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

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.