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Photoreflectance as a non-destructive, room-temperature technique for routine testing of PM–HEMT structures
Materials Science and Engineering B66 (1999) 141 – 145 www.elsevier.com/locate/mseb
Photoreflectance as a non-destructive, room-temperature technique for routine testing of PM–HEMT structures M. Androulidaki a,*, M. Lagadas a, K. Michelakis a, P. Panayotatos a,b a
Foundation for Research and Technology-Hellas (FORTH), IESL, Microelectronics Research Group, P.O. Box 1527, 71110 Heraklion, Crete, Greece b Rutgers, The State Uni6ersity of New Jersey, Department of Electrical & Computer Engineering, 94 Brett Rd., Piscataway, NJ 08854 -8058, USA
Abstract In the context of a comparative study of MBE and MOCVD PM – HEMT structures on 3¦ GaAs substrates, utilization of photoreflectance indicated that the technique can provide substantial information both non-destructively as well as at room temperature. Concentrating on the structure of the photoreflectance trace below 1.4 eV, the technique can provide information on the combined effect of thickness and In composition in the InGaAs quantum well. In particular, photoreflectance was found to be especially useful for mapping the uniformity over a single wafer as well as for mapping the reproducibility from wafer to wafer. Representative, characteristic patterns were consistently observed for MBE-grown layers, distinct from equally characteristic patterns of MOCVD-grown layers. More importantly, these patterns, obtained by room temperature photoreflectance, were found to coincide with those obtained by low temperature photoluminescence. Although the two techniques identify different electronic transitions, room temperature photoreflectance proves to be equally well adapted as an acceptance test for layer uniformity as low temperature photoluminescence. In probing the reason of non-uniformities, however, low temperature photoluminescence does provide more information. Experimental results are presented for both MBE and MOCVD PM – HEMT structures as well as information extracted from their treatment by modeling in the quantum wells. © 1999 Elsevier Science S.A. All rights reserved. Keywords: PM – HEMT; Photoreflectance; Photoluminescence; Uniformity
1. Introduction The non-destructive characterization and qualification of complex semiconductor multilayer structures is crucial for assessing the quality of material intended for use in fabrication of devices and MMICs such as pseudomorphic HEMT (PM – HEMT) structures, based on III–V materials, for high frequency and high power applications. Photoreflectance (PR) [1] is one such technique that can be shown to be particularly efficient, possessing the additional advantage of being a room temperature technique [2] thus simplifying the set up and having cost reduction ramifications in an industrial environment. We have applied the technique [2–4] in a study of the uniformity of AlGaAs/InGaAs/GaAs PM–HEMT structures. The samples were grown by * Corresponding author. Tel +30-81-394105; fax + 30-81-394106. E-mail addresses: [email protected] (M. Androulidaki), [email protected] (P. Panayotatos)
Molecular Beam Epitaxy (MBE) and Metalorganic Chemical Vapor Deposition (MOCVD) on (100) S.I. GaAs substrates. Room temperature PR was used to assess the general uniformity of the quantum well (QW) across each wafer. Low temperature (16 K) photoluminescence (PL) [5–7] measurements were also taken on the same wafer locations as were the PR measurements, in order to validate the PR data. These results were used to further distinguish the parameters affecting QW uniformity such as the indium mole fraction in the InGaAs layer as well as the QW width uniformity, or in other words, the InGaAs layer thickness uniformity.
