Structure characterization of CVD amorphous Si3N4 by pulsed neutron total scattering

The structure factor S(Q) of chemically vapor-deposited (CVD) amorphous Si₃N₄ has been measured by pulsed neutron diffraction over the range of the scattering vector Q from 1–330 nm^?1. The oscillatory behavior in the S(Q) persists up to Q = 300 nm^?1 and there is appreciable small angle scattering intensity. The SiN bond length is l_SiN = 0.1729 nm, and its coordination numbers n_SiN and n_NSi are 3.70 and 2.78 respectively. The bond angles around a Si and a N atom are found to be 109.8 and 121°. Analysis of the small angle scattering intensity shows the existence of voids with an average diameter of about 1 nm and a volume fraction of about 4%, which may stabilize the amorphous structure of Si₃N₄ having rigid covalent bonds due to relaxing the strain energy accumulated in the matrix.     

Transport properties of amorphous germanium prepared by the glow discharge technique

The paper deals with conductivity, thermoelectric power and field effect measurements on amorphous Ge specimens prepared by the decomposition of germane gas in a rf glow discharge. Substrate temperatures T_d of 300, 400 and 500 K were used during deposition. The sign of the thermoelectric power S is negative throughout the temperature range investigated (200–500 K). Above 300 K, the conductivity activation energy in specimens prepared at T_d = 500 K lies between 0.40 and 0.43 eV; it is equal to the gradient of the S versus 1/T curves, suggesting transport in the extended electron states. Below room temperature there is an increasing contribution in all specimens from electron hopping transport in localized states lying about 0.25 eV below ?_C. Both conductivity and thermoelectric power results can be interpreted satisfactorily in terms of these two current paths. Hopping at the Fermi level has not been observed. The preliminary field effect measurements indicate that, as in amorphous Si, ?_f lies near a density of state minimum. The density of states at ?_f is appreciably higher than that in similarly prepared Si specimens.     

Photoluminescence of Si nanocrystallites in different types of matrices

This paper presents the comparative investigation of photoluminescence (PL) and its temperature dependence for rf-magnetron co-sputtered Si-enriched SiO_x systems and amorphous Si films prepared by hot-wire CVD method with Si nanocrystallites of different sizes. It is shown that PL spectra of Si–SiO_x films consist of the five PL bands peaked at 1.30, 1.50, 1.76, 2.05 and 2.32 eV. Amorphous Si films with Si nanocrystallites are characterized by three PL bands only peaked at 1.35, 1.50 and 1.76 eV. The peak position of the 1.50 eV PL band shifts with the change of Si quantum dot sizes and it is attributed to exciton recombination inside of Si quantum dots. The nature of four other PL bands is discussed as well.     

Effect of illumination on hydrogenated amorphous silicon thin films doped with chalcogens

Hydrogenated amorphous silicon thin films doped with chalcogens (Se or S) were prepared by the decomposition of silane (SiH₄) and H₂Se/H₂S gas mixtures in an RF plasma glow discharge on 7059 corning glass at a substrate temperature 230 °C. The illumination measurements were performed on these samples as a function of doping concentration, temperature and optical density. The activation energy varied with doping concentration and is higher in Se-doped than S-doped a-Si:H thin films due to a low defect density. From intensity versus photoconductivity data, it is observed that the addition of Se and S changes the recombination mechanism from monomolecular at low doping concentration films to bimolecular at higher doping levels. The photosensitivity (σ_ph/σ_d) of a-Si, Se:H thin films decreases as the gas ratio H₂Se/SiH₄ increased from 10^?4 to 10^?1, while the photosensitivity of a-Si, S:H thin films increases as the gas ratio H₂S/SiH₄ increased from 6.8 × 10^?7 to 1.0×10^?4.     

