GaN Heteroepitaxy Research Articles

Long-Range Orbital Hybridization in Remote Epitaxy: The Nucleation Mechanism of GaN on Different Substrates via Single-Layer Graphene.

Remote epitaxy is a very promising technique for the preparation of single-crystal thin films of flexibly transferred III-V group semiconductors. However, the epilayer nucleation mechanism of remote epitaxy and the epilayer-substrate interface interactions on both sides of graphene are not well-understood. In this study, remote homo- and heteroepitaxy of GaN nucleation layers (NLs) were performed by metal organic chemical vapor deposition on GaN, sapphire (Al2O3), and AlN substrates with transferred single-layer graphene, respectively. The results show that the interface damage of SLG/GaN at high temperature is difficult for us to achieve the remote homoepitaxy of GaN. Therefore, we explored the nucleation mechanism of remote heteroepitaxy of GaN on SLG/Al2O3 and SLG/AlN substrates. Nucleation density, surface coverage, diffusion coefficient, and scaled nucleation density were used to quantify the differences in nucleation information of GaN grown on different polar substrates. Using high-resolution X-ray diffraction and high-resolution transmission electron microscopy analysis, we revealed the interfacial orientation relationship and atomic arrangement distribution between the GaN NLs and substrates on both sides of the SLG. The electrostatic potential effect and adsorption ability of the substrates were further investigated by first-principles calculations based on density functional theory, revealing the principle that the substrate polarity affects the atomic nucleation density. The partial density of states shows that there is long-range orbital hybridization of the electronic states of the substrate and adsorbed atoms in remote epitaxy, and the crystal properties of the substrate play an important role in the in-plane orientation relationship of the NL and substrate across the SLG. The abovementioned results reveal the nature of remote epitaxy and broaden the perspective for the rapid and large-area preparation of single-crystal GaN films.

Construction of van der Waals substrates for largely mismatched heteroepitaxy systems using first principles

The high density of defects induced by large strains in largely mismatched heteroepitaxy systems, such as AlN and GaN, because of the large lattice and thermal mismatch between substrates and epilayers is the bottleneck problem. Graphene-assisted van der Waals (vdW) heteroepitaxy offers a new opportunity to resolve this problem. However, it suffers from the difficulty of nucleation. Here we theoretically assess the effects of five 2D materials for vdW heteroepitaxy of AlN and GaN, including graphene, hBN, MoS2, gC3N, and gC3N4, and provide physical insights using first-principle calculations. MoS2 and gC3N exhibit significant potential to overcome the shortcomings of graphene owing to their appropriate binding strengths and Al (or Ga) diffusion barriers. Moreover, the interface behavior between the epilayers and the substrates are carefully analyzed. Our findings are helpful not only for obtaining high-quality AlN and GaN films but also for developing new criterions to discover effective 2D materials for vdW heteroepitaxy.

Optimization of N-polar GaN growth on bulk GaN substrate by MOCVD

N-polar GaN samples under different V/III ratio were grown on bulk N-polar GaN substrates by metal organic chemical vapor deposition. The V/III ratio was optimized by adjusting the flow rate of trimethylgallium, meanwhile the flow rate of ammonia is consistent. The samples were characterized by High Resolution X-Ray Diffraction, Raman, photoluminescence and Van-der-Pauw Hall measurement. The best dislocation density of the samples is approximately 3.72 × 106 cm−2 which is three orders of magnitude smaller than the common heteroepitaxy of N-polar GaN. The background carrier concentration of the samples is reduced by about 95% as the flow rate of trimethylgallium decreasing.

Si Complies with GaN to Overcome Thermal Mismatches for the Heteroepitaxy of Thick GaN on Si.

