Epitaxial Si Films Research Articles

Twin-free thermal laser epitaxy of Si on sapphire

The heteroepitaxial growth of silicon (Si) is essential for modern electronics. Our study investigates the potential of thermal laser epitaxy (TLE) for Si epitaxy. A systematic study on the evaporation behavior of Si during TLE identifies and addresses the causes of notable flux rate fluctuations, resulting in a Si flux with ±0.3% stability over time. We also demonstrate heteroepitaxy of Si on c-plane sapphire substrates via TLE. High-temperature substrate preparation combined with deposition at a substrate temperature of 1000 °C produced high-quality epitaxial Si (111) films without twin domains.

Surface Debye temperature determination from LEED: correlation to defects in epitaxial films

Low energy electron diffraction (LEED) peak intensities were measured to estimate surface Debye temperature for epitaxially-grown silicon thin films and for bulk Si (001). Rutherford backscattering spectroscopy (RBS, random and channeling modes) and positron annihilation spectroscopy (PAS) were used to quantify defect density and distribution in the near-surface layers. The surface Debye temperature of bulk Si (001), and 1.0 μm, and 0.6 μm Si on sapphire were measured to be 333 K, 299 K, and 260 K, respectively. RBS and PAS showed that the defect concentration was highest near the film/substrate interface, presumably due to the lattice mismatch, and decreased toward the top surface. The thicker film presented fewer defects in the surface and near-surface layers. We showed that a larger concentration of defects Nd in Si epitaxial films correlates with a lower surface Debye temperature following the empirical relation θD=(365±14)−(8.1±1.5)×10−13Nd for our set of samples. Our study suggests the further development of LEED to estimate near-surface defect concentrations.

Impact of PECVD-prepared interfacial Si and SiGe layers on epitaxial Si films grown by PECVD (200 °C) and APCVD (1130 °C)

The homoepitaxy of Si is particularly interesting for the purpose of kerfless wafer production, for example in the photovoltaic domain. Substrate surface engineering is a key step prior to epitaxial growth, which will affect the quality of the epitaxial layer and its detachment for layer transfer. In this work, we propose two plasma-based surface engineering methods including the deposition of a bilayer homoepitaxial interface and a SiGe heteroepitaxial interface. Their impact on the crystalline quality of epitaxial Si layers grown both by plasma-enhanced chemical vapor deposition (PECVD) at 200 °C and by atmospheric pressure chemical vapor deposition (APCVD) at 1130 °C are explored. Stacking faults are observed in epitaxial Si layers with an ultra-thin epitaxial Si interface layer. For surface engineering method based on the addition of an interfacial heteroepitaxial SiGe layer, a higher interfacial hydrogen content and a better bulk epitaxial Si quality are observed in comparison with interfacial homoepitaxial Si layer.

Implantation of Silicon Ions into Sapphire: Low Doses

After the ion implantation of silicon into sapphire followed by high-temperature annealing, silicon and aluminosilicate precipitates are observed in the surface region of sapphire. X-ray measurements with the mapping of reciprocal space show the presence of a compressive stress with a –0.12% strain in the normal direction, and a tensile stress with a 0.2% strain in the R plane in this region. This reduces the lattice mismatch between sapphire and Si(100) and, thus, can improve the crystal quality of epitaxial Si films grown on such modified sapphire substrates.

Effects of dopant concentration on microstructure and strain states of in-situ phosphorus-doped epitaxial silicon films during dry oxidation

The adjustability of the incorporation and distribution of P dopants in Si materials has led to their application in various electronic and photovoltaic devices. In particular, P-doped Si films have been investigated for dopant-induced phenomena such as oxidation enhancement and dopant segregation during thermal oxidation. Despite their technical as well as scientific importance of dopant behaviors in P-doped Si films, these were hardly observed due to low P incorporation. In this study, we prepared and oxidized P-doped epitaxial Si (Si:P) films grown on the Si(100) substrate using in-situ P-doped deposition technique to incorporate high P concentration up to 9.6% (4.8 × 1021 cm−3) in the films, exceeding solid solubility limits (~3 × 1020 cm−3). We found that in oxidized samples with P concentration greater than 6.2%, defects were generated in the near-surface region of Si:P layers and segregated P was observed at the defected areas. The chemical bonding states of P at the oxidized Si:P film increased in the P 2p peak intensity at the interface between the oxide and the Si:P layer, indicating P segregation at this interface. In the strain states of Si:P films, the decrease in the out-of-plane lattice parameter was observed, while maintaining the constant in-plane lattice parameter, implying strain release due to decreased P concentration in the Si:P layer after dry oxidation.

