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Abstract
Since the discovery of two dimensional (2D) materials, there has been a gold rush for van der Waals integrated 2D material heterostructure based optoelectronic devices. Van der Waals integration involves the physical assembly of the components of the device. In the present work, we extended van der Waals integration from 2D materials to three-dimensional (3D) materials, and herein we uniquely designed a van der Waals contacted light emitting diode based on MoOx staked ZnO/GaN heterostructure. The presence of the MoOx layer between n-type ZnO and p-type GaN leads to the confinement of electrons and an increase in the electron charge density at n-type ZnO. The n-type MoOx, a well-known hole injection layer, favors the availability of holes at the ZnO site, leading to the efficient recombination of electrons and holes at the ZnO site, which results in predominant high-intensity UV-EL emission around 380 nm in both forward and reverse bias.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The invention of PN junction in the early 1950s opens the door for microelectronic and optoelectronic devices. The realization of p-type GaN in the early 1990s makes the high brightness blue light emitting diodes (LED) possible and the GaN p-n junction based LEDs have been commercialized since then [1–4]. The high exciton binding energy of 60 meV makes ZnO a vivid material for LEDs in the past decade. ZnO with the bandgap of 3.37 eV is suitable for realizing blue LEDs and it can be fabricated at a lower cost compared with GaN. Alivov et al. fabricated MBE designed ZnO/GaN LED and observed EL peak at 430 nm [5]. Junseok et al. proposed ZnO nanowire-based ZnO/GaN LED, which displayed EL peak at 460 nm and 560 nm [6]. Wang et al. reported n-ZnO/Al2O3/p-GaN LED with EL emission at both blue and violet regions [7]. X. Zhang et al. reported n-ZnO/AlN/p-GaN LED with a broad EL peak at around 430 nm [8]. C. Xu et al. reported UV emission from n-ZnO/i-MgO/p-GaN LED [9]. H. Hirayama et al. reported AlGaN-based LED with UV emission [10–11].
To achieve ZnO/GaN-based LED with a UV emission, a well-defined epitaxial growth of semiconducting multiple quantum wells or semiconducting heterostructure is highly demanded, which involves expensive material growth techniques, such as MOCVD and MBE [12–15]. The semiconducting heterostructures grown by these expensive techniques are sensitive to defects and impurities at the interface, which could drastically influence the illumination mechanism of the LEDs [16,17]. However, in 3D-3D van der Waals integration, different bulk semiconductors are combining by simple direct physical attachment without any chemical interactions or chemical bonding in between the components, and hence clear and clean interfaces can be achieved. Van der Waals’s integration method allows flexibility in device design. Thus various devices based on van der Waals integration have been achieved, such as LEDs, photodetectors, and solar cells, showing the potential of van der Waals integration in electronic and optoelectronic device fields [18–22]. X. Duan et al. detailed the 2D-2D van der Waals integration, and he extended the van der Waals integration method to 2D-3D and 3D-3D material combinations and he also comprehensively explained its advantages and applications for electronic and optoelectronic devices [23]. Y. Lu and X. Duan et al. reported van der Waals integrated devices based on nanomembranes of 3D materials [24].
MoOx has been widely used as hole selective contact in organic electronics and photovoltaics [25–30]. A. Javey et al. reported MoOx and Si integrated carrier selective layer, which enhanced the performance of the Si-based solar cell. Further, as a hole transport layer, the integration of MoOx with 2D MoS2 realized a new phenomenon and 2D MoS2 p-type transistor [31,32]. In MoOx, the reduced oxidation state of Mo results in the formation of an oxygen defect band below the conduction band, which is responsible for its n-type nature. Thus, MoOx shows semi-metallic behavior with large chemical potential (∼6.9eV) and high work function with a low density of states at the Fermi level.
Herein, we designed a 3D-3D van der Waals contacted Ag/n-type(ZnO/MoOx)/p-GaN/Au heterostructured LED, and the n-type ZnO/MoOx heterostructure was prepared on the 300 nm SiO2 by chemical vapor deposition (CVD), where MoOx is an n-type wide bandgap semiconductor (∼3eV) with high electron affinity (∼6.7eV), and ionization energy (∼9.7eV) and the MoOx is well-known as a hole transport layer and as an electron blocker [33,34]. In the Ag/n-type(ZnO/MoOx)/p-GaN/Au heterostructured LED, the MoOx layer between n-type ZnO and p-type GaN acts as an electron blocker that leads to the confinement of electrons, and an increase in the electron charge density at n-type ZnO and as a hole injection layer MoOx allows the charge flow between p-GaN and ZnO or favors the availability of holes at ZnO site, which results in the high-intensity UV emission around 380 nm originates from the recombination of electrons and holes at the ZnO site in both forward and reverse bias.
