Showing papers by "Oliver Ambacher published in 2000"

Two dimensional electron gases induced by spontaneous and piezoelectric polarization in undoped and doped AlGaN/GaN heterostructures

Abstract: Two dimensional electron gases in Al x Ga 12x N/GaN based heterostructures, suitable for high electron mobility transistors, are induced by strong polarization effects. The sheet carrier concentration and the confinement of the two dimensional electron gases located close to the AlGaN/GaN interface are sensitive to a large number of different physical properties such as polarity, alloy composition, strain, thickness, and doping of the AlGaN barrier. We have investigated these physical properties for undoped and silicon doped transistor structures by a combination of high resolution x-ray diffraction, atomic force microscopy, Hall effect, and capacitance‐voltage profiling measurements. The polarization induced sheet charge bound at the AlGaN/GaN interfaces was calculated from different sets of piezoelectric constants available in the literature. The sheet carrier concentration induced by polarization charges was determined

Influence of crystal polarity on the properties of Pt/GaN Schottky diodes

Abstract: Si-doped epitaxial GaN layers with Ga- and N-face polarity were grown by plasma-induced molecular-beam epitaxy (PIMBE) in order to characterize the influence of polarity on the electrical properties of Pt Schottky diodes. Different barrier heights for Pt onto these two materials are obtained from the dependence of the effective barrier height versus ideality factor, determined by I–V measurements to be 1.1 and 0.9 eV for Ga- and N-face GaN, respectively. C–V measurements confirm the greater barrier heights for Ga-face material. A possible explanation for this behavior can be a different band bending of the conduction and valence band, inferred from the self-consistent solution of the Schrodinger–Poisson equation, including polarization-induced surface and interface charges, which result from the different spontaneous polarization in epitaxial layers with different polarity.

Two-dimensional electron gases in Ga-face and N-face AlGaN/GaN heterostructures grown by plasma-induced molecular beam epitaxy and metalorganic chemical vapor deposition on sapphire

Abstract: We report on the growth of nominally undoped GaN/AlxGa1−xN/GaN (x<0.4) high mobility heterostructures with N-face or Ga-face polarity on sapphire substrates by plasma-induced molecular beam epitaxy (PIMBE) and metalorganic chemical vapor deposition in order to study the formation and electrical transport properties of polarization induced two-dimensional electron gases (2DEGs). By depositing a thin AlN nucleation layer on the sapphire substrates before the growth of a GaN buffer layer by PIMBE, we were able to change the polarity of the wurtzite films from N to Ga face. The switch in the polarity causes a change in the sign of the spontaneous and piezoelectric polarization directed along the c axis of the strained AlGaN barrier. As a consequence the polarization induced 2DEG is confined at different interfaces in heterostructures with different polarities. The transport properties of the 2DEGs in Ga- and N-face heterostructures were investigated by a combination of capacitance–voltage profiling, Hall effect, and Shubnikov-de Haas measurements. Dominant electron scattering mechanisms are studied in order to provide the knowledge necessary for further improvements of the electron transport properties and performance of AlGaN/GaN based “normal” (based on Ga-face heterostructures) and “inverted” (based on N-face heterostructures) high electron mobility transistors.

DX -behavior of Si in AlN

Abstract: In Si doped AlN, a large persistent photoconductivity is found for temperatures below 60 K after exposure to light with photon energies above 1.5 eV. Simultaneously, a persistent electron spin resonance signal is observed with an isotropic g factor of 1.9885 due to an effective mass donor state, while no spin resonance signal is detectable after cooling the sample in the dark. Both observations show that Si undergoes a $\mathrm{DX}$-like metastability in this material. Based on the experimental findings, a detailed configuration diagram is proposed.

