DRIFT MOBILITIES OF ELECTRONS AND HOLES AND SPACE-CHARGE-LIMITED CURRENTS IN AMORPHOUS SELENIUM FILMS. Technical Note No. 5

Technical Report · Fri Sep 01 00:00:00 EDT 1961

OSTI ID:4816123

Hartke, J L

The results of drift mobility and space-charge-limited current measurements made on 20 micron thick films of amorphous selenium were interpreted using the band model of a semiconducting solid with impurity or imperfection levels in the forbidden energy region between the conduction and valence bands. Drift mobilities were measured in the 220 to 300 deg K temperature range by determining the time of transit across the films of carriers which were photoinjected at one surface by 10/sup -8/ second light pulses. The drift mobilities at 300 deg K were 0.165 cm/sup 2//volt sec for holes and 7.8 x 10/sup - 3/ cm/sup 2/volt sec for electrons. The temperature dependence of hole mobility was exp(-- 0.14 ev/kT), and that of electrons was exp(--0.285 ev/kT). The results, which agreed well with those of Spear, indicated that approximately 10/ sup 19/ electron traps were present in a 0.285 ev region just below the conduction band edge, and that about 10/sup 21/ cm/sup -3/ hole traps occupied a 0.14 evaportion of the gap immediately above the valence band edge. It was postulated that the shallow traps were due to imperfections. Electron drift mobilities were reduced by a factor of three when 0.5 mole% arsenic was added to the films, and were about an order of magnitude smaller than those measured in pure films when 2 mole% of arsenic was added. Their temperature dependence was unchanged, suggesting that the arsenic increased the concentration of imperfections which produce shallow electron traps. Pulse heights of the drifting carriers were used to determine the carrier ranges. Hole ranges were 2- 4 x 10/sup -8/ cm/sup 2/volt, and exhibited no observable temperature dependence, suggesting that the number of hole capture centers was relatively temperature independent. Electron ranges were 1-2 x 10/sup -7/ cm/sup 2/volt at 300 deg k and decreased with decreasing temperature, suggesting that the number of electron capture centers was temperature dependent. Gold and telluriuT electrodes were used to inject holes into amorphous selenium, producing space-chargelimited currents at high fields. The observed currents indicated hole capture centers distributed uniformly in the gap for electron energies less than 1 ev above the valence band edge. The densities of these levels were 10/sup 15/ cm/sup -3/ ev/ sup -1/, and the width in energy of the distribution was 0.15 to 1 ev. Electrical conductivities of 10/sup -14/ to 10/sup -16/ ohm/sup -1/ cm/sup were obtained at low fields, and suggested that the material was nearly intrinsic. The levels in the gap would thus have to be neutral to avoid strongly extrinsic conductivity. The lowest value of conductivity agreed well with values extrapolated from conductivities of liquid selenium. Drift of photoinjected carriers in the presence of space-chargelimited currents was used to probe the electric field. The general spatial dependence was that expected of spacecharge- limiting by holes, but could not be explained quantitatively, possibly due to the nature of the techniques used. (auth)

• Research Organization: Illinois. Univ., Urbana. Electrical Engineering Research Lab.
• DOE Contract Number: AF 49(638)-417
• NSA Number: NSA-16-021018
• OSTI ID: 4816123
• Report Number(s): APOSR-1416; AD-267838
• Resource Relation: Other Information: Orig. Receipt Date: 31-DEC-62
• Country of Publication: United States
• Language: English

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METALS, CERAMICS, AND OTHER MATERIALS
ADDITIVES
ARSENIC
ATOMIC MODELS
CAPTURE
CARRIERS
CURRENTS
DEFECTS
DIFFUSION
DISTRIBUTION
ELECTRIC CHARGES
ELECTRIC CONDUCTIVITY
ELECTRIC FIELDS
ELECTRIC POTENTIAL
ELECTRODES
ELECTRONS
ENERGY LEVELS
FILMS
FREQUENCY
GOLD
IMPURITIES
LIQUID METALS
LOW TEMPERATURE
MEASURED VALUES
MOBILITY
MOTION
PULSE ANALYZERS
SELENIUM
SEMICONDUCTORS
SOLIDS
SURFACES
TELLURIUM
TEMPERATURE
THERMAL DIFFUSION
THICKNESS
VALENCE
VELOCITY