Influence of the number of periods on the magnetization reversal process of antiferromagnetically coupled amorphous CoxSi1−x/Si multilayers

The advantage of using amorphous materials for preparing complex magnetic structures, like magnetic/non-magnetic multilayers, is illustrated for the case of the study of the Co–Si system. In particular, the antiferromagnetic AF coupling recently reported for amorphous Co–Si alloys separated by Si spacers and the associated magnetization reversal processes have been studied in (Co_0.75Si_0.25/Si)_n multilayers with different number of periods n. It has been found that the magnetic field corresponding to the onset of the reversal process increases with the number of periods in a linear way. For n > 2, the magnetization stabilizes not only in the AF state but also in other intermediate configurations between both saturated states. At the same time, as n increases, all the processes of formation and breaking of these states get smoothed.     

Low-temperature RPCVD of Si,SiGe alloy,and Si1−yCy films on Si substrates using trisilane (Silcore)

Si homo-epitaxial growth by low-temperature reduced pressure chemical vapor deposition (RPCVD) using trisilane (Si₃H₈) has been investigated. The CVD growth of Si films from trisilane and silane on Si substrates are compared at temperatures between 500 and 950 °C. It is demonstrated that trisilane efficiency increases versus silane's one as the surface temperature decreases. Si epilayers from trisilane, with low surface roughness, are achieved at 600 and 550 °C with a growth rate equal to 12.4 and 4.3 nm min⁻¹, respectively. It is also shown that Si_1−xGe_x layers can be deposited using trisilane chemistry.     

Bonding rearrangement in amorphous silicon nitrides deposited by hot-wire chemical vapor deposition upon thermal annealing

The bonding rearrangement upon thermal annealing of amorphous silicon nitride (a-SiN_x:H) films deposited by hot-wire chemical vapor deposition was studied. A wide range of N/Si atom ratio between 0.5 and 1.6 was obtained for the a-SiN_x:H sample series by varying the source gases ratio only. Evolutions of Si–N, Si–H and N–H bonds upon annealing were found to depend strongly on the N/Si atom ratio of the films. According to the above observations, we propose possible reaction pathways for bonding rearrangement in a-SiN_x:H with different N/Si ratios.     

Growth kinetics and boron doping of very high Ge content SiGe for source/drain engineering

We have studied the in-situ boron doping of high Ge content Si_1?xGe_x layers (x=0.3, 0.4 and 0.5). These layers have been grown at low pressure (20 Torr) and low temperature (600–650 °C) with a heavily chlorinated chemistry on blanket Si(0 0 1) substrates. Such a chemistry yields a full selectivity versus SiO₂ (isolation) and Si₃N₄ (sidewall spacers) on patterned wafers with gate stacks. We have quantified the impact of the diborane flow on the SiGe layer crystalline quality, its resistivity, the SiGe:B growth rate and the apparent Ge concentration. Resistivity values lower than 1 mΩ cm are easily achieved, all the more so for high Ge content layers. The SiGe growth rate increases and the apparent Ge concentration (from X-ray diffraction) decreases as the diborane flow increases. B atoms (much smaller than Si or Ge) indeed partially compensate the compressive strain in the SiGe:B layers. We have also probed the in-situ boron and phosphorus doping of Si at 750 °C, 20 Torr with a heavily chlorinated chemistry. The B ions concentration increases linearly with the diborane flow, then saturates at a value close to 4×10¹⁹ cm^?3. By contrast, the P ions concentration increases sub-linearly with the phosphine flow, with a maximum value close to 9×10¹⁸ cm^?3. Adding diborane (phosphine) to the gaseous mixture leads to a sharp increase (decrease) of the Si:B (the Si:P) growth rates, which has to be taken into account in device layers. All the know-how acquired will be most handy for the formation of in-situ doped recessed or raised sources and drains in metal-oxide semiconductor devices.     

