Growth Of GaN Crystals Research Articles

Growth of bulk GaN crystals by Na flux method

AbstractHigh‐quality and low cost bulk crystals are needed in the field of group III nitride semiconductors in order to develop optical and electrical devices. There are two approaches for the growth of bulk GaN crystal by the Na flux method. One is to grow thick GaN crystal on a seed GaN crystal grown by vapour phase method. The other one is to grow GaN crystal on the seed GaN crystal obtained by spontaneous nucleation. The both methods show advantages and disadvantages, respectively. We also present the effects of the flux composition on the growth habit and the growth rate for fabrication of boule‐shape bulk GaN crystals. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

GaN boules grown by high rate HVPE

AbstractThe most promising approach for GaN substrate fabrication on the base of HVPE consists in the growth of thick GaN crystals and subsequent cutting of these boules into slices followed by conventional surface preparation. This work focusses on a HVPE GaN boule growth process optimized to highest growth rate and the use of a commercially available vertical HVPE reactor from AIXTRON in order to develop a technology of high productivity to enable low‐cost substrates. To this end, GaN crystals up to thicknesses of 6.3 mm have been grown on 2 inch GaN‐on‐sapphire templates produced by MOVPE. The rate in HVPE growth was 450 µm/h and the material properties of the crystals have been evaluated. No degradation of the material quality was found by comparison of structural, optical and transport properties with published data of the state of the art of bulk GaN. Dislocation densities as low as 6x105 cm‐2, high electron mobility of 960 cm2/Vs for a carrier density of 1x1016 cm‐3, low background impurity incorporation of oxygen and silicon, luminescence with well resolved excitonic emission, and a high value of thermal conductivity of 294±44 W/mK were determined. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

High nitrogen pressure solution (HNPS) growth of GaN on 2 inch free standing GaN substrates

Recent results of High Nitrogen Pressure Solution (HNPS) growth of GaN crystals deposited on and separated from 2 inch, and smaller, GaN substrates grown by Hydride Vapor Phase Epitaxy (HVPE) have been presented. The influence of the c-plane bowing in the initial substrate on quality, rate and mode of growth by HNPS method has been analyzed in details.

Ammonothermal growth of high-quality GaN crystals on HVPE template seeds

High quality GaN crystals have been successfully grown by the ammonothermal method in alkaline ammonia solutions using hydride vapor phase epitaxy (HVPE) seeds. The grown crystals, over 1 mmthick, are clear and possess excellent structural and optical properties. The crystalline structure of the as-grown bulk GaN is as good as, or better than the HVPE seeds as measured by high resolution X-ray rocking curves with 100arcsec of full width at half maximum (FWHM) on (002) and 90arcsec on (102) diffractions. The crystal quality is improved through a process of careful seed selection and controlled heating during nucleation, so that the ammonothermal growth replicates the seed crystals on both the nitrogen and gallium faces. The results are confirmed by low temperature photoluminescence spectra resolving donor-bound and free excitons as well as multiple phonon replicas, and further by room temperature cathodoluminescence indicating reduced yellow-band emission. Successful growth of high quality GaN crystals on HVPE seeds will facilitate the scale-up to large area growth by use of large area GaN HVPE templates as seeds.

Molecular beam epitaxy as a method for the growth of free-standing bulk zinc-blende GaN and AlGaN crystals

We have studied the growth of zinc-blende GaN and AlxGa1−xN layers, structures and bulk crystals by molecular beam epitaxy (MBE). MBE is normally regarded as an epitaxial technique for growth of very thin layers with monolayer control of their thickness. However, we have used the MBE technique for bulk crystal growth and have produced GaN layers up to 100μm in thickness. Thick, undoped, cubic GaN films were grown on semi-insulating GaAs (001) substrates by a modified plasma-assisted molecular beam epitaxy (PA-MBE) method and were removed from the GaAs substrate after the growth. The resulting free-standing GaN wafers may be used as substrates for further epitaxy of cubic GaN-based structures and devices. We have demonstrated that the PA-MBE process, we had developed, also allows us to achieve free-standing zinc-blende AlxGa1−xN wafers.

Impact of N- and Ga-face polarity on the incorporation of deep levels in n-type GaN grown by molecular beam epitaxy

Deep levels in N-face and Ga-face n-type GaN grown by plasma-assisted molecular beam epitaxy were detected, analyzed and compared using deep level optical spectroscopy (DLOS) and conventional thermal deep level transient spectroscopy (DLTS), which together enable deep level detection throughout the GaN band gap. A redistribution of band gap states was observed between the two GaN crystal growth polarities but with a similar total trap density. Most significant was a tenfold concentration increase in a trap at EC-0.25 eV that is likely related to nitrogen vacancies for the N-face polarity material, with no significant change for the Ga-vacancy-related level at EC-2.60 eV. The DLOS results suggest that carbon impurities, which generate several GaN band gap states, appear to incorporate differently for both crystal polarities with the potential carbon interstitial at EC-1.28 eV being undetected for N-face material. Finally, low concentrations of several new levels in the N-face n-GaN not previously observed in Ga-face n-GaN were observed.

