GaN Growth Process Research Articles

Microstructural and spectroscopic analysis of epitaxial lateral overgrowth GaN via the self-decomposing hexagonal graphene mask

The use of two-dimensional material like graphene to alleviate lattice mismatch has been an effective way to realize high-quality GaN on heterogeneous substrates. The lack of hanging bonds on the graphene surface provides a new attempt for epitaxial lateral overgrowth (ELOG). In this study, a hexagonal graphene mask was used for the growth of GaN, the graphene mask disappeared during the GaN growth process, but GaN still maintained the ELOG mode, and the threading dislocation density was significantly reduced. Raman and PL spectra demonstrated the stress relaxation in ELOG GaN and showed a stress relaxation of 0.157 GPa at the interface between the substrate and ELOG GaN. This study demonstrates the feasibility and advantages of graphene masks for nitrides and extends the research on stress relaxation of ELOG GaN using a graphene mask.

A study of thermal etching of GaN by atmospheric argon inductively coupled plasma

GaN has a wide application in electronics and optoelectronics due to its excellent electrical properties. The thermal etching process is commonly used in the growth and subsequent process of GaN. As an easy-to-operate technique, atmospheric argon inductively coupled plasma (Ar-ICP) is first applied to the thermal etching of GaN in this study. Through the analysis of surface chemical composition and near-surface plasma spectroscopy, the etching principle and mechanism of the circular etching pit formation are revealed. The maximum material removal rate of Ar-ICP thermal etching can reach 18.72 μm/min. The optical properties of the sliced GaN after the Ar-ICP thermal etching and lapping process are evaluated by photoluminescence (PL), respectively. The PL intensity of the Ar-ICP thermally etched sample increases and the peak position of near-band emission shows a red shift compared to the lapped sample.

High-Energy Computed Tomography as a Prospective Tool for In Situ Monitoring of Mass Transfer Processes inside High-Pressure Reactors-A Case Study on Ammonothermal Bulk Crystal Growth of Nitrides including GaN.

For the fundamental understanding and the technological development of the ammonothermal method for the synthesis and crystal growth of nitrides, an in situ monitoring technique for tracking mass transport of the nitride throughout the entire autoclave volume is desirable. The feasibility of using high-energy computed tomography for this purpose was therefore evaluated using ex situ measurements. Acceleration voltages of 600 kV were estimated to yield suitable transparency in a lab-scale ammonothermal setup for GaN crystal growth designed for up to 300 MPa operating pressure. The total scan duration was estimated to be in the order of 20 to 40 min, which was sufficient given the comparatively slow crystal growth speed in ammonothermal growth. Even shorter scan durations or, alternatively, lower acceleration voltages for improved contrast or reduced X-ray shielding requirements, were estimated to be feasible in the case of ammonoacidic growth, as the lower pressure requirements for this process variant allow for thinned autoclave walls in an adapted setup designed for improved X-ray transparency. Promising nickel-base and cobalt-base alloys for applications in ammonothermal reactors with reduced X-ray absorption in relation to the maximum operating pressure were identified. The applicability for the validation of numerical simulations of the growth process of GaN, in addition to the applicability of the technique to further nitride materials, as well as larger reactors and bulk crystals, were evaluated.

Green light-emitting diodes with improved efficiency by an in situ C-doping GaN current spreading layer

The introduction of an in situ C-doped GaN layer in green light-emitting diodes (LEDs) is successfully realized by optimizing the temperature of the GaN growth process. The C-doped GaN film acts as a current spreading layer for green LEDs, allowing for a more uniform current distribution and consequently an increase in luminous efficiency. At the same time, the insertion of a C-doped GaN layer does not lead to the degradation of the surface morphology as well as the crystalline quality. Electroluminescence results show that the C-doped GaN layer grown at 850°C is appropriate to be used in green LEDs.

Fundamental Studies on Crystallization and Reaching the Equilibrium Shape in Basic Ammonothermal Method: Growth on a Native Lenticular Seed.

