Aerotaxy: gas-phase epitaxy of quasi 1D nanostructures.

Cost- and resource-efficient growth is necessary for many applications of semiconductor nanowires. We here present the design, operational details and theory behind Aerotaxy, a scalable alternative technology for producing quality crystalline nanowires at a remarkably high growth rate and throughput. Using size-controlled Au seed particles and organometallic precursors, Aerotaxy can produce nanowires with perfect crystallinity and controllable dimensions, and the method is suitable to meet industrial production requirements. In this report, we explain why Aerotaxy is an efficient method for fabricating semiconductor nanowires and explain the technical aspects of our custom-built Aerotaxy system. Investigations using SEM (scanning electron microscope), TEM (transmission electron microscope) and other characterization methods are used to support the claim that Aerotaxy is indeed a scalable method capable of producing nanowires with reproducible properties. We have investigated both binary and ternary III-V semiconductor material systems like GaAs and GaAsP. In addition, common aspects of Aerotaxy nanowires deduced from experimental observations are used to validate the Aerotaxy growth model, based on a computational flow dynamics (CFD) approach. We compare the experimental results with the model behaviour to better understand Aerotaxy growth.

Template-assisted vapour-liquid-solid growth of InP nanowires on (001) InP and Si substrates.

We report on the synthesis of vertical InP nanowire arrays on (001) InP and Si substrates using template-assisted vapour-liquid-solid growth. A thick silicon oxide layer was first deposited on the substrates. The samples were then patterned by electron beam lithography and deep dry etching through the oxide layer down to the substrate surface. Gold seed particles were subsequently deposited in the holes of the pattern by the use of pulse electrodeposition. The subsequent growth of nanowires by the vapour-liquid-solid method was guided towards the [001] direction by the patterned oxide template, and displayed a high growth yield with respect to the array of holes in the template. In order to confirm the versatility and robustness of the process, we have also demonstrated guided growth of InP nanowire p-n junctions and InP/InAs/InP nanowire heterostructures on (001) InP substrates. Our results show a promising route to monolithically integrate III-V nanowire heterostructure devices with commercially viable (001) silicon platforms.

Synthesis and Applications of III-V Nanowires.

Low-dimensional semiconductor materials structures, where nanowires are needle-like one-dimensional examples, have developed into one of the most intensely studied fields of science and technology. The subarea described in this review is compound semiconductor nanowires, with the materials covered limited to III-V materials (like GaAs, InAs, GaP, InP,...) and III-nitride materials (GaN, InGaN, AlGaN,...). We review the way in which several innovative synthesis methods constitute the basis for the realization of highly controlled nanowires, and we combine this perspective with one of how the different families of nanowires can contribute to applications. One reason for the very intense research in this field is motivated by what they can offer to main-stream semiconductors, by which ultrahigh performing electronic (e.g., transistors) and photonic (e.g., photovoltaics, photodetectors or LEDs) technologies can be merged with silicon and CMOS. Other important aspects, also covered in the review, deals with synthesis methods that can lead to dramatic reduction of cost of fabrication and opportunities for up-scaling to mass production methods.

Growth kinetics of Ga x In(1-x)P nanowires using triethylgallium as Ga precursor.

Ga x In(1-x)P nanowire arrays are promising for various optoelectronic applications with a tunable band-gap over a wide range. In particular, they are well suited as the top cell in tandem junction solar cell devices. So far, most Ga x In(1-x)P nanowires have been synthesized by the use of trimethylgallium (TMGa). However, particle assisted nanowire growth in metal organic vapor phase epitaxy is typically carried out at relatively low temperatures, where TMGa is not fully pyrolysed. In this work, we developed the growth of Ga x In(1-x)P nanowires using triethylgallium (TEGa) as the Ga precursor, which reduced Ga precursor consumption by about five times compared to TMGa due to the lower homogeneous pyrolysis temperature of TEGa. The versatility of TEGa is shown by synthesis of high yield Ga x In(1-x)P nanowire arrays, with a material composition tunable by the group III input flows, as verified by x-ray diffraction measurements and photoluminescence characterization. The growth dynamics of Ga x In(1-x)P nanowires was assessed by varying the input growth precursor molar fractions and growth temperature, using hydrogen-chloride as in situ etchant. We observed a complex interplay between the precursors. First, trimethylindium (TMIn) inhibits Ga incorporation into the nanowires, resulting in higher In composition in the grown nanowires than in the vapor. Second, the growth rate increases with temperature, indicating a kinetically limited growth, which from nanowire effective binary volume growth rates of InP and GaP can be attributed to the synthesis of GaP in Ga x In(1-x)P. We observed that phosphine has a strong effect on the nanowire growth rate with behavior expected for a unimolecular Langmuir-Hinshelwood mechanism of pyrolysis on a catalytic surface. However, growth rates increase strongly with both TEGa and TMIn precursors as well, indicating the complexity of vapor-liquid-solid growth for ternary materials. One precursor can affect the decomposition of another, and each precursor can affect the wetting properties and catalytic activity of the metal particle.

