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Dislocation lines in indium-doped GaAs crystals observed by infrared light scattering tomography of about 1 μm wavelength radiation
332
Journal of Crystal Growth 88 (1988) 332—340 North-Holland, Amsterdam
DISLOCATION LINES IN INI)IIJM-DOPED GaAs CRYSTALS OBSERVED BY INFRARED LIGHT SCA’fl’ERING TOMOGRAPHY OF ABOUT 1 ~tm WAVELENGTh RADIATION Tomoya OGAWA Department of Physics, Gakushuin University, Mejiro, Toshimaku~Tokyo 171, Japan Received 30 March 1987; manuscript received in final form 14 August 1987
Decorated dislocation lines in GaAs crystals are remarkably well observed by scattering of an infrared laser beam in the 1 ism wavelength range. Some of these dislocation lines are also observed by absorption imaging in the same wavelength region but others are only detected by light scattering. Furthermore, the former lines correspond to the pits etched by molten KOH. Dislocation lines in In-doped GaAs crystals grown by the LEC method from nearly stoichiometric melts are studied in connection with their growth history, where grown-in dislocation lines are bent at growth interfaces to react with each other and then their density is decreased. At the shoulder part of the ingots, many slip dislocations are found, where most dislocation lines are so isolated that they are clearly and individually observed by light scattering tomography without etching.
1. Introduction Light scattering tomography was developed to characterize transparent crystals [1—9],and is useful to observe defects in the crystals with very high image contrast. It can be applied to fairly big specimens such as crystals with dimensions of a few centimeter thickness and a few inches width. Especially, this tomography is very effective for characterization of materials composed of elements with large atomic number, because the absorption coefficient of the X-rays is nearly proportional to the atomic number of composite elements but optical absorption does not have such a relation. For example, GaAs crystals are transparent for JR radiation in the 1 ,sm range and then defects in the crystals are studied by scattering of the infrared radiation even if they are fairly thick [6,7,10], but the crystals should be sliced, polished and etched until about a few hundred pm or much thinner in order to study their defects by X-rays with the Laue arrangement.
2. Experimental procedures In this paper, light scattering images were obtamed by linearly polarized radiation of i.iS pm
from a He—Ne laser with 15 mW and by nonpoiarized multi-mode radiation of 1.32 pm from a YAG laser with 100 mW, and JR absorption imaging was done by a beam passed through a Littrow type monochromator from a halogen lamp (100 W) as shown in fig. 1 [13,14]. To detect scattered light from defects by a PbS—PbO vidicon (Hamamatsu N214) [6,7,13], a beam from the He—Ne laser must be focused into a fine light pencil with a 30 pm diameter in order to increase the light intensity inside the pencil but a broader beam from the YAG laser will generate a good enough intensity to detect the scattered light. By the broader illumination, spatial structures and the configuration of dislocation lines are sometimes recognized more easily when they are decorated, while the tomographic resolution is, of course, lowered. Here broad beam tomography was better for studying the growth history of GaAs crystals because most dislocation lines in the crystals were already decorated by atoms and/or clusters of excess components and/or impurities when they were received. The In-doped GaAs crystal-i was grown as a 2 inch diameter ingot by the LEC method from a slightly As-rich melt without magnetic field and crystal-2 with a 2 inch diameter was grown by LEC from a slightly Ill-component-rich melt un-
0022-0248/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
T. Ogawa
I
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______________________
J
/ Dislocation lines in In-doped GaAs crystals
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_____ 4 ________
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Fig. 1. Schematic drawing of image formation system. For image formation, the infrared radiation due to both absorption of the illumination and scattering of the laser beam from the specimen is focused by an objective lens of 1 : 2.8/50 mm on the photo-sensitive plate of a PbS—PbO vidicon. The signal from the vidicon is digitized by the frame memory and stored into the register of a 16 bit personal computer for accumulation to improve the signal-to-noise ratio in order to get a good image. After the accumulation, the data on the register are successively transferred to 2D 8 inch floppy disk. After the data acquisition for a picture is completed, the data on the floppy disk are transferred to the frame memory through the computer, by which the dark current of the vidicon is completely eliminated and the data are linearly compressed to 8 bits to avoid overflow of data from the frame memory. The data on the memory are modified by the computer to eliminate attenuation of incident laser beam and/or inhomogeneity of the illumination, if this process is necessary.
