[Show abstract][Hide abstract] ABSTRACT: Experimental gain spectra at continuous-wave (CW) operation of laser diodes (LDs) fabricated from c-plane, nonpolar and semipolar GaN-based materials emitting in violet, blue and green spectral regions are presented. Gain spectra were obtained using the Hakki–Paoli method at high resolution. The ability of the setup to resolve the sharp Fabry–Perot longitudinal peaks of the lasers at all injection currents allowed us to accurately measure the current density to reach transparency on the devices characterized in this work, and determine both total losses and differential modal gain curves up to threshold. We present a comparison of transparency current density and modal gain for nonpolar and c-plane LDs in violet and blue regions. The main parameters for the analysis are the internal electric fields (for c-plane lasers), hole effective masses and valence band splittings. In a preliminary analysis, we investigated the gain spectrum of semipolar green LDs and observed a significantly lower total linewidth (homogeneous plus inhomogeneous) than reported elsewhere, which seems to indicate that inhomogeneous broadening is not the main issue governing the evolution toward high performance green LDs.
Semiconductor Science and Technology 01/2012; 27(2). · 1.92 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Fast, bias-induced, surface potential transients (SPTs) and conductance transients (CTs) were simultaneously measured on an AlGaN/GaN high electron mobility transistor. SPTs measured near the drain-side gate edge and CTs have nearly the same shape and are well-fit with two exponentials having room-temperature time constants of 4.2 ms and 36 ms, likely indicating emission from two trap species. Kelvin probe force microscopy was used to measure SPTs. Electrostatic simulations of SPT amplitudes, which account for the measured probe/sample geometry, are consistent with a uniform trapped surface charge density of 7 × 1012 electrons/cm2 extending 200 nm from the drain-side gate edge.
[Show abstract][Hide abstract] ABSTRACT: We have performed low frequency /f noise measurements from 85 K to 450 K to investigate the energy distribution of defects in GaN/AlGaN high electron mobility transistors fabricated in three different processes. The noise is well described by the model of P. Dutta and P. M. Horn, Rev. Mod. Phys. 53, 497 (1981). A peak in the defect energy distribution is observed at ∼0.2 eV for all device types investigated, which we attribute to the reconfiguration of an oxygen DX-like center in AlGaN. An additional peak at an energy >1 eV is observed for devices grown under nitrogen-rich conditions, which we attribute to the reconfiguration energy of negatively charged nitrogen antisites.
[Show abstract][Hide abstract] ABSTRACT: With the emergence of the direct emission green laser diodes based on InGaN technologies, pico projection systems utilizing an all-direct diode light engine will soon be available. Such systems will provide a wide range of benefits over existing light engine technologies. Here we will discuss these benefits and present Soraa's progress on the development of high-efficiency direct diode green and blue lasers based on nonpolar/semipolar InGaN.
SID Symposium Digest of Technical Papers. 06/2011; 42(1).
[Show abstract][Hide abstract] ABSTRACT: We present state-of-the-art performance from green and blue InGaN-based laser diodes fabricated on nonpolar/semipolar substrates. Using these novel crystal orientations, we demonstrate high-power, high-efficiency, and long-lifetime continuous-wave laser operation. For green wavelengths at 520 nm, we report on continuous wave single mode lasing with over 65 mW of output power and wall plug efficiencies over 2%. In the blue regime we describe single-mode lasers operating with over 23% wall-plug-efficiency and with output powers greater than 750 mW. To the best of the author's knowledge, this efficiency represents the highest reported to date for a single-mode blue laser. These InGaNbased devices offer dramatic improvement in performance, size, weight, and cost over conventional gas and solid state lasers for use in defense, biomedicine, and consumer projection displays.
[Show abstract][Hide abstract] ABSTRACT: We report recent progress for InGaN-based lasers diodes fabricated on nonpolar/semipolar substrates. We demonstrate single-mode green lasers operating with >3.5% wall-plug-efficiency and 100mW output power. In the blue regime we demonstrate single-mode lasers with >23% wall-plug-efficiency and high power lasers with >1.4W of output power.
