Diffusion of zinc in indium phosphide at 700°C

Solid-State Electronics, 1974, Vol. 17, pp. 531-538.

Pergamon Press.

Printed in Greal Britain

D I F F U S I O N OF ZINC IN I N D I U M P H O S P H I D E AT 700°C A. HOOPER, B. TUCK and A. J. BAKER Department of Electrical and Electronic Engineering, University of Nottingham, England (Received 22 December 1972; in revised form 26 June 1973)

Abstract--The diffusion of zinc into n-type InP at 700°C is described for a variety of experimental conditions. Radioactive zinc was used and radio-tracer profiles were plotted. Observations were made of the semiconductor surfaces after each diffusion. The p-n junction depths were also measured and some electrical measurements were taken. For some experimental conditions, very high values of zinc surface concentration were found. The samples showing high surface concentrations also exhibited surface features after diffusion, and these were studied using optical microscopy and an X-ray probe microanalyser. In some cases very small changes in diffusion conditions produced large variations in the measured profile. This is attributed to a boundary being crossed in the In/P/Zn ternary phase diagram. A comparison of the radio-tracer and p - n junction measurements shows that the number of zinc atoms in a sample is much greater than the number of shallow acceptors. For diffusions in which excess phosphorus was used, evidence is presented to suggest that the diffusion coefficient varies with zinc concentration at low densities of zinc, but saturates to a constant value at zinc concentrations above l0 ~9cm 3. 1. I N T R O D U C T I O N

Increasing interest has been shown in recent years in the I I I - V semiconductor lnP. In common with most semiconductors of this group, the material is normally prepared n-type, and p - n junctions are made by diffusing in a group II element. It is usually assumed that a group II atom occupies the group III site of the semiconductor, acting as an acceptor. The group II element most commonly used is zinc, and a great deal of work has been carried out investigating the diffusion of zinc in GaAs and GaP. Much less information is available for InP, however. Chang and Casey[1] have presented radiotracer diffusion profiles, plotted for temperatures between 600 and 900°C, using a standard set of diffusion conditions. Other work has been carried out using electrical measurements and p - n junction depths [2-4]. In the work to be described, radio-tracer diffusion profiles were plotted at 700°C for a wide range of experimental conditions. To complement this, observations were made of the semiconductor surfaces at the end of each diffusion, using both the optical microscope and the electron-probe X-ray microanalyser. The p - n junction depths were also measured, and Hall-effect measurements were carried out on a homogeneously-doped sample. 2. EXPERIMENTAL PROCEDURE

The lnP was in the form of thin discs, cut in a (100) orientation, about 1 cm 2 in area and 500/xm

thick. They were all n-type, with a carrier concentration of 6-8 x 1016 c m 3. Diffusion profiles were plotted using radio-tracer Zn 65. This isotope emits y-rays at an energy of 1.1 MeV and has a half-life of 245 days. An InP slice was chemically polished in a 1 per cent solution of bromine in methanol and then put into a clean quartz ampoule with a small amount of zinc. In some of the experiments a small piece of phosphorus was also put into the ampoule. The ampoule was evacuated to 10-.5 torr and sealed to a volume of 12.5-13 cm 3. It was then placed in a three-zone furnace set to 700°C. Full details of the conditions for the experiments are given in the table. At the end of the diffusion the sample was removed and cleaned. Diffusion profiles were plotted by removing layers using the 1 per cent bromine-methanol solution, and counting the activity of the removed material. Layer thicknesses were found by weighing the specimen before and after etching. Before carrying out the profiling procedure a small piece of the semicondoctor was broken off the slice. This piece was used to determine the depth of the p - n junction. At the junction the number of acceptors is equal to the number of donors (i.e. 6-8 × l0 TM cm 3). The concentration of zinc atoms at this depth could be determined by reference to the radio-tracer profile, and so a comparison of the concentration of acceptors with the concentration of zinc atoms could be made. The 531

