 1.5 ±0.2 eV are found, respectively.For a thin deposited Ti layer (< 100 nm), the whole Ti is finally transformed into TiSi2 with 20@ μω cm resistivity. For thicker Ti thicknesses, titanium oxide stops the reaction.

Laser‐induced diffusion by irradiation of silicon dipped into an organic solution of the dopant

A new method for preparation of p‐n junctions is presented. It consists in dipping the silicon to be doped into an organic liquid containing the dopant and irradiating its surface through the liquid with a high‐energy pulsed ruby laser. It is shown that by this treatment the dopant is driven into the crystal and that abrupt junctions with high doping levels can be realized. Using this simple technique, solar cells with AM1 efficiencies above 13% can be prepared.

Thermally stimulated current measurements on silicon junctions produced by implantation of low energy boron ions

Abstract Thermally stimulated current (TSC) measurements have been performed directly on junctions realized by implantation of low energy (15 keV) boron ions into N-type silicon after annealing treatments at different temperatures between 180 and 500°C. The TSC curves have been analysed by a new method based on the measurement of the mean time before re-emission of the trapped carriers and compared with that of Grossweiner. Both methods indicate that the dominant level is a hole trap located at >Ev + 0·25 eV. This defect anneals at 260°C and is believed to be correlated with the vacancy-vacancy association.

Laser‐beam annealing of heavily damaged implanted layers on silicon

The behavior during annealing of heavily doped silicon layers obtained by a high‐current‐density ion implantation, realized by discharge in BF3 atmosphere, is investigated. The annealing is performed by a laser pulse and the surface layers are studied by Rutherford backscattering, SIMS, and conductivity measurements. Comparisons with thermal annealing show the advantage of using laser pulses to restore the original crystallinity.

Evolution of implanted carbon in silicon upon pulsed excimer laser annealing

Formation of epitaxial Si1−yCy substitutional alloy layers on monocrystalline silicon surfaces with y≊1 at. % is reported. The preparation method was carbon ion implantation, followed by KrF excimer laser annealing. Results of Rutherford backscattering (RBS), secondary ion mass spectrometry (SIMS) and infrared absorption analyses are compared. The authors concluded that, up to ∼1 at. % carbon content, the dominant process is nonequilibrium trapping of carbon in substitutional lattice sites upon fast resolidification. Above this concentration the complex carbon redistribution processes are influenced by silicon carbide precipitation in the melt and segregation effects in the near‐surface region.