c-v - and dlts-investigations of pyramid-shaped ge quantum dots embedded in n-type silicon
TRANSCRIPT
C-V - and DLTS-investigations of pyramid-shaped Ge Quantum Dots
embedded in n-type Silicon
Victor-Tapio Rangel-Kuoppa1,a, Alexander Tonkikh2,b, Nikolay Zakharov2,c, Peter Werner2,d, Wolfgang Jantsch1,e
1Institute of Semiconductor- and Solid State Physics, Johannes Kepler Universität, A-4040 Linz,
Austria
2Max-Planck Institute for Microstructure Physics, Weinberg 2 D-06120, Halle, Germany
Keywords: Ge Quantum Dots, Ge Quantum pyramids, CV, DLTS, deep level, activation energy.
Abstract. We investigate self-assembled pyramid-shaped Ge Quantum Dots (QDs) with lateral
dimensions of 15 nm, and heights of 2.5-3 nm. These Ge QDs were grown by Molecular Beam
Epitaxy (MBE) on n-type Si(100) substrates using the Sb-mediated growth mode. The resistivity of
the substrates was about 5 Ωcm. The Si buffer layer below the QDs and the Si capping layer above
them were doped up to 1018 cm
-3 by Sb. Cross-section transmission electron microscopy shows the
QDs and the Sb delta-doped layers. Using standard photolithographic techniques, a 0.3 mm2 Au
Schottky contact was applied to the epilayer, while an Ohmic contact was formed on the back side
of the substrate. Plotting C-2 vs. V plot reveals the nominal doping of 10
18 cm
-3. DLTS studies
revealed two levels with fitted activation energies of 49 meV and 360-390 meV. They are related to
the Sb doping and the Pb interface states, respectively. The simulation suggests a deep level with a
volumetric concentration of 2.55×1015 cm
-3. Multiplying this value by the thickness of the depletion
region obtained from the CV measurements, we find that the deep level capture about 5.8×109
electrons per cm2.
Introduction
Self-assembled Ge Quantum Dots (QDs) embedded in Si are promising candidates for electronic
applications, such as nonvolatile memories and photo-detectors [1-5]. The intention of this
manuscript is to study thoroughly the electrical properties of Ge QDs grown by Molecular Beam
Epitaxy (MBE) on n-type Si(100) substrates using the Sb-mediated growth mode [6]. The
Capacitance Voltage (CV)- and the Deep Level Transient Spectroscopy (DLTS) techniques are
used. This article is divided in the following sections: in Section 2 details about the experimental
growth of the samples are given. Also, structural information about the Ge QDs is provided by
Transmission Electron Microscopy (TEM) characterization. The processing of the samples for the
electrical characterization is also described. In Section 3, the CV and DLTS study is explicated and
commented. Briefly, two signals are observed, one at 160 K and the other one at 260 K. The first
one is explained in terms of the Sb doping, while the second one is attributed to the presence of Pb
interface states. Finally, a summary is given.
Section 2. Experimental growth, morphological analysis and sample preparation.
