bluuebunny/crossref_metadata_2025 huggingface dataset:
Study of highly selective wet gate recess process for Al0.25Ga0.75As/GaAs based pseudomorphic high electron mobility transistors
Authors: Khatri R. , Radhakrishnan K.
Published: August 2004

Etching selectivity of citric acid buffer based etchant was investigated for the gate recess process of a double δ doped Al0.25Ga0.75As/In0.2Ga0.8As/Al0.25Ga0.75As based pseudomorphic high electron mobility transistor epitaxial layer structure grown by molecular beam epitaxy. Optimization of the etchant resulted in a selectivity of as high as 4800 for GaAs over Al0.25Ga0.75As layer. Source-drain current Idss during the gate recess was found to be stable for as long as 1200 s. The gate recess undercut was found to increase laterally as a function of recess time. The pH value of the selective etchant played a major role in the enhancement of the selectivity for wet etching of GaAs on Al0.25Ga0.75As. Morphology of the wet etched GaAs/Al0.25Ga0.75As surface was investigated using atomic force microscopy. The observed surface morphology was excellent, with a rms surface roughness value of about 1 nm. X-ray photoelectron spectroscopy analysis indicated the formation of Al2O3 at the wet etched surface of Al0.25Ga0.75As. The etch rate of doped GaAs for this etchant was 40 Å/s.
Nanomechanical structures with 91 MHz resonance frequency fabricated by local deposition and dry etching
Authors: Kim G. M. , Kawai S. , Nagashio M. , Kawakatsu H. , Brugger J.
Published: August 2004

We report an all-dry, two-step, surface nanoengineering method to fabricate nanomechanical elements without photolithography. It is based on the local deposition through a nanostencil of a well-defined aluminum pattern onto a silicon/silicon-nitride substrate, followed by plasma etching to release the structures. The suspended 100-nm-wide, 2-μm-long, and 300-nm-thick nanolevers and nanobridges have natural resonance frequencies of 50 and 91 MHz, respectively. The fabrication method is scalable to a full wafer and allows for a variety of materials to be structured on arbitrary surfaces, thus opening new types of nanoscale mechanical systems.
Formation of silicon nanocrystals in SiO2 by oxireduction reaction induced by impurity implantation and annealing
Authors: Jacobsohn L. G. , Zanatta A. R. , Nastasi M.
Published: August 2004

Silicon nanocrystals are candidates for complete integration of optical and electronic functions in Si-based devices. In this work, we investigate the feasibility of producing them by a method based on impurity-induced oxireduction of SiO2. Thermodynamic calculations suggested Mg as a suitable impurity due to its chemical strength in oxireducing the SiO2 matrix and simultaneously avoiding the formation of Si-based compounds. The samples were obtained by 1×1017 Mg+/cm2 implantations into fused silica followed by thermal anneal in vacuum. Transmission electron microscopy (TEM), and Raman and photoluminescence measurements confirmed the presence of Si nanocrystals. The average nanocrystal size was evaluated according to the phonon confinement and quantum confinement models relative to the Raman and photoluminescence results, respectively, to be about 10 nm, in agreement with the TEM results.
Fabrication of three-dimensional microstructures by two-dimensional slice by slice approaching via focused ion beam milling
Authors: Fu Yongqi , Bryan Ngoi Kok Ann
Published: August 2004

A direct programming control approach for fabrication of three-dimensional (3D) microstructures by use of focused ion beam (FIB) milling is put forth in this article. The previously reported Vasile mathematical model was simplified by our model of constant dwell time and sputter yield during the milling process. Milling of the 3D microstructures can be transferred to the milling of many discrete two-dimensional (2D) slices with thin and constant thickness. The 3D milling can be completed by the direct FIB 2D milling slice by slice with the same layer thickness. The number of the slices can be determined according to maximum depth of the 3D microstructures. The milling depth for each slice depends on the slice thickness. The simplified model and process parameters can be written in a program code running in the FIB equipped computer. The whole process can be performed automatically after adjusting the focusing status of the ion beam on the sample. It was shown by the experimental results that the method has the advantages of simple mathematical model, less memory space used, and fast milling speed.
Multivariable control of multizone chemical mechanical polishing
Authors: Shiu Sheng-Jyh , Yu Cheng-Ching , Shen Shih-Haur
Published: August 2004

The modeling and multivariable control of the multizone chemical mechanical polishing (CMP) are studied in this work. For a three-zone CMP, the copper thickness across the radial position is measured with an in situ sensor and then the measurements are converted to 60 data points across the radial position. In the process control notation, these are the controlled variables and the manipulated variables are the three pressures applied to each zone. Therefore, this is a 60×3 nonsquare multivariable control problem. In the modeling phase, two-level factorial designs are carried out on all three pressures followed by the least square regression. Thus, a 60×3 steady-state gain matrix is obtained followed by finding dynamic elements according to the gas holdup in each chamber. The control objective is to maintain uniform surface profile using all three manipulated variables. As a result of the nonsquare system, it is not possible to keep all 60 outputs at their set points using only three inputs. The singular value decomposition (SVD) is used to design a nonsquare feedback controller. This SVD controller can be implemented using standard software with little difficulty. The proposed control system was tested on incoming wafers with different surface profiles. Results show that attainable uniformity can be maintained using the proposed SVD controller.
Observation and evaluation of flaked particle behavior in magnetically enhanced reactive ion etching equipment using a dipole ring magnet
Authors: Moriya Tsuyoshi , Nagaike Hiroshi , Denpoh Kazuki , Morimoto Tamotsu , Aomori Masaru , Kawaguchi Shinichi , Shimada Manabu , Okuyama Kikuo
Published: August 2004

