| 1. |
- Darakchieva, Vanya, et al.
(författare)
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Electron accumulation at nonpolar and semipolar surfaces of wurtzite InN from generalized infrared ellipsometry
- 2009
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Ingår i: APPLIED PHYSICS LETTERS. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 95:20, s. 202103-
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Tidskriftsartikel (refereegranskat)abstract
- The free electron properties of nonpolar (1120)-oriented and semipolar (1011)-oriented wurtzite InN films are studied by generalized infrared ellipsometry (GIRSE). We demonstrate the sensitivity of GIRSE to the surface charge accumulation layer and find a distinct surface electron accumulation to occur at all surfaces. The obtained surface electron sheet densities are found to vary from 0.9x10(13) to 2.3x10(14) cm(-2) depending on the surface orientation and bulk electron concentration. The upper limits of the surface electron mobility parameters of 417-644 cm(2)/V s are determined and discussed in the light of electron confinement at the surface.
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| 2. |
- Darakchieva, Vanya, et al.
(författare)
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Structural anisotropy of nonpolar and semipolar InN epitaxial layers
- 2010
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Ingår i: JOURNAL OF APPLIED PHYSICS. - : American Institute of Physics. - 0021-8979 .- 1089-7550. ; 108:7, s. 073529-
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Tidskriftsartikel (refereegranskat)abstract
- We present a detailed study of the structural characteristics of molecular beam epitaxy grown nonpolar InN films with a- and m-plane surface orientations on r-plane sapphire and (100) gamma-LiAlO2, respectively, and semipolar (10 (1) over bar1) InN grown on r-plane sapphire. The on-axis rocking curve (RC) widths were found to exhibit anisotropic dependence on the azimuth angle with minima at InN [0001] for the a-plane films, and maxima at InN [0001] for the m-plane and semipolar films. The different contributions to the RC broadening are analyzed and discussed. The finite size of the crystallites and extended defects are suggested to be the dominant factors determining the RC anisotropy in a-plane InN, while surface roughness and curvature could not play a major role. Furthermore, strategy to reduce the anisotropy and magnitude of the tilt and minimize defect densities in a-plane InN films is suggested. In contrast to the nonpolar films, the semipolar InN was found to contain two domains nucleating on zinc-blende InN(111) A and InN(111) B faces. These two wurtzite domains develop with different growth rates, which was suggested to be a consequence of their different polarity. Both, a- and m-plane InN films have basal stacking fault densities similar or even lower compared to nonpolar InN grown on free-standing GaN substrates, indicating good prospects of heteroepitaxy on foreign substrates for the growth of InN-based devices.
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| 3. |
- Kasic, A., et al.
(författare)
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Phonons and free-carrier properties of binary, ternary, and quaternary group-III nitride layers measured by infrared Spectroscopic Ellipsometry
- 2003
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Ingår i: Physica Status Solidi. C, Current topics in solid state physics. - : Wiley. - 1610-1634 .- 1610-1642. ; 0:6 SPEC. ISS., s. 1750-1769
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- This work reviews recent ellipsometric investigations of the infrared dielectric functions of binary, ternary, and quaternary group-III nitride films. Spectroscopic Ellipsometry in the mid-infrared range is employed for the first time to determine phonon and free-carrier properties of individual group-III nitride heterostructure components, including layers of some ten nanometer thickness. Assuming the effective carrier mass, the free-carrier concentration and mobility parameters can be quantified upon model analysis of the infrared dielectric function. In combination with Hall-effect measurements, the effective carrier masses for wurtzite n- and p-type GaN and n-type InN are obtained. The mode behavior of both the E1(TO) and A1(LO) phonons are determined for ternary compounds. For strain-sensitive phonon modes, the composition and strain dependences of the phonon frequencies are differentiated and quantified. Information on the crystal quality and compositional homogeneity of the films can be extracted from the phonon mode broadening parameters. A comprehensive IR dielectric function database of group-III nitride materials has been established and can be used for the analysis of complex thin-film heterostructures designed for optoelectronic device applications. Information on concentration and mobility of free carriers, thickness, alloy composition, average strain state, and crystal quality of individual sample constituents can be derived. © 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
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| 4. |
- Darakchieva, Vanya, et al.
