- Email: [email protected]
Metalorganic vapor phase epitaxy of Zn1−xFexSe films
j. . . . . . . . C R Y S T A L GROWTH
ELSEVIER
Journal of Crystal Growth 170 (1997) 523-527
Metalorganic vapor phase epitaxy of Znl_xFe Se films J. Peck a, T.J. Mountziaris a,., S. Stoltz b, A. Petrou h, P.G. Mattocks c Department of Chemical Engineering, Center for Electronic and Electro-optic Materials, State UniL'ersiO'of New York, Buffalo. New York 14260, USA Department of Physics, Center for Electronic and Electro-optic Materials, State Unit ersity of New York, Buffalo, New York 14260, USA c Department of Physics, SUNY College at Fredonia, Fredonia, New York 14063, USA
Abstract
Thin single-crystalline films of the diluted magnetic semiconductor (DMS) Zn~_~Fe~Se (0 < x _< 0.22) have recently been grown by metalorganic vapor phase epitaxy (MOVPE) [1]. The films were deposited on GaAs(100) substrates in a vertical axisymmetric stagnation-flow reactor equipped with a specially designed split inlet to minimize pre-reactions between the group II and VI precursors. The precursors were (CH3)2Zn:N(C2Hs) 3, Fe(CO) 5 and H2Se diluted in H 2 carrier gas. The Zn~_,FexSe films were grown at 393°C, 120 Torr and V I / I I = 1.11. These conditions were found to yield very high quality ZnSe films. Film growth rates ranged from 3.54 /xm/h for x = 0.22 to 4.11 /xm/h for x = 0.09 and are significantly (up to an order of magnitude) higher than the typical rates obtained by molecular beam epitaxy (MBE). The epilayers were characterized by X-ray diffraction (XRD), Raman, reflectance, absorption, and X-ray photoelectron spectroscopies (XPS) and by scanning electron microscopy (SEM). It appears that MOVPE is a suitable technique for growing high quality epitaxial Zn I _xFexSe films and is very attractive for efficient growth of thick (several microns) DMS films for Faraday magneto-optical applications.
1. I n t r o d u c t i o n
Diluted magnetic semiconductors (DMS) are I I V I compounds in which a fraction of the group II
cations is randomly substituted by a transition metal ion, such as Fe z+, Co 2+ or Mn 2+ [2,3]. The interest in DMS alloys arises from the hope of manipulating the electronic properties of semiconductors by introducing magnetic interactions. The exchange interaction between the spins of the transition metal ions and those of the band electrons can lead to a dra-
* Corresponding author. Fax: + l 716 645 3822; E-mail: [email protected].
matic enhancement of g-factors (by as much as two orders of magnitude) and to giant Faraday rotations [4-6]. Furthermore, DMS alloys can exhibit negative magnetoresistance, which can reach very high values and lead to an insulator-to-metal transition induced by an increasing external magnetic field. The ultimate objective of this research is to exploit the magnetic-field related effects in DMS films and epitaxial heterostructures for magneto-optical device applications. The first DMS crystals were grown by bulk growth techniques, such as the modified Bridgman method [7]. Over the past ten years, molecular beam epitaxy (MBE) [8], a high vacuum technique that utilizes beams of elemental sources, has been used to grow a variety of DMS films and quantum-well structures
0022-0248/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved PII S0022-0248(96)0063 8-0
524
J. Peck et al. /Journal of Co'stal Growth 170 (1997) 523-527
based on them. From the various DMS materials, the Mn-based ones are better understood than those containing Fe or Co. Zn I xFexSe has been grown in the past by the Bridgman technique [9] in the form of bulk crystals and by MBE in the form of thin films [101. Our group has recently reported the growth of Zn I xFexSe (0 < x < 0.22) epitaxial layers on GaAs(100) substrates by metalorganic vapor phase epitaxy (MOVPE) [1]. MOVPE offers higher flexibility in terms of precursor selection as well as cost-effectiveness compared to MBE. The control on thickness and composition of the epilayers achieved by MOVPE is sufficient for most optoelectronic applications [11]. Furthermore, the film growth rates during MOVPE can be up to an order of magnitude higher than the ones obtained by MBE. As a result, MOVPE is the technique of choice for growing thick (several microns) DMS films for devices, such as magneto-optical modulators based on the giant Faraday rotation of DMS films.
