20 research outputs found
Annual Report 2009 - Institute of Ion Beam Physics and Materials Research
The Institute of Ion Beam Physics and Materials Research (IIM) is one of the six institutes of the Forschungszentrum Dresden-Rossendorf (FZD), and contributes the largest part to its Research Program \"Advanced Materials\", mainly in the fields of semiconductor physics and materials research using ion beams. The institute operates a national and international Ion Beam Center, which, in addition to its own scientific activities, makes available fast ion technologies to universities, other research institutes, and industry. Parts of its activities are also dedicated to exploit the infrared/THz free-electron laser at the 40 MeV superconducting electron accelerator ELBE for condensed matter research. For both facilities the institute holds EU grants for funding access of external users
p-TypeE InAs/GaAs Quantum Dot, Dot-In-Well, and Low-Frequency Noise Properties of Infrared Photodetectors
Several types of p-doped Infrared detectors were studied. These include InAs/GaAs quantum dot (QDIP), and dots-in-well (DWELL) and split off band-based heterojunction detectors. In these structures, IR absorption leading to detection is based on valence-band inter-sublevel hole transitions. For a QDIP and DWELL, at 80 K, two response bands observed at 1.5 â 3 and 3 â 10 ”m were identified as due to optical transitions from the heavy hole to spinâorbit split-off QD level and from the heavy-hole to heavy/light-hole level, respectively. Unlike the n-type with bias dependent spectral response, the p-type hole response displays a well-preserved spectral profile (independent of the applied bias) observed in both QDIP and DWELL detectors. At a response peak of ~ 5.2 ”m, QDIP and DWELL exhibit an external quantum efficiency of 17 % and 9 % respectively. At elevated temperatures between 100 and ~120 K (for QDIP), 130 K (for DWELL), both QDIP and DWELL detectors exhibit a strong far-infrared or terahertz (THz) response up to 70 ”m which show promising potential of p-type QDs for developing THz infrared photodetectors.
Based on the dark current and noise power spectral density analysis, structural parameters such as the numbers of active layers, the surface density of QDs, and the carrier capture or relaxation rate, type of material and electric field are some of the optimization parameters identified to improve the photoconductive and dark current gain of detectors. The capture probability of DWELL is found to be more than two times higher than the corresponding QDIP. Based on the noise analysis, QDs based structures suppressed phonon scattering and enhanced carrier life time or photoconductive gain. Furthermore, in a GaAs/AlxGa1-xAs heterostructure, for a given width of AxlGa1-xAs barrier, the barrier thickness can be varied by varying the Al mole fraction x, which is referred to as a graded barrier. Grading the barrier and optimizing the emitter thickness of GaAs/AlGaAs heterostructures enhance the absorption efficiency, the escape probability and lower the dark current; hence, enhances the responsivity and specific detectivity of detectors.
The two important methods (Arrhenius plot and Temperature Dependent Internal photoemission (TDIPS)) of determining detectors threshold wavelengths or band offsets were compared. For detectors with long threshold wavelength (\u3e\u3e 9.3 ÎŒm), the Arrhenius plot used to extract activation energy leads to energy values with deviation higher than ~ 10 % from the corresponding TDIPS values and results from the temperature dependent Fermi distribution tailing effect and FowlerâNordheim tunneling current. Therefore, TDIPS or other methods, that take the temperature effects on the band offset and FowlerâNordheim tunneling current into account, are needed for a precise band offset characterization of a long threshold wavelength detectors
Heterostructure engineering of quantum dots-in-a-well infrared photodetectors
Three of the most important characteristics of third-generation imaging systems are high operating temperature, multispectral operation, and large format arrays. The quantum dot infrared photodetector technology, owing to the three-dimensional confinement of carriers, the richness of the electronic spectra in quantum dots, and the mature III-V based fabrication technology, satisfy these requirements. This work focuses on quantum dots-in-a-well (DWELL) detectors in which InAs quantum dots are embedded in a compressively strained InGaAs-GaAs quantum well. Barriers separating two stacks of quantum dots can be GaAs, AlGaAs or a combination of different materials, with \u27smart barriers\u27. Motivation for this work is to improve the understanding and the performance of DWELL detectors to achieve high temperature operation and high signal to noise ratio for these detectors for given wavelength requirements, at applied biases compatible with CMOS technology. This