The 5th International Symposium on Advanced Science and Technology of Silicon Materials
(JSPS Si Symposium), Nov. 10-14, 2008, Kona, Hawaii, USA
Absorption Blue Shift in Thin-Film Ge-on-Si Photodiodes Induced by
Post-Growth Annealing
Yasuhiko Ishikawa*, Sungbong Park, Jiro Osaka, Kazumi Wada
Department of Materials Engineering, Graduate School of Engineering, The University of Tokyo,
7-3-1 Hongo, Bunkyo, Tokyo 113-8656, Japan
e-mail: [email protected], [email protected],
[email protected], [email protected]
Abstract
Effect of post-growth annealing has been studied on the absorption properties of near-infrared photodiodes
(PDs) using a thin Ge layer epitaxially grown on Si. The Ge layer was directly grown on Si by ultrahigh-vacuum
chemical vapor deposition at 600°C, and the post-growth annealing was carried out at 800°C, which is known to
be effective for the reduction of threading dislocations in Ge. In the case of no post-growth annealing, a red shift
in the absorption edge was observed for PDs having a thin Ge layer of 90 nm. In comparison with bulk Ge having
the absorption edge at 1550 nm, the edge was shifted to longer wavelength around 1600 nm. This red shift is
similar to PDs with an annealed Ge layer of 600 nm in thickness and ascribed to the direct band gap narrowing
induced by the tensile strain. In contrast to the red shift, the post-growth annealing for PDs having 90-nm-thick
Ge induced a large blue shift to ~1400 nm. A secondary ion mass spectroscopy analysis revealed that the
annealing causes the Si diffusion into Ge with the length as large as 100 nm. This means that the Ge layer below
~100 nm is changed into SiGe alloy. The SiGe formation leads to the band gap broadening responsible for the
absorption blue shift, which overcomes the red shift by the strain-induced band gap narrowing. For the detection
of 1550 nm light using thin Ge layers, such a SiGe formation should be carefully prevented.
I. Introduction
Photodiodes (PDs) using Ge epitaxial layers on Si have been widely studied as the near-infrared detectors
in Si photonics, since Ge shows a large optical absorption due to the direct band gap of 0.8 eV, which corresponds
to the wavelength of 1550 nm. The post-growth annealing at temperatures above ~800°C is one of the key steps to
reduce the threading dislocations in Ge derived from the 4% lattice mismatch [1]. The annealing is also effective
for the expansion of detection range over 1600 nm, as we previously reported for Ge pin PDs with the Ge layer as
thick as 1000 nm [2]; an absorption red shift occurs due to the band gap narrowing by the tensile strain, which is
derived from the thermal expansion mismatch between Ge and Si. So far, such Ge pin PDs on Si have revealed
large responsivities approaching 1 A/W at 1550 nm and the 3 dB bandwidth as large as 30 GHz [3, 4]. In order to
further increase the bandwidth by reducing the transit time of carriers, the Ge thickness should be reduced as
small as 100 nm or below [5].
In this work, effect of the post-growth annealing on the responsivity spectra was studied for PDs with a
90-nm-thick Ge absorption layer. As a result, in contrast to the “red” shift for PDs without the annealing, an
absorption “blue” shift occurs after the annealing when the Ge thickness is reduced to be as thin as 100 nm.
II. Experimental
The samples prepared in this study are composed of p-Ge/i-Si/n-Si structures with the Ge thickness of 90
nm, as in Fig. 1(a). This structure is favorable for thin-film Ge PDs, taking into account the low breakdown field
strength of i-Ge in pin PDs as well as the doping profile control [5]. The photocurrent flows due to the diffusion
of electrons generated in p-Ge towards i-Si. The small conduction band offset at the Ge/Si interface does not
prevent the electron flow.
For the fabrication of p-Ge/i-Si/n-Si PDs, a 500-nm-thick i-Si layer was grown by ultrahigh-vacuum
chemical vapor deposition (UHV-CVD) at 600°C on a 4-inch n+-Si(100) wafer (~0.01 cm), using Si2H6 as the
source gas. Subsequently, an undoped Ge layer was grown as the absorption layer using a two-step growth [1], i.e.,
a ~30-nm-thick Ge buffer layer was grown at a low temperature of 370°C, followed by the high-temperature
growth at 600°C. As the source gas, 9% GeH4 diluted in Ar was used. After the 90-nm-thick Ge growth, a
110-nm-thick Si cap layer was grown. The post-growth annealing was performed at 800°C for 20 min. For the top
p-type doping, a boron ion implantation was made. The boron density in the Si cap is ~1019 cm-3, while the
density is reduced to be ~1018 cm-3 in the Ge layer to increase the diffusion length of photo-generated electrons.
The implanted areas, or the diode areas, were defined by a photolithography. The diodes were square-shaped with
the widths of 20 - 500 μm. The activation annealing was made at 650°C for 30 min. Finally, Al electrodes were
formed. For the light illumination in the responsivity measurements, Al contacts were formed only near the
periphery of the implanted area. The square-shaped illumination areas had the widths of 10 - 490 μm, smaller by
10 μm than the widths of implanted areas.
As the reference, p-Ge/i-Si/n-Si PDs without the post-growth annealing were prepared. Ordinary Ge pin
PDs on p+-Si wafer were also prepared, as in Fig. 1(b), where the Ge thickness was increased to 600 nm. The
details of the fabrication process for the Ge pin PDs have been described elsewhere [6].
