Intra-valence band photocurrent spectroscopy of self-assembled Ge dots in Si

et al. 2001

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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.

Section: Introductionmentioning

confidence: 99%

Space Charge Spectroscopy of Self Assembled Ge Quantum Dots in Si

Asperger

Miesner

et al. 2001

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“…Here, the use of quantum dots could be advantageous since the band offsets are generally larger in dots than in quantum wells. Furthermore, quantum dot infrared photodetectors (QDIPs) are intrinsically suited for normal incidence operation since the confinement potential in dots is three-dimensional [5].In this paper we present a normal incidence mid-infrared photocurrent study on Ge quantum dots embedded in vertical as well as in lateral detector structures. Such lateral intraband photodetectors are typical quantum dot devices since they are based on the three dimensional confinement for carriers.…”

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confidence: 99%

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confidence: 99%

Vertical and Lateral Mid-Infrared Photocurrent Study on Ge Quantum Dots in Si

Miesner

Abstreiter

2001

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In this paper we discuss photocurrent measurements in normal incidence geometry performed on self-assembled Ge dots in Si. The "classical" detector concept with vertical photocurrent is compared to a new one which is based on in-plane photoconductivity and is only realizable with dots. The active region of the samples consists of ten layers of Ge dots formed by self-assembly in the Stranski-Krastanov growth mode with lateral dimensions of about 20 nm and a height of about 1.5 nm. The mid-infrared photocurrent measurements show the typical line shape of bound-to-continuum transitions. The responsivity of the lateral device of 10 mA/W is about twice that of the vertical structure, furthermore the peak maximum at 284 meV is shifted by about 40 meV towards smaller energies. This is explained using a model involving carrier transfer to the modulation doping layer.1. Introduction During the past decade, intense research has been carried out on photodetectors using intra-or intersubband transitions in semiconductor nanostructures [1][2][3]. Such devices may find applications in various fields such as remote sensing or communications. The mainly interesting mid-infrared spectral ranges are between 3-5 mm and 8-12 mm where the earth atmosphere has its major transmission windows. Due to its intrinsic compatibility with standard Si readout circuitry, the Ge/Si material system is particularly interesting for mid-infrared detectors. Furthermore the indirect bandgap of the material is not limiting the optical properties for intraband transitions. Efficient quantum well infrared photodetectors (QWIPs) with absorption wavelength between 6 and 20 mm have been realized using SiGe quantum wells in Si [4]. However, the spectral range between 3 and 5 mm can hardly be covered with quantum wells since the band offsets are too small. Here, the use of quantum dots could be advantageous since the band offsets are generally larger in dots than in quantum wells. Furthermore, quantum dot infrared photodetectors (QDIPs) are intrinsically suited for normal incidence operation since the confinement potential in dots is three-dimensional [5].In this paper we present a normal incidence mid-infrared photocurrent study on Ge quantum dots embedded in vertical as well as in lateral detector structures. Such lateral intraband photodetectors are typical quantum dot devices since they are based on the three dimensional confinement for carriers. On the InAs/GaAs material system, lateral photodetectors have been realized and showed their high potential for efficient normal incidence mid-infrared photodetection [6,7].

“…This may be due to the good knowledge of material parameters and the large accessible range of dot size which can be controlled by substrate temperature, Ge content and coverage. Ge islands with a diameter of about 200 nm down to 10 nm and a height of 10 nm to 1 nm were fabricated just by varying the substrate temperature from about 700 to 500 C. They can be embedded in active Si structures without defects and reveal quantum dot related properties, for example in photocurrent spectroscopy of intra valence band transitions in the mid-infrared spectral range and in admittance spectroscopy [6][7][8]. From these studies, hole localization energies around 350 meV, zero-dimensional (0D) hole quantization energies of about 40 meV and Coulomb blockade energies up to 15 meV were deduced for a dot size of about 20 nm.…”

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confidence: 99%

Self-Ordering of Ge Islands on Si Substrates Mediated by Local Strain Fields

Zhu

Abstreiter

et al. 2001

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We have fabricated linear chains and two-dimensional arrays of Ge islands by self-assembling and self-ordering processes on vicinal Si substrates.Step bunches and stripe-like strain fields caused by underlying Si/SiGe multilayers with periodic wire-like accumulations at step edges induce an alignment of Ge islands. Repulsion of neighboring islands is caused by overlapping short-ranged strain fields surrounding partially strain relaxed dots and defines a minimum island separation. A periodic array of wires forming within the multilayer serves as a self-organized template for twodimensionally ordered Ge islands with a lateral period of 120 nm which is comparable to the island size.In the last decade, the epitaxial self-assembly of semiconductor quantum dots by the Stranski-Krastanov growth mode has developed to one of the most powerful techniques for fabricating quantum structures of high quality. The main material systems used are GaAs/InAs and Si/Ge with a lattice mismatch of about 7% and 4%, respectively. The smaller size and effective electron mass for In(Ga)As dots enhances quantum size effects compared to Ge dots. These features and the efficient optical interband transitions favor In(Ga)As quantum dots for optical fundamental studies and future optoelectronic applications [1,2]. A multitude of studies of the fundamental dot formation processes like nucleation, coalescence, ripening, faceting, shape transitions and ordering of dots, however, were performed on Ge islands on (001)Si substrate [3][4][5]. This may be due to the good knowledge of material parameters and the large accessible range of dot size which can be controlled by substrate temperature, Ge content and coverage. Ge islands with a diameter of about 200 nm down to 10 nm and a height of 10 nm to 1 nm were fabricated just by varying the substrate temperature from about 700 to 500 C. They can be embedded in active Si structures without defects and reveal quantum dot related properties, for example in photocurrent spectroscopy of intra valence band transitions in the mid-infrared spectral range and in admittance spectroscopy [6][7][8]. From these studies, hole localization energies around 350 meV, zero-dimensional (0D) hole quantization energies of about 40 meV and Coulomb blockade energies up to 15 meV were deduced for a dot size of about 20 nm. In this paper, we present a study on self-ordering of Ge islands in one-or two-dimensional arrays. The self-organized process of step-bunching in Si/SiGe multilayers on vicinal Si substrates and Ge island nucleation modified by local strain are combined.