Liquid Encapsulated Czochralski GaAs Research Articles

A Spectrum Image Cathodoluminescence Study of Dislocations in Si-Doped Liquid-Encapsulated Czochralski GaAs Crystals

Dislocations in Si-doped liquid-encapsulated Czochralski GaAs crystals have been studied by spectrum imaging cathodoluminescence. The interaction between the dislocations and the crystal matrix results in complex atmospheres, which can extend several tens of micrometers around the dislocations. The formation of these atmospheres depends on the melt stoichiometry and the doping level. Different atmospheres formed during growth and postgrowth cooling were studied. Their main characteristics and possible scenarios of formation are suggested, taking account of the characteristics of the samples in terms of [As]/[Ga] ratio and doping. Excess As defects seem to play a major role in the formation of the atmospheres.

LEC growth of semi-insulating GaAs crystals in traveling magnetic field generated in a heater–magnet module

For the first time semi-insulating (SI) GaAs single crystals were grown by the liquid encapsulated Czochralski (LEC) method using a traveling magnetic field (TMF) system generated in a heater–magnet module (HMM). The system was developed within the framework of the KRIST MAG ˜ ® project. The HMM, that generates heat and TMF simultaneously, was placed closely around the crucible inside the chamber of the industrial CI 358 puller. Before the growth experiments the induced vertical Lorentz force density F Lz was evaluated by the weight force response of a dummy. First growth experiments at various f/ ϕ ratios ( f—frequency and ϕ—phase shift) were carried out. Interface morphology and temperature fluctuation strengths were analyzed by a striation technique. Etch pit density, carbon, residual impurity and EL2° contents as well as electrical properties of the as-grown TMF–LEC GaAs crystals were measured. A preliminary correlation between crystal qualities and field frequency has been noticed.

Defects in Ga(In)NAs thin films grown by atomic H-assisted molecular beam epitaxy

The vacancy-type defects in Ga1−yInyNxAs1−x dilute nitride films grown by atomic H-assisted molecular beam epitaxy (H-MBE) were investigated. The positron annihilation measurements showed that the densities of vacancy-type defects in GaNxAs1−x (x=0%–1.3%) films grown under an optimized atomic H flux were as low as that for a liquid encapsulated Czochralski (LEC) GaAs substrate. Further, the influence of vacancy-type defects on the crystal quality and optical properties were studied by x-ray diffraction and photoluminescence (PL) measurements. The integrated PL intensity at 77K drastically decreased as N composition was increased, but we found no clear correlation between the density or volume of vacancy defects and optical properties, and the S parameters were nearly constant at a value of ∼0.516 in all Ga1−yInyNxAs1−x films grown by our H-MBE technique.

The Effect of High Temperature Annealing on Schottky Diode Characteristics of CoCr/n-LEC GaAs Contacts

The CoCr/n-GaAs Schottky contacts fabricated were annealed in the temperature range 100 to 300°C for 5 min and 350 to 650°C for 1 min, to attain reproducible and stable Schottky diodes. The thermal annealing proceeding has been seen to be useful for improving the electrical characteristics of CoCr evaporation on Liquid Encapsulated Czochralski (LEC) GaAs. With use of current-voltage (I–V) techniques, the Schottky barrier height Φb and ideality factor n range from 0.85 eV and 1.07 (for as-deposited sample) to 0.90 eV and 1.05 (for 400°C annealing) for 1 min, and 0.82 eV and 1.10 for 550°C. The contact properties of the Schottky diodes deteriorated and became nearly ohmic above 750°C for 1 min.

Comparison of deep levels spectra and electrical properties of GaAs crystals grown by vertical Bridgeman and by liquid encapsulated Czochralski methods

Conducting and semi-insulating crystals of GaAs grown by vertical Bridgeman (VB) and liquid encapsulated Czochralski (LEC) methods were studied by means of deep levels transient spectroscopy, photoinduced current transient spectroscopy, panchromatic microcathodoluminescence, optical absorption and electrical measurements. The most prominent traps observed in both types of crystals were the EL8, EL6, EL5, EL3 and EL2 electron traps. The results suggest that VB crystals are similar to LEC GaAs crystals grown from slightly Ga rich melts. The electrical properties and uniformity of VB crystals were greatly improved by the standard 950 °C annealing procedure.

