In order to improve the crystal quality of GaN-based light emitting devices, photoluminescence (PL) characterization of below-gap states in plasma assisted MBE-grown GaN/AlGaN quantum well (QW) structures has been done by utilizing a below-gap excitation (BGE) light in addition to an above-gap excitation light. The decrease of the band-edge PL intensity clue to the addition of the BGE of 1.17 eV indicates the presence of an energy-matched below-gap state in the two-wavelength excited PL. In continuation to our previous efficiency improvement by applying modulation-doping to GaAs/AlGaAs QW's, we focused on several undoped and Si-doped GaN/AlGaN QW's. Experimental results showed that Si modulation-doping reduces the density of below-gap states in the QW region, hence it is promising for increasing internal quantum efficiency of GaN-based QW's.

This chapter contains sections titled: Introduction Growth of InN by Plasma-Assisted Molecular Beam Epitaxy Introduction InN PAMBE Growth Peculiarities Role of Different Nitrogen Species in PAMBE Maintenance of Stoichiometric Conditions During InN Growth by PAMBE Undoped InN Growth by PAMBE with Different Initial Stages Growth and Epilayer Morphology Interface with Sapphire, XRD Characterization and Hall Measurements Summary Growth of InN by Metalorganic Molecular Beam Epitaxy Introduction MOMBE as a Growth Technique for InN Growth Process Growth System Substrate Preparation Nitridation Nucleation Layer Growth Influence of Growth Parameters on Surface Morphology Influence of Growth Temperature Influence of V/III Ratio Dependence of Structural and Electrical Properties of InN Grown by MOMBE on V/III Ratio Raman Measurements XRD Measurements Hall Measurements Summary Metalorganic Vapor Phase Epitaxy of InN Introduction Experimental Surface Morphology and Growth Rate of MOVPE InN Electrical Properties of MOVPE InN Summary Physical Properties of Hexagonal InN Introduction Lattice Dynamics of Single-Crystalline InN Layers First-Order Raman Scattering Phonon Dispersion in InN Electronic Structure of Single-Crystalline InN Layers Characterization of Samples Absorption and Luminescence in InN Luminescence and Absorption of Crystals with High Electron Concentrations Temperature Dependence of the Luminescence Band Shape Concentration Dependence of PL Band and Absorption Coefficient Photoluminescence Excitation and Photomodulated Reflectance Spectra Optical Spectra of InxGa1–xN Layers Wide-Gap InN-based Samples Postgrowth Treatment of InN Samples Proton Irradiation Summary Conclusions Acknowledgments References

We present results of photoluminescence studies of the band gap of non-intentionally doped single-crystalline hexagonal InN layers and In-rich In(x)Ga(1-x)N alloy layers (0.36 < x < 1). The band gap of InN is found to be close to 0.7 eV. This is much smaller than the values of 1.8 eV to 2.1 eV cited in the current literature. A bowing parameter of b approximate to 2.5 eV allows one to reconcile our and the literature data for the band gap values of In(x)Ga(1-x)N alloys in the entire composition region.

Dielectric layers within III-nitride transistor technology can act either as passivation layers or as gate-dielectric layers. In this paper, we reflect on both issues and present novel approaches of dielectric schemes. In both cases, the elimination of surface traps or, more generally, of surface states is a key issue in obtaining improved device performance. As gate dielectrics, we introduced and investigated thermally and photoelectrochemically generated Al x Ga 2−x O 3 , SiO 2 , the combination of Al x Ga 2−x O 3 and SiO 2 (tandem-dielectric stack), and e-beam-deposited Al 2 O 3 . These dielectric layers serve simultaneously as a passivation layer. In addition, we introduced plasma-enhanced chemical-vapor deposition (PECVD)-deposited SiN x for passivation. The results highlight the importance of passivation and the introduction of gate dielectrics and emphasize the relationship between surface states and improved direct-current (DC) performance. Backed by additional measurements, we proposed a different gateleakage mechanism for heterostructure field-effect transistor (HFET) and metal-oxide semiconductor heterostructure field-effect transistor (MOSHFET) devices.

