Growth Of Epitaxial Layers Research Articles

Nano-Scale Smoothing of Double Layer Porous Si Substrates for Detaching and Fabricating Low Cost, High Efficiency Monocrystalline Thin Film Si Solar Cell By Zone Scanning Annealing

1. Introduction Monocrystalline Si solar cell (c-Si SC) prepared from Si wafer which cut and sliced from Si ingot exhibit kerf loss and also has extra thickness of Si wafer for power generation given absorbance. In this study, we therefore focused on “thin film c-Si SC”. Thin Si is expected to reduce cost and improve open circuit voltage. Fig.1 outlines the preparation of a thin film c-Si SC. A seed layer which also works as a detach layer is fabricated on monocrystalline Si wafer, and then a thin film Si is deposited by epitaxial growth on it and is then detached. However defects of the seed layer are influenced to the crystallinity of epitaxial growth film, which is a key factor of photoelectric conversion efficiency as c-Si SC.A previously proposed low cost fabrication method for c-Si SC uses double layer porous Si (DLPS) as the seed layer for epitaxial growth.1) DLPS consists of a low porosity layer (LPL) on top and a high porosity layer (HPL) at the bottom as a detach layer. Thin film Si is prepared by epitaxial growth on DLPS as a seed layer, and it is possible to prepare thin film c-Si SC and to reuse of the Si wafer by mechanical detachment. Thick and high porosity HPL is needed to improve the detaching process. However, the nonuniform structure of LPL, including surface roughness (Rms) and crystalline structure, which is the base of epitaxial growth affects the quality of the thin Si film. So, a treatment after the formation of DLPS is one approach to achieving a balance between improved efficiency of thin film c-Si SC and the yield of the detaching process. To achieve this balance, we previously proposed the “zone-melting crystallization (ZMC) method2)” (Fig.2) in which amorphous Si is melted and crystallized at the solid-liquid interface by melting Si surface in a line (using lamp line heater) and then scanned in one direction. Previously, SiO2/Si/SiO2 tri-laminar structure films with smooth amorphous Si annealed using high-speed ZMC (7.0 mm/s) and monocrystalline Si film oriented (100) were successfully prepared 3). In this study, the seed layer of DLPS have less crystal defects and smoother surface were fabricated by applying ZMC to retain the structure of HPL while changing only the surface of LPL by using a scanning lamp heater. The dependence of nano-order surface structure on the amount of heat produced by the scanning lamp heater was investigated. 2. Results and Discussion 2-1. DLPS preparation Fig.3 shows cross-section FE-SEM image of DLPS, revealing successful preparation by two-step anodization of monocrystalline Si. Fig.4 shows the dependence of Rms of DLPS on the HPL thickness, revealing that Rms increased increasing HPL thickness. Because thicker HPL makes it easier to detach the epitaxial Si thin film, there is a trade-off between Rms and improving the yield of the detaching process. 2-2. Effect of using ZMC method for DLPS The optimal conditions for ZMC method (i.e., the amount of heat and scan speed) were evaluated by calculating the total amount of heat based on the heat, time and area.Fig.5 shows the dependence of Rms on the total amount of heat, revealing that Rms was minimum when scan speed was 5 mm/s and the total amount of heat was 20 × 103 kJm-2. Rms, Peak-to-valley and pore size on the surface decreased to 0.631 (17%), 2.248 (16%), 27.5 (2.3%) nm, respectively. And then, Rms increased with increasing amount of heat higher than 20 × 103 kJm-2. Rms also depended on scan speed, and tended to increase Rms with too slow scanning. This dependence explains why the porous Si structure was changed by heating; the surface energy decreased by heating, thus promoting surface oxidation by hydrogen desorption. Moreover, although the percentage of upper lamp heating in the total amount of heat when scan speed of it is as small as 1, 5, 15, 26 mm/s is 24, 6.2, 2.1, 1.3%, respectively, Rms is largely influenced by the amount of heat from the upper lamp heater. In summary, Rms can be successfully decreased by adjusting the lamp heater and minimizing the effect of LPL of DLPS.In conclusion, heating the DLPS by the ZMC method is crucial for controlling nano surface structure. Controlling surface oxidization and hydrogen desorption is also crucial. 3. References 1) M. Karim, etal., Nanoscale Research Letters, 9, 348 (2014).2) M. Ihara, etal., Applied Physics Letters 79, 3809 (2001).3) S. Yokoyama, etal., IEEE, 312-315 (2002). Figure 1

