research article stress and grain boundary properties of ...obtained by christie et al. [...

Research ArticleStress and Grain Boundary Properties of GaN Films Prepared byPulsed Laser Deposition Technique

D Ghosh1 S Hussain2 B Ghosh1 R Bhar1 and A K Pal1

1 Department of Instrumentation Science Jadavpur University USIC Building Calcutta 700 032 India2UGC-DAE CSR Kalpakkam Node Kokilamedu 603104 India

Correspondence should be addressed to A K Pal msakp2002yahoocoin

Received 9 January 2014 Accepted 12 February 2014 Published 17 March 2014

Academic Editors Z Jiang and N Martin

Copyright copy 2014 D Ghosh et alThis is an open access article distributed under theCreative CommonsAttribution License whichpermits unrestricted use distribution and reproduction in any medium provided the original work is properly cited

Polycrystalline gallium nitride films were successfully deposited on fused silica substrates by ablating a GaN target using pulsedNd-YAG laser Microstructural studies indicated an increase in the average crystallite size from sim8 nm to sim70 nm with the increase insubstrate temperature from 300K to 873K during deposition The films deposited here were nearly stoichiometric XPS studiesindicated two strong peaks located at sim11166 eV and sim395 eV for Ga2p

32and a N1s core-level peak respectively The films

deposited at substrate temperature above 573K are predominantly zinc blende in nature PL spectra of the films deposited athigher temperatures were dominated by a strong peak at sim32 eV FTIR spectra indicated a strong and broad absorption peakcentered sim520 cmminus1 with two shoulders at sim570 cmminus1 and 584 cmminus1 Characteristic Raman peak at sim531 cmminus1 for the A

1(TO) mode

is observed for all the films Grain boundary trap states varied between 31times 1015 and 7times1015mminus2 while barrier height at the grainboundaries varied between 124meV and 3714meV Stress in the films decreased with the increase in substrate temperature

1 Introduction

Gallium nitride (GaN) is a promising material for appli-cations in optoelectronic devices such as ultraviolet-blue-green light-emitting diodes (LEDs) and laser diodes (LDs)due to its direct wide band gap and good thermal stabilityIt is equally suitable for high-temperature and high-powerelectronic applications Depending on the growth conditionsGaN crystallizes either in the stable hexagonal (wurtzite 120572-phase) or metastable cubic (zinc-blende 120573-phase) polytypesThe prevalent deposition techniques for depositing GaN thinfilms are mainly metal-organic chemical vapor deposition(MOCVD) and molecular beam epitaxy (MBE) Recentlyamorphous and polycrystalline GaN thin films depositedusing magnetron sputtering technique [1ndash7] and laser abla-tion [8ndash10] technique have also been reported

GaN thin films with wurtzite structure were deposited byChen et al [5] by using reactive DC magnetron sputteringtechnique The films exhibited a polycrystalline structurewith a strong (002) orientation and were utilized as activechannel layer to produce top-gate n-type thin-film transistors

(TFTs) Highly textured polycrystalline GaN films havingan average grain size of several hundred angstroms wereobtained by Christie et al [7] using an RF plasma-assistedmolecular beam epitaxy system on quartz substrates Zouet al [6] reported the deposition of GaN films on glasssubstrates by the middle frequency magnetron sputteringmethod Amorphous gallium nitride (a-GaN) films withthicknesses of 5 and 300 nm were deposited by Wang etal [8] on n-Si (100) substrates by pulsed laser deposition(PLD) and their field emission (FE) propertieswere reportedHong et al [9] reported the deposition of GaN films on Si(400) wafers by a pulsed laser deposition technique Theyobserved that although the film and the substrate did not haveany interface epitaxy out-of-plane texture of the film wascontrollable Lopez et al [10] reported a process to produceGaN films activated with Eu3+ ions by the pulsed laserdeposition (PLD) technique The film was found to containsubmicron size crystals with hexagonal shape Sanguino etal [11] deposited GaN thin films on prenitridated c-planesapphire at two substrate temperatures (600∘C and 650∘C) bycyclic pulsed laser deposition Increase in the surfacemobility

Hindawi Publishing CorporationISRN Materials ScienceVolume 2014 Article ID 521701 10 pageshttpdxdoiorg1011552014521701

2 ISRNMaterials Science

for a better crystal growth was associated with increasingsubstrate temperature during deposition Tong et al [12]deposited GaN thin films on Si (111) substrates using PLDassisted by gas discharge The structure surface morphologyand optoelectronic properties of the films were found to bestrongly dependent on the substrate temperature and thelaser incident energy

One of the current issues in GaN research is to deposithigh quality GaN polycrystalline thin films using inexpensivesubstrate and at a relatively lower substrate temperature Trapstates at the grain boundary region and residual stress wouldmodulate the electron transport process in these polycrys-talline films Excepting the attemptmade byChowdhury et al[3] on Be-doped polycrystalline GaN films none of the abovereports addressed the grain boundary phenomena associatedwithGaNpolycrystalline films adequatelyThe density of trapstates at the grain boundary (119876

119905sim 1 times 10

15mminus2) and thebarrier height (119864

119887sim 16 eV) reported by Chowdhury et al

were significantly higher than those one would expect for agood device material However they did not report the stresscontent in the filmsHence it is apparent that the informationonGaN in polycrystalline thin film form reported so far is farfrom being comprehensive and complete Thus there existsstill a definite need to synthesize polycrystalline GaN filmsby appropriate deposition technique so that films containinglesser amount of density of trap states with associated lowerbarrier height and stress can be achieved This would ensureunhindered electron transport in the device grade materialthus synthesised

A successful attempt of synthesizing GaN in polycrys-talline form by ablating GaN (99995) target under con-trolled deposition condition is reported hereThe synthesizedfilms were characterized by measuring optical microstruc-tural photoluminescence and compositional propertiesInformation related to stress and grain boundary propertieshas also been documented

2 Experimental Details

A PLD system evacuated to a level of 10minus7 Torr was used toablate a GaN target (9999 purity Sigma Aldrich) by usinga NdYAG laser (wavelength (120582) = 355 nm pulse duration(120591) = 10 ns frequency (119891) = 10Hz) for depositing GaN filmson fused silica (quartz) and Si (100) substrates (for FTIRmeasurements) GaN films were deposited at four differentsubstrate temperatures 473K 673K 773K and 873K Timeof deposition for all the films was sim9 minutes The laserfluence was maintained at 8 Jcm2 and the laser beam wasfocused with an 119891 = 23 cm glass lens on the target at an angleof 45∘ with respect to the normal The target was rotated at10 rpm to avoid fast drilling The distance between the targetand the substrate was maintained at sim65 cm

Compositional information was obtained by EDAX(Oxford Instrumentsrsquo INCA Energy 250 Microanalysis Sys-tem) and Photoelectron Spectroscopy (XPS) XPS measure-ments were carried out using a Ms SPECS (Germany) make

spectrometer Al-K120572 linewas used as the X-ray source at148674 eV The anode was operated at a voltage of 15 kV andsource power level was set to 400W An Ar ion source wasalso provided for sputter-etch cleaning of specimens It wasoperated at 2 kV and 50120583A This value varied dependingon the sample Spectra were collected using the DLSEGD-PHOIBOS-HSA3500 analyzer The surface morphology ofthe films was studied by Field Emission Scanning ElectronMicroscope (FESEM) (Carl Zeiss SUPRA 55) X-ray diffrac-tion (XRD) studieswere carried out by usingRigakuMiniFlexXRD (0154 nm CuK

120572line) to obtain the microstructural

information Photoluminescence (PL) measurements wererecorded at 300K by using a 300W xenon arc lamp as theemission source A Hamamatsu photomultiplier along witha 14m monochromator was used as the detecting systemFTIR spectrawere recorded in the range of 400ndash4000 cmminus1 byusing a Nicolet-380 FTIR spectrometer Raman spectra wererecorded using Renishaw inVia micro-Raman spectrometerusing 514 nm Argon laser

3 Results and Discussion

31 Microstructural Study GaN films were deposited onfused silica substrates and Si (100) substrate at differentsubstrate temperature (119879

