Contributed paper 101, 109, 117, 125, 129, 135, 142, 150, 164, 173, 188, 195, 203
The history of semiconductor
devices has been characterised
by a constant drive towards lower dimensions in order to increase
density, system functionality and performance. However, this is still
from being comparable with the performance of natural systems, such as
human brain. The challenges facing semiconductor technologies in the
will he to move towards miniaturisation.
The influence of this trend on the quantum sensing of infrared radiation is one example that is elaborated here. A new generation of infrared detectors has been developed by growing layers of different semiconductors with nanometer thicknesses. The resulted bandgap engineered semiconductor has superior performance compared to the bulk material. To enhance this technology further, we plan to move from quantum wells to quantum wire and quantum dots.
High quality fluoride and oxide single crystals for optical, piezoelectric, and other applications have been grown by advanced crystal growth techniques. Corquiriite- and Perovskite-type fluoride single crystals - LiCaAlF6, LiSrAlF6, KMgF3 and BaLiFi3 - have been grown for solid state ultraviolet laser applications, and as window materials for next generation optical lithography. La3Nb0.5Ga5.5O14 and La3Ta0.5Ga5.5O14 piezoelectric single crystals of size and quality comparable to La3Ga5SiO14 (langasite) have been produced. The piezoelectric and device properties of the crystals were investigated. A search for new langasite-type materials was also performed. Promising new structural materials, Al2O3/RE3Al5O12 (RE = rare earth) eutectic fibres, have been grown by the micro-pulling-down technique. Undoped and doped b-Ga2O3 single crystals have been grown by the floating zone technique as promising transparent conductive oxides.
The world at the end of the 20th century has become ”blue”. Indeed, this past decade has witnessed a ”blue rush” towards the development of violet-blue-green light emitting diodes (LEDs) and laser diodes (LDs) based on wide bandgap III-Nitride semiconductors. And the hard work has culminated with, first, the demonstration of commercial high brightness blue and green LEDs and of commercial violet LDs, at the very end of this decade. Thanks to their extraordinary properties, these semiconductor materials have generated a plethora of activity in semiconductor science and technology. Novel approaches are explored daily to improve the current optoelectronics state-of-the-art. Such improvements will extend the usage and the efficiency of new light sources (e.g. white LEDs), support the rising information technology age (e.g. high density optical data storage), and enhance the environmental awareness capabilities of humans (ultraviolet and visible photon detectors and sensors). Such opportunities and many others will be reviewed in this presentation.
The article shows the most important experimental results describing the properties of nitride layers on GaN single crystals. The layers were grown using metalorganic chemical vapour deposition (MOCVD). The growth was monitored by in-situ laser reflectometry. The layers contain very small dislocation density of about 10-103 cm-2 (the same as in the GaN substrates). Morphology and crystallographic quality was examined using atomic force microscopy and X-ray diffraction. The layers have excellent photoluminescent properties what has a direct impact on the optoelectronic device properties.
The main methods of crystal
growth from the melt, the
Czochralski and floating zone, is discussed and compared. Advantages,
and limitations of both methods as well as ways of solving some of the
problems existing during growth of different types of multioxides
The chemical composition of crystals grown by the Czochralski method very often differs from the stoichiometfic composition. Such deviations were found and well documented in a few groups of materials, for example in garnets. Since the deviation is not known for most of the crystals, a simple way to determine the optimum starting composition of the melt will be presented. In order to determine the composition of the melt one should take into account evaporation of a volatile component that dissociates at high temperatures during crystal growth.
Some problems related to the dopant distribution along the crystal growth direction in correlation with segregation coefficient for both methods is discussed. To grow solid solution single crystals by the Czochralski method with a desired concentration of the admixture one has to know segregation coefficients of the components. A few examples of the dopant solubility lomit in different crystal matrices is presented.
