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									EW-MOVPE XIII, Ulm, 2009                                                                                          C.06



                                  GaAs-growth on porous silicon
                              1                      2              1           1                 1
                 M. Wiesner , E. A. Angelopoulos , R. Roßbach , M. Jetter and P. Michler
       1
         Institut für Halbleiteroptik und Funktionelle Grenzflächen, Universität Stuttgart, Allmandring 3,
                                            70569 Stuttgart, Germany
       2
        Institut für Mikroelektronik Stuttgart (IMS CHIPS), Allmandring 30a, 70569 Stuttgart, Germany



                                                     Abstract
          We present investigations of porous silicon as a compliant substrate for gallium arsenide-silicon
      heteroepitaxy. GaAs layers were grown on samples consisting of porous Si layers with different porosities
      and layer thicknesses formed on the surface of bulk Si substrates. The resulting structures were
      investigated by high resolution X-ray diffraction, scanning electron microscopy and photoluminescence
      measurements. An improvement of the crystal quality of GaAs layers grown on specially treated porous Si
      substrates compared to layers grown directly on bulk Si substrates could be shown.


Introduction
    Silicon (Si) is the most commonly used basic material in microelectronics. Outstanding features of Si
substrates are small mass, good thermal conductivity, large wafer diameter and, most of all, low cost.
Disadvantageous however is silicon’s indirect bandgap. For this reason, III-V compounds, primarily gallium
arsenide (GaAs), serve as a basic material for optoelectronic devices. Monolithic integration of both material
systems with the prospect of economically fabricating integrated circuits is highly desirable. Major problems
however are the 4 % lattice mismatch between Si and GaAs, leading to a large density of misfit dislocations,
and also the big difference in thermal expansion coefficients, which causes crack formation due to large
post-growth tensile stress in the GaAs layer. In the last 20 years many efforts have been made to overcome
these difficulties, such as thermal cycle annealing, implementation of a strained layer superlattice, GaAs
overgrowth on GaAs bonded to Si, growth through buffer layers and use of compliant substrates. We follow
the latter approach by using porous silicon (PSi) for the realization of compliant substrates. PSi layers are
considered to be mechanically flexible providing the ability to relax the stress resulting from lattice mismatch
and thermal expansion coefficients by straining the “softer” porous structure.
    Aim of this study was to find the parameters of PSi layers best suitable for GaAs/Si-heteroepitaxy. For
this purpose we characterized PSi layers with different values of thickness and porosity using high resolution
X-ray diffraction (HRXRD) measurements. GaAs layers of different thicknesses were then grown on PSi
samples by metal-organic vapor-phase epitaxy (MOVPE).



Experimental
    PSi layers were fabricated on 6-inch p-type (100) Si wafers by anodization in an electrochemical etching
cell containing an electrolytic solution of 50% w/w hydrofluoric acid (HF) and isopropanol in a 3:1 volume
ratio, the latter used as a wetting agent. The thickness of the layers was measured using scanning electron
microscopy (SEM) and the porosity could then be calculated gravimetrically [1]. Adjustment of thickness and
porosity was carried out by varying the process time, anodization current and wafer doping level, in such a
way that resulted in homogeneous PSi layers with well defined properties. After the PSi formation and a
rinse-dry procedure, half-wafers were subjected to a 5 min thermal annealing step at 1100°C in hydrogen
atmosphere. During this step sintering of the porous material is observed with atoms recrystallizing in an
energetically favorable structure of crystalline Si incorporating spherical vacuum cavities. Both the as-etched
and the annealed PSi/Si samples having PSi thicknesses ranging from 0.5 μm to 10 μm and porosities from
20% to 60% were used as substrates for GaAs growth. Prior to epitaxy, the substrates were dipped in HF to
remove the native oxide.
    The growth of GaAs was carried out by MOVPE in an AIX-200 horizontal reactor setup at low pressure of
100 mbar. The source materials were trimethylgallium (TMGa) and arsine (AsH3).
The samples were characterized by HRXRD, SEM, atomic force microscopy (AFM) and photoluminescence
(PL). PL spectra were recorded at 4 K using 514.5 nm Ar-ion laser as an excitation source.
EW-MOVPE XIII, Ulm, 2009                                                                                                             C.06