The responsivity measurements were performed at room temperature using a monochromatic halogen lamp
with a normal incidence. Secondary ion mass spectroscopy (SIMS) analyses were made for Ge layers with the
thickness of 1000 nm in order to examine the Ge and Si compositions around the interface. X-ray diffraction
(XRD) measurements using a Cu K radiation were also performed for Ge layers with the thickness of 500 nm.
III. Results and Discussion
Figure 2(a) shows typical responsivity spectra taken for p-Ge/i-Si/n-Si PDs with the diode area of 500 500 μm2. The solid lines correspond to the experimentally obtained spectra for PDs with and without the
post-growth annealing, while the dashed line shows the calculated spectrum using the absorption coefficient
reported for bulk Ge [7]. For PD without the post-growth annealing, the responsivity spectrum is similar to the
calculated one, although the absorption edge is slightly red-shifted. On the other hand, the responsivity spectrum
for PD with the post-growth annealing showed a large blue shift with the absorption edge at ~1400 nm.
The observed red shift for PD without the annealing is similar to the Ge pin PD with a thicker (600 nm) Ge,
as in Fig. 2(b), although the annealing was carried out for this pin PD. As we reported previously for Ge as thick
as 1000 nm [2,8], the red shift is ascribed to the direct band gap narrowing of Ge induced by the tensile strain.
During the cooling from the growth/annealing temperature, at which Ge layer on Si should be almost relaxed, a
tensile strain is accumulated due to the thermal expansion mismatch between Ge and Si [8].
Fig. 1. Schematic cross-sections for (a) p-Ge/i-Si/n-Si PD with 90-nm-thick p-Ge absorption layer and (b) pin PD
with 600-nm-thick Ge absorption layer.
(a)
(b)
Fig. 2. Responsivity spectra for (a) p-Ge/i-Si/n-Si PD with 90-nm-thick p-Ge absorption layer and for (b) pin PD
with 600-nm-thick Ge absorption layer.
The strain can be confirmed by XRD measurements. Examples of XRD curves taken for 500-nm-thick Ge
on Si are shown in Fig. 3. For both of the samples with and without the annealing, the Ge (400) peaks are located
at larger diffraction angles than that for unstrained Ge. This indicates the reduction of lattice constant in the
vertical direction, i.e., the presence of in-plane tensile strain. From the peak positions, the tensile strains are found
to be 0.09% for the sample without the annealing (Fig. 3, bottom) and 0.23% for the one with the annealing (Fig.
3, top). These values correspond to the absorption edge of 1570 nm and 1615 nm, respectively (the direct band
gap energies of 0.789eV and 0.768 eV) [8]. Although the strain in the 90-nm-thick Ge on Si has not been
measured directly by XRD, the observed red shift for PD without the post-growth annealing should be ascribed to
the band gap narrowing induced by the tensile strain.
It is important for the annealed sample that the Ge (400) peak in the top left of Fig. 3 possesses a tail
towards larger diffraction angles. This is probably ascribed to the formation of SiGe alloy around the Ge/Si
interface. The SiGe alloy should be responsible for the observed blue shift after the annealing of 90-nm-thick Ge,
since, if the thin Ge layer completely changed into SiGe, the band gap increases, and the absorption edge moves
to the wavelength smaller than 1550 nm. In order to quantitatively investigate the SiGe formation, SIMS profiles
were compared between the Ge layers on Si with and without the post-growth annealing. Here, the results for a
thick Ge (1000 nm) is shown in order to eliminate the effect of the top Si layer on Ge. As in Fig. 4, although the
diffusion of Ge into Si is almost negligible, the annealing causes the Si diffusion into Ge as large as 100 nm. This
means that the thin Ge layer, sandwiched by the Si cap and substrate, is completely changed into SiGe alloy with
a graded composition. Taking account of the SIMS results, the Si composition of SiGe in the present PD should
be several % on average. According to ref. 9, UHV-CVD-grown SiGe with the Si composition of 2.7% (4.2%) has
the direct band gap energy of 0.870 eV (0.905 eV), corresponding to the absorption edge of 1425 nm (1370 nm).
This value agrees well with the observed absorption edge after the annealing. Therefore, the blue shift can be
ascribed to the SiGe formation, causing the band gap broadening. Although a tensile strain may be present in the
SiGe layer, the band gap broadening due to the SiGe formation (~0.1 eV) would overcome the band gap
narrowing due to the tensile strain (~0.03 eV).
For the 1550 nm light detection using thin Ge layers, such a SiGe formation should be prevented. The
growth of high-quality Ge on Si without the post-growth annealing is highly required.
Fig. 3. Typical -2 XRD curves taken for
500-nm-thick Ge on Si. The top curve is for the
sample with annealing, and the bottom curve is for
the sample without annealing.
Fig. 4. Typical SIMS profiles around Ge/Si interface
of 1000-nm-thick Ge on Si. The solid line is for the
sample with annealing, and the dashed line is for the
sample without annealing.
IV. Summary
Effects of post-growth annealing at 800°C were studied on the absorption properties of PDs using a
UHV-CVD-grown thin Ge layer on Si. In the case of no post-growth annealing, a strain-induced red shift in the
absorption edge was observed for PDs having 90-nm-thick Ge, which is similar to PDs with an annealed Ge layer
of 600 nm. In contrast to the red shift, the post-growth annealing induced a large blue shift to ~1400 nm.
According to SIMS and XRD results, the annealing causes the Si diffusion into Ge, forming a wider gap SiGe
alloy responsible for the absorption blue shift. For the detection of 1550 nm light using thin Ge layers, such a
SiGe formation should be carefully prevented.
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