Ge Oxide Research Articles

Ge gate stacks based on Ge oxide interfacial layers and the impact on MOS device properties

Ge gate stacks by exposing oxygen plasma to ALD Al2O3/Ge were proposed.GeOx formed between Al2O3 and Ge significantly reduces interface state density.High electron mobility of 690cm2/Vs in Ge nMOSFETs with EOT of 0.7-0.8nm.High hole mobility of 546cm2/Vs in Ge p-MOSFETs with EOT of 0.7-0.8nm. CMOS utilizing high mobility Ge channels on Si substrates is expected to be one of promising devices for high performance and low power advanced LSIs. Here, one of the most critical issues in Ge MOSFETs is the realization of gates stacks with superior MOS interfaces. We have proposed novel Ge gate stack technology using ultrathin Ge oxide interfacial layers (ILs). The Ge ILs are formed by exposing ECR oxygen plasma to thin Al2O3 on Ge substrates, which is called plasma post oxidation. Here, Al2O3 serves as a sufficient oxygen barrier that suppresses the growth of unnecessarily-thick GeOx IL, thanks to its intrinsic oxygen permeability. The relationship between the GeOx thickness and interface state density (Dit) is evaluated. Also, in order to further reduce EOT, we have introduced a revised version of gate stack formation using HfO2/Al2O3 stacks. The role of ultrathin Al2O3 films on the properties of the gate stacks is studied. As a result, HfO2/Al2O3/GeOx/Ge gate stacks having EOT of 0.72nm as the thinnest value have been realized with the excellent interface quality. High electron mobility of 754 and 690cm2/Vs and hole mobility of 596 and 546cm2/Vs have been obtained in EOT of 0.82 and 0.76nm, respectively.

Structural and optical studies of nanostructured TiO2–Ge multi-layer thin films

This paper reports the effects of annealing on structural and optical properties of nanostructured multi-layer TiO2–Ge thin films. These films were characterized using different techniques such as X-ray diffraction, X-ray reflectivity, Rutherford backscattering (RBS), and Fourier Transform Infrared spectroscopy. Annealing was responsible for pronounced changes in structural and optical properties of these films, as associated with changes in their structures, stoichiometry and stress-state.Three sets of TiO2–Ge multi-layer films were deposited by electron beam evaporation and resistive heating with different Ge layer thickness (5, 10 and 15nm), and TiO2 layer thickness was fixed to 20nm. The films were annealed in air up to 500°C for 2h. RBS studies showed that the layer structure of TiO2–Ge multi-layer films had been formed. The absorption spectra and band gap energy showed a blue shift with decrease in Ge layer thickness. The absorption spectra of these films suggest quantum confinement that increases with annealing temperature before the complete oxidation of Ge. Apparently complete oxidation results in sudden or sharp rise in band gap energy that matches with that of TiO2. RBS study reveals that layered structure of TiO2–Ge multi-layer films is not destroyed by annealing, which may be due to non-wetting behavior of Ge and its oxide with TiO2. These results imply that nanostructured TiO2–Ge multi-layer thin films may be employed as heterojunctions (with tunable band gap energy) based on quantum confinement effects for use in photovoltaics.

Interfacial Reaction Mechanisms in Al2O3/Ge Structure by Oxygen Radical Process

We have investigated the impacts of the oxygen radical process on the interfacial structures and electrical properties of Al2O3/Ge structures to clarify the interfacial reaction mechanisms. At a low process temperature, the oxygen radical process can introduce oxygen atoms to the Al2O3/Ge interface without a thermally activated process in spite of the high barrier property of the oxygen diffusion for the Al2O3 layers. In addition, the oxygen radical process at a low process temperature can relatively suppress the diffusion of Ge atoms from the Ge substrate or GeO molecules from the Al2O3/Ge interface to the surface of the Al2O3 layer. However, at a high process temperature, Ge atoms and/or GeO molecules actively diffuse into the Al2O3 layer during the oxygen radical process as well as the O2 thermal annealing, and the diffusion changes the depth distribution of Ge oxides in the Al2O3/Ge structure. From the analysis of the electrical properties of MOS capacitors, the interface state density (Dit) of the Al2O3/Ge structure decreases not with increasing thickness of the Ge oxide interlayer but with the amount of Ge oxide near the Al2O3/Ge interface. The increase in the amount of the Ge oxide distributed in the Al2O3 layer induces the increase in the capacitance equivalent thickness (CET). The diffusion of Ge into the Al2O3 layer with a high process temperature causes the unexpected increase in CET. Therefore, the oxygen radical process at low temperature effectively decreases Dit of Al/Al2O3/Ge MOS capacitors without increasing CET.

