(Invited) Rare Earth Doped Layers Fabricated By Atomic Layer Deposition
Authors: Khomenkova Larysa , Jolivet Aline , Labbé Christophe , Portier Xavier , Cardin Julien , Gourbilleau Fabrice 
Published: May 2020

Many approaches, developments, structures and materials have been proposed these last years to offer innovative solutions allowing the decrease of the CO2 production and the energy consumption while increasing the efficiency of the properties which are aiming for. Over the last decade, among the approaches developed, a growth process has experienced an important boom related to the possibility it offers on the development of high quality layers. It concerns the Atomic Layer Deposition technique (ALD) that allows growing conformal layer on a substrate having high surface/volume ratio. Although such a technique is well developed and used for microelectronic and photovoltaic applications, the works on photonic devices are still at an early stage. Indeed, for the former, Al2O3, SiNx, HfO2 materials are currently deposited using ALD for microelectronic devices. Besides, the high quality alumina passivation layer is deposited on the Si solar cell bottom that allows the increase of the performance of the solar cell. However, for photonic devices, the crucial issue is to succeed in incorporating and managing the dopant concentration during the process.
ALD process is a, at least, two steps process requiring a precursor containing the element of the layer to grow and oxidation step. The most used sequence for Al2O3 growth is a trymethylaluminium (TMA) precursor pulse followed by an oxidation step using water pulse. For doping such layer, a suitable precursor allowing to grow a dopant atomic layer in a temperature range and under oxidizing conditions that are appropriate with the host matrix growth has to be found.
The objective of this paper is to present our recent investigations on the growth of RE-doped Al2O3 layer by ALD using different RE precursors. For growing such doped layer, four steps process is required corresponding to the TMA precursor/oxidation sequence followed by the RE-precursor/oxidation one. Fabrication conditions using either water, O2 plasma or ozone as oxidant agent have been investigated to find the ALD window leading to a suitable Growth Per Cycle (GPC) rate in a low deposition temperature range. After deposition, different annealing treatments have been applied to optimize the microstructural and the optical properties of the films considering their potential application, notably in photovoltaic domain. The most attention was paid to Tb- and Er-doped Al2O3 thin layers growth and to the optimization of the processing conditions to achieve required optical properties for the photon management in solar cell.
As an example, Er-doped Al2O3 layers have been produced varying the number of Er precursor cycles with respect to the TMA one in order to manage the Er concentration in the layer. Our thin films containing 4.0 to 13 at.% of Er in the host matrix were found to be homogeneous not only prior to the annealing treatment but also after an annealing at 650°C during 15 minutes in nitrogen, conditions that are suitable for solar cell processing. For the highest concentration, phase separation has been observed upon annealing temperature at high temperature. Photoluminescence and photoluminescence excitation experiments have been carried out to study the optical response of such a layer. Indeed, considering that 100nm-thick Al2O3 ALD layer is actually used at the bottom of the Si Cell, succeeding to incorporate optically active Er ions in such a layer and achieving an upconversion process to convert the IR non-absorbed photons by the cell would be a way to increase the Si cell efficiency without modifying a well-established industrial process.