Absorption of terahertz radiation in Ge/Si(001) heterostructures with quantum dots

The terahertz spectra of the dynamic conductivity and radiation absorption coefficient in germanium-silicon heterostructures with arrays of Ge hut clusters (quantum dots) have been measured for the first time in the frequency range of 0.3–1.2 THz at room temperature. It has been found that the effective dynamic conductivity and effective radiation absorption coefficient in the heterostructure due to the presence of germanium quantum dots in it are much larger than the respective quantities of both the bulk Ge single crystal and Ge/Si(001) without arrays of quantum dots. The possible microscopic mechanisms of the detected increase in the absorption in arrays of quantum dots have been discussed.     

Longitudinal conductivity of Ge/Si heterostructures with quantum dots

The conductance along an island layer of Ge quantum dots buried in silicon was investigated. The sizes of the islands varied in the range D ≈ 12−19 nm. It was found that the charge transport is characterized by two activation energies. The first one is associated with the thermal emission of holes from Ge quantum wells into the valence band of Si. The second one is due to the tunneling of holes between islands under Coulomb blockade conditions and is determined by the electrostatic charging energy of a quantum dot. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 6, 423–426 (25 March 1996)     

Atomic structure of Ge quantum dots on the Si(001) surface

In situ morphological investigation of the “105” faceted Ge islands on the Si(001) surface (hut clusters) have been carried out using an ultra high vacuum instrument integrating a high resolution scanning tunnelling microscope and a molecular beam epitaxy vessel. Both species of hut clusters-pyramids and wedges-were found to have the same structure of the “105” facets which was visualized. Structures of vertexes of the pyramidal clusters and ridges of the wedge-shaped clusters were revealed as well and found to be different. This allowed us to propose a crystallographic model of the “105” facets as well as models of the atomic structure of both species of the hut clusters. An inference is made that transitions between the cluster shapes are impossible.     

Phonons in Ge/Si superlattices with Ge quantum dots

Ge/Si superlattices containing Ge quantum dots were prepared by molecular beam epitaxy and studied by resonant Raman scattering. It is shown that these structures possess vibrational properties of both two-and zero-dimensional objects. The folded acoustic phonons observed in the low-frequency region of the spectrum (up to 15th order) are typical for planar superlattices. The acoustic phonon lines overlap with a broad emission continuum that is due to the violation of the wave-vector conservation law by the quantum dots. An analysis of the Ge and Ge-Si optical phonons indicates that the Ge quantum dots are pseudoamorphous and that mixing of the Ge and Si atoms is insignificant. The longitudinal optical phonons undergo a low-frequency shift upon increasing laser excitation energy (2.54–2.71 eV) because of the confinement effect in small-sized quantum dots, which dominate resonant Raman scattering.     

Negative interband photoconductivity in Ge/Si heterostructures with quantum dots of the second type

It was found that irradiating an array of Ge nanoclusters in n-Si with light that induced interband transitions gave rise to negative photoconductivity. This result was explained by localization of equilibrium electrons at the Si/Ge interface in the potential of the nonequilibrium holes trapped on deep states in Ge islands.     

Binding of electron states in multilayer strained Ge/Si heterostructures with type-II quantum dots

Mechanical strains in a multilayer Ge/Si(001) heterostructure with vertically aligned Ge nanoclusters (quantum dots) are calculated using an interatomic potential based on the Keating valence-force-field model. It is found that the nonuniform spatial elastic strain distribution in this medium gives rise to a three-dimensional potential well for electrons in the strained Si layers near Ge nanoclusters. The depth of the potential well reaches 100 meV, and its spatial dimensions are determined by the diameter of the Ge nanoclusters. For a structure consisting of four Ge islands 23 nm in diameter arranged one above another, the electron binding energies in this well and the spatial electron density distribution are determined. The ground state has an s-like symmetry and is characterized by an electron binding energy of ～95 and ～60 meV for the elemental composition of Ge in the nanoclusters c = 1 and c = 0.7, respectively. The existence of bound electron states in the conduction band of strained Si must lead to a relaxation of the selection rules that determine the low efficiency of the radiative recombination in indirect-gap semiconductors. This explains the high value of the oscillator strength observed for the interband transitions in multilayer Ge/Si(001) structures with vertical correlation of the arrangement of Ge nanoclusters.     