Bulk single crystals up to 20 mm in diameter and 40 mm long for LiInS2 and up to 10 mm, 20 mm, respectively, for LiInSe2 have been grown. Their colour changed from colourless to rose for the first one and from yellow to dark red for the other. All crystals have wurtzite-type lattice (Pna21 space group), lattice parameters were determined. A band gap was found to be 3.72 and 3.57 eV forLiInS2 and 3.02, 2.86 eV forLiInSe2 at 80 and 300 K, respectively. Colour variations are due to point defects, first of all to interstitial sulfur, resulting in additional wide absorption bands in the shortwave part of transparency range. For LiInS2 the SHG phase matching conditions were found to he similar for samples of different colour and some difference from Boyd’s predictions of 1973 was shown: for XY plane Dj~ +3° at 2.6 mm and Dj~ -3° at 4-5 mm. Nonlinear susceptibility for LiInS2 was estimated: deff(XY) ~3.4 pm/V relative to Boyd’s value as 10.6 pm/V. A proper illumination gives a photoinduced change ofLiInSe2 colour from dark red to yellow as a result of changes in point defects charge state.
X-ray diffraction and synchrotron x-ray topography methods were used to analyse strain in GaAs layers grown on GaAs am Si substrates by epitaxial lateral overgrowth (ELO) from a liquid phase. We show the laterally overgrown parts of ELO strips adhere to the SiO2 mask which results in their downwards bending. The procedure was found which allows to control adhesion of the layers to the mask by adjusting the vertical growth rate of the layers. For the case of GaAs ELO layers grown on substrates the ELO stripes bend outwards from the mask due to the tensile strain in the GaAs buffer layer. Recent data published on strain in other than GaAs ELO structures are reviewed and compared with our results.
One of the simplest device realisations of the classic particle-in-the-box problem of basic quantum mechanics is the quantum well infrared photodetector(QWIP). In this paper we discuss the effect of focal plane array non-uniformity on the performance, optimisation of the detector design, material growth and processing that has culminated in realisation of large format long-wavelength QWIP cameras, holding forth great promise for many applications in 6-18 micron wavelength range in science, medicine, defence and industry. In addition, we present the recent developments in long-wavelength/very longwavelength dualband QWIP imaging camera for various applications.
The development of devices for
mid-, long-, and very long-wavelength
infrared (IR) detection has benefited greatly from advances in hand-gap
engineering. Recently, there has been great progress in the development
of n-type GaAslAlGaAs quantum well infrared photoconductor (QWIP)
in all three technologically important wavelength windows, p-type
QWIPs represent a viable alternative to n-type GaAs/AlGaAs QWIPs,
the advantage of normal incidence absorption without the need for
couplers. The maturity of the MBE of GaAs/AlGaAs layered materials
the possibility of mass producing low cost, high performance, large
high uniformity, multicolour, high frequency bandwidth, two-dimensional
imaging QWIP arrays.
This paper describes progress in optimising the performance of p-type GaAs/AlGaAs QWIPs through modelling, growth, and characterisation. Using the 8´8 envelope-function approximation (EFA), a number of structures were designed and their optical absorption calculated for comparison with experiment. Samples were grown by MBE based on the theoretical designs and their photoresponse measured, p-type QWIPs were optimised with respect to the well and barrier widths, alloy concentration. and dopant concentration; resonant cavity devices were also fabricated and temperature dependent photoresponse was measured.
The quantum efficiencies and the background-limited (BLIP) detectivities under BLIP conditions of our p-QWIPs are comparable to those of n-QWIPs: however, the responsivities are smaller. For our mid-IR p-QWIPs, the 2D doping densities of (l-2)´1012 cm-2 maximised the BLIP temperature and dark current limited detectivity by operating at around 100 K. At 80 K, the detectivity of the optimum doped sample was 3.5´l011 cmHz1/2/W at 10 V bias. Barrier widths greater than 200 Å were sufficient to impede the tunneling dark current; resonant cavities enhanced absorption fivefold.
