Showing papers by "Oliver Ambacher published in 2003"

Electronics and sensors based on pyroelectric AlGaN/GaN heterostructures – Part B: Sensor applications

Abstract: In the present article recent results concerning sensor applications of AlGaN layers and AlGaN/GaN heterostructures are summarized. The piezoresistive effect in piezoelectric AlGaN layers is investigated and the dependence of the piezoresistive gauge factor on the Al content is attributed to the influence of strain induced piezoelectric fields. An enhancement of this effect is observed in AlGaN/GaN heterostructures resulting in high longitudinal gauge factors. The response of gas sensitive Pt:GaN Schottky diodes to hydrogen and hydrogen containing gases is analyzed up to temperatures of 600 °C and employed to realize gas sensitive field effect transistors which are demonstrated to operate up to 400 °C. In addition, ion sensitive field effect transistors (ISFETs) have also been fabricated on the basis of AlGaN/GaN heterostructures. The GaN surface shows a high pH sensitivity which is attributed to the presence of a thin native metal oxide layer on the surface. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

AlN/diamond heterojunction diodes

Abstract: An aluminum nitride/diamond p–n heterojunction has been realized by plasma-induced molecular-beam epitaxy growth of AlN on (100) diamond. The epitaxial nature of this heterojunction has been confirmed by high-resolution x-ray diffraction. The silicon-doped AlN film (n-type) on the natural boron-doped (p-type) diamond substrate formed a heterobipolar diode with good rectifying properties and surprisingly efficient light emission in the spectral range from 2.7 to 4.8 eV under forward bias. Results concerning the structural, electrical, and optical characterization of the AlN/diamond heterojunction are reported in this letter.

Energy gap and optical properties of InxGa1–xN

Abstract: We present ab initio calculations of the electronic structure and the optical properties of In x Ga 1-x N. They are completed by studies of the strain influence on the alloys. The results are critically discussed in the light of recent experiments. We find an energy gap of InN < 1 eV and a nonparabolic absorption edge. The strong variation of the alloy gap with the In molar fraction is described by a composition-dependent bowing parameter. The tendency of spinodal decomposition is suppressed by biaxial strain. Its extent depends on the realization of strain accommodation.

Electronics and sensors based on pyroelectric AlGaN/GaN heterostructures

Abstract: Electronic transport in semiconductors that possess high internal spontaneous and piezoelectric polarization opens up a new field of pyroelectronics and pyrosensors. The pyroelectric character of group-III-nitrides with wurtzite crystal structure yields a novel degree of freedom in designing and tailoring devices for modern micro- and nanoelectronic applications. Furthermore, spontaneous and piezoelectric polarization induced surface and interface charges can be used to develop very sensitive but robust sensors for the detection of ions, gases and polar liquids. We present a review of both theoretical and experimental studies of spontaneous and piezoelectric polarization present in AlGaN/GaN heterostructures as well as the electronic transport properties of polarization induced two-dimensional electron gases which are formed at the AlGaN/GaN interface due to the difference in the total polarization of two adjacent III-nitride layers. We demonstrate that the two-dimensional electron gases (2DEGs) achieved without modulation doping are very suitable as channel of high electron mobility transistors optimally suited for high power and high frequency applications (PART A) as well as for various kinds of sensors which can be operated in harsh environments (PART B). (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Growth of a-plane InN on r-plane sapphire with a GaN buffer by molecular-beam epitaxy

Abstract: We report heteroepitaxial growth of InN on r-plane sapphire substrates with an AlN nucleation layer and GaN buffer using plasma-assisted molecular-beam epitaxy. The InN film was identified to be nonpolar (1120) a-plane which follows the a-plane GaN buffer. Optical absorption and photoluminescence measurements of this material show that InN has a fundamental band gap of about 0.7 eV, which is also seen for growth on c-plane sapphire. The room-temperature Hall mobility of undoped a-plane InN is around 250 cm2/V s with a carrier concentration around 6×1018 cm−3. We also studied the electrical properties of the a-plane InN as a function of film thickness. In contrast to c-plane InN grown on c-plane sapphire, we did not observe apparent improvement of electrical properties of a-plane InN by growing thicker films.

