H. Hardtdegen - Academia.edu (original) (raw)

Papers by H. Hardtdegen

Semiconductor Science and Technology, 2012

ABSTRACT Micro-photoluminescence (µ-PL) studies were performed on AlGaN/GaN heterostructure field... more ABSTRACT Micro-photoluminescence (µ-PL) studies were performed on AlGaN/GaN heterostructure field effect transistors (HFETs) with different gate-recessing depths. It was found that µ-PL is the method of choice for detecting dry etching damage and simultaneously recording strain and stress in the HFET GaN layer. Lateral sub-µm resolved mapping shows that the strain in the GaN layer after recessing is partially relaxed and non-uniform.

Ion implantation Rare earth X-ray diffraction Rutherford backscattering spectroscopy Secondary io... more Ion implantation Rare earth X-ray diffraction Rutherford backscattering spectroscopy Secondary ion mass spectrometry Magnetic properties of thin films We present a complex study of rare earth elements implanted GaN layers grown by low pressure metalorganic vapor phase epitaxy on c-plane sapphire substrates. Gd, Dy, La and Lu ions were implanted with energies of 200 keV and doses ranging from 5 × 10 13 to 4 × 10 17 atoms.cm − 2. The chemical composition and concentration profiles of ion-implanted layers were studied by secondary ion mass spectrometry and Rutherford back scattering. The structural properties of the layers were characterized by Rutherford back scattering/channeling and X-ray diffraction reciprocal space mapping. Gd implanted layers exhibit ferromagnetic behavior persisting up to~720 K. Since the ferromagnetic behavior was not observed in the case of La and Lu implanted layers, it cannot be attributed to the structural damage of the layer. Based on the fact that the samples are electrically conducting we conclude that the ferromagnetism can be associated with doped electrons mediating the ferromagnetic interaction between local moments on Gd and Dy.

physica status solidi (b), 2005

ABSTRACT In this paper we will first report on the use of real-time determination of wafer temper... more ABSTRACT In this paper we will first report on the use of real-time determination of wafer temperature for transparent substrates. With this method we will study the unintentional influence of growth parameter variations on the surface temperature. The effect on nitride growth optimization will be discussed. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Spintronics VII, 2014

ABSTRACT Due to their strong spin-orbit coupling III-V semiconductor nanowires are excellent cand... more ABSTRACT Due to their strong spin-orbit coupling III-V semiconductor nanowires are excellent candidates for electrical spin manipulation. Therefore, a major goal is to tailor spin-orbit coupling in these devices. Direct electrical spin injection into quasi one-dimensional nanowires is demonstrated. Furthermore, the weak antilocalization effect was investigated in InAs nanowires. The quantum corrections to the conductivity are interpreted by developing a quasi-one-dimensional diffusive model. It turns out that by means of doping and electric gating the spin-lifetimes can be tuned significantly. By creating few-electron quantum dots inside these devices the impact of the confinement on the spin relaxation properties is investigated.

Physical Review B, 2009

ABSTRACT We have studied experimentally the nonequilibrium currents (NECs) induced by sweeping ei... more ABSTRACT We have studied experimentally the nonequilibrium currents (NECs) induced by sweeping either the magnetic field B or the carrier density n(S) of a two-dimensional electron system (2DES). The gated 2DES resided in a modulation-doped GaAs/Al(x)Ga(1-x)As heterostructure and was integrated into a micromechanical cantilever. The NECs provoke a magnetic moment which we have detected via torque magnetometry down to 300 mK. Additional electrical leads allowed for simultaneous magnetotransport measurements. We find a hysteretic behavior of the NECs and a striking asymmetry of the corresponding magnetic moment around integer filling factors nu=hn(S)/eB. Surprisingly, the shape of the hysteresis loops is the same for sweeps of B or n(S) if plotted versus nu. In a certain parameter regime each NEC signal exhibits a characteristic slope which is found to depend only on the filling factor at large B or n(S). Based on a model considering capacitive coupling between 2DES and gate we attribute the slopes to the conductance quantization of the quantum Hall effect. The NECs are found to be limited by the time-dependent buildup of the radial Hall field governed by the gate capacitance. These findings are in contrast to a floating 2DES without a gate where the breakdown of the quantum Hall effect was previously reported to limit the NECs. Our model also explains the observed shape and dependence on temperature as well as sweep rate. The in situ measurement of the longitudinal resistance allows us to directly correlate the magnetic behavior with both the magnetic field and temperature-dependent resistance of the 2DES.

