Low-energy Plasma-enhanced Chemical Vapor Deposition Research Articles

Hole and positron interaction with vacancies and p-type dopants in epitaxially grown silicon

The concentration of vacancies and impurities in semiconductors plays a crucial role in determining their electrical, optical, and thermal properties. This study aims to clarify the nature of the interaction between positrons and ionized p-type impurities, emphasizing the similarities they share with the interaction between holes and this type of impurity. An overall strategy for investigating defects in semiconductor crystals that exhibit a combination of vacancies and p-type impurities is presented. By using positron annihilation spectroscopy, in particular, Doppler broadening of the annihilation radiation, we quantify the concentration of vacancies in epitaxial Si crystals grown by low-energy plasma-enhanced chemical vapor deposition. The vacancy number densities that we find are (1.2 ± 1.0) × 1017 cm−3 and (3.2 ± 1.5) × 1020 cm−3 for growth rates of 0.27 and 4.9 nm/s, respectively. Subsequent extended annealing of the Si samples effectively reduces the vacancy density below the sensitivity threshold of the positron technique. Secondary ion mass spectrometry indicates that the boron doping remains unaffected during the annealing treatment intended for vacancy removal. This study provides valuable insights into the intricate interplay between vacancies and ionized impurities with positrons in semiconductor crystals. The obtained results contribute to advance the control and understanding of material properties in heterostructures by emphasizing the significance of managing vacancy and dopant concentrations.

Near‐Infrared Light Trapping and Avalanche Multiplication in Silicon Epitaxial Microcrystals

AbstractThe chemical vapor deposition of silicon on a patterned silicon substrate leads to the formation of 3D microcrystals, which, due to their inclined top facets and high aspect ratio, produce a light‐trapping effect enhancing the optical absorption in the near‐infrared (NIR). In this work, it is demonstrated that Si microcrystals can form the building blocks of a new class of NIR sensitive photodetectors operating in a linear or avalanche regime. Microcrystal‐based devices are designed by coupling a 2D kinetic‐growth model with a Poisson drift‐diffusion solver and fabricated by combining electron beam lithography and low‐energy plasma‐enhanced chemical vapor deposition (LEPECVD). The optoelectronic properties of microcrystal‐based p–i–n photodiodes are investigated both theoretically and experimentally by means of finite‐difference time‐domain (FDTD) simulations and responsivity measurements. At 1000 nm wavelength, the responsivity of microcrystal‐based devices is six times higher than that of an equivalent mesa diode. Moreover, the photocurrent gains of Si microcrystals operating as an avalanche photodiode (APD), at the same wavelength, reaches 2 × 104 demonstrating the potentialities of substrate patterning, combined with epitaxial growth, for amplified photodetection applications.

