Gallium Nitride Epitaxy Research Articles

Silicon nitride shadowed selective area growth of low defect density vertical GaN mesas via plasma-assisted molecular beam epitaxy

Ion bombardment during inductively coupled plasma reactive-ion etching and ion-implantation introduces irreparable crystalline damage to gallium nitride (GaN) power devices, leading to early breakdown and high leakage current. To circumvent this, a bi-layer selective area growth mask was engineered to grow up to 3.0 µm thick epitaxy of GaN using plasma-assisted molecular beam epitaxy as an ion-damage-free alternative to standard epitaxial processing routes. The masks and regrown architectures are characterized via SEM, conductive-atomic force microscopy (AFM), x-ray photo electron spectroscopy, Raman, and cathodoluminescence. Mask deposition conditions were varied to modulate and minimize the stress induced during thermal cycling. The resulting mesas exhibit low leakage, attributed to naturally terminated sidewalls as measured by an innovative perpendicular AFM measurement of the regrown sidewall. The regrown sidewall exhibited RMS (root mean square) roughness of 1.50 (±0.34) nm and defect density of 1.36 × 106 (±1.11 × 106) cm−2. This work provides a method to eliminate defect-inducing steps from GaN vertical epitaxial processing and stands to enhance GaN as a material platform for high-efficiency power devices.

(Invited) Advanced Electronic Characterization of Wide Band Gap Semiconductor Defects at the Nanoscale

Wide bandgap materials such as gallium nitride (GaN) and gallium oxide (Ga2O3) are currently under development for realizing next-generation high power electronic devices beyond silicon. Vertical structures produced through homoepitaxial growth are preferred for scaling high-voltage devices, which places strict demands on the quality of bulk substrates. In particular, extended defects can propagate during epitaxial growth into the active areas of devices. Therefore, there is a need for identifying defects and determining their electronic properties at relevant nano- to micro- scale dimensions in order to disentangle their combined effects on macroscopic devices. Here, we use a combination of laser-based photoemission electron microscopy (PEEM) and electrical atomic force microscopy (AFM) methodologies to investigate the local electronic properties of surface defects in epitaxially grown GaN and β-Ga2O3. PEEM is a non-scanning electron microscopy technique where photoelectrons emitted by a surface illuminated by deep-ultraviolet light are imaged with nanometer-scale spatial resolution. The emitted photoelectrons are sensitive to the local electronic structure, making PEEM a powerful tool for evaluating electronic changes at length scales relevant for devices, and provides valuable information for identifying “killer” defects which must be minimized during device growth and processing.The difficulty in producing large, low dislocation density bulk GaN substrates has led to longstanding issues with leakage current in diodes and transistor devices. We identify surface defects with different electronic properties in GaN epitaxy on different substrates using PEEM. We observe star-shaped defects on strain-patterned GaN substrates and elongated dark patches on growth ridges on ammonothermally grown GaN substrates. In both cases, the observed electronic changes in PEEM correlate directly with local electrical AFM measurements, revealing that local changes in the electronic structure at these defects results in altered electrical conduction.In contrast, the development of large bulk Ga2O3 substrates has been more successful, yet the identification and influence of various defects on device performance is still largely unknown at this point. The topography of β-Ga2O3 epitaxy varies widely between growth methods and substrate orientations. For example, β-Ga2O3 by hydride vapor physical epitaxy lacks fine structure, yet has surface defects that regularly appear in parallel orientations. In β-Ga2O3 grown on different substrate orientations, we observe variations in the surface electronic properties using PEEM. For certain substrate orientations and growths, we find nanoscale variations in doping, while other orientations display micron-scale electronic inhomogeneity and defects. Our results demonstrate that laser-based PEEM can be a powerful tool for detecting electrically active defects, as well as evaluating the nanoscale electronic homogeneity of wide bandgap semiconductors.

Localized Epitaxial Growth of 402 V Breakdown Voltage Quasi‐Vertical GaN‐on‐Si p‐n Diode on 200 mm‐Diameter Wafers

Localized epitaxy of gallium nitride (GaN) on silicon (Si) wafers is an efficient way to relax elastically the tensile stress generated in the GaN layer after growth, allowing epitaxy of thick layers for the fabrication of vertical power devices operating at high voltage. In this study, a 4.7 μm‐thick GaN layer is grown by metal–organic vapor phase epitaxy on 200 mm‐diameter Si wafers for the fabrication of quasi‐vertical Schottky and p‐n diodes. The uniformity of the doping concentration in the layer is mapped spatially by scanning spreading resistance microscopy, while scanning capacitance microscopy illustrates the differently doped regions in the p‐n diode. The net doping concentration is extracted by capacitance–voltage (C–V) measurements and it is found to be about 3 × 1016 cm−3. On a 140 μm‐diameter quasi‐vertical p‐n diode, destructive breakdown occurs at 402 V, with no periphery protection on the device, demonstrating that localized epitaxy of GaN on Si has great potential for vertical high‐power devices.