Freestanding GaN-substrates and devices

Abstract: Various physical aspects and potential applications of the laser-induced separation of GaN epilayers from their sapphire substrate are reviewed. The effect of short laser pulses on the thermal decomposition of GaN and possible applications of the laser-induced dissociation of GaN for fast etching of this material is discussed. Particular emphasis is placed on the defect-free delamination of large area GaN films with thicknesses ranging from 3 to 300 μm from sapphire substrates. The use of the resulting freestanding GaN films in device technology and homoepitaxy of III-nitrides are outlined. Specific examples are the flip-chip bonding of freestanding InGaN/GaN LEDs to a silicon submount and the production of pseudosubstrates for the homoepitaxy of high quality GaN epilayers. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Growth of cubic InN on r-plane sapphire

Abstract: InN has been grown directly on r-plane sapphire substrates by plasma-enhanced molecular-beam epitaxy. X-ray diffraction investigations have shown that the InN layers consist of a predominant zinc blende (cubic) structure along with a fraction of the wurtzite (hexagonal) phase which content increases with proceeding growth. The lattice constant for zinc blende InN was found to be a=4.986 A. For this unusual growth of a metastable cubic phase on a noncubic substrate an epitaxial relationship was proposed where the metastable zinc blende phase grows directly on the r-plane sapphire while the wurtzite phase arises as the special case of twinning in the cubic structure.

Magnetotransport properties of a polarization-doped three-dimensional electron slab in graded AlGaN

Abstract: Shubnikov--de-Haas oscillation is observed in a polarization-doped three-dimensional electron slab in a graded ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}$ semiconductor layer. The electron slab is generated by the technique of grading the polar semiconductor alloy with spatially changing polarization. Temperature-dependent oscillations allow us to extract an effective mass of ${m}^{*}{=0.21m}_{0}.$ The quantum scattering time measured $({\ensuremath{\tau}}_{q}=0.3\mathrm{ps})$ is close to the transport scattering time $({\ensuremath{\tau}}_{t}=0.34\mathrm{ps}),$ indicating the dominance of short-range scattering. Alloy scattering is determined to be the dominant mechanism-limiting mobility; this enables us to extract an alloy-scattering parameter of ${V}_{0}=1.8\mathrm{eV}$ for the ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}$ material system. Polarization-doping presents an exciting technique for creating electron slabs with widely tunable density and confinement for the study of dimensionality effects on charge transport and collective phenomena.

Spin resonance investigations of Mn 2 + in wurtzite GaN and AlN films

Abstract: High quality Mn-doped GaN and A1N films grown by molecular beam epitaxy have been investigated with X-band electron spin resonance (ESR). The observed resonance patterns are well described by the spin Hamiltonian for isolated 5 5 Mn 2 + centers with electronic spin S=5/2 and nuclear spin I=5/2. Isotropic g factors g =2.000 and hyperfine parameters A=-69 G×gμ B are observed both in GaN and A1N, while the fine-structure parameters vary from D G a N =-218 G×gμ B for strongly strained GaN films to D G a N =-236 G ×gμ B for almost relaxed GaN films, and to D A l N =-648 G×gμ B for relaxed AlN films. At intermediate orientations of the crystalline c axis with respect to the magnetic field, intermixing occurs between the nuclear spin eigenstates due to off-diagonal elements in the spin Hamiltonian, which strongly enhances the transition probabilities of usually forbidden ESR transitions with ‖Δm 1 ‖>0. This is confirmed experimentally as well as by numerical simulations. It is concluded that Mn 2 + impurities are present as isolated, paramagnetic centers at the investigated doping concentration of 10 2 0 cm - 3 , without any evidence for electrostatic or magnetic coupling to extended valence band states, which is a prerequisite of ferromagnetic exchange required for spintronic devices.

