Residual and intentional n-type doping of ZnO thin films grown by metal-organic vaporphase epitaxy on sapphire and ZnO substratesStéphane Brochen, Matthieu Lafossas, Ivan-Christophe Robin, Pierre Ferret, Frédérique Gemain, Julien Pernot,

and Guy Feuillet

Citation: Journal of Applied Physics 115, 113508 (2014); doi: 10.1063/1.4868591 View online: http://dx.doi.org/10.1063/1.4868591 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/115/11?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Photoluminescence and secondary ion mass spectrometry investigation of unintentional doping in epitaxialgermanium thin films grown on III-V compound by metal-organic chemical vapor deposition J. Appl. Phys. 111, 013502 (2012); 10.1063/1.3673538 High resolution photoluminescence spectroscopy of donors in undoped and In-doped ZnO grown bymetalorganic vapor phase epitaxy J. Appl. Phys. 110, 083506 (2011); 10.1063/1.3652854 Influence of thermally diffused aluminum atoms from sapphire substrate on the properties of ZnO epilayersgrown by metal-organic chemical vapor deposition J. Vac. Sci. Technol. A 29, 03A106 (2011); 10.1116/1.3549136 Microstructural compositional, and optical characterization of GaN grown by metal organic vapor phase epitaxyon ZnO epilayers J. Vac. Sci. Technol. B 27, 1655 (2009); 10.1116/1.3137967 Electrical and optical studies of metal organic chemical vapor deposition grown N-doped ZnO films J. Vac. Sci. Technol. B 27, 1705 (2009); 10.1116/1.3110018

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Residual and intentional n-type doping of ZnO thin films grown bymetal-organic vapor phase epitaxy on sapphire and ZnOsubstrates

St�ephane Brochen,1,2,3 Matthieu Lafossas,1 Ivan-Christophe Robin,1 Pierre Ferret,1

Fr�ed�erique Gemain,1 Julien Pernot,2,3,4 and Guy Feuillet11CEA-LETI, MINATEC Campus, 17 rue des Martyrs, F-38054 Grenoble Cedex 9, France2Univ. Grenoble Alpes, Inst. NEEL, F-38042 Grenoble, France3CNRS, Inst. NEEL, F-38042 Grenoble, France4Institut Universitaire de France, 103 boulevard Saint Michel, 75005 Paris, France

(Received 21 November 2013; accepted 4 March 2014; published online 20 March 2014)

ZnO epilayers usually exhibit high n-type residual doping which is one of the reasons behind the

difficulties to dope this material p-type. In this work, we aimed at determining the nature of the

involved impurities and their potential role as dopant in ZnO thin films grown by metalorganic

vapor phase epitaxy (MOVPE) on sapphire and ZnO substrates. In both cases, secondary ion mass

spectroscopy (SIMS) measurements give evidence for a strong diffusion of impurities from the

substrate to the epilayer, especially for silicon and aluminum. In the case of samples grown on

sapphire substrates, aluminum follows Fick’s diffusion law on a wide growth temperature range

(800� 1000 �C). Thus, the saturation solubility and the diffusion coefficient of aluminum in ZnO

single crystals have been determined. Furthermore, the comparison between SIMS impurity and

effective dopant concentrations determined by capacitance-voltage measurements highlights, on

one hand a substitutional mechanism for aluminum diffusion, and on the other hand that silicon

acts as a donor in ZnO and not as an amphoteric impurity. In addition, photoluminescence spectra

exhibit excitonic recombinations at the same energy for aluminum and silicon, indicating that

silicon behaves as an hydrogenic donor in ZnO. Based on these experimental observations, ZnO

thin films with a controlled n-type doping in the 1016 � 1019cm�3 range have been carried out.

These results show that MOVPE growth is fully compatible with the achievement of highly

Al-doped n-type thin films, but also with the growth of materials with low residual doping, which

is a crucial parameter to address ZnO p-type doping issues. VC 2014 AIP Publishing LLC.

[http://dx.doi.org/10.1063/1.4868591]

I. INTRODUCTION

ZnO is a widely studied II–VI semiconductor, especially

for optoelectronic applications like near ultraviolet light emit-

ters or detectors.1 Nevertheless, it is well known that the de-

velopment of an all ZnO optoelectronic device is hampered

by p-type doping difficulties.2,3 A good understanding of the

residual n-type doping appears as an important preliminary

step to try and understand the origin of these difficulties.

Metalorganic vapor phase epitaxy (MOVPE) is a widely

used method for the epitaxial growth of semiconductor thin

films, especially for large scale industrial opto-electronic

applications as light emitting diodes or laser diodes (e.g., for

nitride-related material devices). This vapor phase growth

method, which allows the achievement of ZnO thin films

with a high crystalline quality,4–8 is a relevant method for

doping studies because of a large choice of metalorganic pre-

cursors and a good control of film stoichiometry.

In this paper, we have investigated both residual and

intentional n-type doping, with aluminum impurities, of ZnO

thin films grown by MOVPE. For the sake of comparison, the

ZnO epilayers were grown on sapphire and ZnO substrates.

In this framework, we have combined capacitance-voltage

C(V) measurements and secondary ion mass spectroscopy

(SIMS) in order to investigate the role of some impurities on

the effective dopant concentration profiles. Correlatively,

photoluminescence measurements were also performed on

these samples to investigate the role of donor impurities on

the optical properties of ZnO thin films and especially on

excitonic recombinations in the ZnO near band edge region.

