Low-Temperature PLD-Growth of Ultrathin ZnO Nanowires by Using Zn x Al1−x O and Zn x Ga1−x O Seed Layers
© The Author(s) 2017
Received: 21 December 2016
Accepted: 7 February 2017
Published: 20 February 2017
ZnO nanowires (NWs) are used as building blocks for a wide range of different devices, e.g. light emitters, resonators, and sensors. Integration of the NWs into such structures requires a high level of NWs’ diameter control. Here, we present that the doping concentration of Zn x Al1−x O and Zn x Ga1−x O seed layers has a strong impact on the NW growth and allows to tune the diameter of the NWs by two orders of magnitude down to less than 7 nm. These ultrathin NWs exhibit a well-oriented vertical growth and thus are promising for the investigation of quantum effects. The doping of the ZnO seed layers has also an impact on the deposition temperature which can be reduced down to T≈400∘C. This temperature is much smaller than those typically used for the fabrication of NWs by pulsed laser deposition. A comparison of the NWs indicates a stronger impact of the Ga doping on the NW growth than for the Al doping which we attribute to an impact of the size of the dopants. The optical properties of the NWs were investigated by cathodoluminescence spectroscopy which revealed a high crystalline quality. For the thin nanowires, the emission characteristic is mainly determined by the properties of the surface near region.
The fabrication of nanostructures such as nanowires (NWs) attracted a lot of interest in the last years, since they allow to reduce the size of devices, e.g. light emitters , electromechanical resonators , and the realization of devices with high spatial resolution, e.g. pressure and 3D imaging sensors [3, 4]. The desired geometry of the NWs is determined by the applications. For example, thick NWs with a thickness of several hundreds of nanometer up to few micrometer are preferred for applications which based on the compression of the NWs, e.g. caused by an externally applied pressure, whereas thin NWs (few nanometer thick) are interesting for surface and bending sensitive applications . These wires are interesting also for gas sensors due to their large surface-to-volume ratio . For example, an enhanced sensitivity of thin NWs to oxygen was reported by Fan and Lu . Of special interest are ultrathin NWs with a diameter comparable to the Bohr radius. For such thin ZnO NWs, a dipole moment of about 10 times larger than that in bulk crystal was predicted by Dag et al. . Furthermore, the presence of quantum effects in these wires is expected, which would opens new fields of applications, e.g. formation of topological qubits [9, 10].
However, there are only a limited number of reports which present the growth of ultrathin NW, e.g. by using electro-chemical deposition , metalorganic vapor phase epitaxy deposition [12, 13], carbothermal growth , and hydrothermal growth method . The drawback of these methods is that they require catalysts which can be partly incorporated into the NWs and might have an influence on their optical and electrical properties. Moreover, NWs grown by these techniques are typically not well-oriented. These drawbacks can limit the usability of the NWs and the perfomance of devices. A promising method to overcome the mentioned drawbacks, i.e. the growth of well-oriented verticaly aligned NWs without using a catalyst, is pulsed laser deposition (PLD). The growth of NWs by PLD was introduced about one decade ago  and is now a well-established technique . This growth represents a trade-off between simplicity and controlling of the NWs’ properties [16–18]. However, the obtained NWs have typically a diameter between 60 and 600 nm and thus are much thicker than the Bohr radius of the exciton. Here, we show that the composition of the underlying seed layer has a significant impact on the diameter of the NWs. By using Al- and Ga-doped ZnO seed layers, we were able to achieve well-oriented NWs with diameters less than 7 nm. Furthermore, we show that the choice of the seed layer has also a strong impact on the growth temperature which can be strongly reduced from T≈950∘C, typically used during the PLD process, down to T≈400∘C. This relatively low process temperature can preserve CMOS structure  and thus high-crystalline ZnO can be integrated in these structures by using this growth process.
In order to investigate the NW growth as a function of the Ga and Al concentration of the seed layer, we deposited Zn x Al1−x O and Zn x Ga1−x O thin films with 0≤x≤7 at.% on a-plane sapphire substrate by using a conventional low-pressure PLD process. Thereby, we use an oxygen partial pressure of about p=0.01 mbar and a growth temperature of about 720∘C. The sintered ceramic targets used for the growth of the seed layers were produced by a mixture of ZnO and Al 2 O 3 or rather Ga 2 O 3 powders. The concentration of the dopants in the thin films was measured by energy-dispersive X-ray spectroscopy (EDX) analysis and the concentration was determined to be 0≤x≤7 at.%. Note that in the case of the Zn x Al 1 − x O films, the measurements were performed on reference films grown on Si (100) substrates deposited under the same conditions as the seed layers for the NW growth in order to avoid an incorrect determination of the Al concentration by the sapphire substrate. All seed layers have a thickness of about 200−250 nm which was determined by spectroscopic ellipsometry.
For the fabrication of the NWs, we use a high-pressure PLD process (HP-PLD). A detailed description of this technique is given in ref. . In contrast to the conventional low-pressure PLD process, used for the fabrication of the seed layers, in the HP-PLD, an undoped ZnO target is used and the presence of a background gas flow is required in order to ensure a transport of the ablated particles from the target toward the substrate. Here, we used Ar as background gas (50 sccm) with a partial pressure of 150 mbar and varied the growth temperature in the range of T=400– 950∘C. As we will show later, the growth temperature has a strong impact on the growth of the NWs.
