bluuebunny/crossref_metadata_2025 huggingface dataset:
Threshold Voltage and Field for Metal Filament Formation in Hydrogenated Amorphous Silicon
Authors: Stradins P. , Branz H. M. , Jackson W. B. , Crandall R.S. , Hu J. , Wang Q.
Published: April 2011

Electrical switching due to metallic filament formation in hydrogenated amorphous silicon (a-Si:H) is studied in metal/a-Si:H/metal structures. We examine the effects of a-Si:H switch layer thickness, applied voltage and polarity, metal contact material, and contact interface properties. For switching, the voltage applied to the contacts must be large enough to establish: 1) a minimum threshold voltage of about 2V at the contacts and 2) a bias field of about 1 MV/cm in the bulk. Changing contact material and polarity strongly affects the switching behavior.
Suppression of Staebler-Wronski Effect Induced Electrical Crosstalk in a-Si:H-Based Image Sensors
Authors: Theil Jeremy A.
Published: April 2011

Hydrogenated amorphous silicon photodiodes have been considered for use in array-based image sensors. They promise to significantly reduce the size and cost of CMOS image sensors, while offering the promise of improved pixel sensitivity. However, Staebler-Wronski Effect (SWE) based electrical crosstalk degradation has been a major concern in their acceptance, due to degraded spatial contrast and color fidelity. Since the SWE is a fundamental mechanism of a Si:H, solutions to this issue must look to ways of mitigating the SWE on diode array performance rather than elimination of SWE. In order to study electrical crosstalk, a novel device structure that inhibits light from reaching portions of the a-Si:H/dielectric interface was designed and fabricated to directly measure interpixel leakage currents. Results from these structures indicate that edge leakage can be a significant contributing component to the measured signal. In addition, a CMOS-compatible structure to suppress electrical crosstalk was designed and fabricated. Results from these structures demonstrate suppression of crosstalk up to lateral electric fields of at least 2 x 104 V/cm. Such suppression is adequate for densely packed minimum-size pixel arrays. Aspects of the design and implementation of the structure will also be discussed.
Low Temperature (<150°C) Doping Techniques for Polysilicon TFT's
Authors: Hong W.S. , Kim J.M. , Han S.H. , Lee Y.H. , Kim Y.W. , Lee S.H. , Kim D.Y.
Published: April 2011

Doping of polysilicon (poly-Si) films was performed at a low temperature (<150°C), by using three different dopant incorporation methods: ion shower, dopant layer deposition and plasma immersion. All three techniques were shown to be capable of obtaining sheet resistance values that were smaller than 104 Ω/sq., which were considered to be sufficient to form good source-drain contacts. Also, a sheet resistance value that is as low as 300 Ω/sq. was demonstrated. It was found that the laser energy used for dopant activation was the major parameter to control the sheet resistance of the poly-Si films. The lowest attainable sheet resistance was not affected much by the ion dose, as long as the initial dose is higher than 1015 cm−2. The plasma immersion method was shown to be a good alternative to the ion shower, as the doping could be performed in a relatively short time without causing a structural damage to the poly-Si film.
Silicon-Hydrogen Bonds in Boron and Phosphorous Doped Polycrystalline Silicon Thin Films
Authors: Saleh R. , Nickel N. H.
Published: April 2011

Hydrogen bonding in laser crystallized boron and phosphorous doped polycrystalline silicon is investigated using Raman spectroscopy and hydrogen effusion measurements. During laser crystallization the intensity of the local vibration modes near 2000 and 2100 cm−1 decreases. The intensity of vibration mode at 2000 cm−1 decreases faster than the one at 2100 cm−1. From H effusion measurements, the hydrogen density-of-states (H DOS) distribution is derived. For undoped amorphous silicon the H DOS exhibits two prominent peaks at hydrogen binding energies of E– μH = –1.1 and –1.5 eV. In B doped a-Si:H the peak at –1.1 eV is less pronounced while in P doped a-Si:H the H binding energy increases by about 0.1 eV. In all samples laser crystallization causes an increase of the H binding energy by about 0.2 – 0.3 eV. However, the peaks in the H DOS observed in B-doped samples are preserved during laser crystallization.
CW Argon-ion laser initiated aluminum induced crystallization of amorphous silicon thin films
Authors: Paduru Sampath K. , Abu-safe Husam H. , Naseem Hameed A. , Al-Shariah Adnan , Brown William D.
Published: April 2011

CW Argon-ion laser initiated aluminum induced crystallization (AIC) of RF magnetron sputtered amorphous silicon (a-Si) thin films has been investigated. It was found that lasers could be effectively used to initiate AIC process at very low threshold power densities. An argon-ion laser (λ=514.5 nm) was used to anneal Al/a-Si/glass structures with varying power densities ranging between 55 and 125 W/cm2 and exposure times ranging from 10 to 120 s. X-ray diffraction analysis showed the resulting films to be polycrystalline. The crystallization rate increased both with power density and exposure time. Environmental scanning electron microscopy (ESEM) analysis showed that the surface features change with increasing power density and irradiation time. A dendritic growth pattern was observed in the initial stages of interaction between the films. A strong crystalline Raman peak at around 520 cm-1 was observed in the Raman spectra of the crystallized samples.
Defect Densities, Diffusion Lengths and Device Physics in Nanocrystalline Si:H Solar Cells
Authors: Dalal Vikram L. , Sharma Puneet , Staab David , Noack Max , Han Keqin
Published: April 2011

