Ge Oxide Research Articles

Characterization of the “clean-up” of the oxidized Ge(100) surface by atomic layer deposition

While the “clean-up” effect on III-V substrates has recently been well documented interfacial reactions during atomic layer deposition (ALD) on Ge substrates are not fully explored. The “clean-up” of Ge oxides is studied by interrupting the ALD process following individual precursor pulses for in situ monochromatic x-ray photoelectron spectroscopy analysis. Germanium oxides are found to be reduced by TMA and water, while an interfacial GeON layer is only affected by the initial TMA pulse. Oxide free germanium surfaces behave analogously to a surface with initial native oxides since they are oxidized measurably prior to the first TMA pulse due to residual oxidants in a commercial ALD chamber.

Evolution of SiO 2/Ge/HfO 2(Ge) multilayer structure during high temperature annealing

Use of germanium as a storage medium combined with a high-k dielectric tunneling oxide is of interest for non-volatile memory applications. The device structure consists of a thin HfO 2 tunneling oxide with a Ge layer either in the form of continuous layer or discrete nanocrystals and relatively thicker SiO 2 layer functioning as a control oxide. In this work, we studied interface properties and formation kinetics in SiO 2/Ge/HfO 2(Ge) multilayer structure during deposition and annealing. This material structure was fabricated by magnetron sputtering and studied by depth profiling with XPS and by Raman spectroscopy. It was observed that Ge atoms penetrate into HfO 2 layer during the deposition and segregate out with annealing. This is related to the low solubility of Ge in HfO 2 which is observed in other oxides as well. Therefore, Ge out diffusion might be an advantage in forming well controlled floating gate on top of HfO 2. In addition we observed the Ge oxidation at the interfaces, where HfSiO x formation is also detected.

Coverage Dependence of the Adsorption Structure of Alanine on Ge(100)

The variations with coverage and annealing temperature in the adsorption structure of alanine on Ge(100) have been investigated using high-resolution core-level photoemission spectroscopy (HRCLPES). The C 1s, N 1s, and O 1s core-level spectra at a low initial coverage show that both the carboxyl and amine groups of the alanine molecules participate in bonding with the Ge(100) surface in an "intrarow O-H dissociated and N dative bonded structure". However, at higher coverage we found that in addition to this structure an "O-H dissociation structure" is present. Moreover, we systematically monitored the variation of the bonding features of alanine adsorbed on Ge(100) with annealing temperature and thus were able to track the desorption processes. By analyzing the C 1s, N 1s, and O 1s spectra at 420 K, we conclude that the principal adsorption structure at this temperature is the "O-H dissociation structure" because of the disconnection of Ge-N dative bonding. At higher temperatures, the "O-H dissociation structure" is converted into various fragments such as Ge oxide (or Ge-CO), Ge nitride (Ge cyanide), and Ge carbide.

Surface orientation dependence of interface properties of GeO2/Ge metal-oxide-semiconductor structures fabricated by thermal oxidation

We have fabricated GeO2/Ge interfaces on (100), (110), and (111) orientation substrates by direct thermal oxidation. The x-ray photoelectron spectroscopy analyses suggest that the Ge oxides are composed of GeO2 and have almost the same interfacial structure, independent of the surface orientations. The gate current conduction mechanism through the GeO2/Ge metal-oxide-semiconductor structure is dominated by Fowler–Nordheim tunneling. In addition, the barrier height between Ge and GeO2 is evaluated to be 1.2–1.4 eV. In interface trap density (Dit) measurement by using the low temperature conductance method, the amount of Dit in the conduction band side is also almost the same, while Dit in the valence band side is lowest for the (111) surface. Minimum detectable Dit is lower than 1×1011 eV−1 cm2 for all the orientations. These surface orientation dependences of the GeO2/Ge interface properties are quite different from those of the SiO2/Si interface.

Effects of silicon nitride diffusion barrier on germanium MOS capacitors with HfON gate dielectrics

MOS capacitors with hafnium oxynitride (HfON) gate dielectrics were fabricated on Ge and Si substrates using the RF reactive magnetron sputtering method. A large amount of fixed charges and interface traps exist at the Ge/HfON interface. HRTEM and XPS analyses show that Ge oxides were grown and diffused into HfON after post metal annealing. A Si nitride interfacial layer was inserted between Ge and HfON as diffusion barrier. Using this method, well behaved capacitance-voltage and current-voltage characteristics were obtained. Finally hystereses are compared under different process conditions and possible causes are discussed.

