Polycrystalline Gallium Nitride Research Articles

Selective mask formation and gallium nitride template fabrication on patterned sapphire substrates for light-emitting diodes

Patterned sapphire substrates are composed of multiple planes, leading to the undesirable polycrystalline growth of gallium nitride (GaN) during template deposition. However, patterned sapphire substrates and templates with good crystalline quality would be useful for increasing the light extraction efficiency in light-emitting diodes. In this study, a silicon dioxide (SiO2) mask was selectively introduced onto the lenses of a patterned sapphire substrate to suppress the growth of polycrystalline GaN. The SiO2 mask prevented the negative influence of the patterned sapphire substrate on GaN growth under various conditions. High-resolution x-ray diffraction analysis revealed that the GaN template grew as a single crystal in the presence of the SiO2 mask. Furthermore, the compressive stress generated in the GaN template was relieved due to the suppression of polycrystalline growth.

Thermal Atomic Layer Deposition of Polycrystalline Gallium Nitride

We report the successful preparation of polycrystalline gallium nitride (poly-GaN) layers by thermal atomic layer deposition (ALD) at low temperatures (375–425 °C) from trimethylgallium (TMG) and a...

Epitaxy of Single‐Crystalline GaN Film on CMOS‐Compatible Si(100) Substrate Buffered by Graphene

AbstractFabricating single‐crystalline gallium nitride (GaN)‐based devices on a Si(100) substrate, which is compatible with the mainstream complementary metal‐oxide‐semiconductor circuits, is a prerequisite for next‐generation high‐performance electronics and optoelectronics. However, the direct epitaxy of single‐crystalline GaN on a Si(100) substrate remains challenging due to the asymmetric surface domains of Si(100), which can lead to polycrystalline GaN with a two‐domain structure. Here, by utilizing single‐crystalline graphene as a buffer layer, the epitaxy of a single‐crystalline GaN film on a Si(100) substrate is demonstrated. The in situ treatment of graphene with NH3 can generate sp3 CN bonds, which then triggers the nucleation of nitrides. The one‐atom‐thick single‐crystalline graphene provides an in‐plane driving force to align all GaN domains to form a single crystal. The nucleation mechanisms and domain evolutions are further clarified by surface science exploration and first‐principle calculations. This work lays the foundation for the integration of GaN‐based devices into Si‐based integrated circuits and also broadens the choice for the epitaxy of nitrides on unconventional amorphous or flexible substrates.

Improving Material Quality of Polycrystalline GaN by Manipulating the Etching Time of a Porous AlN Template

A porous template has been reported to reduce defect density and strains, and hence to improve the properties of gallium nitride (GaN) deposited on it. On the other hand, creating a porous aluminum nitride (AlN) template is challenging and, therefore, reports on it are scarce. In this work, the material quality of a polycrystalline GaN layer was improved by manipulating the etching time of the porous AlN template. The porous AlN template was fabricated for 5 and 30 min by ultraviolet (UV)-assisted sodium hydroxide (NaOH)-based electrochemical etching. The 5-min-etched porous AlN template exhibited a better uniformity of the pore distribution than the 30-min-etched porous AlN template. A non-porous AlN was also prepared for a comparison. All samples had gallium oxide (Ga2O3) inclusions in the polycrystalline GaN due to the oxide layer formation during the AlN template etching. However, such inclusions have been successfully removed by dipping the porous AlN template in hydrofluoric acid (HF) solution prior to the GaN layer deposition. Next, the polycrystalline GaN layer was deposited on both templates by an electron beam (e-beam) evaporator, followed by a thermal annealing treatment to promote better crystalline structure. The polycrystalline GaN layer that was deposited on the 5 min-etched porous AlN template showed a good uniform distribution of grains with lower surface roughness and smaller x-ray diffraction (XRD) full-width half maximum (FWHM) compared to other conditions. In addition, the 5-min-etched porous AlN template also showed better electrical properties than its counterparts, which justifies the use of this process for electronic devices.

