Propagation of Nanopores and Formation of Nanoporous Domains during Anodization of n-InP in KOH

Abstract

Anodization of highly doped (1018 cm-3) n-InP in 2 – 5 mol dm-3 KOH under potentiostatic or potentiodynamic conditions results in the formation of a nanoporous sub-surface region. Pores originate from surface pits and an individual, isolated porous domain is formed beneath each pit in the early stages of anodization. Each such domain is separated from the surface by a thin non-porous layer (typically ~40 nm) and is connected to the electrolyte by its pit. Pores emanate from these points along the <111>A crystallographic directions to form domains with the shape of a tetrahedron truncated symmetrically through its center by a plane parallel to the surface of the electrode. We propose a three-step model of electrochemical pore formation: (1) hole generation at pore tips, (2) hole diffusion and (3) electrochemical oxidation of the semiconductor to form etch products. Step 1 determines the overall etch rate. However, if the kinetics of Step 3 are slow relative to Step 2, then etching can occur at preferred crystallographic sites leading to pore propagation in preferential directions.

Preparation and novel photoluminescence properties of the self-supporting nanoporous InP thin films

“…2a. It is reported that pits can be prepared via defect-related electrochemical etching 14,17 . Due to the interface curvature effects and the high electric field…”

Process of Formation of Porous Layers in n-InP

“…The electrode area was typically 0.5 cm 2 . InP wafers with carrier concentrations from 3.410 18 to 6.710 18 cm -3 and etch pit densities of less than 500 cm -2 were ECS Transactions, 77 (4) 67-96 (2017) used. Anodization was carried out in aqueous KOH electrolytes of 5 mol dm -3 .…”

“…It is generally accepted that this limited hole supply is what causes the initiation and propagation of porous etching, with hole supply being enhanced (and hence, porous etching initiated) at defect sites at the surface [6]. The newly formed pore tips then act as sites for the continuous preferential supply of holes [7]. However, the variation in feature size, as well as the morphology observed, as experimental conditions are varied cannot be so readily explained.…”

“…However, at concentrations of 1 mol dm -3 or less, no porous layers were observed and between 1.8 mol dm -3 and 1.0 mol dm -3 a transitional behaviour that exhibits highly porous growth (without the presence of a near surface layer) was observed. Furthermore, we have developed a 'three-step' mechanism [18], based on our results for pore formation in n-InP in KOH, where the variations in pore morphology are due to the competition in kinetics between hole supply, carrier diffusion at the semiconductor surface and electrochemical reactions at the semiconductor-electrolyte interface.…”

Anodic Formation of Nanoporous Indium Phosphide in KOH Electrolytes: Effects of Temperature and Concentration

“…47 Due to the differing etch rates of crystal planes, the formation of tetrahedral etch pits (seen as dove-tailed and v-groove voids, respectively in (011) and (011) cross sections) is observed on the (100) surface of III-V semiconductors. 1,2,38,43,44 We have previously shown that anodization of an n-InP electrode in >2 mol dm −3 KOH results in the formation of a nanoporous InP layer 3,[48][49][50][51] of finite thickness. [52][53][54][55][56] Further investigation showed that the porous region is capped by a thin layer (20-40 nm, depending on the conditions) close to the surface that appears to be unmodified.…”