Journal of Crystal Growth 312 (2010) 2499–2502

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Journal of Crystal Growth

0022-02

doi:10.1

n Corr

E-m

journal homepage: www.elsevier.com/locate/jcrysgro

Recent achievements in AMMONO-bulk method

R. Dwilinski a,n, R. Doradzinski a, J. Garczynski a, L. Sierzputowski a, R. Kucharski a, M. Zajac a,M. Rudzinski b, R. Kudrawiec c, J. Serafinczuk d, W. Strupinski b

a AMMONO sp. z.o.o., Czerwonego Krzyza 2/31, 00-377 Warsaw, Polandb Institute of Electronic Materials Technology, Wolczynska 133, 01-919 Warsaw, Polandc Institute of Physics, Wroclaw University of Technology, Wybrzeze Wyspianskiego 27, 50-370 Wroc!aw, Polandd Faculty of Microsystem Electronics and Photonics, Wroclaw University of Technology, Janiszewskiego 11/17, 50-372 Wroc!aw, Poland

a r t i c l e i n f o

Available online 9 April 2010

Keywords:

A2. Growth from solutions

A2. Metalorganic vapor phase epitaxy

B1. Nitrides

B2. Semiconducting III-V materials

48/$ - see front matter & 2010 Elsevier B.V. A

016/j.jcrysgro.2010.04.001

esponding author. Tel.: +48 601 387 901; fax

ail address: dwilinski@ammono.com (R. Dwil

a b s t r a c t

In this paper we present progress made recently in the development of the growth of truly bulk GaN

crystals by the ammonothermal method in basic environment. High quality 2-in c-plane GaN seeds are

shown. Non-polar wafers can also be cut out from thick GaN crystals grown by ammonothermal

method. Perfect crystallinity manifests in very narrow peaks in X-ray rocking curves (the full width at

half maximum equals about 15 arcsec). GaN epilayers deposited on these substrates exhibit intrinsic

narrow exciton lines, which are very sensitive to the optical selection rules typical for hexagonal

symmetry, proving the truly non-polar character of such AMMONO-GaN substrates. Other challenges

like homogenous insulating properties or high p-type conductivity have been also accomplished by

means of ammonothermal method. Semi-insulating crystals of resistivity up to 1011 O cm and p-type

conductivity within hole concentration up to 1018 cm�3 are already available in diameters up to 1.5-in.

& 2010 Elsevier B.V. All rights reserved.

1. Introduction

Gallium nitride (GaN) has attracted great attention for itsmaterial properties that are useful for applications in short-wavelength optoelectronic and high power electronic devices [1]such as white or colour light emitting diodes, blue laser diodes(LD), UV detectors and high power-high frequency transistors.However, the development of low cost aforementioned devices ofsufficiently high efficiency is limited due to a lack of suitablesubstrates for growing homoepitaxial structures. Under suchcircumstances, the use of truly bulk GaN substrates would be anideal solution for this problem.

So far, satisfactory results were obtained by growth of a thicklayer by different vapor phase epitaxy techniques, for examplehydride vapor phase epitaxy (HVPE) [2]. Substrates produced thisway enabled a breakthrough in real commercialization of opticalstorage media based on blue lasers. However, the resultingstructures still suffer from large dislocation density generatedby the use of non-native seeds. Even after seed separation, suchfree-standing GaN is still highly stressed and bowed. Apart fromHVPE, large progress has been done in development of othermethods of bulk GaN production. For example, many advantages(also related to quality of the samples) in comparison with HVPEwere found in the Na flux method [3] and the high pressure

ll rights reserved.

: +48 22 814 02 07.

inski).

method [4]. However, there are still many problems waiting to besolved, such as poor growth of seeds, heterogeneity, mosaicity,poor scalability or extreme growth conditions. Therefore, weproposed the ammonothermal method in order to overcome thesedifficulties [5–8]. In this paper we describe our recent develop-ments in manufacturing AMMONO-bulk GaN substrates.