Spin resonance investigations of Mn2+ in wurtzite GaN and AIN films

Abstract: High quality Mn-doped GaN and A1N films grown by molecular beam epitaxy have been investigated with X-band electron spin resonance (ESR). The observed resonance patterns are well described by the spin Hamiltonian for isolated 5 5 Mn 2 + centers with electronic spin S=5/2 and nuclear spin I=5/2. Isotropic g factors g =2.000 and hyperfine parameters A=-69 G×gμ B are observed both in GaN and A1N, while the fine-structure parameters vary from D G a N =-218 G×gμ B for strongly strained GaN films to D G a N =-236 G ×gμ B for almost relaxed GaN films, and to D A l N =-648 G×gμ B for relaxed AlN films. At intermediate orientations of the crystalline c axis with respect to the magnetic field, intermixing occurs between the nuclear spin eigenstates due to off-diagonal elements in the spin Hamiltonian, which strongly enhances the transition probabilities of usually forbidden ESR transitions with ‖Δm 1 ‖>0. This is confirmed experimentally as well as by numerical simulations. It is concluded that Mn 2 + impurities are present as isolated, paramagnetic centers at the investigated doping concentration of 10 2 0 cm - 3 , without any evidence for electrostatic or magnetic coupling to extended valence band states, which is a prerequisite of ferromagnetic exchange required for spintronic devices.

Micro-Raman study of electronic properties of inversion domains in GaN-based lateral polarity heterostructures

Abstract: The electronic properties of inversion domains in a GaN-based lateral polarity heterostructure were investigated using micro-Raman spectroscopy. The piezoelectric polarization of each domain was calculated from strain determined via Raman scattering. The free carrier concentration and electron mobility were deduced from the longitudinal optical phonon–plasmon coupled mode. The electron concentration in the N-face domain was slightly higher than that in the Ga-face domain. It appears that during growth, a larger number of donor impurities may have been incorporated into the N-face domain than into the Ga-face domain.

Photoelectron emission microscopy observation of inversion domain boundaries of GaN-based lateral polarity heterostructures

Abstract: An intentionally grown GaN film with laterally patterned Ga- and N-face polarities is studied using in situ UV-photoelectron emission microscopy (PEEM). Before chemical vapor cleaning of the surface, the emission contrast between the Ga- and N-face polarities regions was not significant. However, after cleaning the emission contrast between the different polarity regions was enhanced such that the N-face regions exhibited increased emission over the Ga-face regions. The results indicate that the emission threshold of the N-face region is lower than that of the Ga face. Moreover, bright emission was detected from regions around the inversion domain boundaries of the lateral polarity heterostructure. The PEEM polarity contrast and intense emission from the inversion domain boundary regions are discussed in terms of the built-in lateral field and the surface band bending induced by the polarization bound surface charges.

Correlation between strain, optical and electrical properties of InN grown by MBE

Abstract: The evolution of structural, optical and electrical properties was investigated for thin InN layers on either GaN or AlN buffer layers. Up to a layer thickness around 1 μm a biaxial strain is present in the layers. On AlN buffers the InN layers relax faster than on GaN. Both the interface and the surface affect the electron carrier concentration essentially. Thus, only with layers exceeding a thickness of 1 μm electron densities below 1018 cm−3 can be achieved. Optical spectroscopy indicates a band gap energy below 0.7 eV, however, the position of the absorption edge can be shifted up to 0.5 eV in thin layers. The Moss–Burstein shift due to the high electron concentration has the major influence on these effects. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Growth and characterization of GaN:Mn epitaxial films

Abstract: The oxidation states of Mn in epitaxial GaN films grown by plasma induced molecular beam epitaxy were investigated by electron spin resonance (ESR), elastic recoil detection, superconducting quantum interference device magnetization, and photothermal deflection spectroscopy measurements. Comparison of the measured Mn2+ spin densities with the total Mn concentrations in GaN of about 1020 cm−3 indicates that the majority of Mn is present in the neutral Mn3+ acceptor state. In samples codoped with Si, electrons are transferred to the Mn acceptors, which is observed by a strong increase of the Mn2+ spin densities in ESR. Simultaneously, this charge transfer reduces the characteristic optical absorption features at 1.5 and above 1.8 eV, suggesting their assignment to a Mn3+-related gap state.

Exciton quenching in Pt/GaN Schottky diodes with Ga- and N-face polarity

Abstract: We observed a peculiar dependence of low-temperature electroreflectance spectra of Pt/GaN Schottky diodes with Ga- and N-face polarity on the bias voltage (so-called “rotation” spectra), indicating the quenching of discrete exciton states and the formation of an exciton dead layer (EDL) beneath the gate. Data analysis is carried out using the field-dependent dielectric function of GaN. It yields parameters of excitons for the zero-field limit and the depth of the EDL, as well as the surface band bending and the ionized impurity concentration.

