InAs/InP/InSb Nanowires as Low Capacitance n-n Heterojunction Diodes

Nanowire diodes have been realized by employing an axial heterojunction between InAs and InSb semiconductor materials. The broken-gap band alignment (type III) leads to a strong rectification effect when the current-voltage (I-V) characteristic is inspected at room temperature. The additional insertion of a narrow InP barrier reduces the thermionic contribution, which results in a net decrease of leakage current in the reverse bias with a corresponding enhanced rectification in terms of asymmetry in the I-V characteristics. The investigated diodes compare favorably with the ones realized with p-n heterostructured nanowires, making InAs/InP/InSb devices appealing candidates to be used as building blocks for nanowire-based ultrafast electronics and for the realization of photodetectors in the THz spectral range

Electronic Nanodevices

The start of high-volume production of field-effect transistors with a feature size below 100 nm at the end of the 20th century signaled the transition from microelectronics to nanoelectronics. Since then, downscaling in the semiconductor industry has continued until the recent development of sub-10 nm technologies. The new phenomena and issues as well as the technological challenges of the fabrication and manipulation at the nanoscale have spurred an intense theoretical and experimental research activity. New device structures, operating principles, materials, and measurement techniques have emerged, and new approaches to electronic transport and device modeling have become necessary. Examples are the introduction of vertical MOSFETs in addition to the planar ones to enable the multi-gate approach as well as the development of new tunneling, high-electron mobility, and single-electron devices. The search for new materials such as nanowires, nanotubes, and 2D materials for the transistor channel, dielectrics, and interconnects has been part of the process. New electronic devices, often consisting of nanoscale heterojunctions, have been developed for light emission, transmission, and detection in optoelectronic and photonic systems, as well for new chemical, biological, and environmental sensors. This Special Issue focuses on the design, fabrication, modeling, and demonstration of nanodevices for electronic, optoelectronic, and sensing applications

Scanning Probe Microscopy Conductivity Measurements of InP Nanowires for Solar cells

This thesis is devoted to the analysis of the I-V properties of axial InP pn-junction nanowires made for solar cell application. A novel method has been used to measure the I-V characteristics very accurately and reproducibly, using scanning tunneling microscopy (STM). The STM is used to first image the nanowires from top and then form a low resistive point contact between the STM tip and an individual nanowire, which is still on its growth substrate, in ultrahigh vacuum conditions. This setup is well suited to investigate the I-V characteristics of individual nanowires with high accuracy and statistical relevance. In particular, the I-V curves are first analyzed to evaluate when a low resistive point contact has been established. Then, I-V characteristics of nanowires before and after sample cleaning are obtained in order to compare the effect of the surface oxide layer on the nanowire electric properties. The InP pn-junction nanowires show rectifying behavior with typical ideality factors between 2.5 and 2.6. When the surface oxide is removed from the nanowires by annealing under atomic hydrogen background, the ideality factor slightly improves and the conductivity of the individual nanowires increases dramatically for both reverse and forward bias

High-performance piezo-phototronic multijunction solar cells based on single-type two-dimensional materials

Piezotronics and piezo-phototronics based on the third-generation semiconductor (such as ZnO, GaN, CdS, and monolayer chalcogenides) and two-dimensional materials, have attracted increasing attention due to the coupling characteristic of piezoelectric, photon excitation, and semiconductor properties. Strain can not only induce piezoelectric charges but also modulate bandgap of piezotronic materials. In this paper, we propose a structure of piezo-phototronic multijunction solar cell based on single-type two-dimensional piezoelectric semiconductor materials. By using the theory of detailed balance limit, the open circuit voltage and short circuit current of this piezo-phototronic multijunction solar cell are calculated. The results indicate that power conversion efficiency of the piezo-phototronic multijunction solar cell can theoretically reach to 33%, under the blackbody of temperature 6000K, which is higher than the well-known theoretical Shockley-Queisser limit. This work provides guidance to design the next generation ultra-high performance piezo-phototronic solar cells

Three-Terminal Junctions operating as mixers, frequency doublers and detectors: A broad-band frequency numerical and experimental study at room temperature

The frequency response of nanometric T- and Y-shaped three-terminal junctions (TTJs) is investigated experimentally and numerically. In virtue of the parabolic down-bending of the output voltage of the central branch obtained at room temperature under a push-pull fashion input, we analyze: the low-frequency performance (<1 MHz) of TTJs operating as mixers, their RF capability as doublers up to 4 GHz and detection at 94 GHz. Special attention is paid to the impedance matching and cut-off frequency of the measurement set-up. The numerical study is done by means of Monte Carlo simulations. We illustrate the intrinsic functionality of the device as frequency doubler or rectifier up to THz. The role of the width of the central branch on the highfrequency response is also explored, finding different cut-off frequencies for doubling and detection as a consequence of the diverse working principles of both mechanisms and the particular geometry of the TTJs.ROOTHz (FP7-243845

