In situ doping of silicon carbide semiconductor via epitaxy

Silicon carbide (SiC) is a wide bandgap semiconductor which can operate at high temperatures and resist chemicals and radiation, making it ideal for applications in a range of harsh environments [1]. Among over 200 polytypes of SiC, only cubic silicon carbide (3C-SiC) can be heteroepitaxially grown on Si. However, there are still no commercial 3C-SiC devices available due to its cost and issues with growth and leakage currents [2]. While leakage into the underlying Si can be managed by transferring the 3C-SiC layer to an insulating substrate, this process is difficult to scale and integrate into current technologies [3]

Temperature-dependent photoluminescence characteristics of GeSn epitaxial layers

GeSn epitaxial heterostructures are emerging as prominent candidates for the monolithic integration of light sources on Si substrates. Here we propose a judicious explanation for their temperature-dependent photoluminescence (PL) that is based upon the so far disregarded optical activity of dislocations. By working at the onset of plastic relaxation, which occurs whenever the epilayer releases the strain accumulated during growth on the lattice mismatched substrate, we demonstrate that dislocation nucleation can be explicitly seen in the PL data. Notably, our findings point out that a monotonous thermal PL quenching can be observed in coherent films, in spite of the indirect nature of the GeSn bandgap. Our investigation, therefore, contributes to a deeper understanding of the recombination dynamics in this intriguing group IV alloy and offers insights into crucial phenomena shaping the light emission efficiency

High mobility holes in a strained Ge quantum well grown on a thin and relaxed Si0.4Ge0.6/LT-Si0.4Ge0.6/Si(001) virtual

Epitaxial growth of a compressively strained Ge quantum well (QW) on an ultrathin, 345 nm thick, Si0.4Ge0.6/LT-Si0.4Ge0.6/Si(001) virtual substrate (VS) has been demonstrated. The VS, grown with a low temperature Si0.4Ge0.6 seed layer on a Si(001) substrate, is found to be fully relaxed and the Ge QW is fully strained. The temperature dependence of Hall mobility and carrier density clearly indicates a two-dimensional hole gas in the Ge QW. At room temperature, which is more relevant for electronic devices applications, the samples show a very high Hall mobility of 1235 cm2 V−1 s−1 at a carrier density of 2.36×1012 cm−2

Ballistic one-dimensional holes with strong g-factor anisotropy in germanium

We report experimental evidence of ballistic hole transport in one-dimensional quantum wires gate-defined in a strained SiGe/Ge/SiGe quantum well. At zero magnetic field, we observe conductance plateaus at integer multiples of 2e2/h. At finite magnetic field, the splitting of these plateaus by Zeeman effect reveals largely anisotropic g-factors with absolute values below 1 in the quantum-well plane, and exceeding 10 out-of-plane. This g-factor anisotropy is consistent with a heavy-hole character of the propagating valence-band states, which is in line with a predominant confinement in the growth direction. Remarkably, we observe quantized ballistic conductance in device channels up to 600 nm long. These findings mark an important step toward the realization of novel devices for applications in quantum spintronics

Magnetotransport, structural and optical characterization of p-type modulation doped heterostructures with high Ge content Si[subscript 1-x]Ge[subscript x] channel grown by SS-MBE on Si[subscript 1-y]Ge[subscript y]/Si(001) virtual substrates

