Electronic properties and gap state defect passivation of Si SiO2 nanostructures

Bandgap control of silicon based material provides a promising way towards next generation photovoltaic devices such as tandem solar cells, what can be realized by nanostructures consisting of Si SiO2 quantum wells or superlattices. However, due to increased interface to volume ratios at reduced dimensions, charge carrier recombination and scattering at Si SiO2 interfaces might dominate the photoelectrical properties and gain critical impact on mobility lifetime products amp; 956; amp; 964; and thus internal quantum efficiencies [1]. To circumvent this drawback, the effect of hydrogen treatment on charge carrier recombination and electronic densities of states at the interface of ultrathin oxides layers is analyzed. Samples with structurally and chemically well defined interfaces were prepared by plasma oxidation of crystalline Si with atomic oxygen under ultrahigh vacuum conditions [2]. It is demonstrated, that Si SiO2 interface states can be passivated under appropriate conditions in forming gas H2 N2 and in hydrogen plasma. As a result, the photoelectrical performance of such structures is clearly improved. This is verified by i estimation of mobility lifetime products from photocurrent measurements, ii analysis of interface densities of states by means of surface photovoltage measurements SPV , and iii deducing densities of occupied states in the band gap as elucidated from UV excited constant final state yield spectroscopy CFSYS

Si SiO2 quantum well and quantum dot structures atomic scale preparation and characterization with respect to photovoltaic application

Si SiO2 single quantum wells and quantum dot layers were prepared under ultrahigh vacuum conditions in order to study their structural, chemical and photo electrical properties with respect to a possible application in photovoltaic devices. Particular focus is put on the realization of well defined and abrupt interfaces with low densities of interface gap states. The detection of a photocurrent in these quantum structures is demonstrated. Its spectral dependence correlates with the respective structural properties. Internal quantum efficiencies of photoconductivity and, thus, carrier mobility lifetime products are strongly affected by Si SiO2 interface states and were significantly enhanced upon hydrogen treatment due to passivation of interface gap state

Ultrahigh vacuum preparation and passivation of abrupt SiO2 Si 111 interfaces

Compositionally and structurally abrupt Si SiO2 interfaces were prepared under ultrahigh vacuum conditions by RF plasma oxidation of Si 111 substrates with thermalized neutral oxygen atoms. The chemical, structural and electronic properties of the interface were analyzed and discussed with respect to a possible application in photovoltaic Si SiO2 quantum well structures. The benefits of using neutral atomic oxygen were explored and turned out to be manifold ultrathin SiO2 layers thickness 1 2 nm can be precisely grown, the formation of suboxides is mostly suppressed and abrupt Si SiO2 interfaces are obtained even at moderate substrate temperatures of 300 to 600 C. Due to the perfect Si SiO2 interfaces, the SiO2 layers allow thermal post oxidation treatment up to 1000 C without significant change in oxide thickness or stoichiometry. This is an essential prerequisite for the envisaged realization of Si SiO2 superlattices with high crystallinity and low strain. It was shown that a post oxidation annealing step lowers the strain and disorder at the interface resulting in lower intrinsic density of interface states. A further decrease of the density of interface states was achieved by hydrogen passivation as a result of saturation of dangling bond

Ultra thin silicon oxide layers on crystalline silicon wafers comparison of advanced oxidation techniques with respect tochemically abrupt SiO2 Si interfaces with low defect densities

Six advanced oxidation techniques were analyzed, evaluated and compared with respect to the prepa ration of high quality ultra thin oxide layers on crystalline silicon. The resulting electronic and chemicalSiO2 Si interface properties were determined by a combined x ray photoemission XPS and surface pho tovoltage SPV investigation. Depending on the oxidation technique, chemically abrupt SiO2 Si interfaceswith low densities of interface states were fabricated on c Si either at low temperatures, at short times,or in wet chemical environment, resulting in each case in excellent interface passivation. Moreover, thebeneficial effect of a subsequent forming gas annealing FGA step for the passivation of the SiO2 Si inter face of ultra thin oxide layers has been proven. Chemically abrupt SiO2 Si interfaces have been shown togenerate less interface defect state

Hydrogen passivation of interfacial gap state defects at UHV prepared ultrathin SiO2 layers on Si 111 , Si 110 , and Si 100

A complete in situ process from preparation and hydrogen passivation to interface gap state analysis by near UV photoelectron spectroscopy NUV PES without breaking ultrahigh vacuum UHV conditions is applied to ultrathin oxide layers on Si 111 , 110 , and 100 . RF plasma oxidation with thermalized neutral oxygen atoms allows the growth of homogeneous ultrathin SiO2 layers lt; 2 nm and the preparation of compositionally and structurally abrupt Si SiO2 interfaces with minimal amounts of suboxides ranging from 2 on Si 100 to 4 on Si 110 . The oxide growth is independent of the crystallographic orientation. Appropriate plasma treatment with nearly thermalized hydrogen atoms Ekin lt; 1 eV leads to significant passivation of dangling bonds at the ultrathin SiO2 Si interfaces and is most efficient on Si 100 . In contrast, energetic hydrogen plasma treatment of these interfaces with kinetic energies exceeding 120 eV, which is conventionally applied for polycrystalline Si thin film solar cells, imparts large amounts of energy and deteriorates the electrical properties as is reflected in interface degradation and increased densities of defect state

Ultrathin SiO2 layers on Si 111 preparation, interface gap states and the influence of passivation

Essential prerequisite for successful application of Si SiO2 nanostructures in photovoltaics is the realization of well defined and abrupt interfaces with low densities of interface gap states. Here, a complete in situ process from preparation and hydrogen passivation to interface gap state analysis by near UV photoelectron spectroscopy without breaking ultrahigh vacuum UHV conditions is introduced. It is demonstrated, that by RF plasma oxidation of Si 111 substrates with thermalized neutral oxygen atoms ultrathin SiO2 layers with compositionally and structurally abrupt Si SiO2 interfaces with minimal amount of intermediate oxidation states bridging the transition from Si to SiO2 can be realized. Plasma oxidized samples have significantly lower interface gap states than samples oxidized by thermal oxidation at 850 C. Interface gap state densities were further reduced by in situ hydrogen plasma passivation with nearly thermalized H atoms. The resulting reduction of interface recombination velocity and the increase of effective majority and minority carrier lifetimes are revealed by constant photocurrent measurements and quasi steady state photoconductance, respectivel

Scale alignment : On the role of material culture for urban design

Globalisation has led to the transformation and proliferation of urban borders. Old hierarchies and relations between centres and sub-centres and centres and peripheries are challenged, and new alliances between different places and districts are formed. How can urban design and urban design research meet the challenges of a complex and quickly transforming multi-scalar society? In this article I make three arguments. First, I argue for a territorial perspective that would allow us to better acknowledge how actors of different scales come together and have effects on the level of urban design. Second, I argue that we need to better acknowledge the role of material culture in research on urban design; whereas urban design often benefits from the large-scale perspective of urban morphology, it seldom incorporates the roles of small-scale objects into studies. Finally, I argue that an important task for researchers in urban design is to see how actors of different scales are aligned, and through this alignment produce effects and changes in the ways in which our cities are used