[Show abstract][Hide abstract] ABSTRACT: Using non-c-plane bulk GaN substrates, we demonstrate continuous-wave
single-mode blue-emitting laser diodes operating with over 23% wall plug
efficiency and over 750 mW output power, which represent the highest
values reported to date. Furthermore, we demonstrate continuous-wave 520
nm green-emitting laser diodes with over 60 mW output power and 1.9%
wall plug efficiency. The rapid performance evolution of laser diodes
fabricated on non-c-plane orientations is validation of the benefits
resulting from increased electron--hole overlap, reduced effective hole
mass, and increased design flexibility.
[Show abstract][Hide abstract] ABSTRACT: Deep levels in N-face and Ga-face n-type GaN grown by plasma-assisted molecular beam epitaxy were detected, analyzed and compared using deep level optical spectroscopy (DLOS) and conventional thermal deep level transient spectroscopy (DLTS), which together enable deep level detection throughout the GaN band gap. A redistribution of band gap states was observed between the two GaN crystal growth polarities but with a similar total trap density. Most significant was a tenfold concentration increase in a trap at EC-0.25 eV that is likely related to nitrogen vacancies for the N-face polarity material, with no significant change for the Ga-vacancy-related level at EC-2.60 eV. The DLOS results suggest that carbon impurities, which generate several GaN band gap states, appear to incorporate differently for both crystal polarities with the potential carbon interstitial at EC-1.28 eV being undetected for N-face material. Finally, low concentrations of several new levels in the N-face n-GaN not previously observed in Ga-face n-GaN were observed.
[Show abstract][Hide abstract] ABSTRACT: A new pulsed- IV pulsed-RF cold field-effect transistor (cold-FET) technique is presented to extract the parasitics of AlGaN/GaN HEMTs under various quiescent dc-biasing points. The measurement system implemented with a large signal network analyzer applies the technique of multiple recording to acquire pulsed-RF small-signal S -parameters with no loss of dynamic range as the pulse duty cycle decreases. These cold-FET measurements are performed on unpassivated and silicon nitride (SiN) passivated devices by turning the device off for 1 ?s with a 1% duty cycle to analyze the impact of slow thermal and trapping effects on the device parasitics. The parasitic fringe capacitances extracted are found to be bias independent, except for the gate to drain capacitance in devices without SiN passivation. In unpassivated devices, the drain parasitic resistance is found to rapidly increase with increasing drain bias at negative gate to source voltages. On the contrary, in devices with SiN passivation, the dependence of the resistance with the drain bias voltage is much less significant. A simple physical model is used to fit the functional dependence of the 2-D electron gas (2DEG) concentration upon the gate-to-source and gate-to-drain voltages, which is then proposed for fitting the measured data. The analysis indicates that the variation of the resistance with bias voltage in the device studied with SiN passivation and also for the unpassivated device at V <sub>GS</sub>=0 V is well accounted for by the reduction of the mobility with increased temperature due to self-heating, whereas for the device studied without SiN passivation, the increase of the drain resistance with drain voltages at negative gate bias principally arises from the decrease of the 2DEG population in a narrow region near the gate contact. An equivalent circuit is also introduced to explain the decrease of the source and drain parasitic inductances with increasing drain voltages at large n-
egative gate bias, which is observed in unpassivated devices.
IEEE Transactions on Microwave Theory and Techniques 06/2010; · 2.23 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We present state-of-the-art performance from green, blue, and violet InGaN-based laser diodes fabricated on nonpolar and semipolar GaN substrates. Using these novel crystal orientations, we demonstrate high power, high efficiency, continuous-wave operation from single-lateral-mode electrically pumped laser diodes at wavelengths from 405 nm to 500 nm. Additionally, we present the longest reported continuous-wave lasing demonstration of 525 nm and an output power of over 9 mW at 521 nm. Wall-plug efficiencies of over 25% in the violet region, 17.5% in the blue region, over 5% at 472nm, and 2.2% in the 500 nm range are reported. These InGaN-based devices offer dramatic improvement in size, weight, and cost over conventional gas and solid state lasers and may enable a variety of new applications in defense and security.