Ge QDs were grown via the Sb mediated growth mode, similar to that reported earlier [6]. Before
Ge deposition the Si(100) surface was covered by 1 ML of Sb. Then Ge was deposited at a growth
rate of 0.02 nm/sec. The nominal thickness of the deposited Ge was 1.0 nm. The substrate
Solid State Phenomena Vols. 178-179 (2011) pp 72-75Online available since 2011/Aug/16 at www.scientific.net© (2011) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/SSP.178-179.72
All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP,www.ttp.net. (ID: 132.174.255.116, University of Pittsburgh, Pittsburgh, USA-26/11/14,18:34:15)
temperature throughout the QD formation was 6000C. A test sample, grown in that way, but without
capping of the QDs was investigated by Atomic Force Microscopy (AFM). The AFM image of this
sample is shown in Fig.1a. AFM investigation has revealed that the QD- surface density was about
2 × 1011 cm
-2. Capacitance-Voltage and DLTS measurements were performed on another sample,
where the Sb-mediated Ge QDs were embedded in the MBE grown n-type Si. A buffer layer was
grown below QDs. The thickness of the buffer layer was 190 nm. The thickness of the capping layer
was 70 nm. The background doping of the buffer-and capping layers was done by Sb. Since Sb
segregates intensely throughout Si:Sb epitaxy, an appropriate growth temperature was chosen to
allow Sb incorporation at a level of 1018cm
-3 from the pre-deposited Sb reservoir layer. The
thickness of this reservoir layer was about 1 ML (~7 ×1014 cm
-2). The Sb δ-doped layer was grown
10 nm below the QD layer by deposition of 1 nm thick Si film at decreased substrate temperature to
achieve an appropriate Sb sheet concentration. Cross-section TEM image of this sample is shown in
Fig. 1b.
Fig.1a AFM image of Sb-mediated Ge QDs
grown on Si(100) at 600 0C.
Fig. 1b TEM cross-section image. The Ge QD
layer is dark. A weak strain induced contrast
caused by a Sb delta-doped layer is seen,
10 nm below the Ge layer.
In order to form a Schottky diode within the sample, first the epilayer was capped with 200 nm
SiO2 using Plasma Enhanced Chemical Vapour Deposition (PECVD). An ohmic contact was
formed on the back side of the substrate by depositing 200 nm AuSb and annealing it at 360oC for
5 min. Afterwards, using standard photolithographic techniques, as those described in Ref. 7, a
0.3 mm2 Au Schottky contact was deposited on the epilayer.
Section 3. Capacitance Voltage and DLTS study.
CV measurements done at 296 K on two of the Au-Schottky contacts are shown in Fig. 2 a). The
C-2 vs V analysis and its linear fitting is shown in Fig. 2 b). It confirms the expected 10
18 cm
-3
doping.
Solid State Phenomena Vols. 178-179 73
a) b)
Fig. 2 a) CV measurement of two Au Schottky contacts, done at 296 K, b) C-2 vs V analysis and
linear fitting of curves show in a). The desired 1018 cm
-3 doping is confirmed.
The DLTS study was done using 1 ms pulses, 25 Hz repetition rate and a filling pulse of 0 V.
The sample was glued to the sample holder with Fixogum © to ensure good thermal contact [8].
The reverse bias was varied in the range of -0.1…-1.6 V in steps of 0.1 V. Some of the DLTS
spectra are shown in Fig. 3.
0 50 100 150 200 250 300-50-45-40-35-30-25-20-15-10-505
VR = -1.6 V
VR = -1.1 V
VR = -0.9V
VR = -0.8V
VR = -0.5V
DLTS signal (arb. units)
Temperature (K)
VR = -0.2V
Fig. 3. Cascade plot of the DLTS curve done with 1 ms pulse duration, 25 Hz repetition rate, at
various reverse bias voltages. The pulse bias was 0 V. The zero on the scale is shown for the
measurement done with VR = -1.6 V. Each measurement done at a smaller reverse bias is shifted by
80% of ist amplitude upwards.
