1. Introduction

In recent years, research on ZnO has been spurred by its promising applications in short-wavelength optoelectronic devices. Due to the large exciton binding energy of ~ 60 meV, stimulated emission and laser action at and above room temperature (RT) has been observed in ZnO [1, 2]. In 1998, H. Cao et. al. first reported the random laser action in highly disordered ZnO polycrystalline films and powders [3, 4]. Since then, many groups have worked on the development of optically pumped ZnO film ultraviolet (UV) random lasers [5–12]. Our recent work reported on electrically pumped UV random lasing from ZnO films, using a ZnO-based metal-insulator-semiconductor (MIS) structure on Si [13]. In our ZnO-base MIS devices, there are a number of traps in the insulator layers of SiO_x (x<2) films. However, this imposes complexity on the carrier transport in the devices. Previously, the effects of carrier transport on the random lasing have not been substantially investigated. Therefore, the operating conditions for the random lasing from the ZnO-based MIS devices were ambiguous. In this work, we have investigated the current-voltage (I-V) characteristics and electroluminescence (EL) of the ZnO-based MIS devices in a wide range of forward bias voltages. The results show that above a critical bias voltage the devices exhibit a negative differential resistance (NDR) following a normal I-V characteristic. Moreover, with the increase of bias voltage, the EL from the devices in the normal region is transformed from spontaneous emission into random lasing; while, in the NDR region the EL is transformed from random lasing into very weak spontaneous emission. The mechanism for the effects of carrier transport on the random lasing from the ZnO-based MIS devices has been tentatively explained.

2. Sample fabrication and measurement

In the experiment, ZnO-based MIS (Au/SiO_x/ZnO) structures on Si substrates were fabricated according to the procedures described in our previous report [13, 14]. Briefly, ~ 300 nm thick highly c-axis oriented polycrystalline ZnO films were grown on n-type Si substrates with a resistivity of 3×10^-3 Qcm by reactive dc sputtering. Then, ~ 100 nm thick SiO_x films were deposited onto ZnO films by a sol-gel process. Finally, ~ 20 nm thick semitransparent Au films and ~ 100 nm thick Au films were respectively sputtered onto the SiO_x films and on the backside of silicon substrates. The Au films acting as the electrodes were patterned with ~10 nm diameter circles. The schematic diagram for the ZnO-based MIS device is illustrated in the inset of Fig. 1.

The I-V characteristics of the ZnO-based MIS devices were measured with a Keithley 4200 semiconductor characterization system. The EL spectra for the ZnO-based MIS devices under different dc bias voltages were recorded using an Acton spectraPro 2500i spectrometer with a minimum spectrum resolution of 0.5 Å and an accuracy of ± 2 Å. During the acquisition of EL spectra, a scanning step size of 1 Å was used. All of the measurements were carried out at RT in room atmosphere. To designate the polarity of bias applied on the devices, the forward/reverse bias is referring to Au electrode of gate is connected to positive/negative voltage.

3. Results and discussion

Figure 1 shows the I-V characteristic of a typical ZnO-based MIS device. The forward I-V curve can be clearly divided into three regions in terms of the dependence of the current on the bias voltage. In the range of 0–10.5 V, denoted as a normal region, the current increases rapidly with the voltage. In the range of 10.5–15.0 V the current decreases rapidly, exhibiting an obvious NDR effect with a peak-to-valley current ratio of ~ 9.2. With the further increase of the bias voltage, the current increases slightly, until the device breakdown voltage is reached. As is expected, under reverse bias the current is quite small. The rectification ratio reaches ~ 58 at the bias voltages of ± 10.5 V. Of interest is that the reverse I-V characteristic also exhibits an NDR region. However, the reverse I-V characteristic is not discussed because the device is not electroluminescent under reverse bias.

 

Fig. 1. I-V characteristic of a typical ZnO-based MIS device. The inset shows the schematic diagram of the MIS device.

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Figure 2 shows the RT EL spectra in the wavelength range of 360–400 nm for the ZnO-based MIS device at different forward bias voltages. It should be mentioned that the EL in the visible region is relatively weak. As shown in Fig. 2, the features of EL spectra change significantly with the applied forward bias voltage. At a relatively low voltage of 6.5 V, the EL spectrum reveals a spontaneous UV emission originated from near-band-edge (NBE) recombination. When the forward bias voltage is increased to 8.5 V, sharp peaks appear in the EL spectrum, indicative of the random laser action of the device. The inset of Fig. 2 shows the spectrally integrated EL intensity as a function of the current, which reveals a threshold current of ~ 50 mA for the random lasing. The random lasing becomes stronger with increasing forward bias voltage up to 10.5 V beyond which NDR is exhibited. In the NDR region, the EL from the device still demonstrates random lasing at the bias voltages lower than ~12.5 V. However, the random lasing is weaker compared to that generated in the normal region, as exemplified by the comparison of the spectrum at 11.0 V with that at 10.2 V. At the bias voltages of 12.5 V and above, the EL from the device again turns into the spontaneous emission, which becomes progressively weaker. Figure 3 shows the dependences of spectrally integrated EL intensity and current on the bias voltage. The EL intensity-voltage curve conforms largely to the I-V characteristic, indicating that the EL intensity is substantially determined by the current.

