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Point defect distributions in ZnSe crystals: effects of gravity vector orientation during physical vapor transport growth
Journal of Crystal Growth 204 (1999) 41}51
Point defect distributions in ZnSe crystals: e!ects of gravity vector orientation during physical vapor transport growth Ching-Hua Su *, S. Feth , D. Hirschfeld, T.M. Smith, Ling Jun Wang, M.P. Volz , S.L. Lehoczky Microgravity Science and Applications Department, Science Directorate, NASA/Marshall Space Flight Center, Huntsville, AL 35812, USA Department of Materials and Metallurgical Engineering, New Mexico Institute of Mining and Technology, Socorro, NM 87801, USA Department of Physics, Geology and Astronomy, University of Tennessee at Chattanooga, Chattanooga, TE 37403, USA Received 19 January 1999; accepted 15 March 1999 Communicated by K.W. Benz
Abstract ZnSe crystals were grown by the physical vapor transport technique under horizontal and vertical (stabilized and destabilized) con"gurations. Secondary ion mass spectroscopy and photoluminescence measurements were performed on the grown ZnSe samples to map the distributions of [Si], [Fe], [Cu], [Al] and [Li or Na] impurities as well as Zn vacancy, [< ]. Annealings of ZnSe under controlled Zn pressures were studied to correlate the measured photolumines8 cence emission intensity to the equilibrium Zn partial pressure. In the horizontal grown crystals the segregations of [Si], [Fe], [Al] and [< ] were observed along the gravity vector direction whereas in the vertically stabilized grown crystal 8 the segregation of these point defects was radially symmetrical. No apparent pattern was observed on the measured distributions in the vertically destabilized grown crystal. The observed segregations in the three growth con"gurations were interpreted based on the possible buoyancy-driven convection in the vapor phase. 1999 Elsevier Science B.V. All rights reserved.
1. Inroduction The electrical and optical properties of semiconductors depend on the distributions of impurities (or dopants) and of native point defects (deviation from stoichiometry). A nonuniform electro-optic response across semiconducting wafers can serious-
* Corresponding author. Tel.: #1-256-544-7776; fax: #1256-544-8762. E-mail address: [email protected] (C.-H. Su) Raytheon STX Corporation.
ly limit state-of-the-art device performance and future device applications. Because of the high melting temperature, bulk growth of wide band gap semiconducting compounds was mainly conducted by vapor growth and di!erent variations of the physical vapor transport (PVT) technique have been applied to the growth of ZnSe bulk crystals [1}9]. The results of some #uid dynamic analyses stated that the e!ects of gravity on heat transfer and mass transport in most of the PVT systems were insigni"cant [10]. However, the convective #ows caused by the buoyancy-driven force in the #ow "eld in the vicinity of the growing surface will
0022-0248/99/$ - see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 9 ) 0 0 1 8 7 - 6
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result in phenomena such as strati"cation in the vapor phase as well as interface #uctuation (non-uniform step bunching). This, in turn, will result in the nonuniform distributions of impurities and native point defects in the grown crystals. In the published literatures on vapor growth and characterization of II}VI semiconducting compounds, the e!ects of gravity on the grown crystals were neither examined nor reported. Several vapor growth experiments have been conducted under the low-gravity condition [10}14]. The improvements in the as-grown surface morphology, compositional uniformity in the ternary compounds and structural crystalline quality were reported. Recently, the vapor growth of Cl-doped CdTe was performed in the low-gravity environment during the EUREKA-1 mission [15]. A striking di!erence in the two-dimensional resistivity distribution between the crystal grown on Earth and that grown in low-gravity was observed. The resistivity distribution is essentially related to the Cl doping distribution. For the "rst time the e!ects of gravity on the dopant distribution in the crystals grown by PVT was shown experimentally. In this paper, by comparing the impurities and Zn vacancy distributions determined by secondary ion mass spectroscopy (SIMS) and photoluminescence (PL) measurements in crystals grown under three di!erent growth con"gurations relative to the gravity direction, we have observed, for the "rst time, the e!ects of the gravity vector orientation on the point defect distributions in the ZnSe crystals grown by PVT.
2. Experimental procedure 2.1. Crystal growth The ZnSe crystals were grown in a closed PVT system using a three-zone translational furnace. To study the e!ects of gravity vector orientation on the various properties of the grown crystals the growth experiments were conducted in three con"gurations. With respect to the gravity vector orientation the growth direction made an angle of 03 (VD, vertical destabilized con"guration), 903 (H, hori-
zontal con"guration) and 1803 (VS, vertical stabilized con"guration). The growth furnace and process [8,16}19] as well as the results on ZnSe [20] have been described in detail elsewhere. The samples from four grown boules will be reported here: the horizontally self-seeded grown ZnSe-15H and ZnSe-40H crystals, the vertically (destabilized) self-seeded grown ZnSe-31VD and the vertically (stabilized) seeded-grown ZnSe-16VS crystals. The growth temperature was in the range of 1135} 11443C and the furnace translation rate varied from 2.4 mm/day (ZnSe-16VS) and 3.5 mm/day (ZnSe15H) to 4.9 mm/day (ZnSe-40H and ZnSe-31VD). 2.2. SIMS measurements Four ZnSe samples were prepared for the SIMS measurements. Two of them were used as standards. One standard was the high-purity grade starting material provided by Eagle-Picher, Inc. The other standard was a specimen sliced from a vertically (stabilized) grown crystal, ZnSe-35. Before the SIMS measurements the impurity levels in both standards were analyzed using glow discharge mass spectroscopy (GDMS) by the National Research Council of Canada. Among the 55 trace elements measured most of the impurity levels in the two standards were below 50 ppb (atomic) except for the elements O, Al, Si, Ca, Fe, Cu, Cd and Te. The remaining two samples were wafers sliced from two ZnSe grown boules for the impurity distribution mapping. One sample was sliced axially from the as-grown facet on the side of the crystal, ZnSe-40H, grown horizontally with no contact between the crystal and the ampoule wall [20]. The other sample was sliced perpendicularly to the growth direction from the ZnSe-16VS crystal grown in the vertical stabilized con"guration. Except for the as-grown ZnSe-40H facet surface the other three samples were polished with diamond paste to 0.1 lm particle size. The SIMS analyses were performed at the University of New Mexico/Sandia National Laboratory Advanced Material Laboratory. A Cameca IMS-4f machine was utilized to provide the oxygen ion beam with spatial resolution of 25}30 lm in diameter and 100 lm in depth. The standards were mounted together with the sample in the chamber
