High-pressure Liquid Encapsulated Czochralski Research Articles

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.

3D unsteady analysis of gas turbulent convection during HPLEC InP growth

Three-dimensional (3D) unsteady features of turbulent nitrogen convection in an industrial high-pressure, liquid-encapsulated Czochralski (HPLEC) InP growth system have been studied using a 3D unsteady model based on the large eddy simulation (LES) approach. The conjugated simulation of the gas flow, the encapsulant motion and heat transfer in the crystal has been performed for varying gas pressure. A strong effect of gas convection on the temperature distribution in the crystal, and on the heat loss from the crystal and encapsulant surfaces has been revealed. Non-axisymmetric unsteady fluctuations of the temperature gradients in the crystal, produced by changes of the gas flow, have been observed, which can be responsible for dislocation generation in the growing crystal.

Mechanisms of Thermo-Solutal Transport and Segregation in High-Pressure Liquid-Encapsulated Czochralski Crystal Growth

The mechanism of dopant transport and segregation in high-pressure liquid-encapsulated Czochralski (HPLEC) grown III-V compound crystals (e.g., GaAs, InP) has been numerically studied using an integrated model, MASTRAPP. The model approximates the melt flow in the crucible as a quasi-steady-state, laminar, and axisymmetric flow, but the gas flow is considered as turbulent. Based on the physics of the growth process, a two-time-level scheme has been implemented where the dopant transport and growth are simulated at a smaller time scale while flow and temperature solutions are obtained from quasi-static calculations. Detailed numerical analyses are performed for the conditions of pure crystal rotation, pure thermally driven natural convection, and pure crucible rotation as well as for mixed flow with all of these forces present simultaneously. The dopant transport and segregation in these cases are well correlated to the corresponding melt flow pattern. Very weak radial segregation is predicted for pure crystal rotation because the resulting melt flow leads to a fairly flat solute boundary layer. The natural convection, on the other hand, produces a nonuniform boundary layer along the melt/crystal interface. This leads to a strong radial segregation with a high concentration along the central axis of the crystal. The crucible rotation has a similar effect. The combined effect of all of these flow mechanisms produces a strong radial segregation, whose extent depends on the relative strength of the driving forces. In all of these cases, strong melt flows lead to thin boundary layers that result in decreased longitudinal segregation. The predictions agree well with the experimental observations reported in the literature.

Local and Global Simulations of Bridgman and Liquid-Encapsulated Czochralski Crystal Growth

A curvilinear finite volume-based numerical methodology has been developed that can be effectively used for simulation of the Bridgman and Czochralski (Cz) crystal growth processes. New features of grid generation have been devised and added to the original formulation (Zhang et al., 1995, 1996) to make it suitable for global modeling. The numerical model can account for convection in both the melt and the gas phases, convection/radiation in the furnace, and conduction in all solid components. Results for Bridgman growth show that the flow pattern and interface shape strongly depend on thermal conductivities of the crystal, melt, and ampoule materials. Transient simulations have been performed for the growth of Bismuth crystal in a Bridgman-Stockbarger system and the growth of GaAs crystal using liquid-encapsulated Czochralski (LEC) technique. This is the first time that a global high-pressure LEC model is able to account for convective flows and heat transfer and predict the interface shape and its dynamics.

Characterization of deep centers in undoped semi-insulating GaAs substrates by normalized thermally stimulated current spectroscopy: Comparison of 100 and 150 mm wafers

A well-established characterization method for investigating deep traps in semi-insulating (SI) GaAs is thermally stimulated current (TSC) spectroscopy; however, TSC is not considered to be a quantitative technique because it involves carrier mobility, lifetime, and geometric factors, which are either unknown or poorly known. In this paper, we first show how to quantify a TSC spectrum, by normalizing with infrared (hv = 1.13 eV) photocurrent, and then apply this method (called NTSC) to study the lateral uniformity of the main deep centers across the diameters of undoped SI GaAs wafers. The wafers used in the study include both the standard 100 mm sizes and the new 150 mm variations, and are grown by both the low and high pressure liquid encapsulated Czochralski techniques. The results reveal that the 150 mm wafers have a worse NTSC uniformity for the main traps and a higher degree of compensation, as compared these parameters for the 100 mm wafers. In addition, nonuniformities related to the electric field effects on both the TSC spectrum and the low temperature photocurrent are found in the 150 mm wafer grown by the low pressure technique.

