Development of germanium/silicon integration for near infrared detection

Silicon (Si) is the base material for electronic technologies and is emerging as a very attractive platform for photonic integrated circuits (PICs). PICs allow optical systems to be made more compact with higher performance than discrete optical components. Applications for PICs are in the area of fibre-optic communication, biomedical devices, photovoltaics and imaging. Germanium (Ge), due to its suitable bandgap for telecommunications and its compatibility with Si technology is preferred over III-V compounds as an integrated on-chip detector at near infrared wavelengths. There are two main approaches for Ge/Si integration: through epitaxial growth and through direct wafer bonding. The lattice mismatch of ~4.2% between Ge and Si is the main problem of the former technique which leads to a high density of dislocations while the bond strength and conductivity of the interface are the main challenges of the latter. Both result in trap states which are expected to play a critical role. Understanding the physics of the interface is a key contribution of this thesis. This thesis investigates Ge/Si diodes using these two methods. The effects of interface traps on the static and dynamic performance of Ge/Si avalanche photodetectors have been modelled for the first time. The thesis outlines the original process development and characterization of mesa diodes which were fabricated by transferring a ~700 nm thick layer of p-type Ge onto n-type Si using direct wafer bonding and layer exfoliation. The effects of low temperature annealing on the device performance and on the conductivity of the interface have been investigated. It is shown that the diode ideality factor and the series resistance of the device are reduced after annealing. The carrier transport mechanism is shown to be dominated by generation–recombination before annealing and by direct tunnelling in forward bias and band-to-band tunnelling in reverse bias after annealing. The thesis presents a novel technique to realise photodetectors where one of the substrates is thinned by chemical mechanical polishing (CMP) after bonding the Si-Ge wafers. Based on this technique, Ge/Si detectors with remarkably high responsivities, in excess of 3.5 A/W at 1.55 μm at −2 V, under surface normal illumination have been measured. By performing electrical and optical measurements at various temperatures, the carrier transport through the hetero-interface is analysed by monitoring the Ge band bending from which a detailed band structure of the Ge/Si interface is proposed for the first time. The above unity responsivity of the detectors was explained by light induced potential barrier lowering at the interface. To our knowledge this is the first report of light-gated responsivity for vertically illuminated Ge/Si photodiodes. The wafer bonding approach followed by layer exfoliation or by CMP is a low temperature wafer scale process. In principle, the technique could be extended to other materials such as Ge on GaAs, or Ge on SOI. The unique results reported here are compatible with surface normal illumination and are capable of being integrated with CMOS electronics and readout units in the form of 2D arrays of detectors. One potential future application is a low-cost Si process-compatible near infrared camera.

Fabrication and characterization of doped and porous silicon nanowires as anodes for lithium ion batteries

