Different optical crystals are reviewed from the viewpoint of their optical properties, among which typical crystals survived in the recent information technology (IT) systems. Most of revived or survived crystals are somehow traditional ones because of their established properties. Potential applications of oxide crystals into shorter wavelengths are focused as additional ripple effects of the impetuous deployment of the IT. Some recent activities on developing new crystals are described briefly; GdVO4 with a large walk-off angle as a birefringent crystal, Y2O3 as a new laser host, periodically poled ferroelectric LiNbO3 and LiTaO3 crystals as effective devices of frequency conversion, and traditional sapphire as a potential substrate of GaN-LEDs.
Keyword: oxides, optical crystal, birefringence, magnetooptic, electrooptic, solid-state laser, optical nonlinear, periodically poled crystals, optical fibre communication/information systems.
This paper is a brief summary of the technological development and state-of-the-art performance of quantum cascade lasers (QCLs) produced at the Centre for Quantum Devices. Laser design will be discussed, as well as experimental details of device fabrication. Recent work has focused on the development of high peak and average power QCLs emitting at room temperature and above. Scaling of the output is demonstrated by increasing the number of emitting regions in the waveguide core. At l = 9 µm, over 7 W of peak power has bee demonstrated at room temperature for a single diode, with an average power of 300 mW at 6% duty cycle. At shorter wavelengths, laser development includes the use of highly strain-balanced heterostructures in order to maintain a high conduction band offset and minimize leakage current. At l = 6 µm, utilizing a high reflective coating and epilayer-down mounting of the laser, we have demonstrated 225 mW of average power from a single facet at room temperature. Lastly, these results are put in the perspective of other reported results and possible future directions are discussed.
Keywords: quantum cascade laser, high power, molecular beam epitaxy.
Investigations and comparative analysis of p-to-n type
conductivity conversion processes on the identical samples of vacancy doped
p-CdxHg–xTe (x »
0.2) under ion-beam milling (IBM) and anodic oxide annealing and on the
identical samples of As-doped p-CdxHg1-xTe (x »
0.22) under IBM and anodic oxide annealing have been carried out. The conductivity
type conversion has been observed at the considerable depth of the vacancy
doped material both under IBM or under anodic oxide annealing
while in the case with As-doped material only under IBM.
It was considered that conversion in all these processes was determined
by the mercury interstitial diffusion from corresponding mercury diffusion
source and recombination with its native acceptors – cationic vacancies
(in the first case) or with donor complex formations (in the second one).
It has been shown that in the vacancy-doped p-CdxHg1-xTe
the effective diffusion coefficients for the mercury interstitials that
determines the depth of the converted layer are equal each other at equal
temperatures either under thermal annealing in the saturated mercury vapour
or anodic oxide annealing. It proves the identity of the mercury concentration
in the diffusion source. Absence of the conversion under anodic oxide annealing
in the As-doped p-CdxHg1-xTe is explained by insufficient
Hg concentration in the source and it matches well with necessary condition
for donor complex formation as it takes place under IBM.
Keywords: CdxHg1-xTe, p-n structure, mercury diffusion, ion beam milling, anodic oxide and thermal annealing.
View of basic and specific physical and chemical features of growth and defect formation in mercury cadmium telluride (MCT) heterostructures (HS’s) on GaAs substrates by molecular beam epitaxy (MBE) was made. On the basis of this knowledge a new generation of ultra high vacuum set, ultra-fast ellipsometer of high accuracy and automatic system for control of technological processes was produced for reproducibility of growth MCT Hs’s on substrates up to 4” in diameter. The development of industrially oriented technology of MCT HS’s growth by MBE on GaAs substrates 2” in diameter and without intentional doping is presented. The electrical characteristics of n-type and p-type of MCT HS’s and uniformity of MCT composition over the surface area are excellent. The residual donor and acceptor centres are supposed as hypothetically tellurium atoms in metallic sublattice (“antisite” tellurium) and double-ionised mercury vacancies. The technology of fabricating focal plane arrays is developed. The high quality characteristics of infrared detectors conductance and diode mode are measured. Calculations of detector parameters predicted the improvement in serial resistance and detectivity of infrared diode detectors based on MCT heterostructures with graded composition throughout the thickness.
