GaN is an attractive material for high-power electronics due to its wide bandgap and large breakdown field. Verticalgeometry devices are of interest due to their high blocking voltage and small form factor. One challenge for realizing complex vertical devices is the regrowth of low-leakage-current p-n junctions within selectively defined regions of the wafer. Presently, regrown p-n junctions exhibit higher leakage current than continuously grown p-n junctions, possibly due to impurity incorporation at the regrowth interfaces, which consist of c-plane and non-basal planes. Here, we study the interfacial impurity incorporation induced by various growth interruptions and regrowth conditions on m-plane p-n junctions on free-standing GaN substrates. The following interruption types were investigated: (1) sample in the main MOCVD chamber for 10 min, (2) sample in the MOCVD load lock for 10 min, (3) sample outside the MOCVD for 10 min, and (4) sample outside the MOCVD for one week. Regrowth after the interruptions was performed on two different samples under n-GaN and p-GaN growth conditions, respectively. Secondary ion mass spectrometry (SIMS) analysis indicated interfacial silicon spikes with concentrations ranging from 5e16 cm-3 to 2e18 cm-3 for the n-GaN growth conditions and 2e16 cm-3 to 5e18 cm-3 for the p-GaN growth conditions. Oxygen spikes with concentrations ~1e17 cm-3 were observed at the regrowth interfaces. Carbon impurity levels did not spike at the regrowth interfaces under either set of growth conditions. We have correlated the effects of these interfacial impurities with the reverse leakage current and breakdown voltage of regrown m-plane p-n junctions.
KEYWORDS: Gallium nitride, Diffusion, Electron beams, Switching, Metalorganic chemical vapor deposition, Raman spectroscopy, Luminescence, Temperature metrology, Current controlled current source
GaN is a promising material for a range of high power, high frequency, and high temperature device applications. The wide bandgap of GaN leads to high breakdown voltages and low switching losses. Recently, large HVPE grown GaN substrates have become available for homoepitaxial growth leading to ~103 reduction in dislocation density compared to similarly grown heteroepitaxial films. Device performance, however, is ultimately limited by the transport properties of minority carriers. Measured values for minority carrier diffusion lengths and lifetimes in GaN vary widely1-3, and a recent report suggests flaws in the commonly adopted electron beam induced current (EBIC) method in the planar-collection geometry.1 Here we report on EBIC measurements performed on ~8 micrometer thick GaN grown by MOCVD on 2 inch GaN substrates with a n-type dopant density of ~5×1016 cm-3. We determine the carrier diffusion length using both the conventional EBIC method by varying the electron beam-to-contact lateral distance as well as by varying the electron beam penetration depth and find the extracted values differ by ~10×, with the latter technique yielding the lower diffusion length. We rationalize these results in light of other materials characteristics including Raman and photoluminescence. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.
It is well known that hydrogen passivation of Mg in Mg-doped GaN reduces free hole concentrations. While
there are numerous studies of passivation of Mg in GaN, little work has been reported concerning passivation rates in
AlGaN alloys. We investigated the hydrogen interaction with Mg in nitrides by measuring the intensity of the electron
paramagnetic resonance (EPR) signal associated with the acceptor. The samples were isothermally annealed in
sequential steps ranging from 5 min - 6.6 h between 300 and 700 oC in H2:N2 (7%: 92%) or pure N2. The signal intensity
decreased during the H2N2 anneal and was revived by the N2 anneal as expected; however, the rate at which the intensity
changed was shown to depend on Al concentration. In addition, while all signals were quenched at 700 oC in H2:N2, a
750 oC N2 anneal reactivated only about 30% of the Mg in the alloys and 80% of the intensity in the GaN film. These
data suggest that the rate of passivation and activation of Mg by hydrogen is dependent on the concentration of Al in the
AlxGa-1xN layer. The EPR annealing data could prove to be beneficial in improving p-type optimization in AlGaN
alloys.
