Nanopatterned Si Substrates Research Articles

Morphology of the Surface of Semipolar GaN Layers during Epitaxy on a Nano-Patterned Si Substrate

Morphology of the Surface of Semipolar GaN Layers during Epitaxy on a Nano-Patterned Si Substrate

Selective-area growth of β-Ga2O3 nanowire films on nano-patterned Si(111) substrate by metal-organic chemical vapor deposition

β-Ga2O3 nanowire films were prepared on Si(111) substrate by selective area growth (SAG) using metal-organic chemical vapor deposition (MOCVD). A large-area periodic nano-hole array TiN mask was fabricated. The effects of MOCVD process parameters on SAG were discussed by calculating Ga supersaturation. It was found that the SAG is closely related to the selective area nucleation and inhibition of lateral growth. Transmission electron microscopy confirmed that β-Ga2O3 nanowires have a preferential orientation of the (002) crystal plane. The sample after in-situ O2 annealing showed an x-ray diffraction intensity increase of (400), (002), and (-111) crystal planes. On this basis, P–Si/β‐Ga2O3 nanowire heterojunctions were fabricated and presented high resistance of the films. At a bias voltage of 20 V, the current decreased from 6.14 × 10−6 to 2.25 × 10−6 A. The MOCVD-based SAG paves a novel way to fabricate β-Ga2O3 nanowire films with high surface-to-volume ratio and homogeneity.

Morphology of the surface of semipolar GaN layers during epitaxy on a nano-patterned Si substrate

It has been studied the morphology of the surface of the semipolar gallium nitride layers synthesized on nano-patterned Si(100) or Si(113) substrates with a V-shaped or U-shaped surface profile, respectively. The morphology of the surface of the semipolar layers indicates that the different height-to-width ratio of the GaN(11-22) and GaN(10-11) blocks is associated with a higher growth rate of the GAN(11-22) face than GaN(10-11) and with different growth rates of the semipolar and polar crystal faces during the nucleation of the layer on a nano-patterned substrate. Keywords: surface morphology, semipolar gallium nitride, nano-patterned silicon substrate.

The growth of hexagonal and cubic GaN on a nano-patterned Si(100) substrate

The results of studies of semipolar GaN(10-12) layers synthesized on a nano-patterned Si(100) substrate are presented. It is shown that in the method metalorganic vapor phase epitaxy, the use of a nanorelief consisting of V-shape groove with inclined faces close to the Si(111) plane can lead to the formation of regions of cubic gallium nitride in the nano-groove. Model of the origin of the cubic phase are based on the formation of AlN nuclei in (0001) and (10-10) nano-groove and the conjugation of the AlN(10-10) and c-GaN planes by the “magic mismatch” mechanism.

TEM investigation of microstructure of semipolar GaN layers grown on nano-patterned Si(001) substrates

The synthesis of III-nitride binary compounds on commercial standard (001) silicon wafers by vapour-phase epitaxy is one of the promising directions for III-nitride technology development. However, the difference between crystal symmetry of Si(001) and wurzite (0001) surface structures is challenge that hinders the development. A use of silicon substrates with nano-patterned surface is one of the solutions to the problem. In this paper we present a transmission electron microscopy study of polar and semipolar GaN layers grown by halide vapour-phase epitaxy and metalorganic vapour-phase epitaxy on nano-patterned silicon (001) substrate. Crystallographic orientation relationships between the silicon substrate and GaN layers is identified. For GaN layers grown by metalorganic vapour-phase epitaxy an effect of SiC buffer layer synthesized by original growth method on their microstructure and surface morphology is under consideration.

Influence of surface passivation on the electrical properties of p–i–n GaAsP nanowires

The electrical properties of passivated and non-passivated axial p–i–n junctions in GaAsP nanowires are investigated using electron-beam induced current microscopy. Organized self-catalyzed p–i–n nanowires having different segment lengths are grown by molecular-beam epitaxy on nanopatterned Si substrates. The nanowires are in situ passivated with a GaP shell. The position of the junction is found to be dependent on the length of the upper Be-doped segment evidencing the diffusion of Be atoms from the upper segment to the bottom part of the nanowire. Comparison between non-passivated and passivated nanowires shows a strong enhancement of the collection region after passivation. The results also prove the existence of a p-doped shell around the nanowires formed due to a parasitic radial growth. This shell is depleted in non-passivated nanowires; however, it becomes electrically active after surface passivation modifying the carrier collection pattern.