II. GROWTH AND EXPERIMENTAL DETAILS

The ZnO epilayers were grown by MOVPE in a 6 � 2

in. close coupled shower head Aixtron system using diethyl-

zinc (DEZn) and N2O as a source of zinc and oxygen,

respectively, and with N2 as carrier gas. The system is also

equipped with an O2 line used for annealing steps and also as

another source of oxygen for the low temperature growth re-

gime. In the case of intentionally n-doped samples, trimethyl

aluminum ðC6H18Al2Þ is used as source of aluminum. The

growth temperature is measured using an in-situ pyrometer

system at 633 nm (Laytec epicurveTT system).

The growth conditions are optimized with a VI/II molar

flow ratio of 6700 and with a reactor pressure between 50

and 200 millibars in the temperature range of 500� 1000 �C,

with several growth steps as described in Figure 1; at first

substrate annealing under O2 at 1070 �C during 300 s, then

nucleation layer growth under O2 and DEZn at 520 �C during

1840s (�450nm), followed by an annealing under O2 þ N2O

0021-8979/2014/115(11)/113508/10/$30.00 VC 2014 AIP Publishing LLC115, 113508-1

JOURNAL OF APPLIED PHYSICS 115, 113508 (2014)

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at 965 �C during 300 s. The main growth regime under N2Oand DEZn begins at 935 �C during 1200 s (�150 nm) and

continues with a temperature and a duration noted Tg and tg,

respectively, in the following. The growth duration tg is set

in order to obtain typical thicknesses around a few lm at a

growth temperature Tg varying between 800 and 1000 �C.

The growth conditions for the different samples are given in

Table I (heteroepitaxial thin films), Table II (homoepitaxial

thin films), and Table III (intentionally aluminum doped thin

films).

The impurity concentration profiles were measured on

our sample series by SIMS both in the positive ion detection

mode with a Csþ primary ion bombardment accelerated at

14:5 keV and in the negative ion detection mode with a 02þ

primary ion bombardment accelerated at 5:5 keV (analyses

at Probion SA, France).

The effective dopant concentration profiles ND � NA are

measured by capacitance-voltage C(V) measurements using

a 720 lm diameter double Schottky mercury probe with a

20 Hz� 1 MHz Agilent LCR meter (4284 A) in parallel

mode. In the case of highly Al-doped samples, an oxygen

plasma treatment is performed in order to limit leakage cur-

rent in the Schottky diodes.9 After this treatment, a strong

increase of the rectifying behavior in the I(V) characteristics

is observed (not shown), allowing an accurate determination

of the effective dopant concentration profiles with respect to

the diode cutoff frequency, as described in Ref. 10

The PL spectra were obtained at 10 K by using a

frequency-doubled 244 nm cw Ar laser coupled with a

0:55 m monochromator equipped with a 1800 mm�1 grating.

The excitation conditions are the same for all samples.

III. RESULTS AND DISCUSSIONS

A. Identification of impurities

In a first step, the impurity concentration has been inves-

tigated by secondary ion mass spectroscopy (SIMS) in ZnO

thin films grown by MOVPE both on ZnO and sapphire sub-

strates. Typical SIMS concentration profiles of group III (B,

FIG. 1. MOVPE growth steps conditions: (1) Substrate surface annealing

under O2 at 1070 �C during 300 s; (2) Growth of the nucleation layer under

O2 and DEZn at 520 �C during 1840 s (�450 nm); (3) Annealing of the

nucleation layer under O2 þ N2O at 965 �C during 300 s; (4) Regrowth under

N2O and DEZn at 935 �C during 1200 s (�150 nm); and (5) Main growth re-

gime under N2O and DEZn at Tg and during tg.

TABLE I. Epilayer thickness, growth conditions (duration and temperature), aluminum diffusion parameters (diffusion coefficient DAl and saturation solubility

CsatAl ), and mean value of the effective dopant concentration ND � NA of undoped ZnO samples grown on sapphire substrates.

Sample Thickness (lm) Duration tg sð Þ Temperature Tgð �CÞ DAl ðcm2:s�1Þ CsatAl (cm�3) ND � NA (cm�3)

#1 3.3 13 050 800 7:9� 10�14 2:7� 1018 5:8� 1016

#2 3.4 13 050 860 4:5� 10�13 1:8� 1018 1:1� 1017

#3 2.0 6600 880 3:3� 10�13 2:2� 1018 5:8� 1016

#4 4.8 17 400 930 1:5� 10�12 2:6� 1018 2:1� 1017

#5 1.1 6000 990 7:2� 10�12 3:6� 1018 4:2� 1018

TABLE II. Epilayer thickness, growth conditions (duration and temperature), mean value of the effective dopant concentration ND � NA, and average SIMS

concentration of aluminum and silicon of undoped ZnO thin film (#6) grown on O-polar ZnO substrate (#7).

Sample Thickness lmð Þ Duration tg sð Þ Temperature Tgð �CÞ ND � NA ðcm�3Þ Al½ � ðcm�3Þ Si½ � cm�3ð Þ

#6 3.4 13 050 860 2:7� 1016 1:3� 1015 4:5� 1016

#7 500 … … 1:8� 1017 1:9� 1016 2:0� 1017

TABLE III. Kind of substrate, epilayer thickness, growth conditions (duration and temperature), mean value of the effective dopant concentration ND � NA,

and average SIMS concentration of aluminum of intentionally Al-doped ZnO samples.

