Journal of the Korean Physical Society, Vol. 42, February 2003, pp. S161∼S164

Growth and Impurity Study of GaN Single Crystal Grown by Na Flux

Sang Eon Park

COMTECS Ltd, Advanced Materials Research Laboratory, Daegu 704-702

Chae-Ryong Cho

Korea Basic Science Institute, Busan Branch, Busan 609-735

Sung Kyu Kim and Se-Young Jeong∗

Department of Physics, Pusan National University, Busan 609-735

We have employed a Na flux method to grow GaN single crystal. Two kinds of crucibles were usedto grow GaN crystal. GaN single crystals inside the crucibles exhibit plate-like morphologies. Thedepth profile of elements of GaN crystal was measured by using secondary ion mass spectroscopy.The nature of blue luminescence(BL) and donor-acceptor emissions of GaN single crystal wereinvestigated at low temperature. The BL was eliminated by using InconelTM instead of SUSTM asa crucible. The purity of GaN crystal was strongly dependent on the ingredients of the crucibleused and we notably reduced the quantity of unintentionally doped impurities.

PACS numbers: 81.05.Ea, 81.10.-h, 78.30.Fs. 78.55.CrKeywords: Crystal growth, Na, PL, GaN

I. INTRODUCTION

GaN-based nitride thin films have been developed forblue and ultra-violet laser diodes and detectors by manyresearch groups [1, 2]. These GaN-based optoeletronicand microwave devices are manufactured on sapphiresubstrates. Dislocation density and many defects inthe device are due to the lattice mismatch and ther-mal expansion coefficients between substrates and GaNfilm [3, 4]. Bulk GaN single crystals must be a suit-able substrate for the fabrication of high quality nitride-based light-emitting diodes(LEDs). GaN film possessesa high density of grain boundaries, dislocation densities(1010 ∼ 1011/cm3) and various point defects. Under-standing the nature of these defects and their poten-tial influence on device properties is of increasing impor-tance for optimization of device performance [5]. Thedefects in the band gap of GaN are usually characterizedby luminescence. The yellow luminescence is a broadGaussian-shaped emission centered at 2.2 ∼ 2.3 eV andappears to be a universal feature in GaN grown by metal-organic vapor-phase epitaxy and molecular beam epitaxy[6–10]. The origin of the yellow luminescence emissionis still being debated. At room temperature, the donor-acceptor(DA) emission superimposes on the near bandedge emission. It was reported that DA emission origi-nates from shallow acceptor and shallow donor recombi-nation and their phonon replicas [7,11,12]. In particular,

∗E-mail: syjeong@pusan.ac.kr

the basic material properties of GaN epitaxial layers con-cern PL bands peaked in the red(1.8 eV, Si and Mg co-doped), the yellow(2.2 eV, Si-doped or undoped), andthe blue (2.8 eV, Mg-doped) spectra. The yellow andblue luminescence are often found in undoped material[7,12]. In this paper, we have investigated the character-istics of PL in GaN single crystal grown using SUS andInconel growth cells.

II. EXPERIMENTAL

The crystal growth proceeded by first adjusting thetemperature and pressure of the reactor cells to the de-sired level using the SUS and Inconel growth cells [13,14]. SUS and Inconel were made of 18 %Cr - 8 %Ni and72 %Ni - 15 %Cr - 8 %Fe, respectively. We will representhere SUSTM and InconelTM as trademarks. Both SUSTM

and InconelTM cells contained NaN3 powder, which actedas a source of nitrogen upon heating, and pure(4N) Gametal as reactants. The InconelTM cell( Fe + Cr ∼ 30 %) has less Fe and Cr impurities than the SUSTM cell(Fe+ Cr ∼ 70 %). After processing, the reactor cell was ex-tracted from the reactor. Most of the grown crystal wasin the Na melts. We could not confirm whether the crys-tal nucleated on the growth cell or in the melt near themelt surface. The structure of the grown GaN crystalshas been studied by X-ray diffraction [13]. Secondaryion mass spectroscopy (SIMS) analysis was performedwith a magnetic-sector-based instrument (Cameca IMS-

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Fig. 1. Optical microscope images of GaN crystal grownby (a) SUS cell and (b) Inconel cell.

4f). O2+-ion beams were used as the primary ion sources

to detect positive secondary ions. The primary beam wasaccelerated through 8.0 keV and its intensity was approx-imately 100 nA. The dependence of PL on the temper-ature was measured from 5 K to 300 K for the purposeof examining the optical qualities of the prepared crys-tal using a closed cycle helium cryostat. A He-Cd laser,with 325 nm (55 mW) emission wavelength, was used asthe excitation source.

III. RESULTS AND DISCUSSION

GaN bulk crystal grown inside SUSTM and InconelTM

cells exhibits plate-like morphologies 4 ∼ 5 mm in size,as shown in Fig. 1.