Nanoscale electrical analyses of axial-junction GaAsP nanowires for solar cell applications

Axial p–n and p–i–n junctions in GaAs0.7P0.3 nanowires are demonstrated and analyzed using electron beam induced current microscopy. Organized self-catalyzed nanowire arrays are grown by molecular beam epitaxy on nanopatterned Si substrates. The nanowires are doped using Be and Si impurities to obtain p- and n-type conductivity, respectively. A method to determine the doping type by analyzing the induced current in the vicinity of a Schottky contact is proposed. It is demonstrated that for the applied growth conditions using Ga as a catalyst, Si doping induces an n-type conductivity contrary to the GaAs self-catalyzed nanowire case, where Si was reported to yield a p-type doping. Active axial nanowire p–n junctions having a homogeneous composition along the axis are synthesized and the carrier concentration and minority carrier diffusion lengths are measured. To the best of our knowledge, this is the first report of axial p–n junctions in self-catalyzed GaAsP nanowires.

Nanogap Engineering for Enhanced Transmission of Wire Grid Polarizers in Mid-Wavelength Infrared Region

Wire-grid polarizers (WGPs) have been widely used in various fields, such as polarimetry, imaging, display, spectroscopy, and optical isolation. However, conventional WGPs used in diverse mid-wavelength infrared (MWIR) applications show high reflection losses, which intrinsically arise from high refractive indices of their IR-transmitting substrates, such as silicon (Si) and germanium (Ge). This study demonstrated the enhanced transmittance of a transverse magnetic (TM) wave that surpassed ~80% over the entire MWIR range from 3000 to 5000 nm in a narrow air gap of a WGP, where aluminum (Al) was selectively deposited on a nanopatterned Si substrate using an oblique angle deposition method. Moreover, a higher TM wave transmittance was achieved by reducing the air gaps of the WGPs in the nanopatterns, which were distinctly different from the traditional WGPs comprising metal wires patterned directly on a flat substrate. A finite-difference time-domain simulation was performed to investigate optical properties of the proposed WGPs, which showed that the electric field in the air nanogap was remarkably enhanced. The characteristic performances were further investigated using a combination of an effective medium approximation and an admittance diagram, revealing that the broadband transmission enhancement could be attributed to a combined effect of a strong electric field and a better admittance matching. The approach and results described in this paper hold promise for the design and the fabrication of high-quality WGPs, as well as their numerous applications.

GaAs Solar Cells on Nanopatterned Si Substrates

Integrating III–Vs onto Si is a promising route toward tandem photovoltaics and cost mitigation of III–V substrates. While many III–V/Si photovoltaic integration approaches have been studied, epitaxial growth on Si allows for fewer processing steps compared to other approaches. However, current epitaxial pathways utilize expensive techniques, such as thermal cycle annealing or thick buffer layers to control defect densities, undermining the low-cost goal of integrating III–Vs with Si. Here, we present single-junction GaAs solar cells grown directly on Si using selective area growth as an alternative low-cost technique to control material quality with a much thinner buffer. We demonstrate a 10.4%-efficient GaAs device grown on a V-grooved Si substrate, which achieved an antiphase domain-free III–V/Si interface and a threading dislocation density of 2 × 107 cm –2, despite the lack of a graded buffer. We compare this growth on V-grooved Si to solar cells grown on polished Si with and without a patterned silica buffer layer, which demonstrate efficiencies of 6.5% and 6.8%, respectively; the polished Si was patterned using nanoimprint lithography, which is a low-cost patterning technique compatible with III–V selective area growth. Cracking is found to be a critical challenge that hinders solar cell performance and is exacerbated by the V-grooved Si surface topography. The results in this paper provide a promising pathway toward high-efficiency, low-cost III–V/Si tandem photovoltaics.