Sample Substrate Thickness lmð Þ Duration tg sð Þ Temperature Tgð �CÞ ND � NA ðcm�3Þ Al½ � ðcm�3Þ

#8 Sapphire 4.8 17 000 990 4:9� 1018 8:8� 1018

#9 ZnO (O-face) 2.6 17 000 990 4:5� 1018 7:5� 1018

#10 ZnO (Zn-face) 2.3 17 000 990 2:3� 1018 6:3� 1018

113508-2 Brochen et al. J. Appl. Phys. 115, 113508 (2014)

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Al, Ga, and In), I (H and Li) and IV (Si and C) impurities,

measured on such ZnO epilayers are plotted in Figure 2.

These impurities have been selected because they are sup-

posed to have an important role on the electrical properties

of ZnO samples. For the sake of comparison, the two ZnO

samples in Figure 2 have been grown in the same run, at

a growth temperature Tg ¼ 860 �C and a growth duration tg

¼ 13050 s corresponding to a layer thickness around 3:4 lm.

As shown in Figures 2(a) and 2(b), the concentrations of

hydrogen (H) and carbon (C) impurities appear to be rela-

tively high (>1:0� 1018cm�3) for the two samples analysed

here, both in the layer and the substrate. This could indicate

that the concentration of hydrogen and carbon may originate

from the SIMS measuring chamber and not necessarily from

the ZnO samples analyzed here. Indeed, even if hydrogen is

regarded as a cause of doping in ZnO,11,12 and carbon as an

amphoteric but electrically active impurity in ZnO,13,14 it is

difficult to determine the exact concentration of these ele-

ments in our samples, because they are abundant and diffi-

cult to remove totally from the SIMS chamber, leading to a

high detection limit in the sample.

Inversely, boron (B), indium (In), and gallium (Ga) con-

centrations are relatively low (<5:0� 1015cm�3) and close

to the SIMS detection threshold for both samples. Despite

their significant potential role on the n-type conductivity in

ZnO,15,16 these species are suspected to have a negligible

influence on the electrical properties of the ZnO epilayers

due to their relatively low concentrations.

In the case of the homo-epitaxial sample, the Figure 2(a)

reveals relatively low aluminum (Al) and lithium (Li) con-

centrations in the ZnO epilayer, lower than 5:0� 1015 and

5:0� 1016cm�3, respectively, which slightly diffuse from

the ZnO substrate into the ZnO epilayer. Similarly, the SIMS

silicon (Si) profile provides also evidence that silicon has

also diffused into the epilayer but with a maximum silicon

concentration at the substrate-layer interface larger than

3:0� 1018cm�3. Like carbon, silicon in ZnO is considered

as an amphoteric dopant which could act as a donor if on a

zinc site SiZn or as an acceptor if on an oxygen site SiO.17–19

In the case of the hetero-epitaxial samples grown on sap-

phire, the SIMS profiles shown in Figure 2(b) reveal an

extremely low lithium (Li) concentration, close to the SIMS

detection threshold both in the substrate and in the epitaxial

ZnO layer. On contrary, aluminum (Al) and silicon (Si) SIMS

profiles indicate a strong diffusion of these impurities within

the ZnO thin film, as in the previous homo-epitaxial case, with

concentrations larger than 1:0� 1018cm�3 at the sapphire-ZnO

interface for these two species. Nevertheless, the diffusion pro-

file is less extended for hetero-, than for homo-epitaxial sam-

ples and seems to originate exclusively from the substrate-layer

interface with a silicon concentration around 3:5� 1018cm�3

at the interface for this particular sample. This silicon contami-

nation may result from exposure of the substrate surface to air

and/or from chemo-mechanical polishing residues.

Unfortunately, even if silicon SIMS measurements show

typical diffusion profiles, the silicon concentration at the sub-

strate surface is intrinsically uncontrolled and did not allow

us to perform relevant quantitative analysis of these diffusion

profiles, such as the one we will implement below in the case

of aluminum.

In the case of aluminum, the SIMS concentration profile

plotted in Figure 2(b) also exhibits a typical diffusion profile

spreading over a few micrometers. Moreover, aluminum

impurities appear to come from the sapphire substrate as al-

ready observed by Tang et al.20 also in the case of ZnO thin

films grown by MOVPE on sapphire substrates. In this case,

as we will see in the following, the aluminum concentration

at the Al2O3=ZnO interface results from thermodynamic

equilibrium during MOVPE growth between the zinc oxide

thin film and an infinite reservoir of aluminum, consisting of

the sapphire substrate itself. Using Fick’s diffusion law, this

allows to determine the diffusion coefficient and the satura-

tion solubility of aluminum in ZnO.

B. Aluminum diffusion

In order to further investigate aluminum diffusion in

ZnO during MOVPE growth, the influence of the growth

FIG. 2. Typical SIMS impurity profiles, performed on ZnO layers grown by MOVPE, at the same growth temperature (860 �C), on ZnO (a) and sapphire (b)

substrates.

113508-3 Brochen et al. J. Appl. Phys. 115, 113508 (2014)

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temperature has been studied in the case of thin films grown

on sapphire substrates. Figure 3 shows the aluminum concen-

tration profiles determined by SIMS on samples #1 to #5,

listed in Table I, for growth temperatures from 800 �C to

990 �C and a growth duration between 6000 s and 17400 s cor-

responding to layers thicknesses of a few micrometers. The

SIMS aluminum profiles, in the 1� 1015 � 1� 1018cm�3

concentration range, confirm that aluminum diffused from the

sapphire substrate into the ZnO epilayer for all samples.

The one dimensional Fick’s diffusion law has been

used, assuming a semi-infinite solid in equilibrium with an

infinite aluminum source, in order to describe the aluminum

diffusion profiles as schematically represented in Figure 4.