(författare)
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Hydrogen in InN: A ubiquitous phenomenon in molecular beam epitaxy grown material
- 2010
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Ingår i: APPLIED PHYSICS LETTERS. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 96:8, s. 081907-
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Tidskriftsartikel (refereegranskat)abstract
- We study the unintentional H impurities in relation to the free electron properties of state-of-the-art InN films grown by molecular beam epitaxy (MBE). Enhanced concentrations of H are revealed in the near surface regions of the films, indicating postgrowth surface contamination by H. The near surface hydrogen could not be removed upon thermal annealing and may have significant implications for the surface and bulk free electron properties of InN. The bulk free electron concentrations were found to scale with the bulk H concentrations while no distinct correlation with dislocation density could be inferred, indicating a major role of hydrogen for the unintentional conductivity in MBE InN.
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| 5. |
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| 6. |
- Darakchieva, Vanya, et al.
(författare)
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Unintentional incorporation of H and related structural and free-electron properties of c- and a-plane InN
- 2012
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Ingår i: PHYSICA STATUS SOLIDI A-APPLICATIONS AND MATERIALS SCIENCE. - : Wiley-VCH Verlag Berlin. - 1862-6300. ; 209:1, s. 91-94
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Tidskriftsartikel (refereegranskat)abstract
- In this work, we present a comprehensive study on the hydrogen impurity depth profiles in InN films with polar c-plane and nonpolar a-plane surface orientations in relation to their structural and free-electron properties. We find that the as-grown nonpolar films exhibit generally higher bulk and near-surface H concentrations compared to the polar InN counter-parts. The latter may be partly associated with a change in the growth mode from 2D to 3D and a decrease in the grain size. Thermal annealing leads to a reduction of H concentrations and the intrinsic H levels are influenced by the structural characteristics of the films. The factors allowing reduction of bulk H and free electron concentrations in a-plane films are discussed.
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| 7. |
- Darakchieva, Vanya, et al.
(författare)
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Unintentional incorporation of hydrogen in wurtzite InN with different surface orientations
- 2011
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Ingår i: Journal of Applied Physics. - : American Institute of Physics (AIP). - 0021-8979 .- 1089-7550. ; 110:6, s. 063535-
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Tidskriftsartikel (refereegranskat)abstract
- We have studied hydrogen impurities and related structural properties in state-of-the-art wurtzite InN films with polar, nonpolar, and semipolar surface orientations. The effects of thermal annealing and chemical treatment on the incorporation and stability of H are also discussed. The near-surface and bulk hydrogen concentrations in the as-grown films increase when changing the surface orientation from (0001) to (000 (1) over bar) to (1 (1) over bar 01) and to (11 (2) over bar0), which may be associated with a decrease in the grain size and change of the growth mode from 2D to 3D. Thermal annealing at 350 degrees C in N(2) leads to a reduction of H concentrations and the intrinsic levels of bulk H are found to correlate with the structural quality and defects in the annealed films.
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| 8. |
- Darakchieva, Vanya, et al.
(författare)
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Unravelling the free electron behavior in InN
- 2008
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Ingår i: Optoelectronic and Microelectronic Materials and Devices, 2008. - : IEEE. - 9781424427161 ; , s. 90-97
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Konferensbidrag (refereegranskat)abstract
- Precise measurement of the optical Hall effect in InN using magneto-optical generalized ellipsometry at IR and THz wavelengths, allows us to decouple the surface accumulation and bulk electron densities in InN films by non-contact optical means and further to precisely measure the effective mass and mobilities for polarizations parallel and perpendicular to the optical axis. Studies of InN films with different thicknesses, free electron densities and surface orientations enable an intricate picture of InN free electron properties to emerge. Striking findings on the scaling factors of the bulk electron densities with film thickness further supported by transmission electron microscopy point to an additional thickness dependent doping mechanism unrelated to dislocations. Surface electron accumulation is observed to occur not only at polar but also at non-polar and semi-polar wurtzite InN, and zinc blende InN surfaces. The persistent surface electron density shows a complex behavior with bulk density and surface orientation. This behavior might be exploited for tuning the surface charge in InN.
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| 9. |
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| 10. |
- Lorenz, L, et al.
