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Comparison of structural and optical properties in strained GaInAsP MQW structures grown by MOVPE and MOMBE
Journal of Crystal Growth 209 (2000) 424}430
Comparison of structural and optical properties in strained GaInAsP MQW structures grown by MOVPE and MOMBE P. KroK ner!,",*, H. Baumeister!, J. Rieger!, E. Veuho!!, O. Marti", H. Heinecke" !Inxneon Technologies, Dept. CPR 7, D-81730 Munich, Germany "Department of Experimental Physics, University of Ulm, D-89069 Ulm, Germany
Abstract The growth parameter dependence of the transition from 2D to 3D growth of GaInAsP multiple quantum well (MQW) structures up to e "0.5% tensile-strained barriers was examined. Identical MQW structures with e "1% B W compressively strained wells were grown by metal organic vapor-phase epitaxy (MOVPE) and metal organic molecular beam epitaxy (MOMBE) and characterized by photoluminescence (PL), X-ray di!raction and transmission electron microscopy. Increasing the tensile barrier strain resulted in deteriorated optical and crystalline properties beyond a critical strain limit, which depends on growth temperature. The deterioration originates from lateral layer thickness and strain modulations. Their density, amplitude and thus their e!ect on the optical MQW properties are di!erent for both growth methods. High-quality MOMBE-grown MQW structures up to e "2% compressive well strain and W e "0.5}1% tensile barrier strain could be achieved by inserting thin intermediate layers at each internal interface. The B composition of these intermediate layers has a signi"cant e!ect on MQW material properties. ( 2000 Elsevier Science B.V. All rights reserved.
1. Introduction The quaternary Ga In As P material sysx 1~x y 1~y tem is widely used for the fabrication of optoelectronic devices like high-speed telecommunication laser diodes emitting at 1.3 and 1.55 lm wavelength. Strained layer multiple quantum wells (MQW) in the active zone allow for improved device performance. In order to prevent plastic relaxation through mis"t dislocations of the strained layers, the net strain of the MQW layers should
* Corresponding author. Tel.: #49-89-234-48786; fax: #4989-234-41658. E-mail address: [email protected] (P. KroK ner)
remain below a critical strain limit [1]. This can be achieved by the technique of strain compensation. It is widely accepted that the growth of strained layers in MOMBE [2], MBE [3], GSMBE [4,5] and MOVPE [6}8] is susceptible to a transition from 2D to an anisotropic 3D growth mode, also referred to as wavy layer growth. In particular, tensile-strained layers seem to be susceptible to this e!ect. It can reduce the material quality of the MQW structure severely [2}8], limiting the application of tensile-strained layers and should thus be eliminated. Despite the fact that MOVPE and MOMBE are non-equilibium processes, the development of composition #uctuations is often assumed to be due to spinodal decomposition [2}4,7,8]. Surface selective growth mechanisms [9]
0022-0248/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 9 ) 0 0 5 8 4 - 9
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are a more likely cause for the e!ect. It is the aim of this paper to contribute to the current understanding of this topic by discussing the role of growth temperature and strain in identical MQW structures grown by MOVPE and MOMBE. Furthermore, the insertion of thin intermediate layers (IML) at each internal MQW interface is evaluated as a means to control wavy layer growth. Thus, conclusions for the growth of highly strained MQW structures can be drawn.
2. Experimental procedure Our MQW test structures were grown by MOVPE at 6003C and 6453C or by MOMBE at 5003C. Details of the MOVPE and MOMBE growth systems are published elsewhere [10]. All grown MQW structures have 5 periods, compressively strained GaInAsP or InAsP well layers and tensile-strained GaInAsP barrier layers (Table 1). The layer thicknesses are 8 nm, each. They are embedded between a 200 nm InP bu!er and a 100 nm InP cap layer. (1 0 0) InP substrates 23 misaligned towards S1 1 0T were used. MQW growth rates were 0.9}1.1 lm/h for MOMBE samples with V/III ratios of 5}7. For MOVPE, growth rates were 1.5 lm/h and V/III ratios 130}270. The samples were analyzed by photoluminescence measurements (PL), micro-PL (l-PL) measurements with a spatial resolution below 50 lm, high-resolution X-ray di!raction (HRXRD) and bright-"eld transmission electron microscopy (TEM) using the g"S2 0 0T di!raction vector.
