Evolution of point defects in pulsed-laser-melted Ge1-xSnxprobed by positron annihilation lifetime spectroscopy

Direct-band-gap Germanium-Tin alloys (Ge1-xSnx) with high carrier mobilities are promising materials for nano- and optoelectronics. The concentration of open volume defects in the alloy, such as Sn and Ge vacancies, influences the final device performance. In this article, we present an evaluation of the point defects in molecular-beam-epitaxy grown Ge1-xSnxfilms treated by post-growth nanosecond-range pulsed laser melting (PLM). Doppler broadening - variable energy positron annihilation spectroscopy and variable energy positron annihilation lifetime spectroscopy are used to investigate the defect nanostructure in the Ge1-xSnxfilms exposed to increasing laser energy density. The experimental results, supported with ATomic SUPerposition calculations, evidence that after PLM, the average size of the open volume defects increases, which represents a raise in concentration of vacancy agglomerations, but the overall defect density is reduced as a function of the PLM fluence. At the same time, the positron annihilation spectroscopy analysis provides information about dislocations and Ge vacancies decorated by Sn atoms. Moreover, it is shown that the PLM reduces the strain in the layer, while dislocations are responsible for trapping of Sn and formation of small Sn-rich-clusters.

Band-gap and strain engineering in GeSn alloys using post-growth pulsed laser melting

The pseudomorphic growth of Ge_1-xSn_xon Ge causes in-plane compressive strain, which degrades the superior properties of the Ge_1-xSn_xalloys. Therefore, efficient strain engineering is required. In this article, we present strain and band-gap engineering in Ge_1-xSn_xalloys grown on Ge a virtual substrate using post-growth nanosecond pulsed laser melting (PLM). Micro-Raman and x-ray diffraction (XRD) show that the initial in-plane compressive strain is removed. Moreover, for PLM energy densities higher than 0.5 J cm^-2, the Ge_0.89Sn_0.11layer becomes tensile strained. Simultaneously, as revealed by Rutherford Backscattering spectrometry, cross-sectional transmission electron microscopy investigations and XRD the crystalline quality and Sn-distribution in PLM-treated Ge_0.89Sn_0.11layers are only slightly affected. Additionally, the change of the band structure after PLM is confirmed by low-temperature photoreflectance measurements. The presented results prove that post-growth ns-range PLM is an effective way for band-gap and strain engineering in highly-mismatched alloys.

Ellipsometric characterization of doped Ge0.95Sn0.05 films in the infrared range for plasmonic applications

GeSn as a group-IV material opens up new possibilities for realizing photonic device concepts in Si-compatible fabrication processes. Here we present results of the ellipsometric characterization of highly p- and n-type doped Ge_0.95Sn_0.05 alloys deposited on Si substrates investigated in the wavelength range from 1 to 16 μm. We discuss the suitability of these films for integrated plasmonic applications in the infrared region.

A quantitative MR measure of the fornix in schizophrenia

Some cognitive disturbances accompanying schizophrenia may be due to abnormalities in the thalamus and components of the limbic system. The fornix is an important white-matter relay pathway connecting these structures and is likely to be affected in schizophrenia as well.Magnetic resonance images of the fornix were analyzed in 15 schizophrenic patients and 15 matched comparison group subjects. Fornix volume was compared between the two groups and was also correlated with the volumes of other neuroanatomical structures, as well as with illness presentation, clinical status, and cognitive/psychological measures. There was no significant difference in fornix volume between the two groups. Of note, fornix volume correlated significantly with the volumes of the hippocampus, parahippocampus, and the superior temporal gyrus in the schizophrenic subjects, but not in the controls. Moreover, the correlation between fornix and parahippocampal gyrus volumes differed significantly between the two groups. No association was found between fornix volume and illness presentation or between fornix and cognitive/clinical measures.Results suggest that there are no marked changes in fornix volume in schizophrenia by MRI. The fornix, however, may be part of a network of structures affected in schizophrenia, as indicated by correlated volumetric changes.

Subgenual cingulate cortex volume in first-episode psychosis

OBJECTIVE

Gray matter volume and glucose utilization have been reported to be reduced in the left subgenual cingulate of subjects with familial bipolar or unipolar depression. It is unclear whether these findings are secondary to recurrent illness or are part of a familial/genetic syndrome. The authors' goal was to clarify these findings.

METHOD

Volumetric analyses were performed by using magnetic resonance imaging in 41 patients experiencing their first episode of affective disorder or schizophrenia and in 20 normal comparison subjects.