C.-H. Su et al. / Journal of Crystal Growth 204 (1999) 41}51
and the beginning-of-day and end-of-day analyses of the standards were performed to remove possible error from instrument drift over that particular day. The absolute concentration of a speci"c element in the samples was determined from the relative sample/standard signal ratio of the element and the concentration of that element in the standards determined from GDMS. A total of 13 points and 36 points were mapped on the ZnSe-40H and the ZnSe-16VS wafers, respectively. 2.3. PL measurements and annealings under controlled Zn pressure The surfaces of the PL samples were either asgrown (1 1 0) facet, cleaved (1 1 0) plane or wiresawn surface polished with Al O /H O solution to 0.3 lm particle size. The samples were illuminated with the 364 nm line from a coherent Ar ion laser at a power density of about 0.8 W/cm and a beam size of about 1.5 mm in diameter. The PL spectra were detected by a SPEX 1403 spectrometer with a typical slit width of 200 lm which corresponded to a resolution of 0.32 meV. The samples were maintained between 5 and 10 K and the spectral data were recorded by a personal computer using SPEX DM3000 software. The intensity ratios between various emissions were calculated by taking the ratios of the integrated areas under the particular emission peaks. The PL mapping was accomplished by analyzing the 20 to 90 PL spectra measured on the four sample surfaces at equaldistance grid points. The PL emission designated as I at 2.783 eV has been attributed in the literature to either as an exciton bound to the deep Zn vacancy acceptor [21}26], < , or bound to the deep Cu accep8 tor [23,27}29]. To identify the origin of I ex periments of annealing ZnSe under controlled Zn pressure were performed. In the "rst annealing experiment, ZnSe-4A, the PL spectrum of a (1 1 0)ZnSe wafer provided by Eagle-Picher, Inc. was measured before annealing. Then the 2 mm thick wafer and 3.66 g of Zn (six-nines purity from Johnson Matthey, Inc.) were loaded inside a 12 mm OD, 8 mm ID fused silica ampoule which had been outgassed under vacuum for 16 h. The sealed am-
43
poule was then placed inside a two-zone furnace with ZnSe at 11043C and Zn at 5563C. After 5 h the annealing was terminated by cooling the furnace to room temperature in 4 h. A second annealing experiment, ZnSe-7A, was performed using a 2.75 mm thick ZnSe(1 1 0) wafer and 3.45 g of Zn provided by the same vendors. Prior to seal-o! the ampoule was back "lled with 0.03 atm of Ar to reduce the sublimation rate of ZnSe caused by the applied thermal gradient during annealing. The ZnSe wafer was controlled at 11043C, same as ZnSe-4A, while the Zn reservoir was maintained at 5753C during the second annealing. After 8 h the annealing was terminated by directionally cooling the ampoule from the Zn end to the room temperature environment in 3 min.
3. Results and discussion 3.1. SIMS mappings The concentration contour map of [Si], [Fe] and [Cu] generated by the SIMS on the horizontally grown ZnSe-40H(1 1 0) facet are shown in Fig. 1a}Fig. 1c, respectively, together with the growth direction and gravity direction. From the SIMS results [Si] and [Fe] clearly show a tendency to segregate towards the bottom while [Cu] segregates towards the bottom at the "rst grown region and towards the top at the middle region of the facet. The mappings on the vertically grown ZnSe16VS wafer cut perpendicular to the growth axis are shown in Fig. 2a}Fig. 2c for [Si], [Fe] and [Cu], respectively. The results of [Si] and [Cu] indicate that these impurities tend to segregate radially towards the edge of the wafer. The mapping of [Fe] implies very high concentration at one edge point which might have been caused by contamination. In general, the measured impurity levels, especially [Cu], in ZnSe-40H are lower than that in ZnSe-16VS. This di!erence might be the result of the impurity segregation along the axial growth direction since the mapping on ZnSe-40H covered the area at 1.2}1.7 cm from the "rst-grown tip whereas the mapping on the ZnSe16VS wafer was sliced at 2.0 cm from the "rstgrown tip.
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C.-H. Su et al. / Journal of Crystal Growth 204 (1999) 41}51
Fig. 1. Impurity concentration contour maps generated by SIMS on the ZnSe-40H as-grown facet. (a) [Si], (b) [Fe] and (c) [Cu].
The error in the absolute values for the impurity levels (given in ppb, atomic) in Figs. 1 and 2 is not known. The assumption that the standards are compositionally uniform so that the GDMS results represent the impurity levels at the SIMS spot is the main source of the error. However, the quantitative relative distributions of the impurities should be accurate to the instrument errors associated with SIMS ((5%).
3.2. PL measurements and controlled Zn pressure annealings Fig. 3 shows the broad PL spectrum of the GDMS specimen, ZnSe-35V, and it represents the typical PL spectrum for other samples studied here. The spectrum was dominated by the near band edge emission and no donor}acceptor pairs (DAP) emissions were observed. The emission at 2.802 eV,
C.-H. Su et al. / Journal of Crystal Growth 204 (1999) 41}51
45
Fig. 2. Impurity concentration contour maps generated by SIMS on a ZnSe-16VS wafer cut perpendicular to the growth axis. (a) [Si], (b) [Fe] and (c) [Cu].
designated as F , is associated with the free exciton [30}32]. The I emission at 2.798 eV was attributed to the exciton bound to the substitutional neutral donors such as Al, Ga and In on the Zn site or Cl and F on the Se site [24,25,27,33]. From the GDMS results on this sample the possible donor concentrations measured were: [Al] 1700 ppb, [Ga] 30 ppb, [Cl] 9 ppb, [In] 4 ppb and [F] under detection limit of 5 ppb. Therefore, it was concluded that Al is the most likely donor associated
with the I emission in our grown crystals. As mentioned in Section 2.3 the I emission at 2.783 eV with its LO phonon replicas at 32 meV apart was considered to be either associated with an exciton bound to the deep < or bound to the 8 deep Cu acceptor. Fig. 4 shows the near band edge PL emissions for the sample ZnSe-4A before and after the Zn pressure annealing. The free exciton emission, F , is an intrinsic property of the samples. The intensity
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C.-H. Su et al. / Journal of Crystal Growth 204 (1999) 41}51
crystals. At the same time, the emission of I re duced tremendously from A(I /A(F )"64.5 before annealing to 7.52 after annealing. Assuming [< ] 8 is proportional to A(I )/A(F ), this result con"rms the previous "nding that the I emission is re lated to < [21}26]. The reaction during Zn vapor 8 annealing can be described as the sublimation of a Zn atom from the solid and the formation of a Zn vacancy and a Zn atom in the gas phase
Fig. 3. PL broad spectrum on the vertically (stabilized) grown ZnSe-35V crystal.
