APPLIED PHYSICS LETTERS
VOLUME 76, NUMBER 19
8 MAY 2000
Structure and optical properties of SiÕInAsÕSi layers grown by molecular beam epitaxy on Si substrate N. D. Zakharov,a) P. Werner, and U. Go¨sele Max-Planck Institute of Microstructure Physics, 06120 Halle/Saale, Weinberg 2 Germany
R. Heitz and D. Bimberg Technical University of Berlin, Berlin, Germany
N. N. Ledentsov, V. M. Ustinov, B. V. Volovik, Zh. I. Alferov, N. K. Polyakov, V. N. Petrov, V. A. Egorov, and G. E. Cirlin A.F. Ioffe Phisico-Technical Institute RAS, St. Petersburg, Russia
共Received 29 November 1999; accepted for publication 8 March 2000兲 Epitaxial Si/InAs/Si heterostructure grown on 共001兲 Si substrate by molecular beam epitaxy and annealed at 800 °C was investigated by high resolution transmission electron microscopy. Extensive interdiffusion leads to the formation of an InAs solid solution in the Si cap layer. Additionally, InAs-enriched regions with extensions of ⬃6 nm, which exhibit two kinds of ordering are observed. ¯ ) planes inclined and The ordering of InAs molecules has occurred, respectively, in 共101兲 and (101 ¯ 0) planes parallel to the 关001兴 growth direction. It is attributed to the energy gain from 共110兲 and (11 the reduced number of mixed Si–As and Si–In bonds. The sample show photoluminescence in the 1.3 m region, which is tentatively attributed to the recombination of excitons localized in the ordered regions. © 2000 American Institute of Physics. 关S0003-6951共00兲00719-1兴
The potential benefit from combining the advantageous optical properties and flexibility of III–V semiconductors with silicon technology widely used in microelectronics has attracted great interest for decades. Up to now, researchers have focused on the growth of continuous layers of III–V materials on silicon.1 The large misfit between Si and, e.g., InAs (⫽10.6%) renders the growth of electronic or optical quality material a practically unresolvable problem. More recently, the possibility to exploit the formation of narrow-gap III–V islands on Si substrates has been pointed out.2 Indeed, such small InAs islands on Si共100兲 surface have been observed by scanning tunneling microscopy and highresolution transmission electron microscopy 共HRTEM兲.3,4 HRTEM investigations of capped InAs/Si structures revealed a high density of coherent InAs clusters with typical dimensions in the 3 nm region at the InAs/Si interface for optimized growth conditions.5,6 Such samples exhibit a broad photoluminescence 共PL兲 peak in the 1.3 m region at 10 K.3 Detailed optical investigations of this PL line indicated a k-indirect type II transition, which has been tentatively attributed to excitons localized in the small coherent InAs clusters.7 The extreme small size (⬍3 nm) of these clusters might, however, prevent sufficient carrier localization. The present work presents a detailed structural characterization of such InAs–Si layers providing insight into the origin of the 1.3 m PL peak. The InAs/Si heterostructure was grown by molecular beam epitaxy 共MBE兲 on p-type Si共100兲 substrate using an EP 1203 machine. Conventional evaporation cells were used for In, Si, and As fluxes. The growth rates for InAs and Si were 0.03 and 0.017 nm/s, respectively, and the As4 /In flux ratio was ⬃4. InAs was deposited at a substrate temperature a兲
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of T s ⫽350 °C. For these particular growth conditions, the reflection high energy electron diffraction pattern indicates the two- 共2D兲 to three-dimensional 共3D兲 transition at 1.3 monolayers 共ML兲. The nominal thickness of the deposited InAs was 1.6 ML. Immediately after the InAs deposition, a 10 nm Si cap layer was grown at the same T s ⫽350 °C followed by a 10 min annealing step at 700 °C. Further 40 nm Si cap layer was grown at 700 °C with a final 10 min annealing step at 800 °C to smooth resulting surface. The crystalline quality of the structure and the composition of the grown layers were investigated by techniques of transmission electron microscopy 共TEM兲. The following