Advantages of photoconductive detectors


URALSEMICONDUCTOR, RESEARCH CENTRE

The advantages of photoconductive detectors on the basis of thin films of super pure and stoichiometric lead sulfide.

    For the first time in the world (1985) stoichiometric films of lead sulfide with content of impurities of no more than 10-10 % (usually 10-3%) are received by us. Due to this our oxygen-free thin-film photoconductive detectors are differ on parameters from existing (sensitized in the presence of oxygen) ones and come nearer on reliability and stability of parameters to silicon monocrystal photodiodes and phototransistors. Films of lead sulfide of such quality were received during continuation of our researches of synthesis and properties of films in system CdS-PbS [1-5], where photoconducting films CdxPb1-xS of composition x≤0,16 also had stoicheometry and special cleanliness.
In addition to technical characteristics (see advertising descriptions of single-element and multielement photoconductive detectors ) we would like to pay attention to the following properties of our photoconductive detectors :
1. High reliability and stability of the complex of parameters of photoconductive detectors during long operation, change of parameters within 15 years does not exceed (-10 ~ +20) %. High stability of parameters after affect of  thermal impact.
2. Change of parameters at room temperature (+22 °С) after influence of low and high temperatures (thermal cycles in the range of from  -60°С to +60 °С) does not exceed ±2 % and ±6 % accordingly.
3. Change of parameters of photoconductive detectors depending on temperature is characterized by high repeatability at different thermal cycles. Temperature characteristics of the photoconductive detectors made of one substrate are practically identical. Temperature characteristics of photoconductive detectors made of different substrates but with identical values of the time constant and dark resistance are close also.
For multielement photoconductive detectors the disorder of parameters on elements decreases after thermal cycles, at measurements both at +22 °С, and in a range of temperatures   from -60°С to +60°С.
4. After strong illumination (2000 lux from the «A»-type source — a heating lamp at 2840°К) deterioration of sensitivity and increase of dark resistance does not exceed (3-8) % and is quickly restored. The time constant and sensitivity are restored on 0.98 level for no more than 20 minutes.
It allows realize the dynamic range on optical input over 120 decibels (more than one million times). Actually it means, that on a background of constant radiation of the wall heated up to straw-colored color inside the industrial power boiler it is possible to observe fluctuations of a torch of a flame with energy at million times smaller.
 5. Offered photoconductive detectors differ from known analogues by the following parameters:
 5.1 Maximum of spectral sensitivity is shifted to long-wave region and is near to the range (2,35-2,55) μm, and “read border of sensitivity”  - near to (3,1-3,2) μm (on a level of 10% from maximum) for a one-layer very thin (~0,5 μm instead of 1-2,5 μm) film photoresistor.
 5.2 Quick-acting, i.e. small time constant (50-75) µs (instead of known (200-500) µs);
5.3 High current sensitivity at maximum (2.5µm) of spectral characteristic, 4-7 A/W instead of usual 0.3 A/W;
5.4 Low dark resistance, (25-100) kΩ/ (instead of (300-1500) kΩ/ »);
5.5 Low dark resistance together with high current sensitivity allows take out our photoresistor  on a long (up to 10 m) cable (without preamplifier) to the point necessary for measurement;
6. On detectivity (1.8-3.0)×1010sm×W-1×Hz1/2 our photoconductive detectors occupy middle position in comparison with existing in the world, conceding to the best ones in 1,5-2,5 times.
But high detectivity of known best photoconductive detectors on the basis of lead sulfide is provided due to high values of a  time constant and dark resistance (see item 5.2 and 5.4  before).
An increasing of dark resistance leads, as a rule, to worsening of reliability, i.e. the most sensitive photoconductive detectors are less stable. In serial production it demands long cycles of testing with recurring measurements that sharply increases the cycle of their production and cost.
7. At thermoelectric cooling detectivity of our photoconductive detectors grows faster, than for known, and known  limiting characteristics for photoconductive detectors on the basis of lead sulfide at 196°К, are achieved for our photoconductive detectors already at temperature 225°К, and dark resistance of photoconductive detectors thus does not exceed 600 kΩ/, that essentially reduces electrical hindrances at amplifying a photosignal. Already in the case of two-cascade thermoelectric cooling our photoconductive detectors excels existing ones in all parameters.
 8. Our multielement photoconductive detectors (see the advertising description of the 64-element photoresistor) differs from existing ones by improved uniformity of parameters on elements, so, the disorder of parameters for elements of the 64-element photoresistor is in   ± (3-8) % limits for all parameters of a photoresistor.
Our high-efficiency original technology of production of oxygen-free semi-conductor films of
lead sulfide, and manufacturing on their basis of thin-film photoconductive detectors makes accessible multielement photoconductive detectors of fine quality for all industries and under the acceptable prices. Multielement (64 elements) photoconductive detectors can be made almost the same as silicon microcircuits.
 9. Our technological processes of production of semi-conductor lead sulfide and manufacturing of photoconductive detectors do not require using of expensive in purchase and operation of super pure shops and the vacuum equipment.
10. All above described advantages of our photoconductive detectors allow to develop and serially produce for the industry both known gauges and devices with the improved characteristics, and new ones. Earlier these gauges and devices could not be developed for the purposes of a batch production in view of low reliability and low stability of parameters of photoconductive detectors on the basis of lead sulfide, and also high cost of multielement photoconductive detectors .
In the present prospectus for comparison the data accessible to us under scientific articles and advertising descriptions of photoconductive detectors on the basis of lead sulfide of the following firms were used: Mullard (Great Britain); SBRC and Oriel (USA); Hamamatsu (Japan), and also the books on optoelectronics of  T. Moss [6].

