ZnO-nanofilm/Si-micropillar p-n nanoheterostructure arrays were prepared by growing n-type ZnO onto a p-type nanoporous Si pillar array. Its current-voltage characteristics of nanoheterostructure showed good rectifying behavior with onset voltage of ~1.5 V, forward current density of ~28.7 mA/cm2 at 2.5 V, leakage current density of ~0.15 mA/cm2 and rectifying ratio of ~121 at ± 2.5 V. The electron transport across nanohetreostructure obeys the trap-charge-limit current model. Moreover, strong white light electroluminescence from ZnO-nanofilm/Si-micropillar light-emitting diode (LED) has been achieved, which could open up possibilities to build new ZnO/Si-based highly efficient solid-state lighting devices.
©2012 Optical Society of America
Semiconductor nanoheterostructures have attracted worldwide interests in recent years because they exhibit unique electrical or light-emitting properties which are different from their traditional counterparts [1–5]. In particular, it has been shown that electrically driven light emission was realized with semiconductor nanoheterostructures, where carrier injection occurs across the p-n junction [2–4]. As one of the promising II–VI compound semiconductors, hexagonal zinc oxide (ZnO) has received significant attention in optoelectronics applications due to its superior physical properties, such as wide direct bandgap (3.37 eV), large exciton binding energy (60 meV), high thermal stability and high oxidation resistance in harsh environments. Among semiconductor materials, single-crystal Si (sc-Si) has been a dominant electronic material and considerable efforts have been devoted to it since the observation of the strong visible light emission from Si nanostructures . However, due to its indirect band gap, sc-Si can only give poor near-infrared light emission which make it not an ideal material for efficient Si-based optoelectronic devices. Thus, the exploration of Si-based ZnO nanoheterostructures is great important in developing future optoelectronic nanodevices [2,4,7–9]. n-ZnO has been deposited onto p-type sc-Si substrates via various preparation techniques ranging from pulsed laser deposition , vapor-liquid-solid [5,10], to electrodeposition . And these fabricated ZnO/Si nanoheterostructures have been tested as a possible alternative pathway for many applications, such as light-emitting diode (LED) and photodetectors [4,9–12]. In this paper, we report a novel and general approach to generate sufficient carrier injection and efficient light emission from a ZnO-nanofilm/Si-micropillar nanoheterostructure which was built by employing a nanoporous Si pillar arrays (NSPA) substrate. This new approach for fabricating an efficient, large-area Si-based micropillar white LED array could pave the way for developing Si optoelectronic integrated circuits (OEICs) which could overcome the speed limitation of electrical interconnects and add extra functionalities on Si chips.
Figure 1 shows the scheme of fabricating LED arrays with ZnO-nanofilm/Si-micropillars (ZnO/NSPA) as the p-n nanoheterojunction materials. First, NSPA was prepared by hydrothermally etching (100) oriented, p-type sc-Si wafers (ρ~0.01 Ω cm) in the solution of hydrofluoric acid containing ferric nitrate . N-type ZnO seed nanolayer was grown on surface of NSPA by wet coating technology before prior to the growth of ZnO nanoflim . After uniformly coating NSPA surface with ZnO seed nanolayer, ZnO nanofilms were synthesized by self-catalytic thermal evaporation and vapor-phase transport method in a horizontal quartz tube furnace, in which the analytical reagent ZnO (99.9%) and graphite (99.9%) powders with 1:1 molar ratio were used as source materials. During the reaction, keep the tube under room pressure, and gradually increase the temperature to 800 °C within 30 min, then keep that for 20 min, meanwhile introduce a flow of nitrogen atmosphere (99.99%) with the flow rate of 25 sccm (standard cubic centimeters per minute). Finally, a ~100 nm Ag layer was sputtered on the backside of Si (111) substrate and the resulting sample was annealed at 350 °C for 5 min to form ohmic contact. The top electrode on ZnO surface was built by vacuum sputtering a ~10 nm Au layer after annealling.
Field emission scanning electron microscopy (FE-SEM), transmission electron microscope (TEM), high resolution TEM (HRTEM), X-ray diffraction (XRD), energy dispersive spectroscopy (EDS), I-V and electroluminescence (EL) characteristics were used to examine the structure, elemental components and properties of the fabricated devices.
