Low-pressure Metalorganic Vapor Phase Research Articles

Mg doping profile in III-N light emitting diodes in close proximity to the active region

We have investigated how in (AlGaIn)N quantum well (QW) light emitting diodes, grown by low-pressure metal-organic vapor phase epitaxy, the actual Mg doping profile close to the interface between AlGaN electron barrier and QW active region, known to be affected by both segregation of back diffusion of Mg, can be controlled and adjusted by the appropriate choice of growth parameters. Almost identical Mg doping profiles, and thus electroluminescence efficiencies which are strongly affected by the Mg doping profile, could be achieved for different combinations of Mg precursor flow and growth temperature. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

High critical electric field of AlxGa1−xN p-i-n vertical conducting diodes on n-SiC substrates

We report the current-voltage characteristics of AlxGa1−xN (x=0–0.22) p-i-n vertical conducting diodes grown on n-SiC substrates by low-pressure metal organic vapor phase epitaxy. An increase in the breakdown voltage was experimentally demonstrated with increasing Al composition. The corresponding critical electric fields were calculated to be 2.4MV∕cm for GaN and 3.5MV∕cm for Al0.22Ga0.78N. The critical electric field is proportional to the band gap energy to a power of 2.5. The forward voltage drop also increases with increasing Al composition but it is still as low as 5.2V even in the case of the Al0.22Ga0.78N p-i-n diode.

Stranski-Krastanov GaN∕AlN quantum dots grown by metal organic vapor phase epitaxy

We report a thorough study of the growth of self-assembled GaN∕AlN quantum dots (QDs). These QDs were grown on sapphire substrates using the Stranski-Krastanov (SK) growth mode by means of low-pressure metal organic vapor phase epitaxy. The influence of the V∕III ratio on the QD height, shape, and density is studied. Emissions above (3.8eV) and below (2.8eV) the band gap of GaN are observed for small dots (1.3nm) and large ones (4.4nm), respectively. Special emphasis was given to the SK growth mode transition and the parameters influencing the QD formation. The two-dimensional–three-dimensional transition is studied continuously by photoluminescence mapping showing its high sensitivity to the V∕III ratio.

P-InGaN/n-GaN Vertical Conducting Diodes on n+-SiC Substrate for High Power Electronic Device Applications

We have investigated the current–voltage (I–V) characteristics of p-InGaN/n-GaN vertical conducting diodes grown on n+-SiC substrates by low-pressure metal organic vapor phase epitaxy (MOVPE). The breakdown voltage (VB) of 250 V was obtained with a low on-state resistance (Ron) of 1.28 mΩ cm2 when the n-GaN layer thickness was increased to 1800 nm, leading to the high figure-of-merit, (VB)2/Ron, of 49 MW/cm2. With increasing measurement temperature from room temperature (RT) to 520 K, the on-state resistance decreased due to the reduced contact resistance of the p-InGaN layer, while the breakdown voltage remained almost constant because of fewer defects in the n-GaN layer. These I–V characteristics are preferable for high-power and high-temperature electronic device applications.

Microphotoluminescence of exciton and biexciton around 1.5μm from a single InAs∕InP(001) quantum dot

We report on the fabrication by low-pressure metalorganic vapor phase epitaxy of InAs∕InP(001) quantum dots (QDs) emitting around 1.5μm, and on the observation of microphotoluminescence (μPL) from a single QD in this wavelength range. The QDs are diamond shaped, with a density of 6.3∙109cm−2. μPL experiments were carried out on a QD sample covered with a gold mask containing apertures (diameter of 200nm). Well defined peaks corresponding to the emission of single QDs were recorded between 1.24 and 1.6μm. The analysis of the pump power dependence of their intensity allows us to discriminate between exciton and biexciton emission lines.

In situ monitoring of GaN epitaxial lateral overgrowth by spectroscopic reflectometry

The application of spectroscopic reflectometry to the monitoring of epitaxial lateral overgrowth of GaN in low pressure metalorganic vapor phase epitaxy has been investigated. Real-time vertical and lateral growth rates and hence thickness and wing width of the growing GaN are extracted. A vertical growth enhancement was clearly observed at an early stage, followed by vertical growth suppression until full coalescence was achieved. The lateral to vertical growth ratio was obtained showing clear time dependent characteristics. The observations were explained by considering the mass transport between the growing (0001) facets and the {112¯0} sidewall facets.