2. Film growth Znl_xFexSe films were grown on GaAs(100) substrates, 2 ° misoriented towards (110), in a vertical stagnation-flow MOVPE reactor. The precursors used in this study were: (CH3)2Zn:N(C2H5)3, Fe(CO) 5 and H2Se diluted in H 2 carrier gas. The (CH3)2Zn:N(CzHs) 3 liquid was obtained in a stainless steel bubbler from Air Products and was kept at 17°C during growth. Its vapor pressure at 17°C is 30 Torr and its vapor pressure equation is: log lo P(mmHg) = 8.266 - 1970/T(K). The Fe(CO) 5 liquid was obtained from Aldrich and was further purified by vacuum distillation. It was kept at 0°C during growth. Its vapor pressure at 0°C is 7 Torr and its vapor pressure equation is log 10P(mmHg) = 8.50 - 2097/T(K). The H2Se gas was obtained as a 5% mixture in H 2 from Solkatronic Chemicals (megabit grade). The H 2 carrier was purified using a Pd diffusion cell (Matheson). The gas flow rates were controlled by electronic mass flow controllers (Edwards). Typical flow rates of the H 2 streams were 500 sccm of carrier, 16.6 sccm through the Zn bubbler and 1.5 to 5 sccm through the Fe bubbler. The flow rate of the 5% H2Se in H 2 mixture was 15
sccm. A typical substrate size was 1 × 1 cm. The substrates were cleaned and chemically etched using standard procedures before being loaded into the reactor. They were pre-treated in the reactor by heating them at 560°C and 120 Torr for at least 3 min under hydrogen. The group VI source was turned on at 420°C and was kept on until the susceptor temperature dropped below 340°C after the growth was completed. A specially designed axisymmetric inlet consisting of two co-axial stainless steel tubes ( 1 / 8 and 1 / 2 inch OD) was employed to minimize pre-reactions between the Zn and Se sources [1]. The Se source was supplied through the inner tube, while the Zn and Fe sources and the bulk of the carrier gas were supplied through the annular region. The distance between the outlet of these tubes and the substrate was 1 inch. This distance and the flow rates of the gases can be adjusted to prevent formation of particulates, but still allow complete mixing above the substrate to obtain films with uniform composition. When the Zn and Se sources were premixed 1 ft upstream of the reactor, particulates (presumably ZnSe) were formed in the inlet tube and films could not be grown. The operating conditions used for MOVPE of Znl_xFexSe were identified by growing first ZnSe on GaAs under various conditions and comparing the quality of the epilayers using Raman and reflectance spectroscopies. Very high quality ZnSe epilayers were grown at a susceptor temperature of 393°C, an operating pressure of 120 Torr and a V I / I I ratio of 1.11 to 1 with the mole fraction of the Zn source in the combined inlet stream being 1.3 × 10-3. Znl_xFexSe films were subsequently grown by introducing Fe(CO) 5 vapors into the reactor. The mole fraction of the more reactive Fe source was varied between 2.6 × 10 .5 and 8.7 × 10 .5 in the combined inlet stream to obtain films with varying Fe content.
