Growth, Characterization And Defects Analysis Of ZnTe Crystals

ZnTe is a â…¡-â…¥ compound semiconductor with excellent optical and electrical properties, which make it widely used for THz wave emission and detection, pure green light emitting diode（LED）, laser diode（LD）, and solar cell, etc. However, so far the growth technology for producing large volume high quality ZnTe single crystals is not mature. This thesis reports on the research of the technology of the growth of ZnTe from Te solution. The processing conditions for obtaining large volume single crystal are obtained. The crystalline quality and the optical and electrical properties of the as-grown ZnTe were also characterized,and the generation, morphology and distribution of the defects, such as Te-rich phase, were revealed.First, the specific crystal growth method and technological processes were designed.Temperature gradient solution growth（TGSG） method was used to grow ZnTe directionally in the traditional vertical Bridgman furnace with two-temperature zone, by using tellurium as the solvent to lower the growth temperature, where the molar ratio of the starting materials is Zn:Te=3:7. The melting temperature of the starting materials is about 1060 °C. The temperature was lowered gradually during the growth to maintain a proper degree of supersaturation, to make sure the growth proceeds continuously and stably. Self seeding, i.e. a geometric selection method was applied to obtain large volume single crystal. The crucible pulling rate V and temperature gradient G on the solid/liquid interface were adjusted to obtain a flat crystallization interface. Accelerated crucible rotation technique（ACRT） was used to promote the solute transfer. In and Cr were doped into ZnTe to increase the resistivity.When the crucible pulling rate V is 0.1⁰.5 mm/h, and temperature gradient G is 5¹0K·cm–1, the solute transfer effects is very bad without crucible rotation, resulting in Te inclusions of large size in the as-grown crystals. Monodirectinal rotation of the crucible leads to better solute transfer effect, but the solid/liquid（S/L） interface has still a large degree of concaveness, and the size of the obtained grains is small. Bidirectional rotation of the crucible,combining with larger G（10³0 K·cm–1）, results in much better solute transfer effects. At these conditions, ZnTe ingots of 30 mm in diameter and about 70 mm in length with good homogeneity were successfully obtained. The kinetic conditions during the growth are also very stable, since some of the grains formed in the tip of the ingot can grow continuously into the middle and tail the ingot. The cross-section area of <110> orientated wafers sliced from the ingot can be larger than 20×10 mm2. This growth method has also good impurity rejecteffect, some CrTe that was not incorporated into ZnTe matrix was pushed by S/L interface,and finally concentrated in the end of the ingot.The generation principles of some point and extended（line, face and volume） defects in ZnTe grown from Te solution were discussed. High density of dislocations were introduced by radial gradient, which furthermore promotes the formation of cellular structures, thus, lowers the yield of the ingot.A solution with 3:2:1 volume ratio of HF:H₂O₂:H₂O was found having very strong orientation selectivity, can precisely reveal many kinds of extended defects. It can produce smaller size pits（textures） on the perfect area, regarded as “standard pits”, which is very useful for the analysis of the types and the relative direction of the defect-related pits produced on the same oriented surface. It can also reveal the three-dimensional morphology of Te inclusions fast and accurately, since it readily dissolve Te-rich phase.The mechanisms of the formation, and the size, shape and distribution of Te inclusions were studied. According to their formation mechanisms, Te inclusions in ZnTe can be classified into primary inclusions and secondary inclusions. Primary inclusions formed on S/L interface, the break of the flat interface, faceted interface formed owing to strong growth anisotropy are responsible for the formation of primary inclusions. However, secondary inclusions formed after growth, attributed to the formation of Te precipitates. In addition,cracks owing to the stress, and the thermal migration of Te inclusions also account for the formation of secondary inclusion at anywhere inside ZnTe matrix. The size of Te inclusions will be enlarged owing to Ostwald ripening and the merging process resulted from thermal migration. Both the size and the density of Te inclusions increase gradually from the tip to the end of the ingot. The shape, size and distribution can reveal the thermal field distribution of the ingot to a certain degree. The shape of the polyhedron Te inclusions was found to be a truncated octahedron. The shape formation mechanism of polyhedron Te inclusions was explained from the structural and surface energy aspects, as well as by the growth rate dispersion（GRD） theory. The relative size of the facets of a polyhedron Te inclusion is found to be very sensitive to the local conditions of the matrix.The transmission and reflection spectra obtained by UV-Vis-NIR spectrometer, and the transmission spectra measured by fourier transform infrared spectrometer（FTIR） were used to study the band structures and quality of ZnTe, respectively. The UV-Vis-NIR transmission spectra were found very sensitive to the dopants, Cr- and In-doping results in a red-shift of the absorption edge for about 10²0 nm. However, the reflection spectra are not sensitive to the dopants. Mechanically polish will cause structural damages on the surface of ZnTe, whichleads to the reduction of the intensity and the dispersion tendency of the reflection peaks. The lower energy transitions are more sensitive to the structural damages. The intensity of the reflection peaks of the passivated surfaces were also lowered after passivation. However,conversely, the higher energy transitions were more sensitive to the passivation. FTIR was used to measure the transmittance of ZnTe in the range of 500⁴000 cm-1. Only In-doped ZnTe have the transmittance larger than 60%, whose transmission spectra are also very flat.However, Cr-doping results in a descending type transmission spectra with much lower transmittance. The resistivity of undoped ZnTe is about 102 Ω·cm, In-doping can increase the resistivity greatly to about 108 Ω·cm, however, Cr-doping only increases the resistivity up to1000 Ω·cm. Crystals with less defects, higher resistivity and homogeneity have higher transmittance, as well as higher degree of flatness and narrower shape distribution of the transmittance spectra.Polyhedron and skeleton ZnTe crystals were found in Te-solvent region at the tail of the ingot. Their formation was nourished by the residual solutes. The kinetic formation mechanisms of these crystals were emphasized on, where layer growth mechanism and Berg effect were found critical. Crystals with larger size are more easily to be affected by the kinetic conditions, which in turn leads to crystals with more complex shapes. It was also found that the morphology of ZnTe crystals is of great reference significance for the understanding of the formation of some defects, such as Te inclusions during the crystal growth.