bluuebunny/crossref_metadata_2025 huggingface dataset:
Observations of electron velocity overshoot during high-field transport in AlN
Authors: Collazo Ramón , Schlesser Raoul , Roskowski Amy , Davis Robert F. , Sitar Z.
Published: April 2011

The energy distribution of electrons transported through an intrinsic AlN film was directly measured as a function of the applied electric field. Following the transport, electrons were extracted into vacuum through a semitransparent Au electrode and their energy distribution was measured using an electron spectrometer. The electron energy distribution featured kinetic energies higher than that of completely thermalized electrons. Transport through 80 nm thick layers indicated the onset of quasi-ballistic transport. This was evidenced by symmetric energy distributions centered at energies above the conduction band minimum for fields greater than 530 kV/cm. Drifted Fermi-Dirac energy distributions were fitted to the measured energy distributions, with the energy scale referenced to the bottom of the AlN conduction band. The drift energy and the carrier temperature were obtained as fitting parameters. Overshoots as high as five times the saturation velocity were observed and a transient length of less than 80 nm was deduced. In addition, the velocity-field characteristic was derived from these observations. This is the first experimental demonstration of this kind of transport in AlN.
Time Resolved Optical Studies of InGaN Layers Grown on LGO
Authors: Cheung Maurice , Namkoong Gon , Furis Madalina , Chen Fei , Cartwright Alexander. N. , Doolittle W. Alan , Brown April
Published: April 2011

Radiative recombination processes in bulk InGaN grown by molecular beam epitaxy (MBE) on lithium gallate (LGO or LiGaO2) substrates were investigated using microscopic PL and time-resolved photoluminescence (TRPL). The improved structural quality resulting from a better lattice match of the LGO substrate to III-V nitride materials simplifies these investigations because well-defined composition phases can be analyzed for both homogeneous and phased separated InGaN samples. Epilayers of InGaN intentionally grown with and without indium segregation were studied. X-ray diffraction measurements showed that the homogeneous epilayer was high quality In0.208Ga0.702N and the segregated epilayer exhibited peaks corresponding to both In0.289Ga0.711N and In0.443Ga0.557N indicating the presence of higher In concentration regions in this sample. Spatially resolved photoluminescence spectra confirm the existence of these regions. The photoluminescence intensity decay is non-exponential for both samples and a stretched exponential fit to the decay data confirms the existence of local potential fluctuations in which carriers are localized before recombination.
Femtosecond pump and probe spectroscopy of optical nonlinearities in an InGaN/GaN heterostructure
Authors: Chen Fei , Cartwright A. N.
Published: April 2011

The magnitudes and evolutions of two photoinduced absorption nonlinearities, absorption bleaching and field screening, were compared and investigated by employing electroabsorption and femtosecond pump-probe spectroscopy in a biased InGaN/GaN p-i-n double heterostructure. Steady state electroabsorption measurements indicate the field induced absorption coefficient changes in this structure are caused by the Franz-Keldysh effect. The temporal resolution of the absorption bleaching spectra suggests that the photoinduced carriers rapidly relaxed to the InGaN band edge within several picoseconds. As the applied reverse bias field was increased, the transition of the differential absorption spectral signature from the signature for absorption bleaching to the signature for field screening was observed. The magnitude of the change in absorption due to photoinduced carrier screening of the applied field is quantified and compared to absorption bleaching.
Structural Defects in Mg-doped GaN and AlGaN grown by MOCVD
Authors: Tomiya S. , Goto S. , Takeya M. , Ikeda M.
Published: April 2011

We have investigated the structural defects formed in Mg-doped GaN and AlGaN epitaxial layers grown by metal organic chemical vapor deposition. These defects have an inverse pyramidal shape and appear when the Mg concentration [Mg] is higher than ∼4×1019/cm3. The density of the defects increases as [Mg] increases, but the size of the defects becomes smaller as [Mg] increases. The density of the defects also has a strong correlation with the hydrogen concentration in the epitaxial layers. Transmission electron microscope analysis reveals that the defects have an inversion operation to the matrix and that their boundaries are Mg-rich. We also propose a model for defect formation.
Distinguishing negatively-charged and highly conductive dislocations in gallium nitride using scanning Kelvin probe and conductive atomic force microscopy
Authors: Simpkins Blake S. , Yu Edward T. , Waltereit Patrick , Speck James S.
Published: April 2011

Scanning Kelvin probe microscopy (SKPM) and conductive atomic force microscopy (C-AFM) are used to image surfaces of GaN grown by molecular beam epitaxy (MBE). Numerical simulations are used to assist in the interpretation of SKPM images. Detailed analysis of the same area using both techniques allows imaging of surface potential variations arising from the presence of negatively charged dislocations and dislocation-related current leakage paths. Correlations between the charge state of dislocations, conductivity of leakage current paths, and possibly dislocation type can thereby be established. Approximately 25% of the leakage paths appear to be spatially correlated with negatively charged dislocation features. This is approximately the level of correlation expected due to spatial overlap of randomly distributed, distinct features of the size observed, suggesting that the negatively charged dislocations are distinct from those responsible for localized leakage paths found in GaN. The effects of charged dislocation networks on the local potential profile is modeled and discussed.
Characterization of Dark-Block Defects in Cantilever Epitaxial GaN on Sapphire
Authors: Provencio P. P. , Follstaedt D. M. , Missert N. A. , Koleske D. D. , Mitchell C. C. , Allerman A. A. , Ashby C. I. H.
Published: April 2011

