A. Van Helvoort - Academia.edu (original) (raw)

Papers by A. Van Helvoort

Nano Letters, 2015

Device configurations that enable a unidirectional propagation of carriers in a semiconductor are... more Device configurations that enable a unidirectional propagation of carriers in a semiconductor are fundamental components for electronic and optoelectronic applications. To realize such devices, however, it is generally required to have complex processes to make p-n or Schottky junctions. Here we report on a unidirectional propagation effect due to a self-induced compositional variation in GaAsSb nanowires (NWs). The individual GaAsSb NWs exhibit a highly reproducible rectifying behavior, where the rectifying direction is determined by the NW growth direction. Combining the results from confocal micro-Raman spectroscopy, electron microscopy, and electrical measurements, the origin of the rectifying behavior is found to be associated with a self-induced variation of the Sb and the carrier concentrations in the NW. To demonstrate the usefulness of these GaAsSb NWs for device applications, NW-based photodetectors and logic circuits have been made.

We report on the low temperature micro-photoluminescence (µ-PL) of single GaAs/AlGaAs core-shell ... more We report on the low temperature micro-photoluminescence (µ-PL) of single GaAs/AlGaAs core-shell nanowires (NWs) with axial GaAsSb core-inserts grown by Auassisted molecular beam epitaxy. The GaAs NW core has hexagonal wurtzite (WZ) crystal structure, whereas the GaAsSb NW core-insert has cubic zinc blende (ZB) crystal structure. The upper GaAsSb/GaAs interface contains a few nm thin cubic ZB GaAs segment just before the crystal structure switches back to WZ. By adding a growth interruption (GI) directly after the GaAsSb insert, the ZB GaAs segment in the WZ GaAs barrier can be avoided. This GIinduced crystal structure change of the upper GaAs barrier next to the GaAsSb/GaAs interface makes a large difference on the PL properties from the GaAsSb NW core-inserts. This is believed to be due to that the GaAsSb/GaAs heterojunction band alignment is changed from type II to type I when the GaAs crystal structure is changed from ZB to WZ.

ABSTRACT A method for probing the electrical and structural characteristics of individual as-grow... more ABSTRACT A method for probing the electrical and structural characteristics of individual as-grown III-V nanowires was studied. In-situ electrical characterization was performed in a focused ion beam / scanning electron microscopy system by using a fine nano-manipulator and ion beam assisted deposition. Transmission electron microscopy specimens of probed nanowires are prepared afterwards. This method would potentially allow the correlation of electrical and structural characteristics (e.g. crystal faults such as twinning) of the nanowire-substrate system. The challenge is in contacting the nanowires so that the electrical characteristics of the nanowire-substrate system can be extracted correctly.

Materials Research Society Symposium Proceedings, 2008

Page 1. Optical Properties of Single Wurtzite GaAs Nanowires and GaAs Nanowires with GaAsSb Inser... more Page 1. Optical Properties of Single Wurtzite GaAs Nanowires and GaAs Nanowires with GaAsSb Inserts Thang B. Hoang1, Hailong Zhou1, Anthonysamy F. Moses1, Dasa L. Dheeraj1, ATJ van Helvoort2,Bjørn-Ove Fimland1 and Helge Weman1 ...

Nano letters, 2008

We have demonstrated the growth of a unique wurtzite (WZ) GaAs nanowire (NW) with a zinc blende (... more We have demonstrated the growth of a unique wurtzite (WZ) GaAs nanowire (NW) with a zinc blende (ZB) GaAsSb insert by Au-assisted molecular beam epitaxy. An abrupt interface from the WZ GaAs phase to the ZB GaAsSb phase was observed, whereas an intermediate segment of a 4H polytype GaAs phase was found directly above the ZB GaAsSb insert. A possible mechanism for the different phase transitions is discussed. Furthermore, low temperature microphotoluminescence (micro-PL) measurements showed evidence of quantum confinement of holes in the GaAsSb insert.