At a growth temperature Tg, the concentration of aluminum

Cðx; tgÞ in the epilayer after a growth time tg and along the xdirection can be written as21

C x; tð Þ ¼ CSatAl Tgð Þ 1þ erf � x

2ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiDAl Tgð Þ � tg

p� �� �(1)

with erf the error function, DAlðTgÞ (cm2:s�1) the aluminum

diffusion coefficient, and CsatAl ðTgÞ (cm�3) the saturation solu-

bility of aluminum in ZnO. In this model, the concentration

of aluminum in the reservoir CResAl is supposed to be sufficient

to reach the saturation solubility, i.e., CResAl > Csat

Al , for each

temperature involved here.

This two quantities depend on temperature Tg and can

be described by an Arrhenius law. Indeed, the temperature

dependence of the diffusion coefficient can be written as

DAl Tgð Þ ¼ D0 � exp � Ed

kBTg

� �(2)

where kB is the Boltzmann’s constant, Ed is the activation

energy of the impurity diffusion, and D0 is a constant

(cm2:s�1) depending on the reservoir concentration.

Similarly, the temperature dependence of the saturation solu-

bility can be expressed as

CSatAl Tgð Þ ¼ C0 � exp � Ef

kBTg

� �(3)

where Ef is the formation energy of aluminum on the

involved diffusion site and C0 (cm�3) is the concentration of

these sites. Therefore, Eq. (1) has been used to adjust the alu-

minum diffusion profiles measured by SIMS as shown in

Figure 3. In order to simplify the problem, only the growth

parameters (temperature Tg and duration tg) corresponding to

the main growth step (numbered 5 on Figure 1), have been

considered in Eq. (1). As shown in Figure 3, the use of this

model, based on Fick’s diffusion law, allows us to adequately

describe the aluminum SIMS concentration profiles for this

series of five ZnO samples grown at different temperatures

by MOVPE on sapphire substrates.

Thus, several values of the aluminum diffusion coeffi-

cient DAlðTgÞ and of the aluminum saturation solubility

CSatAl ðTgÞ in the 800� 990 �C temperature range have been

extracted from the fits of the aluminum SIMS concentration

profiles of Figure 3 according to Eq. (1). These values are

listed in Table I and plotted in Figures 5(a) and 5(b). In addi-

tion to our experimental values, data taken from the literature

and obtained by a chemical method in the case of polycrys-

talline ZnO samples22 have been plotted in the same temper-

ature range for comparison.

From the Arrhenius diagrams of the aluminum diffusion

coefficient DAlðTgÞ (Fig. 5(a)), we deduced an activation

energy associated to aluminum diffusion in ZnO single crys-

tal of Ed ¼ 2:71 eV, consistent with the value of 2:74 eV

given in Ref. 22 by V. J. Norman.

Nevertheless, as shown in Figure 5(a), the aluminum

diffusion coefficient is higher by about one order of magni-

tude, for the same temperature range, than the value reported

in Ref. 22. This difference can be explained by the fact that

the diffusion coefficient depends on the aluminum concen-

tration in the reservoir CResAl which therefore appears to be

higher in our case, where this aluminum source consists in

the sapphire substrate itself.

Unlike the diffusion coefficient, the saturation solubility

CSatAl is independent of the aluminum source concentration

CResAl , provided that there is sufficient aluminum in the source

to reach the saturation solubility CSatAl ðTgÞ at the solid-source

interface, i.e., when the aluminum concentration of the

FIG. 3. SIMS aluminum profiles measured on ZnO thin films grown on sap-

phire substrates and listed in Table I (symbols). Adjustment according to

Eq. (1) (solid lines).

FIG. 4. Schematic Fick’s diffusion profile in the case of a material in ther-

modynamic equilibrium with an infinite reservoir of aluminum with concen-

tration CResAl exceeding the aluminum saturation solubility CSat

Al of this

material at a given temperature.

113508-4 Brochen et al. J. Appl. Phys. 115, 113508 (2014)

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source is greater than or equal to the aluminum saturation

solubility at a given temperature (Tg).

Furthermore, the temperature dependence of the alumi-

num saturation solubility CSatAl ðTgÞ, plotted in Figure 5(b), is

also close to the values determined in Ref. 22, except for the

sample with the lower growth temperature. This experimental

observation could be explained by the fact that some growth

steps, as steps numbered 3 and 4 in Figure 1, occur at higher

temperature than the main growth regime, and could affect

the concentration of aluminum at the ZnO/sapphire interface.

Indeed, if considering the saturation solubility CSatAl value

for the sample with the smallest growth temperature Tg

¼ 800 �C (#1), a corresponding temperature from the

Arrhenius plot is found around 920 �C (Fig. 5(b)), which is

close to the high temperature steps numbered 3 and 4 preced-

ing the epilayer main growth step. Thus, even if the main

growth step, numbered 5 on Figure 1, occurred at relatively

low temperature, the aluminum concentration at the

sapphire-ZnO interface seems to be imposed by the preced-

ing steps occurring at higher temperature.

Nevertheless, in excluding this last data point (#1) and

assuming, as we will see in the following parts, a substitutional

diffusion of aluminum on Zn site with C0 ¼ 4:2� 1022cm�3,

a formation energy for substitutional aluminum of Ef

¼ 1:00 eV is obtained, close to the value of 1:08 eV given in

Ref. 22. Thereby, from the activation energy of the diffusion

Ed ¼ 2:82 eV, determined previously, which is the sum of the

formation energy Ef ¼ 1:00 eV of the involved point defect

AlZn and its migration barrier Eb, i.e., Ed ¼ Ef þ Eb,23 we

deduce a migration barrier energy of Eb ¼ 1:71 eV.

