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We demonstrate the suppression of the bulk generationrecombination current in nBn devices based on an InAsSb active layer (AL) and a AlSbAs barrier layer (BL). This leads to much lower dark currents than in conventional InAsSb photodiodes operating at the same temperature. When the BL is p-type, very high doping must be used in the AL (nB p n +). This results in a significant shortening of the device cutoff wavelength due to the Moss-Burstein effect. For an n-type BL, low AL doping can be used (nB n n), yielding a cutoff wavelength of ∼4.1 μm and a dark current close to ∼3 × 10 − 7 A/cm 2 at 150 K. Such a device with a 4-μm-thick AL will exhibit a quantum efficiency (QE) of 70% and background-limited performance operation up to 160 K at f/3. We have made nB n n focal plane array detectors (FPAs) with a 320 × 256 format and a 1.3-μm-thick AL. These FPAs have a 35% QE and a noise equivalent temperature difference of 16 mK at 150 K and f/3. The high performance of our nB n n detectors is closely related to the high quality of the molecular beam epitaxy grown InAsSb AL material. On the basis of the temperature dependence of the diffusion limited dark current, we estimate a minority carrier lifetime of ∼670 ns. C 2011 Society of Photo-Optical Instrumentation Engineers (SPIE).

We study the electronic coupling between two vertically stacked InAs quantum dots, which are embedded in the center of a n-i-n structure. We use a micro-photoluminescence setup to optically isolate a single quantum dot pair and measure the time-averaged photoluminescence under an applied vertical electric field. We find that field tunable coupling between excited states of the two quantum dots leads to charge transfer from one dot to the other. We model the spectra including simultaneously the field dependent charge transfer and exciton capture rates, and the many-body spectra of the quantum dot molecule for different carrier configurations. PACS: 73.21.La, 78.55.Cr, 78.67.Hc The emerging field of quantum computation has attracted great interest over the last few years [1]. Various theoretical schemes were proposed for the implementation of quantum bits (qubits) and quantum gates, using semiconductor quantum dots (QDs) [2]. Specifically, the vertically stacked double QD system was suggested to host a single [3,4] or two qubits [5]. One can then control the coherent two-level system (qubit) with short optical pulses [3,5], by an applied electric field [4,6,7], or by a magnetic field [6]. Once the basic quantum operation in such a system is achieved, scaling up to high-density self-assembled ordered arrays of these units should be feasible [8]. A necessary step towards realization of a single qubit in a QD pair is to achieve electronic (wavefunction) coupling between the two dots. In recent years, several attempts have been made in this direction by comparing samples with different inter-dot spacing [9,10].Clearly, the coupling between the two QDs in the pair is highly sensitive to their relative energy levels. These energies are fixed for each QD by its dimensions and material composition, which are hard to control, especially in the technologically important type of self assembled QDs. However, by varying an electric field across the QD pair one can tune the electronic states of the two QDs into and out of resonance. This method allows one to investigate the electronic coupling between the two dots in a single, specific QD molecule, thus avoiding the difficulty of comparing different molecules from various samples with each other.In this work we study the photoluminescence (PL) spectra of two vertically stacked QDs as a function of excitation intensity and external electric field. We compare the spectra of a single dot (QD atom), an electronically uncoupled QD pair and a coupled QD molecule.For the QD molecule we find that the two dots have a large ground state energy difference and that coupling occurs between their excited states. By tuning the electric field across the molecule we control the transfer of charge between the two dots, which is revealed in the time-averaged emission spectra, in similarity to recent works on charged single QDs [11].

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