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3. Results and discussion 3.1. Strained MQWs without intermediate layers We compared samples of structures A (cf. Table 1) with tensile barrier strains between 0% and 0.5%, grown by MOVPE at 6003C and 6453C as well as by MOMBE at 5003C. The net strain of the MQW stack never exceeded the critical strain limit calculated according to Matthews and Blakeslee [1]. Thus no strain relaxation by the development of dislocation networks occured. This was con"rmed by l-PL measurements where no dark line defects, generally associated with dislocation networks, were observed for any sample [11]. Fig. 1 depicts results of standard PL measurements performed at room temperature. Below 0.3% tensile barrier strain, a low FWHM of 20 meV and high intensities were obtained in MOVPE samples. Beyond 0.3% strain, our data reveal a strain region where a steep increase of the FWHM and a successive drop of the intensity is recognized. At a growth temperature of 6453C it is sharply de"ned and can be located at 0.46%$0.05% tensile strain. These "ndings re#ect another strain limit beyond which the MQW quality deteriorates. Yet, it is di!erent from the Matthews and Blakeslee de"nition of critical strain which delimits the potential development of dislocation networks serving for strain relaxation. The inset of Fig. 1 displays the HRXRD rocking curves of the MOVPE samples grown at 6453C with (a) 0.46% and (b) 0.47% barrier strain. A sharply de"ned satellite peak structure, resulting from a well-de"ned long distance periodicity of the MQW periods can be seen in (a). When the strain
Table 1 Well and barrier layer compositions and strains of grown MQW structures. All barrier layer compositions are GaInAsP (j"1.20 lm equivalent wavelength) MQW structure
Well layer composition (j"1.57 lm)
Well layer strain (compressive) (%)
Barrier layer strain (tensile) (%)
A B C D
GaInAsP GaInAsP InAsP InAsP
1 1 2 2
0}0.5 0.5 0.5 1
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Fig. 1. PL results (room temperature) for MQW samples of structure A with di!erent barrier strains, grown in MOVPE at 6453C and 6003C and in MOMBE at 5003C. Inset: HRXRDrocking curves of MOVPE MQW samples (¹"6453C) marked with (a) and (b) in the graph. TEM images of circled samples are shown in Fig. 2.
limit exceeds, a total loss of the satellite peak structure, as seen in (b), implies a breakdown of the long-distance periodicity. At 6003C growth temperature the tensile strain limit for MQW material deterioration is lowered to 0.3}0.4% with the transition regime being less pronounced than at 6453C. For MOMBE the increase of FWHM with barrier strain is much weaker (Fig. 1), not showing any sharp transition to a structural breakdown. At 0.5% tensile strain the FWHM remains below 40 meV, much lower than obtained by MOVPE. Taking the FWHM as a measure of well thickness and composition homogeneity, at high strains the MQW distortion is less pronounced than in MOVPE. But, for the given structure, even at low
barrier strains of 0.2% the FWHM does not drop below 28 meV. HRXRD rocking curves always showed a well-de"ned satellite peak structure. Yet, between 0.4% and 0.5% strain a severe drop to almost zero occured in PL intensity. In contrast, for MOVPE intensity is reduced by a factor of only 2. Our data suggest a similar critical limit of approximately 0.5% tensile barrier strain for MOMBE and MOVPE structures, beyond which distortions develop rapidly. A similar strain limit has been mentioned by other authors for the onset of wavy layer growth [3,5]. TEM characterization was done in order to link the PL and HRXRD results to the morphology. Figs. 2a and b show TEM images of the two severely degraded MOVPE and MOMBE samples which are marked by circled data points in Fig. 1. (0 11 1) cross sections were prepared since thickness modulations linked to wavy layer growth are anisotropic and run along the [0 1 1] direction [2}5]. Both MOVPE and MOMBE samples show layer thickness modulations and strain "elds, apparent as dark regions in the images. The MOMBE sample pertains an undistorted "rst period of the MQW. Thickness variations start to be visible in the second barrier and are ampli"ed towards the upper periods, leading to severe undulations in the top period. Well and barrier layer thickness modulations preferably display an antiphase relationship, i.e. above a thin well region a thick barrier follows and vice versa. Lateral e!ective strain modulation is the consequence, appearing as dark shadows around layer distortions in the top period of Fig. 2a. This is similar to reports of groups employing related growth technologies i.e. MBE and GSMBE [2,4,5]. Layer thickness and strain modulations are of larger amplitude in the MOVPE sample, forming strong strain "elds of 200}400 nm in length and starting to be visible already in the "rst MQW period. An ampli"cation through the successive periods cannot be registred. A much larger characteristic wavelength K of the modulations in MOVPE samples (K '150 nm) MOVPE is noted, di!erent from MOMBE samples with K (80 nm. This is supported by compariMOMBE sons of the observed characteristic wavelength extracted from literature for samples grown by
P. Kro( ner et al. / Journal of Crystal Growth 209 (2000) 424}430
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Fig. 2. (0 11 1) cross-section TEM images of deteriorated MQW samples marked in Fig. 1 and grown by (a) MOMBE and (b) MOVPE.