We report on the properties of nanocrystalline Si:H solar cells. The solar cells were of the p+nn+ type, with the n+ layer deposited first on a stainless steel substrates. The solar cells were prepared under high hydrogen dilution conditions using either ECR plasma deposition, or VHF diode plasma deposition processes. The deposition pressures were kept low, 5 mTorr in the ECR reactor and 50 mTorr in the VHF reactor. All the solar cells reported showed a high Raman ratio of crystalline to amorphous peaks. Properties such as dark current, deep level defects and shallow doping densities, and hole diffusion lengths were measured in these cells. It was found that the base layer was always n type, but that its doping could be changed by adding ppm levels of B during growth. A sufficient B doping even type converted the base layer to p type. It was found that there was a good one-to-one correlation between the shallow doping and deep level defects, suggesting that the same element, probably oxygen, is responsible for generating both shallow dopants and deep levels. The diffusion length of holes was measured in these cells using quantum efficiency vs. voltage techniques, and it was found that the diffusion length data could be explained very well by invoking trap-controlled recombination statistics. The dark I(V) curves could be represented by a standard diode model for highly crystalline materials, but as the degree of crystallinity was reduced, the diode factor increased. Voltage could be improved by reducing the crystallinity of the layer, but doing so resulted in a decrease in quantum efficiency in the infrared regions of the solar spectrum.
Protocrystalline Silicon at High Rate from Undiluted Silane
Authors: Schropp R.E.I. , Veen M.K. van , Werf C.H.M. van der , Williamson D.L. , Mahan A.H.
Published: April 2011

Hot Wire Chemical Vapor Deposition (HWCVD) is shown to be a fast method for the deposition of protocrystalline silicon films from undiluted silane. Intrinsic silicon-hydrogen films (2 μm thick) have been deposited by HWCVD on plain stainless steel as well as on stainless steel precoated with a n-type doped microcrystalline silicon layer. In X-ray diffraction experiments, the linewidths of the first sharp peak (FSP) were 5.59 ± 0.09 degrees and 5.29 ± 0.11 degrees, respectively, indicating improved medium-range order and a template effect due to the μc-Si:H n-layer. For thinner layers (0.7 μm thick), the linewidths of the FSP were 5.29 ± 0.09 degrees and 5.10 ± 0.09 degrees. These FSPs are as narrow as for optimized i-layers made by H2-diluted plasma deposition, however, at a much higher deposition rate (1 nm/s), at moderate temperature (250°C), and without the use of H2 dilution. In accompanying transmission electron micro-graphs, the layers show a significant concentration of elongated small voids in the growth direction that are not interconnected. Small Angle X-ray Scattering (SAXS) results are consistent with these observations. We suspect that the void nature allows the bulk of the film to be more ordered. The utilization of such layers in n-i- p solar cells on plain stainless steel leads to cells with a remarkably good stability, showing a decrease of the fill factor of less than 10 % during 1500 h of light soaking.
Understanding the structure of Si nanoclusters in a/nc-Si:H films using spherical aberration-corrected transmission electron microscopy
Authors: Perrey Christopher R. , Thompson Siri S. , Lentzen Markus , Kortshagen Uwe , Carter C. Barry
Published: April 2011

Recent work has shown that the electrical properties of hydrogenated amorphous Si films with nanocrystalline inclusions (a/nc-Si:H) make this material a promising candidate for applications in solar cells. The present study applies the technique of spherical aberration-corrected high-resolution transmission electron microscopy for the identification and analysis of the crystalline content of an a/nc-Si:H film. By varying both the spherical aberration of the objective lens and the defocus, regions of crystallinity in the a/nc-Si:H film can be identified. This study reports the analysis of Si nanoparticles of approximately 1.5 nm in size. Some of these nanoparticles contain planar defects, such as twin defects and stacking faults. All particles observed were the same crystal structure as bulk Si, which agrees with theoretical cluster calculations. Beam damage was observed in the amorphous matrix for long electron–beam exposures.
Formation of periodic grain boundary in an Si thin film crystallized by a linearly polarized Nd:YAG pulse laser with an ultra sonic oscillator
Authors: Kaki Hirokazu , Ootani Takehiko , Horita Susumu
Published: April 2011

In order to obtain a large silicon (Si) grain and to control the location of its boundary in a Si film melting-crystallized by a pulse laser, we have proposed to use periodic thermal distribution spontaneously induced by irradiation of a linearly polarized laser beam. We estimated the suitable amorphous Si (a-Si) thickness taking account of multiple reflection theoretically and confirmed it experimentally. Also, we proposed a novel technique to reduce the irradiation pulse number to control the grain boundary location stably in the crystallized Si film, in which the elastic wave was generated on the surface of a-Si film prior to melting-crystallization by using an ultra sonic oscillator. Owing to this technique, we can control the grain boundary location periodically with only 1 pulse irradiation in the crystallized Si film.
A photothermal method of simultaneous determination of ultra-shallow junction depth and abruptness
Authors: Salnick Alex , Nicolaides Lena , Opsal Jon , Jain Amitabh , Rogers Duncan , Robertson Lance
Published: April 2011

Thermal wave (TW) studies of ultra-shallow junctions (USJ) formed by ion implantation into a semiconductor wafer followed by rapid thermal annealing (RTP) are described. It is shown that using the TW technique allows for a simultaneous determination of the most important USJ parameters – depth and profile abruptness. Experimental results for junction depth and abruptness obtained on a set of B+-implanted, RTP-annealed USJ samples show better than 0.99 correlations to the corresponding secondary ion mass spectroscopy (SIMS) data.
Ultra-Shallow Junction Formation Technology from the 130 to the 45 nm node
Authors: Jain Amitabh
Published: April 2011

