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InAs(100) strain layer modulated structures grown by molecular beam epitaxy
606
Surface
CROSS-SECTIONAL OF GaAs/InAs(lCKl) GROWN
TRANSMISSION STRAIN
BY MOLECULAR
ELECTRON
LAYER MODULATED
Science 174 (1986) 6066614 North-Holland. Amsterdam
MICROSCOPY STRUCTURES
BEAM EPITAXY
and B.F. LEWIS,
R. FERNANDEZ,
Jez Propulsron Luhoruro~,, Received
I September
L. ENG and F.J. GRUNTHANER
C’ulifornru fnsriture of Twhnologv,
1985: accepted
for publication
Posodencc. ~‘uli/orniu VI IiN, 1,S.I
16 November
1985
We report some results of cross-secttonal transmission electron microscope (XTEM) studres on GaAs/InAs multiple quantum wells and superlattices grown under a variety of unusual growth conditions via molecular beam epitaxy. These growth conditions cover the usual dynamic shuttering of group III fluxes, interrupted growth, and the first usage of growth under group V controlled growth rate conditions. The modulated structures vary in individual layer thickness from 3 monolayers (ML) to 30 ML and total thickness from 200 A to 1.2 pm. Results show the presence. and consequences, of both intrinsic strain and strain effects due to XTEM specrmen thickness. They also indicate the possibility of realizing good qualtty interfaces in such highly lattice mismatched systems (drr/a = 7%) via the use of some unusual and novel growth conditions.
1. Introduction
Attempts to grow highly lattice mismatched semiconductor modulated structures [l-5] with high structural and chemical integrity have intensified in the past few years employing the techniques of metal-organic chemical vapor deposition (MOCVD) [2,3] and molecular beam epitaxy (MBE) [4.5]. Of particular interest has been the growth of GaAs/In ,Ga, ~, As multiple quantum well (MQW) structures in which the lattice mismatch can reach 7’5%for s = 1. The first successful realization of a GaAs/InAs MQW structure with well-formed interfaces and low defect density grown directly on GaAs substrate was recently reported [4] by us for MBE growth of thin individual layers (4 monolayers (ML) of InAs alternating with 8 monolayers of GaAs). The success of the growth was attributed to an interrupted growth technique [6,7] in which InAs was deposited in submonolayer increments. This interrupted growth approach was employed to allow annealing during the development of 0039-6028/86/$03.50
Physics
Publishing
0 Elsevier Science Publishers B.V. (North-Holland Division) and Yamada Science Foundation
M. Y. Yen et al. / XTEM
studies of GaAs/
InAs structures grown by MBE
601
individual monolayers of InAs thereby enabling thermodyamic accommodation of lattice-mismatch-induced strain. Furthermore, considerations of the kinetics of MBE growth as revealed in computer simulations [8,9] based on the configuration-dependent-reactive-incorporation (CDRI) model and systematic measurements of the dynamics of the intensity in reflection high-energy electron diffraction (RHEED) during GaAs/InAs and InAs/GaAs MBE growth [lO,ll] suggest that growth interruption will also minimize the surface step density for the subsequent deposition. Guided by such considerations of the interplay between kinetic and thermodynamic aspects of MBE growth we have examined growth of GaAs/InAs MQW structures and superlattices over a wide range of growth conditions, including some highly unusual ones, and with individual layer thicknesses up to 30 monolayers. In this paper we present some results of cross-sectional transmission electron microscopy studies of a few of these systems.
