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Abstract
Laser annealing is a usual step in the process to improve the performance of amorphous InGaZnO4 (a-IGZO) thin film transistors (TFTs). However, a high energy laser will induce damage to a-IGZO thin films during annealing. Knowing laser induced damage thresholds (LIDT) and the mechanisms of a-IGZO thin films helps to use appropriate laser energy density during annealing to avoid damage to the thin films and to achieve the best TFTs’ properties. In this article, the ultraviolet laser with a wavelength of 355 nm and a pulse width of 7.7 ns LIDT and damage mechanisms of a-IGZO thin films are reported. The damage morphologies are characterized with optical microscopy and scanning electron microscope and Raman spectra. The electrical and optical properties of a-IGZO thin films are studied. The a-IGZO thin films have LIDT increased from 0.12 J/cm2, 0.16 J/cm2, and 0.23 J/cm2 to 0.24 J/cm2 with absorbance decreased from 22.4%, 18.1%, and 17.3% to 12.3%. The concentrations of oxygen and free carrier (Ne) and thermal conductivity and optical band gap (Eg) and electrical effective mass (m*) are important factors affecting the LIDT of thin films. The thermal conductivity influences the surface temperature and LIDT of thin films. The increased Eg and m* with the decreased Ne in thin films are other important reasons for the increased LIDT. The laser mainly induces thermal damage of thin films with intrinsic processes including avalanche ionization and multi-photon absorption. No apparent phase transformation and lattice expansion exists during laser irradiation for the stable amorphous structures of a-IGZO thin films.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Recently amorphous InGaZnO4 (a-IGZO) thin films have been studied extensively as the active layer of thin film transistors (TFTs) for flat panel display [1]. During depositing post-fabrication furnace annealing is an essential process to achieve high field-effect mobility and stability for a-IGZO TFTs [2]. But furnace annealing has disadvantages of requiring process temperature as high as 400 ℃ which is too high for many flexible substrates and long annealing time of almost several hours [3]. However the laser annealing, including excimer laser [3–9], CO2 laser [2], and solid-state laser [10–11] and HeCd laser [12], are especially promising annealing method for activating a-IGZO TFTs during high throughput fabrication such as flexible process, with advantages of maintaining substrate temperature below 30 ℃ and be suitable for any flexible substrates and with extremely short annealing time of only tens of nanoseconds. Laser annealing is also considered as a promising tool with capability of selective treatment and fine spatial resolution developed for ZnO [9,13] and In2O3 [14] thin films.
However during annealing high energy laser radiation will induce damage of thin films [3,8–9,15–16]. But to our knowledge, for a-IGZO thin films there are no laser induced damage reports till now. The laser damage mechanisms and thresholds of some wide gap semiconductor thin films such as indium tin oxide [17–18] and W-doped In2O3 [19] and GaN [18] thin films irradiated by wavelength at 1064 nm and nanosecond pulse width laser had been reported but seldom for wavelength at 355 nm ultraviolet laser. The 355 nm ultraviolet laser is usually used for annealing a-IGZO TFTs [10] due to the larger photon energy than the band gap of a-IGZO thin film [6].
Knowing the LIDT of a-IGZO thin films can acquire the highest laser energy density used in annealing to avoid damage the thin films and the best TFTs’ performance. In the paper the ultraviolet laser induced damage thresholds (LIDT) and mechanisms for a-IGZO thin films are studied by the ISO 21254-1 standard LIDT measuring method [20] and the damage morphologies characterizing techniques. The effects of optical and electrical properties on the LIDT of thin films are discussed.
2. Experiment details
The a-IGZO thin films were deposited by sputtering from InGaZnO4 target with base pressure of 9.0×10−4 Pa and working pressure of 0.5 Pa and Ar flow rate of 40 SCCM and O2 flow rate (FO2) from 0, 1, and 2 to 3 SCCM and RF power of 200 W and substrate temperature at 300 ℃. All thin films have thickness ∼50 nm. The structure and surface morphology and film composition were studied by a X-ray diffraction (XRD) meter and a Scanning electron microscope (SEM) attached with a Energy Dispersion Spectroscopy (EDS). A UV-Vis spectrometer was used to measure the transmittance of thin films. To examine the defect states in samples the photoluminescence (PL) spectroscopy excited with wavelength of 325 nm laser was carried out at room temperature (RT). To determine the optical constants and thickness of thin films the Spectroscopic ellipsometer (SE) method was used. The carrier transportation properties and Seebeck coefficient (S) of thin films were obtained by a Hall system using the Van der Pauw configuration and S measuring apparatus at RT. LIDT were tested at wavelength of 355 nm and pulse width of 7.7 ns laser in 1-on-1 mode with the ISO 21254-1 standard. The laser damage morphologies were observed by a Nomarski microscope and SEM. The thin film structures after different laser energy irradiation were studied by a Raman spectroscopy.
