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Thermal conversion of GaAs
Solid State Communications Vol. 3, pp. lili-lviii, 1965. Pergamon Press Ltd. Printed in Great Britain.
Abstracts of Important Papers Published in Soviet Physics-Solid State (Fizika Tverdogo Tela) A complete cover~to.covertranslation of the Russian journal is published by the American Institute of Physics approximately six months after the date of original publication.
THERMAL CONVERSION OF GaAs. B. I. Boltacks and F. S. Shishianu (Vol. 7, No. 4, p. 1021). The kin etics of the thermal conversion process was studied (actually the kinetics of thermal acceptor diffusion during annealing), and additional data was obtained on the nature of thermal acceptors. Furthermore, the possibility of extracting copper from GaAs (with assistance from Zn) was demonstrated. The first step was a study of the effect of heating on the concentration and mobility of carriers. The experiments were performed on single crystals of n-type material ~aving an electron concentration of 8 x 1016 cm~and a mobility of 3700 cm’/Vsec. It was found that after each heat-treatment the elecfron concentration slowly~,decreases,reaching a value of 6 x 1016 cm~after heating at 900°C. A subsequent heating at 1000°Cbrings about the thermal conversion of GaAs from n- to p-type. Further heating at still higher temperatures causes an abrupt increase in t~ehole concentration: reaching 3.5 x 1017 cm after heating at 1150°C. The electron mobility decreases as the2/ temperature Vsec after a 900°C is increased treatment). (from 3700 The mobility to 3200 cm 01 holes after thermal conversion remains 240cm’/ Vsec. In observing the migration of the boundary of a p-type region formed during the heating of an n-type specimen, the kinetics of thermal conversion could be studied. The results show that thermal conversion occurs furst of all near a surface or near internal crystal defects. In addition to such large sources of thermal acceptors there are always smaller ones which initiate micro-regions of p-type conductivity. The p-type regions in these experiments were detected by a thermal probe or by the direction of rectification and also by the electrolytic deposition of silver. It is shown that the p-type regions appear first —
dislocation sites. The dimensions of the p-type regions formed by heat treatment depend on the temperature and duration of the treatment: as the latter are Increased, the p-type regions grow until they completely overlap. In examining the samples by transmission of infrared, one Bees that the p-type regions near the surface and near large crystal defects are opaque.* The mechanism of this phenomenon is discussed. The diffusion coefficient of thermal acceptors is evaluated from the spreading of the p-type regions and the migration of p-n transitions near the surface. It is found that in the temperature range of 700-19)°Cits value varies from 3 x 10-8 to 10-6 cm /5cc, which corresponds to the dissusion coefficient of copper in GaAs at these temperatures. From the temperature dependence of the hole concentration (from Hall effect) in the range -170°to 200°C,in uniform samples which had been subjected to thermal conversion at 1140°and 8800C, the energy level of the thermal acceptor was determined. It is located 0. 15 cv above the valence band. Furthermore, after a second heating at a lower temperature (1020°C) and a slowlevel cooling (3 x 10-2 second acceptor appeared 0. 1 deg/sec) eV aboveathe valence band. In order to determine the energies of copper levels in GaAs, a thin layer of Cu was deposited electrolytically on the samples. These were then heated to 880°-1140°C.It was found that the resulting impurity levels depend consider ably on the copper concentration in the sample. It was established that the main reason for the thermal conversion of GaAs is the copper tinpurity in the sample, and therefore, it is extremely important to eliminate Cu from GaAs. A purification method by redifussion is proposed: —
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*For p-type regions at dislocation sites the relationship is more complex.
liv
ABSTRACTS OF PAPERS IN SOVIET PHYSICS-SOLID STATE
the essence of the method consists in creating at the surface of the specimen a “binding boundary” and then heating the sample to an optimal temperahire. Preliminary results are presented on using Zn fro Cu extraction from GaAs.
Vol. 3, No. 7
study of this influence makes it possible to make a series of conclusions with respect to changes of the energy spectrum of the crystals. Bibliography: 10 titles.
