Effect of a Thin Film of Polypolyphosphazene on the pH Response of InP
Authors: Meledje Jean-claude , Gonçalves Anne-Marie , Simon Nathalie , Aureau Damien , Frégnaux Mathieu , Ouattara L , Etcheberry Arnaud 
Published: February 2020

Indium phosphide semiconductor (InP) had a direct band gap of 1.34 eV which was suitable for solar spectrum [1]. Moreover, the low surface recombination velocity [2] made InP a relevant candidate for solar-driven water decomposition. However InP suffered from (photo)corrosion due to dissolved oxygen which was still present [2],[3],[4]. At open circuit O2 behaved as a strong oxidized agent [1]. For n- type, the migration of photogenerated holes to the surface induced a competition between water oxidation reaction and self-oxidation of InP which led to passivated layer.

In this work, passivated layers on InP were monitored by using anodic treatments as well in aqueous electrolyte as in liquid ammonia (atmospheric pressure, -55°C). In aqueous media, at pH 9, the growth and formation of thin and well covering InPO4 -like anodic ﬁlms were evidenced [5]
,
[6]. In liquid ammonia the formation of a phosphazene film was shown on the surface [7],[8]. The perfect coverage of the surface by the phosphazene-like film was revealed by XPS analyses and photoluminescence [9]. The aim of this paper was to quantify the robustness of such passivated layers under photoanodic currents. By using atomic absorption spectroscopy (AAS) techniques, preliminary results showed that the dissolution level of InP was kept to very low or non-detectable level when the surface was recovered by the phosphazene film. The results are very encouraging since it proved that the phosphazene layer avoids InP dissolution that would authorize InP to behave as a photoanode.

In this work, we proceed in the same way with InPO4 -like anodic films but also with hybrids layers formed sequentially in both electrolytes [10]. For each case, stability of InP layers are quantified by AAS after anodic photo-electrochemical treatment.

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