All crystals showed strong and well defined single crys-talline XRD patterns. The XRD pattern confirmed theoriented growth with the (0001) of GaN. The structuralsymmetries of GaN grown by the SUSTM and InconelTM

cell were determined using the φ-scan of (101̄1) peak witha φ-rotation axis parallel to the c-axis of the GaN crys-tals. The six diffraction peaks were observed at regu-lar 60o increments. Although SIMS analysis of an ion-implanted standard sample is necessary to obtain a quan-titative content of impurity, the relative contents can beexplained by the incorporation of impurities due to thedifferent growth cell. The Ga, N, Fe and Cr elements areevaluated by using an O2

+ primary beam. Fig. 2(a) and2(b) shows the SIMS depth profiles for GaN single crys-tals grown by SUSTM and InconelTM cell, respectively.A depth profile analysis was performed for Ga, N andthe other impurities, Fe and Cr. The yield of the ion ofGa and N was constant throughout the depth of GaNsingle crystal.

Anomalous behavior of the Fe and Cr elements yieldedfrom GaN crystal grown using the SUSTM cell wasstronger than for that grown using the InconelTM cell,especially in the surface region. It was verified that theimpurity incorporation depends on the elements of thegrowth cell. The nature of blue-band luminescence (BL)and DA emission of undoped GaN grown using the var-ious crucibles and a flux method was investigated usinga low-temperature PL system. We compared the pho-toluminescence spectra of GaN single crystal grown byusing the SUSTM cell with that by the InconelTM cell.In the case of the SUSTM cell, as shown in Fig. 3, there

are three PL bands usually observed in the range of 2.5∼ 3.5 eV; i) Normally the near-band-edge transition at3.4 eV is observed. ii) Under low temperature from 5 Kto 175 K, an ultraviolet(UV) band is observed at about3.27 eV that involves donor-acceptor pair (DAP) lumi-nescence due to optical transitions from a shallow donorto a shallow acceptor. Shallow DAP emission was fol-lowed by several phonon replicas. iii) Broad blue lumi-nescence centered near 2.9 eV is obtained.

The intensity of the BL emission was even higher thanthat of the near band edge emission. The 2.9 eV PLemission band has revealed a fatigue effect that could beexplained by the metastable nature of the related defect[8]. Various proposals have been made as to the natureof defect responsible for the 2.9 eV band in undopedGaN [7, 12, 15]. Two principal mechanisms have beenproposed. One of them comprises a transition from theconduction band or shallow donor to a deep acceptor,which was assumed to be an isolated VGa(Ga vacancy)or hydrogenated VGa [8]. In an alternative model, the2.9 eV band was attributed to transition from the deepdonor VNMg ( substitution Mg for N vacancy ) to theshallow acceptor [7, 12, 16]. The width of the emissionband related to the deep point defect can be explained bythe strength of the electron-phonon coupling and energyof the zero vibrational state of the defect. The full widthat half-maximum (FWHM) for the blue emission bands

Fig. 2. Typical SIMS depth profiles of Ga, N, Fe and Crfor GaN crystal grown by (a) SUS cell (b) Inconel cell.

Growth and Impurity Study of GaN Single Crystal Grown by Na Flux – Sang Eon Park et al. -S163-

Fig. 3. PL spectra of the GaN crystal grown by a fluxmethod inside SUS cell.

is described by a configuration coordinate model [12,17],as follows ;

W(T) = W(0)

√coth(

~ωoe

2kT) (1)

where ~ωoe is the energy of the excited phonon. The

FWHM of the BL emission was fitted by the above equa-tion with the results of experiments, as shown in Fig. 4.The Gaussian fit yielded each maximum for all the mea-sured samples. The value of W(0) and ~ωo

e is 515 meVand 48.7 meV, respectively. The BL peak may arise fromcomplexes of extended defects and native-point defectsor impurities.

The transition between impurities related to deep levelshows generally a broad peak and quenching of the PLintensity. Thermal quenching of this band begins at T >200K. The impurities related to elements of the SUSTM

cell may explain all the defect-related sub-bandgap PLbands showing strong electron-lattice coupling. The tem-

Fig. 4. Temperature dependence of FWHM of the 2.9 eVband for GaN crystal grown by SUS cell. The blank cycle isa fit using a configuration coordinate (CC) model. The insetshows that the Gaussian fit yielded each maximum for thesample measured at room temperature.

perature dependence of the BL band energy is also notconsistent with Varshni’s equation including the relax-ation of lattice and participation of phonons [18]. In thecase of the InconelTM cell, as shown in Fig. 5, there aretwo PL peaks, the near band to band transition at 3.4eV and DAP luminescence with phonon replica at lowtemperature.