aim has been pursued on three fronts: barrier designs, device designs and material systems. Smart barriers, such as resonant tunneling barriers have been demonstrated to improve the signal to noise ratio of the detector by reducing the dark current significantly, while keeping the photocurrent constant. A systematic experimental study has been conducted for understanding the effect of different types of transitions on the properties of DWELL detectors, which showed that bound to quasibound (B-Q) type of transitions optimize the device performance at moderate bias levels. The performance of B-Q type of architectures has been substantially improved by the use of confinement enhancing (CE) barriers that combine the advantages of high energy barriers, such as low dark current and high signal to noise ratio, with those of low energy barriers, such as high responsivity and longer peak wavelengths at low bias operation. A new type of detector, a quantum dot based quantum cascade detector, has been proposed and implemented. QD-QCD exhibits a strong photovoltaic action, leading to strong performance at zero bias, by the virtue of internal electric field generated by the quantum cascade action in the barrier. The zero bias operation, combined with record low photoconductive gains for any quantum dot detectors, makes QD QCD very attractive for focal plane array applications. For improved understanding, theoretical modeling of quantum dot strain, based on atomistic valence force field method as well as transport simulations of general heterostructure detectors with drift-diffusion model have been developed. The transport simulation results indicate the presence of a strong space charge region forming between the highly n-doped contact regions and non-intentionally doped barrier regions, which makes the internal electric field highly nonlinear in space. This has been verified by systematic experiments, in which effects of this electric field nonlinearity on the device parameters have been studied. This work would enable a device designer to choose different device parameters such as spectral response position and shape, photoconductive gain, response, signal to noise ratio, dark current levels, activation energies etc. This knowledge has been utilized in demonstrating highly sensitive FPAs, as well as high operating temperature imaging (at 140K) with DWELL detectors. State of the art performance has been obtained from different devices at different wavelengths, such as such as a detectivity of 4x1011 cm.Hz1/2W-1 at 77K in a bound to quasibound device with a cutoff wavelength of 8.5 ÎŒm, which is higher than that obtained from state of the art QWIPs. Although the dark current levels are substantially lower than standard QWIPs, and background limited photodetection is at much higher temperature, the focal plane array sensitivities are lower than those of the state of the art QWIPs, by around 10 mK, due to lower quantum efficiency (a factor of 2-3) and higher photoconductive gain. This difference can be eliminated by the use of gratings or shape engineering through the use of submonolayer quantum dots and with smaller photoconductive gains with DWELL detectors
Semiconductor Quantum Structures for Ultraviolet-to-Infrared Multi-Band Radiation Detection
In this work, multi-band (multi-color) detector structures considering different semiconductor device concepts and architectures are presented. Results on detectors operating in ultraviolet-to-infrared regions (UV-to-IR) are discussed. Multi-band detectors are based on quantum dot (QD) structures; which include quantum-dots-in-a-well (DWELL), tunneling quantum dot infrared photodetectors (T-QDIPs), and bi-layer quantum dot infrared photodetectors (Bi-QDIPs); and homo-/heterojunction interfacial workfunction internal photoemission (HIWIP/HEIWIP) structures. QD-based detectors show multi-color characteristics in mid- and far-infrared (MIR/FIR) regions, where as HIWIP/HEIWIP detectors show responses in UV or near-infrared (NIR) regions, and MIR-to-FIR regions. In DWELL structures, InAs QDs are placed in an InGaAs/GaAs quantum well (QW) to introduce photon induced electronic transitions from energy states in the QD to that in QW, leading to multi-color response peaks. One of the DWELL detectors shows response peaks at ⌠6.25, ⌠10.5 and ⌠23.3 ”m. In T-QDIP structures, photoexcited carriers are selectively collected from InGaAs QDs through resonant tunneling, while the dark current is blocked using AlGaAs/InGaAsAlGaAs/ blocking barriers placed in the structure. A two-color T-QDIP with photoresponse peaks at 6 and 17 ”m operating at room temperature and a 6 THz detector operating at 150 K are presented. Bi-QDIPs consist of two layers of InAs QDs with different QD sizes. The detector exhibits three distinct peaks at 5.6, 8.0, and 23.0 ”m. A typical HIWIP/HEIWIP detector structure consists of a single (or series of) doped emitter(s) and