Journal of Crystal Growth 310 (2008) 3149– 3153

1. ARTICLE IN PRESS
Journal of Crystal Growth 310 (2008) 3149– 3153
Contents lists available at ScienceDirect
Journal of Crystal Growth
journal homepage: www.elsevier.com/locate/jcrysgro
Investigation of As-doped ZnO films synthesized via thermal annealing of
ZnSe/GaAs heterostructures
O. Maksimov Ã, B.Z. Liu
Materials Research Institute, Pennsylvania State University, University Park, PA 16802, USA
a r t i c l e in fo abstract
Article history: We synthesized ZnO films via oxidative annealing of ZnSe/GaAs heterostructures and investigated their
Received 29 January 2008 structural and optical properties. Films were polycrystalline, c-axis oriented and exhibited superior
Received in revised form optical properties. In addition, we detected nanometer-size As clusters into the ZnO film and a GaxOy
26 February 2008 layer at the ZnO/GaAs interface. Formation of an interfacial layer can prevent use of this technique for
Accepted 14 March 2008
p-type doping and complicates identification of the origin of p-type response in the annealed ZnO/GaAs
Communicated by R. Fornari
Available online 20 March 2008 heterostructures.
& 2008 Elsevier B.V. All rights reserved.
PACS:
71.55.Gs
81.40.Ef
82.80.Pv
87.64.Bx
Keywords:
A1. Auger electron spectroscopy
A1. p-Type doping
B1. ZnO
B2. Semiconducting II–VI materials
1. Introduction with transition metals [5] coupled with nanosecond-long spin
coherence time measured at low temperatures [6] makes this
There is a broad technological and scientific interest in zinc material extremely promising for future application in spin
oxide (ZnO) due to its unique physical and chemical properties [1]. electronics.
It is a radiatively hard wide band gap semiconductor (EG$3.37 eV) Although high crystalline quality ZnO films were grown using
that can be easily doped n-type. Its band gap energy can be tuned molecular beam epitaxy (MBE), chemical vapor deposition (CVD),
by alloying with MgO and CdO from 7.9 to 2.3 eV [2], covering and pulsed laser deposition (PLD), further progress in this area is
deep-UV to visible regions of the spectrum. It has a much higher slowed down by the difficulties associated with doping ZnO
exciton binding energy, 60 meV, when compared with other wide p-type. It is generally acknowledged that high enough doping
band gap semiconductors like GaN or SiC, leading to the efficient levels are difficult to achieve both due to the background n-type
excitonic transitions at room temperature. In addition, owing to doping originating from the presence of H impurities and point
the availability of native substrates and amenability to wet defects, such as O vacancies and Zn interstitials [7–11], and due to
chemical etching, ZnO is an extremely promising material for the large acceptor activation energies and/or low solubility of
the development of optoelectronic devices, such as ultraviolet and commonly used group V (N, P, As) and group I (Li) dopants. In
visible light-emitting diodes (LEDs) and detectors. ZnO nanos- addition, a slow transition from p-type to n-type conductivity was
tructures (nanoparticles, nanorods, nanobelts, etc.), which can be observed by a number of research groups. It was tentatively
synthesized using inexpensive physical vapor transport techni- assigned either to the acceptor migration from the substitutional
ques, were shown to be extremely promising for application as gas to the interstitial position or to the hydrogen diffusion [12,13].
sensors, due to the large surface to volume ratio [3], and In spite of these difficulties, there are reports of ZnO-doped
microlasers due to the superior optical properties [4]. Further- p-type with group V (N [14,15], P [16,17], As [18–22], and Sb
more, reports of high-temperature ferromagnetism in ZnO doped [23,24]) and group I (Li [25]) elements. Co-doping with
two potential acceptors (N and As) [26] or acceptor and donor
(N and Al) [27,28] was also used. In the case of the acceptor–donor
à Corresponding author. Tel.: +17242956624; fax: +17242956617. co-doping, the improvement is believed to be primarily due to the
E-mail address: maksimov@netzero.net (O. Maksimov). higher solubility of the forming N–Al–N complex. A temperature
0022-0248/$ - see front matter & 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jcrysgro.2008.03.027

2. ARTICLE IN PRESS
3150 O. Maksimov, B.Z. Liu / Journal of Crystal Growth 310 (2008) 3149–3153
modulation epitaxy technique was also applied to achieve
002 GaAs
N-doped p-type ZnO [29]. Here, nitrogen-doped layers were
Intensity (Arb. Units)
grown at low temperature (400 1C) to increase solubility followed
by the deposition of thin undoped layer at high temperature 002
(950 1C) to improve crystalline quality. 3.05
In particular, p-type doping utilizing thermal As diffusion from
Intensity (Arb. Units)
GaAs substrates into the ZnO films [18,21,30–34] and by oxidation
of the ZnTe/GaAs heterostrucutres [35,36] was realized by a number
of research groups. In addition, both n- and p-type ZnO films were
102
synthesized through the annealing of undoped ZnSe crystals in the 12 16 20 24
activated oxygen atmosphere (radical beam gettering epitaxy) ω (deg.)
