Two-dimensional array of self-assembled AlInAs quantum wires S. Francoeur, A. G. Norman, A. Mascarenhas, E. D. Jones, J. L. Reno et al. Citation: Appl. Phys. Lett. 81, 529 (2002); doi: 10.1063/1.1493222 View online: http://dx.doi.org/10.1063/1.1493222 View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v81/i3 Published by the American Institute of Physics.
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APPLIED PHYSICS LETTERS
VOLUME 81, NUMBER 3
15 JULY 2002
Two-dimensional array of self-assembled AlInAs quantum wires S. Francoeur,a) A. G. Norman, and A. Mascarenhas National Renewable Energy Laboratory, Golden, Colorado 80401
E. D. Jones, J. L. Reno, S. R. Lee, and D. M. Follstaedt Sandia National Laboratories, Albuquerque, New Mexico 87185
共Received 26 February 2002; accepted for publication 10 May 2002兲 We present the optical and structural characterization of a two-dimensional array of self-organized AlInAs quantum wires. The structure was created by epitaxially stacking along the 关001兴 direction thin self-assembled, 关100兴-oriented, superlattices separated by homogeneous layers of Al0.48In0.52As. Vertical and lateral self-alignment results in a highly regular array of wires oriented along the 关010兴 direction. The wire cross-sectional dimensions are about 10⫻14.4 nm2 and their density is 1.9 ⫻1011 cm⫺2 . The energy and the nature of the electronic transitions are significantly affected by confinement in two dimensions: 共1兲 a blueshift of about 100 meV is observed and 共2兲 the two lowest energy transitions are both polarized along the 关010兴 direction. For comparison, the two lowest energy transitions of a lateral superlattice with similar characteristics have a heavy- 共polarization along 关010兴兲 and a light-hole character 共polarization along 关100兴兲. Large polarization ratios are measured for both transitions. © 2002 American Institute of Physics. 关DOI: 10.1063/1.1493222兴
quantum wells grown along 关001兴 in which the well is a lateral superlattice similar to sample A and the vertical barrier is simply a homogeneous layer of Al0.48In0.52As. The key to fabrication of a lateral array of wires 共wires parallel to the surface兲 of the nanometer dimensions used here is the use of self-assembled lateral superlattices as building blocks. Lateral superlattices, sometimes referred to as laterally composition modulated structures, are comprised of a periodic variation in alloy composition along a direction perpendicular to the growth direction 共see Ref. 9 for a collection of articles on the subject兲. These superlattices can be fabricated in a variety of semiconductor compounds 共GaInP,1 InGaAs,10 and AlInAs11兲 with either molecular beam epitaxy1,11 共MBE兲 or metalorganic chemical vapor deposition,8,12,13 or both. The main characteristics are the superlattice’s orientation, period, average composition, modulation amplitude and modulation profile. The lateral superlattices in these samples were induced by growing a vertical short-period superlattice 共SPS兲 by MBE. Both structures were grown on an AlInAs homogeneous buffer layer lattice matched on an InP 共001兲 substrate. The short-period superlattice consists of a given number of periods of the following sequence: 1.42 monolayer of AlAs and 2.0 monolayers of InAs. These values were obtained from x-ray diffraction 共see Ref. 14 for more details兲. Sample A is simply a stack of 100 periods of the SPS while sample B is made of 10 periods of a 10-period SPS alternating with a homogeneous alloy of Al0.48In0.52As. Sample B has a 93 nm cap layer of Al0.48In0.52As. As reported previously, the misorientation of the substrate allows control over the direction of the lateral superlattice axis.5,15 For the samples of interest in this study, a 2° offcut along 关101兴 was used to induce a well-defined superlattice axis along 关100兴. Figure 1 shows 共010兲 cross-sectional micrographs of samples A and B taken using the 共002兲 diffraction conditions in dark-field mode. These conditions are preferred for their sensitivity to composition rather than strain. Figure 1共a兲
The use of self-organized phenomena is one of the preferred approaches for the fabrication of semiconductor nanostructures. Using this approach, it is relatively simple to obtain a high density of nanometer scale quantum dots or/and wires without using pattern transfer or patterned growth templates. However, due to the statistical nature of the phenomenon, the electronic properties are strongly affected by the lack of control over the structural properties: self-assembled nanostructures suffer from significant size and shape fluctuations and their nonuniform spatial distribution. Attempts to gain control over the characteristics of these structures led to a better understanding of the surface kinetics during epitaxial growth and, as a result, new and more complex laterally defined structures like lateral superlattices,1 tilted superlattices,2 superlattices of dots,3 arrays of quantum wires,4 and arrays of quantum pillars5 have been achieved. However, just as for their predecessors, these structures also suffer from a lack of spatial uniformity thereby impeding the study of their electronic