1. Introduction

ZnO is a promising semiconductor material for high efficient short-wavelength optoelectronic devices, e.g. short-wavelength light emitting diodes (LEDs) and laser diodes, due to its direct wide band gap (3.37 eV) and large exciton binding energy of 60 meV at room temperature (RT) [1]. It is well known that the heterojunction in the ZnO-based p-n junction LEDs can improve current confinement, leading to a higher electron-hole recombination and electroluminescence (EL) emission efficiency [2]. ZnO can grow epitaxially on the surface of GaN with the same wurtzite crystal structure and nearly same lattice constants, leading to a low in-plane lattice mismatch of about 1.9% [3]. Importantly, the band gap energy of GaN (3.40 eV) at RT matches well with that of ZnO (3.37 eV). Therefore, a lot of work focused on the fabrication of ZnO/GaN p-n heterojunction LEDs. ZnO is intrinsically n-type semiconductor due to its self-compensation of native point defects such as oxygen vacancy (V_O) or zinc interstitial, and the n-ZnO/p-GaN heterojunction LEDs with ultraviolet (UV), blue-violet and blue emission lights [4–6] are reported. Nevertheless, the group-V (N, P, As and Sb) [7–10] and group-I (Li, Na) [11,12] elements can be doped into ZnO to achieve the p-type ZnO, which motivate the researches on the p-ZnO/n-GaN heterojunction LEDs [10,13–16]. In fact, transition metal Cu element, which is adjacent with Zn element in the periodic table of elements, is also a candidate for p-type doping of ZnO [17,18]. And the radii size of Cu⁺ (77 pm) and Cu²⁺ (73 pm) ions is similar with that of Zn²⁺ (74 pm) ion [19]. As a result, the formation energy of Cu substitution on Zn sites (Cu_Zn) is low, indicating that Cu element is suitable for doping ZnO. For examples, Hsu [19] and Kim [20] prepared the p-ZnO:Cu/n-ZnO homojunction nanorod UV photodetector and the p-ZnO:Cu/n-SiC LED, respectively. Dingle [17] and Yan [18] reported that the Cu-doped ZnO has acceptor states of Cu_Zn at 0.45 eV and 0.70 eV above the valance band maximum (VBM). Due to the above characteristics of Cu-doped ZnO and the mature preparation technique of n-GaN film, the investigation on p-ZnO:Cu/n-GaN heterojunction LED is worthy and available.

As one of the important tricolour LED compositing white light LED, red light LED is widely used in alarm, information display and traffic lighting system. Moreover, UV-free red light can avoid damage to plant by UV light [21] and promote the plant photosynthesis [22]. It is worthy of tunning EL wavelength of ZnO/GaN p-n heterojunction LED from UV to UV-free red light on a large scale by doping selected element. The ZnO/GaN p-n heterojunction LED usually emits UV light originated from near-band-edge (NBE) radiative recombination of ZnO and/or GaN [4]. With the aids of defect levels of doped ZnO and the electronic interfacial transition of ZnO/GaN [6,10,23,24], the EL emission of ZnO/GaN p-n heterojunction LED is extended from UV to blue, blue-violet, white lights [4–6,10,24]. In the research [10], our group ever reported that the EL emission of p-ZnO:Sb/n-GaN heterojunction LED can be tuned into UV-free warm white light by the electronic interfacial transition from the conduction band (CB) of n-GaN to the acceptor levels of p-ZnO:Sb due to dopant Sb in ZnO. Early, Alvi [25] ever observed that their white light n-ZnO nanotubes/p-GaN LED after annealed in nitrogen has a strong red emission at 705 nm resulting from V_O. And Zhou [26] reported that the n-ZnO:Mn/n-GaN heterojunction LED emits a strong red-dominant EL emission at 650 nm by formation of a thin oxide blocking nanolayer on the n-ZnO:Mn/n-GaN interface. In this paper, we present a UV-free red light p-ZnO:Cu/n-GaN LED (as shown in Fig. 1), which is fabricated by growing the cross-connected p-ZnO:Cu nanobushes on n-GaN film using chemical vapor deposition (CVD) under oxygen-rich (O-rich) condition. The UV-free red light emission is due to the electronic interfacial transition from the CB of n-GaN to the deep acceptor levels (DALs) of p-ZnO:Cu.

 

Fig. 1 Schematic diagram of the p-ZnO:Cu/n-GaN LED device. The circled inset part shows the schematic of cross-connected structure.