undoped barrier(s), which are placed between two highly doped contact layers. The dual-band response arises from interband transitions of carriers in the undoped barrier and intraband transitions in the doped emitter. Two HIWIP detectors, p-GaAs/GaAs and p-Si/Si, showing interband responses with wavelength thresholds at 0.82 and 1.05 ”m, and intraband responses with zero response thresholds at 70 and 32 ”m, respectively, are presented. HEIWIP detectors based on n-GaN/AlGaN show an interband response in the UV region and intraband response in the 2-14 ”m region. A GaN/AlGaN detector structure consisting of three electrical contacts for separate UV and IR active regions is proposed for simultaneous measurements of the two components of the photocurrent generated by UV and IR radiation
Scanning near-field infrared microspectroscopy on semiconductor structures
Near-field optical microscopy has attracted remarkable attention, as it is the only technique that allows the investigation of local optical properties with a resolution far below the diffraction limit. Especially, the scattering-type near-field optical microscopy allows the nondestructive examination of surfaces without restrictions to the applicable wavelengths. However, its usability is limited by the availability of appropriate light sources. In the context of this work, this limit was overcome by the development of a scattering-type near-field microscope that uses a widely tunable free-electron laser as primary light source.
In the theoretical part, it is shown that an optical near-field contrast can be expected when materials with different dielectric functions are combined. It is derived that these differences yield different scattering cross-sections for the coupled system of the probe and the sample. Those cross-sections define the strength of the near-field signal that can be measured for different materials. Hence, an optical contrast can be expected, when different scattering cross-sections are probed. This principle also applies to vertically stacked or even buried materials, as shown in this thesis experimentally for two sample systems.
In the first example, the different dielectric functions were obtained by locally changing the carrier concentration in silicon by the implantation of boron. It is shown that the concentration of free charge-carriers can be deduced from the near-field contrast between implanted and pure silicon. For this purpose, two different experimental approaches were used, a non-interferometric one by using variable wavelengths and an interferometric one with a fixed wavelength. As those techniques yield complementary information, they can be used to quantitatively determine the effective carrier concentration. Both approaches yield consistent results for the carrier concentration, which excellently agrees with predictions from literature. While the structures of the first system were in the micrometer regime, the capability to probe buried nanostructures is demonstrated at a sample of indium arsenide quantum dots. Those dots are covered by a thick layer of gallium arsenide. For the first time ever, it is shown experimentally that transitions between electron states in single quantum dots can be investigated by near-field microscopy. By monitoring the near-field response of these quantum dots while scanning the wavelength of the incident light beam, it was possible to obtain characteristic near-field signatures of single dots. Near-field contrasts up to 30 % could be measured for resonant excitation of electrons in the conduction band of the indium arsenide dots
Scanning near-field infrared microspectroscopy on semiconductor structures
Near-field optical microscopy has attracted remarkable attention, as it is the only technique that allows the investigation of local optical properties with a resolution far below the diffraction limit. Especially, the scattering-type near-field optical microscopy allows the nondestructive examination of surfaces without restrictions to the applicable wavelengths. However, its usability is limited by the availability of appropriate light sources. In the context of this work, this limit was overcome by the development of a scattering-type near-field microscope that uses a widely tunable free-electron laser as primary light source.
In the theoretical part, it is shown that an optical near-field contrast can be expected when materials with different dielectric functions are combined. It is derived that these differences yield different scattering cross-sections for the coupled system of the probe and the sample. Those cross-sections define the strength of the near-field signal that can be measured for different materials. Hence, an optical contrast can be expected, when different scattering cross-sections are probed. This principle also applies to vertically stacked or even buried materials, as shown in this thesis experimentally for two sample systems.