Zn (s)P< #Zn(g). (1) 8 8 From a statistical thermodynamic analysis of nonstoichiometry in nonmetallic binary compounds [36] the chemical potential of Zn, k , is given by 8 [< ] (2) k "!e !k¹ ln 8 , 8 8 2 where e is the excess Gibbs free energy to create 8 one Zn vacancy. For an ideal vapor phase the chemical potential can be written as k "k¹ ln P #k (¹), (3) 8 8 8 where P is the partial pressure of Zn in atm and 8 k (¹) is the chemical potential of Zn gas at 1 atm 8 and the temperature in question. From Eqs. (2) and (3), we have
Fig. 4. Near band edge PL spectra of the ZnSe-4A sample before and after Zn controlled pressure annealing.
ratio of a speci"c emission to that of the free exciton emission has been adopted by the Si industry [34] and by Isshiki et al. on ZnSe [35] as a measurement of the defect concentration associated with that emission. From the spectra the I intensity in creased as the ratio of the integrated area under the emission changed from A(I )/A(F )"0.58 before annealing to 2.10 after annealing. It is noteworthy that the GDMS results on the ZnSe starting material showed that all the "ve possible I donor con centrations were below 5 ppb. It is evidenced from the PL and GDMS results that during processing at elevated temperature Al was released from the fused silica ampoule and incorporated in the grown
k¹ ln(P [< ])"!H #¹S #k¹ ln 2, (4) 8 8 8 8 where H and S are the enthalpy and excess 8 8 entropy, respectively, to create one Zn vacancy in the ZnSe solid from Zn vapor at 1 atm. According to Eq. (4) the quantity P [< ], or 8 8 P ;A(I )/A(F ), should be invariant for ZnSe an8 nealing at constant temperature but under di!erent P . The vapor pressure of Zn is given by [37] 8 6742 log P (atm)"! #9.874 8 ¹ (K) !1.3555 log ¹ (K).
(5)
Table 1 lists the annealing temperatures of ZnSe and the Zn reservoir, the corresponding Zn over pressure, the measured integrated intensity ratio of PL emissions, A(I )/A(F ) and the product P ; 8 A(I )/A(F ). The last column shows that P ; 8 A(I )/A(F ) is indeed a constant, with an average value of 0.04624, for the two annealing runs with di!erent P . 8
C.-H. Su et al. / Journal of Crystal Growth 204 (1999) 41}51
47
Table 1 The annealing temperatures of ZnSe, ¹ , and the Zn reservoir, ¹ , the corresponding Zn over pressure, P , the measured integrated 81 8 8 intensity ratio of PL emissions, A(I )/A(F ) and the product P ;A(I )/A(F ) 8 Run
¹ (3C) 81
¹ (3C) 8
P (atm) 8
A(I )/A(F )
P ;A(I )/A(F ) 8
ZnSe-4A ZnSe-7A
1104 1104
556 575
6.1;10\ 9.0;10\
7.52 5.18
0.0459 0.0466
Fig. 5. PL intensity ratio contour maps on the ZnSe-40H as-grown facet. (a) A(I )/A(F ) and (b) A(I )/A(F ).
3.3. PL mappings Fig. 5 presents the PL mapping results on the as-grown facet of ZnSe-40H, the same facet as that shown in Fig. 1 for the SIMS results, as well as the gravity direction and the growth direction. The A(I )/A(F ) contour map is given in Fig. 5a which indicates that [Al] segregated radially slightly towards the top and axially towards the "rst grown region. Fig. 5b shows the A(I )/A(F ) contour map on the same surface. The equilibrium local P cal8 culated from P ;A(I )/A(F )"0.04624 corres8 ponds to 6.1;10\ atm for the highest measured A(I )/A(F ), 7.58, and 2.17;10\ atm for A(I )/ A(F ) at 2.13. The map implies that [< ] segre 8 gates radially towards the bottom and axially towards the last grown region. The PL mappings on
the polished ZnSe-15H wafer cut perpendicular to the growth direction are shown in Fig. 6a and Fig. 6b, respectively, for A(I )/A(F ) and A(I )/A(F ). Trends in the radial distributions of [Al] and [< ] 8 similar to that in ZnSe-40H were observed. The near band edge PL emission of the as-grown surface of ZnSe-16VS, the same crystal as that of the SIMS measurements described earlier, is shown in Fig. 7. Besides the F , I and I emissions, the emission of IV W at 2.792 eV was observed. This peak was attributed to the exciton bound to the neutral Li or Na acceptor [21,27,38,39]. Fig. 8a}Fig. 8c show the contour maps of A(I )/A(F ), A(IV W)/A(F ) and A(I )/A(F ), respectively. The mappings indi cate that [Al], [Li or Na] and [< ] segregated 8 radially towards the center. Fig. 9a and Fig. 9b shows the contour maps of A(I )/A(F ) and
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C.-H. Su et al. / Journal of Crystal Growth 204 (1999) 41}51
Fig. 6. PL intensity ratio contour maps on the ZnSe-15H wafer cut perpendicular to the growth axis. (a) A(I )/A(F ) and (b) A(I )/A(F ).
Fig. 7. Near band edge PL spectrum on the as-grown ZnSe16VS surface.