2. Ammonothermal method—general principles and its stateof the art

The ammonothermal method focuses great interest of bothscientific and industrial community due to its merits like growthin equilibrium conditions, growth in a closed system andscalability. The scheme of the crystal growth process in theammonothermal method is as follows: GaN containing feedstockis dissolved in one zone of the high pressure autoclave, thentransported by convection as a result of the temperature gradientto the second zone, where GaN is crystallized on native seeds dueto the supersaturation of solution. The typical temperatures andpressures applied are 0.2–0.5 GPa and 500–600 1C, respectively. Inaddition, the use of the mineralizers is necessary in order toenhance the solubility of GaN in ammonia. AMMONO companydevelops the ammonobasic version of the growth method; alkalimetals or their amides (LiNH2, NaNH2 or KNH2) are used asmineralizers. Although the ammonothermal growth rate can befew dozen times smaller compared with HVPE growth, its perfect

Fig. 2. The thick 1-in AMMONO-GaN crystal (a) and 11 mm�22 mm non-polar

(m-plane) substrate cut out of this crystal (b).

R. Dwilinski et al. / Journal of Crystal Growth 312 (2010) 2499–25022500

scalability (possibility of growing hundreds of crystals in one runsimultaneously) makes the method even more suitable for massproduction. More details about the ammonothermal method aredescribed in our last publications [5] and patents [6].

Recently, large progress has been made in the development ofthe truly bulk GaN growth by the ammonothermal method. Thequality of crystals obtained in this way exceeds in many aspectsthe quality of those grown by the HVPE method. They are strainfree, their crystal lattice is extremely flat (lattice curvatureexceeds 100 m), and their dislocation density is two orders ofmagnitude lower (about 103 cm�2) [5]. The ammonothermallygrown GaN (A-GaN) crystals may then cause another break-through in very high efficiency and low cost LD production.Crystallization conditions do not impose any crystal size limita-tions, besides the size of autoclave, proving a perfect scalability ofthe method. The possibility of using A-GaN crystals for seededgrowth was also confirmed. The loss of quality with increasingthickness was not observed at all (up to any thickness limited onlyby time of the process), enabling the successful production ofnon-polar and semi-polar A-GaN substrates. Moreover, thestructural quality improves, for example when the growth isinitiated on HVPE seed. A wide range of electrical properties wasalso achieved [7]. Tuning the electrical properties will be one ofthe subjects of this paper.

The ammonothermal method enabled growth of c-planeoriented (polar) 1- and 1.5-in substrates [8] that are currentlyon the market. Fig. 1 shows one of our first 2-inch seedmonocrystals, from which 2-in substrates can be sliced. Veryrecently, AMMONO sp. z.o.o. began to increase the populationof such c-plane seeds (Fig. 1) used for manufacturing of 2-inwafers. This may facilitate much more effective production ofoptoelectronic devices grown on truly bulk A-GaN substrate.

However, non-polar, m-plane oriented GaN substrates are evenmore promising and interesting for photonics (green diodes andlasers) [9] due to lack of internal electric fields. Such substrateshave been obtained by the hydride vapor phase epitaxy (HVPE)method [10,11] by slicing the c-direction grown boule along theproper direction, yielding to about one-centimeter square plates.A similar approach was applied to A-GaN crystals [12]. In this casethe larger size of non-polar or semi-polar GaN substrates can beobtained from boule of much smaller diameter, when comparedto 2-in HVPE GaN. Fig. 2a shows the photograph of the thick1-in GaN crystals, from which a 11 mm�22 mm non-polarsubstrate can be made (Fig. 2b). Due to the progress made byour group, this is a bit larger sized than the one publishedvery recently (8 mm�17 mm) [12]. Moreover, production ofsemi-polar, for example (2 0 2 1) plane oriented, wafers is alsoachievable.

