Epitaxial Samples Research Articles

Investigation of Growth Mechanism and Role of H2 in Very Low Temperature Si Epitaxy

Low temperature defect free crystalline silicon epitaxy has been pursued to achieve abrupt doping profiles in metal–oxide–semiconductor field-effect transistor channels and thin film crystalline solar cells. However, growth at low temperatures suffers from low growth rate and crystalline-amorphous transition. Despite promising results reported by Labrune et al. [1], growth mechanism has not been yet fully explained [4]. Silicon epitaxy in highly hydrogen diluted ambient is mainly driven by two proposed roles played by hydrogen: 1.diffusion length enhancement of the ad-species [2], 2. selective etching of the amorphous growth. In the former role, hydrogen enables the ad-species to move for longer distances and the latter role atomic hydrogen breaks weak amorphous Si-Si bonds and leaves the sites open for crystalline growth [3].In this paper we report on epitaxy mechanism and role of hydrogen in achieving high crystal quality growth using ultra-high-vacuum PECVD technique. Silicon epitaxial layers were grown at 250-350oC with low plasma power density (PPD) of 20-100mW/cm2. The experiments were done in high hydrogen to silane dilution ratio (DR) of 50, 30 and 100.Our results show that conditions for growing epitaxial Si at low temperatures are very critical and dependent on one another. Under such condition, amorphous transition happens when plasma power and hydrogen dilution are not in balance. Combination of proper plasma power, hydrogen dilution and temperature makes an experimental space where silicon epitaxy is feasible. Presence of hydrogen on the surface is of high importance and might have a constructive or destructive role in epitaxy. Passivation of Si dangling bonds with two atoms results in shorter diffusion length of ad-species. However, presence of atomic hydrogen can reduce this number and enhance the ad-species mobility. Proper adjustment of plasma power results in sufficient production of abstracting radicals such as SiH3 and H while the surface is not bombarded with high energy radicals.Fig. 1a depicts the transmission electron microscopy (TEM) image of the highest film quality sample that is grown at 50 mW/cm2 PPD and DR of 30. Nucleation at substrate/film is initiated so perfectly that it is very hard to find and discern the interface (Fig. 1b). This has resulted in elimination of proposed nucleation step needed for higher quality growth [5]. Consequently, no stacking faults and dislocations is developed in the film. Crystalline to amorphous transition is not observed due to the absence of stress and saw tooth features (Fig. 1c).Increased surface roughness and accumulation of hydrogen at dislocation centers are known as the reasons for epitaxy breakdown. These are attributed to high energy radical bombardment and atomic hydrogen etching of the surface. The atomic force microscopy images show that surface roughness of the epitaxial samples (0.136 nm) are comparable with bare Si substrate (0.128 nm) while it is higher for the broken epitaxy sample (0.187 nm). The hydrogen content measurement by Fourier transform infrared spectroscopy shows that high quality films have lower percentage of SiH and SiH2 bonds. The sample with broken epitaxy has 0.619% SiH and 0.696% SiH2. While the highest quality sample has only.024% SiH and 0.025% SiH2. These results show that epitaxy breaks if the growth condition is more towards domination of selective amorphous etching over surface diffusion enhancement. Therefore, surface diffusion enhancement has the major role in epitaxial growth mechanism at low temperatures.[1] Labrune et al., EPJ Photovoltaics 3, 30303 (2012).[2] Bray et al., Appl. Phys. Lett. 80, 2356 (2002).[3] Tsai et al., J. Non Cryst. Solids 114, 151 (1989).[4] Johnson et al., Appl. Phys. Lett. 92, 103108 (2008).[5] Mosleh et al. 39th IEEE PVSC, Tampa, FL, (2013).

Enhanced dielectric nonlinearity in epitaxial Pb0.92La0.08Zr0.52Ti0.48O3 thin films

High quality c-axis oriented epitaxial Pb0.92La0.08Zr0.52Ti0.48O3 films were fabricated using pulsed laser deposition on (001) LaAlO3 substrates with conductive LaNiO3 buffers. Besides confirmation of the in-plane and out-of-plane orientations using X-ray diffraction, transmission electron microscopy study has revealed columnar structure across the film thickness with column width around 100 nm. Characterization of ferroelectric properties was carried out in comparison with polycrystalline Pb0.92La0.08Zr0.52Ti0.48O3 films to extract the effect of epitaxial growth. It is found that the ratio between the irreversible Rayleigh parameter and reversible parameter increased up to 0.028 cm/kV at 1 kHz on epitaxial samples, which is more than twice of that on their polycrystalline counterparts. While this ratio decreased to 0.022 cm/kV with increasing frequency to100 kHz, a much less frequency dependence was observed as compared to the polycrystalline case. The epitaxial Pb0.92La0.08Zr0.52Ti0.48O3 films exhibited a higher mobility of domain wall and the higher extrinsic contribution to the dielectric properties, as well as reduced density of defects, indicating that it is promising for tunable and low power consumption devices.