As aluminum is well known as a shallow donor in ZnO,

when substituting for zinc in the lattice, this impurity is

expected to play an important role on the electrical proper-

ties of such epilayers.16,24

C. Identification of residual donors

1. Heteroepitaxial ZnO layers grown on sapphiresubstrates

The previous experimental observations highlight a

strong diffusion of aluminum in the ZnO epilayer during the

MOVPE growth but also an increase of the Al-SIMS concen-

tration near the ZnO epilayer surface (Fig. 3) except for sam-

ple (#3). This experimental observation, maybe due to

surface impurity segregation, is not yet well understood but

implies an impurity concentration higher than 1016cm�3 near

the top of the ZnO epilayer, which is a sufficient concentra-

tion to affect the electrical properties of these samples.

Figure 6 shows the effective dopant concentration pro-

files ND � NA measured by C(V) on samples #1 to #5 com-

pared to aluminum and silicon SIMS concentration profiles.

The depth analyzed under the Schottky contact corresponds

to the extent of the space charge region under reverse bias

and depends on the sample doping range.

In the cases of samples #1 and #2, the concentrations of

silicon shown in Figure 6 are higher than the aluminum con-

centrations in the depth reachable by C(V) measurements. In

these cases, the SIMS profiles of silicon are very close to the

effective dopant concentration profile ND � NA measured by

C(V). This indicates that silicon seems to be responsible for

the residual n-type doping of samples #1 and #2.

In the opposite case of sample #3, the aluminum SIMS

concentration profile is higher than that of silicon and fully

corresponds to the effective dopant concentration profile.

The residual n-type doping is governed here by the diffusion

profile of aluminum from the sapphire into the ZnO epilayer.

This experimental observation confirms that during the diffu-

sion process, aluminum is mainly located on electrically

active sites like substitutional zinc sites AlZn and reveals that

the activation energy of the aluminum diffusion in ZnO

Ed ¼ 2:71 eV determined previously is related to a substitu-

tional diffusion mechanism as suggested by V. J. Norman.22

In the intermediate cases of samples #4 and #5, the

SIMS profiles of aluminum and silicon are in the same con-

centration range for the corresponding depth explored by

C(V) measurement (Fig. 6). The effective dopant concentra-

tion ND � NA then appears to be equal to the sum of the con-

centrations of aluminum and silicon measured by SIMS.

This last observation confirms with regard to results shown

in Figure 6 that both aluminum and silicon have a predomi-

nant role on the residual n-type doping on MOVPE ZnO thin

films analysed here.

Note that near the surface of sample #3 (Figure 6), which

is the less doped sample, the effective dopant concentration

FIG. 5. Arrhenius plots of the aluminum diffusion coefficient DAl and the

aluminum saturation solubility CSatAl determined from Eq. (1) for ZnO thin

films grown on sapphire substrates at various growth temperatures listed in

Table I. Experimental data (symbols) and linear adjustment (doted line)

compared to results from V. J. Norman (dashed line).22

113508-5 Brochen et al. J. Appl. Phys. 115, 113508 (2014)

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profile ND � NA decreases more rapidly than the aluminum

diffusion profile, reaching a doping concentration ND � NA

< ½Al� þ ½Si�. This experimental observation could be

explained by donor compensation by an acceptor concentra-

tion NA around 2:0� 1016cm�3 and seems to indicate the ab-

sence of another donor level in this concentration range.

2. Homoepitaxial ZnO layers

In order to try and minimize impurity diffusion from the

ZnO substrate to the epilayer, a homoepitaxial ZnO thin film

has been grown at relatively low temperature (860 �C) with a

relatively large thickness (�3:5 lm), as reported in Table II.

The homoepitaxial thin film (sample #6) has been grown in a

single step on an O-polar hydrothermal ZnO single crystal

substrate (from Tokyo Denpa).

Similarly to ZnO layers grown on sapphire substrates,

SIMS concentration profiles of aluminum and silicon measured

on ZnO homoepitaxial layer (sample #6) have been compared

to the effective dopant concentration profiles (Fig. 7). In order

to compare residual n-type doping of the ZnO epilayer to the

bulk ZnO substrate, SIMS and C(V) have equally been per-

formed on the rear surface of the ZnO substrate (sample #7).

The SIMS concentration profiles of silicon are in both

cases, ZnO epilayer and substrate, higher than the aluminum

profiles and close to the effective dopant concentration pro-

files measured by C(V) as shown on the Figure 7. This obser-

vation is consistent with the results obtained in Sec. III C 1,

about heteroepitaxial ZnO thin films grown on sapphire sub-

strates, and confirms that silicon impurity acts as a donor dop-

ant in ZnO and can be responsible, at sufficient concentration,

for the residual n-type doping of ZnO samples.

Note that the donor behavior of silicon in ZnO is con-

sistent with density functional theory analysis which predicts

that Si preferentially substitute Zn in ZnO, acting as a

FIG. 6. Aluminum (circles) and silicon (stars) SIMS concentration profiles

compared to C(V) effective dopant concentration profiles ND � NA (squares)

measured on undoped ZnO thin films grown on sapphire substrates and listed in

Table I. The Schottky contact is located at the origin of the x-axis (zero depth).

FIG. 7. Aluminum (circles) and silicon (stars) SIMS concentration profiles

and C(V) effective dopant concentration profiles ND � NA (squares) measured

on undoped ZnO thin films (#6) grown on ZnO substrates (#7) and listed in

Table II. The Schottky contact is located at the origin of the x-axis (zero depth).