A comparative study of defect formation in GaAs nanocrystals selectively grown on nanopatterned and flat Si(001) substrates

Crystal defects present in GaAs nanocrystals ∼15–50 nm in diameter and grown by metal organic vapor phase epitaxy on top of two different nanopatterned Si(001) substrates (nanopillars and nanotips with ∼40–80 nm openings embedded in a SiO2 matrix) and on a planar substrate, have been investigated by means of atomic-resolution aberration-corrected scanning transmission electron microscopy. Conditions of their formation are discussed. The defect analysis of the three GaAs/Si systems reveals a higher defect density in the GaAs crystals grown on nanopillars as compared to those grown on nanotips and the planar substrate, possibly concomitant to the atomic-scale irregularities identified at the patterned Si(001) nanopillars. It is concluded that the misfit strain in the GaAs nanocrystals is fully plastically relaxed while no noticeable substrate compliance effects are observed on any of the studied substrates.

Nanostructuring of Si substrates by a metal-assisted chemical etching and dewetting process.

In this work, we reported on the development of lithography-free technology for the fabrication of nanopatterned Si substrates. The combination of two phenomena, the solid-state dewetting process and metal-assisted wet chemical etching, allowed for fabrication of Si nanocolumns on large areas in a relatively simple way. The process of dewetting the thin metal layer enabled formation of nickel nanoislands, which were used as a shadow mask in the deposition of a catalytic metal pattern. Application of the two-stage dewetting process with the repetition of the metal deposition and annealing step enabled us to obtain a significant increase in the surface coverage ratio and the surface density of the nanoislands. As a catalytic metal, a gold layer was applied in the metal-assisted wet chemical etching process. The obtained columnar nanostructures showed a great verticality and had a high aspect ratio. In the conducted studies, the maximum etching rate (at RT) was higher than 1.2 μm min−1. The etching rate increased with increasing concentration of oxidizing (H2O2) and etching (HF) agent, with a tendency to saturate for more concentrated solutions. The etching rate was significantly higher for Si substrates with a crystallographic orientation (115) than for (111), but there was no privileged direction of etching except for the direction vertical to the substrate. With increasing layer thickness of the catalytic metal a decrease in the metal-assisted wet chemical etching process efficiency was observed. The developed technology allows for fabrication of patterned substrates with a wide range of lateral dimension of nanocolumns and their density.

Direct growth of thick AlN layers on nanopatterned Si substrates by cantilever epitaxy

Here, Demir et al. (Article No. 1600363) demonstrate the pulsed atomic layer epitaxy AlN layers that have been grown on a 200 nm period of nano-patterned Si (111) substrates by a cantilever epitaxy via MOCVD, and compared with AlN layers grown by a maskless lateral epitaxial overgrowth (LEO) on micro-patterned Si (111) substrates. The material quality of 5-10 μm thick AlN grown by LEO is comparable to that of the much thinner layers (2 μm) grown by cantilever epitaxy on the nano-patterned substrates. Back emission ultra violet light emitting diodes (UV LEDs) were fabricated by flip chip bonding to patterned AlN heat sinks, followed by a complete Si (111) substrate removal, demonstrating a peak pulsed power of ~0.7 mW at 344 nm peak emission wave-length. Additionally, this study features the first demonstration of UV LEDs on nano-patterned Si substrates.

Direct growth of thick AlN layers on nanopatterned Si substrates by cantilever epitaxy

AlN layers have been grown on 200 nm period of nanopatterned Si (111) substrates by cantilever epitaxy and compared with AlN layers grown by maskless lateral epitaxial overgrowth (LEO) on micropatterned Si (111) substrates. The material quality of 5–10 µm thick AlN grown by LEO is comparable to that of much thinner layers (2 µm) grown by cantilever epitaxy on the nanopatterned substrates. Indeed, the latter exhibited root mean square (RMS) roughness of 0.65 nm and X-ray diffraction full width at half-maximum (FWHM) of 710 arcsec along the (0002) reflection and 930 arcsec along the (10–15) reflection. The corresponding room temperature photoluminescence spectra was dominated by a sharp band edge peak. Back emission ultra violet light emitting diodes (UV LEDs) were fabricated by flip chip bonding to patterned AlN heat sinks followed by complete Si (111) substrate removal demonstrating a peak pulsed power of ∼0.7 mW at 344 nm peak emission wavelength. The demonstrated UV LEDs were fabricated on a cost effective epitaxial structure grown on the nanopatterned Si substrate with a total thickness of 3.3 µm.

