Undoped Liquid Encapsulated Czochralski GaAs Research Articles

Photorefractive wave mixing in undoped liquid encapsulated Czochralski GaAs at 1.5 μm: Validation of photorefractive modeling

We present photorefractive measurements in undoped GaAs performed at 1.06 μm, 1.32 μm, and at 1.55 μm. Using concentrations of EL20/+ that we determined through optical absorption and electron paramagnetic resonance measurements in the same sample, we show that a single defect model with an electron-hole competition quantitatively explains our results of photorefractive wave mixing.

GaAs and BAs antisite defects in gallium arsenide

Samples of p-type Ga-rich non-stoichiometric undoped liquid-encapsulated Czochralski GaAs have been subjected to 2 MeV electron irradiation, carried out in stages, to cause the Fermi level to move upwards from the valence band. At each stage, infrared absorption measurements were made of the strengths of electronic transitions from levels at 78 meV and 203 meV and vibrational absorption at 601 cm-1 from 11BAs. The results are consistent with an assignment of the three signatures to 11BAs0, BAs-, and BAs2-, in agreement with previous proposals but in disagreement with other recently published results. The commonly held view that the 78/203 meV defect is due to GaAs antisites cannot therefore be considered to be established.

Temperature Dependence of Electron-Beam-Induced Current Image of Semi-Insulating Undoped Liquid-Encapsulated Czochralski GaAs

Semi-insulating undoped liquid-encapsulated Czochralski GaAs was investigated by the electron-beam-induced current method. It is found that defects which cannot be observed at room temperature become visible when the temperature is raised to about 350 K. A simple theoretical consideration on this experimental result was performed.

Lattice spacing measurements around dislocations in an undoped GaAs crystal grown by the liquid-encapsulated Czochralski method

Abstract The lattice spacing in a cellular structure of an undoped liquid-encapsulated Czochralski GaAs crystal has been measured by synchrotron X-ray topography with a (+n, −n, +n) triple-crystal system. Distinguishable lattice spacing variations were observed in dislocation-related regions. One region, neighbouring to the cell walls, shows a small lattice spacing compared with the average value. Another region, where dislocations intersect with the crystal surface inside the cell, shows a large lattice spacing. The lattice spacing variation depends on the dislocation features.

Role of Electron Traps on the Thermal Conversion and Its Suppression for Liquid-Encapsulated Czochralski GaAs Single Crystals

Concentration depth profiles of both carrier and electron traps shallower than EL2 are investigated for undoped liquid-encapsulated Czochralski GaAs single crystals after annealing in vacuum and ambient arsenic vapor pressure with As metal powders. Thermal conversion near the surface of semi-insulating GaAs annealed in vacuum at 900°C is suppressed by annealing with ambient arsenic pressure of 100 Torr. The carrier concentration depth profile of thermally converted GaAs, which is an initially semi-insulating substrate, depends not only on the concentration change of EL2 but also on that of native shallow acceptors and deep donors shallower than EL2.

Infrared absorption properties of the EL2 and the isolated AsGa defects in neutron-transmutation-doped GaAs: Generation of an EL2-like defect.

The EL2 and the isolated ${\mathrm{As}}_{\mathrm{Ga}}$ antisite defects in neutron-transmutation-doped (NTD) GaAs were studied by using the infrared (ir) absorption technique concurrent with thermal annealing. The results show that irradiation with low thermal-neutron doses partially decomposes the EL2 complex in semi-insulating (si) GaAs grown by the liquid-encapsulated Czochralski (LEC) growth technique. On the other hand, a small amount of EL2 is generated in as-grown Ga-rich undoped p-type LEC GaAs. The EL2 defect in low-dose thermal-neutron-irradiated samples (both si and p-type) was found to be stable up to 850 \ifmmode^\circ\else\textdegree\fi{}C. High neutron-irradiation doses, however, completely annihilate EL2 but generate a different EL2-like defect (DL2). The DL2 defect is observed after annealing the high-dose NTD samples for 6 min at 600 \ifmmode^\circ\else\textdegree\fi{}C. The DL2 concentration is observed to be larger than that of EL2 in as-grown LEC si GaAs by a factor of 2.3 or higher. The photoquenching and thermal recovery properties of DL2 and EL2 defects are identical. However, the DL2 defect does not exhibit the same thermal stability or the zero-phonon line of the EL2 defect. Thermal annealing kinetics shows that DL2 is composed of three point defects. The residual absorption (unquenchable component) after photoquenching the EL2 (DL2) defect is interpreted as the photoionization of the isolated ${\mathrm{As}}_{\mathrm{Ga}}$ antisite. The present results support the identification of EL2 as a complex defect and cast further doubt on the validity of the model which identifies EL2 as the isolated ${\mathrm{AS}}_{\mathrm{Ga}}$ antisite defect.