2. Experimental
In our present work, using powder ZnO and MoO3 as source material, we followed the CVD growth method to fabricate the ZnO/MoOx heterostructure thin film on 300 nm SiO2 with a temperature of about 950 °C and a growth period of 45min in the Ar atmosphere (see Fig. S1 in Supplement 1). The CVD of powder MoO3 could result in a layer of sub-stoichiometric MoOx on the silica substrate. Herein, the MoOx layer was chosen to integrate with ZnO and GaN to construct 3D heterostructure LED with pure UV EL.
The fabrication process of Ag/n-ZnO/MoOx/p-GaN/Au LED was the van der Waals integration method, and is a simple, efficient method to achieve semiconductor heterostructures, which involves the direct physical attachment of n-type ZnO/MoOx thin film with p-type GaN. To form ohmic contacts, the silver electrode was deposited on the ZnO/MoOx heterostructure and Au/Ni electrode on the GaN by thermal evaporation. The physically attached Ag/n-type ZnO/MoOx and p-type GaN/Au were clamped tightly, resulting in the completion of the fabrication of LED.
XRD, XPS, and PL characterization of the ZnO/MoOx heterostructure were carried out. The SEM-EDX data of p-GaN has shown in Fig. S2 in Supplement 1 confirms the p-GaN. The electroluminescence (EL) characterization was carried out by using Ocean Optics QE Pro. I-V characterization of Ag/n-ZnO/MoOx/p-GaN/Au LED at both forward and reverse bias was carried out by the Keithley 2400 instrument.
3. Results and discussion
3.1 ZnO/MoOx thin film characterization
Figure 1 shows the schematic picture of the LED device. Figure 2(a) shows the room temperature PL spectrum of ZnO/MoOx thin film. A He-Cd laser (325 nm) was used to excite the sample and the resulted spectrum showed the ZnO characteristic PL peaks at 385 nm, 475 nm, and 497 nm, which exhibits the presence of ZnO in the heterostructure thin film. The low PL intensity is due to the presence of MoOx in ZnO/MoOx heterostructure thin film. Figure 2(b) shows the basic XRD data of polycrystalline thin film prepared by CVD growth process, indicating the polycrystalline nature of ZnO film growth predominantly along (112) and (201) directions with secondary peaks of (100), (002), (101), (103) and (200) orientations. Polycrystalline MoOx with growth orientation is shown in the same figure. Figures 2(c) and (d) shows the XPS data of ZnO/MoOx thin film and the XPS data of MoOx, respectively. In the XPS data of ZnO/MoOx thin film shown in Fig. 2(c), the peaks at appropriate positions in the spectrum for Mo, Zn, and oxygen exactly match with the standard XPS pattern for these elements, which confirms the elemental presence of Zn, Mo, oxygen, and the presence of ZnO in the CVD grown thin film. In the XPS data of MoOx (Fig. 2(d)), the spectra were divided into five peaks: 229.7eV, 235.9 eV, 234.6 eV, 232.7 eV and 231.85 eV. The 229.7 eV peak is from Mo4+3d5/2. The 235.9 eV and 232.7 eV peaks are from Mo6+3d3/2 and Mo6+3d5/2, respectively. The 234.6 eV and 231.85 eV peaks are from Mo5+3d3/2 and Mo5+3d5/2, respectively. This indicates the presence of different oxidation states of Mo in the ZnO/MoOx heterostructure thin film, confirming the presence of MoOx.
Fig. 2. (a) PL of ZnO/MoOx thin film (b) XRD data of polycrystalline (c) XPS data of ZnO/MoOx thin film and (d) the XPS data of MoOx.
The CVD growth of ZnO/MoOx heterostructure thin film on 300nm silicon substrate could be primarily concluded from these characterizations. Figure 3(a) shows I-V characterization of Ag/n-ZnO/MoOx/p-GaN/Au heterostructured LED and Fig. 3(b) shows a plot of reverse bias voltage & electrical current & EL integrated intensity of the ZnO/MoOx/p-GaN LED in reverse bias. The UV emission will not have been realized for n-ZnO/p-GaN heterostructure without the insertion of the MoOx layer, which means the MoOx is a crucial component for the UV EL from the LED.
Fig. 3. (a) I-V characteristics of heterostructure n-type ZnO/MoOx/p-GaN LED. (b) A plot of reverse bias voltage & electrical current & EL integrated intensity of the n-type ZnO/MoOx/p-GaN LED in reverse bias.
3.2 Illumination mechanism of Ag/n-ZnO/MoOx/p-GaN/Au LED in forward bias
Fig. 4. (a) Pure UV emission from the ZnO site with insignificant defect based emission. (b) High intensity, sharp UV EL peak with heat-induced slight wavelength shift. (c), (d) and (e) Illumination mechanism of LED at forward bias.
