GaN based microLED technology is particularly interesting for micro-displays for augmented reality. Beyond that, however, many additional applications exploiting the massively parallel nature of optical channels from microLED arrays are of interest. This includes microLED arrays for chip-based lensless microscopy, optogenetics, low power gas sensing, optical neuromorphic computing, high-speed maskless lithography or massively parallel line-of-sight communication in HPC centers (LIFI), replacing optical fibers. In order to drive large numbers of pixels at high frequencies in customized microLED arrays, the integration with a CMOS backplane chip is required.
The talk will mostly focus on chip processing and hybrid integration techniques for microLEDs and the opportunities for GaN based photonic integration in general, and GaN/CMOS integration in particular. First applications of specially designed microLED arrays in the field of optical neuromorphic computing will also be discussed. Test results include pattern recognition of hand-written digits from the MNIST dataset.
Aluminum nitride (AlN) exhibits large breakdown electric fields and high thermal conductivity which allows for excellent miniaturization and power density in high-power electronics.
In this talk, we investigate normally-off vertical n-channel trench MISFETs, with the channel consisting of nominally undoped graded AlGaN. The graded AlGaN layer creates immobile volume charges, and the lack of impurities reduces impurity scattering. The n-doped drift layer is composed of AlN for optimum electric field management. Contacts are placed on AlGaN for low ohmic resistance. Using TCAD simulations, the physics of device operation is studied. For comparison, a conventional impurity-doped FET without PID is taken as reference device. The simulations encompass calculation of local strain, solution of the Poisson-equation and electron and hole continuity equation on a 2-dimensional cross-section of the device. Transfer characteristics, threshold voltage, on-resistance and electric field are discussed. Surface states and interface charges at the nonpolar trench sidewall are included in the study. Finally, technological implementation and experimental results are discussed.
In this study we present the novel approach of GaN-based high-contrast grating (HCG) mirrors as highly reflective top mirrors in GaN vertical-(external)-cavity surface-emitting lasers [V(E)CSELs]. These mirrors can well be integrated into a conventional process flow for GaN-based VECSEL devices – in contrast to conventional Bragg mirrors. Results of thorough growth experiments performed to reliably fabricate high quality cavity structures with well-defined and fine-tuned optical properties will be reported. The properties of HCG mirrors have been calculated by extensive optical device simulations. Properties of HCG structures have been analyzed experimentally by reflection measurements and results will be discussed with a focus on HCG design, potential material combinations, compatibility with existing process steps in GaN technology and the various ways of integration into VECSEL structures.
In this work we present a new microscope based on Nano-illumination microscopy (NIM), i.e., an innovative technique based on a 2D array of nano-Light-Emitting Diodes (LEDs) used to illuminate a sample. The key point of this method is that the pitch of the LED array fixes the spatial resolution. So, potentially, with LED pitches lower than the diffraction limit, super resolution could be achieved. While nanometer sized LEDs are not available yet, we present a prototype based on optical downscaling of a single 5µm lateral size LED. Extended Field-of-View (FOV) is obtained by mechanical movement with nanopositioners. Aspects of NIM microscopy such as its size, its flexibility in the sensing hardware or its potential for fluorescence, make it a perfect candidate to enhance emerging sensing applications in different fields, but especially life science (medical imaging, genomics, ...). We demonstrate the possibilities of the NIM technique with patterns as well as with biological samples.
MicroLED arrays are arrangements of small, individually addressable LED pixels, which offer the possibility to obtain any desired illumination pattern with a high spatial and temporal control. In this work, we report on the advanced fabrication of micro- and nanoLED arrays and their system integration including electronics and graphical user interface. The combination of the chip-based LED array, electronics and software is called a MicroLED platform.
The light density output, homogeneity and switching behavior of the arrays are investigated. The system allows the modulation of the single light sources with a spatial resolution in the micron range and enables the programming of different light patterns at modulation frequencies up to the MHz range. The fully integrated system with microLED array and electronics is shown during presentation are now available as Structured Micro Illumination Platforms (SMILE) for applications from sensing to microscopy to quantum computing.
This work presents a compact low-cost and straightforward shadow imaging microscopy technique based on spatially resolved nano-illumination instead of spatially resolved detection. Independently addressable nano-LEDs on a regular 2D array provide the resolution of the microscope by illuminating the sample in contact with the LED array and creating a shadow image in a photodetector located on the opposite side. The microscope prototype presented here is composed by a GaN chip with an 8x8 array of 5μm-LEDs with 10 μm pitch light sources and a commercial CMOS image sensor with integrated lens used as a light collector. We describe the microscope prototype and analyze the effect of the sensing area size on image reconstruction.
