Built-in Defects Research Articles

Application of optical emission microscopy for reliability studies in 4H–SiC p+/n−/n+ diodes

An optical emission microscopy technique with spatial and spectral resolution capabilities is applied for stability studies of 4H–SiC material properties. From the example of a 4H–SiC p+/n−/n+ diode imaged at different stages of electrical overstress the mechanism of degrading performance is directly unveiled. We correlate this phenomenon with irreversible structural changes within the active region created by a nonuniform heating related stress. The stress-generated features are interpreted as multiple stacking faults spreading throughout the whole base region and nucleated in the vicinity of built-in defects and process-induced structural deficiencies.

Growth of silicon nanostructures on graphite

Silicon nanostructures such as small clusters, superclusters, and elongated chains, with an average diameter of a few nanometers, have been synthesized by magnetron sputtering on cleaved highly oriented pyrolytic graphite (HOPG). Scanning tunneling microscopy (STM) reveals that flat, defect-poor areas of the HOPG surface are covered with almost uniformly sized silicon clusters of 0.6±0.2 nm, 5.1±1.2 nm, and 15.4±3 nm diameter. Surface regions with defects such as pits and craters, descending a few layers into the graphite surface, are sparsely covered with silicon. Most of the deposited material, with an average diameter of 2 nm, is found to be attached to the monatomic step edges forming the crater rims. A simulation of the growth process, i.e. deposition of silicon atoms onto a surface with built-in defects, and subsequent surface diffusion and aggregation of the adatoms, convincingly reproduces most of the Si nanostructures observed in the STM topographs.

Gain spectroscopy and vertical cavity devices in wide-gap semiconductors

The InGaN quantum-well diode laser is now reaching the threshold of commercial availability as an edge emitting blue/violet device, yet many questions remain concerning the details of its operation. For example, the threshold current density in the best cw InGaN QW lasers is still about one order of magnitude larger than that in the green ZnCdSe QW diode devices, in spite of the materials being quite similar in the effective mass band picture. On the one hand, the nitride semiconductors are remarkably tolerant of built-in crystalline defects at high current densities. On the other, the microscopics of optical gain are significantly influenced by crystalline `disorder’ that emanates from indium compositional anomalies in the active InGaN quantum-well material.

IDDQ Testing Experiments for Various CMOS Logic Design Structures

In this paper, we present two studies. The first study constitutes an assessment of the effectiveness of IDDQ (quiescent power supply current) in detecting transistor-level defects for three CMOS logic design styles. This study was carded out by designing, simulating, fabricating, and testing CMOS devices with built-in defects. The second study involves an assessment of IDDQ in a production-type environment and the effect of bum-in on IDDQ levels. This study was carried out in a production facility. The results show that IDDQ testing can detect some types of defects in precharge and pseudo-NMOS circuits but may require partitioning circuitry for the latter.

Transient power supply current monitoring?A new test method for CMOS VLSI circuits

We present a new method for testing digital CMOS integrated circuits. The new method is based on the following premise: monitor the switching behavior of a circuit as opposed to the output logic state. We use the transient power supply current as a window of observability into the circuit switching behavior. A method for isolating normal switching transients from those which result from defects is introduced. The feasibility of this new testing approach is investigated by conducting several experiments involving the design of integrated circuits with built-in defects, fabrication, and physical testing. The results of these experiments show this new test method to be a promising one for detecting defects that can escape stuck-at testing andI DDQ testing.

Defects in amorphous and solid phase epitaxial silicon

The microstructural morphology of amorphous Si (a-Si) layers deposited in ultrahigh vacuum, as well as crystalline Si grown by solid phase epitaxy (SPE), was studied as a function of Al doping and vapour beam incidence angle. The microstructure of the films was investigated using cross-section transmission electron microscopy. All a-Si layers have a columnar structure, with an average column width of ⩽ 5 nm. The direction of the columns abruptly changes with the change of deposition direction and shows local column tilts and void formation at substrate surface irregularities. These built-in defects in the a-Si films also influence the defect structure in epitaxial Si films grown by SPE. Voids are initially aligned along the column directions and extra voids form owing to irregularities of the columnar structure. Doping of amorphous Si with Al to 10 18−10 20 cm −3 does not leave detectable effects in the amorphous structure itself, but will increase the void density of the re-grown SPE Si layers. Furthermore, segregation of Al resulting in metallic inclusions in the amorphous crystalline interface causes metal induced crystallization of Si at temperatures far below the normal SPE regrowth temperature, thus preventing the formation of single crystalline silicon in a single-step process.