Heteroepitaxial growth of lattice mismatched materials has advanced through the epitaxy of thin coherently strained layers, the strain sharing in virtual and nanoscale substrates, and the growth of thick films with intermediate strain-relaxed buffer layers. However, the thermal mismatch is not completely resolved in highly mismatched systems such as in GaN-on-Si. Here, geometrical effects and surface faceting to dilate thermal stresses at the surface of selectively grown epitaxial GaN layers on Si are exploited. The growth of thick (19 µm), crack-free, and pure GaN layers on Si with the lowest threading dislocation density of 1.1 × 107 cm-2 achieved to date in GaN-on-Si is demonstrated. With these advances, the first vertical GaN metal-insulator-semiconductor field-effect transistors on Si substrates with low leakage currents and high on/off ratios paving the way for a cost-effective high power device paradigm on an Si CMOS platform are demonstrated.

A combined growth process for state-of-the-art GaN on silicon

This study aims to simplify the heteroepitaxy of GaN on Si while keeping state-of-the-art III-nitride materials. The originality of this work is to combine the advantages of both NH3-MBE and metal organic chemical vapor deposition (MOCVD) growth techniques. First, the structural quality of AlN is assessed by AFM, XRD measurements, and TEM. Terraces and atomic step edges are observed by AFM showing a good structural quality of the AlN epilayer. TEM investigation reveals a sharp and well-controlled AlN/Si using NH3-MBE. Then, a 2-μm thick smooth GaN layers are obtained by MOCVD on top of these AlN templates with the use of a SiNx treatment allowing a 3D growth mode able to induce an efficient dislocation filtering to be obtained. GaN structural properties, measured by XRD, AFM, and SEM, are discussed and compared to GaN-on sapphire (Al2O3). In addition, the stress in the GaN layers grown on these AlN templates with or without a SiNx treatment is assessed. GaN epilayers with a dislocation density in the range of few 108 dislocations per cm2 have been achieved using a single AlN buffer layer (no interlayer) and only 2 μm of GaN.

Microstructural and Compositional Characterisation of Electronic Materials

<p class="p1"> </p><p class="p2"><span class="s1"> </span>Microstructural and compositional characterisation of electronic materials in support of the development of GaAs, GaN, and GaSb based multilayer device structures is described. Electron microscopy techniques employing nanometer and sub-nanometer scale imaging capability of structure and chemistry have been widely used to characterise various aspects of electronic and optoelectronic device structures such as InGaAs quantum dots, InGaAs pseudomorphic (pHEMT), and metamorphic (mHEMT) layers and the ohmic metallisation of GaAs and GaN high electron mobility transistors, nichrome thin film resistors, GaN heteroepitaxy on sapphire and silicon substrates, as well as InAs and GaN nanowires. They also established convergent beam electron diffraction techniques for determination of lattice distortions in III-V compound semiconductors, EBSD for crystalline misorientation studies of GaN epilayers and high-angle annular dark field techniques coupled with digital image analysis for the mapping of composition and strain in the nanometric layered structures. Also, <em>in-situ </em>SEM experiments were performed on ohmic metallisation of pHEMT device structures. The established electron microscopy expertise for electronic materials with demonstrated examples is presented.</p>

Ultralow threading dislocation density in GaN epilayer on near-strain-free GaN compliant buffer layer and its applications in hetero-epitaxial LEDs.

High threading dislocation (TD) density in GaN-based devices is a long unresolved problem because of the large lattice mismatch between GaN and the substrate, which causes a major obstacle for the further improvement of next-generation high-efficiency solid-state lighting and high-power electronics. Here, we report InGaN/GaN LEDs with ultralow TD density and improved efficiency on a sapphire substrate, on which a near strain-free GaN compliant buffer layer was grown by remote plasma atomic layer deposition. This “compliant” buffer layer is capable of relaxing strain due to the absorption of misfit dislocations in a region within ~10 nm from the interface, leading to a high-quality overlying GaN epilayer with an unusual TD density as low as 2.2 × 105 cm−2. In addition, this GaN compliant buffer layer exhibits excellent uniformity up to a 6” wafer, revealing a promising means to realize large-area GaN hetero-epitaxy for efficient LEDs and high-power transistors.