[Show abstract][Hide abstract] ABSTRACT: The incorporation of deep level defects in n -type GaN grown by plasma assisted molecular beam epitaxy was studied via systematic adjustment of the nitrogen plasma source power from 150 to 400 W while maintaining a constant V/III beam flux ratio. Deep level optical spectroscopy and conventional thermal deep level transient spectroscopy measurements, which together enable deep level detection throughout the GaN band gap, revealed several deep level concentrations that depend strongly on rf-plasma power. The concentrations of the gallium vacancy deep level at E <sub> C </sub>-2.60 eV and carbon-related point defects with energy levels at E <sub> C </sub>-3.28 and E <sub> C </sub>-1.35 eV are found to be very sensitive to the nitrogen source power, increasing by up to 50 times for a corresponding increase in plasma power from 150 to 400 W. The relation between the concentrations of these traps and plasma power follows an Arrhenius-type behavior and is suggestive of plasma damage associated with the energetics of the constituent active nitrogen species. In contrast, two traps at E <sub> C </sub>-0.86 and E <sub> C </sub>-0.59 eV did not exhibit a systematic dependence on plasma power, with this difference a result of the dislocation-related nature of these defects.
Journal of Applied Physics 04/2010; · 2.21 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We present new advances in green, blue, and violet InGaN-based laser diodes fabricated on nonpolar and semipolar GaN substrates. Using these novel crystal orientations, we report high power, high efficiency, continuous-wave operation from single-lateral-mode electrically pumped laser diodes at wavelengths from 405 nm to 500 nm. Additionally, we present continuous-wave lasing demonstrations out to 523 nm, representing the longest continuous-wave green laser emission reported to date. Wall-plug efficiencies of over 25% in the violet region, 16.2% in the blue region, and 2.2% in the 500 nm range are presented. These InGaN-based devices offer dramatic improvement in size, weight, and cost over conventional gas or solid state lasers and may enable a variety of new applications in defense, biomedical, industrial, and consumer projection displays.
[Show abstract][Hide abstract] ABSTRACT: An additive phase noise measurement system is integrated with a large signal network analyzer (LSNA) and a tunable monochromatic light source. This system is used to measure the additive phase noise characteristics of an unpassivated AlGaN/GaN high electron mobility transistor (HEMT) at 2 GHz under various operating conditions. Illumination with different photon energies, below the AlGaN bandgap, is applied to probe the dependence of the RF additive phase noise on the trap and 2DEG population. Different drain voltages are also used to investigate the bias dependence of the phase noise. From 1 Hz to 10 KHz, an 1/f region is identified in the additive phase noise at 2 GHz, which is indicative of the presence of uniformly distributed traps. Further a decrease in additive phase noise is clearly observed with increasing photon energies below the GaN bandgap. This is due to the decrease of the trap population induced by photon assisted emission of electrons from the trap levels to the conduction band. Further it is found that the additive phase noise at 2 GHz increases at higher drain voltages. Various RF load impedances are also used to further characterize the noise performance of both passivated and unpassivated AlGaN/GaN HEMTs. The larger the drain voltage swing introduced, the more additive phase noise is observed. A degration of additive phase noise is also observed with the unpassivated device compared to the passivated device. Some preliminary results from a physical cyclostationary model are also presented. The observed 1/f noise increase at RF occuring at large bias or in large signal RF operation are attributed to the increase efficiency of the RF upconversion of the trap 1/f occupation fluctuation when the drain resistance increases. This work also demonstrates that the new combined additive phase noise/LSNA testbed developed is a useful tool for characterizing the additive phase noise in transistors/amplifiers under large signal operation.