This work presents a first prototype for a new approach to microscopy: a system basing its resolving power on the light emitters instead of the sensors, without using lenses. This new approach builds on the possibility of making LEDs smaller than current technology sensors, offering a new approach to microscopy we plan on developing towards superresolution. The microscope consists on a SPAD based camera, a 8x8 LED array with 5x5 μm LEDs distributed with a pitch of 10 μm, and discrete driving electronics to control them. We present simulations of the system, as well as the first microscope prototype implementing the method, and the results obtained through it.
A non-invasive optical measurement system based on a broadband light source and color filters has been developed for determining pulse rate and arterial blood oxygen saturation (SaO2). In contrast to classical pulse oximetry using red and infrared LEDs to measure the peripheral capillary oxygen saturation (SpO2), we use color filters in our system. Spectral analysis of human tissue can be easily achieved by combining a tiny color filter matrix and a commercial CMOS/CCD image sensor. During system operation, white LED light illuminates our tissue (e.g., a finger), while a CCD sensor covered by filters detects the light transmitted through that tissue. The CCD sensor is controlled by a Field Programmable Gate Array (FPGA) and a microcontroller. The detected photoplethysmographic (PPG) signal is transferred to a host computer and analyzed with MATLAB. After sensor system calibration, pulse rate and SpO2 can be simply extracted from the PPG signal. The heart rate and SpO2 of different volunteers are then measured simultaneously by commercial pulse oximetry and the proposed sensor system, in which results from both devices show good agreement. To integrate more functions into system, nanostructured color filter matrix containing 15 filters for different wavelengths is designed and fabricated. This filter can be designed to provide transmission peaks over the visible and near-infrared range (i.e., the human tissue optical transparent window) and has a high potential to be fabricated directly on top of pixels of an image sensor.
In this work, we study the optical emission from arrays of InGaN/GaN MQW nanofin and nanorod arrays with sizes ranging from a few micrometers down to sub-wavelength dimensions (i.e., nanometers). Such systems are of interest for developing arrays of single addressable nanoLEDs, which could be used to obtain a visible wavelength super-resolution microscope where the resolution is due to highly localized light spots with sub-wavelength LED-to-LED pitch.
We have used commercial full-wave Maxwell solvers (COMSOL, CST) to calculate the optical field emitted from a single nanoLED in a periodic array for a wavelength of 450 nm. Simulations on 11×11 nanoLED arrays with pitches of 200 nm up to 800 nm and diameters of down to 50 nm have been conducted, in which the dependency of the emission pattern on different structural parameters is studied. In case of small nanoLED array with very narrow pitch, a large optical cross-talk between the activated LED and its neighboring pixels was found. Moreover, in presence of cross-talks, test objects smaller than the LED pitch placed on its surface with influence of near field could potentially be resolved by evaluating the varied emission patterns obtained by different pixel activations. Routes to achieve higher localized optical fields and reduce optical cross-talk have been also investigated by modifying the nanoLED array structures (e.g., by introducing filling material among the LED pixels).
We present a nanometer-scale correlation of the structural, optical, and chemical properties of InGaN/GaN core-shell microrods. The core-shell microrods have been fabricated by metal organic vapor phase epitaxy (MOVPE) on c-plane GaN/sapphire templates covered with a SiO2-mask. The MOVPE process results in a homogeneous selective area growth of n-doped GaN microrods out of the mask openings. Surrounding the n-GaN core, a nominal 5 nm thick GaN shell and 30 nm thick InGaN layer were deposited.
Highly spatially resolved cathodoluminescence (CL) directly performed in a scanning transmission electron microscope (STEM) was applied to analyze the selective Indium incorporation in the thick InGaN shell and the luminescence properties of the individual layers. Cross-sectional STEM analysis reveal a hexagonal geometry of the GaN-core with m-plane side-walls. Directly at the corners of the hexagon a-plane nano-facets with a length of 45 nm are formed. The overgrowth of the GaN core with InGaN leads to a selective formation of Indium-rich domains with triangular cross-section exactly at these nano-facets as evidenced by Z-contrast imaging. Probing the local luminescence properties, the most intense CL emission appears at the m-plane side-facets with 392 nm peak wavelength. As expected, the Indium-rich triangles emit a red-shifted luminescence around 500 nm.
We present a nanometer-scale correlation of the structural, optical, and electronic properties of InGaN/GaN core-shell microrod LEDs: The microrods were fabricated by MOVPE on a GaN/sapphire template covered with an SiO2-mask. Through the mask openings, Si-doped n-GaN cores were grown with high SiH4 flow rate at the base. Subsequently, the SiH4 flow rate was reduced towards the microrod tip to maintain a high surface quality. The Si-doped GaN core was finally encased by an InGaN single quantum well (SQW) followed by an intrinsic GaN layer and a thick Mg-doped p-GaN shell.
