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  • 1.
    Armakavicius, Nerijus
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Knight, Sean Robert
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Kuhne, Philipp
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Stanishev, Vallery
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Tran, Dat
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Richter, Steffen
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Lund Univ, Sweden.
    Papamichail, Alexis
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Stokey, Megan
    Univ Nebraska Lincoln, NE 68588 USA.
    Sorensen, Preston
    Univ Nebraska Lincoln, NE 68588 USA.
    Kilic, Ufuk
    Univ Nebraska Lincoln, NE 68588 USA.
    Schubert, Mathias
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Lund Univ, Sweden; Univ Nebraska Lincoln, NE 68588 USA.
    Paskov, Plamen
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Darakchieva, Vanya
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Lund Univ, Sweden.
    Electron effective mass in GaN revisited: New insights from terahertz and mid-infrared optical Hall effect2024In: APL Materials, E-ISSN 2166-532X, Vol. 12, no 2, article id 021114Article in journal (Refereed)
    Abstract [en]

    Electron effective mass is a fundamental material parameter defining the free charge carrier transport properties, but it is very challenging to be experimentally determined at high temperatures relevant to device operation. In this work, we obtain the electron effective mass parameters in a Si-doped GaN bulk substrate and epitaxial layers from terahertz (THz) and mid-infrared (MIR) optical Hall effect (OHE) measurements in the temperature range of 38-340 K. The OHE data are analyzed using the well-accepted Drude model to account for the free charge carrier contributions. A strong temperature dependence of the electron effective mass parameter in both bulk and epitaxial GaN with values ranging from (0.18 +/- 0.02) m(0) to (0.34 +/- 0.01) m(0) at a low temperature (38 K) and room temperature, respectively, is obtained from the THz OHE analysis. The observed effective mass enhancement with temperature is evaluated and discussed in view of conduction band nonparabolicity, polaron effect, strain, and deviations from the classical Drude behavior. On the other hand, the electron effective mass parameter determined by MIR OHE is found to be temperature independent with a value of (0.200 +/- 0.002) m(0). A possible explanation for the different findings from THz OHE and MIR OHE is proposed. (c) 2024 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/)

  • 2.
    Bittrich, Eva
    et al.
    Leibniz Inst Polymerforsch Dresden eV, Germany.
    Domke, Jari
    Friedrich Schiller Univ Jena, Germany.
    Jehnichen, Dieter
    Leibniz Inst Polymerforsch Dresden eV, Germany.
    Bittrich, Lars
    Leibniz Inst Polymerforsch Dresden eV, Germany.
    Malanin, Mikhail
    Leibniz Inst Polymerforsch Dresden eV, Germany.
    Janke, Andreas
    Leibniz Inst Polymerforsch Dresden eV, Germany.
    Uhlmann, Petra
    Leibniz Inst Polymerforsch Dresden eV, Germany; Univ Nebraska, NE 68588 USA.
    Eichhorn, Klaus-Jochen
    Leibniz Inst Polymerforsch Dresden eV, Germany.
    Papamichail, Alexis
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Stanishev, Vallery
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Darakchieva, Vanya
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Al-Hussein, Mahmoud
    Univ Jordan, Jordan.
    Levichkova, Marieta
    Heliatek GmbH, Germany.
    Fritz, Torsten
    Friedrich Schiller Univ Jena, Germany.
    Walzer, Karsten
    Heliatek GmbH, Germany.
    Morphology of Thin Films of Aromatic Ellagic Acid and Its Hydrogen Bonding Interactions2020In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 124, no 30, p. 16381-16390Article in journal (Refereed)
    Abstract [en]

    Ellagic acid (EA), an antioxidant from fruits or other plants, has recently evoked interest in the field of organic electronics because of its weak electron donor properties. In this work, the preparation of uniaxial pi-stacked EA films by thermal evaporation on different surfaces is reported for the first time. The (102) lattice plane of the pi-electron system was confirmed as the contact plane for one monolayer equivalent on Ag(111) by low-electron energy diffraction. X-ray and atomic force microscopy measurements revealed nanocrystalline grains with an average inplane size of 50 nm and considerably smaller average out-of-plane crystallite sizes (16-25 nm) in films of 16-75 nm thickness. The influence of different substrates was minor compared to the effect of the film thickness. An increase in the in-plane density of grains at larger film thicknesses was deduced from the trend in their uniaxial optical properties. Weak and strong intermolecular H-bonding interactions were identified in the EA crystal lattice, while a surplus of weak H-bonding was observed for the nanocrystallites in thin films, as compared to bulk EA. Finally, EA was coevaporated with the semiconducting thiophene molecule DCV4T-Et-2 to demonstrate principle interactions with a guest molecule by H-bonding analysis. Our results illustrate the feasibility of applying EA films as alignment layers for templating other semiconducting organic films used in organic electronic devices.