119904) The FESEM pictures of four

representative films deposited at 473K 673K 773K and873K are shown in Figures 1(a)ndash1(d) respectively TheFESEM images (Figures 1(a)ndash1(d)) reveal that the films arecompact with well-dispersed polycrystals constituting thefilms It was observed that the grain sizes (119863) increased (insetof Figure 1(b)) with the increase in substrate temperaturesduring deposition The grain size varied between 8 nm to70 nm with increasing substrate temperature It may beindicated here that the films grown here are compact andthe grain size obtained for these films was significantly largerthan those reported for GaN polycrystalline films preparedby sputtering technique [3] Hong et al [9] reported a grainsize sim100ndash300 nm for the GaN films deposited by PLDtechnique on Si (400) wafers kept at 900∘C Comparing thesubstrate temperature during deposition grain size obtainedhere compares favorably with those obtained by Hong et alThe grain size of the films reported in this communicationis larger than that reported by Lopez et al [10] for theEu3+ doped GaN thin films grown on (0001) oriented Al

2O3

substrates by the PLD growth techniqueA representative histogram of a film deposited at 873K

is shown in the inset of Figure 1(d) which shows a narrowdistribution of the grain size Gradual increase in grain sizewas observed for films as the substrate temperature wasincreased from 300K to 623K after which rapid grain growthwas observed XRD pattern of a representative GaN filmdeposited at 119879

119904= 773K is shown in the inset of Figure 1(c) It

may be mentioned here that the XRD patterns are dominatedby sharp peaks corresponding to reflections from (101) (102)and (004) planes of GaN The peak at sim73∘ became sharperfor films deposited at 873K at the expense of other peaksThis would mean that films deposited above 773K becamepreferentially oriented in (004) direction

ISRNMaterials Science 3

100nm

(a)

500 750 10000

20

40

60

80

Gra

in si

ze (n

m)

100nm Ts (K)

(b)

20 40 60 800

25

50

75

100(1

01)

Inte

nsity

(au

)

(102

)

(004

)

100nm

2120579 (deg)

(c)

40 50 60 70 8005

101520

Grain size (nm)100nm

Num

ber o

f gra

ins i

n1120583

m2

(d)

Figure 1 FESEM pictures for four representative GaN films deposited at 119879119904 (a) 473K (b) 673K (inset shows the variation of grain size

with substrate temperature) (c) 723K (inset shows the corresponding XRD trace) and (d) 873K (inset shows the corresponding grain sizedistribution)

32 XPS Studies Figure 2(a) shows the general survey XPSspectrum of a representative GaN film deposited at 873KThe general survey scans of all the films were found to bethe same A low intense peak for C1s could be located atsim28803 eV This is shifted by sim363 eV from its standardvalue towards higher binding energy The survey scan isdominated by two strong peaks at sim1117 eV (Figure 2(b)) and11437 eV (Figure 2(c)) for Ga2p

32 and Ga2p12 core-level

spectra respectively [13 14] The above intense peak sim1117 eVwould correspond to GandashN bond in GaN There are a fewlower intensity peaks for Ga3d

52and Ga3p

12at sim21 eV and

107 eV respectivelyThe peak at 107 eV could be deconvoluted(inset of Figure 2(c)) in two peaks located at sim1162 eV and1095 eV The peak at sim1162 eV could be identified as arisingdue to Ga3p

12and the peak at sim1095 eV is due to Ga3p

32

The peak sim537 eV overlaps with that of O1s Oxygen peakarises due to physisorbed oxygen at the surface Low intensitypeak of N1s around sim395 eV is also observed (Figure 2(d))This peak is associated with the binding energies of N as GandashN [15]

33 Photoluminescence Studies PL spectra recorded at 80Kand at an excitation of 34 eV for the films deposited at 473Kand 873K are shown in Figures 3(a) and 3(b) respectivelyPL spectra for films deposited at lower substrate temperatures(Figure 3(a)) contained three peaks located atsim32 eV 291 eV

and sim263 eV while films deposited above 773K indicateda single peak at sim32 eV (Figure 3(b)) The peak at sim32 eVcould be associated with near band edge luminescencewhile the luminescence peaks at sim263 eV and 291 eV forfilms deposited at lower substrate temperatures could beidentified due to transitions between deep donor and shallowacceptor pairs The origin of the deep donor may be due tonitrogen vacancy or Ga interstitial site-related defects Thedeep acceptor levels arise due to Ga vacancy (119881Ga) or galliumvacancy with oxygen on a nitrogen site complex

34 Band Gap and Optical Constants The absorption coef-ficients (120572) of GaN films were determined by measuringtransmittance and reflectance in these films [16 17] Filmsdeposited at higher substrate temperature were more absorb-ing (Figure 3(c)) than those deposited at lower temperaturesThis may be due to compactness of the films and lessernumber of trap states at the grain boundariesThe absorptioncoefficient (120572) may be written as a function of the incidentphoton energy (ℎ]) as follows

120572 = (

119860

ℎ]) ℎ] minus 119864

119892

119898

(1)

where 119860 is a constant and varies for different transitionsindicated by different values of119898 and119864

119892is the corresponding

band gap of the material


0 200 400 600 800 1000 12000

10

20

30

40

C1s

Relat

ive i

nten

sity

(cs

)

Binding energy (eV)

N1s

O1s

Ga(

l mm

)

O(K

LL)

Ga3

d 52

Ga3

p 12

Ga2p32

Ga2

p 12

(a)

1108 1112 1116 1120 1124

20

30

40

50

60

Relat

ive i

nten

sity

(cs

)

Binding energy (eV)

Ga2p32

(b)

1130 1140 1150

24

28

32

36

Relat

ive i

nten

sity

(cs

)

Binding energy (eV)

100 110 1200

1

2

3

4

Relat

ive i

nten

sity

(cs

)

Binding energy (eV)

Ga3p12

Ga2p12

(c)

375 380 385 390 395 400 405 410

0

1

2

3

4

5

Relat

ive i

nten

sity

(cs

)

Binding energy (eV)

N1s

(d)

Figure 2 XPS spectra of a representative GaN film deposited at 873K (a) general survey (b) Ga2p32

core-level spectra (c) Ga2p12 core-level

spectra (inset shows the spectra for Ga3p12) and (d) N1s core-level spectra

Now (1) may be rewritten as

ln (120572ℎ]) = ln119860 + 119898 ln (ℎ] minus 119864

119892) (2)

[

119889 (ln120572ℎ])119889 (ℎ])

] =

119898

ℎ] minus 119864

119892

(3)

Equation (3) indicates that a plot of 119889[ln(120572ℎ])]119889[ℎ]] versusℎ] will show a divergence at ℎ] = 119864

119892from which a rough

estimate of 119864119892may be obtained and as such by using (2) the

value of 119898 can easily be evaluated from the slope of the plotof ln(120572ℎ]) versus ln(ℎ] minus 119864

119892) Figure 3(d) (inset) shows the

plot of ln(120572ℎ]) versus ln(ℎ] minus 119864

119892) for a representative film

deposited at 873K The value of 119898 was obtained from theslope sim051 which indicates a direct transition to be present inthe film The band gap was determined by extrapolating thelinear portion of the plot of (120572ℎ])2 versus ℎ] (Figure 3(d))and was calculated to be sim327 eV The band gaps obtainedfor films deposited at different substrate temperatures (119879

119904) are

shown in Table 1 and the variation of 119864119892with 119879

119904is shown in

the inset of Figure 3(c) It could be observed that the filmsdeposited at temperatures higher than 573K had band gapsim32 eV to 327 eV Films deposited at lower temperaturesindicated gradual increase in band gap sim29 eV to 311 eV Itmay be noted here that the fall in the transmittance versuswavelength curve (not shown here) was sharper for filmsdeposited at higher substrate temperatures

The optical constants (refractive index 119899 and extinctioncoefficient 119896) of the polycrystalline GaN films were deter-mined from the transmittance versus wavelength traces byusing the Kramers-Kronig (KK) model [18] and the modifiedKKmodel given byXue et al [19] Using the values for the realpart of the refractive index (119899) and the extinction coefficient(119896) the real and complex part of the dielectric constant (120576

1

and 1205762 resp) can be estimated by using the relations

120576

1= 119899

2(120582) minus 119896

2(120582)

120576

2= 2119899 (120582) 119896 (120582)

(4)


300 350 400 450 500 550

0

1

2

3

4

5PL

inte

nsity

(au

)

32 e

V

293

eV

263

eV

120582 (nm)

(a)

350 400 450 5000

2

4

6

8

PL in

tens

ity (a

u)

120582 (nm)

(b)