By using Si(100) with different dopant type (n++-type (As) or p-type (B)), it is shown how metal-assisted chemically (MAC) etched silicon nanowires (Si NWs) can form with rough outer surfaces around a solid NW core for p-type NWs, and a unique, defined mesoporous structure for highly doped n-type NWs. High resolution electron microscopy techniques were used to define the characteristic roughening and mesoporous structure within the NWs and how such structures can form due to a judicious choice of carrier concentration and dopant type. Control of roughness and internal mesoporosity is demonstrated during the formation of Si NWs from highly doped n-type Si(100) during electroless etching through a systematic investigation of etching parameters (etching time, AgNO3 concentration, %HF and temperature). Raman scattering measurements of the transverse optical phonon confirm quantum size effects and phonon scattering in mesoporous wires associated with the etching condition, including quantum confinement effects for the nanocrystallites of Si comprising the internal structure of the mesoporous NWs. Laser power heating of NWs confirms phonon confinement and scattering from internal mesoporosity causing reduced thermal conductivity. The Li+ insertion and extraction characteristics at n-type and p-type Si(100) electrodes with different carrier density and doping type are investigated by cyclic voltammetry and constant current measurements. The insertion and extraction potentials are demonstrated to vary with cycling and the occurrence of an activation effect is shown in n-type electrodes where the charge capacity and voltammetric currents are found to be much higher than p-type electrodes. X-ray photo-electron spectroscopy (XPS) and Raman scattering demonstrate that highly doped n-type Si(100) retains Li as a silicide and converts to an amorphous phase as a two-step phase conversion process. The findings show the succinct dependence of Li insertion and extraction processes for uniformly doped Si(100) single crystals and how the doping type and its effect on the semiconductor-solution interface dominate Li insertion and extraction, composition, crystallinity changes and charge capacity. The effect of dopant, doping density and porosity of MAC etched Si NWs are investigated. The CV response is shown to change in area (current density) with increasing NW length and in profile shape with a changing porosity of the Si NWs. The CV response also changes with scan rate indicative of a transition from intercalation or alloying reactions, to pseudocapactive charge storage at higher scan rates and for p-type NWs. SEM and TEM show a change in structure of the NWs after Li insertion and extraction due to expansion and contraction of the Si NWs. Galvanostatic measurements show the cycling behavior and the Coulombic efficiency of the Si NWs in comparison to their bulk counterparts.

Germanium MOS Capacitors with Hafnium Dioxide and Silicon Dioxide Dielectrics

Germanium (Ge) does not grow a suitable oxide for MOS devices. The Ge/dielectric interface is of prime importance to the operation of photo-detectors and scaled MOSTs. Therefore there is a requirement for deposited or bonded dielectric materials. MOS capacitors have been formed on germanium substrates with three different dielectric materials. Firstly, a thermally grown and bonded silicon dioxide (SiO2) layer, secondly, SiO2 deposited by atmospheric pressure CVD ‘silox’, and thirdly a hafnium oxide (HfO2) high-k dielectric deposited by atomic layer deposition (ALD). Ge wafers used were p-type 1 0 0 2 O cm. C–V measurements have been made on all three types of capacitors to assess the interface quality. ALD HfO2 and silox both display acceptable C–V characteristics. Threshold voltage and maximum and minimum capacitance values closely match expected values found through calculation. However, the bonded SiO2 has non-ideal C–V characteristics, revealing the presence of a high density of interface states. A H2/N2 post metal anneal has a detrimental effect on C–V characteristics of HfO2 and silox dielectrics, causing a shift in the threshold voltage and rise in the minimum capacitance value. In the case of hafnium dioxide, capacitor properties can be improved by performing a plasma nitridation of the Ge surface prior to dielectric deposition.

Analysis of UDP-galactose 4'-epimerase mutations associated with the intermediate form of type III galactosaemia

Type III galactosaemia is a hereditary disease caused by reduced activity in the Leloir pathway enzyme, UDP-galactose 4'-epimerase (GALE). Traditionally, the condition has been divided into two forms-a mild, or peripheral, form and a severe, or generalized, form. Recently it has become apparent that there are disease states which are intermediate between these two extremes. Three mutations associated with this intermediate form (S81R, T150M and P293L) were analysed for their kinetic and structural properties in vitro and their effects on galactose-sensitivity of Saccharomyces cerevisiae cells that were deleted for the yeast GALE homologue Gal10p. All three mutations result in impairment of the kinetic parameters (principally the turnover number, k(cat)) compared with the wild-type enzyme. However, the degree of impairment was mild compared with that seen with the mutation (V94M) associated with the generalized form of epimerase deficiency galactosaemia. None of the three mutations tested affected the ability of the protein to dimerize in solution or its susceptibility to limited proteolysis in vitro. Finally, in the yeast model, each of the mutated patient alleles was able to complement the galactose-sensitivity of gal10 Delta cells as fully as was the wild-type human allele. Furthermore, there was no difference from control in metabolite profile following galactose exposure for any of these strains. Thus we conclude that the subtle biochemical and metabolic abnormalities detected in patients expressing these GALE alleles likely reflect, at least in part, the reduced enzymatic activity of the encoded GALE proteins.

