Luminescence and magnetic resonance in post-hydrogenated microcrystalline silicon

We report photoluminescence, EPR and ODMR studies of plasma hydrogenated LPCVD microcrystalline silicon thin films. Apart from the dangling bond EPR resonance which is similar to that observed in a-Si : H, and possibly the emission at 1.3 eV, we find emission bands and resonance signals quite unlike those observed in normal c-Si and a-Si : H. We conclude that μc-Si : H cannot be regarded purely as crystallites of pure silicon embedded in a matrix of a-Si : H.     

A DLTS study of the effects of boron counterdoping on the gap states in n-type hydrogenated amorphous silicon

The technique of Deep Level Transient Spectroscopy (DLTS) and related space charge measurement techniques have been applied to a series of 300 vppm phosphine (PH₃) doped, 0–44 vppm diborane (B₂H₆) counterdoped samples of hydrogenated amorphous silicon (a-Si:H). The trends in the resulting density of states, g(E), show: (1) A large increase of g(E) at ≈0.4 to 0.6 eV from the valence band, comparable in magnitude to the total number of B atoms introduced; (2) A smaller decrease in the midgap dangling bond states on the order of ≈2.5% of the added B; (3) A drop in the number of occupied shallow states ≈0.5%; and (4) A quantitative assessment of the degree of autocompensation which shows roughly five compensating midgap states removed for every six shallow acceptor states introduced by B.     

Variations of the local density of electron states and short-range order in amorphous hydrated silicon films

Ultrasoft x-ray spectroscopy methods have been used to observed a change in the energy distribution of the silicon valence states after annealing a-Si:H films at 500 °C. This change appears as three distinct maxima in the density of states 3.5, 7.2, and 10.2 eV above the top of the valence band, which indicates ordering of the a-Si:H structural network. The energy distance between the latter two maxima (E?E _v=7.2 and 10.2 eV) supports electron-diffraction data indicating a decrease in the silicon-silicon interatomic distance by 0.2 Å in comparison with the crystal. The presence of a third maximum (E?E _v=3.5 eV) is connected with the change in the hybridization of the s-p-functions of silicon with decrease of the coordination number.     

P-related defects in P-doped a-Si:H

The electronic states of a weak Si-P bond are calculated by applying a simple LCAO MO method to a small cluster of atoms. The energy level of the antibonding state of the weak Si-P bond appears above that of the Si dangling bond in the gap. The ESR hyperfine lines observed in P-doped a-Si:H can be attributed to an electron in this antibonding state. The change of the length of the Si-P bond according to its charged state is suggested to be a possible origin of the Staebler-Wronski effect.     

Transient photocurrent saturation and deep trapping in hydrogenated amorphous silicon

The transient photocurrent following laser impulse excitation has been measured as a function of laser excitation density for several samples of undoped hydrogenated amorphous silicon (a-Si:H). A feature is observed in these data which is interpreted as originating from complete saturation of the occupancy of deep electron traps. The trap density is estimated from this feature and compared to the neutral dangling bond defect density obtained from electron spin resonance; both measurements are of the same magnitude. Estimates of the density of states, g¹(E), using the measured trap density are also presented.     

Evaporated a-Si films with low ESR defect density by hydrogenation under NH3 ambient

a-Si films were obtained by a reactive evaporation of Si atoms with an electron gun, under a NH₃ ambient of 5 × 10^?5 torr. The H introduced amount efficiently reduces the ESR dangling bond density from 4 × 10¹⁹ cm^?3 in pure samples to 2 × 10¹⁷ cm^?3. The hydrogen presence is evidence by both the Si-H infrared bands and the strong spin-density increase with annealing. This increase, beginning at a temperature of 350°C, is attributed to the H effusion. XPS measurements show also an incorporation of bonded nitrogen.     

Recombination centers in phosphorous doped hydrogenated amorphous silicon

Optical absorption below the mobility gap of a-SiH_x:P films is derived from photoconductivity measurements and interpreted in terms of optical transitions from occupied localized states in the exponential valence band tail and dangling bond states 0.8 eV above the valence band edge to unoccupied free electron conduction band states. Collection efficiency measurements of Schottky barrier structures indicate that P doping introduces centers with large capture cross-section for holes.     

