Applied Surface Science 255 (2008) 2057–2062

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Applied Surface Science

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Dependence of N2 pressure on the crystal structure and surface quality of AlN thinfilms deposited via pulsed laser deposition technique at room temperature

Gaurav Shukla, Alika Khare *

Department of Physics, Indian Institute of Technology Guwahati, Guwahati 781039, Assam, India

A R T I C L E I N F O

Article history:

Received 27 February 2008

Received in revised form 25 June 2008

Accepted 26 June 2008

Available online 5 July 2008

PACS:

81.05.Ea

81.15.Fg

61.10.Nz

Keywords:

Pulsed laser deposition

Aluminum nitride (AlN)

Reactive pulse laser deposition

Thin films

A B S T R A C T

In this paper, dependence of N2 pressure on the crystal structure and surface quality of AlN thin films

deposited via pulsed laser ablation of Al target in the environment of N2 at room temperature is reported.

The films were analyzed with AFM, SEM, XRD and FTIR for crystal structure and surface morphology. At

higher N2 pressure, the orientation of AlN is (1 0 1) whereas at lower pressure (0 0 2) orientation was

observed. The dependence of PL spectra of pulsed laser deposited AlN thin films on to the crystal structure

and deposition time is reported. The effect of N2 pressure on FTIR spectra of AlN thin film is also reported

in the paper.

� 2008 Elsevier B.V. All rights reserved.

1. Introduction

High quality AlN thin films are emerging as a versatile materialfor the semiconductor industry [1–3]. AlN has a wide band gap(6.2 eV), high thermal conductivity (320 W/m-K) and highresistivity (1013 V/cm2) and exhibits a negative electron affinity[4]. It is used as a light-emitting device in the green–blue–UVspectral range and as UV photodetector [4–6]. AlN films can alsoserve as the gate dielectric in high voltage, high power electronicdevices [7] and can act as buried dielectric layer in Silicon-On-Insulator (SOI) application [8]. In the literature, various techniquesare reported for the deposition of thin films of AlN, such as metalorganic chemical vapor deposition [9], plasma-assisted molecularbeam epitaxy [10], reactive magnetron sputtering [11] and pulsedlaser deposition (PLD) [12,13]. Pulsed laser deposition has beenproven to be a simple technique to fabricate AlN thin films onsilicon and sapphire substrates [14–19]. With PLD, quality andstoichiometry of the deposited thin films can be easily controlled.In the present work, we report deposition of AlN(0 0 2) and (1 0 1)

* Corresponding author. Tel.: +91 361 2582701/2582705;

fax: +91 361 2582749/2690762.

E-mail address: alika@iitg.ernet.in (A. Khare).

0169-4332/$ – see front matter � 2008 Elsevier B.V. All rights reserved.

doi:10.1016/j.apsusc.2008.06.190

single crystal thin films on single crystalline Si(1 0 0) in pure N2

environment using Al as a target at room temperature.

2. Experimental setup

The experimental setup used to deposit the AlN thin films isshown in Fig. 1. The second harmonic Q switched Nd:YAG laser(Model-Quanta systems-HYL101, 400 mJ/pulse in fundamental with8 ns pulse duration and 10 Hz repetition rate) is focused on to thepure Al target with a lens of focal length of 35 cm. The target wasmounted inside the vacuum chamber through a motorized vacuumfeed through and continuously moved in order to avoid piercingwith the repeated shots of laser. The chamber was initially evacuatedto a base pressure of 10�6 Torr and then filled with N2 gas in thepressure range of 10�2 to 102 Torr. When the high power laser isfocused on to the Al target at 458 in the ambient of N2 gas the Al andNitrogen ions are formed which spreads and undergoes the properdynamics to form the AlN which was deposited on ultra sonicallycleaned single crystalline p-type Silicon(1 0 0) wafers placed parallelto and 2 cm apart from the target. The deposition time was keptfrom 20 to 60 min at room temperature. These films of AlN werescanned with AFM (Model-smena NT-MDT) and SEM (LEO-5500) forsurface morphology and XRD (SEIFERT) for the crystal structure.Optical characterizations were done with Photoluminescence(Thermo-spectronic) and FTIR (Horriba Jobin Yuvon).

Fig. 1. Experimental setup.