Cantilever epitaxy of GaN on sapphire has been augmented by the use of initial facetted GaN growth on narrow sapphire mesas (< 1μm) in order to turn remaining threading dislocations from vertical to horizontal, and thus reduce the overall dislocation density at the surface where devices would be placed. With this modification, isolated non-radiative block-like defect areas have been introduced that hinder optical and electronic material performance. Here we characterize these defects with microscopy, and show that they are arrays of lateral dislocations, with cracks and voids along their centerlines. We deduce that they result when tilted GaN is joined to neighboring oriented material. Their presence is independent of the type of nucleation layer used.
Study of the Origin of Misorientation in GaN Grown by Pendeo-Epitaxy
Authors: Zakharov D. N. , Liliental-Weber Z. , Roskowski A. M. , Einfeldt S. , Davis R. F.
Published: April 2011

Growth of pendeo-epitaxial (PE) layers introduces misorientation between the seed layers and the overgrown wing layers. The origin of this misorientation has been studied by Transmission Electron Microscopy (TEM) using a set of samples in which subsequent procedures utilized in PE were applied, i.e. growth of GaN template, stripe etching, annealing at the growth temperature of the PE layers and final PE growth. It was shown that etching of seed-stripes did not change the type of defects or their distribution. However, heating to the PE growth temperature drastically modified the surface and V-shaped pits were formed. The surface became smooth again after the PE growth took place. Overgrowth of the V-shaped pits resulted in formation of edge threading dislocations over a seed-stripe region with a dislocation density of 8.0×108 cm−2. Formation of new edge dislocations over the seed can have an influence on the misorientation between the PE grown regions.
Characterization of Cubic GaN Films Using An AlN/GaN Ordered Alloy on GaAs (100) by RF-MBE
Authors: Shike Junichi , Shigemori Atsushi , Ishida Koichi , Takahashi Kiyoshi , Kimura Ryuhei
Published: April 2011

High quality cubic GaN films were successfully grown on an AlN/GaN ordered alloy by RF-MBE. AlN/GaN ordered alloy is here employed instead of a AlGaN nucleation layer formed by nitridation of an AlGaAs buffer layer. Dominant cubic GaN epilayer (1.0 μm) growth was confirmed by insitu RHEED observations, AFM, TEM, PL and X-ray diffraction measurements.
Background Impurity Reduction and Iron Doping of Gallium Nitride Wafers
Authors: Vaudo Robert P. , Xu Xueping , Salant Allan D. , Malcarne Joseph A. , Hutchins Edward L. , Brandes George R.
Published: April 2011

Background impurities and the resulting electrical characteristics were studied for GaN wafers grown using hydride vapor phase epitaxy at various growth conditions. The electron concentration was found to decrease with increasing GaN thickness, by orders of magnitude in the first few microns of growth, but continuing gradually for thousands of microns. Physical removal of the backside degenerate layer enabled improved analysis of the electrical properties. Secondary ion mass spectroscopy was used to determine that the presence of oxygen and silicon accounted for the electron concentration for unintentionally n-type doped material. The concentration of oxygen was found to vary more than that of silicon and increased with decreasing growth temperature. The resistivity was measured to be as high as 1 ohm-cm, corresponding to a carrier concentration of 1016 cm−3. Iron was demonstrated to effectively compensate the residual donors and increased the resistivity to greater than 109 ohm-cm at room temperature and greater than 3×105 ohm-cm at 250 °C. An activation energy for the iron-doped GaN was determined by variable temperature resistivity measurements to be 0.51 eV.
Lattice constant variation in GaN:Si layers grown by HVPE
Authors: Usikov A. , Kovalenkov O. V. , Mastro M. M. , Tsvetkov D. V. , Pechnikov A. I. , Soukhoveev V. A. , Shapovalova Y. V. , Gainer G. H.
Published: April 2011

The structural, optical, and electrical properties of HVPE-grown GaN-on-sapphire templates were studied. The c and a lattice constants of the GaN layers were measured by x-ray diffraction. It was observed that the c and a lattice constants vary non-monotonically with Si-doping. The proper selection of Si-doping level and growth conditions resulted in controllable strain relaxation, and thus, influenced defect formation in GaN-on-sapphire templates. It was also observed that HVPE homoepitaxial GaN layers grown on the templates have better crystal quality and surface morphology than the initial templates.
High-Spatial-Resolution Cathodoluminescence Measurement of InGaN
Authors: Kanie Hisashi , Okado Hiroaki , Yoshimura Takaya
Published: April 2011

This paper described observation of cathodoluminescence (CL) of microcrystalline InGaN bulk crystals under a scanning electron microscope (SEM) with a high-spatial-resolution (HR) CL measuring apparatus. HR-CL spectra from facets of InGaN crystals vary from facet to facet and are single peaked. Histogram analysis of the CL peak positions of HR spectra from the facets of the crystals in the area scanned during a low-resolution CL measurement shows a two-peaked form with comparable peak wavelengths. The diffusion length of a generated electron- ho le pair or an exciton from the recombination centers with a higher-energy-level state to that with a lower state is estimated to be 500 nm at the longest by the comparison of two monochromatic HR-CL images of adjoining facets.
Phase Transitions on Gan Surfaces
Authors: Adelmann C. , Lymperakis L. , Brault J. , Mula G. , Neugebauer J. , Daudin B.
Published: April 2011

