Si doping of cubic heteroepitaxial GaN layers studied by Raman scattering

Thin Solid Films 364 (2000) 107±110 www.elsevier.com/locate/tsf

Si doping of cubic heteroepitaxial GaN layers studied by Raman scattering G. Bentoumi a,*, A. Deneuville a, E. Bustarret a, B. Daudin b, G. Feuillet b, E. Martinez b, P. Aboughe-Nze c, Y. Monteil c a

Laboratoire d'Etudes des ProprieÂteÂs Electroniques des Solides, Centre National de la Recherche Scienti®que and Universite Joseph Fourier de Grenoble, BP 166, 38042 Grenoble Cedex 9, France b DeÂpartement de la Recherche Fondamentale sur la MatieÁre CondenseÂe, CEA/Grenoble, SPMM, 17 rue des Martyrs, 38054 Grenoble Cedex 9, France c Laboratoire des MultimateÂriaux et Interfaces, Universite C. Bernard de Lyon, 43 Bd du 11 Novembre 1918, 69622 Villeurbanne Cedex, France

Abstract The room temperature concentration and mobility of electrons introduced by Si-doped cubic GaN ®lms have been derived from their Raman spectra. These ®lms were grown by MBE on cubic SiC thin ®lms deposited by CVD on Si. The Si-doped ®lms has a mobility lower (in the 50 to 210 cm 2 V/s range) than (1650 cm 2 V/ s) the undoped ®lms, attributed to the signi®cant increase upon doping of the hexagonal parasitic volume fraction in the ®lms. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Cubic GaN; Si doping; Raman; Cubic SiC; Mobility

1. Introduction In doped polar semiconductors the LO Raman peak shifts to higher wave numbers when the carrier concentration increases, and widens when the carrier mobility decreases (phonon±plasmon coupling) [1±6]. Epitaxial growth of the metastable cubic phase of GaN has been achieved on various (100) surfaces [7±18]. We use here a cubic SiC [19] thin ®lm as a template grown on the (100) face of a Si wafer. Because the hexagonal phase is more stable, any growth of b -GaN yield a mixture of hexagonal and cubic GaN [17±23]. The residual hexagonal content, as well as the quality of the cubic and of the hexagonal phases can be derived from the analysis of the TO peak of the cubic phase and of the E2 peak of the hexagonal phase [17]. In this work, we use Raman scattering to determine the electron concentration and mobility, the hexagonal phase content, and the quality of Si-doped b -GaN ®lms. We ascribe the signi®cant decrease in the electron mobility as their Si content increases to an increase of their residual hexagonal content.

2. Experimental details Starting from a highly doped Si(100) wafer, there was * Corresponding author. Laboratoire d'Etudes des ProprieÂteÂs Electroniques des Solides, Centre National de la Recherche Scienti®que and Universite Joseph Fourier de Grenoble, BP 166, 38042 Grenoble Cedex 9, France.

®rst a carburation at 11508C of the Si (100) substrate, then the growth of 3 mm-thick SiC at 13008C from a silanepropane (44±56%) mixture and ®nally that of two cubic GaN layers deposited by molecular beam epitaxy (MBE) with a N2 RF plasma source and under Ga-rich growth conditions. The ®rst one, grown at 6208C, 0.44 mm thick, is undoped. The top layer, 1 mm thick, grown at 6408C, is either non-intentionally (nid) or Si-doped with Si cell temperatures of 980, 1030 or 10808C. The micro-Raman backscattering spectra of these GaN samples were excited at room temperature by the unpolarized 514.5 nm radiation of an argon laser (power ,5 mW) and recorded each 0.6 cm 21 with a CCD through a DILOR XY spectrometer. Both the TO (around 552 cm 21, see below) and the LO (around 740 cm 21) modes of the cubic phase were observed. The very ef®cient E2 (around 568 cm 21, see below) mode of the hexagonal phase was seen even at very low residual hexagonal content. The residual hexagonal content was derived from the ratio of the area under the hexagonal E2 and the cubic TO peaks after calibration [17]. The `quality' of the cubic and hexagonal regions was estimated from the full width at half maximum (FWHM) of the TO (cubic) and E2 (hexagonal) peaks. Previous measurements [19] have shown that the III to V ratio changed along one direction on the sample surface and that the hexagonal content increased within the ®lm from the SiC interface to the free surface of the GaN ®lm. For this study, the measurements were performed at the center of each sample, with the laser focus on the free surface.

