InAlAs epitaxial layers grown over InP patterned substrates

December 1994

MaterialsLetters 21 (1994) 371-375

TEM analysis of InGaAs/InAlAs epitaxial layers grown over InP patterned substrates F. Peiro a,b, A. Comet a, J.R. Morante ‘, K. Zekentes ‘, A. Georgakilas ’ aLCMh4, Departamento de Fisica Aplicada i Electrdnica, Universidad Barcelona, Diagonal 645-647, 08028 Barcelona, Spain b Serveis Cientifico-Tecnics, Universidad Barcelona, Lluis So& i Sabaris 1,3, 08028 Barcelona, Spain ’ Foundation for Research and Technology - Hellas, P.O. Box 1527, Heraklion, Crete, Greece

Received 17 May 1994; in final form 20 September 1994; accepted 20 September I994

Abstract The structural characteristics of InAlAs/InGaAs films grown in plasma etched wells by molecular beam epitaxy have been analyzed by transmission electron microscopy. While the density of defects in the layer grown inside the well is comparable to the one obtained in layers grown in non-patterned substrates, a high density of defects, originated by the faceting properties of the growth, is observed near the window’s edge.

Epitaxial growth on patterned substrates is of potential interest in the purpose of building advanced electronic and optoelectronic devices [ 11. Most of the studies have been carried out using liquid phase epitaxy (LPE), vapor phase epitaxy (VPE), metal organic chemical vapor deposition (MOCVD) and metal organic molecular beam epitaxy (MOMBE) techniques and good selective area epitaxial growth has been obtained [ 2,3 1. Conversely, the poor selectivity achieved when using the molecular beam epitaxy technique had restricted its use as selective epitaxial growth technique. However, recent studies of InGaAs layers grown by MBE in InP etched wells prepared by reactive ion etching (RIE) in windows of a Silox mask have indicated that the MBE technology may be used successfully for the development of a InP-based monolithic optoelectronic integrated circuit (OEIC) technology [ 41. The plasma etching gives the advantage of selecting the desired plane of the walls due to its high etching anisotropy, and furthermore may be integrated with deposition techniques in an ultra-high vacuum system. Excellent

InGaAs films can be epitaxially deposited in the wells and the polycrystalline material, which is grown over the Silox mask, is easily removed by a lift off procedure. In this work we present structural characterization by transmission electron microscopy ( TEM ) of InAlAs/InGaAs structures grown by MBE inside InP wells prepared by RIE. Our attention will be devoted to study the two main problems limiting the application of these structures as devices: the presence of defects in the epilayer and the growth habit at the edges of the wells. So, for the first time, we will compare, from planar-view images, the structural characteristics of layers grown on non-patterned substrates and in the well. Concerning the second question, we will present cross-sectional TEM results of the faceted growth occurring at the well edges, showing that a different faceting along the two orthogonal ( 110) directions takes place. Moreover, the growth mode on the vertical walls of the well will also be analyzed. InAlAs/InGaAs heterostructures were grown by MBE on patterned (001) InP substrates. A 120 nm

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F. Peiro et al. / Materials Letters 21 (I 994) 3 71-3 75

thick Silox mask with open rectangular windows of dimensions 1000 urn x 250 urn to 30 urn x 30 pm was used to pattern the InP substrates. InP wells with walls orthogonal to the (00 1) plane were etched through the Silox windows by RIE. In order to assess any differences on the quality of the layers grown on the windows, quarters or halfs of 2 inch patterned wafers were indium bondes on molybdenum substrate holders, side by side with non-patterned InP substrates. The growth was accomplished at a growth rate of 0.85 urn/h and equivalent pressure ratio of V/III = 25. The InP substrate was thermally cleaned, prior to growth under an arsenic background overpressure, at a preheating temperature of 530” C. The thicknesses of the different layers constituting the InGaAs/InAlAs heterostructure were 1.85 pm InGaAs/lO nm InAlAs/ 30 nm InGaAs/O.lS urn InAlAs grown on the InP substrate. The In,Ga, _xAs and Infil, _,As compositions were slightly different from the lattice matched ones (x=0.53 andys0.52, respectively) and a value ofxE0.55 has been calculated from optical spectroscopy experiments [ 41. The plan-view and cross-section TEM observations have been done in a Hitachi 800 NA microscope at 200 kV. In order to see if there exist structural differences between the layers grown on patterned or non-patterned sustrates, we have performed planar-view observations on the (00 1) zone axis, imaging the samples in both g= 220 and g= 220 bright-field two beam conditions. Figs. la and lb show plan-view micrographs of the layers grown on non-patterned and pat-