Semiconductor Science and Technology, 2013

In this work we have investigated the variation of Sb concentration among and within zinc blende ... more In this work we have investigated the variation of Sb concentration among and within zinc blende (ZB) GaAsSb inserts in wurtzite (WZ) GaAs bare-core and WZ GaAs/AlGaAs core-shell nanowires (NWs) grown by Au-assisted molecular beam epitaxy. The Sb concentration variation was related to the optical properties as determined by photoluminescence (PL). The NW structure and the Sb concentration were studied by transmission electron microscopy (TEM), energy dispersive x-ray spectroscopy (EDX) and quantitative high angle annular dark field scanning TEM (HAADF STEM). A clear trend relating the maximum Sb concentration with the insert length was observed: the longer the insert, the higher the Sb concentration. In addition, there are graded Sb concentration gradients both along and across the GaAsSb inserts. The influence of the Sb concentration variation on the PL emission from the GaAsSb inserts was investigated with correlated micro-PL and TEM-EDX on the same single NWs. Based on the PL results and the observed Sb concentration profiles, we propose a qualitative energy band diagram for a typical ZB GaAsSb insert in a WZ GaAs NW for the heterostructured NWs studied here. Type I transitions within the central region of the ZB GaAsSb inserts were found to dominate the insert-related PL emission. Weak type II transitions within the inserts due to the graded Sb concentration were observed as well. Using an existing empirical model, the Sb concentrations were additionally determined from the ground state PL energies (type I transition). For the average Sb concentration, the concentrations based on PL were in agreement with EDX and quantitative HAADF STEM results.

Nanotechnology, 2008

We report the growth of GaAsSb nanowires (NWs) on GaAs(111)B substrates by Au-assisted molecular ... more We report the growth of GaAsSb nanowires (NWs) on GaAs(111)B substrates by Au-assisted molecular beam epitaxy. The structural characteristics of the GaAsSb NWs have been investigated in detail. Their Sb mole fraction was found to be about 25%. Their crystal structure was found to be pure zinc blende (ZB), in contrast to the wurtzite structure observed in GaAs NWs grown under similar conditions. The ZB GaAsSb NWs exhibit rotational twins around their [111]B growth axis, with twin-free segments as long as 500 nm. The total volumes of GaAsSb segments with twinned and un-twinned orientations, respectively, were found to be equal by x-ray diffraction analysis of NW ensembles.

Nanotechnology, 2009

We report the growth of GaAs/AlGaAs core-shell nanowires (NWs) on GaAs(111)B substrates by Au-ass... more We report the growth of GaAs/AlGaAs core-shell nanowires (NWs) on GaAs(111)B substrates by Au-assisted molecular beam epitaxy. Electron microscopy shows the formation of a wurtzite AlGaAs shell structure both in the radial and the axial directions outside a wurtzite GaAs core. With higher Al content, a lower axial and a higher radial growth rate of the AlGaAs shell were observed. Room temperature and low temperature (4.4 K) micro-photoluminescence measurements show a much higher radiative efficiency from the GaAs core after the NW is overgrown with a radial AlGaAs shell.

Nanotechnology, 2013

Control of the crystal phases of GaAs nanowires (NWs) is essential to eliminate the formation of ... more Control of the crystal phases of GaAs nanowires (NWs) is essential to eliminate the formation of stacking faults which deteriorate the optical and electronic properties of the NWs. In addition, the ability to control the crystal phase of NWs provides an opportunity to engineer the band gap without changing the crystal material. We show that the crystal phase of GaAs NWs grown on GaAs(111)B substrates by molecular beam epitaxy using the Au-assisted vapor-liquid-solid growth mechanism can be tuned between wurtzite (WZ) and zinc blende (ZB) by changing the V/III flux ratio. As an example we demonstrate the realization of WZ GaAs NWs with a ZB GaAs insert that has been grown without changing the substrate temperature.