Macro-segregation, dynamics of interface and stresses in high pressure LEC grown crystals

High dislocation density and strong dopant inhomogeneities have been found in high pressure liquid-encapsulated Czochralski (HPLEC) grown crystals. The origin and underlying mechanisms of these defects are attributed to the complex nature of transport phenomena in the HPLEC system. Our integrated computer model (MASTRAPP) can simulate this process by calculating the flow and heat transfer in both the melt and the gas, and thermal-elastic stress in the crystal. In this work, this model has been further extended to investigate the development of thermal stress in the growing crystal and the redistribution of dopant in the melt. The results for InP growth show complex gas flow and heat transfer pattern in the system. Two large stress spots are predicted by the model, one at the edge of the crystal just above the encapsulant layer and the other in the top corner of the crystal. Although the stress always remains largest at the first location, its value decreases as the crystal grows, due to the enhanced cooling of the crystal. A curved crystal/melt interface is also found to introduce high thermal stresses in its vicinity, which may be dangerous because of a high temperature at the interface and thus a low strength of the crystal. The model also predicts both radial and longitudinal dopant segregation in the growing crystal, and shows that the dopant redistribution in the melt is caused by the complex flow pattern in the melt. This is the first time, that a strong radial dopant segregation has been predicted based on a comprehensive flow model for a HPLEC growth.

Transport phenomena in a high pressure crystal growth system: In situ synthesis for InP melt

The physical phenomena underlying the “one-step” in situ synthesis and high pressure growth of indium phosphide crystals are complex. A high resolution computer model based on multizone adaptive grid generation and curvilinear finite volume discretization is used to predict the flow and temperature fields during the synthesis of the InP melt. Simulations are performed for a range of parameters, including Grashof number, crucible rotation, and location of the injector. These parameters affect the gas flow in a high pressure liquid-encapsulated Czochralski (HPLEC) furnace significantly, and have a strong influence on the melt synthesis and its control.

Modeling of high pressure, liquid-encapsulated Czochralski growth of InP crystals

A high resolution numerical scheme based on multizone adaptive grid generation and curvilinear finite volume discretization has been implemented to simulate the high pressure, liquid-encapsulated Czochralski (HPLEC) growth of InP crystals and study the effect of gas recirculation on melt flow and crystal/melt interface shape. The model incorporates flows induced by buoyancy and capillary forces and by crystal and crucible rotations, as well as the radiation heat loss from the melt and the crystal surfaces. It is demonstrated that the thermal interaction between the gas and the melt must be accounted for to predict the interface shape and dynamics accurately. The numerical results demonstrate that the transport phenomena in a high pressure growth system is very complex. The temperature distribution in the crystal and the shape of the melt/crystal interface also change significantly with the size of the crystal. This has a strong influence on dislocations in the crystal as shown by the experiments of Kohiro et al. [J. Crystal Growth 158 (1996) 197] and Jordan et al. [J. Crystal Growth 70 (1984) 555].

Vacuum Bakeout Effect on Ambient Gas in a High Pressure LEC Puller

H2 and CO generation from graphite furnace components in a high-pressure liquid-encapsulated Czochralski (LEC) puller was studied in detail from the standpoint of reproducible contamination to the LEC charge (B2O3 encapsulant/GaAs melt). It has been found that H2 and CO contamination from the graphite furnace components varies widely according to the vacuum level as well as to the high-temperature duration time of the vacuum bakeout process. It has been concluded that controlling the vacuum bakeout condition is the key to achieving reproducible generation of H2 and CO during LEC GaAs single-crystal growth.