3. Results and discussion
The XRD measured result, as shown in Fig. 2(a) , confirmed successful the formation of hexagonal crystalline ZnO (wurtzite-type, space group: P63mc) onto NSPA. As has been demonstrated previously by our group , the NSPA is a micron/nanometer structural composite system with a distinct triple hierarchical structure, i.e. an array composed of micron-sized Si pillars, nanopores densely distributing on the pillars and Si nanocrystallines constructing the pore walls. The average height and top-top distance of the Si pillars is ~3.5 and ~3.6 μm respectively, and the average density of the pillars reaches ~107/cm2 which is the most suitable density according to electrostatic calculation to maximize the emitted current density . These structural features made NSPA an ideal template for assembling Si-based functional pillarlike nanoheterojunction arrays. The FE-SEM images of Fig. 2(b)-2(d) represent the typical surface morphologies ZnO/NSPA. Clearly, the Si pillars were still well-separated and the characteristic of the pillar array was still kept even after ZnO overcoating procedure. From the image of ZnO/NSPA after nc-ZnO growth in Fig. 2(b), the amplified image of an individual pillar in Fig. 2(c) and the cross-sectional image in Fig. 2(d), it can be found that both the pillars and valleys around the Si pillars were covered with nc-ZnO nanoparticles and a continuous grain membrane of ZnO was formed on the surface of NSPA. Judged from the TEM image of one single ZnO-nanofilm/Si-micropillar (Fig. 2(e)), the thickness of ZnO nanofilm coated on Si micropillars was estimated to be 200-300 nm. The HRTEM image shown in Fig. 2(f), taken from the square region marked in Fig. 2(e), indicates that the lattice spacing, measured from the lattice fringes along the grown axis direction of ZnO nanofilm, is ~0.26 nm. It corresponds to the hexagonal structure of ZnO and shows that the preferred growth occurred along the  direction. In order to determine the phase of the crystalline zone, electron diffraction measurement with two-dimensional fast Fourier transform (FFT) mode was carried out at the typical area shown in Fig. 2(f). The result presented in the inset of Fig. 2(f) proved that the spots correspond to the diffraction from the (002) crystal plane of hexagonal phase ZnO. To give a further qualitative evaluation of the elemental distribution on the surface of ZnO/NSPA, EDS measurements were carried out at three random-picked representative sites, which have different geometrical features of the surface and are labeled by A, B and C in the inset in Fig. 2(c). The elemental compositions of any site include three elements exclusively, Si, Zn and O. The atom percentages for Si, Zn and O detected at the top sites of the pillars (site A) were 14.61%, 28.68% and 56.71%, while those detected at the half height sites of the pillars (site B) and at the valley sites surrounding the pillars (site C) were 12.97%, 28.29%, 58.74%, and 21.63%, 24.76%, 53.61%, respectively. Considering the fact that the effective irradiating radius of electron beam spot in the EDS experiment was ~0.5 μm, the coating quantity of ZnO nanofilm grown at above three typical sites of the pillars is consistent. Based on the above analysis, the structure of ZnO/NSPA could be illustrated by the diagrammatic sketch given in Fig. 1(c), here the ZnO nanofilm coverage is quite good throughout the whole NSPA surface. The experiments by room temperature Hall effect disclosed that ZnO nanofilm deposited on p-type NSPA behaved as a n-type semiconductor, and the carrier concentration of ZnO nanofilm and NSPA layer are ~3.7 × 1016 cm−3 and ~1.2 × 1018 cm−3, respectively. This indicated that a complex and unique n-ZnO/p-Si nanoheterojunction array has been prepared.
