Si/Si1-xGex/Si Heterojunction PIN Nanowires Fabricated by Using an Aqueous Electroless Etching Method

Background

Si1−xGex nanowires offer higher carrier mobilities than Si and an optical detection range extending into the infrared. Earlier fabrication routes, including vapor-liquid-solid growth, solution-phase synthesis and lithography-based etching, generally require high temperature or high vacuum, templates and dedicated equipment, and the resulting nanowires can suffer from poor orientation, uncontrollable doping concentration and poor alignment.

Design and approach

In AEE, silver nanoparticles formed by galvanic displacement in an HF/AgNO3 solution catalyze local oxidation of the semiconductor, and HF removes the oxide; repetition of this sequence etches vertical nanowires into the wafer. Because the nanowires are etched from a wafer whose doping is set beforehand and no further dopant diffusion step is applied, the original doping profile is retained. The starting stack consisted of a 300 nm graded Si1−xGex layer (five 60 nm layers with x = 11.8, 16.8, 22.2, 16.8 and 11.8%) grown by ultra-high-vacuum chemical vapor deposition at 550 °C on an n+ Si (100) wafer, capped with a 100 nm boron-doped p-type poly-Si layer (3 ± 0.3 × 10¹⁸ atoms/cm³) grown by thermal CVD at 600 °C. Nanowires were etched in 4.9 M HF with 0.03 M AgNO3 at 60 °C for 10 min, the silver was removed in HNO3, and 150 nm indium-tin-oxide (ITO) top electrodes were sputtered after the nanowires were held in photoresist.

How it was evaluated

Nanowire morphology was examined by field-emission scanning electron microscopy (FE-SEM), and the retention of the Si1−xGex layers was checked by X-ray diffraction (XRD) of the bulk and nanowire structures. Current-voltage characteristics of the nanowire and bulk photodiodes were measured with a source-measure unit in the dark and under fluorescent room light (50 W).

Key results

The nanowires were vertically aligned over a large area with a uniform length of about 5 μm and diameters of 100 to 150 nm, and they were bundled at their tips by capillary forces during drying. Both the bulk and the nanowire structures showed three Si1−xGex (400) peaks at 2θ = 68.48°, 68.68° and 68.94° (x = 22.2, 16.8 and 11.8%), corresponding to out-of-plane lattice constants of 5.476, 5.462 and 5.444 Å, close to the values of 5.48, 5.46 and 5.45 Å obtained from Vegard's rule for the same compositions. The nanowire peaks were weaker and broader (full width at half maximum 0.28°, 0.36° and 0.38°, against 0.2°, 0.28° and 0.32° for the films), which the authors attribute to the gathering of the nanowires at their tips.

Both device types were rectifying. The bulk photodiode showed an on/off ratio of 1 × 10³ and a forward current of 1.0 × 10⁻² A. Under reverse bias, the dark current was 9.2 × 10⁻⁶ A for the nanowire photodiodes and 1.0 × 10⁻² A for the bulk photodiodes. The abstract describes this as a three-order-of-magnitude decrease, whereas the conclusion states four orders of magnitude; the authors attribute the improvement to the larger surface area of the nanowire structure.

For the nanowire photodiodes, the abstract reports a four-order-of-magnitude difference between the dark and light off-currents under reverse bias. For the bulk photodiodes, the abstract states that the two current levels differed by less than one order of magnitude, while the Results section describes the difference as decreasing from two orders of magnitude to approximately a few hundred as the reverse bias increased.

Open questions

The photoresponse was measured under fluorescent room light; the authors propose application of the nanowire photodiodes to infrared detection at a wavelength of 1.55 μm, which was not tested in this paper.

How it connects to other research

The related study by Lieber and co-workers built p-type/intrinsic/n-type (p-i-n) junctions radially in coaxial silicon nanowires and used them as solar cells; here the PIN stack is defined vertically in a wafer before top-down etching, and the nanowires are evaluated as photodiodes rather than power sources.