An AlGaN Light-emitting diode (LED) emitting with a peak wavelength at 291 nm and a radiant power of 0.5 mW @ 100 mA was fabricated on a sapphire substrate. A compact gated fluorescence detection system was built using this LED as the excitation light source. We demonstrate that it provides sufficient power using Terbium enhanced fluorescence to detect subnanomolar concentrations of dipicolinic acid (DPA, 2, 6-pyridinedicarboxylic acid), a substance uniquely present in bacterial spores such as that from B. anthracis, providing a basis for convenient early warning detectors. We also describe initial results from a novel approach for biological aerosol detection using long lived fluorescence from a Europium tagged dye that binds to proteins.
Deep ultraviolet light emitting diodes (LEDs) with emission wavelengths shorter than 300 nm have been grown by metalorganic vapor phase epitaxy. A bottom emitting LED design is used which requires a high-Al content AlxGa1-xN (x = 0.5 - 0.8 ) buffer layer which has sufficient conductivity and is transparent to the quantum well emission wavelength. LEDs were flip chip mounted to a silicon submount which provides for good thermal performance as well as improved light extraction. For large area 1 mm x 1 mm LEDs emitting at 297 nm, an output power as high as 2.25 mW under direct current operation has been demonstrated at 500 mA with a forward voltage of 12.5 volts. For shorter wavelength LEDs emitting at 276 nm, an output power as high as 1.3 mW has been demonstrated under direct current operation at 300 mA with a forward voltage of 9.2 volts. Recent improvements in heterostructure design have resulted in quantum well emission at 276 nm with a peak intensity that is 330 times stronger than the largest sub-bandgap peak. LEDs with emission wavelengths as short as 237 nm have also been demonstrated.
In this paper, we overview the critical materials challenges in the development of AlGaN-based deep ultraviolet light emitting diodes (LEDs) and present our recent advances in the performance of LEDs in the 275-290 nm range. Our primary device design involves a flip-chip, bottom emitting, transparent AlGaN (Al = 47-60%) buffer layer structure with interdigitated contacts. To date, under direct current operation, we have demonstrated greater than 1 mW of output power at 290 nm with 1 mm x 1 mm LEDs, and greater than 0.5 mW output power from LEDs emitting at wavelengths as short at 276 nm. Electroluminescence spectra demonstrate both a main peak from quantum well emission as well as sub-bandgap emission originating from radiative recombination involving deep level states. The heterostructure designs that we have employed have greatly suppressed this deep level emission, resulting in deep level peak intensities that are 40-125X lower than the primary quantum well emission for different LED designs and applied current densities.
Significant advancements have been made in the characterization and understanding of the degradation behavior of the III-V semiconductor materials employed in Vertical Cavity Surface Emitting Laser (VCSEL) diodes. Briefly, for the first time a technique has been developed whereby it is possible to view the entire active region of a solid state laser in a Transmission Electron Microscope (TEM) using a novel Focussed Ion Beam (FIB) prepared plan-view sample geometry. This technique, in conjunction with TEM cross-section imaging has enabled a three-dimensional characterization of several of the degradation mechanisms that lead to laser failure. It is found that there may occur an initial drop in laser power output due to the development of cracks in the upper mirror layers. In later stages of degradation, dislocations are punched out at stress-concentrating sites (e.g. oxide aperture tips) and these dislocations can then extend over the active region in a manner consistent with recombination enhanced dislocation motion. Alternatively, complex three-dimensional dislocation arrays which exhibited dendritic-like growth and which cover the entire active region can nucleate on a single defect.
Vertical-cavity surface-emitting lasers (VCSELs) are uniquely suited for massively parallel interconnects and scannerless imaging applications due to their small size, high efficiency and amiability to formation of high-density 2-dimensional arrays. We have successfully fabricated 4096 element arrays (64×64) containing alternating rows of selectively-oxidized 850 nm VCSELs and resonant-cavity photodetectors (RCPDs) on a 55 micron pitch monolithically integrated on semi-insulating GaAs substrates. We employ a matrix addressable architecture to reduce the input and output electrical connections to the array, where all the VCSELs (or RCPDs) in each row are connected by a common metal trace at the base of their mesas. The columns are connected by metal traces that bridge from mesa top to mesa top, connecting every other row (i.e., only VCSELs or only RCPDs). The design, fabrication and performance of these arrays is discussed.
