Structural and Electrical Characterization of Si-Modfet Structures Grown at High Rates by Lepecvd

Compressively strained Ge channels on relaxed SiGe buffer layers

Low-energy plasma-enhanced chemical vapor deposition for strained Si and Ge heterostructures and devices

Low energy plasma enhanced chemical vapour deposition (LEPECVD) is a deposition technique developed for the epitaxy of Si and SiGe at ultra-high deposition rates. Due to a high current plasma discharge composed of low energy particles, a high plasma enhancement can be obtained without any accompanying plasma induced damage of the wafer surface. The most important application of LEPECVD so far is for compositionally graded relaxed SiGe buffer layers. Such relaxed buffer layers are demonstrated with end composition up to pure Ge and with a growth time below 1 hour. A p-type hetero-MOSFET formed in a SiGe channel compressively strained to a Si0.5Ge0.5 relaxed buffer layer, is demonstrated as one example where the high growth rates of LEPECVD allows the synthesis of devices which cannot be produced with an acceptable throughput with conventional deposition methods. The room temperature effective hole mobility of 760 cm2/Vs obtained on such devices demonstrates a high structural and electrical quality of the LEPECVD material.

Band-edge photoluminescence of SiGe/strained-Si/SiGe type-II quantum wells on Si(100)

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

Resonant tunneling of variously strained Si/Ge xSi 1−x/Si heterostructures

A correlation between bulk leakage current density and threading dislocation density in silicon–germanium mesa-isolated diodes fabricated on relaxed graded buffer layers is presented. Si0.75Ge0.25 p-i-n diodes were grown on SiGe graded buffers with different grading rates. Graded buffers with different grading rates yielded “virtual substrates” with varying densities of threading dislocations. Bulk leakage current densities were differentiated from surface leakage currents by using p-i-n diodes with different areas. We demonstrate that the increase in bulk leakage current density in SiGe p-i-n diodes can be modeled by generation processes assisted by deep levels related to threading dislocations.

Magnetron sputter epitaxy of [formula omitted] heterostructures

In this work, we report a hole mobility of one million in germanium. This extremely high value of 1.1 x 106 cm2V-1s-1 at a carrier sheet density of 3.0 x 1011 cm-2 was observed in a strained Ge quantum well structure grown by reduced-pressure chemical vapor deposition (RP-CVD) and is nearly an order of magnitude higher than previously reported values [1]. Compressively strained Ge offers considerable promise as an alternative channel conduction material to Si because of its previously reported high hole mobility of 120,000 cm2V-1s-1 (at 2 K) [1] in two-dimensional hole gases (2DHG) in modulation-doped SiGe heterostructures [2-4]. Previous studies have used growth techniques such as molecular beam epitaxy (MBE) and low energy plasma enhanced chemical vapor deposition (LEPE-CVD), or a combination of these techniques to take advantage of their low growth rates for accurate control of the epitaxial layer properties. However, recent developments in low temperature epitaxy using reduced-pressure CVD (RP-CVD) now makes it possible using an industrially compatible process to control the thickness of strained epitaxial layers to within a few monolayers [5] and achieve very high levels of purity. The heterostructure studied in this work is shown in Fig. 1. All layers were grown by reduced pressure CVD (RP-CVD) using an ASM Epsilon 2000 reactor and employed a reverse-linearly graded Si0.2Ge0.8 relaxed buffer with low surface roughness (rms value ~ 2 nm, crosshatch spatial wavelength ~ 2 m) and threading dislocation density (≤ 4 x 106 cm-2) [6-7]. The active region included a 20 nm biaxial compressively strained Ge QW, that was grown at low temperature to prevent any relaxation [8]. High resolution x-ray diffraction (HR-XRD) confirmed that the Si0.2Ge0.8 buffer was slightly over-relaxed with respect to the Si substrate and that the Ge channel was fully strained with respect to the buffer. The B-doping density was determined as ~ 1 x 1018 cm-3 using ultra low energy secondary ion mass spectrometry (uleSIMS). Hall bars were fabricated using standard lithography and wet chemical etching while Al contacts were deposited by thermal evaporation and annealed at 425 °C for 20 mins in N2 to ensure ohmic behavior. Resistivity and Hall effect measurements were performed in the temperature range 12 K - 300 K with Fig. 2 showing the Hall mobility and carrier sheet density as a function of temperature. The room temperature Hall mobility was 1200 cm2V-1s-1 and is believed to be dominated by parallel conduction through the MOD heterostructure [2]. However, at 12 K the measured Hall mobility and carrier sheet density were 1.0 x 106 cm2V-1s-1 and 3.0 x 1011 cm-2, respectively and represents the highest hole mobility observed for a 2DHG in strained Ge. Fig. 2 shows that whilst the carrier sheet density saturates, the Hall mobility continues to increase for temperatures below 60 K. Additional experiments at 0.4 K found that the Hall mobility saturated at a value of ~ 1.1 x 106 cm2V-1s-1. At low temperatures, we expect the mobility to be limited by ionized impurity scattering [1]. Our results suggest that the RP-CVD growth process has resulted in both a very low impurity and extremely pure Ge layer, so that even at 12 K ionized impurity scattering is very weak [9] and demonstrates that RP-CVD can be used to grow strained Ge QWs with exceptionally high hole mobilities.

