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Structural and Surface Studies of Undoped Porous GaN Grown onSapphire

 AINORKHILAH MAHMOOD1, 2, a, ZAINURIAH HASSAN1,b,YUSHAMDAN YUSOF1,c, YAM FONG KWONG1, d, CHUAH LEE SIANG3,e 

and NASER MAHMOUD AHMED 1, f  1Nano-Optoelectronics Research and Technology Laboratory, School of Physics, Universiti Sains

Malaysia, 11800 Penang, Malaysia.

2 Department of Applied Sciences, Universiti Teknologi MARA, 13500 Permatang Pauh,Penang, Malaysia.

3Physics Section, School of Distance Education, Universiti Sains Malaysia, 11800 Minden,Penang, Malaysia.

aainor_khilah@yahoo.com.my, bzai@usm.my, cyushamdan@usm.my, dyamfk@usm.my,echuahleesiang@usm.my, f nas_tiji@yahoo.com

Keywords: Porous GaN, Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM),High Resolution X-ray Diffraction (HR-XRD)

Abstract. Owing to its great potential in optoelectronic devices, structural and surface properties of  porous GaN prepared by UV electrochemical etching has been investigated. Scanning electronmicroscopy (SEM), atomic force microscopy (AFM) and high resolution X-ray diffraction (HR-XRD) phi-scan and rocking curves measurements revealed the nature of the pore morphology andnanostructures. SEM micrograph indicated that the shapes of pores for porous sample are nearlyhexagonal. The AFM measurements revealed that the surface roughness increased in the poroussample. X-ray diffraction phi-scan showed that porous GaN sample maintained the epitaxial feature.

Introduction

The wide band gap semiconductor GaN and related materials have received increasing attention inrecent years due to its potential applications for optoelectronic devices operating in the spectralregion from the blue to near-UV and in electronic devices such as high temperature, high power andhigh frequency transistors [1-3]. In addition, the Monte Carlo simulations predict that saturationvelocity in GaN is higher than the values for Si and GaAs [4]. GaN devices are capable to operatenot only at high temperature but also in hostile and harsh environments [5]. Additionally, GaN hasemerged as important materials for high power electronics devices owing to its high breakdownfields [6].

Since the unearthing of porous Si shows augmented luminescence efficiency in 1990 [7], initialefforts to make porous GaN were motivated by the desire to realize similar effect with an ultraviolet(UV) band gap material. Porous GaN necessarily has high surface area, shift of band gap,luminescence intensity enhancement, as well as efficient photoresponse as compared to bulk. Thus,it is possible that porous GaN can be tailored to fabricate new sensing device [8].

Porous GaN can be prepared by dry etching techniques such as ion milling, chemical assisted ion beam etching, reactive ion etching and inductively-coupled plasma reactive ion etching.Conversely, these methods could induce surface damage and moreover they lack the desiredselectivity to the morphology, dopant and composition [9, 10]. The most feasible as a cost effective

method to prepare porous GaN is by electrochemical etching. The main factor in theelectrochemical etching is the electrolyte. Due to their difference in chemical nature, a wide rangeof aqueous electrolytes have been used for GaN. However, it is difficult to identify the optimumetching conditions, e.g. current density, etching time, electrolyte concentration, illuminationconditions, etc. since the quality of GaN surface always differs in various growth runs.

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In the present work, porous GaN was prepared by UV assisted electrochemical etching method.This method was employed in this work due to its advantages such as low processing temperature,low structural damage, process simplicity, versatility, and low processing cost [11]. The sample is

 biased positively and ultra-violet (UV) illumination is used to assist in the generation of electron-hole pairs [8, 12]. In this study, the morphological and structural properties of porous GaN

samples are characterized by scanning electron microscopy (SEM), atomic force microscopy(AFM) and high resolution X-ray diffraction (HR-XRD).

Materials and Methodology

The commercial unintentionally doped (IUD) n-type GaN film grown by metalorganic chemicalvapor deposition (MOCVD) on two inch diameter sapphire (0001) substrate was used in this study.The thickness of GaN film is 6 µm with carrier concentration of ~ 6.05 x 1017 cm-3 as determined byHall effect measurement. The wafer was then cleaved into few pieces. Subsequently boiling aquaregia was used to etch and clean the sample. Porous GaN in this work was generated by UV assistedelectrochemical etching.

In the anodic etching process, platinum was used as cathode electrode; the GaN sample wasconnected to power supply and biased positive with the constant current densities of 20 mA/cm2 for 30 minutes anodization. An ultra-violet (UV) lamp with 500 W and 4 % concentration of KOHelectrolyte were used in this experiment. After chemical treatment, the samples were removed fromthe solution and rinsed with distilled water and dried in ambient air.

