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Hall mobility of electrons in CdS and CdSe layers
Vol. 3, No. 1
ABSTRACTS OF PAPERS IN SOVIET PHYSICS-SOLID STATE
A. G.Stasenko (Vol. 6, No. 11, pp. 3361-3363). The absorption spectrum of CdS was measured in the vacuum U. V. A vacuum spectrometer was used which was calibrated with a “sliding vacuum spark’! For the absorption measurement, the source of radiation was a capillary discharge tube with flowing hydrogen. The specimen was in a vacuum tight compartment and the whole system was pumped down to low4 mm Hg. The spectrum was photographed on an X-ray plate sensitized by an alcohol solution of sodium salicilate. The CdS specimen consisted of thin films evaporated onto a singlecrystal plate of LiF. The absorption of these CdS films was also measured at longer wavelengths in a spectrophotometer. The absorption spectrum shows that the absorption of CdS increases above about 8.3 eV. It is proposed that this increase is due to the transition of 3s electrons into the conduction band of CdS. Absorption bands which peak at ‘7.95 and 9.95 eV are also observed. The absorption peak at 7.95 eV is attributed to transitions A6 - A& El between levels due to Cd atoms in the CdS lattice. The observed shift of the atomic energy levels closer to each other in the crystal field is characterized by a coefficient of “compression” which is equal to the ratio of the transition energy in the free atom Efa to the energy of the absorption peak Em, due to the transition when the atom is in the lattice. Efa P=E,,=
yw=
1.02
The 9.95 eV peak is believed due to transitions from El/2 to two levels Eli2 and an E3 level corresponding to the formation of a 4% d ion in the CdS lattice. The “compression” coefficient for such transitions is equal to p=-
Ef E
11.78 eV ion === max
1.18
It is proposed that the simultaneous occurrence of absorption bands attributable to Cd atoms and to Cd+ ions is due to a mixed type of binding in CdS crystals. Bibliography: 13 titles. HALL MOBILITY OF ELECTRONS IN CdS
.. .
111
AND CdSe LAYERS. I. A. Karpovich and B. N. Zvonkov (Vol. 6, No. 11, pp. 3392-3396). Electron Hall mobility has been studied in pure and indium-doped CdS and CdSe layers having a relatively high conductivity, and in insulating, photosensitive layers activated by copper. The CdS and CdSe layers were prepared by thermal deposition of powdered compounds on glass or quarts substrates heated to 3000C at pressure - 5 10-5 mm Hg. Molybdenum boasts were used for evaporation. Rectangular samples measured 5 X 16 mm2, and 1.5 - 10 CI in thickness. Indium contacts were made by vacuum evaporation, and were ohmic in all cases. Indium impurities were introduced by evaporation of a measured quantity at the same time as the evaporation of the semiconductor. n-Type conductivity was observed in all samples under investigation. The carrier concentration in the doped samples could be varied from values less than 1014 to values of the order of 1017 cm-3 by changing the parameters of the evaporation (speed of evaporation and degassing time of the powder). By introducing indium impurities in the layers it was possible to obtain layers with carrier concentrations from 2 * 1019 and 5 lo18 cm-3 for CdS and Cd&, respectively. Direct current measurements of the Hall emf and electrical conductivity werr made by the usual null method. The following results were obtained: Electron mobility in the undoped and slightly indium-doped layers is limited by barriers, and is lower, by approximately two orders of magnitude, than the mobility in single crystals; the mobility increases exponentially with the rise in temperature and decreases with increase in carrier concentration. It is noted that the physical nature of the barriers in the samples under investigation is not clear, and one may only assert that in well-conducting, “degenerate” layers, and in the insulating photosensitive layers with strong photoexcitation, the influence of the barriers is absent. Under these conditions the Hall mobility of electrons is mainly determined by the usual scattering processes, among which impurity scattering (which is probably due primarily to ionized impurities and structural defects) is of considerable importance. This is supported by the temperature dependences of mobility which do not show, or show only slight decrease in mobility with increase in temperature, which is characteristic for the case where thermal scattering predominatea. Bibliography: 12 titles. l