3. Results and discussion
Figure 1 shows the XRD patterns of a-IGZO thin films. All thin films are amorphous structure. The free carrier concentration (Ne) of a-IGZO thin films were measured by the Hall instrument at room temperature. The carrier type in all investigated thin films is electron. The Ne decreases from 1.3 E18, 1.7 E17, and 3.3 E16 to 3.0 E16 cm−3 when FO2 increased from 0, 1, 2 to 3 SCCM. The following defects formation reaction can be considered in a-IGZO thin films:
The presence of oxygen vacancies (VO) donates free electrons in thin films. The Ne is positive proportional to the concentration of VO (NVO). With the increase of FO2 during deposition the NVO in thin film could be decreased and therefore the Ne in thin film decreases. This conforms with the Hall test result.
Figure 2 shows the optical properties of a-IGZO thin films. The absorbance (A) at wavelength of 355 nm are calculated from A = 1-T-R where T is the transmittance and R is the reflectance measured by spectrometer. The A decreases from 22.4%, 18.1%, and 17.3% to 12.3% for a-IGZO1 to a-IGZO4 thin films which shows that the photons are efficiently and uniformly absorbed throughout the entire a-IGZO film thickness. The absorption coefficients (α) of thin films shown in Fig. 2 (a) are calculated from α= - (ln T) /d where T is the transmittance and d is the film thickness for a-IGZO thin films. The α at wavelength of 355 nm as extracted from Fig. 2 (a) decreases from 46650 cm−1, 26814 cm−1, and 14015 cm−1 to 10238 cm−1 from a-IGZO1 to a-IGZO4 thin films. All α are larger than 104 cm−1 which belongs to the intrinsic high absorption region when electrons transition from the valence to conduction bands in amorphous semiconductors. The optical length (Lopt) or penetration depth for laser at wavelength of 355 nm are calculated by Lopt≈1/α [6] which increases from 214 nm, 373 nm, and 714 nm to 977 nm for a-IGZO1 to a-IGZO4 thin films. Therefore most of the incident beam is absorbed in the entire a-IGZO thin films. Figure 2 (b) shows the band gaps (Eg) of a-IGZO thin films fitted by (α E)1/2∼E [21] where α is the absorption coefficient and E is the photon energy. The Eg are lower than the laser photon energy (3.5 eV) to ensure be intrinsically absorbed by thin films.
The defects within optical band gap of a-IGZO thin films are studied by PL spectra from UV to visible region with excitation wavelength of 325 nm at RT. The thin films have two broad and asymmetric peaks centered at 405 and 460 nm [22] shown in Fig. (3). The main violet emission peaks at 405 nm are attributed to the electron transition from conduction band tail states to valence band tail states [22]. The minor emission at 460 nm are attributed to various defects such as Zn vacancies and interstitials and O vacancies [22]. The PL intensity of near-band-edge emission strongly depends on the Ne of thin films. With the Ne increased from a-IGZO1 to a-IGZO4 thin films, the PL intensity of this peak decreases.
Figure 4 shows the LIDT of a-IGZO thin films measured by laser wavelength of 355 nm and pulse width of 7.7 ns and incidence angle of 0°and laser spot equivalent area of 0.25 mm2 in the 1-on-1 mode with ISO 21254 standard-Laser and laser-related equipment-Test methods for LIDT at RT in 45 ± 5% relative humidity. It is found that the LIDT increases from 0.12 J/cm2, 0.16 J/cm2, and 0.23 J/cm2 to 0.24 J/cm2 with the A decreases from 22.4%, 18.1%, and 17.3% to 12.3%. The three most frequently proposed fundamental damage processes for laser energy be coupled into a thin-film are avalanche ionization, multi-photon ionization, and impurities absorption [23]. Reduction the absorbance and absorption coefficients are crucial to increase the LIDT. Non-stoichiometric and surface inclusions are the typical absorption centers in thin films. Here with the increase of O contents in thin film the Ne and band-edge absorption at wavelength of 355 nm decrease and therefore the LIDT increases.