Applied Physics Letters, 2013

Fluorescence-lifetime imaging microscopy (FLIM) was applied to investigate the donor distribution... more Fluorescence-lifetime imaging microscopy (FLIM) was applied to investigate the donor distribution in SrTiO 3 single crystals. On the surfaces of Nb-and La-doped SrTiO 3 , structures with different fluorescence intensities and lifetimes were found that could be related to different concentrations of Ti 3+ . Furthermore, the inhomogeneous distribution of donors caused a non-uniform conductivity of the surface, which complicates the production of potential electronic devices by the deposition of oxide thin films on top of doped single crystals.

Applied Physics Letters, 2013

Fluorescence-lifetime imaging microscopy (FLIM) was applied to investigate the donor distribution... more Fluorescence-lifetime imaging microscopy (FLIM) was applied to investigate the donor distribution in SrTiO 3 single crystals. On the surfaces of Nb-and La-doped SrTiO 3 , structures with different fluorescence intensities and lifetimes were found that could be related to different concentrations of Ti 3+ . Furthermore, the inhomogeneous distribution of donors caused a non-uniform conductivity of the surface, which complicates the production of potential electronic devices by the deposition of oxide thin films on top of doped single crystals.

Nanotechnology, 2013

We investigated the transport properties of GaAs/InAs core/shell nanowires grown by molecular bea... more We investigated the transport properties of GaAs/InAs core/shell nanowires grown by molecular beam epitaxy. Owing to the band alignment between GaAs and InAs, electrons are accumulated in the InAs shell as long as the shell thickness exceeds 12 nm. By performing simulations using a Schrödinger-Poisson solver, it is confirmed that confined states are present in the InAs shell, which are depleted if the shell thickness is below a threshold value. The existence of a tubular-shaped conductor is proved by performing magnetoconductance measurements at low temperatures. Here, flux periodic conductance oscillations are observed which can be attributed to transport in one-dimensional channels based on angular momentum states.

Nanotechnology, 2013

Important technological steps are discussed and realized for future room-temperature operation of... more 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.

Journal of Applied Physics, 2012

We report on the fabrication and measurements of planar mesoscopic Josephson junctions formed by ... more 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. In 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 superconducting and normal metal terminals. The conductance fluctuation amplitude is found to be about 6 times larger in superconducting contacted nanowires. The enhancement of the conductance fluctuations is attributed to phase-coherent Andreev reflection as well as to the large number of phase-coherent channels due to the large superconducting gap of the Nb electrodes.

Crystal Growth & Design, 2014

The role of Si during the metal−organic vapor phase epitaxy of GaN rods is investigated. Already ... more The role of Si during the metal−organic vapor phase epitaxy of GaN rods is investigated. Already a small amount of Si strongly enhances the vertical growth of GaN. Reactive ion etching experiments show that the inner volume of the rod is much more strongly etched than the m-plane surface layer. Transmission electron microscopy and energy dispersive X-ray spectroscopy measurements reveal that Si is predominiantly incorporated in the surface layer of the m-plane sidewall facets of the rods. The formation of a SiN layer prevents growth on and etching of the m-planes and enhances the mobility of atoms promoting vertical growth. Annealing experiments demonstrate the extraordinary thermal resistivity in comparison to undoped GaN rod structures and GaN layers. The subsequent InGaN quantum well growth on the GaN rods reveals the antisurfactant effect of the SiN layer. A model based on the vapor−liquid−solid growth mode is proposed. The results help to understand the role of Si during growth of GaN rod structures to improve the performance of rod based light emitting and electronic devices.

Physica Status Solidi (C) Current Topics in Solid State Physics, 2009

Materials Research Society Symposium Proceedings, 2012

We present comparative studies of optical properties of GaN nanowires (NWs) obtained by two diffe... more We present comparative studies of optical properties of GaN nanowires (NWs) obtained by two different self-formation techniques: Plasma-Assisted Molecular Beam Epitaxy (PAMBE) growth; and plasma etching of GaN layers deposited by Metal-Organic Vapor Phase Epitaxy (MOVPE). The effects of the coalescence process on grown NW and plasma-induced defects in etched NWs have been studied by photoluminescence (PL) and Raman scattering. In MBE grown NWs, the coalescence-associated defects are extended toward the NW top for intermediate Ga flux. Using High Resolution Electron Microscopy of reactive plasma etching (RIE) NWs, it was found that NWs obtained with an optimal combination of inductive (ICP) and capacitive (RF) plasma are free of extended structural defects. The PL efficiency is strongly increased in plasma etched NWs. However, plasma-induced point defects have to be taken into account for explaining the changes of the PL spectra. Less plasma-induced degradation is observed for high ICP/RF power ratios.