0040-6090/00/$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0040-609 0(99)00911-6

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Fig. 3. The LO Raman peaks from the b -GaN top layers. Fig. 1. Room temperature longitudinal optical (LO) mode Raman backscattering peak for various cubic GaN epilayers grown on cubic SiC templates.

3. Results and discussion Fig. 1 shows the Raman spectra of the samples with the nid and the Si doped b -GaN top layers. The signal from the sample with the nid top layer is nearly symmetrical around 737.5 cm 21. As the Si incorporation increases, the signal becomes wider with additional components on its high wave number side. The best ®ts are obtained by the sum of three Gaussian components, for example for the highest doping level, Fig. 2, centered around 737.5, 742 and 756 cm 21. The ®rst one is ascribed to the LO mode of the undoped b -GaN of the underlying layer. The second one has a wave number close to the 740 cm 21 value ascribed by Yaguchi et al. [9] to the E1(LO) signal from the residual hexagonal phase, here from the underlying b -GaN. The third band comes from the LO mode of b -GaN top layers shifted to higher wave number by the phonon±plasmon coupling. The Fig. 3 shows the ®ts of this third component normalized to the intensity of the LO peak of the undoped under-

lying b -GaN. From nid to the highest doping level, the peak shifts from about 739 up to about 756 cm 21 and widens from about 17 to about 46 cm 21. The intensity of the LO Raman peak is a function of the product of the imaginary part of the opposite of the dielectric permitivity 1(v ) by A(v ) [3] I…v† ˆ A…v†Im…21=1…v††

…1†

In doped semiconductors, 1(v ) and A(v ) include additional components which depend on the plasmon frequency v p and on its damping constant g [3] ! v2p v2LO 2 v2TO …2† 1…v† ˆ11 11 2 2 v…v 1 ig† vTO 2 v2 2 ivG The standard expression of A(v ) is given in Ref. [4]   n A…v† ˆ 1 1 Cv2TO =D 2v2p g v2TO 2 v2     22v2 G v2 1 g2 2 v2p 1 C v2TO = v2LO 2 v2TO h i  v2p …g…v2LO 2 v2TO † 1 G…v2p 2 2v2 †† 1 v2 …v2 1 g2 † g …3† with D ˆ v2p g‰…v2TO 2 v2 †2 1 v2 G2 Š 1 v2 G…v2LO 2 v2TO †…v2 1 g2 †

…4†

Fig. 2. Decomposition of the LO Raman peak into three spectral components in the highest Si-doped cubic GaN epilayer. Full squares are experimental points, lines are results of the least-square deviation ®tting procedure

11 : m * 2 :v p 4p e 2

with

n…cm23 † ˆ

and

m…cm2 =V s† ˆ

8:488 £ 10227 m* …Kg†g

…5† …6†

with v p and g in cm 21. The ®t needs v TO, v LO, G and C (Faust±Henry coef®cient) of the ideal undoped ®lm. C ˆ 0:55 was used because it gives the best ®ts. We used a TO wave number vTO ˆ 553 cm 21 (close to the experimental values of 552 cm 21 of Ref. [8] and 551 cm 21 of Ref. [17]) with a damping constant G ˆ 14 cm 21, a LO wave number vLO ˆ 737:5 cm 21 (close to

G. Bentoumi et al. / Thin Solid Films 364 (2000) 107±110

109

Table 1 The plasmon frequencies and their damping constants from the ®t, and the corresponding carrier concentrations and mobilities Si cell temperature (8C)

v p (cm 21)

n (cm 23)

Nid 980 1030 1080

65 105 145 310

2£ 2.6 £ 5.1 £ 2.4 £

10 16 10 16 10 16 10 17

g (cm 21)

m (cm 2 V/s)