GaN homoepitaxy by metalorganic chemical-vapor deposition on free-standing GaN substrates

Abstract: Lattice-matched GaN layers are grown by metalorganic chemical-vapor deposition on free-standing GaN substrates, which were fabricated by laser-induced liftoff of 300-μm-thick films grown by hydride vapor-phase epitaxy. Pretreatment of the free-standing films before the homoepitaxial growth of GaN involved mechanical polishing of the Ga-face surface and a final Cl-based plasma etch. By a combination of high-resolution x-ray diffraction, atomic-force microscopy, as well as Raman and photoluminescence spectroscopy, the structural and optical properties of the lattice-matched GaN layers are characterized. X-ray diffraction patterns of (0002), (0004), and (0006) reflexes with a full width at half maximum (FWHM) of as low as 20 arcsec are obtained. The dislocation density is determined to be 2×107 cm−2 and the surface morphology is dominated by bilayer steps with terraces of about 200 nm. The lattice mismatch between the GaN substrate and the homoepitaxial layer is below 3×10−5, resulting in a very narrow FWHM ...

High electron mobility AlGaN/GaN heterostructure on (111) Si

Abstract: Room-temperature Hall mobilities exceeding 900 cm2/V s are obtained for AlGaN/GaN heterostructures on (111) Si by single-temperature flow modulation organometallic vapor phase epitaxy. Thin pseudomorphic AlGaN top layers exhibit a 1.5 nm surface roughness and induces a two-dimensional electron gas sheet carrier concentration of 1.0×1013 cm−2. The GaN buffer layer has a background carrier concentration of 1.0×1015 cm−3, 130 arcsec x-ray diffraction full width at half maximum, and a low-temperature photoluminescence linewidth of 10 meV. An AlN nucleation layer provides static electrical isolation between the AlGaN/GaN and the conducting Si substrate. Large crack-free areas of high-crystalline-quality epitaxial material are obtained and have been successfully used for transistor fabrication.

Structural and optical properties of Si-doped GaN

Abstract: Structural and optical properties of Si-doped GaN thin films grown by metal-organic chemical vapor deposition have been studied by means of high resolution x-ray diffraction (XRD), atomic force microscopy, photoluminescence, photothermal deflection spectroscopy, and optical transmission measurements. The incorporation of silicon in the GaN films leads to pronounced tensile stress. The energy position of the neutral donor bound excitonic emission correlates with the measured stress. The stress induced near band gap luminescence shift is estimated to 19^+_-2 meV/GPa. An increasing concentration of dopant impurities in the films leads to asymmetries of the XRD and photoluminescence spectra, which are probably related to a Stress induced inhomogeneous distribution of dopants. Atomic force microscopy observations of surface modulation with increasing silicon doping support this latter statement. Transmission and photothermal deflection spectroscopy measurements are used to determine the band gap energy and Urbach energy of highly doped samples.

Structural properties of AlGaN/GaN heterostructures on Si(111) substrates suitable for high-electron mobility transistors

Abstract: Transmission electron microscopy (TEM) investigations of metal organic vapor phase deposition grown AlxGa1−xN/GaN heterostructures on Si(111) containing an AlN high-temperature buffer layer have been carried out. The structural properties at the interface and in the epilayer as well as the electronic properties suitable for a high electron mobility transistor (HEMT) were analyzed and compared with systems grown on Al2O3(0001). High resolution TEM (HRTEM) at the AlN/Si(111) interface reveals a 1.5–2.7 nm thick amorphous SiNx layer due to the high growth temperature of TAlN=1040 °C. Therefore, a grain-like GaN/AlN region extending 40–60 nm appears and it is subsequently overgrown with (0001) orientated GaN material because of geometrical selection. The residual strain at the AlN/Si(111) interface is estimated to be er=0.3±0.6% by Fourier filtering of HRTEM images and a moire fringe analysis. This indicates almost complete relaxation of the large mismatch f(AlN/Si)=+23.4% which seems to be supported by the S...

