1. Introduction

Field emission display (FED) is considered to be superior to liquid crystal display(LCD) and plasma display panel (PDP) in such aspects as luminous efficiency, brightness, viewing angle and power [1, 2]. Recent years, researches on field emission have been mainly focused on one-dimensional nanostructure materials, such as carbon nanotube, SiCN nanorods, MoO3 nanobelts, ZnO nanowires and so forth [3–7]. Wide spread applications have also been found in large-area flat panel displays due to their unique characteristics: low work function, excellent mechanical strength, high chemical stability, high aspect ratio and high thermal conductivity. However, complication of device structure limits the increasing of fabrication sizes and hinders the development of commercial application of large area display [8]. Also, more importantly, as a vital role in the realization of full-color display, efficient blue electroluminescence (EL) emitters still faces great challenges because of the lack of appropriate synthetic routes to prepare larger band-gap emission materials [9].Nonetheless, using films with either very low electron affinity or an intrinsic nanostructure was proved to better overcome those problems [10]. It was also reported that the electron affinity of Y₂O₃ was estimated to be around 1.7 eV [11–13] and could be applied as a promising material for field emitters because electrons in Y₂O₃ with low electron affinity can be easily extracted from the surface to vacuum. In addition, Y₂O₃ phosphor activated by rare earth ions, such as Eu³⁺, Er³⁺, Yb³⁺ and so on, is recently known as the most promising oxide-based phosphors for FED applications [14, 15].

In this paper, ITO/Y₂O₃/Ag films field emission devices were fabricated using Y₂O₃ films as insulator and the stable, uniform and intense blue EL were obtained when the forward bias was applied to Ag. According to the analysis of surface morphology, luminescence mechanism was illustrated by the cross-section of the devices. In addition, the improvement of the device performance was also discussed.

2. Fabrication

Figure 1 shows the structure diagram of the device. The preparation of that is as follows: The P-type (100) Si wafers were chosen as substrates. Y₂O₃ films (~100nm thickness) were deposited on Si wafers by electron beam evaporation (EVA450) with the vacuum degree of 9*10-5 Pa and subsequently annealed in nitrogen ambient at 1050 °C for 1 hour (h). The indium tin oxide (ITO) layer (~200nm thickness) was deposited on the top of Y₂O₃ layer after annealing by RF Magnetron Sputtering method and Ag was deposited on the back of Si wafers. The thickness of the Ag electrode was approximately 200 nm. ITO and Ag were used as the negative electrode and the positive one, respectively. In addition, in order to optimize device performance, other things being equal, Y₂O₃/In₂O₃ multiple films were deposited on P-type Si wafers to replace Y₂O₃ films.

 

Fig. 1 Structure diagram of the device.

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Surface morphologies of the films were investigated by scanning electron microscopy (SEM). The EL spectra of the devices were measured by ACTON 150 CCD spectrometer and current-voltage (I-V) measurements were carried out using a Keithley 2410.

3. Results and discussion

Figure 2 shows EL spectra and images of the device obtained when the forward bias was applied to 10V, 15V and 20V, respectively. Four sharp emission lines were appeared in the spectral range from ultraviolet to blue, mainly dominated by blue. These sharp lines correspond to the 293.78 nm InI, 316.10 nm InIand 444.82 nm InII and 403.07 nm InIII transitions (according to National Institute of Standards and Technology Database). The half-peak width of all the lines is as low as 4nm, indicating the device has high monochromaticity. When applying voltage to the device, the intensity of the blue EL was enhanced with increasing the forward voltage and the brightness also subsequently increased. That might be attributed to the increase of bias that brought electrons closer to interface leading to enhanced electron scattering of interface traps [16]. Besides, the maximum intensity was also obtained at the forward bias of 20V. The inset shows I-V characteristics of the device. Under a forward bias, the device demonstrated typical I-V diode characteristics and the turn-on voltage was just 10V. The blue luminescence was clearly visible by naked eyes under dark condition and the EL spectrum was recorded. The current increased significantly with the increase of the forward bias. When the voltage was applied to 20V, the corresponding current reached up to approximately 3.5mA. Nevertheless, under a reverse bias, the current of the device was near to zero. The reverse cut-off characteristic demonstrates a good unilateral conductivity of the device. The dependence of the output power of the device on the forward voltage is shown in the Fig. 2(b). It is clearly observed that the output power increases with the forward voltage. It reaches around 0.85 μW when the forward voltage is 20 V.

 

Fig. 2 (a) EL spectra and images of the device under the forward bias of 10V, 15V and 20V, respectively. The inset shows I-V characteristics of the device (Ag electrode is defined as anode); (b) the dependence of the output power of the device on forward voltage.

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The analysis of surface morphology of the films was performed to investigate the origin of blue EL. Figure 3(a) shows the SEM image of Y₂O₃ film after annealing. It was observed that different cracks were formed inside the annealed Y₂O₃ film in nitrogen ambient at high temperature. ITO film was deposited on the top of annealed Y₂O₃ film and the SEM image of ITO film was shown in Fig. 3(b). The surface of ITO film became uneven, and the depressions of varying sizes were distributed over the whole film. It might be attributed to the sputter ITO film was deposited into the cracks of Y₂O₃ film and thus resulted in different degree of depressions. Simultaneously, the depressions also provide strong evidence for the existence of the cracks.

