Chemical bonding, optical constants, and electrical resistivity of sputter-depositedgallium oxide thin filmsC. V. Ramana, E. J. Rubio, C. D. Barraza, A. Miranda Gallardo, Samantha McPeak, Sushma Kotru, and J. T.

Grant

Citation: Journal of Applied Physics 115, 043508 (2014); doi: 10.1063/1.4862186 View online: http://dx.doi.org/10.1063/1.4862186 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/115/4?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Effect of etching with cysteamine assisted phosphoric acid on gallium nitride surface oxide formation J. Appl. Phys. 114, 064907 (2013); 10.1063/1.4817899 Compositional study of vacuum annealed Al doped ZnO thin films obtained by RF magnetron sputtering J. Vac. Sci. Technol. A 29, 051514 (2011); 10.1116/1.3624787 Optical properties of amorphous Ga As 1 x N x film sputtering with different N 2 partial pressures J. Vac. Sci. Technol. A 24, 1714 (2006); 10.1116/1.2217977 Influence of sputtering parameter on the optical and electrical properties of zinc-doped indium oxide thin films J. Vac. Sci. Technol. A 23, 1187 (2005); 10.1116/1.1924473 Optical behavior of reactive sputtered carbon nitride films J. Appl. Phys. 91, 2812 (2002); 10.1063/1.1446224

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Chemical bonding, optical constants, and electrical resistivityof sputter-deposited gallium oxide thin films

C. V. Ramana,1,a) E. J. Rubio,1 C. D. Barraza,1 A. Miranda Gallardo,1 Samantha McPeak,2

Sushma Kotru,2,3 and J. T. Grant41Department of Mechanical Engineering, University of Texas at El Paso, El Paso, Texas 79968, USA2Department of Electrical and Computer Engineering, The University of Alabama, Tuscaloosa,Alabama 35487, USA3Materials for Information and Technology, The University of Alabama, Tuscaloosa, Alabama 35487, USA4Research Institute, University of Dayton, Dayton, Ohio 45469, USA

(Received 9 October 2013; accepted 1 January 2014; published online 24 January 2014)

Gallium oxide (Ga2O3) thin films were made by sputter deposition employing a Ga2O3 ceramic

target for sputtering. The depositions were made over a wide range of substrate temperatures (Ts),

from 25 to 600 �C. The effect of Ts on the chemical bonding, surface morphological characteristics,

optical constants, and electrical properties of the grown films was evaluated using X-ray

photoelectron spectroscopy (XPS), atomic force microscopy (AFM), spectroscopic ellipsometry

(SE), and four-point probe measurements. XPS analyses indicate the binding energies (BE) of the

Ga 2p doublet, i.e., the Ga 2p3/2 and Ga 2p1/2 peaks, are located at 1118.0 and 1145.0 eV,

respectively, characterizing gallium in its highest chemical oxidation state (Ga3þ) in the grown

films. The core level XPS spectra of O 1s indicate that the peak is centered at a BE� 531 eV, which

is also characteristic of Ga-O bonds in the Ga2O3 phase. The granular morphology of the

nanocrystalline Ga2O3 films was evident from AFM measurements, which also indicate that the

surface roughness of the films increases from 0.5 nm to 3.0 nm with increasing Ts. The SE analyses

indicate that the index of refraction (n) of Ga2O3 films increases with increasing Ts due to improved

structural quality and packing density of the films. The n(k) of all the Ga2O3 films follows the

Cauchy’s dispersion relation. The room temperature electrical resistivity was high (�200 X-cm) for

amorphous Ga2O3 films grown at Ts¼RT-300 �C and decreased to �1 X-cm for nanocrystalline

Ga2O3 films grown at Ts� 500–600 �C. A correlation between growth conditions, microstructure,

optical constants, and electrical properties of Ga2O3 films is derived. VC 2014 AIP Publishing LLC.