The intensity of the band-edge emission at 5 K showsseveral tens times higher than that at room temperaturein the case of the GaN grown in the the InconelTM cell,as compared to SUSTM cell. An analysis of the spec-tra from the SUSTM cell and InconelTM cell has shownthat the fine structure is formed by series of sharp peaks,associated with local and lattice phonons, at low temper-ature. The deep centers involved in DA recombinationare suggested to arise from self-compensation and to bevacancy-dopant associates [7]. The blue bands in nom-inally undoped GaN may arise from distant DA pairsinvolving residual impurities, as well as the vacancy as-sociates. The phonon replica in the SUSTM cell may berepeated at the energies related to the vibrational stateof a point defect. Observation of fine structure enablesus to determine the vibrational characteristics of the de-fect in the ground state [12]. In the case of GaN crys-tal grown by InconelTM cell, the energy band gap withtemperature, E(T), may be fitted to the expression ofVarshni’s equation [18,19]

E(T) = E(0)g −α T2

T + β(2)

The parameters α and β are fitting parameters. E(0)g isthe energy of the band gap at 0 K. From the fitting ofVarshni’s equation, the dependence of the temperature ofthe near band edge emission is well defined due to the re-laxation of lattice constant, as shown in the inset of Fig.5. The fitting values of α and β are −4.0×10−4eV/K and-810 K, respectively. In the case of GaN crystal grown bySUSTM cell, the temperature dependence of the energy

Fig. 5. PL spectra of the GaN crystal obtained inside theInconel cell are presented from 5 K to 300 K. The inset showsthe fit of Varshni’s equation according to the near band edgeemission.

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band gap is also consistent with Varshni’s equation. Thefitting values of α and β are −2.0×10−3eV/K and -3500K, respectively. Since β comes out negative, no physicalmeaning can be ascribed to this relation for GaN [19].

IV. SUMMARY

The BL emissions were almost eliminated by simplyexchanging the growth cell for an Inconel cell as the re-sult of reduced quantities of Fe, Cr. From the correlationbetween PL and the impurities of GaN for two kindsof growth cell conditions, it has been verified that theimpurity incorporation should depend on the elementsof cell material. Impurities related to ingredients of thegrowth cell are readily incorporated into GaN single crys-tal grown by Na flux. It is necessary to carry out afurther study on whether the DAP emission originatesfrom recombination between shallow donors and shallowacceptors, such as Fe, Cr, and vacancy-dopant associates.

ACKNOWLEDGMENTS

This work was supported by grant No. R14-2002-029-01000-0 from the ABRL Program of the Korea Science &Engineering Foundation and in part by Korea ResearchFoundation Grant(KRF-2000-DP0105).

REFERENCES

[1] S. Nakamura and G. Fasol, The blue laser diodes -GaN Based Light Emitters and Lasers (Springer-Verlag,Berlin, 1997).

[2] I. Ferguson, M. Schurman and I. Eliashevich, J. KoreanPhys. Soc. 39, S433 (2001).

[3] S. D. Lester, F. A. Ponce, M. G. Craford and D. A.Steigerwald, Appl. Phys. Lett. 66, 1249 (1995).

[4] M. Koike, S. Yamasaki, S. Nagai, N. Koide, S. Asami,H. Amamo and I. Akassaki, Appl. Phys. Lett. 68, 1403(1996).

[5] I. Grzegory, Mat. Sci. Eng. B 82, 30 (2001).[6] S. J. Chung, O. H. Cha, C-H. Hong, E-K. Suh, H. J. Lee,

Y. S. Kim and B. H. Kim, J. Korean Phys. Soc. 37, 1003(2000).

[7] U. Kaufmann, M. Kunzer, H. Obloh, M. Majer, Ch.Manz, A. Ramakrishman and B. Santic, Phys. Rev. B59, 5561 (1999).

[8] S. J. Xu, G. Li, S. j. Chua, X. C. Wang and W. Wang,Appl. Phys. Lett. 72, 2451 (1998).

[9] E. F. Schubert, I. D. Goepfert and J. M. Redwing, Appl.Phys. Lett. 71, 3224 (1997).

[10] S. K. Noh, C. R. Lee, S. E. Park, I. H. Lee, I. H. Choi,S. J. Son, K. Y. Lim and H. J. Lee, J. Korean Phys. Soc.32, 851 (1998).

[11] G. Popovici, G. Y. Xu, A. Botchkarev, W. Kim, H. Tang,A. Salvador, H. Morkoc, R. Strange and J. O. White, J.Appl. Phys. 82, 4020 (1997).

[12] M. A. Reshchikov, F. Shahedipour, R. Y. Korotkov, B.W. Wessels and M. P. Ulmer, J. Appl. Phys. 87, 3351(2000).

[13] C. -R. Cho, S. E. Park, Y. C. Cho and S. -Y. Jeong,Mater. Res. Soc. Symp. Proc. 680E, 9.10 (2001).

[14] H. Yamane, M. Shimada, T. Sekiguchi and F. J. DiSalvo,J. Crystal Growth 186, 8 (1998).

[15] I. K. Shmagin, J. F. Muth, J. H. Lee, R. M. Kolbas, C.M. Balkas, Z. Sitar and R. F. Davis, Appl. Phys. Lett.71, 455 (1997).

[16] M. A. Reshchikov, G. C. Yi and B. W. Wessels, Phys.Rev. B 59, 13176 (1999).

[17] C. C. Klick and J. H. Schulman, Luminescence in Solidsin Solid Sate Physics (Academic Press, New York, 1957),Vol. 5.

[18] Y. P. Varshni, Physica, 34, 149 (1967).[19] B. Monemar, Phys. Rev. B 10, 676 (1974).