Characterization of InGaN thin films using high-resolution x-ray diffraction

Abstract: Wurtzite InGaN thin films grown by metalorganic chemical vapor deposition on sapphire substrates with and without GaN buffer layers are investigated by high-resolution x-ray diffraction measurements. The structural quality, lattice constants, strain, and indium composition of 100 nm thick films with In concentrations up to 33% are evaluated by measuring symmetric (00.2) and asymmetric (20.5) reflexes. The quality of the InGaN layers with widely different biaxial stress is measured and compared. An analytical solution for the determination of the In content of strained epitaxial layers is introduced. The results show that neglecting the strain can result in a severe miscalculation of the In concentration.

The polarization-induced electron gas in a heterostructure

Abstract: Models for the magnitude of the electron density induced by spontaneous and piezoelectric polarization in a polar heterostructure are given. In the case of pure material the induced electrons derive from `ambient' doping associated with surface states and surface ions. Where the donor density is large enough depletion will contribute to the electron gas.

AlGaN-based ultraviolet light detectors with integrated optical filters

Abstract: AlGaN-based ultraviolet light detectors with high responsivities in a narrow range of photon energy (≈0.3 eV) were grown by plasma induced molecular beam epitaxy on sapphire substrates. A thin Al0.4Ga0.6N film acting as an optical filter blocking high energy light was separated by an insulating AlGaN barrier from a third light sensitive Al0.3Ga0.7N layer. By optimizing the alloy compositions in the heterostructure and thickness of the filter layer, a peak responsivity of up to 35 A/W was achieved over a narrow range of wavelength with a peak position at 305 nm. The rejection of visible light response with respect to the peak responsivity was 5 orders of magnitude for a photoconductor device with a filter thickness of 1.4 μm. Decay times of the photoresponse after excimer laser exposure were determined to be between 40 and 330 ns.

Influence of oxygen and methane plasma on the electrical properties of undoped AlGaN/GaN heterostructures for high power transistors

Abstract: Plasma-induced damage often reduces the electrical and optical performance and the lifetime of compound semiconductor devices. We have investigated the effect of oxygen and methane reactive ion etching plasma on the electrical characteristics of nominally undoped GaN/Al 0.25 Ga 0.75 N/GaN high mobility heterostructures on sapphire substrates grown by plasma-induced molecular beam epitaxy and metalorganic chemical vapor deposition (MOCVD). The electrical transport properties of the two-dimensional electron gas in the AlGaN/GaN heterostructures were investigated by a combination of capacitance–voltage profiling and Hall effect measurements. The performance degradation of the heterostructures is attributed to the reduction of the carrier density and mobility due to ion bombardment, which is causing a creation of surface and deep acceptor states. After rapid thermal annealing (RTA) at temperatures between 400°C and 800°C, the electrical properties of the heterostructures exposed at moderated plasma power density and bias were mostly recovered. However, samples exposed at high power density lost the significant part of the electron sheet carrier concentration unrecoverable even after RTA at 800°C.

Optical characterization of Mg-doped GaN films grown by metalorganic chemical vapor phase deposition

Abstract: Scanning electron microscopy, micro-Raman, and photoluminescence (PL) measurements are reported for Mg-doped GaN films grown on (0001) sapphire substrates by low-pressure metalorganic chemical vapor phase deposition. The surface morphology, structural, and optical properties of GaN samples with Mg concentrations ranging from 1019 to 1021 cm−3 have been studied. In the scanning micrographs large triangular pyramids are observed, probably due to stacking fault formation and three-dimensional growth. The density and size of these structures increase with the amount of magnesium incorporated in the samples. In the photoluminescence spectra, intense lines were found at 3.36 and 3.31 eV on the triangular regions, where the presence of cubic inclusions was confirmed by micro-Raman measurements. The excitation dependence and temperature behavior of these lines enable us to identify their excitonic nature. From our study we conclude that the interface region between these defects and the surrounding wurtzite GaN could be responsible for PL lines.