532

A. HOOPER, B. TUCK and A. J. BAKER Table 1. Giving experimental conditions for the diffusions

Curve

Diffusion time (min)

Zinc weight (txg)

I 2 3 4 5 6 7 8 9 10 11 12 13 14 15

10 20 30 30 40 60 180 20 40 120 1080 20 20 20 20

1990 1060 890 635 520 130 24 1030 440 48 10 1040 1030 1030 1020

Phosphorus weight Ozg)

Surface concn, C,, (cm ~) 5.5 x 4.0 × 5.0 x 3.8 x 1-6 × 9.0 x 1.1 z

I000 1030 1020 1000 150 170 350 550

j u n c t i o n d e p t h was d e t e r m i n e d b y c l e a v i n g the s e m i c o n d u c t o r p e r p e n d i c u l a r l y to t h e f a c e s a n d e t c h i n g the c r o s s - s e c t i o n t h u s r e v e a l e d in a 1 : 2 : 6 s o l u t i o n of K O H : KFe(CN)~: H 2 0 f o r 15 sec in the p r e s e n c e of a s t r o n g light. A further comparison between the concentration of zinc a t o m s a n d a c c e p t o r s w a s m a d e b y diffusing a n I n P s a m p l e f o r a sufficiently long time f o r it to b e c o m e h o m o g e n e o u s l y d o p e d with zinc to a level of 7 × l0 t9 c m 3. T h e sample, n o w c o m p l e t e l y p type, was f o r m e d into t h e V a n d e r P a u w configuration b y s a n d - b l a s t i n g a n d a s t a n d a r d Hall-effect m e a s u r e m e n t w a s carried out. This e x p e r i m e n t g a v e a v a l u e f o r the hole c o n c e n t r a t i o n in the specimen. A s s u m i n g all t h e a c c e p t o r s w e r e singly ionised, this was also t h e a c c e p t o r c o n c e n t r a t i o n . A n u m b e r of e x p e r i m e n t s w e r e carried o u t u s i n g n o n - r a d i o a c t i v e zinc. In all o t h e r r e s p e c t s t h e y w e r e identical to the diffusions d e s c r i b e d a b o v e . T h e p u r p o s e of t h e s e e x p e r i m e n t s was to o b s e r v e the s t a t e of t h e s u r f a c e s of the diffused s p e c i m e n s . T w o t e c h n i q u e s w e r e u s e d f o r this. In the first, the diffused s p e c i m e n s w e r e o b s e r v e d u n d e r t h e optical m i c r o s c o p e to see if t h e initially fiat a n d s h i n y surf a c e h a d b e c o m e m a r k e d in a n y way. In t h e s e c o n d t e c h n i q u e , the e l e c t r o n p r o b e X - r a y m i c r o a n a l y s e r was used. T h e m i c r o a n a l y s e r s c a n s a c r o s s a specim e n with an e l e c t r o n b e a m , c a u s i n g X - r a y s to b e e m i t t e d w h i c h are c h a r a c t e r i s t i c of the c h e m i c a l e l e m e n t s m a k i n g u p the surface. By t u n i n g the ins t r u m e n t to d e t e c t the c h a r a c t e r i s t i c r a d i a t i o n f r o m j u s t o n e e l e m e n t , a ' m a p ' of the s u r f a c e c a n b e

I'0X

5"5 × 2.2× 2.0x

2.4x 5.0× 1.2 ×

102' 10~-' 102' 102' 10~'' 10'~ 10'9 102" 10 ~ 10 '~ 10'~ 10:' 102' 10~"

1 . 2 × 10 ~°

Junction depth (xj) (u,m) 24 45 53 No value 68 75 37 13 25 24 10 44 41 22 22

Concn. at x~ (cm ~) 4.0 x 10TM 4-0 × 10'~ 2.5 x 10 '~ 1.0 × 10'" 1.5 × 10 '~ 3.0x 10 '7 1-5 X 10'9 2-5 × 10'~ 4.0× 10'~ 1 . O x 10'" 2.0x 10 '~ 2.6x 10TM 2.0 × 10'~ 2-0 x 10 '~