Nanoepitaxy of InAs∕InP quantum dots by metalorganic vapor phase epitaxy for 1.55μm emitters

We report here on the structural and optical properties of selectively grown InAs∕InP quantum dots. Our approach combines electron-beam lithography, reactive ion etching, and selective low-pressure metalorganic vapor phase epitaxy, which allows the growth of nanometer-scale InAs quantum dots directly on InP substrate and an improved control of their size uniformity and density. These nanogrown InAs dots exhibit a high-efficiency photoluminescence band pointed at 1.55μm at room temperature.

Current-voltage characteristics of p-InGaN∕n-GaN vertical conducting diodes on n+-SiC substrates

p - In Ga N ∕ n - Ga N vertical conducting diodes have been grown on n+-SiC substrates by low-pressure metalorganic vapor phase epitaxy and their current-voltage characteristics have been investigated. The typical forward voltage drop was 3.8–4.0 V at a forward current density of 100A∕cm2 with an on-state resistance of ∼1.3mΩcm2. The ideality factor of the samples was ∼2, meaning that the tunneling current through defects is small enough in these devices. The breakdown voltage (VB) increased with increasing n-GaN layer thickness, while it increased with decreasing carrier concentration of the layer by substituting undoped GaN for n-GaN. When the undoped GaN layer thickness was increased to 1800 nm, the highest breakdown voltage of 305 V was obtained with a low on-state resistance (Ron) of 1.51mΩcm2, leading to the figure-of-merit, (VB)2∕Ron, of 62MW∕cm2.

Effect of annealing ambient flux on InAs islands grown on GaAs(0 0 1) substrates by metal–organic vapor phase epitaxy

Effects of post-growth annealing under dissimilar ambient fluxes on InAs islands grown on GaAs(0 0 1) substrates by low pressure metal–organic vapor phase epitaxy (LP-MOVPE) have been investigated. It is found that InAs islands annealed under different ambient fluxes exhibit different surface morphology evolution. Islands annealed under DMHy flux exhibit dissolving of the 3D island morphology. In contrast, upon annealing under TMSb flux, the 3D island morphology is enhanced with increased island density. The result suggests that we can fine tune the size and density of InAs islands by post-growth annealing under different ambient fluxes and this method is expected to be applicable to the growth of QDs of other material systems. Our findings are discussed in terms of the existence of critical nucleus in heterogeneous growth processes.

InAs epitaxial lateral overgrowth of W masks

The conditions for successful epitaxial lateral overgrowth of W patterns by InAs using low-pressure metalorganic vapour phase epitaxy were studied. The substrates used were InAs (0 0 1) wafers and the range of growth temperatures between 500 and 600 °C. The selective growth was investigated as a function of the V/III-ratio on W-ring test structures and wires oriented in different directions. 100-nm-wide wires, oriented in 30° off from the [ 1 1 ̲ 0 ] -direction, were completely covered with InAs without any void formation and the surface was planarized after deposition of 350 nm at the V/III-ratio of 14. Wires oriented along the [ 1 1 ̲ 0 ] -direction were found to effectively block the lateral overgrowth by the formation of mesa ridges limited by {1 1 0} and {1 1 1}A-planes for V/III-ratios between 14 and 56. All other grating directions showed limited lateral growth. This observed orientation dependence is in general agreement with the overgrowth of GaAs, but shows differences compared to InP.

Polarity control of GaN thin films grown by metalorganic vapor phase epitaxy

Control of the polarity of GaN thin films grown on c-plane sapphire by low-pressure metalorganic vapor phase epitaxy has been achieved. Growth of either polarity was realized by identifying and selecting appropriate substrate pre-treatments and AlN LT-buffer growth conditions. N-face (–c) GaN films were only obtained by pre-nitridation of the sapphire substrate after a H2 anneal. Smooth films of each type of polarity were obtained by using N2 as a transport and dilution gas and a V/III ratio of less than 500. The same growth rate of 1.2 µm/h was observed for both types of polarities under the same growth conditions. Further analysis of the growth conditions demonstrated that by using N2 as the dilution gas the transport was mass-transfer limited. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

