bluuebunny/crossref_metadata_2025 huggingface dataset:
Microscopic Aspects of the Staebler-Wronski Effect
Authors: Stutzmann Martin
Published: April 2011

The microscopic origin and the creation mechanisms of metastable, light-induced defects in hydrogenated amorphous silicon are reviewed. Based on excitonic electron-hole pair recombination, a consistent quantitative description of defect creation kinetics can be obtained, including the experimentally observed differences between continuous wave and pulsed illumination as well as the effect of competing recombination pathways in compensated material. High resolution spin resonance spectra obtained by low-field spin-dependent transport are used to examine the interaction of metastable defects with hydrogen.
A Novel Method for Fabrication of Hyhdrogenated Amorphous Silicon and High Quality Poly-SI Films on the Same Substrate by Employing Excimer Laser
Authors: Choi Kwon-Young , Lee Jong-Wook , Choi Hyoung-Bae , Jeon Jae-Hong , Han Min-Koo , Kim Yong-Sang
Published: April 2011

We have fabricated the a-Si:H film for pixel region and poly-Si film for driver region on the same glass substrate by a rapid thermal process successfully. By employing the halogen lamp, the considerable amount of hydrogen in the PECVD a-Si:H film, which causes the undesirable film ablation due to hydrogen evolution during excimer laser annealing, could be reduced selectively in the peripheral driver area where the a-Si:H film is recrystallized into poly-Si film in order to obtain the high mobility. After rapid thermal annealing, the hydrogen content in a-Si:H films, of which the inherent hydrogen content was found to be about 10–12 at. %, is successfully reduced to less than 5 at. %. The annealing conditions for dehydrogenation are 500, 550, 600 and 650 °C with various halogen lamp irradiation period which are below the point of the glass shrinkage. It should be noted that after rapid thermal annealing, any physical damage in the glass substrate has not been observed.
Application of Hot Wire Deposited Intrinsic Poly-Silicon Films in N-I-P cells and TFTS
Authors: Rath J. K. , Van Zutphen A.J.M.M. , Meiling H. , Schropp R. E. I.
Published: April 2011

Poly-silicon films have been prepared by hot-wire chemical vapour deposition (HWCVD) from hydrogen diluted silane gas at a low temperature (430 °C). The crystalline volume fraction is 95%. The grains have an average size of 70 nm and coalesce completely. The activation energy (0.54 eV) and the low carrier concentration (6.8 × 1010 cm−3) indicate the fully intrinsic nature of the films. The low (<1017 cm−3) defect density, the absence of 2100 cm−1 mode in infrared spectrum, the low activation energy of Hall mobility (0.012 eV) and the ambipolar diffusion length (LD) of 568 nm all indicate that the grain boundaries in the poly-Si:H films are indeed very thin. Preliminary n-i-p cells incorporating a poly-silicon i-layer yielded 3.15% efficiency and a current density of 18.2 mW/cm2 for only a 1.5 μm i-layer. In the solar cell, the conducting path is along the columnar grains ( (220) preferential orientation from XRD) and the carrier transport bypasses the grain boundary defects. This has been confirmed from the c-Si like optical absorption spectrum (measured by Dual Beam Photoconductivity in the cell configuration) at the low energy region. Inverted staggered thin-film transistors incorporating the poly-silicon layers showed transfer and output characteristics similar to those of state of the art a-Si:H TFTs : the saturation mobility is 0.4 cm2/Vs and the off current is approximately 10−11 A. This leads to the conclusion that the silicon near the SiO2 interface (the channel region) is still amorphous. This is illustrated by XTEM.
High-Deposition-Rate a-Si:H Through VHF-CVD of Argon-Diluted Silane
Authors: Meiling H. , Bezemer J. , Schropp R. E. I. , Van Der Weg W. F.
Published: April 2011

We discuss various ways to produce hydrogenated amorphous silicon, a-Si:H, at a high deposition rate. We also present results of our recent study on the structural properties of a-Si:H films deposited at high rates using argon (Ar) dilution of silane in a 50-MHz glow discharge. The results of the depositions with Ar dilution are compared to films deposited from pure silane, SiH4. The deposition rate rd is changed by varying the rf power Prf into the discharge. We focus on the Prf-dependence of the hydrogen (H) bonding configuration and total H content in the film. It is observed that rd saturates at 14 Å/s for pure SiH4, and at 22 Å/s for Ar-diluted SiH4 deposition. Upon increase of Prf the H bonding configuration changes from mostly isolated H to mostly clustered H, and back to mostly isolated H. It is argued that Ar* metastable atoms play an important role in the growth mechanism at intermediate Prf, whereas at high Prf ion bombardment through Ar+ and ions becomes crucial. Two high-rate a-Si:H films are incorporated in thin-film transistors, TFTs. We present their characteristics before and after illumination with calibrated light. It is shown that a-Si:H TFTs with a saturation mobility of 0.7 cm2/Vs can be fabricated, with the complete intrinsic layer deposited at 20 Å/s.
Versatile High Rate Plasma Deposition and Processing with very high Frequency Excitation
Authors: Heintze M.
Published: April 2011

The interest in plasma deposition using very high frequency (VHF) excitation arose after the preparation of a-Si:H at high growth rates was demonstrated. Subsequently the improved process flexibility and the control of material properties offered by the variation of the plasma excitation frequency was recognized. The preparation of amorphous and microcrystalline thin films in a VHF-plasma is described. The increased growth rates have been attributed to an enhancement of film precursor formation at VHF, to the decreased sheath thickness as well as to an enhancement of the surface reactivity by positive ions. Plasma diagnostic investigations show that the parameters mainly affected by the excitation frequency are the ion flux to the electrodes as well as the sheaths potentials and widths, rather than the plasma density.
Study of Microvoids in High-Rate a-Si:H Using Positron Annihilation
Authors: Zou X. , Webb D. P. , Lin S. H. , Lam Y. W. , Chan Y. C. , Hu Y. F. , Beling C. D. , Fung S.
Published: April 2011