2. Results and discussion Several 3¦ MOCVD and MBE-grown single heterostructure AlGaAs/InGaAs/GaAs PM–HEMT wafers were studied by both PR and PL. The InGaAs
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M. Androulidaki et al. / Materials Science and Engineering B66 (1999) 141–145
layer thickness was 100 A, for the MOCVD and 90 A, for the MBE wafers, and the two dimensional electron gas density (Ns) was higher than 2 ×1012 cm − 2 nominally. In samples on which Hall measurements were performed the nominal values were confirmed. A conventional [1,4] experimental setup was used that employs a He–Ne laser of 0.5 W/cm2. PR spectra were taken at predetermined steps across the whole wafer surface, for each wafer, and the measurements were subsequently statistically processed. A typical PR spectrum is shown in Fig. 1. The peak around 1.4 eV is common to all spectra taken from all wafers, and is attributed to the contribution of the GaAs cap and buffer layers. Because of the relatively high value of Ns, the Fermi level falls between the second and the first electron level of the QW which suggests that the latter is completely filled. This was confirmed by the lineshape of the PL spectrum, in particular from the relative intensities of the peaks at E1 and E2 and by the sloping characteristic at energies higher than E1 (Fig. 3) [8]. Thus the dominant transition that is monitored is from the heavy hole state to the second electron level of the QW. Maps of the feature around 1.24 eV are maps of the energy of this transition and are a measure of the uniformity over the wafer of QW factors that affect this transition. Fig. 2 shows typical maps of an MOCVD (left) and an MBE (right) structure. These patterns exhibit a radial symmetry around the center of the wafer in the case of MOCVD and around a point off-center just off the periphery of the wafer, in the case of MBE. Though
the particular results reported in this work focus on MBE- and MOCVD-grown single heterostructures on 3¦ wafers, these reproducible, characteristic patterns of MBE and MOCVD layers were also consistently observed for layers grown both as single heterostructures, normally intended for low noise device applications, as well as for double heterostructures, intended for power device applications. Furthermore, the characteristic MBE pattern was also present in structures grown on both 3¦ and 4¦ substrates and it was further verified by comparing to the grower’s resistivity mapping. These characteristic patterns are consistent with the positioning of the substrates during epitaxial growth: in the case of MOCVD-grown samples the substrates were individually rotated, while in the case of the MBE-grown samples the substrates were placed at the periphery of a rotating plenum. Statistical PR data for several wafers appear in Table 1. For the MOCVD-grown wafers, the transition energy monitored is found to be smaller near the center of each wafer and the variation of this value, measured by the relative standard deviation (RSD) is less than 1% over each wafer. The variation is even lower in the case of MBE-grown wafers. PR measurements were validated by PL. A typical PL spectrum, in this case for sample A, is shown in Fig. 3, where two QW transitions can be observed. PL measurements were taken at the very points on the wafer that the PR measurements had been taken. The range of values in the resulting PL mapping indicates that the energy for both transitions exhibits the same range of variation, namely about 25 meV. The differ-
Fig. 1. Typical room temperature Photoreflectance spectrum of MOCVD-grown PM – HEMT structure A with features attributed to different layers. Inset presents blow-up of InGaAs feature utilized for the determination of the energy of the transition from the heavy hole level to the second electron level in the QW.
M. Androulidaki et al. / Materials Science and Engineering B66 (1999) 141–145
143
Fig. 2. Photoreflectance mapping of similar Single Heterojunction PM – HEMT layers: MOCVD-gown structure B (left) and MBE-grown structure F (right). The average peak and relative standard deviations are 1.236 eV and 0.81% for MOCVD-B and 1.264 eV and 0.24% for MBE-F. The gray scale represents % deviation from the average.
ence of the two transition energies E2 −E1 does not vary significantly across the wafer, ranging from 92 to 97 meV. This indicates that the QW thickness variation is similarly small. In addition, the ratio of the intensities of the peaks at E2 and E1 is constant, indicating constancy of Ns. Furthermore, the shape (broadening) of the feature around the transition, in the PR spectrum this time, remains relatively constant over a single wafer, also indicating no significant variation of Ns. A variational theoretical model was used for the interpretation of the PL results with the above rationale in mind. The model self-consistently solves the Poisson and Schro¨dinger [3,9] (G. Halkias, personal communication) equations by assuming two states inside the QW. According to this model, by assuming that only the width of the QW varies, with the In composition assumed constant, the experimental results of a 5 meV maximum difference of the two transition energies E2 − E1 lead to the conclusion that the full range of variation for the QW thickness would be between 5 and 10 A, over the whole wafer surface. Such a thickness variation, however, would only account for 5 meV of variation in the PR peak at E2 which, in fact, varies by 25 meV. Given the above, the remaining factor that strongly affects the transition variation is the In composition x in the Inx Ga1 − x As well. Assuming x to be the only factor affecting QW uniformity and employing the model indicates that the change of 25 meV in E2, representing the range between the minimum and the maximum over the wafer, is caused by a 0.03 change in x. From the comparison of the PR and PL maps (Fig. 4) it is evident that a similar variation of E2 transition is present in both cases. We thus conclude that, in such a case, one can utilize PR mapping to roughly extract the In compositional variation. For the particular samples the range of 25 meV in the MOCVD samples
represent a mole fraction variation of 0.03, indicating In mole fraction variation of 9 5.8% or compositional variation of 9 1.5% and for the MBE samples the range of 16 meV represents a mole fraction variation of 0.02, indicating In mole fraction variation of 94.2% or compositional variation of 9 1.0%. Such material nonuniformity proved to be too small to propagate nonuniformities in DC and RF characteristics of PMHEMTs processed on these wafers or to suggest the preference of one growth technique over the other for better device uniformity [10]. Indeed DC characteristic mapping with a relative standard deviation of several percent, exhibits no obvious correlation with the PR mapping of much lower relative standard deviation [10]. We concluded, in this case, that the processing non-uniformities are much higher that the layer nonuniformities and thus the former mask the latter at the device level [10]. Nevertheless, PR mapping was adequate to allow reaching several conclusions about the material, such as that growth on 3¦ and 4¦ substrates exhibit the same uniformity, or that MBE and MOCVD are equally acceptable and thus the technique seems to be adequate for assessing general uniformity of the QW in PMHEMT layers.
