Vapour Pressure Controlled Czochralski Method Research Articles

Contributions to the development of crystal growth technologies

An overview of selected contributions to the development of crystal growth technology of the Laudise Prize awardee 2023 is presented. First some results on shaped crystal growth are given, such as layers with eutectic periodic structures, casting of high-resistivity CdTe sheets, growth of in situ core doped laser rods by double die EFG, and Czochralski growth of Si crystals with rectangular cross section. Then the correlation between melt structure and quality of II-VI crystals showing high bond ionicity is discussed. The importance of marked melt overheating for growth of high-quality CdTe and (Cd,Zn)Te crystals is derived. Also the in situ stoichiometry control for reducing intrinsic point defects, precipitates and inclusions is demonstrated by the examples of Bridgman and VGF CdTe growth with Cd extra source and vapor pressure controlled Czochralski method of 6 inch GaAs crystals. Next, the reason and general appearance of dislocation cell patterns in all growing thermomechanical stressed crystals and their possible prevention are explained. The contribution of point defect diffusion for dislocation climb at high temperatures to form globular cells is underlined. Finally, melt growth experiments under travelling magnetic field (TMF) generated in coil-shaped heater-magnet modules around the crucibles are discussed. The importance of global numeric modeling for optimization the TMF convection mode in the melt is accentuated. It is shown that the growing interface can be levelled, the solution flux at vertical liquid phase epitaxy can be homogenized, and the IR transmission and minority carrier live time in 640 kg mc-Si ingots for photovoltaics can be markedly enhanced and homogenize by such external field application. The close cooperation between academics, engineers and industry is particularly emphasized.

Growth and characterization of GaAs crystals produced by the VCz method without boric oxide encapsulation

Unintentionally doped GaAs crystals grown from Ga-rich melts without B 2O 3 encapsulation by the modified vapour pressure controlled Czochralski (VCz) method are analysed. The influence of this growth technique on dislocation density and distribution is presented. The concentration of As precipitates in dependence on the melt composition is shown. Finally, the contents of residual impurities and their influence on some characteristic electrical parameters are discussed. The attention is focused on the feasibility of in situ controlled near-stoichiometric semi-insulating (SI) GaAs crystals with minimized content of As precipitates.

Growth of GaAs crystals from Ga-rich melts by the VCz method without liquid encapsulation

The growth experiments and properties of undoped GaAs crystals grown by the vapour pressure controlled Czochralski (VCz) method without boric oxide encapsulant from Ga-rich melts are presented. Morphological and structural features were investigated by sensitive etching procedures. The precipitate content was analysed by laser scattering tomography. The mean scattering intensity of As precipitates decreases with increasing Ga excess in the melt. An enhanced probability of Ga-rich inclusion incorporation has been observed. In 3-in crystals, the mean etch pit density was decreased down to 5×10 3 cm −2 with improved radial homogeneity. The etch pits did not show cell patterning. Markedly reduced contents of boron and hydrogen have been detected. No oxygen defects and related complexes independently on the carbon concentration were ascertained by local vibrational mode. For the first time, the carbon concentration in the grown crystals was reduced by carefully controlling the CO content of the growth atmosphere in a VCz arrangement without boric oxide encapsulant. Such experiments without B 2O 3 encapsulant are of fundamental character and offer numerous information about the point defect situation in as-grown crystals.

Non-stoichiometric growth of GaAs by the vapour pressure controlled Czochralski (VCz) method without B 2O 3 encapsulation

Results of the growth of semi-insulating GaAs crystals by the VCz method without B 2O 3 encapsulant are presented. Crystals have been grown from Ga-rich and near-stoichiometric melts controlled by As partial pressure in the range from 590°C to 630°C. The CO content in the gas atmosphere has been controlled. We investigated the influence of melt composition, reduced boron incorporation and adjusted carbon concentration. Native and impurity defect concentrations are compared with those of conventional VCz samples.