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2. Experimental

2.1 Fabrication and characterization of materials

The cross-connected ZnO:Cu nanobushes were grown on the commercial n-GaN:Si film with a sapphire substrate in a horizontal tube furnace by CVD method under O-rich condition. Before CVD, the ZnO:Cu growth region on the n-GaN film was coated with 3-5 nm Au thin film using a shadow mask by magnetron sputtering technology. Next, ZnO, graphite and CuO powder (weight ratio 1:1:6.5) were ground together as the source material. A quartz boat with the source material was placed at the heating zone in a horizontal quartz tube. Meanwhile, the n-GaN film with the Au thin film was downstream and close to this quartz boat. The mixture of argon gas of 100 sccm and oxygen gas of 3 sccm passed through the furnace tube where the pressure was maintained at the 7 × 10³ Pa. The CVD process was carried out at 950°C for 20 minutes under this O-rich condition. After the reaction had finished, the furnace will cool down to RT. At last, Grey cross-connected ZnO:Cu nanobushes were grown on the n-GaN film. For comparison, dark-blue undoped ZnO nanorod arrays were also grown on n-GaN film following the same procedure except the source materials without CuO powder.

The morphology, structure, and chemical composition of the samples were characterized by X-ray diffraction (XRD) (SHIMADZU XRD-7000), X-ray photoelectron spectroscopy (XPS) (Kratos AXIS-ULTRA DLD-600W), scanning electron microscopy (SEM) (FEI Nova Nano-SEM 450), high-resolution transmission electron microscope (HRTEM) and energy-dispersive X-ray spectroscopy (EDS) (FEI Titan G2 60-300). Luminescence performances at RT were characterized by photoluminescence (PL) measurement device (Horiba Jobin Yvon, Lab RAM HR800). The PL measurement at 5 K was performed by the Ti:sapphire femtosecond laser (Coherent Mira 900) (exciting wavelength of 350 nm) with an excitation power of 0.20 mW in superconducting magnet cryostat (OXFORD Microstate MO) and was analyzed using monochromator (Shamrock 500i) with EMCCD (Newton 970) under the environment of liquid helium.

2.2 Device fabrication and measurement

We evaporated thermally a Au film with the thickness of 10 nm on the n-GaN:Si film with a sapphire substrate using a shadow mask. Metal In dot was contacted with Au film to obtain Ohmic contact of Au-In/n-GaN. The cross-connected ZnO:Cu nanobushes formed the continuous whole. Metal In dot was directly contacted with this continuous whole of ZnO:Cu to form Ohmic contact of In/p-ZnO:Cu. The I-V characteristic of as-prepared LED was measured using a two-probe station with Keithley 4200 semiconductor parameter analyzer at RT. The EL measurement was also carried out through this two-probe station by applying a direct current (DC) voltage to the device using a source meter (B2901A). The EL spectrum of the as-fabricated LED was collected by a high-sensitivity fiber optic spectrometer with a cooled charge-coupled device detector (AvaSpec-HS1024 × 122TEC-USB2).

3. Results and discussion

The oblique-view SEM images of undoped ZnO and ZnO:Cu are shown in Fig. 2(a) and 2(b), respectively. Figure 2(a) exhibits the highly vertical oriented ZnO nanorod arrays, while Fig. 2(b) exhibits the cross-connected ZnO:Cu nanobushes with a worse aligned orientation. The corresponding enlarged image of selected region and the top view image of ZnO:Cu in the inset of Fig. 2(b) further illustrate this cross-connected nanobushes structure. Obviously, dopant Cu changes the morphology of ZnO. To explore the phase structure of ZnO:Cu, XRD measurement results of undoped ZnO and ZnO:Cu are depicted in Fig. 2(c). No traces of any secondary phases (such as CuO and Cu₂O) are found within the sensitivity of XRD measurement. The XRD peaks of undoped ZnO and ZnO:Cu are both indexed as hexagonal wurtzite ZnO structure (JCPDS36-1451, P6₃mc). The reflected intensity of (002) plane of ZnO:Cu decreases and the intensity of other peaks increases compared to undoped ZnO.

 

Fig. 2 The morphology and structure characterization of the cross-connected p-ZnO:Cu nanobushes. (a) and (b) Large area 30° oblique-view SEM images of the ZnO nanorod arrays and ZnO:Cu nanobushes. The inset of (b) exhibits an enlarged image of selected region and the top view image of ZnO:Cu. (c) XRD comparison of the ZnO nanorod arrays and p-ZnO:Cu nanobushes on n-GaN film with a sapphire substrate. (d) The HRTEM image of a branch from the p-ZnO:Cu nanobushes, proved that the p-ZnO:Cu nanobushes are well crystallized.

