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ZnO, CdS, CdSe and CdTe bulk crystals

№3 The department of photoelectric phenomena
B.M. Bulakh

The significance and the effectiveness of the development. During the last years, attention of many researchers was focused on ZnO due to its perspective application instead of GaN for development of the devices emitted in the ultraviolet-blue spectral region. In comparison with GaN, ZnO has such advantages as high exciton binding energy, high mechanical strength, thermo- and radiation stability, as well as relatively low cost and simplicity of the growing. The main disadvantage of ZnO is the absence of high quality p-type material. Therefore, development of reliable method of production of high quality ZnO crystals is the actual problem.

The main technical parameters. The technology for ZnO growth proposed below allows to produce relatively easy high quality p-type ZnO single cryatsals. Using this technology it is possible to grow perfect bulk crystals of CdS, CdSe, CdTe with the next parameters: the specific resistance in the range of 10⁴ – 10⁷ Ohm·cm for CdS, CdSe and CdTe, and 0,1-50 Ohm·cm for ZnO, the dislocation density of about 10² cm^-2. CdS and CdSe crystals have high photosensitivity (ΔR/R ~ 10⁶). Optical strength of ZnO crystals exceeds 10⁸ W/cm².
    The shape of CdS, CdSe and CdTe is plate- and needle-like. ZnO crystals are transparent, uncolored six-facet prism of 15 cm in length, without visible defects.

The composition gathered from grown crystals

Brief description of the development. The crystals are grown by gas-phase synthesis from initial elements. The scheme of the reactor is shown below. Quarts tube (1) of  50-mm diameter contains two quarts tubes (2) and (4) of 20-mm diameter each. The funnel-shaped opening of tubes (2) and (4) creates the zone of crystallization (3). The ceramic or quarts boat (6) with chalcogenide ZnX is inserted in the tube (2). Argon gas is passed through the tube (2) with flow of 40 sccm/min. Oxygen gas previously H2O moistened in barboter (5) is passed through the tube (4) with the same velocity. The reactor is maintained in furnace that allows to control independently the temperature of ZnX evaporation and temperature of crystallization zone (3). The temperature of crystallization zone (3) during the crystal growth is 1200 °С. The temperature of chalcogenide evaporation depends on initial composition of ZnX. ZnS, ZnSe and ZnTe are used as a chalcogenide ZnX.  The temperature of evaporation is 1200 °С for ZnS, 1100 °С for ZnSe and 1000 °С for ZnTe.

The schema of reactor for ZnO crystal grow.

The argon flow transfers the vapor of chalcogenide ZnX to reaction zone (3) where their oxidation by oxygen flow and water vapor occurs. ZnO crystals grow at the walls of crystallizator. The time of crystallization is 6 hours. Mentioned conditions allow to produce transparent uncolored ZnO crystals with perfect hexagonal prism-like shape with maximal length of 15-20 mm and diameter of 0.5 mm. In addition to needle-like crystals, small amount of plate-like crystals of 0.3 mm thickness is grown.

General view of ZnO crystals.

Area of application. The area of possible crystal application includes different optoelectronic devices (development of photodetectors and light emitting devices) and various scientific researches. In particular, the method of low-temperature purification and doping by different impurities of II-VI compounds was developed.