Characterization of thick HVPE GaN films

Superlattices and Microstructures 36 (2004) 417–424

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Characterization of thick HVPE GaN films

Gérard Noueta,∗, Pierre Ruteranaa, Jun Chenb, Benliang Leic,Haohua Yec, Guanghui Yuc, Ming Qic, Aizhen Lic

aLaboratoire Structures des Interfaces et Fonctionnalité des Couches Minces, UMR CNRS 6176, ENSICAEN,6 Boulevard du Maréchal Juin, 14050 Caen Cedex, France

bLaboratoire Universitaire de Recherche d’Alençon, IUT d’Alençon, 61250 Damigny, FrancecState Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and

Information Technology, Chinese Academy of Sciences, Shanghai 200050, China

Available online 27 October 2004

Abstract

The morphology of the top surface for HVPE GaN layers grown on a MOCVD GaN templatewith a thin LT-AlN interlayer was investigated. This surface is characterized by the formation ofnumerous hillocks associated withscrew dislocations or nanopipes. Their size is large and may reachmore than 1 mm. The rocking curves of the 002 and 102 reflections correspond to a relaxed layer.The HREM images of the as-deposited and annealed interlayers show a perfect atomic structure witha very abrupt AlN/HVPE GaN interface. Thus, the deposition of the LT-AlN layer has promoted thegrowth of an HVPE layer with an excellent crystalline quality.© 2004 Published by Elsevier Ltd

1. Introduction

The nitrides InN, GaN, and AlN are semiconductors with a band gap in the range of0.7–6.3 eV. They are promising materials for applications in the field of optoelectronicdevices, such as in light emitting diodes and laser diodes from red to near ultraviolet,

∗ Corresponding address: Institut des Sciences dela Matiere, et du Rayonnement ESCTM-CRISMAT, UMRCNRS 6508, 6 Boulevard du Maréchal Juin, 14050 Caen Cedex, France. Tel.: +33 2 31 45 26 47; fax: +33 2 3145 26 60.

E-mail address: [email protected] (G. Nouet).

0749-6036/$ - see front matter © 2004 Published by Elsevier Ltddoi:10.1016/j.spmi.2004.09.005

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418 G. Nouet et al. / Superlattices and Microstructures 36 (2004) 417–424

UV detectors, and high temperature and high power devices. Growth of these materials isstill complicated due to the lack of suitable substrates for homoepitaxy. The most commonsubstrate, sapphire, which exhibits a largelattice mismatch, 16%, and a very importantthermal expansion coefficient mismatch of about 34%, gives rise to layers containinghuge densities of threading dislocations, of the order of 109 cm−2. More sophisticatedmethods based on an epitaxial lateral overgrowth process with one or two steps, allow thereaching of lower densities, with records in the region of 5×106 cm−2. In these conditions,the devices are efficient but their lifetime depends strongly on the density of defects inthe GaN film [1]. Recent developments have turned towards the use of a single-crystalbulk material as a substrate to achieve homoepitaxial growth of layers containing fewerdislocations; a material with a density of 105 cm−2 would be considered as excellent. Theclassical route to the production of GaN single crystals needs high hydrostatic pressureof nitrogen, up to 2 GPa, and high temperatures of 1500–1800◦C. The crystals obtainedcontain as few as 102 cm−2 dislocations, but their sizes, in the region of 12 mm, are still alimitation for industrial fabrication of devices [2]. The other possibility is to provide a highquality free standing substrate of GaN layers with a thickness of the order of 300–500µm.HVPE is a growth technique characterized by a very high growth rate, 100–150µm/h.This is a promising technique for industrial production due to its low cost compared tothat of metal–organic chemical vapour deposition (MOCVD) or molecular beam epitaxy(MBE). However, the fast growth of thick layers is still problematic, with the formationof cracks due to the thermal expansion coefficient and lattice parameter mismatch, andreproducibility of the growth of high qualityHVPE layers has not been achieved yet.The experimental conditions ofgrowth differ strongly according to the techniques and thefirst stages may be very important. Improvement of the nucleation in the HVPE techniquehas been obtained by using a variety of processes such as sapphire surface pre-treatment,low temperature buffer layer use, and multiple-step processes [3]. In this paper, we haveinvestigated the influence of a thin AlN interlayer on the morphology and crystallineproperties of the HVPE GaN layers.