 

Fig. 2. RT EL spectra of the ZnO-based MIS device at different forward bias voltages: 6.5, 8.5, 10.2, 11.0 and 12.5 V, respectively. The inset shows the spectrally integrated EL intensity as a function of the current.

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Fig. 3. The dependences of spectrally integrated EL intensity and current on the voltage for the ZnO-based MIS device.

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To understand the NDR of the ZnO-based MIS devices and its influence on the EL is still a challenge. It was reported 50 years ago that both metal-insulator-metal (MIM) and MIS structures exhibited NDR. However, the exact mechanism of the NDR still remains an open issue [15–20]. A recent model presented by R. E Thurstans and D. P Oxley can address all of the reliable experimental evidence including NDR effect for the MIM structure and is fully self-consistent [21]. In the following, our explanation on the NDR of the ZnO-based MIS devices is based on the basic ideas presented in their model. Figures 4(a) and 4(b) show the schematic diagrams for the electron transport within a ZnO-based MIS device under different forward bias voltages. A number of metal islands embedded in the SiO_x layer are formed during a so-called ‘electroforming process’, which occurs once the device is applied with a sufficiently high voltage. As the ZnO-based MIS device is under forward bias, there are two aspects for the electron transport in SiO_x film. For one, as shown in Fig. 4(a), the electrons coming from ZnO film transport in SiO_x film through successive tunneling, mediated by the electroformed metal islands. For the other, certain defects in SiO_x film can act as the traps for electrons, hindering the tunneling of electrons between the two metal islands. It is believed that the tunneling electrons suffer increasing probability of scattering into the traps as the bias increases. In our experiment, when the forward voltage is below 10.5 V, as shown in Fig. 4(a), the tunneling of electron predominates over the trapping of electron. This leads to a normal curved I-V characteristic where the current increases with the bias. In this case, the device is in a low-resistance state. It should be noted that under forward bias the electrons injected into ZnO film from the silicon substrate will firstly accumulate nearby the ZnO/SiO_x interface, as shown in Fig. 4(c), prior to entering into SiO_x film. On the other hand, holes are injected into ZnO via two possible pathways proposed in our previous work [14]. They recombine with electrons near the ZnO/SiO_x interface. The accumulation of electrons and the electron-hole recombination nearby the ZnO/SiO_x interface are crucial for generating UV EL. With the increase of forward bias, more electrons and holes are injected into ZnO and substantial stimulated UV emission from ZnO occurs, which contributes to the optical gain. The UV light propagating in the plane of ZnO film will perform a random walk due to the multiple inter-grain scattering. Through stimulated emission together with multiple scattering, photons of certain wavelengths achieve sufficiently large gain that overcomes the losses. Consequently, the emission spectrum shows laser spiking, indicating the occurrence of random lasing. When the forward bias exceeds 10.5 V, as shown in Fig. 4(b), the tunneling electrons suffer increasing probability of being scattered into traps. The trapped electrons will constitute a negative charge between the tunneling sites, thus raising the interfacial barrier against the tunneling of electrons. As a result, the current lowers, meaning that the device exhibits NDR. In other words, the device is in a high-resistance state. In this case, compared with the case of low-resistance state, fewer electrons accumulate at the ZnO/SiO_x interface. Moreover, the injection of holes into ZnO film is also weakened. As a result, the random lasing from ZnO film is progressively suppressed along with increasing forward bias on the device. Ultimately, the EL turns to be spontaneous emission that is very weak due to the low injection current.

 

Fig. 4. Schematic diagrams of electron transport in SiO_x film: (a) tunneling and (b) trapping of electrons are dominant, respectively. (c) Schematic energy-band diagram for the forward-biased ZnO-based MIS device.

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4. Conclusion

In conclusion, we have systematically investigated the I-V characteristics and EL performance of the ZnO-based MIS device in a wide range of forward bias voltages. It is found that the device exhibits a remarkable NDR effect under sufficiently high forward bias following the normal curved I-V characteristic. In the normal region, the EL from ZnO is transformed from spontaneous emission to random lasing, which increases with the increase of the bias voltage. While, in the NDR region, the EL from ZnO transforms from random lasing to the weak spontaneous emission with the increase of bias voltage. Based on a self-consistent model that takes into account the competition between the tunneling and trapping of electrons in SiO_x film, we have explained the NDR and its effect on the EL of the ZnO-based MIS device. It is believed that this work contributes to the comprehensive understanding of carrier transport and EL performance of the ZnO-based MIS device.