A(I )/A(F ) on the as-grown surface of the vertically grown ZnSe- 31VD. The implication from the mappings is that [Al] and [< ] segregated without an 8 apparent pattern. A factor of 3 in A(I )/A(F ), or [Al], was observed along the gravity direction in the horizontally grown ZnSe-40H crystal. In the vertically (stabilized) grown ZnSe-16VS the A(I )/A(F ) contour was almost radially symmetric with a factor of 2.6 between the edge and the center of the crystal. On the other hand, the calculated local P in equilib8 rium with the crystals given in the above
A(I )/A(F ) mappings should be only valid for the condition that the mapped surfaces were at the temperature of 11043C during growth. All the mapped surfaces, except ZnSe-40H, were sliced perpendicularly to the thermal gradient and, therefore, should be reasonably isothermal and the temperature range was estimated from the degree of supercooling to be 1105 to 11303C. In the case of ZnSe-40H the 5 mm axial distance across the surface was at a temperature gradient of about 203C/cm. However, this gradient only a!ects the accuracy of P in the axial direction and the 8 relative segregation measured in the radially gravitational direction should be valid. Along the radial direction in the growth ampoules a P ratio of about 2 from top to bottom in 8 the horizontal cases and a ratio of 3.5 from edge to center in the vertical stabilized case was observed. 3.4. Discussions on the measured segregations To interpreted the segregations measured by the SIMS and PL mappings the thermophysical properties of the vapor phase such as the identities of the transported vapor species responsible for the incorporation of the measured impurities are
C.-H. Su et al. / Journal of Crystal Growth 204 (1999) 41}51
49
Fig. 8. PL intensity ratio contour maps on the ZnSe-16VS. (a)A(I )/A(F ), (b) A(IV W)/A(F ) and (c) A(I )/A(F ).
needed. Without a detailed knowledge on the transported species the cause of the measured segregations is speculated below. In the horizontal case, in an environment with Zn (atomic weight 65.37) and Se (157.92) as the predominant species the slight trend of Al (26.98) segregated towards the top as measured by PL mapping was probably caused by the strati"cation in the vapor phase due to the buoyancy force. The segregation of Si (atomic weight 28.09) and Fe (55.85) towards the bottom as measured by SIMS implies that the atomic species of Si and Fe may not
be the transported species in the vapor phase. The [Cu] segregation in ZnSe-40H cannot be interpreted easily. Since Cu is known to be a fast di!user at elevated temperature in the wide band gap semiconductors, such as CdTe [40], a simple consideration involving only the vapor phase will not be adequate. The Zn atom is 2.4 times lighter than the Se molecule and, hence, radially, P was high 8 er towards the top of the ampoule which resulted in higher [< ] towards the bottom of the crystal as 8 measured by PL mappings on both ZnSe-40H and ZnSe-15H.
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C.-H. Su et al. / Journal of Crystal Growth 204 (1999) 41}51
Fig. 9. PL intensity ratio contour maps on the as-grown surface of ZnSe-31VD. (a) A(I )/A(F ) and (b) A(I )/A(F ).
In the vertically stabilized growth runs, both SIMS and PL mappings on the ZnSe-16VS samples showed a radially symmetric pattern in the segregations of [Si], [Al], [Li or Na] and [< ] 8 which may have been caused by a convective #ow in the shape of toroid adjacent to the growing surface. On the other hand, in the vertically destabilized case of ZnSe-31VD, due to the enhancement of the convective contribution to the #uid #ow there were no clear patterns in the segregations of [Al] and [< ] as measured by the PL 8 mapping. The axial segregation of [< ], or P , mapped 8 8 by PL on ZnSe-40H shows a factor of 3.5 in the [< ] variation over the 5 mm axial distance with 8 the higher [< ] towards the last grown region. 8 A simple one-dimensional di!usion calculation using the experimental growth conditions (¹ " 11453C, ¹ "11313C, P /P (source)"2.9 8 1 and the measured residual gas and composition [20]) as inputs predicts that the stoichiometry at the grown crystal corresponds to P /P "4.9. 8 1 Hence, due to the limited amount of source material, as the growth proceeded the source material became less Zn-rich and so did the grown crystal - [< ] higher towards the last grown 8 region.
4. Conclusions For the "rst time the e!ects of earth gravity vector orientation on the distribution of point defects ([Si], [Fe], [Cu], [Al], [Li or Na] and [< ]) 8 were observed in the ZnSe samples grown by PVT under horizontal, vertical (stabilized and destabilized) con"gurations using the SIMS and PL mappings. The annealings of ZnSe under controlled Zn pressures con"rmed the origin of the PL I emission and established the correlation be tween measured PL emission intensity and the equilibrium P . In the horizontal grown crystals, 8 segregations along the gravity vector direction were observed whereas in the vertically (stabilized) grown crystal the segregations were radially symmetrical. Furthermore, there were no apparent patterns in the measured distributions in the vertically (destabilized) grown crystal. The observed segregations in the three growth con"gurations were interpreted based on the possible contribution of buoyancy-driven convection in the vapor phase. Acknowledgements Special thanks are due to Dr. M. Wiedenbeck for the technical assistance on the SIMS measurements
C.-H. Su et al. / Journal of Crystal Growth 204 (1999) 41}51
and to National Research Council of Canada for the GDMS analyses. This work was supported by the Microgravity Research Division of the National Aeronautics and Space Administration.
