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Cathodoluminescence study of molecular beam epitaxy (MBE) grown MgZnSSe and BeMgZnSe alloy based heterostructures
Journal of Crystal Growth 201/202 (1999) 481}485
Cathodoluminescence study of molecular beam epitaxy (MBE) grown MgZnSSe and BeMgZnSe alloy based heterostructures V.A. Solov'ev *, S.V. Sorokin , I.V. Sedova , G.N. Mosina , S.V. Ivanov , H.-J. Lugauer, G. Reuscher, M. Keim, A. Waag, G. Landwehr Iowe Physico-Technical Institute of RAS, 194021 St. Petersburg, Russia Physikalisches Institut der Universita( t Wu( rzburg, Am Hubland, D-97074 Wu( rzburg, Germany
Abstract Cathodoluminescence (CL) is shown to be a very e!ective and fast technique for the study of defects and their spatial distributions in ZnSe-based epilayers and heterostructures, including laser diodes grown by MBE on GaAs. A relatively low electron beam primary energy ((20 keV) is found to be most preferable for the correct stacking fault (SF) observation. CL of undoped heterostructures with a CdSe fractional monolayer active region provides useful information on the spatial distribution of large relaxed CdSe-based islands containing nonradiative recombination centers. 1999 Elsevier Science B.V. All rights reserved. PACS: 78.60.Hk; 81.15.Hi Keywords: ZnMgSSe and BeMgZnSe heterostructures; Stacking faults; Cathodoluminescence
Relatively short operating lifetime of ZnSe-based green laser diodes (LDs) fabricated on GaAs by MBE is mainly attributed to the high density of stacking faults (SFs) [1]. Guha et al. showed that SFs act as nonradiative recombination centers and can initiate mis"t dislocations in the quantum well (QW) region of ZnMgSSe/ZnCdSe laser structures [2]. [1 0 0] dark line defects (DLDs), emerging during degradation of ZnMgSSe/ZnSSe/ZnCdSe separate con"nement heterostructure (SCH) QW LDs,
* Corresponding author. Fax: #7-812-247-86-40; e-mail: [email protected]!e.rssi.ru.
also originate from the grown-in defects such as SFs and mis"t dislocations [3]. Therefore, lowering the defect density by the proper control over growth conditions as well as development of defect revealing techniques are of great importance. Among the well-known structural characterization techniques, such as X-ray di!raction, transmission electron microscopy (TEM), and cathodoluminescence (CL), the latter allows one to visualize the nonradiative recombination related defects, which is particularly important for lightemitting devices. Low-temperature CL imaging and spectroscopy were successfully performed for investigation of mis"t dislocations and domains in ZnSe/GaAs(1 0 0) epilayers [4]. CL imaging was
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 8 ) 0 1 3 8 1 - 5
482
V.A. Solov'ev et al. / Journal of Crystal Growth 201/202 (1999) 481}485
Fig. 1. Plan-view TEM (a), CL (b, d) and EPD (c) images of samples 4306 (a, b) and cb954 (c, d).
