Alexander Ulyashin

Hydrides as materials for semiconductor electronics

Systematic studies using density functional theory have shown that some hydrides possess the features of semiconductors. These features include larger fundamental band gap, well dispersed bottom-most conduction band and/or top-most valence band, small electron/hole effective masses and small intrinsic carrier concentration. It is demonstrated that depending upon the composition, hydrides possess a wide range of band gap values and hence they can be regarded as materials for narrow to wide band gap semiconducting applications. The possibility of designing hydride-based p–n junctions, and also their advantages as well as deficiencies compared to existing oxide semiconductors, are discussed. Replacing oxide-based semiconductors by hydrides can help to avoid problems such as formation of an oxide layer, band offsets, large concentration of defect states at the interface between the oxide and semiconductor, etc. Moreover, hydrides can be regarded as an alternative to conventional semiconductors and hence can be used in future-generation electronic devices called “hydride electronics”.

Transmission electron microscopy study of hydrogen defect formation at extended defects in hydrogen plasma treated multicrystalline silicon

Hydrogenation of multicrystalline silicon for solar cell applications is considered to be an effective method of increasing the lifetime by passivating defects and impurities. Hydrogen plasma treated as-cut and chemically etched multicrystalline silicon samples have been studied by electron microscopy in order to investigate hydrogen defect formation at extended bulk defects. In chemically etched samples, the texture of the surface after hydrogen plasma treatment differs between different grains depending on grain orientation. In as-cut samples, hydrogen induced defects are formed on sawing defects that extend up to ∼5 μm below the Si surface. Intragranular defects are also observed in the ∼1 μm subsurface region. The density of defects is higher in as-cut samples than in chemically etched samples and the size of the defects increases with depth. Hydrogen induced structural defects on bulk dislocations and on dislocations in twin grain boundaries and stacking faults are found several microns below the sam...

Characterization of the oxygen distribution in Czochralski silicon using hydrogen-enhanced thermal donor formation

The hydrogen-enhanced thermal donor (TD) formation in Czochralski (Cz) silicon is used for the characterization of the interstitial oxygen distribution by spreading resistance probe (SRP) analysis or by the carrier concentration from capacitance–voltage (C–V) measurements. For as-grown wafers or wafers with a denuded zone, the enhanced TD formation in Cz silicon has been studied by applying a hydrogenation from a plasma. A kinetic model for the hydrogen-enhanced TD formation is presented, and a method for the conversion of the carrier concentration due to TDs into a concentration of interstitial oxygen is proposed. For comparison, infrared spectrometry was applied for the characterization of the oxygen concentration in the samples. On the basis of the proposed model, the analysis by the SRP or C–V measurements of Cz Si samples containing TDs, which were generated with the support of hydrogen, can be used for the quantitative estimation of the distribution of interstitial oxygen in the as-grown wafers as well as, at least qualitatively, of the interstitial oxygen distribution in wafers with denuded zones.

The hydrogen gettering at post-implantation hydrogen plasma treatments of helium- and hydrogen implanted Czochralski silicon

The effect of the gettering of hydrogen at buried defect layers at post-implantation hydrogen plasma treatments and the loss of hydrogen at these layers at high-temperature annealing of helium-implanted (at 300 keV, 1 × 10 16 cm -2 ; at 1 MeV, 1 × 10 15 , 1 × 10 16 and 1 × 10 17 cm -2 ) and, for comparison, of hydrogen-implanted (at 70 keV, 1 × 10 15 , 1 × 10 16 and 3 × 10 16 cm -2 ) Czochralski silicon was studied. The samples were annealed at 450 and 1000°C in flowing nitrogen or hydrogen ambient. As-implanted and annealed samples were treated by a d.c. hydrogen plasma at 150°C. Secondary ion mass spectroscopy (SIMS) was applied to measure the hydrogen concentration profiles of the samples. The buried defect layers, which were created by hydrogen or helium implantation, act as good getter centers for hydrogen at appropriate hydrogen plasma or heat treatments. It is shown that the loss of hydrogen from the buried layers depends on the temperature, the time of annealing and the annealing ambient. The loss of hydrogen at 450°C and the decrease of the hydrogen SIMS signal do not only result from a formation of H 2 molecules captured by cavities and platelets, but also from the penetration of atomic hydrogen into the wafer bulk. This penetration leads to a thermal donor formation, as can be proved by spreading resistance probe analysis.

