Showing papers by "Oliver Ambacher published in 1998"

Growth and applications of Group III-nitrides

Abstract: Recent research results pertaining to InN, GaN and AlN are reviewed, focusing on the different growth techniques of Group III-nitride crystals and epitaxial films, heterostructures and devices. The chemical and thermal stability of epitaxial nitride films is discussed in relation to the problems of deposition processes and the advantages for applications in high-power and high-temperature devices. The development of growth methods like metalorganic chemical vapour deposition and plasma-induced molecular beam epitaxy has resulted in remarkable improvements in the structural, optical and electrical properties. New developments in precursor chemistry, plasma-based nitrogen sources, substrates, the growth of nucleation layers and selective growth are covered. Deposition conditions and methods used to grow alloys for optical bandgap and lattice engineering are introduced. The review is concluded with a description of recent Group III-nitride semiconductor devices such as bright blue and white light-emitting diodes, the first blue-emitting laser, high-power transistors, and a discussion of further applications in surface acoustic wave devices and sensors.

Defect structure of epitaxial GaN films determined by transmission electron microscopy and triple-axis X-ray diffractometry

Abstract: Important structural characteristics (correlation lengths of columnar crystallites, dislocation densities, angles of rotational disorder) of hexagonal GaN grown by metallorganic chemical vapour deposition on c-plane sapphire are determined by transmission electron microscopy and triple-axis X-ray diffractometry. GaN films exhibit an edge dislocation density in the range of 1011 cm−2, a tilt and twist angle of 0.1° and 1.3° and a columnar structure with a lateral and vertical correlation length of 150 and 1000 nm respectively. The determination of correlation lengths and dislocation densites from X-ray patterns was undertaken using two independent evaluation methods which are discussed in detail. It is also shown that triple-axis X-ray diffractometry is a highly suitable technique for the separation of different kinds of structural defects such as edge and screw dislocations that lead to a characteristic broadening of symmetric and asymmetric Bragg reflections. The correlation lengths and dislocat...

Sound velocity of AlxGa1−xN thin films obtained by surface acoustic-wave measurements

Abstract: To determine the sound velocity in wurtzite AlxGa1−xN, we have used surface acoustic-wave (SAW) delay lines on AlxGa1−xN/c-Al2O3. AlxGa1−xN films with compositions from x=0 to x=1 were grown by plasma-induced molecular beam epitaxy. Starting from published data, we fine tuned the values of the elastic moduli used in numerical calculations such that the simulated and measured dispersion of the SAW were in good agreement. Based on these values, the surface and bulk acoustic-wave velocities of single-crystal AlxGa1−xN were determined as functions of the composition. The resulting SAW velocities ranged from 3700 to 5760 m/s for GaN and AlN, respectively.

Negative electron affinity of cesiated p-GaN(0001) surfaces

Abstract: The adsorption of cesium on clean n- and p-GaN(0001)-1×1 surfaces at 150 K was investigated using x-ray photoemission spectroscopy, photoemission spectroscopy with monochromatized He I radiation ultraviolet photoelectron spectroscopy (UPS) and a Kelvin probe (contact potential difference, CPD). The CPD measurements gave work functions of 3.88±0.15 and 3.6±0.15 eV for clean n- and p-GaN(0001) surfaces, respectively. The widths of UPS energy distribution curves yield an ionization energy of 6.8±0.15 eV. Thus, depletion and inversion layers exist at clean surfaces of n- and p-GaN(0001) surfaces, respectively. As a function of Cs coverage, the work function displays the well-known behavior in that it first decreases, passes through a minimum, and eventually reaches a value of 2.1 eV, the work function of metallic cesium. In the submonolayer coverage regime, the ionization energy decreases by 2.3±0.15 eV. At clean p-GaN(0001) surfaces the vacuum level lies by only 0.3 eV above the conduction-band minimum in th...