One of the main materials challenges of the 130 nm silicon technology node was the need to find a processing solution to the anomalous diffusion behavior of ion-implanted dopants known from three decades of research. Reduction of implantation energy no longer proved sufficient when trying to reduce source/drain extension junction depth, increase abruptness, and limit sheet resistance. Spike-annealing, a new process in which ion implanted silicon could be heated rapidly to temperatures required for dopant activation and then cooled down without dwelling at temperature, adequately addressed the scaling requirements of this node. The resulting junctions achieved high dopant concentration values very close to the surface while limiting junction depth. However, this increased the propensity for dopant migration to overlying layers associated with the source/drain spacer. Loss of device performance due to this and other phenomena became a strong motivating factor for further materials research in order to sustain progress through the 130 nm and 90 nm nodes. Complex interactions between various layers have been understood and the resulting developments in spacer materials have enabled high performance devices. The requirements of the 65 and 45 nm nodes stretch spike-annealing to its limit and newer Ultra-High Temperature anneals must be considered.
Optimization of Fluorine Co-implantation for PMOS Source and Drain Extension Formation for 65nm Technology Node
Authors: Graoui H. , Hilkene M. , McComb B. , Castle M. , Felch S. , Cowern N.E.B. , Al-Bayati A. , Tjandra A. , Foad M. A.
Published: April 2011

The main challenges for PMOS ultra shallow junction formation remain the transient enhanced diffusion (TED) and the solid solubility limit of boron in silicon. It has been demonstrated that low energy boron implantation and spike annealing are key in meeting the 90 nm technology node ITRS requirements. To meet the 65 nm technology requirements many studies have used fluorine co-implantation with boron and Si+ or Ge+ pre-amorphization (PAI) and spike annealing. Although using BF+2 can be attractive for its high throughput, self-amorphization and the presence of fluorine, many studies have shown that for the fluorine to successfully reduce TED its energy needs to be well optimized with respect to the boron's, therefore BF+2 does not present the right fluorine/boron energy ratio for the optimum junction formation. In this work we optimize the fluorine energy when a deep or shallow PAI is used. We also demonstrate that the fluorine dose needs to be carefully optimized otherwise a reverse effect can be observed. We will also show that the optimized junction depends less on the Ge+ energies between 2 keV and 20 keV and when HF etch is implemented after Ge+ PAI, improvements in both the junction depth and the sheet resistance are observed.
Suppression of Boron Diffusion by Fluorine Implantation in Preamorphized Silicon
Authors: Impellizzeri Giuliana , dos Santos José H. R. , Mirabella Salvatore , Priolo Francesco , Napolitani Enrico , Carnera Alberto
Published: April 2011

We have investigated the role of fluorine in the reduction of transient enhanced diffusion (TED) and thermal diffusion (TD) of B in preamorphized Si layers implanted with F. For this purpose, we have employed B delta-doped layers, grown by molecular beam epitaxy (MBE), as markers for silicon self-interstitials (Is). We have shown that boron TED decreases with increasing amount of incorporated F up to the complete TED suppression. Furthermore, we have clearly demonstrated that the physical mechanism that suppresses the boron TED is not a B-F chemical bonding, but a strong interaction between F atoms and Is. In addition, we have seen that fluorine strongly reduces B diffusion also under Is thermal equilibrium concentration. Our results clearly show that the presence of F lowers the Is density very effectively, reducing the boron TED as well as the dopant diffusion under equilibrium conditions.
Atomistic Analysis of the Role of Silicon Interstitials in Boron Cluster Dissolution
Authors: Aboy Maria , Pelaz Lourdes , Marqués Luis A. , López Pedro , Barbolla Juan
Published: April 2011

Boron implantation into preamorphized Si, followed by low temperature solid phase epitaxial (SPE) regrowth produces high activation combined with low diffusion. However, in the presence of high B concentrations, the activation obtained after the SPE regrowth only can reach concentrations in the order of a few times 1020 cm−3, and even more deactivation occurs during additional annealing. We have analyzed the role of the Si interstitials injected from the end of range (EOR) damage in B deactivation and reactivation by atomistic simulations. We have shown that the B cluster evolution can be clearly correlated to the evolution of Si interstitial defects at EOR. This is also compatible with B cluster stabilization in the presence of excess Si interstitials, observed in oxidation experiments.
Concentration Dependence of Boron-Interstitial Cluster (BIC) Formation in Silicon-on-Insulator (SOI)
Authors: Saavedra A. F. , Jones K. S. , Radic L. , Law M. E. , Chan K. K.
Published: April 2011

The effect of boron concentration on boron-interstitial cluster (BIC) dissolution was studied in separation by implantation of oxygen (SIMOX) and SOITEC silicon-on- insulator (SOI) materials. SOI substrates having surface silicon thickness of 750 Å and 1450 Å were ion implanted with 11B+ at a energy of 15 keV. The dose of the implant was varied from 3×1014 cm−2 to 1×1015 cm−2 to provide different boron peak concentrations. Anneals were performed at 825 °C in a nitrogen ambient using RTA for short times and furnace anneals for longer times. The retained dose of boron in the surface silicon layer was estimated using simulations from UT-Marlowe 5.0. Hall Effect was used to measure the sheet resistance, mobility, sheet number, and fraction of active boron. At lower concentrations, SOI exhibited significantly lower sheet numbers. As the boron concentration was increased SOI began to approach bulk Si. However, degraded mobility and sheet resistance was observed at all concentrations. Truncating the boron profile over the surface Si/buried oxide interface enhanced the fraction of active boron in the 750 Å SOI. This also lead to a higher fractional activation in 750 Å SOI than bulk Si as the concentration increased due to a loss of interstitials, effectively reducing the BIC population.
Deposition and Integration of a Novel Ultra-Low k (2.2) Material
Authors: Schulberg Michelle T. , Humayun Raashina , Sengupta Archita , Sun Jia-Ning
Published: April 2011

