Growth and Characterizationof High Quality Continuous GaN Filmson Si-Doped Cracked GaN Templates

S. J. Chua (a), M. Hao1) (a), J. Zhang (b), and E. K. Sia (b)

(a) Institute of Materials Research and Engineering, 3 Research Link, Singapore 117602,Singapore

(b) Department of Electrical and Computer Engineering, National University of Singapore,10 Kent Ridge Crescent, Singapore 119260, Singapore

(Received July 14, 2001; accepted August 4, 2001)

Subject classification: 61.10.Kw; 61.72.Ff; 68.37.Lp; 68.55.Jk; 78.55.Cr; 81.15.Gh; S7.14

It was found that continuous GaN films could be grown on cracked Si-doped GaN templates.Plan-view and cross section micrographs showed that no cracks occurred in the top GaN layeralthough it was grown on a cracked Si-doped GaN layer. The continuous GaN films on thecracked Si-doped GaN have been characterized by X-ray diffraction and Raman spectroscopy. Itwas shown that the top GaN film is of high crystal quality. A good surface morphology has beenproven by scanning electron microscopy observations. Transmission electron microscopy analysisshowed that the screw and/or mixed dislocation density in the top GaN film has been greatly re-duced. However, pure edge dislocation density in the film remains unchanged.

Introduction Device quality GaN and related materials are primarily grown on sap-phire substrates by two-step method using metalorganic chemical vapor deposition(MOCVD) [1]. Besides high density of dislocations, the cracking has also been ob-served in GaN films grown on sapphire substrates. The feature of the cracking patternindicates that the cracking takes place under biaxial tensile stress [2, 3]. It is notstraightforward to understand tensile cracking in GaN film on sapphire substrate. Sincethe thermal expansion coefficient of sapphire is larger than that of GaN, after coolingdown, a compressive stress estimated at greater than 1 GPa would be expected todevelop in a GaN film that was grown stress free at the growth temperature (about1000 �C) [4]. Typically, smaller, but still compressive stress is observed [4–8]. Therefore,it is clear that the cracking does not occur during the cooling down process. Recently,in situ wafer curvature measurements have shown that undoped GaN films are in ten-sion at the growth temperature indicating that the cracking may occur during thegrowth [9]. According to the epitaxial relation adopted by GaN on sapphire and thelattice constants, the GaN would be under compression at the growth temperature ifthe GaN were coherently grown on sapphire substrate. Above a critical thickness, thisbiaxial compression can, to a lesser or greater extent, be relaxed by a network of misfitdislocations at the GaN/sapphire interface. However, there is no dislocation mechanismthat would switch the sign of the crystallographic mismatch from compressive to tensile[3]. The physics behind the occurrence of tensile stress in GaN film during growth isstill not clear. It is considered to be a possibility that it is associated with island coales-cence [3, 10]. Many evidences show that the tensile stress in GaN film could be in-

1) Corresponding author; Phone: +65 874 1983; Fax: +65 872 0785; e-mail: ms-hao@imre.org.sg

phys. stat. sol. (a) 188, No. 1, 421–424 (2001)

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creased by Si doping, which could result in a reduction of the critical thickness fromabout 5 mm for undoped GaN to about 2 mm for Si-doped GaN with a Si concentrationof 2 � 1019 cm––3 [3, 8, 10]. The increase in tensile stress caused by Si doping has alsobeen discussed in terms of a crystallite coalescence model because first-principles calcula-tions show that the substitution of Si for Ga in the lattice causes only negligible changes inthe lattice constants [10]. Si doping in GaN is desired not only to form the low resistancen-type conductivity. It was also found that Si doping could reduce the dislocation densityin GaN films and improve the crystal quality [7, 11]. However, the maximum amount of Sidoping appears to be limited by the cracking discussed above. We have found that a highquality continuous GaN film can be grown on cracked Si-doped GaN by carefully select-ing the growth parameters. The properties of crack-free GaN films grown on cracked Si-doped GaN templates will be discussed, based on X-ray diffraction (XRD), Raman scat-tering and transmission electron microscopy (TEM) analysis.

