J. Werner

Germanium-tin p-i-n photodetectors integrated on silicon grown by molecular beam epitaxy

GeSn heterojunction p-i-n diodes with a Sn content of 0.5% are grown with a special low temperature molecular beam epitaxy. The Sn incorporation in Ge is facilitated by a very low temperature growth step in order to suppress Sn surface segregation. Diodes with sharp doping transitions are realized as double mesa structures with a diameter from 1.5 up to 80 μm. An optical responsivity of these GeSn diodes of 0.1 A/W at a wavelength of λ=1.55 μm is measured. In comparison with a pure Ge detector the optical responsivity is increased by factor of 3 as a result of Sn caused band gap reduction.

High bandwidth Ge p-i-n photodetector integrated on Si

The authors present a germanium on silicon p-i-n photodiode for vertical light incidence. For a Ge p-i-n photodetector with a radius of 5μm a 3dB bandwidth of 25GHz is measured at an incident wavelength of 1.55μm and zero external bias. For a modest reverse bias of 2V, the 3dB bandwidth increases to 39GHz. The monolithically integrated devices are grown on Si with solid source molecular beam epitaxy. The complete detector structure consisting of a highly p-doped Ge buried layer, an intrinsic absorption region, and a highly n-doped top contact layer of Ge∕Si is grown in one continuous epitaxial run. A low growth temperature sequence was needed to obtain abrupt doping transitions between the highly doped regions surrounding the intrinsic layer. A theoretical consideration of the 3dB bandwidth of the Ge detector was used to optimize the layer structure. For a photodiode with 5μm mesa radius the maximum theoretical 3dB frequency is 62GHz with an intrinsic region thickness of 307nm.

Texture of Cu(In, Ga)Se2 thin films and nanoscale cathodoluminescence

We investigate the microstructure of Cu(In,Ga)Se2 thin films with different preferred grain orientations (textures). The films are grown by coevaporation from elemental sources. We analyse the electro-optical properties of the films with a cathodoluminescence spectrometer attached to a transmission electron microscope. {112} textured films show sharp contrasts at the grain boundaries, whereas grain boundaries in {220/204} textured films give only very weak contrasts indicating a preferential population of electronically rather inactive grain boundaries.

Laser synthesis of germanium tin alloys on virtual germanium

Synthesis of heteroepitaxial germanium tin (GeSn) alloys using excimer laser processing of a thin 4 nm Sn layer on Ge has been demonstrated and studied. Laser induced rapid heating, subsequent melting, and re-solidification processes at extremely high cooling rates have been experimentally achieved and also simulated numerically to optimize the processing parameters. “In situ” measured sample reflectivity with nanosecond time resolution was used as feedback for the simulations and directly correlated to alloy composition. Detailed characterization of the GeSn alloys after the optimization of the processing conditions indicated substitutional Sn concentration of up to 1% in the Ge matrix.

Spatial inhomogeneities in Cu(In,Ga)Se2 solar cells analyzed by an electron beam induced voltage technique

Spatial variations of the local open circuit voltage in Cu(In,Ga)Se2 solar cells are analyzed by an electron beam induced voltage (EBIV) technique. The major pattern visualized by our EBIV measurements are spatial inhomogeneities on a length scale of between 5 and 20μm. Quantitative evaluation of the EBIV signals shows that the loss of open circuit voltage due to the inhomogeneities is about 100mV. Additional analysis of our samples by energy dispersive x-ray analysis excludes fluctuations of the Ga or Cu content as the source of the inhomogeneities. Instead, the spatial inhomogeneous supply of Na from the glass substrate turns out as a possible origin of inhomogeneities. Spatially resolved secondary ion mass spectroscopy measurements show that the Na content of our Cu(In,Ga)Se2 samples varies between 0.03 and 0.15at.% on a length scale of tens of micrometers.

Room Temperature Direct Band Gap Emission from Ge p-i-n Heterojunction Photodiodes

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.

Ge quantum dot tunneling diode with room temperature negative differential resistance

We present current density-voltage characteristics of Ge quantum dot p+-i-n+ tunneling diodes. The diode structure with Ge quantum dots embedded in the intrinsic region was grown by low temperature molecular beam epitaxy without any postgrowth annealing steps. The quantum dot diodes were fabricated using a low thermal budget fabrication process which preserves the Ge quantum structure. A negative differential resistance at room temperature of a Ge quantum dot tunneling diode was observed. A maximum peak to valley ratio of 1.6 at room temperature was achieved.

Potential variations around grain boundaries in impurity-doped BaSi2 epitaxial films evaluated by Kelvin probe force microscopy

Potential variations around the grain boundaries (GBs) in antimony (Sb)-doped n-type and boron (B)-doped p-type BaSi2 epitaxial films on Si(111) were evaluated by Kelvin probe force microscopy. Sb-doped n-BaSi2 films exhibited positively charged GBs with a downward band bending at the GBs. The average barrier height for holes was approximately 10 meV for an electron concentration n ≈ 1017 cm−3. This downward band bending changed to upward band bending when n was increased to n = 1.8 × 1018 cm−3. In the B-doped p-BaSi2 films, the upward band bending was observed for a hole concentration p ≈ 1018 cm−3. The average barrier height for electrons decreased from approximately 25 to 15 meV when p was increased from p = 2.7 × 1018 to p = 4.0 × 1018 cm−3. These results are explained under the assumption that the position of the Fermi level Ef at GBs depends on the degree of occupancy of defect states at the GBs, while Ef approached the bottom of the conduction band or the top of the valence band in the BaSi2 grain ...

Strain relaxation of metastable SiGe/Si: Investigation with two complementary X-ray techniques

Metastable and strain relaxed SiGe layers with about 20% Ge content have been grown by molecular beam epitaxy on Si substrates at 550 °C. The thickness regime of metastability and the onset of strain relaxation were investigated on dust particle free surfaces obtained by careful chemical cleaning and epitaxy loading under clean room conditions. Compared to earlier results true metastable regime without misfit dislocations was obtained up to 140 nm thickness. The onset of strain relaxation started with heterogeneous nucleation sites of misfit dislocations. X-ray topography proved to be a unique monitoring tool to observe a low density of single dislocations. From these results we suggested to define a critical thickness band with lower bound tcl from dislocation nucleation to an upper bound tco (600 nm in our case) defined by the onset of considerable strain relaxation. The strain relief was measured by X-ray diffraction (reciprocal space mapping) and found to be very abrupt (76% strain relaxation at 800 n...

Si Esaki diodes with high peak to valley current ratios

We report room temperature current voltage characteristics of Si p+-i-n+ Esaki diodes integrated on silicon substrates. The diodes were fabricated by low-temperature molecular beam epitaxy. Very high and abrupt p- and n-type dopant transitions into the 1020 cm−3 ranges are achieved by boron and antimony, respectively. The integrated devices are realized without a postgrowth annealing step. The silicon Esaki diodes show negative differential resistance at room temperature with excellent peak to valley current ratios up to 3.94. A variation in the thickness of the silicon tunneling barrier changes the peak current density over three orders of magnitude.