Increasing demands for faster chip speed and reduced power consumption are driving the semiconductor industry to develop insulating layers with lower dielectric constants. As the dielectric constant of a material is reduced, however, it becomes increasingly difficult to achieve the mechanical strength required to manufacture a multilevel interconnect. A new route to the synthesis of mesoporous silica has been demonstrated on 200 mm wafers. Silicate precursors dissolved in supercritical CO2 are infused into a block copolymer film. The polymer is then removed, but the resulting porous SiO2 replicates its ordered structure, enhancing the strength of the network. Incorporation of alkyl silicates further improves the film properties. Post-treatment to cap residual silanol groups renders the surface of the film hydrophobic and stabilizes it to air exposure. By appropriate choice of the block copolymer and other process parameters, the pore size and density can be varied and k values as low as 1.8 can be achieved. For a film with a dielectric constant of 2.25, the pore size is ∼4 nm. The hardness and modulus are 1.1 GPa and 7.8 GPa, respectively, as measured by nanoindentation. Four-point bend measurements yield fracture energies of 9.8 J/m2. More importantly, the film can withstand chemical mechanical planarization (CMP) using standard oxide polishing conditions.
Polyimide as a Plastic Substrate for the Flexible Organic Electroluminescent Device
Authors: Park Jin-Woo , Kim Youngkyoo , Lim Hyuntaek , Bae Cheol Min , Kim Il , Ando Shinji , Ha Chang-Sik
Published: April 2011

In this work, transparent flexible substrates based on polyimides(PI) with indium tin oxide (ITO) thin films for organic electroluminescent devices have been prepared. 2,2'-bis-(3,4- dicarboxyphenyl) hexafluoropropanedianhydride (6FDA) and 2,2'-bis-(trifluoromethyl)-4,4'- diaminobiphenyl (TFDB) PI films were used for transparent flexible substrates. ITO thin films were prepared at two substrate deposition temperatures of 25°C and 150°C with a typical radio- frequency (r.f.) planar magnetron sputtering system. The sheet resistance and the optical transmission properties of the fluorinated PI substrate were comparable with those for the slide glass substrate. The substrate properties are better when the deposition temperature is higher. SiO2 coating onto the fluorinated PI substrate by sol-gel reaction was also attempted to improve the optical transmission of the ITO/PI substrate. It was found that the coating of SiO2 affected on the morphology of the PI substrates.
Roll-to-Roll Processes for the Manufacturing of Patterned Conductive Electrodes on Flexible Substrates
Authors: Chaug Y.S. , Haubrich Jeanne E. , Sereda Mike , Liang R.C.
Published: April 2011

Simple, cost-effective roll-to-roll processes for the manufacturing of patterned conductor films have been developed. The technique involves printing proprietary masking patterns, depositing conductive material, and stripping away undesired areas. In one approach, a pattern is printed on the plastic substrate with a masking coating. Then, conductive material is deposited on the patterned substrate and the undesired areas are stripped away, leaving behind the patterned electrode structures. In another approach, a first masking pattern is printed, followed by a second masking coating which is repelled by the first masking layer and forms the negative pattern. The first masking pattern is stripped off, a conductive layer is deposited, and the second masking pattern is stripped off, leaving a high resolution conductor pattern on the web. No photolithographic exposure, development or chemical etching is needed. These processes are useful for producing rolls of electrodes with features as small as 25 microns for the roll-to-roll manufacturing of flexible display applications.
Flexible electronics for space applications
Authors: Brandon Erik , West William , Zhou Lisong , Jackson Tom , Theriot Greg , Devine Rod A.B. , Binkley David , Verma Nikhil , Crawford Robert
Published: April 2011

NASA is currently developing a host of deployable structures for the exploration of space. These include balloons, solar sails, space-borne telescopes and membrane-based synthetic aperture radar. Each of these applications is driven by the need for a thin, low mass, large area structure (e.g., polymer-based) which could not be implemented using conventional engineering materials such as metals and alloys. In each case, there is also the need to integrate sensing and control electronics within the structure. However, conventional silicon-based electronics are difficult to integrate with such large, thin structures, due to a variety of concerns including mass, reliability and manufacturing issues. Flexible electronics, particularly thin film transistors (TFTs), are a potentially key enabling technology that may allow the integration of a wide range of sensors and actuators into these types of structures. There are numerous challenges, however, regarding the survivability of such devices during stowage and deployment of the structure, as well as during operation in the harsh environments of space. We have fabricated TFTs on polyimide substrates, and are investigating the durability of these devices with respect to relevant space environments. We are also developing flexible sensor technologies for the integration of distributed sensor networks on large area structures.
Oxidation behavior of TiN with a Ti interlayer on stainless steel
Authors: Shiao Ming-Hua , Wang Ching-Chiun , Su Chien-Ying , Shieu Fuh-Sheng
Published: April 2011

