Physical characteristics and photoluminescence properties of
phosphorous-implanted ZnO thin films
Chin-Ching Lin
a,*, San-Yuan Chen
a, Syh-Yuh Cheng
ba
Department of Materials Science and Engineering, National Chiao-Tung University, 1001 Ta Hsueh Road, Hsinchu 300, Taiwan, ROC
b
Materials Research Laboratories, Industrial Technology Research Institution, Chutung, Taiwan, ROC Available online 13 September 2004
Abstract
ZnO films were implanted with phosphorus in the range from 5 1012to 5 1015cm2
. Effect of phosphorus concentration on structural characteristics and photoelectric behavior of phosphorus-implanted ZnO films under different atmosphere and annealing treatment was investigated. It has been demonstrated that below solubility (1:5 1018ions/cm3
), the defect formation will be dominated by annealing atmosphere and more defects can be formed in oxygen ambient than in nitrogen atmosphere as revealed from PL spectra. However, excess phosphorus doping, above solubility (1:5 1018
ions/cm3), will induce the formation of the phosphide compounds in ZnO films and seriously deteriorate the crystallinity and optical property of the films. However, a high-resistive but not p-type ZnO film is obtained by phosphorus doping.
# 2004 Elsevier B.V. All rights reserved.
PACS: 64.75.þg; 82.65.þr; 68.35.Dv
Keywords: ZnO; Phosphorus implantation; Photoluminescence; Defect chemistry
1. Introduction
ZnO is a wide bandgap semiconductor, which has many optoelectronic applications such as laser diodes (LDs), light emission diodes (LEDs), invisible field effect transistors, and phosphorescent display[1,2]. It has been recognized to grow high-quality both p- and n-type ZnO thin films for the development of ZnO-based optoelectronic devices. However, p-type ZnO is very difficult to develop because of problems such as self-compensating process on doping and low solubi-lity of the dopants [3]. Recently, several research groups proposed the formation of p-type ZnO by
various dopants and doping method [4,5]. Joesph et al. have demonstrated that p-type ZnO films can be fabricated by co-doping Ga and N into ZnO. Moreover, Kim et al. mentioned that p-type ZnO can be also developed by phosphorus doping via thermal activation process [6]. Even though it was reported that p-type ZnO has been developed, no detailed studies were made to investigate the effect of phosphorus doping on the structural change and photoluminescence properties of ZnO films. There-fore, in this work, the effect of various phosphorus concentrations and different annealing conditions on structure and photoluminescence properties of phos-phorus-implanted ZnO films will be focused and discussed.
*Corresponding author. Tel.:þ86-3-571-2121
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.05.255
2. Experimental
The ZnO thin films (150 nm) were deposited on 4 in. diameter Si substrates by rf magnetron sputter-ing, using 99.99% ZnO as a target at a substrate temperature of 50 8C, rf power of 50 W, sputtering pressure of 10 mTorr. The phosphor ions with the fluence ranging from 5 1012to 5 1015cm2were
implanted into the as-grown ZnO films. After ion implantation, the specimens were annealed at 850 and 1000 8C for 20 min in pure oxygen and nitrogen atmospheres. The crystal structure was determined by X-ray diffractometer using Cu Ka radiation. Photo-luminescence measurement was performed by the excitation from 325 nm He–Cd laser at room tem-perature. The depth profile of the implanted ions was measured by secondary ion mass spectrometry (CAMECA IMS-SF). The surface and cross-sectional morphologies of the ZnO:P films was analyzed by scanning electron microscopy (FE-SEM, S-4100)
and transmission electron microscopy (TEM
Philips TECNAI 20). The electrical properties of the doped ZnO films were investigated by van der Pauw method room-temperature Hall measurements with non-sintered indium contacts and magnetic field of 0.315 T.
3. Results and discussion
Fig. 1shows the XRD patterns of the P-implanted (fluence of 5 1012, 1 1014, and 5 1015cm2)
and non-implanted ZnO thin films annealed at 850 8C in nitrogen atmospheres. With an increase of phos-phorus concentration, the (0 0 2)-peak intensity decreased obviously and a weak diffraction peak was observed at the fluence of 5 1015cm2 for
the P-implanted ZnO films annealed at 850 8C in nitrogen. Similar behavior was also observed in the case of oxygen atmosphere. It was believed that the phenomenon is strongly dependent on the solubility limit of the implanted phosphorus in ZnO films. In addition, the (0 0 2) diffraction peak of the ZnO films was shifted towards the direction of smaller 2y angle with the increase of fluence from 5 1012 to 5 1015cm2. According to Bragg Law, the shift
toward smaller 2y direction indicates an increase of the lattice constant that was considered due to the
incorporation of the phosphorus into ZnO matrix to form antisite PZn or phosphide (PO4) compound.
Fig. 2 shows the depth profile of phosphorus-implanted ZnO films with various fluences. As it
Fig. 1. XRD patterns of ZnO films implanted with different phosphorus fluences and annealed at 850 8C in nitrogen.
