P. Würfel

The photovoltaic effect and the charge transport in LiNbO3

Abstract The charge transport giving rise to the photovoltaic effect is compared with the photoconductivity in the photon energy range from 2.5 eV to 5 eV. The photoconductivity at 2.5 eV is activated with about the same activation energy as the Hall mobility, which was measured for reduced samples between room temperature, where it is 1.1×10−3 cm2/Vs, and 200°C. The dependence on temperature, intensity, and time demonstrates different charge transport mechanisms for the short-circuit photocurrent and for the photoconductivity.

The origin of the photo-emf in ferroelectric and non-ferroelectric materials

A photo-emf arises from an optical excitation that is either asymmetric in k-space or occurs at asymmetries in real space, as is shown by a discussion of the Boltzmann transport equation. Excitation asymmetric in k-space leads to the photogalvanic effect. A photo-current is impressed with unpolarized light in the bulk of ferro- and pyroelectric crystals, and with polarized light even in non-pyroelectric piezoelectric crystals. Photon drag and optical rectification are outlined in connection with the photogalvanic effects. Excitation at asymmetries in real space leads to a barrier photo-emf, if both electrons and holes are mobile and if the electric and chemical potentials are not spatially constant in the unilluminated semiconductor. The Dember effect as being based on one mobile charged species only, does not represent a photo-emf. Anomalous photovoltages of a magnitude larger than the band gap, in particular in polytype zinc sulfide, that have been accounted for by a battery of barrier photo cells, are ...

Ferroelectric properties of thin NaNO2–layers

Abstract Thin layers of NaNO2 with a thickness of a few μm were grown from the melt on several substrate materials. They have single-crystalline areas of the order of cm2. They exhibit essentially the same dielectric and ferroelectric properties as bulk NaNO2-crystals. The spontaneous polarization is directed preferably perpendicular to the surface of the layer.

NaNO2 layers as pyroelectric radiation detectors

Abstract Thin NaNO2 layers on a substrate, when provided with contacts and a radiation absorbing layer, form a pyroelectric radiation detector of small heat capacity. The pyroelectric figure of merit for NaNO2 is p/ecp = 5,6 · 10−12 m/V. The frequency response at low frequencies is typical for that of a two-layer structure. At high frequencies, when the temperature wave penetrates only into the pyroelectric layers, the detector behaves as if there were no substrate present. While the frequency dependence of the penetration depth of the temperature wave causes the voltage responsivity to decrease less than proportional to ω−1, the current responsivity is observed for the same reason to increase with increasing frequency.

Charge compensation in vacuum-cleaved ferroelectric triglycine sulfate (TGS)

Cleaving a TGS crystal in vacuum perpendicular to its ferroelectric axis is supposed to create new surfaces where the discontinuity of the spontaneous polarization is uncompensated. The field effect conductance in thin Te films on TGS surfaces, however, gives no indication for a lack of compensation charge at the cleavage face. It is concluded that electrons must be tunneling between both cleavage faces during their separation.

NaNO2 thin films for pyroelectric detector arrays

Abstract Integrated NaNO2 pyroelectric radiation detectors have been made by growing NaNO2-films from the melt directly on silicon. The pyroelectric signal is read out by an integrated circuit in the Si-substrate. The detectors, single element or arrays, can be coupled to the electronics in two ways, either by the ferroelectric field effect at the NaNO2-Si contact, or by separate MOS-FETs. The performance of the electrical coupling by the ferroelectric field effect is strongly limited by the trapping of charge in interface states. The performance of signal transfer to a MOS-FET is more efficient, but suffers from a large capacitive load, if the gate of the MOS-FET is connected to the lower detector electrode formed by metallization on the silicon chip. Impedance matching therefore requires a small thickness of the NaNO2-film. A small thickness, on the other hand, favors the penetration of the temperature wave into the substrate with the accompanying heat loss. Electrical and thermal optimization are thus ...

Ferroelectric field effect of a thin NaNO2-layer on a Si-substrate

Ferroelectric single crystalline NaNO2-layers are grown from the melt onto Si-FET substrates. The pyroelectric signal appearing across the NaNO2-Layer upon a temperature change of the NaNO2 is observed to induce a change in conductance of the source-drain channel in the adjacent Si-FET. The deposition of the NaNO2-layer on the Si-FET constitutes the integration of the pyroelectric NaNO2 radiation sensor with an impedance transformer. This appears as an important step towards the realization of a pyroelectric sensor array integrated into circuits on a Si-substrate for processing the sensor signals.

Investigation of domains in ferroelectric NaNO2 by a laser scanning microscope

Abstract Single-crystalline thin films of ferroelectric NaNO2 have been investigated by a computer-controlled laser scanning microscope, capable of inspecting a 1 × 1 mm2 area with 2 μm resolution, thereby revealing the domain structure and its variations due to an applied electric field. The orientation of the domain walls changes from {100} in untreated samples to {100} and_{101} after application of a voltage. This difference is explained by the small velocity of the {100}- and {101}-walls during polarization reversal.

Ferroelectric field effect in te films on TGS substrates

The conductance of Te films on TGS cleavage faces is strongly affected by the spontaneous polarization of the TGS. At 100 K switching the polarization causes the conductance to change by 11 orders of magnitude. Hall effect measurements show that this ferroelectric field effect changes both the concentration of holes in the Te and their mobility.