In this paper, we have carried out the positron annihilation measurement on high-rate and low-rate a-Si:H thin films deposited by PECVD. By means of the slow positron beam Doppler-broadening technique, the depth profiles of microvoids in a-Si:H have been determined. We have also studied the vacancy-type defect in the surface region in high-rate grown a-Si:H, making comparison between high-rate and low-rate a-Si:H. By plotting S and W parameters in the (S, W) plane, we have shown that the vacancies in all of the high-rate and low-rate deposited intrinsic samples, and in differently doped low-rate samples are of the same nature.
High-Rate PECVD of Low Defect Density a-Si:H on Large Areas
Authors: Röhlecke S. , Steinke O. , Schade F. , Stahr F. , Albert M. , Deltschew R. , Kottwitz A. , Carius R.
Published: April 2011

Industrial production of amorphous silicon solar cells, photoreceptors and several opto-electronic devices requires large area, high-deposition-rate plasma reactors and deposition processes. Non-uniformity of die film thickness and particle generation at high power densities as well as the deposition rate are found to be important limiting factors in large area PECVD.

The deposition was performed in a capacitively-coupled coaxial diode rf glow discharge with large areas (1000 cm2 and 2000 cm2) at 13.56 MHz and 27.12 MHz. We studied the particle generation in the plasma reactor over a wide range of silane concentration (20 % to 100 %) in the SiH4/He mixture. We will present the opto-electronic properties of a-Si:H films and the influence of the substrate bias. The films are characterized by dark- and photoconductivity and by PDS.

It was confirmed through this study that helium dilution is effective in the suppression of powder growth for high-rate deposition up to 18 μm/hr. Special attention was paid to the optimization of reactor design and plasma conditions for the deposition of low density of states a-Si:H (∼1016 cm−3) at deposition rates of up to 18 μm/hr. Darkconductivity was 10−9 S/cm and photoconductivity was about 5.10−4 S/cm.
Bombardment by Reflection - A Novel Method to Prepare High Quality Hydrogenated Amorphous Germanium by Anodic PCVD
Authors: Diez Annette , Drüsedau Twlo P.
Published: April 2011

Deposition of amorphous germanium by anodic PCVD was performed changing the germane/hydrogen dilution ratio from 1/6 to 1/100. Films deposited under high dilution of germane are of a high mechanical density (92.5 % of bulk) and good optoelectronic properties (electrical conductivity at room temperature σ=10−5 (Ω cm)−1, majority carrier mobility-lifetime product at λ=1200 nm ημ=x 10−7 cm2/V). The quality of the films is increased by deposition at increasing pumping speed for hydrogen (lower hydrogen pressure) keeping the other parameters constant. It is suggested that fast atomic hydrogen originating from a backscattering process at the powered electrode bombard the growing film and contribute to the improved quality of the a-Ge:H. Also, the growth kinetics of the films is changed from linear to parabolic dependence on germane flow interchanging deposition from high to low hydrogen pressure. This effect is rationalized in terms of a contribution of energetic atomic hydrogen to the dissociation of germane.
Deposition of Device Quality Amorphous Silicon by Hot-Wire CVD
Authors: Feenstra K. F. , Van Der Werf C. H. M. , Molenbroek E. C. , Schropp R. E. I.
Published: April 2011

In this paper we present the results of the optimization of hydrogenated amorphous silicon films deposited by the hot-wire method in a larger area system. Using a two-wire design, we succeeded in depositing films that exhibit uniform electrical properties over the whole 4” x 4” Corning 7059 glass substrate. At a substrate temperature of 430 °C. and a pressure of 20 μbar we obtained a growth rate of ∼2 nm/s. The temperature of the tungsten filaments was kept at 1850 °C. The values for the photoconductivity and dark conductivity were 8.9×10−6 S/cm and 1.6×10−10 S/cm respectively, whereas the ambipolar diffusion length, as measured with the Steady-State Photocarrier Grating technique (SSPG), amounted to 145 nm. This value is higher than for our device quality glow-discharge (GD) films, which yield devices with efficiencies higher than 10%. The hydrogen content was 9.5%.

We report on the density-of-states (DOS) distribution in the films, which was measured with the techniques of Thermally Stimulated Conductivity (TSC) and Constant Photocurrent Method (CPM). Furthermore, we describe the behavior of the electrical properties on light-induced degradation. Finally, we incorporated these films in solar cells, using conventional GD doped layers. Preliminary SS/n-i-p/ITO devices yielded efficiencies in excess of 3% under 100 mW/cm2 AM 1.5 illumination. Further work concerning the optimization of the interfaces is in progress.
Fast Etching of Amorphous and Microcrystalline Silicon by Hot-Filament Generated Atomic Hydrogen
Authors: Wanka H. N. , Schubert M. B.
Published: April 2011