der a magnetic field along the growth direction [26]. Crystal-i was cut with 5 mm thickness perpendicularly to the [001] growth direction, and was specified as wafer-i. Crystal-2 was cut by (100) planes with 4 mm thickness, which was parallel to the [001] growth direction, and denoted wafer-2. 3. Results and discussions IR absorption and light scattering images were simultaneously taken by the configuration shown in fig. 2a. Fig. 3 is obtained in wafer-i by a slightly oblique illumination of 1 ~tm wavelength and by scattering of 1.15 ~sm radiation from the He—Ne laser [14]. Absorption images of dislo-
cation lines are observed as dark rods due to elongation of the shadow caused by the oblique illumination, and scattered beams by the dislocation lines are observed as white spots in fig. 3 because the laser beam is focused into the fine light pencil with 30 ~.im diameter and then the beam can illuminate only small parts of dislocation lines. The white spots are shifted along the dark rods when the scanning plane of the laser beam is shifted from the top surface to the bottom one of wafer-i [14]. Almost all white spots belong to the dark rods in fig. 3, but the six spots located near position B dothe notformer belongdislocation to any darklines rodsare in the figure. Here classified as type A dislocations and the latter as type B.
334
/ Dislocation lines in In-doped GaAs crystals
T Ogawa Translation of Sample~-...
Translation of -
IR illumination (a)
(b)
Fig. 2. Arrangement to take figs. 3, 5 and 8. (a) Simultaneous observation of IR absorption and light scattering (for figs. 3 and 8). (b) To take the dislocation images along [001] by the tomography (for fig. 5).
Fig. 4 is the etch pit pattern on the (OOi) plane of wafer-i due to molten KOH, where all the etch pits correspond to the white spots of the type A dislocation lines. However, there is no etch pit at
B,
1mm
‘~1
Fig. 3. Picture taken simultaneously by IR absorption and light scattering in wafer-i by the arrangement shown in fig. 2a. The dark rods are absorption images of 1 ~tm radiation due to oblique illumination and the white spots are caused by scattering of a light pencil with 30 ~zm diameter and i.is ~ wavelength from a 15 mW He—Ne laser,
1 mm Fig. 4. Etch pits obtained by molten KOH. The (001) top surface of wafer-i was etched by molten KOH at 300 C. By comparing this figure with fig. 3, it becomes clear that each white spot correlated with a dark rod has complete one-by-one correspondence to the etch pit on this figure.
T. Ogawa
/ Dislocation lines
_______________________________________
_____ —
in In-doped GaAs crystals
335
the arrows in the figure, at 5 mm/h from position D to E and at 4 mm/h after position E [26]. A lot of slip dislocation lines along the [011] direction are found near the shoulder of the crystal, and grown-in dislocation lines are inside the ingot but they do not have any definite directions crystallographically. To get some details about the dislocations, a broad beam tomograph was taken by a beam with 1 mm diameter of 1.32 ,sm radiation from a 100 mW YAG laser. Fig. 7 is a tomographic image taken by the broad beam at the position which is
Fig. 5. Light scattering images of dislocation lines on a (100) plane in wafer-i. The white spots indicate light scatterers on dislocation lines along the growth [001] direction. The image is taken by a fine light pencil from a He—Ne laser and reproduced with positive contrast. Marker represents 1 mm.
