Liquid Encapsulated Czochralski Growth Research Articles

Dislocation bundle formation during liquid encapsulated Czochralski growth of GaAs crystals

Residual dislocations in a P-alloyed and an undoped GaAs crystal grown by the liquid encapsulated Czochralski method were investigated by transmission electron microscopy. In the undoped GaAs, dislocation bundles constructing cell walls and/or lineages consist of at least three different Burgers vectors. In the P-alloyed GaAs with low dislocation density, individual dislocation reactions were observed. Sessile dislocations were found to be formed by an interaction between two kinds of dislocations. The sessile dislocation formation may be a first step for the complicated dislocation structure in the undoped crystal.

Nondestructive measurement of indium content in semi-insulating GaAs substrates and ingots

We report the use of room-temperature photoluminescence to determine low levels of indium in semi-insulating GaAs intended for use as low dislocation material for substrates of integrated circuits. We have compared this technique with other optical methods such as low-temperature (4.2 K) photoluminescence and Raman scattering and find it more convenient for this application. Room-temperature photoluminescence easily allows measurement of In content in InxGa1−xAs in the range of 0.1–2% with a standard deviation of 0.1%, which is sufficient for most practical applications. The technique is nondestructive and rapid. The measurement can be done on polished, sawed, or etched surfaces, or on side surfaces of as-grown ingots without any treatment. To illustrate its use, we have measured variations in the In content of semi-insulating GaAs due to segregation of In in the melt during liquid encapsulated Czochralski growth.

Stoichiometry dependence of electrical activation efficiency in Si implanted layers of undoped, semi-insulating GaAs

Stoichiometry dependences of the electrical activation efficiency in Si implanted layers have been investigated for undoped, semi-insulating (SI) liquid encapsulated Czochralski (LEC) GaAs grown from melts of different compositions. The absolute value of lattice parameter was used as a measure for deviation from stoichiometry in GaAs. Irrespective of the annealing procedures, the electrical activation efficiency tends to increase and saturate to a maximum value as the crystals become As-rich, i.e., the lattice parameter increases from 0.565358 to 0.565364 nm. These results demonstrate that the precise control of stoichiometry greatly improves the reproducibility of LEC growth of undoped SI GaAs suitable for GaAs integrated circuits.

Growth and properties of large-diameter indium lattice-hardened GaAs crystals

The liquid-encapsulated Czochralski (LEC) growth of In-doped GaAs from 3 kg melts in a Melbourn high-pressure puller has resulted in low-dislocation, large-diameter crystals. Post-growth boule annealing at 950°C for 18 h is found to be an effective stress-relief treatment. This allows high wafer yields, comparable to those from undoped GaAs crystals to be obtained from In-doped boules, with the added advantage of greatly improved uniformity in electrical properties. These substrates are semi-insulating, thermally stable ( ρ ≥ 10 7 ω cm and μ H ⩾ 5000 cm 2/V · s with a ± 10% or better radial uniformity), and contain low residual impurity concentrations ( ⩽ mid-10 15 cm -3) as determined by secondary ion mass spectroscopic (SIMS) analysis. In this study the effectiveness of various concentrations of indium in reducing the dislocation density in GaAs have been explored. A comparison of the thoretically calculated thermal stress experienced by a crystal during LEC growth, with observed reductions in dislocation etch pitch density, indicate an apparent 28-fold increase in the critically resolved shear stress (CRSS) of In-doped over undoped GaAs for 8 x 10 19 cm -3 In in the solid. Polished substrates obtained from these crystals show minimal subsurface damage, believed to be related to the increased hardness of this material, and are now approaching high-quality silicon wafers in this respect.

Temperature dependence for the onset of plastic yield in undoped and indium-doped GaAs

The onset of plastic yield in undoped and highly indium-doped GaAs, prepared by liquid encapsulated Czochralski (LEC) growth, has been determined by uniaxial compression tests over the temperature range 250–600 °C. No distinguishable difference in yield stress is observed between undoped and indium-doped GaAs; however, LEC GaAs exhibits a modest increase in yield strength over undoped Bridgman-grown material. The absence of a significant increase in the yield strength of indium-doped over undoped LEC GaAs implies that the mechanism by which indium reduces the dislocation density in GaAs is significantly effective only at temperatures near the melting point.

Photoluminescence study of residual shallow acceptors in liquid-encapsulated Czochralski-grown InP

For nominally undoped InP crystals grown in silica, pBN, ceramic AlN, and pG crucibles by the liquid-encapsulated Czochralski (LEC) technique, residual shallow acceptors are investigated by photoluminescence measurements. Mg and Ca, contained in the InP starting materials, are found to be removed due to reactions with the B2O3 encapsulant. Zn and C, of which Zn was dominant throughout the undoped crystals, are found to be residual acceptors incorporated during LEC growth. The B2O3 encapsulant is identified as one Zn contamination source.

