Showing papers by "Oliver Ambacher published in 2001"
TL;DR: In this paper, a two-dimensional electron gas (2DEG) is induced using the strong spontaneous and piezoelectric polarization inherent in the AlGaN/GaN structures, and three-dimensional nonlinear thermal simulations are made to determine the temperature rise from heat dissipation in various geometries.
Abstract: Undoped AlGaN/GaN structures are used to fabricate high electron mobility transistors (HEMTs). Using the strong spontaneous and piezoelectric polarization inherent in this crystal structure a two-dimensional electron gas (2DEG) is induced. Three-dimensional (3-D) nonlinear thermal simulations are made to determine the temperature rise from heat dissipation in various geometries. Epitaxial growth by MBE and OMVPE are described, reaching electron mobilities of 1500 and 1700 cm/sup 2//Ns, respectively, For electron sheet density near 1/spl times/10/sup 13//cm/sup 2/, Device fabrication is described, including surface passivation used to sharply reduce the problematic current slump (dc to rf dispersion) in these HEMTs. The frequency response, reaching an intrinsic f/sub t/ of 106 GHz for 0.15 /spl mu/m gates, and drain-source breakdown voltage dependence on gate length are presented. Small periphery devices on sapphire substrates have normalized microwave output power of /spl sim/4 W/mm, while large periphery devices have /spl sim/2 W/mm, both thermally limited. Performance, without and with Si/sub 3/N/sub 4/ passivation are presented. On SiC substrates, large periphery devices have electrical limits of 4 W/mm, due in part to the limited development of the substrates.
408 citations
TL;DR: In this paper, the influence of GaN crystal polarity on various properties of epitaxial films and electronic devices is discussed. But the authors focus on the Ga-face polarity.
Abstract: We review the influence of GaN crystal polarity on various properties of epitaxial films and electronic devices. GaN films grown on sapphire by MOCVD or HVPE usually exhibit Ga-face polarity. N-face polarity is obtained either on the backside of such layers after removal from the substrate, or by turning the crystal polarity in MBE growth via a thin AlN buffer layer. In addition to rather obvious differences in their structural and morphological features, Ga- and N-face samples differ also in their electronic properties. Thus, different Schottky barrier heights are observed for both polarities, the position and detailed properties of spontaneously formed two-dimensional electron gases vary with polarity, and the adsorption of gases and ions also show an influence of the two different surfaces. A particular interesting possibility is the growth of lateral polarity heterostructures with predetermined macroscopic domains of different polarity separated by inversion domain boundaries. These structures make use of the crystal polarity as a new degree of freedom for the investigation of electronic properties of III-nitrides and for novel devices.
222 citations
TL;DR: In this article, the electronic properties and the electron affinities of AlxGa1−xN(0001) surfaces were investigated by ultraviolet photoemission spectroscopy (UPS) over the whole composition range.
Abstract: The electronic properties and the electron affinities of AlxGa1−xN(0001) surfaces were investigated by ultraviolet photoemission spectroscopy (UPS) over the whole composition range. The samples were prepared by N-ion sputtering and annealing. Surface cleanliness and stoichiometry were monitored with x-ray photoemission spectroscopy. Samples with high aluminum content showed traces of oxygen which could not be removed by further cleaning cycles. However, we have evidence that the oxygen is located in the bulk and not at the surface. From the UP spectra the ionization energies and electron affinities as a function of composition x were determined. A decrease in electron affinity with increasing aluminum content was found, but the electron affinity remains positive for all x. Thus, earlier predictions of negative electron affinity for high aluminum content were not confirmed.
176 citations
TL;DR: In this article, the performance of a number of relevant exhaust gas components (H 2, HC, CO, NO) was tested with the test gas concentrations and the composition of background gases were chosen to simulate exhaust gas emissions from lean-burn engines.
Abstract: The paper reports on novel gas sensing devices based on III-nitride materials. Both platinum GaN Schottky diodes as well as high-electron-mobility transistors formed from GaN/AIGaN heterostructures with catalytically active platinum gates were investigated. The performance of these devices towards a number of relevant exhaust gas components (H 2 , HC, CO, NO) was tested. The test gas concentrations as well as the composition of background gases were chosen to simulate exhaust gas emissions from lean-burn engines. We found that GaN-based devices with platinum electrodes are mainly sensitive to hydrogen and unsaturated hydrocarbons with a sizeable cross-sensitivity to CO. Furthermore a strikingly dissimilar gas sensing behavior has been observed with respect to saturated hydrocarbons on the one hand and to hydrogen and unsaturated hydrocarbons on the other hand.