In the first example, the different dielectric functions were obtained by locally changing the carrier concentration in silicon by the implantation of boron. It is shown that the concentration of free charge-carriers can be deduced from the near-field contrast between implanted and pure silicon. For this purpose, two different experimental approaches were used, a non-interferometric one by using variable wavelengths and an interferometric one with a fixed wavelength. As those techniques yield complementary information, they can be used to quantitatively determine the effective carrier concentration. Both approaches yield consistent results for the carrier concentration, which excellently agrees with predictions from literature.
While the structures of the first system were in the micrometer regime, the capability to probe buried nanostructures is demonstrated at a sample of indium arsenide quantum dots. Those dots are covered by a thick layer of gallium arsenide. For the first time ever, it is shown experimentally that transitions between electron states in single quantum dots can be investigated by near-field microscopy. By monitoring the near-field response of these quantum dots while scanning the wavelength of the incident light beam, it was possible to obtain characteristic near-field signatures of single dots. Near-field contrasts up to 30 % could be measured for resonant excitation of electrons in the conduction band of the indium arsenide dots.Die optische Nahfeldmikroskopie hat viel Beachtung auf sich gezogen, da sie die einzige Technologie ist, welche die Untersuchung lokaler optischer Eigenschaften mit Auflösungen unterhalb der Beugungsgrenze ermöglicht. Speziell die streuende Nahfeldmikroskopie erlaubt die zerstörungsfreie Untersuchung von OberflĂ€chen ohne EinschrĂ€nkung der verwendbaren WellenlĂ€ngen. Die Nutzung ist jedoch durch das Vorhandensein entsprechender Lichtquellen beschrĂ€nkt. Im Rahmen dieser Arbeit wurde diese BeschrĂ€nkung durch Entwicklung eines streuenden Nahfeldmikroskops ĂŒberwunden, das einen weit stimmbaren Freie-Elektronen-Laser als primĂ€re Lichtquelle benutzt.
Im theoretischen Teil wird gezeigt, dass ein optischer Kontrast erwartet werden kann, wenn Materialien mit unterschiedlichen DielektrizitĂ€tskonstanten kombiniert werden. Es wird hergeleitet, dass diese Unterschiede in unterschiedlichen Streuquerschnitten fĂŒr das gekoppelte System aus Messkopf und Probe resultieren. Diese Streuquerschnitte definieren die StĂ€rke des Nahfeldsignals, welches auf unterschiedlichen Materialien gemessen werden kann. Ein optischer Kontrast kann also erwartet werden, wenn unterschiedliche Streuquerschnitte untersucht werden. Dass dieses Prinzip auch auf ĂŒbereinander geschichtete oder sogar verborgene Strukturen angewendet werden kann, wird in dieser Doktorarbeit an zwei Probensystemen experimentell gezeigt.
Im ersten Beispiel wurden die unterschiedlichen DielektrizitĂ€tskonstanten durch örtliches Ăndern der LadungstrĂ€gerdichte in Silizium durch Bor-Implantation erreicht. Es wird gezeigt, dass die Dichte der freien LadungstrĂ€ger an Hand des optischen Kontrastes zwischen implantiertem und reinem Silizium ermittelt werden kann. Zu diesem Zweck wurden zwei unterschiedliche AnsĂ€tze verwendet, ein nicht-interferometrischer mittels variabler WellenlĂ€ngen und ein interferometrischer mit einer konstanten WellenlĂ€nge. Weil diese Techniken gegensĂ€tzliche Informationen liefern, können sie genutzt werden, um die effektive LadungstrĂ€gerdichte quantitativ zu bestimmen. Beide AnsĂ€tze lieferten konsistente Resultate fĂŒr die TrĂ€gerdichte, welche sehr gut mit den Vorhersagen der Literatur ĂŒbereinstimmt.