RESULTS

The left subgenual cingulate volume of the patients with affective disorder who had a family history of affective disorder was smaller than that of patients with affective disorder with no family history of the illness and the normal comparison subjects. Patients with schizophrenia did not differ from comparison subjects in left subgenual cingulate volume.

CONCLUSIONS

Left subgenual cingulate abnormalities are present at first hospitalization for psychotic affective disorder in patients who have a family history of affective disorder.

MRI anatomy of schizophrenia

Structural magnetic resonance imaging (MRI) data have provided much evidence in support of our current view that schizophrenia is a brain disorder with altered brain structure, and consequently involving more than a simple disturbance in neurotransmission. This review surveys 118 peer-reviewed studies with control group from 1987 to May 1998. Most studies (81%) do not find abnormalities of whole brain/intracranial contents, while lateral ventricle enlargement is reported in 77%, and third ventricle enlargement in 67%. The temporal lobe was the brain parenchymal region with the most consistently documented abnormalities. Volume decreases were found in 62% of 37 studies of whole temporal lobe, and in 81% of 16 studies of the superior temporal gyrus (and in 100% with gray matter separately evaluated). Fully 77% of the 30 studies of the medial temporal lobe reported volume reduction in one or more of its constituent structures (hippocampus, amygdala, parahippocampal gyrus). Despite evidence for frontal lobe functional abnormalities, structural MRI investigations less consistently found abnormalities, with 55% describing volume reduction. It may be that frontal lobe volume changes are small, and near the threshold for MRI detection. The parietal and occipital lobes were much less studied; about half of the studies showed positive findings. Most studies of cortical gray matter (86%) found volume reductions were not diffuse, but more pronounced in certain areas. About two thirds of the studies of subcortical structures of thalamus, corpus callosum and basal ganglia (which tend to increase volume with typical neuroleptics), show positive findings, as do almost all (91%) studies of cavum septi pellucidi (CSP). Most data were consistent with a developmental model, but growing evidence was compatible also with progressive, neurodegenerative features, suggesting a "two-hit" model of schizophrenia, for which a cellular hypothesis is discussed. The relationship of clinical symptoms to MRI findings is reviewed, as is the growing evidence suggesting structural abnormalities differ in affective (bipolar) psychosis and schizophrenia.

Left planum temporale volume reduction in schizophrenia

BACKGROUND

The planum temporale, located on the posterior and superior surface of the temporal lobe, is a brain region thought to be a biological substrate of language and possibly implicated in the pathophysiology of schizophrenia. To investigate further the role of planum temporale abnormalities in schizophrenia, we measured gray matter volume underlying the planum temporale from high spatial resolution magnetic resonance imaging techniques.

METHODS

Sixteen male patients with chronic schizophrenia and 16 control subjects were matched for age, sex, handedness, and parental socioeconomic status. Magnetic resonance imaging images were obtained from a 1.5-T magnet.

RESULTS

Gray matter volume was significantly reduced in the left planum temporale (28.2%) in schizophrenic patients compared with normal controls. Schizophrenic patients showed a reversal of the left greater than right planum temporale asymmetry found in normal controls. Heschl's gyrus (primary auditory cortex) showed no differences between the left and right sides in either group. Of note, the Suspiciousness/Persecution subscale score of the Positive and Negative Syndrome Scale was associated with reduced left planum temporale volume in schizophrenic patients.

CONCLUSIONS

Patients with schizophrenia have reduced left planum temporale gray matter and a reversal of planum temporale asymmetry, which may underlie an impairment in language processing and symptoms of suspiciousness or persecution characteristic of schizophrenia.

Lower left temporal lobe MRI volumes in patients with first-episode schizophrenia compared with psychotic patients with first-episode affective disorder and normal subjects

OBJECTIVE

Magnetic resonance imaging (MRI) studies of schizophrenic patients have revealed structural brain abnormalities, with low volumes of gray matter in the left posterior superior temporal gyrus and in medial temporal lobe structures. However, the specificity to schizophrenia and the roles of chronic morbidity and neuroleptic treatment in these abnormalities remain unclear.

METHOD

Magnetic resonance (1.5-T) scans were obtained from 33 patients with first-episode psychosis and 18 age-matched normal comparison subjects, all right-handed. Sixteen of the patients were diagnosed with affective disorder and 17 with schizophrenia.

RESULTS

Quantitative volumetric analysis showed that the patients with first-episode schizophrenia had significantly smaller gray matter volume in the left posterior superior temporal gyrus than did the patients with first-episode affective psychosis or the comparison subjects, with a significant left-less-than-right asymmetry. The schizophrenic patients also showed a smaller gray matter volume of the left posterior amygdala-hippocampal complex than the comparison subjects. Both the patients with schizophrenia and those with affective psychosis had significant left-less-than-right asymmetry of the posterior amygdala-hippocampal complex.

