Joseph Dresner

Diffusion length of holes in a‐Si:H by the surface photovoltage method

The diffusion length L for holes in undoped a‐Si:H films has been measured by using a variation of the surface photovoltage method. Values of L in the range 0.33–0.45 μ were found for samples prepared at substrate temperatures Ts = 240 °C and Ts = 330 °C. After prolonged illumination, a reduction to L<0.2 μ was observed; the original value of L was restored after annealing at 200 °C.

Growth of MgO films with high secondary electron emission on Al-Mg alloys

We have identified the processes which take place during the formation of MgO layers on the surface of Al-1% Mg alloys by measuring surface composition, electron multiplication, and Mg evaporation. An initial layer of Al2O3 on the alloy plays a vital role in this process: it acts as a sink for Mg, and prevents the inward diffusion of O2 during the heating, thereby localizing the MgO formation on the outer surface. The Mg concentration in the Al2O3 layer increases about 20-fold over its original value, and the Al2O3 is partially reduced. For a given temperature and pressure of oxygen, the MgO growth rate depends on the thickness and crystallinity of the Al2O3. Electron multiplication factors 5 of 10 to 15 are obtained; part of the secondary electrons originate in the underlying Al2O3 layer.

Preparation and properties of n-type ZnTe

Abstract ZnTe/Al crystals were prepared under high argon pressure by cooling a Zn-rich melt from 1280° to 1100°C and abrupt quenching to room temperature. These crystals are n-type with resistances of 105 to 107 ohm cm and electron mobilities from 70 to 350 cm2/Vsec.

Hall mobility for electrons in undoped a‐Si:H

The Hall mobility μH of electrons in undoped a‐Si:H has been measured between 200 and 550 K under illumination and in the dark. There are two distinct regimes: below Tt =360 K is independent of temperature, with a value μH ∼0.1 cm2 V−1 sec−1; above Tt, μH = 7.9 exp (−0.13 eV/kT) cm2 V−1 sec−1. The data can be fitted by a model where there is one set of conducting states for electrons and two possible modes of transport. Above Tt conduction is by thermally activated hopping; below Tt it takes place either by tunneling or by a temperature‐independent jumping process. Conduction does not take place through extended states in the temperature range 200 – 550 K.

The diffusion of holes in undoped amorphous Si :H

Abstract The diffusion length of holes in undoped a-Si : H has been measured by a modified surface-photovoltage method. The a-Si : H was deposited from a d.c. glow discharge in silane. Both field-free and field-assisted hole transport were observed, and could be distinguished by varying the width of the surface depletion layer using different illumination levels. Field-free (thermal) diffusion lengths vary from about 0·3 μm to about 1·1 μm, being a function of substrate temperature during film growth and of the details of the deposition procedure. The hole diffusion length increases with temperature in the range from -30°C to 60°C in the form of a Boltzmann factor with a characteristic energy of 0·22 eV for a film grown at 330°C and of 0·28 eV for a film grown at 250°C. Our results suggest that analyses of a-Si : H solar cells ought not to ignore field-free hole transport.

Amorphous silicon p‐i‐n‐i‐p and n‐i‐p‐i‐n diodes

This letter describes the preparation and electrical characteristics of a‐Si:H p‐i‐n‐i‐p and n‐i‐p‐i‐n thin‐film diodes suitable for driving monochrome liquid crystal displays with more than 500 lines. The symmetrical current‐voltage curves in the reverse breakdown regime can be described by i=i0 exp(E/E0), where E0≂9×104 V/cm. In the range 20–125 °C, the current is thermally activated with an energy of 0.25 eV. The response time to applied voltage pulses is ≤10 μs. The stability of the electrical characteristics is adequate for at least 104 h of operation in a liquid crystal display. Electrical characteristics indicate that the reverse breakdown current is a tunneling current injected into the i layer and that electrons are likely to be dominant.

Photoelectronic properties of CdTe‐electrolyte heterojunctions: Feasibility as solar energy converters

We show that cadmium telluride is a promising material for semiconductor‐electrolyte solar cells. Polycrystalline n‐type CdTe, in contact with an aqueous solution of SnCl2, forms a well‐defined Schottky barrier. The efficiency in sunlight is 4.8%. The cell shows a drift of characteristics with time that may be associated with a low rate of formation of hydroxide on the surface. However, operating stability is relatively good compared with other semiconductor‐electrolyte cells that have been reported in the past.

Feedback multiplier flat-panel television: III&#8212;Ion bombardment, dynode life, and ambient pressure

The gas-ion-feedback electron multiplier has been recognized for some time as capable of yielding a large-area electron source whose output can be spatially controlled on the scale of a picture element. Such a high-gain multiplier array can be conveniently formed using foils of oxidized Al-Mg alloy bonded to glass plates. During operation of the display the MgO dynodes of the electron multiplier are subject to intense ion and electron bombardment. This affects the device through three kinds of interaction. The ultimate life of a dynode is limited by sputtering. However, for the ion doses encountered during 104h of operation, the changes in secondary emission are not related primarily to sputtering damage but rather to the physisorption of the gas atoms on the MgO surface. This entrapment of gas also causes changes in the ambient pressure which must be compensated for. Finally, electron bombardment by itself can cause dissociation of MgO unless the MgO film has been stabilized by making the surface stoichiometric. Stabilized MgO dynodes can withstand a primary electron dose of 5000 C/cm2without damage and a dose of A+or He+ions of about 2 × 1017ions/cm2for an allowable decrease in secondary emission of 30 percent. An analysis of multiplier operation shows that a multiplier life of at least 104h can be expected in argon and helium; helium is the preferred choice because of the higher operating pressure and smaller sputtering damage to the electrodes. The sticking coefficient of 500-eV He ions on clean MgO at 23°C was measured to be 0.4 and it was found that at least six monolayers of He could be pumped into the dynodes. The characteristic time for gas release from the walls τris not constant but depends on the ion energy and on the degree of inward diffusion during the experiment. At 23°C,\tau_{r} > 100h initially for 500-eV ions, and only 25 percent of the gas is recovered after three days. At 90° C,\tau_{r} \approx 1h. These data were used to compute the pressure variations during various operating cycles. It is concluded that a gas supply is needed which is capable of delivering a total quantity of 20 1. torr of He on demand over hundreds of hours at 10-3torr, and which can reabsorb a comparable quantity of gas in about a minute. A pumping speed as small as 0.2 1/s would be sufficient to accomplish this.