Zane A. Shellenbarger

Defect reduction of GaAs epitaxy on Si (001) using selective aspect ratio trapping

Metal-organic chemical vapor deposition growth of GaAs on Si was studied using the selective aspect ratio trapping method. Vertical propagation of threading dislocations generated at the GaAs∕Si interface was suppressed within an initial thin GaAs layer inside SiO2 trenches with aspect ratio >1, leading to defect-free GaAs regions up to 300nm in width. Cross-sectional and plan-view transmission electron microscopies were used to characterize the defect reduction. Material quality was confirmed by room temperature photoluminescence measurements. This approach shows great promise for the fabrication of optoelectronic integrated circuits on Si substrates.

Quaternary InGaAsSb Thermophotovoltaic Diodes

InxGa1-xAsySb1-y thermophotovoltaic (TPV) diodes were grown lattice matched to GaSb substrates by metal-organic vapor phase epitaxy in the bandgap range of EG = 0.5 to 0.6 eV. InGaAsSb TPV diodes, utilizing front-surface spectral control filters, are measured with thermal-to-electric conversion efficiency and power density (PD) of nTPV = 19.7% and PD = 0.58 W/cm2, respectively, for a radiator temperature of Tradiator = 950 degC, diode temperature of Tdiode = 27 degC, and diode bandgap of EG = 0.53 eV. Practical limits to TPV energy conversion efficiency are established using measured recombination coefficients and optical properties of front surface spectral control filters which for 0.53-eV InGaAsSb TPV energy conversion are nTPV = 28% and PD = 0.85 W/cm2 at the above operating temperatures. The most severe performance limits are imposed by 1) diode open-circuit voltage (VOC) limits due to intrinsic Auger recombination and 2) parasitic photon absorption in the inactive regions of the module. Experimentally, the diode VOC is 15% below the practical limit imposed by intrinsic Auger recombination processes. Analysis of InGaAsSb diode electrical performance versus diode architecture indicates that VOC and thus efficiency are limited by extrinsic recombination processes such as through bulk defects

Defect reduction of GaAs/Si epitaxy by aspect ratio trapping

We report on the metallorganic chemical vapor deposition growth of GaAs on patterned Si (001) substrates, which utilizes the aspect ratio trapping method. It was found that when growing GaAs above the SiO2 trenched region, coalescence-induced threading dislocations and stacking faults originated on top of the GaAs/SiO2 interfaces. These defects were found to be indirectly related to the initial defect-trapping process during trenched GaAs growth. Causes of coalescence defect formation and its reduction were experimentally investigated by employing a two-step growth optimization scheme. Improvement of material quality has been characterized by cross-sectional and plan-view transmission electron microscopy and x-ray diffraction.

Enhanced electro-optic effect in GaInAsP–InP three-step quantum wells

We report on the enhanced electro-optic coefficient of GaInAsP three-step quantum wells (3SQW) for high power electrorefraction modulator applications. Measured electro-optic coefficient of the 3SQW is nearly three times higher than the conventional rectangular quantum well (RQW) at λ=1.55 μm. The enhanced electro-optic effect, combined with a low optical absorption coefficient α<1 cm−1 in the 3SQW increases a modulator figure of merit by nearly 36 times, and decreases the power consumption by nearly one order of magnitude compared with a conventional RQW design.

Recent progress in GaInAsSb and InAsSbP photodetectors for mid-infrared wavelengths

Progress on mid-IR photodetectors fabricated by the liquid phase epitaxial growth of GaInAsSb and InAsSbP on GaSb and InAs substrates is reported. GaInAsSb p/n and p-i-n detectors and InAsSbP p/n detector structures were fabricated. Preliminary results indicate that these devices can have higher detectivity with lower cooling requirements than commercially available detectors in the same wavelength range. IR p/n junction detectors made from GaInAsSb and InAsSbP showed cut-off wavelengths of 2.3 micrometers and 2.9 micrometers respectively. Room temperature background noise- limited detectivity of 4 X 1010 cmHz1/2/W GaInAsSb detectors and 4 X 108 cmHz1/2/W for InAsSbP was measured.

Highly linear and efficient phase modulators based on GaInAsP-InP three-step quantum wells

Highly linear and efficient phase modulators based on three-step quantum wells are reported. The spatial separation of electron and hole wave functions in the three-step quantum well leads to enhancement of the linear electro-optic component. In parallel, the quadratic electro-optic component is suppressed using a method based on tailored doping profile. Measured modulation efficiency is 48°∕mm V, and the ratio of linear to quadratic components of the phase modulation is 640 at λ=1560nm. The efficiency is similar to the best reported values for semiconductor modulators at this wavelength, while the linearity is more than one order of magnitude higher.

Reduction of AlGaAs Heterostructure High-Index-Contrast Ridge Waveguide Scattering Loss by Sidewall Smoothing Through Oxygen-Enhanced Wet Thermal Oxidation

We demonstrate the efficacy of oxidation smoothing of sidewall roughness in high-index-contrast AlGaAs heterostructure ridge waveguides via oxygen-enhanced nonselective wet thermal oxidation for reducing scattering loss. Single-mode waveguides of core widths between 1.5 and 2.2 ¿m are fabricated using both the inward growth of a ~ 600-nm sidewall-smoothing native oxide outer cladding and, for comparison, encapsulation of an unoxidized etched ridge with a ~ 600-nm deposited silicon oxide cladding layer. On average, measured loss coefficients are reduced by a factor of 2 with the oxidation smoothing process.

Universality of mode-locked jitter performance

It is shown experimentally that the jitter of actively mode-locked laser pulses is determined by two factors: first, by spontaneous noise associated with cavity loss, and second, by round-trip propagation time. As the round-trip time is increased, a characteristic frequency which defines the high-frequency limit of phase noise decreases. For a comparable round-trip time and cavity loss, the jitter of mode-locked lasers based on diverse gain media, whether semiconductor or erbium ion is universal and independent of the upper-state transition lifetime.

Transient thermal analysis of InGaAsP-InP high-power diode laser arrays with different fill factors

The temperature kinetics of InGaAsP∕InP high-power laser diode arrays with different fill factors was studied experimentally and theoretically. It was shown that except during a short initial period, the laser array heating is determined by the heat flux propagation through the heat spreader. To characterize this heating, we developed a two-dimensional analytical model. Experimentally, the temperature change in the active region was obtained by measuring the laser spectrum’s temporal evolution during a single current pulse. Three distinctive periods in the transient heating process were clearly identified—an initial temperature rise, a square-root-of-time dependence of the active-region temperature increase, and an exponential approach of the active-region temperature to its steady-state value. We demonstrated that in the initial period of time, the heat propagates within the laser bar structure, and the laser bar design (fill factor) strongly affects the active-region temperature rise. In the later perio...

Epitaxial InGaAsP/InP photodiode for registration of InP scintillation

Operation of semiconductor scintillators requires optically tight integration of the photoreceiver system on the surface of the scintillator slab. We have implemented an efficient and fast quaternary InGaAsP pin photodiode, epitaxially grown on the surface of an InP scintillator wafer and sensitive to InP luminescence. The diode is characterized by an extremely low room-temperature dark current, about 1 nA/cm 2 at the reverse bias of 2 V. The low leakage makes possible a sensitive readout circuitry even though the diode has a large area (1 � 1m m 2 ) and therefore large capacitance (50 pF). Results of electrical, optical and radiation testing of the diodes are presented. Detection of individual a-particles and g-photons is demonstrated.