Intraband Photocurrent Research Articles

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.

Electronic states in vertically ordered Ge/Si quantum dots detected by photocurrent spectroscopy

We report on intraband photocurrent spectroscopy of sixfold stacked Ge/Si quantum dots embedded in a Si matrix and aligned along the growth direction. The dots are formed in a shape of pyramids with the average lateral size of 18 nm. The $n$-type heterostructures show broad spectral response ranging from 5 to 20 $\ensuremath{\mu}\mathrm{m}$, depending on the polarization of the incoming infrared light. The normal incidence photocurrent peak centered around 12--15 $\ensuremath{\mu}\mathrm{m}$ is attributed to the transitions from the electron states localized in the Si region adjacent to the dots to continuum states of the Si matrix. The electron confinement is caused by a modification of the conduction band alignment induced by inhomogeneous tensile strain in Si around the buried Ge/Si quantum dots. Using the Ge content and dot shape determined by Raman and scanning tunneling microscopy analysis as input parameters for three-dimensional band structure simulations, a good agreement between measured and calculated electron binding energy is obtained. Photoluminescence spectroscopy and measurements of temperature dependence of dark conductance are used to correlate photocurrent results.

The influence of quantum dot size on the sub-bandgap intraband photocurrent in intermediate band solar cells

The effect of quantum dot (QD) size on the performance of quantum dot intermediate band solar cells is investigated. A numerical model is used to calculate the bound state energy levels and the absorption coefficient of transitions from the ground state to all other states in the conduction band. Comparing with the current state of the art, strong absorption enhancements are found for smaller quantum dots, as well as a better positioning of the energy levels, which is expected to reduce thermal carrier escape. It is concluded that reducing the quantum dot size can increase sub-bandgap photocurrent and improve voltage preservation.

Photocurrent spectroscopy of site-controlled pyramidal quantum dots

Intraband photocurrent spectroscopy of site-controlled pyramidal quantum dots by inserting them into the intrinsic region of n-i-n like quantum dot infrared photodetector structure is reported. The photovoltaic response is observed in the mid-infrared region. A peak responsivity of 0.4 mA/W at 120 meV (λ = 10 μm) is observed at 10 K at −2 V bias. The ability to engineer states in the conduction band of the QDs has been exploited to tune their photocurrent response from 10 μm to 18 μm with a narrow spectral width of Δλ/λ = 0.17.

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 µm. 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 µm 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 µm) is associated with hole transitions from the dots to the nearby Si0.65Ge0.35 layer.

Design Consideration and Demonstration of Resonant-Cavity-Enhanced Quantum Dot Infrared Photodetectors in Mid-Infrared Wavelength Regime (3–5 $\mu{\rm m}$)

Lattice-mismatched-strain-driven, defect-free, 3-D islands, dubbed self-assembled quantum dots (SAQDs), currently provide the most advanced platform for quantum-dot-based devices with successful applications to SAQD lasers and considerable potential for infrared detectors. For the latter, one of the limitations is the short optical path length owing to the difficulties in creation of large numbers of quantum dot layers without formation of structural defects arising from accumulated strain. This limitation can be considerably overcome by creation of an appropriate resonant cavity to enhance the optical field in the SAQD regions. In this paper, we demonstrate resonant-cavity-enhanced quantum dot infrared photodetectors in the mid-infrared (MIR, 3-5 μm ) regime. For effective enhancement, the SAQDs are designed to generate a very narrow peak (Δλ/λ ~ 10%) in the intraband photocurrent response in the MIR range utilizing short period superlattices as the quantum confining layers. Incorporating such SAQD layers at specific enhanced electric field regions within the resonant cavity comprising a two-pair GaAs/air-gap back mirror and a GaAs surface front mirror, we have obtained QDIP detectivity enhancement of ~ 8-12 times.

INTRABAND AUGER EFFECT IN QUANTUM DOT STRUCTURES

Intraband photocurrent and absorption measurements were performed on InAs / InGaAlAs / InP quantum dot structures. To identify the optical transitions, theoretical calculations were performed and showed that the observed PC is generated by a bound to bound transition, where the final state is about 200 meV deep below the conduction band continuum. The reported results strongly suggest that an Auger process plays a fundamental role in generating the observed intraband photocurrent.