MOVPE [7,8] with MBE, GSMBE or MOMBE data [2,3,5]. Comparing the low amplitude of distortions of the MOMBE sample to the larger amplitude in the MOVPE sample, the di!erence in PL FWHM and HRXRD results can be understood. Assuming nonradiative recombination centers to be linked to each modulation period [7], their large density in the MOMBE sample could cause the severe PL intensity drop. In contrast, their density could be lower in the MOVPE samples due to " being larger, allowing for stronger radiative recombination to take place and thus explaining the reduced drop in PL intensity. Models explaining the successive development of the lateral thickness modulations have been described in Refs. [5,6]. Volume strain energy reduction is widely accepted as a driving force for the 3D growth mechanism [12]. In an initially homogeneous layer, some thickness or composition #uctuation can occur, leading to a local variation in strain or composition. As can be seen from surface selective growth (SSG), the element incorporation e$ciencies are sensitive to surface constitution, particularly when growing quaternary materials [9]. When growing on an inhomogeneous surface, lateral variations in element incorporation, lateral growth rate modulations and thus an ampli"cation of the thickness modulations are the consequence. Anisotropic surface di!usion serves as the relevant mass transport mechanism responsible for the
lateral redistribution of the incorporated elements. The rate of modulation development is controlled by the sensitivity of the incorporation e$ciencies on the starting conditions of the grown layer and by the surface di!usion rate and length [13]. The latter can be lowered by increasing the V/III #ux ratio, thus slowing down modulation development [3,5]. Both increasing the growth temperature beyond 6003C in MOVPE and reducing it below 5003C in MOMBE, MBE and GSMBE yield higher e!ective V/III ratios, due to increased cracking e$ciency of hydrides and lower surface desorption of group V atoms, respectively. A higher V/III ratio reduces surface di!usivity and thus modulation development. This is supported by groups who obtained improved morphology for GSMBE and MBE samples with tensile-strained layers at low growth temperatures and high V/III ratios [3,5]. The growth temperature induced lower di!usion distance and rate could account for the smaller characteristic wavelength K observed in MOMBE grown samples as compared to MOVPE. 3.2. Strained MQWs with intermediate layers Given the tensile strain as limiting factor for the MQW quality, a key idea is to study whether the development of 3D growth can be suppressed when providing an improved starting surface for each
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grown layer. Thus, we inserted IML at each internal interface. Using MOMBE, MQW structures B, C and D (cf. Table 1) were grown with InP IML of di!erent thickness. Fig. 3 shows the PL results. Closed symbols mark measurements on MQWs of structure B. The drop in FWHM and rise in intensity with increasing IML thickness point towards the positive e!ect of the InP IML on the MQW morphology. At 2.5 nm thickness, an intensity equal to MQWs without structural damage is obtained. Fig. 4a shows a TEM analysis of the cross section of this sample (encircled closed symbol in Fig. 3). It con"rms that no visible undulations can be recognized and a uniform lateral and periodic vertical structure is obtained. We repeated this experiment for the growth of MQWs with high compressive well strains (structures C and D). Again, InP was used as IML material. As can be seen from the open data points
Fig. 3. PL results of MOMBE grown MQW samples with InP, InAsP and GaInAsP intermediate layers: (black data points) structure B; (white diamonds) structure C; (white circles) structure D. TEM images of circled samples are shown in Fig. 4.
Fig. 4. TEM images of MQW samples with InP intermediate layers marked in Fig. 3. (a) Structure B, (0 11 1) cross section. (b) Structure C, (0 1 1) cross section.
in Fig. 3, a minimum IML thickness of 1.4 nm proved to be necessary to detect PL intensity at all. The defect density, being the source for nonradiative recombination, is further reduced when the thickness is increased. Surprisingly, the FWHM cannot be reduced further below 42 meV for both 0.5% and 1% tensile barrier strain when varying IML thickness. Not depicted in the graph, we con"rmed the increase of the PL FWHM with increasing well strain in smaller steps from 1% to 2%, regardless whether GaInAsP or InAsP was used as well material. This suggests the strain to be the cause of the large FWHM. Using TEM, the (0 1 1) and (0 11 1) cross sections of structure C (marked with a circle in Fig. 3) were analyzed (cf. Fig. 4b). The barrier layers are smooth and of constant thickness, however, the well layers show anisotropic sporadic thickness variations of up to 30% along the [0 11 1] direction with large spacing explaining the large PL FWHM. No phase correlation between thickness variations in adjacent periods can be identi"ed. Those results con"rm that the insertion of InP IML suppresses the wavy layer growth. InP grows particularly in hollows of existing undulations [14], planarizing any starting wavyness and reducing lateral lattice spacing modulations. Thus, successive layers can grow on planar surfaces. The tendency towards ampli"cation of the wavyness is reduced, when the IML is
