V. V. Kirienko

Influence of delta-doping on the performance of Ge/Si quantum-dot mid-infrared photodetectors

The effect of delta-doping on the performance of ten-period Ge/Si quantum-dot (QD) mid-infrared photodetectors (λmax≃3.4 μm) was investigated. Ge QDs fabricated by molecular-beam epitaxy at 500 °C are overgrown with Si at 600 °C. Each Si barrier contains a boron delta-doping layer located near the QD plane to provide holes to the dots. Within the sample set, we examined devices with different positions of the δ-doping layer with respect to the QD plane, different distances between the δ-doping layer and the QD plane, and different doping densities. All detectors show pronounced photovoltaic behavior implying the presence of an internal inversion asymmetry. We observed a reversal of the voltage dependence of responsivity with respect to zero bias when the δ-doping plane is carried from the bottom to the top of the dot layer. This result indicates that the main reason for the asymmetric photoresponse is the existence of a built-in electric field due to the placing dopants in the barriers. Devices with a low...

Ge'Si quantum-dot metal-oxide-semiconductor field-effect transistor

We report on the operation of Si metal–oxide–semiconductor field-effect transistor with an array of ∼103 10 nm diameter Ge self-assembled quantum dots embedded into the active channel. The drain current versus gate voltage characteristics show oscillations caused by Coulomb interaction of holes in the fourfold-degenerate excited state of the dots at T⩽200 K. A dot charging energy of ∼43 meV (i.e., >kT=26 meV at T=300 K) and disorder energy of ∼20 meV are determined from the oscillation period and the temperature dependence study of current maxima, respectively.

Midinfrared photoresponse of Ge quantum dots on a strained Si0.65Ge0.35 layer

We report on intraband photocurrent spectroscopy of Ge self-assembled quantum dots placed on a strained Si0.65Ge0.35 quantum well, which, in turn, is incorporated in a Si matrix. The p-type devices show broad spectral response ranging from 2 to 12 ?m. By a comparison between photocurrent measurements and the hole energy level scheme, as deduced from six-band k p calculations, the two main contributions to the photoresponse are identified. The absorption band between 2 and 4 ?m is attributed to the bound-to-continuum transitions between the bound states of the quantum dots and the continuum states in the Si barrier. The photoresponse at longer wavelength (4?12 ?m) is associated with hole transitions from the dots to the nearby Si0.65Ge0.35 layer.

Pulsed ion-beam induced nucleation and growth of Ge nanocrystals on SiO2

Pulsed low-energy (200eV) ion-beam induced nucleation during Ge deposition on thin SiO2 film was used to form dense homogeneous arrays of Ge nanocrystals. The ion-beam action is shown to stimulate the nucleation of Ge nanocrystals when being applied after thin Ge layer deposition. Temperature and flux variation was used to optimize the nanocrystal size and array density required for memory device. Kinetic Monte Carlo simulation shows that ion impacts open an additional channel of atom displacement from a nanocrystal onto SiO2 surface. This results both in a decrease in the average nanocrystal size and in an increase in nanocrystal density.

Phonon bottleneck in p-type Ge/Si quantum dots

We study the effect of quantum dot size on the mid-infrared photo- and dark current, 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 and the growth temperature during molecular beam epitaxy of Ge/Si(001) system in the Stranski-Krastanov growth mode. In all samples, we observed the general tendency: with decreasing the size of the dots, the dark current and hole capture probability are reduced, while the photoconductive gain and photoresponse are enhanced. Suppression of the hole capture probability in small-sized quantum dots is attributed to a quenched electron-phonon scattering due to phonon bottleneck.

Ge/Si photodiodes and phototransistors with embedded arrays of germanium quantum dots for fiber-optic communication lines

Photodetectors based on Ge/Si multilayer heterostructures with germanium quantum dots are fabricated for use in fiber-optic communication lines operating in the wavelength range 1.30–1.55 μm. These photodetectors can be embedded in an array of photonic circuit elements on a single silicon chip. The sheet density of germanium quantum dots falls in the range from 0.3 × 1012 to 1.0 × 1012 cm−2, and their lateral size is approximately equal to 10 nm. The heterostructures are grown by molecular-beam epitaxy. For a reverse bias of 1 V, the dark current density reaches 2 × 10−5 A/cm2. This value is the lowest in the data on dark current densities available in the literature for Ge/Si photodetectors at room temperature. The quantum efficiency of photodiodes and phototransistors subjected to illumination from the side of the plane of the p-n junctions is found to be 3% at a wavelength of 1.3 μm. It is demonstrated that the maximum quantum efficiency is achieved for edge-illuminated waveguide structures and can be as high as 21 and 16% at wavelengths of 1.3 and 1.5 μm, respectively.

Broadband Ge/SiGe quantum dot photodetector on pseudosubstrate

We report the fabrication and characterization of a ten-period Ge quantum dot photodetector grown on SiGe pseudosubstrate. The detector exhibits tunable photoresponse in both 3- to 5- μ m and 8- to 12- μ m spectral regions with responsivity values up to about 1 mA/W at a bias of −3 V and operates under normal incidence radiation with background limited performance at 100 K. The relative response in the mid- and long-wave atmospheric windows could be controlled through the applied voltage.

