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STRUCTURAL, COMPOSITIONAL, AND OPTICAL PROPERTIES OF ULTRATHIN Si/Ge
SUPERLATTICES
K. Eberl, G. Krötz, R. Zachai, G. Abstreiter
To cite this version:
K. Eberl, G. Krötz, R. Zachai, G. Abstreiter. STRUCTURAL, COMPOSITIONAL, AND OPTICAL
PROPERTIES OF ULTRATHIN Si/Ge SUPERLATTICES. Journal de Physique Colloques, 1987, 48
(C5), pp.C5-329-C5-332. �10.1051/jphyscol:1987570�. �jpa-00226774�
JOURNAL DE PHYSIQUE
Colloque C5, suppl6ment au noll, Tome 48, novembre 1987
STRUCTURAL, COMPOSITIONAL, AND OPTICAL PROPERTIES OF ULTRATHIN Si/Ge SUPERLATTICES
K. EBERL, G. KROTZ, R. ZACHAI and G. ABSTREITER
Physik-Department E16, Technische Universitat Miinchen, 0-8046 Garching, F.R.G.
Abstract
Strained-layer superlattices (SLS) composed of a sequence of ultrathin Si and Ge layers are grown on Ge(ll0) buffer layers by MBE. Crystal- line quality, relaxation of asymmetrically SLS, and interdiffusion are studied in situ by LEED and AES. New optical transitions in the range of 0.7 to 0.8eV are observed with photoluminescence experiments.
Superlattices composed of semiconductors with different intrinsic lattice constant have achieved considerable attention recently [1,21.
Strain distribution and superlattice effects in such artificially ordered crystals provide a large degree of flexibility in tailoring the electronic and optical properties [3,4]. In the case of ultrashort period Si/Ge SLS e.g. quasi direct optical transitions are expected in a host crystal with indirect band gap 151. First evidence of structurally induced optical transitions were reported recently in a Si/Ge (4x4ML) SLS grown on Si(100) substrate using electroreflectance spectroscopy [6]. In this paper we report on the growth of Si/Ge SLS's on Ge(ll0) buffer layers and present the observation of new optical transitions by infrared photoluminescence measurements.
The samples were prepared by MBE at substrate temperatures (Tg ) in the range of 350 to 500°C. The superlattices were grown on Ge buffer layers using GaAs(ll0) cleavage planes as substrates. Growth con- ditions and set-up of the specially designed MBE-system are described elsewhere 173. While there is good lattice matching between Ge and GaAs, the lattice constant of Si is about 4% smaller. Consequently the strain distribution of a Si/Ge SLS on a thick Ge buffer layer is just contrary to the situation on Si substrates. To match the substrate the Si layers are strained tensile whereas the Ge layers are not strained.
To grow high quality Si/Ge SLS's one has to consider two types of critical thicknesses (hc) for pseudomorphic growth, i.e. hc of the in- dividual strained layers and the overall thickness O of an asymmetri- cally SLS (hcSLS). If O exceeds the critical value ,the strain begins to be relaxed by formation of misfit dislocation lines. The transition from pseudomorphic to relaxed growth of pure Si and different Si/Ge SLS's was measured with LEED. The growth process had to be interrupted repeatedly. Variations of the lateral surface lattice constant all changes the LEED-spot seperation. An accuracy 02 about 0.3% is achieved by using a vidicon camera to record the intensity distribu- tion of the diffraction patterns. The relative change in all as a func- tion of coverage O is plotted in Fig.1 for a single Si layer and various Si/Ge SLS's. Pure Si on Ge starts to relax already after 5ML
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987570
JOURNAL DE PHYSIQUE
F i a . 1:
Relative change in lattice constant as a function of the
film thickness of pure Si
-,
layers and several Si/Ge SLS's
--
determined by low energy electron diffraction (LEED)
.