Bibliography: 8 titles. INFLUENCE OF HIGH HYDROSTATIC PRESSURE ON PHOTOCONDUCTION IN MONOCRYSTALLINE CdS. K. Gullamov, N. A. Tikhonilrova and V. M. Frledkin (Vol. 7, No. 4, p. 1037). The influence of high hydrostatic pressure on the photoconduction of monocrystalline CdS was investigated. The work consists of two parts: In the first part the question of the dependence of the intrinsic and phenomenological carrier life-times on pressure is studied theoretically; the second part is experimental. Steady state photocurrent, as well as the phenomenolc~gical lifetime was measured for single crystals in the region up to 15 x ~ atm. The high pressure chamber is shown schematically, representing a conical vessel in a high pressure multiplicator block. A monochromator, yM-2, was used in recording the spectral distribution of the photocurrent. Crystals used were grown from the melt. The phenomenological lifetime of crystals grown from the gas phase was measured by the usual method, using square wave pulses of light with 104 sec rise time. Curves of spectral distribution of thi steady state photocurrent for various pressures, variation of the energy gap with pressure, and the dependence of in a/a0 on pressure (where a = value of the peak photocurrent) show that, for CdS (dEg)/(dP) + 4.7 x jo-6 eV/ 1. atm, It must andbe(d)/(dP) noted that in a/a0 the observed 0.32 xscattering 10~atmof values (dEg)/(dP) and (dEr)/(dP), for a number of other crystals obtained from the melt, is due to the presence of uncontrollable impurities In the crystals. Two sets of measurements are represented graphically for the dependence of in (~r’i)/(1o)on pressure (.r’ — phenomenological lifetime determined for CdS crystal grown from the vapor phase) at low and high light intensities. It follows from the results that the signs of the effect are opposite for the two sets of measurements; The sign for the fjrst set (d)/(dP) in (T’)/ (ia) + (3-6) x 10~atm~and for the second set (d)/(dP) in (.r’)/~) (10. 7-1. 3) x i0~atm~. This data Is compared with the calculated values, It is assumed that, under the influence of pressure, both the intrinsic and the phenomenological lifetimes of the carriers in CdS vary, and the -
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EXPERIMENTAL STUDY OF ENERGY SPECTRUM AND OF EMISSION PROPERTIES OF THIN FILMS OF Ge ON TUNGSTEN. N. V. Mlleshkina and I. L. Sokolskaya (Vol. 7, No. 4, p. 1043). We present the results of a study of the energy distribution of electron emission from atomic layers of Ge on tungsten, and also of the emission properties of thicker layers of Ge on tungsten. The following results have been obtamed: (1) The analysis of the electron energy distribution from Ge films obtained by “hightemperature” migration confirms the hypothesis that there are uncovered regions of tungsten between (Oil) and (001) facets, and this preferential location is demonstrated, (2) It is found that the electron energy spectrum from a monoatomic layer of Ge on w obtained by “low-temperature” migration is composed of two groups of electrons. According to our model, the low energy electrons are valence electrons from the Ge layer, (3) When Ge is evaporated onto tungsten, the emission cur rent drops at most by a factor of 70. When the thickness of the Ge layer is increased, the emission current increases. The spectra from “thick” layers of Ge exhibit two groups of electrons, one of which has energies greater than for bare tungs ten, which can be explained by the occurrance of electronic conduction in thick layers of Ge, and (4) The energy spectrum of electrons from Ge atoms agglomerated intocrustallites) bulk material on top of tungsten (“collars” and differs completely, in shape and half-width, from the spectra obtained both with bare and Ge-coated tungsten. A set of experimental relationships is reported. -
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Bibliography: 5 titles. FEEDBACK EFFECT OF THERMOEMISSION ON SATURATION CURRENT. L. D. Tsendin (Vol. 7, No. 4, p. 1078). This is a theoretical investigation of how the saturation current is affected by a distortion of the distribution function at the surface of a semiconductor thermal cathode. It Is shown that this distortion causes a several-fold decrease in the theoretical saturation current and a change
Abstracts of Important Papers Published in Soviet Physics-Solid State (Fizika Tverdogo Tela) A complete cover~to.covertranslation of the Russian journal is published by the American Institute of Physics approximately six months after the date of original publication.