High quality InN films have been grown on (0001) sapphire substrates by metalorganic molecular beam epitaxy, characterized for crystal structure, chemical composition, and optical properties. Depth profiling indicated a high, increasing oxygen concentration profile towards the volume of the film. Photoluminescence revealed two different, coexisting features: a low energy transition around 0.8 eV, and a high energy feature, peaking near 2 eV. The former band originates from the direct transition in the near surface range. The latter, low intensity band results from the bulk region. Oxynitride formation has been accounted for a spatially varying optical band gap EG, determined to 0.61 eV for pure, defect free material. X-ray diffraction analysis supports defect mediated lattice instability, as proposed recently by a molecular dynamic lattice theory.

We present results of a detailed study of X-ray, photoluminescence, and Raman measurements of hexagonal InN, In-rich InxGa1-xN (0.36 < x < 1) alloys, and InN samples annealed in oxygen. (C) 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

The investigated InGaN ternary films were grown by MOMBE, with the In fraction varying from 6% to 100%. Using XRD and TEM, we determine the crystalline state, the nature of defects inside the layers and their distribution as related to the In concentration. The band gap of the alloys was studied through PL measurements, it is below 1 eV for InN.

Below-gap states in plasma assisted MBE-grown GaN/AlGaN quantum well (QW) structures have been studied by two-wavelength excited photoluminescence (PL). The decrease of the PL intensity with the addition of a below-gap excitation light source of 1.17 eV implies the presence of an energy-matched below-gap state. We have studied several QW structures grown on AlGaN or AlN buffer layers on sapphire substrates. We found improved internal quantum efficiency with increasing the number of QW's and with providing a sufficiently thick AlGaN or AlN buffer layer. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In the present work semiconductor quarter wavelength distributed Bragg reflector (DBR) mirrors have been studied by high resolution transmission electron microscopy (HRTEM). The mirrors have been fabricated monolithically by plasma assisted molecular beam epitaxy (MBE) on sapphire (0001) substrates. The samples are conformed of a large number of AlxGa1−xN/AlN layers with 5.5 and 20.5 periods, both with different aluminium concentration. The samples have been designed utilising spectroscopic ellipsometry (SE) dispersion spectra of previously fabricated single layers. The aim of this work was to determine the distortion of lattice parameters of AlxGa1−xN/AlN epilayers, since this is important for the later production of vertical cavity surface emitting lasers (VCSELs). Distortions of half periods layers were determined from HRTEM techniques and are compared with the distortion determination using an equilibrium theory and high resolution X-ray diffraction (XRD) measurements.

AlxGa1-xN/ AlyGa1-yN Distributed Bragg Reflectors (DBRs) with up to 45 periods have been grown on (0001) sapphire substrates by r.f. plasma-assisted molecular beam epitaxy (PAMBE) with the aid of two Al effusion cells. Several samples were grown with an Al mole fraction varying between 0.38 ≤ x ≤ 1 (0 ≤ y ≤ 0.4) at temperatures of up to 890 °C. In all samples, an AlxGa1-xN buffer layer was used to prevent cracking of the quarter wave stack and improving surface morphology by choosing the Al content so that strain energy in the DBR structure would be compensated. X-ray diffraction (XRD) and scanning electron microscopy (SEM) investigations were performed to determine the thickness of the quarter wave layer periods and the Al mole fraction of corresponding AlxGa1-xN single layers. Room-temperature calibrated reflection and transmission (R&T) measurements were performed. Thus stray and self-absorption of the DBRs were extracted from reflectance and transmittance. The thickness of the quarter wave layers was designed such that the measured peak reflectances appeared between 346 nm to 421 nm. The dispersion data, including refractive indices and absorption coefficients, used in the calculation were extracted from R&T measurements done on the above mentioned AlxGa1-xN single layers.

Wurtzite GaN samples containing one, three and five 4nm thick high temperature (HT) AlN Interlayers (IL) have been grown on (0001) sapphire substrates by plasma-assisted molecular beam epitaxy (PAMBE). N-polar as well as Ga-polar thin films have been characterized by x-ray diffraction (XRD), atomic force microscopy (AFM), transmission electron microscopy (TEM), and electrical measurements. All samples under consideration show excellent AFM rms surface roughness below 1nm. Previously, we published a reduction of the threading dislocation (TD) density by a factor of seven due to the introduction of one AlN-IL. When introducing multiple AlN-IL a reduction by a factor of 5.2 is achieved. Hall measurements show a rise in electron mobility due to possible 2DEG formation at the interface between GaN and the AlN-ILs. Significant growth mode differences between Ga-polar and N-polar samples result in drastically higher electron mobility values for N-polar material. For N-polar samples the exceptional mobility increase from 68 (no AlN-IL) to 707 cm2/Vs (one A1N-IL) as well as the extremely low intrinsic carrier density of 1 × 1017 cm−3 prove the applicability of AlN barriers in inverted FET devices.

