Growth, optical, and electrical properties of nonpolar m-plane ZnO on p-Si substrates with Al2O3 buffer layers

Article abstract of DOI:10.1063/1.3673346

Nonploar m-plane ZnO films were grown on p-Si (111) substrates by using atomic layer deposition. X-ray diffraction and high resolution tunneling electron microscopy measurements showed that the ZnO films were 〈1010〉 oriented, and the crystalline quality o

Full text of DOI:10.1063/1.3673346

Growth, optical, and electrical properties of nonpolar m -plane ZnO on p -Si substrates
with Al2O3 buffer layers
T. Wang, H. Wu, C. Chen, and C. Liu
Citation: Applied Physics Letters 100, 011901 (2012); doi: 10.1063/1.3673346
View online: http://dx.doi.org/10.1063/1.3673346
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/100/1?ver=pdfcov
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APPLIED PHYSICS LETTERS 100, 011901 (2012)
Growth, optical, and electrical properties of nonpolar m-plane
ZnO on p-Si substrates with Al O buffer layers
2
3
a)
T. Wang, H. Wu, C. Chen, and C. Liu
b)
Department of Physics, Key Laboratory of Acoustic and Photonic Materials and Devices of Ministry
of Education, Wuhan University, Wuhan 430072, People’s Republic of China
(Received 11 November 2011; accepted 8 December 2011; published online 3 January 2012)
Nonploar m-plane ZnO films were grown on p-Si (111) substrates by using atomic layer
deposition. X-ray diffraction and high resolution tunneling electron microscopy measurements
ꢀ
showed that the ZnO films were h1010i oriented, and the crystalline quality of ZnO films was
improved with an Al O buffer layer, which significantly enhances the photoluminescence and
2
3
reduces the reverse leakage current of ZnO/Si heterojuction. VC 2012 American Institute of Physics.
doi:10.1063/1.3673346]
[
Recently, zinc oxide (ZnO) has attracted much attention
for its potential applications in the next generation ultra-
violet optoelectronic devices due to its wide direct band gap
were used as the sources of zinc and oxygen, respectively.
Pung et al. had deposited ZnO thin films by ALD, and they
ꢀ
ꢀ
reported that ZnO films obtained at 200 C were (1010)
1
16
dominated. During the deposition, DEZn and H O were
of 3.37 eV and high exciton binding energy of 60 meV. So
far, several deposition techniques have been developed to
2
alternately fed into the chamber using nitrogen as the carrier
gas. After each process, a second purge using nitrogen was
introduced to clean the redundant former precursor. The
thickness of the ZnO film was of 600 nm. The Al2O3 thin
film was also grown by ALD technique using trimethyl
2
grow ZnO thin films, e.g., molecular beam epitaxy (MBE),
3
pulsed laser deposition, chemical vapor deposition, and
4
5
magnetron sputtering. Because of the minimum surface free
6
energy of ZnO (0002) plane, ZnO films deposited by most
techniques are (0002) dominated. As is well known, the elec-
tric field is polarized along the c-axis of wurtzite structures,
caused by the spontaneous and piezoelectric polarizations,
and thus, electrons and holes are segregated in crystal. This
polarized field has negative effects on the performance of
devices such as a decrease of internal quantum efficiency of
aluminum and H O as the precursors. 100 cycles were per-
2
formed, and the Al2O3 thin films were 10 nm thick.
X-ray diffraction (XRD, Bede D1) and high-resolution
transmission electron microscopy (HRTEM, JEOL JEM
2010) were used to evaluate the crystallization, microstruc-
tural, and interfacial properties. The photoluminescence (PL)
(HORIBA LabRAM HR800) spectra and electrical measure-
ments were utilized to analyze the effects of the buffer layer,
in order to estimate the advantages of nonpolar ZnO films.
Figure 1 shows the XRD spectra of the ZnO thin films
with (sample S1) and without Al2O3 buffer layer (sample
S2). The peaks all matched with the standard diffraction pat-
tern of wurtzite ZnO. The results revealed that ZnO thin
7–9
the emitting devices.
