Use of photocurrent-voltage characteristics of MOS structures to determine insulator bulk trapped charge densities and centroids

A new technique is described for determining the density and centroid of trapped space charge in the oxide layer of metal-oxide-semiconductor (MOS) structures. Photo-current-voltage (photo I-V) characteristics for both the metal-oxide and Si-oxide interfaces are used to determine the internal fields due to bulk trapped charge, and hence its density and centroid. Experimental examples for locating both positive and negative charge are presented. For negative charge, an Al-Si0₂-W-SiO₂-Si structure (MOWOS) with a layer of approximately 10¹⁴ W atoms/cm² deposited 80 Å from the Si-SiO₂ interface and charged by electronic internal photoemission is investigated. For positive charge, the location of trapped holes generated by 16.85 eV photons or x-rays in the SiO₂ layer of an MOS structure under a voltage bias is discussed. The photo I-V technique is compared to others in terms of its direct, rapid, minimally perturbing, low current, and low field characteristics.     

Etching characteristics of SiO2 in CHF3 gas plasma

The etching characteristics of SiO₂} have been investigated in the CHF₃} gas plasma using the planar type reactor with the 400 kHz rf power. The etch rate of SiO₂}, the SiO₂} /Si and SiO₂}/resist etch rate ratios, and the deterioration of photoresist films are studied with a variety of etching parameters. The etching characteristics depend strongly on the coupling mode. With the cathode coupling mode, the values of 300å/min and of larger than 100 are obtained for the etch rate of SiO₂} and the SiO₂}/Si etch rate ratio, respectively. Only 8 is given for the SiO₂} /Si etch rate ratio with the anode one. The deterioration of photoresist films less occurs with the cathode coupling mode than with the anode one. The dependences of the etching characteristics on the rf current, gas pressure, gas flow rate, and the electrode separations are also studied some in detail with the cathode coupling mode. Possible explanations for some of the experimental results are discussed.     

Properties of low-temperature (80-300° C) pyrolytic SiO2 on Si and InP

We report on a new low-temperature pyrolytic deposition technology for silicon dioxide. We present data characterizing the electrical and optical properties of this dielectric deposited on Si and InP substrates. The effects of thermal processing are also reported. Deposition of high-quality SiO₂ is achieved by reacting SiH₄ and O₂ at pressures of 2–12 Torr. Reactions occur by pyrolysis only, promoting stoichiometric SiO₂ deposition and good interfacial properties. No plasma- or photo-enhancement is required. Deposition is achieved at temperatures as low as 80° C, the lowest temperature ever reported for pyrolytic SiO₂ deposition. Rates as high as 65 Å/min at 100° C and 100-150 Å/min at 150-300° C are attained. The leakage current densities measured for both Si and InP MIS capacitors (e.g. 10^-9 Å/cm² for 150° C SiO₂) are two to six orders of magnitude lower than values reported for plasma- and photo-enhanced SiO₂ deposited at equivalent temperatures. The high-temperature integrity of this dielectric also makes it a promising annealing cap for group III-V compound semiconductors. Our annealing studies show that SiO₂-capped indium phosphide surfaces remain specular up to 850° C.     

Specific features of the hydride vapor-phase epitaxy of nitride materials on a silicon substrate

Specific features of the growth of the GaN/AlN/Si heterocomposite in which layers of Group-III element nitrides are grown on a silicon substrate by hydride vapor-phase epitaxy are studied. The effect of the temperature at which the AlN buffer layer is grown on diffusion processes at the heterointerfaces and on the quality of the epitaxial layers being grown is considered. It is shown that, with the epitaxial technique used, the buffer layer should be grown at high temperatures (1080°C) because the thickness of the component-mixing region is minimized in this case and abrupt interfaces are formed in the GaN/AlN/Si heterocomposite. The double-stage growth of gallium nitride on the high-temperature AlN buffer layer with a thickness of 300–400 nm makes it possible to obtain GaN layers with thicknesses of up to 0.3 μm without crack formation.     