Approx. amount of zinc in diffused sample (u,g) 900 660 500 45 35 <~ 1 10 8 5 < 1 410 250 13 17

p r o d u c e d with regions c o n t a i n i n g t h e e l e m e n t s h o w i n g up as a r e a s of high intensity. M a p s for indium, p h o s p h o r u s a n d zinc w e r e o b t a i n e d , to disc o v e r h o w t h e s e e l e m e n t s w e r e d i s t r i b u t e d at t h e s u r f a c e , a n d to relate t h e i r d i s t r i b u t i o n s to o b s e r v able s u r f a c e f e a t u r e s . T h e m i c r o p r o b e w a s also u s e d to plot diffusion profiles. A diffused s p e c i m e n was c l e a v e d at rightangles to the s u r f a c e of t h e slice a n d t h e microp r o b e was m a d e to s c a n a c r o s s t h e e x p o s e d crosssection. B y using a piece of p u r e zinc as a s t a n d a r d it was possible to c o n v e r t the resulting plot of X - r a y i n t e n s i t y v s d i s t a n c e to o n e of zinc c o n c e n t r a t i o n vs distance. This is a m u c h less s e n s i t i v e m e t h o d of plotting profiles t h a n t h a t of u s i n g r a d i o - t r a c e r profiles: it h a s a m i n i m u m d e t e c t i o n level for zinc of a b o u t 5 × l 0 t9 c m -3. H o w e v e r , b y giving e s s e n tially the s a m e results as the r a d i o - t r a c e r m e t h o d it p r o v i d e d a v e r y useful c h e c k o n t h e e x p e r i m e n t a l techniques. 3. RESULTS A set of profiles is s h o w n in Fig. I f o r t h e experim e n t s in w h i c h only zinc a n d I n P were p u t iqto t h e a m p o u l e . T h e c u r v e s are plotted against the p a r a m e t e r x/~/t f o r the sake of c o n v e n i e n c e : t h e diffusion t i m e s are g i v e n in the table. T h e profiles c a n b e divided into t w o classes. C u r v e s 1-4 all h a v e a high s u r f a c e c o n c e n t r a t i o n w h i c h c h a n g e s o n l y slightly for different a m o u n t s of zinc in t h e ampoule. Call t h e s e Class 1. B e t w e e n c u r v e s 4 a n d 5, h o w e v e r , a v e r y small c h a n g e in diffusion condi-

533

Diffusion in lnP 1022 L ++

? 10 E o

g o

\

N

10

I 0

0'SxlO -4

xll'-f-,

I'OxlO -4 cm.

1'5 x l O - 4

2'OxlO -4

sec-I/2

Fig. l. Diffusion profiles of zinc in n-type InP for different amounts of zinc in the diffusion ampoule (see table); T = 700°C.

2(b). The features took the form of small spheres. X-ray microprobe pictures for a surface such as that of Fig. 2(b) are shown in Fig. 3. They show the distributions of zinc, phosphorus and indium, respectively, on the same area of surface. There is a strong correspondence between zinc and phosphorus on the surface and a few circular areas which are rich in indium. These last features correspond to the spherical objects seen under the optical microscope. More careful examination of the circular areas shows that, although they are mostly indium, they also contain a core rich in zinc and phosphorus. The samples giving Class 2 profiles maintained the good polished appearance throughout the diffusion experiment and showed no marked surface features at the end of the diffusion. The microanalyser showed a uniform distribution of all three elements over the surfaces. In Fig. 4 is shown a series of profiles taken with approximately 1 mg of phosphorus added to the diffusion ampoule. These profiles can be fitted fairly well by error function complement curves, and show the characteristics of Class 2. The surface concentrations are fairly low and the surfaces of the specimens maintained a high quality throughout 10 21

