Showing papers in "Semiconductor Science and Technology in 2020"

Lateral β-Ga2O3 field effect transistors

Abstract: Beta phase Gallium Oxide (BGO) is an emerging ultra-wide bandgap semiconductor with disruptive potential for ultra-low power loss, high-efficiency power applications. The critical field strength is the key enabling material parameter of BGO which allows sub-micrometer lateral transistor geometry. This property combined with ion-implantation technology and large area native substrates result in exceptionally low conduction losses, faster power switching frequency and even radio frequency power. We present a review of BGO epitaxial materials and lateral field-effect transistors developments, highlight early achievements and discuss engineering solutions with power switching and radio frequency applications in mind.

Degenerate doping in β-Ga2O3 single crystals through Hf-doping

Abstract: n-type conductivity of β-Ga2O3 grown from the melt is typically achieved using Sn and Si. In this paper, we experimentally and computationally investigate Hf doping of β-Ga2O3 single crystals using UV–vis-NIR absorption and Hall effect measurements and hybrid functional calculations. Unintentionally-doped and Hf-doped samples with a nominal concentration of 0.5at% were grown from the melt using vertical gradient freeze and Czochralski method in mixed Ar + O2 atmosphere. We demonstrate Hf dopants, predicted to incorporate on the octahedral GaII site as a shallow donor, achieve degenerate doping in β-Ga2O3 with a measured electron concentration ~2 × 1019 cm−3, mobility 80–65 cm2 V−1 s−1, and resistivity down to 5 mΩ cm in our samples. The concentration of Hf was measured to be 1.3 × 1019 atoms cm−3 using glow discharge mass spectroscopy on doped samples, confirming Hf to be the cause of n-type conductivity (electron concentration ~2 × 1019 cm−3).

Two-step write-verify scheme and impact of the read noise in multilevel RRAM-based inference engine

Abstract: The accuracte cell conductance tuning is critical to realizing multilevel resistive random access memory (RRAM) based compute-in-memory inference engine. To tighten the distribution of the cells of each state, we developed a two-step write-verify scheme within the limited number of iteration, which was tested on a test vehicle based on HfO2 RRAM array to realize 2-bit per cell. The conductance of the cells are gathered in the targeted range within 10 loops of set and reset process for each step. Moreover, the read noise of the RRAM cells is statistically measured and its impact on the upper-bound of analog-to-digital converter (ADC) resolution is predicted. The result shows that the intemediate state cells under relatively high read voltage (e.g. 0.2V) are vulnerable to the read noise. Fortunately, the aggregated read noise along the column will not disturb the output of a 5-bit ADC that is required for 128×128 array with 2-bit per cell.

Non-equilibrium dynamics, materials and structures for hot carrier solar cells: a detailed review

Abstract: Since their advent around the start of the millennium, hot carrier solar cells came into the focus of a broader research community as one of the so-called 3$^{\rm{rd}}$ generation photovoltaic concepts. As an exciting research field enthusiastically covered by an increasing number of publications, some aspects of hot carrier solar cell research, namely data interpretation and conclusions with respect to high efficiency devices, appear to show some discrepancies. It therefore appeared timely to provide a detailed review of current hot carrier solar cell research from the fundamentals of non-equilibrium carrier dynamics to complete solar cells to enable advancement with the knowledge of solid state and semiconductor physics being fully taken into account. In our work, we discuss the hot carrier non-equilibrium dynamics right from the process of hot carrier generation, going beyond the standard 1-dimensional approach, hence exploring phononic and other dynamic limits as occurring in real materials. Thermodynamic modelling of hot carrier solar cells in the literature presented conversion efficiencies from 0.04 to 84 \%. This situation called for an evaluation and a comparison against the Shockley-Queisser efficiency limit. The assessment of characterisation techniques used for dynamic and steady-state detection of hot carrier populations form another part of this review, including to what extent certain data can or should be used in regards to hot carrier solar cells. With this wealth of information, we work through III-V, IV-IV, II-VI, and non-trivial materials which were proposed for hot carrier absorbers in the literature. With the physics and materials considered, we then examine energy-selective contact designs which also have to fulfil the criterion of carrier selectivity. Finally, we look at the whole hot carrier solar cell, departing from the original concept to more feasible designs and qualitatively new approaches.

