Tuesday, January 21, 2014

Seeded growth of GaN single crystals from solution at near atmospheric pressure

1. Introduction
The technological importance of GaN is due to its excellence in high-power, high-frequency, and optoelectronic devices, which are significant for an assortment of applications. The primary hindrance in the successful development of such a variety of III-nitride-based devices is the lack of a native high-purity, single-crystal GaN substrate with a low concentration of point and extended defects. Commercial hydride vapor phase epitaxy (HVPE) GaN substrates contain dislocation densities on the order of 105 cm−2, leading to critical limitations and problems in devices, which can be illustrated, for example, by reduced output and lifetime of III-nitride laser diodes as well as gate leakage current in RF AlGaN/GaN high electron mobility transistors (HEMTs) . The development of a true GaN boule growth technique with the potential for large-volume production will alleviate these issues.
Under nitrogen ambient at atmospheric pressure, gallium nitride is stable until only 1120 K , and it can only be stabilized at higher temperatures by drastically increasing the nitrogen pressure . The standard crystal growth techniques used for the fabrication of semiconductor wafers (Bridgman, Czochralski, etc.) cannot be developed for GaN, because at pressures below 4.5 GPa GaN does not melt but decomposes.
There are two main approaches to bulk GaN growth development—growth from gas phase and growth from liquid phase. At this point, HVPE is the only technique that is able to commercially produce GaN substrates. This technique has been used to grow wafers up to 2 in diameter at relatively high growth rates (over 100 μm/h) . Although these substrates have been used for device fabrication through homoepitaxial growth, the quality of substrates (high dislocation density and bowing) produced by this technique is a major concern for current device development. Even areas with reduced dislocations within the best HVPE samples have dislocation densities of at best 105 cm−2. In addition, the material grown by HVPE has considerable residual stress level within the substrate.
An alternative technique for single crystal growth involves deposition of GaN from the liquid phase, i.e. from solution. Growth from the liquid phase has resulted in GaN single crystals with dislocation densities less than 102 cm−2. High-pressure nitrogen solution growth (HPS) method is carried out using high pressures (greater than 1.0 GPa) and high temperatures (greater than 1400 °C) to dissolve nitrogen into Ga and has been successfully used to grow thin GaN crystal platelets of up to 1.5 cm, laterally.
GaN also has been grown at lower temperatures/pressures (5.0 MPa) by the Na flux method . Addition of Na increases the reactivity and solubility of nitrogen in the Ga and the gaseous nitrogen reacts with the flux/elemental gallium to saturate the solution and deposit crystals. An ammonothermal method has also been developed for GaN growth . This technique requires pressures in the range of 150–400 MPa and growth rates are on the order of 1–2 μm/h.
This work is a continuation of previous efforts and to develop a new technique, which meets critical requirements for the production of low defect density and low-cost GaN substrates. These requirements impose limitations on the direction of the development. Perfection of the bulk material requires crystal growth near equilibrium conditions (i.e. growth from solution). Large crystals suitable for commercial production of high-quality substrates require growth on a seed. In addition, size and cost appropriate for industrial production require growth at moderate temperature and pressure. This article describes a technical approach (Fig. 1a) to dissolve a solid GaN source in the proper solvent to create a solution at moderate pressure and temperature, and then to grow crystals on a seed from the supersaturated solution by applying a temperature gradient.

Fig. 1. Schematics of (a) the technical approach to GaN growth from solution, and (b) growth reactor.