Two peaks are seen, one around 160, the other at 260 K. The simulation for the peak around
260 K shows an activation energy Eact between 360 and 390 meV. This signal was not observed for
measurements done with small reverse biases, like -0.2 V, and started to appear for reverse bias of -
0.5 V and beyond. Studying Pb centers using Laplace DLTS, Dobaczewski et al. reported energy
-20 -16 -12 -8 -4 0100
200
300
400
500
600
700
800
900
Capacitance (pF)
Voltage (V)
296 K
left contact
0 V -> -20V
-20V -> 0 V
right contact
0 V -> - 20V
-20V -> 0 V
-24 -20 -16 -12 -8 -4 0 4-5.0x10
-6
0.0
5.0x10-6
1.0x10-5
1.5x10-5
2.0x10-5
2.5x10-5
3.0x10-5
3.5x10-5
296 K
left contact
0 V -> -20V
-20V -> 0 V
right contact
0 V -> - 20V
-20V -> 0 V
(Capacitance)-2 (pF-2)
Voltage (V)
Linear fit
y=1.77×10-6 - 1.29×10
-6 x
p = 1.04×1018 cm
-3
74 Gettering and Defect Engineering in Semiconductor Technology XIV
levels of 0.38 eV, below the conduction band [9]. They relate them to the Pb0 and Pb1 states, which
are related to the (100), (111) and (110) interface orientations. Thus, it is plausible that our signal
comes from interface states that are formed between the Ge QDs. In fact, some of us have recently
reported the presence of these orientations, on Ge QDs samples grown with slightly smaller Sb delta
doping [10]. The signal around 160 K appears for all reverse biases, and it seems to increase as the
reverse bias increases. Simulations suggest an activation energy between 40 and 70 meV. Sb has
an activation energy of 43 meV [11]. Thus, we attribute this signal to the Sb doping.
In summary, the DLTS study of pyramid-shaped Ge QDs has been reported in this article. The
DLTS study shows the presence of two levels, with activation energies between 40-70 meV and
360-390 meV. The latter level is much deeper than expected for the localization of electrons due to
the dilatational strain in Si adjacent to the Ge islands [12,13] and much lower in concentration: only
3% of the Ge islands seem to have this level. Therefore, in the absence of a microscopic
identification of the origin of those levels we tentatively assign them to structural or chemical
defects associated with the Ge islands, where the Pb centers are a possibility. The localized states
usually seen for Ge islands in n-type material do not appear here. Apparently those levels are too
shallow due to confinement in our samples because of the smaller island size of 15 nm diameter and
2.5 - 3 nm height.
Acknowledgements
Victor-Tapio Rangel-Kuoppa gratefully acknowledges the National Council for Science and
Technology (CONACyT) of México, postdoctoral fellowship 78965, the Fonds zur Förderung der
Wissenschaftlichen Forschung (Vienna, Austria) and the PLATON-SiN project of FFG (Vienna,
Austria), project 20550. Alexander Tonkikh gratefully acknowledges the support of the DFG-
RFBR (project 436/RUS 113).
The authors would like the thank Elisabeth Pachinger, Alma Halilovic, Ursula Kainz, Eckehard
Nusko, Otmar Fuchs, and Stephan Bräuer for expert technical assistance.
References
[1] D. J. Eaglesham and M. Cerullo: Phys. Rev. Lett. Vol. 64 (1990), p. 1943.
[2] G. Abstreiter, P. Schittenhelm, C. Engel, E. Silveira, A. Zrenner, D. Meertens and W. Jager:
Semicond. Sci. Technol. Vol. 11 (1996), p. 1521.
[3] C. M. A. Kapteyn, M. Lion, R. Heitz, D. Bimberg, C. Miesner, T. Asperger, K. Brunner and
G. Abstreiter: Appl. Phys. Lett. Vol. 77 (2000), p. 4169.
[4] C. M. A. Kapteyn, M. Lion, R. Heitz, D. Bimberg, C. Miesner, T. Asperger, K. Brunner and
G. Abstreiter: Phys. Stat. Sol. B. Vol. 224 (2001), p. 261.
[5] V. Lavchiev, R. Holly, G. Chen, F. Schaffler, R. Goldhahn and W. Jantsch: Opt. Lett. Vol. 34
(2009) p. 3785.
[6] A. Tonkikh, N. Zakharov, V. Talalaev and P. Werner: Phys. Stat. Sol. RRL Vol. 4 (2010),
p. 224.
[7] V.-T. Rangel-Kuoppa and A. Conde-Gallardo: Thin Solid Films Vol. 519 (2010), p. 453.