tions causes a large reduction in surface concentration: further reductions in the amount of zinc in the ampoule give rise to corresponding reductions in surface concentration. Let curves 5-7 make up Class 2. The surface concentrations shown by the Class 1 profiles are very high indeed. They are of the order of 5 × 102~ cm-3: this should be compared to the figure of about 4 x l0 = cm 3 for the total concentration of indium and phosphorus atoms in InP! The shapes of curves 1 and 2 are similar and, in fact, are quite close to error function complement curves. Profiles 3 and 4, on the other hand, have points of inflection. The main difference between curves 1 and 2 and curves 3 and 4 is that the latter represent a longer diffusion time, and there is evidence to suggest that the point of inflection does start to appear on Class 1 profiles after a minimum diffusion time. Class 1 samples showed quite marked surface features. For short diffusion times, the features were rectangular in shape. They lined up in an orderly manner (see Fig. 2(a)) and, bearing in mind that the InP surface was (100), this suggests they were related to the crystal orientation of the semiconductor surface. For long diffusion times the surface appearance changed to that shown in Fig.

{

i

I 0 2C

IO 19

E o

.\-

Iol8

=8 N

\ IO L7

1°~6

/

I 2 . 0 x l O -5

[

I

4 . 0 x l O -5

6 . 0 x l O -5

x/,~,

8 . 0 x l 0 -5

cm. sec-/~

Fig. 4. Diffusion profiles of zinc in InP at 700°C using 1 mg of phosphorus in the ampoule and varying amounts of zinc. The curves are error function complements.

534

A. HOOPER, B. TUCK and A. J. BAKER

the experiment. It is interesting to compare profile 2 (Fig. l) with profile 8 (Fig. 4). They are both for 20 rain diffusions, with 1 mg of zinc in the ampoule. The difference is that for profile 8 the ampoule also contained 1 mg of phosphorus. It would appear, then, that the addition of this amount of phosphorus has the effect of changing the subsequent profile from Class l to Class 2. A further series of diffusions was carried out, therefore, all using 1 mg of zinc and weights of phosphorus varying between zero and l rag. The profiles are shown in Fig. 5. Once again it was found that the transition between Class 1 type profiles and Class 2 was discontinuous, a relatively small change in conditions between experiments 13 and 14 providing a large change in the measured profile. An approximate value was calculated for the total weight of zinc inside a specimen at the end of each experiment. This weight was compared with the weight put into the ampoule at the start of the experiment. The data is given in the Table and it demonstrates yet another difference between Class 1 and Class 2 profiles. For Class 1 a large fraction of the original zinc entered the specimens, whereas for Class 2 the fraction is much smaller. The junction-depth measurements gave similar 1022~

[

I

I

I

results for both types of profile. The concentration of zinc atoms at the junction was always greater than the concentration of acceptors. For most of the specimens the factor was in the range I0-I00. This can be compared to the work of Galvanov et al, [4] who, on the basis of electrical measurements, came to the conclusion that the true density of zinc atoms in heavily doped InP exceeds the hole density by one or two orders of magnitude. The result was confirmed by the Hall measurement carried out on the sample homogeneously diffused with zinc to a level of 7 × 10'~ cm 2. The hole concentration was measured at 2-5× 10~cm 3 a factor of about 30 less. In Fig. 6 are shown photographs of junction traces for a diffusion in which excess phosphorus is used and for one in which no phosphorus was used. It can be seen that the former gave a much flatter junction. A similar result was found by Nygren and Pearson [6] when diffusing zinc into GaP. It follows from this that the measurements of junction depth were more accurate for profiles 8-15 than for profiles 1-7. The comparisons of acceptor and zinc concentrations should therefore be more reliable for the former set and there is, in fact, less scatter in the results for profiles 8-15. 4. D I S C U S S I O N

4.1 Solubility and surface effects

1021

l02o

E o

d

~10 ~9

§ N 1018

po,7

v 0

0"5×10 -4

I-0 xlO -4

X/7~,

['Sx 1O-4

[ 2'Ox 10-4

cm .sec-~

Fig. 5. Diffusion profiles of zinc in InP at 700°C using I mg of zinc in the ampoule and varying amounts of phosphorus.

