Continuous GaN Film Research Articles

Epitaxy of single-crystalline GaN films on novel Ca3Ta(Ga0.5Al0.5)3Si2O14 substrates by metal-organic chemical vapor deposition

The langasite (La3Ga5SiO14, LGS) is a nearly lattice-matched substrate for epitaxy of GaN films. However, it is a challenge to grow GaN films on LGS substrates by metal-organic chemical vapor deposition (MOCVD) because LGS is unstable in aggressive gas atmosphere at high temperature of ∼1000 °C. To overcome the problem, this study chooses another LGS family crystal Ca3Ta(Ga0.5Al0.5)3Si2O14 (CTAGS) which has the same crystal structure as LGS and is more stable in high temperature. The morphology of GaN films at different growth stages are characterized and the results show continuous and smooth GaN films could be obtained after growing for 3 h. Single-crystalline GaN films are determined by transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD) measurements. The epitaxial relationship between GaN and CTAGS is GaN(0001)//CTAGS(0001) and GaN(101¯0)//CTAGS(213¯0), the lattice mismatch between GaN and CTAGS is 4.15%. The work is great potential in promoting the development of GaN-based devices.

Growth of continuous GaN films on ZnO buffer layer by chemical vapor deposition for ultraviolet photodetector

Growth of continuous GaN films on ZnO buffer layer by chemical vapor deposition for ultraviolet photodetector

Epitaxial Growth of GaN on Magnetron Sputtered AlN/Hexagonal BN/Sapphire Substrates.

Magnetron sputtering is adopted to deposit ~25 nm thick AlN on the surface of hexagonal BN(h-BN)/sapphire substrates, followed by epitaxial GaN growth on top of the AlN/h-BN/sapphire substrate using a metal–organic chemical vapor deposition system. Compared to GaN grown on the h-BN/sapphire surface directly, this method results in a continuous and smooth GaN film with a smaller root mean square roughness. Besides, the introduction of the sputtered AlN layer reduces the dislocation density of GaN by 35.7%. We provide a pathway of GaN epitaxy on the h-BN surface, which significantly improves its surface morphology and crystal quality.

Growth of GaN on monolayer hexagonal boron nitride by chemical vapor deposition for ultraviolet photodetectors

Chemical vapor deposition (CVD) technology is a simple and flexible method used to prepare high-quality crystalline materials. Traditional CVD technology, based on pre-deposited thin catalyst metal, usually produces nanostructures instead of continuous films. In this work, a continuous GaN film on a monolayer boron nitride (h-BN) insertion layer is demonstrated using CVD technology. The experimental results and theoretical calculations indicate that abundant GaN nanocrystallites are firstly formed at the edges or grain boundaries of the monolayer h-BN by quasi-van der Waals epitaxy. Then, the vapor-solid mechanism will control further growth of the GaN nanocrystallites, causing them to merge into a continuous GaN film. Meanwhile, the CVD-grown GaN ultraviolet detector exhibits a relatively high responsivity with a value of 0.57 A W−1 at 2 V. In this paper, a simple low-cost CVD method is proposed for preparing continuous films on two-dimensional materials for electronic and optoelectronic devices.

GaN-Based LEDs Grown on Graphene-Covered SiO2/Si (100) Substrate

The growth of nitride on large-size and low-cost amorphous substrates has attracted considerable attention for applications in large-scale optoelectronic devices. In this paper, we reported the growth of GaN-based light-emitting diodes (LEDs) on amorphous SiO2 substrate with the use of nanorods and graphene buffer layers by metal organic chemical vapor deposition (MOCVD). The effect of different growth parameters on the morphology and vertical-to-lateral aspect ratio of nanorods was discussed by analyzing growth kinetics. Furthermore, we tuned nanorod coalescence to obtain continuous GaN films with a blue-LED structure by adjusting growth conditions. The GaN films exhibited a hexagonal wurtzite structure and aligned c-axis orientation demonstrated by X-ray diffractometer (XRD), Raman, and transmission electron microscopy (TEM) results. Finally, five-pair InGaN/GaN multi-quantum-wells (MQWs) were grown. The photoluminescence (PL) showed an intense emission peak at 475 nm, and the current–voltage (I-V) curve shows a rectifying behavior with a turn-on voltage of 5.7 V. This work provides a promising fabrication method for the large-area and low-cost GaN-based devices on amorphous substrates and opens up the further possibility of nitride integration with Si (100) complementary metal oxide semiconductor (CMOS) electronics.