Understanding InP Nanowire Array Solar Cell Performance by Nanoprobe-Enabled Single Nanowire Measurements.

III-V solar cells in the nanowire geometry might hold significant synthesis-cost and device-design advantages as compared to thin films and have shown impressive performance improvements in recent years. To continue this development there is a need for characterization techniques giving quick and reliable feedback for growth development. Further, characterization techniques which can improve understanding of the link between nanowire growth conditions, subsequent processing, and solar cell performance are desired. Here, we present the use of a nanoprobe system inside a scanning electron microscope to efficiently contact single nanowires and characterize them in terms of key parameters for solar cell performance. Specifically, we study single as-grown InP nanowires and use electron beam induced current characterization to understand the charge carrier collection properties, and dark current-voltage characteristics to understand the diode recombination characteristics. By correlating the single nanowire measurements to performance of fully processed nanowire array solar cells, we identify how the performance limiting parameters are related to growth and/or processing conditions. We use this understanding to achieve a more than 7-fold improvement in efficiency of our InP nanowire solar cells, grown from a different seed particle pattern than previously reported from our group. The best cell shows a certified efficiency of 15.0%; the highest reported value for a bottom-up synthesized InP nanowire solar cell. We believe the presented approach have significant potential to speed-up the development of nanowire solar cells, as well as other nanowire-based electronic/optoelectronic devices.

Electrical and optical evaluation of n-type doping in In x Ga(1-x)P nanowires.

To harvest the benefits of III-V nanowires in optoelectronic devices, the development of ternary materials with controlled doping is needed. In this work, we performed a systematic study of n-type dopant incorporation in dense In x Ga(1-x)P nanowire arrays using tetraethyl tin (TESn) and hydrogen sulfide (H2S) as dopant precursors. The morphology, crystal structure and material composition of the nanowires were characterized by use of scanning electron microscopy, transmission electron microscopy and energy dispersive x-ray analysis. To investigate the electrical properties, the nanowires were broken off from the substrate and mechanically transferred to thermally oxidized silicon substrates, after which electron beam lithography and metal evaporation were used to define electrical contacts to selected nanowires. Electrical characterization, including four-probe resistivity and Hall effect, as well as back-gated field effect measurements, is combined with photoluminescence spectroscopy to achieve a comprehensive evaluation of the carrier concentration in the doped nanowires. We measure a carrier concentration of ∼1 × 1016 cm-3 in nominally intrinsic nanowires, and the maximum doping level achieved by use of TESn and H2S as dopant precursors using our parameters is measured to be ∼2 × 1018 cm-3, and ∼1 × 1019 cm-3, respectively (by Hall effect measurements). Hence, both TESn and H2S are suitable precursors for a wide range of n-doping levels in In x Ga(1-x)P nanowires needed for optoelectronic devices, grown via the vapor-liquid-solid mode.

Intersubband Quantum Disc-in-Nanowire Photodetectors with Normal-Incidence Response in the Long-Wavelength Infrared.

Semiconductor nanowires have great potential for realizing broadband photodetectors monolithically integrated with silicon. However, the spectral range of such detectors has so far been limited to selected regions in the ultraviolet, visible, and near-infrared regions. Here, we report on the first intersubband nanowire heterostructure array photodetectors exhibiting a spectrally resolved photoresponse from the visible to long-wavelength infrared. In particular, the infrared response from 3 to 20 µm is enabled by intersubband transitions in low-bandgap InAsP quantum discs synthesized axially within InP nanowires. The intriguing optical characteristics, including unexpected sensitivity to normal incident radiation, are explained by excitation of the longitudinal component of optical modes in the photonic crystal formed by the nanostructured portion of the detectors. Our results provide a generalizable insight into how broadband nanowire photodetectors may be designed and how engineered nanowire heterostructures open up new, fascinating opportunities for optoelectronics.