P. Kro( ner et al. / Journal of Crystal Growth 209 (2000) 424}430
su$ciently think. Grilhe [15] stated a tendency of strained lattices towards the development of a wavyness of their surface by means of surface di!usion of atoms. This might explain the thickness modulations seen in Fig. 4b. Since the well strain is very large, they develop rapidly despite planar surfaces. InP IML thus cannot help. We also compared InAsP and GaInAsP as materials for IML growth, using comparable IML thicknesses. Their strain was chosen to be either lattice matched to InP or 0.2% compressive strain which is the average between well and barrier strain for MQW structure B, analogous to Ref. [16]. The obtained PL results for lattice-matched IML inserted in MQW structure B are shown in Fig. 3 (black data points). At this intermediate MQW strain level, InAsP performes equally well as InP, but GaInAsP leads to an increased FWHM and a lower intensity. No improvement could be obtained for 0.2% IML strain. We repeated the comparison at 2% compressive well strain (not depicted in the graph). Now even InAsP IML leads to a larger FWHM and lower intensity. No PL intensity can be measured for GaInAsP IML. Only InP preserves a high PL intensity. Annealed at 6003C and 6503C for 1 h, wavelength shifts of the MQWs of *j(4 nm for InP and InAsP IML, but *j"9.5 nm for GaInAsP IML were observed. InAsp and GaInAsP di!er from the binary material InP because they o!er one or two degrees of freedom to alter their composition, respectively. Grown on an inhomogeneous layer, the material can adjust for present lattice spacing modulations, thereby minimizing its strain energy. The planarization and lattice spacing equalization e!ect is thus reduced. Additionally, when inserting InP IML, growth of each GaInAsP tensile-strained layer starts on binary material which possibly reduces surface selectivity in comparison to growth on GaInAsP. This suggests that surface selective growth mechanisms are the cause for wavy layer growth.
4. Conclusion For MOVPE-grown MQW structures, layer thickness and strain modulations develop rapidly
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beyond a limit of 0.45% tensile barrier strain, resulting in increased PL FWHM. Elevated growth temperatures in MOVPE seem to allow for a better MQW morphology at (0.45% tensile layer strain. In MOMBE a similar strain limit is found. However, it is associated mainly with a sharp drop in PL intensity, rather than a rapid increase in PL FWHM as in MOVPE. We attribute this di!erence to the characteristic wavelength K of the modulations being K '150 nm and K (80 nm. MOVPE MOMBE Thin intermediate InP layers yield a drastic improvement of the MQW quality, even for highly strained structures, whereas InAsP and GaInAsP intermediate layers appear to be less suitable. Thus a much larger range of strain can be accessed in strain-compensated MQW structures when InP intermediate layers are employed. The origin of the wavy layer growth in strained-MQW structures is still controversial, it is probably caused by surface selective growth. Acknowledgements The authors wish to thank Karl-Peter Johansen for helpful discussions. References [1] J. Matthews, A. Blakeslee, J. Cryst. Growth 27 (1974) 118. [2] H. Sugiura, M. Mitsuhara, M. Ogasawara, M. Itoh, H. Kamada, J. Appl. Phys. 81 (3) (1997) 1427. [3] T. Okada, G. Weatherly, J. Cryst. Growth 179 (1997) 339. [4] J. Emery, C. Starck, L. Goldstein, A. Ponchet, A. Rocher, J. Cryst. Growth 127 (1993) 241. [5] A. Ponchet, A. Le Corre, A. Godefroy, S. SalauK n, A. Poudoulec, J. Cryst. Growth 153 (1995) 71. [6] R. Goldman, R. Feenstra, C. Silfvenius, B. Stalnacke, G. Landgren, J. Vac. Sci. Technol. B 15 (4) (1997) 1027. [7] U. Bangert, A. Harvey, V. Wilkinson, C. Dieker, J. Jowett, A. Smith, S. Perrin, C. Gibbins, J. Cryst. Growth 132 (1993) 231. [8] R. Glew, K. Scarrott, A. Briggs, A. Smith, V. Wilkinson, X. Zhou, M. Silver, J. Cryst. Growth 145 (1994) 764. [9] H. Heinecke, G. Davies, Selected area epitaxy, in: J.S. Foord, G. Davies, W. Tsang (Eds.), Chemical Beam Epitaxy and Related Techniques, Wiley, New York, 1997, p. 331. [10] E. Veuho!, J. Cryst. Growth 188 (1998) 231. [11] M. Nakao, H. Oohashi, T. Hirono, H. Kamada, H. Sugiura, J. Appl. Phys. 78 (5) (1995) 3462.
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[12] F. Glas, J. Appl. Phys. 62 (8) (1987) 3201. [13] B. Spencer, P. Voorhees, S. Davis, J. Appl. Phys. 73 (10) (1993) 4955. [14] G. Patriarche, A. Ougazzaden, F. Glas, Appl. Phys. Lett. 69 (1996) 2279.
[15] J. Grilhe, Acta Metall. Mater. 41 (3) (1993) 909. [16] K. Hiramoto, M. Sagawa, S. Fujisaki, T. Toyonaka, Proceedings of the IEEE 24th International Symposium on Compound Semiconductors, San Diego, California, September 8}11, 1997.
