ZNO Defect Emissions

Defects in three different types of ZnO nanostructures before and after annealing under different conditions were studied. The annealing atmosphere and temperature were found to strongly affect the yellow and orange-red defect emissions, while green emission was not significantly affected by annealing. The defect emissions exhibited a strong dependence on the temperature and excitation wavelength, with some defect emissions observable only at low temperatures and for certain excitation wavelengths. The yellow emission in samples prepared by a hydrothermal method is likely due to the presence of OH groups, instead of the commonly assumed interstitial oxygen defect. The green and orange-red emissions are likely due to donor acceptor transitions involving defect complexes, which likely include zinc vacancy complexes in the case of orange-red emissions.

ZnO is a wide bandgap semiconductor which is of interest for a great variety of practical applications, including shortwavelength photonic devices. Therefore, the optical properties of ZnO have been extensively studied. At room temperature, ZnO typically exhibits one emission peak in the UV region due to the recombination of free excitons, and possibly one or more peaks in the visible spectral range which are attributed to defect emissions, but the origin of defect emissions is still not fully clear.

The most commonly observed green defect emission is also the most controversial one, for which various hypotheses have been proposed. There is convincing evidence that the defects involved are located at the surface based on the suppression of this emission by a surface coating and results from polarized luminescence measurements [29], but their chemical nature requires further study.

Yellow and orange-red emissions have been less controversial. It has been proposed that these two emissions may involve similar deep levels but different initial states
(conduction band and shallow donors), and they were also found to exhibit different dependences on the excitation wavelength. The yellow emission is commonly attributed to oxygen interstitial defects, although some impurities such as Li may also play a role. In addition to this common hypothesis, the possible presence of Zn(OH)2 at the surface was identified as a possible reason for the weak UV and the strong visible (broad yellow and green) emission.

The orange-red emission centred at ∼640–650 nm is also commonly attributed to the presence of excess oxygen in the samples, such as oxygen interstitial defects. Other hypotheses include surface dislocations and zinc interstitials. The proposed explanations for the different visible emissions in ZnO are often contradictory, with different defect types proposed to explain the same emission or the same defect types proposed to explain emission in different spectral ranges. In order to study the origins of the different defect emissions in ZnO, we investigated in detail three types of nanostructures, which emit green, yellow and orange-red defect emissions.

Since hydrogen can passivate the defects and consequently enhance the UV emission and reduce the visible emission [32], the worsening of the UV to visible emission ratio for annealing at ∼400 ◦C is likely related to the release of residual hydrogen. It should also be noted that the yellow emission is present only in as-grown nanorods, and it disappears with annealing.

Annealing was also found to affect the decay times of the defect emissions, but not as significantly as in the case of excitonic emission. The decay times of the green, yellow and orange-red emissions measured for different nanostructures were similar, with one dominant component in the 6–10 ns range and one low magnitude contribution with a slower decay, typically in the 30–100 ns range. This is different from a previous report on the very different decay times observed for the yellow and red emissions in ZnO.

Conclusion: It was found that the yellow emission in nanorods prepared by a hydrothermal method can be attributed to Zn(OH)2 or OH groups, instead of the commonly assumed interstitial oxygen defect. The green emission likely originated from surface or grain-boundary defects. The red emission originated from defects related to excess oxygen, possibly involving zinc vacancy complexes.

Defect emissions in ZnO nanostructures, Nanotechnology, 2007
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Varistor

A varistor is an electronic component with a significant non-ohmic current-voltage characteristic. The name is a portmanteau of variable resistor. Varistors are often used to protect circuits against excessive transient voltages by incorporating them into the circuit in such a way that, when triggered, they will shunt the current created by the high voltage away from the sensitive components. A varistor is also known as Voltage Dependent Resistor or VDR. A varistor’s function is to conduct significantly increased current when voltage is excessive.

*Note: only non-ohmic variable resistors are usually called varistors. Other, ohmic types of variable resistor include the potentiometer and the rheostat.


Metal oxide varistor

The most common type of varistor is the Metal Oxide Varistor (MOV). This contains a ceramic mass of zinc oxide grains, in a matrix of other metal oxides (such as small amounts of bismuth, cobalt, manganese) sandwiched between two metal plates (the electrodes). The boundary between each grain and its neighbour forms a diode junction, which allows current to flow in only one direction. The mass of randomly oriented grains is electrically equivalent to a network of back-to-back diode pairs, each pair in parallel with many other pairs. When a small or moderate voltage is applied across the electrodes, only a tiny current flows, caused by reverse leakage through the diode junctions. When a large voltage is applied, the diode junctions break down because of the avalanche effect, and a large current flows. The result of this behaviour is a highly nonlinear current-voltage characteristic, in which the MOV has a high resistance at low voltages and a low resistance at high voltages.

For example, follow-through current as a result of a lightning strike may generate excessive current that permanently damages a varistor. In general, the primary case of varistor breakdown is localized heating caused as an effect of thermal runaway. This is due to a lack of conformality in individual grain-boundary junctions, which leads to the failure of dominant current paths under thermal stress.

Varistors can absorb part of a surge. How much effect this has on risk to connected equipment depends on the equipment and details of the selected varistor. Varistors do not absorb a significant percentage of a lightning strike as energy that must be conducted elsewhere is many orders of magnitude greater than what is absorbed by the small device.

A varistor remains non-conductive as a shunt mode device during normal operation when voltage remains well below its "clamping voltage". If a transient pulse (often measured in joules) is too high, the device may melt, burn, vaporize, or otherwise be damaged or destroyed. This (catastrophic) failure occurs when "Absolute Maximum Ratings" in manufacturer's datasheet are significantly exceeded. Varistor degradation is defined by manufacturer's life expectancy charts using curves that relate current, time, and number of transient pulses. A varistor fully degrades typically when its "clamping voltage" has changed by 10%. A fully degraded varistor remains functional (no catastrophic failure) and is not visually damaged.



Diode
In electronics, a diode is a two-terminal device ( thermionic diodes may also have one or two ancillary terminals for a heater).

Diodes have two active electrodes between which the signal of interest may flow, and most are used for their unidirectional electric current property. The varicap diode is used as an electrically adjustable capacitor.

The directionality of current flow most diodes exhibit is sometimes generically called the rectifying property. The most common function of a diode is to allow an electric current to pass in one direction (called the forward biased condition) and to block the current in the opposite direction (the reverse biased condition). Thus, the diode can be thought of as an electronic version of a check valve.

Real diodes do not display such a perfect on-off directionality but have a more complex non-linear electrical characteristic, which depends on the particular type of diode technology. Diodes also have many other functions in which they are not designed to operate in this on-off manner.

Early diodes included “cat’s whisker” crystals and vacuum tube devices (also called thermionic valves). Today the most common diodes are made from semiconductor materials such as silicon or germanium.