Electrical transport measurement was carried out to quantitatively characterize the physical contact between n-ZnO nanofilms and p-Si micropillars, which is critical in future device fabrication. By applying the bias voltage with a positive Ag electrode to Au electrode in forward direction, the dark current density-voltage (J-V) curve at room temperature for ZnO/NSPA LED device is depicted in Fig. 3 . For clarifying the roles played by NSPA/sc-Si heterostructure and the contacts for both Ag/sc-Si and Au/ZnO, the dark J-V curves were measured and presented in the top inset of Fig. 3. Obviously, no rectification characteristics were observed for NSPA/sc-Si, Ag/sc-Si or Au/ZnO. Therefore, the heterojunction formed between n-ZnO and p-NSPA was the only contributor for the rectification of ZnO/NSPA. As shown in Fig. 3, the forward current densities of ~28.7 mA cm−2 for ZnO/NSPA and ~3.6 mA cm−2 for ZnO/sc-Si at 2.5 V, and a leakage current of ~0.15 mA cm−2 for ZnO/NSPA and ~0.85 mA cm−2 for ZnO/sc-Si at a reverse bias of 3.5 V were observed, respectively. The rectification ratios of the forward to reverse bias current were approximately 121:1 and 7:1 at the bias voltage of ± 2.5 V for ZnO/NSPA and ZnO/sc-Si, respectively. The turn-on voltages of our devices are defined by extrapolating the linear fits in the high current regimes to J = 0 as indicated by the dashed lines in Fig. 3. The turn-on voltage of ZnO/NSPA diode is estimated to be:1.5 V, which is about 3 times smaller than that of ZnO/sc-Si (:3.9 V). Clearly, the rectifying behavior of ZnO/NSPA nanoheterojunction is much better than that of ZnO/sc-Si flat surface. The high forward current density of ZnO/NSPA could be explained by the heterojunction theory. Theoretically, the dark J-V relation for a heterojunction could be described using the Richardson-Schottky diode equation :Eq. (1), the higher J for ZnO/NSPA would require big J0 and small n and Rs values. As for n, which describes the deviation degree of the heterojunction diode from that of an ideal diode, it could be calculated through the following equation transformed from the Eq. (1):Fig. 3 and the Eq. (2), the values of n are calculated to be ~15.4 and ~38.6 for ZnO/NSPA and ZnO/Si respectively, and both far from the ideal value 1. This should be attributed to the high density of the interface states in ZnO/NSPA. Also, the measured J0 for ZnO/NSPA is only ~0.15 mA cm−2 at a reverse bias of 3.5 V, which is very small. In fact, because the current flow is restricted to the narrow pillar region where the strong field occurs, the leakage current is thus greatly reduced. This could also effectively explain why the reverse current density for ZnO/NSPA is much smaller than that of ZnO/sc-Si. The above results indicate that in ZnO/NSPA, both the values of J0 and n would actually go against obtaining a high J value. Therefore, the high J value realized in ZnO/NSPA could only be resulted from its small Rs. Actually, NSPA owns a very large specific surface area, which would surely enlarge the contact interface between ZnO nanofilm and nanoporous Si micropillars and therefore result in the great reduction in Rs. In addition, according to the fundamental electromagnetic theory, a strong field can be induced on top of each nanoporous Si pillar by a small external bias, which would also contribute to the high forward current density . Theoretically, three equations, JV2exp(a/V), Jexp(−eV/nkT) and JVm, characteristic of the tunneling, thermionic, and trap-charge-limited current (TCLC) models, respectively [17–19]. As shown in the bottom inset of Fig. 3, the simulation on the J–V data of ZnO/NSPA showed that the experimental curve could be well fitted by JVm with m = 2.74 and the proportionality constant being 2.24. So the phenomenon of charge transport in the ZnO/NSPA nanoheterojunction could be explained by the TCLC model, which has been usually observed in wide band gap p-n diodes [18,19].