2. Experimental The growth was carried out in a computer-controlled Riber 1001-2 MBE system. The GaAs(lOO) substrate surfaces were prepared following the usual chemical cleaning procedures and mounted on a molybdenum block substrate holder using indium. The oxide layer remaining on the GaAs surface at the end of the chemical cleaning procedure referred to above was alternatively removed by two different techniques. The first of these was the standard practice of themally removing the oxide in situ under an As, flux immediately prior to buffer layer growth. The alternate procedure entailed the removal of the thin passivating oxide by chemical spin etching in a dry box with inert N, atmosphere followed by transfer to the MBE loading chamber through a vacuum interlock. This procedure [12] has been found to provide high-purity GaAs substrate surfaces with lower level of carbon. Good streaked RHEED patterns were observed prior to buffer layer deposition when the substrate was brought to temperatures and As, pressures in the usual range of values for GaAs growth. In all cases, however, a buffer layer of GaAs was grown under the usual As-stabilized (2 x 4) surface growth conditions to obtain yet higher quality surfaces prior to growth of the MQW structures. Throughout the growth sequence, starting with the GaAs buffer layer deposition, the RHEED was monitored using a low light level video camera. The video signal was recorded on a VHS video recorder for subsequent analysis of specular spot intensity behavior and for detailed examination of surface reconstruction during growth. lntensity analysis entailed digitization of the video signal and data analysis on a Varian V76 minicomputer. The electron microscopy studies were carried out using a Philips 420T analytical microscope operated at 120 keV with a line resolution of 2 A. The
specimens for the microscopy were prepared by cleaving the grown samples along (110) planes and cementing two slabs of the grown MQW structure face-to-face. Such composite specimens were then mechanicalIy thinned from the side to a thickness of = 50 ym. They were next thinned to electron transparency using an Ar ion beam incident at very low angle of incidence. The results presented in this paper were obtained employing the (0, 0, 0) and (2, 0. 0) two-beam imaging conditions with the MQW being observed in cross-section (XTEM).
3. Superlattice growth conditions In view of space limitations. we restrict ourselves to presentation of the findings on only three modulated structures out of the many grown under a varied range of growth conditions and examined via XTEM. This range ol growth conditions, motivated by systematic study [10.11.13] of the RHEED intensity dynamics for deposition of InAs on GaAs and GaAs on InAs (in addition to the usual GaAs/GaAs and InAs/InAs depositions), cover growth under such dramatically differing conditions that to facilitate discussion of the results we classify them in the following broad categories. (I) Standard MBE growth: group III controlled growth rate under the usual group V-stabilized surface conditions with dynamic shuttering of Ga and In source fluxes (i.e. no interruption of growth); (2) standafd-interrupted growth: same as in (1) except that growth is interrupted in a periodic fashion after programmed amounts of deposition and the growth front is allowed to heal the step-density distrihution: (3) metal stabilized growth: group III controlled growth rate, but under conditions where the surfaces are metal stabilized: (4) alternating group V and metal stabilized surface conditions with deposition of alternate layers: (5) group V controlled growth rate: growth under conditions such that the growth rate is controlled by the reactive incorporation [8,9] of the group V species: and (6) aIternate group III and group V controlled growth rate: same as (5) except that the growth rate controlling species may be alternated with the two materiais. Finally, any of the growth categories (3) through (6) may be combined with the notion of growth interruption in order to help improve the quality of the growth front for deposition of subsequent layers, and thus help improve the nature of the interface as well as the structural quality of the material. These considerations are particularly important for systems with high shear strain. The group V controlled growth rate cited in (5) and (6) is realized when the arrival rate of the group III species is significantly higher than that for the group V. Naturally, to achieve stoichiometric growth under such conditions. the group III flux is terminated after a predetermined amount of deposition, but this does not necessarily imply growth interruption. We have previously demonstrated (131 the occurrence of group V controlled RHEED
M. Y. Yen et al. / XTEM
studies of GaAs/
InAs structures grown by MBE
609
intensity oscillations under such conditions, which corresponds to direct measurement of the incorporation rate of the arsenic. Systematic studies [13] of GaAs/GaAs growth under these conditions have allowed us to study the As-incorporation rate as a function of As, pressure and substrate temperature, and allow us to calibrate the metal and group V flux, the latter to better than 3%.
4. Transmission Results superlattice
electron microscopy results
of TEM consists
studies on sample of a 3 ML Irk/6
1 are shown ML GaAs
in figs. 1 and 2. The unit cell grown under
Fig. 1. Two-beam, da? .-field image contrast micrograph showing the mid-region of the 3 ML InAs/ ML GaAs superlattices (sample 1 grown under growth conditions noted as category (4)). Note the presence of defect lines and the “banding” effects discussed in the text.