The most remarkable feature of the results is the very high surface concentrations achieved in the Class 1 profiles. Large surface concentrations must always be looked at carefully in this type of experiment because of the possibility of radio-tracer zinc condensing from the vapour onto the sample during cooling. This does occur, and if the count from the initial section is anomalously high, it is ignored (see, for instance, the curves of Fig. 4). The high surface concentrations of the Class I curves do not come into this category, however. The concentrations are obtained if points from deep inside the crystal are extrapolated to the surface. It seems, therefore, that the high solubilities are genuine and that zinc can exist in very high concentration in InP. When zinc is introduced to most III-V semiconductors, it is usually assumed that it occupies the group III site, acting as an acceptor. It is usually assumed, also, that most of the zinc acceptors are singly ionised so that the number of holes is approximately equal to the number of zinc atoms. The results of the previous section show that this is not the case for InP and, in view of the high zinc solubility, this is not at all surprising. The curves of Fig. l and Fig. 3 show that zinc can exist in InP to a

Fig. 2. Typical surfaces p r o d u c e d by the diffusion o f 2 m g zinc into n-type I n P at dissociation p h o s p h o r u s pressure. (a) T = 700°C, t : 10 rain. (b) T = 700°C, t : 250 min. Magnification x 375.

[Facing p. 534]

(a) Zinc (b) Phosphorus (c) Indium

Fig. 3. Elemental distribution o n the surface o f a diffused specimen, similar to that o f Fig. 2(b). T ~ 700°C a n d t = 250 rain. In each case the light areas c o r r e s p o n d to the n a m e d element. Magnification x 160.

Surface

Juncfion

Surface

Junction

Fig. 6. Effect o f adding excess phosphorus on the planarity ofp-n junctions formed by diffusion of zinc in n-type InP. (a) Zinc weight = 440/~g, phosphorus weight = 1 mg. (b) Zinc weight = 500#g, phosphorus weight == 0. T ~ 700°C, t = 40 rain, magnification x 590.

Diffusion in lnP level of 5 × 102t cm 3. If all of this zinc were to occupy indium sites, it would be necessary to replace one indium atom in four by a zinc atom. The results show that at high concentrations only a small fraction of the zinc atoms are electrically active (i.e. acting as shallow acceptors). These are presumably the atoms which do occupy indium sites. A second, inactive, state of the zinc also occurs. More work is required to determine the physical form of this state. The experiments also show that a large fraction of the zinc put into the ampoule at the start of the experiments became incorporated into the Class 1 profiles. The conditions inside the ampoule therefore must have been changing throughout an experiment and it is interesting that the surface concentration in the sample stayed high even though the amount of zinc external to the sample was falling. Figure 1 indicates that the value of the surface concentration is relatively insensitive to the amount of zinc originally put into the ampoule, providing it is more than about 600 p,g. A class 2 curve can be obtained rather than a Class 1 either by decreasing the amount of zinc below 600 ~ g or by adding phosphorus. Class 1 specimens show evidence of a liquid having formed on the surface during diffusion, and also an association of zinc and phosphorus on the surface. Class 2 specimens maintain the high quality surface originally prepared. In order to attempt to explain these results, it is necessary to consider the phase diagram for the l n / P / Z n system at 700°C. This has not yet been determined. Phase diagrams have been determined, however, for the Ga/As/Zn [7] and the Ga/P/Zn [8] systems. In view of the fact that these two systems show very similar features, it seems worth while considering the results of the present work on the assumption that the InP system shows the same qualitative features. Figure 7 shows a phase diagram for a III/V/Zn system. A diagram of this sort is followed by Ga/As/Zn over the temperature range 700-950°C and by Ga/P/Zn in the range 700-1040°C. The regions likely to be relevant to this work are labelled A, B, C, D on Fig. 7. If a system can be described by a point within region A, then it consists of solid Ill-V, a liquid given by the relevant point on the liquidus curve ac and the vapour which is in equilibrium with the liquid. There are thus three components (III, V, Zn) and three phases (solid, liquid, vapour), and the phase rule gives two degrees of freedom for the system. One of these is used up in choosing the temperature to be 700°C. There is one degree of freedom left, so the zinc vapour pressure can be varied by changing the ex-