Thin Solid Films, 2011

Magnetic semiconductors III-V semiconductors Ion implantation Rare earth X-ray diffraction Ruther... more Magnetic semiconductors III-V semiconductors Ion implantation Rare earth X-ray diffraction Rutherford backscattering spectroscopy Secondary ion mass spectrometry Magnetic properties of thin films

Physical Review B, 2006

The magnetoconductivity of GaxIn1-xAs/InP quantum wires with widths in the range of 1220-250nm wa... more The magnetoconductivity of GaxIn1-xAs/InP quantum wires with widths in the range of 1220-250nm was investigated. The finite zero-field spin splitting in our samples gives rise to spin relaxation and weak antilocalization in wide wires. In contrast, for the narrow wires, only weak-localization behavior is seen even though the zero-field spin splitting is independent of wire width. The observed renormalization of

Journal of Experimental and Theoretical Physics, 2013

Journal of Crystal Growth, 2001

ABSTRACT

Journal of Crystal Growth, 2009

ABSTRACT

Journal of Applied Physics, 2008

The effect of a small dose of gamma irradiation on transport characteristics of the two-dimension... more The effect of a small dose of gamma irradiation on transport characteristics of the two-dimensional electron gas ͑2DEG͒ in AlGaN / GaN heterostructures was investigated. It is shown that the carrier concentration remains practically unchanged after an irradiation dose of 10 6 rad, while the 2DEG mobility exhibits a considerable increase. The results are explained within a model that takes into account the relaxation of elastic strains and structural-impurity ordering occurring in the barrier layer under irradiation.

Journal of Applied Physics, 2011

The effect of Si-doping on the morphology, structure, and transport properties of nanowires was i... more 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

Semiconductor Science and Technology, 2012

ABSTRACT Micro-photoluminescence (µ-PL) studies were performed on AlGaN/GaN heterostructure field... more ABSTRACT Micro-photoluminescence (µ-PL) studies were performed on AlGaN/GaN heterostructure field effect transistors (HFETs) with different gate-recessing depths. It was found that µ-PL is the method of choice for detecting dry etching damage and simultaneously recording strain and stress in the HFET GaN layer. Lateral sub-µm resolved mapping shows that the strain in the GaN layer after recessing is partially relaxed and non-uniform.

Ion implantation Rare earth X-ray diffraction Rutherford backscattering spectroscopy Secondary io... more Ion implantation Rare earth X-ray diffraction Rutherford backscattering spectroscopy Secondary ion mass spectrometry Magnetic properties of thin films We present a complex study of rare earth elements implanted GaN layers grown by low pressure metalorganic vapor phase epitaxy on c-plane sapphire substrates. Gd, Dy, La and Lu ions were implanted with energies of 200 keV and doses ranging from 5 × 10 13 to 4 × 10 17 atoms.cm − 2. The chemical composition and concentration profiles of ion-implanted layers were studied by secondary ion mass spectrometry and Rutherford back scattering. The structural properties of the layers were characterized by Rutherford back scattering/channeling and X-ray diffraction reciprocal space mapping. Gd implanted layers exhibit ferromagnetic behavior persisting up to~720 K. Since the ferromagnetic behavior was not observed in the case of La and Lu implanted layers, it cannot be attributed to the structural damage of the layer. Based on the fact that the samples are electrically conducting we conclude that the ferromagnetism can be associated with doped electrons mediating the ferromagnetic interaction between local moments on Gd and Dy.

physica status solidi (b), 2005

ABSTRACT In this paper we will first report on the use of real-time determination of wafer temper... more ABSTRACT In this paper we will first report on the use of real-time determination of wafer temperature for transparent substrates. With this method we will study the unintentional influence of growth parameter variations on the surface temperature. The effect on nitride growth optimization will be discussed. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Spintronics VII, 2014

