T.T. Braggins

Critical parameters in the single‐target sputtering of YBa2Cu3O7

The critical parameters in the single‐target magnetron sputtering of YBa2Cu3O7 have been identified and sufficiently optimized to allow the reproducible deposition of films with Tc’s of ≳90 K and Jc’s of ≫ 106 A/cm2 at 77 K. It was found that during film growth the bombardment of the YBa2Cu3O7 by energetic particles must be minimized and also a stronger oxidizing agent than molecular oxygen must be present to obtain films with these properties. Otherwise, films are deposited that, by x‐ray diffraction and energy dispersive x‐ray spectroscopy analyses, are indistinguishable from the highest‐Tc 1:2:3 stoichiometric material but which have critical temperatures of ≪90 K. Films need not have 1:2:3 overall stoichiometry to have optimum superconducting properties. In such cases the excess elements are present as second‐phase particles.The critical parameters in the single‐target magnetron sputtering of YBa2Cu3O7 have been identified and sufficiently optimized to allow the reproducible deposition of films with Tc’s of ≳90 K and Jc’s of ≫ 106 A/cm2 at 77 K. It was found that during film growth the bombardment of the YBa2Cu3O7 by energetic particles must be minimized and also a stronger oxidizing agent than molecular oxygen must be present to obtain films with these properties. Otherwise, films are deposited that, by x‐ray diffraction and energy dispersive x‐ray spectroscopy analyses, are indistinguishable from the highest‐Tc 1:2:3 stoichiometric material but which have critical temperatures of ≪90 K. Films need not have 1:2:3 overall stoichiometry to have optimum superconducting properties. In such cases the excess elements are present as second‐phase particles.

Compensation of residual boron impurities in extrinsic indium-doped silicon by neutron transmutation of silicon

Infrared‐sensitive focal plane arrays based on extrinsic silicon, which integrate the detection and signal‐processing functions onto a single chip, are currently being developed at several laboratories. For imaging in the 3–5‐μm atmospheric window, highly doped Si : In is a leading candidate due to its spectral range, quantum efficiency, and moderate cooling requirements (50–60 K). The effects of residual boron impurities in the Si : In detector must, however, be compensated by donor concentrations to achieve these operational temperatures, so that precision compensation is a key factor for the production of uniform high‐responsivity detector material. We report here the successful use of thermal‐neutron irradiation for transmuting a small fraction of the silicon atoms into a known concentration of phosphorus donors in order to compensate Si : In detector material. Czochralski‐grown Si : In starting material of 〈100〉 orientation was evaluated by variable‐temperature Hall effect studies to contain NIn=2.5×...

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

Abstract 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 FETs 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.

Meeting device needs through melt growth of large-diameter elemental and compound semiconductors*

High-quality, large-diameter semiconductor wafers are required by the device engineer because of the well-known yield advantages of large-area wafer processing. Yet the growth of large semiconductor single crystals with high compositional purity, low concentrations of stoichiometric and point defects, and high crystalline perfection becomes increasingly difficult as one progresses from elemental Si and Ge, through the III-V compounds to the II-VI compounds. Limitations are imposed by the fundamental thermophysical constants of latent heat, thermal conductivity, and critical resolved shear stress and present significant challenges to the crystal grower. This presentation reviews progress made in the melt growth of these semiconductors as large-diameter crystals.

Impurities in commercial-scale magnetic czochralski silicon: Axial versus transverse magnetic fields

Abstract Magnetic Czochralski (MCZ) silicon crystals, 4 inch diameter, were grown in a HAMCO CG-2000 puller, modified to accommodate either a 5000 G superconducting axial field magnet or a 1500 G transverse field electromagnet. The effect of field strength, crystal and crucible rotation rates on the oxygen concentration and distribution are reported for both field configurations. Results for the axial case demonstrate that the oxygen concentration increases in field strength and crystal rotation rate, while crucible rotation rate has only a small effect. In the case of transverse field, it is shown that the oxygen concentration decreases with increase in field strength and decrease in crucible rotation rate. Crystal rotation rate has negligible effect. The macroscopic radial distribution of oxygen using the transverse field is very uniform under most conditions, compared to the axial field and is comparable to the zero field case. While the application of axial field causes a dramatic increase in the effective segregation coefficient of gallium, no such effects were observed (at least for the low fields used) in the segregation behavior of phosphorous in the transverse field. Probable causes for the various observed differences between the two field configurations are discussed.

