# Intrinsic recombination and interface characterization in "surface-free" GaAs structures

## Abstract

We have conducted a thorough photoluminescence (PL) and PL time-decay study of the interfacial passivating effects of metalorganic chemical vapor deposition prepared Al<inf>0.3</inf>Ga<inf>0.7</inf>As, n<sup>+</sup>-doped GaAs, and Na<inf>2</inf>S surface barriers on epitaxial, high-purity (n<sup>-</sup>) GaAs structures. We observe 300-K radiative lifetimes, in such 10-μm structures, of 2.5 μs, 800 ns, and 150 ns, respectively. These are to be compared with lifetimes of ∼2 ns for a single, "bare" GaAs surface with an Al<inf>0.3</inf>Ga<inf>0.7</inf> As rear surface barrier, and ∼0.5 ns for unpassivated epitaxial GaAs. Accompanying radiative efficiencies are 10<sup>3</sup>-10<sup>4</sup> higher in all of these epibased structures, and 10<sup>2</sup> higher for Na<inf>2</inf>S, than for corresponding bare GaAs surfaces. Further, from detailed PL lifetime studies versus GaAs thickness, we find the lowest interfacial recombination velocities reported for any GaAs/Al<inf>x</inf>Ga<inf>1-x</inf>As structure, to date, of ≲40 cm/s, and, correspondingly, 0-1800 cm/s for n<sup>+</sup>/n<sup>-</sup>/n<sup>+</sup> all-GaAs homostructures. Thus, virtually "surface-free" structures are now achievable. In comparison, we find, at best, ∼5500 cm/s for Na<inf>2</inf>S, and typically 34 000 cm/s for bare GaAs surfaces. We conclude, on the basis of our detailed experimental study of a wide variety of samples, that these values provide truly reliable measures of surface recombination velocities for both surface types. We find an unambiguous determination of surface recombination velocities requires detailed examination of the minority-carrier recombination kinetics versus temperature, together with minority-carrier spatial transport properties. After demonstrating that minority-carrier recombination kinetics in our ideal structures are truly "intrinsic," and thus wholly unaffected by extrinsic processes, we examine the temperature dependence of band-to-band and free-exciton recombination. We fully explain all intrinsic, free-carrier recombination found in each structure, for temperatures of 40-300 K through rate equations appropriate to each structure. Similar low-temperature (1.8-40 K) studies confirm dominant decay proceeds, here, by intrinsic free excitons.