Fig. 2. SIMS depth profile of ZnO films implanted with different phosphorus fluences after annealed at 850 8C in nitrogen atmo-spheres.
can be seen, the secondary ion counts of phosphorus abruptly increase near the side of Si substrate regions. It was believed due to the original implanted phos-phorus and incomplete diffusion in annealing process. This concentration corresponds to the solubility[7]of phosphorus in ZnO film that is determined as 2:5 1017, 1:5 1018, and 8:5 1019ions/cm3 for
the fluence of 5 1012, 1 1014, and 5 1015ions/ cm2, respectively.
Fig. 3shows the SEM images of ZnO films doped with phosphorus and then annealed at 850 8C in nitrogen atmosphere. As shown in Fig. 3(a), both ZnO films with non-implanted and implanted with 5 1012ions/cm2exhibit similar surface morphology
(r.m.s: 2.5 nm). Above that concentration, i.e. 5 1015ions/cm2, Fig. 3(b) illustrates that several
ridge regions are formed in ZnO films (r.m.s.: 5.4 nm). It was postulated that the formation of
the glass-like ridge structure may be related to the excess doping of phosphorus.
In order to investigate the phosphorus doping effect on the crystalline ZnO films, TEM analysis were performed. As shown inFig. 4for ZnO films annealed at 850 8C in nitrogen atmosphere, it was found that the cross-sectional microstructure was clearly divided into two regions: crystalline (columnar shape) and inter-layer (flat-belt) structure. The interinter-layer could be considered as a buffer layer to reduce the stress due to lattice mismatch between ZnO and Si. However, for ZnO films with 1 1014ions/cm2implanted, several
small clusters were observed in the ZnO interlayer in
Fig. 4(b)as marked with arrows. According to energy-dispersive spectrometry (EDS) measurement, those clusters were primarily composed of phosphorus, zinc and oxygen elements that may be related to the formation of glass-like ridge structure.
Fig. 3. SEM images of ZnO films doped with phosphorus at fluences of (a) 5 1012 and (b) 5 1015ions/cm2
and then
annealed at 850 8C in nitrogen atmosphere. Fig. 4. Cross-sectional TEM images of ZnO films annealed at850 8C in nitrogen atmosphere: (a) without phosphorus-implanted; (b) with phosphorus-implanted (fluence: 1 1014ions/cm2
Fig. 5 shows the room-temperature PL spectra of non-implanted and phosphorus-implanted ZnO films annealed at 850 8C in nitrogen atmosphere. The PL behavior for deep-level emission of ZnO films implanted with various fluences of phosphorus is also illustrated in the inset ofFig. 5for comparison. The inset is the deep-level emission of ZnO films implanted with various fluences. It was observed that the peak intensity of the UV emission varies with the concentration of fluence. A very stronger UV peak (378 nm) and a relatively low deep-level emission (545 nm) were obtained for the non-implanted sample. However, as the ZnO films were implanted with different phosphorus fluences and annealed in nitro-gen atmosphere, the UV emission peak of the ZnO films becomes weaker and presents slightly red shift as compared to the non-implanted one. This PL result along with the XRD analysis and surface morphology implies that there should be a solubility limit for phosphorus incorporated into ZnO films. If the implanted concentration is close to the solubility limit, both crystal structure and NBE emission would be strongly influenced and become poor.
On the other hand, as the phosphorus-implanted ZnO films were annealed in O2atmosphere (not shown
here), the UV peak intensity was remarkably decreased compared to that annealed in N2
atmo-sphere. In addition, with increasing phosphorus-implanted concentration up to 1 1014 fluence, a
weaker UV peak accompanied with a stronger deep-level emission around 545 nm was detected in O2atmosphere than that in N2atmosphere. It implies
that more defects were probably induced in ZnO film annealed in oxygen atmosphere than that annealed in nitrogen atmosphere. Therefore, according to above discussion, it was believed that the property deteriora-tion in the phosphorus-implanted ZnO films is corre-lated closely with the formation of defects and some glassy phase (MPO4) as evidenced fromFig. 4(b)of TEM.
In addition, the resistivity and carrier type of non-implanted and phosphorus-non-implanted ZnO films were further investigated by Hall measurement. The non-implanted ZnO film has the resistivity of 52 O cm and exhibits n-type (1:65 1016cm3) characteristics.
However, with phosphorus-implanted fluence of 5 1012ions/cm2, the resistivity of
phosphorus-doping ZnO films increases up to 256 O cm but the carrier concentration approaches to 1:65 1015cm3.
Thus, the conversion of carrier type was never observed that is probably attributed to the formation of phosphide even though the phosphorus element was successfully incorporated into ZnO films.
4. Conclusion
As evidenced from TEM results, the implanted phosphorus in ZnO films tends to react with zinc and oxygen elements to form the clusters and this would induce defects as revealed from PL spectra. Furthermore, it has been demonstrated that below solubility (1:5 1018ions/cm3), the defect formation
is mainly dominated by annealing atmosphere. Above that, both the crystalline quality and optical property will be deteriorated due to the formation of phosphide compounds. Therefore, in this condition, high resistive but not p-type ZnO film is obtained by phosphorus doping.
Acknowledgements
The authors gratefully acknowledge the National Science Council of the Republic of China for its financial support through contract no. NSC-92-2216-E-009-029.
Fig. 5. Dependence of fluence conditions on room-temperature PL spectra for the phosphorus-implanted ZnO films annealed at 850 8C in nitrogen atmospheres. The inset is the deep-level emission of ZnO films implanted with various fluences of phosphorus.
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