In this study, bias mediated tuning of the detection wavelength within the infrared wavelength region is demonstrated for quantum dots-in-a-well (DWELL) infrared photodetectors. In DWELL structures, intersubband transitions in the conduction band occur from a discrete state in the quantum dot to a subband in the quantum well. Compared to "conventional" quantum dot infrared photodetectors, where the transitions take place between different discrete bands in the quantum dots, new possibilities to tune the detection wavelength window are opened up, partly by varying the quantum dot energy levels and partly by adjusting the width and composition of the quantum well. In the DWELL structure used, an asymmetric positioning of the InAs quantum dot layer in a 8 nm wide In0.15Ga0.85As/GaAs QW has been applied which enables tuning of the peak detection wavelength within the long wavelength infrared (LWIR; 8 - 14 µm) region. When the applied bias was reversed, a wavelength shift from 8.5 to 9.5 µm was observed for the peak position in the spectral response. For another DWELL structure, with a well width of 2 nm, the tuning range of the detector could be shifted from the medium wavelength infrared (MWIR; 3-5 µm) region to the LWIR region. With small changes in the applied bias, the peak detection wavelength could be shifted from 5.1 to 8 µm. These tuning properties of DWELL structures could be essential for applications such as modulators and two-colour infrared detection.
We report on a quantum dots-in-a-well infrared photodetector (DWELL QDIP) grown by metal organic vapor phase epitaxy. The DWELL QDIP consisted of ten stacked InAs/In0.15Ga0.85As/GaAs QD layers embedded between n-doped contact layers. The density of the QDs was about 9 x 1010 cm-2 per QD layer. The energy level structure of the DWELL was revealed by optical measurements of interband transitions, and from a comparison with this energy level scheme the origin of the photocurrent peaks could be identified. The main intersubband transition contributing to the photocurrent was associated with the quantum dot ground state to the quantum well excited state transition. The performance of the DWELL QDIPs was evaluated regarding responsivity and dark current for temperatures between 15 K and 77 K. The photocurrent spectrum was dominated by a LWIR peak, with a peak wavelength at 8.4 μm and a full width at half maximum (FWHM) of 1.1 μm. At an operating temperature of 65 K, the peak responsivity was 30 mA/W at an applied bias of 4 V and the dark current was 1.2×10-5 A/cm2. Wavelength tuning from 8.4 μm to 9.5 μm was demonstrated, by reversing the bias of the detector.
Single quantum dots (QDs), based on the InAs/GaAs material system have been characterized by micro-photoluminescence (μPL). The self-organized quantum dots studied are fabricated by the Stransky-Krastanov method, taking advantage of the strain caused by the lattice mismatch between InAs and GaAs. Well-defined narrow excitonic features from individual QDs are monitored in the μPL spectra, upon single or dual tunable laser excitation. The charge state of the quantum dot is revealed from these excitonic lines in the μPL spectra. However, by tuning the laser excitation energy, it is demonstrated that the charge state of the dot can be altered: The distribution of neutral and charged excitons is demonstrated to be extremely sensitive on the laser energy. In addition, with an additional infrared laser, striking changes are induced in the μPL spectra. The results achieved demonstrate the existence of two well-defined excitation energy regions for the main laser, in which the presence of the infrared laser will decrease or increase, respectively, the integrated dot μPL intensity. For excitation above the critical threshold energy of the main laser, the addition of the infrared laser will induce a considerable increase, by up to a factor 5, in the QD emission intensity. At laser excitation below the threshold energy, on the other hand, the QD emission intensity will decrease. This fact is due to reduced carrier capture efficiency into the dot as determined by the internal electric field driven carrier transport. In order to get further insight into the carrier capture process due to the electric field in the vicinity of the QD, the dots have also been subjected to an external electric field
In most optical experiments with QDs, electrically injected or photoexcited carriers are primarily created somewhere in the sample outside the QDs, e.g. in the barriers or in the wetting layer. Consequently, excited carriers undergo a transport in the wetting layer and/or barriers prior to the capture into the QDs. This circumstance highlights the crucial role of the carrier transport and capture processes into the dot for the performance and operation of the dot based devices such as QD lasers, QD infrared detectors and QD memory devices. This transport effect on the optical response of the quantum dots has been investigated by subjecting the carriers to an external electric field in μPL measurements. This external field is formed by application of a lateral field between two top contacts. It is demonstrated that the QD PL signal intensity could be increased several times (>5 times) by optimizing the magnitude of this external field.
We present a study of the radiative recombination in In0.15Ga0.85N/GaN multiple quantum well samples, where the conditions of growth of the InGaN quantum layers were varied in terms of growth temperature and donor doping. The photoluminescence peak position varies strongly (over a range as large as 0.3 eV) with excitation intensity, with donor doping as well as with delay time after pulsed excitation. The peak position is mainly determined by the Stark effect induced by the piezoelectric field. In addition potential fluctuations play an important role, and largely determine the width of the emission. These potential fluctuations may be as large as 0.2 eV in the present samples. Screening effects from donor electrons and excited electron-hole pairs are important, and account for a large part of the spectral shift with donor doping, with excitation intensity and with delay time after pulsed excitation (shifts up to 0.2 eV). We suggest a dominant role of 2D electron- and donor screening in this case, predicting that rather strong localization potentials of short range (of the order 100 angstroms) are present. The possibility that excitons as well as shallow donors are impact ionized by electrons in these rather strong lateral potential fluctuations present at this In composition is discussed in connection with the long decay times observed at all temperatures.
This paper will examine the effect of quantum well confinement on a variety of radiative processes such as impurity binding energies excitonic recombination (both free excitons and bound excitons) and other phenomena observed in bulk material and now being investigated in low dimensional quantum structures. Emphasis will be on one-dimensional confinement provided by quantum wells. Extension to lower dimensional structures will be discussed.