A case study of the use of reflections within the Applied physics and electrical engineering program at Linköping University is presented. Reflections have been used for several years and they are done at four stages in the program, in terms of reflections at the end of the Introductory course in year one, design-implement experiences in year three and five, and a reflection document that is the last component of the Master’s thesis. In the first three stages a project model is used to support the planning and execution of the project, and in the project model the project work ends with a reflection. In the reflection document connected to the Master’s thesis the student reflects upon both the thesis work itself and the entire education program, according to the sections and subsections of the CDIO Syllabus. The paper describes how the reflections are integrated in the program. Experiences from student perspective are collected in a small-scale study via interviews with students from year one and year five. 

In this work, silicon carbide (SiC) coatings were successfully grown by pulsed chemical vapor deposition (CVD). The precursors silicon tetrachloride (SiCl4) and ethylene (C2H4) were not supplied in a continuous flow but were pulsed alternately into the growth chamber with H-2 as a carrier and a purge gas. A typical pulsed CVD cycle was SiCl4 pulse-H-2 purge-C2H4 pulse-H-2 purge. This led to growth of superconformal SiC coatings, which could not be obtained under similar process conditions using a constant flow CVD process. We propose a two-step framework for SiC growth via pulsed CVD. During the SiCl4 pulse, a layer of Si is deposited. In the following C2H4 pulse, this Si layer is carburized, and SiC is formed. The high chlorine surface coverage after the SiCl4 pulse is believed to enable superconformal growth via a growth inhibition effect.

The approaches to conformal and superconformal deposition developed by Abelson and Girolami for a low-temperature, low-pressure chemical vapor deposition (CVD) setting relevant for electronic materials in micrometer or submicrometer scale vias and trenches, are tested here in a high-temperature, moderate pressure CVD setting relevant for hard coatings in millimeter-scale trenches. Conformal and superconformal deposition of polycrystalline silicon carbide (SiC) can be accomplished at deposition temperatures between 950 and 1000 degrees C with precursor partial pressure higher than 20 Pa and an optional minor addition of HCl as a growth inhibitor. The conformal deposition at low temperatures is ascribed to slower kinetics of the precursor consumption along the trench depth, whereas the impact of high precursor partial pressure and addition of inhibitor is attributable to surface site blocking. With the slower kinetics and the site blocking from precursor saturation leading the growth to nearly conformal and the possibly preferential inhibition effect near the opening than at the depth, a superconformal SiC coating with 2.6 times higher thickness at the bottom compared to the top of a 1 mm trench was achieved. Published under an exclusive license by the AVS.

Highly ⟨111⟩-oriented 3C-SiC coatings with a distinct surface morphology consisting of hexagonally shaped pyramidal crystals were prepared by chemical vapor deposition (CVD) using silicon tetrachloride (SiCl4) and toluene (C7H8) at T ≤ 1250 °C and ptot = 10 kPa. In contrast, similar deposition conditions, with methane (CH4) as the carbon precursor, resulted in randomly oriented 3C-SiC coatings with a cauliflower-like surface of SiC crystallites. No excess carbon was detected in the highly ⟨111⟩-oriented 3C-SiC samples despite the use of aromatic hydrocarbons. The difference in the preferred growth orientation of the 3C-SiC coatings deposited by using C7H8 and CH4 as the carbon precursors was explained via quantum chemical calculations of binding energies on various crystal planes. The adsorption energy of C6H6 on the SiC (111) plane was 6 times higher than that on the (110) plane. On the other hand, CH3 exhibited equally strong adsorption on both planes. This suggested that the highly ⟨111⟩-oriented 3C-SiC growth with C7H8 as the carbon precursor, where both C6H6 and CH3 were considered the main active carbon-containing film forming species, was due to the highly preferred adsorption on the (111) plane, while the lower surface energy of the (110) plane controlled the growth orientation in the CH4 process, in which only CH3 contributed to the film deposition. 

SiC multilayer coatings were deposited via thermal chemical vapor deposition (CVD) using silicon tetrachloride (SiCl₄) and various hydrocarbons under identical growth conditions, i.e. at 1100 °C and 10 kPa. The coatings consisted of layers whose preferred growth orientation alternated between random and highly 〈111〉-oriented. The randomly oriented layers were prepared with either methane (CH₄) or ethylene (C₂H₄) as carbon precursor, whereas the highly 〈111〉-oriented layers were grown utilizing toluene (C₇H₈) as carbon precursor. In this work, we demonstrated how to fabricate multilayer coatings with different growth orientations by merely switching between hydrocarbons. Moreover, the success in depositing multilayer coatings on both flat and structured graphite substrates has strengthened the assumption proposed in our previous study that the growth of highly 〈111〉-oriented SiC coatings using C₇H₈ was primarily driven by chemical surface reactions.

