A. A. Bloshkin

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...

Effect of overgrowth temperature on the mid-infrared response of Ge/Si(001) quantum dots

Ge/Si quantum dots fabricated by molecular-beam epitaxy at 500 °C are overgrown with Si at different temperatures Tcap, and their mid-infrared photoresponse is investigated. The photocurrent maximum shifts from 2.3 to 3.9 μm with increasing Tcap from 300 to 750 °C. The best performance is achieved for the detector with Tcap = 600 °C in a photovoltaic mode. At a sample temperature of 90 K and no applied bias, a responsivity of 0.43 mA/W and detectivity of 6.2 × 1010 cmHz1/2/W at λ = 3 μm were measured under normal incidence infrared radiation. The device exhibits very low dark current (Idark = 2 nA/cm2 at T = 90 K and U = −0.2 V) and operates until 200 K.

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.

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.

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...

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.

Strain-induced localization of electrons in layers of the second-type Ge/Si quantum dots

Electronic states in multilayer Ge/Si heterostructures with different periods of the arrangement of layers of Ge quantum dots have been studied by the photocurrent spectroscopy method. It has been found that the binding energy of electrons increases with a decrease in the thickness of a silicon spacer and with the extension of Si layers near the vortices of Ge nanoclusters. The results constitute experimental evidence of the deformation mechanism of the formation of localized electronic states in Ge/Si heterostructures with second-type quantum dots.

On the process of hole trapping in Ge/Si heterostructures with Ge quantum dots

Admittance spectroscopy is used to determine the cross sections and energy levels of holes in Ge/Si heterostructures with Ge quantum dots. The structures are grown by molecular-beam epitaxy. It is established that, in layers of quantum dots produced at low growth temperatures Tg ≤ 450°C, the capture cross section for hole trapping into quantum dots exponentially increases with increasing hole binding energy (the Meyer-Neldel rule), with the same characteristic energy ∼25 eV independent of Tg. It is shown that the Meyer-Neldel rule is violated in structures grown at higher temperatures or in samples treated in hydrogen plasma. In the case of nanoclusters synthesized at low temperatures, the experimental results suggest that charge-carrier trapping into Ge quantum dots proceeds via the electron-phonon mechanism with the participation of structural defects.