Therefore, it is of importance to
grow ZnO thin films without the effects of the polarized elec-
tric field.
In nonpolar ZnO films, electric field along the growth
direction is zero. Therefore, it is beneficial to prepare high-
quality nonpolar ZnO films to improve the properties of the
1
0,11
optoelectronic devices.
that a-plane ZnO films grown on r-plane sapphire substrates
Several studies have reported
ꢀ
films were all h1010i oriented whether with or without
1
2,13
14
by metal-organic chemical vapor deposition
and MBE,
Al2O3 buffer layer. Due to the lowest surface energy of
(0002) plane of ZnO, film grown along h0001i direction
as well as m-plane ZnO films deposited on MgO (001) sub-
15
strates by MBE. However, the efforts to grow nonpolar
ZnO films on Si substrates are very limited. To integrate
ZnO optoelectronic devices with the conventional Si-based
semiconductor devices is very promising.
In this work, we have prepared a heterostructure of non-
polar m-plane ZnO film on p-Si (111) substrate by atomic
layer deposition (ALD). An Al2O3 ultrathin layer was fabri-
cated by ALD as the buffer layer. It was found that the
Al O layer improves not only the crystalline quality of the
2
3
ZnO films, but also the electrical and optical properties of
the p-n heterojunction.
N-type ZnO films were deposited on p-type Si (111)
substrates by ALD (Beneq TSF-200) at a temperature of
ꢀ
2
00 C. Diethyl zinc (DEZn) and deionized water (H2O)
FIG. 1. XRD spectra of the ZnO thin films grown on Si without Al2O3
buffer layer (top) and with the buffer layer (bottom). The diffraction peaks
of ZnO (100) can be observed in both spectra.
a)
Electronic mail: H.Wu@whu.edu.cn.
b)
Electronic mail: Chang.Liu@whu.edu.cn.
0003-6951/2012/100(1)/011901/3/$30.00
100, 011901-1
VC 2012 American Institute of Physics
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1

011901-2
Wang et al.
Appl. Phys. Lett. 100, 011901 (2012)
FIG. 2. (a) TEM image of ZnO/Al2O3/
Si. (b) HRTEM image of the ZnO/
Al2O3/Si interfaces. ZnO is polycrystal
near the interface. (c) HRTEM image of
ZnO in the top region, where ZnO film
ꢀ
is h1010i oriented. The inset shows the
DDPs of the marked regions. (d)
HRTEM image of the ZnO/Si interfaces
of without the buffer layer.
ꢀ
should be faster than that along h1010i direction. However,
With the deposition of Al2O3 buffer layer, the intensity of
band-edge transition increased about 58%, due to the improve-
ment of crystallization and the orientation of the m-plane ZnO
films. Although the deep level emissions were very weak even
without the buffer layer, it was obviously that it still decreased
when Al O buffer layer was introduced, as shown in the inset
ꢀ
no ZnO (0002) peak has been found, indicating the (1010)
oriented growth nature of the nonpolar ZnO films. It is well
known that the ZnO (0002) is a polar plane, which can be
positively charged with zinc ions [Zn-(0001)] at the end or
ꢀ
negatively charged with oxygen ions [O-(0001)] at the end.
2
3
Note the fact that, DEZn might dissociate into ethyl group
ꢀ
of Fig. 3. Note that the two samples were deposited under the
same conditions except for the buffer layer. Hence, the
decrease of the deep level emissions of ZnO films with Al O3
fragments such as CH CH - and CH - at 200 C. Further-
3
3
2
more, these anions might adhere to the positively charged
1
2
6,17
Zn-(0001) polar surface.
hindered direction, and as a result, the ZnO films grew along
Thus, h0002i growth may be
buffer layer was directly suggested to less oxygen vacancies
on the interfacial layer. One reason may be related to the amor-
phous Al O layer which has more hydroxyl than autoxidation
ꢀ
the h1010i direction.