Indium oxide (In2O3) is an important transparent conducting material widely used in optoelectronic applications. Herein, we study the deposition of In2O3 by thermal atomic layer deposition (ALD) using our recently reported indium(iii) triazenide precursor and H2O. A temperature interval with self-limiting growth was found between similar to 270 and 385 degrees C with a growth per cycle of similar to 1.0 angstrom. The deposited films were polycrystalline cubic In2O3 with In : O ratios of 1 : 1.2, and low levels of C and no detectable N impurities. The transmittance of the films was found to be >70% in visible light and the resistivity was found to be 0.2 m omega cm. The high growth rates, low impurities, high optical transmittance, and low resistivity of these films give promise to this process being used for ALD of In2O3 films for future microelectronic displays.

The AlGaN/GaN epitaxial layers were grown by metalorganic chemical vapor deposition on both Ga-face and N-face free-standing GaN substrates, which were fabricated by halide vapor phase epitaxy method. Time-resolved photoluminescence technique was applied to study the optical properties of the grown AlGaN/GaN heterostructures. A strong emission at similar to 3.41 eV has been observed in the Ga-polar AlGaN/GaN structure, which is confirmed to associate to the two-dimensional electron gas. The temporal behavior of the two-dimensional electron gas emission is studied, which correlates well with the recombination processes in an asymmetric triangular potential well formed by an AlGaN/GaN structure grown in [0 0 0 1] direction. (C) 2019 Elsevier B.V. All rights reserved.

Gallium nitride (GaN) semiconductor material can become semi-insulating when doping with carbon. Semi-insulating buffer layers are utilized to prevent leakage currents in GaN high power devices. Carbon is inherently present during chemical vapor deposition (CVD) of GaN from the use of trimethyl gallium (TMGa) as precursor. TMGa decomposes in the gas phase, releasing its methyl groups, which could act as carbon source for doping. It is previously known that the carbon doping levels can be controlled by tuning the CVD process parameters, such as temperature, pressure and precursor flow rates. However, the mechanism for carbon incorporation from TMGa is not yet understood. In this paper, a model for predicting carbon incorporation from TMGa in GaN layers grown by CVD is proposed. The model is based on ab initio quantum chemical calculations of molecular adsorption and reaction energies. Using Computational Fluid Dynamics, with a chemical kinetic model for decomposition of the precursors and reactions in the gas phase, to calculate gas phase compositions at realistic process conditions, together with the proposed model, we obtain good correlations with measurements, for both carbon doping concentrations and growth rates, when varying the inlet NH3/TMGa ratio. When varying temperature (800 – 1050°C), the model overpredicts carbon doping concentrations at the lower temperatures, but predicts growth rates well, and the agreement with measured carbon doping concentrations is good above 1000°C.

AlGaN/GaN high electron mobility transistors (HEMTs) fabricated on a heterostructure grown by metalorganic chemical vapor deposition using analternative method of carbon (C) doping the buffer are characterized. C-doping is achieved by using propane as precursor, as compared to tuningthe growth process parameters to control C-incorporation from the gallium precursor. This approach allows for optimization of the GaN growthconditions without compromising material quality to achieve semi-insulating properties. The HEMTs are evaluated in terms of isolation anddispersion. Good isolation with OFF-state currents of 2 ' 10%6A/mm, breakdown fields of 70V/μm, and low drain induced barrier lowering of0.13mV/V are found. Dispersive effects are examined using pulsed current–voltage measurements. Current collapse and knee walkout effectslimit the maximum output power to 1.3W/mm. With further optimization of the C-doping profile and GaN material quality this method should offer aversatile approach to decrease dispersive effects in GaN HEMTs.

The impact of the sharpness of the AlGaN/GaN interface in high-electron mobility transistors (HEMTs) is investigated. Two structures, one with an optimized AlGaN/GaN interface and another with an unoptimized, were grown using hot-wall metal-organic chemical vapor deposition. The structure with optimized sharpness of the interface shows electron mobility of 1760 cm(2)/V . s as compared with 1660 cm(2)/V . s for the nonoptimized interface. Gated Hall measurements indicate that the sharper interface maintains higher mobility when the electrons are close to the interface compared with the nonoptimized structure, indicating less scattering due to alloy disorder and interface roughness. HEMTs were processed and evaluated. The higher mobility manifests as lower parasitic resistance yielding a better dc and high-frequency performance. A small-signal equivalent model is extracted. The results indicate a lower electron penetration into the buffer in the optimized sample. Pulsed-IV measurements imply that the sharper interface provides less dispersive effects at large drain biases. We speculate that the mobility enhancement seen AlGaN/AlN/GaN structures compared with the AlGaN/GaN case is not only related to the larger conduction band offset but also due to a more welldefined interface minimizing scattering due to alloy disorder and interface roughness.

In this work we employ terahertz (THz) ellipsometry to determine two-dimensional electron gas (2DEG) density, mobility and effective mass in AlGaN/GaN high electron mobility transistor structures grown on 4H-SiC substrates. The effect of the GaN interface exposure to low-flow-rate trimethylaluminum (TMA) on the 2DEG properties is studied. The 2DEG effective mass and sheet density are determined tobe in the range of 0.30-0.32m₀ and 4.3-5.5×1012 cm^–2, respectively. The 2DEG effective mass parameters are found to be higher than the bulk effective mass of GaN, which is discussed in view of 2DEG confinement. It is shown that exposure to TMA flow improves the 2DEG mobility from 2000 cm²/Vs to values above 2200 cm²/Vs. A record mobility of 2332±61 cm²/Vs is determined for the sample with GaN interface exposed to TMA for 30 s. This improvement in mobility is suggested to be due to AlGaN/GaN interface sharpening causing the reduction of interface roughness scattering of electrons in the 2DEG.