141 citations
TL;DR: In this article, high-electron-mobility transistors (HEMTs) based on AlGaN/GaN heterostructures were successfully tested as chemically sensing devices.
Abstract: High-electron-mobility transistors (HEMTs) based on AlGaN/GaN heterostructures were successfully tested as chemically sensing devices. Exposing the unprotected polar GaN surface in the gate area of a HEMT to liquids of different polarity, milliampere changes in the source-drain current could be detected. These sensing effects are likely to arise from chemical interactions with a sheet of ionic charge on the free GaN surface which compensates the electronic charge of a two-dimensional electron gas at a subsurface AlGaN/GaN interface.
136 citations
TL;DR: In this article, the inversion domain boundaries (IDBs) of lateral polarity heterostructures have been spectroscopically imaged at low temperature using high spatial resolution photoluminescence and it is shown that the IDBs are not only optically active, but are more than an order of magnitude brighter than the GaN bulk material.
Abstract: Intentionally grown GaN inversion domain boundaries (IDBs) of lateral polarity heterostructures have been spectroscopically imaged at low temperature using high spatial resolution photoluminescence. It is shown that the IDBs are not only optically active, but are more than an order of magnitude brighter than the GaN bulk material. Our findings are in agreement with calculations predicting that IDBs should not adversely affect near-band-gap photoluminescence due to the absence of midgap electronic states. Typical linewidths are on the order of 10–20 meV, however, features less than 0.6 meV are observed. The boundary emission is found to be neither spectrally nor spatially uniform. Also, a strong polarization dependence of the IDB photoluminescence is measured and determined to be oriented parallel to the boundary between GaN of N- or Ga-face polarity.
63 citations
TL;DR: In this paper, the piezoresistivity of wurtzite AlGaN/GaN layers with different Al contents and electron concentrations grown by plasma induced molecular beam epitaxy is investigated.
Abstract: The piezoresistivity of wurtzite AlxGa1−xN layers with different Al contents and electron concentrations grown by plasma induced molecular beam epitaxy is investigated. A strong increase of the piezoresistivity with increasing Al content and decreasing carrier density is observed. The corresponding piezoresistive gauge factor is negative and its absolute value increases from 3.5 to 25.8 if the Al concentration is increased from x=0 to 0.35. The dependence of the piezoresistive effect on the free electron concentration in the epitaxial layers is calculated by a model which compares the strain induced piezoelectric field to an externally applied gate voltage in field effect transistors. In addition, the strain dependence of the channel conductivity in pseudomorphic, undoped AlGaN/GaN high electron mobility transistors containing a polarization induced two-dimensional electron gas close to the heterointerface, is measured. A remarkably high gauge factor of −85 is observed for these devices.
59 citations
TL;DR: In this paper, the wetting behavior of GaN surfaces with N-face and Ga-face polarity and the influence of different surface treatments was studied by measuring wetting angle of highly purified water by microscopic imaging.
Abstract: The wetting behaviour of GaN surfaces with N-face and Ga-face polarity and the influence of different surface treatments is studied by measuring the wetting angle of highly purified water by microscopic imaging. We found that wet thermal oxidation of the surface leads to a decreased wetting angle indicating an improved wetting behaviour. The presence of Al in AIN or AlGaN leads to a further reduction of the wetting angle, which is attributed to the presence of Al 2 O 3 on the surface. In addition the comparison of Ga- and N-face material revealed a lower wetting angle for all N-face samples. XPS analysis showed the enhanced formation of native oxide on the surface with N-face polarity.
51 citations
TL;DR: In this paper, high-resolution photoluminescence spectra have been measured in high-quality homoepitaxial GaN grown on a free-standing GaN substrate with lower residual strain.
