High-quality GaN Layers Research Articles

Microstructural characterization of AlxGa1−xN/GaN high electron mobility transistor layers on 200 mm Si(111) substrates

Realization of high crystal quality GaN layers on silicon substrates requires a great control over epitaxy as well as detailed characterization of the buried defects and propagating dislocations within the epilayers. In this Letter, we present the microstructural characterization of AlxGa1−xN/GaN high electron mobility transistor (HEMT) structures epitaxially grown on 200 mm Si (111) substrates with superlattice (SL) buffers. The HEMT stack is also composed of an intermediate thick AlxGa1−xN layer sandwiched between the short-period and long-period AlxGa1−xN/AlN SLs. Structural and morphological characteristics of the GaN-based epilayers are studied to assess the quality of the HEMT stack grown on such 200 mm diameter substrates. The detailed microstructural uniformity of the epilayers is addressed by the transmission electron microscopy (TEM) technique. In depth scanning TEM with electron energy loss spectroscopy (EELS) investigations have been carried out to probe the microstructural quality of the HEMT stack comprising of such short- and long-period AlxGa1−xN/AlN SLs. The EELS-plasmon maps are utilized to address the sharp interface characteristics of the SL layers as well as the top active p-GaN/AlxGa1−xN/GaN layers. This work highlights the capability of high-resolution TEM as a complementing characterization method to produce reliable AlxGa1−xN/GaN HEMTs on such a large diameter silicon substrate.

Spalling induced van der Waals lift-off and transfer of 4-in. GaN epitaxial films

Epitaxial lift-off (ELO) of high-quality GaN layers allows for integration with a variety of materials enabling improved performance, reduced costs, and development of new electronics. Of the ELO technologies, two-dimensional (2D) material-based lift-off offers great promise but is still in the early stages of development and has yet to demonstrate the scale and yield of other ELO technologies. Here, we demonstrate the potential of this process's scalability, speed, and yield through epitaxial growth and lift-off of 4-in. GaN films using a 2D boron nitride (BN) van der Waals (vdW) buffer layer. Since the BN layer acts as the growth template and the mechanical release layer, both the quality and adhesion of the GaN layer are correlated with the BN morphology and uniformity. Detailed spectroscopic mapping demonstrates excellent BN uniformity, which translates into growth of high-quality GaN as shown in mapping of the x-ray rock curves (XRCs), atomic force microscopy, and photoluminescence. Scanning transmission electron microscopy and electron energy loss spectroscopy reveal abrupt chemically distinct interfaces between the sapphire, BN, and AlN/GaN layers essential for efficient lift-off. Combined with the BN/GaN vdW heterostructure, Ni spalling is used to efficiently lift-off and transfer a full 4-in. GaN layer. Post transfer characterization of a 1.9 μm thick GaN layer transferred to a SiO2/Si wafer shows a very minimal change in the XRC and photoluminescence. Strain measurements before and after transfer show that the process fully relaxes residual strain formed in the GaN during high-temperature growth. This work highlights the potential for industry scalability of an exciting 2D material-based lift-off technology, which can facilitate higher power and more efficient radio frequency devices.

InGaN/AlInN interface with enhanced holes to improve photoelectrochemical etching and GaN device release

We introduce a novel superlattice structure for releasing GaN-based devices with selective photo-electrochemical (PEC) etching by incorporating a lattice-matched AlInN barrier in an InGaN/GaN sacrificial stack. A dopant-free two-dimensional hole gas is formed at the InGaN/AlInN interface due to the band bending and strong polarization discontinuity, which is revealed in simulations. PEC etching using the four period InGaN/AlInN superlattice exhibits almost three times higher etch rate and smoother etched surfaces when compared to conventional InGaN/GaN release layers. A systematic investigation with different AlInN layer thicknesses shows that a thin AlInN layer is able to achieve smooth surface with uniform etch process during the PEC while thicker AlInN exhibits poorer surface morphology although the etch rate was faster. Furthermore, it is found that using HNO3 as the electrolyte improved the etched surface smoothness compared to KOH when followed by a post-release HCl treatment. This structure will enable the release of high quality GaN layers and the fabrication of novel optical devices.