The most important result of our work is the realization of room temperature EL under a broad range of applied voltage as shown in Fig. 4(a) . No EL was detected under reverse bias. The EL signal becomes detectable when a forward direct current (dc) bias of 5 V is applied across the device with Au as cathode and Ag as anode. The EL spectra of ZnO/NSPA were further measured at various applied forward-bias voltages from 5 to 20 V, as shown in Fig. 4(a). The EL spectra show that the intensities increased while maintained the spectrum shapes consistent even increasing the applied voltage from 5 V up to 20 V. It should be noted that these broad emission spectra overlap with the whole visible spectral region (400-700 nm) throughout all applied voltage range, which means the emission is always white light no matter what voltage applied. Although the absolute emission intensities were not measured but the white light emission was quite strong from our sample and could be observed by the naked eyes even at a lower applied voltage of 5 V. The emission stability could last about 7-8 min persistently when applying a forward bias of 15 V. This indicates that ZnO/NSPA could be a potential light source for future solid-state white LED devices. However, further increase of applied voltage over 24 V, resulted in broken down of the device, and no EL detected. In order to make these emission peaks clearly distinguishable, the EL spectrum at 5 V is fitted to a Gaussian function, as shown in Fig. 4(a) (dotted line), where three emission peaks centered at 411, 502 and 632 nm could be clearly seen. It was reported that the violet emission from undoped ZnO nanorods is related to zinc interstitial (Zni), and the position of the Zni level theoretically locating at ~0.22 eV below the conduction band [20,21]. The violet peak is centered at ~411 nm (3.02 eV), and this agrees well with the transition energy from Zni level to the valence band in ZnO (approximately 3.1 eV) [20–22]. The green emission peak centered at ~502 nm, which has been frequently observed in ZnO-based materials, may come from electron-hole recombination from Zni to oxygen vacancy (Vo) energy level. This can be explained by the full potential linear muffin-tin orbital method, which explains that the position of the Vo level is located at ~2.47 eV below the conduction band [20,23]. Therefore, it is expected that the transition from Zni to Vo level is ~2.25 eV. This agrees approximately with the experimental green EL peak position centered at ~502 nm (2.47 eV). It has been reported that the position of the oxygen interstitials (Oi) level is located approximated at 2.28 eV below the conduction band in ZnO, and it is expected that the band transition from Zni to Oi level is approximately 1.9-2.28 eV [20–22]. This agrees well with the red peak that is centered at ~632 nm (1.97 eV).
In order to elucidate the mechanism of light generation in our heterojunction, the expected energy band diagrams of n-ZnO/p+-NSPA/p+-Si heterojunction at zero and forward bias were shown in Fig. 4(b) and 4(c), respectively. To simplify the analysis, here we neglect the effects of dipole, interfacial state and native silicon oxide layer between the ZnO nanofilm and NSPA substrate. Considering that the electron affinities of Si and ZnO are χSi = 3.95 eV and χZnO = 4.35 eV, and the bandgap energy of Si, NSPA and ZnO are Eg,Si~1.12 eV, Eg,NSPA~2.0 eV and Eg,ZnO~3.26 eV, respectively [13,24], the conduction band offset for electrons is ∆EC = χZnO - χSi = 0.4 eV, while that for holes is ∆EV = Eg,ZnO - Eg,NSPA + ∆EC = 1.7 eV. It is clearly that the band gap energy of NSPA is adjusted in between the band gaps of Si and ZnO, and the active NSPA layer would play a key role in the optical properties due to the large surface area and high interface state density. And the ~0.4 eV conduction band offset works as an energy barrier layer to confine the electrons in the ZnO nanolayer, which could enhance the emission from ZnO nanofilm. As the forward bias applied, holes in the p+-Si substrates firstly, flow into the valence band of NSPA and then flow together with the holes in NSPA layer into the valence band of the ZnO nanofilm, while electrons are injected into the ZnO layer via the Au electrode at an appropriate negative voltage. Then electrons on the conduction band of ZnO may first fall into these empty defect-produced traps and subsequently, either directly recombines with holes injected from NSPA side and produces the emission peak at ~411 nm, or recombines with deep level Vo and Oi and produces the emission peaks at ~502 nm and 632 nm.
In summary, novel ZnO/Si nanoheterostructures were constructed by growing ZnO nanofilms onto NSPA. The J-V curve of ZnO/NSPA heterojunction demonstrates a good rectifying behavior characterized by a high forward current density of ~28.7 mA cm−2 at 2.5 V, and the electron transport across ZnO/NSPA could be dominated by the trap-charge-limited current model. Our designed ZnO/NSPA devices exhibit an electrically driven white light mission under a low voltage, and the EL properties could be tuned effectively by the applied voltage. The EL spectra could be well interpreted based on the energy band diagram. These results might provide a new and simple approach to design and fabricate white light LEDs based on n-ZnO and p-Si nanomaterials in the future.
This work was supported by the Natural Science Foundation of China (Grant No. 11104008), Doctoral Fund of Ministry of Education of China (Grant No. 20090010120014), the Beijing Natural Science Foundation (Grant No. 1103033), the 973 Program (Grant Nos. 2011CBA00503, 2011CB932403), and the 863 Program (Grant No. 2012AA03A609).
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