[37,38]. However, additional effects, such as formation of an
interfacial Zn2As2O7 layer [39], Ga diffusion into ZnO film [40,41], 004GaAs
and Zn diffusion into the GaAs substrate [42] were also reported. 110
Also, isolated As atoms should act either as deep acceptors (As is
incorporated substitutionally at the O position—AsO) and donors x100
101 004
(As is incorporated substitutionally at the Zn position—AsZn) or
remain amphoteric (As is incorporated interstitially—Asi). Thus,
p-type conductivity is explained by the formation of a complex with 30 40 50 60 70
two spontaneous Zn vacancies (AsZnÀ2VZn) [43]. Clearly, data Θ -2Θ (deg.)
interpretation is not straightforward and the origin of the p-type Fig. 1. XRD yÀ2y scan registered from the annealed ZnSe/GaAs heterostructure.
response remains controversial. Hence, we synthesized ZnO films by Inset is the XRD o scan registered for the /0 0 2S ZnO peak.
thermal oxidation of MBE-grown ZnSe/GaAs heterostructures and
investigated their structural and optical properties. A particular /0 0 1S or c-axis preferred orientation is usually observed due
emphasis was dedicated to the film/substrate interface that was to its lowest surface free energy. Some misoriented ZnO grains
examined using Auger electron spectroscopy (AES). (/1 0 1S, /1 0 2S and /11 0S) are also present. There were
observations of Te and ZnO2 inclusions in the oxidized ZnTe and
2. Experimental details ZnxNy films [35,46]. Here, we do not detect any peaks that can be
indexed as ZnSe, Se, or ZnO2 phases suggesting that ZnSe was
High crystalline quality ZnSe films were grown in the Veeco entirely transformed into ZnO by oxidative annealing. The /0 0 2S
MBE system on the epi-ready semi-insulating (0 0 1) GaAs ZnO peak is relatively narrow, with a full-width at half-maximum
substrates. Next, they were oxidized in a horizontal tube furnace (FWHM) of $0.31, which is comparable to the previous reports
by annealing in an oxygen flow for 2 h at 500 1C. Our annealing time [31,49]. This corresponds to the mean grain size of $27 nm
exceeds rapid annealing used for ZnTe films (1–25 min) [35,36] and calculated using the Scherrer formula
is comparable to the long annealing (1–5 h) applied toward ZnS
[44,45] and ZnxNy [46–48] films to assure full transformation of D ¼ 0:89l=b cos y
ZnSe into the ZnO. Thickness of ZnSe films was varied between ˚
where l is the X-ray wavelength (1.5406 A), b is the FWHM of
300 nm and 1 mm. Since similar results were obtained for all of the
the diffraction peak in radians, and y is the Bragg diffraction
films, only the data for a 700-nm thick film is presented.
angle [50].
Structural and optical properties of the annealed films were
Inset of Fig. 1 is the o scan (rocking curve) of the /0 0 2S peak
investigated using X-ray diffraction (XRD), Raman and photolumi-
that shows out-of-plane mosaic spread and serves a good
nescence (PL) spectroscopies. XRD measurements were carried out
indication of crystalline quality. Although it is broader
in yÀ2y and o modes (to determine out-of-plane orientation and
(FWHM$3.051) than measured for the PLD-grown ZnO films
mosaic spread) using a Scintag X2 diffractometer. Raman measure-
(FWHM$1.451) [51], it is comparable to the value reported for
ments were performed in a backscattering geometry using a
ZnO film heavily doped with P (FWHM$3.211) [52]. Thus, dopant
confocal Raman set up (CRM 200, WITec) equipped with an Ar+ -ion
(As) incorporation could be the reason of the rocking curve
laser (a 488 nm line focused with a 40 Â objective was used for
broadening.