Optical second-harmonic generation induced by electric current in graphene on Si and SiC substrates

We find that the flow of direct electric current (dc) through graphene on substrate enhances surface optical second-harmonic generation (SHG) from the graphene/substrate system. The current can enhance surface SHG by about 300% for a chemical-vapor-deposition (CVD) graphene monolayer on a SiO${}_{2}/\mathrm{Si}(001)$ substrate, and by about 25% for an epitaxial four-layer-graphene film on a 3.5\ifmmode^\circ\else\textdegree\fi{}-miscut vicinal SiC(0001) substrate. The enhancement in both the CVD and epitaxial graphene samples is due to electric field-induced SHG, which is produced by the current-associated vertical electric field at the SiO${}_{2}/$Si interface or at the graphene/SiC interface. Measurements of rotational-anisotropy SHG (RA-SH) from both samples revealed that the current-induced SHG varies strongly with the measurement location along the current flow direction. By measuring RA-SH from the vicinal SiC(0001) substrate, we determined all three second-order susceptibility tensor elements (${d}_{33}$ = \ensuremath{-}52.0 pm/V, ${d}_{15}$ = 20.0 pm/V, and ${d}_{31}$ = 18.7 pm/V) that characterize the SHG response of hexagonal SiC at the fundamental wavelength of 740 nm. We further determined the three effective susceptibility tensor elements (${d}_{33}$ = \ensuremath{-}135.8 pm/V, ${d}_{15}$ = 18.5 pm/V, and ${d}_{31}$ = 14.6 pm/V) that characterize the surface SHG from the graphene/vicinal-SiC(0001) sample and finally showed that the current-dependent tensor element ${d}_{33}$ can be enhanced to a large value of ${d}_{33}$ = \ensuremath{-}199.0 pm/V by electric current in epitaxial graphene.

Band Gap Engineering of SnO2 by Epitaxial Strain: Experimental and Theoretical Investigations

The effect of mis-match strain on the structural, electronic, and optical properties in SnO2 epitaxial thin films has been systematically investigated by the experimental and theoretical methods. Our results indicate that the tensile strain exists in the thin film and decreases with the thickness of epitaxial samples. Besides, the optical band gap significantly reduces with increasing the tensile strain in the bc plane. Our hybrid functional calculations present that the narrowing of band gap of SnO2 under tensile strain is due to the weakening of bonding and antibonding split, which results from the disorder of SnO6 octahedra, and the biaxial strain is found to be more efficient than the uniaxial strain for tuning the band gap of SnO2.

Magnetization reversal of in-plane uniaxial Co films and its dependence on epitaxial alignment

This work studies the influence of crystallographic alignment onto magnetization reversal in partially epitaxial Co films. A reproducible growth sequence was devised that allows for the continuous tuning of grain orientation disorder in Co films with uniaxial in-plane anisotropy by the controlled partial suppression of epitaxy. While all stable or meta-stable magnetization states occurring during a magnetic field cycle exhibit a uniform magnetization for fully epitaxial samples, non-uniform states appear for samples with sufficiently high grain orientation disorder. Simultaneously with the occurrence of stable domain states during the magnetization reversal, we observe a qualitative change of the applied field angle dependence of the coercive field. Upon increasing the grain orientation disorder, we observe a disappearance of transient domain wall propagation as the dominating reversal process, which is characterized by an increase of the coercive field for applied field angles away from the easy axis for well-ordered epitaxial samples. Upon reaching a certain disorder threshold level, we also find an anomalous magnetization reversal, which is characterized by a non-monotonic behavior of the remanent magnetization and coercive field as a function of the applied field angle in the vicinity of the nominal hard axis. This anomaly is a collective reversal mode that is caused by disorder-induced frustration and it can be qualitatively and even quantitatively explained by means of a two Stoner-Wohlfarth particle model. Its predictions are furthermore corroborated by Kerr microscopy and by Brillouin light scattering measurements.

