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MOCVD growth of GaN-based high electron mobility transistor structures
Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The present work was to improve the overall quality of GaN-based high electron mobility transistor (HEMT) epitaxial structures grown on semi-insulating (SI) SiC and native GaN substrates, using an approach called bottom-to-top optimization. The bottom-to-top optimization means an entire growth process optimization, from in-situ substrate pretreatment to the epitaxial growth and then the cooling process. Great effort was put to gain the understanding of the influence of growth parameters on material properties and consequently to establish an advanced and reproducible growth process. Many state-of-the-art material properties of GaN-based HEMT structures were achieved in this work, including superior structural integrity of AlN nucleation layers for ultra-low thermal boundary resistance, excellent control of residual impurities, outstanding and nearly-perfect crystalline quality of GaN epilayers grown on SiC and native GaN substrates, respectively, and record-high room temperature 2DEG mobility obtained in simple AlGaN/GaN heterostructures.

The epitaxial growth of the wide bandgap III-nitride epilayers like GaN, AlN,  AlGaN, and InAlN, as well as various GaN-based HEMT structures was all carried out in a hot-wall metalorganic chemical vapor deposition (MOCVD) system. A variety of structural and electrical characterizations were routinely used to provide fast feedback for adjusting growth parameters and developing improved growth processes.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2015. , 59 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1662
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:liu:diva-117138DOI: 10.3384/diss.diva-117138ISBN: 978-91-7519-073-0 (print)OAI: oai:DiVA.org:liu-117138DiVA: diva2:805922
Public defence
2015-05-12, Planck, Fysikhuset, Campus Valla, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2015-04-17 Created: 2015-04-17 Last updated: 2015-04-17Bibliographically approved
List of papers
1. Low thermal resistance of a GaN-on-SiC transistor structure with improved structural properties at the interface
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, 54-58 p.Article 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
Keyword
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
2. Impact of residual carbon on two-dimensional electron gas properties in AlxGa1−xN/GaN heterostructure
Open this publication in new window or tab >>Impact of residual carbon on two-dimensional electron gas properties in AlxGa1−xN/GaN heterostructure
2013 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 102, no 19, 193506- p.Article in journal (Refereed) Published
Abstract [en]

High tuneability of residual carbon doping is developed in a hot-wall metalorganic chemical vapor deposition reactor. Two orders of temperature-tuned carbon concentration, from ∼2 × 1018 cm−3 down to ∼1 × 1016 cm−3, can be effectively controlled in the growth of the GaN buffer layer. Excellent uniformity of two-dimensional electron gas (2DEG) properties in AlxGa1−xN/AlN/GaN heterostructure with very high average carrier density and mobility, 1.1 × 1013 cm−2 and 2035 cm2/V·s, respectively, over 3" semi-insulating SiC substrate is realized with the temperature-tuned carbon doping scheme. Reduction of carbon concentration is evidenced as a key to achieve high 2DEG carrier density and mobility.

Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2013
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-96138 (URN)10.1063/1.4804600 (DOI)000320440800103 ()
Available from: 2013-08-14 Created: 2013-08-14 Last updated: 2017-12-06
3. Room-Temperature mobility above 2200 cm2/V.s of two-dimensional electron gas in a sharp-interface AlGaN/GaN heterostructure
Open this publication in new window or tab >>Room-Temperature mobility above 2200 cm2/V.s of two-dimensional electron gas in a sharp-interface AlGaN/GaN heterostructure
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2015 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 106, no 25, 251601Article in journal (Refereed) Published
Abstract [en]

A high mobility of 2250 cm2/V·s of a two-dimensional electron gas (2DEG) in a metalorganic chemical vapor deposition-grown AlGaN/GaN heterostructure was demonstrated. The mobility enhancement was a result of better electron confinement due to a sharp AlGaN/GaN interface, as confirmed by scanning transmission electron microscopy analysis, not owing to the formation of a traditional thin AlN exclusion layer. Moreover, we found that the electron mobility in the sharp-interface heterostructures can sustain above 2000 cm2/V·s for a wide range of 2DEG densities. Finally, it is promising that the sharp-interface AlGaN/GaN heterostructure would enable low contact resistance fabrication, less impurity-related scattering, and trapping than the AlGaN/AlN/GaN heterostructure, as the high-impurity-contained AlN is removed.

Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2015
National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-117133 (URN)10.1063/1.4922877 (DOI)000357036600005 ()
Available from: 2015-04-17 Created: 2015-04-17 Last updated: 2017-12-04Bibliographically approved
4. Growth optimization of AlGaN/GaN HEMT structure on 100 mm SiC substrate: Utilizing bottom-to-top approach
Open this publication in new window or tab >>Growth optimization of AlGaN/GaN HEMT structure on 100 mm SiC substrate: Utilizing bottom-to-top approach
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

The structure of high electron mobility transistors (HEMTs) based on group-III nitride materials generally consists of three important blocks; a nucleation layer, a semi-insulating (SI) GaN buffer layer, and active layers. In this work, we present an overall growth optimization, which leads to superior crystalline quality and ultra-low thermal boundary resistance (TBR) of a 35-nm AlN nucleation layer, excellent crystalline quality of carbon-doped GaN buffer layer, and high mobility (> 2000 cm2/Vs) of two-dimensional gas (2DEG) in a simple AlGaN/GaN heterostructure grown on a SI SiC substrate.

National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-117134 (URN)
Available from: 2015-04-17 Created: 2015-04-17 Last updated: 2015-04-17Bibliographically approved
5. Metalorganic chemical vapor deposition growth of high-mobility AlGaN/AlN/GaN heterostructures on GaN templates and native GaN substrates
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, 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
6. Dispersive Effects in Microwave AlGaN/AlN/GaN HEMTs With Carbon-Doped Buffer
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, 2162-2169 p.Article 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
Keyword
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
7. Impact of AlGaN/GaN interface sharpness on HEMT performance
Open this publication in new window or tab >>Impact of AlGaN/GaN interface sharpness on HEMT performance
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

The impact of the design and sharpness of the AlGaN/GaN interface in GaN-based HEMTs is investigated. Three structures with different AlGaN/GaN interface properties were grown with hot-wall MOCVD. One structure has a 2-nmthick AlN exclusion layer in between the AlGaN and the GaN, while the other two differ in their sharpness of the Al transition at the AlGaN/GaN interface. The structures with AlN exclusion layer and optimized sharpness of the interface show similar electron mobilities (1760 and 1740 cm2/Vs). HEMTs were processed and evaluated. Gated Hall-measurements indicate that the sharper interface maintains a higher mobility when the electrons are close to the interface compared both to the AlNexclusion layer and the non-optimized structure. The higher mobility manifests as lower parasitic resistance yielding better DC and high frequency performance. Pulsed IV measurements indicate that the sharper interface provide less dispersive effects compared both to the AlN exclusion layer and the optimized interface.

Keyword
GaN HEMT, AlGaN/GaN interface, sharpness
National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-117137 (URN)
Available from: 2015-04-17 Created: 2015-04-17 Last updated: 2015-04-17Bibliographically approved

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