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[18] C.-H. Su, M.P. Volz, D.C. Gillies, F.R. Szofran, S.L. Lehoczky, W. Zhou, M. Dudley, H.-C. Liu, R.F. Brebrick, J.C. Wang, Proceedings of the AIAA 32nd Aerospace Sciences Meeting, Paper 94-9564, 1994. [19] K. Chattopadhyay, S. Feth, H. Chen, A. Burger, C.-H. Su, J. Crystal Growth 191 (1998) 377. [20] C.-H. Su, M.A. George, W. Palosz, S. Feth, S.L. Lehoczky, J. Crystal Growth, submitted for publication. [21] J.L. Merz, K. Nassau, J.W. Shiever, Phys. Rev. B 8 (1973) 1444. [22] J.C. Bouley, P. Blanconnier, A. Herman, Ph. Ged. P. Henoc, J.P. Noblanc, J. Appl. Phys. 46 (1975) 3549. [23] S.-M. Huang, Y. Nozue, K. Igaki, Jpn. J. Appl. Phys. 22 (1983) L420. [24] M. Isshiki, T. Yoshida, K. Igaki, W. Uchida, S. Suto, J. Crystal Growth 72 (1985) 162. [25] M. Isshiki, T. Yoshida, T. Tomizono, S. Satoh, K. Igaki, J. Crystal Growth 73 (1985) 221. [26] E. Tournie, C. Morhain, G. Neu, M. Laugt, C. Ongaretto, J.-P. Faurie, R. Triboulet, J.O. Ndap, J. Appl. Phys. 80 (1996) 2983. [27] P.J. Dean, D.C. Herbert, C.J. Werkhoven, B.J. Fitzpatrick, R.N. Bhargava, Phys. Rev. 23 (1981) 4888. [28] P.J. Dean, B.J. Fitzpatrick, R.N. Bhargava, Phys. Rev. 26 (1982) 2016. [29] P.J. Dean, Phys. Stat. Sol. (a) 81 (1984) 625. [30] K. Yoneda, Y. Hishida, T. Toda, H. Ishii, T. Niina, Appl. Phys. Lett. 45 (1984) 1300. [31] K.P. Giapis, D.C. Lu, K.F. Jensen, Appl. Phys. Lett. 54 (1989) 353. [32] Z. Zhu, G.D. Brownlie, P.J. Thompson, K.K. Prior, B.C. Cavenett, Appl. Phys. Lett. 67 (1995) 3762. [33] J.L. Merz, H. Kukimoto, K. Nassau, J. Shiever, Phys. Rev. B 6 (1972) 545. [34] M. Tajima, T. Masui, D. Itoh, T. Nishino, J. Electrochem. Soc. 137 (1990) 3544. [35] M. Isshiki, K. Masumoto, W. Uchida, S. Satoh, Jpn. J. Appl. Phys. 30 (1991) 515. [36] R.F. Brebrick, J. Solid State Chem. 1 (1969) 88. [37] R.F. Brebrick, H. Liu, High Temp. Mater. Sci. 35 (1996) 215. [38] Z. Zhu, H. Mori, T. Yao, J. Appl. Phys. 73 (1993) 1146. [39] J.M. Depuydt, T.L. Smith, J.E. Potts, H. Cheng, S.K. Mohapatra, J. Crystal Growth 86 (1988) 318. [40] E.D. Jones, J.C. Clark, in: Peter Capper (Ed.), Properties of Narrow Gap Cadmium-Based Compounds, INSPEC, The Institution of Electrical Engineers, London, UK, 1994, p. 472.
Point defect distributions in ZnSe crystals: e!ects of gravity vector orientation during physical vapor transport growth Ching-Hua Su *, S. Feth , D. Hirschfeld, T.M. Smith, Ling Jun Wang, M.P. Volz , S.L. Lehoczky Microgravity Science and Applications Department, Science Directorate, NASA/Marshall Space Flight Center, Huntsville, AL 35812, USA Department of Materials and Metallurgical Engineering, New Mexico Institute of Mining and Technology, Socorro, NM 87801, USA Department of Physics, Geology and Astronomy, University of Tennessee at Chattanooga, Chattanooga, TE 37403, USA Received 19 January 1999; accepted 15 March 1999 Communicated by K.W. Benz
Abstract ZnSe crystals were grown by the physical vapor transport technique under horizontal and vertical (stabilized and destabilized) con"gurations. Secondary ion mass spectroscopy and photoluminescence measurements were performed on the grown ZnSe samples to map the distributions of [Si], [Fe], [Cu], [Al] and [Li or Na] impurities as well as Zn vacancy, [< ]. Annealings of ZnSe under controlled Zn pressures were studied to correlate the measured photolumines8 cence emission intensity to the equilibrium Zn partial pressure. In the horizontal grown crystals the segregations of [Si], [Fe], [Al] and [< ] were observed along the gravity vector direction whereas in the vertically stabilized grown crystal 8 the segregation of these point defects was radially symmetrical. No apparent pattern was observed on the measured distributions in the vertically destabilized grown crystal. The observed segregations in the three growth con"gurations were interpreted based on the possible buoyancy-driven convection in the vapor phase. 1999 Elsevier Science B.V. All rights reserved.
1. Inroduction The electrical and optical properties of semiconductors depend on the distributions of impurities (or dopants) and of native point defects (deviation from stoichiometry). A nonuniform electro-optic response across semiconducting wafers can serious-
* Corresponding author. Tel.: #1-256-544-7776; fax: #1256-544-8762. E-mail address: [email protected] (C.-H. Su) Raytheon STX Corporation.
ly limit state-of-the-art device performance and future device applications. Because of the high melting temperature, bulk growth of wide band gap semiconducting compounds was mainly conducted by vapor growth and di!erent variations of the physical vapor transport (PVT) technique have been applied to the growth of ZnSe bulk crystals [1}9]. The results of some #uid dynamic analyses stated that the e!ects of gravity on heat transfer and mass transport in most of the PVT systems were insigni"cant [10]. However, the convective #ows caused by the buoyancy-driven force in the #ow "eld in the vicinity of the growing surface will
0022-0248/99/$ - see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 9 ) 0 0 1 8 7 - 6
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result in phenomena such as strati"cation in the vapor phase as well as interface #uctuation (non-uniform step bunching). This, in turn, will result in the nonuniform distributions of impurities and native point defects in the grown crystals. In the published literatures on vapor growth and characterization of II}VI semiconducting compounds, the e!ects of gravity on the grown crystals were neither examined nor reported. Several vapor growth experiments have been conducted under the low-gravity condition [10}14]. The improvements in the as-grown surface morphology, compositional uniformity in the ternary compounds and structural crystalline quality were reported. Recently, the vapor growth of Cl-doped CdTe was performed in the low-gravity environment during the EUREKA-1 mission [15]. A striking di!erence in the two-dimensional resistivity distribution between the crystal grown on Earth and that grown in low-gravity was observed. The resistivity distribution is essentially related to the Cl doping distribution. For the "rst time the e!ects of gravity on the dopant distribution in the crystals grown by PVT was shown experimentally. In this paper, by comparing the impurities and Zn vacancy distributions determined by secondary ion mass spectroscopy (SIMS) and photoluminescence (PL) measurements in crystals grown under three di!erent growth con"gurations relative to the gravity direction, we have observed, for the "rst time, the e!ects of the gravity vector orientation on the point defect distributions in the ZnSe crystals grown by PVT.