  • 3.
    Gogova, Daniela
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Ghezellou, Misagh
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Tran, Dat Q.
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Richter, Steffen
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Solid State Physics and NanoLund, Lund University, P. O. Box 118, 221 00 Lund, Sweden.
    Papamichail, Alexis
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    ul-Hassan, Jawad
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Persson, Axel R.
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Persson, Per
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Kordina, Olof
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, Faculty of Science & Engineering.
    Monemar, Bo
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Hilfiker, Matthew
    Department of Electrical and Computer Engineering, University of Nebraska-Lincoln, Lincoln, Nebraska 68588, USA.
    Schubert, Mathias
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Department of Electrical and Computer Engineering, University of Nebraska-Lincoln, Lincoln, Nebraska 68588, USA.
    Paskov, Plamen P.
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Darakchieva, Vanya
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Solid State Physics and NanoLund, Lund University, P. O. Box 118, 221 00 Lund, Sweden.
    Epitaxial growth of β-Ga2O3 by hot-wall MOCVD2022In: AIP Advances, E-ISSN 2158-3226, Vol. 12, no 5, article id 055022Article in journal (Refereed)
    Abstract [en]

    The hot-wall metalorganic chemical vapor deposition (MOCVD) concept, previously shown to enable superior material quality and high performance devices based on wide bandgap semiconductors, such as Ga(Al)N and SiC, has been applied to the epitaxial growth of beta-Ga2O3. Epitaxial beta-Ga2O3 layers at high growth rates (above 1 mu m/h), at low reagent flows, and at reduced growth temperatures (740 degrees C) are demonstrated. A high crystalline quality epitaxial material on a c-plane sapphire substrate is attained as corroborated by a combination of x-ray diffraction, high-resolution scanning transmission electron microscopy, and spectroscopic ellipsometry measurements. The hot-wall MOCVD process is transferred to homoepitaxy, and single-crystalline homoepitaxial beta-Ga2O3 layers are demonstrated with a 201 rocking curve width of 118 arc sec, which is comparable to those of the edge-defined film-fed grown (201) beta-Ga2O3 substrates, indicative of similar dislocation densities for epilayers and substrates. Hence, hot-wall MOCVD is proposed as a prospective growth method to be further explored for the fabrication of beta-Ga2O3.

  • 4.
    Kakanakova-Georgieva, Anelia
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Papamichail, Alexis
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Stanishev, Vallery
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Darakchieva, Vanya
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Lund Univ, Sweden.
    Incorporation of Magnesium into GaN Regulated by Intentionally Large Amounts of Hydrogen during Growth by MOCVD2022In: Physica status solidi. B, Basic research, ISSN 0370-1972, E-ISSN 1521-3951, Vol. 259, no 10, article id 2200137Article in journal (Refereed)
    Abstract [en]

    Herein, metal-organic chemical vapor deposition (MOCVD) of GaN layers doped with Mg atoms to the recognized optimum level of [Mg] approximate to 2 x 10(19) cm(-3) is performed. In a sequence of MOCVD runs, operational conditions, including temperature and flow rate of precursors, are maintained except for intentionally larger flows of hydrogen carrier gas fed into the reactor. By employing the largest hydrogen flow of 25 slm in this study, the performance of the as-grown Mg-doped GaN layers is certified by a room-temperature hole concentration of p approximate to 2 x 10(17) cm(-3) in the absence of any thermal activation treatment. Experimental evidence is delivered that the large amounts of hydrogen during the MOCVD growth can regulate the incorporation of the Mg atoms into GaN in a significant way so that MgH complex can coexist with a dominant and evidently electrically active isolated Mg-Ga acceptor.

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  • 5.
    Knight, Sean Robert
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Solid State Physics and NanoLund, Lund University, Lund, 22100, Sweden.
    Richter, Steffen
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Solid State Physics and NanoLund, Lund University, Lund, 22100, Sweden.
    Papamichail, Alexis
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Kuhne, Philipp
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Armakavicius, Nerijus
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Guo, Shiqi
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, Faculty of Science & Engineering.
    Persson, Axel R.
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Stanishev, Vallery
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Rindert, Viktor
    Solid State Physics and NanoLund, Lund University, Lund, 22100, Sweden.
    Persson, Per O. Å.
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Paskov, Plamen P.
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Schubert, Mathias
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Solid State Physics and NanoLund, Lund University, Lund, 22100, Sweden; Department of Electrical and Computer Engineering, University of Nebraska-Lincoln, Lincoln, 68588, NE, United States.
    Darakchieva, Vanya
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Solid State Physics and NanoLund, Lund University, Lund, 22100, Sweden.
    Room temperature two-dimensional electron gas scattering time, effective mass, and mobility parameters in AlxGa1−xN/GaN heterostructures (0.07 ≤ x ≤ 0.42)2023In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 134, no 18, article id 185701Article in journal (Refereed)
    Abstract [en]

    Al xGa 1−xN/GaN high-electron-mobility transistor (HEMT) structures are key components in electronic devices operating at gigahertz or higher frequencies. In order to optimize such HEMT structures, understanding their electronic response at high frequencies and room temperature is required. Here, we present a study of the room temperature free charge carrier properties of the two-dimensional electron gas (2DEG) in HEMT structures with varying Al content in the Al xGa 1−xN barrier layers between x=0.07 and x=0.42⁠. We discuss and compare 2DEG sheet density, mobility, effective mass, sheet resistance, and scattering times, which are determined by theoretical calculations, contactless Hall effect, capacitance-voltage, Eddy current, and cavity-enhanced terahertz optical Hall effect (THz-OHE) measurements using a low-field permanent magnet (0.6 T). From our THz-OHE results, we observe that the measured mobility reduction from x=0.13 to x=0.42 is driven by the decrease in 2DEG scattering time, and not the change in effective mass. For x<0.42⁠, the 2DEG effective mass is found to be larger than for electrons in bulk GaN, which in turn, contributes to a decrease in the principally achievable mobility. From our theoretical calculations, we find that values close to 0.3 m0 can be explained by the combined effects of conduction band nonparabolicity, polarons, and hybridization of the electron wavefunction through penetration into the barrier layer.

