Ge has made an impact on electronic applications. However, surface passivation is still a critical element in realizing high performance devices. Though a range of passivation techniques have proven adequate for electronics applications, optical applications can be more demanding. Here, we investigate thermal oxidation because it is a relatively simple, low temperature process that has been reported to yield interfacial trap densities in the range of 1E11 eV^-1cm^-2.
We report a simple surface passivation technique via dry thermal oxidation that is shown to improve the radiative recombination efficiency in Ge, as determined by relative photoluminescence (PL) intensity. For this study, undoped Ge wafers were cleaned by solvents and stripped of native oxide using a dilute HF-based solution to realize a bare Ge surface. Samples were then oxidized in a tube furnace at 500°C for one hour in an O2 ambient. To test this passivation technique on processed surfaces, a different set of samples was intentionally damaged by reactive ion etching (RIE) before oxidation. X-ray photoelectron spectroscopy (XPS) was used to confirm the Ge oxide. PL was performed on all samples. Oxidizing an as-received sample resulted in 50% higher peak emission. The improvement in PL suggests a strong reduction in nonradiative recombination. We infer that thermal oxidation produced a higher quality oxide interface, with fewer dangling bonds than native oxide interfaces formed in ambient air. RIE reduced PL intensity to about 40% of the as-received intensity, indicating significant surface damage and formation of recombination sites. These samples were then oxidized, which increased peak PL intensity by 4.5x, equal to or higher than bare oxidized wafers. Although we expected RIE samples to be permanently degraded, several etched wafers had even higher PL after oxidation than the corresponding bare, thermally oxidized wafers. We suspect this may be due to two effects, arising from surface roughening. Either transmission of the 534 nm pump beam into the sample was increased by a graded effective index, or facets of the roughened surface enabled multiple reflections, increasing total absorption. Finally, this low temperature thermal process is post-diffusion CMOS compatible, which satisfies a major prerequisite for materials integration on Si.
Thermal oxidation thus appears to be a successful technique to repair highly defective surfaces, observed from improvements in RIE-damaged Ge.