Investigation of the negative peak in photoinduced transient spectra of semi-insulating gallium arsenide

A common observation in the photoinduced transient (PITS) spectra of semi-insulating GaAs is the appearance of a negative peak, which is anomalous in that both electron and hole traps should give rise to positive peaks. In this paper, it is shown that the negative peak can be explained in terms of charge exchange with the GaAs surface and only occurs in material which displays particular types of current-voltage and current-temperature characteristics. The dependence of this peak on the processing effects of surface passivation, etching, and baking and polishing has been investigated and its sensitivity to variations in incident light intensity is demonstrated. A new variation of PITS, namely gated-PITS has been employed. This technique suppresses the negative peak in the spectrum, allowing transients corresponding to emission from EL2 to be detected in particular undoped liquid encapsulated Czochralski GaAs substrates for the first time.

Influence of boron related defects on activation of silicon implanted into undoped semi-insulating GaAs

The dependence of sheet carrier concentration of a Si-implanted layer on lattice parameters of undoped liquid encapsulated Czochralski GaAs was investigated. Crystals were grown from near-stoichiometric melts with varying boron concentrations. Sheet carrier concentration increases with an increase in lattice parameters due to the reduced boron concentration. An anomalous reduction in the lattice parameter suggests that native defects such as BGa VAs are responsible for the increase in the occupancy of implanted Si atoms on As sites.

Infrared laser scanning microscopy in transmission: A new high-resolution technique for the study of inhomogeneities in bulk GaAs

This letter describes the use of the infrared laser scanning microscope in transmission to observe inhomogeneities in bulk GaAs. The results show that the technique achieves a high lateral resolution of ≂2 μm, and is able to detect low contrast intensity fluctuations down to 0.2%. The images obtained show contrast due to both particle scattering and EL2 absorption. The method is nondestructive, is suitable for bulk specimens, and enables three-dimensional defect distributions to be determined. As examples we show results from both In-doped and undoped liquid encapsulated Czochralski GaAs specimens.

Electron traps in dislocation-free In-alloyed liquid encapsulated Czochralski GaAs and their annealing properties

Electron traps in dislocation-free In-alloyed liquid encapsulated Czochralski (LEC) GaAs have been studied by deep level transient capacitance spectroscopy. Five traps are observed with activation energies ranging from 0.26 to 0.79 eV below the conduction band. The energies are closely equal to the ones observed in undoped LEC GaAs. However, a notable difference between the In-alloyed crystal and the undoped crystal exists in effects of annealing on the deep levels. For the In-alloyed crystal, all levels except EL2(0.79 eV) are unstable under the annealing at 850 °C, while for the undoped crystal, levels EL5(0.41 eV) and EL2 remain stable.

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.

Experimental evaluation of low-frequency oscillations in undoped GaAs to probe deep level parameters

A new method for characterizing undoped, semi-insulating GaAs is presented. The low-frequency current oscillations on undoped liquid encapsulated Czochralski GaAs substrates are used to determine deep level energies, cross sections, and concentrations. A Fourier transform is used to resolve the various frequency components of the signal. The frequency is seen to vary almost four orders of magnitude in a 50-K temperature range. Plotting the temperature dependence of each frequency component in an Arrhenius plot gives activation energies and cross sections corresponding to the deep levels responsible for the oscillations. It is seen that two closely spaced levels cause the low-frequency oscillations. In addition, this technique allows the determination of trap parameters as a function of electric field at values much less than the critical field for intervalley transfer in GaAs.

Multi-Step Rapid Thermal Annealing of Si-Implanted Gaas for Microwave Discrete Devices and Monolithic Integrated Circuits Fabrication

ABSTRACTIn this paper, we report a multi-step rapid thermal annealing process for microwave discrete devices and monolithic integrated circuits fabrication. 2” diameter undoped liquid encapsulated Czochralski GaAs wafers were implanted with 29 Si+ and annealed without capping using incoherent light from high intensity halogen lamps. The annealing was carried out in multiple temperature steps to achieve optimum damage removal and dopant activation. As a result, wafers implanted with mid 1012 cm−2 dose exhibited 85–90% activation efficiency for 100kV implant and nearly 100% activation for 300 kV implant. In comparison with single-stepannealed wafers, multi-step-annealed wafers showed not only higher activation efficiency, but also more uniform activation, higher electron mobility and better device performance.