ABSTRACT Due to their strong spin-orbit coupling III-V semiconductor nanowires are excellent cand... more ABSTRACT Due to their strong spin-orbit coupling III-V semiconductor nanowires are excellent candidates for electrical spin manipulation. Therefore, a major goal is to tailor spin-orbit coupling in these devices. Direct electrical spin injection into quasi one-dimensional nanowires is demonstrated. Furthermore, the weak antilocalization effect was investigated in InAs nanowires. The quantum corrections to the conductivity are interpreted by developing a quasi-one-dimensional diffusive model. It turns out that by means of doping and electric gating the spin-lifetimes can be tuned significantly. By creating few-electron quantum dots inside these devices the impact of the confinement on the spin relaxation properties is investigated.

Physical Review B, 2009

ABSTRACT We have studied experimentally the nonequilibrium currents (NECs) induced by sweeping ei... more ABSTRACT We have studied experimentally the nonequilibrium currents (NECs) induced by sweeping either the magnetic field B or the carrier density n(S) of a two-dimensional electron system (2DES). The gated 2DES resided in a modulation-doped GaAs/Al(x)Ga(1-x)As heterostructure and was integrated into a micromechanical cantilever. The NECs provoke a magnetic moment which we have detected via torque magnetometry down to 300 mK. Additional electrical leads allowed for simultaneous magnetotransport measurements. We find a hysteretic behavior of the NECs and a striking asymmetry of the corresponding magnetic moment around integer filling factors nu=hn(S)/eB. Surprisingly, the shape of the hysteresis loops is the same for sweeps of B or n(S) if plotted versus nu. In a certain parameter regime each NEC signal exhibits a characteristic slope which is found to depend only on the filling factor at large B or n(S). Based on a model considering capacitive coupling between 2DES and gate we attribute the slopes to the conductance quantization of the quantum Hall effect. The NECs are found to be limited by the time-dependent buildup of the radial Hall field governed by the gate capacitance. These findings are in contrast to a floating 2DES without a gate where the breakdown of the quantum Hall effect was previously reported to limit the NECs. Our model also explains the observed shape and dependence on temperature as well as sweep rate. The in situ measurement of the longitudinal resistance allows us to directly correlate the magnetic behavior with both the magnetic field and temperature-dependent resistance of the 2DES.

Applied Physics Letters, 2013

Fluorescence-lifetime imaging microscopy (FLIM) was applied to investigate the donor distribution... more Fluorescence-lifetime imaging microscopy (FLIM) was applied to investigate the donor distribution in SrTiO 3 single crystals. On the surfaces of Nb-and La-doped SrTiO 3 , structures with different fluorescence intensities and lifetimes were found that could be related to different concentrations of Ti 3+ . Furthermore, the inhomogeneous distribution of donors caused a non-uniform conductivity of the surface, which complicates the production of potential electronic devices by the deposition of oxide thin films on top of doped single crystals.

Applied Physics Letters, 2013

Fluorescence-lifetime imaging microscopy (FLIM) was applied to investigate the donor distribution... more Fluorescence-lifetime imaging microscopy (FLIM) was applied to investigate the donor distribution in SrTiO 3 single crystals. On the surfaces of Nb-and La-doped SrTiO 3 , structures with different fluorescence intensities and lifetimes were found that could be related to different concentrations of Ti 3+ . Furthermore, the inhomogeneous distribution of donors caused a non-uniform conductivity of the surface, which complicates the production of potential electronic devices by the deposition of oxide thin films on top of doped single crystals.

Nanotechnology, 2013

We investigated the transport properties of GaAs/InAs core/shell nanowires grown by molecular bea... more We investigated the transport properties of GaAs/InAs core/shell nanowires grown by molecular beam epitaxy. Owing to the band alignment between GaAs and InAs, electrons are accumulated in the InAs shell as long as the shell thickness exceeds 12 nm. By performing simulations using a Schrödinger-Poisson solver, it is confirmed that confined states are present in the InAs shell, which are depleted if the shell thickness is below a threshold value. The existence of a tubular-shaped conductor is proved by performing magnetoconductance measurements at low temperatures. Here, flux periodic conductance oscillations are observed which can be attributed to transport in one-dimensional channels based on angular momentum states.

Nanotechnology, 2013

Important technological steps are discussed and realized for future room-temperature operation of... more 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.

