Andreas Winden

Effect of Si-doping on InAs nanowire transport and morphology

The effect of Si-doping on the morphology, structure, and transport properties of nanowires was investigated. The nanowires were deposited by selective-area metal organic vapor phase epitaxy in an N2 ambient. It is observed that doping systematically affects the nanowire morphology but not the structure of the nanowires. However, the transport properties of the wires are greatly affected. Room-temperature four-terminal measurements show that with an increasing dopant supply the conductivity monotonously increases. For the highest doping level the conductivity is higher by a factor of 25 compared to only intrinsically doped reference nanowires. By means of back-gate field-effect transistor measurements it was confirmed that the doping results in an increased carrier concentration. Temperature dependent resistance measurements reveal, for lower doping concentrations, a thermally activated semiconductor-type increase of the conductivity. In contrast, the nanowires with the highest doping concentration show a...

Nanoimprint and selective-area MOVPE for growth of GaAs/InAs core/shell nanowires

We report on the technology and growth optimization of GaAs/InAs core/shell nanowires. The GaAs nanowire cores were grown selectively by metal organic vapor phase epitaxy (SA-MOVPE) on SiO(2) masked GaAs (111)B templates. These were structured by a complete thermal nanoimprint lithography process, which is presented in detail. The influence of the subsequent InAs shell growth temperature on the shell morphology and crystal structure was investigated by scanning and transmission electron microscopy in order to obtain the desired homogeneous and uniform InAs overgrowth. At the optimal growth temperature, the InAs shell adopted the morphology and crystal structure of the underlying GaAs core and was perfectly uniform.

Direct electro-optical pumping for hybrid CdSe nanocrystal/III-nitride based nano-light-emitting diodes

We propose a device concept for a hybrid nanocrystal/III-nitride based nano-LED. Our approach is based on the direct electro-optical pumping of nanocrystals (secondary excitation) by electrically driven InGaN/GaN nano-LEDs as the primary excitation source. To this end, a universal hybrid optoelectronic platform was developed for a large range of optically active nano- and mesoscopic structures. The advantage of the approach is that the emission of the nanocrystals can be electrically induced without the need of contacting them. The proof of principal was demonstrated for the electro-optical pumping of CdSe nanocrystals. The nano-LEDs with a diameter of 100 nm exhibit a very low current of ∼8 nA at 5 V bias which is several orders of magnitude smaller than for those conventionally used. The leakage currents in the device layout were typically in the range of 8 pA to 20 pA/cm2 at 5 V bias. The photon-photon down conversion efficiency was determined to be 27%. Microphotoluminescence and microelectroluminesce...

Supercurrent in Nb/InAs-nanowire/Nb Josephson junctions

We report on the fabrication and measurements of planar mesoscopic Josephson junctions formed by InAs nanowires coupled to superconducting Nb terminals. The use of Si-doped InAs-nanowires with different bulk carrier concentrations allowed to tune the properties of the junctions. We have studied the junction characteristics as a function of temperature, gate voltage, and magnetic field. For junctions with high doping concentrations in the nanowire, Josephson supercurrent values up to 100 nA are found. Owing to the use of Nb as superconductor, the Josephson coupling persists at temperatures up to 4 K. In all junctions, the critical current monotonously decreased with the magnetic field, which can be explained by a recently developed theoretical model for the proximity effect in ultra-small Josephson junctions. For the low-doped Josephson junctions, a control of the critical current by varying the gate voltage has been demonstrated. We have studied conductance fluctuations in nanowires coupled to superconduc...

Spectral Sensitivity Tuning of Vertical InN Nanopyramid-Based Photodetectors

In this paper, we report on the fabrication of InN nanopyramid-based photodetectors operating within the telecommunication wavelength range. We found that the spectral sensitivity of individually addressable InN nanopyramids can be tuned within an interval of 1550–1750 nm since the band edge luminescence energy correlates with the structure size. According to this, we optimized the nanopatterning technique of SiO2/GaN/sapphire substrates as well as the selective area growth to precisely control the nanostructure dimensions. Furthermore, we developed a technological process to contact InN nanopyramids and to integrate them into a high-frequency device layout. Thus, InN nanopyramid-based photodetectors exhibit a low RC constant, low dark currents below 1 nA, as well as a high responsivity of about 0.2 A/W at a wavelength of 1.55 µm.