Fig. 2. Two-beam. dark-field image contrast of sample 1 under the same diffraction conditiona of fig. 1 taken at a point = 0.75 pm from the substrate/superlattice interface. Note the change scale.
as in
conditions described in category (4) repeated 390 times. Fig. 1 shows a TEM image contrast of a region well into the nearly 1 pm superlattice and fig. 2 (at higher magnification) a region still further away from the starting interface. Separate from the existence of defects such as dislocations and twins in fig. 1, one notes the presence of intensity contrast “bands” over distance scales much larger than the 3 ML InAs/ ML GaAs layers constituting the superlattice. These bands are associated with internal variations of strain in the TEM specimen and are not necessarily a reflection of structural defects of the type noted above. As has been recently shown by Gison et al. [14] through their work on MBE grown Ge,Si, _ 1; superlattices, large variations in the local lattice parameter and modulation thickness can occur in thin samples for
M. Y. Yen et al. / XTEM studies of GaAs/ InAs structures grow by MBE
611
Fig. 3. Two-beam, bright-field image contrast micrograph of 4 ML InAs/ ML GaAs multiple quantum well grown under standard interrupted growth conditions (sample 2 of the text). Note the absence of any significant strain-induced image contrast “banding” effects of the type seen in figs. 1 and 2.
XTEM due to elastic relaxation of strain. This considerably complicates interpretation of the image contrast in such systems and obscures the real quality of the modulated structure. It is also important to note that the degree of the strain-induced effects is significantly dependent upon the total thickness of the superlattice. Figs. 1 and 2 correspond to a superlattice whose total thickness is nefrly 1 pm, even though the individual layer thicknesses are quite small (= 9 A InAs and 17 A GaAs). For much smaller total thickness of the superlattice the strain relaxation effects, and consequently the “banding” effects seen in image contrast studies, are expected to be significantly reduced. T’his is seen to be the case in the image contrast (bright-field) micrograph shown for sample 2 in fig. 3. Sample 2 was grown with growth interruption, but otherwise standard conditions (i.e. category (2) of section 3) and consists of individual InAs and GaAs layers of thicknesses 4 ML and 8 ML respectively, comparable to the 3 ML and 6 ML case of sample 1. The total thickness of this MQW structure is, banding effects. however, only = 200 A. One notes far less strain-induced Note the absence of defects originating at the substrate or dislocations and twins in the MQW structure itself, unlike sample 1. The quality of the
Fig. 4. High-magnification. two-beam, dark-field image of 30 ML InAs/ ML CiitA~ multiple quantum well grown under category (6) growth conditions with alternate group V and group III controlled growth rate (sample 3 of the text).
interfaces is remarkably good. Further details of this MQW structure, which is the first reported successful growth of a high-quality modulated structure involving lattice mismatch as large as = 7%, may be found in ref. [4]. In fig. 4 is shown the dark-field image contrast behavior of sample 3 which represents the highly unusual (to our knowiedge. the first) attempt to grow layers under group V controlled growth rate conditions, in this case InAs only (i.e. category (6) of section 3). The individual layer thicknesses are 30 ML each for InAs and GaAs, which is a thickness significantly larger than the critical thickness for generation of misfit dislocations for a lattice mismatch of = 7%’ estimated from simple theories. The total thickness of this MQW structure consisting of IS periods in all is = 2265 A. Yet no evidence of any significant
M. Y. Yen et al. / XTEM studies of GaAs / InAs structures grown by MBE
level of misfit are seen in fig. in comparison total thickness
613
dislocation density is seen. The strain-induced banding effects 4, but at a level consistent with the intermediate total thickness to samp!e 1 of large thickness (= 1 pm) and sample 2 of small (= 200 A).
5. Summary The results of our cross-sectional transmission electron microscopy examination of GaAs/InAs multiple quantum well and superlattices grown over a range of novel growth conditions, including the highly unusual As-controlled growth rate conditions, reveal several interesting features. The results selected for presentation here are intended to indicate that even for highly lattice mismatched systems such as GaAs/InAs (da/a = 7%), usage of new growth conditions which exploit the differing roles of kinetics and thermodynamics holds the promise of realizing high-quality structures. We have explored a variety of unconventional and novel growth conditions guided by examination of the underlying kinetic and thermodynamic considerations and RHEED intensity behavior. A more detailed account of such investigations will be provided elsewhere. Specifically, these results show that successful growth of good-quality modulated structures even with fairly thick (given the high lattice mismatch) individual layer thicknesses are possible, provided the total thickness is not made too large. Separate from the defects that can be generated when the individual layer thicknesses (for a given lattice mismatch) exceed some critical thickness, it is the total strain in the grown structure that is of critical importance. Thus even structures involving individual layer thicknesses significantly smaller than the corresponding critical thickness for generation of defects can end up with a high defect density due to internal strain relaxation if grown to large total thicknesses. The mechanical effects associated with the thickness of XTEM specimens itself may contribute to some extent to the density of such defects observed in diffraction studies, quite apart from the strain-related banding effects seen in image contrast studies and thereby complicate their interpretation. Electrical and optical properties of such superlattices may thus end up being of a quality higher than one might expect on the basis of a cursory glance at the TEM image contrast alone. Some evidence for this appears to be indicated by our preliminary measurements of the electron mobility of sample 1. We hope to report on such findings in the near future. Acknowledgements This work was supported by the Office of Naval Research (ONR Contract #N00014-83-4-0050) at the University of Southern California. Part of the
research described in this paper was carried out by the Jet Propulsion Laboratory (JPL), California Institute of Technology under contract with the National Aeronautics and Space Administration. It was also supported by ONR and by the University of Southern California via subcontract No. 98531 at JPL.