Simulation and Experimental Study on Anti-reflection Characteristics of Nano-patterned Si Structures for Si Quantum Dot-Based Light-Emitting Devices.

Surface-textured structure is currently an interesting topic since it can efficiently reduce the optical losses in advanced optoelectronic devices via light management. In this work, we built a model in finite-difference time-domain (FDTD) solutions by setting the simulation parameters based on the morphology of the Si nanostructures and compared with the experimental results in order to study the anti-reflection behaviors of the present nano-patterned structures. It is found that the reflectance is gradually reduced by increasing the depth of Si nanostructures which is in well agreement with the experimental observations. The reflectance can be lower than 10 % in the light range from 400 to 850 nm for Si nano-patterned structures with a depth of 150 nm despite the quite low aspect ratio, which can be understood as the formation of gradually changed index layer and the scattering effect of Si nano-patterned structures. By depositing the Si quantum dots/SiO2 multilayers on nano-patterned Si substrate, the reflectance can be further suppressed and the luminescence intensity centered at 820 nm from Si quantum dots is enhanced by 6.6-fold compared with that of flat one, which can be attributed to the improved light extraction efficiency. However, the further etch time causes the reduction of luminescence intensity from Si quantum dots which may ascribe to the serious surface recombination of carriers.

Pt 금속마스크를 이용하여 제작한 나노패턴 Si(111) 기판위에 성장한 GaN 박막 특성

본 연구에서는 Si(111) 기판을 이용하여 고품질의 GaN 박막을 성장하기 위하여 다양한 패턴을 갖는 Si 기판을 제작하였다. Si(111) 기판위에 이온 스퍼터(ion-sputter)를 이용하여 Pt 박막을 증착한 후 열처리(thermal annealing)하여 Pt 금속 마스크를 형성하고 유도 결합 플라즈마 이온 식각(inductively coupled plasma-reactive ion etching, ICP-RIE) 공정을 통하여 기둥(pillar)형태의 나노 패턴된 Si(111) 기판을 제작하였고 리소그래피 공정을 통하여 마이크로 패턴된 Si(111) 기판을 제작하였다. 일반적인 Si(111) 기판, 마이크로 패턴된 Si(111) 기판 및 나노 패턴된 Si(111) 기판위에 유기화학기상증착(metal organic chemical vapor deposition, MOCVD) 방법으로 GaN 박막을 성장하여 표면 특성과 결정성 및 광학적 특성을 분석하였다. 나노 패턴된 Si(111) 기판위에 성장한 GaN 박막은 일반적인Si(111) 기판과 마이크로 패턴된 Si(111) 기판위에 성장한 GaN 박막보다 표면의 균열과 거칠기가 개선되었다. 나노 패턴된 Si(111) 기판위에 성장한 GaN (002)면과 (102)면에 x-선 회절(x-ray diffraction, XRD) 피크의 반폭치(full width at half maximum, FWHM)는 576 arcsec, 828 arcsec으로 다른 두 기판위에 성장한 GaN 박막 보다 가장 낮은 값을 보여 결정성이 향상되었음을 확인하였다. Photoluminescence(PL)의 반폭치는 나노 패턴된 Si(111) 기판위에 성장한 GaN 박막이 46.5 meV으로 다른 기판위에 성장한 GaN 박막과 비교하여 광학적 특성이 향상되었음을 확인하였다. An attempt to grow high quality GaN on silicon substrate using metal organic chemical vapor deposition (MOCVD), herein GaN epitaxial layers were grown on various Si(111) substrates. Thin Platinum layer was deposited on Si(111) substrate using sputtering, followed by thermal annealing to form Pt nano-clusters which act as masking layer during dry-etched with inductively coupled plasma-reactive ion etching to generate nano-patterned Si(111) substrate. In addition, micro-patterned Si(111) substrate with circle shape was also fabricated by using conventional photo-lithography technique. GaN epitaxial layers were subsequently grown on micro-, nano-patterned and conventional Si (111) substrate under identical growth conditions for comparison. The GaN layer grown on nano-patterned Si (111) substrate shows the lowest crack density with mirror-like surface morphology. The FWHM values of XRD rocking curve measured from symmetry (002) and asymmetry (102) planes are 576 arcsec and 828 arcsec, respectively. To corroborate an enhancement of the growth quality, the FWHM value achieved from the photoluminescence spectra also shows the lowest value (46.5 meV) as compare to other grown samples.

