Band Alignment of N- and P- Doped InP at Electrolyte and Ultra High Vacuum Junctions: Fundamental Correlation between the Open Circuit Potential Under Illumination and XPS Photopeak Energy

Abstract

Electrochemistry measurements have provided in-situ studies of surface and interfaces at the semiconductor / electrolyte junction. It has required monitoring the current/voltage responses at the semiconductor/ electrolyte interface associated with impedance measurements. In this work, energy band diagram of InP has been given by determining Fermi level (work function), conduction band (electron affinity) and valence band positions. They have been referred by a theoretical absolute scale with vacuum level as a reference point. The electrochemical potential of electrons is given by Nernst equation according to an electrochemical reference electrode. The solid state scale at 0 eV and the electrochemical scale have been offset from one another by a defined voltage according to the reference electrode. For example 0 eV corresponded to -4.5 V vs. NHE (Normal Hydrogen Electrode) or -4.46 V SCE (Sulfate Calomel Electrode). The contact between InP and the electrolyte acted as a liquid schottky junction providing the equalization of both Fermi levels giving rise to the electrolyte work function. In the dark at open circuit potential conditions (Voc), the equilibrium junction of InP/aqueous electrolyte made InP Fermi level in depletion region. For this band bending, the Boltzmann relation described the distribution of electrons in the space charge region and the electric field was determined by Gauss’ law. Poisson’s equation could be solved within that region to give the Mott–Schottky equation. For both -n and –p doped InP, flat band potentials and the doping levels were deduced from the resulting linear straight lines of the C-2(V) curves. The energy band diagram using –n and –p types InP was accurately deduced. Another interesting interfacial features are the open circuit potentials in dark and under illumination. They reflect on -n and -p type the electronic transfer processes associated to the junction building. Under illumination the variation of Voc (ΔVoc) between both types vs illumination intensity is related to the equilibrium evolution due to hole / electron distribution through the junction under light perturbation. Figure 1 shows the Voc separation from the dark (ΔVoc~ 130mV) and for the highest illumination condition (ΔVoc~ 550mV). Highest separation should be obtained by still increasing the light power. Note that in this case the electrolyte is diluted with sulfuric acid (0.5M), which maintains a perfect oxide free interface. This gives rise to a perfect reversibility of Voc between the dark and illuminated situations contrary to observations which have been already studied in neutral pH [1]. Similar access to the absolute energy diagram of -n and -p InP samples can be observed in Ultra High vacuum condition (UHV). As for liquid junction an absolute energy position towards the Fermi level of an X-ray Photoelectron Spectrocopy spectrometer (XPS) can be observed. Figure 2 shows the binding energy position variation of core levels for -n and -p InP samples. Again the surfaces are oxide free. The variation between -n and -p is around 0.7eV corresponding again to samples submitted to strong illumination due in this case to X-rays flux. Even if the junction has been under high vacuum, persuasive analogies have been observed between the variation of Fermi level at open circuit potential under illumination and their respective relative position by XPS. Specific band bending at interfaces UHV/SC and Liquid/SC are compared and discussed through the data obtained by interfacial electrochemistry and XPS considerations. [1] J. C. Meldeje, A.-M.Gonçalves, N. Simon, L. Ouattara, A. Etcheberry. J. of Electrochem. Soc. 165 (4) H3138-H3142 (2018) Figure 1