Effect of annealing on structural, optical and electrical properties of nanostructured Ge thin films

Ge thin films with a thickness of about 110 nm have been deposited by electron beam evaporation of 99.999% pure Ge powder and annealed in air at 100–500 °C for 2 h. Their optical, electrical and structural properties were studied as a function of annealing temperature. The films are amorphous below an annealing temperature of 400 °C as confirmed by XRD, FESEM and AFM. The films annealed at 400 and 450 °C exhibit X-ray diffraction pattern of Ge with cubic-F structure. The Raman spectrum of the as-deposited film exhibits peak at 298 cm −1, which is left-shifted as compared to that for bulk Ge (i.e. 302 cm −1), indicating nanostructure and quantum confinement in the as-deposited film. The Raman peak shifts further towards lower wavenumbers with annealing temperature. Optical band gap energy of amorphous Ge films changes from 1.1 eV with a substantial increase to ∼1.35 eV on crystallization at 400 and 450 °C and with an abrupt rise to 4.14 eV due to oxidation. The oxidation of Ge has been confirmed by FTIR analysis. The quantum confinement effects cause tailoring of optical band gap energy of Ge thin films making them better absorber of photons for their applications in photo-detectors and solar cells. XRD, FESEM and AFM suggest that the deposited Ge films are composed of nanoparticles in the range of 8–20 nm. The initial surface RMS roughness measured with AFM is 9.56 nm which rises to 12.25 nm with the increase of annealing temperature in the amorphous phase, but reduces to 6.57 nm due to orderedness of the atoms at the surface when crystallization takes place. Electrical resistivity measured as a function of annealing temperature is found to reduce from 460 to 240 Ω-cm in the amorphous phase but drops suddenly to 250 Ω-cm with crystallization at 450 °C. The film shows a steep rise in resistivity to about 22.7 KΩ-cm at 500 °C due to oxidation. RMS roughness and resistivity show almost opposite trends with annealing in the amorphous phase.

Ge nanocrystals embedded in a GeOx matrix formed by thermally annealing of Ge oxide films

The authors investigate the formation of Ge nanocrystals by thermal annealing of substoichiometric GeOx films fabricated by electron-beam evaporation. At the same time, they also monitor the evolution of the GeOx matrix. The phase separation into semiconductor and oxide phases and the evolution of Ge nanocrystals were monitored by a combination of x-ray diffraction (XRD), Raman, and transmission electron microscopy (TEM) measurements. TEM shows spherical particles of sizes in the range of 2–9nm. They infer that an annealing temperature of 500°C is sufficient to generate a reasonable density of Ge nanocrystals in an amorphous GeOx matrix. Both XRD and Raman measurements suggest a simultaneous crystallization of the matrix at an annealing temperature of 600°C.

Ge/GeO2 Interface Control with High Pressure Oxidation for Improving Electrical Characteristics

We have investigated High Pressure Oxidation (HPO) of Ge for improving Ge/GeO2 interface in terms of GeO desorption control. C-V characteristics in HPO-GeO2 MIS capacitors revealed the dramatic improvement with a very small hysteresis and frequency dispersion. Furthermore, the dielectric constant enhancement in GeO2 grown by HPO was observed. A sub-bandgap photo absorption was observed in GeO2 film grown by Atmospheric Pressure Oxidation (APO), while a sharp absorption edge at ~6 eV, corresponding to the bandgap of GeO2, was seen in the film by HPO. Based on the thermodynamic consideration, the effectiveness of HPO and low temperature oxygen annealing (LOA) in terms of the suppression of GeO desorption is discussed.