Keynote; Epitaxy-Based Strain-Engineering Methods for Advanced Devices

In Si microtechnology with the economy of scale slackening its pace, energy efficiency is the technology driver of the future. The replacement of metallic by optical interconnects and the rigorous reduction of the supply voltage (VDD) of logic circuitry are suitable options to drastically reduce power consumption. Thus the integration of optoelectronic components and steep slope devices, like the T-FET, will meet this demands. However, both elements will either need or benefit from a direct band gap material, which is not supplied by Si. Possible solutions are the given by III/V compound or by Ge based semiconductors, such as GeSn alloys grown on Si and combinations thereof. These are challenging tasks and many obstacles have to be overcome, like large lattice mismatch, alloys with compositions far beyond the solid solubility limit, antiphase domains and interface and surface states. In this paper we will discuss several epitaxial methods to address these challenges. Furthermore, basing on profound Si technology, possible fabrication routes for electronic and optoelectronic devices will be discussed. Nanotechnology in combination with advanced selective area epitaxy offer unique paths to fabricate high quality III/V nanostructures on Si. In particular templated selfassembly of vertical core/ shell and lateral nanowires on nanopatterned Si substrates will be discussed. The structural properties have been investigated using high resolution transmission microscopy. Fig. 1 depicts a lateral InAs nanowires grown on Si (110) substrates revealing (111) facets at the InAs/Si interface and rotational twins in the InAs nanowire.Regarding photonic devices the most promising group IV material is Ge, since the conduction band minimum at the G-point of the Brillouin-zone (referred to as G-valley) is placed only ca. 140 meV above the fourfold degenerate indirect L-valley. To band engineer Ge towards a direct bandgap semiconductor, two approaches are very promising, Ge with a large tensile strain (> 1.5%) and relaxed GeSn alloys with Sn concentrations above 10% [1,2]. Here, both systems are discussed, however, special emphasis is put on the growth of GeSn and SiGeSn alloys. These are predicted to have a direct band gap energy of less than 0.5 eV. Thus GeSn provides advantages for optoelectronics as well as for ultralow power T-FETs, since the small direct band gap is beneficial for the realization of large tunnel currents. However, due to the large lattice mismatch to Si, the small solubility of Sn in Ge and the necessity to strain engineer the layers, epitaxy is extremely demanding. Here we introduce an epitaxial growth method using enhanced surface kinetical reactions to provide high quality SiGeSn/GeSn heterostructures on Ge virtual substrates on Si with Sn concentrations above 10%.Fig. 2 shows a TEM cross section of a strained Ge layer embedded in relaxed SiGeSn cladding layers grown on a relaxed GeSn buffer layer. The double heterostructure provides a type I band alignment and includes doping levels for active optoelectronic devices. Structural characterization using X-ray diffraction and RBS to determine strain and crystallographic perfection have been performed. Photoluminescence and absorption spectroscopy give insights into the indirect to direct band gap transition in these materials. Finally, novel concepts for Tunnel-FET´s using III/V and group IV heterostructures will be discussed. The recently achieved strained-Si nanowire TFET exhibiting a subthreshold swing of less than 60 mV and large Ion/Ioff ratios are base for the integration of these heterostructure concepts. SiGeSn alloys potentially promise an energy efficient platform by the integration of ultralow power and optoelectronic devices. [1] Camacho-Aguilera, R. E. et al. An electrically pum-ped germanium laser. Opt. Express 20, 11316–20 (2012). [2] Bauer, M. et al. Ge–Sn semiconductors for band-gap and lattice engineering. Appl. Phys. Lett. 81, 2992 (2002).

