Selective Metalorganic Vapor Phase Epitaxy Research Articles

Influence of adatom migration on wrinkling morphologies of AlGaN/GaN micro-pyramids grown by selective MOVPE**Project supported by the National Natural Science Foundation of China (Grant Nos. 61274039 and 61574173), the National Key Research and Development Program, China (Grant No. 2016YFB0400105), the International Science and Technology Collaboration Program of Guangdong Province, China (Grant No. 2013B051000041), the International Science and Technology Collaboration Program

GaN micro-pyramids with AlGaN capping layer are grown by selective metal–organic–vapor phase epitaxy (MOVPE). Compared with bare GaN micro-pyramids, AlGaN/GaN micro-pyramids show wrinkling morphologies at the bottom of the structure. The formation of those special morphologies is associated with the spontaneously formed AlGaN polycrystalline particles on the dielectric mask, owing to the much higher bond energy of Al–N than that of Ga–N. When the sizes of the polycrystalline particles are larger than 50 nm, the uniform source supply behavior is disturbed, thereby leading to unsymmetrical surface morphology. Analysis reveals that the scale of surface wrinkling is related to the migration length of Ga adatoms along the AlGaN facet. The migration properties of Al and Ga further affect the distribution of Al composition along the sidewalls, characterized by the μ-PL measurement.

Migration characterization of Ga and In adatoms on dielectric surface in selective MOVPE**Project supported by the National Natural Science Foundation of China (Grant Nos. 61274039 and 51177175), the National Basic Research Program of China (Grant No. 2011CB301903), the Ph.D. Programs Foundation of Ministry of Education of China (Grant No. 20110171110021), the International Sci. & Tech. Collaboration Program of China (Grant

Migration characterizations of Ga and In adatoms on the dielectric surface in selective metal organic vapor phase epitaxy (MOVPE) were investigated. In the typical MOVPE environment, the selectivity of growth is preserved for GaN, and the growth rate of GaN micro-pyramids is sensitive to the period of the patterned SiO2 mask. A surface migration induced model was adopted to figure out the effective migration length of Ga adatoms on the dielectric surface. Different from the growth of GaN, the selective area growth of InGaN on the patterned template would induce the deposition of InGaN polycrystalline particles on the patterned SiO2 mask with a long period. It was demonstrated with a scanning electron microscope and energy dispersive spectroscopy that the In adatoms exhibit a shorter migration length on the dielectric surface.

The influence of aluminum content on the surface morphology of heavily doped (Al)GaN mesastrip structures grown by selective metalorganic vapor phase epitaxy

Heavily Si-doped Al x Ga1–x N mesastrip structures were grown by selective MOVPE technology. Al x Ga1–x N:Si mesastructures with x ~ 0.01–0.07 possess a smoother top and more even side surfaces as compared to those in analogous GaN:Si structures. During the growth of mesastructures with x ~ 0.03–0.07, a thin nanocrystalline AlN deposit appears on the Si3N4 mask. This deposit is not formed during the growth of structures with sufficiently low aluminum content.

Proposal and Analysis of Distributed Reflector-Laser Diode Integrated with an Electroabsorption Modulator

A novel integrated laser, that is, a distributed reflector laser diode integrated with an electroabsorption modulator, is proposed to improve the output efficiency, single-mode stability, and chirp. The proposed laser can be realized using the selective metalorganic vapor phase epitaxy technique (that is, control of the width of the insulating mask), and its fabrication process is almost the same as the conventional electroabsorption modulated laser (EML) process except for the asymmetric coupling coefficient structure along the cavity. For our analysis, an accurate time-domain transfer-matrix-based laser model is developed. Based on this model, we perform steady-state and large-signal analyses. The performances of the proposed laser, such as the output power, extinction ratio, and chirp, are compared with those of the EML. Under 10-Gbps NRZ modulation, we can obtain a 30% higher output power and about 50% lower chirp than the conventional EML. In particular, the simulation results show that the chirp provided by the proposed laser can appear to have a longer wavelength side at the leading edge of the pulse and a shorter wavelength side at the falling edge.

