GaN-based Device Structures Research Articles

MOVPE of GaN-based mixed dimensional heterostructures on wafer-scale layered 2D hexagonal boron nitride—A key enabler of III-nitride flexible optoelectronics

We summarize our recent progress in Metal organic vapor phase epitaxy (MOVPE) van der Waals epitaxy of wafer-scale 2D layered hexagonal boron nitride (h-BN) on sapphire and subsequently grown III-N materials. This one step growth process allows for mechanical transfer of GaN-based devices from h-BN on sapphire to various supports. We first review the growth of h-BN on unpatterned and patterned sapphire templates. Second, we describe h-BN growth on dielectric pre-patterned sapphire templates, which enables dicing-free GaN-based device structures’ pick-and-place heterogenous integration of III-N devices. Third, we review the growth of self-assembled 1D GaN-based nanowire light emitting diode (LED) structures on layered 2D h-BN for mechanical transfer of nanowire LEDs. Together, these results illustrate the potential of wafer-scale van der Waals h-BN MOVPE to enhance the III-N device functionality and to improve III-N processing technology.

Novel Scalable Transfer Approach for Discrete III‐Nitride Devices Using Wafer‐Scale Patterned h‐BN/Sapphire Substrate for Pick‐and‐Place Applications

AbstractThe mechanical release of III‐nitride devices using h‐BN is a promising approach for heterogeneous integration. Upscaling this technology for industrial level requires solutions that allow a simple pick‐and‐place technique of selected devices for integration while preserving device performance. An advance that satisfies both of these requirements is demonstrated in this work. It is based on a lateral control of the h‐BN quality, using patterned sapphire with a SiO2 mask, to achieve localized van der Waals epitaxy of high‐quality GaN based device structures. After process fabrication, the devices can be individually picked and placed on a foreign substrate without the need for a dicing step. In addition, this approach could reduce delamination of h‐BN on large diameter substrates because each h‐BN region is smaller, with independent device structures. Discrete InGaN LEDs on h‐BN are grown and fabricated on 2 in. patterned sapphire using a SiO2 mask. A set of devices are selectively released and transferred to flexible aluminum tape. The transferred LEDs exhibit blue light emission around 435 nm. The approach presented here is scalable on any wafer size, can be applied to other types of nitride‐based devices, and can be compatible with commercial pick‐and‐place handlers for mass production.

Wafer-scale controlled exfoliation of metal organic vapor phase epitaxy grown InGaN/GaN multi quantum well structures using low-tack two-dimensional layered h-BN

Recent advances in epitaxial growth have led to the growth of III-nitride devices on 2D layered h-BN. This advance has the potential for wafer-scale transfer to arbitrary substrates, which could improve the thermal management and would allow III-N devices to be used more flexibly in a broader range of applications. We report wafer scale exfoliation of a metal organic vapor phase epitaxy grown InGaN/GaN Multi Quantum Well (MQW) structure from a 5 nm thick h-BN layer that was grown on a 2-inch sapphire substrate. The weak van der Waals bonds between h-BN atomic layers break easily, allowing the MQW structure to be mechanically lifted off from the sapphire substrate using a commercial adhesive tape. This results in the surface roughness of only 1.14 nm on the separated surface. Structural characterizations performed before and after the lift-off confirm the conservation of structural properties after lift-off. Cathodoluminescence at 454 nm was present before lift-off and 458 nm was present after. Electroluminescence near 450 nm from the lifted-off structure has also been observed. These results show that the high crystalline quality ultrathin h-BN serves as an effective sacrificial layer—it maintains performance, while also reducing the GaN buffer thickness and temperature ramps as compared to a conventional two-step growth method. These results support the use of h-BN as a low-tack sacrificial underlying layer for GaN-based device structures and demonstrate the feasibility of large area lift-off and transfer to any template, which is important for industrial scale production.