Photoconductive gain and quantum efficiency of remotely doped Ge/Si quantum dot photodetectors

We study the effect of quantum dot charging on the mid-infrared photocurrent, optical gain, hole capture probability, and absorption quantum efficiency in remotely delta-doped Ge/Si quantum dot photodetectors. The dot occupation with holes is controlled by varying dot and doping densities. From our investigations of samples doped to contain from about one to nine holes per dot we observe an over 10 times gain enhancement and similar suppression of the hole capture probability with increased carrier population. The data are explained by quenching the capture process and increasing the photoexcited hole lifetime due to formation of the repulsive Coulomb potential of the extra holes inside the quantum dots. The normal incidence quantum efficiency is found to be strongly asymmetric with respect to applied bias polarity. Based on the polarization-dependent absorption measurements it is concluded that, at a positive voltage, when holes move toward the nearest δ-doping plane, photocurrent is originated from the bound-to-continuum transitions of holes between the ground state confined in Ge dots and the extended states of the Si matrix. At a negative bias polarity, the photoresponse is caused by optical excitation to a quasibound state confined near the valence band edge with subsequent tunneling to the Si valence band. In a latter case, the possibility of hole transfer into continuum states arises from the electric field generated by charge distributed between quantum dots and delta-doping planes.

Localization of electrons in dome-shaped GeSi/Si islands

We report on intraband photocurrent spectroscopy of dome-shaped GeSi islands embedded in a Si matrix with n+-type bottom and top Si layers. An in-plane polarized photoresponse in the 85–160 meV energy region has been observed and ascribed to the optical excitation of electrons from states confined in the strained Si near the dome apexes to the continuum states of unstrained Si. The electron confinement is caused by a modification of the conduction band alignment induced by inhomogeneous tensile strain in Si around the buried GeSi quantum dots. Sensitivity of the device to the normal incidence radiation proves a zero-dimensional nature of confined electronic wave functions.

Hydrogen passivation of self-assembled Ge/Si quantum dots

We studied the effect of hydrogen plasma treatment on room-temperature photoluminescence ofself-assembled Ge/Si quantum dots by varying temperature and duration of treatment. Hydrogenplasma exposure at 300 C° for 30min was found to result in the improvement in the radiativeefficiency of the Ge quantum dots by one order of magnitude. The enhancement of thephotoluminescence intensity is thought to be due to the passivation of nonradiative centerslocated nearby or inside the dots via formation of Si–H bonds. Infrared absorption spectroscopywas used to correlate photoluminescence results.Keywords: quantum dots, photoluminescence, hydrogen passivation(Some figures may appear in colour only in the online journal)A major drawback in the development of silicon optoelec-tronic integrated circuits is the difficulty in fabricating effi-cient light-emitting devices. Various semiconductor structureshave been studied to design Si-based light emitters operatingon telecom wavelengths, including Si layers on oxidized Sisurface [1], Ge-on-Si structures [2], Er doped Si-basedmaterials [3], and GeSn alloys [4]. The light emission in thetelecom wavelength range of 1.3–1.6μm can be realized inself-assembled Ge/Si quantum dots (QDs) fabricated viaStranski–Krastanov mechanism [5–7]. Small sizes of Geislands in Si matrix, their high areal density, and abruptinterfaces are the crucial issues when ensembles of QDs areconsidered for both device applications and fundamentalphysical studies. The above mentioned requirements can befulfilled with the use of a low-temperature deposition tech-nique [8–10]. However, the low-temperature growth isaccompanied by formation of point defects in either interfaceof QDs or in surrounding Si layers resulting in a degradationof the Ge QD photoluminescence (PL) [9].The most serious problem for the application of Ge/SiQDs up to now is the low luminescence efficiency, especiallyat room temperature. A few approaches have been exploitedto achieve a room-temperature PL, such as manipulating theovergrowth temperatures [11, 12] and the Si spacer thickness[13], doping the Ge nanoislands with antimony [14],formation of Ge/Si QD superlattices [15]. For InAs/GaAsQDs, it has been previously found that improvement in the PLefficiency can be achieved by exposure of samples tohydrogen plasma [16, 17]. The enhancement of the emissionwas supposed to originate from the passivation of non-radiative centers located nearby or inside the dots [18]. Si-based heterostructures can also exhibit point defects mainly inthe form of Si dangling bonds. These dangling bonds act aselectrically active recombination centers for charge carriersand thus cause the optical emission to deteriorate. It has beenclearly demonstrated that introducing atomic hydrogen intoamorphous, crystalline, and polycrystalline Si films results ina reduction of the Si dangling-bond concentration on thesurface and grain boundaries through the formation of Si–Hbonds [19–22].Sobolev et al [23] reported on the enhance-ment of the low-temperature PL from the hydrogen passivatedGe/Si quantum dots. However neither the enhancement factornor the optimal parameters for passivation have been estab-lished. In this paper we study the effect of hydrogen passi-vation on the more practically important room temperatureemission from Ge/Si self-assembled QDs. The optimalhydrogenation temperature and the exposure time, whichensure the enhancement of the dot radiative efficiency by oneorder of magnitude, were determined.