A hot tungsten wire effectively dissociates H2 into atomic hydrogen and thereby facilitates etching and hydrognation of silicon. Hot filament generated atomic hydrogen etches amorphous silicon (a-Si:H) at a rate of up to 27 Å/s and microcrystalline (μc) Si at rates up to 20 Å/s. A large laminar gas flow is the key to high etch rates. It provides for a fast transport of the etch products out of the reaction zone and thereby avoids redeposition. The etch rate increases with pressure and with H2 gas flow. Likewise, the etch rate rises with the filament temperature and saturates at a filament temperature of approximately 2150°C when approaching the maximum H2 dissociation probability. The decrease of the etch rate at higher substrate temperatures is attributed to the loss of the surface coverage by atomic hydrogen. The etch selectivity between a-Si:H and μc-Si drops at elevated substrate temperatures. Boron doping decreases the etch rates both for a-Si:H and μc-Si, whereas phosphorous doping does not significantly affect it. This etch selectivity is caused by a catalytic effect of BH3 on the surface hindering the formation of the main etch product silane. Even for highest etch rates no surface roughening of a-Si:H occurs, however, a bond structure modification of the near surfaces arises, an effect which results in the formation of a nanocrystalline surface layer. The increase of the μc-Si etch rate close to the film substrate interface characterizes the film thickness at which the coalescence of the microcrystalline nuclei starts.
Bidimensional Numerical Analysis of A μc-Si:H P-I-N Photodiode under Local Illumination
Authors: Fantoni A. , Vieira M. , Martins R.
Published: April 2011

The behaviour of a inicrocrystalline p-i-n junction locally illuminated with monochromatic radiation (incident power of 50 mW/cm2) is analysed by means of numerical experiences. The model used for the two-dimensional analysis of the transport properties of a μc-Si:H p-i-n photo-detector is based on the simultaneous solution of the continuity equations for holes and electrons together with the Poisson's equation. The solution is found on a rectangular domain, taking into account the dimension perpendicular to the junction plane and one on the parallel plane. The lateral effects occurring wimin the structure, due to the non-uniformity of the illumination, are outlined. The results we present show that the potential profile has a linear variation from the illuminated to the dark neutral region. The lateral components of the electric field and of the current density vectors reveal to be mainly localised inside the doped layers.
VHF Plasma Deposition of μc-Si p-Layer Materials
Authors: Deng X. , Jones S. J. , Liu T. , Izu M. , Ovshinsky S. R. , Hoffman K.
Published: April 2011

Microcrystalline silicon (μc-Si) p-layers have been widely used in amorphous silicon (a-Si) solar cell research and manufacturing to achieve record high solar cell efficiency. In order to further improve the solar cell performance and achieve wider parameter windows for the process conditions, we studied the deposition of high quality μc-Si p-layer material using a very high frequency (VHF) plasma enhanced CVD process. A design of experiment (DOE) approach was used for the exploration and optimization of deposition parameters. The usage of DOE leads to a quick optimization of the deposition process within a short time frame. In addition, by using a modified VHF deposition process, we have improved the solar cell blue response which leads to a 6–10% improvement in the solar cell efficiency. Such an improvement is likely due to an improved microcrystalline formation in the p-layer.
Influence of Front Contact Material on Silicon Heterojunction Solar Cell Performance
Authors: Rizzolp R. , Galloni R , Summonte C. , Pinghin R. , Centurioni E. , Zignan F. , Desalvo A. , Rava P. , Madan A.
Published: April 2011

The emitter of amorphous/crystalline silicon heterojunction (HJ) solar cells is normally very thin. Consequently, the metal used as a front contact can produce a partial or even total depletion of this layer. As a result, the diffusion potential of the p-n junction deviates from its maximum value. In this paper, we report the results concerning HJ in which either metal dots (Au, Al), semitransparent metal layers, or indium tin oxide (ITO) dots or layers were used as front contact on the same HJ structure, namely (p)a-Si:H / (i)a-Si:H / (n)c-Si / Al. We show that, for thin p-layers, the dark and light J-V characteristics of HJ solar cells depend on the material used as front contact. In particular, we found that the dark saturation current increases if a low work function material is used. This increase is interpreted in terms of p-layer depletion, and is shown to directly influence the J-V characteristics under illumination, producing a reduction of the open circuit voltage of solar cells.
Stability of Single and Tandem Junction A-Si:H Solar Cells Grown using the ECR Process
Authors: Dalal Vikram L. , Maxson Tim , Girvan Robert , Haroon Sohail
Published: April 2011

We report on the fabrication and stability tests of single junction a-Si:H, and tandem junction a-Si:H/a-Si:H solar cells using the ECR process under high hydrogen dilution (H-ECR process). We show that devices with high fill factors can be made using the H-ECR process. We also report on the stability studies of the solar cells under 1 and 2-sun illumination conditions. The solar cells show very little degradation even after 500 hours of illumination under 2 x sunlight illumination.
Modeling and Scaling of a-Si:H and Poly-Si Thin Film Transistors
Authors: Shur M. S. , Slade H. C , Ytterdal T. , Wang L. , Xu Z. , Hack M. , Aflatooni K. , Byun Y. , Chen Y , Froggatt M. , Krishnan A. , Mei P. , Meiling H. , Min B.-H. , Nathan A. , Sherman S. , Stewart M. , Theiss S.
Published: April 2011

We have developed analytic SPICE models for hydrogenated amorphous silicon (a-Si:H) and polysilicon (poly-Si) thin film transistors (TFTs) which accurately model all regimes of operation, are temperature dependent to 150°C, and scale with device dimensions. These models have been presented in [1, 2]. In this work, we compare the current-voltage characteristics predicted by our models with the measured characteristics from TFTs fabricated at different foundries. We compare the extracted device parameters in order to evaluate the robustness of our models and to determine a suitable default parameter set. We also use the models to examine the effects of device scaling for short channel TFTs. The models can be accessed using the circuit simulator AIM-Spice [3], which is available at http://nina.ecse.rpi.edu/aimspice.
Prospects for Active Matrix Displays on Plastic
Authors: Gates Stephen M.
Published: April 2011