Automatic diameter control in high-pressure LEC growth of gallium arsenide

From the technological point of view the main task is to achieve a regular shape, good crystallographical and electrical parameters in the crystal growth process. The GaAs single crystals grown by the technology of Liquid Encapsulated Czochralski (LEC) generally have satisfactory parameters if the process is strictly controlled. In this paper we have tried to investigate the possibility of the accurate control of the diameter of the GaAs crystal in a high pressure Galaxy type puller, made by LPA, France.

Comparison of deep centers in semi-insulating liquid-encapsulated Czochralski and vertical-gradient freeze GaAs

Three-inch, semi-insulating (SI) GaAs, grown by the vertical gradient freeze (VGF) technique, has been studied by IR absorption, temperature-dependent dark current and Hall-effect, thermally stimulated current (TSC), and photoinduced current transient spectroscopy and has been compared with undoped, SI GaAs, both As-rich and Ga-rich, grown by the high-pressure liquid-encapsulated Czochralski method. The results clearly indicate that (1) the VGF GaAs contains less EL2, which suggests a less As-rich crystal stoichiometry; (2) in some VGF samples activation energies of 0.43 or 0.46 eV are deduced from temperature-dependent carrier concentration or resistivity measurements, respectively, and (3) VGF samples often show a thermal quenching behavior in the TSC peak T5. There is evidence to suggest that the 0.43 eV center is related to VAs, and T5 to VGa.

Comparison of Computed and Measured Temperature Distributions in the High‐pressure Liquid Encapsulated Czochralski (HPLEC) Configuration

AbstractThe calculations of the temperature field of the InP growth system are carried out with a stationary FEM/Newton algorithm. With regard to the radiation transparency of the B2O3 layer, the limiting cases transparency and non‐transparency are investigated. With the thermocouples which are inserted into the crystal, the temporal temperature behavior at selected positions of the crystal is recorded. The shapes of the crystal/melt interface of further crystals are analysed by microscopy. Computational and experimental results agree.

On steady‐state modelling of the high‐pressure liquid encapsulated czochralski (HPLEC) crystal growth of InP (Conduction Model)

AbstractThe steady‐state temperature distribution of the growth system is calculated at various stages of the growth process. The results indicate a serious influence of the thermal transparency of the B2O3. The heater power which has to be adjusted in order to achieve a constant diameter growth is calculated as an inverse problem.

Total simulation model of high pressure liquid encapsulated Czochralski crystal growth

A numerical model has been made to determine the effect of various geometric and thermal variables. The model consists of the following three blocks: (1) the the entire furnace system in which entire temperature distributions were calculated considering the effects of radiative heat exchange and gas flow; (2) a crystal and melt system in which temperature distributions and the crystal/melt interface were calculated, (3) thermal stress analysis by which resolved shear stress is obtained. Calculated results were in good agreement with experimental results; the temperature response to the change of heater power, the crystal/melt interface shape which was observed from growth striations and etch-pit densities on 4 inch diameter (100) oriented GaAs wafers.

The low thermal stress HP-LEC growth of 75 mm diameter In-DOPED GaAs crystals

A study is presented of the effects of various thermal environments in a Melbourn high-pressure liquid encapsulated Czochralski puller on the thermal stress experienced by 75 mm diameter GaAs crystals doped with between 5.4×10 19 to 2.1×10 20cm −3 indium. Approaches investigated for obtaining low thermal stress growth conditions include the use of thick (up to 6 cm) B 2O 3 encapsulating layers, a graphite “top hat” enclosure, and a controllable crystal after-heater and encapsulant heat shield. Of these, the latter configuration appears to yield the lowest thermal stress growth conditions using a 2 cm thick B 2O 3 encapsulant layer. Growths performed using this configuration have yielded low-dislocation (<10 3 cm -2) 75 mm diameter GaAs substrates at an indium doping concentration of 1.1×10 20 cm −3. Finite element fluid dynamics modeling techniques have been employed to determine the effect of the after-heater and encapsulant heat shield on the temperature in the growing ingot and boric oxide encapsulant. These results are compared with those obtained for “conventional” growth conditions.