EL2 distributions in doped and undoped liquid encapsulated Czochralski GaAs

We compare the longitudinal and radial distributions of EL2 in undoped semi-insulating and intentionally doped n-type GaAs crystals grown by the liquid encapsulated Czochralski technique. Longitudinal profiles in undoped crystals are controlled by changes in melt stoichiometry as the crystal is pulled from the melt. EL2 profiles along crystals doped above about 1×1017 cm−3, on the other hand, are controlled primarily by the carrier concentration as a result of the suppression of EL2 by free electrons. Radial EL2 profiles are typically W shaped and M shaped in undoped and doped (above threshold) crystals, respectively. The origin of these radial profiles is discussed in terms of residual stress, melt stoichiometry, and the suppression of EL2 by electrons. Our results are also discussed in the light of the antisite model for EL2.

Evidence of the role of boron in undoped GaAs grown by liquid encapsulated Czochralski

We provide experimental evidence of electrical activity correlated with residual boron impurities and native point defects in undoped liquid-encapsulated Czochralski GaAs crystals. In Ga-rich samples containing ≥1017 cm−3 boron, an 0.073-eV acceptor level is observed in which the concentration increases with Ga and B content. An approximately quadratic increase in the concentration of the 0.073-eV defect acceptor is observed with increasing boron concentration, suggesting that a complex involving boron with an intrinsic defect (VAs, Gai, or GaAs ) is responsible for the observed acceptor behavior. No evidence of electrically active boron or boron complexes was found in semi-insulating GaAs pulled from stoichiometric or As-rich melts.

Infrared absorption of the 78-meV acceptor in GaAs

We observe the infrared absorption spectra associated with a residual 78-meV acceptor in undoped liquid encapsulated Czochralski GaAs. The acceptor appears to be associated with an intrinsic defect, most likely the antisite GaAs. This acceptor is the dominant level in material grown from heavily Ga-rich melts. The concentration of the level depends strongly on melt stoichiometry increasing from about 3×1015 cm−3 to 3×1016 cm−3 as the As atom fraction changes from 0.47 to 0.43.

Growth and characterization of large diameter undoped semi-insulating GaAs for direct ion implanted FET technology

The growth of large diameter, semi-insulating GaAs crystals of improved purity by Liquid Encapsulated Czochralski (LEC) pulling from pyrolytic boron nitride (PBN) crucibles and characterization of this material for direct ion implantation technology, is described. Three-inch diameter, 〈100〉-oriented GaAs crystals have been grown in a high pressure Melbourn crystal puller using 3 kg starting charges synthesized in-situ from 6/9s purity elemental gallium and arsenic. Undoped and Cr-doped LEC GaAs crystals pulled from PBN crucibles exhibit bulk resistivities in the 10 7 and 10 8 Ω cm range, respectively. High sensitivity secondary ion mass spectrometry (SIMS) demonstrates that GaAs crystals grown from PBN crucibles contain residual silicon concentrations in the mid 10 14 cm −3 range, compared to concentrations up to the 10 16 cm −3 range for growths in fused silica containers. The residual chromium content in undoped LEC grown GaAs crystals is below the SIMS detection limit for Cr (4 × 10 14 cm −3). The achievement of direct ion implanted channel layers of near-theoretical mobilities is further evidence of the improved purity of undoped, semi-insulating GaAs prepared by LEC/PBN crucible techniques. Direct implant FET channels with (1–1.5) × 10 17 cm −3 peak donor concentrations exhibit channel mobilities of 4,800–5,000 cm 2/V sec in undoped, semi-insulating GaAs substrates, compared with mobilities ranging from 3,700 to 4,500 cm 2/V sec for various Cr-doped GaAs substrates. The concentration of compensating acceptor impurities in semi-insulating GaAs/PBN substrates is estimated to be 1 × 10 16 cm −3 or less, and permits the implantation of 2 × 10 16 cm −3 channels which exhibit mobilities of 5,700 and 12,000 cm 2/V sec at 298K and 77K, respectively. Discrete power FET's which exhibit 0.7 watts/mm output and 8 dB associated gain at 8 GHz have been fabricated using these directly implanted semi-insulating GaAs substrates.

Undoped semi-insulating LEC GaAs: a model and a mechanism

Undoped semi-insulating GaAs grown by the high-pressure liquid encapsulated Czochralski (LEC) method has been produced for use in direct ion implantation in several laboratories. A clear understanding of the factors controlling impurity transport and compensation in these materials has been lacking to date. In this work, detailed characterization has been performed on undoped semi-insulating crystals grown from both SiO2 and PBN crucibles followed by a proposed impurity model and compensation mechanism.