The main advantageous features of laser interference lithography (LIL) technology in fabrication of nano structures and
devices are high resolution, low cost and high efficiency. By using LIL, the graded-index patterns consist of periodic
modified lines (MLs) on the periodic square patterns. Patterns with MLs formed on SiO<sub>2</sub> deposited on GaAs substrate.
The orientation and periodicity of MLs are shown to depend on the configuration of the incident laser beams. Periodic
arrays of holes in GaAs, covered with SiO<sub>2</sub> bubbles, were directly written into the sample within only some minutes. The
diameters of the smallest holes were less than 30 nm. The smallest modification features of the GaAs were less than 5
nm. Four-beam LIL was shown to be a good technology to generate graded-index photonic crystals with square lattice.
High intensity enhancement and sub-wavelength focusing were achieved simultaneously. The results indicate a new lowcost
and high-efficiency way of fabricating planar lens.
Laser interference lithography (LIL) is concerned with the use of interference patterns generated from two or several
coherent beams of laser radiation for the structuring of materials. This paper presents the work on the processes based on
resists and direct writing with laser interference lithography. In the work, a four-beam laser interference system was used
as a submicrometer structuring tool in which a high-energy pulsed, frequency-tripled and TM polarized Nd:YAG laser (355 nm) with a coherent length of 3 m, energy power up to 320 mJ/cm<sup>2</sup>, pulse duration of 8 ns and 10 Hz repetition rate was used as a light source. The experimental results were achieved with 2-beam and 4-beam interference patterning. The processes can be used to define submicron surface relieves in large areas for use in the field of MEMS.
The constraints on dilute-nitride Semiconductor Optical Amplifiers (SOAs) for multi-wavelength amplification have
been evaluated. SOAs have been fabricated by angling the facets of a GaInNAs/GaAs edge emitting laser using gas
enhanced focused ion beam etching. The original laser has been characterized in terms of its temperature dependence and
net modal gain. A full width half maximum (FWHM) of 40nm has been found at 298K. Good temperature stability has
also been found with a value of 0.35nm/K for the lasing wavelength. The good temperature stability of the device has
been explained in terms of the role that the monomolecular recombination plays in the temperature dependence of the
device. The monomolecular recombination has been reported temperature independent having two key effects; reduction
of the temperature performance and reduction of the dynamic performance in terms of an increase in the threshold
current and a decrease of the high speed potential. Iodine gas enhanced focused ion beam etching (GAE-FIB) has been
used for the fabrication of the SOA, the iodine gas increasing the etching rate by a factor of 2.5. The fabrication has been
completed in two steps; in the first one the facets have been angled and in the second step a cross-section procedure has
been employed for smoothing of the facets. Once the SOA has been fabricated its potential for simultaneous multiple
channel amplification has been studied. A flat gain spectrum over a range of 40nm has been obtained. This value and the
wavelength range have good agreement with the net modal gain measured in the original laser device. In addition,
minimum channel interspacing has been achieved with no wavelength degradation.
By adding a little nitrogen in InGaAs / GaAs quantum well (QW), a strong bandgap-bowing in the QW is
caused. However, the incorporation of nitrogen results in lower photonuminescence intensity and wider line
width as a result of increased non-radiative centers. In order to increase the efficiency of radiative
recombination and hence reduce the laser threshold, a post-growth heat treatment has to be applied. Such kind
of heat treatment results in a big blue shift due to interdiffusion and other effects. During growth, in order to
incorporate nitrogen into InGaAs, the growth temperature is much lower than normal InGaAs growth. Large
number of point defects is induced under such low temperature. This is the main cause of the interdiffusion at
the interfaces of InGaAsN / GaAs QW. There are some other facts to cause the blue shift during heat
treatment, such as local neighbourhood redistribution called short range ordered. In our study, different blue
shift behaviors were clearly observed due to different blue shift mechanism. Post-growth heat treatment also
affects the laser performance dramatically. Lower temperature treatment mainly decreases the absorption loss
and higher temperature treatment improves the conductivity of the cladding layers. Different heat treatment
also results in very different burn-in behavior. An optimized heat treatment will be concluded after the
annealing discussion on laser devices. In order to assure longer emission wavelength well as higher
emission efficiency, many efforts have been tried and will be discussed in this paper.
Multi-beam laser interference lithography (MB-LIL) is a rapid and cost-effective maskless optical lithography technique
to parallelly pattern periodic or quasi- periodic micro/nano-structured material over large areas more than square
centimetres. An interference pattern between two or more coherent laser beams is set up and recorded in a recording
material of substrate. This interference pattern consists of a periodic series of geometries representing intensity minima
and maxima. The patterns that can be formed depend on the number and configuration of laser beams. This review
introduces the development and application of MB-LIL system for fabrication of micro/nano-structured material. At first,
it surveys various types of MB-LIL methods by classifying different beam configurations. Then the paper shows some
application results for fabrication 2D/3D micro/nano structure arrays by means of interference patterns with multi-exposed
or directly ablation technique. The patterend micro/nano-structure arrays include crossed diffraction grating
array in photoresist, 3D pattern in polymetric photonic crystals, and magnetic nanoarrays in thin film. Finally, an
innovative four-beam LIL system is introduced, which is being developed within the EC-granted project DELILA.