Hexagonal Boron Nitride as an Intermediate Layer for Gallium Nitride Epitaxial Growth in Near-Ultraviolet Light-Emitting Diodes.

We introduce the development of gallium nitride (GaN) layers by employing graphene and hexagonal boron nitride (h-BN) as intermediary substrates. This study demonstrated the successful growth of GaN with a uniformly smooth surface morphology on h-BN. In order to evaluate the crystallinity of GaN grown on h-BN, a comparison was conducted with GaN grown on a sapphire substrate. Photoluminescence spectroscopy and X-ray diffraction confirmed that the crystallinity of GaN deposited on h-BN was inferior to that of GaN grown on conventional GaN. To validate the practical applicability of the GaN layer grown on h-BN, we subsequently grew an NUV-LED structure and fabricated a device that operated well in optoelectrical performance experiments. Our findings validate the potential usefulness of h-BN to be a substrate in the direct growth of a GaN layer.

A Brief Overview of the Rapid Progress and Proposed Improvements in Gallium Nitride Epitaxy and Process for Third-Generation Semiconductors with Wide Bandgap.

In this paper, we will discuss the rapid progress of third-generation semiconductors with wide bandgap, with a special focus on the gallium nitride (GaN) on silicon (Si). This architecture has high mass-production potential due to its low cost, larger size, and compatibility with CMOS-fab processes. As a result, several improvements have been proposed in terms of epitaxy structure and high electron mobility transistor (HEMT) process, particularly in the enhancement mode (E-mode). IMEC has made significant strides using a 200 mm 8-inch Qromis Substrate Technology (QST®) substrate for breakdown voltage to achieve 650 V in 2020, which was further improved to 1200 V by superlattice and carbon-doped in 2022. In 2016, IMEC adopted VEECO metal-organic chemical vapor deposition (MOCVD) for GaN on Si HEMT epitaxy structure and the process by implementing a three-layer field plate to improve dynamic on-resistance (RON). In 2019, Panasonic HD-GITs plus field version was utilized to effectively improve dynamic RON. Both reliability and dynamic RON have been enhanced by these improvements.

Characterization of unintentional doping in localized epitaxial GaN layers on Si wafers by scanning spreading resistance microscopy

Localized epitaxy of gallium nitride (GaN) on silicon (Si) is studied, with the aim of achieving material compatible with 1200 V vertical devices, in particular an unintentional doping level <1 × 1016 cm−3, which is essential to have high quality devices. In this study, three mask materials (SiN, SiO2, Al2O3) are examined and the unintentional doping concentration in GaN layers grown by metal organic vapor phase epitaxy (MOVPE) is investigated by scanning spreading resistance microscopy (SSRM). The results from SSRM are verified by secondary ions mass spectroscopy (SIMS) and show that an unintentional doping concentration lower than 1016 cm−3 is achieved when Al2O3 is used as the mask material. This value is lower than any previous studies of localized GaN growth, and in addition, this is the first time that SSRM has been used to observe such low doping concentrations in GaN.

Strain Release in GaN Epitaxy on 4° Off-Axis 4H-SiC.

A hybrid field-effect transistor (HyFET), superior for power electronic applications, can be created by harnessing the merits of two representative wide-bandgap semiconductors, gallium nitride (GaN) and silicon carbide (SiC). Yet, the incompactness in the epitaxy techniques hinders the development of the HyFET-GaN is usually grown on on-axis foreign substrates including SiC, whereas SiC homoepitaxy prefers off-axis substrates. This work presents a GaN-based heterostructure epitaxially grown on a conventional 4° off-axis 4H-SiC substrate, which manifests its high quality and suitability for constructing GaN-based high-electron-mobility transistors, thereby suggesting a practical approach to realizing HyFETs. In the meanwhile, a distinct two-step biaxial strain-relaxation process is proposed and studied with comprehensive characterizations.