The wafers (both polar and non-polar) must be properlypolished by mechanical and chemico-mechanical polishing (CMP)

Fig. 1. The 2-in AMMONO-GaN seed crystal. The colour contrast is due to different

reflections of light from the surrounding.

in order to obtain epi-ready surface for epitaxy. Such preparedA-GaN substrates were applied for homoepitaxy by metalo-organic vapor phase epitaxy and epilayers of excellent propertieswere obtained in this way. The results of optical investigations ofthese epilayers will be described in Section 4.

3. Electrical properties.

Besides n-type conductivity, challenges like homogenousinsulating properties or high p-type conductivity have been alsoaccomplished by means of ammonothermal method. The carrierconcentration of the wafers can be controlled by appropriatedoping [7]. n-type (electron concentration n�1018–1019 cm�3,resistivity r¼10�3–10�2 O cm), p-type and semi-insulating sub-strates can be grown via the ammonothermal method, asmeasured by Hall effect experiments and capacitance–voltagemeasurements. Semi-insulating crystals of resistivity up to 1011 Ocm and p-type conductivity within hole concentration p up to1018 cm�3 (p�1018 cm�3, r¼101–102 O cm) are already available

Fig. 3. CER measurement result for n-type (a), semi-insulating (b) and p-type

(c) AMMONO-GaN c-plane substrate.

R. Dwilinski et al. / Journal of Crystal Growth 312 (2010) 2499–2502 2501

in diameters up to 1.5-in. The conductivity was confirmed by thecapacitance–voltage measurements. In addition, the type ofsurface band bending was determined by contactless electrore-flectance (CER) spectroscopy [13].

Fig. 3 shows CER spectra for n-type (a), SI (b) and p-type(c) A-GaN substrate. For n-type band bending (n-type GaN) CERresonance starts from positive signal and then is negative, seeFig. 3a. For semi-insulating (SI) GaN crystal the Fermi level insidethe crystal is located far from the conduction and valence bands,but at the surface the Fermi level is located at the same energy asfor n-type GaN, i.e., about 0.5 eV below the conduction band, sinceit is a feature of semiconductor surface, which does not changewith doping. It means that the surface band bending for SI GaNflattens and can be even p-type. The shape of CER resonance for SIGaN sample confirms this scenario, i.e., the signal is negative atthe beginning, then positive and again negative (Fig. 3b). Thep-type band bending is evidently observed for p-type GaNsubstrates, see Fig. 3c. In this case CER signal is negative at thebeginning and then positive. From these measurements it hasbeen concluded that the surface band bending changes fromn-type samples to p-type samples, as monitored by change of thesign of the CER signal (Fig. 3). Possibility of tuning the electricalproperties suggests that truly bulk AMMONO-GaN crystalsmay find application in both optoelectronics (highly conductivesubstrates) and high power electronics (semi-insulatingsubstrates for High Electron Mobility Transistors production).

4. Non-polar GaN substrates.

The non-polar (m-plane) substrates have been obtainedby the method described in Section 2. Such substrates are

Fig. 4. X-ray rocking curve measured for symmetrical (1 0 �1 0) and asymme-

trical (2 0 �2 1) plane of non-polar (m-plane) substrate.

characterized by extremely narrow rocking curves for bothsymmetrical (1 0 –1 0) and asymmetrical (2 0 –1 0) planes(FWHM¼15 and 19 arcsec, respectively, see Fig. 4), very smalldislocation density (of the order of 104 cm�3) and very goodoptical properties [12].

Very good crystallinity of the substrate resulted also in highquality, stress free homoepitaxial layers. Here, we presentreflectivity and photoluminescence (PL) spectra of the GaN layersgrown on A-GaN non-polar wafers (Fig. 5). In addition, forcomparison, similar data for c-plane epilayers are shown(Fig. 6). The PL measurements were performed at temperatureT¼10 K in a standard configuration using a SI multichanneldetector, 0.55 m length monochromator and YAG laser with anaverage output power of �2 mW (0.1 mJ in one 10 ns pulse). Theexcitation beam of wavelength 266 nm was defocused to adiameter of �2 mm. The same experimental set-up was used tomeasure reflectance spectra. The sample was illuminated bywhite light from a halogen lamp (100 W) at near normal incidencewith various light polarizations to the c- and a-axis.