Germanium Quantum Wells for Non-Linear Integrated Photonics

In recent years, mid-infrared integrated photonics has raised an increasing interest due to the envisioned applications in molecular sensing, environmental monitoring and security. The Silicon-on-insulator (SOI) based technology, operating at wavelengths λ < 3.2 μm has already reached a significant technology readiness level. By leveraging the maturity of the SOI technology and taking advantage of the high index contrast between Si and SiO2 many functionalities such as low loss waveguiding, modulation and frequency comb generation have already been demonstrated. However, the wavelength range is limited by the absorption of the SiO2 layer. Many material platforms, such as III-V semiconductors, halides and chalcogenides are under investigation to fill this gap. Of particular interest is the Ge-rich SiGe-on-Si material platform, thanks to its compatibility to already existing foundry processes and its wide transparency at the wavelengths of interest. Nevertheless, key functionalities such as wavelength conversion, photodetection and high-speed modulation are still missing.A viable way to achieve such advanced functionalities is the exploitation of intersubband transitions (ISBTs) in hole-doped Ge/SiGe quantum wells (QWs). In particular, ISBTs in can be leveraged to engineer an artificial second-order optical nonlinearity in group IV materials, where it is normally absent for symmetry reasons. In this work, we exploit ISBTs in asymmetric coupled Ge/SiGe (ACQWs) (see Fig.1) to achieve highly efficient second harmonic generation at mid-infrared frequencies. The ACQW has been designed with an advanced semi-empirical first neighbor sp3d5s* tight-binding model, which has been used to calculate the second-order nonlinear susceptibility χ(2) (see fig 1b).The ACQW has been then grown by Low-Energy Plasma-Enhanced Chemical Vapor Deposition (LEPECVD) and structurally characterized by x-ray diffraction (XRD) and high-resolution scanning transmission electron microscopy (STEM) . For the optical measurements, samples were cut in a 2 mm single-pass surface-plasmon waveguide with the side facets shaped to 70° with respect to the growth plane and the top facet close to the ACQWs region coated by a Ti/Au layer. Then the samples (cooled at 10 K) have been pumped with a CW quantum cascade laser emitting at λ =10.3 μm. The light coming out from the samples have been then filtered and collected by an MCT detector. The second harmonic emission has been recorded as a function of the input power (see fig. 1c) and a c(2) = 6x104 pm/V has been extracted from the measurement, a two orders of magnitude improvement with respect to the best nonlinear crystals.Finally, the possibility to integrate ACQWs in waveguides has been theoretically investigated. Since artificial nonlinearities based on ISBTs involve real quantum states, as opposite to the virtual quantum states employed in standard nonlinear crystals, significative optical absorption at the pump and at the second harmonic wavelengths must be considered. Therefore, the length of the ACQW waveguide must be carefully designed in order to limit the optical losses. By solving the coupled wave equations with optical absorption at the pump and second harmonic wavelengths, we show that high conversion efficiencies around ≈ 10% can be achieved with waveguide integrated ACQWs for an optimal waveguide length of a ≈200 μm.The integration scheme, shown in fig 2a, consists in a stack of ACQW integrated in a waveguide containing a periodic corrugation, which is necessary to achieve quasi-phase matching between the pump and the second harmonic. Input and output coupling of the light is achieved through adiabatic tapers by integrating the ACQW stack on a Si0.3Ge0.7 waveguide, where the light is injected and from which it is collected and measured. The pump and the second harmonic mode are fairly overlapped in the ACQW region, as shown in fig 2b and 2c, where the distribution of the first TM mode has been simulated using the Lumerical software package.In conclusion, Ge/SiGe ACQW are very promising to expand the functionalities available in MIR integrated photonic circuits, especially in the important spectral region up to 10 μm. In particular, the experimental data obtained from the material characterization, as well as the preliminary studies of waveguide integration show that Ge/SiGe ACQW has the potential to realize highly efficient integrated wavelength converters.Acknowledgments: This work has been supported by Fondazione Cariplo, grant n° 2020-4427. Figure 1

Electrically Tunable Ge/Si VIS-Swir Photodetector

Ge-on-Si photodiodes have been firstly reported more than twenty years ago opening the way for the integration of IR photodetectors on Si. A tremendous development has been done, moving from vertically illuminated, stand-alone devices, to waveguide integrated arrays of photodetectors and CMOS integrated imagers. Usually, the Ge epilayer act as the absorbing material for the NIR radiation, while Si acts only as a substrate. Here we report on a dual-band Ge-on-Si photodetector where light detection can take place both within the Ge epilayer and the underlying Si substrate: the device responsivity can thus be tuned from the VIS to the SWIR spectral range by means of an external bias. Therefore, the presented device can be implemented as a VIR-SWIR CMOS compatible imager.Two photodiodes in a back-to-back configuration are formed by p-doping the back side of a high resistivity Si wafer and by epitaxially growing a p-i-n heterojunction Ge-on-Si layer on the front side (Fig.1a)). An external bias can be applied to the whole layer stack by means of ohmic contacts formed on the top Ge layer and on the wafer backside (Fig.1b)). When a positive bias is applied between the top and bottom contact, the Ge diode is forward biased while the Si diode is reversed biased. In such a configuration the photocurrent generated within the Si substrate by back illuminating the device, will flow through the external circuit and be detected. By reversing the bias polarity, still maintaining a backside illumination, the photocurrent will be generated within the Ge epilayer: in this way it is possible to tune the device responsivity simply by controlling the external bias. A TCAD model has been set up to optimize doping levels, in particular that of the Si substrate, determining the most effective position the p-n junction within the Si photodiode and investigating the role played by substrate thickness[1].According with the simulation outcome, a p-type layer has been formed on the backside of an intrinsic Si wafer by spin-on-dopant obtaining a boron-doped layer with an average concentration of 2×1019cm-3 and a thickness of ≈300nm. The epitaxial growth has been performed by low-energy plasma-enhanced chemical vapor deposition (LEPECVD).Firstly, a 200nm thick phosphorous-doped Si layer has been deposited at 760°C followed by 3 µm thick nominally undoped Ge, grown at 500°C. Six thermal annealing cycles between 600-780°C have been performed in-situ to reduce the threading dislocation density. As a final step 150nm of boron-doped Ge have been deposited.Ge mesas with dimensions ranging between 100-500µm have been realized by standard UV-lithography and dry etching on the Ge-on-Si epitaxial layer. The mesas have been etched down to the intrinsic Si surface to electrically isolate the different devices on the same chip. The top ohmic contacts have been realized by e-beam deposition of a Ti/Al metal stack and lift-off. Different bottom ohmic contact have been tested both by depositing two rectangular stripes of Ti/Au or by depositing a transparent ITO layer on the backside of the chip.Optical characterization was performed by means of a lamp-monochromator set-up. The VIS/SWIR bias dependent dual-band operation of the device is demonstrated in Fig.1c), where the spectral responsivity is shown for two different applied voltages. The peak responsivities are 0.41A/W and 0.63A/W at 960nm and 1520nm, respectively. The specific detectivity D* of the device has been found to be 7·1011cmHz1/2/W and 2·1010cmHz1/2/W in the VIS and NIR, respectively[2].VIS-SWIR single-pixel imaging was performed by mounting the chip on a rastering system comprising a three-axis stage. In this way the pixel can scan the image plane behind a piano-convex lens. Two images, one in the VIS and one in the SWIR, can be obtained by repeating the scanning and changing the polarity of the applied bias at the device Fig.4d). A coloured VIS image has been obtained by repeating the measurements three times with three different optical bandpass filters (450nm, 550nm and 650nm) resulting in three images with R, G, and B components, which have been combined to obtain the final VIS image.In conclusion, we have designed and fabricated a voltage-tunable dual-band photodetector operating in the VIS and SWIR range. The device is based on a couple of p-i-n photodiodes connected back-to-back formed within a Ge-on-Si epitaxial structure. The device architecture enables to electronically select the shorter (400-1100nm) or longer (1000-1600nm) wavelength range with a relatively low applied voltage. VIS-SWIR single-pixel imaging has been performed.[1] A.De Iacovo, et al., Journal of Lightwave Technology, 37(14), 3517(2019).[2] E.Talamas Simola, et al., Opt. Express 27, 8529-8539(2019). Figure 1