We review the status of active matrix display fabrication on plastic substrates, specifically active matrix liquid crystal displays (AM LCDs). Research efforts in the U.S. and Europe are surveyed. Currently available transparent plastics, and active programs to improve these materials are described. Our work on thin film transistor (TFT) fabrication at 125°C is then reported, with a comparison of reactive sputtering and plasma enhanced CVD methods.
Change in The Spectral Shape of the Subgap Absorption in a-SI:H by Photodegradation at 4.2K and its Relation to the Mobility-Lifetime Product
Authors: Stradins P. , Fritzsche H.
Published: April 2011

We have observed a pronounced change in the shape of subgap absorption spectrum α(hν) of a-Si:H due to photodegradation at T=4.2K, in contrast to light exposure at T=300K. During the 4.2K exposure, α(hν) at 1.1≤(h‥)≤1.6eV increases with time as (α(t)-α(0)) ∝t0.3. However, for hν≤1eV the absorption α(hν) stays constant or decreases with exposure at 4.2K. Subsequent anneal causes the subgap absorption at the higher photon energies to decrease monotonically while a (hν ≤ 1.1 eV) increases first and goes through a maximum at T≈350K. This unexpected increase in a(hν) during annealing occurs at the anneal temperatures where the degraded mobility-lifetime product recovers. We suggest that the nonmonotonic recovery of a (hν≤ 1.1eV) during annealing is associated with preferential anneal of deeper defects or with structural relaxations around the defects during anneal causing a shift in their energy levels.
Via-Hole Addressed TFT and Process for Large-Area A-Si:H Electronics
Authors: Gleskova H. , Wagneri S. , Shen D. S.
Published: April 2011

We demonstrate a new technology for RC gate delay reduction, by fabricating an array of amorphous silicon thin-film transistors (a-Si:H TFTs) on a thin glass substrate provided with via holes. All gates are connected through via holes to a metal line that is run on the back side of the substrate. We opened via holes with a diameter of 35 to 50 μm in 50 μm glass foil. For the first time, all TFT pattern definition steps used a process which employs electrophotographic toner masks.
Amorphous Silicon Photodiodes for Replacing Degenerated Photoreceptors in the Human Eye
Authors: Schubert M. B. , Hierzenberger A. , Baumung V. , Wanka H. N. , Nisch W. , Stelzle M. , Zrenner E.
Published: April 2011

An ambitious neurotechnology program has been started in Germany in 1995, in the long run aiming at the realization of visual prostheses for blind people. A broad technological approach has been chosen which besides crystalline silicon microelectronics also involves amorphous silicon photodiodes for subretinal implantation, because thin film technology may be able to offer better solutions to some of the complex problems. Special topics we report on here include the development of low temperature deposition techniques for enabling the use of flexible plastic or bio-degradable substrate foils, the study of protective and bio-compatible coatings, as well as novel contacting and energy supply schemes. The key issue for stimulating retina cells by the use of technically generated photocurrents is an optimum capacitive coupling to these cells. For this purpose we study several contact layers (p-doped a-Si:H, microcrystalline Si, metal-induced crystallization) which provide high perpendicular but at the same time low lateral conductivity, thereby greatly reducing parasitic losses to the surrounding tissue. The photovoltaic mode of operation of the implanted photodiodes may be limited due to shortcomings in the charge transfer to the nerve cells, in which case additional infrared energy has to be coupled into the devices. Local light-induced stimulation can then be realised by using an a-Si:H i-layer as a photoresistor on top of the IR-sensitive solar cell.
Hydrogenated Amorphous Silicon Photoresists for Hgcdte Patterning
Authors: Hollingsworth R. E. , DeHart C. , Wang Li , Dinan J. H. , Johnson J. N.
Published: April 2011

A process to use a hydrogenated amorphous silicon (a-Si:H) film as a dry photoresist mask for plasma etching of HgCdTe has been demonstrated. The a-Si:H films were deposited using standard plasma enhanced chemical vapor deposition with pure silane as the source gas. X-ray photoelectron spectra show that virtually no oxide grows on the surface of an a-Si:H film after 3 hours in air, indicating that it is hydrogen passivated. Ultraviolet light frees hydrogen from the surface and enhances the oxide growth rate. A pattern of 60 micron square pixels was transferred from a contact mask to the surface of an a-Si:H film by ultraviolet enhanced oxidation in air. For the conditions used, the oxide thickness was 0.5–1.0 nm. Hydrogen plasmas were used to develop this pattern by removing the unexposed regions of the film. A hydrogen plasma etch selectivity between oxide and a-Si:H of greater than 500:1 allows patterns as thick as 700 nm to be generated with this very thin oxide. These patterns were transferred into HgCdTe by etching in an electron cyclotron resonance plasma. An etch selectivity between a-Si:H and HgCdTe of greater than 4:1 was observed after etching 2500 nm into the HgCdTe. All of the steps are compatible with processing in vacuum.
Light Induced Effects in A-Si:H Films Alloyed with Sulfur
Authors: Yoon Jong-Hwan , Taylor P. C. , Yan Baojie , Lee Czang-Ho
Published: April 2011