In this paper, a detailed investigation of the basic ammonothermal growth process of GaN is presented. By analyzing the crystallization on a native seed with a lenticular shape, thus with an intentionally varying off-cut, we wanted to answer some basic questions: (i) Which crystallographic planes play the most important role during growth (which planes are formed and which disappear)? (ii) What is the relationship between the growth rates in different crystallographic directions? (iii) What is the influence of the off-cut of the seed on the growth process? Two non-polar slices, namely, and , as well as a basal plane slice of an ammonothermal crystal were analyzed. The examined planes were selectively etched in order to reveal the characteristic features of the growth process. The applied characterization methods included: optical microscopy with Nomarski contrast and ultraviolet illumination, X-ray topography and high-resolution X-ray diffraction, and secondary ion mass spectrometry. The obtained results allowed for creating a growth model of an ammonothermal GaN crystal on a lenticular seed. These findings are of great importance for the general understanding of the basic ammonothermal crystal growth process of GaN.

Study on the stress and mechanism of self-separated GaN grown by Na-flux method

A 2 inch free-standing c-plane GaN wafer was fabricated through in situ self-separation using HVPE-seed crystal etching back (HCEB) by intentionally adjusting the nitrogen pressure in the Na-flux growth process of GaN. First, adjust the nitrogen pressure in the reactor to a lower level to facilitate HCEB to form a large number of voids at the interface between the c-plane HVPE seed and the c-plane Na-flux GaN. After regrowth of approximately 340 μm thick Na-flux GaN, self-separation was achieved during the cooling process. The free-standing GaN wafer was almost stress-free as a result of strain relief by the in situ self-separation process, which was confirmed by room-temperature Raman and low-temperature photoluminescence measurements. It is supposed that the HCEB process can be applied to fabricate high-quality free-standing GaN wafers in the future.

Probing unintentional Fe impurity incorporation in MOCVD homoepitaxy GaN: Toward GaN vertical power devices

Unintentional impurity incorporation in GaN drift layers represents a challenging issue that can limit their potential performance in vertical power devices. In this paper, we focus on studying the origins of Fe impurity incorporation in metal-organic chemical vapor deposition (MOCVD) grown GaN materials. Acting as a compensator in n-type GaN drift layers, Fe impurities can reduce the electron mobility in GaN and limit the lowest controllable doping level. Two sources, the sample cleaning process and growth susceptor, were identified as the main mechanisms of Fe incorporation in the MOCVD GaN growth process. It was found that solvent cleaning of the wafer can introduce significant Fe contamination at the growth interface, which would slowly be incorporated into the GaN epilayer, thus causing background Fe impurity as high as 1017 cm−3 level. Moreover, the Fe impurity in the coating material on the susceptor can introduce additional Fe impurity during the growth process. Our studies revealed that the Fe impurity level could be significantly suppressed by more than two orders when an alternative cleaning process was used and the susceptor surface was fully covered by substrates. Characterization of the Fe impurity concentrations was performed via secondary ion mass spectrometry. The trap level (EC − 0.57) eV from deep-level transient spectroscopy that had previously been attributed to Fe confirmed the carrier compensation effect from Fe. Room temperature Hall mobility as high as 1007 cm2/V s was achieved on the MOCVD grown low-Fe GaN. Results from this work will provide guidance for achieving high purity GaN toward high performance GaN vertical power devices.

(Invited) Nanostructures and Crystal Defects in Thick GaN and SiC Epitaxial Layers for Power Electronic Switches