Trap-limited diffusion of hydrogen in precursor derived amorphous Si–B–C–N-ceramics

The tracer diffusion of hydrogen is studied in precursor derived amorphous Si–C–N and Si–B–C–N ceramics using deuterium as a tracer and secondary ion mass spectrometry (SIMS). Since the amorphous ceramics are separated in carbon rich phases (amorphous carbon and amorphous C(BN)_x, respectively) and silicon rich phases (amorphous Si₃N₄ and amorphous Si_3+(1/4)xC_xN_4−x, respectively) we additionally measured the diffusivities of hydrogen in amorphous carbon, in amorphous SiC and in amorphous C–B–N films. The silicon rich phases are identified as diffusion paths for hydrogen in the precursor derived ceramics. Diffusion of hydrogen in these materials is explained with a trap limited diffusion mechanism with a single trap level. We found activation enthalpies of about 2 eV for the precursor derived ceramics, where the activation enthalpy is the sum of a migration enthalpy and a binding enthalpy. The low values for the pre-exponential factors of less than 10⁻⁷ m²/s can be explained with an appropriate expression for the entropy factor.     

Investigation of gap states in phosphorous-doped ultra-thin a-Si:H by near-UV photoelectron spectroscopy

A variant of photoelectron spectroscopy with near-UV light excitation was established and applied to an n-type doping series of ultra-thin a-Si:H layers (layer thickness ～10 nm). Using this technique, the position of the surface Fermi level E_Fs is obtained and the density of recombination active defect states in the a-Si:H band gap down to ～10¹⁵ states/cm³ can be detected. Defect densities are generally about one order of magnitude higher than in the bulk of thicker (several 100 nm) layers, and the minimum achievable distance of E_Fs from the conduction band is ～360 mV for doping with 10⁴ ppm PH₃. The optimum doping for the fabrication of solar cells is almost one order of magnitude lower. This discrepancy may be explained by enhanced recombination at the a-Si:H/c-Si interface at high doping levels, and in addition by an efficient recombination pathway where charge carriers tunnel from c-Si via a-Si:H band tail states into the a-Si:H and subsequently recombine at dangling bond states.     

Er-doped hydrogenated amorphous silicon: structural and optical properties

The effect of erbium-doping on the structural and optical properties of hydrogenated amorphous silicon (a-Si:H) is investigated. Optical absorption and Raman spectra indicate that erbium doping introduces defect states, and that above a concentration of 0.27 at.%, induces strong structural disorder. The photoluminescence measurements show that erbium doping introduces non-radiative decay paths for carriers in a-Si:H, leading to decrease in both the Er³⁺ and intrinsic a-Si:H luminescence intensity when the Er concentration is increased to more than 0.04 at.%. The results are compared to that of Er-doped crystalline Si, and the possible excitation mechanisms of Er in a-Si:H are discussed.     

Two-dimensional modelling and simulation of hydrogenated amorphous silicon p-n-n solar cell

Two-dimensional device modelling for hydrogenated amorphous silicon p⁺-n-n⁺ solar cell has been carried out by using MEDICI^™ device simulator and the influence of absorber layer thickness, doping concentration, and dangling bond density of states in absorber layer on photo parameters are investigated. A strong correlation between n-type doping and dangling bond density in the absorber layer relative to the stability of the a-Si:H solar cell is observed. An increased stabilized efficiency is obtained when n-type dopant concentration in the absorber layer is higher than optimum value for higher initial efficiency. The window layer (p⁺ layer) of the device is designed with a three layered structure of graded doping for higher device performance. This window layer structure in the a-Si:H p⁺-n-n⁺ cell resulted in higher open circuit voltage and fill factor and hence higher efficiency of the cell. The efficiency of the modified amorphous silicon solar cell structure is found to be 12.85%.     