Abrupt SiGe and Si Profile Fabrication by Introducing C Delta Layer

High quality and steep Si / Si0.5Ge0.5 / Si profile is fabricated by introducing a C delta layer at the interface using reduced pressure chemical vapor deposition system. The Si0.5Ge0.5 and Si layers are deposited by H2-SiH4-GeH4 at 500oC and H2-Si2H6 at 500oC to 575oC, respectively. By introducing a C delta layer at the surface, roughening of the Si0.5Ge0.5 surface is maintained at 575oC due to suppressed surface migration of Si and Ge as well as defect injection into the Si0.5Ge0.5 layer resulting in high crystallinity Si cap layer growth. Adsorbed CH3 species at the surface are preventing the epitaxial Si cap layer growth at 500oC, but it is possible to deposit high quality epitaxial Si at higher temperature because of hydrogen-desorption from adsorbed CH3. Interdiffusion of Si and Ge at the interface is observed at 525oC in the case of sample without C delta layers, but the interdiffusion is suppressed even at 575oC by introducing C delta layers.

Molecular beam epitaxial growth and characterization of InAs layers on GaAs (001) substrate

High-quality InAs epilayers were grown onto GaAs (001) substrate by molecular beam epitaxy. The optimal growth conditions were examined over a wide range of substrate temperature, substrate offcut orientation, and As4/In flux ratio. The surface morphology, electrical and structural properties were investigated by Nomarski optical microscopy, Hall effect measurement, and X-ray diffraction, respectively. It is worth noting that InAs layers grown on GaAs (001) substrate with 2° offcut towards 〈110〉 have better crystalline quality and electrical properties than that grown on GaAs substrate without offcut. The results indicated that the layers grown at 400 °C, with a group V/III flux ratio of 8.5, yielded to the highest electrical quality, with a Hall mobility of 22,420 cm2/Vs at 80 K and 12,970 cm2/Vs at room temperature. It is found that the top part of 5 µm-thick InAs layer exhibits a high Hall mobility of 77,380 and 25,275 cm2/Vs, at 80 and 300 K, respectively.

Growth and optical properties of FePt:BaTiO3 nanocomposite films

FePt:BaTiO3 nanocomposite films consisting of a regular stacking of pure BaTiO3 (BTO) layers and BTO layers embedding FePt nanocrystals were grown on (001) MgO substrates by pulsed laser deposition. Transmission electron microscopy experiments on cross-sectional specimens demonstrated the high quality of the epitaxial growth of the different layers, whatever their composition. The structural analysis also gave evidence of a misfit strain relaxation at the BTO/MgO interface. We report and discuss the optical and pholuminescence properties of the nanocomposite films in comparison with the ones recorded in pure BTO films. Our results emphasize the role of nanostructuration and growth processes on the evolution of the observed properties.

Impact of LT-GaAs layers on crystalline properties of the epitaxial GaAs films grown by MBE on Si substrates

GaAs films with low-temperature GaAs (LT-GaAs) layers were grown by molecular beam epitaxy (MBE) method on vicinal (001) Si substrates oriented 6° off towards [110]. The grown structures were different with the thickness of LT-GaAs layers and its arrangement in the film. The processes of epitaxial layers nucleation and growth were controlled by reflection high energy electron diffraction (RHEED) method. Investigations of crystalline properties of the grown structures were carried out by the methods of X-ray diffraction (XRD) and transmission electron microscopy (TEM). The crystalline perfection of the GaAs films with LT-GaAs layers and the GaAs films without ones was comparable. It was found that in the LT- GaAs/Si layers the arsenic clusters are formed, as it occurs in the LT-GaAs/GaAs system without dislocation. It is shown that large clusters are formed mainly on the dislocations. However, the clusters have practically no effect on the density and the propagation path of threading dislocations. With increasing thickness of LT-GaAs layer the dislocations are partly bent along the LT-GaAs/GaAs interface due to the presence of stresses.

