Temperature-dependent photoluminescence of Ge/Si and Ge 1-ySn y/Si, indicating possible indirect-to-direct bandgap transition at lower Sn content

Temperature (T)-dependent photoluminescence (PL) has been investigated for both p-Ge and n-Ge1-ySny films grown on Si substrates. For the p-Ge, strong direct bandgap (ED) along with weak indirect bandgap related (EID) PL at low temperatures (LTs) and strong ED PL at room temperature (RT) were observed. In contrast, for the n-Ge1-ySny, very strong dominant EID PL at LT and strong ED PL were observed at RT. This T-dependent PL study indicates that the indirect-to-direct bandgap transitions of Ge1-ySny might take place at much lower Sn contents than the theory predicts, suggesting that these Ge1-ySny could become very promising direct bandgap semiconductors

Observation of Heavy- and Light-hole Split Direct Bandgap Photoluminescence from Tensile-strained GeSn (0.03% Sn)

Temperature- (T-) and laser power-dependent photoluminescence (PL) measurements have been made for the tensile-strained, undoped GeSn (0.03% Sn) film grown on Si substrate. The PL results show not only clear strain-split direct bandgap transitions to the light-hole (LH) and heavy-hole (HH) bands at energies of 0.827 and 0.851 eV at 10 K, respectively, but also clearly show both strong direct and indirect bandgap related PL emissions at almost all temperatures, which are rarely observed. This split of PL emissions can be directly observed only at low T and moderate laser power, and the two PL peaks merge into one broad PL peak at room temperature, which is mainly due to the HH PL emission rather than LH transition. The evolution of T-dependent PL results also clearly show the competitive nature between the direct and indirect bandgap related PL transitions as T changes. The PL analysis also indicates that the energy gap reduction in Γ valley could be larger, whereas the bandgap reduction in L valley could be smaller than the theory predicted. As a result, the separation energy between Γ and L valleys (∼86 meV at 300 K) is smaller than theory predicted (125 meV) for this Ge-like sample, which is mainly due to the tensile strain. This finding strongly suggests that the indirect-to-direct bandgap transition of Ge1−ySny could be achieved at much lower Sn concentration than originally anticipated if one utilizes the tensile strain properly. Thus, Ge1−ySny alloys could be attractive materials for the fabrication of direct bandgap Si-based light emitting devices

Direct Bandgap Cross-over Point of Ge\u3csub\u3e1-y\u3c/sub\u3eSn\u3csub\u3ey\u3c/sub\u3e Grown on Si Estimated through Temperature-dependent Photoluminescence Studies

Epitaxial Ge1-ySny (y = 0%–7.5%) alloys grown on either Si or Ge-buffered Si substrates by chemical vapor deposition were studied as a function of Sn content using temperature-dependent photoluminescence (PL). PL emission peaks from both the direct bandgap (Γ-valley) and the indirect bandgap (L-valley) to the valence band (denoted by ED and EID, respectively) were clearly observed at 125 and 175 K for most Ge1-ySny samples studied. At 300 K, however, all of the samples exhibited dominant ED emission with either very weak or no measureable EID emission. At 10 K, ED is dominant only for Ge1-ySny with y \u3e 0.052. From the PL spectra taken at 125 and 175 K, the unstrained indirect and direct bandgap energies were calculated and are plotted as a function of Sn concentration, the results of which show that the indirect-to-direct bandgap transition occurs at ∼6.7% Sn. It is believed that the true indirect-to-direct bandgap cross-over of unstrained Ge1-ySny might also take place at about the same Sn content at room temperature. This observation suggests that these Ge1-ySny alloys could become very promising direct bandgap semiconductor materials, which will be very useful for the development of various new novel Si- and Ge-based infrared optoelectronic devices that can be fully integrated with current technology on a single Si chip

Complementary Metal-oxide Semiconductor-compatible Detector Materials with Enhanced 1550 nm Responsivity via Sn-doping of Ge/Si(100)

Previously developed methods used to grow Ge1−ySny alloys on Si are extended to Sn concentrations in the 1019−1020 cm−3 range. These concentrations are shown to be sufficient to engineer large increases in the responsivity of detectors operating at 1550 nm. The dopant levels of Sn are incorporated at temperatures in the 370–390 °C range, yielding atomically smooth layers devoid of threading defects at high growth rates of 15–30 nm/min. These conditions are far more compatible with complementary metal-oxide semiconductor processing than the high growth and processing temperatures required to achieve the same responsivity via tensile strain in pure Ge on Si. A detailed study of a detector based on a Sn-doped Ge layer with 0.25% (1.1 × 1020 cm−3) Sn range demonstrates the responsivity enhancement and shows much better I-V characteristics than previously fabricated detectors based on Ge1−ySny alloys with y = 0.02

