Liquid Encapsulated Czochralski Technique Research Articles

X-ray double-crystal diffractometry characterization of semi-insulating GaAs

The X-ray double crystal diffractometry method was employed to measure variations in dislocation densities, and normal residual strains in undoped, and indium-doped semi-insulating GaAS wafers grown by the liquid-encapsulated Czochralski technique. Low thermal gradient growth conditions, and indium doping decreased dislocation densities significantly. The distribution of dislocation densities was similar to the variation in EL2 concentrations. Normal residual strains were high in the undoped sample grown under the high thermal gradient growth conditions. The strain values were considerably lower in the undoped sample grown under the low thermal gradient growth conditions. Indium doping increased the strain slightly.

Evidence of native gallium antisite defects in semi-insulating liquid-encapsulated Czochralski-grown GaAs

We report the observation by photoluminescence of the gallium antisite double acceptor in As-rich GaAs grown by the liquid-encapsulated Czochralski technique. In the semi-insulating material both arsenic antisite and gallium antisite defects are present simultaneously which must be taken into account for any electrical compensation model.

Quenching and recovery characteristics of the EL2 defect in GaAs under monochromatic-light illumination.

Quenching of the normal state and recovery from the metastable state of the EL2 defect in semi-insulating GaAs grown by the liquid-encapsulated Czochralski technique were studied under monochromatic-light illumination in the energy range 0.7\ensuremath{\le}h\ensuremath{\nu}\ensuremath{\le}1.51 eV. The photoquenching data show two peaks at 1.125 and 1.30 eV. In addition, light with energy h\ensuremath{\nu}=1.46 eV is found to photoquench the normal state of EL2 in unannealed doped samples within 25 min. On the other hand, the photoinduced recovery data from the metastable state show a complex structure consisting of a broad band around 0.9 eV and multiple sharp peaks between 1.4 eV and the band edge. The magnitude as well as structure of recovery were found to be sample dependent. Both quenching and recovery data are in conflict with the available calculations and predictions reported for the isolated-arsenic-antisite model. The recovery peaks are coincident with the calculated arsenic vacancy (${V}_{\mathrm{As})}$ energy levels. Thus, the present results support the complex models involving ${V}_{\mathrm{As}}$. The optical recovery from the metastable state suggests that EL2 is a charge-controlled bistable defect. The sample dependence of most of the data indicates the existence of interactions between EL2 and other defects and traps.

Gettering of donor impurities by V in GaAs and the growth of semi-insulating crystals

Vanadium added to the GaAs melt getters shallow donor impurities (Si and S) and decreases their concentration in the grown crystals. This gettering is driven by chemical reactions in the melt rather than in the solid. Employing V gettering, we were able to grow reproducibly semi-insulating GaAs by horizontal Bridgman and liquid-encapsulated Czochralski techniques, although V did not introduce any midgap energy levels. The compensation mechanism in these crystals was controlled by the balance between the native midgap donor EL2 and residual shallow acceptors. Vanadium gettering contributed to the reduction of the concentration of shallow donors below the concentration of acceptors. The present findings clarify the long-standing controversy on the role of V in achieving semi-insulating GaAs.

Origin and effects of carbon in low-pressure liquid encapsulated Czochralski GaAs

We have investigated the origin and effects of carbon in semi-insulating GaAs grown by the low-pressure liquid encapsulated Czochralski (L-LEC) technique. The carbon concentration in the seed portion of boules grown from dry B2O3 was found to depend on the delay time between completion of melt compounding and solidification of the seed segment. This behavior for carbon with delay time after compounding demonstrates that the predominant source of carbon in L-LEC crystals is the initial charge material and not the puller ambient or graphitic parts. Effects of carbon on the transport properties of as-grown L-LEC are also observed and discussed. Comparisons are made between measured electrical properties and those calculated from carrier statistics and scattering theory. In addition, the effects of carbon on thermal stability and ion implantation behavior of this material is assessed.

Synchrotron section topographic study of defects in InP substrates and quaternary laser structures

Indium phosphide single crystals grown by the liquid encapsulated Czochralski (LEC) and vertical gradient freeze (VFG) techniques have been studied with X-ray diffraction topography using synchrotron radiation. Section topographs show that while the VGF substrates are of a better quality than the LEC ones, the defect level in epitaxial layers is more strongly determined by the quaternary layer composition and lattice mismatch than by threading dislocations from the substrate.