Recent advances in GaN HEMT development

Abstract: GaN HEMTs show great potential for RF applications. This paper provides an overview on the current status of GaN HEMT technology. It discusses the relevant properties of the AlGaN/GaN material system. Describes the evolution of GaN HEMTs during the last 10 years, and highlights the state of the art performance of these transistors. A comparison with competing HEMT type is made and the prospects of commercial GaN HEMTs are discussed.

GaN polarity domains spatially resolved by x-ray standing wave microscopy

Abstract: The conventional x-ray standing wave (XSW) technique [1, 2] is a phase sensitive and element specific Fourier technique, which is commonly used to analyse single crystals on the millimetre to centimetre scale. Here, we present an advanced microprobe technique based on the XSW method demonstrating that structural analysis can be achieved with chemical sensitivity on a microscopic scale. We apply the XSW microscopy technique to study the crystallographic polarity from inversion domains in GaN-based lateral polarity heterostructures. We focus the x-ray beam by a refractive lens [3] into a micrometre slice and generate the XSW field by Bragg-reflection from the (0002) diffraction planes recording the GaKα fluorescence as a function of the incidence angle. In this first demonstration of microscopic polarity determination with x-rays, we analyse the reversion of polarity across a inversion domain boundary with a spatial resolution of 1.5 μm. The new micro-XSW technique will permit microscopic examinations of the crystalline structure of modern semiconductor devices with chemical sensitivity and structural resolution on the picometre scale.

Photoreflectance studiesof N‐ and Ga‐face AlGaN/GaN heterostructures confininga polarisation induced 2DEG

Abstract: Photoreflectance measurements have been carried out in order to determine the electric field strength F within the topmost layers of Ga-face polarity Al x Ga 1-x N/GaN and N-face polarity GaN/Al x Ga 1-x N/GaN heterostructures containing high-mobile polarisation induced 2DEGs. For both types of samples F decreased from 400 kV/cm at room temperature up to 200 kV/cm when cooling down the structure to T = 5K. Our results strongly emphasise the existence of surface donor and surface acceptor states of the Ga- and N-face samples, respectively. The temperature dependence of F is explained by the change of the piezoelectric and spontaneous polarization. From self-consistent conduction band calculations the bare surface potential was obtained.

The transition from 2D to 3D nanoclusters of silicon carbide on silicon

Abstract: We have evaluated the critical average distance between the nanoclusters of silicon carbide grown by molecular beam epitaxy on silicon and estimated the time of the transition from two-to three-dimensional growth.

Study of inversion domain pyramids formed during the GaN:Mg growth

Abstract: ThestudyofstructuraldefectsinducedbytheintroductionofMgduringthegrowthofMOCVDGaNispresented.Themagnesiumincorporationintothecrystalgrowthnotonlyinduceschangesinthestackingsequencefromhex-agonaltocubicstructures,butalsoinvertstheGaNpolarityfromGa-facetoN-face.Basedonthedifferentsurfacestructureandsurfacemigrationlengthofabsorbingprecursorsforeachpolaritytype(Ga-orN-face),the3DgrowthontopoftheN-facetriangulardefectisdescribed.TheN-facematerialischaracterizedbythreedanglingbondsofni-trogenthatpointuptowardthec-planesurface,enhancingthecrystalgrowthalongthec-axis. 2002ElsevierScienceLtd.Allrightsreserved. Keywords:Inversiondomain;Stackingfault;Polarity 1. IntroductionFurther progress towards advanced optoelectronicdevicesrequiresabetterunderstandingoftheimpurityincorporation, defect formation, and recombinationprocessesinGaNepilayers.Difficultieswithp-dopingofGaNhasslowedthedevelopmentofGaNtechnologyanddelayedtheapplicationofthismaterialinopto-electronic devices. Magnesium is the best candidateamong all possible dopants to obtain p-doping, butthermalannealingneedstobeappliedtodissociateMg–HcomplexesandactivatetheMgatoms[1].Despitetheseproblems,blueLEDshavereachedthecommercialmarket,andsomeCWlasersnowadayshavealifetimeofthousandsofhours[2].However,completeunder-standingofthebehaviorofthisdopantisstillnotsat-isfactory.Thelifetimeoftheseopticaldevicesiscriticallydeterminedbytypicaldefectssuchasstackingfaults,inversiondomain(ID)boundaries,threadingandmisfitdislocations,whichcanactastrapsandrecombinationcenters.Thisisprobablythereasonwhysofewresearchgroupshavebeenabletoproducelaserswithsufficientlylongworkinglifetimes.Inordertoinvestigatethefor-mationofsomestructuraldefectstypicalofMg-dopedGaN films, in this work we present the study ofMOCVDgrownGaNsampleswithMgdopingcon-centrationrangingfrom10