Successful Fabrication of GaN Epitaxial Layer on Non‐Catalytically‐grown Graphene

Sapphire is widely used as a substrate for the growth of GaN epitaxial layer (EPI), but has several drawbacks such as high cost, large lattice mismatch, non‐flexibility, and so on. Here, we first employ graphene directly grown on Si or sapphire substrate as a platform for the growth and lift‐off of GaN‐light‐emitting diode (LED) EPI, useful for not only recycling the substrate but also transferring the GaN‐LED EPI to other flexible substrates. Sequential standard processes of nucleation/recrystallization of GaN seeds and deposition of undoped (u‐) GaN/AlN buffer layer were done on graphene/substrate before the growth of GaN‐LED EPI, accompanied by taping and lift‐off of u‐GaN/AlN or GaN‐LED EPI. This approach can overcome the limitations by the catalytic growth and transfer of graphene, and make the oxygen‐plasma treatment of graphene for the growth of GaN EPI unnecessary.

Epitaxial NbN/AlN/NbN tunnel junctions on Si substrates with TiN buffer layers

We have developed epitaxial NbN/AlN/NbN tunnel junctions on Si (100) substrates with a TiN buffer layer. A 50-nm-thick (200)-oriented TiN thin film was introduced as the buffer layer for epitaxial growth of NbN/AlN/NbN trilayers on Si substrates. The fabricated NbN/AlN/NbN junctions demonstrated excellent tunneling properties with a high gap voltage of 5.5 mV, a large IcRN product of 3.8 mV, a sharp quasiparticle current rise with a ΔVg of 0.4 mV, and a small subgap leakage current. The junction quality factor Rsg/RN was about 23 for the junction with a Jc of 47 A/cm2 and was about 6 for the junction with a Jc of 3.0 kA/cm2. X-ray diffraction and transmission electron microscopy observations showed that the NbN/AlN/NbN trilayers were grown epitaxially on the (200)-orientated TiN buffer layer and had a highly crystalline structure with the (200) orientation.

Bulk Growth of Large Area SiC Crystals

The growth of large diameter silicon carbide (SiC) crystals produced by the physical vapor transport (PVT) method is outlined. Methods to increase the crystal diameters, and to turn these large diameter crystals into substrates that are ready for the epitaxial growth of SiC or other non homogeneous epitaxial layers are discussed. We review the present status of 150 mm and 200 mm substrate quality at Cree, Inc. in terms of crystallinity, dislocation density as well as the final substrate surface quality.

Homoepitaxial Chemical Vapor Deposition of up to 150 μm Thick 4H-SiC Epilayers in a 10×100 mm Batch Reactor

In this paper we present results on the growth of low-doped thick epitaxial layers on 4° off-oriented 4H-SiC using a warm-wall multi-wafer CVD system (Aixtron VP2800WW). Statistical data on doping and thickness of 25 μm to 40 μm layer growth show results similar to standard epilayer growth (5-15 μm). Improvements in thickness and doping uniformity as well as the reduction of epitaxial defects has boosted the quality of 25 μm to 40 μm thick epilayers. Laser light scattering measurements resulted in projected device yields with median values of 83% and 96% for 5×5 mm2 and 2×2 mm2 die size, respectively. This corresponds to a low epitaxial defect density of < 0.75 cm-2 in 25-40 μm thick epilayers. This paper also presents results of 60 μm to 150 μm thick epitaxial layer growth. Excellent results for doping, thickness and carrier lifetime were achieved. As an example results of a fully loaded 10×100mm run with 150 μm thick epilayers are presented. Wafer-to-wafer doping and thickness values of 3.7 % and 3.4% for sigma/mean were accomplished, respectively. Typical average lifetime values of 5 μs to 6 μs were measured on the 150 μm thick layers without post-epi treatments.

