H. Cerva

Impact of copper contamination on the quality of silicon oxides

To study the impact of metal contamination on the quality of gate oxides, (100) silicon wafers were intentionally contaminated with copper from the backside. The in‐diffusion of copper and/or oxidation were performed in a rapid thermal annealing system. Gate oxide areas with low breakdown fields of about 2–3 MV/cm were located in a pinhole detector and correlate very well with the contaminated areas revealed by Secco defect etching. Using various analytical tools the failure mechanism was found to be related to the formation of Cu‐rich precipitates at the SiO2/Si interface. Depending on the in‐diffusion temperature (and hence on the supersaturation of Cu in Si), two different mechanisms were observed: At high supersaturation (1200 °C/30 s) Cu‐rich silicide particles can bend, crack, and finally penetrate the oxide layer. At lower in‐diffusion temperatures (900 °C/60 s) lens‐shaped Cu silicides form at the SiO2/Si interface and reduce the oxide thickness.

Influence of misfit stress on the magnetoresistive properties of La0.7Ca0.3MnO3−δ thin films

For this study La0.7Ca0.3MnO3−δ (LCMO) films of various thicknesses were deposited on SrTiO3(100), LaAlO3(100), and MgO(100) substrates. The substrate–film lattice mismatch causes changes in the microstructure of the films and can therefore influence their magnetoresistive behavior. The structure of the films was characterized by x-ray diffraction and high-resolution transmission electron microscopy. Magnetic and electric properties were determined using standard techniques. Samples grown on SrTiO3 show a strongly distorted growth. A large number of defects are incorporated in the initial LCMO layers. Beginning at a thickness of approximately 500 nm films on these substrates show strong deviations in electric behavior and develop macroscopic cracks. In the case of LaAlO3 substrates the films show only a few lattice defects. The substrate–film lattice mismatch is compensated by large stresses in the LCMO films. The best samples with the least amount of defects are grown on MgO substrates even though the su...

Defect formation in silicon at a mask edge during crystallization of an amorphous implantation layer

Implantation defects under a mask edge were studied by cross‐sectional transmission electron microscopy. An arsenic implantation with a dose of 5×1015 cm−2 as used for source/drain implantations in metal‐oxide‐semiconductor transistor structures completely amorphizes a 63‐nm‐thick silicon surface layer and produces a sharply curved amorphous/crystalline interface under the mask edge. Annealing at 900 °C results in the formation of vacancy‐type dislocation half‐loops or microtwinning on {111} planes under the mask edge. Partial crystallization of the amorphous layer at 500 °C revealed a notch in the amorphous/crystalline interface which is shown to be due to the different epitaxial regrowth rates on the various lattice planes. On further annealing, the above‐mentioned defects are generated when the crystallization fronts on both sides of the notch join.

Nanostructured Nd–Fe–B magnets with enhanced remanence

Nanostructured isotropic Nd–Fe–B magnets with enhanced remanence were produced by hot compaction of mechanically alloyed Nd–Fe–B powders at temperatures of about 600 °C. Phase formation occurred during hot pressing without significant grain growth. The microstructure is a two phase nanocomposite of hard magnetic Nd2Fe14B and soft magnetic α‐Fe with an average grain size of about 20 nm. These small dimensions allow effective exchange interactions between hard and soft magnetic grains and result in an enhancement of the remanence by more than 30% above the Stoner–Wolfarth limit expected for noninteracting single domain particles. So far, the best results are a remanence of 1.0 T, an energy density of 121 kJ/m3, and a coercivity of 4.2 kA/cm for a Nd–Fe–Co–Si–B magnet.

Novel relaxation process in strained Si/Ge superlattices grown on Ge (001)

A new mode of misfit defect formation has been observed for the first time in high quality Si/Ge strained‐layer superlattices grown by molecular beam epitaxy on Ge(001). In order to investigate the transition from coherent to incoherent growth we have studied a set of samples with a varying number of superlattice periods by transmission electron microscopy. High‐resolution lattice imaging reveals that strain relaxation occurs through successive glide of 90° (a/6)〈211〉 Shockley partial dislocations on adjacent {111} planes. The resulting microtwins represent the only relaxation mechanism we observed in the samples.