Interfacial hydrogen incorporation in epitaxial silicon for layer transfer

Recently, epitaxial Si layers have attracted strong attention, particularly in photovoltaics. This successful application depends mainly on the easiness of their transfer to a foreign carrier substrate. Therefore, developing a simple and efficient method to realize the transfer is a key issue. A most delicate point is the lift-off of the epitaxial layer from its parent substrate. In this work, we present a method to weaken the interface based on hydrogen incorporation. We have been able to control the hydrogen content at the interface between the crystalline silicon substrate and the epitaxial films by changing the epitaxial growth conditions. Several bonding techniques have been tested and epitaxial Si films have been transferred successfully via anodic bonding. A hydrogen-assisted transferring mechanism is presented.

Electron-cyclotron resonance Ar plasma-induced electrical activation of B atoms without substrate heating in B doped Si epitaxial films on Si(100)

Low-energy Ar plasma enhanced decomposition of SiH4 and B2H6 enables B-doped Si epitaxial film formation on Si(100) without substrate heating. For the concentration of B atoms and carriers in the B-doped Si films, depth profiles were investigated. At a fixed partial pressure and microwave power for the deposition, increasing tendency of the concentrations from interface to surface and the lower electrical activation ratio of the B atom for the higher B concentration were clarified. The activation ratio was typically below 1% in the higher B concentration region above 1020 cm−3, while it was as high as above 5% up to 100% in the lower B concentration region below 1019 cm−3. By reducing deposition rate in the fixed Ar plasma condition, the activation ratio in the higher B concentration region was apparently improved. Moreover, effect of post Ar plasma irradiation without substrate heating after about 1.2 nm-thick film deposition was examined for the films with the B concentration of 8 × 1019 cm−3 and initial carrier concentration of 2 × 1019 cm−3. By the post Ar plasma irradiation, 200% enhancement in the activation ratio was successfully observed and the carrier concentration reaches as high as 6 × 1019 cm−3. By increasing the post plasma irradiation time, the activation ratio tended initially to increase and then to degrade. This indicates that Ar plasma irradiation possibly induces incorporation of B atoms at substitutional sites even in nanometer-order deep region beneath surface, while deactivation of the B atoms also proceeds gradually and overtakes the activation finally. These results of plasma-enhanced activation process without substrate heating is expected to be utilized for smart fabrication of semiconductor devices with high carrier concentration and abrupt junction interfaces.

High thermoelectric performance in high crystallinity epitaxial Si films containing silicide nanodots with low thermal conductivity

High crystallinity Si films containing silicide nanodots (NDs) were epitaxially grown on Si substrates at high temperature (∼750 °C), where the silicide phase of NDs (metallic α-FeSi2 or semiconductor β-FeSi2) was selectable by tuning the Fe deposition amount. The high crystallinity high-temperature-grown Si films with NDs exhibited lower thermal conductivity (5.4 W m−1 K−1) due to the phonon scattering at the ultrasmall ND interfaces than bulk Si-silicide nanocomposites that have ever been reported. In this ND system with extremely low thermal conductivity, due to the less point defects and high quality ND interface, the thermoelectric power factor (∼28 μW cm−1 K−2) was observed to be the same as the high value of Si films without NDs at room temperature, which is the highest value among Si-silicide bulk nanocomposites ever reported. The simultaneous achievement of a high power factor and low thermal conductivity in the high quality ND system will provide the key for design of high thermoelectric performance of Si-based nanostructured films.

CMOS-Compatible Antimony-Doped Germanium Epilayers for Mid-Infrared Low-Loss High-Plasma-Frequency Plasmonics.

