Tue Nguyen

Oxidation studies of SiGe

We have studied the kinetics and mechanism of oxidation of SiGe alloys deposited epitaxially onto Si substrates by low‐temperature chemical vapor deposition. Ge is shown to enhance oxidation rates by a factor of about 3 in the linear regime, and to be completely rejected from the oxide so that it piles up at the SiO2/SiGe interface. We demonstrate that Ge plays a purely catalytic role, i.e., it enhances the reaction rate while remaining unchanged itself. Electrical properties of the oxides formed under these conditions are presented, as well as microstructures of the oxide/substrate, Ge‐enriched/SiGe substrate, and SiGe/Si substrate interfaces, and x‐ray photoemission studies of the early stages of oxidation. Possible mechanisms are discussed and compared with oxidation of pure silicon.

The relation between positive charge and breakdown in metal‐oxide‐silicon structures

The relation between positive‐charge accumulation at the Si‐SiO2 interface and the occurrence of high‐field breakdown in metal‐oxide‐silicon structures has been investigated. Oxides having different hole‐trapping properties were prepared with the addition of short rapid thermal anneals in O2. Experiments testing hole trapping, high‐field stressing, the initial current transients at constant gate voltage, and breakdown statistics were performed on these oxides to examine the correlation between positive charge and breakdown. The conclusion is that positive‐charge generation is only one of the processes occurring during high‐field stress but is not the main cause for breakdown. Large current increases were observed for oxides that have large hole‐trapping efficiencies, but the current increase is followed by fast current decay. The mechanism causing the current decay was investigated and was found to be an intrinsic mechanism which is related to the neutralization of the positive charge. These processes alw...

Base profile design for high-performance operation of bipolar transistors at liquid-nitrogen temperature

Measurements of thin epitaxial-base polysilicon-emitter n-p-n transistors with increasing base doping show the effects of bandgap narrowing, mobility changes, and carrier freezeout. At room temperature the collector current at low injection is proportional to the integrated base charge, independent of the impurity distribution. At temperatures below 150 K, however, minority injection is dominated by the peak base doping because of the greater effectiveness of bandgap narrowing. When the peak doping in the base approaches 10/sup 19/ cm/sup -3/, the bandgap difference between emitter and base is sufficiently small that the current gain no longer monotonically decreases with lower temperature but instead shows a maximum as low as 180 K. The device design window appears limited at the low-current end by increased base-emitter leakage due to tunneling and by resistance control at the high-current end. Using the measured DC characteristics, circuit delay calculations are made to estimate the performance of an emitter-coupled logic ring oscillator at room and liquid-nitrogen temperatures. It is shown that if the base doping can be raised to 10/sup 19/ cm/sup -3/ while keeping the base thickness constant, the minimum delay at liquid-nitrogen temperature can approach the delay of optimized devices at room temperature. >

Influence of localized latent defects on electrical breakdown of thin insulators

Electrical breakdown in thin SiO/sub 2/ films was measured with different techniques at different electric fields. It is shown that oxide reliability is affected by the presence of latent defects requiring a certain time to develop and evolve towards a destructive stage. As such a time is weakly dependent on applied fields, breakdown is not adequately detected by accelerated tests. It is shown that, due to the localized nature of breakdown, meaningful relationships between measured parameters able to clarify the microscopic nature of oxide failure are not easy to establish. >

High performance operation of silicon bipolar transistors at liquid nitrogen temperature

The feasibility of liquid nitrogen temperature operation of homojunction Si bipolar transistors is examined. Measurements of thin base NPN transistors with increasing base doping show the competing mechanisms of bandgap narrowing with mobility and freeze-out. When the bandgap difference between emitter and base becomes small, a maximum in current gain versus temperature is observed because the increase in mobility dominates at temperatures as low as 180 Kelvin. Delay calculations are made to compare the performance of an ECL ring-oscillator at room and liquid nitrogen temperature. It is shown that little performance degradation occurs if the base doping can be raised to 1 - 2 × 1019cm-3to prevent freeze-out of the base doping, while keeping the base thickness within 1000 Å. The upper limit on base doping appears to be set by increased base-emitter leakage due to tunneling.

