Diffusion of chromium in zinc chalcogenides during hot isostatic pressing

Highlights

• The diffusion of chromium in ZnS and ZnSe under hot isostatic pressing at a temperature of 1000–1330 ° C has been studied.

• The chromium concentration and its distribution over the ZnS, ZnSe samples volume were determined by IR spectroscopy.

• The diffusion coefficients of Cr in ZnS and ZnSe at various temperatures and pressures were determined for the first time.

• It is shown that the temperature dependences of the diffusion coefficients are described by the Arrhenius equation.

Abstract

The diffusion of chromium impurities in zinc chalcogenides (ZnS, ZnSe) during hot isostatic pressing (HIP) in the temperature range T = 1000–1330 °C and pressures P = 100–190 MPa has been investigated. The chromium concentration and its distribution over the sample volume with a step from 5 to 50 μm was determined by FTIR spectroscopy in combination with an IR microscope. The diffusion coefficients of Cr in ZnS and ZnSe at various temperatures and pressures were determined. The diffusion coefficients of Cr at 1300 °C were (7.0 ± 0.3) × 10⁻⁸ cm²/s in ZnSe and (3.4 ± 0.4) × 10⁻⁸ cm²/s in ZnS. It is shown that the temperature dependences are described by the Arrhenius equation, and the isostatic pressure does not affect the value of the diffusion coefficients.

Introduction

The development of efficient media for solid-state tunable lasers is currently one of the most urgent tasks of laser materials science. Lasers on the basis of zinc chalcogenides (ZnSe, ZnS) doped with chromium are of great scientific and practical interest because of their relatively high power, compaction, wide tuning range as well as the ability to operate at room temperature [[1], [2], [3], [4], [5], [6], [7], [8], [9], [10]]. The demand in such radiation sources stems from the possibility of their use in various fields such as remote sensing and monitoring of the atmosphere, environmental pollution control, distance measurement, optical communication, medicine and military technologies [1,2,11].

The best results in terms of the lasing efficiency have been obtained for the Cr²⁺:ZnSe lasers [4], but there is a significant interest in lasers based on doped ZnS [6,[8], [9], [10]]. Compared to zinc selenide, the matrix from zinc sulfide has several potential benefits that may result in high lasing characteristics of Cr²⁺:ZnS-lasers. For example, thermo-optical characteristics of zinc sulfide, namely, the thermal expansion coefficient, thermo-optical constant and thermal conductivity should help to reduce the contribution of the matrix to the magnitude of the induced thermal lens. The ZnS matrix also has a higher mechanical strength and laser damage threshold [12].

The crystals doped with the ions of chromium Cr²⁺ either during the growth (melt and PVD - Physical Vapor Deposition technologies) [[13], [14], [15]] or by post-growth thermal diffusion in the pre-synthesized material can be used as active laser media based on zinc chalcogenides ZnSe and ZnS. Among the known methods for producing crystals of А₂В₆ compounds, doped with transition metals, the diffusion method is the most used because of the wide range of features to provide precision doping [1]. It is just by this method that the working media Cr²⁺:ZnSe, Cr²⁺:ZnS were created on the basis of which the lasers with the highest performance were fabricated [4,8]. Typically, such a doping process is performed by diffusion of the dopant ions during thermal treatment of ZnSe and ZnS using metallic chromium or chromium-containing substances [[6], [7], [8]]. The main problems with preparation of active media based on zinc chalcogenides while doping them with the ions of chromium Cr²⁺ during growth can be attributed to contamination of the material by impurities entering from the apparatus and the difficulty in achieving high concentration of dopant ions in the matrix. In addition, in the process of growth, a large-block crystal structure is formed, which degrades the mechanical properties of optical elements. Therefore, for doping zinc chalcogenides during post-growth thermal diffusion typically the materials are used fabricated by chemical deposition from the vapor phase (Chemical Vapor Deposition, CVD) - CVD-ZnSe and CVD-ZnS since they contain insignificant amounts of impurities, have fine crystalline structure and may have significant dimensions [11].

To control the characteristics of the laser media as well as to select the optimum doping conditions it is necessary to have information on the shape of the concentration profile in the bulk of optical elements making it possible to calculate the effective concentration and diffusion coefficient of the dopant. During diffusion doping, the obtaining of the required concentration profile of the doping ion requires considerable time of high temperature treatment due to the low values of the diffusion coefficient of doping ions in the matrix. In order to increase the diffusion rate, and to decrease the time of treatment some special techniques are used such as creation of induced defects in the material matrix by gamma-activation [16] and annealing in zinc vapors [17,18]. In recent years the hot isostatic pressing (HIP) [[18], [19], [20], [21], [22], [23], [24]] has successfully been used for diffusion doping of zinc chalcogenides allowing the carrying out of doping at higher temperatures (more than 1100°С) and pressure without critical degradation of the optical elements due to any sublimation process. The use of HIP-treatment not only makes it possible to increase the temperature of the diffusion process in comparison with annealing at atmospheric pressure but also reduces the number of defects in the form of pores and inclusions of the second phase scattering the radiation [25].

To determine the diffusion coefficients approaches are used based on mathematical processing of the concentration profiles of dopant in samples [[26], [27], [28], [29], [30], [31]]. As for the data on diffusion coefficients of Cr in ZnS and ZnSe at temperatures above 1100 °C they were not found in literature.

In this regard, the aim of this work was to study the effect of temperature and pressure on the diffusion coefficients of chromium in zinc selenide and zinc sulfide in the range of temperatures 1100–1300 °C and pressures of 100–190 MPa during HIP-treatment.