Nano Letters, 2010

We report on a crystal phase-dependent photoluminescence (PL) polarization effect in individual w... more We report on a crystal phase-dependent photoluminescence (PL) polarization effect in individual wurtzite GaAs nanowires with a zinc blende GaAsSb insert grown by Au-assisted molecular beam epitaxy. The PL emission from the zinc blende GaAsSb insert is strongly polarized along the nanowire axis while the emission from the wurtzite GaAs nanowire is perpendicularly polarized. The results indicate that the crystal phases, through optical selection rules, are playing an important role in the alignment of the PL polarization in nanowires besides the linear polarization induced by the dielectric mismatch. The strong excitation power dependence and long recombination lifetimes (∼4 ns) from the wurtzite GaAs and zinc blende GaAsSb-related PL emission strongly indicate the existence of type II band alignments in the nanowire due to the presence of nanometer thin zinc blende segments and stacking faults in the wurtzite GaAs barrier.

Nano Letters, 2012

The optical properties of the wurtzite (WZ) GaAs crystal phase found in nanowires (NWs) are a hig... more The optical properties of the wurtzite (WZ) GaAs crystal phase found in nanowires (NWs) are a highly controversial topic. Here, we study high-quality pure WZ GaAs/AlGaAs core− shell NWs grown by Au-assisted molecular beam epitaxy (MBE) with microphotoluminescence spectroscopy (μ-PL) and (scanning) transmission electron microscopy on the very same single wire. We determine the room temperature (294 K) WZ GaAs bandgap to be 1.444 eV, which is ∼20 meV larger than in zinc blende (ZB) GaAs, and show that the free exciton emission at 15 K is at 1.516 eV. On the basis of time-and temperature-resolved μ-PL results, we propose a Γ 8 conduction band symmetry in WZ GaAs. We suggest a method for quantifying the optical quality of NWs, taking into consideration the difference between the room and low temperature integrated PL intensity, and demonstrate that Au-assisted GaAs/AlGaAs core−shell NWs can have high PL brightness up to room temperature.

Nano Letters, 2014

We report on the epitaxial growth of large-area position-controlled self-catalyzed GaAs nanowires... more We report on the epitaxial growth of large-area position-controlled self-catalyzed GaAs nanowires (NWs) directly on Si by molecular beam epitaxy (MBE). Nanohole patterns are defined in a SiO 2 mask on 2 in. Si wafers using nanoimprint lithography (NIL) for the growth of positioned GaAs NWs. To optimize the yield of vertical NWs the MBE growth parameter space is tuned, including Ga predeposition time, Ga and As fluxes, growth temperature, and annealing treatment prior to NW growth. In addition, a non-negligible radial growth is observed with increasing growth time and is found to be independent of the As species (i.e., As 2 or As 4 ) and the growth temperatures studied. Cross-sectional transmission electron microscopy analysis of the GaAs NW/Si substrate heterointerface reveals an epitaxial growth where NW base fills the oxide hole opening and eventually extends over the oxide mask. These findings have important implications for NW-based device designs with axial and radial p−n junctions. Finally, NIL positioned GaAs/AlGaAs core−shell heterostructured NWs are grown on Si to study the optical properties of the NWs. Room-temperature photoluminescence spectroscopy of ensembles of as-grown core−shell NWs reveals uniform and high optical quality, as required for the subsequent device applications. The combination of NIL and MBE thereby demonstrates the successful heterogeneous integration of highly uniform GaAs NWs on Si, important for fabricating high throughput, large-area position-controlled NW arrays for various optoelectronic device applications.

Micron, 2013

The Sb concentration in axial GaAs 1−x Sb x inserts of otherwise pure GaAs nanowires has been inv... more The Sb concentration in axial GaAs 1−x Sb x inserts of otherwise pure GaAs nanowires has been investigated with quantitative high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). The Sb concentration was quantified by comparing the experimental image intensities normalized to the incident beam intensity with intensities simulated with a frozen lattice multislice approach. Including static atomic displacements in the simulations was found to be crucial for correct compositional analysis of GaAs 1−x Sb x . HAADF intensities of individual nanowires were analysed both across the nanowires, exploiting their hexagonal cross-sectional shape, and along the evenly thick central part of the nanowires. From the cross-sectional intensity profiles, a decrease in the Sb concentration towards the nanowire outer surfaces was found. The longitudinal intensity profiles revealed a gradual build-up of Sb in the insert. The decrease of the Sb concentration towards the upper interface was either gradual or abrupt, depending on the growth routine chosen. The compositional analysis with quantitative HAADF-STEM was verified by energy dispersive X-ray spectroscopy.