200 400 600 800 1000 12000

2

4

6

8

10

Tran

smiss

ion

()

120582 (nm)

(c)

minus10 minus08 minus06 minus04120

121

122

123

0

2

4

6

8

10

12

14

(120572h)

2(times10

10cm

minus2

eV2)

m = 05

ln(120572h)

ln(h minus Eg)

1 2 3 4 5 6 7 8h (eV)

(d)

Figure 3 PL spectra for films deposited at (a) 473K and (b) 873K (c) transmittance (119879119903) spectra and (d) plots of (120572ℎ])2 versus ℎ] for a

representative GaN film deposited at 873 K Inset of (c) shows variation of band gap (119864

119892) with substrate temperature (119879

119904) and inset of (d)

shows the plot of ln(120572ℎ]) versus ln(ℎ] minus 119864

119892)

The values of 119899 and 119896 obtained as above are shown inFigures 4(a) and 4(b) respectively The refractive indices forthese GaN films were found to vary from 205 to 235 withinthe measured energy range Their dielectric constant variedfrom 375 to 55 as shown in Figures 4(c) and 4(d)

35 FTIR and Raman Studies FTIR spectra for all the filmsdeposited at different substrate temperatures on Si (100) werefound to be dominated by a strong and broad absorptionpeak centered sim520 cmminus1 with two shoulders at sim570 cmminus1and 584 cmminus1 Figure 5(a) shows the FTIR spectra for arepresentative polycrystalline GaN films deposited at 119879

119904=

873K The broad peak sim520 cmminus1 may be ascribed to thepresence of peaks due to GandashN stretchingmode at sim570 cmminus1and 119864

1(LO) phonon or 119860

1(LO) phonon mode at sim584 cmminus1

[20] In addition to these peaks presence of absorption peaksat sim990 cmminus1 and 1521 cmminus1 could be due to the SindashO bondsoriginating from the substrate (silicon)ss

Raman spectra for a mixed cubic and hexagonal GaNare generally governed by the relations between the phononmodes of the two modifications The wurtzite structure ofGaN arises from the zincblende structure by compressingthe crystal along the (111) axis which becomes the 119888-axis ofthe hexagonal phase Also it may happen that changing thestacking of layers along this axis would result in a doublingof the number of atoms in a primitive unit cell This wouldculminate in splitting of 119879

2mode of the cubic phase into

the 119860

1and 119864

1modes of the hexagonal phase Since the

hexagonal distortion is small the above splitting will also begenerally small


400 500 600 700

18

20

22

24

26

473 K573 K673 K

773 K873 K

n

120582 (nm)

(a)

400 500 600 70000

02

04

06

573 K673 K

773 K873 K

k

120582 (nm)

(b)

400 500 600 700

3

4

5

6

7

473 K573 K673 K

773 K873 K

120576 1

120582 (nm)

(c)

400 500 600 70000

05

10

15

20

25

573 K673 K

773 K873 K

120576 2

120582 (nm)

(d)

Figure 4 (a) Plots of 119899 versus 120582 (b) plots of 119896 versus 120582 (c) plots of 1205761versus 120582 and (d) plots of 120576

2versus 120582 of a representative GaN film

GaN films deposited at lower substrate temperaturesare predominantly hexagonal in structure and cubic GaNcontent in the films increases as the deposition temperatureis increased Thus one would expect relevant signatures inRaman modes in such films Figure 5(b) shows the Ramanspectra of a GaN films deposited at 873K The Ramanspectrum showed a first-order Raman active phononmode atsim531 cmminus1 for 119860

1(TO) modes Three broad peaks spreading

over 570 cmminus1 to 680 cmminus1 have their centers at 571 cmminus1616 cmminus1 and 644 cmminus1 corresponding to the first order highfrequency 119864

2 overtone of 119864

1(TO) and 119860

1(TO) overtone

modes respectively The strongest 1198642(high) phonon line in

the spectrum reflects the characteristics of the hexagonal

crystal phase of the GaN film The Raman peak at 672 cmminus1would arise from the optical-phonon branch at the zoneboundary

It is known that at high defect densities wave vec-tor conservation in the Raman scattering process breaksdown and phonons from the entire Brillouin zone can beobserved in the Raman spectrum [21] Thus the Ramanspectrum would reflect the total phonon density of statesdue to disorder-activated Raman scattering (DARS) TheRaman peak at 672 cmminus1 probably arises from the optical-phonon branch at the zone boundary Siegle and coworkers[22] have reported second-order Raman scattering experi-ments on hexagonal and cubic GaN covering the acoustic


500 1000 1500 2000 2500000

004

008

012

016

020

024

Abso

rban

ce (a

u)

520

990

1521

Wavenumber (cmminus1)

(a)

400 500 600 700 800

0

20

40

60

80

64461

6

57153

1

Inte

nsity

(au

)

672

Raman shift (cmminus1)

(b)

Figure 5 (a) FTIR and (b) Raman spectra of a representative GaN film deposited at 873K

and the optical overtone spectral region The hexagonalmodes lying between the first-order peaks at sim616 cmminus1and sim644 cmminus1 could be attributed to the overtone pro-cesses of the highest acoustic-phonon branch either at thezone boundary (119871 point) or from the 119861 mode at the Γ

point

36 Grain Boundary Studies from Below-Band-Gap Absorp-tion Studies GaN films deposited here could be seen toconsist of aggregates of randomly distributed grains andgrain boundary regions Size of the grains increased asthe substrate temperature during deposition was increased(inset of Figure 1(b)) The grain boundary region wouldcontain a large amount of defect states which become chargedafter trapping free carriers from the neighbouring grains Inaddition to these intrinsic defect states at the grain boundaryregions of the GaN films a considerable amount of thermalstress due to the mismatch of thermal expansion coefficientsof the film (120572film) and substrate (120572subs) is bound to appearin these films This would result in fluctuating nature of thepotential [23ndash25] The band gap (119864

119892) would in turn also

have a fluctuating nature This would result in additionalindirect optical transitions below the fundamental band gapof the materialThe details of the processes have already beenreported elsewhere [23ndash25] Following the detailed procedurereported by Maity et al [23] the relevant grain boundaryparameters like density of trap states (119876

119905) and barrier height at

the grain boundary (119864119887) were computed from the theoretical

fit of normalized absorption coefficients 1205721205720below the band

gap regionFigure 6(a) shows representative plots of the experimen-

tal data of normalized absorption coefficients 1205721205720for a rep-

resentative GaN film deposited at 873K It may bementionedhere that the contribution due to the thermal disorder ata specific temperature (at which the optical absorption hasbeen recorded) is fixed the ultimate shape of 120572120572

0versus

(119864

119892minus ℎ]) plot will depend on the amount of the intrinsic

disorder or the density of the defect states present in the filmThe grain boundary parameters (119876

119905and 119864

119887) were estimated

from the best fit theoretical curve (shown in Figure 6(a))corresponding to the experimental data of 120572120572

0versus (119864

119892minus

ℎ]) 1205720being the value of 120572 at 119864

119892 The values of119876

119905and 119864

119887are

given in Table 1Variation of119876

119905and 119864

119887with grain size (ie with substrate

temperature during deposition) is depicted in Figure 6(b) Itcould be observed that the density of trap states (119876

119905) at the

grain boundaries decreased from 7times10

15mminus2 to 31times1015mminus2for films deposited at higher temperatures These values aresmaller than those reported for the rf sputtered films [3] Thebarrier height at the grain boundaries (119864

119887) was significantly

smaller compared to those reported by Chowdhury et al [3]As the films are high resistive the number density of chargecarriers will be small The films would be fully depleted ofcharge carriers when the density of trap states at the grainboundaries is large for films deposited at lower temperaturesAs the number density of trap states decreased for filmsdeposited at higher substrate temperatures probability of thetrap states to get fully filled by the charge carriers will increaseleaving behind the grains partially depleted This has beenreflected by the exponential fall in 119864

119887with the grain size

37 Determination of Stress Earlier reports by various work-ers indicated that the material properties (eg stress strainand barrier height) are very much influenced by the presenceof trap states at the grain boundaries which are in facthighly insulating regions The details of the technique havealready been discussed elsewhere [24] It has been observedin the earlier section that grain boundaries in polycrystallineGaN films effectively control the electron transport as wellas optical absorbance Besides the above grain boundaryeffects mechanical stress due to lattice dilation was seen tomodulate the lattice ordering at the grain and grain boundaryregion This would modulate the below-band-edge opticalabsorption processes as a whole in the form of electrostatic