Thermopower investigation of n- and p-type GaN

Abstract: A comparative investigation of the Hall effect, conductivity, and thermopower properties of molecular-beam-epitaxy-grown GaN is presented. In unintentionally doped n-type GaN, a negligible thermal activation of the thermopower is observed above 300 K. In as-grown GaN:Mg, a thermopower activation energy of 280 meV is observed at high temperatures, as well as a scattering factor $A=3.$ At temperatures below 120 K, the Seebeck coefficient of p-type GaN changes sign and indicates n-type conductivity. These results show that hopping in the acceptor band contributes significantly to the electronic transport properties. After hydrogenation of GaN:Mg, both conductivity and thermopower have an activation energy of 520 meV, which is at variance with the presence of potential fluctuations in the material. This demonstrates that hydrogen passivates Mg-doped GaN by the formation of electrically inactive Mg-H complexes, in contrast to the formation of compensating H-related donors, which should lead to noticeable potential fluctuations.

Absorption of InGaN Single Quantum Wells Determined by Photothermal Deflection Spectroscopy.

Abstract: Photothermal deflection spectroscopy (PDS) is used to study the absorption of GaN/InGaN/GaN double heterostructures in the energy range from 0.6 to 3.8 eV. The heterostructures containing 1–77-nm-thick InGaN single quantum wells were deposited by chemical vapour deposition from organometallic precursors. They are measured to investigate the absorption coefficient, bandgap, indium concentration and fluctuation of the quantum wells. A bandgap increase of hexagonal InxGa1-xN (x≈0.14) of 60 meV is observed with decreasing well thicknesses from 15 to 1 nm. The distribution of In-concentration of the InxGa1-xN layers was estimated from the slope of the absorption coefficient versus photon energy for energies below the bandgap and found to be Gaussian with a full width of half maximum of Δx=0.03.

Polarity determination of a GaN thin film on sapphire (0001) with x-ray standing waves

Abstract: The x-ray standing wave technique was used to determine the polarity of a 1 μm thick GaN film grown by molecular beam epitaxy on an α-Al2O3(0001) single crystal. The standing wave was generated by x-ray diffraction from the GaN film. The Ga Kα fluorescence yield was recorded as a function of incidence angle within the range of the GaN(0002) reflection. Analysis of the data reveals that the film has grown with N polarity, i.e., the nitrogen atoms occupy the top half of the wurtzite (0001) bilayers.

Nitrogen Effusion and Self-Diffusion in Ga14N/Ga15N Isotope Heterostructures

Abstract: Ga14N/Ga15N/Ga14N isotope heterostructures are used to study nitrogen self-diffusion by secondary-ion mass spectrometry and thermally activated decomposition. After interdiffusion of Ga14N and Ga15N layers at temperatures between 770°C and 970°C the diffusion profiles are measured. The isotope heterostructures are particularly well suited for self-diffusion studies because the diffusion takes place at the interface inside the GaN crystal, and therefore the analysis is free from perturbations such as surface electric fields, mechanical stress or chemical potential gradients. The temperature dependence of the nitrogen self-diffusion coefficient (D) in hexagonal GaN was determined to be 1600 cm-2 s-1 exp [(-4.1±0.4) eV/kBT], leading to a self-diffusion entropy SSD of about 10kB. The nitrogen flux through an isotope interface is compared with the nitrogen loss from a free GaN surface in vacuum above the decomposition temperature, to obtain information about the diffusion kinetics relevant for epitaxial growth and high temperature device applications.

Quantitative Transmission Electron Microscopy Investigation of the Relaxation by Misfit Dislocations Confined at the Interface of GaN/Al2O3(0001)

Abstract: A quantitative transmission electron microscopy (TEM) investigation of the relaxation of GaN grown on Al2O3(0001) has been carried out. Terminating {110}-substrate fringes observed at the interface of the highly mismatched system have been characterized and the efficiency of the relaxation was measured. Wurtzite type GaN was grown by plasma induced molecular beam epitaxy (MBE) on the (0001) basal plane of Al2O3. The in-plane orientation between GaN and substrate reveals a high lattice misfit of f=-13.9% and therefore the critical thickness of dislocation formation is reached when the first monolayer of GaN is grown. An expected interfacial relaxation process is characterized by the results of high resolution transmission electron microscopy (HRTEM) which reveals misfit dislocations confined in the GaN/Al2O3 interface region. The quantitative evaluation of the HRTEM images on the one hand and Moire pattern on the other hand shows the effectiveness of the observed relaxation process: here a degree of relaxation δ=(-11.8±0.9)% and a residual strain of er=(-2.1±0.9)% was measured, and it seems that only er causes a dislocation density of estimately 1010 cm-2 in the GaN epilayer.