Highly spatially resolved cathodoluminescence (CL) directly performed in a scanning transmission electron microscope (STEM) was applied to analyze the free-carrier concentration within the Si-doped GaN core and the luminescence properties of the individual functional layers. The CL was supported by Raman spectroscopy directly carried out at the same microrod on the thin TEM-lamella.
The cross-sectional CL of a single microrod resolves the emission of the single layers. CL and Raman measurements reveal a high free-carrier concentration of 7x1019 cm 3 in the bottom part and a decreasing doping level towards the tip of the microrod. Moreover, structural investigations exhibit that initial Si-doping of the core has a strong influence on the formation of extended defects in the overgrown shells. However, we observe the most intense emission coming from the InGaN QW on the non-polar side walls, which shows a strong red shift along the facet in growth direction due to an increased QW thickness accompanied by an increased indium concentration right at the intersection of generated defects and InGaN QW, a red shifted emission appears, which indicates indium clustering.
Today’s InGaN-based white LEDs still suffer from a significant efficiency reduction at elevated current densities, the so-called “Droop”. Core-shell microrods, with quantum wells (QWs) covering their entire surface, enable a tremendous increase in active area scaling with the rod’s aspect ratio. Enlarging the active area on a given footprint area is a viable and cost effective route to mitigate the droop by effectively reducing the local current density. Microrods were grown in a large volume metal-organic vapor phase epitaxy (MOVPE) reactor on GaN-on-sapphire substrates with a thin, patterned SiO2 mask for position control. Out of the mask openings, pencil-shaped n-doped GaN microrod cores were grown under conditions favoring 3D growth. In a second growth step, these cores are covered with a shell containing a quantum well and a p-n junction to form LED structures. The emission from the QWs on the different facets was studied using resonant temperature-dependent photoluminescence (PL) and cathodoluminescence (CL) measurements. The crystal quality of the structures was investigated by transmission electron microscopy (TEM) showing the absence of extended defects like threading dislocations in the 3D core. In order to fabricate LED chips, dedicated processes were developed to accommodate for the special requirements of the 3D geometry. The electrical and optical properties of ensembles of tens of thousands microrods connected in parallel are discussed.
The controlled growth of highly n-doped GaN micro rods is one of the major challenges in the fabrication of recently developed three-dimensional (3D) core-shell light emitting diodes (LEDs). In such structures with a large active area, higher electrical conductivity is needed to achieve higher current density. In this contribution, we introduce high quality heavily-doped GaN:Si micro-rods which are key elements of the newly developed 3D core-shell LEDs. These structures were grown by metal-organic vapor phase epitaxy (MOVPE) using selective area growth (SAG). We employed spatially resolved micro-Raman and micro-photoluminescence (PL) in order to directly determine a free-carrier concentration profile in individual GaN micro-rods. By Raman spectroscopy, we analyze the low-frequency branch of the longitudinal optical (LO)-phonon-plasmon coupled modes and estimate free carrier concentrations from ≈ 2.4 × 1019 cm−3 up to ≈ 1.5 × 1020 cm-3. Furthermore, free carrier concentrations are determined by estimating Fermi energy level from the near band edge emission measured by low-temperature PL. The results from both methods reveal a good consistency.
Low-cost and low-power piezoresistive cantilever resonators with integrated electrothermal heaters are developed to support the sensing module enhancement of the second generation of handheld cantilever-based airborne nanoparticle (NP) detector (CANTOR-2). These sensors are used for direct-reading of exposure to carbon engineered nanoparticles (ENPs) at indoor workplaces. The cantilever structures having various shapes of free ends are created using silicon bulk micromachining technologies (i.e, rectangular, hammer-head, triangular, and U-shaped cantilevers). For a complete wearable CANTOR-2, all components of the proposed detector can be grouped into two main units depending on their packaging placements (i.e., the NP sampler head and the electronics mounted in a handy-format housing). In the NP sampler head, a miniaturized electrophoretic aerosol sampler and a resonant silicon cantilever mass sensor are employed to collect the ENPs from the air stream to the cantilever surfaces and measuring their mass concentration, respectively. After calibration, the detected ENP mass concentrations of CANTOR-2 show a standard deviation from fast mobility particle sizer (FMPS, TSI 3091) of 8–14%.