Characterization of the TiN coatings oxidized in air at temperatures at 600 and 700°C for 30 min was carried out by X-ray diffraction (XRD), atomic force microscopy (AFM), transmission electron microscopy (TEM) and Auger electron spectroscopy (AES). TiN thin films with a Ti interlayer were prepared by hollow cathode discharge ion plating on AISI 304 stainless steel. Both XRD and TEM results show that the TiN coatings and Ti interlayer have columnar structure with (111) and (0002) preferred orientations, respectively. AFM results show the existence of pinholes on the surface of specimens due to electropolishing process of the steel substrate, and the surface roughness (Ra) changes from 3.5 nm for the as-deposited specimen to 11.6 nm after oxidation at 700°. After oxidation, the TiO2 oxide layer formed on the specimen surface was porous and retained the columnar structure as the original TiN coating. The microstructure of the Ti interlayer gradually changed from columnar to polycrystalline structure due to grain growth. The Auger elemental depth profiling indicated that interdiffusion of the Ti interlayer with steel substrate had occurred during the oxidation process.
Nanopatterning on Reconstructed Ceramic Surfaces
Authors: A Shwetha Shetty , Shenoy Vijay B , Carter C. Barry , Ravishankar N.
Published: April 2011

The use of reconstructed ceramic surfaces as templates for nanopatterning has been demonstrated recently. This technique differs from the surface decoration by Au on stepped surfaces of alkali halides which has been a topic of intense research in the past. Some of the intriguing aspects related to the physical origin of the phenomena have been considered here. Based on heterogeneous nucleation of Pt vapor on wedged alumina surfaces, it has been shown that the valley sites are the preferred sites for nucleation. However, the hill sites are decorated by the particles in the present study pointing out to a different physical origin for the formation of the nanoparticles. The role of electrostatic energy reduction on the formation of such nanopatterns is discussed.
Grain Boundary Properties and Grain Growth: Al Foils, Al Films
Authors: Barmak Katayun , Archibald Wayne E. , Rollett Anthony D. , Ta'asan Shlomo , Kinderlehrer David
Published: April 2011

Relative grain boundary energy as a function of misorientation angle has been measured in cube-oriented, i.e., <100> fiber-textured, 120 [.proportional]m-thick Al foil using orientation imaging microscopy and a statistical multiscale method. The energies of low-angle boundaries increase with misorientation angle, in good agreement with the Read-Shockley model. The relative energies of high-angle boundaries exhibit little variation with misorientation. Examination of the grain structure of <111> fiber-textured, 100 nm-thick Al films annealed at 400°C for 0.5-10 h shows 5 and 6 sided grains to be the most frequent, and the fraction of four-sided grains to be significant. The mean number of sides is slightly lower than the expected value of 6 for two- dimensional structures. Of lognormal, gamma and Rayleigh distributions, gamma gives the best fit to the grain size data in the films; however, the difference between gamma and lognormal is small. Grain growth is not self-similar and stagnates after one hour of annealing. The evolution of the grain size distribution with time indicates that the growth stagnation in the films is neither consistent with boundary pinning by grooving nor with conventional treatments of solute drag. Surface, elastic-strain and plastic-strain energy driving forces do not play a significant role in the grain growth and the subsequent stagnation since the films are strongly textured even in the as- deposited state. The steady-state distributions of reduced grain area for two-dimensional, Monte Carlo and partial differential equation based simulations show excellent agreement with each other, even when anisotropic boundary energies are used. However, comparison with experimental distributions reveals a significantly higher population of small grains in the experiments.
Charge coupled cyclotron motion of electrons and holes in InGaAsN epitaxial layers
Authors: Porteanu H. E. , Loginenko O. , Koch F. , Dumitras G. , Geelhaar L. , Riechert H.
Published: April 2011

Time resolved cyclotron resonance measurements are used to investigate more in further detail the effective mass of electrons and holes in InGaAsN epitaxial layers. The In and N content in the alloy are adjusted to yield the latticematching of the epilayers (200 nm thick) to GaAs. A continuous increase of the effective mass of electron and an increase of the resonance associated with holes is observed. Through the evolution of the imaginary part of conductivity as a function of the elapsed time we show that this observation is a coupled cyclotron resonance that may have maxima in the real part of conductivity but should not be necessarily correlated with the “increase” of the effective mass.
Structural and Optical Effects of Capping Layer Material and Growth Rate on the Properties of Self-Assembled InAs Quantum Dot Structures
Authors: Agnello Gabriel , Tokranov Vadim , Yakimov Michael , Lamberti Matthew , Zheng Yuegui , Oktyabrsky Serge
Published: April 2011

In order to develop nanoengineering methods to control electronic spectrum of self-assembled InAs quantum dots (QDs) grown by molecular beam epitaxy, we have utilized atomic force microscopy (AFM), photoluminescence (PL) and TEM methods to investigate the effects of capping layer growth on the physical/chemical properties as well as the optical/electronic performance of QD device structures. Capping layer material choice (or its absence all together) has been found to directly influence QD dimensions (size, height), and subsequently, to affect QD emission wavelength. We report results of QD lateral size and height as well as densities of InAs QDs capped with 2ML (monolayers) of AlAs or GaAs grown at various rates. Our AFM results are complemented by PL measurements, where the optical properties of capped versus non-capped QDs have been explored and direct correspondence between structural differences induced by capping and the electronic/optical properties of QDs is demonstrated. Analysis of the data shows that the results can be explained by two competing surface processes. The first of which is the redistribution of indium between QDs on top of the 2D wetting layer, resulting in the increase of QD size with time. The second effect is the diffusion of indium out of the QDs and onto the top of the capping layer. TEM with multislice image simulation has supported our AFM and PL observations with the demonstration of “indium driven” alloy intermixing in the overlayer as well as significant alloying in the InAs wetting layer.
Electrical Measurement of Recombination Lifetime in Blue Light Emitting Diodes
Authors: Awaah M. A. , Nana R. , Das K.
Published: April 2011