3. Film characterization The results from the analysis of a ZnSe sample and three representative Znl_xFexSe samples are summarized in Table 1. It appears that the growth rate of the Znl_xFe, Se films drops with increasing iron content. However, the growth rate of the pure
J. Peck et al. / Journal of Crystal Growth 170 (1997) 523-527
ZnSe film is the lowest, indicating a possible synergistic effect of the Fe when it is present in small concentrations. All reported growth runs were repeated at least once and the results were found to be reproducible. XRD peaks obtained from a Rigaku Geigerflex 0 - 2 0 diffractometer with C u K a radiation and a curved graphite monochromator were attributed to the (200), (400) and (600) planes of the Zn~_xFexSe epilayers with the same orientation as the substrate. The (400) peak widths (6(20)) of the scans increased with iron concentration to 0.2 ° for sample #4, compared with the instrument limit of 0.06 ° for the GaAs substrate. The peaks moved progressively to lower angles as x increased. The GaAs and ZnSe (400) peaks provided reference points from which to measure this shift. The published value for the variation of the lattice constant, a, of MBE-grown Zn~_xFe~Se films with the Fe mole fraction (x), da/dx=O.058 A [12], leads to the values of x listed in Table 1. These values are in good agreement with those determined from the bandgap measurements described below. Optical characterization of the Znl_xFexSe epilayers was carried out using Raman, reflectance and absorption spectroscopies. The Raman spectra yield information on the vibrational modes associated with the incorporation of Fe in the group II fcc sublattice [13]. Optical absorption and reflectance, on the other hand, measure the bandgap, which depends on the Fe mole fraction [10]. The room-temperature Raman spectrum associated with the zone center optical phonons from sample # 4 is shown in Fig. 1. This spectrum was excited using 100 mW of the 4880 ,~ line of an argon-ion laser. The backscattering z(x',x')~ geometry, for which the LO phonons are allowed, was used. Here x', y' and z correspond to the (110), (]10) and (001) crystallographic direc-
525
2.0.
LO 1
Sample 4 .-~ 1.8. T = 3 0 0 K t-
/ / GaAs 1.6-
/11~° ~.,,
•
1.4-
•~
1.2,
-
lg 1.0.
~ LO 2, TO I
08
2~o
2~o
3~o
Stokes Raman Shift (cm-1) Fig. 1. Raman phonon spectrum of Zn~_xFexSe (x = 0,22) epilayer recorded at 300 K using 100 mW of the 4880 ,~ line of an argon-ion laser.
tions, respectively. The scattered light was analyzed with a Spex triple spectrometer equipped with a cooled CCD multichannel detector array. Here we follow the notation of Mak et al. [13] to label the various Zn t _ xFex Se phonons. In addition to the TO 2 mode at 206 cm - t and the LO~ mode at 258 cm -1 present in ZnSe, a phonon at 229 c m - t labelled as LO 2 is identified as the band mode ZnSe:Fe in the mixed crystal [13]. We note that we were not able to resolve the TO t from the LO 2 mode. This is probably due to the fact that the TO t mode is forbidden in the backscattering geometry from the (100) face and the calculated separation between the two modes is less than 5 c m - t for x < 0.22. The monochromatic beam for the absorption and reflectance spectroscopies is produced by a combination of a 250 W broad-band, tungsten-halogen lamp and a grating spectrometer. The intensity of the transmitted or reflected beam is synchronously detected using a photomultiplier tube operating in cur-
Table 1 Results from the characterization of the Z n S e / G a A s and the Zn I _ x F e xS e / G a A s epilayers grown at 393°C, 120 Torr and V I / I I = 1.11; The mole fraction of the Zn source in the combined feed stream of the reactor was 1.3 X 10 -3 Sample
1 2 3 4
Thickness (/zm)
1.700 1.028 0.973 0.885
Growth time (min)
30 15 15 15
Growth rate ( / z m / h )
3.40 4.11 3.89 3.54
Fe mole fraction, x In feed
In film
0.00 2.6X 10 -5 4.3 X 10 -5 8.7 X 10 -5
0.00 0.09 0.11 0.22
Bandgap (meV)
2800 2815 2840 2898
J. Peck et al. /Journal of Co'~tal Growth 170 (1997) 523-527
526
Sample 1 T = 10K
c
v UJ "O I
'ID
z}9
z~0
zal
z.82
Incident Photon Energy
(eV)
Fig. 2. Reflectance derivative spectrum (d R / d E) of ZnSe calibration film at 10 K.
rent mode. To measure the bandgap of the MOVPEgrown samples using absorption, the GaAs substrate was chemically etched away using a 5 : 1 mixture of 5% NaOH and 30% H202 [14]. The optical transmission is zero for photon energies above the gap; it increases abruptly at the bandgap, Eg, which allows an accurate determination of Eg. Experimentally measured bandgaps of representative samples are listed in Table 1. An alternative method for determining the bandgap is reflectance spectroscopy. The advantage of this technique is that it does not require any sample preparation (such as the removal of the GaAs substrate). The reflectance derivative dR/dE
Sample 2 T= 10K ¢.-
uJ
"o "O
2.80
2.82
2.84
Incident Photon Energy
2.86 (eV)
Fig. 3. Reflectance derivative spectrum (d R / d E) of Zn t - , Fe ~ Se ( x = 0.09) epilayer at 10 K.
