The optical polarization of the in-plane emission of c-plane oriented (In)(Al)GaN multiple quantum well light emitting
diodes in the spectral range from 288 nm to 386 nm has been investigated by electroluminescence measurements. The
intensity of transverse-electric polarized light relative to the transverse-magnetic polarized light decreases with
decreasing emission wavelength. This effect is attributed to the different electronic band structures in the active region of
the light emitting diodes. A changing aluminum and indium mole fraction in the (In)(Al)GaN quantum wells results in a
rearrangement of the valence bands at the Γ-point of the Brillouin zone. For shorter wavelengths the crystal-field splitoff
hole band moves closer to the conduction band relative to the heavy and light hole bands and as a consequence the
transverse-magnetic polarized emission increases. Moreover, the in-plane polarization is shown to depend on the
injection current. The correlation between the in-plane polarization and the injection current has been found to be
different for light emitting diodes with InGaN and (In)AlGaN multiple quantum wells. The results highlight that
polarization effects need to be considered when optimizing the light extraction from ultraviolet light emitting diodes in
the (In)AlGaN materials system.
We describe recent work on InGaN lasers and AlGaN UV LEDs at the Palo Alto Research Center (PARC). The
presentation includes results from InGaN laser diodes in which the usual epitaxial upper cladding layer is replaced with
an evaporated or sputtered non-epitaxial material, such as indium tin oxide, silver, or a silver-palladium-copper alloy [1,
2]. Non-epitaxial cladding layers offer several advantages to long wavelength InGaN laser diodes, such as eliminating
the need to expose vulnerable InGaN active layers to the high temperatures required for growing conventional p-AlGaN
cladding layers subsequent to the active layer growth.
The presentation also discusses our recent results on AlGaN UV LEDs. UV LEDs with 300 micron square geometries
operating at λ = 325 nm exhibit output powers of 13 mW with differential quantum efficiencies of 0.054 W/A measured
under wafer-level, unpackaged condition with no heat sink. LEDs operating at λ = 290 nm under similar test conditions
display output powers of 1.6 mW for large-area 300 μm X 1 mm devices.
Results for long-wavelength emitters are presented for semi-polar InGaN/AlGaN/GaN heterostructures grown on
GaN(1122)/m-sapphire templates by metalorganic chemical vapor deposition. The semi-polar GaN layers were 10 to 25
μm thick and grown by HVPE on sapphire substrates. X-ray diffraction measurements indicated high crystallographic
quality that approaches that of GaN(0001) layers on sapphire. A comparison based on optical pumping experiments,
low- and high-density excitation photoluminescence experiments, and atomic force microscopy is drawn between
InGaN/GaN quantum well laser heterostructures grown by metalorganic vapor phase epitaxy either on either polar
GaN(0001)/c-sapphire or on semi-polar GaN(1122)/m-sapphire. C-plane InGaN/GaN/sapphire structures exhibited low
threshold pump power densities < 500 kW/cm<sup>2</sup> for emission wavelengths up to 450 nm. For laser structures beyond 450
nm the threshold pump power density rapidly increased resulting in a maximum lasing wavelength of 460 nm. Semipolar
InGaN/GaN(1122)/m-sapphire structures showed a factor of 2-4 higher threshold pump power densities at
wavelengths below 440 nm which is partly due to lower crystalline perfection of the semi-polar GaN/sapphire templates.
However, at longer wavelengths > 460 nm the threshold power density for lasing of semi-polar heterostructures is less
than that for c-plane heterostructures which enabled rapid progress to demonstration of lasing at 500 nm wavelength on
semi-polar heterostructures. The absence of V-type defects in semi-polar, long-wavelength InGaN/GaN structures which
are usually present in long-wavelength c-plane InGaN/GaN structures is attributed to this phenomenon.
