Wim Laureys

Nanoscaled interdigitated electrode arrays for biochemical sensors

Nanoscaled interdigitated electrode arrays were made with Deep U.V. lithography. Electrode widths and spacings from 500 nm down to 250 nm were achieved on large active areas (0.5 mm/spl times/1 mm). These electrodes allow for the detection of affinity binding of biomolecular structures (e.g. antigens, DNA) by impedimetric measurements. Such a sensor is developed, theoretically analyzed, experimentally characterized, and is demonstrated as an affinity biosensor.

Fabrication of large area silicon solar cells by rapid thermal processing

Large area n+pp+ solar cells have been fabricated on 10 cm×10 cm pseudo‐quasi‐square CZ silicon wafers (1 Ω cm, p‐type) predominantly used by the photovoltaic (PV) industry. All the high‐temperature steps have been performed by rapid thermal processing (RTP). Emitter formation, back surface field (BSF) formation, and surface oxidation have been performed in just two RTP steps each lasting 50 s. Solar cells of 15% efficiency have been fabricated this way, demonstrating the applicability of this low thermal budget technology to large area, modulable size, industrial quality Si wafers. Furthermore, the rapid thermal oxidation (RTO) is shown to result in good quality thin oxides with Si/SiO2 interface trap densities (Dit)<1011 cm−3 eV−1 near‐midgap.

Selective emitters in Si by single step rapid thermal diffusion for photovoltaic devices

Selective phosphorous diffusion is performed in Si to simultaneously form shallow n/sup +/p junctions of different depths in the submicron range by rapid thermal annealing (RTA). Low temperature (400/spl deg/C) atmospheric pressure chemical vapor deposited (APCVD) phosphosilicate glass (PSG) is used as diffusion source. A wide range of n/sup +/p junctions could be tailored with the same thermal budget by changing only the APCVD-PSG composition. This allows the formation of selectively diffused emitters in different regions of the wafer in one RTA step. 10 cm/spl times/10 cm Cz-Si selective emitter photovoltaic (PV) devices are fabricated this way with high energy conversion efficiencies in the range of 17% to 18%.

Improving low-temperature APCVD SiO 2 passivation by rapid thermal annealing for Si devices

The quality of low-temperature (/spl ap/400/spl deg/C) atmospheric pressure chemical vapor deposited (APCVD) silicon dioxide (SiO/sub 2/) films has been improved by a short time rapid thermal annealing (RTA) step. The RTA step followed by a low temperature (400/spl deg/C) forming gas anneal (FGA) results in a well-passivated Si-SiO/sub 2/ interface, comparable to thermally grown conventional oxides. Efficient and stable surface passivation is obtained by this technique on virgin silicon as well as on photovoltaic devices with diffused (n/sup +/p) emitter surface while maintaining a very low thermal budget. Device parameters are improved by this APCVD/RTA/FGA passivation process.

Rapid thermal annealing of spin-coated phosphoric acid films for shallow junction formation

Rapid thermal annealing (RTA) of spin-coated phosphoric acid (H3PO4) films on silicon substrates has been studied for the formation of shallow junctions. The junctions are characterized by spreading resistance profiling. Device quality, shallow ( 750 °C), below which absorption bands originating from water species are noted. More than 15% efficient, shallow emitter, large-area (10 cm×10 cm) n+pp+ silicon solar cells are fabricated with a short-time processing using this rapid thermal processing technique.

Interaction between bulk and surface passivation mechanisms in thin film solar cells on defected silicon substrates

Common features of thin film silicon (TFSi-) solar cells hydrogenated in a microwave induced remote plasma (MIRP) are studied. The thin films were epitaxially grown on ribbons and multicrystalline silicon (mc-Si). Optimal hydrogenation renditions are presented in relation to the sample conditions such as the surface passivation, the active carrier concentration in the epitaxial layer and the substrate material. The MIRP-hydrogenation is studied in the temperature range of 350-415/spl deg/C. A high passivation efficiency is observed at 400/spl deg/C for all substrates in spite of an unfavourable hydrogenation regime at 375/spl deg/C. In the absence of an oxide during the hydrogenation, an optimal hydrogenation time of 1 hour is observed irrespective of the substrate used. This optimal hydrogenation time is a consequence of the interaction between the incoming atomic hydrogen and the boron dopant in the epitaxial layer.

Rapid thermal processing of conventionally and electromagnetically cast 100 cm/sup 2/ multicrystalline silicon

100 cm/sup 2/ n/sup +/pp/sup +/ solar cells have been fabricated by rapid thermal processing (RTP) on conventionally cast (CC) and electromagnetically cast (EMC) multicrystalline silicon (mc-Si). All thermal steps were carried out by fast-ramp (>30/spl deg/C/s) RTP using tungsten-halogen lamps. Emitter and BSF were simultaneously formed by RTP co-diffusion of phosphorous and boron/or aluminum (50-60 seconds) and surface passivation by rapid thermal oxidation, RTO (40-50 seconds). The all-RTP process resulted in 14.1% and 13.3% efficient cells on CC and EMC mc-Si respectively. The EMC cells, when subjected to an additional plasma hydrogen treatment, improved to give the same efficiency as the CC mc-Si cells. Systematic lifetime measurements performed on these materials show that the degradation in the EMC mc-Si is mainly due to the activated crystallographic defects, responding favourably to hydrogenation treatments but poorly to RT-gettering treatments. On the other hand, significant gettering effects are observed in CC mc-Si.

Solar cell preparation in thin silicon membranes

Solar cells prepared in a thin (/spl ap/30 /spl mu/m) crystalline silicon membrane with a supporting frame allow an evaluation of the potential of c-Si thin film cells on cheap substrates. At the same time, lightweight and more radiation-hard solar cells may have direct applications in space. This paper studies the fabrication process of solar cells in /spl ap/30 /spl mu/m thick p-Si epitaxial layers, incorporating a p/sup ++/-Si etch-stop/back-surface field layer, using KOH etching. Wax, rubber and silicon nitride were tested as masking materials during the etching. It was found that both wax and silicon nitride could be used as materials for the masking of supporting frames for solar cell thinning up to 30 /spl mu/m. However, silicon nitride does not reliably protect the frontside structure.

Comparison of bulk and surface passivation properties of plasma nitrides on Si and SiGe solar cells

In this paper the effects of direct and remote plasma nitrides on the performance of Cz-Si and SiGe bulk cells are analysed. The surface passivation properties of both kinds of nitrides are compared to a thin thermal oxide, grown at high temperatures. Internal quantum efficiency measurements prove that the surface recombination velocities are lowest in case of the remote plasma nitride layer. The extracted values of the surface recombination velocity are as low as 1.5/spl times/10/sup 3/ cm/s for the remote plasma nitride, whereas the value for the thermal oxide is twice as high, comparable to the value obtained for the direct plasma nitride. SiGe-cells show the same tendency, although the blue response is lower in absolute value. However, when using the appropriate SiGe-absorption coefficient for parameter extraction by PC-ID, the surface recombination velocity of the plasma nitrides on SiGe-emitters is comparable to what is found in case of Si. In addition, we also found that for both remote plasma and direct nitride layers, there is a significant enhancement of the red response of all types of cells, compared to the samples without nitride. A comparison was made of the behaviour of the cells, with starting lifetime of 10-30 /spl mu/s, showing that the enhancement of the red response by the use of a plasma nitride is comparable to the beneficial effects of a remote plasma hydrogenation. However, for starting lifetimes lower than 5 /spl mu/s, the separate hydrogenation step brings an additional improvement of the red response compared to the only-nitride case and the effect of the two treatments seems to be cumulative.