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Moreover, the enlarged lattice image of a selected region and its corresponding fast Fourier transform (FFT) pattern are also exhibited in Fig. 2(d), according to a HRTEM image of the corner of a branched ZnO:Cu. The HETEM image and the enlarged lattice image of a selected corner region with the visible lattice fringes demonstrate that the branched ZnO:Cu is well-crystallized wurtzite structure. Two sets of lattices with the same lattice fringe spacing of ~0.28 nm are present in enlarged lattice image of the selected corner region. The FFT pattern of the same region can be indexed as the diffraction spots of ZnO $(10\overline{1}0)$and$(01\overline{1}0)$planes. In addition, the angle of branch-to-stem is measured to be ~60°, just corresponding to the angle between $(10\overline{1}0)$and $(01\overline{1}0)$ planes. Thus, the stem and branch are oriented along the direction of $[10\overline{1}0]$ and $[01\overline{1}0]$, respectively. Consequently, it can be concluded that the introduction of Cu dopant caused the pronounced changes in morphology and inhibited the growth along the direction perpendicular to (0002) plane. This case also existed in the growth of hydrothermal Cu doped ZnO, which was explained using a face-selective crystal growth inhibition mechanism [27].

In addition, the EDS element mappings of ZnO:Cu in Fig. 3(a) elucidate that Cu element is distributed evenly in ZnO:Cu. Furthermore, the EDS spectrum of ZnO:Cu in Fig. 3(b) also presents the signals of Cu, Zn, O and Mo. Among them, the signal of Mo is from the TEM grid of molybdenum. The table in the inset of Fig. 3(b) shows a quantification result of the EDS analysis, which indicated that the ratio of Cu element in ZnO:Cu is roughly 1.9 at%. All those indicate that Cu element exists in ZnO:Cu. Chemical analysis of the ZnO:Cu nanobushes were also performed by XPS. The survey spectrum in Fig. 3(c) reveals the presence of Zn, Cu, O and C. A high resolution scan spectrum of the Cu 2p peak in Fig. 3(d) shows two prominent peaks at 932.7 eV and 952.7 eV corresponding to Cu2p_3/2 and Cu2p_1/2 spin-orbit splitting, respectively. The dominated peak curve can be Gaussian fitted with a major Cu⁺ component and a Cu²⁺ minor component, similar to the result of reported Cu-doped ZnO nanowire [28]. Two strong satellite peaks in the binding energy range of 938–945 eV and 959-964 eV are resulting from electron shakeup [20] in Cu3d band (Cu3d9 configuration) of the cupric oxide (Cu²⁺) with hole states. So Cu exhibited a mixed valence state of Cu⁺ and Cu²⁺. ZnO is a native n-type material. When Zn²⁺ ions were substituted by Cu²⁺, the carrier density of ZnO didn't change significantly [29]. However, the substitution of Zn²⁺ by Cu⁺ is a p-type doping which will increase the hole density of the sample [30].

 

Fig. 3 Characterization on distribution and valence states of Cu in ZnO:Cu. (a) The EDS mapping images of corresponding element O (yellow), Zn (red) and Cu (orange) from a portion of ZnO:Cu nanobranch. (b) The EDS of the corresponding whole nanobranch. The table in the inset of (b) is a quantification result of the EDS analysis. (c) XPS survey spectrum of the p-ZnO:Cu nanobushes/n-GaN film. (d) High resolution scan spectrum of Cu 2p peaks.

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The RT PL spectra of n-GaN, undoped ZnO and ZnO:Cu (spectra of undoped ZnO and ZnO:Cu normalized to green emission peak) are exhibited in Fig. 4(a). It is noted that the peaks of n-GaN (727.1 nm) and undoped ZnO (753.6 nm) are just located at the double wavelength positions of the strong UV emission from n-GaN (363.9 nm) and undoped ZnO (377.0 nm), respectively. In addition to the similar PL line shapes of n-GaN or undoped ZnO (as shown in Fig. 4(a)), these are probably just the 2nd order effect due to the grating spectrometer. Besides, the normalized PL spectra at higher energy are also exhibited in the inset of Fig. 4(a). The NBE emission peak (UV peak) of ZnO:Cu (382 nm, 3.246 eV) is red-shifted by ~0.05 eV compared with that of undoped ZnO (377 nm, 3.289 eV), which is probably due to the formation of shallow acceptor levels in ZnO:Cu. In addition, the RT PL spectrum of ZnO:Cu exhibits a very weak UV emission and an enhanced green emission (533 nm, 2.327 eV). The ratio of green and UV emission intensities (I_G/I_UV) of ZnO:Cu is much larger than that of undoped ZnO. The enhancement of green light and suppression of UV light in ZnO:Cu PL spectrum implies that more Cu-related deep levels are introduced by intentionally doping Cu in ZnO.

