Thin Ge Layer Research Articles

Realization and characterization of thin single crystal Ge films on sapphire

We have successfully produced and characterized thin single crystal Ge films on sapphire substrates (GeOS). Such a GeOS template offers a cost-effective alternative to bulk germanium substrates for applications where only a thin (<2 µm) Ge layer is needed for device operation. The GeOS templates have been realized using the Smart CutTM technique. 100 mm diameter GeOS templates have been manufactured and characterized to compare the Ge thin film properties with bulk Ge. Surface defect inspection, SEM, AFM, defect etching, XRD and Raman spectroscopy were all performed. The results obtained for each characterization technique used have highlighted that the material properties of the transferred thin Ge film were very close to the ones of a bulk Ge reference. An epitaxial AlGaInP/GaInP/AlGaInP double heterostructure was grown atop the GeOS template to demonstrate the template's stability under the conditions encountered in typical device realization. The photoluminescent behavior of this epitaxial structure was nearly identical to that of a similar structure grown on a bulk Ge substrate. The GeOS templates therefore offer a viable alternative to bulk Ge substrates in the fabrication of devices whose operation is compatible with a thin film structure.

Strain relaxation of thin Ge films on Si(001) grown by carbon-mediated epitaxy

Strain relaxation of thin Ge layers grown by carbon-mediated epitaxy at 50°C on Si(001) was investigated for a multi-step growth procedure. Additionally, the impact of the post-growth annealing temperature on the strain relaxation obtained for a single carbon-mediated growth step was analyzed. The degree of relaxation was monitored in situ by reflection high energy electron diffraction and ex situ by high-resolution x-ray diffraction and transmission electron microscopy. For multi-step growth, relaxation occurs mainly during the first cycle (growth and annealing). Full relaxation is already achieved after 3–4 cycles. The relaxation process during a single growth cycle can be divided into three sections: annealing up to 200°C is sufficient to trigger most of the strain relaxation. After annealing up to 450°C, the film lattice parameter already equals the Ge bulk value, though the layer shows a rough surface. Only an annealing step up to 650°C results in a smooth surface and interface. Smooth and fully relaxed Ge films of only 26nm thicknesses were obtained by carbon-mediated growth.

Reduced Pressure CVD Growth of Ge and Ge1−xSnx Alloys

The epitaxial growth of Ge and GeSn alloys on Si(100) by Reduced Pressure Chemical Vapor Deposition is discussed. Particular emphasis is placed on the growth kinetics in the temperature window between 375°C and 475°C. Using Ge2H6 precursor and H2 carrier gas a low activation energy of 0.7 eV was found for the Ge growth which further decreases to 0.5 eV for N2 carrier gas. Thin Ge layers with high crystalline quality are aimed which may be used as buffers for the epitaxial growth of GeSn layers. Furthermore, GeSn alloys with up to 10 at.% Sn are synthesized on Si(100) at growth temperatures of 375°C employing Ge2H6 and SnCl4 precursors. All Ge and GeSn layers exhibit high crystalline quality as revealed by X-ray reciprocal space mapping and are nearly fully strain relaxed.

Epitaxial Ge Growth on Si(111) Covered with Ultrathin SiO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; Films

The epitaxial growth of Ge on Si(111) covered with the 0.3 nm thick SiO2 film is studied by scanning tunneling microscopy. Nanoareas of bare Si in the SiO2 film are prepared by Ge deposition at a temperature in the range of 570℃-650℃ due to the formation of volatile SiO and GeO molecules. The surface morphology of Ge layers grown further at 360℃-500℃ is composed of facets and large flat areas with the Ge(111)-c(2 × 8) reconstruction which is typical of unstrained Ge. Orientations of the facets, which depend on the growth temperature, are identified. The growth at 250℃-300℃ produces continuous epitaxial Ge layers on Si(111). A comparison of the surface morphology of Ge layers grown on bare and SiO2-film covered Si(111) surfaces shows a significantly lower Ge-Si intermixing in the latter case due to a reduction in the lattice strain. The found approach to reduce the strain suggests the opportunity of the thin continuous epitaxial Ge layer formation on Si(111).

