Migration-enhanced Metalorganic Chemical Vapor Deposition Research Articles

Migration-enhanced metal–organic chemical vapor deposition of Al xIn 1− xN/GaN heterostructures ( x>0.75) on c-plane sapphire

The growth and the characterization of Al x In 1− x N/GaN ( x>0.75) heterostructures by migration-enhanced metal–organic chemical vapor deposition are optimized through variations in growth temperature and precursor modulation schemes, resulting in high quality films. Al x In 1− x N/GaN heterostructures were characterized by XRD reciprocal space mapping in order to calculate the relative strain to GaN as a function of indium composition. AFM measurements yielded an rms roughness value of ∼4 Å. By controlling indium incorporation, a maximum 2DEG mobility of ∼1270 cm 2/V s, a maximum sheet charge density of ∼4.1×10 13 cm −2, and a minimum sheet resistance of 179.9 Ω/□ were obtained.

Carrier Lifetimes in GaN Revealed by Studying Photoluminescence Decay in Time and Frequency Domains

The carrier dynamics in GaN epilayer, grown using migration enhanced metalorganic chemical vapor deposition (MEMOCVD), is investigated in a wide range of photoexcitation power density of twelve orders of magnitude. Study of photoluminescence decay lifetimes at extremely low excitation power densities of 0.5 mW/cm2 was performed using frequency-domain lifetime measurements with a UV LED (280 nm) as an excitation source. The carrier lifetimes at high excitation power densities was studied using time-resolved photoluminescence (TR PL) spectroscopy and light-induced transient grating (LITG) technique. The measurements were carried out in the temperature range from 10 to 300 K. At high carrier densities, which are typical for device applications, the room- temperature carrier lifetime of 2 ns was obtained, while a variety of decay components was revealed at low excitation intensities. Trapping and detrapping of the carriers/excitons is shown to play an important role in the carrier dynamics at low carrier densities.

Growth of AlN films and their characterization

AbstractSingle crystal AlN layers have been produced by migration enhanced metal organic chemical vapor deposition (MEMOCVD), hydride vapor phase epitaxy (HVPE) and their combination. The growth was carried out on 2” basal plane sapphire substrates. In MEMOCVD, the duration and waveforms of precursors were varied to achieve better surface mobility and thus better atomic incorporation. It resulted in superior layer quality templates with the narrowest (002) X‐ray rocking curve full width half maximum (FWHM). Such high quality AlN templates were used as seeds for subsequent HVPE growth. Thick films with thickness ranging from 1‐25 µm have been grown by HVPE with growth rates as high as 200 µm/min, highest ever reported. Films grown by the two methods have been extensively characterized by Nomarski microscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), high‐resolution X‐ray diffractometry (HRXRD), and photoluminescence (PL). (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Optical power degradation mechanisms in AlGaN-based 280nm deep ultraviolet light-emitting diodes on sapphire

We present a study of reliability of AlGaN-based 280nm deep ultraviolet light-emitting diodes on sapphire substrate grown by migration-enhanced metal-organic chemical vapor deposition. Two modes of optical power degradation were observed: catastrophic and gradual. The catastrophic degradation is believed to be due to metal alloying at macroscopic defects in the top p layers of the light-emitting diode structure. For the gradual power degradation, two time constants were determined, which were temperature and bias dependent. For the temperature-dependent part, the values of the activation energies and room-temperature degradation rates at dc currents of 10 and 20mA were determined to be 0.23 and 0.27eV and 1.31×10−3and5.93×10−3h−1, respectively.

Time-resolved experimental study of carrier lifetime in GaN epilayers

Time-resolved photoluminescence and light-induced transient grating measurements of GaN epilayers show that the photoluminescence decay can be described by two coupled exponential terms and that carrier mobility and lifetime in GaN epilayers are correlated within the model which accounts for nonradiative carrier recombination predominantly at dislocations. The obtained results demonstrate that migration-enhanced metalorganic chemical vapor deposition (MEMOCVD™) allows for growth of high-quality GaN epilayers on sapphire substrates with the dislocation density close to 108cm−2, carrier lifetime as long as 2 ns, and ambipolar diffusion coefficient of 2.1cm2s−1 corresponding to the hole mobility of approximately 40cm2V−1s−1.

