Monolithic integration of light-emitting diodes and power metal-oxide-semiconductor channel high-electron-mobility transistors for light-emitting power integrated circuits in GaN on sapphire substrate

We report the demonstration of monolithically integrated light-emitting diodes (LEDs) and power metal-oxide-semiconductor channel high-electron-mobility transistors (HEMTs) in GaN. The structure comprised a direct epitaxial integration of layers typical for a GaN-based LED grown directly on top of the layers of a GaN-based HEMT. The layers were then fabricated into a serially connected pair of GaN LED and metal-oxide-semiconductor-gated 0.3 lm-channel HEMT by exposing the LED/HEMT epitaxial layers in selective area etching. The resulting monolithically integrated circuit shows a full gate voltage modulation of the light output power. This demonstrates compatibility of group-III nitride LED and HEMT processes. GaN-based high-power light-emitting diodes (LEDs) used in lighting applications typically require dedicated electronic driver circuits for AC-DC power conversion, current sourcing, and dimming using pulse-width modulation (PWM) or analog current control methods. 1 GaN power switching field-effect transistors (FETs), such as metaloxide-semiconductor (MOS) FETs, high-electron-mobility transistors (HEMTs), and MOS-Channel HEMTs (MOSCHEMTs) have shown outstanding performance in terms of high breakdown voltage (BV), low specific on-resistance, and high operating frequency, 2-4 and can be very useful as output devices for emerging applications of high power-high voltage LED systems. Ultimately, monolithic integration of GaN-based LEDs and GaN power HEMTs can reduce the cost and the size of solid state lighting systems, improve system reliability, and serve as a technology platform for the development of light-emitting power integrated circuits (LEPICs). LEPICs can also play an important role in adding functionalities required for emerging solid state lighting applications such as visible light communication (VLC) and other LED control technologies required for future smart lighting applications

Structural Properties of Eu-Doped GaN Investigated by Raman Spectroscopy

Rare-earth (RE) impurities doped GaN are highly promising candidates for light emitting device applications due to their efficient electroluminescence properties at room temperature. Among those, Eu doped GaN has been identified as an excellent material for the red spectral region due to its strong emission at 620 nm. As a transition internal to the Eu doping atom (4f-4f), light emission originates in a much smaller complex than the more flexibly controllable quantum structures of wells, wires, and dots. This is thought to make the center less susceptible to structural defects and in particular radiation damage in the lattice host. Nevertheless, the lattice host is crucial for providing the excitation in from of free electrons and holes. In this respect, the actual lattice site Eu occupies in the host lattice, i.e. in GaN, is important. A large fraction of Eu atoms are typically inactive which must be attributed to their lattice site and local environment. GaN films implanted with Eu to concentrations of {approx}10{sup 18} cm{sup -3} were subjected to a highly directed beam of 500 keV He{sup +} at a dose of 5 x 10{sup 14} cm{sup -2}. By means of a shadow mask, irradiated and unexposed regions lie very close to each other on the same sample. We used optical and structural analysis to identify the exerted radiation damage. At the full radiation dose, photoluminescence intensity has decayed to {approx}0.01 of its initial value. From the dose dependence of the radiation decay we previously concluded, that this decay is in part due to the destruction of radiative Eu sites [J.W. Tringe, unpublished (2006)]. Along the transition from virgin to irradiated material we analyze the accumulated damage in terms of surface morphology (atomic force microscopy), crystallinity (x-ray diffraction), and phonon dispersion using micro-Raman spectroscopy. In addition to the well-studied E{sub 2}(high) mode, two new vibrational modes at 659 cm{sup -1} and 201 cm{sup -1} were observed in the Eu implanted and annealed sample, prior to He{sup +} irradiation. These modes are either remnants of the implantation damage or related to the Eu impurity. As such they can be indicative of the actual lattice site the Eu atom resides on. After irradiation, broad Raman modes at 300 cm-1 are being observed. This band indicates disorder activated Raman scattering (DARS) due to the radiation damage. An additional narrow mode appears at 672 cm{sup -1}, which can possibly be due to a nitrogen vacancy related vibrational mode. The continuous transition from irradiated to un-irradiated sample allows the direct evolution of radiation damage and its coordinated effects in structural, optical and vibrational properties. By its systematic correlation we anticipate to be able to elucidate the Eu lattice interaction and the processes of radiation damage

Investigation of cracks in GaN films grown by combined hydride and metal organic vapor-phase epitaxial method