References [I] J.M.
Matthews
and A.E. Blake&e.
J. Crystal.
Growth,
27 (1974)
11X: 29 (1975)
273: 32
(1976)265. [2] I.J. Fritz. L.R. Dawson and T.E. Zipperian. J. Vacuum Sci. Technol. RI (19X3) 3X7. [3] T. Fukui and H. Saito. Japan. J. Appl. Phys. 23 (1984) L521. [4] F.J. Grunthaner. M.Y. Yen. R. Fernandez. T.C. Lee. A. Madhukar and B.F. Lewis. Appl. Phys. Letters 46 (1985) 983. [S] M.C. Tamargo. R. Hull. L.H. Greene. J.R. Hayes and A.Y. Cho, Appl. Phys. Letters 46 (1985) 569. [6] F.J. Grunthaner, A. Madhukar, R. Fernandez, T.C. Lee. M.Y. Yen and B.F. Lewis. Presented at the Intern. Superlattice Conf.. August 1984. Urbana. Illinhis; to he published in Superlattices and Microstructures. [7] A. Madhukar, T.C. Lee. M.Y. Yen. P. Chen. J.Y. Kim. S.V. Ghatsas and P.G Newman. Appl. Phys. Letters 46 (1985) 114X. (81 S.V. Ghaisas and A. Madhukar, J. Vacuum Sci. Tech&. 83 (19X5) 540. [9] A. Madhukar and S.V. Ghaisas. Appl. Phys. Letters 47 (1985) 247. [IO] B.F. Lewis. T.C. Lee, F.J. Grunthaner. A. Madhukar, R. Fernandez and J. Maseqtan. J. Vacuum Sci. Technol. B2 (1984) 419. [ll] B.F. Lewis, F.J. Grunthaner. A. Madhukar. T.C. Lee and R. Fernandez, J. Vacuum SCI. Tech&. B3 (1985) 1317. [12] R.P. Vasquez, B.F. Lewis and F.J. Grunthaner. J. Vacuum Sci. Technol. BI (19X3) 791. [13] B.F. Lewts. R. Fernandez, A. Madhukar and F.J. Grunthaner, J. Vacuum Sci. Technol. 84 (1986) 560. [14] J.M. Gibson, R. Hull. J.C. Bean and M.M.J. Treaty. Appl. Phys. Letters 46 (1985) 649.
Surface
CROSS-SECTIONAL OF GaAs/InAs(lCKl) GROWN
TRANSMISSION STRAIN
BY MOLECULAR
ELECTRON
LAYER MODULATED
Science 174 (1986) 6066614 North-Holland. Amsterdam
MICROSCOPY STRUCTURES
BEAM EPITAXY
and B.F. LEWIS,
R. FERNANDEZ,
Jez Propulsron Luhoruro~,, Received
I September
L. ENG and F.J. GRUNTHANER
C’ulifornru fnsriture of Twhnologv,
1985: accepted
for publication
Posodencc. ~‘uli/orniu VI IiN, 1,S.I
16 November
1985
We report some results of cross-secttonal transmission electron microscope (XTEM) studres on GaAs/InAs multiple quantum wells and superlattices grown under a variety of unusual growth conditions via molecular beam epitaxy. These growth conditions cover the usual dynamic shuttering of group III fluxes, interrupted growth, and the first usage of growth under group V controlled growth rate conditions. The modulated structures vary in individual layer thickness from 3 monolayers (ML) to 30 ML and total thickness from 200 A to 1.2 pm. Results show the presence. and consequences, of both intrinsic strain and strain effects due to XTEM specrmen thickness. They also indicate the possibility of realizing good qualtty interfaces in such highly lattice mismatched systems (drr/a = 7%) via the use of some unusual and novel growth conditions.