the positions where the type B dislocation lines are located [17]. The type B dislocation lines were lying on a (ill) plane and were also passing through the top and bottom (001) faces, which was confirmed by successively taking many tomographs with small and equal intervals [14], or by layer-by-layer tomography [5,8]. Fig. 5 shows the dislocation lines observed on a (100) plane of wafer-i by the arrangement shown in fig. 2b. It is clearly confirmed by this photograph that the type A dislocation lines are irregularly decorated by something which may be arsenic atoms and/or clusters on the lines [12,23,25], because excess atoms due to deviation from stoichiometry should be absorbed by dislocation lines and/or make clusters or introduce vacancies of deficient component. Anyway, there are dislocation lines decorated by As atoms or clusters and by the others, because spatial fluctuation of the Ill/V ratio cannot be avoided. However, the lines decorated by As were more numerous than the others or the type A dislocations were more numerous than the type B ones. Fig. 6 shows the tomograph taken by a beam with 30 ~smdiameter from a 15 mW He—Ne laser on a (100) plane of wafer-2. Crystal-2 was grown along [001] at 6 mm/h to position D indicated by
D
E
______________
10 mm Fig. 6. Tomograph on a (100) plane in wafer-2. This tomograph was taken by the fine beam on a (100) plane which is parallel to the [001] growth direction. The growth velocity of the crystal was 6 mm/h at position D and then 5 mm/h from position D to E. After position E, the crystal was grown at 4 mm/h.
336
T Ogawa
/ Dislocation lines
in In-doped GaAs crystals
1mm
____
Fig. 7. Dislocation images obtained by scattering of a broad beam from a 100 mW YAG laser with 1.32 pm wavelength. In order to make clear the dislocation lines in fig. 6, a broad beam tomograph was taken at the position indicated by the white label in the silhouette of wafer-2. Here the dislocation lines at the upper part of the figure are bent to react with each other, especially at the interfaces indicated by a and b. Grown-in dislocations will have a screw component because many of them form helicoids. The four lines inclined by 450 against the growth striations are slip dislocations which may be induced by thermal stress at the growing interface. The dislocations lying on the growth striations will be a sort of misfit dislocations because the concentration of In atoms in the crystal is steeply changed there.
T Ogawa
/ Dislocation lines in In-doped GaAs crystals
indicated by the white label on the right side of the figure. Here we found many types of dislocation lines such as helical and slip ones, and mutual reactions between dislocation lines, At nearly the same place as fig. 7, absorption imaging by a parallel beam illumination of 1 ~tm wavelength radiation was done simultaneously with the broad beam tomography, and is shown in fig. 8. A lot of parallel striations are observed horizontally in fig. 8. They are growth stnations caused by fluctuating incorporation of indium atoms at growing interfaces because the average In concentration in the crystal is estimated as 1020 cm ~ from an As/(Ga + In + As) = 0.499 melt with an In concentration of 1021 cm3. The growth striations are not caused by true absorption but
337
rather by an alternating gradient of refractive index due to the alternating change of In concentration in the wafer. The reason why the black and white striations are observed by gradient of refractive index without absorption is that the gradient induces deflection of propagating beam direction and then makes an intensity modulation by deviation of beam at a position distant from the specimen. This effect is clearly observed when a parallel beam illumination is used for a parallel cut specimen with the gradient as in this observation. In figs. 7 and 8, reactions of dislocation lines are observed. One example is clearly observed at the growth interfaces indicated by a and b in these figures, where the lines are bent at the interfaces
4-I.
1
5:11:
Fig. 8. Broad beam tomograph taken simultaneously with transmitted JR illuniination through wafer-2 at nearly the same position as fig. 7. The horizontal and parallel striations in this figure are growth striations observed by a nearly parallel beam illumination. The sharp white lines are dislocation images caused by the light scattering from a YAG laser with wavelength of 1.32 pm.
338
T Ogawa
/ Dislocation lines in
In-doped GaAs crystals
Fig. 9. Slip dislocations at the shoulder part of the ingot. A broad beam tomograph was taken at the position indicated by the white label on the silhouette in the figure. Thermal stress induced slip on the (lii) planes from the periphery of the ingot. especially at the shoulder part of wafer-2.
