Selective Area Metalorganic Vapor Phase Research Articles

Characterization of the parasitic masking layer formed during GaN SA-MOVPE using PECVD SiO2 masks

X-ray photoelectron spectroscopy (XPS) was used to study the surface composition of the parasitic masking layer formed during selective area metalorganic vapor phase epitaxy (MOVPE) of gallium nitride (GaN) using silicon dioxide (SiO2) masks deposited by plasma enhanced chemical vapor deposition (PECVD). It was shown that non-stoichiometric silicon oxynitride layer (SiOxNy) is responsible for the inhibition of GaN nucleation in the vicinity of the SiO2 mask. Atomic force microscopy (AFM) was applied to examine the surface topography, whereas scanning capacitance microscopy (SCM) and scanning spreading resistance microscopy (SSRM) techniques were used for the purpose of electrical characterization of the parasitic masking layer. The overgrowth of GaN islands through the discontinuities in the SiOxNy film was shown. The SCM imaging showed the presence of a positive electric charge in the parasitic masking layer, indicating the accumulation of defects in the silicon oxynitride film.

Effect of Plasmonic Ag Nanoparticles on Emission Properties of Planar GaN Nanowires.

The combination of plasmonic nanoparticles and semiconductor substrates changes the properties of hybrid structures that can be used for various applications in optoelectronics, photonics, and sensing. Structures formed by colloidal Ag nanoparticles (NPs) with a size of 60 nm and planar GaN nanowires (NWs) have been studied by optical spectroscopy. GaN NWs have been grown using selective-area metalorganic vapor phase epitaxy. A modification of the emission spectra of hybrid structures has been observed. In the vicinity of the Ag NPs, a new emission line appears at 3.36 eV. To explain the experimental results, a model considering the Fröhlich resonance approximation is suggested. The effective medium approach is used to describe the enhancement of emission features near the GaN band gap.

Parasitic masking effect in GaN SA-MOVPE using SiO2 masks deposited by the PECVD technique

The paper reports a phenomenon of parasitic substrate masking during selective area metalorganic vapor phase epitaxy (SA-MOVPE) of gallium nitride (GaN) using silicon dioxide (SiO2) masks deposited by plasma enhanced chemical vapor deposition (PECVD) at pressures above 150 hPa. The discussion of the thermal stability and nitridation process of the SiO2 films is followed by the presentation of the experimental work results focused on the investigation of the relationship between the parasitic masking effect and pressure in the MOVPE reactor chamber. Analysis of the surface morphology is conducted using scanning electron microscopy (SEM) and atomic force microscopy (AFM). A hypothesis was proposed explaining the formation of the deposition-free region that indicates silanimine (HNSi) layer formation as a possible factor responsible for the adverse inhibition of GaN nucleation.

Deformed Honeycomb Lattices of InGaAs Nanowires Grown on Silicon‐on‐Insulator for Photonic Crystal Surface‐Emitting Lasers

AbstractPhotonic crystals can be used to achieve high‐performance surface‐emitting lasers and enable novel photonic topological insulator devices. In this work, a GaAs/InGaAs heterojunction nanowire platform by selective area metalorganic vapor phase epitaxy for such applications is demonstrated. The nanowires are arranged into deformed honeycomb lattices on silicon‐on‐insulator substrate to exploit the quadrupolar photonic band‐edge mode. Core–shell and axial heterostructures are formed with their crystalline properties studied by scanning transmission electron microscopy. Room‐temperature, single mode lasing from both stretched and compressed honeycomb lattices within the telecom‐O band, with lasing threshold as low as 1.25 µJ cm−2 is demonstrated. The potential of using InGaAs nanowire‐based honeycomb lattices for small‐divergence surface‐emitting lasers and topological edge mode lasers is investigated. Finite‐difference time‐domain far field simulations suggest a sub‐10° beam divergence can be achieved thanks to the out‐of‐plane diffraction.