Table 1 Statistical results from PR spectra across several 3¦ wafers
MOCVD MOCVD MOCVD MBE MBE MBE
Wafer c
E2 (eV)
S.D. (meV)
R.S.D.(%)
A B C D E F
1.248 1.236 1.232 1.256 1.259 1.264
7 10 9 2 2.5 3
0.56 0.81 0.73 0.16 0.23 0.24
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M. Androulidaki et al. / Materials Science and Engineering B66 (1999) 141–145
Fig. 3. Typical Photoluminescence spectrum taken at 16 K of MOCVD-grown PM – HEMT structure A and relevant transitions (inset).
Fig. 4. Room temperature Photoreflectance (left) and 16 K Photoluminescence (right) mapping of the same half wafer of MOCVD-grown PM – HEMT structure A. The gray scale represents % deviation from the average.
3. Conclusions In a study of several PM – HEMT structures on MOCVD and MBE wafers, it was found that photoreflectance can be a viable room temperature technique for probing the combined effects of In compositional variation and of layer thickness variation in the QW. A comparison between low temperature PL and room temperature PR maps on the same sample indicates that similar information can be extracted on QW uniformity. Modeling of PL data can further provide estimates of the In composition variation across the wafer. We suggest that PR can be adequately
used as a more economical, faster and simpler room temperature non-destructive technique, for the routine inspection of PM–HEMT structures.
Acknowledgements This work was supported by the European Union DGIII through ESPRIT Project 21315 (GAMMA). We are grateful to our partners at Picogiga and at the ´ lectronique Philips for the supply of Laboratoires d’E complete PMHEMT structures as well as for illuminating discussions. In addition we are indebted to Dr. W.
M. Androulidaki et al. / Materials Science and Engineering B66 (1999) 141–145
Jantz and his colleagues at Fraunhofer Institut Angewandte Festko¨rperphysik for stimulating discussions and exchange of data.
References [1] F. Pollak, H. Qiang, D. Yan, W. Krystek, S. Moneger, Solid State Elec. 38 (6) (1995) 1121–1129. [2] Y. Yin, H. Qiang, D. Yan, F. Pollak, T. Noble, Semic. Sci. Tech. 8 (1993) 1599 – 1604. [3] Y. Yin, H. Qiang, F. Pollak, D. Streit, M. Wojtowicz, Appl. Phys. Lett. 61 (13) (1992) 1579–1581.
145
[4] A. Dimoulas, K. Zekentes, M. Androulidaki, N. Kornelios, C. Michelakis, Z. Chatzopoulos, Appl. Phys. Lett 63 (10) (1993) 1417 – 1419. [5] A. Dodabalapur, V.P. Kesan, D.R. Hinson, D.P. Neikirk, B.G. Streetman, Appl. Phys. Lett 54 (17) (1989) 1675 – 1677. [6] W. Lu, G. Ng, B. Jogal, J. Lee, C. Park, J. Appl. Phys. 82 (3) (1997) 1345 – 1349. [7] J.M. Gilperez, J.L. Sanchez-Rojas, E. Munoz, E. Calleja, J.P.R David, G. Hill, J. Castagne, Appl. Phys. Lett. 61 (10) (1992) 1225 – 1227. [8] C. Colvard, N. Nouri, H. Lee, D. Ackley, Phys. Rev. B. 39 (11) (1989) 8033 – 8036. [9] D.H. Park, K.F. Brennan, J. Appl. Phys. 65 (4) (1989) 1615– 1620. [10] M. Lagadas, K. Michelakis, M. Kayambaki, P. Panayotatos, EXMATEC 98.