A recombination lifetime of approximately 25 ns was extracted from measured reverse recovery storage times in AlGaN/GaN/AlGaN double heterojunction blue light emitting diodes. This experimentally determined lifetime is expected to arise from a combination of radiative and non-radiative processes occurring in the diodes. The non-radiative processes are likely to be due the presence of a high concentration deep-states as identified from the current-voltage and capacitance-voltage measurements. Current-voltage characteristics of these diodes were highly non-ideal as indicated by high values of the ideality factor ranging from 3.0 – 7.0. Logarithmic plots of the forward characteristics indicated a space-charge-limited-current (SCLC) conduction in presence of a high density of “deep-level states” in the active region of the diodes. An analysis of these characteristics yielded an approximate density of these deep-level states as 2 × 1017/cm3. The density of deep-states extracted from capacitance-voltage measurements were in good agreement with that obtained from current-voltage measurements.
Growth and Characterization of ZnO Nanonail
Authors: Seo H. W. , Wang D. , Tzeng Y. , Sathitsuksanoh N. , Tin C. C. , Bozack M. J. , Williams J. R. , Park M.
Published: April 2011

Zinc oxide (ZnO) is an interesting material for short-wavelength optoelectronics due to its wide band gap. The nanostructures of ZnO are also intriguing since a variety of morphology can be obtained by employing different processing parameters. In our laboratory, ZnO nanonails were successfully synthesized at low temperature using a thermal chemical vapor deposition. The morphology of the sample was studied by using scanning electron microscopy. The shape of the nail head can be controlled from hexagon to quasi-circular shape. X-ray diffraction, Raman scattering, photoluminescence spectroscopy were also performed to analyze the ZnO nanonail. Photoluminescence spectroscopy suggested that the defects in the ZnO nanonail and nanobone are of different nature.
Raman spectroscopy of V and Co doped ZnO ceramics and thin films
Authors: Samanta K. , Awasthi N. , Sundarakannan B. , Bhattacharya P. , Katiyar R. S.
Published: April 2011

Lattice dynamical and electronic transition changes due to V and Co doped ZnO have been investigated using optical techniques. Vanadium and Co doped ZnO pellets were prepared using conventional ceramic processing route and thin films were fabricated by pulsed laser deposition. Raman spectra of Zn1-xVxO targets showed many additional peaks in the range of 230 to 350 cm-1 and 750 to 900 cm-1. Integrated intensities of these additional modes decreased with increase of temperature as similar to the host ZnO modes, which precludes electronic Raman scattering to be the origin. Raman peaks for stoichiometric Zn3(VO4)2 and Zn2V2O7 compounds also had additional peaks that can be attributed to the secondary phases formed in the compositions of Zn1-xVxO. Raman spectra of Zn1-xCoxO showed no additional modes besides ZnO modes, however, the intensity of the second order peak at 540 cm-1 was increased due to Co doping. Thin films of Zn1-xCoxO exhibited highly c-axis orientation deposited on (001)Al2O3 substrates. The optical absorption of the films showed that the band gap decreased with increase of Co concentrations at room temperature along with the sub bandgap absorptions due to d-d transitions of Co2+.
Longitudinal Modes in InAlGaAs/AlGaAs High-Power Laser Diodes
Authors: Passmore B. S. , Chua Y. C. , Manasreh M. O. , Tomm J. W.
Published: April 2011

The emission from bias voltage driven In0.06Al0.08Ga0.86As/Al0.3Ga0.7As edge-emitting diode lasers with cavity lengths of 0.6 and 0.9 mm was measured at 290 K using a high resolution Fourier-transform infrared spectrometer. The light-current (L-I) characteristic curves showed that a threshold current of ∼ 0.5 and 0.8 A was needed to reach the stimulated emission in laser diodes with cavity lengths of 0.6 and 0.9 mm, respectively. The laser diode longitudinal modes were observed in the stimulated emission spectrum for each of the devices. These spectra enabled us to calculate the refractive index, total loss, and gain of the active region. On the other hand, the cavity length was determined by utilizing the interferogram spectrum, longitudinal modes spacing, and direct measurements of the laser dimensions. A good agreement between the cavity length values obtained from the three methods was achieved.
Ultrashort Pulse Generation with Semiconductor Modelocked Lasers Using Saturable Absorbers Based on Intersubband Transitions in GaN/AlGaN Quantum Wells
Authors: Ahmad Faisal R. , George Paul , Dawlaty Jahan , Rana Fahan , Shaff William J.
Published: April 2011

We present a novel scheme for generating high energy ultrashort pulses in modelocked semiconductor lasers using a fast saturable absorber based upon intersubband transitions (ISBT) in GaN/AlGaN superlattice. The fast electron relaxation time (∼100–300 fs) in this saturable absorber enables the generation of stable sub-100 fs pulses at the communication wavelength (1.55μm) with high pulse energies (5–10 pJ).
Zener tunneling of light in an optical superlattice
Authors: Ghulinyan Mher , Gaburro Zeno , Pavesi Lorenzo , Oton Claudio J. , Toninelli Costanza , Wiersma Diederik S.
Published: April 2011

We report on the observation of resonant Zener tunnelling of light waves in an optical superlattice. The one dimensional (1D) structures are made in free-standing porous silicon and are designed specifically to exhibit two photonic minibands. A controlled optical path gradient has been maintained over the sample thickness which resulted in tilting of photonic minibands and formation of optical Wannier-Stark ladders. At a certain value of optical gradient the two minibands couple within the extension of the structure and a resonant tunnelling channel through the superlattice forms, resulting in a very high transmission peak. Ultrafast time resolved transmission experiments were performed: excitation of the Wannier-Stark states causes the appearance of photonic Bloch oscillations, which are strongly damped when Zener tunneling modes are excited. The observed phenomenon is the optical analogue of resonant Zener tunnelling in an electronic superlattice.
Characterization of Minority-Carrier Hole Transport in Nitride-Based Light-Emitting Diodes with Optical and Electrical Time-Resolved Techniques
Authors: Kaplar R. J. , Kurtz S. R. , Koleske D. D. , Allerman A. A. , Fischer A. J. , Crawford M. H.
Published: April 2011