Growth of ultrathin barrier InAlGaN/GaN heterostructures with superior properties using sputtered AlN/sapphire templates and optimized group-III injection rate by metalorganic chemical vapor phase deposition

In this paper, we investigate the influence of group-III precursor injection rate on the material and electrical properties of InAlGaN/GaN heterostructures grown by Metalorganic Chemical Vapor Phase Deposition. It is demonstrated that high-quality GaN layers can be achieved by using the sputtered AlN/sapphire templates. During the barrier layer growth, the injection rate of the Trimethylindium plays an important role in the amount of Ga incorporation into the InAlGaN layer, while the variation of Trimethylaluminum has less impact. High mobility of 1800 cm2/(V·s) and high carrier electron density with an ultrathin of 3 nm thickness InAlGaN barrier layer simultaneously maintaining high crystallinity and smooth surface of InAlGaN barrier layer is achieved with sputtered AlN/sapphire templates and optimized group-III injection rate.

Low RF loss and low dislocation density of GaN grown on high-resistivity Si substrates

The conductive channel induced by aluminum (Al) diffusion into high-resistivity Si substrates is one of the main contributors to RF loss of GaN-on-Si RF devices. However, it has been unclear how and when the Al diffuses into the substrates. Here, we study the origins of the Al impurity in the substrates. It’s revealed that trimethylaluminum (TMAl) flow rate during the pretreatment is a considerable contributor to the Al diffusion. By restricting the TMAl preflow rate, high-quality GaN layers with RF loss of 0.3 dB mm−1 at 10 GHz and dislocation density of 1.6 × 109 cm−2 have been achieved on Si substrates.

The study of the AlN/Si(111) epitaxial structures grown by PA MBE via coalescence overgrowth of AlN nanocolumns

The AlN/Si(111) epitaxial structures were synthesized by coalescence overgrowth of AlN nanocolumns using PA MBE technique. Such epitaxial structures can be used as a buffer layer for obtaining high quality AlN and GaN layers. Structural, electrical and chemical properties of these samples were studied. For the first time it was demonstrated that the etching of the obtained type of AlN/Si(111) structures in KOH can become a promising method for obtaining high quality free-standing AlN and GaN.

395 nm GaN-based near-ultraviolet light-emitting diodes on Si substrates with a high wall-plug efficiency of 52.0%@350 mA.

The high-performance 395 nm GaN-based near-ultraviolet (UV) light emitting diodes (LEDs) on Si substrates have been obtained by designing an AlN buffer layer to decrease the dislocations density of the GaN layer. By adopting a multi-layer structure with a high- and low-V/III ratio alternation, a high-quality AlN buffer layer has been obtained with a small full-width at half-maximum (FWHM) for AlN(0002) X-ray rocking curve (XRC) of 648 arcsec and a small root-mean-square roughness of 0.11 nm. By applying the optimized AlN buffer layer, the high-quality GaN layer with GaN(0002) and GaN(10-12) XRC FWHM of 260 and 270 arcsec have been obtained, and the high-performance GaN-based near-UV LED wafers and chips have been fabricated accordingly. The as-fabricated near-UV LED chips exhibit a light output power of 550 mW with a forward voltage of 3.02 V at 350 mA, corresponding to a wall-plug efficiency of 52.0%. These chips with outstanding performance are of paramount importance in the application of curing, sterilization, efficient white lighting, etc.

Influence of carrier gas H2 flow rate on quality of p-type GaN epilayer grown and annealed at lower temperatures**Project supported by the the Science Challenge Project of China (Grant No. TZ2016003), the National Natural Science Foundation of China (Grant Nos. 61674138, 61674139, 61604145, 61574135, 61574134, 61474142, and 61474110), and Beijing Municipal Science and Technology Project (Grant No. Z161100002116037).

In this work, we study the influence of carrier gas H2 flow rate on the quality of p-type GaN grown and annealed at lower temperatures. It is found that the concentration of H atoms in Mg-doped GaN epilayer can effectively decrease with appropriately reducing the carrier gas H2 flow rate, and a high-quality p-type GaN layer could be obtained at a comparatively low annealing temperature by reducing the carrier gas H2 flow rate. Meanwhile, it is found that the intensity and wavelength of DAP peak are changed as the annealing temperature varies, which shows that the thermal annealing has a remarkable effect not only on the activation of acceptors but also on the compensation donors.