An in situ particle monitoring system using laser light scattering method was installed onto a commercially available radio frequency (rf) plasma oxide-etching tool that is enhanced magnetically using a dipole ring magnet. We observed the behavior of particles that flaked off the deposition film. It can be proven that the flaked particles have different trajectories depending on the magnetic field. When no magnetic field is applied, the flaked particles make a reciprocating motion near the grounded electrode due to the positive charge of flaked particles within the sheath. Alternatively, flaked particles enter the bulk plasma in response to a small gradient of electrical potential near the upper electrode when the magnetic field is active. Particles typically move horizontally with vertical vibration in bulk plasma using a magnetic field. The main force acting on the particles is electrostatic in nature.
Comprehensive study of InGaP–AlxGa1−xAs–GaAs composite-emitter heterojunction bipolar transistors with different thickness of AlxGa1−xAs graded layers
Authors: Cheng Shiou-Ying , Chen Chun-Yuan , Chen Jing-Yuh , Chuang Hung-Ming , Yen Chih-Hung , Liu Wen-Chau
Published: August 2004

The characteristics of InGaP–AlxGa1−xAs–GaAs composite-emitter heterojunction bipolar transistors (CEHBTs) with different thickness of AlxGa1−xAs graded layer are comprehensively studied and demonstrated. The thickness of the graded AlxGa1−xAs layer is an important factor to affect device performances. In this work, it is found that CEHBTs with 70 Å∼100 Å AlxGa1−xAs graded layers exhibit better properties due to the absence of potential spike. For comparison, experimentally, two practical CEHBTs with 0 and 100 Å AlxGa1−xAs graded layers are fabricated. Generally, good agreements between experimental results and theoretical simulations are found. It is, therefore, concluded that the CEHBT with an appropriate thickness of the AlxGa1−xAs graded layer offers the promise for analog, digital, and microwave device applications, especially for lower operation voltage and lower power consumption circuits.
High mobility SiGe/Si transistor structures on sapphire substrates using ion implantation
Authors: Alterovitz S. A. , Mueller C. H. , Croke E. T.
Published: August 2004

We report the fabrication of high mobility n-type SiGe/Si transistor structures on sapphire substrates by ion implanting phosphorus ions into strained 10-nm-thick silicon channels. The strained Si channels were sandwiched between Si0.7Ge0.3 layers, which, in turn, were deposited on Si0.7Ge0.3 virtual substrates and graded SiGe buffer layers. After the molecular beam epitaxy film growth process was completed, donors were introduced and activated using ion implantation and postannealing processes. Microstructural characterization of the buffer layer, virtual substrate, and electron conduction structure using high resolution x-ray diffraction, cross-sectional transmission electron microscopy, atomic force microscopy, and secondary ion mass spectroscopy are reported. Room temperature electron mobilities up to 900 cm2/V s at a carrier density of 1.3×1012 cm−2 were measured. Electron concentration appears to be the key factor that determines mobility, with the highest mobility observed for electron densities in the 1–2×1012 cm−2 range.
50 nm InGaAs/InAlAs/GaAs metamorphic high electron mobility transistors using double exposure at 50 kV electron-beam lithography without dielectric support
Authors: Kim S. C. , Lim O. K. , Lee H. S. , Baek T. J. , Shin D.-H. , Rhee J. K.
Published: August 2004

We are presenting a technology to fabricate InGaAs/InAlAs/GaAs metamorphic high electron mobility transistors (MHEMTs) with a gate length of 50 nm without a thin dielectric supporting layer. This technology uses a ZEP520/P(MMA-MAA)/PMMA trilayer by double exposure at 50 kV electron-beam lithography. The fabricated 50 nm MHEMTs with a 70 μm unit gate width and two fingers were characterized through dc and rf measurements. The maximum drain current density and transconductance (gm) were 180 mA/mm and 713 mS/mm, respectively. From rf measurements, we obtained the current gain cut-off frequency of 218.8 GHz.
Annealing effects on the operation stability of ballistic electron emission from electrochemically oxidized nanocrystalline silicon diodes
Authors: Ichihara Tsutomu , Honda Yoshiaki , Baba Toru , Komoda Takuya , Koshida Nobuyoshi
Published: August 2004

To improve the operation life of ballistic electron emission from nanocrystalline silicon diodes, thermal annealing effects have been studied for polycrystalline silicon (poly-Si) based devices. The experimental devices are constructed by sequentially depositing tungsten, anodized nanocrystalline poly-Si (NPS), and gold films upon glass substrates. The NPS films are partially oxidized by electrochemical oxidation treatment, and then annealed at 550 °C in vacuum or in a forming gas ambient. The electron emission efficiency and stability are investigated in relation to the compositional characterization of the respective NPS layers by thermal desorption spectroscopy. It is shown that removal of incorporated H2O molecules and hydrogen-related species from interfacial SiO2 films between silicon nanocrystallites is very effective to suppress interfacial scattering losses of electrons and to enhance the ballistic emission stability for long-term operation.
Solid-phase reaction and Schottky contact properties of Co/n-poly-Si0.84Ge0.16/n-Si (100)
Authors: Wang Guang-Wei , Ru Guo-Ping , Qu Xin-Ping , Li Bing-Zong
Published: August 2004