The Mechanism of Oxidation of SiGe.

We have studied the oxidation mechanism of SiGe alloys, prepared by UHV/CVD. Ge is found to be completely rejected from the oxide and to pile up at the oxide/substrate interface. In the case of thick SiGe layers, this can give rise to very heavily concentrated layers of SiGe (up to 80% Ge) several hundreds of angstroms thick between the oxide and the substrate. The oxidation of Si then proceeds by Si diffusing though the Ge-rich layer. When only a thin layer of Ge is present on top of Si at the start, it acts as a marker, moving, unchanged, with the SiO 2 /Si interface. The rate of oxidation is enhanced in this case. The atomic motion and chemical reactions involved in the process are discussed and possible mechanisms are described to explain the data. Oxidation of a SiGeB sample is also discussed and compared with the previous case.

Excimer laser‐induced deposition of silicon nitride thin films

Silicon nitride films have been prepared by excimer laser photolysis of ammonia/silane or ammonia/disilane mixtures at temperatures in the range 225–625u2009°C in a hot‐walled low‐pressure reactor. The highest‐quality films, deposited at 225u2009°C, have high breakdown‐field strength, Ebd=8.8 Mvu2009cm−1, low midgap interface‐state‐trap densities, Nit=1.7×1011 eV−1u2009cm−2, and a dielectric constant of e=4.8.

Reaction mechanism of cobalt with silicon dioxide

The reaction mechanism of thin cobalt (Co) films with silicon dioxide (SiO2) substrate under rapid thermal annealing conditions has been investigated. Reaction of thin cobalt film (12.5 nm) with a SiO2 substrate is observed in an inert ambient (N2) and in vacuum (∼10−8 Torr). The reaction is manifested by the formation of craterlike depressions on the SiO2 substrate and by the presence of a Co2SiO4 reaction product determined by transmission electron microscopy diffraction patterns. Much less damage is observed with no reaction product observed if the samples are annealed in a forming gas ambient (90% N2/10% H2), the cobalt film is much thicker (150 nm), or the cobalt film is in situ cleaned (e.g., 5 min in 400u2009°C, forming gas ambient) prior to annealing in either inert or vacuum ambient. It is proposed that the presence of oxygen is required in order to initiate the reaction between cobalt and SiO2. The source of the oxygen contaminant, in our studies, is the oxygen on the surface of the cobalt film. The...

Characterization of IC devices fabricated in low temperature (550°c) epitaxy by UHV/CVD technique

Sub-micron insitu doped silicon epitaxial films have been successfully grown at temperatures as low as 550°C by a novel UHV/CVD process. Extensive electrical characterization of test devices fabricated in these films using low temperature (≤ 880°C) processing indicated that the epilayers were of high quality. The n+-p junctions exhibit ideal characteristics with ideality of 1.0, and reverse bias leakage current density of < 2.5 fA µm-2at 5 Volts. Carrier lifetime measurement from MOS capacitors was as high as 160 µsecs. C-V and oxide breakdown data further confirmed the suitability of these layers for VLSI devices.

Influence of sputter gas and sputter pressure on the structure and magnetic properties of Co–Pt–Cr thin films (abstract)

The magnetic properties of high density recording media have been shown to be very sensitive to the film microstructure. We have investigated the influences of different sputter gases (Ar, Xe, Kr) and different sputter pressures (3 to 24 mTorr) on the microstructures and magnetic properties of CoPtCr/Cr thin films. The magnetic properties of the films (coercive field, Hc, coercive squareness, S*, and remanant moment, Mr) were determined using a vibrating sample magnetometer. The microstructures were examined by transmission electron microscopy and x‐ray diffraction. For all three gases studied the microstructures and properties of the CoPtCr films were found to change with sputter pressure in similar manners. Films deposited at the lowest pressure consisted of well connected, equiaxed grains. With only a slight increase in pressure, the grains formed chains separated by small gaps. As the gas pressure increased further, the chains became better defined and the gaps between them widened. The coercive squar...