AlGaN Microfins as Nonpolar UV Emitters Probed by Time-Resolved Cathodoluminescence

Despite the continuous technological progress and recent commercialization of UV light emitting diodes for sterilization and disinfection applications, the performance of solid-state UV emitters still lags far behind that of their InGaN-based counterparts, which emit in the visible blue–green spectrum. Both fundamental physical aspects and material quality restrictions have been discussed as origin of this striking difference. In this study, GaN/AlGaN core–shell microfins with a-plane sidewalls are proposed as a model system to demonstrate the efficiency potential of ultra-low-defect nonpolar UV emitters. Such structures are manufactured by bottom-up selective-area metalorganic vapor phase epitaxy of GaN microfin cores and further overgrowth of the AlGaN active region, employing a compositionally graded GaN/AlGaN short-period superlattice for strain management. Using this approach, mostly crack-free AlGaN quantum wells emitting around 321 nm could be realized with a threading dislocation density of as low as (6 ± 3) × 107 cm–2. The carrier dynamics within the nonpolar AlGaN structures are studied by time-resolved cathodoluminescence spectroscopy, revealing fast radiative recombination lifetimes down to 0.6 ns and distinct signatures of carrier localization at low temperatures. Delayed carrier emission from localized states within the cladding is proposed to be a cause for anomalously high effective carrier lifetimes observed at temperatures below 80 K. By analyzing the characteristic temperature dependence of radiative and nonradiative recombination processes and fitting the lifetime data with an appropriate model across the whole temperature range, a room-temperature internal quantum efficiency of (40 ± 6) % is estimated for the studied sample. The findings corroborate the interest in nonpolar AlGaN structures as highly efficient UV emitters.

Emission color modulation of InGaN/GaN multiple quantum wells by selective area metalorganic vapor phase epitaxy on hexagonal windows

We investigated influences of mask pattern on the emission from InGaN multiple quantum wells through the differences in plane orientation appearing on the multifaceted islands in selective area metalorganic vapor phase epitaxy. Cathodoluminescence mapping confirmed that emission colors changed depending on the crystal plane. Photoluminescence spectroscopy showed that the emission wavelength red-shifted by increasing the mask width. By combining the difference of indium incorporation efficiency depending on the crystal plane and the lateral vapor phase diffusion effect, multiple quantum wells with different emission wavelengths of up to 106 nm were grown simultaneously in the microscale region.

A New Strategy for Selective Area Growth of Highly Uniform InGaAs/InP Multiple Quantum Well Nanowire Arrays for Optoelectronic Device Applications

AbstractIII‐V semiconductor nanowires with quantum wells (QWs) are promising for ultra‐compact light sources and photodetectors from visible to infrared spectral region. However, most of the reported InGaAs/InP QW nanowires are based on the wurtzite phase and exhibit non‐uniform morphology due to the complex heterostructure growth, making it challenging to incorporate multiple‐QWs (MQW) for optoelectronic applications. Here, a new strategy for the growth of InGaAs/InP MQW nanowire arrays by selective area metalorganic vapor phase epitaxy is reported. It is revealed that {110} faceted InP nanowires with mixed zincblende and wurtzite phases can be achieved, forming a critical base for the subsequent growth of highly‐uniform, taper‐free, hexagonal‐shaped MQW nanowire arrays with excellent optical properties. Room‐temperature lasing at the wavelength of ≈1 µm under optical pumping is achieved with a low threshold. By incorporating dopants to form an n+‐i‐n+ structure, InGaAs/InP 40‐QW nanowire array photodetectors are demonstrated with the broadband response (400–1600 nm) and high responsivities of 2175 A W−1 at 980 nm outperforming those of conventional planar InGaAs photodetectors. The results show that the new growth strategy is highly feasible to achieve high‐quality InGaAs/InP MQW nanowires for the development of future optoelectronic devices and integrated photonic systems.

Magnetization switching depending on magnetic fields applied to ferromagnetic MnAs nanodisks selectively-grown on Si (111) substrates

We report on the applied external magnetic field, B, dependence of a magnetic domain structure and magnetization switching in MnAs nanodisks on AlGaAs nanopillar buffers selectively grown on Si (111) substrates partially covered with dielectric SiO2 thin film mask patterns by selective-area metal–organic vapor phase epitaxy. The results on the B dependence of magnetic domain structures observed by magnetic force microscopy show that the ratio, or percentage, of a single magnetic domain is minimized at B = −1.5 kG in the nanodisks with an area of 4 × 104 nm2 or smaller, although the decrease to the minimum of the ratio is markedly small in the case of the nanodisks with an area of 4 × 104 nm2 or larger at B = −0.5 kG. The angle distribution of magnetization directions shows that the magnetization directions markedly tend to be parallel to the ridge directions of the hexagonal nanodisks, i.e., distribute in steps of ∼60° corresponding to the magnetic easy axes of the hexagonal NiAs-type crystal structure. The results suggest that the magnetic domains and coercive force can be tuned by controlling the MnAs nanodisk size.