The Schottky contact of Co, its silicide and germanosilicide on n-poly-Si0.84Ge0.16 layer, was investigated. Amorphous Si0.84Ge0.16 layer was deposited on n-Si (100) substrate by ion beam sputtering (IBS). The layer was doped through thermal diffusion of phosphorus to fabricate n-poly-Si0.84Ge0.16 thin film. The Schottky diodes were formed by deposition of Co on n-poly-Si0.84Ge0.16 by the IBS technique. Solid-phase reaction between Co and n-poly-Si0.84Ge0.16 by rapid thermal annealing (RTA) as a function of temperature was studied. Phase identification and atomic depth profile were characterized by x-ray diffraction and Auger electron spectroscopy, respectively. The current–voltage and capacitance–voltage characteristics of both as-deposited and annealed Co/n-poly-Si0.84Ge0.16 Schottky diodes were investigated. The results reveal that the Schottky barrier height (SBH) keeps nearly constant with the annealing temperature between 300 and 600 °C. The constancy of the SBH confirms the fact that Co and its silicides contacting with the same semiconductor have the close Schottky barrier height.
Sub-10 nm electron beam lithography using cold development of poly(methylmethacrylate)
Authors: Hu Wenchuang (Walter) , Sarveswaran Koshala , Lieberman Marya , Bernstein Gary H.
Published: August 2004

We investigate poly(methylmethacrylate) (PMMA) development processing with cold developers (4–10 °C) for its effect on resolution, resist residue, and pattern quality of sub-10 nm electron beam lithography (EBL). We find that low-temperature development results in higher EBL resolution and improved feature quality. PMMA trenches of 4–8 nm are obtained reproducibly at 30 kV using cold development. Fabrication of single-particle-width Au nanoparticle lines was performed by lift-off. We discuss key factors for formation of PMMA trenches at the sub-10 nm scale.
Characterization of resist-trimming processes by quasi in situ x-ray photoelectron spectroscopy
Authors: Pargon E. , Joubert O. , Xu Songlin , Lill Thorsten
Published: August 2004

We have used x-ray photoelectron spectroscopy (XPS) to investigate the resist-patterns’ transformations after exposure to resist trimming plasmas. The influence of the chemistry, the feed gas ratio, the bias power and the pressure have been studied for a Cl2/O2 and a HBr/O2 trim chemistry. An experimental procedure based on the chemical-topography analysis of resist patterns using XPS has been set up to determine the chemical composition and thickness of the reactive layers that are formed both on tops and sidewalls of the resist features during the resist trimming process. The resist modifications obtained by XPS have been correlated with the process performance (trim rate) obtained by SEM observations. In this study, we show that the main difference between HBr/O2 and Cl2/O2 trimming processes is attributed to the difference in reactivity between Br and Cl radicals with the resist during plasma exposure. We succeed in explaining the impact of the plasma parameters (oxygen percentage in the gas phase, bias power, pressure) on the Cl2/O2 trimming process. A decrease in trim rate is well correlated with the formation of a thick and chlorine-rich layer formed on the resist sidewalls.
Gas phase reaction products during tungsten atomic layer deposition using WF6 and Si2H6
Authors: Grubbs R. K. , Steinmetz N. J. , George S. M.
Published: August 2004

The gas phase reaction products during tungsten (W) atomic layer deposition (ALD) using WF6 and Si2H6 were studied using quadrupole mass spectrometry. The gas phase reactions products were different for the WF6 and Si2H6 reactions. No surface reactions were observed for WF6 exposures at room temperature. The WF6 reaction produced H2, HF and SiF4 at a reaction temperature of 473 K. Mass spectrometer cracking patterns established that SiF4 is the silicon reaction product instead of SiHF3. Auger electron spectroscopy (AES) measurements confirmed that the H2, HF and SiF4 gas phase reaction products during WF6 exposure coincided with the loss of silicon surface species. The Si2H6 reaction showed two separate reaction channels depending on reaction temperature. At room temperature, a temperature insensitive reaction produced SiHF3 and H2 reaction products. A second reaction produced H2 as the reaction product at 473 K. AES measurements confirmed that the SiHF3 and H2 reaction products during Si2H6 exposure were concurrent with the gain of silicon surface species. Together with previous FTIR spectroscopy and AES studies, these mass spectrometer results help to identify the stoichiometry of the surface reactions during the sequential WF6 and Si2H6 exposures that define W ALD.
High-aspect-ratio chemically assisted ion-beam etching for photonic crystals using a high beam voltage-current ratio
Authors: Kotlyar M. V. , O’Faolain L. , Wilson R. , Krauss T. F.
Published: August 2004

We investigate etching conditions for photonic crystals (PhCs) in InGaAsP/InP and AlGaAs/GaAs using a new regime of CAIBE operation. We show that the beam voltage-current ratio is critical in obtaining high material/mask selectivity. For one-dimensional PhCs, i.e., air slots, selectivities of 22:1 and 50:1 were achieved in InP and GaAs, respectively, using a very high beam voltage (about 1500 V) and a low beam current (about 10 mA). Etched features were observed to be very smooth, i.e., edge roughness was low. Two-dimensional PhCs were etched in InGaAsP/InP under similar conditions achieving selectivities up to 27:1 and 34:1 for hole diameters of 170 and 270 nm, respectively.
Rigid organic molds for nanoimprint lithography by replica molding of high glass transition temperature polymers
Authors: Pisignano Dario , D’Amone Stefania , Gigli Giuseppe , Cingolani Roberto
Published: August 2004

