Materials Letters 65 (2011) 1396–1399

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Materials Letters

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Growth of InN layers on Si (111) using ultra thin silicon nitride buffer layerby NPA-MBE

Mahesh Kumar a,b, Mohana K. Rajpalke a, Thirumaleshwara N. Bhat a, Basanta Roul a,b, Neeraj Sinha c,A.T. Kalghatgi b, S.B. Krupanidhi a,⁎a Materials Research Centre, Indian Institute of Science, Bangalore-560012, Indiab Central Research Laboratory, Bharat Electronics, Bangalore-560013, Indiac Office of Principal Scientific Advisor, Government of India, New Delhi-110011, India

⁎ Corresponding author.E-mail address: sbk@mrc.iisc.ernet.in (S.B. Krupanid

0167-577X/$ – see front matter © 2011 Elsevier B.V. Adoi:10.1016/j.matlet.2011.02.012

a b s t r a c t

a r t i c l e i n f o

Article history:Received 12 November 2010Accepted 1 February 2011Available online 20 February 2011

Keyword:Epitaxial growthStructuralRamanSemiconductors

Indium nitride (InN) epilayers have been successfully grown by nitrogen-plasma-assisted molecular beamepitaxy (NPA-MBE) on Si (111) substrates using different buffer layers. Growth of a (0001)-oriented singlecrystalline wurtzite–InN layer was confirmed by high resolution X-ray diffraction (HRXRD). The Ramanstudies show the high crystalline quality and the wurtzite lattice structure of InN films on the Si substrateusing different buffer layers and the InN/β–Si3N4 double buffer layer achieves minimum FWHM of E2 (high)mode. The energy gap of InN films was determined by optical absorption measurement and found to be in therange of ~0.73–0.78 eV with a direct band nature. It is found that a double-buffer technique (InN/β–Si3N4)insures improved crystallinity, smooth surface and good optical properties.

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1. Introduction

Indium nitride (InN) is an interesting and potentially importantsemiconductor material with superior electronic transport properties[1]. Compared to all other group III-nitrides, InN possesses the lowesteffective mass, the highest mobility, the highest saturation velocityand a narrow band gap of 0.7–0.9 eV [2,3]. Such properties of InN asnarrow band gap, infra-red photoluminescence in the practicalimportant optical communication wavelengths, possibility of gener-ation of THz emission and superior electron transport properties arevery promising for numerous applications of this material. The growthof high-quality InN is still difficult, due to the low dissociationtemperature, the high equilibrium vapor pressure of nitrogen over theIndium, and the lack of suitable lattice-matched substrates. Amongthe various substrates, sapphire has been widely used for theheteroepitaxial growth of InN, even if it has an insulating propertyand a large lattice mismatch with InN (~25%). On the other hand, Si(111) can be the best substrate to grow InN because it offers manyattractive advantages such as the smaller lattice mismatch (~8%),good doping properties, and thermal conductivity. The InN epilayersgrowth on Si substrates has been reported recently by MBE andmetalorganic chemical vapor deposition (MOCVD), using differenttypes of buffer layers, such as the low-temperature InN (LT-InN) [4],GaN [4], AlN [5], AlN/Si3N4 double layer [6,7] and AlN/AlGaN/GaN

triple buffer layer [8]. Recently, it was proposed by Li et. al. [9] that anamorphous SiNx layer formed by unintentional nitridation of thesubstrate surface during the first stage of the growth might causedetrimental effects on the grown InN films. To overcome the effect ofamorphous SiNx on growth quality, we formed a single-crystal β-Si3N4

ultrathin layer on Si (111) surface by exposing the surface to radio-frequency (RF) nitrogen plasmawith a high content of nitrogen atomsprior to the low-temperature (LT) InN buffer layer. In this work weperformed a study on the structural, morphological and opticalproperties of the InN epilayers grown on Si substrates with differentbuffer layers.