Effect of Flux Rate Variation at Fixed V/III Ratio on Semi-Polar (112¯2) GaN: Crystal Quality and Surface Morphology Study

We report on the crystal improvement of semi-polar (112¯2) gallium nitride epitaxy layer on m-plane (101¯0) sapphire substrate by changing the flux rate at a fixed V/III ratio. The high-resolution X-ray diffraction (HR-XRD) analysis showed that lower flux rate enhanced the crystal quality of GaN epitaxy with the lowest FWHM values of 394 and 1173 arc seconds at [11¯23] and [11¯00] planes, respectively. In addition, Raman spectroscopy showed that flux rate did not affect the stress state of the GaN crystal. However, atomic force microscopy (AFM) micrograph depicted an anomalous trend where the lowest flux rate produces roughest surface with RMS roughness of 40.41 nm. Further analysis of AFM results on the undulation period length along [11¯23] and [11¯00] directions is carried out. It shows that as the growth rate decreases, the average undulation period along [11¯23] and [11¯00] directions increases from 2.59 µm and 1.90 µm to 3.52 µm and 3.52 µm, respectively. The mechanism for the surface roughening at the lower flux rate is then explained by using the adatom surface diffusion relation L ~ Dτ.

Lattice Polarity Manipulation of Quasi-vdW Epitaxial GaN Films on Graphene Through Interface Atomic Configuration.

Quasi van der Waals epitaxy, a pioneering epitaxy of sp3 -hybridized semiconductor films on sp2 -hybridized 2D materials, provides a way, in principle, to achieve single-crystal epilayers with preferred atom configurations that are free of substrate. Unfortunately, this has not been experimentally confirmed in the case of the hexagonal semiconductor III-nitride epilayer until now. Here, it is reported that the epitaxy of gallium nitride (GaN) on graphene can tune the atom arrangement (lattice polarity) through manipulation of the interface atomic configuration, where GaN films with gallium and nitrogen polarity are achieved by forming CONGa(3) or COGaN(3) configurations, respectively, on artificial CO surface dangling bonds by atomic oxygen pre-irradiation on trilayer graphene. Furthermore, an aluminum nitride buffer/interlayer leads to unique metal polarity due to the formation of an AlON thin layer in a growth environment containing trace amounts of oxygen, which explains the open question of why those reported wurtzite III-nitride films on 2D materials always exhibit metal polarity. The reported atomic modulation through interface manipulation provides an effective model for hexagonal nitride semiconductor layers grown on graphene, which definitely promotes the development of novel semiconductor devices.

Laser‐Assisted Metal–Organic Chemical Vapor Deposition of Gallium Nitride

Ammonia (NH3) is commonly used as group V precursor in gallium nitride (GaN) metalorganic chemical vapor deposition (MOCVD). The high background carbon (C) impurity in MOCVD GaN is related to the low pyrolysis efficiency of NH3, which represents one of the fundamental challenges hindering the development of high purity thick GaN for vertical high power device applications. This work uses a laser-assisted MOCVD (LA-MOCVD) growth technique to address the high-C issue in MOCVD GaN. Carbon dioxide (CO2) laser with wavelength of 9.219 um was utilized to facilitate NH3 decomposition via resonant vibrational excitation. The LA-MOCVD GaN growth rate (as high as 10 um/hr) shows a strong linear relationship with the trimethylgallium (TMGa) flow rate, indicating high effective V/III ratios and hence efficient NH3 decomposition. Pits-free surface morphology of LA-MOCVD GaN was demonstrated for films with growth rate as high as 8.5 um/hr. The background [C] in LA-MOCVD GaN films decreases monotonically as the laser power increases. A low [C] at 5.5E15 cm-3 was achieved in LA-MOCVD GaN film grown with the growth rate of 4 um/hr. Charge transport characterization of LA-MOCVD GaN films reveals high crystalline quality with room temperature mobility >1000 cm2/Vs. LA-MOCVD growth technique provides an enabling route to achieve high quality GaN epitaxy with low-C impurity and fast growth rate simultaneously. This technique can also be extended for epitaxy of other nitride-based semiconductors.