Light-induced effects are studied in hydrogenated amorphous silicon-sulfur alloys (a-SiSx:H) and compared to those that exist in a-Si:H. The a-SiSx:H films were grown by decomposition of pre-mixtures of SiH4 and H2 S. The light-induced effects were monitored using electrical (dark conductivity and photoconductivity, including the constant photocurrent method [CPM]) and optical (photoluminescence) measurements and electron spin resonance. It is found that sulfur alloying results in a significant reduction in the degradation in the dark- and photo-conductivity. For ana-SiSx:H film grown with a gas mixture of H2 S/SiH4= 0.02, there is an increase of over an order of magnitude in the dark conductivity and a small decrease in the photoconductivity after 50 hours of light soaking. The subgap deep defect density as measured by CPM increases with illumination time, following a stretched exponential to saturation. The saturated defect density is an order of magnitude higher than that observed in the annealed state.
Control of Valence States by a Codoping Method in P-Type GaN Materials
Authors: Yamamoto T. , Katayama-Yoshtoa H.
Published: April 2011

We propose a new valence control method, the “codoping method (using both n- and p-type dopants at the same time)”, for the fabrication of low-resistivity p-type GaN crystals based on the ab-initio electronic band structure calculations. We have clarified that while doping of acceptor dopants, BeGa and MgGa, leads to destabilization of the ionic charge distributions in p-type GaN crystals, doping of Sica or ON give rise to p-type doped GaN with high doping levels due to a large decrease in the Madelung energy. The codoping of the n- and p-type dopants (the ratio of their concentrations is 1:2) leads to stabilization of the ionic charge distribution inp-type GaN crystals due to a decrease in the Madelung energy, to result in an increase in the net carrier densities.
Polar-Twinned Defects in LiGaO2 Substrates Lattice Matched with GaN
Authors: Ishii Takao , Tazoh Yasuo , Miyazawa Shintaro
Published: April 2011

We investigated the Czochralski growth of LiGaO2 single crystal for use as a substrates for the epitaxial growth of hexagonal GaN. Crossed lines were observed in mechano-chemically polished {001} wafers sliced from a (001) axis boule. Chemical etching revealed that there exists a difference in chemical stability between two domains separated by a crossed line. Since LiGaO2 single crystal is polar along the c-axis, the formation of multi-domain structure is due to the polarity inversion of the c-axis, that is, polar twinned defects. We found that the GaN thin film on the (001) substrate with a multi-domain structure peeled off and that this is closely related with multi-polarity of substrate.
Substrate Effects on the Growth of InN
Authors: Donovan S. M. , MacKenzie J. D. , Abernathy C. R. , Pearton S. J. , Holloway P. , Ren F. , Zavada J. M. , Chai B.
Published: April 2011

Auger electron spectroscopy was used to examine the nitridation behavior of GaAs, sapphire and lithium aluminate (LAO) substrates exposed to an RF nitrogen plasma. No evidence of nitridation was found for the sapphire and LAO substrates. GaAs substrates did show evidence of nitridation which led to smooth InN surface morphology without the need for a low temperature buffer. Comparable InN films were obtained on sapphire and LAO substrates when a low temperature AlN buffer was used. Hall measurements indicate background carrier concentrations are relatively insensitive to substrate type, though mobilities decreased as surface morphology was improved.
Initial Stages of MOCVD Growth of Gallium Nitride Using a Multi-Step Growth Approach
Authors: Kobayashi J. T. , Kobayashi N. P. , Dapkus P. D. , Zhang X. , Rich D. H.
Published: April 2011

A multilayer buffer layer approach to GaN growth has been developed in which the thermal desorption and mass transport of low temperature buffer layer are minimized by deposition of successive layers at increased temperatures. High quality GaN with featureless surface morphology has been grown on (0001) sapphire substrate by metalorganic chemical vapor deposition using this multilayer buffer layer approach. The lateral growth and coalescence of truncated 3D islands (TTIs) nucleated on low temperature buffer layers at the initial stage of overlayer growth is affected by the thickness of the final buffer layer on which nucleation of TTIs takes place. The effect of the thickness of this buffer layer on the quality of GaN is studied by using scanning electron microscopy, van der Pauw geometry Hall measurements and cathodoluminescence and an optimum value of 400Å is obtained.
Mechanical Properties of Gallium Nitride and Related Materials
Authors: Drory M. D.
Published: April 2011

Basic mechanical properties, such as hardness and fracture toughness are examined for GaN and related materials. The mechanical properties are explored by indentation with a diamond (Vickers) pyramid under less than 20N load. Testing on bulk single crystal GaN indicated hardness and fracture toughness similar to GaP, but with much greater values than GaAs. Data for compound semiconductors are compared with values of a number of substrate materials. The following materials are examined in bulk form: AlN, Al2O3 (sapphire), Al2O3-TiC (ALTIC), GaAs, GaN, GaP, Ge, Si, SiC, ZnS, and ZnSe.
Resonant Raman Scattering in GaN/Al0.15 Ga0.85N and InyGa1-yN/GaN/AlxGa1-xN Heterostructures
Authors: Behr D. , Niebuhr R. , Obloh H. , Wagner J. , Bachem K. H. , Kaufmann U.
Published: April 2011

We report on resonant Raman scattering in Al0.15Ga0.85N/GaN single quantum wells (QWs) and AlxGa1-xN/GaN/lnyGa1-yN heterostructures. By choosing appropriate excitation conditions we could probe selectively the GaN quantum well or the Al0.15Ga0.85N barrier of Al0.15Ga0.85N/GaN single quantum wells. For the InxGa1-xN material system a linear frequency shift of the E2- and A1(LO) phonon mode to lower frequencies was found with increasing In content. The shift was determined to -0.79cm-1 per % In content for the A,(LO) phonon frequency. Resonant excitation of AlxGa1-xN/GaN/InyGa1-YN heterostructures enabled us to detect phonon signals from the InxGa1-xN layer in the heterostructure and to determine its In content.
Electron-Phonon Scattering in Very High Electric Fields
Authors: Ridley B. K.
Published: April 2011

