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Chemical etchants for InAs
NOTES
728
complete information on the behaviour, the real part of w giving the frequency of the transients and the imaginary part giving the exponential growth or decay factor of the envelope. If the method is applied at nodes A and B using the admittance function of expression (2) exactly the same result is obtained as if it is applied at terminals a and b using the impedance function of expression (1). In both cases the equation for determining complex w is Y(w)Z(w)+l
= 0.
Hence the frequency and envelope growth or decay characteristics of the “parallel circuit mode” and the “series circuit mode” are identical and it is not justifiable to distinguish them as separate modes. In particular it is known that the cut-off frequency of the diode is an upper frequency cut-off and this must apply to the “parallel circuit mode” also. The concept of a “conductive lower cut-off frequency” advanced by the authors for the “parallel circuit mode” is therefore invalid for the equivalent circuit analysed. G. H. B. THOMPSON Dept. of Physics University of Oxford Oxford, England
GaAs, have been extensively studied by many workers.(aJ) In the case of InAs, in spite of many references of the chemical etchants available for cleaning and preferential etching, no systematic study has been undertaken. FAUST and SAGAR(~) reported that the etchant 1HNOs : 1HCl is used for polishing, but it was observed that it did not polish the { 111) faces of InAs. As in the case of GaAs, In rich faces of InAs are always the most difficult to polish chemically. Here suitable chemical etchants, for polishing, revealing dislocations on (111) faces and p-n junction delineation in InAs, are discussed. InAs samples for etching studies were prepared by cutting monocrystal in the direction perpendicular to (111). Both sides of the samples were ground and lapped with hand on thick glass plate with Alundum (Al2O3) abrasive (5~ particle size) and distilled water. The samples thus prepared were plane parallel to +2~. The n-type InAs crystals used were boat grown and Czochralski pulled type. The identification of (111) and (iii) faces was done by using the etching technique described by WAREKOIS and R/IETZGER.c4)In the case of the etchant bromine in methyl alcohol (No. 2, Table I), the following two methods were used for polishing the identified { 11 l} faces.
References
3-
1. S. V. JASKOLSKI and T. KORYU ISHII,Solid-State Electron. 8, 869 (1965). 2. H. W. BODE, Network Analysis and Feedback Amplifier Design, D. van Nostrand, New York (1945). .E -:_ Solid-State Electronics Pergamon Press 1966. pp. 728-729. Printed in Great Britain
Chemical
Vol. 9,
etchants for InAs
(Received 3 December 1965) BECAUSE of the increasing interest in using the (111) surfaces of III-V compounds for making semiconductor devices(l), the polishing, removing thin diffusion layers and revealing dislocations of these faces are of great importance. The etchants for one of the most widely used III-V compound,
2-
S c 5 Li
Ii_@5
9/, Braby
1
volume
FIG. 1. Etching rates of InAs as a function of bromine in methyl alcohol.
729
NOTES
Table 1. Chemical etchants for InAs
No.
Etchants
1. Cont. HCl
2.
3. 4. 5.
6.
7.
8. 9.
Principal uses and comments
(a) Preferential etching, etching rate 5 mg/ cm2/min at 75’C* (well defined pyramidal pits).
(b) Cleaning InAs (etching time 12-l 5 sec.) CHsOH : (0.4-l % Polishing { 111) faces i.e. (111) and (ITT) faces, by volume) Brs etching rate l-2.5 p/min.* Etching 40-120 sec. 1HNOs : 1HAc: Etching 30-60 sec. 1HsOz : lHAc1 (a) Identification of (JJi) 25HNOs : 15 HF and (111) surfaces, : 15HAc : 0.3 Brs etch pits on In side. 8 (CP-4) (a) Low surface recombination velocity. (a) p-n junction delinea1HF : 3HNOs tion (etching time : 2H20 l-3 min). (b) dislocations on (111) and (TIT) faces. Good etchant for clean1HNOs : 5&Oa ing etching time l-2 min at 75°C (increasing the HzOa content dislocations on In sides). dislocations on In sides, 1HNOs : 1HsOz : 6 tartaric acid etching time 3-5 min. AgNOs as 5 y0 solution, IHNOs : 2HF : rinse in 1HNOs : lHz0, 2AgNOs preferential etching.
Refs.
(5,6)
t (7) (7) t4)
t
t
(3) (3)
* Approximate etching rate. t Studied by author. $ HAc means glacial acetic acid. 0 In side means (111) surface.