Concrete applications.

    On the basis of our PbS photoconductive detectors for the research and industrial purposes were created and are used the following photodetecting devices:
1. The fire gauge of an open flame of a range (1.0-3.3) µm is noise proof to natural and artificial illumination, has no analogues in the world on a complex of parameters.
2. Fire gauges for detection of a smoke, industrial and room, with very high noise stability to optical and electromagnetic disturbances.
3. Active gauges of crossings of invisible IR-beam for the security signal system and for the industry with very high noise stability to optical disturbances and a low power consumption at voltage of 12V and 24 V.
4. Gauges of the control of a continuity and quality of burning of gas torches of industrial boilers, large power and technological installations (for example, for heating reactors in the chemical industry).
5. Two-channel gauges for the control of pollution of water for systems of removal of slime during water-preparation on thermal power stations.
6. Optical ten-channel devices for the control of process of packing of cigarettes in packs for tobacco factories.
7. Gauges for detection and devices for measurement of length of metal hire on speeds up to 10 m/s with accuracy of 0.05 m at length up to 12 m with use of own radiation of hire heated up to (300-800) °С.
8. Capacity measuring instruments of pulse ultra-violet lasers; multielement photodetectors for measurement of distribution of energy on section of a beam of the laser, tuning of positions of a beam.
9. Photodetecting devices for the devices measuring a concentration of sugar in liquids, and measuring humidity on light reflected from a surface of objects.
10. Photodetecting devices and multielement photoconductive detectors for the multispectral parallel optical analysis in geology, nonferrous metallurgy, for space engineering, and other areas for scientific researches and the control of industrial processes. One and two-element, four-element (including square ones), 16-element, 32-element, 64-element (the advertising description for the 64-element photoresistor is applied) photodetecting devices were used.

Super pure semi-conductor films of lead sulfide,
specification of their fundamental parameters and peculiarity of manufacturing of photo-conducting material for industrial purposes. The method of production of super pure semi-conductor films and it’s opportunities.