Porous semiconductor compounds

Abstract: In this review paper, we present a comparative analysis of the electrochemical dissolution of III-V (InP, GaAs, GaN), II-VI (ZnSe, CdSe) and SiC semiconductor compounds. The resulting morphologies are discussed including those of porous layers and networks of low-dimensional structures such as nanowires, nanobelts, nanomembranes etc. Self-organized phenomena in anodic etching are disclosed, leading to the formation of controlled porous patterns and quasi-ordered distribution of pores. Results of templated electrochemical deposition of metal nanowires, nanotubes and nanodots are summarized. Porosification of some compounds is shown to improve luminescence characteristics as well as to enhance photoconductivity, second harmonic generation and Terahertz emission. Possible applications of porous semiconductor compounds in various areas are discussed.

Design study of the gate-all-around silicon nanosheet MOSFETs

Abstract: The gate-all-around (GAA) silicon nanosheet (SiNS) metal-oxide-semiconductor field-effect transistor (MOSFET) structures have been recognized as excellent candidates to achieve improved power performance and area scaling compared to the current FinFET technologies. Specifically, SiNS structures provide high drive currents due to wide effective channel width (W eff ) while maintaining short-channel control. In this paper, we fabricate a GAA SiNS MOSFET fully surrounded by a gate with a gate length (L G ) of 22 nm, a SiNS width (W NS) of 23 nm, and SiNS thickness (T NS) of 6 nm. In addition, the fabricated GAA SiNS MOSFETs were evaluated for electrostatic characteristics and short-channel effects (SCEs) according to various channel length and width dimensions. We confirmed that the GAA SiNS MOSFET showed similar short-channel controllability regardless of W NS due to the extremely thin T NS. In addition, we analyzed SCEs of GAA SiNS MOSFETs with different T NS through simulation.

InP membrane integrated photonics research

Abstract: Recently a novel photonic integration technology, based on a thin InP-based membrane, is emerging. This technology offers monolithic integration of active and passive functions in a sub-micron thick membrane. The enhanced optical confinement in the membrane results in ultracompact active and passive devices. The membrane also enables approaches to converge with electronics. It has shown high potential in breaking the speed, energy and density bottlenecks in conventional photonic integration technologies. This paper explains the concept of the InP membrane, discusses the versatility of various technology approaches and reviews the recent advancement in this field.

Influence of deposition temperature on amorphous Ga2O3 solar-blind ultraviolet photodetector

Abstract: Solar-blind ultraviolet photodetectors have potential applications in space communication, ozone hole monitoring and missile tracking. Amorphous Ga2O3 (a-Ga2O3) films are deposited by a simple radio frequency magnetron sputtering at different deposition temperatures. Fully transparent devices on a quartz substrate are fabricated with high responsivity, wide detection range and good repeatability. With the increase of Ga2O3 deposited temperature, the concentration of oxygen vacancy increases accordingly, leading to a wide detection range from 250 to 325 nm and high responsivity (138 A W−1 at 5 V bias). The underlying mechanism has been discussed and analyzed. Our results should advance the application of a-Ga2O3-based ultraviolet photodetectors and other relevant devices.

Fast response of UV photodetector based on Ag nanoparticles embedded uniform TiO2 nanotubes array

Abstract: Titanium dioxide (TiO2) has drawn a potential research interest for ultraviolent (UV) photodetector (PD) applications because of its tunable bandgap in UV absorption region, low-absorption coefficient in visible region, n-type semiconducting property, and excellent chemical stabilities. In this study, attempts were made to explore the performances of TiO2 nanostructures such as nanotubes (NTs) and nanorods (NRs) based UV PDs embedded with 23 nm plasmonic silver (Ag) nanoparticles (NPs), which offer the local surface plasmonic resonance or near-field enhancement. The vertical TiO2 NTs and NRs with high uniformity and height of 1 µm were successfully synthesized using simple and low-cost electrochemical anodization and hydrothermal growth techniques, respectively onto Ti substrates. From the x-ray diffraction analysis, it was ascertained that the anatase phase has been formed for NTs, whereas the rutile phase dominated the NRs. All these nanostructures were characterized by various material characterization techniques such as field emission scanning electron microscopy, x-ray photoelectron spectroscopy, UV-vis-NIR and Raman spectroscopy to investigate their surface and structural morphologies and absorption spectra. Efforts were put to fabricate Ag/ (with or without Ag NPs) TiO2 nanostructures/ Ti based UV PDs, where Ag has been utilized as top electrode and Ti served the purpose for bottom electrode. The electrical characteristics such as current-voltage, responsivity, detectivity, external quantum efficiency, rise and decay times were systematically investigated under 365 nm UV light radiance. Among all the UV PDs, TiO2 NTs anchored with the Ag NPs offer better photocurrent, responsivity (1.37 AW-1), detectivity (5.18 x 1010 Jones), external quantum efficiency (465.42%), rise (0.43 s) and decay (0.70 s) times. In order to have better insight on the device operational principle, a band diagram was proposed and it was realized that desorption of oxygen ions, increment of free electron carriers, and localized surface plasmonic effect were responsible for obtaining improved photoresponse.