High-mobility p-channel metal-oxide-semiconductor field-effect transistors on Ge-on-insulator structures formed by lateral liquid-phase epitaxy

High-mobility p-channel metal-oxide-semiconductor field-effect transistors (MOSFETs) were fabricated on germanium-on-insulator (GOI) structures formed by lateral liquid-phase epitaxy (LLPE) from the Si seed areas. It was found that appropriate rapid annealing conditions for LLPE effectively suppress intermixing at the Si seed regions and produce tensile strained single-crystalline Ge layers surrounded by SiO2 microcrucibles. We examined the electrical properties of the thin Ge layers using GOI MOSFETs with back-gate control in the p-type accumulation mode. Excellent transistor performance, such as a low off-leakage current of 1 × 10−7 μA/μm, a high on/off current ratio of 106, and high low-field hole mobility of 480 cm2/Vs, which is 2.8 times higher than that of the reference silicon-on-insulator device, was demonstrated, indicating that the LLPE method provides high-quality local GOI structures and that it is a feasible way to fabricate the next-generation Ge-based devices.

Layer-by-layer analysis of structures containing δ-layers by secondary ion mass spectrometry taking into account the TOF.SIMS-5 depth resolution function

The depth resolution function of the TOF.SIMS-5 secondary ion mass spectrometer has been studied for several types of analyzed structures under different experimental conditions. A numerical algorithm for calculating the depth distribution profile of an element for a given structure model taking into account ion mixing, induced roughness, and information depth has been realized. The values of the depths of the thin near-surface layers (δ-layers) of Ge in a Si matrix and Si in a GaAs matrix were determined by fitting the calculated profiles to experimental ones by varying parameters of the resolution function. It is shown that determination of the depth of the δ-layer from the peak position on the experimental profile yields a large systematic error, account of which noticeably increases the accuracy of estimating the delta-layer’s position.

Photoluminescence Efficiency and Size Distribution of Self Assembled Ge Dots on Porous TiO&lt;SUB&gt;2&lt;/SUB&gt;

For Ge nanodots approximately 20 nm in diameter grown by annealing a thin amorphous Ge layer deposited by molecular beam epitaxy on a mesoporous TiO2 layer on Si(001), photoluminescence (PL) was observed as a wide near-infrared band near 800 meV. Using a tight binding theoretical model, the energy-dependent PL spectrum was transformed into a dependence on dot size. The average dot size determined the peak energy of the PL band and its shape depended on the size distribution, including bandgap enlargement due to quantum confinement. Combining the dot sample PL with an established dependence of emission efficiency on dot diameter, it was possible to derive a dot size distribution and compare it with results obtained independently from atomic force microscopy.

Investigation of Germanium Implanted with Hydrogen for Layer Transfer Applications

The technology for thin Ge layer transfer by hydrogen ion-cut process is characterised in this work. Experiments were carried out to determine suitable hydrogen ion implantation doses in germanium for the low temperature ion cut process by examining the formation of blisters on implanted samples. Raman and Spreading Resistance Profiling (SRP) have been used to analyse defects in germanium caused by hydrogen implants. Bevelling has been used to facilitate probing beyond the laser penetration depth. Results of Raman mapping along the projection area reveal that after post implant annealing at 400 °C, some crystal damage remains, while at 600 °C, the crystal damage has been repaired. SRP shows that some amount of hydrogen acceptor states (~1Î1016 acceptors/cm2) remain after 600 °C. These are thought to be vacancy-related point defect clusters.