Antimony (Sb) heavily-doped germanium (Ge)-on-silicon (Si) epitaxial films are investigated as mid-infrared (MIR) plasmonic materials. Structural, electrical, and optical properties have been improved by proper choice of dopant species (i.e., Sb) and optimization of the growth parameters (i.e., Sb flux and substrate temperature). The increased electron conductivity can be attributed to the elevated carrier concentration (1.5 × 1020 cm-3) and carrier mobility (224 cm2 V-1 s-1) in the Sb-doped Ge epilayers. The measured MIR reflectivities of the Sb-doped Ge films show free-carrier-dependent properties, which leads to tunable real and imaginary parts of permittivities. Localized surface plasmon polaritons of the bowtie antennas fabricated from the Sb-doped Ge films are demonstrated. The fabricated antennas can provide signal enhancement for the molecular vibrational spectroscopy when these vibrational lines are spectrally in proximity to the localized plasmon resonance. These CMOS-compatible Sb-doped Ge epilayers offer a platform to study the interaction of MIR plasmon with nanostructures on chips.

Magnetic and Transport Properties of Co1+<italic>x</italic>FeV1−<italic>x</italic>Si Epitaxial Films Grown by Molecular Beam Epitaxy

We study magnetic and transport properties of Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1+x</sub> FeV <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> Si epitaxial films grown by molecular beam epitaxy. Even though we co-evaporate each element with stoichiometry, Co-rich and V-poor CoFeVSi films are obtained, which results in a relatively large value of the saturation magnetic moment compared with the theoretical value. However, for the nonstoichiometric films, electrical and magnetotransport properties indicate a spin-gapless semiconductor-like electronic band structure.

Effect of P Concentration on Ti Silicide Formation in In‐Situ P Doped Epitaxial Si Films

Herein, the effect of P concentration in highly P‐doped epitaxial Si films on the formation of Ti silicide therein are investigated. Ti films sputter‐deposited on P‐doped Si substrates are annealed at different temperatures, and the thus obtained samples are characterized by X‐ray diffraction (XRD) and transmission electron microscopy. The former technique revealed that the formation of C54 is suppressed by high P concentrations, while the resistivity of Ti silicide are shown to be independent of P concentration when the C54 phase is fully formed under the condition of sufficient thermal budget. Most importantly, this work confirms the occurrence of a silicidation delay in in situ P‐doped epitaxial Si films and demonstrates that this delay increases the temperature of the C49 to C54 phase transition of Ti silicide.

Polarized Raman spectra of β-FeSi2 epitaxial film grown by molecular beam epitaxy

Polarized Raman spectra of a β-FeSi2(100)//Si(001) epitaxial film grown by molecular beam epitaxy were measured to identify the Raman mode of the observed Raman active lines. Twelve of the observed 18 Raman lines showed a clear dependence of the Raman intensity on the crystal rotation angle. By factor group analysis using the orthorhombic symmetry D2h18 of β-FeSi2, five Raman lines (193, 200, 249, 401, 494 cm-1) and seven lines (175, 277, 284, 298, 327, 410, 442 cm-1) were completely assigned to the Ag and B3g modes, respectively. The depolarization ratio of Raman scattering intensities was obtained from polarized Raman spectra measured in two polarization configurations. The values of the depolarization ratio also support the assignment of the Ag and B3g modes in β-FeSi2.