(Invited) Interfacing Graphene with III-Nitrides for Power Device Applications

Full performance of GaN power devices is not attained yet due to their self-heating. Towards solving this issue Silicon Carbide is a common substrate for growth of GaN and AlGaN epitaxial layers. In spite of the superior thermal conductivity of SiC compared to e.g. Sapphire or Silicon, thermal management of GaN based devices is still a challenge. This talk will review recent research appraising the benefits of graphene integration with III-N for high power electronics. The merits of graphene as a 2D carbon made material are well demonstrated. We will present, from a material perspective, the recent progresses in the growth of high quality GaN layers on graphene templates. The latter are obtained via sublimation epitaxy on SiC and this process and the graphene quality will be also discussed in the view of template design and its role in the subsequent CaN and AlN deposition. Furthermore, the possibility and the advantage of templated growth of novel 2D Nitrides will be discussed, both from experimental and theoretical point of view of their formation mechanism. In order to gain full understanding of the new structure formation results from HRTEM and STEM characterization will be shown. Finally, strategies to use graphene for thermal management in high-power AlGaN/GaN transistors will be discussed. References A. Kovacs et al. Graphoepitaxy of highquality GaN layers on graphene/6H-SiC, Adv. Mater. Interfaces 2, 1400230 (2015).F. Giannazzo et al. Phys. Status Solidi A 214, No. 4 (2017)

Anisotropic strain relaxation and high quality AlGaN/GaN heterostructures on Si (110) substrates

We have investigated the growth and relaxation mechanisms of anisotropic lattice misfit strain in AlN and GaN layers on Si (110) substrates. A qualitative model is proposed to explain the relaxation process. It is revealed that the anisotropic misfit strain is quickly relaxed in the low temperature AlN layer by the formation of interface misfit dislocations, small misoriented grains, and lattice distortion. As a result, isotropic properties and atomically smooth surface are observed in the high temperature AlN layer. Based on this isotropic AlN layer, a high quality GaN layer and AlGaN/GaN heterostructures with a high electron mobility of 2160 cm2/(V · s) have been obtained. This work will have important impacts on the understanding of the epitaxy of isotropic semiconductor films on anisotropic substrates.

Low temperature depositions of GaN thin films by plasma-enhanced atomic layer deposition

Metalorganic chemical vapour deposition and molecular beam epitaxy have already been demonstrated to be successful techniques for obtaining high-quality epitaxial GaN layers with low impurity concentrations and pretty good electrical properties. However, high growth temperature employed in both of these methods give rise to some intrinsic defects of the thin films, such as high background-carrier concentrations. As a low-temperature thin film deposition method, plasma-enhanced atomic layer deposition (PE-ALD) has more unique advantages compared to both methods for epitaxial growth of GaN. In this paper, the polycrystalline GaN thin films were fabricated on Si (100) substrates at 150-300℃ by PE-ALD. Trimethylgallium and N2/H2 plasma gas mixture were used as the Ga and N precursors. The growth rate of the thin films was demonstrated by the spectroscopic ellipsometer. The crystal structrue and composition of the GaN thin films were characterized by X-ray diffractometer and X-ray photoelectron spectrometer (XPS). It is showed that the growth window for PE-ALD grown GaN thin films is 210-270℃, where the growth rate remains constant at 0.70 /cycle. And it is known that it is the self-limiting nature of PE-ALD that is ascribed to the plateau of the growth rate. Films grown at relatively higher temperature are polycrystalline with a hexagonal wurtzite structure, while films grown under relatively lower temperature are amorphous. The grazing incidence X-ray diffraction (GIXRD) patterns of the polycrystalline thin films reveal three main peaks located at 2=32.4, 34.6 and 36.9, which are corresponding to the (100), (002) and (101) reflections. It is showed that the Ga, N atoms would get higher energy for more effective migration to positions with lowest energy to form ordered crystalline arrange at higher growth temperature. The XPS results show that all the N elements of the as-grown thin films are in the form of NGa bond, indicating that all the N elements are formed into GaN thin films; and there is a little amount of the Ga elements that exist in GaO bond. The fact that there is no Ga2O3-related peaks in the GIXRD pattern suggests that there is small amount of amorphous Ga2O3 dispersed in the polycrystalline GaN thin films. In the future work, reducing the concentration of the C and O impurities may be achieved by increasing the time of the reaction and plasma pules in the process formula and replacing the inductively coupled plasma with the hollow cathode plasma, respectively.

