W. D. Goodhue

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.

Quantum well oscillators

Oscillations have been observed for the first time from double barrier resonant tunneling structures. By eliminating impurities from the wells, we have been able to increase the tunneling current density by a factor of nearly 100. With the attendant increase in gain and improved impedance match to the resonant circuit, the devices oscillated readily in the negative resistance region. Oscillator output power of 5 μW and frequencies up to 18 GHz have been achieved with a dc to rf efficiency of 2.4% at temperatures as high as 200 K. It is shown that higher frequencies and higher powers can be expected.

Fundamental oscillations up to 200 GHz in resonant tunneling diodes and new estimates of their maximum oscillation frequency from stationary‐state tunneling theory

Fundamental oscillations have been measured up to 200 GHz in resonant‐tunneling diodes at room temperature. Oscillations in the range 102–112 GHz were achieved with diodes mounted in a WR‐6 waveguide resonator, and the peak output power in this range was approximately 5 μW. The same diodes oscillated between 192 and 201 GHz and generated about 0.2 μW when mounted in a WR‐3 resonator. The estimated maximum oscillation frequency ( fmax) for these devices is 244 GHz, assuming the average drift velocity across the depletion layer to be 4×107 cm s−1. This estimate has been obtained from a new phenomenological theory of the negative differential conductance which accounts for the frequency‐dependent spreading resistance and transit‐time delay. The theory is also used to show that diodes having fmax exceeding 600 GHz are feasible simply by modifying the doping profile in the regions on either side of the double‐barrier structure.

Observation of millimeter‐wave oscillations from resonant tunneling diodes and some theoretical considerations of ultimate frequency limits

Recent observations of oscillation frequencies up to 56 GHz in resonant tunneling structures are discussed in relation to calculations by several authors of the ultimate frequency limits of these devices. We find that calculations relying on the Wentzel–Kramers–Brillouin (WKB) approximation give limits well below the observed oscillation frequencies. Two other techniques for calculating the upper frequency limit were found to give more reasonable results. In one method we use the solution of the time‐dependent Schrodinger equation obtained by Kundrotas and Dargys [Phys. Status Solidi B 134, 267 (1986)], while in the other we use the energy width of the transmission function for electrons through the double‐barrier structure. This last technique is believed to be the most accurate since it is based on general results for the lifetime of any resonant state. It gives frequency limits on the order of 1 THz for two recently fabricated structures. It appears that the primary limitation of the oscillation freque...

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.

Picosecond switching time measurement of a resonant tunneling diode

Picosecond bistable operation has been experimentally observed for the first time in a double‐barrier resonant tunneling diode. A rise time of 2 ps was measured using the electro‐optic sampling technique; this is the fastest switching event yet observed for an electronic device. This time domain measurement adds necessary information to the understanding of the transport mechanisms in the resonant tunneling diode and is consistent with switching time limitations computed for the device. It also demonstrates that appropriately designed double‐barrier quantum well diodes have a response time comparable to that of the fastest all‐optical logic elements, and that they may be very useful in high‐speed logic applications.

Large room‐temperature effects from resonant tunneling through AlAs barriers

At room temperature, we have observed negative differential resistance in AlAs double‐barrier structures and a large hysteresis in the current‐voltage characteristic of a stack of five AlAs double‐barrier structures. The peak‐to‐valley ratio of the current was as high as 3.5:1 in a double‐barrier structure. To the best of our knowledge, this is the largest room‐temperature peak‐to‐valley ratio observed to date in a double‐barrier structure and the first report of a room‐temperature hysteresis in a stacked structure. These structures were grown by molecular beam epitaxy using thin AlAs barriers in GaAs. Both the first and second resonances were observed, and are well explained by simple tunneling theory assuming a value of 1.0±0.1 eV for the GaAs‐AlAs conduction‐band discontinuity seen by the tunneling electrons. This value is very close to the difference in conduction‐band energy at the Γ points found by using the accepted values of GaAs and AlAs band gaps with 65% of the band‐gap difference appearing in ...

Stark effect in AlxGa1−xAs/GaAs coupled quantum wells

Optical spectra near the band edges of AlxGa1−xAs/GaAs coupled quantum well structures are found to exhibit rich structure. Under the Stark perturbation, these transitions have behavior remarkably different from those associated with single quantum wells. Positive energy shifts and high sensitivity to electric fields have been observed and interpreted as evidence of well coupling. Results of a simple numerical calculation support this interpretation.

Observation of electric field gradients near field‐emission cathode arrays

The variation of electric field gradient above arrays of field emission cathodes has been investigated using atomic force microscopy. The spatial distribution of electric field gradient was obtained as a function of bias and height. Results show a parabolic relationship between the sample bias and electric field gradient. Furthermore, the height dependence of the field gradient is found to follow a power law relationship. These new results demonstrate that force‐gradient atomic force microscopy is capable of providing a direct visual presentation of the variation of field gradients above submicron‐periodicity field emitter arrays.

Monolithic GaAs/AlGaAs diode laser/deflector devices for light emission normal to the surface

Light emission normal to the surface of a GaAs/AlGaAs wafer has been obtained by fabricating edge‐emitting double‐heterostructure diode lasers with a monolithic 45° deflector adjacent to one of the laser facets. The deflector and adjacent facet were formed by ion beam assisted etching, while the other facet was cleaved. Diode laser/deflector devices with two etched laser facets could be used to fabricate monolithic two‐dimensional laser arrays.