Silicon microprobe tips are fabricated and integrated with piezoresistive cantilever sensors for high-speed surface roughness scanning systems. The fabrication steps of the high-aspect-ratio silicon microprobe tips were started with photolithography and wet etching of potassium hydroxide (KOH) resulting in crystal-dependent micropyramids. Subsequently, thin conformal wear-resistant layer coating of aluminum oxide (Al2O3) was demonstrated on the backside of the piezoresistive cantilever free end using atomic layer deposition (ALD) method in a binary reaction sequence with a low thermal process and precursors of trimethyl aluminum and water. The deposited Al2O3 layer had a thickness of 14 nm. The captured atomic force microscopy (AFM) image exhibits a root mean square deviation of 0.65 nm confirming the deposited Al2O3 surface quality. Furthermore, vacuum-evaporated 30-nm/200-nm-thick Au/Cr layers were patterned by lift-off and served as an etch mask for Al2O3 wet etching and in ICP cryogenic dry etching. By using SF6/O2 plasma during inductively coupled plasma (ICP) cryogenic dry etching, micropillar tips were obtained. From the preliminary friction and wear data, the developed silicon cantilever sensor has been successfully used in 100 fast measurements of 5- mm-long standard artifact surface with a speed of 15 mm/s and forces of 60–100 μN. Moreover, the results yielded by the fabricated silicon cantilever sensor are in very good agreement with those of calibrated profilometer. These tactile sensors are targeted for use in high-aspect-ratio microform metrology.
Energy conversion technologies are aiming to extremely high power capacities per year. Nontoxicity and abundance of
the materials are the key requirements to a sustainable photovoltaic technology. Oxides are among the key materials to
reach these goals. We investigate the influence of thin buffer layers on the performance of an ZnO:Al/buffer/Cu2O solar cells. Introduction of a thin ZnO or Al2O3 buffer layer, grown by thermal ALD, between ZnO:Al and Cu2O resulted in 45% increase of the solar cell efficiency. VPE growth of Cu2O employing elemental copper and pure oxygen as precursor materials is presented. The growth is performed on MgO substrates with the (001) orientation. On- and off- oriented substrates have been employed and the growth results are compared. XRD investigations show the growth of the (110) oriented Cu2O for all temperatures, whereas at a high substrate temperature additional (001) Cu2O growth occurs. An increase of the oxygen partial pressure leads to a more pronounced 2D growth mode, whereby pores between the islands still remain. The implementation of off-axis substrates with 3.5° and 5° does not lead to an improvement of the layer quality. The (110) orientation remains predominant, the grain size decreases and the FWHM of the (220) peak increases. From the AFM images it is concluded, that the (110) surface grows with a tilt angle to the substrate surface.
Vertical silicon nanowire (SiNW) resonators are designed and fabricated in order to assess exposure to aerosol
nanoparticles (NPs). To realize SiNW arrays, nanolithography and inductively coupled plasma (ICP) deep reactive ion
etching (DRIE) at cryogenic temperature are utilized in a top-down fabrication of SiNW arrays which have high aspect
ratios (i.e., up to 34). For nanolithography process, a resist film thickness of 350 nm is applied in a vacuum contact mode
to serve as a mask. A pattern including various diameters and distances for creating pillars is used (i.e., 400 nm up to
5 μm). In dry etching process, the etch rate is set high of 1.5 μm/min to avoid underetching. The etch profiles of Si wires
can be controlled aiming to have either perpendicularly, negatively or positively profiled sidewalls by adjusting the
etching parameters (e.g., temperature and oxygen content). Moreover, to further miniaturize the wire, multiple sacrificial
thermal oxidations and subsequent oxide stripping are used yielding SiNW arrays of 650 nm in diameter and 40 μm in
length. In the resonant frequency test, a piezoelectric shear actuator is integrated with the SiNWs inside a scanning
electron microscope (SEM) chamber. The observation of the SiNW deflections are performed and viewed from the
topside of the SiNWs to reduce the measurement redundancy. Having a high deflection of ~10 μm during its resonant
frequency of 452 kHz and a low mass of 31 pg, the proposed SiNW is potential for assisting the development of a
portable aerosol resonant sensor.