[8] V.-T. Rangel-Kuoppa and G. Chen: Rev. Sci. Instrum. Vol. 81 (2010), p. 036102.
[9] L. Dobaczewski, S. Bernardini, P. Kruszewski, P. K. Hurley, V. P. Markevich, I. D. Hawkins
and A. R. Peaker: Appl. Phys. Lett. Vol. 92 (2008), p. 242104.
[10] A. A. Tonkikh, G. E. Cirlin, V. G. Dubrovskii, V. M. Ustinov and P. Werner, Semiconductors
Vol. 38 (2004), p. 1202.
[11] S. M. Sze, Physics of Semiconductor Devices, third ed. Wiley-Interscience, New York, 1969.
[12] C. Penn, F. Schaffler, G. Bauer and S. Glutsch: Phys. Rev. B Vol. 59 (1999), p. 13314.
[13] M. Brehm, T. Suzuki, T. Fromherz, Z. Zhong, N. Hrauda, F. Hackl, J. Stangl, F. Schaffler and
G. Bauer: New Journal of Physics Vol. 11 (2009), p. 063021.
Solid State Phenomena Vols. 178-179 75
Gettering and Defect Engineering in Semiconductor Technology XIV 10.4028/www.scientific.net/SSP.178-179 C-V - and DLTS-Investigations of Pyramid-Shaped Ge Quantum Dots Embedded in N-Type Silicon 10.4028/www.scientific.net/SSP.178-179.72
DOI References
[2] G. Abstreiter, P. Schittenhelm, C. Engel, E. Silveira, A. Zrenner, D. Meertens and W. Jager: Semicond.
Sci. Technol. Vol. 11 (1996), p.1521.
http://dx.doi.org/10.1088/0268-1242/11/11S/012 [3] C. M. A. Kapteyn, M. Lion, R. Heitz, D. Bimberg, C. Miesner, T. Asperger, K. Brunner and G. Abstreiter:
Appl. Phys. Lett. Vol. 77 (2000), p.4169.
http://dx.doi.org/10.1063/1.1334651 [4] C. M. A. Kapteyn, M. Lion, R. Heitz, D. Bimberg, C. Miesner, T. Asperger, K. Brunner and G. Abstreiter:
Phys. Stat. Sol. B. Vol. 224 (2001), p.261.
http://dx.doi.org/10.1002/1521-3951(200103)224:1<261::AID-PSSB261>3.0.CO;2-3 [6] A. Tonkikh, N. Zakharov, V. Talalaev and P. Werner: Phys. Stat. Sol. RRL Vol. 4 (2010), p.224.
http://dx.doi.org/10.1002/pssr.201004259 [7] V. -T. Rangel-Kuoppa and A. Conde-Gallardo: Thin Solid Films Vol. 519 (2010), p.453.
http://dx.doi.org/10.1016/j.tsf.2010.07.087 [8] V. -T. Rangel-Kuoppa and G. Chen: Rev. Sci. Instrum. Vol. 81 (2010), p.036102.
http://dx.doi.org/10.1063/1.3321563 [9] L. Dobaczewski, S. Bernardini, P. Kruszewski, P. K. Hurley, V. P. Markevich, I. D. Hawkins and A. R.
Peaker: Appl. Phys. Lett. Vol. 92 (2008), p.242104.
http://dx.doi.org/10.1063/1.2939001 [12] C. Penn, F. Schaffler, G. Bauer and S. Glutsch: Phys. Rev. B Vol. 59 (1999), p.13314.
http://dx.doi.org/10.1103/PhysRevB.59.13314 [13] M. Brehm, T. Suzuki, T. Fromherz, Z. Zhong, N. Hrauda, F. Hackl, J. Stangl, F. Schaffler and G. Bauer:
New Journal of Physics Vol. 11 (2009), p.063021.
http://dx.doi.org/10.1088/1367-2630/11/6/063021