Initial stages of ITO/Si interface formation: In situ x-ray photoelectron spectroscopy measurements upon magnetron sputtering and atomistic modelling using density functional theory

Initial stages of indium tin oxide (ITO) growth on a polished Si substrate upon magnetron sputtering were studied experimentally using in-situ x-ray photoelectron spectroscopy measurements. The presence of pure indium and tin, as well as Si bonded to oxygen at the ITO/Si interface were observed. The experimental observations were compared with several atomistic models of ITO/Si interfaces. A periodic model of the ITO/Si interface was constructed, giving detailed information about the local environment at the interface. Molecular dynamics based on density functional theory was performed, showing how metal-oxygen bonds are broken on behalf of silicon-oxygen bonds. These theoretical results support and provide an explanation for the present as well as previous ex-situ and in-situ experimental observations pointing to the creation of metallic In and Sn along with the growth of SiOx at the ITO/Si interface.

Strain-induced modulation of band structure of silicon

This work presents ab initio study of strain-induced modulation of band structure of Si. It is shown that at straining pressures >12GPa, band structure of Si can be turned from indirect to direct. Both the bottommost conduction band and topmost valence band are located at the Γ point. The conduction band minimum at the Γ point of the strained Si is found to be much more dispersive than that at the X point of the unstressed Si. Consequently, electrical conductivity through the Γ valley is suggested to be more superior than the X point of the unstressed Si. Barrier height, which is needed to transfer electrons in the Γ point to X∕L points or from Γ point to X∕L to Γ point have been calculated. The results have been applied to explain peculiarities of electronic structure and light emission of Si based materials containing dislocations and voids.

Electronic device fabrication by simple hydrogen enhanced thermal donor formation processes in p-type Cz-silicon

Abstract The incorporation of hydrogen into p-type Czochralski (Cz) silicon by a plasma results in an enhanced thermal donor (TD) formation. Counter doping by TDs and a rapid p–n junction formation occurs in p-type Si if the acceptor concentration is lower than 10 16 cm −3 . Two process routes are discussed: (1) a one-step process where p–n junctions appear just after a H plasma exposure at 400–450°C; (2) a two-step process where the p–n junction formation requires an annealing at 400–450°C after a plasma treatment at lower temperatures. No dopant incorporation is involved in the processes. A controlled TD formation can be used for a rapid low temperature technology for the fabrication of diodes with deep p–n junctions. The characterization of the samples/devices was done by spreading resistance probe analysis, I ( V )-, and C ( V ) measurements.

Hydrogen redistribution and enhanced thermal donor formation at post implantation annealing of p-type hydrogen implanted Czochralski silicon

Abstract The hydrogen redistribution and the enhanced conversion of the region near the surface of hydrogen implanted p-type Czochralski (Cz) silicon wafers into n-type by thermal donor (TD) formation at low-temperature (450°C) post-implantation annealing have been investigated. For comparison low-temperature (260°C) RF hydrogen plasma treated Cz Si with subsequent annealing at 450°C was studied, too. Spreading resistance probe (SRP) analysis and secondary ion mass spectrometry (SIMS) were used for the samples characterization. It is shown that the hydrogen redistribution and hydrogen enhanced thermal donor formation in hydrogen implanted or hydrogen plasma treated p-type Cz Si leads to the formation of deep p–n junctions after 450°C annealing. The buried defect layer in hydrogen implanted Cz Si samples acts as an effective getter for hydrogen and therefore a delay in the formation of deep p–n junctions was observed as compared to hydrogen plasma treated samples with subsequent annealing at 450°C.

CLASSIFICATION OF HYDRIDES ACCORDING TO FEATURES OF BAND STRUCTURE

Band structure of hydrides has been studied by density functional calculations. From analysis of band structures, it is found that, similar to semiconductors, some hydrides possess open fundamental band gap, and can be classified according to the following three characteristic features. The first is based on the value of the fundamental band gap and, therefore, the hydrides have been classified as narrow or wide band gap materials. The second feature is based on a comparison of the relative location in k space of the bottommost conduction band and topmost valence band (VB). Thus, hydrides can be classified as either direct or indirect band gap materials. The third feature is based on the origin of the topmost valence band and depends on the dominant contribution of s-, p-, and d-electrons to the topmost VB. According to this criterion, hydrides can be classified as type s, p, d or hybridised materials. This classification will be useful in the application of hydrides for the construction and processing of electronic devices within the framework of the recent innovations in ‘hydride electronics’.

Comparative analysis of electronic structure and optical properties of crystalline and amorphous silicon nitrides

We present a study of the electronic structure and optical properties of Si3N4 and Si2N3H in crystalline and amorphous phases by first-principles calculations. We find that besides structural disorder those matrix atoms with dangling and floating bonds contribute to energy levels close to the Fermi level. From a comparative analysis of calculated optical spectra we conclude that the difference in optical properties between crystalline and amorphous silicon nitrides—either hydrogenated or unhydrogenated—is only small. It is present mainly in the energy range close to the fundamental band gap. At larger energies the difference is negligible. It is found that the structural disorder in silicon nitrides investigated does not affect essentially the electronic structure and optical properties of these materials. It is concluded that such amorphous silicon nitrides can be used instead of their crystalline counterparts for various applications in which optical properties of such materials are important.