also used to study II}VI device degradation phenomena [3]. However, there are only few papers devoted to CL investigations of SFs [2,5]. An exchange of the conventional ZnMgSSe solid alloys with novel Be-based ones is expected to result in further improvements of II}VI laser parameters, which is due to the more covalent bond nature and higher lattice rigidity [6]. In this paper we present a detailed study of structural quality of MgZnSSe- and BeMgZnSebased epilayers and heterostructures with the main focus on CL. All the structures were grown by MBE pseudomorphically onto GaAs(1 0 0) substrates. The
growth conditions and composition control for Sand Be-based heterostructures have been reported elsewhere [6,7]. The samples were studied using TEM, CL and etch-pit density (EPD) evaluation. JEOL JEM-7A and JEM-100C microscopes were utilized for plan-view TEM studies. In EPD measurements wet chemical etching by concentrated HCl acid at ¹"603C was used to reveal the defect density. Spectral broad-range CL studies were carried out in a scanning electron microscope (SEM) CamScan Series 4-88 DV100. Plan-view TEM studies have shown that most of the defects are triangular SFs bounded by 11 1 12 Frank-type partial dislocations. Moreover, most of
V.A. Solov'ev et al. / Journal of Crystal Growth 201/202 (1999) 481}485 Table 1 SF density (cm\) values obtained by di!erent evaluation techniques, (TEM) transmission electron microscopy, (CL) cathodoluminescence, (EPD) chemical etching and etch pit counting Samples ZnMgSSe : N 0272 0278 0279 0302 0307
TEM
CL
EPD
10 (10
2;10 (10 2;10 (10 2;10
2;10
3;10
2;10
3;10 7;10
2;10 10
6;10 2;10 4;10 4;10 8;10 10
8;10 10 2;10 2;10 10 2;10
5;10 (10
BeZnSe : N cb948 BeMgZnSe : N cb957 cb958 Laser structures 4306 cb952 cb954 cb955 cb1000 cb1005
5;10
10 8;10
Note: The samples for which numbers begin with `cba are based on BeMgZnSe, the others are based on ZnMgSSe.
the SFs are twins, clearly observed both in TEM and CL images (Fig. 1a and Fig. 1b). Since the SF spreads in the (1 1 1) planes, the size of the twin dark spot features in the CL image enables one to determine the origination depth of certain SF [2]. Usually, it correlates directly with the total thickness of a heterostructure. The EPD technique also reveals the twin-pits attributed to the SF defects by comparison with the relevant CL images (Fig. 1c and Fig. 1d). We have found that correct SF density values can be obtained from CL data only by using appropriate value of the accelerating voltage E . Relative ly large E values &25}40 kV are conventionally used to reduce the e!ect of surface recombination of nonequilibrium carriers by increasing the electron beam penetration depth, which in turn increases the CL signal. We have found that for E (20 kV the CL images re#ect the true spatial distribution of SFs seen as an array of dark spots, with the density of the defects and their mutual
483
disposition being unchanged at di!erent E . For E '20 kV, the features attributed to the SFs dis appear and a weak contrast without any dark spots is observed, which is probably associated with an inhomogeneous impurity distribution in the n>GaAs substrate. We believe that, under the lower energy electron beam excitation CL, the signal is formed by ZnSe-based epilayers only, while for large E , the electron beam penetration depth ex ceeds the total epilayer thickness [4], giving rise to a considerable contribution from radiative carrier recombination in GaAs. Since SFs originate close to or exactly at a "lm/substrate interface, they cannot a!ect the GaAs related luminescence and, hence, cannot produce any contrast in the CL image. A comparative analysis of SF density data obtained by TEM, CL, and EPD technique is summarized in Table 1 including both S- and Be-based structures. A good correlation in the estimation of the SF density between the data obtained by all these techniques, especially between plan-view TEM and CL data, has been established in the 10}10 cm\ range. As concerns the EPD technique, sometimes a considerable discrepancy between the EPD values and both TEM and CL ones has been observed, because the chemical procedure is not very reproducible, especially for multilayer structures. It should be mentioned that for SF density as high as 10 cm\, the images of single defects are not resolved and a complex picture of alternating black and white regions with di!erent form and size (1}5 lm) is observed. All the structures and epilayers presented in the Table 1 were grown under di!erent conditions and initial growth stage procedures, as reported in Ref. [8]. The best result with SF density (as low as 10 cm\) has been obtained for a BeMgZnSe/BeZnSe/ZnSe SCH QW structure containing a 4 monolayer (ML) BeTe bu!er layer at the ZnSe/GaAs interface. We have also studied CL of both S- and Bebased SCH laser structures with a 2.8 ML-CdSe fractional monolayer (FM) active region. It has been found that CL images of the undoped laser structures with thin upper ZnMgSSe cladding layers look like those of structures with extremely large SF densities (Fig. 2a). After etching the upper layers including the FM active regions the CL
484
V.A. Solov'ev et al. / Journal of Crystal Growth 201/202 (1999) 481}485
Fig. 2. CL images (a, b, d) and SEM micrograph (c) of samples cb1000 (b) and 1568: before (a) and after (c, d) removing the upper layers.