Vertical-cavity surface-emitting lasers (VCSELs) are uniquely suited for applications requiring high-density 2-dimensional arrays of lasers, such as massively parallel interconnects or imaging applications. We have successfully fabricated 64x64 arrays containing alternating rows of selectively-oxidized 850 nm VCSELs and resonant-cavity photodetectors (RCPDs) on semi-insulating GaAs. In order to reduce the input/output pin count, we employed a matrix addressable architecture, where all the VCSELs (or RCPDs) in each row are connected by a common metal trace at the base of their mesas. The columns are connected by metal traces that bridge from mesa top to mesa top, connecting every other row (i.e., only VCSELs or only RCPDs). The pitch of devices in the array is 55 microns, and total resistance contributed by the long (up to 3.5 mm) row and column traces is below 50 ohms. The epitaxial design, fabrication and performance of these arrays is discussed.
InxGa1-xAs1-yNy quaternary alloys offer the promise of longer wavelength, >= 1.3 micrometers optical transceivers grown on GaAs substrates. To achieve acceptable radiative efficiencies at 1.3 micrometers , highly- strained InGaAsN quantum wells (x approximately equals 0.4, y approximately equals 0.005) are being developed as laser active regions. By introducing GaAsP layers into the active region for strain-compensation, gain can be increased using multiple InGaAsN quantum wells. In this work, we report the first strain-compensated, 1.3 micrometers InGaAsN MQW lasers. Our devices were grown by metal- organic chemical vapor deposition. Lasers with InGaAsN quantum well active regions are proving superior to lasers constructed with competing active region materials. Under pulsed operation, our 1.3 micrometers InGaAsN lasers displayed negligible blue-shift from the low-injection LED emission, and state-of-the-art characteristic temperature (159 K) was obtained for a 1.3 micrometers laser.
The optical gain spectra for GaInNAs/GaAs quantum wells are computed using a microscopic laser theory. From these spectra, the peak gain and carrier radiative decay rate as functions of carrier density are determined. These dependences allow the study of the lasing threshold current density of GaInNAs/GaAs quantum well structures.
The variation of the value of the linewidth of an excitonic transition in InGaAsN alloys (1% and 2% nitrogen) as a function of hydrostatic pressure using photoluminescence spectroscopy is studied at 4K. The excitonic linewidth increases as a function of pressure until about 100 kbar after which it tends to saturate. This pressure dependent excitonic linewidth is used to derive the pressure variation of the exciton reduced mass using a theoretical formalism based on the premise that the broadening of the excitonic transition is caused primarily by compositional fluctuations in a completely disordered alloy. The linewidth derived ambient pressure masses are compared and found to be in agreement with other mass measurements. The variation of this derived mass is compared with the results from a nearly first-principles approach in which calculations based on the local density approximation to the Kohn-Sham density functional theory are corrected using a small amount of experimental input.
Massively parallel optical interconnects are appropriate to ease the data bandwidth bottleneck that will occur in future computing applications. Vertical cavity surface emitting lasers (VCSELs) are promising sources for emerging 2D optical systems such as free space and guided wave optical interconnects. We discuss the development of high performance VCSEL arrays, including individually addressable and matrix addressable arrays. We also show the characteristics of GaAs microelectronic driver and photoreceiver chips that have been designed to interface with Si-based CMOS circuitry. Finally, the potential of these source and receiver modules for use in free space or guided wave parallel channel optical interconnect architectures will be described.
Vertical cavity surface emitting lasers (VCSELs) which operate in multiple transverse optical modes have been rapidly adopted into present data communication applications which rely on multi-mode optical fiber. However, operation only in the fundamental mode is required for free space interconnects and numerous other emerging VCSEL applications. Two device design strategies for obtaining single mode lasing in VCSELs based on mode selective loss or mode selective gain are reviewed and compared. Mode discrimination is attained with the use of a thick tapered oxide aperture positioned at a longitudinal field null. Mode selective gain is achieved by defining a gain aperture within the VCSEL active region to preferentially support the fundamental mode. VCSELs which exhibit greater than 3 mW of single mode output power at 850 nm with mode suppression ratio greater than 30 dB are reported.