Figure 5 (a) shows the electrical conductivity (σ) of a-IGZO thin films measured in temperature (T) range of 300∼473 K. The Ne at RT decreases from 1.3 E18 cm−3, 1.7 E17 cm−3, and 3.3 E16 cm−3 to 3.0 E16 cm−3 for a-IGZO1 to a-IGZO4 thin films. All thin films have Ne< nMott seen later and belong to non-degenerate semiconductors. The electronic thermal conductivity (κe) are calculated by
where kB is the Boltzmann constant and q is the electron charge and T is the temperature [24]. Figure 5 (b) shows that the κe almost increases from a-IGZO1 to a-IGZO4 thin films with the increase of temperature. The higher thermal conductivity, the better thermal diffusibility, and the lower surface temperature at the same irradiating laser fluence, and therefore the higher LIDT for thin films. The K9 glass substrates with thickness of 1 mm are used for deposition a-IGZO thin films. As calculation, the penetration depth for laser at wavelength of 355 nm are increased from 214 nm, 373 nm, and 714 nm to 977 nm for a-IGZO1 to a-IGZO4 thin films. All thin films have thickness almost 50 nm. Therefore, most of the laser energy transmits into the glass substrate. But the K9 glass substrate has thermal conductivity of 1.4 W/(m. K) [25] which is larger than the a-IGZO thin films calculated here. The laser energy could be rapidly diffused in the substrate. Therefore, the effect of glass substrate on the LIDT could be neglected.Material and laser wavelength and pulse width are important parameters influencing the LIDT of thin films. Observation the morphologies of laser damage sites is an important tool to analyze the breakdown mechanism. Figure 6 shows the laser induced damage morphologies of a-IGZO thin films by microscopic with the same magnification scope at increased laser energy density from 0.1 to 0.5 J/cm2. At each thin film with the increase of laser energy density from 0.1 to 0.5 J/cm2, the size of damage site increases. From the damage morphologies it can be found that the laser mainly induces thermal damage of thin films. No impurities within thin films exist in our experiment, and the damage can be treated with intrinsic processes including avalanche ionization and multi-photon absorption in which the LIDT is proportional to Eg and electron effective mass (m*) of thin films [26]. Figure 7 shows the SEM morphologies and composition of thin films at different laser damage regions with increased energy density. The damage morphologies (Fig. 6(a)–6(e) for a-IGZO1 and Fig. 6(f)–6(j) for a-IGZO4 thin films) exhibit deterministic features of intrinsic optical absorption related thermal degradation. The concentration of oxygen in thin films decreases with the increased laser energy density as measured by EDS. Figure 8 shows the Raman spectra of a-IGZO thin films at the center of laser damage regions in Fig. 6 with the increased laser energy density. In all as-deposited a-IGZO thin films three Raman peaks at 500 cm−1, 615 cm−1 and 1050 cm−1 [27] which are related to the stretching vibrations of octahedrons are found. It is found that no apparent peaks shifts exist with the increase of laser energy density for all thin films. These indicate that no evident phase transformation and lattice expansion existing during laser irradiation of a-IGZO thin films. These may be due to the stable amorphous structures of a-IGZO thin films within the laser induced temperature range in these irradiation.
The m* at Fermi energy level can be extracted from experimental measured S values which are depended on temperature (T) and m* and free carrier concentration (Ne) as [24]:
where kB is the Boltzmann constant and q is the electron charge and h is the Plank constant. The Ne decreases from 1.3 E18 cm−3, 1.7 E17 cm−3, and 3.3 E16 cm−3 to 3.0 E16 cm−3 and S decreases from −66.7µV/K, −68.2µV/K, and −1449.8µV/K to −2192.7 µV/K at RT when FO2 during deposition increased from 0, 1, and 2 to 3 SCCM for a-IGZO thin films. All S values are negative which indicates the n-type conductivity of thin films. The results are consistent with the Hall measurement. The relation of Ne with Eg and extracted m*/m0 where m0 is the static electron mass are showed in Fig. 9 (a). The Mott criterion Nnd<(0.25/a0*)3<Nd [28] (with effective Bohr radius a0*=h2ɛ0ɛs/(πq2 m*) where ɛ0 is the free space dielectric function and ɛs is the static dielectric function of a-IGZO thin film and h is the Plank constant and q is the electron charge and m* is effective electron mass) defines the Mott transition from non-degenerate (indicated by “nd”) to degenerate (indicated by “d”) electron concentrations. For m* and ɛs extracted in Fig. 9 (b) where the dielectric functions of a-IGZO thin films are fitted by the SE method, nMott are calculated to be 4.0E19 cm−3, 1.1E19 cm−3, and 5.70E18 cm−3 and 3.70E18 cm−3 for a-IGZO1 to a-IGZO4 thin films, respectively. The nMott for all thin films are larger than their Ne which indicates that all a-IGZO thin films belong to the non-degenerate semiconductors. For Ne>nMott, the Eg widens with Ne by the Burstein-Moss shift. But the electron-electron and electron-ion scatterings will narrows the Eg. In here all Ne<< nMott which indicates that the Eg widening effect will not occur [28]. Therefore the Eg and m* decrease with Ne as seen in Fig. 9 (a). The increased Eg and m* with the decreased Ne by the increased FO2 during deposition are another important reasons for the increased LIDT of a-IGZO thin films.4. Summary
In conclusion, the ultraviolet laser induced damage thresholds and mechanisms of a-IGZO thin films are researched. The concentrations of oxygen and free carrier, intrinsic the absorption and absorbance, and the thermal conductivity and optical band gap and electrical effective mass are all important factors influencing the LIDT of thin films. The laser mainly induces thermal damage of thin films by avalanche ionization and multi-photon absorption. No apparent phase transformation and lattice expansion exist during laser irradiation for the stable amorphous structures of thin films. The results help to use appropriate laser energy density during laser annealing and achieve better a-IGZO TFTs’ properties for future flexible display application.
Funding
National Natural Science Foundation of China (NSFC) (61674107); Shenzhen Key Lab Fund (ZDSYS 20170228105421966); Science and Technology Plan of Shenzhen (JCYJ20170302150335518).
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