Effects of Atomic Layer Deposition-Al2O3 Interface Layers on Interfacial Properties of Ge Metal–Oxide–Semiconductor Capacitors

We have investigated the interfacial control effects of the atomic layer deposition (ALD)-Al2O3 on the crystalline and electronic characteristics of LaAlO3/Ge metal–oxide–semiconductor (MOS) structures. X-ray photoelectron spectroscopic analysis revealed that Al2O3/Ge interfaces are much more stable than LaAlO3/Ge and La2O3/Ge interfaces for the formation of Ge oxides owing to the interfacial reaction. For the LaAlO3/ALD-Al2O3/Ge structure, Al2O3 interfacial layers with thicknesses less than 1 nm effectively suppress the interfacial reaction between LaAlO3 and Ge. The thickness of Ge oxides formed at the ALD-Al2O3/Ge interface decreases with increasing Al2O3 thickness and the stack structure is thermally stable against postdeposition annealing at 600 °C. These results indicate that interfacial reaction mainly arises during the sputtering-LaAlO3 deposition, not the ALD-Al2O3 deposition. On the other hand, capacitance equivalent oxide thickness decreases with increasing interfacial ALD-Al2O3 thickness, because the formation of Ge oxides at the interface is effectively suppressed by ALD-Al2O3 interfacial layers. It can be concluded that ALD-Al2O3 is promising as an interfacial control layer between Ge substrates and high-k dielectrics.

The role of in situ generated nano-sized metal particles on the coulombic efficiency of MGeO 3 (M = Cu, Fe, and Co) electrodes

To improve the coulombic efficiency of GeO 2 electrode, a Cu-containing ternary metal oxide (CuGeO 3) was prepared and the electrochemical behavior of Cu component was studied. The GeO 2 electrode shows a low coulombic efficiency in the first cycle (43%), which is mainly caused by a poor Ge oxidation kinetics (Ge + 2Li 2O → GeO 2 + 2Li + + 2e −). The X-ray absorption spectroscopy (XAS) data illustrate that the Cu component in CuGeO 3 is converted to nano-sized metallic Cu in the earlier stage of lithiation but idles thereafter. In contrast, the Ge component in CuGeO 3 behaves like the GeO 2 electrode. It is converted to nano-sized Ge by a conversion reaction and further lithiated by alloying reaction. The de-lithiation proceeds in the reverse order. The CuGeO 3 electrode shows a much improved coulombic efficiency (74%) in the first cycle, which is indebted to a facilitated Ge oxidation with a much reduced electrode polarization. This feature has been explained by the favorable roles provided by the in situ generated nano-sized metallic Cu particles that make such an intimate contact with the nano-sized Ge and Li 2O that they can catalyze Li 2O decomposition and provide an electronic conductive network for Ge oxidation. A similar favorable effect was observed with the other ternary oxides (FeGeO 3 and CoGeO 3), wherein the formation of nano-sized metallic Fe and Co can be assumed.

Post metallization annealing study in La 2O 3/Ge MOS structure

The study on post metallization annealing (PMA) in electrical characteristics and interfacial properties of La 2O 3/Ge structures has been conducted. The PMA treatment in N 2 ambient induces the growth of interfacial Ge oxide layer accompanied with decrease of capacitance value and interface trap density. The interface-layer growth is caused by the oxidation of Ge substrate due to the hydroxyl group absorbed in La 2O 3 from the ambient. The metal electrode capping might prevent the hydroxyl from evaporating during annealing, which enhances the interface reaction. On the other hand, leakage current increment has been observed for the sample with PMA in case of using Pt gate electrode. It is due to the diffusion of Pt and/or Ge and a Pt-germanide formation in La 2O 3 film during PMA. This leakage current increment can be suppressed by using Ta or W electrode which has less reactivity with Ge than Pt at high temperature.

Oxidation resistance of thin boron carbo-nitride films on Ge(100) and Ge nanowires

Chemical vapor deposition of thin (< 10 nm) films of amorphous boron carbo-nitride (BC 0.7N 0.08, or BCN) on Ge(100) and Ge nanowire (GeNW) surfaces was studied to determine the ability of BCN to prevent oxidation of Ge. X-ray photoelectron spectroscopy was used to track Ge oxidation of BCN-covered Ge(100) upon exposure to ambient, 50 °C deionized water, and a 250 °C atomic layer deposition HfO 2 process. BCN overlayers incorporate O immediately upon ambient or water exposure, but it is limited to 15% O uptake. If the BCN layer is continuous, the underlying Ge(100) surface is not oxidized despite the incorporation of O into BCN. The minimum continuous BCN film thickness that prevents Ge(100) oxidation is ~ 4 nm. Thinner films (≤ 3.2 nm) permitted Ge(100) oxidation in each of the oxidizing environments studied. GeNWs with a 5.7 nm BCN coating were resistant to oxidation for at least 5 months of ambient exposure. High resolution transmission electron microscopy images of HfO 2/BCN/Ge(100) cross-sections and BCN-coated GeNWs reveal clean, abrupt BCN–Ge(100) interfaces.