spectrum for the ZnSe (x = 0) reference sample #1 is shown in Fig. 2. It contains a strong feature at the bandgap of ZnSe (2.800 eV). The reflectance derivative spectrum from sample # 2 (x = 0.09) is shown in Fig. 3 and indicates a bandgap of 2.815 eV. The increase in the linewidth compared to sample #1 is attributed to alloy fluctuations. The bandgaps determined using reflectance spectroscopy are in very good agreement with the ones obtained using absorption. The morphology of the films was studied by SEM (Hitachi S-800). Micrographs of samples with low Fe concentration (x = 0.09) indicated a very smooth surface. An increase in surface roughness with increased iron content was observed with an "orangepeel" structure [15] at x = 0.11 and 0.22. The observed feature widths in the lateral dimension were about 0.4 and 0.8 /xm for samples with x = 0.11 and 0.22, respectively. Finally, the elemental composition of the epilayers was obtained by XPS analysis (Surface Science Instruments 206 Small Spot) and confirmed the presence of Zn, Se and Fe at concentrations comparable to those obtained by XRD.
4. Conclusions
Zn 1 xFe ~Se epilayers were grown on GaAs(100) substrates by MOVPE using (CH3)2Zn:N(C2Hs) 3, Fe(CO) 5 and H2Se diluted in H 2 as precursors. The incorporation of Fe in the films was confirmed by observing the vibrational modes associated with the incorporation of Fe in the group II fcc sublattice using Raman spectroscopy. Furthermore, XPS analysis indicated the presence of Zn, Fe and Se in the films. The mole fraction of Fe in the films was obtained by measuring the variation of the lattice constant with Fe content using XRD. The bandgap of the films was measured by absorption and reflectance spectroscopies and found to increase with increasing Fe content. SEM analysis of the films indicated a very smooth surface at low Fe content ( x = 0.09) and an increased surface roughness at higher Fe concentration (x = 0.1 ! and 0.22). It appears that MOVPE is a suitable technique for growing high quality Zn I xFe,.Se films for magneto-optical devices. The high growth rates achieved by
J. Peck et al. / Journal of Co'stal Growth 170 (1997) 523-527
MOVPE in comparison to MBE, make MOVPE especially attractive for applications that require thick films, such as Faraday magneto-optical modulators.
Acknowledgements This work has been supported by the Office of the Provost (Multidisciplinary Pilot Project Program) and by the Center for Electronic and Electro-optic Materials of SUNY-Buffalo. We would like to thank Professor H. Luo for helpful discussions, X.W. Lin, T. Overocker and W.Y. Yu for assistance with the experiments and Professor O.T. Beachley, Jr., and D. Rosenblum for the purification of the Fe source. J.P. acknowledges financial support from the NASA Space Grant Fellowship Program.
References [1] T.J. Mountziaris, J. Peck, S. Stoltz, W.Y. Yu, A. Petrou and P.G. Mattocks, Appl. Phys. Lett. 68 (1996) 2270. [2] J.K. Furdyna, J. Appl. Phys. 64 (1988) R29.