The state-of-the-art power switching devices made from SiC and GaN semiconductors contain a high density of crystal defects. Most of these defects are present in starting wafers and some are generated during device processing. There is little conclusive evidence so far on the exact role that the crystal defects play on device performance, manufacturing yield, and more importantly, long-term field-reliability especially when devices are operating under extreme stressful environments. This paper provides a review of the current state-of-the-art of SiC and GaN power semiconductor material technology, and the potential impact crystal defects may have on high-density power switching electronics. Group III-Nitride materials suffer from both extended and point defects, each of which will challenge the material’s application in both vertical and lateral power devices. The extended defects include vertical threading dislocations of both edge and screw type. The latter defects have been shown to be correlated to leakage in vertical two terminal device structures while the influence of the former is still undetermined and remains a critical research issue. Channel surfaces in vertical three terminal devices will also degrade due to vertical threading dislocations. These extended defects occur in all epitaxial layers grown on c-plane substrates (the predominant and largest area substrate type) and are the result of the lack of a high quality substrate bulk material as well as substrate surface. The present substrate technology after multiple efforts (either GaN, SiC or Si) still have a high defect density indicating that low-defect seeds may be non-existing. Point defects influence the background carrier concentration in low-doped layers and various recombination processes. The intrinsic nature of III-N materials makes nitrogen-vacancies a predominant point defect that automatically dopes the crystal n-type, but there are impurities (oxygen and carbon) from most growth environments that also contribute to conductivity. These defects are not well understood and not well controlled. Such defects must be minimized in order to realize thick epitaxial layers with drift regions that will support both high blocking voltages and low on-state resistances. A wide range of materials characterization methods have been applied to further understand these impurities. The semiconductor and dielectric materials used in the construction of power electronics switching devices employed for energy efficiency applications experience extreme electrical and thermal stresses during the on-state as well as when switching high voltages and high currents. For example, in a bipolar type power semiconductor switching device in order to achieve significant conductivity modulation and reduce the drift-region resistance, high-level injection of minority carriers is typical. If the semiconductor contains a high density of crystal defects, as is the case with WBG semiconductors, minority carrier recombination phenomenon can be adversely affected. For example, it has been shown more than a decade ago [3, 4] that basal plane dislocations (BPDs) present in SiC are excited by the energy released from minority carrier recombination process; the result is excess BPD generation and its glide within the semiconductor material that leads to an increase in the drift-region resistance and thermal run-away in a forward biased bipolar junction diode The reported experimental data for high-voltage SiC semiconductor strongly suggests that material defects have profound impact on its characteristics under extreme environments. Since GaN semiconductor has even higher density of material defects, this problem is further exacerbated especially in thick epitaxial GaN layers which are required for power switches beyond 5kV. The integration of the pillar junction scheme with the trench-gate GaN MOSFET processes developed at the University of Maryland will be reported. The thick pillar junction epitaxial layers have been grown by a commercial vendor, using a windowed GaN growth process and/or selective deep etching, followed by GaN regrowth to fabricate columns with opposite carrier polarities. The doping density of the n--pillars under the gate has been specified with background carrier concentration n ≤ 1x1016 cm-3, while the p-pillars will have a higher doping density (p ≥ 1x1018 cm-3) to maximize depletion in the n--pillar. The p-body and n+-source layers are grown by subsequent epitaxial growth across the whole structure. Our current research into optimized trench gate fabrication and gate dielectric processing has been used to fabricate gate trenches into the structure. These devices can be compared to traditional trench MOSFETs with extremely thick blocking layers of comparable thickness to the pillar junction layer in order to determine the role of defects in thick GaN epi layers in terms of limiting device performance and reliability.

Effect of Hydrogen-Ion-Implantation on Self-Separation of Free-Standing Gallium Nitride Wafers Grown By Hvpe

