Electron states at the (1 0 0)Ge/HfO2 Interface

The energy distribution of extended and localized electron states at the Ge/HfO₂ interface is determined by combining the internal photoemission of electrons and holes from Ge into the Hf oxide and AC capacitance/conductance measurements. The inferred offsets of the conduction and valence band at the interface, i.e., 2.0 ± 0.1 and 3.0 ± 0.1 eV, respectively, suggest the possibility to apply the deposited HfO₂ layer as a suitable insulator on Ge. The post-deposition annealing of the Ge/HfO₂ structures in oxygen results in 1 eV reduction of the valence band offset, which is attributed to the growth of a GeO₂ interlayer. However, this treatment enables one to substantially reduce the density of Ge/HfO₂ interface traps, approaching ≈1×10¹² cm⁻² eV⁻¹ near the Ge midgap.     

Influence of passivating interlayer on Ge/HfO2 and Ge/Al2O3 interface band diagrams

The energy band alignment between Ge, HfO₂ and Al₂O₃ was analyzed as influenced by passivating interlayers (ILs) of different composition (GeO₂, Ge₃N₄, Si/SiO_x). From internal photoemission and photoconductivity experiments we found no IL-sensitive dipoles at the Ge/HfO₂ 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 HfO₂ growth using H₂O-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 HfO₂ and Al₂O₃ deposition suppresses the formation of the narrow-gap Ge oxide.     

In situ atomic layer deposition and synchrotron-radiation photoemission study of Al2O3 on pristine n-GaAs(0 0 1)-4 × 6 surface

This work presents the in situ reflection high-energy electron diffraction (RHEED), scanning tunneling microscopy (STM) and synchrotron-radiation photoemission studies for the morphological and interfacial chemical characterization of in situ atomic layer deposited (ALD) Al₂O₃ on pristine molecular beam epitaxy (MBE) grown Ga-rich n-GaAs (1 0 0)-4 × 6 surface. Both the RHEED pattern and STM image demonstrated that the first cycle of ALD-Al₂O₃ process reacted immediately with the GaAs surface. As revealed by in situ synchrotron-radiation photoemission studies, two types of surface As atoms that have excess in charge in the clean surface served as reaction sites with TMA. Two oxidized states were then induced in the As 3d core-level spectra with chemical shifts of +660 meV and +1.03 eV, respectively.     

Effective formation of interface controlled Y2O3 thin film on Si(1 0 0) in a metal-(ferroelectric)-insulator-semiconductor structure

Yttrium was deposited on the chemical oxide of Si and annealed under vacuum to control the interface for the formation of Y₂O₃ as an insulating barrier to construct a metal-ferroelectric-insulator-semiconductor structure. Two different pre-annealing temperatures of 600 and 700 °C were chosen to investigate the effect of the interface state formed after the pre-annealing step on the successive formation of Y₂O₃ insulator and Nd₂Ti₂O₇ (NTO) ferroelectric layer through annealing under an oxygen atmosphere at 800 °C. Pre-anneal treatments of Y-metal/chemical-SiO₂/Si at 600 and 700 °C induced a formation of Y₂O₃ and Y-silicate, respectively. The difference in the pre-anneal temperature induced almost no change in the electrical properties of the Y₂O₃/interface/Si system, but degraded properties were observed in the NTO/Y₂O₃/interface/Si system pre-annealed at 600 °C when compared with the sample pre-annealed at 700 °C. C-V characteristics of the NTO/Y₂O₃/Si structured system showed a clockwise direction of hysteresis, and this gap could be used as a memory window for a ferroelectric-gate. A smaller hysteric gap and electrical breakdown values were observed in the NTO/Y₂O₃/Si system pre-annealed at 600 °C, and this was due to an unintentional distribution of the applied field from the presence of an interfacial layer containing Y-silicate and SiO₂ phases.     

Influence of the oxidation temperature on the non-trigonal Ge dangling bonds at the (1 0 0)Ge/GeO2 interface

GeO₂ was proposed as valuable passivation layer at the surface with Ge to integrate oxide with high dielectric constant in Ge-based logic devices. Hence, the identification of the defects present at different Ge/GeO₂ interfaces becomes a mandatory issue to predict the electrical features of devices based on such materials. High sensitive electrically detected magnetic resonance measurements were performed to study the microstructure of defects occurring at such an interface. The influence of the oxidation temperature on the electrically active paramagnetic traps was investigated.     