Formation of the seed layers for layer-transfer process silicon solar cells by zone-heating recrystallization of porous silicon structures

Thin-film crystalline silicon is promising for photovoltaic application to reduce the cost of photovoltaic energy. Porous silicon structures have been intensively studied as a seed layer for epitaxial growth of thin Si film and layer-transfer process (LTP). In this article, another approach for LTP has been proposed. The seed layers for epitaxial silicon growth have been formed by zone-heating recrystallization of double-layer por-Si structures. The influence of annealing parameters on porous silicon structures was studied. The transformation of por-Si layer to crystalline Si was observed with the formation of smooth continuous surface with the roughness 0.3 nm, peak-to-valley distance around 3.5 nm, and reduced density of pores. The mechanism of the transformation of por-Si surface due to the action of hydrogen in the passivated pores with preventing surface oxidation was proposed.

Growth mechanisms for Si epitaxy on O atomic layers: Impact of O-content and surface structure

The epitaxial growth of Si layers on Si substrates in the presence of O atoms is generally considered a challenge, as O atoms degrade the epitaxial quality by generating defects. Here, we investigate the growth mechanisms for Si epitaxy on O atomic layers (ALs) with different O-contents and structures. O ALs are deposited by ozone (O3) or oxygen (O2) exposure on H-terminated Si at 50°C and 300°C respectively. Epitaxial Si is deposited by chemical vapor deposition using silane (SiH4) at 500°C. After O3 exposure, the O atoms are uniformly distributed in Si-Si dimer/back bonds. This O layer still allows epitaxial seeding of Si. The epitaxial quality is enhanced by lowering the surface distortions due to O atoms and by decreasing the arrival rate of SiH4 reactants, allowing more time for surface diffusion. After O2 exposure, the O atoms are present in the form of SiOx clusters. Regions of hydrogen-terminated Si remain present between the SiOx clusters. The epitaxial seeding of Si in these structures is realized on H-Si regions, and an epitaxial layer grows by a lateral overgrowth mechanism. A breakdown in the epitaxial ordering occurs at a critical Si thickness, presumably by accumulation of surface roughness.

GaN Epitaxial Layer Grown with Conductive Al(x)Ga(1-x)N Buffer Layer on SiC Substrate Using Metal Organic Chemical Vapor Deposition.

This study investigated GaN epitaxial layer growth with a conductive Al(x)Ga(1-x)N buffer layer on n-type 4H-SiC by high-temperature metalorganic chemical vapor deposition (HT-MOCVD). The Al composition of the Al(x)Ga(1-x)N buffer was varied from 0% to 100%. In terms of the crystal quality of the GaN layer, 79% Al was the optimal composition of the Al(x)Ga(1-x)N buffer layer in our experiment. A vertical conductive structure was fabricated to measure the current voltage (I-V) characteristics as a function of Al composition, and the I-V curves showed that the resistance increased with increasing Al concentration of the Al(x)Ga(1-x)N buffer layer.

Growth and characterization of epitaxial aluminum layers on gallium-arsenide substrates for superconducting quantum bits

The quest for a universal quantum computer has renewed interest in the growth of superconducting materials on semiconductor substrates. High-quality superconducting thin films will make it possible to improve the coherence time of superconducting quantum bits (qubits), i.e., to extend the time a qubit can store the amplitude and phase of a quantum state. The electrical losses in superconducting qubits highly depend on the quality of the metal layers the qubits are made from. Here, we report on the epitaxy of single-crystal Al (011) layers on GaAs (001) substrates. Layers with 110 nm thickness were deposited by means of molecular beam epitaxy at low temperature and monitored by in situ reflection high-energy electron diffraction performed simultaneously at four azimuths. The single-crystal nature of the layers was confirmed by ex situ high-resolution x-ray diffraction. Differential interference contrast and atomic force microscopy analysis of the sample’s surface revealed a featureless surface with root mean square roughness of 0.55 nm. A detailed in situ study allowed us to gain insight into the nucleation mechanisms of Al layers on GaAs, highlighting the importance of GaAs surface reconstruction in determining the final Al layer crystallographic orientation and quality. A highly uniform and stable GaAs (001)- reconstruction reproducibly led to a pure Al (011) phase, while an arsenic-rich GaAs (001)- reconstruction yielded polycrystalline films with an Al (111) dominant orientation. The near-atomic smoothness and single-crystal character of Al films on GaAs, in combination with the ability to trench GaAs substrates, could set a new standard for the fabrication of superconducting qubits.