l
FIELD EMISSION MICROSCOPY OF Na ON
ABSTRACTS OF PAPERS IN SOVIET PHYSICS-SOLID STATE
A. G.Stasenko (Vol. 6, No. 11, pp. 3361-3363). The absorption spectrum of CdS was measured in the vacuum U. V. A vacuum spectrometer was used which was calibrated with a “sliding vacuum spark’! For the absorption measurement, the source of radiation was a capillary discharge tube with flowing hydrogen. The specimen was in a vacuum tight compartment and the whole system was pumped down to low4 mm Hg. The spectrum was photographed on an X-ray plate sensitized by an alcohol solution of sodium salicilate. The CdS specimen consisted of thin films evaporated onto a singlecrystal plate of LiF. The absorption of these CdS films was also measured at longer wavelengths in a spectrophotometer. The absorption spectrum shows that the absorption of CdS increases above about 8.3 eV. It is proposed that this increase is due to the transition of 3s electrons into the conduction band of CdS. Absorption bands which peak at ‘7.95 and 9.95 eV are also observed. The absorption peak at 7.95 eV is attributed to transitions A6 - A& El between levels due to Cd atoms in the CdS lattice. The observed shift of the atomic energy levels closer to each other in the crystal field is characterized by a coefficient of “compression” which is equal to the ratio of the transition energy in the free atom Efa to the energy of the absorption peak Em, due to the transition when the atom is in the lattice. Efa P=E,,=
yw=
1.02
The 9.95 eV peak is believed due to transitions from El/2 to two levels Eli2 and an E3 level corresponding to the formation of a 4% d ion in the CdS lattice. The “compression” coefficient for such transitions is equal to p=-
Ef E
11.78 eV ion === max
1.18
It is proposed that the simultaneous occurrence of absorption bands attributable to Cd atoms and to Cd+ ions is due to a mixed type of binding in CdS crystals. Bibliography: 13 titles. HALL MOBILITY OF ELECTRONS IN CdS
.. .
111
AND CdSe LAYERS. I. A. Karpovich and B. N. Zvonkov (Vol. 6, No. 11, pp. 3392-3396). Electron Hall mobility has been studied in pure and indium-doped CdS and CdSe layers having a relatively high conductivity, and in insulating, photosensitive layers activated by copper. The CdS and CdSe layers were prepared by thermal deposition of powdered compounds on glass or quarts substrates heated to 3000C at pressure - 5 10-5 mm Hg. Molybdenum boasts were used for evaporation. Rectangular samples measured 5 X 16 mm2, and 1.5 - 10 CI in thickness. Indium contacts were made by vacuum evaporation, and were ohmic in all cases. Indium impurities were introduced by evaporation of a measured quantity at the same time as the evaporation of the semiconductor. n-Type conductivity was observed in all samples under investigation. The carrier concentration in the doped samples could be varied from values less than 1014 to values of the order of 1017 cm-3 by changing the parameters of the evaporation (speed of evaporation and degassing time of the powder). By introducing indium impurities in the layers it was possible to obtain layers with carrier concentrations from 2 * 1019 and 5 lo18 cm-3 for CdS and Cd&, respectively. Direct current measurements of the Hall emf and electrical conductivity werr made by the usual null method. The following results were obtained: Electron mobility in the undoped and slightly indium-doped layers is limited by barriers, and is lower, by approximately two orders of magnitude, than the mobility in single crystals; the mobility increases exponentially with the rise in temperature and decreases with increase in carrier concentration. It is noted that the physical nature of the barriers in the samples under investigation is not clear, and one may only assert that in well-conducting, “degenerate” layers, and in the insulating photosensitive layers with strong photoexcitation, the influence of the barriers is absent. Under these conditions the Hall mobility of electrons is mainly determined by the usual scattering processes, among which impurity scattering (which is probably due primarily to ionized impurities and structural defects) is of considerable importance. This is supported by the temperature dependences of mobility which do not show, or show only slight decrease in mobility with increase in temperature, which is characteristic for the case where thermal scattering predominatea. Bibliography: 12 titles. l
l
FIELD EMISSION MICROSCOPY OF Na ON