Optical non-desctructive tests for the evaluation of microprecipitates in semiconductors and devices

As semiconductor technologies evolve from micrometre to submicrometre scale the devices become more sensitive to built-in defects which have not generally been considered previously because of their very small size. In particular, the metallurgy of the materials and the various fabrication steps give rise to small aggregates of foreign or excess atoms of nanometre size and a density in the range of 10 10 cm −3. There are actually very few observation techniques available for revealing these flaws inside the materials non-destructively. Of course photon-based techniques fit more consistently with the need for non-invasive and non-destructive inspection. Laser scanning tomography (LST) was the first solution providing three-dimensional information on scattering particles as small as 1 nm in bulk semi-conductors. Numerous investigations have been performed in III–V compounds as well as in silicon where this method, to date, is the only realistic way of studying the generation of oxide-related internal gettering centres. Recent results in the field are summarized. Nevertheless, LST images cannot easily be obtained from regions just below the surface because of excessive scattering from all surface artefacts (defects as well as structures). It follows that specific modifications have to be introduced in the technique, though the basic argument for light-scattering images is still preserved. Ogawa recently showed that silica particles in SIMOX layers can be investigated using a Brewster illumination whereas Fillard and Montgomery have revealed grown-in particles in MOCVD GaAs layers. These techniques are still in progress, along with a new method of particle submicrometre ranging called Nanolidar which is intended to evaluate the position of particles to a very high precision. This makes it possible to explore layered structures and to identify the exact location of the disturbing defects introduced by the epitaxial or annealing processes. The present state of the art is detailed. New calculation methods are tested competitively in a computer simulation of three-dimensional diffraction patterns. It turns out that the Fourier, phase shift method can be used profitably to provide three-dimensional ranging of the particles with a largely submicrometre accuracy.

Identification of defects in amorphous silicon.

Defects in amorphous Si:H are identified by a new atomic model encompassing built-in defects and light-induced defects. The model consists of a metastable localized center having an atom with two possible sites, as in the Chadi and Chang model for DX centers in crystalline GaAs. Specific rehybridizations for two-site atoms produce changes in charge state and bonding that explain a wide range of effects, including n-type or p-type doping or compensation, the Staebler-Wronski effect and its variations with doping, quench-induced defects, and defect spin

Linkage of Efficiency and Stability of a-Si Solar cells

ABSTRACTBy combining the steady-state and transient behaviors of a recently generalized analysis of the kinetics of metastable defects in good amorphous Si:H, it is shown that no treatment can remove all metastable defects. There is always a significant remnant that is proposed to be the observed built-in defects, which are then not a separate species, distinct from the metastable defects. This remnant is due to vibrational breaking of weak bonds in latent defect centers, and cannot be due to recombination of thermally excited carriers. If the stability can be improved by reducing the number of these latent defect centers, this will also reduce the density of built-in defects that control the initial efficiency. Furthermore, there are good reasons to believe that the source of these defects is extrinsic to a-Si:H, so that improvement in both properties may be achievable.

Systemic and prosthodontic treatment to prevent bone resorption in edentulous patients

There are two main factors responsible for loss of the alveolar process under dentures. These are the body chemistry of the patient and the built-in defects of complete dentures. The systemic factors related to bone resorption and correction procedures were described. An occlusal plan for complete dentures that reduces bone resorption was suggested. There are two main factors responsible for loss of the alveolar process under dentures. These are the body chemistry of the patient and the built-in defects of complete dentures. The systemic factors related to bone resorption and correction procedures were described. An occlusal plan for complete dentures that reduces bone resorption was suggested.