MOCVD and characterization of Hf-silicate thin films using HTB and TEMAS

Hafnium silicate (HfSi_xO_y) films were deposited by metal-organic chemical vapor deposition (MOCVD) using a combination of precursors: hafnium tetra-tert-butoxide [Hf(OC(CH₃)₃)₄, HTB] and tetrakis-ethylmethylamino silane [Si(N(C₂H₅)(CH₃))₄, TEMAS]. The activation energy was independent on the ratio of precursor amounts in the surface reaction regime. The grown films showed Hf-rich characteristics and the impurity concentrations were less than 1 at.% (below detection limits). Hafnium silicate films were amorphous up to 700 °C annealing. Hf/(Hf + Si) composition ratio and dielectric constant (k) of the Hf-silicate films decreased by increasing the growth temperature above 270 °C.     

Amorphous to crystalline transition of selenium thin films of different thicknesses

Thin films of amorphous selenium obtained by vacuum evaporation display an increase of “optical gap” E_g^opt with an increase of thickness of the film. From the observed dependence of E_g^opt on the thickness of the film, the influence of the thickness on the gap states is interpreted in terms of the density of states model proposed by Mott and Davis. The amorphous to crystalline transition obtained by heat treatment of the specimen is also investigated. The minimum temperature for an appreciable change in crystallisation determined by the transmission of light through selenium films is also a function of the thickness and binding energy of the films. The crystalline structures resulting from heat treatment at different temperatures have been identified by scanning electron microscopy. The generation of different crystalline structures is reported in terms of the thickness and preparation conditions of the amorphous films.     

Local structure around phosphorus and silicon in the CaO–SiO2–PO2.5 system

The local structure of phosphorus and silicon in the molten CaO–SiO₂–PO_2.5 slag system was investigated by magic angle spinning nuclear magnetic resonance (MAS-NMR). The ³¹P MAS-NMR spectra revealed that phosphorus was present primarily as the monophosphate complex ion PO₄^3?, with a small amount of diphosphate ion also present. Their relative ratio to total phosphorus was independent of the phosphate concentration of the sample. In the case of the ²⁹Si MAS-NMR, the mean number of the non-bridging oxygen atoms associated with tetrahedrally coordinated silicon decreased as the phosphate concentration increased at a fixed CaO/SiO₂ ratio. This indicates that the nonbridging oxygen atoms around the silicon were replaced by bridging oxygen atoms around the phosphorus as the phosphate concentration in the samples increased.To elucidate the basicity dependence of the structure of slag, the relationship between the structure and optical basicity was also investigated. The relative ratio of Qⁿ (Qⁿ means the silicon atoms tetrahedrally bonded with “n” number of bridging oxygen atoms) strongly depends on the optical basicity. These optical basicity dependencies of the structures of phosphorus and silicon can be explained clearly by the basicity equalization concept (Duffy and Ingram, 1976) [12].     

Electron paramagnetic resonance and raman spectroscopy characterization of diamond films fabricated by HF CVD method

Thin polycrystalline diamond films were synthesized on silicon substrate by Hot Filament Chemical Vapor Deposition (HF CVD) technique from a mixture of hydrogen and different content of methyl alcohol. A comparative study on the Electron Paramagnetic Resonance (EPR), Raman spectroscopy and Scanning Electron Microscopy (SEM) were performed. It was shown that EPR signal, Raman spectra and morphology, studied by SEM, strongly depend on the ratio of CH₃OH/H₂ in the HF CVD reactor. The peak‐to‐peak line‐width in EPR signal varies from 0.09 to 0.8 mT depending on diamond quality. The Raman spectra of our diamond film showed, except well defined diamond Raman lines positioned at 1332 cm^‐1 with different Full Width at Half Maximum (FWHM), a broad band having maximum at around 1530 cm^‐1 which is characteristic for amorphous carbon phase. The obtained results show that EPR, SEM and Raman spectroscopy yield complementary results about the defects present in CVD diamond films. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Doping dependence of crystallization growth rate in a-Si CVD films

Crystallization growth rates (V_g) on boron and phosphorus doped a-Si C VD films are obtained using conductivity measurements during isothermal annealings at temperatures 510<T_A<650°C. For boron doping, the associated activation energy of V_g is equal to 2.9 eV in the whole doping range (up to 2 × 10^?3 B₂H₆), whereas V_g increases by a factor 4 in the range 0 ? 7 × 10^?6 and remains almost constant for higher doping. In this low range, the neutral dangling bonds became positively charged and non paramagnetic by electronic compensation with the acceptors, and the E.S.R. signal decreases from 10¹⁹ to 10¹⁷ cm^?3. These results indicate clearly that dangling bonds and their charge state play an important role in the growth rate process.     

