Intrinsic photoluminescence from low temperature deposited zinc oxide thin films as a function of laser and thermal annealing

Abstract

Abstract. An investigation into the modification of low temperature deposited ZnO thin films by different annealing processes has been undertaken using laser, thermal and rapid thermal annealing of 60nm ZnO films deposited by Hi-Target-Utilization-Sputtering. Single pulse laser annealing using a KrF excimer laser ( A = 248nm) over a range of fluences up to 315 mJ/cm 2 demonstrates controlled indepth modification of internal film microstructure and luminescence properties without the film degradation produced by high temperature thermal and RTA processes. Photoluminescence properties show that the ratio of defect related deep level emission (DLE, 450nm -750nm, 2.76eV-1.65eV) to excitonic near band-edge emission (NBE at 381nm, 3.26eV) is directly correlated to processing parameters. Thermal and rapid thermal processing results in the evolution of a strong visible orange/red DLE photoluminescence (with peaks at 590nm, 2.10eV and 670nm, 1.85eV) dominated by defects related to excess oxygen. At higher temperatures, the appearance of a green/yellow emission (530nm, 2.34eV) indicates a transition of the dominant radiative transfer mechanism. In contrast, laser processing removes defect related DLE and produces films with intense NBE luminescence, correlated to the observed formation of large grains (25-40nm IntroductionThin films of ZnO are of interest across a range of optoelectronic and sensor device applications due to ZnO being a wide gap (>3 eV) n-type semiconductor with a high exciton binding energy [1] and a piezoelectric response [2]. Poly crystalline thin films of ZnO are deposited by a variety of physical and chemical vapour methods, with sputtering [1] being a preferred choice for low cost and scalability. However, to achieve the desired thin film properties, particularly for low temperature deposited films, it is critical to control grain microstructure, surface morphology, and internal defects [3]. Techniques that have been previously reported to improve these properties of ZnO thin films include post deposition thermal annealing [4,5], rapid thermal annealing [6,7], and laser annealing [8,9,10]. In this paper we present the results from a comparative study of the effect of all three of these annealing processes on the microstructure, crystallinity and associated intrinsic photoluminescence properties of low temperature sputter deposited ZnO thin films. The results demonstrate that pulsed laser annealing is a powerful tool for the controlled modification of low temperature deposited thin films. In particular, the work presented here explores the effect of a more comprehensive range of laser processing parameters on low temperature ZnO thin films than previously reported. The results

Photoluminescence enhancement of ZnO via coupling with surface plasmons on Al thin films

“…20 The key parameter of this process is the fluence of the applied laser pulse. Specifically, the as-grown sputtered Si/ZnO films of this study do not exhibit initially any PL at all due to their very low crystalline quality.…”

“…In particular, when an 80 nm thick Ag film was used as an interlayer, the ZnO is ablated for fluence as low as 100 mJ/cm 2 , meaning that the ablation due to Ag occurs before the structural modifications of the ZnO, which are responsible for the emergence of NBE of the ZnO layer itself. The penetration depth of the 248 nm laser beam on ZnO has been experimentally calculated to be approximately 120 nm, 20 leading to the 60 nm ZnO thin film being semitransparent to the 248 nm beam and part of the beam's energy to be delivered to the underlying metal; Al is reflective at 248 nm, while Ag is absorbing resulting in different laser-matter interactions at 248 nm, which explains the difference in compatibility of the two interlayer metals with the LA process. The high absorbance of 248 nm light from Ag results in the increase in the Ag temperature and dewetting with subsequent ablation of the ZnO.…”

“…[11][12][13][14] The fabrication of high-quality ZnO films with strong UV or VIS emission is a challenging task; a high-temperature deposition or/and a post-deposition annealing scheme is usually required 15,16 with thermal annealing 17 and rapid thermal annealing, 18 which however are not compatible with temperature sensitive underlayers, such as thin metal layers. Consequently, laser annealing 19,20 (LA), being a cold process, is a favourable alternative for processing such structures. Specifically, Tsakonas et al have presented the superiority of laser annealing, for the fabrication of high-intensity UV emitting ZnO, due to the nature of laser annealing, being an ultra-fast approach, allowing precise delivery of energy to the constituents and being compatible with large area processing.…”

“…Specifically, Tsakonas et al have presented the superiority of laser annealing, for the fabrication of high-intensity UV emitting ZnO, due to the nature of laser annealing, being an ultra-fast approach, allowing precise delivery of energy to the constituents and being compatible with large area processing. 20 They have concluded that LA can control the microstructure, defect density, luminescence properties, surface morphology, and internal strain/ film stress in ZnO. The resultant DLE/NBE emissions are directly correlated with LA processing parameters, consistent with the optical absorption properties of the films, as well as with the structural changes occurring after LA (e.g., the as-deposited ZnO films do not exhibit any PL whatsoever; they are polycrystalline with fine grains, 10-15 nm, and high in-grain defect density; after LA with a fluence of 238 mJ/cm 2 , their grain size was enlarged to 10-40 nm but still with high in-grain defect density and exhibit significant DLE; after LA with a fluence of 295 mJ/cm 2 , their microstructure is transformed to columnar with 60 nm long columns, with a diameter of 15-25 nm, very thin grain boundaries, and a very low defect density, exhibiting very strong NBE).…”

“…The resultant DLE/NBE emissions are directly correlated with LA processing parameters, consistent with the optical absorption properties of the films, as well as with the structural changes occurring after LA (e.g., the as-deposited ZnO films do not exhibit any PL whatsoever; they are polycrystalline with fine grains, 10-15 nm, and high in-grain defect density; after LA with a fluence of 238 mJ/cm 2 , their grain size was enlarged to 10-40 nm but still with high in-grain defect density and exhibit significant DLE; after LA with a fluence of 295 mJ/cm 2 , their microstructure is transformed to columnar with 60 nm long columns, with a diameter of 15-25 nm, very thin grain boundaries, and a very low defect density, exhibiting very strong NBE). 20 However, the NBE emission intensity that can be achieved by LA is still below the desired level for UV light emitting devices; thus, the emission efficiency of ZnO has to be substantially increased. 21 Recently, surface plasmon (SP) excitations at semiconductor/metal (usually metal is Ag) interfaces have been exploited for the enhancement of PL in emitting devices.…”