[http://dx.doi.org/10.1063/1.4862186]

I. INTRODUCTION

Oxide dielectrics have been the subject of numerous

investigations for many years due to their possible device

integration in a wide range of technologies involving elec-

tronics, electro-optics, opto-electronics, and magneto-

electronics. Gallium oxide (Ga2O3), which is the stable oxide

of Ga, is one of the wide band gap oxides1–6 with potential

applications in luminescent phosphors,7 high temperature

sensors,8–11 antireflection coatings, and solar cells.10,12

Ga2O3 has been recognized as a deep-ultraviolet transparent

conducting oxide (UV–TCO),1–5,13 which makes the mate-

rial interesting for transparent electrode applications in

UV optoelectronics.6,14–16 While conventional transparent

oxides, such as In2O3, SnO2, ZnO, and ITO, are opaque in

the UV region due to their small band gaps (�3 eV), Ga2O3

exhibits a wide band gap (�5 eV) and high transparency in

the UV region.4,6,12–16

Gallium oxide can crystallize in five different crystal

structures: a, b, c, d, and e phases.17,18 Among these phase,

b-Ga2O3 is the most stable with a melting point of

1780 �C.10,17 The crystal structure of b-Ga2O3 is base

centered monoclinic (space group C2/m)17,18 where the oxy-

gen ions are in a distorted cubic packing arrangement and

the gallium ions are in distorted tetrahedral and octahedral

sites.18 The lattice parameters of b-Ga2O3 are

a¼ 12.2140(3) A, b¼ 3.0371(9) A, c¼ 5.7981(9) A, and

b¼ 103.83�.19–21 Despite great promise, studies on

b-Ga2O3 are rather scarce compared to other well explored

wide band gap oxides. For thin films grown using either

physical or chemical vapor deposition methods, properties

and phenomena of b-Ga2O3 films depend on the processing

conditions such as base pressure,9 growth temperature,22–24

reactive pressure (if any), deposition rate, and annealing

conditions. Therefore, the controlled growth and manipula-

tion of b-Ga2O3 at nanoscale dimensions has important

technological implications. Specifically, the ultramicrostruc-

ture in terms of the chemical valence state of Ga and the

surface chemistry of the grown films strongly influences the

electronic properties and hence their device applications.

Most importantly, determination of the index of refraction

(n) and its functional dependence on the wavelength is im-

portant for transparent oxides, such as Ga2O3 in this case.

Furthermore, for thin films and nanomaterials, the “n” val-

ues are sensitive to the microstructure; n is influenced by

various factors such as surface/interface structure, crystal

quality, packing density, lattice parameters, lattice strain,

a)Author to whom correspondence should be addressed. Email:

rvchintalapalle@utep.edu. Tel.: 915-747-8690. Fax: 915-747-5015

0021-8979/2014/115(4)/043508/9/$30.00 VC 2014 AIP Publishing LLC115, 043508-1

JOURNAL OF APPLIED PHYSICS 115, 043508 (2014)

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defect structure, and chemical composition. On the other

hand, the index of refraction is the most important physical

quantity, which is connected to most of the optical and

electro-optic phenomena in matter and is useful for engi-

neering modern electronic and optical devices. The present

work was, therefore, performed on sputter-deposited Ga ox-

ide films made by sputter-deposition at various temperatures

in order to probe their surface chemistry, Ga-O bonding,

and optical constants. Combined X-ray photoelectron spec-

troscopy (XPS), atomic force microscopy (AFM), and spec-

troscopic ellipsometry (SE) measurements allowed us to

determine the chemical composition, morphology, and

optical constants of Ga2O3 films. The results obtained are

presented and the effects of growth temperature and micro-

structure on the index of refraction profiles and electrical

conductivity of Ga2O3 films are discussed in this paper.

II. EXPERIMENTS

A. Fabrication

Gallium oxide films were deposited onto silicon (Si)

(100) wafers by radio-frequency magnetron sputtering. All

the substrates were thoroughly cleaned using the RCA

(Radio Corporation of America) procedure. Briefly, the RCA

cleaning procedure removes organic, alkali ions, and heavy

metal contaminants present on the surface of the

substrate. The cleaning procedure has three major steps. (1)