Drastic Reduction of Dislocation Density in Semipolar (1122) GaN Stripe Crystal on Si Substrate by Dual Selective Metal–Organic Vapor Phase Epitaxy

A drastic reduction of the dislocation density in a semipolar (1122) GaN stripe on a patterned Si substrate was achieved by the two-step selective growth of a GaN stripe. After depositing a SiO2 mask on the (1122) and (0001) faces of a GaN stripe grown on a (113) Si substrate, GaN was regrown only on the (1122) face. The dislocation density estimated from the dark-spot density in a cathodoluminescence (CL) image greatly decreased from 4.0×108 to 1.0×105/cm2 in the regrowth region. A transmission electron microscopy (TEM) image also verified that there were no dislocations at the regrowth interfaces.

Drastic Reduction of Dislocation Density in Semipolar (1122) GaN Stripe Crystal on Si Substrate by Dual Selective Metal–Organic Vapor Phase Epitaxy

A drastic reduction of the dislocation density in a semipolar (1122) GaN stripe on a patterned Si substrate was achieved by the two-step selective growth of a GaN stripe. After depositing a SiO2 mask on the (1122) and (0001) faces of a GaN stripe grown on a (113) Si substrate, GaN was regrown only on the (1122) face. The dislocation density estimated from the dark-spot density in a cathodoluminescence (CL) image greatly decreased from 4.0×108 to 1.0×105/cm2 in the regrowth region. A transmission electron microscopy (TEM) image also verified that there were no dislocations at the regrowth interfaces.

MOVPE growth and properties of GaN on (1 1 1)Si using an AlInN intermediate layer

Using an AlInN intermediate layer, GaN was grown on (1 1 1)Si substrate by selective metalorganic vapor phase epitaxy. The variation of the surface morphology was investigated as a function of the In composition and thickness of the AlInN layer. It was found that the In composition in the AlInN layer was a function of the growth temperature and thickness. Because of the small band offset at the AlInN/Si hetero-interface, we have achieved a low series resistance of the order of 9 Ω (0.0036 Ω cm 2) across the GaN/AlInN/AlN/Si layer structure.

Demonstration of planar thick InP layers by selective MOVPE

We studied the selective metal-organic vapor-phase epitaxy of thick InP bulk layers. The work focused on the obtention of planar thick layers by an adjustment of the growth conditions. We showed that reduced pressure and temperature in the reactor allowed to decrease the sharpness of the thickness profiles in the vicinity of the mask. This approach is consistent with the vapor phase diffusion model and the kinetic theory. Thick InP layers generally show huge overgrowths at the edges of the dielectric stripes. These overgrowths were suppressed by reducing the growth rate. All samples’ thickness profiles were characterized by means of optical interferometer microscopy and surface profiler. Scanning electronic microscopy was used in the observation of the edge overgrowths and highlighted the complexity and anisotropy of the growth at these edges.

Wavelength Switching Using GaInAs/InP MQW Variable Refractive-Index Arrayed Waveguides by Thermo-Optic Effect

We have successfully demonstrated a GaInAs/InP multiple quantum well (MQW)-based wavelength switch composed of the straight arrayed waveguide with linearly varying refractive index distribution by changing the refractive index using thermo-optic effect. Since optical path length differences between waveguides in the array were achieved through refractive index differences that were controlled by SiO2 mask design in selective metal-organic vapor phase epitaxy (MOVPE), wavelength demultiplexing, and the output port switching in each wavelength of light by the refractive index change in the array waveguides through the thermooptic effect were achieved. We have obtained the wavelength switching and the change of transmission spectra in each output ports.

Series resistance in a GaN/AlGaN/n‐Si structure grown by MOVPE

GaN film was grown on a (111)Si substrate using AlGaN intermediate layer by selective metalorganic vapor phase epitaxy. The series resistance of the GaN/AlGaN/Si diode was evaluated as a function of the thickness and the alloy composition of the AlGaN intermediate layer. The resistance was decreased by reducing the thickness of the intermediate layer but showed a minimum value. The minimum resistance was evaluated as a function of the alloy composition. Assuming an exponential increase of the minimum resistance as a function of the conduction band offset in the AlGaN/Si interface, the resistance for a GaN/Si hetero interface was estimated to be 0.0018 Ω cm2. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Al doping in (1−101)GaN films grown on patterned (001)Si substrate

The effect of Al doping on crystalline and optical properties of semipolar (1−101)GaN was investigated. The samples were grown on a patterned (001)Si substrate by selective metal-organic vapor phase epitaxy. The x-ray analyses showed that the strain in the (1−101)GaN layer is reduced substantially by the Al doping. Moreover, the cathode-luminescence (CL) intensity of the band edge emission band was enhanced and the linewidth became narrow. The CL images showed the reduction of dislocation density. These results show that the small amount of Al atoms in GaN improves the crystalline and optical properties. The results are attributed to the dislocation pinning and solution hardening effect due to Al atoms.