Lateral polarity heterostructures by overgrowth of patterned AlxGa1-xN nucleation layers

Abstract: In this study thin AlxGa1−xN nucleation layers on sapphire were patterned and overgrown by plasma-induced molecular beam epitaxy (PIMBE) and metalorganic chemical vapor deposition (MOCVD) to obtain adjacent regions of GaN and AlGaN/GaN heterostructures with different polarities. The role of polarity on the structural and electrical properties of epitaxial layers and AlGaN/GaN heterostructures was investigated for samples grown on patterned AlN or GaN nucleation layers. Epitaxial GaN and AlGaN/GaN heterostructures grown on Al-face AlN or N- face GaN nucleation layers were found to be Ga-face or N-face, respectively, independent of the technique used for the overgrowth.

Polarization Induced Effects in AlGaN/GaN Heterostructures

Abstract: Two-dimensional hole and electron gases in wurtzite GaN/Al Ga 1xN/ GaN heterostructures areinduced by strong polarization induced effects. The sheet carrier concentration and the confinement of the two-dimensional carrier gases located close to one of the AlGaN/GaN interfaces are sensitive to a high number of different physical properties such as polarity, alloy composition, strain, thickness, and doping. We have investigated the structural quality, the carrier concentration profiles, and electrical transport properties by a combination of high resolution X-ray diffraction, Hall effect, and C—V profiling measurements. The investigated heterostructures with Nand Ga-face polarity were grown by metalorganic vapor phase or plasma induced molecular beam epitaxy covering a broad range of alloy compositions and barrier thickness. By comparison of theoretical and experimental results we demonstrate that the formation of two-dimensional hole and electron gases in GaN/AlGaN/GaN heterostructures both rely on the difference of the polarization between the AlGaN and the GaN layer. In addition the role of polarity on the carrier accumulation at different interfaces in nand p-doped heterostructures will be discussed in detail.

GaN/SiC heterojunction bipolar transistors

Abstract: Heterojunction bipolar transistors made from GaN emitter and SiC base and collector regions are desired for high power, broad bandwidth microwave amplifiers. Their most critical element is the quality of the base-emitter junction. High efficiency emitter injection requires an interface between these lattice-mismatched materials, which does not produce significant leakage current due to defects, in addition to having desirable band-offsets. An important factor in understanding GaN/SiC heterojunction rectification is accounting for spontaneous polarization and piezoelectric effects. Theory is presented which shows that a strained GaN/SiC junction will form a 2D hole gas in the base, while an unstrained junction will form a 2D-electron gas. Extending the critical thickness of GaN grown on SiC using patterned area growth might permit improved interface properties.

Laser-Induced Liftoff And Laser Patterning Of Large Free-Standing GaN Substrates

Abstract: Free-standing GaN crystals are produced from 200-300 µn thick GaN films grown on 2 inch sapphire substrates by hydride vapor phase epitaxy. The GaN films are separated from the growth substrate by laser-induced liftoff, using a pulsed laser to thermally decompose a thin layer of GaN close to the film-substrate interface. The free-standing films are polished and used for the homoepitaxial growth of high quality GaN layers by metalorganic chemical vapor deposition. The structural and optical properties of the homoepitaxial films in comparison to layers grown on sapphire are significantly improved, mainly because of lower dislocation density and surface roughness as low as 5×106 cm2 and 0.2 nm, respectively. Laser-induced thermal decomposition is also applied to achieve etching of GaN. At exposures of 500 mJ/cm2 with 355 nm light, etch rates of up to 90 nm for one pulse are obtained. Illumination with an interference grating is used to produce trenches as narrow as 100 nm or sinusoidal surface patterns with a period of 260 nm. Such surface morphologies are very useful for the processing of anti-reflection coatings or distributed Bragg reflectors.