Room‐temperature ferromagnetism in V‐doped GaN thin films grown by MOCVD

AbstractV‐doped GaN thin films were grown on c‐sapphire substrate by metal organic chemical vapour deposition method (MOCVD). We have used vanadium tetrachloride (VCl4) to intentionally incorporate vanadium (V) during the crystal growth of GaN. X‐ray diffraction measurements revealed no secondary phase in the samples.Magnetic experiments using superconducting quantum interference device (SQUID) showed clear hysteresis loop in magnetization versus applied field (M –H) curves for V‐doped GaN films. The ferromagnetic behavior was evidenced at 300 K, implying the Curie temperature to be over 300 K. Strong and broad blue‐luminescent band (centered at 2.6 eV) is induced by the V doping in GaN. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Growth of GaN crystals by Na flux LPE method

AbstractHigh‐quality and low‐cost bulk crystals are needed in the field of group III nitride semiconductors in order to develop optical and electrical devices. Controlling the nucleation phenomenon and solution condition during growth is important for growth of bulk GaN crystal by the Na flux method. The carbon doping into the solution and the forced induction of solution flow are effective to control them. We also present some attempts for improving the quality of seed substrate for fabrication of boule‐sized bulk GaN crystals.

Monolithic micro-fabrication of Si micro-electro-mechanical structure with GaN light emitting diode

Si micro-electro-mechanical device with GaN light emitting diode (LED) is monolithically fabricated. The GaN-LED layer was grown by molecular beam epitaxy on Si wafer and the basic properties of the LED were tested. The GaN/Si wafer was micromachined using deep reactive ion etching to fabricate Si electrostatic comb-drive actuator. A light distribution variable device with Si actuator was fabricated from the grown wafer. From those experiments, it is shown that the combination of the GaN crystal growth on Si wafer and the Si micromachining is valuable for a new kind of optical micro-systems.

Pseudohalide vapour growth of thick GaN layers

The synthesis and growth of crystalline GaN from elemental gallium and ammonia as feeding materials has been investigated. Experimental observations lead to the suggestion that the presence of carbon drastically changes the chemistry of the GaN formation. It is supposed that graphite parts react with ammonia yielding the pseudohalide hydrogen cyanide, HCN. This acts as a transporting agent for Ga, forming volatile Ga(CN) ( g) . Crystalline GaN is deposited either by the thermal decomposition of gallium monocyanide or by a reaction that is completely analogous to that of the common HVPE growth technique with the only difference that HCl is replaced by HCN. The different chemical reactions involved in this “pseudohalide vapour phase epitaxy” (PVPE) process are considered from a thermodynamic point of view. The potential of the PVPE as a disputable alternative method for the growth of bulk GaN crystals is still to be investigated.

A Freestanding GaN/HfO $_{\bf 2}$ Membrane Grown by Molecular Beam Epitaxy for GaN–Si Hybrid MEMS

Combination of GaN light source and Si-microelectromechanical systems (MEMSs) is a promising hybrid structure for optical MEMS. As one of GaN-Si hybrid structures, a freestanding GaN/HfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> membrane was fabricated on Si substrate. Unlike conventional GaN membrane on Si substrate, the fabricated membrane had a tensile stress by using the HfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> layer. Therefore, the GaN/HfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> membrane was flat enough to be useful for several MEMS. The GaN crystal was grown by molecular beam epitaxy on the HfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> layer deposited on Si substrate. The surface of the HfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> layer was nitrified before GaN crystal growth, and thus, a part of HfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> surface was changed to HfN, the lattice of which matched well to that of GaN. The characteristics of the GaN crystal grown on the nitrified HfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> layer were also investigated.

LPE Growth of Bulk GaN Crystal by Alkali-Metal Flux Method

We succeeded in growing a GaN single crystal substrate with diameter of about two inches using the Na flux method. Our success is due to the development of a new apparatus for growing large GaN single crystals. The crystal grown in this study has a low dislocation density of 2.3×105 (cm-2), The secondary ion mass spectrometry (SIMS) technique demonstrates that the Na element is difficult to be taken in the crystal in both the + and – c directions, resulting in a Na concentration of 4.2 × 1014 (cm-3) in the crystal. Our success in growing a two-inch GaN substrate with a low impurity content and low dislocation density should pave the way for the Na flux method to become a practical application.