Exciton hopping and nonradiative decay in AlGaN epilayers

Monte Carlo simulation of phonon-assisted localized exciton hopping has been employed to describe the photoluminescence linewidth variation with temperature and to reveal band potential profile of ternary AlGaN epilayers with different carrier lifetimes. The lifetimes of 30 and 190 ps were experimentally determined in the layers with AlN buffers grown by conventional metal-organic chemical vapor deposition (MOCVD) and by migration-enhanced MOCVD (MEMOCVD™), respectively. The potential profile in AlGaN is shown to consist of double-scaled fluctuations. Exciton hopping in Al0.26Ga0.74N occurs within the random potential fluctuations (on the scale σ≈19meV) in isolated low-potential regions with the average localization energy dispersed on the scale Γ≈19meV. Such a pattern of band potential profile was found to be independent on the growth technique used for the deposition of their AlN buffer layers. This implies that the large difference in carrier lifetimes estimated in the AlGaN epilayers with the same Al content is caused by different densities of nonradiative recombination centers rather than by carrier localization in the potential fluctuations.

AlGaN Deep-Ultraviolet Light-Emitting Diodes

We report on the development of AlGaN based deep ultraviolet (DUV) light-emitting diodes (LEDs) grown by migration-enhanced metalorganic chemical vapor deposition (MEMOCVD). Improved quality of AlGaN has allowed us to achieve milliwatt-power at wavelengths ranging from 365 to 265 nm. For 295 and 280 nm LEDs, record CW powers with wall-plug-efficiency approaching 1.0% were realized. The CW power reached 1.2 and 1.0 mW at 20 mA for 280 and 295 nm LEDs, respectively. A multiple-chip package (UV lamp) emitted CW power of 11 mW at the wavelength of 280 nm. Under pulse operation, the 280 nm UV lamp produced power as high as 56 mW. The CW power levels at 20 mA were 0.5, 0.25 and 0.15 mW for a single-chip 275, 270 and 265 nm LEDs, respectively. A 265 nm UV lamp exhibited a record high CW power exceeding 1.5 mW. The applications of these DUV LEDs in bio-agents detection have been demonstrated and the preliminary results will be presented.

N‐Al 0.75 Ga 0.25 N epilayers for 250 nm emission ultraviolet light emitting diodes

We have developed a unique approach combining migration-enhanced metalorganic chemical vapor deposition (MEMOCVD) high temperature AlN buffer layers and AlGaN/AlN superlattices (SLs) to yield high quality ( HQ) AlGaN layers for 250 nm LEDs. Symmetric/asymmetric X-ray diffraction (XRD) and room temperature photoluminescence measurements were used to study the high-structural and optical quality. The (002) and (114) rocking curve full width at half–maximum (FWHM) of 1.4 µm n-Al0.75Ga0.25N grown over AlGaN/AlN buffer were 143 and 565 arcsec, respectively. Crack-free Al0.75Ga0.25N layers with electron concentration as high as 1 × 1018 cm–3 and Hall mobility about 50 cm2/V.s were successfully grown and used for sub-milliwatt power (0.12 mW at a pulse pump current of 300 mA) 250 nm deep ultraviolet light emitting diodes (UVLEDs). In addition, for comparison, we prepared n-AlGaN only using high temperature AlN without SLs inserted. The experiments show that the AlGaN/AlN SLs inserted play a crucial role in improving structural and optical quality of high Al-composition AlGaN epilayers. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Reliability and degradation modes of 280 nm deep UV LEDs on sapphire

AbstractIn this paper, we report a study of the degradation of AlGaN-based 280 nm LEDs, which were grown on sapphire substrates using migration-enhanced metalorganic chemical vapor deposition process (MEMOCVD). Electroluminescence (EL), atomic force microscopy (AFM), cathodoluminescence (CL), and scanning electron microscopy (SEM) observations showed that the degradation of deep UV LEDs generally fell into two categories: catastrophic degradation and gradual degradation. The catastrophic degradation was found to be mostly caused by the non-uniformity of surface morphology. The gradual power reduction had two characteristic time constants indicating two possible degradation mechanisms as found from temperature and bias dependent LED power degradation measurements. The faster time constant was bias dependent and virtually constant with temperature whereas the second time constant (slower) varied exponentially with junction temperature. For this temperature dependent part, the activation energies of degradation were determined to be 0.23 eV and 0.27 eV under injected current density of 100 A/cm2 and 200 A/cm2 respectively.