2
3
Figure 2 shows the cross-sectional TEM images of the
ZnO film on the Al O buffer layer. The orientation relation-
SiO2, leading to compact ZnO films. Another reason might be
the oxygen atoms which can be removed from the amorphous
Al O layer and occupy the oxygen vacancies at the initial
2
3
ship of Si and ZnO is Si(111)//ZnO(100). An amorphous layer
about 1 nm thick on the Si substrate was attributed to SiO2
due to the autoxidation in the air. It should be noted that the
interfacial of ZnO is polycrtstal, which could be observed in
Figs. 2(b) and 2(d). The nano-crystallites can be clearly found
in the interfacial region in Figs. 2(b) and 2(d). With the amor-
phous buffer layer of Al O , the size of the nano-crystallites
2
3
18
stage of the growth of ZnO films. The few oxygen vacancies
also lead to the better crystallization. Therefore, the deep level
emissions at around 500 nm due to the oxygen vacancies
decreased, and the band-edge transition increased after intro-
ducing the Al O buffer layer.
2
3
Figure 4 shows a typical current-voltage (I-V) character-
istic of the n-ZnO/Al2O3/p-Si heterostructures. To achieve
the Ohmic contacts, the indium and aluminum electrodes
2
3
became bigger which can be observed from Figs. 2(b) and
(d). The improvement of the crystallization and the orienta-
2
2
tion may lead to the increase of the nano-crystals size. During
the growth, it may be suggested that the small particles gath-
with the area of 1 mm were welded on the surfaces of ZnO
and Si layers, respectively. The I-V curve was measured by
changing the bias voltage from þ5 to ꢂ5 V. The area of p-n
ꢀ
ered together and grew along h1010i, as proved in the Fig.
2
2(c), in which the digital diffraction patterns (DDPs) obtained
junction was about 6 mm , and a diode-like rectification
by fast Fourier transformation (FFT) of the marked region,
corresponding to the top region of ZnO film, indexed and
behavior appeared. Under the reverse bias of ꢂ5 V, the leak-
age current of the m-plane ZnO/p-Si without the buffer layer
was about 3 lA while that with the buffer was about 0.3 lA.
The amorphous Al2O3 layer helped to decrease the leakage
current due to its insulating properties. Therefore, the 10 nm
ꢀ
determined the wurtzite structure of ZnO with a h1010i orien-
tation. Moreover, the HRTEM graph in Fig. 2(d) obviously
showed that the size and the crystallization of the grains grad-
ually became with increasing thickness better.
Figure 3 shows the PL spectra of the ZnO films with and
without Al O buffer layer. The ratio of band-edge transition
2
3
to deep level emissions was above 60. As is known, the deep
level emissions were from zinc interstitials or oxygen vacan-
cies. Very weak deep level emissions indicated few zinc
interstitials and oxygen vacancies in the m-plane ZnO films.
It was due to the principle of ALD as shown in the following
16
reactions:
ꢁ
ꢁ
ZnOH þ ZnðCH2CH3Þ ! ZnOZnðCH2CH3Þ þ C2H6
2
ꢁ
ꢁ
ZnðCH CH Þ þ H O ! ZnOH þ C H
2
3
2
2
6
FIG. 3. (Color online) The PL spectra of the ZnO films with and without
Al2O3 buffer layer. The inset shows the logarithmic intensity spectra.
Sample S1 has a stronger free-exciton emission and a weaker deep level
emission.
Because of the self-limiting growth of the ALD technique,
ZnO films with few zinc interstitials and oxygen vacancies
were obtained.
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011901-3
Wang et al.
Appl. Phys. Lett. 100, 011901 (2012)
This work is supported by the NSFC under Grant Nos.
0904116, 11074192, 11175135, and J0830310.