Structural and electrical properties of Er-doped HfO2 and of its interface with Ge (0 0 1)

Er-doped HfO₂ thin films with Er content ranging from 0% to 15% are deposited by atomic layer deposition on native oxide free Ge(001). The crystallographic phase is investigated by X-ray diffraction and is found to depend on the Er%. The cubic fluorite structure develops on Ge for Er% as low as 4% and is stable after annealing at 400 °C in N₂. Microstrain increases with increasing the Er content within the fluorite structure. Time of flight secondary ion mass and electron energy loss spectroscopy evidence a Ge diffusion from the substrate that results in the formation of a Ge-rich interfacial region which does not present a structural discontinuity with the oxide. The diffusion of Ge is enhanced by the annealing and causes a reordering of the crystal lattice. In annealed films the interface defect density measured by low temperature conductance measurements is found to decrease with decreasing the Er content.     

Charge trapping and interface characteristics in normally-off Al2O3/GaN-MOSFETs

Normally-off GaN-MOSFETs with Al₂O₃ gate dielectric have been fabricated and characterized. The Al₂O₃ layer is deposited by ALD and annealed under various temperatures. The saturation drain current of 330 mA/mm and the maximum transconductance of 32 mS/mm in the saturation region are not significantly modified after annealing. The subthreshold slope and the low-field mobility value are improved from 642 to 347 mV/dec and from 50 to 55 cm² V⁻¹ s⁻¹, respectively. The I_D-V_G curve shows hysteresis due to oxide trapped charge in the Al₂O₃ before annealing. The amount of hysteresis reduces with the increase of annealing temperature up to 750 °C. The Al₂O₃ layer starts to crystallize at a temperature of 850 °C and its insulating property deteriorates.     

Photoemission study of the Si(1 1 1)-native SiO2/copper phthalocyanine (CuPc) ultra-thin film interface

The Ultraviolet and X-ray Photoemission Spectroscopy (UPS, XPS) investigation was done to examine the interface formation between deposited copper phthalocyanine (CuPc) thin films and covered with native oxide n- and p-type silicon Si(1 1 1) substrates. The UPS results indicated the existence of small interface dipole effect for very first layer of CuPc deposited on both types of substrates. The dipoles were oriented differently depending on silicon conductivity type. In this paper we present that near the inorganic/organic interface the phthalocyanine’s molecular orbital levels shift downwards 0.20 ± 0.05 eV in the case of n-Si substrate and upwards 0.25 ± 0.05 eV for p-Si indicating the different displacement of the negative charge within the interface region. This tendency was also confirmed by conducted XPS study of the core levels. It is highly probable that band bending-like shift is provoked by the continuous change of CuPc molecule orientation induced by interface polarization layer.     

Admittance spectroscopy of traps at the interfaces of (1 0 0)Si with Al2O3, ZrO2, and HfO2

Admittance (ac) measurements were carried out to determine the interface trap density (D_it) as a function of energy E in the Si bandgap at interfaces of Si with different insulating oxides (Al₂O₃, ZrO₂, HfO₂). The results are compared to those of the conventional thermal SiO₂/Si interface. The results show that a significant portion of the interface trap density in the as-deposited and de-hydrogenated samples is related to the amphoteric Si dangling bond defects (P_b0 -centers). The D_it is much enhanced for the Al-containing insulators as compared to Si/SiO₂ but can be reduced by annealing in O₂. As to annealing in H₂, efficient passivation of P_b0 centers by hydrogen is achieved for Si/ZrO₂ and Si/HfO₂ interfaces, yet it fails for Si/Al-containing oxide entities. Among the insulators studied, the results suggest HfO₂ to be the best choice of an alternative insulator.     