527
[3] N. Samarth and J.K. Furdyna, Proc. IEEE 78 (1990) 990. [4] J.A. Gaj, R.R. Galazka and M. Nawrocki, Solid State Commun. 25 (1978) 193. [5] D.K. Bartholomew, J.K. Furdyna and A.K. Ramdas, Phys. Rev. B 34 (1986) 6943. [6] L.P. Fu, T. Schmiedel, A. Petrou, J. Warnock and B.T. Jonker, Appl. Phys. Lett. 60 (1992) 583. [7] A. Pajaczkowska, Progr. Crystal Growth Characterization 1 (1978) 289. [8] L.A. Kolodziejski, R.L. Gunshor, N. Otsuka, S. Datta, W.M. Becker and A.V. Nurmikko, IEEE J. Quantum Electron. QE-22 (1986) 1785. [9] A. Twardowski, M. yon Ortenberg and M. Demianiuk, J. Crystal Growth 72 (1985) 401. [10] B.T. Jonker, J.J. Krebs, S.B. Qadri and G.A. Prinz, Appl. Phys. Lett. 50 (1987) 848. [11] T.F. Kuech, Proc. IEEE 80 (1992) 1609. [12] B.T. Jonker, J.J. Krebs, G.A. Prinz, X. Liu, A. Petrou and L. Salamanca-Young, Mater. Res. Soc. Syrup. Proc. 151 (1989) 151. [13] C.-L. Mak, R. Sooryakumar, B.T. Jonker and G.A. Prinz, Phys. Rev. B 45 (1992) 3344. [14] E. Kurtz, S. Einfeldt, J. Nurmberger, S. Zerlauth, D. Hommel and G. Landwehr, Phys. Status Solidi (b) 187 (1995) 393. [15] P.J. Wright, P.J. Parbrook, B. Cockayne, A.C. Jones, E.D. Orrel, K.P. O'Donnell and B. Henderson, J. Crystal Growth 94 (1989) 441.
ELSEVIER
Journal of Crystal Growth 170 (1997) 523-527
Metalorganic vapor phase epitaxy of Znl_xFe Se films J. Peck a, T.J. Mountziaris a,., S. Stoltz b, A. Petrou h, P.G. Mattocks c Department of Chemical Engineering, Center for Electronic and Electro-optic Materials, State UniL'ersiO'of New York, Buffalo. New York 14260, USA Department of Physics, Center for Electronic and Electro-optic Materials, State Unit ersity of New York, Buffalo, New York 14260, USA c Department of Physics, SUNY College at Fredonia, Fredonia, New York 14063, USA
Abstract
Thin single-crystalline films of the diluted magnetic semiconductor (DMS) Zn~_~Fe~Se (0 < x _< 0.22) have recently been grown by metalorganic vapor phase epitaxy (MOVPE) [1]. The films were deposited on GaAs(100) substrates in a vertical axisymmetric stagnation-flow reactor equipped with a specially designed split inlet to minimize pre-reactions between the group II and VI precursors. The precursors were (CH3)2Zn:N(C2Hs) 3, Fe(CO) 5 and H2Se diluted in H 2 carrier gas. The Zn~_,FexSe films were grown at 393°C, 120 Torr and V I / I I = 1.11. These conditions were found to yield very high quality ZnSe films. Film growth rates ranged from 3.54 /xm/h for x = 0.22 to 4.11 /xm/h for x = 0.09 and are significantly (up to an order of magnitude) higher than the typical rates obtained by molecular beam epitaxy (MBE). The epilayers were characterized by X-ray diffraction (XRD), Raman, reflectance, absorption, and X-ray photoelectron spectroscopies (XPS) and by scanning electron microscopy (SEM). It appears that MOVPE is a suitable technique for growing high quality epitaxial Zn I _xFexSe films and is very attractive for efficient growth of thick (several microns) DMS films for Faraday magneto-optical applications.
1. I n t r o d u c t i o n
Diluted magnetic semiconductors (DMS) are I I V I compounds in which a fraction of the group II
cations is randomly substituted by a transition metal ion, such as Fe z+, Co 2+ or Mn 2+ [2,3]. The interest in DMS alloys arises from the hope of manipulating the electronic properties of semiconductors by introducing magnetic interactions. The exchange interaction between the spins of the transition metal ions and those of the band electrons can lead to a dra-
* Corresponding author. Fax: + l 716 645 3822; E-mail: [email protected].