Autonomous liquid encapsulated Czochralski (LEC) growth of single crystal GaAs by “intelligent” digital control

An “intelligent” digital growth control approach developed at Arizona State University (ASU) which combines deterministic and heuristic techniques allows virtually total automation of the high-pressure LEC process for GaAs growth. A number of 4 kg, 3 inch semi-insulating GaAs crystals have been grown reproducibly on an adapted Cambridge Instruments CI 358 puller without any significant operator intervention, with process yields of more than 75% single crystals. Crystals produced with the digital system were superior in diameter control, compared to conventionally grown crystals, and exhibited a better uniformity of their electrical and crystalline properties.

Whole-ingot annealed semi-insulating GaAs substrates for low-noise microwave amplifiers

The quality of liquid-encapsulated Czochralski (LEC) grown GaAs substrates critically affects the final low-noise microwave device and circuit performance as evidenced by comparing Si-implanted undoped, In-alloyed, and whole-ingot annealed semi-insulating substrates. We investigated differences in Si-implant activation, electrical profiles, and uniformity of material, device, and circuit parameters. The best noise figure of 1.33 dB at 10 GHz was measured on a 0.5-µm low-noise FET fabricated on the high-pressure whole-ingot annealed LEC wafer. A noise figure of 2.0 dB with associated gain of 24 dB at 10 GHz was achieved for a monolithic two-stage low-noise amplifier (LNA) fabricated on the standard high-pressure LEC substrate.

Low dislocation, semi-insulating In-doped GaAs crystals

Elemental indium doping of GaAs melts to a concentration in the 10 20 cm -3 range was found to be highly effective in reducing dislocation densities in large diameter GaAs crystals grown by the high-pressure liquid encapsulated Czochralski technique. Nominally 50 mm diameter 〈 100 〉-grown crystals exhibit dislocation densities of less than 500 cm -2 over 80% of the central crystal diameter compared to densities greater than 10 4 cm -2 in undoped GaAs crystals. In other respects, In-doped GaAs grown from stoichiometric or slightly As-rich melts are indistinguishable from undoped GaAs, showing stable resistivities in the 10 7 to 10 8 ohm cm range, measured mobilities approaching 5000 cm 2/V·s and only slightly modified 29Si implantation characteristics.

A direct comparison of LEC GaAs grown using low- and high-pressure techniques

A direct comparison has been made between the properties and implant behavior of undoped semi-insulating (SI) low- and high-pressure liquid encapsulated Czochralski (LEC) GaAs. Although the properties of high-pressure LEC had previously been well characterized, this is the first detailed study of the quality of low-pressure LEC material. It is shown that both material types are similar in structural quality, purity, and in the characteristics of conducting layers formed by direct ion implantation. Arsenic-rich melts and B 2 O 3 with greater than 500-ppm H 2 O content are required for obtaining reproducible and stable SI behavior growing from quartz crucibles.

Low noise GaAs metal-semiconductor field-effect transistor made by ion implantation

Low noise GaAs metal-semiconductor field-effect transitors (MESFET’s) have been made by direct ion implantation into high pressure liquid encapsulated Czochralski (HPLEC) undoped substrates and unintentionally doped high resistivity buffer layers. A noise figure of 1.3 dB with 10.3-dB associated gain has been achieved at 12 GHz, and a 2.2-dB noise figure with 7.4-dB associated gain has been measured at 18 GHz with a 0.6-μm gate length by 300-μm gate width FET device, fabricated from a HPLEC undoped substrate. Comparable dc and rf performance has also been achieved for devices fabricated from undoped buffer layers. Equivalent circuit elements have been evaluated in an effort to understand the critical parameters necessary to achieve these results.