We discuss the relationship between microstructure and luminescence efficiency for heteroepitaxial films of GaN grown on c-axis sapphire substrates by metalorganic chemical-vapor deposition. We directly characterize the correlation between threading dislocations as observed by transmission electron microscopy, surface morphology as observed by atomic force microscopy, and wavelength-resolved cathodoluminescence imaging. We show that the inhomogeneity in the luminescence intensity of these films near band edge can be accounted for by a simple model where nonradiative recombination at threading dislocations causes a deficiency of minority carriers and results in dark regions of the epilayer. An upper bound for average diffusion length is estimated to be 250 nm.

Correlation of cathodoluminescence inhomogeneity with microstructural defects in epitaxial gan grown by metalorganic chemical-vapor deposition

Semiconducting nanocrystalline thin films of n-Fe2O3 were synthesized by a spray−pyrolytic method. These films were used for the photoelectrochemical splitting of water to hydrogen and oxygen gases. The rates of photoelectrochemical splitting of water at these thin-film electrodes were found to depend on the spray time, substrate temperature, solvent composition in the spray solution, and the concentration of the spray solution. The maximum photocurrent density of 3.7 mA cm-2 at 0.7 V/saturated calomel electrode (SCE) was obtained at the n-Fe2O3 film synthesized using the optimum condition of substrate temperature of 350 °C, the spray time of 60 s, and the spray solution of 0.11 M FeCl3 in 100% ethanol. The band gap energy of this film was found 2.05 eV. The flatband potential of −0.74 V/SCE and the apparent donor density of 2.2 × 1020 cm-3 were found from the Mott−Schottky plots at the AC frequency of 1000 Hz. The n-Fe2O3 films synthesized using the optimum conditions gave rise to a total conversion effi...

Photoelectrochemical splitting of water at nanocrystalline n-fe2o3 thin-film electrodes

Two-dimensional (2D) materials, particularly black phosphorus (bP), have demonstrated themselves to be excellent candidates for high-performance infrared photodetectors and transistors. However, high-quality bP can be obtained only via mechanical exfoliation from high-temperature- and high-pressure-grown bulk crystals and degrades rapidly when exposed to ambient conditions. Here, we report solution-synthesized and air-stable quasi-2D tellurium (Te) nanoflakes for short-wave infrared (SWIR) photodetectors. We perform comprehensive optical characterization via polarization-resolved transmission and reflection measurements and report the absorbance and complex refractive index of Te crystals. It is found that this material is an indirect semiconductor with a band gap of 0.31 eV. From temperature-dependent electrical measurements, we confirm this band-gap value and find that 12 nm thick Te nanoflakes show high hole mobilities of 450 and 1430 cm2 V-1 s-1 at 300 and 77 K, respectively. Finally, we demonstrate that despite its indirect band gap, Te can be utilized for high-performance SWIR photodetectors by employing optical cavity substrates consisting of Au/Al2O3 to dramatically increase the absorption in the semiconductor. By changing the thickness of the Al2O3 cavity, the peak responsivity of Te photoconductors can be tuned from 1.4 μm (13 A/W) to 2.4 μm (8 A/W) with a cutoff wavelength of 3.4 μm, fully capturing the SWIR band. An optimized room-temperature specific detectivity ( D*) of 2 × 109 cm Hz1/2 W-1 is obtained at a wavelength of 1.7 μm.

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Solution-Synthesized High-Mobility Tellurium Nanoflakes for Short-Wave Infrared Photodetectors.

For high-bit-rate long-haul fiber optic communications, the avalanche photodiode (APD) is frequently the photodetector of choice owing to its internal gain, which provides a sensitivity margin relative to PIN photodiodes. APDs can achieve 5-10-dB better sensitivity than PINs, provided that the multiplication noise is low and the gain-bandwidth product is sufficiently high. In the past decade, the performance of APDs for optical fiber communication systems has improved as a result of improvements in materials and the development of advanced device structures. This paper presents a brief review of APD fundamentals and describes some of the significant advances

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Recent Advances in Telecommunications Avalanche Photodiodes

In this Topical Review, we survey the state-of-the-art of single photon detectors based on avalanche diodes fabricated in the InGaAsP materials system for photon counting at near infrared wavelengths in the range from 0.9–1.6 µm. The fundamental trade-off between photon detection efficiency and dark count rate can now be managed with performance that adequately serves many applications, with low dark count rates of ∼1 kHz having been demonstrated at photon detection efficiencies of 20% for 25 µm diameter fiber-coupled devices with thermoelectric cooling. Timing jitter of less than 50 ps has been achieved, although device uniformity is shown to be essential in obtaining good jitter performance. Progress is also reported towards resolving the limitations imposed on photon counting rate by afterpulsing, with at least 50 MHz repetition frequencies demonstrated for 1 ns gated operation with afterpulsing limited to the range of 1–5%. We also present a discussion of future trends and challenges related to these ...

Advances in InGaAsP-based avalanche diode single photon detectors

Distinct improvements in avalanche‐gain and dark‐current characteristics have been made in InGaAsP heterostructure APD’s. A planar‐type p‐n junction is formed in an InP window layer, separated from a light‐absorbing InGaAsP layer. This APD structure has yielded an avalanche gain of 3000 and a dark‐current density as low a 1 μA/cm2 at 0.5 Vb.

InGaAsP heterostructure avalanche photodiodes with high avalanche gain

Correlations between photoluminescent decay time and dislocation density were studied in undoped liquid phase epitaxial GaP. Dislocations strongly reduce the photoluminescent decay time, especially in samples in which nonradiative recombination times (τ0) due to shunt paths other than dislocations are long. Dislocations were found to correspond to photoluminescence dark spots whose diameter is correlated with τ0. A quantitative model is presented, which is in good qualitative agreement with the observed photoluminescent−decay−time−versus−dislocation−density relationships.

Yoshishige Matsumoto

Papers

InGaAsP heterostructure avalanche photodiodes with high avalanche gain

Effects of dislocations on photoluminescent properties in liquid phase epitaxial GaP