Growth Of GaN Epilayers Research Articles

Graphene-assisted remote epitaxy wrinkle-free GaN films on flexible mica

When GaN films were directly deposited onto fluorophlogopite mica substrates, spontaneous wrinkling was frequently observed in GaN films. Because of the non-uniform interface adhesion between GaN and mica, two types of delamination phenomena have been observed by scanning electron microscopy. The maximum thickness of wrinkle-free GaN films on mica substrates was ∼200 nm. Interface pinning effect occurred due to the dangling bonds caused by the damage to mica during the deposition of GaN films. To avoid the formation of dangling bonds, the surface of mica was shielded by a graphene interlayer which was transferred via a wet method. Wrinkles were eradicated after the formation of sliding contact between GaN films and fully graphene-covered mica substrates. Despite the presence of the graphene lattice, the growth of GaN epilayer remained governed by the potential field of mica, indicating that remote epitaxy occurred on a 2D mica support substrate.

(Invited) Prospects for MOCVD Growth of GaN for 10kV Power PN Diodes

GaN wide-bandgap semiconductors have superior electronic material properties compared to Si and even SiC that will enable the foundational diodes and transistors for the next revolution in power management technology. To date, GaN pn diodes with 33 μm thick, lightly n-type doped drift layers have been demonstrated with breakdown voltages of 5 kV. [1] However, applications requiring higher blocking voltages could operate with greater efficiency and higher frequency if GaN based diodes and transistors could be proven. Simulations indicate that GaN pn diodes with blocking voltages of 10 kV require drift layers at least 50 μm thick and with carrier concentration less than 3x1015 cm-3. Achieving such demanding such material requirements presents significant challenges for GaN growth processesHydride Vapor Phase Epitaxy (HVPE) is the growth process of choice for very thick GaN layers due to its high growth rate (>50 μm/hr) and is a prevalent method for producing free-standing N+ GaN substrates. This growth technique and its use of carbon free precursors is a natural choice for growth of 50+ μm thick drift layers needed for 10 kV pn diodes. However, Si from the quartz growth chamber used in conventional hot-wall HVPE system leads to difficulty in controllably achieving low carrier concentrations and thus new materials and equipment designs for the growth chamber are being developed. [2] Furthermore, it is challenging to maintain smooth surface morphology of thick HPVE grown GaN epilayers and to grow p-type GaN epilayers that are needed for continuously grown pn diodes. Anode formation by epitaxial regrowth using Metal-Organic Chemical Vapor Deposition (MOCVD) or Mg implantation and annealing have yet to demonstrate multi-kilovolt diodes with reverse leakage currents equal to the best, continuously grown diodes grown by MOCVD.The low growth rate (< ~5 μm/hr) for MOCVD and the use of carbon containing precursors make it a suspect process for producing the > 50 μm thick, lightly doped (< 3x1015 cm-3) drift layers needed for 10 kV diodes. However, large substrate capacity, high up-time MOCVD reactors running highly-efficient, mature manufacturing processes developed for commercial production of visible LEDs could prove attractive for GaN pn diodes when compared to immature, quartz free HVPE reactors that to date have much lower wafer capacity. Further increasing the growth rate of the HPVE process offers diminishing returns in reducing the overall production cycle time, while modest advances in the growth rate of the MOCVD process would significantly reduce in cycle time.Since the lower growth rate of MOCVD might not be disqualifying, the concern shifts to controlling electron concentration at the low levels (< 3x1015 cm-3) in GaN epilayers where the background carbon concentration is at a similar level due to the use of carbon containing precursors. In this presentation we will discuss MOCVD growth and Si doping of GaN epilayers up to 60 μm thick and the properties of pn diodes with 50 μm thick drift layers. We find that carrier concentration is linear with Si flow down to 1x1015 cm-3, the limit of the mercury probe capacitance voltage system, and that the carrier concentration is independent of epilayer thickness from a few microns to 60 microns showing that the MOCVD process is stable for growth of thick drift layers required for 10 kV diodes. We find that controlling the surface morphology is the greatest challenge to producing 10kV-class pn diode devices and discuss how various elements such as surface mis-cut and substrate vendor affect surface morphology. While more work is needed, our studies suggest that MOCVD growth is capable of producing epilayers required for GaN pn power diodes with breakdown voltages of 10 kV.The information, data, or work presented herein was funded in part by the Advanced Research Projects Agency – Energy (ARPA-E), U.S. Department of Energy under the PNDIODES and OPEN+ programs directed by Dr. Isik Kizilyalli. Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA-0003525. H. Ohta et al., J. J. Appl. Phys., 57, 04FG09 (2018).H. Fujikura et al., Appl Phys. Lett. 117, 012103 (2020).