Abstract: High-resolution photoluminescence spectra have been measured in high-quality homoepitaxial GaN grown on a free-standing GaN substrate with lower residual strain than in previous work. Unusually strong and well-resolved excitonic lines were observed. Based on free- and bound exciton transitions some important GaN parameters are derived. The Arrhenius plot of the free A exciton recombination yields a binding energy of 24.7 meV. Based on this datum, an accurate value for the band-gap energy, EG(4.3 K) = 3.506 eV, can be given. From the donor bound excitons and their “two-electron” satellites, the exciton localization energy and donor ionization energy are deduced. Finally, estimates of the electron and hole masses have been obtained within the effective mass approximation.
44 citations
TL;DR: In this paper, an isotropic g factor of 19475 was obtained for zinc-blende GaN and wurtzite GaN alloys, and the g tensors of the silicon effective mass donor in these materials were determined experimentally.
Abstract: Electron spin resonance experiments were performed on Si-doped wurtzite and zinc-blende GaN and Si-doped wurtzite ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}$ alloys with $x=015,$ 032, 052, 075, and 1 For zinc-blende GaN, an isotropic g factor of 19475 is found The g tensors of the silicon effective mass donor in wurtzite GaN and ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}$ alloys are determined experimentally, and for $xl038$ are identical within experimental error to tensors observed in unintentionally n-type-doped material With increasing Al content, the g values increase and the g-tensor anisotropy is reduced The g values are calculated within a five-band $k\ensuremath{\cdot}p$ model in the cubic approximation To account for the isotropic g factor of wurtzite AlN, a nonvanishing spin-orbit coupling of the higher ${\ensuremath{\Gamma}}_{5}$ conduction bands of the order of 50 meV is necessary To describe the anisotropy at lower Al content, a full seven-band $k\ensuremath{\cdot}p$ model for the wurtzite crystal structure is investigated The model shows that the anisotropy in this material system is also due to coupling to higher conduction bands rather than to valence bands Additional estimates for the momentum matrix elements and the interband mixing coefficients are discussed
40 citations
TL;DR: In this paper, the authors report photoluminescence (PL) and optically detected magnetic resonance (ODMR) measurements on magnesium-doped GaN samples grown by metal-organic chemical vapor deposition, molecular beam epitaxy, and high-pressure--high-temperature synthesis.
Abstract: We report photoluminescence (PL) and optically detected magnetic resonance (ODMR) measurements on magnesium-doped GaN samples grown by metal-organic chemical vapor deposition, molecular beam epitaxy, and high-pressure--high-temperature synthesis. The samples exhibit at least three luminescence bands in the red-to-infrared spectral range with maxima at \ensuremath{\approx}1.75, \ensuremath{\approx}1.55, and below 1.4 eV. ODMR on these emission bands reveals two deep defects with isotropic g values of 2.001 and 2.006 and linewidths of 4--5 and 18--32 mT, respectively. Spectrally resolved ODMR experiments suggest that a donor-to-deep defect recombination is responsible for the transitions at 1.75 eV, while an acceptor-to-deep defect transition causes the PL bands with lower energy. The deep centers involved are attributed to defects with energy levels in the lower part of the band gap but close to the midgap region.
Abstract: Plasma-induced molecular beam epitaxial AlInGaN heterostructures have been characterized by spatial resolved cathodoluminescence and x-ray energy dispersive microanalysis. Competitive incorporation of Al and In has been observed, with the formation of In-rich regions, showing enhanced luminescence around surface pinholes. These island-like In-rich regions are favored by growth at lower temperature due to the higher incorporation of indium into the alloy. The elastic strain relaxation associated to pinhole formation induces preferential local indium incorporation. The diffusion of carriers to these areas with reduced band gap enhances the luminescence emission of the quaternary film. The width and intensity of the luminescence appear to be sensitive to the mismatch between the quaternary film and the GaN layer below.
TL;DR: In this article, both the magnitude and the characteristic frequency of the generation-recombination noise power density in Si-doped aluminum nitride (AlN:Si) were found to be thermally activated.
Abstract: Generation–recombination noise is observed in Si-doped aluminum nitride (AlN:Si). Both the magnitude and the characteristic frequency of the generation–recombination noise power density are found to be thermally activated. Using a model based on charge carrier number fluctuations in a two-level system, transition energies and potential barriers of the DX center formed by Si donors in AlN are quantitatively determined.
