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Striation etching of undoped, semi-insulating lec-grown GaAs
459
Journal of Crystal Growth 57(1982)459—461 North-Holland Publishing Company
LE’VrER TO THE EDITORS STRIATION ETCHING OF UNDOPED, SEMI-INSULATING LEC-GROWN GaAs Shintaro MIYAZAWA Musashino Electrical Communication Laboratory, Nippon Telegraph and Telephone Public Corporation, Pvfusashuio-shi, Tokyo 180. Japan
Received 16 September 1981~manuscript received in final form 4 November 1981
Growth striations in a LEC-grown, <100) oriented, undoped, semi-insulating GaAs bulk crystal were succesfully revealed on an as-cleaved (110) face by successive etching with 3 HNO 3 + 4 1120 + HF solution and AB-etchant. The solid—liquid interface shape was determined by striation etching.
The Liquid Encapsulation Czochralski (LEC) grown, undoped, semi-insulating GaAs substrates, which can be ion implanted directly, are of considerable interest in FET technology [1,2]. The GaAs IC fabrication requires high doping uniformity of the implant. This uniformity is, however, strongly affected by crystal quality or inhomogeneity in the substrate. The crystal growth method causes inhomogeneities, such as impurity striations, due to temperature fluctuation during pulling. Since growth striations represent a useful built-in record of the solid—liquid interface shape at any point on the crystal, they are widely employed in studying defects and inhomogeneities related to the interface shape. Presently, there are no observations on growth striations in undoped or Cr-doped semi-insulating GaAs bulk grown by LEC. This is thought to be one of the obstacles in characterizing the crystal and evaluating doping uniformity after ion implantation. This letter reports, for the first time, a chemical etching process for evaluating growth striations in an LEC-grown, undoped, semi-insulating GaAs bulk crystal. An appropriately 375 p.m thick (100) wafer was prepared from the LEC-grown, <100) oriented GaAs boule. The wafer was taken from the upper part of the boule, near the so-called crystal shoulder part. The Cr concentration in the wafer was not detected with a spark-source mass spectrometer 0022-0248/82/0000—0000/$02.75
©
(SSMS), because of the detection limit (0.1 wt ppm). The electrical resistivity at room temperature was greater than 106 ~ cm. Growth striations were succesfully revealed by a chemical etching process as follows. The (110) face was cleaved from the wafer. The as-cleaved face was etched, first, for about 2 mm in a solution consisting of HNO3, H20 and HF (first etchant). A typical mixing ratio was 3 :4: 1 by volume. After etching, the specimen was well rinsed with deionized water and was subsequently etched with the well-known AB-etchant [3] (second etchant) for 5—10 mm. Both etchings were carried out at room temperature. Since the (110) face was freshly as-cleaved and unpolished, any artifacts were not introduced. Figs. la—ic show etched features at the central, middle and two-thirds points of the cleaved (110) face, respectively, taken under a differential optical microscope. Roughly spaced striations running diagonally across the cleaved face are clearly observed. The irregular spacing for the striations was roughly estimated to be about 10 to 60 p.m. In this etching process, the pre-etching with the first etchant was subtle. Neither the first etchant nor the second etchant did reveal striations. However, they delineated crystal defects, such as dislocalions. An interesting feature was that growth striations were not revealed on high dislocation density areas, as shown in fig. lc. Etched roughness due to dislocations blanks out a weakly etched pattern of
1982 North-Holland
460
S.