We present a novel optoelectronic package incorporating Vertical-Cavity Surface-Emitting Laser (VCSEL) arrays with built-in power monitors. The power monitor consists of a thin film amorphous silicon p-i-n photodetector that is fabricated on glass. Sets of micro-machined springs for electrical contacting are also fabricated in the same process on the same glass substrate. The springs are made by sputtering, masking, and releasing a stress-engineered conductive thin film. The stress-engineered film is patterned into electrical routing wires whose ends curl up into compliant springs when released from the substrate. Hybrid packages are formed by pressing the micro-machined springs against individual contact pads of the GaAs VCSEL array in a flip-chip assembly process. The power monitor is designed so it lies directly in front of the laser array in the path of the light after module assembly. Although only about 2% of the laser power is absorbed by the sensor, a large signal to noise ratio is retained because of the sensor’s extremely low dark current. Our typical laser output powers of about 1 mW at wavelengths of 811 nm produces power monitor photocurrents of 0.5 to 1 μA, which, for our detectors, correspond to dynamic ranges of over five orders of magnitude.
We report on out-of-plane micro-machined inductors exhibiting record high quality factors (Q) on silicon integrated circuits. The coils are made by three-dimensional self-assembly of stress-engineered structures fabricated with standard semiconductor batch processing techniques. Coils fabricated on low resistance CMOS-compatible silicon exhibit quality factors of over 70 at 1 GHz. BiCMOS test oscillators utilizing these micro-machined coils show significant phase noise reduction over similar oscillators using conventional spiral coils.
We report on a novel flip-chip packaging technology capable of interconnecting devices packed at very high density. The process utilizes micro-machined cantilevers for establishing electrical contact, where package assembly is performed at room temperature without solder. The cantilevers, called micro-springs, are fabricated by sputtering, masking, and releasing a stress-engineered conductive thin film on a quartz substrate. The film is patterned into electrical routing wires whose ends are released from the substrate. Upon release, the film stress forces the ends to curl up into compliant springs. Packages are formed by pressing the micro-springs against a set of device contact pads, much like probing pads using tungsten needle probes. The connections between springs and contact pads are anchored by an encapsulating acrylic adhesive. We utilize this packaging technology to interconnect 200-element arrays of independently addressable VCSELs with 4 micrometer-wide pads on 6 micrometer pitch to silicon CMOS driver chips with equally dense output lines. Tests show the technology produces good contacts with excellent robustness.
A new type of compliant interconnect derived from a thin metal film fabricated with a controlled stress profile is being developed for flip- flop interconnects and probing devices. Interconnections have been demonstrated on lateral pitches as tight as 6 microns. The interconnect is highly elastic and can provide up to hundreds of microns of vertical compliance.
We disclose a method of eliminating the polarization instability in laterally-oxidized vertical-cavity surface- emitting lasers. By employing an appropriately-shaped device aperture, we are able to make the lasers operate in a single polarization direction through their entire L-I curve.
We report on our efforts to develop a laser printbar consisting of a very dense array of independently addressable laterally-oxidized top-emitting VCSELs. In order to maintain wafer planarity for easy electrical routing, the buried oxidation layer in our structure is accessed through small via holes instead of a more typical mesa etch. Unlike most VCSELs, our devices utilize transparent indium-tin- oxide top contacts that allow for a more compact device design. The 200-element array we fabricated has a linear density of one device every 3 micrometers .
In this work, we demonstrate fusion of GaAs-based laser structures to GaN-based light-emitting diode (LED) heterostructures. Successful operation of red and infrared lasers fused to functioning GaN LEDs is achieved. A single heterostructure consisting of AlGaInAs/AlGaAs quantum well (QW) and GaInP/AlGaInP QW laser diode structures was grown by low-pressure organometallic vapor phase epitaxy (OMVPE) on GaAs substrates. The GaN LED structure was grown by OMVPE on an A-face sapphire substrate. The heterostructures were fused at 650 degrees Celsius in an H<SUB>2</SUB> ambient, while under uniaxial pressure. To fabricate the lasers, the GaAs substrate was selectively etched, leaving the red and infrared QW laser stack structure on GaN. Ridge waveguide QW lasers and GaN LEDs were fabricated with the fused epilayers. Infrared, AlGaInAs QW lasers (4 X 500 micrometer), operated with a threshold current (I<SUB>th</SUB>) of 40 mA and external differential quantum efficiency ((eta) <SUB>d</SUB>) of 11.5%/facet at about 821 nm. Red, GaInP QW lasers (4 X 500 micrometer), operated with a I<SUB>th</SUB> of 118 mA and (eta) <SUB>d</SUB> of 18.7%/facet at about 660 nm. The adjacent InGaN/GaN LED emitted at 446 nm.