Dingkun Ren

Feasibility of room-temperature mid-wavelength infrared photodetectors using InAsSb nanostructured photoabsorbers

Compound semiconductor mid-wavelength infrared photodetectors operating at room temperature are the sensors of choice for demanding applications such as thermal imaging, heat-seeking, and spectroscopy. However, those detectors suffer from high dark current and thus normally require additional cooling accessories. In this work, we argue for the fundamental feasibility that by using nanowires coupled with plasmonic nano-antennae as photoabsorbers, the dark current can be largely reduced compared with typical planar devices. To demonstrate the idea, we simulate the device characteristics, such as dark current, responsivity, and detectivity, of InAsSb0.07 nanowire photodetectors, and compare those properties with the best research InAs photovoltaic diodes. The results show that the designed nanowire detectors offer over one-order lower dark current and enable a peak detectivity of 7.0×1010 cm Hz1/2W-1 at 3.5 μm. We believe this work will provide a guidance to the design of nanowire-based MWIR photodetectors and stimulate additional experimental and theoretical research studies.

Numerical analysis of nanowire surface recombination using a three-dimensional transient model

To characterize surface recombination of nanowires, time-resolved photoluminescence (TRPL) is commonly implemented to correlate measured lifetime with the nonradiative effect at surface. In this work, we develop a threedimensional transient model to perform a numerical analysis of surface recombination for InGaAs nanowires on GaAs substrates. By mimicking a complete TRPL measurement process, we computationally calculate optical generation and emission of InGaAs nanowires, and numerically probe the carrier dynamics inside nanowires. It is found that the TRPL spectra are determined by a complex convolution of surface recombination velocity and incident wavelengths. In addition, we show that due to the three-dimensional geometry of nanowire, using a typical analytical equation to extract surface recombination velocity might be no longer valid. We believe these results provide an alternative approach for the computational analysis of TRPL measurements and surface properties for three-dimensional nanostructured devices.

Selective-area nanowire photodetectors: from near to mid-wavelength infrared (Conference Presentation)

Semiconductor nanowires are frequently highlighted as promising building blocks for next-generation optoelectronic devices. In this study, we explore infrared photodetectors based on selective-area nanowire arrays, spanning the wavelength spectrum from near-infrared (NIR) to mid-wavelength infrared (MWIR). Examples of these nanowire detectors include: NIR GaAs photodiodes, NIR InGaAs avalanche photodetectors (APDs), NIR InGaAs-GaAs single-photon photodiodes (SPADs), short-wavelength infrared (SWIR) InAs photodiodes, and MWIR InAsSb photodiodes. The small fill factor of nanowire arrays, i.e., the small junction area, is advantageous as it causes significant suppression of dark current, which further decreases the noise level and increases the detectivity. In addition, by utilizing metal nanostructures as 3D plasmonic gratings, we can enhance optical absorption in nanowires through excitation of surface plasmonic waves at metal-nanowire interfaces. Our work shows that, through proper design and fabrication, nanowire-based photodetectors can demonstrate equivalent or better performance compared to their planar device counterparts.

Catalyst-free selective-area epitaxy of GaAs nanowires by metal-organic chemical vapor deposition using triethylgallium

We demonstrate catalyst-free growth of GaAs nanowires by selective-area metal-organic chemical vapor deposition (MOCVD) on GaAs and silicon substrates using a triethylgallium (TEGa) precursor. Two-temperature growth of GaAs nanowires-nucleation at low temperature followed by nanowire elongation at high temperature-almost completely suppresses the radial overgrowth of nanowires on GaAs substrates while exhibiting a vertical growth yield of almost 100%. A 100% growth yield is also achieved on silicon substrates by terminating Si(111) surfaces by arsenic prior to the nanowire growth and optimizing the growth temperature. Compared with trimethylgallium (TMGa) which has been exclusively employed in the vapor-solid phase growth of GaAs nanowires by MOCVD, the proposed growth technique using TEGa is advantageous because of lower growth temperature and fully suppressed radial overgrowth. It is also known that GaAs grown by TEGa induce less impurity incorporation compared with TMGa, and therefore the proposed method could be a building block for GaAs nanowire-based high-performance optoelectronic and nanoelectronic devices on both III-V and silicon platforms.

A three-dimensional insight into correlation between carrier lifetime and surface recombination velocity for nanowires

The performance of nanowire-based devices is predominantly affected by nonradiative recombination on their surfaces, or sidewalls, due to large surface-to-volume ratios. A common approach to quantitatively characterize surface recombination is to implement time-resolved photoluminescence to correlate surface recombination velocity with measured minority carrier lifetime by a conventional analytical equation. However, after using numerical simulations based on a three-dimensional (3D) transient model, we assert that the correlation between minority carrier lifetime and surface recombination velocity is dependent on a more complex combination of factors, including nanowire geometry, energy-band alignment, and spatial carrier diffusion in 3D. To demonstrate this assertion, we use three cases-GaAs nanowires, InGaAs nanowires, and InGaAs inserts embedded in GaAs nanowires-and numerically calculate the carrier lifetimes by varying the surface recombination velocities. Using this information, we then investigate the intrinsic carrier dynamics within those 3D structures. We argue that the conventional analytical approach to determining surface recombination in nanowires is of limited applicability, and that a comprehensive computation in 3D can provide more accurate analysis. Our study provides a solid theoretical foundation to further understand surface characteristics and carrier dynamics for 3D nanostructured materials.

Optical spectroscopy of p-GaAs nanopillars on Si for monolithic integrated light sources

In this work, we study the optical properties and emission dynamics of the novel nanostructure p-GaAs nanopillars (NPs) on Si. The integration of III-V optoelectronics on Si substrates is essential for next-generation high-speed communications. NPs on Si are good candidates as gain media in monolithically integrated small-scale lasers on silicon. In order to develop this technology, an in-depth knowledge of the NP structure is necessary to resolve its optimal optical properties. The optical characterization which has been carried out consists of the emission analysis for different NP geometries. We measured NPs with different combinations of pitch (of the order of a few μm) and diameter (of the order of tens of nm). A comparison of intensities for the various NPs provides us with the most efficient geometry. The quality of the crystal grown has been studied from temperature-dependent photoluminescence (PL). A red shift and a significant reduction of the intensity of the NP emission are observed with an increase in temperature. The results also show the presence of two non-radiative recombination channels when the intensity peaks at different temperatures are analyzed with the activation energy function.