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Surfaces, Interfaces, and Applications
Engineering high-k/SiGe interface with ALD oxide for selective GeOx reduction Mahmut S. Kavrik, Peter Ercius, Joanna Cheung, Kechao Tang, Qingxiao Wang, Bernd Fruhberger, Moon J. Kim, Yuan Taur, Paul C McIntyre, and Andrew C. Kummel ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b22362 • Publication Date (Web): 02 Apr 2019 Downloaded from http://pubs.acs.org on April 2, 2019
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ACS Applied Materials & Interfaces
Engineering high-k/SiGe interface with ALD oxide for selective GeOx reduction
Mahmut S. Kavrik1, Peter Ercius2, Joanna Cheung1, Kechao Tang3, Qingxiao Wang4, Bernd Fruhberger5, Moon Kim4, Yuan Taur1, Paul C. McIntyre3 and Andrew C. Kummel1*
1Materials
Science and Engineering, University of California San Diego, La Jolla, California,
92093, United States 2National
Center for Electron Microscopy, Molecular Foundry, Lawrence Berkeley National
Laboratory, Berkeley, California, 94720, United States 3Materials
Science and Engineering, Stanford University, Stanford, California, 94305, United
States 4Material
Science and Engineering, University of Texas, Dallas, Texas 75080-3021, United States
5California
Institute for Telecommunications and Information Technology, University of California San
Diego, La Jolla, CA 92093, United States Corresponding Author: Andrew Kummel,
[email protected] KEYWORDS: SiGe CMOS, low-power electronics, high-mobility transistor, high-k dielectrics, Al2O3, HfO2 interface trap charge, atomic layer deposition
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Abstract Suppression of electronic defects induced by GeOx at the high-k gate oxide/SiGe interface is critical for implementation of high mobility SiGe channels in CMOS technology. Theoretical and experimental studies have shown that a low defect density interface can be formed with an SiOxrich interlayer on SiGe. Experimental studies in literature indicates better interface formation with Al2O3 in contrast to HfO2 on SiGe however the mechanism behind this is not well understood. In this study, the mechanism of forming a low defect density interface between Al2O3/SiGe is investigated using atomic layer deposited (ALD) Al2O3 insertion into or on top of ALD HfO2 gate oxides. To elucidate the mechanism, correlations are made between the defect density determined by impedance measurements and the chemical and physical structure of the interface determined by high resolution scanning transmission electron microscopy and electron energy loss spectroscopy (STEM – EELS). Compositional analysis reveals an SiOx rich interlayer for both Al2O3/SiGe and HfO2/SiGe interfaces with insertion of Al2O3 into or on top of the HfO2 oxide. The data is consistent with the Al2O3 insertion inducing decomposition of the GeOx from the interface to form an electrically passive, SiOx rich interface on SiGe. This mechanism shows that nanolaminate gate oxide chemistry cannot be interpreted as resulting from a simple layer by layer ideal ALD process because the precursor or its reaction products can diffuse though the oxide during growth and react at the semiconductor interface. This result shows that in scaled CMOS, remote oxide ALD (oxide ALD on top of the gate oxide) can be used to suppress electronic defects at gate-oxide semiconductor interfaces by oxygen scavenging.
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ACS Applied Materials & Interfaces
Introduction SiGe alloys are employed as stressor layers in mainstream complementary metal–oxide– semiconductor (CMOS) transistors and are being investigated as p-type field effect transistor (FET) channels due to their high mobility1 and ease of integration into CMOS2. Thermally stable HfO2 gate oxides with high dielectric constants reduce CMOS device power consumption3-4. SiGe p-FETs with high-k gate dielectrics which have low defect interfaces can provide better electrostatic control of the channel and higher drive current for low gate bias voltage. Conversely, a high density of interface defects between the high k gate oxide and the SiGe channel degrades device performance metrics such as subthreshold slope and reduces the on/off current ratio5. The main challenge for implementing SiGe FETs is the binary atom termination (Si-Ge) of the surface which results in formation of SiGeOx mixed oxides and dangling bonds on both Si and Ge atoms6-8. GeOx and associated dangling bonds are the main sources of defects producing interface trapped charge (Dit), while SiOx is a stable oxide that forms a nearly defect-free interface according to theoretical calculations9. Previously, several techniques such as nitride and sulfur passivation on Si0.7Ge0.3(001) were studied with Al2O3 gate oxides and reduction in the interface defect density via suppression of GeOx formation was reported 10-11. However similar low defect density interfaces could not be established with HfO2 gate oxide. This is because oxygen containing species such as excess H2O, OH, and/or O can diffuse through HfO2 during atomic layer deposition (ALD), forming GeOx defects on the SiGe surface; in addition, the nature of HfO2 allows diffusion of Ge and GeOx to the top surface of the oxide as a result of reaction with HfO2 and GeO2 decomposition11-14 Recently, HfO2/SiGe interfaces formed with Al2O3-HfO2 nanolaminate gate dielectric stacks were found to have a low interface state density, and it was hypothesized that the mechanism was
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reduction of GeOx out-diffusion during ALD15. Theoretical DFT models of the amorphous HfO2/Si0.5Ge0.5(001) interface have shown that low-defect interfaces may be formed even before hydrogen passivation with short anneals ( 4×; the lowest interface defect density of 0.67 ×1012 eV-1cm-2 is obtained with 50 cycles of Al2O3.