In recent years, free-standing Gallium Nitride (FS-GaN) wafers were utilized as substrates for high-brightness light-emitting-diodes (HB-LED), LASER diodes (LD) and power devices due to their characteristics, such as 3 times higher thermal conductivity than sapphire substrates and higher breakdown voltage than conventional Si substrates. The FS-GaN wafers grown by hydride vapor phase epitaxy (HVPE) demonstrated lower dislocation density (DD) than that of GaN wafers grown by metal-organic chemical vapor deposition (MOCVD) on sapphire substrates (template GaN wafer). However, one of big challenges for their commercializing is the crack free FS-GaN wafers during the HVPE GaN growth, which resulted in lowering the productivity and increasing the fabrication cost. The main cause of the cracking is the stress induced by the differences in thermal expansion coefficients (TEC) and lattice constants of sapphire and GaN. To overcome this issue, the design of GaN growth process with self-separation is the key technology. The self-separation technique consists of two key points, one is separation enhancement and the other is blocking edge-poly-GaN growth. In this study, we investigated the effect of hydrogen-ion-implantation on self-separation of FS-GaN wafers. In addition, the blocking effect of edge grinding of template GaN wafers was also investigated. Two-inch-diameter template GaN wafers with the GaN layer thickness of 5 μm were prepared and the hydrogen-ions were implanted with various dose conditions to enhance self-separation of FS-GaN wafers from the sapphires. The edges of the implanted template GaN wafers were then ground to eliminate the polished edge surfaces to deter the poly-GaN growth. 800-μm-thick GaN layers were grown by HVPE on the template GaN wafers, and the FS-GaN wafers were successfully self-separated during the cool-down process. Fig. 1 shows the edge images of the GaN on sapphire substrates after 300-μm-thick GaN layer was grown on the template GaN substrates. When the edge-grinding was utilized to template GaN substrate, the poly-GaN was grown less than that of the substrate without edge-grinding. Also, the poly-GaN was easily removed at the edge of the substrate by edge-grinding without additional process. Fig. 2 shows the hydrogen-ion-band inside the MOCVD-GaN layer with three different hydrogen-ion dose conditions. The sample with hydrogen-ion dose of 2.25 E17 H+/cm2 showed larger size of the hydrogen cavity than the samples with hydrogen-ion dose of 1.75 E17 and 2.0 E17 H+/cm2. Fig. 3 shows the wafer images after 800-μm-thick GaN grown on the edge-ground template GaN with three different hydrogen-ion-implantation conditions. The FS-GaN wafers with hydrogen-ion dose of 1.75 E17 and 2.0 E17 H+/cm2 were successfully self-separated from the sapphire substrates, while the sample with hydrogen-ion-dose of 2.25 E17 H+/cm2 was self-separated around 63% of the area. In our presentation, we will report how we achieved the self-separation of FS-GaN wafers without cracking by hydrogen-ion-implantation into the template GaN substrates, and discuss about the crystal quality of the grown FS-GaN samples. * This work was financially supported by the Brain Korea 21 Plus Program in 2018. Figure 1

Study on Dislocation Annihilation Mechanism of the High-Quality GaN Grown on Sputtered AlN/PSS and Its Application in Green Light-Emitting Diodes

GaN was grown on the sputtered AlN/patterned sapphire substrate under two growth modes by metal–organic chemical vapor deposition, which was named as “rising tide” and “tsunami” growth modes, respectively, due to different characteristics of the GaN growth process. High-quality GaN epilayer was obtained under “tsunami” growth mode, and the full-width at half-maximums of GaN (002)/(102) high-resolution X-ray diffraction rocking curves were 58/90 arcsec. The green InGaN/GaN light-emitting diodes fabricated on GaN under “tsunami” growth mode exhibited both higher light output power and external quantum efficiency. By monitoring the GaN films at different growth stages using the scanning electron microscope and the transmission electron microscope as well as cathodoluminescence, the dislocation annihilation mechanisms were researched. Under “tsunami” growth mode, GaN grew into the shape of a truncated pyramid that promoted dislocations originated from flat area bend toward the inclined planes, and it was noteworthy that the propagation of dislocations in grains on the conical surface was inhibited. While under “rising tide” growth mode, the dislocations on the conical surface had chances to extend.

Influence of dislocations on the thermal decomposition of GaN

The thermal annealing process is widely used in the growth and subsequent process of GaN, but the mechanism of thermal decomposition is unclear. A series of GaN templates were annealed at 1040 °C for different time. It is found that dislocations play an important role in both the origin of thermal decomposition and the evolution of surface morphology. The thermal decomposition rate of GaN without dislocations is much lower than that with dislocations. Thermal annealing could be used as a new method to assess the dislocation density of GaN. This study provides the underlying insights needed to study the thermostability of intrinsic GaN.