Results
    Prior to the core GaAs-growth experiments, compliant                                                  004-ReflectionPorous Si 2µm
substrate samples with different combinations of PSi layer                                                       Porosity 60%




                                                                     Normalized log intensity (arb. u.)
thickness and porosity were subjected to HRXRD measurements                                                      FWHM 35"
in order to evaluate the crystalline properties of PSi. Figure 1                                                 ∆ω= 352"        Substrate
                                                                                                                            -3   FWHM 18''
compares the rocking curve of the 004-reflection from as-etched                                                  ∆a/a = 2.4*10
samples with a PSi layer thickness of 2 µm and different degree
of porosity. For 20% porosity, the relative change of the lattice
constant with respect to the Si substrate is ∆a/a=2.2*10 . An
                                                           -4

increase by one order of magnitude is observed for the sample                                               Porosity 30%
with 60% porosity. The Bragg peak originating from the PSi layer                                            FWHM 24"             Substrate
shows a full width half maximum (FWHM) which is comparable                                                  ∆ω= 50"              FWHM 22''
to the one from the bulk Si substrate, thus demonstrating the                                                          -4
                                                                                                            ∆a/a = 3.4*10
excellent crystalline quality of the top porous layer and the
suitability of the samples for GaAs epitaxial overgrowth.

    The initial nucleation layer plays an important role in the             Porosity 20%
crystalline quality of the final GaAs layer. GaAs growth on Si              FWHM 16"
                                                                            ∆ω= 32"                Substrate
starts with formation of 3D islands (Volmer-Weber growth) [2],                                     FWHM 15"
                                                                                        -4
turning into a quasi-2D growth mechanism (Frank-van der                     ∆a/a = 2.2*10
Merve). For optimal growth results, these islands should be
small and grow at a high density, preventing them from                 -400          -200      0            200
becoming defective even before they coalesce. In order to find
the best PSi parameters for the nucleation, 20 nm GaAs were                           ∆ω(arcsec)
grown on different substrates and analyzed by AFM. The growth Figure 1 Rocking curves from as-etched
temperature was set to 400°C, as is typically done for nucleation PSi/Si samples for different values of
layers. AFM results show no clear tendency which porosity and porosity
porous layer thickness might be favorable. However there is a
significant difference between layers grown on annealed and as-etched compliant substrates, as shown in
figure 2 for a sample with a PSi layer with 20% porosity and a thickness of 2 µm. The GaAs nucleation layer
grown on the annealed substrate shows a much smoother surface with a roughness of 0.97 nm, in contrast
to 2.64 nm on the as-etched substrate.




   (a)                                             (b)
Figure 2 AFM pictures of a 20 nm GaAs nucleation layer on porous Silicon (a) as-etched (b) annealed.
EW-MOVPE XIII, Ulm, 2009                                                                                C.06




    The most widely used technique for epitaxy of GaAs on Si is the so-called two-step growth [3]. This
method implies growth of a nucleation layer at low temperature (400°C), followed by the main layer at
temperatures of about 600-750°C. We chose a thickness for the nucleation layer of 20 nm and then grew 1
µm GaAs on different compliant substrate samples, both annealed and as-etched. SEM pictures of these
structures show a similar result as the investigation of the nucleation. While GaAs layers on the as-etched
samples do not show an improvement in crystal quality compared to layers grown on bare Si substrates (w/o
porous layer), layers on annealed samples clearly exhibit less defects. This result is confirmed by HRXRD.
The rocking curve FWHM of GaAs on an annealed compliant substrate shows a decrease of about 40’’
compared to that on an as-etched, for all porosities and PSi layer thicknesses. Best FWHM (267’’) was
achieved for the substrate with PSi 10 µm /20% porosity. For bulk Si, the best value reported for 1 µm GaAs
on (100) oriented substrate (4.5° towards <001>) is 189’’ [4].
 PL investigations also showed a significantly higher intensity on annealed substrates. For all samples the PL
FWHM was in a range between 30 and 40 meV and did not show any trend in relation to PSi thickness or
porosity.