2. Experimental

Thick GaN films were grown in a horizontal HVPE reactor, where HCl was introducedinto the Ga source region to produce GaCl as the group III precursor and NH3

was introduced into the growth region as the group V precursor for GaN growth.Sapphire substrates with (0001) orientation were degreased using organic solvents(trichloroethylene, acetone, and methanol) in an ultrasonic cleaner and rinsed withdeionized water. HVPE growth of GaN was performed on an MOCVD template with anLT-AlN interlayer. The template is obtained with a traditional two-step growth, LT-GaNwas used as a low temperature buffer layer, and the GaN epilayer is grown at 1060◦C.The LT-AlN interlayer was prepared by RF-MBE at 400◦C with a 4 nm/min growth rate;the RF power is 450 W. N2 gas is the III source and the flow rate is 0.8 sccm. The totalthickness of the as-grown LT-AlN layer is expected to be about 20 nm. Then, a 14µm thickGaN layer was deposited at 1050◦C on theLT-AlN layer. The HCl flow rate is 15 sccmwith 200 sccm N2 as the carrier gas; the NH3 flow rate is 600 sccm with 200 sccm N2

G. Nouet et al. / Superlattices and Microstructures 36 (2004) 417–424 419

Fig. 1. Optical microstructure of the top surface of the HVPE thick GaN layer without annealing of the AlNinterlayer.

as the carrier gas; an additional 2000 sccm of N2 was used as the maincarrier gas duringgrowth. The growth rate is about 7µm/h. Possible modification of the AlN interlayer wasverified after annealing at high temperature, 1050◦C, for 20 min.

The crystalline quality of the HVPE layers was characterized by means of highresolution X-ray diffraction (HRXRD) using the GaN symmetrical (002) and asymmetrical(102) reflections. The topography of the top surface of the HVPE GaN epitaxial layer wasstudied by means of optical microscopy. Finally, the atomic structure was analysed bytransmission electron microscopy (TEM/HREM) of cross-sectional specimens along the〈112̄0〉 orientation of GaN in order to observe the interfaces edge on.

3. Results and discussion

The optical microscope analysis of the top surface of the specimens, with or withoutannealing of the AlN interlayer, shows the formation of numerous hillocks (Figs. 1and2).Generally, they present a pyramidal growth but, in some cases, some similarity witha Frank–Read source of the double-ended type is observed (Fig. 3). This is a possiblemechanism for the multiplication of the dislocations [4]. The density of the hillocks is ofthe same order of magnitude: 1.5 × 103 (as-deposited AlN) and 2× 103 cm−2 (annealedAlN). The diameter of the largest hillocks may reach 1 mm. The formation of thesehillocks is associated with screw dislocations and nanopipes. Actually, the nanopipe isformed according to Frank’s theory by the increase of the diameter of the dislocationcore to equilibrate the strain energy of the line with the surface energy of the internalinterfaces [5]. Thus, it is expected that the densityof screw dislocations is of the sameorder of magnitude as that of the hillocks. Usually, growth controlled by screw dislocationsleads to the formation of a growth spiral around the dislocation line. However, in thecase of the interaction of two spirals of the same Burgers vector but of opposite sign,the spiral disappears and only terraces without a visible centre are observed [6]. Moreover,the shape of the spiral or terrace may be polygonal with the same symmetry as the surface


Fig. 2. Optical microstructure of the top surface of the HVPE thick GaN layer with annealing of the AlN interlayer.

Fig. 3. Optical microstructure of the top surface of the HVPE thick GaN layer without annealing of the AlNinterlayer, showing some similarity with a Frank–Read source.

