THE JOURNAL OF
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PHYSICAL CHEMISTRY (Registered in U. S. Patcnt 0ffir.o)
VOLUME 61
(0 Copyright, 1957, by the American Chemical Society)
NUMBER 5
MAY 27, 1957
DISSOCIATION PRESSURE AND COHESIVE ENERGY O F I N D I U M PHOSPHIDE BY KURTWEISER RCA Laboratories, Radio Corporation of America, Princeton, N . J . Received Juiu 67, 1966
The dissociation pressure of solid InP has been measured between 700 and 1045’. From the temperature dependence of the va or ressure and the known heat of va oriaation of indium and heat of dissociation of Pc molecules, the cohesive energy o?Inl! was calculated to be 154,000 cal.f)mole.
I. Introduction
ld c
pressure was determined by measuring the total premre
over the solid investigated. Compared to the method deIn view Of the current interest in the semi-con- scribed below, this procedure suffers from the possible error ducting properties of InP,’ a measurement of Its of the compound having an appreciable vapor pressure so dissociation pressure as a function of temperature that the total pressure does not necessarily equal the disis of considerable technological importance. Fur- s o $ ~ t ~ r ~ experiments, ~ ~ ~ ~ cryetala ~ t h ofe high purity thermore, such measurements together with exist- I n P (the impurity content, as determined from electrical ing data on the heat of vaporization of indium and measurements, was less than one part in 10% were sealed the heat of dissociation of phosphorus enable one into an evacuated quartz tube, as shown schematically in 1. ”he bulb containing the crystals W a s made of to calculate the cohesive energy of the Compound, Fig. heavy wall tubing (ap r. 2.5 mm. wall thickness) in order i.e.9 the energy necessary to dissociate to mlnimize both the $an er of explosions and temperature into the gaseous atoms of its components. fluctuations inside the b u b . To determine a pressure, the
crystals were kept in the “front furnace” at a controlled “dccomposition temperature” TD,while the temperature in the “rear furnace” was decreased in intervals. A t each interval, tern erature equilibration was permitted (6 to 10 minutes), a n i then a short blast of air was directed at the tip through the tube J . If the lowered the temperature sufficiently, a condensate of hosphorus in the form of a few droplets would be induced. the temperature of the “rear furnace” was hi her than the condensation temperature To of thethephosp%orus, Mest.uf air the was condensate turned off. disAs appeared rapidly after
11. Experimental Methods and Results
Crystala of ~~pheated in a sealed, evacuated vial, part1 decompose until the pressure of the vapor in the vial aqua$ the dissociation pressure of the compound. since, at the temperatures investigated, the phosphorus pressure is always many orders of magnitude larger than the vapor pressure of indium,: the dissociat,ion pressure of ~~p is essentiall equal to its phosphorus pressure. The plosphorus pressure over solid indium phosphide was measured by the method of the “dew-point,*’ by determining the temperature at which the vapor pressure was the disappear more persist when Tc was of pure phosphorus is equal to the dissociation pressure of the slOwlYp and compound. This method has long been uRed for the study reached; below To the condensate would spread rapidly. has not been a plied The condensation temperature can be established in thia of aqueous solut,ionsa but to measurements of the dissociation pressures of phospkdes. manner with an accuracy Of better than loo,and the dissorn the comprehensive study of many other hosphides ciation pressure of I n P at TD can then be determined from Pressure data Of PhosPhoruS at T C * 8 The airthe undertaken by Biltz and his collaborators4 the &sociation blast method prevented supersaturation of the phosphorus as well as permitting an accurate determination of (1) (a) H. Welker, J . Electronics, [I] I, 181 (1955); (b) 0.a. ~ ~ 1vapor, berth and H. Weiss, 2. Natwjorschung, loa, 616 (1955); (c) 0.0. the dew-point by gving warnin$ of its approach. The determination of the dissociation pressures at the Folberth, Ibid.. loa, 602 (1955): (d) F. Oswald, ibid., sa, 181 (1954); two lowest temperatures investigated was carried out by a (e) H. Welker, ibid., Sa, 248 (1968); (f) 76,744 (1952); ( g ) Physics, slightly modified procedure, since the dew-points of phosXX, No. 11. 893 (1954). (2) L. L. Quill. “Chemistry and Metallurgy of Miscellaneous phorus fell below room temperature. The tip Of the vial Materiale Chemistry,” Ch. 111, McCraw-Hill Book CO., New York, was kept in a beaker full of water, first slightly below room temperature, and then at ice temperature. The temperaN . Y., 1950.
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(3) See for examrde, J. N. Friend, “Textbook of Physical Chemistry,” Vol. 11, J. B. Lippincott, 1935, p. 125. (4) W. Biltz, Z.physik. Chsm., 189A, 10 (1941).
(5) ( a ) F. 8. Dainton. Tmne. Faraday Sac., 48, 912 (1960); “Handbook of Physics & Chemistry,” 1966-1956.
513
(b)
KURTWEISER
514
tip, and b occaaiondy heating ;the tip to drive off the sublimate.' &he occasional conversion of the white phosphorus condensate to the red modification resulted in an uncertainty aa to the proper vapor pressure to be assigned to the condeneate. If, however, the condensate induced by the air blast was kept at a minimum, the difficulty was avoided since the phosphorue would evaporate before it had a chance ta . convert to the red form. For the data shown in Fig. 2, on1 those measurements were used in which the a pemance a n d disa pearance of the condensate were read& visible, and in wfhh no conversion to red phosphorus took place.
I
I/
Fig. 1.--.Schematic diagram of apparatus for determining dissociation pressures of InP. L
\ ..
-
Ipo 1I.C).
Fig. 2.-Dissociation
pressure of InP from 70O0 t o 1045'.
ture of the drystals, TD,was raised gradually until a condensate of phosphorus appeared at the tip. The decomposition temperature corresponding to a given "dew-point" was thus determined. The results of four se arate runs are shown in Fig. 2, where the logarithm of t i e dissociation pressure is plotted against the reciprocal of the absolute temperature. For each run, a new quartz vial and-different crystals were used. The data represented true equillbrium values since the same results were obtained when the data were taken with increasing or with decreasing temperature. The two main difficulties encountered during the ex erimenta were due to the presence of an indium phospxide layer on the walls of the vial, and to the occasional convcrsion of white to red phosphofus. The indium phosphide layer was due to slow sublimation of the compound, and obscured the appearance of the phosphorus condensate. The trouble was overcome by focusing a strong lamp on the
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III. Discussion It is a well known result of thermodynamics' that the temperature dependence of the equilibrium constant for a chemical reaction at constant volume can be expressed by the equation In K, D / R T + AI?'&') lnT + const. (1) provided that A& is constant over the temperature range investigated. In this equation, K , represents the equilibrium constant a t constant volume] EO is the heat of the reaction over the temperature range investigated] Acov is the difference in heat capacity at constant volume between products and reactants] R is the gas constant, and T is the absolute temperature. Since for a typical chemical reaction, EO is of the order of 10 to 100 kcaL1 while Ac, is of the order of calories per degrees] a plot of In K against 1/T may appear linear over a considerable temperature range. The chemical equilibria studied here can be represented by the equations InP. I rIn1 I/J'l(g) (24
-
In1
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Vol. 61
+ y.InP.
+
(1n.y.InP)i where 0
<