The glass transition of polycarbonate resins of high softness temperature is exploited to realize rigid polymeric replicas of master patterns, with no need for antisticking layers and applied pressure. Such replicas enable the transfer of patterns onto polymers having a lower glass-transition temperature by nanoimprint lithography. As a demonstration, we show the pattern transfer onto poly(methylmethacrylate), which demonstrates good fidelity and remarkable simplicity of the process.
Study of structural and microstructural properties of AlN films deposited on silicon and quartz substrates for surface acoustic wave devices
Authors: Assouar M. B. , Elmazria O. , Elhakiki M. , Alnot P.
Published: August 2004

Piezoelectric aluminum nitride thin films were deposited by reactive dc magnetron sputtering under various experimental conditions on a silicon (100) substrates. These films structural and microstructural properties were studied using x-ray diffraction (XRD), selected area electron diffraction (SAED), and transmission electron microscopy (TEM) in order to determine the optimum growth conditions that improve the piezoelectric coupling. A highly c-axis oriented AlN film (002) was identified from XRD and SAED. An average grain column size of about 40 nm was determined from TEM. The root-mean-square surface roughness measured using atomic-force microscopy was less than 7 nm. The optimum experimental parameters found for AlN deposition on Si(100) substrates were then extrapolated to its deposition on a ST-cut quartz substrate for the investigation of the influence of substrates on the surface acoustic wave (SAW) propagation. The AlN/Si and AlN/ST-quartz SAW devices were formed using the photolithography process. Their operating characteristics, phase velocity, temperature coefficient of frequency, and electromechanical coefficient (K2) were measured and analyzed. Experimental results show that the generalized SAW (GSAW) as well as the pseudosurface acoustic wave were excited in this structure. Their phase velocities are, respectively, 3245 m/s and 5117 m/s. The SAW device realized from the AlN/ST-cut quartz structure exhibits higher phase velocity and higher K2 compared to those formed on the ST-quartz substrate.
Mn and Be codoped GaAs for high hole concentration by low-temperature migration-enhanced epitaxy
Authors: Onomitsu Koji , Fukui Hideo , Maeda Takashi , Hirayama Yoshiro , Horikoshi Yoshiji
Published: August 2004

We report the effect of codoping of Mn–Be and the modified doping on an increment of the total hole concentration. Both the Mn-doped and Be-doped samples exhibit hole concentrations higher than 1×1019 cm−3 at room temperature. In contrast, the Mn–Be codoped sample shows a remarkable decrease in its hole concentration and mobility. Therefore, the resistivity is increased by more than 102 times of the Be-doped sample. This dramatic deterioration is probably caused by the complex defects due to the codoping. We performed the spatially separated doping of Mn and Be in GaAs and found that the hole concentration increases and the mobility shows no decrease for this sample. It is expected that the complex defect concentration is decreased considerably.
Soft lithography molding of polymer integrated optical devices: Reduction of the background residue
Authors: Paloczi George T. , Huang Yanyi , Scheuer Jacob , Yariv Amnon
Published: August 2004

Soft lithography molding is a promising technique for patterning polymer integrated optical devices, however the presence of a background residue has the potential to limit the usefulness of this technique. We present the soft lithography technique for fabricating polymer waveguides. Several effects of the background residue are investigated numerically, including the modal properties of an individual waveguide, the coupling ratio of a directional coupler, and the radiation loss in a waveguide bend. Experimentally, the residue is found to be reduced through dilution of the core polymer solution. We find that the force with which the soft mold is depressed on the substrate does not appreciably affect the waveguide thickness or the residue thickness. Optical microscope images show that the residue is thinnest next to the waveguide.
Nonorthogonal wafer dicing for waveguide, microelectromechanical systems, and nanotechnology applications
Authors: Pau S. , Taylor J. A. , Pai C. S.
Published: August 2004

We propose a technique to dice wafer using photolithography and deep reactive ion etch. We demonstrate our technique by dicing an eight inch wafer into 1.8×104 pieces of arbitrary shape and size. Our idea is suitable in dicing nano-chips and non-rectangular devices such as waveguide and microelectromechanical systems.
Schottky junction properties on high quality boron-doped homoepitaxial diamond thin films
Authors: Chen Y. G. , Ogura M. , Okushi H.
Published: August 2004

Experiments of annealing effect of boron-doped homoepitaxial diamond film on Schottky junction properties have been conducted to investigate the origin of the “pinning” states in oxidized surface of diamond. Metals of Al, Ni, Au, and Pt are evaporated onto boron-doped diamond films with post thermal annealing less than 800 °C or higher than 900 °C. The current–voltage (I–V) characteristics of Schottky junctions for low-temperature annealing show excellent rectification behavior, indicating the existence of the “pinning” states. After high-temperature annealing, the I–V curves show worse rectification or ohmic properties depending on the metal work function, suggesting the decrement of the density of “pinning” state. Capacitance–voltage measurements for the Schottky diodes show the strong thermal annealing effects on surface region property of the diamond film only after high temperature annealing. A possible explanation is the oxygen-related “pinning” states existing in the oxidized-surface of diamond are annealed out partially during high temperature annealing.
Impurity-induced disordering in AlGaInP superlattices studied using cross-sectional scanning tunneling microscopy
Authors: Teng K. S. , Brown M. R. , Wilks S. P. , Sobiesierski A. , Smowton P. M. , Blood P.
Published: August 2004