Recent experimental and theoretical studies highlighted the importance of the growing surface structure on the final morphology of GaN. Actually, optimum morphology is achieved by growth in presence of a Ga bilayer adsorbed on the GaN surface. The threshold fluxes limiting the region of the Ga bilayer adsorption have been measured as a function of the GaN substrate temperature, giving rise to a Ga adsorption phase diagram. The Ga flux limiting the regions in the adsorption phase diagram exhibit a linear behavior in an Arrhenius plot. However, both energy activation (about 5 eV) and prefactor (in the 1025 range) are surprinsingly high. These questions were adressed by studying the adsorption/desorption of Ga adatoms and small islands (consisting of 2 and 3 Ga adatoms) on the Ga bilayer surface employing first principle density functional theory calculations. We find a desorption barrier of 2.1 eV and a binding energy between two Ga atoms of approximately 0.3 eV. Using these numbers we derived a simple growth model (based on rate equations). An analysis of the experimental data with the model revealed the origin of the large difference in the activation energies and the unusually large prefactor. We find that the nucleation of the droplets cannot be described by a simple Arrhenius behavior (as commonly assumed to fit experimental data) but that the nucleation energy is temperature dependent.
GaN epilayers and AlGaN/GaN multiple quantum wells grown on freestanding [1100] oriented GaN substrates
Authors: Chen C. Q. , Gaevski M. E. , Sun W. H. , Kuokstis E. , Yang J. W. , Simin G. , Khan M. A , Maruska H. P. , Hill D. W. , Chou M. M. C. , Gallagher J. J. , Chai B. H , Song J. H. , Ryu M. Y. , Yu P. W.
Published: April 2011

We report on the homoepitaxial growth of GaN on freestanding [1100] oriented GaN substrates using metalorganic chemical vapor deposition. A proper pretreatment of the substrates was found to be essential for the GaN homoepitaxy. The influence of growth conditions such as V/III molar-ratio and temperature on the surface morphology and optical properties of epilayers was investigated. Optimized pretreatment and growth conditions led to high quality [1100] oriented GaN epilayers with a smooth surface morphology and strong band-edge emission. These layers also exhibited strong room temperature stimulated emission under high intensity pulsed optical pumping. Based on these GaN epilayer, AlGaN/GaN multiple quantum wells have been grown on the freestanding M-plane GaN. Photoluminescence data confirm that built-in electric field for M-plane structures is very weak, and this situation results in a stronger PL intensity in comparison with C-plane multiple quantum wells in tests at low excitation level.
Role of polarization in the photoluminescence of C- and M-plane oriented GaN/AlGaN multiple quantum wells
Authors: Kuokstis E. , Chen C. Q. , Gaevski M. E. , Sun W. H. , Yang J. W. , Simin G. , Asif Khan M. , Maruska H. P. , Hill D. W. , Chou M. M. C. , Gallagher J. J. , Chai B. H.
Published: April 2011

The comparative study of photoluminescence (PL) dynamics of wurtzite-type GaN/AlGaN multiple quantum wells (MQWs) fabricated using low-pressure metalorganic chemical vapor deposition technique over GaN coated [0001]-sapphire (C-plane) and single crystalline [1100]-oriented freestanding GaN (M-plane) substrates is presented. The MQWs on C-plane sapphire at low excitation exhibited much lower (∼30 times) PL intensity in comparison with M-plane samples. The C-plane MQWs showed a strong excitation intensity-induced PL spectrum line blueshift (up to 140 meV). Meanwhile identical MQW structures on M-plane substrates demonstrated no PL peak shifts indicating an absence of polarization fields. At higher excitation (>50 kW/cm2) the PL intensity and spectra peak positions for both the C- and the M-plane MQWs become nearly the same and do not differ with subsequent increase of pumping. Theoretical analysis and comparison with PL experimental data revealed strong (up to ∼1.2–1.3 MV/cm) built-in electrostatic fields in the C-plane structures whereas M-plane structures are almost non-polar.
Cathodoluminescence of MBE-grown cubic AlGaN/GaN multi-quantum wells on GaAs (001) substrates
Authors: As D. J. , Potthast S. , Köhler U. , Khartchenko A. , Lischka K.
Published: April 2011

Cubic phase GaN/AlxGa1-xN Multi Quantum Well structures were grown by rf-plasma assisted molecular beam epitaxy (MBE) on GaAs (001) substrates. X-ray measurements showed a high phase purity of the epilayers and revealed an Aluminum incorporation between 9 % and 49 %, respectively. The QW luminescence was tuned between 3.25 eV and 3.4 eV by means of the variation of QW barrier Aluminum content and QW width. Strong Cathodoluminescence (CL) from the GaN QWs and the underlying cubic AlxGa1-xN bulk material was observed at room temperature. The spatial localization of the QW emission was unambiguously determined by depth-resolved CL measurements. Combined with a model of energy-dependent penetration, diffusion, and recombination, these variations indicate a value of about 20 nm for the minority carrier diffusion length within the AlxGa1-xN confinement layer. The assignment of AlxGa1-xN bulk and GaN luminescence was further supported by employing a simple effective-mass quantum mechanical model.
Microscopic Description of Radiative Recombinations in InGaN/GaN Quantum Systems
Authors: Morel Aurelien , Lefebvre Pierre , Taliercio Thierry , Gil Bernard , Grandjean Nicolas , Damilano Benjamin , Massies Jean
Published: April 2011

Recombination dynamics in a variety of InGaN/GaN quantum systems has been studied by time resolved photoluminescence (PL). We have discovered that the time-decay of PL exhibits a scaling law: the nonexponential shape of this decay is preserved for quantum wells and quantum boxes of various sizes while their decay time varies over several orders of magnitude. To explain these results, we propose an original model for electron-hole pair recombination in these systems, combining the effects of internal electric fields and of carrier localization on a nanometer-scale. These two intricate effects imply a separate localization of electrons and holes. Such a microscopic description accounts very well for both the decays shape and the scaling law.
Dielectric Function of “Narrow” Band Gap InN
Authors: Goldhahn R. , Shokhovets S. , Cimalla V. , Spiess L. , Ecke G. , Ambacher O. , Furthmüller J. , Bechstedt F. , Lu H. , Schaff W. J.
Published: April 2011