Photoluminescence in polysilane alloys

Polysilane alloys are prepared by the rf glow discharge of disilane. The microscopic structure, which depends on substrate temperature, is discussed by measuring the infrared absorption spectrum and dangling bond density. Temperature dependence of the photoluminescence of polysilane alloys is measured and compared with that of conventional a-Si:H film. Based on the microscopic structure and calculated energy level for polysilane molecules, a three dimensional quantum well model is proposed for the band structure of polysilane alloys. This model is consistent with photoluminescence and related optical properties.     

Accumulation and annealing of implantation damage in a-Si:H

Hydrogenated amorphous silicon (a-Si:H) films have been irradiated with H⁺, B⁺, P⁺, and Ar⁺ ion beams. The accumulation and the annealing of irradiation-induced defects has been investigated through a series of electronic transport and PDS measurements. We find that for all projectiles damage accumulation is dominated by atomic displacement collisions with the damage saturating for energy transfers in excess of about 10 eV/target atom. Annealing at elevated temperatures causes the conductivity of doped and irradiated a-Si:H films to increase according to stretched exponential decay curves. All annealing parameters derivable from such fits scale with the energy originally dissipated into atomic displacement collisions. For energy transfers up to 10 eV/target atom the activation energy for annealing increases up to a saturation value and, at the same time, an increasing fraction of the irradiation-induced defects becomes stable against annealing at moderate temperatures (T _a<250° C). We discuss these results with respect to damage accumulation data in crystalline silicon (c-Si) and with regard to the annealing of metastable defects in a-Si:H.     

Photon energy dependence of SW effect in a-Si:H films

In order to explore the creation process of Staebler-Wronski (SW) defect in a-Si: H films. We investigated the effect of monochromatic light exposure with different photon energies: 2.54, 1.96, 1.17 and 0.95 eV. The experimental results show that threshold energy for SW defect creation is around 1.17 eV.     

EPR investigation of a-Si:H aerosol particles formed under silane thermal decomposition

The dangling bond defects were investigated in a-Si:H particles formed under silane thermal decomposition in flow reactor. EPR together with hydrogen evolution method were used. The experimental results allowed us to conclude that there are two kinds of dangling bond defects in a-Si:H aerosol particles. The defects of the first kind are localized on the surface of interconnected microvoids and microchannels (surface dangling bonds) and those of the second kind are embedded in amorphous silicon network (volume dangling bonds). The thermal equilibration of dangling bonds and temperature dependence of equilibrium dangling bond concentration were investigated. It was found that at temperatures > 400 K the dangling bond concentrationNApplied Magnetic Resonance _s reversibly depends on sample temperature. The volume dangling bond concentration increases with temperature increasing (the effective activation energy of dangling bond formationU > 0), and the surface dangling bond concentration decreases with temperature increasing (U < 0). It has been found that EPR line is considerably asymmetric for samples with high hydrogen content and for low hydrogen content the EPR line is weakly asymmetric. A conclusion was drawn that the asymmetry degree depends on amorphous silicon lattice distortions. This conclusion has been confirmed by EPR spectra simulations.     

Strongly phonon-bottlenecked spin-lattice relaxation of dangling bonds in sputtered a-Si evidenced by chemical etching

CW saturation ESR experiments at 20.9 and 9.0 GHz have been carried out between 4.2 and 300 K on a-Si films sputtered on fused silica substrates. The ESR signal (of g = 2.0050 ± 0.0001) ascribed to dangling bonds in the a-Si network shows up to be strongly saturatable (onset of saturation at K-band microwave power P_S ? 1μW at 4.2 K). Submitting the a-Si film subsequently to a SiO₂ chemical etchant drastically decreases (P_S increased by more than a factor 1500) the saturatability from where a strongly phonon-bottlenecked (PBN) spin—lattice relaxation regime is concluded to exist on the non-etched films. This PBN is ascribed to O₂ contamination, enhancing the saturatability in two steps.     

A study of built-in potential ina-Si solar cells by means of back-surface reflected electroabsorption

The electroreflectance (ER) signal has been studied for the purpose of identifying the built-in field in practical amorphous silicon (a-Si∶H) solar cells. Through both theoretical and experimental considerations, it has been confirmed that the ER signal essentially comes from the light which is reflected at the back surface and hence experiences the internal electric field within thea-Si∶H layer. By analyzing the ER signal, which is really the back-surface reflected electroabsorption signal, the built-in potentialV _bcan be evaluated. This method has been applied to various types ofp-i-n junctiona-Si solar cells.V _bof a usual homojunction solar cell was about 0.85 V. Increases ofV _bby 50≈130mV have been found in heterojunction solar cells constructed withp-type amorphous silicon carbide (a-SiC∶H) and/orn-type microcrystalline silicon (μc-Si) as compared with homojunctionp-i-n solar cells. Moreover, a clear dependence ofV _bon the substrate materials has been observed. These experimental results are described in connection with cell performances.     