Epitaxial growth of Dy2O3 thin films on epitaxial Dy-germanide films on Ge(0 0 1) substrates

Ultra-thin films of Dy are grown on Ge(0 0 1) substrates by molecular beam deposition near room temperature and immediately annealed for solid phase epitaxy at higher temperatures, leading to the formation of DyGe_x films. Thin films of Dy₂O₃ are grown on the DyGe_x film on Ge(0 0 1) substrates by molecular beam epitaxy. Streaky reflection high energy electron diffraction (RHEED) patterns reveal that epitaxial DyGe_x films grow on Ge(0 0 1) substrates with flat surfaces. X-ray diffraction (XRD) spectrum suggests the growth of an orthorhombic phase of DyGe_x films with (0 0 1) orientations. After the growth of Dy₂O₃ films, there is a change in RHEED patterns to spotty features, revealing the growth of 3D crystalline islands. XRD spectrum shows the presence of a cubic phase with (1 0 0) and (1 1 1) orientations. Atomic force microscopy image shows that the surface morphology of Dy₂O₃ films is smooth with a root mean square roughness of 10 Å.     

Direct measurement of interfacial structure in epitaxial Gd2O3 on GaAs (0 0 1) using scanning tunneling microscopy

The epitaxial growth of Gd₂O₃ on GaAs (0 0 1) has given a low interfacial density of states, resulting in the demonstration of the first inversion-channel GaAs metal-oxide-semiconductor field-effect transistor. Motivated by the significance of this discovery, in this work, cross-sectional scanning tunneling microscopy is employed herein to obtain precise structural and electronic information on these epitaxial films and interfaces. At the interface, the interfacial stacking of Gd₂O₃ films is directly correlated with the stacking sequence of the substrate GaAs. Additionally, from the local electronic states across the gate oxides, the spatial extent of the GaAs wavefunctions into the oxide dielectric may suggest a minimum Gd₂O₃ thickness to be of bulk properties.     

Density functional theory study of first-layer adsorption of ZrO2 and HfO2 on Ge(1 0 0)

Density functional theory was used to performed a survey of transition metal oxide (MO₂ = ZrO₂, HfO₂) ordered molecular adsorbate bonding configurations on the Ge(1 0 0)-4 × 2 surface. Surface binding geometries of metal-down (O-M-Ge) and oxygen-down (M-O-Ge) were considered, including both adsorbate and displacement geometries of M-O-Ge. Calculated enthalpies of adsorption show that bonding geometries with metal-Ge bonds (O-M-Ge) are essentially degenerate with oxygen-Ge bonding (M-O-Ge). Calculated electronic structures indicate that adsorbate surface bonding geometries of the form O-M-Ge tend to create a metallic interfaces, while M-O-Ge geometries produce, in general, much more favorable electronic structures. Hydrogen passivation of both oxygen and metal dangling bonds was found to improve the electronic structure of both types of MO₂ adsorbate systems, and induced the opening of true semiconducting band gaps for the adsorbate-type M-O-Ge geometries. Shifts observed in the DOS minima for both O-M-Ge and M-O-Ge adsorbate geometries are consistent with surface band bending induced by the adsorbate films, where such band bending extends much further into the Ge substrate than can be modeled by the Ge slabs used in this work.     

Optimization of Al2O3/ZrO2 nanolaminate structure for thin-film encapsulation of OLEDs

We report thin-film moisture barriers based on Al₂O₃/ZrO₂ nanolaminates grown by ALD for an encapsulation of OLEDs. In order to optimize the moisture-barrier performance of the nanolaminates, the most important factors affecting the performance were sought by measuring WVTR of the nanolaminates via an electrical Ca test. We found out that both the number of interfaces in the nanolaminates and the thickness of ZrO₂ in a unit layer were responsible for the performance. By optimizing the nanolaminate structure, the moisture-barrier performance was enhanced up to 350% from a single layer of the same thickness. The WVTR of 30-nm-thick optimized nanolaminate barrier was 2 × 10⁻⁴ g/(m² day) or less at ambient condition. A storage-lifetime measurement of an OLED with a 100-nm-thick encapsulation layer showed that it could exceed 70,000 h if stored at ambient condition.     

Electrical and interfacial characteristics of nanolaminate (Al2O3/ZrO2/Al2O3) gate stack on fully depleted SiGe-on-insulator

The structural and electrical characteristics of a novel nanolaminate Al₂O₃/ZrO₂/Al₂O₃ high-k gate stack together with the interfacial layer (IL) formed on SiGe-on-insulator (SGOI) substrate have been investigated. A clear layered Al₂O₃ (2.5 nm)/ZrO₂ (4.5 nm)/Al₂O₃ (2.5 nm) structure and an IL (2.5 nm) are observed by high-resolution transmission electron microscopy. X-ray photoelectron spectroscopy measurements indicate that the IL contains Al-silicate without Ge atom incorporation. A well-behaved C–V behavior with no hysteresis shows the absence of Ge pileup or Ge segregation at the gate stack/SiGe interface.     

