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Forsberg, Urban
Publications (10 of 85) 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
Bergsten, J., Chen, J.-T., Gustafsson, S., Malmros, A., Forsberg, U., Thorsell, M., . . . Rorsman, N. (2016). Performance Enhancement of Microwave GaN HEMTs Without an AlN-Exclusion Layer Using an Optimized AlGaN/GaN Interface Growth Process. IEEE Transactions on Electron Devices, 63(1), 333-338
Open this publication in new window or tab >>Performance Enhancement of Microwave GaN HEMTs Without an AlN-Exclusion Layer Using an Optimized AlGaN/GaN Interface Growth Process
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2016 (English)In: IEEE Transactions on Electron Devices, ISSN 0018-9383, E-ISSN 1557-9646, Vol. 63, no 1, p. 333-338Article in journal (Refereed) Published
Abstract [en]

The impact of the sharpness of the AlGaN/GaN interface in high-electron mobility transistors (HEMTs) is investigated. Two structures, one with an optimized AlGaN/GaN interface and another with an unoptimized, were grown using hot-wall metal-organic chemical vapor deposition. The structure with optimized sharpness of the interface shows electron mobility of 1760 cm(2)/V . s as compared with 1660 cm(2)/V . s for the nonoptimized interface. Gated Hall measurements indicate that the sharper interface maintains higher mobility when the electrons are close to the interface compared with the nonoptimized structure, indicating less scattering due to alloy disorder and interface roughness. HEMTs were processed and evaluated. The higher mobility manifests as lower parasitic resistance yielding a better dc and high-frequency performance. A small-signal equivalent model is extracted. The results indicate a lower electron penetration into the buffer in the optimized sample. Pulsed-IV measurements imply that the sharper interface provides less dispersive effects at large drain biases. We speculate that the mobility enhancement seen AlGaN/AlN/GaN structures compared with the AlGaN/GaN case is not only related to the larger conduction band offset but also due to a more welldefined interface minimizing scattering due to alloy disorder and interface roughness.

Place, publisher, year, edition, pages
IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, 2016
Keywords
AlGaN/GaN interface; GaN high-electron mobility transistor (HEMT); interface sharpness
National Category
Chemical Sciences
Identifiers
urn:nbn:se:liu:diva-124488 (URN)10.1109/TED.2015.2501838 (DOI)000367259600043 ()
Note

Funding Agencies|Swedish Governmental Agency for Innovation Systems; Swedish Defence Materiel Administration; Swedish Foundation for Strategic Research

Available from: 2016-02-02 Created: 2016-02-01 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
Gustafsson, S., Chen, J.-T., Bergsten, J., Forsberg, U., Thorsell, M., Janzén, E. & Rorsman, N. (2015). Dispersive Effects in Microwave AlGaN/AlN/GaN HEMTs With Carbon-Doped Buffer. IEEE Transactions on Electron Devices, 62(7), 2162-2169
Open this publication in new window or tab >>Dispersive Effects in Microwave AlGaN/AlN/GaN HEMTs With Carbon-Doped Buffer
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2015 (English)In: IEEE Transactions on Electron Devices, ISSN 0018-9383, E-ISSN 1557-9646, Vol. 62, no 7, p. 2162-2169Article in journal (Refereed) Published
Abstract [en]

Aluminium gallium nitride (AlGaN)/GaN high-electron mobility transistor performance is to a large extent affected by the buffer design, which, in this paper, is varied using different levels of carbon incorporation. Three epitaxial structures have been fabricated: 1) two with uniform carbon doping profile but different carbon concentration and 2) one with a stepped doping profile. The epitaxial structures have been grown on 4H-SiC using hot-wall metal-organic chemical vapor deposition with residual carbon doping. The leakage currents in OFF-state at 10 V drain voltage were in the same order of magnitude (10-4 A/mm) for the high-doped and stepped-doped buffer. The high-doped material had a current collapse (CC) of 78.8% compared with 16.1% for the stepped-doped material under dynamic I-V conditions. The low-doped material had low CC (5.2%) but poor buffer isolation. Trap characterization revealed that the high-doped material had two trap levels at 0.15 and 0.59 eV, and the low-doped material had one trap level at 0.59 eV.

