Liquid Encapsulated Czochralski Research Articles

Impurities related micro-defects in GaSb crystal grown by LEC method

The epitaxial growth of devices based on gallium antimonide (GaSb) is negatively impacted by defects related to impurities in the crystal. Liquid encapsulated czochralski (LEC) technology was used to grow 2-inch Te-doped GaSb (100) single crystal ingots, which were then processed into polished wafers in order to investigate the origins and consequences of defects related to impurities. Polished GaSb wafers were found to have surface micro-defects. Energy dispersive x-ray spectroscopy (EDAX) and scanning electron microscopy (SEM) measurements indicate that the micro-defects are associated with carbon and oxygen impurities. Glow discharge mass spectrometry (GDMS) is used to identify the origins of carbon and oxygen impurities. Based on the findings, an optimization process was developed to enhance the crystal quality, and a micro-defect-free GaSb single crystal was produced.

Phosphorus escaping phenomena during the growth of InP crystal by in-situ liquid-encapsulated Czochralski method and P-rich-related defects in InP crystal

Phosphorus escaping phenomena during the growth of InP crystal by in-situ liquid-encapsulated Czochralski method and P-rich-related defects in InP crystal

Численное моделирование процесса выращивания монокристаллов методом Чохральского в квазистационарном приближении

We consider the steady-state growth of a single crystal by the liquid encapsulated Czochralski method. The mathematical model accounts for heat transfer in the crystal, melt and encapsulant, radiative heat exchange between the crucible and the surrounding furnace, formation of the growth interface and the melt/encapsulant interface meniscus, release of the latent heat during the phase transition. The time history of the growth is reconstructed from a sequence of steady state calculations with decreasing melt depth. The numerical method is based on explicit interface tracking. The employed computational procedure ensures global conservation of thermal energy. The response of prototype growth system for changes in pull rate or heater temperature is examined numerically.

Stability Analysis of Thermocapillary Convection of B<sub>2</sub>O<sub>3</sub>/Sapphire Melt in an Annular Pool

Aiming at the thermocapillary convection stability of sapphire crystal grown by liquid-encapsulated Czochralski method, by non-linear numerical simulation, obtained the flow function and temperature distribution of R-Z cross section, as well as the velocity and temperature distribution at liquid-liquid interface and monitoring point of B2O3/sapphire melt in annular two liquid system, covered with solid upper wall and in microgravity. By means of linear stability analysis, obtained the neutral stability curve and critical stability parameters of the system, and revealed the temperature fluctuation of the liquid-liquid interface. The calculated results of B2O3/sapphire melt were compared with 5cSt silicone oil/HT-70. The results show that under the same geometrical conditions, the flow of B2O3/sapphire melt system is more unstable than 5cSt silicone oil/HT-70, there are two unstable flow patterns, radial three-dimensional steady flow cell and hydrothermal waves near the hot wall. The larger the ratio of Pr number of upper and lower fluid layers is, the better the effect of restraining the flow of lower fluid layers is.

Residual stress distribution and flatness of dislocation-free Te-GaSb (100) substrate

Raman spectroscopy has been used to evaluate residual stress distribution across GaSb single crystal wafers with different Te doping concentrations grown by liquid encapsulated Czochralski (LEC) method. Undoped GaSb wafers grown by LEC and vertical temperature gradient freezing method were used as reference wafers for comparison analysis. The residual stress increases but its distribution uniformity improves in LEC-GaSb wafers with the concentration of Te dopant increasing. Moreover, annealing at temperature 650 °C can effectively increase its distribution uniformity and results in an improvement of the flatness. The results also suggest that the flatness of GaSb wafers is better when Te doping concentration is controlled within a certain range.

Evaluation of LEC and VGF-InAs substrates through surface defect characterization and epitaxy growth

Undoped InAs (100) substrate wafers prepared by liquid encapsulated Czochralski (LEC) and vertical temperature gradient freezing (VGF) methods have been studied and compared in their epi-ready state. The substrate surface defects have been analyzed by surface scanner KLA candela, Energy Dispersive X-Ray Spectroscopy (EDAX), Total-reflection X-ray fluorescence spectroscopy (TXRF), with an aim to find the contrast between LEC-InAs and VGF-InAs substrates. It is found that the VGF wafers have lower surface point defects and good stoichiometry compared with the LEC-wafer. VGF-InAs is more favorable to LEC-InAs for oxide thermal desorption and midwavelength infrared（MWIR）epitaxial layer growth with low defect density.