Structural changes in amorphous GexSiOy on the way to nanocrystal formation

Temperature induced changes of the local chemical structure of bulk amorphous GexSiOy are studied by Ge K-edge x-ray absorption near-edge spectroscopy and Si L2/3-edge x-ray Raman scattering spectroscopy. Different processes are revealed which lead to formation of Ge regions embedded in a Si oxide matrix due to different initial structures of as-prepared samples, depending on their Ge/Si/O ratio and temperature treatment, eventually resulting in the occurrence of nanocrystals. Here, disproportionation of GeOx and SiOx regions and/or reduction of Ge oxides by pure Si or by a surrounding Si sub-oxide matrix can be employed to tune the size of Ge nanocrystals along with the chemical composition of the embedding matrix. This is important for the optimization of the electronic and luminescent properties of the material.

(Invited) GOI Substrates: Fabrication and Characterization

Wafer bonding is an attractive process to creat new structures and materials, alternative to conventional bulk-Si substrates, in the form of substrates for the advanced metal-oxide-semiconductor field effect transistors (MOSFETs). A germanium on insulator (GOI) substrate attracts much attention because it makes the best use of the higher carrier mobility advantages compared with Si, and is suitable to further downscaling of Ge MOSFET dimensions. However, careful consideration is given especially to the bonded interface between Ge and buried oxide, which should have a device grade quality. In this work, we fabricate GOI substrates by using a wafer bonding method. Atomic structures, chemistry, and electrical properties of the Ge/BOX interface are also characterized.

Kinetics of thermally oxidation of Ge(100) surface

Thermal oxidation of a Ge(100) surface was investigated by using spectroscopic ellipsometry (SE) and x-ray photoelectron spectroscopy (XPS). Ge oxide was grown in the temperature range of 375 to 550°C in dry-O2 ambience at atmospheric pressure. Although the Ge-oxide growth rate shows a linear relationship in a log-log plot at a fixed temperature, and the slope indicates an enhancement of GeO desorption at oxidation temperatures over 490°C. The GeO desorption was also confirmed from the XPS analysis of the Si surface which was oxidized simultaneously with the Ge(100) surface. Thus, the Ge thermal oxidation at atmospheric pressure cannot be explained simply by the Deal-Grove model, in which the contribution of thermal desorption of Ge monoxide must be taken into account.

Evidence of GeO volatilization and its effect on the characteristics of HfO2 grown on a Ge substrate

HfO2 films are deposited by atomic layer deposition (ALD) using tetrakis ethylmethylamino hafnium (TEMAH) as the hafnium precursor, while O3 or H2O is used as the oxygen precursor. After annealing at 500 °C in nitrogen, the thickness of Ge oxide's interfacial layer decreases, and the presence of GeO is observed at the H2O-based HfO2 interface due to GeO volatilization, while it is not observed for the O3-based HfO2. The difference is attributed to the residue hydroxyl groups or H2O molecules in H2O-based HfO2 hydrolyzing GeO2 and forming GeO, whereas GeO is only formed by the typical reaction mechanism between GeO2 and the Ge substrate for O3-based HfO2 after annealing. The volatilization of GeO deteriorates the characteristics of the high-κ films after annealing, which has effects on the variation of valence band offset and the C—V characteristics of HfO2/Ge after annealing. The results are confirmed by X-ray photoelectron spectroscopy (XPS) and electrical measurements.

Atomic layer-by-layer oxidation of Ge (100) and (111) surfaces by plasma post oxidation of Al2O3/Ge structures

The ultrathin GeOx/Ge interfaces formed on Ge (100) and (111) surfaces by applying plasma post oxidation to thin Al2O3/Ge structures are characterized in detail using X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy. It is found that the XPS signals assigned to Ge 1+ and the 2+ states in the GeOx layers by post plasma oxidation have oscillating behaviors on Ge (100) surfaces in a period of ∼0.3 nm with an increase in the GeOx thickness. Additionally, the oscillations of the signals assigned to Ge 1+ and 2+ states show opposite phase to each other. The similar oscillation behaviors are also confirmed on Ge (111) surfaces for Ge 1+ and 3+ states in a period of ∼0.5 nm. These phenomena can be strongly regarded as an evidence of the atomic layer-by-layer oxidation of GeOx/Ge interfaces on Ge (100) and (111) surfaces.