images of such structures reveal SF densities of about 10 cm\ (Fig. 2c and Fig. 2d), which are typical for similar structures without CdSe FM, grown under the same conditions. Unlike the undoped structures, the FM SCH diode structures with p}n junctions in the vicinity of the FM insertion give CL images with expected SF densities (Fig. 2b). The observed disagreement is associated with the peculiarities of the CL signal formation in the structures with and without p}n junction. For the undoped structures the nonequilibrium carriers generated in the thin top cladding recombine in the FM active region giving the main contribution to the CL emission. Plan-view TEM studies of 2}3 ML thick CdSe/ZnSe FM structures [9], demonstrating the formation of strain-induced self-organizing 15}40 nm CdSe-based dot-like islands, have also shown that the largest relaxed CdSe islands are nonuniformly distributed across the sur-
face and form clusters with characteristic spacings of 1}2 lm. Therefore, the CL contrast observed in such undoped FM structures is believed to be associated with nonradiative recombination of carriers in the clusters of CdSe relaxed islands. For the diode structures, radiative recombination from the FM active region with CdSe islands is strongly suppressed by the build-in electric "eld of the p}n junction. In these structures the CL image is formed in the upper BeMgZnSe cladding and BeTe/BeSe superlattice (for the case of Be-based structures) and provides information about the real SF densities. This work is supported in part by RFBR Grants and by the Program of Ministry of Sciences of RF `Physics of Solid State Nanostructuresa, as well as the Bayerische Forschungsstiftung and the Deutsche Forschungsgemeinschaft.
V.A. Solov'ev et al. / Journal of Crystal Growth 201/202 (1999) 481}485
References [1] A. Ishibashi, J. Crystal Growth 159 (1996) 555. [2] S. Guha, J.M. DePuydt, J. Qiu, G.E. Ho#er, M.A. Haase, B.J. Wu, H. Cheng, Appl. Phys. Lett. 63 (1993) 3023. [3] S. Guha, H. Cheng, M.A. Haase, J.M. DePuydt, J. Qiu, B.J. Wu, G.E. Ho#er, Appl. Phys. Lett. 65 (1994) 801. [4] H.T. Lin, D.H. Rich, D.B. Wittry, J. Appl. Phys. 75 (1994) 8080. [5] L.H. Kuo, L. Salamanca-Riba, B.J. Wu, G. Ho#er, J.M. DePuydt, H. Cheng, Appl. Phys. Lett. 67 (1995) 3298. [6] A. Waag, F. Fisher, K. SchuK ll, T. Baron, H.-J. Lugauer, Th. Litz, U. Zehnder, W. Ossau, T. Gerhard, M. Keim,
485
G. Reuscher, G. Landwehr, Appl. Phys. Lett. 70 (1997) 1. [7] S. Ivanov, S. Sorokin, I. Krestnikov, N. Faleev, B. Ber, I. Sedova, P. Kop'ev, J. Crystal Growth 184/185 (1998) 70. [8] S.V. Ivanov, R.N. Kyutt, G.N. Mosina, L.M. Sorokin, S.V. Sorokin, P.S. Kop'ev, Proc. 23rd Int. Symp. Compound Semiconductors, St. Petersburg, 1996, Inst. Phys. Conf. Ch. 3, Ser. No. 155, 1997, p. 223. [9] A.A. Toropov, S.V. Ivanov, T.V. Shubina, S.V. Sorokin, A.V. Lebedev, A.A. Sitnikova, P.S. Kop'ev, M. Willander, G. Posina, J.P. Bergman, B. Monemar, Proc. Int. Symp. Formation, Physics and Device Application of Quantum Dot Structures, 1998, Sapporo, Jpn. J. Appl. Phys., to be published.