Journal of Physics: Conference Series, 2011

To correlate optical properties to structural characteristics, we developed a robust strategy for... more To correlate optical properties to structural characteristics, we developed a robust strategy for characterizing the same individual heterostructured semiconductor nanowires (NWs) by alternating low temperature micro-photoluminescence (µ-PL), low voltage scanning (transmission) electron microscopy and conventional transmission electron microscopy. The NWs used in this work were wurtzite GaAs core with zinc blende GaAsSb axial insert and AlGaAs radial shell grown by molecular beam epitaxy. The series of experiments demonstrated that high energy (200 kV) electrons are detrimental for the optical properties, whereas medium energy (5-30 kV) electrons do not affect the PL response. Thus, such medium energy electrons can be used to select NWs for correlated optical-structural studies prior to µ-PL or in NW device processing. The correlation between the three main µ-PL bands and crystal phases of different compositions, present in this heterostructure, is demonstrated for selected NWs. The positions where a NW fractures during specimen preparation can considerably affect the PL spectra of the NW. The effects of crystal-phase variations and lattice defects on the optical properties are discussed. The established strategy can be applied to other nanosized electro-optical materials, and other characterization tools can be incorporated into this routine.

Journal of Physics: Conference Series, 2010

ABSTRACT Synthesis and application of semiconductor nanowires (NWs) require detailed understandin... more ABSTRACT Synthesis and application of semiconductor nanowires (NWs) require detailed understanding of their structures. Transmission electron microscopy (TEM) has proven to be the most used structural characterisation tool for these nanostructures. For the characterisation of more complex heterostructured NWs, in this study axial (i.e. with "insert") or radial (i.e. "core-shell") III-V NWs, we use dark field TEM and high angle annular dark field scanning TEM (HAADF-STEM) as imaging modes. For the GaAs/GaAsSb/GaAs axial heterostructured NWs, dark field TEM exploits the structural difference between wurtzite GaAs NW and zinc blende GaAsSb insert, resulting in an excellent contrast between the GaAs part and the GaAsSb insert of the NW. In addition, twins, stacking faults and polytypes of the GaAs phase can easily be identified. For the radial GaAs/AlGaAs core-shell NWs where both core and shell have the wurtzite structure, HAADF-STEM is found to be a very useful technique.

Journal of Crystal Growth, 2013

We report on the growth, structural and electrical characterizations of Be-doped GaAs nanowires (... more We report on the growth, structural and electrical characterizations of Be-doped GaAs nanowires (NWs) grown by the Au-and Ga-assisted vapour-liquid-solid techniques using molecular beam epitaxy. The growth rate of Be-doped GaAs NWs grown by the Au-assisted technique is observed to be lower as compared to the growth rate of undoped GaAs NWs grown under identical conditions. However, no effect on either the growth rate or the morphology of NWs has been observed for Be-doped GaAs NWs grown by the Ga-assisted technique with the same Be flux as used for the Au-assisted technique. Electrical characterization reveals that the Ga-assisted grown NWs show more consistent, symmetric current-voltage (I-V) characteristics with higher electrical current than the Au-assisted grown NWs. Finally, we show that ohmic contacts to Be-doped Ga-assisted NWs can be achieved either by post-annealing the metal-contacted NW or increasing the doping concentration during the NW growth.

IOP Conference Series: Materials Science and Engineering, 2009

We report on the low temperature micro-photoluminescence (mu-PL) of single GaAs/AlGaAs core-shell... more We report on the low temperature micro-photoluminescence (mu-PL) of single GaAs/AlGaAs core-shell nanowires (NWs) with axial GaAsSb core-inserts grown by Au-assisted molecular beam epitaxy. The GaAs NW core has hexagonal wurtzite (WZ) crystal structure, whereas the GaAsSb NW core-insert has cubic zinc blende (ZB) crystal structure. The upper GaAsSb/GaAs interface contains a few nm thin cubic ZB GaAs segment just

Microscopy and Microanalysis, 2014