Ammonothermal Method Research Articles

High-temperature acidic ammonothermal method for GaN crystal growth

We developed high-temperature autoclaves which are capable of growing GaN crystals at temperature <850°C and pressure <150MPa. A high-temperature acidic ammonothermal method with acidic mineralizer revealed retrograde solubility of GaN, and yielded GaN crystals with XRC-FWHM values of around 30″.

Structural and optical investigation of non-polar (1-100) GaN grown by the ammonothermal method

We studied the structural and optical properties of state-of-the-art non-polar bulk GaN grown by the ammonothermal method. The investigated samples have an extremely low dislocation density (DD) of less than 5 × 104 cm−2, which results in very narrow high-resolution x-ray rocking curves. The a and c lattice parameters of these stress-free GaN samples were precisely determined by using an x-ray diffraction technique based on the modified Bond method. The obtained values are compared to the lattice parameters of free-standing GaN from different methods and sources. The observed differences are discussed in terms of free-electron concentrations, point defects, and DD. Micro Raman spectroscopy revealed a very narrow phonon linewidth and negligible built-in strain in accordance with the high-resolution x-ray diffraction data. The optical transitions were investigated by cathodoluminescence measurements. The analysis of the experimental data clearly demonstrates the excellent crystalline perfection of ammonothermal GaN material and its potential for fabrication of non-polar substrates for homoepitaxial growth of GaN based device structures.

InGaN laser diodes with reduced AlGaN cladding thickness fabricated on GaN plasmonic substrate

We demonstrate InGaN laser diodes with substantially reduced thickness of their bottom AlGaN cladding grown on plasmonic GaN substrate. The electron concentration in plasmonic substrate grown by ammonothermal method was of the order of 1020 cm−3, which corresponds to the refractive index reduction by 0.7% when comparing to undoped GaN. We were able to reduce the thickness of AlGaN bottom claddings from the initial 800 nm down to 400 nm without any worsening of laser threshold current or near-field patterns. The results are in agreement with the optical modeling of the InGaN laser diode structure fabricated on plasmonic GaN substrate.

Surface morphology study of basic ammonothermal GaN grown on non-polar GaN seed crystals of varying surface orientations from m-plane to a-plane

GaN crystals were grown on hydride vapor phase epitaxy (HVPE) GaN seed crystals with non-polar surface orientations varying between on-axis m-plane and a-plane using the basic ammonothermal method. Three different surface morphology regimes were observed with the surface features including mounds, slate-like morphologies, and pyramidal ‘spikes’ composed of (0001) and non-polar microfacets. A macroscopic off-orientation of the non-steady-state surface and newly appearing steady-state growth surfaces towards [0001¯] by approximately 1° was observed with geometric constraints suggesting an off-orientation of the observed {10–10} microfacets towards [0001¯] by approximately 1° or greater.

Cathodoluminescence Study of Ammonothermal GaN Crystals

GaN crystals grown in supercritical ammonia by the ammonothermal method were studied by cathodoluminescence (CL), both in image and spectrally resolved modes. The main extended defects and the incorporation of point defects and impurities in different growth sectors were revealed. The influence of the seeds, the role of the growth planes and the changes in the crystal quality during the growth run are discussed.

Observation of superconductivity at 30∼46K in AxFe2Se2(A = Li, Na, Ba, Sr, Ca, Yb and Eu)

New iron selenide superconductors by intercalating smaller-sized alkali metals (Li, Na) and alkaline earths using high-temperature routes have been pursued ever since the discovery of superconductivity at about 30 K in KFe2Se2, but all have failed so far. Here we demonstrate that a series of superconductors with enhanced Tc = 30∼46 K can be obtained by intercalating metals, Li, Na, Ba, Sr, Ca, Yb, and Eu in between FeSe layers by the ammonothermal method at room temperature. Analysis on their powder X-ray diffraction patterns reveals that all the main phases can be indexed based on body-centered tetragonal lattices with a∼3.755–3.831 Å while c∼15.99–20.54 Å. Resistivities show the corresponding sharp transitions at 45 K and 39 K for NaFe2Se2 and Ba0.8Fe2Se2, respectively, confirming their bulk superconductivity. These findings provide a new starting point for studying the properties of these superconductors and an effective synthetic route for the exploration of new superconductors as well.