Modifying the surface of a titanium alloy with an electron beam and a-C:H:SiOx coating deposition to reduce hemolysis in cardiac assist devices

The use of implantable mechanical pumps to support stable heart function saves lives to a large number of patients. The priority goal of research in this area is to develop advanced pumps that will ensure minimal injuries and fractures of blood cells. A combined method of modifying the surface of disk pump parts made of titanium alloy VT-6 has been developed to reduce blood hemolysis. The method includes material pretreatment with a high-current low-energy electron beam and plasma-enhanced chemical vapor deposition of a-C:H:SiOx coating. The effect of such electron beam parameters as the energy density and the number of pulses on the surface morphology, hardness, plasticity index, friction coefficient and wear rate of the titanium alloy was studied. The effect of the 1.6-μm-thick a-C:H:SiOx coating on the surface morphology, mechanical and tribological properties of VT-6 samples subjected to preliminary electron beam processing was also investigated. Parts of the disk pump were modified according to the proposed technology and pump was tested on the degree of red blood cell destruction. After modifying the surface of pump parts, the destruction of red blood cells did not occur during the 4-h experiment, unlike a similar pump without surface modification. It is shown that the physical properties of the surface are one of the most important factors affecting hemolysis in mechanical pumps to support stable heart function.

Three-dimensional SiGe/Si heterostructures: Switching the dislocation sign by substrate under-etching

We present a joint theoretical and experimental analysis of the dislocation distribution in graded epitaxial SiGe crystals grown on under-etched Si pillars by low-energy plasma-enhanced chemical vapor deposition. Dislocation dynamics simulations are used to investigate preferential positioning of ${60}^{\ensuremath{\circ}}$ dislocations introduced in the system to release the lattice misfit strain. Coupling to a finite-element solver is exploited to allow for the exact numerical treatment of the stress fields in the presence of a complex distribution of free surfaces. The results show that, by suitably under-etching the Si pillars, it is possible to reverse the sign of the Burgers vector of the dislocations. This helps explaining differences in the experimentally observed distribution of dislocations in SiGe crystals grown on vertical and under-etched pillars, leading to a strong reduction of defects in the latter case. The agreement between simulations and experiments is not simply qualitative: the predicted number of defects generated by multiplication processes in tall crystals is indeed fully consistent with the measured one.