2. Experimental procedure
Growth experiments were carried out in a custom-designed growth reactor heated with a vertical tube furnace from Mellen. The reactor allows the crucible with the charge to be loaded from the bottom and is configured for top-seeded growth (Fig. 1b). A multi-component solvent was developed to dissolve the GaN source and, subsequently, to grow GaN single crystals from the solution. Preparation of the multi-component solvent was performed in a separate reactor, similar in design to the growth reactor. The materials used for the solvent preparation have purity higher than 4 N. Handling of the solvent components, which are sensitive to moisture and oxygen, and charging the crucible, which has an inner diameter of 17 mm, were carried out in a glove box under nitrogen atmosphere with O2 and H2O content below 1 ppm. Polycrystalline GaN aggregate in the bottom of the crucible served as a source for the top-seeded runs. A layer of the solvent was placed on the top of the source for these runs (Fig. 2a). After charging in the glove box the crucible and charge were transferred from the glove box to the growth reactor. The crucible position in the reactor with respect to the heater (Fig. 1b) was chosen to induce an axial thermal gradient inside of the solution. After loading, the reactor was pumped and purged to create a pure nitrogen atmosphere. Purified house nitrogen supplied the ambient during the experiments. The gallium nitride seed was dipped and kept inside the solution at the growth temperature and pressure for 50–80 h, then it was removed and the crucible was cooled. Growth runs were conducted at nitrogen pressure of 0.23–0.25 MPa and temperature of 800 °C. The crucible contents and seeds were washed in DI water and hydrochloric acid to dissolve the remaining compounds. Optical microscopy, micro Raman scattering (μRS), X-ray diffraction (XRD) and photoluminescence (PL) spectroscopy were used to characterize the GaN crystals.

Fig. 2. Schematic view and optical images of (a) the crucible with GaN source, solvent and HVPE GaN polycrystalline aggregate as seed, (b) the seed—HVPE GaN polycrystalline aggregate before growth run, (c) the seed—HVPE GaN polycrystalline aggregate after growth run with grown GaN crystals, (d) and (e) zoomed images of the crystals shown in the (c) arrow indicates m direction.

Micro Raman measurements were performed at room temperature in the backscattering geometry, in order to characterize the structural quality of the sample. The 1064.1 nm line of a Nd:YAG laser was focused to a 2 μm diameter spot and used for excitation, while the scattered light was analyzed by a triple spectrometer TRIVISTA 557 equipped with an OMA V 1024-1.7 liquid-nitrogen-cooled InGaAs linear array detector. The FWHM spectral resolution of the system was 0.1 cm−1.
For XRD rocking curve measurements of the freestanding crystals a Blake double-crystal diffractometer equipped with a Si (1 0 0) beam conditioner and configured for the 004 diffraction with CuKα1 radiation was used. Compared to typical diffractometers equipped with 4-bounce monochromators, the double-crystal instrument provides nearly one order of magnitude higher beam intensity at the sample and a much better resolution (5 versus 12 arcsec). The beam size at sample was 80 μm wide and 200 μm high in cross-section.
Low-temperature PL (LT-PL) measurements were performed to evaluate the optical and electronic quality of the grown crystals. To maintain a temperature of 5 K during the LT-PL measurements, samples were placed in a continuous helium flow cryostat. Samples were excited with the 325 nm line from a He–Cd laser, and laser excitation intensity was maintained at 0.8 mW using neutral density filters. Light emitted by the sample was dispersed using a double-grating spectrometer fitted with 1800 groves/mm. The spectra were acquired using a UV-sensitive GaAs photomultiplier coupled to a computer-controlled photon counter.
3. Results and discussion
The critical part of the solution growth approach is the choice of solvent. The solvent should efficiently dissolve the GaN source and create a solution at moderate temperatures and pressures at which GaN is stable. That solvent has been developed to satisfy these criteria at near atmospheric pressure. As a result, GaN has been grown on a GaN seed utilizing the proper solvent at a growth temperature of 800 °C and pressures of 0.2–0.3 MPa. It should be noted that GaN was also grown at lower pressures, but this was not explored based on the developed technique, which is designed to maintain a small overpressure of nitrogen in the growth ambient.
As previously mentioned, the growth from the solution on a seed at moderate pressure and temperature is one of the key features of the described technique. Demonstration of this aspect was the main objective of this work. The first top-seeded growth runs were conducted using HVPE GaN polycrystalline aggregates (Kyma Technologies, Inc.) as seeds (Fig. 2). The shape of the polycrystalline pieces facilitates easy mounting and makes it simple to observe dipping into the solution. This type of seed also gives an opportunity to study the nucleation and GaN growth on different crystal faces and in different crystallographic directions during one growth run. Fig. 2b shows such a polycrystalline seed before the growth run. The seed is mounted with Ta wire and dipped partly into the solution as shown schematically in Fig. 2a. Due to the crucible position inside of the furnace (Fig. 1b), the temperature of the seed was slightly lower than the GaN source. After 68 h of exposure to the solution at 800 °C and 0.24 MPa, the seed was removed. After cleaning the remaining solution from the seed, grown crystals of different orientations were found on the immersed portion of the seed. Most of the crystals formed as an epitaxial expansion of the crystallites of the aggregate (Fig. 2c). Some crystals were nucleated as twins on the very edge of the crystallites and developed as freestanding crystals (Fig. 2d). Most of the crystals grew epitaxially, with the most high growth rates in the non-polar directions (Fig. 2e). All of the grown crystals were transparent and colorless, with well-defined hexagonal morphology.
Examination of the grown crystals with μRS spectroscopy in the z(xy,xy)  id="" style='orphans: auto;widows: auto;-webkit-text-stroke-width: 0px; word-spacing:-.15ex' alt="View the MathML source" xmlns:xoe="http://www.elsevier.com/xml/xoe/dtd" class=imgLazyJSB border=0 title="View the MathML source" data-inlimgeid=1-s2.0-S0022024808004545-si1.gif data-loaded=true v:shapes="_x0000_i1025"> geometry showed that the first order allowed E21, E22 and A1(LO) phonons with full-width at half-maximum (FWHM) of 0.26, 3.1 and 6.9 cm−1, respectively (Fig. 3). The sharp line widths indicate high structural quality and low impurity concentrations.

