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Studying femtosecond-laser hyperdoping by controlling surface morphology
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Image of FIG. 1.
FIG. 1.

Fabrication procedures. (a) To measure the I–V properties: (Step 1) a layer of thermal oxide (thickness 150 nm) was grown and a layer of Al (thickness 1 μm) was deposited atop the Si wafer; square openings (100 × 100 μm2) were then etched into these layers. Hyperdoping is performed via irradiation with a single fs-laser pulse centered on the exposed area; the Al acts as a mirror, reflecting the laser pulse from the surrounding area. (Step 2) The Al is etched away; a portion of the SiO2 layer is also etched away, leaving an insulating layer appropriate for (Step 3) deposition of Al contacts in a geometry for probing the electronic properties of the junction. The microscope image on the right shows an overhead view of the device. (b) To measure the Hall effect: (Step 1) hyperdoping is performed on a SOI wafer; afterward (Step 2), the silicon device layer is etched away except for a 100 × 100 μm2 area at the center of the laser-irradiated areas; finally, (Step 3) Ti/Ni/Ag contacts (20/20/500 nm) were deposited in a van der Pauw geometry.16 An overhead scanning electron microscope image of sample prepared for Hall measurements is shown on the right.

Image of FIG. 2.
FIG. 2.

(a) SIMS measurements of fs-laser doped silicon at several different fluences. (b) The implanted sulfur dose plotted against fluence indicates thatsulfur is measurable above the background following irradiation with laser fluences just higher than the measured melting threshold of (2.1 ± 0.5) kJ m−2 (vertical lines). The shaded area indicates the detection limit of the SIMS measurement due to oxygen contamination.

Image of FIG. 3.
FIG. 3.

Cross-sectional bright field TEM images of laser-doped (8 kJ m−2) silicon reveal that (a) an amorphous layer (α–Si) resting atop the crystalline (c-Si) substrate directly after laser-irradiation and that (b) recrystallization occurs following a 30-min thermal anneal (975 K). The tungsten is deposited during FIB sample preparation. Microdiffraction confirms that single-pulse laser-doping produces (c) an amorphous surface resting atop (d) the monocrystalline substrate. Thermal annealing yields significant recrystallization of the amorphized volume.

Image of FIG. 4.
FIG. 4.

Current-voltage properties of the junction between the laser-doped region (8 kJ m−2) and the p-type substrate (polarity of measurement shown in Fig. 1(a)). Current-voltage properties are linear directly following laser-irradiation, but exhibit rectification following a thermal anneal, indicating activation of the sulfur donors, and/or healing of the junction defects.

Image of FIG. 5.
FIG. 5.

Chemical and structural characterization of laser-hyperdoped silicon. (a) Atomic force microscope image of the laser-irradiated surface. (b) Bright-field TEM image showing an amorphous silicon layer extending 66 ± 7 nm from the surface, with the crystalline substrate underneath. Inset shows selected area diffraction of the laser-melted region inset. (c) Depth profile of the sulfur concentration. The measured concentration exceeds the solid solubility limit of sulfur in crystalline silicon at depths no greater than 90 ± 14 nm. (d) High-magnification TEM image revealing the amorphous and crystalline interference patterns of the surface region and substrate, respectively.


Generic image for table
Table I.

Summary of Hall effect measurements shown alongside the sulfur dose (measured with SIMS). Where not marked, all quantities are ±1 in the last significant digit except fluence, which is subject to a 20% uncertainty.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Studying femtosecond-laser hyperdoping by controlling surface morphology