In the RF (13.56 MHz) silane plasma, recently the SiH3 radical density was measured using infrared diode laser absorption spectroscopy, and the correlation between the SiH3 radical density and the growth rate of hydrogenated amorphous silicon thin film was investigated. The SiH2 radical density was also measured using modified laser induced fluorescence spectroscopy and intracavity laser absorption spectroscopy. Those measurement methods and main results are reviewed here.

The formation of clusters in glow discharge plasma processing is of great concern with respect to the production yield. Their appearance, trapping and transport in silane plasmas have been the subject of several publications, but little is known about the mechanism of their formation and growth and how to avoid them in intense discharges used for high rate deposition of amorphous silicon. We present mass spectrometric and light scattering data and theoretical modelling which show that the formation of clusters in a clean silane discharge (total impurity ≈ 10 ppm) is due to a sequential growth of higher silanes SinH2n+2 with a strong, catastrophic-like onset starting from pentasilane. A selfconsistent mechanistic model will be presented together with a discussion of alternative ionic mechanisms. Several possibilities of how to avoid the cluster formation at high deposition rates of a-Si will be discussed and documented.

The growth of polycrystalline silicon thin films fabricated from fluorinated precursors SiFnHm (n+m≤3) on SiO 2 substrates was examined in detail by real time ellipsometry. The volume fraction of crystal in the film was within 50 vol.% on the average when it was grown continuously on glass. A low density amorphous layer of 300Å thick was formed in the early stage of the growth. The crystallinity, however, was improved with an increase in accumulated film thickness. The layer-by-layer technique of alternating the deposition of very thin film and the exposure to hydrogen plasma was effective on the promotion of crystallization. Optimal conditions of both the deposition and hydrogen plasma treatments were also established by in situ ellipsometry.

We have studied the microstructure of μc-Si:H/ZnO and μc-Si:H/a-Si:H interfaces in reactive magnetron sputtering (RMS) using in situ spectroscopic ellipsometry (SE). For (μc- Si:H deposited on ZnO, the real time ellipsometry trajectory shows that there is 20 volume % void apparent inside the first 100Å film. Then the film becomes densified, the initial void layer propagates as a surface layer, and its thickness decreases to 53Å when the bulk film reaches 200Å. Both unhydrogenated and hydrogenated silicon films deposited on ZnO show similar nucleation and coalescence behavior. This indicates that the atomic hydrogen rich plasma under μc-Si growth conditions doesn't reduce the ZnO substrate. For the μc-Si:H/a-Si:H study, we also observe the delayed crystallite nucleation after a ∼ 100Å high H content interface layer forms. Part of this layer results from H implanted into the a-Si:H substrate (∼ 45Å deep). The thickness of this interface layer decreases as the crystallite nucleation begins. This is the first direct experimental evidence showing sub-surface μc-Si:H formation.

The microstructural evolution of a-Si1-xCx:H with an optical gap of 1.95 eV, prepared by plasma-enhanced chemical vapor deposition (PECVD), has been studied versus gas phase H2- dilution by real time spectroscopic ellipsometry. As the H2/(CH4+SiH4) flow ratio is increased to 24, the monolayer-scale features of the growth process suggest an enhancement in precursor diffusion on substrate and film surfaces. Such features include a reduction in nucleation density, extensive surface smoothening during coalescence, and an increase the structural stability and density of the final film. We suggest a causal connection between these characteristics, and the photoelectronic properties of the film, which also improve with H2-dilution. Potentially detrimental effects of H2 dilution when a-Si1-xCx:H is deposited on TCO’s, including metal contamination at interfaces with Sn02 and H-diffusion into ZnO, are also characterized.

A quadripole mass spectrometer was installed on the line of a PCVD system to measure the depletion fraction of silane and the monosilicon radicals in the discharges. The paper presents a method of measuring the depletion fraction of silane when setting the mass spectrometer at the high ionization voltage. The experimental results were compared and accordant with those reported before [1,2,3].

In this study we characterize hydrogen diffusion and reaction processes in the near-surface (top 200 Å) of a-Si:H that lead to network equilibration under standard conditions of plasma-enhanced chemical vapor deposition (PECVD). Real time spectroscopic ellipsometry (SE) is used to provide continuous kinetic information on the near-surface conversion of Si-Si to Si-H bonds during exposure of in situ-prepared films at 250°C to filament-generated atomic H. We have found that for optimum PECVD a-Si:H, the formation of additional Si-H bonds is limited by the capture of H at trapping sites, and the rapid diffusion process (D>10-14 cm2/s) by which H reaches the site is not detected optically. Deep trapping occurs at a rate of ∼10 3 s-1 under our filament conditions, estimated to generate ∼1020 cm-3 mobile H in the near-surface of the film. Finally, more than 1021 cm-3 additional H atoms are trapped with emission rates <2×10-7 s-1, suggesting trap depths >2.0 eV. Shallower traps are also detected at lower concentration.