1. Introduction
Attempts to grow highly lattice mismatched semiconductor modulated structures [l-5] with high structural and chemical integrity have intensified in the past few years employing the techniques of metal-organic chemical vapor deposition (MOCVD) [2,3] and molecular beam epitaxy (MBE) [4.5]. Of particular interest has been the growth of GaAs/In ,Ga, ~, As multiple quantum well (MQW) structures in which the lattice mismatch can reach 7’5%for s = 1. The first successful realization of a GaAs/InAs MQW structure with well-formed interfaces and low defect density grown directly on GaAs substrate was recently reported [4] by us for MBE growth of thin individual layers (4 monolayers (ML) of InAs alternating with 8 monolayers of GaAs). The success of the growth was attributed to an interrupted growth technique [6,7] in which InAs was deposited in submonolayer increments. This interrupted growth approach was employed to allow annealing during the development of 0039-6028/86/$03.50
Physics
Publishing
0 Elsevier Science Publishers B.V. (North-Holland Division) and Yamada Science Foundation
M. Y. Yen et al. / XTEM
studies of GaAs/
InAs structures grown by MBE
601
individual monolayers of InAs thereby enabling thermodyamic accommodation of lattice-mismatch-induced strain. Furthermore, considerations of the kinetics of MBE growth as revealed in computer simulations [8,9] based on the configuration-dependent-reactive-incorporation (CDRI) model and systematic measurements of the dynamics of the intensity in reflection high-energy electron diffraction (RHEED) during GaAs/InAs and InAs/GaAs MBE growth [lO,ll] suggest that growth interruption will also minimize the surface step density for the subsequent deposition. Guided by such considerations of the interplay between kinetic and thermodynamic aspects of MBE growth we have examined growth of GaAs/InAs MQW structures and superlattices over a wide range of growth conditions, including some highly unusual ones, and with individual layer thicknesses up to 30 monolayers. In this paper we present some results of cross-sectional transmission electron microscopy studies of a few of these systems.
2. Experimental The growth was carried out in a computer-controlled Riber 1001-2 MBE system. The GaAs(lOO) substrate surfaces were prepared following the usual chemical cleaning procedures and mounted on a molybdenum block substrate holder using indium. The oxide layer remaining on the GaAs surface at the end of the chemical cleaning procedure referred to above was alternatively removed by two different techniques. The first of these was the standard practice of themally removing the oxide in situ under an As, flux immediately prior to buffer layer growth. The alternate procedure entailed the removal of the thin passivating oxide by chemical spin etching in a dry box with inert N, atmosphere followed by transfer to the MBE loading chamber through a vacuum interlock. This procedure [12] has been found to provide high-purity GaAs substrate surfaces with lower level of carbon. Good streaked RHEED patterns were observed prior to buffer layer deposition when the substrate was brought to temperatures and As, pressures in the usual range of values for GaAs growth. In all cases, however, a buffer layer of GaAs was grown under the usual As-stabilized (2 x 4) surface growth conditions to obtain yet higher quality surfaces prior to growth of the MQW structures. Throughout the growth sequence, starting with the GaAs buffer layer deposition, the RHEED was monitored using a low light level video camera. The video signal was recorded on a VHS video recorder for subsequent analysis of specular spot intensity behavior and for detailed examination of surface reconstruction during growth. lntensity analysis entailed digitization of the video signal and data analysis on a Varian V76 minicomputer. The electron microscopy studies were carried out using a Philips 420T analytical microscope operated at 120 keV with a line resolution of 2 A. The
specimens for the microscopy were prepared by cleaving the grown samples along (110) planes and cementing two slabs of the grown MQW structure face-to-face. Such composite specimens were then mechanicalIy thinned from the side to a thickness of = 50 ym. They were next thinned to electron transparency using an Ar ion beam incident at very low angle of incidence. The results presented in this paper were obtained employing the (0, 0, 0) and (2, 0. 0) two-beam imaging conditions with the MQW being observed in cross-section (XTEM).
