To investigate the nano-LED, the electrical properties and optical properties of the devices have been tested. Emission of light has been observed from the ZnO nanostructure when bias is applied to the device. The Current-Voltage characteristic of the nano-LED has been tested. The spectrum of the light has been measured in order to analyze the source of emitted light. The intensity of the light with respect to different voltage levels has been measured as well, which proves that the nano-LED device can possibly be used as a single photon source. In this chapter, the details of the experiment methods will be presented and the results from the experiments will be shown.
3.1 Electroluminescence from Nano-LED
As a light source, the nano-LED is electrically biased for the light-emitting test.
To test the electroluminescence (EL) from the nano-LED, a Keithley 2400 Source Meter is used to apply current to the device. On the device, the Ni layer serves as the electrodes to the ZnO nanostructure between the nanogap. Two probes connected to the Keithley land on the two sides of the nanogap and the current passes the ZnO nanostructure through the Ni electrodes. Figure 3-1-1 is the picture of a S2 nanogap device under test taken by an optical microscope with the magnification of 100. The cross-sectional view of the device under test is shown in Figure 3-1-2.
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Figure 3-1-1. Device under test under optical microscope.
Figure 3-1-2. Cross-sectional view of the device under test.
When the device is under bias, high electric field exists between the tips and carrier transport will happen at the metal-semiconductor interface. Only the ZnO nanostructure between the nanogap is electrically active and electroluminescence may happen from the nanostructure.
The result of the electroluminescence test is shown in Figure 3-1-3.
Figure 3-1-3. Electroluminescence of the ZnO nanostructure between the nanogap.
In the test, the Keithley 2400 Source Meter provides a current of 10Î¼A to the ZnO nanostructure through the probes. When light of the microscope is turned off, emission of light is observed right from the position of the nanogap, which proves the happening of electroluminescence of the ZnO nanostructure between the gap.
3.2 Spectrum of the Light Emitted from Nano-LED
ZnO is a II-VI direct bandgap semiconductor59. The emission spectra of ZnO strongly depend on the preparation methods and the growth conditions. In this research, the X-ray diffraction (XRD) data (Figure 3-2-1) of the oxidized Zn film has shown that the ZnO prepared by thermal oxidation of Zn possess a polycrystalline wurtzite crystal structure. However, since the width of the gap is only several tens of nanometers, the size of the synthesized ZnO nanostructure could be smaller than the mean grain size of the polycrystalline ZnO film. The ZnO nanostructure may have the structure of single crystal, which allows radiative recombination to occur in the direct band gap, thus may explains the electroluminescence. To find out the source of the photon emission, the spectrum of the light has been measured.
Figure 3-2-1. X-ray diffraction data of the thermally oxidized Zn film.
An Acton MicroSpec 2300i monochromator with a Princeton Instruments Cascade 512B (CCD 97) camera is used to get the spectrum of the light emitted from the nano-LED.
The working principle of the monochromator is explained in Figure 3-2-2. A polychromatic light is aimed at the entrance slit of the monochromator. When the light encounters the grating inside, it is dispersed and each wavelength reflects from the grating at a slightly different angle. The grating rotates slowly and the dispersed light is reimaged so that individual wavelengths could be directed to the exit slit and detected by the CCD camera. By comparing the intensity of the light of different wavelengths, the spectrum can be obtained.
Figure 3-2-2. Working principle of monochromator.
The setup of the experiment is shown in Figure 3-2-3. The sample devices are bonded on an acrylic plate that has two testing probes mounted on it. After adjusting the positions of the probes under optical microscope, the plate is mounted on an optical stage. The stage can be moved back and forth to adjust the focus of the image to the camera. The desired device can be located on the screen by moving the stage left and right. The stage with the sample on it is then put into a big box that is covered with aluminium foil (Figure 3-2-3 (a)). The probes are connected to the Keithley outside the box through wires (Figure 3-2-3 (c)). A lens with the magnification of 20 is used to acquire the image of the focused device. The light emitted from the sample will pass through the lens to the monochromator and finally reaches the CCD camera (Figure 3-2-3 (b)). During the experiment, the box is covered by a piece of black cloth to reduce the interference from the light outside (Figure 3-2-3 (d)).
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Figure 3-2-3. Experiment setup of the spectrum test: (a) device is fixed on the stage inside the box; (b) top view of the sample in the box; (c) device is connected
to Keithley; (d) box is covered during the measurement.
