Nanostructured surfaces

Peculiarities of polished surface formation of PbTe single crystals at the chemical etching by (NH4)2Cr2O7 + HBr + ethylene glycol compositions.
Malanych G.P.
Semiconductor Chemistry Department, V.Ye. Lashkaryov Institute of Semiconductor Physics of the NAS of Ukraine. 
Prospect Nauki, 41, Kyiv-03028, Ukraine.
E-mail: galya.malanich@gmail.com

The presence of a deformation layer on the surface of PbTe-based thermoelectric elements reduces the thermoelectric efficiency of such modules and contributes to their degradation. Therefore, an important technological operation is the chemical removing of the disturbed near-surface layer. To prepare a surface of the required quality, chemical-mechanical polishing (CMP) in specially selected etchants is usually used. Bromine-based etchants are one of the most effective polishing solutions.
The main purpose of this work is to investigate the process of chemical treatment of the PbTe crystals surfaces by [(NH4)2Cr2O7 + HBr + ethylene glycol] / ethylene glycol solutions and to establish the surface state after chemical etching using atomic force microscopy.
The single-crystalline ingots of PbTe, grown by the Bridgman method, were cut into 5 × 7 × 1.5 mm size wafers using the string cutting. The cutting-induced surface deformation layer was partially removed by mechanical grinding with aqueous slurries of M10 to M5 and ASM 1/0 abrasive powders. The disturbed during cutting and grinding layer with the thickness of 100-150 μm was removed by CMP using etchant at a polishing rate of 50 μm·min–1. CMP was carried out on a glass polisher covered with lawn cloth at a continuous supply of etchant (2-3 ml·min–1) for 3 min and a pressure on the plates in 2-3 kPa. The dissolution rate of crystals was defined using the electronic indicator TESA DIGICO 400. Morphological study of the polished surface of PbTe was spent on scanning probe microscope NanoScope IIIa Dimension 3000TM (Digital Instruments, USA).
Some quantity of the viscous component (ethylene glycol) was added to the basic solution B [(NH4)2Cr2O7 + HBr + ethylene glycol] to diminish the polishing rate and to improve the surface properties. Using such dilution, it is possible to change the CMP rate within the interval from 1 to ~ 50 μm·min–1.
As a result of the chemical-mechanical polishing, the PbTe single crystal polished surface is formed with Ra < 5 nm.