Melt-back Etching Research Articles

Impact of buffer growth on crystalline quality of GaN grown on Si(111) substrates

AbstractIn this work we focus on the impact of different buffer and nucleation layers on the corresponding crystalline quality of gallium nitride (GaN), grown by metal‐organic chemical vapour deposition (MOCVD). In situ curvature measurements, X‐ray diffraction (XRD) and transmission electron microscopy (TEM) are used for advanced characterization. The influence of various growth modes on meltback etching and cracking is analyzed. Also the effect of a silicon nitride (SiN) mask on the growth of GaN and its coalescence is investigated. Furthermore, associated potential consequences for the growth of aluminium nitride (AlN) interlayers are examined to obtain a homogeneous surface without cracks and with good crystalline quality. Our studies indicate that an incomplete coalesced GaN surface located underneath the AlN interlayer leads to an increased defect density. Additionally we studied the influence of growth temperature at nucleation on the material quality and the process stability. Finally we demonstrate interlayer induced compressive strain during GaN growth with an XRD rocking‐curve full width half maximum (FWHM) as low as 450 arc seconds for the $10{\bar {1}}2$ reflection.

AlGaN/GaN-heterostructures on (111) 3C-SiC/Si pseudo substrates for high frequency applications

We present the realization of high electron mobility transistors (HEMTs) based on AlGaN/GaN heterostructures, which were grown on silicon substrates using an ultrathin SiC transition layer. The growth of AlGaN/GaN heterostructures on 3C-SiC(111)/Si(111) was performed using metalorganic chemical vapour deposition (MOCVD). The 3C-SiC(111) transition layer was realized by low pressure CVD and prevented Ga-induced meltback etching and Si-outdiffusion in the subsequent MOCVD growth. The two-dimensional electron gas (2DEG) formed at the AlGaN/GaN interface showed an electron sheet density of 1.5 × 10 13 cm − 3 and a mobility of 870 cm 2/Vs. The HEMTs DC and RF characteristics were analysed and showed a peak cut-off frequency as high as 29 GHz for a 250 nm gate length.

Reduction of threading dislocations in GaN on in-situ meltback-etched Si substrates

We report a novel growth technique of GaN films on Si substrates using a metalorganic chemical vapor deposition. First, Ga droplets are deposited on a Si substrate by feeding trimethylgallium. And then the substrate is heated at 1080 °C, resulting in the formation of recesses on its surface by meltback etching. Finally, a GaN film is grown on the Ga-induced meltback-etched surface using a high-temperature-grown AlN intermediate layer. After the growth of the GaN film, 0.5–1-μm-diameter pits were observed on the GaN surface. A cathodoluminescence image reveals that low-threading-dislocation-density regions were successfully grown around the pits.

2H-AlGaN/GaN HEMTs on 3C-SiC(111)/Si(111) Substrates

We present the realization of high electron mobility transistors (HEMTs) based on AlGaN/GaN heterostructures grown on silicon substrates using a SiC transition layer. The growth of AlGaN/GaN heterostructures on Si (111) was performed using metalorganic chemical vapour deposition (MOCVD). The (111) SiC transition layer was realized by low pressure CVD and prevented Ga-induced meltback etching and Si-outdiffusion in the subsequent MOCVD growth. The two-dimensional electron gas (2DEG) formed at the AlGaN/GaN interface showed an electron sheet density of 1.5x1013 cm-3 and a mobility of 870 cm²/Vs proving the high structural quality of the heterostructure. Device processing was done using electron beam lithography. DC and RF characteristics were analysed and showed a peak cut-off frequency as high as 6 GHz for a 1.2 µm gate HEMT.

Growth of low defect density GaP layers on Si substrates within the critical thickness by optimized shutter sequence and post-growth annealing

We have developed a growth procedure for realizing a low defect density GaP layer on an Si substrate. The growth procedure consists of two parts. One is the post-growth annealing for the annihilation of stacking faults (SFs). We have investigated an annihilation mechanism with molecular beam epitaxy grown GaP layers. 1-monolayer-thick SFs typically generate from the GaP/Si interface in a non-annealed GaP layer. In a 700 °C annealed GaP layer, generation points of these SFs tend to shift toward the GaP surface. In a 730 °C annealed GaP layer, SFs density is effectively decreased. These results suggest that SFs are annihilated through the climb motion of two partial dislocations during the post-growth annealing. Another one is the optimized shutter sequence for migration enhanced epitaxy. We have revealed that it is effective for the suppression of both three-dimensional growth and melt-back etching to increase in a stepwise manner the number of supplied Ga atoms per cycle. As a result, the generation of threading dislocations and pits is remarkably suppressed. A root mean square surface roughness of 0.13 nm is obtained within the critical thickness. We have estimated etch pit density (EPD) to be ∼7×10 5 cm −2 with a GaPN/GaP/Si structure. To the best of our knowledge, this value is same as that of commercially available GaP substrates and is the lowest one in the EPD of GaP/Si heteroepitaxy.