3. Superlattice growth conditions In view of space limitations. we restrict ourselves to presentation of the findings on only three modulated structures out of the many grown under a varied range of growth conditions and examined via XTEM. This range ol growth conditions, motivated by systematic study [10.11.13] of the RHEED intensity dynamics for deposition of InAs on GaAs and GaAs on InAs (in addition to the usual GaAs/GaAs and InAs/InAs depositions), cover growth under such dramatically differing conditions that to facilitate discussion of the results we classify them in the following broad categories. (I) Standard MBE growth: group III controlled growth rate under the usual group V-stabilized surface conditions with dynamic shuttering of Ga and In source fluxes (i.e. no interruption of growth); (2) standafd-interrupted growth: same as in (1) except that growth is interrupted in a periodic fashion after programmed amounts of deposition and the growth front is allowed to heal the step-density distrihution: (3) metal stabilized growth: group III controlled growth rate, but under conditions where the surfaces are metal stabilized: (4) alternating group V and metal stabilized surface conditions with deposition of alternate layers: (5) group V controlled growth rate: growth under conditions such that the growth rate is controlled by the reactive incorporation [8,9] of the group V species: and (6) aIternate group III and group V controlled growth rate: same as (5) except that the growth rate controlling species may be alternated with the two materiais. Finally, any of the growth categories (3) through (6) may be combined with the notion of growth interruption in order to help improve the quality of the growth front for deposition of subsequent layers, and thus help improve the nature of the interface as well as the structural quality of the material. These considerations are particularly important for systems with high shear strain. The group V controlled growth rate cited in (5) and (6) is realized when the arrival rate of the group III species is significantly higher than that for the group V. Naturally, to achieve stoichiometric growth under such conditions. the group III flux is terminated after a predetermined amount of deposition, but this does not necessarily imply growth interruption. We have previously demonstrated (131 the occurrence of group V controlled RHEED
M. Y. Yen et al. / XTEM
studies of GaAs/
InAs structures grown by MBE
609
intensity oscillations under such conditions, which corresponds to direct measurement of the incorporation rate of the arsenic. Systematic studies [13] of GaAs/GaAs growth under these conditions have allowed us to study the As-incorporation rate as a function of As, pressure and substrate temperature, and allow us to calibrate the metal and group V flux, the latter to better than 3%.
4. Transmission Results superlattice
electron microscopy results
of TEM consists
studies on sample of a 3 ML Irk/6
1 are shown ML GaAs
in figs. 1 and 2. The unit cell grown under
Fig. 1. Two-beam, da? .-field image contrast micrograph showing the mid-region of the 3 ML InAs/ ML GaAs superlattices (sample 1 grown under growth conditions noted as category (4)). Note the presence of defect lines and the “banding” effects discussed in the text.
Fig. 2. Two-beam. dark-field image contrast of sample 1 under the same diffraction conditiona of fig. 1 taken at a point = 0.75 pm from the substrate/superlattice interface. Note the change scale.
as in
conditions described in category (4) repeated 390 times. Fig. 1 shows a TEM image contrast of a region well into the nearly 1 pm superlattice and fig. 2 (at higher magnification) a region still further away from the starting interface. Separate from the existence of defects such as dislocations and twins in fig. 1, one notes the presence of intensity contrast “bands” over distance scales much larger than the 3 ML InAs/ ML GaAs layers constituting the superlattice. These bands are associated with internal variations of strain in the TEM specimen and are not necessarily a reflection of structural defects of the type noted above. As has been recently shown by Gison et al. [14] through their work on MBE grown Ge,Si, _ 1; superlattices, large variations in the local lattice parameter and modulation thickness can occur in thin samples for
M. Y. Yen et al. / XTEM studies of GaAs/ InAs structures grow by MBE
611
Fig. 3. Two-beam, bright-field image contrast micrograph of 4 ML InAs/ ML GaAs multiple quantum well grown under standard interrupted growth conditions (sample 2 of the text). Note the absence of any significant strain-induced image contrast “banding” effects of the type seen in figs. 1 and 2.