TL;DR: In this paper, an energy shift of the excitonic emission lines towards higher energies was observed, indicating the presence of residual compressive strain in the GaN layer, from which biaxial compressive stresses ranging between 0.34 and 1.7 GPa were deduced.
Abstract: Ga-face AlGaN/GaN heterostructures with different sheet carrier concentrations have been studied by photoluminescence and Raman spectroscopy. Compared to bulk GaN, an energy shift of the excitonic emission lines towards higher energies was observed, indicating the presence of residual compressive strain in the GaN layer. This strain was confirmed by the shift of the E2 Raman line, from which biaxial compressive stresses ranging between 0.34 and 1.7 GPa were deduced. The spontaneous and piezoelectric polarizations for each layer of the heterostructures have been also calculated. The analysis of these quantities clarified the influence of the residual stress on the sheet electron concentration (ns). Possible causes for the discrepancies between the calculated and experimentally determined sheet carrier densities are briefly discussed.
TL;DR: Ionization energies and electron affinities of clean AlxGa1-xN(0001) surfaces were investigated by ultraviolet photoemission spectroscopy over the whole composition range as discussed by the authors.
Abstract: Ionization energies and electron affinities of clean AlxGa1-xN(0001) surfaces were investigated by ultraviolet photoemission spectroscopy over the whole composition range. The samples were cleaned with cycles of N + -ion sputtering and annealing partly within a Ga atom flux. The ionization energy is measured as 6.5 eV and is almost independent of the aluminum content in good agreement with the general chemical trend. The electron affinity decreases linearly with composition from 3.1 eV for GaN to 0.25 eV for AlN. No evidence for negative electron affinity at AlN(0001) surfaces was found. Adsorption of oxygen at room temperature leads to a significant increase of the ionization energy and electron affinity. With AlN(0001) surfaces, an oxygen uptake of 0.6 monolayers is observed after exposures of 10 8 Langmuirs and the ionization energy increases by approximately 2 eV.
TL;DR: In this article, the response of Pt-GaN Schottky diodes towards hydrogen and hydrogen containing gases is analyzed and their gas sensitivity is characterized from room temperature up to 600°C, and the sheet carrier density of a two dimensional electron gas confined at the heterointerface in AlGaN/GaN high electron mobility transistors is shown to be significantly influenced by changes in the electronic properties of the device surface.
Abstract: The present paper gives an overview over different sensor applications of GaN thin films and AlGaN/GaN heterostructures. The response of Pt-GaN Schottky diodes towards hydrogen and hydrogen containing gases is analysed and their gas sensitivity is characterized from room temperature up to 600°C. In addition, the sheet carrier density of a two dimensional electron gas confined at the heterointerface in AlGaN/GaN high electron mobility transistors (HEMTs) is shown to be significantly influenced by changes in the electronic properties of the device surface. This effect is successfully exploited for the realization of ion detectors and sensors for fluid monitoring based on AlGaN/GaN HEMTs with non-metalized gate areas. Promising possibilities of fabricating monolithically integrated sensor devices for wireless signal transmission are demonstrated by the realization of SAW devices on epitaxial AlN-films.
TL;DR: In this article, the authors performed Shubnikov-de Haas (SdH) and Hall effect measurements to investigate the electronic transport properties of polarisation induced 2DEGs in Al x Ga 1-x N/GaN heterostructures with alloy compositions between x = 0.10 and 0.35 and sheet carrier concentrations of up to n s = 1.05 × 10 13 cm -2.
Abstract: We have performed Shubnikov-de Haas (SdH) and Hall effect measurements to investigate the electronic transport properties of polarisation induced 2DEGs in Al x Ga 1-x N/GaN heterostructures with alloy compositions between x = 0.10 and 0.35 and sheet carrier concentrations of up to n s = 1.05 × 10 13 cm -2 . From SdH measurements of 2DEGs with sheet carrier concentrations of 2.1 × 10 12 and 4.6 × 10 12 cm -2 , effective electron masses were determined to be 0.24 and 0.207m 0 , respectively. In addition, angle resolved SdH measurements were performed to evaluate the effective g-factor from the angle of zero oscillation amplitude. Including the measured electron masses, the g-factors were calculated to be 2.11 and 2.47, respectively. In order to identify the main electron scattering mechanism we determined the ratio between transport- and quantum scattering times as a function of sheet carrier concentration. We observe a significant decrease of the τ t /τ q ratio with increasing 2DEG carrier concentration, indicating a transition from a dominant small angle to large angle scattering mechanism when n s exceeds 7 × 10 12 cm -2 .