ZrB₂ films were deposited on 900 °C-preheated or non-preheated GaN(0001) surfaces by direct current magnetron sputtering from a compound target. Analytical transmission electron microscopy and scanning transmission electron microscopy with energy dispersive X-ray spectroscopy and electron energy loss spectroscopy revealed a 0001 fiber textured ZrB₂ film growth following the formation of a ~2 nm thick amorphous BN layer onto the GaN(0001) at a substrate temperature of 900 °C. The amorphous BN layer remains when the substrate temperature is lowered to 500 °C or when the preheating step is removed from the process and results in the growth of polycrystalline ZrB₂ films. The ZrB₂ growth phenomena on GaN(0001) is compared to on 4H-SiC(0001), Si(111), and Al₂O₃(0001) substrates, which yield epitaxial film growth. The decomposition of the GaN surface during vacuum processing during BN interfacial layer formation is found to impede epitaxial growth of ZrB₂.

The creation of a semi insulating (SI) buffer layer in AlGaN/GaN High Electron Mobility Transistor (HEMT) devices is crucial for preventing a current path beneath the two-dimensional electron gas (2DEG). In this investigation, we evaluate the use of a gaseous carbon gas precursor, propane, for creating a SI GaN buffer layer in a HEMT structure. The carbon doped profile, using propane gas, is a two stepped profile with a high carbon doping (1.5 x 10(18) cm(-3)) epitaxial layer closest to the substrate and a lower doped layer (3 x 10(16) cm(-3)) closest to the 2DEG channel. Secondary Ion Mass Spectrometry measurement shows a uniform incorporation versus depth, and no memory effect from carbon doping can be seen. The high carbon doping (1.5 x 10(18) cm(-3)) does not influence the surface morphology, and a roughness root-mean-square value of 0.43 nm is obtained from Atomic Force Microscopy. High resolution X-ray diffraction measurements show very sharp peaks and no structural degradation can be seen related to the heavy carbon doped layer. HEMTs are fabricated and show an extremely low drain induced barrier lowering value of 0.1 mV/V, demonstrating an excellent buffer isolation. The carbon doped GaN buffer layer using propane gas is compared to samples using carbon from the trimethylgallium molecule, showing equally low leakage currents, demonstrating the capability of growing highly resistive buffer layers using a gaseous carbon source. (C) 2015 AIP Publishing LLC.

Aluminium gallium nitride (AlGaN)/GaN high-electron mobility transistor performance is to a large extent affected by the buffer design, which, in this paper, is varied using different levels of carbon incorporation. Three epitaxial structures have been fabricated: 1) two with uniform carbon doping profile but different carbon concentration and 2) one with a stepped doping profile. The epitaxial structures have been grown on 4H-SiC using hot-wall metal-organic chemical vapor deposition with residual carbon doping. The leakage currents in OFF-state at 10 V drain voltage were in the same order of magnitude (10^-4 A/mm) for the high-doped and stepped-doped buffer. The high-doped material had a current collapse (CC) of 78.8% compared with 16.1% for the stepped-doped material under dynamic I-V conditions. The low-doped material had low CC (5.2%) but poor buffer isolation. Trap characterization revealed that the high-doped material had two trap levels at 0.15 and 0.59 eV, and the low-doped material had one trap level at 0.59 eV.

The creation of a semi-insulating (SI) buffer layer in AlGaN/GaN HEMT devices is crucial for preventing a current path beneath the two-dimensional electron gas (2DEG). Here we evaluate the use of a carbon precursor, propane, for creating a SI GaN buffer layer. The carbon doping profile obtained from SIMS measurement shows a very uniform incorporation versus depth and no significant memory effect from carbon doping is seen, allowing for the creation of a very abrupt profile. The high carbon doping (1.5×1018 cm-3) does not influence the surface morphology. HRXRD ω rocking curve showed a FWHM of 200 arcsec of the (0002) and 261 arcsec for (10-12) reflection of the GaN, respectively. HEMT devices were processed on the epitaxial layers. An extremely low drain induced barrier lowering value of 0.1 mV/V was measured for a HEMT with a gate length of 0.2 𝜇m. This demonstrates the capability of growing a highly resistive buffer layer using intentional carbon doping.