Time-resolved photoluminescence characterization of GaAs nanowire arrays on native substrate.

Time-resolved photoluminescence (TRPL) measurements of nanowires (NWs) are often carried out on broken-off NWs in order to avoid the ensemble effects as well as substrate contribution. However, the development of NW-array solar cells could benefit from non-destructive optical characterization to allow faster feedback and further device processing. With this work, we show that different NW array and substrate spectral behaviors with delay time and excitation power can be used to determine which part of the sample dominates the detected spectrum. Here, we evaluate TRPL characterization of dense periodic as-grown GaAs NW arrays on a p-type GaAs substrate, including a sample with uncapped GaAs NWs and several samples passivated with AlGaAs radial shell of varied composition and thickness. We observe a strong spectral overlap of substrate and NW signals and find that the NWs can absorb part of the substrate luminescence signal, thus resulting in a modified substrate signal. The level of absorption depends on the NW-array geometry, making a deconvolution of the NW signal very difficult. By studying TRPL of substrate-only and as-grown NWs at 770 and 400 nm excitation wavelengths, we find a difference in spectral behavior with delay time and excitation power that can be used to assess whether the signal is dominated by the NWs. We find that the NW signal dominates with 400 nm excitation wavelength, where we observe two different types of excitation power dependence for the NWs capped with high and low Al composition shells. Finally, from the excitation power dependence of the peak TRPL signal, we extract an estimate of background carrier concentration in the NWs.

Room-temperature InP/InAsP Quantum Discs-in-Nanowire Infrared Photodetectors.

The possibility to engineer nanowire heterostructures with large bandgap variations is particularly interesting for technologically important broadband photodetector applications. Here we report on a combined study of design, fabrication, and optoelectronic properties of infrared photodetectors comprising four million n+-i-n+ InP nanowires periodically ordered in arrays. The nanowires were grown by metal-organic vapor phase epitaxy on InP substrates, with either a single or 20 InAsP quantum discs embedded in the i-segment. By Zn compensation of the residual n-dopants in the i-segment, the room-temperature dark current is strongly suppressed to a level of pA/NW at 1 V bias. The low dark current is manifested in the spectrally resolved photocurrent measurements, which reveal strong photocurrent contributions from the InAsP quantum discs at room temperature with a threshold wavelength of about 2.0 µm and a bias-tunable responsivity reaching 7 A/W@1.38 µm at 2 V bias. Two different processing schemes were implemented to study the effects of radial self-gating in the nanowires induced by the nanowire/SiOx/ITO wrap-gate geometry. Summarized, our results show that properly designed axial InP/InAsP nanowire heterostructures are promising candidates for broadband photodetectors.

Bias-dependent spectral tuning in InP nanowire-based photodetectors.

Nanowire array ensembles contacted in a vertical geometry are extensively studied and considered strong candidates for next generations of industrial scale optoelectronics. Key challenges in this development deal with optimization of the doping profile of the nanowires and the interface between nanowires and transparent top contact. Here we report on photodetection characteristics associated with doping profile variations in InP nanowire array photodetectors. Bias-dependent tuning of the spectral shape of the responsivity is observed which is attributed to a Schottky-like contact at the nanowire-ITO interface. Angular dependent responsivity measurements, compared with simulated absorption spectra, support this conclusion. Furthermore, electrical simulations unravel the role of possible self-gating effects in the nanowires induced by the ITO/SiO x wrap-gate geometry. Finally, we discuss possible reasons for the observed low saturation current at large forward biases.

InxGa1-xP Nanowire Growth Dynamics Strongly Affected by Doping Using Diethylzinc.

Semiconductor nanowires are versatile building blocks for optoelectronic devices, in part because nanowires offer an increased freedom in material design due to relaxed constraints on lattice matching during the epitaxial growth. This enables the growth of ternary alloy nanowires in which the bandgap is tunable over a large energy range, desirable for optoelectronic devices. However, little is known about the effects of doping in the ternary nanowire materials, a prerequisite for applications. Here we present a study of p-doping of InxGa1-xP nanowires and show that the growth dynamics are strongly affected when diethylzinc is used as a dopant precursor. Specifically, using in situ optical reflectometry and high-resolution transmission electron microscopy we show that the doping results in a smaller nanowire diameter, a more predominant zincblende crystal structure, a more Ga-rich composition, and an increased axial growth rate. We attribute these effects to changes in seed particle wetting angle and increased TMGa pyrolysis efficiency upon introducing diethylzinc. Lastly, we demonstrate degenerate p-doping levels in InxGa1-xP nanowires by the realization of an Esaki tunnel diode. Our findings provide insights into the growth dynamics of ternary alloy nanowires during doping, thus potentially enabling the realization of such nanowires with high compositional homogeneity and controlled doping for high-performance optoelectronics devices.