Microscopic mechanisms of Si(111) surface nitridation and energetics of Si3N4/Si(111) interface

Nitridation of silicon surface is an important step in GaN or AlN nanostructures growth process which determines quality of fabricated devices. In this work modeling of gas-surface reaction of the Si(111) surface nitridation was performed using density functional theory calculations. We show that interface structure corresponding to first stoichiometric Si3N4 monolayer may be generated in a great number of different ways by means of vacancy-assisted mechanism. The nearly continuum distribution of total energies may be associated with such structures formed after exothermic reaction with considerable energy gain. Nevertheless founded global minimum corresponds to abrupt interface and crystalline-like Si3N4 monolayer structure. Suggested mechanism of silicon nitride film growth is based on the calculated energy barriers for diffusion of volatile species such as N, N2, SiN and SiN2 which are building blocks for the growth of Si3N4 film. At first stage atomic nitrogen diffusion from vacuum to substrate occurs. Then silicon vacancies in substrate and volatile SiN radicals are formed. The back diffusion of SiN from substrate to Si3N4-vacuum side is the main mechanism responsible for silicon nitride film growth.

Spatial distribution of crystalline quality in N-type GaN grown on patterned sapphire substrate

The epitaxial layers of n-type GaN were grown on both planar and patterned sapphire substrate (PSS) by metal organic chemical vapor deposition. By comparing the epitaxial layers grown on planar substrate, GaN grown on PSS exhibited many improvements both on surface morphology and crystalline quality according to the characterization of atoms force microscopy, and high resolution X-ray diffraction. Spatially resolved micro-Raman scattering results were performed for mapping the spatial variations in crystalline quality of the n-type GaN grown on PSS. According to the variations on the intensity and the full width at half maximum of GaN E2 (high) peaks, the crystalline quality improvement occurred in the lateral growth regions which correspond to center region of the pyramid patterns. We proposed that the bending of dislocations during the lateral growth plays an important role in the spatial variations of GaN crystalline quality. Cross sectional transmission electron microscope and spatial cathodoluminescence mapping results further supported the explanation of the dislocation inhibition during the growth process of GaN grown on PSS.

Heteroepitaxial growth of GaN on vertical Si{110} sidewalls formed on trench-etched Si(001) substrates

A maskless Si trench structure was developed to integrate crystallographically non-polar GaN microstructures with semi-polar facets on Si(001). GaN “fins” were preferentially grown by MOCVD on Si{110} trench sidewalls formed by deep reactive ion etching (DRIE) of Si(001) such that GaN(0001)//Si{110} and GaN(10-10)//Si(001), resulting in a non-polar crystal structure with respect to the Si(001) substrate surface. No masking layer was required to prevent GaN growth on the Si(001) top surface of the trenches, instead, it was found that GaN nucleated preferentially on the Si{110} trench sidewalls. GaN was also observed to nucleate at the top corner of the trenches due to Si etching and exposure of high-index Si facets during the pre-growth H2 anneal. This undesired GaN nucleation was successfully suppressed by reducing the H2 anneal time and/or increasing the growth temperature and decreasing the precursor V/III to enhance Ga-adatom diffusion. Cross-sectional TEM studies confirmed that the GaN fins were crystallographically non-polar with respect to the Si(001) substrate surface and were bounded by semi-polar and non-polar facets. The reported Si fabrication and GaN growth process shows promise for the integration of non-polar and semi-polar GaN microstructures on industry standard Si(001) substrates.

First‐principles investigation of the GaN growth process in carbon‐added Na‐flux method

AbstractWe hypothesized that a nitrogen transportation process mediated by cyanide (CN) ions causes the observed increase in growth rate by carbon addition in the Na‐flux growth of GaN. Using first‐principles calculations, we investigated the probability of the nitrogen transportation process from the viewpoint of change of free energy. In addition, to verify whether the CN can separate into N and C atoms, we estimated the C–N bond energy on the GaN crystal surface by the blue‐moon method using constrained molecular dynamics simulations.