The effect of doping an (Al0.3Ga0.7)0.5In0.5P/Al0.5In0.5P superlattice was investigated using cross-sectional scanning tunneling microscopy (XSTM). Two superlattices of different doping concentration were studied; namely, a superlattice where both well and barriers were doped, and a superlattice with only the barriers doped. Interdiffusion and impurity-induced layer disordering at the well and barrier interfaces is reported for both superlattice structures. However, the extent of interdiffusion across the interfaces, when only the barrier was doped, was measured to be at least 2.5 times more than the superlattice with both well and barriers doped. The considerable extent of Al–Ga interdiffusion in the barrier doped superlattice is thought to be caused by Zn diffusion at the (AlxGa1−x)0.5In0.5P heterostructures during Zn activation at 690 °C. This activation process encourages the segregation of the acceptor Zn out from the AlInP barrier layers and into AlGaInP well layers. In addition, this process is also known to enhance Al and Ga diffusion rates. XSTM images also show alloy clustering at both differently doped superlattices. Interdiffusion, alloy clustering, and interface roughness observed at the AlGaInP superlattices would have a detrimental effect on the suitability of a multiquantum barrier structure, based on this material system, to improve quantum confinement in red laser devices.
Optical properties of wurtzite and zinc-blende GaN/AlN quantum dots
Authors: Fonoberov Vladimir A. , Balandin Alexander A.
Published: August 2004

We investigate theoretically and compare optical properties of wurtzite and zinc-blende GaN/AlN quantum dots with heights from 1.5 to 4.5 nm. The quantum dot size corresponds to the strong quantum confinement regime. It has been established that the built-in piezoelectric field at the GaN/AlN interface governs optical properties of wurtzite quantum dots while having a small effect on zinc-blende quantum dots. The strain field strongly modifies the excitonic states in both wurtzite and zinc-blende GaN/AlN quantum dots. It has been shown that the radiative lifetime dependence on the quantum dot height is very different in the zinc-blende and wurtzite quantum dots. The excitonic optical properties of GaN/AlN quantum dots calculated using our model are in good agreement with available experimental data. Reported theoretical results for the optical spectra of GaN/AlN quantum dots can be used for interpretation of experimental data and optimization of the quantum dot structures for optoelectronic applications.
Interpretation of current transport properties at Ni/n–GaN Schottky interfaces
Authors: Sawada T. , Kimura N. , Imai K. , Suzuki K. , Tanahashi K.
Published: August 2004

Current transport properties at Ni/n–GaN Schottky interfaces formed on oxide-etched or thermally oxidized surfaces are studied by current–voltage–temperature (I–V–T) and capacitance–voltage measurements. The results support existence of surface patches with low Schottky barrier height (SBH), which cause a leakage current of the Schottky structures. Based on “surface patch” model, the fraction of the total patch area of 10−4–10−5 and the SBH lowering of 0.4 eV within patches are deduced for the oxide-etched Ni/n–GaN metalorganic chemical vapor deposition (MOCVD) samples. The influence of surface patches was found much larger for the present Ni/n–GaN molecular beam epitaxy (MBE) samples. The inclusion of N-polarity domains in the Ga-polarity layer seems to be main origin of the patches for the MBE-grown samples. Ni/n–GaN Schottky samples formed on dry-oxidized or wet-oxidized MOCVD layers represented reduction of the effective SBH with negligible increase of the n value, indicating that an intermediary native oxide between Ni and n-GaN degrades the SBH. I–V–T characteristics showed that the leakage current due to surface patches is comparable with that of the oxide-etched sample. Concerning thermal stability of the bulk GaN (MOCVD), the impurity concentration in the layer increased after wet oxidation above 500 °C, although little change was observed after wet oxidation at temperatures below 400 °C. Finally, a postfabrication annealing in nitrogen at 400 °C led to increase of the effective SBH even for the oxidized Ni/n–GaN Schottky samples.
Analysis of interface electronic structure in InxGa1−xN/GaN heterostructures
Authors: Zhang H. , Miller E. J. , Yu E. T. , Poblenz C. , Speck J. S.
Published: August 2004

Capacitance–voltage profiling was used to measure interfacial polarization charge densities and conduction-band offsets at InxGa1−xN/GaN heterojunction interfaces for x=0.054 and 0.09. A variant of the conventional analysis technique used to deduce interface charge density and band-offset values from capacitance–voltage data was developed and applied. Conduction-band offsets of 0.09±0.07 and 0.22±0.05 eV are obtained for x=0.054 and 0.09, respectively. Polarization charge densities derived from these measurements are (1.80±0.32)×1012 and (4.38±0.36)×1012 e/cm2 for x=0.054 and 0.09, respectively. These values are somewhat lower than those predicted theoretically, but are in good agreement with values inferred from a substantial body of optical data reported for InxGa1−xN/GaN quantum-well structures.
In situ monitoring of Zn* and Mg* species during helicon-wave-excited-plasma sputtering epitaxy of ZnO and Mg0.06Zn0.94O films
Authors: Koyama T. , Ohmori T. , Shibata N. , Onuma T. , Chichibu S. F.
Published: August 2004

