Fabrication and characterization of UV-LEDs

This thesis investigates the emerging technology of ultraviolet light-emitting diodes (UV LEDs) based on III-nitride materials. Despite the incredible improvement of these devices over the past decade, if compared with visible LEDs based on the same semiconductor system, UV LEDs still suffer from a much reduced efficiency that severely limits their potential. The technological issues responsible for this problem have been analysed and possible solutions and mitigation strategies have been proposed, both at growth and fabrication level. In particular, the n-type doping of the AlGaN materials used in the cladding layers of these devices has been optimized, for AlN concentrations in the range of 50–85%. The transport mechanism in these materials has also been studied, and the presence of a significant impurity conduction at room temperature has been detected; the consequences of this fact on the doping optimization have been highlighted. A deep-UV LED for space application emitting below 250 nm, and a near-UV LED emitting at 340 nm—in whose active region an InAlN alloy has been used in place of the more common AlGaN—have both been successfully demonstrated. The use of micron-sized emitters has been investigated with the aim of improving switching characteristics and light-extraction efficiency of these devices. An optical bandwidth of over 20 MHz has been demonstrated for the deep-UV LED, as required by the funding agency. Thanks to the optimization work performed on the reflective sidewalls of the micro-emitters, an increase of light-extraction efficiency up to four times was shown

Doping of III-nitride materials

In this review paper we will report the current state of research regarding the doping of III-nitride materials and their alloys. GaN is a mature material with both n-type and p-type doping relatively well understood, and while n-GaN is easily achieved, p-type doping requires much more care. There are significant efforts to extend the composition range that can be controllably doped for AlGaInN alloys. This would allow application in shorter and longer wavelength optoelectronics as well as extending power electronic devices. It is found that doping of AlGaN and InGaN alloys with low-gallium-content has particular challenges, especially for p-materials and these issues are described

Fast growth of smooth AlN in a 3 x 2 showerhead-type vertical flow MOVPE reactor

The conditions required for a high growth rate of AlN in a 3 × 2″ showerhead-type vertical flow metalorganic vapor phase epitaxy (MOVPE) reactor are studied. It is found that at the standard growth conditions (low V/III, 50 mbar, 1110 °C, H2), the growth rate linearly increases with the trimethylaluminium (TMAl) flow rate until about 280 μmol min−1 with some drop of precursor utilization efficiency at higher pressures. While the pre-reaction of TMAl with NH3 at 140 μmol min−1 of TMAl is still not a major issue, it is not possible, however, to maintain a smooth AlN surface morphology during this “fast” growth. To suppress the surface morphology deterioration, the growth pressure requires optimization. An increase of the growth pressure, to 75 mbar, is found to be critical to grow 20+ μm of smooth AlN at a rate of about 3.6 μm h−1 on bulk AlN substrates

InAlN-based LEDs emitting in the near-UV region

Fully functional InAlN-based ultraviolet LEDs emitting at 340–350 nm were demonstrated for the first time; detailed electrical and optical characterization is presented and discussed. Results from the measurements at pulsed conditions are in agreement with the attribution of the dominant electroluminescence peak to near-band-edge emission. The composition of the AlGaN barriers was chosen to give the same internal polarization field as that of the InAlN wells. A simulation study of this polarization-matched heterostructure shows a significant increase in the electron-hole overlap integral if compared with a standard AlGaN/AlGaN active region having the same level of carrier confinement. Limitations and problems of these preliminary devices are also presented, and possible future work aimed at increasing their efficiency is discussed

Significant contribution from impurity-band transport to the room temperature conductivity of silicon-doped AlGaN

Silicon-doped n-type (0 0 0 1) AlGaN materials with 60% and 85% AlN content were studied close to the doping condition that gives the lowest resistivity (Si/III ratios in the ranges 2.8–34  ×  10−5 and 1.3–6.6  ×  10−5, respectively). Temperature-dependent conductivity and Hall-effect measurements showed that, apart from the diffusion-like transport in the conduction band, a significant amount of the conductivity was due to phonon-assisted hopping among localized states in the impurity band, which became almost completely degenerate in the most doped sample of the Al0.6Ga0.4N series. In the doping range explored, impurity-band transport was not only dominant at low temperature, but also significant at room-temperature, with contributions to the total conductivity up to 46% for the most conductive sample. We show that, as a consequence of this fact, the measurements of Hall carrier concentration and Hall mobility using the usual single-channel approach are not reliable, even at high temperatures. We propose a simple method to separate the contributions of the two channels. Our model, although only approximate, can be used to gain insight into the doping mechanism: particularly it shows that the room-temperature free-electron concentration in the conduction band of the Al0.6Ga0.4N material reaches its maximum at about 1.6  ×  1018 cm−3, well below the value that would have been obtained with the standard single-channel analysis of the data. This maximum is already achieved at dopant concentrations lower than the one that gives the best conductivity. However, further increase of the doping levels are required to enhance the impurity-band channel, with concentrations of the carriers participating in this type of transport that increase from 2.1  ×  1018 cm−3 up to 4.3  ×  1018 cm−3. For the Al0.85Ga0.15N, even though it was not possible to estimate the actual carrier concentrations, our measurements suggest that a significant impurity-band channel is present also in this material

