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Li, Xun
Publications (10 of 16) Show all publications
Danielsson, Ö., Li, X., Ojamäe, L., Janzén, E., Pedersen, H. & Forsberg, U. (2016). A model for carbon incorporation from trimethyl gallium in chemical vapor deposition of gallium nitride. Journal of Materials Chemistry, 4(4), 863-871
Open this publication in new window or tab >>A model for carbon incorporation from trimethyl gallium in chemical vapor deposition of gallium nitride
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2016 (English)In: Journal of Materials Chemistry, ISSN 0959-9428, E-ISSN 1364-5501, Vol. 4, no 4, p. 863-871Article in journal (Refereed) Published
Abstract [en]

Gallium nitride (GaN) semiconductor material can become semi-insulating when doping with carbon. Semi-insulating buffer layers are utilized to prevent leakage currents in GaN high power devices. Carbon is inherently present during chemical vapor deposition (CVD) of GaN from the use of trimethyl gallium (TMGa) as precursor. TMGa decomposes in the gas phase, releasing its methyl groups, which could act as carbon source for doping. It is previously known that the carbon doping levels can be controlled by tuning the CVD process parameters, such as temperature, pressure and precursor flow rates. However, the mechanism for carbon incorporation from TMGa is not yet understood. In this paper, a model for predicting carbon incorporation from TMGa in GaN layers grown by CVD is proposed. The model is based on ab initio quantum chemical calculations of molecular adsorption and reaction energies. Using Computational Fluid Dynamics, with a chemical kinetic model for decomposition of the precursors and reactions in the gas phase, to calculate gas phase compositions at realistic process conditions, together with the proposed model, we obtain good correlations with measurements, for both carbon doping concentrations and growth rates, when varying the inlet NH3/TMGa ratio. When varying temperature (800 – 1050°C), the model overpredicts carbon doping concentrations at the lower temperatures, but predicts growth rates well, and the agreement with measured carbon doping concentrations is good above 1000°C.

Place, publisher, year, edition, pages
Royal Society of Chemistry, 2016
National Category
Physical Sciences Physical Chemistry
Identifiers
urn:nbn:se:liu:diva-118113 (URN)10.1039/c5tc03989d (DOI)000368839700027 ()
Note

Funding agencies: Swedish Foundation for Strategic Research (SSF); Swedish Defence Material Administration (FMV)

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Available from: 2015-05-22 Created: 2015-05-22 Last updated: 2017-12-04Bibliographically approved
Bergsten, J., Li, X., Nilsson, D., Danielsson, Ö., Pedersen, H., Janzén, E., . . . Rorsman, N. (2016). AlGaN/GaN high electron mobility transistors with intentionally doped GaN buffer using propane as carbon precursor. Japanese Journal of Applied Physics, 55, 05FK02-1-05FK02-4, Article ID 05FK02.
Open this publication in new window or tab >>AlGaN/GaN high electron mobility transistors with intentionally doped GaN buffer using propane as carbon precursor
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2016 (English)In: Japanese Journal of Applied Physics, ISSN 0021-4922, E-ISSN 1347-4065, Vol. 55, p. 05FK02-1-05FK02-4, article id 05FK02Article in journal (Refereed) Published
Abstract [en]

AlGaN/GaN high electron mobility transistors (HEMTs) fabricated on a heterostructure grown by metalorganic chemical vapor deposition using analternative method of carbon (C) doping the buffer are characterized. C-doping is achieved by using propane as precursor, as compared to tuningthe growth process parameters to control C-incorporation from the gallium precursor. This approach allows for optimization of the GaN growthconditions without compromising material quality to achieve semi-insulating properties. The HEMTs are evaluated in terms of isolation anddispersion. Good isolation with OFF-state currents of 2 ' 10%6A/mm, breakdown fields of 70V/μm, and low drain induced barrier lowering of0.13mV/V are found. Dispersive effects are examined using pulsed current–voltage measurements. Current collapse and knee walkout effectslimit the maximum output power to 1.3W/mm. With further optimization of the C-doping profile and GaN material quality this method should offer aversatile approach to decrease dispersive effects in GaN HEMTs.