The crystalline quality of AlGaN/GaN heterostructures was improved by optimization of surface pretreatment of the SiC substrate in a hot-wall metal-organic chemical vapor deposition reactor. X-ray photoelectron spectroscopy measurements revealed that oxygen- and carbon-related contaminants were still present on the SiC surface treated at 1200 °C in H₂ ambience, which hinders growth of thin AlN nucleation layers with high crystalline quality. As the H₂ pretreatment temperature increased to 1240 °C, the crystalline quality of the 105 nm thick AlN nucleation layers in the studied series reached an optimal value in terms of full width at half-maximum of the rocking curves of the (002) and (105) peaks of 64 and 447 arcsec, respectively. The improvement of the AlN growth also consequently facilitated a growth of the GaN buffer layers with high crystalline quality. The rocking curves of the GaN (002) and (102) peaks were thus improved from 209 and 276 arcsec to 149 and 194 arcsec, respectively. In addition to a correlation between the thermal resistance and the structural quality of an AlN nucleation layer, we found that the microstructural disorder of the SiC surface and the morphological defects of the AlN nucleation layers to be responsible for a substantial thermal resistance. Moreover, in order to decrease the thermal resistance in the GaN/SiC interfacial region, the thickness of the AlN nucleation layer was then reduced to 35 nm, which was shown sufficient to grow AlGaN/GaN heterostructures with high crystalline quality. Finally, with the 35 nm thick high-quality AlN nucleation layer a record low thermal boundary resistance of 1.3×10⁻⁸ m² K/W, measured at an elevated temperature of 160 °C, in a GaN-on-SiC transistor structure was achieved.

Severe surface decomposition of semi-insulating (SI) GaN templates occurred in high-temperature H2 atmosphere prior to epitaxial growth in a metalorganic chemical vapor deposition system. A two-step heating process with a surface stabilization technique was developed to preserve the GaN template surface. Utilizing the optimized heating process, a high two-dimensional electron gas mobility ∼2000 cm²/V·s was obtained in a thin AlGaN/AlN/GaN heterostructure with an only 100-nm-thick GaN spacer layer homoepitaxially grown on the GaN template. This technique was also demonstrated viable for native GaN substrates to stabilize the surface facilitating two-dimensional growth of GaN layers. Very high residual silicon and oxygen concentrations were found up to ∼1 × 10²⁰cm⁻³ at the interface between the GaN epilayer and the native GaN substrate. Capacitance-voltage measurements confirmed that the residual carbon doping controlled by growth conditions of the GaN epilayer can be used to successfully compensate the donor-like impurities. State-of-the-art structural properties of a high-mobility AlGaN/AlN/GaN heterostructure was then realized on a 1 × 1 cm² SI native GaN substrate; the full width at half maximum of the X-ray rocking curves of the GaN (002) and (102) peaks are only 21 and 14 arc sec, respectively. The surface morphology of the heterostructure shows uniform parallel bilayer steps, and no morphological defects were noticeable over the entire epi-wafer.

Comparative studies have been made on AlGaN/GaN epitaxial layers grown by metalorganic chemical vapor deposition on both Ga- and N-face free-standing GaN substrates fabricated by halide vapor phase epitaxy. By time-resolved photoluminescence studies, we conclude that two-dimensional electron gas (2DEG) only appears for heterostructures grown on Ga-face. We studied the temporal behavior of the 2DEG emission, which correlates well with recombination processes in an asymmetric triangular potential well formed by an AlGaN/GaN structure grown in [0001] direction.

Methane (CH4), ethylene (C2H4), acetylene (C2H2), propane (C3H8), iso-butane (i-C4H10), and trimethylamine [N(CH3)(3)] have been investigated as precursors for intentional carbon doping of (0001) GaN in chemical vapor deposition. The carbon precursors were studied by comparing the efficiency of carbon incorporation in GaN together with their influence on morphology and structural quality of carbon doped GaN. The unsaturated hydrocarbons C2H4 and C2H2 were found to be more suitable for carbon doping than the saturated ones, with higher carbon incorporation efficiency and a reduced effect on the quality of the GaN epitaxial layers. The results indicate that the C2H2 molecule as a direct precursor, or formed by the gas phase chemistry, is a key species for carbon doping without degrading the GaN quality; however, the CH3 species should be avoided in the carbon doping chemistry.

GaN layers were homoepitaxially grown on N-face free-standing GaN substrates using a hot-wall metalorganic chemical vapor deposition method. By using optimized growth parameters, layers with a smooth morphology were obtained. The crystalline quality of epilayers was studied by a high resolution X-ray diffraction technique and compared to the substrates. Optical properties of the epilayers studied by low temperature time-resolved photoluminescence have shown longer recombination time for donor-bound exciton compared to the substrates. (C) 2014 Elsevier B.V. All rights reserved.

A high mobility of 2250 cm²/V·s of a two-dimensional electron gas (2DEG) in a metalorganic chemical vapor deposition-grown AlGaN/GaN heterostructure was demonstrated. The mobility enhancement was a result of better electron confinement due to a sharp AlGaN/GaN interface, as confirmed by scanning transmission electron microscopy analysis, not owing to the formation of a traditional thin AlN exclusion layer. Moreover, we found that the electron mobility in the sharp-interface heterostructures can sustain above 2000 cm²/V·s for a wide range of 2DEG densities. Finally, it is promising that the sharp-interface AlGaN/GaN heterostructure would enable low contact resistance fabrication, less impurity-related scattering, and trapping than the AlGaN/AlN/GaN heterostructure, as the high-impurity-contained AlN is removed.