Spatial distribution of elastic deformations in Ge/Si structures with quantum dots

A method is developed for calculating the elastic deformation in coherently strained heterostructures on the basis of the valence force field (VFF) model using the Green’s function of the “atomistic” elastic problem. The spatial distribution of the elastic deformations in a Ge/Si system with pyramidal Ge quantum dots buried in a Si matrix is investigated theoretically. The deformation distribution in and around the pyramids is determined. Near quantum dots, the region near the tip of the pyramid is most strongly intensely. Inside quantum dots the region of the vertex is most relaxed, and the most strained section lies on the contour of the pyramid base. Compression occurs in the plane of the pyramid base inside quantum dots, and stretching occurs along the vertical direction. The picture is reversed near quantum dots: stretching occurs in the lateral direction and compression in the vertical direction. It is shown that the local deformations and their spatial distribution are essentially independent (to within the scaling) of the size of the quantum dots for 10–15 nm pyramid bases.     

Excitons in Ge/Si double quantum dots

The spatial structure of excitons and the oscillator strength characterizing the intensity of interband optical transitions in vertically coupled Ge/Si quantum dots have been theoretically studied. It has been found that the probability of the exciton transition under certain conditions (the sizes of the quantum dots, the separation of the dots) can be much larger (up to a factor of 5) than the value for the case of single quantum dots. It is expected that the results will make it possible to approach the creation of efficient light-emitting and photoreceiving devices based on Si and Ge indirect-band semiconductors.     

Folded acoustic phonons in Si/Ge superlattices with Ge quantum dots

The spectra of Raman scattering by folded acoustic phonons in Si/Ge superlattices with pseudomorphic layers of Ge quantum dots (QDs) grown by low-temperature (T = 250°C) molecular beam epitaxy are studied. New features of the folded phonon lines related to the resonant enhancement and unusual intensity ratio of the doublet lines that cannot be explained by the existing theory have been observed. The observed modes are shown to be related to the vibrations localized to the QDs and induced by the folded phonons of the Si spacer layers. The calculations performed in the model of a one-dimensional chain of atoms have allowed the nature of the localization of acoustic phonons attributable to a modification of the phonon spectrum of a thin QD layer to be explained. The observed intensity ratio of the folded phonon doublet lines is caused by asymmetry of the relief of the QD layers.     

Luminescence study of Si/Ge quantum dots

We present a photoluminescence (PL) study of Ge quantum dots embedded in Si. Two different types of recombination processes related to the Ge quantum dots are observed in temperature-dependent PL measurements. The Ge dot-related luminescence peak near 0.80 eV is ascribed to the spatially indirect recombination in the type-II band lineup, while a high-energy peak near 0.85 eV has its origin in the spatially direct recombination. A transition from the spatially indirect to the spatially direct recombination is observed as the temperature is increased. The PL dependence of the excitation power shows an upshift of the Ge quantum dot emission energy with increasing excitation power density. The blueshift is ascribed to band bending at the type-II Si/Ge interface at high carrier densities. Comparison is made with results derived from measurements on uncapped samples. For these uncapped samples, no energy shifts due to excitation power or temperatures are observed in contrast to the capped samples.     

Second-harmonic spectroscopy of Ge/Si(001) and Si1-xGex(001)/Si(001)

0.9 Ge_0.1(001)/Si(001) films with SH photon energies 3.1<2hν<3.5 eV near the bulk E₁ critical point of Si(001) or Si_0.9Ge_0.1(001). Ge was deposited on Si(001) by using atomic layer epitaxy cycles with GeH₄ or Ge₂H₆ deposition at 410 K followed by hydrogen desorption. As Ge coverage increased from 0 to 2 monolayers the SH signal increased uniformly by a factor of seven with no detectable shift in the silicon E₁ resonant peak position. SH signals from Si_0.9Ge_0.1(001)/Si(001) were also stronger than those from intrinsic Si(001). Hydrogen termination of the Si_0.9Ge_0.1(001) and Ge/Si(001) surfaces strongly quenched the SH signals, which is similar to the reported trend on H/Si(001). We attribute the stronger signals from Ge-containingsurfaces to the stronger SH polarizability of asymmetric Ge-Si and Ge-Ge dimers compared to Si-Si dimers. Hydrogen termination symmetrizes all dimers, thus quenching the SH polarizability of all of the surfaces investigated. Received: 13 October 1998 / Revised version: 18 January 1999     

掺杂对多层Ge/Si(001)量子点光致发光的影响

利用超高真空化学气相沉积设备, 在Si (001) 衬底上外延生长了多个四层Ge/Si量子点样品. 通过原位掺杂的方法, 对不同样品中的Ge/Si量子点分别进行了未掺杂、磷掺杂和硼掺杂. 相比未掺杂的样品, 磷掺杂不影响Ge/Si量子点的表面形貌, 但可以有效增强其室温光致发光; 而硼掺杂会增强Ge/Si量子点的合并, 降低小尺寸Ge/Si量子点的密度, 但其光致发光会减弱. 磷掺杂增强Ge/Si量子点光致发光的原因是, 磷掺杂为Ge/Si量子点提供了更多参与辐射复合的电子. 关键词： Ge/Si量子点 磷掺杂 光致发光     