Uniformity of 3-in., semi-insulating, vertical-gradient-freeze GaAs wafers

We have evaluated the uniformity in [EL2], dislocation (or etch-pit) density (EPD), resistivity, mobility, and carrier concentration for 3-in., semi-insulating GaAs wafers grown by the vertical-gradient-freeze (VGF) technique. Although slight W or U patterns were observed in [EL2] and EPD along the 〈110〉 directions, for the first time, nevertheless the overall uniformity was excellent, and comparable to that in the best In-doped and whole-boule-annealed ingots grown by the liquid-encapsulated Czochralski (LEC) technique. Based on results from implant-activation studies on LEC wafers, it is estimated that the measured nonuniformities in EPD and [EL2] for the VGF wafers would contribute only about 1% to implant-activation-efficiency nonuniformities in Si-implanted wafers designed for field-effect transistor applications.

Observation of microdefects in indium-doped GaAs crystals by preferential etching and infrared light scattering tomography

Microdefects in indium-doped GaAs crystals grown by the liquid encapsulated Czochralski (LEC) technique were investigated by using preferential Abrahams-Buiocchi etching (AB etching) and infrared light scattering tomography. Elliptical pits were observed on AB etched (100) surfaces in dislocation-free regions at the front of the ingot, not observed at the tail. The elliptical pits are considered to correspond to microprecipitates. Furthermore, an attempt to improve the homogeneity along the growth direction, the effect of heat treatment on the density of the defects, was investigated.

Wafer-level correlations of EL2, dislocation density, and FET saturation current at various processing stages

The neutral deep-donor density [EL2]0, and dislocation density,ρD, are measured on adjacent, semi-insulating GaAs wafers, grown by both high-pressure (HP) and low-pres-sure (LP) liquid-encapsulated Czochralski (LEC) techniques; also, other nearby wafers from each boule are used for low-noise, field-effect-transistor (FET) fabrication. Dense data maps (at least 3500 points per wafer per parameter) are then visually and math-ematically compared for [EL2]0,ρD,Iu, Ir, and Ig where the latter three quantities rep-resent the unrecessed-ungated, recessed-ungated, and gated saturation currents, re-spectively, for ion-implanted, 0.5 ]smm × 300 µm FET’s. For theparticular wafers and processing used in this study, the following conclusions can be drawn: (1) onall of the wafers, materials (EL2 andρD) non-uniformities are correlated with at least some of theIu non-uniformities; (2) onsome of the wafers, materials non-uniformities follow all the way through toIg, but on others, the gate-recess step itself introduces much stronger non-uniformities; (3) the HP-LEC wafers give slightly higherIu’s than the LP-LEC waf-ers; and (4) [EL2]0 is a better predictor ofIu than isρD.

The impact of various annealing methods on silicon implant activation properties for GaAs crystals

Wafers obtained from undoped, semi-insulating GaAs crystals grown by liquid-encapsulated Czochralski and vertical gradient freeze techniques were implanted with 29Si at a dose of 3×1012 ions/cm2 and an energy of 60 keV. Activation of the implanted region was carried out by furnace or rapid thermal annealing methods, with and without the use of an SiO2 encapsulant. Profiles of the electron concentration and distribution were obtained by the capacitance-voltage method. Significant variations in the donor distributions were found to be dependent upon the annealing technique employed. The dissimilarities in implant activation properties for GaAs wafers grown by the two techniques, and subjected to identical annealing cycles, have been attributed to variations in melt stoichiometry and native point defects.

Photoquenching and photoinduced-recovery properties of the EL2 defect in GaAs: Evidence against the identification of EL2 with the isolated AsGa defect.

Photoquenching of the stable state and photoinduced recovery from the metastable configuration of the $\mathrm{EL}2$ defect in semi-insulating GaAs grown by liquid-encapsulated Czochralski technique were studied by use of the infrared absorption technique concurrent with monochromatic light irradiation in the energy range of $0.7\ensuremath{\le}h\ensuremath{\nu}\ensuremath{\le}1.5$ eV. The monochromatic light intensity was \ensuremath{\le}2 mW/${\mathrm{cm}}^{2}$. The photoquenching data show two peaks at 1.125 and 1.30 eV while the photoinduced recovery data exhibit a complex structure consisting of a broad band around 0.9 eV and a set of multiple sharp peaks between 1.44 and 1.5 eV. The present results cannot be explained in terms of the available calculations and predictions reported for the isolated arsenic antisite when it undergoes a symmetry-distortion transition.