Investigation of thick GaAs:Cr pixel sensors for X-ray imaging applications

The new generation of synchrotron radiation sources sets advanced requirements in the performance of hybrid pixel detectors. A critical point in the design of such detectors is the choice of the sensor material. Chromium-compensated Gallium Arsenide (GaAs:Cr) is a promising candidate for use in pixel detectors, targeting imaging applications in the intermediate energy range 20–50 keV. In this work, thick GaAs:Cr sensors grown recently using the Liquid Encapsulated Czochralski (LEC) method were bonded to Timepix chips. The performance of the sensors was characterised by using various X-ray sources in view of imaging applications. The uniform illumination of the sensors reveals inhomogeneities due to the presence of crystal defects in the microscopic level. Using a micro-focused X-ray beam we extract sensitivity maps and the mobility-lifetime product at the pixel level. The results obtained confirm that the non-uniformities observed at the sensor level are directly linked with the microscopic defects. Overall, the efficient use of such sensors in X-ray imaging applications relies on their stability over irradiation-time.

Influence of melt convection on distribution of indium inclusions in liquid-encapsulated Czochralski-grown indium phosphide crystals

Indium (In) inclusions have been found in 〈100〉 indium phosphide (InP) single crystals grown in In-rich melt by the liquid-encapsulated Czochralski (LEC) method. Two kinds of In inclusions with different morphologies, one is lath-like and the other is polyhedral, have been observed. Most of the In inclusions are lath-like and their long sides are always parallel to the $$[0\overline{1}1]$$ or $$[0{\overline{1}}{\overline{1}}]$$ orientation. In (100) InP wafers, these inclusions mainly distribute in two regions viz., center of the wafer and annular belt at a certain distance from the center. These special distributions of In inclusions in InP crystals were found in our experiments for the first time. In order to clarify the cause of the special distributions of In inclusions, numerical simulations have been carried out on the melt convection in the process of InP crystal growth. The results show that the rotations of crystal and crucible significantly affect the number and direction of convection cells in the melt, which have a great influence on the enrichment and distribution of excess In at the solid–liquid interface. Different melt convections modify the radial distributions of excess In atoms near the solid–liquid interface and result in different radial distributions of In inclusions in the crystals.

Evaluation of residual stress in InP and InAs (100) substrates obtained from single crystals grown by LEC and VGF methods

Raman spectroscopy has been used to characterize and compare residual stress distribution across wafers sliced from un-doped InP and InAs single crystals grown by liquid encapsulated Czochralski (LEC) and vertical temperature gradient freezing (VGF) methods, respectively. Both VGF-InP and VGF-InAs wafers exhibit greater residual stress but even distribution across the wafer than their LEC wafers. There is an inverse distribution correlation between the residual stress and the Full width at half maximum (FWHM) of Raman peak for the wafer grown by LEC or VGF method. The flatness results suggest that VGF-grown wafers generally have better flatness parameters but with higher residual stress than LEC-grown wafers.

A Comparison of Defects Between InAs Single Crystals Grown by LEC and VGF Methods

InAs single crystals grown by the liquid-encapsulated Czochralski (LEC) method and vertical gradient freezing (VGF) are studied by low-temperature photoluminescence spectroscopy, infrared transmission and reflectance spectroscopy, double-crystal x-ray diffraction and Hall effect measurement, respectively. A properly controlled etching solution is used to reveal beautiful square dislocation etch pits in the crystals. In addition to extremely low dislocation density, the concentration of native defects in the VGF-InAs single crystals is much lower than that in LEC-InAs, giving VGF-InAs better electrical and optical properties. The nature of the defects in InAs single crystals is discussed by considering the variation in stoichiometry and environment during the crystal growth processes.

Mechanism of free electron concentration saturation phenomenon in Te-GaSb single crystal**Project supported by the National Natural Science Foundation of China (Grant Nos. 61474104 and 61504131).

Te-doped GaSb single crystal grown by the liquid encapsulated Czochralski (LEC) method exhibits a lag of compensating progress and a maximum carrier concentration around 8×1017 cm−3. The reason for this phenomenon has been investigated by a quantity concentration evaluation of the Te donor and native acceptor. The results of glow discharge mass spectrometry (GDMS) and Hall measurement suggest that the acceptor concentration increases with the increase of Te doping concentration, resulting in the enhancement of electrical compensation and free electron concentration reduction. The acceptor concentration variation is further demonstrated by photoluminescence spectra and explained by the principle of Fermi level dependent defect formation energy.