The results of studying the growth of self-assembled Ge(Si) islands on relaxed Si1−xGex/Si(001) buffer layers (x≈25%), with a low surface roughness are reported. It is shown that the growth of self-assembled islands on the buffer SiGe layers is qualitatively similar to the growth of islands on the Si (001) surface. It is found that a variation in the surface morphology (the transition from dome-to hut-shaped islands) in the case of island growth on the relaxed SiGe buffer layers occurs at a higher temperature than for the Ge(Si)/Si(001) islands. This effect can be caused by both a lesser mismatch between the crystal lattices of an island and the buffer layer and a somewhat higher surface density of islands, when they are grown on an SiGe buffer layer.

Based on calculations within the density functional theory and an analysis of low-temperature photoluminescence spectra, the structure of electron–hole liquid in shallow Si/Si_1 – _ x Ge_ x Si (100) quantum wells 5 nm wide with germanium content x = 3–5.5% is studied. It is shown that the energy of quasi-two-dimensional electron–hole liquid localized in quantum wells for this composition range as a function of carrier concentration exhibits two local minima. The position of the deeper (major) minimum depends on the quantum well design and controls properties of quasi-two-dimensional electron–hole liquid at low temperatures. For the series of Si/Si_1 – _ x Ge_ x Si quantum wells, modification of properties of electron–hole liquid was experimentally shown, which can be interpreted as a change of the major minimum due to an increases in the germanium concentration in the Si_1 – _ x Ge_ x layer. The effect of the multicomponent composition (electrons, light and heavy holes) of the electron–hole liquid on low-temperature photoluminescence spectra of Si/Si_1 ‒ _ x Ge_ x Si quantum wells is discussed.

A method to obtain high-quality strain–relaxed SiGe buffer layers on Si(001) substrates is presented. In this method, the strain relaxation of the SiGe layer is performed using a two-step procedure. Firstly, a low-temperature-grown SiGe layer, whose surface is covered by a thin Si cap layer, is thermally annealed. At this stage, the strain is incompletely relaxed and an atomically flat surface can be realized. Then, a second SiGe layer is grown on the first layer to achieve further strain relaxation. Transmission electron microscopy has clearly revealed that dislocations are dispersively introduced into the first SiGe/Si substrate interface and thus no pile up of dislocations occurs. The formation of a periodic undulation on the growth surface of the second SiGe layer is the key to inducing a drastic reduction in the threading dislocation density.

We discuss the mechanism of efficient photoluminescence (PL) from Si quantum dots and wells. Luminescence properties of SiO2-capped Si nanocrystals are different from those of H-terminated Si nanocrystals, but are very similar to those of Si quantum wells sandwiched by SiO2 layers. The size-dependence of PL properties and resonantly excited PL spectra of SiCb-capped Si dots and wells indicate that excitons are localized near the interface between the crystalline Si core and surface oxide layer, and the strong coupling of electronic and vibrational excitations causes the broad PL spectrum. The exciton localization plays an essential role in the efficient luminescence process in nanoscale Si/SiO2 systems.

Controlling optical polarization of {11–22} semipolar multiple quantum wells using relaxed underlying InGaN buffer layers

An integrated technology computer aided simulation framework is used for the first time to predict the reliability (degradation) of substrate-induced strained-Si channel heterojunction field effect transistors on relaxed Silicon-Germanium buffer layer with ultra-thin SiO2 and high-k gate stacks. State-of-the-art four-state nonradiative multiphonon model is used for the degradation studies. Single defects and trap studies have been taken up on devices subjected to negative voltage stressing at an elevated temperature. Threshold voltage shift (due to charge capture and emission processes) in virtually fabricated devices has been studied in detail. For the first time, non radiative multiphonon model is used to explain the degradation mechanisms (oxide defects dominating the partial recovery of threshold voltage after stressing) in strained-Si channel heterojunction field effect transistors. It is shown that degradation in strained-Si channel device on relaxed-SiGe buffer is more compared to its Si-channel counterpart.