Very high frequency (VHF) glow discharges are employed for high rate a-Si:H deposition while maintaining good optoelectronic materials properties. A more efficient radical generation, either due to higher electron densities or an enhanced high energy electron tail is generally assumed as the mechanism. We have investigated a VHF a-Si:H deposition plasma between 40 and 250MHz by optical emission spectroscopy (OES), mass spectroscopy (MS), ion energy measurements and electrical impedance analysis. The present study shows that the increase of the deposition rate with frequency is essentially due to an enhanced ion flux to the growth surface.

Amorphous hydrogenated germanium a-Ge:H was deposited by very high frequency glow discharge (VHF-GD) at frequencies between 25 and 220MHz, low pressure (2.5Pa) and high deposition rates (≤4Á /s) on both electrodes of a parallel plate reactor. The films are comparable to material deposited on the powered electrode of a conventional RF-GD. Mobility-lifetime products ηµτ for photogenerated charge carriers around 10-7cm2/V and a Fermi level position at EC-EF≈400meV indicate good opto-electronic properties. The IR spectra show that the samples are free of oxydization and incorporate slightly more voids than the RF material. Measurements of ion energy Ei and flux Фi in the substrate plane show that “hard&” preparation conditions are obtained comparable to the deposition on the powered electrode of an RF-GD.

A series of hydrogenated amorphous silicon carbide (a-Si1–xCx:H) films was deposited by rf glow discharge deposition using various pressures, electrode spacings and hydrogen dilution ratios. We found that improvement of the structure by hydrogen dilution is more effective when a large electrode spacing is applied. In the case of undiluted a-SiC:H, the product of pressure and electrode spacing appears to be the important parameter. Dilution causes an increase of the photoconductivity. The band gap decreases but increases again for highly diluted samples. A striking result is that the Fourier transform infra-red spectroscopy (FTIR) bands assigned to CHX and SiHx increase upon dilution when a small electrode spacing is applied, although the hydrogen content is reduced. It is shown that this is due to an increase of the density of the films and to an increase of the amount of carbon built into the bulk instead of into voids. The combination of decreasing hydrogen content, void fraction and increasing amount of carbon atoms into the bulk explains the behaviour of the photoconductivity and band gap as a function of H2 dilution.

Effective bandgap modifications of hydrogenated silicon are demonstrated in this work. These films were obtained by (combined) hydrogen dilution and atomic hydrogen treatments during the deposition. The Si—H bonding configurations were characterized by NMR measurements. The hydrogen dilution tends to create a very sharp line-shape in the NMR spectra as the dilution ratio is increased and the addition of atomic hydrogen treatment can produce the same NMR spectra even at a lower temperature. The optical bandgap of these Si:H samples showed the same tendency that the bandgap narrowed as both the dilution ratio was increased and the addition of atomic hydrogen treatment.

In-situ and ex-situ transient photoconductivity measurements of intrinsic a-Si:H films deposited from He-diluted SiH4 are presented. It is shown that material with good optoelectronic properties can be deposited at high deposition rates. The films can already be characterized during the deposition. It is shown that material with fairly different properties can be deposited with a relative low defect density.

Poly-Si thin films with grains 100–200 nm in dia. showing a highly ordered texture were grown from fluorinated precursors, SiFnHm (n+m=3), on a glass substrate at 300–400 °C with the aid of atomic hydrogen. According to the in situ observation by ellipsometry, the reconstruction was undergone in a solid phase stimulated by impinging atomic hydrogens within a thin layer of about 10 nm thick owing to the strong chemical interaction of the pair of H and F in Si-network. Both H and F were released efficiently from the network to the levels of 2 × 1020 cm−3 and (2−5) × 1019 cm−3, respectively. Dangling bonds were also efficiently passivated down to 4 × 1016 cm−3 with hydrogens diffused through the network. P-doped films showing electrical conductivity of 10−2 S/cm (300 °K) with the activation energy of 0.24 eV was obtained by alternately repeating the deposition of thin layer and the treatment with atomic hydrogens.

We present a method to reduce the defect density in hydrogenated amorphous silicon (a-Si:H) deposited at low substrate temperatures similar to those used for device fabrication . Film-growth precursors are energized by a heated mesh to enhance their surface diffusion coefficient and this enables them to saturate more surface dangling bonds.

Electron transport properties of a-Si:H prepared in a remote hydrogen plasma deposition reactor (RHPD) at TD = 400°C were investigated in the temperature regime 110 K ≤ T ≤ 300 K by time-of-flight and post-transit spectroscopy experiments. Based on these data the conduction-band-tail state distribution was calculated. In the energy range 85 meV ≤ Ec- E ≤ 350 meV below the mobility edge Ec the tail is well described by an exponential distribution with a characteristic energy of ≃ 21 meV. Deeper in the mobility gap (Ec-E > 350 meV) the tail smoothly passes over into the defect density which is approximately six orders of magnitude smaller than at the mobility edge. Comparisons with data deduced on conventionally prepared a-Si:H (RF-, DC-glow discharge) at TD = 230 °C show that electron transport and the conduction band tail of the RHPD material are comparable.