Perfect Separation of Hybrid Orientation Structure of CeO2(100) and (110) Regions Grown on SOI Substrates with Lithographically Formed Trenches

Using magnetron sputtering with low energy electron beam irradiation, epitaxial growth of CeO2 layers on Si(100) substrates has been studied for the application to microelectronics. We have found that orientation selective epitaxial (OSE) growth of CeO2(100) and (110) layers on Si(100) is capable by controlling surface potential distribution. With the aim of application to hybrid orientation technology for higher speed CMOS devices, we are studying the hybrid orientation structure of the CeO2(100) and (110) regions on Si(100) substrates using electron beam-induced OSE growth by reactive magnetron sputtering. Two separate areas of growth are seen, with CeO2(100) layers found to grow in areas irradiated by electrons during the growth process, and the CeO2(110) layers growing in the areas without irradiation. The lateral orientation mapping by X-ray diffraction measurements reveal the existence of transition regions between these two orientation areas. The width of the transition region is found to be considerably large and decrease proportionally as the logarithm of the underlying Si substrate resistivity. To make a breakthrough in the limitation in reduction of the transition region width, we propose a new method of OSE growth on silicon on insulator (SOI) substrates with lithographically formed trenches. The trenches prevent spread of the potential distribution to the neighboring Si islands. We report the experimental results showing perfect isolation between OSE grown regions having different crystallographic orientations, optimizing the Si layer thickness of SOI and the cross-sectional geometry of the trenches. Figure 1

On a reduction in cracking upon the growth of AlN on Si substrates by hydride vapor-phase epitaxy

The main problem of the epitaxial growth of thick AlN layers on a Si substrate consists in the formation of cracks, which complicates the application of structures of this kind in the fabrication of semiconductor devices. The possibility of obtaining crack-free AlN layers with a thickness exceeding 1 μm and a mirror- smooth surface by hydride vapor-phase epitaxy is demonstrated. The properties of the layers are studied by X-diffraction analysis, optical and scanning electron microscopy, and Raman spectroscopy.

Enzyme-assisted growth of nacreous CaCO3/polymer hybrid nanolaminates via the formation of mineral bridges

Laminated nanostructures in nacre have been adopted as models in the fabrication of strong, tough synthetic nanocomposites. However, the utilization of CaCO3 biominerals in these composites is limited by the complexity of the synthesis method for nanosized biominerals. In this study, we use the enzymatic reaction of urease to generate a nanoscale CaCO3 thin film to prepare CaCO3/polymer hybrid nanolaminates. Additional layers of CaCO3 thin film are consecutively grown over the base CaCO3 layer with the intercalation of organic layers. The morphology and crystallinity of the added CaCO3 layers depend strongly on the thickness of the organic layer coated on the underlying CaCO3 layer. When the organic layer is less than 20nm thick, the amorphous CaCO3 layer is spontaneously transformed into crystalline calcite layer during the growth process. We also observe crystalline continuity between adjacent CaCO3 layers through interconnecting mineral bridges. The formation of these mineral bridges is crucial to the epitaxial growth of CaCO3 layers, similar to the formation of natural nacre.

Self-Assembly of Tetraphenyldibenzoperiflanthene (DBP) Films on Ag(111) in the Monolayer Regime.