Long-Range Lateral Dopant Diffusion in Tungsten Silicide Layers

Novel diode test structures have been manufactured to characterize long-range dopant diffusion in tungsten silicide layers. A tungsten silicide to p-type silicon contact has been characterized as a Schottky barrier rectifying contact with a silicide work function of 4.8 eV. Long-range diffusion of boron for an anneal at 900 °C for 30 min has been shown to alter this contact to become ohmic. Long-range diffusion of phosphorus with a similar anneal alters the contact to become a bipolar n-p diode. Bipolar diode action is demonstrated experimentally for anneal schedules of 30 min at 900 °C, indicating long-range diffusion of phosphorus (~38 µm), SIMS analysis shows dopant redistribution is adversely affected by segregation to the silicide/oxide interface. The concept of conduit diffusion has been demonstrated experimentally for application in advanced bipolar transistor technology. © 2009 IEEE.

Data on carotid intima-media thickness and lipoprotein subclasses in type 1 diabetes from the Diabetes Control and Complications Trial and the Epidemiology of Diabetes Interventions and Complications (DCCT/EDIC)

Type 1 diabetes (T1DM) is associated with increased risk of macrovascular complications. We examined longitudinal associations of serum conventional lipids and nuclear magnetic resonance (NMR)-determined lipoprotein subclasses with carotid intima-media thickness (IMT) in adults with T1DM (n=455) enrolled in the Diabetes Control and Complications Trial (DCCT). Data on serum lipids and lipoproteins were collected at DCCT baseline (1983-89) and were correlated with common and internal carotid IMT determined by ultrasonography during the observational follow-up of the DCCT, the Epidemiology of Diabetes Interventions and Complications (EDIC) study, at EDIC 'Year 1' (199-1996) and EDIC 'Year 6' (1998-2000). This article contains data on the associations of DCCT baseline lipoprotein profiles (NMR-based VLDL & chylomicrons, IDL/LDL and HDL subclasses and 'conventional' total, LDL-, HDL-, non-HDL-cholesterol and triglycerides) with carotid IMT at EDIC Years 1 and 6, stratified by gender. The data are supplemental to our original research article describing detailed associations of DCCT baseline lipids and lipoprotein profiles with EDIC Year 12 carotid IMT (Basu et al. in press) [1].

Growth and Characterization of ZnO based Heterojunction diodes and ZnO Nanostructuresby Pulsed Laser Ablation