1
1
D. M. Bagnall, Y. F. Chen, Z. Zhu, T. Yao, S. Koyama, M. Y. Shen, and
T. Goto, Appl. Phys. Lett. 70, 2230 (1997).
2
X. L. Du, Z. X. Mei, Z. L. Liu, Y. Guo, T. C. Zhang, Y. N. Hou, Z. Zhang,
Q. K. Xue, and A. Y. Kuznetsov, Adv. Mater. 21, 4625 (2009).
B. J. Jin, S. Im, and S. Y. Lee, Thin Solid Films 366, 107 (2000).
T. M. Barnes, J. Leaf, C. Fry, and C. A. Wolden, J. Cryst. Growth 274,
3
4
412 (2005).
S. H. Jeong, B. S. Kim, and B. T. Lee, Appl. Phys. Lett. 82, 2625 (2003).
N. Fujimura, T. Nishihara, S. Goto, J. Xu, and T. Ito, J. Cryst. Growth
130, 269 (1993).
5
6
7
8
9
0
A. Chakraborty, H. Xing, M. D. Craven, S. Keller, T. Mates, J. S. Speck,
S. P. DenBaars, and U. K. Mishra, J. Appl. Phys. 96, 4494 (2004).
F. Bernardini, V. Fiorentini, and D. Vanderbilt, Phys. Rev. B 56, 10024
FIG. 4. (Color online) I-V curves of the samples S1 and S2. A rectification
behavior can be observed. The inset shows the sketch map of our measure-
ment configuration.
(
1997).
X. L. Wu, G. G. Siu, C. L. Fu, and H. C. Ong, Appl. Phys. Lett. 78, 2285
2001).
(
Al2O3 buffer layer effectively prevented the reverse current
cross the heterojunction. However, under the forward bias,
the threshold voltage of the sample S1 layer was about 2 V,
which was only slightly higher than that of the sample S2
1
P. Waltereit, O. Brandt, A. Trampert, H. T. Grahn, J. Menniger, M.
Ramsteiner, M. Reiche, and K. H. Ploog, Nature 406, 865 (2000).
Y. J. Sun, O. Brandt, S. Cronenberg, S. Dhar, H. T. Grahn, K. H. Ploog, P.
Waltereit, and J. S. Speck, Phys. Rev. B 67, 041306 (2003).
C. R. Gorla, N. W. Emanetoglu, S. Liang, W. E. Mayo, Y. Lu, M.
Wraback, and H. Shen, J. Appl. Phys. 85, 2595 (1999).
T. Abe, Y. Kashiwaba, S. Onodera, F. Masuoka, A. Nakagawa, H. Endo,
and I. Niikura, J. Cryst. Growth 298, 457 (2007).
S. Han, S. Hong, J. Lee, J. Song, Y. Nam, S. Chang, T. Minegishi, and T.
Yao, J. Cryst. Growth 309, 121 (2007).
E. Cagin, J. Yang, W. Wang, J. D. Phillips, S. K. Hong, J. W. Lee, and J.
Y. Lee, Appl. Phys. Lett. 92, 233505 (2008).
11
1
2
(about 1.6 V). The Al O buffer layer was also the reason to
the increase of the threshold voltage.
2 3
13
In summary, we have fabricated a heterojunction of non-
ploar m-plane ZnO films on p-Si (111) substrates by ALD.
Al2O3 buffer layer was introduced to improve not only the
crystalline quality but also the optical and electrical proper-
ties. With the Al O buffer layer, a nearly 58% increase of
1
1
1
1
4
5
6
7
2
3
S.-Y. Pung, K.-L. Choy, X. Hou, and C. Shan, Nanotechnology 19,
435609 (2008).
the PL intensity is observed. Furthermore, the leakage cur-
rent of the m-plane ZnO on p-Si is reduced by one order.
This work provides a way to integrate nonpolar ZnO on p-Si
for optoelectronic devices.
Z. R. Tian, J. A. Voigt, J. Liu, B. McKenzie, M. J. McDermott, M. A.
Rodriguez, H. Konishi, and H. Xu, Nature Mater. 2, 821 (2003).
Y. T. Chen, C. L. Cheng, and Y. F. Chen, Nanotechnology 19, 445707
(2008).
18
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