  • 6.
    Knight, Sean Robert
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Lund Univ, Sweden; Lund Univ, Sweden.
    Richter, Steffen
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Lund Univ, Sweden; Lund Univ, Sweden.
    Papamichail, Alexis
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Stokey, Megan
    Univ Nebraska, NE 68588 USA.
    Korlacki, Rafal
    Univ Nebraska, NE 68588 USA.
    Stanishev, Vallery
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Kuhne, Philipp
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Schubert, Mathias
    Lund Univ, Sweden; Lund Univ, Sweden.
    Darakchieva, Vanya
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Lund Univ, Sweden; Lund Univ, Sweden.
    Terahertz permittivity parameters of monoclinic single crystal lutetium oxyorthosilicate2024In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 124, no 3, article id 032101Article in journal (Refereed)
    Abstract [en]

    The anisotropic permittivity parameters of monoclinic single crystal lutetium oxyorthosilicate, Lu2SiO5 (LSO), have been determined in the terahertz spectral range. Using terahertz generalized spectroscopic ellipsometry (THz-GSE), we obtained the THz permittivities along the a, b, and c? crystal directions, which correspond to the ea; eb, and ec? on-diagonal tensor elements. The associated off diagonal tensor element eac? was also determined experimentally, which is required to describe LSO's optical response in the monoclinic a-c crystallographic plane. From the four tensor elements obtained in the model fit, we calculate the direction of the principal dielectric axes in the a-c plane. We find good agreementwhen comparing THz-GSE permittivities to the static permittivity tensors from previous infrared and density functional theory studies.

  • 7.
    Korlacki, R.
    et al.
    Univ Nebraska, NE 68588 USA.
    Stokey, M.
    Univ Nebraska, NE 68588 USA.
    Mock, A.
    NRC Res Associateship Programs, DC 20001 USA.
    Knight, S.
    Univ Nebraska, NE 68588 USA.
    Papamichail, Alexis
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Darakchieva, Vanya
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Schubert, Mathias
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Univ Nebraska, NE 68588 USA; Leibniz Inst Polymer Res Dresden, Germany.
    Strain and stress relationships for optical phonon modes in monoclinic crystals with beta-Ga2O3 as an example2020In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 102, no 18, article id 180101Article in journal (Refereed)
    Abstract [en]

    Strain-stress relationships for physical properties are of interest for heteroepitaxial material systems, where strain and stress are inherent due to thermal expansion and lattice mismatch. We report linear perturbation theory strain and stress relationships for optical phonon modes in monoclinic crystals for strain and stress situations which maintain the monoclinic symmetry of the crystal. By using symmetry group analysis and phonon frequencies obtained under various deformation scenarios from density-functional perturbation theory calculations on beta-Ga2O3, we obtain four strain and four stress potential parameters for each phonon mode. We demonstrate that these parameters are sufficient to describe the frequency shift of the modes regardless of the stress or strain pattern which maintain the monoclinic symmetry of the crystal. The deformation potentials can be used together with experimentally determined phonon frequency parameters from Raman or infrared spectroscopy to evaluate the state of strain or stress of beta-Ga2O3, for example, in epitaxial heterostructures.

  • 8.
    Kuhne, Philipp
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Armakavicius, Nerijus
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Papamichail, Alexis
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Tran, Dat
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Stanishev, Vallery
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Schubert, Mathias
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Univ Nebraska Lincoln, NE 68588 USA.
    Paskov, Plamen
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Darakchieva, Vanya
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Lund Univ, Sweden.
    Enhancement of 2DEG effective mass in AlN/Al0.78Ga0.22N high electron mobility transistor structure determined by THz optical Hall effect2022In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 120, no 25, article id 253102Article in journal (Refereed)
    Abstract [en]

    We report on the free charge carrier properties of a two-dimensional electron gas (2DEG) in an AlN/AlxGa1-xN high electron mobility transistor structure with a high aluminum content (x = 0.78). The 2DEG sheet density N s = ( 7.3 +/- 0.7 ) x 10 12 cm(-2), sheet mobility mu s = ( 270 +/- 40 ) cm(2)/(Vs), sheet resistance R- s = ( 3200 +/- 500 ) omega/ ?, and effective mass m( eff) = ( 0.63 +/- 0.04 ) m( 0) at low temperatures ( T = 5 K ) are determined by terahertz (THz) optical Hall effect measurements. The experimental 2DEG mobility in the channel is found within the expected range, and the sheet carrier density is in good agreement with self-consistent Poisson-Schrodinger calculations. However, a significant increase in the effective mass of 2DEG electrons at low temperatures is found in comparison with the respective value in bulk Al0.78Ga22N ( m( eff) = 0.334 m( 0)). Possible mechanisms for the enhanced 2DEG effective mass parameter are discussed and quantified using self-consistent Poisson-Schrodinger calculations .Published under an exclusive license by AIP Publishing.

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  • 9. Order onlineBuy this publication >>
    Papamichail, Alexis
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Hot-wall MOCVD for advanced GaN HEMT structures and improved p-type doping2023Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    The transition to energy efficient smart grid and wireless communication with improved capacity require the development and optimization of next generation semiconductor technologies and electronic device components. Indium nitride (InN), gallium nitride (GaN) and aluminum nitride (AlN) compounds and their alloys are direct bandgap semiconductors with bandgap energies ranging from 0.7 to 6.0 eV, facilitating their utilization in optoelectronics and photonics. The GaN-based blue light-emitting diodes (LEDs) have enabled efficient and energy saving lighting, for which the Nobel Prize in Physics 2014 was awarded. GaN and AlN have high critical electric fields, high saturation carrier velocities and high thermal conductivities, which make them promising candidates for replacing silicon (Si) in next-generation power devices. The polarization-induced two-dimensional electron gas (2DEG), formed at the interface of AlGaN and GaN has enabled GaN-based high electron mobility transistors (HEMTs). These devices are suitable for high-power (HP) switching, power amplification and high-frequency (HF) applications in the millimeter-wave range up to THz frequencies. As such, HEMTs are suitable for next-generation 5G and 6G communication systems, radars, satellites, and a plethora of other related applications.