Time-resolved x-ray excited optical luminescence in InGaN/GaN multiple quantum well structures

Synchrotron-based x-ray radiation has been utilized to measure time-resolved x-ray excited optical luminescence (TR-XEOL) from InGaN/GaN multiple quantum well (MQW) structures. Excess carrier recombination lifetimes were determined for MQWs grown on conventional c-plane as well as on non-polar m-plane substrates. In addition, the simultaneous measurement of XEOL and x-ray fluorescence reveals an interaction between inner-core excitations of Si impurities and bound exciton recombination in doped GaN-based device structures. Furthermore, the TR-XEOL characterization technique were also applied to InGaN/GaN MQWs grown on GaN inverted pyramid structures.

Application of ZnO to Passivate the GaN-based Device Structures

AbstractThe fabrication of high-resistivity ZnO-based thin films lattice-matched to AlGaN/GaN structures has been developed. It relies on low-temperature reactive sputter deposition of ZnO:Sb from ZnSb target. Taking into account the hygroscopic nature of ZnO surface, an additional coating by Si3N4 films is applied to ensure the humidity protecition. The developped passivation suppresses leakage currents in Schottky diods, and substantially improves output characteristics of AlGaN/GaN HEMT.

III-N Epitaxial Growth for Nitride Devices

AbstractVarious GaN-based device structures were grown on (0001) sapphire and 6H-SiC substrates by metalorganic chemical vapor deposition. The device structures of this study include a variety of p-n junction-based devices, such as InGaN/GaN multiple-quantum-well green light emitting diodes, GaN p-i-n vertical rectifiers, and GaN/InGaN heterojunction bipolar transistors. This paper describes the epitaxial growth and device performance characteristics of these device structures. We have developed state-of-of-the-art growth techniques for the materials that are critical for high-performance electronic and optoelectronic devices. High-performance InGaN HBTs, high-voltage GaN rectifiers and long-wavelength green LEDs have been epitaxially grown, fabricated, and characterized. The details of the material growth, device fabrication, and device characterization will be presented.

The Application of High Pressure in Physics and Technology of III-V Nitrides

Due to high bonding energy of N 2 molecule, the III-V semiconducting nitrides, especially GaN and InN require high N 2 pressure to be stable at high temperatures necessary for growth of high quality single crystals. Physical properties of GaN-Ga(I)-N 2 system are discussed in the paper. On the basis of the experimental equilibrium p-T-x data and the quantum-mechanical modeling of interaction of N 2 molecule with liquid Ga surface, the conditions for crystallization of GaN were established. The crystals obtained under high pressure are of the best structural quality, having dislocation density as low as 10-100 cm - 2 which is several orders of magnitude better than in any other crystals of GaN. The method allows to grow both n-type substrate crystals for optoelectronics and highly resistive crystals for electronic applications. The physical properties of the pressure grown GaN measured to characterize both point defects and extended defects in the crystal lattice are discussed in the paper. A special attention is paid to the application of high pressure to reveal the nature of the point defects in the crystals and electric fields in GaN-based quantum structures. Due to their very high structural quality, the pressure grown crystals are excellent substrates for epitaxial growth of quantum structures. It opens new possibilities for optoelectronic devices, especially short wavelength high power lasers and efficient UV light emitting diodes. This is due to the strong reduction in dislocation densities in relation to existing structures (10 6 -10 8 cm - 2 ) which are grown on strongly mismatched sapphire and SiC substrates. The experimental results on the epitaxial growth and physical properties of GaN-based device structures supporting above conclusions are discussed in the paper. The current development of blue laser technology in High Pressure Research Center is shortly reviewed.

Cracking of GaN on sapphire from etch-process-induced nonuniformity in residual thermal stress

An experiment was performed to explain the appearance of cracks along mesa structures during the processing of GaN device layers grown on sapphire substrates. Micro-Raman spectroscopy was used to measure the position-dependent stress in the GaN layer. We show evidence that the stress at the interface with the substrate may be larger along the mesa structures than that of the as-grown layer, and that this increase in stress can be enough to induce cracks along mesa structures during processing. We report on the formation of cracks that propagate guided by the nonuniformity of the stress induced by the formation of mesa structures in the GaN layer, independent of crystal direction. The understanding of cracking mechanisms has implications in GaN-based device structures that require heteroepitaxial growth of layers with different lattice size and thermal expansion coefficients.

