Christopher M. Thrush

Thermopower enhancement in PbTe with Pb precipitates

The thermoelectric power of polycrystalline PbTe samples containing nanometer-sized precipitates of Pb metal is enhanced over that of bulk PbTe. Samples of PbTe containing excess Pb and Ag were prepared using conventional metallurgical heat treatments. These samples are shown, by x-ray diffraction, by microscopy, and by the presence of a superconductive transition, to contain Pb precipitates with sizes on the order of 30–40nm. The thermopower enhancement is related to an increase in the energy dependence of the relaxation time, as evidenced by a complete set of measurements of thermoelectric and thermomagnetic transport coefficients.

Narrow-gap semiconductor magnetic-field sensors and applications

Narrow-gap semiconductors have been used for decades in the fabrication of magnetic field sensors, such as magnetoresistors and Hall sensors. Magnetic field sensors are, in turn, used in conjunction with permanent magnets to make contactless potentiometers and rotary encoders. This sensing technology offers the most reliable way to convert a mechanical movement into an electrical signal, and is widespread in automotive applications. Recent developments in the growth of thin epitaxial layers of InAs and InSb on semiinsulating GaAs or InP substrates have resulted in the development of magnetoresistors with excellent sensitivity and operating temperatures up to 285 degrees C. Magnetoresistors and Hall sensors require a very thin active semiconductor region, a high carrier density and a high room-temperature mobility. The best materials are narrow-gap III-V compounds. 2DEG layers in InSb and InAs would be ideally suited for these devices. The accumulation layer at the surface of InAs has been used to make magnetoresistors, Hall sensors and magnetotransistors. n-type doped thin InSb films are used to make magnetoresistors that outperform Si-based Hall sensors, even with integrated amplification. The authors describe device design criteria, materials requirements and a direct comparison of the three types of galvanomagnetic devices, magnetoresistors, Hall sensors and magnetotransistors, made from the same material. They compare the output of different magnetic field sensing technologies, such as Si and GaAs Hall sensors, and NiFe-based magnetoresistors, with InSb magnetoresistors.

Wavelength coverage of lead‐europium‐selenide‐telluride diode lasers

Diode lasers made from a new semiconductor material, Pb1−xEuxSeyTe1−y, have recently been developed for sensor applications. This material is grown by molecular beam epitaxy and may be lattice matched to PbTe substrates. Double heterojunction mesa stripe diode lasers have been fabricated and the laser emission energy determined as a function of composition. At 80 K, the emission energy follows the relation E (in eV)=0.219+5.51x. With the range of active region compositions so far explored (up to x=0.046, y=0.054), the wavelength range 6.6–2.6 μm can be covered under pulsed conditions (6.6–3.8 μm cw). Laser operation up to 190 K pulsed, 147 K cw, has been attained with up to 1‐mW single mode output power. These devices are useful for spectroscopic measurements of molecules in a gas or adsorbed onto surfaces.

Temperature stable Hall effect sensors

Magnetic field sensors are needed for high-accuracy position, angle, force, strain, torque, and current flow measurements. Molecular beam epitaxy was used to grow tellurium-doped indium-gallium antimonide thin films. Hall effect sensors made from these films have been studied for their magnetic sensitivity and thermal stability. For a range of alloy composition near In/sub 0.8/Ga/sub 0.2/Sb and n-type doping levels near 2/spl times/10/sup 17/ cm/sup -3/, high magnetic sensitivity from -40/spl deg/C to +200/spl deg/C was found with a resolution of better than /spl plusmn/0.5% over the entire temperature range.

Two‐dimensional electron gas magnetic field sensors

We describe the use of accumulation layers of electron charge in applications as magnetoresistive devices. We consider two such systems: an InGaAs/InP heterostructure in which we identify a two‐dimensional electron gas from the observation of the quantum Hall effect, and InAs films, in which a strong surface accumulation of charge is inferred from depth profiling studies of the galvanomagnetic coefficients. Magnetoresistive devices fabricated from these materials exhibit outstanding field sensitivity and temperature stability due to the existence of electrons of relatively high density and mobility in the accumulation regions. We also model the magnetosensitivity of our devices.

