Marshall J. Cohen

The mechanism of Schottky‐barrier formation in polyacetylene

An investigation of metal‐polyacetylene contacts by using x‐ray photoemission spectroscopy is reported. For undoped p‐type polyacetylene, Mg metal formed a rectifying contact with a Schottky‐barrier height of ≳0.6 eV; Au metal formed a pure ohmic contact. Changes in band bending in the polyacetylene with metal deposition were directly observed by x‐ray photoemission spectroscopy and correlated with I‐V transport measurements. Our results indicate that the mechanism for Schottky‐barrier formation in polyacetylene is the electrostatic match of work functions at the metal‐polyacetylene interface and that there are no intrinsic or extrinsic filled interface states within the polyacetylene band gap.

Effect of heterojunction spike on the quantum efficiency of an AlGaAs/GaAs heterojunction charge coupled device

The ’’spike’’ in the conduction resulting from the band‐gap discontinuity in a p‐ GaAs/n‐Al0.3Ga0.7As heterojunction is observed via the blocking of photoexcited electron flow from the p‐GaAs to the n‐Al0.3Ga0.7As. The spike height deduced from photocurrent versus bias results is 0.14 eV above the conduction‐band minimum of p‐GaAs. This value is 0.13 eV lower than that predicted by theory and suggests a positive interface charge density of 4×1011 cm−2. The lowered barrier does not appear to reduce the electron collection in the charge coupled device operation under low light level illumination.

(SN)x‐GaAs polymer‐semiconductor solar cells

We report the first solar cell whose junction is formed by a polymer‐semiconductor interface. Open‐circuit voltages, Voc≳0.7 V, have been observed on cells consisting of a thin film of polymeric sulfur‐nitride, (SN)x, deposited on GaAs. This is an enhancement of more than 40% over the Voc commonly measured with metal‐GaAs solar cells. Initial efforts have resulted in efficiencies ≳6% without antireflection coatings.

A backside‐illuminated imaging AlGaAs/GaAs charge‐coupled device

A glass‐supported, backside‐illuminated AlGaAs/GaAs heterojunction charge‐coupled device (CCD) imager is reported. The CCD structure was grown by liquid phase epitaxy on a GaAs substrate. The top epi‐layer was bonded to glass and the GaAs substrate completely removed. A ten‐pixel three‐phase Schottky gate CCD was fabricated on the glass‐supported layer. The CCD was successfully operated as a line imager with the photosignal entering through the support glass.

GaAs charge coupled devices for high speed signal processing applications

The use of GaAs Schottky gate charge coupled devices (CCDs) clocking at frequencies from 102MHz to 103MHz in signal processing applications is described. Included are discussions of the device physics which allows GHz clocking frequencies, the current state of the technology, and efforts in progress at Rockwell International to further develop the technology.

Low dark current glass bonded AlGaAs/GaAs Schottky gate imaging CCD

The fabrication and characterization of a glass bonded, backside illuminated, AlGaAs/GaAs heterojunction CCD imager is reported. The dark current of these devices is found to be <1 nA/(cm²which indicates that the glass bonding procedure does not adversely effect the dark current device performance. To isolate process induced degradation of the CCD Schottky gates, diodes were fabricated on n-Al<inf>x</inf>Ga<inf>1-x</inf>As (0 ≤ × ≤ 0.55) by e-beam evaporation of Cr/AuGe, Ti/Pt/Au and Cr/Au. The data indicate that φ<inf>Bn</inf>(Cr) ≤ φ<inf>Bn</inf>(Ti) for all solid compositions examined. Low temperature annealing causes a degradation of the Cr/AuGe and Ti/Pt/Au devices; however, the characteristics of the Cr/Au diodes improve significantly

Effect of proton bombardment isolation on the DC and transient characteristics of Schottky barriers on Al x Ga 1-x As

The dark current of Schottky barrier diodes on n-AlGaAs/p-GaAs/n+GaAs structures isolated with a proton bombardment guard ring is measured as a function of proton dose. It was found that the optimum proton dose is near 1014cm-2which resulted in a dark current of 130 pA/cm2. The DC leakage currents are much greater than the dark currents under charge integration conditions and can be explained as a hole current from the p-GaAs to the Schottky metal.

Solar Cell Characterization at Rockwell International

The Solar Cell Test Facilities at Rockwell International are described. The facilities enable the characterization of cells in the dark and under a variety of illumination conditions. A basic automated measurements system has been built around an Analog Devices MACSYM II minicomputer. Computerized dark I-V and C-V tests allow automatic determination of barrier height, diffusion potentials, doping profiles, diode n-factors, and extrapolated reverse saturation currents. The same system is used to measure the illuminated I-V characteristics and determine ISC, VOC , F.F., and locate the maximum power point. The facility also contains a computerize spectrophotometer which performs simultaneous measurements of either absolute spectral response and reflectivity or transmissivity and reflectivity. Concentrator solar cells can be characterized at up to 200X insolation using a chopped, concentrated Schoeffel solar simulator. A unique, high concentration test facility utilizing sunlight has also been built.

AIGaAs/GaAs Heterojunction Charge-Coupled Devices (CCDs) For Visible/Near Infrared Imaging Applications

The use of AlGaAs/GaAs Schottky gate, heterojunction CCDs for visible/NIR imagers offers a number of distinct advantages over the use of silicon MOS homojunction CCDs, several of which are of particular importance for space applications. The use of the heteroepitaxial technology possible with the GaAs/AlAs alloy system allows the design of devices each of whose layers is optimized to a specific functions. A CCD imager, for example, has a wide bandgap electrically inactive AlGaAs layer for optical access to the device, a narrow bandgap p-GaAs layer as an optical absorber for good response to the visible-NIR spectrum, end an intermediate bandgap n-AlGaAs CCD channel for charge transfer and low dark current. adapting glass bonding technology originally developed for III-V photocathodes, a backside illuminated CCD can be realized which eliminates optical obscuration from the CCD gates. Finally, the fundamental properties of the AlGaAs/GaAs system together with the use of Schottky barrier gates eliminates cross-talk and blooming and results in a device which is intrinsically radiation hard. During this presentation, the above considerations will be discussed in detail and the status of the development program at Rockwell International will be summarized. Important demonstrations including glass bonded, backside illuminated imaging and the observation of room temperature dark currents <100 pA/cm2 will be descirbed.

Observation of two-dimensional charge transfer in GaAs

The first observation of two-dimensional (x-y) charge tranfer in a GaAs charge coupled device (CCD) is reported. A 32×32 bit 4-phase CCD array with serial read-in and read-out linear multiplexer CCDs was fabricated using Schottky barrier gate technology. Two dimensional charge transfer was verified by optically generating a charge packet in the array and transferring it to the read-out multiplexer. This result demonstrates the feasibility of designing GaAs CCD arrays for signal processing, memory and imaging applications.