Place, publisher, year, edition, pages
Institute of Physics Publishing (IOPP), 2016
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:liu:diva-128077 (URN)10.7567/JJAP.55.05FK02 (DOI)000374697600081 ()
Funder
Swedish Foundation for Strategic Research Swedish Research Council
Available from: 2016-05-16 Created: 2016-05-16 Last updated: 2017-11-30
Tengdelius, L., Lu, J., Forsberg, U., Li, X., Hultman, L., Janzén, E. & Högberg, H. (2016). ZrB2 thin films deposited on GaN(0001) by magnetron sputtering from a ZrB2 target. Journal of Crystal Growth, 453, 71-76
Open this publication in new window or tab >>ZrB2 thin films deposited on GaN(0001) by magnetron sputtering from a ZrB2 target
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2016 (English)In: Journal of Crystal Growth, ISSN 0022-0248, E-ISSN 1873-5002, Vol. 453, p. 71-76Article in journal (Refereed) Published
Abstract [en]

ZrB2 films were deposited on 900 °C-preheated or non-preheated GaN(0001) surfaces by direct current magnetron sputtering from a compound target. Analytical transmission electron microscopy and scanning transmission electron microscopy with energy dispersive X-ray spectroscopy and electron energy loss spectroscopy revealed a 0001 fiber textured ZrB2 film growth following the formation of a ~2 nm thick amorphous BN layer onto the GaN(0001) at a substrate temperature of 900 °C. The amorphous BN layer remains when the substrate temperature is lowered to 500 °C or when the preheating step is removed from the process and results in the growth of polycrystalline ZrB2 films. The ZrB2 growth phenomena on GaN(0001) is compared to on 4H-SiC(0001), Si(111), and Al2O3(0001) substrates, which yield epitaxial film growth. The decomposition of the GaN surface during vacuum processing during BN interfacial layer formation is found to impede epitaxial growth of ZrB2.

Place, publisher, year, edition, pages
Elsevier, 2016
Keywords
A1. X-ray diffraction; A1. Interfaces; A1. Energy-dispersive X-ray spectroscopy; A1. Electron energy loss spectroscopy; A3. Physical vapor deposition processes; B1. Borides
National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-130920 (URN)10.1016/j.jcrysgro.2016.08.011 (DOI)000386984000012 ()
Available from: 2016-08-31 Created: 2016-08-31 Last updated: 2017-11-21Bibliographically approved
Li, X., Bergsten, J., Nilsson, D., Danielsson, Ö., Pedersen, H., Rorsman, N., . . . Forsberg, U. (2015). Carbon doped GaN buffer layer using propane for high electron mobility transistor applications: Growth and device results. Applied Physics Letters, 107(26), 262105
Open this publication in new window or tab >>Carbon doped GaN buffer layer using propane for high electron mobility transistor applications: Growth and device results
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2015 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 107, no 26, p. 262105-Article in journal (Refereed) Published
Abstract [en]

The creation of a semi insulating (SI) buffer layer in AlGaN/GaN High Electron Mobility Transistor (HEMT) devices is crucial for preventing a current path beneath the two-dimensional electron gas (2DEG). In this investigation, we evaluate the use of a gaseous carbon gas precursor, propane, for creating a SI GaN buffer layer in a HEMT structure. The carbon doped profile, using propane gas, is a two stepped profile with a high carbon doping (1.5 x 10(18) cm(-3)) epitaxial layer closest to the substrate and a lower doped layer (3 x 10(16) cm(-3)) closest to the 2DEG channel. Secondary Ion Mass Spectrometry measurement shows a uniform incorporation versus depth, and no memory effect from carbon doping can be seen. The high carbon doping (1.5 x 10(18) cm(-3)) does not influence the surface morphology, and a roughness root-mean-square value of 0.43 nm is obtained from Atomic Force Microscopy. High resolution X-ray diffraction measurements show very sharp peaks and no structural degradation can be seen related to the heavy carbon doped layer. HEMTs are fabricated and show an extremely low drain induced barrier lowering value of 0.1 mV/V, demonstrating an excellent buffer isolation. The carbon doped GaN buffer layer using propane gas is compared to samples using carbon from the trimethylgallium molecule, showing equally low leakage currents, demonstrating the capability of growing highly resistive buffer layers using a gaseous carbon source. (C) 2015 AIP Publishing LLC.