GaN Nanowire Schottky Barrier Diodes

A new concept of vertical gallium nitride (GaN) Schottky barrier diode based on nanowire (NW) structures and the principle of dielectric REduced SURface Field (RESURF) is proposed in this paper. High-threading dislocation density in GaN epitaxy grown on foreign substrates has hindered the development and commercialization of vertical GaN power devices. The proposed NW structure, previously explored for LEDs offers an opportunity to reduce defect density and fabricate low cost vertical GaN power devices on silicon (Si) substrates. In this paper, we investigate the static characteristics of high-voltage GaN NW Schottky diodes using 3-D TCAD device simulation. The NW architecture theoretically achieves blocking voltages upward of 700 V with very low specific on-resistance. Two different methods of device fabrication are discussed. Preliminary experimental results are reported on device samples fabricated using one of the proposed methods. The fabricated Schottky diodes exhibit a breakdown voltage of around 100 V and no signs of current collapse. Although more work is needed to further explore the nano-GaN concept, the preliminary results indicate that superior tradeoff between the breakdown voltage and specific on-resistance can be achieved, all on a vertical architecture and a foreign substrate. The proposed NW approach has the potential to deliver low cost reliable GaN power devices, circumventing the limitations of today's high electron mobility transistors (HEMTs) technology and vertical GaN on GaN devices

Influence of free radical surface activation on Si/SiC heterogeneous integration by direct wafer bonding

In this study, a surface activated bonding method using remote plasma is applied to realize the direct wafer bonding of Si and SiC. A comparison of different surface treatments is reported. Hydrophilic and hydrophobic wafers have been exposed to in-situ argon and nitrogen radicals generated by remote plasma for surface activation before bonding. A comparison of the bonding yield and surface condition has been conducted and analyzed as a function of the surface treatments. It has been shown that N2 plasma leads to the highest yield of > 97 %, strongest bond of > 360 N and interfacial layer (IL) thickness of ~1.5 nm

Thermal stability of crystallographic planes of GaN nanocolumns and their overgrowth by metal organic vapor phase epitaxy

Thermal annealing of top−down fabricated GaN nanocolumns (NCs) was investigated over a wide range of temperatures for ammonia-rich atmospheres of both nitrogen and hydrogen. It was found that in contrast to the annealing of planar GaN layers, where surface morphology change is governed purely by material decomposition, reshaping of GaN NCs is strongly affected by competition between different crystallographic facets, which in turn depends on ambient atmosphere and temperature. A qualitative mechanism explaining the observed behavior has been proposed. On the basis of the analysis of these annealing results, growth conditions suitable for either predominantly lateral expansion of the NCs turning their sidewalls into six well-defined vertical m-plane facets, or, vice versa, their infilling from the base regions between the NCs were determined. GaN NC arrays of increased filling factors as compared to the as top−down fabricated ones have been demonstrated using these optimized growth conditions

Significant contribution from impurity-band transport to the room temperature conductivity of silicon-doped AlGaN

Silicon-doped n-type (0 0 0 1) AlGaN materials with 60% and 85% AlN content were studied close to the doping condition that gives the lowest resistivity (Si/III ratios in the ranges 2.8–34  ×  10−5 and 1.3–6.6  ×  10−5, respectively). Temperature-dependent conductivity and Hall-effect measurements showed that, apart from the diffusion-like transport in the conduction band, a significant amount of the conductivity was due to phonon-assisted hopping among localized states in the impurity band, which became almost completely degenerate in the most doped sample of the Al0.6Ga0.4N series. In the doping range explored, impurity-band transport was not only dominant at low temperature, but also significant at room-temperature, with contributions to the total conductivity up to 46% for the most conductive sample. We show that, as a consequence of this fact, the measurements of Hall carrier concentration and Hall mobility using the usual single-channel approach are not reliable, even at high temperatures. We propose a simple method to separate the contributions of the two channels. Our model, although only approximate, can be used to gain insight into the doping mechanism: particularly it shows that the room-temperature free-electron concentration in the conduction band of the Al0.6Ga0.4N material reaches its maximum at about 1.6  ×  1018 cm−3, well below the value that would have been obtained with the standard single-channel analysis of the data. This maximum is already achieved at dopant concentrations lower than the one that gives the best conductivity. However, further increase of the doping levels are required to enhance the impurity-band channel, with concentrations of the carriers participating in this type of transport that increase from 2.1  ×  1018 cm−3 up to 4.3  ×  1018 cm−3. For the Al0.85Ga0.15N, even though it was not possible to estimate the actual carrier concentrations, our measurements suggest that a significant impurity-band channel is present also in this material

Reduction of threading dislocation density in top-down fabricated GaN nanocolumns via their lateral overgrowth by MOCVD

Reduction of threading dislocation density in top-down fabricated GaN nanocolumns (NCs) via their successive lateral shrinkage by anisotropic wet etch and lateral overgrowth by metalorganic chemical vapor deposition is studied by transmission electron microscopy. The fabrication process involves a combination of dry and wet etches to produce NC arrays of a low fill factor (<5%), which are then annealed and laterally overgrown to increase the array fill factor to around 20%–30%. The resulting NC arrays show a reduction in threading dislocation density of at least 25 times, allowing for the reduction in material volume due to the array fill factor, with dislocations being observed to bend into the voids between NCs during the overgrowth process