Low-temperature conductance of the weak junction in InAs nanowire in the field of AFM scanning gate

We have investigated the conductance of an InAs nanowire in the presence of an electrical potential created by an AFM scanning gate at liquid helium temperature. The influence of the direction of a local electrical field on the tunneling rate through a weak junction in the InAs wire is clearly observed. To explain this behavior, the redistribution of the electrons among conductive channels in the wire must be taken into account. We have confirmed that the pattern of Coulomb blockade diamonds gives the same result for the ratio of quantum dot sizes as that revealed by scanning gate imaging.

Vertically integrated (Ga, In)N nanostructures for future single photon emitters operating in the telecommunication wavelength range

Important technological steps are discussed and realized for future room-temperature operation of III-nitride single photon emitters. First, the growth technology of positioned single pyramidal InN nanostructures capped by Mg-doped GaN is presented. The optimization of their optical characteristics towards narrowband emission in the telecommunication wavelength range is demonstrated. In addition, a device concept and technology was developed so that the nanostructures became singularly addressable. It was found that the nanopyramids emit in the telecommunication wavelength range if their size is chosen appropriately. A p-GaN contacting layer was successfully produced as a cap to the InN pyramids and the top p-contact was achievable using an intrinsically conductive polymer PEDOT:PSS, allowing a 25% increase in light transmittance compared to standard Ni/Au contact technology. Single nanopyramids were successfully integrated into a high-frequency device layout. These decisive technology steps provide a promising route to electrically driven and room-temperature operating InN based single photon emitters in the telecommunication wavelength range.

Preparation of Ohmic contacts to GaAs/AlGaAs-core/shell-nanowires

Ohmic contacts to GaAs/AlGaAs core/shell nanowires are prepared by using a Ni/AuGe/Ni/Au layer system. In contrast to Ohmic contacts to planar GaAs/AlGaAs layer systems here, relatively low alloying temperatures are used in cylindrical geometry. Lowest resistances are found for annealing temperatures of 320 °C and 340 °C. For annealing temperatures exceeding 360 °C, the nanowires degraded completely. Nanowires annealed under optimized conditions preserved their Ohmic characteristics even down to temperatures of 4 K.

Highly Transparent Conducting Polymer Top Contacts for Future III--Nitride Based Single Photon Emitters

In this paper we report on a simple conductive polymer based contacting technology for III–nitride based nanostructures with respect to the electrical operation within the telecommunication wavelength range. Singularly addressable InN/GaN pyramidal nanostructures were selectively grown by metalorganic vapour phase epitaxy (MOVPE) and subsequently integrated into a high-frequency device layout for future ultrafast electro-optical operation. The employment of the p-conducting polymer poly(3,4-ethylenedioxythiophene)–poly(styrene sulfonate) (PEDOT:PSS) is found to increase the light transmittance up to 89% at a wavelength of 1550 nm compared to 72% in the case of a conventional Ni/Au thin layer top contact. DC measurements using a quasi operation mode for 1000 h reveal no degradation and only a moderate increase of the dark currents. Thus, conducting polymer technology shows tremendous potential for future highly efficient and reliable room temperature operation of nitride based single photon emitters (SPEs).

Quantum dots in InAs nanowires induced by surface potential fluctuations

Back-gated InAs nanowire field-effect transistors are studied focusing on the formation of intrinsic quantum dots, i.e. dots not intentionally defined by electrodes. Such dots have been studied before, but the suggested explanations for their origin leave some open questions, which are addressed here. Stability diagrams of samples with different doping levels are recorded at electron temperatures below 200 mK, allowing us to estimate the number and size of the dots as well as the type of connection, i.e. in series or in parallel. We discuss several potential physical origins of the dots and conclude that they are most probably induced by potential fluctuations at the nanowire surface. Additionally, we show that via gate voltage and doping, the samples can be tuned to different regimes of Coulomb blockade.