Influence of metal thickness to sensitivity of Pt/GaN Schottky diodes for gas sensing applications

Abstract: Hydrogen gas sensors based on Pt/GaN Schottky diode structures were fabricated and their responses to hydrogen were studied. These diodes were fabricated on Si doped GaN layer (Nd = 1×1017). Three sets of diodes were fabricated with 80 A, 240 A and 400 A of Pt for the Schottky contacts. The electronic performances of 0.25 × 0.25 mm devices were tested in up to 1 % H2 gas in synthetic air (79% N2, 21% O2) by volume. The devices were operated in constant current mode in a forward bias condition. The change in voltage was monitored with the diodes exposed to hydrogen and to dry air at varying temperatures. The responses increased as the thickness of the Schottky metal contact decreased at any given temperature up to 310 °C. The trend of increasing response with decreasing thickness was also observed in 0.5 × 0.5 mm and 1.0 × 1.0 mm size Schottky diodes. SEM studies of the microstructure showed that the thinner Pt devices had higher grain boundary densities. The increase in sensitivity with decreasing thickness points to the dissociation of molecular hydrogen on the surface, the diffusion of atomic hydrogen through the Pt grain boundaries and the adsorption of hydrogen to the surface as a possible mechanism of sensing of hydrogen by Schottky diodes.

High-Resolution XRD Investigations of the Strain Reduction in 3C-SiC Thin Films Grown on Si (111) Substrates