Arsenic silicide formation by oxidation of arsenic implanted silicon

Wet oxidations of (100) silicon implanted with an arsenic dose of 2 × 10¹⁶ cm⁻² and an energy of 30 keV were carried out in the temperature range between 600 and 900° C. The oxidation rate is increased on the arsenic implanted samples up to a factor of 2000 as compared to undoped samples. During these oxidations the arsenic suicide phase AsSi is precipitated at the oxide/silicon interface. After short oxidation times at 600° C, a continuous AsSi layer is found. It is dissolved during extended oxidation times and finally almost all As is incorporated in the oxide. After 900° C oxidations, substantial AsSi crystallites remain at the Si/SiO₂ interface. They are still observed up to the larg-est oxide thickness grown (2.3 μm). The AsSi phase and the distribution of the im-planted arsenic were analyzed by TEM, SIMS and XRF measurements.     

Lithography with silicon ions

Some applications of focussed ion beams are discussed with special emphasis on the lithography aspects. Experimental data on the ranges of H⁺, Be⁺ and Si⁺⁺ in polymethylmethacrylate (PMMA) are presented. The sensitivity of PMMA to Si⁺⁺ ions is measured to be 0.35 μC/cm². It is shown by replication through a transmission mask that Si⁺⁺ ions with low to moderate incident energies, such as found in focussed ion beam systems, can be used for submicrometer lithography in single and multi-layer resist systems. This paper was presented at the 25th Electronic Materials Conference, Burlington, Vermont, June 1983     

A sliding wafer-OMVPE scheme for fabricating subnanometer superlattices

A sliding wafer-OMVPE (Organometallic Vapor Phase Epitaxy) reactor suitable for growing superlattices is developed. The reactor is a two-channel horizontal reactor with a susceptor placed across the two channels. A slider is used to transport the substrate by sliding it along the susceptor surface from one channel to the other. This scheme makes it possible to set the temperature of the susceptor in each channel independently by utilizing the skin effect of radio wave in graphite. The performance of this scheme is demonstrated by growing a superlattice of ten periods of GaAs(3.5 Å)/AlAs(7 Å).     

Characterization of oxidized a-SiHx surfaces used in mis structures

Atomically clean and oxidized surfaces of a-SiH_x films, prepared by plasma decomposition of SiH₄ in a capacitative reactor, were studied using Auger and UV photoemission spectroscopy. The oxides were characterized in terms of the fine structure of the Si_LVV Auger transition. We also studied the electrical properties of MIS solar cell structures in which the a-SiH_x film was given the same treatment as the samples for surface studies and correlated the results of the two kinds of measurements. For thin (< 20 Å) oxides the photodiode open circuit voltage increases linearly with oxide thickness. This increase is found to be due to an increase in the barrier height and the built-in potential upon oxidation.     

Photoluminescence of InxGa1-xAs-GaAs strained-layer superlattices

Photoluminescence of In_xGa_1-xAs-GaAs strained-layer superlattices (SLS's) grown by molecular beam epitaxy (MBE) is investigated. Highly strained SLS's composed of layers differing in their bulk lattice constants by as much as 2.7% are examined over the temperature range 20K-300K. Photoluminescence (PL) spectra for several In_0.28Ga_0.72AsGaAs SLS's are presented, providing data relating effective band gap and PL intensity to temperature and layer thickness. These data suggest a critical (maximum) alloy layer thickness for optical quality material in the range of 80å-100å for crystals with x = 0.28 and an In_xGa_1-xAs/GaAs layer thickness ratio of L_z/L_B = 1.3. Results of PL experiments on In_0.38Ga_{0 62}As-GaAs SLS's are also presented, and the effects of lattice misfit at the SLS/substrate interface upon the optical quality of these SLS's is examined.     

Simulation of Terahertz Generation from Lateral Diffusion Currents in Semiconductor Devices

In this paper we model the carrier dynamics and resulting THz emission from lateral diffusion currents within a semiconductor device which has been partially masked by a metallic mask. We present a numerical 1D model and a 1D Monte Carlo simulation which both demonstrate that regardless of the excitation laser spot shape we do not expect to see measurable THz emission in the direction of the optical pump propagation from lateral diffusion currents. Experimentally such devices do produce strong THz emission. We analytically investigate the role of the metal mask and we found that it suppresses the emission of dipoles that are in a region that is less than a wavelength away from the interface. The results from the numerical model are also included in a finite element analysis model of the geometry which predicts THz emission if and only if the metal mask is present.     