535 Zn

a

m-2~

e~

Fig. 7. Ternary phase diagram for a III/V/Zn system. perimental conditions. Since the solubility of zinc is primarily fixed by the ambient zinc pressure, this means that the surface concentration can be varied. The same story is true for region D except that the liquid is now given by a point on the liquid de. If the system is within region B, four phases coexist. These are solid III-V, solid ZmV2, a liquid given by point c, and the vapour phase. The phase rule indicates only one degree of freedom in this case, and this is used up in setting the temperature. It follows that all points in region B have exactly the same phases: moving about within the region merely alters the relative amounts. All points in the region have the same vapour pressures of indium, phosphorus and zinc, and so all diffusions carried out within the region will have the same surface concentration of zinc. Region C resembles B in all respects except that the liquid phase is given by the point d rather than c. Based on Fig. 7, a tentative explanation for the diffusion results can be given. It must first be noted that a diagram such as Fig. 7 refers only to the liquid and solid phases, so that any material in the vapour is not included. Now group V elements are more volatile than group III, so if a pure piece of InP is heated, the system of Fig. 7 moves to the indium side of InP on the base-line, because more phosphorus than indium has entered the vapour phase. In this case, the system consists of solid lnP in equilibrium with liquid a. If a piece of zinc is added, the system is therefore described by a point just above InP on the diagram, and a little to the left. If the amount of zinc is sufficient, this puts the system in region B: such a point is marked X on

536

A. HOOPER, B. TUCK and A. J. BAKER

the diagram. It must be borne in mind that the weight of zinc used in these experiments is small compared to the weight of InP, so X must be rather close to the InP point. If the amount of zinc is varied, the vapour pressures inside the diffusion ampoule do not change at all, providing the point X remains within region B. Since the surface concentration of the diffusion profile is fixed by these external pressures, all such diffusions will have the same surface concentration. It is possible, therefore, that the relatively constant surface concentration of profiles I-4 is due to the experimental conditions not removing the system from region B. If the amount of zinc in the ampoule is sufficiently reduced, the boundary between regions A and B will be crossed so that the system is described by a point such as X'. The tie-fine passing through X ' will touch the liquidus at b, say. In crossing the boundary, therefore, the liquid phase is changed from zinc-rich (point c) to indium-rich (point b). It is reasonable to assume that a corresponding reduction in zinc vapour pressure would then take place, leading to a discontinuous drop in the surface concentration of zinc in the sample. The change which occurs between profiles 4 and 5 might well be due to this cause. A very similar effect has been reported when diffusing zinc into GaAsg: a discontinuous change in the zinc surface concentration occurred for a small change in diffusion conditions. This could be directly related to the system changing from one region of the Ga/As/Zn phase diagram to another. A very similar explanation can be used to account for the fact that adding phosphorus to the tube also brings about the change from group 1 profiles to group 2. In this case the phosphorus sends the system from the monovariant region B to region D (passing briefly through C), and the liquid phase changes from a composition given by the point c to one on the liquidus de. Again a zinc-rich liquid is exchanged for a zinc-deficient one, and the zinc vapour pressure in the ampoule is reduced. This might explain the discontinuity shown in Fig. 5. The X-ray microprobe results suggested that two different phases had formed on the surface of the group I specimens. Small spheres containing all three elements were found, and distributed over the rest of the surface was a phase showing a positive correlation between zinc and phosphorus (Fig. 3). These effects both fit in with the ideas expressed above: the first could be liquid c at diffusion temperature, and the zinc-phosphorus phase could be the Zn~P~, which is predicted.

