Technology and physicochemical properties of thin films and wires based on lead telluride for thermoelectric energy converters

Code: 10.820.05.09.UF
Title: Technology and physicochemical properties of thin films and wires based on lead telluride for thermoelectric energy converters
Programme: Bilateral project Moldova - Ukraine
Execution period: 2010-2011
Institutions: Ghitu Institute of Electronic Engineering and Nanotechnologies
Project Leader: Meglei Dragosh, dr., associated professor (docent)
Participants: Laboratory of Electronics of Low Dimensional Structures



A Process for Growing Single Crystals of Lead Telluride Doped with Impurities

The simplest and most common method of growing lead telluride single crystals is the Bridgman method. The setup for growing single crystals via this method consists of an ordinary furnace with three resistive heating zones. The upper zone is at a temperature of 20-30°C above the solidus of the phase diagram of the PbTe alloy, which ensures a complete melting of the components and provides favorable conditions for the homogenization of the melt; the middle zone serves to maintain a guaranteed crystallization front and to control it.

Single crystals are grown in a purified and graphitized quartz tube to prevent the interaction of the molten material with the glass during the solidification of the material.

For the synthesis and growth of single crystals, we used pure parent materials (Pb C0000 and Te purified via zone recrystallization) with a total weight of 30-50 g. For the growth, we used tubes with a length of 100-140 mm and a diameter of 13-18 mm. Using this method, we obtained single crystals with a diameter up to 18 mm.

A Process for the Preparation of Doped Lead Telluride Microwires

Experiments on the preparation of PbTe microwires were performed using liquid phase casting. In comparison with the known method for the preparation of metallic microwires, the use of a semiconductor material causes a number of technological features. First, the resistance of semiconductor materials is much higher than that of metals. To melt the material by a high-frequency field, it must be preheated via a thermal method. It is necessary to prevent the penetration of undesirable impurities into the semiconductor material from the glass and the environment. This is a major problem, because the physical properties of a semiconductor material dramatically change after doping with respect to metallic microwires.

The quality of the resulting microwires was determined visually under a NEOFOT-30 microscope and during the study of the microstructure of longitudinal and transverse sections. The results showed that, along the microwire, there are many cracks, which are caused by high rates of crystallization of the microwire and different coefficients of linear expansion of the PbTe material and the glass envelope. High rates of crystallization lead to high degrees of supercooling in the melt and, as a consequence, to the formation of spontaneous crystallization centers, which also lead to the appearance of microcracks. These microcracks can be removed by the zone recrystallization of the microwire.

PbTe microwires with the highest quality can be obtained via filling quartz capillaries in a vacuum under an inert gas pressure; the method is as follows: an evacuated quartz tube that contains a required amount of the semiconductor material, with dead-end quartz capillaries located over it, is placed in a high-temperature furnace so that the capillaries and the semiconductor material are in the working zone of the furnace. After the melting of the material and the immersion of the capillaries into the melt, an excess pressure P = (2 – 12) x 102 torr is formed in the tube owing to the inert gas; as a consequence, the capillaries are filled with the material. After that, an oriented recrystallization of the melt inside the capillaries is carried out.

High-quality PbTe microwires were obtained under the following conditions: a growth rate of 1-10 cm/h, a temperature gradient of 10-15 deg/cm, a maximum temperature of the melt maintained at a level of 10-15 deg higher than the solidification temperature, and an excess gas pressure of (6.8 - 49) x 104 Pa.