Comparison of structural and optical properties in strained GaInAsP MQW structures grown by MOVPE and MOMBE P. KroK ner!,",*, H. Baumeister!, J. Rieger!, E. Veuho!!, O. Marti", H. Heinecke" !Inxneon Technologies, Dept. CPR 7, D-81730 Munich, Germany "Department of Experimental Physics, University of Ulm, D-89069 Ulm, Germany
Abstract The growth parameter dependence of the transition from 2D to 3D growth of GaInAsP multiple quantum well (MQW) structures up to e "0.5% tensile-strained barriers was examined. Identical MQW structures with e "1% B W compressively strained wells were grown by metal organic vapor-phase epitaxy (MOVPE) and metal organic molecular beam epitaxy (MOMBE) and characterized by photoluminescence (PL), X-ray di!raction and transmission electron microscopy. Increasing the tensile barrier strain resulted in deteriorated optical and crystalline properties beyond a critical strain limit, which depends on growth temperature. The deterioration originates from lateral layer thickness and strain modulations. Their density, amplitude and thus their e!ect on the optical MQW properties are di!erent for both growth methods. High-quality MOMBE-grown MQW structures up to e "2% compressive well strain and W e "0.5}1% tensile barrier strain could be achieved by inserting thin intermediate layers at each internal interface. The B composition of these intermediate layers has a signi"cant e!ect on MQW material properties. ( 2000 Elsevier Science B.V. All rights reserved.
1. Introduction The quaternary Ga In As P material sysx 1~x y 1~y tem is widely used for the fabrication of optoelectronic devices like high-speed telecommunication laser diodes emitting at 1.3 and 1.55 lm wavelength. Strained layer multiple quantum wells (MQW) in the active zone allow for improved device performance. In order to prevent plastic relaxation through mis"t dislocations of the strained layers, the net strain of the MQW layers should
* Corresponding author. Tel.: #49-89-234-48786; fax: #4989-234-41658. E-mail address: [email protected] (P. KroK ner)
remain below a critical strain limit [1]. This can be achieved by the technique of strain compensation. It is widely accepted that the growth of strained layers in MOMBE [2], MBE [3], GSMBE [4,5] and MOVPE [6}8] is susceptible to a transition from 2D to an anisotropic 3D growth mode, also referred to as wavy layer growth. In particular, tensile-strained layers seem to be susceptible to this e!ect. It can reduce the material quality of the MQW structure severely [2}8], limiting the application of tensile-strained layers and should thus be eliminated. Despite the fact that MOVPE and MOMBE are non-equilibium processes, the development of composition #uctuations is often assumed to be due to spinodal decomposition [2}4,7,8]. Surface selective growth mechanisms [9]
0022-0248/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 9 ) 0 0 5 8 4 - 9
P. Kro( ner et al. / Journal of Crystal Growth 209 (2000) 424}430
are a more likely cause for the e!ect. It is the aim of this paper to contribute to the current understanding of this topic by discussing the role of growth temperature and strain in identical MQW structures grown by MOVPE and MOMBE. Furthermore, the insertion of thin intermediate layers (IML) at each internal MQW interface is evaluated as a means to control wavy layer growth. Thus, conclusions for the growth of highly strained MQW structures can be drawn.
2. Experimental procedure Our MQW test structures were grown by MOVPE at 6003C and 6453C or by MOMBE at 5003C. Details of the MOVPE and MOMBE growth systems are published elsewhere [10]. All grown MQW structures have 5 periods, compressively strained GaInAsP or InAsP well layers and tensile-strained GaInAsP barrier layers (Table 1). The layer thicknesses are 8 nm, each. They are embedded between a 200 nm InP bu!er and a 100 nm InP cap layer. (1 0 0) InP substrates 23 misaligned towards S1 1 0T were used. MQW growth rates were 0.9}1.1 lm/h for MOMBE samples with V/III ratios of 5}7. For MOVPE, growth rates were 1.5 lm/h and V/III ratios 130}270. The samples were analyzed by photoluminescence measurements (PL), micro-PL (l-PL) measurements with a spatial resolution below 50 lm, high-resolution X-ray di!raction (HRXRD) and bright-"eld transmission electron microscopy (TEM) using the g"S2 0 0T di!raction vector.