Feature size control using surface reconstruction temperature in block copolymer lithography for InAs nanowire growth

Here we present a method to control the size of the openings in hexagonally organized BCP thin films of poly(styrene)-block-poly(4-vinylpyridine) (PS-b-P4VP) by using surface reconstruction. The surface reconstruction is based on selective swelling of the P4VP block in ethanol, and its extraction to the surface of the film, resulting in pores upon drying. We found that the BCP pore diameter increases with ethanol immersion temperature. In our case, the temperature range 18 to 60 °C allowed fine-tuning of the pore size between 14 and 22 nm. A conclusion is that even though the molecular weight of the respective polymer blocks is fixed, the PS-b-P4VP pore diameter can be tuned by controlling temperature during surface reconstruction. These results can be used for BCP-based nanofabrication in general, and for vertical nanowire growth in particular, where high pattern density and diameter control are of importance. Finally, we demonstrate successful growth of indium arsenide InAs vertical nanowires by selective-area metal-organic vapor phase epitaxy (MOVPE), using a silicon nitride mask patterned by the proposed PS-b-P4VP surface reconstruction lithography method.

Impact of InGaAs carrier collection quantum well on the performance of InAs QD active region lasers fabricated by diblock copolymer lithography and selective area epitaxy

Quantum dot (QD) laser diodes, where the active region consists of wetting-layer-free InAs QDs, were demonstrated by block copolymer (BCP) lithography and subsequent selective area metal-organic vapor phase epitaxy, which results in the QD density of ∼4 × 1010 cm2. In this work, we show that an In0.1Ga0.9As QW located in close proximity to the wetting-layer-free InAs QDs leads to an enhanced carrier injection into the QDs, allowing for lasing at room temperature (RT). Devices employing InAs QDs grown with and without In0.1Ga0.9As QW carrier collection layer exhibit lasing at 80 K, while only the lasers grown with the In0.1Ga0.9As QW exhibits lasing at RT. Driving current-dependent electroluminescence measurements reveals a low carrier injection into the QDs in the device grown without In0.1Ga0.9As QW carrier collection layer. In addition, the appearance of lasing emission, corresponding to high energy transitions, indicates a relatively flat gain profile for this InAs QD active region grown by SA-MOVPE. In addition, EL measurements at 80 K on devices employing varying thickness (dInAs) QDs indicate the peak emission wavelength varies from 920 nm (dInAs ∼ 1.5 nm) to 974 nm (dInAs = 3 nm), although the devices employing dInAs ∼ 3 nm did not exhibit lasing, possibly because of a significant degree of strain relaxation for this QD thickness.

3D GaN Fins as a Versatile Platform for a‐Plane‐Based Devices

GaN fins on GaN‐on‐sapphire templates are fabricated by continuous mode selective area metalorganic vapor phase epitaxy. The fins exhibit high aspect ratios and smooth nonpolar a‐plane sidewalls with an ultra‐low threading dislocation density of a few 105 cm−2 making them ideally suited for optoelectronic to electronic applications. A detailed analysis of the inner structure of GaN fins is provided by the help of marker layer experiments and correlation of results from fins fabricated under different growth conditions, leading to the development of a growth model to explain the final geometry and optical as well as electrical properties of these high aspect ratio fins. Distinctly different material properties for the central and outer parts of the fins are detected. Whereas the outer sidewalls represent high quality GaN surfaces with very low defect densities, a strong quenching of near band edge emission (NBE) in the central part of the fins is accompanied by heavy compensation of free electrons. A possible explanation is the incorporation of excessive point defects, like intrinsic defects or carbon impurity. The sidewall regions, however, prove to be highly suitable for device applications due to their strong NBE emission, low dislocation density, and high free carrier concentration.