High infrared responsivity Indium-doped silicon detector material compensated by neutron transmutation

The use of the neutron transmutation for producing precisely compensated, extrinsic idium-doped, silicon detector material of high infrared responsivity is reported. Highly indium-doped silicon crystals containing (1 to 3) × 1017cm-3indium concentrations and residual acceptors in the low 1012cm-3have been grown by float-zone doping. The high purity obtained by this growth technique enables very low net donor compensation densities to be achieved by neutron irradiation in a reactor. Transmuted phosphorus concentrations ranging from (1 to 20) × 1012cm-3have been investigated and compensation densities,N_{D} - N_{A}, as low as 2 × 1012cm-3have been achieved in irradiated samples after suitable damage annealing. Residual radioactivity due to transmuted indium isotopes approaches negligibly low levels for the neutron fluences required with high purity float-zone Si:In material. Significant improvements in infrared detector performance have been demonstrated with neutron compensated indium-doped silicon. Peak responsivities up to 100 A/W at 50 K and 103-V/cm detector bias have been measured, corresponding to dc photoconductive gains in the 30 to 40 range and mobility-lifetime products > 10-3cm2/V. Additional studies indicate that the detector responsivity, which is adversely affected by high-temperature CCD fabrication processes, can be restored significantly by phosphorus gettering techniques.

Growth and characterization of Indium-doped silicon for extrinsic IR detectors

Experimental growth and characterization studies of extrinsic indium-doped silicon for 3- to 5-µm focal-plane array applications have been carried out. Large, 2- and 3-in-diameter, -oriented indium-doped silicon crystals were prepared by Czochralski crystal pulling. The growth conditions affecting crystalline perfection, maximum dopant concentration and uniformity, and the residual shallow acceptor impurity content in grown crystals were investigated. In addition, effects of carbon content on the concentration of the 0.11-eV defect level in indium-doped silicon have been studied. The results demonstrate that near dislocation-free crystals containing indium concentrations up to 5 × 1017cm-3can be achieved at low growth rates to delay the onset of constitutional supercooling, while unwanted boron and aluminum impurities can be maintained at levels approaching 1 × 1013cm-3, when high purity synthetic-quartz crucibles are utilized to minimize melt contamination. Phosphorus-compensated indium-doped infrared (IR) detector performance and test photodetectors show peak responsivities up to 10 A/W (5.9 µm, 1000 V/cm, 50 K). Monolithic CCD test structures fabricated on Czochralski indium-doped silicon substrates show lower responsivities, however, due to detrimental effects of high-temperature CCD processing.

YBCO and LSCO Films Grown by Off-Axis Sputtering

Using off-axis, dc magnetron sputtering we have produced YBa2Cu3O7 (YBCO) and La1.85Sr0.15CuO4 (LSCO) films at temperatures of 650–700°C which have a critical temperature, normal-state conductivity, critical current density, rf surface resistance, c-axis growth orientation, and surface structure comparable to the best films produced by laser ablation, co-evaporation in activated oxygen, or other sputtering techniques. In contrast to the other techniques, off-axis sputtering has simultaneously produced uniform film properties across a 2-inch diameter substrate holder and used stoichiometric oxide targets. The use of stoichiometric targets permitted YBCO films to be easily integrated with other epitaxial oxide films over this relatively large area. Low-energy electron diffraction showed that the 1:2:3 structure was maintained within the length of one unit cell from the film surface for YBCO and PrBa2Cu3O7 (PrBCO) films and multilayers.

Chapter 1 High-Purity LEC Growth and Direct Implantation of GaAs for Monolithic Microwave Circuits†

Publisher Summary This chapter discusses the establishment of a reproducible gallium arsenide (GaAs) materials base to realize the full potential of direct ion implantation as a reliable, cost-effective fabrication technology of high-performance GaAs metal–semiconductor field effect transistor (MESFET) devices and integrated circuits (IC). The considerable efforts directed at improving basic GaAs materials and processes result from the strong interdependence of high-frequency GaAs circuit performance upon substrate quality. Many of the conventional wafer preparation techniques used today in silicon have been applied on a laboratory scale to large liquid-encapsulated Czochralski (LEC)-grown GaAs crystals. The low-breakage processing of GaAs demands the development of special handling techniques based on the automated cassette and wafer transport methods that are being utilized in silicon IC manufacturing.

Meeting device needs through melt growth of large-diameter elemental and compound semiconductors☆

Abstract High-quality, large-diameter semiconductor wafers are required by the device engineer because of the well-known yield advantages of large-area wafer processing. Yet the growth of large semiconductor single crystals with high compositional purity, low concentrations of stoichiometric and point defects, and high crystalline perfection becomes increasingly difficult as one progresses from elemental Si and Ge, through the III-Vs to the II-VI compounds. For example, dislocation-free Si crystals can be grown up to at least eight-inch diameter, GaAs crystals up to four-inch diameter but containing high dislocation densities, while CdTe can only be prepared as large-grain polycrystalline two-inch diameter ingots. Limitations are imposed by the fundamental thermophysical constants of latent heat, thermal conductivity, and critical resolved shear stress, which present significant challenges to the crystal grower. This presentation reviews progress made in the melt growth of these semiconductors as large-diameter crystals. How well the current needs of the device community are being met, and new potential device opportunities made possible as a result of the progress in crystal growth, will be illustrated.