00 02 04 06 08 1000

02

04

06

08

10

Experimental plot

120572120572

0

120572M120572M0120572E120572E0

120572M120572M0 + 120572E120572E0

Eg minus h (eV)

(a)

10

15

20

25

30

35

40

0 10 20 30 40 50 60 70

3

4

5

6

7

Grain size (nm)

Eb

(meV

)

Eb

Qt

Qt

(times10

15cm

minus2)

(b)

0 10 20 30 40 50 60 70010

015

020

025

030

035

0403 4 5 6 7

Stre

ss (G

Pa)

Grain size (nm)

Grain sizeQt

1015) cmminus2Qt (

(c)

Figure 6 (a) Variation of 1205721205720versus (119864

119892minusℎ]) for a representative GaN film deposited at 873K -998771- theoretical best fit curves (b) Variation

of trap state density (119876119905) and barrier height (119864

119887) with grain size and (c) variation of stress (119878) with grain size (mdash◼mdash) and density of trap state

(119876119905) (mdash998771mdash) at grain boundary region for GaN films deposited at 873K

fluctuations of the band edge [26ndash28] and hence informationon the stress could also be generated from the above

The disorders in the film would result in considerableamount of indirect optical transitions for photon energy ℎ] lt119864

119892which in turnwould produce a broad absorption band tail

in contrast to the sharp absorption edge produced in a single-crystalline material The specific shape of the absorptionband tail which depends on the initial and final states ofthe transitions concerned will be determined by the intrinsicstrain (120575119886119886)gb The intrinsic strain could be determined bythe density of the defect states at the grain boundary regionof the film since the other contributions in the total strain

would remain constant for a particular substrate material at aspecific temperature

Now considering contributions from both the thermaland grain boundary effects the strain (120575119886119886) and stress (119878) inthe GaN films may be expressed as

(

120575119886

119886

)

total= (

120575119886

119886

)

gb+ (

120575119886

119886

)

th

119878total = 119878gb + 119878th

(5)

Here 119878gb (120575119886119886)gb and 119878th (120575119886119886)th correspond to thegrain boundary and thermal components of stress and strain


Table 1 Different quantities related to GaN films obtained from thisstudy

Sample numberand substratetemperature(119879119904)

119864

119892

(eV)

Grain sizefrom SEM

(nm)

Λ

119889

(nm)119876

119905

(1015 mminus2)119864

119887

(meV)Stress(GPa)

GaN-1473K 290 8 383 7 372 037

GaN-2573K 311 12 355 66 332 031

GaN-3673K 321 14 332 60 262 025

GaN-4773K 324 35 304 52 181 017

GaN-5873K 327 70 289 31 124 011

respectivelyThe thermal stress arises when the film is cooledafter deposition The stress due to grain boundaries will ingeneral be proportional to the total grain boundary areawhich in turn would depend inversely on the average grainsize [25 28 29] We have considered the lattice dilation atfinite temperature as due to phonon vibration and defectstates in the polycrystalline GaN films [23 25 27]

From the experimentally obtained values of residualstrain (120575119886119886) obtained as above and by knowing the standardvalues of Youngrsquos modulus119884 (GPa) and the Poissonrsquos ratio (120574)for a given film material the true stress (119878) can be obtainedfrom (6) as shown below

119878 = [

119884

1 minus 120574

](

120575119886

119886

) (6)

Stresses in the films thus evaluated are shown in Figure 6(c)It may be noted that the stress decreased from 038GPa to01 GPa for films deposited at higher temperature that is withincrease in grain size (Table 1 andFigure 6(c))This result is incommensurate with the fact that at a reasonable depositionrate the adatoms will have increased mobility with increasedsubstrate temperature during deposition to facilitate graingrowth Further with increased grain size grain boundarieswould decrease resulting in decrease in the density of trapstates

4 Conclusion

Polycrystalline films of GaN were deposited with differentgrain sizes by varying the substrate temperature during pulselaser deposition Grain size varied between 8 to 70 nm asthe substrate temperature during sputtering changed from300K to 873K XPS studies indicated two strong peakslocated at sim11166 eV and sim395 eV for Ga2p

32 and N1s core-level spectra respectively Photoluminescence measurementat 300K exhibited a strong peak at sim291 eV followed by a lowintensity peak for band edge luminescence at sim320 eV Theluminescence at sim263 eV could be identified due to transi-tions between deep donor and shallow acceptor pairs FTIRspectra were dominated by a strong and broad absorption

peak centered sim520 cmminus1 with two shoulders at sim570 cmminus1and 584 cmminus1 First-order Raman active phonon mode atsim513 cmminus1 for 119860

1(TO) modes was observed Additionally

three broad peaks spreading over 570 cmminus1 to 680 cmminus1 thatappeared in Raman spectra have their centers at 570 cmminus1614 cmminus1 and 644 cmminus1 corresponding to the first-orderhigh frequency119864

2 overtone of119864

1(TO) and119860

1(TO) overtone

modes respectively Density of trap states at the grain bound-ary (119876

119905) changed from 7 times 10

minus15mminus2 to 31 times 10

minus15mminus2 andthe barrier height (119864

119887) decreased from 3717meV to 124meV

as the grain size increased with the increase in depositiontemperature The stress decreased from 038GPa to 01 GPafor films deposited at higher temperature

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

Acknowledgments

The authors wish to thank the Department of Science andTechnology Government of India for sanctioning financialassistance under PURSE Scheme of Jadavpur Universityfor executing this programme D Ghosh and B Ghoshwish to thank the Department of Science and TechnologyGovernment of India and UGC-DAE-CSR respectively forgranting them the fellowships

References

[1] E C Knox-Davies S R P Silva and J M Shannon ldquoPropertiesof nanocrystalline GaN films deposited by reactive sputteringrdquoDiamond and Related Materials vol 12 no 8 pp 1417ndash14212003

[2] T Maruyama and H Miyake ldquoGallium nitride thin filmsdeposited by radio-frequencymagnetron sputteringrdquo Journal ofVacuum Science and Technology A Vacuum Surfaces and Filmsvol 24 no 4 pp 1096ndash1099 2006

[3] M P Chowdhury R K Roy B R Chakraborty and A K PalldquoBeryllium-doped polycrystalline GaN films optical and grainboundary propertiesrdquoThin Solid Films vol 491 no 1-2 pp 29ndash37 2005

[4] K Kubota Y Kobayashi and K Fujimoto ldquoPreparation andproperties of III-V nitride thin filmsrdquo Journal of Applied Physicsvol 66 no 7 pp 2984ndash2988 1989

[5] R Chen W Zhou and H S Kwok ldquoTop-gate thin-filmtransistors based onGaN channel layerrdquoApplied Physics Lettersvol 100 no 2 Article ID 022111 2012

[6] CWZouH JWangM L Yin et al ldquoPreparation ofGaNfilmson glass substrates by middle frequency magnetron sputteringrdquoJournal of Crystal Growth vol 311 no 2 pp 223ndash227 2009

[7] V A Christie S I Liem R J Reeves V J Kennedy A Mark-witz and S M Durbin ldquoCharacterisation of polycrystallinegallium nitride grown by plasma-assisted evaporationrdquo CurrentApplied Physics vol 4 no 2ndash4 pp 225ndash228 2004

[8] F Y Wang R Z Wang W Zhao S XueMei W Bo and Y HuildquoField emission properties of amorphous GaN ultrathin filmsfabricated by pulsed laser depositionrdquo Science in China SeriesF Information Sciences vol 52 no 10 pp 1947ndash1952 2009


[9] J Hong Y Chang Y Ding Z L Wang and R L SnyderldquoGrowth of GaN films with controlled out-of-plane texture onSi wafersrdquoThin Solid Films vol 519 no 11 pp 3608ndash3611 2011

[10] N P Lopez J H Tao J McKittrick et al ldquoEu3+ activatedGaN thin films grown on sapphire by pulsed laser depositionrdquoPhysica Status Solidi (C) Current Topics in Solid State Physicsvol 5 no 6 pp 1756ndash1758 2008

[11] P Sanguino M Niehus L V Melo et al ldquoCharacterisation ofGaN films grown on sapphire by low-temperature cyclic pulsedlaser depositionnitrogen rf plasmardquo Solid-State Electronics vol47 no 3 pp 559ndash563 2003

