THE JOURNAL OF
P H Y S I C A L CHEMISTRY (Registered in U. S. Patent Office)
VOLUME59
(Copyright, 1955 by the American Chemical Society)
NUMBER9
SEPTEMBER 20, 1955 ~~
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THE THERMAL DECOMPOSITION OF GERMANE. I. KINETICS BY KENZITAMARU, MICHELBOUDART A N D HUGHTAYLOR Frick Chemical Laboratory, Princeton University, Princeton, N . J . Received February 66,1965
The kinetics of decomposition of germane have been studied by a static method. The reaction rate is dependent only on the partial pressure of germane, being independent of that of hydrogen. The reaction is apparently first order a t higher pressures, decreasing to zero order at low pressures of germane. The experimental results can be explained by assuming concurrent first- and zero-order reactions with activation energies of 51.4 and 41.2 kcal., respectively. By changing the specific surface area of the reaction vessel it was concluded that the zero-order reaction occurs on the deposited germanium surface, while the first-order reaction is homogeneous. For the surface reaction it is concluded that the germanium surface is covered by adsorbed molecules, radicals and atoms decomposing a t the measured rate. Oxygen affects the decomposition process markedly. Contamination with oxygen accelerates the zero-order process and lowers the activation energy to 38.2 kcal. The oxygen appears to remain a t the germanium surface even with subsequent germane deposition. Arsine accelerates the rate of germane decompoAition on germanium surfaces although arsine itself is decomposed extremely slowly on germanium surfaces a t 302O, much more slowly than on antimony or arsenic surfaces.
The thermal decomposition of germane to yield germanium films and molecular hydrogen occurs slowly below 280" but measurably rapidly a t somewhat higher temperatures. During the decomposition the surface of germanium is constantly renewed by fresh deposition. The reaction therefore offers the possibility of kinetic studies on clean elemental surfaces. It offers also the possibility of studying the influence of contaminants, for example, oxygen and arsenic on such films. There is thus the possibility of correlating catalytic action with the known effects of foreign elements on the electrical properties of germanium. Hogness and Johnson' studied the kinetics of the decomposition of germane in 1932. They found rates proportional to the one-third power of the germane pressure a t higher temperatures with an inhibition by hydrogen in a lower temperature range. They developed an equation for their kinetics based upon the Langmuir adsorption theory which assumed a three-point contact between germane moleculesand the germanium surface. We decided to restudy the reaction with the extremely pure germanium now available as source of the germane, and to examine the effect of foreign elements deliberately introduced to the germanium. Experimental The decomposition of germane was studied by a static (1) T. R. Hogness and W. C. Johnson, J . Am. Chem. Soe., 64, 3583 ( 1932).
method. Since hydrogen is the only gaseous constituent resulting from the decomposition, the rate of the reaction was followed by observing the total pressure of the system which, on completion of decomposition, was twice the initial pressure. From the total pressure one may readily calculate the pressure of the undecomposed hydride and the pressure of the hydrogen resulting from the decomposition. Apparatus.-A cylindrical Pyrex glass vessel, which had been carefully cleaned, was used for the reaction vessel. The inside diameter of the vessel was 2.7 em. and its volume was 67 cc. The temperature of the reaction vessel was controlled satisfactorily using vapor baths of mercury, diphenylamine or acenaphthene under various constant pressures. The reaction vessel was attached to a mercury manometer by means of capillary tubing and during the reaction its pressure was followed by the manometer, keeping the volume constant. Between the reaction vessel and the manometer, a solid carbon dioxide trap was used to prevent the entry of vapor of mercury, grease or other im urities. gefore the reactant was introduced into the reaction vessel a vacuum of less than mm. was obtained by means of a mercury diffusion pump backed up by a Cenco Hy-Vac oil pump. Preparation of Germane.-Germane was prepared from germanium dioxide.2 First GeClr was obtained by boiling GeOz with pure concentrated HCleS GeCl4 (b.p. 86.5') was separated from concentrated HCl by means of a sepa(2) This oxide was obtained from the Bell Telephone Laboratories. I t was essentially free from the following impurities which were probably present in the materials of Hogness: traces of As, Si, and Sn, possibly totaling 0.03%. There might have been bare traces of AI, Ag, Ca, Cr, Cu, Fe, Ge, Mg, M n , Na, Pb. (3) This HCI was obtained by distilling "chemically pure" HC1, of which the maximum impurities are: free CI, 0.0000; sulfites, 0.00008; sulfates, 0.00008: hy. met. (as P b ) , 0.0001: Fe, 0.00001; As, 0.000001; "I, 0.0003; residue on ignition, 0.0004%.
