The work deals with studies of the grain size and surface state effect on photoelectric and transport properties of
PbTe(In) films in the temperature range from 4.2 K up to 200 K under irradiation of a blackbody source and terahertz
laser pulses. The PbTe(In) films were deposited on insulating substrates kept at the temperatures TS equal to -120 (see manuscript)
250C. AFM, SEM, Auger spectroscopy and X-ray diffraction were used to study the film microstructure. Increase of the
TS value led to mean grain size growth from 60 up to 300 nm. All films had a column-like structure with the columns
nearly perpendicular to the substrate plane. It is shown that microstructure of the films strongly affects the
photoconductivity character in the terahertz region of the spectrum. Positive persistent photoresponse is observed at low
temperatures in the polycrystalline films. For these films transport and photoelectric properties are determined by the
grain volume and impurity state specifics. Nanocrystalline films have all features of non-homogeneous systems with
band modulation. For these films only negative photoconductivity is observed in the whole temperature range. Possible
mechanisms of the photoresponse formation are discussed.
Photoelectric, transport and optical properties are studied for nanostructured PbTe(In) films. Synthesis of the films was
performed using evaporation of a target source to a glass substrate. The films have column-like structure with a mean
grain size varying from about 60 nm to 170 nm depending on the substrate temperature. Analysis of the data obtained
revealed that the conductivity of the films is determined by two mechanisms: charge transport along the inversion
channels at the grain surface and activation through barriers at the grain boundary. Persistent photoconductivity appears
in the films below T = 150 K. The frequency dependence of the relative photoresponse has a pronounced maximum. The
photoresponse in the ac mode may be by two orders of magnitude higher than in the dc measurements.
Lead salt materials are of high interest for midinfrared optical emitters and detectors for molecular spectroscopy. The IV-VI narrow gap semiconductors have a multivalley band structure with band extrema at the L point of the Brillioun zone. Due to the favorable mirrorlike band structure, the nonradiative Auger recombination is reduced by one or two orders of magnitude below that of narrow gap III-V and II-VI semiconductor compounds1. The photoluminescence in the midinfrared range for PbSe film structures, excited by a semiconductor laser diode, is investigated.
The PbSe films were prepared by Physical Vapor Deposition (PVD) using an electron gun. A PbSe crystal doped with 0.1 at% Bi was used as a source for the fabrication of thin layers. Starting from the assumption that the rate of nucleation is a predominate factor in determining grain size, thin films were fabricated on substrates that had been maintained at various temperatures of deposition process2. Amorphous glass and Kapton polyimide film was used as substrate. The growth rate was 0.2 nm/s. Films were thermally treated at high oxygen pressure in a heated encapsulated system. Microstructure has been studied using XRD, AFM and HRSEM.
For PbSe structures photoluminescence at temperature as high as 300 K is demonstrated.
Lead chalcogenide thin films have found their way in a variety of applications over the last three decades. IR-detectors
based on these materials, for instance, are commercially used in microelectronics, medicine and for military applications.
This work is concerned with the effect of the nanostructure on the photoelectric properties of n-type In-doped PbTe thin
films. Nanostructured thin films were prepared by varying the rate of nucleation as a function of the nature and
temperature of the substrates. The broken bonds at the grain boundaries generate acceptor states in n-type films, capture
electrons from the interior of the grains and give rise to p-type inversion layers between adjacent grains. A model, based
on the assumption the current is exclusively due to the motion of holes in the inversion channels along grain boundaries
is proposed to explain temperature dependences of photoelectric properties. It was demonstrated such structure is
optimal for maximizing their photoconductivity due the separation of electron-hole pairs on grain boundaries and
consequently increasing the carrier life time. This approach allows designing IR-detectors based on nanocrystalline PbTe
films with high sensitivity at wavelength up to 4-5 &mgr;m that do not require cryogenic cooling.
The work of a MEMS-technology compatible thin-film thermoelectric IR radiation sensor with multiple PbTe
thermocouples is simulated. The sensitivity up to 350 V/W is predicted. PbTe thin films are grown on the BaF2(111)
buffered Si(111) substrates, using the hot-wall-beam epitaxy technique. Indium and zinc ion implantation is applied to
transform the as-grown p-PbTe films into the n-type thermocouple counterpart films. With a dose of 10l6 cm-2 and
exposition time of 3-4 h, a 4-μm-thick film is homogeneously reversed, retaining the high epitaxial quality.