Room Temperature Direct Band-Gap Emission from an Unstrained Ge P-I-N LED on Si

We present a novel Ge on Si based LED with unstrained i-Ge active region. The device operates at room temperature and emits photons with energy of 0.8 eV. It basically resembles a p-i-n structure formed on a sub-micrometer thin Ge layer. The Ge layer has been grown on Si substrate by utilizing thin virtual buffer, so it becomes stress free but with high threading dislocation density. We show that such forward biased diode generates strong emission, caused by direct band to band transition in Ge. Using an InSb based detector we were able to analyze the emission spectrum in a broad energy range. We show that at low and moderate currents, features belonging to the direct and the indirect band to band electronic transitions are present which are characteristic for Ge. Clearly dominating is the direct transition related peak. Due to the missing stress-related red shift this peak appears close to the desired communication wave length of 1.55 μm. The dependence of radiation intensity on the excitation current follows a power low with exponent of 1.7, indicating that the recombination rate of the competitive nonradiative processes is relatively low. At high excitation currents features appear in the low energetic part of the spectrum. All results presented here are discussed in view of the outcome from measurements on Ge high quality bulk material. The role of the dislocation in the Ge films is discussed.

Near-infrared–ultraviolet absorption cross sections for Ge nanocrystals in SiO2 thin films: Effects of shape and layer structure

Measured absolute optical absorption cross sections of 2–6 nm Ge nanocrystals embedded in SiO2 in the near-infrared–ultraviolet spectral range are reported. Thin layers of Ge sandwiched between SiO2 layers were prepared in a multilayered configuration, which upon heat treatment at 800 °C led to multilayers of Ge nanocrystals with a narrow size distribution. Four samples with different nanocrystal average sizes were prepared by this technique. In addition, samples containing Ge nanocrystals randomly distributed in SiO2 films were also prepared in order to clarify the effects of the multilayer configuration on the optical properties. The size distribution and density of the nanocrystals were obtained from transmission electron microscopy investigations and Rutherford backscattering spectrometry measurements. In combination with this structural information the nanocrystal absorption cross sections were obtained from optical transmission measurements, which were corrected for thin film interference effects by a self-consistent procedure. The measurements clearly show that the nanocrystal absorption per atom in the 1.5–5 eV spectral range is smaller than in bulk Ge atoms regardless of the nanocrystal size and that the absorption cross sections per atom for the nanocrystals are largely independent of the nanocrystal size. Moreover, we find that the absorption cross sections for small concentrations of Ge nanocrystals embedded in SiO2 are quantitatively well described by the Mie theory using bulk values for the dielectric function of Ge in the spectral range investigated. Hence quantum confinement effects have only minor if any influence on the dominant features of the absorption spectra for spherical Ge nanocrystals embedded in SiO2. For Ge nanocrystals ordered in multilayers we find that the experimental absorption cross section for the larger and more close packed nanocrystals exceeds that for similar concentrations of nanocrystals randomly distributed in the films, whereas this effect is not observed for multilayered samples with smaller and less densely packed nanocrystals. The combined results show that for all the Ge nanocrystals studied the nanocrystal shape and the matrix in which the nanocrystals are embedded, as well as the nanocrystal configuration play an important role in the optical response of the Ge nanocrystals and that these effects dominate the effects of quantum confinement in the absorption from spherical Ge nanocrystals embedded in SiO2.

Optical Properties of Germanium Dots Self-Assembled on Porous TiO2 Templates

Self-assembled Ge quantum dots were formed by in-situ thermal annealing of a thin amorphous Ge layer deposited by molecular beam epitaxy on a thin porous TiO2 layer grown on SiO2 on Si(001). For samples with dot diameters ranging from 10 to 35 nm, the dot photoluminescence (PL) appeared primarily as a wide near-infrared band peaked near 800 meV. The peak energy of the PL band reflects the average dot size and its shape depends on the dot size distribution. Using a tight binding bandgap model, we have analyzed the PL spectrum in terms of the dot size distribution. The observed size distribution determined from transmission electron and atomic force microscopy allowed the determination of the nonlinear increase in the PL efficiency with decreasing dot diameter. Knowing this generic PL efficiency, we show that it is possible to evaluate the size distribution of Ge dots from their PL energy dependence.