Evidence for transient composition variations at GaAs/Ga1−xAlxAs heterostructure interfaces prepared by metal-organic chemical vapour deposition

The composition profiles of GaAs/Ga_1?xAl_xAs heterostructures prepared in two different Metal-Organic Chemical Vapour Deposition (MOCVD) reactors have been studied. Transmission electron microscopy (TEM). and Auger and secondary ion mass spectrometry (SIMS) sputter profiling results are in good agreement and interface widths below 20 å have been achieved. Significant new results on transient phenomena have been obtained. showing that large and sometimes very sharp excursions in composition can be associated with valve switching actions. Direct evidence is presented that gas pressure or flow transients can occur during valve operations. and it is suggested that such instabilities are the cause of the effects observed.     

Room temperature oxidation of lead-indium alloy films

The kinetics of growth of thin (14 to 40Å) oxide layers on lead-indium alloys was investigated ellipsometrically, using: 3000Å thick films at 23°C; and oxygen exposures at 760 torr for times ranging from five minutes to five days. Assuming that the oxide layer is comprised of a two-phase mixture of PbO and In₂O₃ having a negligible extinction coefficient made it possible to estimate the oxide composition from the ellipsometrically-obtained oxide refractive index. Under these oxidizing conditions, the volume fraction of PbO in the oxide mixture decreases from a value of unity for pure lead to zero for alloys containing more than 30 at. percent In, in agreement with the Auger Electron Spectroscopy results of Chou and coworkers. The oxidation rate equals a exp (X₁/X), where α and X₁ will be seen to vary complexly with alloy composition. A theoretical explanation of these results is also presented.     

Implementation of AES Key Schedule Using Look-Ahead Technique

The commencement of decryption process of Advanced Encryption Standard (AES) algorithm is dependent on availability of the last round key. In this paper, we propose a look-ahead technique for increasing the speed of implementation of AES key schedule using which the last round key can be made available fast. The other round keys can also be computed in a parallel path using the proposed technique. Applications such as key search engines need to be agile to key changes for decrypting given encrypted messages using all the keys in the available key space so that fast decryption is possible. The FPGA implementation results using Xilinx XC5VLX85 are also provided.     

Optical Properties of 3D Printable Plastics in the THz Regime and their Application for 3D Printed THz Optics

We characterize the terahertz (THz) properties of several materials which can be used for fused material deposition 3D printing. We identify Polystyrene as a material which shows a promising compromise between printability and THz transparency. Furthermore, printed THz lenses are presented and characterized.     

Characterization of a rapid thermal anneal TiNxOy/TiSi2 contact barrier

In this paper, the physical and electrical properties of a TiN_xO_y/TiSi₂ dual layer contact barrier are reported. The TiN_xO_y/TiSi₂ barrier was formed by rapidly annealing a Ti thin film on Si in an N₂ ambient. During this process, the Ti film surface reacts with N₂ to form a TiN_xO_y skin layer and the bulk of the Ti film reacts with Si to form an underlying TiSi₂ layer. The influences of rapid thermal anneal (RTA) conditions on the TiN_xO_y layer were investigated by varying the RTA temperature from 600 to 1100° C and cycle duration from 30 to 100 s. It is found that the resulting TiN_xO_y and TiSi₂ layer thicknesses are dependent on RTA temperature and the starting Ti thickness. For a starting Ti thickness of 500Å, 150Å thick TiN_xO_y and 800Å thick TiSi₂ are obtained after an RTA at 900° C for 30 s. The TiN_xO_y thickness is limited by a fast diffusion of Si into Ti to form TiSi₂. When a Ti film is deposited on SiO₂, Ti starts to react with SiO₂ from 600° C and a significant reduction of the SiO₂ thickness is observed after an RTA at 900° C. The resulting layer is composed of a surface TiN_xO_y layer followed by a complex layer of titanium oxide and titanium suicide. In addition, when Ti is depos-ited on TiSi₂, thicker TiN_xO_y and TiSi₂ layers are obtained after RTA. This is because the TiSi₂ layer retards the diffusion of Si from the underlying substrate into the Ti layer. NMOSFETs were fabricated using the TiN_xO_y/TiSi₂ as a contact barrier formed by RTA at 900° C for 30 s and a significant reduction of contact resistance was obtained. In addition, electromigration test at a high current density indicated that a significant improvement in mean time to failure (MTF) has been obtained with the barrier.     