Keywords: MCT, MBE, growth GaAs, Si, ellipsometer, heterostructures, infrared detectors, photoconductors, photodiodes, focal plane arrays, p-n junctions.
Photoluminescence (PL) spectra and excitation spectra
(PLE) (under steady-state conditions), time resolved spectra (PL-TRS) and
decay curves of photoluminescence (PL-DC) in micro- and nanosecond range
(under pulsed operation) at different temperatures (10 K-room) on anodically
etched boron – doped silicon are presented. PLE shows that visible PL is
excited by light from UV region. PL and PL-TRS exhibit multiband structure
and can be decomposed as a sum of few Gaussians. PL-DCs have multiexponential
shape. Relaxation times depend on wavelength of the observation. To explain
our
results we assumed a model in which the multibarrier
structure is formed by larger Si crystallites or wires (quantum well) surrounded
by Si crystallites with diameters in the nanometer range (barrier region).
The visible photoluminescence originates from radiative recombination between
discrete energy levels in a quantum well.
Keywords: porous silicon, photoluminescence, decay times, time resolved spectra, excitation spectra.
Performance of various possible designs of 400-nm nitride
vertical-cavity surface-emitting lasers (VCSELs) has been analysed with
the aid of the advanced three-dimensional (3D) thermal-electrical-optical-gain
self-consistent threshold simulation. It has been demonstrated that it
is practically impossible to reach the fundamental-mode operation in nitride
VCSELs of the traditional design with two ring contacts. To enhance this
desired operation, uniformity of current injection into VCSEL active regions
should be dramatically improved. Therefore we focused our research on designs
with tunnel junctions and/or a semitransparent contact. In particular,
it has been proved that the design with two cascading active regions, two
tunnel junctions and a semitransparent contact may offer the most promising
room-temperature performance characteristics for both pulse and continuous-wave
operation. In particular, this design offers high mode selectivity with
distinct fundamental transverse mode domination. Our simulations reveal,
that the thickness and localization of a semitransparent contact as
well as localization of active regions and tunnel junctions
are crucial for a successful construction designing.
Keywords: computer simulation, nitride VCSEL, tunnel junction, ITO contact.
Modern communication in absolute secrecy requires creation
of new intrinsically secure quantum communication channels. It is particularly
necessary during the first connection between two parties establishing
then in assumed unconditional security the secret cryptographic key which
is supposed to be used afterwards during normal information exchanging.
This new emerging field of quantum information technology is based on a
new type of light sources, in which numbers of emitted photons can be carefully
controlled. Especially advantageous are sources of single photons emitted
at strictly predetermined moments, so called single-photon devices. Then
any possible eavesdropper activity will be followed by some unavoidable
disturbance which alerts both communication parties to an event. In the
present paper, the Purcell effect associated with enhancement of spontaneous
emission coupled to a resonator is explained, methods used to produce streams
of antibunched photons are given, mechanisms applied to control carrier
injection into quantum dots are shown and some possible designs of single-photon
devices are presented and described. These devices are based on taking
advantage of both the Purcell effect
and the atom-like energy spectrum of quantum dots.
Keywords: quantum cryptography, single-photon devices, communication systems, secrecy in communication.
New structures aiming at controlling ferromagnetic properties of diluted magnetic semiconductors quantum wells are presented. The carrier density is monitored by applying voltage in p-i-n diode or adjusting a distance between quantum well and surface. Surface doping was successfully applied to obtain samples with CdMnTe quantum well with up to 9.3% Mn concentration.
Keywords: molecular beam epitaxy, quantum wells, magnetic materials, semiconducting II-VI materials.
The advanced three-dimensional fully self-consistent optical-electrical-thermal-gain
model of the 1.3-µm
(GaIn)(NAs)/GaAs vertical-cavity surface-emitting laser
(VCSEL) has been developed to simulate its room-temperature (RT) continuous-wave
(CW) performance characteristics and to enable its structure optimisation.
The standard GaInNAs VCSEL structure with an intracavity-contacted configuration
exhibits very nonuniform current injection into its active region, whereas
a uniform current injection is important in long-wavelength VCSELs for
low threshold, high-efficiency and stable-mode operation. Therefore we
decided to insert an additional tunnel junction within the active-region
neighbourhood.
