V. A. Armbrister

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.

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.

Nucleation and growth of ordered groups of SiGe quantum dots

An approach for the formation of ordered groups of Ge nanoislands (quantum dots, QDs) upon epitaxial growth on the surface of a heterostructure constituted by a Si (100) substrate having preliminarily formed seeds in the form of disk-like SiGe nanomounds is developed. It is found that the observed arrangement of QDs within a group is due to the anisotropic elastic-strain energy distribution on the surface of a SiGe nanomound, namely, to the existence of four local energy minima arranged in an ordered manner along the [100] and [010] directions with respect to the seed center. Multilayer structures with vertically aligned QD groups are grown using the suggested approach. The crystal structure and the elemental composition of the spatially ordered nanostructures are examined by transmission electron microscopy, X-ray diffraction analysis, and Raman spectroscopy.

Plasmon polariton enhanced mid-infrared photodetectors based on Ge quantum dots in Si

Quantum dot based infrared (IR) photodetectors (QDIPs) have the potential to provide meaningful advances to the next generation of imaging systems due to their sensitivity to normal incidence radiation, large optical gain, low dark currents, and high operating temperature. SiGe-based QDIPs are of particular interest as they are compatible with silicon integration technology but suffer from the low absorption coefficient and hence small photoresponse in the mid-wavelength IR region. Here, we report on the plasmonic enhanced Ge/Si QDIPs with tailorable wavelength optical response and polarization selectivity. Ge/Si heterostructures with self-assembled Ge quantum dots are monolithically integrated with periodic two-dimensional arrays of subwavelength holes (2DHAs) perforated in gold films to convert the incident electromagnetic IR radiation into the surface plasmon polariton (SPP) waves. The resonant responsivity of the plasmonic detector at a wavelength of 5.4 μm shows an enhancement of up to thirty times o...

Dense arrays of Ge nanoclusters induced by low-energy ion-beam assisted deposition on Si02 films

Ge islands less then 10 nm in base diameter and with a number density of about 8×1011 cm2 were created on Si02 films by low-energy ion-beam assisted deposition in high vacuum. The structures obtained were analyzed by Electron Spectroscopy for Chemical Analysis, Atomic Force Microscopy and High Resolution Electron Microscopy. It was found that due to desorption at 300-375 °C less than 50% of Ge deposited remains at the surface. Only pulse regime of ion-beam action results in formation of nanoclusters. It is suggested that the simultaneous nucleation of Ge islands at pulse ion-beam action is the main reason of high homogeneity of size distribution of Ge nanoislands.

Surface plasmon dispersion in a mid-infrared Ge/Si quantum dot photodetector coupled with a perforated gold metasurface

The photodetection improvement previously observed in mid-infrared (IR) quantum dot photodetectors (QDIPs) coupled with periodic metal metasurfaces is usually attributed to the surface light trapping and confinement due to generation of surface plasmon waves (SPWs). In the present work, a Ge/Si QDIP integrated with a metal plasmonic structure is fabricated to experimentally measure the photoresponse enhancement and verify that this enhancement is caused by the excitation of the mid-IR surface plasmons. A 50 nm-thick gold film perforated with a 1.2 μm-period two-dimensional square array of subwavelength holes is employed as a plasmonic coupler to convert the incident electromagnetic IR radiation into SPWs. Measurements of the polarization and angular dependencies of the photoresponse allow us to determine the dispersion of plasmon modes. We find that experimental dispersion relations agree well with that derived from a computer simulation for fundamental plasmon resonance, which indicates that the photodetection improvement in the mid-IR spectral region is actually caused by the excitations of surface plasmon Bloch waves.The photodetection improvement previously observed in mid-infrared (IR) quantum dot photodetectors (QDIPs) coupled with periodic metal metasurfaces is usually attributed to the surface light trapping and confinement due to generation of surface plasmon waves (SPWs). In the present work, a Ge/Si QDIP integrated with a metal plasmonic structure is fabricated to experimentally measure the photoresponse enhancement and verify that this enhancement is caused by the excitation of the mid-IR surface plasmons. A 50 nm-thick gold film perforated with a 1.2 μm-period two-dimensional square array of subwavelength holes is employed as a plasmonic coupler to convert the incident electromagnetic IR radiation into SPWs. Measurements of the polarization and angular dependencies of the photoresponse allow us to determine the dispersion of plasmon modes. We find that experimental dispersion relations agree well with that derived from a computer simulation for fundamental plasmon resonance, which indicates that the photodet...

Photovoltaic Ge/SiGe quantum dot mid-infrared photodetector enhanced by surface plasmons

We report the fabrication and characterization of a multilayer Ge quantum dot detector grown on Si1-xGex virtual substrate (x = 0.18) for photovoltaic mid-wave infrared photodetection. Detector displays an over 100% photovoltaic response enhancement as compared to a conventional Ge/Si device due to smaller hole effective mass in the SiGe barriers. A further enhancement in sensitivity is achieved by excitation of surface plasmon polariton waves in a Ge/SiGe photodetector coupled with a two-dimensional plasmonic structure. The plasmonic resonance induced photocurrent enhancement is found to be larger when the incident infrared light illuminates the detector from its substrate side. At zero bias and 90 K, the responsivity of 40 mA/W and peak detectivity of 1.4 × 1011 cm·Hz1/2/W are determined at a wavelength of 4 µm.