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I.3. Quantum Dot Infrared Photodetectors (QDIPs) and Other Device Demonstrations

Our interests and focus lie in demonstration of methodologies for, and realization of, the highest quality electronic and optoelectronic devices of the time. Our contributions cover:

(a) Quantum Dot Infrared Photodetectors (QDIPs) [Current Focus]

(b) Quantum Dot Lasers

(a) Quantum Dot Infrared Photodetectors (QDIPs):
Development of high performance infra-red (IR) detectors in the range of mid-wavelength IR (MWIR, 3-5 micron) and long-wavelength IR (LWIR, 8-12 micron) attract considerable interest for variety of fields, such as biomedical applications (e.g., cancer detection), military applications (e.g., intercontinental ballistic missile defense), and night vision, etc. Amongst a few types of IR detectors in these wavelength ranges, a QDIP has several advantages over other IR detectors (e.g., quantum well infrared photodetectors (QWIPs) or HgCdTe (MCT) detectors) because of unique electronic properties in QDs in which electrons are 3-dimensionally confined. For instance, QDs shows much higher absorption of the normal incident photons than QWs, and are expected to have higher quantum efficiency owing to long carrier lifetimes of excited electrons, which, in turn, will reduce dark current to enable the high temperature operation of IR detectors.


Figure 1 Schematic structure of InAs/InGaAs/GaAs QDIP.	Figure 2 Schematic operation principle of InAs/InGaAs/GaAs QDIP.

We have been one of the leading research groups in the field of QDIPs to have presented comprehensive studies of QDIP characteristics and its performances from structural aspects to device aspects on the strength of our accumulated knowledge of QD properties in terms of their growth kintics, structural properties, and optical properties under the collaboration with Prof. Joe Campbell's group (Univ. Virginia). We have shown one of the first demonstrations of normal incidence response of QDIP [26-28] with dot-in-well (DWELL) structure [27,28]. Utilizing our unique method to selectively manipulate a particular QD energy level [I.1.(b)(ii)77], voltage controllable two-color QDIP [29, 30, 32] and one of the best LWIR QDIP [I.1.(b)(ii)76, 33, 35] have been demonstrated, as well as we shed light on the interconnections between QD energy structures and QDIP device characteristics [26-35].


Figure 3 Demonstration of voltage tunable two-color photoresponse.	Figure 4 Energy levels of QDs in voltage tunable two-color QDIP.

           On top of the above extensive studies on QDIP, we are currently tackling on a remaining challenge in the field of QDIP and also in other QD devices, that is realization of rightly doped high quality (low defect density) multiple QD (MQD) structures. Since SAQDs are inherently strained, many number of QD layers give rise to accumulated strain in the structure, which eventually introduces dislocations and degrades QD optical properties and electronic properties of MQD structures. At the same time, as the MQD structure become thicker, doping condition needs to be refined accordingly to manipulate QD energy levels with respect to Fermi energy to optimize electron occupancy in QDs.

           So far we have created 20-layer of MQD structure with a minimal defect density by utilizing strain relief InGaAs layer and unimodal shallow distribution of InAs 3D islands. Figure 5 shows a cross-sectional TEM image (dark field, g = (220)) of 20 stacks of 2.0ML InAs QDs capped with 20ML InGaAs and 130ML GaAs. QD formation is clearly observed up to the 20th QD layer, and its density is constant throughout entire 20 layers. No dislocation is observed within the examined area. This indicates the dislocation density < 10⁷ /cm².

           This high quality 20-period QDIP shows much lower dark current and also lower photocurrent than a 10-period QDIP (Figure 6) by ~3 order of magnitude, which implies that lower electron density in MQD region of the 20-period QDIP. We have attributed this low electron density to its higher built-in potential in the band bending, which makes QD energy levels far above Fermi energy with respect to kBT, from the estimation of the QDs ionization energy and activation energy deduced from temperature dependent photocurrent and dark current. We have also studied that various doping conditions such as n-p-n structure or thicker spacer layers can reduce dark current significantly (by ~9 order of magnitude), which can be also understood from the energy level relationships between QD energies and Fermi energy.


Figure 5 Cross sectional TEM image of 20-period QDIP	Figure 6 Dark current of 10- and 20-period QDIP

(b) Self Assembled Quantum Dots:

We have demonstrated one of the first quantum dot lasers in 1996. For details, see [24].

Inverted HEMTs (high electron mobility transistors, 1988) [1], high power MISFETs (1992) [4,5], high peak-to-valley ratio and high peak current density RTDs (resonant tunneling diodes, 1990) [6,9], high contrast ratio normal and inverted-geometry multiple quantum well (MQW) spatial light modulators (SLMs 1989-91) [10,11,14,15,16,18,20] the first integrated RTD, FET, and SLM based opto-electronic switch (1992) [22], the first monolithically integrated heterojunction phototransistor and semiconductor spatial light modulator [16].

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