Direct van der Waals epitaxy of stress-free GaN films on PECVD grown graphene

Residual stress is generated in GaN epitaxial layers due to the mismatch during GaN epitaxy on sapphire using the traditional method. Therefore, the use of graphene to reduce residual stress and dislocation densities in GaN epitaxy has become an important research direction. However, growing a stress-free GaN film on graphene substrate remains challenge. In this work, we directly grew graphene on sapphire via plasma enhanced chemical vapor deposition (PECVD) to obtain an epitaxial graphene with characteristic orientation, and ultra-low stress GaN films can then be obtained through metal organic chemical vapor deposition (MOCVD) assisted with the sputtering AlN buffer layer. Through this method, we successfully obtained continuous and flat GaN films with ultra-low biaxial compressive stress (0.023 GPa) without the complicated stress engineering during epitaxial growth. First principle calculation was employed to confirm that the characteristic orientation of epitaxial graphene is crucial to release the stress in GaN. The obtained GaN films can also be easily transferred because of small van der Waals force on graphene. The transferred GaN heterojunction was directly fabricated into a metal-insulator-semiconductor (MIS) device from which typical electrical properties can be obtained. Our work reveals the stress-releasing mechanism and excellent stress-releasing effect of graphene and provides a new epitaxial strategy to guide crystallographic epitaxy.

Effects of N₂ Partial Pressure on Growth, Structure, and Optical Properties of GaN Nanorods Deposited by Liquid-Target Reactive Magnetron Sputter Epitaxy.

GaN nanorods, essentially free from crystal defects and exhibiting very sharp band-edge luminescence, have been grown by reactive direct-current magnetron sputter epitaxy onto Si (111) substrates at a low working pressure of 5 mTorr. Upon diluting the reactive N2 working gas with a small amount of Ar (0.5 mTorr), we observed an increase in the nanorod aspect ratio from 8 to ~35, a decrease in the average diameter from 74 to 35 nm, and a two-fold increase in nanorod density. With further dilution (Ar = 2.5 mTorr), the aspect ratio decreased to 14, while the diameter increased to 60 nm and the nanorod density increased to a maximum of 2.4 × 109 cm−2. Yet, lower N2 partial pressures eventually led to the growth of continuous GaN films. The observed morphological dependence on N2 partial pressure is explained by a change from N-rich to Ga-rich growth conditions, combined with reduced GaN-poisoning of the Ga-target as the N2 gas pressure is reduced. Nanorods grown at 2.5 mTorr N2 partial pressure exhibited a high intensity 4 K photoluminescence neutral donor bound exciton transitions (D0XA) peak at ~3.479 eV with a full-width-at-half-maximum of 1.7 meV. High-resolution transmission electron microscopy corroborated the excellent crystalline quality of the nanorods.

Threading dislocation reduction in three-dimensionally grown GaN islands on Si (111) substrate with AlN/AlGaN buffer layers

Three-dimensional GaN island growth without any masks was first introduced under high pressure in metalorganic chemical vapor deposition after the growth of AlN and AlGaN buffer layers on Si (111) substrate, followed by two-dimensional GaN growth to form a continuous GaN film with improvement of the crystalline quality and surface smoothness. X-ray diffraction and cross-sectional scanning transmission electron microscopy analyses show that a high-quality GaN film can be achieved by bending of edge threading dislocations (TDs) and the formation of dislocation half-loops. It is observed that most of edge TDs bend 90° from the growth direction along c-axis, whereas mixed TDs bend about 30° towards the inclined sidewall facets of the islands. Consequently, a 1.2 μm thick GaN epitaxial film with a low threading dislocation density of 2.5 × 108 cm−2 and a smooth surface of 0. 38 nm roughness can be achieved on Si substrate.