Ultrahigh sensitivity SIMS analysis of oxygen in silicon

We report on the detection of very low oxygen concentration in silicon by a secondary‐ion mass spectrometry (SIMS) method. Using a magnetic IMS 6F Cameca SIMS spectrometer and applying a very high primary Cs+ ion flux, prolonged presputtering, extensive vacuum chamber baking, titanium sublimation pump, and an LN trap, we have reached a detection limit of ~2 × 1015 O atoms/cm3 in chemical vapor deposition epitaxial Si films. This value appears to be at least 10 times lower than in any published or unpublished source known to the authors, including the reference sensitivities listed by the instrument manufacturer. Most likely, the key improvement that has allowed us to drive the detection limit to 1015 at/cm3 is the use of an ion pump in the analysis chamber. The working pressure in our analysis chamber is ~10−10 Thorr, ie, 1 decade lower than that the commercially equipped with a turbo pump. This paper demonstrates optimized analytical conditions for the oxygen measurements in Si, as a function of depth: (i) Very shallow profiles are practically impossible to measure accurately because of native oxide at the surface. (ii) Shallow‐to‐medium range profiles, up to ~20 μm, are the most amenable to SIMS measurements. (iii) Medium‐to‐deep (~20‐50 μm) range is required to follow interdiffusion and segregation in epitaxial layers when the oxygen‐free layer is grown on a CZ Si substrate. (iv) Extremely deep profiles, up to full thickness of the wafer, definitely necessitate beveling.

High mobility single-crystalline-like silicon thin films on inexpensive flexible metal foils by plasma enhanced chemical vapor deposition

In this report, we demonstrate heteroepitaxial growth of single-crystalline-like Si thin films on inexpensive and lightweight flexible metal foil substrates using radio-frequency plasma-enhanced chemical vapor deposition (RF-PECVD). Single-crystalline-like Ge thin film on ion-beam textured templates on metal foils served as epitaxy enabling substrates for Si growth. The epitaxial Si films were oriented along (004) direction and strongly biaxially-textured with narrow in-plane and out-of-plane spreads. Surface morphology was granular with 300–400 nm diameter grains having very low-angle grain boundaries (average ∼ 0.5°). Raman spectroscopy revealed near 100% crystallinity and high structural quality, comparable to bulk c-Si wafer. Controllable in-situ gas phase doping was carried out to achieve n- and p-type Si films with high electron and hole mobilities of 230 and 65 cm2/V-s respectively and carrier concentrations suitable for electronic device application. Direct deposition of high mobility single-crystalline-like Si thin films on flexible and inexpensive metal foils can be a potential route towards roll-to-roll scalable manufacturing of high-performance flexible electronics applications.

Critical effect of nanometer-size surface roughness of a porous Si seed layer on the defect density of epitaxial Si films for solar cells by rapid vapor deposition

Monocrystalline, low defect Si thin films fabricated via 1-minute vapor deposition on a double porous layer treated by zone heating recrystallization.

Bunches of misfit dislocations on the onset of relaxation of Si0.4Ge0.6/Si(001) epitaxial films revealed by high-resolution x-ray diffraction

The experimental x-ray diffraction patterns of a Si0.4Ge0.6/Si(001) epitaxial film with a low density of misfit dislocations are modeled by the Monte Carlo method. It is shown that an inhomogeneous distribution of 60° dislocations with dislocations arranged in bunches is needed to explain the experiment correctly. As a result of the dislocation bunching, the positions of the x-ray diffraction peaks do not correspond to the average dislocation density but reveal less than a half of the actual relaxation.

Monocrystalline Si Thin Film for Solar Cells Grown on Double Layer Porous Si Treated By Zone Heating Recrystallization