Zirconium diboride (ZrB2) thin films have been deposited on 4H-SiC(0001) substrates by direct current magnetron sputtering from a compound target. The effect of deposition temperature (500-900 degrees C) and sputtering power (100-400 W) on the composition and structure of the films have been investigated. Electron microscopy and X-ray diffraction reveal that high sputtering power values and high deposition temperatures are favorable to enhance the crystalline order of the epitaxial 0001 oriented films. X-ray photoelectron spectroscopy shows that the composition of the films is near-stoichiometric for all deposition temperatures and for high sputtering power values of 300 W and 400 W, whereas under-stoichiometric films arc obtained when applying 100 W or 200 W. Decreasing the deposition temperature, or in particular the sputtering power, result in higher C and O impurity levels. The resistivity of the films was evaluated by four-point-probe measurements and found to scale with the amount of O impurities in the films. The lowest resistivity value obtained is 130 mu Omega cm, which makes the ZrB2 films interesting as an electrical contact material. (C) 2015 Elsevier B.V. All rights reserved.

The early stages of InGaN/GaN quantum wells growth for In reduced conditions have been investigated for varying thickness and composition of the wells. The structures were studied by monochromated STEM–VEELS spectrum imaging at high spatial resolution. It is found that beyond a critical well thickness and composition, quantum dots (>20 nm) are formed inside the well. These are buried by compositionally graded InGaN, which is formed as GaN is grown while residual In is incorporated into the growing structure. It is proposed that these dots may act as carrier localization centers inside the quantum wells.

ZrB₂ thin films have been synthesized using direct current magnetron sputtering from a ZrB₂ compound target onto 4H-SiC(0001) and Si(100) substrates kept at different temperatures (no heating, 400 °C, and 550 °C), and substrate bias voltage (-20 V to -80 V). Time-of-flight energy elastic recoil detection analysis shows that all the films are near stoichiometric and have a low degree of contaminants, with O being the most abundant (< 1 at.%). The films are crystalline, and their crystallographic orientation changes from 0001 to a more random orientation with increased deposition temperature. X-ray diffraction pole figures and selected area electron diffraction patterns of the films deposited without heating reveal a fiber-texture growth. Four point probe measurements show typical resistivity values of the films ranging from ~95 to 200 μΩcm, decreasing with increased growth temperature and substrate bias.

Semiconductor quantum dots (QDs) have been demonstrated viable for the emission of single photons on demand during the past decade. However, the synthesis of QDs emitting photons with pre-defined and deterministic polarization vectors has proven arduous. The access of linearly-polarized photons is essential for various applications. In this report, a novel concept to directly generate linearly-polarized photons is presented. This concept is based on InGaN QDs grown on top of elongated GaN hexagonal pyramids, by which predefined orientations herald the polarization vectors of the emitted photons from the QDs. This growth scheme should allow fabrication of ultracompact arrays of photon emitters, with a controlled polarization direction for each individual QD emitter.

Epitaxial ZrB₂ thin films were deposited at a temperature of 900 °C on 4H-SiC(0001) and Si(111) substrates by magnetron sputtering from a ZrB₂ source at high rate ~80 nm/min. The films were analyzed by thin film X-ray diffraction including pole figure measurements and reciprocal space mapping as well as high resolution electron microscopy.

Hexagonal GaN pyramids have been fabricated by hot-wall metal organic chemical vapor deposition (hot-wall MOCVD) and the growth evolution have been studied. It was concluded that the pyramid growth can be divided into two regimes separated by the adsorption kinetics of the {1101} surfaces of the pyramids. In the adsorption regime, the pyramids grow simultaneously in the <1101> and [0001] -directions. In the zero-adsorption regime the pyramids grow only in the [0001] direction. Thus the pyramid growth ceases when the (0001) facet growth has been terminated. Large arrays consisting of highly uniform pyramids with apex radii of 3 nm or less were achieved in the zeroadsorption regime. The growth-regime type was concluded to have a large impact on the uniformity degradation of the pyramids, and their optical properties. The impacts of threading dislocations which enter the pyramid from underneath are also discussed.

InGaN quantum dots (QDs) formed on top of GaN pyramids have been fabricated by means of selective area growth employing hot wall MOCVD. Upon regrowth of a patterned substrate, the growth will solely occur in the holes, which evolve into epitaxially grown wurtzite based pyramids. These pyramids are subsequently overgrown by a thin optically active InGaN well. The QDs are preferably nucleating at the apices of the pyramids as evidenced by the transmission electron microscopy (TEM). The emission from these QDs have been monitored by means of microphotoluminescence (µPL), in which single emission lines have been detected with a sub-meV line width. The µPL measurements undoubtedly reveal that the QDs are located in the apexes of the pyramids, since the sharp emission peaks can only be monitored as the excitation laser is focused on the apices in the µPL. It is also demonstrated that the emission energy can be changed in a controlled way by altering the growth conditions, like the growth temperature and/or composition, for the InGaN layers. The tip of the GaN pyramid is on the nm scale and can be made sharp or slightly truncated. TEM analysis combined with µPL results strongly indicate that the Stranski-Krastanow growth modepreferably is taking place at the microscopic c-plane truncation of the GaN pyramid. Single emission lines with a high degree of polarization is a common feature observed for individual QDs. This emission remains unchanged with increasing the excitation power and sample temperature. An in-plane elongated QD forming a shallow potential with an equal number of electrons and holes is proposed to explain the observed characteristics of merely a single exciton emission with a high degree of polarization.