Removal of insoluble organic contaminants using 5:1:1

H2O/H2O2/NH4OH solution. (2) Removal of ionic and heavy

metal atomic components using a solution of 6:1:1

H2O/H2O2/HCl solution. (3) Removal of native oxide by

buffered oxide etching solution. Finally, the substrates were

dried with nitrogen before introducing them into the vacuum

chamber, which was initially evacuated to a base pressure of

�10�6 Torr. A gallium oxide target (Plasmaterials Inc.) of

5 cm diameter and 99.999% purity was employed for sputter-

ing. The Ga2O3-target was placed on a 5 cm sputter gun,

placed at a distance of 8 cm from the substrate. A sputtering

power of 40 W was initially applied to the target while intro-

ducing high purity argon (Ar) into the chamber to ignite the

plasma. Once the plasma was ignited, the power was

increased to 100 W for reactive deposition. The flow of Ar

was controlled using an MKS mass flow meter. Before each

deposition, the Ga2O3-target was pre-sputtered for 10 min

with the shutter closed. The deposition was carried out for

30 min. The thickness of the films was �40 nm. The samples

were deposited at different temperatures in the range of

25–600 �C. Substrate rotation was maintained during the

entire deposition time to ensure uniform coverage on the

substrate surface.

B. Characterization

1. X-ray photoelectron spectroscopy

The chemical composition of the Ga2O3 films was

determined using X-XPS. The samples were mounted on a

stainless steel sample holder, held down with double-sided

carbon tape, and analyzed using a calibrated Physical

Electronics PHI 5700 XPS system with a monochromated

Al K-a (1486.7 eV) source operating at 350 W at

�1� 10�9 torr and an angle of emission of 45�. Survey and

multiplex (multiple high resolution regions) scans were

taken before and after sputtering with Ne ions at 4 keV for

5 min. Survey spectra were obtained over the binding

energy (BE) range of 0–1200 eV with an energy step of

0.80 eV, a total time of 0.58 s per data point, a dwell time

of 20 ms, and an analyzer pass energy of 187.85 eV.

Multiplex spectra were obtained with an energy step of

0.125 eV and analyzer pass energy of 29.35 eV. Peaks

scanned in multiplex mode were Ga 2p, Si 2p, O 1 s, and C

1 s with a total time per data point of 0.85 s, 1.7 s, 0.85 s,

and 1.7 s, respectively. The Ga 2p region was chosen

instead of the more commonly used Ga 3d region to avoid

interference of the Ga 3d peaks with the O 2 s peak.24,25

The chemical shifts of the Ga 2p and Ga 3d peaks are simi-

lar.26 Probing the Ga 2p region, therefore, also provides in-

formation on the Ga-O chemical bonding and Ga valence

state in the grown films. After data acquisition, survey spec-

tra were analyzed using CasaXPS V2.3.16 to obtain atomic

composition. Peak areas were calculated using Shirley back-

ground subtraction and concentrations were obtained using

sensitivity factors for the instrument.

2. Atomic force microscopy

Surface morphology of the Ga2O3 films was studied

employing AFM using a Veeco Multimode scanning probe

microscope with a Nanoscope V controller. AFM images

were acquired using the tapping mode at a scanning fre-

quency of 1.53 Hz. Silicon non-contact cantilevers

(Nanoandmore, USA) were used to acquire the tapping

mode images. These uncoated silicon cantilevers are

125 lm long, 30 lm wide, and 4 lm thick, with a radius of

curvature of �8 nm. The resonance frequency of the tips

was �320 kHz. Tip quality was qualitatively assessed by

the clarity and presence of artifacts in the acquired images.

The calibration standard used to calibrate the scanner con-

sisted of platinum-coated, 200 nm-tall silicon columns

spaced at 10 lm intervals on centers. The columns have a

length of 5 nm on a side. AFM images (512 scan lines and

512 pixels per scan line) were acquired with an integral

gain of approximately 2 and an amplitude setpoint of

300 mV. The drive amplitude varied between 30 and

180 mV while the proportional gain varied from 4 to 25.

The images were then subjected to a 3rd order flattening

procedure using the Bruker NanoScope Analysis V1.40

software to remove the non-linear background artifact intro-

duced by the piezo scanner. Following the flattening proce-

dure, the surface roughness was quantified over five areas

of 1 lm� 1 lm on the 5 lm scan size images.

3. Spectroscopic ellipsometry

The optical properties and surface/interface characteris-

tics were probed by SE. The measurements were made using

a W. A. Woollam VVASE spectroscopic ellipsometer oper-

ating in the wavelength range of 250–1350 nm with a step

size of 2 nm and at angles of incidence of 65�, 70�, and 75�,near the Brewster’s angle of silicon. The number of

043508-2 Ramana et al. J. Appl. Phys. 115, 043508 (2014)

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revolutions per measurement was set at 20. The ellipsometry

data analysis was performed using commercially available

WVASE32 software.27

4. Electrical resistivity

The four-point Van der Pauw method was used to mea-

sure the resistivity of the films.28 Four electrical contacts on

the sample were made using a two part conducting epoxy

and silver wires.