Photoluminescene study acceptor defects in lightly doped n type GaSb single crystals

Lightly Te-doped GaSb samples grown by the liquid encapsulated Czochralski (LEC) method have been studied by Hall measurements and low-temperature PL spectroscopy. The results suggest that acceptor-related antisite is the dominant defect in n-type GaSb with low Te-doping concentration. As the Te concentration increases, gallium vacancy related defects become the main acceptor. A new band of around 665 meV is observed in the GaSb sample with the lowest Te-doping concentration. The variation of the acceptor defects and their influence on the electronic and optical property on the n-GaSb single crystal are discussed based on the results.

Resonant electronic Raman scattering of below-gap states in molecular-beam epitaxy grown and liquid-encapsulated Czochralski grown GaAs

Resonant electronic Raman (ER) scattering is used to compare the below-gap excitations in molecular-beam epitaxially grown GaAs and in undoped semi-insulating GaAs substrates. The measurement geometry was designed to eliminate common measurement artifacts caused by the high optical transmission below the fundamental absorption edge. In epitaxial GaAs, ER is a clear Raman signal from the two-electron transitions of donors, eliminating an ambiguity encountered in previous results. In semi-insulating GaAs, ER occurs in a much broader dispersive band well below the bound exciton energies. The difference in the two materials may be due to the occupation of the substrate acceptor states in the presence of the midgap state EL2.

Crystal growth of sapphire for substrates for high‐brightness, light emitting diodes

Sapphire has emerged as the preferred substrate for high‐brightness, light emitting diodes (HB‐LEDs) made with a GaN epilayer. While the growth of most crystalline materials for electronic and laser applications normally converges onto a single, preferred method—Czochralski growth for silicon, lithium niobate and YAG (yttrium aluminum garnet), vertical gradient freeze (VGF) and Liquid Encapsulated Czochralski (LEC) for GaAs crystals (depending upon current density and device application), vapor transport for SiC, for example—a wide range of processes are employed for sapphire growth. These include Czochralski, Kyropoulos, EFG (Edge‐defined, Film‐fed Growth), Bagdasarov, classical Bridgman and several variants of Bridgman such as HEM (Heat Exchanger Method) and CHES (Controlled Heat Extraction System).This variety of approaches to grow crystals of what is basically the same material reflects a balance between acceptable defect levels, the costs of downstream fabrication steps such as wire sawing, and the wafer diameters demanded by the market. Each process has advantages and disadvantages. The Kyropoulos method, for example, probably yields crystals with the lowest defect levels, while EFG eliminates the wire sawing stage of wafer fabrication which is a major cost factor. Large sapphire crystals grown by HEM are frequently oxygen deficient, and a‐axis Czochralski crystals have higher dislocation densities than Kyropoulos boules.Facet formation on the c‐plane is an important criterion in selecting the growth methodology for sapphire. However, we will show that melt grown sapphire crystals have a curved interface that passes through other facet planes, and this can impact the growth process.In this paper we will summarize the various crystal growth methods currently employed to grow sapphire, discuss the strengths and weaknesses of each approach and also discuss the often overlooked thermodynamic aspects of this high‐temperature crystal growth process. We will attempt to show the long‐term trends in the evolution of the various growth processes and predict the ultimate winner of the contest to supply substrates.

Chromium compensated gallium arsenide detectors for X-ray and γ-ray spectroscopic imaging

Semi-insulating GaAs material of 500μm thickness grown using the Liquid Encapsulated Czochralski (LEC) method has been compensated with chromium to produce high resistivity single crystals suitable for spectroscopic imaging applications. Results are presented for the performance of three small pixel detectors each with 80×80 pixels on a 250μm pitch, fabricated with metal contacts and bonded to a spectroscopic imaging ASIC. Current–voltage measurements demonstrated a material resistivity of 2.5×109Ωcm at room temperature. At an optimised bias voltage, the average energy resolution at 60keV (FWHM) was in the range 2.8–3.3keV per pixel. An analysis of the voltage dependent X-ray spectroscopy suggests that the electron mobility lifetime (μτe) for each detector is in the range 2.1–4.5×10−5cm2V−1. The spectroscopic imaging capability of the detectors is also demonstrated in X-ray absorption spectroscopy measurements.