2. Experimental procedure 2.1. Crystal growth The ZnSe crystals were grown in a closed PVT system using a three-zone translational furnace. To study the e!ects of gravity vector orientation on the various properties of the grown crystals the growth experiments were conducted in three con"gurations. With respect to the gravity vector orientation the growth direction made an angle of 03 (VD, vertical destabilized con"guration), 903 (H, hori-
zontal con"guration) and 1803 (VS, vertical stabilized con"guration). The growth furnace and process [8,16}19] as well as the results on ZnSe [20] have been described in detail elsewhere. The samples from four grown boules will be reported here: the horizontally self-seeded grown ZnSe-15H and ZnSe-40H crystals, the vertically (destabilized) self-seeded grown ZnSe-31VD and the vertically (stabilized) seeded-grown ZnSe-16VS crystals. The growth temperature was in the range of 1135} 11443C and the furnace translation rate varied from 2.4 mm/day (ZnSe-16VS) and 3.5 mm/day (ZnSe15H) to 4.9 mm/day (ZnSe-40H and ZnSe-31VD). 2.2. SIMS measurements Four ZnSe samples were prepared for the SIMS measurements. Two of them were used as standards. One standard was the high-purity grade starting material provided by Eagle-Picher, Inc. The other standard was a specimen sliced from a vertically (stabilized) grown crystal, ZnSe-35. Before the SIMS measurements the impurity levels in both standards were analyzed using glow discharge mass spectroscopy (GDMS) by the National Research Council of Canada. Among the 55 trace elements measured most of the impurity levels in the two standards were below 50 ppb (atomic) except for the elements O, Al, Si, Ca, Fe, Cu, Cd and Te. The remaining two samples were wafers sliced from two ZnSe grown boules for the impurity distribution mapping. One sample was sliced axially from the as-grown facet on the side of the crystal, ZnSe-40H, grown horizontally with no contact between the crystal and the ampoule wall [20]. The other sample was sliced perpendicularly to the growth direction from the ZnSe-16VS crystal grown in the vertical stabilized con"guration. Except for the as-grown ZnSe-40H facet surface the other three samples were polished with diamond paste to 0.1 lm particle size. The SIMS analyses were performed at the University of New Mexico/Sandia National Laboratory Advanced Material Laboratory. A Cameca IMS-4f machine was utilized to provide the oxygen ion beam with spatial resolution of 25}30 lm in diameter and 100 lm in depth. The standards were mounted together with the sample in the chamber
C.-H. Su et al. / Journal of Crystal Growth 204 (1999) 41}51
and the beginning-of-day and end-of-day analyses of the standards were performed to remove possible error from instrument drift over that particular day. The absolute concentration of a speci"c element in the samples was determined from the relative sample/standard signal ratio of the element and the concentration of that element in the standards determined from GDMS. A total of 13 points and 36 points were mapped on the ZnSe-40H and the ZnSe-16VS wafers, respectively. 2.3. PL measurements and annealings under controlled Zn pressure The surfaces of the PL samples were either asgrown (1 1 0) facet, cleaved (1 1 0) plane or wiresawn surface polished with Al O /H O solution to 0.3 lm particle size. The samples were illuminated with the 364 nm line from a coherent Ar ion laser at a power density of about 0.8 W/cm and a beam size of about 1.5 mm in diameter. The PL spectra were detected by a SPEX 1403 spectrometer with a typical slit width of 200 lm which corresponded to a resolution of 0.32 meV. The samples were maintained between 5 and 10 K and the spectral data were recorded by a personal computer using SPEX DM3000 software. The intensity ratios between various emissions were calculated by taking the ratios of the integrated areas under the particular emission peaks. The PL mapping was accomplished by analyzing the 20 to 90 PL spectra measured on the four sample surfaces at equaldistance grid points. The PL emission designated as I at 2.783 eV has been attributed in the literature to either as an exciton bound to the deep Zn vacancy acceptor [21}26], < , or bound to the deep Cu accep8 tor [23,27}29]. To identify the origin of I ex periments of annealing ZnSe under controlled Zn pressure were performed. In the "rst annealing experiment, ZnSe-4A, the PL spectrum of a (1 1 0)ZnSe wafer provided by Eagle-Picher, Inc. was measured before annealing. Then the 2 mm thick wafer and 3.66 g of Zn (six-nines purity from Johnson Matthey, Inc.) were loaded inside a 12 mm OD, 8 mm ID fused silica ampoule which had been outgassed under vacuum for 16 h. The sealed am-
43
poule was then placed inside a two-zone furnace with ZnSe at 11043C and Zn at 5563C. After 5 h the annealing was terminated by cooling the furnace to room temperature in 4 h. A second annealing experiment, ZnSe-7A, was performed using a 2.75 mm thick ZnSe(1 1 0) wafer and 3.45 g of Zn provided by the same vendors. Prior to seal-o! the ampoule was back "lled with 0.03 atm of Ar to reduce the sublimation rate of ZnSe caused by the applied thermal gradient during annealing. The ZnSe wafer was controlled at 11043C, same as ZnSe-4A, while the Zn reservoir was maintained at 5753C during the second annealing. After 8 h the annealing was terminated by directionally cooling the ampoule from the Zn end to the room temperature environment in 3 min.
3. Results and discussion 3.1. SIMS mappings The concentration contour map of [Si], [Fe] and [Cu] generated by the SIMS on the horizontally grown ZnSe-40H(1 1 0) facet are shown in Fig. 1a}Fig. 1c, respectively, together with the growth direction and gravity direction. From the SIMS results [Si] and [Fe] clearly show a tendency to segregate towards the bottom while [Cu] segregates towards the bottom at the "rst grown region and towards the top at the middle region of the facet. The mappings on the vertically grown ZnSe16VS wafer cut perpendicular to the growth axis are shown in Fig. 2a}Fig. 2c for [Si], [Fe] and [Cu], respectively. The results of [Si] and [Cu] indicate that these impurities tend to segregate radially towards the edge of the wafer. The mapping of [Fe] implies very high concentration at one edge point which might have been caused by contamination. In general, the measured impurity levels, especially [Cu], in ZnSe-40H are lower than that in ZnSe-16VS. This di!erence might be the result of the impurity segregation along the axial growth direction since the mapping on ZnSe-40H covered the area at 1.2}1.7 cm from the "rst-grown tip whereas the mapping on the ZnSe16VS wafer was sliced at 2.0 cm from the "rstgrown tip.
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C.-H. Su et al. / Journal of Crystal Growth 204 (1999) 41}51
Fig. 1. Impurity concentration contour maps generated by SIMS on the ZnSe-40H as-grown facet. (a) [Si], (b) [Fe] and (c) [Cu].
The error in the absolute values for the impurity levels (given in ppb, atomic) in Figs. 1 and 2 is not known. The assumption that the standards are compositionally uniform so that the GDMS results represent the impurity levels at the SIMS spot is the main source of the error. However, the quantitative relative distributions of the impurities should be accurate to the instrument errors associated with SIMS ((5%).