HVPE Growth of GaN Layers on Cleaved β-Ga<sub>2</sub>O<sub>3</sub> Substrates

GaN epitaxial layers were successfully grown by hydride vapour phase epitaxy (HVPE) on β-Ga2O3 substrates produced by cleaving. The initial stages of GaN epitaxial growth on β-Ga2O3 were studied by scanning electron microscopy (SEM) and x-ray diffraction analysis (XRD). The nucleation and the transition from the nucleation layer to a continuous GaN film were studied. It was found that the growth starts with formation of small crystallites on the substrate surface. As the growth continues, crystallites transform into pyramidal islands which increase in size and merge together. It was found that the structural quality of the GaN layers rapidly improves with increasing thickness. The full width at half maximum of x-ray ω rocking curves for (0002) peak decreased from 1370 to 540 arcsec as the deposition time was increased from 30 to 120 sec. This corresponds to the variation of the nominal layer thickness from 250 nm to 1000 nm.

Strain states in GaN films grown on Si(111) and Si(110) substrates using a thin AlN/GaN superlattice interlayer

AbstractWe report the characterization results of the strain states in GaN films grown both on Si(111) and Si(110) substrates by applying a thin AlN/GaN superlattice structure (SLs) as an interlayer (IL). The strain states in the GaN films grown on Si(111) and Si(110) substrates are compared. It is demonstrated that the modulation effect by the SL ILs on the strain state in the GaN film is universal although different behaviours on the Si(111) and Si(110) substrate are observed due to the different lattice symmetry relationship between the substrate and the epilayer. By using the technique, we succeeded in growing thick continuous GaN films (> 3 µm), which are necessary for the high‐power, high‐voltage device applications. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Nanopore morphology in porous GaN template and its effect on the LEDs emission

GaN grown on sapphire is electrochemically etched in HF and in KOH. Etching in HF results in a network of nanopillars while that etched in KOH results in a network of pores. The higher density of voids from the network of pores shows the highest strain relaxation for a 1.2 µm thick GaN overgrown on the porous templates. In general, a light-emitting diode (LED) on the porous templates gives about 1.5 times higher intensity and a spectral envelop shift towards the red due to a higher In incorporation. The higher intensity is attributed to enhanced light extraction due to light scattering at the voids formed from the pores and improved material quality with dislocation reduction. The formation of larger overgrowth GaN islands which merges to give a continuous GaN film over the porous template reduced the dislocation density and also accounted for higher strain relaxation for the growth of the quantum dots (QDs) and quantum well layers. This reduced the extent of peak shift of LEDs grown on porous GaN template and improved its performance.

Modulation of Strain States in GaN Films by a Thin AlN/GaN Superlattice Interlayer Grown on Si(110) Substrates

The effect of a thin AlN/GaN superlattice structure (SLs) interlayer on the strain properties in GaN films grown on Si(110) substrates is investigated. By comparing the strain states in GaN films without the interlayer, it is found that the strain in GaN films can be modulated by the SLs interlayer, which can be converted from a tensile state to a compressive one. The realization of the compressive strain state in the GaN film results in the suppression of the crack generation in the thick GaN growth, which is a general problem in the GaN growth on Si substrates. Using this simple technique, we successfully grow a crack-free continuous GaN film exceeding 4 µm thick. Therefore, the SLs interlayer is a promising structure in growing crack-free thick GaN on the Si substrate for optic and electronic device applications.