T. Ogawa
/
Dislocation lines in In-doped GaAs crystals
and react with each other, so as to decrease their density. Other examples are the helical lines in figs. 7 and 8. They are caused by climb and prismatic glide of some dislocation segments because, when a screw dislocation is curved by absorption of some vacancies and/or excess atoms, an edge component is introduced into the dislocation by the curving. The lines with 450 inclination against the growth striations are slip dislocation lines along [011] on (lii) planes which are generated at growth interfaces because of high temperature gradient and fluctuation at the interfaces. The slip dislocation lines along [011] on (iii) planes are dominantly observed near the shoulder of wafer-2 as shown in fig. 6. Fig. 9 is taken at the position indicated by the white label in the right part of the figure. These lines are induced from the periphery of the ingot by thermal stresses. There are several dislocation lines on the growth interfaces in the lower right part of fig. 9, some of which are similar to misfit dislocations on the interfaces and some others are bent towards the growth direction and become changed into grownin dislocation lines.
Acknowledgements The present author wishes to express his cordial thanks to Dr. T. Katsumata Optoelectronic Joint Research Laboratory, and Dr. N. Toyoda, Sumitomo Electric Industries, for their helpful support, and to Dr. J. Matsui, Nippon Electric Co., Central Research Laboratory, for suggestive discussions about the dislocations. This study is supported by Research-in-Aids for Developmental Scientific Research and for Special Project Research on “Alloy Semiconductor Physics and Electronics” from the Ministry of Education, Science and Culture of Japan ~,
References [1] K. [2] [3] [4] [5] [6] [7]
4. Conclusions [8]
Light scattering tomography at the 1 p~mwavelength range is very useful for the observation of dislocation lines and other defects such as inclusions in In-doped GaAs crystals. Here the dislocations which were observed by both the light scattering and the absorption imaging at the wavelength range were also detected by molten KOH etching but some other dislocation lines were detected only by light scattering. This difference will be caused by the difference of decoration atoms on the dislocation lines. Some grown-in dislocation lines were bent at growth interfaces and reacted with each other, by which the density of the lines was decreased. Many slip dislocations on {111} planes were observed along <011), which were induced by thermal stresses at the shoulder part and positions where the diameter of the ingot was abruptly changed when the crystals were grown by the LEC method.
339
[9]
Moriya and T. Ogawa, J. Crystal Growth 44 (1978) 53. K. Moriya and T. Ogawa, Phil. Mag. A41 (1980) 191. K. Moriya and T. Ogawa, Phil. Mag. A44 (1981) 1085. K. Moriya and T. Ogawa, J. Crystal Growth 60 (1982) 1. K. Moriya and T. Ogawa, Japan. J. Crystal Growth 58 (1982) 115. K. Moriya and T. Ogawa, Japan. J. Appl. Phys. 22 (1983) L207. T. Ogawa, Infrared tomography for detection of lattice defects in Ill—V compound crystals, in: Defect Recogrntion and Image Processing in Ill—V Compounds (DRIP 1985), Ed. J.P. Fillard (Elsevier, Amsterdam, 1985) p. 1. S. Kuma, Y. Otoki and K. Kurata, Stereographic observation of dislocations in GaAs crystals by infrared light scattering [7], p. 19. K. Moriya,tomography, Observationref. of micro-defects in GaAs crystals
by infrared light scattering, ref. [7], p. 27. [10] T. Katsumata, T. Obokata, M. Nakajima and T. Fukuda, Observations of inhomogeneities in dislocated and Indoped dislocation free GaAs crystals by newly developed video-enhanced IR topography, ref. [7], p. 149. [11] M.S. Skolmck, Infrared imaging of large diameter GaAs crystals, ref. [7], p. 165. [12] S. Clark, M.R. Brozel and D.J. Stirland, Observations of single dislocations in GaAs by infrared microscopy, ref. [7],Ogawa p. 201. and N. Nango, Rev. Sci. Instr. 57 (1986) 1135. [13] T. [14] T. Ogawa, Japan. J. Appl. Phys. 25 (1986) L316. [15] T. Ogawa, Japan. J. Appl. Phys. 25 (1986) L916. [161Y. Otoki, M. Nakamori, R. Nakazono and S. Kuma, Mechamsum of annealing of semi-insulating GaAs, in: Semi-Insulating Ill—V Materials, Eds. H. Kukimoto and S. Miyazawa (Omsha, Tokyo, and Elsevier, Amsterdam, 1986) p. 285. *
Presently at Toshiba Co.