LP MOVPE growth and characterization of high Al content Al xGa 1−xN epilayers

High quality Al x Ga 1− x N layers have been grown on sapphire (0 0 0 1) by low pressure metalorganic vapour phase epitaxy (LP MOVPE) at 1180 °C. The aluminum content covers a wide composition range varying from 0 to 65%. In order to ameliore the quality of these epilayers especially for high aluminum content ( X s Al >0.35), growth conditions are carefully optimized. The growth rate of AlGaN increases linearly with increasing group III elements partial pressure. The reduction of the growth pressure from 50 to 20×10 −3 atm induces an increase of the growth rate and raises the solid Al content from 19.6 to 53.3%. Additionally, a linear relationship (slope about 1) is observed between gas and solid aluminum content in the whole range of Al content in AlGaN alloys grown at 20×10 −3 atm. The structural properties of Al x Ga 1− x N layers, with different aluminum composition, are characterized by double X-ray diffraction (DDX) showing a high quality of our epilayers. A degradation of the crystalline quality of Al x Ga 1− x N films is observed with increasing aluminum content. The optical properties of these layers are investigated by photoluminescence measurements at low temperature (12 K). With increasing the Al content, the principal peak (neutral donor bound exciton) shifts to higher energies and the full width at half maximum (FWHM) increases significantly.

Effect of annealing on photoluminescence and microstructures of InGaN/GaN multi-quantum well with Mg-doped p-type GaN

InGaN/GaN multi-quantum well structure with Mg-doped p-type GaN was grown by low-pressure metalorganic vapour phase epitaxy. After rapid-thermal-annealing at 700 and 900 degrees C, both the red-shift and the blue-shift of the photoluminescence (PL) peak, the decreased and the enhancement of the PL intensity were observed. The transmission electron microscopic images showed that InGaN multi-quantum-dots-like (MQD-like) structures with dimensions less than 5×10nm were formed in InGaN wells. The changes of PL spectra could be tentatively attributed to the competition between the red-shift mechanism of the quantum-confined Stark effect and the blue-shift mechanism of the quantum size effect due to MQD-like structures.

Growth of InGaAsP based asymmetric quantum well infrared photodetector using metalorganic vapor phase epitaxy

A lattice-matched InyGa1−yAszP1−z∕InwGa1−wAsvP1−v∕InxGa1−xAs asymmetric step quantum well infrared photodetector grown by low-pressure metalorganic vapor phase epitaxy (MOVPE) using N2 carrier with tertiarybutylarsine (TBA) and tertiarybutylphosphine (TBP) is reported. The spectral responsivity of the detector has its peak at a wavelength of 10.7 μm with a peak responsivity of 0.19 A/W under 0.8 V bias at 25 K. A maximum peak detectivity of 1.9×109cmHz1∕2∕W was achieved under 0.6 V bias at 25 K. The measured activation energy using thermionic emission of carriers is found to be about 81 meV. This work demonstrates the fabrication of InP based quantum well infrared detectors using MOVPE with TBA and TBP sources with performance comparable to that achieved using molecular beam epitaxy.

External quantum efficiency of Pt/n-GaN Schottky diodes in the spectral range 5–500 nm

The external quantum efficiency in the spectral wavelength range 5–500 nm of a large active area Pt/n -type GaN Schottky photodiode that exhibits low reverse bias leakage current, is reported. The Schottky photodiodes were fabricated from n − / n + epitaxial layers grown by low pressure metalorganic vapour phase epitaxy on single crystal c-plane sapphire. The current–voltage ( I – V ) characteristics of several 0.25 cm 2 devices are presented together with the capacitance–voltage ( C – V ) characteristics of one of these devices. A leakage current as low as 14 pA at 0.5 V reverse bias is reported, for a 0.25 cm 2 diode. The ultraviolet quantum efficiency measurements show that the diodes can be used as radiation hard detectors for the 5–365 nm spectral range without the use of visible blocking filters. A peak responsivity of 77.5 mA/W at 320 nm is reported for one of the fabricated devices, corresponding to a spectral detectivity, D * = 1.5 × 1 0 14 cm Hz 1 / 2 W - 1 . The average detectivity between 250 and 350 nm, for the same device, is reported to be D ¯ * = 1.3 × 1 0 14 cm Hz 1 / 2 W - 1 . The spatial responsivity uniformity variation was established, using H 2 Lyman-α radiation, to be ±3% across the surface of a typical 0.25 cm 2 diode.