Fabrication de cellules triple-jonction à multi-terminaux

Dans l’industrie du photovoltaïque à concentration, les cellules utilisées sont des cellules à jonctions multiples vu qu’elles permettent d’atteindre des efficacités élevées. Celles qui nous intéressent sont des cellules monolithiques triple-jonction à base d’InGaP/ InGaAs/Ge pouvant atteindre un rendement plus que 40 % sous concentration. Communément, on dépose deux contacts sur la face avant et en dessous de la face arrière qui constituent consécutivement le contact émetteur et le contact base, d’où l’architecture à deux terminaux. Dans cette architecture, les sous-cellules sont électriquement interconnectées en série. Si on veut améliorer les performances de la cellule triple-jonction, il est primordial d’avoir des informations électriques sur chaque sous-cellule, d’où la nécessité de les caractériser individuellement. Cependant, avec l’architecture à deux-terminaux, les caractérisations des sous-cellules ne sont pas évidentes. D’où l’idée de fabriquer des cellules triple-jonction à multi-terminaux telles que les sous-cellules sont électriquement indépendantes, ce qui permet d’avoir un accès direct à chacune d’elles et les caractériser sur une base individuelle. Dans le cadre de cette recherche, un procédé de gravure chimique sélective des différentes couches d’empilement à base d’InGaP/InGaAs/Ge a été développé en utilisant des solutions chimiques à base de H2SO4/H2O2/H2O et HCl/ H3PO4. Ce procédé a visé à réaliser une architecture permettant, par la suite, le dépôt des contacts métalliques Pd/Ge/Ti/Pd/Al, Pt/Ti/Au, Cu/Pt/Ti/Au and Ni/Au sur chacune des sous-cellules en utilisant les techniques de fabrication souvent utilisées telles que la photolithographie et l’évaporation. Finalement, des caractérisations en obscurité des différentes sous-cellules indépendantes électriquement ont été effectuées afin de montrer la fiabilité du procédé de fabrication proposé.Abstract : In the concentrated photovoltaic industry, cells often used are multi-junction cells as they can achieve high efficiencies. Cells of interest are monolithic triple-junction cells based on InGaP/InGaAs/Ge that can achieve efficiencies higher than 40% under concentration. Commonly, two contacts are deposited on the front side and underneath the back side which consecutively constitutes the emitter contact and the base contact, hence the two-terminal architecture. In this architecture, the sub-cells are electrically interconnected in series. To improve the performance of the triple-junction cell, it is essential to have electrical information about each sub-cell, hence the need to characterize them individually. However, with the two-terminal architecture, characterizations of sub-cells are not straightforward. Hence, the idea of fabricating multi-terminal triple-junction cells such that sub-cells are electrically independent, allowing direct access to each of them and characterizing them on an individual basis. In this research, a process for wet selective etching of the InGaP/InGaAs/Ge’s stack layers was developed using chemical solutions based on H2SO4/H2O2/H2O and HCl/H3PO4. This process aimed to achieve an architecture allowing the subsequent deposition of Pd/Ge/Ti/Pd/Al, Pt/Ti/Au, Cu/Pt/Ti/Au and Ni/Au metal contacts on each of the sub-cells using commonly used fabrication techniques such as photolithography and evaporation. Finally, dark characterisations of different electrically independent sub-cells were carried out to test the reliability of the proposed fabrication process

Challenges for the future of tandem photovoltaics on the path to terawatt levels: A technology review

As the photovoltaic sector approaches 1 TW in cumulative installed capacity, we provide an overview of the current challenges to achieve further technological improvements. On the raw materials side, we see no fundamental limitation to expansion in capacity of the current market technologies, even though basic estimates predict that the PV sector will become the largest consumer of Ag in the world after 2030. On the other hand, recent market data on PV costs indicates that the largest cost fraction is now infrastructure and area-related, and nearly independent of the core cell technology. Therefore, additional value adding is likely to proceed via an increase in energy yield metrics such as the power density and/or efficiency of the PV module. However, current market technologies are near their fundamental detailed balance efficiency limits. The transition to multijunction PV in tandem configurations is regarded as the most promising path to surpass this limitation and increase the power per unit area of PV modules. So far, each specific multijunction concept faces particular obstacles that have prevented their upscaling, but the field is rapidly improving. In this review work, we provide a global comparison between the different types of multijunction concepts, including III-Vs, Si-based tandems and the emergence of perovskite/Si devices. Coupled with analyses of new notable developments in the field, we discuss the challenges common to different multijunction cell architectures, and the specific challenges of each type of device, both on a cell level and on a module integration level. From the analysis, we conclude that several tandem concepts are nearing the disruption level where a breakthrough into mainstream PV is possible.Comment: 50 pages, 24 Figure