properties. This lack of control has been limiting the optical characterization of lateral superlattices, an essential component of the wire array presented in this work, to photoluminescence spectroscopy. Recently, lateral superlattices of improved structural integrity have been achieved, allowing other complementary spectroscopy techniques like modulation spectroscopy6 to be used to characterize the nature of the optical transitions in AlInAs lateral superlattices.7,8 We present the structural and optical characterization of an AlInAs quantum wire array. The structural characterization was done using transmission electron microscopy and the electronic transitions were studied using electroreflectance spectroscopy. Results from two samples are presented, compared and discussed. Sample A is a thick layer comprised of a self-assembled AlInAs lateral superlattice whose axis is oriented along 关100兴. Sample B consists of multiple a兲
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529
© 2002 American Institute of Physics
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Francoeur et al.
Appl. Phys. Lett., Vol. 81, No. 3, 15 July 2002
FIG. 2. Electromodulated reflectance of sample A. A fit to the experimental spectra was used to determine the transition energies.
FIG. 1. 共010兲 cross-sectional transmission electron microscopy image taken in the 共002兲 dark-field diffraction conditions of 共a兲 sample A and 共b兲 sample B.
shows the cross section of the thick lateral superlattice. A well-defined, slightly tilted, lateral sequence of vertical bright and dark columns is observed. The contrast in the image is generated by the difference in composition: bright and dark correspond to In and Al rich regions, respectively. Each column is in fact elongated perpendicular to the plane of the cross section, making this structure a lateral superlattice. The composition profile along 关100兴 is best approximated by a sine function. Plan-view images of A can be found in Ref. 15 and are very similar to the plan-view images of B 共not shown兲. Figure 1共b兲 shows the wire array 共sample B兲. Two composition modulations are evident: vertical modulation, along 关001兴, formed by a 10-period vertical superlattice 共that is comprised of alternating lateral superlattices and an Al0.48In0.52As separation layer兲 and selforganized lateral modulation, along 关100兴. As seen from the contrast between In and Al rich regions in Fig. 1共b兲, the superposition of the vertical and lateral superlattices produces a regular array of quantum wires with a density of 1.9⫻1011 cm⫺2 . The approximate cross-sectional size of each wire is 10⫻14.4 nm2 and the composition profile is a
sine function along 关100兴 and a step function along 关001兴. As can be seen, the vertical alignment of the structure is good. The strain field of the buried lateral superlattices penetrates the homogeneous alloy 共and may induce slight composition modulation in it兲 and aligns the subsequent superlattice such that the strain energy is minimized. This effect enhances the alignment of the array and makes the periodicity more uniform.3 The properties of the lateral superlattices in both samples were also studied with x-ray diffraction. Superlattice satellites appeared next to the 共002兲 diffraction peak displaced along 关100兴. The position and intensity of these satellites for samples A and B are comparable. This indicates that the periodicity and amplitude of the composition modulation in both samples are similar.14 The optical transitions in the two samples differ considerably due to the additional vertical confinement added in the second sample. Modulated electroreflectance is used to characterize the interband transitions and the spectra measured are fitted to the third derivative of the dielectric function in the low-field regime.6 300 K electro-reflectance spectra were recorded as a function of the polarization direction of the incident light and the reflection was recorded in quasinormal geometry. The modulated field was applied using a contactless transparent electrode. Figure 2 shows the electroreflectance spectra of sample A. Four transitions can easily be observed. The two unpolarized transitions on the high energy side are related to the InP substrate and the Al0.48In0.52As buffer layer. Their signal intensity is equally divided in the 关100兴 and 关010兴 polarizations. The transitions of interest are the ones on the low energy side. The In-rich quantum wells under compressive (A) strain are at the origin of these transitions. E (A) 1 and E 2 , at 1.066 and 1.148 eV, are both strongly polarized along 关010兴 and 关100兴, respectively. The anisotropy between the polarizations is a direct consequence of the reduced symmetry of the superlattice and the degeneracy between the valence bands that is lifted under compressive strain. Due to the nature of the light-hole and heavy-hole band-edge wave functions, the dipole moments of the optical transitions are polarization dependent. Taking into consideration the direction of observation 关001兴, with respect to the superlattice axis 关100兴, tran-
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Francoeur et al.