801
K. TAMARU, M. BOUDART AND H. TAYLOR
802
ratory funnel and was distilled again, taking the distillates between 86 and 88". In a nitrogen atmosphere, GeC14 was dropped into a solution of LiA1HP4in ethylene glycol-dimethyl ether which had been cooled by liquid nitrogen. The reaction vessel was allowed to warm up slowly. The germane produced was caught in a liquid nitrogen trap after passing through ascarite to remove HCl. The hydride was purified by distilling several times between solid carbon dioxide and liquid nitrogen.
Experimental Results Decomposition of Germane at 302".-Germane was decomposed at 302" a t various initial pressures. The reaction proceeded with good reproducibility from t.he first experiment on the glass vessel. The results are shown in Fig. 1. All the curves in Fig. 1 are superimposable at corresponding pressures of germane, which shows that the rate of the decomposition is only dependent upon the partial pressure of germane, being independent of hydrogen.
0
200 300 Time, min. Fig. l.-GeHd decomposition a t 302'. 100
.
50
100
150
200
1000 (for 0 )1500 2000 Time, min. Fig. 3.-GeH4 decomposition a t 278": 0, in open vessel (bottom legend); in vessel filled with glass wool (top legend).
0
L
where ko and kl are the velocity constants of the zero- and the first-order reactions, respectively. On integration
?
where po is the initial pressure. Figures 4 and 5 show the applicability of equation 2 to the ab-
+o+ 0
I
I
0
Fig. 4.-GeHd
+ &-
I
20 Time, min. decomposition a t 330': calculated.
40
+, observed;
0,
100
200 300 400 Time, min. Fig, 2.-GeH4 decomposition a t various tempoeratures: V, 278'; 0, 302"; +, 314'; 0 , 330 0
Theoretical.-The experimental results showed that the decomposition was apparently first order a t higher pressures, but the order decreased to zero with decreasing pressure. This suggested that two kinds of reactions, both first and zero order, are taking place simultaneously, with the former predominant a t higher pressure while the latter predominates a t lower pressure. Thus, the following equation was assumed
400
Effect of Temperature upon the Reaction Rate.The kinetics of the decomposition was studied at various temperatures, which are shown in Figs. 2 and 3. Here also the reaction rates were only dependent upon the partial pressure of germane.
0
VOl. 59
500
+,
(4) This hydride was obtained from Metal Hydrides, Incorporated.
0
Fig. 5.-GeH4
100
200 Time, min. decomposition at 302': calculated.
300
+, observed;
400
f 'I
0,
served data, where the results of the decomposition a t 330 and 302" are shown, taking ko and kl as 0.248 cm./min., 0.0403 rnin.-l for the experiment a t 330°, and 0.0296 cm./min., and 0.00510 min.-' for that a t 302", respectively. Table I shows the calculated values of ko and kl for experiments a t four different temperatures and the dependence of the reaction rate constants upon temperature is shown in Fig. G . Applying Arrhenius' equation, and the method of least squares, activation energies of 41.9 and 51.4 kcal./mole were obtained for the zero- and the first-order reactions, respectively.
Sept., 1955
KINETICS OF THERMALIDECOMPOSITION OF GERMANE
803
-3.
1.6
1.7
1.8 1.9 1/T x 10s. Pig. 6.-Dependence of GeH4 decomposition rate upon temperature: 0 = k l ; 0 = ko.
1.8
Fig. 8.-GeH4
1.9 2.0 l / T x 103. decomposition rates on Ge a t various temperatures.