Comparison of the Electroluminescence of Blue and Deep-UV Light-Emitting Diodes at Elevated Temperatures

AbstractThe performance of InGaN and AlGaN-based blue (465nm) and deep ultraviolet (UV) (280 nm) light-emitting diodes (LEDs) at elevated temperatures (25-175 °C) were investigated. As a result of uniform high-Al content AlGaN alloys yielded by migration-enhanced metalorganic chemical vapor deposition, the deep-UV LED showed dominant band-edge emission, much smaller alloy broadening and weaker localization effects as compared to the InGaN LED. Strong carrier localization was retained in the blue LED up to 175 °C, leading to temperature-independent emission intensity at low-energy tails. The UV LED, however, showed a much more rapid decrease in light output with increasing temperature. The characteristic temperature was 37 K, compared to 270 K for the blue LED. These findings implicate the lack of localization effects in AlGaN alloys as one of the causal factors in the poor thermal performance of the deep UV LED and suggest that increasing carrier confining potentials will provide a critical means to improve its thermal stability.

Impact of Small Miscuts of (0001) Sapphire on the Growth of AlxGa1-xN/AlN

AbstractIn this paper, using chemical etching, atomic force microscope (AFM) and High- resolution X-ray diffraction (HRXRD), we report a study of the effect of various small miscuts of (0001) sapphire substrate (<1°) and the way to further improve the material quality. A set of AlN epilayers and AlN/AlxGa1-xN Superlattices (SLs) were grown by Migration-enhanced Metalorganic Chemical Vapor Deposition (MEMOCVD) on vicinal (0001) sapphire substrates. The threading dislocation density was found to be very sensitive to the miscut angles. The etch pit density reduced to 7×106 cm-2 for normal-oriented (0°-off) from the starting value of 7×107 cm-2 for 0.5°-off. We found the surface morphologies can be easily controlled by the different substrate miscut angles. The 1-2 Monolayers (MLs) step flow morphology for normal- oriented substrate changed to step bunches of 10 MLs height for 0.5°-off substrate. Correspondingly, AFM Root Mean Square (RMS) increased from 1.52 to 9.15 Å with a 5um×5um scan. This finding may help enhance the quality of full structure UVLED material and eventually improve the lifetime of UVLEDs.

AlGaN-based 280nm light-emitting diodes with continuous-wave power exceeding 1mW at 25mA

Optimization of the migration-enhanced metalorganic chemical vapor deposition and further optimization of the contact and active layer design for 280nm light-emitting diodes resulted in large improvement of cw and pulsed output power and in a superior spectrum purity. The ratio of the main peak to the background luminescence determined by the detection system is higher than 2000:1 at 20mA dc. The on-wafer cw power was measured to be 255μW at 20mA dc. The power popped up exceeding 1mW for a packaged device under 25mA dc and 9mW under pulse 200mA. The maximum wall-plug-efficiency of 0.67% was obtained for the packaged device at 25mA dc.

High quality InN/GaN heterostructures grown by migration enhanced metalorganic chemical vapor deposition

We report on the structural properties and optical and electrical characteristics of InN epitaxial layers grown on highly resistive GaN templates using migration enhanced metalorganic chemical vapor deposition (MEMOCVD). The material quality of InN improved significantly for the layer thickness larger than 150 nm. The highest extracted value of the room temperature electron mobility was close to 850 cm2/V s for samples with electron carrier concentration of ∼4×1018 cm−3. The measured dependence of the electron mobility on electron concentration is interpreted using the model accounting for ionized impurity scattering, polar optical scattering, and compensation. The MEMOCVD-grown material exhibited stronger photoluminescence (PL) compared to InN deposited using conventional metalorganic chemical vapor deposition. Room temperature PL spectra were similar to InN grown using molecular beam epitaxy (MBE) with peak emission at 0.8 eV. The obtained results demonstrate the potential of the MEMOCVD technique for deposition of high quality InN epitaxial layers at reduced temperatures comparable to those used in MBE growth.

Metalorganic Chemical Vapor Deposition of Non-polar III-Nitride Films over a-plane SiC Substrates

ABSTRACTWe report progress in growing non-polar a-plane III-nitride films and heterostructures over a-plane 4H-SiC. a-plane SiC is more closely lattice-matched to a-plane GaN than is r-plane sapphire. Consequently, better structural quality a-plane nitride films may result over a-plane SiC substrates. By migration enhanced metalorganic chemical vapor deposition (MEMOCVD), an atomically smooth (1120)AlN layer with RMS roughness of 0.3nm was obtained. From the results of XRD, the structural defects in the AlN layer on SiC substrates were strongly reduced compared to those grown on r-plane sapphire. Also by applying our selective area lateral epitaxy (SALE) growth procedure, we achieved high structural and optical quality a-plane GaN films on 4H-SiC with RMS roughness only 0.4nm. Therefore, non-polar III-nitride films and heterostructures on SiC substrates are promising building blocks for realizing high performance polarization-free devices.