Stress distribution of 12 μm thick crack free continuous GaN on patterned Si(110) substrate

AbstractThe stress distribution on crack free thick continuous GaN film (12 µm) grown by Metal organic chemical vapour deposition (MOCVD) on the arrays of different sizes of the patterned silicon substrate is investigated by micro‐Raman (µRaman) spectroscopy. On the largest crack free mesa (400 μm) both µ‐photoluminescence (µPL) at low temperature and µRaman measurements are performed. The µRaman shift of the GaN E2 mode shows the U‐shape in‐plane stress distribution across the mesa. The center of the mesa is under tensile stress and it relaxes near the corner and edges. A similar trend is observed also from the µPL spectra. The size of the mesa, the trench height and the trench width of the patterned silicon are varied to study the stress of the thick epitaxial crack free GaN layer. The size and trench height of the mesa have a large influence on the GaN film stress but the trench width does not show any significant effect. The maximum stress is saturated for the large sizes of mesas. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Selective growth of GaN nanodots and nanostripes on 6H‐SiC substrates by metal organic vapor phase epitaxy

AbstractGaN nanodots and nanostripes with smooth sidewall surfaces have been selectively grown on 6H‐SiC substrates by metal organic vapor phase epitaxy. By varying the growth reactor pressure, we have been able to grow either isolated nanostructures or laterally overgrown structures. As confirmed by the Raman scattering and X‐ray diffraction techniques, the nanostructures have no influence of step bunching that occurs in the unmasked area of the continuous GaN film. The frequency shift of the E2 optical phonons shows that the residual strain in the nanostripes is relaxed compared to the continuous GaN film. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Improved optical performance of GaN grown on pattered sapphire substrate

An improved GaN film with low dislocation density was grown on a C-face patterned sapphire substrate (PSS) by metalorganic chemical vapor deposition (MOCVD). The vapor phase epitaxy starts from the regions with no etched pits and then spreads laterally to form a continuous GaN film. The properties of the GaN film have been investigated by double crystal X-ray diffraction (DCXRD), atomic force microscopy (AFM) and photoluminescence (PL), respectively. The full-width at half-maximum (FWHM) of the X-ray diffraction curves (XRCs) for the GaN film grown on PSS in the (0002) plane and the (101̄2) plane are as low as 312.80 arcsec and 298.08 acrsec, respectively. The root mean square (RMS) of the GaN film grown on PSS is 0.233 nm and the intensity of the PL peak is comparatively strong.

Effects of surface treatment for sapphire substrate on gallium nitride films

High quality GaN films with low dislocation density have been successfully grown on the c-plane sapphire substrate, which the sapphire is etched by molted KOH solution. The solid-phase epitaxy starts from the regions with no etched pits where there are some micro-sidesteps to help to nucleate and then spreads laterally by increasing V/III ratio to form continuous GaN films. The crystal quality of epilayer is improved with the lengthening of etching time, but when the etching time is prolonged to some extent, the size of etching pit is too large to coalesce of two wings, the epilayer crystal quality is decreased, so the etching time for sapphire substrate has crucial effect on the epilayer crystal quality. The full widths at half maximum of the X-ray diffraction curves for the GaN films grown on the sapphire substrate, which has been etched for 50 min, in (0 0 0 2) plane and ( 1 0 1 ¯ 2 ) plane are as low as 202.68 arcsec and 300.24 arcsec, respectively, and the etch-pit density (EPD) of those etched in molten KOH is reduced down to 3.1 × 10 5 cm −2. The epilayer exhibits atomically smooth surface, whose root mean square roughness is found to be 0.184 nm. Compared with the conventional epitaxial lateral overgrown technique. The technique is much simpler, the voids and crystallographic tilt can also be overcome.

High-quality GaN films grown on surface treated sapphire substrate

High-quality GaN films with low dislocation density have been successfully grown on the c-plane specially treated surface of the sapphire substrate. The vapour phase epitaxy starts from the regions with no etched pits where there are some micro-sidesteps to help to nucleate and then spreads laterally with increasing V/III ratio to form continuous GaN films. The full-widths at half-maximum of the x-ray diffraction curves for the GaN films grown on the treated surface of the sapphire substrate in the (0 0 0 2) plane and the (1 0 −1 2) plane are as low as 208.80 arcsec and 320.76 acrsec, respectively, and the etch-pit density of those etched in molten KOH is reduced to 5.5 × 105 cm−2. The surface of the epilayer exhibits atomically smooth, whose root mean square roughness is found to be 0.2863 nm.The epilayer grown on the treated surface of the sapphire substrate exhibits excellent optical quality, in which the yellow luminescence is nearly invisible in the photoluminescence spectrum. Compared with the conventional epitaxial lateral overgrowth technique, the technology is much simpler and the crystallographic tilt can also be greatly decreased.