Growth of wurtzite Al x Ga1-x P nanowire shells and characterization by Raman spectroscopy.

The phonon energies of AlGaP in wurtzite crystal structure are generally not known, as opposed to their zincblende counterparts, because AlGaP crystallizes in zincblende phase in bulk and thin films structures. However, in nanowires AlGaP can be grown in wurtzite crystal structure. In this work we have grown wurtzite GaP/AlGaP/GaP core-shell nanowires by use of MOVPE. After developing suitable growth conditions, the Al composition was determined by STEM-EDX measurements and the wurtzite AlGaP phonon energies by Raman spectroscopy. Raman measurements show a peak shift with increasing Al composition in the AlGaP shell. We find that the phonon energies for wurtzite AlGaP are slightly lower than for zincblende AlGaP. Our results can be used to determine the Al composition in wurtzite AlGaP by Raman scattering.

In situ characterization of nanowire dimensions and growth dynamics by optical reflectance.

Optical reflectometry is commonly used as an accurate and noninvasive characterization tool when growing planar semiconductor layers. However, thin-film analysis schemes cannot be directly applied to nanowire systems due to their complex optical response. Here, we report on reliable in situ characterization of nanowire growth with high accuracy using optical reflectance spectra for analysis. The method makes it possible to determine the nanowire length, diameter, and growth rate in situ in real time with high resolution. We demonstrate the method's versatility by using the optical reflectance data for determining nanowire dimensions on both particle-assisted and selective-area grown nanowires. To indicate the full potential of in situ characterization of nanowire synthesis we evaluate the growth dynamics of InP nanowires in the presence of the p-type dopant precursor diethylzinc. We observe that the growth rate is strongly affected by the diethylzinc. At low diethylzinc flows, the growth rate decreases monotonously while higher flows lead to an initially increasing growth rate. From these in situ characterization data, we conclude that the surface migration length of adatom species is affected strongly by the addition of diethylzinc. We believe that this characterization method will become a standard tool for in situ growth monitoring and aid in elucidating the complex growth dynamics often exhibited during nanowire growth.

Structural Properties of wurtzite InP-InGaAs nanowire core-shell heterostructures.

We report on growth and characterization of wurtzite InP-In(1-x)Ga(x)As core-shell nanowire heterostructures. A range of nanowire structures with different Ga concentration in the shell was characterized with transmission electron microscopy and X-ray diffraction. We found that the main part of the nanowires has a pure wurtzite crystal structure, with occasional stacking faults occurring only at the top and bottom. This allowed us to determine the structural properties of wurtzite In(1-x)Ga(x)As. The InP-In(1-x)Ga(x)As core-shell nanowires show a triangular and hexagonal facet structure of {1100} and {101̅0} planes. X-ray diffraction measurements showed that the core and the shell are pseudomorphic along the c-axis, and the strained axial lattice constant is closer to the relaxed In(1-x)Ga(x)As shell. Microphotoluminescence measurements of the nanowires show emission in the infrared regime, which makes them suitable for applications in optical communication.

A comparative study of absorption in vertically and laterally oriented InP core-shell nanowire photovoltaic devices.

We have compared the absorption in InP core-shell nanowire p-i-n junctions in lateral and vertical orientation. Arrays of vertical core-shell nanowires with 400 nm pitch and 280 nm diameter, as well as corresponding lateral single core-shell nanowires, were configured as photovoltaic devices. The photovoltaic characteristics of the samples, measured under 1 sun illumination, showed a higher absorption in lateral single nanowires compared to that in individual vertical nanowires, arranged in arrays with 400 nm pitch. Electromagnetic modeling of the structures confirmed the experimental observations and showed that the absorption in a vertical nanowire in an array depends strongly on the array pitch. The modeling demonstrated that, depending on the array pitch, absorption in a vertical nanowire can be lower or higher than that in a lateral nanowire with equal absorption predicted at a pitch of 510 nm for our nanowire geometry. The technology described in this Letter facilitates quantitative comparison of absorption in laterally and vertically oriented core-shell nanowire p-i-n junctions and can aid in the design, optimization, and performance evaluation of nanowire-based core-shell photovoltaic devices.