The impressive performance improvements of laterally oxidized VCSELs come at the expense of increased fabrication complexity for 2-dimensional arrays. Since the epitaxial layers to be wet-thermally oxidized must be exposed, non-planarity can be an issue. This is particularly important in that electrical contact to both the anode and cathode of the diode must be brought out to a package. We have investigated four fabrication sequences suitable for the fabrication of 2- dimensional VCSEL arrays. These techniques include: mesa etched polymer planarized, mesa etched bridge contacted, mesa etched oxide isolated (where the electrical trace is isolated from the substrate during the oxidation) and oxide/implant isolation (oxidation through small via holes) all of which result in VCSELs with outstanding performance. The suitability of these processes for manufacturing are assessed relative to oxidation uniformity, device capacitance, and structural ruggedness for packaging.
We have designed and assembled two generations of integrated micro-optical systems that deliver pump light and detect broadband laser-induced fluorescence in micro-fluidic separation systems employing electrochromatography. The goal is to maintain the sensitivity attainable with larger, tabletop machines while decreasing package size and increasing throughput. One type of micro-optical system uses vertical-cavity surface-emitting lasers (VCSELs) as the excitation source. Light from the VCSELs is relayed with four-level surface relief diffractive optical elements (DOEs) and delivered to the chemical volume through substrate-mode propagation. Indirect fluorescence from dye- quenched chemical species is collected and collimated with a high numerical aperture DOE. A filter blocks the excitation wavelength, and the resulting signal is detected as the chemical separation proceeds. Variations of this original design include changing the combination of reflective and transmissive DOEs and optimizing the high numerical aperture DOE with a rotationally symmetric iterative discrete on-axis algorithm. We will discuss the result of these implemented optimizations.
We describe the design and microfabrication of an extremely compact optical system as a key element in an integrated capillary-channel electrochromatograph with laser induced fluorescence detection. The optical design uses substrate-mode propagation within the fused silica substrate. The optical system includes a vertical cavity surface-emitting laser (VCSEL) array, two high performance microlenses and a commercial photodetector. The microlenses are multilevel diffractive optics patterned by electron beam lithography and etched by reactive ion etching in fused silica. Two generations of optical subsystems are described. The first generation design is integrated directly onto the capillary channel-containing substrate with a 6 mm separation between the VCSEL and photodetector. The second generation design separates the optical system onto its own module and the source to detector length is further compressed to 3.5 mm. The systems are designed for indirect fluorescence detection using infrared dyes. The first generation design has been tested with a 750 nm VCSEL exciting a 10-4 M solution of CY-7 dye. The observed signal-to-noise ratio of better than 100:1 demonstrates that the background signal from scattered pump light is low despite the compact size of the optical system and meets the system sensitivity requirements.
The fabrication and performance of selectively oxidized 850 nm vertical cavity surface emitting laser (VCSEL) diodes which emit through transparent GaP substrates is reported. Emission through the substrate is advantageous for many VCSEL configurations, such as for the incorporation of optical elements in the substrate or flip-chip integration to microelectronic circuitry. The short wavelength bottom- emitting VCSELs are fabricated by wafer fusion using an inert gas low temperature annealing process. The electrical characteristics of n- and p-type GaAs/GaAs and GaAs/GaP wafer bonded interfaces have been examined to optimize the annealing temperature. A significant reduction of the current-voltage characteristics of the VCSELs bonded to GaP substrates has been achieved whereby the bottom-emitting VCSELs show similar threshold voltage as compared to top- emitting lasers.