(a) The lapped samples were etched in freshly prepared solutions having 04-l per cent by volume bromine in methyl alcohol. Figure 1 shows the etching rates of InAs as a function of bromine in methyl alcohol. For light etching concentrations of this order of bromine were best. The samples were turned over during the process of etching so that each side spent equal times in contact with the bottom of the vessel.
(b) In this method the lapped samples were polished by lapping them by hand on a glass plate covered with a sheet of fine cloth soaked with the bromine in methyl alcohol solution. Concentrations up to few per cent of bromine (5-10 per cent by volume) were found to be very effective. Apart from high polish and no edge rounding up, this method prevented the occasional formation of etch pits. This method is also useful for polishing the particular surface before junction delineation. In the above methods, the samples after polishing were rinsed with methyl alcohol to remove the etchant. For p-n junction delineation studies, the n-type InAs samples diffused with Zn were used. Before chemical etching for junction delineation the surface was polished by the second method discussed above. The surface of the samples were then etched in a solution consisting of 1 part HF, 3 parts HNOs and 2 parts distilled water, for l-3 min. The surface was never allowed to be in partial contact with air during etching and was immediately rinsed with water. By this etchant (No. 6, Table 1) it was possible to obtain sharp delineation of the p-n junctions. The behaviour of other chemical etchants given in Table 1 were re-examined with the view of conducting a systematic and comparative study of the various etchants for InAs. Acknowledgements-The author is thankful to Prof. B. I. BOLTAKS, Institute of Semiconductor, Leningrad for his encouragement and to Mr. S. I. REMBEZAfor his help.
Solid State Physics Laboratory Lucknow Road Delhi-7, India
B.
L. SHARMA
References 1. G. DE MARS, Semicond. Prod. 2, 24 (1959). 2. M. V. SULLIVAN and G. A. KOLB, J. Electrochem. Sot. 110, 585 (1963). 3. J. W. FAUST, JR., Compound Semiconductors, Vol. 1, p. 455. (Ed. R. K. WILLARDSONet al.), Reinhold, New York (1962). 4. E. P. WAREKOIS and P. H. METZGER, J. appl. Phys. 30,960 (1959). 5. J. W. FAUST JR., and A. SAGAR, J. appl. Phys. 31, 331 (1960). 6. L. BERNSTEIN,J. Electrochem. Sot. 109, 270 (1962). 7. N. A. GORYIJNOVAand S. I. RA~AUTSAN,Dokl. Akad. Nauk SSSR, 121,848 (1958).
728
complete information on the behaviour, the real part of w giving the frequency of the transients and the imaginary part giving the exponential growth or decay factor of the envelope. If the method is applied at nodes A and B using the admittance function of expression (2) exactly the same result is obtained as if it is applied at terminals a and b using the impedance function of expression (1). In both cases the equation for determining complex w is Y(w)Z(w)+l
= 0.
Hence the frequency and envelope growth or decay characteristics of the “parallel circuit mode” and the “series circuit mode” are identical and it is not justifiable to distinguish them as separate modes. In particular it is known that the cut-off frequency of the diode is an upper frequency cut-off and this must apply to the “parallel circuit mode” also. The concept of a “conductive lower cut-off frequency” advanced by the authors for the “parallel circuit mode” is therefore invalid for the equivalent circuit analysed. G. H. B. THOMPSON Dept. of Physics University of Oxford Oxford, England
GaAs, have been extensively studied by many workers.(aJ) In the case of InAs, in spite of many references of the chemical etchants available for cleaning and preferential etching, no systematic study has been undertaken. FAUST and SAGAR(~) reported that the etchant 1HNOs : 1HCl is used for polishing, but it was observed that it did not polish the { 111) faces of InAs. As in the case of GaAs, In rich faces of InAs are always the most difficult to polish chemically. Here suitable chemical etchants, for polishing, revealing dislocations on (111) faces and p-n junction delineation in InAs, are discussed. InAs samples for etching studies were prepared by cutting monocrystal in the direction perpendicular to (111). Both sides of the samples were ground and lapped with hand on thick glass plate with Alundum (Al2O3) abrasive (5~ particle size) and distilled water. The samples thus prepared were plane parallel to +2~. The n-type InAs crystals used were boat grown and Czochralski pulled type. The identification of (111) and (iii) faces was done by using the etching technique described by WAREKOIS and R/IETZGER.c4)In the case of the etchant bromine in methyl alcohol (No. 2, Table I), the following two methods were used for polishing the identified { 11 l} faces.