Nanopillar-based structures hold promise as highly sensitive resonant mass sensors for a new generation of aerosol
nanoparticle (NP) detecting devices because of their very small masses. In this work, the possible use of a silicon
nanopillar (SiNPL) array as a nanoparticle sensor is investigated. The sensor structures are created and simulated using a
finite element modeling (FEM) tool of COMSOL Multiphysics 4.3 to study the resonant characteristics and the
sensitivity of the SiNPL for femtogram NP mass detection. Instead of using 2D plate models or simple single 3D
cylindrical pillar models, FEM is performed with SiNPLs in 3D structures based on the real geometry of experimental
SiNPL arrays employing a piezoelectric stack for resonant excitation. In order to achieve an optimal structure and
investigate the etching effect on the fabricated resonators, SiNPLs with different designs of meshes, sidewall profiles,
lengths, and diameters are simulated and analyzed. To validate the FEM results, fabricated SiNPLs with a high aspect
ratio of ~60 are employed and characterized in resonant frequency measurements. SiNPLs are mounted onto a
piezoactuator inside a scanning electron microscope (SEM) chamber which can excite SiNPLs into lateral vibration. The
measured resonant frequencies of the SiNPLs with diameters about 650 nm and heights about 40 μm range from 434.63
kHz to 458.21 kHz, which agree well with those simulated by FEM. Furthermore, the deflection of a SiNPL can be
enhanced by increasing the applied piezoactuator voltage. By depositing different NPs (i.e., carbon, TiO2, SiO2, Ag, and
Au NPs) on the SiNPLs, the decrease of the resonant frequency is clearly shown confirming their potential to be used as
airborne NP mass sensor with femtogram resolution level.
A closed loop circuit capable of tracking resonant frequencies for MEMS-based piezoresistive cantilever resonators is
developed in this work. The proposed closed-loop system is mainly based on a phase locked loop (PLL) circuit. In order
to lock onto the resonant frequency of the resonator, an actuation signal generated from a voltage-controlled oscillator
(VCO) is locked to the phase of the input reference signal of the cantilever sensor. In addition to the PLL component, an
instrumentation amplifier and an active low pass filter (LPF) are connected to the system for gaining the amplitude and
reducing the noise of the cantilever output signals. The LPF can transform a rectangular signal into a sinusoidal signal
with voltage amplitudes ranging from 5 to 10 V which are sufficient for a piezoactuator input (i.e., maintaining a large
output signal of the cantilever sensor). To demonstrate the functionality of the system, a self-sensing silicon cantilever
resonator with a built-in piezoresistive Wheatstone bridge is fabricated and integrated with the circuit. A piezoactuator is
utilized for actuating the cantilever into resonance. Implementation of this closed loop system is used to track the
resonant frequency of a silicon cantilever-based sensor resonating at 9.4 kHz under a cross-sensitivity test of ambient
temperature. The changes of the resonant frequency are interpreted using a frequency counter connected to the system.
From the experimental results, the temperature sensitivity and coefficient of the employed sensor are 0.3 Hz/°C and 32.8
ppm/°C, respectively. The frequency stability of the system can reach up to 0.08 Hz. The development of this system
will enable real-time nanoparticle monitoring systems and provide a miniaturization of the instrumentation modules for
cantilever-based nanoparticle detectors.
The development of low-cost and low-power MEMS-based cantilever sensors for possible application in hand-held
airborne ultrafine particle monitors is described in this work. The proposed resonant sensors are realized by silicon bulk
micromachining technology with electrothermal excitation, piezoresistive frequency readout, and electrostatic particle
collection elements integrated and constructed in the same sensor fabrication process step of boron diffusion. Built-in
heating resistor and full Wheatstone bridge are set close to the cantilever clamp end for effective excitation and sensing,
respectively, of beam deflection. Meanwhile, the particle collection electrode is located at the cantilever free end. A 300
μm-thick, phosphorus-doped silicon bulk wafer is used instead of silicon-on-insulator (SOI) as the starting material for
the sensors to reduce the fabrication costs. To etch and release the cantilevers from the substrate, inductively coupled
plasma (ICP) cryogenic dry etching is utilized. By controlling the etching parameters (e.g., temperature, oxygen content,
and duration), cantilever structures with thicknesses down to 10 - 20 μm are yielded. In the sensor characterization, the
heating resistor is heated and generating thermal waves which induce thermal expansion and further cause mechanical
bending strain in the out-of-plane direction. A resonant frequency of 114.08 ± 0.04 kHz and a quality factor of 1302 ±
267 are measured in air for a fabricated rectangular cantilever (500x100x13.5 μm3). Owing to its low power
consumption of a few milliwatts, this electrothermal cantilever is suitable for replacing the current external piezoelectric
stack actuator in the next generation of the miniaturized cantilever-based nanoparticle detector (CANTOR).
A method for die-attach based on sintering of micro- and nano-silver-particles, which is stable in harsh environments, was described. A modified flip-chip bonder providing high placement accuracy was used for precise pick and place die-attach. Components of sensors designed for data logging during deep drilling, i.e., a MEMS vibration sensor and a MOEMS pressure sensor, were assembled and tested at temperatures up to 250°C. Shear tests of bonded devices were performed before and after temperature load. Bonded silicon-on-insulator Wheatstone bridges and GaP-PD were tested by temperature cycling (50 cycles from 100°C up to 250°C).