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Photoreflectance as a non-destructive, room-temperature technique for routine testing of PM–HEMT structures M. Androulidaki a,*, M. Lagadas a, K. Michelakis a, P. Panayotatos a,b a
Foundation for Research and Technology-Hellas (FORTH), IESL, Microelectronics Research Group, P.O. Box 1527, 71110 Heraklion, Crete, Greece b Rutgers, The State Uni6ersity of New Jersey, Department of Electrical & Computer Engineering, 94 Brett Rd., Piscataway, NJ 08854 -8058, USA
Abstract In the context of a comparative study of MBE and MOCVD PM – HEMT structures on 3¦ GaAs substrates, utilization of photoreflectance indicated that the technique can provide substantial information both non-destructively as well as at room temperature. Concentrating on the structure of the photoreflectance trace below 1.4 eV, the technique can provide information on the combined effect of thickness and In composition in the InGaAs quantum well. In particular, photoreflectance was found to be especially useful for mapping the uniformity over a single wafer as well as for mapping the reproducibility from wafer to wafer. Representative, characteristic patterns were consistently observed for MBE-grown layers, distinct from equally characteristic patterns of MOCVD-grown layers. More importantly, these patterns, obtained by room temperature photoreflectance, were found to coincide with those obtained by low temperature photoluminescence. Although the two techniques identify different electronic transitions, room temperature photoreflectance proves to be equally well adapted as an acceptance test for layer uniformity as low temperature photoluminescence. In probing the reason of non-uniformities, however, low temperature photoluminescence does provide more information. Experimental results are presented for both MBE and MOCVD PM – HEMT structures as well as information extracted from their treatment by modeling in the quantum wells. © 1999 Elsevier Science S.A. All rights reserved. Keywords: PM – HEMT; Photoreflectance; Photoluminescence; Uniformity
1. Introduction The non-destructive characterization and qualification of complex semiconductor multilayer structures is crucial for assessing the quality of material intended for use in fabrication of devices and MMICs such as pseudomorphic HEMT (PM – HEMT) structures, based on III–V materials, for high frequency and high power applications. Photoreflectance (PR) [1] is one such technique that can be shown to be particularly efficient, possessing the additional advantage of being a room temperature technique [2] thus simplifying the set up and having cost reduction ramifications in an industrial environment. We have applied the technique [2–4] in a study of the uniformity of AlGaAs/InGaAs/GaAs PM–HEMT structures. The samples were grown by * Corresponding author. Tel +30-81-394105; fax + 30-81-394106. E-mail addresses: [email protected] (M. Androulidaki), [email protected] (P. Panayotatos)
Molecular Beam Epitaxy (MBE) and Metalorganic Chemical Vapor Deposition (MOCVD) on (100) S.I. GaAs substrates. Room temperature PR was used to assess the general uniformity of the quantum well (QW) across each wafer. Low temperature (16 K) photoluminescence (PL) [5–7] measurements were also taken on the same wafer locations as were the PR measurements, in order to validate the PR data. These results were used to further distinguish the parameters affecting QW uniformity such as the indium mole fraction in the InGaAs layer as well as the QW width uniformity, or in other words, the InGaAs layer thickness uniformity.
