Speaker
Description
The increasing world energy demand in combination with the dependency on limited fossil fuels results in a lot of stress on global climate and the geopolitical situation. Most energy scenarios emphasise the importance of innovations not only on the generating side e.g. by renewable energy, but also on the consumer side e.g. by more energy efficient lighting or personal electronics devices. Recently, there has been much interest in zinc sulfide (ZnS) due to its applications as a buffer layer in thin film photovoltaic (PV). Being a II-VI semiconductor with a wide band gap which can be doped n- and p-type, ZnS is also an interesting candidate for transparent conducting films (TCF)s which are needed for light emitting diodes (LEDs). Since both these targeted devices require uniform coatings with precise thickness control, Atomic Layer Deposition (ALD) is an ideal deposition technique. ALD is a self-limited deposition method that is characterized by alternating exposure of the growing film to chemical precursors, resulting in the sequential deposition of (sub)monolayers over the exposed sample surface. The main advantages of ALD are atomic level control over layer thickness, low deposition temperatures and conformal and pin-hole free coating. One of the earliest reported ALD processes was in fact ALD of ZnS from elemental Zn and S.
In this work we report on a plasma enhanced ALD process for ZnS. Thin films were deposited in a home-built pump-type ALD reactor by using diethylzinc (DEZ) in combination with H2S or H2S/Ar-plasma as reactants. The substrates were Si(100) wafers covered with 100 nm thermally grown SiO2 or glass slides. Argon diluted H2S-Plasma was used instead of a pure H2S-Plasma in order to minimize the exposure of the ALD reactor to the highly reactive S radicals. The plasma was generated remotely from the substrate by RF inductive coupling at 200 Watt. The substrate temperature was varied from 60°C to 300°C. Thin film growth rate was monitored in-situ by spectroscopic ellipsometry while the structural and optical properties were characterised ex-situ using X-ray diffraction (XRD), X-ray fluorescence (XRF), X-ray reflectivity (XRR), X-ray photoelectron spectroscopy (XPS) and UV/Vis spectroscopy.
Saturation was observed for both H2S and H2S/Ar-plasma pulse times longer than 3s proofing that it is a true ALD process. Comparing the ZnS film thickness as a function of ALD cycles, it can be seen that the Ar/H2S-plasma enhanced process nucleates slightly earlier than the thermal process. The growth per cycle (GPC) of the thermal ALD process decreases by more than 75% when increasing the deposition temperature from 60°C to 300°C. The GPC of the plasma enhanced ALD process is much less dependent on substrate temperature. This expansion of the ALD temperature window can lead to a better device integration or better matching of temperature windows for ALD of ternary compounds. Both thermal and plasma enhanced ALD leads to crystalline ZnS films with similar XRD patterns, indicating mainly the cubic zinc blende phase.
