Terahertz detection with tunneling quantum dot intersublevel photodetector
X. H. Su, J. Yang, P. Bhattacharya, G. Ariyawansa, A. G. U. Perera
2006
Applied Physics Letters
The characteristics of a tunnel quantum dot intersublevel photodetector, designed for the absorption of terahertz radiation, are described. The absorption region consists of self-organized In 0.6 Al 0.4 As/ GaAs quantum dots with tailored electronic properties. Devices exhibit spectral response from 20 to 75 m ͑ϳ4 THz͒ with peak at ϳ50 m. The peak responsivity and specific detectivity of the device are 0.45 A / W and 10 8 cm Hz 1/2 / W, respectively, at 4.6 K for an applied bias of 1 V.
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... to terahertz radiation is observed up to 150 K. With the advent of suitable terahertz sources, 1,2 it is imperative to develop the detection technology in this spectral range. Like any other electromagnetic radiation, terahertz waves can be detected by coherent or incoherent techniques. Most coherent detection schemes utilize frequency conversion. [3] [4] [5] Examples are Schottky diode mixers, nonlinear optical crystals or coatings and gated photoconductive antennas, or switches. Coherent techniques generally provide good sensitivity, but require a high degree of sophistication and instrumentation. The simplest incoherent detectors are heat based, such as bolometers or those made with pyroelectric crystals. These are usually slow and are generally built for low temperature operation. Semiconductor and heterojunction-based schemes that have been characterized include doped Ge detectors, 6 which provide sensitivity at very low temperatures ͑ϳ4 K͒, photoconductive detectors triggered by femtosecond optical pulses, 7 heterojunction interfacial work function internal photoemission detectors, 8 and high-electron mobility transistors operating in the plasma-wave regime. 9 More recently, quantum-confinement based detectors 10 have generated interest. These employ heterostructures similar to quantum-well infrared photodetectors ͑QWIPs͒ and quantum cascade lasers. Quantum dot infrared photodetectors ͑QDIPs͒, consisting of a multilayered self-organized In͑Ga, Al͒As/ Ga͑Al͒As quantum dot active region, have emerged as a technology capable of detecting light across a broad range of infrared ͑IR͒ wavelengths. [11] [12] [13] [14] The advantages of QDIPs result from three-dimensional carrier confinement in quantum dots. The associated advantages include ͑i͒ intrinsic sensitivity to normal-incidence light, ͑ii͒ long lifetime of photoexcited electrons due to reduced electron-phonon scattering, and ͑iii͒ low dark current due to three-dimensional quantum confinement and reduced thermionic emission. We recently demonstrated a QDIP in which the dark and photocurrents were decoupled by the incorporation of a double-barrier resonant tunneling heterostructure with each quantum dot layer. The tunnel barrier resonantly transmitted the photoexcited electrons, but blocked most of the dark current composed of electrons with a broad energy distribution. The resulting device-the tunnel QDIP ͑Refs. 15 and 16͒-demonstrated far infrared ͑FIR͒ operation ͑ peak =17 m͒ at room temperature with ultralow dark current. In In͑Ga͒As/ GaAs quantum dots, the intersublevel energy spacings or the energy difference between the dot and continuum states is normally 40-60 meV, which corresponds to the mid-IR and FIR wavelength ranges. In fact, the longest cutoff wavelength reported for detection with QDIPs is less than 25 m. 13 Therefore, the dot heterostructure and/or the dot size need to be engineered for detection at longer wavelengths and in the terahertz range. In this letter we report the performance characteristics of tunnel QDIPs, incorporating In 0.6 Al 0.4 As/ GaAs self-organized quantum dots of reduced size in the active region, which exhibit spectral response with peak and cutoff wavelengths of 50 and 75 m ͑ϳ4.0 THz͒, respectively. The conduction band diagram of an In 0.6 Al 0.4 As/ GaAs quantum dot layer and the associated resonant tunnel heterostructure are shown in Fig. 1͑a͒ . A single 60 Å thick Al 0.1 Ga 0.9 As barrier is incorporated before each dot layer to form a quantum well with well-defined final states for the photoexcited electrons. The width of the well region and the composition of the barrier can be varied to tune the final states in resonance with the resonant state of the double barrier heterostructure. For detection of terahertz radiation, the energy spacing between the confined state in the dot and the quasibound states in the well has to be of the order of 10 meV or less. This transition is illustrated in Fig. 1͑a͒ . To achieve this, we have grown In 0.6 Al 0.4 As/ GaAs quantum dots in the active region of the devices, instead of the more conventional InAs dots. Incorporation of Al into the dot material serves two purposes. First, due to the larger band gap of InAlAs, compared to InAs, the bound state energies are closer to the GaAs barrier energy, and hence to the quasibound states in the well. Second, due to the smaller migration rate of Al adatoms on the growing surface during epitaxy, the Al-containing islands ͑dots͒ are smaller in size compared to InAs dots and the dot confined states are higher in energy. In this study, the dot size in the devices was also varied, by the growth parameters, to tune the absorption frequency. Finally, the density of In 0.6 Al 0.4 As dots ͑ϳ3 ϫ 10 11 cm −2 ͒ is generally an order of magnitude larger than a͒ FAX: ͑734͒763
doi:10.1063/1.2233808
fatcat:4qqwvtoocjftpckynwb3etf7ly