THERMAL CONVERSION OF GaAs. B. I. Boltacks and F. S. Shishianu (Vol. 7, No. 4, p. 1021). The kin etics of the thermal conversion process was studied (actually the kinetics of thermal acceptor diffusion during annealing), and additional data was obtained on the nature of thermal acceptors. Furthermore, the possibility of extracting copper from GaAs (with assistance from Zn) was demonstrated. The first step was a study of the effect of heating on the concentration and mobility of carriers. The experiments were performed on single crystals of n-type material ~aving an electron concentration of 8 x 1016 cm~and a mobility of 3700 cm’/Vsec. It was found that after each heat-treatment the elecfron concentration slowly~,decreases,reaching a value of 6 x 1016 cm~after heating at 900°C. A subsequent heating at 1000°Cbrings about the thermal conversion of GaAs from n- to p-type. Further heating at still higher temperatures causes an abrupt increase in t~ehole concentration: reaching 3.5 x 1017 cm after heating at 1150°C. The electron mobility decreases as the2/ temperature Vsec after a 900°C is increased treatment). (from 3700 The mobility to 3200 cm 01 holes after thermal conversion remains 240cm’/ Vsec. In observing the migration of the boundary of a p-type region formed during the heating of an n-type specimen, the kinetics of thermal conversion could be studied. The results show that thermal conversion occurs furst of all near a surface or near internal crystal defects. In addition to such large sources of thermal acceptors there are always smaller ones which initiate micro-regions of p-type conductivity. The p-type regions in these experiments were detected by a thermal probe or by the direction of rectification and also by the electrolytic deposition of silver. It is shown that the p-type regions appear first —
dislocation sites. The dimensions of the p-type regions formed by heat treatment depend on the temperature and duration of the treatment: as the latter are Increased, the p-type regions grow until they completely overlap. In examining the samples by transmission of infrared, one Bees that the p-type regions near the surface and near large crystal defects are opaque.* The mechanism of this phenomenon is discussed. The diffusion coefficient of thermal acceptors is evaluated from the spreading of the p-type regions and the migration of p-n transitions near the surface. It is found that in the temperature range of 700-19)°Cits value varies from 3 x 10-8 to 10-6 cm /5cc, which corresponds to the dissusion coefficient of copper in GaAs at these temperatures. From the temperature dependence of the hole concentration (from Hall effect) in the range -170°to 200°C,in uniform samples which had been subjected to thermal conversion at 1140°and 8800C, the energy level of the thermal acceptor was determined. It is located 0. 15 cv above the valence band. Furthermore, after a second heating at a lower temperature (1020°C) and a slowlevel cooling (3 x 10-2 second acceptor appeared 0. 1 deg/sec) eV aboveathe valence band. In order to determine the energies of copper levels in GaAs, a thin layer of Cu was deposited electrolytically on the samples. These were then heated to 880°-1140°C.It was found that the resulting impurity levels depend consider ably on the copper concentration in the sample. It was established that the main reason for the thermal conversion of GaAs is the copper tinpurity in the sample, and therefore, it is extremely important to eliminate Cu from GaAs. A purification method by redifussion is proposed: —
___________
*For p-type regions at dislocation sites the relationship is more complex.
liv
ABSTRACTS OF PAPERS IN SOVIET PHYSICS-SOLID STATE
the essence of the method consists in creating at the surface of the specimen a “binding boundary” and then heating the sample to an optimal temperahire. Preliminary results are presented on using Zn fro Cu extraction from GaAs.
Vol. 3, No. 7
study of this influence makes it possible to make a series of conclusions with respect to changes of the energy spectrum of the crystals. Bibliography: 10 titles.