Results and discussion
By using Al- and Ga-doped ZnO seed layers, the NW growth with respect to the deposition temperature is different compared to that one observed for the undoped films. In the case of Al-doped layers, for the highest growth temperature of T≈950∘C, we observed a well-oriented growth of vertically aligned NWs only for Al concentration of x=3.5 at.% with a diameter of about 80 nm and an aspect ratio of 30 (see Additional file 1: Figure S1 and Additional file 2: Figure S2). For the other concentrations, the NW growth is suppressed and only the growth of a honeycomb-like structure is obtained. These honeycomb-like structures were not observed on the undoped seed layers. By reducing the temperature to T≈700∘C, NWs were obtained on the seed layer with a concentration of x=7 at.% only and the growth on the seed layers with x≤3.5 at.% is suppressed. Surprisingly, a further reduction of the growth temperature leads to a decrease of the Al concentration which supports the growth of NWs, i.e. at T≈600∘C, NWs are obtained for x=3.5 and x=2 at.% whereas for T≈400∘C, the growth is observed on the seed layers with x=1 and 2 at.%. Note, all NWs are well-oriented vertically, and for T≈400∘C the NWs have typically a diameter of d≤7 nm and an aspect ratio of 45. For the concentrations which does not support the growth of NWs at a given temperature, we observe the growth of a honeycomb-like structure.
The growth on the Ga-doped seed layers is slightly different at T≈950∘C compared to that one observed on the Zn x Al 1 − x O layers. At this growth temperature, the NW growth is suppressed for the seed layer with a doping concentration of x=3.5 at.%. However, for x=7 at.% an elongated pyramidal structure was obtained. For the other concentrations and growth temperatures, the results are similar to that one obtained on the Al-doped seed layers. Remarkably, on the Ga-doped seed layer with x=1 at.% at T≈400∘C a high density of ultrathin NWs with a diameter of x≤7 nm and an aspect ratio of about 100 were obtained.
An impact of the Al concentration on the NW growth was also observed by Käbisch et al. . They attributed this behavior to a change of the surface polarity from an O-terminated surface of the undoped ZnO layer to a Zn-terminated one of the doped layers. In the last case, honeycomb-like structures were obtained instead of NWs. The observed growth of the honeycomb structure for the Al- and Ga-doped layers in our experiments would also indicate such a change of the surface polarity. In order to verify this change, we determined the polarity of the seed layers by using an etching method described by Mariano and Hanneman . Accordingly, we etched the seed layers in a diluted HCl acid with a concentration of 1:100 for 30 s. The etching rate was found to be about 1 nm per second by using spectroscopic ellipsometry. Only for the undoped seed layer as well as for Al-doped layers with x=1 at.%, we found an O-terminated surface which manifested itself by a pyramidal structure after the etching whereas for the other seed layers we found a crater-like structure which indicates a Zn-termination. However, in contrast to the results obtained by Käbisch et al., we were able to grow NWs on the Zn-terminated seed layers even at high temperature (Fig. 2). Thus, the change of the polarity of the surface seems to be unlikely for the difference of the observed growth behavior.
The surface roughness and the crystal quality seem to be also unlikely for the observed behavior. On the one hand, from XRD measurements, we can conlude that the grain size is similar for all Ga-doped ZnO films whereas for the Al-doped ZnO films, the grain size increases with increasing Al concentration. However, the observed growth is similar for both kinds of seed layers and approximately the same NW diameter can be achieved on these seed layers which is in contrast to the results obtained by Ting et al. . They observed that the diameter of the NWs increases with the grain size of the seed layer. On the other hand, neither an increase of the diameter of the NWs with decreasing surface roughness as reported by Ghayour et al.  nor another correlations were found. Note that a change of the surface roughness and of the grain size caused by the elevated temperature during the growth process can be excluded since XRD and AFM measurements performed on reference samples and annealed under deposition conditions do not show a significant change of the surface roughness and grain size.
A change of the observed morphology can be caused by a variation of the growth mechanism [18, 29]. A possible reason of such variation might be a change of the free energy given by Δ F=F m +F i −F s [29, 30]. Here F m ,F i , and F s represent the free energy of the incoming material, the interface, and the surface, respectively. However, the experimental determination of the free energy components of the HP-PLD process is challenging.
To summarize, we have shown that the doping concentration of the Zn x Al1−x O and Zn x Ga1−x O has a strong impact on the NW growth. In doing so, we were able to tune the diameter by two orders and are able to grow vertically well-oriented ultrathin NWs. These ultrathin NWs are promising for the investigation of quantum effects of the electronic structure. Furthermore, these NWs can be achieved at temperatures of T≈400∘C which is suitable for the preservation of CMOS structures. The choice of the doping material of the seed layer influences also the morphology of the NWs. For the Zn x Ga 1 − x O, we observe an increase of the NW diameter and a decrease of the aspect ratio, whereas for the Zn x Al 1 − x O seed layers, these properties are almost constant. The optical properties of the NWs were investigated by CL experiments. For the thick NWs, we observe a broadening of the donor bound exciton of about 2 meV which reflects the high crystalline quality of the NWs. With decreasing diameter, the broadening of the emission peaks increases which we attribute to the strong increase of the surface-to-volume ratio for the thin NWs.
This work was funded by the European Commission as part of the Project PiezoMAT in the FP7 Framework Programme (grant no. 611019). We thank Prof. Dr. Reinhard Denecke and Claudia Wöckel (Wilhelm-Ostwald-Institut, Universität Leipzig) for fruitful discussions. We acknowledge support from the German Research Foundation (DFG) and Universität Leipzig within the program of Open Access Publishing.
All authors contributed equally to the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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