matic enhancement of g-factors (by as much as two orders of magnitude) and to giant Faraday rotations [4-6]. Furthermore, DMS alloys can exhibit negative magnetoresistance, which can reach very high values and lead to an insulator-to-metal transition induced by an increasing external magnetic field. The ultimate objective of this research is to exploit the magnetic-field related effects in DMS films and epitaxial heterostructures for magneto-optical device applications. The first DMS crystals were grown by bulk growth techniques, such as the modified Bridgman method [7]. Over the past ten years, molecular beam epitaxy (MBE) [8], a high vacuum technique that utilizes beams of elemental sources, has been used to grow a variety of DMS films and quantum-well structures
0022-0248/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved PII S0022-0248(96)0063 8-0
524
J. Peck et al. /Journal of Co'stal Growth 170 (1997) 523-527
based on them. From the various DMS materials, the Mn-based ones are better understood than those containing Fe or Co. Zn I xFexSe has been grown in the past by the Bridgman technique [9] in the form of bulk crystals and by MBE in the form of thin films [101. Our group has recently reported the growth of Zn I xFexSe (0 < x < 0.22) epitaxial layers on GaAs(100) substrates by metalorganic vapor phase epitaxy (MOVPE) [1]. MOVPE offers higher flexibility in terms of precursor selection as well as cost-effectiveness compared to MBE. The control on thickness and composition of the epilayers achieved by MOVPE is sufficient for most optoelectronic applications [11]. Furthermore, the film growth rates during MOVPE can be up to an order of magnitude higher than the ones obtained by MBE. As a result, MOVPE is the technique of choice for growing thick (several microns) DMS films for devices, such as magneto-optical modulators based on the giant Faraday rotation of DMS films.
2. Film growth Znl_xFexSe films were grown on GaAs(100) substrates, 2 ° misoriented towards (110), in a vertical stagnation-flow MOVPE reactor. The precursors used in this study were: (CH3)2Zn:N(C2H5)3, Fe(CO) 5 and H2Se diluted in H 2 carrier gas. The (CH3)2Zn:N(CzHs) 3 liquid was obtained in a stainless steel bubbler from Air Products and was kept at 17°C during growth. Its vapor pressure at 17°C is 30 Torr and its vapor pressure equation is: log lo P(mmHg) = 8.266 - 1970/T(K). The Fe(CO) 5 liquid was obtained from Aldrich and was further purified by vacuum distillation. It was kept at 0°C during growth. Its vapor pressure at 0°C is 7 Torr and its vapor pressure equation is log 10P(mmHg) = 8.50 - 2097/T(K). The H2Se gas was obtained as a 5% mixture in H 2 from Solkatronic Chemicals (megabit grade). The H 2 carrier was purified using a Pd diffusion cell (Matheson). The gas flow rates were controlled by electronic mass flow controllers (Edwards). Typical flow rates of the H 2 streams were 500 sccm of carrier, 16.6 sccm through the Zn bubbler and 1.5 to 5 sccm through the Fe bubbler. The flow rate of the 5% H2Se in H 2 mixture was 15
sccm. A typical substrate size was 1 × 1 cm. The substrates were cleaned and chemically etched using standard procedures before being loaded into the reactor. They were pre-treated in the reactor by heating them at 560°C and 120 Torr for at least 3 min under hydrogen. The group VI source was turned on at 420°C and was kept on until the susceptor temperature dropped below 340°C after the growth was completed. A specially designed axisymmetric inlet consisting of two co-axial stainless steel tubes ( 1 / 8 and 1 / 2 inch OD) was employed to minimize pre-reactions between the Zn and Se sources [1]. The Se source was supplied through the inner tube, while the Zn and Fe sources and the bulk of the carrier gas were supplied through the annular region. The distance between the outlet of these tubes and the substrate was 1 inch. This distance and the flow rates of the gases can be adjusted to prevent formation of particulates, but still allow complete mixing above the substrate to obtain films with uniform composition. When the Zn and Se sources were premixed 1 ft upstream of the reactor, particulates (presumably ZnSe) were formed in the inlet tube and films could not be grown. The operating conditions used for MOVPE of Znl_xFexSe were identified by growing first ZnSe on GaAs under various conditions and comparing the quality of the epilayers using Raman and reflectance spectroscopies. Very high quality ZnSe epilayers were grown at a susceptor temperature of 393°C, an operating pressure of 120 Torr and a V I / I I ratio of 1.11 to 1 with the mole fraction of the Zn source in the combined inlet stream being 1.3 × 10-3. Znl_xFexSe films were subsequently grown by introducing Fe(CO) 5 vapors into the reactor. The mole fraction of the more reactive Fe source was varied between 2.6 × 10 .5 and 8.7 × 10 .5 in the combined inlet stream to obtain films with varying Fe content.