physica status solidi (b)Volume 230, Issue 2 p. R4-R6 Rapid Research Note Band Gap of InN and In-Rich InxGa1—xN alloys (0.36 < x < 1) V.Yu. Davydov, V.Yu. Davydov [email protected] Ioffe Physico-Technical Institute, Russian Academy of Science, Polytekhnicheskaya 26, 194021 St. Petersburg, RussiaSearch for more papers by this authorA.A. Klochikhin, A.A. Klochikhin Ioffe Physico-Technical Institute, Russian Academy of Science, Polytekhnicheskaya 26, 194021 St. Petersburg, RussiaSearch for more papers by this authorV.V. Emtsev, V.V. Emtsev Ioffe Physico-Technical Institute, Russian Academy of Science, Polytekhnicheskaya 26, 194021 St. Petersburg, RussiaSearch for more papers by this authorS.V. Ivanov, S.V. Ivanov Ioffe Physico-Technical Institute, Russian Academy of Science, Polytekhnicheskaya 26, 194021 St. Petersburg, RussiaSearch for more papers by this authorV.V. Vekshin, V.V. Vekshin Ioffe Physico-Technical Institute, Russian Academy of Science, Polytekhnicheskaya 26, 194021 St. Petersburg, RussiaSearch for more papers by this authorF. Bechstedt, F. Bechstedt Institut für Festkörpertheorie and Theoretische Optik, Friedrich-Schiller-Universität Jena, Max-Wien-Platz 1, 07743 Jena, GermanySearch for more papers by this authorJ. Furthmüller, J. Furthmüller Institut für Festkörpertheorie and Theoretische Optik, Friedrich-Schiller-Universität Jena, Max-Wien-Platz 1, 07743 Jena, GermanySearch for more papers by this authorH. Harima, H. Harima Department of Electronics and Information Science, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, JapanSearch for more papers by this authorA.V. Mudryi, A.V. Mudryi Institute of Solid State and Semiconductor Physics, Belarus Academy of Sciences, Brovki 17, 220072 Minsk, BelarusSearch for more papers by this authorA. Hashimoto, A. Hashimoto Department of Electronics Engineering, Fukui University, Bunkyo, Fukui 910-8507, JapanSearch for more papers by this authorA. Yamamoto, A. Yamamoto Department of Electronics Engineering, Fukui University, Bunkyo, Fukui 910-8507, JapanSearch for more papers by this authorJ. Aderhold, J. Aderhold LfI, University of Hannover, Schneiderberg 32, 30167 Hannover, GermanySearch for more papers by this authorJ. Graul, J. Graul LfI, University of Hannover, Schneiderberg 32, 30167 Hannover, GermanySearch for more papers by this authorE.E. Haller, E.E. Haller University of California and Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USASearch for more papers by this author V.Yu. Davydov, V.Yu. Davydov [email protected] Ioffe Physico-Technical Institute, Russian Academy of Science, Polytekhnicheskaya 26, 194021 St. Petersburg, RussiaSearch for more papers by this authorA.A. Klochikhin, A.A. Klochikhin Ioffe Physico-Technical Institute, Russian Academy of Science, Polytekhnicheskaya 26, 194021 St. Petersburg, RussiaSearch for more papers by this authorV.V. Emtsev, V.V. Emtsev Ioffe Physico-Technical Institute, Russian Academy of Science, Polytekhnicheskaya 26, 194021 St. Petersburg, RussiaSearch for more papers by this authorS.V. Ivanov, S.V. Ivanov Ioffe Physico-Technical Institute, Russian Academy of Science, Polytekhnicheskaya 26, 194021 St. Petersburg, RussiaSearch for more papers by this authorV.V. Vekshin, V.V. Vekshin Ioffe Physico-Technical Institute, Russian Academy of Science, Polytekhnicheskaya 