In this method, voltage and current are measured

using eight different combinations of the four contact

points on the sample. This means that in the first set of

measurements, current is applied to one pair of electrodes

and voltage is measured across the other two electrodes

and then the process is repeated for other contact configu-

rations. A current of 2 mA was applied to the sample using

a Keithley 220 programmable current source. A Keithley

196 sensitive digital voltmeter was used to read the volt-

age and a 196 DMM system was used to read the current

between two contacts. A Keithley 2001 switch was used to

switch between contacts. The meters were connected to a

computer and LABVIEW data acquisition system via the

IEEE-488 GPIB for automatic data recording. The final re-

sistivity was calculated using the Van der Pauw principle

taking into account the sample thickness, the shape factor,

and other limitations.28

III. RESULTS

A. Chemical bonding—x-ray photoelectronspectroscopy

Structural and surface morphology characterization of

Ga2O3 films was performed employing X-ray diffraction

(XRD) and scanning electron microscopy (SEM), and the

results were reported elsewhere.29 The chemical valence

state and surface chemistry of the grown Ga2O3 films was

analyzed by XPS. The XPS survey spectra recorded for

as-grown and Neþ sputtered Ga-oxide films are shown in

Fig. 1. The data are shown in the BE range of 0–1200 eV for

two representative samples grown at RT and 500 �C pre-

sented in the upper and lower panels, respectively (Fig. 1).

For the as-grown Ga-oxide films, all the spectral features,

except the C 1s level, are attributed to the constituent ele-

ment core levels or Auger lines. The C 1s signal is due to

hydrocarbons adsorbed on the film surface during air expo-

sure during transfer from the growth chamber to the XPS

instrument. It should be noted that the C 1s signal is com-

pletely absent in the XPS spectra of Neþ bombarded films

for both the samples shown. This observation confirms the

removal of hydrocarbon contamination from the surface. It is

clear from the XPS data that the spectral components related

to constituent elements become more intense as a result of

ion-bombardment of the film surfaces. The absence of a Si

substrate peak is due to the fact that the deposited Ga-oxide

FIG. 1. XPS survey scans of represen-

tative Ga2O3 films grown at different

substrate temperatures. The survey

scans obtained for films grown at RT

(upper panel) and 500 �C (lower panel)

are shown. The data shown are from

as-grown and Neþ sputtered samples.

The removal of surface carbon species

upon sputtering is evident.

043508-3 Ramana et al. J. Appl. Phys. 115, 043508 (2014)

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films cover the Si surface and are sufficiently thick to prevent

detection of photoelectrons from the substrate.

The core level photoelectron spectra of the Ga 2p doublet

are shown in Fig. 2. The high resolution XPS spectra are

shown for as-grown (left panel) and Neþ sputtered (right

panel) samples. The BE values of these Ga 2p3/2 and Ga 2p1/2

peaks are 1118.0 and 1145.0 eV, respectively; they are almost

constant and do not exhibit any change or shift as a function

of substrate temperature during growth. Furthermore, no

appreciable change is noted in the BE values of these peaks

in as-grown and Neþ sputtered samples. However, a very

small variation in full-width at half-maximum is noted for

these peaks in Neþ sputtered samples compared to as-grown

samples. The BE values of Ga 2p levels for metallic Ga are:

