Dieker, Ch.

Hartmann, A.

Jäger, W.

Lüth, H.

Schmidt, K.

Vescan, L.

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Optical and structural investigation of SiGe/Si quantum wells L. Vescan, A. Hartmann, K. Schmidt, Ch. Dieker, and H. LLjth Institute for Thin Films and Ion Technology, Research Center Jiilich, 5170-Jiilich, Germany W. Jager Institute for Solid State Physics, Research Center Jiilich, 5170-Jiilich, Germany (Received 27 November 1991; accepted for publication 26 February 1992) In this letter we report photoluminescence and structural results obtained on asymmetrically strained S&Ge&Si single and multiple quantum wells epitaxially grown by low pressure chemical vapor deposition. Well-resolved peaks were obtained which can be attributed to quantum well excitons and their transversal optical phonon replica. A good correlation between peak properties and structure results was found. From the photoluminescence peak energies a valence band offset of 0.27 eV and an effective hole mass of-O.25 were estimated. Photoluminescence (PL) is a powerful method widely used in later years to study the electronic states in quantum wells. For SiGe/Si heterostructures there have been several reports of PL for various samples from unstrained bulk to atomic layer superlattices. For unstrained bulk Sir-,Ge, alloys Weber and Alonso’ presented a detailed study of the near-band-gap PL. Sharp lines were detected in unstrained Sir-,Ge, alloys grown by liquid phase epitaxy.* Broad PL peaks were detected on molecular beam epitaxy (MBE) grown single layers.3,4 Intense PL was reported by Noel et LzI.~ for MBE-grown single (SQW) and multiple quantum wells (MQW). There are several reports on strained Si,- Ge, superlattices grown by MBE.‘-7 In particular, it has been predicted that for certain values of m and n and spe- cific strain conditions, direct or quasi-direct band-gap ma- terial should be obtained in these superlattices.s In the present study, SiGe/Si SQWs and MQWs were epitaxially grown by low pressure chemical vapor deposi- tion (LPCVD) and characterized by PL, Rutherford back- scattering (RBS), and transmission electron microscopy (TEM). We observe several well-defined PL lines at low temperatures, with linewidths of -7-30 meV depending on quantum well (QW) thickness. The comparison be- tween measured and calculated PL energies allowed us to estimate the valence-band offset (AE,) and the effective mass for holes (mf). The QW structures were deposited on p-type Si ( 100) wafers, 2000 fl cm. The epitaxy was carried out at 700 “C! and 0.1 Torr in a LPCVD system9 using as source gases dichlorosilane and germane. Usual growth rates were for Si 2 nm/min and-for Sio.7Ge0.3 1.8 nm/min.” The following layer sequences were chosen: for the MQWs: -260 nm Sio.g3Geo.07 buffer and then the MQWs . . consrstmg of Sio.,Geo,3 wells and Si barriers (see Table I) and for SQWs: - 1 pm Si layer, a Sio.7Gq.3 SQW layer, and a SiessGeco2 cap layer (see Table II). The thickness of the QWs was determined by RBS using 1.4 MeV 4He’ ions, tilting the sample 8 1” to the He+ beam and fitting the measured data with calculated ones using the program RUMP.~~ TEM characterization of the SQW and MQW structures was performed with a JEOL 4000FX electron microscope at 400 kV on plan-view and cross-section specimens. The PL was measured at 4.2 K using a Fourier transform spectrometer with a Ge p-i-n diode cooled with liquid nitrogen as a detector. The sam- ples were optically excited by Ar+ iaser light with a power density of - 10 W/cm*. In Fig. 1 typical PL spectra from Sio.,Gec3/Si MQWs of various well thicknesses are presented, together with the PL spectrum of the Si substrate itself. Three main features are observed in the PL spectra of the MQWs. We group them into lines labeled from a to d, lines D, and lines from the substrate. Visible in the pure-substrate PL spectrum is a complex of exciton lines. This consists of free exciton lines (FE) with phonon participation (TO, LO, TA, TO + Or) and bound excitons without phonon and its phonon replica. ‘* The broad feature labeled EHD is due to