High tuneability of residual carbon doping is developed in a hot-wall metalorganic chemical vapor deposition reactor. Two orders of temperature-tuned carbon concentration, from ∼2 × 10¹⁸ cm⁻³ down to ∼1 × 10¹⁶ cm⁻³, can be effectively controlled in the growth of the GaN buffer layer. Excellent uniformity of two-dimensional electron gas (2DEG) properties in Al_xGa_1−xN/AlN/GaN heterostructure with very high average carrier density and mobility, 1.1 × 10¹³ cm⁻² and 2035 cm²/V·s, respectively, over 3" semi-insulating SiC substrate is realized with the temperature-tuned carbon doping scheme. Reduction of carbon concentration is evidenced as a key to achieve high 2DEG carrier density and mobility.

Impressive power densities have been demonstrated for GaN-on-SiC based high-power high-frequency transistors, although further gains can be achieved by further minimizing the device thermal resistance. A significant 10-30% contribution to the total device thermal resistance originates from the high defect density AlN nucleation layer at the GaN/SiC interface. This thermal resistance contribution was successfully reduced by performing systematic growth optimization, investigating growth parameters including: Substrate pretreatment temperature, growth temperature and deposition time. Interfacial thermal resistance, characterized by time resolved Raman thermography measurements AlGaN/GaN HEMT structures, were minimized by using a substrate pretreatment and growth temperature of 1200 °C. Reducing the AlN thickness from 105 nm (3.3×10^-8W/m²K) to 35 nm (3.3×10^-8 W/m²K), led to a ~2.5× interfacial thermal resistance reduction and the lowest value reported for a standard AlGaN/GaN HEMT structure.

The emission of a charged exciton in an InGaN quantum dot (QD) on top of a GaN pyramid is identified experimentally. The intensity of the charged exciton exhibits the expected competition with that of the single exciton, as observed in temperature-dependent micro-photoluminescence measurements, performed with different excitation energies. The non-zero charge state of this complex is further supported by time resolved micro-photoluminescence measurements, which excludes neutral alternatives of biexciton. The potential fluctuations in the vicinity of the QD that localizes the charge carriers are proposed to be responsible for the unequal supply of electrons and holes into the QD.

Issues of major relevance to the n-type conductivity of Al_0.77Ga_0.23N associated with Si and O incorporation, their shallow donor or deep donor level behavior, and carrier compensation are elucidated by allying (i) study of Si and O incorporation kinetics at high process temperature and low growth rate, and (ii) electron paramagnetic resonance measurements. The Al_0.77Ga_0.23N composition correlates to that Al content for which a drastic reduction of the conductivity of Al_xGa_1−xN is commonly reported. We note the incorporation of carbon, the role of which for the transport properties of Al_xGa_1−xN has not been widely discussed.

Growing InGaN quantum dots (QDs) at the apex of hexagonal GaN pyramids is an elegant approach to achieve a deterministic positioning of QDs. Despite similar synthesis procedures by metal–organic chemical vapor deposition, the optical properties of the QDs reported in the literature vary drastically. The QDs tend to exhibit either narrow or broad emission lines in the micro-photoluminescence spectra. By coupled microstructural and optical investigations, the QDs giving rise to narrow emission lines were concluded to nucleate in association with a (0001) facet at the apex of the GaN pyramid.

Morphological variations of gallium polar (0001) oriented hexagonal GaN pyramids grown by hotwall metal organic chemical vapor deposition under various growth conditions are investigated. The stability of the semi-polar {1102} and non-polar {1100} facets are particularly discussed. The presence of the {1102} facets near the apex of the pyramid was found to be controllable by tuning the absolute flow rate of ammonia during the growth. Vertical non-polar {1100} facets appeared ingallium rich-conditions which automatically were created when the growth time was prolonged beyond pyramid completion. The result was attributed to a gallium passivation of the {1100} surface.

Optical characterization of single quantum dots (QDs) by means of micro-photoluminescence (mu PL) will be reviewed. Both QDs formed in the Stranski-Krastanov mode as well as dots in the apex of pyramidal structures will be presented. For InGaAs/GaAs dots, several excitonic features with different charge states will be demonstrated. By varying the magnitude of an external electric or magnetic field and/or the temperature, it has been demonstrated that the transportation of carriers is affected and accordingly the charge state of a single QD can be tuned. In addition, we have shown that the charge state of the QD can be controlled also by pure optical means, i.e. by altering the photo excitation conditions. Based on the experience of the developed InAs/GaAs QD system, similar methods have been applied on the InGaN/GaN QD system. less thanbrgreater than less thanbrgreater thanThe coupling of LO phonons to the QD emission is experimentally examined for both charged and neutral excitons in single InGaAs/GaAs QDs in the apex of pyramidal structures. It is shown that the positively charged exciton exhibits a significantly weaker LO phonon coupling in the mu PL spectra than the neutral and negatively charged species, a fact, which is in consistency with model simulations performed.