1117.0 and 1144.0 eV for Ga 2p3/2 and Ga 2p1/2, respectively,

and are indicated by the dashed lines in Fig. 2.24 Compared to

metallic gallium, the grown Ga-oxide films show a positive

shift in the BE (1.0 eV). It should be noted that the BE shift

occurs due to the redistribution of electronic charge around

the constituent atoms.30–32 Therefore, the differences in Ga

chemical bonding cause a BE shift that can be used to extract

information about the oxidation state of Ga in the grown

films. Based on these considerations, it can be concluded that

the chemical state of Ga in the films is in the highest valence

state (Ga3þ) without any of the reported intermediate valence

states.24,33

The core level photoelectron spectra of O 1s also

revealed interesting features about the surface chemistry of

the grown Ga-oxide films. The high resolution XPS spectra

of O 1s core levels are shown for as-grown (left panel) and

Neþ sputtered (right panel) samples in Fig. 3. The O 1s peak

is asymmetric in as-grown samples. While the main peak is

centered at a BE �531 eV, there is a small shoulder at a

BE� 532 eV. As shown in Fig. 4 for a representative sam-

ples grown at RT, the O 1s peak for as-grown Ga-oxide films

can be resolved into two components. The most intense peak

is located at 531.0 eV and the less intense peak is at

532.3 eV, representing the different chemistries of oxygen.

The less intense contribution at a BE of 532.3 eV disappears

in all the samples upon Neþ sputtering of the surface. With

Neþ sputtering of the surface, the O 1s peak changes into a

single, sharp peak centered at a BE of 531.0 eV. These fea-

tures can be understood as follows. It has been reported that

the O 1s peak in Ga-oxide occurs at a BE� 531 eV, which

FIG. 2. Ga 2p core level spectra of

Ga2O3 thin films grown at various sub-

strate temperatures. The core level

spectra obtained from as-grown and

Neþ sputtered samples are shown. The

vertical lines indicate the BE positions

for Ga 2p levels in gallium metal and

serve as reference to indicate the posi-

tive shift of the Ga 2p lines in the

grown Ga-oxide films.

FIG. 3. O 1s core level spectra of

Ga2O3 thin films grown at various sub-

strate temperatures. The core level

spectra obtained from as-grown and

Neþ sputtered samples are shown.

043508-4 Ramana et al. J. Appl. Phys. 115, 043508 (2014)

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characterizes Ga-O bonding with the highest oxidation

state of Ga (Ga3þ). The less intense contribution at a

BE� 532.3 eV can be attributed to either carbonyl (oxygen

bonded to carbon) or hydroxyl (oxygen bonded to hydrogen)

groups, which are adsorbed onto the surface.34,35 However,

it was noted in the survey XPS spectra (Fig. 1) that the C 1s

signal was present on all of the as-grown Ga-oxide samples.

The presence of C 1s disappeared upon Neþ sputtering of the

surface. Based on these observations, it is clear that the

minor contribution that appears in O 1s spectra is due to the

surface C-O or hydroxide bonds, which disappeared on sput-

tering away the few top-most surface layers.

B. Surface characteristics—atomic force microscopy

The AFM results for the Ga2O3 samples grown at

Ts� 500 �C are shown in Fig. 5. The topographic images are

shown along with the corresponding 3D surface images. It is

remarkable to note the evident nano-particulate morphology

the film surfaces, where the conically shaped grains are dis-

tributed uniformly all over the film surface. Samples grown

at Ts< 500 �C (not shown) are mostly amorphous in nature

and no crystalline grains are presented on the film surfaces.

This observation is in agreement with the XRD data, which

indicate that Ga2O3 films grown at Ts< 500 �C were amor-

phous in nature while those grown at Ts� 500 �C are

nano-crystalline. It is evident from Fig. 5 that the grain-size

increases with increasing growth temperature. The size

increases from �20 nm to �32 nm with increasing growth

temperature from 500 �C to 600 �C. The effect of growth

temperature is also reflected in the case of root-mean-square

surface roughness values, which increased from 1 nm to

�3 nm with increasing temperature from 500 to 600 �C. The

surface roughness values of the amorphous films were not

significant. For instance, the set of films grown at

Ts< 500 �C exhibit a surface roughness of <1 nm.

C. Optical constants—spectroscopic ellipsometry

Optical constants of the Ga2O3 films were primarily

probed by SE, which measures the relative changes in the

amplitude and phase of the linearly polarized monochro-

matic incident light upon oblique reflection from the sample

surface. The experimental parameters obtained by SE are the

angles W (azimuth) and D (phase change), which are related

to the microstructure and optical properties, defined by36–40

q ¼ Rp=Rs ¼ tan W exp iDð Þ; (1)

FIG. 4. Peak fitting analysis of the O 1s core-level peak in as-grown Ga-ox-

ide films fabricated at RT. It is evident that the peak can be resolved into

two separate components, which are located at 531.0 eV and 532.3 eV with

high and low intensities, respectively.

