Controlled Doping Methods for Radial p/n Junctions in Silicon

Abstract

P/n and n/p junctions with depths of 200 nm to several micrometers have been created in flat silicon substrates as well as on 3D microstructures by means of a variety of methods, including solid source dotation (SSD), low‐pressure chemical vapor deposition (LPCVD), atmospheric pressure chemical vapor deposition, and plasma‐enhanced chemical vapor deposition. Radial junctions in Si micropillars are inspected by optical and scanning electron micro­scopies, using a CrO3‐based staining solution, which enables visualization of the junction depth. When applying identical‐doping parameters to flat substrates, ball grooving, followed by staining and optical microscopy, yields similar junction depth values as high‐resolution scanning electron microscopy imaging on stained cross‐sections and secondary ion mass spectrometry depth profilometry. For the investigated 3D microstructures, doping based on SSD and LPCVD give uniform and conformal junctions. Junctions made with SSD‐boron doping and CVD‐phosphorus doping could be accurately predicted with a model based on Fick's diffusion law. 3D‐microstructured silicon pillar arrays show an increased efficiency for sunlight capturing. The functionality of micropillar arrays with radial junctions is evidenced by improved short‐circuit current densities and photovoltaic efficiencies compared with flat surfaces, for both n‐ and p‐type wafers (average pillar arrays efficiencies of 9.4% and 11%, respectively, compared with 8.3% and 6.4% for the flat samples).

Effects of Pillar Height and Junction Depth on the Performance of Radially Doped Silicon Pillar Arrays for Solar Energy Applications

“…[ 7 ] Junction depths were experimentally verifi ed by ball grooving and staining experiments on fi ve different fl at samples (140, 790, 900, 1170, and 1550 nm). Since the phosphorus surface concentration upon drive-in is relatively low, transport of phosphorus into silicon can be described by Fick's law of diffusion.…”

“…[ 7 ] In summary, p-type silicon {100} substrates (5-10 Ω cm, 525 µm thickness, single side polished, Cz-grown, Okmetic Finland) were covered with 100 nm silicon nitride (SiN x ). On the front side, after removal of SiN x , the substrate was patterned with arrays of hexagonally packed dots (4 µm diameter, 6 µm pitch, 0.5 × 0.5 cm 2 cell size on a specimen of 2 cm × 2 cm) by standard photolithography (Olin 907-17 photoresist).…”

“…Values of the junction depth were predicted by FEM (COMSOL Multiphysics 4.2) simulations and verifi ed with ball grooving and staining. [ 7 ] Atomic Layer Deposition : To reduce the refl ectivity of the micropillar arrays, a layer (42 nm) of aluminum oxide (Al 2 O 3 ) was conformally coated on several samples, using a thermal ALD process at 100 °C (Picosun P-300S reactor). Trimethyl-aluminum (TMA) and H 2 O were used as precursors and N 2 was the carrier gas, yielding a growth rate of 0.83 Å per cycle (after initial growth stabilization).…”

Electrochemical coating of micro-structured silicon for photoelectrochemical water splitting

“…Chakthranont et al used so-called black silicon which shows a very high light absorption and is composed of needle-shaped (tapered) nanowires, in their case ~3 μm length, ~200 nm average diameter and ~300 nm spacing. We assume that the microwires in our system do absorb more light than flat equivalents as proven by Elbersen et al [46,47] and Kelzenberg et al [25] but believe that mass transfer overcompensates this effect and leads to an overall underperformance of the microwire structures compared to flat samples. In contrast, the positive light trapping effect of microwires overcompensated the negative diffusion effect in the system reported by Chakthranont et al as these authors used smaller microwire dimensions leading to a significantly shorter ion diffusion path length.…”

“…[25,46] For instance, Elbersen et al have studied the effects of microwire length on photovoltaic activity of silicon microwires with radial p/n junctions and a spacing of 2 µm and found that all tested microwires with a length between 10 µm and 60 µm outperformed flat equivalents under perpendicularly oriented illumination. [47] However, Boettcher et al prepared similarly sized microwire arrays with a radial n + /p junction, deposited platinum on the tops and tested them for photocatalytic hydrogen evolution.…”