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T Ogawa
/ Dislocation lines in In-doped GaAs crystals
[17] T. Ogawa, N. Toyoda and S. Nishine, Correlation of etch pits with infrared-light scattering and absorption images in indium doped LEC GaAs crystals, ref. [16], p. 213. [181 T. Katsumata, H. Okada, T. Kikuta, T. Fukuda and T. Ogawa, Microdefects in LEC GaAs observed with the infrared scattering and their correlation with the infrared transmission image, ref. [16], p. 145. [19] M.R. Brozel and M.S. Skolnick, Near band edge “reverse contrast” images in GaAs, ref. [16], p. 109. [20] M.S. Skolnick, M.R. Brozel, AD. Pitt and J. Maguire, Absorption and scattering processes in LEC GaAs crystals, ref. [16], p. 139. [21] M.R. Brozel, S. Clark and D.J. Stirland, Interaction between the deep level defect EL2 and edge dislocations in semi-insulating GaAs, ref. [16], p. 133.
[22] S. Miyazawa, Is a dislocation-free crystal the most promising for GaAs ICs?, ref. [16], p. 3. [23] AG. Cullis, P.D. Augustus and D.J. Stirland, J. Appl. Phys. 51 (1980) 2556. [24] K. Sumino, Dislocations in GaAs crystals, in: Defects and Properties of Semiconductors: Defect Engineering, Eds. J. Chikawa, K. Sumino and K. Wada (KTK Sci. Pub!., Reidel, Dordrecht, 1987) p. 3. [25] K. Tomrzawa, K. Sassa, Y. Shimanuki and J. Nishizawa, Dislocations in GaAs crystals grown by As pressure controlled Cz method, ref. [24], p. 25. [26] T. Katsumata, H. Okada, T. Kimura and T. Fukuda, J. App!. Phys. 60 (1986) 3105.
Journal of Crystal Growth 88 (1988) 332—340 North-Holland, Amsterdam
DISLOCATION LINES IN INI)IIJM-DOPED GaAs CRYSTALS OBSERVED BY INFRARED LIGHT SCA’fl’ERING TOMOGRAPHY OF ABOUT 1 ~tm WAVELENGTh RADIATION Tomoya OGAWA Department of Physics, Gakushuin University, Mejiro, Toshimaku~Tokyo 171, Japan Received 30 March 1987; manuscript received in final form 14 August 1987
Decorated dislocation lines in GaAs crystals are remarkably well observed by scattering of an infrared laser beam in the 1 ism wavelength range. Some of these dislocation lines are also observed by absorption imaging in the same wavelength region but others are only detected by light scattering. Furthermore, the former lines correspond to the pits etched by molten KOH. Dislocation lines in In-doped GaAs crystals grown by the LEC method from nearly stoichiometric melts are studied in connection with their growth history, where grown-in dislocation lines are bent at growth interfaces to react with each other and then their density is decreased. At the shoulder part of the ingots, many slip dislocations are found, where most dislocation lines are so isolated that they are clearly and individually observed by light scattering tomography without etching.
1. Introduction Light scattering tomography was developed to characterize transparent crystals [1—9],and is useful to observe defects in the crystals with very high image contrast. It can be applied to fairly big specimens such as crystals with dimensions of a few centimeter thickness and a few inches width. Especially, this tomography is very effective for characterization of materials composed of elements with large atomic number, because the absorption coefficient of the X-rays is nearly proportional to the atomic number of composite elements but optical absorption does not have such a relation. For example, GaAs crystals are transparent for JR radiation in the 1 ,sm range and then defects in the crystals are studied by scattering of the infrared radiation even if they are fairly thick [6,7,10], but the crystals should be sliced, polished and etched until about a few hundred pm or much thinner in order to study their defects by X-rays with the Laue arrangement.
