Thin Germanium Research Articles

Low Temperature (180°C) Growth of Smooth Surface Germanium Epilayers on Silicon Substrates Using Electron Cyclotron Resonance Chemical Vapor Deposition

This paper describes a new method to grow thin germanium (Ge) epilayers (40 nm) on c-Si substrates at a low growth temperature of 180°C using electron cyclotron resonance chemical vapor deposition (ECR-CVD) process. The full width at half maximum (FWHM) of the Ge (004) in X-ray diffraction pattern and the compressive stain in a Ge epilayer of 683 arcsec and 0.12% can be achieved. Moreover, the Ge/Si interface is observed by transmission electron microscopy to demonstrate the epitaxial growth of Ge on Si and the surface roughness is 0.342 nm. The thin-thickness and smooth surface of Ge epilayer grown on Si in this study is suitable to be a virtual substrate for developing the low cost and high efficiency III-V/Si tandem solar cells in our opinion. Furthermore, the low temperature process can not only decrease costs but can also reduce the restriction of high temperature processes on device manufacturing.

Strong absorption and selective emission from engineered metals with dielectric coatings

We demonstrate strong-to-perfect absorption across a wide range of mid-infrared wavelengths (5-12µm) using a two-layer system consisting of heavily-doped silicon and a thin high-index germanium dielectric layer. We demonstrate spectral control of the absorption resonance by varying the thickness of the dielectric layer. The absorption resonance is shown to be largely polarization-independent and angle-invariant. Upon heating, we observe selective thermal emission from our materials. Experimental data is compared to an analytical model of our structures with strong agreement.

Mobility enhancement of top contact pentacene based organic thin film transistor with bi-layer GeO/Au electrodes

The enhancement of the charge injection and field effect mobility by inserting a thin (5 nm) germanium oxide (GeO) interlayer between the Au electrode and pentacene layer in a top contact pentacene based organic thin-film transistor (OTFTs) was reported. In comparison with the pentacene-based OTFT with only-Au electrode, the device performance has been considerably improved, which exhibits the highest field effect mobility of 0.96 cm2/Vs. The improvement was attributed to significant reduction of barrier height at Au/pentacene interfaces and smoothed surface of pentacene layer after inserting a thin GeO layer.

Formation of Thin Germanium Dioxide Film with a High-Quality Interface Using a Direct Neutral Beam Oxidation Process

Formation of Thin Germanium Dioxide Film with a High-Quality Interface Using a Direct Neutral Beam Oxidation Process

Formation of Thin Germanium Dioxide Film with a High-Quality Interface Using a Direct Neutral Beam Oxidation Process

A damage-free and low-temperature neutral beam oxidation (NBO) process is used to directly form a thin germanium dioxide (GeO2) film. A GeO2 film with only a small amount of suboxide is formed even at a low substrate temperature of 300 °C because of the extremely low-activation-energy oxidation owing to bombardment with 5-eV-energy oxygen neutral beams. We combined the NBO process with hydrogen radical native oxide removal treatment to form high-quality GeO2 films, and our fabricated Al2O3/GeO2/Ge gate stack has an extremely low interface state density (Dit) of less than 1×1011 cm-2 eV-1.

Low activation energy, high-quality oxidation of Si and Ge using neutral beam

In this letter, we investigated the mechanism that forms thin silicon and germanium oxide films with a high-quality interface using a low-temperature neutral beam oxidation (NBO) process. Because NBO has high reactivity due to bombardment by energetic oxygen-neutral beams even at low substrate temperatures, we found that an extremely low activation energy for the atomic layer oxidation reaction could be achieved during the process itself. As a result, there was little suboxide at the interface between the oxide films and the semiconductor, and device characteristics with a high performance were observed.