Improved performance of MOVPE-grown GaInNAs quantum wells by control of interfacial strain

A comprehensive study to achieve optically efficient GaInNAs quantum wells (QWs) grown by low-pressure metalorganic vapor phase epitaxy (MOVPE) has been accomplished. In order to extend emission wavelength over 1.3 μm effectively, GaInNAs QW structures with multiple barriers of GaNAs/InGaAs were proposed. Significant improvement of photoluminescence efficiencies and easy controllability of emission wavelengths over the 1.3 μm range were demonstrated with the proposed structure. It is believed that the wavelengths emitted from our employed QW structures are correlated to the amount of strain, determined by the strain competition between InGaAs (compressive) and GaNAs (tensile) layers. As N contents increased in the QWs, the emission wavelength was able to extend up to 1.38 μm without deteriorating spectral width.

Growth and structural properties of thick InAs films on GaAs with low-pressure metalorganic vapor phase epitaxy

High quality InAs films are grown on GaAs (0 0 1) substrates by low-pressure metalorganic vapor phase epitaxy. It is found that the combination of InAs buffer grown under low temperature and InAs epilayer grown under high temperature is essential to obtain high quality InAs films. The role of buffer layer is discussed. AFM observation on thick InAs film surface shows that step-flow growth mode takes place. The crystalline quality of InAs is characterized by transmission electron microscopy and X-ray diffraction. The crystalline quality improves with increase in film thickness. Residual strain due to thermal mismatch between InAs layers and GaAs substrates is investigated with X-ray diffraction.

Metalorganic vapor phase epitaxy growth of AlGaInAs quantum dots on (1 0 0) GaAs substrate

AlGaInAs quantum dots were grown on GaAs (100) substrate with low-pressure metalorganic vapor phase epitaxy. Growth temperature and trimethylaluminum input was varied to investigate their effects on quantum dot density and size. A model was proposed, the results agreed well with experiment. The morphological characteristics were interpreted in terms of adatom migration and nucleus stability, as well as pyrolysis of input precursors. The optimal growth temperature and aluminum composition in the vapor phase were estimated in order to achieve high quantum dot density. Under optimal growth conditions, quantum dot density can be as high as 2×1010cm−2, about 1.5 times of aluminum-free quantum dots grown at the same temperature.

MOVPE Growth and Fabrication of 1.3μm High Power InGaAsP-InP Polarization-Insensitive Superluminescent Diodes with Complex Strained Quantum Wells

High power polarization-insensitive InGaAsP-InP multiple quantum well (MQW) superluminescent diodes (SLD's) emitting at 1.3μm were investigated. A combination of tensile strained and compressively strained quantum wells called complex strained MQW were used in a single active layer in order to obtain polarization insensitivity. Low-pressure metalorganic chemical vapor phase epitaxy was used for crystal growth. High resolution X-ray diffraction and photoluminescence spectra showed excellent crystal quality. The SLD's were fabricated to ridge waveguide structure with 7° tilted cavity, the two facets were coated with two layers anti-reflection TiO2/SiO2 films, residual facet reflectivity was found to be less than 0.04%. The SLD's exhibited a up to 18.8 mW optical output power and less than 1 dB polarization dependence of output power with a less than 0.5 dB optical spectra modulation at 250 mA.

Group III‐nitride distributed Bragg reflectors and resonant cavities grown on bulk GaN crystals by metalorganic vapour phase epitaxy

Series of ten-, twenty-, and thirty-period (Al,Ga)N-based distributed Bragg reflectors (DBR) have been grown by low-pressure metalorganic vapour phase epitaxy on bulk GaN substrates. Respective mirror target wavelength reflectances of 54, 72, and 86%, as well as stop band-widths lie in nearly exact agreement with the simulations. The DBR are found to be crack-free if pseudomorphically grown on partially relaxed (Ga,In)N compliance layers. In a second series GaN lambda-cavities are introduced into the mirror structures forming nominally symmetric Fabry-Perot resonators sandwiched either between the air/GaN-interface and an underlying AlxGa1−xN/GaN-Bragg mirror, or sandwiched between a top-and a bottom-DBR with various numbers of AlxGa1−xN/GaN quarter-wave layers. Measured reflectance spectra of samples without top-DBR agree well with the simulations, whereas reflectance spectra of samples with top-DBR remain significantly below the simulations. Upon introduction of AlxGa1−xN/GaN-based bottom-DBR with 90% target wavelength reflectance up to 15.5% light extraction are predicted for encapsulated light emitting diodes. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)