MOVPE growth and cathodoluminescence properties of GAN microcrystal co-doped with Zn and Si

A (1 1 1) Si micro-facet was fabricated by KOH anisotropic etching on a (0 0 1) Si substrate. On the (1 1 1) Si facet, a GaN:Zn, Si-microcrystal was grown by selective metal-organic vapor phase epitaxy using an AlN intermediate layer and covered with an AlN capping layer. The size of the AlN/GaN/AlN microcrystal was determined by that of the (1 1 1) Si facet on the substrate. The optical properties of sample were evaluated by cathodoluminescence (CL) measurement. The CL intensity of DAP emission band was enhanced by reducing the crystal size. On the other hand, in the case of samples without AlN capping layer, the CL intensity was decreased with the smaller crystals. This result suggests that the optical confinement does enhance the CL intensity in the AlN/GaN/AlN sample.

Mg doping in (1 1¯ 0 1)GaN grown on a 7° off-axis (0 0 1)Si substrate by selective MOVPE

Mg doping was attempted in (1 1¯ 0 1)GaN grown on a patterned (0 0 1)Si by selective metal-organic vapor phase epitaxy (MOVPE). The source material for the Mg doping was EtCp 2Mg and the electrical properties were studied on samples with different doping levels. All samples showed p-type conduction. However, the hole concentration was decreased by the Mg doping at low doping levels followed by an increase at high doping levels. The activation energy was around 106–130 meV depending on the doping level, which was larger than that found in undoped or carbon-doped samples. The AFM images showed gradual change in the surface structure in accordance with the Mg-doping level. The variation of the optical spectra was discussed in relation to the Mg-doping levels.

Near-red emission from site-controlled pyramidal InGaN quantum dots

We have fabricated InGaN nanostructures on top of GaN hexagonal pyramids by selective metalorganic vapor-phase epitaxy. With this approach, we are able to exactly control the position of the emitting quantum dot, which is an essential requirement for functionalized single-photon emitters. The emission properties as well as the relaxation and recombination mechanisms were investigated using spectroscopic methods. Regions of different confinement were identified, with the photoluminescence emission from the InGaN quantum dots around 2.03eV and a decay time of 1.4ns. The constant temperature behavior of the radiative decay time confirms its zero-dimensional character. Spatially resolved cathodoluminescence measurements attribute this emission to the apex of the pyramid.

Four-channel 10-Gb/s operation of AlGaInAs-MQW-BH-DFB-LD array for 1.3-/spl mu/m CWDM systems

We have developed a four-channel AlGaInAs multiple quantum well, buried heterostructure, distributed feedback laser diode (MQW BH-DFB-LD) array for 1.3-/spl mu/m coarse wavelength division multiplexing (CWDM) systems. Fine detuning control by using a narrow-stripe selective metalorganic vapor-phase epitaxy (MOVPE) technique and an electron beam (EB) lithography technique enables 10-Gb/s operations in all channels at 85/spl deg/C. We obtained throughputs of more than 40 Gb/s in total. We confirmed low crosstalk operations for a 250-/spl mu/m pitch array. This is an attractive feature for fabricating compact transmitter modules.

Wavelength Demultiplexing and Optical Deflection in Variable Refractive-Index Waveguide Array Based on Selectively Grown GaInAs/InP MQW Structure

A GaInAs/InP multiple quantum well (MQW)-based wavelength demultiplexer composed of a waveguide array in which the refractive index varies across the array yielded successful results of wavelength demultiplexing and optical deflection. Since optical path length differences between waveguides in the array are achieved through refractive-index differences controlled by the SiO 2 mask design in selective metal-organic vapor phase epitaxy (MOVPE), a straight waveguide grating can be formed with reduced optical propagation losses. A straight waveguide array device with a 1.4% refractive-index difference was fabricated. The fabrication of a preliminary wavelength demultiplexer was also achieved, for which a wavelength separation with an approximately 25 nm spacing and free spectral range (FSR) of approximately 100 nm were obtained. Moreover, an optical deflector was investigated and primitive deflection was achieved at 1460 and 1490 nm incident wavelengths.

