D.K. Schroder

Free carrier absorption in silicon

Free carrier absorption in heavily doped layers reduces the useful photon flux in the photoconductive region of extrinsic Si infrared detectors. A simple theory is developed which predicts the transmissivity of such layers as a function of their sheet resistance and the wavelength of the radiation. Experimental data over the 2.5-20 µm wavelength and 5-500 Ω/square sheet resistance range are given for both diffused and ion-implanted layers and also for polysilicon gases. The temperature dependence of both transmissivity and sheet resistance is investigated from 20 to 300 K.

Fabrication and some applications of large-area silicon field emission arrays

Abstract A method of fabricating large-area arrays of sharply-pointed field emitters at densities up to 1·5 × 10 5 per cm 2 from single crystal silicon wafers is described. The point emitters are formed by etch-undercutting a precision oxide pattern which is delineated on the silicon surface by projection photolithography. Observations indicate that emitters with very small tip dimensions in the 200Arange are formed. In the presence of an external electric field, such as produced by a voltage applied to a closely-spaced, planar anode, multiple-emitter arrays are shown to field-emit electrons uniformly over areas up to 3 cm dia. Two important applications currently being explored, are discussed: (1) High resistivity, p -Si has been utilized to develop experimental field emission photocathodes with which field emission imaging has been demonstrated. These photoemitters exhibit very high photo-sensitivities at visible and near i.r. wavelengths. For example, at 0·86 μm, the measured quantum efficiency is 25 per cent which is about five-times higher than the red-sensitive S-20 photocathode and comparable to the highest reported sensitivities of the III–V photosurfaces; (2) N -type emitter arrays show considerable promise as high current, cold cathodes and total emission currents of 1/4 A from 1 cm 2 areas of 100 Ω-cm n -type emitters have been obtained. Measurements were made under pulse conditions because of anode dissipation considerations.

The semiconductor field-emission photocathode

The recently developed large-area field-emission photocathode is described. It consists of a finely spaced array of point emitters fabricated by etching of p-type silicon or other semiconductor. Uniform emission over areas of 6-7 cm2have been obtained. For Si, the spectral response extends from 0.4 to 1.1 µm. Quantum yields of 25 percent at 0.86 µm have been measured, which is about five times the value reported for the extended S-20 photocathode and comparable to the best III-V photoemitters. Calculations indicate that quantum yields of up to 40 percent at 0.86 µm and 28 percent at 0.9 µm are attainable with the present photocathode structures. For low dark current densities, photocathode cooling to temperatures approaching 77 K must be employed at present. The dark current is shown to be dominated by surface-generated electrons in the space-chargeregion of the emitters. Effects of phosphorus gettering and annealing treatments on dark current are discussed, and the spatial frequency response of the device is determined. The results of a computer study show that the field intensification factor of p-semiconductor field emitters behaves quite differently from that of metallic emitters.

Characterization of current transport in MNOS structures with complementary tunneling emitter bipolar transistors

We have extended the work of previous investigators and studied current transport in thin- (10-20 Å) and thick-(80 Å) oxide MNOS structures with complementary tunneling emitter bipolar transistors. These devices are fabricated with ion-implanted p-n and n-p junctions to distinguish the dominant carrier species in the insulator. The dominant species in thin-oxide devices is hole transport, comprising about 99 percent of the emitter current. The hole transport is suppressed in the thick-oxide structures, where the dominant carriers are electrons. Electron impact ionization multiplication is observed in thick-oxide structures.

The doping concentrations of indium‐doped silicon measured by Hall, C‐V, and junction‐breakdown techniques

Hall‐effect measurements are routinely used to determine the doping concentrations in semiconductor material. We show that for Si : In such measurements give erroneously high results. The Hall concentrations, determined by curve fitting, are typically twice those determined from C‐V and junction‐breakdown measurements, which measure the doping concentrations directly. We consider the latter data more reliable.

Transparent gate silicon photodetectors

Optical and electrical properties of silicon imaging devices with In 2 O 3 -SnO 2 gates are reported. The optically transparent electrically conducting films were sputtered from both glass and metal targets. Quantum efficiencies of such devices are significantly better than photodiodes or polysilicon gate devices. The electrical properties such as dark current and interface state density are as low as conventional aluminum gate devices after appropriate anneals.

Bulk and optical generation parameters measured with the pulsed MOS capacitor

Theory and experiments are presented for the pulsed MOS capacitor when carrier generation is constant. This condition obtains when quasi-neutral bulk-region generation dominates over that in the space-charge region or when generation is due to an external excitation mechanism. It is shown how the diffusion length, surface generation velocity, and external flux can be obtained from the pulsed C-i response. This extends the usefulness of the pulsed MOS-C to narrow-gap semiconductors, such as germanium, as well as to optical generation measurements.

Experimental confirmation of the Fowler‐Nordheim law for large‐area field emitter arrays

It is well known that single field emitters obey the Fowler‐Nordheim (F‐N) relationship. We show here that large‐area emitters, with as many as 106 emitters participating in the emission process, also obey the F‐N law, giving straight lines on such plots.

An explanation for the anomalous impurity concentrations in Si as measured by the Hall effect

It is shown that inclusion of a temperature variation of impurity activation energies in the analysis of Hall data can explain reported discrepancies in measurements of impurity concentrations in Si. The required variation of activation energies is reasonably well accounted for by screening theory; a very good fit can be obtained if one includes in addition a slight temperature variation in the Hall factor.

The spatial variation of the charged gold concentration in silicon p-n step junctions

The spatial variations of the three charged gold states in the transition region of silicon p+n and n+p step junctions are calculated. The exact solution is obtained at thermal equilibrium. Under reverse applied bias, exact solution is not possible and approximate solutions using staircased charge distributions are obtained. Under reverse bias, most of the gold atoms in the transition region are in the negative or acceptor charge state for both p+n and n+p junctions. This is due to the dominance of the hole emission process at the neutral gold centers over all other electron and hole emission processes of the three charge states at 300°K. Since the shallow-level impurities in these two types of junctions are of opposite charge and the deep-level gold impurity is mainly in the negative charge state, quite different electrical behaviors are expected from these two junction types. In work to be described elsewhere, some of these differences have been demonstrated experimentally, such as the breakdown voltage and the impedance, and quantitatively correlated with theory using the spatial variations obtained in this paper.