PEALD-deposited crystalline GaN films on Si (100) substrates with sharp interfaces**Project supported by the Fundamental Research Funds for the Central Universities (Grant Nos. FRF-BR-16-018A, FRF-TP-17-022A1, and FRF-TP-17-069A1), the National Natural Science Foundation of China (Grant Nos. 61274134 and 51402064), USTB Start-up Program (Grant No. 06105033), China Postdoctoral Science Foundation (Grant No. 2018M631333), Beijing Natural Science Foundation (Grant Nos. 2184112 and

Polycrystalline gallium nitride (GaN) thin films were deposited on Si (100) substrates via plasma-enhanced atomic layer deposition (PEALD) under optimal deposition parameters. In this work, we focus on the research of the GaN/Si (100) interfacial properties. The x-ray reflectivity measurements show the clearly-resolved fringes for all the as-grown GaN films, which reveals a perfectly smooth interface between the GaN film and Si (100), and this feature of sharp interface is further confirmed by high resolution transmission electron microscopy (HRTEM). However, an amorphous interfacial layer (∼2 nm) can be observed from the HRTEM images, and is determined to be mixture of GaxOy and GaN by x-ray photoelectron spectroscopy. To investigate the effect of this interlayer on the GaN growth, an AlN buffer layer was employed for GaN deposition. No interlayer is observed between GaN and AlN, and GaN shows better crystallization and lower oxygen impurity during the initial growth stage than the GaN with an interlayer.

(Invited) A Comparative Study of Low-Temperature III-V Nitrides ALD in Thermal and Radical-Enhanced Modes

In the field of III-V nitrides, nowadays research is strongly focused on electron devices employing epitaxially grown gallium nitride (GaN) and aluminum gallium nitride (AlGaN) layers, as certain properties of these materials are superior to silicon. Still, the production of GaN/AlGaN in monocrystalline form leads to a high cost. Developing the polycrystalline films with cheaper production techniques may pave the way to more widespread adoption of these nitrides in microelectronics and lighting. Atomic layer deposition (ALD) allows in this light for less stringent (compared to epitaxial techniques) growth requirements, enabling reduced process temperatures and direct growth on a large variety of substrates, e.g., on Si and SiO2. ALD combines accurate thickness control with a high degree of uniformity and conformality. Despite of the growing number of publications w.r.t. various applications of the ALD poly-nitrides, the question if they exhibit competitive advantages for being implemented in electron devices remains open. It is generally accepted that due to a sufficient chemical activity of ammonia (NH3) with surface methyl groups (-CH3) after a trimethylaluminum (TMA) pulse, ALD of AlN readily occurs in purely thermal mode at low temperatures (< 370 oC). This is opposite to thermal deposition of GaN films from trimethylgallium (TMG) and NH3 precursors. It is reported that due to a low chemical activity of NH3 with the film surface after a TMG pulse, low-temperature deposition hardly occurs in purely thermal regime. The majority of reports on low-temperature deposition of GaN have therefore used additional means of activation (e.g., plasma) to dissociate NH3, thereby enhancing the reactivity with TMG-exposed surface by means of forming NHx radicals. This work reports on the deposition of GaN films at a temperature of 400 oC, in a purely thermal chemical mode using TMG and NH3, and compares that with plasma-enhanced GaN ALD. The demonstrated plasma-free process can be beneficial for industrial-scale batch-type reactors where uniform radical delivery is hard to achieve. The proposed approach utilizes the strong adduct-formation chemistry of the Lewis-acid TMG and the Lewis-base NH3 [1, 2] on the growth surface. Beyond a specific temperature, the TMG:NH3 surface adduct is expected to dissociate into Ga-NH2-Ga linkage, forming the first monolayer of GaN in a self-limiting fashion. Alternating pulses of TMG and NH3 with in-between purges therefore result in film growth from a sequence of such self-limiting surface reactions. The growth per cycle (GPC) has shown an increase with NH3 partial pressure [3], in accordance with the adduct reaction kinetics. This pressure-dependence of the growth has been in-situ monitored by spectroscopic ellipsometry (SE), revealing a maximum GPC value of 0.045 nm/cycle. The grazing incidence X-ray diffraction (GIXRD) scans confirmed polycrystalline wurtzitic structure of the GaN. The compositional depth profile obtained by X-ray photoelectron spectroscopy (XPS) revealed the formation of a stoichiometric GaN. The efficient removal of methyl groups (i.e., carbon) from the films has been confirmed by the low carbon content (1 at. %). Increasing the temperature from 400 oC to 550 oC activated the thermal decomposition of TMG, suppressing the adduct formation kinetics, breaking the mechanism of carbon removal from the films and thereby increasing the carbon content up to 10 at. %. The research efforts were further directed towards ALD of boron nitride (BN) films from BBr3 and NH3 precursors, exploring the role of adduct-formation chemistry in that process as well. The performed comparison between radical-enhanced and pure-thermal deposition of BN and GaN layers will be shown in the presentation, clarifying the film formation mechanisms. The purely thermal radical-free deposition of poly-GaN films will be highlighted, focussing on the growth mechanism, ALD window, and film properties. Ref.: [1] M. E. Bartram, T. A. Michalske, J. W. Rogers, Jr. and R. T. Paine, Chem. Mater. 5, 1424 (1993). [2] J. Randall Creighton and George T. Wang, J. Phys. Chem. A 109, 10554 (2005). [3] S. Banerjee, S. Dutta, A.A.I. Aarnink, J. Schmitz, D. J. Gravesteijn and A. Y. Kovalgin, “Purely thermal deposition of polycrystalline gallium nitride films at 400 oC”, submitted to ALD 2018 conference.