    Despite the immense efforts in the field, several material related issues still hinder the full exploitation of the unique properties of GaN-based semiconductors in HF and HP electronic applications. These limitations and challenges are related among others to: i) poor efficiency of p-type doping in GaN, ii) lack of linearity in AlGaN/GaN HEMTs used in low-noise RF amplifiers and, iii) MOCVD growth related difficulties in achieving ultra-thin and high Alcontent AlGaN barrier layers with compositionally sharp Al profiles in AlGaN/GaN HEMTs for HF applications.

    In this PhD thesis, we address the abovementioned issues by exploiting the hot-wall MOCVD combined with extensive material characterization. Main results can be grouped as follows:

    i) state-of-art p-GaN with room-temperature free-hole concentrations in the low 1018 cm-3 range and mobilities of ~10 cm2/Vs has been developed via in-situ doping. A comprehensive understanding of the growth process and its limiting factors, as related to magnesium (Mg), hydrogen (H) and carbon (C) incorporation in GaN is established. Further improvement of p-type doping in as-grown GaN:Mg is achieved by using GaN/AlN/4H-SiC templates and/or by modifying the gas environment in the growth reactor through the introduction of high amounts of hydrogen (H2) in the process. Using advanced scanning transmission electron microscopy (STEM) in combination with electron energy loss spectroscopy (EELS) we have established an improved comprehensive model of the pyramidal inversion domain defects (PIDs) in relation to the ambient matrix. First experimental evidence that Mg is present at all interfaces between PID and matrix allows for more accurate evaluation of Mg segregated at the PID, necessary for understanding the main limiting factor for p-type conductivity in GaN against alternative compensating donor or passivation sources.

    ii) Compositionally graded AlGaN channel layers in AlGaN/(Al)GaN HEMTs with various types of compositional grading have been developed, and graded channel devices were compared with conventional AlGaN/GaN HEMT indicating improved linearity. The first large signal measurements in Europe of a graded channel AlGaN/GaN HEMT has been carried out demonstrating improved linearity figure of merit IM3 by 10 dB compared to conventional Fe-doped GaN buffer devices. These results are showing state-of-the-art performance and pave the way for novel highly linear GaN receivers.

    iii) Ultrathin (sub-10nm) and high Al-content (>50%) AlGaN barrier GaN HEMT structures have been developed with 2DEG carrier densities ~1.1×1013 cm-2 and mobilities ~1700 cm2/Vs. Advanced characterization with atomic precision involving STEM and energy dispersive X-ray spectroscopy (EDS), has allowed experimental determination of the Al profiles and has revealed deviations from the nominally intended structures. Such deviations are found also in different source materials including commercial HEMT epistructures grown by MOCVD. The implications of the Al-profile deviations are critically analyzed in terms of 2DEG properties and device fabrication and performance. The capabilities and the limitations of MOCVD processes, related to growth of compositionally sharp and ultrathin high-Al-content AlGaN layers on GaN have been evaluated and their prospects in HF have been assessed.

    List of papers
    1. Mg-doping and free-hole properties of hot-wall MOCVD GaN
    Open this publication in new window or tab >>Mg-doping and free-hole properties of hot-wall MOCVD GaN
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    2022 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 131, no 18, article id 185704Article in journal (Refereed) Published
    Abstract [en]

    The hot-wall metal-organic chemical vapor deposition (MOCVD), previously shown to enable superior III-nitride material quality and high performance devices, has been explored for Mg doping of GaN. We have investigated the Mg incorporation in a wide doping range ( 2.45 x 10( 18) cm(-3) up to 1.10 x 10(20) cm(-3)) and demonstrate GaN:Mg with low background impurity concentrations under optimized growth conditions. Dopant and impurity levels are discussed in view of Ga supersaturation, which provides a unified concept to explain the complexity of growth conditions impact on Mg acceptor incorporation and compensation. The results are analyzed in relation to the extended defects, revealed by scanning transmission electron microscopy, x-ray diffraction, and surface morphology, and in correlation with the electrical properties obtained by Hall effect and capacitance-voltage (C-V) measurements. This allows to establish a comprehensive picture of GaN:Mg growth by hot-wall MOCVD providing guidance for growth parameters optimization depending on the targeted application. We show that substantially lower H concentration as compared to Mg acceptors can be achieved in GaN:Mg without any in situ or post-growth annealing resulting in p-type conductivity in as-grown material. State-of-the-art p-GaN layers with a low resistivity and a high free-hole density (0.77 omega cm and 8.4 x 10( 17) cm(-3), respectively) are obtained after post-growth annealing demonstrating the viability of hot-wall MOCVD for growth of power electronic device structures. (C)2022 Author(s).