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3. Results and discussion 3.1. Strained MQWs without intermediate layers We compared samples of structures A (cf. Table 1) with tensile barrier strains between 0% and 0.5%, grown by MOVPE at 6003C and 6453C as well as by MOMBE at 5003C. The net strain of the MQW stack never exceeded the critical strain limit calculated according to Matthews and Blakeslee [1]. Thus no strain relaxation by the development of dislocation networks occured. This was con"rmed by l-PL measurements where no dark line defects, generally associated with dislocation networks, were observed for any sample [11]. Fig. 1 depicts results of standard PL measurements performed at room temperature. Below 0.3% tensile barrier strain, a low FWHM of 20 meV and high intensities were obtained in MOVPE samples. Beyond 0.3% strain, our data reveal a strain region where a steep increase of the FWHM and a successive drop of the intensity is recognized. At a growth temperature of 6453C it is sharply de"ned and can be located at 0.46%$0.05% tensile strain. These "ndings re#ect another strain limit beyond which the MQW quality deteriorates. Yet, it is di!erent from the Matthews and Blakeslee de"nition of critical strain which delimits the potential development of dislocation networks serving for strain relaxation. The inset of Fig. 1 displays the HRXRD rocking curves of the MOVPE samples grown at 6453C with (a) 0.46% and (b) 0.47% barrier strain. A sharply de"ned satellite peak structure, resulting from a well-de"ned long distance periodicity of the MQW periods can be seen in (a). When the strain
Table 1 Well and barrier layer compositions and strains of grown MQW structures. All barrier layer compositions are GaInAsP (j"1.20 lm equivalent wavelength) MQW structure
Well layer composition (j"1.57 lm)
Well layer strain (compressive) (%)
Barrier layer strain (tensile) (%)
A B C D
GaInAsP GaInAsP InAsP InAsP
1 1 2 2
0}0.5 0.5 0.5 1
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Fig. 1. PL results (room temperature) for MQW samples of structure A with di!erent barrier strains, grown in MOVPE at 6453C and 6003C and in MOMBE at 5003C. Inset: HRXRDrocking curves of MOVPE MQW samples (¹"6453C) marked with (a) and (b) in the graph. TEM images of circled samples are shown in Fig. 2.
limit exceeds, a total loss of the satellite peak structure, as seen in (b), implies a breakdown of the long-distance periodicity. At 6003C growth temperature the tensile strain limit for MQW material deterioration is lowered to 0.3}0.4% with the transition regime being less pronounced than at 6453C. For MOMBE the increase of FWHM with barrier strain is much weaker (Fig. 1), not showing any sharp transition to a structural breakdown. At 0.5% tensile strain the FWHM remains below 40 meV, much lower than obtained by MOVPE. Taking the FWHM as a measure of well thickness and composition homogeneity, at high strains the MQW distortion is less pronounced than in MOVPE. But, for the given structure, even at low
barrier strains of 0.2% the FWHM does not drop below 28 meV. HRXRD rocking curves always showed a well-de"ned satellite peak structure. Yet, between 0.4% and 0.5% strain a severe drop to almost zero occured in PL intensity. In contrast, for MOVPE intensity is reduced by a factor of only 2. Our data suggest a similar critical limit of approximately 0.5% tensile barrier strain for MOMBE and MOVPE structures, beyond which distortions develop rapidly. A similar strain limit has been mentioned by other authors for the onset of wavy layer growth [3,5]. TEM characterization was done in order to link the PL and HRXRD results to the morphology. Figs. 2a and b show TEM images of the two severely degraded MOVPE and MOMBE samples which are marked by circled data points in Fig. 1. (0 11 1) cross sections were prepared since thickness modulations linked to wavy layer growth are anisotropic and run along the [0 1 1] direction [2}5]. Both MOVPE and MOMBE samples show layer thickness modulations and strain "elds, apparent as dark regions in the images. The MOMBE sample pertains an undistorted "rst period of the MQW. Thickness variations start to be visible in the second barrier and are ampli"ed towards the upper periods, leading to severe undulations in the top period. Well and barrier layer thickness modulations preferably display an antiphase relationship, i.e. above a thin well region a thick barrier follows and vice versa. Lateral e!ective strain modulation is the consequence, appearing as dark shadows around layer distortions in the top period of Fig. 2a. This is similar to reports of groups employing related growth technologies i.e. MBE and GSMBE [2,4,5]. Layer thickness and strain modulations are of larger amplitude in the MOVPE sample, forming strong strain "elds of 200}400 nm in length and starting to be visible already in the "rst MQW period. An ampli"cation through the successive periods cannot be registred. A much larger characteristic wavelength K of the modulations in MOVPE samples (K '150 nm) MOVPE is noted, di!erent from MOMBE samples with K (80 nm. This is supported by compariMOMBE sons of the observed characteristic wavelength extracted from literature for samples grown by
P. Kro( ner et al. / Journal of Crystal Growth 209 (2000) 424}430
427
Fig. 2. (0 11 1) cross-section TEM images of deteriorated MQW samples marked in Fig. 1 and grown by (a) MOMBE and (b) MOVPE.