Bibliography: 8 titles. INFLUENCE OF HIGH HYDROSTATIC PRESSURE ON PHOTOCONDUCTION IN MONOCRYSTALLINE CdS. K. Gullamov, N. A. Tikhonilrova and V. M. Frledkin (Vol. 7, No. 4, p. 1037). The influence of high hydrostatic pressure on the photoconduction of monocrystalline CdS was investigated. The work consists of two parts: In the first part the question of the dependence of the intrinsic and phenomenological carrier life-times on pressure is studied theoretically; the second part is experimental. Steady state photocurrent, as well as the phenomenolc~gical lifetime was measured for single crystals in the region up to 15 x ~ atm. The high pressure chamber is shown schematically, representing a conical vessel in a high pressure multiplicator block. A monochromator, yM-2, was used in recording the spectral distribution of the photocurrent. Crystals used were grown from the melt. The phenomenological lifetime of crystals grown from the gas phase was measured by the usual method, using square wave pulses of light with 104 sec rise time. Curves of spectral distribution of thi steady state photocurrent for various pressures, variation of the energy gap with pressure, and the dependence of in a/a0 on pressure (where a = value of the peak photocurrent) show that, for CdS (dEg)/(dP) + 4.7 x jo-6 eV/ 1. atm, It must andbe(d)/(dP) noted that in a/a0 the observed 0.32 xscattering 10~atmof values (dEg)/(dP) and (dEr)/(dP), for a number of other crystals obtained from the melt, is due to the presence of uncontrollable impurities In the crystals. Two sets of measurements are represented graphically for the dependence of in (~r’i)/(1o)on pressure (.r’ — phenomenological lifetime determined for CdS crystal grown from the vapor phase) at low and high light intensities. It follows from the results that the signs of the effect are opposite for the two sets of measurements; The sign for the fjrst set (d)/(dP) in (T’)/ (ia) + (3-6) x 10~atm~and for the second set (d)/(dP) in (.r’)/~) (10. 7-1. 3) x i0~atm~. This data Is compared with the calculated values, It is assumed that, under the influence of pressure, both the intrinsic and the phenomenological lifetimes of the carriers in CdS vary, and the -
-
EXPERIMENTAL STUDY OF ENERGY SPECTRUM AND OF EMISSION PROPERTIES OF THIN FILMS OF Ge ON TUNGSTEN. N. V. Mlleshkina and I. L. Sokolskaya (Vol. 7, No. 4, p. 1043). We present the results of a study of the energy distribution of electron emission from atomic layers of Ge on tungsten, and also of the emission properties of thicker layers of Ge on tungsten. The following results have been obtamed: (1) The analysis of the electron energy distribution from Ge films obtained by “hightemperature” migration confirms the hypothesis that there are uncovered regions of tungsten between (Oil) and (001) facets, and this preferential location is demonstrated, (2) It is found that the electron energy spectrum from a monoatomic layer of Ge on w obtained by “low-temperature” migration is composed of two groups of electrons. According to our model, the low energy electrons are valence electrons from the Ge layer, (3) When Ge is evaporated onto tungsten, the emission cur rent drops at most by a factor of 70. When the thickness of the Ge layer is increased, the emission current increases. The spectra from “thick” layers of Ge exhibit two groups of electrons, one of which has energies greater than for bare tungs ten, which can be explained by the occurrance of electronic conduction in thick layers of Ge, and (4) The energy spectrum of electrons from Ge atoms agglomerated intocrustallites) bulk material on top of tungsten (“collars” and differs completely, in shape and half-width, from the spectra obtained both with bare and Ge-coated tungsten. A set of experimental relationships is reported. -
-
Bibliography: 5 titles. FEEDBACK EFFECT OF THERMOEMISSION ON SATURATION CURRENT. L. D. Tsendin (Vol. 7, No. 4, p. 1078). This is a theoretical investigation of how the saturation current is affected by a distortion of the distribution function at the surface of a semiconductor thermal cathode. It Is shown that this distortion causes a several-fold decrease in the theoretical saturation current and a change