Cathodoluminescence study of molecular beam epitaxy (MBE) grown MgZnSSe and BeMgZnSe alloy based heterostructures V.A. Solov'ev *, S.V. Sorokin , I.V. Sedova , G.N. Mosina , S.V. Ivanov , H.-J. Lugauer, G. Reuscher, M. Keim, A. Waag, G. Landwehr Iowe Physico-Technical Institute of RAS, 194021 St. Petersburg, Russia Physikalisches Institut der Universita( t Wu( rzburg, Am Hubland, D-97074 Wu( rzburg, Germany
Abstract Cathodoluminescence (CL) is shown to be a very e!ective and fast technique for the study of defects and their spatial distributions in ZnSe-based epilayers and heterostructures, including laser diodes grown by MBE on GaAs. A relatively low electron beam primary energy ((20 keV) is found to be most preferable for the correct stacking fault (SF) observation. CL of undoped heterostructures with a CdSe fractional monolayer active region provides useful information on the spatial distribution of large relaxed CdSe-based islands containing nonradiative recombination centers. 1999 Elsevier Science B.V. All rights reserved. PACS: 78.60.Hk; 81.15.Hi Keywords: ZnMgSSe and BeMgZnSe heterostructures; Stacking faults; Cathodoluminescence
Relatively short operating lifetime of ZnSe-based green laser diodes (LDs) fabricated on GaAs by MBE is mainly attributed to the high density of stacking faults (SFs) [1]. Guha et al. showed that SFs act as nonradiative recombination centers and can initiate mis"t dislocations in the quantum well (QW) region of ZnMgSSe/ZnCdSe laser structures [2]. [1 0 0] dark line defects (DLDs), emerging during degradation of ZnMgSSe/ZnSSe/ZnCdSe separate con"nement heterostructure (SCH) QW LDs,
* Corresponding author. Fax: #7-812-247-86-40; e-mail: [email protected]!e.rssi.ru.
also originate from the grown-in defects such as SFs and mis"t dislocations [3]. Therefore, lowering the defect density by the proper control over growth conditions as well as development of defect revealing techniques are of great importance. Among the well-known structural characterization techniques, such as X-ray di!raction, transmission electron microscopy (TEM), and cathodoluminescence (CL), the latter allows one to visualize the nonradiative recombination related defects, which is particularly important for lightemitting devices. Low-temperature CL imaging and spectroscopy were successfully performed for investigation of mis"t dislocations and domains in ZnSe/GaAs(1 0 0) epilayers [4]. CL imaging was
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 8 ) 0 1 3 8 1 - 5
482
V.A. Solov'ev et al. / Journal of Crystal Growth 201/202 (1999) 481}485
Fig. 1. Plan-view TEM (a), CL (b, d) and EPD (c) images of samples 4306 (a, b) and cb954 (c, d).
