Quantum Dot Infrared Photodetectors: Photoresponse Enhancement Due to Potential Barriers
© Mitin et al. 2010
Received: 19 July 2010
Accepted: 16 August 2010
Published: 31 August 2010
Potential barriers around quantum dots (QDs) play a key role in kinetics of photoelectrons. These barriers are always created, when electrons from dopants outside QDs fill the dots. Potential barriers suppress the capture processes of photoelectrons and increase the photoresponse. To directly investigate the effect of potential barriers on photoelectron kinetics, we fabricated several QD structures with different positions of dopants and various levels of doping. The potential barriers as a function of doping and dopant positions have been determined using nextnano3 software. We experimentally investigated the photoresponse to IR radiation as a function of the radiation frequency and voltage bias. We also measured the dark current in these QD structures. Our investigations show that the photoresponse increases ~30 times as the height of potential barriers changes from 30 to 130 meV.
Many optoelectronic devices are based on the phenomenon of photoconductivity in which a material becomes more electrically conductive due to photocarriers created by electromagnetic radiation. Photocarriers contribute to the electric current until they are trapped by impurities and/or defects. Long photocarrier lifetime would substantially improve the operation of optoelectronic devices, such as IR and THz detectors and solar cells. New nanostructured materials that provide long photocarrier lifetime at room temperatures would significantly increase the commercial market for infrared and terahertz technologies.
Initial hopes related to QD nanostructures were associated with the "phonon bottleneck" concept, which assumes that the phonon-assisted bound-to-bound transitions in QDs are prohibited, unless the energy between two discrete levels matches the phonon energy . According to this concept, the intrinsic electron relaxation in quasi-1D nano-objects, such as QDs, was anticipated to be significantly slower than in 2D and 3D structures. However, the phonon bottleneck model completely ignores interaction between electrons and corresponding modification of electron states. It is not surprising that the experimentally measured phonon-mediated electron relaxation turned out to be much faster than it is expected in the phonon bottleneck concept . Recent investigations  unambiguously demonstrated that the actual intradot kinetics is completely opposite to what can be expected for weakly interacting electrons and phonons. In reality, strong coupling between electrons and longitudinal optical (LO) phonons leads to the formation of the polaron states, which decay due to the interaction of LO phonons with acoustical phonons. Such kinetics results in strong energy and temperature dependences of the electron relaxation. For example, for 14 meV transition, the relaxation time reduced from 1.5 ns at 10 K to 560 ps at 30 K, and further to 260 ps at 50 K. At room temperatures, the polaron decay time is observed in the range of 2–30 ps, depending on the electron energy . Thus, after numerous experiments with various QD structures, no true phonon bottleneck has been found [3–5].
Another possibility to suppress photoelectron relaxation and to increase a photoelectron lifetime is related with the interdot kinetics. In theoretical works [6–9], we proposed to suppress the capture processes by means of potential barriers in specially engineered QD structures. Potential barriers are always created, when electrons populating the dots are taken from the specific areas located relatively far from the dots. Changing the position of dopants and doping level, one can manage the potential barriers around dots and control the photoelectron capture processes [8, 9].
In this work we present direct experimental demonstration of strong effects of potential barriers on photoresponse of QD structures. The paper is organized in the following way. In the next section we describe the fabrication of the QD structures with different positions of dopants and various doping levels. After that we present results of measurements of the photoresponse and noise characteristics of these structures. Then, using the nextnano3 software we calculate the dot population and potential barriers around dots as a function of dopant positions and doping levels. Finally, we discuss the photoresponse enhancement in terms of potential barriers around QDs.
Several standard quantum dot infrared photodetector (QDIP) structures have been grown by the Riber Compact 21 MBE (molecular beam epitaxy) in AlGaAs matrix materials. For a given amount of In, adding aluminum to the nucleation layer and the surrounding matrix decreases the surface mobility of In and, thus, increases the dot density and decreases the dot size, when compared to InAs QDs grown onto GaAs surfaces and embedded in the binary compound GaAs only. In addition to the dot size and density distribution, an increase in the aluminum concentration also increases the conduction band offset. This shifts the center energy (decreases the wavelength) of the detector from the THz to the MID-IR region. In addition, the dark current is further reduced by increasing the aluminum concentration of the matrix material.