13 Si on Ge-buffer
2 4 6 10 100 1000
coverage
(a)
as indicated by the arrow. The dashed-dotted curves characterize the measured development of 6all / a ~ e for Six Gel
-
alloys [7] corresponding to the mean concentration of Si and Ge in the different Si/Ge super- lattices, i. e. x = ds i / (ds i + d ~ e ) with d in ML. Dashed lines indicate the limiting case of complete. relaxation of the respective overlayer.For example the equilibrium lattice constant for a superlattice com- posed of a sequence of 3ML Si and 9ML Ge corresponds to a Si. 2 3 Ge. 7 s
alloy [ 8 ]
.
The lattic constant of Si. z s Ge. 7 s is nearly 1% smaller thana c e . That means beyond hcSLS the Si layers relax partly to about 3%
extention whereas the Ge layers become 1% biaxially compressed. The results in Fig. 1 demonstrate that the Si layers in an Si/Ge SLS must be smaller than 5ML and the hc S L is approximately equal to hc of the corresponding Six Gel
-
alloy. The Ge (110) surf ace of the buffer layer shows a sharp and intensive (1x3) reconstruction pattern which changes to an (1x1) relaxation already after deposition of 2ML Si. The shift of the diffraction beams for 0>5ML is concomittand with a significant lowering of the crystal quality. For dsi<5ML the (1x3) pattern returns after deposition of 4ML Ge. Consequently during the preparation of an SLS composed of a sequence of 3ML Si and 9MLGe alternating (1x1) and (1x3) LEED-patterns are observed. In figure 2 angular profiles across the (01)-beam in [I101 and [001] direction for several surfaces are shown. The insert represents the unit mesh of the topmost layer of an ideal (110) diamond surface. The first profiles are obtained from the surface of an almost completely relaxed Si.zsGe.7~ film.angular profile 11101 dihmmd
of the (01)-spot [OOil
Ge buffer I1101
I1iOl
-
Iooil-
c o v e r a g e in monolayers
F i a . 3: Ratio of the AES intensi- F i a . 2: Angular profiles across ties of the Si(92eV) and Ge(47eV) the (01)-LEED spot in two direc- line as a function of the coverage tions for three different samples. in the second period of a (3x9)ML The electron energy is 40eV. Si/Ge superlattice.
They are considerably broadened, which is caused by the dislocati2n network created at coverages beyond ho. Additional broadening in [001]
direction is probably due to the atomic structure of the (110) surface as discussed in [ 6 1 . Surprisingly good crystal quality is reached for the 100 period (3x9) Si/Ge SLS. The overall thickness is =2370A, i.e.
this structure is also relaxed. For comparision the profil across the (01) -beam of the Ge buffer layer are also given in Fig. 2. The much better quality of the superlattice surface compared to the alloy is probably due to trapping of dislocation lines at the many superlattice interfaces. Superlattice buffer layers have also been introduced successfully for the growth of GaAs on Si [9].
Auger electron spectroscopy (AES) is used to determine the concen- tration profiles of Si and Ge in the (3x9) SLS's for various growth temperatures. The AES-measurements were also performed between growth intervalls. For quantitative analysis we evaluated the usual peak to peak hight of the Ge(47eV) line, the Si(92eV) line and corresponding Auger transitions between 1100 and 1620eV in the derivative spectra.
Fig. 3 shows the ratio between the intensity of the low energy Si and Ge Auger-line normalized to the bulk intensities as an function of O for two different Te 's. The full line indicates the calculated ideal ratio for sharp interfaces without any interdiffusion between the in- dividual layers. The dashed curve is expected for strong intermixing during growth with a mean Si fraction of only 35% in the nominal Si layers and graded alloy in the "GeW-layer. The sample grown at Tg=500°C is approximately described by this curve. In the LEED-pattern of this structure we notice enhanced surface roughness with increasing coverage. It should be mentioned, however, that the Auger-results can't be interpreted only in terms of 3-dimensional growth. Conside- rable diffusion between the impinging Si or Ge atoms on the surface of the inverse topmost layer seems to occur. Segregation on the other hand turns out to be neglectible as can be seen by comparing high energy and low energy Auger lines which have an escape depth of 25A and 5A respectively. For Tgs470°C the interfaces are rather sharp even after 100 periods. The upper limit of Ge in the 3ML Si-film is estimated to be about 20% for these lower growth temperatures.