2. Experimental procedures In this paper, light scattering images were obtamed by linearly polarized radiation of i.iS pm
from a He—Ne laser with 15 mW and by nonpoiarized multi-mode radiation of 1.32 pm from a YAG laser with 100 mW, and JR absorption imaging was done by a beam passed through a Littrow type monochromator from a halogen lamp (100 W) as shown in fig. 1 [13,14]. To detect scattered light from defects by a PbS—PbO vidicon (Hamamatsu N214) [6,7,13], a beam from the He—Ne laser must be focused into a fine light pencil with a 30 pm diameter in order to increase the light intensity inside the pencil but a broader beam from the YAG laser will generate a good enough intensity to detect the scattered light. By the broader illumination, spatial structures and the configuration of dislocation lines are sometimes recognized more easily when they are decorated, while the tomographic resolution is, of course, lowered. Here broad beam tomography was better for studying the growth history of GaAs crystals because most dislocation lines in the crystals were already decorated by atoms and/or clusters of excess components and/or impurities when they were received. The In-doped GaAs crystal-i was grown as a 2 inch diameter ingot by the LEC method from a slightly As-rich melt without magnetic field and crystal-2 with a 2 inch diameter was grown by LEC from a slightly Ill-component-rich melt un-
0022-0248/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
T. Ogawa
I
16 bit Personal
I
640K Computer byte with Memory
______________________
J
/ Dislocation lines in In-doped GaAs crystals
~~~ter Video
333
Color CRTImage Recorder
Psuedo-colo Unit
8” Floppy Disk~ Printer
~Memo~Pb~bO
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Vidicon Camera
F—
_________________ Key Board
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F
Vide ~er
/SamPle
-LI~II1
_______
_____ 4 ________
______________
I YAG Laser
I
~-
Quartz Prism
Moving Stage amp
(Littrow’s type monochrometer)
Fig. 1. Schematic drawing of image formation system. For image formation, the infrared radiation due to both absorption of the illumination and scattering of the laser beam from the specimen is focused by an objective lens of 1 : 2.8/50 mm on the photo-sensitive plate of a PbS—PbO vidicon. The signal from the vidicon is digitized by the frame memory and stored into the register of a 16 bit personal computer for accumulation to improve the signal-to-noise ratio in order to get a good image. After the accumulation, the data on the register are successively transferred to 2D 8 inch floppy disk. After the data acquisition for a picture is completed, the data on the floppy disk are transferred to the frame memory through the computer, by which the dark current of the vidicon is completely eliminated and the data are linearly compressed to 8 bits to avoid overflow of data from the frame memory. The data on the memory are modified by the computer to eliminate attenuation of incident laser beam and/or inhomogeneity of the illumination, if this process is necessary.
der a magnetic field along the growth direction [26]. Crystal-i was cut with 5 mm thickness perpendicularly to the [001] growth direction, and was specified as wafer-i. Crystal-2 was cut by (100) planes with 4 mm thickness, which was parallel to the [001] growth direction, and denoted wafer-2. 3. Results and discussions IR absorption and light scattering images were simultaneously taken by the configuration shown in fig. 2a. Fig. 3 is obtained in wafer-i by a slightly oblique illumination of 1 ~tm wavelength and by scattering of 1.15 ~sm radiation from the He—Ne laser [14]. Absorption images of dislo-
cation lines are observed as dark rods due to elongation of the shadow caused by the oblique illumination, and scattered beams by the dislocation lines are observed as white spots in fig. 3 because the laser beam is focused into the fine light pencil with 30 ~.im diameter and then the beam can illuminate only small parts of dislocation lines. The white spots are shifted along the dark rods when the scanning plane of the laser beam is shifted from the top surface to the bottom one of wafer-i [14]. Almost all white spots belong to the dark rods in fig. 3, but the six spots located near position B dothe notformer belongdislocation to any darklines rodsare in the figure. Here classified as type A dislocations and the latter as type B.
334
/ Dislocation lines in In-doped GaAs crystals
T Ogawa Translation of Sample~-...
Translation of -
IR illumination (a)
(b)
Fig. 2. Arrangement to take figs. 3, 5 and 8. (a) Simultaneous observation of IR absorption and light scattering (for figs. 3 and 8). (b) To take the dislocation images along [001] by the tomography (for fig. 5).