The image that acquired by the CCD camera through the optical lens and the monochromator can be seen in the Winspec software program in the computer. Figure 3-2-4 shows a focused S2 device with probes landing on it. The picture taken by the CCD camera has relatively low brightness. To make the image clear, the profile of the device and position of the probes are depicted.
Figure 3-2-4. Image of the S2 device taken by CCD camera.
Figure 3-2-5 shows the electroluminescence from the position of the ZnO nanostructure of a S2 device when the whole box is covered with black cloth.
Figure 3-2-5. Image of the electroluminescence of the ZnO nanostructure
taken by the CCD camera.
In the experiment, the light of wavelengths ranging from 200nm to 1000nm is set to be guided to the CCD camera. The test result is shown in Figure 3-2-6. It shows the spectrum of the light obtained from the nano-LED under four different current levels.
Figure 3-2-6. Spectrum of the light emitted from the device at different current levels.
The electroluminescence spectrum of the nano-LED is in the visible band ranging from 450nm to 1000nm. The major wavelengths are between 580nm and 850nm. When the current applied to the device increases, the intensity of the light increases, but the wavelengths are still the same. The visual spectrum is attributed to some intrinsic and extrinsic defects such as oxygen vacancies and Zn vacancies in ZnO. The mechanism of the visible wavelengths emitted from the device will be analyzed in the next chapter.
3.3 Intensity of the Light Emitted from Nano-LED at Different Voltage Levels
The intensity of the light emitted from the nano-LED is very low. Since the intensity of light is related to the number of photons, the nano-LED can be used to generate small amount of photons or even single photon. With number of photons that emitted per second from the device measured, if the frequency of the applied bias is high enough, it is possible for the nano-LED to emit only one single photon at a time. As the smallest unit of quantum computing, a single photon can store quantum information and it is potentially free from decoherence. Single photon emitters are essential components for realizing optical quantum computing6.
With the same setup as in the spectrum test, the intensity of the emitted light with respect to different levels of applied voltage is measured. The voltage ranges from 10 to 40V. When the bias voltage is further increased, the ZnO nanostructure between the silicon tips may be broken by the high electric current that passes through it. At each measurement point, the light is integrated for several seconds. The values of the intensity obtained by the camera are then normalized to the intensity per second. The test result is shown in Figure 3-3-1.
Figure 3-3-1. Measured intensity of the light emitted from the device
at different voltages.
The turn-on voltage of the nano-LED for light emission is ~12V. The intensity in the semi-log plot has two near linear slopes. It indicates that the light intensities are exponentially increasing. The break point voltage may result from the carrier injection saturation at one junction. The inset in Figure 3-3-1 shows the photon rate converted from the measured intensity. The photon rate at 12.5V is estimated to be ~ 9000/s. When an ultra-short pulse near the turn on voltage is applied to the device, emission of one photon at a time could be possible.
3.4 I-V Curve Analysis of Nano-LED
In electronics, the current-voltage characteristic of a device is the relationship between the voltage across it and the current passes through it. The I-V characteristics of an electrical element can be used to determine a deviceâ€™s fundamental parameters and to analyze its behavior in electrical circuits. The shape of the I-V curve is determined by the transport of charge inside the device. For a diode, the current increases exponentially with forward bias while the current becomes negligible with reverse bias73. To explore the electronic structure of the nano-LED, the I-V curve of the device needs to be measured.
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In a similar method as of the EL test, the I-V curve is measured by a HP4140B pA Meter/DC Voltage Source. The applied voltage from the source ranges from -25V to 25V with an increment of 1V and the value of the current is measured at each test point. The test result has been plotted in Figure 3-4-1.
Figure 3-4-1. Measured I-V curve of the ZnO nanostructure between the nanogap.
The measured I-V data is fitted by the PKUMSM program74. As can be seen in the figure, the exponential curve shows typical characteristics of a diode structure. The semiconductor parameters of the device can be extracted from the fit. The values are listed in Table 3-4-1, where and are barrier heights of the junctions, R is the resistance of the nanostructure, is the doping concentration, and is the carrier mobility.
Table 3-4-1. Extracted electrical parameters of ZnO nanostructure.
The extracted barrier heights are asymmetrical and the values deviate from the calculated barrier heights, which may result from the formation of the Ni-Zn alloy during the thermal oxidation process to convert Zn to ZnO.