Ultralow temperature ramping rate of LT to HT for the growth of high quality Ge epilayer on Si (1 0 0) by RPCVD

An approach to grow high quality strain-relaxed Ge on silicon (100) substrate has been proposed using a modified two-step growth method by reduced pressure chemical vapor deposition system. The modified Ge growth method consists of the low temperature growth at 400°C, intermediate temperature growth from 400 to 600°C with low ramping rate and high temperature growth at 600°C. The post-growth anneal in hydrogen at 800°C was introduced immediately after the Ge growth. For the Ge epitaxial film grown with low temperature ramping rate of 6°C/min, the root mean square roughness is 0.621nm in 10×10μm2 scan field, and the threading dislocation density (TDD) is 3×106cm−2 with the film thickness of about 1.3μm. Furthermore, in contrast to the epitaxial Ge sample grown with the high temperature ramping rate of 140°C/min, intermediate low temperature ramping rate from 400 to 600°C is advantageous in terms of TDD reduction, as well as maintaining smooth surface morphology in Ge epilayer.

Vapour-Phase and Solid-Phase Epitaxy of Silicon on Solid-Phase Crystallised Seed Layers for Solar Cells Application

Vapour-phase and solid-phase epitaxy are used for thickening of a solid-phase crystallised silicon seed layer on glass. Cross-sectional transmission microscope images confirm that a transfer of crystallographic information has taken place from the seed layer into the epilayers. X-ray diffraction, scanning electron microscopy, and transmission electron microscopy reveal that the density of planar defects (mainly on{111}plains) in the vapour-phase epitaxial sample is much higher than in the solid-phase epitaxial sample. These planar defects can act as recombination centres for free-charge carriers. Consequently, PC1D modelled minority carrier diffusion length in the vapour-phase grown epilayer is 50% shorter than that in the solid-phase grown epilayer. As a result, a solar cell grown by solid-phase epitaxy achieves open circuit voltage of 468 mV, short circuit current of 9.17 mA/cm2, and photovoltaic conversion efficiency at 2.75% which are all higher than those of the solar cell grown by vapour-phase epitaxy on the same seed layer, 400 mV, 7.28 mA/cm2, 1.69%, respectively. It proves that solid-phase epitaxy is more suitable for the solar cell growth on the solid-phase crystallised silicon seed layer than vapour-phase epitaxy.

Snapshots of non-equilibrium Dirac carrier distributions in graphene

The optical properties of graphene are made unique by the linear band structure and the vanishing density of states at the Dirac point. It has been proposed that even in the absence of a bandgap, a relaxation bottleneck at the Dirac point may allow for population inversion and lasing at arbitrarily long wavelengths. Furthermore, efficient carrier multiplication by impact ionization has been discussed in the context of light harvesting applications. However, all of these effects are difficult to test quantitatively by measuring the transient optical properties alone, as these only indirectly reflect the energy- and momentum-dependent carrier distributions. Here, we use time- and angle-resolved photoemission spectroscopy with femtosecond extreme-ultraviolet pulses to directly probe the non-equilibrium response of Dirac electrons near the K-point of the Brillouin zone. In lightly hole-doped epitaxial graphene samples, we explore excitation in the mid- and near-infrared, both below and above the minimum photon energy for direct interband transitions. Whereas excitation in the mid-infrared results only in heating of the equilibrium carrier distribution, interband excitations give rise to population inversion, suggesting that terahertz lasing may be possible. However, in neither excitation regime do we find any indication of carrier multiplication, questioning the applicability of graphene for light harvesting.

Ultrathin low temperature Si0.75Ge0.25/Si buffer layer for the growth of high quality Ge epilayer on Si (100) by RPCVD

An approach to grow high quality strain-relaxed Ge by reduced pressure chemical vapor deposition system has been proposed in this paper. Prior to epitaxial growth high quality Ge layer, an ultrathin Si0.75Ge0.25/Si superlattice buffer layer with thickness of 54nm and a 460nm Ge seed layer were deposited successively at low temperature. Then an 840nm thickness Ge layer was grown at high temperature for faster deposition rate. After achieving high-temperature annealing at hydrogen atmosphere, the root-mean-square roughness of the epitaxial Ge lowers to 0.61nm at 10×10μm2 scan area, and the threading dislocation density is 1×106cm−2. Micro-Raman spectra manifests the extremely uniform distribution of tensile strain in Ge layer at room temperature, which can be contributed to the higher thermal expansion coefficient of Ge than that of Si. In contrast to the epitaxial Ge sample without buffer layer, the crystal quality and Hall hole mobility of the Ge epilayer incorporated ultrathin Si0.75Ge0.25/Si buffer improve significantly, indicating that the ultrathin low temperature Si0.75Ge0.25/Si superlattice buffer layer plays an important role on fabricating high quality Ge epilayer.