TABLE I Fig. 6. The kinetics a t 278" are shown as the zeroTHEREACTION VELOCITY CONSTANTS ( k o AND k l ) AT FOUR order line in Fig. 3. DIFFERENT TEMPERATURES These facts suggest that the zero-order reaction T,'C.
330 314 302 278
ko, om./min.
0.248 ,0864 ,0296 ,00908
kl, m k - 1
0.0403 .0111 .00510 ,000658
Effect of the Specific Surface Area of the Reaction Vessel.-The specific surface area of the reaction vessel was changed to study its effect upon the reaction velocity, by varying the shape of the reaction vessel or putting pieces of glass tubing or glass wool into it. The results are shown in Fig. 7, where the surface area-volume ratios are 1.6, ca. 4, ca. 4, and more than 7.5 cm.-I for the curves I, 11, I11 and IV, respectively, and curve V was obtained by putting glass wool in the reaction vessel. It can be seen that as the surface area-volume ratio increases, the zero-order reaction becomes more predominant until, a t sufficiently high ratios, the zero-order reaction is alone important.
takes place on the germanium surface, while the first-order reaction is a homogeneous gas reaction. Effect of Oxygen upon the Germane Decomposition.-When 1 em. of oxygen was put into the reaction vessel before germane was introduced, it changed the decomposition rate quite remarkably. The oxygen caused an explosive reaction with germane, and the remaining germane decomposed as a zero-order reaction a t a much faster rate than usual, shown by the crosses in Fig. 9, in com30 I
5 20 2
% * e,
2 10
l 0 0
0 Fig. 7.-GeH4
200 400 Time, min. decomposition in various reaction vessels a t 302".
With glass wool in the reaction vessel, germane was decomposed at different temperatures, and the temperature coefficient of the. zero-order rate as shown in Fig. 8, yields an activation energy of 41.2 kcal./mole. This value agrees well with the activation energy for the zero-order reaction in
50
100 150 Time, min. Fig. 9.-Effect of oxygen upon GeH4 decomposition at 312': V, without oxygen; 0 , oxygen contaminated surface; f,additional oxygen.
parison with the triangles for rates on a clean germanium surface. After the reaction, the reaction vessel was evacuated and pure germane was introduced. This also showed reproducible zero-order reactions as shown by the circles in Fig. 9. With this vessel germane decompositions were repeated without oxgyen and the decompositions were quite
K. TAMARU, M. BOUDART AND H. TAYLOR
804
reproducible zero-order reactions, as shown in Figs. 9 and 10. When oxygen was again added, the reaction rate increased a bit.
On the clean germanium Curve I Curve I1
VOl. 59 ki, min. -1
ko, crn./min.
0.0115 ,0118
0.0606 ,108 .I72
.0102
The data show the increase of the heterogeneous reaction rates and almost constant homogeneous reaction rates.
100 200 300 Time, min. Fig. IO.-GeH4 decomposition on the oxygen contaminated Ge surface at 278". 0
The effect of temperature on the reaction on an oxygen-contaminated surface is shown in Fig. 11, from which an activation energy of 38.2 kca1.l mole is obtained. This value is smaller than that of the reaction on the clean germanium surface.
(I
s 3 -1
I
-2 I
1.7
I 1.8
I
1.9
I / T X lo3. Fig. ll.-GeH4 decomposition on the oxygen contaminated Ge surface at various temperatures.