Study of carrier concentration in single InP nanowires by luminescence and Hall measurements.

The free electron carrier concentrations in single InP core-shell nanowires are determined by micro-photoluminescence, cathodoluminescence (CL) and Hall effect measurements. The results from luminescence measurements were obtained by solving the Fermi-Dirac integral, as well as by analyzing the peak full width at half maximum (FWHM). Furthermore, the platform used for Hall effect measurements, combined with spot mode CL spectroscopy, is used to determine the carrier concentrations at specific positions along single nanowires. The results obtained via luminescence measurements provide an accurate and rapid feedback technique for the epitaxial development of doping incorporation in nanowires. The technique has been employed on several series of samples in which growth parameters, such as V/III-ratio, temperature and dopant flows, were investigated in an optimization procedure. The correlation between the Hall effect and luminescence measurements for extracting the carrier concentration of different samples were in excellent agreement.

III-V nanowire synthesis by use of electrodeposited gold particles.

Semiconductor nanowires are great candidates for building novel electronic devices. Considering the cost of fabricating such devices, substrate reuse and gold consumption are the main concerns. Here we report on implementation of high throughput gold electrodeposition for selective deposition of metal seed particles in arrays defined by lithography for nanowire synthesis. By use of this method, a reduction in gold consumption by a factor of at least 300 was achieved, as compared to conventional thermal evaporation for the same pattern. Because this method also facilitates substrate reuse, a significantly reduced cost of the final device is expected. We investigate the morphology, crystallography, and optical properties of InP and GaAs nanowires grown from electrodeposited gold seed particles and compare them with the properties of nanowires grown from seed particles defined by thermal evaporation of gold. We find that nanowire synthesis, as well as the material properties of the grown nanowires are comparable and quite independent of the gold deposition technique. On the basis of these results, electrodeposition is proposed as a key technology for large-scale fabrication of nanowire-based devices.

Synthesis of doped InP core-shell nanowires evaluated using hall effect measurements.

InP core-shell nanowire pn-junctions doped with Zn and Sn have been investigated in terms of growth morphology and shell carrier concentration. The carrier concentrations were evaluated using spatially resolved Hall effect measurements and show improved homogeneity compared to previous investigations, attributed to the use of Sn as the n-type dopant. Anisotropies in the growth rate of different facets are found for different doping levels that in turn affects the migration of Sn and In on the nanowire surface. A route for increasing the In migration length to obtain a more homogeneous shell thickness is presented.

Optical characterization of InAs quantum wells and dots grown radially on wurtzite InP nanowires.

Correlated micro-photoluminescence (µPL) and cathodoluminescence (CL) measurements are reported for single core-shell InP-InAs wurtzite nanowires grown using metal-organic vapor phase epitaxy. Samples covering a radial InAs shell thickness of 1-12 ML were investigated. The effective masses for the wurtzite material were determined from the transition energy dependence of the InAs shell thickness, using a model based on linear deformation potential theory. InP cores with segments of mixed zincblende and wurtzite, on which quantum dots nucleated selectively, were also investigated. Narrow peaks were observed by µPL and the spatial origin of the emission was identified with CL imaging.

Optical far-field method with subwavelength accuracy for the determination of nanostructure dimensions in large-area samples.

The physical, chemical, and biological properties of nanostructures depend strongly on their geometrical dimensions. Here we present a fast, noninvasive, simple-to-perform, purely optical method that is capable of characterizing nanostructure dimensions over large areas with an accuracy comparable to that of scanning electron microscopy. This far-field method is based on the analysis of unique fingerprints in experimentally measured reflectance spectra using full three-dimensional optical modeling. We demonstrate the strength of our method on large-area (millimeter-sized) arrays of vertical InP nanowires, for which we simultaneously determine the diameter and length as well as cross-sample morphological variations thereof. Explicitly, the diameter is determined with an accuracy better than 10 nm and the length with an accuracy better than 30 nm. The method is versatile and robust, and we believe that it will provide a powerful and standardized measurement technique for large-area nanostructure arrays suitable for both research and industrial applications.