Investigation of the critical layer thickness in elastically strained InGaAs/GaAlAs quantum wells by photoluminescence and transmission electron microscopy

We have studied strained InGaAs quantum wells with GaAlAs barriers, grown by molecular beam epitaxy, varying In content and well thickness. Photoluminescence measurements were made at 12 K. The appearance of a characteristic additional exciton‐like photoluminescence peak indicates the transition between elastically strained and relaxed layers. This transition was also observed by the occurrence of misfit dislocations in the corresponding transmission electron microsope (TEM) images. Layer thicknesses and In content were also determined by TEM. The results give a critical layer thickness of 29±0.5 nm at an In content of 32±2%. This value lies about a factor of 3–4 above the critical layer thickness calculated by Matthews and Blakeslee [J. Cryst. Growth 27, 118 (1974)].

Structure and magnetoresistive properties in La–manganite thin films

This study investigates the structure of perovskite thin films and its influence on their colossal magnetoresistance (CMR) properties. Epitaxial thin films of perovskite manganites La1−xBxMnO3−δ (B=Ca,Sr) were prepared on SrTiO3 (100) substrates using on- and off-axis pulsed laser deposition (PLD) techniques. X-ray diffraction, resistance and magnetoresistance measurements, as well as high-resolution transmission electron microscopy (HRTEM) investigations were carried out. HRTEM observations reveal epitaxial growth for the first few layers of all prepared samples. Thicker on-axis prepared films grow with a large number of defects, whereas off-axis prepared samples grow in a columnar structure. Since the magnetic properties in systems with double-exchange interaction are very sensitive to the local structure it has great influence on the electronic properties.

Ferroelectric strontium bismuth tantalate thin films deposited by metalorganic chemical vapour deposition (MOCVD)

Thin films of Sr 1-x Bi 2+x Ta 2 O 9 (SBT) have been deposited by metalorganic chemical vapor deposition (MOCVD) on 150 mm Si wafers with Pt/Ti electrodes. The choice of Bi precursor significantly affects the process; film homogeneity is significantly improved when using a β-diketonate Bi precursor in combination with compatible Sr and Ta precursors. A highly repeatable process has been developed, with good run-to-run composition and thickness control. Effects of Bi volatility have been investigated in annealing experiments that show the onset of Bi loss at ∼570°C at reduced pressure (1-10 Torr). Film properties relevant to integrated ferroelectric random access (Fe RAMS) memories have also been characterized. Remenant polarizations (2P r ) up to 24 μC cm -2 have been obtained at 5 V, with 90% saturation of 2P r at 1.5 V and a coercive voltage of 0.52 V for a 140 mn film. Electrical leakage current density values were < 2×10 -8 A cm -2 at 1.5 V. Fatigue endurance has been measured to 10 11 cycles with < 10% degradation in switched charge.

InP on Si(111): Accommodation of lattice mismatch and structural properties

The growth of InP by low‐pressure metalorganic chemical vapor deposition on vicinal Si(111), misoriented 3° towards [112], is reported. By double crystal x‐ray diffraction an order of magnitude improvement in structural quality is measured as compared to InP on Si(001). From high resolution electron microscopy near the interface a heavily twinned region of 25 nm in thickness is found with microtwins having their twin planes parallel to the Si(111) surface. In this region the 8% misfit between InP and Si is completely accommodated in one dimension by partial dislocations having Burgers vectors a/6〈112〉 (Schockley partial dislocations) in the (111) surface plane which are associated with the formation of the thin twinned region close to the interface. Above this zone the crystallographic orientations of the InP are identical to those of the Si substrate.

Effect of compressive and tensile strain on misfit dislocation injection in SiGe epitaxial layers

The relaxation behavior of short‐period Si/Ge superlattices and SixGe1−x alloy layers under compressive and tensile strain field is compared experimentally by means of transmission electron microscopy as well as theoretically on the basis of a half‐loop dislocation nucleation model. It was found that misfit dislocations in tensily strained layers grown on Ge(001) substrates are imperfect and of the 90° Shockley type provided some critical misfit fc is exceeded. Subsequent nucleation and glide of these partial dislocations on adjacent {111} glide planes leads to the formation of stacking faults and microtwins. In the low misfit regime (f<fc), the nucleation of 60° perfect dislocations is energetically favorable. In contrast, misfit dislocations in layers which experience a compressive strain field within the (001) growth plane are generally of the 60° type. In this case the critical thickness for coherent growth is found to be substantially enlarged with respect to the inverse strain situation where microt...