Fig. 3. Room temperature micro Raman spectrum of the GaN crystal grown on the polycrystalline GaN seed in the backscatter geometry.

High crystallinity of the grown crystals was also confirmed by XRD. Note that the intensities represented on the rocking curve are actual count rates. The sample tilt was then adjusted for optimum rocking curve breadth and peak intensity. FWHM of about 16 arcsec was obtained for the (0 0 0 4) rocking curve, excluding the additional dispersion and convolution corrections that would only enhance this number slightly (Fig. 4).

Fig. 4. XRD rocking curve of the (0 0 0 4) reflection for the GaN crystal grown on the polycrystalline GaN seed.

The position and intensity of the PL spectral peaks provide information about the type and concentration of the impurities, respectively. A dominant peak at 3.47 eV was observed in the spectra of crystals grown on polycrystalline aggregate seed (Fig. 5a) and has been attributed to excitons bound to neutral shallow donor impurities (XD0 or D0X) . High crystalline quality of the grown crystals was verified by improved XD0 line shape and linewidth, as compared with that of the seed. The nature of the dominant peak at 3.42 eV, observed on the spectrum of the seed (indicated with a question mark), is not yet determined and is under investigation. Reduction of the yellow band (YB) intensity, shown in Fig. 5b, is consistent with lower native defects and/or residual impurity concentration in the growth crystal , as compared with that of the seed.


Fig. 5. (a) LT–PL spectra of crystals grown on polycrystalline aggregate seed, (b) reduction of yellow band in the grown crystals compared to the seed.

Growth runs were performed using HVPE GaN templates as a seed to verify the prospect of the homoepitaxial growth on single-crystal GaN seeds. The configuration of these runs was similar to those conducted with polycrystalline GaN seeds. The seed was mounted with Ta wire and partly immersed into the solution when the temperature in the solution reached 800 °C. A small positive temperature difference between the seed and GaN source was induced by the chosen location of the crucible inside of the furnace as shown in Fig. 1b. An epitaxially grown GaN layer was observed on the part of the template immersed into the solution for 80 h (Fig. 6). The part of the seed with the epitaxially grown layer showed enhanced transparency due to N-face surface improvement as seen in Fig. 6a.

Fig. 6. Images of the HVPE GaN template used as a seed with epitaxially grown GaN layer: (a) nitrogen face, (b) gallium face.

