Seref Kalem

Black silicon with high density and high aspect ratio nanowhiskers

The physical properties of black silicon (b-Si) formed on Si wafers by reactive ion etching in chlorine plasma are reported in an attempt to clarify the formation mechanism and the origin of the observed optical and electrical phenomena, which are promising for a variety of applications. The b-Si consisting of high density and high aspect ratio sub-micron length whiskers or pillars with tip diameters of well under 3 nm exhibits strong photoluminescence (PL) both in the visible and the infrared, which is interpreted in conjunction with defects, confinement effects and near band-edge emission. Structural analysis indicates that the whiskers are all crystalline and encapsulated by a thin Si oxide layer. The infrared vibrational spectrum of Si-O-Si bondings in terms of transverse-optic (TO) and longitudinal-optic (LO) phonons indicates that disorder induced LO-TO optical mode coupling can be an effective tool in assessing the structural quality of the b-Si. The same phonons are likely coupled to electrons in visible region PL transitions. Field emission properties of these nanoscopic features are demonstrated indicating the influence of the tip shape on the emission. Overall properties are discussed in terms of the surface morphology of the nanowhiskers.

Controlled thinning and surface smoothening of silicon nanopillars

A convenient method has been developed to thin electron beam fabricated silicon nanopillars under controlled surface manipulation by transforming the surface of the pillars to an oxide shell layer followed by the growth of sacrificial ammonium silicon fluoride coating. The results show the formation of an oxide shell and a silicon core without significantly changing the original length and shape of the pillars. The oxide shell layer thickness can be controlled from a few nanometers up to a few hundred nanometers. While downsizing in diameter, smooth Si pillar surfaces of less than 10 nm roughness within 2 microm were produced after exposure to vapors of HF and HNO3 mixture as evidenced by transmission electron microscopy (TEM) analysis. The attempt to expose for long durations leads to the growth of a thick oxide whose strain effect on pillars can be assessed by coupled LO-TO vibrational modes of Si-O bonds. Photoluminescence (PL) of the pillar structures which have been downsized exhibits visible and infrared emissions, which are attributable to microscopic pillars and to the confinement of excited carriers in the Si core, respectively. The formation of smooth core-shell structures while reducing the diameter of the Si pillars has a potential in fabricating nanoscale electronic devices and functional components.

Photoluminescence from silicon nanoparticles embedded in ammonium silicon hexafluoride

Silicon (Si) nanoparticles (NPs) were synthesized by transforming a Si wafer surface to ammonium silicon hexafluoride (ASH) or (NH(4))(2)SiF(6) under acid vapor treatment. Si-NPs which were found to be embedded within the polycrystalline (ASH) layer exhibit a strong green-orange photoluminescence (PL). Differential PL measurements revealed a major double component spectrum consisting of a broad band associated with the ASH-Si wafer interfacial porous oxide layer and a high energy band attributable to Si-NPs embedded in the ASH. The origin of the latter emission can be explained in terms of quantum/spatial confinement effects probably mediated by oxygen related defects in or around Si-NPs. Although Si-NPs are derived from the interface they are much smaller in size than those embedded within the interfacial porous oxide layer (SiO(x), x > 1.5). Transmission electron microscopy (TEM) combined with Raman scattering and Fourier transformed infrared (FTIR) analysis confirmed the presence of Si-NP and Si-O bondings pointing to the role of oxygen related defects in a porous/amorphous structure. The presence of oxygen of up to 4.5 at.% in the (NH(4))(2)SiF(6) layer was confirmed by energy dispersive spectroscopy (EDS) analysis.