The process and mechanism of the GaN nanoparticles formed by nitridation of β-Ga2O3 crystal

The process of conversion from [Formula: see text]-Ga2O3 single crystal to gallium nitride (GaN) in an atmosphere of NH3 by chemical vapor deposition is investigated. The surface morphology and microstructure of the GaN nanoparticles are observed by scanning electron microscope, which indicates that the growth of GaN is via the Volmer–Weber mechanism. The [Formula: see text]-Ga2O3 is firstly evaporated at high temperature to form the porous layer, followed by the surface-defect induced GaN nucleation formation. The crystalline structure and epitaxial relationship of the GaN nanoparticles are investigated by X-ray diffraction (XRD) via [Formula: see text]–[Formula: see text], showing GaN (0002) and (0004) diffraction peaks in the XRD spectra. It is concluded that the polycrystalline GaN film with hexagonal structure has a strong c-axis preferential orientation.

On the solubility of gallium nitride in supercritical ammonia–sodium solutions

Due to the disparity between observed gallium nitride (GaN) growth under conditions for which literature reports normal solubility, GaN solubility in supercritical NH3-Na containing solutions was re-evaluated. Isothermal gravimetric experiments on polycrystalline GaN were performed in the temperature range (T =415–650°C) for which retrograde growth of GaN routinely occurs (P≈ 200MPa, molar NH3:Na fill ratio =20:1). Two previously-unreported error contributions to the gravimetric determination of GaN solubility were identified: Ga-alloying of exposed Ni-containing components, and the presence of a dense, Ga-absorbing Na-rich, second phase under these conditions. Due to the inability to measure Ga-alloying of the exposed autoclave wall for each experiment, considerable scatter was introduced in the refined GaN solubility curve. No clear dependence of GaN solubility on temperature was resolvable, while most solubility values were determined to be within a band of 0.03–0.10mol% GaN, normalized by fill NH3.

Electron Enhanced Growth of Crystalline Gallium Nitride Thin Films at Room Temperature and 100 °C Using Sequential Surface Reactions.