Growth temperatures on the substrate surface are monitored by infrared pyrometer. The surface temperature affects the density, quality, and size of the quantum dots that are formed and has to be constant, reproducible, and adjustable to control the QD formation. Typical growth temperatures are 500 ± 10°C as read by the pyrometer.
Dopant concentration (×1011 cm-2)
Number of electrons per QD
Barrier height (meV)
Middle of AlGaAs layers
Middle of AlGaAs layers
Middle of AlGaAs layers
InAs dots grown on GaAs and AlGaAs surfaces form in approximately 2.2 monolayers of InAs growth. During the normal growth of layers, the substrate is rotated at 30 RPMs to insure uniform thickness of layers.
The shown images are taken at different positions on a 3-inch wafer with respect to the indium effusion cell. The image size is 3 × 3 μm per image; the distance between images is half an inch, moving closer to the indium cell by going from left to right and from top to the bottom. Further adjustments to growth rates and times allow the properties of the quantum dots to be adjusted. The size of the dots and the matrix material used to embed the QDs determine the wavelength at which the QDIPs operate.
The vertical QDIP structure was processed by standard optical lithography, etching, and metallization techniques. To investigate the electron transport in QDIP structures, square 100 × 100 μm2 mesas with alloyed Ni/Ge/Au/Ni/Au high-quality ohmic contacts were formed. Top and bottom contacts were deposited on the highly Si-doped GaAs layers and followed by rapid thermal annealing at 430°C for 40 s. Positive bias polarity corresponds to a positive voltage, which is applied to the top contacts.
Photoresponse and Dark Current
For low-temperature optical measurements, each sample was mounted inside a helium continuous-flow cryostat. The current–voltage characteristics were recorded with Keithley 2602 Multimeter. Our measurements have been done with 10-7 A/cm2 accuracy. The spectral response of our QDIP structures was measured using a Bruker Optics Vertex 70 Fourier transform infrared (FTIR) spectrometer and a low noise current mode 7265 DSP Lock-in Amplifier.
We also found that the dark current density for sample B46 is higher by two orders of magnitude than that for samples B44 and B45. The sample B46 with the modulation doping has the highest dark current density and the lowest gain. The sample B44 with the silicon Delta doping just before InAs QD layer and the sample B45 with Si doping directly in the middle of each AlGaAs barrier layer show almost the same values of the dark currents.
We investigated the photocurrent in our samples as a function of optical pumping. In these measurements, we used a red light-emitting diode (LED) as a source of the optical radiation. The corresponding energy of photons, ~2 eV, is higher than QDIP's intersubband transitions and InAs energy gap. The energy of IR photoexcitations was tuned to the resonance absorption, i.e. to 380 meV in accordance with the data in Figure 3. The optical power of LED was calibrated by using a silicon power sensor.
Modeling: Potential Barriers in QD Structures
The analysis of optical and electrical properties of the grown samples was done using nextnano3 software . This versatile software allows for simulation of multilayer structures combined of different materials with realistic geometries in three dimensions. The simulation tool solves self-consistently Schrödinger, Poisson, and current equations. The conduction and valence bands are defined within single-band or multiband k.p envelope. In this modeling, we used well-established material parameters for the simulated structures: the effective mass of electron, m*, in γ-valley of GaAs is 0.067 and in Al0.22Ga0.78As, m*, is 0.085. Effects of strain [11, 12] were included in simulations.
Using this software, the three-dimensional bandstructures of the grown samples were obtained. Analyzing two-dimensional slices of conduction band profiles, the heights of potential barriers that divide neighboring QDs were defined.
In this work we investigated the effects of the potential barriers around QDs on the photoresponse of QDIPs. We found that in accordance with our theoretical conclusions [6–9], the potential barriers substantially suppress photoelectron capture and enhance the photoresponse.
This work was supported by AFOSR, the research of Antipov was also supported by NSF under Grant No DMR 0907126.
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