Single Crystalline GaN Tiles Grown on Si (111) Substrates by Confined Lateral Guided Growth to Eliminate Wafer Bowing

A novel concept termed confined lateral guided growth (CLGG) has been demonstrated to prepare single crystalline GaN microtiles with a goal to create high crystalline quality and strain‐free GaN on large‐sized Si (111) substrates. Uniform array of hexagonal GaN tiles has been created with a diameter of larger than 30 μm for each tile. As revealed by the surface pits density of around 2 × 108 cm−2, the density of dislocations in these GaN tiles has been significantly reduced compared with that obtained typically from a GaN heteroepitaxy on Si (111). The strain in GaN tiles has been dramatically reduced to nearly zero. This approach offers a route to eliminate wafer‐bowing for growing high crystalline quality GaN on Si, especially when the substrate size scales to 12 in and beyond. The growth behavior and mechanism within these confined growth masks are investigated by both finite element modeling and experimental studying, indicating that CLGG is dominated by gas phase diffusion.

Potentialities of AlGaN/GaN Heterostructures Grown on 2°-Off 4H-SiC Substrates

So far, most of the SiC homoepitaxy has been realized on 8-deg-off and 4-deg-off substrates, whereas GaN heteroepitaxy is done on SiC on-axis (up to 0.3 deg off). As 6-inch SiC wafers are being introduced into the market, a decrease of the substrate off-cut to 2 deg for SiC homoepitaxy is desirable to reduce the manufacturing costs. If both, GaN heteroepitaxy and SiC homoepitaxy are successful on 2-deg-off SiC substrates, this would pave the way to monolithic integration of both kinds of devices, as well as to obtain cheap insulating SiC substrates for AlGaN/GaN microwave and THz devices. In this work, we present our present status of AlGaN/GaN growth on SiC 2-deg off. Comparing to the on-axis situation, we obtained similar structural (XRD and TEM data) and electrical characteristics, but not morphological ones. Therefore, we propose two ways of a decrease of AlGaN surface roughness when grown on SiC 2 deg-off: i) by planarization, ii) by lateral patterning.

Ellipsometric study of GaN/AlN/Si(111) heterostructures grown by molecular beam epitaxy

GaN and related structures attracted a great interest in the recent years for electronic and optoelectronic applications due to their promising properties. GaN is grown popularly on foreign substrates like sapphire and SiC. However, silicon due to its favourable properties attended the great attention of material scientists and researchers to utilize as substrate for heteroepitaxy of GaN based structures and devices. Silicon substrates are low cost, available in large diameters and have well characterized thermal and electrical properties. In this study, GaN/AlN/Si(111) heterostructures were grown by molecular beam epitaxy. We performed x-ray diffraction spectroscopy and spectroscopic ellipsometry on these samples to study their structural and optical properties. XRD measurements performed on these samples revealed the presence of high quality GaN films as well as the presence of AlN buffer layer with the following miller indices: GaN (002), GaN (004), GaN (006) and GaN (110) along with Si peak of phase (111). The ellipsometric data obtained were used to characterize the GaN/Si samples as a function of film thickness. Refractive index, extinction coefficient and dielectric constant were calculated by the measured data.

Phonon scattering in strained transition layers for GaN heteroepitaxy

Strained transition layers, which are common for heteroepitaxial growth of functional semiconductors on foreign substrates, include high defect densities that impair heat conduction. Here, we measure the thermal resistances of AlN transition layers for GaN on Si and SiC substrates in the temperature range $300<T<550$ K using time-domain thermoreflectance. We propose a model for the effective resistance of such transition films, which accounts for the coupled effects of phonon scattering on defects and the two interfaces. The data are consistent with this model using point defects at concentrations near 10${}^{20}$ cm${}^{\ensuremath{-}3}$ and transmission coefficients based on the diffuse mismatch model. The data can be also described using lower transmission coefficients and eliminating the defects in the AlN. The data and modeling support the hypothesis that point defect scattering in the AlN film dominates the resistance, but may also be consistent with a high presence of near-interfacial defects in the bounding films.