    Place, publisher, year, edition, pages
    AIP Publishing, 2022
    National Category
    Materials Chemistry
    Identifiers
    urn:nbn:se:liu:diva-185485 (URN)10.1063/5.0089406 (DOI)000796003000002 ()
    Note

    Funding Agencies|Swedish Governmental Agency for Innovation Systems (VINNOVA) under the Competence Center Program [2016-05190]; Linkoping University; Chalmers University of Technology; Ericsson; Epiluvac; FMV; Gotmic; Hexagem; Hitachi Energy; On Semiconductor; Saab; SweGaN; UMS; Swedish Research Council VR [2016-00889]; Swedish Foundation for Strategic Research [RIF14-055, RIF14-0074, EM16-0024]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University [2009-00971]; KAW Foundation

    Available from: 2022-06-01 Created: 2022-06-01 Last updated: 2024-03-01
    2. Incorporation of Magnesium into GaN Regulated by Intentionally Large Amounts of Hydrogen during Growth by MOCVD
    Open this publication in new window or tab >>Incorporation of Magnesium into GaN Regulated by Intentionally Large Amounts of Hydrogen during Growth by MOCVD
    2022 (English)In: Physica status solidi. B, Basic research, ISSN 0370-1972, E-ISSN 1521-3951, Vol. 259, no 10, article id 2200137Article in journal (Refereed) Published
    Abstract [en]

    Herein, metal-organic chemical vapor deposition (MOCVD) of GaN layers doped with Mg atoms to the recognized optimum level of [Mg] approximate to 2 x 10(19) cm(-3) is performed. In a sequence of MOCVD runs, operational conditions, including temperature and flow rate of precursors, are maintained except for intentionally larger flows of hydrogen carrier gas fed into the reactor. By employing the largest hydrogen flow of 25 slm in this study, the performance of the as-grown Mg-doped GaN layers is certified by a room-temperature hole concentration of p approximate to 2 x 10(17) cm(-3) in the absence of any thermal activation treatment. Experimental evidence is delivered that the large amounts of hydrogen during the MOCVD growth can regulate the incorporation of the Mg atoms into GaN in a significant way so that MgH complex can coexist with a dominant and evidently electrically active isolated Mg-Ga acceptor.

    Place, publisher, year, edition, pages
    Wiley-V C H Verlag GMBH, 2022
    Keywords
    gallium nitride; hydrogen; metal-organic chemical vapor deposition; p-type doping
    National Category
    Condensed Matter Physics
    Identifiers
    urn:nbn:se:liu:diva-186495 (URN)10.1002/pssb.202200137 (DOI)000811116800001 ()
    Note

    Funding Agencies|Swedish Governmental Agency for Innovation Systems (VINNOVA) under the Competence Center Program [2016-05190]; Linkoping University; Chalmers University of technology; Ericsson; Epiluvac; FMV; Gotmic; Hexagem; Hitachi Energy; On Semiconductor; Saab; SweGaN; UMS; Swedish Research Council VR [2016-00889]; Swedish Foundation for Strategic Research [RIF14-055, EM16-0024]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University, Faculty Grant SFO Mat LiU [2009-00971]

    Available from: 2022-06-28 Created: 2022-06-28 Last updated: 2024-03-01Bibliographically approved
    3. Mg segregation at inclined facets of pyramidal inversion domains in GaN:Mg
    Open this publication in new window or tab >>Mg segregation at inclined facets of pyramidal inversion domains in GaN:Mg
    2022 (English)In: Scientific Reports, E-ISSN 2045-2322, Vol. 12, no 1, article id 17987Article in journal (Refereed) Published
    Abstract [en]

    Structural defects in Mg-doped GaN were analyzed using high-resolution scanning transmission electron microscopy combined with electron energy loss spectroscopy. The defects, in the shape of inverted pyramids, appear at high concentrations of incorporated Mg, which also lead to a reduction in free-hole concentration in Mg doped GaN. Detailed analysis pinpoints the arrangement of atoms in and around the defects and verify the presence of a well-defined layer of Mg at all facets, including the inclined facets. Our observations have resulted in a model of the pyramid-shaped defect, including structural displacements and compositional replacements, which is verified by image simulations. Finally, the total concentration of Mg atoms bound to these defects were evaluated, enabling a correlation between inactive and defect-bound dopants.

    Place, publisher, year, edition, pages
    Nature Portfolio, 2022
    National Category
    Condensed Matter Physics
    Identifiers
    urn:nbn:se:liu:diva-189936 (URN)10.1038/s41598-022-22622-1 (DOI)000874705000046 ()36289429 (PubMedID)
    Note

    Funding Agencies|Linkoping University

    Available from: 2022-11-15 Created: 2022-11-15 Last updated: 2023-12-28
    4. Tuning composition in graded AlGaN channel HEMTs toward improved linearity for low-noise radio-frequency amplifiers
    Open this publication in new window or tab >>Tuning composition in graded AlGaN channel HEMTs toward improved linearity for low-noise radio-frequency amplifiers
    Show others...
    2023 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 122, no 15, article id 153501Article in journal (Refereed) Published
    Abstract [en]

    Compositionally graded channel AlGaN/GaN high electron mobility transistors (HEMTs) offer a promising route to improve device linearity, which is necessary for low-noise radio-frequency amplifiers. In this work, we demonstrate different grading profiles of a 10-nm-thick AlxGa1-xN channel from x = 0 to x = 0.1 using hot-wall metal-organic chemical vapor deposition (MOCVD). The growth process is developed by optimizing the channel grading and the channel-to-barrier transition. For this purpose, the Al-profiles and the interface sharpness, as determined from scanning transmission electron microscopy combined with energy-dispersive x-ray spectroscopy, are correlated with specific MOCVD process parameters. The results are linked to the channel properties (electron density, electron mobility, and sheet resistance) obtained by contactless Hall and terahertz optical Hall effect measurements coupled with simulations from solving self-consistently Poisson and Schrodinger equations. The impact of incorporating a thin AlN interlayer between the graded channel and the barrier layer on the HEMT properties is investigated and discussed. The optimized graded channel HEMT structure is found to have similarly high electron density (similar to 9 x 10(12) cm(-2)) as the non-graded conventional structure, though the mobility drops from similar to 2360 cm(2)/V s in the conventional to similar to 960 cm(2)/V s in the graded structure. The transconductance g(m) of the linearly graded channel HEMTs is shown to be flatter with smaller g(m) and g(m) as compared to the conventional non-graded channel HEMT implying improved device linearity. (c) 2023 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