The influence of the Al pre-deposition on the properties of AlN buffer layer and GaN layer grown on Si (1 1 1) substrate

The influence of Al pre-deposition on the properties of AlN buffer layer and GaN layer grown on Si (1 1 1) substrate by metalorganic chemical vapor deposition (MOCVD) has been systematically studied. Compared with the sample without Al pre-deposition, optimum Al pre-deposition time could improve the AlN buffer layer crystal quality and reduce the root mean square (RMS) roughness. Whereas, overlong Al-deposition time deteriorated the AlN crystal quality and Al-deposition patterns could be found. Cracks and melt-back etching patterns appeared in the GaN layer grown without Al pre-deposition. With suitable Al-deposition time, crack-free 2.0 μm GaN was obtained and the full-width at half-maximum (FWHM) of (0 0 2) plane measured by double crystal X-ray diffraction (DCXRD) was as low as 482 arcsec. However, overlong Al-deposition time would result in a great deal of cracks, and the crystal quality of GaN layer deteriorated. The surface of GaN layer became rough in the region where the Al-deposition patterns were formed due to overlong Al-deposition time.

Growth of pit-free GaP on Si by suppression of a surface reaction at an initial growth stage

We investigated the surface morphology of 50-nm-thick GaP layers grown on Si substrates, by atomic force microscopy. Pits with a density of about 10 8 cm −2 were observed at the surface of GaP layers grown by conventional migration-enhanced epitaxy (MEE). The pit diameter increased with increasing the growth temperature in the range from 440 °C up to 540 °C. Transmission electron microscope (TEM) observations revealed that the origin of the pits was the melt-back etching of the Si surface by Ga droplets. The Ga droplets are thought to form during the initial MEE growth on a P-prelayer. By changing the conventional MEE growth mode into molecular beam epitaxy growth mode for the initial 2 monolayers, the melt-back etching was suppressed. As a result, a pit-free GaP layer was successfully grown on a Si substrate with a root-mean-square surface roughness of 0.2 nm. In addition, no stacking faults were observed in cross-sectional TEM images. Finally, anti-phase domains completely self-annihilated within 10 nm from the interface.

Thermal stability of metal organic vapor phase epitaxy grown AlInN

AlInN layers with a thickness of 100nm were grown by metal organic vapor phase epitaxy on GaN buffer layers on Si(111) substrates. By varying the growth temperature, In and NH3 flows, and reactor pressure, three series with different In contents were produced and thermally treated in the temperature range from 30to960°C. The as grown and annealed layers were investigated by x-ray diffraction in standard and grazing incidence geometry. Nearly lattice matched samples with an indium concentration of 17%–18% show long time stability at annealing temperatures as high as 960°C. At higher temperatures, the onset of severe Ga–Si meltback etching prevents further measurements. Nonlattice matched samples consist of pseudomorphic and relaxed parts. In the latter, a redistribution and loss of indium is observed upon annealing.

Growth of crack-free GaN on Si(1 1 1) with graded AlGaN buffer layers

Hexagonal GaN films on Si(1 1 1) substrates have been grown by metalorganic vapor phase epitaxy (MOVPE). High-temperature AlN buffers provided a reliable diffusion barrier to avoid meltback etching. By introducing a thick, graded AlGaN buffer layer, the critical thickness for cracking has been increased to at least 2 μ m . The films have been characterized by optical microscopy, transmission electron microscopy (TEM) and X-ray diffraction.

Growth and characterization of In 0.28Ga 0.72N/GaN multiple-quantum wells on Si(1 1 1)