electron microscopes were exploited: Philips CM20T and JEOL JEM-4000 EX operating at 200 and 400 kV, respectively. Commercial software package ‘MacTempas’ was used for computer simulations of HRTEM images.8 PL spectra were measured at a temperature of 7 K using argon laser excitation. Typical cross-section and plan-view images of the investigated structure are shown in Figs. 1共a兲 and 1共b兲, respectively. The plan-view image shows the good structural quality of the sample with a relatively low density (1 ⫻108 1/cm2) of structural defects, marked A in Fig. 1共b兲. These defects 共A兲 are located at the InAs/Si interface and do not penetrate into Si cap layer. The nature of the contrast features B in Fig. 1共a兲 is analyzed below. First, the average InAs concentration in the Si cap layer can be estimated from selected area diffraction 共SAD兲 taken at once from substrate and cap layer in cross-sections sample as shown in Fig. 1共a兲. Such SAD pattern reveal a splitting of reflections in 关001兴 direction perpendicular to the layer surface 关Fig. 1共c兲兴. This splitting is attributed to a tetragonal distortion of Si cap layer due to the formation of an InAs solid solution. The magnitude of the observed splitting of the
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FIG. 1. 共a兲 关010兴 cross-section TEM image. (2a⫻1a) ordered InAs regions are marked as B. Black circle SAD indicate the region from what the selected area diffraction was taken. 共b兲 Plan view 关001兴 TEM image. Defects marked as A in are situated at the interface. 共c兲 Selected area diffraction 共SAD兲 from cross-section sample. Diffraction was taken from the region marked by black circle in 共a兲. Sample was oriented along 关010兴. ⌬ ⫽⌬g/ 兩 g兩 , gÄ(206)-diffraction vector, ⌬g-the change in g.
reflection 共206兲 measured in SAD corresponds to a tetragonal distortion ⌬⫽⌬g/g (206) ⫽⌬a/a Si⫽0.007. The volume of 3 (1⫹⌬a/a Si). It immediately the distorted unit cell is V t ⫽a Si follows that average cubic unit cell parameter of the solid solution a ss is given by: 1/3 a ss ⫽V 1/3 t ⫽a Si共 1⫹⌬a/a Si 兲 .
共1兲
Taking a ss as to be linearly dependent on the InAs concentration in the Si matrix, it follows: C InAs⫽ 共 a ss ⫺a Si兲 / 共 a InAs⫺a Si兲 ,
共2兲
where C InAs concentration of InAs in Si cap layer, a Si ,a InAs-unit cell parameters of Si and InAs, respectively. Substitution of ⌬a/a Si⫽0.007 into Eqs. 共1兲, 共2兲 gives C InAs ⫽0.004. Thus the averaged composition of the cap layer can be written as Si0.996共InAs兲0.004 . Second, in high resolution cross-sectional images the dark regions marked by B in Fig. 1共a兲 reveal a doubling of periodicity of 兵002其 lattice planes in 关110兴 or 关 ¯1 10兴 directions 关Fig. 2共a兲兴. It leads to appearance of diffuse maxima situated halfway between ⫾(220) matrix reflections in the Fourier transformed image 共FFT兲 关see insert in Fig. 2共a兲兴. This result can be interpreted as a partial ordering of InAs in Si. A possible idealized model of such an ordering is shown in Fig. 2共b兲 where InAs occupies every other atomic 共101兲 plane inclined by 45° to the surface. Let us define the twodimensional ordering as (na⫻mb), where n,m-integer numbers and a,b periodicities in two perpendicular directions without ordering. In our case these directions are ¯ ) and a⫽b⫽a Si/2&⫽0.192 nm. It gives (2a (101),(101 ⫻1a). A HREM image 关Fig. 2共c兲兴 simulated on the basis of this structural model shows that the darker rows correspond to InAs atomic rows. It is obvious that the contrast calculated for an ideal ordering is in a qualitative agreement with the experimental image. These partially ordered regions can also be observed at low magnification using diffraction contrast
FIG. 2. 共a兲 关010兴 cross-section HRTEM image. Doubled periodicity 0.38 nm is marked. Fourier transformation of the image is inserted in upper right corner. Diffuse maxima on the halfway between central beam and ⫾(202) reflections indicate to the (2a⫻1a) ordering. 共b兲 Idealized model of (2a ⫻1a) ordering. 共c兲 Theoretical image calculated according to the model in 共b兲. Unit cell is outlined.
