HRXRD Study of Epitaxial GaN Films Grown on ZnO

HRXRD Study of Epitaxial GaN Films Grown on ZnO Buffer Layers by Magnetron Sputtering P. Mohanta, D. Singh, R. Kumar, T. Ganguli, R. S. Srinivasa et al. Citation: AIP Conf. Proc. 1349, 675 (2011); doi: 10.1063/1.3606038 View online: http://dx.doi.org/10.1063/1.3606038 View Table of Contents: http://proceedings.aip.org/dbt/dbt.jsp?KEY=APCPCS&Volume=1349&Issue=1 Published by the American Institute of Physics.

Related Articles Electron distribution and scattering in InAs films on low-k flexible substrates Appl. Phys. Lett. 100, 232103 (2012) Influence of flow rate on different properties of diamond-like nanocomposite thin films grown by PECVD AIP Advances 2, 022132 (2012) Direct assembly of three-dimensional mesh plasmonic rolls Appl. Phys. Lett. 100, 193107 (2012) Spectroscopic ellipsometry model for optical constant of NiSi formed on silicon-on-insulator substrates J. Appl. Phys. 111, 093501 (2012) A formation mechanism for ultra-thin nanotwins in highly textured Cu/Ni multilayers J. Appl. Phys. 111, 073526 (2012)

Additional information on AIP Conf. Proc. Journal Homepage: http://proceedings.aip.org/ Journal Information: http://proceedings.aip.org/about/about_the_proceedings Top downloads: http://proceedings.aip.org/dbt/most_downloaded.jsp?KEY=APCPCS Information for Authors: http://proceedings.aip.org/authors/information_for_authors

Downloaded 08 Jun 2012 to 59.162.23.79. Redistribution subject to AIP license or copyright; see http://proceedings.aip.org/about/rights_permissions

HRXRD Study of Epitaxial GaN Films Grown on ZnO Buffer Layers by Magnetron Sputtering P. Mohanta, D. Singh, R. Kumar#, T. Ganguli#, R. S. Srinivasa †, S. S. Major* Center For Research in Nano-Technology and Science, †Department of Metallurgical Engineering and Materials Science,*Department of Physics, Indian Institute of Technology Bombay, Mumbai – 400076, India # Semiconductor Laser Section, RRCAT, Indore-452013, India Email: [email protected] Abstract. GaN films were deposited on ZnO buffer layers of d ifferent thicknesses over sapphire substrates by reactive sputtering of a GaAs target in 100 % nitrogen at 700 oC. HRXRD measurements and the corresponding micro-structural analysis show that with increase in buffer layer thickness from 25 to 200 nm, the coherence length along the growth direction remains unaffected. In contrast, along lateral direction, it decreases drastically with increase in buffer layer thickness. Over a narrow range of buffer layer thickness (50-100 nm), high quality epitaxial GaN films were grown with crystallite tilt of 0.6 - 0.7 o and microstrain of 6 -7 x 10 -4. Keywords: GaN, Epitaxy, Magnetron sputtering, HRXRD PACS: 81.15.cd, 81.15.-z

at a pressure of 8 10-3 mbar and flow rate of 5 SCCM was used as the sputtering-cum-reactive gas. The details have been described elsewhere [5]. HRXRD measurements were carried out in phi ( ), omega ( ) and -2 scan geometries using PANalytical X’Pert MRD System.

INTRODUCTION High quality GaN films are important because of their applications in a wide range of optoelectronic and microelectronic devices. Device quality GaN films are usually grown epitaxially, using MOCVD or MBE techniques. However, during the last few years magnetron sputtering has also been utilized to deposit epitaxial GaN films on sapphire substrates [1]. There are also a few reports on the growth of epitaxial GaN films on ZnO buffer layers (BL) over sapphire substrates by MOCVD [2], MBE [3] and PLD [4]. The improvement in structural quality of these films has been attributed to the fact that both ZnO and GaN have hexagonal wurtzite structure, with a lattice mismatch ~1.8%. In the present work, GaN films were deposited on ZnO BL of varying thickness over sapphire by reactive sputtering of a GaAs target in 100 % nitrogen. Micro-structural characterization of the GaN films has been carried out by high resolution X-ray diffraction (HRXRD).

RESULT AND DISCUSSION GaN (101) reflection

3500

Intensity (a.u.)

2800

2100

1400

700

0 -100

-50

0

50

(degree)

100

150

200

250

FIGURE 1.

scan of (101) reflection from GaN film deposited on 100 nm ZnO BL over sapphire.