GaN-based substrates and optoelectronic materials and devices

In order to solve the problems of GaN heteroepitaxy on sapphire substrate, some techniques were explored. Freestanding GaN substrates have been made by hydride vapor phase epitaxy (HVPE), laser lift-off (LLO), and chemical mechanical polishing techniques. Wafer bending and cracking in the HVPE growth were partly settled by pulsed flow modulation method. High-crystal quality was established for 1.2 mm thick GaN substrate by X-ray diffraction measurement, in which the full width of half maximum values were 72, 110 arcsec for (102), (002) peaks. A novel micro-size patterned sapphire substrate (PSS) and a nano PSS were also fabricated. High-power vertical structure light emitting diodes (VSLEDs) have been developed by Au–Sn eutectic wafer bonding, homemade micro-area LLO, and light extraction structure preparation. The high-injection-level active region with low temperature GaN sandwiched layers was used for low-efficiency droop. The light output power of VSLED was achieved as 400 mW driven at 350 mA, and the dominant wavelength is about 460 nm. The structures and properties of strain modulated superlattices (SLs) and quantum wells as well as advanced simulation of carriers transport across the electron blocking layer were investigated in laser diodes. The hole concentration was achieved as high as 1.6 × 1018 cm−3 in AlGaN/GaN SLs:Mg by inserting an AlN layer. High-quality AlGaN epilayers and structures were grown by MOCVD. Some device structures of UV LEDs and detectors were demonstrated. The emission wavelength of 262 nm UV LED has been successfully fabricated. At last, high-quality InN and InGaN materials for solar cell were grown by boundary-temperature-controlled epitaxy and growth-temperature-controlled epitaxy. Hall-effect measurement showed a recorded electron mobility of 3,280 cm2/(V s) and a residual electron concentration of 1.47 × 1017 cm−3 at 300 K.

Effect of Si Doping on the Crystal Structure of HVPE Grown Boron Phosphide

Boron Phosphide (BP) layers have been grown on Si (100) as a substrate for cubic GaN heteroepitaxy. Si heavy doping was attempted to reduce the threading dislocation density in the BP. To observe the effect on dislocations in BP by Si doping, we used cross-sectional transmission electron microscopy (XTEM). Also, Glow Discharge-Optical Emission Spectroscopy (GDOES) were used to evaluate the effect of Si doping. For the samples under the conditions (a) and (e), we found the formation of pits at BP surface and an increasing in the proportion of threading dislocations with an angle of 90 ° to the surface. We could not see a significant change in the threading dislocation density by Si doping under the conditions used in this paper.

Using the kinetic Wulff plot to design and control nonpolar and semipolar GaN heteroepitaxy

For nonpolar and semipolar orientations of GaN heteroepitaxially grown on sapphire substrates, the development of growth procedures to improve surface morphology and microstructure has been driven in a largely empirical way. This work attempts to comprehensively link the intrinsic properties of GaN faceted growth, across orientations, in order to understand, design and control growth methods for nonpolar (1 1 2 0) GaN and semipolar (1 1 2 2) GaN on foreign substrates. This is done by constructing a comprehensive series of kinetic Wulff plots (or v-plots) by monitoring the advances of convex and concave facets in selective area growth. A methodology is developed to apply the experimentally determined v-plots to the interpretation and design of evolution dynamics in nucleation and island coalescence. This methodology offers a cohesive and rational model for GaN heteroepitaxy along polar, nonpolar and semipolar orientations, and is broadly extensible to the heteroepitaxy of other materials. We demonstrate furthermore that the control of morphological evolution, based on invoking a detailed knowledge of the v-plots, holds a key to the reduction of microstructural defects through effective bending of dislocations and blocking of stacking faults. The status and outlook of semipolar and nonpolar GaN growth on sapphire substrates will be presented.