excitation), Actron spectrometer, and a charge-couple device
Fig. 2 is the Raman spectrum collected from the annealed
camera (Andor DV401-BV CCD). PL measurements were performed
ZnSe/GaAs heterostructure. As is expected for a highly textured
at room temperature. Luminescence was excited with a 266 nm line
film measured in a backscattering geometry, only EHigh (435 cmÀ1)
2
of a pulsed Nd:YAG laser (Microchip NanoUV-266, JDS Uniphase),
and ALO (569 cmÀ1) modes are observed [53]. ALO peak is much
1 1
spectrally resolved through a spectrometer (ISA, Edison), and
more intense, when compared with EHigh and has a characteristic
2
detected with a photomultiplier tube.
asymmetric shape with a low-energy tail. An increase in intensity
The morphology was investigated with scanning electron
of an ALO peak was reported for the N-doped ZnO [54] while
1
microscopy (SEM) in the field emission SEM (JEOL 6700F). The
similar asymmetry was recorded for a Sb-doped ZnO [55]. Thus,
surface and in-depth composition analysis was performed by AES
shape and intensity of ALO mode can serve an indication of dopant
1
in the Physical Electronics 670 filed emission scanning Auger
incorporation.
nanoprobe using a 10 KeV and 10 nA electron beam. The samples
In addition to the two lines originating from the ZnO film, three
were 301 tilted with respect to the electron beam during analysis.
more are present at a low-energy side of the spectrum. An intense
Three KV Ar ion sputtering was used for depth profiling.
peak at 292 cmÀ1 is the LO mode from the GaAs substrate. Two
other peaks recorded at 199 and 257 cmÀ1 require more attention.
3. Results and discussion They can be assigned as Eg and A1g modes of the crystalline As [56]
indicating there are nanometer-size As clusters dispersed into the
Fig. 1 is a yÀ2y scan collected from the annealed ZnSe film. ZnO layer. Absence of the As peaks in the XRD spectrum can be
It reveals that the film is a highly textured /0 0 2S ZnO. Such explained by their small volume fraction.

3. ARTICLE IN PRESS
O. Maksimov, B.Z. Liu / Journal of Crystal Growth 310 (2008) 3149–3153 3151
A1g (As) 3.262 eV
7000
Intensity (Arb. Units)
LO (GaAs)
Eg (As)
165 meV
LO (ZnO)
Intensity (Arb. Units)
A1
6000
E2high (ZnO)
3.5 3.0 2.5 2.0
Energy (eV)
Fig. 4. PL spectrum collected at room temperature from the annealed ZnSe/GaAs
heterostructure.
5000
Si KLL
O KLL
200 400 600 Zn LMM
Raman Shift (cm-1)
Intensity (Arb. Units)
Fig. 2. Raman spectrum collected from the annealed ZnSe/GaAs heterostructure. As LMM
C LMM
200 400 600 800 1000 1200 1400 1600
Kinetic Energy (eV)
Fig. 5. AES survery spectrum from the surface of the annealed ZnSe/GaAs
heterostructure.
Fig. 5 shows a surface spectrum acquired prior to sputtering. In
addition to the expected Zn, O, and As, Si and C are found. While C
is from the hydrocarbon contamination unavoidable during the
Fig. 3. SEM image of the annealed ZnSe/GaAs heterostructure.
sample handling, Si contamination may come from the wall of the
quartz tube during annealing. Both C and Si are present only on
Fig. 3 is the SEM micrograph showing the surface of the ZnO the surface. Se or Ga are not observed, indicating these elements
film. It is composed of dense grains with uniform size distribution are either not present or below the detection limit of the
as is expected for a polycrystalline film. technique. The presence of As on the surface indicates possible
Fig. 4 is the PL spectrum collected at room temperature. It is diffusion of As into ZnO, which is confirmed by the depth profile,
dominated by an intense, narrow (FWHM$165 meV) band edge as shown in Fig. 6. It can be seen that As is uniformly distributed
emission line at $3.262 eV. No significant deep-level emission, within the ZnO film (Region I), with a concentration of roughly 8.5
usually originated from the point defects such as Zn vacancies atomic percents. Thermally activated As diffusion from the GaAs
(VZn), Zn interstitials (Zni), and oxygen vacancies (VO), [1] is substrate into the ZnO film was previously detected using
present at $2.4 eV. Thus, although the film is polycrystalline, secondary ion mass spectroscopy (SIMS) [18,30,35,41,57]. Arsenic
individual ZnO grains are close to stoichiometry and of high content depended on the deposition/annealing conditions with up
optical quality. We should note that superior optical properties to 1021 cmÀ3 As atoms usually incorporated into ZnO. By
were reported for the ZnO films obtained via oxidative annealing comparison, our As concentration is higher, probably, due to the
of ZnS [44,45] and Zn3N2 [47]. long annealing time. Since it is above the solubility limit,

4. ARTICLE IN PRESS
3152 O. Maksimov, B.Z. Liu / Journal of Crystal Growth 310 (2008) 3149–3153
80 this layer should have a nanocrystalline or amorphous structure.