We present the results of H diffusion studies on glow discharge (GD) and hot wire (HW) deposited a-Si:H films studied by infrared (IR) spectroscopy. In this technique, a-Si:H films deposited on crystalline silicon are annealed isochronally for fixed times, and after each anneal the decrease in the amount of SiH infrared absorption is carefully measured. We then incorporate these data into the rate equation for loss of SiH bonds due to annealing, enabling the determination of an activation energy to break these bonds and place the H in a site that cannot be measured by IR. The activation energies for device quality GD films are similar to those found in the a-Si:H diffusion literature, suggesting that diffusion parameters can be measured by this method. On the other hand, a similar analysis of data for HW films containing similar H contents is not clear cut. GD and HW samples with lower H content, deposited at higher Ts, are also examined. We discuss these results in the context of film microstructure.

We have developed a low-temperature (plasma-free) thermal-CVD method for the purpose of eliminating the plasma-CVD method on the amorphous-silicon thin-film transistor (a-Si TFT) process. The motivation comes from the fact that, in the plasma-CVD method, there arc many shortcomings originating from inherent properties of plasma, and that only the CVD method has potentially no drawbacks among various alternative methods. High deposition temperature, the most serious technological barrier in the present a-Si and silicon- nitridc (SiN) CVD methods, has been overcome by the use of high silicon-hydridcs and high nitrogen-hydrides for source gases. Electronic properties of the CVD-produced a-Si film arc improved by post-hydrogenation. Excimer-laser annealing is the next generation technology for improving dramatically the electronic properties of the CVD-produced Si and SiN films. This paper reviews these novel technologies developed in our group aiming at the TFT process, and also the CVD-produced TFT characteristics.

Hydrogenated amorphous silicon, a-Si:H, is deposited from silane (SiH4) and hydrogen (H2) using a tungsten wire at low filament temperatures (Tfil = 1200°C) by catalytic chemical vapor deposition.

The deposition rate increases monotonically with the depositions pressures and shows a maximum at an H2 : SiH4 flow ratio of unity. Vanishingly small deposition rates were observed for silane-only depositions and for H2-to-SiH4 flow ratios of 2.5 and above. The optoelectronic properties show complex dependence on substrate temperature (Tsub). Three intervals of Tsub with distinct optoelectronic were observed: as Tsub increases from 180 to 220°C, the optical bandgap, EgTauc increases from 1.9 to 2.4eV, the dark conductivity,σd, decreases from 10-10 to 10-15 Ω-1cm-1and the photoconductivity, σph, decreases from 10-5 to 10-10Ω-1cm-1 (region (i)). As Tsub increases from 220 to 250 °C, EgTauc decreases to 1.8eV and the photosensitivity, σph/σd decreases to ~ 1 due to an increase of both σd and σph (region (ii)). Throughout these two regions, the photoconductivity γ factor remains between 0.6 and 0.9 and the activation energy of the dark conductivity, Ea,σd, remains between 0.7 and 0.9eV. Above 250°C, the σph and σd remain approximately constant at 10-4 Ω-1cm-1 and γ decreases to below 0.5 and Ea,σd ~ 0.3eV.

This paper investigates the effects of ion implantation and annealing for pure (a-Si) and hydrogenated amorphous silicon (a-Si:H). The photocarrier lifetime in as-deposited a-Si:H decreases from ≥200 to 3 ps after 1 MeV Si+ implantation to doses exceeding 1014/cm2. A comparison with relaxed a-Si suggests that damage generation in a-Si:H merely arises from displacements in the silicon network. Annealing of ion-damaged a-Si:H at 200-500 °C recovers the carrier lifetime to 60-100 ps as a result of hydrogen passivation of electrical defects. However, Raman spectroscopy shows that hydrogen does not significantly enhance long-range network relaxations during annealing. This implies that thermal treatments of ion-implanted a-Si:H can not fully recover the as-deposited state.

Chemical annealing, the periodic exposure of the growing amorphous silicon surface to H radicals, has evoked much interest because of its potential for modifying the film properties during the crucial process of sub-surface restructuring. We report the results of measurements of H content in chemically annealed a-Si:H and a-Si:H,F films.

We prepared a-Si:H and a-Si:H,F in a dc-triode reactor from SiH4 and SiF4 plus H2, respectively. We chemically anneal with a H2 discharge by switching off the Si bearing gas and switching on the H2 flow after 7 s of deposition. We measure the H content with a differential pressure effusion apparatus. We also report the opto-electronic properties of these films, including the defect density after light-soaking.

The key parameter of chemical annealing is the anneal duty cycle, which we specify as the fraction of the time that the H2 discharge is on. In the silane samples, we find that the H content is independent of chemical annealing. For the fluorinated samples we observe that the H content starts out at about 5 at.% without chemical annealing, rises to 10-15 at.% at 0.5 to 0.7 annealing duty cycle, and then decays to 6-7 at.% at 0.9 duty cycle. Although not fully understood, we offer evidence that these effects are due to the physics of the glow-discharge rather than surface reactions.