2. Experimental

The samples used for this study were grown by NPA-MBE system(OMICRON) equipped with a radio frequency plasma source. Theundoped Si (111) substrates were chemically cleaned followed bydipping in 5% HF to remove the surface native oxide. The substrateswere thermally cleaned at 900 °C for 1 h in ultra-high vacuum. Insample (a) InN layers of thickness 280 nm were grown directly on Siat 450 °C without buffer layer, in sample (b) the LT-InN buffer layersof thickness 30 nm were grown at 400 °C followed by 250 nm of InNfilms at 450 °C. In samples (c) and (d), these were exposed tonitrogen plasma at 530 °C and 700 °C, respectively for 30 min.Afterwards, LT-InN buffer layer of thickness 30 nm was grown at400 °C followed by 250 nm of InN films at 450 °C. In sample (e) firstwe have grown single-crystal β-Si3N4 ultrathin layer on Si (111)surface by exposing the surface to RF nitrogen plasma with a high

20 30 40 50 70 80602 (degree) Azimuth angle (degree)

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Fig. 1. (i) HRXRD 2θ-ω scans of InN films on the Si (111) substrate (ii) In-plane Phi-scan measured from InN epilayers and the Si (111) substrate showing a sixfold and a threefoldsymmetry, respectively.

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content of nitrogen atoms and details of growth conditions canbe found elsewhere [10], then an LT-InN buffer layer of thickness30 nm was grown at 400 °C followed by 250 nm of InN films at450 °C. For all the samples, Indium effusion cell temperature waskept at 780 °C and the corresponding beam equivalent pressure(BEP) was maintained at 2.1×10−7 mbar. Nitrogen flow rate andplasma power were kept at 1.0 sccm and 350 W, respectively forthe nitridation, buffer layers and InN growth subsequently. Thestructural characterization and surface morphologies of the sampleswere carried out by HRXRD and atomic force microscopy (AFM),respectively. Besides, samples were characterized by micro-Ramanspectroscopy using a 514 nm line of the Ar+ ion laser and opticalabsorption spectra were measured at room temperature by BrukerIFS 66v/s vaccum fourier transform interferometer.

Fig. 2. FWHM of Raman E2 (high) modes and FWHM of XRD rocking curves of samples(a) to (e).

3. Results and discussion

Fig. 1(i) shows the HRXRD 2θ-ωscans of the InN films grown on theSi (111) substrate and it can be seen that except the substrate peak,only a strong (0002) InN diffracted peak at 2θ=31.32 and a weak(0004) peak at 2θ=65.32 were present, establishing the epitaxialnature of InN thin film highly oriented along the [0001] direction ofthe wurtzite InN. An in-plane Phi scan was also taken by rotating thesample around its surface-normal direction to investigate the in-planealignment of the InN film. As shown in Fig. 1(ii) the diffraction peaksfrom the (10–11) plane of InN were observed at 60° intervals, clearlyconfirming the hexagonal structure of the InN epilayer. In addition,the (1–11) plane of the Si substrate showed a threefold symmetry asexpected. The crystalline quality of InN epilayers grown on the Si(111) substrate was determined by full width half maximum(FWHM)of HRXRD rocking curves of (0002) symmetry planes of InN epilayers,

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Fig. 3. Room temperature micro-Raman spectra of InN films.

Fig. 4. (i) 1×1 μm2 AFM images of InN on the Si (111) substrate for samples (a), (b), (c), (d) and (e). (ii) The surface roughness (RMS) of InN films grown by using different buffer layers.

1398 M. Kumar et al. / Materials Letters 65 (2011) 1396–1399

as shown in Fig. 2. The FWHM of rocking curve decreased when LT-InN buffer layer was used compared to without buffer layer, butincreased when low temperature nitridation layer (530 °C) followedby LT-InN buffer layer was used. High temperature nitridation(700 °C) followed by LT-InN buffer layer drastically reduces theFWHM and LT-InN/β-Si3N4 double buffer layers achieve theminimumFWHM of around ~32′. The crystalline quality and lattice structure ofthe InN film was further investigated by Raman spectroscopy. Fig. 3shows a room-temperature Raman spectrum of the InN films andactive modes of InN can be clearly identified as follows: the wurtzite–InN E2 (high) mode at 489 cm−1, E1 (TO) mode at 474 cm−1, A1 (TO)mode at 445 cm−1 and A1 (LO)mode at 589 cm−1 [11,12]. The FWHMof the wurtzite–InN E2 (high) mode was determined by this spectrumand shown in Fig. 2. The sample (e) which is grown using the LT-InN/β-Si3N4 double buffer layer achieves theminimum FWHMof E2 (high)

mode. The Raman results confirmed the high crystalline quality andwurtzite lattice structure of InN films on the Si substrate and supportsthe observation by XRD data.