the electron-hole-droplet. All these substrate lines were ob- served, with one exception, in the MQW spectra, too, but reduced in intensity. The exception is the EHD line, which is absent in all MQWs. First, we will discuss-lines a to d (see also Table I). Line d is well resolved in the 1 nm MQW, in the 1.2 nm sample it lies in the region of substrate lines, in the 1.5 and 2 nm samples it is again well defined. The line c overlaps the substrate lines in the 1 nm sample, but for thicker wells it is again well separated from the substrate lines. The dominant characteristic of all four lines a to d is their shift to lower energy as the well thickness increases, which un- doubtedly can be correlated with transitions of electrons from the condition band to QW-confined holes..The line- width of lines a to d is for MQWs up to 1.5 nm relatively small (7-10 meV), while broadening occurs for the 2 nm MQW. Another interesting characteristic is that lines a and c seem to be correlated. Their energy difference is - 57 TABLE I. Data for MQW samples: thickness d (in nm) and PL peaks. For all MQWs: buffer x=0.07, d=260 nm; QW: x=0.3; n is the number of periods. MQW PL lines (meV) d SK3 4i n a b c d d-b c-a 1.0 _ 12 10 1031 1051 1089 1109 58 58 1.0 12 20 1031 1050 1089 1109 58 58 1.2 14 20 1021 1034 1078 1093 59 57 I.5 13 20 1008 1017 1064 1075 58 56 2.0 14 20 984 1026 1041 57 2183 Appl. Phys.Lett.60 (18),4 May1992 0003-6951/92/182183-03$03.00 @ 1992 American Institute of Physics 2183 Downloaded 31 Jan 2008 to 134.94.122.9. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jspTABLE II. Data for SQW samples: thickness d and energies of PL lines; cap layer: x=0.02; SQW: x=0.3. Can layer SQW PL lines/meV d/nm d&m a b c d d-b 360 2.2 917 1018 1034 57 500 2.6 912 963 51 580 2.7 894 947 53 330 3.5 888 938 50 17 5.0 865 920 55 13 5.8 959 914 55 meV, while the same is valid for the line pair b and d having an energy difference of -58 meV. The lines D, and D, do not shift when the well thick- nesses increase, but they become more intense for thicker wells. Their energetic position coincides with the disloca- tion lines D1 (0.8 12 eV) and D, (0.875 eV) identified in deformed silicon13 and were also observed in relaxed SiGe.“’ Figure 2(a) presents a MQW structure with wells of 1.5 nm thickness in a cross-section TEM micrograph showing a high degree of perfection characterized by layers of homogeneous thickness and abrupt and planar inter- faces. This demonstrates, that this MQW structure is fully strained. A network of misfit dislocations has been identi- fied at the buffer-substrate interface, visible as localized strained regions, which are undoubtedly related to the D1 and D2 lines in the PL spectrum of this sample. Figure 2(b) shows a cross-section TEM micrograph of a 2 nm MQW structure displaying regions of high structural per- fection and regions within the individual wells with a con- tinuously changing well thickness and/or composition, re- sulting in increasing bulging of the multilayers in growth direction. Misfit dislocations are present only at the buffer- b d “1 DI D2 1.0 ’ energy (eV) FIG. 1. PL spectra of S&Ge&‘Si MQW and SQW structures with dif- ferent well thicknesses and of the substrate alone, measured at T-4.2 K. .L.A..: .-__ “. _..._ ._ FIG. 2. Cross-section TEM micrograph of 20-period MQW structures grown on S&.s3Geee7 buffers; bright field images were taken with electron beam parallel to interfaces near a (110) projection. The corresponding PL are shown in Fig. 1. Nominal well width: (a) 1.5 nm and (b) 2 nm. substrate interface, but not at all at the well interfaces and no threading dislocations were observed. The formation of such regions (lateral extension - 100-500 nm) was ob- served for all samples with nominal thickness of the SiGe wells > 2 nm. It also occurred in SQWs, if this layer thick- ness is exceeded. The SQWs investigated have a nominal well thickness of 2.2 to 6 nm (see Table II). In Fig. 1 PL spectra of two SQW structures are shown. Similar to the MQWs, the QW-related lines b to d of the SQW samples shift to lower energy as the well