Atomic species in the sputtered plumes from pure ZnO and ZnO–MgO (94:6 mol %) targets were identified by means of plume emission spectroscopy (PES) during helicon-wave-excited-plasma sputtering epitaxy. Dominant Zn and Mg species detected by PES were excited neutral zinc (Zn*) and magnesium (Mg*), respectively. Their densities were uniquely controlled by the target bias (Vt) that accelerates the velocity of Ar cations, keeping the gross plasma density constant. The growth rate was principally limited by the densities of Zn* and Mg*, as well as nonradiative species such as Zn–O and Mg–O, and the MgO molar fraction x in the MgxZn1−xO epilayers nearly agreed with that of the target between 500 and 650 °C. Structural qualities of the epilayers were significantly improved by the use of nearly lattice-matched substrates having proper surface arrangements. The a-axis-locked single-domain (0001) MgxZn1−xO epitaxy was accomplished on an uniaxially quasi-lattice-matched (112̄0) Al2O3 substrate. The epilayers had atomically smooth surfaces, and 0.26-nm-high monolayer steps were observed in ZnO epilayers. The ZnO and Mg0.06Zn0.94O epilayers exhibited a predominant near-band-edge photoluminescence peak at room-temperature (3.28 and 3.36 eV, respectively).
Interfacial properties of ultra thin ZrxSi1−xO2 with compositional gradation grown on Si(100) using Zr[N(C2H5)2]4 and Si(OC4H9)4
Authors: Kim Jaehyun , Yong Kijung
Published: August 2004

Zr x Si 1−x O 2 films were deposited by using Zr[N(C2H5)2]4 and Si(OC4H9)4. Composition (x) of a 4 nm thick ZrxSi1−xO2 was investigated by Zr 3d, Si 2p, and O 1s x-ray photoelectron spectroscopy depth profiles. The Zr/(Zr+Si) ratio gradationally changed from ∼0.1 at the silicate film surface to ∼0.67 at the ZrxSi1−xO2-Si interface during Ar+ sputtering. An atomically flat interface with no sub-SiO2 interfacial layers was observed. The dielectric constants were approximately 9 for both Zr-silicate films as-deposited and annealed at 500 °C in oxygen ambient. When annealed in oxygen ambient, the flat band approached the ideal value in C–V curve. The leakage current density of the Zr-silicate films as-deposited and annealed at 500 °C was ∼5×10−4 and ∼3×10−8 A/cm2, respectively, at a bias of 1.0 V.
Nanoscale current transport in epitaxial SrTiO3 on n+-Si investigated with conductive atomic force microscopy
Authors: Schaadt D. M. , Yu E. T. , Vaithyanathan V. , Schlom D. G.
Published: August 2004

We have used conductive atomic force microscopy to image the nanoscale current distribution in SrTiO3 grown epitaxially on n+-Si by molecular beam epitaxy. Topographic and current images were obtained simultaneously in contact mode with a bias voltage applied to the sample. Topographic images show a flat surface with a roughness of about 0.5 nm. Current images show small areas with local current flow on the order of pA for voltages larger than ∼2 V in forward bias and larger than ∼4 V in reverse bias. Histograms of the magnitude of the electrical current show a relatively narrow log-normal distribution, suggesting a common current mechanism with a Gaussian distribution in a parameter on which the current depends exponentially. Analysis of current images and histograms over a range of bias voltages suggests thermionic emission as the dominant current mechanism, rather than conduction associated with localized defects such as pin-holes, threading dislocations or grain boundaries. The analysis yields a barrier height of ∼0.5–0.6 eV with and a relative dielectric constant of 5–15, which is in reasonable agreement with previous reports using a dead layer model.
High temperature limitations for GaN growth by rf-plasma assisted molecular beam epitaxy: Effects of active nitrogen species, surface polarity, hydrogen, and excess Ga-overpressure
Authors: VanMil B. L. , Guo Huicheng , Holbert L. J. , Lee Kyoungnae , Myers T. H. , Liu Ting , Korakakis D.
Published: August 2004

The temperature used for growth of GaN by molecular beam epitaxy is ultimately limited by the greatly reduced growth rate related to thermal decomposition. This limiting temperature apparently varies from group to group. Factors influencing thermal decomposition are growth species (atomic versus metastable molecular nitrogen), surface polarity (N- versus Ga-polar), the presence of atomic hydrogen, and varying Ga-overpressure. Surface polarity and growth species are the predominant influence determining the onset of thermal decomposition. There are indications that the use of a significant Ga-overpressure can suppress decomposition allowing for an increase in obtainable growth temperatures for a given polarity. Electrical properties are shown to be strongly influenced by Ga-overpressure and thermal decomposition.
Real time spectroscopic ellipsometry study during the growth of nanocrystalline nitride protective coatings
Authors: Aouadi S. M. , Debessai M. , Tomek R. , Maeruf T.
Published: August 2004

Real time spectroscopic ellipsometry (RTSE) was employed to study the optical, electronic, and transport properties of various classes of nanocrystalline nitride coatings grown by reactive unbalanced magnetron-sputtering. The investigated materials include TaZrN and NbZrN solid solutions, and nc-ZrN/Ag and nc-ZrN/Ni nanocomposites. Film properties were tailored by varying film composition. The following models were used to interpret the ellipsometric data: (a) effective medium theories describing the heterostructures in terms of their constituent materials and (b) a combined Drude–Lorentz model describing the optical response of the conduction and valence electrons. A correlation was found between the optical data and the chemical composition using the linear and Bruggeman effective medium approximations for the solid solution and the nanocomposite materials, respectively. The Drude–Lorentz model provided extensive information regarding the optical and electronic properties of the films (plasma frequency, electron mean free path, electrical resistivity, and band structure). For example, a correlation was made between the conduction electron mean free path and the grain size in nanocomposite films. RTSE findings were confirmed using x-ray diffraction, transmission electron microscopy, and x-ray photoelectron spectroscopy.
Structural and electrical properties of HfOxNy and HfO2 gate dielectrics in TaN gated nMOSCAP and nMOSFET devices
Authors: Choi Kyu-Jeong , Kim Jeon-Ho , Yoon Soon-Gil , Shin Woong-Chul
Published: August 2004