 

Fig. 3 (a) SEM image of Y₂O₃ film annealed in nitrogen ambient at 1050°C for 1h; (b) SEM image of ITO film deposited on the top of annealed Y₂O₃ film.

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According to the above results, luminescence mechanism was able to be illustrated by the cross-section of the devices (Fig. 4). The blue part corresponding to Y₂O₃ film acts as insulation layer. The upper left of the figure displays an enlarged Y₂O₃ film crack. The sputter ITO film was deposited into the cracks, but might not fill the entire crack. Unfilled space creates a vacuum. When a negative voltage was applied to ITO and the opposite was applied to Ag, ITO film inside the crack induced the negative charges and Y₂O₃ film induced the positive charges. Unfilled space was converted into strong electric field due to the distance with nanoscale between the positive and negative charges. The strong electric field lowered surface potential barrier [17, 18]. Thus the electrons of ITO were ionized under the electric field. The kinetic energy of the ionized electrons was acquired by K-shell electrons of indium atoms: one part was used to break away from their nuclei, and the other transformed into the kinetic energy of escape electrons. The electrons of atoms outer shell (usually an L- or M-shell) filled the K-shell vacancies when the electrons escaped. The energy released in this transition with the photons emitted (characteristic radiation) [19]. Hence indium ions characteristic spectrum was identified as a result of such inner-electron transitions and the explanation is consistent with the experiment results of blue EL.

 

Fig. 4 The cross-section of the device and partial enlarged drawing of the Y₂O₃ film crack was shown in the upper left.

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In order to improve brightness and stability of the device, Y₂O₃/In₂O₃ multiple films were attempted to replace single Y₂O₃ films as the insulation layer. 10 periods of Y₂O₃/In₂O₃ films were alternatively deposited on P-type Si substrates by e-beam evaporation. The thickness of each Y₂O₃ and In₂O₃ layer were 9 and 1 nm, respectively. The as-deposited Y₂O₃/In₂O₃ multiple films were annealed in nitrogen ambient at 1050°C for 1 h. As a result, In³⁺ ions were doped in Y₂O₃. The single Y₂O₃ films were also treated with the same annealing condition and electrode sputtering. Figure 5(a) shows the EL spectra and images of two devices at a forward bias voltage of 20V. Both the spectra and images display that luminescence intensity and brightness with the doping of In₂O₃ almost doubled the single Y₂O₃ films. And the peak positions almost have no shift. The I-V characteristics of both devices are given in the inset. The turn-on voltage decreased and the current increased significantly, on account of the doping of In³⁺ ions. This might be due to different radii of the doping In³⁺ and Y³⁺ leading to the decrease of grain sizes. Compared to Fig. 3(a), the SEM image of annealed Y₂O₃/In₂O₃ multiple films [Fig. 5(c)] demonstrates smaller grain sizes and larger cracks. The increased density of field emitters with smaller grains led to the enhancement of EL intensity and brightness. In addition, the CIE chromaticity coordinates of two devices were calculated to be (0.16, 0.03) and (0.16, 0.02) [Fig. 5(b)] based on their EL spectra, which is located in the deep blue region. Compared with the National Television System Committee (NTSC) standard blue (CIE (x, y) coordinates of (0.14, 0.08)), the devices with smaller CIE y-value possess lower the power consumption [20].

 

Fig. 5 (a) EL spectra and images of the devices with different insulation layers annealed in nitrogen ambient at 1050°C for 1h (the corresponding forward bias: 20V) .The inset shows I-V characteristics of both devices; (b) The corresponding CIE 1931 chromaticity coordinates of the devices;(c) SEM image of Y₂O₃/In₂O₃ multiple films annealed in nitrogen ambient at 1050°C for 1h.

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4. Conclusions

Y₂O₃ films were deposited on Si wafers by electron beam evaporation and annealed in nitrogen ambient at 1050 °C. ITO was deposited on the top of the annealed Y₂O₃ film and Ag was deposited on the back of Si wafers. When applying the forward bias to Ag electrode and the reverse bias to ITO. The intense and sharp lines are found at 293.78 nm InI, 316.10 nm InI, 444.82 nm InII and 403.07 nm InIII transitions. The half-peak width of four lines is only as low as 4nm. According to the analysis of surface morphology, luminescence mechanism was illustrated by cross-section of the devices. Under the action of strong electric field, the K-shell electrons gained kinetic energy and part of them escaped. The electrons of atoms outer shell filled the K-shell vacancies and the energy released in this transition with the occurrence of characteristic radiation of indium ions. Besides, the devices demonstrate typical I-V diode characteristics under the forward bias and the turn-on voltage is 10V. The maximum intensity was obtained at 20V. In addition, Y₂O₃/In₂O₃ multiple films were attempted to replace single Y₂O₃ films as the insulation layer to optimize device performance. It is clearly observed that the EL intensity and luminescent brightness with the doping of In₂O₃ almost doubled the single Y₂O₃ films. The CIE chromaticity coordinates were also calculated to be (0.16, 0.03) and (0.16, 0.02), respectively and the smaller CIE y-value means the lower the power consumption of the devices. Finally, we also concluded that the novel blue devices can meet the demand of field emission display in stability and coincidence of electronic emission and easily realize large area display. However, the luminescent mechanism is complex and needs further studies.