M. I. Flik

Infrared refractive indices of LaAlO 3 , LaGaO 3 , and NdGaO 3

We discuss the experimental and theoretical studies of the infrared optical constants of three perovskite materials: LaAlO3, LaGaO3, and NdGaO3. These materials are commonly used as substrates for depositing high-temperature superconducting films. The transmittance and the reflectance are measured with a Fourier-transform infrared spectrometer. At frequencies from 1000 to 10 000 cm−1, for which the substrates are transparent, the refractive index is extracted directly from the measured transmittance and reflectance. Kramers–Kronig relations are employed to yield the refractive index of these materials at frequencies from 100 to 1000 cm−1. We also use a Lorentz phonon oscillator model to obtain the refractive index by fitting the measured data to the model function. The advantages and limitations of each method are discussed. The refractive index for the LaAlO3 substrate is also obtained at 10 and 78 K with a cryogenic accessory. The results are compared with those reported by other research groups. This research provides information necessary for the quantitative study of the optical properties of thin superconducting films.

Predicted absorptance of YBa/sub 2/Cu/sub 3/O/sub 7//YSZ/Si multilayer structures for infrared detectors

High-T/sub c/ superconducting YBa/sub 2/Cu/sub 3/O/sub 7/ films deposited on silicon substrates with yttria-stabilized-zirconia (YSZ) buffer layers have significant potential for radiation detectors. The infrared absorptance of a YBa/sub 2/Cu/sub 3/O/sub 7//YSZ/Si multilayer structure, which strongly influences the detector performance and depends on the layer thicknesses, is analyzed here. The electromagnetic field inside the layers is determined based on the Maxwell equations and the optical constants of these materials. The film thicknesses yielding maximum absorptance are obtained for different wavelengths. The spatial distribution of power flux is determined by integrating the Poynting vector over a finite spectral region. The analytical method presented in this study is recommended for the radiative design of high-T/sub c/ superconducting detectors based on absorbing multilayer structures.<<ETX>>

Thermal isolation of high-temperature superconducting thin films using silicon wafer bonding and micromachining

Using a new micromachining technology, thermally isolated thin films of high-temperature superconductor have been microfabricated. The intended application for these structures is in infrared bolometers. A silicon wafer bonding process produces a low thermal mass island of single-crystal silicon on a silicon nitride membrane which provides thermal isolation. The silicon can act as a seed for the epitaxial growth of YBa/sub 2/Cu/sub 3/O/sub 7/ on a yttria-stabilized zirconia buffer layer. This paper describes the overall concept of the thermally isolated device, and demonstrates that the micromachined structure can be fabricated with high-quality superconducting films. >

Optimized design of far-infrared Fabry-Perot resonators fabricated from YBa/sub 2/Cu/sub 3/O/sub 7/

The transmittance of a carefully designed YBa/sub 2/Cu/sub 3/O/sub 7/ Fabry-Perot resonator is predicted analytically. The substrate thickness is shown to strongly influence the transmittance of the YBa/sub 2/Cu/sub 3/O/sub 7/-on-MgO Fabry-Perot resonator. The peak transmittance and finesse of YBa/sub 2/Cu/sub 3/O/sub 7/-on-MgO Fabry-Perot resonators fabricated from the best YBa/sub 2/Cu/sub 3/O/sub 7/ material available are comparable with those of the best metal mesh resonators in the far-infrared spectral region. It is noted that a reduction of the fraction of residual normal-state electrons in YBa/sub 2/Cu/sub 3/O/sub 7/ films will permit the construction of far-infrared Fabry-Perot resonators superior to any existing device.<<ETX>>

Radiative substrate heating for high-Tc superconducting thin-film deposition : film-growth-induced temperature variation

We have examined the problem of substrate temperature changes during the growth of YBa2Cu3O7−x (YBCO) thin films on LaAlO3 when the substrate is radiatively heated. Substrate temperature variations sufficient to degrade film quality occur unless the heater temperature is controlled during growth. At a heater temperature of 900 °C, the temperature of a LaAlO3 substrate was measured to be only 540 °C, while a similar substrate with a 2000‐A YBCO film reached 640 °C. The experimental data and calculations indicate that the heater temperature must be as high as 1240 °C to heat LaAlO3 to 730 °C, and must be decreased by over 200 °C during the first 500 A of film deposition to maintain a constant substrate temperature during film growth. This study shows the need for an in situ noncontact substrate temperature measurement technique.

Epifilm thickness measurements using Fourier transform infrared spectroscopy: Effect of refractive index dispersion and refractive index measurement

Procedures and results of refractive index measurements using a Fourier transform infrared spectrometer are reported. These measurements were performed on both lightly and heavily doped silicon samples over the midinfrared (2.5–25 μm) spectrum region. A strong dependence of refractive index as a function of substrate dopant concentration was observed. Moreover, it was observed that the refractive index of heavily doped silicon also varies significantly with wavelength. Furthermore, it was also observed that the refractive index of silicon decreases with increasing wafer temperature for long wavelengths. Finally, the effect of refractive index dispersion on epifilm thickness measurement was examined. The results suggest that the spectral dispersion of the refractive index cannot be neglected for epifilm thickness measurements.

In situ substrate temperature measurement in high‐Tc superconducting film deposition

An adaptively calibrated pyrometer (ACP) is employed to measure the substrate surface temperature during the YBa2Cu3O7−δ film growth process. The film‐thickness‐dependent emittance of the film‐substrate composite is determined from thin‐film optics for instantaneous calibration of the pyrometer. The optical constants of the LaAlO3 substrate and the YBa2Cu3O7−δ film are measured at high temperatures using a Fourier transform infrared spectrometer with a special sample holder that controls the sample temperature and oxygen pressure. The accuracy and precision of the ACP are demonstrated through both in situ and ex situ measurements. The present study facilitates exact and reproducible temperature control in high‐Tc superconducting film growth processes.

Substrate temperature measurement for growth of large-area high-T/sub c/ superconducting films

The substrate temperature has been measured and controlled by an adaptively calibrated pyrometer during the YBa/sub 2/Cu/sub 3/O/sub 7/ film deposition process. The experimental results show that a feedback control system based on this pyrometer can maintain the substrate temperature accurately at a set value. Thermal problems associated with the fabrication of multilayer structures are discussed. The study demonstrates accurate substrate temperature measurement and control for large-area film growth, facilitating the fabrication of high-quality superconducting electronic devices.<<ETX>>

Photometric accuracy of FTIR spectrometers

High photometric accuracy is essential for precise measurements of optical properties. The effect of source radiation power on the photometric accuracy is a mechanism of growing importance, since modern FTIR spectrometers possess strong sources for high signal-to-noise ratio. A related problem is radiation incident on the detector from a high-temperature sample. This paper presents an analysis of the photometric error in the reflectance measurements for samples at high temperatures. The maximum temperature that can be investigated as a function of the required photometric accuracy is determined for several dielectric crystals and film- substrate composites.