3. Film characterization The results from the analysis of a ZnSe sample and three representative Znl_xFexSe samples are summarized in Table 1. It appears that the growth rate of the Znl_xFe, Se films drops with increasing iron content. However, the growth rate of the pure
J. Peck et al. / Journal of Crystal Growth 170 (1997) 523-527
ZnSe film is the lowest, indicating a possible synergistic effect of the Fe when it is present in small concentrations. All reported growth runs were repeated at least once and the results were found to be reproducible. XRD peaks obtained from a Rigaku Geigerflex 0 - 2 0 diffractometer with C u K a radiation and a curved graphite monochromator were attributed to the (200), (400) and (600) planes of the Zn~_xFexSe epilayers with the same orientation as the substrate. The (400) peak widths (6(20)) of the scans increased with iron concentration to 0.2 ° for sample #4, compared with the instrument limit of 0.06 ° for the GaAs substrate. The peaks moved progressively to lower angles as x increased. The GaAs and ZnSe (400) peaks provided reference points from which to measure this shift. The published value for the variation of the lattice constant, a, of MBE-grown Zn~_xFe~Se films with the Fe mole fraction (x), da/dx=O.058 A [12], leads to the values of x listed in Table 1. These values are in good agreement with those determined from the bandgap measurements described below. Optical characterization of the Znl_xFexSe epilayers was carried out using Raman, reflectance and absorption spectroscopies. The Raman spectra yield information on the vibrational modes associated with the incorporation of Fe in the group II fcc sublattice [13]. Optical absorption and reflectance, on the other hand, measure the bandgap, which depends on the Fe mole fraction [10]. The room-temperature Raman spectrum associated with the zone center optical phonons from sample # 4 is shown in Fig. 1. This spectrum was excited using 100 mW of the 4880 ,~ line of an argon-ion laser. The backscattering z(x',x')~ geometry, for which the LO phonons are allowed, was used. Here x', y' and z correspond to the (110), (]10) and (001) crystallographic direc-
525
2.0.
LO 1
Sample 4 .-~ 1.8. T = 3 0 0 K t-
/ / GaAs 1.6-
/11~° ~.,,
•
1.4-
•~
1.2,
-
lg 1.0.
~ LO 2, TO I
08
2~o
2~o
3~o
Stokes Raman Shift (cm-1) Fig. 1. Raman phonon spectrum of Zn~_xFexSe (x = 0,22) epilayer recorded at 300 K using 100 mW of the 4880 ,~ line of an argon-ion laser.
tions, respectively. The scattered light was analyzed with a Spex triple spectrometer equipped with a cooled CCD multichannel detector array. Here we follow the notation of Mak et al. [13] to label the various Zn t _ xFex Se phonons. In addition to the TO 2 mode at 206 cm - t and the LO~ mode at 258 cm -1 present in ZnSe, a phonon at 229 c m - t labelled as LO 2 is identified as the band mode ZnSe:Fe in the mixed crystal [13]. We note that we were not able to resolve the TO t from the LO 2 mode. This is probably due to the fact that the TO t mode is forbidden in the backscattering geometry from the (100) face and the calculated separation between the two modes is less than 5 c m - t for x < 0.22. The monochromatic beam for the absorption and reflectance spectroscopies is produced by a combination of a 250 W broad-band, tungsten-halogen lamp and a grating spectrometer. The intensity of the transmitted or reflected beam is synchronously detected using a photomultiplier tube operating in cur-
Table 1 Results from the characterization of the Z n S e / G a A s and the Zn I _ x F e xS e / G a A s epilayers grown at 393°C, 120 Torr and V I / I I = 1.11; The mole fraction of the Zn source in the combined feed stream of the reactor was 1.3 X 10 -3 Sample
1 2 3 4
Thickness (/zm)
1.700 1.028 0.973 0.885
Growth time (min)
30 15 15 15
Growth rate ( / z m / h )
3.40 4.11 3.89 3.54
Fe mole fraction, x In feed
In film
0.00 2.6X 10 -5 4.3 X 10 -5 8.7 X 10 -5
0.00 0.09 0.11 0.22
Bandgap (meV)
2800 2815 2840 2898
J. Peck et al. /Journal of Co'~tal Growth 170 (1997) 523-527
526
Sample 1 T = 10K
c
v UJ "O I
'ID
z}9
z~0
zal
z.82
Incident Photon Energy
(eV)
Fig. 2. Reflectance derivative spectrum (d R / d E) of ZnSe calibration film at 10 K.