Photoluminescence bands of deep centres in neutron-transmutation-doped GaAs

Deep centres in neutron-transmutation-doped semi-insulating GaAs grown by the liquid-encapsulated Czochralski (LEC) technique were studied using the photoluminescence technique. Two photoluminescence bands, PL1 and PL2, were observed at 1.26 and 1.0 eV from the band edge, respectively. The Ge-photoluminescence band is also observed. The Franck-Condon shift was estimated for PL1 and PL2 by utilising the temperature dependence of the linewidth and the configuration coordinate model. The PL1 atomic structure is speculated to be more complex than that of PL2. Both PL1 and PL2 are related to the impurities introduced by the neutron transmutation doping.

Existence of interstitialcy Zn atoms in GaAs:Zn grown by the liquid-encapsulated Czochralski technique

Using the extended x-ray absorption fine structure method, we have studied the local structures around Zn atoms in Zn-doped GaAs crystal grown by the liquid-encapsulated Czochralski technique. Zn atoms, whose concentration is about 2.4×1018 cm−3, have the first nearest neighbor atoms at a very short distance of about 1.95 Å and the second nearest neighbor atoms at a distance of about 3.15 Å. The Zn atom configuration forms ‘‘interstitialcy’’, not substitutional, tetrahedral, or hexagonal sites. As the Zn concentration increases, however, Zn atoms have a tendency to occupy substitutional sites.

Electron-irradiation effects on the infrared absorption properties of the EL2 defect in GaAs.

Infrared absorption measurements of electron-irradiated GaAs grown by the liquid-encapsulated Czochralski technique were used to study the electron irradiation effects on the EL2 defect. Based on the photoquenching and thermal recovery signatures, the quenchable component of the ir absorption spectra is attributed to the EL2 defect. It is found that EL2 is unaffected by 2.1-MeV electron irradiation up to the maximum dose used in the present study. It is proposed that the fluctuation of the EL2 concentration in samples exposed to different irradiation levels is primarily due to the compensation mechanism rather than the dissociation of the EL2 complex.

Growth and defects of In-doped LEC GaP crystals

In-doped GaP crystals have been grown by the liquid encapsulated Czochralski technique and characterized by preferential etching. The effective distribution coefficient of indium is 0.14. Growth cells resulting from constitutional supercooling were observed in the heavily In-doped crystals. The fraction of melt solidified at which growth cells appear is proportional to the inverse of the In concentration in the initial melt. Growth cells did not form when the In concentration in the initial melt was less than 0.7 at%. (111)B wafers etched by RC solution showed D-pits and S-pits. No DL-pits were found. The dislocation density (D-pits density) decreases with increasing indium content. A large dislocation-free area can be obtained in the heavily In-doped wafers. However, the S-pits density in In-doped wafers is about 10 5 cm -2 and is higher than that in undoped wafers.

The Double Donor Issue of the EL2 Defect in GaAs.

AbstractPhoto-induced changes in the infrared (IR) absorption spectrum of the EL2 defect in undoped and lightly alloyed unannealed GaAs materials grown by the liquid-encapsulated Czochralski technique were observed under a monochromatic light irradiation. These changes were attributed to the change of the charge states of EL2. The spectrum which is believed to be due to the charged EL2 exhibits a complex structure with two peaks at 1.07 and 1.32 eV and a shoulder at 0.94 eV. The EL2+/2+ → EL20/+ and EL20/+ → EL2+/2+ transitions were obtained by illuminating the sample with 0.7 ≤ hv ≤ 0.95 eV and 1.3 ≤ hv ≤1.5 eV, respectively. The transformation EL2+/2+ ↔ EL20/+ can be achieved in less than 10 sec and can be repeatedly switched back and forth between the two states. Fourier-transform photoconductivity (FTPC) measurements were performed on a large number of samples both semi-insulating and n-type. Two broad peaks were observed in the FTPC spectra with thresholds at 0.80 and 0.95 eV. The photoquenching and thermal recovery properties of these two peaks are identical to those of EL2. Both IR and FTPC results suggest strongly that EL2 is a double donor.