temed substrates, respectively. The prevailing type of defects are stacking faults and threading dislocations; the densities being very similar in both cases (Table 1). There exists also an asymmetry between the images taken with g=220 and g=220, which reveal a higher density of stacking faults on {i11) than on {ii 1}. The origin of this asymmetry has been related to the differences on Peierls barriers along the ( 110) directions, for face cubic centered crystals [ 5 1. The single crystal edge characteristics can be seen in the TEM micrographs of Fig. 2, which have been obtained in ( 110) cross-sectional specimens. The different faceting along the two ( 110) directions is evident. So, whereas {114) and {11S} facets appear in the [ 1TO] cross section (Fig. 2a), {11 l} facets are present in the [ 1 lo] cross section (Fig. 2b). We can also observe that a part of the InP well is covered completely by the single crystal InGaAs/InAlAs deposition and the faceting edges start appearing only on the top half of the epilayer. Moreover, we can see the presence of a polycrystalline InGaAs/InAlAs layer developing over the Silox mask, indicating that a phase selective growth occurs (the deposited material is single crystalline on top of the exposed InP semiconductor inside the windows and polycrystalline on top of the Silox mask). Energy dispersive Xray measurements have shown [6] that the deposited alloy composition is similar in both the mask and window regions. A careful observation of Fig. 2 reveals that alloys also grow on the walls, resulting in a “V” like shape of deposited layers at the edges. The

Fig. 1. g= 220 plan view images of the layers grown (a) on the non-patterned InP and (b) inside the window of the patterned substrate. We observe essentially threading dislocations and stacking faults, with similar densities in both cases.

F. Peiro et al. /Materials Letters 21 (1994) 371-37s

313

Table 1 Density of stacking faults, psw and threading dislocations, pTD,measured under the reflections g=O22 and g=O2?! independently Sample

non-patterned patterned

PSF

ho

g= 022

g=o22

g=o22

g=o22

3.4x 10’ 1.3x 10’

7.1x106 8.5X lo6

2.3x lo8 1.2x 108

2.9x lo8 2.1 x lo8

presence of a strong contrast on the InP substrate near the edge is also noticeable, indicating the existence of a high strain. When this region is imaged at higher magnification (Fig. 3 ), we can observe a high density of defects, preferentially following the { 111) planes. In fact, these defects have been generated in the intersection of different facets (between the layers grown inside the window and on the walls), during the growth due to the strains associated with misregistry of the lattice planes. Fig. 4 shows the [ 1 lo] cross-sectional image of the InAlAsfInGaAs herostructure grown in the window of the patterned substrate, but far from the well edge. This image has been obtained with g= 200, which is the best diffraction condition to achieve the greatest contrast between the InAlAs and InGaAs layers. From our measurements, we can deduce that the layer thicknesses are exactly the same as in the non-patterned sample, so there is no change in the growth rate in regions far from the edge, with respect to the growth rate on non-patterned substrates. Conversely, when the thicknesses are measured on the wall, the growth rate is approximately a half of the growth rate value over the (00 1) substrate orientation. The configuration of the layers grown in the window can be interpreted taking into account the kinetic process of adatoms: incorporation, desorption and surface migration. According to the model of Ohtsuka et al. [ 7 1, the MBE growth on a planar surface depends on the orientation between the direction of the flux that is fixed to a certain substrate orientation and the direction normal to the surface. Taking into account the anisotropy of growth rate and the surface migration length until desorption as main parameters, they show that facets are formed for those orientations where the growth rate has local minima. In this model, they explain the minima of growth rate obtained for low-index surfaces such as (001) and ( 111) by the rate-limiting nucleation process of two-