The studied non-polar films exhibit narrow exciton lines(corresponding to intrinsic free exciton transitions FXA, FXB andFXC), which are very sensitive to the optical selection rules typicalfor hexagonal symmetry (Fig. 5a). For unpolarized light and whenthe electric vector is perpendicular to the c-axis, three wellresolved free exciton A, B and C transitions (FXA, FXB and FXC,respectively) are observed in reflectance spectrum at energies ofEFXA¼3.4776, EFXB¼3.4827 and EFXC¼3.5015 eV, which aretypical for homoepitaxial epilayers [13,14]. But when the electricvector is parallel to the c-axis the FXA transition is not observed inreflectance spectrum. This observation means that our crystals

Fig. 5. Reflectance (a) and photoluminescence (b) of MOVPE epilayer grown on

non-polar AMMONO-GaN substrate. The inset shows the PL spectra measured in a

wide energy range.

Fig. 6. Reflectance (a) and photoluminescence (b) of MOVPE epilayer grown on

polar c-plane AMMONO-GaN substrate. The inset shows the PL spectra measured

in a wide energy range.

R. Dwilinski et al. / Journal of Crystal Growth 312 (2010) 2499–25022502

have an ideal hexagonal symmetry without any local structuralimperfections, which could destroy this symmetry and change theoptical selection rules. This feature proves the truly non-polarcharacter of the studied material and distinguishes it from polarepilayers. As shown in Fig. 6a, reflectance spectra are not sensitiveto the light polarization in case of GaN epilayers grown on c-planesubstrate (contrary to non-polar films).

It should be mentioned here that some stacking faults (SFs)appear very often for GaN epilayers. Their presence is manifestedin PL spectra by an emission band at 3.42 eV [15]. It has beenfound that no emission band is observed around 3.42 eV in PLspectra (Fig. 5b), which means that the number of SFs is negligiblysmall in our epilayers. Second, it is observed that the energygap-related PL band is composed of a strong bound exciton line(transition labeled as I) and relatively strong FXA and FXB lines. Thepresence of free exciton recombination in low temperature PL spectraproves the high optical quality of GaN epilayers. Moreover, as insets ofFigs. 5 and 6 show, the excitonic recombination at �3.48 eVdominates the PL spectra, being much stronger than the donor–acceptor-pair transition (DAP) and parasitic ‘yellow’ emission band(2.2 eV), which is typical of carrier recombination via native pointdefects and impurities in GaN.

5. Conclusions

The excellent structural properties of crystals, combinedwith advantages of the method itself (seed-multiplication,repeatability and scalability), allow us to claim that AMMONO-Bulk Method is an extremely promising technology for fabricatinghigh-quality, large diameter GaN substrates. Confirmed highscalability of the method (2-in seeds are available) will play acrucial role in further improving its cost-effectiveness whenimplemented in the industrial scale. The wide spectrum ofelectrical properties suggests that truly bulk AMMONO-GaNcrystals may find application in both optoelectronics (highlyconductive substrates) and high power electronics (semi-insulat-ing substrates for High Electron Mobility Transistors production).

Moreover, as no deterioration of the quality within crystalthickness was observed, the growth of sufficient sized non-polarm-plane substrates has been succeeded. Such substrates eliminatethe problem of internal piezoelectric field, which reduces theefficiency of devices, and are essential for green lasers fabrication.We have succeeded in performing homoepitaxy by MOVPEresulting in high quality, strain free GaN epilayer, which wasdocumented by sharp exciton lines at energies typical ofunstrained hexagonal GaN crystals.

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