Enhanced growth rate for ammonothermal gallium nitride crystal growth using ammonium iodide mineralizer

High-temperature gallium nitride (GaN) crystal growth using the acidic ammonothermal method with ammonium iodide (NH4I) as a mineralizer was investigated. The growth rate reached 105μm/day, which was much higher than that previously achieved using acidic ammonothermal methods, and exceeded 100μm/day, which is the minimum growth rate required for industrial applications. When NH4I was used as a mineralizer, high-speed crystal growth was achieved at a relatively low pressure compared to the case of using an ammonium chloride (NH4Cl) as a mineralizer.

Femtosecond-laser-driven photoelectron-gun for time-resolved cathodoluminescence measurement of GaN

A rear-excitation femtosecond-laser-driven photoelectron gun (PE-gun) is developed for measuring time-resolved cathodoluminescence (TRCL) spectrum of wide bandgap materials and structures such as semiconductors and phosphors. The maximum quantum efficiency of a 20-nm-thick Au photocathode excited using a frequency-tripled Al(2)O(3):Ti laser under a rear-excitation configuration is 3.6×10(-6), which is a reasonable value for a PE-gun. When the distance between the front edge of the PE-gun and the observation point is 10 mm, the narrowest electron-beam (e-beam) diameter is 19 μm, which corresponds to one tenth of the laser-beam diameter and is comparable to the initial e-beam diameter of a typical W hair-pin filament of thermionic electron-gun. From the results of TRCL measurements on the freestanding GaN grown by the ammonothermal method and a GaN homoepitaxial film grown by metalorganic vapor phase epitaxy, overall response time for the present TRCL system is estimated to be 8 ps. The value is the same as that of time-resolved photoluminescence measurement using the same excitation laser pulses, meaning that the time-resolution is simply limited by the streak-camera, not by the PE-gun performance. The result of numerical simulation on the temporal e-beam broadening caused by the space-charge-effect suggests that the present PE-gun can be used as a pulsed e-beam source for spatio-time-resolved cathodoluminescence, when equipped in a scanning electron microscope.

Improving the purity of GaN grown by the ammonothermal method with in-autoclave gas-phase acidic mineralizer synthesis

In-autoclave synthesis of a gas-phase acidic mineralizer was investigated for high-purity GaN growth by the ammonothermal (AT) method. To reduce oxygen contamination of GaN from highly hygroscopic NH4Cl powder, purified NH3 and HCl gases were introduced sequentially fed into a Pt-lined autoclave to synthesize NH4Cl within the autoclave. The autoclave was pre-charged with GaN seed wafers and polycrystalline precursors, and carefully dehydrated under dynamic vacuum. Because of the decrease in oxygen concentration, the lattice parameter approached the intrinsic value. The Ga-polar layers exhibit a near-band-edge emission peak at room temperature.

Non-polar and semi-polar ammonothermal GaN substrates

In this paper we review the developments of producing non-polar (i.e. m-plane and a-plane) and semi-polar (i.e. (20.1)-plane) wafers by ammonothermal method. The growth method and polishing results are described. We succeeded in producing 26 mm × 26 mm non- and semi-polar wafers. These wafers possess outstanding structural and optical properties, with threading dislocation density of the order of 104 cm−3. Detailed studies of homoepitaxial layers, as well as AlGaN heterostructures are also presented, showing the potential of studied ammonothermal substrates in the fabrication of optoelectronic devices.