Large-bandgap materials can support very high electric fields (>lMV/cm) without breaking down. The possibility then exists for an electron in a conduction band to become quasi-localized in Wannier-Stark states, a possibility that depends on the scattering rate being less than the Bloch oscillation frequency. If this condition is met the scattering rate itself is affected and the description of transport must be changed from the usual model in which the electron is assumed to be virtually free. Here, we examine the feasibility of obtaining this condition in GaN using a simple three-dimensional tight-binding model for the bandstructure and taking the dominant scattering mechanism to be the polar and non-polar interaction with optical phonons and short-wavelength acoustic phonons.
Optical-Gain Measurements on GaN and Alx Ga1-xN Heterostructures
Authors: Eckey L. , Holst J. , Kutzer V. , Hoffmann A. , Broser I. , Ambacher O. , Stutzmann M. , Amano H. , Akasaki I.
Published: April 2011

Optical gain processes in thin GaN and AlGaN are compared by means of gain spectroscopy using the stripe length method and high-excitation photoluminescence, both performed at various densities and temperatures. We find that inelastic excitonic scattering processes and biexciton decay are important at low temperatures and low excitation densities Both materials are similar in that increasing the excitation density results in gain spectra dominated by the electron-hole plasma and phonon-assisted band-to-band recombination. These also prevail at high temperatures.
Hrem and CBED Studies of Polarity of Nitride Layers with Prismatic Defects Grown Over SiC
Authors: Vermaut P. , Ruterana P. , Nouet G. , Salvador A. , Morkog H.
Published: April 2011

The polarity of GaN films and their AlN buffer layer grown on (0001)si 6H-SiC by an electron cyclotron resonance plasma enhanced molecular beam epitaxy has been investigated by convergent beam electron diffraction (CBED) and high resolution electron microscopy (HREM). The experimental results are in good agreement with the simulations and allow to determine that the free surfaces of the GaN and AlN layers are Ga and Al-terminated respectively. Moreover, (1210) prismatic planar defects observed in the AlN layers have been identified as stacking faults and observations in different areas of the specimens have shown that the layers are unipolar.
Valence Band Physics in Wurtzite GaN
Authors: Azuhata T. , Sota T. , Chichibu S. , Kuramata A. , Horino K. , Yamaguchi M. , Yagi T. , Nakamura S.
Published: April 2011

We present a summary of recent progress towards the understanding of the valence-band physics in wurtzite GaN. Systematic studies have been performed on the strain dependence of the free exciton resonance energies by photoreflectance measurements using well-characterized samples. Analyzing the experimental data with the Hamiltonian appropriate for the valence bands, the values have been determined of the crystal field splitting, the spin-orbit splitting, the shear deformation potential constants, and the energy gap in the unstrained crystal. Discussions are given on the strain dependence of the energy gaps, of the effective masses, and of the binding energies for the free exciton ground states as well as on the valence band parameters. Using the obtained values and the generalized Elliott formula, the fundamental optical absorption spectra obtained experimentally were analyzed. The values of the elastic stiffness constants, which play a crucial role to determine the shear deformation potential constants, are also given.
Electrical and Optical Properties of InGaN/AlGaN Double Heterostructure Blue Light-Emitting Diodes
Authors: Yang K. , Shi H. T. , Shen B. , Zhang R. , Chen Z. Z. , Chen P. , Zheng Y. D.
Published: April 2011

In this paper, we studied the electrical and optical characteristics of Nichia double heterostructure blue light-emitting diodes, with In0.06Ga0.94N:Zn, Si active layer, at 77 and 300 K. Measurement of the forward bias current-voltage behavior of the device demonstrates a departure from the Shockley model of p-n diodes, and it is observed that the dominant mechanism of carrier transport across the junction is associated with carrier tunneling. Electroluminescence experiments of the devices were performed. We obtained an emission peak located at 2.80 eV, and a relatively weaker short-wavelength peak of 3.2 eV. A significant blue shifts of the optical emission peak which is consistent with the tunneling character of electrical characteristics was observed. Furthermore, we studied the properties of electroluminescence under various pulsed currents, and a degradation in I-V characteristics and a low resistance ohmic short were observed.
Effect of Hydrogen Chloride on the Capacitance-Voltage Characteristics of MOCVD-Grown AlN/6H-SiC Mis Structures
Authors: Tin C. C. , Gichuhi A. , Bozack M. J. , Shannon C. G. , Teh C. K.
Published: April 2011

Aluminum nitride (AlN) is a promising material as gate insulator for 6H-SiC metal-insulator-semiconductor (MlS)-based devices. Using metalorganic chemical vapor deposition (MOCVD)-grown AlN, we have recently fabricated Au/AlN/6H-SiC MIS structures with different AlN/6H-SiC interfacial characteristics depending on the AlN growth procedures. We have also found that the use of hydrogen chloride gas is effective in improving the capacitance-voltage characteristics of the AlN/6H-SiC structure. The reason for such improvement is not well understood and several possible mechanisms for such improvement include factors such as substrate surface morphology and surface contaminants. In this paper, we will examine the relationship between surface morphology of the substrates and the capacitance-voltage characteristics of Au/AlN/6H-SiC structures.
Theory of Gain in Group-HI Nitride Lasers
Authors: Chow W. W. , Wright A. F. , Girndt A. , Jahnke F. , Koch S. W.
Published: April 2011

A microscopic theory of gain in a group-III nitride quantum well laser is presented. The approach, which treats carrier correlations at the level of quantum kinetic theory, gives a consistent account of plasma and excitonic effects in an inhomogeneously broadened system.
Growth and Characterization of Thermal Oxides on Gallium Nitride
Authors: Wolter Scott D. , Mohney Suzanne E. , Venugopalan Hari , Waltemyer Debra L. , Luther Brian P.
Published: April 2011

Little information is available about the thermal oxidation of GaN. Since GaN is of interest for high temperature electronics, knowledge of the stability of GaN in potentially oxidizing environments would be useful. Furthermore, evaluation of the characteristics of the thermal oxide will provide information needed for assessing the potential of this oxide in processing or device applications.