Material Properties, Thermal Stabilities and Electrical Characteristics of Ge MOS Devices, Depending on Oxidation States of Ge Oxide: Monoxide [GeO(II)] and Dioxide [GeO2(IV)

AbstractGe monoxide [GeO(II)] and dioxide [GeO2(IV)], which are selectively formed on Ge substrate by controlling pH and redox potential in pretreatment solution, are confirmed by XPS. ΔEC in GeO(II)/Ge and GeO2(IV)/Ge are almost the same, whereas ΔEV in GeO(II)/Ge is smaller than that in GeO2(IV)/Ge, resulting in smaller Eg of GeO(II). GeO(g) desorption is suppressed in LaAlO3/Ge gate stack, whereas GeO(g) desorbs through LaAlO3 layer when there is an intentional interfacial GeO(II) layer, leading to a large increase in Jg. GeO(g) desorption temperature in Ge oxide/Ge gate stacks decreases with the increase in the ratio of GeO(II) in Ge oxide and is independent of the oxidation techniques. Since GeO(g) desorption is accompanied by H2O(g) desorption, a new model to explain the GeO(g) desorption phenomena is proposed, in which Ge(OH)2 decomposes into GeO(g) and H2O(g). Highly effective etching methods of Ge oxide, using HCl solution and HCl vapor at higher temperature than boiling point of Ge (hydro-)chloride are demonstrated.

Thermodynamics and Kinetics for Suppression of GeO Desorption by High Pressure Oxidation of Ge

AbstractWe analyze a main scheme for the suppression of GeO desorption by the high pressure oxidation which drastically improve the electrical quality of Ge/GeO2 capacitors. The inherent driving force for GeO to form at the Ge/GeO2 interface and to diffuse toward the GeO2 surface was realized by the concentration gradient in the GeO2 film, which was obtained from the thermodynamic calculation. Kinetic consideration based on the comparison with Si/SiO2 stacks suggests that GeO desorption at the GeO2 surface is the rate-limiting process under passive oxidation conditions. When O2 pressure is increased by high pressure oxidation, the vapor pressure of GeO at the GeO2 surface is reduced, restricting GeO desorption at the GeO2 surface.

Formation of Ge nanocrystals and evolution of the oxide matrix in as-deposited and annealed LPCVD SiGeO films

SiGeO films were deposited by LPCVD using Si 2H 6, GeH 4 and O 2 as reactive gases and furnace annealed to segregate the possible excess of Si and Ge in the form of nanocrystals embedded in an oxide matrix. For low GeH 4:Si 2H 6 flow ratios and deposition temperatures of 450 ∘C or lower, the deposited film consists of a SiO 2 matrix incorporating Ge. No Ge oxides and no nanocrystals are detected. After annealing of the samples with SiO 2 matrices at temperatures of 600 ∘C or higher, quasi-spherical isolated Ge nanocrystals with diameters ranging from 4.5 to 9 nm and homogeneously distributed throughout the whole film thickness are formed. In the samples deposited with low GeH 4:Si 2H 6 flow ratios, the original SiO 2 matrix holds its composition.

Interface Charge Induced p-Type Characteristics of Aligned Si1−xGex Nanowires

This study reports the electrical transport characteristics of Si(1-x)Gex (x=0-0.3) nanowires. Nanowires with diameters of 50-100 nm were grown on Si substrates. The valence band spectra from the nanowires indicate that energy band gap modulation is readily achievable using the Ge content. The structural characterization showed that the native oxide of the Si(1-x)Gex nanowires was dominated by SiO2; however, the interfaces between the nanowire and the SiO2 layer consisted of a mixture of Si and Ge oxides. The electrical characterization of a nanowire field effect transistor showed p-type behavior in all Si(1-x)Gex compositions due to the Ge-O and Si-O-Ge bonds at the interface and, accordingly, the accumulation of holes in the level filled with electrons. The interfacial bonds also dominate the mobility and on- and off-current ratio. The large interfacial area of the nanowire, together with the trapped negative interface charge, creates an appearance of p-type characteristics in the Si(1-x)Gex alloy system. Surface or interface structural control, as well as compositional modulation, would be critical in realizing high-performance Si(1-x)Gex nanowire devices.