26, 194021 St. Petersburg, RussiaSearch for more papers by this authorF. Bechstedt, F. Bechstedt Institut für Festkörpertheorie and Theoretische Optik, Friedrich-Schiller-Universität Jena, Max-Wien-Platz 1, 07743 Jena, GermanySearch for more papers by this authorJ. Furthmüller, J. Furthmüller Institut für Festkörpertheorie and Theoretische Optik, Friedrich-Schiller-Universität Jena, Max-Wien-Platz 1, 07743 Jena, GermanySearch for more papers by this authorH. Harima, H. Harima Department of Electronics and Information Science, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, JapanSearch for more papers by this authorA.V. Mudryi, A.V. Mudryi Institute of Solid State and Semiconductor Physics, Belarus Academy of Sciences, Brovki 17, 220072 Minsk, BelarusSearch for more papers by this authorA. Hashimoto, A. Hashimoto Department of Electronics Engineering, Fukui University, Bunkyo, Fukui 910-8507, JapanSearch for more papers by this authorA. Yamamoto, A. Yamamoto Department of Electronics Engineering, Fukui University, Bunkyo, Fukui 910-8507, JapanSearch for more papers by this authorJ. Aderhold, J. Aderhold LfI, University of Hannover, Schneiderberg 32, 30167 Hannover, GermanySearch for more papers by this authorJ. Graul, J. Graul LfI, University of Hannover, Schneiderberg 32, 30167 Hannover, GermanySearch for more papers by this authorE.E. Haller, E.E. Haller University of California and Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USASearch for more papers by this author First published: 08 April 2002 https://doi.org/10.1002/1521-3951(200204)230:23.0.CO;2-ZCitations: 240AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onEmailFacebookTwitterLinkedInRedditWechat References [1] V.Yu. Davydov, A.A. Klochikhin et al., phys. stat. sol. (b) 229, R1 (2002). 10.1002/1521-3951(200202)229:33.0.CO;2-O CASWeb of Science®Google Scholar [2] T.L. Tansley and C.P. Foley, J. Appl. Phys. 59, 3241 (1986). 10.1063/1.336906 CASWeb of Science®Google Scholar [3] E. Kurimoto, H. Harima et al., phys. stat. sol. (b) 228, 1 (2001). 10.1002/1521-3951(200111)228:1<1::AID-PSSB1>3.0.CO;2-U CASWeb of Science®Google Scholar [4] J. Aderhold, V.Yu. Davydov, F. Fedler, H. Klausing et al., J. Cryst. Growth 222, 701 (2001). 10.1016/S0022-0248(00)00986-6 CASWeb of Science®Google Scholar [5] V.V. Mamutin, V.A. Vekshin, V.Yu. Davydov et al., phys. stat. sol. (a) 176, 247 (1999). 10.1002/(SICI)1521-396X(199911)176:1<247::AID-PSSA247>3.0.CO;2-I CASWeb of Science®Google Scholar [6] E. Burstein, Phys. Rev. 93, 632 (1954). 10.1103/PhysRev.93.632 CASWeb of Science®Google Scholar [7] A.P. Levanyuk and V.V. Osipov, Sov. Phys. – Usp. 24, 187 (1981) [Usp. Fiz. Nauk 133, 427 (1981)]. 10.1070/PU1981v024n03ABEH004770 Google Scholar [8] M.H. Kim, J.K. 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High reflectivity (>90%) distributed Bragg reflectors (DBR) have been successfully produced utilizing the AlGaN/AlN material system. We present reflectivity and XRD data of Ga-polar AlxGa1—xN/AlN Bragg reflectors grown on sapphire. High peak reflectivities between 54% (5.5 period mirror) and 97% (25.5 period mirror) combined with large reflectivity FWHM of 30 nm have been found. All reflectors have been designed by ex-situ spectroscopic ellipsometry (SE) data of respective reference samples.