Optimization of the growth of GaN epitaxial layers in an indigenously developed MOVPE system

Growth of GaN epitaxial layers is optimized in an indigenously developed nitride metal organic vapour phase epitaxy (MOVPE) system. The motivation of present work is to develop a simple MOVPE reactor which is though affordable but can deliver GaN epilayers of desired quality. The design and fabrication methodology of nitride MOVPE system is briefly discussed. MOVPE growth of GaN epilayers is carried out through a two-step growth process where influence of the growth parameters of low temperature buffer layer on the crystalline properties of high temperature GaN epilayer is discussed in detail. Optimum values of the growth temperature, annealing duration and thickness of GaN buffer layer are achieved. High resolution X-ray diffraction measurements (HRXRD) confirmed that the crystalline quality of GaN epitaxial layers is reasonable. A good surface morphology of the optimized sample is strongly corroborated by the results obtained from photoluminescence (PL) and HRXRD techniques. Usefulness of a systematic Williamson-Hall analysis in the optimization of growth of GaN epitaxial layers is demonstrated where a high value of lateral coherence length of GaN buffer layer is found to be a key parameter. Such an optimization process leads to the good crystalline quality of GaN epitaxial layers with low dislocation density. It is found that the high temperature GaN epilayers grown on the optimized GaN buffer layer are compressively stressed which is also revealed by the PL measurements.

Development of polarity inversion in a GaN waveguide structure for modal phase matching

In this work, we report on the fabrication of a GaN/AlGaN waveguiding structure dedicated to modal phase matching, where GaN waveguide has planar polarity inversion. First, we optimized the growth conditions for the AlGaN epilayer. Second, on top of the AlGaN epilayer, we fabricated the waveguiding structure starting with the growth of the Ga-polar GaN epilayer followed by atomic layer deposition (ALD) of an Al $$_{2}$$ O $$_{3}$$ layer, then, continuing with the growth of N-polar GaN epilayer. We tested several layer thicknesses, but with 20 nm we managed to inverse the GaN polarity from Ga to N. To confirm the N-polarity, we etched the GaN epilayer surface in aqueous KOH solution. We performed out-of-plane (0002) and in-plane (11–20) X-ray diffraction and rocking curve measurements to estimate the crystalline quality of the AlGaN epilayer, Ga- and N-polar GaN epilayer. Atomic force microscopy measurement lets us evaluate the epilayer surface morphology and roughness. Optical and scanning electron microscopy inspection revealed characteristic hexagonal N-polar GaN epilayer surface. We used high-resolution transmission electron microscopy to investigate the crystallinity and orientation of the Ga- and N-polar GaN epilayer, also the Al $$_{2}$$ O $$_{3}$$ ALD layer, the interface quality of the waveguide structure.

Resistive heater element made of highly durable TaC-coated graphite for high-temperature and highly corrosive processes: application to MOCVD GaN epitaxial growth

Ultra-thick TaC-coated graphite prepared via a novel process was verified for use as a highly durable heater material in high-temperature and highly corrosive process environments such as those for SiC, GaN, and AlN bulk/epitaxial growth. As fundamental data for heater design, the temperature dependence of the electrical resistivity of the ultra-thick TaC layer was assessed. Estimation based on resistivity data indicates that the TaC-coated graphite heater element requires a slightly higher electrical current than that for conventional heaters. A coil-shaped ultra-thick TaC-coated heater for a MOCVD GaN growth system was fabricated using the proposed process. It was found to successfully heat the system to the target temperature for GaN epilayer growth. The quality of the MOCVD-grown GaN epilayers obtained with the TaC-coated heater was confirmed to be almost identical to that of those obtained with a conventional heater in terms of crystallinity, thickness homogeneity, intrinsic defects, impurity incorporation, and contamination.