Fig. 4 is the etch pit pattern on the (OOi) plane of wafer-i due to molten KOH, where all the etch pits correspond to the white spots of the type A dislocation lines. However, there is no etch pit at
B,
1mm
‘~1
Fig. 3. Picture taken simultaneously by IR absorption and light scattering in wafer-i by the arrangement shown in fig. 2a. The dark rods are absorption images of 1 ~tm radiation due to oblique illumination and the white spots are caused by scattering of a light pencil with 30 ~zm diameter and i.is ~ wavelength from a 15 mW He—Ne laser,
1 mm Fig. 4. Etch pits obtained by molten KOH. The (001) top surface of wafer-i was etched by molten KOH at 300 C. By comparing this figure with fig. 3, it becomes clear that each white spot correlated with a dark rod has complete one-by-one correspondence to the etch pit on this figure.
T. Ogawa
/ Dislocation lines
_______________________________________
_____ —
in In-doped GaAs crystals
335
the arrows in the figure, at 5 mm/h from position D to E and at 4 mm/h after position E [26]. A lot of slip dislocation lines along the [011] direction are found near the shoulder of the crystal, and grown-in dislocation lines are inside the ingot but they do not have any definite directions crystallographically. To get some details about the dislocations, a broad beam tomograph was taken by a beam with 1 mm diameter of 1.32 ,sm radiation from a 100 mW YAG laser. Fig. 7 is a tomographic image taken by the broad beam at the position which is
Fig. 5. Light scattering images of dislocation lines on a (100) plane in wafer-i. The white spots indicate light scatterers on dislocation lines along the growth [001] direction. The image is taken by a fine light pencil from a He—Ne laser and reproduced with positive contrast. Marker represents 1 mm.
the positions where the type B dislocation lines are located [17]. The type B dislocation lines were lying on a (ill) plane and were also passing through the top and bottom (001) faces, which was confirmed by successively taking many tomographs with small and equal intervals [14], or by layer-by-layer tomography [5,8]. Fig. 5 shows the dislocation lines observed on a (100) plane of wafer-i by the arrangement shown in fig. 2b. It is clearly confirmed by this photograph that the type A dislocation lines are irregularly decorated by something which may be arsenic atoms and/or clusters on the lines [12,23,25], because excess atoms due to deviation from stoichiometry should be absorbed by dislocation lines and/or make clusters or introduce vacancies of deficient component. Anyway, there are dislocation lines decorated by As atoms or clusters and by the others, because spatial fluctuation of the Ill/V ratio cannot be avoided. However, the lines decorated by As were more numerous than the others or the type A dislocations were more numerous than the type B ones. Fig. 6 shows the tomograph taken by a beam with 30 ~smdiameter from a 15 mW He—Ne laser on a (100) plane of wafer-2. Crystal-2 was grown along [001] at 6 mm/h to position D indicated by
D
E
______________
10 mm Fig. 6. Tomograph on a (100) plane in wafer-2. This tomograph was taken by the fine beam on a (100) plane which is parallel to the [001] growth direction. The growth velocity of the crystal was 6 mm/h at position D and then 5 mm/h from position D to E. After position E, the crystal was grown at 4 mm/h.
336
T Ogawa
/ Dislocation lines
in In-doped GaAs crystals
1mm
____
Fig. 7. Dislocation images obtained by scattering of a broad beam from a 100 mW YAG laser with 1.32 pm wavelength. In order to make clear the dislocation lines in fig. 6, a broad beam tomograph was taken at the position indicated by the white label in the silhouette of wafer-2. Here the dislocation lines at the upper part of the figure are bent to react with each other, especially at the interfaces indicated by a and b. Grown-in dislocations will have a screw component because many of them form helicoids. The four lines inclined by 450 against the growth striations are slip dislocations which may be induced by thermal stress at the growing interface. The dislocations lying on the growth striations will be a sort of misfit dislocations because the concentration of In atoms in the crystal is steeply changed there.