This thesis is a report on experimental investigations of magnetotransport, structural and optical properties of p-type modulation doped (MOD) heterostructures with Sit-xGex channel of high Ge content (0.6<x<l) grown on Sit_yGey/Si(OOI) virtual substrate (VS). The active layers of MOD heterostructures were grown by solid source molecular beam epitaxy (SSMBE). The VSs were grown either by SS-MBE or low-pressure chemical vapour deposition (LP-CVD). The influence of thermal annealing on magnetotransport, structural and optical properties of Sit-xGexlSit-yGey heterostructures was studied by performing the post growth furnace thermal annealing (FTA) treatments in the temperature range of 600-900C for 30min and rapid thermal annealing (RTA) treatments at temperature 750C for 30sec. Structural and optical analysis of p-type MOD Sit-xGex!Si1-yGey heterostructures involved the techniques of cross-sectional transmission electron microscopy, ultra low energy secondary ion mass spectrometry, photoluminescence spectroscopy, micro-Raman spectroscopy and scanning white-light interferometry. From the combinations of experimental results obtained by these techniques the Ge composition in the SiGe heteroepilayers, their thicknesses, state of strain in the heteroepilayers and dislocations microstructure in VSs were obtained. After post growth thermal annealing treatments were observed broadening of the Si1-xGex channel accompanied with the reduction of Ge content in the channel and smearing of Sit-xGex/Sit_yGey interfaces. The Sio.7Geo.3 on low-temperature Si butTer VSs with very good structural properties were designed and grown by SS-MBE. These include: relatively thin 850nm total thickness of VS, 4-6nm Peak-to-Valley values of surface roughness, less than lOscm-2 threading dislocations density and more than 95% degree of relaxation in the top layers of VS. The Hall mobility and sheet carrier density of as-grown and annealed p-type MOD Sit-xGex/Sil-yGey heterostructures were obtained by a combination of resistivity and Hall etTect measurements in the temperature range of 9-300K. The FTA at 600C for 30min was seen to have a negligible etTect on the Hall mobility and sheet carrier density. Increasing the annealing temperature resulted in pronounced successive increases of Hall mobility accompanied by the opposite behaviour of sheet carrier density. Each sample had the optimum FTA temperature corresponded to the maximum Hall mobility. After RTA at 750C for 30sec the increase of Hall mobility for researched samples was observed as well. The highest mobility (at sheet carrier density) of 2DHG measured at 9K was observed for sample containing Ge channel grown on thick Sio.4Geo.6 linearly graded VS and corresponds to 14855cm2.y-I·s-l (2.87. 10 1 2cm-2). The highest Hall mobility (at sheet carrier density) measured at 293K was observed for Sio.2Geo.slSio.6sGeo.3s heterostructure after FT A at 750C for 30min and corresponds to 1776cm2.y-I·s-t (2.37·1013cm-2). To extract the drift mobility and sheet carrier density of 2DHG at temperatures up to 300K, magnetotransport measurements in magnetic fields up to II T were performed on several heterostructures. The data were analyzed by technique of Maximum-Entropy Mobility Spectrum Analysis. The highest drift mobility (at sheet carrier density) of2DHG at 290K was obtained for the Sio.2Geo.slSio.6sGeo.3s heterostructure after FTA at 750C and corresponds to 3607 cm2·V- I·s-1 (4.94.1012cm-2). Low temperature magnetotransport measurements down to 350 mK and at magnetic fields up to 11 T were carried out on several heterostructures. From the temperature dependence of the Shubnikov-de Haas oscillations observed at temperatures below 20K were extracted followed parameters of 2DHG, etTective mass, sheet carrier density, transport and quantum scattering times, and related parameters. For the Sio.osGeo.9s1Sio.37Geo.63 heterostructure was obtained the lowest hole etTective mass m*=O.l5·mo and the highest transport to quantum scattering times ratio a=2.18

Single step silicon carbide heteroepitaxy on a silicon wafer at reduced temperature

A single step growth approach for wafer-scale homogeneous cubic silicon carbide (3C-SiC) heteroepitaxy, using chemical vapour deposition (CVD), on a silicon (Si) substrate is demonstrated. Residual biaxial tensile strain causing a wafer bow is eliminated in the 3C-SiC epilayer via in-situ defects engineering and heteroepitaxy at reduced temperature. Thermal mismatch between the 3C-SiC epilayer and substrate is minimised by a substantial reduction of growth temperature, down to ∼1000 °C. Heteroepitaxy of high quality, fully relaxed 3C-SiC epilayers with minimal wafer bow is demonstrated, made possible by careful process optimisation. Unusually very high growth rate of 3C-SiC of > 10 µm/hr is achieved. At the same time the epilayer is free from any other silicon carbide (SiC) polytype inclusions. Moreover, the reduced growth temperature unlocks the ability to deposit high quality 3C-SiC epilayers within traditional Si-based cold walled CVD reactors, enabling the growth of such thin films on unprecedently high volumes and wafer diameters up to 300 mm and above

Complex quantum transport in a modulation doped strained Ge quantum well heterostructure with a high mobility 2D hole gas

The complex quantum transport of a strained Ge quantum well (QW) modulation doped heterostructure with two types of mobile carriers has been observed. The two dimensional hole gas (2DHG) in the Ge QW exhibits an exceptionally high mobility of 780 000 cm2/Vs at temperatures below 10 K. Through analysis of Shubnikov de-Haas oscillations in the magnetoresistance of this 2DHG below 2 K, the hole effective mass is found to be 0.065 m0. Anomalous conductance peaks are observed at higher fields which deviate from standard Shubnikov de-Haas and quantum Hall effect behaviour due to conduction via multiple carrier types. Despite this complex behaviour, analysis using a transport model with two conductive channels explains this behaviour and allows key physical parameters such as the carrier effective mass, transport, and quantum lifetimes and conductivity of the electrically active layers to be extracted. This finding is important for electronic device applications, since inclusion of highly doped interlayers which are electrically active, for enhancement of, for example, room temperature carrier mobility, does not prevent analysis of quantum transport in a QW

Bunches of misfit dislocations on the onset of relaxation of Si0.4_{0.4}Ge0.6_{0.6}/Si(001) epitaxial films revealed by high-resolution x-ray diffraction

The experimental x-ray diffraction patterns of a Si$_{0.4}$Ge$_{0.6}$/Si(001) epitaxial film with a low density of misfit dislocations are modeled by the Monte Carlo method. It is shown that an inhomogeneous distribution of 60$^\circ$ dislocations with dislocations arranged in bunches is needed to explain the experiment correctly. As a result of the dislocation bunching, the positions of the x-ray diffraction peaks do not correspond to the average dislocation density but reveal less than a half of the actual relaxation