Impact of ammonium sulfide solution on electronic properties and ambient stability of germanium surfaces: towards Ge-based microelectronic devices

The monolayer adsorption of sulfur on Ge(100) surfaces from aqueous (NH4)2S solution is an approach to saturate, i.e., to passivate broken surface bonds and hence to reduce the electrical and chemical activity of this semiconductor surface. Despite its importance in view of developing Ge-based microelectronic devices, we still lack a fundamental understanding of how this treatment modifies the electrical and chemical properties of the Ge surface. In this study, the electronic properties and ambient stability of sulfurized p-type Ge surfaces are investigated using a variety of complementary spectroscopic techniques. Based on these results we evaluate the degree of electrical and chemical passivation that can be achieved by sulfur adsorption from (NH4)2S solution. We find that sulfur atoms chemically bind to Ge surface atoms within the first few seconds after immersion in solution. Saturation is achieved after approximately 30 s at a maximum sulfur coverage below half of a monolayer. The Ge–S bonds have a partial ionic character, causing depletion of the majority charge carriers near the surface. The band gap measured at the surface exhibits a lower density of surface states compared to the clean Ge surface, indicating that the S/Ge surface is electrically passivated. The Ge–S bonds are preserved upon moderate exposure to ambient conditions (ca. 2 hours), but a small fraction of the sulfur is oxidized. The steady increase of the oxygen coverage with increasing exposure time suggests a growth of Ge oxides, indicating limited resistance of the sulfurized Ge surfaces to oxidizing conditions.

Ge(100)-2×1表面の超音速酸素分子線による室温酸化促進

Ge(100)-2×1表面の超音速酸素分子線による室温酸化促進

Band alignment in Ge/GeOx/HfO2/TiO2 heterojunctions as measured by hard x-ray photoelectron spectroscopy

We investigate the interlayer (IL) thickness dependence of band offsets in a germanium based bilayer metal-oxide-semiconductor sandwich with an amorphous plasma enhanced atomic layer deposited (PE-ALD) HfO2 IL and PE-ALD grown TiO2 high k gate dielectric using hard x-ray photoelectron spectroscopy. The native Ge oxide shifts to higher oxidation state as the thickness of the IL layer was increased. The Hf 4f core line shows a broadening with increasing thickness, indicating the formation of Hf-Ge germanate. We observed a deviation from the bulk offset for films with ultra thin layers of HfO2.

Study of electron transport characteristics through self-aligned Si-based quantum dots

Self-aligned Si-based quantum dots (QDs) with an ultra-thin oxide interlayer were spontaneously formed on ∼1.0-nm-thick thermally grown SiO2/Si(100) by a process sequence that consists of Si-QDs formation by controlling low-pressure chemical vapor deposition (LPCVD) using pure Si2H6, selective Ge-LPCVD, thermal oxidation of the dots, thermal desorption of Ge oxide, and subsequent formation of the Si-QDs. After formation of Al back electrode, electron transport properties through the aligned dots structures so-prepared were characterized by employing atomic force microscopy with a conductive cantilever. The tunneling current through the aligned dots exhibited a clear current bump and negative differential conductance at room temperature with a peak current to valley ratio as high as 100 at around the resonance voltage as a result of resonant tunneling mediated by the quantized energy levels of the dots.

Effect of nitrogen on structural stability of bismuth doped GeTe films under thermal treatment

We investigated the microstructures and electrical properties of 8.4% nitrogen-doped GeBi(6at.%)Te and GeBi(6at.%)Te films that were thermally annealed in air atmosphere. Despite annealing in air, GeBi(6at.%)Te films showed only phase transition from cubic to rhombohedral phases. However, the Ge oxide, island shaped metallic Te, and Ge–Bi–Te phases were generated in 8.4% nitrogen-doped GeBi(6at.%)Te films.

Evolution of the Structural and Electrical Properties of GeTe Under Different Annealing Conditions

GeTe films were found to exhibit chemical, structural, and electrical changes depending on the crystal structure and annealing atmosphere. Amorphous GeTe was converted to cubic crystalline GeTe after annealing treatment at 400°C. Subsequent annealing in air induced phase transitions in the cubic GeTe, resulting in the formation of Ge oxide as the major phase and metallic Te as the minor phase. However, GeTe films deposited at 200°C maintained the rhombohedral structure regardless of the annealing time or atmosphere.

Structural properties of Ge nanocrystals synthesized by a PVD nanocluster source

In this study, the synthesis of Ge nanoparticles by a physical vapour deposition (PVD) nanocluster source has been investigated. We typically obtain Ge nanoparticles with a mean diameter between 4 and 9 nm. The microstructure of Ge nanoparticles was widely studied by high resolution transmission electron microscopy, energy filtered transmission electron microscopy, X-ray diffraction and Raman spectroscopy. The main result is the capability of the PVD process to synthesize well crystallized Ge nanocrystals at room temperature and without any post thermal annealing. It was found from Raman analysis that these room temperature deposited nanoparticles are composed of a Ge crystalline core and a sub-nm thick amorphous Ge shell. A low temperature annealing (450 °C) during 1 h under vacuum leads to the reorganization of the surface atoms while longer annealing seems to promote the formation of a Ge oxide shell around the nanoparticles.

