Correlation between nitrogen and carbon incorporation into MOVPE ZnO at various oxidizing conditions
Introduction
During recent years ZnO has gained a significant recognition on behalf of its unique properties that can enhance performance of optoelectronic, spintronic and renewable energy conversion devices. Specifically, progress in p-type doping [1], [2] and light-emitting diode manufacturing [3] trigger further development in the field. Interestingly, undoped ZnO normally exhibits an n-type conductivity and some consensus has been reached explaining this behavior by a combination of intrinsic defects (such as zinc interstitials, oxygen vacancies, etc.) and/or donor acting impurities (such as hydrogen, aluminum, gallium, etc.). On the other hand, there is a great deal of controversy associated with converting ZnO to p-type in spite of a host of successful demonstrations reported in literature (see, e.g. a review by Look and Claflin [4]). In most of the cases acceptor action of group I and V elements substituting Zn or O atoms, respectively, were investigated. Considering a “simple” extrinsic acceptor-type defect substituting a matrix atom in ZnO (building of complexes having acceptor properties have been reported in literature too [4]), nitrogen (NO−) used to be regarded as the best candidate due to its low ionization energy and size similarity to oxygen [5]. However, the issue is that nitrogen doping in ZnO may also result in donor-like defects, specifically interstitial nitrogen (Ni+) and molecular nitrogen on oxygen site ((N2)O+) [6], [7]. In addition, NO− state can be readily passivated by hydrogen [8] and/or compensated by intrinsic defects [9], and the diversity of nitrogen behavior often leads to contradicting results reported by different authors. Anyhow, the most respected results on nitrogen-induced n- to p-type ZnO conversion were reported using in-situ doping during molecular beam epitaxy [2]. In its turn, metal organic vapor phase epitaxy (MOVPE) is a well-established industrially compatible method synthesizing semiconductor device structures and it has also been widely applied for ZnO synthesis in general and testing nitrogen doping in particular [10]. However, because of carbon contaminations associated with using metal-organics during the MOVPE synthesis, electronic properties associated with atomic/molecular nitrogen can be modified due to building complexes with carbon [11], [12], [13]. Since carbon contaminations are practically inevitable during MOVPE synthesis and may build a serious roadblock for using this technology for controllable p-type doping of ZnO, more studies on interactions between nitrogen and carbon in ZnO are strongly motivated.
In the present paper, we report on a correlation between nitrogen and carbon incorporation into ZnO films during MOVPE synthesis using three different oxidizing conditions in a sense of carbon-to-nitrogen ratios during the oxidation stage: (i) tertiary butanol (t-BuOH) only, (ii) t-BuOH+lower amount of N2O and (iii) N2O only—while diethylzinc (DEZ) is used for supplying Zn.
Section snippets
Experimental
ZnO films were deposited on c-plane Al2O3 and p-type (∼2 Ω cm) Si(1 1 1) substrates by MOVPE (TiTan, EMF Ltd.). The substrates were cleaned using acetone and ethanol in an ultrasonic bath, followed by an etch process in H2SO4:H2O2:H2O=1:1:6 solution. Si substrates were in addition dipped in HF solution to remove a native SiO2 layer. Finally, the Al2O3 and Si substrates were washed with deionized water and dried by N2 immediately before inserting into the chamber. DEZ was used for supplying Zn
Results and discussion
Fig. 1 shows XRD 2θ scan for samples A–E (Table 1) in a region of 30–75°. All the ZnO films exhibit preferential (0 0 0 1) orientation. Indeed, only (0 0 0 2) and (0 0 0 4) ZnO-related diffraction peaks are observed indicating reasonable crystalline quality of the films, Fig. 1. Nitrogen concentration versus depth profiles through samples A–E are shown in Fig. 2. Firstly, N concentration in sample A is likely to be below the detection limit except for the regions in the vicinity of the surface/interface
Conclusions
Nitrogen-doped ZnO films with preferential (0 0 0 1) orientation were synthesized on c-Al2O3 and Si substrates by metal organic vapor phase epitaxy (MOVPE) using tertiary butanol (t-BuOH) and/or N2O as oxidizers for diethylzinc (DEZ). A striking correlation between nitrogen and carbon incorporation into ZnO was revealed by concentration versus depth profiling employing SIMS, consistently with recently reported simulations of nitrogen–carbon complexing. The correlation is observed independently on
Acknowledgment
Financial support for our work provided by the Research Council of Norway (RCN) through “Understanding ZnO” FRINAT and “OxideNano” SUP projects is gratefully acknowledged.
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