Experiments Two samples used in this study were grown in a low-pressure MOCVDreactor equipped with an in situ reflectivity measurement system. Ammonia was usedas nitrogen source. Trimethylgallium was flowing into the reactor by using hydrogen asa carrier gas. Si doping was achieved by introducing silane into the gas stream. Thedeposition procedure for sample 963 was sequential growth of: (i) a low temperature(LT) GaN buffer on (0001) sapphire substrate at 520 �C, (ii) 1 mm high temperature(HT) undoped GaN layer at 1020 �C, and (iii) 3 mm HT Si-doped GaN layer with a Siconcentration over 1019 cm––3. The chamber pressure was kept at 200 Torr. The crackingalong {1100} cleavage plane occurred in the Si-doped layer of this sample. For sample960, following the deposition of the layers exactly the same as for sample 963, a 2 mmundoped GaN was grown by using another set of growth parameters which can en-hance the degree of lateral growth.

Results and Discussions As illustrated in Fig. 1a2), the array of cracks can be clearlyidentified in the sample 963. It is also very clear that most of the cracks extend to thetop surface. Figure 1b is an optical micrograph of sample 960. By focusing the micro-scope through the thickness of the film, the cracks in sample 960 can be graduallybrought into and then out of focus indicating that the cracks lie in the Si-doped layer.Figure 1c is a cross-section optical microgragh of sample 960. Please note that twopiceses of the sample are pasted together by epoxy. It is very clear that the cracks donot penetrate into the top surface. Figure 1d is a plan-view scanning electron micro-scopy (SEM) picture for sample 960. This picture has almost the same scale as that of

422 S. J. Chua et al.: High Quality GaN Films on Si-Doped Cracked GaN Templates

2) Colour figure is published online (www.physica-status-solidi.com).

20 �m(a) 20 �m

(b)20 �m

(d)

20 �m

(c)

Fig. 1 (colour). a) Optical micrograph of sample 963; b) optical micrograph of sample 960;c) cross-section micrograph of sample 960; d) SEM picture of sample 960

Fig. 1b. The cracks are completely invisible in Fig. 1d, further indicating that there areno cracks in the top undoped GaN layer of sample 960. SEM investigations have alsoconfirmed that the crack-free undoped GaN film grown on cracked Si-doped GaN hasa very good surface morphology.XRD 2q–w scan with a very narrow slit can give information about the strain and

the distribution of lattice constant in the crystal. In this study, the XRD curves wererecorded in 2q–w scan mode for both sample 963 and sample 960 with various diffrac-tions. Four Ge crystals were used as monochromator for X-ray source. A 1/32 degreeslit was used in the measurements. As shown in Fig. 2a, for asymmetric (1102) diffrac-tion, the sample 963 (cracked Si-doped GaN) gives a very broad peak with a very longtail in the low angle side indicating that the lattice relaxation occurred near the cracks.Since uncracked film is compressively strained, the in-plane lattice constant a will be-come larger after the lattice relaxation. The extent of this kind of relaxation is, cer-tainly, dependent on the distance from the cracks [10]. This explains why there is a verylong tail in the low angle side of the 2q–w scan curve for (1102) diffraction of thesample 963. The (1102) diffraction curve from sample 960 is shown in Fig. 2b. For thecontinuous GaN film grown on the cracked Si-doped GaN, the XRD signal will comefrom both top continuous GaN film and the underneath cracked Si-doped GaN. Com-pared with the sample 963, the high-angle part of the 2q–w scan curve becomes sharpfor sample 960. That means the peak from the continuous GaN is much narrower, andit was superimposed on the broad peak from the underneath cracked Si-doped GaN inthe high angle side. So, it can be concluded that the continuous GaN film on thecracked Si-doped GaN is still in compressive strain at room temperature. However, theRaman scattering studies show that the compressive stress in these continuous GaNlayers is negligible.According to the Poisson relation, the c lattice constant will decrease with increasing a

lattice constant if lattice relaxation occurs. The XRD 2q–q scan curve for (0002) diffrac-tion is only related to c lattice constant. Therefore, for cracked Si-doped GaN, the (0002)2q–w scan curve will have a shoulder in the high angle side. After having a continuousGaN layer on the cracked Si-doped GaN, the low angle part of the curve will becomesharp. These results have been proved by the experiments (the curves are not shown).Figure 3 shows dark-field TEM pictures of a crack-free GaN film grown on a cracked