GaN Heteroepitaxy on Strain-Engineered (111) Si/Si1−xGex

The metalorganic chemical vapor deposition growth of GaN on strained Si/Si1−xGex epilayers on (111) Si substrates was investigated. A multi-beam optical stress sensor (MOSS) was used for in situ stress measurements during growth of the entire heterostructure. MOSS was initially used to measure the extent of stress relaxation during the growth of constant-composition Si1−xGex layers on Si. The results compared favorably to that obtained by post-growth high-resolution x-ray diffraction. MOSS was also used to monitor stress during the growth of thin, tensile-strained Si on relaxed Si0.95Ge0.05/compositionally graded epilayers. The tensile-strained Si/Si1−xGex epilayers were then used as virtual substrates for the growth of GaN epilayers using a thin (90 nm) AlN buffer layer. GaN grown on tensile-strained Si exhibited a higher initial compressive stress and reduced final tensile stress compared to films grown directly on (111) Si, resulting in a lower crack density in the GaN along with a reduced density of threading dislocations. These results suggest that strain engineering of the Si surface prior to growth may provide an alternative method to improve the quality of GaN grown on (111) Si.

Nitride surface chemistry influence on band offsets at epitaxial oxide/GaN interfaces

GaN surface and near-surface chemistry influence on band offsets of oxide overlayers is demonstrated through X-ray photoelectron spectroscopy measurements using epitaxial (111)-oriented MgO films on (0001)-oriented Ga-polar GaN as a case study. For identical cleaning and MgO growth conditions, GaN subsurface oxygen impurities influence the GaN bare surface band bending and the ultimate band offset to MgO heterolayers. As the GaN surface oxygen concentration increases from an atomic concentration of 0.9% to 3.4%, the valence band offset to MgO decreases from 1.68 eV to 1.29 eV, respectively. This study highlights the sensitivity of the oxide/nitride interface electronic structure to GaN epilayer preparation and growth conditions.

A detailed study of AlN and GaN grown on silicon-on-porous silicon substrate

In this study, we investigate the growth of AlN and GaN epilayers on silicon-on-porous silicon (SOP) as a compliant substrate. The porous silicon layers is obtained by electrochemical etching of silicon wafers in HF based solutions. Different porosities (10–30%) and porous layer thicknesses (3–30 μm) have been investigated. In order to obtain a single orientation GaN, a Si cap layer is grown on porous silicon by chemical vapour deposition leading to a smooth epi-ready surface. Then, the subsequent growth of AlN by molecular beam epitaxy and GaN by metal-organic chemical vapour deposition is investigated by atomic force microscopy and X-ray diffraction. The epilayers grown on SOP show similar surface morphology and structural quality to state of the art GaN layers grown on bare silicon. In addition, strain in the GaN layers grown on SOP and Si has been investigated. Interestingly, the results indicate that a slight compliant effect is achieved on SOP substrate with the thicker porous layer.

Effect of double superlattice interlayers on growth of thick GaN epilayers on Si(110) substrates by metalorganic chemical vapor deposition

The effect of double thin AlN/GaN superlattice interlayers (SL ILs) on the growth of thick GaN epilayers by metalorganic chemical vapor deposition (MOCVD) on Si(110) substrates is investigated. It is found that the GaN middle layer (GaN layer between the two SL ILs) can affect the strain state of the GaN epilayer. By comparison with the case of a single SL IL, it is shown that the double SL ILs can have a stronger compressive effect on the GaN epilayer grown on it, which results in lower residual tensile strain in the GaN film after the growth. By optimizing the GaN middle layer thickness, a 4-µm-thick crack-free GaN epilayer is successfully achieved. By this simple technique, it is expected that high-quality crack-free thick GaN can be grown on Si substrates for optical and electronic device applications.

Effect of Top-Region Area of Flat-Top Pyramid Patterned Sapphire Substrate on the Optoelectronic Performance of GaN-Based Light-Emitting Diodes

The flat-top pyramid patterned sapphire substrates (FTP-PSSs) have been prepared for the growth of GaN epilayers and the fabrication of lateral-type light-emitting diodes (LEDs) with an emission wavelength of approximately 470 nm. Three kinds of FTP-PSSs, which were denoted as FTP-PSS-A, FTP-PSS-B, and FTP-PSS-C, respectively, were formed through the sequential wet etching processes. The diameters of circle areas on the top regions of these three FTP-PSSs were 1, 2, and 3 μm, respectively. Based on the X-ray diffraction results, the full-width at half-maximum values of rocking curves at (002) plane for the GaN epilayers grown on conventional sapphire substrate (CSS), FTP-PSS-A, FTP-PSS-B, and FTP-PSS-C were 412, 238, 346, and 357 arcsec, while these values at (102) plane were 593, 327, 352, and 372 arcsec, respectively. The SpeCLED-Ratro simulation results reveal that the LED prepared on FTP-PSS-A has the highest light extraction efficiency than that of the other devices. At an injection current of 350 mA, the output powers of LEDs fabricated on CSS, FTP-PSS-A, FTP-PSS-B, and FTP-PSS-C were 157, 254, 241, and 233 mW, respectively. The results indicate that both the crystal quality of GaN epilayer and the light extraction of LED can be improved via the use of FTP-PSS, especially for the FTP-PSS-A.