Three-dimensionally-architectured GaN light emitting crystals

We demonstrate the epitaxial growth of three-dimensional (3D) GaN single crystal arrays through metal–organic vapor phase epitaxy (MOVPE) on lattice-matched ZnO templates that were achieved via hydrothermal growth in which we controlled the position, size and morphology of the layer. To prevent collapse of the crystals during MOVPE growth, graphene sheets were employed as a protection/mask layer, and initial GaN growth was performed at a relatively low temperature under hydrogen-depleted conditions. Temperature-dependent growth behaviors of GaN crystals on diverse types of ZnO templates can facilitate the control of hexagonal crystal shapes including pencil, tent and plate shapes. Furthermore, a 3D light emitting crystal array was demonstrated through subsequent growth of high-quality n-type and p-type GaN layers.

Graphene integration with nitride semiconductors for high power and high frequency electronics

Group III nitride semiconductors (III-N), including GaN, AlN, InN, and their alloys, are currently the materials of choice for many applications in optoelectronics (light-emitting diodes, laser diodes), and high-power and high-frequency transistors. Due to its attractive electrical, optical, mechanical, and thermal properties, graphene (Gr) integration with III-N technology has been considered in the last few years, in order to address some of the major issues which still limit the performances of GaN-based devices. To date, most of the studies have been focused on the use of Gr as transparent conductive electrode (TCE) to improve current spreading from top electrodes and light extraction in GaN-LEDs. This paper will review recent works evaluating the benefits of Gr integration with III-N for high power and high frequency electronics. From the materials side, recent progresses in the growth of high quality GaN layers on Gr templates and in the deposition of Gr on III-N substrates and templates will be presented. From the applications side, strategies to use Gr for thermal management in high-power AlGaN/GaN transistors will be discussed. Finally, recent proposals of implementing new ultra-high-frequency (THz) transistors, such as the Gr base hot electron transistor (GBHET), by Gr integration with III-N will be highlighted.

AlGaN/GaN High Electron Mobility Transistor Grown and Fabricated on Lattice Matched Metallic Layers

AlGaN/GaN high electron mobility transistors (HEMTs) showed its promising features in high power and high frequency applications such as inverter units in hybrid electric vehicles, advanced radar systems, and satellite-based communication networks with its high density of sheet carrier concentration, high electron mobility, and radiation hardness. In order to lower the cost of HEMTs, larger diameters substrates are needed. Currently, the HEMT structures are grown on non-native sapphire, SiC and Si substrates or on native substrate of vapor phase epitaxy (VPE) GaN wafers. HEMTs grown on sapphire substrate suffer from poor heat dissipation and higher defect density due to the relative larger lattice mismatch to GaN. SiC has a very good thermal conductivity and is less lattice mismatch to GaN, but high quality semi-insulating SiC substrates are quite expensive. Although, Si substrates are fair low cost and available for larger diameter substrates, due to larger lattice mismatch to GaN, it requires to grow thick complex AlGaN buffers prior to the growth of GaN layer to reduce the threading dislocation density. These thick AlGaN layers are very defective and thermal resistive, which will hinder device heat dissipation. VPE grown GaN wafers are limited to 2-3” diameter and very expensive. Recently, it was reported that high quality GaN layers were grown by RF plasma-assisted molecular beam epitaxy (PA-MBE) or metal organic chemical vapor deposition (MOCVD) on high-quality, single crystal ZrTi refractory metal alloys deposited by plasma sputtering on c-plane sapphire and Si substrates. These ZrTi refractory metal alloys are not only lattice match to GaN, but have a similar coefficient of thermal expansion (CTE) as the CTE of GaN [1]. By employing these ZrTi alloys, there is no need to grow thick AlGaN buffer, both photonic and electronic GaN based device structure can be grown on large area Si substrates to achieve low cost GaN based device fabrication. In this work, we have demonstrated AlGaN/GaN HEMTs grown on ZrTi refractory metal alloys, which were deposited on both Si and sapphire substrates. HEMTs with a gate dimension of 1 µm × 100 um were fabricated and characterized. Both dc and rf performance of AlGaN/GaN HEMTs fabricated on the ZrTi metal alloys will be presented. 1. “Epitaxial Growth of High Quality GaN Films on Lattice Matched Metallic Layers”, A.M. Dabiran, F. Machuca, I. De, and R. Weiss, ECS Trans. 66, 113-117 (2015).