Acidic ammonothermal growth of bulk GaN crystals

Acidic ammonothermal growth of bulk GaN crystals

Effect of carrier gas on the surface morphology of V-doped GaN layers

Vanadium-doped GaN (GaN:V) have been elaborated by metalorganic chemical vapour deposition (MOCVD). We have used vanadium tetrachloride (VCl4) to intentionally incorporate vanadium (V) during the crystal growth of GaN. The films were grown on sapphire substrate with tow procedures. A series of layers were elaborated under nitrogen (N2) and another under hydrogen (H2). For the growth of GaN:V in hydrogen atmospheric, we have used the SiN treatment consisting of an exposure of sapphire substrate to a mixture of ammonia (NH3) and silane (SiH4). In-situ laser reflectometry analysis show that the surface morphology of layers depends on VCl4 flow rate and the growth conditions. The experiments show that the quality of the grown layers (as measured with X-ray diffractometer (XRD), scanning electron microscopy (SEM) and photoluminescence (PL) increases under N2.

Solvothermal Processes for Nitride Synthesis: Examples of Li3GaN2 and Graphitic C3N4 Elaboration

When precursors decompose or react in the presence of a solvent in a closed system at a temperature higher than the solvent’s boiling point the reaction is called a solvothermal process. This reaction can be carried out either in supercritical or subcritical conditions, in homogeneous or heterogeneous systems, pressure and temperature being both key parameters. As the main interest of such processes is the enhancement of chemical reactivity, solvothermal reactions have been widely involved for nitride elaboration. We report two examples relative to solvothermal syntheses of nitrides. The first one deals with the elaboration of Li3GaN2: this ionic nitride has been successfully synthesized, structurally characterized and tested as nutrient for the ammonothermal GaN crystal growth. The second one is related to the elaboration of a well-crystallized graphitic carbon nitride (g-C3N4) aimed to be developed as a precursor for conversion towards dense CNx phases.

GaN‐Si‐MEMS structure fabricated from nano‐column GaN quantum well crystal grown on Si substrate

AbstractNanocolumn‐crystallized InGaN/GaN quantum well crystals were deposited on Si (111) substrate. Photoluminescence measurement demonstrated a wide emission wavelength from about 400 nm to 700 nm. We propose a new light source device combined with Micro‐Electro‐Mechanical Systems (MEMS). The proposed device is monolithically composed of the GaN light source structures and Si MEMS. The direction of the light beam emitted from an array of the light source can be changed by a MEMS beam steering mechanism. A micro‐stage with comb actuators was fabricated from the Si substrate. The InGaN/GaN quantum well film is patterned on the micro‐stage. Basic researches on the growth of GaN crystals on Si substrate were also carried out. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Lithographically defined carbon growth templates for ELOG of GaN

We report the initial use of lithographically defined carbon growth templates for use as an epitaxial lateral overgrowth (ELOG) mask for metalorganic chemical vapor deposition (MOCVD) heteroepitaxial GaN on sapphire. Interferometric lithography is used to define high aspect ratio structures in SU-8, which are then pyrolyzed in a reducing atmosphere up to 1200 °C. The resist structures convert to amorphous carbon, shrinking 80% in the vertical direction and 53% in the horizontal direction, but maintain their pattern geometry and adhesion to the substrate. These templates are capable of surviving GaN nucleation layer growth temperatures (∼530 °C), GaN crystal growth and high-temperature annealing up to 1050 °C. This new approach to ELOG offers several advantages, requiring fewer processing steps, and favorable selectivity tendencies as well as the capability to create growth masks which are difficult or impossible to fabricate using a top–down etching approach.

Crystal Growth of GaN by the Reaction of Ga&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt; with Li&lt;sub&gt;3&lt;/sub&gt;N in Liquid Ga

Crystal Growth of GaN by the Reaction of Ga&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt; with Li&lt;sub&gt;3&lt;/sub&gt;N in Liquid Ga

Growth of GaN crystals from chlorine-free gas phase

The aim of this research is the development of an efficient chlorine-free growth process for GaN bulk crystals. Ga evaporated from a heated melt is transported to the substrate where it reacts with reactive nitrogen generated from the decomposition of ammonia to form gallium nitride. Numerical simulation was used to optimise the set-up geometry and growth regime. It was observed that growth rate and layer quality are influenced by typical growth parameters such as temperature, total pressure and ammonia flow, as well as by source composition, crucible material and flow conditions in the growth area. Initially, mm-sized crystallites were deposited on the seed holder with growth rates well above 10 μm/h, which proved the potential of the method. Recently, the work was focused on the deposition on various single crystalline substrates. Epitaxial layers up to a bulk-like thickness of 280 μm were achieved on GaN–sapphire templates.

Physico-chemical features of the acid ammonothermal growth of GaN

Growth of GaN crystals by the ammonothermal route is both challenging and promising. We discuss the impact of the chemical environment on the nucleation of GaN through the acidic ammonothermal route at temperatures ⩽550 °C. The solubility of GaN in supercritical ammonia containing the acidic mineralizer ammonium chloride (NH 4Cl) is investigated. Low-temperature photoluminescence shows that improved supersaturation yields ammonothermal GaN with high-optical quality comparable to hydride vapor-phase epitaxy (HVPE)-grown GaN.