Solid-state broadband lasers emitting in the 2-4 μm region are of interest for a variety of applications. However,
conventional lasers for these wavelengths have limitations, such as the complexity and cryogenic operation. Recently,
alternative materials for the mid-IR lasers--Cr2+-doped II-VI chalcogenide crystals-have been proposed. The RT mid-
IR absorption in these crystals is due to the effective Cr2+ intracenter transitions. We present the results of an
investigation of the Cr2+ crystals. The crystals were grown from the CdSe source by seeded physical vapor
transport in the helium atmosphere with a wafer of CdSexS1-x as a seed. A CrSe source was used for the growth-time
doping. The changing sulfur content along the growth axis was determined by energy dispersive x-ray microanalysis. An
absorption peak in the IR range of 1.5-2.2 μm due to the Cr2+ intracenter transition 5T2->5E of ions was revealed. The
maximum peak absorption was determined to be 4.5 cm-1. The concentration of the 2+ ions in the samples calculated
from the IR absorption was found to vary fkom 1x1017 to 3x1018 cm-3. With the sulfur content, the intracenter absorption
peak shifts to shorter wavelengths.
Recently efficient room temperature lasing material for this IR range has been demonstrated in II-VI semiconductors doped with transition metals. These dopants incorporated into CdSe substitute metallic atoms and create deep levels in the band gap. The room temperature mid-infrared absorption in CdSe:Cr is observed due to the intracenter transtion 5T2→5E of Cr2+ ions. These tetrahedrally-coordinated ions are especially attractive as laser centers on account of high luminescence quantum yields for emission in the 2-3 μm range. Electrical and optical measurements are obtained with CdSe single crystals doped with chromium from a gas soruce CrSe over a wide temperature range (500-1050°C). These processes are intended to control the concentrations of the impurity and intrinsic defects. The low temperature annealing of CdSe crystals in CrSe atmosphere allows obtaining high electron mobility up to 9000 cm2/Vs at 80 K and dmeonstrates the low native defect concentration. A high temperature annealing gives a rise of electron concentration with decreased mobility. Optical absorption measurements show that at the high annealing temperature effective doping with Cr takes place. The impurity absorption beyond the absorption edge is interpreted by the excitation of Cr2+ and Cr1+ deep levels. Mid-infrared absorptin spectrum in 1.2-3.8 μm was measured. A discovered absorption peak at 1.9 μm corresponds to the intracenter transition 5T2→5E of Cr2+ ions. The intensity of this peak increases with increasing annealing temperature due to the growth of the impurity concentration. The maximal absorption coefficient in peak has a value of 4 cm-1 Cr2+ concentration in samples was calculated by using the peak absorption coefficient and it was varied from 1017 to 2×1018 cm-3.
AIVBVI (chalcogenides of IV-group elements) semiconductors are well-known materials having applications in infrared optoelectronics. Major efforts in the field of physics of the condensed matter are aimed at elucidating the influence of defects and impurities on the properties of semiconductor materials. In many cases, impurities can dramatically change the properties of semiconductor materials. Pb1-x-ySnxGeyTe is a well- known narrow-band-gap semiconductor with usually high (n>=1016 cm-3) intrinsic defects. The introduction of indium into these solid solutions establishes an impurity level that is a function of the Sn and Ge concentration and of temperature. For x>0.22, the In impurity level lies within the forbidden band (dielectric state) and exhibits the attendant sharp drop in the electron density. In the dielectric state (temperatures below 25 K) the In-doped alloys are unique in that they are photosensitive with a decrease in the electrical resistivity and an increase of carrier lifetime by several orders of magnitude (10-3-104 s) depending on the temperature. The long relaxation time of the non- equilibrium concentration of electrons, manifested as persistent photoconductivity (PPC) effect. One of the advantages of PPC effect is the possibility of increasing the signal/noise ratio by an increase of the charge integration time. Therefore one of the major challenges in developing array photodetectors operating in the television standard with a frame time ~ carrier lifetime is to insure signal accumulation in each element of the array. As it is seen we have unequal possibility to realize novel type of array photodetectors over a wide range of the IR spectrum (up to 30 micrometers ), in which each element provides radiation detection and its accumulation.
KEYWORDS: Signal detection, Carbon dioxide lasers, Absorption, Carbon monoxide, Gas lasers, Semiconductor lasers, Group IV semiconductors, N-type semiconductors, Semiconductors, Lead
Our study is concerned with the photo-thermovoltaic effects caused by the absorption of CO2 laser light in narrow gap AIVBVI semiconductors. We report on results of experimental study of photoresponse induced in n-type and p- type lead telluride with Ni contacts. We show that in the case of ohmic contacts (n-PbTe-Ni and p-PbTe-Ni at 300K) the detected signal originated from thermoemf due to created crystal lattice temperature gradient. In the case of p- PbTe-Ni at 80K we have Schottky contacts and the photoresponse consists at least of two components: fast and great in value photoemf- due to carrier generation resulting from two-photon absorption, and slow as well as of lower magnitude thermoemf.
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