[12] X L Tong Q G Zheng S L Hu Y X Qin and Z H DingldquoStructural characterization and optoelectronic properties ofGaN thin films on Si(111) substrates using pulsed laser depo-sition assisted by gas dischargerdquo Applied Physics A MaterialsScience and Processing vol 79 no 8 pp 1959ndash1963 2004

[13] J F Moulder W F Stickle P E Sobol and K D BombenHandbook of X-Ray Photoelectron Spectoscopy Edited by JChastain Perkin-Elmer Eden Prairia Minn USA 1992

[14] D Briggs and M P Seah Practical Surface Analysis JohnWileyamp Sons New York NY USA 1979

[15] S I Bahn C M Lee S J Lee J I Lee C S Kim and SK Noh ldquoDetermination of the Al mole fraction in epitaxialALXGa1minusxNGaN (x lt 025) heterostructuresrdquo Journal of theKorean Physical Society vol 43 no 3 pp 381ndash385 2003

[16] J C Manifacier J Gasiot and J P Fillard ldquoA simple method forthe determination of the optical constants n k and the thicknessof a weakly absorbing thin filmrdquo Journal of Physics E ScientificInstruments vol 9 no 11 pp 1002ndash1004 1976

[17] D Bhattacharyya S Chaudhuri A K Pal and S K Bhat-tacharya ldquoBandgap and optical transitions in thin films fromreflectance measurementsrdquo Vacuum vol 43 no 4 pp 313ndash3161992

[18] J I Pankove Optical Processes in Semiconductors Prentice-Hall Englewood Cliffs NJ USA

[19] S W Xue X T Zu W G Zheng H X Deng and Z XiangldquoEffects of Al doping concentration on optical parameters ofZnOAl thin films by sol-gel techniquerdquo Physica B CondensedMatter vol 381 no 1-2 pp 209ndash213 2006

[20] M Mizushima T Kato Y Honda M Yamaguchi and NSawaki ldquoInfrared reflectance in GaNAlGaN triangular stripesgrown on Si(111) substrates by MOVPErdquo Journal of the KoreanPhysical Society vol 42 pp S750ndashS752 2003

[21] W Limmer W Ritter R Sauer B Mensching C Liu andB Rauschenbach ldquoRaman scattering in ion-implanted GaNrdquoApplied Physics Letters vol 72 no 20 pp 2589ndash2591 1998

[22] H Siegle G Kaczmarczyk L Filippidis A P LitvinchukA Hoffmann and C Thomsen ldquoZone-boundary phononsin hexagonal and cubic GaNrdquo Physical Review BmdashCondensedMatter andMaterials Physics vol 55 no 11 pp 7000ndash7004 1997

[23] A B Maity S Chaudhuri and A K Pal ldquoModification ofthe absorption edge due to grain boundaries and mechanicalstresses in polycrystalline semiconductor filmsrdquo Physica StatusSolidi (B) vol 183 no 1 pp 185ndash191 1994

[24] A B Maity M Basu S Chaudhuri and A K Pal ldquoStress andmicrohardness in polycrystalline thin films from below-band-gap absorption studiesrdquo Journal of Physics D Applied Physicsvol 28 no 12 pp 2547ndash2553 1995

[25] A B Maity D Bhattacharyya S Chaudhuri and A K PalldquoAbsorption tail of polycrystalline semiconductor filmsrdquo Vac-uum vol 46 no 3 pp 319ndash322 1995

[26] V I Gavrilenko ldquoElectronic structure and optical properties ofpolycrystalline cubic semiconductorsrdquo Physica Status Solidi (B)vol 139 no 2 pp 457ndash466 1987

[27] J Szczyrbowski ldquoOn the shape of the absorption tail inamorphous solidsrdquo Physica Status Solidi (B) vol 96 no 2 pp769ndash774 1979

[28] J Tauc ldquoAbsorption edge and internal electric fields in amor-phous semiconductorsrdquo Materials Research Bulletin vol 5 no8 pp 721ndash729 1970

[29] R W Hoffman ldquoStresses in thin films the relevance of grainboundaries and impuritiesrdquoThin Solid Films vol 34 no 2 pp185ndash190 1976

Submit your manuscripts athttpwwwhindawicom

ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

CorrosionInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Polymer ScienceInternational Journal of



CeramicsJournal of


CompositesJournal of

NanoparticlesJournal of



International Journal of

Biomaterials


NanoscienceJournal of

TextilesHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Journal of

NanotechnologyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of

CrystallographyJournal of


The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014


CoatingsJournal of

Advances in

Materials Science and EngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Smart Materials Research



MetallurgyJournal of


BioMed Research International

MaterialsJournal of


Nano

materials


Journal ofNanomaterials


for a better crystal growth was associated with increasingsubstrate temperature during deposition Tong et al [12]deposited GaN thin films on Si (111) substrates using PLDassisted by gas discharge The structure surface morphologyand optoelectronic properties of the films were found to bestrongly dependent on the substrate temperature and thelaser incident energy

One of the current issues in GaN research is to deposithigh quality GaN polycrystalline thin films using inexpensivesubstrate and at a relatively lower substrate temperature Trapstates at the grain boundary region and residual stress wouldmodulate the electron transport process in these polycrys-talline films Excepting the attemptmade byChowdhury et al[3] on Be-doped polycrystalline GaN films none of the abovereports addressed the grain boundary phenomena associatedwithGaNpolycrystalline films adequatelyThe density of trapstates at the grain boundary (119876

119905sim 1 times 10

15mminus2) and thebarrier height (119864

119887sim 16 eV) reported by Chowdhury et al

were significantly higher than those one would expect for agood device material However they did not report the stresscontent in the filmsHence it is apparent that the informationonGaN in polycrystalline thin film form reported so far is farfrom being comprehensive and complete Thus there existsstill a definite need to synthesize polycrystalline GaN filmsby appropriate deposition technique so that films containinglesser amount of density of trap states with associated lowerbarrier height and stress can be achieved This would ensureunhindered electron transport in the device grade materialthus synthesised

A successful attempt of synthesizing GaN in polycrys-talline form by ablating GaN (99995) target under con-trolled deposition condition is reported hereThe synthesizedfilms were characterized by measuring optical microstruc-tural photoluminescence and compositional propertiesInformation related to stress and grain boundary propertieshas also been documented

2 Experimental Details

A PLD system evacuated to a level of 10minus7 Torr was used toablate a GaN target (9999 purity Sigma Aldrich) by usinga NdYAG laser (wavelength (120582) = 355 nm pulse duration(120591) = 10 ns frequency (119891) = 10Hz) for depositing GaN filmson fused silica (quartz) and Si (100) substrates (for FTIRmeasurements) GaN films were deposited at four differentsubstrate temperatures 473K 673K 773K and 873K Timeof deposition for all the films was sim9 minutes The laserfluence was maintained at 8 Jcm2 and the laser beam wasfocused with an 119891 = 23 cm glass lens on the target at an angleof 45∘ with respect to the normal The target was rotated at10 rpm to avoid fast drilling The distance between the targetand the substrate was maintained at sim65 cm

Compositional information was obtained by EDAX(Oxford Instrumentsrsquo INCA Energy 250 Microanalysis Sys-tem) and Photoelectron Spectroscopy (XPS) XPS measure-ments were carried out using a Ms SPECS (Germany) make

spectrometer Al-K120572 linewas used as the X-ray source at148674 eV The anode was operated at a voltage of 15 kV andsource power level was set to 400W An Ar ion source wasalso provided for sputter-etch cleaning of specimens It wasoperated at 2 kV and 50120583A This value varied dependingon the sample Spectra were collected using the DLSEGD-PHOIBOS-HSA3500 analyzer The surface morphology ofthe films was studied by Field Emission Scanning ElectronMicroscope (FESEM) (Carl Zeiss SUPRA 55) X-ray diffrac-tion (XRD) studieswere carried out by usingRigakuMiniFlexXRD (0154 nm CuK

120572line) to obtain the microstructural

information Photoluminescence (PL) measurements wererecorded at 300K by using a 300W xenon arc lamp as theemission source A Hamamatsu photomultiplier along witha 14m monochromator was used as the detecting systemFTIR spectrawere recorded in the range of 400ndash4000 cmminus1 byusing a Nicolet-380 FTIR spectrometer Raman spectra wererecorded using Renishaw inVia micro-Raman spectrometerusing 514 nm Argon laser