2. Results and discussion Several 3¦ MOCVD and MBE-grown single heterostructure AlGaAs/InGaAs/GaAs PM–HEMT wafers were studied by both PR and PL. The InGaAs
0921-5107/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 9 9 ) 0 0 1 3 4 - 8
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layer thickness was 100 A, for the MOCVD and 90 A, for the MBE wafers, and the two dimensional electron gas density (Ns) was higher than 2 ×1012 cm − 2 nominally. In samples on which Hall measurements were performed the nominal values were confirmed. A conventional [1,4] experimental setup was used that employs a He–Ne laser of 0.5 W/cm2. PR spectra were taken at predetermined steps across the whole wafer surface, for each wafer, and the measurements were subsequently statistically processed. A typical PR spectrum is shown in Fig. 1. The peak around 1.4 eV is common to all spectra taken from all wafers, and is attributed to the contribution of the GaAs cap and buffer layers. Because of the relatively high value of Ns, the Fermi level falls between the second and the first electron level of the QW which suggests that the latter is completely filled. This was confirmed by the lineshape of the PL spectrum, in particular from the relative intensities of the peaks at E1 and E2 and by the sloping characteristic at energies higher than E1 (Fig. 3) [8]. Thus the dominant transition that is monitored is from the heavy hole state to the second electron level of the QW. Maps of the feature around 1.24 eV are maps of the energy of this transition and are a measure of the uniformity over the wafer of QW factors that affect this transition. Fig. 2 shows typical maps of an MOCVD (left) and an MBE (right) structure. These patterns exhibit a radial symmetry around the center of the wafer in the case of MOCVD and around a point off-center just off the periphery of the wafer, in the case of MBE. Though
the particular results reported in this work focus on MBE- and MOCVD-grown single heterostructures on 3¦ wafers, these reproducible, characteristic patterns of MBE and MOCVD layers were also consistently observed for layers grown both as single heterostructures, normally intended for low noise device applications, as well as for double heterostructures, intended for power device applications. Furthermore, the characteristic MBE pattern was also present in structures grown on both 3¦ and 4¦ substrates and it was further verified by comparing to the grower’s resistivity mapping. These characteristic patterns are consistent with the positioning of the substrates during epitaxial growth: in the case of MOCVD-grown samples the substrates were individually rotated, while in the case of the MBE-grown samples the substrates were placed at the periphery of a rotating plenum. Statistical PR data for several wafers appear in Table 1. For the MOCVD-grown wafers, the transition energy monitored is found to be smaller near the center of each wafer and the variation of this value, measured by the relative standard deviation (RSD) is less than 1% over each wafer. The variation is even lower in the case of MBE-grown wafers. PR measurements were validated by PL. A typical PL spectrum, in this case for sample A, is shown in Fig. 3, where two QW transitions can be observed. PL measurements were taken at the very points on the wafer that the PR measurements had been taken. The range of values in the resulting PL mapping indicates that the energy for both transitions exhibits the same range of variation, namely about 25 meV. The differ-
Fig. 1. Typical room temperature Photoreflectance spectrum of MOCVD-grown PM – HEMT structure A with features attributed to different layers. Inset presents blow-up of InGaAs feature utilized for the determination of the energy of the transition from the heavy hole level to the second electron level in the QW.
M. Androulidaki et al. / Materials Science and Engineering B66 (1999) 141–145
143
Fig. 2. Photoreflectance mapping of similar Single Heterojunction PM – HEMT layers: MOCVD-gown structure B (left) and MBE-grown structure F (right). The average peak and relative standard deviations are 1.236 eV and 0.81% for MOCVD-B and 1.264 eV and 0.24% for MBE-F. The gray scale represents % deviation from the average.
ence of the two transition energies E2 −E1 does not vary significantly across the wafer, ranging from 92 to 97 meV. This indicates that the QW thickness variation is similarly small. In addition, the ratio of the intensities of the peaks at E2 and E1 is constant, indicating constancy of Ns. Furthermore, the shape (broadening) of the feature around the transition, in the PR spectrum this time, remains relatively constant over a single wafer, also indicating no significant variation of Ns. A variational theoretical model was used for the interpretation of the PL results with the above rationale in mind. The model self-consistently solves the Poisson and Schro¨dinger [3,9] (G. Halkias, personal communication) equations by assuming two states inside the QW. According to this model, by assuming that only the width of the QW varies, with the In composition assumed constant, the experimental results of a 5 meV maximum difference of the two transition energies E2 − E1 lead to the conclusion that the full range of variation for the QW thickness would be between 5 and 10 A, over the whole wafer surface. Such a thickness variation, however, would only account for 5 meV of variation in the PR peak at E2 which, in fact, varies by 25 meV. Given the above, the remaining factor that strongly affects the transition variation is the In composition x in the Inx Ga1 − x As well. Assuming x to be the only factor affecting QW uniformity and employing the model indicates that the change of 25 meV in E2, representing the range between the minimum and the maximum over the wafer, is caused by a 0.03 change in x. From the comparison of the PR and PL maps (Fig. 4) it is evident that a similar variation of E2 transition is present in both cases. We thus conclude that, in such a case, one can utilize PR mapping to roughly extract the In compositional variation. For the particular samples the range of 25 meV in the MOCVD samples
represent a mole fraction variation of 0.03, indicating In mole fraction variation of 9 5.8% or compositional variation of 9 1.5% and for the MBE samples the range of 16 meV represents a mole fraction variation of 0.02, indicating In mole fraction variation of 94.2% or compositional variation of 9 1.0%. Such material nonuniformity proved to be too small to propagate nonuniformities in DC and RF characteristics of PMHEMTs processed on these wafers or to suggest the preference of one growth technique over the other for better device uniformity [10]. Indeed DC characteristic mapping with a relative standard deviation of several percent, exhibits no obvious correlation with the PR mapping of much lower relative standard deviation [10]. We concluded, in this case, that the processing non-uniformities are much higher that the layer nonuniformities and thus the former mask the latter at the device level [10]. Nevertheless, PR mapping was adequate to allow reaching several conclusions about the material, such as that growth on 3¦ and 4¦ substrates exhibit the same uniformity, or that MBE and MOCVD are equally acceptable and thus the technique seems to be adequate for assessing general uniformity of the QW in PMHEMT layers.