Region I Region II Region III Thus, oxidation of ZnSe is not self-limiting process at this film
Approx. concentration (atom%)
thickness (o1 mm) and further oxidation of GaAs substrate occurs.
60 Furthermore, As diffusion into the ZnO layer can be facilitated by
O
the oxidation process through the anion exchange observed in the
Zn
Ga ZnSe/GaAs system [59,60]. We should also mention that Ga2O3
40 As layer was detected at the ZnO/(Cu, In)GaSe2 [61] and ZnO/GaN
interfaces [62,63]. Since much higher temperatures (41000 1C)
are required to promote reaction between ZnO and Ga2O3 [64], we
do not expect formation of more complex phases like ZnGa2O4.
20
Still, GaxOy interfacial layer can prevent use of oxidative annealing
for p-type doping and complicates identification of the origin of
p-type response in the annealed ZnO/GaAs heterostructures. For
0
example, Zn-doped Ga2O3 is a p-type wide band gap semicon-
Sputtering time (s)
ductor (EG$4.8 eV) [65,66]. Then, electroluminescence may be
Fig. 6. Depth profile of the annealed ZnSe(700 nm)/GaAs heterostructure. due to the hole injection from the p-type Ga2O3 into the ZnO.
Quantification is performed by applying instrument-default relative sensitivity While GaxOy interfacial layer does not form in many cases, for
factors to the integrated peak areas. The slight off-stoichiometry may arise either
example when a few mm thick ZnTe is used [35,36], some of the
from the deviation of the instrument-default relative sensitivity factors from the
real values, or from other factors like preferential sputtering.
previous reports of successful p-type doping [18] and device
fabrication [31–34] using As diffusion into the ZnO/GaAs hetero-
structures have to be revisited since very limited structural
characterization is reported. Clearly, SIMS data cannot be used as
the proof of substitutional or interstitial As incorporation into the
ZnO alloy. Instead, As can be present in the nanometer-size
clusters. Finally, detailed structural characterization should be
performed in each case with particular attention being dedicated
dN (E)/dE (a.u.)
toward the ZnO/GaAs interface to exclude interfacial layer
Ga in formation.
Region III
Ga in
Region II 4. Conclusions
We synthesized ZnO films via oxidative annealing of ZnSe/
GaAs heterostructures and investigated their optical and structur-
al properties using a wide range of techniques. Films were highly
textured and exhibited sharp band edge PL at room temperature.
1065 1070 1075 1080 1085 We observed that As diffusion into ZnO layer is accompanied by
Kinetic Energy (eV) the formation of nanometer-size As clusters within the ZnO
film and a GaxOy layer at the ZnO/GaAs interface. Such a layer
Fig. 7. The Ga LMM spectra acquired at different depths from the annealed
complicates identification of the origin of p-type response in the
ZnSe/GaAs heterostructure.
annealed ZnO/GaAs heterostructures and can prevent use of
oxidative annealing for p-type doping.
formation of As clusters, detected using Raman spectroscopy, is
not surprising. Due to the presence of clusters, we cannot judge on
Acknowledgments
the amount of As incorporated into ZnO alloy at the substitutional
(AsO and AsZn) and interstitial (Asi) positions and on the efficiency
of thermal activated diffusion as a doping technique. This work was partially supported by the Department of the
Navy, Office of Naval Research under Grant N00014-07-1-0460.
Fig. 6 also shows that the zinc/oxygen ratio remains constant
($0.65) within the ZnO film in Region I, indicating uniform Any opinions, findings and conclusions or recommendations
oxidation of ZnSe film during annealing. The slight off-stoichio- expressed in this material are those of the authors and do not
metry may arise either from the deviation of the instrument- necessarily reflect the views of the Office of Naval Research.
default relative sensitivity factors from the real values, or from I would also like to thank Dr. Nitin Samarth (Pennsylvania State
other factors like preferential sputtering. Notice that Ga is not University) for providing ZnSe films. The authors acknowledge use
present within the ZnO film, and low concentration of Ga in of facilities at the PSU Site of the NSF NNIN under Agree-
Region I is from the background noise. ment#0335765.
It is interesting to see an interfacial layer (Region II) formed
between the ZnO film and the GaAs substrate, as shown in Fig. 6. References
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