Laser‐Assisted Metal–Organic Chemical Vapor Deposition of Gallium Nitride

Ammonia (NH3) is commonly used as group‐V precursor in gallium nitride (GaN) metal–organic chemical vapor deposition (MOCVD). The high background carbon (C) impurity in MOCVD GaN is related to the low decomposition efficiency of NH3, which represents one of the fundamental challenges hindering the development of high‐purity thick GaN for vertical high‐power device applications. This work uses a laser‐assisted MOCVD (LA‐MOCVD) growth technique to address the high‐C issue in MOCVD GaN. A carbon dioxide (CO2) laser with a wavelength of 9.219 μm is utilized to facilitate NH3 decomposition via resonant vibrational excitation. The LA‐MOCVD GaN growth rate (GR; as high as 10 μm h−1) shows a strong linear relationship with the trimethylgallium (TMGa) flow rate, indicating high effective V/III ratios and, hence, efficient NH3 decomposition. [C] in LA‐MOCVD GaN films decreases monotonically, as the laser power increases. A low [C] at 5.5 × 1015 cm−3 is achieved in the LA‐MOCVD GaN film grown with GR of 4 μm h−1. LA‐MOCVD GaN films reveal high crystalline quality with room‐temperature mobility of &gt;1000 cm2 V−1 s−1. LA‐MOCVD provides an enabling route to achieve high‐quality GaN epitaxy with low‐C and fast GR simultaneously. This technique can be extended for epitaxy of other nitride‐based semiconductors.

Reactor materials for high purity HVPE GaN growth: a thermodynamic analysis

The chemical resistance of quartz, aluminum oxide, boron nitride, molybdenum, and platinum in the environment of the hydride vapor phase epitaxy of gallium nitride was estimated by chemical equilibrium calculation. The interaction of materials with hydrogen, hydrogen chloride, chlorine, ammonia, and gallium chlorides was analyzed. The conditions in which the use of each material is permissible were determined.

Predicting the Conditions for the Vapor-Phase Epitaxy of the III–V Compounds

Chloride-hydride epitaxy is the main vapor-phase technique for forming layers of functional homostructures and heterostructures for microelectronics and optoelectronics. At present, nanoheterostructures are obtained by MOS-hydride and molecular epitaxy; the molecular-layering technique is currently being perfected. The occurrence of new materials requires long-term development of the optimal technological conditions for their fabrication and the mathematical, physical, and other principles for modeling these processes. The chloride-hydride method continues to be perfected for forming relatively thick layers of functional heterostructures. The work outlines the fundamentals of physicochemical modeling by the example of chloride-hydride epitaxy. A physicochemical model of the variation in the technological modes of the vapor-phase epitaxy of different compounds under the corresponding conditions, which facilitate the formation of compounds with the same degree of disorder, is discussed. Equations for predicting the conditions for the epitaxy of other materials of the same group by the well-developed technology of a material have been derived. The obtained regularities can be used to optimize the chloride-hydride epitaxy of gallium phosphide and solid solutions based on it. The calculated conditions for gallium-nitride epitaxy are shown to be in good agreement with the conditions of real technological developments made by other authors.

Liquid Phase Epitaxy of Gallium Nitride

In this work, liquid phase epitaxy of gallium nitride (GaN) has been achieved using pulsed plasma nitridation of molten Ga films. Typically, continuous exposure of Ga to nitrogen plasma results in ...

Epitaxy of GaN in high aspect ratio nanoscale holes over silicon substrate

Dislocation filtering in gallium nitride (GaN) by epitaxial growth through patterned nanoscale holes is studied. GaN grown from extremely high aspect ratio holes by metalorganic chemical vapor deposition is examined by transmission electron microscopy and high-resolution transmission electron microscopy. This selective area epitaxial growth method with a reduced epitaxy area and an increased depth to width ratio of holes leads to effective filtering of dislocations within the hole and improves the quality of GaN significantly.

(Invited) Simulation Study of High Voltage Vertical GaN Nanowire Field Effect Transistors