EXPERIMENTAL DETAILS ZnO BL of different thicknesses (25, 50, 100 and 200 nm) were deposited on sapphire substrates by rf magnetron reactive sputtering of a zinc target in argonoxygen ambient. Subsequently, 500-600 nm thick GaN films were deposited by rf magnetron reactive sputtering of a 3 inch GaAs target with 100 % nitrogen

A typical scan of asymmetric (101) reflection for the GaN film deposited on ~100 nm ZnO BL over sapphire substrate is shown in figure 1. The presence of six peaks in scan reveals six fold symmetry of the hexagonal lattice, indicating in-

Solid State Physics, Proceedings of the 55th DAE Solid State Physics Symposium 2010 AIP Conf. Proc. 1349, 675-676 (2011); doi: 10.1063/1.3606038 © 2011 American Institute of Physics 978-0-7354-0905-7/$30.00

675


plane orientation and hence, suggests that the GaN film grows epitaxially on ZnO buffer suffer substrate. The ω and ω-2θ scans for the symmetric (0002), (0004) and (0006) reflections were carried out on allthe GaN films deposited on ZnO BL of different thicknesses. A typical ω and ω-2θ scans of (002) reflection are shown in figure 2. The corresponding Williamson-Hall plots of qx vs q and qz vs q are shown in figure 3. The lateral coherence length (LCL)and tilt were estimated from the intercept and slope of the linear qx vs q plots and are listed in table 1.

0.040

Intensity (a.u.)

GaN/ZnO (50 nm)/S

FWHM=0.688o

GaN/ZnO (100 nm)/S

FWHM=0.693o

GaN/ZnO (200 nm)/S

-8

-6

-4

-2

qx(Å-1) qz(Å-1)

0.00340

2

4

Intensity (a.u.)

-0.4

-0.2

GaN/ZnO (25 nm)/S GaN/ZnO (50 nm)/S GaN/ZnO (100 nm)/S GaN/ZnO (200 nm)/S

6

8

0.2

0.45

0.60

0.75

0.90

-1

1.05

1.20

78

0.69

60

48

0.59

55

35

0.69

56

7

2.11

54

1.84 x 10-3 7.77 x 10-4 6.20 x 10-4 3.01 x 10-3

thickness of buffer layers, the lateral coherence length ( lateral crystallite size) decreases drastically, in case of thicker buffer layers. Further, buffer layers of 50 -100 nm shows small values of crystallite tilt and microstrain. It is noticed that if the buffer layer is too thin or too thick, it cannot relax the strain which is due to large differences between the lattice constants and thermal coefficients. The reductions in the quality of the GaN epilayers with ZnO BL of 200 nm thickness and above have been consistently observed and a detailed investigation is being carried out. Overall, it can be concluded that the thickness of the ZnO BL has a significant effect on the epitaxial quality of GaN films and the optimal thickness of ZnO BL is 50 - 100 nm for the high quality epitaxial growth of GaN on sapphire substrates.

FWHM=0.107o

0.0

0.30

TABLE 1. Characteristics parameters of GaN films deposited on ZnO BL over sapphire substrate. Sample LCL Tilt VCL Micro details (nm) (degree) (nm) strain

FWHM=0.079o

GaN/ZnO(200 nm)/S

0.15

symmetric scans along (a) ω -axis (q x), (b) ω-2θ -axis (qz) for (0002), (0004) and (0006) reflections.

FWHM=0.083o

GaN/ZnO(100 nm)/S

(002)

0.00170

FIGURE 3. Williamson-Hall plots, obtained from the

FWHM=0.081o

GaN/ZnO(50 nm)/S

(006) (004)

Reciprocal lattice vector q ( Å )

(degree) GaN/ZnO(25 nm)/S

b

0.00255

0.00000 0.00

(002) reflection

b

(002)

0.00085

FWHM=2.848o

0

0.016

(004)

0.000

FWHM=0.720o

GaN/ZnO (25 nm)/S

(006)

GaN/ZnO(25 nm)/S GaN/ZnO(50 nm)/S GaN/ZnO(100 nm)/S GaN/ZnO(200 nm)/S

0.024

0.008

(002) reflection

a

a

0.032

0.4

(degree)

FIGURE 2. (a) ω and (b) ω-2θ scans of (0002) reflections for GaN films deposited on ZnO BL. It is also found that the crystallite tilt is smallest for the films deposited on a 50 nm BL and attains a large value of 2.11o for the film deposited on 200 nm BL. Similarly, the vertical coherence length (VCL) and micro-strain were estimated from the intercept and slope of the linear Williamson-Hall plots of qz vs q and are listed in table 1. The vertical coherence lengths remain practically same as the buffer layer thickness increases. The micro-strain in the films deposited on 50 and 100 nm BL are 7.77x10-4 and 6.20 x 10-4 respectively, which are much smaller than the values of 1.84 x10-3 and 3.01x10-3 for the films deposited on 25 and 200 nm BL. Micro-strain values in the range of (2-4) x10-4 have been reported for epitaxial GaN films on sapphire substrate grown by MOCVD [2]. The above results show that while the vertical coherence length practically unaffected by the

REFERENCES 1. 2. 3. 4. 5.

Y. Daigo and N. Mutsukura, Thin Solid Films 483, 3842 (2005). R. Chierchia, T. Bottcher, H. Heinke, S. Figge and D. Hommel, J. Appl. Phys. 93, 8918-8925 (2003). G. Popovici, W. Kim, A. Botchkarev, H. Morkoc and J. Solomon, Appl. Phys. Lett. 71, 3385-3388 (1997). X. W. Sun, R. F. Xiao and H.S. Kwok, J. Appl. Phys. 84, 5776 -5779 (1998). B. S. Yadav, S. Singh, T. Ganguly, R. Kumar, S.S. Major and R. S. Srinivisa, Thin Solid Films 517, 488493 (2008).

676