Predicting Size Distributions of Ge Nanodots from Their Photoluminescence

Self-assembled Ge nanodots were fabricated by in situ thermal annealing of a thin amorphous Ge layer deposited by molecular beam epitaxy on a mesoporous layer on Si(001). With average dot diameters of about , the dot photoluminescence (PL) appeared primarily as a wide near-infrared band near . Employing a tight binding theoretical model, we have transformed the energy-dependent PL spectrum into a dependence on dot size. The peak energy of the PL band reflects the average dot size and its shape depends on the dot size distribution, including, for the smaller diameter dots, a bandgap enlargement due to quantum confinement. Using the dot sample PL and our previously established dependence of the PL quantum efficiency on dot diameter, we show that it is possible to make a limited prediction of the size distribution of Ge dots, which we compare with the actual size distribution obtained from atomic force microscopy.

(Invited) Si, SiGe, Ge, and III-V Semiconductor Nanomembranes and Nanowires Enabled by SiGe Epitaxy

SiGe epitaxy in conjuction with selective etching lends itself as an excellent method for manufacturing of nanomembranes and nanowires for a variety of semiconductor materials and nano-geometries. Early device application of this technique has been demonstrated for the device architecture of Silicon-On-Nothing (SON) MOSFET and localized SON MOSFET allowing accommodation of a SOI-type MOSFET on a bulk Si wafer. In the meantime, this approach has been successfully extended to multichannel and multi-nanowire devices. Together with the availability of thin Ge layers (e.g. by virtue of Ge condensation), SiGe epitaxy opens up the possibility of a heterogenous co-integration of high efficiency III-V multijunction solar cells, III-V complementary logic, thermovoltaic device structures, lasers, modulators and detectors with Si-based electronics collocated on the same Si substrate. SiGeC epitaxial layers offer an additional engineering window for lattice mismatch and stress control, as well for achieving desirable selectivities of the required selective etches. Furthermore, the nanomembranes and nanowires made available by SiGe epitaxy and selective etches on ubiquitous Si substrates are excellent vehicles for exploration of the differences between the material properties of 3D (bulk)) and 2D (nanomembranes) and 1D (nanowire) systems.

Local intermixing on Ge/Si heterostructures at low temperature growth

We report experimental investigation on a series of strained Ge/Si heterostructure with various Ge thicknesses (the order of nanometers) grown at low temperatures (260 °C). In addition to the conventional uniform intermixing at the Ge/Si interface for structures with thin Ge layer, local intermixing characterized by a pattern structure is observed for structures with thick Ge layer. The pattern is formed beneath the Ge layer with an island shape and exhibits a Ge concentration dependent profile. From the analysis, it shows that the growth temperature and the strain energy stored in the Ge layer play a dominated role for the observation. In comparison with the conventional high temperature growth, this investigation shows that the self-assembly process is suppressed at low growth temperatures and intermixing plays a dominant role for strain relaxation.

Growth of high quality InP layers in STI trenches on miscut Si (0 0 1) substrates

In this work, we report the selective area epitaxial growth of high quality InP in shallow trench isolation (STI) structures on Si (0 0 1) substrates 6° miscut toward (1 1 1) using a thin Ge buffer layer. We studied the impact of growth rates and steric hindrance effects on the nano-twin formation at the STI side walls. It was found that a too high growth rate induces more nano-twins in the layer and results in InP crystal distortion. The STI side wall tapering angle and the substrate miscut angle induced streric hindrance between the InP facets and the STI side walls also contribute to defect formation. In the [1̄ 1 0] orientated trenches, when the STI side wall tapering angle is larger than 10°, crystal distortion was observed while the substrate miscut angle has no significant impact on the InP defect formation. In the [1̄ 1 0] trenches, both the increased STI tapering angle and the substrate miscut angle induce high density of defects. With a small STI tapering angle and a thin Ge layer, we obtained extended defect free InP in the top region of the [1 1 0] trenches with aspect ratio larger than 2.