Strain-induced step bunching in orientation-controlled GaN on Si

We report a technique for the fabrication of high-quality GaN-on-silicon (Si) substrates for use in various power applications. GaN epitaxial layers were generated on Si(111) vicinal faces that had been previously covered with a thin coating of Al2O3 to control the orientation of the AlN seed layers. We obtained orientation-controlled GaN layers and found a linear relationship between the GaN c-axis and Si[111] tilt angles. As a result, the threading dislocation density in the AlN seed layer was reduced and high-quality GaN layers were generated. The X-ray rocking curves for these layers exhibited full width at half maximum values of 390′′ and 550′′ for the (004) and (114) reflections, respectively. Significant step bunching was observed on a GaN(0001) vicinal face produced using this technique, attributed to strain-induced attractive interactions between steps. Thus, by controlling the strain near the surface layer, we achieved the step flow growth of GaN on Si.

Influence of laser repetition rate on the structural and optical properties of GaN layers grown on sapphire (0001) by laser molecular beam epitaxy

High-quality GaN layers were grown on sapphire (0001) substrates using laser molecular beam epitaxy (LMBE) by laser ablating a solid GaN target at different laser repetition rates (10–40 Hz) under a constant supply of r.f. nitrogen plasma. The effect of laser repetition rate on the structural and optical properties of GaN layers was systematically studied using high-resolution X-ray diffraction (HRXRD), field emission scanning electron microscopy, atomic force microscopy, Raman spectroscopy and photoluminescence (PL) spectroscopy. High-resolution X-ray rocking curve measurements revealed highly c-axis oriented GaN layers on sapphire grown at 30 Hz with a calculated screw dislocation density of ~1.42 × 107 cm−2, whereas the GaN layers grown at 10 or 40 Hz consisted the screw dislocation density in the range of 108–109 cm−2. Surface morphological analysis revealed a change in grain size as well as surface roughness as a function of laser repetition rate and is explained on the basis of growth kinetics. Vibrational Raman spectroscopy revealed that the GaN layer grown at 10 Hz shows an in-plane compressive stress of ~1 GPa, while the film grown at 30 Hz exhibits a minimum stress of ~0.3 GPa. The PL measurements show a highly luminescent band-to-band emission of GaN at 3.44 eV for the 10 Hz grown highly strained GaN layer and at 3.41 eV for the less strained film grown at 30 Hz along with a broad defect band emission centered around 2.28 eV. It is found that the GaN layers grown at 30 Hz have excellent structural and optical properties. We expect that the less strained thin and highly oriented GaN film grown by LMBE can further be utilized for developing prodigious low-temperature-grown nitride-based multilayer structures and devices.

Zero lattice mismatch and twin-free single crystalline ScN buffer layers for GaN growth on silicon

We report the growth of thin ScN layers deposited by plasma-assisted molecular beam epitaxy on Sc2O3/Y2O3/Si(111) substrates. Using x-ray diffraction, Raman spectroscopy, and transmission electron microscopy, we find that ScN films grown at 600 °C are single crystalline, twin-free with rock-salt crystal structure, and exhibit a direct optical band gap of 2.2 eV. A high degree of crystalline perfection and a very good lattice matching between ScN and GaN (misfit < 0.1%) makes the ScN/Sc2O3/Y2O3 buffer system a very promising template for the growth of high quality GaN layers on silicon.

GaN heterostructures with diamond and graphene

The full performance of GaN devices for high power applications is not exploited due to their self-heating. Possible solutions are the integration of materials with high heat conductivity i.e., single crystalline diamond and graphene layers. We report the growth of single crystalline (0001)-oriented GaN thin films on (100), (110) and (111) diamond single crystals studied by transmission electron microscopy (TEM) in cross-sections. As for graphene, we show a high quality GaN layer that was deposited on patterned graphene layers and 6H-SiC. The atomic structures of the interfaces in the heterostructure are studied using aberration-corrected scanning TEM combined with energy dispersive x-ray and electron energy-loss spectroscopy.