Fabrication and properties of double perovskite SrLaVRuO6

Polycrystalline bulk and epitaxial thin film samples of double-perovskite SrLaVRuO6 were prepared, and their magnetic and electrical properties were investigated. Structural characterization on the bulk samples has revealed that a double-perovskite phase can be formed in coexistence with the secondary phase of nonmagnetic semiconducting Sr3V2O8. Magnetic and electrical investigations have shown that the double-perovskite SrLaVRuO6 compound is an antiferromagnetic metal. Moreover, it has been found that epitaxy using a lattice-matched substrate leads to suppression of the Sr3V2O8 secondary phase and promoting growth of phase-pure double-perovskite SrLaVRuO6.

Structural, electronic, and magnetic behavior of two dimensional epitaxial Fe3O4/TiN/Si(100) system

Epitaxial Fe3O4 thin films are deposited at 400, 500, 600 °C on TiN buffered Si(100) using reactive magnetron sputtering. In-situ reflection high energy electron diffraction confirmed the growth to be two-dimensional (2-D) at 500–600 °C and X-ray reflectivity revealed a sharp interface (∼0.4 nm). Growth temperature effect on structural, electronic, and magnetic behavior was probed by X-ray diffraction, Raman, and magnetization studies. Small value of deformation potential (0.19 eV/Å) and electron phonon coupling constant (0.71), sharp Verwey transition with large jump at 119 K in magnetization during field cooled warming and early saturation at 2.5 kOe affirmed the superior magnetic quality of epitaxial Fe3O4 samples.

Epitaxial graphene morphologies probed by weak (anti)-localization

We show how the weak field magneto-conductance can be used as a tool to characterize epitaxial graphene samples grown from the C or the Si face of silicon carbide, with mobilities ranging from 120 to 12 000 cm2/(V·s). Depending on the growth conditions, we observe anti-localization and/or localization, which can be understood in term of weak-localization related to quantum interferences. The inferred characteristic diffusion lengths are in agreement with the scanning tunneling microscopy and the theoretical model which describe the “pure” mono-layer and bilayer of graphene [MacCann et al., Phys. Rev. Lett. 97, 146805 (2006)].

Thermoelectric Properties of Undoped and Si-doped Bulk GaN

ABSTRACTIn this study, thermoelectric properties of bulk and epitaxy GaN with various doping concentration are investigated. Seebeck coefficients decreased with the increase of carrier concentration for both bulk and epitaxial GaN samples, and the Seebeck coefficients of epitaxial GaN samples are found to be larger than that of bulk GaN samples in the similar carrier density due to the higher dislocation scattering. For epitaxial samples, a high power factor of 4.72 × 10-4 W/m-K2 is observed. The power factors of the bulk GaN samples are in the range of from 0.315× 10-4W/m-K2 to 0.354× 10-4W/m-K2 due to the low Seebeck coefficients.

Photoinduced current transient spectroscopy of defect levels in CuInSe2 and CuGaSe2 epitaxial and polycrystalline layers

Photoinduced current transient spectroscopy (PITS) was used to study defect level spectra in Cu(In,Ga)Se2 chalcopyrite compounds used as absorbers in solar cells. We show that for CuGaSe2 PITS provides similar results as modulated photocurrent (MPC) spectroscopy, while in the case of CuInSe2 much better results are achieved with PITS due to the lack of background illumination necessary in the MPC. We found four defect levels common for both CuInSe2 and CuGaSe2 (three electron traps with one of them acting as a recombination centre and one very deep hole trap). All those deep levels were observed both in the epitaxial and polycrystalline samples so we attributed them to bulk structural defects.

The CuInSe2–CuIn3Se5 defect compound interface: Electronic structure and band alignment

The interface formation between stoichiometric chalcopyrite CuInSe2 and the copper-deficient defect phase CuIn3Se5 is investigated by in situ photoelectron spectroscopy. The use of epitaxial samples allows for the preparation of highly defined surfaces and accurate analysis of the electronic structure. Valence band structures measured with synchrotron-based photoelectron spectroscopy are in agreement with density functional theory. We observe a lowering of the top valence band of CuIn3Se5 of 0.29 eV with respect to CuInSe2. The increased optical gap for copper-deficient material leads to aligned conduction bands.

