Crystallographic tilting of AlN/GaN layers on miscut Si (111) substrates

Li Wang n, Fusheng Huang, Zhiyong Cui, Qin Wu, Wen Liu, Changda Zheng, Qinghua Mao,Chuanbing Xiong, Fengyi JiangQ1

National Engineering Research Center for LED on Si Substrate, Nanchang University, No. 235 East Nanjing Road, Jiangxi Province, Nanchang 330047, PR China

a r t i c l e i n f o

Article history:Received 18 July 2013Accepted 10 October 2013

Keywords:Epitaxial growthSemiconductorsCrystal growthChemical vapor deposition

a b s t r a c t

We report on the crystallographic tilting of AlN/GaN layers grown on Si (111) substrates with differentmiscut angles toward [110] direction. The inclination angle of the Si (111), AlN (002) and GaN (002)planes to the sample surface was measured by high resolution X-ray diffraction. We found that the tiltangles of the AlN and GaN layers with respect to the Si (111) plane are significantly larger than thatpredicted by the classical Nagai model. We proposed that in-plane mismatch played an important role inthe tilt epitaxy of AlN/GaN layers on miscut Si (111) substrate.

& 2013 Published by Elsevier B.V.

1. Introduction

Miscut substrates are commonly used in heteroepitaxy. Thereason for the miscut may be intentional, such as to create vicinalsurfaces with well-ordered step–terrace distributions along highsymmetry directions of the crystalline lattice, or the miscut maybe an unintended consequence of a wafer cutting and polishingprocess. Heteroepitaxial layers grown on miscut substrates gen-erally exhibit a crystallographic tilt with respect to the substrate orunderlying layers. This effect is called the Nagai tilt, because it wasfirst observed and discussed by Nagai [1]. The Nagai tilt has beeninvestigated intensively for GaAs and GaP based materials [1–4],due to its important impact on many aspects of the materialproperty.

Growth of III-nitride materials has been one of the mostattractive research fields in the semiconductor research commu-nity over the past two decades, due to their wide applications insolid state lighting and high temperature, and high power electro-nic devices. Nitride films are usually grown on highly mismatchedheterosubstrate such as sapphire, SiC and Si. In Nagai's model, thetilt angle depends on the miscut angle and the mismatch betweenthe layer and the substrate [1,2]. So the Nagai tilt would have asignificant effect in nitride epitaxial layers. However, to date, littleattention has been paid to this effect in these materials. Huang andco-workers reported that AlN/GaN layers on SiC substrates obeyedthe Nagai model strictly but the situation for sapphire/GaN systemis more complicated [5,6]. To explain the observed tilt angle ofGaN layers on miscut sapphire substrate, Huang proposed an

extended model which included the effect of step bunching andvertical misfit dislocation formation. Krysko observed that the tiltangle between InGaN layer and the miscut GaN substrate is 10%smaller compared to the value predicted by the Nagai model [7].

Si (111) is another attractive substrate for GaN based materialsgrowth, due to its availability of large high-quality wafers, lowprice, and good thermal conductivity. However, because of theexcessively large mismatch and the Ga melt back problem [8],direct growth of GaN on Si is very difficult. To obtain high qualityGaN layer for fabrication of light emitting or other electronicdevices, the AlN/GaN buffer structure is widely adopted [9]. TheAlN layer serves as a wetting layer for GaN and can prevent Gamelt back etching. Both the in-plane and the out-of-planemismatch between Si and AlN or GaN are very large, so a greattilting is expected to occur for the III-nitride layers on Si (111).Revealing this effect will be helpful for understanding the growthkinetics and strain relief mechanism of III-nitrides on Si (111)substrate. In this paper, we will report our experimental observa-tion of the crystallographic tilting of AlN/GaN layers on Si (111)substrates with different miscuts, and discuss the role of in-planemismatch on the epitaxial tilt of these layers.

2. Experiments

Three Si (111) substrates with nominal miscut angles of 01, 11and 21 toward [110] direction, respectively, were used in this study.The substrate surface was patterned with 700�700 trench-mesastructure by lithography and dry etching process. Before growth,the Si wafers were cleaned by a standard RCA cleaning method.Then the three substrates were put into a Thomas Swan CCS7 in.�2 in. reactor and the growth procedure was as follows. First,

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n Corresponding author. Tel.: þ86 791 88317916.E-mail address: wl@ncu.edu.cn (L. Wang).

