Suppression of randomness of electrically pumped random lasing from light-emitting devices based on ZnO films

In this work, the randomness of electrically pumped random laser (RL) from ZnO-based metal-insulator-semiconductor (MIS) structured light-emitting device (LED) is significantly suppressed, by using appropriately patterned hydrothermal ZnO film with large crystal grains as the light-emitting layer. The hydrothermal ZnO film on silicon substrate, with the crystal grains sized over 500 nm, is first patterned into a number of square blocks separated by streets by using laser direct writing photolithography. Based on such a patterned ZnO film, the MIS (Au/SiO₂/ZnO) structured LEDs are prepared on silicon substrates. Under the same injection current, the LED with the patterned ZnO film exhibits much fewer RL modes than that with the non-patterned ZnO film and, moreover, the former displays ever-fewer RL modes with the the decrease of block size. Besides, the wavelength of the strongest RL mode from the LED with the patterned ZnO film fluctuates in a much narrower range than that with the non-patterned ZnO film. It is worth mentioning that the LED with the patterned hydrothermal ZnO film can even be pumped into the single-mode RL under the desirable conditions such as low injection current and small patterned blocks. Moreover, the comparative investigation indicates that the LED with the large-grain hydrothermal ZnO film exhibits the smaller RL threshold current than that with the small-grain sputtered ZnO film, and the former has fewer RL modes and a higher output lasing power than the latter under the same injection current. As for the physical mechanism behind the aforementioned results, it is analyzed as follows. Regarding the LED with the patterned ZnO film, on the one hand, due to the limited numbers of crystal grains and grain boundaries within a single block, the multiple optical scattering is remarkably suppressed. Then, the paths through which the net optical gain and therefore the lasing action can be achieved via multiple optical scattering are much fewer than those in the case of the non-patterned ZnO film. On the other hand, due to optical gain competition among different RL modes occurring within the limited space of a single block, the RL modes with significant spatial overlap cannot lase simultaneously. For the two-fold reasons as mentioned above, the LED exhibits ever-fewer RL modes with the decrease of the size of blocks. Moreover, the inter-block optical coupling enables the optical gain competition among different RL modes to be more violent within a single block, leading to further reduction of RL modes.