Selective-area growth of magnetic MnAs nanodisks on Si (1 1 1) substrates using multiple types of dielectric masks

We report on selective-area metal-organic vapor phase epitaxy and structural characterizations of magnetic MnAs nanodisk structures after the AlGaAs buffer layer growth on Si (1 1 1) substrates using multiple types of mask materials and designs, which are mask patterns (I) and (II) of SiO2 and SiON removing (I) the mask materials outside 100 × 100 µm2 square regions with periodical circular openings and (II) only the periodical circular openings within the 100 × 100 µm2 square regions. The results of structural characterizations show that two key issues, which are the unintentional growth of MnSi alloy in Si substrates and MnAs alloy with approximately 17% of Si atoms on nanodisks, are typically observed in the case of Si substrates with mask pattern (I) of SiO2. On the other hand, these two key issues are not observed in the case of Si substrates with mask pattern (II) of SiON. These results suggest that mask pattern (II) of SiON are effective to prevent the unintentional reactions of Mn atoms with Si substrates for growing MnAs nanodisks with a high degree of uniformity.

Magnetic domain structure and domain wall analysis of ferromagnetic MnAs nanodisks selectively-grown on Si (111) substrates for spintronic applications

We report on the experimental and analytic results on magnetic domain and domain wall structures of MnAs nanodisks on AlGaAs nanopillar buffers selectively grown on Si (111) substrates partially covered with dielectric SiO2 thin film mask patterns using selective-area metal-organic vapor phase epitaxy. The results on the size dependence of the magnetic domain structure in MnAs nanodisks investigated by magnetic force microscopy show that a single domain is predominant in the MnAs nanodisks with an area of approximately 3 × 104 nm2 or less. It is also indicated that in the nanodisks with an area of approximately 6 × 104 nm2 or more, multiple domains, in particular, two magnetic domain structures with a 180° domain wall, are predominant. In addition, in the case of nanodisks with multiple domains, not only Néel walls but also Bloch walls are possibly formed, according to the detailed analyses of the magnetic force microscope images obtained. These results suggest that the magnetic domains and domain walls can be tuned by the control of the MnAs nanodisk size making them interesting nanostructures for spintronic applications.

Growth and characterization of GaAs nanowires on Ge(1 1 1) substrates by selective-area MOVPE

We report the growth of GaAs nanowires (NWs) on a Ge substrate using selective-area metal-organic vapor phase epitaxy (SA-MOVPE) for solar cell applications. Vertical GaAs NWs were aligned on a non-polar Ge(1 1 1) substrate by implementing As surface treatment and buffer layer growth. The transmission electron microscope (TEM) showed the occurrence of a twin in the NW and no dislocation at the GaAs/Ge interface. Photoluminescence spectra suggested luminescence from a type-II quantum well structure originated from the twin and defect-levels due to Ga vacancies and Ge interdiffusion from the Ge substrate.

Selective-area growth of pulse-doped InAs nanowires on Si and vertical transistor application

III-V compound semiconductors are promising channel materials for the future low-power and high-performance transistor because of their high electron/hole mobility. Here, we report on the integration of vertical InAs nanowire (NW)-channels on Si by selective-area metalorganic vapor phase epitaxy (MOVPE) with a pulse doping technique and demonstration of an InAs NW vertical surrounding-gate transistors. The device had a small subthreshold slope of 68 mV/decade, a normalized transconductance of 0.25 μS/μm, and on/off ratio of around 107. The axial junction with the pulse doping effectively suppressed off-state leakage current resulting in good electrostatic gate control.

Efficient Green Emission from Wurtzite Al xIn1- xP Nanowires.

Direct band gap III–V semiconductors, emitting efficiently in the amber–green region of the visible spectrum, are still missing, causing loss in efficiency in light emitting diodes operating in this region, a phenomenon known as the “green gap”. Novel geometries and crystal symmetries however show strong promise in overcoming this limit. Here we develop a novel material system, consisting of wurtzite AlxIn1–xP nanowires, which is predicted to have a direct band gap in the green region. The nanowires are grown with selective area metalorganic vapor phase epitaxy and show wurtzite crystal purity from transmission electron microscopy. We show strong light emission at room temperature between the near-infrared 875 nm (1.42 eV) and the “pure green” 555 nm (2.23 eV). We investigate the band structure of wurtzite AlxIn1–xP using time-resolved and temperature-dependent photoluminescence measurements and compare the experimental results with density functional theory simulations, obtaining excellent agreement. Our work paves the way for high-efficiency green light emitting diodes based on wurtzite III-phosphide nanowires.