Please cite this article as: Wang L, et al. Crystallographic tilting of AlN/GaN layers on miscut Si (111) substrates. Mater Lett (2013), http://dx.doi.org/10.1016/j.matlet.2013.10.036i

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the substrates were heated to 1100 1C in H2 ambient for a 20-minthermal cleaning. Next, a 1000 Å thick AlN layer was deposited at1060 1C as a buffer. Then, a full-structure LED film containing a3 μm thick Si doped n type GaN layer and six quantum wells wasdeposited.

The structural properties of the samples were studied by a highresolution X-ray diffractometer (Panalytical X'pert PRO). For eachsample, rocking curves were measured at azimuths of 01, 901, 1801and 2701, respectively, by rotating the sample about the azimuthalaxis. At each azimuth, the detector was located to the 2θ positionof Si (111), GaN (002) and AlN (002) planes, respectively, to recordthe rocking curves for these planes. So a total of 36 rocking curveswere measured. From the peak position of these rocking curves,we calculated the inclination angle of all the measured planes withrespect to the sample surface and obtained the tilt angles of theepitaxial layers. To check the error of the measurements, weremounted and re-measured the nominal 01 off sample for severaltimes. The calculated results show that the error is within70.0151.

3. Results and discussion

Fig. 1 shows the rocking curves of Si (111), AlN (002) and GaN(002) planes of the 21 miscut sample. From Fig. 1 we can see thatthe peak positions of all the planes are shifted from theirtheoretical values and varies with the azimuth, indicating aninclination of these planes to the sample surface. The inclinationangle Φ of a measured plane can be calculated by [10]

ΦO ¼ ðφ0�φ180Þ=2 ð1Þ

Φ? ¼ ðφ90�φ270Þ=2 ð2Þ

tan 2Φ¼ tan 2ΦOþ tan 2Φ? ð3Þwhere φ is the peak position of the measured plane at differentazimuths, and ΦO and Φ? are the inclination components in the

direction parallel and perpendicular to the goniometer plane,respectively. The epitaxial tilt can be deduced from the differencein the inclination of the Si (111), AlN (002) and GaN (002) planes.

We listed all the calculated inclination angles and tilt angles inTable 1. The Nagai model is a widely used model for the analysis ofepitaxial tilt, which can be expressed as [1,2]

tan ΔΦ= tan Φ¼ ðCL�CSÞ=CS ð4Þ

where ΔΦ is the tilt angle while CL and CS are the vertical latticeconstants of the epitaxial layer and the substrate, respectively. Themodel predicts that the tilt direction will be away from the surfacenormal (positive tilt) in the case of CL4CS and toward the surfacenormal (negative tilt) in the case of CLoCS. In our case, CL equalsthe bilayer thickness of AlN (2.491 Å) or GaN (2.593 Å) [6] while CSequals the bilayer thickness of the Si substrate (3.135 Å).

Comparing the data listed in Table 1, one can find that all themeasured tilt angles are significantly larger than that predicted byEq. (4). Huang [6] reported that the tilt angle of the GaN layer withrespect to the sapphire substrate is related to the step configura-tion on the substrate surface. When the miscut of the sapphiresubstrate is small, the tilt angle obeys the Nagai model. If themiscut is large enough, the steps on the sapphire substrate surfacetends to bunch up and form multi-bilayer steps, which lead to theformation of vertical misfit dislocation and cause the deviation ofthe tilt angle from the classical Nagai model. However, Huang'smodel is not applicable in our case, because it is well known thatthe surface steps on Si (111) substrate under temperature higherthan 860 1C are single bilayer steps [11]. Inspecting the data shownin Table 1, one can find that although the difference between themeasured tilt angles and the predicted values is quite large, thetan ΔΦ/tan Φ values for both AlN and GaN are approximatelyinvariant for all the samples. This implies that the deviation isirrelevant to the amount of miscut but is related to the mismatch.As described in Refs. [1,2,6], the Nagai model only included theout-of-plane mismatch. However, for epitaxy of GaN, the layerthickness required for full strain relaxation can be up to a fewmicrometers and the strain tends to be relaxed gradually [5]. Thelarge in-plane mismatch between AlN and Si (111) should not beneglected due to the tetragonal deformation. The in-plane latticeconstants of AlN and Si (111) are 3.112 Å and 3.84 Å, respectively.When the AlN layer pseudomorphically grows on the Si (111) plane,the vertical strain εzz¼�0.136 in the AlN layer can be calculatedusing the constants from Ref. [12]. Then, the CL for a strained AlNlayer can be calculated as CL¼(1�0.136)�2.491¼2.152 Å. Thus, thetotal vertical mismatch (CL�CS)/CS¼31.4% is obtained. This value is in