Transparent conducting oxides (TCO’s) have been known and used for technologically important applications for more than 50 years. The oxide materials such as In2O3, SnO2 and impurity doped SnO2: Sb, SnO2: F and In2O3: Sn (indium tin oxide) were primarily used as TCO’s. Indium based oxides had been widely used as TCO’s for the past few decades. But the current increase in the cost of indium and scarcity of this material created the difficulty in obtaining low cost TCO’s. Hence the search for alternative TCO material has been a topic of active research for the last few decades. This resulted in the development of various binary and ternary compounds. But the advantages of using binary oxides are the easiness to control the composition and deposition parameters. ZnO has been identified as the one of the promising candidate for transparent electronic applications owing to its exciting optoelectronic properties. Some optoelectronics applications of ZnO overlap with that of GaN, another wide band gap semiconductor which is widely used for the production of green, blue-violet and white light emitting devices. However ZnO has some advantages over GaN among which are the availability of fairly high quality ZnO bulk single crystals and large excitonic binding energy. ZnO also has much simpler crystal-growth technology, resulting in a potentially lower cost for ZnO based devices. Most of the TCO’s are n-type semiconductors and are utilized as transparent electrodes in variety of commercial applications such as photovoltaics, electrochromic windows, flat panel displays. TCO’s provide a great potential for realizing diverse range of active functions, novel functions can be integrated into the materials according to the requirement. However the application of TCO’s has been restricted to transparent electrodes, ii notwithstanding the fact that TCO’s are n-type semiconductors. The basic reason is the lack of p-type TCO, many of the active functions in semiconductor originate from the nature of pn-junction. In 1997, H. Kawazoe et al reported the CuAlO2 as the first p-type TCO along with the chemical design concept for the exploration of other p-type TCO’s. This has led to the fabrication of all transparent diode and transistors. Fabrication of nanostructures of TCO has been a focus of an ever-increasing number of researchers world wide, mainly due to their unique optical and electronic properties which makes them ideal for a wide spectrum of applications ranging from flexible displays, quantum well lasers to in vivo biological imaging and therapeutic agents. ZnO is a highly multifunctional material system with highly promising application potential for UV light emitting diodes, diode lasers, sensors, etc. ZnO nanocrystals and nanorods doped with transition metal impurities have also attracted great interest, recently, for their spin-electronic applications This thesis summarizes the results on the growth and characterization of ZnO based diodes and nanostructures by pulsed laser ablation. Various ZnO based heterojunction diodes have been fabricated using pulsed laser deposition (PLD) and their electrical characteristics were interpreted using existing models. Pulsed laser ablation has been employed to fabricate ZnO quantum dots, ZnO nanorods and ZnMgO/ZnO multiple quantum well structures with the aim of studying the luminescent properties.

Electrical Characteristics of n-ZnO/p-Si Heterojunction Diodes Grown by Pulsed Laser Deposition at Different Oxygen Pressures

Heterojunction diodes of n-type ZnO/p-type silicon (100) were fabricated by 12 pulsed laser deposition of ZnO films on p-Si substrates in oxygen ambient at 13 different pressures. These heterojunctions were found to be rectifying with a 14 maximum forward-to-reverse current ratio of about 1,000 in the applied 15 voltage range of -5 V to +5 V. The turn-on voltage of the heterojunctions was 16 found to depend on the ambient oxygen pressure during the growth of the ZnO 17 film. The current density–voltage characteristics and the variation of the 18 series resistance of the n-ZnO/p-Si heterojunctions were found to be in line 19 with the Anderson model and Burstein-Moss (BM) shift.

Transparent p-AgCoO2/n-ZnO diode heterojunction fabricated by pulsed laser deposition

Transparent diode heterojunction on ITO coated glass substrates was fabricated using p-type AgCoO2 and n-type ZnO films by pulsed laser deposition (PLD). The PLD of AgCoO2 thin films was carried out using the pelletized sintered target of AgCoO2 powder, which was synthesized in-house by the hydrothermal process. The band gap of these thin films was found to be ~3.89 eV and they had transmission of~55% in the visible spectral region. Although Hall measurements could only indicate mixed carrier type conduction but thermoelectric power measurements of Seebeck coefficient confirmed the p-type conductivity of the grown AgCoO2 films. The PLD grown ZnO films showed a band gap of ~3.28 eV, an average optical transmission of ~85% and n-type carrier density of~4.6×1019 cm− 3. The junction between p-AgCoO2 and n-ZnO was found to be rectifying. The ratio of forward current to the reverse current was about 7 at 1.5 V. The diode ideality factor was much greater than 2.

Transcriptional regulators of the GAD acid stress island are carried by effector protein-encoding prophages and indirectly control type III secretion in enterohemorrhagic Escherichia coli O157:H7