Infrared absorption of electron irradiation induced deep defects in semi-insulating GaAs

Electron irradiation induced defects in semi-insulating GaAs grown by the liquid-encapsulated Czochralski technique were studied using infrared absorption spectroscopy. A broad peak (P1) and a shoulder (P2) were observed in the infrared absorption spectra of the irradiated materials at 0.98 and 0.78 eV, respectively. The electron-phonon coupling strength as well as the Franck–Condon shift of P1 was estimated from the temperature dependence of the linewidth. The annealing kinetics between 375 and 450 °C show that the P1 defect vanishes by a long-range migration process with an enthalpy of 0.78±0.02 eV. A speculation for the atomic structure of P1 is presented.

Growth and Characterization of GaAs Single Crystals

From a commercial viewpoint, gallium arsenide (GaAs) is currently the leading member of the III-V compound family. Oriented substrates, cut and polished from single-crystal boules, form the materials foundation for a rapidly maturing technology of high speed and high frequency electronic devices and circuits. The initial thrust of GaAs applications was in high powered lasers and light-emitting diodes (LEDs) fabricated on n-type (Si-doped) GaAs wafers grown by the horizontal Bridgman technique. One of the important benefits of using GaAs is the high electron mobility compared to Si. This property has driven the development of low noise and power field-effect transistors (FETs) for microwave applications. The semi-insulating substrate requirement (>107 Ω-cm) was initially met by chromium doping. Currently, the interest is focused on MMIC (microwave monolithic integrated circuits), MIMIC (millimeter microwave ICs), analog ICs for lightwave transmitters and receivers, and digital ICs. The digital circuits have been realized with ion-implanted FETs, selectively doped heterostructure transistors (SDHTs), and heterostructure bipolar transistors (HBTs). Presently, most of the semi-insulating (SI) material processed by the industry is nominally undoped, and grown by the liquid encapsulated Czochralski (LEC) technique. The SI behavior is attained via a delicate balance of deep EL2 donors and carbon acceptors, avoiding chromium in order to eliminate the anomalous out-diffusion and type-conversion associated with this dopant.GaAs wafers up to 4 inches in diameter, with electrical properties homogenized by whole ingot annealing, are currently available from U.S. domestic and overseas suppliers. However, the overall quality is compromised by the large dislocation densities, varying 104 – 105/cm2.

Dislocation reduction in large-diameter LEC GaAs growth: I. Low gradient growth and indium doping

The effects of passive heat shielding, ambient pressure control, and indium doping on the properties of large-diameter, semi-insulating GaAs crystals grown by the liquid encapsulated Czochralski (LEC) technique have been investigated. Dislocation densities in the “ring” region of undoped crystals as low as 3700 cm -2 were achieved by low gradient growth compared to levels of over 20,000 in standard LEC material. A reduction in operating pressure from 300 to 150 psi led to further reductions to levels as low as 2800 cm -2. Effectively dislocation-free material with dislocation densities ranging from 10 to 2000 cm -2 was grown in the low gradient configuration by isovalent doping with indium at concentration levels of (6–9) × 10 19 cm -3. A new phenomenon of critical isovalent doping was identified, which establishes a critical indium concentration for minimum dislocation density. Critical doping is explained in terms of competing effects of lattice “hardening” and reduced thermal transport due to indium. It was found that background impurity and point defect concentrations remain substantially unaffected by passive shielding, although some differences between the properties of undoped and indium doped crystals were identified. Finally, from the standpoint of crystal manufacturing, it was demonstrated that wafer yield per crystal of an isovalent hardening growth process would be at least 30% lower than a comparable undoped growth process due to the limitation of constitutional supercooling.

Inclusion-like defects in InP substrates and related defects in heteroepitaxial and Zn diffused layers

InP substrates grown by the liquid encapsulated Czochralski technique often contain typical inclusion-like defects which consist of a central core from which dislocation loops are punched out. The correlation between these defects present in InP substrates and the crystal quality in InGaAsP and InGaAs epilayers and Zn diffused InP layers has been studied. It has been found that the InGaAsP layers grown by liquid phase epitaxy exhibit roughly circular large hillocks, up to 100 μm in diameter, related to the inclusion-like defects; moreover, the propagation of dislocation loops from the substrate into the epilayers has been demonstrated. As for InGaAs layers grown by the molecular beam epitaxy, defects elongated in the [110] direction, up to 50 μm in length and 100 μm in width, have been observed to be in a one-to-one correspondence with the inclusion-like defects. Finally, misfit dislocations within both InGaAs/InP and InGaAsP/InP heterostructures and cracks within Zn diffused InP layers have been found to originate preferentially at the inclusion-like defects.