Spectroscopic ellipsometry studies in the energy range from 0.7 up to 5.5 eV were carried out in order to determine the dielectric function (DF) of ‘narrow’ band gap (< 1 eV) single-crystalline InN films grown by molecular beam epitaxy on sapphire substrates. The imaginary part of the DF is characterized by a strong increase immediately above the band gap and then by a nearly constant value up to 4 eV. Pronounced structures above 4 eV are attributed to transitions along the L-M direction in the Brillouin-zone as a comparison with first-principles calculations indicates. In contrast, sputtered layers (band gap ∼1.9 eV) studied for comparison show a completely different spectral shape of the DF. Finally, DF's of high In-content InGaN alloys are presented, providing further evidence that InN is a “narrow” band gap semiconductor.
Electron Microprobe and Photoluminescence Analysis of Europium-Doped Gallium Nitride Light Emitters
Authors: Martin R. W. , Dalmasso S. , O'Donnell K. P. , Nakanishi Y. , Wakahara A. , Yoshida A.
Published: April 2011

Rare-earth doped GaN structures offer potential for optical devices emitting in the visible region [1,2]. We describe a study of MOVPE grown GaN-on-sapphire epilayers implanted with Europium ions, producing characteristic red emission lines between 540 and 680 nm due to intra-4f(n) electron transitions. As-implanted and subsequently annealed samples are investigated using a combination of wavelength dispersive x-ray analysis (WDX), electron microscopy, cathodoluminescence (CL) and photoluminescence (PL). WDX is shown to be a powerful technique for quantifying rare-earth concentrations in GaN, with varying electron beam voltages allowing a degree of depth profiling, further enhanced by the simultaneous collection of room temperature luminescence (CL) from the analysed region [3]. The intensities of the sharp lines observed in the luminescence spectrum are compared to the doping density (between 1014 – 1015 cm−2) and the Eu content measured by WDX, using a Eu-doped glass standard. Differences observed in the luminescence spectra produced by laser and electron beam excitation will be discussed along with the importance of the annealing conditions, which “heal” defects visible in the electron micrographs.
Efficient GaN-based Micro-LED Arrays
Authors: Choi H. W. , Jeon C. W. , Dawson M. D. , Edwards P. R. , Martin R. W.
Published: April 2011

Highly efficient, two-dimensional arrays of parallel-addressed InGaN blue microLEDs with individual element diameters of 8, 12 and 20μm have been fabricated. In order to overcome the difficulty of interconnecting multiple device elements with sufficient step-height coverage for contact metallisation, a novel scheme involving the etching of sloped-sidewalls has been developed. The devices have I-V characteristics similar to those of broad-area reference LEDs fabricated from the same wafer, and give superior (3mW) light output in the forward direction to the reference LEDs, despite much lower active area. The external efficiencies of the micro-LED arrays improve as the dimensions of the individual elements are scaled down. This is attributed to scattering at the etched sidewalls of in-plane propagating photons into the forward direction.
Growth and Fabrication of High Reverse Breakdown Heterojunction n-Gan: p-6H-Sic Diodes
Authors: Sampath A. V. , Bhattacharyya A. , Singh R. , Eddy C. R. , Lamarre P. , Stacey W. F. , Morris R. S. , Moustakas T. D.
Published: April 2011

Wide band gap semiconductors are attractive for developing high power switching devices because of their ability to operate at both higher temperatures as well as higher frequencies than conventional Si. In this paper we report on the growth and fabrication of GaN/SiC np heterojunction diodes by depositing Si doped n-GaN films by plasma-assisted molecular beam epitaxy directly on SiC without the use of GaN or AlN buffers.

Careful ex-situ and in-situ preparation of the Si terminated 6H- SiC surface was necessary to produce high quality diodes. Vertical circular diodes were fabricated with sizes varying from 200 microns to 1mm in diameter. Mesas were formed by ICP etching of the MBE deposited n-GaN layer using Cl2. A Ti/Al/Ni/Au metal stack was employed as an n-ohmic contact to the GaN layer and an Al/Ti/Au metal stack was employed as a backside p-ohmic contact to the 6H- SiC layer. The diodes were characterized by I-V and C-V measurements. The 1 mm diameter diodes exhibited almost ideal behavior under forward bias with an ideality factor of 1.6, and a reverse saturation current of 10–19 A/cm2. Under reverse bias, these devices were driven up to 1000 V with a measured leakage current of 5×10–7 A. and a dynamic resistance varying from 1010 to 109 ohms with increasing reverse bias. The built-in potential in these n-p heterojunctions was determined from C-V measurements to be 2.25 V. From these values we determined that the heterojunction is of Type II with conduction and valence bands offsets calculated to be 0.65 and 1.1 eV respectively.
Segregation effects and bandgap engineering in InGaN quantum-well heterostructures
Authors: Bulashevich Kirill A. , Karpov Sergey Yu. , Talalaev Roman A. , Evstratov Igor Yu. , Makarov Yuri N.
Published: April 2011

The analysis of In surface segregation and its impact on the composition profile and light emission spectra of the InGaN single quantum well heterostructures grown by Metalorganic Vapor Phase Epitaxy (MOVPE) is carried out by coupled solution of the Poisson and Schrödinger equations. Effective methods of controlling the composition profile, indium predeposition and temperature ramping during the cap layer growth are considered in terms of surface segregation model. General trends in spectra transformation upon the forward bias variation and their correlations with the quantum well electronic structure are discussed.
AlGaN/GaN Metal-Oxide-Semiconductor Heterostructure Field-Effect Transistors (MOSHFETs) with the Delta-Doped Barrier Layer
Authors: Fan Z. Y. , Li J. , Lin J. Y. , Jiang H. X. , Liu Y. , Bardwell J. A. , Webb J. B. , Tang H.
Published: April 2011