Sn-induced low-temperature growth of Ge nanowire electrodes with a large lithium storage capacity

We herein present the synthesis of germanium (Ge) nanowires on Au-catalyzed low-temperature substrates using a simple thermal Ge/Sn co-evaporation method. Incorporation of a low-melting point metal (Sn) enables the efficient delivery of Ge vapor to the substrate, even at a source temperature below 600 °C. The as-synthesized nanowires were found to be a core/shell heterostructure, exhibiting a uniform single crystalline Ge sheathed within a thin amorphous germanium suboxide (GeO(x)) layer. Furthermore, these high-density Ge nanowires grown directly on metal current collectors can offer good electrical connection and easy strain relaxation due to huge volume expansion during Li ion insertion/extraction. Therefore, the self-supported Ge nanowire electrodes provided excellent large capacity with little fading upon cycling (a capacity of ∼900 mA h g(-1) at 1C rate).

Structural, Vibrational, and Optical Characterization of Anodically Electroplated Germanium Thin Films

Germanium thin films were anodically electrodeposited from ethylenediamine solutions of which contain the well known deltahedral Zintl ions. The anodes were -doped Si(100) wafers or ITO-covered glass slides. The films were studied for their optical and structural properties by Raman spectroscopy, TEM, and luminescence measurements. The Raman spectra of the films show peaks shifted to lower frequencies compared to the same peaks in bulk germanium and indicate nanocrystalline structure for the films. This is further corroborated by the TEM observations. Laser-induced crystallization at selected areas of the films results in significant growth of the nanocrystals. The films exhibit photoluminescence in the visible light range. This has been attributed to a combination of nanocrystallinity and surface defects. Overall, the optical and structural properties of the anodically electrodeposited thin germanium films can be effectively adjusted by tuning the electrodeposition parameters.

SEM/EDS study of metal-assisted oxide desorption

Strongly-enhanced desorption of a thick (100 nm) silicon oxide layer by the pre-sputtering of a thin germanium surface film was observed under high-temperature vacuum annealing conditions. High-resolution SEM imaging reveals that germanium nanoislands are first formed on the sample surface, and that these then act as nucleation centres for the formation of voids in the oxide, leading to a rapid desorption of the silicon oxide layer. EDS analysis of the silicon surface after oxide decomposition shows that the introduced germanium impurities are fully consumed in this desorption process.

Stranski–Krastanow growth of multilayer In(Ga)As/GaAs QDs on Germanium substrate

We have described Stranski–Krastanow growth of multilayer In(Ga)As/GaAs QDs on Ge substrate by MBE. The growth technique includes deposition of a thin germanium buffer layer followed by migration-enhanced epitaxy (MEE) grown GaAs layer at 350°C. The MEE layer was overgrown by a thin low-temperature (475°C) grown GaAs layer with a subsequent deposition of a thick GaAs layer grown at 590°C. The sample was characterized by AFM, cross-sectional TEM and temperature-dependent PL measurements. The AFM shows dense formation of QDs with no undulation in the wetting layer. The XTEM image confirms that the sample is free from structural defects. The 8 K PL emission exhibits a 1051 nm peak, which is similar to the control sample consisting of In(Ga)As/GaAs QDs grown on GaAs substrate, but the observed emission intensity is lower. The similar slopes of Arrhenius plot of the integrated PL intensity for the as-grown QD sample grown on Ge substrate as well as for a reference QD sample grown on GaAs substrate are found to be identical, indicating a similar carrier emission process for both the samples. This in turn indicates coherent formation of QDs on Ge substrate. We presume due to the accumulated strain associated with the self-assembled growth of nanostructures on Ge that nonradiative recombination centers are introduced in the GaAs barrier in between the QD layers, which in turn degrades the overall optical quality of the sample.

Ultra-thin ultra-smooth and low-loss silver films on a germanium wetting layer

We demonstrate a method to fabricate ultra-thin, ultra-smooth and low-loss silver (Ag) films using a very thin germanium (Ge) layer as a wetting material and a rapid post-annealing treatment. The addition of a Ge wetting layer greatly reduces the surface roughness of Ag films deposited on a glass substrate by electron-beam evaporation. The percolation threshold of Ag films and the minimal thickness of a uniformly continuous Ag film were significantly reduced using a Ge wetting layer in the fabrication. A rapid post-annealing treatment is demonstrated to reduce the loss of the ultra-thin Ag film to the ideal values allowed by the quantum size effect in smaller grains. Using the same wetting method, we have also extended our studies to ultra-smooth silver-silica lamellar composite films with ultra-thin Ag sublayers.