P>Type III secretion (T3S) plays a pivotal role in the colonization of ruminant hosts by Enterohemorrhagic Escherichia coli (EHEC). The T3S system translocates effector proteins into host cells to promote bacterial attachment and persistence. The repertoire and variation in prophage regions underpins differences in the pathogenesis and epidemiology of EHEC strains. In this study, we have used a collection of deletions in cryptic prophages and EHEC O157 O-islands to screen for novel regulators of T3S. Using this approach we have identified a family of homologous AraC-like regulators that indirectly repress T3S. These prophage-encoded secretion regulator genes (psr) are found exclusively on prophages and are associated with effector loci and the T3S activating Pch family of regulators. Transcriptional profiling, mutagenesis and DNA binding studies were used to show that these regulators usurp the conserved GAD acid stress resistance system to regulate T3S by increasing the expression of GadE (YhiE) and YhiF and that this regulation follows attachment to bovine epithelial cells. We further demonstrate that PsrA and effectors encoded within cryptic prophage CP933-N are required for persistence in a ruminant model of colonization.

Thermoelectric properties of BiOCu1-xMxSe (M = Cd and Zn)

Doping of BiOCuSe at the copper site with divalent cadmium and zinc cations has been investigated. Analysis of the powder X-ray diffraction data indicates that the ZrCuSiAs structure of BiOCuSe is retained up to substitution levels of 10 and 5 at.% for Cd2+ and Zn2+, respectively. Substitution of monovalent Cu+ with divalent Cd2+ or Zn2+ leads to an increase in the magnitude of the electrical resistivity and the Seebeck coefficient. All synthesized materials behave as p-type semiconductors.

Synthesis, characterisation and thermoelectric properties of the oxytelluride Bi2O2Te

Bi2O2Te was synthesised from a stoichiometric mixture of Bi, Bi2O3 and Te by a solid state reaction. Analysis of powder X-ray diffraction data indicates that this material crystallises in the anti-ThCr2Si2 structure type (space group I4/mmm), with lattice parameters a = 3.98025(4) and c = 12.70391(16) Å. The electrical and thermal transport properties of Bi2O2Te were investigated as a function of temperature over the temperature range 300 ≤ T/K ≤ 665. These measurements indicate that Bi2O2Te is an n-type semiconductor, with a band gap of 0.23 eV. The thermal conductivity of Bi2O2Te is remarkably low for a crystalline material, with a value of only 0.91 W m-1 K-1 at room temperature.

Propriedades Físicas do Semicondutor Bi2Te3

Thermoelectric Refrigerators (TEC Thermoelectric Cooling) are solid-state heat pumps used in applications where stabilization of temperature cycles or cooling below the room temperature are required. TEC are based on thermoelectric devices, and these in turn, are based on the Peltier effect, which is the production of a difference in temperature when an electric current is applied to a junction formed by two non-similar materials. This is one of the three thermoelectric effects and is a typical semiconductor junction phenomenon. The thermoelectric efficiency, known as Z thermoelectric or merit figure is a parameter that measures the quality of a thermoelectric device. It depends directly on electrical conductivity and inversely on the thermal conductivity. Therefore, good thermoelectric devices have typically high values of electrical conductivity and low values of thermal conductivity. One of the most common materials in the composition of thermoelectric devices is the semiconductor bismuth telluride (Bi2Te3) and its alloys. Peltier plates made up by crystals of semiconductor P-type and N-type are commercially available for various applications in thermoelectric systems. In this work, we characterize the electrical properties of bismuth telluride through conductivity/resistivity of the material, and X-rays power diffraction and magnetoresistance measurements. The results were compared with values taken from specific literature. Moreover, two techniques of material preparation, and applications in refrigerators, are discussed

Sintering of tin oxide and its applications in electronics and processing of high purity optical glasses

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Photoelectrochemical properties of sol-gel Nb2O5 films

Structural, optical, electro and photoelectrochemical properties of amorphous and crystalline sol-gel Nb2O5 coatings have been determined. The coatings are n-type semiconductor with indirect allowed transition and present an overall low quantum efficiency (phi < 4%) for UV light to electric conversion. The photoconducting behavior of the coatings is discussed within the framework of the Gartner and Sodergren models. Improvement can be foreseen if Nb2O5 coatings can be made of 10-20 nm size nanoparticles.