References
3-
1. S. V. JASKOLSKI and T. KORYU ISHII,Solid-State Electron. 8, 869 (1965). 2. H. W. BODE, Network Analysis and Feedback Amplifier Design, D. van Nostrand, New York (1945). .E -:_ Solid-State Electronics Pergamon Press 1966. pp. 728-729. Printed in Great Britain
Chemical
Vol. 9,
etchants for InAs
(Received 3 December 1965) BECAUSE of the increasing interest in using the (111) surfaces of III-V compounds for making semiconductor devices(l), the polishing, removing thin diffusion layers and revealing dislocations of these faces are of great importance. The etchants for one of the most widely used III-V compound,
2-
S c 5 Li
Ii_@5
9/, Braby
1
volume
FIG. 1. Etching rates of InAs as a function of bromine in methyl alcohol.
729
NOTES
Table 1. Chemical etchants for InAs
No.
Etchants
1. Cont. HCl
2.
3. 4. 5.
6.
7.
8. 9.
Principal uses and comments
(a) Preferential etching, etching rate 5 mg/ cm2/min at 75’C* (well defined pyramidal pits).
(b) Cleaning InAs (etching time 12-l 5 sec.) CHsOH : (0.4-l % Polishing { 111) faces i.e. (111) and (ITT) faces, by volume) Brs etching rate l-2.5 p/min.* Etching 40-120 sec. 1HNOs : 1HAc: Etching 30-60 sec. 1HsOz : lHAc1 (a) Identification of (JJi) 25HNOs : 15 HF and (111) surfaces, : 15HAc : 0.3 Brs etch pits on In side. 8 (CP-4) (a) Low surface recombination velocity. (a) p-n junction delinea1HF : 3HNOs tion (etching time : 2H20 l-3 min). (b) dislocations on (111) and (TIT) faces. Good etchant for clean1HNOs : 5&Oa ing etching time l-2 min at 75°C (increasing the HzOa content dislocations on In sides). dislocations on In sides, 1HNOs : 1HsOz : 6 tartaric acid etching time 3-5 min. AgNOs as 5 y0 solution, IHNOs : 2HF : rinse in 1HNOs : lHz0, 2AgNOs preferential etching.
Refs.
(5,6)
t (7) (7) t4)
t
t
(3) (3)
* Approximate etching rate. t Studied by author. $ HAc means glacial acetic acid. 0 In side means (111) surface.
(a) The lapped samples were etched in freshly prepared solutions having 04-l per cent by volume bromine in methyl alcohol. Figure 1 shows the etching rates of InAs as a function of bromine in methyl alcohol. For light etching concentrations of this order of bromine were best. The samples were turned over during the process of etching so that each side spent equal times in contact with the bottom of the vessel.
(b) In this method the lapped samples were polished by lapping them by hand on a glass plate covered with a sheet of fine cloth soaked with the bromine in methyl alcohol solution. Concentrations up to few per cent of bromine (5-10 per cent by volume) were found to be very effective. Apart from high polish and no edge rounding up, this method prevented the occasional formation of etch pits. This method is also useful for polishing the particular surface before junction delineation. In the above methods, the samples after polishing were rinsed with methyl alcohol to remove the etchant. For p-n junction delineation studies, the n-type InAs samples diffused with Zn were used. Before chemical etching for junction delineation the surface was polished by the second method discussed above. The surface of the samples were then etched in a solution consisting of 1 part HF, 3 parts HNOs and 2 parts distilled water, for l-3 min. The surface was never allowed to be in partial contact with air during etching and was immediately rinsed with water. By this etchant (No. 6, Table 1) it was possible to obtain sharp delineation of the p-n junctions. The behaviour of other chemical etchants given in Table 1 were re-examined with the view of conducting a systematic and comparative study of the various etchants for InAs. Acknowledgements-The author is thankful to Prof. B. I. BOLTAKS, Institute of Semiconductor, Leningrad for his encouragement and to Mr. S. I. REMBEZAfor his help.
Solid State Physics Laboratory Lucknow Road Delhi-7, India
B.
L. SHARMA
References 1. G. DE MARS, Semicond. Prod. 2, 24 (1959). 2. M. V. SULLIVAN and G. A. KOLB, J. Electrochem. Sot. 110, 585 (1963). 3. J. W. FAUST, JR., Compound Semiconductors, Vol. 1, p. 455. (Ed. R. K. WILLARDSONet al.), Reinhold, New York (1962). 4. E. P. WAREKOIS and P. H. METZGER, J. appl. Phys. 30,960 (1959). 5. J. W. FAUST JR., and A. SAGAR, J. appl. Phys. 31, 331 (1960). 6. L. BERNSTEIN,J. Electrochem. Sot. 109, 270 (1962). 7. N. A. GORYIJNOVAand S. I. RA~AUTSAN,Dokl. Akad. Nauk SSSR, 121,848 (1958).