    Place, publisher, year, edition, pages
    AIP Publishing, 2023
    National Category
    Condensed Matter Physics
    Identifiers
    urn:nbn:se:liu:diva-193705 (URN)10.1063/5.0141517 (DOI)000967612400009 ()
    Note

    Funding Agencies|Swedish Governmental Agency for Innovation Systems (VINNOVA); Lund University; Linkoeping University; Chalmers University of Technology [2022-03139]; Ericsson; Epiluvac; FMV; Gotmic; Hexagem; Hitachi Energy; On Semiconductor; Region Skane SAAB; SweGaN; Volvo Cars; UMS; Swedish Research Council VR; Swedish Foundation for Strategic Research; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoeping University; KAW Foundation [2016-00889, 2022-04812]; Swedish Research Council and the Foundation [RIF14-055, EM16-0024, STP19-0008]; [2009-00971]; [2021-00171]; [RIF21-0026]

    Available from: 2023-05-15 Created: 2023-05-15 Last updated: 2023-12-28
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  • 10. Order onlineBuy this publication >>
    Papamichail, Alexis
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    P-type and polarization doping of GaN in hot-wall MOCVD2022Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    The devolopment of group-III nitride semiconductor technology continues to expand rapidly over the last two decades. The indium nitride (InN), gallium nitride (GaN) and aluminum nitride (AlN) compounds and their alloys are direct bandgap semiconductors with a wide bandgap range, spanning from infrared(IR) to deep-ultraviolet (UV), enabling their utilization in optoelectronic industry. The GaN-based light-emitting diode (LED) is already the commercial solution for efficient and energy saving lighting. Additionally, the physical properties of these materials such as the high critical electric field, the high saturation carrier velocity and the high thermal conductivity, make them promising candidates for replacing silicon (Si), and other wide-bandgap semiconductors such as silicon carbide (SiC) in power devices. More importantly, the polarization-induced two-dimensional electron gas (2DEG), forming at the interfaces of these semiconductors, led to the fabrication of the GaN-based high electron mobility transistor (HEMT). This device is suitable for high power (HP) switching, power amplifiers and high frequency (HF) applications in the millimeter-wave range up to THz frequencies. As such, HEMTs are suitable for 5G communication systems, radars, satellites and a plethora of other related applications.

    Achieving the efficient GaN blue LED (Nobel Prize in Physics 2014), came as a result of (partially) solving several material issues of which, p-type GaN was of crucial importance. Since 1992, a lot of effort is being dedicated on the understanding and overcoming of the limitations hindering efficient p-type conductivity and low hole mobility in metal-organic chemical vapor deposition (MOCVD) grown p-GaN. The limitations arise from the fact that magnesium (Mg) is the only efficient p-type dopant for GaN so far and only a very small percentage ∼2% of the incorporated Mg is active at room temperature. More limitations come from its solubility in GaN and the crystal quality deterioration and formation of inversion domains (IDs) at high doping levels. Free-hole concentrations in the low 1018 cm-3 range with mobilities at ∼10 cm2V-1s-1 demonstrate the state-of-art in MOCVD grown p-GaN, still leaving a wide window for improvement. Another intensively investigated topic is related to the aluminum gallium nitride (AlGaN)/GaN HEMTs. High electron density and mobility of the 2DEG in the range of 1013 cm-2 and ∼2400 cm2V-1s-1 respectively, are reported. Interface engineering, addition of interlayers and backbarriers are only some of the modifications introduced at the basic AlGaN/GaN HEMT structure in order to achieve the aforementioned values. Nevertheless, fundamental phenomena can still be revealed by special characterization techniques and provide a deeper understanding on the causal factors of theHEMT’s macroscopic properties.

    The main research results presented in this licentiate thesis are organized in three papers:

    In paper I we perform an in-depth investigation of the Mg-doped GaN growth by hot-wall MOCVD. We strive for exploiting any possible advantages of the hot-wall MOCVD on the growth of high-quality p-GaN relevant for use in HP devices. Additionally, we aim to gain a comprehensive understanding of the growth process and its limiting factors. The effects of growth conditions on the Mg, hydrogen (H) and carbon (C) incorporation in GaN are approached from the gallium (Ga)-supersaturation point of view. Control of the bis(cyclopentadienyl) magnesium (Cp2Mg)/trimethylgallium(TMGa) ratio, the V/III ratio and the growth temperature, resulted in high quality p-GaN growth on AlN/4H-SiC templates, showing state-of-the-art electrical properties. In paper II, we manage to increase the free-hole concentrations in as-grown GaN:Mg in two different ways, either by growing the GaN:Mg layer on a GaN/AlN/4H-SiC template, or by modifying the gas environment of the growth. It is shown that using a GaN/AlN/4H-SiC template results in higher carrier concentration and large improvement of the as-grown p-GaN resistivity. More importantly, the high amount of hydrogen (H2) flow during GaN:Mg growth, results in higher amount of non-passivated Mg in the as-grown layers allowing for high free-hole concentration and significantly lower resistivity in the as-grown p-GaN. Paper III focuses on the effect of aluminum (Al)-content variation in the barrier layer of AlGaN/GaN HEMTs. The THz-optical Hall effect (OHE) measurements revealed a peak of the 2DEG mobility followed by a decrease above certain value of Al%. We correlate this effect with the electron effective mass (meff) variation and draw conclusions about the mobility limiting mechanisms. In the low-Al regime, the mobility decreases because of the increase in meff while, in the high-Al regime, the mobility is limited by the lower carrier scattering time.