Si-doped GaN template. From the pictures, taken at a diffraction vector of (0002), one

phys. stat. sol. (a) 188, No. 1 (2001) 423

Fig. 2. XRD 2q–w scan curves of a) sample 963 and b) sample 960 with a scattering vector of(1102)

can clearly find that the dislocations in the top GaN are much less those that in theunderneath Si-doped GaN layer. Only screw or mixed dislocations have the contrast asthis condition. So we can say that the screw and/or mixed dislocation density has beengreatly reduced in the top GaN grown on the cracked Si-doped GaN templates. TEMpicture of Fig. 3b was taken around a crack line in the underneath Si-doped GaN. It isvery interesting to note that the dislocations near the crack are bound together overthe crack line in the top GaN layer. The reduction of dislocation density was not foundin the TEM pictures with a diffraction vector of (1210). That means that pure edgedislocation density in the top GaN layer remains unchanged.

Conclusion It was found that continuous GaN films can be grown on cracked Si-dopedGaN templates. SEM observations confirmed that there are no cracks in the GaN layerand this layer has a very good surface morphology. High crystal quality of this GaN layerhas been proved by both XRD and Raman spectroscopy. TEM analysis showed that thescrew and/or mixed dislocation density in this GaN layer has been greatly reduced. How-ever, pure edge dislocation density in the film remains unchanged. It is very promisingthat this continuous GaN layer can be used as a template to grow any device structure,and the underneath cracked Si-doped GaN layer may help to release the stress during thedevice structure growth and to avoid the cracking in the device layers.

References

[1] S. Nakamura, The Blue Laser Diode: GaN Based Light Emitters and Lasers, Springer-Verlag,Berlin 1997.

[2] N. Itoh, J. C. Rhee, T. Kawabata, and S. Koike, J. Appl. Phys. 58, 1828 (1985).[3] E. V. Etzkorn and D. R. Clarke, J. Appl. Phys. 89, 1025 (2001).[4] J. W. Ager III, T. Suski, S. Ruvimov, J. Krueger, G. Conti, E. R. Weber, M. D. Bremser, R. F.

Davis, and C. P. Kuo, Mater. Res. Soc. Symp. Proc. 449, 775 (1997).[5] K. Hiramatsu, T. Detchprohm, and I. Akaksaki, Jpn. J. Appl. Phys. 32, 1528 (1993).[6] C. Kisielowski, J. Kruger, S. Ruvimov, T. Suski, J. W. Ager III, E. Jones, Z. Liliental-Weber,

M. Rubin, E. R. Weber, M. D. Bremser, and R. F. Davis, Phys. Rev. B 54, 17745 (1996).[7] S. Ruvimov Z. Liliental-Weber, T. Suski, J. W. Ager III, J. Washburn, J. Krueger, C. Kisie-

lowski, E. R. Weber, H. Amano, and I. Akasaki, Appl. Phys. Lett. 69, 990 (1996).[8] I. Lee, I. Choi, C. Lee, E. Shin, D. Kim, S. K. Noh, S. Son, K. Lim, and H. J. Lee, J. Appl. Phys.

83, 5787 (1998).[9] S. Hearne, E. Chason, J. Han, J. A. Floro, J. Figiel, J. Hunter, H. Amano, and I. S. T. Tsong,

Appl. Phys. Lett. 73, 356 (1999).[10] L. T. Romano, C. G. Van de Walle, J. W. Ager III, W. Gotz, and R. S. Kern, J. Appl. Phys. 87,

7745 (2000).[11] H. Hirayama, M. Ainoya, A. Kinoshita, A. Hirata, and Y. Aoyagi, Mater. Res. Soc. Symp.

Proc. 639, G1.3.1 (2001).

424 S. J. Chua et al.: High Quality GaN Films on Si-Doped Cracked GaN Templates

Top GaN

Cracked GaN

1 µm

(a)

Top GaN

Cracked GaN

1 �mcrack

(b)Fig. 3. Dark field TEM pictures ofdifferent areas of a continuousGaN film grown on cracked Si-doped GaN. The diffraction vectoris (0002) for both pictures. Acracking line can be seen in b)