Catalytic behavior of metallic particles in anisotropic etching of Ge(100) surfaces in water mediated by dissolved oxygen

The authors demonstrate that Ge(100) surfaces containing metallic particles are etched anisotropically in water. This originates from the catalytic reduction of dissolved oxygen (O2) in water to water molecules (H2O) on the metallic particles, which is followed by the enhanced oxidation of Ge around the particles. The soluble nature of Ge oxide (GeO2) in water promotes the formation of inverted pyramidal etch pits composed of (111) microfacets. On the basis of the results, the authors propose strategies for avoiding unwanted surface roughening during the wet cleaning of Ge.

Development of Metallic Hermetic Sealing for MEMS Packaging for Harsh Environment Applications

Hermetic sealing of microelectromechanical system sensors is indispensable to ensure their reliable operation and also to provide protection during fabrication. This work proposes two prospective candidates for hermetic sealing for rugged environment applications, i.e., Al-Ge and Pt-In. Al-Ge was chosen due to its compatibility with complementary metal–oxide–semiconductor technology. Pt-In possesses the highest remelting temperature among all the solder systems, which is desired for high-temperature applications in both the energy and aerospace industries. The various bonding parameters for Al-Ge eutectic bonding and Pt-In transient liquid-phase (TLP) bonding have been optimized, and their influence on the bond quality is reported. Optimization of bonding parameters has been carried out with the objective of ensuring void-free bonds. A new configuration for stacking Al-Ge thin films has been demonstrated to tackle the issue of loss of Ge prior to bonding, since native Ge oxides are soluble in deionized water. The impact of solid-state aging prior to Al-Ge eutectic bonding has been investigated. The method of tailoring the phases in the Pt-In joint is also discussed. The prospects and constraints of eutectic and TLP bonding from the hermeticity perspective are discussed in detail. Furthermore, changes in the microstructure under aging at 300°C up to 500 h and the resulting influence on the mechanical properties are presented. The overall finding of this work is that Al-Ge can achieve better mechanical and hermetic performance for high-temperature applications.

Oxygen transport and GeO2 stability during thermal oxidation of Ge

Oxygen transport during thermal oxidation of Ge and desorption of the formed Ge oxide are investigated. Higher oxidation temperatures and lower oxygen pressures promote GeO desorption. An appreciable fraction of oxidized Ge desorbs during the growth of a GeO2 layer. The interplay between oxygen desorption and incorporation results in the exchange of O originally present in GeO2 by O from the gas phase throughout the oxide layer. This process is mediated by O vacancies generated at the GeO2/Ge interface. The formation of a substoichiometric oxide is shown to have direct relation with the GeO desorption.

Desorption of Ge Species during Thermal Oxidation of Ge and Annealing of HfO2/GeO2 Stacks

Thermal stability of HfO2/GeO2 stacks was investigated. These structures were stable on Si up to 600 oC following annealing in N2. Samples prepared on Ge yielded direct evidence of migration and loss of both Ge and O. This contrasting behavior is a result of GeO formation at the GeO2/Ge interface. Desorption of Ge was also observed during thermal oxidation of this semiconductor. The amounts of desorbed Ge were related to the oxygen pressure. Oxygen isotopic tracing results evidenced a strong interaction of oxygen from the gas phase with the already formed GeO2 layer.

Electroluminescence from One-Dimensionally Self-Aligned Si-Based Quantum Dots with High Areal Dot Density

Self-aligned Si-based quantum dots (QDs) with an areal density as high as ∼1013 cm-2 have been successfully fabricated on GeH4-adsorbed ultrathin SiO2 by the process sequence consisting of Si-QDs formation by controlling low-pressure chemical vapor deposition (LPCVD) using pure Si2H6, selective Ge-LPCVD from 5% GeH4 diluted with He, thermal oxidation of the dots, thermal desorption of Ge oxide, and subsequent formation of the Si-QDs. In semitransparent Au-gate diodes with self-aligned dots so-prepared, when carriers were injected to the self-aligned Si-QDs from the n-Si(100) substrate for electrons and from the Au top electrode for holes, electroluminescence (EL) in the near-infrared region at room temperature becomes observable with an increase in current at positive biases over a threshold voltage as low as ∼1.2 V at the Au top electrode. Note that, in the case of an areal dot density of ∼1013 cm-2, the EL threshold voltage was reduced down to ∼60% of that of ∼1011 cm-2 and emission intensity was enhanced markedly by a factor of ∼425 in comparison with the case of ∼1011 cm-2 under the same current density. This is clear evidence of not only an increase in radiative recombination rate in the self-aligned structure but also an improvement of recombination efficiency due to a decrease in current leakage with increasing dot density.