dimensional islands. Once the islands are formed, they spread laterally acorss the surface until they meet each other and complete one monolayer. When the growth plane is not a low-index surface, an array of atomic steps which act as adsorption sites of adatoms is formed. Since those steps flow continuously and do not disappear with the evolution of gorwth, the growth rate increases relative to the low-index surface. This change of growth mode from nucleation assisted process to step flow process would explain the development of facets in the edges of our structures. In our case, the faster growing planes are the (114) and ( 118 ) ones, in agreement with other experimental results [ 8,9]. Another interesting point to be explained is the difference between the faceting in the two ( 110) orthogonal directions. As commented previously, one important parameter affecting the growth is the surface diffusion length of the adatoms. In the model of growth occurring by step flow on the vicinal surface, the group III atoms are not incorporated into the epitaxial layer immediately after the arrival to the surface, but they move a distance equal to the diffusion length before incorporation occurs. Measurements of the change of growth mode from 2D nucleation to step flow [lo] and of the influence of the growth temperature on the diffusion length [ 111 have shown that the diffusion length of group III atoms along the [ 1TO] direction is about four times larger than along the [ 1 lo] direction. So, the different faceting occurring between the two orthogonal ( 110) directions can be explained by the different surface diffusion length of the group III atoms in these directions. The V-shaped structure observed in Fig. 3 develops as the epilayers grown on the walls and on the facet planes intersect. At this point, a high density of threading dislocations and stacking faults are formed. However, far from the edge, the growth of the layer is unaffected by the influence of the growth on the

F. Peiro et al. 1 MaterialsLetters21(1994) 371-375

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Fig. 2 (a). [ 1iO] cross sectional view of the patterned sample. This bright field m~cro~aph shows the border effect for the [ 1f0 ] side of the window (orthogonal to the paper). The scheme illustrates the preferential growth on ( 114) and ( 118) facets. 2(b). Bright field image of the window in the ( 110) zone axis (the [ 1lo] side of the window is orthogonal to the paper). The shape of the intersection (small square in the scheme) is slightly different from that shown in (a). The growth takes place in { I 11)facets.

walls and, as observed by comparison of Figs. 1a and Ib, a layer with the same density of defects as the one grown on non-patterned substrates is obtained. In our case, the lower density of defects expected when using patterned substrates [ 121 is not observed, probably due to the large dimensions of our windows.

In summary our work indicates that InGaAs/InAlAs st~ctures can be developed in RIE wells of patterned InP substrates having similar characteristics as the structures grown on non-patterned substrates. The high deffective region observed near the window’s edges is originated by the faceting properties

F. Peiro et al. / Materials L.&ten 21 (I 994) 3 71-3 75

Fig. 3. Detail of the defects developed at the intersection the (001) epilayer and the layer growing in the wall.

between

of MBE growth over small structures. The different diffusion length of the atoms along the two orthogonal directions ( 110) would explain the different faceting obtained in these directions.

Acknowledgement This work has been partly funded by Spanish CICYT “Program National de Materiales” under the project MAT93-0564.

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P.K. Skevington, C.L. French and J.S. Foord, Mater. Sci. Eng. B 9 ( 1991) 93. [ 210. Kayser, J. Crystal Growth 107 ( 199 1) 989.

315

Fig. 4. Cross sectional image of the heterostructure grown on the patterned sample. The thicknesses of the layers are 128 nm of InAlAs, 28 nm of InGaAs and 3.5 nm of InAlAs. No differences have been observed with respect to those measured in layers grown in non-patterned substrates.

[3] J.S.C. Chang, K.W. Carey, J.E. Turner and L.A. Hodge, J. Electron. Mater. 19 ( 1990) 345. [4] A. Georgakilas, A. Christou, P. Lefebvre, J. Allegre, K. Zekentes and G. Halkias, Appl. Phys. Letters 61 ( 1992) 798. [5] B.A. Fox and W.A. Jesser, J. Appl. Phys. 68 (1990) 2739. [ 61 A. Georgakilas, K. Zekentes, K. Tsagaraki, P. Lefebvre, J. Allegre and A. Christou, in: 4th International Conference on InP and Related Materials, Newport, RI (IEEE, New York, 1992) pp. 101-104. [7] M. Ohtsuka and A. Suzuki, J. Crystal Growth 95 (1989) 55. [8] T. Yuasa, M. Mannoh, T. Yamada, S. Naritsuka, K. Shinozaki and M. Ishii, J. Appl. Phys. 62 ( 1987) 764. [ 9 ] J. Stephen, P.L. Derry, S. Margalit and A. Yariv, Appl. Phys. Letters 47 (1985) 712. [lo] K. Ohta, T. Koyima and T. Nakagawa. J. Cryst. Growth 95 (1989) 71. [ 111 M. Hata, T. Isu, A. Watanabe and Y. Katayama, J. Vacuum Sci. Technol. B 8 (1990) 692. [ 121 S. Guha, A. Madhukar and L. Chen, Appl. Phys. Letters 56 ( 1990) 2304.