 

Fig. 4 PL characterizations of ZnO:Cu and undoped ZnO. (a) Normalized PL spectra comparison of undoped ZnO, ZnO:Cu and n-GaN at RT. The inset presents the normalized PL spectra comparison at high energy. (b) Normalized low temperature (5K) PL spectra comparison of undoped ZnO and ZnO:Cu. The inset presents the normalized low temperature (5K) PL spectra at high energy.

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In order to further investigate the optical emission and defects of ZnO:Cu, the normalized low temperature (5 K) PL spectra of ZnO:Cu and undoped ZnO are shown in Fig. 4(b). The inset of Fig. 4(b) also shows the normalized PL spectra of ZnO:Cu and undoped ZnO in the range of 3.0-3.5 eV. The low temperature PL spectrum of ZnO presents a strong UV peak at 3.365 eV, which is NBE emission of exciton bound to donors (D⁰X) [10]. However, the low temperature PL spectrum of ZnO:Cu exhibits four weak new emission peaks at 3.380 eV, 3.311 eV, 3.238 eV and 3.166 eV, respectively. The first peak at 3.380 eV is attributed to free exciton (FX) emission [31]. The second peak at 3.311 eV, which ever appeared in N, P and Sb doped p-ZnO [7,8,32], corresponds with the free electronic radiative transition to shallow acceptors (E_A) of stack defects resulting from Cu dopant (FA) [33]. Moreover, Wenckstern [34] also reported that in homoepitaxial pulsed laser deposition grown phosphorus-doped ZnO layers, the region with stack defects is p-type conductivity, whereas the region without stack defects is n-type conductivity. It implies that intentionally doping Cu in ZnO induced more Cu-related stack defects which promoted the formation of p-type ZnO:Cu. The third and fourth peak at 3.166 eV and 3.238 eV are ~2 × 72 meV and 72 meV away from the third peak at 3.311 eV, respectively. Thus the third and fourth peak are assigned to the second and first longitudinal optical (LO) phonon replicas [35] of the FA, respectively. In addition, the spectrum of ZnO:Cu nanobushes also exhibits an enhanced, and red-shifted structured green emission (SGE) [17,36] peak. Intentionally doping Cu in ZnO under O-rich condition usually causes more Cu-related DALs and point defect of V_Zn. Yan [18] reported that DALs of Cu_Zn under O-rich growth condition was located at 0.70 eV above the VBM of ZnO. Thus, the SGE corresponds to the electronic transition related to deep acceptor of Cu⁺_Zn or/and Cu²⁺_Zn. Additionally, Zinc vacancies (V_Zn) are also the DALs at 0.87 eV above the VBM [37]. A transition from CB (or a shallow donor) to the DALs of V_Zn also causes O-rich unstructured green emission at 2.3 eV in ZnO [38]. Hence, this enhanced red-shifted SGE of ZnO:Cu should be due to the V_Zn and Cu_Zn under O-rich growth condition. Unstructured green emission related to V_Zn is probably masked by the strong Cu-related structured green band in the same spectral region.

The Ohmic contact on n-GaN film is fabricated by thermally evaporating 10 nm thick Au film through a shadow mask. Metal In dot is directly contacted with Au film. The cross-connected p-ZnO:Cu nanobushes formed a whole continuous structure, which is in favor of current spreading. So we can use just a small In dot as electrode to contact with the whole cross-connected ZnO:Cu nanobushes. The linear I-V relations shown in the inset of Fig. 5(a) indicate that Ohmic contacts on two parts are achieved. Figure 5(a) shows I-V characteristics of this p-ZnO:Cu/n-GaN heterojunction LED device. The non-linear I-V curve exhibits an rectifying diode-like behavior, which suggests that the p-ZnO:Cu/n-GaN heterojunction is formed.