113508-6 Brochen et al. J. Appl. Phys. 115, 113508 (2014)

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shallow donor.17–19 However, the most favourable associated

Si impurity form is expected to be the double donor Si2þZn

charge state. This appears to be in contradiction both with

our observations and with other experimental works involv-

ing an effective single donor behaviour of silicon in

ZnO,25–27 which may be due to a more complex form of Si-

related shallow donor levels in ZnO.

Moreover, from C(V) measurements the residual n-type

doping is about one order of magnitude less in the ZnO epi-

layer than in the ZnO hydrothermal substrate with effective

dopant concentrations about ND � NA ¼ 2:7� 1016cm�3 and

ND � NA ¼ 1:8� 1017cm�3, respectively.

Notice furthermore that the concentration of acceptor lev-

els can be estimated, in considering aluminum and silicon as

majority donors, by NA ¼ ½Al� þ ½Si� � ðND � NAÞ ¼ 1:9�1016cm�3 which is close to the previously estimated value,

of about 2:0� 1016cm�3, in the case of heteroepitaxial ZnO

thin films grown on sapphire substrates (sample #3). This

acceptor concentration NA includes all the acceptor levels

located below the Fermi level, associated for example to the

presence of intrinsic acceptor defects,23 and appears to be

lower in both homo- and hetero-epitaxial layers, than the value

estimated in a previous work on hydrothermal samples.28

Nevertheless, the same estimation in the case of the ZnO

substrate (#7) leads to an acceptor density NA ¼ 3:9�1016cm�3 which is certainly underestimated, due to the

presence of an additional donor level not linked to aluminum

and silicon impurities. Indeed, as already discussed in Ref. 28,

the coexistence of a deep and a shallow donor level with con-

centrations in the same order of magnitude as the compensa-

tion leads to a more realistic value of the acceptor density NA

in hydrothermal samples, higher than 1� 1017cm�3.

Moreover, this low compensation value in MOVPE epi-

layers is also consistent with transport measurements using

f-MEMSA analysis performed on a MOVPE homo-epitaxial

thin film, indicating a high carrier mobility channel in the

epilayer associated to a lower acceptor concentration NA in

the epilayer than in the ZnO substrate.29

D. Intentional n-type doping with aluminum

Having determined above that aluminum impurities dif-

fusing from the sapphire substrate could be responsible for

the residual n-type doping of the ZnO epilayers, we now turn

to aluminum intentional in-situ doping during the MOVPE

growth with trimethyl-aluminum ðC6H18Al2Þ used as source

of Al-dopant.

Moreover, in order to investigate the influence of the

substrate on aluminum incorporation during growth, three

ZnO thin films have been grown in the same run using three

different substrates as listed in Table III. Sample #8 has been

grown on a c-plane sapphire substrate while samples #9 and

#10 have been grown on O-polar and Zn-polar ZnO sub-

strates, respectively.

According to the results discussed previously about the

temperature dependence of the saturation solubility CSatAl (Fig.

5(b)), the n-doped ZnO thin films have been grown during

tg ¼ 17000 s at a growth temperature Tg ¼ 990 �C, as listed

in Table III, which corresponds to a concentration of alumi-

num in substitutional site estimated around 5:0� 1018cm�3.

After a preliminary study consisting in calibrating the alu-

minum incorporation by SIMS measurements as a function of

aluminum precursor flow (not shown), intentionally n-doped

ZnO thin films have been grown with a targeted aluminum

concentration around 5:0� 1018cm�3, according to the alumi-

num saturation solubility set by the growth temperature.

Figure 8 shows both the average aluminum SIMS con-

centrations (solid line) and the effective dopant concentra-

tion profiles (symbols) measured on the three intentionally

n-doped ZnO thin films (samples #8 to #10). In the three

cases, the aluminum SIMS concentration profiles reveal a

flat aluminum incorporation during the growth in the whole

epilayer thickness (not shown), but with a slight difference

of concentration as a function of the substrate used, as

reported in Table III.

The aluminum incorporation in the ZnO thin films, plot-

ted in Figure 8 and reported in Table III, appears to be higher

FIG. 8. Average aluminum SIMS con-

centration (solid lines), effective dop-

ant concentration profiles ND � NA

(symbols), and aluminum saturation

solubility (dashed lines) at a growth

temperature Tg ¼ 990 �C for intention-

ally in-situ Al doped ZnO thin films

grown by MOVPE on c-plane Al2O3

substrate (#8), on O-polar ZnO sub-

strate (#9), and on Zn-polar ZnO sub-

strate (#10) as listed in Table III. The

Schottky contact is located at the ori-

gin of the x-axis (zero depth).

113508-7 Brochen et al. J. Appl. Phys. 115, 113508 (2014)

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in the case of sapphire substrates (sample #8) and is slightly

lower in the case of Zn-polar ZnO substrate (sample #10)

than for the oxygen polarity (sample #9), but stays in the

same doping range, a little higher than the targeted alumi-

num concentration.

Furthermore, the effective dopant concentration profiles

measured by C(V) and also plotted in Figure 8 confirm the

influence of the substrate on the doping efficiency. ZnO thin

films grown on sapphire and O-plane ZnO substrates (sam-

ples #8 and #9) exhibit an equivalent effective dopant con-

centration around 5:0� 1018 cm�3 as reported in Table III.

Sample #10 grown on Zn-polar ZnO substrate presents an

effective dopant concentration significantly smaller, around

2:0� 1018cm�3, indicating a poorer activation of aluminum

as a donor center in case of zinc polarity ZnO substrate.