Abstract: In this work the biaxial stress of 3C-SiC thin films epitaxia lly grown on Si(111) substrates has been investigated by using x-ray diffraction methods. The influence of the resulting strain on the electrical properties of SiC/Si heterojunctions was an lyzed. Different methods to prepare the surface prior to the SiC deposition were compared: (i) ex situ carbonization, (ii) interface modification by deposition of Ge prior to epitaxial growth and (iii) annealing of the silicon surface. The x-ray measurements revealed the lowest strain in ex situ carbonized samples, showing a transition from tensile to compressive strain when off-axis substrates were used. The highest strain appeared in SiC layers grown on a thin Ge intermediate layer whi ch was deposited prior to SiC growth without an additional annealing step of the substrate. The strai n in the SiC layer is directly correlated with the reverse current through the heterojunction. Introduction Epitaxially grown mismatched semiconductor heterostructures are of increasing importance for microand optoelectronic devices or circuits. Lattice mismatched layers can be elastically strained by pseudomorphic growth on the substrate. Alternatively the strain can be relieved by relaxation of the epilayer due to formation of misfit dislocations resulting in a n in-plane lattice parameter of the epitaxial film close to that of the bulk material. If epitaxia l l yers of 3C-SiC are grown on Si substrates the large mismatch in the lattice constants and the thermal expansion coefficients lead to a substantial residual tensile strain. A significant part of the 20% mismatch in lattice constants can be released by the formation of a dislocation network. However, the mis match in thermal expansion coefficients of SiC and Si introduces an additional strain into the s yst m during the cooling down process after growth. This strain results in a strong degradation of the layer properties and a wafer warpage, limiting the use of SiC/Si hetrostructures for device a pplications and as pseudo substrate for the deposition of group III-nitrides. In this work we analyze the e ffect of different techniques to minimize the residual strain of the SiC layers and to improve the structural and electrical properties of the grown heterostructures. Experimental The 3C-SiC thin films (thickness ~120 nm) were grown by solid-source mol cular-beam epitaxy (MBE) on (111)-oriented onand off-axis Si crystal wafers at a substrate temperature of 1000°C with a growth rate around 1 nm/min. Prior to epitaxial growth the Si( 111) substrates were prepared by different methods. The first method uses an ex vacuo carbonization process at 1280°C in a propane-hydrogen atmosphere inside a rapid thermal processing (RTP) sy stem [1]. The MBE Materials Science Forum Online: 2003-09-15 ISSN: 1662-9752, Vols. 433-436, pp 233-236 doi:10.4028/www.scientific.net/MSF.433-436.233 © 2003 Trans Tech Publications Ltd, Switzerland All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications Ltd, www.scientific.net. (Semanticscholar.org-12/03/20,10:56:45) preparation starts with a low-energy hydrogen plasma treatment [ 2] resulting in a hydrogenated surface with a rms roughness below 1 nm. After an annealing step at 782 °C it transforms into a smooth surface with a (7x7) reconstruction. Carbon was deposited at 325 °C f ollowed by a gradual increasing of the substrate temperature up to the final SiC growt h temperature [3]. In an alternative method, up to 1 monolayer germanium was deposited on the clean silicon surf ace prior to the start of the MBE conversion process [3]. High-resolution x-ray diffraction measurements and reciprocal spac e m pping were performed in a Philips high-resolution diffractometer with Cu Kα1 radiation and a spotsize of 0.8 x 0.8 mm . The sample was positioned in an Eulerian – type cradle, where position optim ization of the scattering vector was facilitated by independent variation of the incident angle (ω), the diffraction angle (2 θ), the angles of rotation around the surface normal ( φ), and around an in-plane horizontal direction (ψ). Particularly for the angles ω und 2θ, the position precision of the goniometer was 2.5 x 10 -4 and 5 x 10 degrees, respectively. The reciprocal space map consist of a se ries of 2θ-ω scans performed at different values of ω around the (111) and (224) SiC reflections. As there is a large lat tice mismatch between the SiC epilayer and the Si substrate (Fig.1), the absolute measurement of lattice parameters (residual strain) by detecting symmetric as well as asym metric reflections is necessary. Results and discussion Typical reciprocal space maps of the (111) and (224) SiC reflections are hown in Fig. 2 and 3. The results are summarized in Table 1. All the SiC layers grown on MBE prepared substrates have a tensile stress, however, the value is strongly influenced by the prepa ration technique. Deposition of one ML Ge prior to the SiC deposition can decrease the residual str ain in the grown SiC layers. However, the preparation of the surface prior to the Ge deposition is e ssential for such an improvement. After the deposition of Ge on a non-annealed Si surface, whic h appears hydrogen terminated and has a roughness of a few monolayers, the residual strain in the SiC lay r is increased. This might be a favourable situation for the application of the thin Si C layers as templates for the subsequent growth of group III-nitrides. The residual strain in the SiC layers can be minimized by an ex situ RTP carbonization. On offaxis substrates even a transition from the usually observed tensile str a n to compressive strain could be achieved. A similar behaviour of SiC on Si layers was previously observed after RTP carbonization [4]. In our case this change in the strain might be due t o a combination of an additional interface defect creation due to the the second annealing of the substrates in the MBE and an interface arrangement between SiC and Si depending on the ff-angle. On the off-axis substrates the SiC crystallites are growing already slightly tilted to the substrate reducing the mismatch between layer and substrate (Fig. 2a) while on on-axis substrates the (111) plane of the layer grows parallel to the (111) plane of the substrate (Fig. 2b). The two “extreme” cases are shown in Fig. 3. Fig.1: Vertical section through the reciprocal latt ices of the(111) – oriented Si substrate and the 3C -SiC (111) epilayer. SiC (224) SiC (115)

Charge Transfer at the Mn Acceptor Level in GaN

Abstract: MBE-grown GaN : Mn layers with Mn doping concentrations around 1020 cm−3 were investigated by photoconductivity measurements. From electron spin resonance (ESR), Mn is known to be mostly present in the neutral Mn3+ or Mn2+ + h+ state, which leads to a reassignment of the known optical absorption features to charge transfer from Mn3+, either by direct photoionization at about 1.8 eV or by a photothermal ionization process via an excited state (Mn3+)* at 1.42 V higher internal energy than the Mn3+ ground state. It is proposed that the Mn3+/Mn2+ acceptor level is located about 1.8 eV above the valence band edge of GaN so that the nature of the acceptor wavefunction is very different from an effective-mass-like state such as the Mn2+ + h+ complex in GaAs : Mn. According to these experimental results, the realization of carrier-mediated ferromagnetism becomes rather unlikely in not co-doped GaN : Mn.