Studies of first- and second-order Raman scattering in hexagonal ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}$ alloys are reported. The dependences of frequencies of all Raman-allowed optical phonons versus Al content are traced in detail in the entire composition range. The one-mode behavior of LO phonons and the two-mode behavior of the other phonons is established. It is shown that the composition dependences of ${A}_{1}(\mathrm{TO}),$ ${A}_{1}(\mathrm{LO}),$ ${E}_{1}(\mathrm{LO}),$ and ${E}_{2}(\mathrm{low})$ phonon energies are convenient tools for the quantitative characterization of the Al content in ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}$ alloys. The energy position of the ${B}_{1}(\mathrm{high})$ silent mode is proposed. A narrow gap separating the dispersion regions of transverse and longitudinal optical phonons is revealed in the phonon density-of-state function. The composition dependence of the phonon line broadening is investigated experimentally and theoretically. It is shown that the broadening is due to elastic phonon scattering by the composition fluctuations. A theoretical approach is used where the statistical and dynamical aspects of the phonon scattering are treated separately. The type, size, and number of the fluctuations responsible for the phonon line broadening are estimated. The theory is qualitatively consistent with the observed composition dependences.

We present the first GaN based MOSFET with wet chemical processed gate oxide. The oxide was grown photoelectrochemically (PEC) in KOH based aqueous solutions and was determined to be AlxGa2−xO3. This process offers low surface damage. The gate contact for our created PEC-MOSHFET (metal oxide semiconductor heterostructure field effect transistor) was fabricated by e-beam evaporation of tungsten on the AlxGa2−xO3 layer, followed by a lithographic step and wet etch by H2O2. Source and drain contacts were placed by the liftoff technique using Ti/Al. Peak values for the mutual conductance (gm) are 64 mSmm−1 for MOVPE (metalorganic vapour pressure epitaxy) structures with 2DEG mobility of 190 cm2V−1s−1. We achieve a maximum drain current IDmax of 540 mAmm−1 for the PEC-MOSHFET. The results obtained for transistor operation are compared to other gate dielectrics such as SiO2 with different pre-treatments and to a conventional HFET with a Ni/Au Schottky gate. Depletion starts at threshold voltages Vth of −4 V in the case of the PEC-MOSHFET, for the conventional HFET structure Vth is about −9 V and for the SiO2-MOSHFETs it varies between −11.5 and −14 V depending on the wet chemical pre-treatment. Leakage currents depend on device isolation and on gate currents, which are lowest for the SiO2-MOSHFETs (∼2 pA) and several orders of magnitude bigger for the HFET (∼4 μA). Gate currents for the PEC-MOSHFET depend on the oxide growth and vary between microamperes and a few picoamperes.

This work reports on the influence of the surface and the gate length on the performance of AlGaN/ GaN based Hetero Field Effect Transistors (HFETs). Differently NH4Sx treated surfaces result in variation of the drain current I-Dmax of more then 100%. Gate recessing by photoelectrochemical treatment changes the threshold voltage V-th but affects the drain current little. Next, the reduction of the gate length increases the I-Dmax further by more than 60%. The I-Dmax values for the transistors are 350 mA mm(-1) for the NH4Sx-treated, 850 mA for the untreated, and 1.43 A mm(-1) for the one with a 0.2 mum gate length. The corresponding transconductances g(m) are 66, 150, and 280 mS mm(-1), respectively. Surface analysis with Auger Electron Spectroscopy (AES) and contact characterization (TLM) reveals, that the NH4Sx treatment removes the native oxide and increases the contact resistance as well. Therefore we attribute the increase Of I-Dmax and g(m) mainly to a beneficial behavior of gallium-oxide at the surface on the sheet carrier density n(s) of the 2DEG at the heterointerface.

A survey of most recent studies of optical absorption, photoluminescence, photoluminescence excitation, and photomodulated reflectance spectra of single-crystalline hexagonal InN layers is presented. The samples studied were undoped n-type InN with electron concentrations between 6 x 10(18) and 4 x 10(19) cm(-3). It has been found that hexagonal InN is a narrow-gap semiconductor with a band gap of about 0.7 eV, which is much lower than the band gap cited in the literature. We also describe optical investigations of In-rich InxGa1-xN alloy layers (0.36 < x < 1) which have shown that the bowing parameter of b similar to 2.5 eV allows one to reconcile our results and the literature data for the band gap of InxGa1-xN alloys over the entire composition region. Special attention is paid to the effects of post-growth treatment of InN crystals. It is shown that annealing in vacuum leads to a decrease in electron concentration and considerable homogenization of the optical characteristics of InN samples, At the same time, annealing in an oxygen atmosphere leads to formation of optically transparent alloys of InN-In2O3 type, the band gap of which reaches approximately 2 eV at an oxygen concentration of about 20%. It is evident from photoluminescence spectra that the samples saturated partially by oxygen still contain fragments of InN of mesoscopic size.