Dislocation core structures in Si-doped GaN

Aberration-corrected scanning transmission electron microscopy was used to investigate the core structures of threading dislocations in plan-view geometry of GaN films with a range of Si-doping levels and dislocation densities ranging between (5 ± 1) × 108 and (10 ± 1) × 109 cm−2. All a-type (edge) dislocation core structures in all samples formed 5/7-atom ring core structures, whereas all (a + c)-type (mixed) dislocations formed either double 5/6-atom, dissociated 7/4/8/4/9-atom, or dissociated 7/4/8/4/8/4/9-atom core structures. This shows that Si-doping does not affect threading dislocation core structures in GaN. However, electron beam damage at 300 keV produces 4-atom ring structures for (a + c)-type cores in Si-doped GaN.This work was funded in part by the Cambridge Commonwealth trust, St. John's College, British Federation of Women Graduates and the EPSRC. M.A.M. acknowledges the support from the Royal Society through a University Research Fellowship. Additional support was provided by the EPSRC through the UK National Facility for Aberration-Corrected STEM (SuperSTEM).This is the author accepted manuscript. The final version is available from AIP via http://dx.doi.org/10.1063/1.493745

Segregation of In to dislocations in InGaN.

Dislocations are one-dimensional topological defects that occur frequently in functional thin film materials and that are known to degrade the performance of InxGa1-xN-based optoelectronic devices. Here, we show that large local deviations in alloy composition and atomic structure are expected to occur in and around dislocation cores in InxGa(1-x)N alloy thin films. We present energy-dispersive X-ray spectroscopy data supporting this result. The methods presented here are also widely applicable for predicting composition fluctuations associated with strain fields in other inorganic functional material thin films.This work was funded in part by the Cambridge Commonwealth trust, St. John’s College and the EPSRC. SKR is funded through the Cambridge-India Partnership Fund and Indian Institute of Technology Bombay via a scholarship. MAM acknowledges support from the Royal Society through a University Research Fellowship. Additional support was provided by the EPSRC through the UK National Facility for Aberration-Corrected STEM (SuperSTEM). The Titan 80- 200kV ChemiSTEMTM was funded through HM Government (UK) and is associated with the capabilities of the University of Manchester Nuclear Manufacturing (NUMAN) capabilities. SJH acknowledges funding from the Defence Treat Reduction Agency (DTRA) USA (grant number HDTRA1-12-1-0013).This is the accepted manuscript. The final version is available at http://pubs.acs.org/doi/abs/10.1021/nl5036513

Effect of local structural order on the doping in hydrogenated amorphous silicon (a-Si:H)

Doping in a-Si:H has been studied in the light of local structural order present in these films and the consequent modification in this upon dopant incorporation. It is seen that the local structural order is a very important parameter to understand the doping mechanism and variation in this conelates well with the variation in doping efficiency with increasing dopant concentration and the increase in the defect density. Interesting results on the compensated samples are also reported and discussed

Gas Phase Chemistry Study During Deposition of a-Si: H and μc-Si: H Films by HWCVD using Quadrupole Mass Spectrometry

Amorphous and microcrystalline silicon films were deposited by HWCVD under different deposition conditions and the gas phase chemistry was studied by in situ Quadrupole Mass Spectrometry. Attempt is made to correlate the properties of the films with the gas phase chemistry during deposition. Interestingly, unlike in PECVD, partial pressure of H2 is higher than any other species during deposition of a-Si:H as well as μc-Si:H. Effect of hydrogen dilution on film properties and on concentration of various chemical species in the gas phase is studied. For low hydrogen dilution [H2]/ [SiH4] from 0 to 1 (where [SiH4] is 10 sccm), all films deposited are amorphous with photoconductivity gain of ∼ 106. During deposition of these amorphous films SiH2 was dominant in gas phase next to [H2]. Interestingly [Si]/[SiH2] ratio increases from 0.4 to 0.5 as dilution increased from 0 to 1, and further to more than 1 for higher hydrogen dilution leading to [Si] dominance. At hydrogen dilution ratio 20, consequently films deposited were microcrystalline