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4
4 SiGe/HfO2 / 0 Al2O3
SiGe/ 0 Al2O3 /HfO2
SiGe/HfO2 / 1 Al2O3
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SiGe / Al2O3 / HfO2 : 0.92
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Figure 4. Interface defect density distributions across the band gap for MOSCAP devices calculated with the full interface state model. Interface defects at SiGe- oxide interface decrease by insertion of Al2O3 layers before HfO2 (a) and after HfO2 (b) gate oxide. (c) Comparison of interface defects variation at SiGe- oxide interface by insertion of Al2O3 layers into HfO2 gate oxide. (d) Interface defect density decreases by increase in Al2O3 thickness. For 50 cycles of Al2O3 , peak Dit reduces to 6×1011 eV-1cm-2 and integrated defects across the bandgap is as low as 0.19×1011 eV-1cm-2.
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Ni
Ni
Ni
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HfO2
HfO2
Al2O3
~0.9nm
~1.1nm
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~0.8nm
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SiGe TEM
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Figure 5. STEM HAADF (high-angle annular dark-field), BF (bright field) images of (a) control HfO2 , (b) HfO2/Al2O3/SiGe bilayer, (c) Al2O3/HfO2/SiGe bilayer, (d) and Al2O3 – HfO2 Nanolaminate MOSCAPs. In these images, oxide structures and regions are defined according to z contrast. The interfacial layer between SiGe and oxide indicated with black and white arrows on corresponding STEM – HAADF, STEM - BF and TEM image. Note in (b) the interlayer consists of both SiGeOx and Al2O3, so it appears thicker than the control device in (a). In comparison to control device of HfO2/SiGe, bilayer (c) and NL (d) shows thinner interface consistent with remote Al2O3 insertion reducing IL.
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Intensity [a.u.]
1.0
200keV
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HfCl4 + H2O
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HfO2
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Figure 6. STEM- EELS compositional analysis of MOSCAP devices. EELS experiment performed at 80keV and 200keV as indicated. The inset drawings illustrate corresponding gate stack structure along with the ALD chemistry above it. The compositions of the elements are averages area of ~6 x 0.2 nm parallel to sample surface. The red dashed line intercepts the half peak values of the O signals and indicate the SiGe HfO2 interface. Black and green arrows denote Si and Ge composition on the SiGe surfaces respectively. Blue arrow indicates the Al composition in the oxide. AlOx-HfO2 interdiffusion is seen for bilayer samples regardless of the initial structure and confirmed with raw data analysis in Fig 6 and Fig. S5. This interdiffusion is prominent for EELS analysis at 80keV. In comparison to a-b, devices in c-f show lower Ge/Si ratio at the intersection with red dashed lines indicates Si rich interface formation with Al2O3 incorporation into HfO2. Ni interdiffusion is seen in devices a and b and Al2O3 insertion into HfO2 impedes the Ni diffusion as seen in c-f.
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(a)
(c)(b)
(a) (b) Hf
5.0 nm 5.4 nm 5.8 nm
1000
Hf
Al Intensity [a.u.]
Al Intensity [a.u.]
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ACS Applied Materials & Interfaces
5.2 nm 5.6 nm 6.0 nm
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1500
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8.2 nm 8.6 nm 9.0 nm
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Figure 7. STEM- EELS compositional analysis of Ni/Al2O3/HfO2 /SiGe MOSCAP device. Raw EELS data taken at 200 keV from sample in Fig. 6e is shown in a 3D semi-log graph (a) with the energy axis indicating the electron energy loss and corresponding intensity in arbitrary units. The axis labeled with distance indicates location of the electron beam on sample. The colored consecutive black and light blue lines indicate electron energy loss for the given location on samples and two colors chosen to enhance the image contrast. Each data line projects energy loss averaged from areas of 5 × 0.2nm parallel to the sample surface. The peaks appear on the graphs corresponds to Si K edge (1839 eV), Ge L edge 1217 eV, Hf M edge 1662 eV, O K edge 532 eV, Al K edge 1560 eV, Ni L edge 885 eV. The blue arrow indicates SiOx interface formation between SiGe and HfO2. Pink and red arrows indicate the Ge and Si compositions on SiGe surface. The Ge signal decays earlier than Si as it approaches the HfO2 layer. Black arrows denote Al composition across the oxide. Al2O3 insertion onto HfO2 in bilayer structure forms intermixing by Al diffusion. To increase the visibility of Al peak and inter diffusion, semi-log 2D graph of raw EELS data with 1-2K energy loss range is presented in graph b (SiGe/HfO2 interface region) and c (HfO2/Ni interface region) Two graphs prepared with offsets introduced between each curve to improve visibility.
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(a)
Ni Al2O3
Ni
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Ni
Ni
Al2O3 Al2O3
Al2O3
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SiGe