.t!ii ~i:awa
/
.S!r,atuui etclung ~I undoped, semi-insulating LEC-grown GaAs
_____ _____
______________________________________________ —.
[a
striations. At the crystal periphery, the dislocation density increases considerably, which has been theoretically analyzed by Jordan et al. [4]. Growth striation; were not distinguished at the crystal Since striations give a solid—liquid interface shape, the growing interface shape of the crystal examined is concluded to be strongly concave toward the melt. By observing the whole area of the etched (110) cleaved face, the solid—liquid interface shape can be given as shown in fig. ld. At the mid-point (b), the angle between the pulling <100) axis and the striation;, i.e. solid—liquid in-
[b
-
HNO3
~
to be about 72°, concave The growth striations as well as the solid—liquid interface shape may affect the electrical properties and the FET performances on the (100) face, because unintentionally doped or residual impurities are located along the striations [5]. No traces of growth striation; were detected after double etching on a mechano—chemically polished (100) face, perpendicular to the (110) face examined. However, irregularities in FET performances fabricated on the Si ion implanted (100) face were clearly observed, related to the growth striation; and the solid—liquid interface. The results will be published in a separate paper [6]. In conclusion, the successive etching with 3 ~
+ 4H2O + HF solution and AB-etchant could reveal growth striation;, for the first time, on a freshly cleaved (110) face of a LEC-grown, undoped, semi-insulating GaAs bulk crystal. The solid—liquid interface shape during pulling was then determined to be concave toward the melt in
c (100)
)(
J \~
this experiment. The author would like to express his thanks to Dr. M. Ohmori for his encouragement, and also to Drs. S. Akai and K. Tada, Sumitomo Electric Industries Ltd., for fruitful discussions.
Fig. I. (a)—(c) Growth striations revealed on a (110) cleaved
d
c b
a
face. (d) Sketch indicating the solid—liquid interface shape, where the arrows a, b and c correspond to etched pictures, respectively. The scale on the picture is o.5 mm.
S. Mivazawa
/
Striation etching of undoped, semi-insulating LEC.grown GaAs
461
References
[4J AS. Jordan, R. Caruso and AR. von Neida, Bell System
[I) RD. Fairman, R.T. Chen, J.R. Oliver and D.R. Chen.
[5] J.C. Brice, The Growth of Crystals from Liquids (NorthHolland. Amsterdam, 1973) ch. 4, p. 158. [6] Y. Nanishi, H. Yamazaki, T. Mizutani and S. Miyazawa. in: Proc. 1981 Intern. Symp. on GaAs and Related Cornpounds, VIII-2, Oiso, Japan, 1981 (to be published).
Tech. J. 59 (1980) 593, IEEE Trans. Electron Devices ED-28 (1981) 135. [2] R.N. Thomas, H.M. Hobgodd, G.W. Eldridge, D.L. Barrett and T.T. Braggins, Solid-State Electron. 24 (1981) 387. [3] M.S. Abrahams and C.J. Buiocchi, J. Appl. Phys. 36 (1965) 2855.
Journal of Crystal Growth 57(1982)459—461 North-Holland Publishing Company
LE’VrER TO THE EDITORS STRIATION ETCHING OF UNDOPED, SEMI-INSULATING LEC-GROWN GaAs Shintaro MIYAZAWA Musashino Electrical Communication Laboratory, Nippon Telegraph and Telephone Public Corporation, Pvfusashuio-shi, Tokyo 180. Japan
Received 16 September 1981~manuscript received in final form 4 November 1981
Growth striations in a LEC-grown, <100) oriented, undoped, semi-insulating GaAs bulk crystal were succesfully revealed on an as-cleaved (110) face by successive etching with 3 HNO 3 + 4 1120 + HF solution and AB-etchant. The solid—liquid interface shape was determined by striation etching.
The Liquid Encapsulation Czochralski (LEC) grown, undoped, semi-insulating GaAs substrates, which can be ion implanted directly, are of considerable interest in FET technology [1,2]. The GaAs IC fabrication requires high doping uniformity of the implant. This uniformity is, however, strongly affected by crystal quality or inhomogeneity in the substrate. The crystal growth method causes inhomogeneities, such as impurity striations, due to temperature fluctuation during pulling. Since growth striations represent a useful built-in record of the solid—liquid interface shape at any point on the crystal, they are widely employed in studying defects and inhomogeneities related to the interface shape. Presently, there are no observations on growth striations in undoped or Cr-doped semi-insulating GaAs bulk grown by LEC. This is thought to be one of the obstacles in characterizing the crystal and evaluating doping uniformity after ion implantation. This letter reports, for the first time, a chemical etching process for evaluating growth striations in an LEC-grown, undoped, semi-insulating GaAs bulk crystal. An appropriately 375 p.m thick (100) wafer was prepared from the LEC-grown, <100) oriented GaAs boule. The wafer was taken from the upper part of the boule, near the so-called crystal shoulder part. The Cr concentration in the wafer was not detected with a spark-source mass spectrometer 0022-0248/82/0000—0000/$02.75
©
(SSMS), because of the detection limit (0.1 wt ppm). The electrical resistivity at room temperature was greater than 106 ~ cm. Growth striations were succesfully revealed by a chemical etching process as follows. The (110) face was cleaved from the wafer. The as-cleaved face was etched, first, for about 2 mm in a solution consisting of HNO3, H20 and HF (first etchant). A typical mixing ratio was 3 :4: 1 by volume. After etching, the specimen was well rinsed with deionized water and was subsequently etched with the well-known AB-etchant [3] (second etchant) for 5—10 mm. Both etchings were carried out at room temperature. Since the (110) face was freshly as-cleaved and unpolished, any artifacts were not introduced. Figs. la—ic show etched features at the central, middle and two-thirds points of the cleaved (110) face, respectively, taken under a differential optical microscope. Roughly spaced striations running diagonally across the cleaved face are clearly observed. The irregular spacing for the striations was roughly estimated to be about 10 to 60 p.m. In this etching process, the pre-etching with the first etchant was subtle. Neither the first etchant nor the second etchant did reveal striations. However, they delineated crystal defects, such as dislocalions. An interesting feature was that growth striations were not revealed on high dislocation density areas, as shown in fig. lc. Etched roughness due to dislocations blanks out a weakly etched pattern of
1982 North-Holland
460
S.