In forward bias, the p-type GaN is connected to the positive terminal and n-type ZnO/MoOx thin-film is connected to the negative terminal of the externally applied voltage source. Figures 4(a) and (b) show EL of LED at high and very high external applied voltages respectively. The MoOx layer between p-type GaN and n-type ZnO acts as an electron blocking layer, which strongly blocks electron flow from n-type ZnO to p-type GaN. Since there is no considerable electron transfer from n-type ZnO to p-type GaN and no recombination, no EL can result from the LED at relatively low applied forward voltages as shown in Fig. 4(d). At relatively high applied forward voltages, the Fermi energy level of the p-GaN moves downward and the MoOx, a known hole injection layer, which supports the hole injection from p-type GaN to the n-type ZnO and leads to the near band edge recombination of holes and electrons at ZnO site, resulting in the EL with the wavelength of 380 nm as shown in Figs. 4(d) and (e). Thus, in forward bias, the EL from LED originated from the recombination of electrons and holes at the ZnO/MoOx site. The EL intensity of the LED increases with the increase in applied forward bias voltages.
3.3 Illumination mechanism of Ag/n-ZnO/MoOx/p-GaN/Au LED in reverse bias
Fig. 5. (a) Pure UV emission from the ZnO site and shows the slight redshift in the EL emission at higher applied voltages. (b) Defects based EL emission from the LED with a broad peak and the peak center at 600 nm. The weak increment of defect based EL intensity with an increase in applied voltages. Saturation in the defect based EL intensity from the LED at different applied voltages observed. (c) and (d) The illumination mechanism of LED at reverse bias.
In reverse bias, n-type ZnO/MoOx thin film is connected to the positive terminal and p-type GaN is connected to the negative terminal of the external voltage source. In applied reverse bias voltages, the band alignment at the interface between n-type ZnO/MoOx and p-type GaN semiconductors is an important factor to affect the EL of LED. The energy level of the conduction band of MoOx is very close to the energy level of the valence band of p-type GaN, which favors the electron tunneling from p-type GaN to the n-type ZnO, as shown in Fig. 5.
Due to the nearly broken gap band alignment between n-type ZnO/MoOx and p-type GaN, even at relatively low applied voltages (10V), electrons from the valence band of p-type GaN could tunnel to the conduction band of ZnO thin film. These tunneling electrons recombine with the holes at the ZnO site, resulting in defects based broadband EL with peak center at 600nm from the LED as shown in Fig. 5(b). At relatively high applied reverse bias voltages (30 V), the number of electrons tunneling from p-GaN to the ZnO increases or in other words electron charge density increases at ZnO and the tunneling electrons possess higher energy, hence these electrons occupy higher energy levels in ZnO. Thus the tunneling electrons recombine with available holes at ZnO results in 380 nm high-intensity UV-EL as shown in Figs. 5(a), (c), and (d). The UV-EL peak intensity increases with a further increase in applied voltage (see Fig. S3 in Supplement 1). At high applied voltages, defect based broadband light emission reaches saturation and hardly shows the rise even with an increase in applied voltage. Moreover, at high applied voltages, heat-induced slight bandgap shift or slight wavelength shift of EL peak could be observed.
4. Conclusion
In conclusion, 3d-3d van der Waals contacted Ag/n-type(ZnO/MoOx)/p-GaN/Au heterostructured LED has fabricated and the characterization (XRD, XPS, PL, and SEM-EDX) of these semiconductor heterostructures were carried out. In the n-type ZnO/MoOx/p-GaN LED, the van der Waals integration of the n-type & p-type semiconductors, which allows a relatively clear and clean interface of semiconductor heterostructure with very limited interfacial imperfections. I-V characterization of n-type ZnO/MoOx/p-GaN heterostructured LED shows good rectifying behavior in forward and reverse bias, which further confirms the n-type nature of ZnO/MoOx heterostructure.
The n-ZnO/MoOx/p-GaN heterostructured LED with UV emission was achieved in both forward and reverse bias voltages. At forward applied voltages, MoOx acts as a potential barrier for electrons (an electron blocking layer), which leads to the confinement of electrons at n-type ZnO. At higher forward applied voltages, hole injection from p-type GaN to the n-type ZnO leads to the recombination of holes and electrons at n-type ZnO, resulting in UV emission at the wavelength of 380 nm. At reverse bias voltages, the nearly broken gap band alignment between n-ZnO/MoOx and p-GaN results in the tunneling of the electrons from the valence band of p-type GaN to the conduction band of n-type ZnO and leading to the recombination of electrons and holes at ZnO site and further resulting in UV emission.
Funding
National Natural Science Foundation of China (51202216, 51502264, 61774135); Special Foundation of Young Professor of Zhejiang University (2013QNA5007).
Acknowledgments
S. S. Lin thanks the support from the National Natural Science Foundation of China (No. 51202216, No.51502264 and No. 61774135) and Special Foundation of Young Professor of Zhejiang University (Grant No. 2013QNA5007). N. K Manjunath mainly contributes to this work.
Disclosures
The authors declare no conflicts of interest.
See Supplement 1 for supporting content.
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