Low temperature annealing effects on the performance of proton irradiated GaAs detectors

Semi-insulating, undoped, Liquid Encapsulated Czochralski (SI-U LEC) GaAs detectors have been irradiated with 24 GeV/c protons at the fluence of 5.6×10 13 p/cm 2. The detector charge collection efficiency, cce, for both electrons and holes is remarkably reduced after irradiation while the reverse current increases. The effect of annealing the detectors at temperatures ranging from 220 °C to 280 °C has been seen to reduce the reverse current and to increase the electron cce, while the recovery of the hole cce is negligible in irradiated detectors. Deep electron traps have been followed in their evolution with the heat treatment temperature by P-DLTS and C-V measurements. They recover by increasing the heat treatment temperature and this can explain the restoration observed in electron cce.

X-ray imaging using a 320/spl times/240 hybrid GaAs pixel detector

We present room temperature measurements on 200 /spl mu/m thick GaAs pixel detectors, which were hybridized to silicon readout circuits. The whole detector array contains 320/spl times/240 square shaped pixel with a pitch of 38 /spl mu/m and is based on semi-insulating liquid-encapsulated Czochralski (LEC) GaAs material. After fabricating and dicing, the detector chips were indium bump flip chip bonded to CMOS readout circuits based on charge integration and finally evaluated. This readout chip was originally designed for the readout of flip chip bonded infrared detectors, but appears to be suitable for X-ray applications as well. A bias voltage between 50 V and 100 V was sufficient to operate the detector at room temperature. The detector array did respond to X-ray radiation by an increase in current due to production of electron hole pairs by the ionization processes. Images of various objects and slit patterns were acquired by using a standard X-ray source for dental imaging. The new X-ray hybrid detector was analyzed with respect to its imaging properties. Due to the high absorption coefficient for X-rays in GaAs and the small pixel size, the sensor shows a high modulation transfer function up to the Nyquist frequency.

A study of the dislocations in Si-doped GaAs comparing diluted Sirtl light etching, electron-beam-induced current, and micro-Raman techniques

Impurity atmospheres around dislocations have been studied in n-type Si-doped liquid encapsulated Czochralski (LEC) GaAs substrates by micro-Raman spectroscopy, diluted Sirtl-like etching with light (DSL) method, and electron-beam-induced current (EBIC). A complete morphological study of the recombinative atmospheres revealed by photoetching was achieved by phase stepping microscopy (PSM), which is an optical interferometry technique allowing to obtain the surface topography with a high vertical resolution (in the nanometer range). The minority carrier diffusion length was measured by EBIC at different points of the atmospheres. Structural distortion at the regions surrounding the dislocation core were observed by micro-Raman spectroscopy. The carrier depletion depth and the recombination of the photogenerated carriers were also studied by Raman spectroscopy, obtaining a good agreement with the EBIC data and the photoetching rates. Impurity gettering and diffusion and defect reactions involving As interstitials are assumed to play a major role in the formation of the recombinative atmospheres.

Thermal stability of NiTi alloy contacts on n-type liquid encapsulated Czochralski GaAs

The electrical characteristics of NiTi/n-GaAs Schottky barrier diodes annealed at temperatures from 100 to 300°C for 5 min and from 350 to 650°C for 1 min have been investigated as a function of annealing temperature, with the use of current–voltage ( I– V) techniques. The Schottky barrier height Φ b and ideality factor n range from 0.57 eV and 1.06 for as-deposited sample to 0.84 eV and 1.06 for 450°C annealing for 1 min, and 0.80 eV and 1.10 for 500°C. The barrier height for these contacts increases with increasing annealing temperature. The thermal annealing proceeding has been seen to be useful for improving the electrical characteristics of NiTi evaporation on Liquid Encapsulated Czochralski (LEC) GaAs. This can be ascribed to thermal stability of the NiTi alloy/n-GaAs SBDs. The contact properties of the Schottky diodes deteriorated and became nearly ohmic above 650°C for 1 min.

Influence of defect inhomogeneities on the Hall mobility and concentration in undoped GaAs

We report the investigation of the steady-state Hall mobility and carrier concentration behavior depending on extrinsic light excitation intensity, wavelength and temperature in n-type liquid-encapsulated Czochralski (LEC) GaAs crystals. The carrier concentration of the samples measured in the dark ranged from 108 to 3×1011 cm−3, and the Hall mobility showed a pronounced minimum in the concentration region 109–1011 cm−3. The measured dependencies demonstrated peculiarities in this region which could not be explained by a homogeneous semiconductor model. We demonstrate that the observed effects are caused by the complex influence of the deep levels in the band gap and of the defect inhomogeneities related to the cellular structure of dislocations. The effect of inhomogeneities is intensified in the samples from the intermediate concentration region 109–1011 cm−3. It is related to the specific compensation conditions, which take place when the Fermi level appears between the EL2 level and the negative-U double oxygen-related donor level EC−0.43 eV. The influence of nonuniformities can be modified significantly in a nonunique way by changing the excitation conditions. The numerical modeling according to the effective medium theory qualitatively confirms the influence of the mesoscopic inhomogeneities with different effective activation energies associated with the cellular distribution of the EL2 defect and oxygen-related donor EL3. Different activation energies of the cells and their walls may sometimes cause the intensified effect of inhomogeneities with increasing temperature. Nevertheless, the experimental results imply that the observed phenomena are actually more complicated and the appearance of the additional scattering associated with the change of defect structure most probably should be taken into account.