Comparative determination of absolute Raman scattering efficiencies and application to GaN

Abstract: Absolute Raman scattering efficiencies for first-order light scattering of optical zone-center phonons were determined for cubic and hexagonal GaN. An improved method for the determination of scattering efficiencies by comparative measurement with a standard substance such as BaF2 was used to minimize the experimental errors substantially. © 1998 John Wiley & Sons, Ltd.

Time-resolved photoluminescence study of excitons in hexagonal gan layers grown on sapphire

Abstract: We performed time-resolved and continuous wave photoluminescence on two samples of hexagonal GaN, one with free exciton emission and the other without. For the sample with free exciton emission, very different decay dynamics are observed between the front and backside emission. We find that the strain caused by the lattice mismatch between the sapphire substrate and the GaN film has a large influence on the population decay of the sample with free exciton emission and a minor influence on the decay properties of the sample dominated by bound exciton emission. A polariton picture is used to describe the observed behavior.

Analysis of composition fluctuations on an atomic scale in Al0.25Ga0.75N by high-resolution transmission electron microscopy

Abstract: Composition fluctuations in the Al0.25Ga0.75N layer of an AlGaN/GaN transistor structure grown by plasma induced molecular beam epitaxy on Al2O3(0001) at a growth temperature of 870 °C were studied by digital analysis of lattice images (DALI) of high-resolution transmission electron microscopy (HRTEM) cross-section images. DALI exploits the linear dependence of the lattice parameters on the Al content by applying Vegard’s law. Detecting the distances between intensity maxima positions in the micrograph which can be considered as a fingerprint of the local lattice parameters quantitatively derives composition profiles on an atomic scale. In the HRTEM cross-section image different areas were observed in the Al0.25Ga0.75N layer with either homogeneous or “striped” contrast. In the striped areas the analyses indicate a strong periodic decomposition with a period of 1 nm consisting of 1 ML Al0.8Ga0.2N and about 3 ML Al0.07Ga0.93N. The regions with homogeneous contrast do not exhibit significant composition flu...

Microstructural characterisation of nanocrystalline GaN prepared by detonations of gallium azides

Abstract: High quality nanoscale, phase-pure hexagonal gallium nitride (GaN) crystallites have been synthesized by the thermal induced detonation of molecular precursors of the type (R3N)Ga(N3)3 (R=CH3, C2H5, etc.). The method allows the control of the particle size regime from 2 to about 1000 nm. X-ray diffraction and Rietveld simulations revealed an anisotropic platelet-like shape of the particles. The obtained GaN material was as well characterized by transmission electron microscopy and electron diffraction, photoluminescence spectroscopy, SEM, IR, RAMAN, thermal gas effusion/mass spectrometry, thermal analysis, elemental analysis. Gas absorption measurements (BET method) showed a specific surface area of about 90 m2 · g−1. © 1998 John Wiley & Sons, Ltd.

Nanoscale Hexagonal Gallium Nitride from Single Molecule Precursors: Microstructure and Crystallite Size Dependent Photoluminescence

Abstract: High quality nanoscale, phase-pure hexagonal gallium nitride crystallites have been synthesized by using the thermally induced explosion of the molecular precursor (Et 3 N)Ga(N 3 ) 3 . The method allows the control of the particle size from 2 to 24 nm. X-ray diffraction and Rietveld simulations revealed a platelet-like shape of the larger particles in contrast to the rather spherical smaller crystallites. An increasing blue shift of the high energy luminescence with decreasing crystallite size was found (up to 4.4 cV at 5 K).