Place, publisher, year, edition, pages
AMER INST PHYSICS, 2015
National Category
Chemical Sciences
Identifiers
urn:nbn:se:liu:diva-125161 (URN)10.1063/1.4937575 (DOI)000368442300020 ()
Note

Funding Agencies|Swedish Defense Materiel Administration (FMV); Swedish Foundation for Strategic Research (SSF)

Available from: 2016-02-15 Created: 2016-02-15 Last updated: 2017-11-30
Li, X. (2015). CVD solutions for new directions in SiC and GaN epitaxy. (Doctoral dissertation). Linköping: Linköping University Electronic Press
Open this publication in new window or tab >>CVD solutions for new directions in SiC and GaN epitaxy
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis aims to develop a chemical vapor deposition (CVD) process for the new directions in both silicon carbon (SiC) and gallium nitride (GaN) epitaxial growth. The properties of the grown epitaxial layers are investigated in detail in order to have a deep understanding.

SiC is a promising wide band gap semiconductor material which could be utilized for fabricating high-power and high-frequency devices. 3C-SiC is the only polytype with a cubic structure and has superior physical properties over other common SiC polytypes, such as high hole/electron mobility and low interface trap density with oxide. Due to lack of commercial native substrates, 3C-SiC is mainly grown on the cheap silicon (Si) substrates. However, there’s a large mismatch in both lattice constants and thermal expansion coefficients leading to a high density of defects in the epitaxial layers. In paper 1, the new CVD solution for growing high quality double-position-boundaries free 3C-SiC using on-axis 4H-SiC substrates is presented. Reproducible growth parameters, including temperature, C/Si ratio, ramp-up condition, Si/H2 ratio, N2 addition and pressure, are covered in this study.

GaN is another attractive wide band gap semiconductor for power devices and optoelectronic applications. In the GaN-based transistors, carbon is often exploited to dope the buffer layer to be semi-insulating in order to isolate the device active region from the substrate. The conventional way is to use the carbon atoms on the gallium precursor and control the incorporation by tuning the process parameters, e.g. temperature, pressure. However, there’s a risk of obtaining bad morphology and thickness uniformity if the CVD process is not operated in an optimal condition. In addition, carbon source from the graphite insulation and improper coated graphite susceptor may also contribute to the doping in a CVD reactor, which is very difficult to be controlled in a reproducible way. Therefore, in paper 2, intentional carbon doping of (0001) GaN using six hydrocarbon precursors, i.e. methane (CH4), ethylene (C2H4), acetylene (C2H2), propane (C3H8), iso-butane (i-C4H10) and trimethylamine (N(CH3)3), have been explored. In paper 3, propane is chosen for carbon doping when growing the high electron mobility transistor (HEMT) structure on a quarter of 3-inch 4H-SiC wafer. The quality of epitaxial layer and fabricated devices is evaluated. In paper 4, the behaviour of carbon doping using carbon atoms from the gallium precursor, trimethylgallium (Ga(CH3)3), is explained by thermochemical and quantum chemical modelling and compared with the experimental results.

GaN is commonly grown on foreign substrates, such as sapphire (Al2O3), Si and SiC, resulting in high stress and high threading dislocation densities. Hence, bulk GaN substrates are preferred for epitaxy. In paper 5, the morphological, structural and luminescence properties of GaN epitaxial layers grown on N-face free-standing GaN substrates are studied since the N-face GaN has advantageous characteristics compared to the Ga-face GaN. In paper 6, time-resolved photoluminescence (TRPL) technique is used to study the properties of AlGaN/GaN epitaxial layers grown on both Ga-face and N-face free-standing GaN substrates. A PL line located at ~3.41 eV is only emerged on the sample grown on the Ga-face substrate, which is suggested to associate with two-dimensional electron gas (2DEG) emission.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2015. p. 50
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1654
Keywords
CVD, SiC, GaN, epitaxy
National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-117878 (URN)10.3384/diss.diva-117878 (DOI)978-91-7519-084-6 (ISBN)
Public defence
2015-06-11, Schrödinger, Fysikhuset, Campus Valla, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2015-05-22 Created: 2015-05-12 Last updated: 2019-11-15Bibliographically approved
Li, X., Bergsten, J., Nilsson, D., Danielsson, Ö., Pedersen, H., Rorsman, N., . . . Forsberg, U. (2015). Intentionally carbon doped GaN buffer layer for HEMT application: growth and device results.
Open this publication in new window or tab >>Intentionally carbon doped GaN buffer layer for HEMT application: growth and device results
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2015 (English)Manuscript (preprint) (Other academic)
Abstract [en]