Exceptional thermal strain reduction by a tilting pillar architecture: Suspended Ge layers on Si (001)

In this paper we present the exceptional thermal strain release provided by micrometric Si pillar arrays to Ge epitaxial patches suspended on them, for different pillar aspect ratios and patch sizes. By combining 3D and 2D Finite Element Method simulations, low-energy plasma-enhanced chemical vapor deposition on patterned Si substrates, μ-Raman, μ-photoluminescence and XRD measurements, we provide a quantitative and consistent picture of this effect with the patch sizes. Strain relaxation up to 85% of the value for the corresponding planar films can be obtained for a squared patch 100μm in size. Finally, the enhanced thermal strain relaxation is analytically explained in terms of the Si pillar lateral tilting, critically dependent on the pillar aspect ratio, very similarly to the well-known case of a deflected beam. Our results are transferable to any material deposited, or wafer bonded at high temperature, on any patterned substrate: wafer bowing can be controlled by micrometric patterned features well within the present capabilities of deep reactive ion etching.

Reduced-Pressure Chemical Vapor Deposition Growth of Isolated Ge Crystals and Suspended Layers on Micrometric Si Pillars.

In this work, we demonstrate the growth of Ge crystals and suspended continuous layers on Si(001) substrates deeply patterned in high aspect-ratio pillars. The material deposition was carried out in a commercial reduced-pressure chemical vapor deposition reactor, thus extending the "vertical-heteroepitaxy" technique developed by using the peculiar low-energy plasma-enhanced chemical vapor deposition reactor, to widely available epitaxial tools. The growth process was thoroughly analyzed, from the formation of small initial seeds to the final coalescence into a continuous suspended layer, by means of scanning and transmission electron microscopy, X-ray diffraction, and μ-Raman spectroscopy. The preoxidation of the Si pillar sidewalls and the addition of hydrochloric gas in the reactants proved to be key to achieve highly selective Ge growth on the pillars top only, which, in turn, is needed to promote the formation of a continuous Ge layer. Thanks to continuum growth models, we were able to single out the different roles played by thermodynamics and kinetics in the deposition dynamics. We believe that our findings will open the way to the low-cost realization of tens of micrometers thick heteroepitaxial layer (e.g., Ge, SiC, and GaAs) on Si having high crystal quality.

Strain Engineering in Highly Mismatched SiGe/Si Heterostructures

In this work we present an innovative approach to realise coherent, highly-mismatched 3-dimensional heterostructures on substrates patterned at the micrometre-scale. The approach is based on the out-of-equilibrium deposition of SiGe alloys graded at an exceptionally shallow grading rate (GR) of 1.5%µm−1 by low energy plasma enhanced chemical vapour deposition (LEPECVD). Fully coherent SiGe/Si crystals up to 6µm in width were achieved as confirmed by defect etching and transmission electron microscopy (TEM) analyses. The experimental results are supported by calculations of the energy for dislocation formation which indicate that elastic relaxation is energetically favoured over plastic relaxation in the narrower heterostructures. X-ray diffraction measurements show that the SiGe crystals are strain-free irrespective of the stress relieving mechanism which changes from elastic to plastic by increasing their width. The impact of dislocations on the SiGe crystal quality is analysed by comparing the width of X-ray diffraction peaks as a function of the heterostructure size.

Temperature-controlled coalescence during the growth of Ge crystals on deeply patterned Si substrates

A method for growing suspended Ge films on micron-sized Si pillars in Si(001) is discussed. In [C.V. Falub et al., Science 335 (2012) 1330] vertically aligned three-dimensional Ge crystals, separated by a few tens of nanometers, were obtained by depositing several micrometers of Ge using Low-Energy Plasma-Enhanced Chemical Vapor Deposition. Here a different regime of high growth temperature is exploited in order to induce the merging of the crystals into a connected structure eventually forming a continuous, two-dimensional film. The mechanisms leading to such a behavior are discussed with the aid of an effective model of crystal growth. Both the effects of deposition and curvature-driven surface diffusion are considered to reproduce the main features of coalescence. The key enabling role of high temperature is identified with the activation of the diffusion process on a time scale competitive with the deposition rate. We demonstrate the versatility of the deposition process, which allows to switch between the formation of individual crystals and a continuous suspended film simply by tuning the growth temperature.