In this study, thick GaN epilayers and GaN powders were exposed to dry air at 450°C, 750°C, 900°C, 925°C, 950°C, and 1000°C for periods of 1 to 25 hours. Following oxidation, the epilayers were analyzed by x-ray photoelectron spectroscopy and glancing incidence x-ray diffraction, and the powders were analyzed by conventional x-ray diffraction. For both the GaN films and powders, significant oxidation was observed at 900°C, and the oxide was identified as monoclinic β-Ga2O3. Oxidation in dry air resulted in roughening of the oxide/GaN interface and oxide surface. In the temperature range 900°C to 1000°C, linear kinetics were observed for times up to 10 hours indicating an interfacial reaction mechanism as the rate limiting step for oxidation. An apparent activation energy of ∼72 kcal/mole was determined for this process.
Surface Preparation and Growth Condition Dependence of Cubic GaN Layer on (001) GaAs by Hydride Vapor Phase Epitaxy
Authors: Tsuchiya H. , Sunaba K. , Yonemura S. , Suemasu T. , Hasegawa F.
Published: April 2011

GaN buffer layers and thick GaN layers were grown on (001) GaAs substrates by hydride vapor phase epitaxy. The ratio of cubic to hexagonal components in the grown layer was estimated from the ratio of the integrated X-ray diffraction intensities of the cubic (002) plane and hexagonal (1011) planes measured by w scan. The optimum growth conditions were thermal cleaning at 600°C, growth temperature of 500°C and thickness of 30 nm for the buffer layer, and the Will ratio of 300 for thick GaN growth at 800°C. Cubic component in the layer grown with those conditions was more than 85% and strong cubic photoluminescence emission was observed at 377 nm (3.28 eV).
MBE-Growth of Strain Engineered GaN Thin Films Utilizing a Surfactant
Authors: Klockenbrink R. , Kim Y. , Leung M. S.H. , Kisielowski C. , Krüger J. , Sudhir G. S. , Rubin M. , Weber E. R.
Published: April 2011

GaN films were grown on sapphire substrates at temperatures below 725 °C utilizing a Constricted Glow Discharge plasma source. A three dimensional growth mode is observed at such low growth temperatures resulting in films that are composed of individual but oriented grains. The strain that originates from the growth on the lattice mismatched substrate with a different thermal expansion coefficient is utilized to influence the thin film growth. The strain can be largely altered by the growth of suitable buffer layers. Thereby, optical and structural film properties can be engineered. It is argued that the surface diffusion of Ga ad-atoms is affected by engineering the strain. Alternatively, surface diffusion can be influenced by surfactants. It is demonstrated that the use of bismuth as a surfactant allows to modify the surface morphology of the GaN films that reflects the size of the grains in the films. The results suggest that a substantial increase of the oriented grain sizes in the films is possible while maintaining a low growth temperature.
Quasi-Thermodynamic Analysis of Metalorganic Vapor Phase Epitaxy of GaN
Authors: Duan Shukun , Lu Dacheng
Published: April 2011

A thermodynamic analysis of GaN grown by MOVPE has been proposed based on quasi-thermodynamic equilibrium established on the solid-vapor interface. Phase diagrams for the MOVPE growth of GaN using TEGa and NH3 has been calculated. The phase diagram is consists of four phases regions: the region for single condensed phase of GaN, the region for double condensed phase of GaN (s) +Ga(l), the etching region with Ga droplets and the etching region without Ga droplets. The effect of growth temperature, reactor pressure, content of carrier gas, deposition ratio of NH3 and V/III ratio upon growth of GaN using MOVPE has been studied.
Modeling and Model Validation for RTP Design
Authors: Fordham M. , Pan M. , Hu J. , Sorrell F. Y.
Published: April 2011

Spectral and spectral band irradiance sensors are developed to measure the spatial heat flux distribution from a rapid thermal processor (RTP) lamp bank. They are used in conjunction with a Sensarray thermocouple wafer to evaluate the thermal uniformity of one axially symmetric lamp bank from a 3 zone RTP system. Measured performance of the lamp bank differs sharply from predictions obtained using a numerical ray trace model. Discrepancies between experimental measurements and model results are resolved by blackening a beveled (focusing) surface introduced to shield process lamp bases from overheating. Further modeling confirms that the bevel causes undesirable focusing of lamp radiation on the planar wafer surface. The shape of the focused component of the irradiance profile is highly dependent on the lamp radiance spatial distribution assumed in the model.
Integrated Rapid Thermal CVD Processing Solutions for 0.18–0.25μm Technologies
Authors: Gilboa H. , Gilboa Y. E. , Atzmon Z. , Levy S. , Spilberg H. , Bransky E. , Doitel Z. , Thakur R.P.S. , Weimer R. A. , Deboer S. , Pan J.
Published: April 2011