The effect of a small amount of oxygen was studied, putting 1/10 mm. oxygen in a new vessel. This is shown by the curve I in Fig. 12. Curve I1 in Fig. 12 was obtained by putting 2/10 mm. oxygen into the vessel. The rates of the first- and the zero-order reactions, that is, kl and ko, in this case, are :
50 100 150 Time, min. Fig. l2.-Effect of small amounts of oxygen on GeH4 decomposition a t 312": v, with increasing amounts of oxygen I and 11. 0
T o make sure that part of the zero-order reaction in Fig. 1 is not due to an extremely small amount of air left in the reaction vessel, a new reaction vessel was flushed with oxygen-free nitrogen and was evacuated very carefully. The same result was obtained as in Fig. 1. Even when the vessel was flushed with oxygen, before it was evacuated, the result was quite the same. Effect of Arsine upon the Germane Decomposition.-When a small amount of arsine was mixed with the germane, the decomposition of germane was slightly affected. The over-all reaction order was decreased probably due to a faster zero-order reaction and slower first-order reaction. When arsine was introduced into a reaction vessel which is covered by germanium film, it decomposed extremely slowly a t 302" as shown in Fig. Germane-arsine mixtures were decomposed in a reaction vessel with glass wool and the heterogeneous decomposition rate of germane in presence of arsine was measured. The data, shown in Table 11, indicate that germane, when mixed with arsine, decomposes a little faster than germane alone. The activation energy for the reaction was roughly estimated from Table 11. It shows almost the same, or slightly lower, activation energy than with pure germane. ( 5 ) It is interesting t o note that arsine decomposes fairly rapidly on antimony surfaces at 278', decomposes more slowly on arsenic surfaces, and t h e slowest on germanium surfaces.
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KINETICSOF THERMAL DECOMPOSITION OF GERMANE
Sept., 1955
EFFECTOF
805
agree fairly well, if the roughness of the surface is TABLEI1 taken into consideration. ARSINE O N THE GERMANE DECOMPOSITION It seems t o be most likely that the germanium The reaction rate is in cm./min.
Temp.,
10.20/,
OC.
0% AsHa
4.6%AsHa
228 255 267
0.0088 .0620 .160
0.0117 .OS25 .156
278 294
.339 1.105
.363
ASHI
12% AsHa
0.216 .217 0.523(280")
Discussion Reaction Mechanism.-The fact that the catalytic decomposition of germane is a zero-order reaction suggests that the surface of the germanium is almost covered by adsorbed molecules, radicals or atoms. In this case, according to the statistical mechanical treatment of rate processes, the rate of the reaction can be expressed as6 (3)
where G is the number of sites per unit surface area and E is the activation energy of the reaction. The observed value of E is equal to 41.2 kcal./ mole and thus the calculated reaction rate from equation 3 is equal to 1.0 X l O l a molecules per second per unit surface area. This assumes that the number of sites G is equal to 1015which is the number of germanium atoms on unit surface area. On the other hand the observed reaction rate a t 302" is equal t o 5.2 X 1013 molecules per second per unit surface area, assuming that the surface is smooth. The observed and calculated values
surface is almost covered by GeH, radicals or chemisorbed hydrogen during the reaction, and the desorption of the latter measures the rate of the over-all reaction. As to the effect of oxygen upon the germane decomposition, the good reproducibility of the germane decomposition on the oxygen-contaminated surface suggests that the oxygen always stays on the germanium surface, migrating through the deposited germanium to the surface. The increase of the reaction rate by oxygen contamination (ko is about 17 times larger a t 302") is due to the lower activation energy of the reaction, as the 3.0 kca1.l mole decrease of the activation energy corresponds to a 14 times faster reaction rate. This increase in reaction rate due to oxygen contamination of the germanium surface is in marked contrast to the inhibitory action of oxygen on iron synthetic ammonia catalysts in ammonia synthesis and decomposition. Acknowledgment.-The preceding work was carried out with the assistance of a post-doctoral fellowship kindly provided by the Shell Fellowship Committee of the Shell Companies Foundation, Inc. It also forms part of a program on Solid State Properties and Catalytic Activity supported by the Office of Naval Research N6onr-27018. For this support we wish to express our appreciation and thanks. DISCUSSION MAX BENDER.-YOUmade reference to "absorbed" oxygen on the deposing germanium not being covered by the Ge
that further is deposited. Rather it stays on top as the ger(6) C% S. Glasstone, et al., "The Theory of Rate Processes," McGraw-Hill Book Co., 1941, p. 376; or, replacing T with Tmel-T/Tm, manium film increases in thickness, L e . , bubbles through. Would you expect that the hydrogen still associated with the u = Ge k T m / h e-Bobsa'RT, where Tm is the median of the temperature germanium film, also does the same as the film builds up? range.