Mass Production of High Brightness Blue and Green LEDs by MOCVD

As more advances are made in the performance of GaN-based devices, a trend toward the use of large-scale MOCVD reactors for epitaxial growth of GaN-based device structures is clear. In this paper we describe the use of Emcore's SpectraBlueTM reactor for large-scale manufacturing of blue and green LEDs. The high throughput growth of GaN-based LEDs is demonstrated without compromising LED uniformity or overall performance. In-situ control of key parameters critical to the production of high quality LEDs, such as buffer layer growth is now feasible using in-situ reflectance spectroscopy. Film properties as well as LED device performance are discussed.

Properties of GaN Homoepitaxial Layers Grown on GaN Epitaxial Wafers

ABSTRACTThe lack of GaN substrates is a limiting factor for the development of III-V nitride devices.Recently we proposed to use GaN/SiC epitaxial wafers, consisting of thin GaN layer depositedby hydride vapor phase epitaxy (HVPE) on SiC wafer, as substrates for subsequent growth of IIIVnitrides and devices development. These wafers are attractive to be used as substrates for GaNdevice fabrication because the GaN-based device structures can be grown on these wafers byhomoepitaxy without any buffer layer. Due to high SiC thermoconductivity and cleavagepossibility, these wafers are especially attractive for high-power electronic and optoelectronicapplications. In this paper, we focus on crystal structure, optical and electrical properties of GaNhomoepitaxial layers and p-n structures grown by HVPE on GaN/SiC epitaxial wafers. Newtypes of III-V nitride epitaxial wafers are described, insulating GaN/SiC epitaxial wafers andAIN/SiC epitaxial wafers.

Electrochemical Etching in the GaN-Based Technology

ABSTRACTIn this study, both single undoped GaN epilayers and GaN-based device structures was treated by electrochemical etching in the dilute water solution of KOH or NaOH. Our investigations showed that in the undoped GaN epilayers grown by MOCVD the electrical and optical properties were nonuniform in depth. In this case, high defective and high conductive sublayer adjacent to the substrate was revealed by the electrochemical etching. This high conductive region was proved to condition the results of Hall effect measurements. Electrolyte etching of i-n GaN-based device structures grown by HVPE gave rise to significant increasing of the electroluminescence intensity. Influence of electrochemical etching on luminescence properties of the device structure is discussed.

Reactive Ion Etching of Boron Nitride and Gallium Nitride Materials in C12/Ar and BCl3/Cl2/Ar Chemistries

ABSTRACTBoron nitride (BN) and gallium nitride (GaN) are known as superior semiconductor materials for high power and high temperature applications. Undoped BN layers grown using ion beam and electron cyclotron resonance (ECR) assisted physical deposition on conductive GaN films have demonstrated good insulating properties. These films are thus good candidates as thin insulating layers in high temperature GaN-based device structures such as MIS diodes and MISFETs due to their close thermal expansion coefficient. In order to address the device processing issue, reactive ion etching (RIE) tests were performed on these films. Using Cl2/Ar chemistry, etch rates up to 600 Å/min were measured. These rates were found to increase linearly with increasing rf power and Cl2 flow rate. GaN layers grown by gas source MBE were also dry etched, resulting in smooth sidewalls. Etch rates up to 1,400 Å/min were achieved at 200 W rf power (-280 V d.c. bias) in a BCl3/Cl2/Ar chemistry; this is the highest RIE rate reported up to now for GaN. Using Cl2/Ar and BCl3/Cl2/Ar for BN and GaN respectively, etch selectivities in excess of 5:1 can be obtained. Finally, preliminary Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) results on residue deposition and surface composition changes as a function of the different etch conditions are presented.