InGaAsN is a new semiconductor alloy system with the remarkable property that the inclusion of only 2% nitrogen reduces the bandgap by more than 30%. In order to help understand the physical origin of this extreme deviation from the typically observed nearly linear dependence of alloy properties on concentration, we have investigated the pressure dependence of the excited state energies using both experimental and theoretical methods. We report measurements of the low temperature photoluminescence energy of the material for pressures between ambient and 110 kbar. We describe a simple, density-functional-theory-based approach to calculating the pressure dependence of low lying excitation energies for low concentration alloys. The theoretically predicted pressure dependence of the bandgap is in excellent agreement with the experimental data. Based on the results of our calculations, we suggest an explanation for the strongly non-linear pressure dependence of the bandgap that, surprisingly, does not involve a nitrogen impurity band. Additionally, conduction-band mass measurements, measured by three different techniques, will be described and finally, the magnetoluminescence determined pressure coefficient for the conduction-band mass is measured.
Mid-IR LEDs are being developed for use in chemical sensor systems. As-rich, InAsSb heterostructures are particularly suited for optical emitters in the mid-IR region. We are investigating both InAsSb-InAs multiple quantum well (MQW) and InAsSb-InAsP strained layer superlattice (SLS) structures for use as the active region for light emitting diodes. The addition of phosphorus to the InAs barriers increase the light and heavy hole splitting and hence reduces non-radiative Auger recombination and provides for better electron and hole confinement int eh InAsSb quantum well. Low temperature photoluminescence (PL) emission from MQW structures is observed between 3.2 to 6.0 micrometers for InAsSb wells between 70 to 100 angstrom and antimony more fractions between 0.04 to 0.18. Room temperature PL has been observed to 6.4 micrometers in MQW structures. The additional confinement by InAsP barriers results in low temperature PL being observed over a narrower range for the similar well thicknesses with antimony mole fractions between 0.10 to 0.24. Room temperature photoluminescence was observed to 5.8 micrometers in SLS structures. The addition of a p-AlAsSb layer between the n-type active region and a p-GaAsSb contact layer improves electron confinement of the active region and increases output power by a factor of 4. Simple LED emitters have been fabricated which exhibit an average power at room temperature of > 100 (mu) W at 4.0 micrometers for SLS active regions. These LEDs have been sued to detect CO2 concentrations down to 24 ppm in a first generation, non- cryogenic sensor system. We will report on the development of novel LED device designs that are expected to lead to further improvements in output power.
We have demonstrated room-temperature CW operation of type- II quantum cascade (QC) light emitting diodes at 4.2 micrometers using InAs/InGaSb/InAlSb type-II quantum wells. The type-II QC configuration utilizes sequential multiple photon emissions in a staircase of coupled type-II quantum wells. The device was grown by molecular beam epitaxy on a p-type GaSb substrate and was composed of 20 periods of active regions separated by digitally graded quantum well injection regions. The maximum average output power is about 250 (mu) W at 80 K, and 140 (mu) W at 300 K at a repetition rate of 1 kHz with a duty cycle of 50%.
Mid-infrared (3 - 5 micrometers ) lasers and LED's are being developed for use in chemical sensor systems. As-rich, InAsSb heterostructures display unique electronic properties that are beneficial to the performance of these midwave infrared emitters. The metal-organic chemical vapor depositions growth of AlAs1-xSbx cladding layers and InAsSb/InAsP superlattice active regions are described. A regrowth technique has been used to fabricate gain-guided, injection lasers using undoped (p-type) AlAs0.16Sb0.84 for optical confinement. In device studies, we demonstrate lasers and LEDs utilizing the semi-metal properties of a p-GaAsSb/n-InAs heterojunction as a source for injection of electrons into the active region of emitters. This avoids the difficulties associated with n- type doping of AlAsSb cladding layers required for conventional p-n junction lasers and also provides a means for construction of active regions with multiple gain stages. Gain guided injected lasers employing a strained InAsSb/InAs multi-quantum well active region operated up to 210 K in pulsed mode, with an emission wavelength of 3.8 - 3.9 micrometers . A characteristic temperature of 40 K was observed to 140 K and 29 K from 140 K to 210 K. An optically pumped laser with an InAsSb/InAsP superlattice active region is also described. The maximum operating temperature of this 3.7 micrometers laser was 240 K.
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