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  • 11.
    Papamichail, Alexis
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Kakanakova-Gueorguieva, Anelia
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Sveinbjörnsson, Einar
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Univ Iceland, Iceland.
    Persson, Axel
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Hult, B.
    Chalmers Univ Technol, Sweden.
    Rorsman, N.
    Chalmers Univ Technol, Sweden.
    Stanishev, Vallery
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Le, Son Phuong
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Persson, Per O A
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Nawaz, M.
    Hitachi Energy, Sweden.
    Chen, Jr-Tai
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. SweGaN AB, Olaus Magnus vag 48A, SE-58330 Linkoping, Sweden.
    Paskov, Plamen
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Darakchieva, Vanya
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Lund Univ, Sweden.
    Mg-doping and free-hole properties of hot-wall MOCVD GaN2022In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 131, no 18, article id 185704Article in journal (Refereed)
    Abstract [en]

    The hot-wall metal-organic chemical vapor deposition (MOCVD), previously shown to enable superior III-nitride material quality and high performance devices, has been explored for Mg doping of GaN. We have investigated the Mg incorporation in a wide doping range ( 2.45 x 10( 18) cm(-3) up to 1.10 x 10(20) cm(-3)) and demonstrate GaN:Mg with low background impurity concentrations under optimized growth conditions. Dopant and impurity levels are discussed in view of Ga supersaturation, which provides a unified concept to explain the complexity of growth conditions impact on Mg acceptor incorporation and compensation. The results are analyzed in relation to the extended defects, revealed by scanning transmission electron microscopy, x-ray diffraction, and surface morphology, and in correlation with the electrical properties obtained by Hall effect and capacitance-voltage (C-V) measurements. This allows to establish a comprehensive picture of GaN:Mg growth by hot-wall MOCVD providing guidance for growth parameters optimization depending on the targeted application. We show that substantially lower H concentration as compared to Mg acceptors can be achieved in GaN:Mg without any in situ or post-growth annealing resulting in p-type conductivity in as-grown material. State-of-the-art p-GaN layers with a low resistivity and a high free-hole density (0.77 omega cm and 8.4 x 10( 17) cm(-3), respectively) are obtained after post-growth annealing demonstrating the viability of hot-wall MOCVD for growth of power electronic device structures. (C)2022 Author(s).

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  • 12.
    Papamichail, Alexis
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Persson, Axel
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Richter, Steffen
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Lund Univ, Sweden.
    Kuhne, Philipp
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Stanishev, Vallery
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Persson, Per O A
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Del Castillo, R. Ferrand-Drake
    Chalmers Univ Technol, Sweden.
    Thorsell, M.
    Chalmers Univ Technol, Sweden; Saab AB, Sweden.
    Hjelmgren, H.
    Chalmers Univ Technol, Sweden.
    Paskov, Plamen
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Rorsman, N.
    Chalmers Univ Technol, Sweden.
    Darakchieva, Vanya
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Lund Univ, Sweden.
    Tuning composition in graded AlGaN channel HEMTs toward improved linearity for low-noise radio-frequency amplifiers2023In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 122, no 15, article id 153501Article in journal (Refereed)
    Abstract [en]

    Compositionally graded channel AlGaN/GaN high electron mobility transistors (HEMTs) offer a promising route to improve device linearity, which is necessary for low-noise radio-frequency amplifiers. In this work, we demonstrate different grading profiles of a 10-nm-thick AlxGa1-xN channel from x = 0 to x = 0.1 using hot-wall metal-organic chemical vapor deposition (MOCVD). The growth process is developed by optimizing the channel grading and the channel-to-barrier transition. For this purpose, the Al-profiles and the interface sharpness, as determined from scanning transmission electron microscopy combined with energy-dispersive x-ray spectroscopy, are correlated with specific MOCVD process parameters. The results are linked to the channel properties (electron density, electron mobility, and sheet resistance) obtained by contactless Hall and terahertz optical Hall effect measurements coupled with simulations from solving self-consistently Poisson and Schrodinger equations. The impact of incorporating a thin AlN interlayer between the graded channel and the barrier layer on the HEMT properties is investigated and discussed. The optimized graded channel HEMT structure is found to have similarly high electron density (similar to 9 x 10(12) cm(-2)) as the non-graded conventional structure, though the mobility drops from similar to 2360 cm(2)/V s in the conventional to similar to 960 cm(2)/V s in the graded structure. The transconductance g(m) of the linearly graded channel HEMTs is shown to be flatter with smaller g(m) and g(m) as compared to the conventional non-graded channel HEMT implying improved device linearity. (c) 2023 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

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  • 13.
    Persson, Axel
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Papamichail, Alexis
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Darakchieva, Vanya
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Lund Univ, Sweden.
    Persson, Per O Å
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Mg segregation at inclined facets of pyramidal inversion domains in GaN:Mg2022In: Scientific Reports, E-ISSN 2045-2322, Vol. 12, no 1, article id 17987Article in journal (Refereed)
    Abstract [en]

    Structural defects in Mg-doped GaN were analyzed using high-resolution scanning transmission electron microscopy combined with electron energy loss spectroscopy. The defects, in the shape of inverted pyramids, appear at high concentrations of incorporated Mg, which also lead to a reduction in free-hole concentration in Mg doped GaN. Detailed analysis pinpoints the arrangement of atoms in and around the defects and verify the presence of a well-defined layer of Mg at all facets, including the inclined facets. Our observations have resulted in a model of the pyramid-shaped defect, including structural displacements and compositional replacements, which is verified by image simulations. Finally, the total concentration of Mg atoms bound to these defects were evaluated, enabling a correlation between inactive and defect-bound dopants.

