B.C. Burkey

Optical properties of phosphorus‐doped polycrystalline silicon layers

The index of refraction and the absorption coefficient of low‐pressure chemical vapor deposited polycrystalline silicon (poly Si) implanted with various doses of phosphorus were obtained by means of transmittance and reflectance measurements. The refractive index of poly Si is relatively insensitive to doping in the visible region of the spectrum and agrees quite well with published values for single‐crystal Si. The absorption coefficient of these films decreases with doping in the visible. Nevertheless, the absorption coefficient corresponding to the highest‐doped film (N≊3×1020/cm3) is almost twice the published values for lightly doped single‐crystal Si. In the infrared, the index of refraction decreases systematically and the absorption coefficient increases systematically with doping. This dependence of the optical parameters on doping is attributed to the presence of free carriers. A free‐carrier dispersion/absorption model is applied to the measured refractive index in the region 1.2–1.8 μm and to ...

The pinned photodiode for an interline-transfer CCD image sensor

A pinned photodiode has been developed for use in an interline-transfer CCD. This photoelement has excellent blue response and high charge capacity. Both modeling and experimental results will be presented, including process considerations necessary to avoid unwanted barriers at the diode/transfer-gate edge.

Persistent photoconductivity in donor‐doped Cd1−xZnxTe

Phototransport measurements on chlorine‐doped Cd1−xZnxTe crystals with 0⩽x⩽0.35 show a persistent impurity photoconductivity below 140 °K. The thermal activation energy of the defect center responsible for both the dark carriers and photocarriers increases from −0.045 eV for x=0 to +0.27 eV for x=0.25 (negative implies above the conduction‐band edge), resulting in a decrease of the dark free‐carrier density with increasing Zn content. This permits an increasing light‐to‐dark‐conductivity ratio with increasing Zn content, reaching 2.5×105 for x=0.25. The increase of the mobility under photoexcitation indicates the center to be a double acceptor. Thermal emission rate, photoconductivity decay, and optical cross‐section measurements are also reported.

Steady-state photocarrier collection in silicon imaging devices

Solid-state imagers lose resolution when photocarriers generated in one imaging site diffuse to a nearby site where they are collected. These processes are modeled by solving the steady-state diffusion equation for minority carriers. A source term represents the absorption of photons and the generation of photocarriers, and a linear term represents the loss of photocarriers by recombination. This is equivalent to studying the Helmholtz equation with an inhomogeneous term. The problem is simplified when the light source has symmetry. A line source or a cylindrically symmetric source leads to a two-dimensional problem. The approach of Seib, Crowell, and Labuda allows a solution by quadrature if the further assumption of a smooth top boundary is made. We calculate the integrated normal flux over each imaging site to see how many carriers diffuse from under the illuminated site to another site. We compare our predicted line- and point-spread functions to those measured on imagers and find reasonable agreement. This allows us to extract minority-carrier diffusion lengths. Further calculations show how the diffusion of carriers depends on the photon wavelength and the pixel size. We generalize Seibs approach and apply it to a solid-state imager covered with color filters. This allows us to see the extent of color mixing due to carrier diffusion. We also discuss a finite-difference solution of the diffusion equation that employs the method of conjugate gradients. This approach is useful for problems where the top boundary is not smooth.

A model for charge transfer in buried-channel charge-coupled devices at low temperature

Charge transfer in buried-channel charge-coupled devices (CCDs) is explored with a one-dimensional numerical model which describes the capture and emission of electrons from a shallow donor level in silicon through the use of the Shockley-Read-Hall generation-recombination theory. Incorporated in the model are the three-dimensional Poole-Frenkel barrier lowering theory of A. K. Jonscher (1967) and J. L. Hartke (1968) and the low-temperature form of Poissons equation. Reasonable agreement of the model with experimental data taken from the buried-channel CCDs of a PtSi Schottky barrier infrared image sensor is found. Moreover, the value for the capture cross section of electrons to the shallow phosphorus level in silicon inferred from the model follows the cascade theory for capture by M. Lax (1959) and agrees roughly with determinations made by other experimenters. >

Threshold voltage in short-channel MOS devices

The threshold voltage in short-channel MOS transistors was investigated by use of a two-dimensional numerical solution of Poissons equation and experimental measurements on devices of 5.15-, 3.15-, and 2.15-µm channel length. The assumption of constant equipotential surface in the oxide implicit in the charge-sharing technique is not valid in devices of shorter Channel lengths and at larger operating voltages. The numerical determination of the threshold voltage from the two-dimensional analysis agrees with experimental results. Unlike previous work, the charge-sharing model was investigated from an electric-field point of view. The inadequacies of the charge-sharing model are elucidated qualitatively and quantitatively.

Optical and electrical properties of heavily phosphorus‐doped epitaxial silicon layers

The index of refraction and the absorption coefficient of epitaxial Si films were measured as a function of wavelength (0.4–0.75 μm) and with phosphorus doping as parameter. These optical constants, obtained from reflectance and transmittance measurements of epitaxial Si films on sapphire substrates, were compared with literature values and with recent measurements of heavily P‐doped poly‐Si films. In heavily P‐doped films (near the solid solubility) of poly and epi Si the absorption coefficients are about the same. Also, the Hall mobilities were measured as a function of carrier concentration in the range 6×1019–4×1020/cm3 and compared with the mobilities derived from infrared reflectance measurements on both poly and epi Si.

A 1-Megapixel, progressive-scan image sensor with antiblooming control and lag-free operation

A 1024-pixel*1024-pixel interline charge-coupled device (IL CCD) image sensor has been developed. It incorporates antiblooming and electronic exposure control while eliminating lag and obtaining a high responsivity. The novel features of this device include a noninterlaced, or progressive-scan, architecture and dual-horizontal registers that can be used to clock out the image area by one or two lines at a time. These features make it well suited for applications demanding high-resolution-image capture from a single, high-speed scan. The progressive-scan architecture of this device covers the same resolution in an electronic-camera application as that of a 2-million-element, interlaced device. >

Monte Carlo simulation of the photoelectron crosstalk in silicon imaging devices

The Monte Carlo method is used to evaluate the extent of the crosstalk in solid-state imagers. The calculations are performed in three dimensions and are in excellent agreement with experiment. The Monte Carlo method is used because it handles adjacent regions that either collect or reflect minority carriers.

A 1/3" format image sensor with refractory metal light shield for color video applications

The authors report results obtained on a full-color interline transfer CCD (charged-coupled device) image sensor with pixel dimensions of 8.6 mu m(H)*6.8 mu m(V) using 1.2- mu m design rules and a two-phase, single-polysilicon-per-phase technology. In order to reduce image smear and to provide suitable topography for integral color filters, a refractory light shield with a flowed glass overlayer was incorporated. The basic sensor and pixel architecture is shown. Image smear as a percent of full well, measured with 10% vertical illumination at saturated intensity, is shown as a function of wavelength. Smear is lowest at short wavelengths but is at an acceptable level for applications with controlled illumination.<<ETX>>