XTEM due to elastic relaxation of strain. This considerably complicates interpretation of the image contrast in such systems and obscures the real quality of the modulated structure. It is also important to note that the degree of the strain-induced effects is significantly dependent upon the total thickness of the superlattice. Figs. 1 and 2 correspond to a superlattice whose total thickness is nefrly 1 pm, even though the individual layer thicknesses are quite small (= 9 A InAs and 17 A GaAs). For much smaller total thickness of the superlattice the strain relaxation effects, and consequently the “banding” effects seen in image contrast studies, are expected to be significantly reduced. T’his is seen to be the case in the image contrast (bright-field) micrograph shown for sample 2 in fig. 3. Sample 2 was grown with growth interruption, but otherwise standard conditions (i.e. category (2) of section 3) and consists of individual InAs and GaAs layers of thicknesses 4 ML and 8 ML respectively, comparable to the 3 ML and 6 ML case of sample 1. The total thickness of this MQW structure is, banding effects. however, only = 200 A. One notes far less strain-induced Note the absence of defects originating at the substrate or dislocations and twins in the MQW structure itself, unlike sample 1. The quality of the
Fig. 4. High-magnification. two-beam, dark-field image of 30 ML InAs/ ML CiitA~ multiple quantum well grown under category (6) growth conditions with alternate group V and group III controlled growth rate (sample 3 of the text).
interfaces is remarkably good. Further details of this MQW structure, which is the first reported successful growth of a high-quality modulated structure involving lattice mismatch as large as = 7%, may be found in ref. [4]. In fig. 4 is shown the dark-field image contrast behavior of sample 3 which represents the highly unusual (to our knowiedge. the first) attempt to grow layers under group V controlled growth rate conditions, in this case InAs only (i.e. category (6) of section 3). The individual layer thicknesses are 30 ML each for InAs and GaAs, which is a thickness significantly larger than the critical thickness for generation of misfit dislocations for a lattice mismatch of = 7%’ estimated from simple theories. The total thickness of this MQW structure consisting of IS periods in all is = 2265 A. Yet no evidence of any significant
M. Y. Yen et al. / XTEM studies of GaAs / InAs structures grown by MBE
level of misfit are seen in fig. in comparison total thickness
613
dislocation density is seen. The strain-induced banding effects 4, but at a level consistent with the intermediate total thickness to samp!e 1 of large thickness (= 1 pm) and sample 2 of small (= 200 A).
5. Summary The results of our cross-sectional transmission electron microscopy examination of GaAs/InAs multiple quantum well and superlattices grown over a range of novel growth conditions, including the highly unusual As-controlled growth rate conditions, reveal several interesting features. The results selected for presentation here are intended to indicate that even for highly lattice mismatched systems such as GaAs/InAs (da/a = 7%), usage of new growth conditions which exploit the differing roles of kinetics and thermodynamics holds the promise of realizing high-quality structures. We have explored a variety of unconventional and novel growth conditions guided by examination of the underlying kinetic and thermodynamic considerations and RHEED intensity behavior. A more detailed account of such investigations will be provided elsewhere. Specifically, these results show that successful growth of good-quality modulated structures even with fairly thick (given the high lattice mismatch) individual layer thicknesses are possible, provided the total thickness is not made too large. Separate from the defects that can be generated when the individual layer thicknesses (for a given lattice mismatch) exceed some critical thickness, it is the total strain in the grown structure that is of critical importance. Thus even structures involving individual layer thicknesses significantly smaller than the corresponding critical thickness for generation of defects can end up with a high defect density due to internal strain relaxation if grown to large total thicknesses. The mechanical effects associated with the thickness of XTEM specimens itself may contribute to some extent to the density of such defects observed in diffraction studies, quite apart from the strain-related banding effects seen in image contrast studies and thereby complicate their interpretation. Electrical and optical properties of such superlattices may thus end up being of a quality higher than one might expect on the basis of a cursory glance at the TEM image contrast alone. Some evidence for this appears to be indicated by our preliminary measurements of the electron mobility of sample 1. We hope to report on such findings in the near future. Acknowledgements This work was supported by the Office of Naval Research (ONR Contract #N00014-83-4-0050) at the University of Southern California. Part of the
research described in this paper was carried out by the Jet Propulsion Laboratory (JPL), California Institute of Technology under contract with the National Aeronautics and Space Administration. It was also supported by ONR and by the University of Southern California via subcontract No. 98531 at JPL.
References [I] J.M.
Matthews
and A.E. Blake&e.
J. Crystal.
Growth,
27 (1974)
11X: 29 (1975)
273: 32
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