Avalanche or carrier-multiplication effect, based on impact ionization processes in semiconductors, has a great potential for enhancing the performance of photodetector and solar cells. However, in practical applications, it suffers from high threshold energy, reducing the advantages of carrier multiplication. Here, we report on a low-threshold avalanche effect in a stepwise WSe₂ structure, in which the combination of weak electron-phonon scattering and high electric fields leads to a low-loss carrier acceleration and multiplication. Owing to this effect, the room-temperature threshold energy approaches the fundamental limit, E_thre ≈ E_g, where E_g is the bandgap of the semiconductor. Our findings offer an alternative perspective on the design and fabrication of future avalanche and hot-carrier photovoltaic devices.

In modern electronics and optoelectronics, hot electron behaviors are highly concerned, as they determine the performance limit of a device or system, like the associated thermal or power constraint of chips and the Shockley-Queisser limit for solar cell efficiency. To date, however, the manipulation of hot electrons has been mostly based on conceptual interpretations rather than a direct observation. The problem arises from a fundamental fact that energy-differential electrons are mixed up in real-space, making it hard to distinguish them from each other by standard measurements. Here we demonstrate a distinct approach to artificially (spatially) separate hot electrons from cold ones in semiconductor nanowire transistors, which thus offers a unique opportunity to observe and modulate electron occupied state, energy, mobility and even path. Such a process is accomplished through the scanning-photocurrent-microscopy measurements by activating the intervalley-scattering events and 1D charge-neutrality rule. Findings here may provide a new degree of freedom in manipulating non-equilibrium electrons for both electronic and optoelectronic applications.

Titanium nitride (TiN), with its tunable work function, serves as an electrode metal in the scaling fin-type field-effect transistor and plays the key role for low threshold operation. Measuring the effective work function of thin TiN films is desirable for rapid evaluation before device fabrication. In this work, Kelvin probe force microscopy is applied to study the impact of various factors on the surface potential of TiN films with an uncertainty below 30 mV. By scraping and gauging the potential evolvement of TiN in different circumstances, it is revealed that the surface effect is the major obstacle to determine the work function of the as-deposited TiN. For thick films, the potential drops over 530 mV for a fresh TiN surface relative to that of an aged one. For TiN films thinner than 5 nm, the potential changes by 290 mV due to surface oxidation. This enables a quantitative assessment on the effective work function as well as the surface charge density of TiN films.

Periodic pillars of semiconductor in sub-wavelength size can serve multiple roles as diffracting, trapping and absorbing light for effective photoelectric conversion which has been intensively studied in the visible range. Here, we design and fabricate the micro-pillar arrays of AlGaAs/GaAs multi quantum wells(QWs) for high performance detection of long wavelength infrared light. Compared to its planar counterpart, the array offers 5.1 times intensified absorption at peak wavelength of 8.7 µm with 4 times shrinked electrical area. It’s illustrated by simulation that the normal incident light is guided in the pillars by HE₁₁ resonant cavity mode to form strengthened Ez electrical field, which enables the inter-subband transition of n-type QWs. Moreover, the thick active region of dielectric cavity that contains 50 periods of QWs with fairly low doping concentration will be beneficial to the optical and electrical merits of the detectors. This study demonstrates an inclusive scheme to substantially raise the signal to ratio of infrared detection with all-semiconductor photonic structures.

Efficient coupling in broad wavelength range is desirable for wide-spectrum infrared light detection, yet this is a challenge for intersubband transition in semiconductor quantum wells (QWs). High-Q cavities mostly intensify the absorption at peak wavelengths but with shrinking bandwidth. Here, we propose a novel approach to expand the operating spectral range of the Quantum Well Infrared Photodetectors (QWIPs). By processing the QWs into asymmetric micro-pillar array structure, the device demonstrates a substantial enhancement in spectral response across the wavelength from 7.1 µm to 12.3 µm with guided mode resonance (GMR) effects. The blackbody responsivity is then increased by 3 times compared to that of the 45^° polished edge-coupled counterpart. Meanwhile, the dark current density remains unchanged after the deep etching process, which will benefit the electrical performance of the detector with reduced volume duty ratio. In contrast to the symmetric micro-pillar array that contains simple resonance mode, the detectivity of QWIP in asymmetric pillar structure is found to be improved by 2-4 times within the range of 9.5 µm to 15 µm.