(Invited) Ion Implantation into GaN and Implanted GaN Power Transistors

GaN is a promising material for high-power, high-frequency and high-efficiency electron devices owing to their high saturation electron velocity and large break-down electric field (Eb ). The heavily doped layer with a low sheet resistance is strongly desired to reduce the power loss of GaN device and to increase the transconductance and maximum saturation current through the decrease of series resistances in devices. To reduce the series resistances in a device, a heavily doped epitaxial layer is often grown on the active layer, which would be removed by a dry etching technique to fabricate the recess structure [1]. On the other hand ion implantation is also one of very common technology to make lightly and highly selective doping area in Si [2] and SiC [3,4] devices and it is crucial for making high performance devices. We have previously demonstrated Si ion implantation for GaN HEMTs [5] and MESFETs [6] to reduce the contact resistance and fabricate a lightly-doped channel region. In addition, we have recently demonstrated normally-off GaN MISFETs with Si ion implantation into p-type GaN [7] and N ion implantation for the device isolation [8]. Si implanted MISFETs have achieved the high threshold voltage (Vth ) of +3.4 V [9]. Moreover, we have succeeded to fabricate selective p-GaN regions with Mg ion implantation and this result made it possible to control the Vth of GaN MISFETs from -5 V to +1.5 V. These results indicate a definite availability for power switching device applications. As known well, it is very difficult to form a p-type conductive layer with ion implantation on GaN. The GaN-based devices are usually fabricated on either a sapphire substrate or a Si substrate with a large diameter and at low cost. However these substrates cause many defects in GaN due to large lattice mismatch between epitaxial layers and the substrate. Such defects of high threading dislocation density over 109 /cm2 cause substantial current leakage and such layers would be deteriorated by the high temperature annealing for activation of the implanted Mg and create donor-like defects which compensates the Mg acceptors. P-type conversion by Mg ion implantation was attempted using high quality n--GaN epitaxial layer grown on a free-standing GaN substrate. Low temperature PL spectra showed Mg-acceptor related emissions. Vertical diodes showed clear rectifying I-V characteristics. UV and blue-green light emission was observed at forward biased condition. Positive Hall coefficients were observed by van der Pauw measurements. These results indicate the p-type conversion was possible using high quality GaN layers grown on free-standing GaN substrates, which can lead new device structures and processing for future GaN power devices. [1] B.R. Park, et al., Semisonductor Science and technology, Vol. 28, No. 12, (2013) [2] E. Suekawa, et al., Proceeding of ISPSD & ICs, 11.12, 249 – 252 (1998) [3] S.H. Ryu, et al., IEEE Electron Device Letters, Vol. 22, No. 3, 124 – 126, March (2001) [4] K.W. Chu, et al., IEEE Electron Device Letters, Vol. 34, No. 2, 286 – 288, February (2013) [5] K.Nomoto, T. Nakamura, et al., IEEE Electron Device Letters, Vol. 28, No. 11, 939 – 941, November (2007) [6] K.Nomoto, T. Nakamura, et al., Proceeding of MRS Fall Meeting, Vol. 892 (2005) [7] S. Gu, K. Nomoto, T. Nakamura, et al., physica status solidi (c), Vol. 10, Issue. 5, 820 – 823, May (2013) [8] H. Kasai, T. Nakamura, et al., physica status solidi (c), Vol. 11, Issue. 3-4, 914 – 917, April (2014) [9] S. Taguchi, K. Nomoto, T. Nakamura, et al., physica status solidi (c), Vol. 9, Issue. 3-4, 858 – 860, March (2012)

Homoepitaxial growth of a-plane GaN layers by reaction between Ga2O vapor and NH3 gas

Growth of high-quality a-plane GaN layers was performed by reaction between Ga2O vapor and NH3 gas at a high temperature. Smooth a-plane GaN epitaxial layers were obtained on a-plane GaN seed substrates sliced from thick c-plane GaN crystals. Growth rate increased with increasing Ga2O partial pressure. An a-plane GaN layer with a growth rate of 48 µm/h was obtained. The X-ray rocking curve (XRC) measurement showed that the full widths at half maximum (FWHMs) of GaN with the incident beam parallel and perpendicular to the [0001] direction were 29–43 and 29–42 arcsec, respectively. Secondary ion mass spectrometry (SIMS) measurement revealed that oxygen concentration decreased at a high temperature. These results suggest that growth of a-GaN layers using Ga2O vapor and NH3 gas at a high temperature enables the generation of high-quality crystals.