Effects of indium lattice hardening upon the growth and structural properties of large-diameter, semi-insulating GaAs crystals

The high-pressure liquid encapsulated Czochralski growth of indium lattice-hardened GaAs, from 3 kg melts, has resulted in low-dislocation, large-diameter crystals which exhibit thermally stable, semi-insulating properties. Post-growth boule annealing is found to be an effective stress-relief treatment, which assures high wafer yields and extremely uniform electrical properties. Observed reductions in dislocation density for mid 1019 cm−3 In-doped GaAs substrates indicate an apparent 28-fold increase in the critically resolved shear stress of this material over undoped GaAs near the melting point. Polished substrates obtained from these crystals exhibit very little subsurface damage, approaching high-quality silicon wafers in this respect.

LEC growth and characterization of Ga doped InP crystals

Ga doped InP crystals are grown by the liquid encapsulated Czochralski (LEC) method. From impurity analyses of the grown crystals, effective segregation coefficients of Ga in InP, ke (Ga), are determined and the diffusion coefficient of Ga in the melt is calculated using growth rate dependence of the ke(Ga). The influence of high Ga concentration doping on single crystalline growth is investigated by X-ray topography. The results show that doped concentrations higher than 1020/cm3 in the top of the crystals lead to polycrystalline growth, probably due to large misfit lattice strain gradients.

Finite Element Analysis of a Thermal‐Capillary Model for Liquid Encapsulated Czochralski Growth

A two‐dimensional model for the coupled effects of heat transfer and capillarity in a liquid encapsulated Czochralski growth system is analyzed by solving the full free‐boundary problem describing the temperature field in each phase, the shapes of the melt/solid and fluid/fluid interfaces, and the radius of a steadily growing crystal. Solutions are based on a finite element analysis with Newton iteration for all the variables. Heat transfer in the melt is taken to be dominated by conduction, and radiation to a uniform ambient is included for a transparent encapsulant. Calculations for a model system give reasonable predictions of crystal size and axial temperature gradient. The results are most sensitive to radiation through the encapsulant.

Low dislocation density, large diameter, liquid encapsulated Czochralski growth of GaAs

We have grown 70 nm diameter, 1200 g, 〈100〉 oriented GaAs single crystals in a low thermal gradient environment. Without intentional doping we have obtained average KOH etch pit densities (EPD) of 5000 cm -2 over 70% of the wafer area, and throughout 75% of the ingot length. This material is semi-insulating with a resistivity of 2 × 10 7 Ω cm. By doping with Si to give free carrier concentrations in the range of 1 to 5 × 10 18 cm -3, we reproducibly obtain average EPD's of <200 cm -2. Near the bottom of the ingot, wafers are virtually dislocation free except very near the wafer edge. The distribution of etch pits across the wafer is in general agreement with the thermoelastic stress model: high at the edges and in the center, but with an annular region essentially dislocation free (Si-doped). The axial distribution, however, does not fit this model: generally high at the seed and tail ends of the crystal and low through the bulk of the ingot. Transmission X-ray topographs confirm the nature and extent of the dislocation distribution.

Double doped low etch pit density InP with reduced optical absorption

Liquid encapsulated Czochralski growth of low etch pit density, low optical absorption InP single crystals has been achieved by double doping with sulfur (donor) and zinc (acceptor) impurities. The etch pit density is controlled by the total impurity content N A + N D while the free carrier concentration, which determines the optical absorption, is controlled by the difference N D − N A of the active sulfur and zinc concentrations. Crystals having etch pit densities as low as 100 cm -2 and optical absorption as low as 1 cm -1 at 1.3 μm are reported.

An Analysis of the Derivative Weight-Gain Signal From Measured Crystal Shape: Implications for Diameter Control of GaAs

A commercially viable GaAs device technology for field-effect transistors, integrated circuits, and lasers is critically dependent on the availability of high-quality, single-crystal boules with controlled diameter. We have modeled a diameter control scheme based on the monitoring of crystal weight for liquid-encapsulated Czochralski (LEC) growth of GaAs. The presence of the B <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> O <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> liquid encapsulant and significant capillary forces make the direct interpretation of the weight-gain signal and its time derivative (DWGS) more complicated in comparison with pulled materials such as oxide crystals. We have formulated a realistic model for the LEC growth of axisymmetric crystals and have derived the differential equation relating the time evolution of the DWGS to radius and length. We show that the magnitude of the DWGS at the crystal's “shoulder” is inversely related to the radius of curvature. Furthermore, the meniscus by itself gives rise to a precursor or early warning in the signal, which means that the maximum in DWGS precedes the maximum in shape by a few hundred seconds. The existence of a secondary maximum or aftershock in the signal that is the sole consequence of liquid encapsulation is also demonstrated. Excellent agreement has been obtained between DWGS and the signal predicted from the measured shape of a grown crystal. Thus, prospects for automatic diameter control are encouraging.