Minority Carrier Diffusion Length in AlGaN: A Combined Electron Beam Induced Current and Transmission Microscopy Study

Abstract: Combined electron beam induced current and transmission electron microscopy measurements have been performed on both undoped and Si-doped AlGaN epitaxial films with aluminium contents x ranging from x=0 to x=0.79. TEM analysis shows the distribution and type of structural defects, the content of cubic phase and the decomposition phenomena strongly to depend on the Al content of the layers. The influence of the cubic phase on the electronic properties of the samples is discussed taking into account the high piezoelectric effect present in these alloys. The diffusion length of minority carriers has been determined by EBIC measurements and a lower limit of the lifetime has been estimated.

High Power, Broadband, Linear, Solid State Amplifier.

Abstract: : Large periphery AlGaN/GaN HEMT's gave normalized power > 2 W/mm, and > 4 W/mm on sapphire and SiC substrates, respectively. A new processing method, using photolithography for all steps except for the gate, is being developed to reduce cost and raise throughput and reproducibility. The properties of the two-dimensional electron gas in undoped HEMT structures are compared with theory and explained by a combination of spontaneous and piezoelectric polarization, and by interface roughness. Static induction transistor processing has been developed. Circuits for broad band amplifiers are simulated, and a dual-gate cascade power stage has experimentally yielded 8 db higher gain. A non-linear model for HEMT power devices has been successfully developed. Theoretical models for electron transport in high electric fields have been compared with measured frequency response. Initial measurements of 1/f noise are being made. OMVPE growth has been developed to reach approx. 1,600 sq cm/V-s electron mobility with +/- 7% thickness variation. MBE growth has reached the same electron mobility with +/- 3% thickness variation and is able to deposit a 25 A GaN cap layer for chemical protection. Growth by OMVPE on SiC substrates is underway but requires further optimization to reduce deep donors in the AlGaN barrier.

Normal and Inverted Algan/Gan Based Piezoelectric Field effect Transistors Grown by Plasma Induced Molecular Beam Epitaxy

Abstract: High quality Ga-face and N-face AlGaN/GaN based heterostructures have been grown by plasma induced molecular beam epitaxy. By using Ga-face material we are able to fabricate conventional heterojunction field effect transistors. Because the N-face material confines electrons at a different heterojunction, the resulting transistors are called inverted. The Ga-face structures use a high temperature AIN nucleation layer to establish the polarity. Structures from these materials, relying only on polarization induced interface charge effects to create the two-dimensional electron gases, are used to confirm the polarity of the material as well as test the electrical properties of the layers. The resulting sheet concentrations of the two dimensional electron gases agree very well with the piezoelectric theory for this materials system. Hall mobilities of the two-dimensional gases for the N-face structures are as high as 1150 cm2/Vs and 3440 cm2/Vs for 300 K and 77 K respectively, while the Ga-face structures yield room temperature mobilities of 1190 cm2/Vs. Both structures were then fabricated into transistors and characterized. The inverted transistors, which were fabricated from the N-face material, yielded a maximum transconductance of 130 mS/mm and a current density of 905 mA/mm. Microwave measurements gave an ft of 7 GHz and an fmax of 12 GHz for a gate length of 1 μm. The normal transistors, fabricated from the Ga-face material, produced a maximum transconductance of 247 mS/mm and a current density of 938 mA/mm. Microwave measurements gave an ft of 50 GHz and an fmax of 97 GHz for a gate length of 0.25 μm. This shows that using plasma induced molecular beam epitaxy N-face and Ga(A1)-face AlGaN/GaN heterostructures can be grown with structural and electrical properties very suitable for high power field effect transistors.

Interface Effects on the Persistent Photoconductivity in Thin GaN and AlGaN Films

Abstract: Thin films of GaN and its alloy AlGaN are investigated with respect to their properties of the persistent photoconductivity (PPC). In this work, we show that the film-substrate interface plays an important role for the metastable electrical effect. Strongly absorbed bandgap light causes an increase of photoconductivity which is about one order of magnitude higher when the sample is illuminated from the substrate side near the interface than from the growth side. To access the interface properties at the substrate, we use temperature-dependent Hall effect measurements. The smallest PPC effect was observed for the GaN film with the best interface properties grown on SiC.