Flow instability driven by the combined temperature gradient and counter rotation of crucible and crystal in a liquid-encapsulated Czochralski configuration

The combined effects of temperature gradient and counter rotation of the crucible and crystal on the flow instability in a liquid-encapsulated Czochralski configuration are investigated through a series of direct numerical simulations. The two immiscible fluids of 5cSt silicone oil and HT-70 are considered in this work. Results show that the basic flow is in a steady state and is presented as rich flow structures depending on the rotation Reynolds numbers of the crucible and crystal. When the Marangoni number exceeds a threshold value, the unsteady multi-cellular structures are developed. The oscillatory flow behaves as fluctuation waves propagating from the crystal/fluid interface to the crucible sidewall. The amplitudes of the velocity and temperature fluctuations decrease with the increase of the crystal rotation rate, but increase with the crucible rotation rate. The critical conditions for the onset of the flow instability are obtained. The stability diagram indicates that the rotation of the crucible has a destabilizing effect on the flow, but the crystal rotation can depress the flow instability when the crystal rotation Reynolds number exceeds a certain value.

Distribution of Carbon in Large Diameter Semi-Insulating Gallium Arsenide Grown by Liquid Encapsulated Czochralski Technique

Carbon impurity concentration and dislocation density were investigated with optical microscopy and Fourier transform infrared absorption spectrometer in radial direction of large diameter (6-inch) undoped semi-insulating Gallium Arsenide (SI-GaAs) grown by liquid encapsulated Czochralski (LEC). The experimental results showed that their distributions are both “W”-shaped along wafer diameter, which is relatively higher on the center and lower near the center, but highest on the edge of the wafer. The nonuniformity distribution of thermal stress from growth process leads to the “W”-shaped distribution of dislocations in radial direction. The adsorption of matrix elastic strain field around dislocations induces the “W”-shaped distribution of carbon impurity. Dislocations adsorb carbon impurity and carbon impurity decorates dislocations. Dislocation density distribution affects carbon behavior.

The Dislocation Density Distribution of Different Types of Doped InP Wafers

It is of great significance to study on the dislocation density distribution of different types of doped indium phosphide (InP) crystal wafers for fabricating high-quality InP single crystal with low dislocation density. In the paper, a new etched pits density mapping (EPD mapping) measurement is introduced to measure the dislocation density distribution of the S-doped, Fe-doped and non-doped InP wafers pulled by high-pressure liquid encapsulated Czochralski (HP-LEC) technique. Test results show that in the three types of InP wafers， the S-doped InP wafer’s dislocation density is lowest and uniformity is best; the non-doped InP wafer’s dislocation density is maximum and uniformity is the worst; the Fe-doped InP wafer’s dislocation density and uniformity are between the S-doped and non-doped InP wafers. In the paper, the measurement results are analyzed in detail also from the faces of the doping and crystal growth process and thermodynamic mechanism. This study shows that in addition to traditional methods, using reasonable doping process can also effectively reduce the dislocation in the crystal, enhance the lattice strength and improve the uniformity of the InP single crystal.

D215 GaAs単結晶成長に及ぼす融液流動および温度分布の影響(OS-2:複合対流)

2D computer simulation assists in the decision of growth parameters to reduce the crystal defects of GaAs crystal in liquid encapsulated Czochralski (LEC) method. In this paper, we numerically investigated the reason why the 450mm-long crystal has more crystal defects than the 350mm-long crystal by comparing heat exchange, temperature distribution and velocity vector in the source melt at several stage of the growth. The simulation results indicated that large heat release from crystal and small heat release from crucible bottom are advantageous to the crystal growth with few defects. It is also necessary that forced convection and natural convection of GaAs melt is delicately controlled by the crucible rotation rate and the temperature distribution.

Influence of Silicon and Boron Doping on the Thermal Conductivity of N-GaAs Single Crystals

Thermal conductivity of three different GaAs crystals, one grown by Liquid Encapsulated Czochralski technique and the other two samples grown by Vertical Bridgman technique have been experimentally measured at temperatures 310 K to 360 K. Two crystals are undoped and semi insulating and one crystal is conductive which is doped with Si and B(Silicon ≈ 4x1015 cm-3 and Boron ≈ 8x 1015cm-3). It has been experimentally observed that doping of silicon and boron gives rise to an increase in thermal conductivity.