The tunnel junction is shown to enhance effectively hole
injection via a lateral electron current, with only a modest increase (a
small penalty) in voltage drop and series resistance compared to standard
devices.
Keywords: GaInNAs laser, GaInNAs VCSEL, simulation of performance characteristics, tunnel junction.
The performance of very long wavelength infrared (VLWIR)
HgCdTe photodiodes at temperatures ranging from 77 K up to 150 K is presented.
The effect of inherent and excess current mechanisms on quantum efficiency
and dynamic resistance-area RA product is analysed. Different methods of
determining the ideality factor are shown and among them the one based
on the use of RA product versus bias voltage proves to be the most reliable.
At higher temperatures, however, the calculated ideality
factor does not give any useful information about the
nature of the p-n junction current due to significant influence of the
series and shunt resistances. A comparison of the experimental data with
the results of analytical and numerical calculations shows that the photodiodes
with cut-off wavelength up to 14.5 µm are diffusion-limited at temperatures
exceeding 100 K.
Keywords: HgCdTe photodiodes, heterostructure, ideality factor, R0A product, quantum efficiency.
The surface microrelief of CdHgTe layers grown by molecular
beam epitaxy (MBE) method has been studied by means of atomicforce microscopy.
A periodic surface microrelief in the form of an ordered system of extended
waves with the characteristic period 0.1–0.2 µm has been detected
on epilayers grown at increased temperatures. Angular dependencies of the
conductivity at 77 K have been measured and the conductivity anisotropy
has been detected with a minimum in the direction transverse to microrelief
waves. A feature of the transmission spectrum and the spectrum change after
film annealing are observed.
It is assumed that walls growing in the direction from
the substrate to the surface are formed under microrelief wave slopes.
Such structure can cause the observed feature of the transmission spectrum
if the adjacent walls have different composition. In this work a calculation
of spectral characteristics taking into account the influence of variable
gap composition and nonuniformity of the composition through the depth
has been carried out.
Keyword: CdHgTe layers, MBE, surface microrelief, spectral characteristics.
The French aerospace agency is involved in the realization
of compact solid-state coherent sources, such as optical parametric oscillators
(OPO), using new materials, such as highly non-linearly efficient semiconductors
(ZnSe, GaAs or InP). However, since these materials are optically isotropic,
they require new phase-matching techniques. We report the quasi-phase matched
difference frequency generation in isotropic semiconductors using total
internal reflection. We made use of large Fresnel birefringence at reflection
between the signal and idler wave outputs of an OPO. Large tunability (between
8 and
13 µm) is demonstrated. Agreement between theoretical
expectation and experimental results is excellent.
Keywords: non-linear optics, difference frequency generation, mid-infrared light, quasi-phase matching, isotropic semiconductors, Fresnel birefringence.
Platinum silicide Schottky barrier detectors (SBD) and GeSi/Si-based heterojunction internal photoemission (HIP) detectors are widely used for application in the infrared spectral range. The increase in cutoff wavelength and responsivity of PtSi/Si photodevices is possible by formation of heavily-doped thin layer near to the semiconductor surface. The cutoff wavelength of GexSi1–x/Si-based HIPdetectors depends on x and concentration of boron in GeSi. In this report, the threshold properties of these detectors are considered. The dependencies of spectral detectivities and NETD on cutoff wavelength are calculated for various parameters of SBD and HIP detectors. It is shown that optimal NETD of a SBD and HIP detectors is possible for certain cutoff wavelength and temperature of detectors and depends on storage capacity. Also opportunity of formation of heavily-doped nanolayer in SBD detectors used by short-pulse recoil implantation of boron was studied.
Keywords: PtSi/Si barrier, heavily-doped nanolayers, SBD detectors, HIP detectors, GeSi/Si heterojunctions.
The objectives of this investigation are structural and physical characteristics of the n-Si1–xGex/n(p)-Si heterojunction under strong elastic deformation of Si1–xGex layers which gives rise to misfit dislocations in the heteroboundary region; the factors playing the main role in formation of the band structure of the system; the use of transmission electron microscopy and optical methods for determination of the phenomena connected with misfit dislocations in the grown epitaxial structure; the electrical characteristics of diode structures and the process of electron-hole recombination via dislocation states in a heterojunction.
Keywords: heterojunction, silicon, solid solution, misfit dislocation, photoluminescence, electrical properties.