FIG. 5. AFM images of sputter depos-

ited Ga2O3 films; (a) sample grown at

500 �C, (b) sample grown at 600 �C.

043508-5 Ramana et al. J. Appl. Phys. 115, 043508 (2014)

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where Rp and Rs are the complex reflection coefficients of

the light polarized parallel and perpendicular to the plane of

incidence, respectively. The spectral dependencies of the

ellipsometric parameters, W and D, determined for Ga2O3

films grown at various temperatures are shown in Fig. 6. The

spectral dependencies of ellipsometric parameters W (azi-

muth) and D (phase change) can be fitted with appropriate

models to extract film thickness and the optical constants,

i.e., the refractive index (n) and extinction coefficient (k),

based on the best fit between experimental and simulated

spectra.36,37 The curves obtained for Ga2O3 films indicate

(Fig. 6) a reasonable agreement between the experimental

and simulation data. The Levenberg-Marquardt regression

algorithm was used for minimizing the mean-squared error

(MSE)36

MSE ¼ 1

2N �M

Xn

i¼1

Wexp: � Wcalc:ð Þrexp

Wi

( )224

þDexp: � Dcalc:

� �rexp

Di

( )2 #; (2)

where Wexp, Wcalc and Dexp, Dcalc are the measured (experi-

mental) and calculated ellipsometry functions, N is the num-

ber of measured W, D pairs, M is the number of fitted

parameters in the optical model, and r are standard devia-

tions of the experimental data points.

Extracting meaningful physical information from ellips-

ometry requires the construction of an optical model of the

sample, which generally accounts a number of distinct layers

with individual optical dispersions. Interfaces between these

layers are optical boundaries at which light is refracted and

reflected according to the Fresnel relations. The stack model

used to simulate the spectra for the purpose of determining

the optical constants of Ga2O3 films is schematically shown

in Fig. 7. The model contains, from top, the Ga2O3 film, the

SiO2 interface, and the Si substrate. The surface roughness

was also considered in order to accurately fit the experimen-

tal data. A Cauchy dispersion model for the refractive index

was employed to deduce film thickness, roughness values,

and refractive index (n) dispersion. The Cauchy dispersion

model is40

n ¼ n1 þA

k2þ B

k4: (3)

FIG. 6. The spectral dependence of W and D for Ga2O3 films grown at various temperatures. The experimental data obtained and modeling curves are shown.

FIG. 7. The stack model of the Ga2O3 sample constructed for ellipsometry

data analysis.

043508-6 Ramana et al. J. Appl. Phys. 115, 043508 (2014)

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The microstructure information, specifically, film thick-

ness, roughness, and interfacial oxide thickness, were

allowed to vary and determined from SE analysis. The varia-

tion of film thickness from 43 nm to 32 nm was noted as a

function of growth temperature. It is evident that the film

thickness is more or less constant for Ts¼ 25–400 �C at

which point a decreasing trend prevails. This can be under-

stood based on the fact that the films grown at Ts< 500 �Care amorphous while those grown at Ts� 500 �C were

nano-crystalline.29 The observed thickness decrease from

44 nm to 32 nm is, thus, due to Ga2O3 film crystallization

and densification with increasing substrate temperature.

The dispersion profiles of index of refraction (n) deter-

mined from SE data for Ga2O3 films are shown in Fig. 8.

The “n” dispersion curves also indicate a sharp increase at

shorter wavelengths corresponding to fundamental absorp-

tion of energy across the band gap. The range of n-values

and the observed behavior agree well with those reported in

the literature for electron beam deposited Ga2O3 films.15

However, the effect of growth temperature is evident in the

dispersion curves (Fig. 8), where there is an increase in “n”values with increasing Ts. While the increasing trend of “n”values with increasing Ts from very beginning is clear, the

dispersion curves of Ga2O3 films grown at higher tempera-

ture (500–600 �C) are distinctly separated from those of sam-

ples grown at lower temperatures, indicating their

characteristic optical quality difference.