The effects of radiation on gallium arsenide radiation detectors

Semi-insulating, undoped, Liquid Encapsulated Czochralski (SI-U LEC) GaAs detectors have been irradiated with 1 MeV neutrons, 24 GeV/ c protons, and 300 MeV/ c pions. The maximum fluences used were 6 × 10 14, 3 × 10 14, and 1.8 × 10 14 particles/cm 2, respectively. For all three types of irradiation, the charge collection efficiencies (cce) of the detector are reduced due to the reduction in the electron and hole mean free paths. Pion and proton irradiations produce a greater reduction in cce than neutron irradiation, with the pions having the greatest effect. The effect of annealing the detectors at room temperature, at 200°C and at 450°C with a flash lamp have been shown to reduce the leakage current and increase the cce of the irradiated detectors. The flash-lamp anneal produced the greatest increase in the cce from 26% to 70% by increasing the mean free path of the electrons. Two indium-doped samples were irradiated with 24 GeV/ c protons and demonstrated no improvement over SI-U GaAs with respect to post-irradiation cce.

Enhancement of the Hall mobility in undoped GaAs with low carrier concentration by light excitation

We report the investigation of the steady-state Hall mobility behavior upon extrinsic light excitation in n-type liquid-encapsulated Czochralski GaAs crystals. The carrier concentration of the samples measured in the dark ranged from 108 to 3×1011 cm−3. The Hall mobility demonstrated a pronounced minimum in the concentration region 109–1011 cm−3. In samples of this region a significant increase of the Hall mobility from ⩾1460 to 6300–7800 cm2 V s could be induced by light, in some cases without an increase of the measured carrier concentration. Such behavior was explained by reduction of the mesoscopic nonuniformities related to the cellular structure of dislocations by the carriers generated from defect levels in the band gap.

Microanalytical characterization of inclusions in Cr-doped LEC GaAs

Inclusions of different morphologies were observed by Infrared (IR) microscopy and Scanning Electron Microscopy (SEM) in the last-to-freeze region of a Cr-doped GaAs single crystal grown by the Liquid Encapsulated Czochralski (LEC) technique.

Heat-Treatment Study of Deep-Level Defects in Semi-Insulating Liquid-Encapsulated Czochralski Gallium Arsenide Substrates

Deep-level defects in semi-insulating (S.I.) liquid-encapsulated Czochralski (LEC) GaAs substrates were studied using the thermally stimulated current (TSC) technique. From the heat-treatment temperature dependence of the TSC signal intensity and electrical properties, the thermal behavior of the detected defects with respect to the heat treatment and their correlation to the electrical properties of the substrate were clarified for the first time. The change in the resistivity was closely related to the changes in the TSC signal intensity of T3 (trap depth: 0.31 eV), T6 (0.58 eV) and T x (0.29 eV). The net concentration of these defects changed by 4×1014 cm-3 after heat treatment at 800°C, leading to a threefold increase of the resistivity. Precise thermal control of substrates is very important for consistently obtaining high-quality GaAs substrates.

Melt-solid interface shape in LEC GaAs crystals: comparison between calculated and experimentally observed shapes

Two GaAs crystals have been grown by the liquid-encapsulated Czochralski (LEC) technique under different experimental conditions with the aim of revealing the melt-solid interface during growth. The interface shapes have been evidenced through revealing the growth striations in longitudinal cross sections of the ingot, making use of a photoactivated chemical etching. Measurements of the curvature radii of the various striations have been made by means of a Nomarski-equipped metallographic microscope. To investigate the role of the different growth parameters on the interface shape, the growth processes have been simulated through a model based upon the thermal capillary theory, which is able to describe the temperature field and the interface shapes within the LEC system. The calculated interface shapes have been satisfactorily compared with the experimental ones for undoped crystals. The role of the most significant growth parameters on the interface shape, such as the heater temperature, the crucible-bottom insulation, the encapsulant volume and the pull rate, have been finally investigated through a sensitivity analysis.