WĂ€hrend die Strukturen im ersten Beispiel im Mikrometer-Bereich lagen, wird die Möglichkeit, verborgene Nanostrukturen zu untersuchen, an Hand einer Probe mit Indiumarsenid Quantenpunkten demonstriert. Diese sind von einer dicken Schicht Galliumarsenid bedeckt. Zum ersten Mal wird experimentell gezeigt, dass ĂbergĂ€nge zwischen ElektronenzustĂ€nden in einzelnen Quantenpunkten mit Nahfeldmikroskopie untersucht werden können. Durch die Messung der Nahfeld-Antwort der Quantenpunkte unter Ănderung der WellenlĂ€nge des eingestrahlten Lichtes war es möglich, charakteristische Nahfeld-Signaturen der einzelnen Quantenpunkte zu erhalten. Nahfeld-Kontraste bis zu 30 Prozent konnten fĂŒr die resonante Anregung der Elektronen im Leitungsband der Indiumarsenid Punkte beobachtet werden
Annual Report 2012 - Institute of Ion Beam Physics and Materials Research
In 2012 the HZDR, and in consequence also the Institute of Ion Beam Physics and Materials Research (IIM) including its Ion Beam Center (IBC), has undergone a scientific evaluation. The evaluation committee composed of the Scientific Advisory Board and numerous external experts in our field of research concluded that âthe overall quality of the scientific work is excellentâ, that âthere are an impressive number of young scientists working enthusiastically on a variety of high-level projectsâ and that âthe choice of these projects represents a clear underlying strategy and visionâ. We feel honored and are proud that the external view on our scientific achievements is that extraordinary. In view of this outstanding result we would like to express our gratitude to all our staff members for their commitment and efforts!
In the past year, we continued our integration into the Helmholtz Association of German Research Centers (HGF) with our Institute mostly active in the research area âMatterâ, but also involved in a number of activities in the research area âEnergyâ. In this respect, many consultations were held with the Helmholtz centers contributing to common research areas to precisely define the role we will play in the newly established HGF program âFrom Matter to Materials and Lifeâ (see schematic below). Our IBC has been recognized as a large-scale user facility for ion beam analysis and modification of materials, i.e., specializing on materials science. In particular, the IBC plays a prominent role in the recently approved Helmholtz Energy Materials Characterization Platform (HEMCP), which mainly concentrates on the development of dedicated analytical tools for the characterization of materials required for future energy technologies. The successes achieved by the IBC allows us to invest 7200 k⏠to further improve and strengthen the ion beam capabilities at the Institute. In addition to this infrastructure-related grant, we were also successful in our funding application for the establishment of the International Helmholtz Research School for Nanoelectronic Networks (IHRS NANONET), aiming at promoting the next generation of leading scientists in the field of nanoelectronics. The IHRS NANONET is coordinated by our Institute and offers a well-structured PhD program to outstanding students of all nationalities with emphasis on interdisciplinary research and comprehensive training in technical and professional skills
Sensing and Imaging using Laser Feedback Interferometry with Quantum Cascade Lasers
Quantum cascade lasers (QCLs) are high-power sources of coherent radiation in the midinfrared and terahertz (THz) bands. Laser feedback interferometry (LFI) is one of the simplest coherent techniques, for which the emission source can also play the role of a highly-sensitive detector. The combination of QCLs and LFI is particularly attractive for sensing applications, notably in the THz band where it provides a high-speed high-sensitivity detection mechanism which inherently suppresses unwanted background radiation. LFI with QCLs has been demonstrated for a wide range of applications, including the measurement of internal laser characteristics, trace gas detection, materials analysis, biomedical imaging, and near-field imaging. This article provides an overview of the QCLs and the LFI technique, and reviews the state of the art in LFI sensing using QCLs
Annual Report 2013 - Institute of Ion Beam Physics and Materials Research
The year 2013 was the third year of HZDR as a member of the Helmholtz Association (HGF), and we have made progress of integrating ourselves into this research environment of national Research centers. In particular, we were preparing for the evaluation in the framework of the so-called program oriented funding (POF), which will hopefully provide us with a stable funding for the next five years (2015 â 2019). In particular, last fall we have submitted a large proposal in collaboration with several other research centers. The actual evaluation will take place this spring. Most of our activities are assigned to the program âFrom Matter to Materials and Lifeâ (within the research area âMatterâ). A large fraction of this program is related to the operation of large-scale research infrastructures (or user facilities), one of which is our Ion Beam Center (IBC). The second large part of our research is labelled âin-house researchâ, reflecting the work driven through our researchers without external users, but still mostly utilizing our large-scale facilities such as the IBC, and, to a lesser extent, the free-electron laser. Our in-house research is performed in three so-called research themes, as depicted in the schematic below. What is missing there for simplicity is a small part of our activities in the program âNuclear Waste Management and Safetyâ (within the research area âEnergyâ)
Recommended from our members
Laterally confined THz sources and graphene based THz optics
The region between the infrared and microwave region in the electromagnetic spectrum, the Terahertz (THz) gap, provides an exciting opportunity for future wireless communications as this band has been under utilised. This doctoral work takes a two-pronged approach into closing the THz gap with low-dimensional materials. The first attempt addresses the need for a compact THz source that can operate at room temperature. The second approach addresses the need to build optical elements such as filters and modulators in the THz spectrum.