Low energy electrons may provide mechanisms to enhance thin film growth at low temperatures. As a proof of concept, this work demonstrated the deposition of gallium nitride (GaN) films over areas of ∼5 cm2 at room temperature and 100 °C using electrons with a low energy of 50 eV from an electron flood gun. The GaN films were deposited on Si(111) wafers using a cycle of reactions similar to the sequence employed for GaN atomic layer deposition (ALD). Trimethylgallium (Ga(CH3)3, TMG), hydrogen (H) radicals and ammonia (NH3) were employed as the reactants with electron exposures included in the reaction cycle after the TMG/H and NH3 exposures. A number of ex situ techniques were then employed to analyze the GaN films. Spectroscopic ellipsometry measurements revealed that the GaN films grew linearly with the number of reaction cycles. Linear growth rates of up to 1.3 Å/ cycle were obtained from the surface areas receiving the highest electron fluxes. Grazing incidence X-ray diffraction analysis revealed polycrystalline GaN films with the wurtzite crystal structure. Transmission electron microscopy (TEM) images showed crystalline grains with diameters between 2 and 10 nm depending on the growth temperature. X-ray photoelectron spectroscopy depth-profiling displayed no oxygen contamination when the GaN films were capped with Al prior to atmospheric exposure. However, the carbon concentrations in the GaN films were 10-35 at. %. The mechanism for the low temperature GaN growth is believed to result from the electron stimulated desorption (ESD) of hydrogen. Hydrogen ESD yields dangling bonds that facilitate Ga-N bond formation. Mass spectrometry measurements performed concurrently with the reaction cycles revealed increases in the pressure of H2 and various GaN etch products during the electron beam exposures. The amount of H2 and GaN etch products increased with electron beam energy from 25 to 200 eV. These results indicate that the GaN growth occurs with competing GaN etching during the reaction cycles.

Investigation of the surface anisotropy of polycrystalline gallium nitride coating using a tunnel microscope equipped with a tip of a boron-doped diamond

The structural features of gallium nitride polycrystalline coating formed on a silicon oxide substrate have been considered. It has been shown experimentally that as crystals grow, not only the surface morphology, but crystalline structure and electrophysical parameters of crystals change also. The topography of crystals surfaces at the initial stage of the coating deposition has been found to form due to a diffusion mass transfer.

Characteristics of GaN Thin Films Using Magnetron Sputtering System

The paper presents a polycrystalline GaN thin film with a hexagonal wurtzite structure under the optimized sputtering conditions of 40 W RF power, 5 mT working pressure, using pure nitrogen gas with a substrate temperature of 700°C. The study examines the effects of surface disorders and incorporates it in the thin films characteristics. A radio frequency (RF) Ultra High Vacuum (UHV) Magnetron Sputtering System has been used for the deposition of Gallium Nitride (GaN) on silicon, sapphire and glass substrates with different parameters. The power is varied from 40 W to 50 W, and the pressure from 4 mT to 15 mT. The effects of the RF sputtering powers and gas pressures on the structural properties are investigated experimentally. Sputtering at a lower RF power of 15 W does increase the N atomic percentage, however the deposition rate is substantially slower and the films are amorphous. GaN deposited on both silicon and sapphire wafer resulted in thin films close to stoichiometric once the N2 concentration is 60% or higher. It is also observed that the substrate cooling/heating effects improve the quality of the thin films with fewer defects present at the surface of the GaN epi-structure.

Growth and optical properties of gallium nitride film on glass substrates

AbstractIn this study, gallium nitride (GaN) films were grown on the quartz glass substrates using metal organic chemical vapor deposition with AlN buffer layer. The AlN layer was deposited using rf‐magnetron sputtering. Although the direct nucleation of GaN on quartz glass substrate is difficult, with AlN as the nucleation layer, a large‐area polycrystalline GaN film can be obtained. The structural and optical properties were investigated. The results show that the GaN film has a preferential orientation along (0002). There are mainly three emission regions in the photoluminescence spectra, including the band‐edge, green (2.45‐2.55eV) and yellow (2.0‐2.35 eV) emission regions. The origin and temperature dependence of the peak position, full‐width of half‐magnitude and intensity were also discussed. The findings may help to understand the growth and improve the properties of GaN film grown on amorphous glass substrates. (© 2015 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Thermal expansion of polycrystalline gallium nitride: an X‐ray diffraction study

Gallium nitride is studied at ambient and nonambient temperatures by powder X‐ray diffraction followed by Rietveld refinement of the structure. The structure is reported for the ambient temperature on the basis of laboratory data. The diffraction data collected using a synchrotron beam serve for derivation of the lattice parameter and thermal expansion dependencies on temperature. The variation of unit‐cell size on temperature was studied in detail in a broad range 11 K &lt; T &lt; 1073 K. The obtained dependencies of a, c and V and thermal expansion coefficients are smooth, and the results are consistent with previously collected data. The c/a ratio shows an almost linear decreasing trend with rising temperature. The magnitude of thermal expansion obtained in the described experiments is similar to a recently reported theoretical one. Copyright © 2015 John Wiley &amp; Sons, Ltd.

