CVD and Epitaxy

The deposition of crystalline silicon is a central competence in all thin-film concepts that exploit this material as their photovoltaically active solar cell layer. Chemical vapor deposition (CVD) using chlorosilane as a silicon precursor has long been the focus of our work in this field. This gas is chemically decomposed in hydrogen at temperatures of up to 1300 °C and, when the process is properly controlled, forms a high quality layer of silicon. Skillful selection of process temperatures, gases and substrates enables silicon layers generated in this way to perform a variety of tasks.

Examples of these include fine crystalline encapsulation or seed layers, monocrystalline absorption layers with low doping concentrations and highly doped emitter coatings with a thickness of up to just a few hundred nanometers for both thin-film and wafer solar cells. A crystalline silicon template is required for such emitter coatings (e.g. a thin silicon film).

In future we will continue to work on silicon epitaxy methods for growing high quality layers on atypical substrates, e.g. growth on porous silicon and dielectric layer overgrowth.

In addition to silicon deposition, we also excel at dry etching silicon with HCl gas – CVE – and we research external impurity gettering with HCl gas.

The process underpinning deposition is well known from the field of microelectronics. However, radical adjustments are necessary when it is used in photovoltaics: instead of ultra-pure, defect-free layers, what are needed are throughput rates over 100 times higher and markedly lower costs, but greater numbers of layer defects will be tolerated in return. Our research centers directly on this process of adjustment. Here, we not only develop new deposition processes, e.g. for layers with variable doping concentrations, but also the high throughput systems needed.

Main areas of research focus:

  • Reflectors suitable for epitaxy
  • In-line epitaxy for p-type and n-type c-Si thin-film solar cells
  • Epitaxial silicon emitters (p-type and n-type)
  • Developing lab and high throughput reactors for silicon epitaxy

 

Labs and equipment:

  • Lab-model epitaxial reactors (RTCVD100/RTCVD160)
  • High throughput epitaxial reactors (ConCVD/ProConCVD)
  • Several wet chemistry labs for preparing substrates
  • Spreading resistance profiling (SRP) for determining doping profiles
  • Multicore simulation computer for FLUENT simulations