Cracks appeared in GaN epitaxial layers which were grown by a novel method combining metal organic vapor-phase epitaxy (MOCVD) and hydride vapor-phase epitaxy (HVPE) in one chamber. The origin of cracks in a 22-μm thick GaN film was fully investigated by high-resolution X-ray diffraction (XRD), micro-Raman spectra, and scanning electron microscopy (SEM). Many cracks under the surface were first observed by SEM after etching for 10 min. By investigating the cross section of the sample with high-resolution micro-Raman spectra, the distribution of the stress along the depth was determined. From the interface of the film/substrate to the top surface of the film, several turnings were found. A large compressive stress existed at the interface. The stress went down as the detecting area was moved up from the interface to the overlayer, and it was maintained at a large value for a long depth area. Then it went down again, and it finally increased near the top surface. The cross-section of the film was observed after cleaving and etching for 2 min. It was found that the crystal quality of the healed part was nearly the same as the uncracked region. This indicated that cracking occurred in the growth, when the tensile stress accumulated and reached the critical value. Moreover, the cracks would heal because of high lateral growth rate

Structural dynamics of GaN microcrystals in evolutionary selection selective area growth probed by X-ray microdiffraction

A method to grow high quality, single crystalline semiconductor material irrespective of the substrate would allow a cost-effective improvement to functionality and performance of optoelectronic devices. Recently, a novel type of substrate-insensitive growth process called Evolutionary Selection Selective Area Growth (ES-SAG) has been proposed. Here we report the use of X-ray microdiffraction to study the structural properties of GaN microcrystals grown by ES-SAG. Utilizing high resolution in both direct and reciprocal spaces, we have unraveled structural dynamics of GaN microcrystals in growth structures of different dimensions. It has been found that the geometric proportions of the growth constrictions play an important role: 2.6â.Î 1/4m and 4.5â.Î 1/4m wide growth tunnels favor the evolutionary selection mechanism, contrary to the case of 8.6â.Î 1/4m growth tunnels. It was also found that GaN microcrystal ensembles are dominated by slight tensile strain irrespective of growth tunnel shape

Growth of Crack-Free thick AlGaN Layer and its Application to GaN-Based Laser Diode

In the field of group-III nitrides, hetero-epitaxial growth has been one of the most important key technologies. A thick layer of AlGaN alloy with higher AlN molar fraction is difficult to grow on sapphire substrate, because the alloy layer is easily cracked. It is thought that one cause of generating cracks is a large lattice mismatch between an AlGaN and a GaN, when AlGaN is grown on the underlying GaN layer. We have achieved crack-free Al0.07Ga0.93N layer with the thickness of more than 1μm using underlying Al0.05Ga0.95N layer. The underlying Al0.05Ga0.95N layer was grown directly on sapphire by using the low-temperature-deposited buffer layer (LT-buffer layer). Since a lattice mismatch between the underlying Al0.05Ga0.95N layer and upper Al0.07Ga0.93N layer is relatively small, the generation of cracks is thought to be suppressed. This technology is applied to a GaN-based laser diode structure, in which thick n-Al0.07Ga0.93N cladding layer grown on the Al0.05Ga0.95N layer, improves optical confinement and single-robe far field pattern in vertical direction.</jats:p

Growth of Crack-Free Thick AlGaN Layer and its Application to GaN-Based Laser Diode

AbstractIn the field of group-III nitrides, hetero-epitaxial growth has been one of the most important key technologies. A thick layer of AlGaN alloy with higher AlN molar fraction is difficult to grow on sapphire substrate, because the alloy layer is easily cracked. It is thought that one cause of generating cracks is a large lattice mismatch between an AlGaN and a GaN, when AlGaN is grown on the underlying GaN layer. We have achieved crack-free Al0.07Ga0.93N layer with the thickness of more than 1mm using underlying Al0.05Ga0.95N layer. The underlying Al0.05Ga0.95N layer was grown directly on sapphire by using the lowtemperature-deposited buffer layer (LT-buffer layer). Since a lattice mismatch between the underlying Al0.05Ga0.95N layer and upper Al0.07Ga0.93N layer is relatively small, the generation of cracks is thought to be suppressed. This technology is applied to a GaN-based laser diode structure, in which thick n-Al0.07Ga0.93N cladding layer grown on the Al0.05Ga0.95N layer, improves optical confinement and single-robe far field pattern in vertical direction.</jats:p