The recent development in monocrystalline Si solar cell technology is significant both for reducing the cost and for developing the power generation efficiency. The reduction of Si thickness is one of the key to achieve the further cost reduction. Bottom up technics such as epitaxy and layer transfer process is promising approach to get less than 50 µm-thick Si thin film. Si layer is deposited epitaxially on a seed monocrystalline wafer with a thin sacrificial layer, and then an epitaxial thin film is detached from the seed layer (Fig. 1). This process on double layer porous Si (DLPS), a top low porosity seed layer and a bottom high porosity sacrificial layer, have been widely studied, but there are two challenging issues to be solved. One is the long process time of epitaxy by chemical vapor deposition and another is the quality of the epitaxial film. Rapid vapor deposition (RVD) have been realized to grow epitaxial Si layer at ~10 µm/min deposition rate by the evaporation of Si source with >2000 °C [1]. On the other hand, the crystallographic quality of epitaxial film will depend on the roughness of the seed layer because such roughness will make lattice mismatch during the crystal growth. The effect of the surface roughness is supposed to be larger when the film growth rate get bigger, so to control the roughness become more important to obtain the high quality Si thin film with high speed, low cost process such as RVD. We have developed the zone-heating recrystallization (ZHR) method, the high-speed scanning lamp heating below the melting point of Si, for the selective heat treatment of the Si surface (Fig. 2) [2]. By applying ZHR to the epitaxy and layer transfer process on DLPS by RVD, we obtained the 9 µm-thick monocrystalline Si thin film with only 1-min epitaxy [3]. In this work, we controlled nanometer-size roughness of DLPS surface by ZHR and investigated the properties of Si thin film grown with RVD. To investigate the quality of thin film Si by both XRD and electron spin resonance (ESR), RVD were carried out with intrinsic Si wafer as Si source to avoid the signal of electron from the dopant. ~9 µm intrinsic Si thin films were grown and they could be easily detached by transferring to scotch tape. The Si films with different root mean square roughness (Rms), pore size and pore number density which were grown on DLPS with/without ZHR in several condition were picked up and analyzed. DLPS were formed on p-type monocrystalline Si (100) wafer by two-step anodization. ZHR were carried out with different input power into bottom heater and scanning rate of top lamp heater, and DLPS with different Rms (0.23-0.39 nm) observed by AFM were obtained as described in the previous report [3]. In Si film grown by RVD on the DLPS by each ZHR condition, in-plane XRD Φ-scan of (022) diffraction showed clear four fold symmetry, which proves that the epitaxial monocrystalline film of Si(100) obtained regardless of ZHR conditions. The effect of the surface roughness of DLPS on the crystallographic quality of epitaxial Si films will be discussed. In summary, the epitaxial monocrystalline film by RVD was obtained by the selective heat treatment of Si surface of DLPS by ZHR. The intrinsic monocrystalline film with 9 µm by RVD on DLPS could be easily detached from Si wafer to evaluate crystallographic quality by ESR. [1] Y. Yamasaki et al., CrystEngComm 18, 2016. 3404. [2] A. Lukianov et al., Appl. Phys. Lett. 108, 2016, 1. [3] C. Takazawa et al., ECS Trans. 75, 2017, 11. Figure 1

Effect of disorder on the resistivity of CoFeCrAl films

Structural and electron-transport properties of thin films of the ferrimagnetic Heusler compound CoFeCrAl have been investigated to elucidate structure-property relationships. The alloy is, ideally, a spin-gapless semiconductor, but structural disorder destroys the spin-gapless character and drastically alters the transport behavior. Two types of CoFeCrAl films were grown by magnetron sputtering deposition at 973 K, namely polycrystalline films on Si substrates and epitaxial films on MgO (001) substrates. The resistivity decreases with increasing temperature, with relatively small temperature coefficients of –0.19 μΩcm/K for the polycrystalline films and –0.12 μΩcm/K for the epitaxial films. The residual resistivity of the polycrystalline films deposited on Si is higher than that of the epitaxial film deposited on MgO, indicating that the polycrystalline films behave as so-called dirty metals.

Probing lattice vibration and strain states in highly phosphorus-doped epitaxial Si films

We investigated the lattice vibration and strain states in highly P-doped epitaxial Si films using Raman scattering and X-ray diffraction (XRD) measurements.

Electrical properties and B depth profiles of in-situ B doped Si films grown by ECR Ar plasma CVD without substrate heating

In-situ B doping in epitaxial and amorphous Si films was investigated by low-energy electron-cyclotron-resonance (ECR) Ar plasma chemical vapor deposition (CVD) by using SiH4 and B2H6 without substrate heating. For a B-doped epitaxial Si film, high carrier concentration of 7.1×1019cm−3 with Hall mobility of 19cm2V−1s−1 was measured at room temperature. Moreover, good rectifying characteristics was obtained for a Si p+/n junction diode. Especially for exploration of a heavily B-doped Si film, depth profiling results show that B concentration tends to be smaller near the initial substrate surface. Finally, effective control of B segregation by atomic-order B pre-deposition or by Si buffer deposition on initial Si(100) has been demonstrated to achieve heavy B doping in whole region of deposited Si film.