Development of GaN wafers for solid-state lighting via the ammonothermal method

In order for solid-state lighting to replace existing light bulbs, high power LEDs will be required to handle more current than conventional LEDs. When current densities in high power LEDs become comparable to that of Laser Diodes (LDs), GaN substrates will be critically useful for device reliability. Due to its significant scalability, the ammonothermal growth of bulk GaN could provide cost competitive wafers for high power LEDs.Our team has focused on developing the basic ammonothermal growth method in small 1” internal diameter prototype autoclaves capable of accommodating multiple crystals simultaneously. We have made considerable improvements in the crystal quality particularly in transparency and structural parameters. By optimizing the growth process we have improved the coloration and transparency of the crystals from a black/brown to semi-transparent yellow. We have improved the absorption coefficient at 450nm from 30.5cm−1 to 8cm−1 yielding semi-transparent crystals. Currently, we can reliably achieve a full width half maximum (FWHM) of X-ray 002 reflection between 100 and 300arcs. The crystals have a low dislocation density less than 10−6cm−2 and are n-type with a resistivity of approximately ρ∼10−2Ωcm. In this presentation we will discuss improvements that we have made to provide a more suitable substrate for future high power LEDs.

Transparency of GaN substrates in the mid‐infrared spectral range

AbstractTransparency of truly bulk GaN substrates obtained by ammonothermal method was measured in the mid infrared spectral range by Fourier spectroscopy. The same measurements were performed for GaN templates grown by meatlorganic vapour phase epitaxy on sapphire and SiC substrates. It has been clearly observed that truly bulk GaN substrates are transparent up to ∼7 µm whereas GaN templates grown on sapphire and SiC are transparent only up to ∼6.5 and ∼5.5 µm, respectively, due to non‐transparency of sapphire and SiC in this spectral range. It has been shown that the transparency “cut off” at ∼7 µm for GaN crystals results from the absorption of light by the second harmonic of optical phonons, which is very significant due to the strong electron‐phonon coupling in this material. Also it has been clearly presented that the absorption “cut‐off” in the infrared spectral region can be easily tuned by increasing of free carrier concentration in GaN. It was observed that the infrared transparency can be shifted from ∼7 to ∼2 µm or even shorter wavelengths when the electron concentration is increased up to ∼1019 cm‐3. (© 2012 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

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.

Ammonothermal GaN substrates: Growth accomplishments and applications

AbstractWe are presenting some physical and chemical basis of ammonothermal method of bulk gallium nitride (GaN) synthesis in ammonobasic route. The substrates of polar, non‐polar, and semi‐polar orientation can be obtained by this method. Excellent structural and wide spectrum of electrical parameters of truly bulk GaN crystals obtained this way are revealed. Large progress in manufacturing of large size (1‐in.) non‐polar ammonothermal crystals is reported. A preliminary results on performance of the devices grown on ammonothermal GaN substrates will be shown.

Structural and optical properties of GaN crystals grown by the ammonothermal technique

AbstractHigh quality GaN crystals grown by the ammonothermal method are reported. The grown crystals possess excellent structural and optical properties. As confirmed by X‐ray diffractions, the crystals have very low overall Bragg peak broadening and particularly low twist variance. X‐ray rocking curves indicate the ammonothermal growth is replicating or improving from the seed crystal, with as low as 65 arc‐sec FWHM on (002) when using a 1×3 mm2 unslitted X‐ray spot size. Room temperature PL indicates the material has almost no yellow‐band emission although the yellow emission peak increased after annealing of the sample at 900 °C. The availability of yellow‐band free ammonothermal wafers offers a unique opportunity to study the origin of the yellow‐band emission in GaN (© 2011 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Preparation and selective etch of ammonothermal GaN wafers for defect evaluation

AbstractGrowth of bulk gallium nitride by the ammonothermal method results in thick, faceted specimens presenting polar and non‐polar surfaces. In order to investigate defect density, samples need to be cut, polished, and etched. The polishing method must remove surface damage or etch tests will be inaccurate. We examined the efficacy of different polishing and etching methods on the c, a, and m planes. Polishing methods included fixed‐grit and chemo‐mechanical solutions; results were measured by atomic force microscopy (AFM). Three etchants were tried: 85% hot phosphoric acid, hot KOH‐NaOH eutectic, and photo‐enhanced chemical (PEC) etching. The eutectic etch resulted in repeatable counts on the gallium face. The non‐polar planes did not reveal etch pits (© 2011 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