becomes thicker, and for wells > 2.5 nm only lines d and b are detected, with an energy difference of - 55 meV. No dislocation lines were detected for all SQWs investigated (see Table II). Similar to the MQWs, inho- mogeneous regions are also observed by TEM for all SQWs investigated, but no misfit dislocations at the interfaces and no threading dislocations are detected which explains the absence of the D1 and D2 lines in the PL spectra of the SQWs. We can conclude, that for the SQW and MQW structures based on asymmetrically strained Si,,Gee3/Si the inhomogeneities in the SiGe layers were obviously the cause for the broadening of the PL lines b and d and the disappearance of lines a and c. We start the discussion with the PL lines a and b. Tables I and II show, that these lines lie 50-58 meV below the c and d lines. In bulk Sit-,Ge, alloys Weber and Alonso’ identified the TOsi-si phonon with 58 meV and the 2184 Appl. Phys. Let, Vol. 60, No. 18, 4 May 1092 Vescan et al. 2184 Downloaded 31 Jan 2008 to 134.94.122.9. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp1.10 V E 1.05 .- + .- ul 0 0. 1.00 x RJ zi 0.95 -I CL 0.90 0.85 ? ‘\ \ \ ‘\ 1. ‘i k. ‘x_ ‘. -__ ‘A SQW MQW t;:$II;~:; 1. .> l d-lines h A c-lines I I I I .....7-1-T‘ ---T--- 0 1 2 3 4 5 6 7 8 9 10 well thickness (nm) FIG. 3. Calculated and experimental PL peaks as a function of the well thickness. Only no-phonon QW exciton lines c and dare represented. (a) 0, A MQW, (b) 0,A SQW. TOsi-oe with 49 meV. This allows us to interpret lines a and b as TO replicas of the PL lines c and d, respectively. The c and d lines we assign to no-phonon transitions due to QW excitons, splitted into a doublet. Doublet splitting in the photoluminescence spectra of III-V heterostructures was explained as being due to 1 monolayer thickness vari- ation if the lateral extension of the thickness fluctuations is greater than the Bohr radius of the confined excitons.t4 In our case, the QW exciton lines c and d he -20 meV apart, which is equivalent to -2 monolayers thickness variation. This was observed for well thicknesses <2 nm. In order to analyze the no-phonon QW-exciton lines more quantitatively, we calculate the quantized state ener- gies EI in the QW using different values for rnt in the well and the barrier. No dependence on strain of rn$ was taken into account. The energetic position of the PL lines is equal to Eg- AE,+Ei and is represented in Fig. 3 as a function of the well thickness for i= 1. Here Eg is the gap for cubic Si. Also represented are the QW-exciton lines c and d. A reasonable fit was obtained with mf=0.25 m and AEV =0.27 eV. The disappearance of splitting for wells > 2.5 nm and broadening of the QW-exciton line d is due to an increase of thickness fluctuations, which is in agreement with the TEM results. In conclusion, the photoluminescence of asymmetri- cally strained Si,,Gee3/Si MQW and SQW structures is due to QW excitons and their TO replicas indicating the indirect character of this system. It was possible to esti- mate a valence band offset of 0.27 eV and an effective hole mass of 0.25 m. Splitting of the no-phonon QW-exciton lines is very probably due to thickness fluctuations. In ad- dition, we found that the PL dislocation lines D1 and D2 are well correlated with misfit dislocations at the buffer substrate interface. It is a pleasure to acknowledge helpful conversations with R. Carius, K. Wolter, H. P. Tang, and S. Mantl and the technical assistance of K. Wambach. ‘J. Weber and M. I. Alonso, Phys. Rev. B. 40, 5683 (1989). ‘P. 0. Hansson, J. H. Werner, L. Tapfer, L. P. Tilly, and E. Bauser, J. 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I4 J. Christen, Festkijrperprobleme 30, 239 ( 1990). 2185 Appl. Phys. Let, Vol. 60, No. 18, 4 May 1002 Vescan et al. 2185 Downloaded 31 Jan 2008 to 134.94.122.9. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp

Optical and structural investigation of SiGe/Si quantum wells

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Optical and structural investigation of SiGe/Si quantum wells

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