Abstract
matrices was examined. The spectroscopic analyses and formation mechanism show that the origin of strong light emission with nanosecond decay time is attributed to the effective surface termination and the new nanostructure of pc-Ge with diameter less than 4 nm which forms from reduction reaction of germanium oxides at low annealing temperature.
Introduction
Room temperature visible light emission from nanometer size structure of Si or Ge such as porous silicon (p-Si)t'z) or microcrystals3-5) has been reported. Recently, many researchers are interested in this visible light emission from nanostructure of indirect band gap semiconductors and propose some mechanisms such as a quantum dot or wire theory.
Introduction
We have observed visible photoluminescence (PL) at room temperature from Ge microcrystals (pc-Ge) of the average size of about 6 nm which was made by a radio freq-uency (rQ magnetron co-sputtering metlrod and annealing.a) The PL spectrum showed very broad and had the peak at 57O nm. [n our preliminary study, this visible light emission and large blue shift of the band gap were explained by simple three dimensional quantum confinement theory. However, recently we confirmed that the PL spectrum does not show a simple shift depending on the microcrystal size which can be estimated by three-dimensional confinement theory 6) ln this study, in order to make the mechanism of light emission from the pc-Ge system clear, we examine a formation mechanism and crystallographic properties of the pc-Ge system, PL spectrum, PL decay time, and the excitation spectrum as a function of the size. On the basis of these data, the mechanism of PL from pc-Ge system will be discussed.
Experimental
The samples were prepared onto a silicon wafer or a polished quartz glass by an rf-magnetron cosputtering method. The sputtering target used was 4n's pure SiO2 (100mm in diameter) with 6-9 chips of 6 n'sGe. The sputtering conditions were an Ar partial pressure of 0.26Pa and an applied rf-power of Figure 3 shows the HREM image of the pc-Ge formed in a SiO2 glassy matrix. It was observed that the morphology of pc-Ge changes remarkably, when the size is less than 4 nm. In almost pc-Ge of the size more than 6nm in Fig.3 (b) and new phase of Pc-Ge9'10). Figure 4 shows the PL spectrum obtained from the samples annealed at 300,600, 800oC for 30 min.
Light emission properties of 1tc-Ge system
These PL were.excited by 488-nm Ar ion laser. The . 11)"
Light emission properties of 1tc-Ge system
average sizes -6f pc-Ge in those samples were 4.2, 6.0 and 6.5 nm, respectively. It is observed that the PL peak energy showed weak dependence on the average size of the distribution. On the other hand, the PL intensity increased remarkably with decreasing the average size. Qe Pl.properties for four different average sizes are given in Table I . The PL intensity for the average size of 4.2 nm is about 10 times than that for the average size of 6 nm. In the case of the average size of 14.4 nm, the intensity was very small and comparable to that of the as-deposited sample. The PL intensity was found to be proportinal to the volume fraction Va of pc-Ge with the diameter less than 4 nm. This shows that prc-Ge with the diameter less than 4 nm contributes to the strong PL.
Light emission properties of 1tc-Ge system
The time constant of PL decay are also listed in Table I . The decay was found to show a single exponential process. The time constant was determined to be less than Lns, which was not related to the average size of the pc-Ge system. This ns PL decay implies that the indirect-gap semiconductor Ge begins to have a character of direct optical transition as the size decreases. In the excitation spectrum, it was observed that the peak energy of the excitation spectrum for the PL peak is near the PL peak energy and the stokes shift was small (about 0.08eV).
Light emission properties of 1tc-Ge system
The temperature dependence of PL intensity and spectrum of the average size of 4.2 nm was examined. The PL spectrum does not depend on the temperature, but the PL intensity slightly depends on the temperature. The Arrhenius plots of the intensity gives about 7 meV of the binding energy of excitons Lett. 216-649-1547. 2) V.I*hman and J.Goele: Appl. Phys. Lett. 216-649-1547) S.Furukawa and T.Miyasato: Phys. Rev. 838 (1988) 

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