Space-charge-limited currents: Refinements in analysis and applications to a-Si1−xGex:H alloys

Abstract

A new algorithm is described for deriving the density of states N(E) from the Fermi energy EF upwards toward the conduction band edge. This refinement in the analysis of space-charge-limited currents (SCLC) enables the accurate determination of N(E) by implicitly accounting for the spatial variations of physical quantities across the thickness of the diode. SCLC is measured in NiCr/n+/a-Si1−xGex:H/Pt diode structures. For a-Si:H samples, SCLC values for N(EF) are compared to those derived from admittance measurements on the same diodes. The two determinations agree in samples where 1016<N(EF) <1018 eV−1 cm−3. Arguments are presented that densities of states between 3×1014 and 1016 eV−1 cm−3 found by SCLC methods are more accurate than higher densities found from admittance measurements. Structure in N(E) inferred from a number of investigations is discussed. SCLC in sputtered a-Si0.7Ge0.3:H is also investigated, as a function of hydrogen content cH, optical gap, and photoluminescence intensity IPL. In this alloy increasing cH causes N(EF) to decrease, to a minimum of 3×1016 for cH=14 at. %. IPL increases inversely with N(EF), confirming the sensitivity of SCLC to bulk nonradiative recombination centers. It is concluded that the SCLC measurement and analysis constitute a relatively simple, straightforward, and generally applicable method of obtaining the density of states in the gap of amorphous semiconductors.