The evolution of integrated single-wafer processing for high-temperature applications in the front end of the line (FEOL) occurred with the advancements in single-wafer rapid thermal processing and its acceptance as a manufacturing technology. The Integra RTCVD cluster tool for high-temperature applications features wafer cleaning, rapid thermal processing and single wafer chemical vapor deposition steps. The paper presents integrated vapor phase clean and RTCVD applications for FLASH memory gate stack and DRAM cell.
3.3μm Pyrometry In Single Sided Rta From 400–700°C Using In-Situ Measurement Of Reflection And Transmission
Authors: Marcy D. L. , Chial S. , Beneš M. , Sturm J. C.
Published: April 2011

Pyrometry of silicon wafers under 700°C at wavelengths over 1μm is difficult because lightly doped wafers become partially transparent. In this work, a modified commercial RTCVD reactor with 8” wafer capability was used to study the temperature measurement of Si wafers over the range of 400–700°C using top and bottom pyrometric detectors. We present initial results on measurements of both reflection and transmission in-silu to determine emissivity at 3.3μm. For heavily doped wafers emissivity was independent of temperature and the measured temperature by pyrometry agreed well with that measured by thermocouple for 400–700°C. For lightly doped wafers, emissivity was temperature dependent due to the increased transparency of the wafer at low temperatures. Using fixed emissivity, the measured temperature severely underestimates the actual temperature below 550°C. By calculating emissivity from the measured reflection and transmission, accurate temperature measurement was achieved from 400–700°C without any a priori knowledge of the wafer.
Physical and Electrical Properties of Defects Formed in Rapid Thermal Processing
Authors: Takáno Yukio , Kakumoto Katsunori , Funakoshi Tomoya
Published: April 2011

RTP will be replaced with some of the conventional thermal processing employed in ULSI fabrication lines in near future. We show at first the device characteristics demanded for next generation DRAM which is a typical example of ULSIs and some issues to satisfy the demands. Next we show some candidates for RTP in the ULSI processes and discuss difference between RTP and the conventional thermal processes. We think one of the largest difference is the quenching Si wafers after short time annealing and by the quenching the deep levels due to fast diffusing atoms and point defects in Si are introduced. Experimental results of N2 and Cu related deep levels are shown as the examples of the deep levels induced by the quenching in Si. Finally, we propose the gettering method for them in RTP.
A Reversible Effect of Rapid Thermal Annealing of Indium in Ion Implanted Silicon
Authors: Krasnobaev L. Y. , Cuomo J. J.
Published: April 2011

Si was implanted with 115In+ ions with a dose exceeding the value of the In solid solubility. Rapid thermal annealing with W-halogen lamp was done sequentially with the wafer facing in two different positions relative to the source of the light irradiation. It has been experimentally found that short and long wavelength components of incoherent light act in a different manner on the In atom activation in Si during RTA process. The effect of In activation and deactivation caused by the RTA treatment is reversible in the temperature region of 440–1020°C. The effect of In activation and deactivation exists after thirty RTA pulses.
Electrical Characteristics of New LDD Poly-Si TFT with MIS-Alignment Tolerant Structure for AMLCDs
Authors: Min Byung-Hyuk , Kanicki Jerzy
Published: April 2011

A new LDD poly-Si TFT structure having symmetrical electrical characteristics independent of the process induced mis-alignment is described in this paper. Based on the experimental results, we have established there is no difference between the forward and reverse characteristics and a low leakage current, comparable to a conventional LDD poly-Si TFT, has been maintained for this new poly-Si TFTs. The maximum ON/OFF current ratio of about 1×108 is obtained for the LDD length of 1.0 μm. In addition, the kink effect in the output characteristics is remarkably improved in the new TFTs in comparison to the conventional non-LDD single- or dual-gate TFTs.
Photo and Thermal Stability of Chlorine Doped Amorphous Silicon TFTs
Authors: Kim J. H. , Choi J. H. , Kim C. W. , Souk J. H.
Published: April 2011

The Cl doped amorphous silicon film was deposited from SiH4, SiH2C12 and H2 gas mixture by PECVD process and their properties with various SiH2C12 flow rate is discussed. Controlling the amount of Cl doped was crucial for acquiring high stability TFT without lowering on current. Amorphous silicon and Cl doped amorphous silicon double layer TFT structure, with satisfactory on current and throughput, proved to be more stable than conventional amorphous silicon TFT in photo state and in elevated temperatures.
Photoluminescent Mn-Doped ZNS Nanoclusters Synthesized within Block Copolymer Nanoreactors
Authors: Kane R. S. , Cohen R. E. , Silbey R. J. , Kuno M. , Bawendi M. G.
Published: April 2011

Mn-doped ZnS nanoclusters were synthesized within microphase-separated films of diblock copolymers containing carboxylic acid units on one of the blocks. Zinc was selectively sequestered into the acid-containing domains by treating the films with diethylzinc or an aqueous zinc acetate solution. Manganese was loaded by subsequent treatment of the films with aqueous manganese acetate solutions. The manganese ions displaced a fraction of the zinc ions, and the extent of loading was controlled by varying the loading time, and the concentration of the manganese acetate solution. The extent of loading was tracked using ICP discharge emission spectroscopy. The doped nanoclusters were formed by subsequent treatment with H2S. The size of the doped ZnS nanoclusters could be varied by carrying out the H2S exposure in the presence of a coordinating base. Photoluminescence measurements revealed that the doped nanoclusters were photoluminescent, and showed the characteristic manganese emission. The universal doped cluster synthesis scheme used is quite general and can be easily modified to produce other doped nanocluster clusters, such as Tb-doped ZnS, or Mn-doped CdS.