G. Shukla, A. Khare / Applied Surface Science 255 (2008) 2057–20622058

3. Results and discussion

3.1. XRD analysis

XRD measurements for AlN thin films deposited at various N2

pressures are shown in Fig. 2. Fig. 2(a) and (b) shows XRD

Fig. 2. XRD measurement for AlN thin films deposited at various N2 p

measurements for films deposited at relatively high N2 pressure,100 and 10 Torr respectively. At the higher pressure depositedAlN thin film is highly (1 0 1) oriented. Fig. 2(c) and 2(d) showsXRD measurements for films deposited at relatively low N2

pressure, 10�1 and 5 � 10�2 Torr respectively. The preferredorientation of AlN thin film is (0 0 2) at low pressure. It is also

ressure. (a) 100 Torr; (b) 10 Torr; (c) 10�1 Torr; (d) 5 � 10�2 Torr.

Fig. 3. AFM scanned images of AlN thin films deposited at various N2 pressures: (a) 100 Torr (2 mm � 2 mm); (b) 10 Torr (5 mm � 5 mm); (c) 10�1 Torr (3 mm � 3 mm); (d)

5 � 10�2 Torr (3 mm � 3 mm).

G. Shukla, A. Khare / Applied Surface Science 255 (2008) 2057–2062 2059

observed that deposition of metallic Al and Al2O3 occurs at low N2

pressure. Impurities such as oxygen play a major role at lowambient pressure leading to formation of Al2O3. The FWHMof AlN(1 0 1) and AlN(0 0 2) peak, increases with decreasein N2 pressure. Hence average particle size increases withincrease in N2 pressure for reactive pulsed laser deposited AlNthin films which is also supported by the surface morphologydescribed below.

3.2. Surface morphology (AFM/SEM analysis)

The AFM scans of AlN thin films deposited at various N2

pressure is shown in Fig. 3. Fig. 3(a) and (b) corresponds to AlN thinfilms deposited at higher N2 pressure of 100 and 10 Torrrespectively. It shows the formation of pebble like structure.Fig. 3(c) and (d) corresponds to AFM scan of films deposited atlower N2 pressure of 10�1 and 5 � 10�2 Torr respectively. Thedeposition at lower N2 pressure confirms the nanostructuredgrowth of AlN.

The increase in particle size with N2 pressure is due tocoagulation of particulates at higher pressure. Surface roughnessof deposited AlN thin films decreases with decrease in N2

pressure, from 150 nm rms value for AlN thin films depositedat 100 Torr N2 pressure to 20–30 nm rms value for AlN thinfilms deposited at 5 � 10�2 Torr. This improved surfacequality is due to more uniform expansion of plasma at lowerN2 pressure.

SEM images for AlN thin films deposited at various N2 pressureis shown in Fig. 4. Fig. 4(a)–(d) corresponds to AlN thin filmsdeposited at higher N2 pressure, 100, 50, 10 and 1 Torrrespectively. Fig. 3(e) and (f) corresponds to scanning of filmsdeposited at lower N2 pressure, 10�1 and 5 � 10�2 Torr respec-tively. The SEM analysis also confirms the dependence of surface

morphology on N2 pressure and average particle size decreases atlower pressure.

There is initial columnar growth observed at 10�1 Torr N2

pressure that becomes prominent for AlN thin films deposited at5 � 10�2 Torr N2 pressure. Due to columnar growth at5 � 10�2 Torr N2 pressure, formation of smooth micro cratersand hillocks can be observed on top surface of AlN thin films inFig. 4(f). Fig. 5(a) and (b) shows cross-sectional images of AlN thinfilms deposited at 10�1 and 5 � 10�2 Torr N2 pressure respectively.Fig. 5(b) confirms the c-axis (0 0 2) columnar growth of AlN thinfilms. The thickness of deposited AlN thin films was measured to be3.8 mm at 10�1 Torr and 3.2 mm at 5 � 10�2 Torr.

The dependence of surface quality and orientation of thin filmon to the nitrogen pressure can be explained in terms of plasmadynamics. At low N2 pressure, the mean free path of theconstituent particles of plasma is higher and hence higher kineticenergy can be imparted to the deposited atoms/ions and moleculesat the growing surface of the thin film. Particle bombardment withsufficient kinetic energy results in augmented mobility of thedeposited species thereby rearranging the film surfaces underequilibrium and filling the voids. The wurzite phase of AlN is morestable due to its close-packed stacking than the zincblende phaseand the (0 0 2) orientation have the lowest surface energy [20–22].As a result, islands corresponding to the lowest surface energy cannucleate and grow in an orientation parallel to the substrate toreduce its free energy favouring the growth along (0 0 2)orientation of AlN film. At higher pressures, increased numberof collisions in gaseous phase may lead to a variation in the growthdirection of AlN thin films.

Increase in particle size and thickness can be attributed to thefact that at higher pressure the expansion of plasma is less andhence mobility of reactants decreases and simultaneously particledensity increases. Further c-axis oriented AlN thin films grown by

Fig. 4. SEM images of AlN thin films deposited at various pressures; (a) 100 Torr; (b) 50 Torr; (c) 10 Torr; (d) 1 Torr; (e) 10�1 Torr; (f) 5 � 10�2 Torr.