It remains to explain why the Class 2 profiles were associated with good surfaces, i.e. why no trace of liquid ac was found on the surface after diffusions 5, 6, 7 and why no trace of liquid de was found after diffusions 8-11, 14 and 15. The two points are best considered separately, but the situation can be compared in each case with the wellinvestigated Ga]As/Zn system. When zinc is diffused into GaAs with no excess arsenic used, the system can be described by a point on the gallium side of the phase diagram because more arsenic goes into the vapour phase than gallium. In this case, also, no trace of liquid is normally seen on the surface after diffusion. This might be due to one of two reasons. First, the liquid might form somewhere other than on the specimen surface. It is also possible, however, that the amount of liquid formed is so small that it passes unobserved. Data is available to calculate the weight of Ga/As/Zn liquid formed for a given set of experimental conditions:[10] it can be shown that for a wide range of conditions the amount is very small. For experiments in which a group V element is added to the ampoule, a different effect becomes important. For the Ga]As/Zn system the liquid becomes arsenic-rich (i.e. liquidus de in Fig. 7) and the equilibrium vapour pressure of arsenic over such a liquid can be very high. If the amounts of zinc and arsenic in the tube are small then there is insufficient material to create a high vapour pressure and the liquid cannot form. The number of phases is reduced to two and all of the added zinc and arsenic goes into the vapour phase. Since no liquid forms, the high quality surface of the specimen is maintained throughout the diffusion. This argument has been presented in a quantitative manner for the GaAs system in a previous paper[l I], Both of these explanations for Ga/As/Zn should carry over to the In/P/Zn system. A liquid must form in the ampoule for diffusions on the indiumrich side of the phase diagram, but it seems likely that the amount might be very small. It is most unlikely that a phosphorus-rich liquid would be able to form in the diffusion ampoule and maintain the high phosphorus pressure that such a liquid would require. This last argument implies that further useful information can be extracted from profiles 8-1 I. It is probable that all of the zinc and phosphorus added to the diffusion ampoule went into the vapour phase for these experiments. The vapour pressures of these two elements are therefore known and profiles 8-11 represent a series of experiments in which the phosphorus pressure was kept constant

Diffusion in InP and the zinc pressure was varied. A graph can therefore be plotted of solubility of zinc in InP (i.e. surface concentration) against external zinc vapour pressure, and this is done in Fig. 8(a). At low pressures the zinc surface concentration increases super-linearly with pressure, but at high zinc pressures, the dependance becomes sublinear. This change is probably associated with an increasing proportion of the zinc atoms entering the crystal in the non-active form. 4.2 Diffusion Once again, it is the values of diffusion coefficient for profiles 8-11 which are most useful, since the vapour pressures are known for these experiments. Profiles 8, 9 and l0 fit error function complement curves quite well, so a single value of diffusion coefficient, D, can be fitted to each. The fact that an error function complement curve can be fitted to profile 8 implies that the coefficient varies only slightly over the range for which the profile is plotted, i.e. 10t8 cm-3-102° cm -3. The same value of D should be given by profiles 9 and 10, therefore, and this is approximately the case, as shown in Fig. 8(b): all three give a value of about 3 × 10-'° cm 2 sec '. Profile 11 lies outside this range. It does not give such a good fit to an error function complement, but the best one gives a value of D which is much lower than that for the other three curves. This value is also plotted on Fig. 8(b). Figure 8 indicates that the diffusion coefficient varies quite sharply for low concentrations but saturates between 1 0 t9 c m 3 and 102ocm '. The saturation effect may well be due to an increasing proportion of the dissolved zinc entering in the inactive form. The Class 1 profiles are interesting because although the surface concentrations are about the