technique 关see features B in Fig. 1共a兲兴. In this case, the image contrast 共dark regions兲 results from variations of the extinction distance. The size of (2a⫻1a) coherent ordered regions (⭓6 nm) is about 2 times larger than the size of coherent InAs clusters formed at the InAs/Si interface and described in a former letter.5 Additionally, a special kind of ordering
FIG. 3. HRTEM plan view image. Sample was milled from substrate side. Thus, the image demonstrates the near surface structure of cap layer. 3⫻1 ¯ 0 兴 is seen 共B兲. Fourier transformation of the (3a⫻1a) ordering along 关 11 image is inserted. Maxima B situated on ⫾1/3 g 关 g⫽(220) 兴 indicate to the (3a⫻1a) ordering. Downloaded 19 Feb 2004 to 195.37.184.165. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
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InAs/Si solution is formed in the Si cap layer. The formation of such a InAs/Si solid solution results from an extensive diffusion of InAs into the Si cap layer during annealing at 800 °C. According to our measurements the average concentration of InAs in the Si cap layer can reach C InAs⫽0.4%. In addition to the random distribution of InAs, two similar kinds of ordering have been observed. The (2a⫻1a) ordered regions have a size of ⬃6 nm in diameter. They are formed due to condensation of InAs molecules in every second 共110兲 atomic layer. As a result the free energy of the system gets lower due to decreasing the number of mixed Si–As and –In bonds. The appearance of broad PL band at 1.3 m can be correlated to the formation of InAs/Si ordered regions.
FIG. 4. PL spectrum of the investigated sample showing a broad luminescence at 1.25 m. The peak at 1.1 m correlates to emission from the Si bulk.
was found in the near surface region of cap layer, where ¯ 0) planes perpendicular to the InAs occupies 共110兲 and (11 layer surface 共Fig. 3兲. Here, however, periodicity is larger with only every third atomic plane occupied by InAs. This kind of ordering (3a⫻1a) is usually extended over rather large areas being approximately several tens of nm in diameter. Figure 4 depicts a low temperature PL spectrum of the investigated sample revealing a broad PL peak in the 1.3 m region. Recently, this luminescence has been studied in detail in a different sample, suggesting a k-indirect type II transition in the epitaxial layer.7 Such a transition is indeed expected for coherent InAs clusters observed near the InAs/Si interface. However, the small size (⬃3 nm) of such clusters might be to small for sufficient carrier localization. As shown above, the Si-cap layer is actually a Si–InAs solid solution with (2a⫻1a) and (3a⫻1a) ordered InAs-rich regions. The ordered regions with a high InAs concentration 共and therefore smaller band gap兲 and a size of ⬃6 nm, can provide sufficient carrier localization to explain the observed 1.3 m emission. The incorporation of InAs molecules into the Si is expected to shift the relative positions of the conduction and valence bands leading to a quantum structure. In conclusion, we have demonstrated that InAs can be epitaxially incorporated in silicon by MBE growth. A solid
This work was partially supported by INTAS Grant No. 94-0242, Russian Foundation for Basic Research 共Grant No. 99-02-16799 and 98-02-18317兲, National Program Physics on solid-state nanostructures and National Program ‘‘Advanced devices for micro- and nanoelectronics’’ in a frame of the Project No. 02.04.5.1.40.E.46.
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