Place, publisher, year, edition, pages
IEEE Press, 2015
Keywords
Gallium nitride, HEMTs, trap levels, current collapse, dispersion
National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-117136 (URN)10.1109/TED.2015.2428613 (DOI)000356457900014 ()
Available from: 2015-04-17 Created: 2015-04-17 Last updated: 2017-12-04Bibliographically 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
Chen, J.-T., Pomeroy, J. W., Rorsman, N., Xia, C., Virojanadara, C., Forsberg, U., . . . Janzén, E. (2015). Low thermal resistance of a GaN-on-SiC transistor structure with improved structural properties at the interface. Journal of Crystal Growth, 428, 54-58
Open this publication in new window or tab >>Low thermal resistance of a GaN-on-SiC transistor structure with improved structural properties at the interface
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2015 (English)In: Journal of Crystal Growth, ISSN 0022-0248, E-ISSN 1873-5002, Vol. 428, p. 54-58Article in journal (Refereed) Published
Abstract [en]

The crystalline quality of AlGaN/GaN heterostructures was improved by optimization of surface pretreatment of the SiC substrate in a hot-wall metal-organic chemical vapor deposition reactor. X-ray photoelectron spectroscopy measurements revealed that oxygen- and carbon-related contaminants were still present on the SiC surface treated at 1200 °C in H2 ambience, which hinders growth of thin AlN nucleation layers with high crystalline quality. As the H2 pretreatment temperature increased to 1240 °C, the crystalline quality of the 105 nm thick AlN nucleation layers in the studied series reached an optimal value in terms of full width at half-maximum of the rocking curves of the (002) and (105) peaks of 64 and 447 arcsec, respectively. The improvement of the AlN growth also consequently facilitated a growth of the GaN buffer layers with high crystalline quality. The rocking curves of the GaN (002) and (102) peaks were thus improved from 209 and 276 arcsec to 149 and 194 arcsec, respectively. In addition to a correlation between the thermal resistance and the structural quality of an AlN nucleation layer, we found that the microstructural disorder of the SiC surface and the morphological defects of the AlN nucleation layers to be responsible for a substantial thermal resistance. Moreover, in order to decrease the thermal resistance in the GaN/SiC interfacial region, the thickness of the AlN nucleation layer was then reduced to 35 nm, which was shown sufficient to grow AlGaN/GaN heterostructures with high crystalline quality. Finally, with the 35 nm thick high-quality AlN nucleation layer a record low thermal boundary resistance of 1.3×10−8 m2 K/W, measured at an elevated temperature of 160 °C, in a GaN-on-SiC transistor structure was achieved.

Place, publisher, year, edition, pages
Elsevier, 2015
Keywords
Heat transfer; Metalorganic chemical vapor deposition; Nitrides; High electron mobility transistors
National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-117132 (URN)10.1016/j.jcrysgro.2015.07.021 (DOI)000360501200009 ()
Available from: 2015-04-17 Created: 2015-04-17 Last updated: 2017-12-04Bibliographically approved
Chen, J.-T., Hsu, C.-W., Forsberg, U. & Janzén, E. (2015). Metalorganic chemical vapor deposition growth of high-mobility AlGaN/AlN/GaN heterostructures on GaN templates and native GaN substrates. Journal of Applied Physics, 117(8), Article ID 085301.
Open this publication in new window or tab >>Metalorganic chemical vapor deposition growth of high-mobility AlGaN/AlN/GaN heterostructures on GaN templates and native GaN substrates
2015 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 117, no 8, article id 085301Article in journal (Refereed) Published
Abstract [en]

Severe surface decomposition of semi-insulating (SI) GaN templates occurred in high-temperature H2 atmosphere prior to epitaxial growth in a metalorganic chemical vapor deposition system. A two-step heating process with a surface stabilization technique was developed to preserve the GaN template surface. Utilizing the optimized heating process, a high two-dimensional electron gas mobility ∼2000 cm2/V·s was obtained in a thin AlGaN/AlN/GaN heterostructure with an only 100-nm-thick GaN spacer layer homoepitaxially grown on the GaN template. This technique was also demonstrated viable for native GaN substrates to stabilize the surface facilitating two-dimensional growth of GaN layers. Very high residual silicon and oxygen concentrations were found up to ∼1 × 1020 cm−3 at the interface between the GaN epilayer and the native GaN substrate. Capacitance-voltage measurements confirmed that the residual carbon doping controlled by growth conditions of the GaN epilayer can be used to successfully compensate the donor-like impurities. State-of-the-art structural properties of a high-mobility AlGaN/AlN/GaN heterostructure was then realized on a 1 × 1 cm2 SI native GaN substrate; the full width at half maximum of the X-ray rocking curves of the GaN (002) and (102) peaks are only 21 and 14 arc sec, respectively. The surface morphology of the heterostructure shows uniform parallel bilayer steps, and no morphological defects were noticeable over the entire epi-wafer.

Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2015
National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-117135 (URN)10.1063/1.4913223 (DOI)000351132500059 ()
Note

The authors would like to acknowledge the support from the Swedish Foundation for Strategic Research.

Available from: 2015-04-17 Created: 2015-04-17 Last updated: 2017-12-04Bibliographically 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
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