Etching behaviour at n-type indium antimonide electrodes

Electroanalytical Chemistry and Interfacial Electrochemistry, 42 (1973) 105-110

105

@? Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands

E T C H I N G BEHAVIOUR AT n-TYPE I N D I U M A N T I M O N I D E ELECTRODES

A. A. ISMAIL and T. M. SALEM

National Research Centre, Dokki, Cairo (V.A.R.) (Received 8th May 1972; in revised form 4th August 1972)

INTRODUCTION

Indium antimonide as an intermetallic compound III-V is a zinc blend type of structure of two types of [III] surfaces, i.e. one terminating with group III atoms (A surface) and the second terminating with group V atoms (B surface). This type of orientation of the crystal shows marked differences in the physical and chemical properties of these surfaces. It has been found that the B surfaces of InSb are far more reactive than the A surfaces to oxidizing (electrophilic) agents 1. This difference in reactivity has been shown to be responsible for the fact that dislocations and etch pits appear on A surfaces and not on B surfaces. Etching plays a vital role in the preparation and characterization of crystals. It helps in cleaning surfaces and in revealing dislocations and other imperfections which affect the strength and electrical properties of the crystals. This study was undertaken for the purpose of elucidating the physical and chemical processes that determine the etching behaviour of the [III] surfaces of the indium antimonide single crystal. Two types of etchants were employed: preferential etchants leading to the formation of etch figures and a fast non-preferential etchant resulting in the formation of dislocation etch pits on an otherwise chemically polished surface. The systems HF, HNO3, H 2 0 and Fe 3 + ions in HC1 were chosen for this study. EXPERIMENTAL

Single crystals of indium antimonide prepared by Dr. Parker (Texas Instruments Incorporated, Research Building, Dallas, Texas) were used in this investigatior~. Thin rectangular slabs 2 were cut from single crystal InSb parallel to [111] faces with a tungsten wire saw. The dimensions of the slab were 20 x 10 x 0.5 mm. The crystals were oriented by X-ray techniques before cutting, so that the two large faces of the slabs corresponded to the [111] of the A and B structures. A coat of polystyrene, applied in toluene solution to a lapped--but not polished surface, seemed to give a completely adequate protection. The coating was generally applied over the edges of the face under study in an attempt to minimize edge effect, since there were almost certainly edges or steps present in the initial surface proper. The slabs were etched one at a time, in 10 cm 3 of a solution in a small polyethylene beaker. Agitation was provided by an electric stirring motor equipped

106

A. A. ISMA1L, T. M. SALEM

with a polyethylene paddle. The reaction was quenched at the appropriate time with a large volume of water. The slabs were then rinsed in distilled water, dried and weighed. All slabs were etched three times, and each etch was performed in a fresh portion of the same solution. The kinetics of the etching system were studied as a function of the composition of the etchant at a constant initial temperature (20°C). Etching rates were obtained by measuring the loss of weight of a sample during a measured time of etching. Weights were determined to 0.1 mg and the etching period was selected to remove an appropriate weight from the specimen. The reagents used in this study were the normally available concentrated acids, i.e. 70% H N O 3 and 40% HF. (A) THE SYSTEM HF, HNO3, H 2 0

Data relating to the amount dissolved of the single crystal indium antimonide (III) of structures A and B were plotted as a function of the time of immersion in a wide composition range (10%-100% of an acid) of the system HF, HNO3 and H20. Figures 1 and 2 represent the rate of etching as a function of the amount of acids and water added. It can be seen that the rate of etching remains constant for quite a long period and also that the A planes develop in each case they being less reactive than the B planes. The shapes of the curves are not changed whatever the amount of water added. To the left of the maximum the etch rates are dependent on the hydrofluoric acid concentration and the amount of added water. A fresh solution of concentrated nitric acid does not etch indium antimonide chemically at an ap1

400-500" ~

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0,HNO3 20 100 80 Fig. 1. Rate of etching

40 6'0 Vol/ ml vs.

60 4'0

80 HF100 2'0

soln. composition for InSb(III). Water content: (1) 0, (2) 15, (3) 25, (4) 50%

107

ETCHING BEHAVIOUR AT n-TYPE In-Sb

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100-

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70

60

50

40

30

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Vol / ml Fig. 2. Rate of etching vs. soln. composition for InSb(iii). Water content: (1) 0, (2) 15, (3) 25, (4) 50°/£.

preciable rate. Nitric acid is initially present in a considerable excess but on decreasing the amounts of that acid the rate is increased. Etching rates increase with increased concentrations of H F up to a m a x i m u m at 25% H F ; the etching rate then decreases with further increase of H F concentration. The only difference observed between structures A and B in these curves is an increase of the etch rate in the case of the antimony surface at all concentrations. Gatos and Lavine 1 came to this conclusion when investigating the etch rate of indium antimonide. The average current density between local anode and cathode areas during chemical etching can be estimated from the rate of etching 2. Assuming that the surface while etching is half anode and half cathode and the indium antimonide in both faces goes into solution with a valency of 3, the average corrosion current density in A cm -2 is given by: i=2Aed where A is the etch rate in cm s -1, e is the electrochemical equivalent in C g - 1 and d is the density in g c m - 3 . The factor 2 arises from the assumption that during uniform chemical etching, the surface is on the average half anode and half cathode. F r o m Table 1, it can be seen that the m a x i m u m corrosion current is for structure B and that the corrosion current decreases with increase of the water content of the solution. The m a x i m u m rate of etching occurs when the ratio of H F to H N O 3 in the solution is 1:3 Figs. 1 and 2.

