Journal of Non-Crystalline Solids 137&138 (1991) 255-258 North-Holland JOURNA L OF NON.CRYSTALLINE SOLIDS SPIN-DEPENDENT TRANSPORT AND RECOMBINATION IN a-SI:H KLAUS LIPS AND WALTHER FUHS Fachbereich Physik und Wissenschaftliches Zentrum ffir Materialwissenschaften, Philipps-Universitftt Marburg, Renthof 5, W-3550 Marburg, F.R. Germany We report on a study of spin--dependent photoconductivity in undoped a-Si:H. Applying the microwave modulation technique we observe in addition to the two already known e-db and h resonances an unusually broad quenching signal (g ~ 2.008, AHfwhm = 210 G). We varied the defect density Nd systematically by electron bombardment and light soaking and found a pronounced dependence of the signal intensity of this line on Nd. This behavior is consistent with an assignment to the recombination of exchange coupled e-h pairs which are localized in the band tails. We present first results obtained on pin structures which confirm the current ideas about carrier collection in these devices. 1. INTRODUCTION Studies of the spin---dependent photoconductivity (SDPC) have led to a widely accepted model for the recombination mechanism in undoped a-Si:H 1. Such experiments are carried out by recording the change of the photocurrent when the sample is brought into mi- crowave resonance. Allowing to identify recombination channels and the participating states, SDPC is a power- ful method to study the recombination kinetics. In the past various groups have used this method 1-6 and con- firmed that the dominating recombination center is the Si-dangling bond. Recently an additional signal has been observed in undoped a-Si:H which is about 200 G broad 4--6 and reminds of a similar resonance often observed in ODMR 7'8. In this report we concentrate on two aspects: the influence of the defect density Nd in particular on the broad resonance and the application of the method to pin structures. 2. EXPERIMENTS AND DISCUSSION Undoped a-Si:H films were prepared by the glow discharge technique at standard conditions which give films of high quality. Nd in these films was varied sys- tematically by prolonged illumination (1.92 eV, 3 W/cm2), by bombardment with electrons of high energy (2 MeV, 4 K) and successive anneals between 300 K and 470 K. Nd was measured by photocurrent spectroscopy (CPM). The samples were mounted in the cavity of an X-band spectrometer (Bruker ESP 300) and illuminated through the slots in the cavity walls. The change of the photocurrent was detected by lock-in technique. Two different modulation methods were used: (a) magnetic field modulation like in ESR giving the derivative of the microwave induced changes of Oph and (b) microwave power modulation which directly gives the change of ~rph. Method (a) allows a better evaluation of the g-values whereas (b) enables a better analysis of very broad resonances. Fig. 1 shows the SDPC spectra of undoped a-Si:H. The spectrum which was taken in-phase with the modu- lation at T = 160 K can be deconvoluted into two quenching signals Q1 and Q2 (Fig. la). The Qt line (g = 2.0050, AHpp = 9 G) has been assigned to tunnel- ing transitions of bandtall electrons into neutral dan- gling bonds (e--D0). The Q2 resonance (g = 2.01, AHpp = 20 G) has been attributed to the spin-depen- dent diffusion of holes in the valence-band tail which occurs before the final recombination step takes place by a transition to a doubly occupied dangling bond (h-D-). These two spin-dependent processes have al- ready been identified in the early work of Dersch et al. 1. Undoped a-Si:H exhibits a third unusually broad quenching signal, Q3, which can be detected when the magnetic field range is extended and the microwave 0022-3093/91/$03.50 © 1991 - Elsevier Science Publishers B.V. All rights reserved.