Control of direct band gap emission of bulk germanium by mechanical tensile strain

We show that the recombination energy of the direct band gap photoluminescence (PL) of germanium can be controlled by an external mechanical stress. The stress is provided by an apparatus commonly used for bulge or blister test. An energy redshift up to 60 meV is demonstrated for the room temperature PL of a thin germanium membrane (125 nm wavelength shift from 1535 to 1660 nm). This PL shift is correlated with the in-plane tensile strain generated in the film. A biaxial tensile strain larger than 0.6% is achieved by this method. This mechanical strain allows to approach the direct band gap condition for germanium which is of tremendous importance to achieve lasing with this material.

Germanium-oxide-coated carbon nanotubes

An ultrathin coating of individual multi-wall carbon nanotubes with germanium oxidewas developed. Coated carbon nanotubes were prepared by in situ gelation oftetramethoxy germanium, forming an amorphous germanium oxide layer. Theresulting germanium oxide/carbon nanotube nanocables were characterized bytransmission electron microscopy (TEM), TEM/energy-dispersive x-ray spectroscopy,x-ray photoelectron spectroscopy (XPS) and current–voltage measurements. The∼10 nm thin germanium oxide layer provides excellent electrical insulating properties forindividual carbon nanotubes.

Solid phase epitaxy of amorphous Ge on Si in N2 atmosphere

We demonstrate a straightforward and economical way to obtain smooth germanium layers of high quality on silicon. Thin amorphous germanium layers deposited by plasma enhanced chemical vapor deposition on Si(111) substrates are transformed into single crystalline and smooth layers by solid phase epitaxy in N2 atmosphere. The crystal orientation of the substrate has a clear influence on the crystal quality. This is most likely due to a different growth mode, namely, layer-by-layer for Si(111) and three-dimensional growth for Si(001). The amorphous germanium layer can roughen during annealing due to mobile atoms on the surface. This can be effectively suppressed by annealing in N2 ambient. Electrical measurements show high charge mobility.

Study of ion emission from a germanium crystal surface under impact of fast Pb ions in channeling conditions

A thin germanium crystal has been irradiated at GANIL by Pb beams of 29 MeV/A (charge state Q in = 56 and 72) and of 5.6 MeV/A ( Q in = 28). The induced ion emission from the sample entrance surface was studied, impact per impact, as a function of Q in, velocity v in and energy loss Δ E in the crystal. The Pb ions transmitted through the crystal were analyzed in charge ( Q out) and energy using the SPEG spectrometer. The emitted ionized species were detected and analyzed in mass by a time-Of-flight multianode detector (LAG). Channeling was used to select peculiar Δ E values in Ge and hence peculiar Pb ion trajectories close to the emitting entrance surface. The experiment was performed in standard vacuum. No Ge emission was found. The dominating emitted species are H + and hydrocarbon ions originating from the contamination layer on top of the crystal. The mean value 〈 M〉 of the number of detected species per incoming Pb ion (multiplicity) varies as ( Q in/ v in ) p , with p values in agreement with previous results. We have clearly observed an influence of the energy deposition Δ E in Ge on the emission from the top contamination layer. When selecting increasing values of Δ E, we observed a rather slow increase of 〈 M〉. On the contrary, the probabilities of high multiplicity values, which are essentially connected to fragmentation after emission, strongly increase with Δ E.

