C.J. Vineis

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.

In-situ reflectance monitoring of GaSb substrate oxide desorption

The use of specular reflectance to monitor GaSb substrate oxide desorption in-situ is reported. Substrates were loaded into the organometallic vapor phase epitaxy reactor either as-received (epi-ready) or after receiving a solvent degrease, acid etch and rinse. A variety of surface preparations and anneal conditions were investigated. HCL was used as the etchant, and in certain cases was followed by an additional etch in Br{sub 2}-HCl-HNO{sub 3}-CH{sub 3}COOH for comparison. Rinse comparisons included 2-propanol, methanol, and deionized water. Substrates were heated to either 525, 550, or 575 C. Features observed in the in-situ reflectance associated with the oxide desorption process were interpreted based on the starting oxide chemistry and thickness. Based on in-situ reflectance and ex-situ atomic force microscopy data, a recommendation on a reproducible GaSb substrate preparation technique suitable for high-quality epitaxial growth is suggested.

In-situ monitoring of GaSb, GaInAsSb, and AlGaAsSb

The suitability of the wavelength range provided by silicon photodiode detector arrays for monitoring the spectral reflectance during epitaxial growth of GaSb, AlGaAsSb, and GaInAsSb, which have cutoff wavelengths at 25 degree C of 1.7, 1.2, and 2.3 um, respectively, is demonstrated. These alloys were grown lattice matched to GaSb in a vertical rotating-disk reactor, which was modified to accommodate near normal reflectance without affecting epilayer uniformity, By using a virtual interface model, the growth rate and complex refractive index at the growth temperature are extracted for these alloys over the 600 to 1000 nm spectral range. Excellent agreement is obtained between the extracted growth rate and that determined by ex-situ measurement.

InP-based single-photon detector arrays with asynchronous readout integrated circuits

We have developed and demonstrated a high-duty-cycle asynchronous InGaAsP-based photon counting detector system with near-ideal Poisson response, room-temperature operation, and nanosecond timing resolution for near-infrared applications. The detector is based on an array of Geiger-mode avalanche photodiodes coupled to a custom integrated circuit that provides for lossless readout via an asynchronous, nongated architecture. We present results showing Poisson response for incident photon flux rates up to 10 million photons per second and multiple photons per 3-ns timing bin.

Evolution of surface structure and phase separation in GaInAsSb

Atomic force microscopy was used to study changes in the surface step structure of GaInAsSb layers with varying degrees of phase separation. The layers were grown by organometallic vapor phase epitaxy on (001) GaSb substrates with 2{sup o} miscut angles toward (-1-11)A, (1-11)B, and (101). Alloy decomposition was observed by contrast modulations in plan-view transmission electron microscopy, and broadening in x-ray diffraction and photoluminescence peaks. GaInAsSb layers with a minimal degree of phase separation exhibit a step-bunched step structure. A gradual degradation in the periodicity of the step structure is observed as the alloy decomposes into GaAs- and InSb-rich regions. The surface eventually develops trenches to accommodate the local strain associated with composition variations, which are on the order of a few percent. The surface composition is affected by substrate miscut angle, and although phase separation cannot be eliminated, its extent can be reduced by growing on substrates miscut toward (1-11)B.

Design and reliability of mesa-etched InP-based Geiger-mode avalanche photodiodes

Design modifications to InP-based Geiger-mode avalanche photodiodes are described that improve reliability. Geiger-mode aging at multiple conditions can cause significant degradation in some design variants while linear mode (below breakdown) aging does not.

Self-organized vertical superlattices in epitaxial GaInAsSb

Self-organized superlattices are observed in GaInAsSb epilayers grown nominally lattice matched to vicinal GaSb substrates. The natural superlattice (NSL) is detected at the onset of growth and is inclined by an additional 4° with respect to the (001) terrace of the vicinal GaSb substrate. This tilted NSL intersects the surface of the epilayer, and the NSL period is geometrically correlated with the periodicity of surface undulations. While the underlying driving force for this phase separation arises from solution thermodyamics, the mechanism for the self-organized microstructure is related to local strain associated with surface undulations. By using a template with surface undulations, the tilted NSL can be induced in layers with alloy compositions that normally do not exhibit this self-organized microstructure under typical growth conditions.

Geiger-mode avalanche photodiodes for laser communications and laser radar

Arrays of photon-counting avalanche photodiodes (APDs) enable laser-communications and laser-radar receivers with unprecedented sensitivity at 1.06-mum wavelength. Near room temperature, the best detectors have: 50% photon detection efficiency, 30-kHz dark count rate, and a 1-mus reset time to avoid after-pulsing. Arrays with 64 elements were fabricated in the InGaAsP/InP materials system and were bump-bonded to a custom CMOS integrated circuit (IC) with a novel nonblocking architecture to continuously report both time-of-arrival for incoming photons as well as their spatial location on the array. Larger arrays with 1024 elements were mated to custom read-out ICs that report time and location data at lower duty cycles (typically 1-5%) and are appropriate for pulsed laser-radar systems

Intra-Cavity Terahertz Difference-Frequency Generation in Quantum Cascade Lasers

We demonstrate intra-cavity terahertz difference-frequency generation in quantum cascade lasers. A two-wavelength quantum cascade laser with monolithically integrated optical nonlinearity emitting at 7.6 and 8.7 µm was used to generate difference frequency at 60 µm.

Predicting afterpulsing in free-running, Geiger-mode avalanche photodiodes at 1.06 μm wavelength

An operational model of afterpulsing in Geiger-mode avalanche photodiodes is used to explore sources of afterpulsing in the devices and predicts dark-noise behavior in free-running mode. Predictions of the model are compared with experiment.