Degenerate Parallel Conducting Layer and Conductivity Type Conversion Observed from \u3ci\u3ep\u3c/i\u3e-Ge\u3csub\u3e1 - y\u3c/sub\u3eSn\u3csub\u3ey\u3c/sub\u3e (y = 0.06%) Grown on \u3ci\u3en\u3c/i\u3e-Si Substrate

Electrical properties of p-Ge1−ySny (y = 0.06%) grown on n-Si substrate were investigated through temperature-dependent Hall-effect measurements. It was found that there exists a degenerate parallel conducting layer in Ge1−ySny/Si and a second, deeper acceptor in addition to a shallow acceptor. This parallel conducting layer dominates the electrical properties of the Ge1−ySny layer below 50 K and also significantly affects those properties at higher temperatures. Additionally, a conductivity type conversion from p to n was observed around 370 K for this sample. A two-layer conducting model was used to extract the carrier concentration and mobility of the Ge1−ySny layer alone

Meteorological drivers of respiratory syncytial virus infections in Singapore

Meteorological drivers are known to affect transmissibility of respiratory viruses including respiratory syncytial virus (RSV), but there are few studies quantifying the role of these drivers. We used daily RSV hospitalization data to estimate the daily effective reproduction number (Rt), a real-time measure of transmissibility, and examined its relationship with environmental drivers in Singapore from 2005 through 2015. We used multivariable regression models to quantify the proportion of the variance in Rt explained by each meteorological driver. After constructing a basic model for RSV seasonality, we found that by adding meteorological variables into this model we were able to explain a further 15% of the variance in RSV transmissibility. Lower and higher value of mean temperature, diurnal temperature range (DTR), precipitation and relative humidity were associated with increased RSV transmissibility, while higher value of maximum wind speed was correlated with decreased RSV transmissibility. We found that a number of meteorological drivers were associated with RSV transmissibility. While indoor conditions may differ from ambient outdoor conditions, our findings are indicative of a role of ambient temperature, humidity and wind speed in affecting RSV transmission that could be biological or could reflect indirect effects via the consequences on time spent indoors.Published versio

Magnetic Properties of Transition Metal-Implanted Zno Nanotips Grown On Sapphire and Quartz

ZnO nanotips, grown on c-Al2O3 and quartz, were implanted variously with 200 keV Fe or Mn ions to a dose level of 5×1016cm−2. The magnetic properties of these samples were measured using a superconducting quantum interference device (SQUID) magnetometer. Fe-implanted ZnO nanotips grown on c-Al2O3 showed a coercive field width of 209 Oe and a remanent field of 12% of the saturation magnetization (2.3×10−5emu) at 300K for a sample annealed at 700°C for 20 minutes. The field-cooled and the zero-field-cooled magnetization measurements also showed evidence of ferromagnetism in this sample with an estimated Curie temperature of around 350 K. The Mn-implanted ZnO nanotips grown on c-Al2O3 showed superparamagnetism resulting from the dominance of a spin-glass phase. The ZnO nanotips grown on quartz and implanted with Fe or Mn showed signs of ferromagnetism, but neither was consistent

Magnetic and Electronic Properties of a Mn Delta-doping GaN Layer

The magnetization curve as a function of the magnetic field at 5 K showed that the magnetization of the Mn delta-doped (Ga0.995Mn0.005)N thin films was significantly enhanced in comparison with that of the conventionally-doped (Ga0.995Mn0.005)N thin films. The magnetization curve as a function of the temperature showed that the Curie temperature of the Mn delta-doped (Ga0.995Mn0.005)N thin films was above room temperature. The theoretical electronic results showed that Ga vacancies near the Mn delta-doping layer were likely to cause p-type conductance, indicative of an enhancement of the magnetic properties in (Ga1-xMnx)N thin films

Burden, etiology, and risk factors of respiratory virus infections among symptomatic preterm infants in the tropics : a retrospective single-center cohort study

The burden of respiratory viral infections (RVIs) among preterm infants in the first few years of life, especially those living in the tropics with year-long transmissions of respiratory viruses, remains unknown. We aimed to describe the clinical epidemiology and associated risk factors for RVIs among symptomatic preterm infants ≤32 weeks up to 2 years of life