Raman scattering of individual aluminum nitride (AlN) nanowires is investigated systematically. The axial direction of single nanowire can be rapidly verified by polarized Raman scattering. The angular dependencies of E-2(high) mode show strongly anisotropic behavior in smaller nanowires, which results from optical antenna effect. Raman enhancement (RE) per unit volume of E-2(high) increases with decreasing diameter of nanowires. Compared to the thin film, similar to 200-fold increase of RE is observed in AlN nanowires having diameter less than 50 nm, which is far beyond the quantum confinement regime. Such a large RE can be attributed to the effects of resonant cavity and stimulated Raman scattering.

The temperature-dependence of free-charge carrier mobility, sheet density, and effective mass of a two-dimensional electron gas in a AlGaN/GaN heterostructure deposited on SiC substrate is determined using the THz optical Hall effect in the spectral range from 0.22 to 0.32 THz for temperatures from 1.5 to 300 K. The THz optical Hall-effect measurements are combined with room temperature mid-infrared spectroscopic ellipsometry measurements to determine the layer thickness, phonon mode, and free-charge carrier parameters of the heterostructure constituents. An increase of the electron effective mass from (0.22 +/- 0.01)m(0) at 1.5 K to (0.36 +/- 0.03)m(0) at 300 K is observed, which is indicative for a reduction in spatial confinement of the two-dimensional electron gas at room temperature. The temperature-dependence of the mobility and the sheet density is in good agreement with electrical measurements reported in the literature.

We report on the cathodoluminescence (CL) of nominally undoped semi-insulating GaN layers grown by hot-wall metal-organic chemical vapor deposition (MOCVD) at a threefold increase of the growth rate limited by the TMGa flow. The growth kinetics is such, that C is the only background impurity in the layers with controllably increasing concentration from 5 x 10(16) to 6 x 10(17) cm(-3), while other background impurities, H. O and Si, are essentially at the SIMS detection levels. The hot-wall MOCVD is not an ordinary approach to GaN growth process and this study corroborates a more perceptive outlook on the C incorporation in GaN and any potential C-incorporation-mediated luminescence, including the observed here blue luminescence (BL) at similar to 417 nm, and the yellow luminescence (YL) with shifting peak position towards shorter wavelengths, similar to 555-543-525 nm.

A time-resolved micro-Raman technique was developed to probe the transient voltage in the GaN buffer layer of AlGaN/GaN heterostructure devices. The transient potential distribution under Ohmic contacts of devices behaved like a capacitance-resistance coupled network, with a decrease in amplitude and phase shift of the potential as a function of operating voltage frequency. This phenomenon was used to extract a value of 0.6 M Omega/square for sheet resistance of the AIN nucleation layer at the GaN/SiC interface from the characteristic RC value of the network. This demonstrates the effectiveness of this voltage probe technique as a non-invasive method of characterizing nucleation layers.

Fabrication of single InGaN quantum dots (QDs) on top of GaN micropyramids is reported. The formation of single QDs is evidenced by showing single sub-millielectronvolt emission lines in microphotoluminescence (mu PL) spectra. Tunable QD emission energy by varying the growth temperature of the InGaN layers is also demonstrated. From mu PL, it is evident that the QDs are located in the apexes of the pyramids. The fact that the emission lines of the QDs are linear polarized in a preferred direction implies that the apexes induce unidirected anisotropic fields to the QDs. The single emission lines remain unchanged with increasing the excitation power and/or crystal temperature. An in-plane elongated QD forming a shallow potential with an equal number of trapped electrons and holes is proposed to explain the absence of other exciton complexes.

Capacitance-voltage [C(V)] measurements of metal-insulator-semiconductor-heterostructure capacitors are used to investigate the interface between silicon nitride passivation and AlGaN/AlN/GaN heterostructure material. AlGaN/AlN/GaN samples having different silicon nitride passivating layers, deposited using three different deposition techniques, are evaluated. Different interface state distributions result in large differences in the C(V) characteristics. A method to extract fixed charge as well as traps from the C(V) characteristics is presented. Rough estimates of the emission time constants of the traps can be extracted by careful analysis of the C(V) characteristics. The fixed charge is positive for all samples, with a density varying between 1.3 x 10(12) and 7.1 x 10(12) cm(-2). For the traps, the peak density of interface states is varying between 16 x 10(12) and 31 x 10(12) cm(-2) eV(-1) for the three samples. It is concluded that, of the deposition methods investigated in this report, the low pressure chemical vapor deposited silicon nitride passivation shows the most promising results with regards to low densities of interface states.