Intraband Auger effect in InAs/InGaAlAs/InP quantum dot structures

Intraband photocurrent and absorption measurements were performed on InAs/InGaAlAs/InP quantum dot structures. A full three-dimensional theoretical model has been employed to identify the observed photocurrent as a bound to bound transition, where the final state is about 200 meV deep below the conduction band continuum. The reported results strongly suggest that an Auger process plays a fundamental role in generating the observed intraband photocurrent.

Evidence for an Intraband Auger Effect in InAs/InGaAlAs/InP Quantum Dot Structures

InAs quantum dots grown on InGaAlAs lattice matched to InP have been investigated for use in quantum dot infrared photodetectors. Intraband photocurrent of samples with different doping levels have been measured and with the help of a 3D effective mass calculation, the main absorption peak has been identified. Moreover, the reported results strongly suggest that an Auger process plays a fundamental role in generating the observed intraband photocurrent.

Intraband Auger effect in InAs∕InGaAlAs∕InP quantum dot structures

InAs quantum dot structures grown on InGaAlAs have been investigated for midinfrared photodetection. Intraband photocurrent and absorption measurements, together with a full three-dimensional theoretical modeling revealed that a bound-to-bound optical transition, where the final state is about 200meV deep below the conduction band continuum, is responsible for the photogenerated current. The reported results strongly suggest that an Auger process plays a fundamental role in generating the observed intraband photocurrent. Photoluminescence and interband photocurrent spectra of the same structures further support the reached conclusions.

Room temperature demonstration of GaN∕AlN quantum dot intraband infrared photodetector at fiber-optics communication wavelength

We fabricated a communication wavelength photodetector based on intraband transition in GaN∕AlN self-assembled quantum dot heterostructures. The quantum dot photodetector is based on in-plane transport and has a room temperature spectral peak responsivity of 8mA∕W at wavelength of 1.41μm. We use multipass waveguide geometry to show that the polarization sensitive optical absorption spectrum of the heterostructure is nearly the same as its photocurrent spectral response. This establishes that the detector’s response is due to the presence of quantum dots in its active layer. We use photoluminescence, transmission, and intraband photocurrent spectroscopy to consistently describe the alignment between the energy levels of the quantum dots and that of the wetting layer.

Interband and intraband photocurrent of self-assembled InAs/InAlAs/InP nanostructures

The interband and intraband photocurrent properties of InAs/InAlAs/InP nanostructureshave been studied. The doping effect on the photoluminescence properties of the quantumdots and the anisotropy of the quantum wire interband photocurrent properties arepresented and discussed. With the help of interband excitation, an intraband photocurrentsignal of the InAs nanostructures is observed. With the increase of the interband excitationpower, the intraband photocurrent signal first increases and then decreases, which can beexplained by the variance of the ground state occupation of the InAs nanostructures andthe change of the mobility and lifetime of the electrons. The temperature dependence of theintraband photocurrent signal of the InAs nanostructures is also investigated.

Intraband and interband photocurrent spectroscopy and induced dipole moments of InAs/GaAs(001) quantum dots in n–i–n photodetector structures

We have investigated normal-incidence intra- and interband spectra of self-assembled steep InAs/GaAs(001) quantum dots (QDs) with an average height of ∼8.0 nm and average base width of ∼21 nm placed in n–i(QDs)–n photodetector structures. The ground state occupation of the QDs in the n–i(QDs)–n configuration is examined and used to assign observed intraband transitions. A photovoltaic effect in intraband photocurrent is observed and shown to arise from induced dipole moments. Stark shift in interband photocurrent spectroscopy reveals the presence and direction of interband transition induced dipoles, making this study the first to determine both intra- and interband dipoles in the same ensemble of QDs.

Normal incidence InAs/AlxGa1−xAs quantum dot infrared photodetectors with undoped active region

We have performed a comprehensive investigation of n-type quantum dot infrared photodetectors (QDIPs) based on InAs/GaAs epitaxical island quantum dots (QDs) grown via the innovative punctuated island growth technique. The structural properties of the QDs were investigated with cross-sectional transmission electron microscopy and atomic force microscopy. The electronic properties of the QDs inserted in QDIP devices were investigated with photoluminescence (PL), PL excitation, and intra- and inter-band photocurrent spectroscopy. The influence of AlGaAs layers inserted into the QDIP active regions on the performance of dark current and inter- and intra-band photocurrent was examined. Initial results on intra-band responsivity and detectivity of these QDIPs at 77 K with undoped active region show promise for application.