Spectroscopic determination of the position of the ferm-level in doped amorphous hydrogenated silicon

The variation of the Fermi energy E_F of _a-Si:H with boron or phosphorus doping has been determined by means of photoelectron spectroscopy. The results are shown to be consistent with those obtained from the activation energy of the conductivity. The p-type charge carriers are concentrated in a region 0.3 eV above the spectroscopically determined valence band edge.     

Order parameters in a-Si systems

We have used Raman scattering to characterize the change in structural order for three types of a-Si materials: fluorinated, a-Si:F:H; hydrogenated, a-Si:H; and sputtered, a-Si (sput) under heat-treatment. The narrower TO linewidth, the higher TO-phonon frequency, the lower TA-phonon frequency, and the lower “crystallization temperature”, all indicate an increase in the local order in the amorphous phase for a-SiH, and particularly for a-Si:F:H. This improved structural order is consistent with the reduction of electronic gap-states.     

Electronic structure of Si(111) surfaces

We report on new angle-resolved photoemission studies of Si(111) 2 × 1 and 7 × 7 surfaces. The emission from the 2 × 1 surface shows much structure. For normal emission the energy positions are insensitive to the photon energy in the range 19–27 eV. The emission has been interpreted as a probe of the surface density of states, SDOS, including both surface states, resonances and bulk-like states. The SDOS was also calculated as a function of parallel momentum k_∥ for a model of the Si(111) 2 × 1 surface obtained from energy minimization considerations. We identify emission from the dangling bond band, which has a positive dispersion of 0.6 eV, and also emission from surface resonances which have some character of the compressed and stretched back bonds. There are also other predicted surface resonances that correspond to experimental peaks which have not been identified in previous work. Except for the dangling bond band, the surface resonances are limited in k_∥ space, so that it is not possible to follow these resonance bands over all angles. Maximum intensity for the normal emission from the dangling bond is obtained at 23 eV, while the emission from the lowest s-like states monotonically increases towards 30 eV photon energy. When annealing the cleaved 2 × 1 surface to the 7 × 7 reconstructed surface, the spectra broaden significantly. The intensity of the dangling bond decreases and we see a very small metallic edge.     

Estimation of localized state distribution profiles in undoped and doped a-Si:H by measuring space-charge-limited current

The characteristic temperature which characterizes the localized states of amorphous silicon-hydrogen (a-Si:H) has been obtained by measuring space-charge-limited (SCL) currents in planar structure gap cells for undoped and nitrogen doped films. The SCL current is confirmed by measuring the critical voltage as a function of electrode gap spacing. The characteristic temperature obtained is 1060 ± 150 K for undoped a-Si:H films. The value of nitrogen doped film prepared under the condition of 0.4% N₂ gas partial pressure becomes less than 400 K.     

Picosecond recombination of photo-excited carriers in sputtered a-Si:H

The decay kinetics of photo-excited carriers in r.f. sputtered a-Si:H have been investigated on a picosecond time scale. Decay curves were in agreement with a two-step recombination process for a-Si:H. Results are compared to previous measurements on glow-discharge a-Si:H.     

Room temperature deposition of amorphous SiC thin films using low energy ion bombardment

Silicon carbide (SiC) films are prepared by single- and dual-ion beam sputtering deposition at room temperature, respectively. An assisting argon ion beam (ion energy Ei=150 eV) bombards directly the substrate surface to modify the SiC film surface. The thin films are characterized by the Fourier transform infrared spectroscopy (FTIR) and the Raman spectra. With assisting ion beam bombardment, the density of the Si–C bond in the film increases. Meanwhile, the excess carbon or the size of the sp² bonded clusters and the amorphous Si (a-Si) phase decrease. These results indicate that the composition of the films is mainly Si–C bond. UV-vis transmission shows that the E_opt increases steadily from 1.85 eV for the amorphous SiC (a-SiC) films without bombardment to about 2.29 eV for those with assisting ion beam bombardment.