3 Results and Discussion

31 Microstructural Study GaN films were deposited onfused silica substrates and Si (100) substrate at differentsubstrate temperature (119879

119904) The FESEM pictures of four

representative films deposited at 473K 673K 773K and873K are shown in Figures 1(a)ndash1(d) respectively TheFESEM images (Figures 1(a)ndash1(d)) reveal that the films arecompact with well-dispersed polycrystals constituting thefilms It was observed that the grain sizes (119863) increased (insetof Figure 1(b)) with the increase in substrate temperaturesduring deposition The grain size varied between 8 nm to70 nm with increasing substrate temperature It may beindicated here that the films grown here are compact andthe grain size obtained for these films was significantly largerthan those reported for GaN polycrystalline films preparedby sputtering technique [3] Hong et al [9] reported a grainsize sim100ndash300 nm for the GaN films deposited by PLDtechnique on Si (400) wafers kept at 900∘C Comparing thesubstrate temperature during deposition grain size obtainedhere compares favorably with those obtained by Hong et alThe grain size of the films reported in this communicationis larger than that reported by Lopez et al [10] for theEu3+ doped GaN thin films grown on (0001) oriented Al

2O3

substrates by the PLD growth techniqueA representative histogram of a film deposited at 873K

is shown in the inset of Figure 1(d) which shows a narrowdistribution of the grain size Gradual increase in grain sizewas observed for films as the substrate temperature wasincreased from 300K to 623K after which rapid grain growthwas observed XRD pattern of a representative GaN filmdeposited at 119879

119904= 773K is shown in the inset of Figure 1(c) It

may be mentioned here that the XRD patterns are dominatedby sharp peaks corresponding to reflections from (101) (102)and (004) planes of GaN The peak at sim73∘ became sharperfor films deposited at 873K at the expense of other peaksThis would mean that films deposited above 773K becamepreferentially oriented in (004) direction


100nm

(a)

500 750 10000

20

40

60

80

Gra

in si

ze (n

m)

100nm Ts (K)

(b)

20 40 60 800

25

50

75

100(1

01)

Inte

nsity

(au

)

(102

)

(004

)

100nm

2120579 (deg)

(c)

40 50 60 70 8005

101520


Num

ber o

f gra

ins i

n1120583

m2

(d)






52and Ga3p

12at sim21 eV and



32





120572 = (

119860

ℎ]) ℎ] minus 119864

119892

119898

(1)





0 200 400 600 800 1000 12000

10

20

30

40

C1s

Relat

ive i

nten

sity

(cs

)

Binding energy (eV)

N1s

O1s

Ga(

l mm

)

O(K

LL)

Ga3

d 52

Ga3

p 12

Ga2p32

Ga2

p 12

(a)

1108 1112 1116 1120 1124

20

30

40

50

60

Relat

ive i

nten

sity

(cs

)

Binding energy (eV)

Ga2p32

(b)

1130 1140 1150

24

28

32

36

Relat

ive i

nten

sity

(cs

)

Binding energy (eV)

100 110 1200

1

2

3

4

Relat

ive i

nten

sity

(cs

)

Binding energy (eV)

Ga3p12

Ga2p12

(c)

375 380 385 390 395 400 405 410

0

1

2

3

4

5

Relat

ive i

nten

sity

(cs

)

Binding energy (eV)

N1s

(d)





ln (120572ℎ]) = ln119860 + 119898 ln (ℎ] minus 119864

119892) (2)

[

119889 (ln120572ℎ])119889 (ℎ])

] =

119898

ℎ] minus 119864

119892

(3)









119904) are


119904is shown in



1


120576

1= 119899

2(120582) minus 119896

2(120582)

120576

2= 2119899 (120582) 119896 (120582)

(4)


300 350 400 450 500 550

0

1

2

3

4

5PL

inte

nsity

(au

)

32 e

V

293

eV

263

eV

120582 (nm)

(a)

350 400 450 5000

2

4

6

8

PL in

tens

ity (a

u)

120582 (nm)

(b)

200 400 600 800 1000 12000

2

4

6

8

10

Tran

smiss

ion

()

120582 (nm)

(c)


121

122

123

0

2

4

6

8

10

12

14

(120572h)

2(times10

10cm

minus2

eV2)

m = 05

ln(120572h)

ln(h minus Eg)

1 2 3 4 5 6 7 8h (eV)

(d)






119892)



119904=







the 119860

1and 119864




400 500 600 700

18

20

22

24

26

473 K573 K673 K

773 K873 K

n

120582 (nm)

(a)

400 500 600 70000

02

04

06

573 K673 K

773 K873 K

k

120582 (nm)

(b)

400 500 600 700

3

4

5

6

7

473 K573 K673 K

773 K873 K

120576 1

120582 (nm)

(c)

400 500 600 70000

05

10

15

20

25

573 K673 K

773 K873 K

120576 2

120582 (nm)

(d)







1(TO) and 119860

1(TO) overtone






500 1000 1500 2000 2500000

004

008

012

016

020

024

Abso

rban

ce (a

u)

520

990

1521


(a)

400 500 600 700 800

0

20

40

60

80

64461

6

57153

1

Inte

nsity

(au

)

672


(b)



point










0versus

(119864



119905and 119864



0versus (119864

119892minus



119905and 119864

119887are


119905and 119864



119905) at the








00 02 04 06 08 1000

02

04

06

08

10

Experimental plot

120572120572

0

120572M120572M0120572E120572E0

120572M120572M0 + 120572E120572E0

Eg minus h (eV)

(a)

10

15

20

25

30

35

40

0 10 20 30 40 50 60 70

3

4

5

6

7

Grain size (nm)

Eb

(meV

)

Eb

Qt

Qt

(times10

15cm

minus2)

(b)

0 10 20 30 40 50 60 70010

015

020

025

030

035

0403 4 5 6 7

Stre

ss (G

Pa)

Grain size (nm)

Grain sizeQt

1015) cmminus2Qt (

(c)












(

120575119886

119886

)

total= (

120575119886

119886

)

gb+ (

120575119886

119886

)

th

119878total = 119878gb + 119878th

(5)





119864

119892

(eV)

Grain sizefrom SEM

(nm)

Λ

119889

(nm)119876

119905

(1015 mminus2)119864

119887

(meV)Stress(GPa)

GaN-1473K 290 8 383 7 372 037

GaN-2573K 311 12 355 66 332 031

GaN-3673K 321 14 332 60 262 025

GaN-4773K 324 35 304 52 181 017

GaN-5873K 327 70 289 31 124 011



119878 = [

119884

1 minus 120574

](

120575119886

119886

) (6)


4 Conclusion






2 overtone of119864

1(TO) and119860

1(TO) overtone









Acknowledgments


References






































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




100nm

(a)

500 750 10000

20

40

60

80

Gra

in si

ze (n

m)

100nm Ts (K)

(b)

20 40 60 800

25

50

75

100(1

01)

Inte

nsity

(au

)

(102

)

(004

)

100nm

2120579 (deg)

(c)

40 50 60 70 8005

101520


Num

ber o

f gra

ins i

n1120583

m2

(d)






52and Ga3p

12at sim21 eV and



32





120572 = (

119860

ℎ]) ℎ] minus 119864

119892

119898

(1)





0 200 400 600 800 1000 12000

10

20

30

40

C1s

Relat

ive i

nten

sity

(cs

)

Binding energy (eV)

N1s

O1s

Ga(

l mm

)

O(K

LL)

Ga3

d 52

Ga3

p 12

Ga2p32

Ga2

p 12

(a)

1108 1112 1116 1120 1124

20

30

40

50

60

Relat

ive i

nten

sity

(cs

)

Binding energy (eV)

Ga2p32

(b)

1130 1140 1150

24

28

32

36

Relat

ive i

nten

sity

(cs

)

Binding energy (eV)

100 110 1200

1

2

3

4

Relat

ive i

nten

sity

(cs

)

Binding energy (eV)

Ga3p12

Ga2p12

(c)

375 380 385 390 395 400 405 410

0

1

2

3

4

5

Relat

ive i

nten

sity

(cs

)

Binding energy (eV)

N1s

(d)





ln (120572ℎ]) = ln119860 + 119898 ln (ℎ] minus 119864

119892) (2)

[

119889 (ln120572ℎ])119889 (ℎ])

] =

119898

ℎ] minus 119864

119892

(3)