Journal of Applied Physics, 2012

We report on the fabrication and measurements of planar mesoscopic Josephson junctions formed by ... more 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. In 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 superconducting and normal metal terminals. The conductance fluctuation amplitude is found to be about 6 times larger in superconducting contacted nanowires. The enhancement of the conductance fluctuations is attributed to phase-coherent Andreev reflection as well as to the large number of phase-coherent channels due to the large superconducting gap of the Nb electrodes.

Crystal Growth & Design, 2014

The role of Si during the metal−organic vapor phase epitaxy of GaN rods is investigated. Already ... more The role of Si during the metal−organic vapor phase epitaxy of GaN rods is investigated. Already a small amount of Si strongly enhances the vertical growth of GaN. Reactive ion etching experiments show that the inner volume of the rod is much more strongly etched than the m-plane surface layer. Transmission electron microscopy and energy dispersive X-ray spectroscopy measurements reveal that Si is predominiantly incorporated in the surface layer of the m-plane sidewall facets of the rods. The formation of a SiN layer prevents growth on and etching of the m-planes and enhances the mobility of atoms promoting vertical growth. Annealing experiments demonstrate the extraordinary thermal resistivity in comparison to undoped GaN rod structures and GaN layers. The subsequent InGaN quantum well growth on the GaN rods reveals the antisurfactant effect of the SiN layer. A model based on the vapor−liquid−solid growth mode is proposed. The results help to understand the role of Si during growth of GaN rod structures to improve the performance of rod based light emitting and electronic devices.

Physica Status Solidi (C) Current Topics in Solid State Physics, 2009

Materials Research Society Symposium Proceedings, 2012

We present comparative studies of optical properties of GaN nanowires (NWs) obtained by two diffe... more We present comparative studies of optical properties of GaN nanowires (NWs) obtained by two different self-formation techniques: Plasma-Assisted Molecular Beam Epitaxy (PAMBE) growth; and plasma etching of GaN layers deposited by Metal-Organic Vapor Phase Epitaxy (MOVPE). The effects of the coalescence process on grown NW and plasma-induced defects in etched NWs have been studied by photoluminescence (PL) and Raman scattering. In MBE grown NWs, the coalescence-associated defects are extended toward the NW top for intermediate Ga flux. Using High Resolution Electron Microscopy of reactive plasma etching (RIE) NWs, it was found that NWs obtained with an optimal combination of inductive (ICP) and capacitive (RF) plasma are free of extended structural defects. The PL efficiency is strongly increased in plasma etched NWs. However, plasma-induced point defects have to be taken into account for explaining the changes of the PL spectra. Less plasma-induced degradation is observed for high ICP/RF power ratios.

Thin Solid Films, 2011

Magnetic semiconductors III-V semiconductors Ion implantation Rare earth X-ray diffraction Ruther... more Magnetic semiconductors III-V semiconductors Ion implantation Rare earth X-ray diffraction Rutherford backscattering spectroscopy Secondary ion mass spectrometry Magnetic properties of thin films

Physical Review B, 2006

The magnetoconductivity of GaxIn1-xAs/InP quantum wires with widths in the range of 1220-250nm wa... more The magnetoconductivity of GaxIn1-xAs/InP quantum wires with widths in the range of 1220-250nm was investigated. The finite zero-field spin splitting in our samples gives rise to spin relaxation and weak antilocalization in wide wires. In contrast, for the narrow wires, only weak-localization behavior is seen even though the zero-field spin splitting is independent of wire width. The observed renormalization of

Journal of Experimental and Theoretical Physics, 2013

Journal of Crystal Growth, 2001

ABSTRACT

Journal of Crystal Growth, 2009

ABSTRACT

Journal of Applied Physics, 2008

The effect of a small dose of gamma irradiation on transport characteristics of the two-dimension... more The effect of a small dose of gamma irradiation on transport characteristics of the two-dimensional electron gas ͑2DEG͒ in AlGaN / GaN heterostructures was investigated. It is shown that the carrier concentration remains practically unchanged after an irradiation dose of 10 6 rad, while the 2DEG mobility exhibits a considerable increase. The results are explained within a model that takes into account the relaxation of elastic strains and structural-impurity ordering occurring in the barrier layer under irradiation.

Journal of Applied Physics, 2011

The effect of Si-doping on the morphology, structure, and transport properties of nanowires was i... more 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