Light Trapping and Down‐Shifting Effect of Periodically Nanopatterned Si‐Quantum‐Dot‐Based Structures for Enhanced Photovoltaic Properties

Periodically nanopatterned Si structures have been prepared by using a nanosphere lithography technique. The formed nanopatterned structures exhibit good anti‐reflection and enhanced optical absorption characteristics. The mean surface reflectance weighted by AM1.5 solar spectrum (300–1200 nm) is as low as 5%. By depositing Si quantum dot/SiO2 multilayers (MLs) on the nanopatterned Si substrate, the optical absorption is higher than 90%, which is significantly improved compared with the same multilayers deposited on flat Si substrate. Furthermore, the prototype n‐Si/Si quantum dot/SiO2 MLs/p‐Si heterojunction solar cells has been fabricated, and it is found that the external quantum efficiency is obviously enhanced for nanopatterned cell in a wide spectral range compared with the flat cell. The corresponding short‐circuit current density is increased from 25.5 mA cm−2 for flat cell to 29.0 mA cm−2 for nano‐patterned one. The improvement of cell performance can be attributed both to the reduced light loss and the down‐shifting effect of Si quantum dots/SiO2 MLs by forming periodically nanopatterned structures.

Size effects of nano-pattern in Si(1 1 1) substrate on the selective growth behavior of GaN nanowires by MOCVD

We report on the size effects of nano-patterned Si(111) substrates on the selective growth of GaN nanowires (NWs) using metal organic chemical vapor deposition. The nano-patterns on Si(111) substrates were fabricated by etching process of Au nano-droplets. The size of nano-patterns fabricated on Si(111) substrates were corresponding to size of Au nano-droplets, and the diameter of GaN NWs grown on nano-patterns was similar to the size of nano-pattern. Dense and well-oriented GaN NWs were grown on Si(111) substrates corresponding to the nano-patterns with an average diameter of about 50nm. However, only a few GaN bulk grains, and mixed phase of a few NWs and bulk crystal of GaN were grown on the nano-patterned Si(111) having too small and large diameter, respectively, compare to the nano-patterns with diameter of 50nm. Our results suggested that the selective growth of GaN NWs is strongly affected by the size of nano-patterns and its related mechanism.

Polycrystalline Ni thin films on nanopatterned Si substrates: From highly conformal to nonconformal anisotropic growth

The growth of polycrystalline Ni thin films on nanorippled Si templates is investigated in situ by grazing incidence small angle x-ray scattering as well as ex situ by atomic force microscopy and cross-sectional transmission electron microscopy. The templates have been fabricated by low-energy ion sputtering which leads to the spontaneous formation of a periodic ripple pattern with about 35 nm periodicity and about 3 nm peak-to-peak height. Highly conformal growth of the Ni film is observed under normal incidence deposition with the film surface perfectly replicating the substrate morphology up to a film thickness of at least 120 nm. Grazing incidence deposition perpendicular to the ripple orientation leads to the formation of one-dimensional nanowires on one side of the ripples due to geometrical shadowing. At a film thickness of about 10 nm, a transition to anisotropic columnar growth with rapidly decreasing conformity is observed. In this regime, the nanowires act as growth seeds for the columns and further geometrical shadowing leads to a film consisting of rows of tilted columns.

Selective area growth of GaN nanowires using metalorganic chemical vapor deposition on nano-patterned Si(111) formed by the etching of nano-sized Au droplets

We report on the selective area growth of GaN nanowires (NWs) on nano-patterned Si(111) substrates by metalorganic chemical vapor deposition. The nano-patterns were fabricated by the oxidation of Si followed by the etching process of Au nano-droplets. The size of formed nano-pattern on Si(111) substrate was corresponding to the size of Au nano-droplet, and the diameter of GaN NWs grown was similar to the diameter of fabricated nano-pattern. The interesting phenomenon of using the nano-patterned Si(111) substrates is the formation of very clear substrate surface even after the growth of GaN NWs. However, in the case of GaN NWs grown using Au nano-droplets, there was several nanoparticles including GaN bulk grains on the Si(111) substrates. The smooth surface morphology of nano-patterned Si(111) substrates was attributed to the presence of SiO 2 layer which prevents the formation of unnecessary GaN particles during the GaN NW growth. Therefore, we believe that nano-patterning method of Si(111) which was obtained by the oxidation of Si(111) substrate and subsequent Au etching process can be utilized to grow high-quality GaN NWs and its related nano-device applications.