Influence of substrate geometry on the distribution and stress on Ge nanocrystals in silicon oxide matrix

Synthesis of germanium (Ge) nanocrystals embedded in silicon oxide matrix was carried out on cosputtered Ge plus silicon oxide films deposited on the surface of silicon (Si) wafer etched with V- or U-grooves. The V- or U-grooves were fabricated via the laser interference lithography technique. We found that the substrate geometry has a significant influence on the distribution of the Ge nanocrystals in the silicon oxide matrix. The variation in the distribution of the nanocrystals in the silicon oxide matrix may be due to defective oxide regions caused by sputter deposited silicon oxide film on the V- or U-grooves or may also be related to the different strain fields in the oxide layer.

Fabrication of multilayered Ge nanocrystals by magnetron sputtering and annealing

Multilayered Ge nanocrystals embedded in Si and Ge oxide films have been fabricated on Si substrateby a (SiO2+Ge)/(SiO2+GeO2) superlattice approach, using an rf magnetron sputtering technique with aGe+SiO2 composite target and subsequent thermal annealing inN2 ambientat 750 °C for 5 min. X-ray diffraction (XRD) measurements indicated the formation of Genanocrystals with an average size estimated to be 9.8 nm. Raman scatteringspectra showed a peak of the Ge–Ge vibrational mode shifted downwards to298.8 cm−1, which was caused by quantum confinement of phonons in the Ge nanocrystals. X-rayphotoemission spectroscopy (XPS) analysis demonstrated that the Ge chemical state is mainlyGe0 in the(SiO2+Ge) layerand Ge4+ in the (SiO2+GeO2) layer in the superlattice structure. Transmission electronmicroscopy (TEM) revealed that Ge nanocrystals were confined in(SiO2+Ge) layers, and had good crystallinity. This superlattice approach significantly improved boththe size uniformity of Ge nanocrystals and their uniformity of spacing on the‘Z’ growth direction compared with the conventional Ge-ncs fabrication method using a single and thickSiO2 matrix film.

Atomic Layer Deposition of High-k Dielectric Layers on Ge and III-V MOS Channels

Ge and III-V semiconductors are potential high performance channel materials for future CMOS devices. In this work, we have studied Atomic Layer Deposition (ALD) of high-k dielectric layers on Ge and GaAs substrates. We focus at the effect of the oxidant (H2O, O3, O2, O2 plasma) during gate stack formation. GeO2, obtained by Ge oxidation in O2 or O3, is a promising passivation layer. The germanium oxide thickness can be scaled down below 1 nm, but such thin layers contain Ge in oxidation states lower than 4+. Still, electrical results indicate that small amounts of Ge in oxidation states lower than 4+ are not detrimental for device performance. Partial intermixing was observed for high-k dielectric and GeO2 or GaAsOx, suggesting possible correlations in the ALD growth mechanisms on Ge and GaAs substrates.

Influence of passivating interlayer on Ge/HfO 2 and Ge/Al 2O 3 interface band diagrams

The energy band alignment between Ge, HfO 2 and Al 2O 3 was analyzed as influenced by passivating interlayers (ILs) of different composition (GeO 2, Ge 3N 4, Si/SiO x ). From internal photoemission and photoconductivity experiments we found no IL-sensitive dipoles at the Ge/HfO 2 interfaces, the latter being universally characterized by conduction and valence band offsets of 2.1 and 3.0 eV, respectively. However, in the case of HfO 2 growth using H 2O-based atomic layer deposition, the Ge oxide IL appears to have a narrower bandgap, 4.3 eV, than the 5.4–5.9 eV gap of bulk germania. Accordingly, formation of this IL yields significantly reduced barriers for hole and, particularly, electron injection from Ge into the insulator. Changing to a H-free process for HfO 2 and Al 2O 3 deposition suppresses the formation of the narrow-gap Ge oxide.