There is a significant lack of knowledge about the fundamental electronic properties of InP. As part of a continuing effort to characterize InP, we have employed transient photoluminescence to measure the minority carrier lifetime on n-type and p-type InP wafers. Our measurements show that unprocessed InP wafers have very high minority carrier lifetimes. Lifetimes of 200 nS and 700 nS were observed for lightly-doped p-and n-type material respectively. Lifetimes over 5 nS were found in heavily doped ntype material.
INTRODUCTlONThere is considerable debate about fundamental electronic parameters of InP such as intrinsic carrier concentration, minority carrier lifetime, and heavy doping effects. Good measurements of fundamental material parameters are required to validate theoretical models and reconcile the theory with the results of solar cell performance.InP solar cells have demonstrated efficiencies as high as 19.2% A M O . However, InP n/p solar cells still have substantially lower efficiencies than theoretical models suggest should be achievable.'eV The major problems limiting efficiency are poor material properties. N-type emitters have a surface recombination velocity (SRV) as high as 107cm/sec, and the minority carrier lifetime is nearly an order of magnitude lower than the radiative limit. We have measured the minority carrier lifetime as a function of doping on n-type and p-type InP wafers. The general conclusion reached is the minority carrier lifetimes required for an optimum InP solar cell are observed on the unprocessed InP wafers. THEORY Transient photoluminescence (PL) has been used extensively to determine the minority carrier lifetime in n-and p-type 111-V semiconductors. Our measurements were made on InP wafers used asreceived, sometimes after a chemical thinning step, but with no additional processing. In most cases the wafer thickness (T) satisfied the condition that T >>Ln,, (the minority carrier diffusion length). This allows us to use the semi-infinite slab approximation in this study.Theoretical expressions for the decay of photoluminescence intensity are found in the literaturerss and will not be derived here. Equation 1 (Ref 5) is the theoretical relationship of photoluminescent intensity ( I& vs. time, assuming a high front-surface recombination velocity (SRV), weak self-absorption and a semi-infinite thick sample:Here D is the minority-carrier diffusivity, a is the absorption coefficient of the incident laser light which we took to be between 1-0.5~104 cm-1 for the 904 nm laser7, C is a constant related to collection efficiency, and z is the observed lifetime. The equation shows an exponential decay with time constant z, modulated by the expression in parentheses, which is related to the recombination at the surface. the luminescence at the long wavelength end of the photoluminescence spectrum.Self absorption effects are minimized by measuring
EXPERIMENTAL METHODThe apparatus used to record PL vs.t is unique. It was assembled at considerably lower cost than conventional PL systems and i...