3.2. PL measurements and controlled Zn pressure annealings Fig. 3 shows the broad PL spectrum of the GDMS specimen, ZnSe-35V, and it represents the typical PL spectrum for other samples studied here. The spectrum was dominated by the near band edge emission and no donor}acceptor pairs (DAP) emissions were observed. The emission at 2.802 eV,
C.-H. Su et al. / Journal of Crystal Growth 204 (1999) 41}51
45
Fig. 2. Impurity concentration contour maps generated by SIMS on a ZnSe-16VS wafer cut perpendicular to the growth axis. (a) [Si], (b) [Fe] and (c) [Cu].
designated as F , is associated with the free exciton [30}32]. The I emission at 2.798 eV was attributed to the exciton bound to the substitutional neutral donors such as Al, Ga and In on the Zn site or Cl and F on the Se site [24,25,27,33]. From the GDMS results on this sample the possible donor concentrations measured were: [Al] 1700 ppb, [Ga] 30 ppb, [Cl] 9 ppb, [In] 4 ppb and [F] under detection limit of 5 ppb. Therefore, it was concluded that Al is the most likely donor associated
with the I emission in our grown crystals. As mentioned in Section 2.3 the I emission at 2.783 eV with its LO phonon replicas at 32 meV apart was considered to be either associated with an exciton bound to the deep < or bound to the 8 deep Cu acceptor. Fig. 4 shows the near band edge PL emissions for the sample ZnSe-4A before and after the Zn pressure annealing. The free exciton emission, F , is an intrinsic property of the samples. The intensity
46
C.-H. Su et al. / Journal of Crystal Growth 204 (1999) 41}51
crystals. At the same time, the emission of I re duced tremendously from A(I /A(F )"64.5 before annealing to 7.52 after annealing. Assuming [< ] 8 is proportional to A(I )/A(F ), this result con"rms the previous "nding that the I emission is re lated to < [21}26]. The reaction during Zn vapor 8 annealing can be described as the sublimation of a Zn atom from the solid and the formation of a Zn vacancy and a Zn atom in the gas phase
Fig. 3. PL broad spectrum on the vertically (stabilized) grown ZnSe-35V crystal.
Zn (s)P< #Zn(g). (1) 8 8 From a statistical thermodynamic analysis of nonstoichiometry in nonmetallic binary compounds [36] the chemical potential of Zn, k , is given by 8 [< ] (2) k "!e !k¹ ln 8 , 8 8 2 where e is the excess Gibbs free energy to create 8 one Zn vacancy. For an ideal vapor phase the chemical potential can be written as k "k¹ ln P #k (¹), (3) 8 8 8 where P is the partial pressure of Zn in atm and 8 k (¹) is the chemical potential of Zn gas at 1 atm 8 and the temperature in question. From Eqs. (2) and (3), we have
Fig. 4. Near band edge PL spectra of the ZnSe-4A sample before and after Zn controlled pressure annealing.
ratio of a speci"c emission to that of the free exciton emission has been adopted by the Si industry [34] and by Isshiki et al. on ZnSe [35] as a measurement of the defect concentration associated with that emission. From the spectra the I intensity in creased as the ratio of the integrated area under the emission changed from A(I )/A(F )"0.58 before annealing to 2.10 after annealing. It is noteworthy that the GDMS results on the ZnSe starting material showed that all the "ve possible I donor con centrations were below 5 ppb. It is evidenced from the PL and GDMS results that during processing at elevated temperature Al was released from the fused silica ampoule and incorporated in the grown
k¹ ln(P [< ])"!H #¹S #k¹ ln 2, (4) 8 8 8 8 where H and S are the enthalpy and excess 8 8 entropy, respectively, to create one Zn vacancy in the ZnSe solid from Zn vapor at 1 atm. According to Eq. (4) the quantity P [< ], or 8 8 P ;A(I )/A(F ), should be invariant for ZnSe an8 nealing at constant temperature but under di!erent P . The vapor pressure of Zn is given by [37] 8 6742 log P (atm)"! #9.874 8 ¹ (K) !1.3555 log ¹ (K).
(5)
Table 1 lists the annealing temperatures of ZnSe and the Zn reservoir, the corresponding Zn over pressure, the measured integrated intensity ratio of PL emissions, A(I )/A(F ) and the product P ; 8 A(I )/A(F ). The last column shows that P ; 8 A(I )/A(F ) is indeed a constant, with an average value of 0.04624, for the two annealing runs with di!erent P . 8
C.-H. Su et al. / Journal of Crystal Growth 204 (1999) 41}51
47
Table 1 The annealing temperatures of ZnSe, ¹ , and the Zn reservoir, ¹ , the corresponding Zn over pressure, P , the measured integrated 81 8 8 intensity ratio of PL emissions, A(I )/A(F ) and the product P ;A(I )/A(F ) 8 Run
¹ (3C) 81
¹ (3C) 8
P (atm) 8
A(I )/A(F )
P ;A(I )/A(F ) 8
ZnSe-4A ZnSe-7A
1104 1104
556 575
6.1;10\ 9.0;10\
7.52 5.18
0.0459 0.0466
Fig. 5. PL intensity ratio contour maps on the ZnSe-40H as-grown facet. (a) A(I )/A(F ) and (b) A(I )/A(F ).
3.3. PL mappings Fig. 5 presents the PL mapping results on the as-grown facet of ZnSe-40H, the same facet as that shown in Fig. 1 for the SIMS results, as well as the gravity direction and the growth direction. The A(I )/A(F ) contour map is given in Fig. 5a which indicates that [Al] segregated radially slightly towards the top and axially towards the "rst grown region. Fig. 5b shows the A(I )/A(F ) contour map on the same surface. The equilibrium local P cal8 culated from P ;A(I )/A(F )"0.04624 corres8 ponds to 6.1;10\ atm for the highest measured A(I )/A(F ), 7.58, and 2.17;10\ atm for A(I )/ A(F ) at 2.13. The map implies that [< ] segre 8 gates radially towards the bottom and axially towards the last grown region. The PL mappings on
the polished ZnSe-15H wafer cut perpendicular to the growth direction are shown in Fig. 6a and Fig. 6b, respectively, for A(I )/A(F ) and A(I )/A(F ). Trends in the radial distributions of [Al] and [< ] 8 similar to that in ZnSe-40H were observed. The near band edge PL emission of the as-grown surface of ZnSe-16VS, the same crystal as that of the SIMS measurements described earlier, is shown in Fig. 7. Besides the F , I and I emissions, the emission of IV W at 2.792 eV was observed. This peak was attributed to the exciton bound to the neutral Li or Na acceptor [21,27,38,39]. Fig. 8a}Fig. 8c show the contour maps of A(I )/A(F ), A(IV W)/A(F ) and A(I )/A(F ), respectively. The mappings indi cate that [Al], [Li or Na] and [< ] segregated 8 radially towards the center. Fig. 9a and Fig. 9b shows the contour maps of A(I )/A(F ) and
48
C.-H. Su et al. / Journal of Crystal Growth 204 (1999) 41}51
Fig. 6. PL intensity ratio contour maps on the ZnSe-15H wafer cut perpendicular to the growth axis. (a) A(I )/A(F ) and (b) A(I )/A(F ).