Tetraphenyldibenzoperiflanthene (DBP) is a promising candidate as a component of highly efficient organic photovoltaic cells and organic light-emitting diodes. The structural properties of thin films of this particular lander-type molecule on Ag(111) were investigated by complementary techniques. Highly ordered structures were obtained, and their mutual alignment was characterized by means of low-energy electron diffraction (LEED). Scanning tunneling microscopy (STM) images reveal two slightly different arrangements within the first monolayer (ML), both describable as specific herringbone patterns with two molecules per unit cell whose dibenzoperiflanthene framework is parallel to the surface. In contrast, single DBP molecules in the second ML were imaged with much higher intramolecular resolution, resembling the shape of the frontier orbitals in the gas phase as calculated by means of density functional theory (DFT). Further deposition leads to the growth of highly ordered bilayer islands on top of the first ML with identical unit cell dimensions and orientation but slightly inclined molecules. This suggests that the first ML acts as a template for the epitaxial growth of further layers. Simultaneously, a significant number of second-layer molecules mainly located at step edges or scattered over narrow terraces do not form highly ordered aggregates.

Formation of two-dimensional SiNx layers on GaN nonpolar surfaces

Density functional theory calculations were performed to study the adsorption and incorporation of silicon atoms over GaN (112̅0) and GaN(101̅0) aiming to gain insight into epitaxial growth of two-dimensional layers of silicon nitride (SiNx) on the GaN nonpolar surfaces. Results show that the adsorption of silicon atoms on the bridge sites is energetically more favored as compared with other possible adsorption sites. Formation energy calculations indicate that the silicon impurity atoms prefer to incorporate in second or deeper GaN surface layers. However, it is concluded that the incorporation of Si atoms in the Ga-substitutional site is energetically more favorable compared with the N-substitution and Si-adsorption on the top layers. At full coverage, calculations show the formation of two-dimensional SiNx layers on the GaN nonpolar surfaces over the entire range of Ga chemical potentials, in agreement with experimental observations. In addition, the electronic structure of the GaN nonpolar surfaces with Si doping was analyzed and compared with those of undoped surfaces.

Dynamic atomic layer epitaxy of InN on/in +c-GaN matrix: Effect of “In+N” coverage and capping timing by GaN layer on effective InN thickness

The growth front in the self-organizing and self-limiting epitaxy of ∼1 monolayer (ML)-thick InN wells on/in +c-GaN matrix by molecular beam epitaxy (MBE) has been studied in detail, with special attention given to the behavior and role of the N atoms. The growth temperatures of interest are above 600 °C, far higher than the typical upper critical temperature of 500 °C in MBE. It was confirmed that 2 ML-thick InN wells can be frozen/inserted in GaN matrix at 620 °C, but it was found that N atoms at the growth front tend to selectively re-evaporate more quickly than In atoms at temperatures higher than 650 °C. As a result, the effective thickness of inserted InN wells in the GaN matrix at 660–670 °C were basically 1 ML or sub-ML, even though they were capped by a GaN barrier at the time of 2 ML “In+N” coverage. Furthermore, it was found that the N atoms located below In atoms in the dynamic atomic layer epitaxy growth front had remarkably weaker bonding to the +c-GaN surface.

Integration of InP and InGaAs on 300 mm Si Wafers Using Chemical Mechanical Planarization

Integration of III-V high mobility channel materials in complementary metal oxide semiconductors (CMOS) and III-V photonic materials for integrated light sources on Si substrates requires low defect density III-V buffer layers in order to enable epitaxial growth of high crystal quality active layers. For the fabrication of In0.53Ga0.47As n-channel MOSFET on Si, a lattice matched InP buffer layer is one of the most effective approaches when used in combination with the aspect ratio trapping technique, an integration method known for reducing the density of defects formed during relaxation of strain induced by the lattice mismatch between InP and Si. The InP buffer should be planarized in order to improve thickness uniformity and roughness before subsequent deposition of active layers. In this work we discuss the development of InP planarization on 300 mm Si wafers and investigate slurry composition effects on the final oxide loss and condition of the InP surface. To further explore viability of this approach we deposited an epitaxial In0.53Ga0.47As n-MOS channel layer on top of the planarized InP buffer.