AUGER DEPTH PROFILING AND FACTOR ANALYSIS OF SPUTTER INDUCED ALTERED LAYERS IN SiC

Abstract: The thickness of the altered layer created by ion bombardment of the 6H silicon carbide single crystal was determined by means of Auger depth proling combined with factor analysis. After pre-bombardment of the surface until the steady state by argon ions with energies 1, 2 and 4 keV, the micro proles of the altered layers were recorded by sputtering with low energy argon ions of 300 eV. As the position and shape of the carbon Auger signal depend on the perfection of the crystalline structure, they were used for depth prole evaluation by factor analysis. In this way the depth proles of the damaged surface region could be estimated in dependence on the ion energy. The thickness of the altered layer of SiC bombarded with keV Ar ions using an incident angle of 80 was obtained. K e y w o r d s: sputtering, altered layer, AES, depth proling, implantation, silicon carbide, factor analysis Various steps in device technology as well as a number of surface sensitive analytical techniques often employ sputtering of the surfaces under study by ions. A surface sensitive analytical technique like Auger electron spectroscopy (AES) in combination with low energy ion sputtering is a valuable tool for high resolution depth proling. However, sputtering of the surface by energy ions often brings about changes in morphology, structure an composition of the subsurface region, giving rise to an altered layer with characteristics dieren t from those of the bulk. The thickness of the altered layer and its properties depend on the ion beam parameters (energy, ion species, angle of incidence) as well as on the investigated material itself. AES is a well suited method for investigations of the sputter induced compositional micro proles close to the surface [1] and for thickness determination of the altered layer. One of the methods how to investigate the properties of such altered layers is sputter depth proling of the altered layer under conditions of very shallow ion damage, ie, with a grazing incidence angle and/or with a use of low energy ions (some hundreds of eV) [2]. The present paper reports on investigations of the altered layers of SiC single crystal surfaces due to ion bombardment. Compositional changes at the silicon carbide surface caused by low energy ion bombardment are not well understood up to date. Publications with very different results can be found in the literature reporting on preferential sputtering and accompanying surface compositional changes of the SiC surface. Surface enrichment in silicon [3, 4, 5], restored stoichiometry [6, 7] and also carbon enriched surfaces [8, 9] after sputtering with argon were reported. These discrepancies strongly motivate evaluation of the measured data in order to get a detailed picture of the silicon carbide surface after sputtering.

Sputtering of SiC with low energy He and Ar ions under grazing incidence

Abstract: The effect of low energy sputtering under grazing incidence upon the surface composition of SiC was investigated by Auger electron spectroscopy. The energy of the sputtering projectiles (He, Ar) varied from 200 to 1500 eV. Peak shifts to the higher energies with increasing argon ion energy were observed for all silicon and carbon Auger transitions. These shifts were explained by enhanced damage of the surface region within the sampling depth of the Auger electrons. The insensitivity of the Auger peak position to the energy of helium ions indicates that the damage state in the surface region does not change with the increasing energy of helium ions. An increase of the carbon concentration with the decrease of the argon energy was observed. The experiments were accompanied by dynamic Monte Carlo simulations by the TRIDYN code.

Method of producing a light-emitting diode

Abstract: A method of separating two layers of material from one another in such a way that the two separated layers of material are essentially fully preserved. An interface between the two layers of material at which the layers of material are to be separated, or a region in the vicinity of the interface, is exposed to electromagnetic radiation through one of the two layers of material. The electromagnetic radiation is absorbed at the interface or in the region in the vicinity of the interface and the absorbed radiation energy induces a decomposition of material at the interface.