537

same, the profiles are quite different in shape. Such a feature suggests a non-equilibrium behaviour within the crystal which does not occur in Class 2 diffusions. Curves of the type 1--4 (Fig. 1) are quite reproducible and it seems likely that the basic difference in shape between curves I and 2, on the one hand, and curves 3 and 4, on the other, is due to the difference in diffusion times. The behaviour can again be compared to that of zinc diffusing in GaAs. Here, also, diffusion profiles containing points of inflection are found. It has been shown that the shape of these curves is time-dependant, and the effect has been explained[12] in terms of the vacancy concentration within the semiconductor varying with time. Presumably the occurrence of the effect in Class 1 diffusions is associated with the high zinc concentration in those experiments. 5. CONCLUSIONS (a) Diffusion profiles of zinc in InP have been plotted and observations made of the surfaces of the samples after diffusion. Discontinuities are found in the results in the sense that small variations in diffusion conditions can give rise to large changes in the profile. A comparison of the results with the cases of zinc diffusion in GaAs and GaP suggests that the discontinuities are due to the system moving from one region of the ternary phase diagram to another. (b) Comparison of the profiles with junction-depth measurements and electrical measurements indicates that at high zinc concentrations the zinc density is a factor of about 30 higher than the acceptor concentration. The zinc therefore exists in the semiconductor in an inactive form as well as in an active form. (c) Profiles taken with excess phosphorus in the tube suggest that for this condition the diffusion coefficient increases with T ed

a)

y

t02o

% d IO Ig

8o

E

(b)

o x~

10-9

a

10-~

10-2 Zinc

pressure Pzn,

I0 -I atmos

I0 3

10-2 Zinc

pressure

i,O-i P~,

atmos

Fig. 8. (a) Surface concentration against zinc vapour pressure for profiles 8-11. (b) Diffusion coefficient against zinc vapour pressure for profiles 8-11.

538

A. HOOPER, B. TUCK and A. J. BAKER

zinc c o n c e n t r a t i o n b e l o w a b o u t 1019c m 3 b u t satur a t e s a b o v e this value. (d) It is s u g g e s t e d t h a t a n o n e q u i l i b r i u m p r o c e s s affects the diffusion m e c h a n ism d u r i n g Class 1 diffusions. Acknowledgements--We should like to thank Thorn Lighting Ltd. for financial support of the work, and also R. Hall, K. Brown and D. Norman of Thorn for a number of valuable discussions. A. Hooper is supported by an S.R.C. research studentship. REFERENCES

I. L. L. Chang and H. C. Casey, Solid-St. Electron. 7, 481 (1964). 2. R. M. Kundukhov, S. G. Metreveli and N. V. Siukaev, Soy. Phys. Semiconductors, 1,765 (1967).

3. K. Weiser, R. S. Levitt, M. I. Nathan, G. Burns and J. Woodall, Trans. Metall. Soc. AIME, 230, 271 (1964). 4. V. V. Galvanov, S. G. Metreveli, N. V. Siukaev and S. P. Starosel'tseva, Soy. Phys. Semiconductors, 3, 94 (1969). 5. L. J. Van Der Pauw, Phil. Res. Rep. 13, l, (1958). 6. S. F. Nyg~en and G. L. Pearson, J. Electrochem. Soc. 116, 648 (1969). 7. M. B. Panish, J. Phys. Chem. Solids 27, 291 (1966). 8. M. B. Panish, J. Electrochern. Soc. 113, 224 (1966). 9. K. K. Shih, J. W. Allen and G. L. Pearson, J. Phys. Chem. Solids 29, 379 (1968). 10. K. K. Shih, J. W. Allen and G. L. Pearson, J. Phys. Chem. Solids 29, 367 (1968). 11. M. A. H. Kadhim and B. Tuck, J. Mat. Sci. 7, 68 (1972). 12. B. Tuck and M. A. H. Kadhim, J. Mat. Sci. 7, 585 (1972).