Formation of germanates on germanium by chemical vapor treatment

Optical properties of dislocations free state-of-the-art Germanium(Ge) and Germanium-oninsulator(GeOI) wafers have been characterized using Fourier transformed infrared spectroscopy at oblique incidence, attenuated total reflectance, laser Raman scattering, linear and nonlinear optical transmission. In n-type Ge, in addition to vibrational modes observed in intrinsic(i) Ge, a band at 535cm-1 which is likely due to carbon and a strong peak at 668 cm-1 were observed at non-normal incidence. Despite the strong heavy hole to light hole absorption band at low energies, the 668 cm-1 peak was also observed in p-Ge. The appearance of new bands and the enhancement in band strength are in general observed in both type of wafers at oblique incidence. GeOI exhibits a strong disorder induced LO-TO coupling mode which can only be observed at non-normal incidence. Optical absorption at the near bang edge reveals the presence of doping related disorder and band shrinkage, which is supported also by Ge-Ge one-phonon line broadening at 301 cm-1. Different nonlinear optical absorption behavior was observed in n-Ge, p-Ge and GeOI wafers. The p-Ge becomes transparent to CO2 laser line at 10.6 micrometer, while transmitted power decreases in n-Ge with increasing UV-VIS pump power.The surface of a single crystal Germanium wafer was transformed to fluoride and oxide crystals upon exposure to a vapor of HF and HNO3 chemical mixture. Ellipsometry. X-ray, SEM and photoluminescence were used to investigate the physical properties of the resultant surface structure The analysis indicates that the transformation results in a polycrystalline hexagonal ammonium fluogermanates and a hexagonal alpha-Germanium oxide Clusters with a preferential crystal growth orientation in direction The fluogermanates grow particularly around the germanium oxide Clusters as evidenced by electron dispersive spectroscopy profiling Local vibrational mode analysis confirm the presence of N-H and Ge-F vibrational modes of NH4+ and GeF6- ions. The vibrational modes at around 840 cm(-1) is significative of GeOx stretching bands originating from the partial coverage surface oxide formed together with the fluogermanates and clusters oil the Germanium Electronic band structure as probed by ellipsometry is typical of Ge and any discrepancy was associated with disorder induced band tailing effects originating possibly from the effect of oxide clustering.

Deep Acceptor Levels in Molecular Beam Epitaxial High Purity p-Type GaAs

Deep acceptor impurities in high-purity, unintentionally p-type doped GaAs epilayers grown by molecular beam epitaxy have been investigated by variable temperature Hall effect measurements. The experimental results were analyzed in detail by using the grand partition function formalism assuming multiple acceptor levels with both single and double occupancy. It is shown that p-type conduction is originated from the presence of a residual shallow acceptor and several deep acceptor levels. For the samples having relatively high concentration of shallow acceptors, deep aeceptor states with the ionization energies of about 90 and 200 meV are determined, which are likely associated with the presence of double acceptor centers. In the high purity samples, however, deeper aceeptor levels are required to account for the data.

Controlling photon emission from silicon for photonic applications

The importance of a photon source that would be compatible with silicon circuitry is crucial for data communication networks. A photon source with energies ranging from UV to near infrared can be activated in Si as originationg from defects related to dislocations, vacancies, strain induced band edge transitions and quantum confinement effects. Using an etching method developed in this work, one can also enhance selectively the UV-VIS, band edge emission and emissions at telecom wavelengths, which are tunable depending on surface treatment. Deuterium D2O etching favors near infrared emission with a characteristic single peak at 1320 nm at room temperature. The result offers an exciting solution to advanced microelectronics The method involves the treatment of Si surface by deuterium Deuterium containing acid vapor, resulting in a layer that emits at 1320 nm. Etching without deuterium, a strong band edge emission can be induced at 1150 nm or an emission at 1550 nm can be created depending on the engineered surface structure of silicon. Schottky diodes fabricated on treated surfaces exhibit a strong rectifying characteristics in both cases.

Infrared photoluminescence from Si/Ge nanowire grown Si wafers

This paper investigates the enhancement of room temperature (RT) infrared (IR) photoluminescence (PL) from Si/Ge nanowire (NW) grown silicon (Si) wafers. The NW grown wafers were treated by an acid atmosphere consisting of vapor of hydrofluoric HF and HNO3 chemical mixture. The treatment modifies the surface particularly at defect sites such as pits, dislocations and stacking faults as well as NW surfaces by etching and forming oxides. This process can induce a passivated crystalline Si surface where band-to-band (BB) emission is the dominant property. Strong signals are observed at sub-band gap energies when the treatment results in disordered surface with oxygen related defects. IR PL is a competitive property between the Si BB transition and sub-gap emission which is mainly attributable to defects and partly to Ge dots. The enhancement in BB and deep-level PL was discussed in terms of strain, impurities, Ge dots and oxygen diffusion. The results demonstrate the effectiveness of the method in enhancing and tuning IR PL properties for possible applications.

Optical response of Si/Ge superlattices with embedded Ge dots

A method was provided for treating the optical response of Si/Ge superlattices (SL) with embedded Ge dots. Spectroscopic ellipsometry (SE) measurement at room temperature was used to investigate optical and electronic properties of Si/Ge SL which were grown on silicon (Si) wafers having <;111>; crystallographic orientation. The results of the SE analysis between 1.2 eV and 5.2 eV indicate that the SL system can effectively be described using interdiffusion/intermixing model by assuming a multicrystalline Si and Si1-xGex intermixing layers. The optical transitions exhibit Si, Ge and alloying related critical points.