[Show abstract][Hide abstract] ABSTRACT: A study of the effects of traps on the RF characteristics of AlGaN/GaN HEMTs is conducted using small and large-signal microwave measurements with deep-level optical spectroscopy. Different variations of the drain current swing are observed for illuminations of different photon energies. The time evolution of the current swing in transient measurements enabled to determine the optical transient time-constant and the equilibrium relaxation time-constant. The dependence of the S-parameters upon the illumination yielded results consistent with the variation of the drain current swing under illumination. A variation in the small signal transconductance and drain conductance was extracted at 2 GHz which presumably arises from the variation of the drain resistance under illumination. In addition, it was verified that the SiN passivation greatly helps in mitigating the RF performance degradation arising from deep trapping centers.
[Show abstract][Hide abstract] ABSTRACT: Electrically generated spin polarization is probed directly in bulk GaN using Kerr rotation spectroscopy. A series of n-type GaN epilayers are grown in the wurtzite phase both by molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD) with a variety of doping densities chosen to broadly modulate the transverse spin lifetime, T2*. The spin polarization is characterized as a function of electrical excitation energy over a range of temperatures. Despite weak spin-orbit interactions in GaN, a current-induced spin polarization (CISP) is observed in the material at temperatures of up to 200 K. Comment: 16 pages, 3 figures
[Show abstract][Hide abstract] ABSTRACT: We observe sharp spectral lines, at energies which are higher than the bulk GaN band gap, in the photoluminescence and photoluminescence excitation spectra of GaN/AlGaN heterointerfaces grown by molecular beam epitaxy. The spectra and their temperature dependence are in accord with the Fermi edge singularity expected for two dimensional electron gas systems. The associated localized hole energy in the AlGaN interface side was extracted directly from the spectra.
[Show abstract][Hide abstract] ABSTRACT: The incorporation of deep level defects in n-type GaN grown by ammonia-based molecular beam epitaxy (MBE) is studied via systematic adjustment of the NH3/Ga flux ratio. Deep level optical and transient spectroscopies, which together enable deep level detection throughout the GaN bandgap, reveal defect states whose individual concentrations vary with the NH3/Ga flux ratio. A general trend of lower concentration for deep levels at EC−3.28, EC−1.28, EC−0.62, and EC−0.25 eV with higher NH3/Ga flux ratio was observed, with the strongest reduction at the EC−0.25 eV level, consistent with expectations for a VN-related defect. The known CN impurity state at EC−3.28 eV and suspected CI-related state at EC−1.28 eV also showed a moderate decrease in concentration at the higher NH3/Ga flux ratio. In contrast, the VGa-related defect at EC−2.62 eV was insensitive to the NH3/Ga flux ratio over the range studied here. Taken together, ammonia-MBE GaN has deep level defects with different sensitivities in flux ratios suggestive of independent physical sources. However, the total trap concentrations were significantly reduced for higher NH3/Ga flux ratios in n-type GaN grown by ammonia-MBE under the range of growth conditions used in this study, suggesting that higher NH3/Ga flux ratios will generate higher electronic quality GaN material when using ammonia-based MBE for device applications.
[Show abstract][Hide abstract] ABSTRACT: The impact of growth conditions on the surface morphology and structural properties of ammonia molecular beam epitaxy GaN buffers layers on SiC substrates was investigated. The threading dislocation (TD) density was found to decrease with decreasing NH3:Ga flux ratio, which corresponded to an increase in surface roughness and reduction in residual compressive lattice mismatch stress. Furthermore, the dislocation density and compressive stress decreased for increasing buffer thickness. TD inclination was proposed to account for these observations. Optimized surface morphologies were realized at high NH3:Ga flux ratios and were characterized by monolayer-high steps, spiral hillocks, and pyramidal mounds, with rms roughness of ∼ 1.0 nm over 2×2 μm2 atomic force microscopy images. Smooth surface morphologies were realized over a large range of growth temperatures and fluxes, and growth rates of up to 1 μm/h were achieved. TD densities in the buffers as low as 3×109 cm−2 were demonstrated. These buffers were highly insulating and were used in recently reported AlGaN/GaN HEMTs with power densities of >11 W/mm at 4 and 10 GHz.
Journal of Applied Physics 05/2008; 103(9):093529-093529-7. · 2.21 Impact Factor