Nanoscale epitaxial overgrowth process and properties of GaN layers on Si (111) substrates

Nanoscale epitaxial overgrowth has been explored to realize continuous specular GaN films on patterned SiO2∕GaN∕Si (111) substrates. We have employed both polystyrene-based nanosphere and interferometric lithographies to form the nanohole array patterns and then subsequent regrowth of GaN is carried out by metalorganic chemical vapor deposition. The nanoscale epitaxial overgrowth process of GaN layers is studied by scanning and transmission electron microscopy measurements. Optical spectroscopic methods such as microphotoluminescence and micro-Raman scattering show an improvement of the optical and crystalline quality in such overgrown GaN layers when compared to GaN simultaneously grown on bulk Si (111) without patterning. Realization of such thicker and good quality GaN layer would be useful to achieve III-nitride-based optoelectronic integration on Si substrates.

GaN epilayers on nanopatterned GaN/Si(1 1 1) templates: Structural and optical characterization

Template-based nanoscale epitaxy has been explored to realize high-quality GaN on Si(1 1 1) substrates. We have employed polystyrene-based nanosphere lithography to form the nano-hole array patterns on GaN/Si(1 1 1) template and then, subsequent regrowth of GaN is carried out by metalorganic chemical vapor deposition (MOCVD). During the initial growth stage of GaN on such nanopatterned substrates, we have observed formation of nanoislands with hexagonal pyramid shape due to selective area epitaxy. With further epitaxial regrowth, these nanoislands coalesce and form continuous GaN film. The overgrown GaN on patterned and non-patterned regions is characterized by high-resolution X-ray diffraction (HRXRD) and high-spatial resolution optical spectroscopic methods. Micro-photoluminescence (PL), micro-Raman scattering and scanning electron microscopy (SEM) have been used to assess the microstructural and optical properties of GaN. Combined PL and Raman data analyses show improved optical quality when compared to GaN simultaneously grown on non-patterned bulk Si(1 1 1). Such thicker GaN templates would be useful to achieve III-nitride-based opto- and electronic devices integrated on Si substrates.

Characterization of High Quality Continuous GaN Films Grown on Si-Doped Cracked GaN Template

AbstractIn this paper, deep level defects in high quality continuous GaN films grown over a cracked Si-doped GaN template has been studied using digital deep level transient spectroscopy (DLTS) and transmission electron microscopy (TEM). From TEM observation, it is found that the density of pure screw dislocations have been effectively suppressed while pure edge dislocations remained in substantial quantity. From DLTS measurement, trap levels at Ec -ET ∼ 0.11-0.12 eV, 0.24-0.27 eV, 0.60-0.63 eV were detected in the high quality GaN layer. DLTS measurement was also carried out on the underlying cracked Si-doped GaN template after the top high quality continuous GaN film was removed by plasma etching. An additional defect level at Ec-Et ∼ 0.37 eV was detected which we attributed to defect decoration at screw dislocation. Both the trap levels Ec-ET ∼ 0.24–0.27 eV, 0.60-0.63 eV are believed to originate from mixed screw/edge dislocation based on observation of the logarithmic capture behavior. Trap level at Ec -ET ∼ 0.24-0.27eV, however, experiences a more drastic increase in transient capacitance (i.e. in trap concentration) compared to that of Ec -ET ∼ 0.60-0.63 eV after plasma etching, illustrating that the latter is related to a higher proportion of edge dislocation. The 0.11-0.12 eV trap level, which exhibits an exponential capture kinetic, is believed to be related to nitrogen vacancies. This high quality continuous GaN layer can be used as a template to grow any device structure and the underneath cracked Si-doped GaN layer may help to release stress for the top continuous GaN layer. This can bring about a cracked free epilayer for subsequent device fabrication.