The creation of a semi-insulating (SI) buffer layer in AlGaN/GaN HEMT devices is crucial for preventing a current path beneath the two-dimensional electron gas (2DEG). Here we evaluate the use of a carbon precursor, propane, for creating a SI GaN buffer layer. The carbon doping profile obtained from SIMS measurement shows a very uniform incorporation versus depth and no significant memory effect from carbon doping is seen, allowing for the creation of a very abrupt profile. The high carbon doping (1.5×1018 cm-3) does not influence the surface morphology. HRXRD ω rocking curve showed a FWHM of 200 arcsec of the (0002) and 261 arcsec for (10-12) reflection of the GaN, respectively. HEMT devices were processed on the epitaxial layers. An extremely low drain induced barrier lowering value of 0.1 mV/V was measured for a HEMT with a gate length of 0.2 𝜇m. This demonstrates the capability of growing a highly resistive buffer layer using intentional carbon doping.

National Category
Physical Sciences Physical Chemistry
Identifiers
urn:nbn:se:liu:diva-118112 (URN)
Available from: 2015-05-22 Created: 2015-05-22 Last updated: 2016-08-31Bibliographically approved
Li, X., Hemmingsson, C., Forsberg, U., Janzén, E. & Pozina, G. (2015). Optical properties of AlGaN/GaN epitaxial layers grown on free-standing Ga-face and N-face GaN substrates.
Open this publication in new window or tab >>Optical properties of AlGaN/GaN epitaxial layers grown on free-standing Ga-face and N-face GaN substrates
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2015 (English)Manuscript (preprint) (Other academic)
Abstract [en]

Comparative studies have been made on AlGaN/GaN epitaxial layers grown by metalorganic chemical vapor deposition on both Ga- and N-face free-standing GaN substrates fabricated by halide vapor phase epitaxy. By time-resolved photoluminescence studies, we conclude that two-dimensional electron gas (2DEG) only appears for heterostructures grown on Ga-face. We studied the temporal behavior of the 2DEG emission, which correlates well with recombination processes in an asymmetric triangular potential well formed by an AlGaN/GaN structure grown in [0001] direction.

National Category
Physical Sciences Physical Chemistry
Identifiers
urn:nbn:se:liu:diva-118114 (URN)
Available from: 2015-05-22 Created: 2015-05-22 Last updated: 2015-09-22Bibliographically approved
Li, X., Danielsson, Ö., Pedersen, H., Janzén, E. & Forsberg, U. (2015). Precursors for carbon doping of GaN in chemical vapor deposition. Journal of Vacuum Science & Technology B, 33(2), 021208
Open this publication in new window or tab >>Precursors for carbon doping of GaN in chemical vapor deposition
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2015 (English)In: Journal of Vacuum Science & Technology B, ISSN 1071-1023, E-ISSN 1520-8567, Vol. 33, no 2, p. 021208-Article in journal (Refereed) Published
Abstract [en]

Methane (CH4), ethylene (C2H4), acetylene (C2H2), propane (C3H8), iso-butane (i-C4H10), and trimethylamine [N(CH3)(3)] have been investigated as precursors for intentional carbon doping of (0001) GaN in chemical vapor deposition. The carbon precursors were studied by comparing the efficiency of carbon incorporation in GaN together with their influence on morphology and structural quality of carbon doped GaN. The unsaturated hydrocarbons C2H4 and C2H2 were found to be more suitable for carbon doping than the saturated ones, with higher carbon incorporation efficiency and a reduced effect on the quality of the GaN epitaxial layers. The results indicate that the C2H2 molecule as a direct precursor, or formed by the gas phase chemistry, is a key species for carbon doping without degrading the GaN quality; however, the CH3 species should be avoided in the carbon doping chemistry.

Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2015
National Category
Chemical Sciences
Identifiers
urn:nbn:se:liu:diva-117385 (URN)10.1116/1.4914316 (DOI)000351751100024 ()
Note

Funding Agencies|Swedish Foundation for Strategic Research (SSF); Swedish Defence Materiel Administration (FMV)

Available from: 2015-04-24 Created: 2015-04-24 Last updated: 2017-12-04
Li, X., Hemmingsson, C., Forsberg, U., Janzén, E. & Pozina, G. (2015). Properties of GaN layers grown on N-face free-standing GaN substrates. Journal of Crystal Growth, 413, 81-85
Open this publication in new window or tab >>Properties of GaN layers grown on N-face free-standing GaN substrates
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2015 (English)In: Journal of Crystal Growth, ISSN 0022-0248, E-ISSN 1873-5002, Vol. 413, p. 81-85Article in journal (Refereed) Published
Abstract [en]

GaN layers were homoepitaxially grown on N-face free-standing GaN substrates using a hot-wall metalorganic chemical vapor deposition method. By using optimized growth parameters, layers with a smooth morphology were obtained. The crystalline quality of epilayers was studied by a high resolution X-ray diffraction technique and compared to the substrates. Optical properties of the epilayers studied by low temperature time-resolved photoluminescence have shown longer recombination time for donor-bound exciton compared to the substrates. (C) 2014 Elsevier B.V. All rights reserved.

Place, publisher, year, edition, pages
Elsevier, 2015
Keywords
Characterization; Metalorganic chemical vapor deposition; Nitrides; Semiconducting III-V materials; Hot-wall epitaxy
National Category
Chemical Sciences Physical Sciences
Identifiers
urn:nbn:se:liu:diva-114562 (URN)10.1016/j.jcrysgro.2014.11.020 (DOI)000348037000015 ()
Note

Funding Agencies|Swedish Energy Agency; Swedish Research Council

Available from: 2015-03-02 Created: 2015-02-26 Last updated: 2017-12-04
Li, X., Jacobson, H., Boulle, A., Chaussende, D. & Henry, A. (2014). Double-Position-Boundaries Free 3C-SiC Epitaxial Layers Grown on On-Axis 4H-SiC. ECS Journal of Solid State Science and Technology, 3(4), P75-P81
Open this publication in new window or tab >>Double-Position-Boundaries Free 3C-SiC Epitaxial Layers Grown on On-Axis 4H-SiC
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2014 (English)In: ECS Journal of Solid State Science and Technology, ISSN 2162-8769, E-ISSN 2162-8777, Vol. 3, no 4, p. P75-P81Article in journal (Refereed) Published
Abstract [en]

High quality double-position-boundaries free 3C-SiC epilayers have been successfully grown on on-axis (0001) 4H-SiC by chemical vapor deposition at optimized conditions as observed with optical microscopy and X-ray diffraction. The effect of the growth parameters, including temperature, C/Si ratio, ramp-up condition, Si/H-2 ratio, N-2 addition and pressure, on the quality of the grown layers is investigated. Different techniques, including microscopic and spectroscopic techniques, are used to characterize the epilayers. High resolution X-ray diffraction shows 2 theta-omega curve with full width at half maximum of only 16 arcsec for the (111) reflection detected from a 35 mu m thick 3C-SiC layer, showing the good structural quality of the layer. Reciprocal space maps confirm the absence of double-position-boundaries in a large depth of the layers. Low temperature photoluminescence measurement shows clear near-bandgap emission with sharp and single peaks, which further verifies the high quality of the epilayers.

Place, publisher, year, edition, pages
Electrochemical Society, 2014
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-106298 (URN)10.1149/2.012404jss (DOI)000333570300008 ()
Available from: 2014-05-06 Created: 2014-05-05 Last updated: 2017-12-05Bibliographically approved
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