Mechanisms of self-organization of Ge/Si(0 0 1) quantum dots

The effect of vertical ordering in superlattices of self-assembled Ge/Si(0 0 1) quantum dots was investigated by a combination of structural and optical characterizations via in situ reflection high-energy electron diffraction, transmission electron microscopy, atomic force microscopy and photoluminescence spectroscopy. We show that the vertical ordering observed in quantum-dot superlattices is characterized not only by the alignment of islands along the growth direction but also by a reduction of the critical thickness. The better the vertical ordering is, the more pronounced the reduction of the critical thickness will be. Such an evolution of the critical thickness could be explained by elastic strain fields induced by buried islands and propagated through the spacer layers. An important result issued from this work is the realization of superlattices in which dots can have equal size in all layers. On the other hand, experiments performed on the transformation of the island shape versus the spacer layer thickness suggest that preferential nucleation induced by surface roughness may be the main mechanism responsible for the vertical ordering observed in quantum-dot superlattices.     

Spectrum of electron-hole states of the Si/Ge structure with Ge quantum dots

The lateral photoconductivity spectra of Si/Ge multilayer structures with Ge quantum dots of various sizes are investigated. We observed optical transition lines between the hole levels of quantum dots and electronic states of Si. This enabled us to construct a detailed energy level diagram of the electron-hole spectrum of the Si/Ge structures. It is shown that the hole levels of Ge quantum dots are successfully described by the “quantum box” model using the actual sizes of Ge islands. It I found that the position of the longwavelength photosensitivity boundary of Si/Ge structures with Ge quantum dots can be controlled by changing the growth parameters.     

Optical phonons in Ge quantum dots obtained on Si(111)

The Raman light scattering from optical phonons of Ge quantum dots grown by molecular beam epitaxy on a Si(111) surface is studied. A series of Raman lines related to the quantization of phonon spectrum is observed. It is shown that phonon frequencies are adequately described in terms of the elastic properties and the dispersion of the optical phonons of bulk Ge. The strain experienced by the Ge quantum dots is estimated.     

Micro-Raman scattering by laser-modified structures with Ge/Si quantum dots

Structures with self-assembled Ge/Si quantum dots grown by molecular-beam epitaxy are exposed to pulsed radiation of a picosecond laser. Changes in the vibrational spectrum of nanostructures under an external action are studied by Raman spectroscopy. An analysis of the Raman spectra measured with a micron spatial resolution along the exposed region indicates a mixing of Ge and Si atoms and a change in the induced mechanical stresses in quantum dots.     

Optical absorption spectra of Si with Ge quantum dots

The results of calculations of optical absorption spectra of silicon containing Ge nanoclusters of spherical shape and different size are reported. The optical transitions from the Ge cluster levels to the silicon bulk energy band states are analyzed.     

Suppression of hole relaxation in small Ge/Si quantum dots

We study the effect of quantum dot size on the mid-infrared photocurrent, photoconductive gain, and hole capture probability in ten-period p-type Ge/Si quantum dot heterostructures. The dot dimensions are varied by changing the Ge coverage during molecular beam epitaxy of Ge/Si(001) system in the Stranski–Krastanov growth mode while keeping the deposition temperature to be the same. A device with smaller dots is found to exhibit a lower capture probability and a higher photoconductive gain and photoresponse. The integrated responsivity in the mid-wave atmospheric window (λ = (3–5) μm) is improved by a factor of about 8 when the average in-plane dot dimension changes from 18 to 11 nm. The decrease in the dot size is expected to reduce the carrier relaxation rate due to phonon bottleneck by providing strong zero-dimensional quantum mechanical confinement.     

Spin states of holes in Ge/Si nanowire quantum dots

We investigate tunable hole quantum dots defined by surface gating Ge/Si core-shell nanowire heterostructures. In single level Coulomb-blockade transport measurements at low temperatures spin doublets are found, which become sequentially filled by holes. Magnetotransport measurements allow us to extract a g factor g approximately 2 close to the value of a free spin-1/2 particle in the case of the smallest dot. In less confined quantum dots smaller g factor values are observed. This indicates a lifting of the expected strong spin-orbit interaction effects in the valence band for holes confined in small enough quantum dots. By comparing the excitation spectrum with the addition spectrum we tentatively identify a hole exchange interaction strength chi approximately 130 microeV.