Structural properties of porous GaN have been investigated by SEM (JOEL JSM-6460LV) performed at 10 kV. Energy dispersive X-ray analysis (EDX) was used to identify the elementalcomposition of the sample. AFM (Dimension EDGE Bruker), via non-contact tapping mode wasused to determine the surface morphology and surface roughness of the sample at room temperature

and atmospheric pressure. HR-XRD (PANalytical X’pert PRO MRD) was used to assess thecrystalline quality.

Results & Discussion

Scanning electron microscopy (SEM) images of the as grown and porous GaN samples were shownin Fig.1. At low magnification, it can be clearly seen from Fig. 1(b) that the porous samplecomposed of large quantities of high quality hexagonal like. It appears that etching first occurs atthe centre of grain structures with the grain boundaries remain un-etched as grain boundaries aremostly defined by threading dislocation [13]. This leads to the formation of hexagonal trenches withsize ranges from 20 to 180 nm [14]. At high magnification, further observation as shown in Fig.

1(c) demonstrates that the nanopores possess relatively rough surface. Furthermore, it can beobserved that the pores are not distributed uniformly. On the contrary, it is interesting to note thatthe porous GaN prepared by the electrochemical etching method does not always produce similar surface morphology. Several groups [14-19] fabricated porous GaN by anodic etching; porous GaN

 produced by Hartono et al  [13] was covered with hexagonal type of pores, meanwhile GaNfabricated by Yam et al [19] was covered with star, elongated, triangular and squarish type of pores.Other than this, the average pore size was more sensitive to the concentration of electrolyte ascompared to the applied voltage.

The atomic force microscope pictures shown in Fig. 2 exhibited the surface roughness and surfacemorphology of the as grown and porous GaN samples. The scanned area was commonly set at 5 µm

x 5 µm. There were randomly hexagonal shapes and dark spots on the porous GaN surface of thesample as shown in Fig. 2(c). On the other hand, the surface roughness measured in root meansquare (RMS) was an important parameter to definite surface/interface roughness. The rmsroughness of the as grown and porous samples was found to be 5.21 and 60.08 nm, respectively,

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indicating that the surface roughness increased in the porous GaN samples. It is noteworthy that, theresults obtained from both SEM and AFM measurements were in perfect agreement for the asgrown and porous GaN as seen in Fig. 1.

Fig. 1: SEM images of different samples, (a) as grown, (b) porous GaN, and (c) porous GaN for high magnification

Fig. 2: AFM micrographs of the as grown and porous GaN samples showing different surfacetopography, (a) 2D as grown image, (b) 3D as grown image, (c) 2D porous image, and (d) 3D

 porous image

The crystalline quality and lattice parameters were assessed and determined by using HR-XRD. Fig.

3 shows the Phi-scan on asymmetric GaN (101 2) reflection plane. Six-fold azimuthal symmetrycould be clearly observed for both as-grown and porous GaN samples. Six-fold symmetric isconsistent with the wurtzite (hexagonal) crystal structure [20].

The XRD measurements performed on the symmetric (0002) incorporated with asymmetric (101 2)scan mode was a reliable technique to characterize the crystal quality of the porous samples whichwas reflected in the FWHM of the peaks [21]. The defects within the structure of the porous sample

caused significant broadening in both the symmetric (0002) and asymmetric (10 1 2) scan modes,which have been fitted with widths of 345.6 arcsec and 540.0 arcsec, respectively. Moreover, it is

interesting to note that the measured width of the (101 2) rocking curve was larger than that of the(0002) rocking curve for both samples. The broadening of the asymmetric diffractions was

(a) (b)

(c) (d)

(a) (b) (c)

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indicative of a defective structure with large pure edge threading dislocation (TD) content, in view

of the fact that (0002) peak only broadened by screw or mixed TD whereas the (10 1 2) peak was broadened by all the TDs [22]. For the (0002) scan mode, the lattice parameter c of the as grownand porous GaN were determined to be 5.1831 Å and 5.1827 Å, respectively. Meanwhile, from the

(101 2) scan mode, the lattice parameter a of the as grown and porous GaN were determined to be3.1804 Å and 3.1759 Å, respectively. The change of lattice constant a and c indirectly measured thedegree of distortion of hexagonal cell, this could be observed from the change of  c/a ratio as

 presented in Table 1. Fig. 4 illustrates very clearly FWHM for porous GaN. 

Fig. 3: X-ray diffraction phi-scan profiles of as grown and porous GaN using (101 2) plane.

