Lead selenide layers were obtained by thermal evaporation in vacuum on thermally oxidized silicon. As-deposited layers present no response to infrared radiation and an activation procedure is mandatory. The sensitization process described in this work consists of a thermal treatment developed in two different stages: The first step in an iodine and oxygen rich atmosphere at 220 °C, and the second one at a higher temperature (450 °C) on air. After this treatment, the developed photodetectors show high sensitivity to medium wavelength infrared radiation at room temperature. We have analyzed the structural, compositional, electrical, and morphological changes of the films occurred during the activation process. After processing many films, we conclude that iodine plays a key role in the PbSe sensitization. This halogen behaves as a transport agent during the PbSe recrystallization process, and promotes a fast growth of PbSe microcrystals. Oxygen is trapped into the PbSe lattice during the recrystallization p...

Role of halogens in the mechanism of sensitization of uncooled PbSe infrared photodetectors

We report low-hysteresis, ambipolar bottom gold contact, colloidal PbSe nanowire (NW) field-effect transistors (FETs) by chemically modifying the silicon dioxide (SiO2) gate dielectric surface to overcome carrier trapping at the NW-gate dielectric interface While water bound to silanol groups at the SiO2 surface are believed to give rise to hysteresis in FETs of a wide range of nanoscale materials, we show that dehydration and silanization are insufficient in reducing PbSe NW FET hysteresis Encapsulating PbSe NW FETs in cured poly(methyl) methacrylate (PMMA), dehydrates and uniquely passivates the SiO2 surface, to form low-hysteresis FETs Annealing predominantly p-type ambipolar PbSe NW FETs switches the FET behavior to predominantly n-type ambipolar, both with and without PMMA passivation Heating the PbSe NW devices desorbs surface bound oxygen, even present in the atmosphere of an inert glovebox Upon cooling, overtime oxygen readsorption switches the FET polarity to predominantly p-type ambipolar b

Flexible, low-voltage, and low-hysteresis PbSe nanowire field-effect transistors.

PbSe photodetector arrays for IR sensors

Copper doped zinc telluride (ZnTe:Cu) thin films have been synthesized by an electrodeposition technique from acidic aqueous bath containing ZnSO 4 , TeO 2 and CuSO 4 . The reaction mechanism has been studied by cyclic voltammetry to identify the deposition potential of ZnTe and ZnTe:Cu. X-ray diffraction as well as SEM techniques have been employed to investigate the structure and surface morphology of as-deposited and doped films. Optical properties, such as transmission, refractive index and band gap have been analyzed. The drastic change in resistivity has been observed due to incorporation of Cu dopent and the results are discussed in detail.

Synthesis and characterization of copper doped zinc telluride thin films

Using colloidal CdSe nanowire (NW) field-effect transistors (FETs), we demonstrated the dependence of carrier transport on surface stoichiometry by chemically manipulating the atomic composition of the NW surface. A mild, room-temperature, wet-chemical process was devised to introduce cadmium, selenium, or sulfur adatoms at the surface of the NWs in completed devices. Changes in surface composition were tested for by energy dispersive spectroscopy and inductively coupled plasma-atomic emission spectroscopy and through the use of the vibrational reporter thiocyanate. We found that treatment with cadmium acetate enhances electron currents, while treatment with sodium selenide or sodium sulfide suppressed them. The efficacy of doping CdSe NWs through subsequent thermal diffusion of indium was highly dependent on the surface composition. While selenium-enriched CdSe NW FETs were characterized by little to no electron currents, when combined with indium, they yielded semimetallic devices. Sulfur-enriched, indi...

Solution-based stoichiometric control over charge transport in nanocrystalline CdSe devices.