These comparisons of the aluminum SIMS concentra-

tion ½Al� with the effective dopant concentrations ND � NA

lead to a doping efficiency ðND � NAÞ=½Al� about 56%, 60%,

and 37% for samples #8, #9, and #10, respectively. It is im-

portant to note here that these very high effective dopant

concentrations (ND � NA > 1� 1018cm�3) have been suc-

cessfully measured by C(V) thanks to the use of preliminary

oxygen plasma surface treatments, inducing an increase of

the rectifying behaviour of the Schottky diode.9

Taking into account the results obtained in the previous

section about aluminum diffusion from sapphire, the satura-

tion solubility of substitutional aluminum in ZnO CSatAl ðTgÞ

has also been reported in Figure 8 for a growth temperature

Tg ¼ 990 �C identical for all three samples. As shown in

Figure 8, the effective dopant concentrations in samples #8

and #9 are very close to the aluminum saturation solubility

CSatAl ð990 �CÞ unlike sample #10 in which the effective dopant

concentration is approximately twice lower.

For a SIMS aluminum concentration around 9:0�1018cm�3, as in sample #8 grown on sapphire substrate,

the temperature dependence of the substitutional aluminum

saturation solubility CSatAl ðTgÞ (Fig. 5(b)) indicates that in

order to increase the doping efficiency, the growth tempera-

ture should be higher than in this experiment, i.e., around

1070 �C rather than 990 �C. Indeed, the effective aluminum

concentration will not exceed the substitutional aluminum

concentration dictated by the growth temperature Tg.

Otherwise, beyond the saturation solubility, the excess alu-

minum atoms are not incorporated into the ZnO lattice on

zinc sites and seems to form electrically inactive centres30,31

which can furthermore induce crystal disorder.

E. Optical properties

Beyond the comparison that we carried out previously

between the concentration of aluminum and silicon impur-

ities, as determined from SIMS measurements, and the effec-

tive dopant concentration profiles ND � NA, as determined

from C(V) measurements, we undertook a comparison of the

optical and electrical properties of the different samples. For

this purpose, photoluminescence (PL) measurements have

been performed on samples with an effective donor concen-

trations ND � NA in the 1016 � 1019cm�3 doping range.

The PL spectra obtained from ZnO thin films with

increasing doping concentration are plotted in Figure 9, in

the 3.10–3:45 eV near band edge region. The spectra were

obtained with the same excitation conditions of 3Wcm�2 at a

temperature of 10 K and without normalization so that the

PL intensities of the different emissions can directly be com-

pared. In this energy range, the PL spectra are dominated by

excitonic recombinations, with free excitons (FX) or excitons

bound to neutral donors (D0X),32 followed by longitudinal

optical (LO) phonon replica with an energy separation of

72 meV.33

For all samples, the donor bound A-exciton D0XA emit-

ting at 3.361 eV, and assigned to the presence of an hydro-

genic donor as aluminum (I6)33 dominates the near band edge

emission but with intensities and widths varying with the do-

nor concentration. Note that whatever the respective concen-

tration of silicon and aluminum, the donor bound A-exciton

emission D0XA is at the same energy, indicating that both alu-

minum and silicon behave as hydrogenic donors.

Furthermore, the A-free exciton (FXA) emission at

3:375 eV is seen only for the homoepitaxial sample #6 with

FIG. 9. Photoluminescence measurements performed at 10 K on ZnO thin

films grown by MOVPE on both sapphire and ZnO substrates and containing

an increasing amount of effective dopant concentration ND � NA determined

by CðV) measurements as listed in Tables I–III. Dashed and doted lines indi-

cate the position of excitonic recombinations as described in the text.

113508-8 Brochen et al. J. Appl. Phys. 115, 113508 (2014)

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the lowest residual doping level of 2� 1016cm�3. For this

sample, the LO phonon replica of the A-free exciton are also

clearly seen as well as the emission from the excited state of

the A-free exciton (n¼ 2) at 3:42 eV. For the other samples

with a higher donor concentration, the optical signatures of

the free exciton are no longer seen. Only the LO phonon rep-

lica of the A-free exciton (FXA � LO) are still seen for sam-

ples #1, #2, and #4 which have a donor concentration lower

than 2:0� 1017cm�3.

For the less doped samples (#6, #1, #2, and #4), with a

donor concentration lower than 2:0� 1017cm�3, the D0XA

and the D0XB emissions can be distinguished whereas this is

not the case for samples #10, #9, and #8 with a donor con-

centration higher than 2:0� 1018cm�3. For those samples

the only optical signatures seen are the D0XA emission and

its LO phonon replica.

Moreover, the full width at half maximum (FWHM) of

the D0XA photoluminescence emission (determined assum-

ing Gaussian distributions as illustrated by the dashed-doted

lines in Figure 9) on each samples analysed here and

reported in Figure 10 increases with the effective dopant

concentration measured by C(V). This bandwidth broadening

DE can be expressed as,34,35

DE ¼ DE0 þ DEI�B (4)

where DE0 is a constant associated in part to the intrinsic

linewidth at T¼ 0 and the thermal broadening at 10 K, and

DEI�B is the widening of the D0XA emission due to the impu-

rity band broadening DED with the increase of the dopant

concentration ND as36

DED �q2

e

4peNDð Þ

13 (5)

where qe is the electronic charge and e ¼ 7:8e0 is the static

dielectric constant in ZnO.33 Applying Haynes’ rule which

correlates the neutral donor-bound exciton DD0XA emission

energy with the donor ionization energy ED as DD0XA

¼ aDED, we can roughly estimate

DEI�B � aq2

e

4peNDð Þ

13: (6)

From experimental data adjustment (Fig. 9) and neglecting

the concentration of acceptors as ND � NA � ND, values of

DE0 ¼ 2:8 meV (at 10 K) and of a ¼ 0:2 are obtained. This

values of a ¼ 0:2 is consistent with Haynes’s constant values

a ¼ 0:2� 0:3 in ZnO.1 As shown in Figure 10, this simple

description allows to describe adequately the evolution of the

D0XA FWHM in this 3� 1016 � 5� 1018cm�3 doping range.