Understanding and controlling heteroepitaxy with the kinetic Wulff plot: A case study with GaN

This work represents a comprehensive attempt to correlate the heteroepitaxial dynamics in experiments with fundamental principles in crystal growth using the kinetic Wulff plot (or v-plot). Selective area growth is employed to monitor the advances of convex and concave facets toward the construction of a comprehensive v-plot as a guidepost for GaN heteroepitaxy. A procedure is developed to apply the experimentally determined kinetic Wulff plots to the interpretation and the design of evolution dynamics in nucleation and island coalescence. This procedure offers a cohesive and rational model for GaN heteroepitaxy on polar, nonpolar, and semipolar orientations and is broadly extensible to other heteroepitaxial material systems. We demonstrate furthermore that the control of morphological evolution, based on invoking a detailed knowledge of the v-plots, holds a key to the reduction of microstructural defects through effective bending of dislocations and geometrical blocking of stacking faults, paving a way to device-quality heteroepitaxial nonpolar and semipolar GaN materials.

Growth of A-plane GaN on (0 1 0) LiGaO 2 by plasma-assisted MBE

The (0 1 0) surface of LiGaO 2 is closely lattice matched to A-plane ( 1 1 2 ¯ 0 ) GaN making it an interesting candidate as a substrate for heteroepitaxy of non-polar GaN. We demonstrate successful, first-time growth of A-plane GaN on (0 1 0) LiGaO 2 using plasma-assisted molecular beam epitaxy. Structural and morphological analysis is performed using X-ray and reflective high energy electron diffraction, scanning electron and atomic force microscopy. Very high phase purity of A-plane GaN is shown. Apart from defects of the epitaxial film originating from substrate scratches, the film is smooth and shows an rms roughness of 10 nm over an area of 8 × 8 μ m 2 .

Synthesis and characterization of heteroepitaxial GaN films on Si(111)

We report crack-free and single-crystalline wurtzite GaN heteroepitaxy layers have been grown on Si (111) substrate by metal-organic chemical vapor deposition(MOCVD). Synthesized GaN epilayer was characterized by X-ray diffraction(XRD), atomic force microscope (AFM) and Raman spectrum. The test results show that the GaN crystal reveals a wurtzite structure with the <0001> crystal orientation and XRD ω-scans showed a full width at half maximum (FWHM) of around 583 arcsec for GaN grown on Si substrate with an HT-AlN buffer layer. In addition, the Raman peaks of E 2 high and A 1(LO) phonon mode in GaN films have an obvious redshit comparing to bulk GaN eigen-frequency, which most likely due to tensile strain in GaN layers. But the AO phonon mode of Si has a blueshit which shows that the Si substrate suffered a compressive strain. And we report that the AlN buffer layer plays a role for releasing the residual stress in GaN films.

Scanning capacitance microscopy studies of GaN grown by epitaxial layer overgrowth

Epitaxial layer overgrowth (ELOG) is a common technique for dislocation density reduction in GaN heteroepitaxy. Here, scanning capacitance microscopy is used to study the variations in unintentional doping arising from the ELOG process and reveals facet-dependent incorporation of n-type dopants during the initial regrowth of GaN, and then p-type doping arising from the use of bis(cyclopentadienyl)magnesium to enhance lateral growth during the coalescence stage.

Understanding nonpolar GaN growth through kinetic Wulff plots

In this paper we provide explanations to the complex growth phenomena of GaN heteroepitaxy on nonpolar orientations using the concept of kinetic Wulff plots (or v-plots). Quantitative mapping of kinetic Wulff plots in polar, semipolar, and nonpolar angles are achieved using a differential measurement technique from selective area growth. An accurate knowledge of the topography of kinetic Wulff plots serves as an important stepping stone toward model-based control of nonpolar GaN growth. Examples are illustrated to correlate growth dynamics based on the kinetic Wulff plots with commonly observed features, including anisotropic nucleation islands, highly striated surfaces, and pentagonal or triangular pits.

AlN and GaN hetero epitaxy on Si substrate using activity modulation migration-enhanced MBE

AlN and GaN hetero epitaxy on Si substrate using activity modulation migration-enhanced MBE