We present a computational study on the anisotropic luminescence and the efficiency of a core-shell type nanowire
LED based on GaN with InGaN active quantum wells. The physical simulator used for analyzing this device
integrates a multidimensional drift-diffusion transport solver and a k · p Schr¨odinger problem solver for quantization
effects and luminescence. The solution of both problems is coupled to achieve self-consistency. Using this
solver we investigate the effect of dimensions, design of quantum wells, and current injection on the efficiency and
luminescence of the core-shell nanowire LED. The anisotropy of the luminescence and re-absorption is analyzed
with respect to the external efficiency of the LED. From the results we derive strategies for design optimization.
Thermoelectric modules convert thermal energy into electrical energy and vice versa. At present bismuth telluride is the
most widely commercial used material for thermoelectric energy conversion. There are many applications where bismuth
telluride modules are installed, mainly for refrigeration. However, bismuth telluride as material for energy generation in
large scale has some disadvantages. Its availability is limited, it is hot stable at higher temperatures (>250°C) and
manufacturing cost is relatively high. An alternative material for energy conversion in the future could be silicon. The
technological processing of silicon is well advanced due to the rapid development of microelectronics in recent years.
Silicon is largely available and environmentally friendly. The operating temperature of silicon thermoelectric generators
can be much higher than of bismuth telluride. Today silicon is rarely used as a thermoelectric material because of its high
thermal conductivity. In order to use silicon as an efficient thermoelectric material, it is necessary to reduce its thermal
conductivity, while maintaining high electrical conductivity and high Seebeck coefficient. This can be done by
nanostructuring into arrays of pillars. Fabrication of silicon pillars using ICP-cryogenic dry etching (Inductive Coupled
Plasma) will be described. Their uniform height of the pillars allows simultaneous connecting of all pillars of an array.
The pillars have diameters down to 180 nm and their height was selected between 1 micron and 10 microns.
Measurement of electrical resistance of single silicon pillars will be presented which is done in a scanning electron
microscope (SEM) equipped with nanomanipulators. Furthermore, measurement of thermal conductivity of single pillars
with different diameters using the 3ω method will be shown.
For future measurements while depth drilling, commercial sensors are required for a temperature range from -40 up to
300 °C. Conventional piezoresistive silicon sensors cannot be used at higher temperatures due to an exponential increase
of leakage currents which results in a drop of the bridge voltage. A well-known procedure to expand the temperature
range of silicon sensors and to reduce leakage currents is to employ Silicon-On-Insulator (SOI) instead of standard wafer
material. Diffused resistors can be operated up to 200 °C, but show the same problems beyond due to leakage of the p-njunction.
Our approach is to use p-SOI where resistors as well as interconnects are defined by etching down to the oxide
layer. Leakage is suppressed and the temperature dependence of the bridges is very low (TCR = (2.6 ± 0.1) μV/K@1 mA
up to 400 °C).
The design and process flow will be presented in detail. The characteristics of Wheatstone bridges made of silicon, n-
SOI, and p-SOI will be shown for temperatures up to 300 °C. Besides, thermal FEM-simulations will be described
revealing the effect of stress between silicon and the silicon-oxide layer during temperature cycling.
A silicon cantilever with slender geometry based Micro Electro Mechanical System (MEMS) for nanoparticles mass
detection is presented in this work. The cantilever is actuated using a piezoactuator at the bottom end of the cantilever
supporting frame. The oscillation of the microcantilever is detected by a self-sensing method utilizing an integrated full
Wheatstone bridge as a piezoresistive strain gauge for signal read out. Fabricated piezoresistive cantilevers of 1.5 mm
long, 30 μm wide and 25 μm thick have been employed. This self-sensing cantilever is used due to its simplicity,
portability, high-sensitivity and low-cost batch microfabrication. In order to investigate air pollution sampling, a
nanoparticles collection test of the piezoresistive cantilever sensor is performed in a sealed glass chamber with a stable
carbon aerosol inside. The function principle of cantilever sensor is based on detecting the resonance frequency shift that
is directly induced by an additional carbon nanoparticles mass deposited on it. The deposition of particles is enhanced by
an electrostatic field. The frequency measurement is performed off-line under normal atmospheric conditions, before and
after carbon nanoparticles sampling. The calculated equivalent mass-induced resonance frequency shift of the
experiment is measured to be 11.78 ± 0.01 ng and a mass sensitivity of 8.33 Hz/ng is obtained. The proposed sensor
exhibits an effective mass of 2.63 μg, a resonance frequency of 43.92 kHz, and a quality factor of 1230.68 ± 78.67.