Reducing Zn diffusion in single axial junction InP nanowire solar cells for improved performance

In this work axial n-i-p junction InP nanowires were grown by selective-area metal organic vapor phase epitaxy (SA-MOVPE) technique with the growth sequence starting from n-segment. The optical properties and carrier lifetimes of the n-, i- and p-type segments were studied and compared using time-resolved photoluminescence (PL) and cathodoluminescence (CL) measurements. We demonstrate for the first time that CL is capable of resolving the electrical profile of the nanowires, namely the varied lengths of the n-, i- and p-segments, providing a simple and effective approach for nanowire growth calibration and optimization. The CL result was further confirmed by electron beam induced current (EBIC) and photocurrent mapping measurements performed from the fabricated single nanowire solar cell devices. It is revealed that despite a non-optimized device structure (very long n-region and short i-region), the n-i-p nanowire solar cells show improved power conversion efficiency (PCE) than the previously reported p-i-n (growth starts with p-segment) single nanowire solar cells due to reduced p-type dopant (Zn) diffusion during the growth of n-i-p solar cell structure.

High In-content InGaN nano-pyramids: Tuning crystal homogeneity by optimized nucleation of GaN seeds

Uniform arrays of submicron hexagonal InGaN pyramids with high morphological and material homogeneity, reaching an indium composition of 20%, are presented in this work. The pyramids were grown by selective area metal-organic vapor phase epitaxy and nucleated from small openings in a SiN mask. The growth selectivity was accurately controlled with diffusion lengths of the gallium and indium species, more than 1 μm on the SiN surface. High material homogeneity of the pyramids was achieved by inserting a precisely formed GaN pyramidal seed prior to InGaN growth, leading to the growth of well-shaped InGaN pyramids delimited by six equivalent 101¯1 facets. Further analysis reveals a variation in the indium composition to be mediated by competing InGaN growth on two types of crystal planes, 101¯1 and (0001). Typically, the InGaN growth on 101¯1 planes is much slower than on the (0001) plane. The formation of the (0001) plane and the growth of InGaN on it were found to be dependent on the morphology of the GaN seeds. We propose growth of InGaN pyramids seeded by 101¯1-faceted GaN pyramids as a mean to avoid InGaN material grown on the otherwise formed (0001) plane, leading to a significant reduction of variations in the indium composition in the InGaN pyramids. The InGaN pyramids in this work can be used as a high-quality template for optoelectronic devices having indium-rich active layers, with a potential of reaching green, yellow, and red emissions for LEDs.

Analyses of magnetization switching and magnetic domains in lateral MnAs nanowires in combination with structural characterization

We report on structural and magnetic characterization results of lateral MnAs nanowires formed by selective-area metal–organic vapor phase epitaxy. The lateral MnAs nanowires observed by backscattered and secondary electron microscopies have a high aspect ratio (i.e., 2.6 µm long and 360 nm wide) and well-defined crystal facets. The external magnetic field dependence in magnetic force microscopy observations of the lateral nanowires reveals that single and multiple magnetic domains were formed in the lateral nanowires. This study suggests that magnetic domains are tuned by the structural control of lateral nanowires. However, no magnetization switching is observed under a relatively high external magnetic field of 5000 G, which should be enough for the magnetization switching of the lateral nanowires, possibly owing to the magnetization switching of the magnetic force microscope tip itself used in this study during observations.

Single-photon emission from InAsP quantum dots embedded in density-controlled InP nanowires

We attempted to control the density and size of InP-based nanowires (NWs) and nanowire quantum dots (NW-QDs) during selective-area metalorganic vapor phase epitaxy. InP nanowire arrays with a 5 µm pitch and an average NW diameter d of 67 nm were successfully grown by optimization of growth conditions. InAsP quantum dots were embedded in these density-controlled InP NW arrays, and clear single-photon emission and exciton-biexciton cascaded emission were confirmed by excitation-dependent photoluminescence and photon correlation measurements.