Note furthermore that for the intentionally Al-doped

samples #8, #9, and #10, the FWHM of the D0XA emission

has to be interpreted carefully because of an effective donors

concentration around 5� 1018cm�3, close to the expected

ZnO metal-insulator transition, and beyond which screening

by free electrons could affects directly the excitonic recom-

bination energy.37

Moreover, complementary X-ray diffraction measure-

ments (not shown) exclude the possible contribution of an in-

homogeneous strain on the increase of the D0XA FWHM

with the dopant concentration.

Moreover, since the donor concentration ND reported in

Figure 10 represents both the concentration of aluminum and

silicon, as demonstrated previously from the comparison of

C(V) and SIMS measurement, the broadening of the D0XA

photoluminescence bandwidth also suggests that Si behaves

like a donor in ZnO. Indeed, in the case of samples #1, #2,

and #6, where silicon is the major impurity, the D0XA recom-

bination occurs at the same energy than the one expected for

aluminum at 3:361 eV and labelled I6 in the literature.1,33

Nonetheless, this assumption would deserve to be confirmed

by other experiments, as the study of ZnO samples intention-

ally doped with silicon.

These optical results are in good agreement with what

was observed for non intentionally doped homoepitaxially

grown ZnO layers38 and confirm the doping evolution found

by C(V) and SIMS measurements as previously described.

IV. CONCLUSION

In the present study, the identification and the role of

some impurities on the electrical and optical properties of

ZnO thin films grown by metalorganic vapor phase epitaxy

(MOVPE) on both sapphire and ZnO substrates have been

investigated by employing complementary characterization

techniques, such as secondary ion mass spectroscopy

(SIMS), capacitance-voltage characteristics C(V), and photo-

luminescence (PL).

In a first step, SIMS analyses have revealed a strong im-

purity diffusion from the substrate to the epilayer during the

MOVPE growth. In the homo-epitaxial case, impurities

involved in these diffusion processes originate from residual

ZnO bulk impurities, such as lithium, aluminum, and silicon,

in addition to silicon surface contaminations. In the hetero-

epitaxial case, impurity diffusion concerns silicon, essen-

tially from surface contamination, and aluminum from the

sapphire substrate itself, resulting in typical diffusion profiles

spreading over a few micrometers.

Applying Fick’s diffusion law in the particular case of

samples grown on sapphire substrates to describe the

FIG. 10. Experimental (symbols) and simulated (solid line) photolumines-

cence bandwidth of the D0XA recombination at 10 K, according to Eq. (4)

with DE0 ¼ 2:75 meV (dashed line).

113508-9 Brochen et al. J. Appl. Phys. 115, 113508 (2014)

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aluminum SIMS profiles for various growth temperatures,

and assuming the Al2O3 substrate as an infinite reservoir of

aluminum, the saturation solubility and the diffusion coeffi-

cient of aluminum in ZnO have been determined in the

[800 �C� 1000 �C] temperature range (Fig. 5).

In a second step, the correspondence between the effec-

tive dopant concentration profiles determined by C(V) mea-

surement and the SIMS impurity concentration shows on the

one hand that aluminum is mainly located on electrically

active donor sites in the ZnO lattice, such as AlZn. These ex-

perimental observations indicate a substitutional diffusion

mechanism for aluminum from the sapphire substrate to the

epilayer during the MOVPE growth. On the other hand, the

impurity/dopant concentration profiles comparison also

demonstrates that silicon does not behave as an amphoteric

impurity in ZnO, but only as a donor. Complementary photo-

luminescence measurements confirmed furthermore that,

similarly to aluminum, silicon acts as a shallow donor in

ZnO. Thus, these two impurities have been identified as

responsible for the background doping concentration in ZnO

epilayers.

As aluminum and silicon impurities diffuse from the

substrate and/or from its surface into the ZnO epilayer during

the MOVPE growth, low residual doping has been achieved

in decreasing the growth temperature and/or increasing the

layer thickness. Thereby, samples with an effective residual

dopant concentration ND � NA in the 2� 3� 1016 cm�3 and

5� 6� 1016cm�3 range have been reached in the case of

homo-epitaxial and hetero-epitaxial samples, respectively.

This controlled and low residual doping is a key parameter

to find the best conditions for easier p-type doping in further

studies.

Moreover, intentional in-situ aluminum doping using

trimethyl-aluminum ðC6H18Al2Þ as a source, has been

achieved up to ND � NA ¼ 5� 1018cm�3 for a growth tem-

perature of 990 �C, consistent with the aluminum saturation

solubility, as determined previously from the aluminum

Fick’s diffusion profiles. Moreover, the Arrhenius plot of

this saturation solubility indicates that an increase of the

temperature growth is needed in order to increase the substi-

tutional aluminum concentration and thereby the doping

level in the case of large aluminum precursor flow.

ACKNOWLEDGMENTS

The authors would like to thank M. Leroux for the criti-

cal reading of the manuscript. This work was supported by

the French National Research Agency (ANR) through the

ANR MATETPRO DeFiZnO Project (No. MAPR09-

442955) and through Carnot Funding.

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