Terahertz quantum cascade lasers (THz QCLs) are one of the most compact, powerful sources of coherent radiation that bridge the terahertz gap. However, their cryogenic requirements for operation limit the scope of the applications. This is because of the electron-electron scattering and heating of the 2-dimensional free electron gas which leads to significant optical phonon scattering of the hot electrons. Theoretical studies in laterally confined QCL structures have predicted enhanced lifetime of the upper state through suppression of the non-radiative intersubband relaxation of carriers, which leads to lower threshold, and higher temperature performance. Lithographically defined vertical nanopillar arrays with electrostatic radius less than tens of nm offer a possible route to achieve lateral confinement, which can be integrated into QCL structures. A typical gain medium in a QCL consists of at least 100 repeat periods, with a thickness of 6-14 micron. For practical implementation of the top-down approach, restrictions are imposed by aspect ratios that can be achieved in present dry-etching systems. Typically, for sub-200 nm radius pillars, the thickness ranges from 1-3.5 micron. It is therefore necessary to work with THz QCLs based on 3-4 quantum well active regions, so as to maximise the number of repeat periods (hence gain) within an ultra-thin active region.
After an introductory chapter, Chapter 2 presents a theoretical treatise on the realistic electrostatic potential in a lithographically defined nanopillar by scaling from a single quantum well (resonant tunnelling diode) to a THz QCL. Chapter 2 also discusses, the effect of lateral confinement on the intersubband states and the plasmonic mode in a THz QCL. One of the key experimental challenges in scaling down from QCLs to quantum-dot cascade lasers is the electrical injection into the nanopillars. This involves insulation and planarisation of the high aspect-ratio nanopillar arrays. Furthermore, the choice of the planarising layer is critical since it determines the loss of any optical mode. This experimental challenge is solved in Chapter 3. Chapter 4 presents the electro-optic performance of low-repeat period QCLs with an active region thickness that is less than 3.5 micron.
Another topic of recent interest in the THz optics community is plasmonics in graphene. This is because the bound electromagnetic modes (plasmons) are tightly confined to the surface and can also be tuned with carrier concentration. Plasmonic resonance at terahertz frequencies can be achieved by gating graphene grown via chemical vapour
deposition (CVD) to a high carrier concentration. THz time domain spectroscopy of such gated monolayer graphene shows resonance features around 1.6 THz superimposed on the Drude-like frequency response of graphene which may be related to the inherent poly-crystallinity of CVD graphene. Chapter 5 discusses these results, as an understanding of these features is necessary for the development of future THz optical elements based on CVD graphene. Chapter 5 finally describes how the gate tunability of THz transmission through graphene can be exploited to indirectly modulate a THz QCL.
Chapter 6 presents ideas from this doctoral work, which can be developed in future to address the issues of enhanced temperature performance of THz QCLs and to realise realistic THz devices based on graphene