Room temperature direct band gap emission is observed for Si-substrate-based Ge p-i-n heterojunction photodiode structures operated under forward bias. Comparisons of electroluminescence with photoluminescence spectra allow separating emission from intrinsic Ge (0.8？eV) and highly doped Ge (0.73？eV). Electroluminescence stems from carrier injection into the intrinsic layer, whereas photoluminescence originates from the highly n-doped top layer because the exciting visible laser wavelength is strongly absorbed in Ge. High doping levels led to an apparent band gap narrowing from carrier-impurity interaction. The emission shifts to higher wavelengths with increasing current level which is explained by device heating. The heterostructure layer sequence and the light emitting device are similar to earlier presented photodetectors. This is an important aspect for monolithic integration of silicon microelectronics and silicon photonics. 1. Introduction Progress in Si-based photonics from Ge/Si heterostructures attracts worldwide attention [1]. Photonics and optoelectronics play an essential role in many areas of applications [2] as in telecommunication, information technology, and optical interconnect systems. Integration of Si-based microelectronics and optoelectronic devices would be greatly enhanced if similar facilities and technologies can be used. One approach is the development of optoelectronic components based on Si compatible materials. Waveguiding in silicon-on-insulator (SOI) structures, high speed detecting by Ge/Si heterostructure devices [3, 4], and signal modulation by interferometric principles were demonstrated in the last decades. In the last years, small area absorption modulators based on electric field modification of material properties (quantum-confined Stark effect, Franz-Keldysh effect) were developed [5]. However, the realization of light emitters with a Si base is still challenging. Due to the fact that it is an indirect semiconductor with a relatively large direct band gap (3.2？eV), Si itself is not suited for the development of light emitters. An alternative and compatible material is Ge. Ge is also an indirect semiconductor but the direct band gap of 0.8？eV is only slightly larger than the indirect one ( ？meV). Furthermore, 0.8？eV corresponds to the required communication wavelength of 1,550？nm [6]. The small energy difference between the direct and the indirect band gap should open the possibility for light emitting devices with reasonable quantum efficiencies. The direct gap photoluminescence of Ge at a temperature of 2？K was
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