(författare)
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Hydrogen In Group‐III Nitrides: An Ion Beam Analysis Study
- 2011
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Ingår i: AIP Conference Proceedings, Volume 1336. - : AIP. - 9780735408913 ; , s. 310-313
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Konferensbidrag (refereegranskat)abstract
- The doping mechanisms of InN, a promising material for novel optoelectronic and electronic devices, are still not well understood. Unintentional hydrogen doping is one possibility that could explain the unintentional n‐type conductivity in high‐quality nominally undoped InN films. We measured a series of state‐of‐the‐art InN samples grown by molecular beam epitaxy with 2 MeV 4He‐ERDA and RBS, showing the presence of relatively high amounts of hydrogen not only at the surface, but also in a deeper layer. Strong depletion of hydrogen due to the analysing beam was observed and taken into account in the analysis. Here, we report on the details of the analysis and show how the results correlate with the free‐electron concentrations of the samples.
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| 11. |
- Schoehe, S., et al.
(författare)
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Free-charge carrier parameters of n-type, p-type and compensated InN:Mg determined by infrared spectroscopic ellipsometry
- 2014
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Ingår i: Thin Solid Films. - : Elsevier. - 0040-6090 .- 1879-2731. ; 571, s. 384-388
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Tidskriftsartikel (refereegranskat)abstract
- Infrared spectroscopic ellipsometry is applied to investigate the free-charge carrier properties of Mg-doped InN films. Two representative sets of In-polar InN grown by molecular beam epitaxy with Mg concentrations ranging from 1.2 x 10(17) cm(-3) to 8 x 10(20) cm(-3) are compared. P-type conductivity is indicated for the Mg concentration range of 1 x 10(18) cm(-3) to 9 x 10(19) cm(-3) from a systematic investigation of the longitudinal optical phonon plasmon broadening and the mobility parameter in dependence of the Mg concentration. A parameterized model that accounts for the phonon-plasmon coupling is applied to determine the free-charge carrier concentration and mobility parameters in the doped bulk InN layer as well as the GaN template and undoped InN buffer layer for each sample. The free-charge carrier properties in the second sample set are consistent with the results determined in a comprehensive analysis of the first sample set reported earlier [Schoche et al., J. Appl. Phys. 113, 013502 (2013)]. In the second set, two samples with Mg concentration of 2.3 x 10(20) cm(-3) are identified as compensated n-type InN with very low electron concentrations which are suitable for further investigation of intrinsic material properties that are typically governed by high electron concentrations even in undoped InN. The compensated n-type InN samples can be clearly distinguished from the p-type conductive material of similar plasma frequencies by strongly reduced phonon plasmon broadening.
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| 12. |
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| 13. |
- Xie, Mengyao, et al.
(författare)
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Effect of Mg doping on the structural and free-charge carrier properties of InN
- 2014
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 115:16, s. 163504-
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Tidskriftsartikel (refereegranskat)abstract
- We study the structural and free-charge carrier properties of two sets of InN films grown by molecular beam epitaxy doped with different Mg concentrations from 1x1018 cm-3 to 3.9x1021 cm-3. We determine the effect of Mg doping on surface morphology, lattice parameters, structural characteristics and carrier properties. We show that infrared spectroscopic ellipsometry can be used to evidence successful p-type doping in InN, which is an important issue in InN. High resolution X-ray diffraction, combined with atomic force microscopy measurements reveals a drastic decrease in structural quality of the film for Mg concentrations above 1020 cm-3, accompanied with a significant increase in surface roughness. In addition, a decrease of the c-lattice parameter and an increase of the a-lattice parameter are found with increasing Mg concentration. Different contributions to the strain are discussed and it is suggested that the incorporation of Mg leads to a change of growth mode and generation of tensile growth strain. At high Mg concentrations zinc-blende InN inclusions appear which are suggested to originate from higher densities of stacking faults. Infrared spectroscopic ellipsometry analysis shows a reduced LPP-coupling, manifested as a characteristic dip in the IRSE data, and qualitatively different broadening behavior for Mg concentrations between 1.1x1018 cm−3 and 2.9x1019 cm−3 indicate the existence of a p-type conducting bulk InN layer for these Mg concentrations.
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