also used to study II}VI device degradation phenomena [3]. However, there are only few papers devoted to CL investigations of SFs [2,5]. An exchange of the conventional ZnMgSSe solid alloys with novel Be-based ones is expected to result in further improvements of II}VI laser parameters, which is due to the more covalent bond nature and higher lattice rigidity [6]. In this paper we present a detailed study of structural quality of MgZnSSe- and BeMgZnSebased epilayers and heterostructures with the main focus on CL. All the structures were grown by MBE pseudomorphically onto GaAs(1 0 0) substrates. The
growth conditions and composition control for Sand Be-based heterostructures have been reported elsewhere [6,7]. The samples were studied using TEM, CL and etch-pit density (EPD) evaluation. JEOL JEM-7A and JEM-100C microscopes were utilized for plan-view TEM studies. In EPD measurements wet chemical etching by concentrated HCl acid at ¹"603C was used to reveal the defect density. Spectral broad-range CL studies were carried out in a scanning electron microscope (SEM) CamScan Series 4-88 DV100. Plan-view TEM studies have shown that most of the defects are triangular SFs bounded by 11 1 12 Frank-type partial dislocations. Moreover, most of
V.A. Solov'ev et al. / Journal of Crystal Growth 201/202 (1999) 481}485 Table 1 SF density (cm\) values obtained by di!erent evaluation techniques, (TEM) transmission electron microscopy, (CL) cathodoluminescence, (EPD) chemical etching and etch pit counting Samples ZnMgSSe : N 0272 0278 0279 0302 0307
TEM
CL
EPD
10 (10
2;10 (10 2;10 (10 2;10
2;10
3;10
2;10
3;10 7;10
2;10 10
6;10 2;10 4;10 4;10 8;10 10
8;10 10 2;10 2;10 10 2;10
5;10 (10
BeZnSe : N cb948 BeMgZnSe : N cb957 cb958 Laser structures 4306 cb952 cb954 cb955 cb1000 cb1005
5;10
10 8;10
Note: The samples for which numbers begin with `cba are based on BeMgZnSe, the others are based on ZnMgSSe.
the SFs are twins, clearly observed both in TEM and CL images (Fig. 1a and Fig. 1b). Since the SF spreads in the (1 1 1) planes, the size of the twin dark spot features in the CL image enables one to determine the origination depth of certain SF [2]. Usually, it correlates directly with the total thickness of a heterostructure. The EPD technique also reveals the twin-pits attributed to the SF defects by comparison with the relevant CL images (Fig. 1c and Fig. 1d). We have found that correct SF density values can be obtained from CL data only by using appropriate value of the accelerating voltage E . Relative ly large E values &25}40 kV are conventionally used to reduce the e!ect of surface recombination of nonequilibrium carriers by increasing the electron beam penetration depth, which in turn increases the CL signal. We have found that for E (20 kV the CL images re#ect the true spatial distribution of SFs seen as an array of dark spots, with the density of the defects and their mutual
483
disposition being unchanged at di!erent E . For E '20 kV, the features attributed to the SFs dis appear and a weak contrast without any dark spots is observed, which is probably associated with an inhomogeneous impurity distribution in the n>GaAs substrate. We believe that, under the lower energy electron beam excitation CL, the signal is formed by ZnSe-based epilayers only, while for large E , the electron beam penetration depth ex ceeds the total epilayer thickness [4], giving rise to a considerable contribution from radiative carrier recombination in GaAs. Since SFs originate close to or exactly at a "lm/substrate interface, they cannot a!ect the GaAs related luminescence and, hence, cannot produce any contrast in the CL image. A comparative analysis of SF density data obtained by TEM, CL, and EPD technique is summarized in Table 1 including both S- and Be-based structures. A good correlation in the estimation of the SF density between the data obtained by all these techniques, especially between plan-view TEM and CL data, has been established in the 10}10 cm\ range. As concerns the EPD technique, sometimes a considerable discrepancy between the EPD values and both TEM and CL ones has been observed, because the chemical procedure is not very reproducible, especially for multilayer structures. It should be mentioned that for SF density as high as 10 cm\, the images of single defects are not resolved and a complex picture of alternating black and white regions with di!erent form and size (1}5 lm) is observed. All the structures and epilayers presented in the Table 1 were grown under di!erent conditions and initial growth stage procedures, as reported in Ref. [8]. The best result with SF density (as low as 10 cm\) has been obtained for a BeMgZnSe/BeZnSe/ZnSe SCH QW structure containing a 4 monolayer (ML) BeTe bu!er layer at the ZnSe/GaAs interface. We have also studied CL of both S- and Bebased SCH laser structures with a 2.8 ML-CdSe fractional monolayer (FM) active region. It has been found that CL images of the undoped laser structures with thin upper ZnMgSSe cladding layers look like those of structures with extremely large SF densities (Fig. 2a). After etching the upper layers including the FM active regions the CL
484
V.A. Solov'ev et al. / Journal of Crystal Growth 201/202 (1999) 481}485
Fig. 2. CL images (a, b, d) and SEM micrograph (c) of samples cb1000 (b) and 1568: before (a) and after (c, d) removing the upper layers.