GaN epitaxial lateral overgrowth on laser-textured sapphire

Laser-patterned (0001) sapphire has been evaluated as a substrate to perform GaN epitaxial lateral overgrowth (ELO) by metal-organic chemical vapor deposition (MOCVD). Several conditions of trench ablation were tested by varying scanning speed and laser fluence of a 266-nm picosecond laser. Growth of GaN epilayers was characterized by micro photoluminescence and cathodoluminescence (CL) spectroscopy. The spectra revealed a strain relaxation in the epitaxially overgrown GaN layer with improved quality expressed by enhanced intensity and redshift of the bandgap luminescence (from 3.44 to 3.40 eV). The bandgap against yellow luminescence (YL) intensity ratio also revealed that a defect-depleted area exists at the edge of the trench.

Effect of ZnO buffer layer thickness on the epitaxial growth of GaN by reactive magnetron sputtering

Epitaxial GaN films were grown on sputtered ZnO buffer layers of thickness 25–200nm over c-plane sapphire by reactive sputtering of GaAs in nitrogen at 700°C. The epitaxial quality and microstructure have been studied by high resolution X-ray diffraction in phi (ϕ) and omega (ω) scan geometries. The surface morphology of epilayers was studied by atomic force microscopy and scanning electron microscopy and their crystalline quality was assessed by Raman spectroscopy. These studies have shown that ZnO buffer layers of 50–100nm facilitate growth of GaN epilayers of high crystalline quality, compared to those grown on thinner and thicker ZnO buffer layers.

(Invited) Scalable GaN-on-Silicon Using Rare Earth Oxide Buffer Layers

Growth of GaN epilayers on silicon (111) is achieved by using a rare earth oxide buffer layer technology. The use of the REO material system allows strain engineering of the silicon wafers prior to GaN growth in order to mitigate stresses formed during the growth and subsequent cooling process. Alloying of oxides with more than one rare earth element allows compositional and lattice parameter control of the REO material and thereby imparting different strains into the silicon wafer. The wafer bow of the REO/Si heterostructures is also a found to be dependent on the thickness of the oxide layers grown. Growth of GaN on REO/Si templates result in lower bow compared to growth on silicon wafers alone.

Influence of controlled growth rate on tilt mosaic microstructures of nonpolar a-plane GaN epilayers grown on r-plane sapphire

The influence of the epitaxial growth rate on the surface morphology, as well as the crystallinity and optical properties of nonpolar a-plane GaN epilayers grown on r-plane sapphire substrate, has been investigated by x-ray diffraction and photoluminescence studies. The a-plane GaN epilayer grown at faster growth rates revealed a horizontal-pillar-shaped morphology with triangular-pits having sharp corners. While the epilayer grown at a faster growth rate showed a greater difference in the ω tilt-offset angle and line-width value between the on- and off-axes from x-ray diffraction, the a-plane GaN grown at the relatively slower growth rate showed a flat morphology with few pits and small ω tilt-offset difference. Growth of nonpolar a-plane GaN epilayer was optimized, and the effect of the growth rate of the a-plane GaN epilayer and the reason for the difference in the ω tilt-offset line-width value between the c- and m-direction mosaicity of x-ray diffraction were analyzed.

The effects of sapphire nitridation on GaN growth by metalorganic chemical vapour deposition

In situ optical reflectivity measurements are employed to monitor the GaN epilayer growth process above a low-temperature GaN buffer layer on a c-plane sapphire substrate by metalorganic chemical vapour deposition. It is found that the lateral growth of the GaN islands and their coalescence are promoted in the initial growth stage if optimized nitridation time and temperature are selected when the substrate is pre-exposed to ammonia. As confirmed by atomic force microscopy observations, the quality of the GaN epilayers is closely dependent on the surface morphology of the nitridated buffer layer, especially grain size and nucleation density.