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Fig. 1. Rocking curves of Si (111), AlN (002) and GaN (002) planes of the 21 miscutsample measured at different azimuths.

Table 1Calculated inclination angles and tilt angles. Predicted values are calculated by theclassical Nagai model.

Sample 01 Miscut 11 Miscut 21 Miscut

ΦSi (111) 0.22 1.151 2.123ΦAlN (002) 0.146 0.683 1.351ΦGaN (002) 0.152 0.711 1.39ΔΦAlN/Si �0.074 �0.468 �0.772ΔΦGaN/Si �0.068 �0.44 �0.733tan ΔΦAlN/tan ΦSi (111) (%) �33.50 �40.60 �36.40tan ΔΦGaN/tan ΦSi (111) (%) �31.00 �38.20 �34.50Vertical mismatchAlN/Si (%) 20.50GaN/Si (%) 17.30Predicted tiltAlN/Si �0.045 �0.235 �0.434GaN/Si �0.038 �0.199 �0.367

The unit of the angles in the table is deg.

L. Wang et al. / Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎2

Please cite this article as: Wang L, et al. Crystallographic tilting of AlN/GaN layers on miscut Si (111) substrates. Mater Lett (2013), http://dx.doi.org/10.1016/j.matlet.2013.10.036i

fairly coinciding with the measured tan ΔΦAlN/tan ΦSi (111) values inour experiment.

Based on the above discussion, the epitaxial tilt of AlN and GaNlayer on Si (111) substrate can be depicted as in Fig. 2. At the initialstage of AlN growth, the layer is pseudomorphically grown on theterraces of the Si (111) substrate, the bilayer thickness of AlN iscoherent to CS (Si (111) bilayer thickness) at the step and relaxes toCL (tetragonally deformed AlN bilayer thickness) over the length ofa terrace, then the AlN layer is tilted due to the difference of CS andCL. When the layer thickness exceeds the critical thickness, misfitdislocation begins to form and gradually relaxes the strain.Because the dominant dislocations in GaN are edge dislocationswith a Burgers vector lying in the basal plane [13], they will notcause a tilt of the layer. So the tilt formed at the initial growthstage is permanently preserved in the whole epitaxial layer. Itshould be noted that because the high temperature grown AlN/Si(111) samples usually show an amorphous layer at the interfaceafter growth [14], a direct observation of the atomic arrangementnear the interface of these samples is inaccessible. Hu reportedthat the amorphous layer was induced by the large stress at theinterface when thick GaN was grown [15]. This means that the AlNlayers were under great strain at the early stage of growth, whichis in agreement with our result.

4. Conclusions

We found that the tilt angles of AlN and GaN layers withrespect to the Si (111) plane are significantly larger than thatpredicted by the classical Nagai model. We proposed that in-planemismatch played an important role in the tilt epitaxy of AlN andGaN layers on miscut Si (111) substrate through tetragonaldeformation of the lattice.

Acknowledgments

This study is supported by the National High-TechnologyResearch and Development Program of China under Grant No.2011AA03A101, the National Natural Science Foundation of Chinaunder Grant No. 51072076, and the National Key TechnologyResearch and Development Program of China under Grant No.2011BAE32B01.

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Fig. 2. Schematic diagram of tilting of AlN layer on Si (111) substrate. The tiltformed by the strained AlN layers near the interface is preserved in the wholeepitaxial layer, regardless the introduction of misfit dislocation during the con-sequent growth.

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Please cite this article as: Wang L, et al. Crystallographic tilting of AlN/GaN layers on miscut Si (111) substrates. Mater Lett (2013), http://dx.doi.org/10.1016/j.matlet.2013.10.036i