Roles of Hydrogen in Amorphous Oxide Semiconductor In-Ga-Zn-O: Comparison of Conventional and Ultra-High-Vacuum Sputtering

Abstract

We investigated roles of hydrogen on physical properties of a-IGZO films and thin-film transistors (TFTs) by comparing standard and ultra-high vacuum (UHV) sputtering systems. It was confirmed that the impurity hydrogens come mainly from the residual gas in the deposition chamber and the molecules adsorbed to the surface of the sputtering target. It was found impurity hydrogen has unfavorable effects as follows; (i) enhances selective Zn desorption during film deposition, and (ii) weakens chemical bonds of the resulting film, causing temperature instability. On the other hand, the UHV a-IGZO films with less hydrogen had low density and exhibited structural instability, suggesting that some hydrogens would have a favorable effect to enhance structural relaxation rate and to form denser and more stable structures during film deposition at room temperature. The revealed hydrogen effects are discussed in relation to those in amorphous silicon and silicon dioxide.Amorphous oxide semiconductors represented by amorphous InGa-Zn-O (a-IGZO) have superior properties for thin-film transistors (TFTs) such as large electron mobilities > 10 cm 2 /(Vs), good uniformity, stability, and low-temperature fabrication process; 1 therefore, a-IGZO TFTs are used in large-area, high-resolution and/or highframe rate liquid-crystal displays (LCDs) and organic light-emitting diode displays (OLEDs). Although the high mobility is obtained for a-IGZO TFTs even fabricated at room temperature (RT) without thermal treatments, post-deposition thermal annealing e.g. at 300-400 • C is required to obtain satisfactory good uniformity and stability. [2][3][4][5] In particular, addition of appropriate amount of water in the annealing atmosphere produces further improved TFTs (wet O 2 annealing). 6 It is reported that the wet O 2 annealing reduced donor states and weak oxygen-metal chemical bonds, 6,7 the latter of which was confirmed by suppressed desorption of H 2 O and metal-related species in a lowtemperature region 100-300 • C. Unexpectedly, it also revealed that the wet O 2 annealing suppressed desorption of hydrogen although the wet O 2 annealing uses H 2 O in the annealing atmosphere. It has also been reported that a-IGZO films contain high-density impurity hydrogens at 10 20 -10 21 cm −3 , which exist mainly as -OH groups. 8,9 On the other hand, it has also been reported that post-deposition hydrogen treatments (e.g., thermal annealing in a hydrogen-containing atmosphere, 10 hydrogen plasma, 11-13 and proton implantation) 14 generate shallow donor states and increase free electrons and electronic conductivity, which is supported also by theoretical calculations; 15 therefore, hydrogen treatments e.g. by subsequent deposition of aSiN x :H have been proposed to fabricate co-planar and self-alignment structure TFTs. 11 To explain these results, we proposed a model that the impurity hydrogens in unannealed a-IGZO would work as shallow donors, but the generated free electrons are trapped and compensated by excess oxygens introduced during film depos...

Highly hydrogen-sensitive thermal desorption spectroscopy system for quantitative analysis of low hydrogen concentration (∼1 × 1016 atoms/cm3) in thin-film samples

“…Before deposition, the Si substrates were pre-annealed at 800 °C for 1 h in the deposition chamber, as described in Section VI and S2 for the optical absorption spectrum). These composition and optical results are similar to those of previously reported a-IGZO thin films fabricated using an UHV sputtering chamber [16,25]. , which is ~2 orders of magnitude higher than that of SIMS and RNRA, within a 10% error.…”

“…Pa was employed for film deposition to minimize the hydrogen impurity concentration in the a-IGZO thin films because hydrogen concentration in the a-IGZO thin films largely depends on the base pressure of the sputtering deposition chamber [16]. Before deposition, the Si substrates were pre-annealed at 800 °C for 1 h in the deposition chamber, as described in Section VI and S2 for the optical absorption spectrum).…”

“…In addition to their effect on representative Si-and GaAs-based semiconductor devices [1,6], the role of hydrogen impurities in oxide semiconductors such as ZnO has also attracted considerable attention. Even a low hydrogen concentration (~10 16 atoms/cm 3 ) in oxide semiconductors has been reported to result in unintentional n-type donors, thereby seriously affecting the electron transport properties and device performance [3]. To address these issues, highly hydrogen-sensitive (HHS) quantitative analyses of semiconductors have been performed using techniques including electron spin resonance [7], infrared spectroscopy [8,9], Raman scattering spectroscopy [9], secondary ion mass spectrometry (SIMS) [10,11], resonance nuclear reaction analysis (RNRA) [10,12], and thermal desorption spectroscopy (TDS) [13 -15].…”

“…Thermally desorbed hydrogen from samples is generally detected using a quadrupole mass spectrometer (QMS), i.e., a simple technique. Because the hydrogen detection limit for thin films of a commercially available TDS is ~10 19 atoms/cm 3 (i.e., one order of magnitude higher than that of SIMS and RNRA), a low hydrogen concentration in thin films (~10 18 atoms/cm 3 ), which was successfully determined using SIMS, could not be detected using a commercially available UHV-TDS system, as reported in [16]. Therefore, the development of a SIMS or RNRA system with higher hydrogen sensitivity would be more appropriate to achieve extremely high hydrogen-sensitive detection for thin films.…”

Beneficial effect of hydrogen in aluminum oxide deposited through the atomic layer deposition method on the electrical properties of an indium–gallium–zinc oxide thin-film transistor

“…The first can be negligible when a passivation layer is adopted, but the other sources occur very often during the fabrication of oxide TFTs. Hydrogen is incorporated from the residuals in the chamber and the target surface even in a high-vacuum condition [9]. In addition, the incorporation of hydrogen into the active layer is inevitable during the deposition of the adjoining layers, such as the gate insulator, passivation, etch-stop, and inter-dielectric layers.…”