The fabrication and characterization of AlGaN/GaN metal-oxide-semiconductor heterostructure field-effect transistors (MOSHFETs) with the δ-doped barrier are reported. The incorporation of the SiO2 insulated-gate and the δ-doped barrier into HFET structures reduces the gate leakage and improves the 2D channel carrier mobility. The device has a high drain-current-driving and gate-control capabilities as well as a very high gate-drain breakdown voltage of 200 V, a cutoff frequency of 15 GHz and a maximum frequency of oscillation of 34 GHz for a gate length of 1 μm. These characteristics indicate a great potential of this structure for high-power-microwave applications.
AlGaN/GaN Heterostructure Field-Effect Transistors with Back-Doping Design for High-Power Applications: High Current Density with High Transconductance Characteristics
Authors: Maeda Narihiko , Tsubaki Kotaro , Saitoh Tadashi , Tawara Takehiko , Kobayashi Naoki
Published: April 2011

Electron transport properties and DC device characteristics have been examined in the AlGaN/GaN heterostructure field-effect transistors (HFETs) with back-doping design that makes it possible to obtain high two-dimensional electron gas (2DEG) densities even for the devices with thin AlGaN barrier layers. In the back-doping design, an asymmetric double-heterostructure is employed, and donor atoms are doped not only in the surface-side AlGaN layer but also in the underlying AlGaN layer. In this structure, electrons are efficiently supplied also from the back-doped AlGaN barrier layer to the GaN channel and merged into a single 2DEG layer, with the help of the negative polarization charges at the heterointerface between the GaN channel and the underlying AlGaN barrier layer. By using back-doping design, very high 2DEG densities around 3×1013 cm−2 has been achieved in the Al0.3Ga0.7N/GaN HFET whose barrier layer (Al0.3Ga0.7N) is designed to be as thin as 120 Å. An HFET with the gate-length of 1.5 μm has exhibited a high current density of 1.2 A/mm and a high transconductance of 200 mS/mm, which is ascribed to high 2DEG densities and thin barrier layers in these devices. HFETs with the back-doping design are thus promising for high-power applications.
Effect of Al-content and Post Deposition Annealing on the Electrical Properties of Ultra-thin HfAlxOy Layers
Authors: Carter R. J. , Tsai W. , Young E. , Caymax M. , Maes J. W. , Chen P. J. , Delabie A. , Zhao C. , DeGendt S. , Heyns M.
Published: April 2011

Scaled HfAlxOy/SiO2 stacks down to 1.5 nm EOT have been achieved. Although the addition of Al to the HfO2 matrix can be beneficial, it is observed that the benefit of using a Hf-aluminate is compromised if the film has a high Al-content. This is observed in terms of a dielectric constant close to that of pure Al2O3 (∼ 9) and a large amount of negative fixed charge in the film (∼ 1012 cm-2). Using oxygen post deposition anneals we have been able to reduce flatband voltage shifts associated with fixed charge as well as CV hysteresis. In terms of scaling, the benefit of using a high-k material is compromised if a SiO2 layer is also present in the gate stack. Therefore, it is necessary to perform an O2 PDA at moderate temperatures or in low O2 partial pressures in order to control the thickness of the interfacial oxide layer.
2D Dopant Profiling for Advanced Process Control
Authors: Wang Xiang-Dong , Xie Qianghua , Hooker Joe , Lu Shifeng , Lee J. J. , Tobin Phil , Liu Wei , Cross Linda
Published: April 2011

As the CMOS device dimensions continue to shrink, it is more and more critical to control the process parameters during mass production of advanced VLSI chips in order to achieve high yield and profitability. 2D dopant characterization is one of the critical techniques to resolve manufacturing excursions. A quick access to dopant distribution, especially precise delineation of p-n junction would readily provide critical information for many manufacturing issues, as well as device design and process development. Here we present our approaches to some of those issues with available techniques. The main techniques we used are dopant selective etching (DSE) and scanning probe microscopy based electrical measurements including scanning capacitance microscopy (SCM) and scanning spread resistance microscopy (SSRM). These techniques provided complementary results and showed strengths in solving different issues. We have successfully delineated junction of CMOS devices with 0.13 μm technology with source/drain extensions. Other applications, including diode leakage, well-well isolation, and buried layer delineation with the combination of these methods are presented.
SiGe pMOSFETs Fabricated on Limited Area SiGe Virtual Substrates
Authors: Waite Andrew , Straube Urs , Lloyd Neil , Croucher Sally , Teng Tang Yue , Rong Bifeng , Evans Alan , Grasby Tim , Whall Terry , Parker Evan
Published: April 2011

Silicon germanium pMOSFETs with channel lengths down to 0.4m have been fabricated on limited area silicon germanium virtual substrates. The devices have a 5nm thick Si0.3Ge0.7 active layer grown by MBE on top of relaxed Si0.7Ge0.3 virtual substrate. Virtual substrates were grown on top of 10μm square silicon pillars defined by etching trenches around their perimeter into the original silicon substrate. This limits the area of the growth zone, which in turn promotes the relaxation of the virtual substrate. Electrical measurements on 2μm long channel devices show that the maximum mobility in the strained SiGe devices is 211cm2V-1cm-1, compared to 104cm2V-1cm-1 for silicon reference devices. This increase in hole mobility increases the current drive of 0.4m devices measured at Vgt=-2V, Vds=-2.5V from 154μA/m to 192μA/μm.
Effect of Nature of the Precursor on Crystallinity and Microstructure of MOCVD-Grown ZrO2 Thin Films
Authors: Dharmaprakash M. S. , Shivashankar S. A.
Published: April 2011