Photoluminescence Efficiency of Ge Dots Self-Assembled on SiO2 and TiO2 Films

Self-assembled Ge nanodots were formed by in-situ thermal annealing of a thin amorphous Ge layer deposited by molecular beam epitaxy on a thin SiO2 or TiO2 layer on Si(001). The dot photoluminescence (PL) appeared primarily as a wide near-infrared band peaked near 800 meV. Using both the k•p and tight binding theoretical models, we have analyzed the PL spectrum in terms of the dot size distribution required to reproduce the observed asymmetric band shape. The peak energy of the PL band reflects the average dot size and its shape depends on the dot size distribution. The observed size distribution determined from transmission electron and atomic force microscopy allowed the determination of the nonlinear increase in the PL quantum efficiency with decreasing dot diameter. In addition, we show it is possible to evaluate the size distribution of Ge dots from their PL energy dependence.

Detection efficiency for an ultraviolet photon counting imaging system based on the Ge induction readout

The detection efficiency, assumed proportional to the count rate, of an ultraviolet photon counting imaging system based on the Ge induction readout is studied. For the Ge induction readout, the system count rate with different Ge layer resistances, Ge layer thicknesses and ceramic thicknesses are tested under different light intensities. The test results show that the system count rate with a high resistance in the Ge layer is lower than that with a low resistance in the Ge layer; further, the imaging area decreases as the Ge layer resistance increases under high photon flux. Moreover the system count rate with a thick Ge layer is lower than that with a thin Ge layer. In addition, the system count rate with 2-mm ceramic is higher than that with 1.5- and 3-mm ceramic. The above observation should be considered when designing an ultraviolet photon counting imaging detector based on the Ge induction readout.

Controllable Dresselhaus field in microscopically inversion-symmetric quantum wells

We have investigated theoretically a method to control the Dresselhaus effective magnetic field, using crystallographically inversion-symmetric quantum wells, whose well layer consists of a compound semiconductor and a thin Ge layer at the centre of the well. The Dresselhaus field vanishes, because of the inversion symmetry of the crystal structure, and is induced by electric field applied perpendicular to the well. Sps* tight-binding calculation shows that the Dresselhaus field is proportional to the applied electric field, and is comparable to the Rashba field for wells with appropriate width. As a result, we can externally control the strength of the total effective magnetic field without changing its shape.

Impact of the H 2 anneal on the structural and optical properties of thin and thick Ge layers on Si; Low temperature surface passivation of Ge by Si

Using a low temperature/high temperature strategy, we have grown thin (0.27 μm) and thick (2.45 μm) Ge layers on Si(0 0 1) substrates that we have submitted to various constant temperature (750 °C) or cyclic (750 °C/890 °C) H 2 anneals, the objective being to identify those yielding the smoothest surfaces, the lowest threading dislocations densities (TDDs) and the highest near infra-red optical absorptions. The best trade-off for thin layers was 750 °C, 60 min H 2 anneals. Using longer duration 750 °C anneals and especially 750 °C/890 °C cyclic anneals indeed yielded rougher surfaces and vastly degraded optical absorption (deleterious formation of GeSi alloys). By contrast, short 750 °C/890 °C thermal cyclings yielded the best metrics in thick Ge layers (while being at the same time the best in terms of throughput): root mean square surface roughness around 0.8 nm, TDD around 10 7 cm −2, slightly tensily-strained layers (which a plus for optical absorption as the absorption edge is shifted to higher wavelengths), a limited penetration of Si into Ge (and thus absorption coefficients at 1.3 and 1.55 μm almost equal to those of as-grown layers), etc. We have also described the low temperature (450 °C/525 °C) process that we have developed to passivate Ge surfaces thanks to SiH 4 prior to gate stack deposition. Si layer thickness should be below 20 Å in order to have conformal deposition. A transition of the growth front to 3 dimensions has indeed been evidenced for 20 Å and higher.

Low-voltage high-speed thin-film-Ge surface PIN photodetectors integrated on Si waveguide

In this article, we demonstrate the thin-film-Ge surface PIN photodetectors. With thin Ge layer (~120nm including buffer layers), the devices couple well with the incoming Si waveguide and achieve high responsivity due to the surface layout of the highly-doped p+/n+ region. By shrinking the p+/n+ spacing of the device, the device exhibits high speed of ~20GHz at the bias of −1V.