(Fig. 4). As the height separating the different terraces is related to the magnitude of theBurgers vector of the screw dislocation or to a multiple of it, the associated steps are notdiscernible in the optical microscope. HVPE growth is carried out at high temperature, sothe mobility of the species on the surface is very high and a mechanism based on a flowmode with propagation of a step on the vicinal surface is generally favoured. However,the observation of the hillocks correspondsto a mechanism of growth controlled by thescrew dislocations. The competition betweenthe two mechanisms depends strongly on thenumber of nucleation sites and the structure of the initial surface on which the growthstarts is thus the relevant factor. When the number of nucleation sites is large enough as aconsequence of the roughness of the surface, a mosaic morphology is expected. In contrast,


Fig. 4. A hillock producing hexagonal symmetry.

Fig. 5. Stacking of the MOCVD GaN layer, AlN interlayer, and HVPE layer without annealing.

the decrease of thenumber of nucleation sites on a smooth surface promotes the growth ofa spiral controlled by the screw dislocations.

Theθ–2θ scan shows for both specimens the 002 reflection and its harmonics; no extrareflection was found confirming the favoured growth along thec-axis. The crystallinequality was determined with the rocking curves; theω-scans give as the FWHM (FullWidth at Half-Maximum) for the specimen without annealing of the AlN interlayer:574 arcsec (002 reflection) and 739 arcsec (102 reflection); and for the specimen withannealing: 482 arcsec and 469 arcsec, respectively. This improvement should be partlyrelated to the relaxation of strain in the thick HVPE GaN layer.

In TEM, an image at low magnification shows the different layers deposited on thesapphire substrate: the MOCVD layer (thickness: 3600 nm), the AlN interlayer (thickness:17 nm), and the initial part of the HVPE layer (thickness: 15µm). The MOCVD layercontains basal stacking faultsand threading dislocations (Fig. 5). At the beginning of the


Fig. 6. The concentration of defects on the top of the AlN interlayer without annealing.

Fig. 7. An HREM image of the AlN interlayer without annealing showing a dislocation (s) crossing the AlNinterlayer.

HVPE layer growth, thenucleation zone contains a rather high density of dislocations,up to 50 nm, and this density decreases fast (Fig. 6). Many basal stacking faults are stillpresent in the HVPE GaN layer.

This microstructure is similar to that obtained in the HVPE GaN layer with an AlNbuffer layer deposited at 1000◦C [7]. At high magnification, the atomic structure may beimaged, showing that the AlN interlayer is well crystallized and that the screw dislocationscoming from the MOCVD GaN layer and crossing it are the main defects. Moreover,the interface between the LT-AlN interlayer and theHVPE GaN layer is perfectly abrupt(Fig. 7).

For the second specimen withthe AlN interlayer annealedat high temperature, thegeneral microstructure is retained, and the thickness of the AlN interlayer is now of 20 nm.Its atomic structure is similar (Fig. 8) but moirefringes are present in some parts (Fig. 9).The annealing could have slightly modified theAlN/HVPE GaN interface, but the variation


Fig. 8. The microstructure of the layersafter annealing of the AlN interlayer.

Fig. 9. The formation of moire fringes in some parts of the AlN interlayer after annealing.

of the HWHM of the rocking curve for the 102 reflection favours a decrease of the densityof edge dislocations in the HVPE thick GaN layer.

4. Summary

HVPE thick GaN layers deposited on an AlN interlayer are characterized by numeroushillocks on the top surface corresponding to a growth controlled by the screw dislocationsresulting in the formation of large grains. As shown by the rocking curves and the HREMimages, they display excellent structural properties.

Acknowledgements

This work is supported by the National High Technology Research and DevelopmentProject of China (Grant No 2001AA311100 and 2002AA305304), and the CNRS/ASCChine2003 project No 14915.


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Characterization of thick HVPE GaN films