D. Electrical resistivity

Figure 9 shows the room temperature resistivity of the

films grown at various deposition temperatures. Films grown

at lower temperatures are seen to exhibit higher resistivity,

the values being approximately 200–210 X-cm for films

grown up to 300 �C. A sharp decrease in resistivity is

observed for samples prepared at 400 �C. As the growth

temperature increases, the resistivity is seen to decrease fur-

ther. The sample grown at 600 �C exhibits the lowest resis-

tivity of 1 X-cm.

IV. DISCUSSION

The emphasis in this work is to establish the direct rela-

tionship between the index of refraction (“n”) and structural

quality of sputter-deposited Ga2O3 films grown under vari-

able growth temperature (Ts). It must be emphasized that the

optical properties are sensitive to the film microstructure.

Various factors such as surface/interface structure, grain

size, crystal quality, lattice parameters, defect structure, and

chemical composition influence the optical properties of ox-

ide films. The growth temperature, which is an important

thermodynamic parameter, plays an important role,

besides the reactive oxygen pressure, in deciding the micro-

structure as well as properties of materials resulting from

sputter-deposition. Therefore, the XPS, AFM, and SE results

along with electrical resistivity variation are considered in

order to derive the functional relationship between the sur-

face physics, morphology, and optical properties of Ga2O3

films.

The effect of Ts on the crystal structure, phase, grain

size, and band gap of Ga2O3 films is summarized in Table I.

The crystal structure, phase, and band gap were evaluated

in our earlier work.29 Briefly, Ga2O3 films grown at

Ts¼RT-400 �C were amorphous (a-Ga2O3). A substrate

temperature of 500 �C is critical to the onset of film crystalli-

zation with the b-Ga2O3 phase formation. Therefore, films

grown at Ts� 500 �C crystallize in the b-Ga2O3 phase. The

grain size determined varied from 15 to 35 nm with increas-

ing Ts. Now, we focus our attention on the XPS results to es-

tablish the surface chemistry of the Ga2O3 films. XPS

analyses indicate that the films are nearly stoichiometric and

Ga exists in its highest chemical valence state. The BE val-

ues of Ga 2p3/2 and Ga 2p1/2 peaks (1118.0 and 1145.0 eV,

respectively) and the BE shift from Ga0 (1 eV) characterizes

the Ga in its highest chemical valence state (Ga3þ) in all the

films, almost without any appreciable change, as a function

FIG. 8. The dispersion profiles of index of refraction of Ga2O3 films grown

at various Ts. The sharp increase in index of refraction at k’ 400 nm is due

to fundamental absorption across the band gap of the films. The distinct dis-

persion curves as a function of Ts indicate the differences in structural qual-

ity of Ga2O3 films.

FIG. 9. Electrical resistivity variation of Ga2O3 films with Ts. It can be noted

that the resistivity of amorphous Ga2O3 films is generally high and exhibits

a sharp decrease for nanocrystalline Ga2O3 films.

043508-7 Ramana et al. J. Appl. Phys. 115, 043508 (2014)

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of Ts. This is important from the view point of electronic

structure since reduced chemical valence states of Ga can

create defects, which in turn alter the optical and electrical

properties. Furthermore, the chemical stoichiometry analyses

made by XPS in this work, and RBS results reported else-

where29 are in good agreement with each other. Also, these

chemical studies rule out the possible compositional defects

or reduced Ga states and their influence on the optical and

electrical properties of Ga2O3 films.

Finally, the observed variation in the index of refraction

and electrical resistivity of Ga-oxide films can be explained

based on the microstructure variation of Ga2O3 films with

Ts. A simple model can be formulated to explain the

observed functional relationship between the optical con-

stants and Ts in Ga2O3 films. Evident from the SE results,

the dispersion of optical constants depend on the growth

temperature. XRD measurements demonstrated that the

Ga2O3 films are completely amorphous at Ts¼ 25–500 �C at

which point amorphous to nano-crystalline structural trans-

formation occurs.29 The observed increase in “n” values

when Ga2O3 films are grown at higher temperatures can,

therefore, be attributed to the improved structural order and

packing density of the films. Furthermore, while surface

roughening can induce scattering losses, AFM data evi-

denced the rms roughness of the films are not significant in

Ga2O3 films as a function of Ts. Usually, rough film surfaces

result in increased light-scattering losses at the interface.