Wavelength demultiplexer using GaInAs-InP MQW-based variable refractive-index arrayed waveguides fabricated by selective MOVPE

A GaInAs-InP multiple quantum well (MQW)-based wavelength demultiplexer composed of an arrayed waveguide in which the refractive index varies across the array was fabricated. Since optical path length differences between waveguides in the array are achieved through refractive-index differences that are controlled by SiO/sub 2/ mask design in selective metal-organic vapor phase epitaxy (MOVPE), straight waveguide gratings having reduced optical propagation losses can be achieved. Furthermore, by employing MQW waveguides, variations in the refractive index may be induced through an applied electric field, allowing the device to manipulate wavelengths dynamically. A straight arrayed waveguide device having a 1.4% difference in refractive index was fabricated using an asymmetric side mask via a single selective MOVPE growth. The achievement of a diffraction angle difference of 4.40/spl deg/ between wavelengths of 1520 and 1580 nm was confirmed experimentally. In addition, a preliminary wavelength demultiplexer with a wavelength separation of approximately 25 nm and a free spectral range (FSR) of approximately 100 nm was also fabricated.

1.3-/spl mu/m AlGaInAs strain compensated MQW-buried-heterostructure lasers for uncooled 10-gb/s operation

We have successfully fabricated 1.3-/spl mu/m AlGaInAs strain-compensated multiple-quantum-well (MQW) buried-heterostructure (BH) lasers by narrow-stripe selective metalorganic vapor-phase epitaxy. Based on the optimization of AlGaInAs strain compensated MQW and the Al-oxidation-free BH process, we obtained a low-threshold current of 12.5 mA and a relaxation frequency of more than 10 GHz at 85/spl deg/C for Fabry-Perot lasers. For distributed feedback lasers, we demonstrated a 10-Gb/s operation and transmission of over 16 Km for a single mode fiber at 100/spl deg/C. Furthermore, a record-low 25.8-mA/sub p-p/ modulation current for a 10-Gb/s modulation at 100/spl deg/C was demonstrated with shorter cavity and high grating-coupling coefficient. A median life of more than 1/spl times/10/sup 5/ h at 85/spl deg/C was estimated after an aging test of over 5000 h for these lasers. These superior characteristics at high temperatures were achieved by the combination of the high differential gain of AlGaInAs strain compensated MQW and the BH structure.

Optical and electrical properties of (1-101)GaN grown on a 7° off-axis (001)Si substrate

Uniform growth of (1-101)GaN was performed on coalesced stripes of GaN which had been prepared by selective metalorganic vapor phase epitaxy on a 7° off-axis (001)Si substrate via an AlN intermediate layer. The cathodoluminescence spectra at 4 K exhibited a donor bound excitonic emission at 358 nm followed by defect-related emission peaks at 363, 371, and 376 nm. The 363 and 376 emission bands are observed upon the coalescence region. The Hall measurements exhibited p-type conduction at 80–300 K (the hole carrier density 6.3×1012 cm−2 and hole mobility 278 cm2/V s at 100 K). The activation energy of the acceptor was estimated to be 60 meV. The possible origin of the p-type conduction is discussed in relation to the unintentionally doped carbon.

The compositional non‐uniformity in an AlGaN capping layer of AlGaN/GaN pyramid grown on a (111) Si substrate by selective MOVPE

The alloy composition of an AlGaN film grown on a GaN hexagonal pyramidal structure is studied by cathode luminescence spectroscopy. The samples are made by selective area growth mode of metal organic vapour phase epitaxy (MOVPE). It was found that the Al composition is lower than the nominal composition at the bottom of the pyramid, while it was higher than the nominal composition near the top of the pyramid. Analysing the composition as a function of the position on the facets, the diffusion length of Ga was estimated to be 310 nm and 915 nm on the (0001) and facet, respectively. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)