Fabrication and characterization of ultra-fast Si-based detectors for near-infrared wavelengths

This thesis presents two different concepts for the fabrication of ultrafast metal-semiconductor-metal (MSM) photodetectors, which are to be used in the near-infrared wavelength regime and which are compatible to silicon processing techniques. To achieve this goal, we have grown Si-Si$_{1-x}$Ge$_{x}$ undulating layer superlattices with x=0.39 and 0.45 by molecular beam epitaxy (MBE) an top of epitaxial implanted COSi$_{2}$ layers and fabricated "vertical" MSM detectors. The devices show a quantum efficiency of 5% for the wavelength of 1320 nm and 0.9 % for 1550 nm. We performed time response measurements, using a Ti:sapphire laser and an optical parametric oscillator which generates ultrafast pulses at infrared wavelengths. An electrical response time of 11.6 ps füll width at half maximum (FWHM) was obtained at a wavelength of 1300 nm. At 1550 nm a response time of 9.4 ps was measured. In a second approach, we have grown pure Ge by MBE an Si(111). The sensitive volumes are 270 nm thick Ge films. Interdigitated Cr metal top electrodes of 1.5 - 3 $\mu$m spacing and identical finger width form Schottky contacts to the Ge film. These detectors show a response time of 12.5 ps füll width at half maximum both at 1300 nm and 1550 nm. The temporal response is limited by the transit time of the carriers between the electrodes

Optoelectronic properties of high-Si-content-Ge 1 −x – y Si x Sn y /Ge 1− x Sn x /Ge 1− x–y Si x Sn y double heterostructure

The optoelectronic properties of Ge1−x−ySixSny/Ge1−xSnx/Ge1−x−ySixSny doubleheterostructures pseudomorphically grown on a Ge substrate were investigated. Thephotoluminescence (PL) intensity of the sample with Ge0.66Si0.23Sn0.11 cladding layers is threetimes larger compared to PL from structure with a Ge cladding layer, which can be attributed tohigher energy band offsets at both conduction and valence band edges at theGe0.91Sn0.09/Ge0.66Si0.23Sn0.11 interface. The PL spectrum of the sample with theGe0.66Si0.23Sn0.11 cladding layer at room temperature can be deconvoluted into four components,and the origins of these components can be assigned to direct and indirect transitions bymeasuring the temperature dependence of each component’s intensity. In addition, we examinedthe formation and characterization of strain-relaxed Ge1−x−ySixSny/Ge1−xSnx/Ge1−x−ySixSnydouble heterostructures to relieve the compressive strain in the Ge1−xSnx layer. Stacking faultswere observed in the Ge1−xSnx and Ge1−x−ySixSny layers. The PL peak intensity of the strainrelaxedGe1−xSnx layer decreases by a factor of 1/20 compared to the PL peak intensity of thedouble heterostructure pseudomorphically grown on a Ge(001) substrate. In addition, PLintensity can be increased by post-deposition annealing owing to decreasing defects

Structural Property Study for GeSn Thin Films

The structural properties of GeSn thin films with different Sn concentrations and thicknesses grown on Ge (001) by molecular beam epitaxy (MBE) and on Ge-buffered Si (001) wafers by chemical vapor deposition (CVD) were analyzed through high resolution X-ray diffraction and cross-sectional transmission electron microscopy. Two-dimensional reciprocal space maps around the asymmetric (224) reflection were collected by X-ray diffraction for both the whole structures and the GeSn epilayers. The broadenings of the features of the GeSn epilayers with different relaxations in the ω direction, along the ω-2θ direction and parallel to the surface were investigated. The dislocations were identified by transmission electron microscopy. Threading dislocations were found in MBE grown GeSn layers, but not in the CVD grown ones. The point defects and dislocations were two possible reasons for the poor optical properties in the GeSn alloys grown by MB

Epitaxial Growth of Ge1-xSnx by REduced Pressure CVD Using SnC14 and Ge2H6

The epitaxial growth of Ge and GeSn on Si(100) is studied in the low temperature regime by reduced pressure chemical vapor deposition using showerhead technology. Most emphasis is placed on the growth kinetics in the low temperature regime of 375oC to 500oC which is characterized by surface limited reactions. Using Ge2H6 precursor low activation energy of 0.7 eV was determined for H2 carrier gas which further decreases to 0.5 eV if N2 is used. GeSn layers with 10 and 6.5 at.% Sn are obtained at growth temperatures of 375oC and 400oC respectively, using Ge2H6 and SnCl4 as precursors. All Ge and GeSn layers are of high crystalline quality.</jats:p

Vertical GeSn/Ge Heterostructure Gate-All-Around Nanowire p-MOSFETs

A process for the fabrication of vertical gate-all-around (GAA) nanowire p-FETs with diameters of down to 20 nm based on Ge and GeSn/Ge-heterostructures is presented. The resulting Ge-based devices exhibit a low subthreshold slope (SS) of 66 mV/dec, a low drain-induced barrier lowering of 35 mV/V and an Ion/Ioff-ratio of 2.1×10^6 for devices with a diameter of 20 nm. Using a GeSn/Ge-heterostructure with GeSn as the top layer and source of the device, the on-current was increased by ~32%. With these results the high potential of incorporation of GeSn into Ge-MOSFET technology is demonstrated