Au/CaF2/nSi(1 1 1) tunnel emitter phototransistor

Owing to the improved growth technology of the 1-2 nm calcium fluoride films on silicon, tunnel metal-insulator-semiconductor transistors have been fabricated. Measured output characteristics show both saturation and active mode of operation. Estimated value of current gain exceeds 1000 approaching the theoretically estimated value in this system. This result supports the candidacy of calcium fluoride for being a vital dielectric in silicon-based functional electronics.     

Origin of electron mobility enhancement in (1 1 1)-oriented InGaAs channel metal-insulator-semiconductor field-effect-transistors

Electrical and physical characteristics of the Al₂O₃/InGaAs interfaces with (1 1 1)A and (1 0 0) orientations were investigated in an attempt to understand the origin of electron mobility enhancement in the (1 1 1)A-channel metal-insulator-semiconductor field-effect-transistor. The (1 1 1)A interface has less As atoms of high oxidation states as probed by X-ray photoelectron spectroscopy. The electrical measurements showed that energy distribution of the interface traps for the (1 1 1)A interface is shifted toward the conduction band as compared to that for the (1 0 0) interface. Laterally-compressed cross-section transmission electron microscopy images showed that the characteristic lengths of the interface roughness are different between the (1 1 1)A and (1 0 0) interfaces. The contributions of the Coulomb and roughness scattering mechanisms are discussed based on the experimental results.     

Comparative study of AlGaN/GaN metal-oxide-semiconductor heterostructure field-effect transistors with Al2O3 and HfO2 gate oxide

AlGaN/GaN metal-oxide-semiconductor heterostructure field-effect transistors (MOSHFETs) with Al₂O₃ gate oxide which was deposited by atomic layer deposition (ALD) were fabricated and their performance was then compared with that of AlGaN/GaN MOSHFETs with HfO₂ gate oxide. The capacitance (C)-voltage (V) curve of the Al₂O₃/GaN MOS diodes showed a lower hysteresis and lower interface state density than the C-V curve of the HfO₂/GaN diodes, indicating better quality of the Al₂O₃/GaN interface. The saturation of drain current in the I_D-V_GS relation of the Al₂O₃ AlGaN/GaN MOSHFETs was not as pronounced as that of the HfO₂ AlGaN/GaN MOSHFETs. The gate leakage current of the Al₂O₃ MOSHFET was five to eight orders of magnitude smaller than that of the HfO₂ MOSHFETs.     

Stack engineering of low-temperature-processing Al2O3 dielectrics prepared by nitric acid oxidation for MOS structure

The electrical characteristics of low-temperature-processing Al₂O₃ films were studied. With an anodization SiO₂ film as a buffer layer, Al₂O₃ dielectric was grown on it by oxidizing an ultra-thin aluminum film in nitric acid, followed by a surface DAC-ANO compensation. The significant development is, when the Al₂O₃ film fabrication of this experiment was repeated, which means one more same Al₂O₃ layer deposition, the sample demonstrated satisfactory electrical properties.     

Crystalline orientation dependence of electrical properties of Mn Germanide/Ge(1 1 1) and (0 0 1) Schottky contacts

We have investigated the crystalline orientation dependence of the electrical properties of Mn germanide/Ge(1 1 1) and (0 0 1) Schottky contacts. We prepared epitaxial and polycrystalline Mn₅Ge₃ layers on Ge(1 1 1) and (0 0 1) substrates, respectively. The Schottky barrier height (SBH) estimated from the current density-voltage characteristics for epitaxial Mn₅Ge₃/Ge(1 1 1) is as low as 0.30 eV, while the SBH of polycrystalline Mn₅Ge₃/Ge(0 0 1) is higher than 0.56 eV. On the other hand, the SBH estimated from capacitance-voltage characteristics are higher than 0.6 eV for both samples. The difference of these SBHs can be explained by the local carrier conduction through the small area with the low SBH regions in the epitaxial Mn₅Ge₃/Ge(1 1 1) contact. This result suggests the possibility that the lowering SBH takes place due to Fermi level depinning in epitaxial germanide/Ge(1 1 1) contacts.