These results and analysis indicate that the proposed self-sensing piezoresistive silicon cantilever can offer the necessary
potential for a mobile nanoparticles monitor.
Nanoparticles are easily attracted by surfaces. This sticking behavior makes it difficult to clean contaminated samples.
Some complex approaches have already shown efficiencies in the range of 90%. However, a simple and cost efficient
method was still missing. A commonly used silicone for soft lithography, PDMS, is able to mold a given surface. This
property was used to cover surface-bonded particles from all other sides. After hardening the PDMS, particles are still
embedded. A separation of silicone and sample disjoins also the particles from the surface. After this procedure, samples
are clean again. This method was first tested with carbon particles on Si surfaces and Si pillar samples with aspect ratios
up to 10. Experiments were done using 2 inch wafers, which, however, is not a size limitation for this method.
We achieved to etch nano- and deep structures in silicon using ICP-cryogenic dry etching process. We etched nanopores
and nanocantilevers with an etch rate of 13 nm/min, nanopillars with an etch rate of 2.8 μm/min - 4.0 μm/min, membrane
and cantilever structures with an etch rate of 4 μm/min and 3 μm/min, respectively. Nanopores and nanocantilevers
are interesting structures for Bionanoelectronics. Nanopillars can be used as substrates/templates for the MOCVD
growth of GaN nanoLEDs. They are the basic constituents of a nanoparticle balance and also of a thermoelectric generator.
For the joining of the silicon wafers of the thermoelectric generator the low temperature joining technique can be
used. Cantilevers can be used for sensing, e.g. as tactile cantilevers. They can be used also as resonator for mass sensing
even in the subnanogram region. The actuation of the resonator can be done by using piezoelectric thin films on the
cantilevers. The mass detection depends on the resonance frequency shift caused by loaded mass on the cantilevers. Such
cantilevers are robust and easy to produce. The deep etching in silicon was done by using a photoresist mask and
creating perpendicular and smooth sidewalls.
An optically pumped ZnO distributed feedback laser operating at 383 nm has been designed, fabricated and
characterized. Single mode operation was observed for a wide temperature range between 10 and 270 K. In order to
avoid technologically difficult etching of ZnO, a 3rd order diffraction grating was dry-etched into an additional 120 nm-thick
Si3N4 layer deposited on the ZnO active region. The spectral linewidth of the laser emission was 0.4 nm, whereas
an optical pump threshold intensity of 0.12 MW/cm2 and a peak output power of 14 mW were seen. The temperature
tuning coefficient of the ZnO refractive index was determined from wavelength vs. temperature measurements; a value
of 9 × 10-5 K-1 was found, in good agreement with literature values.
We present an experimental data, which demonstrate a basically new mechanism of carrier radiative recombination in semiconductor heterostructures-recombination via Tamm-like interface states. Bright line was observed in photoluminescence spectra of periodical ZnSe/BeTe heterostructures at the energies, which correspond to the optical transitions between electron and hole Tamm-like interface states in studied heterosystem. Photoluminescence via Tamm-like interface states was observed for wide range excitation densities in the temperature range from 15K to 160K. It was found that for short-period ZnSe/BeTe heterostructures at low temperatures and at low excitation densities photoluminescence via Tamm-like interface states is much stronger than conventional interband radiative recombination.
Optical properties of ZnO doped with Mn and V were studied. Zn(Mn)O layers were grown by peroxide MBE, and Zn(V)O was prepared by high-dose ion implantation of bulk ZnO prepared by hydrothermal technique. The Zn(Mn)O layers containing up to 50% of Mn were characterized by high-resolution x-ray diffraction, photoluminescence, and optical absorption. A blue shift of the band edge revealed from optical absorption measurements points to the incorporation of at least a part of Mn atoms on the lattice sites. An increase in the Zn(Mn)O band gap and an enhancement of the broad below band gap absorption associated with Mn ions were observed with increasing Mn composition. Correlating structural and optical transmission data, we suggest that the band edge of Zn(Mn)O rises linearly with the amount of Mn ions substituting Zn on the lattice sites. Photoluminescence of ZnO moderately doped with Mn shows several emission lines (the strongest ones are located at 3.34 and 3.36 eV). Surprisingly, no shift in the near-band-edge emission (3.36 eV) was detected in the photoluminescence data. Photoluminescence excitation studies revealed that the near-band-edge peak and the peak centered around 3.34 eV have different origin. Most probably, the second line is due to Mn intracenter transitions. Photoluminescence studies of ZnO bulk samples implanted with V+ have revealed that thermal annealing at 800 °C restores to a large extent the optical quality of the material. A new emission line centered at 3.307 eV has been found in the photoluminescence spectrum of the highly conductive samples implanted with a V dose of 1 × 1016 cm-2.