This paper presents a theoretical analysis of formation of 4-beam laser interference patterns for nanolithography.
Parameters of 4-beam interference patterns including the pattern amplitude, period, orientation and uniformity were
discussed. Analytical expressions were obtained for the spatial distribution of radiation of the interfering beams as a
function of their amplitudes, phases, angles of incidence on the sample, and polarization planes with computer
simulation and experimental results.
Beryllium incorporation in InGaAsN quantum well improves the optical properties of this dilute nitride material significantly. After annealing, the intensity of the photoluminescence of this new dilute nitride material (InGaAsNBe) is about 20 times higher and its wavelength is even 25 nm longer. After a certain time of this heat treatment, the photoluminescence quenched slowly for InGaAsN structures because of the strain relaxation due to the thermal activation. The photoluminescence of InGaAsNBe increased rapidly and show no saturation even after a very long time of annealing. Beryllium incorporation in InGaAs which was grew at the same temperature as dilute nitrides also improves the optic properties. But the improvement for InGaAsNBe is 10 times more than for InGaAsBe. Laser processing based on the new InGaAsNBe structures resulted in one half of the threshold current density compare to conventional InGaAsN.
Beryllium was incorporated in InGaAsN single quantum well (SQW). Comparing with the conventional InGaAsN SQW structures, photoluminescence (PL) investigations show a significant improvement. After 3000 sec of annealing at 700 °C, the PL peak area is about 20 times higher while the wavelength keeps 25 nm longer. After 800 sec of this annealing, the PL quenched slowly for the conventional structures because of the strain relaxation, while the PL of the new structures increased rapidly and show no saturation after 3000 sec of annealing. Laser processing based on the new InGaAsN structures resulted in one half of the threshold current density compare to conventional InGaAsN.
High power and single mode InGaAsN ridge waveguide lasers were developed. The pulsed the maximum output power was 240 mW at room temperature (RT). The threshold was 15 mA at 20°C. The ridge waveguide laser could work beyond 120°C. For cw operation, the lasers show a maximum output up to 40 mW RT. The broad area lasers using the same materials has been working under continuous-wave operation at constant current (80% of maximum output) for more than 42,800 device-hours at 30°C with as-cleaved facets. They are still working well.
Before processing the InGaAsN/GaAs edge emitting lasers, post-growth rapid thermal annealing (RTA) was applied on the wafer. Different RTA results in different threshold current density (J<sub>th</sub>). RTA at 720°C reduces the J<sub>th</sub> significantly but keeps the linear fit slope of J<sub>th</sub> vs 1/L (L is the cavity length). It indicates that RTA at 720°C can decrease the absorption losses. High temperature RTA at 890°C can dramatically decrease the linear fit slope, which indicates that the carrier conductivity is improved dramatically even the RTA time is only one second.
We report on the growth of GaInNAs materials and lasers by molecular beam epitaxy (MBE) using a rf-plasma source. Optimal GaInNAs quantum well (QW) structures have been designed and grown in order to achieve the brightest and narrowest photoluminescence (PL) spectra beyond 1.30 um. State-of-the-art GaInNAs/GaAs SQW lasers operating at 1.32 um have been demonstrated. For a broad area oxide stripe, uncoated Fabry-Perot laser with a cavity length of 1600 um, the threshold current density is 546 A/cm2 at room temperature. Optical output up to 40 mW per facet under continuous wave operation was achieved for these uncoated lasers at room temperature.
We report the growth of GaInAsN heterostructures on GaAs substrates by conventional molecular beam epitaxy (MBE) using a radio frequency plasma source. Lattice-matched bulk samples and several strained single quantum well (SQW) and multiple quantum well (MQW) structures were grown. The QWs were sandwiched between two GaAsN strain-compensating layers (SCL) and AlGaAs cladding layers. By the aid of SCLs the photoluminescence (PL) wavelength red-shifted as much as 88 nm with the same intensity. GaInAsN strain-mediating layers (SML), having less strain than QW, were also used to obtain red shift and improved luminescence properties. The structures were studied by room temperature (RT) PL, x-ray diffraction (XRD) measurements and atomic force microscopy (AFM). The indium and nitrogen compositions of the QWs varied from 34 to 38 % and 1.3 to 3.5 %, respectively. Most of the studied structures showed PL peak wavelength at over 1.3 mm. Depending on the structure and thermal annealing treatment conditions the wavelength blue shifted up to 55 nm and intensity increased ~45 times. Furthermore, an AFM image of a five QW sample showed very smooth surface indicating together with PL measurements that high quality MQWs can be realized. In addition, 1.32-micrometers continuous-wave GaInAsN edge-emitting lasers were demonstrated.