The HfOxNy films grown at 300 °C in plasma and N2 ambient using hafnium tertiary-butoxide (Hf[OC(CH3)3]4) as the precursor in the absence of O2 were prepared to improve the thermal stability of HfO2-based gate dielectrics. The HfOxNy films showed a 200 °C higher crystallization temperature and more suppression in the growth of an interfacial layer than that of HfO2 films when films were annealed at 700 °C in a nitrogen ambient. The HfOxNy films showed an improvement of interface charge trap densities by a forming gas annealing treatment, resulting in much more improvement in drive current and subthreshold swing (S.S.) than those of HfO2 films.
Surface and grain boundary scattering studied in beveled polycrystalline thin copper films
Authors: Zhang W. , Brongersma S. H. , Clarysse T. , Terzieva V. , Rosseel E. , Vandervorst W. , Maex K.
Published: August 2004

The inverse relationship between film thickness and electrical resistivity of metallic films is usually studied by depositing a series of films with different thickness and measuring their sheet resistance with a four-point probe. However, the structure and uniformity of polycrystalline thin films typically depend on thickness, rendering it difficult to establish the dominant electron scattering mechanism. In order to circumvent the uniformity issue we now use beveled films to establish the thickness dependent resistivity of thin copper films. Here the resistivity of films down to a few nm’s can be studied without, e.g., a percolation effect. Additionally, the comparison of data obtained on samples where beveling took place either before or after anneal is used to study the impact of grain size on resistivity. It is shown that the normally observed strong increase in resistivity is dominated by grain boundary scattering. However, the influence of surface scattering can be clearly observed when thick films are thinned after a high temperature anneal in which case the grain size is considerably larger than the electron mean free path of copper at room temperature.
Generation of nano-sized free standing single crystal silicon particles
Authors: Dong Y. , Bapat A. , Hilchie S. , Kortshagen U. , Campbell S. A.
Published: August 2004

A system has been designed to generate monodisperse, single crystal silicon nanoparticles. The particles are generated using a SiH4/H2 mixture in a high density plasma. An aerodynamic lens assembly focuses and size selects the particles. A high voltage plate is used to accelerate the charged particles such that they can be injected into a high temperature annealing tube to be heated. The shape and structure of the particles are changed in the annealing tube. Single crystals are obtained.
Surface potential and morphology issues of annealed (HfO2)x(SiO2)1−x gate oxides
Authors: Ludeke R. , Lysaght P. , Cartier E. , Gusev E. , Chudzik M. , Foran B. , Bersuker G.
Published: August 2004

The surface morphology and surface potential variations of annealed (HfO2)x(SiO2)1−x films were investigated by noncontact atomic force microscopy (AFM) in ultrahigh vacuum. Additional modes of data acquisition included contact potential difference (CPD) and differential capacitance. Two types of samples were investigated. The first, a set consisting of 4 nm thick samples with (HfO2)x(SiO2)1−x compositions of x=0.4, 0.6, and 0.8, were annealed at 1000 °C for 10 s in N2 gas. The second, a 2.2 nm thick sample of composition (HfO2)0.78(SiO2)0.22 was annealed in vacuum at 50 °C intervals from 850 to 1000 °C. The anneals resulted in a microstructure consisting of phase-separated HfO2 crystallites and amorphous silica, as observed in high resolution transmission electron microscope (HRTEM) images. The crystallites appear to be responsible for most of the morphology observed with the AFM, with surface features for the hafnium rich x=0.6 and 0.8 compositions generally agreeing with the crystal sizes observed by HRTEM. The AFM images for the x=0.4 sample showed substantially broader features than the 5 nm crystallites seen by HRTEM, with evidence for inclusions of low dielectric constant (κ) material, presumably silica, on portions of the surface. The vacuum annealed sample showed an initial trend to lower roughness and CPD fluctuation range, with a minimum in both (rms roughness=0.077 nm and ΔCPD=0.2 V) for a 10 s anneal at 900 °C. Thereafter both measures increased substantially. The 1000 °C vacuum annealed sample compared favorably in structure, roughness, and to a lesser extent in the CPD fluctuation range with the 1000 °C N2 annealed sample. The N2 annealed samples for x=0.4 and x=0.8 exhibited CPD fluctuations as large as 0.4 V, with a smaller value of 0.22 V observed for the x=0.6 sample. CPD fluctuations consist of a small amplitude substructure that correlated with the microstructural features of the surface, superimposed on long range CPD fluctuations (20–>50 nm) unrelated to any surface features. Their origin is speculative, but could be associated with bulk and/or interface fluctuations in the density of trapped charge. Their potential adverse impact on device performance is discussed.
Interfacial strain-induced self-organization in semiconductor dielectric gate stacks. II. Strain-relief at internal dielectric interfaces between SiO2 and alternative gate dielectrics
Authors: Lucovsky G. , Maria J. P. , Phillips J. C.
Published: August 2004