Integration of GaN Crystals on Micropatterned Si(0 0 1) Substrates by Plasma-Assisted Molecular Beam Epitaxy

We present an innovative approach to integrate arrays of isolated, strain-free GaN crystals on patterned Si substrates. First, micrometer-sized pillars are patterned onto Si(0 0 1) substrates. Subsequently, 2.5 μm Si substrates are deposited by low-energy plasma-enhanced chemical vapor deposition, forming crystals mostly bounded by {1 1 1}, {1 1 3}, and {15 3 23} facets. Plasma-assisted molecular beam epitaxy is then used for GaN deposition. GaN crystals with slanted {0 0 0 1} facets having a root-mean-square surface roughness of 0.7 nm are obtained for a deposited material thickness of >3 μm. Microphotoluminescence measurements performed at room and cryogenic temperature show no yellow luminescence and a neutral donor-bound A exciton transition at 3.471 eV (10 K) with a full width at half-maximum of 10 meV. Microphotoluminescence and micro-Raman spectra reveal that GaN grown on Si pillars is strain-free. Our results indicate that the shape of GaN crystals can be tuned by the pattern periodicity and that ...

Group-IV midinfrared plasmonics

The use of heavily doped semiconductors to achieve plasma frequencies in the mid-IR has been recently proposed as a promising way to obtain high-quality and tunable plasmonic materials. We introduce a plasmonic platform based on epitaxial n-type Ge grown on standard Si wafers by means of low-energy plasma-enhanced chemical vapor deposition. Due to the large carrier concentration achieved with P dopants and to the compatibility with the existing CMOS technology, SiGe plasmonics hold promises for mid-IR applications in optoelectronics, IR detection, sensing, and light harvesting. As a representative example, we show simulations of mid-IR plasmonic waveguides based on the experimentally retrieved dielectric constants of the grown materials.

Acoustoelectric effects in very high-mobility p-SiGe/Ge/SiGe heterostructure at low temperatures in high magnetic fields

The contactless Surface Acoustic Wave (SAW) technique was implemented to probe the high-frequency (ac) conductivity in a high-mobility p-SiGe/Ge/SiGe structure in the integer quantum Hall (IQHE) regime. The structure was grown by low-energy plasma-enhanced chemical vapor deposition and comprised a two-dimensional channel formed in a compressively strained Ge layer. It was investigated at temperatures of 0.3–5.8 K and magnetic fields up to 18 T at various SAW intensities. In the IQHE regime, in minima of the conductivity oscillations with small filling factors, holes are localized. The ac conductivity is of the hopping nature and can be described within the “two-site” model. Furthermore, the dependence of the ac conductivity on the electric field of the SAW was determined. The manifestation of non-linear effects is interpreted in terms of nonlinear percolation-based conductivity.

Ge/SiGe superlattices for nanostructured thermoelectric modules

Thermoelectrics are presently used in a number of applications for both turning heat into electricity and also for using electricity to produce cooling. Mature Si/SiGe and Ge/SiGe heteroepitaxial growth technology would allow highly efficient thermoelectric materials to be engineered, which would be compatible and integrable with complementary metal oxide silicon micropower circuits used in autonomous systems. A high thermoelectric figure of merit requires that electrical conductivity be maintained while thermal conductivity is reduced; thermoelectric figures of merit can be improved with respect to bulk thermoelectric materials by fabricating low-dimensional structures which enhance the density of states near the Fermi level and through phonon scattering at heterointerfaces. We have grown and characterized Ge-rich Ge/SiGe/Si superlattices for nanofabricated thermoelectric generators. Low-energy plasma-enhanced chemical vapor deposition has been used to obtain nanoscale-heterostructured material which is several microns thick. Crystal quality and strain control have been investigated by means of high resolution X-ray diffraction. High-resolution transmission electron microscopy images confirm the material and interface quality. Electrical conductivity has been characterized by the mobility spectrum technique.

Power Factor Characterization of Ge/SiGe Thermoelectric Superlattices at 300 K

To accurately characterize the efficiency of thermoelectric materials and characterize the maximum power that they can produce, a device using micro/nanofabrication techniques has been developed, enabling all three properties included in the figure of merit, ZT, of a thermoelectric material to be measured using a single device. The fabrication and testing of the device are presented. The electrical conductivity and Seebeck coefficient of Ge/SiGe heterostructures grown by low-energy plasma-enhanced chemical vapor deposition are used for demonstration. Experimental results as a function of quantum well width are presented, demonstrating power factors up to 6.02 ± 0.05 mW m−1 K−2 at 300 K. Modeling and physical characterization demonstrate that these results are presently limited by high threading dislocation density.