Hf O 2 -based InP n-channel metal-oxide-semiconductor field-effect transistors and metal-oxide-semiconductor capacitors using a germanium interfacial passivation layer

In this letter, we present our experimental results of HfO2-based n-channel metal-oxide-semiconductor field-effect transistors (MOSFETs) and metal-oxide-semiconductor capacitors (MOSCAPs) on indium phosphide (InP) substrates using a thin germanium (Ge) interfacial passivation layer (IPL). We found that MOSCAPs on n-InP substrates showed good C-V characteristics such as a small capacitance equivalent thickness (14Å), a small frequency dispersion (<10% and <200mV), and a low dielectric leakage current (∼5×10−4A∕cm2 at Vg=1.5V), whereas MOSCAPs on p-InP exhibited poor characteristics, implying severe Fermi level pinning. It was also found that InP was more vulnerable to a high temperature process such that C-V curves showed a characteristic “bump” and inversion capacitance at relatively high frequencies. From n-channel MOSFETs on a semi-insulating InP substrate using Ge IPL, HfO2, and TaN gate electrodes, excellent electrical characteristics such as a large transconductance (9.3mS∕mm) and large drain currents (12.3mA∕mm at Vd=2V and Vg=Vth+2V) were achieved, which are comparable to other works.

A study of metal-oxide-semiconductor capacitors on GaAs, In0.53Ga0.47As, InAs, and InSb substrates using a germanium interfacial passivation layer

In this letter, we present electrical characteristics of HfO2-based metal-oxide-semiconductor capacitors (MOSCAPs) on n- and p-type GaAs, In0.53Ga0.47As, InAs, and InSb substrates, along with the effect of a thin germanium interfacial passivation layer. We found that MOSCAPs on all n-type substrates showed good C-V characteristics with small frequency dispersion (<10% and <200 mV). However, MOSCAPs on p-type GaAs and In0.53Ga0.47As substrates exhibited poor C-V characteristics implying severe Fermi level pinning, as has also been seen for p-type InP substrate. On the other hand, MOSCAPs on p-type InAs and InSb substrates, known as smaller bandgap materials, showed good C-V characteristics. We also present plausible mechanism for Fermi level pinning and interface characteristics.

X-ray photoelectron and auger spectroscopy analysis of Ge:Mn-based magnetic semiconductor layers

Thin (∼60 nm) germanium layers supersaturated with a manganese impurity of 10–16 at % have been studied by x-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). The layers have been fabricated by pulsed laser deposition onto a semi-insulating single-crystal GaAs substrate. The results of XPS analysis of the Ge:Mn layers reveal a change in the line shape of germanium and manganese (2p) in the surface region compared to deeper layers, which indicates a transition from the oxidized form of the base material (Ge2+ and Ge1+) and impurity (Mn2+) near the surface to the unoxidized state of germanium (Ge 0) and manganese (Mn0) in the interior of the layer. The XPS spectra of the valence electrons of the Ge:Mn structure indicate that the density of states in the valence band of the ferromagnetic Ge:Mn structures is caused not only by mechanical mixing of germanium and manganese. The composition of heterogeneous inclusions in Ge:Mn films has been studied using scanning Auger microscopy.

Characterization and comparison of X-ray focusing optics for ultrafast X-ray diffraction experiments

We have characterized and compared the performance of different types of multi-layer optics for the focusing of femtosecond X-ray pulses. Using X-ray pulses at 8 keV, from a laser-driven plasma source we have measured the spatial distribution of the diffracted X-rays directly after and in the focal plane of the various X-ray optical devices. For a Montel optic with 7.3× magnification we obtained the largest number of focused X-ray photons per unit angle. The performance of this optic in the X-ray diffraction experiment on a thin germanium film is demonstrated.

Characterization of Threading Dislocations in Thin Germanium Layers by Defect Etching: Toward Chromium and HF-Free Solution

Defect etching is a fast and simple technique for the revelation of defects in single-crystalline materials. We propose here three different chemistries that allow accurate monitoring of the density of threading dislocations in germanium. This work especially focuses on solutions that have low etch rates and high selectivity in order to work on thin layers. Moreover, solutions that do not contain any carcinogenic nor HF acid are proposed in order to reduce the impact on environment and the risks for the user. A comparison of the etch rate and the selectivity toward defects is given as a function of the composition of the etching solutions (components and concentrations). A complete procedure for the proper determination of threading dislocation densities is also presented.