This article applies bond constraint theory to develop a scaling relationship for the density of bond-strain induced defects such as fixed charge localized at internal dielectric interfaces. The magnitude of this charge scales with the square of the difference between the average number of bonds/atom of the interface constituents such as SiO2 and Si3N4, or ZrO2 or HfO2. Consistent with equilibrium phase diagrams of the SiO2–Zr(Hf)O2 binary alloy systems, interfaces between: (i) SiO2 and (ii) ZrO2, HfO2, and Zr and Hf silicate alloys exhibit a strain-induced self-organization after annealing to temperatures of ∼600–800 °C producing a diphasic interfacial transition region comprised of ZrO2 encapsulated by SiO2. This reduces fixed charge by more than 1 order of magnitude. In marked contrast, and also consistent with differences in their equilibrium phase diagrams, strain-induced self-organization does not occur for temperatures up to at least 1000 °C at interfaces between: (i) SiO2 and Al2O3 and (ii) SiO2 and Si oxynitride alloys due to binary alloy compound phases with congruent melting points that prevent formation of the diphasic interfacial transition region comprised of the end member oxide dielectrics.
Interfacial strain-induced self-organization in semiconductor dielectric gate stacks. I. Strain relief at the Si–SiO2 interface
Authors: Lucovsky G. , Phillips J. C.
Published: August 2004

It has been demonstrated that Si–SiO2 interfaces in field effect transistors are not atomically abrupt, but instead contain: (i) an interfacial transition region ∼0.5 nm thick with an average SiO composition as well as (ii) a strained or defective region in the Si substrate that is of similar extent. The strain profile across these interfacial transition regions, compressive in the SiO2 and tensile in the Si substrate, results from a combination of growth induced strain, as well as differences between the linear expansion coefficients of SiO2 and the substrate Si. Two high-temperature transitions modify the strain profile, and the transition region bonding at the Si–SiO2 interface. The first is a visco-elastic relaxation in the SiO2 occurring at ∼1000 °C, and the second is associated with bonding changes within the interfacial transition region occurring at ∼900 °C. This article uses spectroscopic studies to identify the chemical bonding changes within the interfacial transitions region that occur after 900 °C annealing in an inert ambient. The physical and chemical forces that drive these changes are addressed from two perspectives: (i) reactions kinetics and (ii) bond constraint theory. Finally the effects of strain relief on device performance and reliability are discussed.
Low-temperature growth of GaN layers on (0001)6H–SiC by compound source molecular beam epitaxy
Authors: Honda Tohru , Hama Masaki , Aoki Yohta , Akiyama Miwako , Obinata Naoyuki , Kawanishi Hideo
Published: August 2004

The low-temperature growth of GaN is required to prevent cracks due to thermal expansion. The lower limit of the temperature of the GaN growth by compound source molecular beam epitaxy (CS-MBE) was estimated using the results of reflection high-energy electron diffraction and atomic force microscopy. The lower limit of the temperature of GaN growth by CS-MBE was investigated and found to be below 450 °C. The lower limit is due to the migration of atoms at the surface and the re-evaporation of excess Ga atoms.
Analysis and control of excess leakage currents in nitride-based Schottky diodes based on thin surface barrier model
Authors: Kotani Junji , Hashizume Tamotsu , Hasegawa Hideki
Published: August 2004

Using a rigorous computer simulation program for current transport through a Schottky barrier with an arbitrary potential profile, the leakage current mechanism in GaN and AlGaN Schottky diodes was investigated on the basis of the thin surface barrier (TSB) model recently proposed by the authors’ group. Computer simulation assuming various possible defect density distributions was carried out to reproduce the measured temperature dependent current voltage (I–V)-temperature characteristics of the GaN and AlGaN Schottky diodes which showed excessive reverse leakage. By assuming exponentially decaying distributions from surface for defect donors with energy depth of 0.25 eV for GaN and 0.37 eV for Al0.15Ga0.85N, I–V curves measured by our group as well as reported in the literatures were almost completely reproduced both in forward and reverse direction over a wide temperature range. The defect donors are proposed to be N vacancies or their related complexes that are formed during metal deposition. The result confirms the validity of the TSB model. From the viewpoint of the TSB model, attempts were also made to suppress leakage currents. It was found that a low-energy electrochemical metal deposition process and a metal–insulator–semiconductor Schottky structure using an ultrathin Al2O3 film by electron cyclotron resonance oxidation of Al film were remarkably effective in reducing excess leakage currents due to reduction of defect deep donors.
Controlled decoherence of a charge qubit in a double quantum dot
Authors: Fujisawa Toshimasa , Hayashi Toshiaki , Hirayama Yoshiro
Published: August 2004

We investigate coherent time evolution of a charge qubit in a semiconductor double quantum dot. A high-speed voltage pulse controls the energy bias and strength of the decoherence of the system. The qubit is effectively isolated from the electrodes in the Coulomb blockade regime, while it is affected by dissipative tunneling processes in the single-electron tunneling regime. We discuss the importance of the controlled decoherence by means of density matrix simulations.
Nanoscale fluctuations in the distribution of dopant atoms: Dopant-induced dots and roughness of electronic interfaces
Authors: Ebert Ph. , Jäger N. D. , Urban K. , Weber E. R.
Published: August 2004

We investigated the effect of nanoscale fluctuations in the distribution of dopant atoms on the roughness of electronic interfaces of p–n junctions and on the formation of electronically separated dopant-induced dots. It is shown that the screening fields around each charged dopant atom and a clustering of dopant atoms induce two separate roughness components, which are much larger than the roughness of the underlying metallurgical interface. This leads to charge carrier depleted zones extending locally through the entire nominally homogeneously doped layer for layer thicknesses close to the cluster dimension. The results demonstrate that the clustering of dopants strongly limits the size of future devices by forming electronically separated dopant-induced dots, which introduce drastic changes in the current–voltage characteristics.