G. Wohl

Ge-on-Si vertical incidence photodiodes with 39-GHz bandwidth

Vertical-incidence Germanium photodiodes grown on thin strain-relaxed buffers on Silicon substrates are reported. For a mesa-type detector with a diameter of 10 /spl mu/m, a resistance-capacitance-limited 3-dB bandwidth of 25.1 GHz at an incident wavelength of 1552 nm and zero external bias has been measured. At a reverse bias of 2 V, the bandwidth is 38.9 GHz. The detector comprises a 300-nm-thick intrinsic region, and thus, has the potential for easy integration with Si circuitry and exhibits zero bias external quantum efficiencies of 23%, 16%, and 2.8% at 850, 1298, and 1552 nm, respectively.

Characterization of self-assembled Ge islands on Si(100) by atomic force microscopy and transmission electron microscopy

Abstract We present an alternative starting point of the fabrication of nanostructures for electronic devices by using self-assembling structures. One way for the growth of self-assembling structures as quantum dot (QD) arrays is based on the formation of coherently strained Ge islands on Si and requires controlling of a defined island growth (Stranski–Krastanov). For this reason we carried out systematic quantitative investigations of the growth of Ge islands. Stacks of two layers of Ge islands with a Si spacer were grown on a Si buffer and characterized by atomic force microscopy (AFM) and transmission electron microscopy (TEM). In the first series the Ge layer thickness was varied at a constant growth temperature and in the second series the growth temperature was varied for a constant Ge layer thickness. Many of the results described in this paper confirm the expected growth behavior of Ge islands, i.e. the Stranski–Krastanov growth mode and the increasing island density with decreasing growth temperature. However, two new aspects of the island growth were found. Small Ge islands are already formed from 2.15 monolayers (ML) of Ge at high growth temperatures. At lower growth temperatures (≤645°C, 6.2 ML coverage), few large islands with defects and a high density of small coherent islands are observed simultaneously. TEM studies of cross-sectional and plan-view samples reveal that the small islands are elongated along (100) directions. Reasons for the formation of the two kinds of islands are discussed.

Relaxed Si0.7Ge0.3 buffer layers grown on patterned silicon substrates for SiGe n-channel HMOSFETs

Abstract For a successful fabrication of hetero-MOSFETs (HMOSFETs) the compatibility of the epitaxial growth of the heterostructures with the standard CMOS process is an important factor. For this reason, we demonstrate for the first time the growth of relaxed SiGe buffer layers with a Ge content up to 30% by differential molecular beam epitaxy (MBE) after the definition of the active device areas of n-channel HMOSFETs by lateral oxide isolation. The influence of lateral dimensions and the composition of the epitaxial layer stacks on the surface topography, relaxation and defect density has been investigated. Scanning electron microscopy (SEM) and differential interference contrast (DIC) micrographs were applied to characterize the quality of the differential epitaxy and the surface topography. The degree of relaxation in the patterned areas was determined using micro-Raman spectroscopy. The analysis of the dislocations was carried out by transmission electron microscopy (TEM). The reduction of the surface cross-hatch with decreasing lateral dimensions was clearly seen. Further, we found, that the increase of the relaxation corresponds to a higher density of misfit dislocations confined at the interface SiGe buffer/Si buffer. To our surprise, we also observed a strong influence of the pure Si channel (10 nm tensile-strained Si layer) on the SiGe buffer layer. This indicates that the investigation of the quality of the relaxed SiGe buffer layers should be only done with the whole epitaxial layer stack of the device structure.

Ge-on-Si pin-photodiodes for vertical and in-plane detection of 1300 to 1580 nm light

Ge-on-Si pin-photodiodes for vertical and in-plane detection are presented. The devices are grown on an 31 nm ultrathin, strain relaxed buffer (SRB) layer. The vertical photodiode exhibits a zero-bias responsivity of 117 mA/W and a bandwidth of 1.5 GHz at a wavelength of 1298 nm. In comparison, the detector for in-plane detection has a zero-bias responsivity of 70 mA/W and a bandwidth of 4.4 GHz. For a bias voltage of -2 V a bandwidth of 6.2 GHz has been measured. At 1580 nm the photo responsivity of the lateral photodiode is higher (26 mA/W) than of the vertical photodiode (19 mA/W). The dislocation density probably may limit the bandwidth.

Fully relaxed Si0.7Ge0.3 buffers grown on patterned silicon substrates for hetero-CMOS transistors

For a successful fabrication of hetero-CMOS transistors the compatibility of the epitaxial growth of the heterostructures with the standard CMOS process is an important factor. A very promising integration approach utilizes the realization of heterostructures in a limited area, so-called differential epitaxy. In this paper, we report on new results of fully strain-relaxed SiGe buffers (SRB) with a thickness of about 750 nm, i.e. on the order of the thickness of the field oxide in the standard CMOS technology. The SiGe SRB layers completed by active device layer stacks were grown by several growth concepts, linearly and step-graded buffers and in comparison a method using a low-temperature epitaxial Si (LTE-Si) starting layer. The influence of lateral dimensions and of the different growth concepts on the surface morphology, relaxation and defect density has been investigated. Differential interference contrast (DIC) micrographs and atomic force measurements (AFM) were applied to characterize the surface topography. The relaxation measurements in the patterned areas were performed using micro-Raman spectroscopy. The analysis of the dislocations was carried out by transmission electron microscopy (TEM). Two main results should be emphasized: Full relaxation (also in the small window) was only achieved for the SiGe buffer with 50-nm LTE-Si and TEM images show different dislocation structures for a graded buffer and the buffer with 50-nm LTE-Si.

SiGe pin-photodetectors integrated on silicon substrates for optical fiber links

100% Ge pin-photodetectors grown on SiGe strain relaxed buffer (SRB) layers are presented, For integrated detectors the SRB layer growth as well as the subsequent SiGe photodiode technology processing must be compatible with standard CMOS technology. DC photoresponsivities of 145 mA/W at 1.3 /spl mu/m and 25 mA/W at 1.55 /spl mu/m can be achieved. In first experiments the 100% Ge pin-photodetector exhibits an RC limited 3-dB opto-electrical bandwidth of 0.9 GHz.

40-Gbit/s Ge-Si photodiodes

Ge-pin-photodiodes monolithically integrated on strain-relaxed buffers are presented. For easy on-wafer testing mesa-type photodiodes are designed for vertical light incidence. At the telecommunication wavelengths of 850 nm, 1298 nm and 1550 nm zero-bias external quantum efficiencies of 23%, 16% and 2.8% are measured. An optoelectronic 3 dB-bandwidth, limited by the RC-product, has been measured up to 38.9 GHz