LT-PL spectra covering the spectral range between 2.85 and 3.55 eV for four regions of the homoepitaxially grown layer are shown in Fig. 7a. The probed regions are the unpolished nitrogen-face, polished gallium-face, and the homoepitaxial layer grown simultaneously on both faces. Similar to the crystals grown on the polycrystalline seed, LT-PL revealed a dominant peak at 3.47 eV for both Ga- and N-faces. The intense luminescence band observed in the spectral range at 3.0–3.3 eV is associated with the recombination process involving donor–acceptor pair (DAP) with zero-phonon line (ZPL) at 3.27 eV, and phonon replicas at 3.18 eV (1LO-DAP) and at 3.09 eV (2LO-DAP). The PL spectra of the Ga-face show reduction of DAP band intensity, which suggests lower incorporation of shallow acceptors, relative to the N-face of the grown layer and the template. Fig. 7b shows the PL spectra covering the region 1.75–2.75 eV. Both faces of the layer showed a decrease in the YB intensities (2.2–2.4 eV) as compared to that of the template, suggesting a decrease in the concentration of the native defects and/or residual impurities.

Fig. 7. (a) LT–PL spectra of epitaxially grown GaN layer, (b) reduction of yellow band in the grown layer compared to the seed.

Fig. 8 displays an ω–2θ space map of the symmetric (0 0 0 4) reflection for the Ga-face (a) as-received GaN seed and (b) GaN grown crystal. The nearly 100 μm thick homoepitaxially grown layer showed a two order-of-magnitude reduction in the FWHM of the (0 0 0 4) XRD diffraction peak. FWHM of the X-ray rocking curve measured on both the Ga- and N-face of the sample are 111 and 127 arcsec, respectively, compared to 2.15° and 2.45° for the Ga- and N-face of the GaN seed, respectively.

Fig. 8. ω–2θ space map of the symmetric (0 0 0 4) reflection for the (a) as-received GaN seed and (b) Ga-face of the GaN grown crystal.

As a test for the new growth technique, runs with GaN seeds at the bottom of the crucible and GaN source on the top of the solvent were conducted (Fig. 9). In this case a temperature gradient opposite to the top-seeded arrangement should be applied. In the present system, the axial temperature gradient in the solution can be slightly varied only by changing the crucible height along the vertical heater axis (Fig. 1a), and for the bottom-seeded runs the crucible was located at the lower part of the heater. Pressure and temperature for these runs were the same as for the top-seeded runs. Only dissolution of the HVPE template at the bottom of the crucible was observed in the first runs. Images of the seed before the run (Fig. 9a) and after the run (Fig. 9c) visibly show the high degree of the seed dissolution at the bottom of the crucible. The size of the seed was found to be nearly three times smaller after this run, demonstrating the extent to which the developed solution is able to dissolve GaN. Initial bottom-seeded runs revealed that it is more difficult for this arrangement than for the top-seeded arrangement to establish the correct temperature gradient. Moving the crucible along the vertical heater axis does not allow sufficient control of the temperature gradient within the highly thermoconductive solution located inside the crucible, which is itself a similar high thermally conductive material. Minor variations in the temperature gradient using only the vertical heat distribution of the furnace cause dissolution of the HVPE template as observed for a seed at the bottom of the crucible. When the correct temperature difference between GaN source and seed was established, an epitaxial layer of the GaN was grown on the seed located at the bottom of the crucible (Fig. 9d). Runs in this configuration clearly demonstrate that the developed multi-component solvent successfully dissolves GaN and forms a suitable solution, and temperature gradients create the driving force for continuous dissolution of the GaN source and the epitaxial growth of GaN on a seed. These experiments also guided on-going modifications of the reactor heating system to enable more flexibility to create and control a required axial temperature gradient inside the solution.

Fig. 9. Optical images and schematic view of the (a) HVPE GaN template as a seed before growth run, (b) crucible with the charge for the bottom-seeded growth, (c) the same HVPE GaN template partly dissolved during growth run when T2T1, (d) GaN seed with grown GaN layer when T2<T1.

4. Conclusion
High-quality GaN single crystals have been grown for the first time from solution on a GaN seed at nitrogen pressures less than 0.25 MPa and at 800 °C. The high crystallinity and purity of the grown crystals were confirmed by μRS, PL and XRD measurements. The essential feature of this new growth technique is the ability to produce high-quality crystals even without tuning the growth parameters. These results highlight the potential capability of this method.

Source: Journal of Crystal Growth

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