The large biaxial strain in elastically accomodated layers drastically changes the band gaps and offsets in the SLS [1,3]. To investigate the fundamental band gaps of Si/Ge SLS we have performed low temperature photoluminescence (PL) experiments. Spectra of three samples are shown in the energy range from 0.7 to 0.9eV (Fig. 4)
.
The SLS structures differ in their number of periods, one having 15 periods (O<hcSLs) and the other containing of 100 periods (O>hCSLS). The difference in the observed PL peak values (=2OmeV) is probably due to the different strain distributions in the relaxed and unrelaxed SLS. The reference sample, which contains only the Ge buffer layer, but no SLS, exhibits no pronounced structure in the investigated range. We have also measured the luminescence of a corresponding relaxed alloy layer. Such a random alloy exhibits a much broader luminescence with a peak at around 0.85eV and a sharp high energy cutoff at about 0.91eV, which is close to the fundamental band gap of Sio.
2 a Geo . 7 a [lo].
The lumines- cence experiments indicate that the (3x9) Si/Ge SLS band gap is reduced in energy by more than 100 meV compared with the random alloy of the same average composition.We have also tried to get a reliable picture of band alignment and. band gap of the examined SLS by combining recent theoretical results on the valence band offsets El11 with deformation potential theory. The strain induced effects due to spin orbit coupling have been neglected. Because of the slight intermixing, determined from AES , we assumed (3x9) Lio . 9 Geo.
1 /Ge SLS.
For the unrelaxed SLS the splitting of the band edges only occurs in the "Si" layers. Growth along the [I101 direction leads to a splitting of both the A and L conduction band minima. In Fig. 5 only those band edges are depicted, which form the lowest quantum wells.
(25-332 JOURNAL DE PHYSIQUE
0.7 0.73 0.8 085 0.9
energy (eV1
Fia. 4: PL of three samples:
(1) : (3x9) SLS 100 periods
( 2 ) : (3x9) SLS 15 periods
(3) : G e b u f f e r layer
Fia. 5: Band alignments for (a) : (3x9) SLS 15 periods (b) : (3x9) SLS 100 periods
For the unrelaxed sample (Fig. 5a) the energy difference between the top of the valence band in the Ge layer and the bottom of the con- duction band in the Sio. sGeo. I layer is about 480meV. The conduction band offset between the 4-fold A-minima in "Si" and the 6-fold A-mini- ma in Ge is about 440 meV. The potential well for holes in Ge has a height of approximately 700 meV. Taking into account the formation of minibands, the effective superlattice band gap is in reasonable agreement with the observed luminescence energy.
In the 100 period SLS both the "Si" and Ge layers are strained (Fig.
5b). The band gap between the layers is reduced by about 3OmeV, com- pared with the unrelaxed sample. This is probably responsible for the experimentally observed shift of the luminescence peak. A more accu- rate comparision, however, requires the consideration of impurity and phonon assisted effects on the photoluminescence as well as self- consistent superlattice band structure calculations, which are not yet available. Si/Ge superlattices grown along the [110] direction are not to be expected to turn into semiconductors having direct band gaps, because the energetically lowest 4 fold conduction band minimum in Si is at finite k~ in the two-dimensional Brillouinzone. This is dif- ferrent for samples grown on (100) surfaces [1,3,12]. To achieve large band offsets in the conduction band, the Si layers should be strained, no matter what growth direction is used.
Acknowledaements: The work has been supported by the Siemens AG and by Stiftung Volkswagenwerk.
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