119904) are


119904is shown in



1


120576

1= 119899

2(120582) minus 119896

2(120582)

120576

2= 2119899 (120582) 119896 (120582)

(4)


300 350 400 450 500 550

0

1

2

3

4

5PL

inte

nsity

(au

)

32 e

V

293

eV

263

eV

120582 (nm)

(a)

350 400 450 5000

2

4

6

8

PL in

tens

ity (a

u)

120582 (nm)

(b)

200 400 600 800 1000 12000

2

4

6

8

10

Tran

smiss

ion

()

120582 (nm)

(c)


121

122

123

0

2

4

6

8

10

12

14

(120572h)

2(times10

10cm

minus2

eV2)

m = 05

ln(120572h)

ln(h minus Eg)

1 2 3 4 5 6 7 8h (eV)

(d)






119892)



119904=







the 119860

1and 119864




400 500 600 700

18

20

22

24

26

473 K573 K673 K

773 K873 K

n

120582 (nm)

(a)

400 500 600 70000

02

04

06

573 K673 K

773 K873 K

k

120582 (nm)

(b)

400 500 600 700

3

4

5

6

7

473 K573 K673 K

773 K873 K

120576 1

120582 (nm)

(c)

400 500 600 70000

05

10

15

20

25

573 K673 K

773 K873 K

120576 2

120582 (nm)

(d)







1(TO) and 119860

1(TO) overtone






500 1000 1500 2000 2500000

004

008

012

016

020

024

Abso

rban

ce (a

u)

520

990

1521


(a)

400 500 600 700 800

0

20

40

60

80

64461

6

57153

1

Inte

nsity

(au

)

672


(b)



point










0versus

(119864



119905and 119864



0versus (119864

119892minus



119905and 119864

119887are


119905and 119864



119905) at the








00 02 04 06 08 1000

02

04

06

08

10

Experimental plot

120572120572

0

120572M120572M0120572E120572E0

120572M120572M0 + 120572E120572E0

Eg minus h (eV)

(a)

10

15

20

25

30

35

40

0 10 20 30 40 50 60 70

3

4

5

6

7

Grain size (nm)

Eb

(meV

)

Eb

Qt

Qt

(times10

15cm

minus2)

(b)

0 10 20 30 40 50 60 70010

015

020

025

030

035

0403 4 5 6 7

Stre

ss (G

Pa)

Grain size (nm)

Grain sizeQt

1015) cmminus2Qt (

(c)












(

120575119886

119886

)

total= (

120575119886

119886

)

gb+ (

120575119886

119886

)

th

119878total = 119878gb + 119878th

(5)





119864

119892

(eV)

Grain sizefrom SEM

(nm)

Λ

119889

(nm)119876

119905

(1015 mminus2)119864

119887

(meV)Stress(GPa)

GaN-1473K 290 8 383 7 372 037

GaN-2573K 311 12 355 66 332 031

GaN-3673K 321 14 332 60 262 025

GaN-4773K 324 35 304 52 181 017

GaN-5873K 327 70 289 31 124 011



119878 = [

119884

1 minus 120574

](

120575119886

119886

) (6)


4 Conclusion






2 overtone of119864

1(TO) and119860

1(TO) overtone









Acknowledgments


References






































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




0 200 400 600 800 1000 12000

10

20

30

40

C1s

Relat

ive i

nten

sity

(cs

)

Binding energy (eV)

N1s

O1s

Ga(

l mm

)

O(K

LL)

Ga3

d 52

Ga3

p 12

Ga2p32

Ga2

p 12

(a)

1108 1112 1116 1120 1124

20

30

40

50

60

Relat

ive i

nten

sity

(cs

)

Binding energy (eV)

Ga2p32

(b)

1130 1140 1150

24

28

32

36

Relat

ive i

nten

sity

(cs

)

Binding energy (eV)

100 110 1200

1

2

3

4

Relat

ive i

nten

sity

(cs

)

Binding energy (eV)

Ga3p12

Ga2p12

(c)

375 380 385 390 395 400 405 410

0

1

2

3

4

5

Relat

ive i

nten

sity

(cs

)

Binding energy (eV)

N1s

(d)





ln (120572ℎ]) = ln119860 + 119898 ln (ℎ] minus 119864

119892) (2)

[

119889 (ln120572ℎ])119889 (ℎ])

] =

119898

ℎ] minus 119864

119892

(3)









119904) are


119904is shown in



1


120576

1= 119899

2(120582) minus 119896

2(120582)

120576

2= 2119899 (120582) 119896 (120582)

(4)


300 350 400 450 500 550

0

1

2

3

4

5PL

inte

nsity

(au

)

32 e

V

293

eV

263

eV

120582 (nm)

(a)

350 400 450 5000

2

4

6

8

PL in

tens

ity (a

u)

120582 (nm)

(b)

200 400 600 800 1000 12000

2

4

6

8

10

Tran

smiss

ion

()

120582 (nm)

(c)


121

122

123

0

2

4

6

8

10

12

14

(120572h)

2(times10

10cm

minus2

eV2)

m = 05

ln(120572h)

ln(h minus Eg)

1 2 3 4 5 6 7 8h (eV)

(d)






119892)



119904=







the 119860

1and 119864




400 500 600 700

18

20

22

24

26

473 K573 K673 K

773 K873 K

n

120582 (nm)

(a)

400 500 600 70000

02

04

06

573 K673 K

773 K873 K

k

120582 (nm)

(b)

400 500 600 700

3

4

5

6

7

473 K573 K673 K

773 K873 K

120576 1

120582 (nm)

(c)

400 500 600 70000

05

10

15

20

25

573 K673 K

773 K873 K

120576 2

120582 (nm)

(d)







1(TO) and 119860

1(TO) overtone






500 1000 1500 2000 2500000

004

008

012

016

020

024

Abso

rban

ce (a

u)

520

990

1521


(a)

400 500 600 700 800

0

20

40

60

80

64461

6

57153

1

Inte

nsity

(au

)

672


(b)



point










0versus

(119864



119905and 119864



0versus (119864

119892minus



119905and 119864

119887are


119905and 119864



119905) at the








00 02 04 06 08 1000

02

04

06

08

10

Experimental plot

120572120572

0

120572M120572M0120572E120572E0

120572M120572M0 + 120572E120572E0

Eg minus h (eV)

(a)

10

15

20

25

30

35

40

0 10 20 30 40 50 60 70

3

4

5

6

7

Grain size (nm)

Eb

(meV

)

Eb

Qt

Qt

(times10

15cm

minus2)

(b)

0 10 20 30 40 50 60 70010

015

020

025

030

035

0403 4 5 6 7

Stre

ss (G

Pa)

Grain size (nm)

Grain sizeQt

1015) cmminus2Qt (

(c)












(

120575119886

119886

)

total= (

120575119886

119886

)

gb+ (

120575119886

119886

)

th

119878total = 119878gb + 119878th

(5)





119864

119892

(eV)

Grain sizefrom SEM

(nm)

Λ

119889

(nm)119876

119905

(1015 mminus2)119864

119887

(meV)Stress(GPa)

GaN-1473K 290 8 383 7 372 037

GaN-2573K 311 12 355 66 332 031

GaN-3673K 321 14 332 60 262 025

GaN-4773K 324 35 304 52 181 017

GaN-5873K 327 70 289 31 124 011



119878 = [

119884

1 minus 120574

](

120575119886

119886

) (6)


4 Conclusion






2 overtone of119864

1(TO) and119860

1(TO) overtone









Acknowledgments


References






































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




300 350 400 450 500 550

0

1

2

3

4

5PL

inte

nsity

(au

)

32 e

V

293

eV

263

eV

120582 (nm)

(a)

350 400 450 5000

2

4

6

8

PL in

tens

ity (a

u)

120582 (nm)

(b)

200 400 600 800 1000 12000

2

4

6

8

10

Tran

smiss

ion

()

120582 (nm)

(c)


121

122

123

0

2

4

6

8

10

12

14

(120572h)

2(times10

10cm

minus2

eV2)

m = 05

ln(120572h)

ln(h minus Eg)

1 2 3 4 5 6 7 8h (eV)

(d)






119892)



119904=







the 119860

1and 119864




400 500 600 700

18

20

22

24

26

473 K573 K673 K

773 K873 K

n

120582 (nm)

(a)

400 500 600 70000

02

04

06

573 K673 K

773 K873 K

k

120582 (nm)

