Structural Changes and Hydrogen Motion in A-SI:H Observed by Proton Nmr

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Jonathan Baugh*, Daxing Han*, Qi Wang** and Yue Wu*

*Department of Physics & Astronomy, University of North Carolina, Chapel Hill, NC 27599-3255, [email protected] "**NationalRenewable Energy Laboratory, 1617 Cole Blvd, Golden, CO 80401 ABSTRACT Proton nuclear magnetic resonance (NMR) is applied to investigate hydrogen dynamics and microstructures in a-Si:H. In addition to the generic broad and narrow lines observed in all aSi:H, an additional narrow line (about 1 kHz wide) is observed as the temperature is raised above RT. This narrow line is shifted to the up-field by about 4 ppm with respect to the generic narrow line of a few kHz in width. Below 150'C, the change of the proton spectrum with temperature is reversible; the hydrogen associated with this additional narrow line is shown to originate from hydrogen originally associated with the broad line. The spin-lattice relaxation time T1 of this up-field shifted narrow line is about 0.4 s. This short T 1, along with its small linewidth, suggests that this line is associated with molecular hydrogen, possibly trapped in sites of atomic dimensions. INTRODUCTION It is widely believed that hydrogen is involved in the generation of metastable defects upon light soaking [1], an effect generally referred to as the Staebler-Wronski effect (SWE). After over 20 years of intensive investigation, the microscopic mechanism of the SWE remains unclear [1, 2]. Microscopic models of SWE often invoke hydrogen in explaining the metastability of the light-induced defects [3-5]. This idea is further supported by the observation of light-induced hydrogen diffusion in a-Si:H [6, 7].

In addition, thermal

equilibrium defect generation has been observed above an equilibrium temperature Teq with an activation energy of about 0.3 eV [4, 8, 9]. Below Teq, which is around 2000 C in intrinsic aSi:H [9], the system is out of thermal equilibrium with a frozen-in defect concentration as a result of the slow kinetics below Teq attributed to hydrogen glass transition [10]. This idea is supported by observations which establish a close relationship between hydrogen diffusion and thermal equilibrium defect generation [10]. Hydrogen diffusion measures hydrogen displacement over a macroscopic length scale and the concept of a hydrogen glass describes the average behavior of hydrogen kinetics. However, proton NMR studies clearly show that hydrogen in device quality a-Si:H exists either as isolated Si-H in the dilute phase, which gives rise to a narrow peak of a few kHz, or as Si-H clusters which give rise to a broad peak of over 25 kHz [11, 12]. It is not yet clear what role such hydrogen microstructure plays in the metastability of a-Si:H although some models of SWE incorporate explicitly such hydrogen microstructure in the theory [4]. Obviously, it would be very informative for understanding the SWE to investigate hydrogen dynamics of both the dilute and clustered Si-H around Teq using NMR. In addition to the dilute and clustered phases, a few percent of the incorporated hydrogen in device quali