InGaN mini‐laser diode arrays with cw output power of 500 mW

AbstractOne of the required features of nitride‐based laser diodes is the possibility to obtain high output powers from devices, which is essential in e.g. RGB displays, lithography and high speed printing. This can be realized by means of multi emitter structures. We report on fabrication of a mini laser array, integrated on one laser chip and consisting of three emitters of the dimensions of 700 μm x 3 μm. The emitters structures was fabricated by MOVPE on top of the low defect density bulk GaN crystal obtained by ammonothermal method. We tested devices characterized by various pitches from 12 to 80 μm obtaining very similar results. Contrary to our previous study showing strong inter‐emitter coupling we obtained independent emission from each stripe. This fact is a result of better vertical mode confinement and the suppression of substrate mode leakage. These latter effects were the reason for emitters coupling, as we reported previously. The new devices produced up to 500 mW of optical power, this value was limited by the thermal resistance of the whole package. (© 2011 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Contactless electroreflectance of AlGaN/GaN heterostructures deposited on c-, a-, m-, and (20.1)-plane GaN bulk substrates grown by ammonothermal method

Room temperature contactless electroreflectance (CER) has been applied to study optical transitions and the distribution of the built-in electric field in AlGaN/GaN heterostructures grown on c-, a-, m-, and (20.1)-plane GaN substrates obtained by the ammonothermal method. It has been clearly shown that polarization effects in the AlGaN/GaN heterostructures grown on the c-plane lead to a strong built-in electric field in the AlGaN layer. The aforementioned field was determined to be ∼0.43 MV/cm from the period of Franz–Keldysh oscillations (FKOs). In addition, polarization effects lead to the formation of a two dimensional electron gas at the AlGaN/GaN interface, which screens the band bending modulation in the GaN buffer layer, and, therefore, GaN-related excitonic transitions are not observed for this heterostructure. Such features/effects are also not observed in the AlGaN/GaN heterostructures grown on nonpolar and semipolar GaN substrates because any strong polarization effects are not expected in this case. For these heterostructures, very strong and sharp GaN excitonic resonances are clearly visible in CER spectra. The resonances are very similar to the excitonic transitions observed for the GaN epilayers deposited on nonpolar and semipolar substrates. Moreover, there is a very weak AlGaN-resonance without FKO for nonpolar and semipolar heterostructures instead of the strong AlGaN-related FKO, which is typical of polar AlGaN/GaN heterostructures.

Improvement of structural quality in the initial stage of GaN growth by basic ammonothermal method

Study on initial growth of GaN revealed evolutional improvement of grain structure in the initial stage of ammonothermal growth. GaN crystals grown on N-polar surface of GaN platelets for various growth durations were characterized with the X-ray diffraction and Nomarski microscopy. The full width half maximum (FWHM) of X-ray rocking curve from 002 reflection became larger than that of a seed crystal at approximately 100 μm, followed by improvement in thickness larger than approximately 200 μm. On the other hand, FWHM of X-ray rocking curve from 201 reflection did not change. The Nomarski microscope showed larger hillocks for thicker layer indicating coalescence of grains. These results imply that GaN crystal quality in the initial stage was improved through reduction of c-axis tilting associated with coalescence of grains in the grown layer.

Application of contactless electroreflectance to study the epi readiness of m-plane GaN substrates obtained by ammonothermal method

The authors have applied contactless electroreflectance (CER) spectroscopy to study the epi readiness of m-plane GaN substrates obtained by the ammonothermal method. It has been clearly observed that the CER resonance, which is related to the energy gap transition, appears for samples with a well-polished surface. The sharpness of this resonance is directly related to the surface quality. The broadening of energy gap transition can be used as a parameter to quantify the surface quality. For samples polished with optimal conditions, this broadening (γpol) is close to the broadening observed for the cleaved GaN surface (γclev) with m-plane orientation (150–190 vs 135 meV). The quality of the polishing process can be evaluated by analyzing the γclev/γpol ratio, where γclev/γpol=1 corresponds to an excellent polishing process. In the authors’ case, this ratio has been determined to be close to 1 for well-polished samples.