Spectral response of lateral optical anisotropy of periodic undoped type-II ZnSe/BeTe heterostructures with nonequivalent interfaces has been studied by spectroscopic ellipsometry. The spectra revealed two types of features corresponding to optical transitions with energies lying in the bandgap. The position of features of the first type does not depend on the heterostructure period. Features of the second type shift toward lower energies with decreasing period of the heterostructure. This behavior is explained in terms of a model taking into account the existence of electronic and hole interface states, as well as of a mixed-type interface state.
The observation of optical gain at the trion transition of n-doped ZnSe quantum wells is reported. The specific optical coupling between the trion and electron band gives rise to stimulated emission on the low-energy wing of the trion photoluminescence band without degeneracy and inversion in the total particle numbers. Gain values as large as 104 cm-1 are found for excitation intensities of some kW/cm2. A calculation of the absorption-gain crossover photon energy based on a kinetically determined equilibrium of excitons, trions and electrons with a common carrier gas temperature describes the experimental data well.
Ferromagnetic Ga1-xMnxAs films containing up to 5.1 at%Mn were grown by low-temperature MBE. The structural, electrical, and magnetic properties of the layers are reported. At x > 0.01, the materials show a ferromagnetic behavior. The Curie temperature reaches 80 K at 5.1at% Mn. We propose the use of a n+-GaAs/p+-GaMnAs Esaki-diode (ferromagnetic Esaki-diode, FED) to provide injection of spin-polarized electrons via interband tunneling. Under reverse bias, spin-polarized electrons at the Fermi level in the valence band of GaMnAs tunnel to the conduction band of GaAs in contrast to the injection of spin-polarized holes used before.
An additional approach to further improve the reliability of ZnSe based devices is to use beryllium containing II-VI compounds. BeS, BeSe and BeTe are characterized by a considerable amount of covalent bonding and a high bond energy. This distinguishes these materials from the conventional ionic wide gap II-VI semiconductors like ZnSe, ZnTe or CdTe. Recently, thin film structures using Be- compounds have been fabricated and characterized. It became clear that--besides the application aspects--these materials are also very interesting from a more fundamental point of view. Using Be-containing II-VI compounds, ionic and covalent lattice matched II-VI materials can be combined in quantum well structures. The type II band alignment of BeTe and ZnSe gives additional freedom in the band gap engineering, and it is possible to grow lattice method quaternaries of low polarity onto silicon. Here, basic principles of Be containing II-VI compounds will be described, and the potential of these novel materials will be discussed.
Beryllium containing ZnSe-based compound semiconductors introduce substantial additional degrees of freedom for the design of wide gap II-VI heterostructures. Interesting aspects are the lattice matching of BeTe with its high lying valence band and high p-type dopability for the growth of graded gap contacts capable of carrying high current densities, as well as the expected strengthening of quaternary beryllium compounds like BeMgZnSe as compared to the II-VI materials used on the basis of ZnMgSSe. They have a large covalency and therefore large bond energy. The covalency of BeSe e.g. is expected to be as high as the one of GaN. The fabrication of light emitting devices like LEDs and laser diodes is reported.
We report a pronounced effect on an additional hole confinement in the electron Coulomb potential on giant splitting and oscillator strengths of the exciton Zeeman patterns in CdTe/(Cd,Mn)Te quantum well structures. Measuring oscillator strength and Zeeman splitting as a function of the quantum well width and/or an external magnetic field by resonance reflection spectroscopy we demonstrate the transition from a hole subjected to a net potential well to that confined to the electron Coulomb potential occurring in quantum wells thicker than 30 angstroms. This transition is also found in superlattices where the electron wave function changes its character from three- to two-dimensional with an increase of the superlattices period. Analyzing the magnetooptical data taken above and below the transition we find the hole confinement in the electron Coulomb potential to decrease a hole wave function penetration into semimagnetic barriers to decrease the strength of the exchange interaction of holes with magnetic ions.
Interest in CdTe field effect transistors and multi-gated devices stems from the fact that CdTe is lattice matched to HgCdTe. As a consequence it may be possible to develop a monolithic technology that combines HgCdTe infrared focal plane arrays with on-board signal processing based on CdTe devices. Although CdTe metal-semiconductor field effect transistors have only recently been fabricated rapid improvement in device performance has been achieved. All the devices reported have been fabricated from CdTe:In epilayers grown by Photoassisted Molecular Beam Epitaxy. We report on devices having gold Schottky barrier with reverse breakdown voltages as high as 28. 0 V and ideality factors near 1. 7. These MESFETs exhibit good depleting mode action.
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