Electrical Properties of the Base‐Substrate Junction in Freestanding Core‐Shell Nanowires

Abstract

structure through selective adaptation of the material composition, [2] and the possibility of integrating heterocontacts for tunnel junctions, [3] charge-selective contacts, [4] or tunnel barriers, [5,6] a multitude of applications can be targeted. In order to realize these types of device components, special attention must be paid to their surfaces and interfaces, since their electrical properties can be strongly influenced by surface termination and the sequence of layer deposition, especially in the case of core-shell structures. [7] In this context, various groups have developed and investigated epitaxially grown ternary III-V NWs, in particular with III-III-V compositions, such as axial NWs comprising AlGaAs, [8] AlGaP, [9] InGaAs, [10] or radial NWs utilizing AlGaAs. [11][12][13][14][15] III-V based core-shell NWs incorporating p-n or p-i-n junctions have enabled device designs with improved properties, leading to an increase in quantum efficiency and a reduction of non-radiative recombination losses (which in turn increases the effective minority charge carrier lifetimes). This results in improved optoelectronic performance, e.g., enhanced photoluminescence emission. [16] However, these vapor liquid solid (VLS)-grown, multinary III-III-V NWs may exhibit heterogeneous compositions along the NWs as well as in radial direction, caused, e.g., by spontaneous elemental segregation, carry-over effects, or growth Well-defined hetero-interfaces with controlled properties are crucial for any highperformance, semiconductor-based, (opto-)electronic device. They are particularly important for device structures on the nanoscale with increased interfacial areas. Utilizing a ultrahigh-vacuum based multi-tip scanning tunneling microscope, this work reveals inadvertent conductivity channels between the nanowire (NW) base and the substrate, when measuring individual vertical core-shell III-V-semiconductor NWs. For that, four-terminal probing is applied on freestanding, epitaxially grown coaxial p-GaAs/i-GaInP/n-GaInP NWs without the need of nanoscale lithography or deposition of electrical contacts. This advanced analysis, carried out after composition-selective wet chemical etching, reveals a substantially degraded electrical performance of the freestanding NWs compared to detached ones. In an electron beam induced current mode of the nanosensor, charge separation at the substrate-to-NW base junction is demonstrated. An energy dispersive X-ray spectroscopic linescan shows an unintended compositional change of the epitaxially grown NW toward the planar layers caused by different incorporation mechanisms of Ga and In at the NW base. This approach provides direct insight into the NW-substrate transition area and leads to a model of the conductivity channels at the NW base, which should, in principle, be considered in the fabrication of all NW heterostructures grown bottom-up on heterogeneous substrate materials.

Integration of Multijunction Absorbers and Catalysts for Efficient Solar‐Driven Artificial Leaf Structures: A Physical and Materials Science Perspective

“…Although there have been impressive advancements in nanowire growth and designs, the efficiency of light-to-current and current-to-light transformation in devices such as nanowire-based PEC and solar cells, light emitters, and detectors remains limited, clearly not superior to the planar counterparts, because of huge losses during electronic transport due to current leakage pathways. [407][408][409][410] This can be due to inadvertent conductivity channels in the nanowire arrangement, as shown in Figure 25, [411] when four-terminal probing was applied on free-standing, p-i-n (core-shell-shell) nanowires, or due to insufficient charge carrier separation due to defect-assisted tunneling. A similar problem holds for the bipolar heterojunction transistor that suffers from excessive leakage in the base-collector and base-emitter junction inhibiting a net current gain of the transistor.…”

“…This results in complex growth mechanisms [413] due to imperfect doping profile and the evolution of defects and modified and anisotropic charge transport. Second, doping during nanowire growth is physicochemically different from layer doping [411,414] and is taking place at substantially different growth parameters. Third, the surfaceto-volume-ratio and contributions from surface transport in axial junctions are much increased in nanowires compared to layer structures.…”

Impact of the Tip-to-Semiconductor Contact in the Electrical Characterization of Nanowires

“…In addition, the spatial resolution when applying TLM is very low, in particular for NW structures, and the measurement of free-standing NWs is not possible at all, including the determination of the NW-to-substrate interface resistance. This can be remedied by tip-based measurement methods such as conductive atomic force microscopy (C-AFM) − or multitip scanning tunneling microscopy (MT-STM). ,− Only tip-based measurement methods provide suitable nanoscale resolution for electrical measurement; here, a clear definition of the contact behavior is a requirement for the interpretation of measurement results of tip-based measurement methods.…”

“…This can be remedied by tip-based measurement methods such as conductive atomic force microscopy (C-AFM) 26 − 28 or multitip scanning tunneling microscopy (MT-STM). 25 , 29 − 31 Only tip-based measurement methods provide suitable nanoscale resolution for electrical measurement; here, a clear definition of the contact behavior is a requirement for the interpretation of measurement results of tip-based measurement methods.…”