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  • 14.
    Zhang, Hengfang
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Chen, Jr-Tai
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. SweGaN AB, Olaus Magnus vag, S-58330 Linkoping, Sweden.
    Papamichail, Alexis
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Persson, Ingemar
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Paskov, Plamen
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Darakchieva, Vanya
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Lund Univ, Sweden.
    High-quality N-polar GaN optimization by multi-step temperature growth process2023In: Journal of Crystal Growth, ISSN 0022-0248, E-ISSN 1873-5002, Vol. 603, article id 127002Article in journal (Refereed)
    Abstract [en]

    We report growth optimization of Nitrogen (N)-polar GaN epitaxial layers by hot-wall metal-organic vapor phase epitaxy on 4H-SiC (0001) with a misorientation angle of 4 degrees towards the [1120] direction. We find that when using a 2-step temperature process for the N-polar GaN growth, step bunching is persistent for a wide range of growth rates (7 nm/min to 49 nm/min) and V/III ratios (251 to 3774). This phenomenon is analyzed in terms of anisotropic step-flow growth and the Ehrlich-Schwoebel barrier, and their effects on the surface step height and step width. The N-polar GaN growth is further optimized by using 3-step and 4-step temperature processes and the layers are compared to those using the 2-step temperature process in terms of surface morphology and defect densities. It is shown that a significantly improved surface morphology with a root mean square of 1.4 nm and with low dislocation densities (screw dislocation density of 2.8 x 108 cm-2 and edge dislocation density of 1.3 x 109 cm-2) can be achieved for 4-step temperature process. The optimized growth conditions allow to overcome the step-bunching problem. The results are further discussed in view of Ga supersaturation and a general growth strategy for high-quality N-polar GaN growth is proposed.

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  • 15.
    Zhang, Hengfang
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Persson, Ingemar
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Chen, Jr-Tai
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Papamichail, Alexis
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Tran, Dat
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Persson, Per O A
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Paskov, Plamen P.
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Darakchieva, Vanya
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Lund Univ, Sweden.
    Polarity Control by Inversion Domain Suppression in N-Polar III-Nitride Heterostructures2023In: Crystal Growth & Design, ISSN 1528-7483, E-ISSN 1528-7505, Vol. 23, no 2, p. 1049-1056Article in journal (Refereed)
    Abstract [en]

    Nitrogen-polar III-nitride heterostructures offer advantages over metal-polar structures in high frequency and high power applications. However, polarity control in III-nitrides is difficult to achieve as a result of unintentional polarity inversion domains (IDs). Herein, we present a comprehensive structural investigation with both atomic detail and thermodynamic analysis of the polarity evolution in low-and high-temperature AlN layers on on-axis and 4 degrees off-axis carbon-face 4H-SiC (000 (1) over bar) grown by hot-wall metal organic chemical vapor deposition. A polarity control strategy has been developed by variation of thermodynamic Al supersaturation and substrate misorientation angle in order to achieve the desired growth mode and polarity. We demonstrate that IDs are completely suppressed for high-temperature AlN nucleation layers when a step-flow growth mode is achieved on the off-axis substrates. We employ this approach to demonstrate high quality N-polar epitaxial AlGaN/GaN/AlN heterostructures.

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  • 16.
    Zhang, Hengfang
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Persson, Ingemar
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Papamichail, Alexis
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Chen, Jr-Tai
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. SweGaN AB, Olaus Magnus Vag 48A, S-58330 Linkoping, Sweden.
    Persson, Per O A
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Paskov, Plamen
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Darakchieva, Vanya
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Lund Univ, Sweden.
    On the polarity determination and polarity inversion in nitrogen-polar group III-nitride layers grown on SiC2022In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 131, no 5, article id 055701Article in journal (Refereed)
    Abstract [en]

    We investigate the interfaces and polarity domains at the atomic scale in epitaxial AlN and GaN/AlN grown by hot-wall metal organic chemical vapor epitaxy on the carbon face of SiC. X-ray diffraction, potassium hydroxide (KOH) wet chemical etching, and scanning transmission electron microscopy combined provide an in-depth understanding of polarity evolution with the film thickness, which is crucial to optimize growth. The AlN grown in a 3D mode is found to exhibit N-polar pyramid-type structures at the AlN-SiC interface. However, a mixed N-polar and Al-polar region with Al-polarity domination along with inverted pyramid-type structures evolve with increasing film thickness. We identify inclined inversion domain boundaries and propose that incorporation of oxygen on the & lang;40-41 & rang; facets of the N-polar pyramids causes the polarity inversion. We find that mixed-polar AlN is common and easily etched and remains undetected by solely relying on KOH etching. Atomic scale electron microscopy is, therefore, needed to accurately determine the polarity. The polarity of GaN grown on mixed-polar AlN is further shown to undergo complex evolution with the film thickness, which is discussed in the light of growth mechanisms and polarity determination methods.

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