Fig. 7. Near band edge PL spectrum on the as-grown ZnSe16VS surface.
A(I )/A(F ) on the as-grown surface of the vertically grown ZnSe- 31VD. The implication from the mappings is that [Al] and [< ] segregated without an 8 apparent pattern. A factor of 3 in A(I )/A(F ), or [Al], was observed along the gravity direction in the horizontally grown ZnSe-40H crystal. In the vertically (stabilized) grown ZnSe-16VS the A(I )/A(F ) contour was almost radially symmetric with a factor of 2.6 between the edge and the center of the crystal. On the other hand, the calculated local P in equilib8 rium with the crystals given in the above
A(I )/A(F ) mappings should be only valid for the condition that the mapped surfaces were at the temperature of 11043C during growth. All the mapped surfaces, except ZnSe-40H, were sliced perpendicularly to the thermal gradient and, therefore, should be reasonably isothermal and the temperature range was estimated from the degree of supercooling to be 1105 to 11303C. In the case of ZnSe-40H the 5 mm axial distance across the surface was at a temperature gradient of about 203C/cm. However, this gradient only a!ects the accuracy of P in the axial direction and the 8 relative segregation measured in the radially gravitational direction should be valid. Along the radial direction in the growth ampoules a P ratio of about 2 from top to bottom in 8 the horizontal cases and a ratio of 3.5 from edge to center in the vertical stabilized case was observed. 3.4. Discussions on the measured segregations To interpreted the segregations measured by the SIMS and PL mappings the thermophysical properties of the vapor phase such as the identities of the transported vapor species responsible for the incorporation of the measured impurities are
C.-H. Su et al. / Journal of Crystal Growth 204 (1999) 41}51
49
Fig. 8. PL intensity ratio contour maps on the ZnSe-16VS. (a)A(I )/A(F ), (b) A(IV W)/A(F ) and (c) A(I )/A(F ).
needed. Without a detailed knowledge on the transported species the cause of the measured segregations is speculated below. In the horizontal case, in an environment with Zn (atomic weight 65.37) and Se (157.92) as the predominant species the slight trend of Al (26.98) segregated towards the top as measured by PL mapping was probably caused by the strati"cation in the vapor phase due to the buoyancy force. The segregation of Si (atomic weight 28.09) and Fe (55.85) towards the bottom as measured by SIMS implies that the atomic species of Si and Fe may not
be the transported species in the vapor phase. The [Cu] segregation in ZnSe-40H cannot be interpreted easily. Since Cu is known to be a fast di!user at elevated temperature in the wide band gap semiconductors, such as CdTe [40], a simple consideration involving only the vapor phase will not be adequate. The Zn atom is 2.4 times lighter than the Se molecule and, hence, radially, P was high 8 er towards the top of the ampoule which resulted in higher [< ] towards the bottom of the crystal as 8 measured by PL mappings on both ZnSe-40H and ZnSe-15H.
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Fig. 9. PL intensity ratio contour maps on the as-grown surface of ZnSe-31VD. (a) A(I )/A(F ) and (b) A(I )/A(F ).
In the vertically stabilized growth runs, both SIMS and PL mappings on the ZnSe-16VS samples showed a radially symmetric pattern in the segregations of [Si], [Al], [Li or Na] and [< ] 8 which may have been caused by a convective #ow in the shape of toroid adjacent to the growing surface. On the other hand, in the vertically destabilized case of ZnSe-31VD, due to the enhancement of the convective contribution to the #uid #ow there were no clear patterns in the segregations of [Al] and [< ] as measured by the PL 8 mapping. The axial segregation of [< ], or P , mapped 8 8 by PL on ZnSe-40H shows a factor of 3.5 in the [< ] variation over the 5 mm axial distance with 8 the higher [< ] towards the last grown region. 8 A simple one-dimensional di!usion calculation using the experimental growth conditions (¹ " 11453C, ¹ "11313C, P /P (source)"2.9 8 1 and the measured residual gas and composition [20]) as inputs predicts that the stoichiometry at the grown crystal corresponds to P /P "4.9. 8 1 Hence, due to the limited amount of source material, as the growth proceeded the source material became less Zn-rich and so did the grown crystal - [< ] higher towards the last grown 8 region.
4. Conclusions For the "rst time the e!ects of earth gravity vector orientation on the distribution of point defects ([Si], [Fe], [Cu], [Al], [Li or Na] and [< ]) 8 were observed in the ZnSe samples grown by PVT under horizontal, vertical (stabilized and destabilized) con"gurations using the SIMS and PL mappings. The annealings of ZnSe under controlled Zn pressures con"rmed the origin of the PL I emission and established the correlation be tween measured PL emission intensity and the equilibrium P . In the horizontal grown crystals, 8 segregations along the gravity vector direction were observed whereas in the vertically (stabilized) grown crystal the segregations were radially symmetrical. Furthermore, there were no apparent patterns in the measured distributions in the vertically (destabilized) grown crystal. The observed segregations in the three growth con"gurations were interpreted based on the possible contribution of buoyancy-driven convection in the vapor phase. Acknowledgements Special thanks are due to Dr. M. Wiedenbeck for the technical assistance on the SIMS measurements
C.-H. Su et al. / Journal of Crystal Growth 204 (1999) 41}51
and to National Research Council of Canada for the GDMS analyses. This work was supported by the Microgravity Research Division of the National Aeronautics and Space Administration.
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