Low Temperature Surface Passivation Of Silicon Solar Cells

Low temperature surface passivation is a process that has a potential to reduce the input energy cost of the solar cell with minimum modification of the manufacturing bed, while keeping the efficiency, and life of the cells within acceptable range of values. In this review, low temperature deposition methods of SiO2, Al2O3, a-Si:H, silicon nano particles (NPs), and organic materials, are considered. Surface recombination velocities, defect densities, stability of these passivating layers are discussed along with the mechanisms of passivation on Si surface

Low Temperature Surface Passivation Of Silicon Solar Cells

Low temperature surface passivation is a process that has a potential to reduce the input energy cost of the solar cell with minimum modification of the manufacturing bed, while keeping the efficiency, and life of the cells within acceptable range of values. In this review, low temperature deposition methods of SiO2, Al2O3, a-Si:H, silicon nano particles (NPs), and organic materials, are considered. Surface recombination velocities, defect densities, stability of these passivating layers are discussed along with the mechanisms of passivation on Si surface

Dependence of effective doping on structural order in hydrogenated amorphous silicon

Electrical and structural measurements have been made on intrinsic and phosphorus doped a-Si&#166;H films deposited by R.F. glow discharge at different substrate temperatures (T<SUB>s</SUB>) in the range (50-400&#176;C). Effective doping, parameterised as ratio of dark conductivity of the doped film to that of the intrinsic film deposited at the same temperature is maximum around 250&#176;C. Gap state density inferred from DLTS results shows an increase beyond 250&#176;C. The average bond angle deviation &#916;&#952; in the amorphous network, is also minimum around 250&#176;C and increases for higher T<SUB>s</SUB>. The results bring out a good correlation between effective doping and the bond angle deviation, an important element of the short range order in the amorphous network

Systematic study of the process parameters affecting hydrogen plasma passivation of polycrystalline silicon and polycrystalline silicon solar cells

We report detailed studies of hydrogen passivation of polycrystalline silicon wafers by RF plasma and the effect of various process parameters on the extent of passivation as seen through electrical measurements. Similar studies made on polysilicon solar cells are also reported. The need for such studies is emphasized via some interesting results which show an excessive dependence of effective passivation on the process conditions such as processing time, substrate temperature, gas pressure and RF power which seems to have been overlooked by earlier workers. Our results reveal that there is an optimum for almost all the above parameters which yield the most effective passivation of the grain boundaries in polycrystalline silicon. Physically, therefore, it implies that optimum hydrogen incorporation in a particular bonding configuration is a necessary condition to achieve best passivation. Results of samples passivated for several hours indicate the presence of yet another process along with the normal hydrogen diffusion. Possibilities of bond breaking or hydrogen incorporation in different bonding configurations cannot be completely ruled out. The dependence of effective passivation on these process conditions is also revealed by the systematic studies of variation in average carrier concentration, charge carrier mobility and resistivity with substrate temperature, RF power and gas pressure. Polycrystalline silicon solar cells passivated with the optimum process conditions show significant improvement in the efficiency and fill factor. Presence of hydrogen was confirmed by quadrupole mass spectrometry studies by heating the passivated samples

Effect of rf power on the structure and related gap states in hydrogenated amorphous silicon

The intricate relationship between the structure and processing parameters in a-Si:H is not well understood. The effect of variation in rf power on the structural, optical and electronic properties of glow discharge deposited hydrogenated amorphous silicon (a-Si:H) has been studied using deep level transient spectroscopy (DLTS), photoluminescence (PL), spectroscopic ellipsometry and Raman spectroscopy. The DLTS results show a predominant increase in the gap state density around 0.5 eV below the conduction band, which is also supported by the decrease in the intensity of the PL peak around 0.8 eV with rf power. Ellipsometry results show an overall decrease in the value of &#949;2, while the energy corresponding to the &#949;2 peak position remains constant. Significant modifications of spectral features are observed in the Raman spectra of film deposited at higher rf powers. These results suggest that the a-Si:H films grown at higher rf power are less dense and have larger vacancy or void concentration. It is proposed that such structural defects lead to the observed energy levels in the gap and associated physical properties of the a-Si:H films