We report on the growth and characterization of In 0.28Ga 0.72N/GaN multiple-quantum wells (MQW) on GaN/Si(1 1 1) by metalorganic vapor phase epitaxy. First, the GaN epitaxy was grown on Si(1 1 1) using AlGaN/AlN composite buffer layer. Even though the optimized AlGaN/AlN CBL improved the quality of GaN on Si and reduced melt-back etching during growth, the surface image of GaN epitaxy observed by SEM showed some cracks, which were resulted from the large difference in thermal expansion coefficient between GaN and Si. The full-width at half-maximum (FWHM) of the double-crystal X-ray rocking curve for GaN(0 0 0 2) was 886 arcsec. Cross-sectional transmission electron microscopy (XTEM) shows that the predominant crystallographic defects in the GaN are threading dislocations. Next, In 0.28Ga 0.72N/GaN MQW structures were grown on this high-quality GaN/Si(1 1 1). From the XTEM image, it was found that the typical V-defects of uncapped In 0.28Ga 0.72N/GaN MQW structure originate from threading dislocations. Indium-rich InGaN precipitates were readily detected in the In 0.28Ga 0.72N/GaN MQW by high-resolution transmission electron microscopy image. The photoluminescence (PL) spectrum at room temperature from the In 0.28Ga 0.72N/GaN MQW on Si(1 1 1) was separated into two peaks of strong intensity, whose FWHMs are 22 nm for the peak at 500 and 24 nm for the peak at 525 nm. The PL peak at longer wavelength side is believed to come from the In-rich phases by the interdiffusion of indium in the InGaN well layers.

Surface irregularities of MBE grown cubic GaN layers

Cubic GaN layers are grown by molecular beam epitaxy on (0 0 1)GaAs substrates. The influence of intentional deviations from stoichiometric growth conditions on the structural homogeneity of the epitaxial layers and the GaN/GaAs interface was studied. Optical micrographs and AFM-images of the epilayers grown in a Ga-stabilised regime reveal the existence of different types of surface irregularities. We conclude that the irregularities observed are the result of successively melt-back etching in GaN and GaAs and solution growth within Ga-droplets due to the change of the saturation conditions of the liquid Ga-phase on the surface of the growing film.

Thermal stability of GaN on (1 1 1) Si substrate

We studied thermal annealing effects of low-temperature-grown GaN (LT-GaN) on Si substrate by use of the two step growth technique. Difficulties were encountered when LT-GaN films were annealed up to the temperature of 1050°C on Si substrate. However, the LT-GaN on sapphire was stable after the same annealing condition. The decomposition of LT-GaN and the wavy surface were observed. Thus, thermal stability of LT-GaN on Si was lower than that on sapphire. To improve this low thermal stability, LT-GaN on Si was coated by middle-temperature-grown GaN (MT-GaN). However, swellings were observed on the surface after annealing to the temperature of 1050°C and there were hollows, which indicated that the corrosion of Si occurred below the temperature of the melting point of Si, under swellings. Energy dispersive X-ray (EDX) analysis indicated that a large amount of Ga was detected in swellings. Ga is responsible for the corrosion of Si substrate and the possible reason for the formation of hollows was the meltback etching of Si by Ga.

Selective meltback etching of GaN layers in liquid-phase electroepitaxial technique

Abstract Meltback etching and selective meltback etching of GaN layers have been achieved using both liquid-phase epitaxy (LPE) and liquid-phase electroepitaxy (LPEE) techniques with GaAs solutions. Both the uniformity and the etching rate (∼ 2.4 μm/h) were improved using the LPEE technique.

Melt-back etching of GaN

Melt-back etching of GaN is proposed as a novel etching technique and is found to be prospective. The highest etching rate was 1.2 μm/h at a temperature of 1050°C. The etched surface was as good as the surface of an as-grown GaN crystal when the etching temperature is equal to or lower than 900°C, providing a practical etching rate of 0.1 μm/h.

Meltback etching and regrowth of GaAs/AlGaAs in liquid phase epitaxy for fabrication of microlens

Meltback etching was performed both on planar and selective areas. In the case of planar etching, we could see the effect of convection. Such a tendency was observed also for selective etching when the degree of undersaturation was large. By varying the mask opening area, composition of the melt, and etching time, a precise control of the etched shape was possible and a hemispherical shape was obtained. Large anisotropic meltback behavior was apparent when the melt contained no aluminium content. But as the added amount of aluminium was increased, the etched shape became circular. Regrowth characteristics showed gallium capturing phenomenon when the amount of supersaturated arsenic was large. By reducing the supersaturated arsenic content, a successful growth could be obtained.

Fabrication and characterization of lateral InP/InGaAsP heterojunctions and bipolar transistors

We have investigated the fabrication of lateral InP/InGaAsP heterojunctions using both wet chemical and in situ melt-back etching and regrowth to form the device junctions. The current/voltage characteristics of the melt-back-etched and regrown heterojunctions exhibit ideality factors as low as 1.25. In addition, we have fabricated lateral heterojunction bipolar transistors with 2 μm base widths which exhibit a current gain of 6. These results indicate that regrown heterojunctions have adequate injection efficiency to form the active region of devices.