G. Shukla, A. Khare / Applied Surface Science 255 (2008) 2057–20622060

PLD at N2 pressure of 5 � 10�2 Torr have found to have much bettersurface morphology compared with films deposited at much loweror higher pressure. There is no deposition of AlN below 10�2 Torr ofN2 pressure. Probably the intensity used in the present experimentis low enough so as not to dissociate/breakdown N2 gas moleculesbelow 10�2 Torr. Therefore one concludes that optimum N2

pressure for deposition of (0 0 2) oriented AlN thin films at roomtemperature is of the range of 10�2 Torr.

Fig. 5. Cross-sectional SEM images of AlN thin film

3.3. Photoluminescence

Photoluminescence spectra of reactive pulsed laser depositedAlN thin films recorded using Photoluminescence spectrometer(Thermo-spectronic Aminco Bowman Series 2) with 250 nmexcitation wavelength at room temperature is shown in Fig. 6(a)and (b). Fig. 6(a) shows the PL of AlN thin films deposited for20 min at different pressures and Fig. 6(b) shows PL of AlN thin

s deposited at: (a) 10�1 Torr; (b) 5 � 10�2 Torr.

Fig. 6. (a) Photoluminescence of AlN thin films deposited at various N2 pressure. (b)

Photoluminescence of AlN thin films with different deposition time.

G. Shukla, A. Khare / Applied Surface Science 255 (2008) 2057–2062 2061

films deposited for different deposition time. Further increase in PLintensity is observed with increase in deposition time andinterestingly with decrease in N2 pressure. A broad emissionspectrum ranges from 270 to 471 nm is observed [22,23] withmajor peaks situated around 286 nm (�4.33 eV) and 362 nm

Fig. 7. FTIR spectra of AlN thin films and Si substrate.

(�3.42 eV). The PL emission in the visible region of AlN thin films isdue to the impurities and surface defects. The impurities produceoxygen point defects (OþN), nitrogen vacancies (VN) and variousdefect complexes (V3�

Al � 3� OþN). The visible emission may resultfrom radiative recombination of a photogenerated hole with anelectron occupying the nitrogen vacancies and/or from transitionbetween the shallow level of VN and deep level of (V3�

Al � 3� OþN)defect complexes [24–26].

Increase in PL intensity at low N2 pressure as shown in Fig. 6(a)can be attributed to the higher oxygen point defect [23]. Theincrease in PL intensity with deposition time as shown in Fig. 5(b)is due to increase in film thickness for higher deposition time. Shiftof emission peak towards higher wavelength region is observedwith increase in deposition time and N2 pressure due to the higherparticulate size at higher N2 pressure and deposition time.

3.4. FTIR measurements

Fig. 7 shows FTIR measurements on AlN thin films depositedthrough PLD. Graph A in Fig. 7 shows FTIR spectra of AlN thin filmsdeposited at 10�1 Torr of N2 pressure and graph B shows that of at100 Torr N2 pressure. FTIR spectra of Si(1 0 0) substrate is alsoshown by Graph C. The dominant absorption peak at�674 cm�1 inFTIR spectra is corresponds to the AlN transverse optical [E1(TO)]phonon mode [27,28]. The peak at 611 cm�1 is due to Si substrate.AlN A1(TO) mode is also expected at 610 cm�1 but not visible dueto overlapping with Si peak. It is observed that FWHM of AlN[E1(TO)] mode for films deposited at lower N2 pressure is smallcompared to those deposited at higher pressure. This could be dueto increase in disorder at higher pressure. The FTIR spectra are ingood agreement with the XRD and SEM results, which confirm theformation of large AlN crystallites at high N2 pressure.

4. Conclusion

Highly c-axis oriented AlN thin films have been deposited usingreactive pulsed laser deposition at room temperature. At high N2

pressure highly oriented (1 0 1) AlN films were grown with largecrystallites size. It is observed that orientation of deposited AlNthin films changes from (0 0 2) preferred orientation at low N2

pressure to (1 0 1) preferred orientation at high N2 pressure.Surface morphology of deposited films was dependent on N2

pressure. It is found that average particle size increases with N2

pressure and nanostructured growth of AlN(0 0 2) thin filmsthrough reactive pulsed laser deposition can be achieved at low(<10�1 Torr) N2 pressure. Broad Photoluminescence spectrum isobserved in 270–471 nm range. In FTIR spectra AlN [E1(TO)] modeis observed at 674 cm�1 confirming formation of AlN and change inFWHM of AlN [E1(TO)] mode with N2 pressure is also observed.

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