    For the first time in the world (1985-1993) the industrial technology of production of thin semi-conductor films of super pure and stoichiometric lead sulfide, in which the content of electrically active impurities does not exceed 10-10%, by the method of chemical deposition from aqueous solutions was created by us.
The presence of own (not impuritical) conductivity of carriers is measured and shown in a range of temperatures  (130-350)°К at thermal width of the known forbidden zone of 0.34 eV. Correct measurements of conductivity lower than 130°К are impossible owing to transformation at these temperatures of our semiconductor into isolator, namely, at cooling from 300°К up to 140°К conductivity of our lead sulfide decreases (is frozen) in more than 10000 (ten thousands) times.
According to the shift of long-wave edge of photoconductivity the temperature factor of change of width of the forbidden zone is measured. For a temperature interval (330-195) °К the value 1.6×10-4 eV/K instead of known (4-5)×10-4 eV/K is obtained. In view of the optical width of the forbidden zone measured by us at 0.39 eV at 300°К, it allows to calculate (to extrapolate) the value of the width of the forbidden zone for super pure and stoichiometric  lead sulfide at 0° К, equal to 0.34 eV (instead of known 0,286 eV).
Thus, two basic fundamental parameters for widely known industrial semi-conductor material (what lead sulfide is), namely, the temperature factor of change of the width of the forbidden zone and the width of the forbidden zone at 0°К, do not coincide with known ones. It is caused by that all previous measurements of semi-conductor parameters for lead sulfide  were carried out on the material, in best samples of which the content of impurities and a deviation from stoichiometricity was about 10-3%, about what the researchers (T.Moss, D.Bode, E.Patli) always specified [6,7].
This discrepancy of fundamental parameters, alongside with direct measurement of conductivity, is one more proof, that we really managed to improve cleanliness of known semi-conductor material in TEN THOUSAND TIMES.
The obtained thin film (0.3-1.2) µm of lead sulfide with own conductivity at  Т > 130°К direct for manufacturing receivers of radiation, namely photoconductive detectors , is unsuitable because of low photo-sensitivity.
For manufacturing of films with the improved photoconductivity the base film of lead sulfide is alloyed directly during its manufacturing, but thus widely known oxygen for this purpose is not used. Alloying carries out a problem of neutralization of the centres of re-combination, the not charged defects (dispositions) which are very easily formed in very fragile super pure material (microcrystals of a film). Alloying, figuratively and shortly, plays a role of iron armature which is entered in fragile concrete for reception of ferro-concrete with fine operational properties.
At alloying the time constant of our lead sulfide is increased from (10-15) µs to (60-70) µs at 300°К, photoconductivity is increased up to 10 times, value of temperature factor of the change of the width of the forbidden zone is increased something (to ~2,5×10-4 eV/K) also. In all range of temperatures (140-330) °К for our photoconducting film of lead sulfide its own time of re-combination under incident radiation is realized. The time of own recombination of carriers of ideal PbS under incident radiation at 295°K according to theoretical calculations must be about 63 μs, just that was obtained. And, the most important, the semi-conductor material loses the fragility. Its improved photoconducting properties are not worsened at repeated thermal cycles in a range of (195-330) °К and long (not less than 3 months) work at the increased temperature +60°С at day and normal (+22 °С) at night. The first samples of ours PbS films keep the unique photoconductivity and parameters already over 20 years.
So, the technology of our super pure photoconducting lead sulfide can be used for business lot (hundred thousand photoconductive detectors per one year) manufactures with the purpose of satisfaction of requirement of the world market in reliable and cheap photodetectors of a range (0.3-3.2) µm.
Except for lead sulfide we have developed earlier [4,5] a lot of semi-conductor films of threefold super structural compounds in system of cadmium sulfide — lead sulfide with widths of the forbidden zone of 0.46 eV, 0.53 eV, 0.66 eV, 0.73 eV. Superstructural threefold compounds are completely ordered phase of a solid solution, for example Cd1Pb15S16-CdxPb1-xS (x=0,0625) with the width of the forbidden zone 0,53 eV. At low (≤100°C) temperature continuous solid solutions chemically deposited from water solutions can not be obtained.
Films of the received threefold compounds also are super pure and stoichiometric, and can be used both for manufacturing of photoconductive detectors , and optical filters. Parameters of photoconductive detectors are intermediate between lead sulfide and cadmium, except for a time constant which for all compounds does not exceed 200 µs. These photoconductive detectors were used in space engineering and for manufacturing of high-speed multichannel optrons (15 µs) with the open optical channel.
During development of the whole family of super pure semi-conductor films in threefold system of lead and cadmium sulfides, we create an original scientific-experimental method of material science of super pure semiconductors. This method can be used, according to preliminary experimental data, both for production of a threefold material of lead selenide-type and cadmium selenide-type, and semi-conductor materials of fourfold compounds of similar composition and structure.
In our opinion, the opportunity of use of the created method in material science of magnetic and ceramic materials with necessary properties for application is not excluded.

References:

1. Mukhamedyarov R.D., Stuk V.I., Blinov O.J., Zhukov V.N., Kitaev G.A. «Installation for measurement of threshold parameters of photodetectors» .Devices and engineering of experiment, 1976, №6, p. 234.
2. V.I. Stuk, R.D.Mukhamedyarov, V.N. Zhukov, O.U. Blinov «The Modulator of a radiant flow with the synchronous engine», The optic-mechanical industry, 1977, №3.
3. R.D.Mukhamedyarov, V.I. Stuk, G.P.Fadina, G.A.Kitaev «Optoelectronic devices on the basis of high-speed thin-film photoconductive detectors CdxPb1-xS». Electronic engineering. Ser. 10. Release 3 ( 15 ). Microelectronic devices. 1979
4. R.D. Mukhamedyarov, G.A.Kitaev. «Parameters of semi-conductor superstructural compounds Cd1Pb15S16 and Cd5Pb27S32 ». Letters to GTP, volume 6, iss. 21., 1980, «Science», Leningrad branch.
5. Mukhamedyarov R.D., Kitaev G.A., Markova V.M., Stuk V.I. «The research of the kinetic of growth of semi-conductor films CdхPb1-хS at chemical deposition from aqueous solutions». Inorganic materials, volume 17, №10, Moscow, News of the Academy of sciences. 1981.
6. T.Moss, G.Barell, B.Ellis. “Semi-conductor optoelectronics”, tr. from engl, «World» (1976), 430 p.
7. U.I.Ravich, B.A.Efimova, I.A.Smirnov «Methods of research of semiconductors in application to lead chalkogenides PbTe, PbSe and PbS», iss.»Science», Moscow, 1968.