Growth of semi-insulating GaAs crystals in low temperature gradients by using the Vapour Pressure Controlled Czochralski Method (VCz)

The present paper gives a review on fundamentals, modelling, growth, structural and electrical properties of semi-insulating GaAs single crystals, grown in low temperature gradients by the Vapour Pressure Controlled Czochralski Method (VCz), with diameters from 75 up to 150 mm. Special attention is drawn to the investigation of the temperature-fields inside the growing crystals (and thus thermoelastic stress). Additionally, the influence of convective transport of heat within meit and inert gas is investigated by both experiment and modelling. Thermodynamic aspects of arsenic pressure control within the inner VCz chamber as well as the special experimental and technological challenges are discussed. High quality 100 mm (4-inch) crystals with EPD < 10 4 cm −2 and low as-grown residual strain are presented. Very low carbon concentrations of ≈10 14 cm −3 were obtained for the first time in VCz crystals. This material, as one of the challengers to conventional LEC material, is able to meet similar electrical specifications whilst showing improved structural quality and better parameter homogeneity even in the as-grown state. Initial studies of a VCz crystal grown without boric oxide encapsulant is presented.

Temperature field simulation and correlation to the structural quality of semi-insulating GaAs crystals grown by the vapour pressure controlled Czochralski method (VCz)

Recent results of global thermal field modelling using the software programs CrysVUN++ and STHAMAS are presented for 3- and 4-inch VCz SI GaAs growth assemblies. For the first time global simulations including the gas and melt convection in a VCz arrangement are shown. In contrast to conventional LEC the impact of gas convection on the temperature distribution in the growing crystal is not dominant in the VCz case. Contrary to that melt convection cannot be neglected and needs to be controlled by crucible and crystal rotation rates. Simulations were used to optimise the temperature fields in the VCz arrangements and thus, to reduce the von Mises stress within the growing crystal. Doing this, the EPD could be reduced by approximately one order of magnitude compared to conventional LEC. Minimum EPD's were found to be in the range of some 10 3 cm −2 for 4″ crystals. The observed nearly flat (slightly convex) interfaces are in agreement with the simulations. The achieved good radial uniformity of the carbon incorporation and electrical parameters is due to the improved thermal boundary conditions.

Digital Etching of InP by Intermittent Injection of Phosphorous Precursors in Ultra-high Vacuum

Intermittent injections of phosphorous precursors, i.e. tris-dimethylaminophosphorus (TDMAP) and tertiarybutylphosphine (TBP), have been applied for the selective etching of InP (001) substrates in order to study surface adsorption/decomposition mechanism of phosphorous precursors on the InP surface in ultra-high vacuum. Digital etching, in which the etching depth is controlled by the number of injection cycles of etching gases and the etch rate is independent of the injection time and pressure of the etching gas in a self-limiting fashion, is achieved for both precursors. The etch rate is well described by a modified Langmuir-type equation. The TBP molecules stick seven times faster than the TDMAP molecules on the InP surface and both precursors prevent the decomposition of InP resulting from the phosphorus loss during an evacuation time of over 5 s. On S-doped substrate grown by a vapor pressure controlled Czochralski method, the activation energy of the digital etching with TBP significantly decreases to 7 kcal/mol. Therefore, excellent uniformity and controllability of the etch rate is expected. A specular etched surface is achieved and well controlled digital etching is realized at an etching temperature as low as 304°C.

Bulk growth of GaAs An overview

The III–V compound GaAs is of rising importance for opto- and microelectronics, especially, for LEDs and LDs and for high-frequency devices like HBTs, HEMTs and MMICs, respectively. The device performance and degradation mechanisms are critically influenced by bulk properties. Hence, considerable efforts are aimed at a reduction of crystal imperfections and improvement of the uniformity of physical properties connected with the need to increase the crystal diameter. This goal is not easy to attain because of the well-known proportionality between crystal diameter and dislocation density. The efforts focused on a reduction of the dislocation density (below 10 4 cm −3 at least) by reducing the non-linearities of the thermal field in LEC growth have led to the development of fully encapsulated and vapour pressure controlled Czochralski method (FEC and VCZ, respectively). A second line to the same objective has been the improvement of the vertical Bridgman (VB) and vertical gradient freezing (VGF) methods to commercial maturity. The state of art, pros and cons, and the developments of the growth methods to be expected in future are summarized in the present paper. Some fundamental problems of heat transfer, dislocation dynamics (polygonization) and non-stoichiometry related growth phenomena are discussed more in detail.