30 200 325 870

1650 210 130 50

the experimental value of 738 cm 21 of Yaguchi et al. [9] in MOVPE ®lms). The ®ts gave the values of v p and g of Table 1, from which n and m were derived through Eqs. (5) and (6), respectively. We use m* ˆ 0:22 m0 as effective mass of electrons, the same than in hexagonal GaN because that in b -GaN is still unknown. The resulting estimated carrier concentration increased from 2 £ 1016 cm 23 for the nid ®lm to 2:4 £ 1017 cm 23 for the highest doping level, while the mobilities decreased from 1650 to 50 cm 2 V/ s as shown in Fig. 4. In the case of the nid ®lm, the present estimates compare favorably with Hall effect results which have been reported for growth of b -GaN on cubic SiC (n ˆ 1019 cm 23 in an early paper by Liu et al. [8], then 4 £ 1017 cm 23 with a mobility of 760 cm 2 V/ s in a later work [11]) or on GaAs (n ˆ 4 £ 1017 cm 23 and m ˆ 20 cm 2 V/ s by MOCVD [24], and by MBE p ˆ 1 £ 1013 cm 23 and m ˆ 350 cm 2 V/ s under N-rich conditions or n ˆ 7 £ 1013 cm 23 and m ˆ 100 cm 2 V/ s under Ga-rich conditions [25]). Upon doping, the maximum carrier concentration deduced here from Raman spectroscopy was weaker and with lower mobilities than the published Hall effect results for Si doping of the b -GaN deposited on cubic SiC [11] (1:5 £ 1018 to 3 £ 1020 cm 23 with mobilities from 500 down to 60 cm 2 V/s). Although the different measurement methods and the choice of an effective mass value may explain in part such discrepancies, these might as well be tentatively ascribed to

Fig. 4. Transport parameters in Si-doped b -GaN epilayers: Hall effect measurements from the literature, and our data from Raman scattering spectra.

Fig. 5. At the foot of the Raman spectrum from the silicon substrate, two Raman contributions originating from the cubic (TO mode) and the hexagonal (E2 mode) volume fractions of a Si-doped b -GaN sample.

different defect concentrations in the various ®lms, as suggested by the variation of the FWHM of the TO mode. Fig. 5 shows the TO mode region of the Raman spectrum for the sample with the highest doping level. Beside the dominant signal from Si around 520 cm 21, one sees not only the TO peak of the cubic phase around 552 cm 21, but also a shoulder around 568 cm 21 coming from the residual hexagonal phase (and second order Raman band from Si centered around 624±640 cm 21 which is not taken into account in this ®t). In striking agreement with the behavior of the LO peak, the FWHM of the TO peak rises abruptly (from 10.4 to 14.5 cm 21, see dotted line Fig. 6) as the electron concentration increases from 2 £ 1016 (nid) to 2:6 £ 1016 cm 23, the lowest Si incorporation level. However, the FWHM of the TO feature decreased from 14.5 to 11 cm 21 as the doping level was further increased, whereas the microscopic mobility deduced from the LO width parameter kept on decreasing, as seen in Fig. 4. The latter trend might also originate from a higher scattering by the parasitic hexagonal phase, the volume fraction of which can be estimated after calibration [26] from the ratio of the area under the hexagonal E2 component to the cubic TO Raman peak. The residual hexagonal content versus the electron concentration is shown as full line on Fig. 6. Starting from 0.085% for the nid GaN, the parasitic phase concentration increased abruptly to 0.45% for a free carrier concentration of 2:6 £ 1016 cm 23 and then to 0.65% for a 2:4 £ 1017 cm 23. According to the present Raman results, the mobility of our ®lms seems thus to be sensitive to both the `quality' of the cubic phase, and to the concentration of the parasitic hexagonal phase. While our nid ®lms are optimized and give a high electron mobility, our Si doped ®lms remain with low electron mobilities and need further optimization of their quality and hexagonal content. 4. Conclusion The ®t of the LO mode Raman peak of the samples

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References

Fig. 6. Variations of the FWHM of the TO Raman peak (dotted line) and of the hexagonal content (solid line) as a function of the electron concentration.

yielded three Gaussian components ascribed, respectively to the undoped underlying b -GaN layer, to the E1(LO) mode of its residual hexagonal content, and to the shifted LO mode of the b -GaN top layer. Electron concentrations and mobilities were deduced from the ®ts of the shifted LO mode of the top b -GaN layer. For nid ®lm, this method yielded a rather weak residual electron concentration (2 £ 1016 cm 23) with a high mobility (1650 cm 2 V/s). The FWHM of the TO mode of the cubic phase indicated a good structural `quality' with a weak residual parasitic hexagonal content (0.085%). Si incorporation resulting in a slightly higher electron concentration (2:6 £ 1016 cm 23) induced a drastic decrease of the electron mobility (down to 210 cm 2 V/s), with a poorer quality of the cubic phase (FWHM of the TO peak increasing from 10.4 to 14.5 cm 21), and a drastic increase of the concentration of the parasitic hexagonal phase (from 0.085 to 0.45%). Further doping improved the b -GaN quality, but increased its parasitic phase content, and resulted in lower mobilities. Therefore, we ascribe the low carrier mobilities in Si-doped b -GaN ®lms to scattering by an increasing amount of hexagonal regions. Acknowledgements We are indebted to Dr L. Abello who granted some of us access to the Raman spectrometer.

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