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A. A. ISMAIL, T. M. SALEM

TABLE 1 Composition

(1) (2) (3) (4)

2.5 2.5 2.5 2.5

HF+7.5 HF+7.5 HF+7.5 HF+7.5

HNO 3 H N O 3 + 1.5 H20 HNO3+2.5 HzO H N O 3 + 5 H20

Rate o f etching o f lnSb structure/ mg cm- 2 s - 1

Corrosion current densi~y/A cm 2

A

B

A

B

8.3 7 2.5 0.83

11.5 7.5 3.8 1

21.1 17.5 6.5 1.9

26.32 18 9.2 2.4

The action of nitric acid on the indium face is slower, and In 3 + is formed with evolution of nitric oxide. This continues until a maximum is reached at a concentration of 75% of nitric acid. On increasing the HF, the acid attacks the indium nitrate in the solution forming indium fluoride. The latter in the presence of water gives insoluble indium hydroxide. Simultaneously, the etch rate decreases. However, the change in the reaction rate depends on the proportion of HF to HNO3 at a given constant level of added water. I n + H N O 3 + 3 HF --> N O + 2 H 2 0 + I n F 3 If a semiconductor such as InSb with an antimony facet is chemically etched in H N O 3 - H F acid mixture, antimony in the presence of a high percentage of nitric acid (75%) dissolves with the formation of the trioxide 3. The oxide is attacked by HF forming the soluble salt SbF 3. This probably continues until a maximum is reached. On increasing the HF concentration, the fluoride salts function as a Lewis base forming (SbF4)-, and a decrease in the etching rate is observed. The

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vs.

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Fig. 4. Rate of etching

vs.

concn, of HC1. ( Q ) InSb(III), (©) InSb(iii}.

8

1'0

1'2

109

E T C H I N G BEHAVIOUR AT n-TYPE In Sb

reaction at the anode sites may be represented by: Sb+HNO3+3

HF ~ SbF3+NO+2

H20

(B) THE SYSTEM Fe 3+ AND HC1

The relation between the a m o u n t dissolved of the single crystals of structures A and B to the time of immersion is linear in a wide composition range of Fe 3 ÷ in 7 M HC1 acid. The rate of etching (slope) remains constant over a long period of time. This behaviour was observed for both structure faces of the indium antimonide. Figure 3 shows the change of etching rate with concentration of Fe 3 ÷ for a constant amount of hydrochloric acid for the two structures A and B of InSb. The rate of etching changes linearly with increasing concentration of Fe 3 + ions.

Fig. 5. Single crystal InSb; structure A. 0.5 N Fe 3+ + 7 M HC1, 30 rain. Fig. 6. Single crystal InSb; structure B. 0.5 N Fe 3+ + 7 M HC1, 30 rain.

Fig. 7. Single crystal InSb; structue A. H F + H N O 3 + A c O H ,

10 s.

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A. A. ISMAIL, T. M. SALEM

F i g u r e 4 is a plot of the etch rate as a function of the concentration of hydrochloric acid for a constant amount of Fe 3 + ions. The curves show a maximum etching rate at a concentration of HC1 7.8 M for the two structures and it can be seen that the rate of etching is greater in the case of structure B. To confirm these results, the etch patterns of the two types of tetrahedron are shown in Figs. 5 and 6 using a solution of 0.5 N Fe 3 ÷ and 7 M HC1 as etchant for various times (10, 20, 30, 60 min) at 25°C. Dislocation etch pits develop on the A surface of the single crystal indium antimonide electrode (Fig. 5) and also in time the etch pits grow and merge into each other. At B surfaces, the etchants act preferentially but do not reveal dislocation etch pits (Fig. 6). Figure 7 is obtained for structure A using the etchant H F + 2 H N O 3 + 1 AcOH for 10 s at 25°C when dislocation etch pits occurred. SUMMARY

Single crystal indium antimonide surfaces have beeri etched by two types of etchants: preferential etchants and a fast non-preferential etchant. The behaviour of the two facets have been examined and the formation of dislocation etch pits considered. Dislocation etch pits are developed on surface A (III) of the single crystal indium antimonide electrode while at surface B (ill) etchants acting preferentially do not reveal dislocations. The rates of etching and the average corrosion current density for structures A and B are calculated. The etch patterns of the two facets of the single crystal are also given. REFERENCES 1 H. C. Gatos and M. C. Lavine, J. Electrochem. Soc., 107 (1960) 433. 2 D. R. Turner, J. Electrochem. Soe., 107 (1960) 810. 3 F. Cotton and G. Wilkinson, Advanced Inorganic Chemistry, Wiley, New York, 1962, pp. 377, 378, 382.