Growth of GaN epilayers on c-, m-, a-, and (20.1)-plane GaN bulk substrates obtained by ammonothermal method

GaN epilayers were grown by metalorganic chemical vapor deposition (MOCVD) on c-, m-, a-, and (20.1)-plane GaN substrates obtained by the ammonothermal method. The influence of (i) the surface preparation of substrates, (ii) MOCVD growth parameters, and (iii) the crystallographic orientation of substrates on the structural and optical properties of GaN epilayers was investigated and carefully analyzed. It was observed that the polishing of substrates and their misorientation have strong impact on the quality of GaN epilayers grown on these substrates. The MOCVD growth process was optimized for epilayers grown on m-plane GaN substrates. The best structural and optical properties were achieved for epilayers deposited at 1075 °C and the total reactor pressure of 50 mbar. These conditions were applied to grow GaN epilayers on substrates with other ( c-, a-, and (20.1)-plane) crystallographic orientations in the same MOCVD process. Particularly good optical properties were obtained for GaN epilayers deposited on polar and non-polar ( m- and a-plane) substrates, whereas slightly worse optical properties were observed for epilayers deposited on the semi-polar substrate. It therefore means that MOCVD growth conditions optimized for a given crystallographic direction ( m-plane direction in this case) work rather well also for other crystallographic directions.

The influence of substrate etched on the quality of GaN epilayers

We report the growth of GaN epilayers on the sapphire substrate etched by MOCVD. Sapphire substrate is etched by H 3PO 4 and NaOH. The Raman scattering spectroscopy and photoetching analyses show that the substrate etched can effectively decrease the residual stress and the dislocations density in these epilayers. The X-ray diffraction analysis shows the process can reduce the value of the FWHM. Therefore, the quality of GaN epilayers should be improved by substrate etched.

Growth of GaN Epilayers on Defect-Selective Etched Nanoporous GaN Templates Generated by Electrochemical Etching

In this work, a defect-selective etched nanoporous GaN was formed using electrochemical etching in oxalic acid and employed as a template for the overgrowth of high quality GaN epilayers by metal/organic chemical vapor deposition. To illustrate the influence of the nanoporous GaN on the subsequent overgrown GaN epilayer, high resolution X-ray diffraction (XRD) and transmission electron microscopy (TEM) were measured. The linewidth values of the XRD rocking curve for an overgrown GaN epilayer decreased both for the (002) symmetry plane and (102) asymmetry plane. The TEM observations indicated that the GaN epilayers overgrown on the nanoporous GaN templates had fewer threading dislocations (TDs) due to dislocation bending and had annihilated in the form of dislocation loops. The results indicate that the use of nanoporous GaN as a template is an effective way to reduce the density of TDs in GaN epilayers.

Growth of thick, continuous GaN layers on 4-in. Si substrates by metalorganic chemical vapor deposition

We report on the growth of thick GaN epilayers on 4-in. Si(1 1 1) substrates by metalorganic chemical vapor deposition. Using intercalated AlN layers that contribute to counterbalance the tensile strain induced by the thermal mismatch between gallium nitride and the silicon substrate, up to 6.7 μm thick crack-free group III-nitride layers have been grown. Root mean-squares surface roughness of 0.5 nm, threading dislocation densities of 1.1×10 9 cm −2, as well as X-ray diffraction (XRD) full widths at half-maximum (FWHM) of 406 arcsec for the GaN(0 0 2) and of 1148 arcsec for the GaN(3 0 2) reflection have been measured. The donor bound exciton has a low-temperature photoluminescence line width of 12 meV. The correlation between the threading dislocation density and XRD FWHM, as well as the correlation between the wafer curvature and the GaN in-plane stress is discussed. An increase of the tensile stress is observed upon n-type doping of GaN by silicon.

High quality GaN epilayers grown on Si (1 1 1) with thin nonlinearly composition-graded Al xGa 1− xN interlayers via metal-organic chemical vapor deposition

The growth of GaN epilayers on AlN/Si (1 1 1) templates with thin nonlinearly composition-graded Al x Ga 1− x N interlayers via metal-organic chemical vapor deposition (MOCVD) is reported in this work. The composition-graded Al x Ga 1− x N interlayer was achieved by simply changing the growth condition of AlN to that of GaN layers during a fixed time. The surface morphology, crystalline quality and stress of GaN films have been investigated with different Al x Ga 1− x N interlayer thickness, 280 nm, 460 nm, 575 nm, and 750 nm. It was found that the properties of GaN films are highly dependent on the Al x Ga 1− x N interlayer thickness. High quality crack-free GaN films up to 1.2 μm can be achieved with 460 nm thick Al x Ga 1− x N layer.