Lithographically defined low dimensional SiGe nanostripes as silicon stressors

The introduction of strain in semiconductors is a well-known technique for increasing their conductivity and thus for enhancing the performance of silicon-based electronic devices. In the present work, we investigate the strain induced in the Si substrate by linear SiGe/Si structures with a width less than 100 nm. By varying the Ge content and geometrical parameters, it is possible to maximize the strain in the Si substrate without detrimental plastic relaxation in the SiGe stripes. The structures were defined by electron-beam lithography from strained SiGe deposited epitaxially by low-energy plasma-enhanced chemical vapor deposition. The strain in the heterostructures has been characterized by a combination of finite-element modeling, x-ray diffraction, and μRaman spectroscopy techniques. We show that nano-patterning induces an anisotropic strain relaxation in the SiGe stripe with a simultaneous strong compression of the Si substrate.

Quantum-confined Stark effect at 13 μm in Ge/Si_035Ge_065 quantum-well structure

Room-temperature quantum-confined Stark effect in a Ge/SiGe quantum-well structure is reported at the wavelength of 1.3 μm. The operating wavelength is tuned by the use of strain engineering. Low-energy plasma-enhanced chemical vapor deposition is used to grow 20 periods of strain-compensated quantum wells (8 nm Ge well and 12 nm Si(0.35)Ge(0.65) barrier) on Si(0.21)Ge(0.79) virtual substrate. The fraction of light absorbed per well allows for a strong modulation around 1.3 μm. The half-width at half-maximum of the excitonic peak of only 12 meV allows for a discussion on physical mechanisms limiting the performances of such devices.

Photoreflectance study of direct-gap interband transitions in Ge/SiGe multiple quantum wells with Ge-rich barriers

Photoreflectance (PR) was used to study the direct-gap interband transitions in strain-compensated Ge/SiGe multiple quantum well (MQW) structures with Ge-rich SiGe barriers grown by low-energy plasma-enhanced chemical vapor deposition. The PR spectra revealed a wide range of possible optical transitions in the MQW structure. Detailed lineshape fits to the PR spectra and comparison to a theoretical calculation based on the envelope-function approximation with conduction band-offset and stain compensation factor as adjust parameters led to the identification of various interband transitions. The results demonstrated that PR is a powerful technique for nondestructive optical characterization of Ge/SiGe MQW structures.

Plasma Composition by Mass Spectrometry in a Ar-SiH4-H2 LEPECVD Process During nc-Si Deposition

Mass spectrometry has been used to assess plasma composition during a low-energy plasma-enhanced chemical vapor deposition (LEPECVD) process using argon-silane-hydrogen (Ar-SiH4-H2) gas mixtures with input flows of 50 sccm Ar, 2–20 sccm SiH4 and 0–50 sccm H2 at total pressures of 1–4 Pa. Energy-integrated ion densities, residual gas analysis and threshold ionization mass spectrometry have been used to characterize the transition from amorphous (a-Si) to nano-crystalline silicon (nc-Si) deposition at constant LEPECVD operating parameters. While relative ion densities have a marked decrease with H2 input, the densities of SiHx (x < 4) radicals show evolution trends depending on the SiH4 and H2 input. For conditions leading to nc-Si growth a turning point is reached above which SiH is the main radical. Observed SiHx density trends with H2 input are explained based on kinetic reaction rates calculated from previously obtained Langmuir probe data.

Ge-rich islands grown on patterned Si substrates by low-energy plasma-enhanced chemicalvapour deposition

Si1 − xGex islands grown on Si patterned substrates have received considerable attention during thelast decade for potential applications in microelectronics and optoelectronics. In thiswork we propose a new methodology to grow Ge-rich islands using a chemicalvapour deposition technique. Electron-beam lithography is used to pre-pattern Sisubstrates, creating material traps. Epitaxial deposition of thin Ge films by low-energyplasma-enhanced chemical vapour deposition then leads to the formation of Ge-richSi1 − xGex islands(x > 0.8) with a homogeneous size distribution, precisely positioned with respect tothe substrate pattern. The island morphology was characterized by atomicforce microscopy, and the Ge content and strain in the islands was studied byμRaman spectroscopy. This characterization indicates a uniform distribution of islands with high Gecontent and low strain: this suggests that the relatively high growth rate (0.1 nm s − 1) and lowtemperature (650 °C) used is able to limit Si intermixing, while maintaining a long enough adatom diffusionlength to prevent nucleation of islands outside pits. This offers the novel possibility of usingthese Ge-rich islands to induce strain in a Si cap.