(b)

400 500 600 700

3

4

5

6

7

473 K573 K673 K

773 K873 K

120576 1

120582 (nm)

(c)

400 500 600 70000

05

10

15

20

25

573 K673 K

773 K873 K

120576 2

120582 (nm)

(d)







1(TO) and 119860

1(TO) overtone






500 1000 1500 2000 2500000

004

008

012

016

020

024

Abso

rban

ce (a

u)

520

990

1521


(a)

400 500 600 700 800

0

20

40

60

80

64461

6

57153

1

Inte

nsity

(au

)

672


(b)



point










0versus

(119864



119905and 119864



0versus (119864

119892minus



119905and 119864

119887are


119905and 119864



119905) at the








00 02 04 06 08 1000

02

04

06

08

10

Experimental plot

120572120572

0

120572M120572M0120572E120572E0

120572M120572M0 + 120572E120572E0

Eg minus h (eV)

(a)

10

15

20

25

30

35

40

0 10 20 30 40 50 60 70

3

4

5

6

7

Grain size (nm)

Eb

(meV

)

Eb

Qt

Qt

(times10

15cm

minus2)

(b)

0 10 20 30 40 50 60 70010

015

020

025

030

035

0403 4 5 6 7

Stre

ss (G

Pa)

Grain size (nm)

Grain sizeQt

1015) cmminus2Qt (

(c)












(

120575119886

119886

)

total= (

120575119886

119886

)

gb+ (

120575119886

119886

)

th

119878total = 119878gb + 119878th

(5)





119864

119892

(eV)

Grain sizefrom SEM

(nm)

Λ

119889

(nm)119876

119905

(1015 mminus2)119864

119887

(meV)Stress(GPa)

GaN-1473K 290 8 383 7 372 037

GaN-2573K 311 12 355 66 332 031

GaN-3673K 321 14 332 60 262 025

GaN-4773K 324 35 304 52 181 017

GaN-5873K 327 70 289 31 124 011



119878 = [

119884

1 minus 120574

](

120575119886

119886

) (6)


4 Conclusion






2 overtone of119864

1(TO) and119860

1(TO) overtone









Acknowledgments


References






































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




400 500 600 700

18

20

22

24

26

473 K573 K673 K

773 K873 K

n

120582 (nm)

(a)

400 500 600 70000

02

04

06

573 K673 K

773 K873 K

k

120582 (nm)

(b)

400 500 600 700

3

4

5

6

7

473 K573 K673 K

773 K873 K

120576 1

120582 (nm)

(c)

400 500 600 70000

05

10

15

20

25

573 K673 K

773 K873 K

120576 2

120582 (nm)

(d)







1(TO) and 119860

1(TO) overtone






500 1000 1500 2000 2500000

004

008

012

016

020

024

Abso

rban

ce (a

u)

520

990

1521


(a)

400 500 600 700 800

0

20

40

60

80

64461

6

57153

1

Inte

nsity

(au

)

672


(b)



point










0versus

(119864



119905and 119864



0versus (119864

119892minus



119905and 119864

119887are


119905and 119864



119905) at the








00 02 04 06 08 1000

02

04

06

08

10

Experimental plot

120572120572

0

120572M120572M0120572E120572E0

120572M120572M0 + 120572E120572E0

Eg minus h (eV)

(a)

10

15

20

25

30

35

40

0 10 20 30 40 50 60 70

3

4

5

6

7

Grain size (nm)

Eb

(meV

)

Eb

Qt

Qt

(times10

15cm

minus2)

(b)

0 10 20 30 40 50 60 70010

015

020

025

030

035

0403 4 5 6 7

Stre

ss (G

Pa)

Grain size (nm)

Grain sizeQt

1015) cmminus2Qt (

(c)












(

120575119886

119886

)

total= (

120575119886

119886

)

gb+ (

120575119886

119886

)

th

119878total = 119878gb + 119878th

(5)





119864

119892

(eV)

Grain sizefrom SEM

(nm)

Λ

119889

(nm)119876

119905

(1015 mminus2)119864

119887

(meV)Stress(GPa)

GaN-1473K 290 8 383 7 372 037

GaN-2573K 311 12 355 66 332 031

GaN-3673K 321 14 332 60 262 025

GaN-4773K 324 35 304 52 181 017

GaN-5873K 327 70 289 31 124 011



119878 = [

119884

1 minus 120574

](

120575119886

119886

) (6)


4 Conclusion






2 overtone of119864

1(TO) and119860

1(TO) overtone









Acknowledgments


References






































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




500 1000 1500 2000 2500000

004

008

012

016

020

024

Abso

rban

ce (a

u)

520

990

1521


(a)

400 500 600 700 800

0

20

40

60

80

64461

6

57153

1

Inte

nsity

(au

)

672


(b)



point










0versus

(119864



119905and 119864



0versus (119864

119892minus



119905and 119864

119887are


119905and 119864



119905) at the








00 02 04 06 08 1000

02

04

06

08

10

Experimental plot

120572120572

0

120572M120572M0120572E120572E0

120572M120572M0 + 120572E120572E0

Eg minus h (eV)

(a)

10

15

20

25

30

35

40

0 10 20 30 40 50 60 70

3

4

5

6

7

Grain size (nm)

Eb

(meV

)

Eb

Qt

Qt

(times10

15cm

minus2)

(b)

0 10 20 30 40 50 60 70010

015

020

025

030

035

0403 4 5 6 7

Stre

ss (G

Pa)

Grain size (nm)

Grain sizeQt

1015) cmminus2Qt (

(c)












(

120575119886

119886

)

total= (

120575119886

119886

)

gb+ (

120575119886

119886

)

th

119878total = 119878gb + 119878th

(5)





119864

119892

(eV)

Grain sizefrom SEM

(nm)

Λ

119889

(nm)119876

119905

(1015 mminus2)119864

119887

(meV)Stress(GPa)

GaN-1473K 290 8 383 7 372 037

GaN-2573K 311 12 355 66 332 031

GaN-3673K 321 14 332 60 262 025

GaN-4773K 324 35 304 52 181 017

GaN-5873K 327 70 289 31 124 011



119878 = [

119884

1 minus 120574

](

120575119886

119886

) (6)


4 Conclusion






2 overtone of119864

1(TO) and119860

1(TO) overtone









Acknowledgments


References






































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




00 02 04 06 08 1000

02

04

06

08

10

Experimental plot

120572120572

0

120572M120572M0120572E120572E0

120572M120572M0 + 120572E120572E0

Eg minus h (eV)

(a)

10

15

20

25

30

35

40

0 10 20 30 40 50 60 70

3

4

5

6

7

Grain size (nm)

Eb

(meV

)

Eb

Qt

Qt

(times10

15cm

minus2)

(b)

0 10 20 30 40 50 60 70010

015

020

025

030

035

0403 4 5 6 7

Stre

ss (G

Pa)

Grain size (nm)

Grain sizeQt

1015) cmminus2Qt (

(c)












(

120575119886

119886

)

total= (

120575119886

119886

)

gb+ (

120575119886

119886

)

th

119878total = 119878gb + 119878th

(5)





119864

119892

(eV)

Grain sizefrom SEM

(nm)

Λ

119889

(nm)119876

119905

(1015 mminus2)119864

119887

(meV)Stress(GPa)

GaN-1473K 290 8 383 7 372 037

GaN-2573K 311 12 355 66 332 031

GaN-3673K 321 14 332 60 262 025

GaN-4773K 324 35 304 52 181 017

GaN-5873K 327 70 289 31 124 011



119878 = [

119884

1 minus 120574

](

120575119886

119886

) (6)


4 Conclusion






2 overtone of119864

1(TO) and119860

1(TO) overtone









Acknowledgments


References






































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials






119864

119892

(eV)

Grain sizefrom SEM

(nm)

Λ

119889

(nm)119876

119905

(1015 mminus2)119864

119887

(meV)Stress(GPa)

GaN-1473K 290 8 383 7 372 037

GaN-2573K 311 12 355 66 332 031

GaN-3673K 321 14 332 60 262 025

GaN-4773K 324 35 304 52 181 017

GaN-5873K 327 70 289 31 124 011



119878 = [

119884

1 minus 120574

](

120575119886

119886

) (6)


4 Conclusion






2 overtone of119864

1(TO) and119860

1(TO) overtone









Acknowledgments


References






































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials
































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials



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