J. C. Knights

Microstructure of plasma‐deposited a‐Si : H films

Using transmission and scanning electron microscopy, it is shown that plasma deposition of amorphous silicon hydrogen films from silane or silane/argon mixtures proceeds via nucleation and growth of islands of average lateral dimensions ∼100 A. If these islands do not coalesce into a homogeneous film, subsequent growth produces columnar morphology with low‐density interstitial regions. There is a strong correlation between the columnar structure and the presence of nonradiative recombination centers.

Film formation mechanisms in the plasma deposition of hydrogenated amorphous silicon

By studying the step coverage of plasma‐deposited amorphous silicon and germanium on patterned substrates, we find that the film formation process under device‐quality deposition conditions has a substantial component that behaves like a surface rate‐limited chemical vapor deposition process, while conditions producing defective material are associated with a much more physical‐vapor‐deposition‐like process. An explanation involving surface reactions of SiHx radicals is proposed.

Growth morphology and defects in plasma-deposited a-Si:H films

Abstract Structural studies of plasma-deposited a-Si:H are presented that show that a major class of defect is an anisotropic density fluctuation. Studies of the hydrogen environment suggest that an inhomogeneous hydrogen distribution is associated with these fluctuations. From considerations of the deposition chemistry and nucleation theory a model is proposed to describe the film growth process and its relationship to defects.

Effects of inert gas dilution of silane on plasma‐deposited a‐Si:H films

Electrical, optical, and structural characterization of hydrogenated amorphous silicon films plasma‐deposited from mixtures of SiH4 with different inert‐gas diluents reveals substantial differences in a number of properties. A general trend of increasing defect density with atomic weight of the inert gas is observed. Of specific interest to device applications is the observation that high deposition rates can be achieved concurrently with low defect densities when helium is used as a deluent.

Optically induced electron spin resonance in doped amorphous silicon

Abstract We report the first observations of optically induced electron spin resonance signals in doped and undoped amorphous silicon. We also report the observation of equilibrium surface or interface spin densities ∼ 10 13 cm −2 and volume spin densities ∼ 6 × 10 15 cm −3 . The small number of spins observed in equilibrium compared to the large optically induced spin density shows that most electrons are spin paired in equilibrium. We conclude that this implies a very small mean effective correlation energy, U ∼ kT .

Electronic and structural properties of plasma-deposited a-Si:O:H - The story of O2☆

Abstract Measurements of luminescence, optical absorption and infrared vibrational spectra are reported for thin films plasma-deposited from SiH4/O2/Ar mixtures. The addition of oxygen causes the optical absorption edge and luminescence peak position to shift monotonically to higher photon energy. Increases in the luminescence peak width and in the temperature of thermal quenching result in room temperature luminescence in the visible at high oxygen contents.

Optical absorption spectra of surface or interface states in hydrogenated amorphous silicon

The optical absorption of doped and undoped hydrogenated amorphous silicon (a‐Si:H) films ranging from 5 nm to 10 μm was measured using photothermal deflection spectroscopy. The absorption spectra show that there is a high defect layer associated with the surface or interface of the film. From comparison of defect absorption and dangling bond spin densities, it is found that a‐Si:H films which have ∼1015 bulk defects/cm3 exhibit surface or interface layers with ∼1012 dangling bonds/cm2.

Spin-dependent photoconductivity in n-type and p-type amorphous silicon

Abstract It is found that, as in crystalline silicon, the photoconductivity in amorphous silicon prepared by glow discharge is strongly spin-dependent. In this material, the position of the Fermi level can be smoothly varied by phosphorous or boron doping and the magnitude of the spin-dependent recombination has been measured as a function of the doping: it is found to have a large maximum when the material is intrinsic. The similarities with the spin-dependent effects in crystalline and dislocation silicon suggest that the recombination process in amorphous silicon is the same as in the crystalline material and that dislocation-like centres are responsible for the spin-dependent recombination properties in both materials.

Proton nmr studies of annealed plasma-deposited amorphous Si:H films

Abstract Proton magnetic resonance data are presented for plasma-deposited amorphous Si:H as a function of annealing temperature up to 650°C. The data indicate that hydrogen diffuses internally before major evolution occurs, that transfer of hydrogen occurs from a heavily clustered phase to a dilute phase coincident with evolution and that evolution occurs initially from the heavily clustered phase. Comparison with infrared data indicates that the heavily clustered phase can be either SiH x ( x = 2,3) or SiH.

Hydrogen in amorphous semiconductors

Abstract Over the past 5 years, there has been growing interest in a class of amorphous semiconductors deposited in thin-film form in the presence of hydrogen. The interest has derived primarily from certain electronic properties, such as the ability to control the Fermi level by substitutional doping, that make the materials potential candidates for solar photovoltaic energy conversion and thin-film device applications. These same properties have also made the materials attractive “test-beds” for basic research into electronic processes and defect states in amorphous semiconductors.