Wavelength stabilized high pulse power 48 emitter laser bars for automotive light detection and ranging application

Abstract: Diode lasers generating optical pulses with high peak power and lengths in the nanosecond range are key components for LiDAR (Light Detection and Ranging) systems, e.g., for autonomous driving and object detection. We present here an internally wavelength stabilized distributed Bragg reflector (DBR) broad area (BA) laser bar with 48 emitters. The vertical structure based on AlGaAs (confinement and cladding layers) and InGaAs (active quantum well) is specifically optimized for wavelength-stabilized pulsed operation applying a surface Bragg grating with high reflectivity. The bar is electrically driven by a new in-house developed high-speed driver based on GaN transistors providing current pulses with amplitudes of up to 1000 A and a repetition frequency of 10 kHz. The generated 4 ns to 10 ns long optical pulses are nearly rectangular shaped and reach a pulse peak power in excess of 600 Watts at 25°C. The optical spectrum with a centre wavelength of about 900 nm has a width of 0.15 nm (FWHM) with a side mode suppression ratio > 30 dB.

Source-All-Around Tunnel Field-Effect Transistor (SAA-TFET): Proposal and Design

Abstract: In this paper, a new source-all-around tunnel field-effect transistor (SAA-TFET) is proposed and investigated by using TCAD simulation. The tunneling junction in the SAA-TFET is divided laterally and vertically with respect to the channel direction which provides a relatively large tunneling junction area. An n+ pocket design is also introduced around the source to enhance tunneling rates and improve the device characteristics. In addition, the gate and n+ pocket region also overlap in the vertical and the lateral directions resulting in an enhanced electric field and, in turn, the ON-state current of the SAA-TFET is highly increased compared with the conventional TFET. Promising results in terms of DC (ION, IOFF, ON/OFF current ratio and SS) and analog (cutoff frequency) performance are obtained for low (VDD = 0.5 V) and high (VDD = 1 V) supply voltages.

Material-to-device performance correlation for AlGaN-based solar-blind p-i-n photodiodes

Abstract: We report on crystalline quality-to-device performance correlation for self-powered Al0.40Ga0.60N, p-i-n ultraviolet (UV) photodetectors on c-plane sapphire. The active p-i-n detector stack was grown over an AlN buffer. Careful optimization of the nucleation density on growth surface helped achieve a two-orders and one-order of magnitude reduction in the screw and edge dislocation density in the buffer layer, respectively. This resulted in a nine-orders of magnitude reduction in the reverse leakage current from 4.3 mA to 4.2 pA (at 10 V). Correspondingly, a thirteen-fold enhancement in the zero-bias external quantum efficiency (EQE) from 3.4 to 45.5, when measured under 289 nm front-illumination was also observed. The detector epi-stack grown over the optimal AlN buffer layers led to the realization of high-performance p-i-n detectors with a dark current density below 4 nA cm-2 at 10 V and a zero-bias EQE of 74.7 under back-illumination. This is one of the highest zero-bias EQE reported for solar-blind detectors realized on template-free and mask-free III-nitrides grown using metal organic chemical vapor deposition on any substrate. The deep-UV-to-visible rejection ratio exceeded 106 while the deep-UV-to-near UV rejection exceeded 103. The thermal-noise limited detectivity was estimated to be 4 1014 cm Hz1/2 W-1. Hopping conduction along screw dislocation-mediated localized trap states was found to be the dominant carrier transport mechanism in the samples exhibiting high reverse leakage. For these samples, the responsivity (photocurrent) exhibited an exponential variation with reverse bias and a nonlinear variation with input optical power. This is explained using a hole-trapping associated gain mechanism and its impact on the transient characteristics of the detectors is investigated. A 6 1 linear array of the highest EQE detectors was realized and detector performance parameters were found to be comparable before and after wire bonding. This study is expected to enhance the understanding of III-nitrides-based vertical, self-powered detectors and benefit the development of high-performance, focal plane arrays using less complicated growth techniques. © 2020 IOP Publishing Ltd.