MOVPE [7,8] with MBE, GSMBE or MOMBE data [2,3,5]. Comparing the low amplitude of distortions of the MOMBE sample to the larger amplitude in the MOVPE sample, the di!erence in PL FWHM and HRXRD results can be understood. Assuming nonradiative recombination centers to be linked to each modulation period [7], their large density in the MOMBE sample could cause the severe PL intensity drop. In contrast, their density could be lower in the MOVPE samples due to " being larger, allowing for stronger radiative recombination to take place and thus explaining the reduced drop in PL intensity. Models explaining the successive development of the lateral thickness modulations have been described in Refs. [5,6]. Volume strain energy reduction is widely accepted as a driving force for the 3D growth mechanism [12]. In an initially homogeneous layer, some thickness or composition #uctuation can occur, leading to a local variation in strain or composition. As can be seen from surface selective growth (SSG), the element incorporation e$ciencies are sensitive to surface constitution, particularly when growing quaternary materials [9]. When growing on an inhomogeneous surface, lateral variations in element incorporation, lateral growth rate modulations and thus an ampli"cation of the thickness modulations are the consequence. Anisotropic surface di!usion serves as the relevant mass transport mechanism responsible for the
lateral redistribution of the incorporated elements. The rate of modulation development is controlled by the sensitivity of the incorporation e$ciencies on the starting conditions of the grown layer and by the surface di!usion rate and length [13]. The latter can be lowered by increasing the V/III #ux ratio, thus slowing down modulation development [3,5]. Both increasing the growth temperature beyond 6003C in MOVPE and reducing it below 5003C in MOMBE, MBE and GSMBE yield higher e!ective V/III ratios, due to increased cracking e$ciency of hydrides and lower surface desorption of group V atoms, respectively. A higher V/III ratio reduces surface di!usivity and thus modulation development. This is supported by groups who obtained improved morphology for GSMBE and MBE samples with tensile-strained layers at low growth temperatures and high V/III ratios [3,5]. The growth temperature induced lower di!usion distance and rate could account for the smaller characteristic wavelength K observed in MOMBE grown samples as compared to MOVPE. 3.2. Strained MQWs with intermediate layers Given the tensile strain as limiting factor for the MQW quality, a key idea is to study whether the development of 3D growth can be suppressed when providing an improved starting surface for each
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grown layer. Thus, we inserted IML at each internal interface. Using MOMBE, MQW structures B, C and D (cf. Table 1) were grown with InP IML of di!erent thickness. Fig. 3 shows the PL results. Closed symbols mark measurements on MQWs of structure B. The drop in FWHM and rise in intensity with increasing IML thickness point towards the positive e!ect of the InP IML on the MQW morphology. At 2.5 nm thickness, an intensity equal to MQWs without structural damage is obtained. Fig. 4a shows a TEM analysis of the cross section of this sample (encircled closed symbol in Fig. 3). It con"rms that no visible undulations can be recognized and a uniform lateral and periodic vertical structure is obtained. We repeated this experiment for the growth of MQWs with high compressive well strains (structures C and D). Again, InP was used as IML material. As can be seen from the open data points
Fig. 3. PL results of MOMBE grown MQW samples with InP, InAsP and GaInAsP intermediate layers: (black data points) structure B; (white diamonds) structure C; (white circles) structure D. TEM images of circled samples are shown in Fig. 4.
Fig. 4. TEM images of MQW samples with InP intermediate layers marked in Fig. 3. (a) Structure B, (0 11 1) cross section. (b) Structure C, (0 1 1) cross section.
in Fig. 3, a minimum IML thickness of 1.4 nm proved to be necessary to detect PL intensity at all. The defect density, being the source for nonradiative recombination, is further reduced when the thickness is increased. Surprisingly, the FWHM cannot be reduced further below 42 meV for both 0.5% and 1% tensile barrier strain when varying IML thickness. Not depicted in the graph, we con"rmed the increase of the PL FWHM with increasing well strain in smaller steps from 1% to 2%, regardless whether GaInAsP or InAsP was used as well material. This suggests the strain to be the cause of the large FWHM. Using TEM, the (0 1 1) and (0 11 1) cross sections of structure C (marked with a circle in Fig. 3) were analyzed (cf. Fig. 4b). The barrier layers are smooth and of constant thickness, however, the well layers show anisotropic sporadic thickness variations of up to 30% along the [0 11 1] direction with large spacing explaining the large PL FWHM. No phase correlation between thickness variations in adjacent periods can be identi"ed. Those results con"rm that the insertion of InP IML suppresses the wavy layer growth. InP grows particularly in hollows of existing undulations [14], planarizing any starting wavyness and reducing lateral lattice spacing modulations. Thus, successive layers can grow on planar surfaces. The tendency towards ampli"cation of the wavyness is reduced, when the IML is