rent mode. To measure the bandgap of the MOVPEgrown samples using absorption, the GaAs substrate was chemically etched away using a 5 : 1 mixture of 5% NaOH and 30% H202 [14]. The optical transmission is zero for photon energies above the gap; it increases abruptly at the bandgap, Eg, which allows an accurate determination of Eg. Experimentally measured bandgaps of representative samples are listed in Table 1. An alternative method for determining the bandgap is reflectance spectroscopy. The advantage of this technique is that it does not require any sample preparation (such as the removal of the GaAs substrate). The reflectance derivative dR/dE
Sample 2 T= 10K ¢.-
uJ
"o "O
2.80
2.82
2.84
Incident Photon Energy
2.86 (eV)
Fig. 3. Reflectance derivative spectrum (d R / d E) of Zn t - , Fe ~ Se ( x = 0.09) epilayer at 10 K.
spectrum for the ZnSe (x = 0) reference sample #1 is shown in Fig. 2. It contains a strong feature at the bandgap of ZnSe (2.800 eV). The reflectance derivative spectrum from sample # 2 (x = 0.09) is shown in Fig. 3 and indicates a bandgap of 2.815 eV. The increase in the linewidth compared to sample #1 is attributed to alloy fluctuations. The bandgaps determined using reflectance spectroscopy are in very good agreement with the ones obtained using absorption. The morphology of the films was studied by SEM (Hitachi S-800). Micrographs of samples with low Fe concentration (x = 0.09) indicated a very smooth surface. An increase in surface roughness with increased iron content was observed with an "orangepeel" structure [15] at x = 0.11 and 0.22. The observed feature widths in the lateral dimension were about 0.4 and 0.8 /xm for samples with x = 0.11 and 0.22, respectively. Finally, the elemental composition of the epilayers was obtained by XPS analysis (Surface Science Instruments 206 Small Spot) and confirmed the presence of Zn, Se and Fe at concentrations comparable to those obtained by XRD.
4. Conclusions
Zn 1 xFe ~Se epilayers were grown on GaAs(100) substrates by MOVPE using (CH3)2Zn:N(C2Hs) 3, Fe(CO) 5 and H2Se diluted in H 2 as precursors. The incorporation of Fe in the films was confirmed by observing the vibrational modes associated with the incorporation of Fe in the group II fcc sublattice using Raman spectroscopy. Furthermore, XPS analysis indicated the presence of Zn, Fe and Se in the films. The mole fraction of Fe in the films was obtained by measuring the variation of the lattice constant with Fe content using XRD. The bandgap of the films was measured by absorption and reflectance spectroscopies and found to increase with increasing Fe content. SEM analysis of the films indicated a very smooth surface at low Fe content ( x = 0.09) and an increased surface roughness at higher Fe concentration (x = 0.1 ! and 0.22). It appears that MOVPE is a suitable technique for growing high quality Zn I xFe,.Se films for magneto-optical devices. The high growth rates achieved by
J. Peck et al. / Journal of Co'stal Growth 170 (1997) 523-527
MOVPE in comparison to MBE, make MOVPE especially attractive for applications that require thick films, such as Faraday magneto-optical modulators.
Acknowledgements This work has been supported by the Office of the Provost (Multidisciplinary Pilot Project Program) and by the Center for Electronic and Electro-optic Materials of SUNY-Buffalo. We would like to thank Professor H. Luo for helpful discussions, X.W. Lin, T. Overocker and W.Y. Yu for assistance with the experiments and Professor O.T. Beachley, Jr., and D. Rosenblum for the purification of the Fe source. J.P. acknowledges financial support from the NASA Space Grant Fellowship Program.
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