Thin films of PbTe of different thicknesses have been prepared on glass substrates at room temperature by vacuum deposition. It is found that the electrical resistivity of the air-exposed films is much higher (by about 2 to 3 orders of magnitude) than that of the as-grown (unexposed) thin films. The electrical resistivity temperature behaviours of both the air-exposed and as-grown (unexposed) thin films of PbTe are different but both show hysteresis behaviour during successive heating-cooling cycles. These observations can be explained by considering that the desorption of absorbed gas molecules (mainly oxygen) and creation of defects at higher temperatures during heating influence the electrical conduction. Further, the time factor involved in gas desorption-adsorption can cause the observed hysteresis in temperature-dependent conduction behaviour. The as-grown (unexposed) thin-film conductivity exhibits the expected reciprocal thickness dependence due to the thickness effect, but the air-exposed film conductivity does not. This can be explained to be due to the complete masking of the thickness effect by the gas adsorption effect in air-exposed film conductivity. The reciprocal thickness dependence observed in the case of unexposed film conductivity has been explained by the 'effective mean free path' model. The low value of the 'grain boundary' mean free path obtained by the analysis points to the fact that in polycrystalline films, grain boundary scattering is extensive and controls the film conductivity.

http://iopscience.iop.org/article/10.1088/0022-3727/22/1/023/pdf

Electrical conductivity of air-exposed and unexposed lead telluride thin films-temperature and size effects

Lead telluride and lead selenide films were prepared by the thermal evaporation technique on clean glass substrates held at room temperature in a vacuum of 3\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}5}$ Torr. The resistance of the films was measured at different air pressures. The resistance of the as-grown films was found to be of the order of a kilohm but increased with increasing pressure. A resistance maximum was observed at a particular air pressure after which the resistance decreased with increasing pressure. An indirect method, namely, the measurement of the conductivity variation as a function of air pressure, has been used to estimate the extent of air adsorption and the adsorption isotherms have been drawn on this basis. The dependence of adsorption on thickness is also discussed.

Electrical-conductivity changes in PbTe and PbSe films on exposure to the atmosphere.

PbTe thin films of thicknesses ranging from 400 to 4000 A have been prepared by vacuum evaporation at a pressure of 5×10−5 Torr on clean glass substrates held at room temperature. The thermoelectromotive force of these films has been measured in the temperature range 300–600 K. It is found that thermoelectric power, SF varies anomalously with temperature, being constant at lower temperatures, and rapidly decreasing at higher temperatures. SF is found to be positive indicating that the samples are p type. The anomalous behavior is explained by assuming that at higher temperatures additional donor levels are generated due to creation and ionization of defects in the system.

Anomalous temperature dependence of thermoelectric power of PbTe thin films

PbSe thin films of thicknesses in the range 20 to about 170 nm have been prepared on glass substrates held at room temperature by thermal evaporation of the bulk alloy at a pressure of 5×10−5 torr. The thermoelectric power of these films has been evaluated as a function of temperature in the range 300 to 500 K from the thermal e.m.f. data. It was found that the thermoelectric power of all the films initially increases with increasing temperature, then reaches a maximum, and, with a further increase in temperature, decreases rapidly and also changes sign. The possible reasons for this peculiar behaviour have been given. It is also found that there is no systematic variation of thermoelectric power with thickness of the films. This is probably due to completely specular scattering at the external and internal surfaces of PbSe thin films or to changes in stoichiometry of different films.

Temperature variation of thermoelectric power of vacuum deposited PbSe thin films and its thickness dependence

A two‐layer model to explain the thickness dependence of conductivity and thermoelectric power of semiconducting thin films has been developed assuming that the film is a parallel combination of resistances of the three layers: The first is the interior ‘‘grain‐boundary’’ layer, and the other two, outer layers on opposite sides, whose conductivities are altered by the band bending (and is also affected by surface‐gas interactions). The equations obtained in this model lead to an inverse thickness dependence of both the conductivity and thermoelectric power of thin films. The model is applied to analyze the conductivity and thermoelectric variation with thickness of PbTe thin films and the parameters Ug, the energy dependence of the ‘‘grain‐boundary’’ mean free path lg, and σg the surface conductivity, have been evaluated.

A two-layer model to explain the thickness dependence of conductivity and thermoelectric power of semiconductor thin films and application of the model to PbTe thin films

K. Seetharama Bhat

Papers

Electrical conductivity of air-exposed and unexposed lead telluride thin films-temperature and size effects

Electrical-conductivity changes in PbTe and PbSe films on exposure to the atmosphere.

Anomalous temperature dependence of thermoelectric power of PbTe thin films

Temperature variation of thermoelectric power of vacuum deposited PbSe thin films and its thickness dependence

A two-layer model to explain the thickness dependence of conductivity and thermoelectric power of semiconductor thin films and application of the model to PbTe thin films