Forward-to-reverse bias step-recovery measurements were performed on In.07Ga.93N/GaN and Al.36Ga.64N/Al.46Ga.54N quantum-well (QW) light-emitting diodes grown on sapphire. With the QW sampling the minority-carrier hole density at a single position, distinctive two-phase optical decay curves were observed. Using diffusion equation solutions to self-consistently model both the electrical and optical responses, hole transport parameters τp = 758 ± 44 ns, Lp = 588 ± 45 nm, and μp = 0.18 ± 0.02 cm2/Vs were obtained for GaN. The mobility was thermally activated with an activation energy of 52 meV, suggesting trap-modulated transport. Optical measurements of sub-bandgap peaks exhibited slow responses approaching the bulk lifetime. For Al.46Ga.54N, a longer lifetime of τp = 3.0 μs was observed, and the diffusion length was shorter, Lp ≈ 280 nm. Mobility was an order of magnitude smaller than in GaN, μp ≈ 10−2 cm2/Vs, and was insensitive to temperature, suggesting hole transport through a network of defects.
Optimization of GaN Channel Conductivity in AlGaN/GaN HFET Structures Grown by MOVPE
Authors: Hubbard S.M. , Zhao G. , Pavlidis D. , Cho E. , Sutton W.
Published: April 2011

Optimization of GaN channel conductivity in AlGaN/GaN Heterojunction Field Effect Transistor (HFET) structures was performed using High Resistivity (HR) GaN templates grown by Metal-organic Vapor Phase Epitaxy (MOVPE). The GaN sheet resistance was tuned using final nucleation layer (NL) annealing temperature. Using an annealing temperature of 1033°C, GaN with sheet resistance of 10 Ω/sq was achieved, comparable to that of Fe-doped GaN. X-Ray Diffraction (XRD) and Photoluminescence (PL) analysis show that the high resistance GaN is achieved due to compensating acceptor levels introduced through edge-type threading dislocations. XRD analysis also shows optimization of annealing temperature provided a means to maximize GaN sheet resistance without significantly degrading material quality. AlGaN/GaN HFET layers grown using HR GaN templates gave surface and interface roughness of 14 and 7 Å, respectively. The 2DEG Hall mobility and sheet charge of HFETs grown using HR GaN templates was comparable to similar layers grown using unintentionally doped (UID) GaN templates.
Electrical and Optical Properties of 1 MeV-electron irradiated AlxGa1-xN
Authors: Hogsed Michael R. , Ahoujja Mo , Ryu Mee-Yi , Yeo Yung Kee , Petrosky James C. , Hengehold Robert L.
Published: April 2011

The optical and electrical properties of Si doped GaN and Al0.20Ga0.80N films irradiated with 1 MeV electrons at a fluence of 1×1017 cm−2 are investigated using cathodoluminescence (CL), variable-temperature Hall-effect, and deep level transient spectroscopy (DLTS) measurements. The CL spectra measured at 6 K show peak luminescence intensity of the near band edge decreases, on average, by 50% after electron irradiation, indicating the creation of non-radiative recombination centers which are stable at room temperature. At room temperature, the free carrier concentration decreases significantly in both the GaN and AlGaN samples following the irradiation, and the carrier removal rate depends strongly on the initial carrier concentration. DLTS measurements show three electron traps in the as-grown Al0.20Ga0.80N. Following 1 MeV electron irradiation of the Al0.20Ga0.80N sample, three additional electron traps labeled R2, R3, and R4 are observed. The first two traps appear to correspond to radiation-induced traps reported in GaN while the latter appears to be unique to AlGaN.
Reduction of dislocation density in AlGaN with high AlN molar fraction by using a rugged AlN epilayer
Authors: Ishiga Akira , Onishi Takashi , Liu Yuhuai , Haraguchi Masaya , Kuwano Noriyuki , Shibata Tomohiko , Tanaka Mitsuhiro , Miyake Hideto , Hiramatsu Kazumasa
Published: April 2011

A method for making AlGaN with a high AlN molar fraction and low dislocation density is needed for fabricating deep ultraviolet emitters and detectors. In this study, we reduced the dislocation density in AlGaN over a large surface area by using low-pressure MOVPE on a continuously rugged epitaxial AlN substrate. The AlN molar fraction of the AlxGa1-xN was × = 0.51, and atomic steps in the surface were clearly observed with an atomic force microscope (AFM). In addition, the dislocation density was estimated to be 8.8 × 107 cm−2, which is two orders of magnitude lower than that of AlGaN grown on a flat AlN epitaxial layer. Our results indicate that the dislocation density of AlGaN can be greatly reduced by using a rugged AlN epitaxial substrate with continuously inclined facet.
The effect of periodic silane burst on the properties of GaN on Si (111) substrates
Authors: Zang K. Y. , Chua S. J. , Thompson C. V. , Wang L. S. , Tripathy S. , Chow S. Y.
Published: April 2011