Characterization of lattice spacing and inclination around cells in liquid-encapsulated Czochralski GaAs crystals using a 100×100 μm2 incident x-ray beam

Lattice spacing and inclination around cells were investigated precisely for both undoped high etch-pit-densities (EPD) liquid-encapsulated Czochralski (LEC) GaAs crystal of the order of 104 cm−2 and undoped As-ambient LEC (As-LEC) crystal of low EPD of the order of 103 cm−2. Measurement of the lattice spacing and the inclination was carried out by the triple crystal method using a 100×100 μm2 size incident beam of high-intensity synchrotron radiation. It was revealed that the lattice spacing varied by almost Δd, on the order of 10−4 Å, due to the cellular structure of the high EPD crystal. In addition, the low EPD GaAs crystal grown by the As-LEC method showed considerably less variation in lattice spacing and inclination than the high EPD LEC GaAs crystal.

Photo-induced current spectroscopy study on semi-insulating LEC GaAs

Semi-insulating GaAs grown by the liquid-encapsulated Czochralski (LEC) method was investigated by means of photo-induced current transient spectroscopy (PICTS). Surface and bulk contributions in the PICTS signal are compared by studying sandwich and planar samples. It is shown that, when using PICTS on materials with a depletion region, it is possible to identify the nature of traps. The used criterion is discussed. In addition the effects of the excitation light wavelength upon positive and negative bias PICTS spectra are reported.

Series resistance calculation for the Metal-Insulator-Semiconductor Schottky barrier diodes

An accurate way of determining the series resistance Rs of Schottky Barrier Diodes (SBDs) with and without the interfacial oxide layer using forward current-voltage (I–V) characteristics is discussed both theoretically and experimentally by taking into account the applied voltage drop across the interfacial layerVi. For the experimental discussion, the forward biasI–V characteristics of the SBDs with and without the oxide layer fabricated by LEC (the Liquid-Encapsulated Czochralski) GaAs were performed. The SBD without the oxide layer was fabricated to confirm a novel calculation method. For the theoretical discussion, an expression ofVi was obtained by considering effects of the layer thickness and the interface state density parameters on forward biasI–V of the SBDs. The valueRs of the SBD with interfacial oxide layer was seen to be larger than that of the SBD without the interfacial oxide layer due to contribution of this layer to the series resistance. According to the obtained theoretical formula, the value ofVi for the SBD with the oxide layer was calculated and it was subtracted from the applied voltage values V and then the value ofRs was recalculated. Thus, it has been shown that this new value ofRs is in much closer agreement with that determined for the SBD without the oxide layer as predicted. Furthermore, the curves of the interface states energy distribution of each sample are determined. It was concluded that the shape of the density distribution curve and order of magnitude of the density of the interface states in the considered energy range are in close agreement with those obtained by others for Au/n-GaAs Schottky diodes by Schottky capacitance spectroscopy.

Effect of crystal shape on etch-pit-density distribution in undoped low dislocation density GaAs crystals grown by the As-ambient liquid encapsulated Czochralski technique

The dislocation density for conventional liquid encapsulated Czochralski (LEC) GaAs crystals such as those of 3–4 inch in diameter is too high to clarify the dislocation generating conditions. Therefore, growing almost dislocation free crystals and clarifying the dislocation generating conditions individually is necessary before the crystal size is enlarged. To reduce the dislocation density, crystals were grown by the As-ambient LEC (As-LEC) technique under low temperature gradient along the pulling direction. An etch pit density (EPD) at the center of the 20 mm diameter crystal of the order of 10 3 cm −2 was obtained. The EPD distribution and crystal shape in the pulling direction correlate for low EPD crystals and show that crystal shape control is necessary for obtaining lower EPD crystals. The EPD distribution perpendicular to the pulling axis, however, exhibited the so-called “W”-shape. Even though the temperature gradient along the pulling axis was reduced, dislocations which were considered to be generated by excessive radial thermal gradient, remained in the crystals.

Modeling and identification of the liquid encapsulated Czochralski GaAs process for control

This paper develops an analytic model of the macroscopic dynamics of the liquid encapsulated Czochralski (LEC) GaAs growth process together with a series of system identification experiments that confirm the validity of the model for control system design and analysis. The model combines the meniscus dynamics with a low-order model of the thermal dynamics of the process. the effective heat transfer coefficients used in the model are determined from a steady-state finite-element model of the process. The identification experiments confirm that the model gives accurate predictions of the dominant process dynamics and high frequency phase characteristics. Although the model gain at high frequencies is less accurate, the error can be identified and accommodated in the control system design and analysis process.