Therefore, the critical temperature (500 �C) noted in the

XRD measurements seems to promote the structural order

and increases the mobility of ad-atoms to join together to

increase the packing density of the material in the films lead-

ing to observed increase in “n” values. Such behavior was

also noted in sputter-deposited Y2O3 films.39

The effect of microstructure was also reflected in the

electrical resistivity analysis of the Ga2O3 films. The electrical

resistivity usually becomes higher with grain size reduction

due to the increasing grain boundary volume and associated

impedance to the flow of charge carriers.41,42 If the crystallite

size is smaller than the electron mean free path, grain bound-

ary scattering dominates and hence the higher electrical resis-

tivity. Furthermore, the electrical resistivity is also very

sensitive to lattice imperfections in solids, such as vacancies

and dislocations, which are often present in nanocrystalline

materials.41 In addition to that, lattice strain and their distor-

tions can affect the charge mobility causing a decrease in con-

ductivity.41,42 The room-temperature electrical resistivity

variation with Ts observed for Ga2O3 films (Fig. 9) can be

explained taking these factors into consideration. Ga2O3 films

grown at Ts¼RT-300 �C are completely amorphous as evi-

denced in the XRD studies. The randomness or disordered

structure of the films, therefore, accounts for the observed

high electrical resistivity of amorphous Ga2O3 films. The re-

sistivity decrease with increasing Ts can be attributed to the

increasing crystalline nature of Ga2O3 films. The observed

sharp decrease in the electrical resistivity for Ga2O3 films

grown at higher temperatures seems to be a result of the com-

bined effects of the grain-size increase and the phase transfor-

mation from amorphous-Ga2O3 to b-Ga2O3 films. Thus, this

trend in resistivity is expected and can be attributed to better

crystallinity, the presence of larger grains and fewer grain

boundaries in the samples prepared at higher temperatures.

Furthermore, the electrical results are in agreement with the

results obtained from AFM and XRD studies.

V. CONCLUSIONS

Gallium oxide films were produced by sputter deposi-

tion onto Si(100) substrates. The chemical bonding in the

grown films was probed by XPS. The Ga 2p core level spec-

tra and analyses indicate that the Ga 2p3/2 and Ga 2p1/2 peaks

are located at BE values of 1118.0 and 1145.0 eV, respec-

tively, in all the grown Ga-oxide films. The positive shift in

the BE values (compared to elemental Ga) indicate they are

characteristic of gallium in the Ga3þ state. The O 1s core

level XPS spectra from as-grown samples exhibit the pres-

ence of surface carbonyl or hydroxyl groups, which could be

removed by ion sputtering of the top-most surface layers.

The core level XPS spectra of O 1s coupled with the Ga 2p

core level data indicate the presence of the Ga2O3 phase in

the films. AFM revealed the granular morphology of the

nanocrystalline Ga2O3 films. The index of refraction of

Ga2O3 films increases with increasing Ts while the n-disper-

sion obeys the Cauchy dispersion relation. The n-Ts direct

relationship is explained based on the structural transforma-

tion, improved structural order, and enhanced packing den-

sity of the Ga2O3 films with increasing growth temperature.

The room temperature electrical resistivity measurements

demonstrated the direct relationship between crystal struc-

ture and electrical properties. The resistivity was high (�200

X-cm) for amorphous Ga2O3 films and it sharply decreased

at the amorphous-to-crystalline film formation at higher Ts.

ACKNOWLEDGMENTS

This material was based on the work supported by the

Department of Energy under Award Number DE-PS26-

08NT00198-00. C.V.R. also acknowledges with pleasure the

support from the National Science Foundation (NSF) with

NSF-PREM Grant # DMR-1205302. One of us (A.M.G.) is

thankful to the Campus Office of Undergraduate Research

Initiative (COURI) at the University of Texas at El Paso for

the funding and opportunity for research experience.

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TABLE I. Summary of the effect of growth temperature on the crystal struc-

ture, phase, grain size, and band gap of Ga2O3 films.29

Growth

temp. (�C)

Crystal structure

and phase

Ga/O

ratio

Band gap

(eV)

Gran size

(nm)

25 Amorphous 1.6 5.17 …

200 Amorphous 1.6 5.02 …

300 Amorphous 1.5 5.01 14

400 Amorphous 1.5 4.98 16

500 Monoclinic (b-Phase) 1.5 4.99 20

600 Monoclinic (b-Phase) 1.5 4.96 35

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