Co/Ni(111) superlattices studied by microscopy, x-ray absorption, andab initiocalculations

We explore the origins of perpendicular magnetic anisotropy in epitaxial and textured Co/Ni(111) superlattices using a combination of thin-film growth, structural characterization, x-ray magnetic circular dichroism (XMCD), and ab initio calculations. Transmission electron microscopy and x-ray diffraction experiments allow us to show that the bulk magnetoelastic contribution to the total magnetic energy is small compared to the interface anisotropy. The magnetic properties are studied by using XMCD at the Co and Ni L 2,3 edges. Hysteresis loops performed at the Co L 3 edge confirm the perpendicular magnetization for Co thicknesses up to four monolayers. The spin and orbital moments were deduced using the XMCD sum rules. The results are explained by considering two kinds of magnetic moments for Co, distinguishing the interfaces from the rest of the layers. Both effective spin and orbital moments of Co atoms are found to be enhanced at the Co-Ni interfaces, whereas the magnetic moment of Co surrounded by Co is similar to the bulk value. Ab initio calculations allow us to show a strong enhancement of the dipole operator contribution on Co atoms at the interface that is partly responsible for this high effective spin moment at the interface. Such a moment enhancement is not observed for Ni, with the dipole operator contribution being close to zero. Finally, we observed a very surprising proportionality between the effective spin and orbital moments, independent of the absorption edge or deposition technique used. We assign this peculiar behavior to the fact that the magnetic dipole operator involved in the sum rules is closely linked to the increase of the Co orbital moment at the interface. Based on XMCD results obtained on both molecular beam epitaxy and sputter-deposited samples, this link allows us to show the extreme sensitivity of the perpendicular anisotropy with the chemical ordering at the interface.

Resonant Excitation of GrapheneK-Phonon and Intra-Landau-Level Excitons in Magneto-Optical Spectroscopy

Precise infrared magnetotransmission experiments have been performed in magnetic fields up to 32T on a series of multilayer epitaxial graphene samples. We observe changes in the spectral features and broadening of the main cyclotron transition when the incoming photon energy is in resonance with the lowest Landau level separation and the energy of a K point optical phonon. We have developed a theory that explains and quantitatively reproduces the frequency and magnetic field dependence of the phenomenon as the absorption of a photon together with the simultaneous creation of an intervalley, intra-Landau-level exciton, and a K phonon.

The influence of substrate morphology on thickness uniformity and unintentional doping of epitaxial graphene on SiC

A pivotal issue for the fabrication of electronic devices on epitaxial graphene on SiC is controlling the number of layers and reducing localized thickness inhomogeneities. Of equal importance is to understand what governs the unintentional doping of the graphene from the substrate. The influence of substrate surface topography on these two issues was studied by work function measurements and local surface potential mapping. The carrier concentration and the uniformity of epitaxial graphene samples grown under identical conditions and on substrates of nominally identical orientation were both found to depend strongly on the terrace width of the SiC substrate after growth.

Evidences of electrochemical graphene functionalization and substrate dependence by Raman and scanning tunneling spectroscopies

Electrochemical functionalization and possible hydrogenation of treated epitaxial graphene samples on 6H-SiC are presented. To attract H+ ions to react with the exposed working cathode, a 10% sulfuric acid electrolyte was used with a Pt counter anode. Functionalization was determined using Raman spectroscopy and measured by a marked increase in I(D)/I(G) ratio and introduction of C-H bond peak at ∼2930 cm−1. There was also a marked increase in fluorescence background, which clearly differentiates functionalization from lattice damage in the graphene. Quantifying the fluorescence, we estimate that H-incorporation as high as 50% was achieved based on results on hydrocarbons, although other functional groups cannot be excluded. We further distinguished these functionalization signatures from lattice damage through measurements on nanocrystalline graphene on a and m plane SiC, which displayed very different surface morphologies and no measureable fluorescence. Finally, we show that the extent of functionalization is strongly substrate dependent by using samples cut from three semi-insulating 6H-SiC substrates with similar resistivity but orientations varying from on-axis (∼0.02°), 0.5° to 1.0° off-axis. This functionalization was found to be thermally reversible at ∼1000 °C. Scanning tunneling spectroscopy indicates the presence of sp3-like localized states not present in the starting graphene, further supporting the assertion that functionalization has occurred.

Evidence of Electrochemical Graphene Functionalization by Raman Spectroscopy

Electrochemical functionalization of treated epitaxial graphene samples on Si-face 6H-SiC are presented in this work. Three semi-insulating 6H-SiC substrates cut from different boules with varying off cut angle (on axis, 0.5° and 1.0° degrees off axis in the [112‾0] direction) were diced into 10mm x 10mm samples and quality EG grown on top. A home-build electrochemical cell was used with current applied though a 10% H2SO4 solution, with a Pt wire and exposed graphene as the anode and cathode respectively. Functionalization was determined using Raman spectroscopy and measured by an increase in D/G ratio, increase in fluorescence background and introduction of C-H bond peak at ~2930 cm-1. Components of the Raman spectra before and after functionalization of all samples used were analyzed to show a substrate dependent effect on functionalization with values such as D/G ratio and normalized fluorescence slope varying between the substrates.