Appl. Phys. Lett., Vol. 81, No. 3, 15 July 2002
FIG. 3. Electromodulated reflectance of sample B. The dotted line shows the baseline produced by the InP transition. The inset shows the experimental lines shape and a fit 共dotted line兲 after subtraction of the baseline.
sitions with a dominant heavy-hole character are predominantly polarized along 关010兴 whereas transitions with a dominant light-hole character are mostly polarized parallel to and E (A) are assigned the superlattice axis.16 Therefore E (A) 1 2 to transitions having a dominant heavy- and a light-hole character, respectively. The energy and assignment of these transitions are in very good agreement with the results of an eight band k"p calculation for a modulation amplitude of ⬃15%. This model satisfactorily reproduces the energy of the heavy- and light-hole transitions. Figure 3 shows the electroreflectance spectra measured for sample B. Again, four transitions are observed. The transitions of the InP substrate and AlInAs buffer or cap are unpolarized. Two other transitions are observed in the low energy region of these spectra. To model these transitions, we subtract from the two spectra 关(⌬R/R) [010] and (⌬R/R) [100] 兴 the baseline created by the strong InP substrate transition. This baseline is shown by the dotted line. The inset shows the signal measured for both polarizations after subtraction of this baseline. The dotted line shown in the inset shows a perfect fit to the 关010兴 polarization line shape. The modeling yields two transitions located at 1.169 and 1.255 eV with an uncertainty of ⫾5 meV. They are labeled and E (B) E (B) 1 2 , respectively. The lowest energy transition is in agreement with the 300 K photoluminescence transition at 1.167 eV having a full width at half maximum of 59 meV with 共not shown兲. We notice an important blueshift of E (B) 1 respect to E (A) 1 . As noted above, the lateral superlattices in both samples have very similar characteristics. Therefore, the 103 meV shift toward high energy is a direct result of the transition from a two- to a one-dimensional quantum structure. The analysis of the nature of these two transitions is more complex than that for the preceding case. The first complication results from the multiaxial nature of the strain. While the major component of the strain originates from the difference in lattice constant between the wire and its lateral barrier, the vertical mismatch between the wire and the vertical barrier must also be taken into account for an accurate analysis. Second, the reduced symmetry significantly mixes the light- and heavy-hole character even at the zone’s center.
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Therefore the light- and heavy-hole nomenclature used for the previous sample is not appropriate. Nonetheless, the lowest energy transition of a quantum wire under compression has a maximum transition probability for polarized light along the wire axis, i.e., along 关010兴, in agreement with our observations. A discussion of the nature of the second transition would require the help of a model to quantitatively describe the geometry of the structure, the strain, and the material parameters. However, the rotation of the polarization direction of the second transition in sample B demonstrates that a one-dimensional structure was achieved and, as a consequence, the nature and optical selection rules of the transitions were affected. In summary, the fabrication and characterization of an array of self-assembled AlInAs quantum wires was described. Electroreflectance spectroscopy was used to study the optical transitions. In comparing the results between a lateral superlattice and the quantum wire array, we observe distinct changes in the energy and polarization dependence of the lowest energy transitions. The addition of vertical confinement strongly alters the valence band states. While the lowest energy transition has a similar polarization direction in both samples, the polarization direction of the second transition is reversed in the wire array, indicating that the nature of the transition might be different. It appears that this is a promising approach for fabricating an array of selfassembled quantum wires.
1
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