Ge quantum dots in Si were fabricated by molecular beam epitaxy in the Stranski Krastanow growth mode at low substrate temperatures and were investigated by optical and electrical spectroscopy. The dot size is about 20 nm in width and 2 nm in height for a substrate temperature of about 510 C. The effective valence band structure of such Si/Ge quantum dots is consistently analyzed by type-II interband photoluminescence transitions between electrons in the Si host and holes localized within the Ge dots. by bound-to-quasibound intra-valence band transitions of localized holes in the mid-infrared spectral range and by C-F and admittance spectroscopy. Holes localized in the dot ground states reveal in ionization energy of about 350 meV corresponding to the effective Si/Ge dot valence band offset. The energy separation of zero-dimensional ground and first excited levels clue to lateral confinement is about 40 meV, and the Coulomb charging energy for the second hole within the ground state is about 15 meV. A slow thermal excitation rate of holes out of the dots at low temperature strongly affects optoelectronic properties like photocurrent response, The impact of elastic and electronic coupling of dots in self-aligned stacks on band structure is discussed. (C) 2002 Elsevier Science B.V. All rights reserved.

We present a brief survey of vertical transistor devices fabricated by the cleaved-edge overgrowth technique. Different device types are realized using different transistor substrates grown by molecular beam epitaxy. These substrates mainly vary in the layer sequence and thickness between the source/drain contacts. Common to all designs is the vertical gate structure overgrown on a cleavage plane of the substrates. By biasing the gate a two-dimensional electron system of tunable density is induced between source/drain. We study the DC transport properties of long channel (source–drain distance ∼1μm) as well as short-channel (source–drain distance ∼50nm) devices. Also the choice of the source/drain isolation (a superlattice or a p+-δ-doping or a heterobarrier) affects the characteristic device behavior.

We studied the vertical correlation of small hut-like Ge islands in Si–Ge multilayers grown by molecular beam epitaxy (MBE) at 510°C in the modified Stranski–Krastanov growth mode. The fundamental, structural and optical properties of stacked Ge islands embedded in Si are analyzed by in-situ RHEED, TEM, photoluminescence (PL), and photocurrent (PC) spectroscopy. Ge islands of about 20 nm width and 2 nm height reveal a reduction in the critical Ge coverage for correlated growth for Si spacer widths below about 14 nm. This value is much smaller than observed for dome-like islands of about 80 nm size deposited at higher temperatures. Such a scaling of island separation for stacked growth with island diameter is suggested by finite element simulations of elastic strain relaxation in islands. The correlation affects island size, island density, and local strain fields which may have a direct impact on band offsets and the electronic coupling of stacked islands. This is important for possible application in devices like NIR Si–Ge dot photodetectors and tunneling structures.

Emission of holes from self-organized Ge quantum dots (QDs) embedded in Si Schottky diodes is studied by time-resolved capacitance spectroscopy (DLTS). The DLTS signal is rather broad and depends strongly on the filling and detection bias conditions. The observed dependence is interpreted in terms of carrier emission from many-hole states of the QDs. The activation energies obtained from the DLTS measurements are a function of the amount of stored charge and the position of the fermi level in the QDs.

Admittance spectroscopy was used to investigate the density of states in self assembled Ge quantum dots (QDs) of different size embedded in Si Schottky diodes. From the admittance results, activation energies of hole in the QDs have been determined as a function of the external bias which shifts the Fermi level with respect to the energy states in the QDs. The activation energy of a quantum well sample remains constant up to 6 V bias voltage. Large Ge dots (70 nm diameter) show a continuum of activation energies and a low continuous averaged density of states. In small Ge dots (20 nm diameter) a discrete energy level structure with level separations of 40 to 4 meV are observed. They are attributed to strongly quantum confined hole states with significant Coulomb blockade energies.

The emission of holes from charged self-organized Ge quantum dots (QDs) with a diameter of similar to 70 nm embedded in Si is investigated using time-resolved capacitance spectroscopy (DLTS). A broad DLTS signal is observed due to thermal activation of carriers confined to the QDs. The DLTS spectrum and the activation energies strongly depend on the degree of filling of the QDs. The activation energy is found to decrease as the amount of charge stored in the QDs increases. We estimate a room-temperature hole retention time in the order of 1 mus.

This paper describes the simulations of vertical cleaved-edge overgrowth field effect transistors (CEO-FETs). For the simulation the device simulator SIMBA is used, which is capable to handle complex device geometries as well as several physical models represented by certain sets of partial differential equations. With a multidimensional solution of the Poisson equation the Schrodinger equation is solved either in one or in two dimensions according to the confinement of the electrons in the area where quantum mechanical effects are expected. As a new feature the involvement of a hydrodynamic (HD) transport model is implemented to include non-equilibrium transport phenomena in extremely short channels. The experimental results are compared with the simulated data of this device.

We investigate the electrical properties of self-assembled Ge islands embedded in Si Schottky diode structures by means of capacitance–voltage measurements and admittance spectroscopy. The Ge islands form at T=550 °C by self-assembly in the Stranski–Krastanow growth mode with an area density of 4.5×109 cm−2. Their diameter and height are 70 and 6.5 nm, respectively. A linear increase of the thermal activation energy observed in voltage-dependent admittance spectroscopy shows that the ensemble of Ge islands has a low, continuous, averaged density of states.

Hole emission from self-organized Ge quantum dots with a diameter of ∼70 nm in a Si matrix is investigated by time-resolved capacitance spectroscopy [deep level transient spectroscopy (DLTS)]. A complex DLTS signal is observed and explained in terms of thermally activated emission from localized many-particle states. In particular, a gradually decreasing activation energy is found with increasing hole population. A qualitative understanding of the DLTS signal and the observed activation energies is achieved in terms of many-particle states determined by quantization and Coulomb charging.

Self-assembled Ge quantum dots (QDs) embedded in Si-Schottky diodes were studied using admittance spectroscopy. A sample with one layer of Ge QDs embedded in p-Si was grown on a p+-substrate by molecular beam epitaxy and was processed into a Schottky diode structure. Activation energies have been determined as a function of the external bias voltage, which shifts the Fermi level in the sample. Two discrete activation energies of 318 and 303 meV have been extracted in the region of high bias voltage. They are identified as single and double charged QD ground state which is split up by a Coulomb charging energy. At approximately 40 meV lower activation energies several discrete levels attributed to excited dot states with a smaller energy separation of typical 5 meV are observed.