T Ogawa
/ Dislocation lines in In-doped GaAs crystals
indicated by the white label on the right side of the figure. Here we found many types of dislocation lines such as helical and slip ones, and mutual reactions between dislocation lines, At nearly the same place as fig. 7, absorption imaging by a parallel beam illumination of 1 ~tm wavelength radiation was done simultaneously with the broad beam tomography, and is shown in fig. 8. A lot of parallel striations are observed horizontally in fig. 8. They are growth stnations caused by fluctuating incorporation of indium atoms at growing interfaces because the average In concentration in the crystal is estimated as 1020 cm ~ from an As/(Ga + In + As) = 0.499 melt with an In concentration of 1021 cm3. The growth striations are not caused by true absorption but
337
rather by an alternating gradient of refractive index due to the alternating change of In concentration in the wafer. The reason why the black and white striations are observed by gradient of refractive index without absorption is that the gradient induces deflection of propagating beam direction and then makes an intensity modulation by deviation of beam at a position distant from the specimen. This effect is clearly observed when a parallel beam illumination is used for a parallel cut specimen with the gradient as in this observation. In figs. 7 and 8, reactions of dislocation lines are observed. One example is clearly observed at the growth interfaces indicated by a and b in these figures, where the lines are bent at the interfaces
4-I.
1
5:11:
Fig. 8. Broad beam tomograph taken simultaneously with transmitted JR illuniination through wafer-2 at nearly the same position as fig. 7. The horizontal and parallel striations in this figure are growth striations observed by a nearly parallel beam illumination. The sharp white lines are dislocation images caused by the light scattering from a YAG laser with wavelength of 1.32 pm.
338
T Ogawa
/ Dislocation lines in
In-doped GaAs crystals
Fig. 9. Slip dislocations at the shoulder part of the ingot. A broad beam tomograph was taken at the position indicated by the white label on the silhouette in the figure. Thermal stress induced slip on the (lii) planes from the periphery of the ingot. especially at the shoulder part of wafer-2.
T. Ogawa
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Dislocation lines in In-doped GaAs crystals
and react with each other, so as to decrease their density. Other examples are the helical lines in figs. 7 and 8. They are caused by climb and prismatic glide of some dislocation segments because, when a screw dislocation is curved by absorption of some vacancies and/or excess atoms, an edge component is introduced into the dislocation by the curving. The lines with 450 inclination against the growth striations are slip dislocation lines along [011] on (lii) planes which are generated at growth interfaces because of high temperature gradient and fluctuation at the interfaces. The slip dislocation lines along [011] on (iii) planes are dominantly observed near the shoulder of wafer-2 as shown in fig. 6. Fig. 9 is taken at the position indicated by the white label in the right part of the figure. These lines are induced from the periphery of the ingot by thermal stresses. There are several dislocation lines on the growth interfaces in the lower right part of fig. 9, some of which are similar to misfit dislocations on the interfaces and some others are bent towards the growth direction and become changed into grownin dislocation lines.
Acknowledgements The present author wishes to express his cordial thanks to Dr. T. Katsumata Optoelectronic Joint Research Laboratory, and Dr. N. Toyoda, Sumitomo Electric Industries, for their helpful support, and to Dr. J. Matsui, Nippon Electric Co., Central Research Laboratory, for suggestive discussions about the dislocations. This study is supported by Research-in-Aids for Developmental Scientific Research and for Special Project Research on “Alloy Semiconductor Physics and Electronics” from the Ministry of Education, Science and Culture of Japan ~,
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4. Conclusions [8]
Light scattering tomography at the 1 p~mwavelength range is very useful for the observation of dislocation lines and other defects such as inclusions in In-doped GaAs crystals. Here the dislocations which were observed by both the light scattering and the absorption imaging at the wavelength range were also detected by molten KOH etching but some other dislocation lines were detected only by light scattering. This difference will be caused by the difference of decoration atoms on the dislocation lines. Some grown-in dislocation lines were bent at growth interfaces and reacted with each other, by which the density of the lines was decreased. Many slip dislocations on {111} planes were observed along <011), which were induced by thermal stresses at the shoulder part and positions where the diameter of the ingot was abruptly changed when the crystals were grown by the LEC method.
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