TL;DR: The radiative recombination in Ga-face Al 0.30 Ga 0.70 N/GaN single heterostructures (SHs) was studied by photoluminescence (PL) measurements as mentioned in this paper.
Abstract: The radiative recombination in Ga-face Al 0.30 Ga 0.70 N/GaN single heterostructures (SHs) was studied by photoluminescence (PL) measurements. An energy shift of the excitonic transitions toward higher energies was observed, indicating the presence of residual compressive strain in the GaN layer. In addition to these exciton lines, a broad band energetically localized between the exciton lines and the LO-phonon replica was noticed in the undoped SH. From its energy position, excitation power dependence, as well as temperature behaviour, we have attributed this luminescence to the H -band (HB), which is representative of the two-dimensional electron gas (2DEG) recombination.
TL;DR: The electronic noise properties of Si-doped AlN and Al0.3Ga0.7N are investigated in this article, where the potential energy barriers for capture into and emission from the ground state are quantitatively determined from the noise measurements.
Abstract: The electronic noise properties of Si-doped AlN and Al0.3Ga0.7N are investigated. In AlN : Si, generation–recombination (g–r) noise is observed and shown to be linked to DX-centers. The potential energy barriers for capture into and emission from the DX− ground state are quantitatively determined from the noise measurements. In Al0.3Ga0.7N:Si, in addition to 1/f noise, we find two g–r noise processes. However, an unambiguous identification of their origin proves to be difficult.
TL;DR: In this paper, the photoluminescence spectrum of a high quality homoepitaxial GaN film has been measured as a function of temperature and the energy shift has successfully fitted in that temperature range by means of the Bose-Einstein expression instead of Varshni's relationship.
Abstract: The photoluminescence spectrum of a high quality homoepitaxial GaN film has been measured as a function of temperature. As temperature increases the recombination of free excitons dominates the spectra. Their energy shift has successfully fitted in that temperature range by means of the Bose-Einstein expression instead of Varshni's relationship. Values for the parameters of both semi-empirical relations describing the energy shift are reported and compared with the literature.
01 Jan 2001
TL;DR: Pt-GaN Schottky devices with catalytically active platinum (Pt) electrodes were investigated in this article, where the response of these devices towards a number of relevant exhaust gas components such as H2, HC, CO, NO and NO2 was tested.
Abstract: Pt-GaN Schottky devices with catalytically active platinum (Pt) electrodes were investigated. Some measurements were also performed on high-electron-mobility transistors based on GaN/AlGaN heterostructures. The response of these devices towards a number of relevant exhaust gas components such as H2, HC, CO, NO and NO2 was tested. Sensitivity and cross sensitivity profiles were found to depend on temperature, the porosity of the Pt gates as well as on the polarity of the underlying GaN material.
TL;DR: In this paper, different barrier heights for Pt Schottky diodes evaporated onto Ga- and N-face GaN were determined from the dependence of the effective barrier height versus ideality factor by I-V measurements to 1.15 eV and 0.80 eV, respectively.
Abstract: Thick GaN films, grown by hydride vapor phase epitaxy (HVPE), were separated from their sapphire substrate with a laser-induced lift-off process. After cleaning and polishing, these films offer the most direct way to investigate and compare the influence of crystal polarity on the electronic properties of Ga-face and N-face surfaces, respectively. Different barrier heights for Pt Schottky diodes evaporated onto Ga- and N-face GaN are determined from the dependence of the effective barrier height versus ideality factor by I-V measurements to 1.15 eV and 0.80 eV, respectively. The charge neutrality condition at the surface is modified by the spontaneous polarization due to the polarization induced bound sheet charge. This effect has to be included in the electronegativity concept of metal induced gap states (MIGS) and can also be illustrated by different band bending of the conduction and valence band, inferred from the self-consistent solution of the Schrodinger-Poisson equation. Furthermore, temperature dependent I-V characteristics are compared to simulated behavior of Schottky diodes, exhibiting excellent agreement in forward direction, but showing deviations in the reverse current.