The periodic silane burst technique was employed during metalorganic chemical vapor deposition of epitaxial GaN on AlN buffer layers grown on Si (111). Periodic silicon delta doping during growth of both the AlN and GaN layers led to growth of GaN films with decreased tensile stresses and decreased threading dislocation densities, as well as films with improved quality as indicated by x-ray diffraction, micro-Raman spectroscopy, atomic force microscopy, and transmission electron microscopy. The possible mechanism of the reduction of tensile stress and the dislocation density is discussed in the paper.
Normally-off operation GaN HFET using a thin AlGaN layer for low loss switching devices
Authors: Ikeda Nariaki , Kato Kazuo , Li Jiang , Hataya Kohji , Yoshida Seikoh
Published: April 2011

We report on the novel normally-off GaN-based heterojunction field effect transistors (HFETs) on a Si substrate. The AlGaN/AlN/GaN heterostructure was grown using a metalorganic chemical vapor deposition (MOCVD). The HFET for a normally-off operation was fabricated using a precisely controlled thin-AlGaN layer as an electron supply layer. As a result, the HFET was operated at the condition of the positive gate bias. We also characterized the enlarged gate-width devices. The breakdown voltage of FET was over 300 V. A normally-off operation using GaN based HFETs with a thin-AlGaN/AlN/GaN heterostructure on the silicon substrate were thus confirmed for the first time.
P-type GaN epitaxial layers and AlGaN/GaN heterostructures with high hole concentration and mobility grown by HVPE
Authors: Usikov A. , Kovalenkov O. , Ivantsov V. , Sukhoveev V. , Dmitriev V. , Shmidt N. , Poloskin D. , Petrov V. , Ratnikov V.
Published: April 2011

In this paper we report p-GaN growth by hydride vapor phase epitaxy (HVPE) on sapphire substrates. Mg or Zn impurities were used for doping. Layer thickness ranged from 2 to 5 microns. For both impurities, as-grown GaN layers had p-type conductivity. Concentration NA-ND was varied from 1016 to 1018 cm−3. An annealing procedure at 750°C in argon ambient typically increased the concentration NA-ND in 1.5–3.5 times. For Mg doped GaN layers, room temperature hole mobility of 80 cm2V−1s−1 was measured by conventional Van Der Pau Hall effect technique for material having hole concentration of about 1x1018 cm−3. Initial results on highly electrically conducting p-type AlGaN/GaN heterostructures doped with Zn are also reported.
Silicon-based Integrated Optics: Waveguide Technology to Microphotonics
Authors: Janz Siegfried , Bogdanov Alexei , Cheben Pavel , Delâge André , Lamontagne Boris , Picard Marie-Joseé , Xu Dan-Xia , Yap Kuan Pei , Ye Winnie N.
Published: April 2011

Using a waveguide spectrometer chip as an example, we describe how high index contrast waveguides systems such as silicon-on-insulator can be combined with microphotonic design rules to extend the performance of waveguide devices. The challenges arising in the implementation of silicon microphotonic technology are discussed, and recent work addressing the issues of waveguide coupling, polarization sensitivity, waveguide loss and massively parallel data acquisition is reviewed.
Pump-probe experiments on low loss silica waveguides containing Si nanocrystals
Authors: Navarro-Urrios D. , Daldosso N. , Melchiorri M. , Sbrana F. , Pavesi L. , García C. , Garrido B. , Pellegrino P. , Morante J. R. , Scheid E. , Sarrabayrouse G.
Published: April 2011

Rib-loaded silica waveguides containing Si nanocrystals were grown by quadruple implantation of Si ions into a 2 μm-thick thermally-grown SiO2 layer. The thickness of the resulting flat-profile active region was about 300 nm, with a 9.5% Si excess (determined by X-ray photoelectron spectroscopy). Complete phase separation and nanocrystal formation was assured by annealing at 1100 °C, and studied by means of optical tools such as Raman and luminescence. The rib-loaded structure of the waveguides was fabricated by photolithographic and reactive ion etching processes, with patterned rib widths ranging from 1 to 8 μm. Efficient light propagation was observed when end-fire coupling a probe signal both at 633 nm and 780 nm into the waveguides, with attenuation losses as low as 11 dB/cm. Signal amplification experiments, with pulsed and continuous wave (CW) top pumping, have shown increased signal absorption when the pump power is raised. This couples with the lack of any fast component in the time decay of the amplified spontaneous emissions as measured by ns pulsed pumping Variable Stripe Length (VSL) experiments. These two phenomena are interpreted as due to the lack of stimulated emission in these nanocrystalline systems.
Optical properties of silicates co-doped with Si and Er3+ ions
Authors: Lebour Y. , Pellegrino P. , Moreno J. A. , Garcia C. , Garrido B. , Morante J.R. , Prassas M.
Published: April 2011

We have characterized the mechanism of energy transfer from Si nanoparticles to Er3+ ions in different silicate glasses, namely soda-lime and aluminium silicates, and made the comparison with pure silica. By means of a multi-implantation scheme we have formed a 350 nm thick glass layer with a uniform Si excess (5% or 15% atomic excess) and an Er distribution. Several Er doses were chosen so that the resulting Er peak concentration could vary from 2 × 1019 up to 6 × 1020 cm−3. Fused silica wafers coimplanted in the same conditions were used as a reference material in order to compare the different efficiency and mechanisms of Er emission as a function of the host silicate composition. Thermal treatment at low temperature has been performed to enhance the photoluminescence around 1540 nm. Large PL emission, compared to structures doped only with Er, has been successfully detected in all co-implanted glasses, with similar intensity. Moreover, we have measured PL lifetimes from 2.5 to 12 ms (depending on the Er dose and Si excess), and estimated an Er effective excitation cross-section of the order of 1 × 10−17 cm2.