K. M. Molvar

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.

GaN avalanche photodiodes operating in linear-gain mode and Geiger mode

For solar-blind ultraviolet detection, AlGaN avalanche photodiodes (APDs) that operate in Geiger mode can outperform conventional AlGaN photodiodes in sensitivity and should compare favorably to photomultiplier tubes. Toward this goal, we report GaN APDs that operate in the linear-gain mode and in the Geiger mode. The APDs were fabricated from high-quality GaN epitaxial layers grown by hydride vapor phase epitaxy. The GaN layer structure consisted of a Zn-doped /spl pi/ layer, an unintentionally doped n layer, and a Si-doped n+ layer-all on top of a thick GaN unintentionally doped n layer on a sapphire substrate. Capacitance-voltage (C-V) measurements on photodiodes fabricated from some of these layers show that field strengths between 3 and 4 MV/cm are sustainable in the depletion region at voltages slightly below the observed breakdown of /spl sim/80 V. Both mesa-etched and planar devices exhibited avalanche gains of 10 in linear-gain mode and /spl sim/10/sup 6/ in Geiger mode when top illuminated with a 325 nm HeCd laser. Raster measurements of the photoresponse show highly uniform response in gain mode that becomes slightly more inhomogeneous in Geiger mode.

InGaAsP/InP avalanche photodiodes for photon counting at 1.06 μm

Geiger-mode (photon-counting) operation at 1.06 μm has been demonstrated with InGaAsP/InP avalanche photodiodes operated at room temperature. A photon detection efficiency of 33% was measured on uncoated detectors, representing an internal avalanche probability of 60%. Under identical bias conditions a dark count rate as low as 1.7 MHz was measured at 290 K, consistent with a primary dark current of <0.3 pA. Dark count rates drop by approximately 50–200× by cooling the detectors to 210 K (−63 °C).

GaN avalanche photodiodes grown by hydride vapor-phase epitaxy

Avalanche photodiodes have been demonstrated utilizing GaN grown by hydride vapor-phase epitaxy. Spatially uniform gain regions were achieved in devices fabricated on low-defect-density GaN layers that exhibit no microplasma behavior. A uniform multiplication gain up to 10 has been measured in the 320–360 nm wavelength range. The external quantum efficiency at unity gain is measured to be 35%. The electric field in the avalanche region has been determined from high-voltage C–V measurements to be ∼1.6 MV/cm at the onset of the multiplication gain. Electric fields as high as 4 MV/cm have been measured in these devices. Response times are found to be less than 5 μs, limited by the measurement system.

Afterpulsing in Geiger-mode avalanche photodiodes for 1.06μm wavelength

We consider the phenomenon of afterpulsing in avalanche photodiodes (APDs) operating in gated and free-running Geiger mode. An operational model of afterpulsing and other noise characteristics of APDs predicts the noise behavior observed in the free-running mode. We also use gated-mode data to investigate possible sources of afterpulsing in these devices. For 30-μm-diam, 1.06-μm-wavelength InGaAsP∕InP APDs operated at 290K and 4V overbias, we obtained a dominant trap lifetime of τd=0.32μs, a trap energy of 0.11eV, and a baseline dark count rate 245kHz.

Three-dimensional metallodielectric photonic crystals exhibiting resonant infrared stop bands

Using standard microelectronic techniques, we have fabricated arrays of infrared metallodielectric photonic crystals (IR MDPCs) on silicon substrates. The metallic “atoms” are located on a three-dimensional (100)-oriented face-centered-cubic lattice. Resonant stop-band characteristics have been measured with rejection levels of up to 20 dB and widths of up to 83% of the center frequency. We demonstrate structures with stop bands across the midinfrared wavelength range from 2 to 12 μm. Angular studies of the photonic stop bands show an insensitivity to incident angle for some of the structures. The IR MDPC results are compared with measurements made on microwave-scale MDPC structures to help in understanding the infrared results.

Ultraviolet photon counting with GaN avalanche photodiodes

Photon counting, utilizing Geiger-mode avalanche response, has been demonstrated at 300 K in avalanche photodiodes fabricated in GaN grown by hydride vapor-phase epitaxy. Measurements have been made using both passive-quench and time-gated modes of operation. The two important figures of merit for photon-counting applications, photon detection efficiency (PDE) and dark count rate, were measured. A maximum PDE of 13% was measured at 325 nm with a dark count rate of 400 kHz. Typical mesa-etched devices exhibit a parasitic shunt leakage current of less than 20 nA at 90% of breakdown voltage.

New self-aligned planar resonant-tunneling diodes for monolithic circuits

Resonant-tunneling diodes (RTDs) with a new planar configuration have been fabricated with a new self-aligned process that is compatible with that of silicon integrated-circuits technology. The size of the RTD is determined by a shallow boron implant, and the individual RTDs are isolated by a deep proton implant. There is no deep mesa etch. Because of the self-alignment nature of the process, the peak current and voltage of the RTDs are highly uniform. The mean of the standard deviation of the peak current for 4-/spl mu/m/sup 2/ RTDs is 2.3% and the smallest RTDs fabricated are less than 1 /spl mu/m/sup 2/.

A quasioptically stabilized resonant-tunneling-diode oscillator for the millimeter- and submillimeter-wave regions

A semiconfocal open-cavity resonator has been used to stabilize a resonant-tunneling-diode waveguide oscillator at frequencies near 100 GHz. The high quality factor of the open cavity resulted in a linewidth of approximately 10 kHz at 10 dB the peak, which is about 100 times narrower than the linewidth of an unstabilized waveguide oscillator. This technique is well suited for resonant-tunnelling-diode oscillators in the submillimeter-wave region. >

High-speed resonant-tunneling diodes made from the In0.53Ga0.47As/AlAs material system

New double-barrier resonant-tunneling diodes have been fabricated in the pseudomorphic In0.53Ga0.47As/AlAs material system that have peak current densities exceeding 1x105 A cm-2 and peak-to-valley current ratios of approximately 10 at room temperature. One of these diodes yielded oscillations up to 125 GHz, but did not oscillate at higher frequencies because of a large device capacitance. A device with a much lower capacitance is estimated to have a maximum oscillation frequency of 932 GHz and a voltage rise time of 1.5 ps in switching from the peak bias point to the valley bias point. Other reported In0.53Ga0.47As/AlAs diodes are analyzed and yield theoretical maximum oscillation frequencies over 1 THz and rise times as low as 0.3 ps.