C. D. Parker

Resonant tunneling through quantum wells at frequencies up to 2.5 THz

Resonant tunneling through a single quantum well of GaAs has been observed. The current singularity and negative resistance region are dramatically improved over previous results, and detecting and mixing have been carried out at frequencies as high as 2.5 THz. Resonant tunneling features are visible in the conductance‐voltage curve at room temperature and become quite pronounced in the I‐V curves at low temperature. The high‐frequency results, measured with far IR lasers, prove that the charge transport is faster than about 10− 1 3 s. It may now be possible to construct practical nonlinear devices using quantum wells at millimeter and submillimeter wavelengths.

Oscillations up to 712 GHz in InAs/AlSb resonant‐tunneling diodes

Oscillations have been obtained at frequencies from 100 to 712 GHz in InAs/AlSb double‐barrier resonant‐tunneling diodes at room temperature. The measured power density at 360 GHz was 90 W cm−2, which is 50 times that generated by GaAs/AlAs diodes at essentially the same frequency. The oscillation at 712 GHz represents the highest frequency reported to date from a solid‐state electronic oscillator at room temperature.

Oscillations up to 420 GHz in GaAs/AlAs resonant tunneling diodes

We report room‐temperature oscillations up to frequencies of 420 GHz in a GaAs resonant tunneling diode containing two 1.1‐nm‐thick AlAs barriers. These results are consistent with a recently proposed equivalent circuit model for these diodes in which an inductance accounts for the temporal delay associated with the quasibound‐state lifetime. They are also in accordance with a generalized impedance model, described here, that includes the effect of the transit time delay across the depletion layer. Although the peak‐to‐valley ratio of the 420 GHz diode is only 1.5:1 at room temperature, we show that its speed is limited by the parasitic series resistance rather than by the low negative conductance. A threefold reduction in this resistance, along with a comparable increase in the peak‐to‐valley ratio, should allow oscillations up to about 1 THz.

Far‐ir heterodyne radiometric measurements with quasioptical Schottky diode mixers

We have made heterodyne radiometric measurements with GaAs Schottky diode mixers, mounted in a corner‐reflector configuration, over the spectral range 170 μm to 1 mm. At 400 μm, system noise temperatures of 9700 K DSB (NEP=1.4×10−19 W/Hz) and mixer noise temperatures of 5900 K have been achieved. This same quasioptical mixer has also been used to generate 10−7 W of tunable radiation suitable for spectroscopic applications.

Effect of quasibound‐state lifetime on the oscillation power of resonant tunneling diodes

A new equivalent circuit is derived for the double‐barrier resonant tunneling diode. An essential feature of this circuit is the addition of an inductance in series with the differential conductance G of the device. The magnitude of the inductance is τN/G where τN is the lifetime of the (Nth) quasibound state through which all of the conduction current is assumed to flow. This circuit model is used to derive values of theoretical oscillator power that are in much better agreement with experimental results than theoretical predictions made without the inductance. The conclusion is drawn that the response of the double‐barrier structure to a time varying potential is consistent with the coherent picture of resonant tunneling.

Millimeter‐band oscillations based on resonant tunneling in a double‐barrier diode at room temperature

A double‐barrier diode at room temperature has yielded oscillations with fundamental frequencies up to 56 GHz and second harmonics up to 87 GHz. The output powers at these frequencies were about 60 and 18 μW, respectively. These results are attributed to a recent improvement in the material parameters of the device and to the integration of the device into a waveguide resonator. The most successful diode to date has thin (∼1.5 nm) AlAs barriers, a 4.5‐nm‐wide GaAs quantum well, and 2×1017 cm−3 doping concentration in the n‐GaAs outside the barriers. This particular diode is expected to oscillate at frequencies higher than those achieved by any reported p‐n tunnel diode.

Submillimeter detection and mixing using Schottky diodes

Schottky diodes have been used for the first time as harmonic mixers in the 0.1–1.0‐mm wavelength region. Beat notes between the 33rd harmonic of a 74‐GHz V‐band klystron and 118.8‐μ laser radiation are observed directly without the need of narrow‐band synchronous detection. The demonstrated performance of these room‐temperature diodes as wide‐band or heterodyne detectors of submillimeter radiation and their rugged construction make them superior to current point contact devices.

Growth and characterization of high current density, high-speed InAs/AlSb resonant tunneling diodes

High quality resonant tunneling diodes have been fabricated from the InAs/AlSb material system (InAs quantum well and cladding layers, AlSb barriers) on (100)GaAs substrates. A diode with a 6.4‐nm‐thick InAs quantum well and 1.5‐nm‐thick AlSb barriers yielded a room‐temperature peak current density of 3.7×105 A cm−2 and peak‐to‐valley current ratio of 3.2. This corresponds to an available current density of 2.6×105 A cm−2, which is comparable to that of the best In0.53Ga0.47As/AlAs diodes grown on lattice‐matched substrates and is three times higher than that of the best GaAs/AlAs diode reported to date. These results were obtained in spite of a 7.2% lattice mismatch between the InAs epilayers and the GaAs substrates, which leads to a measured threading dislocation density of roughly 109 cm−2. The experimental peak voltage and current density are in good agreement with theoretical calculations based on a stationary‐state transport model with a two‐band envelope function approximation.

Electrical, crystallographic, and optical properties of ArF laser modified diamond surfaces

Pulses of 193 nm radiation from an ArF laser with energies exceeding 0.5 J/cm2 have been shown to modify 40–60 nm thick layers of {100} and {110} oriented diamond surfaces. These layers exhibit highly anisotropic electrical and optical properties which have principal in‐plane axes along the 〈110〉 directions. The minimum resistance is (4–10)×10−4 Ω cm, and minimum in the optical transmittance and maximum in the reflectance occur when the electric field vector of the incident polarized light is aligned along the low resistance direction. Transmission electron microscopy indicates that the modified layer primarily consists of unidentified graphite‐like carbon phases embedded in diamond. The first‐order electron diffraction spots correspond to lattice spacings of 0.123, 0.305, and 0.334 nm. The modified layer is stable at 1800 °C, forms ohmic contacts to type IIb diamond, and supports epitaxial diamond growth.

FAR‐INFRARED PHOTOCONDUCTIVITY IN HIGH‐PURITY EPITAXIAL GaAs

Extrinsic far‐infrared photoconductivity has been observed at 4.2°K in high‐purity n‐type epitaxial layers of GaAs grown on Cr‐doped semi‐insulating GaAs substrates. Measurements of the responsivity and noise in the detection system at 300 Hz in a 1‐Hz bandwidth yield an NEP of 1.2 × 10−11 W at 195 μ, 1.4 × 10−12 W at 337 μ and 6 × 10−11 W at 902 μ. The time constant of the detector has been determined to be shorter than 1 μsec using a Ge avalanche modulator to chop the incident radiation. A time constant of about 5 nsec has been measured using impact impurity ionization in the GaAs.