    Furthermore, the quality of samples grown by thermal cyclic growth (TCG) [5] was investigated.
According to this method, first a 1.8 µm thick GaAs layer was grown similarly to the two-step growth method
described above. Next, a temperature cycle step was performed, during which thin GaAs films were
deposited at alternating low (400°C) and standard (700°C) temperatures. Each low temperature step was
followed by an annealing sequence at 790°C. The idea of this cyclic growth is to introduce dislocations
during the low temperature growth which may interact with existing dislocations in the layer below during the
subsequent annealing steps. After this cyclic growth a thick GaAs layer (1.6 µm) was deposited, resulting in
a total thickness of the sample of 3.6 µm.
These samples show a substantial decrease in
                                                           (004)-GaAs-FWHM (arcsec)




rocking curve FWHM of up to 70’’ compared to                  360
samples grown by two-step method (Fig. 3). This is                 Porosity 20 %
partly also a consequence of the increased thickness          340      as-etched
                                                                       annealed
of the GaAs layers on the TCG samples, which                  320
decreases the dislocation density. Again, layers
grown on annealed substrates show better results.             300
                                                                              1 µm Two-step growth
Best FWHM (218’’) was achieved for a PSi thickness            280
of 10 µm and 20% porosity. Figure 4 shows SEM
pictures of this sample and, for comparison, of GaAs          260
grown on bulk Si using the same recipe. Looking at                   3.6 µm TCG
                                                              240
the cross section it can be seen that there are
considerable fewer defects in the layer grown on the          220
porous sample. Furthermore, the surface is much                       bulk Si        2µm         10µm
smoother and shows broad coalescent areas. PL
                                                                   Thickness of porous Si layer (µm)
intensity of TCG samples is generally increased,
while PL FWHM show a slight decrease of about 5 Figure 3 Comparison of rocking curve FWHM for
meV, compared to two-step growth results.               1µm two-step growth and 3.6 µm thermal cyclic
    For a detailed analysis of the dislocation density, growth
samples have to be investigated by transmission
electron microscopy.
EW-MOVPE XIII, Ulm, 2009                                                                              C.06




           Figure 4 SEM pictures of GaAs layers grown by thermal cyclic growth on bulk Si
           (upper row) and on porous Si (lower row). Shown is the cross section (left) and the
           surface (right), respectively.


Conclusion / Outlook
    Compliant substrates consisting of porous silicon surface layers were investigated as substrate material
for GaAs epitaxy. GaAs layers were deposited by two-step growth as well as by thermal cyclic growth. It
could be demonstrated that GaAs layers grown on annealed porous layers show better crystal quality and
fewer defects than layers grown on bulk Si. Rocking curve FWHM for a 3.6 µm thick GaAs layer down to
218’’ could be reached.
    Nucleation and growth parameters will be further optimized. In addition, samples with an AlGaAs double-
heterostructure on top of the GaAs layer will be grown to perform time-resolved PL measurements, which
give a deeper insight on structural defects.


Acknowledgement
The authors would like to thank E. Kohler for technical assistance with the MOVPE system, A. Fuoss for
sample preparation and S. Ferwana for his help during the fabrication of the PSi layers.

References
[1] E. A. Angelopoulos, S. Ferwana, M. Zimmermann, E. Penteker, J. N. Burghartz, in Proc. 11th Annu.
Work. SAFE, pp. 445-448, The Netherlands, 2008.
[2] D. K. Biegelsen, F. A. Ponce, A. J. Smith, J. C. Tramontana, J. Appl. Phys. 61 (1987) 1856.
[3] M. Akijama, Y. Kawarada, K. Kaminishi, J. Cryst. Growth. 68 (1984) 21.
[4] D. A. Vinokurov at al., Sov. Phys. Semicond. 25 (1991) 617.
[5] R. Dieter, “Defekte bei der Heteroepitaxie von Galliumarsenid auf Sillizium”, Dissertation, Universität
Stuttgart (Juni 1993).

								
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