A practical RFID authentication mechanism for digital television

As information technology continuously progresses, more applied technologies are developed, such as radio frequency identification (RFID). In this paper, we propose a novel digital television (DTV) structure that uses RFID for encryption. RFID is widely used for various applications because of its advantages such as an extended lifetime and security, and it is less affected by environmental constraints. The proposed protocol uses RFID for encryption to withstand many attacks that the traditional system is vulnerable to, such as impersonation attack, replay attack and smart card cloning. Compared with other protocols, the proposed protocol is more secure and efficient. Thus, our proposed protocol makes the DTV framework more complete and secure.     

A comparison of MOS gate structures formed by depositing polycrystalline silicon on thin oxides using conventional low pressure chemical vapor deposition and rapid thermal chemical vapor deposition

MOS structures with 80Å-200Å thick gate oxides were fabricated using polycrystalline silicon gate electrodes deposited by rapid thermal chemical vapor deposition (RTCVD) and by conventional chemical vapor deposition (LPCVD). Polycrystalline silicon doping was achieved by BF₂ or As implantation followed by rapid thermal annealing (RTA). The Q-C method was employed to study the electrical properties of the capacitors through high and low frequencyC- V profiles. The electrical properties of the MOS structures show that the devices fabricated using RTCVD polycrystalline Si are comparable in quality to those with LPCVD polycrystalline silicon gate electrodes. However, the results also indicate that boron diffusion through the thin oxide is a problem regardless of the deposition technique used for polycrystalline silicon. Boron penetration is accompanied by a shift in threshold voltage, inversion layer capacitance and a high density of midgap interface traps. Dopant diffusion in polycrystalline silicon and through the thin gate oxides was also studied by secondary ion mass spectroscopy (SIMS). The findings of the SIMS analysis correlate well with the electrical measurements. The results indicate that significant boron diffusion can occur through an 80A oxide if an RTA temperature higher than 1000° C is used. On the other hand, both SIMS and electrical measurements suggest that As penetration into the substrate is negligible even at temperatures as high as 1100° C.     

Simultaneous prediction of density and moisture content of wood by terahertz time domain spectroscopy

In this study, demonstration of simultaneous prediction of solid wood density and moisture content, both of which are critical in manufacturing operations, of 4 species (Aspen, Birch, Hemlock and Maple) was accomplished using terahertz time-domain spectroscopy (THz-TDS). THz measurements of wood at various moisture contents were taken for two orientations of the THz field (parallel and perpendicular) with respect to the visible grain. The real and imaginary parts of the dielectric function averaged over the frequency range of 0.1 to 0.2 THz had strong correlation with density and moisture content of the wood. We extend a model that has been applied previously to oven-dry wood to include the effects of moisture below the fiber saturation point by combining two effective medium models, which allows the dielectric function of water, air and oven-dry cell wall material to be modeled to give an effective dielectric function for the wood. A strong correlation between measured and predicted values for density and moisture content were observed.     

Ant Optimized Link Quality for Ad Hoc on Demand Distance Vector

The wireless interfaces in Mobile Ad-Hoc Networks (MANETs) have limited transmission range and traffic is relayed through intermediate nodes to ensure intra node communication. Routing plays an important role in network reliability and performance. MANET’s characteristics like mobility and resource constraints adversely affect routing performance. A new MANET routing method based on Ad hoc On- demand Distance Vector (AODV) and Ant Colony Optimization is proposed in this paper. Ad hoc networks can emulate achievement of complex solutions with limited intelligence and individual capacity as seen in ant communities. A new link quality metric enhances AODV routing algorithm to enable it to handle link quality to evaluate routes between nodes.