The control of mechanical bow for GaN substrate grown by HVPE with relatively longer radius of lattice curvature

AbstractWe are going to develop GaN growth process by HVPE method, which shows the near null value of free standing (FS)‐GaN wafer bowing after laser lift‐off (LLO) process in order to reduce the production cost of GaN substrate. The 309 to 318 µm thick HVPE grown GaN layers on sapphire shows 574 to 669 µm of as grown bows, and –299 to 95 µm of bow after LLO process. The 440 µm thick HVPE grown GaN layer on sapphire shows 693 µm of as grown bow, and –4 µm of bow after LLO process. From this GaN template, we could get the 356 µm thick FS‐GaN wafer with 2 µm of bow after polishing. This GaN substrate shows at least 43.87 m of the radius of lattice curvature evaluated by 9‐points measuring of (004) rocking curve along center line parallel to the main flat plane. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Nucleation mechanism of GaN growth on wet etched pattern sapphire substrates

Patterned sapphire substrates (PSS) are generally applied and have been proved to be effective in the increasing light extraction efficiency and crystal quality of GaN-based LEDs. The time evolution growth of GaN on triangle platform shaped PSS with an n-plane inclined surface and triangle cone shaped PSS with a part r-plane surface has been performed to research the nucleation and 3D growth mechanisms of GaN grown on the PSS. After the low temperature GaN growth process and high temperature ramping process, the small islands of GaN rotate and gather on the n-plane surface near the ridges of the triangle platform shaped PSS. Then the next 850 seconds high temperature growth process of GaN shows a 3D behavior. From the observations of SEM, it is believed that , and are the three preferred growth directions for GaN and the (1−10k) and (0001) planes are the two preferable planes during the GaN 3D growth.

High quality N-polar GaN two-dimensional growth on c-plane sapphire by metalorganic vapor phase epitaxy

We report the growth of atomically smooth N-polar GaN on c-plane sapphire by metalorganic vapor phase epitaxy. A two-step growth technique was adopted; low-temperature growth of GaN buffer before high-temperature GaN growth. The complete two-dimensional N-polar GaN growth process was recorded by in situ reflectance. The phase composition of the low-temperature GaN was examined by X-ray diffraction pole figure measurements. The thickness of the low-temperature GaN buffer dramatically affected the crystalline and electronic properties of the N-polar GaN. A very small full width at half maximum for the (0002) X-ray rocking curve, 51arcs, was obtained for 700-nm-thick N-polarity GaN by optimizing the buffer thickness.

One-step graphene coating of heteroepitaxial GaN films

Today, state-of-the-art III-Ns technology has been focused on the growth of c-plane nitrides by metal–organic chemical vapor deposition (MOCVD) using a conventional two-step growth process. Here we show that the use of graphene as a coating layer allows the one-step growth of heteroepitaxial GaN films on sapphire in a MOCVD reactor, simplifying the GaN growth process. It is found that the graphene coating improves the wetting between GaN and sapphire, and, with as little as ∼0.6 nm of graphene coating, the overgrown GaN layer on sapphire becomes continuous and flat. With increasing thickness of the graphene coating, the structural and optical properties of one-step grown GaN films gradually transition towards those of GaN films grown by a conventional two-step growth method. The InGaN/GaN multiple quantum well structure grown on a GaN/graphene/sapphire heterosystem shows a high internal quantum efficiency, allowing the use of one-step grown GaN films as ‘pseudo-substrates’ in optoelectronic devices. The introduction of graphene as a coating layer provides an atomic playground for metal adatoms and simplifies the III-Ns growth process, making it potentially very useful as a means to grow other heteroepitaxial films on arbitrary substrates with lattice and thermal mismatch.

In situ monitoring technologies for ammonthermal reactors

AbstractWe developed an optical cell which can be operated at real ammonthermal reaction conditions under high pressure (200MPa, 300MPa optional) and temperatures up to 650 °C. First video optical and spectroscopical studies were made to demonstrate the feasibility of in situ monitoring of the ammonothermal growth process of GaN with the optical cell. So far unknown parameters like e.g. flow velocity of the fluid could be investigated by simple imaging of particle movement. The in situ optical cell offers access to spectroscopic investigations or direct temperature measurements. First measurements show light absorption of chemical species in the reaction fluid in the UV to near IR range which changes with temperature and pressure implying that more than one intermediate species is present. The application of this optical cell is not restricted to a particular ammonothermal route but can be applied to either ammonothermal variant. An additional development was a reactor with five type K thermocouples to measure the temperature distribution directly in the reaction chamber up to 650 °C and 300 MPa. The flexibility of the thermocouples allows the measurements at nearly any point in the fluid. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)