We report on the hot-wall MOCVD growth of Mg-doped AlxGa1-xN layers with an Al content as high as x similar to 0.85. After subjecting the layers to post-growth in-situ annealing in nitrogen in the growth reactor, a room temperature resistivity of 7 k Omega cm was obtained indicating an enhanced p-type conductivity compared to published data for AlxGa1-xN layers with a lower Al content of x similar to 0.70 and a room temperature resistivity of about 10 k Omega cm. It is believed that the enhanced p-type conductivity is a result of reduced compensation by native defects through growth conditions enabled by the distinct hot-wall MOCVD system.

AlxGa1-xN multiple quantum wells (MQW) were grown on AlN epilayer grown on 4H-SiC substrate. The growth was performed without interruption in a horizontal hot-wall MOCVD reactor using a mixture of hydrogen and nitrogen as carrier gases. The precursors were ammonia, trimethylaluminum and trimethylgallium. Results obtained from X-ray diffraction and infra-red reflectance were used to obtain the composition of the films when growing simple AlxGa1 xN layer. Visible reflectance was used to evaluate the thickness of the films. Finally the MQW parameters as thicknesses and composition variation were obtained by scanning transmission electron microscopy and demonstrated an agreement with the growth parameters used

We demonstrated successful growth of AIN at a temperature of 1200 degrees C in a set of hot-wall MOCVD systems with the possibility of straightforward scaling up the process on larger wafer areas to meet the demand of device technologies. We outlined several aspects of the carefully optimized design and process parameters with relevance to achievement of a high overall growth rate (1 and up to 2 mu m/h), efficiency, and uniformity, which to a great extent depends on how consumption of growth-limiting species by gas-phase adduct formation can actively be prevented. Mixing of the precursors upstream from the deposition area facilitates uniform epitaxial growth, while the greater uniformity of substrate temperature inherent to the hot-wall reactor and rotation of the wafer are of fundamental importance for layer-growth uniformity. The AIN layer thickness can be controlled with an accuracy of +/- 1.3% on 2 in. wafers. The low-temperature cathodoluminescence spectrum of the AIN epitaxial material is strongly dominated by the intense near band-gap deep UV emission at about 208 nm.

The inherent advantages of the hot-wall metal organic chemical vapor deposition (MOCVD) reactor (low temperature gradients, less bowing of the wafer during growth, efficient precursor cracking) compared to a cold-wall reactor make it easier to obtain uniform growth. However, arcing may occur in the growth chamber during growth, which deteriorates the properties of the grown material. By inserting insulating pyrolytic BN (PBN) stripes in the growth chamber we have completely eliminated this problem. Using this novel approach we have grown highly uniform, advanced high electron mobility transistor (HEMT) structures on 4 semi-insulating (SI) SiC substrates with gas-foil rotation of the substrate. The nonuniformities of sheet resistance and epilayer thickness are typically less than 3% over the wafer. The room temperature hall mobility of the 2DEG is well above 2000 cm(2)/V s and the sheet resistance about 270 Omega/sqr.

Currently, up to 50% of the channel temperature in AlGaN/GaN electronic devices is due to the thermal-boundary resistance (TBR) associated with the nucleation layer (NL) needed between GaN and SiC substrates for high-quality heteroepitaxy. Using 3-D time-resolved Raman thermography, it is shown that modifying the NL used for GaN on SiC epitaxy from the metal-organic chemical vapor deposition (MOCVD)-grown standard AIN-NL to a hot-wall MOCVD-grown AIN-NL reduces NL TBR by 25%, resulting in similar to 10% reduction of the operating temperature of AlGaN/GaN HEMTs. Considering the exponential relationship between device lifetime and temperature, lower TBR NLs open new opportunities for improving the reliability of AlGaN/GaN devices.

AlGaN/AlN/GaN high electron mobility transistor heterostructures grown by metal-organic chemical vapor deposition have been studied by temperature dependent time-resolved photoluminescence. The AlGaN-related emission is found to be sensitive to the excitation power and to the built-in internal electric field. In addition, this emission shows a shift to higher energy with the reduction in the excitation density, which is rather unusual. Using a self-consistent calculation of the band potential profile, we suggest a recombination mechanism for the AlGaN-related emission involving electrons confined in the triangular AlGaN quantum well and holes weakly localized due to potential fluctuations.

Epitaxial layers of AlGaN were grown by hot-wall MOCVD and their surfaces wet chemically etched with phosphorous acid. The as-grown surfaces and the development of the etched surfaces after 10 and 20 min of etching were studied with atomic force microscopy (AFM) and CL. In the as-grown layers growth features may be resolved while the RMS is as low as 1.4 Å in a scan area of 2×2 μm. Surfaces etched for 10 min had developed etch pits and a low RMS roughness of 7 Å indicating a uniform quality of the layers. Micrometer scale hexagonal features were observed after 20 min of etching. In some cases a deep hexagonal etch pit is observed in the centre of the hexagonal feature with a 30° rotation to each other, suggesting that the origin is substrate-induced defects. © 2006 Elsevier B.V. All rights reserved.