2DEGs and 2DHGs Induced by Spontaneous and Piezoelectric Polarization in AlGaN/GaN Heterostructures

Abstract: Two dimensional hole and electron gases in wurtzite GaN/AlxGa1-xN/GaN heterostructures are induced by strong polarization induced effects. The sheet carrier concentration and the confinement of the two dimensional carrier gases located close to one of the AlGaN/GaN interfaces are sensitive to a high number of different physical properties such as polarity, alloy composition, strain, thickness and doping. We have investigated the structural quality, the carrier concentration profiles and electrical transport properties by a combination of high resolution x- ray diffraction, Hall effect and C-V profiling measurements. The investigated heterostructures with N- and Ga-face polarity were grown by metalorganic vapor phase or plasma induced molecular beam epitaxy covering a broad range of alloy compositions and barrier thickness. By comparison of theoretical and experimental results we demonstrate that the formation of two dimensional hole and electron gases in GaN/AlGaN/GaN heterostructures both rely on the difference of the polarization between the AlGaN and the GaN layer. In addition the role of polarity on the carrier accumulation at different interfaces in n- and p-doped heterostructures will be discussed in detail

Magnetic resonance investigations of defects in Ga14N and Ga15N

Abstract: The influence of the nitrogen nuclear spin on the optically detected magnetic resonance and electron spin resonance signatures of the intrinsic shallow donor and a deep defect causing the characteristic yellow luminescence have been studied on wurtzite GaN epitaxial layers grown by plasma induced molecular beam epitaxy with isotopically pure 14N and 15N. In particular, the linewidth of the deep defect signal is observed to be independent of the nitrogen isotope. The missing effect of the different nuclear spin properties of the 14N and 15N isotopes is discussed in view of current microscopic models for the yellow luminescence in GaN.

Power limits of polarization-induced AlGaN/GaN HEMT's

Abstract: The present and predicted limits on microwave power performance of undoped AlGaN/GaN HEMTs are presented, based on measured frequency response and drain-source breakdown voltage, both as functions of gate length. The spontaneous and piezoelectric polarization that induce the 2DEG in these HEMTs are covered. Process methods, including Si/sub 3/N/sub 4/ passivation are included. Thermal simulation results are shown for heat dissipation that limits channel temperature to 300/spl deg/C. Microwave cw power density limits of 12.5 W/mm at 10 GHz are predicted for class A operation on thick SiC substrates.

Growth of quaternary AlInGaN/GaN Heterostructures by Plasma Induced Molecular Beam Epitaxy with high In Concentration

Abstract: Growth of AlInGaN/GaN heterostructures on sapphire substrates was achieved by plasma induced molecular beam epitaxy. Different alloy compositions were obtained by varying the growth temperature with constant Al, In, Ga and N fluxes. The In content in the alloy, measured by Rutherford Backscattering Spectroscopy, increased from 0.4% to 14.5% when the substrate temperature was decreased from 775 to 665°C. X-Ray reciprocal space maps of asymmetric AlInGaN (2.05) reflexes were used to measure the lattice constants and to verify the lattice match between the quaternary alloy and the GaN buffer layers.

Polarization induced 2D hole gas in GaN/AlGaN heterostructures

Abstract: The generation of high density 2D hole gases is crucial for further progress in the field of electronic and optoelectronic nitride devices. In this paper, we present results of C-V profiling measurements for N-face GaN/AlGaN heterostructures and systematic theoretical studies of Mg- doped GaN/AlGaN gated heterostructures and superlattices. Our calculations are based on a self- consistent solution of the multiband k.p Schrodinger and Poisson equation and reveal that the hole 2D sheet density is mainly determined by the polarization induced interface charges. For an Aluminium concentration of 30%, the induced hole density in the heterostructure can reach values up to 1.5×1013 cm−2. In the GaN/AlGaN superlattices, the hole sheet density increases with the superlattice period and saturates for a period of 40 nm at a value of 1.5×1013 cm−2.