Fig. 4(i) shows 1 μm×1 μm AFM images of the InN films forsamples (a)–(e), respectively. The root mean square (RMS) roughnessof the InN films is drastically decreased from 14.64 to 5.63 nm whenthe LT-InN buffer layerwas used. Fig. 4(ii) shows the RMS values of theInN films for samples (a) to (e). The nitridation layer grown at lowtemperature (530 °C) gives a smoother surface compared to hightemperature nitridation layer which is grown at 700 °C but crystallin-ity is poorer. The sample (e) which is grown using the LT-InN/β-Si3N4

double buffer layer achieves good crystalline aswell as smooth surfacemorphology.

Fig. 5 shows the absorption spectra of the InN films measured atroom temperature. The nature of the optical interband transition and

Fig. 5. Behavior of (αE)2 as a function of photon energy for samples (a) to (e). Solid linesare extrapolations to the energy axis (dotted lines) to determine the absorption edges.

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value of the energy gap Eg can be determined using the relationderived independently by Tauc et. al. [13] as α(E)E=A(E−Eg)

m,wherem=1/2 for direct transitions andm=2 for indirect transitionsrespectively. A is a disorder parameter nearly independent of thephoton energy E. Eg is approximately taken as the energy band gap.Thus the value of the optical band gap for InN is obtained by plotting(αE)1/m versus E in the high absorption range followed by extrapo-lating the linear region of the plots to (αE)1/m=0. The analysis of ourdata shows that the plots of (αE)1/m versus E gives a linear relationwhich is best fitted withm=1/2. This indicates that the nature of thefundamental interband transition in InN films is direct-allowed. Theoptical band gaps of InN are obtained between ~0.73 to 0.78 eV forsamples (a) to (e) and are shown by the dashed line in Fig. 5, whichclearly demonstrates the characteristic feature of a direct band gapabsorption without ambiguity. In recent literature, the optical bandgap values are reported from 0.65 to 0.85 eV by different researchgroups [1,6]. It is found that the blueshift of absorption edge can beinduced by background electron concentration, and the higherelectron concentration brings the larger blueshift, due to a possibleBurstein–Moss effect.

The double-buffer technique (InN/β-Si3N4) gives good crystallineand smooth surface of InN epilayers, and could be due to followingtwo reasons. Firstly, β-Si3N4 has a lattice constant of 7.602 Å alongthe a axis [14], which is just a little larger than twice that of InN

(a=3.548 Å). If we count twice of the lattice constant of InN as anominal lattice constant, there will be a less lattice mismatch com-pared with that between silicon and InN. In this case, the presenceof silicon nitride greatly reduces the possibility of inducing a highconcentration of dislocations and provides a better platform thansilicon for the growth of InN buffer layer. Secondly, before thenitridation process, we thermally etched the substrate at 900 ° C toremove the passivation layer, which freed enough dangling bonds ofSi existing on the substrate surface. So the silicon nitride is readilyformed on the silicon substrate. In the sameway, the enough danglingbonds of nitrogen on the nitridation layer at high temperature alsoprovided a good platform for the growth of InN buffer layer, whichconsequently offered a good template for the epitaxy of InN epilayers.

4. Conclusion

The structural and optical properties of the InN epilayers on Si withdifferent buffer layers were studied. The narrowest value of theFWHM of XRD rocking curve is observed at ~32′ with the LT-InN/β-Si3N4 double buffer layers. The Raman results confirmed the highcrystalline quality and wurtzite lattice structure of InN films on the Sisubstrate using different buffer layers and the LT-InN/β-Si3N4 doublebuffer layer achieves a minimum FWHM of E2 (high) mode. Theenergy gap of InN films was determined by optical absorption mea-surement and estimated to be in the range of ~0.73–0.78 eV.

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