Growth and characterization of indium antimonide doped with lead telluride

A PbTe dopant source has been used to grow n‐type InSb using the molecular beam epitaxy growth technique. From Auger electron spectroscopy studies, no surface segregation of tellurium is observed up to ∼1019 cm−3 doping levels. The correlation between the PbTe flux used during growth and the electron density in the grown films is very good, suggesting that the incorporation of tellurium is near unity. From secondary‐ion mass spectroscopy (SIMS) studies, no lead could be detected in the films, even when grown at temperatures as low as 280 °C. This suggests that the lead rapidly re‐evaporates from the surface during growth. SIMS depth profiles of tellurium show evidence of solid‐state diffusion at 360 °C with a diffusion coefficient ∼10−13 cm2 s−1. Six‐probe Hall measurements of carrier transport gave room‐temperature mobilities as high as 51 300 cm2 V−1 s−1 at an electron density of 2.9×1016 cm−3 (54 300 at an electron density of 1.9×1016 cm−3 at 110 K) for a film of 4.0‐μm thickness on an InP substrate. O...

Nonlinear optical properties of molecular beam epitaxy grown Bi1-xSbx

We discuss the first investigation of Bi1−xSbx as an infrared nonlinear optical material. Nondegenerate four‐wave mixing experiments at CO2 laser wavelengths yield a large nonlinearity (χ(3)≊3×10−4 esu) which does not saturate at power densities up to 0.5 MW/cm2. Both the ambient and substrate interfaces of the film are highly reflective and the etalon they form is found to have a large effect on the transmission and reflectivity spectra of the as‐grown films. This suggests the possibility that constructive interference of the film’s internal optical fields could be used to considerably enhance the nonlinear signal.

Side optical cavity, single quantum well diode laser

Abstract Large optical cavity single quantum well PbEuSeTe diode lasers recently attained the highest temperatures yet observed to our knowledge for long wavelength (g > 3)m) diode lasers. Current studies of PbTe/PbEuSeTe quantum wells by transport and luminescence techniques suggest that most of the band edge offset at a PbTe/PbEuSeTe heterojunction is in the valence band. Thus a 3odified, side optical cavity single quantum well structure was grown in which the PbTe quantum well active region was placed at one side of the large optical cavity to maximize the potential barrier and, therefore, limit the electron leakage out of the quantum well. This resulted in the lowest threshold currents we have yet observed in these devices — 10 mA at 80 K and 120 mA (1.6 KA/cm2) at 160 K under CW conditions. This device lased at up to 175 K CW (g = 4.47)m), which is the highest CW lasing temperature that we have observed so far. These results support the finding that the conduction band offset is relatively small in PbTe/PbEuSeTe heterojunctions.

Magnetotransport and magneto‐optical properties of δ‐doped InSb

Results of Shubnikov–de Haas (SdH), cyclotron resonance (CR), and Hall‐effect measurements on δ‐doped InSb:Si films grown by molecular‐beam epitaxy on insulating InP substrates are reported. The investigation covers samples with sheet densities of Si dopant atoms ranging from 1×1011 to 1×1013 cm−2, temperatures from 4.2 to 300 K, and fields from 0 to 7 T. The SdH oscillations show that the samples contain electrons of two‐dimensional nature, occupying multiple subbands. The effective masses obtained from the CR data correspond well to the subband occupation densities. The Hall measurements as well as the CR experiments also give evidence for the presence of additional electrons, with the conduction‐band‐edge mass m*=0.014m0 of bulk InSb, which exist presumably in the bulk of the films.

Stripe geometry lead‐telluride diode lasers grown by molecular beam epitaxy

Lead‐telluride homojunction diode lasers suitable for ultrahigh resolution spectroscopy have been fabricated using molecular‐beam epitaxial (MBE) growth and a mesa stripe geometry. A low‐MBE substrate growth temperature of 260 °C was used to minimize interdiffusion during growth. These lasers operate up to 115 K cw with emission in the 6.5–5.0‐μm wavelength range. The threshold current density follows the relation J=J0 exp(T/T0) up to 100 K, with J0=110 A/cm2 and T0=26.0 K. Thermal resistance effects cause a departure from this relationship at higher temperatures. The current‐voltage relationship as a function of temperature indicates that tunneling currents contribute significantly to the threshold current density. Single‐fundamental mode emission is obtained from 20‐μm‐wide mesa stripes at high temperature, whereas a higher order mode is observed with 35‐μm‐wide stripes. Analysis of the near and far field patterns indicates that the lasing mode is index guided at high temperatures, and gain guided at lo...