In the present work, we report the deposition of zirconia thin films on Si(100) at various substrate temperatures by low-pressure metalorganic chemical vapor deposition (MOCVD). Three different zirconium complexes, viz., tetrakis(2,4-pentadionato)zirconium(IV), [Zr(pd)4], tetrakis(2,2,6,6-tetramethyl-3,5-heptadionato)zirconium(IV), [Zr(thd)4], and tetrakis(t-butyl-3-oxo-butanoato)zirconium(IV), [Zr(tbob)4] are used as precursors. The relationship between the molecular structures of the precursors and their thermal properties, as examined by TG/DTA is presented. The films deposited using these precursors have distinctly different morphology, though all of them are of the cubic phase. The films grown from Zr(thd)4 are well crystallized, showing faceted growth at 575°C, whereas the films grown from Zr(pd)4 and Zr(tbob)4 are not well crystallized, and display cracks. These differences in the observed microstructure may be attributed to the different chemical decomposition pathways of the precursors during the film growth, which influence the nucleation and the growth processes. This is also evidenced by the different kinetics of growth from these three precursors under otherwise identical CVD conditions. The details of thin film deposition, and film microstructure analysis by XRD and SEM is presented. The dielectric behavior of the films deposited from different precursors, as studied by C-V measurements, are compared.
Growth and Physical Properties of MOCVD-Deposited Hafnium Oxide Films and Their Properties on Silicon
Authors: Van Elshocht S. , Caymax M. , De Gendt S. , Conard T. , Pétry J. , Claes M. , Witters T. , Zhao C. , Brijs B. , Richard O. , Bender H. , Vandervorst W. , Carter R. , Kluth J. , Daté L. , Pique D. , Heyns M. M.
Published: April 2011

This paper discusses metal organic chemical vapor deposited (MOCVD) HfO2 layers using tetrakis(diethylamido)hafnium (TDEAH) as precursor. We have studied the influence of the starting surface and deposition temperature on the growth kinetics and physical properties of the HfO2 layers. Important characteristics such as crystalline state, density, and organic contamination in the layers were found to be dependent on these parameters.

Typical for this deposition process is the formation of an interfacial layer underneath the high-k layer. Its composition and thickness, affecting scaling of the equivalent oxide thickness, are shown to be closely related to the HfO2 process parameters mentioned above.

Finally, we will show electrical results for HfO2/polySi gate stacks indicating the effect for deposition temperature.
Effectiveness of Plasma Nitrided Silicon Oxynitride as a Barrier Layer between High k Materials and Si Substrates
Authors: Lai Yi-Sheng , Chen J. S.
Published: April 2011

In this work, low temperature (300 – 450°C) plasma nitridation was conducted using N2O and NH3 atmosphere to produce an ultrathin SiOxNy layer. The bonding structure, distribution, and quantity of nitrogen and their effects on the growth kinetic of SiOxNy layers are studied by X-ray photoelectron spectroscopy (XPS). It is found that nitrogen atoms pile at the SiOxNy/Si interface at the very beginning of plasma N2O and NH3 nitridation. Due to the lack of additional Si source and low diffusivity of O or N atoms at low temperature, plasma nitridation will form a self-limited growth of SiOxNy layer. Thermal stability of the interlayer between ultrathin Ta2O5 films on bare Si, plasma N2O nitrided Si, and plasma NH3 nitrided Si is also studied.
Physical-Chemical Evolution upon Thermal Treatments of Al2O3, HfO2 and Al/Hf Composite Materials Deposited by ALCVD™
Authors: Crivelli B. , Alessandri M. , Alberici S. , Cazzaniga F. , Dekadjevi D. , Maes J. W. , Ottaviani G. , Pavia G. , Queirolo G. , Santucci S. , Zanderigo F.
Published: April 2011

This paper presents a systematic investigation of thermal stability of high-k materials deposited on RCA cleaned wafers by ALCVD™ in an ASM Pulsar™ 2000 reactor. Physical-chemical evolution of Al2O3, HfO2 and Al/Hf composite materials (nanolaminate and aluminates) was studied considering two types of thermal treatments: quenched vacuum anneals from 300°C to 900°C and furnace atmospheric processes in N2 or O2 at 850°C and 900°C. Material crystallization and changes in film structure were studied by means of TEM, XRD, XRR, XRF, RBS and TOF-SIMS. Non-contact electrical measurements were used to detect modification in EOT and fixed charge. Al2O3 was found still amorphous at 900°C. Not so for HfO2 that crystallized in monoclinic phase at a temperature between 300–400°C. Crystallization temperature and possible phase separation of Al/Hf composite materials were found to be a function of Al2O3 content and film type. In most of these samples, however, a chemical evolution was detected in addition to the above reported crystallization phenomena. All the achieved results demonstrate that depending on thermal treatment conditions, ALCVD™ high-k stability does not only concern phase transition effects but also a transformation of the “SiO2/high-k” system into “doped-SiO2/silicate” stack.
Influence of Substrate Temperature During Sputter Deposition on the Subsequent Formation of Titanium Disilicide
Authors: Özcan A. S. , Ludwig K. F. , Cabral C. , Lavoie C. , Harper J. M. E.
Published: April 2011