A Nanostructured GaN Device Architecture for Power Applications Z. John Shen, Gourab Sabui, Vitaly Z. Zubialevich, Mary White, Pietro Pampili, Peter J. Parbrook, Mathew McLaren, Miryam Arredondo-Arechavala Gallium nitride (GaN) has emerged as a promising material for development of power semiconductor devices owing to its high critical electric field, large bandgap and superior transport properties. The unique properties of GaN have prompted realization of power devices both in a lateral (high electron mobility transistors (HEMTs)) and a vertical layout (bulk GaN on GaN devices). GaN power devices show extremely low conduction and switching losses and holds the key to extremely low-loss and high-efficiency power converters of the future. However, GaN power devices have been plagued with several inherent drawbacks preventing a ubiquitous adoption of GaN as the material of choice for power switches. The most critical trade-off has been the choice of substrate for the growth of GaN epitaxy: a high performance, high-cost native substrate against a low-cost, non-native substrate with associated reliability issues. For GaN to thrive as a superior successor to Si, a low cost, high performance epitaxy with improved reliability is expected moving forward. Current lateral HEMTs and vertical GaN on GaN devices are inadequate to address this trade-off. A novel nanostructured approach to GaN power devices is proposed in this paper. The unique property of GaN nanowires is its ability to elastically relax in the lateral direction, and accommodate lattice mismatch with non-native substrate through pseudomorphic growth without dislocation formation as compared to thin-film heterostructures. Experimental studies have shown self-induced GaN NWs grown with a high surface to volume ratio on a Si substrate have a non-polar side face and is virtually free of threading dislocations and other structural defects. Power devices grown on a nano-GaN epitaxy theoretically have the potential to bypass the reliability concerns associated with a non-native substrate but still deliver comparable or even superior performance, albeit at a low-cost. Schottky barrier diodes are designed and fabricated based on the nano-GaN concept. Fabricated diodes grown on a Sapphire substrate show distinct rectifying properties blocking voltages of 100 V. Despite repetitive biasing, the devices did not show any sign of current collapse. Design optimization of the nanowire Schottky barrier diode (NWSBD) is performed using 3D TCAD drift-diffusion simulations and semiconductor-oxide interaction was utilized to push the performance of the NWSBDs beyond the unipolar limit of GaN. The optimized NWSBD show the potential to block voltages upwards of 650 V working under present fabrication constraints of doping concentrations and pillar height. Simulated characteristics predict superior VRB 2/RON FoM, outperforming both the lateral HEMTs and the vertical GaN on GaN devices. A fully controlled, three terminal nanowire field effect transistor (NWFET) is also investigated using 3D TCAD drift-diffusion modeling. The NWFET can be operated in both normally-off and normally-on modes based on design and requirements. The 3D gate around the nanowires coupled with a strong dielectric REduced SURface Field (RESURF) effect allows this architecture to block voltages beyond the 1D unipolar material limit of GaN in the reverse bias. With proper design, highly desirable linear voltage to pillar height scaling can be achieved for the NWFET. Breakdown voltages of more than 900 V is predicted, with superior VRB 2/RON and QGD x RDS(ON) figure of merits. Despite lacking a body-diode, the NWFETs being quasi-symmetrical enables current conduction in the 3rd quadrant itself by self-commutated reverse conduction with a very low voltage drop. The nanowire architecture holds great potential to produce power rectifiers and controlled FETs with high performance, improved reliability and ruggedness and in a cost-effective way.

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.

Non-polar GaN film growth on (0 1 0) gallium oxide substrate by metal organic chemical vapor deposition

Abstract

MOVPE growth of GaN on 6-inch SOI-substrates: effect of substrate parameters on layer quality and strain

We demonstrate that higher crystalline quality, lower strain and improved electrical characteristics can be achieved in gallium nitride (GaN) epitaxy by using a silicon-on-insulator (SOI) substrate compared to a bulk silicon (Si) substrate. GaN layers were grown by metal–organic vapor phase epitaxy on 6-inch bulk Si and SOI wafers using the standard step graded AlGaN and AlN approach. The GaN layers grown on SOI exhibited lower strain according to x-ray diffraction analysis. Defect selective etching measurements suggested that the use of SOI substrate for GaN epitaxy reduces the dislocation density approximately by a factor of two. Furthermore, growth on SOI substrate allows one to use a significantly thinner AlGaN buffer compared to bulk Si. Synchrotron radiation x-ray topography analysis confirmed that the stress relief mechanism in GaN on SOI epitaxy is the formation of a dislocation network to the SOI device Si layer. In addition, the buried oxide layer significantly improves the vertical leakage characteristics as the onset of the breakdown is delayed by approximately 400 V. These results show that the GaN on the SOI platform is promising for power electronics applications.

Possibility of the use of intermediate carbidsiliconoxide nanolayers on polydiamond substrates for gallium nitride layers epitaxy

The results of using carbidsiliconoxide (a-C:SiO1.5) films with a thickness of 30–60 nm, produced by the pyrolysis annealing of oligomethylsilseskvioksana (CH3–SiO1.5)n with cyclolinear (staircased) molecular structure, as intermediate films in the hydride vapor phase epitaxy of gallium nitride on polycrystalline CVD-diamond substrates are presented. In the pyrolysis annealing of (CH3–SiO1.5)n films in an atmosphere of nitrogen at a temperature of 1060°C, methyl radicals are carbonized to yield carbon atoms chemically bound to silicon. In turn, these atoms form a SiC monolayer on the surface of a-C:SiO1.5 films via covalent bonding with silicon. It is shown that GaN islands grow on such an intermediate layer on CVD-polydiamond substrates in the process of hydride vapor phase epitaxy in a vertical reactor from the GaCl–NH3–N2 gas mixture.