 

Fig. 5 Characterizations of the red LED device. (a) The I–V characteristics of this p-ZnO:Cu/n-GaN heterojunction LED at RT. (b) The EL spectra of the device under different forward bias voltages. (c) Photo galleries of RT EL from p-ZnO:Cu/n-GaN LED at the forward bias voltage from 10 V to 20 V. (d) Schematic energy band diagram of p-ZnO:Cu/n-GaN can explain the light emission in EL. (e) A plot of the maximum of integrated EL intensities and currents vs the forward bias voltages.

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As shown in Fig. 5(b), the EL spectra of the p-ZnO:Cu/n-GaN LED under various different forward bias voltages exhibit two main red emission peaks: (I) 677 nm, (II) 745 nm and a weak infrared emission peak: (III) 916 nm. The intensity of the red and infrared emission peaks increases with the increase of the forward bias voltage. Meanwhile, the location of those peaks keeps constant, which indicates the electrical and thermal stability of this UV-free red light LED. The UV-free red EL spectrum at forward bias voltage of 3.5 V is shown in the inset of Fig. 5(b). Furthermore, the UV-free red EL emission can be clearly seen with the naked eye in a dark environment. Considering this, we presented the photos of red EL emission under different voltages from 10 V to 20 V in Fig. 5(c). When the applied forward bias voltage increases, the emitting light becomes brighter. And the emission area of UV-free red LED are also enlarged.

To further understand the origin of the UV-free red EL lights, the energy band diagram of p-ZnO:Cu/n-GaN heterojunction is illustrated in Fig. 5(d), which is assigned to type-II alignment. The EL spectrum of p-ZnO:Cu/n-GaN LED exhibits no UV light, but the dominated red lights (peak I: 677 nm, 1.831 eV; peak II: 745 nm, 1.665 eV). It illustrates that the UV light resulting from NBE emission of ZnO and/or GaN is inhibited in EL emission. In addition, there is no red emission peak for ZnO:Cu and GaN, but only green emission peak (533 nm, 2.327 eV) in PL spectrum of p-ZnO:Cu/n-GaN (as shown in Fig. 4(a)). When the forward bias voltage is applied at p-ZnO:Cu/n-GaN heterojunction LED, the CB of GaN will be lower compared with that of p-ZnO:Cu due to the built-in voltage. Thus, the dominated red EL peak I (677 nm, 1.831 eV) and peak II (745 nm, 1.665 eV) will be easily induced by a whole electronic interfacial transitions from CB of n-GaN to the DALs related to Cu_Zn and V_Zn in p-ZnO:Cu, respectively. Furthermore, the emission energy difference (0.165 eV) between red peak I (1.831 eV) and peak II (1.665 eV) was nearly consistent with the difference (0.170 eV) between the DALs of Cu_Zn at 0.70 eV [18] and of V_Zn at 0.87 eV [37] above the VBM in p-ZnO:Cu. The UV-free red EL light of the p-ZnO:Cu/n-GaN LED is well understood by this kind of electronic interfacial transitions, which is also reported in literatures [6,10,23,24,39]. The peak III (916 nm, 1.354 eV) is probably from n-GaN, which is derived from the intra-3d-shell transitions (${}^{4}T{}_1(G)\to {}^{6}A{}_1(S)$) of omnipresent iron trace impurity Fe³⁺ (3d5) inside the n-GaN [40].

Plots of the integrated EL intensity (dotted line) and current (black line) vs the applied bias voltages are shown in Fig. 5(e). Two curves with the similar tendency suggest that the UV-free red EL emission process involved carrier transport through this heterojunction [41]. Moreover, the turn-on voltage in the I-V plot is below 3.5 V.

4. Conclusions

The cross-connected p-ZnO:Cu nanobushes were grown on n-GaN film by CVD under O-rich condition. Both RT and low temperature (5K) PL spectra of ZnO:Cu illustrated that UV emission of ZnO was suppressed and the green emission was enhanced. This cross-connected p-ZnO:Cu nanobushes formed the inner-linked continuous structure, which resulted in the enlargement of the emission area of the the p-ZnO:Cu/n-GaN LED with the increase of the forward bias voltage. The p-ZnO:Cu/n-GaN LED emitted stable UV-free red light (677 nm and 745 nm) in EL measurement. The EL light of this LED is tuned from UV of ZnO/GaN to UV-free red by the electronic interfacial transition from the CB of n-GaN to the DALs of p-ZnO:Cu. This research provides an approach to fabricate red LEDs.