We examine how the substrate temperature during Ti film sputter deposition influences the subsequent texture formation in TiSi2 thin films. Titanium films of 32 nm thickness were sputtered onto Si(001) at elevated substrate temperatures varying between 100 °C and 900 °C. After the depositions, in situ x-ray diffraction (XRD) measurements were performed to study the thin film reactions in real time, as the samples were annealed. The XRD results show that the substrate temperature significantly influences the texture of the initial Ti film as well as the texture of the resulting C54-phase TiSi2. The preferred Ti orientation gradually changes from (002) to (101) fiber texture as the deposition temperature increases up to 500 °C. Films deposited at 600 °C transformed into the C49 phase during deposition while films deposited at 700 °C and higher temperatures transformed into the C54 phase during deposition. The series of deposited films was annealed up to 1000 °C in He to complete the C54 phase formation while monitoring the texture evolution in situ using a position sensitive x-ray detector. The XRD results show that the final C54 phase texture changes from a dominant (311) orientation normal to the substrate to a (010) orientation for substrate temperatures between 600 °C and 700 °C. The C49-C54 phase transformation temperature is also lowered for these deposition temperatures. Ex situ pole figure analysis of the film deposited at 700 °C confirms the dominant C54 (010) texture and shows an in-plane orientation with C54 [001] ∥ Si [110]. For substrate temperatures between 800 °C and 900 °C, the C54 texture changes dramatically. In this case, θ - 2 θ scans do not show a preferred C54 orientation, but pole figure analysis indicates weak inplane orientations.
First-principles Study of Electronic and Dielectric Properties of ZrO2 and HfO2
Authors: Zhao Xinyuan , Vanderbilt David
Published: April 2011

Using density-functional theory with ultrasoft pseudopotentials, we previously investigated the structural and electronic properties of the low-pressure (cubic, tetragonal, and monoclinic) phases of ZrO2 and HfO2, in order to elucidate phonon modes, Born effective charge tensors, and especially the lattice dielectric response in these phases. We now extend this previous work by carrying out similar calculations on the two high-pressure orthorhombic phases, and by providing density-of-states and band-gap information on all polymorphs. Our results show that the electronic structures and dielectric responses are strongly phase-dependent. In particular, the monoclinic phases of ZrO2 and HfO2 are found to have a strongly anisotropic dielectric tensor and a rather small orientational average () compared to the two other low-pressure phases. Our calculations show that is even smaller in the orthorhombic phases.
Structural Quality And Electrical Behavior Of Epitaxial High-κ Y2O3 / Si(001)
Authors: Vellianitis G. , Apostolopoulos G. , Dimoulas A. , Argyropoulos K. , Mereu B. , Scholz R. , Alexe M. , Hooker J. C.
Published: April 2011

Y2O3 thin films were grown directly on Si (001) by MBE and annealed in-situ under UHV at various annealing temperatures. The samples were investigated in-situ by RHEED and ex-situ by HRTEM. A 7 to 15 Å thick non-uniform interfacial amorphous layer is observed in the as-grown sample. After annealing at 490°C under UHV for 30 minutes the amorphous layer is reduced and a sharp Y2O3/Si interface is obtained. At higher annealing temperatures, YSi2 islands start to form at the Y2O3/Si interface. I-V measurements performed on generic MIS structures show that the annealed samples exhibit higher leakage current density than the as-grown sample, due to reduction of the wide band gap interfacial layer. Leakage current densities in annealed samples remain below 1A/cm2, which is acceptable for future high-κ transistor fabrication.
Electro-optic Mach-Zehnder modulators with polycrystalline BaTiO3 thin films on MgO
Authors: Petraru A. , Schubert J. , Schmid M. , Trithaveesak O. , Buchal Ch.
Published: April 2011

The optical and electro-optical properties of epitaxially grown thin films of ferroelectric BaTiO3 on MgO substrates have been established and high quality Mach-Zehnder waveguide modulators have been demonstrated. As a next step towards the integration of ferroelectric thin films on different substrates, we have modified the growth conditions by lowering the growth temperature to study polycrystalline, but still highly transparent BaTiO3 (BTO) films. Polycrystalline BTO on MgO substrates has been grown by pulsed laser deposition (PLD). The growth temperature was reduced from 800 °C to 400 °C at an oxygen pressure of 2×10-3 mbar. Although polycrystalline, the BTO is still birefringent with no= 2.32 and ne = 2.30. Ridge waveguides have been formed by ion beam etching. The estimated waveguide propagation loss is 4 dB/cm at 633 nm. Electro-optic Mach-Zehnder modulators have been realized. Using 3 mm long electrodes with a spacing of 10 μm, a Vπ voltage of 14 V was obtained at 633 nm wavelength. This is half of the observed effective electro-optic coefficient measured at epitaxial BTO films. At 1.5 μm wavelength similar results were observed.
Fabrication and Characterization of Lead Silicate Doped PZT Thin Films
Authors: Karasawa Junichi , Hamada Yasuaki , Ohashi Koji , Kijima Takeshi , Natori Eiji , Shimoda Tatsuya
Published: April 2011

The crystal structure, surface morphology and electrical properties of PbSiOx (PSO) doped Pb(Zr,Ti)O3 (PZT) thin films were systematically investigated. The starting solutions of PSO-doped PZT were prepared by mixing sol-gel solutions of PZT and PSO. RTA-calcined thin films were crystallized by means of an oxygen partial pressure controlled post-annealing process. After the calcining process, lightly doped PZT thin films containing an amount of PSO dopant less than 1.0 at.% exhibit (111)-preferred orientation, and fine grains having a diameter of 50 nm and column-like structure. As the amount of PSO dopant is increased to 5.0 at.%, the (111) peaks becomes obscure and the (100) peaks together with the minor (110) peaks becomes dominant. The column-like structure changes into a plate-like structure having coarse grains whose diameter exceeds 500 nm. Further increasing the level of PSO dopant to 20.0 at.% causes the appearance of a pyrochlore phase having an extremely smooth surface without apparent grains. On the other hand, after the post-annealing process, the pyrochlore phase seen in the heavily doped PZT remarkably disappears, replaced by a perovskite phase together with PSO. The remnant polarization (Pr) gradually decreases as the amount of PSO dopant increases. The break down voltage, fatigue, imprint and other reliability parameters are improved within the proper doping range.
Growth of Zr Substituted Barium Titanate Thin Films from the Vapor Phase
Authors: Ganster R. , Hoffmann-Eifert S. , Waser R.
Published: April 2011