images of such structures reveal SF densities of about 10 cm\ (Fig. 2c and Fig. 2d), which are typical for similar structures without CdSe FM, grown under the same conditions. Unlike the undoped structures, the FM SCH diode structures with p}n junctions in the vicinity of the FM insertion give CL images with expected SF densities (Fig. 2b). The observed disagreement is associated with the peculiarities of the CL signal formation in the structures with and without p}n junction. For the undoped structures the nonequilibrium carriers generated in the thin top cladding recombine in the FM active region giving the main contribution to the CL emission. Plan-view TEM studies of 2}3 ML thick CdSe/ZnSe FM structures [9], demonstrating the formation of strain-induced self-organizing 15}40 nm CdSe-based dot-like islands, have also shown that the largest relaxed CdSe islands are nonuniformly distributed across the sur-
face and form clusters with characteristic spacings of 1}2 lm. Therefore, the CL contrast observed in such undoped FM structures is believed to be associated with nonradiative recombination of carriers in the clusters of CdSe relaxed islands. For the diode structures, radiative recombination from the FM active region with CdSe islands is strongly suppressed by the build-in electric "eld of the p}n junction. In these structures the CL image is formed in the upper BeMgZnSe cladding and BeTe/BeSe superlattice (for the case of Be-based structures) and provides information about the real SF densities. This work is supported in part by RFBR Grants and by the Program of Ministry of Sciences of RF `Physics of Solid State Nanostructuresa, as well as the Bayerische Forschungsstiftung and the Deutsche Forschungsgemeinschaft.
V.A. Solov'ev et al. / Journal of Crystal Growth 201/202 (1999) 481}485
References [1] A. Ishibashi, J. Crystal Growth 159 (1996) 555. [2] S. Guha, J.M. DePuydt, J. Qiu, G.E. Ho#er, M.A. Haase, B.J. Wu, H. Cheng, Appl. Phys. Lett. 63 (1993) 3023. [3] S. Guha, H. Cheng, M.A. Haase, J.M. DePuydt, J. Qiu, B.J. Wu, G.E. Ho#er, Appl. Phys. Lett. 65 (1994) 801. [4] H.T. Lin, D.H. Rich, D.B. Wittry, J. Appl. Phys. 75 (1994) 8080. [5] L.H. Kuo, L. Salamanca-Riba, B.J. Wu, G. Ho#er, J.M. DePuydt, H. Cheng, Appl. Phys. Lett. 67 (1995) 3298. [6] A. Waag, F. Fisher, K. SchuK ll, T. Baron, H.-J. Lugauer, Th. Litz, U. Zehnder, W. Ossau, T. Gerhard, M. Keim,
485
G. Reuscher, G. Landwehr, Appl. Phys. Lett. 70 (1997) 1. [7] S. Ivanov, S. Sorokin, I. Krestnikov, N. Faleev, B. Ber, I. Sedova, P. Kop'ev, J. Crystal Growth 184/185 (1998) 70. [8] S.V. Ivanov, R.N. Kyutt, G.N. Mosina, L.M. Sorokin, S.V. Sorokin, P.S. Kop'ev, Proc. 23rd Int. Symp. Compound Semiconductors, St. Petersburg, 1996, Inst. Phys. Conf. Ch. 3, Ser. No. 155, 1997, p. 223. [9] A.A. Toropov, S.V. Ivanov, T.V. Shubina, S.V. Sorokin, A.V. Lebedev, A.A. Sitnikova, P.S. Kop'ev, M. Willander, G. Posina, J.P. Bergman, B. Monemar, Proc. Int. Symp. Formation, Physics and Device Application of Quantum Dot Structures, 1998, Sapporo, Jpn. J. Appl. Phys., to be published.