.t!ii ~i:awa
/
.S!r,atuui etclung ~I undoped, semi-insulating LEC-grown GaAs
_____ _____
______________________________________________ —.
[a
striations. At the crystal periphery, the dislocation density increases considerably, which has been theoretically analyzed by Jordan et al. [4]. Growth striation; were not distinguished at the crystal Since striations give a solid—liquid interface shape, the growing interface shape of the crystal examined is concluded to be strongly concave toward the melt. By observing the whole area of the etched (110) cleaved face, the solid—liquid interface shape can be given as shown in fig. ld. At the mid-point (b), the angle between the pulling <100) axis and the striation;, i.e. solid—liquid in-
[b
-
HNO3
~
to be about 72°, concave The growth striations as well as the solid—liquid interface shape may affect the electrical properties and the FET performances on the (100) face, because unintentionally doped or residual impurities are located along the striations [5]. No traces of growth striation; were detected after double etching on a mechano—chemically polished (100) face, perpendicular to the (110) face examined. However, irregularities in FET performances fabricated on the Si ion implanted (100) face were clearly observed, related to the growth striation; and the solid—liquid interface. The results will be published in a separate paper [6]. In conclusion, the successive etching with 3 ~
+ 4H2O + HF solution and AB-etchant could reveal growth striation;, for the first time, on a freshly cleaved (110) face of a LEC-grown, undoped, semi-insulating GaAs bulk crystal. The solid—liquid interface shape during pulling was then determined to be concave toward the melt in
c (100)
)(
J \~
this experiment. The author would like to express his thanks to Dr. M. Ohmori for his encouragement, and also to Drs. S. Akai and K. Tada, Sumitomo Electric Industries Ltd., for fruitful discussions.
Fig. I. (a)—(c) Growth striations revealed on a (110) cleaved
d
c b
a
face. (d) Sketch indicating the solid—liquid interface shape, where the arrows a, b and c correspond to etched pictures, respectively. The scale on the picture is o.5 mm.
S. Mivazawa
/
Striation etching of undoped, semi-insulating LEC.grown GaAs
461
References
[4J AS. Jordan, R. Caruso and AR. von Neida, Bell System
[I) RD. Fairman, R.T. Chen, J.R. Oliver and D.R. Chen.
[5] J.C. Brice, The Growth of Crystals from Liquids (NorthHolland. Amsterdam, 1973) ch. 4, p. 158. [6] Y. Nanishi, H. Yamazaki, T. Mizutani and S. Miyazawa. in: Proc. 1981 Intern. Symp. on GaAs and Related Cornpounds, VIII-2, Oiso, Japan, 1981 (to be published).
Tech. J. 59 (1980) 593, IEEE Trans. Electron Devices ED-28 (1981) 135. [2] R.N. Thomas, H.M. Hobgodd, G.W. Eldridge, D.L. Barrett and T.T. Braggins, Solid-State Electron. 24 (1981) 387. [3] M.S. Abrahams and C.J. Buiocchi, J. Appl. Phys. 36 (1965) 2855.