Dependence of chemical composition and bonding of amorphous SiC on deposition temperature and the choice of substrate

Amorphous SiC has superior mechanical, chemical, electrical, and optical properties which are process dependent. In this study, the impact of deposition temperature and substrate choice on the chemical composition and bonding of deposited amorphous SiC is investigated, both 6 in. single-crystalline Si and oxide covered Si wafers were used as substrates. The deposition was performed in a standard low-pressure chemical vapour deposition reactor, methylsilane was used as the single precursor, and deposition temperature was set at 600 and 650 °C. XPS analyses were employed to investigate the chemical composition, Si/C ratio, and chemical bonding of deposited amorphous SiC. The results demonstrate that these properties varied with deposition temperature, and the impact of substrate on them became minor when deposition temperature was raised up from 600 °C to 650 °C. Nearly stoichiometric amorphous SiC with higher impurity concentration was deposited on crystalline Si substrate at 600 °C. Slightly carbon rich amorphous SiC films with much lower impurity concentration were prepared at 650 °C on both kinds of substrates. Tetrahedral Si-C bonds were found to be the dominant bonds in all deposited amorphous SiC. No contribution from Si-H/Si-Si but from sp² and sp³ C-C/C-H bonds was identified.     

The validity of the double layer model on the amorphous semiconductor surface including the bulk localized state within the gap

Under the two assumptions that the origin of surface states may be different from that of bulk localized states within the gap and the density of surface states is sufficiently high, the validity of the double layer model on the amorphous semiconductor surface is investigated in comparison with the case of a crystal. It is suggested that the criteria concerning the double layer should be determined by the relative value of the surface states to that of bulk localized states. The existence of the double layer can be confirmed when the bulk localized state density n(E_f) is smaller than 10¹⁹ cm^?3 eV^?1. When n(E_f) is high at about 10²⁰ cm^?3 eV^?1, surface states cannot be distinguished from the localized states within the gap. This double layer model is strongly supported by the results of previous experiments by others who have measured the dependence of the Schottky barrier height on the work function of metal and the dependence of the surface potential on the preparation conditions of a-SiH.     

In-process monitoring and control of doping gas concentration during vapor phase silicon epitaxial growth by using a flame photometric detector

An in-process monitoring and control method of the doping gas concentration during epitaxial growth of Si was developed. A flame photometric detector (FPD) can be used as a monitor for the PH₃ and B₂H₆ dopant concentrations in the injected doping gases. A combination of this dopant monitor with an automatic control system of the silicon source (SiHCl₃) gas concentration using an infrared spectrophotometer as a monitor, makes possible an automatic in-process control of the concentrations of dopant and of silicon source gas supplied to the reactor. The present system provides an accurate and reproducible control of impurity concentrations in Si epitaxial layers. Good correlation between the monitored signal (or the doping gas concentration) and the impurity concentration incorporated into the growth layers was confirmed for PH₃ (n-type) and B₂H₆ (p-type) doping. For the B₂H₆ doping, a divergence from the linear relationship between the doping gas concentration and the impurity concentration in the layers was observed in the level of acceptor concentration below about 10¹⁵ atoms/cm³. The transient response of the present system was measured by growing epitaxial layers with increasing and decreasing step-changes in the dopant gas flow during continuous deposition of the layers. Some interesting, but complicated, transient responses of impurity concentration in the growth layer were observed. The responses are different between the PH₃ doping and the B₂H₆ doping, and also different between increasing and decreasing steps especially for the B₂H₆ doping.