CONCLUSIONS

These findings suggest that temporal lobe abnormalities are present at the first hospitalization for schizophrenia and that low volume of the left posterior superior temporal gyrus gray matter is specific to schizophrenia compared with affective disorder.

MRI study of cavum septi pellucidi in schizophrenia, affective disorder, and schizotypal personality disorder

OBJECTIVE

A cavum between the septi pellucidi may reflect neurodevelopmental anomalies in midline structures of the brain. The authors examined cavum septi pellucidi in subjects with schizophrenia, affective disorder, and schizotypal personality disorder and in normal subjects.

METHOD

Thirty schizophrenic patients (15 chronic, 15 first-episode), 16 patients with affective disorder (first-episode), 21 patients with schizotypal personality disorder, and 46 normal subjects were evaluated with magnetic resonance imaging. Cavum septi pellucidi was assessed by counting the number of 1.5-mm slices containing cavum septi pellucidi.

RESULTS

The presence or absence of cavum septi pellucidi did not differentiate among groups. However, the prevalence of abnormal cavum septi pellucidi (i.e., cavum septi pellucidi contained on four or more slices) was 30.4% for schizophrenic patients (36.4% for chronic, 25.0% for first-episode), 20.0% for patients with affective disorder, 18.8% for patients with schizotypal personality disorder, and 10.3% for normal subjects. When the authors used the Nopoulos et al. criteria for rating cavum septi pellucidi, which omitted borderline cases with cavum septi pellucidi on three slices, the prevalence of abnormal cavum septi pellucidi increased to 35.0% for schizophrenia (40.0% for chronic, 30.0% for first-episode), 25.0% for affective disorder, 27.3% for schizotypal personality disorder, and 13.0% for normal subjects. There was a statistically significant difference in ratings between schizophrenic and normal subjects.

CONCLUSIONS

The results suggest that alterations in midline structures during the course of neurodevelopment may play a role in the pathogenesis of schizophrenia.

First-episode schizophrenic psychosis differs from first-episode affective psychosis and controls in P300 amplitude over left temporal lobe

BACKGROUND

Schizophrenia is associated with central (sagittal) midline reductions of the P300 cognitive event-related potential and topographic asymmetry of P300, with reduced left temporal voltage. This P300 asymmetry is, in turn, linked to tissue volume asymmetry in the posterior superior temporal gyrus. However, it is unknown whether P300 asymmetry is specific to schizophrenia and whether central and lateral P300 abnormalities are due to chronic morbidity, neuroleptic medication, and/or hospitalization, or whether they are present at the onset of illness.

METHODS

P300 was recorded in first-episode schizophrenia, first-episode affective psychosis, and control subjects (n = 14 per group). Subjects silently counted rare (15%) target tones (1.5 kHz) among trains of standard tones (1.0 kHz). Averages were constructed from brain responses to target tones.

RESULTS

Peak amplitude of P300 and integrated voltage over 300 to 400 milliseconds were significantly different between first-episode schizophrenics and controls over the posterior sagittal midline of the head. First-episode schizophrenics displayed smaller amplitudes over the left temporal lobe than first-episode affective psychotics and controls, but the groups showed no differences over the right temporal lobe.

CONCLUSIONS

Left-sided P300 abnormality in first-episode schizophrenia relative to first-episode affective psychosis and controls suggests that P300 asymmetry is specific to schizophrenic psychosis and present at initial hospitalization. This P300 asymmetry suggests left temporal lobe dysfunction at the onset of schizophrenia.

Parcellation of the human prefrontal cortex using MRI

A methodology was developed for dividing prefrontal cortical gray matter into insular, orbital, inferior, middle, superior, cingulate, and frontal pole regions using anatomical criteria. This methodology was developed as a follow-up to one that measured whole prefrontal gray and white matter volumes in schizophrenic and control subjects. This study showed no overall volume differences in prefrontal cortex between schizophrenic and control subjects. The parcellation of prefrontal cortex was done to increase the probability of detecting abnormalities that were circumscribed to a particular portion of the region. A 1.5 Tesla magnet was used to acquire contiguous 1.5-mm coronal slices of the entire brain. Volumes were then measured in a group of right-handed male (n = 15) subjects. Gray matter was parcellated using criteria that were mainly based on gross anatomy, as visualized in 3-dimensional renderings of the brain. Reliability of the parcellation scheme was very high (r(i) = 0.80 and above). This methodology should be useful in the study of cortical pathology in a number of neurological disorders, including schizophrenia.