Digital Alloy-Grown InAs/GaAs Short-Period Superlattices with Tunable Band Gaps for Short-Wavelength Infrared Photodetection.

The InGaAs lattice-matched to InP has been widely deployed as the absorption material in short-wavelength infrared photodetection applications such as imaging and optical communications. Here, a series of digital alloy (DA)-grown InAs/GaAs short-period superlattices were investigated to extend the absorption spectral range. The scanning transmission electron microscopy, high-resolution X-ray diffraction, and atomic force microscopy measurements exhibit good material quality, while the photoluminescence (PL) spectra demonstrate a wide band gap tunability for the InGaAs obtained via the DA growth technique. The photoluminescence peak can be effectively shifted from 1690 nm (0.734 eV) for conventional random alloy (RA) InGaAs to 1950 nm (0.636 eV) for 8 monolayer (ML) DA InGaAs at room temperature. The complete set of optical constants of DA InGaAs has been extracted via the ellipsometry technique, showing the absorption coefficients of 398, 831, and 1230 cm-1 at 2 µm for 6, 8, and 10 ML DA InGaAs, respectively. As the period thickness increases for DA InGaAs, a red shift at the absorption edge can be observed. Furthermore, the simulated band structures of DA InGaAs via an environment-dependent tight binding model agree well with the measured photoluminescence peaks, which is advantageous for a physical understanding of band structure engineering via the DA growth technique. These investigations and results pave the way for the future utilization of the DA-grown InAs/GaAs short-period superlattices as a promising absorption material choice to extend the photodetector response beyond the cutoff wavelength of random alloy InGaAs.

Thermal dissipation enhancement in flip-chip bonded uni-traveling carrier photodiodes.

The thermal properties of modified uni-traveling carrier (MUTC) photodiode flip-chip bonded to AlN and diamond are simulated. The thermal impedance of InGaAs is the primary source of internal heating. An n-down epitaxial structure is designed to improve thermal dissipation. Compared to the conventional p-down configuration, the n-down MUTCs bonded to diamond, or AlN submounts achieved 145% and 110% improvement in dissipated power density at thermal failure, respectively. The improved thermal characteristics presage higher RF output power before thermal failure.

Anomalous excess noise behavior in thick Al0.85Ga0.15As0.56Sb0.44 avalanche photodiodes.

Al0.85Ga0.15As0.56Sb0.44 has recently attracted significant research interest as a material for 1550 nm low-noise short-wave infrared (SWIR) avalanche photodiodes (APDs) due to the very wide ratio between its electron and hole ionization coefficients. This work reports new experimental excess noise data for thick Al0.85Ga0.15As0.56Sb0.44 PIN and NIP structures, measuring low noise at significantly higher multiplication values than previously reported (F = 2.2 at M = 38). These results disagree with the classical McIntyre excess noise theory, which overestimates the expected noise based on the ionization coefficients reported for this alloy. Even the addition of 'dead space' effects cannot account for these discrepancies. The only way to explain the low excess noise observed is to conclude that the spatial probability distributions for impact ionization of electrons and holes in this material follows a Weibull-Fréchet distribution function even at relatively low electric-fields. Knowledge of the ionization coefficients alone is no longer sufficient to predict the excess noise properties of this material system and consequently the electric-field dependent electron and hole ionization probability distributions are extracted for this alloy.

Frequency behavior of AlInAsSb nBn photodetectors and the development of an equivalent circuit model.

We report the frequency response of Al0.3InAsSb/Al0.7InAsSb nBn photodetectors. The 3-dB bandwidth of the devices varies from ∼ 150 MHz to ∼ 700 MHz with different device diameters and saturates with bias voltage immediately after the device turn on. A new equivalent circuit model is developed to explain the frequency behavior of nBn photodetectors. The simulated bandwidth based on the new equivalent circuit model agrees well with the bandwidth and the microwave scattering parameter measurements. The analysis reveals that the limiting factor of the bandwidth of the nBn photodetector is the large diffusion capacitance caused by the minority carrier lifetime and the device area. Additionally, the bandwidth of the nBn photodetector is barely affected by the photocurrent, which is found to be caused by the barrier structure in the nBn photodetector.