Use of an asymmetric pulse profile for higher crystalline volumes from excimer laser crystallization of amorphous silicon

Abstract

An excimer laser is used to crystallize amorphous silicon on glass to nanocrystalline silicon, yielding higher crystalline volumes than reported earlier, by modifying the laser pulse profile used for crystallization at a given energy density. An asymmetric, shorter pulse profile, as opposed to the conventional Gaussian profile retains the desirable gradual leading edge of the Gaussian pulse for controlled evolution of hydrogen, while increasing the peak energy. The resultant films show an increased surface roughness along with higher crystalline volumes, which may be beneficial for photovoltaics and electron field emission cold cathodes. © 2007 American Institute of Physics. ͓DOI: 10.1063/1.2731664͔ Excimer lasers have been utilized for the crystallization of hydrogenated amorphous silicon ͑a-Si: H͒ for electronic applications 1,2 with well established crystallization physics.3-7 Excimer lasers typically operate in the ultraviolet and hence photons are absorbed by the silicon thin films within a few nanometers of the surface. Melting and solidifying occur on a nanosecond time scale, often without affecting the underlying substrate. This technique enables the use of inexpensive substrates, such as glass, which are highly preferable for low cost, large-area electronic devices. Films with adequate thickness for light harvesting crystallize in the partial melting regime.2,3 The properties of crystallized silicon depend on a number of factors, with laser pulse shape being one of the most significant. Conventionally, a Gaussian profile along the long axis of the pulse has been reported to be best suited for controlled evolution of hydrogen during crystallization. 8,9 Our previous report shows typical energy densities for crystallization with the crystalline volume and surface roughness comparisons of the resultant films for different film thicknesses.2 Here, we report a significant reduction of crystallization laser energy density required to yield a given crystalline volume by modifying the Gaussian pulse profile, while retaining the controlled evolution of hydrogen from a-Si: H films.Chemical vapor deposited 300 and 500 nm thick a-Si: H on a 100 nm thick silicon nitride, capped Corning 1737 glass was used for the experiments. The films contain 10% atomic hydrogen, as specified by the deposition facility. A KrF Lambda Physik excimer laser ͑LPX 210i͒, operating at 248 nm with a 25 ns full width at half maximum pulse duration, was used for crystallization. The a-Si: H films on glass were crystallized in vacuum, at a base pressure of 5 ϫ 10 −4 mbar, by scanning along the laser pulse. The scanning speed for the experiment was 2.5 mm s −1 with pulse repetition rate maintained at 50 Hz. Experimental details are as similar to that used for crystallization of a-Si: H with the Gaussian pulse profile in Ref. 2, for comparison purposes, apart from the laser pulse profile and maximum energy density used. A series of incremental energy densities of up to 200 mJ cm −2 per pulse was used for crystallization, with 25 mJ cm −2 increm...

Electrical properties of pulsed UV laser irradiated amorphous carbon

“…12 Pulsed UV laser annealing has been used to crystallize hydrogenated amorphous silicon. [13][14][15] The merit of pulsed laser annealing ͑PLA͒ with UV photons is that the laser is absorbed close to the surface and the effect to the substrate is minimal compared to thermal annealing in a furnace.…”

“…PLA of deposited samples was performed with the same laser in a stainless steel vacuum chamber with a quartz window, mounted on an automated translation stage. An 8 ϫ 3 mm 2 asymmetric pulse 15 was scanned along the film at 2 mm/ s at 20 Hz repetition rate. Energy densities of 30, 60, 90, and 120 mJ/ cm 2 were used at a base pressure of 3 ϫ 10 −4 Torr.…”

Preparation, structure, and properties of montmorillonite/cellulose II/natural rubber nanocomposites

“…In solids, this change in frequency (Raman shift) is essentially due to interactions between the individual photons and Raman active phonons (quantized lattice vibrations). By studying the Raman shift and the shape of the Raman spectrum it is possible to determine the degree of crystallinity24 achieved in a laser annealed sample. Scanning electron microscopy (SEM) analysis of surface topology was carried out using a Philips XL30 ESEM with a field emission electron gun.…”

Reversible increase of photocurrents in excimer laser-crystallized silicon solar cells

“…[6] The samples were crystallized with 40, 80, 120 and 160 mJcm -2 laser energy densities. 60 nm thick aluminium contacts were evaporated on to the silicon surface after a buffered hydrofluoric acid dip for 20 seconds to remove native oxide layer, resulting in devices with a 5-8 mm 2 area.…”