We report on the growth of Ba(Ti1-xZrx)O3 thin films on Pt(111) / TiO2 / SiO2 / Si substrates by means of metal-organic chemical vapor deposition (MOCVD) using liquid precursors. The MOCVD system consists of an AIXTRON AIX-200 horizontal reactor with a TriJet® vaporizer. In the multi-source injection system the different single element precursor solutions were introduced separately in a pulse mode. The focus of our investigations lies on the correlation between processing conditions, growth rate, and film properties, namely stoichiometry, crystal structure, and surface morphology. Dense, polycrystalline Ba(Ti0.63Zr0.37)O3 films were successfully grown on platinum coated silicon substrates at temperatures around 630°C.
Parametric Optimization of a Sol-Gel Process for the Synthesis of Highly-Oriented (Pb, Ba)TiO3 Thin Films
Authors: Yang Wein-Duo , Pillai Suresh C. , Boland Stacey W. , Haile Sossina M.
Published: April 2011

A sol-gel process incorporating lead acetate trihydrate, barium acetate and titanium isopropoxide as precursors, acetylacetone as a chelating agent, and ethylene glycol as a solvent was used to prepare solid solution (Pb0.5,Ba0.5)TiO3. Water content, annealing temperature and heating rate were varied and the resulting effects on material properties were studied using TGA/DSC, FTIR, FESEM and X-ray diffraction. Crystallization of the perovskite structure occurred at a temperature as low as 450°C. Thin films were prepared by spin coating on (100) MgO using the optimized precursor sol. Pyrolysis and anneal temperatures were varied and the resultant film properties investigated. The pyrolysis temperature used to dry the film was found to strongly affect orientation. Under optimized conditions, highly oriented films were obtained at a crystallization temperature of 600°C.
Development of Improved Precursors for the MOCVD of Bismuth Titanate
Authors: Williams Paul A. , Jones Anthony C. , Tobin Neil L. , Marshall Paul A. , Chalker Paul R. , Davies Hywel O. , Smith Lesley M.
Published: April 2011

Bismuth titanate thin films have been grown by liquid injection MOCVD using Bi(mmp)3 in combination with the new Ti precursors Ti(OPri)2(mmp)2 and Ti(mmp)4 (mmp = OCMe2CH2OMe). Films were grown on Si(100) substrates over the temperature range 300 – 600°C, and were shown to consist predominantly of the Bi4Ti3O12 phase at substrate temperatures > 500°C.
Ferroelectric Bi4Ti3O12–SrBi4Ti4O15 Intergrowth Thin Films Prepared by Pulsed Laser Deposition
Authors: Shibuya Akira , Noda Minoru , Okuyama Masanori
Published: April 2011

C axis-oriented Bi4Ti3O12–SrBi4Ti4O15 (B IT-SB Ti) intergrowth epitaxial ferroelectric thin films have been grown by pulsed laser deposition (PLD) method on MgO (001) and SrTiO3 (001) substrates. The epitaxial growth of BIT-SBTi intergrowth thin films were confirmed by X-ray diffraction (XRD) θ-2θ scan, pole figure plots and reciprocal space mappings. The c axis lattice constant of the BIT-SBTi intergrowth thin film is very close to that of made up of regular stacking of one-halves of the unit cells of Bi4Ti3O12 (3.296 nm) and SrBi4Ti4O15 (4.189 nm). The annealed BIT-SBTi thin film on Pt/Ti/SiO2/Si substrate shows intergrowth structure, too, and exhibits superior ferroelectricity that the values of 2Pr and 2Ec are 32.0 μC/cm2 and 190 kV/cm, respectively. The annealed BIT-SBTi film shows that the degradation of switching charge after 1×1010 switching cycles was 16.5%. This ferroelectric enhancement is attributed to strain of pseudo-perovskite layers interacting through Bi2O2 layer. The dielectric constant and dielectric loss of the annealed BIT-SBTi film were 433 and 0.037, respectively.
Field Induced Dielectric Properties of 0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 Thin Films
Authors: Laha Apurba , Saha S. , Krupanidhi S. B.
Published: April 2011

DC electric field induced dielectric properties of 0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 (PMN-PT) thin films were studied as a function of frequency at different temperatures. It was observed that the dielectric constant (ε) and dissipation factor (tanδ) were decreased in presence of bias field. The temperature of dielectric maxima was found to increase with increasing bias level. The low temperature (<Tm) frequency dispersion of dielectric permittivity was suppressed with the application of dc bias. After a certain bias voltage the relaxor property of films was disappeared i.e. the films exhibited normal ferroelectric behavior. Since the absence of long range interaction among the nanopolar clusters in PMN and its family is believed to be the origin of relaxor behavior, disappearance of relaxor nature in PMN-PT (70/30) films could be attributed to manifestation of long-range order at higher bias voltage. This was observed in the temperature dependence of dielectric constant i.e. the films neither exhibited any frequency dispersion in the temperature of dielectric maximum (Tm) nor showed any diffused phase transition. The relaxor property of PMN-PT thin films was studied in terms of diffused phase transition together with frequency dispersion of the temperature of dielectric maximum (Tm). Vogel-Fulcher relation was used to analyze the frequency dependence of temperature of dielectric maximum.