INJECTION LUMINESCENCE IN AMORPHOUS SILICON p+-i-n+ JUNCTIONS

We present a detailed study of electroluminescence (EL) spectra and + + EL quantum efficiency in well characterised a-Si p i a junctions under forward bias. The PL characteristics of the i region were probed using laser excitation. Some factors controlling EL efficiency are discussed. EL and PL recombination models are compared and discussed. Introduction EL in amorphous silicon has received little attention apart from brief reports by Pankove and Carlson (1) and Street et a1 (2). These authors observed a broad emission peak near 1.3eV in p-i-n and Schottky junctions. In this paper we present a detailed study of EL spectra and quantum efficiency in a-Si junctions deposited on stainless steel substrates. The PL behaviour of the i region was also probed using a laser beam. Experimental All the junctions were prepared at Dundee by the glow discharge method and their electrical characteristics have been fully described elgekkere (3). A typical device configuration is shown in Fig 1. A boron doped p layer about 150nm thick (diborane-silane ratio 2 0.01) was grown onto polished stainless steel, followed by 5 700nm of undoped material, and a phosphorus doped n+ layer (% 50nm 2 thick, phosphine-silane ratio 2 0.003). A semitransparent Au electrode (% O.lcm ) was evaporated onto the n+ layer to provide electrical contact. Junction thicknesses were also measured in some cases by hydrogen profiling (4). EL spectra were measured under forward bias, and PL spectra under zero bias, using a cooled Ge detector. Results EL emission was measured for forward dark currents in the range 50-1000pA and forward voltages V between 6 and 12V, at temperatures between 100 and 200K. In this temperature an3 voltage range, the forward current (iinj) varies as V : where p varies from 6 to 8 as T decreases. The high power law implies space charge and contact effects. i. . is much larger than the thermal equilibrium current and is due to carriers injePfJd from both contacts (9). Figure 2 shows a typical EL spectrum from a junction at 140K. The EL emission peak is weakly structured and is centred around 0.9eV. In contrast the PL spectrum is strongly modulated by interference fringes and the centroid is dispfaced % 0.2eV to higher energies. PL excitation was carried out through the top (n ) contact using Argon laser light at 2.4eV. The EL emission shows a small spectral shift to lower energies between 125 and 200K but no observable change in linewidth. Figure 3 shows a log-log plot of the total EL intensity as a function of i. . ; the intensity varies as iV . where v varies from 1.1 to 1.4 as T increases (~ik"~3, in inset). Fig 4 comparJs the T dependence of nEL and qpL , using 2.4eV laser excitation for the latter. Discussion -$t the doping levels used in the n+ and p+ regions of these junctions 'lpL is % 10 strongly suggesting the EL occurs in the i region. In spite of the complex dependence of i on Vf, the EL intensity varies almost linearly with i (Fig 3) implying a cons&zat EL efficiency. in j Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1981498