Table 1 : The diffraction peak positions and lattice constants of the as grown and porous GaNsamples measured by HR-XRD

Sample(0002) (101 2) Lattice constant (Å)

Peak (°)

FWHM(arcsec)

Peak (°) 

FWHM(arcsec)

c a c/a ratio 

As grown 17.291 331.2 24.087 511.2 5.1831 3.1804 1.6207Porous GaN 17.293 345.6 24.105 540.0 5.1827 3.1759 1.6319

Fig.4 XRD rocking curves of as grown and porous GaN using (101 2) plane

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Summary

In conclusion, the studies showed that porosity could influence the structural and surfacemorphology of the GaN. SEM images suggested that etching has significant effect on the size andshape of the pores. AFM measurements exhibited that surface roughness increased in porous

sample. XRD measurements showed that the FWHM of symmetric (0002) and asymmetric (101 2)

increased for the porous sample; however, the amount of increase was relatively small.

Acknowledgement

Support from FRGS 203/PFIZIK/6711143, USM Postgraduate Grant (USM-RU-PRGS)1001/PFIZIK/843012 and Universiti Teknologi MARA is gratefully acknowledged.

References[1] S.C. Jain, M. Willander, J. Narayan and R. Van Overstraeten: Journal of Applied Physics Vol.

87 (2000), p. 965-1006.[2] D. Feiler, R.S. Williams, A.A. Talin, H. Yoon and M.S. Goorsky: Journal of Crystal Growth

Vol. 171 (1997), p. 12-20.

[3] G. Landwehr, A. Waag, F. Fischer, H.J. Lugauer and K. Schüll: Physica E: Low-dimensionalSystems and Nanostructures Vol. 3 (1998), p. 158-168.

[4] B. Gelmont, K. Kim and M. Shur: Journal of Applied Physics Vol. 74 (1993), p. 1818-1821.[5] J.-Y. Duboz and M.A. Khan, in: B. Gil (Ed.), Group III Nitrides Semiconductors Compounds,

Clarendon Press, Oxford, 1998, pp. 343-390.[6] S.J. Pearton, F. Ren, A.P. Zhang and K.P. Lee: Materials Science and Engineering: R: Reports

Vol. 30 (2000), p. 55-212.[7] L.T. Canham: Applied Physics Letters Vol. 57 (1990), p. 1046-1048.[8] X. Li, Y.-W. Kim, P.W. Bohn and I. Adesida: Applied Physics Letters Vol. 80 (2002), p. 980-

982.[9] C.Y. I. Adesida, A. T. Ping, F. Khan, L. T. Romano, and G. Bulman: MRS Internet J. Nitride

Semiconductor Res. Vol. 4S1 (1999), p.[10] C. Youtsey, I. Adesida, L.T. Romano and G. Bulman: Applied Physics Letters Vol. 72

(1998), p. 560-562.[11] Y. Alifragis, G. Konstantinidis, A. Georgakilas and N. Chaniotakis: Electroanalysis Vol. 17

(2005), p. 527-531.[12] D.J. Diaz, T.L. Williamson, I. Adesida, P.W. Bohn and R.J. Molnar: Journal of Applied

Physics Vol. 94 (2003), p. 7526-7534.[13] H. Hartono, C. B.Soh, S.J. Chua and E.A. Fitzgerald: Phys. Stat. Sol. (c) Vol. 4 (2007), p.

2572.[14] A. Mahmood, N.M. Ahmed, Z. Hassan, F.K. Yam, S.K.M. Bakhori, Y. Yusof and L.S.

Chuah: Advanced Materials Research Vol. 364 (2012), p. 90-94.[15] L.S. Chuah, Z. Hassan, C.W. Chin and H.A. Hassan: World Academy of Science,Engineering and Technology Vol. 55 (2009), p. 16.

[16] A. Mahmood, Z. Hassan, F.K. Yam and L.S. Chuah: Optoelectron. Adv. Mater. Rapid Comm.Vol. 4 (2010), p. 1316-1320.

[17] A. Mahmood, Z. Hassan, Y.F. Kwong, S.K.M. Bakhori and C.L. Siang: AIP ConferenceProceedings Vol. 1341 (2011), p. 45-47.

[18] A.P. Vajpeyi, S.J. Chua, S. Tripathy, E.A. Fitzgerald, W. Liu, P. Chen and L.S. Wang:Electrochemical and Solid-State Letters Vol. 8 (2005), p. G85-G88.

[19] F.K. Yam, Z. Hassan and S.S. Ng: Thin Solid Films Vol. 515 (2007), p. 3469-3474.[20] A. Nahhas, H.K. Kim and J. Blachere: Applied Physics Letters Vol. 78 (2001), p. 1511-1513.

[21] B. Heying, X.H. Wu, S. Keller, Y. Li, D. Kapolnek, B.P. Keller, S.P. DenBaars and J.S.Speck: Applied Physics Letters Vol. 68 (1996), p. 643-645.[22] S. Korçak, M.K. Öztürk, S. Çörekçi, B. Akaoğlu, H. Yu, M. Çakmak, S. Sağlam, S. Özçelik 

and E. Özbay: Surface Science Vol. 601 (2007), p. 3892-3897.

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