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su$ciently think. Grilhe [15] stated a tendency of strained lattices towards the development of a wavyness of their surface by means of surface di!usion of atoms. This might explain the thickness modulations seen in Fig. 4b. Since the well strain is very large, they develop rapidly despite planar surfaces. InP IML thus cannot help. We also compared InAsP and GaInAsP as materials for IML growth, using comparable IML thicknesses. Their strain was chosen to be either lattice matched to InP or 0.2% compressive strain which is the average between well and barrier strain for MQW structure B, analogous to Ref. [16]. The obtained PL results for lattice-matched IML inserted in MQW structure B are shown in Fig. 3 (black data points). At this intermediate MQW strain level, InAsP performes equally well as InP, but GaInAsP leads to an increased FWHM and a lower intensity. No improvement could be obtained for 0.2% IML strain. We repeated the comparison at 2% compressive well strain (not depicted in the graph). Now even InAsP IML leads to a larger FWHM and lower intensity. No PL intensity can be measured for GaInAsP IML. Only InP preserves a high PL intensity. Annealed at 6003C and 6503C for 1 h, wavelength shifts of the MQWs of *j(4 nm for InP and InAsP IML, but *j"9.5 nm for GaInAsP IML were observed. InAsp and GaInAsP di!er from the binary material InP because they o!er one or two degrees of freedom to alter their composition, respectively. Grown on an inhomogeneous layer, the material can adjust for present lattice spacing modulations, thereby minimizing its strain energy. The planarization and lattice spacing equalization e!ect is thus reduced. Additionally, when inserting InP IML, growth of each GaInAsP tensile-strained layer starts on binary material which possibly reduces surface selectivity in comparison to growth on GaInAsP. This suggests that surface selective growth mechanisms are the cause for wavy layer growth.
4. Conclusion For MOVPE-grown MQW structures, layer thickness and strain modulations develop rapidly
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beyond a limit of 0.45% tensile barrier strain, resulting in increased PL FWHM. Elevated growth temperatures in MOVPE seem to allow for a better MQW morphology at (0.45% tensile layer strain. In MOMBE a similar strain limit is found. However, it is associated mainly with a sharp drop in PL intensity, rather than a rapid increase in PL FWHM as in MOVPE. We attribute this di!erence to the characteristic wavelength K of the modulations being K '150 nm and K (80 nm. MOVPE MOMBE Thin intermediate InP layers yield a drastic improvement of the MQW quality, even for highly strained structures, whereas InAsP and GaInAsP intermediate layers appear to be less suitable. Thus a much larger range of strain can be accessed in strain-compensated MQW structures when InP intermediate layers are employed. The origin of the wavy layer growth in strained-MQW structures is still controversial, it is probably caused by surface selective growth. Acknowledgements The authors wish to thank Karl-Peter Johansen for helpful discussions. References [1] J. Matthews, A. Blakeslee, J. Cryst. Growth 27 (1974) 118. [2] H. Sugiura, M. Mitsuhara, M. Ogasawara, M. Itoh, H. Kamada, J. Appl. Phys. 81 (3) (1997) 1427. [3] T. Okada, G. Weatherly, J. Cryst. Growth 179 (1997) 339. [4] J. Emery, C. Starck, L. Goldstein, A. Ponchet, A. Rocher, J. Cryst. Growth 127 (1993) 241. [5] A. Ponchet, A. Le Corre, A. Godefroy, S. SalauK n, A. Poudoulec, J. Cryst. Growth 153 (1995) 71. [6] R. Goldman, R. Feenstra, C. Silfvenius, B. Stalnacke, G. Landgren, J. Vac. Sci. Technol. B 15 (4) (1997) 1027. [7] U. Bangert, A. Harvey, V. Wilkinson, C. Dieker, J. Jowett, A. Smith, S. Perrin, C. Gibbins, J. Cryst. Growth 132 (1993) 231. [8] R. Glew, K. Scarrott, A. Briggs, A. Smith, V. Wilkinson, X. Zhou, M. Silver, J. Cryst. Growth 145 (1994) 764. [9] H. Heinecke, G. Davies, Selected area epitaxy, in: J.S. Foord, G. Davies, W. Tsang (Eds.), Chemical Beam Epitaxy and Related Techniques, Wiley, New York, 1997, p. 331. [10] E. Veuho!, J. Cryst. Growth 188 (1998) 231. [11] M. Nakao, H. Oohashi, T. Hirono, H. Kamada, H. Sugiura, J. Appl. Phys. 78 (5) (1995) 3462.
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[12] F. Glas, J. Appl. Phys. 62 (8) (1987) 3201. [13] B. Spencer, P. Voorhees, S. Davis, J. Appl. Phys. 73 (10) (1993) 4955. [14] G. Patriarche, A. Ougazzaden, F. Glas, Appl. Phys. Lett. 69 (1996) 2279.
[15] J. Grilhe, Acta Metall. Mater. 41 (3) (1993) 909. [16] K. Hiramoto, M. Sagawa, S. Fujisaki, T. Toyonaka, Proceedings of the IEEE 24th International Symposium on Compound Semiconductors, San Diego, California, September 8}11, 1997.