LEC growth of Te-doped GaSb single crystals with uniform carrier concentration distribution

Large diameter (35–50 mm) Te-doped 〈100〉 GaSb single crystals have been successfully grown from a non-stoichiometric melt by the liquid encapsulated Czochralski (LEC) method. The X-ray transmission topograph shows that the 〈100〉 crystals have extremely low dislocation densities (less than 1 cm −2), weak striations and no facets. Hall measurements show that the carrier concentration (7.1×10 18 cm −3) distribution is within ±5% deviation over (100) wafers.

Formation of dislocations during liquid encapsulated Czochralski growth of GaAs single crystals

Results of calculations on thermal stresses induced during pulling of GaAs ingots are reported. Comparison between these calculations and our experimental results shows that the dislocations nucleation in crystals during growth cannot be directly related to a constant value of the critical resolved shear stress (CRSS). According to experiments performed on other materials, it is concluded that the CRSS can strongly depend upon the initial state of the material, and particularly upon temperature and native defects concentration and mobility.

Liquid-encapsulated Czochralski growth of InP crystals

This paper reports the results of our research on the growth of InP crystals used to provide high-quality substrates for GaInAsP/InP diode lasers and detectors. The crystals are grown by the liquid-encapsulated Czochralski (LEC) method using high-purity polycrystalline starting material that is synthesized from the elements in sealed fused-silica ampoules by the horizontal Bridgman method. By making a detailed study of the effects of changes in the LEC growth parameters, we have shown that the probability of twinning is decreased by increasing the temperature gradient at the crystal-melt interface. On the basis of this study we have developed a growth procedure that produces a high yield of twin-free boules. The temperature gradient required to obtain twin-free growth is significantly higher for crystals that are heavily doped with Zn (>1 × 10 18 cm -3) than for those doped with lower concentrations of Zn or with other impurities.

High-purity semi-insulating GaAs material for monolithic microwave integrated circuits

Liquid-Encapsulated Czochralski (LEC) growth of large-diameter bulk GaAs crystals from pyrolytic boron nitride (PBN) crucibles has been shown to yield high crystal purity, stable high resistivities, and predictable direct ion-implantation characteristics. Undoped (≲low 1014cm-3chromium) and lightly Cr-doped (low 1015cm-3range) -GaAs crystals, synthesized and pulled from PBN crucibles contain residual shallow donor impurities typically in the mid 1014cm-3, exhibit bulk resistivities above 107Ω . cm, and maintain the high sheet resistances required for IC fabrication (>106Ω/□) after implantation anneal. Direct29Si channel implants exhibit uniform (± 5 percent) and predictable LSS profiles, high donor activation (75 percent), and 4800- to 5000-cm2/V . s mobility at the (1 to 1.5) × 1017cm-3peak doping utilized for power FET's. It has also been established that LEC crystals can provide the large-area, round

A novel encapsulant material for LEC growth of GaSb

Single crystals of GaSb have been grown by a liquid-encapsulated Czochralski (LEC) technique using a novel encapsulant material. A eutectic composition in the NaCl-KCl binary system was used as the encapsulant. A significant feature was that the oxide film on the GaSb melt pealed off in thin flakes in the encapsulant. Twin-free GaSb single crystals grown on <111> B axis were successfully obtained. Electrical properties were N = (1.0−1.5) × 10 17 cm -3 and μ = 690−740 cm 2 V -1 s -1 at room temperature, exhibiting p-type conduction.

Viewing port window for crystal pullers

A viewing port window that prevents volatile material from being deposited on it during crystal growth has been developed. It has been used successfully for the Liquid Encapsulated Czochralski (LEC) growth of gallium arsenide.

Deep hole traps in n-type liquid encapsulated Czochralski GaP

Cathodoluminescence and photoluminescence time-response measurements have been used to study the prominent luminescence bands in n-type GaP grown by the liquid-encapsulated Czochralski (LEC) method. In as-grown crystals, two deep hole traps are identified. An isoelectronic hole trap, ≃0.4 eV above the valence band edge, produces red emission (1.72 eV) due to bound-exciton and pair recombination, while infrared luminescence (1.51 eV) is produced by pair recombination between a deep (≃0.7 eV) acceptorlike center and the donor dopant. In certain heat-treated samples, an additional structured luminescence band is produced at 2.07 eV, which also apparently results from recombination at an isoelectronic center. Resolution of the no-phonon lines of this emission suggests that an exciton is bound by 186 meV to the impurity center created by annealing. Although no positive identification of the chemical nature of these defects has been possible, we find that our results are not inconsistent with previously suggested models in which group-VI donors are complexed with gallium vacancies. Comparisons are also made with similar luminescence bands and defect levels observed in GaP grown by lower-temperature methods. We conclude that the hole traps studied here are not unique to the LEC material, but are apparently greatly enhanced by the LEC growth conditions.