Growth of Phosphazene Film Onto Undoped n-InP
Authors: Rakotoarimanana Patie Cendra , Fregnaux Mathieu , Etcheberry Arnaud , Goncalves Anne-Marie 
Published: July 2021

An anodic electrochemical treatment in liquid ammonia (-55°C, Patm) provides the formation of an ultrathin film onto p-InP (1017 atoms.cm-3) and n-InP (1018 atoms.cm-3). This overlayer exhibits a polyphosphazene like structure (H2N–P=NH) which presents a very specific spectral signature when analyzed by X-ray photoelectron spectroscopy (XPS[1]). Based on the atomic surface ratios measured, the film growth would require a nucleation step which would be followed by a phosphazene coalescence phenomenon in the two dimensions of the surface[2]. After this anodic treatment no evolution of the surface under air exposure is observed (> 1 year) [3]. This passivation layer, electrochemically obtained, appears promising for InP incorporation into (opto)electronic devices.
Despite the reproducibility of the results, questions still arise, such as the anodic charge which widely varies according to the electrochemical process used. An anodic charge around 7 mC.cm-2 is required by cyclic voltametric1 whereas only 0,5 mC.cm-2 is enough by galvanometry[4] and a charge higher than 3 mC.cm-2 is observed by potentiometry[5]. Some disparities are also observed on XPS atomic surface ratios of the phosphazene like structure. For instance, the N to P133eV atomic ratio is expected to be 2 (see formula above). However, the value obtained on some of the passivated samples is slightly higher.
In order to figure out these variations, the n-type is reinforced but with low doping carriers (1015atoms.cm-3) and under low photon flux. A very different behavior from n-InP (1018 atoms.cm-3) is observed by both cyclic voltametry and galvanometry. Under illumination, holes involved in the anodic process are directly proportional to the photon flux which explains the relevance of n type. n-InP (1015 atoms.cm-3) provides a photoanodic current at lower potential due to its larger space layer where an effective separation of photogenerated electron–hole pairs occurs. A larger range of potential is then available to explore the anodic treatment preventing holes by tunnel effect. By cyclic voltametry under low photon flux, onto n-InP (1015 atoms.cm-3) a constant photoanodic current is observed whereas an anodic wave was observed onto n-InP with higher doping level (1018 atoms.cm-3). As the cycles progress, the rising front of the photocurrent moves from -0.5 V to 1.2 V vs SRE while the intensity of the photocurrent remains constant.
In the same illumination conditions, the galvanometry onto n-InP (1015 atoms.cm-3) required a current magnified by 200 (».500 mC.cm-2) compared to n-InP (1018 atoms.cm-3).Onto undopped n-InP the current efficiency gives then a poor performance in comparison with n-InP at higher doping level.
Whatever the process used, the phosphazen film is detected by XPS. However, by galvanometry, the N to P133eV XPS atomic ratio is close to 3. A fundamental question is raised about the holes engaged during the anodic process by galvanometry.

[1] D. Aureau, M. Fregnaux, A. M. Gonçalves, A. Etcheberry. Surface and Interface Analysis (2018) 1-5.

[2] A.-M. Gonçalves C. Njel, D. Aureau, A. Etcheberry Thin Solid Films 538 (2013) 21–24

[3] A.-M. Goncalves, N. Mezailles, C. Mathieu, P. Le Floch, A. Etcheberry. ´Chem. Mat., 22, (2010) 3114-3120

[4] A.-M. Goncalves, C. Njel, D. Aureau, A. Etcheberry. Appl. Surf. Sci.391 (2017) 44–48

[5] C. Njel, A.-M. Goncalves, A. Etcheberry, Electrochim. Acta 139 (2014) 152–156