Table 1 Statistical results from PR spectra across several 3¦ wafers
MOCVD MOCVD MOCVD MBE MBE MBE
Wafer c
E2 (eV)
S.D. (meV)
R.S.D.(%)
A B C D E F
1.248 1.236 1.232 1.256 1.259 1.264
7 10 9 2 2.5 3
0.56 0.81 0.73 0.16 0.23 0.24
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M. Androulidaki et al. / Materials Science and Engineering B66 (1999) 141–145
Fig. 3. Typical Photoluminescence spectrum taken at 16 K of MOCVD-grown PM – HEMT structure A and relevant transitions (inset).
Fig. 4. Room temperature Photoreflectance (left) and 16 K Photoluminescence (right) mapping of the same half wafer of MOCVD-grown PM – HEMT structure A. The gray scale represents % deviation from the average.
3. Conclusions In a study of several PM – HEMT structures on MOCVD and MBE wafers, it was found that photoreflectance can be a viable room temperature technique for probing the combined effects of In compositional variation and of layer thickness variation in the QW. A comparison between low temperature PL and room temperature PR maps on the same sample indicates that similar information can be extracted on QW uniformity. Modeling of PL data can further provide estimates of the In composition variation across the wafer. We suggest that PR can be adequately
used as a more economical, faster and simpler room temperature non-destructive technique, for the routine inspection of PM–HEMT structures.
Acknowledgements This work was supported by the European Union DGIII through ESPRIT Project 21315 (GAMMA). We are grateful to our partners at Picogiga and at the ´ lectronique Philips for the supply of Laboratoires d’E complete PMHEMT structures as well as for illuminating discussions. In addition we are indebted to Dr. W.
M. Androulidaki et al. / Materials Science and Engineering B66 (1999) 141–145
Jantz and his colleagues at Fraunhofer Institut Angewandte Festko¨rperphysik for stimulating discussions and exchange of data.
References [1] F. Pollak, H. Qiang, D. Yan, W. Krystek, S. Moneger, Solid State Elec. 38 (6) (1995) 1121–1129. [2] Y. Yin, H. Qiang, D. Yan, F. Pollak, T. Noble, Semic. Sci. Tech. 8 (1993) 1599 – 1604. [3] Y. Yin, H. Qiang, F. Pollak, D. Streit, M. Wojtowicz, Appl. Phys. Lett. 61 (13) (1992) 1579–1581.
145
[4] A. Dimoulas, K. Zekentes, M. Androulidaki, N. Kornelios, C. Michelakis, Z. Chatzopoulos, Appl. Phys. Lett 63 (10) (1993) 1417 – 1419. [5] A. Dodabalapur, V.P. Kesan, D.R. Hinson, D.P. Neikirk, B.G. Streetman, Appl. Phys. Lett 54 (17) (1989) 1675 – 1677. [6] W. Lu, G. Ng, B. Jogal, J. Lee, C. Park, J. Appl. Phys. 82 (3) (1997) 1345 – 1349. [7] J.M. Gilperez, J.L. Sanchez-Rojas, E. Munoz, E. Calleja, J.P.R David, G. Hill, J. Castagne, Appl. Phys. Lett. 61 (10) (1992) 1225 – 1227. [8] C. Colvard, N. Nouri, H. Lee, D. Ackley, Phys. Rev. B. 39 (11) (1989) 8033 – 8036. [9] D.H. Park, K.F. Brennan, J. Appl. Phys. 65 (4) (1989) 1615– 1620. [10] M. Lagadas, K. Michelakis, M. Kayambaki, P. Panayotatos, EXMATEC 98.
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