Stress and Grain Boundary Properties of GaN Films Prepared by Pulsed Laser Deposition Technique

Polycrystalline gallium nitride films were successfully deposited on fused silica substrates by ablating a GaN target using pulsed Nd-YAG laser. Microstructural studies indicated an increase in the average crystallite size from ~8 nm to ~70 nm with the increase in substrate temperature from 300 K to 873 K during deposition. The films deposited here were nearly stoichiometric. XPS studies indicated two strong peaks located at ~1116.6 eV and ~395 eV for Ga2p3/2 and a N1s core-level peak, respectively. The films deposited at substrate temperature above 573 K are predominantly zinc blende in nature. PL spectra of the films deposited at higher temperatures were dominated by a strong peak at ~3.2 eV. FTIR spectra indicated a strong and broad absorption peak centered ~520 cm−1 with two shoulders at ~570 cm−1 and 584 cm−1. Characteristic Raman peak at ~531 cm−1 for the A1(TO) mode is observed for all the films. Grain boundary trap states varied between 3.1×1015 and 7×1015 m−2, while barrier height at the grain boundaries varied between 12.4 meV and 37.14 meV. Stress in the films decreased with the increase in substrate temperature.

Acidic ammonothermal growth of GaN crystals using GaN powder as a nutrient

Ammonothermal crystal growth of gallium nitride (GaN) was realized for the first time using GaN powder as a nutrient with growth rates of about 61 μm per day on both Ga and N faces. The GaN powder was synthesized by ammonolysis of Ga metal in the presence of NH4I. The crystal quality of the as-grown GaN wafers was characterized by scanning electron microscope (SEM), X-ray rocking curve (XRC) and photoluminescence (PL) measurements. We examined the effect of different nutrients, including polycrystalline GaN, Ga metal, and GaN powder, on the acidic ammonothermal crystal growth of GaN using an NH4I-based mineralizer. Our results suggest that growth rate and crystal quality of GaN depend largely on the type of nutrient. Polycrystalline GaN as a nutrient can afford high growth rates of up to 150 and 237 μm per day on Ga and N faces, respectively. Growth rates up of to 33 μm per day were achieved using Ga metal as a nutrient. However, SEM, XRC, and PL measurements of GaN crystals grown using different nutrients indicate that Ga metal and GaN powder nutrients can provide better crystal quality of GaN in terms of surface morphology and crystal uniformity than polycrystalline GaN.

Effect of halogen species of acidic mineralizer on solubility of GaN in supercritical ammonia

The solubility of gallium nitride (GaN) in supercritical ammonia with either ammonium bromide (NH 4Br) or ammonium iodide (NH 4I) as a mineralizer was evaluated by measuring the weight loss of polycrystalline GaN on dissolution. The solubility of GaN with these mineralizers was compared with that of ammonium chloride (NH 4Cl). The temperature dependence of the GaN solubility with different mineralizers was in the order NH 4I>NH 4Br>NH 4Cl. At high temperatures a high level of supersaturation was obtained with NH 4I at a low-temperature difference to NH 4Cl. These results suggest that use of NH 4I as a mineralizer could increase the crystal growth rate of GaN.

Structural and Electrical Characterization of GaN Thin Films on Si(100)

The Gallium Nitride (GaN) layers grown on silicon substrates by electron beam evaporation technique. X- ray diffraction revealed that polycrystalline GaN was obtained indicating the enhance crystallinity of the films with annealing temperature at 600oC. Crystalline quality of the GaN films was determined by Scanning Electron Microscopy (SEM). The crystalline size increases with increasing annealing temperature. The fab- ricated MIS structures were characterized using Capacitance-Voltage (C-V) measurements, the capacitance remains nearly constant over a large range in higher negative as well as over a large range in higher positive gate voltages and Current-Voltage (I-V) measurements shows low forward and reverse current possibly due to high density defect formation in the thin layer of gallium nitride during its growth.The film is characterized by X-Ray photoelectron spectroscopy (XPS). The XPS spectra show that formation of pure GaN with- out presence of elemental gallium and Ga2O3 in this film.