The paper presents progress in infrared (IR) detector technologies during two hundred history of their development. Classification of two types of infrared detectors (photon detectors and thermal detectors) is done on the basis of their principle of operation. The overview of infrared systems and detectors is presented. Recent progress in different IR technologies is described from a historical point of view. Discussion is focused mainly on current and the most rapidly developing detectors: HgCdTe heterostruclure photodiodes, quantum well AlGaAs/GaAs photoresistors, and thermal detectors. The outlook for near-future trends in IR technologies is also presented.
Design and fabrication of GaSb/InGaAsSb/AlGaAsSb mid-infrared photodetectors
T.T. Piotrowski, A. Piotrowska, E. Kaminska, M. Piskorski, E. Papis, K. Golaszewska, J. Katcki, J. Ratajczak, J. Adamczewska, A. Wawro, J. Piotrowski, Z. Orman, J. Pawluczyk, and Z. Nowak
The paper reports on the design and fabrication of LPE-grown GaSb/n-InxGa1-xAsySb1-y/p-AlxGa1-xAsySb1-y heterojuction photodetectors operating in the 2-2.4 mm wavelength region. Experiments on LPE growth of high-x-content quaternaries as well as optimisation of device processing has been carried out. LPE growth at T » 530°C enabled obtaining lattice matched heterostructures with 19% indium in the active layer In0.19Ga0.81As0.16Sb0.84/Al0.24Ga0.76As0.04Sb0.96 and photodetectors with lc = 2.25 um. By increasing the temperature of epitaxial growth to 590°C In0.23Ga0.77As0.18Sb0.82/Al0.30Ga0.70As0.03Sb0.97 heterostructures (with 23% indium content) suitable for photodetectors with lc = 2.35 um have been obtained. Mesa-type photodiodes were fabricated by RIE in CCl4/H2 plasma and passivated electrochemically in (NH4)2S. These devices are characterised by differential resistance area product up to 400 Wcm2 and the detectivity in the range the range 3´1010-2´1011 cmHz1/2/W, in dependence on the photodiode active area and cut-off wavelength.
Wigner crystal and other insulating phases of two-dimensional electrons in high magnetic fields
A concise review of the history and of the hasic physics of the Wigner crystal (electron solid) in two-dimensional electron systems in semiconductor heterostructures is given. The results of our experimental study on the formation and of the properties of the magnetic field-induced insulating phase, supposed to he a Wigner crystal, in a two-dimensional electron gas in In0.53Ga0.47As/InP heterostructures are surveyed. These structures are characterised by a much greater disorder potential than AlGaAs/GaAs heterostructures due to the inherent alloy disorder in the InGaAs layer supporting the two-dimensional electron gas. In high magnetic fields, helow a Landau level filling factor of 0.4-0.5 divergent resistivity, non-linear current-voltage characteristics with a threshold, and a transition from non-activated to activated transport were observed. A model is proposed for the Wigner crystal-like ordering of the two-dimensional electron gas in this system with large disorder.
Some properties of fullerenes and carbon nanotubes
P. Byszewski and Z. Klusek
Experiments on preparation of C60ONCFn cycloadduct (Fn = ferrocene) and C60(Cocp2)3 charge transfer complexes are described and their properties analysed. The ferrocene derivative is bound to C60 at the 6-6 bond by a heterocyclic oxygen-nitrogen-carbon ring. The experimental results are compared with the results of modelling using semiempirical quantum chemistry PM3 model. Scanning tunnelling microscopy and spectroscopy has been used to observe variation of electronic structure along capped carbon nanotubes deposited on freshly cleaved HOPG(000l) surface. The electronic structure has been derived from measured dependence of tunnelling conductance dI/dV on the applied voltage. Evolution of dI/dV(V) along the nanotube are discussed in terms of existing theories of nanotubes and quantum chemistry calculations.