Correlation between transient decays in wide gap semiconductors: Photoluminescence vs. photocurrent

Recently, we introduced the Thermalization–Recombination (TR) Model to explain the ubiquitous broad subgap photoluminescence bands in wide bandgap semiconductors [M. Niehus, R. Schwarz, Phys. Status Solidi C 3 (2006) 1637]. The model describes the competition between the thermalization and recombination dynamics of excess carriers trapped in localized states distributed exponentially in energy. In this contribution, we confronted the theoretical and qualitative predictions of the TR Model with experimental results of transient photoluminescence (TPL) of pulsed laser deposited (PLD) polycrystalline gallium nitride (GaN) and polycrystalline zinc oxide (ZnO). The TPL results are compared with transient photocurrent (TPC) measurements in order to highlight the relation between TPL and TPC, as well as similarities in the relaxation dynamics of GaN and ZnO. The general features of the transient decays for both materials can be explained within the framework of the TR model, which is shown to offer significant inside into the relaxation dynamics of wide gap semiconductor materials .

Effect of Surface and Growth Conditions for Formation of Textured Polycrystalline GaN Crystals by Reactive N2 Plasma

The surface and growth conditions for the formation of textured polycrystalline bulk gallium nitride (GaN) crystals under nitrogen (N2) reactive plasma were investigated. Langmuir probe measurements were used to determine the electron density values near the Ga surface at different target locations and ranged from (6–16)×1010/cm2. Optical spectroscopy measurements showed reactive gallium (Ga) and nitrogen species present in the plasma. When the target was bombarded with highly energetic ions, peaks of sputtered Ga at 403 and 417 nm wavelengths were observed in the optical spectroscopy measurements. The top Ga surface showed increased roughening and the resulting crystals were of poor crystalline quality, with fresh Ga leftovers. Smoother crystals were formed when the Ga target was shielded from the direct bombardment of highly energetic N2 plasma, with no sputtered Ga peaks observed during the bombardment. X-ray diffraction (XRD) analysis results revealed well-oriented crystals at the (0002) reflection of GaN. This proved that the shielding of highly energetic ions to decrease roughening of the surface and allowing more neutrals for the reaction were effective in forming well-oriented bulk GaN crystals.

The properties and deposition process of GaN films grown by reactive sputtering at low temperatures

Polycrystalline gallium nitride films, 100nmto1μm thick, were deposited under a range of conditions. Substrate electrode temperatures during sputtering were varied from room temperature to 450°C, the pressure from 0.15to6.0Pa, the nitrogen fraction of the deposition atmosphere from 10% to 100% and the target bias from −400to−1800V. The deposition rates as functions of these conditions are in the range 0.5–25nm∕min. The growth rate is considered to be controlled respectively by the thermally activated desorption from the substrate, changes in the mean free path and concentration of gas particles, differences between the sputter yields of Ga and GaN in Ar and N2, and changes in the ion current and sputter yields. The films are generally columnar, with the grain size increasing with film thickness. The most crystalline films were grown at mid range temperatures, low N2 concentrations, and low target biases, and the most disordered were grown at low pressures. The latter two cases suggest that decreasing the energy of particles incident on the film during deposition results in a more ordered film. The biaxial stress is compressive and shows an increasing trend with the target bias and N2 concentration, reaching 4.7GPa at 75% N2. Oxygen contamination of 3–30at.% has a major effect on the optical properties of the films, increasing the band gap values from 3.02to&amp;gt;4.0eV and the Urbach tail energies from around 150to840meV and decreasing the refractive index from 2.46 to 2.03. At a 40% N2 deposition fraction, the N:Ga ratio is more or less constant at 1:1. Since the absolute oxygen incorporation rate changes very little, it is the relative film deposition rate which determines the final oxygen concentration. Excess Ga at low N2 concentrations causes a decrease in the band gap and an increase in the Urbach tail energy.